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.     

Magnesium Titanate Spinel: A Ceramic Phase for Immobilization of Technetium-99 from Radioactive Wastes

Technetium-99 can be dissolved in Mg₂TiO₄ spinels to form solid solutions with low dissolution rates. These materials are possible host phases for ⁹9Tc in ceramic nuclear waste forms .     

Microstructural Characterization of High-Thermal-Conductivity Aluminum Nitride Ceramic

An aluminum nitride (AlN) ceramic with a thermal conductivity value of 272 W·(m·K)⁻¹, which is as high as the experimentally measured thermal conductivity of an AlN single crystal, was successfully fabricated by firing at 1900°C with a sintering aid of 1 mol% Y₂O₃ under a reducing N₂ atmosphere for 100 h. Oxygen concentrations were determined to be 0.02 and 0.03 mass% in the grains and in the grain-boundary phases, respectively. Neither stacking fault in the grains nor crystalline phase in the grain-boundary regions was found by transmission electron microscopy. An amorphous phase possessing yttrium and oxygen elements was detected between the grains as thin films with a thickness of <1 nm. Because the amount of grain-boundary phase was small, the high-thermal conductivity of the ceramic was attributable to the low oxygen concentration in the AlN grains.     

Effect of CaO on the Thermal Conductivity of Aluminum Nitride

The effect of CaO on the thermal conductivity of aluminum nitride pressureless sintered with 3 wt% Y₂O₃ as a sintering aid was investigated. Over the composition range of 0 to 2.0 wt% additions, CaO decreased the thermal conductivity of the sintered parts by 10%. CaO doping rendered the secondary oxide phases more wetting and thus with a greater tendency to penetrate along the grain boundaries. Furthermore, CaO segregation to the grain boundaries was observed even on those grain boundaries apparently free of secondary phases. These microstructural changes disrupted the connectivity of the high thermal conductivity AIN grains and were the main factors contributing to the decrease in the thermal conductivity of the ceramic parts. CaO additions to samples doped with SiO₂ had the opposite effect, increasing the thermal conductivity. CaO removed SiO₂ from the AIN grains and incorporated it into the oxide second phases, most likely through charge-compensating substitutions Ca²⁺+ Si⁴⁺ for Y³⁺ and/or Al³⁺. Thus, AIN samples containing both SiO₂ and CaO had higher thermal conductivity than those containing comparable amounts of SiO₂ alone.     

Thermal Fracture of Ceramic Materials Under Quasi-Static Thermal Stresses (Ring Test)

The ring test consists in heating a pile of ceramic rings, each 2 in. in outside diameter, 1 in. in inside diameter, and 1½2 in. long, from the inside by a heating element and cooling it from the outside by a calorimetric chamber. The center rings of the pile have radial heat flow. The radial temperature gradient in these center rings can be measured and also the number of calories flowing through their unit surface area. Under equilibrium conditions these measurements yield the thermal conductivity of the ring material. The radial temperature gradient produces thermal stresses in the rings. If the gradient is increased slowly so as to maintain always approximate equilibrium conditions, there will be a maximum gradient which causes failure. The maximum gradient determined by thermal fracture and the thermal conductivity can be used to compute the two thermal stress resistivity constants, R and R ¹, of the material. R and R ¹ have been determined for five ceramic materials: porous TiO₂, dense TiO₂, steatite, cordierite, and spodumene.     

Boron Nitride–Aluminum Nitride Ceramic Composites Fabricated by Transient Plastic Phase Processing

BN–AlN ceramic composites have been successfully fabricated by a novel process referred to as transient plastic phase processing (TPPP). The process used BN as both the reactant phase and the matrix and Al as the transient plastic phase. The products AlN and AlB₁₂ were regarded as the reinforcing phases. With the addition of Al powder in BN, both the mechanical and thermal properties were improved. Relatively high green density (2.03 g/cm³, 82.0% of theoretical density (TD)) and as-sintered density (2.18 g/cm³, 92.6%TD), high bending strength (106 MPa), and high thermal conductivity (72 W/(m·K)) were attained for one kind of BN–AlN composite. A low thermal expansion coefficient of 2.0 × 10⁻⁶/K was also achieved.     

