T-4059
MELT PROCESSING OF THALLIUM-BASED HIGH Tc SUPERCONDUCTORS
by
Steven B. Fellows ProQuest Number: 10783729
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A thesis submitted to the Faculty and the Board of
Trustees of the Colorado School of Mines in partial fulfillment of requirements for the degree of Master of Science (Materials Science).
Golden, Colorado
Signed: ifer-TC^sr— Steven B. Fellows
Approve
Dr. Baki Yarar Thesis Advisor
Golden, Colorado
Dr. John J. Moore Professor and Director Materials Science
ii T-4059
Abstract
The processing and characterization of thallium-barium- cuprate superconductors were investigated. The primary objective was to optimize conditions under which a patterned thick film superconductor could be built on a polycrystalline alumina substrate. As part of this study a number of other problems including the development of a thallium detection and determination apparatus for air-borne particulates, as well as the shielding of RF-radiation by superconductor shells were also investigated. An apparatus to continuously monitor the level of thallium in the environment was built. The rate of response of the apparatus is affected by the flow rates in the circuit; however, when this is fixed at constant rate its response to airborne thallium becomes constant. Para- phenetidine solution at a pH of 3.1 can be used to identify thallium and linear Beer's law behavior is observed up to a concentration of 2.5 x 10-4 M.
The preparation of barium copper oxide powder as a precursor to superconducting material was investigated, and can be prepared with an overall ratio of Ba:Cu of 1:3 in the pure form. The patterning and melting of cuprate designs on alumina was also investigated. Stencil printing techniques
iii T-4059
can be used for this purpose. Napthenic acid provides the best binding characteristics for barium cuprate on alumina since it remains as a liquid at room temperature and thickens upon sitting; upon removal of the mask it retains the shape of the mask.
A procedure that utilizes the vaporization of thallium oxide onto substrates of cuprate melted on alumina as well as an apparatus that supports a substrate over powdered thallium oxide was designed and used to prepare the superconducting material. Such a superconductor is prepared by subjecting barium cuprate layers to thallium oxide vapors for 11 minutes at 890-900°C followed by cooling in air to room temperature.
Thallium-barium-cuprate superconductors prepared in the above manner were characterized by SEM, XRD, resistance versus temperature determinations, and thermoelectric power measurements. The critical temperature varied from 75 to 90
K. The results of the characterizations indicated that the material obtained is identical to that prepared by other groups using alternative techniques. The shielding of radio frequencies by thallium-barium- cuprate superconductors were explored. Two types of prototype devices were prepared: a cubical shell, and a hollow cylinder. The results indicated partial shielding of
iv T-4059
electromagnetic radiation at frequencies between 300 and
22500 kHz, in the radio frequency band.
v T-4059
Table of Contents
Page
Abstract...... iii List of Figures...... ix
List of Tables...... xvii
Acknowledgments...... xix Chapter 1. Introduction...... 1
History of Research...... 1
Theory...... 8
The Thallium Superconducting System...... 13
Types of Thallium Superconductors...... 13 Preparation...... 13 Measuring the Critical Temperature...... 18
The Meissner Effect...... 23
Thermoelectric Power...... 25
X-ray Diffraction...... 30
Applications...... 35 2. Literature Survey...... 40 3. Objectives...... 42
4. Thallium Detection Apparatus 4 9
Spectrophotometric Determination of Thallium.... 49
Experimental Procedure, Results, and Discussion. 53
vi T-4059
Chapter Page
Conclusions...... 66 5. Preparation of Barium Cuprate...... 67 Introduction...... 67
Experimental Procedure...... 67
Results and Discussion...... 68
Conclusion...... 73
6 . Preparation of a Cuprate Design on Alumina...... 74 Introduction...... 7 4
Experimental Procedure...... 75 Results and Discussion...... 76
Conclusions...... 92
7. Vapor Deposition of Thallium Oxide...... 93
Introduction...... 93 Experimental Procedure...... 94 Tl-Vaporization Apparatus [final design]...... 96
Results and Discussion...... 98 Conclusion...... 122
8 . Shielding effects...... 123
Introduction...... 123 Experimental Procedure...... 125 Results and Discussion...... 128
Conclusion...... 131
9. Summary of Conclusions...... 132
vii T-4059
Chapter Page
10. Suggestions for Future Research...... 135
Bibliography...... 137 Appendices A. Thallium Handling and Safety...... 151
B. Literature Survey...... 156 C. Additional data on cuprate melted on alumina...... 224
C-l. XRD data of barium cuprate on alumina...... 225
C-2. Preliminary study of Ba2 Cu3 C>5 on alumina 234 D. Resistance versus temperature curves...... 243 E. The Benefit of Using Nitrogen instead of Helium... 256
viii T-4059
List of Figures Figure Page
1. Organization of thesis...... 2
2. Phase diagram of a type-I superconductor...... 11 3. Phase diagram of a type-II superconductor...... 12 4. Resistance versus temperature curves of
• TlBaCu3 0 5 #5 +x and A Tl2 Ba2 CU3 0 5 #5 +x [53]...... 20 5. Probe used in Resistance versus temperature
apparatus...... 21
6 . Plot showing the Meissner effect of a small single
crystal with a composition of Tl2 Ca2 Ba2 Cu3 0 x ...... 24 7. Thermoelectric power versus temperature apparatus... 28
8 . Probe used in thermoelectric power versus temperature apparatus...... 2 9
9. Crystal structure of thallium-barium-copper-oxide
superconductors. Top: T1:Ba:Cu=l:2:1 Bottom: T1: Ba: Cu=2 : 2 :1...... 31 10. Josephson junction. Insulator width is on the order
of angstroms...... 36 11. SQUID, (Superconducting Quantum Interference Device).
Two Josephson junctions in parallel connected by a
superconducting path...... 38 12. Outline of cuprate and superconductor preparation,
and rf shielding device studies...... 45
ix T-4059
Figure Page 13. Preparing cuprate powder...... 46
14. Melting cuprate onto alumina to produce a
superconductor...... 47
15. Depositing thallium onto substrates to make
superconductors...... 4 8 16. Beer’s law plot...... 51 17. Absorbance versus wavelength for thallium in paraphenetidine solution...... 55
18. Absorbance of thallium/para-phenetidine complex in solutions of various pH values: OpH=2.2, o pH=2.5,
□ pH=2.9, a p h =2.95, ■pH=3.1, * pH=3.35, + pH=3.55.. 56
19. Beer’s law plot of thallium in para-phenetidine at 560 nm...... 58
20. Thallium detection apparatus...... 59
21. Photograph of thallium detection apparatus...... 61
22. Absorbance of 1x10"^ M thallium in the thallium detection apparatus...... 62 23. Figure 24. Absorbance of a solution containing
thallium superconductor material and para
phenetidine. The amount of thallium superconductor
material used in each sample: <> 0.0395 g, o 0.0215 g,
□ 0.0053 g, A 0.0088 g ...... 65
x T-4059
Figure Page 24. SEM photomicrograph of barium cuprate powder...... 69 25. EDX trace of barium cuprate powder...... 70 26. XRD trace of barium cuprate powder corresponding to
a stoichiometry of BaCu3 C>4 . It also shows that
there are no detectable impurities...... 72 27. Shapes placed on alumina by stencil printing techniques...... 7 9
28. Composition of Ba-Cuprate melts on an alumina
substrate a) cross section, b) analytical data,
and c) profile composition [71]...... 81
29. Summary of EDX data for one layer of cuprate
melted onto aluminaiH 930°C, ♦ 940°C, and D 950°C.. 82
30. Photomicrographs of BaiCu3 0 x melted on alumina..... 84 31. Summary of EDX data for two layers of cuprate
melted onto alumina:D 930°C, □ 940°C, and ♦ 950°C.. 85
32. Photomicrograph of second layer of BaiCu3 0 x melted
onto alumina...... 8 6
33. Photomicrograph of second layer of BaiCu3 0 x melted
onto alumina...... 88 34. Photomicrograph of Ba^C^C^ second layer material
deposited using a slurry and then melting the
cuprate...... 89
xi T-4059
Figure Page
35. Photomicrograph of second layer of Ba^Cu3 0 4 melted onto alumina using cuprate-slurry and then sintering the cuprate...... 91 36. Vapor phase reaction apparatus (Design - I)...... 95
37. Cuprate-Tl, vapor phase interaction apparatus
(design II)...... 97 38. Sintered cuprate material with thallium deposited... 99 39. SEM photomicrograph of Tl-cuprate superconductor... 100 40. EDX of a typical Tl-cuprate. T1:Ba:Cu=l.45:1:3 . . . . 101
41. Resistance versus temperature curve for thallium
superconductor...... 1 0 2
42. Thermoelectric power versus temperature for a thallium superconductor...... 103 43. Comparison of XRD data obtained (bottom) to
literature (top)...... 104 44. Examples of cuprate melted on alumina...... 106
45. Resistance versus temperature curve. Sample:
thallium deposited on cuprate melted on alumina.... 107 46. Resistance versus temperature curve for thallium
superconductor. Onset = 90 K, Tc= 7 6 K ...... 109
47. Resistance versus temperature curve for thallium
superconductor. Onset = 125 K, Tc= 99 K ...... 110
xii T-4059
Figure Page
48. XRD trace of the thallium superconductor prepared
by depositing thallium at 900°C...... Ill 49. SEM photomicrograph of thallium deposited on barium cuprate at 900°C for 7 minutes. Copper oxide crystals are visible...... 113
50. SEM photomicrograph of superconducting material.
The stoichiometry is Tl^ b ^Ba^Cu^ . 3 0 x ...... 114 51. SEM photomicrograph of superconducting material.
The stoichiometry is TI3 # 7 BaiCu]_ # 3 0 x ...... 115 52. EDX data of the thallium superconductor,
T1 1. 6BalCul. 3°x...... 1 1 6 53. EDX data of the thallium superconductor,
T1 3.7BalCu1.3°x...... 1 1 7 54. SEM photomicrograph of a thallium structure observed when the thallium is deposited at 890°C... 118 55. Resistance versus temperature curve for thallium
superconducting material prepared at 890°C. Onset
at 90 K with a Tc=75 K. [Tli # gBaiCui. 3 0x ]...... 119
56. Resistance versus temperature curve for thallium superconducting material prepared at 890°C. Onset
at 90 K with a Tc=75 K [Tl3 # yBaiCU! # 3 Ox ]...... 120
xiii T-4059
Figure Page 57. Thermoelectric power versus temperature curve for
thallium superconductors (using melted cuprate on
alumina)...... 1 2 1 58. Design of the cubical shell [inset: hollow cylinder design]...... 126 59. Diagram of the circuit used to test the
shielding effectiveness of the cubical shell and
hollow cylinder used in this study...... 127
60. Effective shielding of the cubical shell...... 129 61. Effective shielding of a hollow cylinder at 116, 967, and 2900 kHz (top to bottom)...... 130
62. XRD trace of alumina wafer...... 226
63. XRD trace of single layer of barium cuprate on
alumina...... 227
64. XRD trace of second layer of barium cuprate applied without using napthenic acid, on alumina... 228 65. XRD trace of second layer of barium cuprate
applied by using napthenic acid, on alumina...... 229
6 6 . Barium concentration of Ba2 Cu3 C>5 melted at 930°C
onto alumina...... 237
67. Photomicrographs of a single layer of Ba2 Cu3 0 5 on alumina...... 238
xiv T-4059
Figure Page
6 8 . Summary of EDX data for two layers of Ba2 Cu3 C>5
melted onto alumina: □ 920°C,+ 930°C,
and O 940°C...... 239
69. Photomicrograph of a second layer of Ba2 CU3 C>5 melted on alumina...... 240
70. Photomicrograph of Ba2 Cu3 0 5 second layer material deposited using a slurry and then melting the cuprate...... 241
71. Photomicrograph of second layer of Ba2 Cu3 C>5
melted onto alumina using cuprate-slurry and then sintering the cuprate...... 242 72. Resistance versus temperature curve for thallium
oxide deposited at 880°C for 7 minutes on a
barium cuprate-alumina..substrate...... 24 6
73. Resistance versus temperature curve for thallium
oxide deposited at 880°C for 8 minutes on a barium cuprate-alumina..substrate...... 247 74. Resistance versus temperature curve for thallium oxide deposited at 880°C for 9 minutes on a
barium cuprate-alumina substrate...... 248
75. Resistance versus temperature curve for thallium
oxide deposited at 880°C for 10 minutes on a barium cuprate-alumina substrate...... 24 9
xv T-4059
Figure Page 76. Resistance versus temperature curve for thallium
oxide deposited at 880°C for 11 minutes on a barium cuprate-alumina substrate...... 250
77. Resistance versus temperature curve for thallium
oxide deposited at 880°C for 12.5 minutes on a barium cuprate-alumina..substrate...... 251 78. Resistance versus temperature curve for thallium
oxide deposited at 880°C for 13 minutes on a barium cuprate-alumina..substrate...... 252
79. Resistance versus temperature curve for thallium
oxide deposited at 890°C for 11.2 minutes on a barium cuprate-alumina substrate...... 253 80. Resistance versus temperature curve for thallium
oxide deposited at 890°C for 14 minutes on a barium cuprate-alumina substrate...... 254
81. Resistance versus temperature curve for thallium
oxide deposited at 890°C for 16 minutes on a barium cuprate-alumina substrate...... 255
xvi T-4059
List of Tables Table Page 1. Progress in superconductivity till the mid 1970s.... 5 2. Some of the new high Tc superconductors...... 9 3. Thallium superconducting compounds and their
critical temperatures...... 14
4. Interatomic distances of 121 thallium superconductors...... 33 5. Peak angles of diffraction of barium cuprate powder (20 degrees)...... 71
6 . The types of masks examined to apply barium
cuprate to alumina...... 77
7. The effects of different binders for putting
cuprate on alumina 7 8
8 . Data for the second layer of BaiCu3 0 4 deposited on
melted cuprate using the slurry method...... 90 9. D-spacings and peak heights of XRD trace of
alumina wafer (Figure 62)...... 230
10. D-spacings and peak heights of XRD trace of single layer of barium cuprate on alumina (Figure 63)...... 231
xvii T-4059
Table Page
11. D-spacings and peak heights of XRD trace of second layer of barium cuprate applied without using napthenic acid, on alumina (Figure 64)...... 232 12. D-spacings and peak heights of XRD trace of second layer of barium cuprate applied by using
napthenic acid, on alumina (Figure 65)...... 233
13. Data for the second layer of Ba2 Cu3 C>5 deposited on melted cuprate using the slurry method...... 235 14. Resistance versus temperature data for optimizing the deposition of thallium oxide...... 245
xviii T-4059
Acknowledgments
I would like thank my advisors, Dr. Baki Yarar and Dr. John U. Trefny for their technical support, advice, and patience during the project and completion of the thesis. I would also like to thank Drs. John J. Moore and David
G. Wirth for being members of my thesis committee.
Appreciation also goes to:
Mr. Luigi A. Greco and Dr. Abdelkader Outzhorhit for their design of the AC resistance apparatus, and help in obtaining the data Mr. Ivan A. Cornejo for providing the thermoelectric power versus temperature data presented in this thesis
Mr. Robert McGrew for his assistance and advice in obtaining the SEM/EDX data Finally, I would like to thank the Colorado Center for Advanced Ceramics and the Coors Ceramics Company, whose funding made this thesis possible.
xix T-4059 1
Chapter 1. Introduction
Until several years ago, the phenomena of superconductivity was only found in a few materials at very low temperatures. These temperatures, in the range of 0-23 K can only be produced using liquid hydrogen and helium, which are expensive. The discovery in 198 6 of a new family of high temperature superconductors made it possible to use liquid nitrogen, which is readily available at acceptable cost. Superconductivity is the "lack of resistance to electrical current when cooled to low temperatures" [1 ] .
Figure 1 shows a flowsheet of the organization of this thesis.
History of" the Research
At one time, it was believed that zero resistance in materials was impossible. At the time of the discovery of superconductivity, 1911, it was known that the resistance of pure metals decreased linearly with temperature, but the properties below that of liquid nitrogen, especially below
25K, were still a mystery [2]. By extrapolating the linearity of the resistance of the metal one would find that zero resistance occurred above absolute zero Kelvin. Onnes 4059
Introduction
Literature Review
Obj ectives
Thallium detection apparatus
Preparation of barium cuprate
Preparing cuprate design on alumina
Vapor deposition of thallium oxide
Shielding effects
Conclusion
Recommendations for future work
Bibliography
Figure 1. Organization of Thesis T-4059 3
believed that negative resistance had no meaning, and zero resistance was inadmissible because it had no analogy at that time [2 ]. However, other scientists had shown accomplishments in measuring the properties of metals in liquid hydrogen and they noticed sudden decreases in the resistance of the metals at the low temperatures. Onnes, believing that zero resistance would only occur at absolute zero, decided to investigate the low temperature properties of pure metals. After getting results showing resistance approaching a minimum above zero in platinum and gold, he believed that pure metals would show infinite conductivity. He was able to distill pure mercury and by freezing it in small capillaries made a wire. Cooling this down to 4.2 K he achieved a resistance less than a millionth of the resistance at the melting point of mercury. Accordingly he called this state superconductivity [2]. Onnes received the
Nobel Prize in 1913 for the liquefaction of helium , one of the results of his study [3]. The period between 1911 and 1945 is characterized by many investigations but no practical applications. Before World War II, only ten laboratories in the world could produce liquid helium.
