Bulk Conductivity and Defect Chemistry of Acceptor-Doped Strontium Titanate in the Quenched State

The grain bulk conductivity of acceptor-doped SrTiO₃ ceramics was investigated by the impedance analysis method after quenching from high-temperature equilibria. The influence of the oxygen partial pressure during equilibration, P ^Eq_O2, is described in terms of a defect model for titanates. From a comparison between the experimental results and the predictions of this model, a predominant ionic conductivity is concluded for a wide P ^Eq_O2 range (approximately 10⁻¹¹ to 10⁵ Pa). The influence of the ionization energy of the acceptors and of possible defect association is discussed.     

Nonstoichiometry and Mixed Conduction in α-Ta2O5

Electrical conductivity of single-crystal and polycrystalline Ta₂O₅ was measured as a function of composition, temperature, and oxygen partial pressure ( P _O2) by both dc and ac complex impedance methods. A defect model based on the assumption of predominant disorder on the oxygen sublattice is proposed to explain the observed nonstoichiometry of α-Ta₂O₅. The defect model is consistent with the observed P _O2-independent ionic conductivity and the P ^−1/4_O2-dependent electronic conductivity. Blocking of the ionic component was observed at low P _O2. Both the electronic and ionic components of conductivity were found to decrease with time. This aging phenomena is related to destabilization of the a phase.     

Influence of Oxygen Partial Pressure on the Quasi-Ternary System Cr─Mn─Ti Oxide

The quasi-ternary system Cr─Mn─Ti oxide was investigated at 1000°C under oxygen partial pressures ranging from 0.21 bar to 10^-21 bar (1 bar = 10⁵ Pa). X-ray diffraction analysis was used to identify phases and determine lattice parameters. The positions of phase boundaries as a function of oxygen partial pressure were measured using the emf method. The spinel MnCr₂O₄ may be regarded as the most interesting compound in this system. Part of the chromium can be replaced by trivalent titanium at low oxygen partial pressures and by trivalent manganese at high pressures, and the formation of a limited solid solution with the spinel Mn₂TiO₄ is possible in all cases. As a result, a coherent single-phase spinel region exists over the entire oxygen partial pressure range at 1000°C.     

Space-Charge Development in Glass

Data are given that graphically demonstrate space-charge build-up in a glass containing alkali and in a glass relatively free of alkali. The observed potential distributions show a decided asymmetry and have large potential drops near the electrodes with a linear potential change near the center of the sample. The observations qualitatively fit the expected theoretical distributions, according to Proctor and Sutton, for material having cations mobile and anions essentially immobile. Crude estimates arising from comparison with the theory yield carrier concentration near 2 × 10¹⁶ per cm³. mobility of the cation near 3 × 10^-6 cm.² per volt second, and diffusion constant near 2 × 10^-6 cm.² per second. These mobility and diffusion values are two orders of magnitude larger than values computed from conductivity data or computed from measurements of the diffusion coefficient for sodium cations, both in simple alkali silicates. According to the theory, these higher values are perhaps reasonable, since they are associated with only the more mobile portion of the cation population.     

Iron-Excess Manganese Ferrite: Electrical Conductivity and Cation Distributions

The electronic conductivity and thermoelectric power of a magnetic ferrite of composition Mn_0.863Fe_2.137O₄ have been measured as functions of oxygen partial pressure and temperature. Within the single-phase region of the spinel ferrite, both electrical properties are essentially independent of the oxygen partial pressure, implying that the electronic charge carrier concentration is governed by the molecularity rather than by oxygen nonstoichiometry. The electrical conduction behavior is successfully interpreted on the basis of a small polaron model, yielding a hopping energy of 0.363 ± 0.008 eV and a drift mobility of 0.05 to 0.10 cm²/(V·s) in the temperature range of 900 to 1300 K.     

