18 W. Dressler: R. riedel covalence)bonded SiCA(sp" hybridization of Si) leads to the 2 H type whereas the doping with and CSi,(sp'hybridization of C)tetrahedra N and P forces the crystallization of the cubic (Figs 3 and 4). These tetrahedra are arranged in B-SiC. In contrast to the above mentioned irre slanes having common edges and one apex in versibility of the b/ la-transformation under the next plane of tetrahedra connecting the standard conditions Kieffer, Jepps and Page stacks. If the stacking sequence of the tetra- describe the reversible transformation of a-siC hedra is ABC a cubic zinc blend structure, des-(6 H)to B-Sic(3 C) by increasing the N2-pres gnated as B-SiC(Fig. 4), results, whilst the sure. sequence ABAB provides a hexagonal wurtzite Besides the synthetic SiC produced by carbo- structure,denoted as a-SiC (Fig 3). The hex- thermal reduction of SiO2, vapour phase reac gonal or rhombohedral a-SiC exists in many tions or thermal decomposition of silanes or polytypes(most frequent polymorphs: 2 H, 4 H, carbosilane some natural deposits are known 6 H and 15 R)composed of intermixed more a-SiC can be found in association with diamond complex arrangements of the tetrahedra planes in iron meteorites of the Canyon Diablo type resulting in large periods of stacking. The most and is denoted as Moissanite. Further natural common hexagonal a-SiC polytype 6 H can occurences are located in Bohemian volcanic derived from the cubic form by insertion of a breccias and Siberian Kimberlites. The cubic rotation(111> twin boundary after every three B-SiC polymorph was struck in the Green-River layers so that after six sequences the initial layer district, USA. position is obtained again. Despite this struc The industrial production of a-silicon carbide tural difference the density of all Sic-poly- is performed by the Acheson-process, -75a morphs is constant at 3. 17 gcm carbothermal reduction of Sio2. Using a graph The thermodynamic stability, the conditions ite electrode surrounded by a Sic rim for the of formation and the phase transformations of electrical coupling, a mixture of quartz sand or the Sic polymorphs have been intensively inves- crushed quartzite (58-65%0), graphite, petrol tigated by Knippenberg, Kieffer, Page, eum coke or ash-free anthracite(35-42%), Jepps and Page%, o and Heine. Controversly sodium chloride (1-2%)and wood chips to previous investigations, postulating B-SiC as (0. 5-1%)as additives is fused at temperatures a stable low temperature modification Knippen- between 2200 and 2400C, whereby the follow berg reported on B-SiC formation not only at ing reaction takes place low temperatures of about 1400oC but also at higher temperatures. Above 2000C surface dif- SiO, +3C-2200-240> SiC+2CO fusion leads to the irreversible transformation of B-Sic to the 6 H polytype of a-SiC. This 528 kJ mol- SiC behavior indicates that B-sic is a metastable SiC-modification which is only formed at lower The resulting a-SiC is coarse grained (Fig. 5), temperatures owing to the very small self diffu- has to be milled to the desired grit size and is sion coefficients of Si and C in SiC. Addition- divided into several qualities depending on the ally, the free enthalpy of B/oz-transformation amount of impurities. The inner part having a acting as the driving force of tra formation green color contains the purest material. The amounts to only -2 kJ mol- at T=2000 K amount of carbon, aluminium and other impur Heine?also calculated the hexagonal a-poly- ities increases continuously with the distance to types to be thermodynamically stable (lower the core and is accompanied by a change of the free energy) in comparison to the cubic B-SiC. SiC color from green to black. In order to Here, the formation of B-SiC is explained by the reduce the amount of metallic residues the pro- energetically preferred building up of parallel duced SiC powder is washed and leached. Sub ayers instead of the incorporation of twin sequently, the excess carbon is oxidized at lay boundaries by the insertion of antiparallel ori- 400C and the resulting oxide layer is removed ented layers typical for the a-SiC-structure by hydrofluoric acid Moreover, Jepps and P d p B-SiC can be produced by a modified Ache- showed that the addition of B stabilizes the 6H son- process at temperatures in the range of polytype whereas Al enhances the formation of 1500-1800C where a fine grained B-Sic is the 4 H polymorph. The presence of Al andn formed via a solid phase reaction. Gas phase18 W. Dressier, R. Riedel covalence) bonded SiC4 (sp 3 hybridization of Si) and CSi 4 (sp 3 hybridization of C) tetrahedra (Figs 3 and 4). These tetrahedra are arranged in planes having common edges and one apex in the next plane of tetrahedra connecting the stacks. If the stacking sequence of the tetra￾hedra is ABC a cubic zinc blend structure, des￾ignated as //-SIC (Fig. 4), results, whilst the sequence ABAB provides a hexagonal wurtzite structure, denoted as a-SiC (Fig. 3). The hex￾agonal or rhombohedral a-SiC exists in many polytypes (most frequent polymorphs: 2 H, 4 H, 6 H and 15 R) composed of intermixed more complex arrangements of the tetrahedra planes resulting in large periods of stacking. The most common hexagonal a-SiC polytype 6 H can be derived from the cubic form by insertion of a rotation (111) twin boundary after every three layers so that after six sequences the initial layer position is obtained again. Despite this struc￾tural difference the density of all SiC-poly￾morphs is constant at 3.17 g cm -3. The thermodynamic stability, the conditions of formation and the phase transformations of the SiC polymorphs have been intensively inves￾tigated by Knippenberg, "6 Kieffer, 67 Page, 68 Jepps and Page 6''~'' and Heine. 73 Controversly to previous investigations, postulating //-SIC as a stable low temperature modification Knippen￾berg"" reported on//-SIC formation not only at low temperatures of about 1400°C but also at higher temperatures. Above 2000°C surface dif￾fusion leads to the irreversible transformation of //-SIC to the 6 H polytype of a-SiC. This behavior indicates that //-SIC is a metastable SiC-modification which is only formed at lower temperatures owing to the very small self diffu￾sion coefficients of Si and C in SiC. Addition￾ally, the free enthalpy of ///a-transformation acting as the driving force of transformation amounts to only -2kJ mol ' at T--2000K. 72 Heine ~' also calculated the hexagonal ~-poly￾types to be thermodynamically stable (lower free energy) in comparison to the cubic//-SIC. Here, the formation of//-SIC is explained by the energetically preferred building up of parallel layers instead of the incorporation of twin boundaries by the insertion of antiparallel ori￾ented layers typical for the a-SiC-structure. Moreover, Jepps and Page 6''~'' and Page °8 showed that the addition of B stabilizes the 6 H polytype whereas A1 enhances the formation of the 4 H polymorph. The presence of A1 and N leads to the 2 H type whereas the doping with N and P forces the crystallization of the cubic //-SIC. In contrast to the above mentioned irre￾versibility of the ///a-transformation under standard conditions Kieffer, 67 Jepps and Page 69 describe the reversible transformation of a-SiC (6 H) to//-SIC (3 C) by increasing the N2-pres￾sure. Besides the synthetic SiC produced by carbo￾thermal reduction of SiO2, vapour phase reac￾tions or thermal decomposition of silanes or carbosilanes some natural deposits are known. a-SiC can be found in association with diamond in iron meteorites of the Canyon Diablo type and is denoted as Moissanite. Further natural occurences are located in Boehemian volcanic breccias and Siberian Kimberlites. The cubic /~-SiC polymorph was struck in the Green-River district, USA. The industrial production of a-silicon carbide is performed by the Acheson-process, 73-~5 a carbothermal reduction of SiO2. Using a graph￾ite electrode surrounded by a SiC rim for the electrical coupling, a mixture of quartz sand or crushed quartzite (58-65%), graphite, petrol￾eum coke or ash-free anthracite (35-42%), sodium chloride (1-2%) and wood chips (0.5-1%) as additives is fused at temperatures between 2200 and 2400°C, whereby the follow￾ing reaction takes place: SiO2 + 3C 22,,o 2400°( ~ ~ SiC + 2CO -528 kJ mol-' SiC. (5) The resulting a-SiC is coarse grained (Fig. 5), has to be milled to the desired grit size and is divided into several qualities depending on the amount of impurities. The inner part having a green color contains the purest material. The amount of carbon, aluminium and other impur￾ities increases continuously with the distance to the core and is accompanied by a change of the SiC color from green to black. In order to reduce the amount of metallic residues the pro￾duced SiC powder is washed and leached. Sub￾sequently, the excess carbon is oxidized at 400°C and the resulting oxide layer is removed by hydrofluoric acid. //-SIC can be produced by a modified Ache￾son-process at temperatures in the range of 1500-1800°C where a fine grained //-SIC is formed via a solid phase reaction. 5"~" Gas phase