High Thermal Conductivity Silicon Nitride Ceramic
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High Thermal
Conductivity Silicon Nitride Ceramic
Kiyoshi Hirao, Koji Watari, Hiroyuki Hayashi, and Mikito Kitayama
Processing and Microstructure of Nonoxide Ceramics
Introduction 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 m1 K1,1,2 a great deal of effort has been focused on the development of nonoxide polycrystalline ceramics with high thermal conductivity. SiC and AlN ceramics with high thermal conductivities of over 200 W m1 K1 have been successfully fabricated, and they are used commercially as high thermal conductivity substrates, heatsinks, and so on. However, widespread use of these materials is still restricted, owing to the low reliability that arises from their poor mechanical properties. In addition, application of SiC ceramics is further limited by their low electric resistance and high dielectric constant. Hence, electrical industries actively search
tivity of over 200 W m1 K1 at room temperature. These facts, combined with the excellent mechanical properties of Si3N4 ceramic, make it a serious candidate for high-performance substrates. This article summarizes recent results on the thermal conductivity of Si3N4 and discusses the extrinsic factors governing the thermal conductivity of Si3N4 ceramic in terms of microstructural parameters such as lattice defects in single-crystal Si3N4 and the morphology of grain-boundary secondary phases, comparing Si3N4 with the cases of SiC and AlN ceramics. In addition, high thermal conductivities achieved in textured Si3N4 are also introduced.
for alternative materials with both high thermal conductivity and superior mechanical properties in order to manage the everincreasing heat generated by electric devices. Silicon nitride (Si3N4) ceramic, on the other hand, is well known as a hightemperature structural ceramic having high strength and fracture toughness. The excellent mechanical properties of Si3N4 result from its unique microstructure, which is composed of hexagonal, rodlike grains, bonded together and reinforcing each other.3–5 Recently, a Si3N4 ceramic with thermal conductivities of over 100 W m1 K1 was reported.6–9 In addition, considering the similarity of chemical bonding and crystal structure of Si3N4 to those of SiC and AlN, Haggerty and Lightfoot10 pointed out that single-crystal Si3N4 has a high intrinsic thermal conduc-
In general, because of their low selfdiffusion coefficients even at high temperatures, nonoxide ceramics with high covalent bonding, such as Si3N4, AlN, and SiC, are densified with the aid of sintering additives. SiC is generally densified through solid-phase sintering by adding small amounts of additives such as B, C,11 and Be compounds,12 although liquid-phase sintering of SiC using oxides or carbides has recently attracted attention as a method for developing anisotropic grains that act as reinforcements.13 On the other hand, AlN and Si3N4 are densified by liquidphase sintering to lower the si
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