Effects of Fast Neutron Irradiation on Thermal Conductivity of Li2O and LiAlO2

Li₂O and LiAlO₂ are two candidates for solid breeder materials in the United States' Fusion Power Program. Critical to breeder design efforts are thermophysical data, the bulk of which have only recently become available, for un-irradiated lithium ceramics. This paper expands the current limited data base by presenting thermal conductivity data between 373 and 1173 K for both materials following fast neutron irradiation. Samples were irradiated at 773 to 1173 K to lithium burnups ≤11.5 × 10²⁰ captures/cm³. Comparisons are made between these data and those from unirradiated archive samples of these same materials.     

Kinetics and Mechanism of Hot Corrosion of γ-Y2Si2O7 in Thin-Film Na2SO4 Molten Salt

γ-Y₂Si₂O₇ is a promising candidate material both for high-temperature structural applications and as an environmental/thermal barrier coating material due to its unique properties such as high melting point, machinability, thermal stability, low linear thermal expansion coefficient (3.9 × 10⁻⁶/K, 200°–1300°C), and low thermal conductivity (<3.0 W/m·K above 300°C). The hot corrosion behavior of γ-Y₂Si₂O₇ in thin-film molten Na₂SO₄ at 850°–1000°C for 20 h in flowing air was investigated using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS). γ-Y₂Si₂O₇ exhibited good resistance against Na₂SO₄ molten salt. The kinetic curves were well fitted by a paralinear equation: the linear part was caused by the evaporation of Na₂SO₄ and the parabolic part came from gas products evolved from the hotcorrosion reaction. A thin silica film formed under the corrosion scale was the key factor for retarding the hot corrosion. The apparent activation energy for the corrosion of γ-Y₂Si₂O₇ in Na₂SO₄ molten salt with flowing air was evaluated to be 255 kJ/mol.     

Phase Separation of Alkoxy-derived (Ti,Sn)O2 Oriented Thin Films

Oriented (Ti,Sn)O₂ thin films with modulated microstructure were successfully synthesized on sapphire substrates by using sol–gel processing combined with spinodal decomposition. The degree of orientation of (Ti_0.5Sn_0.5)O₂ thin films increased in the following order: sapphire (0001), (11     0), and (01     2). (Ti_0.5Sn_0.5)O₂ thin films underwent spinodal decomposition at 900°C by annealing. The variation of the 2theta value of the 202 reflection of (Ti_0.5Sn_0.5)O₂ films showed the typical behavior of spinodal decomposition. The rate of spinodal decomposition of the (Ti_0.5Sn_0.5)O₂ films on sapphire (11     0) was faster than that on sapphire (01     2) substrates. The characteristic modulated microstructure was observed for the spinodally decomposed (Ti_0.5Sn_0.5)O₂ films on sapphire (01     2) substrates by transmission electron microscopy. (Ti_0.3Sn_0.7)O₂ films on sapphire (01     2) substrates were binodally decomposed during annealing at 1300°C.     

Ta2O5/Nb2O5 and Y2O3 Co-doped Zirconias for Thermal Barrier Coatings

Zirconia doped with 3.2–4.2 mol% (6–8 wt%) yttria (3–4YSZ) is currently the material of choice for thermal barrier coating topcoats. The present study examines the ZrO₂-Y₂O₃-Ta₂O₅/Nb₂O₅ systems for potential alternative chemistries that would overcome the limitations of the 3–4YSZ. A rationale for choosing specific compositions based on the effect of defect chemistry on the thermal conductivity and phase stability in zirconia-based systems is presented. The results show that it is possible to produce stable (for up to 200 h at 1000°–1500°C), single (tetragonal) or dual (tetragonal + cubic) phase chemistries that have thermal conductivity that is as low (1.8–2.8W/m K) as the 3–4YSZ, a wide range of elastic moduli (150–232 GPa), and a similar mean coefficient of thermal expansion at 1000°C. The chemistries can be plasma sprayed without change in composition or deleterious effects to phase stability. Preliminary burner rig testing results on one of the compositions are also presented.     