Electrical Conductivity of Epitaxial SrTiO3 Thin Films as a Function of Oxygen Partial Pressure and Temperature

SrTiO₃ (100) epitaxial films with thicknesses of 3, 1 μm, and 250 nm were prepared on MgO (100) substrates by pulsed-laser deposition. The electrical conductivities of the thin films were systematically investigated as a function of temperature and ambient oxygen partial pressure. This was made possible by using a specially designed measurement setup, allowing the reliable determination of resistances of up to 25 GΩ in the temperature range of 600°–1000°C under continuously adjustable oxygen partial pressures ranging from 10⁻²⁰ to 1 bar. The capabilities of the measurement setup were tested thoroughly by measuring a SrTiO₃ single crystal. The well-known characteristics, e.g., the decline of the conductivity with a slope of –1/4 under reducing conditions and the opposite +1/4 behavior in oxidizing atmospheres, are found in the log(σ)–log( p O₂) profiles of the epitaxial films. However, the p -type conductivity decreases, and the n -type conductivity increases with decreasing film thickness. This phenomenon is attributed to the charge carrier redistribution in the surface space charge layers. Owing to the high surface-to-volume ratio, the space charge layers play an important role in thin films.     

Effects of Active Oxidation on the Flexural Strength of α-Silicon Carbide

The effects of oxygen partial pressure ( P _o2) on the oxidation behavior and room-temperature flexural strength of sintered α-SiC were investigated. Groups of flexure bars were exposed at 1400°C to flowing Ar containing various levels of oxygen ( P _o2 ranging from 7.5 × 10⁻⁷ to 1.5 × 10⁻⁴ MPa). The changes in weight, flexural strength, and surface morphology of the samples were strongly influenced by the P _o2 level. When the P _o2 was higher than 3 × 10⁻⁵ MPa, SiO₂ was formed on the surface (i.e., passive oxidation occurred) and the strengths of the samples were not significantly affected. However, when the P _o2 was lower than 2 × 10⁻⁵ MPa, material loss occurred (active oxidation), decreasing the weight and strength of the samples. Both the reduction in strength and the weight loss resulting from active oxidation were proportional to the P _o2. An approximately 50% reduction in strength was observed in the SiC after oxidation for 20 h at a P _o2 of 1.5 × 10⁻⁵ MPa, a level that is slightly lower than the P _o2 at which the transition from active to passive oxidation occurs. Large pits formed during exposure were responsible for the reduction in strength.     

Effect of Al2O3 Additions on the Thermal Expansion Behavior of a Li2O-ZnO-SiO2 Glass-Ceramic

The effect of Al₂O₃ substitution on the thermal expansion behavior of a Li₂O-ZnO-SiO₂ glass-ceramic was investigated using differential thermal analysis, X-ray diffractometry, and dilatometry. The coefficient of thermal expansion of the original Al₂O₃-free glass-ceramic measured between 20° and 900°C decreased from 12.4 × 10^-6°C^-1 to 6.3 × 10^-6°C^-1 with Al₂O₃ substitution for ZnO up to 11 wt%. The results were correlated to the changes in the phase assemblage with Al₂O₃ addition.     

绝缘衬底上化学气相沉积法生长石墨烯材料

利用化学气相沉积法,在Si衬底、蓝宝石衬底和SiC衬底上生长石墨烯材料,研究石墨烯的表面形貌、缺陷、晶体质量和电学特性。原子力显微镜、光学显微镜和拉曼光谱测试表明,Si₃N₄覆盖层可以有效抑制3C-SiC缓冲层的形成;低温生长有利于保持材料表面的平整度,高温生长有利于提高材料的晶体质量。5.08 cm蓝宝石衬底上石墨烯材料,室温下非接触Hall测试迁移超过1000 cm²·V^-1·s^-1,方块电阻不均匀性为2.6%。相对于Si衬底和蓝宝石衬底,SiC衬底上生长石墨烯材料的表面形态学更好,缺陷更低,晶体质量和电学特性更好,迁移率最高为4900 cm²·V^-1·s^-1。     

Microstructure and Dielectric Properties of Textured SrTiO3 Thin Films

We investigate the relationship between microstructure and dielectric properties of textured SrTiO₃ thin films deposited by radio-frequency magnetron sputtering on epitaxial Pt electrodes on sapphire substrates. The microstructures of Pt electrodes and SrTiO₃ films are studied by transmission electron microscopy, atomic force microscopy, and X-ray diffraction. SrTiO₃ films grown on as-deposited and annealed Pt electrodes, respectively, consist of a mixture of (111)- and (110)-oriented grains. Temperature-dependent dielectric measurements show that differences in texture and microstructure are reflected in the Curie–Weiss behavior of the SrTiO₃ films. Phenomenological models that account for the effects of thermal mismatch strain on the dielectric behavior are developed for different film textures. The models predict that at a given temperature, paraelectric (111)-oriented films of SrTiO₃ on tensile substrates will have a higher Curie–Weiss temperature and a greater dielectric constant than (110)-oriented films or bulk SrTiO₃. The experimental dielectric behavior is compared with the predictions from theory, and different contributions, such as interfacial layers, film stress, and microstructure, to the Curie–Weiss behavior are discussed.     

