Mechanically activated synthesis of ultrafine rods of HfB₂ and milling induced phase transformation of monocrystalline anatase particles

Abstract

The mechanically activated synthesis of hafnium diboride HfB₂ from partially hydrated hafnium tetrachloride “HfCl₄” is first described. Monocrystalline rods with submicron to micron lengths and a diameter of about 100 nm are synthesized by annealing, at 1373 K, of powder mixtures of “HfCl₄” and boron ground with steel tools. The monocrystalline rods grow parallel to the c-axis of the HfB₂ structure from iron-rich grains of the activated powder and are defect-free. Facetted nanometer-sized single crystals are obtained instead when magnesium is added to the starting mixtures. The fractureless transformation of single-crystal anatase particles with different initial sizes into orthorhombic TiO₂-II by milling is then described. Milling yields either monocrystalline anatase particles coated with a layer of nanograins of TiO₂-II (grain size ∼10 nm) or fully transformed anatase particles according to the initial diameters of ∼150 nm and ∼25 nm respectively. The relevance of a milling parameter, namely the average power injected per unit volume of powder trapped during a collision, is finally emphasized.