Three major discoveries were made in this period.
In 1933 Meissner and Ochsenfeld [4] found that a solid T-4059 4
cylinder of lead or tin, situated in a uniform applied magnetic field, expelled the magnetic flux as the cylinder was taken into the superconductive state by lowering the temperature of the system. This is observed as a magnet hovering or spinning above a superconductor. Recently, photographs of YBa2 CU3 C>7 _x (YBCO) sitting in liquid nitrogen with a magnet hovering above it have been used to generate popular interest and support for superconductivity research [5].
In 1934 Ehrenfest, Rutgers, Gorter, and Casimir provided a thermodynamic description of superconductivity [3]. H. and F. London proposed a theory which predicted that the magnetic field would not disappear at the surface of a superconductor, but should fall off exponentially with distance into the metal [6 ]. After World War II the production of helium increased, thus making it easier to study superconductivity. The possibility of studying and using superconductors ceased to be a problem except for the cost of helium. Additional theories of superconductivity were developed [7, 8 ]. Bardeen, Cooper, and Schrieffer developed the fundamental theory to describe superconductivity in 1957, which led to a
Nobel Prize [7]. Superconducting alloys were discovered that had critical temperatures as high as 23.2 K. Table 1 T-4059 5
lists the major alloys and temperatures at which they
display superconductivity (critical temperatures) and years of the discovery. The next period which was from 1973 to 1986, was
characterized by problems in both physics and materials
Table 1. Progress in superconductivity till the mid 1970s[5]
Material Critical Temperature Year discovered
Hg 4.1 K 1911 Pb 7.2 1913
Nb 9.2 1930
Nb3Sn 18.1 1954
Nb3 (Al_75Ge.25) 2 0 . 2 1 1966
Nb3Ga 20.3 1971
Nb3Ge 23.2 1973
science. Methods for modifying the parameters in
superconductors were studied. The major objective was the
search for higher critical temperatures [3]. Johannes Bednorz and Karl Muller of IBM Zurich Research Laboratories submitted a paper on April 17, 1986 to the journal
Zeitschrift fur Physik in which they claimed to have T-4059 6
discovered a 30K superconductor in a compound of lanthanum, barium, copper, and oxygen [9]. The paper was not published until September and the first public forums were not held until December of that year. By December Kitazawa of the University of Tokyo and Chu of the University of Houston had also obtained evidence of superconductivity in the new family of oxides. At this time the interest was beginning to grow worldwide.
During the month of December the record transition temperature was pushed to 52K by Chu and his coworkers [9]. On February 16, 1987 Chu announced at a press conference that his group had achieved superconductivity above 77 Kelvin with a compound containing yttrium. This discovery was significant because liquid nitrogen could now be used as a coolant instead of liquid helium. The benefits of using liquid nitrogen are discussed in Appendix E. Since then dozens of compounds have been discovered that have transition temperatures above 77 K. A lot of the excitement had to do with the fact that these new compounds are ceramic oxides containing copper, and that all previous discoveries had involved metallic compounds. Table 2 lists a fraction of these compounds. In early 1988 Sheng and Hermann of the University of
Arkansas made the discovery of superconductivity in the T-4059 7
TIBaCuO system [10]. Zero resistance occurred at 81 K and sharp drops in resistance began above 90K. Sheng and
Hermann examined the methods and similarities among the new high temperature superconductors and were able to formulate two methods to be used in the search for new materials. It was immediately apparent after the discovery of YBCO (superconductors composed of only yttrium, barium, copper, and oxygen) that substitution of other rare earths for yttrium would also produce superconductors. It was known that a small concentration of paramagnetic ions significantly depresses or destroys superconductivity in metallic superconductors. Therefore the first route was to choose rare earths with little or no magnetic moment, ie. Lu(+3) and Yb(+3). The second approach was to choose materials with radii close to that of Y regardless of the magnetic moment.
After Sheng and Hermann's group substituted all the rare earths they came up with three conclusions [10]. First, the substitution of other magnetic rare earths for Y in YBCO did not influence the transition temperature significantly.
Secondly, the ionic radii of the rare earths were important to superconductive behavior of the RE-Ba2 Cu3 0 7 _x (RE=rare earth, includes Y (YBCO) but also Eu, see Table 2) compounds. Third, the valence of the substituted rare earth T-4059 8
was crucial to superconductivity of the RE-Ba2 Cu3 0 7 _x compounds. Ce, Pr, and Tb, are elements with +3 and +4 valence. These did not form superconductive compounds. Therefore it was suspected that the valences and radii of the rare earths were important and one could expect other elements with a +3 valence and similar atomic radius to be good substitutes. From the periodic table one sees that the group IIIA elements have a +3 valence. Thallium is the heaviest element in Group IIIA and has a radius of 0.95 A. The ionic radii of the +3 rare earths vary from 0.85 A to 1.016 A , closely matching the radius of Eu^+. Eu forms a superconductive compound with a critical temperature of 92 K (see Table 2). With this in mind Sheng and Hermann studied the thallium superconducting system [1 0 ] .
Theory
Theories describing the theory of superconductors in general are numerous although, the one due to Bardeen, Cooper, and Schrieffer (BCS) covers low Tc superconductors to everyone's satisfaction [6,7,13,14]. However, superconductivity is recognized mainly as a phenomenon associated with zero resistance to electrical flow in materials. Meissner discovered that when a superconductor T-4059 9
Table 2. Some of the new high Tc superconductors [9]
Material Tc Y.sBa?Cu30y 80 K
YbBaCaCu30y 81
YbBaSrCU30y 81
YBaCaCu^Oy 82
YSrCaCu^Oy 82
YSrBaCu^Oy 85
SmBa^Cu^Oy 85
GdBa^Cu^Oy 86
YbBapCU30y 90
Y.5Sc.sBa^CU30y 90
DyBa^Cu^Oy 91
EuBa^Cu^Oy 92
HoBa^Cu^Oy 92
Y.sLu.sBa^Cu.^Oy 92
YBa2Cu30y 94
Thallium Compounds >81K (highest 127K) T-4059 10
is cooled below a certain temperature (the critical temperature, Tc) in a magnetic field, the magnetic field inside the superconductor becomes zero.
This is different from a perfect conductor. If a perfect conductor is placed in an external magnetic field and then cooled below a critical temperature, the induced magnetic field inside the conductor does not become zero. A superconductor presents the same behavior regardless of the order of cooling and applying a magnetic field, whereas a perfect conductor displays different properties if this sequence is altered.
Superconductors are classified as either type I or II, depending upon their behavior in externally applied magnetic fields. Figures 2 and 3 show phase diagrams of type I and II superconductors, respectively. Type I superconductors expel the magnetic field completely until they become normal materials. Type II superconductors will show complete magnetic field repulsion until Hci . Between
HC 1 and the upper critical field, Hc2 , the magnetic field starts to penetrate the material. Since the material is still superconducting, it shows an incomplete Meissner-
Oschenfeld effect, and this phase is called the Shubnikov phase. Bulk superconductivity disappears at HC2 , where there is complete penetration of the material by the iue . hs iga fatp- superconductor type-Iaof diagram Phase 2. Figure T-4059
Induced Magnetic Field Temperature 11 T-4059 iue . hs iga fatp-I superconductor type-IIaof diagram Phase 3. Figure Magnetic Field cl c c3 2 phase Shubnikov Meissner-Ochsenfeld phase eprtr, T Temperature, 12 T-4059 13
magnetic field. However, some evidence of surface superconductivity may exist to a higher critical field called HC 3 [14]. Thallium superconductors conform to type-II behavior [14] .
The Thallium Superconducting System
Types of Thallium Superconductors Many different types of thallium superconductors have been discovered as can be seen from Table 3. As of this writing the thallium superconductors are included among the highest critical temperatures.
Schilling, et al. [75], has recently announced mercury based superconductors with a Tc~133 K.
Preparation
There are a number of different ways of preparing thallium superconductors; these methods were adapted from the preparation of other superconductors. Two types of superconductors are made: bulk and film. Thin films are usually less than an 0.5 mm thickness but it usually depends upon the process and the investigator as to whether it is classified as thin or thick film. T-4059 14
Table 3. Some thallium superconducting compounds and their
critical temperatures [1 1 , 16]
Material TC TlSr^CuO^ 50
TlBaCU3 0 x -81
TlBa2 CaCu2 0 7 90
TlSr?CaCu;>07 90
Tl2 Ba2 Cu1 0g 90
TlBa^Ca^Cu^Og 1 1 0
Tl2 Ba2 CaCu2Ofi 1 1 0
Tl2 Ba2 Ca3 Cu4 Oi0 119
TlBa2 Ca3 Cu4 0 T 1 2 2
Tl?Ba3 Ca3 Cu3 0 1 n 1 2 2
TlSr^Ca^C^Og 1 2 2
TlBa2 CapCU3 0 !o -127
Most bulk materials are prepared by a sintering process using pressed pellets. In this method the materials are mixed in stoichiometric portions, ground with mortar and pestle, pelletized and sintered at temperatures as high as T-4059 15
900°C under oxygen for periods of time as long as 36 hours [15]. Hermann and Sheng, who discovered the thallium superconductors had to modify this procedure because TI2 O3 has a relatively low melting point, 717°C, and starts to decompose at 100°C [15, 17]. A typical procedure involved first mixing the correct amounts of BaCC>3 and CuO and then grinding them in an agate mortar and pestle. This was then heated in air for more than 24 hours at 925°C to obtain a uniform black powder.
The powder was either BaCu3 C>4 or Ba2 Cu3 C>5 depending on the initial stoichiometry. The barium-copper-oxide powder was then mixed with a stoichiometric amount of TI2 O3 , completely ground and then pressed into a pellet. The pellet was heated in the range of 880-910°C for 2-5 minutes in flowing oxygen [1 0 ].
Their observations were that the thallium did three things: part volatized as black smoke, part became a light yellow liquid, and part reacted with Ba-Cu oxide to form a black porous material [10]. The material displayed superconductivity in liquid nitrogen (the Tc > 77K).
Other investigators have examined the thallium systems and noted that the addition of calcium raised the critical temperature. Many of these thallium superconductors (some containing calcium and others not) have been prepared as T-4059 16
thin films. The ability to do this is necessary for their use in many applications, especially in the microelectronics industry in integrated circuits and for devices taking advantage of quantum effects. Two methods of thin film preparation are discussed below: rf sputtering and sequential electron-beam evaporation.
In rf sputtering, a pellet of the material to be deposited is heated in an atmospheric mixture of oxygen and an inert gas like argon. When an rf field is established in the space above the pellet, this process produces a 'dome' or plasma of ions above the pellet. If a substrate is placed near the plasma dome and heated, particles from the pellet will deposit on the substrate. This procedure allows fairly thin films to be produced.
Qiu and Shih of the Electrical Engineering Department of McGill University and Moreau and Courbess at IMRI, National Research Council have prepared TIBaCaCuO films using this method [18]. They produced a target (the pellet from which the plasma is made) by sintering stoichiometric amounts of BaCOg, CaCC>3 , and CuO at 850°C and then pelletizing under pressure. They then sintered the materials. The apparatus used was an rf magnetron sputtering gun, the diameter of the target was 5 cm, the T-4059 17
incident rf power was 80 W and the distance between the target and substrates was 4 cm. Glass slides and polished
ZrC>2 were used as substrates. The atmosphere was 95-5 Ar-C>2 with a total pressure of 2 mtorr. Then TI2 O3 was placed on top of the materials and heat treated to allow the T1 to diffuse into the BaCaCuO. TI2 O3 was not sputtered because in sputtering the whole inside of the apparatus is covered with the target material and the investigators did not want to contaminate their system [18].
The second method, sequential electron beam evaporation, has been used by Ginley and coworkers at Sandia National Laboratories to produce TIBaCaCuO layered films.
Substrates of SrTiC>3 , sapphire and yttria-stabilized cubic zirconia was used in the formation of the TIBaCaCuO films. The substrate was positioned 10 cm above a Leybold-
Hereau ES V- 6 electron beam evaporator, which was equipped with a four pocket crucible containing Tl, Ba, Ca, and Cu.
The atmosphere of the system was oxygen and a base pressure of 1 . 5x 10“5 mbar was maintained. In this apparatus an electron beam strikes each pocket for a given time to produce a layer of the material as a substrate. Ginley, et al., deposited 25 layers starting and ending with Cu in a sequence Cu-Ba-Ca-Tl so as to achieve a net stoichiometry of
Tl2 CaBa2 Cu2 • The layers were deposited as follows:Cu at T-4059 18
2A/sec to 151 A, Ba at 5A/sec to 564 A. Ca at 5.2 A/sec to 200 A and T1 at 3A/sec to 333 A. They then annealed the material under T1 and oxygen atmosphere (in an enclosed gold crucible) to produce a superconducting thin film.
Measuring the Critical Temperature The method for determining the critical temperature is to cool from room temperature while measuring the resistance across the material. The resistance will usually suddenly drop down to zero at the superconducting state. At the beginning of the steep drop is the onset temperature. The highest temperature of zero resistance is considered the critical temperature. One also has to work at very small (negligible) current densities.
The resistance is usually measured using the four-probe method. The contact area is kept to a minimum and pressed with a known force onto the material. A typical spacing is
1 mm between the probes. Many investigators paste the leads to the superconductor with silver, thus eliminating the need to specify a pressure. The probes are spaced linearly with the current through the outside leads and the voltage drop measured across the two inside leads. For equally-spaced probes, distance d apart, voltage, V, and current, /, the conductivity is: T-4059 19
Figure 4 is an example of resistance versus temperature curves of TlBaCu3 C>5 # 5 +x and Tl2 Ba2 CU3 C>5 # 5 +x. Figure 5 shows the probe. The samples were mounted on a copper platform, which contained a silicon diode thermometer. The thermometer, calibrated in the range 4-300 K, had four leads for electrical connections: two for a temperature dependent output voltage, and two for a constant current source (i.e. 10 M ) ■ The output voltage was converted to a temperature using a polynomial fitted to calibration data.
The thermometer was mounted into probe with "Apiezon thermal grease". It was located on the bottom of the clamp in order to accurately measure the temperature. A resistor-heater controlled the temperature of the sample. #40 enamel coated copper wire was soldered to posts which were insulated from the copper platform, and the wires ran through a stainless steel tube to the top of the apparatus that had a flange with vacuum and electrical connections. The probe was insulated from ground by a block of "Plexiglas".
The probe was surrounded by a copper sheath to protect the sample from thermal shock and external electromagnetic T-4059 20
0.4
0.3
0) O c o in *in 4) Q:
SO 100 150 200 250 300 Temperature K
Figure 4. Resistance versus temperature curves of
• TlBaCu3 0 5 .5 +x and A Tl2 Ba2 Cu3 0 5 .5+x [53] T-4059 21
To data acquisition and computer
Vacuum pump
Pyrex Dewer Stainless steel flask tube Insulator
Resistor heater
Diode Probe- temperature sensor Sample Copper sheath Connections Connections for diode for sample Dewer flask
Figure 5. Probe used in resistance versus temperature
apparatus. T-4059 22
fields during measurement as well as a vacuum environment.
The sample was held to the probe by double stick tape. Four copper leads were attached to the sample with silver paint, and small samples were held in place by a clamp. The wires were soldered to the posts on the probe. After the sample had been mounted and the probe sealed it was inserted into a Pyrex tube which was used to hold liquid nitrogen (see Figure 5). Temperatures as low as 64 K could be obtained by pumping out the gas above the liquid nitrogen in the Pyrex tube. A second dewar insulated the Pyrex tube.
An EG&G Princeton Applied Research model 5205 lock-in amplifier was used to provide an alternating current to take the resistance measurements in order to increase the signal- to-noise ratio. A Hewlett-Packard 241 oscillator, at 300 Hz, was used to provide alternating current to the outer leads on the sample. A current-limiting resistor was placed in series to reduce the current to 2 0 ?A. The oscillator also provided a reference voltage to the lock-in amplifier, which was used to measure the root-mean-square AC voltage produced across the inner leads of the sample. A constant current was needed. However, in some samples the current varied with temperature, especially when the resistance was on the order of the current limiting resistance. A Keithley T-4059 23
177 microvoltmeter was used to monitor the current by measuring the voltage produced across a 1 0 0 or 1 0 0 0 ohm resistor in series with the current limiting resistor.