Thermophysical Properties of ZrB2 and ZrB2–SiC Ceramics

Thermophysical properties were investigated for zirconium diboride (ZrB₂) and ZrB₂–30 vol% silicon carbide (SiC) ceramics. Thermal conductivities were calculated from measured thermal diffusivities, heat capacities, and densities. The thermal conductivity of ZrB₂ increased from 56 W (m K)⁻¹ at room temperature to 67 W (m K)⁻¹ at 1675 K, whereas the thermal conductivity of ZrB₂–SiC decreased from 62 to 56 W (m K)⁻¹ over the same temperature range. Electron and phonon contributions to thermal conductivity were determined using electrical resistivity measurements and were used, along with grain size models, to explain the observed trends. The results are compared with previously reported thermal conductivities for ZrB₂ and ZrB₂–SiC.     

Thermal Expansion of CeO2, Ho2O3, and Lu2O3 from 100° to 300°K by an X-Ray Method

Thermal expansion of CeO₂, Ho₂O₈, and Lu₂O₃ was determined from 100° to 300°K by a back-reflection X-ray technique. The variation of thermal expansion with temperature is the same as that of specific heat for CeO₂ and Ho₂O₃; these oxides obey the Grueneisen model of thermal expansion in the temperature range studied.     

Spark Plasma Sintered Silicon Nitride Ceramics with High Thermal Conductivity Using MgSiN2 as Additives

Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si₃N₄ powders as raw materials and MgSiN₂ as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si₃N₄ phase left in the sintered material. α-Si₃N₄ was completely transformed to β-Si₃N₄ after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)⁻¹ was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si₃N₄ ceramics with high thermal conductivity when appropriate additives are used.     

Carbon Thermal Diffusion in the UC2-C System

Alpha uranium dicarbide disks coated with pyrolytic carbon migrate, relative to the carbon, up a temperature gradient, leaving behind a layer of rejected graphite. A model for the migration process based on solution of carbon at the hot UC₂-C interface, transport across the UC₂ phase by thermal diffusion, and rejection as graphite at the cool UC₂-C interface was developed. The rate of migration may be described by: The apparent activation energy for migration is 76.5±9 kcal/mol, compared to a literature value for carbon self-diffusion in _α-UC₂ of 90.2 kcal/mol. This difference in temperature dependence may be considered preliminary evidence that the heat of transport decreases with increasing temperature. The average value of the heat of transport, calculated using literature values for carbon self-diffusion in _α-UC₂, is 92±27 kcal/mol.     

Na2SO4-lnduced Corrosion of Si3N4 Coated with Chemically Vapor Deposited Ta2O5

Hot isostatically pressed Si₃N₄ was coated with chemically vapor-deposited Ta₂O₅, and subjected to oxidative and corrosive environments to determine the feasibility of using a Ta₂O₅ coating for protecting Si₃N₄ from hot corrosion. The coated structure was relatively stable at 1000^deg;C in pure O₂. However, the Ta₂O₅-Si₃N₄ system became unstable in an environment containing Na₂SO₄ and O₂ at 1000^deg;C because (1) Ta₂O₅ and Na₂SO₄ reacted rapidly to form NaTaO₃ and (2) subsequently NaTaO₃ interacted destructively with the underlying Si₃N₄ substrate to form a molten phase.     

