Enhancing the Flux Pinning of High Temperature

ENHANCING THE FLUX PINNING OF HIGH TEMPERATURE

SUPERCONDUCTING YTTRIUM BARIUM COPPER OXIDE THIN FILMS

Dissertation

Submitted to

The School of Engineering of the

UNIVERSITY OF DAYTON

In Partial Fulfillment of the Requirements for

The Degree of

Doctor of Philosophy in Engineering

Mary Ann Patricia Sebastian, M.S.

UNIVERSITY OF DAYTON

Dayton, OH

August 2017

ENHANCING THE FLUX PINNING OF HIGH TEMPERATURE

SUPERCONDUCTING YTTRIUM BARIUM COPPER OXIDE THIN FILMS

Name: Sebastian, Mary Ann Patricia

APPROVED BY:

______P. Terrence Murray, Ph.D. Timothy J. Haugan, Ph.D. Advisory Committee Chair Committee Member Professor Research Physicist Chemical and Materials Engineering AFRL/RQQM

______Daniel P. Kramer, Ph.D. Christopher Muratore, Ph.D Committee Member Committee Member Professor Professor Chemical and Materials Engineering Chemical and Materials Engineering

______Robert J. Wilkens, Ph.D., P.E. Eddy M. Rojas, Ph.D., M.A., P.E. Associate Dean for Research and Innovation Dean, School of Engineering Professor School of Engineering

Mary Ann Patricia Sebastian

2017

iii ABSTRACT

ENHANCING THE FLUX PINNING OF HIGH TEMPERATURE

SUPERCONDUCTING YTTRIUM BARIUM COPPER OXIDE THIN FILMS

Name: Sebastian, Mary Ann Patricia University of Dayton

Advisor: Dr. P. Terrence Murray, Ph.D.

Superconductors’ unique properties of zero resistance to direct current at their critical temperatures and high current density have led to many applications in communications, electric power infrastructure, medicine, and transportation. Yttrium barium copper oxide, YBa2Cu3O7-δ, (YBCO) is a Type II superconductor, whose thin film’s high current density results from pinning centers associated with point defects from oxygen vacancies, and with twin and grain boundaries. Addition of second-phase inclusions enhances flux pinning and current density by incorporating additional pinning centers. This research systematically studies the effect of nanoparticle pinning with the addition of an insulating, nonreactive phase of Y2BaCuO5 (Y211). While many previous studies focused on single phase additions, the addition of several phases simultaneously shows promise in improving current density by combining different pinning mechanisms.

This research systematically studies the following mixed phase additions to YBCO targets to produce thin films by pulsed laser deposition (PLD): YBCO + BaZrO3 + Y2O3,.

iv YBCO + BaHfO3 + Y2O3, YBCO + BaSnO3 + Y2O3, and YBCO + BaSnO3 + Y211. Thin films are prepared by pulsed laser deposition on LaAlO₃ and SrTiO₃ substrates

Processing parameters vary the volume percent of dopants present in the target and the deposition temperatures of the films to optimize critical current densities. Results and comparisons of flux pinning mechanisms, current densities, critical temperatures, and microstructures will be presented in detail. In short, the 10 vol. % Y211 doped YBCO films achieved the highest current density, and coincidently also possessed the least amount of lattice mismatch and the least amount of difference of thermal expansion coefficients between the dopant and YBCO. Mathematical modeling will address the strong anisotropic and weak isotropic flux pinning contributions of the doped YBCO films. The Y211 doped YBCO films were the only dopant system studied which increased both the isotropic weak and anisotropic strong flux pinning contributions. This work contributes to a greater understanding for future optimizations of YBCO doped films with pinning landscapes tailored for high current and high field applications at various field orientations.

Dedicated to my family

vi ACKNOWLEDGEMENTS

I would like to express my special thanks to my committee chair, Dr. Paul T.

Murray, for all of his guidance and encouragement advising me during this dissertation journey. Special thanks are also in order for all of my committee members for their time and dedication: Dr. Timothy Haugan, Dr. Daniel Kramer, and Dr. Christopher Muratore.

I also would like to express my appreciation to Dr. Daniel Eylon, whose advice started me on this journey. A big thank you also to Dr. Paul Barnes and Dr. Timothy Haugan, for the opportunity to research superconductors at Air Force Research Laboratory/

RQQM, and to Dr. Haugan for all of his mentorship and wisdom on the subject of superconductivity the past several years. Many thanks also to contractors from UDRI:

Mr. Charles Ebbing and Mr. John Murphy, for all of their technical expertise and assistance in keeping the lab running smoothly. Special thanks to my co-workers at

AFRL/RQQM, who offered research advice through the years: namely Dr. Thomas

Bullard for his superconductivity flux pinning expertise, Dr. Michael Susner for assistance with XRD, and Dr. George Panasyuk for his contribution with the mathematical modeling. I would also like to recognize several groups who aided in the characterization studies for my research: specifically Dr. Haiyan Wang’s research team formerly from Texas A&M University, and now at Purdue University, for the high quality TEM contributions and collaboration (National Science Foundation DMR-

1565822); and from AFRL/RX MCF: Mr. Scott Apt and Mr. John Kelley for their

vii training in utilizing the Sirion for SEM, and Ms. Kathleen Shugart for her assistance with the Quanta and EDS analysis. Special thanks is also extended to Dr. Judy Wu from the

University of Kansas, for her advice and expertise in the field of superconductivity, and her research team for their collaboration with the angular current density data (ARO contract No. ARO-W911NF-16-1-0029, and NSF contracts Nos. NSF-DMR-1337737 and NSF-DMR-1508494). I would like to recognize that this research was funded by

AFRL/ Aerospace Systems Directorate and the Air Force Office of Scientific Research

(LRIR No. 14RQ08COR). Lastly, but most importantly, I would like to thank God, for the gift of life and all the blessings it has entailed; my parents for instilling in me the love of learning; my husband and love of my life, Mark, for all of his continued support, encouragement and love; and my children for their love and sacrifice of some “mommy time” the last few years.

viii

TABLE OF CONTENTS

ABSTRACT ...... iv

DEDICATION ...... vi

ACKNOWLEDGEMENTS ...... vii

LIST OF FIGURES ...... xii

LIST OF TABLES ...... xviii

LIST OF ABBREVIATIONS / SYMBOLS ...... xx

CHAPTER I. PROPOSAL ...... 1

CHAPTER II. HISTORY OF SUPERCONDUCTIVITY ...... 5

Summary ...... 5

Discovery of Superconductivity ...... 6

Superconductivity Theory ...... 11

Quest for New Superconductors ...... 16

Superconductor Applications ...... 20

CHAPTER III. THIN FILM BACKGROUND INFORMATION ...... 28

Summary ...... 28

Thin Film Growth ...... 30

Pulsed Laser Deposition ...... 31

Film Growth Mechanism ...... 33

Lattice Mismatch ...... 35

XRD Analysis: Grain Size, Grain Orientation, Film Stresss ...... 41

Film Strain, Flux Pinning, & Current Density Relationship ...... 45

CHAPTER IV. SUMMARY OF PAST RESEARCH & BASIS OF FUTURE

RESEARCH ...... 46

Summary ...... 46

CHAPTER V. EXPERIMENTAL PROCEDURES ...... 53

Summary ...... 53

Target Preparation ...... 53

Substrate Preparation ...... 57

Pulsed Laser Deposition (PLD) ...... 58

Current Density Measurements and Critical Temperatures ...... 60

Film Characterization ...... 62

Scanning Electron Microscope (SEM) ...... 62

..... Transmission Electron Microscope (TEM) ...... 62

X-ray Diffraction (XRD) ...... 62

Profilometer ...... 62

CHAPTER VI. RESULTS ...... 63

1. .... Y211 DOPED YBCO FILMS ...... 63

Summary ...... 63

2. .... BZO + Y2O3 DOPED YBCO FILMS ...... 76

Summary ...... 76

3. .... BHO + Y2O3 DOPED YBCO FILMS ...... 87

Summary ...... 87

4. ... BSO + Y2O3 DOPED YBCO FILMS ...... 95

Summary ...... 95

5. .... BSO + Y211 Doped YBCO Films ...... 103

Summary ...... 103

6. .... Pinning System Comparison and Current Density Mathematical Modeling .... 107

Summary ...... 107

CHAPTER VII. CONCLUSION AND FUTURE WORK ...... 117

REFERENCES ...... 120

LIST OF FIGURES

Figure 1. Resistance vs. Temperature for non-superconducting metal and a Type I Superconductor (4) ...... 10

Figure 2. Magnetic Field vs. Temperature for Type II Superconductor (5) ...... 11

Figure 3. Resistivity vs. Temperature for Type II Superconductor (6) ...... 11

Figure 4. Meissner Effect ...... 11

Figure 5. Superconductors and Their Critical Temperatures Dept. of Energy, Basic Energy Science ...... 19

Figure 6. Simple epitaxial alignments for cubic films (f) on a cubic substrate (s). (a) a0(f) = a0(s); (b) 2a0(f) = a0(s); (c) 21/2a0(f) = a0(s); (d) Illustration of tilted-layer epitaxy between film atoms (above) and vicinal planes of substrates (4) ...... 37

Figure 7. Schematic composite of crystal defects in epitaxial films. 1, Threading edge dislocations; 2, interfacial misfit dislocations; 3, threading screw dislocation; 4,growth spiral; 5, stacking fault in film; 6, stacking fault in substrate; 7, oval defect; 8, hillock; 9, precipitate or void (4)...... 39

Figure 8. Model of a polycrystalline thin film consisting of randomly oriented polygonal grains. Surface energies associated with the substrate interface, grain boundary and upper film surface are shown. (Bottom) Same film displaying preferred orientation or texture. Note: Dashed arrows represent a measure of crystallographic orientation (4)...... 40

xii Figure 9. (a) Flux line interaction with a columnar pin. (b) Cylindrical square model of potential vortex binding energy, where U varies from U0 to U(T) due to thermal fluctuations (33)...... 51

Figure 10. Lindberg box furnace utilized for drying powders and sintering target...... 55

Figure 11. Branson 3210 ultrasonic cleaner used in AFRL research laboratory...... 57

Figure 12. Heater block used in research...... 58

Figure 13. Excimer laser and pld chamber used in research...... 59

Figure 14. (a) in-house sputter-rig Kansan University, (b) Quantum Design Evercool II vsm-ppms used in current density measurements, (c) parallel bridge mask 20µm and 40µm width and 500µm length...... 60

Figure 15. Current density as a function of applied field: (a) YBCO with 10 vol. % Y211 measured at 65K for films deposited at deposition temperatures ranging from 775 – 840°C. (b) YBCO with 10 vol. % Y211 measured at 65 K for films deposited at repetition rates of 2, 4, and 6 Hz. (c) various vol. % Y211 additions at 65 K and 5 K (d) log-log plot of current density verses applied field at 65K for YBCO and 10 vol. % Y211 doped YBCO...... 67

Figure 16. (a) Current density for optimized 10 vol.% Y211 doped YBCO films measured at 65 K (blue curve), 50 K (red curve), 20 K (purple curve), and 5 K ( teal curve). YBCO curves are in black. (b) Corresponding pinning force curves. (c), (d) Angular current density for optimized 10 vol. % Y211

doped YBCO measured at 77K and 65K. Angular Jct courtesy of J. Wu’s research group University of Kansas...... 68

Figure 17. Tc-onset (K) vs. deposition temp. (˚C) for YBCO and vol. % Y211additions. . 68

Figure 18. (a) - (j): SEM images of YBCO and various volume % Y211 doped films on STO substrates at magnifications of 20K and 50K respectively: (a), (b) YBCO deposited at 790˚C. (c), (d) 5 vol. % Y211 deposited at 835 ˚C. (TJ2592A) (e), (f) 10 vol. % Y211 deposited at 835 ˚C (MR064A)...... 69

xiii

Figure 19. (a) - (d) TEM images of vol.% Y211 doped YBCO films: (a) - (d) 5 vol.% Y211 doped YBCO, (e) - (f) 10 vol.% Y211 doped YBCO, (g) - (h) Fourier transform analysis of 5 and 10 vol.% Y211 doped YBCO respectively. Courtesy of H. Wang’s research group Texas A & M University...... 71

Figure 20. (a) XRD 2Theta omega scans for 5, 10, and 15 vol. % Y211 doped YBCO films. (b) XRD rocking curve scans of YBCO (005) peak for 5, 10, and 15 vol. % Y211 doped YBCO films...... 73

Figure 21. Comparison of current density verses various volume % of BZO and Y211 doped YBCO films produced at AFRL/RQQM, WPAFB...... 74

Figure 22. Current density as a function of applied field measured at 77, 65, 20, and

5K for YBCO100-(x+3) BZO x (x = 2, 4, 6 vol.%) Y2O3 = 3 vol.% at: (a), (b), (c) deposition temperature of 810 ˚C, (d), (e), (f) deposition temperature of 825˚C. (g) current density as a function of applied field measured at 65 and 5 K for single doped 2 vol.% BZO and YBCO film and double doped 2

vol.% BZO + 3 vol.% Y2O3 YBCO film (STO substrate and 825˚C deposition temp). Note * signifies matching field for single doped film. (h) pinning force curves for optimized 2 vol.% BZO + 3 vol.% Y2O3 doped YBCO filmat 65, 50, 20, and 5K and for YBCO film shown as black curve...... 80

Figure 23. Angular dependence of current density measured at (a) 77K and (b) 65K at

1T, 3T, 5T, and 9T for 2 vol. % BZO + 3 vol. % Y2O3 doped YBCO film...... 81

Figure 24. Critical temperatures (Tc) for 2, 4, and 6 vol. % BZO with 3 vol. % Y2O3 doped YBCO films, deposited at 810 ˚C and 825 ˚C. Reference Tc for undoped YBCO films shown as black circles...... 81

Figure 25. (a) XRD 2Theta omega scans for 2, 4, and 6 vol. % BZO with 3 vol. %

Y2O3 doped YBCO films. (b) XRD rocking curve scans of YBCO (005) peak for 2, 4, and 6 vol. % BZO with 3 vol. % Y2O3 doped YBCO films...... 82

Figure 26. SEM 20kX and 50kX respectively: (a), (b) YBCO (c) (d) 2 vol.% BZO + 3

vol % Y2O3 doped YBCO on STO substrate at 810 ˚C deposition temperature;

xiv

(e), (f) 2 vol.% BZO + 3 vol.% Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature; (g), (h) 2 vol. % BZO doped YBCO on STO substrate at 825 ˚C deposition temperature...... 84

Figure 27. SEM 20kX and 50kX respectively: (a), (b) 4 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 810 ˚C deposition temperature. (c), (d) 4

vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature...... 85

Figure 28. SEM 20kX and 50kX respectively: (a), (b) 6 vol.% BZO + 3 vol.% Y2O3 doped YBCO on STO substrate at 810 ˚C deposition temperature; (c), (d) 6

vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature...... 85

Figure 29. TEM intermediate and high resolution images respectively:

(a), (b) 2 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature. (c), (d) 4 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature. Courtesy of H. Wang’s research group Texas A & M University. (e) 4 vol. % BZO +

3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature. (f) 6 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature. (e) and (f) courtesy of Nanjing University in collaboration with University of Kansas...... 86

Figure 30. Current density versus applied field: (a), (b) for 4 vol.% BHO + 3 vol. % Y2O3 doped YBCO thin films on LAO substrate at various deposition temperatures, measured at 65K and 5K, (c) for various percentages of BHO + 3 vol. % Y2O3 doped YBCO thin films on STO substrates at810˚C deposition temperature, measured at 65K and 5K, (d) for optimized 2 vol. % BHO + 3 vol.% Y2O3 doped YBCO thin films measured at 65K (blue curve), 50K (red curve), 20K (purple curve), and 5K (teal curve). YBCO current density curve at 65K is in black. (e) corresponding pinning force curves for optimized 2 vol. % BHO + 3 vol. % Y2O3 doped YBCO film. (f) current density vs. applied field for 2 vol. % BHO doped YBCO films (red curves) and for 2 vol. % BHO + 3 vol. % Y2O3 (blue curves) at 65K and 5K. . 90

Figure 31. (a) XRD 2Theta omega scans for 2, 4, and 6 vol. % BHO with 3 vol. %

Y2O3 doped YBCO films. (b) XRD rocking curve scans of YBCO (005) peak for 2, 4, and 6 vol. % BHO with 3 vol. % Y2O3 doped YBCO films...... 91

Figure 32. SEM images 20kX and 50kX respectively: (a),(b) 2 vol.% BHO + YBCO film, STO substrate, 810 ˚C deposition temp.; (c),(d) 2 vol.% BHO + 3

vol.% Y2O3 + YBCO films, STO substrate, 810 ˚C deposition temp.; (e), (f) 4 vol.% BHO + 3 vol.% Y2O 3+ YBCO films, STO substrate, 810 ˚C deposition temp.; (g),(h) 6 vol.% BHO + 3 vol.% Y2O3 + YBCO films, STO substrate, 810 ˚C deposition temp.; (i),(j) 4 vol. % BHO + 3 vol. %

Y2O3 + YBCO films, LAO substrate, 825 ˚C deposition temp...... 93

Figure 33. TEM images of 2 vol. % BHO + 3 vol. % Y2O3 + YBCO on STO substrate at 810 ˚C deposition temperature (a) bright field, (b) dark field. Courtesy H. Wang Group Texas A & M...... 94

and 5K for YBCO1-x-y BSOx (Y2O3)y x=vol.%, y=3vol.% and 795˚C deposition temperature, (d) Current density as a function of applied field

comparison of single doped BSO and double doped BSO + Y2O3YBCO films measured at 65K. Note matching field peak at 3T in both single doped and DD film.10 mol % BSO is 4 vol. % BSO ...... 97

Figure 35. (a) Current density for optimized 5 vol. % BSO + 3 vol.% Y2O3 doped YBCO films measured at 65 K (blue curve), 50 K (red curve), 20 K (purple curve), and 5 K ( teal curve). YBCO curves are in black. (b) Corresponding pinning force curves...... 98

Figure 36. Tc-onset (K) versus deposition temperature (°C) for YBCO1-x-yBSOx (Y2O3)y, x = vol. %, y= 3 vol. %...... 98

Figure 37. SEM images of YBCO+M films at magnification of 20K: (a) YBCO

deposited at 775 ⁰C, (b). 5 % BSO + 3 % Y2O3 deposited at 795 ⁰C...... 100

Figure 38. TEM of YBCO doped film with 5 vol. % BSO +3 vol. % Y2O3 deposited at 795 ⁰C ...... 100

xvi

Figure 39. (a) XRD 2Theta omega scans for 2, 4, and 6 vol. % BSO with 3 vol. %

Y2O3 doped YBCO films. (b) XRD rocking curve scans of YBCO (005) peak for 2, 4, and 6 vol. % BSO with 3 vol. % Y2O3 doped YBCO films...... 101

Figure 40. (a) Current density as a function of applied field for YBCO with 10 vol. % BSO + 3 vol. % Y211 measured at 65 K for various deposition temperatures, (b) Current density as a function of applied field for YBCO with 10 vol. % BSO + 3 vol. % Y211 at 790 ˚C deposition temperature, measured at 65 K (blue), 50 K (red), 20 K (purple), and 5 K (teal), (c)

Tc-onset (K) versus deposition temperature (°C) for YBCO with 10 vol. % BSO + 3 vol. % Y211...... 105

Figure 41. UHR SEM 20kX and 50kX for film deposited with 10 vol. % BSO + 3 vol. % Y211 doped YBCO target on LAO substrate at deposition temperature of 790 ˚C...... 106

Figure 42. TEM for 10 vol. % BSO + 3 vol. % Y211 doped YBCO film on LAO deposited at 790 ˚C. Courtesy of H. Wang’s research group at Texas A & M University. White = LAO, Red = BSO, Blue = YBCO, Yellow = Y211...... 106

Figure 43. Current density vs. applied field for films of YBCO, 10 vol.% Y211, 5

vol.% BSO + 3 vol.% Y2O3, 10 vol.% BSO + 3 vol.% Y211, 2 vol.% BZO + 3 vol.% Y2O3, 2 vol. % BHO + 3 vol.% Y2O3 : (a)at 65 K, (b)at 5 K...... 110

Figure 44. TEM images of doped YBCO films: (a) 5 vol.% Y221. (b) 5 vol.%

BSO + 3 vol.% Y2O3. (c) 2 vol.% BZO + 3 vol.% Y2O3. (d) 10 vol.% BSO + 3 vol.% Y211. (e) 2 vol.% BHO + 3 vol.% Y2O3...... 111

Figure 45 Comparison of strong anisotropic (---) and weak isotropic(—) pinning for

(a)YBCO, (b)BZO + Y2O3 + YBCO, (c)Y211+ YBCO, (d)BSO + Y2O3 + YBCO, (e)BSO + Y211 + YBCO, (f) BHO + Y2O3 + YBCO. (a)– (b) inset graphs: model fit (—) and experimental data points (*) ...... 115

xvii LIST OF TABLES

Table I. Lattice Parameters & Lattice Mismatch with respect to YBCO...... 36

Table II. Target Powders Sources ...... 55

Table III: Target compositions by volume percent. Weight %’s are as batched...... 56

Table IV. Laser Deposition Parameters for Various Doped YBCO Targets...... 59

Table V. FWHM and c-lattice parameters calculated from XRD analysis for 5, 10, and 15 vol. % Y211 doped YBCO films ...... 74

Table VI. FWHM and c-lattice parameters calculated from XRD analysis for 2, 4, and 6 vol. % BZO with 3 vol. % Y2O3 doped YBCO films...... 83

Table VII. Critical Temperatures for 2, 4, and 6 vol. % BHO + 3 vol. % Y2O3 doped YBCO films on STO substrates at 810 ˚C deposition temperatures...... 89

Table VIII. FWHM and c-lattice parameters calculated from XRD analysis for 2, 4, and 6 vol. % BHO with 3 vol. % Y2O3 doped YBCO films...... 92

Table IX. Critical Temperatures for 3, 5, and 10 vol. % BSO + 3 vol. % Y2O3 doped YBCO films on STO substrates at 795 ˚C deposition temperatures...... 99

Table X. FWHM and c-lattice parameters calculated from XRD analysis for 3, 5, and 10 vol. % BSO with 3 vol. % Y2O3 doped YBCO films...... 102

Table XI. Optimal concentration and PLD conditions for Y211 doped YBCO films and the BZO, BHO, and BSO double doped YBCO films...... 109

xviii

Table XII. Overall lattice mismatch for dopants with YBCO ...... 111

Table XIII. C- Lattice for YBCO (005) for Doped YBCO Films ...... 111

Table XIV. Gnuplot fitting parameters for mathematical model of current density temperature dependence...... 113

Table XV. Thermal expansion coefficents...... 116

xix

LIST OF ABBREVIATIONS / SYMBOLS

a Substrate Lattice Parameter ao(s) Lattice Parameter Substrate ao(f) Lattice Parameter Film b Burgers Vector Magnitude

B Magnetic Field

Bexp Experimental FWHM

Binstrument FWHM Due to the XRD Instrumentation

Bsize FWHM Due to Crystallite Size

Bstrain FWHM Due to Strain

°C Temperature Celsius d Lattice Spacing dc Critical Film Thickness

D Diffusion Rate

Ds Surface Diffusion

Dv Volume Weighted Crystallite Size e Strain

E Electron Charge

Edes Desorption Energy f AC Frequency

F Lattice Mismatch

F Deposition Flux

FWHM Full Width Half Maximum h Plank’s Constant

I Current

Ic Current Density

Jc Critical Current Density k Boltzmann’s Constant

K Scherrer Constant .87-1.0

L Step Terrace Spacing

Np Amount of Material Deposited During Laser Pulse

PLD Pulsed Laser Deposition

Q Heat

Q AC Loss

R Resistance

R Breakthrough Sweep Rate tc Time for Island Coalescence

T Temperature

Tc Critical Temperature

Tcf Final Critical Temperature w Filament Diameter

Tco Onset Critical Temperature

α Substrate’s Miscut Angle

xxi

β Integral Breadth of a Reflection in Radians 2휃 Located at 2휃

Φ0 Quantized Magnetic Flux

θ Angle of Radiation in XRD

ψ Tilt Angle of Film in XRD Measurement

|Ψ|2 Number of Superconducting Carriers per Unit Volume

κ Ginzberg Landau Parameter

λ Penetration Depth of Magnetic Field

λ Spacing Between Dendritic Island Growth Sites

λ Wavelength of Radiation

ν Frequency

ξ Coherence Length of Superconductor

τ Time Between Laser Pulses

τres Residence Time

xxii

CHAPTER I

PROPOSAL

In July 1987, President Ronald Reagan announced the start of an Eleven Point

Superconductivity Initiative, which encompassed utilizing superconductivity for energy independence and efficiency. Superconductors possess unique properties, which include

“zero resistance to direct current, extremely high current carrying density, extremely low resistance at high frequencies, extremely low signal dispersion, high sensitivity to magnetic field, exclusion of externally applied magnetic field, rapid single flux quantum transfer, close to speed of light signal transmission” (1). These unique properties have led to many applications in communications, electric power infrastructure, medicine, scientific research, and transportation (2). Application challenges involve determining what is the maximum current density and maximum magnetic field that can be utilized; both of these are increased through pinning centers. Pinning centers improve current density and irreversible field by strengthening pinning force, and preventing vortices motion within type II superconductors. Studying the effect of additional pinning centers on the anisotropic and isotropic contributions to strong and weak flux pinning, leads to a better understanding of their influence on increasing the isotropic behavior, which is beneficial for many applications, such as motors and generators. The goal of this research is to add nanophase defects to yttrium barium copper oxide (YBCO) films to

enhance their flux pinning and increase their current densities, and to evaluate their impact on anisotropic strong and isotropic weak flux pinning.

