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SiC ceramics were prepared from a ␤–SiC powder doped with two different sintering additives—a mixture of La2O3 and Y2O3 and a mixture of Al2O3 and Y2O3—by hot pressing and annealing. Their microstructures, phase compositions, lattice oxygen contents, and thermal conductivities were evaluated. The SiC doped with rare-earth oxides attained thermal conductivities in excess of 200 W/(m K); however, the SiC doped with additives containing alumina had thermal conductivities lower than 71 W/(m K). The high thermal conductivity of the rare-earth-oxide-doped SiC was attributed to the low oxygen content in SiC lattice, high SiC–SiC contiguity, and lack of ␤– to ␣–SiC polytypic transformation. The low thermal conductivity of the alumina-doped SiC was attributed to the point defects resulting from the dissolution of Al2O3 into SiC lattice and the occurrence of polytypic transformation. I. INTRODUCTION

Silicon carbide is one of the most promising hightemperature structural materials due to its excellent properties such as chemical stability, high hardness, wear resistance, and heat resistance, which are mostly attributed to the strong covalent Si–C bond. However, this strong covalency and the low self-diffusion coefficient make the preparation of dense SiC ceramics virtually impossible without the aid of sintering additives. Depending on the type of the sintering additive used, densification of SiC can proceed either by a solidstate-sintering mechanism where typical additives are boron and carbon or boron carbide1,2 or by a liquidphase-sintering mechanism where additives usually are aluminum-element-containing oxides3–8 or nonoxides.9–11 Compared with solid-state sintering, liquidphase sintering not only allows densification at lower temperatures, but also offers a possibility to develop in situ toughened SiC by controlling secondary phase chemistry and microstructure. Recently, liquidphase-sintering of SiC has been the subject of intensive research, and some liquid-phase-sintered SiC (LPS-SiC) possessing superior mechanical properties relative to solid-state-sintered SiC has been developed.3–11 On the other hand, SiC has another attractive property, high thermal conductivity. According to Slack’s estimation,12 pure SiC single crystal has a room-temperature thermal conductivity of 490 W/(m K). However, polycrystalline SiC ceramics have much lower thermal conductivities due to random orientation of grains, lattice impurities, and structural defects within grains, and secondary phases with poorer conductivity at grain boundaries. To date, the highest room-temperature 1854

http://journals.cambridge.org

J. Mater. Res., Vol. 18, No. 8, Aug 2003 Downloaded: 23 Mar 2015

thermal conductivity of sintered SiC reported was 270 W/(m K), which was achieved in a solid-statesintered SiC with BeO as an additive.13,14 However, the application of this SiC has been greatly limited due to the extreme toxicity of BeO. Therefore, the search for alternative sintering additives for SiC that not only can bring about high thermal conductivity of the