The effect of co-doping with transition metals (Fe, Ni, and Sm) on the thermoelectric properties of Al-doped ZnO (AZO) ceramics was studied. The electrical conductivity r of AZO was significantly (12%) increased by Ni co-doping, while an unfavorable deterioration in r was observed for Fe-or Sm-co-doped AZO. Hall-effect measurements indicated that the electron mobility of AZO decreased due to co-doping in all samples. Only the Ni-co-doped AZO sample showed significant enhancement in electron density, resulting in its black color. The thermal conductivity j decreased drastically due to Ni or Sm co-doping of AZO, while only a small change was observed for Fe co-doping of AZO. The j value at 1073 K for Ni-co-doped AZO was 77% of that for AZO. A dimensionless figure of merit ZT = 0.126 was attained at 1073 K for Ni-co-doped AZO, representing an improvement over that of conventional AZO by a factor of 1.50.

This study measured the Vickers hardness of the sintered refractory carbides B 4 C, Mo 2 C, NbC, TiC, V 8 C 7 , W 2 C, WC, WC-SiC, and ZrC over a wide range of test forces between 0.49 and 196 N. The results showed an indentation size effect (ISE), with hardness values that increased with decreasing test force for the carbides. The test force dependence of the hardness was analyzed by the proportional specimen resistance (PSR) model. The value of the a 1 term that reflects the elastic resistance in the model was obtained for the carbides and compared with their measured elastic, shear, and bulk moduli. It was not clear what type of modulus related strongly to the a 1 term. There was a strict correlation between the hardness at an infinite test force and at a test force of 9.8 N. However, the hardness at a test force of 0.49 N a little poorly reflected the hardness at an infinite test force.

Tungsten hemicarbide W 2 C with no or small amounts of W or WC was prepared by reaction-sintering from W and WC powders using a resistance-heated hot-pressing technique called spark plasma sintering. The product phases, density, microstructure, elastic moduli, hardness, and fracture toughness of the sintered bodies were determined. The stable region of the W 2 C phase had a narrow carbon content below 1860 C. W 2 C had a Poisson's ratio of 0.286, and its Young's modulus at zero porosity was determined to be 444 GPa from the true density and bulk density dependence. These values suggest that W 2 C has elastic deformation behavior similar to that of W.

Composites of WC and SiC whisker (0-30 vol% SiC) were prepared at 1550 to 1800 C by using a resistance-heated hot pressing technique called spark plasma sintering. The composites obtained were examined for reaction products and microstructure, and were characterized for mechanical properties. The addition of small amounts of SiC induced the marked grain growth of WC, and drove the densification of WC. Above 10 vol% SiC, dispersed SiC phases impeded the grain growth of WC. Increasing sintering temperature made SiC whiskers thick and lowered the aspect ratio of whiskers. The hardness of composites decreased with increasing average grain size of WC. The Young's modulus of dense composites was decreased with the SiC content. The addition of 3 to 5 vol % SiC greatly increased the fracture toughness of WC.

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Tungsten carbide-silicon carbide (WC-SiC) composites were pressure-sintered with a resistance-heated hot-pressing machine at a sintering temperature of 1600 C. The dense sintered bodies were obtained by the SiC addition ranging from 2-10 mol%. Below 4.85 mol% SiC, WC grains grew abnormally, exhibiting high aspect ratios and intersecting one another. There were no preferential orientations for the abnormal WC grains, which had an irregular plate-like morphology with a thickness of about 3 mm and lengths ranging from 50-100 mm. The Vickers hardness decreased with increasing SiC up to 4.85 mol% and increased above this concentration. The Vickers hardness in the range of 2-4.85 mol% SiC was much lower than that of pure WC, 25.3 GPa, and had a constant value of 20.5 GPa above 7.5 mol% SiC. The fracture toughness increased with the addition of SiC, but large amounts of SiC decreased the fracture toughness. The fracture toughness of the WC-SiC composites was higher than that of the pure WC. [

Binderless tungsten carbide (WC) with added carbon was sintered at 1800°C using a resistance-heated hot-pressing machine. Dense binderless WCs were obtained in the range from 0.25 to 0.3 mass% C, consisting of only a WC phase. The constituent phase transition with increasing carbon addition was WC + W 2 C, WC alone, and WC + residual C. Very fine WC grains were formed in the presence of W 2 C below 0.25 mass% C. When binderless WCs consisted of a WC single phase, larger WC grains were observed. While a high hardness value, more than 23.9 GPa, was measured for binderless WCs below 0.20 mass% C, the hardness decreased markedly in the range from 0.25 to 0.3 mass% C, corresponding to significant WC grain growth. A HallPetch-like relationship was confirmed between the hardness value and the grain size for dense binderless WC.