Electrical and Magnetic Properties of the System SrFeO3–BiFeO3

Perovskite phases containing iron in both the tetravalent and trivalent states were prepared by equilibration at high oxygen pressures. Small bismuth additions to SrFeO₃ resulted in high conductivity (10¹ to 10³ (ohm-cm)^–1 at 26°C) and antiferromagnetic order below 130°K. Distortion to tetragonal symmetry occurs approximately at the composition Bi_0.3Sr_0.7FeO_2.87, but higher bismuth concentrations (up to 80 mole %) give rise to a second cubic phase region which shows decreased conductivity (∼10^–5 (ohm-cm)^–1) and weak ferromagnetism with ordering temperatures higher than 26°C. Rhombohedral specimens Bi_0.9Sr_0.1FeO_2.96 and BiFeO_3.01 are anti-ferromagnetic and have comparatively low conductivity.     

Fiber Debonding in Residually Stressed Brittle Matrix Composites

The competition between initial fiber debonding versus fiber failure marks a crucial event of the microstructural failure process in fiber-reinforced brittle matrix composites. In this study, the role of a thermal residual stress field on the debonding conditions is examined theoretically and analytically. The analysis is based on two critical observations, the first being that the mechanics at the tip of a kink crack are driven only by the singularity at the main crack tip. Following from the first is the second observation that any thermal stress effects on the debonding criteria should enter only through the phase angle ψ ^T of the total stress intensity factor at the main crack tip. In general, this stress intensity factor has a thermal as well as a mechanical load contribution. It is shown that when the thermal and mechanical stress intensities, K ^R and K ^t , respectively, are in phase , i.e., ψ ^R =ψ ^t , the existing debonding conditions are universal and can be used even in the presence of thermal loads. On the contrary, when K ^R and K ^t are out of phase , i.e., ψ ^R ≠ψ ^t , events such as the delamination of thick films or debonding of inclined aligned fibers in brittle matrix composites become sensitive to the presence of the thermal stresses.     

High-Temperature Hall Measurements on Cerium Dioxide Thin Films

Simultaneous Hall and conductivity measurements have been performed on sputtered polycrystalline thin films and on bulk ceramic specimens of nearly stoichiometric CeO₂ in the temperature range between 900° and 1040°C. The measurements have been performed in air using low-frequency alternating current. In the case of the bulk ceramic specimens, an upper limit for the carrier mobility of ≤0.2 cm²/(V·s) has been obtained, which is in accordance with data from the literature for bulk samples. The conductivity of the thin films (l/1Ω·m) at 1000°C) is in accordance with data from the literature for bulk ceramics. The carrier density derived from the Hall measurements (3 × 10¹⁶/cm³ at 1000°C) increases with increasing temperature, whereas the Hall mobility (4 cm²(V·s) at 1000°C) decreases with increasing temperature. These values differ from literature data for bulk ceramic specimens. The difference may be duelo the small grain diameters (∼200 nm) in the 1-μm-thick thin films.     

Dissolution of a Sphene Glass-Ceramic, and of Its Component Sphene and Glass Phases, in Ca-Na-Cl Brines

Leaching experiments for up to 569 d have been performed to examine the dissolution in Ca-Na-Cl groundwater of a sphene (CaTiSiO₅)-based glass-ceramic, a candidate material for immobilizing nuclear fuel recycle wastes. The experiments involved leaching of samples of a simulated-waste-loaded glass-ceramic, doped with ²²Na or ⁴⁵Ca, in a synthetic groundwater at 25° and 100°C. The results are compared with those from separate leaching experiments with ²²Na- or ⁴⁵Ca-doped samples of aluminosilicate glass and ceramic sphene, representing the component phases of the glass-ceramic. The comparison supports a model in which the glass-ceramic dissolution rate may be approximately derived from the weighted average of the separate dissolution rates of its component glass and ceramic phases. No synergistic effects in the leaching of the glass and ceramic phases in the glass-ceramic were found.     

X-ray Diffraction and 151Eu Mössbauer Spectroscopic Study of the Hf1−xEuxO2−x/2(0 ≤x≤ 1.0) System

