Production of α-rhombohedral boron by amorphous boron crystallization

Some characteristic features of the technology of producing alpha -rhombohedral boron by amorphous powder crystallization were considered. Powder samples were heated in a high-temperature resistance furnace at 8 degreesC/min to 1000 degreesC and then the rate was reduced to 2 degreesC/min. After the above heating rates, pyrolytic amorphous boron transforms to alpha -rhombohedral boron when held for 90 min at 1170 degreesC in either argon atmosphere or vacuum, Electrolytic amorphous boron starts to crystallize at 1150 degreesC to beta -rhombohedral boron without going through the intermediate alpha -rhombohedral boron phase. The investigation of the influence of impurities on the crystallization of amorphous boron showed that carbon was more unfavorable than metallic impurities, preventing the formation of the alpha -rhombohedral boron phase. Though the powders of pyrolytic and electrolytic amorphous boron are distinguished by purity (electrolytic boron is more contaminated), the investigations performed lead to the speculation that the crystallization process is influenced by the shortrange order of the atoms of the initial powders rather than by the impurity content. (C) 2000 Academic Press.