Innovation in Ceramic Science and Engineering

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Authors: Mrityunjay Singh, Michael C. Halbig
Abstract: Robust bonding and integration technologies are critically needed for the successful implementation of silicon carbide based components and systems in a wide variety of aerospace and ground based applications. These technologies include bonding of silicon carbide to silicon carbide as well as silicon carbide to metallic systems. A diffusion bonding based approach has been utilized for joining of silicon carbide (SiC) to silicon carbide sub-elements for a micro-electro-mechanical systems lean direct injector (MEMS LDI) application. The objective is to join SiC sub-elements to from a leak-free injector that has complex internal passages for the flow and mixing of fuel and air. A previous bonding approach relied upon silica glass-based interlayers that were non-uniform and not leak free. In the newly developed joining approach, titanium foils and physically vapor deposited titanium coatings were used to form diffusion bonds between SiC materials using hot pressing. Microscopy results show the formation of well adhered diffusion bonds. Initial tests show that the bond strength is much higher than required for the component system. Benefits of the joining technology are fabrication of leak free joints with high temperature and mechanical capability.
Authors: Zheng Fei Chen, Y.C. Su, Yi Bing Cheng
Abstract: Reaction sintering of boron carbide represents an attractive densification process. In this work, sintering mechanisms of silicon carbide and boron carbide composites were studied. Mixed boron carbide/graphite mixtures were sintered in a vacuumed graphite furnace between 1380 and 1450oC. The samples were in contact with bulk silicon metal which melts at 1410oC. Reaction sequence of the composition was investigated by X-ray diffraction, SEM and TEM. It was found that a reaction between molten silicon and B4C occurred and the reaction produced silicon carbide and silicon-containing boron carbide. Dense composites can be achieved by pressureless sintering at 1450oC and the final microstructure consists of silicon carbide, boron carbide, silicon-containing boron carbide and residual silicon at grain boundaries.
Authors: Kozue Matsukawa, Masamitsu Imai, Toyohiko Yano
Abstract: Monazite(LaPO4)-coated alumina-fiber/alumina-YAG (Y3Al5O12) matrix composites were fabricated by in-situ coating of monazite followed by hot-pressing, and the effects of coating and sintering condition on mechanical properties of the composite were examined. Alumina powder and YAG powder (weight ratio, 95:5) were used as raw materials for green sheets, which was fabricated by tape casting technique. Monazite was synthesized by the in-situ reaction of La(NO3) solution with H3PO4 on the surface of fibers. After slurry infiltration into the coated fiber bundles, the fiber cloths were laminated with the green sheets alternately, then they were heat-treated, finally sintered by hot-pressing at various temperatures. The mechanical properties of the composites were changed by the fabrication conditions. Non-brittleness of the composites reduced with the increase of sintering temperature. The composites sintered at 1200oC showed the highest Weibull modulus and pseudo-ductility.
Authors: Koji Matsui, Takanori Yamakawa, Masato Uehara, Naoya Enomoto, Junichi Hojo
Abstract: Small amounts of Al2O3 were added to fine zirconia powder by different ways: powder mixing, hydrolysis of alkoxide, and homogeneous precipitation. During a constant rate heating process, the Al2O3 addition slightly raised the starting temperature of densification of powder compact, and the densification was remarkably stimulated by Al2O3 at temperatures above about 1100oC. According to an isothermal analysis of densification, the densification rate was retarded by Al2O3 addition just after the start of sintering and then the densification rate increased significantly during sintering compared to Al2O3-free powder. These results mean that Al2O3 particles pinned the shrinkage of zirconia powder compact at the initial stage, and diffuse toward zirconia surface to enhance the sintering. The sintering mechanism was explained by the grain-boundary diffusion for the Al2O3-free powder and the volume diffusion for Al2O3-added powder. When the Al2O3 was added to zirconia powder by homogeneous precipitation and alkoxide methods, the densification rate was more stimulated compared to powder mixing method. The sintering mechanism did not change by the way for Al2O3 addition. The Al2O3 addition by the chemical process tended to enhance the grain growth of zirconia, while the uniform microstructure was achieved because of homogeneous addition of Al2O3 by these chemical processes.
Authors: Shiro Shimada, Shuichi Kawano
Abstract: TiO2 coating on Si3N4-based materials powders (Si3N4, α-Y-sialon, β-sialon) was accomplished by heating the powder suspension containing the precursor (1.0 vol% H2O + Ti(O-i-C3H7)4) from 15 to 40 °C. The success of homogenous coating was confirmed by TEM observation. TiO2 coated on the powder is nitrided with NH3 gas at 800 oC to TiN with uniform particle size 10–20 nm. Spark plasma sintering of composite TiN/ Si3N4-based particles at 1400 - 1600 °C yielded the composite ceramics with a relative density > 96% without sintering additives. Sintered TiN/ Si3N4-based composite ceramics containing 17.5 - 25 vol.% TiN showed a higher electrical conductivity > 103 −1 cm-1, enabling these ceramics to be suitable for electric discharge machining.