Data acquisition was done with a Hewlett-Packard Vectra computer, which used BASIC programs, and associated instrumentation measured the voltage in the silicon diode thermometer. The voltage across the sample was measured by a Keithley 181 nanovoltmeter, interfaced with the Vectra computer and connected to the lock-in amplifier’s monitor output. The program in the computer calculated the resistance and temperature and stored the information on disk.
The Meissner Effect
The Meissner effect can be used to get quantitative information because the effect of the field repulsion occurs in the superconducting state and not in the normal state.
Figure 6 is the Meissner susceptibility versus temperature curve for Tl2 Ba2 Ca2 CU3 0 x obtained by Ginley, et al. [12].
The vertical axis is the degree of flux expulsion. The diamagnetism due to the flux expulsion increased quickly as the temperature decreased below 100K. The maximum at 5K is -0.018 emu/g (in 25 Oe). The susceptibility (Xg) was determined to be -7.2x10“^ cm^/g. Figure Figure T-4059 Moment emu/g 0.00 0.02 0.01 6 Po soig h esnrefc o a ml single small a of effect Meissner the showing . Plot - rsa wt cmoiino T2aB2u0 [ Tl2Ca2Ba2Cu30x of composition a with crystal eprtr K Temperature 50 75 100 125 24 12 ]
T-4059 25
Equation 2 is a relationship between the volume fraction if) and the susceptibility. / = -4npXB (2) where p is the density. The value of / was determined to be 0.07.
Thermoelectric Power Thermoelectric power versus temperature measurements were conducted on an appratus at CSM. Measurements were taken by Mr. Ivan Cornejo, of the Department of
Metallurgical and Materials Engineering. An expression for thermoelectric power of a material at an average temperature is derived from the equation that describes the thermoelectric effect for non-equilibrium systems: E = SVT (3) where E is the electric field due to a temperature gradient ( V D in a material, and S, a tensor called the Seebeck coefficient, is the thermoelectric power of the material. Experimentally this equation becomes:
E = S— (4) cbc where E and S are scalar values, and dT is the temperature difference along a length, dx. Integrating over the length, T-4059 26
dx, one obtains the potential difference along the path, A V:
A V = -\X2Edl = -\T2SdT (5) JXl JTX where T2 is the temperature at point X2, and Tl is the temperature at point Xj. Integrating along the line of the lead and the sample, one obtains:
AV = -£sTdT-£ssdT-j£sTdT (6) where s T is the thermoelectric power of the leads, % is the thermoelectric power of the sample, Th is the temperature of the hot side, Tc is the temperature of the cold side, and To is a reference temperature. This equation can be solved as: S V = ST{Tc-Th) + Ss{Th-Tc) (7)
The temperature difference, A71, is defined as:
A T=Th-Tc (8 ) and defining an average temperature, Taverage, is defined as:
T +T Taverage ave = ^ e- (9)
Solving for Ss we get:
$S ^average) ~ + ^ a v e r a g e ) ( 1 0
Figure 7 shows the apparatus, and Figure 8 shows the probe.
Equation 10 was used twice, to determine S from voltage across the chromel leads (see Figure 8 ) and then from the T-4059 27
voltage across the constantan leads. The average of these two calcuations determined the thermoelectric power of the sample.
The probe consisted of a phenolic resin block containing two copper blocks. The distance between the blocks could be controlled and samples were mounted in between. Each copper block had a chromel-constantan thermocouple, soldered flush to the block's top, which were used to measure the voltage across the mounted samples as well as the temperature of the blocks. Small pieces of pressed indium metal, which provided good thermal and electrical contacts between the sample and the blocks, were held with silver paint on the ends of the copper blocks. A sheet of Mylar with a copper strip underneath was positioned between the phenolic resin holder and the copper blocks, such that there was thermal conductivity between the blocks to obtain small and controllable temperature differences between them. A sample clamp made of a Telfon sheet with an aluminum plate above held the sample in place by pressure, by adjustable nylon screws. A resistor-heater was used to produce the required temperature gradient.
The probe was attached to a brass flange, which contained a thermocouple to get a room-temperature reference. The assembly was contained in a brass vacuum T-4059 28
Data acquisition and computer
Vacuum pump Copper block for temperature reference
Brass Dewer flask
.Probe
■Dewer flask
Figure 7. Thermoelectric power versus temperature apparatus T-4059 29
.Screws
Aluminum sheet Teflon sheet
Sample Resistor Copper block heater
Rosin jDlock
Constantan -Constantan
Chromel Chromel
Figure 8 . Probe used in thermoelectric power versus
temperature apparatus T-4059 30
chamber which was kept under vacumm during the course of the test. A valve was used for rapid cooling of the probe by introducing helium and for venting gases.
The data were acquired by interfacing a Keithley 195 Sytem DMM and a 702 scanner with an Apple II computer. The computer controlled the scanner for a particular voltage, which was read by the DMM and processed for the necessary data.
The samples were cooled, under vacuum, to approximately
200 K. The chamber was then isolated from the pump and cooled helium gas introduced to reach a final temperature of
7 6 K. The helium was then pumped out of the chamber and the computer set to collect data. A temperature difference of approximately 5.5 K was produced across the sample by the resistor-heater.
X-ray Diffraction Analysis of the structure of thallium-barium-copper- oxide has been reported by researchers [19, 20]. There are two stoichiometries of Tl:Ba:Cu: 2:2:1 and 1:2:1.
Investigators have reported that 221 crystals are tetragonal and orthorhombic [2 0 ]. Figure 9 shows a picture of the structure of 221 crystals. The Cu-0 octahedra are stretched to about 39%. T-4059 31
Copper
Thallium
Oxygen
Barium
Figure 9. Crystal structure of thallium-barium-copper-oxide
superconductors. Top: T1:Ba:Cu=l:2:1
Bottom: T1:Ba:Cu=2:2:1 T-4059 32
The Tl^+ ions also form an octahedral environment with the oxygen atoms. The Tl-0 lattice consists of double layers of octahedra joined at the edges and linked to the Cu-0 octahedra by common corners along the [0 0 1 ] direction.
The Cu-0 layers alternate with double Tl-0 layers. Two
Tl-0 layers are displaced relative to each other by half of the diagonal of the tetragonal cell base. Ba^-+ cations are located between a Cu-0 layer and a double Tl-0 layer. The Cu-0 octahedra are joined at the corners forming planar layers.
Kolesnikov, et al. [19], reported that there is an isomorphic replacement of copper atoms with some thallium atoms. This is due to the similarity in size between Cu+1 (0.98 A) and Tl^+ (1.05 A) atoms. Also there is a slight displacement of oxygen atoms associated with cells that have the T1 atoms replaced by Cu atoms. This lengthens the Cu-0 bond.
Kolesnikov, et al., attribute electron holes to the defects in the Tl-0 planes and suggests that the superconductivity depends on the ordering of these defects in the crystal. Tetragonal crystals are superconductive while octahedral crystals are not, which may be explained by this ordering.
Table 4 lists the interatomic bond distances that T-4059 33
Kolesnikov, et al., found in their research. The data represent specifically different bonds in the structure. In the orthorhombic distortion the apexes of the Cu-0 octahedra
are slightly displaced. The Ba cations are located between a Cu-0 layer and a double Tl-0 layer.
Figure 9 also shows the structure of 121 thallium
barium copper oxide superconductors. Researchers have studied this structure extensively: The Cu is coordinated with oxygen in tetragonal pyramids. The Cu-0 octahedra can
Table 4. Interatomic distances of 121 thallium superconductor
Bond Distances (A)
Tl-0 1.997 2.030 2.445 2.889
Ba-0 2.7371 2.847 2.98
Cu-0 1.84 1.87 2.82 1.9343 2.724
be viewed as double pyramids and are linked at their corners and oriented with their distorted bases parallel to the a/b plane. The apexes of the pyramids are stretched about 11% in comparison to their base. The Cu-0 squares (base of the pyramids) are also oriented parallel to the a/b plane and
form a two dimensional grid. T-4059 34
The thallium oxygen polyhedra are linked to the CuO tetragonal pyramids. The TIO5 octahedral have very little distortion and are joined by their edges in the a/b plane. There can be an oxygen deficit in the CuO tetragonal pyramids which can lead to an octahedral coordination around the copper atoms and the orientation of the CuO pyramids with their bases perpendicular to the a/b direction.
However, there is no oxygen deficit in the oxygen atom sites around the thallium atoms. Ba^+ ions lie between the T1 and Cu polyhedra. The coordination is between eight and twelve since there are oxygen deficient sites around the barium.
Studies of the 2223 phase (Tl2 Ba2 Ca2 Cu3 0 x) [21] have uncovered a well ordered tetragonal or pseudotetragonal unit cell with a=5.399±0. 0 0 2 A and c=36.25±0.02 A. A diagnostic feature is a strong peak representing the 0 0 2 (hkl) planes with a spacing of 18 A. The 2122 phase has a pseudotetragonal unit-cell with dimensions of 5.44 x 5.44 x 29.5 A. The spacing of 0 0 2 planes (the corresponding peaks in the intensity show up on the XRD data as a prominent feature of the superconducting compound) is 14.8A. Hazen and coworkers of the Carnegie
1 Superconductors in the same system are usually referred to by their stoichiometric ratios. Oxygen, however, is not included. T-4059 35
Institute suggest that the structure of 2223 is similar to
2 1 2 2 but with the copper oxygen planes separated by calcium [2 1 ]. These are probably some of the simplest structures of the thallium based superconductors. The more complicated compounds have correspondingly complicated structures.
Applications
Before concluding this chapter some applications for superconductors need to be mentioned. High field magnets made from superconductors can be made much smaller and more efficient. Switches and memory elements can be made from superconductors by taking advantage of their ability to become normal in high magnetic fields and superconducting in low fields.
One of the important applications is for a magnetometer called a SQUID (Superconducting Quantum Interference
Device). It uses two Josephson junctions in parallel. A Josephson junction is a strip of superconductor with a very thin insulator placed through it (see Figure 10).
Solymar and Walsh discuss four important properties of the Josephson junction as [14] : 1. For low-enough currents there can be a current across the insulator without any accompanying voltage; the T-4059 36
Insulator
‘Superconductor
Figure 10. Josephson junction. Insulator width is
on the order of angstroms. T-4059 37
insulator turns into a superconductor. The reason is that
Cooper pairs tunnel across. 2. For larger currents there are finite voltages across the insulator.
3. A direct transition may be caused between the Josephson characteristics and the 'normal' tunneling characteristics by the application of a small magnetic field. 4. When two Josephson junctions are connected in parallel (SQUID) (Figure 11) the maximum supercurrent that can flow across them is a periodic function of the magnetic flux:
/max = 2 1J,COS (11 V^o J
Using a SQUID of loop area 1 cm^, a magnetic field as low as 10-H Wb/m^ can be measured. This can be used to detect submarines as they perturb the Earth's magnetic field. Also, they can be used to detect the strength and orientation of the paramagnetism in clay-like pottery in order to date the pottery [14].
There are other applications for superconductors; however, the most exciting one is computers. Work is being done already to make transistor-like devices using T-4059 38
Superconducting path X
x
Josephson junction
Figure 11. SQUID, (Superconducting Quantum Interference
Device). Two Josephson junctions in parallel connected by a superconducting path. T-4059 39
superconductors. With the advent of liquid nitrogen level superconductors the development of superconducting computers is coming. The big push is for room temperature superconductors, but so far thallium superconductors (~127 K) [11] and most recently, mercury superconductors at ~133 K [75] (confirmed in the last three months) lie closest. Liquid nitrogen is relatively cheap and if the critical temperatureis high enough so that when current is passed through the material never goes above its Tc, electronic devices can be built.
Superconductivity was discovered in 1911; high temperature superconductors discovered in 1986 and thallium superconductors in 1988. The pace at which the field picked up in the last half decade was extremely fast and progress will lead to superconductors at even higher critical temperatures. T-4059 40
Chapter 2. Literature Review
A literature review was made of the research done on thallium superconductors. The conclusions reached from the literature study with the associated references are as follows:
1. Thallium based superconductors have the highest critical temperature, above the boiling temperature of liquid nitrogen [10, 28, 29, 42, 38, 53, 54].
2. Thallium based superconductors can be prepared by a solid state reaction of powders that contain their constitute elements, ie. TI2 O3 , BaO, CuO, and CaO. The structures of the products have been illucidated and well documented [24, 27, 34, 35, 36, 37, 40, 43, 44, 57, 58, 59, 61, 63, 64].
3. Thallium based superconductors can also be prepared by using techniques that involve the vaporization of one of the constituent materials, eg. TI2 O3 , which then interacts with a solid substrate such as BaCuO [22, 42, 45, 46, 47, 48, 51, 52, 55, 56, 62] . T-4059 41
4. Cracks and a rough surface on thin-film superconductors affects the critical temperature. A smooth, crack-free surface is desirable for the highest critical temperature [23, 25, 26, 39, 49] .
5. Thallium superconductors display the Meissner effect. This can be used to shield against electromagnetic radiation such as radiofrequencies. [30, 41]
6 . Superconductor products can be patterned by screen (or stencil) printing methods [31, 33, 50, 60, 62].
7. Thallium is very toxic and precautions need to be taken to provide a safe working environment [65, 6 6 ].
An outline of topics and information with an up-to-date literature survey is given in Appendix B. T-4059 42
Chapter 3. Objectives
The primary objective of this study is to optimize conditions under which a patterned thick film superconductor could be built on a commonly available substrate such as polycrystalline alumina (OC-AI2 O3 ) .
During the implementation of this study a number of problems were addressed.
1. Due to the toxicity of thallium an apparatus was needed to detect dangerous concentrations of air-borne thallium. Thallium is toxic above the level of 0.1 mg per kg of human body weight. Air-borne products that arise from the handling and heating of thallium compounds will contain thallium oxide particulates. The concentration of thallium would be monitored by the absorbence of light through a chemical that gives a color indication in the presence of thallium.
2. A thallium vapor deposition apparatus was designed. The approach was to vaporize thallium oxide onto a substrate with barium cuprate. The barium cuprate would be suspended above the thallium oxide. The device would also be designed T-4059 43
to allow the quick insertion and removal of the substrate with the cuprate before and after exposure to thallium vapor, allowing precise timing of the exposure to thallium vapors.
3. In order to make a device with thallium superconductors a method to put specific shapes on the surface of the substrate would be sought. Stencil printing, a technique used to make integrated circuits, involves the cutting of a design in a mask to be put over a substrate. After material is deposited and the mask removed the remaining material on the substrate is in the shape of the design.
4. It was discovered that there is an interaction with the alumina substrate used and the cuprate material. Depositing thallium on this layer was not producing a superconductor. A procedure to get the desired morphology and stoichiometry in a cuprate for producing superconductors layer would be sought.
5. The thallium superconductor produced in the vapor phase deposition apparatus would be characterized with respect to Resistance versus Temperature, thermoelectric power, SEM
(scanning electron microscope), and XRD (X-ray diffraction). T-4059 44
Also, the temperature for vaporizing thallium oxide would be optimized.
6 . A device using the techniques developed would be made.
The device would test the ability of the superconductor to shield against electromagnetic radiation.
Figures 12, 13, 14, and 15 show flowcharts of the procedures implemented in this study. Figure 12 shows the order of studies and characterization methods used in this research. Figure 13 shows the procedure for preparing cuprate powder to be used to make superconductors. Figure 14 shows the procedure for depositing shaped cuprates onto alumina substrates. The procedures outlined in Figures 13 and 14 were a result of exploring objectives 3 and 4 discussed above. Figure 15 is the procedure for depositing thallium oxide onto a substrate, as proposed in objective 5. T-4059 45
Cuprate preparation
Cuprate processing
Screenprinting patterns Flakes on alumina
Characterization Characterization
T1 vaporization apparatus design
T1 superconductor T1 superconductors flakes on alumina
Characterization Characterization
Rf shield devices
Cubical shell Hollow cylinder
Characterization Characterization 10 7,10
Characterization methods 1. SEM, 2. EDX, 3. XRD, 4. Optical microscopy, 5. Melting behavior, 6. Image analysis, 7. Meissner effect, 8. Resistance versus temperature, 9. Thermoelectric power versus temperature, 10. Effective shielding
Figure 12. Outline of cuprate and superconductor
preparation, and rf shielding device studies T-4059 46
Mixing of BaO and CuO
Grinding
Pelletization, 700 kg/cnv
Sintering (900 C, 0 :680 ml/min 8 hrs
No Second sintering
Yes
Grinding
Sieving
No Powder < -35 pm
Yes
Storage
Figure 13. Procedure for the preparation of cuprate powder T-4059 47
Powder on alumina
Melting at 930°C, 680ml/min. 0 , 6 min.
Prepare cuprate/napthenic acid slurry
Mask preparation Slurry preparation
Cuprate coated substrate
Partial drying at room temperature
Heating at 180 C-190 C, 680ml/min. 0 , 30 min.
