UNIVERSITY OF CINCINNATI

______, 20 _____

I,______, hereby submit this as part of the requirements for the degree of:

______in: ______It is entitled: ______

Approved by: ______Effects of Processing on PTCR Systems with Barium and Oxide additions in the near stoichiometric region

A thesis submitted in partial fulfillment for the degree of

MASTER OF SCIENCE

in the

Department of Material Science and Engineering University of Cincinnati

2002

by

Srinivas Subramaniam B.E., Bangalore University, Bangalore, India, 1995

Committee chair: Prof. Rodney Roseman ABSTRACT

The effects of processing BaTiO 3 powders with and Titanium

Oxide additions have been studied, in an attempt to understand and control microstructure evolution and PTCR properties in these materials. The base powders used had Ba/Ti ratios of 0.994 and 1.0038. Barium oxide and titanium oxide additions were made to modify the existing stoichiometry of these powders, shifting them towards the near stoichiometric region. Additions were made in the form of direct oxide additions or metallorganic precursor (titanium isopropoxide) as the latter method results in finer particle sizes of the oxide addition. The calculated stoichiometries ranged from 0.9988 to

1.0013. A Ti-excess system (Ba/Ti ratio 0.9890) was also prepared by adding titanium isopropoxide to Ti-excess barium titanate (Ba/Ti ratio 0.994) powders, in order to compare its properties with those of the near stoichiometric systems. TiO 2 additions do not have any effect on the solution behavior of the system being insoluble in aqueous media. BaO additions have a marked effect on the solution behavior of the system with a tendency to increase the pH of the solution. This effect is directly related to the amount of

BaO addition in the system with larger amounts of BaO leading to higher observed pH values. TEM studies on powders with TiO 2 additions show distinct double-layer formations on the particle surfaces suggesting preferential dissolution from the BaTiO 3 particle surfaces. Powders with BaO additions however exhibit a single layer formation on the particle surfaces indicative of BaO dissolution and redeposition during the solution processing and drying stages, limiting preferential dissolution from BaTiO 3 particles.

Undoped systems with BaO direct oxide additions exhibit bimodal large grained microstructures. Systems with TiO 2 direct oxide additions while similar in microstructure

- 2 - had regions of inhomogeneous growth which were attributed to poor dissolution characteristics and agglomeration of TiO2 powders in solution. Near stoichiometric doped systems with direct oxide as well as titanium isopropoxide additions display bimodal microstructure characteristics, caused by the complex mechanisms induced in these systems. The donor doped Ti-excess systems with titanium isopropoxide additions however exhibited coarse grained microstructures with large amounts of grain boundary second phases. Donor doped near stoichiometric systems with oxide additions exhibit low room temperature resistivities with a poor PTCR rise of ~=2 order rise in resistivity. The donor doped Ti-excess systems processed with titanium isopropoxide additions exhibits a large PTC rise of ~4 orders. Higher porosity coupled with low grain boundary coherence and large amounts of liquid phases in the grain boundary regions in this material appear to be responsible for the enhanced PTC properties suggesting these mechanisms and their control to be dominant in obtaining high rise PTC systems.

- 3 -

- 4 - Acknowledgement

I wish to dedicate this work to my parents and brother who have loved and supported me all my life. I also am deeply indebted to my friends in Bangalore who gave me the strength and confidence to pursue my dreams. I wish to thank my advisor Dr. Roseman for his advice and guidance in this work. I will always be grateful to him for his support and confidence in my abilities. Niloy, my friend and colleague has been a valuable source of information and insight into the world of materials. I am grateful to Dr. Buchanan and Dr. Sekhar for their generous advice and support. Douglas Bowling our previous manager and Tonya Ship at the Advanced Materials Characterization center provided me with valuable assistance during the experimental stages of the research.

A major part of the characterization work done in this study was carried out at the Advanced Materials Characterization Center, University of Cincinnati. Working at the Center has been a fruitful experience.

I wish to acknowledge Dr. Alan Dozier and Mr. Larry Rice of the electron microscopy facility at the University of Kentucky, Lexington for their assistance with TEM instrumentation and facilities.

Lastly, I would like to thank all my friends and family for helping me through this journey.

- 5 - TABLE OF CONTENTS

1 LITERATURE REVIEW ...... - 10 - 1.1 FUNDAMENTALS...... - 10 - 1.2 FERROELECTRICS...... - 11 - 1.3 BARIUM TITANATE...... - 13 - 1.4 CONDUCTION IN BATIO3...... - 17 - 1.5 PTCR EFFECT ...... - 21 - 1.6 PROCESSING EFFECTS...... - 29 - 1.6.1 Dissolution Studies...... - 30 - 1.6.2 Thermodynamics ...... - 31 - 1.6.3 in ternary and more complex ...... - 32 - 1.6.4 Kinetics...... - 34 - 1.6.5 Dissolution Mechanisms in BaTiO3-type materials...... - 35 - 1.7 SINTERING BEHAVIOR AND MICROSTRUCTURE EFFECTS IN BATIO3...... - 45 - 1.7.1 General Sintering Theory...... - 45 - 1.7.2 Solid state sintering...... - 47 - 1.7.3 Liquid phase sintering...... - 47 - 1.7.4 Sintering behavior of BaTiO3...... - 50 - 1.7.5 Abnormal grain growth...... - 53 - 1.7.6 Sintering behavior and microstructures in Ba-excess BaTiO3...... - 54 - 1.7.7 Sintering behavior and microstructures in Ti-excess BaTiO3...... - 55 - 1.8 MOTIVATION FOR RESEARCH...... - 56 - 2 EXPERIMENTAL PROCEDURES ...... - 59 - 2.1 RAW MATERIALS...... - 59 - 2.2 SAMPLE PREPARATION...... - 61 - 2.3 PROCESSING OF SYSTEMS WITH TITANIUM ISOPROPOXIDE ADDITIONS...... - 63 - 2.4 MEASUREMENT AND CHARACTERIZATION...... - 63 - 2.4.1 pH Measurements...... - 63 - 2.4.2 Transmission Electron Microscopy (TEM)...... - 65 - 2.4.3 Scanning Electron Microscopy (SEM) ...... - 65 - 2.4.4 Density Measurement...... - 66 - 2.4.5 Electrodes...... - 66 - 2.4.6 Resistance and PTCR Measurement...... - 66 - RESULTS AND DISCUSSIONS ...... - 68 -

3 PH STUDIES ...... - 68 - 3.1 PH STUDIES ON BASE SYSTEMS...... - 69 - 3.2 PH STUDIES ON SYSTEMS WITH OXIDE ADDITIONS...... - 71 - 3.2.1 Ba-excess powders with TiO2 additions...... - 71 - 3.2.2 Ti-excess powders with BaO additions...... - 71 - 3.3 PH STUDIES OF DOPED SYSTEMS...... - 75 - 3.3.1 Base systems with dopant additions...... - 75 - 3.3.2 Doped systems with oxide additions...... - 75 - 3.4 GENERAL TRENDS IN P H STUDIES...... - 81 - 3.5 SUMMARY OF P H STUDIES...... - 84 - 4 TEM STUDIES OF POWDERS ...... - 85 - 4.1 AS-RECEIVED POWDERS...... - 85 - 4.2 BA-EXCESS POWDERS WITH TIO2 ADDITION...... - 85 - 4.3 TI-EXCESS POWDERS WITH BAO ADDITIONS...... - 88 - 4.4 GENERAL TRENDS IN TEM STUDIES...... - 88 -

- 6 - 4.5 SUMMARY OF TEM STUDIES...... - 90 - 5 RESULTS AND DISCUSSIONS OF SINTERED MICROSTRUCTURES ...... - 91 - 5.1 BASE SYSTEMS...... - 91 - 5.1.1 Undoped base systems...... - 91 - 5.1.2 Donor-doped Base BaTiO3 systems ...... - 93 - 5.2 MICROSTRUCTURES OF SYSTEMS WITH OXIDE ADDITIONS...... - 93 - 5.2.1 Undoped Ba-excess systems with TiO2 additions...... - 97 - 5.2.2 Undoped Ti-excess systems with BaO additions...... - 100 - 5.2.3 Doped Ba-excess systems with TiO2 additions...... - 103 - 5.2.4 Doped Ti-excess systems with BaO additions...... - 106 - 5.2.5 Doped systems with Titanium Isopropoxide additions...... - 106 - 5.3 GENERAL TRENDS IN MICROSTRUCTURE STUDIES...... - 110 - 5.4 SUMMARY OF MICROSTRUCTURE RESULTS...... - 114 - 6 RESULTS AND DISCUSSIONS OF ELECTRICAL STUDIES ...... - 115 - 6.1 PTCR EFFECT IN BASE SYST EMS...... - 115 - 6.2 SYSTEMS WITH OXIDE ADDITIONS...... - 117 - 6.2.1 Stoichiometric effect on PTCR behavior in systems with oxide additions...... - 117 - 6.2.2 PTCR properties of systems with Ti-isopropoxide additions...... - 121 - 6.3 GENERAL TRENDS IN PTCR STUDIES...... - 124 - 6.4 SUMMARY OF PTCR STUDIES...... - 126 - 7 CONCLUSIONS ...... - 127 -

8 FUTURE WORK...... - 129 -

9 REFERENCES ...... - 131 -

- 7 - TABLE OF FIGURES 8 Figure 1. Properties of single -crystal BaTiO3: (a) unit-cell distortion of the polymorphs (b) lattice dimensions versus temperature.8...... - 14 - Figure 2. (a) The unit cell of BaTiO3, 2 (b) Approximate ion displacements in the cubic-tetragonal 6 distortion in BaTiO3...... - 16 - Figure 3. Resistivity-Temperature characteristics showing the narrow range of dopant concentration which results in low resistivity, semiconduction.22...... - 22 - 26 Figure 4. Typical resistivity behavior of BaTiO3-type PTCR materials ...... - 24 - Figure 5. Heywang PTCR model-a two dimensional potential barrier at grain boundaries.27...... - 26 - Figure 6. Jonker PTCR model- domains at a grain boundary between two crystallites of different orientation in compensating surface charges. 30 ...... - 28 - Table 1. The stability of the MOH(Z-1)+ species of cations.37 ...... - 33 - Table 2. Categorization of oxides and the relation to dissolution processes.38...... - 36 - Figure 7. dissolution mechanism. (a) Initial surface. (b) Ca2+ dissolution from the surface (c) Base-catalyzed hydrolysis of titanate lattice. (d) Further Ca2+ dissolution.41...... - 40 - Figure 8. SEM photomicrograph (400X) of the as fired surface of TAM HPB milled at a pH of 6 before drying and fired at 1310°C.42...... - 41 - Figure 9. Barium leaching during milling as a function of solution pH.42...... - 41 - Figure 10. The amount of exaggerated grain growth as estimated from micrographs of the as fired surface versus processing conditions.42...... - 44 - Figure 11. a, b, and c. Effect of milling on the barium titanate grain structure.42...... - 44 - Figure 12. Schematic of liquid configuration on solid substrate. 47...... - 49 - 52 Figure 13. The phase diagram of BaO-TiO2 system...... - 52 - Figure 14. SEM micrographs showing morphology and particle size of BaTiO3 powders...... - 60 - Figure 15. Flow Chart for processing BaTiO3 powders...... - 62 - Figure 16. SEM images showing morphology and particle size of TiO2 particles...... - 64 - Figure 17. Representative plot of solution pH versus time for base undoped systems...... - 70 - Figure 18. Representative plot of solution pH versus time for Ba-excess systems with TiO2 additions..- 72 - Figure 19. Representative plots of solution pH versus time for undoped Ti-excess systems with varying amounts of BaO additions...... - 73 - Figure 20. Representative plot of solution pH versus time for base systems with 0.24 m/o Y2O3 showing the effect of yttria nitrate dopant incorporation on pH of the system...... - 76 - Figure 21. Representative plot of doped Ba-excess systems with TiO2 additions. Arrow indicates point at which yttria nitrate dopant addition had been made...... - 77 - Figure 22. Figure of solution pH versus time for doped Ti-excess systems with BaO additions. Effects of varying amounts of BaO additions are shown in the plot...... - 79 - Figure 23. Comparison plot showing pH versus time for undoped base systems, Ba -excess systems with TiO2 additions and Ti-excess systems with BaO additions...... - 82 - Figure 24. Comparison plot of solution pH versus time for doped base systems with TiO2 additions and BaO additions...... - 83 - Figures 25. (a) and (b) showing TEM images of factory powders prior to milling...... - 86 - Figures 26. (a) and (b) TEM images of particles processed with TiO2 additions showing the double layer formation on the particle surfaces...... - 87 - Figure 27. (a) and (b) TEM images of particles processed using BaO additions showing a distinct single layer on the surfaces...... - 89 - Figure 28. Base Ti-excess( Ba/Ti ratio 0.994) samples sintered at 1320°C(a) and 1375°C(b)...... - 92 - Figure 29. Base Ba -excess(Ba/Ti ratio 1.0038) samples sinterd at 1320°C(a) and 1375°C(b)...... - 92 - Figure 30. Donor doped base Ti-excess( Ba/Ti ratio 0.994) samples sintered at 1320°C(a) and 1375°C(b). - 94 - Figure 31. Donor doped base Ba-excess(Ba/Ti ratio 1.0038) samples sintered at 1320°C(a) and 1375°C(b)...... - 94 - Table 3. Results for undoped and donor doped base systems with Ti-excess (Ba/Ti ratio 0.994) and Ba - excess (Ba/Ti ratio 1.0038) powders ...... - 95 - Figure 32. Ba/Ti ratio 0.9988 sintered at 1320°C (a) and 1375°C (b) ...... - 98 - Figure 33. Ba/Ti ratio 1.0009 sintered at 1320°C (a) and 1375°C (b) ...... - 98 -

- 8 - Figure 34. Ba/ Ti ratio 1.0013 sintered at 1320°C (a) and 1375°C (b)...... - 98 - Figures 35. (a), (b), and (c). Undoped systems processed with TiO2 additions showing inhomogeneous regions...... - 99 - Table 4 Results for doped and undoped systems with TiO2 additions...... - 101 - Figure 36. Ba/Ti ratio 0.9988 sintered at 1320°C (a) and 1375°C (b) ...... - 102 - Figure 37. Ba/Ti ratio 1.0009 sintered at 1320°C (a) and 1375°C (b) ...... - 102 - Figure 38. Ba/ Ti ratio 1.0013 sintered at 1320°C (a) and 1375°C (b)...... - 102 - Table 5 Results for doped and undoped systems with BaO additions...... - 104 - Figure 39. Ba/Ti ratio 0.9988 sintered at 1320°C (a) and 1375°C (b) ...... - 105 - Figure 40. Ba/Ti ratio 1.0009 sintered at 1320°C (a) and 1375°C (b) ...... - 105 - Figure 41. Ba/ Ti ratio 1.0013 sintered at 1320°C (a) and 1375°C (b)...... - 105 - Figure 42. Ba/Ti ratio 0.9988 sintered at 1320°C (a) and 1375°C (b) ...... - 107 - Figure 43. Ba/Ti ratio 1.0009 sintered at 1320°C (a) and 1375°C (b) ...... - 107 - Figure 44. Ba/ Ti ratio 1.0013 sintered at 1320°C (a) and 1375°C (b)...... - 107 - Figure 45. Ba/Ti ratio 0.9988 at 1320°C (a) and 1375°C (b)...... - 108 - Figure 46. Ba/Ti ratio 0.9890 at 1320°C (a) and 1375°C (b)...... - 108 - Table 6 Results for systems processed with titanium isopropoxide additions ...... - 111 - Figure 47. Blended system processed with 50mol% 0.994, 50 mol% 1.0038 powder...... - 113 - Figure 48. Near stoichiometric system with direct oxide addition...... - 113 - Figure 49. Near stoichiometric system with isopropoxide addition...... - 113 - Figure 50. Comparison of PTCR effect in base Ba -excess (Ba/Ti ratio 1.0038) and Ti-excess(Ba/Ti ratio 0.994) systems. Samples sintered at 1375°C...... - 116 - Figure 51. Etched microstructure of base Ba-excess sample sintered at 1375°C...... - 116 - Figure 52. Etched microstructure of base Ti-excess sample sintered at 1375°C...... - 116 - Figure 53. Comparison of samples with TiO2 and BaO direct oxide additions. System with BaO addition has Ba/Ti ratio of 0.9988. additions System with TiO2 addition has Ba/Ti ratio of 1.0013. Both samples have been sintered at 1375°C...... - 118 - Figure 54. Comparison of samples with oxide additions to blended system sintered at1375°C. Ba/Ti ratio 0.9988 ...... - 119 - Figure 55. Etched microstructure of Ba-excess + 34mg TiO2 sintered at 1375°C ...... - 119 - Figure 56. Etched microstructure of Ti-excess + 63mg BaO sintered at 1375°C ...... - 119 - Figure 57. Comparison of systems with oxide additions to blended system sintered at 1375°C. Ba/Ti ratio 1.0013 ...... - 120 - Figure 58. Comparison of systems with Titanium isopropoxide additions. Ti-excess system with TiO2 addition has Ba/Ti ratio 0.9890. Ba-excess system with TiO2 addition has Ba/Ti ratio 0.9988. Both sintered at 1375°C ...... - 122 - Figure 59. Etched microstructure of Ba-excess sample with 34mg TiO2 addition from Titanium- isopropoxide sintered at 1375°C ...... - 122 - Figure 60. Etched microstructure of Ti-excess sample with 34mg TiO2 addition from Titanium- isopropoxide sintered at 1375°C ...... - 122 - Figure 61. Comparison of systems processed by different routes. All systems have Ba/Ti ratio 0.9988 and have been sintered at 1375°C...... - 125 -

