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).     

Extended X-ray Absorption Fine Structure (EXAFS) Study on the local Environment around Copper in Low Thermal Expansion Copper Aluminosilicate Glasses

The local environment around copper in the Cu₂O·Al₂O₃·4SiO₃ and CuO·Al₂O₃·4SiO₂ glasses was investigated using the extended X-ray absorption fine structure (EXAFS) technique. It has been found that each Cu(I) in the former glass is coordinated to two oxygens through covalent Cu(I)-O bonds. This is the primary reason for its low thermal expansion coefficient (∼10 × 10⁻⁷ K⁻¹). In the latter glass, which was made by heating the former glass at 600°C in air, each Cu(II) is coordinated to four oxygens and the bonds are ionic in character, contributing to the increase in the thermal expansion coefficient to ∼30 × 10⁻⁷ K⁻¹.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

On the Decomposition of Synthetic Gibbsite Studied by Neutron Thermodiffractometry

The thermal decomposition mechanism of synthetic Al(OH)₃ (gibbsite) was studied in situ by neutron thermodiffractometry in an ambient atmosphere from room temperature to 600°C with 50°C steps. Gibbsite decomposed to yield AlO·(OH) (boehmite) and then poorly crystallized χ-Al₂O₃. Rietveld analysis was used to refine the cell parameters' variation of gibbsite and its thermal expansion coefficients were obtained: for the a -axis: 15±1 × 10⁻⁶ K⁻¹, for b : 10±2 × 10⁻⁶ K⁻¹, and for c : 17±2 × 10⁻⁶ K⁻¹.     

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).     

Preparation and Characterization of Aluminum Borate

Aluminum borate, 9Al₂O₃·2B₂O₃ or Al₁₈B₄O₃₃, was synthesized by the reaction of stoichiometric amounts of α-Al₂O₃ and B₂O₃. The Al₁₈B₄O₃₃ material was formed into a dense ceramic by pressureless sintering with CaO, MgO, or CaAl₂B₂O₇ additives. The material was characterized by low bulk density, moderate coefficient of thermal expansion (3 × 10⁻⁶/°C to 5 × 10⁻⁶/°C), moderate strength (210 to 324 MPa), and low dielectric constant.     

Evaporation of Zinc Oxide from Spinel Solid Solutions in Vacuum

The mechanism for the evaporation of ZnO from powders of (Zn₀₂Co_0.8)O·Al₂O₃ (ZCA) and (Zn_0.2Ni_0.8)O·Al₂O₃ (ZNA) spinels was studied at 1335° to 1500°C in vacuum of 3×10⁻⁵ to 10⁻⁴ torr. The evaporation of ZnO occurred in two stages: at a constant rate in the first and a decreasing rate in the second. The rate-determining process was analyzed as a decomposition reaction at the first stage and as the conjugated process of decomposition reaction and diffusion in the solid at the second. The evaporation rate constant and surface emissivity showed a similar dependence on temperature. The ratio of the concentration of ZnO at the surface to the initial concentration was =0.7 when the first stage of evaporation changed to the second.     

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).     

Tribological Behavior of Ti2SC at Ambient and Elevated Temperatures

The tribological properties of Ti₂SC were investigated at ambient temperatures and 550°C against Ni-based superalloys Inconel 718 (Inc718) and alumina (Al₂O₃) counterparts. The tests were performed using a tab-on-disk method at 1 m/s and 3N (≈0.08 MPa). At room temperature, against the superalloy, the coefficient of friction, μ, was ∼0.6, and at ∼8 × 10⁻⁴ mm³·(N·m)⁻¹ the specific wear rate (SWRs), was high. However, against Al₂O₃, at ∼5 × 10⁻⁵ mm³·(N·m)⁻¹ and ∼0.3, the SWRs and μ were significantly lower, which was presumably related to more intensive tribo-oxidation at the contact points. At 550°C, the Ti₂SC/Inc718 and Al₂O₃ tribocouples demonstrated comparable μ's of ∼0.35–0.5 and SWRs of ∼7–8 × 10⁻⁵ mm³·(N·m)⁻¹. At 550°C, all tribosurfaces were covered by X-ray amorphous oxide tribofilms. At present, Ti₂SC is the only member of a family of the layered ternary carbides and nitrides (MAX phases) that can be used as a tribo-partner against Al₂O₃ in the wide temperature range from ambient to 550°C.     

