High-Temperature X-ray Diffraction Study of the Thermal Expansion and Stability of Nanocrystalline VB2

-The thermal expansion of nano- and microcrystalline VB2 powders has been studied by high-temperature X-ray diffraction in the temperature range 300-1473 K. The thermal expansion coefficient (TEC) of nanocrystalline VB2 has been determined for the first time in the temperature range 300-1473 K and shown to vary linearly with temperature. The TEC of nanocrystalline VB2 has been shown to exceed that of its microcrystalline analog, which is attributable to the larger anharmonicity of atomic vibrations in the nanocrystals. The thermal expansion of nanocrystalline VB2 has been shown to be anisotropic: the c-axis TEC of the material exceeds its a-axis TEC, which is due to the anisotropy in bond strength along the axes of its hexagonal unit cell. As the temperature is raised, the distinction between the TECs along the crystallographic axes of nanocrystalline VB2 decreases, indicating that the B-B covalent bonds in the boride layers of nanocrystalline VB2 become weaker with increasing temperature. At temperatures of up to 1273 K, the crystallite size of nanocrystalline VB2 remains constant at 10-12 nm.