Preparation of Boron Nitride and Boron Carbide by Thermal Treatment of Boric Acid-Glycerin Condensation Product as a Precursor

Boron-containing ceramics were synthesized by the heat treatment of the condensation product of boric acid and glycerin in N2 or Ar. The condensation product was a transparent, glassy solid. It was found that the dehydration was completed and the formation of a compound with the structure (C3H5O3B)n containing B-O-C bonding was revealed. The heat treatment of the condensation product between 900°C and 1250°C yielded boric oxide and amorphous carbon regardless of gas used. When it was heated in N2 above 1300°C, boron nitride (BN) and boron carbide (B4C) were detected by XRD and IR analyses. The crystallinity of BN and B4C increased with the rise of heating temperature. However, heating time affected only the crystallinity of BN. The crystallite size of BN calculated by the Scherrer equation was about 50Å at 1300°C and 100Å at 1400°C. When the condensation product was heated in Ar above 1300°C, the formation of B4C and graphite was observed. For comparison, a physical mixture of B2O3 and carbon black was heated in N2 at 1400°C for 2h. Though the formation of B4C was confirmed, its crystallinity was very low and X-ray peaks assigned to BN were not observed. Thus, the reduction and the subsequent carbide formation and nitridation of an organoborate compound were considerably different from those of the physical mixture.