Effect of Ambient gas and temperature on crystallization of boron nitride spheres prepared by vapor phase pyrolysis of Ammonia Borane

Synthesis of boron nitride spheres (BNS) was achieved by vapor phase pyrolysis of ammonia borane (BH₃NH₃) using two independently temperature-controlled furnaces in a glove box filled with N ₂. The BNS were heated at 1300°-1700°C in flowing NH ₃, N₂, or Ar by multistep heat treatment. The sizes of the BNS could be controlled by heating BH₃NH₃ at different rates (1°C/min: 300-800 nm, 5°C/min: 300 nm-1.2 μm and 10°C/min: 300 nm-1.8 μm). The microstructures of BNS heated in different ambient gases were observed using transmission electron microscopy with selected area electron diffraction. NH₃ gas produced BNS with well-crystallized surface shells and amorphous cores whereas N₂ and Ar gases crystallized the entire BNS. It is evident that of these three ambient gases (NH₃, N₂, and Ar), N₂ gas significantly enhances the crystallization of BN with randomly oriented grains.