Thermophysical Properties of Perovskite-Type Strontium Cerate and Zirconate

Polycrystalline samples of strontium series perovskite-type oxides, SrCeO₃ and SrZrO₃, were prepared and the thermophysical properties were measured. The chemical compositions of the samples do not deviate from the stoichiometric composition. The oxygen-to-metal (O/M) ratios of SrCeO₃ and SrZrO₃ are 3.00±0.03 and 2.98±0.01, respectively. The average linear thermal expansion coefficients are 1.11 × 10⁻⁵ K⁻¹ for SrCeO₃ and 9.69 × 10⁻⁶ K⁻¹ for SrZrO₃ in the temperature range between 300 and 1000 K. The melting temperatures T _m of SrCeO₃ and SrZrO₃ are 2266 and 2883 K, respectively. The longitudinal and shear sound velocities were measured by an ultrasonic pulse–echo method at room temperature in air, which enables to evaluate the elastic moduli and Debye temperature. The heat capacity was measured by using a differential scanning calorimeter in high-purity argon atmosphere. The thermal diffusivity was measured by a laser flash method in vacuum. The thermal conductivities of SrCeO₃ and SrZrO₃ at room temperature are 2.95 and 4.06 W·m⁻¹·K⁻¹, respectively.     

Microstructural and Thermoelectric Characteristics of Zinc Oxide-Based Thermoelectric Materials Fabricated Using a Spark Plasma Sintering Process

M-doped zinc oxide (ZnO) (M=Al and/or Ni) thermoelectric materials were fully densified at a temperature lower than 1000°C using a spark plasma sintering technique and their microstructural evolution and thermoelectric characteristics were investigated. The addition of Al₂O₃ reduced the surface evaporation of pure ZnO and suppressed grain growth by the formation of a secondary phase. The addition of NiO promoted the formation of a solid solution with the ZnO crystal structure and caused severe grain growth. The co-addition of Al₂O₃ and NiO produced a homogeneous microstructure with a good grain boundary distribution. The microstructural characteristics induced by the co-addition of Al₂O₃ and NiO have a major role in increasing the electrical conductivity and decreasing the thermal conductivity, resulting from an increase in carrier concentration and the phonon scattering effect, respectively, and therefore improving the thermoelectric properties. The ZnO specimen, which was sintered at 1000°C with the co-addition of Al₂O₃ and NiO, exhibited a ZT value of 0.6 × 10⁻³ K⁻¹, electrical conductivity of 1.7 × 10⁻⁴Ω⁻¹·m⁻¹, the thermal conductivity of 5.16 W·(m·K)⁻¹, and Seebeck coefficient of −425.4 μV/K at 900°C. The ZT value obtained respects the 30% increase compared with the previously reported value, 0.4 × 10⁻³ K⁻¹, in the literature.     

Effect of Grain Boundaries on Thermal Conductivity of Silicon Carbide Ceramic at 5 to 1300 K

The thermal conductivity of a SiC ceramic was measured as 270 W·m⁻¹·K⁻¹ at room temperature. At low temperatures ( T < 25 K), the decrease in the conductivity was proportional to T ³ on a logarithmic scale, which indicated that the conductivity was controlled by boundaries. The calculated phonon mean free path in the ceramic increased with decreased temperature, but was limited to ∼4 μm, a length almost equal to the grain size, at temperatures below 30 K. We concluded that the thermal conductivity of the ceramic below 30 K was influenced significantly by grain boundaries and grain junctions.     

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).     

Thermal Expansion Anisotropy of Ceria-Stabilized Tetragonal Zirconia

Ceria-stabilized tetragonal zirconia polycrystals were obtained by thermal treatment of amorphous powder prepared by the sol–gel method. Detailed XRD profile analysis was employed to study microstructural disorder and crystallite size and shape; in particular, no fluctuation of stoichiometry was found, the main cause of disorder being attributable to dislocations. Thermal expansion measurements were carried out by high-temperature XRD at 294, 473, 673, 873, and 1073 K using silicon as an internal standard. Thermal expansion coefficients are anisotropic and changes in the stabilizer content have little effect on them. A mean value, α _a = 10.6 × 10⁻⁶ (K⁻¹) and α _c = 13.5 × 10⁻⁶ (K⁻¹), can be assumed for Zr_{1− x} Ce _x O₂ with x in the range 0.12–0.18.     