- 9 - Literature Review

1.1 Dielectric Fundamentals

Dielectrics are defined as those materials, which are electrically insulating, and form electric dipoles in the presence of an electric field.1-4The charge separation resulting from the formation of dipoles can occur on an atomic or molecular level. Dielectric materials are used in a variety of applications such as and substrates. The general formula for capacitance is given by the equation:

Q = CV (1)

Where Q is the charge stored, C is the capacitance and V is the voltage applied across the . The capacitance of a parallel plate capacitor with a vacuum between the electrodes is given by:

C0 = e0 A / d (2)

-12 Where e0 is the permittivity of vacuum (8.85 ×10 F/m), A is the area of the electrodes and d is the distance between the electrodes. The capacitance of a dielectric medium between the electrodes is given by:

C = e A / d (3)

Where e is the permittivity of the medium. The dielectric constant K is defined as:

K = e / e0 (4)

In the presence of an externally applied electric field the dipoles in dielectric materials tend to align themselves in the direction of the field. This phenomenon of polarization can be described by the equation:

D = e0E + P (5)

- 10 - Where D is the dielectric displacement, e0E is the dielectric displacement in vacuum and P is the polarization in the material. From the above equation we can see that the dielectric displacement is directly proportional to the applied field (E). The polarization (P) is proportional to the field as shown by the equation:

P = e0E (K-1) = e0E? (6)

Where ? is termed the dielectric susceptibility of the material and is a measure of the polarizability of the material. The polarization in most materials follows a linear relationship with the dielectric susceptibility. Ferroelectrics are an exception to this rule exhibiting a non-linear polarization - susceptibility relationship. This non-linearity is attributed to the spontaneous polarization mechanisms found in ferroelectrics.

Polarization and dielectric constants in the material are dependent on a number of parameters, the most important parameter being the frequency dependency of the individual polarization mechanisms (ionic, electronic, space charge, molecular). Other factors affecting polarization and dielectric constant in the material are the temperature, density, dopant substitutions, impurities and grain size of the material.

1.2 Ferroelectrics

Dielectric materials can be divided into different classes. A common occurrence found in most materials is the phenomenon of electrostriction. These materials, in the presence of a field display a change in the physical dimension of the material, caused by the displacement of charge. Piezoelectric materials display this property and further, on applying a mechanical stress to the piezoelectric material it will display polarization caused due to the displacement of charge centers from their original positions. This property of piezoelectrics is attributed to the lack of center of symmetry in the crystal.

- 11 - Pyroelectrics are materials in the piezoelectric family which have spontaneous polarization in the crystal. The polarization behavior in pyroelectrics is temperature dependent in the absence of field and stress effects. Ferroelectrics are a sub group of pyroelectric materials with the major difference being the ability that the spontaneous polarization in ferroelectrics is reversible in at least one direction.

Megaw5 has summed up the characteristic features of ferroelectric materials based on the work carried out on Rochelle salt in which the phenomenon of was first discovered by Valasek5. A summary of these properties is given below.

(1) They possess a dielectric hysterisis loop, indicating reversible spontaneous polarization

(2) They show disappearance of hysteresis at a certain temperature, the Curie point

(3) They have a domain structure, which may be visible

(4) They have a high dielectric constant, rising to a peak at the Curie point.

(5) The falling-off of their dielectric constant above the Curie point follows a Curie-

Wiess law.

(6) They possess a psuedosymmetric structure

(7) Their symmetry place them in a polar class

(8) They have a transition at the Curie point to a form of higher symmetry

(9) The Curie point is raised ( or a lower Curie point lowered) by the application of a biasing field

(10) There is a sudden appearance of surface charges at the transition

Point (1) is the most important characteristic of a ferroelectric material and based on this the definition of a ferroelectric is given as follows:

- 12 - “ A ferroelectric is a crystal possessing reversible polarization, as shown by a dielectric hysterisis loop”. The Curie-Wiess behavior of a ferroelectric material above the

Curie point (TC) is given by the equation:

? = C / (T- TC) (7)

Where ? is the susceptibility and C is the Curie constant. Dielectric and ferroelectric properties in materials are greatly dependent on microstructure and grain size considerations, these factors influence the development of domains and their subsequent polarization behavior in the crystal. These considerations will be looked at in greater detail with regard to the ferroelectric behavior of barium titanate.

1.3 Barium Titanate

The prototypical electronic material, barium titanate is one of the most well known materials used in research and industrial applications. The ferroelectric behavior of barium titanate was discovered by von Hippel5 and co-workers in America

(1946) and independently in Russia by Wul and Goldman5 (1945, 1946) to whom the first publication is due.

Barium titanate is isostructural with the mineral perovskite (CaTiO 3) and is referred to as a perovskite.6 This structure is found to be stable between the Curie point at

125°C and 1460°C7 above which a hexagonal non-ferroelectric structure is found. Below the125°C it undergoes three phase transitions accompanied by structural changes from cubic to tetragonal to orthorhombic and to rhombohedral. These transitions occur at

125°C, -5°C and -90°C respectively. These transformations are shown in Figure 1 (a).

Figure 1 (b) shows the corresponding changes in the lattice parameters for these transformations.

- 13 -

1 (a)

1 (b)

8 Figure 1. Properties of single-crystal BaTiO 3: (a) unit-cell distortion of the polymorphs (b) lattice dimensions versus temperature.8

- 14 - The 125°C transition is generally considered the most important in barium titanate. Figure 2 (a) shows the schematic representation of a cubic unit cell of BaTiO 3.

The Ba2+ and O2- ions combine to form a close-packed cubic structure with the smaller

Ti4+ ions occupying the octahedral interstices. The O2- ions are in 4-fold coordination with the Ba2+ ions. Each Ba2+ ion is surrounded by 12 O2- ions and the smaller highly charged Ti4+ ions are in 6-fold coordination with the O2- ions. This structure is observed in a number of perovskite oxides including CaTiO 3, SrTiO 3, CaZrO3, PbTiO 3 and

7 PbZrO3.

On cooling below 125°C the cubic structure assumes a tetragonal structure with a lower symmetry. This transformation elongates the unit cell in the c-direction and

8,9 contracts it in the a-directions. Tetragonal BaTiO 3 is polar and ferroelectric. Figure 2

(b) shows the displacements of the Ti4+ and O2- ions from their equilibrium positions in the tetragonal structure. These displacements result in development of dipole moments and spontaneous polarization. Each cell will then be associated with a permanent ionic dipole. The cubic symmetry at the phase transition allows dipoles to be formed in any of the six available directions in the newly formed elongated c-axis. Domains are regions within which the direction of polarization is the same.

The second displacive transformation in BaTiO 3 occurs at -5°C resulting in an orthorhombic structure. The final transformation occurs at -90°C resulting in a rhombohedral structure. It is useful to visualize these transformations based on the direction of the dipole in unit cell. In the tetragonal structure the dipole orientation is parallel to the c-axis of the unit cell. The dipole orientation in the orthorhombic state is parallel to the face diagonals in the unit cell while in the orthorhombic state the dipole is

- 15 -

2 (a)

2 (b)

Figure 2. (a) The unit cell of BaTiO 3, 2 (b) Approximate ion displacements in the cubic- 6 tetragonal distortion in BaTiO 3.

- 16 - aligned in the direction of the unit cell body diagonals.

1.4 Conduction in BaTiO 3

The conductivity of a material can be described as a function of energy gap between the valence and conduction bands in the material.2 Metals in general have high conductivities due to the overlap between the valence and conduction band. Insulators on the other hand have a wide band gap requiring higher energies for electron accession into the conduction bands. Semiconductors are an intermediate class of materials in which the value of the band gap is lower than those of insulators and thus conduction can occur by excitation of electrons into the conduction band. Further band gap modification in semiconductors can be effectively accomplished. This is extremely useful in designing and fabrication of devices with specific properties.

10 At room temperature BaTiO 3 is an insulator with a band gap of 3.3 eV which is very large compared to typical semiconductor materials such as pure silicon (1.1eV) and

11 germanium (0.67 eV). This leads to pure BaTiO 3 being an insulating material at room temperature. However semiconduction in BaTiO 3 can be achieved by various means such as sintering under reducing atmosphere and dopant additions. Topics dealing with semiconduction in BaTiO 3 are discussed in the following section.

Defect mechanisms play a major part in determining the conductivities in ceramic materials. Two types of defects found are intrinsic and extrinsic defects. Intrinsic defects are defects inherent in the pure material and are also knows as thermal defects. Intrinsic

6 defects can be of the Schottky or the Frankel type. Defect studies in pure BaTiO3 have shown vacancies to be the main defect mechanism.12 Further, simple considerations of

- 17 - the high packing density in the perovskite unit cell would lead to interstitial defects being unlikely.

Intrinsic conduction has been shown to occur in BaTiO 3 materials equilibrated in low partial oxygen pressures. N-type conduction at room temperatures in these materials is given by the equation (8)10:

.. OO = VO + ½ O2(g) + 2e’ (at low PO2) (8)

Under high oxygen partial pressures P-type conduction is expected to occur and has been observed at elevated temperatures10. At room temperature however P-type conduction has not been observed and the material is insulating. The conduction mechanism at high oxygen partial pressures is given below:

· ½ O2 = OO + VBa” + 2h (at high PO2) (9)

Intrinsic conduction mechanisms in BaTiO 3 are difficult to control due to the complex defect structures and uncertainties in impurities and stoichiometry of the material. Conduction in undoped BaTiO 3 materials arises out of a combination of oxygen vacancies, cation vacancies and associated electron-holes. The difficulties associated with intrinsic conduction mechanisms and their control favor conduction modes associated with extrinsic defects.

Compared to intrinsic defects in BaTiO 3, extrinsic defects are more important and effective in obtaining suitable semiconducting behavior. Extrinsic defects are formed by the incorporation of ‘foreign atoms ‘or dopants in the BaTiO 3 crystal lattice at either the

Ba or Ti sites. Two types of dopants commonly used are donors and acceptors. Donor dopants have a higher oxidation state than ions which they are substituted for in the

13 3+ 3+ 3+ 3+ 3+ lattice. Donors typically used in BaTiO 3 areY , Nd , La , Gd and Ce which are

- 18 - substituted for the large Ba2+ ions, while Nb5+ and W6+ are used as donors for substitution in Ti4+ sites.10,13 Acceptor dopants have lower ionic charge than the ions which they substitute in the lattice. In these cases Na+ and K+ ions cab act as acceptors for Ba2+ ions while Gd3+ and Mg2+ will do the same for Ti4+ sites.

Charge neutrality must be maintained in all cases and the charge compensation mechanism occurring in each case decides the resulting conducting or insulating behavior. Charge compensation in the case of donor can take place by either electronic compensation or vacancy compensation mechanisms.14 Electronic compensation at room temperatures leads to conduction while vacancy compensation is expected to result in insulating characteristics. Acceptor doping leads to compensation by oxygen vacancies and a corresponding increase in hole concentration.14

Trivalent ions are commonly used as dopants in semiconducting BaTiO 3 . Dependent on site incorporation (Ba or Ti) the following defect chemistries can occur as shown in equations 10, 11 and 1210:

. M2O3 + BaTiO 3 = 2MBa + 2e’ + BaO + 1/2 O2(g) (electronic compensation) (10)

. M2O3 +BaTiO 3 = 2MBa + VBa” + BaO (cation vacancy compensation) (11)

.. M2O3 +BaTiO 3 = 2MTi’ +VO + 2TiO 2 (anion vacancy compensation) (12)

Calculations have shown that electronic compensation is favored over cation compensation for donor doping at Ba sites. Cation vacancy compensation mechanisms are likely only at high (> ~0.25 mol%) dopant concentrations and high PO2. Oxygen vacancy compensation is favored over hole compensation in the case of Ti site substitution.15

An interesting occurrence in trivalent ions of intermediate ionic size between Ba2+

- 19 - and Ti4+ ionic radii is the operation of a self compensation mechanism. The equation for self compensation is shown below

. M2O3 + [BaTiO 3]lattice = MTi’ + MBa + BaTiO 3 (13)

Self compensation has been noticed in materials doped with Y3+ which has an ionic radius of 0.93Å between the ionic radii of Ti4+ and Ba2+. Stoichiometric variations in BaTiO 3 can affect the dopant behavior where in the Ti-excess materials it acts as a donor, whereas in the presence of Ba-excess, acceptor behavior is induced.15,16 This self compensation behavior has been suggested as a possible explanation for resistivity

15 dependence on donor concentration in BaTiO 3 ceramics.

As seen from the previous paragraphs, conductivity in BaTiO 3 ceramics is controlled by charge carrier type, carrier mobility and carrier concentrations. Electronic compensation mechanisms result in the lowest resistivities between 0.5 and 5.0 ohm

17 cm. Optimally donor doped BaTiO 3 is dominated by electron compensation. These samples display a dark blue color which is believed to arise from the reduction of Ti4+ to

Ti3+. Polaron conduction is said to be associated with this behavior where splitting of 3d energy levels into sublevels results in reduction of Ti4+ to Ti3+ with charge transfer taking place through the 3d conduction band.18-20 The Ti3+ polaron creates a lattice distortion, strain field and polarization field around the neighboring lattice. Charge hopping mechanisms may allow for electron excitation from the Ti3+ to a neighboring

Ti4+ site.21 The energies associated with polaron conduction are lower compared with the energy required for electron excitation into conduction bands, thus favoring polaron conduction modes in these materials. The donor compensation equation for polaron compensation is shown below:

- 20 - 3+ . M + BaTiO 3 = MBa + TiTi’ (polaron conduction) (14)

Figure 3 shows a “U” shaped resistivity-concentration curve for Ce3+ donor doped

22 BaTiO 3. It can be seen from the figure that for dopant concentration levels ~0.3atom% a sharp rise in resistivity is noticed. Electronic compensation modes are replaced by cation vacancy compensation causing the sharp resistivity rise. A number of explanations have been proposed for this change in compensation modes. Wernicke attributes the change in compensation modes to grain growth inhibition caused by overdoping. These changes along with insufficient sintering could affect donor site occupancy with donors occupying both Ba2+ and Ti4+ sites resulting in a compensation mode shift.23 Another interesting theory proposed by Anderson is based on dominance of compensation modes dependent on the dopant concentration. At low concentrations donor spacing in the lattice will be relatively large. Vacancy (VBa) compensation will be unable to effectively neutralize the donor charge, favoring electronic compensation. With increasing dopant concentration donor spacing will decrease favoring vacancy (VBa) compensation modes.

1.5 PTCR effect

The positive temperature coefficient of resistance (PTCR) effect is a phenomenon which is observed in donor doped semiconducting polycrystalline BaTiO 3 ceramics. A transition from semiconducting to insulating behavior occurs at the Tc of the material with a jump in the resistivity by several orders of magnitude. Semiconduction at room temperature has been attributed to the presence of ferroelectric spontaneous depolarization charges in the grain boundary and polaron conduction modes, which arise from electronic compensation mechanisms in donor doped materials. Acceptor doped materials do not display PTC characteristics. PTCR behavior has not been noticed in

- 21 -

Figure 3. Resistivity-Temperature characteristics showing the narrow range of dopant concentration which results in low resistivity, semiconduction.22

- 22 - single crystals24 and thus is thought to arise from grain boundary interactions in optimally processed polycrystalline material. Figure 4 shows the PTCR effect.

This phenomenon has been extensively used in industry with applications as PTC providing over-current protection in electronic circuitry. Other applications of

PTCR materials are in flow sensing and temperature sensing devices.25 PTC materials in industry may attain as many as eight orders of rise in resistivity from room temperature resistivity.

Various models have been put forth in attempting to demystify the PTCR phenomenon. To date there is no unified theory, which explains this phenomenon. A number of factors make this a difficult task. The PTCR phenomenon is a complex interaction of mechanisms involving ceramic semi-conduction, grain boundary effects, ferroelectric crystal effects, defect mechanisms, microstructure dependence, to name a few. A unified theory, which takes into consideration all the interdependent parameters and provides good correlation with experiment, is indeed a difficult task. Further complications arise out of contrasting explanations and models found in literature.

