Variability of the Sr/Ti Ratio in SrTiO3

TiO₂ is observed as a second phase in SrTiO₃ having Sr/Ti ≤ 0.995 (≥0.5 mol% excess TiO₂). The effect of excess TiO₂ on the equilibrium electrical conductivity at 1000°C is consistent with a solubility of <0.1 mol% TiO₂ with the formation of unassociated oxygen vacancies. More Ti0₂ is retained in solid solution when samples are quenched from the sintering temperature rather than furnace-cooled. The effect of excess SrO on the equilibrium electrical conductivity also indicates some solid solubility.     

Electrical Properties of Cerium-Doped BaTiO3

The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO₃ was studied in terms of oxygen partial pressure, P (O₂), and composition. In (Ba_1−xCe _x )TiO₃, the conductivity follows the −1/4 power dependence of P (O₂) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P (O₂) where electron compensation prevails at low P (O₂). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO₃, indicating the substitution of Ce as Ce⁴⁺ for Ti⁴⁺ in Ba(Ti_1−yCe _y )O₃. The Curie temperature ( T _c) was systematically lowered when Ce³⁺ was incorporated into Ba²⁺ sites, whereas the substitution of Ce⁴⁺ for Ti⁴⁺ sites resulted in no change in this parameter.     

Final Sintering of Cr2O3

The effect of oxygen activity on the sintering of high-purity Cr₂O₃ is shown. Theoretical density was approached at the equilibrium O₂ partial pressure needed to maintain the Cr₂O₃ phase ( P _o2=2×10⁻¹² atm). The presence of N₂ in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O₂ activity, correlated with the changing predominant volatile species in the Cr-O system.     

Phase Equilibrium in the System PbO-TiO2–ZrO2

Phase equilibrium relations in the system PbO–TiO₂–ZrO₂ were studied by quenching in the range where the PbO content is 50 mole % and more. Isotherms were examined at 1100°, 1200°, and 1300°C and tie lines were determined between the liquid and solid solution in equilibrium. The incongruent melting point of PbZrO₃ was 1570°C and the equilibrium between liquid, PbO-type solid, and PbZrO₃ is peritectic. Pb(Zr,Ti)O₃ solid solutions containing more than 14 mole % PbZrO₃ decomposed to liquid, ZrO₂, and Pb(Zr,Ti)O₃ and the decomposition temperature rises from 1340° to 1570°C with increasing PbZrO₃ content. The system PbTiO₃–PbZrO₃ should not be treated as a binary, but as a section of the ternary system.     

Sintering Chromium Oxide with the Aid of TiO2

The influence of additives on the sintering behavior of Cr₂O₃ was studied. The oxygen pressure and temperature at which the sintering rate becomes significant are dramatically altered by adding ∼ 1 wt% TiO₂; the sintering temperature decreases, whereas the oxygen pressure for appreciable densification increases. At temperatures as low as 1280°C, densities of 92% of theoretical were obtained. Densification was examined as a function of temperature, oxygen pressure, and TiO₂ concentration. Several other transition metal oxides were tested for their ability to enhance sintering; none was successful.     

Effect of Sintering Atmosphere on Isolated Pores During the Liquid-Phase Sintering of MgO-CaMgSiO4

The shrinkage behavior of isolated pores during the liquidphase sintering of MgO-CaMgSiO₄ at 1650°C in O₂ and N₂ atmospheres has been studied. When 90MgO- 10Ca MgSiO₄ specimens containing artificially produced large spherical pores are sintered in O₂, the liquid and grains flow into the pores as oxygen diffuses out. When sintered in N₂ the pores remain intact even after a long time, because the N₂ gas entrapped in them does not diffuse out. The effect of the sintering atmosphere has also been studied in a fine powder mixture of 80MgO 20CaMgSiO₄ composition. Changing the atmosphere from O₂ to N₂ during the sintering treatment reduces the porosity, probably because of the enhanced oxygen diffusion from the pores. The pores grow when the sintering atmosphere is changed from N₂ to O₂, probably because of oxygen diffusion into the pores from the specimen surface. The practical implication of these results is that changing the atmosphere from O₂ to air during the liquid-phase sintering of oxide ceramics can greatly reduce the porosity.     

Titania as a Sintering Additive in Indium Oxide Ceramics

The influence of TiO₂ additives on the sintering behavior of In₂O₃ ceramics has been investigated. TiO₂ increases the densification rate, decreases the grain growth during the intermediate stage of sintering, and hinders the pore/boundary breakaway that can affect the final stage of sintering. For a given grain size, TiO₂ shifts the grain size/density trajectory toward higher densities. TiO₂ mainly acts by a second-phase mechanism, but it also may decrease the decomposition rate of In₂O₃.     