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.     

Pore Drag and Pore-Boundary Separation in Alumina

Microdesigned interfacial pore structures were used to study pore drag and pore-boundary separation in Al₂O₃. This approach allows the creation of pore arrays containing pores of controlled size and spacing at well-defined singlecrystal seed/polycrystalline matrix interfaces, and enables experimental determination of the peak pore velocity. From the peak pore velocity, values of the surface diffusion coefficient pertinent to sintering can be extracted. At 1600°C, the surface diffusion coefficient is ∼1 × 10⁻⁷ cm²/s for undoped Al₂O₃ and ∼4 × 10⁻⁷ cm²/s for MgO-doped Al₂O₃. The values appear to be insensitive to the seed orientation for the two seed orientations studied. The results suggest a strong influence of pore spacing on the separation condition in undoped Al₂O₃, and a diminished influence in MgO-doped Al₂O₃. Quantitative agreement between theoretically predicted and experimentally observed separation/attachment conditions was obtained.     

Low-Temperature Fabrication of Cordierite Ceramics from Kaolinite and Magnesium Hydroxide Mixtures with Boron Oxide Additions

Thermal reactions of mixtures of ultrafine particles of magnesium hydroxide (Mg(OH)₂) and kaolinite in a composition of MgO:Al₂O₃:2SiO₂ were investigated to obtain dense cordierite ceramics at temperatures <1000°C. While heating the mixture of kaolinite and Mg(OH)₂ with the equivalent of 2 mass% of boron oxide (B₂O₃) (in the form of magnesium borate, 2MgOB₂O₃), an amorphous phase formed at a temperature of ∼850°C after thermal decomposition. Firing the mixture at a temperature of 900°C yielded dense ceramics with an apparent porosity of almost zero. The addition of B₂O₃ promoted the densification at 850°-900°C and accelerated the crystallization of alpha-cordierite. The specimen with 3 mass% of B₂O₃ that was fired at a temperature of 950°C showed a linear thermal expansion coefficient of ∼3 × 10⁻⁶ K⁻¹, a bending strength of >200 MPa, and a relative dielectric constant of 5.5 at 1 MHz. These cordierite ceramics may be used as substrate materials for semiconductor interconnection applications.     

Preparation of Ca3Co4O9 and Improvement of its Thermoelectric Properties by Spark Plasma Sintering

The precursor powders of Ca₃Co₄O₉ were synthesized by a sol–gel method. The results of X-ray diffraction and thermogravimetric and differential thermal analyses patterns indicate that pure Ca₃Co₄O₉ powders could be obtained by calcining the precursor at 800°C for 2 h. High dense Ca₃Co₄O₉ ceramic samples (∼99% of theoretical density) were prepared by the spark plasma sintering (SPS) method. Compared with the conventional sintering (CS), the SPS samples exhibit much higher electrical conductivity and power factor which are respectively about 118 S/cm and 3.51 × 10⁻⁴ W·(m·K²)⁻¹. The SPS method is greatly effective for improving the thermoelectric properties of Ca₃Co₄O₉ oxide ceramics.     

Interdiffusion and Association Phenomena in the System NiO-Al2O3

The rate of formation of NiAl₂O₄ by reaction between single crystals of NiO and Al₂O₃ can be described by k = 1.1 × 10⁴ exp (−108,000 ± 5,000/ RT ) cm²/s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity-vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al³⁺ ions diffuse as [Al_Ni· V_Ni]'pairs. The diffusion coefficient of pairs, D_p , obeys the equation 6.6 × 10⁻² exp (−54,000 ± 3,000/ RT ) cm²/s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al³⁺ dissolved in the spinel.     

Thermal Expansion of Calcium Monoaluminate

The linear and the crystal-lattice thermal expansions of monoclinic CaO·Al₂O₃ were determined. The average coefficient α of linear thermal expansion between 22° and 1200°C is 7.7 × 10⁻⁶/°C. The crystal lattice expands anisotropically with expansion coefficients along the principal axes of the strain ellipsoid of 9.8, 7.3, and 5.9 × 10⁻⁶/°C for the same temperature interval.     