Mask removal
Sintering at 900°C, 680 ml/min. 02, 36 hours
Furnace cooling
Storage for later Tl-superconductor
Figure 14. Procedure for the melting of cuprate onto alumina
to produce a superconductor. T-4059 48
Apparatus cover in furnace
Temperature to 900 C, 680 ml/min. 0
Thallium oxide in tray
Substrate on tray, with cuprate side facing thalium oxide
Tray residence in furnace, 11 min.
Tray removal from furnace, place under cover, and cool in air
Figure 15. Deposition of thallium onto substrates to make
superconductors T-4059 49
Chapter 4. Thallium Detection Apparatus
Industrial exposure to thallium and thallium compounds constitutes a severe health risk to workers. An understanding of its toxicity and the ways it enters the body allow an effective prevention of contamination and illness. In this project the available safety precautions were first explored and then a way to detect dangerous levels of airborne thallium was developed. Appendix A discusses the health effects of thallium and the precautions taken in the laboratory. In order to further protect the workers some method is needed to detect dangerous levels of thallium in the environment if this were to ever occur.
Spectrophotometrie Determination of Thallium
The approach was to find a visible reaction that can also be used to determine the concentration of thallium.
For this purpose a spectrophotometer was used to determine the concentration of a complex ion of thallium. A spectrophotometer measures the percent transmittance of light through an aqueous sample. The percent transmittance is related to concentration by the Beer's Law equation: T-4059 50
log = sbc (12) where I is the intensity of light transmitted through the sample, IQ is the intensity of light incident on the sample.
is the transmittance. sis the molar absorptivity, which is a function of the instrument used, ion detected, and wavelength, b is the path length and c is the concentration of the ion.
If Beer's law is obeyed a plot of absorbance versus concentration will be linear passing through the origin (see Figure 16). However Beer's law is only applicable up to a specific concentration (c in Figure 16) for each system, where the plot becomes nonlinear. The most accurate determinations of concentration are done at the linear range of the curve. There are three types of deviations represented by the non-linear portion of the from the Beer's Law plot: real, chemical, and instrumental. Real deviations are due to the changes in refractive index which occur with changes in concentration. Beer's law assumes a constant refractive index. Chemical deviations are due to the shift in equilibrium involving the absorbing species. Shifts in the concentration of the species being determined might not T-4059
Absorbance 0 0 iue 6 Be' law 16.plot Beer'sFigure Concentration 51 T-4059 52
result in a proportional change in the final concentration of that species due to equilibrium with other species.
Beer's Law is applicable only for absorption of light of a single frequency [67]. Para-phenetidine was chosen as a suitable compound to form a complex ion with thallium [6 8 ]. The reaction for thallium and para-phenetidine is shown in equation 13.
NH, NH
+2 + 3 + + T1 + H (13)
According to Iijima [6 8 ] this produces a purple solution and follows Beer1s-law-behavior.
Acidic solutions were chosen as the media to desolve thallium. The exact pH which gives maximum absorbance was investigated. The proper wavelength for maximum absorbance was examined. Then the Beer’s law plot was found for thallium at this wavelength. Finally an apparatus was constructed that allowed the monitoring of thallium over time and the absorbance versus time at different concentrations was examined. T-4059 53
Experimental Procedure, Results, and Discussion A 2 percent para-phenetidine solution was prepared in methanol. Then TI2 (SO4 ) 3 • 7^ 0 was prepared by using TI2 O3 and H2 SO4 . HNO3 was examined as a possible reagent, to form
T1 (1 0 3 )3 ; however, there was no color reaction of the thallium nitrate crystals in para-phenetidine solution.
TI2 (SO4 )3 •7 H2 O dissociates quickly, which allows the easy reaction of Tl8+ ions with para-phenetidine, simulating the introduction of hot oxidized thallium in an industrial environment. 0.7255 grams of TI2 (SO4 )3 •7 H2 O were dissolved in 100 ml of 0.1 M HC1 solution.
The optimum wavelength was found by searching for the wavelength that the maximum amount of absorption occurs for the same concentration. First, a pH=3.1 solution was prepared by mixing 50 ml of 0.1 M potassium hydrogen phthalate, KHC8 H4 0 4 , with 1 8 *8 ml of the Tl2 (SO4 )3 •7H20/HC1 solution. 20 ml of this solution were put in a cuvette and
4 drops of 2 percent para-phenetidine solution was added.
The amount of thallium was 404 ppm; by adding excess thallium the reaction, (equation 13) was driven to the right, ensuring the formation of complex ion. The percent transmittance was measured in a spectrophotometer from 325 to 84 0 nm. T-4059 54
Figure 17 is the absorbance versus wavelength of thallium in para-phenetidine solution at pH=3.1. There is a single peak at 560 nm indicating this is the optimum wavelength for absorbance. The optimum pH at which the complex ion was the most stable was also established. For this purpose, buffer solutions were prepared in the range of pH=2.2-3.7 by mixing
0.1 M KHC3H4O4 with appropriate amounts of
TI2 (SO4 )3 •7H2O/HCI solution and 0.1 M HC1 solution. The concentration of thallium was approximately 285 ppm in each solution. The percent transmittance was observed for 30 minutes. PH=3.1 was selected.
Figure 18 compares the absorbance versus time for solutions of different pH. Iijima [6 8 ] chose a pH of 3.1 so particular attention was made to this choice. At pH values other than 3.55 or 3.1 the absorbance was unstable at long times. PH=3.1 was chosen over pH 3.55 for two reasons.
PH=3.1 apparently was stable at an earlier time than pH=3.55, which would be favorable for earlier warning.
Also, thallium has a higher solubility in more acidic solutions, so a compromise was made for a slightly lower response in order to have a higher driving force for the dissociation of thallium oxide.
Finally, a Beer's law plot was prepared. This was done Figure 17. Absorbance versus wavelength for thallium in thallium for wavelength versus 17. Absorbance Figure T-4059
Absorbance, abs. units 0.2 0.4 0.6 0.8 250 aapeeiie solution para-phenetidine
350
450 aeegh nm Wavelength,
550
650
750
850 55 iue 8 Asrac ftalu/aapeeiie complex thallium/para-phenetidine of 18. Absorbance Figure T-4059
Absorbance, abs. units 0.2 0.4 0.8 2.2 opH=2 . 5,pH=2 □ . ■pH=3.1, 9, ApH=2.95, nsltos fvrosp aus OpH=2.2, values:pH ofvarious solutions in p=.5 +pH=3.55.x pH=3.35, 0 10 20 Time, min. 30 40 50 60 56 T-4059 57
by mixing one solution of 0.1 M KHC8H4O4 /O.I M HC1 with a solution of 0.1 M HCI/TI2 (SO4 )3 in order to get a varying concentrations in solutions at pH=3.1. The concentration of thallium varied from 0 to 198 ppm. Four drops of 2 percent para-phenetidine solution were added and the transmittance of light was observed at 560 nm. A Beer's law plot was obtained and parameters were set for the operation of an apparatus. Figure 19 shows the Beer's law plot at pH=3.1. The detection limit is 51 ppm of thallium, which is the end of the linear portion of the curve. The molar absorptivity at 560 nm is 1.079xl0“2 ppm--*- cm-^*- (see equation 12) .
An apparatus was designed and built. The apparatus was optimized in order to get all flow rates synchronized. Figure 2 0 shows a schematic diagram of the apparatus that was designed. Thallium contaminated air would be pumped by the air pump through the flow meter into a reactor vessel. The pH of the solution was determined to stay constant for 24 hours of continuous flow of air bubbles through the solution. A three opening 250 ml flask was used for the reactor vessel. The total solution in the system was 280 ml. A condenser was used to ensure that any liquid evaporated by the flow of air was returned to the flask. Figure 19. Beer's law plot of thallium in para-phenetidine inpara-phenetidine thallium of law plot 19. Beer's Figure T-4059
Absorbance, abs. units 0.2 0.4 0.6 0.8 1.2 0 1 0 t 560nm.at 1 2 ocnrto l 1 1 M Concentration 10 1Tl, x 3 4 5 6 7 -4 8 9 10 58 4059
Flow meter
Peristaltic pump Air pump Environment
Condenser
Surge tank Reactor vessel
•Peristaltic pump
Feed line Gravity flow
Spectrophotometrie detector
Incident beam into solution Alarms Recorder not blocked by feed line
Figure 20. Thallium detection apparatus T-4059 60
This would prevent a false reading of high thallium content due to the decrease in the quantity of solvent. The action of air bubbling into the reactor vessel mixed the solution for the reaction to take place. Thallium laden solution is pumped to the spectrophotometer detector where the percent transmittance is measured. Alarms and recorders can be hooked up at this point to alert users of a high thallium concentration and dangerous conditions over time. The thallium solution overflows by gravity into a surge tank and is recycled back to the reactor vessel via a peristaltic pump. The whole apparatus except the spectrophotometer is housed in a closed box to prevent the degradation of para-phenetidine with light. Figure 21 is a photograph of the apparatus. Figure 22 shows the absorbance versus time of a 1x10"^ M solution of thallium. There is a steady rise to a peak at
45 minutes with a decrease in absorbance until 120 minutes, when the experiment was stopped.
The increase is due to the slow addition of thallium solution to the spectrophotometer. The spectrophotometer contains a clear solution at the beginning of the experiment which dilutes the complex ion. The system needs time to come to equilibrium. The decrease is after the thirty minutes it takes to react with pH=3.1 to form a steady- T-4059 Figure 21: Photograph of the thallium detection apparatus thalliumthedetection of Photograph 21:Figure JL IH 61 iue 2 Asrac f lxlO of 22. Absorbance Figure T-4059
Absorbance, abs. units 0.15 0.05 0.25 0.35 0.1 0.2 0.3 detection apparatus detection 0 20 40 Time, min. -4 M thallium in the thallium thallium theinthallium M 60 80 100 120 62 T-4059 63
state absorbance. Therefore, the decrease is due to a dilution with the excess clear solution that is in the surge tank. In order to better control the experiment all the thallium was added immediately. Only after the whole system comes to steady state conditions can the spectrophotometer truly reflect the concentration of thallium in solution. Steady state conditions exist when the reaction comes to equilibrium and the amount of thallium in each tank is constant.
This is determined by the speed of the peristaltic pump. In this experiment fairly slow flow rates were used. The pumps had a maximum flowrate of 2.3 ml/min (136 ml/h) and were usually run at about 10% below maximum speed. Both pumps are synchronized by the presene of a surge tank in the flow circuit.
The absorbtion of barium and copper were tested to see if they would interfere with the absorbance of the thallium- phenetidine complex. Barium oxide and copper oxide are soluble in acidic aqueous solutions. To test each oxide a solution was prepared: 50 ml of 0.1 M KHC8H4O4 , potassium hydrogen phthalate, 18.8 ml of 0.1 M HCl (for pH=3.1 solution). Approximately 0.02 grams of powder was added to each solution and dissolved. Twenty milliliters of each solution was added to a cuvette and four drops of 2 percent T-4059 64
para-phenetidine solution were added. The absorbance of each solution was tested from 350 to 825 nm. The absorbances, at 560 nm for, barium and copper were 0.013 and
0.004, respectively. The concentration of barium and copper were 2 x 10~3 M and 4 x 10~3 M. The spectrophotometer gave a reading of 1.52 with a thallium concentration of 2 x 10“3 M. Barium, indicating the highest absorbance, would indicate less than 1 percent of this reading. Therefore any interference from Ba and Cu were considered negligable.
The response of the apparatus to thallium superconductor was also tested. Figure 23 shows the response of the apparatus to various amounts of thallium superconductor. 280 milliliters of solution was prepared and put into the apparatus. The flowrates were set and allowed to come to equilibrium. Pieces of a thallium superconductor were added directly to the reactor vessel, and the response of the spectrophotometer was observed. The response increased with the increasing concentration of the thallium superconductor, as expected. However, in a heated environment, the thallium is removed from the superconductor instead of the whole superconductor vaporizing. Therefore, in an operational setting the apparatus will not be dissolving thallium superconductor but just the vapors coming from the furnace. This experiment shows that if Figure 23. Absorbance of a solution containing thallium thallium containing solution a of 23. Absorbance Figure T-4059
Absorbance, abs. units 0.1 0.2 0.3 0.4 0.5 0.6 0 0 005 g, A 0.0053 □ g. 0.0088 used in each sample: 0 g,ineach 0.0215used o0.0395g, The amount of thallium superconductor material material superconductor thallium ofamount The superconductor material and para-phenetidine. and para-phenetidine. material superconductor 10 20 30 Time, min. 050 40 60 080 70 90 100 T-4059 66
thallium-based superconductors were present in the air they would not release thallium at levels detectable by the apparatus.
Conclusions As a result of this part of the study the following conclusions were revealed: 1. Para-phenetidine solution at a pH of 3.1 can be used to identify thallium. In this environment the molar absorptivity of the para-phenetidine-Tl complex (with thallium) at 560 nm is 2205 gmoles 1~1 cm“l. 2. Linear Beer's law behavior is observed up to a concentration of 2.5 x 10-4 M.
3. An apparatus that continuously monitors the level of thallium in the environment was built.
4. The response of the apparatus to concentration changes is affected by the flow rates in the circuit; however, when this is fixed at constant rate its response to airborne thallium becomes constant. T-4059 67
Chapter 5. Preparation of Barium Cuprate
Introduction The approach of vaporizing thallium oxide onto a precursor material involves the prior preparation of the precursor. In this project barium cuprate was used as the precursor material to form thallium superconductors.
Hermann [10], used two different starting compositions of Ba:Cu, ie. Ba:Cu=l:3 and Ba:Cu=2:3. Since superconducting material results from either starting material, Ba:Cu=l:3 was used in this study. Such cuprates have been extensively studied by researches in the area of high temperature superconductors [10, 53-55,69]. The approach was to sinter pellets of barium cuprate. Two processes were involved in this: compaction of the powders into a pellet and the sintering of it. Compaction converts a powder into a shaped mass with sufficient strength for subsequent handling. This is usually done by pressing a powder into a pellet using uniaxial pressurization in a die.
Experimental Procedure
Barium copper oxide was prepared in the following manner. Barium oxide, (BaO), and copper oxide, (CuO), T-4059 68
powder were weighed in the ratio Ba:Cu=l:3. They were mixed and ground in a mortar and pestle. The powders were placed into a pellet mold and pressed at 680 atm for one minute. The pellet was placed in a cool furnace and heated to 900°C under an oxygen gas flowrate of 236 ml/min. It was held at this temperature for eight hours and then allowed to cool in the furnace to room temperature.
The pellet was reground in a mortar and pestle and the powder pressed back into a pellet. The pellet was sintered again for eight hours at 900°C under the same regime of oxygen flow and allowed to cool in the furnace again.
The product was finally ground in a mortar and pestle and screened. The -35 micron size fraction was placed in an all glass vial which was kept in a dessicator.
Results and Discussion
Figure 24 is a photomicrograph of barium cuprate powder. It shows an even distribution of particles, without any distinctive crystalline shapes.
Figure 25 is a typical EDX spectrum of the barium cuprate sample showing the emitted x-ray energies from 0 to 10.24 keV. The atomic percentage of barium and copper were determined to be 24.96% and 75.04%, respectively, T-4059 69
Figure 24. SEM photomicrograph of barium cuprate powder. T-4059
0.000 VFS 2048 10.240 200
Figure 25. EDX trace of barium cuprate powder. T-4059 71
Figure 26 is a typical trace of the x-ray diffraction of a powder sample of the same cuprate. The angles of
diffraction are listed in Table 5.
Table 5. Peak angles of diffraction of barium cuprate powder (20 Degrees)
Primary Secondary
29.18 35.52 38.76 28.97 30.04 31.90
34.56 36.28 37.08
38.54 42.27 43.06
45.19 51.04 52.90
53.56 55.96 63.93
67.52 72.84 74.96
The peak at 29.18° indicates BaCuC>2 • Two smaller peaks at
51.04 and 52.90 20 degrees also correspond to BaCuC>2 . The other two large peaks at 35.53 and 38.7 6 degrees correspond to CuO. This indicates that a barium cuprate sample with a
Ba:Cu stoichiometry of 1:3 consists of BaCuC>2 and CuO from the extra Cu. These data are consistent with the findings of other investigators [69] who found that the overall T-4059 72
100
00
60
40
20 Jljl 0 12.65 505 Ufa 42.40 505 505 ?05 82,00
2 0 degrees
Figure 26. XRD trace of barium cuprate powder
corresponding to a stoichiometry of BaCu3 0 4 . It shows that there are no detectable impurities. T-4059 73
stoichiometry of Ba:Cu=l:3 is sufficient for obtaining superconductors with appropriate compositions.
Conclusion
This part of the study showed that barium copper oxide powder can be prepared with an overall ratio of Ba:Cu of 1:3. T-4059 74
Chapter 6. Preparation of a Cuprate Design on Alumina
Introduction A typical electronic device requires the precise positioning of regions of conducting and nonconducting material. This applies to microelectronics as well as large scale applications. A specific layout pattern is needed, which is applied in a series of steps to fabricate circuits.
