NANO AND MICROPARTICLES of HfB<sub>2</sub>: THERMAL-EXPANSION COEFFICIENT AND ITS ANISOTROPY

Abstract

The paper deals with the influence of a dimension factor on the thermal-expansion coefficient (TEC) of hafnium diboride. Nano-sized and microcrystalline hafnium diboride is investigated by method of X-ray diffraction in the temperature range of 300–1500 K. The size of nanocrystal HfB 2 with coherent scattering is 15 nanometers and remained constant during heating. The analysis of temperature dependence of the nano and microcrystalline HfB 2 parameters shows the non-linear growth of the cell metrics with increase in temperature. For the first time, the TEC of nano and microcrystalline HfB2 in the directions of crystallographic axes a and c are defined. The analytical expressions of temperature dependences of nano and microcrystalline HfB 2 of the cell parameters are received in the form of 2 degree polynomials. At the linear approximation of temperature dependence of the lattice parameters (i.e. at lack of temperature dependence of TEC), the TEC of microcrystalline HfB 2 in the studied temperature range are αa = 7.37 · 10 –6 and αс = 7.48 · 10 –6 K –1 for axes 0a and 0c respectively. The TEC of microcrystalline HfB 2 calculated according to X-ray diffraction data corresponds to TEC calculated by a dilatometric technique α = 7.49 · 10 –6 K –1 . At linear approximation of temperature dependence of the lattice parameters, the TEC of the nanocrystal HfB 2 are αa = 7.40 · 10 –6 and αс = 9.88 · 10 –6 K –1 for axes 0a and 0c respectively. The paper shows that the TEC of HfB 2 in nanocrystalline state is greater than the TEC of microcrystalline one. The difference between the TEC of nano and microcrystalline HfB2 are bound with increase in the surface energy of material with increase in dispersion. The paper finds the anisotropy of thermal expansion both micro and nanocrystal HfB 2 . The TEC on the axis 0c is higher than the TEC on the axis 0a. The anisotropy of TEC is explained taking into account the lengths and the nature of interconnections in crystalline structure of HfB 2 . The essential anisotropy of TEC in nanodimensional HfB 2 indicates the domination of the atomic fluctuations anharmonicity growth in nanocrystals in the direction of the axis 0c. The results obtained can be employed to create new environmentally friendly materials for the needs of alternative power engineering.