Effect of Sintering Atmosphere on Densification of MgO-Doped Al2O3

The densification behavior of Al₂O₃-MgO (0.1 wt%) has been studied in O₂ and N₂ atmospheres. Powder compacts have been sintered at 1600°C for 0.5 to 8 h. For some specimens the sintering atmosphere has been changed after 30 min of sintering. Irrespective of sintering atmosphere, sintered densities are approximately the same up to 99% relative density, implying that the capillary pressure effect dominates the atmosphere effect for most of the densification stage. During extended sintering treatment the density of specimens sintered in O₂ becomes higher than that in N₂. When the sintering atmosphere is changed from O₂ to N₂, enhanced densification results, and vice versa. Such an effect of sintering atmosphere is explained by the diffusiveness of gases entrapped in pores, as well as by oxygen potential differences inside and outside of the specimen. Differences in grain growth rate in various atmospheres are discussed on the basis of different densification rates.     

Compositional Change and Compositional Fluctuation in Pb(Zr,Ti)O3 Containing Excess PbO

Compositional changes which take place during sintering of Pb(Zr,Ti)O₃ (PZT) containing excess PbO were studied. The excess PbO forms a liquid phase during the sintering process. The solubility of the TiO₂ component of PZT in liquid PbO is higher than that of ZrO₂ component. Thus, if an excess PbO exists, the composition of PZT phase shifts towards the Ti-lean side. A change in the lattice constants due to this compositional change was actually observed. Coexistence of tetragonal and rhombohedral phases, due to a compositional fluctuation caused by excess PbO, was observed near the morphotropic phase boundary. When PZT containing excess PbO was sintered at 1100°C, a compositional fluctuation occurred early in the process and then decreased with sintering time. These phenomena have agreed with a result of computer simulation of dissolution of TiO₂ component in PZT phase into liquid PbO phase.     

Effects of Sintering Atmosphere on Densification Behavior and Piezoelectric Properties of Pb(Ni1/3Nb2/3)O3–PbZrO3 Ceramics

Densification of polycrystalline Pb(Ni_1/3Nb_2/3)O₃–PbTiO₃–PbZrO₃ (PNN–PT–PZ) specimens was enhanced as the partial pressure of O₂ in the sintering atmosphere was increased. This observation was attributed to the increase in the internal pressure of a closed pore due to the thermal decomposition of PbO at a low partial pressure of O₂. The relative dielectric permittivity (ε_r), d ₃₃, k _p, and grain size of sintered specimens were also increased as the partial pressure of O₂ in the sintering atmosphere was increased. The observed dependence of piezoelectric properties on the partial pressure of O₂ was discussed in terms of the enhanced formation of the A-site vacancy ( V "_Pb) or the suppression of the B-site defect ( V ¨_O) as the oxygen potential increased.     

Enhanced Shrinkage of Lanthanum Strontium Manganite (La0.90Sr0.10MnO3+δ) Resulting from Thermal and Oxygen Partial Pressure Cycling

Exposure of (La_0.90Sr_0.10)_0.98MnO_3+δ (LSM-10) to repeated oxygen partial pressure cycles (air/10 ppm O₂) resulted in enhanced densification rates, similar to behavior shown previously due to thermal cycling. Shrinkage rates in the temperature range 700°–1000°C were orders of magnitude higher than Makipirtti–Meng model estimations based on stepwise isothermal dilatometry results at a high temperature. A maximum in enhanced shrinkage due to oxygen partial pressure cycling occurred at 900°C. Shrinkage was the greatest when LSM-10 bars that were first equilibrated in air were exposed to gas flows of lower oxygen fugacity than in the reverse direction. The former creates transient cation and oxygen vacancies well above the equilibrium concentration, resulting in enhanced mobility. These vacancies annihilate as Schottky equilibria are reestablished, whereas the latter condition does not lead to excess vacancy concentrations.     

Mechanism of ZrTiO4 Synthesis by Mechanochemical Processing of TiO2 and ZrO2

High-energy ball milling initiates a solid-state reaction in an equimolar mixture of TiO₂ and ZrO₂. The first stage of ball milling induced the transformation of anatase TiO₂ to high-pressure phase TiO₂ (II), isostructural with ZrTiO₄. The formation of solid solutions monoclinic ZrO₂/TiO₂ and TiO₂ (II)/ZrO₂ was observed in the intermediate stage. Afterward, a nanosized ZrTiO₄ phase was formed in the milled product from the TiO₂ (II)/ZrO₂ solid solution. The sintering of the milled product at a temperature <1100°C was examined in situ by Raman spectroscopy. The full solid-state reaction toward ZrTiO₄ ceramic is completed at a temperature considerably lower than reported in the literature.     