Stencil printing, also called lithography, is the process used to make designs on a surface. Often more than one step is used to complete a complex design. The placement of a pattern on a wafer can be done in a single step by directly placing a pattern of material. However, the simplest of procedures are usually broken down into more than one step. In the first step, a mask is generated. Then, using the mask a pattern is applied to the substrate. The sophistication of the mask increases with complexity of the design of the circuit. This project focused on the simplest circuit designs, so the simplest form of masks were pursued.
The approach was in two stages. First, to find a material to use as a mask. The criteria of a good mask are that no particles from the mask are left on the substrate, and the mask can define the shape of the device without T-4059 75
leakage under the mask. This includes the process of removing the mask, which may disturb the shape of the device. The mask cannot interact with material being deposited. Also the mask must not have defects, pinholes, intrusions, protrusions, and spots. The mask must also be durable, so that it can be removed easily. The approach in this study was to use barium cuprate to form the design, and then thallium oxide would be deposited to the whole wafer via vapor deposition. Only the areas with the barium copper oxide would be superconducting.
The substrate material were alumina wafers obtained from Coors.
Experimental Procedure
Three types of masking material were examined using polyethylene glycol as the binding agent. On three separate wafers of alumina, cardboard (fastened by a paper clip), clear plastic scotch tape, and Scotch brand 3M tape was placed. A hole was cut out to form the shape of a "U" . Cuprate was mixed with binder to make a thick slurry. Then the slurry was applied to the wafer. The wafers were put in a dessicator and allowed to sit overnight. The next day the templates were removed and the wafer heated to 550°C. T-4059 76
Due to problems with the polyethylene glycol, the next step was to find a more suitable binder. Three other binders were examined: Carbowax 4000, glycerin, and napthenic acid. In each experiment the cuprate was first mixed with the binder, and then applied to a masked wafer.
A minimum amount of binder was used; just enough so that the cuprate formed a slurry that could be applied with a spatula. The wafers after drying of the cuprate slurry overnight were baked at various temperatures from 80 to 980 degrees Celsius under flowing oxygen and observations were made.
Results and Discussion
The first step was to determine a proper masking material. Table 6 summarized the results.
The second step was to determine the best binder. Table
7 summarizes the results.
Figure 27 shows some of the cuprate shapes placed on alumina. T-4059 77
Table 6 . The types of masks examined in order to apply barium cuprate to alumina
Retained Design Mask Adhered to Wafer After Heating
Cardboard No No
Scotch Plastic Tape Not Strongly No, some spreading
Scotch 3M Tape Yes, strongly Yes 4059
Table 7. The effects of different binders for placing cuprate on alumina
Binder Condition After Room Retains Shape Relatively Sticks to Wafer Heating at 200- Temp. After Low Temp. Smooth After Low Temp. 500°C Liquid Heating «200°C) Surface Heating
Glycerin Cracked, did not Yes Yes Yes No stick to wafer
Polyethylene Stuck to surface. Yes Yes No Yes Glycol Uneven shape.
CarboWax 4000 Heated to 900°C. No No No Yes Did not stick to surfacer Napthenic Retains shape. Yes Yes Yes Yes Acid Stuck to wafer. T-4059 79
n n E
Figure 27. Shapes placed on alumina by stencil printing
technique. T-4059 80
The surface stoichiometry and morphology were not truly adequate for preparation of a superconductor. Others [69] have studied the interactions of alumina with barium cuprates at temperature conditions similar to those to be used in superconductor production. The results in Figure 28 indicate that cuprates do interact with alumina substrates forming a barium-rich layer which may be as thick as 5 micrometers away from the wall. Beyond this distance the cuprate becomes the predominating layer. These data also indicate conditions of cuprate-substrate adhesion by mutual interaction. This is a desirable adhesion of thick cuprate and superconductor films onto alumina substrates.
The stoichiometry of a single layer of cuprate melted on alumina was examined. Small amounts (approximately 0.02 grams) of cuprate material were placed on an alumina wafer and melted at high temperatures for varying times. Oxygen flow was kept at 680 ml/min. Figure 29 shows the results. The average concentration of barium was about 37% over the entire range of times. The minimum amount of time was determined to be where the cuprate had mostly melted to the alumina and could not be removed by simple or weak physical means (scraping with a spatula). The maximum time range was determined to be 10 to 14 minutes, where higher temperatures would allow shorter times of heat treatment to be used. The T-4059 81
Al Al+Ba + Cu Crack
LOCATION 1 2 1 4 9 4 7
DISTANCE 0. 1.5 1 S 4.5 9 11»
C o w . Al M 7t 45 24 7 12 20 80 Al Cu 60 40 Ba 20 AlBaCu. Figure 28. Composition of Ba-Cuprate melts on an alumina substrate a) cross section, b) analytical data, and c) profile composition [74] T-4059 82 100 75 e 1 8 50 o o E X3 & 25 0 1 0 12 Tim«, mln. Figure 29. Summary of EDX data for one layer of cuprate melted onto alumina:H 930°C, ♦ 940°C, and D 950°C T-4059 83 optimum time and temperature for depositoin of a first layer of cuprate was determined to be 940°C for 4 minutes and 930°C for 5-6.25 minutes. Figure 30 shows photomicrographs of the material for BaiCu3 0 x at the two temperature-time runs. They were chosen for their smoothest stoichiometry (indicated by the white line across the SEM photomicrographs). Since the stoichiometry of samples in this range were fairly close to each other morphology was taken as the variable for further study. Using the methods described above for producing a single layer of cuprate on alumina, many such samples were prepared. Then, a second layer of cuprate (approximately 0 . 0 2 grams) was placed on top and this material melted at various temperatures and times. Figure 31 shows the results for BaiCu3 0 x . The data clearly indicates that the barium stoichiometry is still high. All the samples displayed the same change in barium concentration as the single layer samples. Figure 32 shows a photomicrograph of the Ba^Cu3 0 x that had the closest number (29.63%) to that of the desired stoichiometry (25%). The white line near the top and the cracks also indicate that the surface morphology has not improved, either. T-4059 84 Figure 30. Photomicrographs of BaiCu3 0 x melted on alumina iue 1 Smayo D aa o w lyr o cuprate of layers fortwo data of EDX Summary 31.Figure T-4059 Barium Concentration, % melted onto alumina onto melted n 950°C ♦ and 100 100 -T 25 - Time, min Time, :D :D 930°C, □ 940°C, 930°C, □ 85 T-4059 Figure 32. Photomicrograph of second layer of Ba]_Cu30x melted onto alumina. T-4059 87 Figure 33 shows another example of Ba]_Cu3 0 x second layer material that is much smoother than that shown in Figure 32, but still contains small cracks and a much worse stoichiometry (32.25%). The final approach was to use a napthenic acid slurry as a second layer. The first layer of cuprate melted on alumina was prepared as before, but the second layer was deposited using a slurry of napthenic acid and cuprate. The slurry for BaiCu3 0 x was mixed in 2 . 2 to 1 cuprate to napthenic acid weight proportions. The slurry was applied to the single layer cuprate and allowed to set. The napthenic acid was removed by heat treating the material at 180-190°C for 30 minutes under an oxygen flowrate of 680 ml/min. The cuprate was then melted and Table 8 lists the results. None of the runs indicated the proper stoichiometry. Figure 34 shows a photomicrograph of the material, indicating no improvement in morphology. Since melting the cuprate produced cracks, a different approach was taken to repair the surface. Instead of melting the second layer as before, a lower temperature was used, 890°C-900°C, and the material sintered under oxygen (680 ml/min.) for 39 hours. T-4059 88 Figure 33. Photomicrograph of second layer of Ba]_Cu30x melted onto alumina. T-4059 89 Figure 34. Photomicrograph of Ba]_Cu304 second layer material deposited using a slurry and then melting the cuprate. T-4059 90 Table 8 . Data for the second layer of BaiCu3 C>4 deposited on melted cuprate using the slurry method Temperature, °C Time, min. Barium concentration, % 930 2 32.68 940 1 31.98 940 2.25 32.62 950 1 33. 69 Figure 35 shows a photomicrograph of BaiCu3 0 x . The barium concentration of the final material was 36.7 6 %. The second layer also indicates a good morphology for producing a superconductor. A series of XRD traces were obtained of alumina, and the various stages in the study of putting cuprate on alumina. They are shown in Appendix C-l. the data shows that the method of using a slurry and then sintering produces an improvement in the ordering of the surface structures, which is favorable for superconductor production. Similar experiments to produce a cuprate layer on alumina were performed with Ba2 Cu3 0 5 powder. The results T-4059 91 Figure 35. Photomicrograph of second layer of Ba]_Cu3 0 4 melted onto alumina using cuprate-slurry and then sintering the cuprate T-4059 92 are in Appendix C-2. There was no marked improvement using this stoichiometry over that of Ba]_Cu3 0 4 . Preliminary work with making thallium superconductors showed that BaiCu3 C>4 was a good starting material and so subsequent experiments to make thallium superconductors were done solely with BaiCU3 0 4 powder. Conclusions This study for the preparation of a cuprate design on alumina allowed the following conclusions to be revealed: 1) Scotch 3M tape can be used as a mask to put a cuprate design on alumina. 2) Napthenic acid provides the best binding characteristics for barium cuprate on alumina. It remains a liquid at room temperature but thickens upon sitting and upon removal of the mask it retains the shape of the mask. It is removed within 30 minutes at 180-190°C. The surface is relatively smooth and it sticks well enough to the wafer for further processing. 3) Applying a cuprate-napthenic acid slurry to a layer of melted cuprate on alumina provides a crack-free surface favorable for superconductor production. Once the napthenic acid is removed, the cuprate is sintered at 900°C for 36 hours. T-4059 93 Chapter 7. Vapor Deposition of Thallium Oxide Introduction Hermann, et al. [55] have prepared thallium superconductors by vaporizing TI2 O3 onto barium calcium copper oxide. He found that TI2 O3 [mp=717°C] evaporates at T>717°C , and the vapor reacts with solid Ba-Ca-Cu oxides, forming thallium superconductors. His technique has simplified the making of thallium superconductors to simply the making of barium cuprates. Thallium superconductors can then easily be made in the form of bulk materials. Additionally, the vaporizing of thallium oxide can be carried out in suitable reactors, thus making the fabrication of thallium superconductors more practical. In this study two designs of a thallium vapor deposition apparatus were tested. The first design tested the probability of the hypothesis being true, "that a thallium superconductor could be prepared in the this fashion." The second design was an optimization of the vapor phase deposition apparatus. Cuprate flakes and melted cuprates on alumina were exposed to thallium oxide vapors to prepare superconductors. T-4059 94 Experimental Procedure Flakes of barium cuprate powder were prepared by pressing the powder on a platinum tray with a spatula and sintering for seven minutes at 920°C under flowing oxygen. Figure 36 shows the first design which was used to test the viability of vaporizing thallium to make superconductors, similar to what had been done by others [55]. Thallium oxide was placed at the bottom of a ceramic crucible (a). A brass screen (b) sat on the crucible cup and supported the cuprate flake. The small crucible was placed inside a larger ceramic crucible so that the end of an oxygen delivery tube could be lowered into it to maintain a Tl2 0 3 (g)+ 0 2 (g) atmosphere for the reaction to occur. Ba2 Ca2 Cu3 0 x powder had been obtained from Hermann, and used in this first design. The cuprate flake was prepared from this powder and set on the brass screen. It was exposed to thallium oxide vapors for 10.5 minutes under an oxygen flowrate of 2040 ml/min. The flake exhibited superconducting properties when it was tested for the Meissner effect in liquid nitrogen. This showed that the vapor process did work as reported by Hermann, and now T-4059 95 Oxygen Outer crucible Figure 36. Vapor phase reaction apparatus (Design I) T-4059 96 the step was to apply it to producing thallium superconductors from cuprates on alumina substrates. T1-Vaporization Apparatus [final design] The next step was to redesign the vapor deposition apparatus to optimize superconductor preparation. Figure 37 shows the design. All components were made from alumina trays manufactured by Coors Ceramics. A large tray had one end cut off, a hole drilled in its bottom, and placed in the furnace, at room temperature, bottom side up. The oxygen inlet tube was placed through the hole. A smaller tray was used as a cover to hold the thallium oxide. The substrate was supported by either of the two trays or by pieces of alumina suspended from the walls of these trays (as in the case with flakes of cuprate) [see inset of Figure 37]. For a typical run TI2 O3 was placed in the tray and the substrate suspended above it and the furnace brought to the operating temperature (890°C-900°C) . After the TI2 O3 was vaporized for 11 minutes the tray was removed from the furnace and allowed to cool outside the furnace. Figure 15 shows the optimal conditions. T-4059 97 O xygon Inlot Tuba Flake f- S cm I 9 cm 15 cm Thallium (III) Oxida 6 cm Small walor sits on th«s« adgas with Cuprata stda toward Thallium (III) Oxida. 0.8 cm 1.3 cm 4.5 cm 4 cm Larga walcr sits on thasa adgas with Cuprata sida down. 10 cm 1.5 cm Figure 37. Cuprate-Tl, vapor phase interaction apparatus (Design II) T-4059 98 Once superconductors prepared from flakes were characterized the cuprates melted on alumina were used. Results and Discussion In a series of experiments sintered cuprate powder flakes were used in accomplishing vapor phase reactions. Figures 38 and 39 show SEM photomicrographs of the sintered cuprate material-based superconductor product. Figure 40 is the data obtained by EDX showing a ratio of Tl:Ba:Cu=l.45:1:3. Figure 41 is a plot of resistance versus temperature for such a cuprate flake. The data shows that the onset temperature is 90°K and Tc=78 K ± 5 K. Figure 42 is a plot of thermoelectric power versus temperature for a similar cuprate flake. The data indicate a smooth transition to zero thermoelectric power at 80 K. One does not observe a sharp drop in thermoelectric power, possibly due to the porous nature of the flake as was shown by SEM. Figure 43 shows an XRD trace for the thallium superconductor flake prepared in our laboratory [bottom] as compared to the XRD trace of a bulk thallium superconductor reported by Hermann [10, 54] [top of Figure 43]. There is a Figure 38. Sintered cuprate material with thallium thallium with cupratematerial Sintered 38.Figure T-4059 m * deposited. 99 T-4059 100 Figure 39. SEM photomicrograph of Tl-cuprate superconductor. T-4059 101 ZAF CORRECTION 15.00 KV 33.73 Degs No. o-f Iterations 2 K C Z 3 CAI [FI [ZAF] ATOM.'/. UT.7. TL-ti 0.411 1.103 1.113 0.99? 1.228 26.41 46.93 BA-L 0.221 1.834 1 .08? 0.995 1 .121 19.33 23.08 CU-K 0.366 0.854 1 .031 1 .000 0.880 54.26 29.93 * ~ High Absorbance TN-5500 Colorado School o-F Mines / JEOL Cursors 0.000keV = 0 0.000 VFS 10.240 Figure 40. EDX of a typical Tl-cuprate. T 1 :B a :Cu-1.45:1:3 102 T-4059 2.80 -q 2.60 £ 2.40 _c O - 2.20 © o c 0 2.00 co CO © CK 0.00 50.00 100.00 150.00 200.00 250.00 Ie mpera lure , K Figure 41. Resistance versus temperature curve for thallium superconductor. iue 2 Temeeti oe ess eprtr fr a for temperature versus power Thermoelectric 42. Figure Thermopower, nV/K T-4059 -4 -3 - -1 2 - - - - 20 40 thallium superconductor. thallium eprtr, K Temperature, 60 K 103 T-4059 104 I I I-----!