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.     

A Hydrogen-Permselective Silicon Oxycarbide Membrane Derived from Polydimethylsilane

A novel microporous membrane has been prepared by modifying an asymmetric SiC support using polydimethylsilane (PMS). One membrane, pyrolyzed at 573 K, possesses H₂/N₂ selectivity of ∼100 and H₂ permeance of 8.9 × 10⁻⁸ mol·m⁻²·s⁻¹·Pa⁻¹ at 473 K. The other membrane, pyrolyzed at 873 K, has a lower H₂/N₂ selectivity of ∼40 and lower H₂ permeance of 4.9 × 10⁻⁹ mol·m⁻²·s⁻¹·Pa⁻¹ at 473 K. Permeation in both membranes exhibits the characteristics of activated diffusion at elevated temperatures. A higher pyrolysis temperature results in a less-permeable membrane. The support preparation and the thermal events of PMS are analyzed. The effects of pyrolysis temperature on membrane property are discussed.     

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.     

Titanate Ceramics for the Stabilization of Partially Reprocessed Nuclear Fuel Elements

A titanate ceramic designed to immobilize high-level waste generated by Amine reprocessing of heavy-water reactor fuel and fabricated by uniaxial hot-pressing was characterized. X-ray diffraction and selected-area electron diffraction were used to identify a six-phase assemblage consisting of betafite, a hollandite structure type, perovskite, uraninite, hibonite, and Magnéli phases. Secondary electron imaging of polished surfaces revealed many microvoids consistent with a measured density of 5.25 g · cm⁻² (∼90% of theoretical density). The waste form was chemically heterogeneous at the hundreds of micrometers scale, as backscattered electron imaging and energy-dispersive X-ray spectroscopy indicated that regions rich in either uranium or titanium oxides were common. Grain sizes ranged from 0.1 to 0.5 μm. All crystals were faceted with the exception of anhedral uraninite grains. Dissolution experiments conducted at 90°C in distilled water gave uranium and titanium loss rates which were solubility limited (10⁻¹ to 10⁻⁴ g · m⁻²· d⁻¹), while cesium dissolution, which was not contrained by solubility limits, was more rapid (23.3 g · m⁻²· d⁻¹). Hydrothermal testing at 150°C in distilled water resulted in precipitation of uranium(VI) titanate (UTiO₅) and brookite (TiO₂).     

Pyroelectric and Electrostrictive Properties of (1 –x–y) PZN ·xBT ·yPT Ceramic Solid Solutions

The pyroelectric and electrostrictive properties of lead zinc niobate–lead titanate–barium titanate (PZN–BT–PT) ceramic solid solution were investigated. These properties of the (1 – x )PZN · x BT series were qualitatively explained with a composition fluctuation model. The pyroelectric depolarization temperatures of (1 – x – y )PZN · x BT · y PT ceramics were utilized to select compositions for room-temperature electrostrictive applications. Among them, 0.85PZN · 0.10BT · 0.05PT ceramic with Q ₁₁= 0.018 m⁴/ C², Q ₁₂=−0.0085 m⁴/C², s ₂ at 25 kV/cm =−6.1 × 10⁻⁴, T _max= 75°C at 1kHz, and T _t= 27°C shows optimum properties for micropositioner applications.     

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.     

Particle Size Dependence of the Electrical Conductivity of NaCl

The ac and dc conductivities of single-crystal and polycrystalline NaCl were measured as a function of both temperature and particle size. The ac conductivity results for single-crystal NaCl agreed well with the literature: intrinsic activation energy = 1.86 ev; extrinsic, impurity-controlled range = 0.74 ev; extrinsic, association range = 1.16 ev; and the intrinsic-extrinsic knee in the curve was at 10³/ T ∼ 1.4°K⁻¹ and σ₀∼ 6 × 10⁻⁸ ohm⁻¹ cm⁻¹. In the intrinsic range, however, the total conductivity (σ₀) was the sum of two ionic contributions: a steady state, nonblocked contribution (σ_θ and a blocked contribution (σ₀—σ_θ). The activation energy for the dc steady state conductivity was 1.6 ev. When the extrinsic, impurity-controlled contribution to the total conductivity was made insignificant by anion doping, the same 1.6 ev was the activation energy for the intrinsic ac conductivity at low temperatures. The data for the polycrystalline samples showed that ac conductivity increased inversely with particle size and dc steady state conductivity increased only slightly, if any, with decreasing particle size. It is postulated that the steady state conductivity is the result of the nonblocked ionic transport of sodium ions and that the ac portion of the total conductivity is due to the movement of chlorine ions which are blocked, giving rise to the polarization phenomenon. The increase in the ac conductivity with decreasing particle size is correlated with the enhanced movement of Cl⁻ in the subgrain boundary region, as has been previously shown by diffusion measurements.     