Yttrium barium copper oxide, YBa2Cu3O7-δ, (YBCO) is a Type II superconductor.

Its critical temperature of 93K is significant because it is above the boiling point of liquid nitrogen, which is 77K. The superconducting current travels along the copper oxide planes (3). YBCO thin film’s high current density results from pinning centers associated with point defects from oxygen vacancies and associated with twin and grain boundaries

(4). Addition of nano-sized second-phase inclusions to YBCO thin films enhances flux pinning by incorporating additional pinning centers, resulting in an increase in current densities (Jc) (5).

Previous studies conducted have focused on single phase additions of many materials including: Y2BaCuO5 (Y211) nanoparticles, Y2O3 nanoparticles, BaSnO3

(BSO) nanorods, and BaZrO3 (BZO) nanorods. Y211 nanoparticles have been included in thin films by pulsed laser deposition of single targets consisting of YBCO and Y211, and as multilayer films by alternating targets of YBCO and Y211 during the deposition.

While the Y211 multilayer films produced better pinning at fields less than 3 – 4 T, the simplicity of Y211 doping of a single target in PLD thin films for enhancing pinning is attractive(6). This proposal seeks to systematically determine the optimal deposition temperature and study the effect of volume percent of a mixed Y211/YBCO target on flux pinning, induced strain, microstructure, and current densities achieved at various temperature and applied field conditions, of the thin films produced by pulsed laser deposition.

While many previous studies focused on single phase additions, the addition of several phases simultaneously shows promise in improving current density by combining different pinning mechanisms. Previous research has focused on combining the artificial pinning centers resulting from barium zirconium oxide (BZO) nanorods and yttrium oxide (Y2O3) nanoparticles (7). It is hypothesized that the combination of BZO and Y2O3 with YBCO influences the lattice strain in the film. This proposal encompasses the effect of the addition of insulating, nonreactive phases of BZO and Y2O3. Processing parameters will vary the target composition volume percent of BZO from 2 – 6 vol. %, while maintaining 3 vol. % Y2O3, and the remaining vol. % YBCO. Pulsed laser deposition will be utilized to produce thin films on LaAlO3 (LAO) and SrTiO3 (STO) substrates at deposition temperatures of 810˚C and 825˚C. Comparisons of strong and weak flux pinning mechanisms, current densities, critical temperatures, and microstructures of the resulting films will be analyzed.

Since addition of BaSnO3 (BSO) nanorods has already achieved higher pinning force densities than BZO nanorods, it is of interest to study if the addition of Y2O3 nanoparticle pinning with BSO further enhances the overall flux pinning landscape (8).

To our knowledge, the effects of combining BSO and Y2O3 additions to YBCO were not researched yet. This proposal will determine the optimal deposition temperature and the optimal combination of a mixed BSO and Y2O3 target for thin films produced via PLD.

Processing parameters will vary the BSO concentration of 3, 5, and 10 vol. %, while maintaining Y2O3 constant at 3 vol. %, and the remaining vol. % YBCO. Data will encompass flux pinning, microstructure, and current density analysis. This research also will investigate the effects on current densities of combining BaSnO3 nanorods and Y211

nanoparticles, specifically conducting a deposition temperature study with a mixed target consisting of 10 vol. % BSO, 3 vol. % Y211, and 87 vol. % YBCO.

In addition, since hafnium, is found below zirconium on the periodic table, it is also of interest to also investigate the effects of doping YBCO with BaHfO3, (BHO), and

Y2O3. Processing parameters will vary the volume percent of BHO from 2, 4, and 6 vol.

%, maintaining Y2O3 constant at 3 vol. %, and the remaining vol. % YBCO. Depositions will be done at temperatures of 790, 810, 825, and 840 °C, to determine the optimum deposition temperature.

A comparison study of the proposed research will entail microstructure analysis utilizing a scanning electron microscope (SEM) & transmission electron microscope

(TEM), and x-ray diffraction (XRD). Current densities and critical temperatures will be measured via the Quantum Design Physical Properties Measurement System (PPMS) with a vibrating sample magnetometer (VSM) probe. Mathematical modeling will address the strong anisotropic and weak isotropic flux pinning contributions of the doped

YBCO films (9)

CHAPTER II

HISTORY OF SUPERCONDUCTIVITY

SUMMARY The discovery of superconductivity began with the study of electricity. In 1720, a British scientist, Stephen Gray, first demonstrated that only some materials conduct electricity. In 1751, Benjamin Franklin proposed a theory leading to materials being classified as conductors and insulators, with electricity flowing from positive to negative. At the turn of the twentieth century, three theories on how the resistance of metals changed with temperature existed. Kelvin proposed that the electrons would freeze, and the resistance would rise as temperature was decreased. Matthiessen believed that resistance would decrease to a low value with decreasing temperature. Dewar thought that the resistance would decrease to zero at the temperature of absolute zero. On April 8, 1911, Heike Onnes found mercury to be superconducting a 4.2 K. In other words, the resistance of mercury was zero at a critical temperature (Tc) of 4.2 K. Onnes henceforth became known as the father of superconductivity. There are two categories of superconductors: Type I superconductors conduct at room temperature and their resistance sharply goes to zero at their superconducting critical temperature; Type II superconductors have higher critical temperature than Type I superconductors and also exhibit a mixed state where some of the external magnetic field does penetrate the surface. This field then becomes trapped in lattice vortices. In 1933 a German scientist, Walther Meissner, discovered that superconductors work by repelling magnetic fields. The resulting screening currents cause a magnetic field on the exterior of the superconductor that opposes the initial applied magnetic field and the force of gravity. This causes the magnet to be repulsed and levitate over the superconductor, and is referred to as the Meissner effect. The London brothers coined the distance the magnetic field penetrated the surface of the superconductor as the London penetration depth. In 1950, The Ginzburg - Landau theory of superconductivity explained superconductivity as an equilibrium system at minimum energy with│Ψ│2 equal to the number of superconducting carriers per unit volume. The

Ginzburg Landau parameter is defined as the ratio of the penetration depth λ of the magnetic field to the coherence length ξ of the superconductor. Alexi Abrikov earned the 2003 Novel Prize for determining that electrons form superconducting pairs at low temperatures, and that the magnetic field penetrates the superconductor in single tubes of non- superconducting material, called vortices, which are arranged in a low energy triangular lattice pattern. In 1973 Bardeen, Cooper, and Schrieffer were awarded the Nobel Prize for developing the BCS theory, which adequately explained all the properties of superconductors. The BCS theory states that the superconducting electrons form cooper pairs, with a coherence length referring to the distance between the electrons in the cooper pairs. In the 1980’s, the quest for new superconductors continued with the discovery of YBCO with a superconducting temperature of 93K, by Paul Chu. The launch of the Eleven Point Superconducting Initiative in 1987 propelled future research, such as the superconducting pnictides, and MgB2. While superconductors may not be completely understood, their unique properties have led to many current and future applications in communications, electric power infrastructure, medicine, scientific research, and transportation. These unique properties include “zero resistance to direct current, extremely high current carrying density, extremely low resistance at high frequencies, extremely low signal dispersion, high sensitivity to magnetic field, exclusion of externally applied magnetic field, rapid single flux quantum transfer, close to speed of light signal transmission.” (Superconductivity present and future applications.2009) It generally takes more than twenty years to commercialize new lab discoveries. Partnerships between government and industry can increase funding, while balancing the risk. Economical cryogenics play a role in developing and promoting LTS and HTS technology.

DISCOVERY OF SUPERCONDUCTIVITY

The discovery of superconductivity begins with the study of electricity. In 1720,

Stephen Gray, a British scientist, first demonstrated that some materials conduct electricity while others do not. Using hemp thread, he was able to conduct electricity for

233 meters. Charles Franḉois de Cisternay du Fay proposed in 1734 a “two fluid” theory insulator, with electricity flowing from positive to negative. Joseph Priestly, a British natural philosopher, was the first to attempt to estimate the electrical conductivity of with attraction and repulsion of different materials, to explain the phenomena. In contrast,

Benjamin Franklin in 1751, proposed a one fluid theory involving positive and negative charges. This formed the basis of a material being classified as a conductor or

metals. In 1776, Henry Cavendish, a British scientist, was the first to recognize a relationship between electrical conductivity and temperature. He realized this by accurately measuring the relative resistance of iron wire in salt solutions. By 1821,

Humphry Davy, a British chemist, also confirmed the relationship between electrical conductivity and temperature, and also established a connection with the material’s length, surface area, and weight. Five years later, the French scientist, Antoine Cesar

Becquerel, constructed the first reference of the conductivity of nine metals with respect to copper. Emil Khristianovich Lenz, a Russian physicist credited with developing

Joule’s Law (Q α RI2, Q = heat Joules, R = resistance ohms, I = current amperes), developed in 1833 a mathematical model for the temperature dependence of conductivity.

After measuring the electrical conductivity of silver, copper, brass, iron, and platinum at

2 various temperatures, he developed an empirical model: γn = x + yn + zn , where γn = electrical conductivity at a temperature n, x = electrical conductivity at 0 °C and y, z are coefficients for the specific material. This was confirmed a decade later by German physicist and mathematician, Johann Heinrich Müller. A decade later, Alexandre-Edmon

Becuqerel, a French physicist, followed in his father’s footsteps, and studied the effect of heating on metals and liquids. He surmised that the change in resistance per unit temperature (dr/dt) was unique for each material. In 1848, Rev. Sir Thomas Romney

Robinson, an Irish astronomer and physicist, proposed a relationship between conduction, temperature, and the molecular forces in matter. Adam Frederik Oluf Arndtsen, a some errors in Lenz’s experiment due to the contact between the metal wires and warm liquid,

he found a proportional resistance increasing with temperature relationship. Upon studying Arndtsen’s work, Rudolf Julius Emanuel Clausius proposed that the resistance of a material at a temperature is a function of its freezing point resistance, and proportional to its absolute temperature. Almost 25 years later, Zygmunt Florenty

Wrόblewski, first proposed the concept of superconductivity; the electrical resistance of metals would be zero at a temperature of absolute zero. The technology at the time only enabled him to conduct experiments at -200 °C, thereby not enabling him to test his hypothesis. In 1860, Ernst Werner Siemens, an inventor and industrialist, used mercury as a standard due to its lack of impurities, to calculate the conductivity of metals at different temperatures. His results agreed with Arndsten. Augustus Matthiessen, a British chemist, also recognized the effects of impurities on conductivity, and studied 200 alloys.

His data also agreed with Lenz’s, aside from different coefficients due to more accurate data collecting. C.W. Siemens, a German engineer testing telegraph cable from Malta to

Alexandria, contemplated the danger of heat generation in the cable. This led to studies of platinum at high temperatures, and the proposal in 1871 of using resistance of metals to measure temperature. He also suggested the relationship that resistance was proportional to the velocity of vibrations of the material’s atoms, and correspondingly to the absolute temperature. The French physicist, Louis-Paul Cailetet, was able to liquefy oxygen in

1885. With his colleague, Edmon Bouty, he studied the conductivity of mercury, silver, magnesium, tin, copper, iron, and platinum at temperatures from zero to -123°C. In the first published reference to the possibility of superconductivity he wrote that it was “very probable that the resistance would become extremely small and therefore the conductivity very great at temperatures below -200 °C.” Ten years later, James Dewar, a British

physicist, measured the conductivities of eight metals and seven alloys at -200 °C. He concluded upon extrapolation that the resistance would be zero at a temperature of absolute zero (1).

At the turn of the twentieth century, three theories on how the resistance of metals changed with temperature existed. Kelvin proposed that the electrons would freeze, and the resistance would rise as temperature was decreased. Matthiessen believed that resistance would decrease to a low value with decreasing temperature. Dewar thought that the resistance would decrease to zero at the temperature of absolute zero. In July

1908, Heike Kamerlingh Onnes succeeded in liquefying sixty cm3 of helium at his low temperature physics laboratory in Leiden. Onnes was very much influenced by Johannes

Van der Waals, and his law of corresponding states, which explained the role of intermolecular forces, van der waals forces, between atoms of a gas. Onnes continued to perfect increasing liquid helium in larger amounts, and storing it in a cryostat for experiments. He decided to turn his attention to investigating the resistance of metals at absolute zero. It was already known that the resistance of metals decreases with decreasing temperature. This occurs because the atoms of the solid metal vibrate less at a lower temperature, which results in less deflection and scattering of the electrons. Onnes chose to study mercury, since it could be highly purified through distillation. On April 8,

1911, he found mercury to be superconducting a 4.2 K. In other words, the resistance of mercury was zero at a critical temperature (Tc) of 4.2 K (figure 1). Onnes henceforth became known as the father of superconductivity. In 1912, tin was found to have a superconducting critical temperature of 3.7 K, followed by lead with a superconducting temperature of 6K (2). Onnes presented his vision of using supermagnets to create

magnetic fields of 10 tesla, at the 1913 International Institute of Refrigeration. To his

dismay, in 1914 he found that pure metals remain superconducting only in weak fields.

Research on superconductivity continued at the Leiden lab under its next director, de

Haas. In the 1920’s, de Haas, found that eutectic lead bismuth alloys superconduct in

fields of up to two tesla. In 1930 a former researcher and crystal grower at Leiden, Lev

Shubnikov, returned to Kharkov, Soviet Union to continue his research. Shubnikov

observed that while the alloy single crystal Pb-Tl lost its diamagnetism at a low critical

field, normal resistance appeared at a higher critical field. This was the first realization of

Type II superconductivity of alloys, as opposed to the Type I superconductivity of pure

metals. Unfortunately this breakthrough was unknown for decades, as Shubnikov was

accused of espionage and shot in 1937 (3).

There are two categories of superconductors. Type I consists of metals and

metalloids that conduct at room temperature and whose resistance sharply goes to zero at

their superconducting critical temperature, Tc (Fig. 1).

Type II superconductors have higher Tc than Type I

and also exhibit a mixed state where some of the

external magnetic field does penetrate the surface.

Figure 1. Resistance vs. Temperature for This field then becomes trapped in lattice vortices. non-superconducting metal and a Type I Superconductor (4) The mixed state results in an onset critical

temperature Tco and a final critical temperature Tcf.

Tco occurs when the resistivity verses temperature curve begins to change to

superconducting behavior. At Tcf , the dc resistivity is zero. The Tc is chosen to be the

average of Tco and Tcf (Fig.2-3) (4-6).

Figure 2. Magnetic Field vs. Temperature Figure 3. Resistivity vs. Temperature for Type for Type II Superconductor (5) II Superconductor (6)

SUPERCONDUCTOR THEORY

In 1933 a German scientist, Walther Meissner, discovered that superconductors work by repelling magnetic fields (7). In a diamagnetic material, there are no dipoles in the absence of a magnetic field. Once a magnetic field is applied, the induced dipoles align opposite to the applied field (8). The resulting induced currents on the superconductor’s surface are referred to as screening currents because they “… screen the interior of the superconductor from the externally applied magnetic field.” The screening currents cause a magnetic field on the exterior of the superconductor that opposes the initial applied magnetic field and the force of gravity. This causes the magnet to be repulsed and levitate over the superconductor.

This is known as the Meissner effect (Fig. 4) (2).

The properties of superconductors resulting from their diamagnetism, allowed for a thermodynamic

Figure 4. Meissner Effect (12)

explanation and distinction between superconductors and conductors (3).

Fritz and Heinz London sought to explain the relationship between superconducting current and applied magnetic field, after finding refuge from the anti-

Semitism of Germany in the 1930’s at Oxford. Highly influenced by the new theory of quantum mechanics with the Bohr hydrogen atom model and the Schrödinger equation’s wave function describing the energy states of electrons, the London brothers proposed that “macroscopic quantum phenomena” described the superconducting electrons in a stationary state. This meant that the magnetic flux penetrating the superconductor would be quantized (Φ0 = h / e, h= Planck’s constant, e = electron charge) (9). Magnetic flux quantization was experimentally confirmed in the 1960’s. The London brothers also coined the distance the magnetic field penetrated the surface of the superconductor as the

London penetration depth, and it is “…related to the mass, charge, and number of superconducting carriers.”

After World War II, research in superconductivity continued. Herbert Fröhlich surmised the effect of crystal lattice vibrations on superconductivity. Emanuel Maxwell,

National Bureau of Standards, and Bernard Serin, Rutgers University, experimentally confirmed Fröhlich’s belief with isotopes of tin. They found that the superconducting transition temperature was “…inversely proportional to the square root of the mass of the atoms.” In the 1950’s, Brian Pippard, of Cambridge, measured the surface resistance of superconductors at microwave frequencies. He proposed that there is a transition from superconducting to non-superconducting over a coherence length, rather than a sharp interface.

In 1950 in the Soviet Union, Lev Landau and Vitaly Ginzburg published the

Ginzburg- Landau theory of superconductivity. They took the perspective of an equilibrium system at minimum energy with│ Ψ│2 equal to the number of superconducting carriers per unit volume (2). The interface energy switched from positive for the superconducting state to negative for the non-superconducting state, when the Ginzburg Landau parameter κ › 1/√2. The Ginzburg Landau parameter is defined as the ratio of the penetration depth λ of the magnetic field to the coherence length ξ of the superconductor (3). No carriers were present above the critical temperature and energy was saved if the carriers were spread out. The solution to the differential equation Ψ provided the London equations, penetration depth, and coherence length. It also agreed with experimental data when the charge was taken as two electrons (2). The Ginzburg-

Landau theory confirmed Shubnikov’s previous research on the difference between Type

I and Type II superconductors (3).

Alexi Abrikov, of the USSR Academy of Sciences, extended the Ginzburg-

Landau theory in 1953. He reasoned that the electrons form superconducting pairs at low temperatures because this arrangement is at a lower energy level. The energy saved is called the superconducting condensation energy. Excluding a magnetic field costs energy, whereas allowing the small penetration of magnetic field saves energy. The superconductivity wave function, Ψ, “decays to zero” over the coherence length. In a

Type I superconductor, the coherence length is greater than the penetration depth. The energy cost of lost superconductivity at the interface is greater than the energy saved by field penetration. Therefore, Type I superconductors prefer no interface. In a Type II superconductor, the penetration depth is greater than the coherence length. The energy

used to destroy superconductivity at the surface is less than the energy gained by penetration of the field. Therefore, Type II superconductors prefer many interfaces between the superconducting and non-superconducting state. In Type II superconductors, the magnetic field penetrates in single tubes of non-superconducting material, called vortices. The vortices contain a quantum of magnetic flux each, and are surrounded by electric current that shields the surrounding superconductor. The vortices repel each other and arrange in a triangular lattice pattern, which is the lowest energy arrangement.

Because of this, Type II superconductors are referred to as being a mixed state, or

Shubnikov phase, in deference to his research over 30 years prior. Initially lacking

Landau’s approval, Abrikov’s research was not published until 1957. The triangular vortices pattern was verified experimentally in the 1960’s, and earned Abrikov the 2003

Nobel Prize.

In the 1950’s, John Bardeen, University of Illinois, began contemplating a theory of superconductivity with post doctorate Leon Cooper, and PhD student Robert

Schrieffer. Cooper proposed that the electrons are paired, called Cooper pairs.

Reflecting on Fröhlich’s work, the team theorized that the crystal lattice vibrations, phonons, played a role in superconductivity. While the liked charge electrons repel each other, coulomb repulsion, they also attract the surrounding positive ions, deforming the lattice structure. Since the positive ions are heavier than the electrons, they take longer to move back to their original position as the first electron leaves. This results in an excess positive charge that attracts the second electron. The distance between the electrons in the pair is called the coherence length. In a metal that is a good conductor, the interaction between the lattice vibrations, phonons, and electrons is weak; whereas in a

superconductor, the interaction is strong. This explains why at high temperature, greater lattice vibrations cause more scattering of electrons, resulting in greater resistance and heating and a loss of superconductivity; an effect that is decreased as temperature is lowered. When a superconducting cooper pair scatters from a phonon interaction, the individual electrons of the pair adjust their individual momentums, and they continue to move in the current direction. Thermodynamically, the superconducting current requires less energy and is more favorable. The superconducting wave function has one value for all of the electrons, referred to as a coherent state. The superconducting energy gap is the amount of energy needed to overcome the binding energy, and breakup the cooper pair of electrons. The superconductor energy gap was confirmed in 1956 by Rolfe

Glover and Michael Tinkham. They showed that superconductors absorbed infrared energy above a certain value, but not below it. This energy gap is related to the strength of the superconductor and the number of cooper pairs. When a superconductor is heated and the thermal energy breaks apart cooper pairs of electrons, the energy gap is continually decreased to zero. At that point, the transition temperature, there are no pairs left to break up and no pairs forming. The BCS theory adequately explained all the properties of superconductors, and was published in Physical Review in 1957, and in

1973 Bardeen, Cooper, and Schrieffer were awarded the Nobel Prize for their work.

In 1960, Ivan Giaver, at General Electric U.S., created a superconductor tunnel junction consisting of two superconductors separated by an insulating layer. The electrons tunnel through the insulating layer. In 1963, Brian Josephson of Cambridge, developed the concept of the Josephson junction. The Josephson junction consisted of two superconducting layers, separated by a thin non-superconducting layer, known as a

weak link. Philip Anderson created the first Josephson junction in 1963 at Bell Labs. The

Josephson junction proved useful in generating microwaves and as a standard for voltage measurements. A steady voltage applied to the junction caused the electrons to oscillate as in alternating current. This occurs in the microwave region, as 1 mV produced a frequency of 486 GHz, and enables the junction to generate microwaves. Since frequency is easily measured and controlled, an array of Josephson junctions can be used as a volt standard and to calibrate voltmeters. It also allows for the accurate measurement of the ratio of electron charge to Planck’s constant. Two Josephson junctions can also be combined in a parallel circuit to form a SQUID, superconducting quantum interference device that senses magnetic fields.