Complexity notwithstanding, a broad understanding of the PTCR effect is necessary if one must attempt to work with and improve on existing materials and technology. Further an understanding of the PTCR effect can help in looking for new applications for these unique materials. The following sections will deal with the PTCR effect based on models, which have been widely accepted as the basis of the PTCR effect. These models are based on the work of Hewyang27 and Jonker.28 Mention will be made where found necessary of work, which attempts to build on the above models.

Discrepancies in these models will also be noted. More detailed explanations can be

- 23 -

26 Figure 4. Typical resistivity behavior of BaTiO 3-type PTCR materials .

- 24 - found in literature.

The widely accepted Heywang27 model assumes the formation of acceptor states at the grain boundaries of the semi-conducting grains. Acceptor states at the grain boundaries in semi-conducting grains result in a space charge layer at the boundaries. The barrier potential developed at the space charge region is a function of the dielectric constant at the grain boundary. The influence of space charges on the barrier in a grain boundary layer is shown in Figure 5.29 The potential barrier height can be determined from the equation:

2 2 f = e ns / 8 eNd (15)

Where f is the barrier height, e the electronic charge, ns the density of trapped electrons, e the dielectric constant of the grain boundary and Nd is the bulk dopant concentration. The dependence of dielectric constant e on the temperature beyond Tc is given by the Curie-Wiess dependence

? = C / (T-Tc) (16)

where ? = e-1

From equation (16) we can see that with increasing temperature beyond Tc the dielectric constant e will decrease resulting in an increase in the potential barrier heightf.

The resistivity dependence on the barrier height is given by the equation

R = R0 exp(f/kT) (17) where R is the resistance of the grain boundary, R0 is constant, k is the Boltzman constant and T the temperature. The drastic rise in resistivity and hence the PTCR can be are related to the rapid fall off in the dielectric constant past Tc. This can be seen as the origin of the PTCR effect.

- 25 -

Figure 5. Heywang PTCR model-a two dimensional potential barrier at grain boundaries.27

- 26 - The Heywang27 model is based on a two dimensional layer of acceptor states being present at the grain boundary. Hewyang assumes “BaTiO 3 to be a linear and isotropic dielectric” which in the case of BaTiO 3 would only occur in the paraelectric cubic state in the material beyond Tc. This assumption limits the usefulness of the model to only beyond the Tc stage. The low room temperature resistivities cannot be sufficiently explained by the Heywang model. The Jonker28 model which builds on the work by

Heywang provides a good explanation for low resistivity semiconducting behavior in the material below the Tc.

The Jonker model is based on the ferroelectric behavior of BaTiO 3 below Tc. The spontaneous polarization in domains near the grain boundary regions neutralize built up charge at the boundaries allowing for charge transport across the grains. Figure 6 shows the effect of domains at the grain boundary regions. The normal component of the spontaneous polarization in the domain acts to compensate space charge buildup at the boundary affecting a decrease in the potential barrier height.30 The barrier height is now given by the equation:

2 2 2 f = (e ns – ?PN ) / 8eNd (18)

where PN is the spontaneous polarization all other values being the same as before. The importance of the Jonker refinement is the fact that equation (18) is of a more general nature which not only takes into consideration below Tc behavior of the material based on the spontaneous polarization in the ferroelectric state but that it is also applicable in predicting above Tc behavior in the material where Ps will become 0 in a non-ferroelectric state.

A number of the refinements which have been made to these existing theories on

- 27 -

Figure 6. Jonker PTCR model- domains at a grain boundary between two crystallites of different orientation in compensating surface charges. 30

- 28 - the PTCR effect. Daniels and Wernicke31 proposed an excess of barium cation vacancies to be responsible for acceptor states generated at the grain boundaries. These acceptor states are thought to extend in a three dimensional manner, forming inward from the grain boundary, unlike the original two dimensional Hewyang model. Desu and Payne32 have suggested a change in the compensation mechanism at the grain boundaries caused by donor segregation at the boundaries. Roseman33,34 has shown the existence of fine domain, no domain regions near the grain boundary in annealed samples, suggesting that electronic charge transport occurs across the grain boundaries through the fine domain regions. The abrupt rise in resistivity in annealed samples is attributed to the breakup of the fine domain regions above Tc.

1.6 Processing effects

The previous sections have given a background on barium titanate, the material its properties and the PTCR phenomenon and its applications in these materials. Obtaining a material with a high PTCR rise and good room temperature properties is dependent on a number of factors. Careful selection of starting powders and dopant additions in the right concentrations are necessary to obtain the required properties. Milling processes and their control lead to fine particle size and homogeneous distribution of dopants and additives.

Sintering parameters must also be carefully chosen and controlled to obtain optimal density and uniform microstructure in the final material.

Processing PTCR materials require careful control over the above mentioned factors. An understanding of the mechanisms involved in each stage of processing is critical in obtaining materials with the desired properties. The following sections will deal with these areas related to processing effects on the systems. Processing effects can

- 29 - be divided into two major areas namely dissolution effects, and sintering and microstructure effects.

Dissolution effects look at the changes occurring in the particles during solution processes such as milling. Dissolution occurring in the system can affect the surface states of the particles. These effects are influenced by a number of factors such as the milling system, the additives involved, temperature, etc. It is necessary to understand dissolution in ceramic systems in order to optimize process parameters and obtain final powders with homogenous properties.

Densification and grain growth during high temperature processes determine the final microstructures of the material which can be largely said to determine the electrical and mechanical properties of the material. Sintering and microstructure studies look at the behavior of the powder compacts during high temperature firing. In order to obtain materials with uniform microstructures and properties it is important to look at its behavior during these stages.

1.6.1 Dissolution Studies

Aqueous dissolution mechanisms in perovskite oxides are extremely complex.

These mechanisms are highly sensitive to surface reactivities, dopants or impurities, solution concentration, pH and temperature variations. Changes in these conditions can cause instabilities in the oxides and their properties. This following sections will focus on obtaining an enhanced understanding of dissolution phenomena in perovskite oxides, with the ultimate aim of improving the processing and electronic properties of barium titanate based materials. Dissolution in various perovskite systems along with the proposed mechanisms will be reviewed.

- 30 - The following section will look at the dissolution of ceramic oxides with a focus on perovskite dissolution. Perovskite dissolution mechanisms are complex and are very often due to the vastly differing properties of metal cations in the lattice. A brief review of the thermodynamic and kinetics involved is given below.

1.6.2 Thermodynamics

Phase diagrams of systems containing metal oxides, water, and dissolved chemicals in the fluid phase are of a complex nature. However considering the only solutes to be mineral acids or bases, the thermodynamic information can be represented

35,36 accurately by means of solubility curves. For a MO n oxide, the solubility product

35 quotient Qs is derived from equation (19) :

+ 2n+ MOn(s) + 2n H = M (aq) + n H2O (19)

where

2n+ + Qs = [ M ] / [H ] (20)

Square brackets represent the molar concentration of the species. True constants must be written in terms of ionic activities. It must however be kept in mind that the above equation is a very simplified approach to solubility in aqueous systems. Hydration in real systems are complex reactions involving monomeric ions and polymerization equilibria. The total solubility ‘S’ is given by equation (21)35:

(2ny-x)+ S = SySx y [My(OH)x ] (21)

Thus the total solubility can be determined, given the hydrolysis quotients of relating concentrations of all relevant species to the concentration of M2n+. Equation (21) takes into account polynuclear hydrolysis of the cation, which is critical to understanding hydrolysis mechanisms of cations. The solubility product constant Ks is a simple

- 31 - correlation of Qs corrected for activity coefficients and is related to the charge 2n and the

2n+ size of M (represented by the interatomic distance d in MO2). The equation is expressed as 35:

2 Log Ks = -5.6-2nA – 11.0[(2n) /d] (22)

Where ‘A’ is an empirical constant used in classifying ions based on their ‘hard’ and ‘soft’ nature. Highly non-hydrolyzable ions exhibit high and negative values of ‘A’.

As the soft character of the ion (linked to the degree of covalency of the M-O bonds in the oxide) increases, ‘A’ values become less negative. Table 1, below, classifies various metal ions into groups giving values of ‘A’.37

1.6.3 Solubility in ternary and more complex oxides

In a four component system containing MO, NO, H2O and HCl as with aqueous mineral solutions at a fixed temperature and pressure, the phase relationship is given by

F=3-P.35 Therefore not more than two solid phases can coexist at equilibrium. Thus the concentrations of M2n+ and N2m+ are fixed by this relationship. Increase in concentration of one phase will lead a decrease in the concentration of the other phase. The solubility of

35 MNOn+m will be given by the equilibrium equation (23) :

+ 2n+ 2m+ MNOn+m (s) + 2(n+m)H = M + N + (n+m) H2O (23)

where

2n+ 2m+ + 2(n+m) Ks = [M ][ N ]/ [H ] (24) Ternary oxides with useful properties are generally combinations of low soluble binary oxides MOn with other more soluble metal oxides NOm. This leads to preferential leaching of NOm and formation of other solid phases.35

- 32 -

Table 1. The stability of the MOH(Z-1)+ species of cations.37

- 33 - 1.6.4 Kinetics

Kinetic considerations in dissolution mechanisms are of paramount importance due to the inherent lack of stability of oxides in aqueous solutions.38 Different theories are required to explain kinetics operating between initial, advanced and near-saturation conditions in different oxide categories. Further distinctions need to be made in fast and slow rates in the same category. Kinetic dissolution mechanisms have been classified into

5 regimes. A brief description of these regimes is given below.38

Regime I: dissloution kinetics occurs under near-saturation (equilibrium) conditions. Redeposition conditions are taken into consideration and the Nernst theory is applicable under these conditions. Regime II: diffusion kinetics are characterized by dependence of rate on stirring speed. In this regime the rate is proportional to the diffusion coefficient of the diffusing species. Regime III: rates are in the range of 10-5-

10-8mol m-2 sec-1. These are characteristic of fast dissolving semiconducting oxides.

Important parameters controlling the kinetics are the ionicity of surface atoms, valance states of cations, surface hydration, as well as properties of the solution like pH, redox potential and ionic strength. Regime IV: characterized by rates in the range of 10-8-10-11 mol m-2 sec-1 and found in slowly-dissolving semiconducting oxides. The rate control mechanism is the charge transfer process to the surface in order for ion formation of metals and oxides to occur before going into solution. Controlling properties for this regime are the ion formation at surface defect sites and solution redox potential. Regime

V: dissolution characteristics are exhibited by covalent insulating oxides such as TiO 2,

SiO 2 and Al2O3, which dissolve faster in alkaline solutions. The acidic dissolution rates are slow <10-11 mol m-2 sec-1. The control mechanism is the hydroxide ion attack rate

- 34 - leading to the rupture of the metal-oxygen bond, i.e., base-catalyzed hydrolysis. Major dissolution mechanisms referred to in these kinetic regimes are shown in Table 2, below.

Aqueous dissolution kinetics are strongly affected by surface area/ volume ratios.

According to Blesa35 heterogeneous reactions proceed at rates that are proportional to the available instantaneous surface area S:

-(dn/dt) = R S (25) n being the number of moles of the solid phase undergoing transformation and R the specific rate.

1.6.5 Dissolution Mechanisms in BaTiO 3-type materials

Nesbit et al.39 have discussed the thermodynamic stability and kinetics of dissolution of perovskite materials in aqueous solutions containing dissolved minerals and the naturally present CO2. This dissolved CO2 can lead to instability in CaTiO 3, shown by equation (26):

CaTiO 3 + CO2 (g) ? TiO 2 + CaCO3 (26)

For this equilibrium relation

?GR° = -RT ln K = + RT ln PCO2 (27)

The other dissolution reaction proposed is the dissolution of perovskite in water, given by equation (28):

+ 2+ CaTiO 3 + H ? Ca (aq) + TiO 2 + H2O(l) (28)

Both ?GR and K have been calculated where

Log K = log [Ca2+]/ [H+] (29)

The leaching experiments were performed on samples of: 1) single crystal Ca-

39 perovskite; 2) Ca-perovskite mineral, and; 3) synthesized SrTiO 3 and BaTiO 3 samples.

- 35 -

Table 2. Categorization of oxides and the relation to dissolution processes.38

- 36 - Leaching experiments were conducted over a range of temperatures from 25 to 300°C using stainless steel autoclaves. Results show a strong dependence of leach rates of all types of samples on temperature, with rates at 100°C being markedly larger than rates of similar samples at higher (300°C) and lower (25°C) temperatures. The results also show good correlation between leach rates for the crystalline perovskite samples, with all having the same order of magnitude. There is, however, a marked difference between leach rates of the single crystal and polycrystalline mineral sample of Ca-, although XRD peaks show good comparison. Though the authors39 are unable to explain this difference in leach rates, some interesting explanations are proposed to account for the increased leach rates at 100°C. Thermodynamic equilibrium concentrations of Ca2+ ions in the solution, at 100° and 300°C are 7x106ppm and 200ppm, respectively. The large leach rates can be attributed to the larger equilibrium concentration at 100°C.

Another important factor in low leach rates at higher temperatures is attributed to the lower solubility of Ca(OH)2, which precipitates out of solution. Similar observations have been made on Ba and Sr perovskites with the help of SEM techniques. X-ray diffraction and electron spectroscopy techniques have confirmed these observations.39

Myhra et al.40 have come to conclusions similar to those of Nesbitt et al.39 confirming the fact that the presence of CO2 in water, along with hydrothermal conditions, leads to leaching of surface layers in titanate materials, resulting in the formation of aqueous carbonates. The leached surface resembles a TiO 2 type surface.

Further conclusions based on the dissolution mechanism state that on attaining equilibrium the TiO 2-like surface has a high stability in aqueous solutions.

Dissolution studies of carried out by Turner et al.41 examine two

- 37 - proposed dissolution models. The first model proposes dissolution based on Ca2+ leaching via ion-exchange from the TiO 2 lattice resulting in breakdown of the lattice and formation of an amorphous TiOH film. The second model is based on complete

2+ dissolution of the CaTiO 3 lattice resulting in Ca ion release into the solution and preferential nucleation of TiO 2 crystals on underlying perovskite sites. Hydrothermal investigations of leaching have provided conclusive evidence of the second model. The authors, using TEM studies on leaching in ambient to 100°C temperatures have shown the formation of the amorphous layer predicted by the first model, thus conclusively proving its existence.

Dissolution mechanisms involving hydrolysis of the cation species are generally attributed to an ion-exchange reaction with H+ in the solution.41 Base-catalyzed

- hydrolysis whereby OH reacts with the TiO 2 lattice can also result in dissolution. The authors41 have proposed a model for perovskite dissolution taking into account pH conditions, Ca2+ equilibrium concentration and temperature effects on dissolution mechanisms. This convincingly relates the two models of dissolution proposed earlier.

The proposed dissolution mechanism is shown Figure 7. The covalently bonded titanate lattice contains Ca2+ ions within the interstices (Fig7(a)). In aqueous solution with the solution concentration of Ca2+ sufficiently low (= 10-5 mol dm-3) the Ca2+ ions will be released into solution (Fig.7(b)). At a sufficiently high pH (9.5) base-catalyzed hydrolysis occurs resulting in the dissolution of the Ti-O lattice and possible hydroxylation

(Fig.7(c)). The breakdown of the titanate lattice results in further dissolution of the Ca2+ ions into solution (Fig.7(d)). Hydrothermal conditions cause the nucleation of Ti(OH)x species and formation of anatase and brookite. Epithermal conditions (ambient to 100°C)

- 38 - however do not allow formation of TiO 2 resulting in an amorphous Ti(OH)x layer precipitating out of solution due to the low solubility of the hydroxide. This amorphous film is sufficient to prevent further Ca2+ dissolution into solution. This mechanism suggests that the TiOH surface is not necessarily a highly stable surface as suggested by

Myhra et al.40, but the stability of the surface is dependent on temperature, eqilibrium concentrations of the cations, and solution pH.

The literature reviewed thus far has examined perovskite dissolution mechanisms, emphasizing their stability in natural aqueous environments. Preferential dissolution in perovskite oxides could also lead to changes in stoichiometry at surfaces, in effect altering the processing behavior and properties of the materials.42,43 For example

Titanium excess BaTiO 3 materials can exhibit abnormal grain grown during sintering, as shown in Figure 8.

