Electrical Conductivity of Zirconia Stabilized with Scandia and Yttria

Electrical conductivity of zirconia stabilized with scandia and yttria (Sc₂O₃+Y₂O₃= 8 mol%) has been measured by the complex impedance method in the temperature range 573 to 1173 K. With increasing Sc₂O₃ concentration, electrical conductivity increases at temperatures above 640 K, but it decreases below this temperature. Electrical conductivity in the electrolytes examined is a result of two processes: an activation energy of 59 to 79 kJ·mol⁻¹ predominant at high temperatures and an activation energy of 109 to 125 kJ·mol⁻¹ predominant at low temperatures.     

High Thermal Conductivity in Silicon Nitride with Anisotropie Microstructure

Silicon nitride was fabricated by tape casting of α-Si₃N₄ powder with 5 wt% Y₂O₃ and 5 vol% rodlike β-Si₃N₄ seed particles, followed by tape stacking, hot pressing under 40 MPa, and annealing at 1850°C for 2-66 h under a nitrogen pressure of 0.9 MPa. Silicon nitrides fabricated by this procedure exhibited a highly anisotropic microstructure with large elongated grains (developed from seed particles) uniaxially oriented parallel to the casting direction. Thermal conductivities parallel to the grain alignment were much higher than those measured in other directions and exhibited high values of up to 120 W/(m.K). The anisotropic thermal conductivity of the specimen could be explained by the rule of mixture, considering that large elongated grains developed from seeds have higher thermal conductivity than a small-grained matrix.     

碳化硅材料热导率计算研究进展

从声子散射机制出发,介绍了Si C热导率的温度特性和微观导热机理。综述了Si C单晶热导率的2种主要计算方法。Boltzmann-弛豫时间近似(RTA)适用于各个温度段的热导率计算,而分子动力学方法更适用于高温热导率计算。分子动力学方法相比于Boltzmann-RTA方法的优点在于它可以考虑所有高次项的非谐作用。介绍了3种Si C陶瓷热导率近似计算模型,包括界面热阻模型、Debye-Callaway模型及多相系统热导率模型。下一步研究的主要方向仍然是优化计算模型及减少拟合参数。     

Thermal Diffusivity/Conductivity of Alumina—Silicon Carbide Composites

Thermal diffusivity and conductivity values for several Al₂O₃-SiC whisker composites were determined. The thermal diffusivity values spanned the range from 373 to 1473 K, and thermal conductivity data wre obtained between 305 and 365 K. The thermal diffusivity decreased with increasing temperature and increased with SiC-whisker content. An estimate of the thermal conductivity of the whiskers was obtained from the direct thermal conductivity measurements, but attempts to derive whisker conductivity values from the thermal diffusivity data were not successful because the laser flash method lacks the required accuracy and precision. Specimens were subjected to two different thermal quench experiments to investigate the effect of thermal history on diffusivity. In the most severe case, multiple 1073- to 373-K quenches, radial cracks were observed in the test specimens; however, there was no change in diffusivity. The lack of sensitivity to thermal cycling appears to be related to the sample size.     

Thermal Conductivity/Diffusivity of Silicon Carbide Whisker Reinforced Mullite

The thermal conductivity and diffusivity of silicon carbide whisker reinforced mullite was shown to increase with whisker content. This effect was much greater for vapor-liquid-solid (VLS) whiskers than for rice-hull (RH) whiskers. This suggests that the thermal conductivity for the VLS whiskers was significantly higher than for the RH whiskers. Due to preferred orientation of the whiskers, thermal conductivity and diffusivity of the composite samples exhibited significant anisotropy.     

Toughening Behavior of Silicon Carbide with Additions of Yttria and Alumina

The fracture-toughness-determining mechanism of silicon carbide with additions of yttria and alumina was studied. Observations of indentation crack profiles revealed that significant crack deflection had occurred. Median deflection angles increased with increased volume fractions of the second phases, which was accompanied by increased fracture toughness.     

