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

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 similar to10 nm) or fully transformed anatase particles according to the initial diameters of similar to150 nm and similar to25 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. (C) 2004 Kluwer Academic Publishers.