Das B-C-system im kinetischen bildungsbereich: Pyrolytische bildung kohlenstoffreicher B-C-Phasen

Abstract

In the kinetically controlled temperature range of formation (900–1200 °C) the primitive rhombohedral lattice of B 13C 2 (space group R 3 ̄ m ) dominates. All phases of higher C content are derived from this lattice. In detail, the following regions can be defined, in order of increasing C content: (i) thermodynamically stable B 13C 2 (14.4 % C, d = 2.48 g/ ml). (ii) thermodynamically less stable B 13C 2 with interstitial C atoms and a small phase width: (B 13C 2)C (B 13C 2C 0.95-1, 20.1–20.4 % C, d = 2.52 g/ ml). The B 13C 2 lattice cannot take up more carbon: there is no formation of B 4C (21.7 % C, d = 2.512) but of graphite, (iii) In this case (B 13C 2)C and graphite form a very close association of high density, hardness and phase width, which is similar to a compound: (B 13C 2)C-·C graphite (31.4–35.4 % C, B 2.04-2.42C, d = 2.44−2.47 g/ ml). (iv) At 36 % C there appears a sharp boundary to the soft, loose conglomerate of (B 13C 2)C and pyro-graphite ( d = 1.36-1.87 g/ ml). (v) Starting from the broad boundary region at 84–90 % C the densities of hard pyrolysis products, again formed, increase from d = 2.16 g/ ml up to 2.21 g/ml (the density of pyrographite). As in the B-C phase diagram, under similar thermodynamical conditions (>2200 °C) there are no (kinetically favoured metastable) B-C phases between 35 and 100 % carbon. Also in the kinetic range of formation of B-C phases, the B 12 icosahedron predominates over the layer structure of graphite; the boron atoms do not form other coordination polyhedrons.