The lack of thermodynamic stability in the BaTiO 3 system during aqueous processing has been observed by a number of researchers.38-43 This can severely affect processing and properties of barium titanate ceramics and perovskite type oxide materials in general. The equation for BaTiO 3 dissolution under alkaline conditions is shown below

2+ - BaTiO 3 + H2O = Ba (aq) + TiO 2 (rutile) + 2OH (30)

Under acidic conditions dissolution will occur by the following reaction

+ 2+ BaTiO 3 + 2H = Ba + TiO 2 (rutile) + H2O (31)

The estimation of dissolved Ba2+ ions can be made by setting the activities of neutrals and solid species to one. This equation is then given by

[Ba2+][OH-]2 =K (32)

- 39 -

(a)

(b)

(c)

(d)

Figure 7. Perovskite dissolution mechanism. (a) Initial surface. (b) Ca2+ dissolution from the surface (c) Base-catalyzed hydrolysis of titanate lattice. (d) Further Ca2+ dissolution.41

- 40 -

Figure 8. SEM photomicrograph (400X) of the as fired surface of TAM HPB milled at a pH of 6 before drying and fired at 1310°C.42

Figure 9. Barium leaching during milling as a function of solution pH.42

- 41 - and

Log K = Log[Ba2+] – 2Log[OH-] (33)

Or

Log[Ba2+] = 20.08 – 2pH (34)

From the point of view of an electroceramist, dissolution studies are of great importance, as an insufficient understanding of these mechanisms can cause adverse effects during processing, leading to drastic changes in the electronic properties of these

42 materials. Anderson et al. have studied the surface chemistry of BaTiO 3 and effects of processing additives on barium dissolution and microstructure in the sintered material.

An important aspect considered is the pH effect on dissolution. Data shown in Figure 9 clearly points to the increased dissolution of barium at low pH values (=6).42Barium dissolution reduces considerably at higher pH (˜12) but never completely stops. This shows good agreement with the studies carried out by Turner et al.41, thus giving good validity to the proposed dissolution mechanism.41Dissolution is unobserved when non- aqueous milling media are used for processing.42 Further, aqueous binders are shown to cause highly exaggerated grain-growth compared to materials processed with non- aqueous binders. These authors note that aqueous binders increase the barium dissolution in solution as compared with non-aqueous acrylic binders. A graph of exaggerated grain growth versus processing conditions is shown in Figure 10.42

Exaggerated grain-growth in sintered materials is attributed to the possible interactions of redeposited Ba ions with the Ti rich surface (in unfiltered processed samples) and the subsequent creation of a liquid phase during sintering.42 Samples which have been filtered before drying show a lesser tendency towards exaggerated grain-

- 42 - growth. Milling effects on the particle surface are shown schematically in Figure 11.42

The authors42 have not taken into consideration the carbonate impurity concentration and its adverse effects on dissolution as shown by Nesbit39 and Myhra.40

Blanco et al.43,44 have also studied the dissolution mechanism in detail from a pH point of view, concentrating on the effects of stoichiometry and aqueous interactions in

BaTiO 3. An important point made in these studies is the effect of carbonate impurities present in the powder. Carbonates can be present in concentrations as high as 3% and can significantly affect the solution chemistry of the titanate powders. Industrial grade powders have been used to conduct the leaching studies, with one of the powders used containing Nb as a dopant. This allowed the comparison of dissolution in undoped and doped powders. Leaching experiments were carried out on the powders as well as sintered discs of the various powders. Conclusions reached by the authors are similar to those reached by Turner et al.41 Surface depletion to the extent of 20nm is noticed in the leached sintered disks of the powder, while powders subjected to leaching do not show this tendency. This suppression of hydrolysis in the powders is attributed to the high concentration of Ba2+ present in the solution released by carbonate contamination of the powders. The study goes so far as to attribute dissolution of Ba2+ in the solution to the carbonate impurity and not due to the dissolution of the BaTiO 3. Another interesting observation made by the authors is the lowered dissolution of the Nb-doped sintered discs. This different behavior was attributed to grain boundary related phenomenon affecting Ba2+ depletion at the boundaries.

- 43 -

Figure 10. The amount of exaggerated grain growth as estimated from micrographs of the as fired surface versus processing conditions.42

Figure 11. a, b, and c. Effect of milling on the barium titanate grain structure.42

- 44 - 1.7 Sintering behavior and microstructure effects in BaTiO3

The previous sections have dealt processing effects occurring in BaTiO 3 materials during the solution processing stages and the effects of dissolution on these materials.

The following section will look at the sintering behavior found in BaTiO 3 materials and their effect on the microstructure evolution and growth and properties.

One of the most important stages in processing PTCR materials is the sintering stage. Processed powders are compacted into a porous compact and fired at high temperatures to obtain a high-density body with the required grain size and microstructure. Depending on the processing methods further heat treatment cycles may be performed to modify the defect states and internal stresses in the material. These treatments generally do not affect the grain size and microstructure obtained during sintering.45 Several studies have shown a direct relation between the microstructure and final material properties closely linking the two. Thus sintering processes are a critical step in determining the final properties of the material. PTCR materials can be very sensitive to minor changes during processing. Dopant additions, their distribution and incorporation playing an important part during sintering and can seriously affect the microstructure and properties of the material. An understanding of sintering mechanisms can help in improving microstructures and material properties. Stricter control can also lead to better reliability and life in these materials.

1.7.1 General Sintering Theory

Sintering can be described very broadly as a high temperature process during which significant physical and chemical changes occur, resulting in the transformation of

- 45 - low strength, porous compact into a dense body. Densification and grain growth are two important characteristics which occur during sintering.

The major driving force in operation during sintering is the reduction of free energy in the system caused by the overall decrease in surface area. This in turn is the result of densification occurring due to the reduction of the solid vapor interface.46 The major processes said to occur during sintering are the change in shape and size of the grains as well as the pores in the system.

The two most widely employed mechanisms in sintering are, solid state sintering and liquid phase sintering. Solid state sintering as the name implies is the result of sintering materials through solid-solid interactions in the compact. Liquid phase sintering on the other hand relies on the generation of a liquid on the material during sintering and relies on the liquid phase to obtain the final microstructures in the liquid. The liquid phase generated can result either from variation in the stoichiometry of the system as is common with Ti-rich BaTiO 3 systems, or on the effect of dopant addition which form liquid at low temperatures. Given below are some important points which can improve the sintering behavior of the system:

1) Selection of high purity starting powders with small particle sizes and narrow distribution. The particles should be non-agglomerated and equiaxed.

2) Selection of suitable dopant additions which enhance the densification trend in the system and act as grain growth inhibitiors is important.

3) Choice of suitable systems for solution processing is necessary. The system chosen must provide uniform distribution of dopants and additives, while minimizing preferential dissolution mechanisms allowing for homogenous particle surfaces.

- 46 - 4) Defect free green body compacts

5) An effective sintering schedule which can suitably modify densification and grain growth kinetics.

The following sections will elaborate further on solid state and liquid phase sintering.

1.7.2 Solid state sintering

Three stages are said to occur during solid state sintering. The initial stage leads to shrinkage and neck-growth facilitated by a collusion of material transport mechanisms which have been mentioned earlier. This is followed by the intermediate stage during which the material assumes the form of a compact with continuously linked pores, all allowing for outward diffusion of gases to take place. The final stage involves sintering of the compact in which pore closure has occurred and the material no longer contains a continuously linked pore network. Densification mechanisms in solid state sintering are attributed to the latter two stages with volume diffusion and boundary diffusion processes being the primary transport processes involved in densification.

1.7.3 Liquid phase sintering

Liquid phase sintering requires the generation of an optimum amount of liquid in the system. This can be achieved by stoichiometric variation on the system as in the case of liquid phase sintering in Ti-rich BaTiO 3 systems. Liquid phase generation can also be attained by additions of liquid inducing dopants to the system.

The liquid generated must have the necessary wetting characteristics in order for uniform distribution and wetting of the particles in the system. Figure 12 is a schematic

- 47 - of the wetting behavior exhibited by a liquid. The contact angle equation given below can be used to determine the wetting characteristics of the liquid:

Cos? = (?sv - ?sl) / ?lv (35)

Where ?sv is the solid-vapor interface energy, ?sl is the solid-liquid interface energy and ?lv the liquid-vapor interface energy. A low contact angle is indicative of good wetting behavior in the system, with uniform liquid distribution in the system and good spreading of liquid on the particle surfaces. Ideal wetting occurs with a contact angle of zero. Systems which exhibit large contact angles exhibit poor wetting characteristics with little or no spreading of the liquid and liquid accumulation in isolated regions, resulting in poor liquid phase sintering characteristics.

Liquid phase sintering mechanisms are considerably different from those exhibited by solid state sintering. Assuming good wetting behavior has been established sintering is said to occur through the rearrangement, solution-reprecipitation and coalescence in this order. The rearrangement phase is facilitated by liquid penetration into and around the particles. The liquid penetration allows for break-up of larger particles and dissolution of edges and particle-particle bridges. Capillary forces acting on the particles leads to better packing and arrangement during liquid phase sintering. The rearrangement and packing is also influenced by the amount of liquid phase generated and the solubility of the solid phase in the liquid. The solution-reprecipitation stage involves particle transfer by dissolution of small particles into the liquid and reprecipitaion on the larger particles. Diffusion occurring through the liquid phase is extremely rapid and can 100-1000 times quicker that solid-state diffusion processes.47

Reduction in the number of particles leads to densification. Coalescence and grain growth

- 48 -

Figure 12. Schematic of liquid configuration on solid substrate. 47

- 49 - mechanisms occur during the final densification stages.

Control and prediction of liquid phase sintering behavior is a difficult task owing to the difficulties present in understanding the liquid behavior, compositional changes occurring in the liquid and the nature of interaction between the solid and the liquid in system.47 Empirical studies on specific systems and their liquid phase behaviors are necessary in order to obtain consistent sintering behavior in the system.

1.7.4 Sintering behavior of BaTiO3

Microstructural development and sintering behavior of BaTiO 3 has been the topic of considerable research due to its industrial significance. The behaviors of these materials are very sensitive to compositional parameters such as the stoichiometry, dopant type and concentration and on the impurities in the starting materials. Further the sintering behavior can be influenced by factors such as atmosphere, temperature, and heating rates.

The sintering behavior of BaTiO 3 materials will be dealt with in the following sections. BaTiO 3 materials exhibit a close relationship to the Ba/Ti ratio of the material.

Distinct sintering characteristics are found in Ba-rich (Ba/Ti ratio >1.0) and Ti-rich

(Ba/Ti ratio< 1.0). The following sections will thus focus on understanding the microstructural development and sintering behaviors of these systems separately.

The BaO-TiO 2 phase diagram is shown in Figure 13 shows the formation of

BaTiO 3. It is important to understand the effects of slight excesses of BaO or TiO 2 in this region. A slight excess of TiO 2 leads to the formation of Ti-rich liquid phase Ba6Ti17O40 above the eutectic temperature of 1320°C. Slight Ba-excess material at temperature below 1450°C however leads to the formation of an insoluble second phase Ba2TiO 4.

- 50 - These excess effects can lead serious alterations in the sintering behavior of the materials as will be discussed below. Further the quantities involved in triggering these mechanisms are extremely small. For example, systems with = 0.1mol% BaO may be

48 sufficient in formation of the insoluble Ba2TiO 4 second phase in Ba-excess materials.

Abnormal grain growth during sintering can be disruptive to the microstructure evolution of the system. Being a high occurrence phenomenon in BaTiO 3 materials, it is appropriate to devote some attention to it. Dopant materials play an important part in most BaTiO 3 systems exhibiting useful properties and applications. The effects of dopants will be discussed where they appear to be of consequence.

A clear definition of excess behavior is necessary before moving to the sintering

49-51 behavior in BaTiO 3 materials. Previous research has shown that materials which are close to stoichiometry, in the range 0.997 = Ba/Ti = 1.005, exhibit sintering behaviors uncharacteristic to the attributed Ba/Ti ratio (Ba-rich or Ti-rich). It is expected that within this range, localized regions may not exhibit Ba/Ti ratios representative of the overall system. It is likely that small Ti-excess areas may co-exist with small Ba-excess areas in these localized environments giving rise to the unexpected sintering behavior within this narrow range. Liu and Roseman49 showed that true Ba-excess behavior may only be expected in systems with Ba/Ti ratios >1.005. It is important that these ‘close to stoichiometric effects’ must be kept in mind, as most of the Ba/Ti ratios looked at in the present study are in this stoichiometric ‘gray region’. This in mind, an understanding of the basic sintering behaviors exhibited in true Ba-rich and Ti-rich systems can be useful in comprehending the sintering behaviors of near stoichiometric systems.

- 51 -

52 Figure 13. The phase diagram of BaO-TiO 2 system.

- 52 - 1.7.5 Abnormal grain growth

The term abnormal grain growth generally refers to microstructures exhibiting discontinuous grain growth characteristics in which a small fraction of grains grow to a large size while consuming the uniform small grained microstructure. It is also referred to as bimodal growth and is easily distinguished by a microstructure exhibiting large grains in a small grained matrix.

Abnormal grain growth is generally attributed to Ti-rich BaTiO 3 systems where the formation of a liquid phase Ba6Ti17O40 above the 1320°C eutectic is thought to trigger rapid densification and discontinuous grain growth.53

A useful analogy can be made to metals where normal grain growth mechanism are characterized by a primary recrystallization mechanism while abnormal grain growth is attributed to a secondary recrystallization process. The driving force for secondary recrystallization is thought to be the lower surface energies attained by the large grains when compared with the higher surface energy smaller grains.

While abnormal grain growth behavior is usually attributed to liquid phase sintering effects in BaTiO 3, recent studies have shown that abnormal grain growth can occur well below the eutectic temperature via solid state sintering and diffusion mechanisms.54,55A preferential twinning mechanism has been suggested whereby lowering of surface energies in the single or doubly twinned (111) and (100) grains is said to assist in abnormal grain growth.54,56

Abnormal growth can also result in grain growth inhibition leading to extremely fine grained microstructures. This has been seen in Ti-rich systems which have been sintered below the eutectic. It has been suggested by Rios et. al. that, below the eutectic

- 53 - excess TiO 2 can act as a pinning force on the surrounding grains suppressing normal grain growth behavior.48,57

It is important to note that while abnormal grain growth is thought to result in bimodal microstructures, it is possible to obtain a uniform microstructure similar to one obtained by normal grain growth, the distinction being the operation of secondary recrystallization processes during abnormal grain growth modes. It is possible to obtain uniform microstructures through abnormal growth processes by control and distribution of seed grains. Zhang, Roseman & Mukherjee has shown that another effective means of controlling abnormal behavior is by the use of blended systems.51

1.7.6 Sintering behavior and microstructures in Ba-excess BaTiO 3

Ba-excess BaTiO 3 systems in general can be characterized by normal grain growth with uniform microstructure development below 1400°C. The barium orthotitanate Ba2TiO 4 second phase is thought to act as a grain growth suppressor. BaO even in amounts in the =0.1mol% range can be effective in suppressing abnormal grain growth and enhancing densification. Typical solid state sintering behavior is found to exist in these systems.

Donor doped Ba-excess systems also exhibit inhibited grain growth suggesting a combination of Ba-excess and donor inhibitions taking place in the system. Further doped

Ba-excess systems exhibit highly insulating behaviors, which is thought to be caused by poor donor incorporation, due to the solid state sintering mechanisms in play. It has been found that addition of liquid phase generating dopants such as SiO 2 result in a reversal to semiconducting behavior. Doped Ba-excess systems with SiO 2 addition show semiconducting behavior and PTC characteristics.58

- 54 - 1.7.7 Sintering behavior and microstructures in Ti-excess BaTiO3

While abnormal grain growth is considered to be a inherent characteristic in Ti- excess BaTiO 3 systems, samples sintered below the eutectic around 1240-1280°C show normal grain growth behavior.59 This suggests that normal grain growth characteristics exist in Ti-excess systems at the initial sintering stages. Samples sintered between 1320-

1350°C in which sintering has been interrupted show bimodal microstructures with large grains embedded in the fine grained matrix.59 Abnormal growth characteristics appear to dominate in this range of temperatures taking over from normal grain growth modes.

Samples sintered above 1350°C exhibit uniform microstructure with large rounded grains. This may result from the large grains in the system consuming the fine grained matrix and impinging upon each other. The final grain sizes are inversely proportional to the logarithm of the heating rate, which is characteristic of secondary recrystallization.

An interesting behavior found in donor-doped Ti-excess systems is that, regardless of dopant type, a uniform fine grained microstructure is observed around a critical donor concentration range ~0.3mol%. Around the optimum donor concentration grains are in the 1µm range.60,61 Abnormal grain growth is completely suppressed.

Further the materials exhibit good electrical behavior, being semiconducting around room temperatures and exhibiting good PTC characteristics. This behavior suggests that uniform dopant incorporation has been achieved in the system. The liquid phase sintering mechanisms in Ti-excess systems appear to have a beneficial effect with regard to dopant distribution.

