Effect of calcium carbonate particle size on formation and morphology of calcium hexaboride powder synthesized from condensed boric acid-poly(vinyl alcohol) product

Abstract Calcium hexaboride (CaB 6 ) powder was synthesized by carbothermal reduction via the transient boron carbide (B 4 C) formation starting from a condensed boric acid (H 3 BO 3 )-poly(vinyl alcohol) (PVA) product. The effect of the size of calcium carbonate (CaCO 3 ) particles on the formation behavior of CaB 6 and the obtained particle morphology was investigated in this study. CaB 6 powder was prepared using CaCO 3 powders with microsize or nanosize particles. The CaB 6 formation reaction was accelerated at lower temperature and shorter heat treatment time when using nanosize CaCO 3 particles. The complete formation of CaB 6 was achieved at 1400 °C for 3 h in an Ar flow. Furthermore, CaB 6 powder with nanosize particles was obtained. The precursor powder obtained using nanosize CaCO 3 particles transiently formed fine B 4 C and calcium borate particles, which are reactive species of CaB 6 , leading to the facile formation of fine CaB 6 particles.