Fracture Toughness Determination of A12O3 Using Four-Point-Bend Specimens with Straight-Through and Chevron Notches

Fracture toughness of a sintered A1₂O₃ was determined with four-point-bend specimens having either straight-through or chevron notches. For the straight-through notched specimens, measured K _Ic decreased with decreasing notch width. For the smallest notch width (66 μm) K _Ic= 3.42±0.13 MN m^−¾. For specimens with chevron notches, a crack initiates and extends from the tip of the notch under increasing load. K _Ic is calculated from the maximum load without measuring crack length, under the assumption that the derivative of the compliance is the same as that for a specimen with a straight-through crack. A refined calculation accounts for the truncated chevron crack shape at maximum load using Bluhm's slice model. For the chevronnotch configuration, a value of K _Ic= 3.49±0.11 MN m^−¾ was measured, which appears to be independent of the initial notch length a ₀ (distance from the crack mouth to the tip of the triangular notch). An effect of a ₁ (length of the chevron notch at the surface) on K _Ic was observed, independent of whether the calculation of K _Ic was based on the straight-through crack assumption or on the slice model.     

Evaporation of Antimony Trioxide from Glass Melts

The transpiration method was used to study the evaporation of Sb₂O₃ from a glass melt with the composition 70SiO₂·15K₂O·15CaO·MgO (in wt%) at 1200° to 1300°C. The glass contained about 0.9 wt% Sb₂O₃. Assuming the monomer Sb₂O₃ is the species of evaporation, the saturation vapor pressures could be calculated with ΔH_V=218±20 kJ·mol⁻¹ and ΔS_V=128±10 J·mol⁻¹·K⁻¹.     

Kinetics of Enthalpy Relaxation at the Glass Transition in Ternary Tellurite Glasses

The kinetics of enthalpy relaxation (recovery) at the glass transition in x K₂O·(20− x )MgO·80TeO₂ glasses has been examined from heat capacity measurements using differential scanning calorimetry to clarify the features of the structural relaxation in ternary TeO₂-based glasses. Ternary glasses such as 10K₂O·10MgO·80TeO₂ show high thermal resistance against crystallization compared with binary glasses. The degree of fragility m estimated from the activation energy for viscous flow E _η and the glass transition temperature T _g is m = 55–62, indicating a fragile character in TeO₂-based glasses. Large heat capacity changes of 43.1–48.2 J·mol⁻¹·K⁻¹ are also observed at the glass transition. The activation energy for enthalpy relaxation Δ H is evaluated from the cooling rate dependence of the limiting fictive temperature, and values of Δ H = 897–1268 kJ·mol⁻¹ are obtained. Negative deviation from additivity in Δ H is also observed. Values of the Kovacs–Aklonis–Huchinson–Ramos (KAHR) parameter θ estimated from Δ H and T _g are 0.33–0.42 K⁻¹. It has been proposed that ternary glasses have more homogeneous and constrained glass structure compared with binary glasses.     

Simple Mathematical Model for the Electrical Conductivity of Highly Porous Ceramics

A novel approach has been proposed to describe the relationship between the conductivity and relative density of highly porous materials. As a first approximation, porous material was represented by a uniaxial string of spheres along the direction of the potential gradient. Then, a string of spheres was remodeled into a rotating body of sine-wave functions, f ( x ) = (1 + r ₀)/2 + [(1 – r ₀)/2] sin (π x / c ) for 0 ≤ x < c and f (π x ) = (1 + r ₀)/2 + [(1 – r ₀)/2] sin {π( x + 1 − 2 c )/(l − c )} for c ≤ x < 1, where the former represents the shape of a sphere, the latter that of the bottleneck between neighboring spheres, and r ₀ denotes the ratio of the minimum diameter at the bottleneck to the maximum diameter of the rotating body. It was shown that the calculated relationships reproduced the reported experimental results for the relationship between the porosity and conductivity of La_0.5Sr_0.5CoO₃, BaF₂, and (ZrO₂)_0.9(Y₂O₃)_0.1. The relative conductivity to the bulk material was close to zero at 45–60% relative density, which is the density of green wares. It steeply increased with an increase in the relative density and then gradually approached that of the bulk material.     