---- 1---- 1---- 0 10 20 30 AO 50 60 70 20 degrees 12. 22. 32. 52. 62. 72. 2 0 degrees Figure 43. Comparison of XRD data obtained (bottom) to literature data (top). T-4059 105 close match between the two traces except for a peak at 20=37° only showing up in the bottom trace. This extra peak is attributed to the excess unreacted cuprate present in a flake, as opposed to a bulk superconductor. It corresponds to a secondary peak in the barium cuprate powder given in Figure 26. These data show that thallium superconducting material can be prepared using the vapor phase method with this apparatus. For the runs with cuprate melted on alumina sheets, at first a single layer of cuprate was melted on alumina and exposed to thallium oxide vapor in order to prove the hypothesis that a single layer of cuprate was inadequate for superconductor production. Figure 44 shows the type of melted cuprate on alumina pieces produced. The cuprate was laid out on an alumina wafer and pressed by hand. A furnace was preheated to 940°C and an oxygen flowrate of 680 ml/min was used in the furnace. The cuprate was melted onto the alumina by insertion into the furnace for 7 minutes. The wafer was removed and cooled in air. Thallium was deposited by the vaporization method onto this melted cuprate and then characterized using the resistance versus temperature method to determine whether a superconductor was produced. Figure 45 is a resistance T-4059 106 ■ H Figure 44. Examples of cuprate melted on alumina. T-4059 107 40.00 35.00 £ 30.00 _c O 25.00 •> 0 o C 20.00 (D -uJ> 0 f 5.00 0 0 CK 10.00 5.00 »i i » i i rn i'i »TTf 11 r r iTTHTI 11 l'f 11 1 M'l I I HTl'i l Mtt ; j m m i i n 0.00 50.00 100.00 150.00 200.00 250.00 300.00 T e mperature , K Figure 45. Resistance versus temperature curve. Sample: thallium deposited on cuprate, melted on alumina. T-4059 108 versus temperature curve for this material. It indicates semiconducting behavior up to about 95 K. A sharp drop at 7 8 K indicates that a superconducting transition had started. SEM examination showed the presence of large cracks. This shows that repair of the cuprate layer is needed to produce a good thallium superconductor. Repair of this layer, by using a second layer of barium cuprate prepared using a cuprate-napthenic acid slurry, was discussed in the previous chapter. Therefore, thallium was deposited on repaired cuprates melted on alumina. The deposition condition of thallium for the production of a superconductor product were optimized by running a series of tests at 880-900°C. Figures 46 and 47 show resistance versus temperature curves for typical superconductors prepared in this manner at 890°C and 900°C. Appendix D summarizes data for tests at 880°C and 890°C. The critical temperature for such products varies from 75 K to 99 K. An XRD trace of such a product is shown in Figure 48. In addition to the thallium superconductor phase signals, strong signals were obtained from the other phases in the material: copper oxide crystals, cuprate, and alumina. Data analysis showed that the formation of copper oxide crystals were the primary cause of the heterogeneities at this temperature. These were confirmed by SEM and EDX data. iue 6 Rssac ess eprtr uv fr thallium for curve temperature versus Resistance 46. Figure T-4059 Res is tan ce , Oh ms 0 2 00- 0 .0 4 6 8 . . . . 00 0 0 0 0 00-1 00 100 100 200 5.0 0.0 350.00 300.00 250.00 200.00 150.00 100.00 50.00 superconductor. Onset = 90 K, Tc= 76 K. 76 Tc= K, 90 = Onset superconductor. - - e mp aue K nature, Te p© m n r T T i T mi | m irn i i j i i i n i i i r i i n i TT 109 T-4059 110 3.00 0) E 2 .5 0 - j C O 2 .0 0 - 1 .5 0 - 0) 1 .0 0 - 0) 0 cv 0 .5 0 - 0 . 00 50.00 100.00 150.00 200.00 250.00 300.00 T e mp er a t u r e , K Figure 47. Resistance versus temperature curve for thallium superconductor. Onset = 125 K, Tc= 99 K. T-4059 111 21943 17554 - 13166 ■ 8777 . 4389 • | -- - ^ 1 5070060700 ISM 8 0 .0 0 2 0 degrees Figure 48. XRD trace of the thallium superconductor prepared by depositing thallium at 900°C. T-4059 112 Figure 49, a photomicrograph of a cuprate sample after seven minutes of exposure to thallium vapor, shows these copper oxide crystals. Results from the cuprate study indicated that one could obtain similar material if the temperature for melting the first layer of cuprate and sintering the second layer were lowered to that described earlier, ie: 930°C and 890°C. Photomicrographs of these superconducting materials are given in Figures 50 and 51. The EDX spectra of these superconductors, shown in Figures 52 and 53, gave the stoichiometries: Tl]_ # gBaiCU]_ # 3 <0X and TI3 # 7 Ba]_Cui. 3 0 x, respectively. Figure 54 shows a thallium structure on one of the samples and EDX data gave a stoichiometry of Tli. gBaiCui. 9 <0X . The superconductors still show some heterogeneity, but the improvement over a 1 0 degree change indicates that the deposition temperature is very narrow. Resistance versus temperature data for the Tli. 6BalCul # 3 0 x and TI3 # 7 BaiCu]_ # 3 <0X superconductors are given in Figures 55 and 56, respectively. The transition temperature is broad with an onset around 95 K and a critical temperature of 75 K. Figure 57 is a typical thermoelectric power versus temperature curve for these superconductors, showing a T-4059 113 Figure 49. SEM photomicrograph of thallium deposited on barium cuprate at 900°C for 7 minutes. Copper oxide crystals are visible. T-4059 114 Figure 50. SEM photomicrograph of superconducting material. The stoichiometry is Tl]_ # gBaiCui, 3 0 x . T-4059 115 Figure 51. SEM photomicrograph of superconducting material. The stoichiometry is TI3 # 7Ba]_Cui # 30x . T-4059 116 0.000 VFS = 2048 10.240 1 0 0 SQ: QUANTIFY REF.S EDS:TLM EDSsBAL EDSsCUK ZAF CORRECTION 15.00 KU 41.01 Degs No. of Iterations 2 K CZI CA3 CF3 tZAFI ATOM.’/. UT.7. TL-M 0.560 1.070 1.074 0.99? 1.149 39.84 60.38 BA-L 8.206 1.001 1.699 6.997 1.093 20.35 21.20 CU-K 0.233 6.824 1.029 1.000 6.848 39.31 18.50 * - Hioh Absorbance Figure 52. EDX data of the thallium superconductor, Til.6B31CU!_ 30x . 117 T-4059 0.000 VFS = 2048 10.240 1 0 0 SQ: QUANTIFY REF.S EOS:TLM EDSsBAL EDSsCUK 2AF CORRECTION 15.00 KW 41.66 Degs No. of Iterations 2 K CZ3 CA3 CF3 GZAF3 ATOM.’/. WT.7. TL-M 0.755 1.036 1.041 0.99? 1.079 61.83 77.49 BA-L 0.137 0.967 1.127 0.998 1.089 16.88 14.21 CU-K 0.106 0.793 1.030 1.090 8.817 21.29 8.30 * - High Absorbance Figure 53. EDX data of the-thallium superconductor, T1 3.7BalCu1.3°x* T-4059 118 Figure 54. SEM photomicrograph of a thallium structure observed when the thallium is deposited at 890°C. 119 400.00 200.00 J00.00 T e mpera lure , K , lure mpera e T 100.00 I I I I I I T I 'I 'I I T I I I I I I - - - superconducting material superconductingmaterial prepared at 890°C. 0.00 Onset at 90 K with a Tc=75 K. . Onset K. with K at 90 Tc=75 a [Tli#gBaiCui.30x ] 00 00 00 00 ...... 0 4. 00- 4. 2 2.00n 0 8.00 6 1 1 SUJ LjQ ‘ 0OUQ') S] S T-4059 Figure 55. Figure Resistance 55. versus temperature curve for thallium T-4059 120 12.00 0) E 10.00 c~ o . - o- 8 00 0) O c 4 . 0 0- 0) 0 Q3 2 .0 0 ^ 0 . 00 i i i i H '-n i | i i i'i i i i i i | i i i i i i i i i | i i i i i ' ' i ' I ' ' ' ' ' ' ' i ' I 1...... 11 50.00 100.00 150.00 200.00 250.00 300.00 350.00 Temperature, K Figure 56. Resistance versus temperature curve for thallium superconducting material prepared at 890°C. Onset at 90 K with a Tc=75 K [TI3#7BaiCui#30x ] . iue 7 Temeeti oe ess eprtr curve temperature versuspower Thermoelectric 57.Figure Thermopower/ |iV/K T-4059 - -2 1 - cuprate on alumina). on cuprate o hlimsprodcos (usingmelted superconductors thallium for 0 5 oo 100 eprtr, K Temperature, 150 200 0 5 2 co 121 0 0 3 T-4059 122 sharp transition beginning at 90 K with a critical temperature of 80 K. Conclusion Thallium superconductors can be prepared using cuprate melted on alumina. The conditions are vaporizing thallium oxide for 11 minutes at 890-900°C and then cooling in air. The critical temperature is at 75 K. T-4059 123 Chapter 8. Shielding effects Introduction High temperature superconductors have numerous applications, levitation and shielding are of particular interest [30, 72]. The shielding effects of low temperature superconductors have been used extensively at 4-23 K, to shield electronic components that are sensitive to AC and DC magnetic fields. High Tc superconductors should be possible to use in the shielding of much larger areas since cooling problems would be less prohibitive. When the magnetic field is smaller than the critical field, Bc, of a superconductor there is no flux penetration into the interior of the superconductor. More generally, and even at higher fields, magnetic shielding of rf fields is achieved by screening currents circulating in a boundary layer at the surface of the superconductor. Electromagnetic radiation is composed of electric and magnetic components. The energy flux of the wave is described by a vector quantity, S, the Poynting vector: S = ExH (14) where E and H are the vector quantities of the electric and magnetic fields, respectively. The magnetic and electric T-4059 124 fields at any point in the wave vary with time, as does the Poynting vector [73]. Preliminary experiments were performed as part of this study to test the ability of thallium superconductor films to shield against the magnetic components of radio frequency waves. Radio frequencies are electromagnetic radiation in the wavelength range from 1 m to 10,000 m. They are of particular interest since they are used as broadcast frequencies, and can interfere with sensitive electronics. Many electrical devices also generate such waves. Devices sensitive to radio frequencies can be shielded, with non superconducting materials, by Faraday cages, or metal shells, but they present size as well as construction limitations. With superconductors, shielding can be built into circuits at the same size level of the circuit components and superconductors do not have to be grounded, thus eliminating the need for a common ground for all of the components of an effective shield. Hollow cylinders of high temperature superconductors have been demonstrated as shields [72]. This part of the study examines the shielding effects of a cubical shell and of a hollow cylinder based on thallium-barium-cuprate superconductors. T-4059 125 Experimental Procedure Two devices were prepared to test the shielding effects of thallium-barium-copper oxide: a cubical shell, and a hollow tube. Figure 58 shows the design of the cubical shell. The stencil printing method described above was used to put thallium superconductor layers on ten wafers of alumina which were used to assemble it. A second design was made using a hollow cylinder made of bulk Tl2 Ba2 CuiC>5 . A diagram of the cylinder, with dimensions, is given in the inset of Figure 58. Both devices were tested using the experimental apparatus shown in Figure 59. Radio frequency waves were generated in the input coil by the frequency generator (General Radio Company Bridge Oscillator, Type 1330-A) . The power in the circuit was determined by measuring the voltage and current using a multimeter, and the frequency measured by a frequency counter. The voltage in the output coil (at liquid nitrogen temperature) was measured by an oscilloscope. The cubical box enclosed the output coil, and both coils were aligned as shown in the figure. The hollow cylinder also enclosed the output coil, which was aligned parallel to the input coil (see inset, Figure 59). T-4059 126 3.0 cm 2.5 cm i i 0 . 8 cm Figure 58. Design of the cubical shell [inset: hollow cylinder design] T-4059 127 Frequency Oscilliscope Multimeter counter Input Trigger Copper coil Frequency generator Shiel Liquid nitrogen Shield Copper coil Shield Figure 59. Diagram of the circuit used to test the shielding effectiveness of the cubical shell and hollow cylinder used in this study. T-4059 128 Results and Discussion Figure 60 shows the results of the shielding effectiveness of the cubical shell where it is seen that the effective shielding increases as the frequency goes up, except with a drop at 2900 kHz. This could be due to a resonance in the "indirect" coupling between wires in the circuit. Since the shielding was never stronger than 14% the rf field was apparently strong enough to always partially penetrate the superconductor. Additionally, noise and the very low response of the oscilloscope at lower frequencies made determination of the effective shielding difficult. The hollow cylinder displayed Meissner effect at the temperature of liquid nitrogen. The shielding effects were tested with the coils in parallel. Figure 61 shows the results of the shielding at four different frequencies. At the higher frequencies, there appears to be more effective shielding. T-4059 iue 0 Efcie hedn ftecbcl shell thecubicalof shielding 60. Effective Figure Percent Shielding, % 12 14 10 0 2 4 6 8 10 34 116 347 F kHz RF, 973 2900 9200 22500 129 T-4059 130 Power, mVv Power, mW Power, mW Figure 61. Effective shielding of a hollow cylinder at 116, 967, and 2900 kHz (top to bottom). T-4059 131 The results show that even at higher power levels the cylinder provides better shielding at the higher frequencies. The primary source of error in this series of experiments was that the ends of the cylinder were open thus creating incomplete shielding. Conclusion These have been preliminary experiments trying to demonstrate the shielding effectiveness of thallium superconductors and not the primary thrust of this thesis. More work needs to be done for a complete characterization and, attempts at improvement. In these experiments, superconducting cubical shells and cylinders have shown partial shielding of electromagnetic radiation at frequencies between 300 and 22500 kHz, in the radio frequency band. T-4059 132 Chapter 9. Summary of Conclusions The following conclusions were revealed in this study: 1. An apparatus to continuously monitor the level of thallium in the environment was built. The rate of response of the apparatus is affected by the flow rates in the circuit; however, when this is fixed at constant rate its response to airborne thallium becomes constant. 2. Para-phenetidine solution at a pH of 3.1 can be used to identify thallium. Linear Beer’s law behavior is observed up to a concentration of 2.5 x 10-4 M. 3. The preparation of barium copper oxide powder as a precursor to superconducting material was investigated. Cuprate powder can be prepared with an overall ratio of Ba:Cu of 1:3 without any detectable impurities. 4. The patterning and melting of cuprate designs on alumina was investigated. Scotch 3M tape can be used as a mask for the design. Napthenic acid provides the best binding characteristics for barium cuprate on alumina. It remains a liquid at room temperature but thickens upon sitting and upon removal of the mask it retains the shape of T-4059 133 the mask. It is removed within 30 minutes at 180-190°C. Applying a cuprate-napthenic acid slurry to a layer of melted cuprate on alumina provides a crack-free surface favorable for superconductor production. Once the napthenic acid is removed, the cuprate is sintered at 900°C for 36 hours. The surface is relatively smooth and adheres well enough to the wafer for further processing. 5. A procedure that utilizes the vaporization of thallium oxide onto substrates of cuprate melted on alumina was developed. An apparatus that supports a substrate over powdered thallium oxide was designed and used to prepare the superconducting material. The conditions are vaporizing thallium oxide for 11 minutes at 890-900°C and then cooling in air. 6 . Thallium-barium-cuprate superconductors prepared in the above manner were characterized by SEM, XRD, resistance versus temperature determinations, and thermoelectric power measurements. The critical temperature varied from 75 to 90 K. The results of the characterizations indicated that the material obtained is identical to that prepared by other groups using alternative techniques. T-4059 134 7. The shielding of radio frequencies by thallium-barium- cuprate superconductors were explored. Two types of prototype devices were prepared: a cubical shell, and a hollow cylinder. The results indicated partial shielding of electromagnetic radiation at frequencies between 300 and 22500 kHz, in the radio frequency band; however, the possibility that partial penetration by the magnetic field(s) generated in the experiment, as well as "indirect" coupling between wires in the apparatus is not to be overlooked. T-4059 135 Chapter 10. Suggestions for Future Research 1. In this study Barium copper oxide was used as the precursor material to make superconductors. Other researchers have shown that the addition of calcium increases the critical temperature up to 124 K. Adding calcium to the cuprate powder in stoichiometric proportions would possibly increase the critical temperature and the effective shielding of the superconducting device. Therefore, future research could include work on TlCaBaCuOx . 