Influence of Oxygen Partial Pressure on Properties of Semiconducting Barium Titanate

Abstract

June 1 !1B3Journal of 7'he American Ceramic (3) At GOO'C, tlic large specimens showed a relatively large increase in density (1%) for runs up to 20 minutes. Devitrificatioii occurred along the edges of the specimens when rum were made for 45 minutes. X-ray investigation indicated the formation of coesite. As the specimens were not sealed in the platinum, some water vapor may have catalyzed the reaction. No bircfringencc mcasuremcnt could be made, as the cracks were too numerous; the specimens were definitely distorted.When small specimens were used, the following results were observed:(1) At 400"C, thc curve showing the increase in density versus time reached an asymptotic value much lower than in the previous experiments at the Same temperature. A slight birefringence was noted.(2) At 6OO0C, the trend of the curve was similar to that obtained previously a t the same temperature. The rate of coesite formation is small.A uniform pressure applied to a glass specimen should not introduce any double refraction in the material. Waxler and Ada1ns7 reported that no birefringence results when glasses and polymers are subjected to hydrostatic pressures. The birefringence found when silica is compressed a t temperatures shows that the pressure is not isostatic. The slight change of shape of the specimen shows that flow takes place. The shearing stresses at each temperature are not of the same magnitude (a large scatter of density data is found a t high temperatures; there is more uniformity a t low temperatures) and depend also on time. It sceiiis that the magnitude of the shearing stresses differs according to the high-pressure device and to the experimental cell design; when powders are used, shearing, effects a t the point of contact of particles should be considered. This explains the large discrepancy between the results obtained by various workers. The results reported here agree with Bridgman,s who in a later experiment concluded that plastic shearing deformation plays an essential role in the increase in density; a similar conclusion was reached by Mackenzie and L a f~r c e .~ The changes obtained in this study at low temperature may have been so small because a better approximation of isostatic pressure can be achieved in the tetrahedral anvil apparatus at low temperature. It seems to the writer that fused silica cannot be compacted irreversibly by hydrostatic means. Neutron-irradiated silica reverts to ordinary fused silica during annealing. In their study of the effect of pressure (up to 120 kbars) a t room temperature on optical absorption spectra of transition metal ions in glass, Drickamer and Tischerlo show that the effects are reversible on the release of pressure except for phosphate glasses, where irreversible behavior occurs only above 50 kbars. Weir et al." also reported that following the release of pressure the refractive index of glass recovers its initial value after a few hours.7R. M. Waxler and L. H. Adams, "Effect of Hydrostatic large positive resistivity-temperature anomaly (PTC) which occurs a...

Influences of Ba/Ti Ratios on the Positive Temperature Coefficient of Resistivity Effect of Y-Doped BaTiO3-(Bi1/2Na1/2)TiO3 Ceramics

“…There is only a slight increase at the T c of ∼175°C. This is consistent with the traditional BT PTCR ceramics when sintered in a reducing atmosphere 13 . It has been known that the reduction–reoxidation method can also be used to fabricate BT–BNT ceramics 14,15 .…”

“…This is consistent with the traditional BT PTCR ceramics when sintered in a reducing atmosphere. 13 It has been known that the reductionreoxidation method can also be used to fabricate BT-BNT ceramics. 14,15 In this work, the sintered BBNT5 was reoxidized at 11001C for 2 h in air, which also shows a remarkable PTCR effect of 3.2 orders of magnitude (as shown in the inset of Fig.…”

Local variability in PTC thermistor grain boundary structures

“…In this model the rapid increase in grain boundary resistivity above T C is explained in terms of the combined effects of a sudden decrease in permittivity, the presence of filled acceptor traps at the grain boundaries within a segregated layer of acceptor ions (Heywang, 1971) or associated with adsorbed oxygen (Kuwabara, 1984;Tien and Carlson, 1963), and the loss of ferroelectric polarization, which together allow the development of back-to-back opposed electrostatic barriers at the grain boundaries. The structures so formed are termed double Schottky barrier (DSB) structures.…”

Structure and Dielectric Properties of Eu-Doped Barium Titanate Ceramics

“…RE ions are trivalent in most of their compounds, 14,15 and generally occupy Ba sites of BaTiO 3 . Charge equilibrium may be compensated by electrons, [16][17][18] A-site Ba vacancies, 19,20 B-site Ti vacancies 1,2,21,22 or the 3d 1 Ti 3ࢪ mode, [23][24][25] due to the different preparation techniques and ratios of starting compositions.…”