Fracture of Isotropic and Textured Ba Hexaferrite

The effects of texture and temperature on the fracture toughness of an oriented Ba hexaferrite were studied. An isotropic ferrite was also measured. Anisotropy of K_le at room temperature in the oriented ferrite was a factor of 3, with the transgranular fracture having a K_lc of 2.83 MN-m^−3/2. Toughness decreased with increasing temperature, exhibiting two regions of behavior: a low-temperature brittle elastic fracture region of — (dK_lc/dT) of 10-⁻⁴ MN · m^−3/2. °C⁻¹ and a distinct high-temperature region of 10-⁻² MN · m^−3/2. °C⁻¹ which exhibited 100% grain-boundary fracture.     

Phase Stability and Ionic Conductivity of Solid Solutions with NASICON Structure

Formation of heterovalent Zr-substituted solid solutions (up to 7 mol%) for Yb³⁺ in Na₆Yb₃(PO₄)₅ and LiNa₅Yb₃(PO₄)₅ complex phosphates was studied by ceramic technique at 950°C. Obtained samples were investigated with X-ray powder diffraction, infrared, and impedance spectroscopy. Zr-substituted (7 mol%) Na₆Yb₃(PO₄)₅ has ionic conductivity of 1.6·10⁻² S/cm at 300°C and 4.8·10⁻⁵ S/cm at room temperature. An updated version of phase diagram for ScPO₄–Na₃PO₄–Li₃PO₄ quasi-ternary system was provided.     

High-Temperature Hydroxylation of Mullite

A plane-parallel, polished, 0.9 mm thick, single-crystal (001) plate of 2:1 mullite was treated for 6 h at 1600°C in an Ar/H₂O (90/10) gas mixture at 100 kPa. Optical microscopy studies and infrared (IR) reflection spectroscopy studies of the lattice vibrations yielded no evidence for change with respect to the untreated reference crystal. However, IR absorption spectroscopy showed that structurally bound OH groups were formed by the heat treatment in the Ar/H₂O gas mixture. IR absorption depth profile analysis showed a rather homogeneous OH distribution through the crystal. Five different hydroxyl groups were separated according to dipole orientations and peak positions: E ‖ a , ω _{a 1}= 3447 cm⁻¹, ω _{a 2}= 3579 cm⁻¹; E ‖ b , ω _{b 1}= 3456 cm⁻¹, ω _{b 2}= 3544 cm⁻¹; and E ‖ c , ω _{c 1}= 3498 cm⁻¹. All IR peaks were strongly broadened (between 90 and 150 cm⁻¹) because of a distribution in O-H binding distances caused by the real structure of mullite.     

Electroceramics from Source Materials via Molecular Intermediates: PbTiO3 from TiO2 via [Ti(catecholate)3]2-

The precursor [NH₄]₂[Ti(catecholate)₃] · 2H₂O is known to react with Ba(OH)₂· 8H₂O in an acid/base process that generates Ba[Ti(catecholate)₃] · 3H₂O, a compound which undergoes low-temperatue calcination to produce BaTiO₃ powder. Attempts to develop similar routes to PbTiO₃ have been frustrated, since lead(II) hydroxide does not exist. The amphoteric yellow PbO and the basic oxide, Pb₆O(OH)₆⁴⁺, are both insufficiently basic to react with [NH₄]₂[Ti(catecholate)₃] · 2H₂O. Based on the large sizes of both the [Ti(catecholate)₃]^2- anion and the Pb²⁺ cation, a precipitation method has been developed in which lead nitrate and [NH₄]₂[Ti(catecholate)₃] · 2H₂O are added together in an aqueous medium causing precipitation and leaving only NH₄NO₃ in solution. The lead-titanium-catecholate complex that forms in this manner undergoes low-temperature pyrolysis to produce PbTiO₃. SEM indicates a submicrometer ultimate crystallite size.