Thermal Diffusivity/Conductivity of Magnesium Oxide/Silicon Carbide Composites

SiC-particle-reinforced MgO composites have been fabricated by hot pressing, and the thermal diffusivities of the composites measured in the temperature range 200–1000°C using a laser flash technique. The thermal conductivity of the composites was calculated by multiplying the diffusivity with density and with heat capacity. The Eshelby inclusion model has been examined, and an equation suitable for particulate composites with porosity has been derived using the multiphase Eshelby model. The model also considers the interfacial thermal condition. Good agreement was obtained between the predictions and the experimental results of the thermal conductivity of the composites, even for various levels of porosity in the composites. Crystal defects, observed in the composites, influenced the thermal conductivity, resulting in a deviation from isothermal interfacial condition. This was reflected in the interfacial thermal parameter,β used in the modeling, and the predicted value of β was in the range of 3–10, depending on the thermal conductivity of SiC used for the calculations.     

Microstructure and Fracture Toughness of Hot-Pressed Silicon Carbide Reinforced with Silicon Carbide Whisker

The effects of β-SiC whisker addition on the microstructural evolution and fracture toughness ( K _IC) of hot-pressed SiC were investigated. Most of the whiskers added disappeared during the densifcation process by transformation into the α-phase. The remaining whiskers acted as nuclei for grain growth, resulting in the formation of large tabular grains around the whiskers. The tabular grains around the whiskers were believed to be formed because of the extreme anisotropy of the interfacial energy between α- and β-SiC. The K _IC of the material was improved significantly by the whisker addition. The increase in the K _IC was attributed to crack bridging followed by grain pullout as a result of the formation of tabular grains in a fine matrix.     

Phase Transformation and Thermal Conductivity of Hot-Pressed Silicon Carbide Containing Alumina and Carbon

Phase transformation and thermal conductivity of hot-pressed β-SiC with Al₂O₃ and carbon additions were studied. Densification rate was a complex function of both Al₂O₃ and carbon. Simultaneous additions of Al₂O₃ and carbon accelerated the 3C → 4H phase transformation. Carbon additions lowered the thermal conductivity of the compact as did the high-temperature hot-pressing. The 3C → 4H transformation and the thermal conductivity were deduced to be related to each other.     

Multiscale Genome Modeling for Predicting the Thermal Conductivity of Silicon Carbide Ceramics

Silicon carbide (SiC) ceramics have been widely used in industry due to its high thermal conductivity. Understanding the relations between the microstructure and the thermal conductivity of SiC ceramics is critical for improving the efficiency of heat removal in heat sink applications. In this paper, a multiscale model is proposed to predict the thermal conductivity of SiC ceramics by bridging atomistic simulations and continuum model via a materials genome model. Interatomic potentials are developed using ab initio calculations to achieve more accurate molecular dynamics (MD) simulations. Interfacial thermal conductivities with various additive compositions are predicted by nonequilibrium MD simulations. A homogenized materials genome model with the calculated interfacial thermal properties is used in a continuum model to predict the effective thermal conductivity of SiC ceramics. The effects of grain size, additive compositions, and temperature are also studied. The good agreement found between prediction results and experimental measurements validates the capabilities of the proposed multiscale genome model in understanding and improving the thermal transport characteristics of SiC ceramics.     

Deposition and Microstructure of Vapor-Deposited Silicon Carbide

Vapor deposition of Sic from methyltrichlorosilane in a fluidized bed and the microstructure of the deposit were studied over a range of deposition temperatures, carrier gas flow rates, and reactant fluxes. The rate-determining factor for the deposition of Sic was the rate of supply of reactant. The microstructure of vapor-deposited Sic was primarily dependent on the deposition temperature; however, carrier gas flow rate and reactant flux had a secondary influence on microstructure. At low temperatures and high carrier gas flow rates, laminar deposits containing excess silicon were produced. At higher temperatures and lower carrier gas flow rates deposits were characterized by faulted columnar grains. The grain diameter increased from about LP at 1400°C to about 15 μ at 1800°C. The grain size also increased, but less markedly, with increasing reactant flux. The deposits characterized by columnar grains were predominantly β-Sic with traces of a-SiC and excess carbon.     