Thermal Degradation Behavior of WOP2O7/Phenanthroline Complex

An intercalation compound of WOP₂O₇/phenanthroline and a mixture of WOP₂O₇/carbon black were prepared to have equal carbon contents. Both samples were heated below 1400°C under a nitrogen atmosphere and their thermal degradation behaviors compared, with the following results: (1) CO gas evolved through carbothermal reduction at a temperature 150°C lower in the intercalation compound than in the WOP₂O₇/carbon black mixture. (2) Both WP and W formed at temperatures approximately 100°C lower in the intercalation compound than in the mixture. (3) Thermal degradation occurred homogeneously throughout the intercalation compound particles, and the particle morphology remained favorable after thermal degradation.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

High-Temperature Neutron Diffraction of the AlN–Al2O3–Y2O3 System

The importance of aluminum nitride (AlN) stems from its application in microelectronics as a substrate material due to high thermal conductivity, high electrical resistance, mechanical strength and hardness, thermal durability, and chemical stability. Yttria (Y₂O₃) is the best additive for AlN sintering. AlN densifies by a liquid-phase mechanism, where the surface oxide, Al₂O₃, reacts with Y₂O₃ to form an Y-Al-O-N liquid that promotes particle rearrangement and densification. Construction of the phase relations in this multicomponent system is essential for optimizing the properties of AlN. The ternary phase diagram of the AlN–Al₂O₃–Y₂O₃ was developed by Gibbs energy minimization using interpolation procedures based on modeling the binary subsystems. This paper aims at testing the resultant understanding experimentally at selected compositions using in situ high-temperature neutron diffractometry. These experimental results agree with the thermodynamic calculations of AlN–Al₂O₃–Y₂O₃. The ternary phase diagram has been constructed for the first time in this work. High-temperature neutron diffractometry has permitted real time measurement of the reactions involved in this ternary system, especially to determine the temperature range for each reaction, which would have been difficult to establish by other means.     

Optical Properties of Er-Doped Al2O3–SiO2 Films Prepared by a Modified Sol–Gel Process

Er-doped Al₂O₃–SiO₂ (1/9 in mol ratio of Al₂O₃/SiO₂) thin films were prepared by using a modified sol–gel process. The modified process entails the precipitation and digestion of Er(OH)₃, obtained from the reaction between Er ions and NH₄OH in solution. Thin films were deposited on Si wafers by using a spin coating technique (3000 rpm) and the coated films were heat treated at different temperatures for 1 h in an oxygen-purged furnace. All the films were structurally characterized by the X-ray diffraction technique using Cu K α radiation. Refractive indices and the morphologies of the films were studied using a spectroscopic phase modulated ellipsometer and atomic force microscopy, respectively. It was observed that the films were crack free and of about 0.4 μm thickness in a single spin coating and both the lifetime and the photoluminescence intensity of Er ions increased with increasing the annealing temperature. The luminescence properties of the Er-doped Al₂O₃–SiO₂ made by a conventional and our modified doping process were compared and discussed from the stand point of peak intensities and lifetimes as a function of annealing temperatures. It is to be noted here that our modified process was found to be more effective in reducing the clustering of Er ions in Al₂O₃–SiO₂ materials as compared to that of the conventional method.     

Spectroscopic and Thermal Properties of Ga2S3–Na2S–CsCl Glasses

The synthesis and properties of the vitreous system (0.75− x )Ga₂S₃–0.25Na₂S– x CsCl, with x varying from 0.1 to 0.2, are presented. Thermal, optical, and structural properties such as density, viscosity, thermal expansion coefficient, glass transition temperature, softening point temperature, refractive index, and absorption coefficient were measured using several techniques: X-ray diffraction, Raman scattering, differential thermal analysis, thermal mechanical analysis, and absorption spectroscopy. This glass system presents a high third-order non-linear optical susceptibility that can be significantly increased by increasing the CsCl content without affecting the low phonon frequency.     

Mechanical Properties/Database of Plasma-Sprayed ZrO2-8wt% Y2O3 Thermal Barrier Coatings

Mechanical behavior of free-standing, plasma-sprayed ZrO₂-8wt% Y₂O₃ thermal barrier coatings, including strength, fracture toughness, fatigue, constitutive relation, elastic modulus, and directionality, have been determined under various loading-specimen configurations. This paper presents and describes a summary of mechanical properties of the plasma-sprayed coating material to provide them as a design database.     

