STRUCTURAL AND SURFACE CHARACTERIZATION OF ULTRAFINE IRON CARBIDE PARTICLES GENERATED BY LASER PYROLYSIS .1. HIGH-TEMPERATURE HE TREATMENT

Three samples of passivated ultrafine iron carbide particles, synthesi zed by laser induced pyrolysis of gaseous precursors, were examined by x-ray diffraction, x-ray photoelectron spectroscopy, and thermogravim etry/mass spectroscopy at temperatures as high as 600-degrees-C under a He atmosphere. The approximate 6-7 nm diam particles began to sinter at temperatures between 300 and 400-degrees-C and formed a complex mi xture of carbide, oxide and metallic phases. Preservation of the carbi de structure on heating was dependent on the purity of the carbide, an d the concentration of oxygen and carbon in the particles. A topotacti c transformation from Fe7C3 to Fe0.98O was observed and is discussed r elative to the as-synthesized/passivated particles composition and str ucture. This topotaxy leads to the selective formation of gamma-Fe2O3 from specific ultrafine particle carbides.