Synthesis and characterization of nanophase Group 6 metal (M) and metal carbide (M2C) powders by chemical reduction methods

Abstract

The group 6 metal chlorides CrCl[sub 3], MoCl[sub 3], MoCl[sub 4], and WCl[sub 4] were reduced in toluene solution with NaBEt[sub 3]H at room temperature to form the corresponding metal colloids in high yield. When the same metal chlorides were reduced in THF solution with LiBEt[sub 3]H and NaBEt[sub 3]H metal carbides (M[sub 2]C) were formed in approximately 95% yield. The metal and metal carbide colloids were shown to be comprised of 1-5-nm-sized particles by TEM. The metal and metal carbide powders derived from the colloids by centrifuging and washing with various solvents were characterized with SEM, TEM, EDS, and ED which showed that the powders were comprised of agglomerates of the primary 1-5-nm-sized crystallite. X-ray powder diffraction studies of these black powders exhibited broad peaks for the s-prepared powders at room temperature where the crystallite size estimated from the broadening analysis generally corresponded to the primary particle size as determined by TEM. Variable-temperature X-ray powder diffraction allowed unambiguous identification of the crystalline metal and metal carbide phases present. Carbon and hydrogen combustion elemental analysis was also used to identify the metal and metal carbide materials. Thermogravimetric analysis in air provided information on the oxidation behavior of these materials andmore » also enabled distinction between the formation of metals versus metal carbides, M[sub 2]C. Experiments to determine the exact origin of the carbide carbon were inconclusive. Treatment of nanocrystalline W with THF and THF/LiBet[sub 3]H solutions did not result in further reaction to form the metal carbide. 38 refs., 10 figs.« less