Carbon forms, carbide yield and impurity-driven nonstoichiometry of plasma-generated β-silicon carbide nanopowders

Abstract

Silicon carbide nanopowders are chemically complex materials containing various carbon forms and impurities. The current study quantifies the chemical complexity of these nanopowders in terms of their carbide phase nonstoichiometry and yield. The investigated samples include a range of β-silicon carbide nanopowders (BET surface area = 30–160 m2 g−1) produced from thermal plasma processes. The samples were characterized as to their carbon forms (temperature-programmed oxidation, Raman spectroscopy, HRTEM), total carbon content (combustion analysis), oxygen and nitrogen contents (inert gas fusion), surface elemental composition and bonding (XPS), crystallographic phases (XRD). The nanopowders contain widely varying amounts (0.5–19%) of carbon in noncarbidic forms (free and organic). However, the carbide phase of the nanopowders is invariably carbon deficient with the C:Si atomic ratio ranging from 0.89 to 0.95. The extent of carbon deficiency is linked to the level of oxygen and nitrogen impurities that substitute for carbon in the carbide structure. The substitution scenarios involve high temperature dissolution of nitrogen into the carbide lattice and room-temperature oxidation of particle surfaces to form an amorphous oxycarbide. Uncontrolled surface oxidation and contamination by carbonaceous deposits reduce the SiC yield of the nanopowders (76–91%) from those of ultrafine and micrometer-sized counterparts. Since the majority of contamination occurs on the nanopowder surface, the yield tends to decrease with decreasing particle size. The obtained results emphasize the importance of explicit control over the chemical impurities and particle surface conditions for fabricating the stoichiometrically balanced SiC nanopowders in high yield.