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.     

Effect of Water Vapor on Dielectric Loss in MgO

A dielectric loss study of porous MgO indicates that H₂O absorption on MgO probably leads to the formation of surface defects as well as hydroxide ions. The ac conductivity, σ_ac followed the equation σ_ac=σ₁ + KP _H2O^0.27 from 550° to 800°C when the water partial pressure was varied between 7 × 10⁻⁵ and 3 × 10⁻² atm.     

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.     

Synthesis and Characterization of a Remarkable Ceramic: Ti3SiC2

Polycrystalline bulk samples of Ti₃SiC₂ were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10⁻⁸ and 2 × 10⁻⁵ kg²-m⁻⁴.s⁻¹. The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 10⁶Ω⁻¹.m⁻¹, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10⁻⁶°C⁻¹, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti₃SiC₂ does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.     

Effect of Oxide Defect Structure on the Electrical Properties of ZrO2

The defect structure of monoclinic ZrO₂ was studied by measuring the transfer numbers and electrical conductivity as functions of O₂ pressure and temperature. The data suggest a defect structure of doubly ionized oxygen vacancies at low pressures, i.e. <10⁻¹⁹ atm, and singly ionized oxygen interstitials at pressures >10⁻⁹ atm. Zirconia is primarily an ionic conductor below #700°C and an electronic conductor at 700° to 1000°C for 10⁻²²≤P_o2≤1 atm.     

High-Temperature Thermophysical Properties of Uranium Monocarbide

Thermal conductivity ( k ), electrical resistivity ( p ), total hemispherical emittance (ε_t), and normal spectral emittance (ε_0.65μ) of dense, arc-cast uranium monocarbide (5.3 wt % total carbon) were measured in the temperature range 1150° to 2050°K. The results were as follows: k , 0.057 cal/sec-cm-deg, 1200° < T < 2050°K, probable error ± 0.002; p, 20.4 × 10⁻⁶+ 114.8 T × 10⁻⁹ ohm-cm, 1175° < T < 2050°K, probable error ± 1.7 × 10⁻⁶; ε_t0.42, 1250° < T < 1980°K, probable error ± 0.02; ε_0.65 0.539 – 0.02 T × 10⁻³ 1150° < T < 1890°K, probable error ± 0.02. Experimental methods are discussed and error sources are analyzed. Uranium monocarbide exhibited typical metallic behavior in its thermophysical properties.     

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 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.     

Sintering and Properties of Monazite-Type CePO4

Monazite-type CePO₄ powder (average grain size 0.3 μm) was dry-pressed to disks or bars. The green compacts began to sinter above 950°C. Relative density ≧ 99% and apparent porosity <1% were achieved when the specimens were sintered at 1500°C for 1 h in air. The linear thermal expansion coefficient and thermal conductivity of the CePO₄ ceramics were 9 × 10⁻⁶/°C to 11 × 10⁻⁶/°C (200° to 1300°C) and 1.81 W/(m · K) (500°C), respectively. Bending strength of the ceramics (average grain size 4 μm) was 174 ± 28 MPa (room temperature). The CePO₄ ceramics were cracked or decomposed by acidic or alkaline aqueous solutions at high temperatures.     

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.     

Control of the Thermal Expansion of Strontium-Doped Lanthanum Chromite Perovskites by B-site Doping for High-Temperature Solid Oxide Fuel Cell Separators

The thermal expansion of La_0.9Sr_0.1Cr_{1- x} M _x O₃(M = Mg, Al, Ti, Mn, Fe, Co, Ni; 0 ≤ x ≤ 0.1) perovskites has been studied in oxidizing and reducing atmospheres in the temperature range from 50° to 1000°C. Cobalt doping of La_0.9Sr_0.1CrO₃was an effective way of increasing the average linear thermal expansion coefficient (TEC), whereas titanium doping showed a negative effect. No effect on the TECs was observed for the B-site dopants in perovskites with the remaining dopants. Linear thermal expansion behavior was observed in the La_0.9Sr_0.1Cr_{1- x} M _x O₃ perovskites with doping of ≥1 mol% aluminum or 10 mol% cobalt. TECs of La_0.9Sr_0.1Cr_0.96Co_0.02Al_0.02O₃ were 10.5 × 10⁻⁶/°C in air, 10.7 × 10⁻⁶/°C under He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 11.8 × 10⁻⁶/°C in H₂ atmosphere.     

Electrical Conduction in Nano-Structured La0.9Sr0.1Al0.85Co0.05Mg0.1O3 Perovskite Oxide

Nanocrystalline La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ oxide powder was synthesized by a citrate–nitrate auto-ignition process and characterized by thermal analysis, X-ray diffraction, and impedance spectroscopy measurements. Nanocrystalline (50–100 nm) powder with perovskite structure could be produced at 900°C by this process. The powder could be sintered to a density more than 96% of the theoretical density at 1550°C. Impedance measurements on the sintered samples unequivocally established the potential of this process in developing nanostructured lanthanum aluminate-based oxides. The sintered La_0.9Sr_0.1Al_0.85Co_0.05Mg_0.1O₃ sample exhibited a conductivity of 2.40 × 10⁻² S/cm in air at 1000°C compared with 4.9 × 10⁻³ S/cm exhibited by La_0.9Sr_0.1Al_0.85Mg_0.15O₃.     

Characterization of Material for Low-Temperature Sintered Multilayer Ceramic Substrates

The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd-Ag, Au, or Cu) during sintering. In this study, SiO₂, CaO, B₂O₃, and MgO were used as additives to Al₂O₃ to decrease the firing temperature by liquid-phase sintering. Compositions with 18.0 and 22.5 wt% B₂O₃ were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10⁻⁴ (1 MHz), insulating resistivity > 10¹⁴ω cm, coefficient of thermal expansion ∼ 7.0 × 10⁻⁶/°, and thermal conductivity ∼ 4.1 W/(m · K).     