Control of Electrical Resistivity in Silicon Carbide Ceramics Sintered with Aluminum Nitride and Yttria

The electrical properties of β‐SiC ceramics were found to be adjustable through appropriate AlN–Y₂O₃ codoping. Polycrystalline β‐SiC specimens were obtained by hot pressing silicon carbide (SiC) powder mixtures containing AlN and Y₂O₃ as sintering additives in a nitrogen atmosphere. The electrical resistivity of the SiC specimens, which exhibited n‐type character, increased with AlN doping and decreased with Y₂O₃ doping. The increase in resistivity is attributed to Al‐derived acceptors trapping carriers excited from the N‐derived donors. The results suggest that the electrical resistivity of the β‐SiC ceramics may be varied in the 10⁴–10^?3 Ω·cm range by manipulating the compensation of the two impurity states. The photoluminescence (PL) spectrum of the specimens was found to evolve with the addition of dopants. The presence of N‐donor and Al‐acceptor states within the band gap of 3C–SiC could be identified by analyzing the PL data.     

Effect of Yttria on the Thermal Conductivity of Aluminum Nitride

The effect of yttria additions up to 10 wt% on the thermal conductivity of pressureless-sintered aluminum nitride was investigated. Additions of up to 3 wt% increased the thermal conductivity to values around 160 W/(m·K). Additions higher than 5 wt% slowly decreased the thermal conductivity because of the increasing volume fraction of the low-conductivity yttrium aluminate phases. In all cases samples containing the same yttria content with the binder burned out in nitrogen had higher thermal conductivity than those with the binder burned out in air. Comparisons are made to previous studies containing additives of silica and calcia to aluminum nitride.     

碳化硅木质陶瓷的显微结构及力学性能

以汉麻秆芯碳化后的碳粉为原料,分别采用注浆和干压成型工艺制备素坯,通过反应烧结制备出碳化硅木质陶瓷.研究了注浆成型工艺中悬浮稳定剂的种类和添加量对浆料性能的影响.采用激光共聚焦显微镜、扫描电子显微镜和X射线衍射仪等分析了碳化硅木质陶瓷的显微结构、物相组成及力学性能.结果表明:采用注浆成型制备的碳化硅木质陶瓷力学性能优异,实测的游离硅含量同理论计算结果一致,说明渗硅过程中硅碳反应充分,烧结体显微硬度、弯曲强度、弹性模量和断裂韧性分别为22.3 GPa、397 MPa、290 GPa和3.0 MPa·m1/2.     

Microstructure and Grain-Boundary Composition of Hot-Pressed Silicon Nitride With Yttria and Alumina

The microstructure of two hot-pressed silicon nitrides containing Y₂O₃ and Al₂O₃ was examined by electron microscopy, electron diffraction, and quantitative, energy-dispersive X-ray microanalysis. A crystalline second phase was identified in the material with additives of 5 wt% Y₂O₃+2 wt% Al₂O₃, as a solid solution of nitrogen mellilite and alumina. An amorphous third phase as narrow as 2 nm is discerned at all grain boundaries of this material by high-resolution dark-field and lattice imaging. The second phase in a material with additives of S wt% Y₂O₃+5 wt% Al₂O₃ was found to be amorphous. Some of the additional alumina additive appears in solid solution with silicon nitride. In situ hot-stage experiments in a high-voltage electron microscope show that the amorphous phase volatilizes above 1200°C, leaving a skeleton of Si₃N₄ grains linked by the mellilite crystals at triple points. The results show that intergranular glassy phases cannot be eliminated by the Y₂O₃/Al₂O₃ fluxing.     

Thermal Conductivity of Gas-Pressure-Sintered Silicon Nitride

Si₃N₄ with high thermal conductivity (120 W/(m^.K)) was developed by promoting grain growth and selecting a suitable additive system in terms of composition and amount. β-Si₃N₄ doped with Y₂O₃-Nd₂O₃ (YN system) or Y₂O₃-A1₂O₃ (YA system) was sintered at 1700°-2000°C. Thermal conductivity increased with increased sintering temperature because of decreased two-grain junctions, as a result of grain growth. The effect of the additive amount on thermal conductivity with the YN system was rather small because increased additive formed multigrain junctions. On the other hand, with the YA system, thermal conductivity considerably decreased with increased additive amount because the aluminum and oxygen in the YA system dissolved into β-Si₃N₄ grains to form a β-SiAlON solid solution, which acted as a point defect for phonon scattering. The key processsing parameters for high thermal conductivity of Si₃N₄ were the sintering temperature and additive composition.     

