Electrical Properties and Defect Structure of Y2O3

Guarded measurements of the electrical conductivity of high-purity, polycrystalline Y₂O₃ in thermodynamic equilibrium with the gas phase were made under controlled temperature and oxygen partial pressure conditions. Data are presented as isobars from 1200° to 1600°C, and as isotherms from oxygen partial pressures of 10⁻¹ to 10⁻¹⁷ atm. The ionic contribution to the total conductivity, determined by the blocking electrode polarization technique, was less than 1% over the entire range of temperatures and oxygen partial pressures studied. Yttria is shown to be an amphoteric semiconductor with the region of predominant hole conduction shifting to higher pressures at higher temperatures. In the region of p -type conduction, the conductivity is represented by the expression σ= 1.3 × 10³ p O₂^3/16 exp (-1.94/kT). The observed pressure dependence is attributed to the predominance of fully ionized yttrium vacancies. Yttria is shown to be a mixed conductor below 900°C.     

Electrical Properties and Defect Structure of HfO2

The defect structure of high-purity, polycrystalline HfO₂ was investigated by measuring the oxygen partial pressure dependence of the electrical conductivity and the sample weight. From 1000° to 1500°C and above oxygen partial pressures of 10 ⁻⁶, the conductivity is electronic and proportional to p o₂^1/5. The predominant defect is completely ionized hafnium vacancies. At lower oxygen partial pressures a broad shallow minimum in the lower temperature conductivity isotherms indicates the presence of an oxygen pressure independent source of electronic charge carriers. By combining the weight change and conductivity data, mobility values were found to vary from 1.6 × 10⁻³ to 3 × 10⁻⁴ cm²/V-sec. The activation energies for the hole mobilities were calculated to be 0.2 ev above 1300° C and 0.7 ev below this temperature.     

Electrical Properties and Defect Structure of a (0.13YO1.5·0.87ThO2) Electrolyte

High-density sintered disks of the composition 0.13YO_1.5·0.87ThO₂ are shown to be mixed conductors at high oxygen pressures (>10⁻⁶ atm) by electrical conductivity and electrochemical cell measurements. The ac and dc conductivity measurements were made between 900° and 1600°C over a wide range of oxygen partial pressures. A blocking-electrode polarization technique for determining transference numbers was not applicable at high oxygen pressures but appeared to work at the lower pressures, indicating a transition to n -type behavior. The electrochemical cell measurements show essentially completely ionic behavior at low oxygen pressure but indicate at least 0.1% electronic contribution at 10⁻¹³ atm at 1000°C. The lower oxygen pressure limit for completely ionic behavior has not been determined but extends below the equilibrium pressures of an Mn-MnO₂, Cr-Cr₂O₃ electrochemical cell at 1000°C.     

Thermal Stability of Barium Ferrite (BaFe12O19)

Compositions in the system Fe₂O3-FeO-BaO in the vicinity of the compound BaFe₁₂O₁₉ were studied at temperatures from 1300° to 1550°C and oxygen pressures from 10⁻² to 10² atm. Equilibrium relations involving several barium ferrous ferrites are described. Barium ferrite can be crystallized congruently from the melt at 40 atm oxygen pressure and 15400°C.     

Electrical Properties of PrFeO3 and Pr2NiO4

The electrical conductivity of PrFeO₃ and Pr₂NiO₄ was investigated at 300° thd 1000°C and at oxygen partial pressures of 1 to 10⁻²⁰ atm and the phase relations and nonstoichiometry of these materials were studied. The results suggest that PrFeO₃ is a semiconductor exhibiting intrinsic behavior at T>300°C and P_o2 >10⁻⁵ atm. The conductivity of Pr₂NiO₄ depends on P_o2 and is thus related to deviations from stoichiometry. These results for Pr₂NiO₃ raise questions about the validity of the suggested semiconductor-to-metal transition model for explaining the electrical properties of La₂NiO₃ and Nd₂Ni0₄.     

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.     

Effect of Hydrogen-Water Atmospheres on Corrosion and Flexural Strength of Sintered α-Silicon Carbide

Sintered α-SiC was exposed for 10 h to H₂ containing various partial pressures of H₂O ( P _H2O from 5×10⁻⁶ to 2×10⁻² atm; 1 atm≅10⁵ Pa) at 1300° and 1400°C. Weight loss, surface morphology, and room-temperature flexural strength were strongly dependent on P _H2O. The strength of the SiC was not significantly affected by exposure to dry H₂ at a P _H2O of 5×10⁻⁶ atm; and following exposure at P _H2O >5×10⁻³ atm, the strength was even higher than that of the as-received material. The increase in strength is thought to be the result of crack blunting associated with SiO₂ formation at crack tips. However, after exposure in an intermediate range of water vapor pressures (1×10⁻⁵< P _H2O <1×10⁻³ atm), significant decreases in strength were observed. At a P _H2O of about 1×10⁻⁴ atm, the flexural strength decreased approximately 30% and 50% after exposure at 1300° and 1400°C, respectively. The decrease in strength is attributed to surface defects caused by corrosion in the form of grain-boundary attack and the formation of pits. The rates of weight loss and microstructural changes on the exposed surfaces correlated well with the observed strength changes.     

