Dense ␤-Si 3 N 4 with various Y 2 O 3 /SiO 2 additive ratios were fabricated by hot pressing and subsequent annealing. The thermal conductivity of the sintered bodies increased as the Y 2 O 3 /SiO 2 ratio increased. The oxygen contents in the ␤-Si 3 N 4 crystal lattice of these samples were determined using hot-gas extraction and electron spin resonance techniques. A good correlation between the lattice oxygen content and the thermal resistivity was observed. The relationship between the microstructure, grain-boundary phase, lattice oxygen content, and thermal conductivity of ␤-Si 3 N 4 that was sintered at various Y 2 O 3 /SiO 2 additive ratios has been clarified.

Calculations based on a simple modified Wiener's model for thermal conductivity of a composite material predict that the thermal conductivity of ␤-Si 3 N 4 decreases quickly as the grain-boundary film thickness increases within a range of a few tenths of a nanometer and also that it initially increases steeply with increased grain size, then reaches almost constant values. Because of the faceted nature of the ␤-Si 3 N 4 crystal, the "average" grain-boundary film thickness is much greater than that in equilibrium. The present study demonstrates both theoretically and experimentally that grain growth alone cannot improve the thermal conductivity of ␤-Si 3 N 4 .

Since the confirmation that nonmetallic single crystals with a diamond-like structure, such as SiC, BP, and AlN, have high intrinsic thermal conductivities of over 300 W m−1 K−1,1,2 a great deal of effort has been focused on the development of nonoxide polycrystalline ceramics with high thermal conductivity.

Controlled-geometry cavities were introduced into the m{101 0} plane of undoped sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding the etched sapphire to an undoped high-purity polycrystalline alumina. Pore-boundary separation during growth of the sapphire seed into the polycrystal entrapped the pores within the single crystal. Pores with an equivalent spherical radius of Ϸ1 m reached a quasi-equilibrium shape after prolonged anneals at 1600°and 1800°C. The introduction of mechanically induced surface defects accelerated pore shape equilibration. The Wulff shape of undoped alumina was determined by characterizing the shape and facet structure of these equilibrated internal pores using optical microscopy, scanning electron microscopy, and atomic force microscopy. The observed planes in the Wulff shape of undoped alumina, c(0001), r{1 012}, s{11 01}, a{112 0}, and p{112 3} planes, were consistent with those reported by Choi et al.; however, a different energy sequence is inferred. The absence of the m-plane in the Wulff shape is consistent with other experimental studies, but inconsistent with those lattice simulations that predict the m-plane to be one of the lowest energy planes in pure alumina. A comparison of Wulff shapes at 1600°and 1800°C suggests that the surface energy of undoped alumina becomes more isotropic as temperature increases.