Diffusion Kinetics of B Element in Zr-B Solid State Reaction

Abstract

Zr metal bulks were isothermally reacted with B powder at 800-1630 °C for 2-16 h to form ZrB₂ layers on the Zr bulk surfaces. Diffusion kinetics of B element in association with Zr + B solid state reaction was investigated by means of XRD, SEM and EDS. Nano-sized ZrB₂ grains without formation of a discernible layer were detected on the Zr surface after reaction at 800 °C for 2 h. When the reaction temperature was increased, continuous layers consisting of faceted ZrB₂ grains were formed at temperatures ranging from 1200 to 1500 °C. The thickness of the ZrB₂ product layers showed a parabolic dependence on time t and an exponential dependence on temperature T via − 1/RT. The Zr + B reaction calculated a diffusion activation energy G = 152.0 ± 62.4 kJ/mol for B diffusion in ZrB₂. The diffusion coefficient of B element was experimentally determined to be \(D{ } = 1743.4 \times 10^{ - 12} {\text{ exp}}\left( {{\raise0.7ex\hbox{${ - 152027.6}$} \!\mathord{\left/ {\vphantom {{ - 152027.6} {{\text{R}}T}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${{\text{R}}T}$}}} \right)\).