- 55 - 1.8 Motivation for Research

Industrial processing of PTCR materials generally tends towards working with Ti- excess BaTiO 3 away from the stoichiometric region. The reasons being the sensitivities of these materials in the stoichiometric region and their unpredictability during processing and sintering lead to abnormal grain growth and poor microstructure and electrical properties of the resulting material. Ti-excess materials result in liquid phase sintering mechanisms dominating and providing good dopant distribution and lower temperatures. Further these materials are generally processed with liquid phase inducing dopants and acceptor dopants which can attain a PTC rise as high as ~8 orders from an initial resistivity of 10-100Ocm.26 Ti-excess materials however have coarse grained bimodal microstructures with large amounts of second phases present and low densities.

It has been suggested in literature that uniform high density fine grained microstructures can result in PTC materials with improved electrical properties and higher withstand voltages, which can be used in thick and devices with strict requirements and specifications. Previous research carried out in our group has shown that blended systems in the near stoichiometric region can result in fine grained microstructures with high densities.50,51 These systems however displayed poor PTC properties with =1order rise in resistance. This study aims to study processing and control of BaTiO 3 powders by BaO and TiO 2 oxide additions with the ultimate aim of fabricating uniform fine grained high- density materials with good PTCR response characteristics.

Dissolution phenomena in BaTiO 3 has been discussed. The effects of surfaces, solution concentrations, dopant additions and pH variations on the system have been discussed. Controlling the surface states of particles during solution processes is very

- 56 - important as these surfaces determine the behavior of the particle during high temperature processes. It is important to understand the effect of dopants on the system during solution processing. Available literature on dissolution in dopant-modified systems is limited. Further few studies actually look at dissolution phenomenon from an electronic- ceramic processing point of view. An understanding of dissolution in dopant-modified systems and their effects on the final properties is important in order to effectively fabricate PTCR materials with good properties and which can be fabricated on a consistent basis.

Sintering and microstructure evolution in the BaTiO 3 is critical in order to achieve desired properties. Abnormal grain growth can occur in these materials, resulting in the loss of these properties. Stoichiometry plays a major role in determining the sintering behavior of these systems. Further the true stoichiometry in localized regions in the system can be very different from the overall stoichiometry.58This can result in unexpected sintering behavior and microstructure evolution. Dopants additions and incorporation in the system is an important area which must also be considered due to their ramifications on the microstructure and properties of the material.

Modifying the behavior of BaTiO 3 powders by BaO and TiO 2 oxide additions could lead to effective control during processing. The effects of these additions could lead to not only beneficial dissolution trends, preventing potentially harmful preferential dissolution from occurring, but also provide better to control over high temperature processes. This study has sought to determine the effects of oxide additions on the processing and properties of barium titanate materials within a narrow range of stoichiometric variations. Effects of starting particle sizes and dopant additions have also

- 57 - been studied. It has also sought to replicate uniform high density microstructures obtained in blended systems50,51 while trying to enhance the PTC properties of the resulting materials.

pH studies during solution processes have addressed the dissolution mechanisms found in these systems. TEM analysis of the dried powders has tried to confirm these trends and observe the final surface states of powders prior to high temperature processes.

Sintering studies have looked at the microstructure evolution in modified systems using optical and SEM techniques, on as sintered surfaces and chemically etched surfaces.

Lastly electrical studies of these materials have tried to relate the processing and microstructures to the final properties observed in these systems. Comparisons between near stoichiometric systems, blended systems50,51 and Ti-excess systems have been made whenever it was found necessary to understand the effects of processing on resulting microstructures and properties in these systems.

- 58 - 2 Experimental Procedures

2.1 Raw Materials

Initial powders used in these investigations were high purity TICON HPB (TAM

Ceramics, Inc., Niagara Falls, NY) BaTiO 3 with Ba/Ti ratios of 0.994 and 1.0038. These powders were processed in the pure form with and without the addition of dopant (yttria nitrate) to obtain the base systems. Stoichiometric variations to the base powders were made using high purity titanium oxide, barium oxide and titanium isopropoxide. Batches of theoretical stoichiometries 0.9988, 0.9890, 1.0009 and 1.0013 were prepared by these additions to the base BaTiO 3 powders. The dopant used was yittria nitrate. The effect of particle size on the distribution of oxide additions and their effect on sintering behavior and electrical properties was analyzed by preparing batches in which Titanium isopropoxide was used as the oxide precursor.

Impurity concentrations in BaTiO 3 powders can be detrimental to the reproducibility and electrical behavior of the subsequently processed PTCR materials.

The Ticon-HPB BaTiO 3 is a high-purity oxalate derived powder. All samples were prepared using the same series of BaTiO 3 powders, the effect is expected to be the same for all samples to minimize variations in results.

The average particle sizes of BaTiO 3 as-received powders was in the range of

0.5~1.5mm. The typical morphology of raw powders is shown in the SEM micrograph of

Figure 14 (a). After ball milling, the average particle sizes of all batches were reduced to

=1mm (Figure 14 (b)).

- 59 -

14 (a). BaTiO 3 powder as-received

14 (b). BaTiO 3 powders after ball milling (12 hours)

Figure 14. SEM micrographs showing morphology and particle size of BaTiO 3 powders.

- 60 - The nitrate was dissolved in a 60 % Isopropanol/ 40 % DI water solution and analyzed for Y2O3 concentration per unit volume of solution to ensure accurate control of dopant additions. This was accomplished by weighing a 10ml nitrate solution in a Pt crucible and slowly heating to 550°C and holding for 60 minutes, as it has been previously found through DTA analysis that the nitrates are driven off during this heat treatment50, leaving the oxide powder. The remaining residue was then weighed and the oxide concentration was calculated.

2.2 Sample Preparation

The method of processing BaTiO 3 using oxide additions is shown in the flow chart of Figure 15. Compared to the standard processing of donor and acceptor modified

BaTiO 3, the raw materials were made by mixing Ti-excess BaTiO 3 (Ba/Ti ratio <1) or

Ba-excess BaTiO 3 (Ba/Ti ratio >1) along with high purity BaO or TiO 2 in order to achieve the desired Ba/Ti ratio. The batches were ball milled using ZrO2 media, in a 60 %

Isopropanol/ 40 % DI water solution. 0.06ml (60µL) Darvan 821A was added as a dispersant with the aid of an Eppendorf pipette accurate to 10µL. After the mixture was wet ball milled for 12 hours, the dopant additives were incorporated as a nitrate solution,

0.3g Carbowax 4000 was added as a binder and ball milling was performed for an additional 30 minutes.

Powders were pressed uniaxially under 15ksi pressure, into discs (F10 x 2 mm3).

The pellets were then sintered within the temperature range of 1320-1375°C for 2 hours in air with a normal heating rate of 10°C/min. All samples were held at 550°C for 60 minutes to enable nitrate decomposition and binder burnout.

- 61 -

Experimental Procedure

Ba or Ti-excess BaTiO 3 BaO/TiO 2/Ti-isopropoxide

pH measurements Dopant Addition Ball Milling

Stirring

Drying

TEM analysis of Powders 1320-1375C, 2hrs Dry Pressing Temperature 550C, 1hr Binder Burnout 600C, 0.1hr

Sintering

Time

Microst ructure Density Electrical Characterization Measurement Measurement

Figure 15. Flow Chart for processing BaTiO 3 powders.

- 62 - The sintered samples were furnace cooled rapidly by shutting off the furnace power, which resulted in an initial cooling rate of ~20°C/min down to ~800°C, upon which a slower rate developed. The initial, faster cooling rate was most important in order to avoid significant diffusion and segregation of dopants and defects.

2.3 Processing of systems with Titanium Isopropoxide additions

A slightly different method was adopted for systems processed with Titanium isopropoxide. Organo-metallic precursors can be sensitive to minor variations in the process. Further, the sequence of processing steps can be a determining factor in obtaining homogeneous distribution and fine particle size in the system. In order to obtain a fine colloidal suspension of TiO 2 particles, Titanium isopropoxide must be dissolved in isopropanol. Addition of water to the solution results in the hydrolytic decomposition of the propoxide, forming a fine colloidal suspension of TiO 2 particles.

Figure 16 (a) shows an SEM micrograph of TiO 2 particles obtained by this method. The particle sizes obtained by this method are in the 0.2µm range. In contrast, particle sizes ranged between 1 µ and 3 µ for the direct oxide addition method. The SEM image in

Figure 16 (b) shows the size of the TiO 2 powders used in the direct oxide addition systems.

2.4 Measurement and Characterization

2.4.1 pH Measurements

In order to observe the dissolution characteristics of the various batches during milling, pH measurements were made at one hour intervals until completion of the milling. The milled systems were then transferred to contaminant free jars and further

- 63 -

16 (a). TiO 2 particles obtained Ti-isopropoxide

16 (b). TiO 2 particles used in direct oxide additions

Figure 16. SEM images showing morphology and particle size of TiO 2 particles

- 64 - stirred for 3 days with pH measurements being made at 24 hour intervals in order to observe any further trends in dissolution. Measurements were made using a Corning 313 series pH meter using a KCl refillable electrode with a pH measurement range between 0 and 14. A two point calibration procedure was adopted and carried out between measurements in order to obtain consistent readings.

2.4.2 Transmission Electron Microscopy (TEM)

The surfaces of powder particles milled under conditions of varying pH exhibit different surface characteristics. TEM studies of these particles were made to determine the differences between the base powders, batches processed using BaO additions and batches processed using TiO 2 additions. The powders were dry mounted on Lasec Carbon grids. The use of solvents to dissolve the powders was not adopted, to minimize alterations to the particle surface chemistry. The instrument used was a JEOL series

2000FX TEM.

2.4.3 Scanning Electron Microscopy (SEM)

Scanning electron microscopy was used to observe the microstructures of the as- sintered surfaces. The samples were metallized with Au-Pd by a Ladd Research

Industries series 40000 evaporator. A Hitachi S-4000 series FEGSEM was used to analyze the as sintered microstructures. The grain size was estimated from the SEM micrographs using the line intercept technique.

Selected samples were polished down to 0.1µm using diamond paste. These were then chemically etched using a 5% HCL, 5% HNO3, 1% HF solution in order to study the grain and domain boundaries. Samples were etched for 0.5 to 2 minutes and immediately

- 65 - washed with DI water. The as-etched microstructures were then analyzed using an FEI

XL-30 ESEM-FEG.

2.4.4 Density Measurement

The density of the sintered pellets was determined geometrically. The diameter

(d) and thickness (t) were measured at several points on each pellet and an average taken.

The density was calculated by equation D=4W/d2t, where D=density and W=weight of the sample in grams. Percentage theoretical density was calculated as the ratio of the sintered density to the theoretical density.

2.4.5 Electrodes

For resistance and PTCR measurements, the sintered discs were hand polished with 600 grit SiC in order to obtain plane parallel surface and in some cases, was further polished to 0.1µm smoothness using diamond paste. Aluminum electrodes were evaporated(8) onto the surfaces for all investigations, where a surface resistance <1O was obtained. A secondary heat treatment was carried out on all doped samples which were used for electrical testing as it has been observed from previous studies58 that this treatment si able to reduce the room temperature resistivities of the samples without significantly affecting the resistivity jump at Tc. This treatment involved inserting the samples into a furnace pre-heated to 600ºC, holding for a total of 10 minutes (samples were turned over after 5 minutes), followed by cooling at room temperature.

2.4.6 Resistance and PTCR Measurement

Resistance measurements were made using a Hewlett-Packard (HP) E3612A D.C. constant voltage source at 0.02 volts in series with the sample and a Keithley 2000 multimeter. PTCR measurement was carried out in a temperature range from 22°C to

- 66 - 200°C at a heating rate of ~5°C/min. Log resistivity was plotted as a function of temperature.

- 67 - Results and Discussions

3 pH studies

The following section deals with the results of pH studies carried out on the base systems and systems with oxide additions. These studies were carried out on both undoped and doped systems. Before proceeding to the results of these studies, the processing and measurement techniques involved need to be clarified.

Ceramic powders are milled in semi-aqueous systems containing de-ionized water and alcohols according to predetermined ratios. The reason being wholly aqueous processing would lead to extended drying times. Pure alcohol systems, though in use are generally found in tape-casting type processes. Further, environmental and cost considerations need to be taken into account when large quantities of organics are generated as waste by-products. Another important point to be noted here is the fact that ceramic systems require a number of additives in the form of deflocculants, dopants, binders and plasticizers. These additives may be soluble in either a polar or non-polar medium requiring the presence of both organic as well as polar solvents. The system used in the following experiments was a blend of 60 % Isopropanol/ 40 % DI water. Dissolved species can seriously affect the reliability and validity of data, especially in the case of pH measurements. Further since this study attempts to correlate pH based information to dissolution of the studied species, namely barium and titanium, and the effects of oxide additions and dopants on the barium titanate present, the presence of dissolved impurity species would lead to large errors in the results and lack of validity of the conclusions.

Studies on perovskite dissolution39 and the dissolution of Barium Titanate43 have shown that dissolved carbonate impurities can lead to preferential leaching of the large cation in

- 68 - the system. This requires that the processing system chosen be free from such dissolved species.

In this study, the 60% Isopropanol / 40% DI water systems were made with freshly prepared de-ionized water in the 18 Mega-ohm purity range. In systems with titanium isopropoxide additions, where in-situ preparation of the milling systems was required, fresh batches of de-ionized water were used. Carbonate impurities present in the system can be expected to have resulted wholly from carbonates in the powders. This study neglects the effect of carbonate impurities in the powders, as the starting powders used have been stored under identical conditions and carbonate impurities, if present are expected to be of similar amounts in all batches.

This study assumes that dissolution effects present in the system are wholly due to aqueous dissolution and the non-polar organic solvent in the system does not in any manner impede dissolution. This assumption is not wholly without basis as studies on processing barium titanate powders in alcohol systems show virtually no leaching of barium after milling.42

3.1 pH studies on base systems

Figure 17 shows a representative plot of the variation in pH vs processing time for the Barium-excess (Ba/Ti ratio = 1.0038) and Titanium-excess (Ba/Ti ratio = 0.994) base powders processed in 60 % Isopropanol/ 40 % DI water systems. pH measurements were made at 1 hour intervals during the 12 hr milling period and subsequently at 24 hour intervals during the stirring stage. The systems were in aqueous media for a total of 72 hours. From the graph it can be seen that there is a slight rise in the pH of the systems during the initial stages of processing. From there on the system attains

- 69 -

14

13

12 Undoped base system 11

10 Solution pH 9

8

7 1 10 100 Time in Hours

Figure 17. Representative plot of solution pH versus time for base undoped systems.

- 70 - a fairly steady state with little or no variation in the pH through the remainder of the processing period. It was also noticed that the addition of the non-polar binder 30 minutes before completion of the milling process does not affect solution pH. The pH trends followed by both the Ba-excess and Ti-excess system are almost overlapping. Both systems remain in the same pH range through the whole process time, maintaining values between 8 and 10, tending towards mildly alkaline to alkaline solution behavior.

3.2 pH studies on systems with oxide additions

3.2.1 Ba-excess powders with TiO2 additions

Figure 18 shows a representative plot of systems which have been milled using

Ba-excess powders to which varying amounts of TiO 2 powders additions (16mg to 35mg) with particle sizes of ~1µm have been made to alter the Ba/Ti ratio of the system. The pH of the solution closely follows the trend of the base systems maintaining pH values in the alkaline range between 8 and 10, with the pH increasing slightly during the initial period of milling and attaining stability through the remaining period in solution.

3.2.2 Ti-excess powders with BaO additions

Figure 19 shows the comparison of systems which have been processed using Ti- excess powders and BaO powder additions to vary the Ba/Ti ratio. The amounts range from 60mg to 100mg and particle sizes are similar to those of the TiO 2 oxide additions. It can clearly be noticed that the systems processed with BaO additions show a distinctly higher solution pH in the 12-13 range for all the systems. The pH follows an upward trend during the initial milling and then remains fairly constant throughout the remaining processing time. Another important point to note is the fact that the pH is affected by the

- 71 -

14

13

12 Ba-excess(1.0038) + TiO2 11

10 Solution pH 9

8

7 1 10 100

Time in Hours

Figure 18. Representative plot of solution pH versus time for Ba-excess systems with TiO 2 additions

- 72 -

14

13

12

11 Ti-excess(0.994) + 90mg BaO 10 Ti-excess(0.994) + 60mg BaO Solution pH 9

8

7

1 10 100 Time in Hours

Figure 19. Representative plots of solution pH versus time for undoped Ti-excess systems with varying amounts of BaO additions

- 73 - amount of BaO added to it. Systems which have 90mg or more BaO additions ( Ba/Ti ratios 1.0009 and 1.0013) appear to display a higher pH through the processing range compared to the system with 60mg BaO addition (Ba/Ti ratio 0.9988) which displays a lower pH value in the 12 range.

