Environmental scanning electron microscopy studies of diffusion mechanism of boron particle combustion

Abstract

This investigation was performed to resolve long-term contradicting theories regarding the mechanisms which govern the species diffusion across the liquid B2O3 layer covering a single boron particle during the combustion of boron. An environmental scanning electron microscope (ElectroScan E-3) was used to observed the liquefaction characteristics of the boron oxide layer and to examine boron dissolution and species diffusion processes in real time. Using a hot stage, crystalline boron particles were heated from 25 to 950°C in O2, H2O, or Ar envirorments. Pure B2O3 particles were also heated in an O2 environment and examined. In situ observations showed that the diffusion of dissolved boron into molten B2O(3(l)) is much more dominant at elevated temperatures than the diffusion of gaseous O2 through the B2O(3(l)) layer. Dissolution of solid boron into the boron oxide layer caused the liquefaction of boron particles at relatively low temperatures (940°C). The chemical composition of liquid boron oxide, coated on the surface of boron particles, was identified as a polymeric vitreous (BO)(n) complex through the reaction between dissolved boron and molten B2O(3(l)).