2. The para-phenetidine solution gave a color indication in the presence of thallium. A solid material that changes color with the presence of airborne thallium could be investigated. Future research could, for example, study dye papers saturated with a para-phenetidine solution and carried as badges by workers. 3. Thallium oxide was easily vaporized onto a variety of differently shaped cuprates such as flakes, wafers and cylinders, to make superconductors. Further research could vaporize thallium oxide onto more complex shapes made out of sintered cuprates, ie. coils and toroids, and study their properties and applications. T-4059 136 4. The method to make thallium superconductors was broken down into two large batch steps: preparation of the cuprate precursor, and then vapor deposition of thallium oxide. Future researchers could explore a way of making this a more continuous process. 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R., Esevier, New York, 1992, pp.337-48. 75. Schilling, A., et al., "Superconductivity above 130 K in the Hg-Ba-Ca-Cu-0 system", Nature, 363, 56-8, 1993. T-4059 151 Appendix A Thallium Handling and Safety T-4059 152 Appendix A. Thallium Handling and Safety The effects of thallium on the human body vary with the dosage. At small doses, parasthesis [nerve damage] and ataxia [loss of motor control] are the dominant symptoms. Parasthesis starts in the fingers and toes, and may progress to atrophy of the muscles [cells dying], peripheral neuropathy [nerve lesions], tremors, mental aberrations as well as changes in the state of consciousness occur. At larger doses, hemorrhaging in the gastro-intestinal tract, headaches, abdominal pains, vomiting, constipation or diarrhea can occur. Exposure to thallium will often cause the skin to dry and become scaly. Fingernails can develop white transverse bands about a month after exposure. Also, bluish line can develop on the gums. [65] The US government uses a value called the TLV, Threshold Limit Value, to identify the maximum exposure a worker can have. TLV is the time weighted average concentration for a normal eight hour workday and forty hour work week. TLV is the amount of material that workers may be repeatedly exposed, day after day, without recordable adverse effect. The TLV for thallium is 0.1 mg/m^' of body volume. T-4059 153 Thallium can be breathed in as part of dust or fumes. It can also enter through the skin if in soluble forms, or from contact on mucous membranes such as eyes, and nose. It exits the body through fecal and urine elimination. Thallium occurs naturally in the environment and normal doses found in a human are as follows: hair:0.007-0.051 ppm, fingernails: 0.051 ppm, urine: lxl0 “4 ppm; thallium can also be found in foods and in animals. Thallotoxicosis, thallium poisoning, can be treated by various drugs. The most successful is Prussian Blue, Fe7c18N18*IOH2 O, which causes no side effects when given at the proper dosage. Thallium poisoning can be completely cured and except in the most severe cases the effects are reversed.[66] To prevent contamination of workers and surroundings in the laboratory, proper safety procedures as follows were implemented in our laboratory: 1) Protective clothing was worn when using thallium. This included: laboratory coat, gloves, plexiglass face shield, and a face mask. The respirator filter on the face masks had NIOSH/MSHA (National Institute of Occupational Safety and Health/Mine Safety and Health Administration) designation of TC-210-132. This is a HEPA (High Efficiency Particulate Absolute) filter designed to remove particles T-4059 154 from the air flowing through it. When inserting samples into a furnace a double set of gloves were worn. A soft surgical glove for the inside to prevent exposure to thallium and a heavy glove for the outside to protect against heat were also used. Since the outside of the heavy gloves were exposed to thallium vapor, care was taken to always handle the heavy gloves while wearing the surgical ones. 2) The furnace that heated the thallium was placed in a fume hood equipped with a HEPA filter. 3) The tools and materials in the work area that could be exposed to thallium were kept to a minimum. 4) The blower of the fume hood was operated twenty four hours per day, seven days a week. Keeping a negative pressure in the hood, prevented dust particles of thallium oxide from contaminating the rest of the laboratory. 5) All tools and materials exposed to thallium were segregated from non-contaminated areas. 6) All waste material contaminated by thallium was segregated so that they can be disposed of properly. All liquids were placed in sealed plastic containers. All solid materials except bulk powders were placed in a double lined plastic bag. Bulk powders were placed in a sealed glass jar. All waste was disposed of by contacting the T-4059 155 Environmental Health and Safety officials of the Colorado School of Mines. 7) The amount of thallium used per experiment was kept to a minimum. 8) The length of time that thallium oxide was vaporized was kept to a minimum. 9) The thallium vapor deposition apparatus was designed with the idea of reducing the flow of gas, therefore minimizing thallium vapor released. T-4059 156 Appendix B Literature Survey T-4059 157 Appendix B. Literature Survey The following is an outline of topics and a listing of references associated with thallium superconductor research. A search of the Chemical Abstracts on-line database was conducted at the Colorado School of Mines using the keywords "thallium", "Tl", and "supercond" up to July 1991. The outline lists topics discussed in the abstracts of these references (the first 75 references). A second search covering dates from January 1991 to June 1993 was done, and these references are numbered 76 to 287 in this appendix. The numbers in brackets indicate these references. I. Overviews of thallium superconductor research A. History of the research of thallium superconductors 1. Review of the history of thallium superconductors [24] 2. Summarizing discovery of TBCCO (Thallium- barium-calcium-copper-oxide) superconductors [22] 3. Review of new compounds with Tc at 125 K [30] 4. Process of discovering a compound with Tl-Ba-Ca-Cu=l:3:4:6 that has Tc=162 K is reviewed [21] T-4059 158 B. Comparing thallium superconductors to other types 1. Advantages of using TBCCO (and disadvantages) over other materials [32] 2. TBCCO advantages and disadvantages [17] C. Review of the classification of the thallium compounds with respect to other superconductors and materials [2] II. Processing of thallium compounds A. Reviews of processing procedures 1. Processing review [5] 2. Preparation study [28] 3. Discussing heat treating of TBCCO [70] 4. Procedure for making ceramic TBCCO superconductors is described. [71] B. Articles covering specific procedures 1. Review of conditions for thin film TBCCO sputtering [9] 2. Review of deposition by molecular beam epitaxy [9] 3. Bulk specimens and special annealing a. Preparing TBCCO by 2 stage annealing [49] b. Synthesis of high Tc superconductors with Tl-Ba-Ca-Cu=l:2:n-1:n (n=2-5) [47] T-4059 159 c. Preparing TCBCO by 2 stage process prequenching and annealing [4 6] 3. Preparation of TBCCO by organometallic precursors [50] 4. Synthesis of TBCCO (120 K) with flux exclusion > 90% of perfect diamagnetizm [72] 5. Preparation of Tl-Ba-Ca-Cu=2133, 2212, and 2223 (Tc at 155K) [69] 6. Optimum processing variables and XRD of Tl-Ba-Ca-Cu=2133, 2212, 2223 [69] 7. Formation of Tl-Ba-Ca-Cu=1245 with five Cu-0 layers and a single Tl-0 layer [56] 8. Predominance of 2223 phase found in processing of thalium superconductors [4 6] D. Articles discussing the optimization of processing conditions 1. Formation of 162 K sample depends on starting stoichiometry, sintering temperature, and cooling rate [53] 2. Review of preparation reaction temperatures, compositions, and reaction mechanisms [31] 3. Article discussing the optimal TBCCO starting composition [67] T-4059 160 III. Articles discussing the theory and models of superconductivity in the thallium compounds A. Role of chemistry in the synthesis of T1 superconductors [12] B. Articles discussing the relationship of the structure 1. Structure model proposed [72] 2. Review of the structure to property relations [23] 3. Articles discussing the relationship between structure and Tc a. Tc increases with increasing number of Cu layers (up to a maximum Tc at n=4) [47] b. Proposal that Tc cannot exceed 164 K for compounds with a certain structure [54] c. Formalism for determining the critical temperatures of thallium superconductors [54] d. Correlation of Tc with the bonding in thallium superconductor structures [1] e. Correlation of structure changes to changes in the critical temperature [3] f. Analysis of crystal structures of Tl-Ba-Ca-Cu=l:2:n-l:n (n=2-5)[47] T-4059 161 g. Structure and superconducting properties in TBCCO [65] h. Review of structure and superconducting properties [31] i. Crystal structure and critical current density as function of Cu valence [30] j . Structure correlation of superconducting compounds[20] k. Chemistry and structure characterization of TBCCO [16] C. Articles discussing models for the understanding of superconducting properties 1. Review of models of superconducting compounds of thallium [27] 2. Suggestion of correlations to describe magnetic conditions in the Cu02 layers in the normal state [48] 3. Effects of nonstoichiometry problems on superconducting properties [2] 4. Understanding properties of superconductors by treating them as intercalation compounds [44] 5. Correlations that were developed for the temperature dependence of penetration depth Tl-Ba-Ca-Cu=2:2:2:3 [31] T-4059 162 6. Review of correlations for Tc correlation [20] IV. Articles discussing data and results from research into thallium superconductors A. General studies performed on thallium superconductors 1. Superconducting properties of materials with Tl-Ba-Ca- Cu=2:2:n-l:n [33] 2. Properties of Tl-Ba-Ca-Cu=2-x:2:n :n+1 (n=0, 1, 2) [57] 3. Characterization study of resistivity, susceptibility, XRD, SEM, and EDX [71] 4. Properties sensitive to sintering temperature and composition of oxides [73] 5. Thallium superconductor with Tl-Ba-Ca- Cu=2:1:1.5:2:3 is mentioned [18] 6. Thallium superconductor with Tl-Ba-Ca- Cu=l.8:1:1.5:3 is mentioned [18] 7. Thallium superconductor with Tl-Ba-Ca- Cu=2:l:2:2 at 105 K [28] 8. Thallium superconductor with Tl-Ba-Ca- Cu=2:2:2:3 at 125 K [28] 9. Review of properties of TBCCO [8] B. Articles reporting specific penetration depth data 1. Penetration depth for Tl-Ba-Ca-Cu=2:2:2:3 is T-4059 163 2520 ± 100 A [31] 2. Penetration depth of thallium-barium-copper- oxide superconductor (TBCO) is 1700 ± 100 A [36] 3. Penetration depth for superconductors when subjected to a magnetic field parallel to (001) [45] C. Articles discussing the structure and morphology 1. Structure of Tl-Ba-Ca-Cu=2:2:n-1:n [52] 2. Electronic structure of TBCO [39] 3. Article reporting that the Tl-0 layers are covalently bonded [39,40] 4. Article reporting surface structures in samples of TBCCO (flake-like structures with steps) [41] 5. Observing flux line lattice in TBCCO [45] 6. Structure of Tl-Ba-Ca-Cu=l:2:3:4 sample that has Tc of 162 K [53] 7. Crystal structure of Tl-Ba-Ca-Cu=l:2:3:4 [58] 8. Tl-Ba-Ca-Cu=l:2:3:4 phase has space group of P4/mmm [58] 9. Phases and structure of TBCCO were reported at 120.3 and 90.0 K [64] 10. Grain boundaries in TBCCO are discussed [17] 11. A study of structures discovered by researchers [28] 12. Review of structure [7] 13. Survey of structures with respect to Cu02 sheets [1] 14. Review of crystal growth in TBCCO superconductors [2 6] 15. Review of structural studies of TBCCO [15] 16. Article reports T1 compound properties strongly characteristic of granular superconductors [60] 17. Morphology and microstructure of sputtered TBCCO [43] Articles discussing the chemical composition of thallium superconductors 1. Chemical composition of Tl-Ba-Ca-Cu=2:2:n-1:n [34] 2. Article reporting that the Tl-Ba-Ca-Cu=l:2:3 phase has family of Tl-Ba-Ca-Cu=l:2:n-1:n (n=l,2,3,4,5, . . .) [58] 3. Review of crystal chemistry [2] 4. Composition review [20] 5. Crystal chemistry of TBCCO review 29 ] 6. Crystal chemistry study of T-4059 165 Tl-Ba-Ca-Cu-0=2-n:2 :n:n+l:2n+6 (n=0, 1, 2) material [57] E. Article discussing the specific heat of TBCCO [35, 51] F. Articles discussing tunneling in thallium superconductors 1. Tunneling data of TBCCO [38] 2. Tunneling I-V characteristic curves of TBCCO [37] G. Article discussing ultrasonic attenuation measurements made on T1 superconductors (Tc=110 K) [42] H. Articles reporting Tc and resistance data 1. Range of critical temperatures in TCBCO [46] 2. Resistance of TBCCO [51] 3. Tc up to 162 K of Tl-Ba-Ca-Cu=l:3:4 : 6 multiphase samples [53] 4. Superconducting states at 100 to 125 K in TBCCO [63] 5. Stable superconducting (Tc>100 K) in Tl-Ca/Ba-Cu-0 [74] 6. Superconductivity of Tl-Ba-Ca-Cu=2:2:0:3 and 1:1:0:3 were observed at Tc=81 K [75] 7. Transition temp at 116 K for TBCCO thin T-4059 166 film [1] 8. Tc controlled by the number of Cu-0 planes [33] I. Articles reporting magnetic measurements 1. DC magnetic measurements on TCBCO [48] 2. Field cooling Meissner effect of TBCCO [51] 3. Isothermal magnetization of TBCCO [51] 4. Magnetization properties of TBCCO in low magnetic fields [60] 5. Article reporting that T1 hysteresis(magnetization) is similar to that of superconducting ring [60] 6. Hall effect in TBCCO has been measured [66] 7. Meissner effect above/below in Thallium superconductors [68] 8. Review of Meissner effect studies [4] 9. Upper critical fields [6] 10. Review of critical current density [6] 11. Review of upper critical fields of thallium superconductors [6] 12. Review of critical current density [6] J. Articles reporting susceptibility data 1. AC susceptibility of TBCCO [51] 2. Article reporting AC susceptibility that support superconductivity with high critical T-4059 167 temperature [73] K. NMR studies 1. T1 NMR shifts and relaxation in TBCCO are reported [52] 2. NMR study of T1 superconductor [19] 3. NMR measurement of Tl-Ba-Ca-Cu=2:2:2:3 are summarized [11] L. XRD studies 1. X-ray diffraction study of Tl-Ba-Ca-Cu=2- x :2:n:n+1 (n=0,1,2) [57] 2. Review of various reports of electron diffraction used to study modulated structures [13] M. IR spectra of TBCCO [59] N. Raman studies 1. Raman measurements on Tl-Ba-Ca-Cu-0=2:n-1:2:n :4+2n (n=l,2,3) [61] 2. Article reports raman spectra of Tl-Ba-Ca-Cu=2:2:0:1 display a systematic variation with the number of Cu02 layers [61] 3. Article reports raman spectra of T-4059 168 Tl-Ba-Ca-Cu=2:2:0:1 is similar to YBCO [61] 0. Thermoelectric power studies 1. Thermoelectric power of Tl compounds. [62] 2. Article reporting that thermoelectric power is positive and decreases with increasing temperature in Bi and Tl materials [62] V. Applications of thallium superconductors A. Review of fabrication techniques to make devices [5] B. Review of the advances in thin film technologies [4] C. Articles discussing wires and related structures 1. Filament technology [4] 2. Tape technology [4] 3. Fabrication of wires [6] Sources 1. 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Supercond., 1989), 95-7, 1990. 276. Ye, H. J., et al., "Infrared properties of the high-Tc superconductor thallium barium calcium copper oxide (T12Ba2CaCu208)", J. Mater. Res., 5(8), 1616-19, 1990. 277. Yoshida, Y., et al., "Properties of thallium -2223 superconducting thick films formed by plasma spraying ", Adv. Supercond. Ill, Proc. Int. Symp. Supercond., 3rd, T-4059 221 Meeting Datel990, 639-41, Ed: Kajimura, K., and Hayakawa, H., Springer, Tokyo, Japan, 1991. 278. Young, M., et al., " Superconducting thallium barium calcium copper oxide thin films ”, AIP Conf. Proc., 251(Supercond. Its Appl.), 76-83, 1992. 