QUEST FOR NEW SUPERCONDUCTORS

Bern Matthias systematically investigated new superconducting compounds in the

1950’s. He recognized that many superconducting alloys have an A15 structure and the importance of a particular number of outer valence electrons in an atom. In 1954,

Matthias found niobium tin to have a critical temperature of 18 K. Magnets made from niobium tin wire could generate nine Tesla by 1960.

In search of new superconductors in the 1980’s J. George Bednorz and K. Alex

Müller began investigating materials whose lattice vibrations coupled strongly with their electrons. In particular, they were looking at materials that exhibited the John Teller effect where the oxygen ions are distorted in the presence of magnetic ions. After noticing that a French research group found a perovskite material containing barium, lanthanum, copper, and oxygen conducted at -100 °C, they decided to vary the ratio of barium ions +2 and lanthanum ions +3 in order to change the charge on the copper ions

from +2 to +3. The extra positive charge on the copper acts as a hole, and is mobile within the crystal structure. In 1986, they found a combination to be superconducting up to 30 K, and published their results in the once popular but now obscure journal

Zeitschrift früPhysik. Bednorz and Müller chose this journal since publishing in a more prestigious journal would take more time, and also run the risk of their work being duplicated and published elsewhere first.

Bednorz and Müller’s work caught the eye of Paul Chu, University of Houston.

He duplicated the work and found that increasing pressure also increased the critical temperature. Meanwhile Koichi Kitazawa, University of Tokyo, identified the material as 2-1-4 phase (La,Ba)2CuO4. Chu then decided that rather than increasing pressure, to substitute lanthanum with a smaller element, yttrium. Working in collaboration with the

University of Alabama in January 1987, he found the resulting compound YBCO to have a superconducting critical temperature of 93K. This was significant because it was above the boiling point of liquid nitrogen, which is 77K. Chu’s work was published in Physical

Review Letters.

In July 1987, President Ronald Reagan announced the start of an Eleven Point

Superconductivity Initiative, which encompassed utilizing superconductivity for energy independence and efficiency, while also being better for the environment. The quest for making new superconductors continued in labs throughout the world. The new oxide materials were inherently brittle. Quality control of the solid state materials proved difficult with the effect of variables such as impurities and oxygen stoichiometry, which depended on optimization of furnace temperature and atmosphere. Progress continued with several new superconductors including bismuth copper oxide with a critical

temperature of 110 K in Jan. 1988, thallium copper oxide with a critical temperature of

120 K in Feb. 1988, and mercury copper oxide with a critical temperature of 135 K in

1993. The high critical temperature superconductors have in common layers of copper and oxygen atoms that are separated by other atoms, where the superconductivity arises from the electrons “…hopping in the Cu-O planes.” Doping, which substitutes atoms between the copper-oxygen planes, changes the material from being magnetic to superconducting, with optimization resulting in the highest critical temperatures.

The discoveries in superconductivity in the 1980’s, led to the testing of other materials, namely carbon, magnesium boride, and the pnictides. Allotropes of carbon include diamond, graphite, and the fullerene C60. In the compound K3C60, potassium is in the interstitial sites with C60 in a face centered cubic lattice. Substituting cesium for potassium expands the lattice since cesium is larger than potassium. The expanded lattice causes the electrons of the fullerene to overlap less. The smaller energy level distribution of these electrons results in more electrons superconducting, and a critical temperature of

38K. Some work has also been done on organic superconductors, with critical temperatures around 12K.

In January 2001, Jun Akimitsu of Tokyo announced a new superconductor that had been overlooked for many years, magnesium boride, MgB2. Both magnesium and boride are light elements, located at the top of the periodic chart. Since lattice vibrational frequency is inversely proportional to the square root of atomic mass, the superconductivity involves a large energy. High vibrational frequencies result in high critical temperatures. The magnesium atoms are present in hexagonal layers that are interwoven with the honeycomb layers of boron atoms. The boron atoms have two

dimensional bonding with electrons within the honeycomb layers and three dimensional bonding connecting the honeycomb layers. This results in two superconducting energy gaps. MgB2 has the added advantage of being very inexpensive.

Another new group of superconductors that may involve more than one energy gap are pnictides. Pnictides refer to compounds composed of a pnictogen, which refers to the elements in the column of the periodic table containing nitrogen, phosphorus, arsenic, etc. In 2007, Hideo Hosono found that the pnictide LaO Fe P has a critical temperature of 3 K. The discovery was significant because the compound contained iron, Fe, rather than copper, Cu. In 2008, La(O,F)FeAs was found to have a Tc of 26 K, and

Sm(O,F)FeAs a Tc of 55 K. Like MgB2, a layered structure in involved, consisting of

FeAs layers interwoven with SmO layers. One major drawback of this superconductor is the hazardous nature of arsenic. A summary of the superconductors discovered to date with their corresponding critical temperatures is presented in figure 5.

Figure 5. Superconductors and Their Critical Temperatures Dept. of Energy, Basic Energy Science

SUPERCONDUCTOR APPLICATIONS

One thing that the superconductors discovered since the 1980’s have in common is that current accepted theory still does not understand completely why they work, nor can it predict the superconductive behavior. BCS theory applies to critical temperatures of about 20K. Other theories involving phonons and tunneling between layers remain to be proven (2). While superconductors may not be completely understood, their unique properties have led to many current and future applications in communications, electric power infrastructure, medicine, scientific research, and transportation. These unique properties include “zero resistance to direct current, extremely high current carrying density, extremely low resistance at high frequencies, extremely low signal dispersion, high sensitivity to magnetic field, exclusion of externally applied magnetic field, rapid single flux quantum transfer, close to speed of light signal transmission.”

(Superconductivity present and future applications.2009) Applications requiring zero resistance and high current densities include magnets, passive microwave devices, interconnects in microelectronics, and electrical energy transport in cables. Applications involving Josephson tunneling include microwave detectors and mixers, physical measurements with SQUIDS, fast logic and memory circuits in computers, and plasma.

Applications requiring high current densities and high critical magnetic fields Hc2 include uses in the electrical power industry, transportation, medicine, and in high energy physics

(10). The main challenges facing commercialization of superconductor applications consist of cost, refrigeration needs, reliability, and acceptance.

Applications in the electrical industry include HTS (high temperature superconducting) cables, fault current limiters, transformers, and energy storage. HTS

wire carries one hundred times the current of copper wires. The first generation multifilament HTS wire became commercially available in 2006. Second generation

(2G) HTS wire makes use of coated conductor architecture and thin film manufacturing processing has lowered cost. HTS cables carry three to five times the power of copper cables. Currently there are three in grid demos in the United States. The world’s first

HTS power transmission cable system in a commercial power grid generates 574 MW of electricity and can power 300,000 homes. Fault current limiters (FCLs) deal with current surges from faults or short circuits resulting from weather or accidents. These current surges can damage the grid if they exceed the rating of circuit breakers, switchgears, and bus distribution transformers. When overcurrent occurs, the HTS FCLs switch from a superconducting state to a resisting state to bring the current down to a safe level. HTS transformers generate less heat waste than traditional oil cooled voltage transformers.

HTS transformers are cooled by nitrogen, which is abundant, environmentally friendly, and safe. They also have the capability of overload operating at peak demands. In wind energy, direct drive wind generators use a stator design with HTS wire replacing the copper wire in the rotor. A MW drive utilizing HTS wire weighs one third less. HTS generators have the capability of doubling the power capacity of the system, and lowering the cost of wind energy. In the electrical industry, energy storage is needed at times to supply a “burst” of power to maintain “grid reliability.” Superconducting magnetic energy storage, SMES, already utilizes low temperature superconductors for stability and reliability of the grid. Currently demonstration units with flywheels employ HTS self- centering bearings to decrease friction losses from 3-5% per hour to less than .1% per hour. Government promotion of demos and pilot projects, utilizing clean energy funds,

will enhance the pathway to commercialization of superconductor applications in the electrical industry.

In the transportation industry, superconductors have the ability to reduce weight and fuel use, resulting in increased range and better efficiency. From the nineteenth century electric street cars to the twentieth century hybrid cars and high speed electric trains, transportation systems have been influenced by balancing the cost of oil with convenience and flexibility. The resulting increased demand on the power grid also needs to be entered into the equation. Magnetically levitated trains use powerful LTS magnets to float over the track, and reach speeds of 500 kmph (11). Maglev trains are currently in use in China, Japan, and South Korea. China’s Shanghai “trans rapid system” began operation in April 2004. It makes 115 daily seven minute trips over nineteen miles, with an average speed of 165 mph. In Aichi, Japan, the low speed HSST “Linimi” opened in

March 2005 for the 2005 World Expo. It is an urban maglev that covers 5.6 miles with nine stations. It’s average speed in 62 mph (12). Superconductors have also found a use in ship propulsion. In the last twenty years, electric propulsion design systems have become more flexible and efficient, combining propulsion with other electrical needs.

Almost 100% of new commercial ships now use electric propulsion; one example being the cruise ship Queen Mary 2. In the year 2000, the U.S. navy decided to move towards an all-electric fleet. HTS motors and generators weigh one third less, are quieter, and more efficient than their copper wire counterparts. Also HTS cables, due to their decreased weight and high current density properties, have been used in demos to replace copper degassing coils on military ships. More research on high temperature superconductors to lower cooling costs will promote use in the transportation industry.

Other important applications of superconducting magnets in the field of medicine include magnetic resonance imaging (MRI), magnetocardiography (MCG), and nuclear magnetic resonance (NMR). MRI provides soft tissue images. It is radiation free and decreases the need for exploratory surgery. It works since the nucleus of atoms has a spin that aligns to an external field at a frequency proportional to the field strength. With a pulse of a particular radio frequency, the spinning nucleus first absorbs, and then releases energy in a millisecond. The relaxation time is different for healthy and diseased tissue. Rapid gradient fields superimposed on the main field provides position information and computer generated images. The first MRI was made in the 1970’s, and currently there are 20,000 in use in the world, the number of which increases 10% annually. The superconducting magnet provides a stable, homogeneous magnetic field, allowing high resolution images. Other variations of MRI include functional magnetic resonance imaging (FMRI), MRI guidance imagery, magnetic resonance spectroscopy, and ultra- low field MRI (ULF-MRI). FMRI provides a “sequence of fast images to study dynamic changes, primarily blood flow rates,” thereby showing activation of brain regions. MRI guidance imagery aids in surgical removal of tumors. Magnetic resonance spectroscopy is used to determine the chemical composition of tumors, and to diagnose and monitor epilepsy. ULF-MRI utilizing a field 10,000 times less than a superconducting magnet, requires a very sensitive superconducting detector called a

SQUID, superconducting quantum interference device. The SQUID provides better contrast in imaging breast and prostate tumors. The lower cost of the ULF-MRI system would allow initial screening uses and more availability of this medical technology.

Magneto encephalography (MEG) also referred to as magnetic source imaging (MSI),

employs an array of SQUID detectors to map the brain, according to magnetic signals. It can show the location of epileptic seizures and brain tumors, and is the only non-invasive method that can be used during pregnancy to determine the brain activity of the baby.

MEG is not widely used due to its initial cost. Future availability of MEG is contingent on installation at research hospitals to establish a large data base to demonstrate to insurance its usefulness. Currently electroencephalography (EEG) is used due to its low cost. However, moisture in the scalp and skull thickness distorts the images. MEG does not require skin contact, and provides undistorted images. Future use could increase by combining ULF-MRI with MEG to lower the cost and increase the data base registration.

Magnetocardiography (MCG), also referred to as heat magnetic field imaging (MFI) can detect heart and coronary artery disease and ischemia. It has the advantages over EKG of being non-invasive with no electrode-skin contact, being capable of detecting artery disease, and having signal strength related to the distance between the heart and the detector. This last capability allows it to detect the unborn baby’s heartbeat, without signal saturation from the mother’s heartbeat. MCG faces the same use challenge as

MEG; it needs the development of a large clinical diagnostic database to show its benefits to insurers and medicare.

LTS superconducting magnets also provide the high strength, homogeneous, and stable magnetic field required for the resolution and precision necessary in nuclear magnetic resonance (NMR) spectroscopy. This analytical technique won the Nobel Prize in 1952, and is widely used in biology, chemistry, materials science, medicine, and physics. It has been used as a diagnostic aid in coordinating the size and concentration of the lipoprotein that carries cholesterol to the correct medications dosage. It is also very

beneficial in determining the sequence and structure of amino acids in proteins, to determine biochemical properties and target new drugs to specific proteins.

Superconductors have also been used in industrial production to filter impurities and provide efficient induction heating. Superconducting magnets are currently being used in kaolin clay processing. The U.S. is the largest manufacturer and exporter of kaolin clay, mostly from Georgia. Colored impurities that are ferromagnetic and paramagnetic are removed by passing slurry through a tube containing stainless steel packing. The tube is magnetized to remove the impurities. This is followed with a back- flush with the magnetic field off. The superconductor magnet is more efficient than the copper magnet, providing 5T at 200 W/hr., for a 95% savings. This is in contrast to 2T and 400kW/hr. energy power requirement for the copper magnet. The superconductor magnet is also more efficient since the throughput is directly related to the field strength.

HTS induction heaters are also being used to heat aluminum, copper, and brass billets prior to machining. They provide a thermal rating of .25 MW- 2MW, and consist of HTS coils and compact chillers that chill to 30K. HTS induction heaters operate more efficiently and require less maintenance.

Three other uses of superconductors involve high energy physics and space research. The Large Hadron Collider (LHC) in Geneva, Switzerland has a 27 km circumference. It is used to collide protons with 7 trillion electron volts of energy. It utilizes thousands of superconducting magnets in two rings to guide the particles, and also utilizes superconducting magnets in detectors. The largest main dipoles used to guide the particles contain over 1500 tons of superconducting wire. The International

Thermonuclear Experimental Reactor has been built with the goal of producing

electricity from fusion. Superconducting magnets will be used to contain the high temperature plasma of fusion. Cost saving uses in space exploration, include “magnetic actuators, magnetic refrigeration, magnetically assisted propulsion, and space based magnetic plasma confinement.

Superconductors have also been utilized in the communications, instrumentation, and electronics industry. In analog wireless communication, antennas on cell towers detect signals, which are then divided into channels with corresponding signal strength loss. HTS filters provide a sharp filter to prevent the overlap of channels, which would increase the range of the base station and increase the number of channels processed.

Currently over 10,000 HTS filter systems are in use. Analog to digital convertors comprised of superconductors have the capability to digitize a wide band of signal without analog pre-processing. It also has the capability to provide a universal system that adapts to translate any signal with appropriate software. This convertor application is not yet commercially available. A superconducting all digital receiver has been made with a superconducting integrated circuit (IC), using niobium, instead of silicon. Eleven thousand Josephson junctions in a Rapid Single Flux Quantum (RSFQ) circuit move magnetic pulses in a picosecond. Government supported demos are needed to support this technology being adapted by communication companies.

Instrumentation applications of superconductors comprise the use of SQUIDS, and their ability to detect magnetic fields very sensitively. Sensors use SQUIDS to detect faint electromagnetic signals with the ability to discriminate between various frequencies.

They are currently being used at the NASA Radio Observatory in Owens Valley,

California. The SQUIDS can also be used to provide infrared images, such as the SCUB-

2 infrared camera currently at the James Clerk Maxwell telescope in Hawaii. As mentioned previously, Josephson junctions act as a convertor between frequency and voltage. This enables it to be used as a standard for measurement of the volt unit, and subsequent calibrations of lab equipment. Superconducting digitizers are also used in radar on navy ships, allowing it to distinguish between a sea-skimming missile’s small radar signal and the signal resulting from background waves, rain, shoreline, and jammers.

Lastly, superconducting electronics have a role to play in high speed computing, with their properties of low gate power and ultra-high speed making it potentially one hundred times faster than silicon. Rapid Single Fire Quantum (RSFQ) circuitry has already surpassed the 20 GHz speed barrier. This is in comparison to current processor speeds of 3-4 GHz, which is limited to more gates and faster clocks resulting in chip heating. The National Security Agency has identified superconductive technology as a role player in petaflop computations, one million billion operations per second.

It generally takes more than twenty years to commercialize new lab discoveries.

Partnerships between government and industry can increase funding, while balancing the risk. Economical cryogenics play a role in developing and promoting LTS and HTS technology (11).

CHAPTER III

THIN FILM BACKGROUND INFORMATION

SUMMARY Thin films are grown by the adsorption of adatoms on a substrate surface. The structure of the film is determined by the mobility of adatoms once they are adsorbed on the surface. The adatom’s mobility is a result of its kinetic energy and arrival rate at the substrate surface, and the relative interaction energies of the film and substrate. Film-film interaction and film-substrate interaction dictates growth mode. Resulting films can be amorphous, polycrystalline, or epitaxial. Thin films can be grown in a variety of ways, which differ in how the atoms or ions are removed from the source and the amount of kinetic energy these atoms are thereby imparted with. Pulsed laser deposition (PLD) has been used to produce quality, stoichiometric thin films. The KrF laser has a wavelength of 248nm, and provides approximately 5 eV per photon, enabling the chemical bonds to be broken without overheating the target. The laser fluence (Joule/cm2), substrate temperature, substrate surface, distance from the target to the substrate, and background pressure all influence the nucleation and kinetics of the film growth. Target splashing can be minimized by employing alower laser energy and smoother targets. Rastering the laser and rotating the target results in more uniform film thickness. Film growth can occur by three different mechanisms: step-flow growth, layer by layer growth, and 3D growth. Film-film interaction and film-substrate interaction dictates growth mode. The step-flow mechanism of film growth results in smooth, uniform growth of the film. Step-flow requires that the time between pulses (τ) be large compared to the diffusion time on the step. Step - flow growth occurs with deposition at higher temperatures, since increased temperature increases diffusion and mobility of the adatoms. Whereas the energy of the laser, pulse rate of the laser, temperature of the substrate, and the distance of the substrate from the heater block effect the nucleation and growth of the film, the choice of substrate and the dopants chosen in YBCO targets possess various degrees of lattice mismatch. The degree of mismatch effects epitaxial

growth of doped YBCO films, the sharpness of interfaces, and the resulting strain in the films. The amount of strain is released by defects in the crystal structure. These defects have a direct impact on current densities and critical temperatures of the thin films produced. This lattice mismatch, along with the different coefficients of thermal expansion for different materials, results in almost all thin films possessing internal stress, without applied external forces. Because of this, it is usually desired to minimize lattice mismatch and choose materials of similar thermal conductivities and expansion coefficients to promote epitaxial growth of thin films. XRD analysis provides information in regards to the grain size, grain orientation within films, and the presence of stresses within the film. The Scherrer formula provides average crystallite size, while neglecting strain effects. Strain in thin films is a culmination of microstrains and macrostrains. Microstrain is the average strain within a single grain. It occurs on the atomic level due to defect distribution, inhomogeneity of the film, and heterogeneous phases. Residual macrostrain is the average strain over a macroscopic region. It results from lattice mismatch at the film/substrate interface and residual thermal stress. Within a crystalline structure, uniform strain causes the unit cell to expand or contract isotropically. This changes the unit cell parameters and results in a shift in the xrd peaks. No broadening of the peaks occurs with uniform strain. With non-uniform strain, there is a systematic shift of atoms from their ideal positions. The non-uniform strain can be due to point defects, plastic deformation, or poor crystallinity, and results in peak broadening. A Williamson – Hall plot of Bexp cos θ verses sin θ can be utilized to separate the size and strain components. A horizontal line indicates that size broadening is the main contributor to the width of the peaks. If strain is the main contributor, the line will have a slope other than zero. The sin2Ψ method is based on the simple case of in-plane stress, or biaxial model, where the stress perpendicular to the film surface is zero. A sin2Ψ plot of lattice parameter a or the lattice spacing d verses sin2Ψ shows the relationship between the tilting angle Ψ of the sample and the stress in the film. A positive slope indicates tensile stress, whereas a negative slope indicates a compressive stress in the film. While x-ray diffraction provides information on the “macrotexture” of many grains, electron backscatter (EBSD) with SEM can provide information on the “microtexture” by imaging one grain at a time. Strain has been shown experimentally to effect the vortex pinning of high temperature superconductors. A. Llordés, et al., found that the non-coherent interface between the BZO dopant and YBCO predominantly controlled the amount of non- uniform nanostrain and isotropic vortex pinning when compared to the dopant concentration effect. However a maximum dopant concentration exists beyond which the current density begins to decrease, despite the strain enhanced vortex pinning.

THIN FILM GROWTH

Thin films are grown by the adsorption of atoms on a substrate surface. The adsorbed atom is referred to as an adatom. The structure of the film is determined by the mobility of atoms once they are adsorbed on the surface. Chemical adsorption has a desorption energy Edes of approximately 1.5 eV, whereas physical adsorption has a desorption energy of about .1 eV. The adatom’s mobility is a result of its kinetic energy and arrival rate at the substrate surface, and the relative interaction energies of the film and substrate. A high initial kinetic energy, a high substrate temperature, and a small interaction with the substrate, enhances diffusion across the surface. Adatoms with no mobility produce amorphous films. Residence time refers to the amount of time that the atom stays on the surface. It is related to the frequency (휈), desorption energy (Edes), temperature (T), and Boltzmann’s constant (k), according to the equation 휏푟푒푠 =

1 ( ) 푒퐸푑푒푠/푘푇. 휈 Film-film interaction and film-substrate interaction dictates growth mode.

If the interaction between the film and substrate is greater than the interaction between each layer of film, island growth results. If the interaction between the layers of the film, and the film and the substrate are the same, layer-by-layer growth results. Combinations of interactions result in layer plus island growth. Resulting films can be amorphous, polycrystalline, or epitaxial. Polycrystalline films possess no long range order and are comprised of random single crystals with different orientations, referred to as texture.

Amorphous films are not crystalline and do not have long-range order due to the adatoms’ lack of mobility. Epitaxial films result from depositing a mono-crystalline film on a mono-crystalline substrate. The film has a lattice structure and orientation like the substrate it is deposited on. Thin films can be grown in a variety of ways, which differ in

how the atoms or ions are removed from the source and the amount of kinetic energy these atoms are thereby imparted with. Pulsed laser deposition (PLD) has been used to produce quality, stoichiometric thin films (1). PLD was first utilized in 1965 to produce semiconductor and dielectric films with a ruby laser. A milestone for the technique was reached when it was used for “…in situ growth of epitaxial high-temperature superconductor films in 1987 at Bell Communications Research” (2). A PLD system under ultra-high vacuum (UHV), that deposits layer by layer film growth while being monitored by reflection high energy electron diffraction (RHEED), is often referred to as laser molecular beam epitaxy (MBE)(2).

PULSED LASER DEPOSITION

Pulsed laser deposition (PLD), i.e. pulsed laser ablation, utilizes a focused pulsed laser beam to strike a target within a vacuum chamber. The type of laser determines the amount of kinetic energy that the atoms ablated from the target have. IR lasers, i.e.

