Silicon carbide (SiC) has a range of useful physical, mechanical and electronic properties that make it a promising material for next-generation electronic devices. Careful consideration of the thermal conditions in which SiC [0001] is grown has resulted in improvements in crystal diameter and quality: the quantity of macroscopic defects such as hollow core dislocations (micropipes), inclusions, small-angle boundaries and long-range lattice warp has been reduced. But some macroscopic defects (about 1-10 cm(-2)) and a large density of elementary dislocations (approximately 10(4) cm(-2)), such as edge, basal plane and screw dislocations, remain within the crystal, and have so far prevented the realization of high-efficiency, reliable electronic devices in SiC (refs 12-16). Here we report a method, inspired by the dislocation structure of SiC grown perpendicular to the c-axis (a-face growth), to reduce the number of dislocations in SiC single crystals by two to three orders of magnitude, rendering them virtually dislocation-free. These substrates will promote the development of high-power SiC devices and reduce energy losses of the resulting electrical systems.

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Directionally solidified B 4 C-TiB 2 composites were prepared by a Floating Zone method. TiB 2 phases in a rod shape were continuously connected in the B 4 C matrix. The c-axes of TiB 2 and B 4 C phases were perpendicular and tilted 22 • to the growth direction, respectively. The (101) and (120) planes of the B 4 C were in parallel to the (001) and (100) planes of TiB 2 , respectively. The electrical conductivity of the composite parallel to the growth direction (σ ) was greater than monolithic B 4 C by a factor of 100 to 1000. The thermal conductivity of the composite parallel to the growth direction (κ ) was about one and a half times as high as that of B 4 C. The anisotropy of electrical and thermal conductivity were basically explained by a mixing law using the values of B 4 C and TiB 2 . The microhardness of the composite was almost the same as that of B 4 C. The electric discharge machining of the composite was possible owing to the enhancement of electrical conductivity.

Directionally solidified B 4 C-SiC composites were prepared by a Floating Zone method. The lamellar texture was observed at 53 mol%SiC. The c-axis of B 4 C phase was tilted 20 • to the growth direction. The (102) plane and [121] direction of the B 4 C phase were parallel to the (311) plane and [121] direction of the SiC phase, respectively. The thermal conductivity of the composite parallel to the growth direction (κ ) was about twice as great as that of monolithic B 4 C. The anisotropy of electrical conductivity and thermal conductivity were explained by a mixing law using the values of B 4 C and SiC. The mass gain due to oxidation was about 1/3 to 1/5 less than that of monolithic B 4 C at 1023 K. The surface perpendicular to the growth direction showed slightly better oxidation resistance than that parallel to the growth direction. The microhardness of the composite was almost the same as that of B 4 C.

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Boron suboxide (B 6 O) sintered bodies were prepared by a solid state reaction and a hot pressing method. The thermoelectric properties of B 6 O were compared with those of B 4 C. The electrical conductivity was smaller than that of B 4 C, and the Seebeck coefficient was twice as large as that of B 4 C indicating p-type conduction. The hopping conduction of electronic charge carriers was suggested from the temperature dependencies of the electrical conductivity and mobility. The thermal conductivity was greater than that of B 4 C. The thermoelectric dimensionless figure-of-merit increased with increasing temperature, and was 0.62 × 10 −3 at 1000 K. This value was almost in agreement with that of B 4 C.

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