3.2.2.1 Dissolution behavior in systems with Oxide additions

The highly alkaline tendency exhibited during solution processing of systems with

BaO additions can be attributed to the dissolution of BaO present in the system. BaO is a highly ionic oxide compared with TiO 2 which is partially covalently bonded and is insoluble in aqueous solutions37 (see Figure 18). In solutions with BaO additions the dissolution of barium from its oxide leads to an increased cation equilibrium concentration. The dissolution kinetics of BaO has a drastic effect on the overall behavior of the system. Being highly ionic, BaO dissolution is extremely rapid and once the equilibrium concentration of Ba2+ ions is attained, further dissolution is minimal. This

2+ effectively prevents dissolution of Ba ions from the BaTiO 3 particles. In the case of titanium oxide additions this will not be the case, as TiO 2, being a covalently bonded oxide, will not readily dissolve in aqueous solutions exhibiting neutral to mildly alkaline

2+ behavior. This in turn will lead to Ba leaching from the BaTiO 3 particle surfaces. The dissolution rate here is dependent on exposure of new BaTiO 3 surfaces to the solution.

This will lead to systems with TiO 2 additions exhibiting dissolution and processing trends similar to the base BaTiO 3 systems.

- 74 - 3.3 pH studies of Doped systems

3.3.1 Base systems with dopant additions

A representative plot of pH vs time for base systems to which yttrium nitrate additions have been made is shown in Figure 20. These additions were made 30 minutes before completion of the milling process along with the binder additions. These systems behave similar to the base systems upto the point of dopant / binder additions, where a drastic drop in the pH is noticed. As can be seen from the plot, addition of the nitrate causes the solution pH to drop drastically to values in the 5-6 range. This change in the pH of the system can be wholly attributed to the addition of the nitrate, since the un- doped systems show no variation in pH on addition of binders. What is also seen is the gradual rise in pH during further stirring of the solution over the 72 hour range, which was not the case in the un-doped base systems. Dopant nitrate additions cause a drastic lowering of the pH in the solution due to the high individual ionic activity of the nitrate ions in the solution.37 This causes further dissolution of the barium titanate particles resulting in the gradual rise in pH seen in Figure 20.

3.3.2 Doped systems with oxide additions

3.3.2.1 Systems with TiO2 additions

In comparison to the doped base systems Figure 21 shows similar trends in the pH versus time plots of systems processed using Ba-excess powders to which TiO 2 additions have been made. Nitrate additions were made to obtain a final dopant concentration of

0.24 m/o yttria in the system. The plot follows the trend shown by the doped base systems. At the point of nitrate addition in the system there can be seen once again the

- 75 -

14

13

12 Dopant incorporation 11

10 9

Solution pH 8

7

6

5 1 10 100 Time in Hours

Figure 20. Representative plot of solution pH versus time for base systems with 0.24 m/o Y2O3 showing the effect of yttria nitrate dopant incorporation on pH of the system

- 76 -

14

13

12 Dopant incorporation 11

10 9

Solution pH 8

7

6

5 1 10 100 Time in Hours

Figure 21. Representative plot of doped Ba-excess systems with TiO 2 additions. Arrow indicates point at which yttria nitrate dopant addition had been made.

- 77 - drastic drop in the pH of the solution. The pH behavior in doped systems with TiO 2 additions can be said to mimic those of the base systems with dopant additions (refer to

Figure 20).

Based on the fact that TiO 2 additions do not affect the dissolution and pH of both the un-doped and the doped systems over a wide range from acidic ~5 to alkaline ~8, it can be argued from the standpoint of solution processing that the concentration of TiO 2 will not in any manner affect the dissolution behavior of the system.

3.3.2.2 Doped systems with BaO additions

Figure 22 indicates the trend followed by doped systems with BaO additions. The system pH again follows the trend similar to undoped systems with BaO additions upto the point of the dopant incorporation. A pH in the highly alkaline range (~12-14) is maintained in the system. On addition of the dopant the pH drops into the mildly alkaline

~8 range where it stabilizes. This behavior can be contrasted with those of base systems and systems with TiO 2 additions where the latter types are found to drop well into the acidic ~5 range on dopant additon. An important point to notice here is that the amount of

BaO additions to the system can be shown to determine the stabilization level of the solution with systems having higher amounts of BaO stabilizing at a higher pH.

This behavior of systems with BaO additions can be positively advantageous in obtaining process control during aqueous stages of processing where preferential dissolution at particle surfaces can result in modified surface states. This in turn can affect the sintering, microstructural and electrical behavior of the system. BaO additions have been shown to maintain the processing pH of the system continuously in the

- 78 -

14

12 Dopant incorporation

10

Ti-excess+ 60mg BaO 8

6

Solution pH Ba -excess+90mg BaO 4

2

0 1 10 100

Time in Hours

Figure 22. Figure of solution pH versus time for doped Ti-excess systems with BaO additions. Effects of varying amounts of BaO additions are shown in the plot

- 79 - Highly alkaline range which can limit or drastically reduce preferential dissolution occurring in the BaTiO 3 particles. It can be seen that the amount of BaO additions are low enough to prevent formation of second phase effects in the system. SEM micro graphs of systems with BaO additions show negligible presence of second phases at the grain-boundaries as will be shown in section dealing with microstructural results (Figures

36-38 and Figures 42-44).

3.3.2.3 Doped systems with Titanium Isopropoxide additions

In order to understand the effect of starting particle size of the oxide additions on processing, doped systems were also prepared with TiO 2 additions (= 0.2 µm) from titanium isopropoxide precursor as compared TiO 2 oxide powder additions (~1µm). Two systems were prepared. One similar to previous systems prepared which used a Ba-excess

BaTiO 3 powder (Ba/Ti ratio 1.0038) with TiO 2 obtaining a final Ba/Ti ratio 0.9988. A second system was prepared using the Ti-excess powder (Ba/Ti ratio 0.994) to which an identical weight of titanium isopropoxide precursor was added bringing the final Ba/Ti ratio to 0.9890. The powders were milled for 12 hours during which time the pH behavior was observed to be similar to previous systems with direct TiO 2 additions as shown in

Figure 21. Addition of 0.24m/o yttria in the form of a nitrate solution caused a drop in the system pH which again followed the behavior of the doped systems with TiO 2 additions.

These results show that systems which were prepared with oxide additions and those prepared using metallorganic precursors follow similar pH.

- 80 - 3.4 General Trends in pH studies

Undoped base systems maintain mildly alkaline pH levels during processing which would lead to barium dissolution at the particle surfaces. Both Ti-excess as well as

Ba-excess systems follow similar trends (see Figure 23). The gradual pH rise during milling of the systems could be caused by further dissolution from newly exposed surfaces.

Doped base systems affect a drastic lowering in the pH on addition of the dopant, going into the acidic pH range (see Figure 24) which can lead to further barium dissolution. Undoped and doped systems with TiO 2 (Figures 23 and 24) additions closely follow the pH trends set by the base undoped and doped systems. This suggests that TiO 2 additions do not affect aqueous dissolution mechanisms in BaTiO 3 systems. This trend is confirmed in systems which have been prepared by means of titanium isopropoxide addition.

Undoped systems with BaO additions exhibit marked differences in solution behavior with the pH being highly alkaline. The amount of BaO added has a direct effect on the pH of the system with small BaO additions maintaining lower pH values compared with larger BaO additions. BaO additions can be used in doped systems to partially negate effects of nitrate ion activity during processing, reducing barium dissolution from

BaTiO 3 powders.

pH studies carried out on doped systems with oxide additions in the form of metallorganic precursors shows that the method of oxide additions does not affect the general pH trend observed in the system during processing. This trend may be extended to systems without dopant additions. Although the pH trends in the metallorganic

- 81 -

14

13

12

11 Systems with BaO additions

10

Solution pH

9

8 Base systems and systems with TiO2 additions

7 1 10 100 Time in hours

Figure 23. Comparison plot showing pH versus time for undoped base systems, Ba- excess systems with TiO 2 additions and Ti-excess systems with BaO additions

- 82 -

13

12 Dopant incorporation 11 Doped Ti-excess systems with 10 BaO addition

9

Solution pH 8

7 Doped base systems and Ba-

excess systems with TiO2 6 addition

5 1 10 100

Time in Hours

Figure 24. Comparison plot of solution pH versus time for doped base systems with TiO 2 additions and BaO additions.

- 83 - precursor systems are similar to systems with direct oxide additions, particle distribution and homogenization will be more uniform in these systems due to the smaller particle sizes in these systems. Further since these particles are in the form of a colloidal suspension, settling and segregation effects will be considerably minimized in these systems.

3.5 Summary of pH studies

· Base systems and systems processed with TiO 2 additions exhibit similar mildly alkaline pH trends during processing.

· Systems with BaO additions exhibit a highly alkaline pH during processing with the pH being directly dependent on the amount of BaO additions present in the system.

· Dopant additions in the form of nitrate solution have the effect of drastically lowering processing pH in all systems which were observed.

- 84 - 4 TEM Studies of powders

TEM analysis of the powders was carried out as follows. As-received factory powders with a Ba/Ti ratio of 0.994 were mounted and analyzed for a comparison with the processed powders. Powders which had been processed with TiO 2 additions and BaO additions were mounted and analyzed.

4.1 As-received Powders

Figures 25 (a) and (b) show TEM bright-field images of factory powders. There is a distinct edge, which distinguishes the particle surface from the rest of the picture. It can also be seen from the images that there does not appear to be any gradations on the immediate surface, which indicates the homogeneous nature of the bulk particle upto the surface.

4.2 Ba-excess powders with TiO2 addition

Figures 26 (a) and (b) show the images of particles which have been processed in solution with TiO 2 additions. A distinct double layer can be noticed on the particle surfaces. It is suspected that, this double layer formation may be attributed to preferential dissolution mechanisms occurring in the solution where Ba2+ ions are released from the barium titanate particle resulting in a Ti-rich layer on the particle surfaces. Drying results in re-deposition of Ba2+ions onto the particle surfaces resulting in the distinctive double layer noticed in Figure 26. Further studies need to be carried out confirming the composition of the double layer at the particle surfaces. In some particles this double layer appears to be discontinuous and jagged. Further the layers appear to be non uniform

- 85 -

25 (a)

25 (b)

Figures 25. (a) and (b) showing TEM images of factory powders prior to milling

- 86 -

26 (a) Bulk

Ti- rich layer Ba-rich layer

26 (b)

Figures 26. (a) and (b) TEM images of particles processed with TiO 2 additions showing the double layer formation on the particle surfaces

- 87 - and varying in thickness. This could be due to the violent agitation during the milling and sieving processes, causing the particles to be broken up and also disturbing the double layer formed on their surfaces. The thickness of the double layer estimated from the images is ~10nm with each layer being ~5nm thick. Certain regions on the particle surface can be observed with lattice fringes running to the interface. This is followed by two distinct inter-layers present on the surface which appear be amorphous in nature. A number of micrographs taken of various samples, all indicate similar trends

4.3 Ti-excess powders with BaO additions

Figure 27 (a) and (b) shows bright field TEM images of Ti-excess powders processed with BaO additions. A distinct single layer can be noticed enveloping the particle surface suggesting that BaO additions have prevented preferential dissolution from occurring at the barium titanate particle surfaces, by going into solution and creating a high barium concentration in solution. The Ba ions have then redeposited on the particle surfaces creating the single layer shown in Figure 27. The layer thickness estimated from the images and is ~5nm.

4.4 General Trends in TEM studies

TEM studies on processed powders clearly show the artifacts of dissolution processes occurring in the system during processing. These observations are a direct visual corroboration of the effects suggested by theory and pH studies. Factory powders which are processed by a chemical route show no indication of single or double layers on the particles surfaces.

- 88 -

. Ba-rich layer

Bulk 27 (a)

27 (b)

Figure 27. (a) and (b) TEM images of particles processed using BaO additions showing a distinct single layer on the surfaces

- 89 - BaTiO 3 powders which have been processed with TiO 2 additions show distinct a

2+ double layer formation suggesting the dissolution of Ba ions from the BaTiO 3 particles creating a Ti-rich layer. Drying results in redeposition of Ba2+ ions on the surfaces of the particles forming the distinct double layer seen in the images. Double layer formation is also expected to occur in base systems during solution processing, due to their similar solution behavior with systems containing TiO 2 additions

Systems with BaO additions show the formation of a single layer on the particle surface. Barium dissolution from the BaO addition will prevent dissolution from barium titanate particles. Drying results in the deposition of Ba ions onto the particle surfaces with the formation of a single layer.

4.5 Summary of TEM studies

· Factory powders are homogeneous upto the bulk termination of the particles.

· BaTiO 3 powders processed with TiO 2 additions exhibit double layer formations on particle surfaces resulting from preferential dissolution of Barium from the BaTiO 3 particle surfaces

· Powders with BaO additions exhibit a single layer formation, resulting from BaO deposition on the particle surfaces, and prevention of preferential dissolution in BaTiO 3 particles

- 90 - 5 Results and Discussions of Sintered Microstructures

5.1 Base systems

Base systems refer to systems which have been processed without oxide additions. They include undoped and donor doped BaTiO 3 with Ba/Ti ratios of 0.994 and

1.0038. These systems have been used to understand the sintering behavior occurring in pure systems with Ba-excess behavior and Ti-excess behavior. These systems provide a comparison with systems to which oxide additions have been made.

5.1.1 Undoped base systems

The microstructure of as-sintered surfaces of base Ti-excess(Ba/Ti ratio 0.994) sample sintered from 1320°C to1375°C are shown in figure 28 (a) and (b). The microstructure of samples sintered at 1320° C is bimodal with Dmax = 10mm and Dmin =

2mm. As the sintering temperature is increased the microstructure maintains a bimodal trend with larger grain sizes and with Davg= 14mm at 1375°C.

The sintered microstructures of base Ba-excess (Ba/Ti ratio 1.0038) samples are shown in figure 29 (a) and (b) under similar sintering conditions. The microstructures are considerably different from the Ti-excess samples with all sample exhibiting very large grains ranging from Davg= 21mm grains in samples sintered at 1320°C to Davg=32mm in

1375°C sintered samples.

Ba-excess samples exhibit large grains with less porosity (~94% theoretical density), fewer open pores and highly faceted angular grain boundaries. Ti-excess samples however have higher porosity (<92% theoretical density) and more rounded grains (Refer to Table 3 for density values).

- 91 -

28 (a) 28 (b)

29 (a) 29 (b)

Figure 28. Base Ti-excess( Ba/Ti ratio 0.994) samples sintered at 1320°C(a) and 1375°C(b). Figure 29. Base Ba-excess(Ba/Ti ratio 1.0038) samples sinterd at 1320°C(a) and 1375°C(b).

- 92 - 5.1.2 Donor-doped Base BaTiO 3 systems

Donor doped base systems have been prepared by adding 0.24 m/o Y2O3 to the base powders with Ba/Ti ratios of 0.994 and 1.0038 powders and sintering under the same conditions as those of the undoped BaTiO 3 systems, the temperature being from

1320°C to 1375°C (as shown in Figures 30 and 31). Both sintered samples exhibit semiconducting behavior with typical dark blue color, as discussed subsequently. The donor-doped Ti-excess (0.994) samples sintered at 1320°C were found to have bimodal microstructures with avg. Dmax=~30mm and avg. Dmin=1mm. There is a tendency towards more uniformity in grain size as the sintering temperatures increase, with samples sintered at 1375°C having Davg=~13mm, although regions of bimodality, small grains are randomly distributed. Donor-doped Ba-excess samples however exhibit uniform microstructures with Davg=~4mm at 1320°C sintering temperature. Ba-excess samples have consistently higher theoretical densities (~97%) while all Ti-excess samples had densities in the range of 86%-89%. The experimental data for base systems is shown

Table 3.

5.2 Microstructures of systems with oxide additions

The following section discusses the sintered microstructures developed in batches with oxide additions. BaO additions made to Ti-excess (Ba/Ti ratio 0.994) powders in varying amounts were expected to have an overall effect on the Ba/Ti ratio of the powder.

Similarly TiO 2 additions in direct oxide form as well as in the form of titanium- isopropoxide were made to Ba-excess (Ba/Ti ratio 1.0038) powders. This was done for

- 93 -

30 (a) 30 (b)

31 (a) 31 (b)

Figure 30. Donor doped base Ti-excess( Ba/Ti ratio 0.994) samples sintered at 1320°C(a) and 1375°C(b). Figure 31. Donor doped base Ba-excess(Ba/Ti ratio 1.0038) samples sintered at 1320°C(a) and 1375°C(b).