YAG, have a wavelength of 1.06 휇푚 , which supplies ~1푒푉 per photon. It causes more target heating than UV lasers and visible lasers. UV lasers, i.e. KrF Excimer, are the most commonly used, and have a wavelength of 248 nm. They provide 5eV per photon, which is enough to break chemical bonds and results in less target heating. Visible lasers have a wavelength of 532 nm, and result in photochemical ablation of the target. The chamber can be pumped down to an ultra-high vacuum or a background gas can be utilized depending on the type of film being deposited. When the laser beam hits the target, the energy is converted first to electronic excitation, followed by thermal, chemical and mechanical energy. This results in evaporation, ablation, plasma formation, and exfoliation of the target. The beam penetrates the target at a depth related to the

beam wavelength and the index of refraction of the material from which the target is made, and is generally 10nm (1). This penetration depth is the most important effect of the laser’s wavelength. A shallow penetration depth minimizes subsurface boiling and resulting particulates on the thin film surface (2). The beam generates an electric field that removes electrons from the target material. These electrons collide with atoms in the target, transfer energy to the lattice, which in turn heats the target material. This heating leads to both vaporization and ablation of the target. The ablated atoms are of a stoichiometric composition and form a plasma plume that expands towards the substrate, as a result of the coulomb explosion and recoil at the target. One problem that can occur is referred to as target splashing. This happens when melting occurs deep within the target, causing molten liquid droplets to be expelled and attach to the substrate film. They typically appear spherical on SEM pictures. Target splashing can be minimized by employing a lower laser energy and smoother targets. Targets can be made smoother by monthly sanding. Placing the substrate above the target also minimizes this effect.

Angular distribution of the ablated atoms can also cause the film thickness not to be uniform. Rastering the laser and rotating the target can minimize this (1). Pre-ablation of the target for a certain amount of time before allowing deposition to occur, also allows all of the ablated atoms of different masses to “expand with an identical angular distribution

(2).”

Interaction of the newly formed plume and the laser beam results in the plume species being heated to a greater extent, especially when using excimer lasers which have a long pulse duration of tens of nanoseconds. Pressure in the chamber influences the shape of the plume, with a vacuum of 10-6 torr resulting in a narrow shape. Increasing

background pressure results in collisions with the atoms of the plume, thereby decreasing their kinetic energy. This then prevents the atoms from sputtering film that has already been deposited on the substrate, decreases defects in the film, and helps to preserve the stoichiometry of the film (1). Research in this dissertation utilized a vacuum of 10-6 torr and an oxygen background of 300 mtorr. Utilizing a laser fluence of two Joules/cm2 keeps the energy of the particles below 50 eV and decreases the amount of sputtering that occurs, in comparison to utilizing a higher laser fluence of 4 J/cm2. Placing the substrate farther away than the stopping distance of the plume also limits sputtering of the growing film. The laser fluence (Joule/cm2), substrate temperature, substrate surface, distance from the target to the substrate, and background pressure all influence the nucleation and kinetics of the film growth (3). In PLD the large supersaturation that occurs on the substrate causes a large nucleation density to occur, and increases the smoothness of the film.

FILM GROWTH MECHANISM

Film growth can occur by three different mechanisms: step-flow growth, layer by layer growth, and 3D growth (1). In a simple model of film growth, adatoms that reach a substrate are mobile with a diffusivity, D, until they reach a lower energy site. These lower energy sites can occur at defects in the substrate surface, or site that increase the adatoms’ atomic coordination number. This causes an adatom to immobilize at steps.

Two adatoms form an island. The substrate surface consists of step terraces that are spaced a distance L ≈ a / α, where a is the substrate’s lattice parameter and α is the substrate’s miscut angle measured in radians. When the deposition flux (F) is low, and the diffusion rate (D) is high, adatoms migrate to the steps without island nucleation,

resulting in step-flow. With PLD, the deposition flux F = Np / τ, where Np is the amount of material deposited during a laser pulse, and τ is the time between laser pulses. Step- flow requires that the time between pulses (τ) be large compared to the diffusion time on

2 the step. In other words, F < 2Np D/ L . (2) Step - flow growth occurs with deposition at higher temperatures, since increased temperature increases diffusion and mobility of the adatoms. (1)From the previous equation, step-flow is also dependent on the substrate miscut angle. The step-flow mechanism of film growth results in smooth, uniform growth of the film (2).

Layer by layer growth involves island growth until the islands coalesce, with subsequent pits filled in by deposited atoms (1). With PLD, saturation of the nucleation sites occurs with the first laser pulse. The spacing between the dendritic island growth sites is λ, which is related to the deposition flux and diffusion rate. The spacing between the islands (λ) must be less than the spacing of the step terraces (L) if layer by layer growth is preferred over step growth. The time between the initial and second pulse allows smoothing of the island shapes. Ripening, which is the coalescence of the smaller islands into larger islands is undesirable initially, since it allows growth to begin at a second layer earlier. Perfect layer by layer growth has not yet been achieved via PLD.

(2) 3D growth is characterized by island nucleation on top of a previously formed island.

Growth does not occur layer by layer, and results in surface roughness (1).

With PLD, it has been found that crystallization and interlayer transfer both occur within microseconds during the plume-substrate interaction. Efficient growth of the film begins by manipulating laser parameters to cause a high nucleation of islands with the

2 first laser pulse. The optimum island density is approximated by (1 / λ ) ≈ (1/Ds tc), where

tc is the time for island nucleation to coalescence, known as ripening, and Ds is the diffusion on the surface. Therefore it is advantageous to suppress ripening by filling the first layer very quickly. This can be achieved by having successive laser pulses deposit less material, and maintaining the deposition rate by increasing the repetition rate of the laser (2).

LATTICE MISMATCH

Optimum YBCO thin film growth via pulsed laser deposition depends on several variables, including laser energy, pulse rate, distance of substrate from the target, temperature of the deposition, composition of the target, and the choice of substrate.

Whereas the energy of the laser, pulse rate of the laser, temperature of the substrate, and the distance of the substrate from the heater block effect the nucleation and growth of the film, the choice of substrate and the dopants chosen in YBCO targets possess various degrees of lattice mismatch. The degree of mismatch effects epitaxial growth of doped

YBCO films, the sharpness of interfaces, and the resulting strain in the films. The amount of strain is released by defects in the crystal structure. These defects have a direct impact on current densities and critical temperatures of the thin films produced.

The term epitaxy comes from the Greek root words epi, meaning placed on, and taxis, meaning arrangement. Epitaxy refers to a crystalline film on a crystalline substrate.

Heteroepitaxy describes the case when the film and substrate are composed of different materials. Different crystalline materials possess different lattice parameters. The difference in lattice parameters is referred to as lattice mismatch. Lattice parameters for various compounds utilized in this research are found in Table I. The lattice parameters

pdf’s are from the Joint Committee on Powder Diffraction Standards (jcpds), and can be found at http://www.icdd.com/products/index.html,

Table I. Lattice Parameters & Lattice Mismatch with respect to YBCO.

Crystal Lattice Lattice Lattice Overall Structure Parameter Mismatch to Mismatch Average (Å) YBCO wrt to YBCO Mismatch ab-avg. wrt c-axis YBCO Perovskite a = 3.825 PDF #40-0169 b = 3.886 c = 11.66

Y211 Othorhombic a = 7.132 Y211a/YBCOa Y211b/YBCO -0.15% b-avg = -7.5% PDF #38-1434 b = 12.18 = +4.5% Y211c/YBCOa c = 5.659 b-avg

=-2.1% (2:1unit cell) or -27% (2:3unit cell)

Y2O3 Cubic 10.604 -2.8% -9.1% -5.6% PDF #43-1036 45˚ rotation

BaHfO3 Cubic 4.161 7.9% 7.1% 7.5% PDF # Pm-3m(221) 00-024-0102 BaSnO3 Cubic 4.116 6.8% 5.9% 6.3% PDF # 00-015-0780 BaZrO3 Cubic 4.193 8.8% 7.9% 8.3% PDF # Pm-3m 00-006-0399 LaAlO3 Cubic 3.7913 -1.65% -1.65% PDF # Perovskite 01-073-3684 Pm-3m(221)

SrTiO3 Cubic 3.903 1.24% 1.24% PDF # Pm-3m(221) 01-070-8508

Lattice mismatch (f) can be calculated by the following equation: f = [ao(s) - ao(f)] / ao(f); where ao(s) is the lattice parameter of the substrate, ao(f) is the lattice parameter of the film. This lattice mismatch, along with the different coefficients of thermal expansion for different materials, results in almost all thin films possessing internal stress, without applied external forces. Because of this, it is usually desired to minimize lattice mismatch and choose materials of similar thermal conductivities to promote epitaxial growth of thin films. If the lattice mismatch is a positive number, the initial film layers will be in tension, while the substrate will be in compression. If the lattice mismatch is a negative number, the initial film layers will be in compression, while the substrate will be in tension. The shrinkage of the film in compression leads to surface roughening of the film. Also if ao(s) ≅ √2 ao(f), the film growth will rotate 45° with respect to the substrate (Fig. 6).

Epitaxial film growth on the vicinal surface of a miscut substrate of 2-3°, with conditions of high temperature and low fluxes, allows adatoms to nucleate at a step edge.

This results in step flow growth. When the lattice mismatch between the film and the substrate is less than 9%, the film grows pseudomorphically, with a strained layer accommodating the mismatch. The pseudomorphic film growth results in coherent epilayers. This continues until a critical film thickness is reached. When the critical film thickness is in the range of a few thousand angstroms, it can be related to the burgers vector magnitude (b) and the lattice mismatch by the following equation: dc ≅ b/2f. In other words, pseudomorphic growth will continue until the total amount of misfit, dcf , exceeds half of the unit cell length. Above the critical film thickness, the misfit strain is released by dislocations in the crystal structure of the film. Dislocation line defects occur as edge or screw defects. Five types of defects can occur in epitaxial films: defects propagating from the substrate, stacking faults, dislocation loops, low angle grain boundaries, and misfit dislocations. Screw dislocations originating at the substrate surface continue in the film as adatoms nucleate at the “…ledge sites of a spiral staircase.” The rest of the film grows epitaxially and defect free. Stacking faults involve changes in the order of the stacking plane, for example ABCABC. Stacking faults occur at dislocations and at oxide precipitates at the substrate interface. The nuclei containing stacking faults coalesce with those lacking stacking faults, to form an inverted pyramid as the film grows. Dislocation loop defects occur in films that contain a high level of dopants or impurities. Dislocations that are stacked vertically create small angle grain boundaries. Grain boundaries occur at the interface between two differently oriented crystal grains. Grain boundaries in thin films tend to be reactive, and also impede

current flow. Misfit dislocations lie in planes that are parallel to the interface. They originate from threading dislocations in planes that intersect the interface. Misfit dislocations generally form by stress concentrating particulates since large misfits of 1-

2% are necessary for nucleation. The various defects are shown in Fig. 7.

Abnormal grain growth leads to different crystal orientations of the grains (Fig.

8). The grains of the film form parallel to the film surface, whereas the columnar morphology forms perpendicular to the film surface. Grain growth in films stops when the grain size is two to three times the film thickness. If one grain orientation occurs more often, as opposed to a random orientation of the grains, the film is referred to as being textured with a preferred orientation. Consequently, an amorphous film has no texture. Twinning and “competition between surface and strain energy” leads to texture.

Low strain energy oriented grains are preferred over higher strain energy grains. The

substrate composition and roughness effects the film texture because it influences the adatom diffusion lengths. Adatom mobility due to higher deposition temperatures also effects the film texture. Thin films grown by evaporation methods possess less texture than those grown from sputtering and PLD. While x-ray diffraction provides information on the “macrotexture” of many grains, electron backscatter (EBSD) with

SEM can provide information on the “microtexture” by imaging one grain at a time.

As mentioned previously, when the lattice mismatch between the film and the substrate is less than 9%, the film grows pseudomorphically, with a strained layer accommodating the mismatch. If the lattice mismatch is a positive number, the initial film layers will be in tension, while the substrate will be in compression. If the lattice mismatch is a negative number, the initial film layers will be in compression, while the substrate will be in tension. The degree of mismatch effects epitaxial growth of doped

XRD ANALYSIS: GRAIN SIZE, GRAIN ORIENTATION, FILM STRESS

XRD analysis provides information in regards to the grain size, grain orientation within films, and the presence of stresses within the film. When the lattice mismatch between the substrate and film is less than 15%, epitaxial growth occurs consisting of columnar grains with strong texture. Texture refers to the preferred orientation of the grains, as opposed to a random orientation (5). The simplest way to assess the texture of a film is to use a symmetric diffraction geometry in a Bragg Brentano arrangement.

Symmetric refers to the incident and diffracted beam making the same angle with respect to the film surface, and is accomplished by having the detector move at twice the angular speed (2θ) when compared to the incident beam (θ). This allows diffraction within grains that are parallel to the film’s surface. The intensity of the peaks corresponds to the volume percent of the grains diffracting at each corresponding peak angle. Asymmetric geometry is another type of xrd geometry that is useful in determining the residual stress

within the film. With asymmetric geometry, the incident beam is less than ten degrees with respect to the sample surface, while the detector moves while remaining centered on the sample. Resulting reflections are from grains oriented at different tilts with respect to the sample surface. It allows for a study of residual stresses by comparing the interplanar spacing of planes at different orientations within the film. The asymmetric geometry can be utilized with the Seemann-Bohlin (SB) diffractometer and with Parallel beam optics

(PB). Stress analysis can also be accomplished with the BB diffractometer by decoupling the sample and detector axes (5). Single line profile analysis of integral breadth, the ratio between peak area and full width half max (FWHM), provides quick information on profile broadening.

In XRD analysis, peak broadening can result from instrument effects, finite crystallite size, strain, and extended defects. Size and strain information can be obtained by utilizing the Scherrer formula, integral breadth analysis, and peak shape analysis. The

Scherrer formula provides average crystallite size, while neglecting strain effects.

Integral breadth methods provide average values of size and strain, in contrast to peak shape methods, which provide information on the distributions of size and strain.

The Scherrer formula relates crystallite size to peak broadening:

퐾휆 Dv = 훽푐표푠휃; Dv = volume weighted crystallite size

K = Scherrer constant .87-1.0 휆 = wavelength of radiation

훽 = integral breadth of a reflection in radians 2휃 located at 2휃. The Scherrer formula can be used to estimate the size of domains between defects in the crystal (6).

Scherrer formula size effects grain domain size is inversely proportional to the peak profile width. It is less reliable as the profile width decreases (size range less than

200 nm), and gives good results for a size range of 200 – 500 nm (7).

Strain in thin films is a culmination of microstrains and macrostrains.

Microstrain is the average strain within a single grain. It occurs on the atomic level due to defect distribution, inhomogeneity of the film, and heterogeneous phases. Residual macrostrain is the average strain over a macroscopic region. It results from lattice mismatch at the film/substrate interface and residual thermal stress. Thermal stresses arise during cooling from high deposition temperatures in PLD, from differences in thermal conductivity between the substrate and the thin film material. XRD cannot give information on the microstresses within individual grains. XRD provides information on

“pseudo-macro stress”, which is why XRD stress tests do not always agree with mechanical strain tests (5).

With X-ray residual stress analysis (XRSA) the crystalline grains behave as a

“microscopic strain gauge”. The strain field is calculated by measuring the interplanar spacing with the aid of Bragg’s law, with the grains oriented in different directions with respect to the surface. Macrostrain causes a change in dhkl , which relates to a corresponding shift in xrd Bragg reflections (7).

By definition, strain is the change in length divided by the original length. The strain may be uniform or non-uniform. Within a crystalline structure, uniform strain causes the unit cell to expand or contract isotropically. This changes the unit cell parameters and results in a shift in the xrd peaks. No broadening of the peaks occurs with

uniform strain. With non-uniform strain, there is a systematic shift of atoms from their ideal positions. The non-uniform strain can be due to point defects, plastic deformation, or poor crystallinity, and results in peak broadening (6).

Following Hooke’s Law, the load causes a change in lattice spacing, d, which changes the diffraction peak positions (8). The strain, e, is the average change in the lattice spacing δd over the diffracting volume: e = δd /d. From Bragg’s law, 2(d + - δd) sin (θ +- δθ) = λ. The fluctuation of δd causes line broadening δθ (5). However line broadening is a sum of crystallite size, strain, and instrument effects.

For Lorentzian peak shape:

Bexp = Bsize + Bstrain + Binst; Bexp = experimental FWHM

Bsize = FWHM due to crystallite size

Bstrain = FWHM due to strain

Binst = FWHM due to the XRD instrumentation.

For Gaussian peak shape:

2 2 2 2 Bexp = Bsize + Bstrain + Binst

Instrument broadening effects can be subtracted by measuring the full width half maximum, (FWHM), using a powder sample of the film constituents, since the powder has small grain size and does not have strain broadening effects. Williamson – Hall plot of Bexp cos θ verses sin θ can be utilized to separate the size and strain components. A horizontal line indicates that size broadening is the main contributor to the width of the peaks. If strain is the main contributor, the line will have a slope other than zero (9).

The sin2Ψ method is based on the simple case of in-plane stress, or biaxial model, where the stress perpendicular to the film surface is zero. A sin2Ψ plot of lattice parameter a or the lattice spacing d verses sin2Ψ shows the relationship between the tilting angle Ψ of the sample and the stress in the film (8). The lattice parameter is attained by measuring the d spacing of a specific family of planes at different sample inclinations of Ψ, while rotating the sample perpendicular to the film surface in the ω direction. It is recommended to look at higher order reflections at higher diffraction angles for the greatest sensitivity of changes in peak position. A positive slope indicates tensile stress, whereas a negative slope indicates a compressive stress in the film (5). If the d spacing for Ψ < 0 is not the same as for Ψ > 0, it is referred to as psi splitting, and indicates triaxial stresses are present. An oscillatory curve results from a highly textured film (9).

FILM STRAIN, FLUX PINNING, & CURRENT DENSITY RELATIONSHIP

Strain has been shown experimentally to effect the vortex pinning of high temperature superconductors. A. Llordés, et al., found that the non-coherent interface between the BZO dopant and YBCO predominantly controlled the amount of non- uniform nanostrain and isotropic vortex pinning when compared to the dopant concentration effect. However a maximum dopant concentration exists beyond which the current density begins to decrease, despite the strain enhanced vortex pinning (10). Van der Laan, et al. have also found experimentally that grain boundary tensile strain decreases current density (11). Other research has shown that twin boundaries form to alleviate strain due to lattice mismatch between YBCO film and STO substrates; and that an increase in twin boundaries resulted in an increase in current density (12)

CHAPTER IV

SUMMARY OF PAST RESEARCH & BASIS OF FUTURE RESEARCH

SUMMARY In Type II superconductors, the magnetic field penetrates in single tubes of non-superconducting material, called vortices. A fluxoid is the circulating vortices of current and the flux within the vortices. Defects within the superconducting film pin the vortices; i.e. the defects prevent the vortices from moving. Addition of nanophase defects enhances flux pinning and thereby increases the current densities of YBCO thin films. These additions in the thin films are classified as one, two, and three dimensional (1D,2D, and 3D) artificial pinning centers (APC). Previous studies conducted have focused on single phase additions of many materials including: Y2BaCuO5 (Y211) nanoparticles, Y2O3 nanoparticles, BaSnO3 (BSO) nanorods, Ba2YNbO6 (BYNO) nanorods, and BaZrO3 (BZO) nanoparticles or nanorods. Thin films can be produced utilizing various pulsed laser deposition (PLD) techniques: including the use of a mixed target composition, the use of individual targets with a multilayer PLD technique, and the use of a mixed YBCO and 2nd phase target with the addition of a pie sector, and incorporating decorated substrates. This chapter discusses in detail previous research conducted and the basis of future research incorporating artificial pinning centers, and the optimization of processing parameters to produce superconducting thin films with high current densities. The 3D-APC of Y211 exhibits a smaller overall average lattice mismatch with YBCO, when compared to other inclusions, such as Y2O3, BSO, and BZO. Past research showed that doping targets of YBCO with 1.6 wt. % Y211 nanoparticles resulted in thin films with slightly higher current Jc at higher fields than pure YBCO films. YBCO doped films have been produced with both mixed doped targets and with the multilayer film approach. The simplicity of Y211 doping of a single target in PLD of thin films for enhancing pinning is attractive. It is of interest to investigate and optimize the addition of various volume percents of Y211 on flux pinning, current densities, and induced strain in YBCO thin films. Previous research has also focused on combining the artificial pinning centers resulting from BZO nanorods and Y2O3 nanoparticles, however a detailed optimization of volume percents of BZO dopants combined with Y2O3 is lacking. Since addition of BaSnO3

nanorods has already achieved higher pinning force densities than BZO nanorods, it is of interest to study if the addition of Y2O3 nanoparticle pinning with BSO further enhances the overall flux pinning landscape. This research also will investigate the effects on current densities of combining Y211 nanoparticles and BaSnO3 nanorods. The flux pinning mechanism for BaHfO3 and Y2O3 double doped YBCO films will also bestudied. The theoretical temperature dependence of the current density, Jc(T), will be mathematically modeled with respect to the isotropic weak and anisotropic strong pinning contributions, for each system studied.

Recall that in Type II superconductors, the magnetic field penetrates in single tubes of non-superconducting material, called vortices. The vortices contain a quantum of magnetic flux each, and are surrounded by electric current that shields the surrounding superconductor. The vortices repel each other and arrange in a triangular lattice pattern, which is the lowest energy arrangement. Because of this, Type II superconductors are referred to as being a mixed state. A fluxoid is the circulating vortices of current and the flux within the vortices. Defects within the superconducting film pin the vortices; i.e. the defects prevent the vortices from moving. Flux creep occurs when the fluxoids overcome the small pinning force and at that point superconductivity is lost. The goal is to add nanophase defects to enhance flux pinning and thereby increase current densities of

YBCO thin films. YBCO thin film’s high current density results from pinning centers associated with point defects from oxygen vacancies and associated with twin and grain boundaries. Addition of nano-sized second-phase inclusions to YBa2Cu3O7 (YBCO) superconducting thin films enhances flux pinning by incorporating additional pinning centers, resulting in an increase in current densities (Jc) (1,1-24). Previous studies conducted have focused on single phase additions of many materials including:

Y2BaCuO5 (Y211) nanoparticles, Y2O3 nanoparticles, BaSnO3 (BSO) nanorods,

Ba2YNbO6 (BYNO) nanorods, and BaZrO3 (BZO) nanoparticles or nanorods (3-6,8-10).

These nano-sized additions are accomplished through various pulsed laser deposition

(PLD) techniques: including the use of a mixed target composition, the use of individual targets with a multilayer PLD technique, and the use of a mixed YBCO and 2nd phase target with the addition of a pie sector incorporating Y2O3,, and incorporating decorated substrates (1-24).

Nanophase additions can also be classified as one, two, and three dimensional artificial pinning centers (1D - APC, 2D - APC, 3D - APC), with 1D - APC’s displaying linear defects such as BSO and BZO nanorods. Nanosize 2D – APC’s display planar defects, and include “... small angle grain boundaries, anti-phase boundaries, and surfaces of large precipitates.” Nanosize 3D – APC’s result from nanoparticles, such as Y2O3 and

Y211 (7).

In particular, the 3D-APC of Y211 exhibits a smaller overall average lattice mismatch with YBCO, when compared to other inclusions, such as Y2O3, BSO, and BZO, as evidenced in Table 1. Lattice mismatch, along with the different coefficients of thermal expansion for different materials, results in almost all thin films possessing internal stress, without applied external forces. Because of this, it is usually desired to minimize lattice mismatch and choose materials of similar thermal conductivities to promote epitaxial growth of thin films (25). Past research showed that doping targets of

YBCO with 1.6 wt. % Y211 nanoparticles resulted in thin films with slightly higher current Jc at higher fields than pure YBCO films (17). Previous research also confirmed that Y211 nanoparticles at 5 vol. % prove to be efficient and strong 3D pinning centers throughout the film thickness (18,19). Y211 nanoparticles have been included in thin films by pulsed laser deposition of single targets consisting of YBCO and Y211, and as

multilayer films by alternating targets of YBCO and Y211 during the deposition (20-24).

