Preparation of AlC<sub>4</sub>B<sub>24</sub> and Al<sub>3</sub>C<sub>2</sub>B<sub>48</sub> Crystals

Abstract

AlC4B24 and Al3C2B48 crystals were prepared by the reaction between carbon and boron powders using a molten aluminium flux in an argon atmosphere. The conditions for obtaining these crystals with a relatively large size were established. As-grown AlC4B24 and Al3C2B48 crystals were used for the study of oxidation in air at high temperatures, and also measurements of density, unit cell dimensions and Vickers microhardness. AlC4B24 crystals were obtained together with crystals of Al3C2B48 or Al4C3. Al3C2B48 were prepared as twinned crystals of A and B phases, without accompanying α-AlB12, β-AlB12 type, γ-AlB12, AlC4B24 or Al4C3 crystals. The optimum conditions for growing AlC4B24 single crystals and Al3C2B48 crystals are the following atomic ratios of starting materials (C/B, Al/B), soaking temperature and soaking time; C/B=9.60/24, Al/B=144.24/24, 1500°C and 5h for AlC4B24 and C/B=0.24/24-0.48/24, Al/B=144.24/24, 1300°C and 5h for Al3C2B48, respectively. Under these conditions, AlC4B24 single crystals, having a black color and metallic luster, were generally obtained in the form of a bipyramidal shape. On the other hand, the twinned crystals of Al3C2B48 were yellowish brown and semitransparent, having a nearly spherical polyhedral shape with smooth planes. The largest crystals prepared have the maximum dimensions of about 2.1mm×2.1mm×2.2mm (for Al3C2B24) and 1.1mm×1.1mm×1.2mm (for AlC4B24). Unit cell dimensions, Vickers microhardness (Hv) and densities (dm) are as follows:AlC4B24; a=8.892(2), b=9.112(1), c=5.692(2) (A), V=461.1(3) (A3). Hv=2500-2690kgf/mm2, dm=2.55(1)g/cm3 Al3C2B48; for A phase, a=12.377(2), b=12.627(1), c=5.079(1) (A), V=793.8(1) (A3)-a=12.363(2), b=12.616(1), c=5.102(1) (A), V=795.8(2) (A3) for B phase, a=6.166(2), b=12.635(2), c=10.156(2) (A), V=791.2(2) (A3)-a=6.181(2), b=12.622(1), c=10.161(1) (A), V=792.7(2) (A3). Hv=2570-3050kgf/mm2, dm=2.59(2)g/cm3The oxidation reaction in air of AlC4B24 and Al3C2B48 crystals began to proceed at about 760°C and 710°C, respectively, and their oxidation products were found to be 9Al2O3⋅2B2O3 (orthorhombic system) and B2O3 (hexagonal system) phases.