Interfacial void formation in born filaments

Abstract

Experiments have shown that when boron filaments are heated in an environment containing a reactive species such as oxygen, formation of boron oxide on the surface and subsequent vaporization causes removal of boron atoms from the surface. Removal of this material results in a contraction of the filament which is due to redistribution of internal stresses and anelastic recovery. The magnitude of the contraction observed is found to be dependent upon the concentration of oxygen in the annealing atmosphere. Examination of the interior of the filaments showed massive void formation around the core and hence separation of cue and sheath. An empirical relationship was constructed to fit the contraction data based on an exponential relationship. Activation energies were determined and were reasonable when compared with similar studies A possible mechanism for the observed phenomena is that there is a net outward diffusion of boron atoms to the surface, and an inward diffusion of vacancies which coalesce at the interface between sheath and core to form the voids. The disordered structure of the vapour-deposited boron (i.e. microcrystalline with crystallite size 2 to 4 nm and highly disordered) is such that it contains many vacancies.