Tracer diffusion studies in NiO bicrystals and polycrystals

Abstract

The tracer self-diffusion of nickel in two NiO bicrystals and a polycrystalline specimen at 700° C has been studied using sectioning and autoradiography. The two bicrystal boundaries were 〈110〉 symmetrical tilt boundaries with misorientations of 39.5° and 99.5°, respectively. After diffusion measurements the bicrystals were fractured and the boundaries studied by a variety of surface analysis techniques. The tracer penetration profiles were analysed in terms of lattice diffusion and diffusion along low angle boundaries composed of dislocation arrays introduced by mechanical polishing. The coefficients for lattice and low angle boundary diffusion are in acceptable agreement with previous determinations. Diffusion along high angle boundaries could not be detected by profiling, but could be detected by autoradiography in some boundaries after annealing at high temperature (1400° C) before the diffusion anneal. Autoradiography also revealed that the boundary diffusion was not homogeneous and was sensitive to both thermal history and boundary geometry. The bicrystal boundaries were found to be extensively contaminated by calcium and silicon compounds. It is concluded that high angle boundary diffusion in the present experiments is dominated by these impurities and their distribution. The implications for other measurements of grain boundary properties are discussed and the need for chemical characterization of the actual boundaries used in measurements of boundary properties is emphasized.