Grain Growth During Sintering of Donor-Doped BaTiO3

Grain growth of donor-doped BaTiO₃ in the presence of KF b'quid phase was studied. The results showed that the composition of the liquid phase present during sintering had a pronounced influence on grain growth and on the formation of semiconducting donor-doped BaTiO₃.     

Effects of Powder Processing on the Green Compacts of High-Purity BaTiO3

Two processing methods have been used to prepare different green compacts of high-purity BaTiO₃. The substructures of agglomerates have been examined by SEM, and the strength of the agglomerates was interpreted in a plot of the logarithm of the pressure versus the relative density, which is consistent with the structures of cut surfaces of green compacts. The Hg-penetration results have been used to compare the uniformity of the powder compacts. The wer method (wet milling and subsequent wet pressing) was used to produce uniform green compacts.     

Initial Sintering of MnXO-Al2O3

Effects of manganese oxide on the initial sintering kinetics of compacts of 5-μ diameter alumina were studied by isothermal shrinkage measurements from 1450° to 1650°C. The observed rates were characterized by assuming a volume-diffusion mechanism. Variations in sintering rate with both oxygen partial pressure in the furnace and impurity concentration, as well as the observed activation energy, indicated oxygen-ion diffusion was the rate-limiting step during densification.     

Release of Oxygen During the Sintering of Doped BaTiO3 Ceramics

The release of oxygen during the sintering of Sb₂O₃-doped BaTiO₃ ceramics containing excess TiO₂ was measured using a mass spectrometer. The amount of oxygen released is proportional to the dopant concentration in the product phase. The evolution of oxygen during sintering was attributed to dissolution of the oxidized form of doped BaTiO₃ in the reacting mixture and simultaneous re-crystallization of the- reduced form.     

Reaction Rates of BaTiO3 and SrTiO3

The temperature dependence of the reaction rates in a 50(BaTiO₃)-50(SrTiO₃) ceramic mixture (wt%) was studied by high-temperature X-ray diffraction. The data were fitted to two theoretical models and times for complete reactions and activation energy were calculated. Use of the results in electronic material applications is discussed.     

Solubility of TiO2 in BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO₃ specimens with a small excess of TiO₂ (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO₂-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO₂ content, Ba₆Ti₁₇O₄₀. The results indicate that the solubility of TiO₂ in BaTiO₃ is <0.1 mol%.     

Crystal Chemistry and Dielectric Properties in the Systems BaTiO3-UO2 and BaTiO3-BaUO3

Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO₂ in solid solution; its structure is then cubic at room temperature. With BaUO₃ additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO₂ goes into solid solution in BaTiO₃ but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO₂. The dielectric constant of BaTiO₃ ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO₂ but declines rapidly above this level. The dielectric constant of BaUO₃ is about two orders of magnitude lower than that of BaTiO₃, and additions of BaUO₃ invariably lower the dielectric constant of BaTiO₃.     

Nonstoichiometry in Undoped BaTiO3

Equilibrium electrical conductivity data for large-grained, poly crystalline, undoped BaTiO₃, as a function of temperature, 750° to 1000°C, and oxygen partial pressure, 10⁻²⁰< P _O2<10⁻¹ MPa, were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The latter are invariably present in sufficient excess to control the defect concentrations through the compensating oxygen vacancies, except under the most severely reducing conditions. Singly ionized oxygen vacancies play no discernible role in the defect chemistry of BaTiO₃ within this experimental range. The derived accidental acceptor content has a slight temperature dependence which may reflect some small amount of defect association. Deviation of the conductivity minima from the ideal shape yields a small P _O2-independent conductivity contribution, which is tentatively identified as oxygen vacancy conduction.     

PTCR Effect in BaTiO3

Calculations of bulk and surface defect energies were used to develop a model of the grain structure of doped BaTiO₃, Segregation processes are expected as a consequence of thermo-dynamic and kinetic factors and result in the development of n-i-n junctions at intergranular contacts, thus leading to the observed PTCR behavior of polycrystalline materials.     