Influence of Oxygen Activity on the Sintering of MgCr2O4

The sintering behavior of MgCr₂O₄ powder compacts was investigated as a function of temperature, time, and oxygen activity. The results show that MgCr₂O₄ cannot be densified to >70% of theoretical density at temperatures up to 1700°C if the oxygen activity exceeds 10⁻⁶ atm. The oxygen activity must be decreased to <10⁻¹⁰ atm before densities exceeding 90% of theoretical can be achieved. Weight loss and X-ray data indicated that maximum density occurred at an oxygen activity just above that where MgCr₂O₄ becomes unstable.     

Thermogravimetric Study of the M-Co-O System: I, M = Ta and Nb at 1200°C

Phase equilibria in the Ta-Co-O and Nb-Co-O systems have been studied at 1200°C at oxygen partial pressures from 10^−0.68 to 10^−13.50 atm for the former and from 10^−0.68 to 10^−13.30 atm for the latter. In both systems, M₂CoO₆ and M₂Co₄O₉ are stable ternary compounds under the experimental conditions, and a new phase, Nb₅Co₂O₁₄, has been identified. The Ta-Co-O system is simple, whereas the Nb-Co-O system is somewhat more complicated because of the extra phase. The lattice constants of the ternary compounds have been determined and compared with previous values. The standard Gibbs energies of reactions have been determined using oxygen partial pressures in equilibrium with three solid phases.     

Thermal Expansion Behavior of Titanium-Doped La(Sr)CrO3 Solid Oxide Fuel Cell Interconnects

La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10⁻⁶/°C in air, 10.5 × 10⁻⁶/°C under a He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 10.9 × 10⁻⁶/°C in a H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁹ atm at 1000°C). La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect.     

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.     

Phase Equilibria in the Spinel Region of the System FeO-Fe2O3-Al2O3

Phase relations in the spinel region of the system FeO-Fe₂O₃-Al₂O₃ were determined in CO₂ at 1300°, 1400°, and 15000°C and for partial oxygen pressures of 4 × 10⁻⁷ and 7 × 10⁻¹⁰ atmospheres at 15OO°C. The spinel field extends continuously from Fe₃O_4-x to FeAl₂O_4+z.     

Oxygen Non-Stoichiometry and Thermal–Chemical Expansion of Ce0.8Y0.2O1.9−δ Electrolytes by Neutron Diffraction

A dense tubular solid electrolyte with the composition Ce_0.8Y_0.2O_1.9−δ (CY20) was prepared. In situ time-of-flight neutron powder diffraction (TOF-ND) was performed at 900°C in the oxygen partial pressure p O₂ range from 10⁻¹–10⁻¹⁸ atm, and TOF-ND data were analyzed by the Rietveld method. Diffraction data showed that the lattice parameter moderately increased with decreasing p O₂ in the range of p O₂>10⁻¹⁴ atm, while a dramatic expansion (∼0.6%) of the fluorite structure occurred at a p O₂ of 10⁻¹⁸ atm. By refining all reasonable structural parameters, an approximately linear relationship between lattice parameter and oxygen vacancy δ was observed, resulting in ɛ_c/δ=0.08 and corresponding to δ=0.10 at a p O₂ of 10⁻¹⁸ atm, all in agreement with the data published in the literature. The relative change in lattice parameter Δ a / a followed a −1/4 power relation with p O₂ in a low- p O₂ regime. As several (often strongly correlated) structural parameters can affect the intensities in ND profiles, care was taken to select refinement variables. It was found that O atom thermal factors for CY20 increased as the oxygen vacancy concentration and lattice expansion increased.     

Ractions of SiC with H2/H2O/Ar Mixtures at 1300°C

The reactions of a sintered α-SiC with 5% H₂/H₂O/Ar at 1300°C were studied. Thermomchemical modeling indicates that three reaction regions are expected, depending on the initial water vapor or equivalently oxygen content of the gas stream. A high oxygen content ( P (O₂) > 10⁻²² atm) leads to a SiO₂ formation. This generally forms as a protective film and limits consumption of the SiC (passive oxidation). An intermediate oxygen content (10⁻²² atm > P (O₂) > 10⁻²⁶ atm) leads to SiO and CO formation. These gaseous products can lead to rapid consumption of the SiC (active oxidation). Thermogravimetric studies in this intermediate region gave reaction rates which appear to be controlled by H₂O gas-phase transport to the sample and reacted microstructures showed extensive grain-boundary attack in this region. Finally, a very low oxygen content ( P (O₂) < 10⁻²⁶ atm) is thermochemically predicted to lead to selective removal of carbon and formation of free silicon. Experimentally low weight losses and iron silicides are observed in this region. The iron silicides are attributed to reaction of free silicon and iron impurities in the system.     

