Self-diffusion of boron in TiB2

Abstract

Self-diffusion studies of boron in polycrystalline TiB2 were carried out as a function of temperature, using a specially designed experiment with stable B10 tracers, B−enriched11 TiB2 samples, and secondary ion mass spectrometry for depth profiling. The diffusivities were extracted from the isotope depth profiles in the range between 950 and 1600 °C. They obey an Arrhenius behavior with an activation enthalpy of about ΔH=2.2 eV and a preexponential factor of D0=4×10−12 m2/s. Interpolation of the diffusivities to the melting point of 3225 °C reveals a very low value of about D(Tm)≈10−15 m2/s, which reflects the covalent bonds present in the material. A possible explanation for the low values obtained for D0 and ΔH is the assumption of a diffusion mechanism via vacancies, where in addition to thermal vacancies a substantial concentration of structural vacancies are present. The possible influence of grain boundaries and of the anisotropic crystal structure on the results is discussed together with crystallographic diffusion paths.