Conversion of Polycrystalline Al2O3 into Single-Crystal Sapphire by Abnormal Grain Growth

In a given batch more than 30%–40% of polycrystalline, MgO-doped Al₂O₃ tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4–10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c -axes oriented ∼ 90° and 45° to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many "curved" boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10⁻¹⁰ m³/(N·s), representing the highest value reported so far in Al₂O₃ and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La₂O₃, or ZrO₂ was added in concentrations of several hundred ppm.     

High-Temperature Hall Measurements on BaSnO3Ceramics

Simultaneous Hall and conductivity measurements were performed in situ between 650° and 1050°C on n-type semiconducting BaSnO₃ceramics. The variation of the Hall mobility and the Hall carrier density as a function of oxygen partial pressure between 10² and 10⁵ Pa and of temperature was investigated. At temperatures below 900°C the conductivity exhibits a dependence on temperature and oxygen partial pressure which is mainly determined by variations of the Hall mobility. Above 900°C most of the significant dependence is due to a variation in carrier density. Furthermore, a simple defect model assuming doubly ionized oxygen vacancies and acceptor impurities is discussed for BaSnO₃.     

High-Temperature Phase Relations in Selected MnO-CrOx-Containing Systems

Phase relations in the pseudobinary MnO-CrO _x system were studied by the reaction of the individual oxides in the temperature range 1400°-1750°C under a reducing atmosphere with a CO:CO₂ volume ratio of 4.8, yielding oxygen partial pressures in the range 10^-9.98-10^-6.97 atm. In the pseudoternary MnO-CrO _x -containing systems that constitute the pseudoquaternary MnO-CrO _x -CaO-SiO₂ system, phase relations were determined only at 1500°C under an oxygen partial pressure of 10^−8.99 atm. Characteristic of these MnO-CrO _x -containing systems was the dominance of the (Mn,Cr)₃O₄ spinel phase.     

Electrical Properties of Gallium-Doped Zinc Oxide Films Prepared by RF Sputtering

Ga-doped polycrystalline ZnO films on glass substrates were prepared by sputtering the targets, which had been prepared by sintering disks consisting of ZnO powder and various amounts of Ga₂O₃, to investigate the effects of gallium doping and sputtering conditions on electrical properties. Optimizing the RF power density, argon gas pressure, and gallium content, transparent Ga-doped ZnO films with resistivity less than 10⁻³Ω·cm were obtained. Electron concentrations for undoped and Ga-doped ZnO films were on the order of 10¹⁸ and 10²¹/cm³, respectively. The Ga-doped ZnO films became degenerate when the electron concentration exceeded ∼ 10¹⁹/cm³, and the optical band gap increased with increasing carrier concentration because of the increase of Fermi energy in the conduction band.     

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.     

High-Temperature Active Oxidation of Chemically Vapor-Deposited Silicon Carbide in CO─CO2 Atmosphere

Active oxidation behavior of CVD-SiC in CO─CO₂ atmospheres was investigated using a thermogravimetric technique in the temperature range between 1823 and 1923 K. The gas pressure ratio, P _CO2/ P _CO, was controlled between 10⁻⁴ and 10⁻¹ at 0.1 MPa. Active oxidation rates (mass loss rates) showed maxima at a certain value of P _CO2/ P _CO, ( P _CO2/ P _CO )^*, In a P _CO2/ P _CO region lower than the ( P _CO2/ P _CO)^* a carbon layer was formed on the SiC surface. In a P _CO2/ P _CO region higher than the ( P _CO2/ P _CO)^*, silica particles or a porous silica layer was observed on the SiC surface.     

