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Aluminum nitride (AlN) ceramics were sintered with spark plasma sintering (SPS) technique using Y 2 O 3 (Sm 2 O 3 )–CaF 2 as the sintering additives. The purification effect of sintering additives, dielectric properties, microstructure and polarization mechanism of AlN ceramics were investigated. The densification process curves indicate that both sintering additive and special SPS sintering mechanism are conductive to reducing the sintering beginning temperature. The results of lattice parameters of AlN ceramics suggest that suitable sintering additives have prominent purification effect on the crystal lattice of AlN ceramics. Dielectric constant increases with Y 2 O 3 (Sm 2 O 3 ) content increasing, and the AlN ceramic containing 2 wt.% Sm 2 O 3 -1 wt.% CaF 2 exhibits the lowest dielectric loss. Oxygen impurities, dislocation and amorphous layer in the crystal structure of AlN ceramics increase dielectric loss of AlN ceramics. The space charge polarization caused by secondary phases and other defects might be the main polarization mechanism of AlN ceramics. [ABSTRACT FROM AUTHOR]

Abstract: Microstructures of short-carbon-fiber-reinforced silicon carbide composites, prepared by hot-pressing with MgO–Al2O3–Y2O3 as sintering additives, were investigated by means of X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. The results showed that the composites could be densified at a relatively low temperature of 1800 °C via the liquid-phase-sintering mechanism. The amorphous interphase in the composites not only avoided the direct contact of the fibers with the matrix, but also improved the fiber/matrix bonding, so they could improve the densification of the composites and avoid the degeneration of the carbon fiber. The nano silicon carbide derived from polycarbosilane, could play a role of improving the relative density of the composites. [Copyright &y& Elsevier]