Processing and Thermal Conductivity of Sintered Reaction-Bonded Silicon Nitride: (II) Effects of Magnesium Compound and Yttria Additives

The effects of the magnesium compound and yttria additives on the processing, microstructure, and thermal conductivity of sintered reaction-bonded silicon (Si) nitride (SRBSN) were investigated using two additive compositions of Y₂O₃–MgO and Y₂O₃–MgSiN₂, and a high-purity coarse Si powder as the starting powder. The replacement of MgO by MgSiN₂ leads to the different characteristics in RBSN after complete nitridation at 1400°C for 8 h, such as a higher β-Si₃N₄ content but finer β-Si₃N₄ grains with a rod-like shape, different crystalline secondary phases, lower nitrided density, and coarser porous structure. The densification, α→β phase transformation, crystalline secondary phase, and microstructure during the post-sintering were investigated in detail. For both cases, the similar microstructure observed suggests that the β-Si₃N₄ nuclei in RBSN may play a dominant role in the microstructural evolution of SRBSN rather than the intergranular glassy chemistry during post-sintering. It is found that the SRBSN materials exhibit an increase in the thermal conductivity from ∼110 to ∼133 (Wm·K)⁻¹ for both cases with the increased time from 6 to 24 h at 1900°C, but there is almost no difference in the thermal conductivity between them, which can be explained by the similar microstructure. The present investigation reveals that as second additives, the MgO is as effective as the MgSiN₂ for enhancing the thermal conductivity of SRBSN.     

Evaluation of Yttria Coated High Density Graphite with Silicon Carbide Interlayer for Uranium Melting Applications

Yttrium Oxide (Y₂O₃) deposited over High Density Graphite (HDG) by Atmospheric Plasma Spray (APS) process is highly desirable as a chemical barrier coating for reusable Uranium (U) melting crucibles in the pyrochemical reprocessing of spent metallic fuels. In the present study, an oxidation protective Silicon Carbide (SiC) interlayer coating over HDG has been achieved by pack cementation process. The high-temperature oxidation resistance and resistance to thermal fatigue failure of Y₂O₃ coating with and without SiC interlayer were evaluated by performing repeated thermal cycling studies at 1450, 1500 and 1550?°C. The durability performance of Y₂O₃ coating with SiC interlayer in the actual working environment was simulated by performing U melting studies using miniature size HDG coated crucibles. The microstructural, chemical and phase characterization of coatings prior and post thermal cycle failure were carried out by SEM/EDS and XRD techniques. It is observed that the SiC interlayer developed by novel pack cementation technique for the Y₂O₃ top coat extended the thermal cycling and life of the coating with U melting in inert argon gas environment significantly. The occurrence of micro-cracking over Y₂O₃ top coat with SiC interlayer perceived after 33, 30 and 25 thermal cycles at 1450, 1500 and 1550?°C, respectively.     

Processing, Microstructure, and Wear Behavior of Silicon Nitride Hot-Pressed with Alumina and Yttria

Commercial silicon nitride powder with A1₂O₃ and Y₂O₃ additives was hot-pressed to complete density. The resulting microstructure contained elongated grains with no trace of remaining α-Si₃N₄. The aspect ratio of the elongated grains increased with increasing soak time at a fixed hot-pressing temperature. X-ray diffraction analysis showed that the crystalline phase in the hot-pressed samples was β-sialon (Si_6−zAl_zO_zN_8−z) with z values that increased with soak time. The fracture strength and fracture toughness of the samples increased as the aspect ratio of the grains increased. The Vickers hardness decreased slightly as the soak time was increased, which was attributed to a grain size effect. Wear tests of silicon nitride against silicon nitride were conducted on a reciprocating pin-on-disk apparatus with paraffin oil as a lubricant. Correlation studies of wear with microstructure and mechanical properties were performed. The wear rate increased rapidly with increasing soak time in spite of the increased strength and toughness. This was attributed to increased third-body wear caused by pullout of pieces from the wear surface. The pullout mechanism was not conclusively identified. However, TEM examination showed clear evidence of dislocation motion under the wear scar. Grain boundary microstresses caused by the anisotropic thermal expansion and elastic properties of the elongated grains may have contributed to the observed pullout.     

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.