Wetting in an Electronic Packaging Ceramic System: II, Wetting of Alumina by a Silicate Glass Melt under Controlled pO2 Conditions

The wetting of polycrystalline alumina by a colored, calciamagnesia aluminosilicae glass was found to be dependent on temperatutre between 1300° and 1500°C, but independent of gas atmosphere effects. Neither the oxygen partial pressure, oveer the range of 10^-6 to 10^-10 Pa, the gas buffer system (Co/CO₂ or H₂/H₂O), nor pre-equilibration of the substrate surface with the atmosphere at tge exoperimental temperature before solid-liuid interface formation affected the stable contact angle. An initial drop in contact angle the stable contact angle. An initial drop in contact angle occurring within the first hour is attributed to repaid dissolution of alumina and the formation of a stable glass/alumina interface. The contact angle after an 8-h isothermal hold decreased from 1300° to 1500°C. The solid-liquid interfacial energy, μM_S1, controls the wetting behavior. Changes in μM_s1 are attributed to he breakup of the silica network as temperature increases.     

Defect Chemistry and Electrical Properties of Thallium Oxide Single Crystals

Ehdectrical resistivity and Hall voltage were measured between 4.2 and 300 K on T1₂O₃ crystals annealehd at 550°C for 24 h under oxygen pressures of 2×10⁴ to 10⁷ Pa. The carrier concentration varied from 7.97×10²⁰ to 5.08×10²⁰ cm⁻³, the low-temperature Hall mobility from 131 to 189 cm²/V.s, and the Fermi level from 7.1×10⁴ to 5.05×10⁴ J/mol above the bottom of the conduction band as P ₀₂ was increased from 2×10⁴ to 10⁷ Pa. The dependence of Fermi level on carrier concentration and P _0l was consistent with a parabolic density-of-states function describing the conduction band. Over the entire region of oxygen pressure investigated, Fermi-Dirac statistics were required to describe the dependence of carrier concentration on P ₀₂.     

High-Temperature Wetting of Sapphire by Aluminum

Sessile drop studies of molten aluminum on single-crystal sapphire substrates were conducted to investigate the effects of atmosphere on contact angle, substrate reactions, and interfacial crystal growth. Unlike previous investigations performed briefly in a vacuum environment in a temperature range within 600°C of the aluminum melting point, these experiments were conducted at higher temperatures (1200° to 1600°C) and at 1-atm total pressure over longer experimental times to more closely approach equilibrium conditions. A continuously flowing buffered gas system utilizing high-purity metered mixtures of hydrogen and helium in combination with a thoria ceramic electrolyte sensor were employed to achieve variations of the oxygen partial pressure from 10⁻¹⁹ to 10⁻¹⁵ atm while continuously maintaining the total pressure at 1 atm. At constant temperature, it was found that neither the oxygen partial pressure nor the crystallographic orientation of the sapphire substrate had a significant effect on the observed contact angles. A continuous decrease of acute contact angles and a single reaction ring characterized the 8-h experiments without the alternating spreading and contracting behavior repeatedly reported in the literature. This phenomenon can be attributed to the lower rate of metal evaporation and interfacial reaction at the higher total gas pressure and yet extremely low oxygen partial pressure of these experiments. Profilometric analysis of sapphire substrates subsequent to the removal of the quenched sessile drops indicates a reduction in metal–solid interaction due to the closer approach to equilibrium than in previous studies. An epitaxial orientation with respect to the substrate was observed in α-alumina crystallite formation at the metal–ceramic interface. Experimental evidence suggests that it was formed by a nucleation and growth process during the cooling period.     

Transference Numbers for In-Plane Carrier Conduction in Thin Film Nanostructured Gadolinia-Doped Ceria Under Varying Oxygen Partial Pressure

We demonstrate a modified Hebb–Wagner approach to quantitatively estimate transference numbers for carrier conduction in thin film oxide conductors using blocking electrodes in an in-plane geometry. We report ionic transference numbers, t _i, for gadolinia-doped ceria (GDC) thin films, a model mixed ion–electron conductor, at 973K and oxygen partial pressure ranging from 0.21 atm down to approximately 10⁻²² atm. Our results indicate that GDC reaches the electrolytic regime ( t _i=0.5) at an oxygen partial pressure of 5 × 10⁻¹⁹ atm at 973K. This approach may be useful for understanding carrier transport mechanisms in low-dimensional oxide heterostructures with specific relevance to nanostructured energy materials.