- 94 -

Table 3. Results for undoped and donor doped base systems with Ti-excess (Ba/Ti ratio 0.994) and Ba-excess (Ba/Ti ratio 1.0038) powders

Sintering Room Ba/Ti Theoretical Addition pH Temperature Grainsize (µm) Temperature Ratio Density% (°C) Resistivity (Ocm)

Min Max

1320 bimodal(2-10) 88

- 8.83 9.28 1350 7 92 Insulator

1375 14 85 0.994 1320 bimodal(1-30) 89 6.E+02

0.24 m/o 5.72 9.36 1350 bimodal(1-50) 86 7.E+02 Y2O3

1375 bimodal(1-15) 86 4.E+03

1320 21 94

- 8.93 9.15 1350 30 90 Insulator

1375 32 94 1.0038 1320 4 97 1.E+03

0.24 m/o 5.75 9.27 1350 4 97 6.E+02 Y2O3

1375 6 97 6.E+02

- 95 - both doped and undoped samples. One exception was a doped system prepared using Ti- excess (Ba/Ti ratio 0.994) powders to which titanium isopropoxide additions were made to obtain a final Ba/Ti ratio of 0.9890 away from the near stoichiometric region

The Ba/Ti ratios attempted by these additions were 0.9988, 1.0009 and 1.0013.

The reason for varying the Ba/Ti ratios in such a narrow region was to try and understand the sintering behavior of powders which are very close to stoichiometry. A Ti-excess

Ba/Ti ratio of 0.9890 was also prepared in order to compare the microstructures and properties exhibited by this Ti-excess system with those of the near stoichiometric systems. Studies have shown that when the Ba/Ti ratio is very close to stoichiometry, the analyzed ratio may not exactly reflect the real local environment.58 It is possible within this narrow region (Ba/Ti ratio < 1.006) for small Ti-excess regions to coexist with small

Ba-excess regions. This can play an important role during sensitive sintering stages.

Further it was sought to compare microstructures of theoretically identical Ba/Ti ratios which were arrived at by different processing routes (e.g. Ba-excess BaTiO 3 + TiO 2 addition = Ba/Ti ratio of 0.9988 = Ti-excess BaTiO 3 + BaO addition). Another important reason for working with the above mentioned Ba/Ti ratios was to form a comparison with microstructures of blended systems.50 Earlier work has been carried out pertaining to the study of controlling abnormal grain growth in BaTiO 3 systems by means of blending and processing Ba-excess and Ti-excess BaTiO 3 powders to obtain powders with intermediate stoichiometries.50 The present study makes an attempt at understanding the sintering behavior and properties of systems modified with oxide additions and their effect on PTC behavior. It also tries to relate dissolution related processes during milling to the final obtained microstructures and properties. Effects of starting particle size and distribution

- 96 - have also been looked at. These studies were carried out by substituting direct oxide additions with titanium isopropoxide additions which form well dispersed colloidal suspensions of TiO 2 particles.

5.2.1 Undoped Ba-excess systems with TiO2 additions

Figures 32, 33 and 34 show the microstructures of Ba-excess systems to which

TiO 2 additions have been made to obtain theoretical Ba/Ti ratios of 0.9988, 1.0009 and

1.0013. Samples with Ba/Ti ratio 0.9988 are bimodal (Refer to figure 32 (a)) at temperatures around the eutectic with large grains (Dmax=25mm) embedded in a fine grained matrix (Dmin=1mm). Sintering at higher temperatures produces more uniform grains with samples sintered at 1375°C exhibiting Davg=28mm (Figure 32 (b))and densities around 95%. The grains are rounded with some amount of open pores distinguishable in the SEM images. Systems with Ba/Ti ratio 1.0009 exhibit lesser bimodality during sintering near the eutectic having a grain size Davg=33mm (Figure 33

(a)). At higher temperatures a tendency towards large grain bimodality is noticed with

Dmax > 45mm in samples sintered at 1375°C (Figure 33 (b)). Samples with Ba/Ti ratio of

1.0013 show similar sintering behavior with large grain bimodality around the eutectic temperature (Figures 34 (a)) and exhibiting larger grains at the higher temperatures with

Dmax> 45mm (Figure 34 (b)). Both systems also exhibit rounded grains with low porosities. These microstructures above the eutectic suggest a liquid phase mechanism involved during the sintering process. Undoped samples with TiO 2 additions have theoretical densities >95%.

In general the above microstructures are representative of each sample. Yet, the samples had local pockets of inhomogenities. As shown in Figure 35 these 100mm wide

- 97 -

32 (a) 32 (b)

33 (a) 33 (b)

34 (a) 32 (b)

Undoped systems processed with TiO 2 additions. Figure 32. Ba/Ti ratio 0.9988 sintered at 1320°C (a) and 1375°C (b). Figure 33. Ba/Ti ratio 1.0009 sintered at 1320°C (a) and 1375°C (b). Figure 34. Ba/ Ti ratio 1.0013 sintered at 1320°C (a) and 1375°C (b).

- 98 -

35 (a)

35 (b)

35 (c)

Figures 35. (a), (b), and (c). Undoped systems processed with TiO2 additions showing inhomogeneous regions.

- 99 - regions are characterized by particles of sizes <1mm, having high porosity. Figures 35 (a),

(b) and (c) shows SEM images of these inhomogeneous regions from samples with TiO 2 additions at various sintering temperatures. It is possible that these regions may stem from inhomogeneous distribution of the TiO 2 particle during milling due to its insolubility in the pH range studied here. The TiO 2 particles may have segregated during the drying process forming regions with high TiO 2 content, affecting the sintering in these areas. Energy Dispersive Spectroscopic analysis of these regions was carried out in an attempt to prove the existence of TiO 2 rich regions. The results were found to be inconclusive. It is possible that the inhomogenities could have been generated by sub- surface TiO 2 particles or by the presence of low amount of TiO 2, below the detection threshold of EDS which could not have been detected by the beam as these surface sensitive techniques are limited by the penetration depth of the electron beam, which does not exceed 1-3mm depending on the operating parameters of the instrument. Data for undoped Ba-excess systems with TiO 2 are given in Table 4.

5.2.2 Undoped Ti-excess systems with BaO additions

Figures 36, 37 and 38, show the microstructures of Ti-excess systems to which

BaO additions have been made to obtain theoretical Ba/Ti ratios of 0.9988, 1.0009 and

1.0013. Figures 36 (a) and (b) show the microstructures of samples with Ba/Ti ratios of

0.9988. Samples sintered at 1320°C are of a fairly uniform grain size with Davg=12mm.

There is an increase in the average grain size as the sintering temperature is increased.

Grains of samples sintered above the eutectic temperatures appear to be rounded.

Bimodal grain size distribution is noticed in samples sintered at 1375°C. Samples with

Ba/Ti ratios of 1.0009 and 1.0013 appear to be similar over the range of sintering

- 100 -

Table 4 Results for doped and undoped systems with TiO 2 additions

Sintering Room Ba/Ti Theoretical Addition pH Temperature Grainsize (µm) Temperature Ratio Density% (°C) Resistivity (Ocm)

Min Max

1320 bimodal(1-25) 95

32.1mg TiO2 8.76 9.16 1350 26 95 Insulator

1375 28 95 0.9988 1320 bimodal(1-20) 97 7.E+05 34.2mg TiO2+ 0.24 5.73 9.23 1350 bimodal(2-25) 95 7.E+03 m/o Y2O3 1375 bimodal(2-30) 94 2.E+03

1320 33 97

19.5mg TiO2 8.96 9.37 1350 43 96 Insulator

1375 > 45 96 1.0009 1320 bimodal(1-10) 94 1.E+04 23.6mg TiO2+ 0.24 5.79 9.13 1350 bimodal(1-20) 92 1.E+04 m/o Y2O3 1375 bimodal(4-20) 95 3.E+04

1320 22 96

16.7mg TiO2 8.9 9.26 1350 36 90 Insulator

1375 46 97 1.0013 1320 bimodal(1-10) 93 6.E+02 16.8mg TiO2+ 0.24 5.72 9.09 1350 bimodal(1-15) 94 9.E+02 m/o Y2O3 1375 bimodal(1-20) 95 7.E+02

- 101 -

36 (a) 36 (b)

37 (a) 37 (b)

38 (a) 38 (b)

Undoped systems processed with BaO additions Figure 36. Ba/Ti ratio 0.9988 sintered at 1320°C (a) and 1375°C (b). Figure 37. Ba/Ti ratio 1.0009 sintered at 1320°C (a) and 1375°C (b). Figure 38. Ba/ Ti ratio 1.0013 sintered at 1320°C (a) and 1375°C (b).

- 102 - temperatures (refer to Figures 37 and 38). Near the eutectic temperature the grains are faceted with Davg=~20mm. At sintering temperature of 1350°C there appears to be liquid phase sintering occurring similar to Ba-excess samples with TiO2 additions which were discussed in the previous section. This leads to the formation of grains with soft rounded edges. At sintering temperature of 1375°C, these samples have exaggerated grains Dmax>

50mm suggesting large grain bimodal behavior. Complete results for systems with BaO additions are given in Table 5.

An important point to note in this case is the absence of inhomogeneous regions which were seen in systems with TiO 2 additions. This indicates the homogeneous distribution of BaO in the systems during milling. BaO being a highly ionic oxide has higher solubility in aqueous systems compared with the covalently bonded TiO 2, readily dissolving in the solution during milling and precipitating uniformly on the particle surfaces during drying.

5.2.3 Doped Ba-excess systems with TiO2 additions

Figures 39, 40 and 41 show the microstructures of Ba-excess systems with TiO 2 additions having Ba/Ti ratios of 0.9988, 1.0009 and 1.0013, which have been doped with

0.24 m/o Y2O3. A general trend exhibited in these systems is a tendency towards bimodality with Dmax> 25mm and Dmin<2mm (see Table 4). The microstructures can be described as large grains embedded in a fine grained matrix. The dopant has a decidedly refining effect on grain size with no grains being larger than 30mm in size. It is concluded that complex coupling effects of oxide additions along with dopant segregation and inhomogeneous distribution lead to severely abnormal grain growth tendencies which seem to override temperature effects in the observed range between 1320° and 1375°C. A

- 103 -

Table 5 Results for doped and undoped systems with BaO additions

Sintering Room Ba/Ti Theoretical Addition pH Temperature Grainsize (µm) Temperature Ratio Density% (°C) Resistivity (Ocm)

Min Max

1320 12 91

63.3mg BaO 11.18 12.26 1350 15 90 Insulator

bimodal(15- 1375 87 >70) 0.9988 bimodal(1- 1320 93 1.E+05 >20) 63.4mg BaO+ 0.24 7.79 12.45 1350 bimodal(2-20) 94 7.E+03 m/o Y2O3 1375 bimodal(2-12) 93 9.E+02

1320 16 94

92.6mg BaO 12.79 13.25 1350 23 95 Insulator

bimodal(15- 1375 95 >100) 1.0009 1320 bimodal(1-10) 94 6.E+03 91.4mg BaO+ 0.24 8 12.73 1350 bimodal(1-15) 96 - m/o Y2O3 1375 bimodal(2-15) 93 3.E+03

1320 19 94

96.9mg BaO 12.85 13.31 1350 24 97 Insulator

bimodal(15- 1375 96 >60) 1.0013 1320 3 95 9.E+02 99.0mgBaO + 0.24 8.86 12.84 1350 bimodal(2-25) 98 1.E+04 m/o Y2O3 1375 bimodal(2-20) 96 7.E+03

- 104 -

39(a) 39 (b)

40 (a) 40 (b)

41 (b) 41 (b)

Donor doped systems processed with TiO 2 additions Figure 39. Ba/Ti ratio 0.9988 sintered at 1320°C (a) and 1375°C (b). Figure 40. Ba/Ti ratio 1.0009 sintered at 1320°C (a) and 1375°C (b). Figure 41. Ba/ Ti ratio 1.0013 sintered at 1320°C (a) and 1375°C (b).

- 105 - 5.2.4 Doped Ti-excess systems with BaO additions

Figures 42, 43 and 44 show the microstructures of donor doped Ti-excess systems with BaO additions. The donor dopant added is 0.24m/o of Yttria. The Ba/Ti ratios in these systems were adjusted to maintain the same ratios as the undoped systems with

BaO additions. The microstructures are once again similar to those obtained for doped systems with TiO 2 additions (see Table 5). Large grains embedded in a fine grained matrix are observed with Dmax=~20mm and Dmin=~1mm with grain size of the fine grained matrix increasing with increased sintering temperatures. Although the final microstructures appear to be similar in doped systems with TiO 2 and BaO additions, it is suspected that these microstructures may be arrived at in different manners. BaO additions could have resulted in neutralizing liquid phase sintering mechanisms.

5.2.5 Doped systems with Titanium Isopropoxide additions

In order to observe the effect of particle size of the oxide additions on sintering and microstructure behavior, donor doped batches with TiO 2 additions in the form of titanium isopropoxide were prepared and sintered at 1320°C and 1375°C. The microstructures for samples with Ba/Ti ratio 0.9988 sintered at 1320°C and 1375°C, respectively, are shown in Figure 45 (a) and (b). Around the eutectic (1320°C), the particles do not exhibit significant growth. Rios et al.55 attributed this microstructure effect to a strong pinning force effect present in a Ti-rich region. It is likely that solid state sintering in the presence of the Ti-excess(resulting from the isopropoxide addition) exerts a pinning force which results in the small grained microstructure with Davg

5 <0.5mm. The samples are light yellow in color and are exhibit high RT resistivities (~10

Ocm) resulting from poor dopant incorporation. Samples sintered at 1375°C exhibit

- 106 -

42 (a) 42 (b)

43 (a) 43 (b)

44 (a) 44 (b)

Doped systems processed with BaO additions Figure 42. Ba/Ti ratio 0.9988 sintered at 1320°C (a) and 1375°C (b). Figure 43. Ba/Ti ratio 1.0009 sintered at 1320°C (a) and 1375°C (b). Figure 44. Ba/ Ti ratio 1.0013 sintered at 1320°C (a) and 1375°C (b).

- 107 -

45 (a) 45 (b)

46 (a) 46 (b)

Systems with Ti-isopropoxide additions Figure 45. Ba/Ti ratio 0.9988 at 1320°C (a) and 1375°C (b). Figure 46. Ba/Ti ratio 0.9890 at 1320°C (a) and 1375°C (b).

- 108 - bimodal microstructures with Dmin = ~3 mm and Dmax = ~20 mm. The microstructures appear to be similar to samples which have been processed using direct TiO 2 additions.

There appears to be an overall decrease in the bimodality observed with fewer abnormal grains appearing over the sample surface, suggesting that the smaller starting particle sizes in the titanium isopropoxide additions have had a positive effect in improving liquid distribution and obtaining a more uniform microstructure. The samples are grey in color and semi-conducting (~102 Ocm). Samples sintered at 1320°C have a density ~97% while the samples sintered at 1375°C have a lower density ~92%.

The sintered microstructures of samples with Ti-excess Ba/Ti ratio 0.9890 at

1320°C and 1375°C are shown in Figure 46 (a) and (b) respectively. This systems was prepared using Ti-excess (Ba/Ti ratio 0.994) powders to which Ti-isopropoxide additions were made. Both samples are blue and semi-conducting. The sample sintered at 1320°C has a bimodal microstructure with Dmin=2mm and Dmax=20mm. Large amounts of Ti- excess second phase in the form of platelets can be noticed suggesting a liquid phase sintering mechanism. The bimodal microstructure may be due to large amounts of liquid phase generated by the large Ti-excess present in the sample as compared to almost no second phase presence in previous near stoichiometric systems with oxide additions (see

Figures 39-44). The sample sintered at 1375°C while still bimodal has shown considerable improvement with Dmin= 5mm and Dmax = 20mm. Due to the higher sintering temperature excessive amounts of liquid phase have been generated as can be seen from

Figure 46 b. Evidence showing the presence of liquid phase will be in subsequent sections. Samples sintered at 1320°C have a low density ~85% while those sintered at

- 109 - 1375°C have a density ~92%. Results for systems processed with Ti-I additions are given in Table 6.

5.3 General trends in microstructure studies

Undoped systems with TiO 2 additions results in bimodal microstructures around the eutectic. Microstructures tending towards large grain bimodal are observed above the eutectic, at 1350 and 1375°C. In systems with TiO 2 additions inhomogeneous regions with extremely small grains are observed, which could be due to uneven distribution and large initial size of the TiO 2 particles. Doped systems with TiO2 additions exhibit bimodal grain size distribution. This may possibly be a result of the inhomogeneous distribution of TiO 2 particles as seen in the un-doped systems. The resulting non-uniform liquid phase distribution in the system may be a likely cause for abnormal grain growth characteristics displayed across the board in doped systems with TiO 2 additions. Samples of both doped and undoped systems with TiO 2 additions exhibit theoretical densities

~95% (refer to Table 4).

Undoped systems with BaO additions show fairly uniform grain growth in all systems with varying Ba/Ti ratios at lower sintering temperatures. A large grain bimodal tendency similar to systems with TiO 2 addition at the higher temperature of 1375°C is also noticed in these systems. An important observation in these systems is the lack of small grained inhomogenous regions suggesting uniform distribution of BaO additions in the powders during milling. Doped samples however continue to exhibit severe bimodal tendencies. BaO additions in the system may have a quenching effect on the liquid phase behavior in the system leading to uneven dopant distribution and bimodality. Doped and undoped systems with BaO additions have densities >90% (refer to Table 5).