279. Yuan, J., et al., "Study of electronic structure in single thallium -oxygen sheet superconductors by EELS ", High Temp. Supercond., Proc. LT-19 Satell. Conf., Meeting Date 1990,S193-S195, Ed: Evetts, J., Hilger, Bristol, UK, 1991. 280. Yuan, J., et al., "Study of electronic structure in single thallium -oxygen sheet superconductors by EELS", Supercond. Sci. Technol., 4 (IS), S193-S195, 1991. 281. Zeng, W. S., et al., "The charge transfer investigation: x-ray photoelectron spectroscopy(XPS) on superconducting thallium calcium barium copper oxide (T12Ca2Ba2Cu3O10) thin film ", Solid State Commun., 84(5), 493-6, 1992. 282. Zeng, W., et al., "X-ray photoelectron spectroscopy on superconductin thallium calcium barium copper oxide (T12CaBa2Cu208) thin film", Chin. Phys. Lett., 8 (6 ), 314-17, 1991. T-4059 222 283. Zetterer, T., et al., "Anomalies of infrared active phonons in superconducting thallium barium calcium copper oxide (T12Ba2Ca2Cu3O10)", Springer Ser. Solid-State Sci., 99(Electron. Prop. High-Tc Supercond. Relat. Compd.), 298-301, 1990. 284. Zetterer, T., et al., "Anomalous behavior of phonons in superconductin thallium barium calcium copper oxide (T12Ba2Ca2Cu3O10) detected by far-infrared spectroscopy", Phys. Rev. B: Condens. Matter, 41(13-B), 9499-501, 1990. 285. Zhdanov, I. I., et al., "Interlayer Cu02 antiferromagnetic spin correlations in thallium -based superconducting thallium barium calcium copper oxide (T12Ba2CanCun+106+2n) and thallium lead strontium calcium yttrium copper oxide (T10.5PbO.5Sr2CaO.8Y0.2Cu207) oxides. Copper-63 and thallium-205 NMR and spin-lattice relaxation data ", Physica C (Amsterdam), 183(4-6), 247- 51, 1991. 286. Zheng, G. Q., et al., "NQR study of high-Tc superconductor thallium strontium calcium copper oxide (TlSr2CaCu207-.delta.) ", Physica C (Amsterdam), 185- 189(Pt. 2), 765-6, 1991. T-4059 223 287. Zhou, W., et al., "Surface atomic images of superconducting thallium lead strontium calcium copper oxide ( (Tl,Pb)Sr2Ca2Cu309) ", Physica C (Amsterdam), 202(3-4), 335-44, 1992. T-4059 224 Appendix C Additional data on cuprate melted on alumina T-4059 225 Appendix C-l. XRD data of barium cuprate on alumina Figures 62 to 65 show XRD traces, obtained in this study, of cuprate (initial stoichiometry of powder: BaiCu3 0 4 ) on alumina. Figure 62 is the XRD trace of the alumina substrate. Figures 63 and 64 are the XRD traces of the melted cuprate single layer and the second layer cuprate used without acid, respectively. One notices that the peaks are fairly similar indicating that while some smoothing took place not much improvement was made in the structure. Figure 65 is the XRD of the cuprate layer produced using napthenic acid. Comparing this series of peaks with the ones produced in Figures 63 and 64 one can see that there are fewer peaks and a wider difference between the larger and smaller peaks. This indicates a more ordered structure than that produced by the other methods, which is favorable to the production of a superconductor. Tables 9-12 list the d-spacings and peak heights shown in Figures 62-65. T-4059 226 2147 1718 • 1288 .. 859 - 429 • 3 0 . 7 0. 20 degrees 2147 1718 1288 859 429 7 0 .0 0 7 5 .0 0 20 degrees Figure 62. XRD trace of alumina wafer T-4059 227 480 384 288 .. 192 rWv* 2 6 . 5 0 .6 0 20 degrees 480 384 288 r 192 5 6. 6 2. 7 4. 2 0 degrees Figure 63. XRD trace of single layer of barium cuprate on alumina T-4059 228 460 368 - 276 184 26. 32. 44. 5 8. 20 degrees 460 1I 368 CO J 276 33 1 *1 184 i 1 1 l 22 25 ^ l ^ l L9 92 8 ■— ------1—------1------56.40 62.1)0 68.00 74.00 800 20 degrees Figure 64. XRD trace of second layer of barium cuprate applied without using napthenic acid, on alumina T-4059 229 268 214 161 107 54 0 26. 32. 3 0 0 44. 2 0 degrees 268 214 ■ 161 . 107 ■ 54 ■ 56. 68.00 74.00 2 0 degrees Figure 65. XRD trace of second layer of barium cuprate applied by using napthenic acid, on alumina T-4059 230 Table 9. D-spacings and peak heights of XRD trace of alumina wafer (Figure 62) Peak Number 20 Degrees D-spacing A Height 1 21.90 4.055 38 2 25.56 3.482 943 3 35.15 2.551 2100 4 37.78 2.379 533 5 41.67 2.166 93 6 43.37 2.085 1526 7 44.83 2.020 39 8 46.28 1.960 41 9 52.57 1.740 675 10 57.52 1. 601 2147 11 59.71 1.548 62 12 61.31 1.511 379 13 65.29 1.428 67 14 66.56 1.404 731 15 68.26 1.373 836 16 70.41 1.326 44 17 74.33 1.275 77 18 76.95 1.238 1090 19 77.03 1.237 312 T-4059 231 Table 10. D-spacings and peak heights of XRD trace of single layer of barium cuprate on alumina (Figure 63) Peak Number 26 Degrees D-spacing A Height 1 22.73 3. 909 56 2 25.20 3.531 449 3 32.42 2 . 759 272 4 34.87 2.571 480 5 36.26 2.476 124 6 36. 94 2.432 65 7 37.60 2.390 118 8 39. 03 2.306 384 9 41.48 2.175 214 10 43.18 2. 093 268 11 45.75 1.982 69 12 46.52 1. 951 141 13 48.91 1. 861 191 14 49. 37 1.844 91 15 51.60 1.770 77 16 52.41 1.745 191 17 52.90 1.729 319 18 53.55 1.710 97 19 57.34 1. 606 435 20 59.27 1.558 221 21 59.91 1. 543 119 22 61.11 1.515 113 23 62.27 1. 490 105 24 63.46 1.465 161 25 64.05 1.453 115 26 66.37 1.407 176 27 68. 05 1.377 194 28 72.60 1.301 125 29 73.105 1.293 176 30 76.735 1.241 198 31 77. 85 1.226 203 T-4059 232 Table 11. D-spacings and peak heights of XRD trace of second layer of barium cuprate applied without using napthenic acid, on alumina (Figure 64) - — e Peak Number 26 Degrees D-spacing A Height 1 22.74 3.907 52 2 25.19 3.533 401 3 25. 62 3.475 145 4 28.42 3.138 52 5 29.28 3. 047 67 6 30. 09 2.968 56 7 32.41 2.760 116 8 34.87 2.571 378 9 36.29 2.473 126 10 37.56 2.393 117 11 39. 01 2 . 307 442 12 41.47 2.176 175 13 43.16 2 . 094 264 14 45.74 1.982 79 15 46.52 1. 951 133 16 48 . 91 1.861 196 17 51. 64 1.769 102 18 52. 90 1.730 460 19 53. 55 1.710 102 20 57.32 1. 606 326 21 58.26 1.583 88 22 59.27 1.558 138 23 59. 90 1.543 105 24 61.06 1.516 107 25 63.43 1.465 111 26 63.99 1.454 107 27 66.37 1.407 149 28 68.05 1.377 180 29 72. 37 1.305 137 30 73. 02 1.295 211 31 75. 65 1.256 146 32 76.74 1.241 195 33 77. 85 1.226 242 34 79.16 1.209 128 T-4059 233 Table 12. D-spacings and peak heights of XRD trace of second layer of barium cuprate applied by using napthenic acid, on alumina (Figure 65) Peak Number 26 Degrees D-spacing A Height 1 22.82 3.895 39 2 25.24 3.526 161 3 25.77 3.454 78 4 29.33 3. 042 46 5 30.23 2.954 38 6 32.49 2.754 118 7 34.69 2.584 157 8 35.46 2.529 51 9 36.36 2.469 75 10 37.10 2.421 42 11 39.05 2.305 241 12 41.56 2.171 102 13 45.71 1. 983 57 14 46.53 1.950 87 15 48.91 1.861 100 16 51.70 1.767 64 17 52. 90 1.729 268 18 53. 65 1.707 83 19 58.52 1.576 66 20 59.36 1.556 124 21 60. 84 1.521 69 22 62.35 1.488 83 23 63.47 1.465 125 24 64.10 1.452 108 25 66.29 1.409 81 26 67. 92 1.379 116 27 72.27 1.306 98 28 73.10 1.293 100 29 73.65 1.285 106 30 75.78 1.254 125 31 77.79 1.227 137 T-4059 234 Appendix C-2. Preliminary study of Ba2Cu3 In addition to studying the melting of Ba]_Cu3 C>4 on alumina, the melting of Ba2 Cu3 C>5 on alumina was also studied. Ba2 Cu3 C>5 powder was prepared by the procedure shown in Figure 13. Figure 66 shows the results of melting Ba2 CU3 C>4 on alumina at 930°C. The average concentration of barium was about 37%. The optimum time for deposition of a layer of Ba2 Cu3 0 5 is 930°C for 3 to 4.5 minutes. Figure 67 is a photomicrograph of Ba2 Cu3 0 5 on alumina showing cracks in the surface. Similar to experiments on BaiCu3 0 4 , steps were taken to repair this layer. A second layer of Ba2 Cu3 C>5 powder was melted onto the cuprate-alumina substrate. Figure 68 shows that the results for a second layer of Ba2 Cu3 C>5 melted onto alumina are similar to that of BaiCU3 C>4 , that all the samples showed the same high stoichiometry. Figure 69 is a photomicrograph of this second layer, showing cracks and an uneven surface. The sample shown in Figure 69 had a barium concentration of 31.11%, which was very close to that of the barium concentration in two layer BaiCugC^ samples, deposited without using napthenic acid. Ba2 Cu3 C>5 was then deposited onto a cuprate-alumina substrate using the slurry method, shown as part of Figure T-4059 235 14. However, the layer was melted, and not sintered once the napthenic acid was removed. Table 13 shows the data for barium concentration obtained by this procedure. Ba2 Cu3 C>5 was mixed with napthenic acid in 1.9:1 proportions, respectively. Table 13. Data for the second layer of Ba2 CU3 C>5 deposited on melted cuprate using the slurry method Temperature, °C Time, min. Barium concentration, % 920 3 40.92 920 4 37.71 930 2 28.77 940 2 31. 68 Figure 70 shows photomicrographs of Ba2 Cu3 C>5 deposited in this manner. However, while the stoichiometry was correct the morphology was very poor. Then, the cuprate deposited via a slurry was sintered for 24 hours and the results shown in Figure 71. Even though the stoichiometry is wrong, the morphology is fairly good. T-4059 236 However, it was decided to use BaiCu3 0 4 powder in the production of superconductors in order to eliminate the number of variables in optimizing the production of a thallium superconductor. T-4059 237 100 75 * 1 ? § 50 1 E xa 2 25 0 2 4 6 8 10 12 14 T im *, min* Figure 66. Barium concentration of Ba2 Cu3 C>5 melted at 930°C onto alumina T-4059 238 Figure 67. Photomicrographs of a single layer of Ba2 Cu3 0 5 on alumina Figure 68. Summary of EDX data for two layers of Ba of layers two for data EDX of Summary 68. Figure T-4059 Barium Concentration, % etdot auia □ 2°, 930°C, 920°C,# □ alumina: onto melted and a 940°C- a and 100 25 50 75 0 2 4 8 6 i ,min. m a, Tim 10 12 2 CU 3 C >5 239 T-4059 240 Figure 69. Photomicrograph of a second layer of Ba2 Cu3 C>5 melted on alumina T-4059 241 Figure 70. Photomicrograph of Ba2 Cu3 0 5 second layer material deposited using a slurry and then melting the cuprate. T-4059 242 Figure 71. Photomicrograph of second layer of Ba2 Cu3 0 5 melted onto alumina using cuprate-slurry and then sintering the cuprate T-4059 243 Appendix D . Resistance versus temperature curves T-4059 244 Appendix D. Resistance versus temperature curves This section shows resistance versus temperature curves for materials that had thallium oxide deposited at 880°C and 890°C during this study. Table 14 summarizes the data and lists the corresponding figures. At 890°C thallium superconductors were made by depositing thallium from 11 to 16 minutes. At 880°C there is no conclusive data indicating a superconductive state above 56 K. Figures 73, 75, and 77 all show sharp transitions indicating that some part of the material may be superconducting. However, the transition is quite small, when compared to the transition for the materials deposited at 890°C, and the resistance never goes to zero. Also, these deposition times are scattered with times by which materials displayed no sharp drop in resistance at low temperatures. The conclusion from this is that the temperature should not be dropped below 890°C for the production of thallium superconductors. T-4059 245 Table 14. Resistance versus temperature data for optimizing the deposition of thallium oxide Figure Number Deposition Deposition Superconductive Temperature, Time, min. with Tc > 76 K? °C 72 880 7 No 73 880 8 NO 74 880 9 No 75 880 10 No 76 880 11 No 77 880 12.5 No 78 880 13 No 79 890 1 1 . 2 Yes 80 890 14 Yes 81 890 16 Yes iue 2 Rssac ess eprtr cre o thallium for curve temperature versus Resistance 72. Figure T-4059 Re s is tan ce , Oh me 82 50- .5 2 8 - 0 .5 3 8 . 0- 3.0 8 84.00- —I,,, . 0 — 0 00 100 100 200 200 300.00 250.00 200.00 150.00 100.00 50.00 barium cuprate-alumina substrate cuprate-alumina barium xd eoie t 8° fr iue o a on 7minutes for 880°C at deposited oxide . -|-r , i [ i i m i i i i i i i i i r i h i i i r i i i i | i i i i i i i i i | i i i m i i' i i [ i i | , , r - | i- , ..r T r at e, K , re tu ra e p m e T 246 T-4059 247 36.00-1 35,50 35.00 34.50 N 34 . 00 T I T I I'l I I 11 I T I I I 11 I l "H I T I- T IT I t~| 11' I n I I M I T I I I I I I M [ 'I T I T I T m 50.00 100.00 150.00 200.00 250.00 300.00 350.00 T e mp gr a t u r e , K Figure 73. Resistance versus temperature curve for thallium oxide deposited at 880°C for 8 minutes on a barium cuprate-alumina substrate iue 4 Rssac ess eprtr cre o thallium for curve temperature versus Resistance 74. Figure T-4059 Res is ta n c e 9 Oh ms .00 r i 0 0 0. 0 0 0. 600. . . 20 40 50. 00 - ^ barium cuprate-alumina substrate cuprate-alumina barium oxide deposited at 880°C for 9 minutes on a on 9minutes for 880°C at deposited oxide t i ' T i i i i | i i ii i i i i i | i i i i i i i i i | i i i i i i i i i | i i i i i i i i i ^ i i i i r i i i i ] 1 00. 00 at e, K , re tu ra e p m e T 1 50. 200. 00 00 250. 300. 00 00' 350. 00 248 T-4059 249 36.80 n 36.40- Q) 36.00- 0) 35. 60 - 35.20- 34 . 80 i i i rn i i i | r~n i i i i ri | i i i i i i i i i | i i i i ii i i i | i i i i i i i i i | ii n i i i rr 50. 00 1 00. 00 1 50. 00 200. 00 250. 00 300. 00 350. 00 T e mp era lu re , K Figure 75. Resistance versus temperature curve for thallium oxide deposited at 880°C for 10 minutes on a barium cuprate-alumina substrate T-4059 250 0.06 0 .0 6 - CD 0. 05- co 0. 05 - 0 .0 4 - 0. 04 | i i i i i i i i i | i i i i i i i i i | i i i i i i i i i | i i i rrrr i r-[-1 i i m i r rT'f r m i i rrr 50.00 100.00 150.00 200.00 250.00 300.00 350.00 T e mp er a t u r e , K Figure 76. Resistance versus temperature curve for thallium oxide deposited at 880°C for 11 minutes on a barium cuprate-alumina substrate iue 7 Rssac ess eprtr cre o thallium for curve temperature versus Resistance 77. Figure T-4059 Res Is tan ce , Oh ms 00- 0 .0 1 5 48. 00 | i 00 48. i i i i i i i i | i i i i i i i i i | i i i i i i i i i - p i i n t i i i . - 0 0.0 5 00- .0 9 4 - 0 .0 2 5 53.00 0 0 0.0 5.0 0 0200 300 350.00 300.00 200 . 250.00 00 150.00 100.00 00 50. aimcpaeauia substrate cuprate-alumina barium oxide deposited at 880°C for 12.5 minutes on a on minutes 12.5 for 880°C at deposited oxide T g p erature K , e r u t mp a r e |-1 i i i i i i r r | i i i i i ir n. 251 T-4059 252 0. 40 CO £ : t—^ 0 . ~ o- ” o : ® 0 -20^ CO CO 0 0 . 00 50.00 100.00 150.00 200.00 250.00 300.00 350.00 T e mp er a t u r e , K Figure 78. Resistance versus temperature curve for thallium oxide deposited at 880°C for 13 minutes on a barium cuprate-alumina substrate T-4059 253 12.00 £ 10.00 _C O 8.00 CD O o 6 *00 -*J> CD 4. 00 CD 0 O' 2.00 0 . 00 rr 0.00 100.00 200.00 300.00 400.00 T e mp er a t u r e , K Figure 79. Resistance versus temperature curve for thallium oxide deposited at 890°C for 11.2 minutes on a barium cuprate-alumina substrate T-4059 254 20.00 co c CO 0. 00 I'ffTi TTViyn i i i i i i i | i i i i i n r T"j~n i i i i i i i | i i i r i r i r rp i i i r i i i r 50.00 100.00 150.00 200.00 250.00 300.00 350.00 Tempera lure , K Figure 80. Resistance versus temperature curve for thallium oxide deposited at 890°C for 14 minutes on a barium cuprate-alumina substrate T-4059 255 1 60. 00 -f 0) Q 120.00 ^ CO 0) q/ 40. 00 0. 00 iTi i ii i 'ii y n i i i i r1 i | i i ii ii ii i | i ! ii i m irp"! i i i i ■ i r i i i ~i i i i i i i1 50.00 100.00 150.00 200.00 250.00 300.00 350.00 Temperature, K Figure 81. Resistance versus temperature curve for thallium oxide deposited at 890°C for 16 minutes on a barium cuprate-alumina substrate T-4059 256 Appendix E . The Benefit of Using Nitrogen instead of Helium T-4059 257 Appendix E. The Benefit of Using Nitrogen instead of Helium A lot of attention has been paid to the new high temperature superconductors because they have a critical temperatures higher than the temperature of liquid nitrogen at a pressure of one atmospheric. The advantage of liquid nitrogen over that of liquid helium is "economical". Helium can only be obtained from wells where it is entrapped with petroleum as a very small fraction of air. Nitrogen is obtained from air where it is quite abundant, 71 mole %. Table 15 compares the heat of vaporization and cost of liquid nitrogen to that of liquid helium. The costs were obtained from General Air Supply Company of Denver, Colorado in July 1993. Table 15. Comparing the cooling power and cost of liquid nitrogen to liquid helium Material Hv cal/mole cost $/liter He (liq) 20.74 8.39 N? (liq) 667.5 1.18 T-4059 258 The heat of vaporization of nitrogen is 32 times greater than that of liquid helium which means that for a given system, approximately 32 times more helium has to be used (if ideal fluids are assumed). Thus, liquid He is 224 times more expensive than liquid N2 . Also, this does not include the cost of the extra energy required to get the system to 4.2 K for helium and the cryogenic system required to stay at that temperature.