Special features of preparation of nanosized hafnium diboride of different dispersity

Abstract

X-ray diffraction analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, thermogravimetry, and elemental analysis have been applied to study the products of interaction of powder hafnium with fine-crystalline boron in the Na 2 B 4 O 7 ionic melt at 600-850°C and of HfCl 4 with NaBH 4 at 300-700°C.Keywords: hafnium diboride, nanosized particle, ionic melt, sodium tetraborate, sodium borohydride, hafnium tetrachloride Nanosized hafnium diboride is a typical nanosized boride of a group IV metal. Similarly to zirconium diboride, hafnium diboride can be prepared starting from the volatile borohydride decomposing upon heating following scheme (1) [1][2][3][4][5]. t M(BH 4 ) 4 → MB 2 + B 2 H 6 + 5H 2 (M = Zr, Hf).(1)Nanosized hafnium diboride can be also prepared via interaction of HfCl 4 with NaBH 4 at 600°С under inert [6,7] or via high-temperature carbothermal reduction of a mixture of HfO 2 and В 2 О 3 [8,9].We have earlier demonstrated the possibility of preparation of nanosized titanium and zirconium diborides upon interaction of titanium (zirconium) with boron in the Na 2 B 4 O 7 ionic melt [10][11][12]. Detailed study of this process is important in view of extending the knowledge on the influence of particles size on physico-chemical, mechanical, and other properties of various materials [13].In this work we studied two routes to prepare nanosized HfB 2 : via interaction of boron and hafnium powders in Na 2 B 4 O 7 ionic melt at 600-850°С (a) and via reaction of HfCl 4 with NaBH 4 at 300-700°С (b). In the latter case, in contrast to Ref.[6], initial heating of the reaction mixture was conducted in vacuum (0.133 Pa) rather than in an inert atmosphere. Table 1 show the conditions and the outcomes of interaction of equimolar amounts of powders of hafnium (0.83 g, 0.0046 mol) and boron (0.1 g, 0.0092 mol) in the presence of excess of Na 2 B 4 O 7 at 600-850°С. The results revealed that hafnium diboride was formed at ≥750°С, above the tetraborate melting point. The composition of hafnium diboride isolated from the reaction mixture (route a) as determined by chemical analysis and X-ray energy dispersive analysis was of HfB 2.02-2.03. Figures 1 and 2 andAccording to X-ray diffraction studies (Fig. 1), hafnium diboride formed crystals of hexagonal system, the lattice parameters being listed in Table 1; they well coincided with the reference data (a 0 = 0.3142 and c 0 = 0.3477 nm [14]).Analysis of the diffraction peaks in the range of 2θ = 15°-110° using the Scherrer equation (2) revealed that the coherent scattering area of HfB 2 D hkl was of ≈55 nm. D hkl = kλ/β hkl cos θ hkl .(2)In Eq.(2), k stands for anisotropy parameter taken equal to 0.9, λ is the X-ray radiation wavelength [λ(CuK α ) = 1.54178 Å], θ is diffraction angle, and β is half-width of the diffraction peak (rad).According to the scanning electron microscopy data, particles of the HfB 2 were of 50-55 nm (Fig. 2).