Powder X-ray diffractometry (XRD) and ¹⁵¹Eu Mössbauer spectroscopy were performed for samples prepared in the temperature range 1500–1500°C for the hafnia–europia (HfO₂–Eu₂O₃) system Eu _x Hf_{1− x} O_{2− x /2}(0 ≤ x ≤ 1.0). The XRD results showed that two types of solid solution phases formed in the composition range 0.25 ≤ x ≤ 0.725: an ordered pyrochlore-type phase in the middle-composition range (0.45 < x < 0.575) and a disordered fluorite-type phase, enveloping the pyrochlore-type phase on both sides, in the composition ranges 0.25 ≤ x ≤ 0.40 and 0.60 ≤ x ≤ 0.725. ¹⁵¹Eu Mössbauer spectroscopy sensitively probes local environmental changes around trivalent europium (Eu³⁺) caused by the formation of these solid solution phases. In addition to the broad, single Mössbauer spectra observed in this study for the Eu³⁺, the values of isomer shift (IS) exhibited a pronounced minimum in the neighborhood of the stoichiometric pyrochlore phase ( x ≈ 0.5). Such IS behavior of Eu³⁺ was interpreted based on the XRD crystallographic information that the ordered pyrochlore phase has a longer (in fact, the longest) average Eu–O bond length than those of the disordered fluorite phases on both sides or the monoclinic (and C-type) Eu₂O₃at x = 1.0.     

Ionic Conductivity in Fused Silica: II, Steady-State Behavior

Direct-current ionic conductivity measurements on type-I doped fused silica glasses were carried out in blocking and nonblocking modes between 300° and 800°C. The analysis of conductivity in steady state showed that the behavior was non-Arrhenius in nature and that In (conductivity) followed a second-degree polynomial of 1/ T. The normalized conductivity for Li⁺ and Na⁺ ions at a particular temperature increased with the alumina/alkali ratio up to about 8 and appeared to level thereafter. This trend was similar to that observed for viscosity in the same glasses published by Bihuniak et al. ¹ For K⁺ electrolysis, no significant trends in the normalized conductivity versus alumina/alkali ratio could be observed. The presence of nonlinearity in Arrhenius behavior could be ascribed to the combined effect of two types of sites present in fused silica: SiO⁻ and AIO₄⁻ sites, where the AIO₄⁻ sites presumably have a lower binding energy for the Li⁺ Na⁺, and K⁺ ions than do the SiO⁻ sites. It is argued that, unlike alkali aluminosilicates, the SiO⁻ sites in fused silica do not disappear at an alumina/alkali ratio of 1, and only reach a sturation level at a ratio of about 8, a conclusion also reached by Bihuniak et al. ² using randomwalk simulation.     

α-Decay Damage Effects in Curium-Doped Titanate Ceramic Containing Sodium-Free High-Level Nuclear Waste

A polyphase titanate ceramic incorporating sodium-free simulated high-level nuclear waste was doped with 0.91 wt% of ²⁴⁴Cm to accelerate the effects of long-term self-irradiation arising from α decays. The ceramic included three main constituent minerals: hollandite, perovskite, and zirconolite, with some minor phases. Although hollandite showed the broadening of its X-ray diffraction lines and small lattice parameter changes during damage ingrowth, the unit cell was substantially unaltered. Perovskite and zirconolite, which are the primary hosts of curium, showed 2.7% and 2.6% expansions, respectively, of their unit cell volumes after a dose of 12 × 10¹⁷α decays.g^-1 Volume swelling due to damage ingrowth caused an exponential (almost linear) decrease in density, which reached 1.7% after a dose of 12.4 × 10¹⁷α decays.g^-1. Leach tests on samples that had incurred doses of 2.0 × 10¹⁷ and 4.5 × 10¹⁷ a decays g^-1 showed that the rates of dissolution of cesium and barium were similar to analogous leach rates from the equivalent cold ceramic, while strontium and calcium leach rates were 2–15 times higher. Although the curium, molybdenum, strontium, and calcium leach fates in the present material were similar to those in the curium-doped sodium-bearing titanate ceramic reported previously, the cesium leach rate was 3–8 times lower.     

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.     

Mechanical and Thermal Properties of Antiperovskite Ti3AlC Prepared by an In Situ Reaction/Hot-Pressing Route

Bulk Ti₃AlC ceramic containing 2.68 wt% TiC was prepared by an in situ reaction/hot-pressing route. The reaction path, microstructure, mechanical and thermal properties were systematically investigated. At room temperature Vickers hardness of Ti₃AlC ceramic is 7.8 GPa. The flexural strength, compressive strength, and fracture toughness are 182, 708 MPa, and 2.6 MPa·m^1/2, respectively. Its apparent Young's modulus, shear modulus, bulk modulus and Possion's ratio are 208.9, 83.4, 140.4 GPa, and 0.25 at room temperature. Apparent Young's modulus decreases slowly with the increasing temperature, and at 1210°C the modulus is 170 GPa. The average coefficient of thermal expansion of Ti₃AlC ceramic is about 10.1 × 10⁻⁶ K⁻¹ in the temperature range of 150°–1200°C. Both the molar heat capacity and thermal conductivity increase with an increase in the temperature. At 300 and 1373 K, the molar heat capacities are 87 and 143·J·(mol·K)⁻¹, while the thermal conductivities are 8.19 and 15.6 W·(m·K)⁻¹, respectively.     

