Synthesis of Fine Vanadium-Carbide (VC0.88) Powder Using Carbon Nanofiber

Аннотация

The synthesis of fine vanadium-carbide (VC_0.88) powder is considered. To produce the vanadium carbide, vanadium(III) oxide is reduced by means of carbon nanofiber in an induction furnace with an argon atmosphere. The carbon nanofiber is produced by the catalytic decomposition of light hydrocarbons. The specific surface of the carbon nanofiber is very high: ~150000 m²/kg, as against ~50000 m²/kg for soot. The impurity content in the carbon nanofiber is 1 wt %. By analysis of the phase diagram of the V–C system, the batch composition and the upper temperature limit in carbide formation may be determined such that vanadium carbide is formed as powder. Thermodynamic analysis yields the initial temperature at which the vanadium( III) oxide is reduced in a furnace with different CO pressures. The characteristics of the vanadium carbide are determined by the following methods: X-ray phase and elementary analysis; pycnometric analysis; scanning electron microscopy with local energy dispersion X-ray microanalysis (EDX); low-temperature nitrogen adsorption with subsequent determination of the specific surface by the BET method; sedimentation analysis; and synchronous thermogravimetric analysis and differential scanning calorimetry (TG/DSC). The material obtained with optimal reduction parameters consists of a single phase: vanadium carbide VC_0.88. The powder particles are predominantly clumped together in aggregates. The mean size of the particles and aggregates is 9.2–9.4 μm, with a broad size distribution. The specific surface of the samples is 1800–2400 m²/kg. Oxidation of vanadium carbide begins at about 430°C and is practically over at 830°C. Optimal synthesis requires stoichiometric proportions of the reagents in the production of vanadium carbide VC_0.88 at 1500–1600°C, with 20-min holding. In this process, carbon nanofiber effectively produces the carbide as the reduction product. The vanadium(III) oxide is reduced practically completely to VC_0.88.