The invention relates to a process for preparing X-ray amorphous and nanocrystalline metal powders of groups IB to VIIIB of the Periodic Table.
Relatively new methods for preparing very fine metal particles comprise metal vaporization (S. C. Davis and K. J. Klabunde, Chem. Rev. 1982, 82, 153-208), electrolytic processes (N. Ibl, Chem. Ing.-Techn. 1964, 36, 601-609) and the reduction of metal halides with alkali metals (R. D. Rieke, Organometallics, 1983, 2, 377) or anthracene-activated magnesium (DE 35 41 633). Also known is the reduction of metal salts with alkali metal borohydrides in aqueous medium to give metal borides (N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press 1986, p. 190). The coreduction of iron and cobalt salts in water leads to an Fe/Co/B-alloy having the composition Fe.sub.44 Co.sub.19 B.sub.37 (J. v. Wonterghem, St. Morup, C. J. W. Koch, St. W. Charles, St. Wells, Nature, 1986, 322, 622).
Sodium borohydride is likewise the reducing agent in producing heat-resistant, amorphous ferromagnetic powders according to DE-A 36 21 624, with salts of iron, cobalt, nickel and chromium being reduced in sodium hydroxide solution in the presence of complexing agents and xylene being added as oxidation-inhibiting phase.
DE-A 40 24 205 teaches a process for preparing metal-magnesium compounds and their use for preparing finely divided, possibly amorphous metal and alloy powders or intermetallic compounds by a non-metallurgical, i.e. wet chemical, route under mild conditions, with the metal-magnesium alloys being obtainable by reacting metal halides of the metals of groups IIIA-VA, VB-VIIB, VIIIB, IB and IIB of the Periodic Table (M.sup.1 X.sub.m) or metals M.sup.1 or M.sup.1 hydrides with magnesium hydride (MgH.sub.2), hydridomagnesium halides (HMgX), organomagnesium compounds and/or metallic magnesium in an organic solvent, in the presence or absence of anthracene or its derivatives, magnesium halides, organic halides and/or quinuclidine as activators. The MgH.sub.2 used here is a particularly active magnesium hydride containing complex catalysts (MgH.sub.2 * according to EP-B 0 003 564 or MgH.sub.2 ' according to DE-C 37 22 993). Stoichiometric reaction of the metal-magnesium compounds with the corresponding metal halides gives amorphous metal powders.
WO 90/07012 is concerned with a process for preparing a metal, an alloy or a ceramic, in which the metal compound to be reduced is subjected to mechanical activation either alone or in the presence of at least one reducing agent. For the mechanical activation, preference is given to using high-energy milling which leads not only to the particle size reduction usual in milling processes but also to a welding together of the powder particles. Bearing in mind the extreme energy consumption, the material wear associated therewith and the usually very long reaction times (mostly more than 24 hours), this process does not provide an economical solution to the production of fine metal powders.
EP-A-0 423 627 teaches a process for preparing finely divided, microcrystalline to amorphous metal or alloy powders and colloidal metals or alloys in organic solvents, in which metal salts, either individually or as a mixture, are reacted in inert organic solvents with alkali metal hydrides or alkaline earth metal hydrides which are kept in solution in the organic medium by means of organoboron or organogallium complexing agents, or with tetraalkylammonium triorganohydridoborate. The process claimed is expensive due to the use of elaborate boron or gallium complexes and in addition frequently gives metal powders having not inconsiderable boron contents.