Authors: Qiang Shen, Z.D. Wei, Mei Juan Li, Lian Meng Zhang
Abstract: AlN ceramics doped with yttrium oxide (Y2O3) as the sintering additive were prepared via the spark plasma sintering (SPS) technique. The sintering behaviors and densification mechanism were mainly investigated. The results showed that Y2O3 addition could promote the AlN densification. Y2O3-doped AlN samples could be densified at low temperatures of 1600-1700oC in 20-25 minutes. The AlN samples were characterized with homogeneous microstructure. The Y-Al-O compounds were created on the grain boundaries due to the reactions between Y2O3 and Al2O3 on AlN particle surface. With increasing the sintering temperature, AlN grains grew up, and the location of grain boundaries as well as the phase compositions changed. The Y/Al ratio in the aluminates increased, from Y3Al5O12 to YAlO3 and to Y4Al2O9. High-density, the growth of AlN grains and the homogenous dispersion of boundary phase were helpful to improve the thermal conductivity of AlN ceramics. The thermal conductivity of 122Wm-1K-1 for the 4.0 mass%Y2O3-doped AlN sample was reached.
Authors: Thanakorn Wasanapiarnpong, Shigetaka Wada, Masamitsu Imai, Toyohiko Yano
Abstract: Silicon nitride (Si3N4) ceramics have been interested for electrical substrate applications, because the ceramics can be made highly mechanical strength, fracture toughness, electrical resistivity and high thermal conductivity. Generally, relatively large amount of additives are required to obtain dense Si3N4 ceramics. During sintering, additives react with SiO2 including surface oxide of Si3N4 raw powder to form a liquid phase. Most of liquid phase changed into glassy phase during cooling down. In this study, Si3N4 ceramics were fabricated by gas pressure sintering. Yttrium oxide (Y2O3), silica (SiO2), and magnesia (MgO) were used for liquid-phase-enhanced sintering process. Dense materials were sintered by this process, but their thermal conductivities were not so high (30-40 W/m·K). Therefore, post-sintering heat-treatment process was performed to reduce the excess amount of glassy phase. An additive system (3 mass% SiO2 with 3 mass% MgO and 1-5 mass% Y2O3) was selected as the sintering aid. These ceramics could be sintered to almost full density at relatively low temperature as 1650oC for 2 h under 0.1 MPa-N2 without packing powder. The resulting materials have high bending strength, about 1 GPa, when 5mass% of Y2O3 was added. Based on the creation of low temperature pressureless sintering without packing powder, a novel two-step sintering (once firing) was proposed. The two-step sintering conducted by sintered at 1650oC under 0.1 MPa-N2 for 2 h for densification in the first step. Followed by heated up to and kept at 1950oC for 8 h under 1.0 MPa-N2 in the second step. The Si3N4 ceramics could be fabricated with relatively high thermal conductivity of 90 W/m·K. Mass loss, microstructure, mechanical properties, oxygen content and chemical composition were discussed.
Authors: Chun Fen Wang, Qun Wang, Sheng Cong Liufu, Qin Yao, Li Dong Chen
Abstract: High dense Bi2Te3 nanowire arrays were fabricated in porous anodic alumina (PAA) by electrochemical deposition. A macro-integration measurement was used to study the thermoelectric properties of a superimposed layer of Bi2Te3/PAA structure. In this macro-integration system, meaningful amounts of heat will transport along Bi2Te3 nanowire arrays, and so the measurement errors of micro-current and micro-temperature difference of individual nanowire can be eliminated. The influences of wire diameter, area fraction of wires and interface thermal resistance in the sandwich structure on the measurement accuracy of Seebeck coefficient and electrical conductivity of Bi2Te3/Al2O3 system were discussed. The experimental electrical conductivity is close to the theoretically calculated value. Further improvement in eliminating the interface thermal resistance would bring more reliable result in Seebeck coefficient measurement. The macro-integration measurement is a practical method to evaluate the thermoelectric properties of thermoelectric nanowire arrays.
Authors: Jacques G. Noudem, S. Lemonnier, M. Prevel, E.S. Reddy, E. Guilmeau, C. Goupil
Abstract: In this work we describe (i) the fabrication of thermoelectric modules based on oxide bulk and foam materials of Ca3Co4O9 and Ca0.95Sm0.05MnO3(ii) and the metal to ceramic contacts preparation. The open porous foam structures of thermoelectric materials can result in designing efficient thermoelectric modules for waste heat sources involving gaseous and liquid media. The possibility of direct large area physical contact of thermoelectric foam elements with hot media will make them efficient electric power generators. The open porous thermoelectric materials with holes can be a good candidate to confine phonons (lattice vibrations) in order to reduce the thermal conductivity if the pores can be made sufficiently small. The performances of the modules were evaluated and possible factors limiting their theoretical performance are discussed. A parameter representing the quality of the modules termed as manufacturing factor (MF) representing the cumulative effect of various factors involved in the fabrication process is introduced and evaluated for the modules and compared to the reported modules.
Authors: Nittaya Keawprak, Rong Tu, Takashi Goto
Abstract: Calcium ruthenates were prepared in different ratios of Ru to Ca (RRu/Ca = 0.5~1.4) by spark plasma sintering. CaRuO3 in a single phase was obtained at RRu/Ca = 1.0. At RRu/Ca < 1.0, a mixture of CaRuO3 and CaO was obtained, whereas CaRuO3 with second phase of RuO2 was obtained at RRu/Ca > 1.0. The density at RRu/Ca < 1.0 were 80-85% and slightly increased with increasing RRu/Ca. The density significantly increased up to 95% with increasing RRu/Ca from 1.1 to 1.4, suggesting that the second phase of RuO2 was effective to densify CaRuO3. The density of CaRuO3 in a single phase was 82% at most. The lattice parameters of CaRuO3 decreased with increasing RRu/Ca from 0.7 to 1.0, showing a nonstoichiometric range of Ca1+δRuO3+δ.

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