While the Y211 multilayer films produced better pinning at fields less than 3 – 4 T, the simplicity of Y211 doping of a single target in PLD thin films for enhancing pinning is attractive (22). It is of interest to investigate and optimize the addition of various volume percents of Y211 on flux pinning, current densities, and induced strain in YBCO thin films.

Previous research has also focused on combining the artificial pinning centers resulting from BZO nanorods and Y2O3 nanoparticles. Liu utilized a 1.5 vol. % BZO and

YBCO target to produce films on LAO decorated with Y2O3 nano-islands(12).

Optimization of film thickness for films consisting of YBCO with BZO and Y2O3 has also been studied, utilizing a 5 mol % BZO and 5 mol % Y2O3 target (15). It is hypothesized that the combination of BZO and Y2O3 with YBCO influences the lattice strain in the film. This research seeks to explore the effects on current density of doping a

YBCO target with 2, 4, and 6 volume percent BZO along with 3 vol. percent Y2O3, with the remaining volume percent YBCO. Since addition of BaSnO3 nanorods has already achieved higher pinning force densities than BZO nanorods (7), it is of interest to study if the addition of Y2O3 nanoparticle pinning with BSO further enhances the overall flux pinning landscape (9). Addition of Y2O3 nanoparticles results in an ab in-plane lattice mismatch of -5.6%, whereas addition of BSO nanorods results in an ab in-plane lattice mismatch of +6.3% (26). This research seeks to explore the effects of cancelling the stresses due to the lattice mismatch, and possible benefits on current densities by combining Y2O3 nanoparticles and BaSnO3 nanorods. To our knowledge, the effects of combining BSO and Y2O3 additions to YBCO were not researched yet. This research also

will investigate the effects on current densities of combining Y211 nanoparticles and

BaSnO3 nanorods.

Since hafnium, is found below zirconium on the periodic table, it is also of interest to also investigate the effects of doping YBCO with BaHfO3 , (BHO), and Y2O3.

S. Sato, et al., researched the effect of 1.5 and 3.0 wt. % BHO doped YBCO on the microwave surface resistance in high dc magnetic fields (27). M. Sieger, et al., studied the effect of 2, 4, and 6 mol.% BHO doped YBCO targets for deposition of thick films

(2µm) on STO and on Ni9W tapes with a PLD-Y2O3/YSZ/CeO2 buffer layer. (28) M.

Watanebe, et al., utilized 1.5, 2.0, and 3.0 wt. % BHO, with remaining percentage

YBCO, targets to study the effect of BHO on current densities of YBCO thin films (29).

This research proposes to investigate the pinning effects resulting from targets comprised of 2, 4, and 6 volume percent BHO with 3 volume percent Y2O3, and the remaining volume percent YBCO.

Lattice mismatch is also an important consideration in choosing a substrate for epitaxial growth of YBCO films. LaAlO3 (LAO) and SrTiO3 (STO) substrates will be utilized in the majority of the research.

Flux pinning centers can also be categorized as weak isotropic pinning centers and strong anisotropic pinning centers, each of which can be mathematically modelled.

Weak isotropic pinning centers result from defects due to oxygen vacancies and

is-weak nanoparticle additions. They contribute to the isotropic current density, (Jc (푇)) for any direction of external magnetic field. Anisotropic strong pinning centers result from intrinsic pinning and extrinsic pinning. The intrinsic anisotropic pinning occurs along the copper oxide planes. The extrinsic pinning occurs due to linear and planar defects

parallel to the ab plane, such as stacking faults, and from defects aligned in the c direction. Examples of defects aligned in the c direction include threading dislocations, twin boundaries, and grain dislocations. Anisotropic strong pinning contributes to the

an-strong anisotropic critical current density, (Jc (푇)), and depends on the direction of the applied external magnetic field. The theoretical temperature dependence of the current density, Jc(T), can be mathematically modeled with respect to the isotropic weak and anisotropic strong pinning contributions according to the following equation:

is-w eak an-strong 2 Jc(T) = Jc (0) exp (-T / T0) + Jc (0) exp (-3(T / T+))

Isotropic Weak Pinning Contribution Anisotropic Strong Pinning Contribution

Jc(0) = current density at 0 K (extrapolated) T = temperature (K)

T0, T+ = curve fitting parameters associated with the energy of the defects.

(30-33) .

The anisotropic strong contribution is based on columnar pin line disorder, modeled as a

cylindrical well of radius c0 and solving the Ginzburg - Landeau equations, with T1 < T < Tdp. T1 is Figure 9. (a) Flux line interaction with a columnar pin. (b) Cylindrical square model of potential vortex binding energy, the characteristic pinning where U varies from U0 to U(T) due to thermal fluctuations (33). temperature and Tdp is the depinning temperature. T+ is the energy scale of the pin, which

is √͂ε1U0b0 . U0 is the vortex binding energy per unit length, which is determined by

2 solving the Ginzburg - Landeau equations. ε = (ϕ0 / 4πλab) , where ϕ0 is the magnetic

-7 2 fluxoid (2.07 X 10 gauss-cm ), and λab = London penetration depth of field at the superconductor surface. The anisotropic parameter, ε, determines the critical field Hc1.

The remaining variable in the formula for T+, b0, determines critical field, Hc2. b0 is the

max{c0, √2ξab }. In this formula, ξab is the superconducting coherence length, from BCS

theory, and is calculated by ξab = ħvf / πΔ, where ħ is the reduced Plank’s constant, Δ is the superconducting energy gap, and vf is the fermi velocity. Alternatively, ξab can be

ħ2 determined from the Ginzburg – Landeau equations to be √ ⁄2푚|훼| , where m is the mass of two electrons, and 훼 is a parameter (32-33).

The isotropic weak contribution’s exponential decrease is due to the inefficiency of point like defects (due to oxygen vacancies) to respond to and pin vortex motion due to thermal activation. It manifests as small bundle pinning. In the isotropic equation, T0 ≈ Uc

/ ln(t/t0), where Uc is the collective pinning activation energy and t/t0 is the time decay of the current density (31-33).

CHAPTER V

EXPERIMENTAL PROCEDURES

SUMMARY Experimental procedures involve pressing and sintering until the appropriate density is reached, pure YBCO and doped YBCO targets, which are to be used for pulsed laser deposition (PLD). Dopants utilized

vary the percentages of BaSnO3, Y2O3, BaZrO3, BaHfO3, and Y211incorporated into the YBCO targets. A Lambda Physik LPX 300

KrF excimer laser is utilized to produce thin films on LaAlO3 (LAO) and SrTiO3 (STO) single crystal substrates. Magnetic current densities (Jcm) and critical transition temperatures are measured with a Quantum Design Evercool II Physical Properties Measurement System (PPMS) with a vibrating sample magnetometer (VSM) probe. Angular transport current densities were also measured with a Quantum Design Physical Properties Measurement System (PPMS). Film characterization is attained via utilization of a scanning electron microscope (SEM), a Transmission Electron Microscope (TEM), and x-ray diffraction (XRD). Film thicknesses were measured with a KLA Tencor profilometer.

TARGET PREPARATION

Targets were produced via solid state processing of commercial powders of

YBCO, BaSnO3, Y2O3, BaZrO3, and Y211, which were dried in a box furnace (Fig. 10) for 8 hours at 450 ˚C. Sources of powders are listed in Table II. BaHfO3 was synthesized from BaCO3 and HfO2 commercial powder, which were also dried in a box furnace for 12 hours at 400 ˚C. BaCO3 (12.097g) and HfO2 (12.904g) powder was mixed with an agate mortar and pestle, followed by pressing with a Carver die press, into a 1.25 inch diameter

pellet. The pellet was initially heat treated and followed by regrinding and repressing of the pellet three times, at temperatures of 800, 830, and 860 ˚C. XRD results showed an incomplete reaction of BaCO3 and HfO2. The ground powders were further heated in powder form in a box furnace using the following heat treatment: 1. Ramp rate 150 ˚C/ hr to 1000 ˚C. Hold at 1000 ˚C for 2 hours. This step converted BaCO3 to BaO and CO2. 2.

Ramp rate 150 ˚C/ hr to 1400 ˚C. Hold at 1400 ˚C for 8 hours. This step reacts BaO with

HfO2 to form BaHfO3 (1). The dried powders were measured and mixed with an agate mortar and pestle to comprise compositions listed in Table III. The mixtures were then individually pressed utilizing a Carver die press with 1.25 inch and 1.125 inch diameter dies and a pressure of 1000 psi. The targets were then sintered at 850 ˚C for 60 hours and

920 ˚C for 156 hours. The target comprised of 10 vol. % BaSnO3 was re-sintered at 850

˚C for 30 hours and 920 ˚C for 78 hours to achieve a higher level of densification. Final densities of the three targets ranged between 86% and 93% theoretical densities (TD). A

YBCO only target was prepared by similar conditions, with final TD of 93.0%.

Figure 10. Lindberg box furnace utilized for drying powders and sintering target.

Table II. Target Powders Sources

Powders Source YBCO Nexans (Y123), Lot # WA 14-13

BaSnO3 Cerao Specialty Inorganics (BSO) #B-1032, lot #71069-A-1

BaZrO3 SCI Eng. Mtls. Lot #905-0022sp, P0560400000035 (BZO)

Y2O3 Alfa AESAR stock# 40759, lot# P4598A, CAS#1314-36-9

Y2BaCuO5 Y211: Superconductive Components Inc. P00ASCP2110030, Lot #6353sep4

HfO2 Alfa AESAR stock # 35666, CAS #10255-23-1

BaCO3 Alfa AESAR stock # 10645, CAS #513-77-9

Table III: Target compositions by volume percent. Weight %’s are as batched.

Theoretical Target YBCO BSO BZO BHO Y O Y211 2 3 Density % % % % % % % Vol. Wt. Vol. Wt. Vol. Wt. Vol. Wt. Vol. Wt. Vol. Wt. TS146 95.0 95.1 5.0 4.9 92.0

TS147 90.0 90.3 10.0 9.7 90.2

TS148 85.0 85.4 15.0 14.6 89.1

TS169 100.0 100.0 90.7

TS187 87.0 86.4 10.0 11.3 3.0 2.4 85.5

TS188 92.0 91.9 5.0 5.7 3.0 2.4 88.8

TS189 94.0 94.2 3.0 3.4 3.0 2.4 92.6

TS190 100.0 94.9

TS191 100.0 94.1

TS192 87.0 85.9 10.0 11.2 3.0 2.9 87.9

TS193 92.5 92.7 7.5 7.3 93.6

TS194 87.5 87.8 12.5 12.2 93.1

TS196 98.0 98.3 2.0 1.7 92.4

TS201 95.0 95.9 2.0 1.7 3.0 2.4 94.9

TS202 93.0 94.1 4.0 3.5 3.0 2.4 90.4

TS203 91.0 92.3 6.0 5.3 3.0 2.4 86.4

TS204 95.0 95.0 2.0 2.6 3.0 2.4 91.4

TS205 93.0 92.5 4.0 5.1 3.0 2.4 87.4

TS206 91.0 90.0 6.0 7.7 3.0 2.3 82.5

TS208 98.0 97.4 2.0 2.6 92.1

SUBSTRATE PREPARATION

Thin films were produced using pulsed laser deposition on LaAlO3 (LAO) and

SrTiO3 (STO) single crystal substrates, having dimensions of 3.2 mm x 3.2 mm x 0.5 mm and 4.0 mm x 10.0 mm x 0.5 mm. Substrates were cleaned via an ultrasonic cleaner for five minutes with acetone, followed by five minutes with isopropyl alcohol (Fig.11). The substrates were mounted to a YBCO coated heater block with colloidal silver paint

(Fig.12). The heater block was then inserted into the PLD chamber and pumper down to a

10-06 torr vacuum overnight.

Figure 11. Branson 3210 ultrasonic cleaner used in AFRL research laboratory.

Figure 12. Heater block used in research.

Pulsed Laser Deposition (PLD)

A Lambda Physik LPX 300 KrF excimer laser (λ=248nm) with a fluence of approximately 1.6 J/cm2, was utilized for the depositions. Laser deposition parameters for the various doped targets are stipulated in Table IV. After deposition, the films were annealed at 500 ⁰C in an oxygen atmosphere for a 30 minute dwell time.

Figure 13. Excimer laser and pld chamber used in research.

Table IV. Laser Deposition Parameters for Various Doped YBCO Targets.

Target Deposition O2 Partial Laser Laser Deposition Composition Temperature Pressure Energy Repetition Time (calibrated) Rate (minutes) (˚C) (mtorr) (mJ) (Hz)

YBCO 790 300 450 4 18

YBCO + Y211 805, 825, 835 300 450 4 18

YBCO + BSO 770, 780, 795, 300 450 4 18 + Y2O3 815, 825

YBCO + 745, 760, 775, 300 450 4 20 BSO + Y211 790, 805

YBCO + 810, 825 300 450 8 7 BZO + Y2O3

YBCO + 790, 810, 825, 300 450 8 7 840 BHO + Y2O3

CURRENT DENSITY MEASUREMENTS AND CRITICAL TEMPERATURES

Magnetic current density (Jcm) and critical transition temperature (Tc) were measured with a Quantum Design Physical Properties Measurement System (PPMS) with a vibrating sample magnetometer (VSM) probe (Fig.14). The VSM data was attained for conditions of 77 K, 65 K, 20 K, and 5 K with an applied field varied from 0-9 T for H //

c-axis of the films.

(a)

(c)

(b)

Figure 14 (a) in-house sputter-rig Kansan University, (b) Quantum Design Evercool II vsm-ppms used in current density measurements, (c) parallel bridge mask 20µm and 40µm width and 500µm length.

Tc-onset temperatures of the films were attained from zero-field-cooled measurements. The measured magnetic data was used to determine the current densities by utilizing a simplified form or the Bean Critical State Model (2-4):

3 2 Jcm = 30Δm/da (A/cm ) Δm = change in magnetization (emu) d = film thickness (cm) a = side length of square films (cm)

Transport current density measurements were conducted through collaboration with Dr.

Judy Wu’s research group at University of Kansas. Silver contacts were sputtered through a metal shadow mask on the films, to reduce the contact resistance to the microbridges. Microbridges of ≈ 20 µm width and ≈500 µm length were patterned utilizing photolithography. Film surfaces were spin coated with 351 photoresist, exposed at 500W UV for 70 s., developed for 50 s., followed by etching for 2 min. with 0.05%

HNO3. Platinum wires were attached to the microbridges with indium for the electrical connections. A Keithley 2430 1KW pulsed current source meter and HP 34420 nanovoltmeter were used to attain the transport current density. LabVIEW was used to control the increasing amplitude of the current pulse train and synchronize the resulting voltage detected across the film. A short 50 ms pulse width and a long time interval of

3000 – 4000 ms minimized heating. The current density was determined using a 1

µV/cm criteria. A Quantum Design Evercool II Physical Property Measurement System

(PPMS) was employed to measure the transport current density as a function of temperature, magnetic field (0-9T), and angle θ between the field and the c-axis of the film, maintaining the field perpendicular to the current. Transport current densities were measured at θ = 0˚ (corresponding to H // c), 45˚, and 90˚ (corresponding to H// ab).

FILM CHARACTERIZATION

SCANNING ELECTRON MICROSCOPE (SEM)

A FEI Sirion Scanning Electron Microscope (SEM) with ultra-high resolution at 5 kV and a spot size of 3 was used to obtain images of the films’ surface.

TRANSMISSION ELECTRON MICROSCOPE (TEM)

Cross-sectional Transmission Electron Microscopy (XTEM) images and Scanning

Transmission Electron Microscopy image (STEM) were obtained using a FEI Tecnai F20 analytical microscope under the acceleration voltage of 200KV. TEM work was courtesy of H. Wang’s group at Texas A & M University.

X-RAY DIFFRACTION (XRD)

Lattice parameters were determined by x-ray diffraction analysis utilizing the

Bruker D8 Discover diffractometer and Rigaku DMAX 2500. (High resolution, Cu tube

1.5418 Å, Power: 40kV, 40mA; 2T: 5-70˚; rocking curves for YBCO (005))

PROFILOMETER

Film thicknesses were determined by etching the thin film with nitric acid and measuring the stair step thickness with a KLA Tencor D-120. Two percent nitric acid was used to etch low volume % doped films (ex. 5 vol. % Y211 doped YBCO films) for 30 to

40 seconds. Ten percent nitric acid was used to etch films that were more difficult to etch

(4 vol. % BZO + 3 vol. % Y2O3 doped YBCO films) for 1 minute 30 seconds.

CHAPTER VI

RESULTS

1. Y211 DOPED YBCO FILMS

SUMMARY Addition of second-phase nanosize defects to YBa₂Cu₃O₇-δ (YBCO) superconductor thin films is known to enhance flux pinning and increase

current densities (Jc). The addition of Y2BaCuO5 (Y211) was studied previously in (Y211/YBCO)N multilayer structures, and in Y211+YBCO films deposited from pie-shaped targets (7-11) . This research systematically studies the effect of Y211 addition in thin films deposited by

pulsed laser deposition from YBCO1-xY211x (x = 0 - 15 vol. %) single targets, at temperatures of 785 - 840 °C. Interestingly, the resulting size of Y211 particles is 20 to 40 nm, in contrast to 10 to 15 nm in previous studies of Y211 and 5-10 nm for other 2nd-phase defect additions; and the

number density is reduced. A slight increase of Jc(H,T) was achieved, compared to previous optimization studies. Experimental data shows

optimized Jc(H) for H < 6 T at 65 K for a deposition temperature of 835- 840 ˚C and a repetition rate of 4 Hz respectively, for the optimal composition of 10 vol. % Y211. The pinning advantage and increased current densities resulting from the addition of 10 vol. % Y211 is maintained regardless of the applied field ranging from H = 0-8 T, and regardless of the temperature ranging from 5 K to 65 K. The optimized

films have a Tc-onset temperature of 89.7 K. Results of x-ray diffraction analysis confirm epitaxial, c-axis oriented film growth. Experimental results illustrate that a significantly higher percentage of 10 vol. % Y211

can be incorporated into YBCO films without degradation of Jcm, in contrast to previous research results from additions of BZO to YBCO films produced at our lab. Sebastian, Mary Ann P., et al. "Optimizing Flux Pinning of YBa₂Cu₃O₇-δ (YBCO) Thin Films with Unique Large

Nanoparticle Size and High Concentration of Y2BaCuO5 (Y211)

Additions." IEEE Trans. on Appl. Supercond., vol. 23, no. 3, p.8002104, 2013.

The artificial pinning center of Y211 exhibits a smaller overall average lattice mismatch with YBCO, when compared to other inclusions, such as Y2O3, BSO, and BZO, as evidenced in Table 1. Lattice mismatch, along with the different coefficients of thermal expansion for different materials, results in almost all thin films possessing internal stress, without applied external forces. Because of this, it is usually desired to minimize lattice mismatch and choose materials of similar thermal conductivities to promote epitaxial growth of thin films (1). The low degree of mismatch between the

Y211 nanoparticles and the epitaxial YBCO film lends to a sharper interface, as will be presented in the TEM results discussion. In other words, the YBCO serves as a substrate for the dopant, Y211. Threading dislocations exist in YBCO growing films due to the rotational relaxation of elastic strains. The highest surface energy exists at the exits of the threading dislocation cores. The high mobility of Y211 due to deposition temperatures, allows Y211 growth to begin at YBCO dislocations and lower the surface free energy (2).

The sharper interface between YBCO and the Y211 allows for larger particle growth. The

Y211 particle continues to grow until the limits of entropy, energy, and strain are reached. Misfit strain is released by dislocations in the crystal structure of the film (1). It is hypothesized that the larger size Y211 nanoparticles result in a high number of stacking faults in the surrounding YBCO lattice. This increase in stacking faults correlates with a higher current density (3). The Y211 nanoparticles also stop the movement of dislocations in YBCO associated with Lorentz forces. In pinning the dislocations, the flux vortices are consequently pinned also.

Past research showed that doping targets of YBCO with 1.6 wt. % Y211 nanoparticles resulted in thin films with slightly higher current Jc at higher fields than pure YBCO films (4). Previous research also confirmed that Y211 nanoparticles at 5 vol.

% prove to be efficient and strong 3D pinning centers throughout the film thickness (5),

(6). Y211 nanoparticles have been included in thin films by pulsed laser deposition of single targets consisting of YBCO and Y211, and as multilayer films by alternating targets of YBCO and Y211 during the deposition (7), (8), (9), (10), (11). While the Y211 multilayer films produced better pinning at fields less than 3 – 4 T, the simplicity of

Y211 doping of a single target in PLD thin films for enhancing pinning is attractive (9). It is of interest to investigate and optimize the addition of various volume percent of Y211 on flux pinning, current densities, and induced strain in YBCO thin films.

This research seeks to explore the effects on current density of doping a YBCO target with 5, 10, and 15 volume percent Y211 with the remaining volume percent

YBCO. Films were produced on STO and LAO substrates via PLD using an energy of

450 mJ, repetition rates of 2, 4, and 6 Hz, and a 300 mtorr O2 atmosphere, followed by annealing at 500 ˚C in an O2 atmosphere for 30 minutes. Resulting film thicknesses were

300, 275, and 312 nm for the 5, 10, and 15 vol. % Y211 doped YBCO films respectively.

From Fig. 15(a), (b): experimental data shows optimized Jc(H) for H < 6 T at 65 K for a deposition temperature of 835-840 ˚C and a repetition rate of 4 Hz respectively, for the optimal composition of 10 vol. % Y211. Fig. 15(c) depicts current density as a function of applied field for 5 K and 65 K, and illustrates that the pinning advantage and increased current densities resulting from the addition of 10 vol. % Y211 is maintained regardless of the applied field ranging from H = 0-8 T, and regardless of the temperature ranging

from 5 K to 65 K. The log-log plot shown in Fig. 15(d) results in an α = 0.40 and 0.27 for

YBCO and 10% Y211 respectively at 65K with an applied field between H = 0.1 - 1 T;

-α where α is the factor from Jc ∞ H (4). The resulting magnetic current densities and pinning forces for a range of temperatures and applied fields for the optimized 10 vol. %

Y211 film and their angular transport current densities are shown in Fig. 16. In Fig. 16,

Theta = 0˚ corresponds to the field H applied parallel to the c direction, and Theta =

90˚corresponds to the field H applied parallel to the a-b plane. As expected, the current density at 90˚ is greater than that at 0˚ because the CuO planes which carry the current lie in the ab direction. The difference in the magnitudes of the current density at 0˚ and 90˚ decreases at lower temperatures as seen when comparing the curves for Jct’s measured at

77K for 1 T and 5 T with those curves for Jct’s measured at 65K for 1 T and 5 T. This signifies more isotropic behavior in regards to current density and flux pinning at lower temperatures. As illustrated in Fig. 17, a Tc-onset temperature of 89.7 K for the optimized film was attained from zero-field-cooled measurements via the VSM-PPMS and is comparable to that of pure YBCO films.

(a) (b)

Figure 17. Tc-onset (K) verses deposition temperature (˚C) for YBCO and vol. % Y211 additions.

Microstructure studies included both SEM and TEM analysis. Referring to Fig.