Fabrication of Microcellular Ceramics Using Gaseous Carbon Dioxide

A microcellular ceramic with cell densities >10⁹ cells/cm³ and cells <10 μm was made with a preceramic mixture of polycarbosilane and polysiloxane. The preceramic compact was saturated with gaseous CO₂, a large number of cells were nucleated and grown by using a thermodynamic instability induced by a rapid pressure drop, and the microcellular preceramic was transformed into a microcellular ceramic by pyrolysis.     

Effective Thermal Conductivity of a Packed Bed of Hollow Zirconia Microspheres, under Vacuum and under 100 kPa of Argon

Measurements have been made of the effective thermal conductivity of a packed bed of hollow, yttria-stabilized zirconia microspheres, under vacuum and under 100 kPa of argon gas. Above 1400 K the spheres begin to sinter together. Before this occurs, the conductivity is given under vacuum by A ₁ T ³+ A ₂ with A ₁= 2 × 10⁻¹¹ W · m⁻¹· K⁻⁴ and A ₂= 0.01 W · m⁻¹· K⁻¹. The thermal conductivity increases strongly with both the gas pressure and the degree of sintering of the spheres. The measured values can be fitted reasonably well by a model developed by Takegoshi et al. These results may have some applicability to the development of high-temperature thermal insulation.     

Characterization of the Thermally Contracting Tungstates Ta22W4O67, Ta2WO8, and Ta16W18O94

An investigation of the properties of high-purity (>99 wt%) tantalum tungstates (Ta₂₂W₄O₆₇, Ta, WO₈, and Ta₁₆W₁₈O₉₄) included determination of density (bulk and theoretical), refined lattice constants, maximum use temperatures, micro-hardness, heat capacity, thermal expansion (contraction) and diffusivity, calculated thermal conductivity, and electrical resistivity. Usable to ∼ 1700 K in air or inert atmospheres, these tantalum tungstates have theoretical densities of 7.3 to 8.5 g/cm³, are relatively soft (120 to 655 kg/mm² hardnesses), and are electrical insulators (6× 10³ to 2× 10⁸Ω^.cm resistivities). The distinguishing properties of the materials are their thermal expansion (average CTE values from + 0.6×10⁻⁸/K to −5.1× 10⁻⁶/K at 293 to 1273 K), thermal expansion hysteresis with minimal observable microcracking, and thermal diffusivity     

Annealing of Irradiation-Induced Thermal Conductivity Changes in ThO2-1.3 wt% UO2

The thermal conductivities of sintered pellets of ThO₂-1.3 wt% U0₂ were measured at 60°C before and after irradiation. The irradiation temperature was below 156°C, and the exposures varied from 3.1 × 10¹⁴ to 4.7 × lo^L7 fissions/cm³. Each fission fragment damaged a region of 2.2 × 10^-16 cm³ with the reduction in conductivity saturating by about 10¹⁷ fissions/cm³. Samples having exposures from 10¹⁵ to 10¹⁶ fissions/cm³ were annealed isothermally at 651 °C or isochronally from 300° to 1200° C to study the annealing of damage. Most of the annealing occurred between 500° and 900°C. The width of this interval plus the slow isothermal annealing suggest that the damage is annealed by a number of single order processes with a spectrum of activation energies from 1.8 to 3.9 eV or, less probably, by a high order process with an activation energy of 3.55 ± 0.4 eV.     

Phase Equilibria at Liquidus Temperatures in the System MgO-FeO-Fe2O3-SiO2

Liquidus temperatures are presented for mixtures in the system MgO-FeO-Fe₂O₃-SiO₂. The standard quenching technique was modified for work under controlled atmospheres of varying O₂ pressures. Data were obtained for the temperature range 1159° to 1775°C., and with O₂ pressures ranging from 1 to 10^-8.⁹ atm. Approximate compositions of crystalline phases were determined, and paths of equilibrium crystallization were derived for selected mixtures under idealized conditions. Application of the phase diagrams to steel-plant refractories problems is indicated.