- 110 -

Table 6 Results for systems processed with titanium isopropoxide additions Sintering Room Ba/Ti Theoretical Addition pH Temperature Grainsize (µm) Temperature Ratio Density% (°C) Resistivity (Ocm) Min Max 34.2mg 1320 <0.5 97 5.E+05 0.9988 TiO 2+ 0.24 5.57 8.98 m/o Y2O3 1375 bimodal(3-20) 92 9.E+02 34.2mg 1320 bimodal(2-20) 85 3.E+02 0.989 TiO 2+ 0.24 5.5 9.02 m/o Y2O3 1375 bimodal(5-20) 92 8.E+02

- 111 - Ba-excess systems which have been processed with titanium isopropoxide additions (Ba/Ti ratio 0.9988), exhibit little growth around the eutectic temperature possibly due to the effect of Ti-excess pinning mechanisms on samples sintered around the eutectic resulting in extremely small grained high density (~97%) with high RT resistivities. Samples sintered at 1375°C are bimodal and similar to samples processed with direct TiO 2 additions. Smaller particles generated by titanium isopropoxide additions appear to have reduced the amount of abnormal growth in the samples, while not completely eliminating it. These samples are semi-conducting and have a lower density

(~90%).

Donor doped Ti-excess systems with titanium isopropoxide additions (Ba/Ti ratio

0.9890) exhibit typical Ti-excess behavior with the formation of liquid phase leading to bimodality at lower temperatures with low density (~85%) with some improvement in microstructure and higher density (~93%) at higher temperature (refer to Table 6).

A microstructure comparison of systems processed under different condition is shown in Figures 47-49. Near stoichiometric blended systems exhibit uniform (Figure 47) high density microstructures, while systems processed with oxide additions (Figures 48 and 49) are bimodal.

- 112 -

47 48

·

49

Microstructures of donor doped near stoichiometric systems with Ba/Ti ratio 0.9988 under different processing conditions sintered at 1375°C. Figure 47. Blended system processed with 50mol% 0.994, 50 mol% 1.0038 powder. Figure 48. Near stoichiometric system with direct oxide addition Figure 49. Near stoichiometric system with isopropoxide addition

- 113 - 5.4 Summary of microstructure results

· Undoped systems processed with BaO and TiO 2 additions have large grained microstructures tending to large grain bimodality at the higher temperatures. Undoped systems with TiO 2 direct oxide additions result in inhomogeneous regions being observed. High densities are found in these systems

· Donor doped near stoichiometric systems processed with BaO and TiO 2 additions exhibit bimodal microstructures resulting from complex interactions occurring during sintering.

· Donor doped Ti-excess systems exhibit coarse grained microstructures with lower densities and significant amounts of liquid phase presence.

- 114 - 6 Results and Discussions of Electrical studies

6.1 PTCR effect in base systems

The PTCR effect observed in the base donor-doped systems is shown in Figure

50. This effect has been achieved using only 0.24m/o Y2O3 donor dopant. Donor doped base Ti-excess samples (Ba/Ti ratio 0.9988) exhibit bimodal microstructures coupled with low densities (~86%-89%). These samples show ~3 order of magnitude PTCR rise.

Donor doped base Ba-excess samples (Ba/Ti ratio 1.0038) have uniform small grained microstructures (~4-6µm), with high densities (~93-97%), exhibit less than one order of magnitude in rise in resistivity.

A number of factors may be responsible for these observations. The Ti-rich systems exhibit liquid phase sintering behavior above the eutectic. Dopant incorporation is enhanced62 resulting in lower RT resistivities. Lower densities and insulating second phase regions surrounding the grain boundaries result in an improved PTCR rise (refer to

Figure 52). In the case of Ba-rich systems the dopant incorporation is reduced due to lower amounts or no liquid phase. Dopant segregation at the boundaries, along with the decrease in the amount of liquid phase, results in inhibition of grain growth. Suppression of grain growth during sintering has been shown to improve densification as pore elimination mechanisms are enhanced63. Microstructure refinement is expected and can be seen by the uniform grains and high densities in these samples (Figures 31 (a) and

(b)). High density microstructures are representative of greater grain to grain contact and higher grain boundary volume (see Figure 51). High grain boundary coherence results in improved electron flow between grains.33,34 Varying degrees of semi-conduction may be

- 115 -

1.E+07

1.E+06

Ocm 1.E+05 Ba-excess

1.E+04

1.E+03 Resistivity 1.E+02

1.E+01 Ti-excess

1.E+00 0 50 100 150 200 250

Temperature °C

Figure 50. Comparison of PTCR effect in base Ba-excess (Ba/Ti ratio 1.0038) and Ti- excess(Ba/Ti ratio 0.994) systems. Samples sintered at 1375°C.

51 52

Figure 51. Etched microstructure of base Ba-excess sample sintered at 1375°C. Figure 52. Etched microstructure of base Ti-excess sample sintered at 1375°C.

- 116 - observed at room temperature. Previous studies have shown an inverse relationship between density and PTCR properties in the material64,65 with higher PTCR properties being obtained in materials with low densities, higher concentration of insulating or amorphous phases or increasing porosity present.

6.2 Systems with oxide additions

The PTCR effect in systems which have been processed with direct oxide additions are significantly affected with less than 2 order of magnitude change in resistivity occurring. This effect is found regardless of the oxide (BaO or TiO 2) addition and method of incorporation (direct oxide or metallorganic).

Figure 53 shows the comparison plot of the PTCR effect on optimally donor doped systems with BaO and TiO 2 additions carried out by the direct oxide method. It is interesting to note that both systems appear to have similar room temperature resistivity behavior. At Tc the system processed with TiO 2 additions attains a higher resistivity ?max caused by the improved liquid phase characteristics which would be expected due to the excess Ti in the system, from the TiO 2 addition.

6.2.1 Stoichiometric effect on PTCR behavior in systems with oxide additions

Figures 54 and 57 show the PTCR plots of near stoichiometric systems with Ba/Ti ratios 0.9988 and 1.0013 respectively. All these batches have the same final calculated

Ba/Ti ratio. Only the manner in which this has been arrived at is different. Each figure shows the comparison of PTCR curves for systems with TiO 2 additions, BaO additions and that of the blended system with identical Ba/Ti ratio.50 Blended systems are prepared

- 117 -

1.E+07

1.E+06

Ba-excess + 34mg TiO 2

Ocm 1.E+05

1.E+04

1.E+03 Resistivity

1.E+02 Ti-excess + 99mg BaO

1.E+01

1.E+00

0 50 100 150 200 250

Temperature °C

Figure 53. Comparison of samples with TiO 2 and BaO direct oxide additions. System with BaO addition has Ba/Ti ratio of 0.9988. additions System with TiO 2 addition has Ba/Ti ratio of 1.0013. Both samples have been sintered at 1375°C

- 118 -

1.E+07

1.E+06

1.E+05 Ba-excess + 34mg TiO 2 Ocm Ti-excess + 63mg BaO 1.E+04

1.E+03 Resistivity 1.E+02

1.E+01 Blended system

1.E+00 0 50 100 150 200 250

Temperature °C

Figure 54. Comparison of samples with oxide additions to blended system sintered at1375°C. Ba/Ti ratio 0.9988

55 56

Figure 55. Etched microstructure of Ba-excess + 34mg TiO 2 sintered at 1375°C Figure 56. Etched microstructure of Ti-excess + 63mg BaO sintered at 1375°C

- 119 -

1.E+07

1.E+06 Ti-excess + 99mg BaO

Ba-excess + 17mg TiO 2

Ocm 1.E+05

1.E+04

1.E+03 Resistivity 1.E+02

Blended system 1.E+01

1.E+00 0 50 100 150 200 250

Temperature °C

Figure 57. Comparison of systems with oxide additions to blended system sintered at 1375°C. Ba/Ti ratio 1.0013

- 120 - by mixing barium titanate powders of with Ba/Ti ratios in a fixed proportion resulting in an modified Ba/Ti ratio. Some very interesting observations may be made based on the comparisons between the two figures. It can be clearly seen that the blended systems have lower room temperature resistivity behaviors with a very low PTC effect of less than one order of magnitude. Systems with oxide additions in general have higher resistivities at room temperature with a modest PTCR effect of ~= 2 orders of magnitude.

The amount of BaO and TiO 2 additions have a marked effect on room temperature behavior and PTC rise characteristics in these systems. Increasing oxide additions of the respective oxide result in higher room temperature resistivities and lower PTC rise in the samples. It is likely that complex interactions during sintering along with inherent inhomogenities present in the system and little or no liquid phase result in degrading the

PTC properties in systems processed with oxide addtions.

Polished and etched surfaces of doped samples with BaO and TiO 2 additions respectively are shown in Figures 55 and 56. The bimodal nature of these microstructures can be clearly seen in the images. Further the high degree of grain boundary coherency appears to have resulted in the poor PTC properties exhibited in these systems.

6.2.2 PTCR properties of systems with Ti-isopropoxide additions

Figure 58 shows the comparison of optimally processed systems with titanium isopropoxide additions. The Ba-excess system with the titanium isopropoxide addition

(Ba/Ti ratio 0.9988) sintered at 1375°C is semiconducting (~900 Ocm) and exhibits a gradual rise in resistivity at Tc of ~2 orders. This system was found to exhibit bimodal

- 121 -

1.E+07

1.E+06 Ti-excess + 34mg TiO 2

1.E+05 Ocm 1.E+04

1.E+03

Resistivity Ba-excess + 34mg TiO 2 1.E+02

1.E+01

1.E+00

0 50 100 150 200 250

Temperature °C

Figure 58. Comparison of systems with Titanium isopropoxide additions. Ti-excess system with TiO 2 addition has Ba/Ti ratio 0.9890. Ba-excess system with TiO 2 addition has Ba/Ti ratio 0.9988. Both sintered at 1375°C

59 60

Figure 59. Etched microstructure of Ba-excess sample with 34mg TiO 2 addition from Titanium-isopropoxide sintered at 1375°C Figure 60. Etched microstructure of Ti-excess sample with 34mg TiO 2 addition from Titanium-isopropoxide sintered at 1375°C

- 122 - tendencies (3 µm- 20 µm) with a density of ~92. The rather gradual PTC rise may be attributed to the bimodal microstructure in the system. Figure 59 of the etched surface of the sample shows a microstructure similar to ones found in samples with oxide additions showing a high degree of grain boundary coherence and the lack of second phases in the grain boundary regions. As seen in samples with direct oxide additions these microstructures have resulted in poor PTC characteristics.

In Ti-excess systems excessive liquid phase has been generated by the titanium isopropoxide additions. Large amounts of second phase regions surrounding the grain boundaries are noticed in these samples. The amount of second phase present increases with sintering temperatures. Semiconduction is noticed in samples sintered at both

1320°C( ~300 Ocm) and 1375°C(~800 Ocm). Samples sintered at 1320°C show a PTC rise of ~3 orders. In samples sintered at 1375°C however the PTC behavior is more pronounced with an almost vertical rise in resistivity at TC and a PTC rise of ~4 orders.

This abrupt rise may be attributed to the the excessive second phase generaterated at this temperature. Figure 60 of the etched surface shows large amounts of second phase liquid along portions of the grain boundaries. Also noticeable is the large grain size of the sample and the resulting higher porosity in the microstructure. These characteristics clearly show that coarse microstructures result in large polar grains and low RT resistivities while the insulating second phases in grain boundary regions produce an almost vertical rise in resistance at Tc. Samples sintered at 1320°C show a PTC rise of ~3 orders. In samples sintered at 1375°C however the PTC behavior is more pronounced with an almost vertical rise in resistivity at TC.and a PTC rise of ~4 orders. This abrupt rise may be attributed to the the excessive second phase generaterated at this temperature.

- 123 - 6.3 General trends in PTCR studies

Figure 61 shows a comparison of the PTC effect in blended systems and systems with oxide additions in the near stoichiometric range. Also shown is the PTC rise of a high Ti-excess system. Donor doped near stoichiometric systems processed with oxide additions have diffuse PTCR properties with ~2 order rise in resitivity which is greater than that found in stoichiometric blended systems. Near stoichiometric systems with isopropoxide additions appear similar to direct oxide systems with PTCR behavior of ~2 orders. All the above systems exhibit microstructures with a high grain boundary coherency and a lack of second phases in the grain boundaries.

Ti-excess systems which have been processed with oxide additions exhibit the best PTC response characteristics in this study with a rise of ~4 orders suggesting a distinct correlation between the amount of second phases present in the samples to the effective PTC rise. These studies seem to suggest the need for coarse microstructures and the generous presence of second phases in order to achieve high PTC rise characteristics in these systems.

- 124 -

1.E+07

1.E+06 Ti-excess System

Ocm 1.E+05

1.E+04 Near Stoichiometric system with oxide addition 1.E+03 Resistivity 1.E+02 Near Stoichiometric blended 1.E+01 system

1.E+00 0 50 100 150 200 250

Temperature °C

Figure 61. Comparison of systems processed by different routes. All systems have Ba/Ti ratio 0.9988 and have been sintered at 1375°C

- 125 - 6.4 Summary of PTCR studies

· Donor doped near stoichiometric systems with oxide additions tend to exhibit poor PTCR properties with a diffuse rise ~=2 orders of magnitude. These systems are characterized by bimodal microstructures exhibiting high grain boundary coherency.

· Ti-excess systems processed with oxide additions exhibit good PTC rise characteristics with an almost vertical rise of ~4 orders. The presence of coarse grained microstructures in conjunction with large liquid phase presence and lower densities are responsible for enhanced PTC properties in these systems.

- 126 - 7 Conclusions

The studies carried out point to a number of significant results of value from both industrial and research standpoints. Processing trends show that the sensitivity of oxide systems to aqueous solutions, the nature of dopant additions, effects of particle size and the resulting sintering behavior can drastically affect final microstructures and properties in these materials.

Dissolution mechanisms in barium titanate powders have been found to be sensitive to solution pH with systems processed in acidic to mildly alkaline ranges exhibiting preferential dissolution as observed by the double layer formation in TEM.

Systems processed in highly alkaline pH (= 12) are relatively free from artifacts of preferential dissolution. Nitrate dopants have a marked influence on solution processes drastically reducing pH in the system and in some cases going into the acidic range.

While the effects of solution processes, taken by themselves, appear to have far reaching consequences, it has been found through these studies that the dominant mechanisms involved in influencing microstructure and property characteristics in these materials occurs during the high temperature sintering processes. High temperature processes were found to override inhomogenities resulting from solution processes, as indicated by the type of microstructures and properties developed in solution processed near stoichiometric systems as well as Ti-excess systems.

A significant finding in this study is the importance of coarse grained microstructures and liquid phase presence in obtaining good PTC properties. Ti-excess with coarse microstructures and significant liquid phase presence in the grain boundary regions exhibit a PTC rise of ~4 orders with the rise being almost vertical. Near

- 127 - stoichiometric systems however exhibit poor PTC rise characteristics. These systems exhibit bimodal microstructures with high grain boundary coherence and minimal presence of second phases.

Working in the near stoichiometric region can be advantageous in obtaining uniform high density microstructures by means of blended systems. Achieving high density uniform microstructure characteristics through other processing routes has met with limited success. On attempting to replicate these properties by means of simpler and more economical means such as processing with oxide additions, one observes the complex interactions of system inhomogenities and process sensitivities, that result in dominance of abnormal growth mechanisms and bimodal microstructures which are unsuitable for PTC applications. Stoichiometric blended systems with their superior microstructures and high densities need to be studied further as these systems may have valuable uses in dielectric materials and device technologies.

- 128 - 8 Future Work

· TEM EDS analysis of bi-layer and monolayer formations on particle surfaces using JEOL FX 2010 TEM could conclusively prove the existence and elemental composition of the particle surfaces. This can be used in developing a broad mechanism to better understand the dissolution trends occurring in complex multi component oxide systems.

· Study microstructure and properties of systems which have been filtered after processing with oxide additions. Comparisons with unfiltered systems can help in understanding the surface behaviors of particles during sintering and their effects on PTC properties.

· Metallorganic oxide precursors show favourable properties during solution processing due to the fine particle sizes generated. Nitrate ions introduced into solution in order for dopant incorporation appear to have negative effects on the solution processes. Dopant incorporation by less intrusive techniques such as metallorganic additions and additions of salts with lower ionic activities need to be looked at.

· Non aqueous alcohol based processing systems need to be studied in more detail as the polar nature of aqueous systems tend to be unsuitable for producing powders of stringent specifications and uniform particle surface characteristics.

· Liquid phase generation during sintering is a critical phenomenon controlling grain growth, densification, dopant distribution and electrical properties in the material. A detailed understanding of liquid phase evolution and

- 129 - effective means of controlling the liquid phase during sintering can lead to superior microstructures and materials properties.

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