Thermal Conductivity of Li2O·Al2O3·nSiO2 Glass-Ceramics between 5 and 100 K

The thermal conductivity of Li₂O·Al₂O₃· n SiO₂ glass-ceramics is studied for n = 4, 6, 8, 10 between 5 and 100 K. A monotonic increase in conductivity is observed for all samples. This behavior is different from that of glassy counterparts which exhibit a plateau in thermal conductivity between 10 and 20 K. It is also observed that while the conductivity of glass-ceramics is lower than that of glasses at low temperatures, the situation is reversed at higher temperatures. A crossover occurs around 40 K for all studied samples. The glass-ceramic behavior is interpreted in the light of the acoustic mismatch theory of Little. At low temperatures, the thermal boundary resistance that exists at the crystalline-amorphous mismatch is high and the thermal conductivity is low. At higher temperatures, the boundary resistance is very small and the high conductivity is mainly due to the crystalline region within the amorphous structure.     

Preparation of TiO2-Coated Al2O3 Particles by Chemical Vapor Deposition in a Rotary Reactor

A process of coating Al₂O₃ particles with TiO₂ by hydrolysis of Ti(OC₄H₉)₄ using chemical vapor deposition in a rotary reactor has been developed. The process resulted in (1) a coating film of TiO₂ which was compact and uniform with the fraction of TiO₂ being 0.1%–10.0% and (2) an amorphous TiO₂ coating at a low reaction temperature converted to anatase at a reaction temperature higher than 673 K.     

Thermal Expansion of Polycrystalline Ti3SiC2 in the 25°–1400°C Temperature Range

We measured the volume thermal expansion of Ti₃SiC₂ from 25° to 1400°C using high-temperature X-ray diffraction using a resistive heated cell. A piece of molybdenum foil with a 250 μm hole contained the sample material (Ti₃SiC₂+Pt). Thermal expansion of the polycrystalline sample was measured under a constant argon flow to prevent oxidation of Ti₃SiC₂ and the molybdenum heater. From the lattice parameters of platinum (internal standard), we calculated the temperature by using thermal expansion data published in the literature. The molar volume change of Ti₃SiC₂ as a function of temperature in °C is given by: V _M (cm³/mol)=43.20 (2)+9.0 (5) × 10⁻⁴ T +1.8(4) × 10⁻⁷ T ². The temperature variation of the volumetric thermal expansion coefficient is given by: α_v (°C⁻¹)=2.095 (1) × 10⁻⁵+7.700 (1) × 10⁻⁹ T . Furthermore, the results indicate that the thermal expansion anisotropy of Ti₃SiC₂ is quite mild in accordance with previous work.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.     

Joining of ZrB2-Based Ultra-High-Temperature Ceramic Composites to Cu–Clad–Molybdenum for Advanced Aerospace Applications

Three hot-pressed ZrB₂-based ultra-high-temperature ceramic composites (UHTCC), ZrB₂–SiC_p (ZS), ZrB₂–SiC_p–C (ZSC), and ZrB₂–SCS9A (SiC fiber)–SiC_p (ZSS), were joined to Cu–clad–Mo using AgCuTi brazes ( T _L∼1073–1173K) and Pd-base brazes ( T _L∼1493–1513K). More extensive chemical interactions occurred in Pd-base joints than in AgCuTi-base joints. The Pd-braze region displayed higher hardness in joints made using ZS than ZSS and ZSC. Residual stress calculations point toward negative strain energy up to ∼23% clad layer thickness because α_{Cu–clad–Mo}<α_ZS (α=coefficients of thermal expansion). Above this thickness, α_{Cu–clad–Mo}>α_ZS, strain energy is positive, and it increases with increasing thickness. Projected reductions in the thermal resistance highlight the benefits of joining the UHTCC to Cu–clad–Mo.