18, comparisons of SEM images at 20K and 50K magnification produced from targets composed of composition 5 and 10 vol. % Y211 addition, show the corresponding larger grain sizes, increased pores, and defects changing due to Y211 addition.

(a) (b)

1 μm

(e) (f)

TEM analysis in Fig. 19, provides comparison of the microstructure of 5 vol. % and 10 vol. % Y211 doped YBCO. TEM micrographs illustrate remarkable large particle size of 30 to 40 nm (Fig.19(c)-19(e)). Analysis suggests that the Y211 dopants introduce compressive stress in the YBCO lattice (Fig.19 (d)). Fourier transform analysis reveals

that the misfit dislocation density in the Y-211 phase is higher for the 5 vol. % Y211 (Fig.

19 (f)) film than for the 10 vol. % Y211 film (Fig. 19(g)). However, the misfit dislocation density is lower in the YBCO phase of the 5 vol. % Y211 film, compared to the 10 vol. %

Y211 film. For the 5 vol. % Y211 film, the misfit dislocations that prefer to penetrate in the Y211 phase are located approximately 4 nm away from the interface. The linear dislocation density is ~1/nm, with the average spacing between adjacent misfit dislocations in the Y211 phase ~2 nm. No significant sign of impact on the YBCO properties can be found. For the 10 vol. % Y211 film, more misfit dislocations are generated in the Y211 phase and accumulate 4.5 nm away from the interface area, with a linear misfit dislocation density of ~1.5/nm. The average spacing between adjacent misfit dislocations is ~2.5 nm. The 10 % Y211 doped YBCO film shows higher effects on the

YBCO lattice properties. To summarize the TEM analysis, while misfit dislocations are present in the YBCO lattice, the dislocations predominate in the Y211 phases. Plane buckling and tilting in the YBCO lattice around the Y211 phases may provide additional flux pinning. YBCO lattices appear less stressed as the distance from Y211 particles increases.

(a) (b)

(e) (f)

(g) (h)

Figure 19. (a) - (d) TEM images of vol. % Y211 doped YBCO films: (a) - (d) 5 vol. % Y211 doped YBCO, (e) - (f) 10 vol. % Y211 doped YBCO, (g) - (h) Fourier transform analysis of 5 and 10 vol. % Y211 doped YBCO respectively. Courtesy of H. Wang’s research group Texas A & M University.

Results of x-ray diffraction analysis in Fig. 20 illustrate that the films are epitaxial c-axis aligned and provide information on the d spacing for the YBCO (005) plane based

on the Bragg equation (12)(13). Peak shift left indicates macrostrain and peak broadening indicates non-uniform strain. A FWHM > 0.2 indicates a broad peak.

Uniform strain is perhaps more important in increasing Jc, and agrees with the idea of the importance of a clean interface between the dopants and YBCO. XRD analysis in Table

V shows that the c- lattice parameter decreases with addition of Y211 dopants to YBCO films. This reaffirms the TEM analysis suggesting compressive stresses in the YBCO lattice, which causes the c – lattice parameter to shorten.

(a)

(b) )

Figure 20. (a) XRD 2Theta omega scans for 5, 10, and 15 vol. % Y211 doped YBCO films. (b) XRD rocking curve scans of YBCO (005) peak for 5, 10, and 15 vol. % Y211 doped YBCO films.

Table V. FWHM and c-lattice parameters calculated from XRD analysis for 5, 10, and 15 vol. % Y211 doped YBCO films. Film # Composition Deposition FWHM β c lattice Temp. ˚C (005) parameter (005) YBCO YBCO peak (Å) MR053 5 vol.% Y211 825 ˚C 0.509 0.595 11.607

MR002 10 vol.% Y211 825 ˚C 0.394 0.508 11.652

MR030 15 vol.% Y211 825 ˚C 0.455 0.564 11.638

2.5E+06

Y211 single-target 825-840°C 2.0E+06

) 1.5E+06 2 (A/cm 1.0E+06 cm J YBCO 775 C 5.0E+05 65K 3T single-target 810-825°C H //c-axis BZO 0.0E+00 0 5 10 15 20

Nanoparticle Volume %

Figure 21. Comparison of current density verses various volume % of BZO and Y211 doped YBCO films produced at AFRL/RQQM, WPAFB.

Experimental results illustrate that a significantly higher percentage of 10 vol. %

Y211 can be incorporated into YBCO films without degradation of Jcm, in contrast to previous research results from additions of BZO to YBCO films produced at our lab.

(Fig. 21) (11). Referring to Fig. 19, TEM analysis illustrates the extraordinary large particle size of 30-40 nm, and that a remarkable clean interface exists between Y211 and

YBCO, resulting in less stress in the structure. This clean interface is a direct result of the low degree of mismatch between Y211 and YBCO. The Y211 adatom mobility from high deposition temperatures, allows it to migrate to the YBCO threading dislocation cores. This lowers the high surface energy initially present at these dislocations (2). The

Y211 particles continue to grow until entropy, energy, and strain limits are reached. This resulted in much larger diameter particle size growth (30-40 nm), when compared to studies of previous nanoparticle growth, such as BZO (4-6 nm) (14). Whereas in YBCO thin films, vortices are weakly pinned at dislocations and point defects from oxygen vacancies and twin boundaries, doping with Y211 incorporates strong pinning centers.

(15). The Y211 nanoparticles stop the dislocation movement in YBCO films and is analogous to stopping crack progression by stopping dislocation movement in material science. In general, materials are stronger in compression than tension. It would be interesting to see if this also holds in regards to pinning force. The experimental data presented should be helpful for a theoretical investigation of a possible new-regime of collective pinning responsible for the increased pinning of the vortex lattice.

While many previous studies focused on single phase additions, the addition of several phases simultaneously shows promise in improving current density by combining different pinning mechanisms. This proposal seeks to systematically study the following mixed phase additions to YBCO targets to produce thin films by PLD: YBCO + BaZrO3

+ Y2O3, YBCO + BaHfO3 + Y2O3, YBCO + BaSnO3 + Y2O3, and YBCO + BaSnO3 +

Y211.

2. BZO + Y2O3 DOPED YBCO FILMS

SUMMARY Previous research has also focused on utilizing the artificial pinning centers resulting from BZO nanorods. It is hypothesized that the

combination of BZO nanorods and Y2O3 nanoparticles with YBCO influences the lattice strain in the film. This research seeks to explore the effects on current density of doping a YBCO target with 2, 4, and 6 volume

percent BZO along with 3 vol. percent Y2O3, with the remaining volume percent YBCO. Films were produced via PLD on STO substrates at deposition temperatures of 810 and 825˚C. Current densities were measured via a vsm-ppms for several temperatures from 77K-5K with applied fields of 1T to 9T. The highest current density was attained with

the YBCO film doped with 2 vol. percent BZO + 3 vol. percent Y2O3 at a deposition temperature of 825 ˚C. Interestingly, the double doped 2 vol. %

BZO with Y2O3 outperforms the single doped 2 vol. % BZO when applied fields exceed 4T. The flattening of the current density curve for the double doped film verses the single doped film signifies an increased isotropic behavior, which is confirmed with angular current density measurements. As the temperature decreases and the applied field increases, the magnitude of the strong pinning due to the dopants at 0˚surpasses the magnitude of the intrinsic pinning at 90˚. Collaborative research with Dr. Judy Wu’s group also showed this benefit of increased isotropic behavior in current density resulting from the utilization of double doping with BZO

and Y2O3 (1) . The critical temperatures were degraded at the higher deposition temperature, and for the optimized 2 vol. % double doped BZO film. This corresponds to increased stacking faults seen as deposition temperature increases and is also typical of a tradeoff between increased current densities and lower critical temperatures (2). Microstructure studies employing xrd analysis, SEM and TEM clearly show nice epitaxial

film growth, and confirm BZO c-axis alignment and the presence of Y2O3. S. Chen, M. A. Sebastian, B. Gautam, J. Wilt, T. Haugan, Z. Xing, J. Wu, “Enhancement of Isotropic Pinning Force in YBCO Films with BaZrO3 Nanorods and Y2O3 Nanoparticles,” IEEE Transactions on Applied Superconductivity, Vol. 27, No. 4, June 2017 J. Wu, J. Shi, F.J. Baca, R. Emergo, A. Elliot, J. Wilt, M. A. Sebastian, T. Haugan, C. V. Varanasi, "Probing Microscopic Strain Interplay Due to Impurity Doping and Vicinal Growth and its Effect on Pinning Landscape in YBCO Films," IEEE Transactions on Applied Superconductivity, Vol. 25, No. 3, June 2015

Previous research has also focused on combining the artificial pinning centers resulting from BZO nanorods (3) (4) (5) and Y2O3 nanoparticles (6). Liu utilized a 1.5 vol. % BZO and YBCO target to produce films on LAO decorated with Y2O3 nano- islands. (7). Optimization of film thickness for films consisting of YBCO with BZO and

Y2O3 has also been studied, utilizing a 5 mol. % BZO and 5 mol% Y2O3 target (8). It is hypothesized that the combination of BZO and Y2O3 with YBCO influences the lattice strain in the film. This research seeks to explore the effects on current density of doping a

YBCO target with 2, 4, and 6 volume percent BZO along with 3 vol. percent Y2O3, with the remaining volume percent YBCO. Films were produced on STO substrates via PLD using an energy of 450 mJ, a repetition rate of 8Hz, and a 300 mtorr O2 atmosphere, followed by annealing at 500 ˚C in an O2 atmosphere for 30 minutes. Resulting film thicknesses were 290, 247, and 256 nm for the 2, 4, and 6 vol. % double doped BZO +

Y2O3 respectively. Single doped BZO films were 135 nm thick.

The experimental results in Fig. 22 show the resulting current densities verses applied field at 77, 65, 20, and 5K for YBCO films with varying 2, 4, and 6 volume percent BZO and 3 volume percent Y2O3, at two different deposition temperatures of

810˚C and 825˚C. Fig. 22 (a) – (c) show current density for double doped film grown at

810 ˚C, while Fig. 22 (d) – (f) show current density for double doped films grown at 825

˚C. Looking at Fig. 22 (a) - (c), the 2 vol. % double doped BZO films (810 ˚C deposition temperature) attained a higher current density than YBCO films at 65K, but not at the lower temperatures of 20 and 5K. In contrast, the 2 vol. % BZO double doped films grown at 825 ˚C attained higher current densities than YBCO at both high and low temperatures. Comparing these graphs show that for all of the temperatures 77K-5K,

the highest current density was attained with the YBCO film doped with 2 vol. percent

BZO + 3 vol. percent Y2O3 at a deposition temperature of 825 ˚C. Interestingly, in Fig.

22 (g), the double doped 2 vol. % BZO with Y2O3 outperforms the single doped 2 vol. %

BZO when applied fields exceed 4 tesla. The single doped 2 vol. % film has a matching field of 3 T for 65K, as indicated by the *. The flattening of the current density curve for the double doped film verses the single doped film signifies an increased isotropic behavior, which is confirmed with angular current density measurements in Fig. 23. In the angular current density graphs shown in Fig. 23, Theta = 0˚ corresponds to the field H applied parallel to the c direction, and Theta = 90˚corresponds to the field H applied parallel to the a-b plane. As the temperature decreases and the applied field increases, the magnitude of the strong pinning due to the dopants at 0˚surpasses the magnitude of the intrinsic pinning at 90˚. Collaborative research with Dr. Judy Wu’s group also showed this benefit of increased isotropic behavior in current density resulting from the utilization of double doping with BZO and Y2O3 (1). Fig. 24 shows the variation in critical temperature verses vol. % BZO dopants in YBCO films, also doped with 3 vol. %

Y2O3, at deposition temperatures of 810 ˚C and 825 ˚C. The critical temperatures were degraded at the higher deposition temperature, and for the optimized 2 vol. % double doped BZO film. This corresponds to increased stacking faults seen as deposition temperature increases and is also typical of a tradeoff between increased current densities and lower critical temperatures (2).

Microstructure studies involved xrd analysis, along with SEM and TEM. From

Fig. 25 (a), the 2 Theta omega scan clearly shows nice epitaxial film growth, and confirms BZO c-axis alignment and the presence of Y2O3. The rocking curves were

measured for the YBCO (005) peak for each of the doped films. Recall that peak shift indicates macrostrain, and peak broadening indicates non-uniform strain. A FWHM >

0.2 defines a broad peak. Referring to Fig. 25 (b) and Table VI, the non-uniform strain increased as the volume percent BZO increased the double doped films, until some stress relief occurred at 6 vol. % BZO.

(a) (b)

(c)

(e) (d)

(f)

(g) (h)

Figure 22. Current density as a function of applied field measured at 77, 65, 20, and 5K for YBCO100-(x+3) BZO x (x = 2, 4, 6 vol. %) Y2O3 = 3 vol. % at: (a), (b), (c) deposition temperature of 810 ˚C, (d), (e), (f) deposition temperature of 825˚C. (g) current density as a function of applied field measured at 65 and 5 K for single doped 2 vol. % BZO and YBCO film and double doped 2 vol.% BZO + 3 vol.% Y2O3 YBCO film (STO substrate and 825˚C deposition temperature). Note * signifies matching field for single doped film. (h) corresponding pinning force curves for optimized 2 vol. % BZO + 3 vol. % Y2O3 doped YBCO film at 65, 50, 20, and 5K and for YBCO film shown as black curve.

(a)

(b)

Figure 23. Angular dependence of current density measured at (a) 77K and (b) 65K at 1T, 3T, 5T, and 9T for 2 vol. % BZO + 3 vol. % Y2O3 doped YBCO film.

Deposition Temperature 810 ˚C

Deposition Temperature 825 ˚C

Figure 24. Critical temperatures (Tc) for 2, 4, and 6 vol. % BZO with 3 vol. % Y2O3 doped YBCO films, deposited at 810 ˚C and 825 ˚C. Reference Tc for undoped YBCO films shown as black circles.

(a)

(001)

(b)

Figure 25. (a) XRD 2Theta omega scans for 2, 4, and 6 vol. % BZO with 3 vol. % Y2O3 doped YBCO films. (b) XRD rocking curve scans of YBCO (005) peak for 2, 4, and 6 vol. % BZO with 3 vol. % Y2O3 doped YBCO films.

Table VI. FWHM and c-lattice parameters calculated from XRD analysis for 2, 4, and 6 vol. % BZO with 3 vol. % Y2O3 doped YBCO films. Film # Composition Deposition FWHM β c lattice Temp. ˚C (005) parameter (005) YBCO YBCO peak (Å) TJ2734 2 vol.% BZO + 825 ˚C 0.839 0.988 11.727

3% Y2O3 TJ2712 4 vol.% BZO + 825 ˚C 0.842 1.026 11.693

3% Y2O3

TJ2768 6 vol.% BZO + 825 ˚C 0.489 0.665 11.732

3% Y2O3

From the SEM micrographs in Fig. 26, one can see how the surface of the film doped with 2 vol. % BZO + 3 vol. % doped YBCO (c,d) contains an increased number of pores and defects in comparison to the YBCO (a,b) film. The effect of the addition of

3 vol. % Y2O3 also has a remarkable impact on the film surface, which can be seen by comparing Fig. 26 (e,f), the double doped 2 vol. % BZO to Fig. 26 (g,h), the single doped

2 vol. % BZO film. Interestingly in Fig. 26(h), within the pores of the film, square shapes are imaged, and are apparently the cross sections of BaZrO3 nanorods, which have a cubic unit cell perovskite structure (9). The number of pores and defects also increases with increasing deposition temperature, due to the increased adatom mobility (e,f). This effect of temperature is also shown with the 4 vol. % BZO double doped film in Fig. 27.

In comparing Fig. 26, 27, and 28, the defects and pores and grains also increase with the composition of volume % BZO, with evidence of a-axis film growth associated with the basket weave pattern shown in Fig. 28(a) (10). TEM in Fig. 29, shows the increased

dopants present in comparing the 2, 4, and 6 volume % double doped BZO films, and especially the impact on the BZO nanorod dimensions (1).

(a) (b)

(e) (f)

(g) (h)

Figure 26. SEM 20kX and 50kX respectively: (a), (b) YBCO (c) (d) 2 vol.% BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 810 ˚C deposition temperature; (e), (f) 2 vol.% BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature; (g), (h) 2 vol. % BZO doped YBCO on STO substrate at 825 ˚C deposition temperature.

(a) (b)

Figure 27. SEM 20kX and 50kX respectively: (a), (b) 4 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 810 ˚C deposition temperature. (c), (d) 4 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature.

(a) (b)

Figure 28. SEM 20kX and 50kX respectively: (a), (b) 6 vol.% BZO + 3 vol.% Y2O3 doped YBCO on STO substrate at 810 ˚C deposition temperature; (c), (d) 6 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature.

(a) (b)

dopants

STO

5 nm 5 nm

YBCO

STO

(e) (f)

50 nm 50 nm

Figure 29 TEM intermediate and high resolution images respectively: (a), (b)

2 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature. (c), (d) 4 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature. Courtesy of H. Wang’s research group Texas A & M University. (e) 4 vol. % BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature. (f) 6 vol. %

BZO + 3 vol. % Y2O3 doped YBCO on STO substrate at 825 ˚C deposition temperature. (e) and (f) courtesy of Nanjing University in collaboration with University of Kansas.

3. BHO + Y2O3 DOPED YBCO FILMS

SUMMARY Since hafnium, is found below zirconium on the periodic table, it is also of interest to also investigate the effects of doping YBCO with BaHfO3, (BHO), and Y2O3. Previous research in the field has only looked at small weight and mole percentages of BHO single doped YBCO targets. This research investigates the pinning effects resulting from targets comprised of 2, 4, and 6 volume percent BHO with 3 volume percent Y2O3, and the remaining volume percent YBCO. Films were deposited via PLD on LAO and STO substrates. Current densities and critical temperatures were measured with the Quantum Design vsm-ppms. An initial deposition temperature study was conducted with films produced from the 4 vol. % BHO double doped YBCO target on LAO substrates, at temperatures of 790, 810, 825, and 840 ˚C. The highest current densities are attained with the optimum deposition temperature of 810 ˚C for high temperature applications (65K) and 790 ˚C for low temperature applications (5K).This was followed by depositions varying the volume % BHO double doped films at the deposition temperature of 810 ˚C. The highest current densities are attained with an optimum dopant concentration of 2 vol.% BHO + 3 vol. % Y2O3 at 65K and fields less than 5T, and with 4 vol.% BHO + 3 vol. % Y2O3 at 65 K and fields greater than 5T. For operating temperatures less than 65K, the 2 vol. % BHO double doped film provides the highest current densities. Fig. 27 (d), (e) show the current densities and corresponding pinning forces for the optimized 2 vol. % BHO double doped YBCO film. Tc gradually decreases from 89.2 K to 85.8 as dopant concentration increases which is typical of a tradeoff between increased current densities and lower critical temperatures. Microstructure studies employing xrd analysis, SEM and TEM clearly show nice epitaxial film growth, and confirm BHO c-axis alignment and the presence of Y2O3.

Since hafnium, is found below zirconium on the periodic table, it is also of interest to also investigate the effects of doping YBCO with BaHfO3 (BHO), and Y2O3. S.

Sato, et al., researched the effect of 1.5 and 3.0 wt. % BHO doped YBCO on the microwave surface resistance in high dc magnetic fields (1). M. Sieger, et al., studied the effect of 2, 4, and 6 mol. % BHO doped YBCO targets for deposition of thick films

(2µm) on STO and on Ni9W tapes with a PLD-Y2O3/YSZ/CeO2 buffer layer. (2) M.

Watanebe, et al., utilized 1.5, 2.0, and 3.0 wt. % BHO, with remaining percentage

YBCO, targets to study the effect of BHO on current densities of YBCO thin films (3).

This research proposes to investigate the pinning effects resulting from targets comprised of 2, 4, and 6 volume percent BHO with 3 volume percent Y2O3, and the remaining volume percent YBCO. Films were produced on STO and LAO substrates via PLD using an energy of 450 mJ, a repetition rate of 8Hz, and a 300 mtorr O2 atmosphere, followed by annealing at 500 ˚C in an O2 atmosphere for 30 minutes. Resulting film thicknesses were158, 162, and 196 nm for the 2, 4, and 6 vol. % double doped BHO +

Y2O3 respectively. Single doped BHO films were 169 nm thick.

Current densities were measured with the Quantum Design vsm-ppms. Fig. 30(a) and (b) show resulting current densities measured at 65K and 5K with fields ramped from

1T to 9T for 4 vol.% BHO + 3 vol. % Y2O3 double doped YBCO thin film on LAO substrate at various deposition temperatures. The highest current densities are attained with the optimum deposition temperature of 810 ˚C for high temperature applications

(65K) and 790 ˚C for low temperature applications (5K). Fig. 30(c) shows resulting current densities for 2,4, and 6 vol.% BHO + 3 vol. % Y2O3 double doped YBCO thin films on STO substrates that were grown at 810˚C deposition temperature, measured at

65K and 5K, with fields ramped from 1T to 9T. The highest current densities are attained with an optimum dopant concentration of 2 vol.% BHO + 3 vol. % Y2O3 at 65K and fields less than 5T, and with 4 vol.% BHO + 3 vol. % Y2O3 at 65 K and fields greater than 5T. For operating temperatures less than 65K, the 2 vol. % BHO double doped film provides the highest current densities. Fig. 30(d), (e) show the current densities and corresponding pinning forces for the optimized 2 vol. % BHO double doped YBCO film.

A comparison of current density vs. applied field for 2 vol. % BHO single doped YBCO films (red curves) and for 2 vol. % BHO + 3 vol. % Y2O3 double doped (blue curves) at

65K and 5K can be seen in Fig. 30(f), where the double doped BHO film performs better at 65K, and conversely the single doped BHO film performs better at 5K. Table VII depicts how the Tc gradually decreases from 89.2 K to 85.8 as dopant concentration increases which is typical of a tradeoff between increased current densities and lower critical temperatures.

Table VII. Critical Temperatures for 2, 4, and 6 vol. % BHO + 3 vol. % Y2O3 doped YBCO films on STO substrates at 810 ˚C deposition temperatures. YBCO 2 % BHO + 4 % BHO + 6 % BHO +

STO substrate 3 % Y2O3 + 3 % Y2O3 + 3 % Y2O3 + 95 % YBCO 93 % YBCO 91 % YBCO STO substrate STO substrate STO substrate

Tc (K) 89.2 88.4 87.1 85.8

Microstructure studies involved xrd analysis, along with SEM and TEM. From

Fig. 31 (a), the 2 Theta omega scan clearly shows nice epitaxial film growth, and confirms BHO c-axis alignment and the presence of Y2O3. The rocking curves were measured for the YBCO (005) peak for each of the doped films. Recall that peak shift indicates macro-strain, and peak broadening indicates non-uniform strain. A FWHM >

0.2 defines a broad peak. The non-uniform micro-strain increases with increased BHO dopant concentration as seen in Fig. 31(b) and Table VIII.

(a) (b)

(e) (f)

Figure 30. Current density versus applied field: (a), (b) for 4 vol.% BHO + 3 vol. % Y2O3 doped YBCO thin films on LAO substrate at various deposition temperatures, measured at 65K and 5K, (c) for various percentages of BHO + 3 vol. % Y2O3 doped YBCO thin films on STO substrates at 810˚C deposition temperature, measured at 65K and 5K, (d) for optimized 2 vol. % BHO + 3 vol.% Y2O3 doped YBCO thin films measured at 65K (blue curve), 50K (red curve), 20K (purple curve), and 5K (teal curve). YBCO current density curve at 65K is in black. (e) corresponding pinning force curves for optimized 2 vol. % BHO + 3 vol. % Y2O3 doped YBCO film. (f) current density vs. applied field for 2 vol. % BHO doped YBCO films (red curves) and for 2 vol. % BHO + 3 vol. % Y2O3 (blue curves) at 65K and 5K.

