Low-temperature synthesis of boride powders by controlling microstructure in precursor using organic compounds

The carbothermal reduction of boron oxide (B2O3) is an important process for the synthesis of boride powders. As a low-temperature synthesis method for boron carbide (B4C) powder by carbothermal reduction, we focused on an approach using a condensed product prepared from boric acid (H3BO3) and an organic compound with a number of hydroxyl groups (a polyol) such as glycerin, mannitol, or poly(vinyl alcohol). A borate ester bond was formed by the dehydration condensation of H3BO3 and a polyol, leading to the homogeneous dispersion of the boron and carbon sources at the molecular level. The thermal decomposition of a condensed H3BO3-polyol product in air was performed to control the amount of carbon to the stoichiometric C/B2O3 ratio required for carbothermal reduction. Within the thermally decomposed product consisting of B2O3 and carbon components (B4C precursor), a B2O3/carbon structure at the nanometer scale was formed. The improved dispersibility and homogeneity of the B2O3/carbon microstructure accelerated the B4C formation at a lower temperature. Consequently, crystalline B4C powder with little free carbon was synthesized by heat treatment at a low temperature of 1200°C in an Ar flow. This low-temperature synthesis approach was applied to the low-temperature synthesis of other boride powders, i.e., boron nitride and calcium hexaboride powders.