First-Principles Theoretical Studies and Nanocalorimetry Experiments on Solid-State Alloying of Zr-B

The thermodynamics and kinetics of the solid-state alloying of Zr-B, underlying a variety of synthesis processes of the ultrahigh-temperature ceramic ZrB2, are widely unknown. We investigate the energetics, diffusion kinetics, and structural evolution of this system using firstprinciples computational methods. We identify the diffusion pathways in the interpenetrating network of interstitial sites for a single B atom and demonstrate a dominant rate-controlling step from the octahedral to the crowdion site that is distinct from the conventional mechanism of octahedral-tetrahedral transition in hexagonal close-packed structures. In the intermediate compounds ZrBx, 0 < x <= 2, the diffusivity of B is highly dependent on the composition while reaching a minimum for ZrB. The activation barrier of diffusion in ZrB2 is in good agreement with nanocalorimetry measurements performed on Zr/B reactive nanolaminates.