ENVIRONMENTAL SCANNING ELECTRON-MICROSCOPY STUDIES OF DIFFUSION MECHANISM OF BORON PARTICLE COMBUSTION

This investigation was performed to resolve longterm contradicting the ories regarding the mechanisms which govern the species diffusion acro ss the liquid B2O3 layer covering a single boron particle during the c ombustion of boron. An environmental scanning electron microscope (Ele ctroScan E-3) was used to observe the liquefaction characteristics of the boron oxide layer and to examine boron dissolution and species dif fusion processes in real time. Using a hot stage, crystalline boron pa rticles were heated from 25 to 950 degrees C in O-2, H2O, or Ar enviro nments. Pure B2O3 particles were also heated in an O-2 environment and examined. In situ observations showed that the diffusion of dissolved boron into molten B2O3(1) is much more dominant at elevated temperatu res than the diffusion of gaseous O-2 through the B2O3(1) layer. Disso lution of solid boron into the boron oxide layer caused the liquefacti on of boron particles at relatively low temperatures (940 degrees C). The chemical composition of liquid boron oxide, coated on the surface of boron particles, was identified as a polymeric vitreous (BO)(n) com plex through the reaction between dissolved boron and molten B2O3(1).