(a)

(b)

Figure 31. (a) XRD 2Theta omega scans for 2, 4, and 6 vol. % BHO with 3 vol. % Y2O3 doped YBCO films. (b) XRD rocking curve scans of YBCO (005) peak for 2, 4, and 6 vol. % BHO with 3 vol. % Y2O3 doped YBCO films.

Table VIII. FWHM and c-lattice parameters calculated from XRD analysis for 2, 4, and 6 vol. % BHO with 3 vol. % Y2O3 doped YBCO films. Film # Composition Deposition FWHM β c lattice Temp. ˚C (005) parameter (005) YBCO YBCO peak (Å) TJ2939 2 vol.% BHO + 810 ˚C 0.353 0.457 11.778

3% Y2O3

TJ2919 4 vol.% BHO + 810 ˚C 0.759 1.003 11.757

3% Y2O3

TJ2943 6 vol.% BHO + 810 ˚C 0.567 0.853 11.734 3% Y2O3

From the SEM micrographs in Fig. 32, the effect of the addition of 3 vol. % Y2O3 to the 2 volume % BHO doped YBCO films has a remarkable impact on the film surface, which can be seen by comparing Fig. 32 (c,d), the double doped 2 vol. % BHO, to Fig. 32

(a,b), the single doped 2 vol. % BHO film. In comparing Fig. 32 (c)-(h), the defects and pores and grains also increase with the increase in the composition of volume % BHO in the films. In contrast to films grown on STO substrates at 810˚C, the 4 vol. % BHO double doped YBCO film grown on a LAO substrate and at the higher deposition temperature of 825 ˚C, resulted in a-axis growth as evidenced by basketweave pattern in

SEM micrograph. This a-axis growth impedes the current in the CuO planes, resulting in the lower current densities measured in Fig. 30(a). The TEM bright field and dark field images in Fig. 33 clearly show nanoparticles within the 2 vol. % BHO + 3 vol. % Y2O3 doped YBCO film.

(a) (b)

(e) (f)

(g) (h)

(i) (j)

Figure 32. SEM images 20kX and 50kX respectively: (a),(b) 2 vol.% BHO + YBCO film, STO substrate, 810 ˚C deposition temp.; (c),(d) 2 vol. % BHO + 3 vol. % Y2O3 + YBCO films, STO substrate, 810 ˚C deposition temp.; (e), (f) 4 vol. % BHO + 3 vol. % Y2O 3+ YBCO films, STO substrate, 810 ˚C deposition temp.; (g),(h) 6 vol. % BHO + 3 vol. % Y2O3 + YBCO films, STO substrate, 810 ˚C deposition temp.; (i),(j) 4 vol. % BHO + 3 vol. % Y2O3 + YBCO films, LAO substrate, 825 ˚C deposition temp.

(a) (b)

Figure 33. TEM images of 2 vol. % BHO + 3 vol. % Y2O3 + YBCO on STO substrate at 810 ˚C deposition temperature (a) bright field, (b) dark field. Courtesy H. Wang Group Texas A & M.

4. BSO + Y2O3 DOPED YBCO FILMS

SUMMARY Addition of nanophase defects to YBa₂Cu₃O₇ (YBCO) superconductor thin films enhances flux pinning, resulting in an increase in transport current densities (Jct). While previous studies focused on single-phase additions, the addition of several phases simultaneously has shown strong improvements by combining different flux pinning mechanisms. This research further explores the effect of mixed phase nanoparticle pinning, with the addition of insulating, nonreactive phases of BaSnO₃ and Y₂O₃. Processing parameters vary the BaSnO₃ concentration of 3, 5, and 10 vol. %, while maintaining Y2O₃ constant at 3 vol. %. Pulsed laser deposition produces films on LaAlO₃ and SrTiO₃ substrates at deposition temperatures of 750-815 °C. Current density is measured for fields ranging from H = 0-9 T with H // c, and temperatures from 5-77 K, providing a detailed picture of pinning effects. Optimized results of flux pinning, magnetic current densities Jcm (H, T), critical transition temperatures (Tc), lattice parameters, and microstructures are presented. The highest current density is achieved with films produced from the 5 vol. % BSO + 3 vol. % Y2O3 doped YBCO target at deposition temperature or 795˚C. Tc-onset increases with increasing deposition temperature for all three varying volume percents of BSO (Fig.33). Tc gradually decreases from 89.2 K to 85.7K as dopant concentration increases which is typical of a tradeoff between increased current densities and lower critical temperatures (TableVIII). Microstructure studies employing xrd analysis, SEM and TEM clearly show nice epitaxial film growth, and confirm BSO c-axis alignment and the presence of Y2O3. The rocking curves were measured for the YBCO (005) peak and reveal that the non-uniform micro-strain increases with increased BSO dopant concentration as seen in Fig. 36 (b) and Table IX. Results of x-ray diffraction analysis provide information on the d spacing for the YBCO (005) plane based on the Bragg equation. The superconducting thin film produced at 795˚C with 5 vol. % BSO, has the longest c value of 11.753 Å (Table IX). This correlates with the previous results: a longer c length, presumably due to stress accommodation of BSO nanorod mismatch, a corresponding higher current density due to an increase of pinning centers, and a decrease in Tc-onset due to a large vol. % addition of BSO. M. Sebastian, et al., “ Optimizing flux pinning of YBCO superconductor with BaSnO3 + Y2O3 dual mixed phase additions,” IEEE Trans. on Appl. Supercond., vol. 25, no. 3, p.8002104, 2015.

Since addition of BaSnO3 nanorods has already achieved higher pinning force densities than BZO nanorods (1), it is of interest to study if the addition of Y2O3 nanoparticle pinning with BSO further enhances the overall flux pinning landscape (2).

Addition of Y2O3 nanoparticles results in an over-all lattice mismatch of -5.6%, whereas addition of BSO nanorods results in an over-all lattice mismatch of +6.3% (3). This research seeks to explore the effects of cancelling the stresses due to the lattice mismatch, and possible benefits on current densities by combining Y2O3 nanoparticles and BaSnO3 nanorods. Films were produced on STO and LAO substrates via PLD using an energy of

450 mJ, a repetition rate of 4Hz, and a 300 mtorr O2 atmosphere, followed by annealing at 500 ˚C in an O2 atmosphere for 30 minutes. Resulting film thicknesses were 194, 258, and 263 nm for the 3, 5, and 10 vol. % double doped BSO + Y2O3 respectively. Single doped BSO films were 270 nm thick. From Fig. 34 (a), current density as a function of applied field at 65 K, is optimized for 5 vol. % BSO addition at a deposition temperature of 795°C. The log-log plot in Fig. 34(b) results in an α = 0.5 and 0.37 for the YBCO film and YBCO doped with 5 vol. % BSO + 3 vol. % Y2O3 respectively (4). From Fig. 34(c), current density as a function of applied field at 65K and 5 K for films deposited at 795°C is optimized at a BSO concentration of 5 vol. %. Fig. 34 also illustrates that the pinning advantage and increased current densities resulting from the addition of 5 vol. % BSO and 3 vol. % Y2O3 are more advantageous for applied fields at 65 K; and this advantage over pure YBCO diminishes at fields applied at the lower temperatures of 20 K and 5 K.

Fig. 35 shows the current densities of the optimized 5 vol. % BSO + 3vol. % Y2O3 doped YBCO film measured at various temperatures, and the corresponding pinning force curves. Critical onset temperatures (Tc-onset) were attained as zero field cooled

measurements via VSM-PPMS. Fig. 36 shows how the Tc-onset increases with deposition temperature. Table IX shows how the Tc-onset decreases with the increase in dopant concentration.

(a) (b)

Figure 34. (a) Current density as a function of applied field for YBCO with 5 vol. % BSO + 3 vol. % Y₂O₃ on STO substrate measured at 65 K for various deposition temperatures, (b) Log-log plot of current density as a function of applied field for YBCO with 5 vol. % BSO + 3 vol. % Y₂O₃ measured at 77K, (c) Current density as a function of applied field at 65K and 5K for YBCO1-x-y BSOx (Y2O3)y x=vol.%, y=3vol.% and 795˚C deposition temperature, (d) Current density as a function of applied field comparison of single doped BSO and double doped BSO + Y2O3YBCO films measured at 65K. Note matching field peak at 3T in both single doped and DD film.10 mol % BSO is 4 vol. % BSO

(a) (b)

Figure 36. Tc-onset (K) versus deposition temperature (°C) for YBCO1-x-yBSOx (Y2O3)y, x = vol. %, y= 3 vol. %.

Table IX. Critical Temperatures for 3, 5, and 10 vol. % BSO + 3 vol. % Y2O3 doped YBCO films on STO substrates at 795 ˚C deposition temperatures.

YBCO 3 % BSO + 5 % BSO + 10 % BSO +

STO substrate 3 % Y2O3 + 3 % Y2O3 + 3 % Y2O3 +

94 % YBCO 92 % YBCO 87 % YBCO

STO substrate STO substrate STO substrate

Tc (K) 89.2 88.1 87.1 85.7

Microstructure studies comparing scanning electron microscopy pictures of pure

YBCO films versus films produced from targets composed of composition 5vol% BSO +

3 vol. % Y2O3 + 92 vol. % YBCO, show the corresponding larger grain sizes and defects due to BSO and Y2O3 (Fig. 37). The outgrowth particles in Fig. 37(b) could be due to either surface splashing particulates from the target or a,b-axis outgrowths (5). FEI

Quanta 650 EDS analysis confirmed the outgrowths consist of copper and oxygen. STEM and XTEM micrographs of the YBCO sample with 5 vol. % BSO + 3 vol. % Y2O3 deposited at 795 ˚C, both taken under the zone axis of STO(010), are shown in Fig. 38.

Fig. 38(c) shows the STEM image under high angle annular dark field mode, also called

Z-contrast images, where the contrast is proportional to Z2. It clearly shows the nanorods of BSO in the YBCO matrix with several big Y2O3 nanoparticles in the view area. The average rod diameter is around 10 nm. The average particle size is around 20 nm. A high resolution XTEM image (Fig. 38(a)) shows a representative Y2O3 nanoparticle epitaxially grown in the YBCO matrix. Similarly, a typical epitaxial BSO nanopillar is shown in

Fig. 38(b) to demonstrate its high quality epitaxial growth in YBCO.

(a) (b)

Figure 37. SEM images of YBCO+M films at magnification of 20K: (a) YBCO deposited at 775 ⁰C, (b). 5 % BSO + 3 % Y2O3 deposited at 795 ⁰C.

(a) (b) (a) (b) (c)

Figure 38. TEM of YBCO doped film with 5 vol. % BSO +3 vol. % Y2O3 deposited at 795 ⁰C (a) XTEM 400K image of nanoparticle, (b) XTEM 690K image of presumed BSO nanorod (c)STEM image at 225K magnification of overall thin film structure. Courtesy of Texas A & M University

From Fig. 39 (a), the 2 Theta omega scan clearly shows nice epitaxial film growth, and confirms BSO c-axis alignment and the presence of Y2O3. The rocking curves were measured for the YBCO (005) peak for each of the doped films. Recall that peak shift indicates macro-strain, and peak broadening indicates non-uniform strain. A

FWHM > 0.2 defines a broad peak. The non-uniform micro-strain increases with increased BSO dopant concentration as seen in Fig. 39 (b) and Table X. Results of x-ray diffraction analysis provide information on the d spacing for the YBCO (005) plane based on the Bragg equation (3). The superconducting thin film produced at 795˚C with 5 vol. % BSO, has the longest c value of 11.753 Å (Table X). This correlates with the previous results: a longer c length, presumably due to stress accommodation of BSO

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nanorod mismatch; a corresponding higher current density due to an increase of pinning centers; and a strong decrease in Tc-onset due to a large vol. % addition of BSO.

(a)

(b)

Figure 39. (a) XRD 2Theta omega scans for 2, 4, and 6 vol. % BSO with 3 vol. % Y2O3 doped YBCO films. (b) XRD rocking curve scans of YBCO (005) peak for 2, 4, and 6 vol. % BSO with 3 vol. % Y2O3 doped YBCO films.

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Table X. FWHM and c-lattice parameters calculated from XRD analysis for 3, 5, and 10 vol. % BSO with 3 vol. % Y2O3 doped YBCO films.

Film # Composition Deposition FWHM β c lattice Temp. ˚C (005) parameter (005) YBCO YBCO peak (Å) TJ2434 3 vol.% BSO 795˚C 0.423 0.548 11.721

+ 3% Y2O3

TJ2425 5 vol.% BSO 795˚C 0.446 0.547 11.753 + 3% Y2O3

TJ2413 10 vol.% BSO 795˚C 0.722 0.856 11.752

+ 3% Y2O3

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5. BSO + Y211 DOPED YBCO FILMS

SUMMARY This research further explores the effect of mixed phase nanoparticle pinning, with the addition of insulating, nonreactive phases of BaSnO₃(BSO), and Y₂BaCuO5 (Y211). The target produced consists of 10 vol. % BSO, 3 vol. % Y211, and 87 vol. % YBCO. Pulsed laser deposition produced films on LaAlO₃ substrates for a deposition temperature study that varied the deposition temperatures from 745-805 °C. Current density is measured for fields ranging from H = 0-9 T with H // c, and temperatures from 5-77 K, providing a detailed picture of pinning effects. Optimized results of flux pinning, magnetic current densities Jcm (H, T), critical transition temperatures (Tc), lattice parameters, and microstructures are presented. Optimization of Jcm occurs at a deposition temperature of 790 ˚C for a concentration of 10 vol. % BSO & 3 vol. % Y211. Interestingly, the film deposited at 790 ˚C, shows better performance at higher fields than pure YBCO films. Doping YBCO films with BSO & Y211 increased the strong pinning contribution to Jc at temperatures below 50K. Similar to the BSO + Y2O3 doped films, an interesting trend of increasing onset critical temperature with the increasing of deposition temperature is observed. Microstructure studies on the 10 vol. % BSO + 3 vol. % Y211 film included UHR SEM, TEM. From the UHR SEM, larger grain sizes and defects due to the dopants are evident. The outgrowth particles, similar to those seen in the BSO + Y2O3 doped YBCO films could be due to either surface splashing particulates from the target or a,b-axis outgrowths (1). EDS analysis confirmed the outgrowths consist of copper and oxygen. HR TEM and FFT analysis of the YBCO sample produced with the 10 vol. % BSO + 3 vol. % Y211 doped YBCO target deposited at 790˚C, identify epitaxial YBCO and BSO nanorods, and the presence of Y211. These films did not perform as well due to the higher optimal deposition temperature or 835˚C for Y211 doped YBCO films, as found in previously stated research. The results from the BSO + Y211 doped films were initially utilized to develop a mathematical model for the temperature dependence of the current density, Jc(T), with respect to the isotropic weak and anisotropic strong pinning contributions. This model was then applied to the other doped YBCO films studied.

As an addition to the BSO + Y2O3 doped YBCO study, it is of interest to investigate the effect of combining BSO and Y211 on the pinning landscape, given that

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Y211 has a smaller lattice mismatch to YBCO than Y2O3 (Table 1). A target was produced consisting of 10 vol. % BSO + 3 vol. % Y211 + 87 vol. % YBCO. Films were produced via PLD varying the deposition temperatures from 745 to 805 °C. From Fig. 40

(a), optimization of Jcm occurs at a deposition temperature of 790 ˚C for films produced from the 10 vol. % BSO & 3 vol. % Y211 doped YBCO target. Fig. 40(b) shows the critical current densities measured at 65, 50, 20, and 5K for the film deposited at 790 ˚C.

Interestingly, the film deposited at 790 ˚C, shows better performance at higher fields than pure YBCO films. Doping YBCO films with BSO & Y211 increased the strong pinning contribution to Jc at temperatures below 50K. From Fig. 40 (c), an interesting trend of increasing onset critical temperature with the increasing of deposition temperature is observed.

Microstructure studies on the 10 vol. % BSO + 3 vol. % Y211 film included UHR

SEM, TEM and xrd analysis. From the UHR SEM in Fig. 42, larger grain sizes and defects due to the dopants are evident. The outgrowth particles, similar to those seen in the BSO + Y2O3 doped YBCO films could be due to either surface splashing particulates from the target or a,b-axis outgrowths (1). EDS with a FEI Quanta 650 confirmed that the outgrowths consist of copper and oxygen. HR TEM and FFT analysis of the YBCO sample with 10 vol. % BSO + 3 vol. % Y211 deposited at 790, identify epitaxial YBCO and BSO nanorods, and the presence of Y211in Fig. 42, with YBCO shown in red, BSO shown in blue, Y211 shown in yellow, and LAO in white. The results from the BSO +

Y211 doped films were initially employed to develop a mathematical model for the temperature dependence of the current density, Jc (T), with respect to the isotropic weak

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and anisotropic strong pinning contributions. This model was then applied to the other doped YBCO films studied.

(a) (b)

(c)

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Figure 41. UHR SEM 20kX and 50kX for film deposited with 10 vol. % BSO + 3 vol. % Y211 doped YBCO target on LAO substrate at deposition temperature of 790 ˚C.

(b) (a)

Y211 Y211 YBCO BSO

middle bottom

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6. PINNING SYSTEM COMPARISON AND CURRENT DENSITY MATHEMATICAL MODELING

SUMMARY As previously mentioned, addition of nano-sized secondary inclusions to YBCO superconducting thin films enhances flux pinning, resulting in an increase in current density. An important consideration in the epitaxial growth of superconducting thin films involves lattice mismatch. Lattice mismatch, along with the different coefficients of thermal expansion for different materials, results in almost all thin films possessing internal stress, without applied external forces. Because of this, it is usually desired to minimize lattice mismatch and choose materials of similar thermal conductivities to promote epitaxial growth of thin films (2). In particular, the 3D-APC of Y211 exhibits a smaller overall average lattice mismatch with YBCO, when compared to other inclusions, such as

Y2O3, BSO, and BZO. It is of interest to compare the optimized Y211 doped YBCO films to optimized films with dual additions, whose choice minimizes the overall lattice mismatch and resulting stress in the films. The optimized thin film doped with Y211 provided the highest current density at 65 K and 5 K, with magnetic field applied parallel to the c

direction. This performance is followed by the BSO + Y2O3, the BZO + Y2O3, BHO + Y2O3, and the BSO + Y211 doped films, respectively. The performance in current density indirectly correlates with the overall lattice mismatch. XRD c - lattice calculations confirm that a negative lattice mismatch between the dopant and YBCO puts the c - lattice of the YBCO (005) in compression, whereas a positive lattice mismatch results in tension. This is also confirmed by TEM analysis discussed in previous chapters. Despite the fact that adding nano-sized insertions enhances flux pinning due to additional defects increasing the current density, a large

mismatch between YBCO and inserted nano-phases decreases Jc. Thus, we have two competing factors. As it occurs, a very small mismatch between YBCO and Y211 essentially does not impede the increase of the current density due to this inclusion. Interestingly, the Y211 doped films which have the least lattice mismatch, shows greater than 50 % increase in both the anisotropic strong and isotropic weak current density contributions. For the rest of the phases studied, the mismatches are large and their detrimental effect outweighs the positive effect of the stronger flux pinning

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on the current density. The theoretical temperature dependence of the

current density, Jc(T), can be mathematically modeled with respect to the isotropic weak and anisotropic strong pinning contributions based on a theoretical model developed by Blatter & Nelson. The Gnuplot program was utilized to determine model parameters from experimentally collected data. M.A. Sebastian, et al., “Study of the Flux Pinning Landscape of

YBCO Thin Films with Single and Mixed Phase Additions BaMO3 + Z: M = Hf, Sn, Zr and Z = Y2O3, Y211,” IEEE Transactions on Applied Superconductivity, Vol. 27, No. 4, June 2017

As previously mentioned, addition of nano-sized secondary inclusions to YBCO superconducting thin films enhances flux pinning, resulting in an increase in current density. An important consideration in the epitaxial growth of superconducting thin films involves lattice mismatch. Lattice mismatch, along with the different coefficients of thermal expansion for different materials, results in almost all thin films possessing internal stress, without applied external forces. Because of this, it is usually desired to minimize lattice mismatch and choose materials of similar thermal conductivities to promote epitaxial growth of thin films (1). In particular, the 3D-APC of Y211 exhibits a smaller overall average lattice mismatch with YBCO, when compared to other inclusions, such as Y2O3, BSO, and BZO (Table I: jcpds pdf #00-040-0169 (YBCO), #00-041-

1105(Y2O3), #00-38-1434 (Y211), #00-03-0632 (BZO), #00-024-0102 (BHO), #00-015-

0780 (BSO), #01-073-3684 (LAO), #01-070-8508 (STO)). It is of interest to compare the optimized Y211 doped YBCO films to optimized films with dual additions, whose choice minimizes the overall lattice mismatch and resulting stress in the films. Optimal target compositions and PLD parameters for the doped YBCO films studied are shown in Table

XI. From Fig. 43, the optimized thin film doped with Y211 provided the highest current

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density at 65 K and 5 K, with magnetic field applied parallel to the c direction. This performance is followed by the BSO + Y2O3, the BZO + Y2O3, BHO + Y2O3, and the

BSO + Y211 doped films, respectively. The performance in current

Table XI. Optimal concentration and PLD conditions for Y211 doped YBCO films and the BZO, BHO, and BSO double doped YBCO films.

YBCO 10 % 2 % BZO + 5 % BSO + 10 % BSO + 2 % BHO + Doped Y211 Targets 3 % Y2O3 3 % Y2O3 3 % Y211 3 % Y2O3

Laser Rep. 4 8 4 4 8 Rate (Hz)

Deposition 835 825 795 790 810 Temp. (°C)

density indirectly correlates with the overall lattice mismatch, shown in Table XII. In

Table XIII, XRD c - lattice calculations confirm that a negative lattice mismatch between the dopant and YBCO puts the c - lattice of the YBCO (005) in compression, whereas a positive lattice mismatch results in tension. TEM imaging in Fig. 44 clearly shows the respective nano-rods and nano-particles in each of the YBCO doped films. Despite the fact that adding nano-sized insertions enhances flux pinning due to additional defects increasing the current density, a large mismatch between YBCO and inserted nano- phases decreases Jc. Thus, we have two competing factors. As it occurs, a very small mismatch between YBCO and Y211 essentially does not impede the increase of the current density due to this inclusion. Interestingly, the Y211 doped films which have the least lattice mismatch, shows greater than 50 % increase in both the anisotropic strong and isotropic weak current density contributions, see Fig. 45(c). For the rest of the phases

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studied, the mismatches are large and their(a) detrimental(b) effect outweighs the positive effect of the stronger flux pinning on the current density.