Nonstoichiometry in Acceptor-Doped BaTiO3

The effect of added Al, as an acceptor impurity, on the equilibrium electrical conductivity of large-grained, polycrystalline BaTiO₃, is consistent with a previously proposed defect model which involves only doubly ionized oxygen vacancies, electrons, holes, and acceptor impurities. The behavior is an extension of that of undoped BaTiO₃, in which an accidental net acceptor excess already plays an important role. Comparison of the derived active acceptor content with the amount of added Al indicates that Al is <50% effective in creating acceptor levels. The magnitude of a small Po₂-independent conductivity component, necessary to fit the observed conductivity minima, increases with added Al content. This is consistent with a contribution from extrinsic oxygen vacancy conduction.     

Initial Sintering of α-Cr2O3

Since the difference between oxygen-ion and cation diffusion coefficients is greater for α-Cr₂O₃ than for α-Fe₂O₃ or α-Al₂O₃, a study of initial-sintering kinetics was undertaken to show unequivocally which species is rate controlling. Fine powders of α-Cr₂O₃, obtained by thermal decomposition of reagent-grade (NH₄)₂Cr₂O₇, were lightly compacted and their isothermal rates of shrinkage were determined between 1050° and 1300°C. Resultant data follow volume-diffusion sintering models, and calculated diffusion coefficients agree with, those measured for oxygen ions in α-Cr₂O₃. There is little evidence that oxygen diffusion along grain boundaries becomes so enhanced that chromium ions are left in control of the process.     

Effect of TiO2 on Initial Sintering of Al2O3

Alpha alumina with additions of TiO₂ sintered more rapidly than "pure" alumina. The rate of initial sintering increased approximately exponentially with titania concentration up to a percentage beyond which the rate of sintering remained approximately constant or decreased slightly with additional titania. The concentration which produces the maximum rate of sintering is thought to be the solubility limit of TiO₂ in Al₂O₃. For alumina particles larger than about 2 μm, the kinetic process was mainly grain-boundary diffusion. With smaller particles, volume diffusion increased. The "solubility limit" increased with decreasing particle size, indicating an excess surface concentration of TiO₂. The data may be interpreted in terms of a region of enhanced diffusion at the grain boundary that increases with TiO₂ concentration. With small alumina particles, this region is large enough to become a significant portion of the volume of the particle, and the small particles appear to sinter by volume diffusion kinetics, but the diffusion coefficient corresponds to an enhanced diffusion coefficient.     

Defect Chemistry of BaTiO3 with Additions of CaTiO3

The substitution of up to 5% Ca²⁺ for Ba²⁺ in BaTiO₃ results in a shift in the oxygen pressure dependence of the equilibrium electrical conductivity that is in the same direction as that caused by addition of acceptor impurities such as Al³⁺ or Ca²⁺ substituted for Ti⁴⁺. In contrast to the latter effect, however, the shape of the conductivity plot is not changed, the conductivity value at the conductivity minimum is not affected, and the amount of the shift increases with decreasing temperature of measurement. It is shown that the shift is primarily due to an increase in the enthalpy of reduction and a decrease in the enthalpy of oxidation as increasing amounts of Ba²⁺ are replaced by Ca²⁺.     

Influence of Interdiffusion on Solid Solution Formation and Sintering in the System BaTiO3-SrTiO3

Formation of BaTiO₃-SrTiO₃ solid solution during sintering of powder mixtures is characterized by preferential diffusion of Ba²⁺ ions. As a consequence, several nonequilibrium phases are temporarily formed; they were identified by X-ray and microprobe analysis. Eutectic liquid appears below 1300°C, which may explain exaggerated grain growth during sintering of BaTiO₃-SrTiO₃ mixtures. Disturbance in neck growth and Kirkendall-type porosity hamper densification in the heterogeneous system as compared with the pure titanates.     

Solubility of BaO in BaTiO3

The solubility and mode of incorporation for BaO in BaTiO₃ were studied by X-ray powder diffraction, scanning and transmission electron microscopy, electron probe microanalysis, and equilibrium electrical conductivity measurements. The presence of barium orthotitanate, Ba₂TiO₄, as a second phase for samples containing >0.1 mol% excess BaO was confirmed by direct microscopic examination. There was no evidence to support the incorporation of excess BaO into BaTiO₃ by a Ruddlesden-Popper type of superlattice ordering mechanism. Measurement of the equilibrium electrical conductivity showed no detectable shift in the conductivity profile resulting from excess BaO, thus setting an upper limit of 100 ppm for the solubility of BaO in BaTiO₃.