Electrical Conduction in Single-Crystal and Polycrystalline Al2O3 at High Temperatures

The electrical conductivity and ion/electron transference numbers in Al₃O₃ were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O₂ partial pressure over single-crystal Al₂O₃ at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O₂ (1 atm) the conductivity above 1400°C was σ≃3×10³ exp (–80 kcal/ RT ) Ω⁻¹ cm⁻¹, which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al₂O₃ and MgO-doped Al₂O₃ showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O₂ partial pressure (below 10⁻¹⁰ atm).     

In Situ Reaction Synthesis, Electrical and Thermal, and Mechanical Properties of Nb4AlC3

In this work, a bulk Nb₄AlC₃ ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb₄AlC₃ were systematically investigated. The thermal expansion coefficient was determined as 7.2 × 10⁻⁶ K⁻¹ in the temperature range of 200°–1100°C. The thermal conductivity of Nb₄AlC₃ increased from 13.5 W·(m·K)⁻¹ at room temperature to 21.2 W·(m·K)⁻¹ at 1227°C, and the electrical conductivity decreased from 3.35 × 10⁶ to 1.13 × 10⁶Ω⁻¹·m⁻¹ in a temperature range of 5–300 K. Nb₄AlC₃ possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa·m^1/2. Most significantly, Nb₄AlC₃ could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580°C was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400°C.     

Thermal Expansion of Some Cubic Refractory Compounds of Thorium

The thermal expansion of a number of cubic refractory compounds of thorium was measured by high-temperature X-ray diffraction. The coefficients of thermal expansion range from 7.5 to 8.2 × 10⁻⁶/°C for ThN, ThC_0.84, ThP_0.71, ThP_0.61, and Th₃P₄ between 900° and 1300°, 1450°, 1750°, 1750°, and 1150°, respectively; the coefficients for ThS, ThB₆, and ThO₂ are in the range of 11.2 to 12.0 × 10⁻⁶/°C between 900° and 1400°, 1700°, and 2100°, respectively. The results are discussed in relation to other properties of these compounds.     

Metastable Crystal Structure of Strontium- and Magnesium-Substituted LaGaO3

The metastable crystal structure of strontium- and magnesium-substituted LaGaO₃ (LSGM) was studied at room and intermediate temperatures using powder X-ray diffractometry and Rietveld refinement analysis. With increased strontium and magnesium content, phase transitions were found to occur from orthorhombic (space group Pbnm ) to rhombohedral (space group R [Threemacr] c ) at the composition La_0.825Sr_0.175Ga_0.825Mg_0.175O_2.825 and, eventually, to cubic (space group Pm [Threemacr] m ) at the composition La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8. At 500°C in air and at constant strontium and magnesium content, a phase transformation from orthorhombic (space group Pbnm ) to cubic (space group Pm [Threemacr] m ) was observed. For the orthorhombic modification, thermal expansion coefficients were determined to be α _a ,ortho = 10.81 × 10⁻⁶ K⁻¹, α _b ,ortho = 9.77 × 10⁻⁶ K⁻¹, and α _c ,ortho = 9.83 × 10⁻⁶ K⁻¹ (25°–400°C), and for the cubic modification to be α_cubic= 13.67 × 10⁻⁶ K⁻¹ (500°–1000°C).     

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.     

Minimization of Parasitic Currents in High-Temperature Conductivity Measurements on High-Resistivity Insulators

An experimental setup and novel measurement technique are described which allow reliable conductivity measurements to be made at conductivities as low as 10⁻¹⁷Ω⁻¹.cm⁻¹ and temperatures up to at least 1300°C. This capability is of particular interest for conductivity measurements on high-resistivity insulators over a large range of temperatures. This technique has been used to measure the conductivity of single-crystal alumina from 400° to 1300°C in a 10⁻⁷ torr (∼1.3 × 10⁻⁵ Pa) vacuum, equivalent to an oxygen partial pressure of about 10⁻⁸ torr (∼1.3 × 10⁻¹¹ atm or ∼ 1.3 × 10⁻⁶ Pa). Surface and gas-phase conductance are determined as a function of temperature, and the requirements for their minimization are described. A key requirement is a very low voltage between the volume guard and the guarded electrode. The effect of leakage currents due to the sample fixture, electrical feedthroughs, and electronic instrumentation are also evaluated, and proper design features to make these effects negligible are outlined.     

Anisotropic Thermal Expansion of β-Ca2SiO4 Monoclinic Crystal

Crystals of β-Ca₂SiO₄ (space group P 12₁/ n 1) were examined by high-temperature powder X-ray diffractometry to determine the change in unit-cell dimensions with temperature up to 645°C. The temperature dependence of the principal expansion coefficients (α_i) found from the matrix algebra analysis was as follows: α₁= 20.492 × 10⁻⁶+ 16.490 × 10⁻⁹ ( T - 25)°C⁻¹, α₂= 7.494 × 10⁻⁶+ 5.168 × 10⁻⁹( T - 25)°C⁻¹, α₃=−0.842 × 10⁻⁶− 1.497 × 10⁻⁹( T - 25)°C⁻¹. The expansion coefficient α₁, nearly along [302] was approximately 3 times α₂ along the b -axis. Very small contraction (α₃) occurred nearly along [     01]. The volume changes upon martensitic transformations of β↔α_L' were very small, and the strain accommodation would be almost complete. This is consistent with the thermoelasticity.