(a)

(b)

Figure 43. Current density verses applied field for films of YBCO, 10 vol. % Y211, 5 vol. % BSO + 3 vol. % Y2O3, 10 vol. % BSO + 3 vol. % Y211, 2 vol. % BZO + 3 vol. % Y2O3, 2 vol. % BHO + 3 vol.% Y2O3 : (a) at 65 K, (b) at 5 K

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Table XII. Overall lattice mismatch for dopants with YBCO

Dopants Overall Lattice Mismatch %

Y211 -0.15

BSO + Y2O3 6.3, -5.6

BHO + Y2O3 7.5, -5.6

BZO + Y2O3 8.3, -5.6

BSO + Y211 6.3, -0.15

Table XIII. C- Lattice for YBCO (005) for Doped YBCO Films YBCO Y211 BSO BZO BHO + + + Y2O3 Y2O3 Y2O3

C Lattice 11.707 11.652 11.753 11.736 11.778 YBCO (005) (Å)

Y211 Y2O3 50 nm

Figure 44. TEM images of doped YBCO films: (a) 5 vol. % Y221. (b) 5 vol. % BSO + 3 vol. % Y2O3. (c) 2 vol. % BZO + 3 vol. % Y2O3. (d) 10 vol. % BSO + 3 vol. % Y211. (e) 2 vol. % BHO + 3 vol. % Y2O3.

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The theoretical temperature dependence of the current density, Jc(T), can be mathematically modeled with respect to the isotropic weak and anisotropic strong pinning contributions according to the following equation: (2-6)

is-weak an-strong + 2 Jc(T) = Jc (0) exp (-T / T0) + Jc (0) exp (-3(T / T ))

Isotropic Contribution Anisotropic Contribution Weak Flux Pinning Strong Flux Pinning

Jc(0) = current density at 0 K (extrapolated) T = temperature (K)

T0, T+ = curve fitting parameters associated with the energy of the defects.

The anisotropic strong contribution is based on columnar pin line disorder, modeled as a cylindrical well and solving the Ginzburg - Landau equations. The isotropic weak contribution’s exponential decrease is due to the inefficiency of point like defects

(due to oxygen vacancies) to respond to and pin vortex motion due to thermal activation.

It manifests as small bundle pinning (2-4). The Gnuplot program was utilized to model the temperature dependence of the current density, from the films’ current density data measured at a field H // c direction of 3 tesla at 77, 65, 60, 50, 30, 20, 10, and 5 K with the vsm-ppms. Gnuplot modeling is courtesy of Dr. John Panasyuk (AFRL/RQQI). The

Gnuplot fitting parameters can be found in Table XIV, with the modeling curves shown in Fig. (45).

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Table XIV. Gnuplot fitting parameters for mathematical model of current density temperature dependence. iso-weak an-strong Jc (0) Jc (0) T0 T+ (A/cm2) (A/cm2) (K) (K)

YBCO on LAO 1.182E+07 0.685E+07 14.32 75.78

YBCO on STO 1.28E+07 0.81 E+07 25.17 26.56

10 vol. % Y211 1.956E+07 1.607E+07 14.43 79.47

10 vol. % BSO 0.445E+07 0.799E+07 28.18 52.99 + 3 vol. % Y211

5 vol. % BSO + 1.004E+07 0.856E+07 18.76 75.43

3 vol. % Y2O3

2 vol. % BZO + 0.940E+07 0.973E+07 17.93 68.06

3 vol. % Y2O3

2 vol. % BHO + 0.790E+07 0.757E+07 17.29 74.38

3 vol. % Y2O3

Based on the model developed by Blatter & Nelson, current density temperature dependence and isotropic and anisotropic flux pinning contributions can be successfully modeled; this gives a visual representation of how various dopants change the weak and strong pinning contributions (Fig. 45). The inset graphs in Fig. 45 show the good fit between the model and the experimental data for the films studied. The fitting

is-weak an-strong parameters T0, T+, Jc , and Jc depend on many factors: the method of creating the YBCO film, the choice of substrate, the number, chemical structure, and volume concentration of added phases, etc. From the model equation, the exponential cutoff for

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the anisotropic component is sharper than for the isotropic component, and for a large enough T (even for T < Tc ≈ 92 K) the isotropic component may dominate, which can be shown by comparison of the isotropic and anisotropic contributions. This indeed occurred for the BSO and Y211 doped YBCO film, which was grown on LAO, as is illustrated in

Fig. 45(e) at T > 45 K. Films shown in Fig. 45 (a,b,c,d,f) were grown on STO. The lattice mismatch between substrate choice and various dopants when compared to YBCO can vary from - 0.15 to + 9 %. The effects of substrate choice (LAO vs. STO) and nano- additions result in overall strain, which limits the maximum volume additions to attain maximum current density. The c-lattice parameter for the YBCO (005) peak, was calculated from XRD analysis. The c-lattice calculations for the various doped films confirm that a negative lattice mismatch between the dopant and YBCO puts the c-lattice of the YBCO (005) in compression, whereas a positive lattice mismatch results in tension. From a materials science aspect and point of view, one can consider that the

Y211 nanoparticles stop the dislocation movement in YBCO films and is analogous to stopping crack progression by stopping dislocation movement. In general, materials are stronger in compression than tension.

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(a) (b)

(d) (c)

(e) (f)

Figure 45 Comparison of strong anisotropic (---) and weak isotropic(—) pinning for (a) YBCO, (b) BZO + Y2O3 + YBCO, (c) Y211+ YBCO, (d) BSO + Y2O3 + YBCO, (e) BSO + Y211 + YBCO, (f) BHO + Y2O3 + YBCO. (a)– (b) inset graphs: model fit (—) and experimental data points (*)

Interestingly, the Y211 doped films, which have the least lattice mismatch, shows >

50 % increase in both the anisotropic strong and isotropic weak current density contributions, as seen in Fig. 45 (c). Different thermal expansion coefficients also play a role in film stress. Thermal expansion coefficients can be found in Table (XV).

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Interestingly, Y211 also has a thermal expansion coefficient closest to YBCO, compared to the other dopant systems.

Table XV. Thermal expansion coefficents.

YBCO 13.7 E-06 / K (7) 13.4 E-06 / K (8) Y211 12.5 E-06 / K (7) Y2O3 8.1E-06 / K (9) BaZrO3 7.0 E-06 / K (7) BaSnO3 9.6 E-06 / K (10) BaHfO3 7.97 E-06 / K (11) SrTiO3 10.8 E-06 / K (7) 11.0 E-06 / K (8)

In regards to the angular Jc measurements for the Y211 doped film discussed in a previous chapter, the difference in the magnitudes of the current density at 0˚ and 90˚ decreases at lower temperatures as seen when comparing the curves for Jct’s measured at

77K for 1 T and 5 T with those curves for Jct’s measured at 65K for 1 T and 5 T. This signifies more isotropic behavior in regards to current density and flux pinning at lower temperatures, and this increase in isotropic behavior is also seen in the mathematical modeling curve in Fig.45(c).

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CHAPTER VII

CONCLUSION AND FUTURE WORK

Superconductors’ unique properties of zero resistance to direct current at their critical temperatures and high current density have led to many applications in communications, electric power infrastructure, medicine, and transportation. Yttrium barium copper oxide, YBa2Cu3O7-δ, (YBCO) is a Type II superconductor, whose thin film’s high current density results from pinning centers associated with point defects from oxygen vacancies, and with twin and grain boundaries. Addition of second-phase inclusions enhances flux pinning and current density by incorporating additional pinning centers. This research systematically studied the effect of nanoparticle pinning with the addition of an insulating, nonreactive phase of Y2BaCuO5 (Y211). While many previous studies focused on single phase additions, the addition of several phases simultaneously shows promise in improving current density by combining different pinning mechanisms.

This research systematically studied the following mixed phase additions to YBCO targets to produce thin films by pulsed laser deposition (PLD): YBCO + BaZrO3 + Y2O3,.

YBCO + BaHfO3 + Y2O3, YBCO + BaSnO3 + Y2O3, and YBCO + BaSnO3 + Y211. Thin films were prepared by pulsed laser deposition on LaAlO₃ and SrTiO₃ substrates

Processing parameters varied the volume percent of dopants present in the target and the deposition temperatures of the films to optimize critical current densities, with the applied magnetic field varied from 0-9 Tesla, and temperature varied from 5-77K. Each

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of the dopant systems studied optimized at different conditions due to their intrinsic properties, two of which are lattice mismatch and thermal expansion coefficients. The

Y211 doped YBCO thin films were optimized at 10 vol.% at a deposition temperature of

835 ˚C. The BaZrO3 double doped YBCO films were optimized at 2 vol.% BZO + 3 vol.% Y2O3 at a deposition temperature of 825˚C. The BaHfO3 double doped YBCO films were optimized at 2 vol. % BHO + 3 vol. % Y2O3 at a deposition temperature of

810˚C. The BaSnO3 double doped YBCO films were optimized at 5 vol. % BSO + 3 vol.

% Y2O3 at a deposition temperature of 795 ˚C. A temperature study on 10 vol. % BSO +

3 vol. % Y211 had an optimized deposition temperature of 795 ˚C. The optimized Y211 doped films achieved the highest current densities, and incidentally, also had the least lattice mismatch and a thermal expansion coefficient closest to that of YBCO, when compared to the other dopants studied. The Y211 nanoparticles were able to grow to a very large size and with a very clean interface with the YBCO film, as evidenced by

TEM imaging. XRD confirmed a shorter c-lattice parameter for the YBCO (005) peak, which agreed with TEM analysis of the Y211 nanoparticles introducing compressive stress in the films. This was in contrast to the other dopants studied, which resulted in longer c-lattice parameters for the YBCO (005), signifying tension in the films. The isotropic weak and anisotropic strong flux pinning contributions to the current density were mathematically modeled for each of the optimized films studied, providing a visual comparison of the flux pinning. The optimized Y211 doped YBCO film was the only system to increase both the isotropic weak and anisotropic strong contribution to current density.

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Future work could progress along several avenues, whether it be further analyzing the flux pinning systems studied or investigating new dopants. All of the dopant systems discussed herein could be further evaluated at higher magnetic fields of up to 35T. More in depth TEM analysis to analyze defects through fast fourier transform could be done on the BSO, BZO, and BHO + Y2O3 films, to further compare different dopants resulting in compressive and tensile strain within the films. This would compare defect density within the film to the Y211 doped films, for which fast fourier transform was already calculated for. Further investigation of a correlation between the strength of the films and increased pinning force could be accomplished by analyzing the fracture toughness of the films by nano-indentation. The experimental data hence forth presented should be helpful for a theoretical investigation of a possible new-regime of collective pinning responsible for the increased pinning of the vortex lattice of the 10 vol.% Y211 doped YBCO thin films..

This work illustrates the impact of dopants on the microstructure and collective pinning of the YBCO films, and contributes to a greater understanding for future optimizations of

YBCO doped films with pinning landscapes tailored for high current and high field applications at various field orientations.

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(10) Wee SH, Goyal A, Zuev YL, Cantoni C, Selvamanickam V, Specht ED. Formation of self-assembled double –perovskite Ba2YNbO6 nanocolumns and their contribution to flux pinning and Jc in Nb- doped YBa2Cu3O7-δ films. Appl Phys Express. 2010;3(2):p.023101-1. (11) Solovyov V, Li Q, Si W, Maiorov B, Haugan TJ, MacManus- Driscoll JL, et al. Influence of defect-induced biaxial strain on flux pinning in thick YBa2Cu3O7 layers. Phys Rev B. Sept. 12, 2012;86(9):094511-1. (12) Liu Y, Du G. Preparation and flux-pinning properties of multilayered yttrium barium copper oxide thin films containing alternating barium zirconate and yttria nanostructures. J Electron Mater. 2011 07;40(7):1512-1516. (13) Baca FJ, Haugan TJ, Barnes PN, Holesinger TG, Maiorov B, Lu R, et al. Interactive growth effects of rare-earth nanoparticles on nanorod formation in YBa2Cu3Ox thin films. Adv Funct Mater. 2013;23(38):4826-4831. (14) Sebastian MAP, Reichart JN, Burke JL, Brunke LB, Haugan TJ, Chen-Fong Tsai CF, et al. Optimizing flux pinning of YBCO superconductor with dual mixed phase additions. IEEE Trans Appl Supercond. 2013;23(3):8002104-8002104. (15) Zhou H, Maiorov B, Baily SA, Dowden PC, Kennison JA, Stan L, et al. Thickness dependence of critical current density in YBa2Cu3O7−δ films with BaZrO3 and Y2O3 additions. Supercond Sci Technol. August 2009;22(8):085013-1. (16) Feldmann DM, Holesinger TG, Maiorov B, Foltyn SB, Coulter JY, Apodaca I. Improved flux pinning in YBa2Cu3O7 with nanorods of the double perovskite Ba2YNbO6 . Supercond Sci Technol. Sept. 2010;23(9):095004-1.

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(24) Haugan TJ, et.al. Superconducting properties of (Mx/YBa2Cu3O7-d)N Multilayer films with variable layer thickness x. J Electron Mater. 2007;36(10):1234-1242. (25) Ohring M. Materials science of thin films. 2nd ed. San Diego, CA: Academic Press; 2002. (26) XRD jcpds, available at http://www.icdd.com/products/pdf4.htm. (27) Sato S, Honma T, Takahashi S, Sato K, Watanabe M, Ichikawa K, et al. Introducing artificial pinning centers into YBCO thin films to improve surface resistance in a DC magnetic field. IEEE Trans Appl Supercond. 2013;23(3). (28) Sieger M, Hanisch J, Pahlke P, Sparing M, Gaitzsch U, Iida K, et al. BaHfO3-doped thick YBa2Cu3O7-δ films on highly alloyed textured Ni-W tapes. IEEE Trans Appl Supercond. 2015 06;25(3):6602604 (4 pp.). (29) Experimental study in the development of HTS NMR probe. 11th European Conference on Applied Superconductivity, EUCAS 2013, September 15, 2013 - September 19; 2013; Genoa, Italy: Institute of Physics Publishing; 2014.

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(30) Gutiérrez J, Llordés A, Gazquez J, Gibert M, Roma N, Ricart S, et al. Strong isotropic flux pinning in solution-derived YBa2Cu3O7-x nanocomposite superconductor films. Nat Mater. 2007;6:367. (31) Guan L, Zhang D, Li X, Li Z. Role of pulse repetition rate in film growth of pulsed laser deposition. Nucl Instrum Methods Phys Res B: Beam Interactions with Materials and Atoms. 2008 1;266(1):57-62. (32) Blatter G, Feigel'man MV, Geshkenbein VB, Larkin AI, Vinokur VM. Vortices in high-temperature superconductors. Rev.Mod.Phys. 1994 Oct;66(4):1125-1388. (33) Ertaş D, Nelson DR. Irreversibility, mechanical entanglement and thermal melting in superconducting vortex crystals with point impurities. Physica C Supercond. 1996;272(1):79-86.

Chapter 5 (1) Suzuki S, Shiobara R, Higashiyama K, Suzuki F. Hitachi's leading superconducting technologies. Hitachi Review 1999;48(5):290-292.

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Chapter 6

1. Y211 DOPED YBCO FILMS (1) Ohring M. Materials science of thin films. 2nd ed. San Diego, CA: Academic Press; 2002. (2) Cherpak Y, Svetchnikov V, Semenov A, Moskaliuk V, Tretiatchenko C, Flis V, et al. On the mechanism of thickness dependence of the critical current density in HTS cuprate epitaxial films. Journ of Phys: Conference Series. 2008;97(1):012259. (3) Wang J, Kwon J, Yoon J, Wang H, Haugan T, Baca F, et al. Flux pinning in YBa2Cu3O7-δ thin film samples linked to stacking fault density. Appl Phys Lett. 2008;92(8):2507. (4) Foltyn SR, Civale L, MacManus-Driscoll J, Jia QX, Maiorov B, Wang H, et al. Materials science challenges for high-temperature superconducting wire. Nat Mater. 2007 09;6(9):631-642. (5) Peurla M, Huhtinen H, Paturi P, Stepanov YP, Raittila J, Laiho R. YBCO films prepared by PLD using nanocrystalline targets doped with BaZrO3 or Y211. IEEE Trans Appl Supercond. 2005;15(2):3050-3053.

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(6) Kametani F, Chen Z, Kim S, Gurevich A, Larbalestier D. Microstructural investigation of the most efficient vortex pinning in a superconducting YBa2Cu3O7 thin film. Microsc and Microanal. 2008;14(Supplement S2):342-343. (7) Kim SI, Kametani F, Chen Z, Gurevich A, Larbalestier DC. On the through-thickness critical current density of an YBa2Cu3O7-x film containing a high density of insulating, vortex-pinning nanoprecipitation. Appl Phys Lett. 2007;90(252502):1-7. (8) Chen Z, Kametani F, Gurevich A, Larbalestier D. Pinning, thermally activated depinning and their importance for tuning the nanoprecipitate size and density in high Jc YBa2Cu3O7-x films. Phys C Supercond. 2009;469(23-24):2021-2028. (9) Barnes PN, Kell JW, Harrison BC, Haugan TJ, Burke JL, Varanasi CV. Nanoparticulate flux pinning centers for YBa2Cu3 O7-d films. IEEE Trans Appl Supercond, 2007;17(2):3717-3719. (10) Haugan T, Barnes PN, Wheeler R, Meisenkothen F, Sumption M. Addition of nanoparticle dispersions to enhance flux pinning of the YBa2Cu3O7-x superconductor. Nat. 2004 08/19;430(7002):867-870.

(11) Haugan TJ, et al. Superconducting Properties of (Mx/YBa2Cu3O7-dy)N multilayer films with variable layer thickness x. J Electron Mater. 2007;36(10):1234-1242. (12) XRD jcpds files, Available at: http://www.icdd.com/products/pdf4.htm. (13) Cullity B, Stock S. Elements of X-Ray Diffraction, 3rd ed. Englewood Cliffs, NJ, USA: Prentice Hall; 2001. (14) Baca FJ, Haugan TJ, Barnes PN, Holesinger TG, Maiorov B, Lu R, et al. Interactive Growth effects of rare-earth nanoparticles on nanorod formation in YBa2Cu3Ox thin films. Adv Funct Mater. 2013;23(38):4826-4831. (15) Blatter G, Feigel'Man M, Geshkenbein V, Larkin A, Vinokur VM. Vortices in high- temperature superconductors. Rev Mod Phys. 1994;66(4):1125.

2. BZO + Y2O3 DOPED YBCO FILMS

(1) Chen S, Sebastian MA, Gautam B, Wilt J, Haugan T, Xing Z, et al. Enhancement of isotropic pinning force in YBCO films with BaZrO3 nanorods and Y2O3 nanoparticles. IEEE Trans Appl Supercond. 2017;27(4):1-5. (2) Wang J, Kwon J, Yoon J, Wang H, Haugan T, Baca F, et al. Flux pinning in YBa2Cu3O7-δ thin film samples linked to stacking fault density. Appl Phys Lett. 2008;92(8):2507. (3) Haugan TJ, Barnes PN, Campbell TA, Pierce NA, Baca FJ, Maartense I. Flux pinning of Y-Ba-Cu-O films doped with BaZrO3 nanoparticles by multilayer and single target methods. IEEE Trans Appl Supercond. 2007;17(2):3724-3728.

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(4) Maiorov B, Baily SA, Zhou H, Ugurlu O, Kennison JA, Dowden PC, et al. Synergetic combination of different types of defect to optimize pinning landscape using BaZrO3 doped YBa2Cu3O7. Nat Mater. 2009 05;8(5):398-404. (5) Gutiérrez J, Llordés A, Gázquez J, Gibert M, Romà N, Ricart S, et al. Strong isotropic flux pinning in solution-derived YBa2Cu3O7-x nanocomposite superconductor films. Nat Mater. 2007 05;6(5):367-373. (6) Baca FJ, Haugan TJ, Barnes PN, Holesinger TG, Maiorov B, Lu R, et al. Interactive growth effects of rare-earth nanoparticles on nanorod formation in YBa2Cu3Ox thin films. Adv Funct Mater. 2013;23(38):4826-4831. (7) Liu Y, Du G. Preparation and flux-pinning properties of multilayered yttrium barium copper oxide thin films containing alternating barium zirconate and yttria nanostructures. J Electron Mater. 2011 07;40(7):1512-1516. (8) Zhou H, Maiorov B, Baily SA, Dowden PC, Kennison JA, Stan L, et al. Thickness dependence of critical current density in YBa2Cu3O7−δ films with BaZrO3 and Y2O3 addition. Supercond Sci Technol. August 2009;22(8):085013-1. (9) Macario LR, Moreira ML, Andres J, Longo E. An efficient microwave-assisted hydrothermal synthesis of BaZrO3 microcrystals: growth mechanism and photoluminescence emissions. Cryst Eng Comm. 2010;12(11):3612-3619. (10) Chang CC, Wu XD, Ramesh R, Xi XX, Ravi TS, Venkatesan T, et al. Origin of surface roughness for c-axis oriented Y- Ba-Cu-O superconducting films. Appl Phys Lett. 1990 10/22;57(17):1814.

3. BHO + Y2O3 DOPED YBCO FILMS

(1) Sato S, Honma T, Takahashi S, Sato K, Watanabe M, Ichikawa K, et al. Introducing artificial pinning centers into YBCO thin films to improve surface resistance in a DC magnetic field. IEEE Trans Appl Supercond. 2013;23(3).

(2) Sieger M, Hanisch J, Pahlke P, Sparing M, Gaitzsch U, Iida K, et al. BaHfO3-Doped thick YBa2Cu3O7-δ films on highly alloyed textured Ni-W tapes. IEEE Trans Appl Supercond. 2015 06;25(3):6602604 (4 pp.). (3) Experimental study in the development of HTS NMR probe. 11th European Conference on Applied Superconductivity, EUCAS 2013, September 15, 2013 - September 19; 2013; Genoa, Italy: Institute of Physics Publishing; 2014.

4. BSO + Y2O3 DOPED YBCO FILMS

(1) Matsumoto K, Mele P. Artificial pinning center technology to enhance vortex pinning in YBCO coated conductors. Supercond Sci Technol. 2010 01;23(1):014001-014001.

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(2) Maiorov B, Baily SA, Zhou H, Ugurlu O, Kennison JA, Dowden PC, et al. Synergetic combination of different types of defect to optimize pinning landscape using BaZrO3 doped YBa2Cu3O7. Nat Mater. 2009 05;8(5):398-404. (3) XRD jcpds available at http://www.icdd.com/products/pdf4.htm. (4) Foltyn SR, Civale L, MacManus-Driscoll J, Jia QX, Maiorov B, Wang H, et al. Materials science challenges for high-temperature superconducting wire. Nat Mater. 2007 09;6(9):631-642. (5) Ramesh R, Inam A, Hwang DM, Sands TD, Chang CC, Hart DL. Surface outgrowth problem in c-axis oriented Y-Ba-Cu-O superconducting thin films. Appl Phys Lett. 1991 04/08;58(14):1557.

5. BSO + Y211 DOPED YBCO FILMS (1) Ramesh R, Inam A, Hwang DM, Sands TD, Chang CC, Hart DL. Surface outgrowth problem in c-axis oriented Y-Ba-Cu-O superconducting thin films. Appl Phys Lett. 1991 04/08;58(14):1557.

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(8) Kawashima J, Yamada Y, Hirabayashi I. Critical thickness and effective thermal expansion coefficient of YBCO crystalline fil. Phys C. 1998;306(-):114-118. (9) Goodfellow metals, alloys, compounds, ceramics, polymers, composites catalogue. 1993/1994; Available at: http://www.goodfellow.com/E/Yttrium-Oxide.html. Accessed July, 2017. (10) Bẻvillon Ẻ, Chesnaud A, Yanzhong W, Dezanneau G, Geneste G. Theoretical and experimental study of the structural, dynamical and dielectric properties of perovskite BaSnO3. J Phys Condens Matter. 2008;20(14):145217. (11) Dogan F, Kumta P N. Advances in electronic and electrochemical ceramics : proceedings of the 107th annual meeting of the American Ceramic Society, Baltimore, Maryland, USA 2005, Ceramic Transactions. 2012.

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