Effect of SiC on Interfacial Reaction and Sintering Mechanism of TiB2

Compacts of TiB₂ with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB₂. The oxygen content of the raw TiB₂ powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO₂ and B₂O₃. Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB₂ samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO₂+ SiC → SiO₂+ TiC. Therefore, the improved sinterability of TiB₂ resulted from the SiO₂ liquid phase that was formed during sintering when the raw TiB₂ powder had 1.5 wt% oxygen.     

TiO2 Produced by Vapor-Phase Oxygenolysis of TiCI4

The formation of TiO₂ powders by oxygenolysis of TiCI₄ was studied with emphasis on the effects of reaction conditions on the particle size of the products. The particle size of TiO₂(anatase) decreased with increasing reaction temperature or O₂concentration and with decreasing TiCI₄ concentration. The results are compared with those for the oxygenolysis of AlBr₃and SiCI₄. It was found that the reactivity of metal halides with O₂ is closely related to the ease of dissociation of the first halogen atom.     

Phase Relations in the Pseudobinary System Na2TiSi4O11-Na2Ti2Si2O9

The quenching technique was used to study subliquidus and subsolidus phase relations in the pseudobinary system Na₂ Ti₂Si₂ O₁₁-Na₂ Ti₂ Si₂ O₉. Both narsarukite (Na₂TiSi₄O₁₁) and lorenzenite (Na₂Ti₂Si₂O₉) melt incongruently. Narsarsukite melts at 911°±°C to SiO₂+liquid, with the liquidus at 1016°C. Lorenzenite melts at 910°±5°C to Na₂ Ti₆ O₁₃+liquid; Na₂ Ti₆ O₁₃ reacts with liquid to form TiO₂ and is thus consumed by 985°±5°C. The liquidus occurs at 1252°C.     

Auger Electron Spectroscopy Analysis of Cross-Section Surface of Oxidized Titanium Carbide Single Crystal

A TiC monocrystal was heated by three thermal cycles with isotherm at 1108 K while exposed to Ar/O₂ mixtures respectively with O₂ contents of 239, 1.0, and 324 ppm. The reactivity was detected with a homemade device based on two identical solid electrolyte oxygen sensors connected to a quadrupole mass spectrometer (QMS). The oxidized sample was cleaved by impact bending under high vacuum and the cross section investigated by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) techniques. Both area and profile AES spectra were acquired by using a 70 nm diameter beam. Spectral changes have been analyzed to identify chemical species present at the TiC/TiO₂ interface. A model for high-temperature oxidation of TiC has been proposed. It implies oxygen diffusion through a protective TiO₂ layer and the existence of two inner interfaces: TiC/TiC_{1− x} O _x and TiC_{1− x} O _x /TiO₂.     

Microwave Dielectrics in the (La1/2Na1/2)TiO3–Ca(Fe1/2Nb1/2)O3 System

Dielectric properties of the system (1 − x)(La_1/2Na_1/2)TiO₃– _x Ca(Fe_1/2Nb_1/2)O₃, where 0.4 # x # 0.6, have been investigated at microwave frequencies. The temperature coefficient of resonant frequency (τ_f), nearly 0 ppm/°C, was realized at x = 0.58. These ceramics had perovskite structure and showed relatively low dielectric losses. A new dielectric material applicable to microwave devices having Q · f of 12000–14000 GHz and a dielectric constant (ε_r) of 59–60 has been obtained at 1300–1350°C for 5–15 h sintering.     

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.     

Densification and Microstructural Development of SrTiO3 Sintered with V2O5

Precipitation of TiO₂ occurs during the sintering of SrTiO₃ with V₂O₅ added as a liquid-phase sintering agent. Satisfactory densification can be obtained at 1250°C when using a high content of V₂O₅ during sintering. However, a microstructure of fine grains and large pores results along with the precipitation of TiO₂. The precipitation of TiO₂ can be repressed by the addition of excess SrO. A well-sintered microstructure with superior densification can thus be obtained at 125O°C from specimens sintered with a low content of V₂O₅ and an appropriate amount of excess SrO.     

Effect of CuO on the Sintering Behavior and Dielectric Characteristics of Titanium Dioxide

The sintering behavior and dielectric characteristics of a titanium dioxide (TiO₂) were investigated by adding CuO. The addition of CuO lowered the sintering temperature of the TiO₂. At a given temperature, the densification and grain growth rate for a TiO₂ with CuO were enhanced, compared with those of pure TiO₂. The results suggest that CuO acts as an activator for the sintering of TiO₂. Dielectric constant and loss were also examined for both pure and CuO added TiO₂ samples. The variations of dielectric constant and loss were discussed in terms of grain size, porosity, and oxygen deficiency.