Gravimetric Determination of Defect Concentrations in NiO

Thermogravimetric measurements were made on NiO from 800° to 1100°C over the oxygen pressure range 10⁻¹ to 10⁻⁴ atm. On the basis of complementary conductivity measurements showing a P (O₂)^1/5 oxygen pressure dependence, it is proposed that the predominant defects are described by an electroneutrality condition involving doubly ionized metal vacancies, impurities, and electron holes, 2[ V"_M ]=[ F_M ]+ p. It is shown that for this defect model, the weight change relative to a low oxygen pressure reference weight is a measure of the effective vacancy concentration, defined as [ V"_M ]_eff≡[ V"_M ]-≡[ F_M ], and therefore has the same oxygen pressure dependence as the electron hole concentration followed in the conductivity measurements. The expression [ V"_M ]_eff=0.168 P (O₂)^1/5 exp (−0.86±0.15/ kT ) is derived to express the effective vacancy concentration in NiO. The probable effective impurity content of the specimens used is calculated.     

Oxidation State of Chromium in CaO–Al2O3–CrOx–SiO2 Melts under Strongly Reducing Conditions at 1500°C

Equilibrium ratios Cr²⁺/Cr³⁺ of chromium oxide dissolved in CaO–chromium oxide–Al₂O₃–SiO₂ melts have been determined by analysis of samples equilibrated at 1500°C under strongly reducing conditions ( p _o2= 10^−9.56 to 10^−12.50 atm). The majority of the chromium is divalent (Cr²⁺) under these conditions and Cr²⁺/Cr³⁺ ratios at given constant oxygen pressures decrease with increasing basicity of the melts, expressed as CaO/SiO₂ ratios. In addition, Cr²⁺/Cr³⁺ ratios, at a given CaO/SiO₂ ratio, are relatively unaffected by the amount of Al₂O₃ present.     

Electrical Properties and Defect Structure of ThO2

The electrical conductivity and thermoelectric power of highpurity polycrystalline ThO₂ in thermodynamic equilibrium with the gas phase were measured as a function of temperature from 1000° to 1600°C and as a function of oxygen partial pressure from 1 to 10⁻²² atm. An n -type electronic contribution to the conductivity is observed above 1400°C at low oxygen pressures. An analytic solution is presented for the oxygen pressure dependence of the total conductivity in the mixed ionicelectron hole conduction region observed at higher oxygen pressures. The activation energies for p -type and ionic conduction are 1.0 and 0.9 eV, respectively. The combined conductivity and thermal emf data give a lower limit of ∼6 cm²/V-s for the electron hole mobility.     

Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.     

Oxidation of Thin Sheet Reaction-Sintered Silicon Nitride

Oxidation of reaction-sintered silicon nitride was studied in damp air. The formation of "passive" silica films was investigated at 1 atm and 700 to 1100°C and some limited work on weight loss behavior was performed in vacuo of 10⁻⁸ to 10⁻⁵ atm at 1050 to 1200°C. Passive behavior was dominated by reaction in the pore network. Oxidation was extensive at 900 to 1000° but slight at 700 to 800°C. At 1100°C a protective skin limited reaction. Weight loss in vacuo was slight at 1050°C. The vacuum pressure required to suppress the weight loss increased from 4 to 5 × 10⁻⁷ atm at 1050° to 1.5 to 2.5 × 10⁻⁵ atm at 1200°C.     

Electrical Conductivity of the Solid Solutions XNiO + (1 − X)Tm2O3 (0.01 X 0.05)

NiO-doped Tm₂O₃ systems (NDT) containing 1, 3, and 5 mol% NiO were found to be solid solutions by X-ray diffraction (XRD) analysis. The lattice parameters ( a ) were obtained by the Nelson-Riley method, and the values decreased with increasing dopant content. Thermal analysis showed that no phase transition occurred in the temperature range covered in this experiment. The electrical conductivities were measured in the range of temperatures from 400° to 1100°C and of oxygen partial pressures from 1 × 10⁻⁵ to 2 × 10⁻¹ atm. The conductivity increases with temperature but there is a break in the conductivity curve, dividing it into two temperature regions. At the high temperatures of 700° to 1100°C, the activation energy ( E_a ) and oxygen partial pressure dependence of conductivity are found experimentally to be 1.4 to 1.5 eV and 1/ n = 1/5.4, and the possible defects and charge carriers are suggested to be metal vacancies and electron holes, respectively. At the lower temperatures of 400° to 600°C, the E_a and 1/ n values obtained are 0.8 to 0.9 eV and 1/ n = 1/7.2 to 1/8.8, respectively. At the lower temperature, the NDT system seems to display a mixed conduction involving ionic conductivity due to diffusion of oxygen.