Kinetics of NiCr2O4 Formation and Diffusion of Cr3+ Ions in NiO

The reaction kinetics for NiCr₂O₄ formation and the diffusion of Cr³⁺ ions into single-crystal NiO were studied between 1300° and 1600°C in air. The experimental activation energy for NiCr₂O₄ formation was about 83 kcal/mol. After incubation, NiCr₂O₄ formed by a diffusion-controlled process. The origin of pores at the NiO/NiCr₂O₄ interface is discussed. The concentration profiles of Cr³⁺ in NiO were linear because the interdiffusion coefficient was directly proportional to the mol fraction Cr³⁺. Theoretical considerations indicate that the interdiffusion coefficient equals 3/2 the self-diffusion coefficient of Cr³⁺, which is rate-determining. The interdiffusion coefficient at 1 mol% Cr₂O₃ can be expressed as =4×10⁻³ exp (−55,000/RT) cm² s⁻¹.     

Characterization of Thermoelectric Metal Oxide Elements Prepared by the Pulse Electric-Current Sintering Method

Thermoelectric elements consisting of the layered polycrystalline materials of Al-doped ZnO and NaCo₂O₄ were prepared using the pulse electric-current sintering (PECS) method at 900°C for 3 min. Direct contact between the polycrystalline Al-doped ZnO and the NaCo₂O₄ was obtained in a single-step process for the stacked powders. The electrical conductivities of the polycrystalline materials prepared by PECS were higher than those of materials prepared by conventional sintering, despite their porous structure. The thermoelectric voltage of the 1-mol%-Al-doped ZnO and NaCo₂O₄ polycrystalline element (measuring ∼6 mm × 3 mm × 15 mm) was 83 mV at d T = 500 K, when the junction of the elements was at 800°C.     

Contactless Density Measurement of Liquid Nd-Doped 50%CaO–50%Al2O3

The density of neodymium-doped calcium aluminate (<1 mol% Nd₂O₃·50% CaO·50% Al₂O₃) liquid was measured over a wide temperature range using an electrostatic levitation furnace. The density was obtained using an UV-based imaging technique that allowed excellent illumination throughout all phases of processing, including elevated temperatures. Over the 1560–2000 K temperature range, the density could be expressed as ρ( T ) = 2.83 × 10³– 0.21( T – T _m) (kg·m⁻³) (±2%) with T _m= 1878 K, which yielded a volume coefficient of thermal expansion α( T ) = 7.5 × 10⁻⁵ K⁻¹.     

Microstructure and High-Temperature Mechanical Behavior of Alumina/Alumina–Yttria-Stabilized Tetragonal Zirconia Multilayer Composites

Layered composites of alternate layers of pure Al₂O₃(thickness of 125 μ m) and 85 vol% Al₂O₃-15 vol% ZrO₂ that was stabilized with 3 mol% Y₂O₃(thickness of 400 μ m) were obtained by sequential slip casting and then fired at either 1550° or 1700°C. Constant-strain-rate tests were conducted on these materials in air at 1400°C at an initial strain rate of 2 × 10^-5 s^-1. The load axis was applied both parallel and perpendicular to the layer interfaces. Catastrophic failure occurred for the composite that was fired at 1700°C, because of the coalescence of cavities that had developed in grain boundaries of the Al₂O₃ layers. In comparison, the composite that was fired at 1550°C demonstrated the ductility of the Al₂O₃+YTZP layer, but at a flow stress level that was determined by the Al₂O₃ layer.     

Thermal Expansion of SrY2O4

Crystals of SrY₂O₄ (space group Pnam ) were examined by high-temperature powder X-ray diffractometry to determine the changes in unit-cell dimensions with temperature. The individual cell dimensions linearly increased with increasing temperature up to 1473 K. The expansion coefficients (K⁻¹) were 1.263(8) × 10⁻⁵ along the a- axis, 7.46(6) × 10⁻⁶ along the b- axis, and 9.93(10) × 10⁻⁶ along the c- axis. The coefficient of mean linear expansion was 1.001(8) × 10⁻⁵ K⁻¹.