Tungsten nanopowder synthesis from tungsten hexachloride in hydrogen-containing thermal plasma of electric arc plasma torch

Tungsten-based materials are widely used in various technical applications. Plasma chemical synthesis with thermal plasma as a highly concentrated heating source is an effective method for producing nanoscale powders of tungsten, tungsten carbide, and W – Ni – Fe and W – Cu tungsten pseudo-alloys. Oxide compounds are mainly used as raw materials, which imposes restrictions on the use of hydrogen as a reducing agent. Tungsten halides are utilized to produce tungsten layers in CVD processes; the use of tungsten halides for tungsten nanopowders synthesis is not mentioned in the literature. In this work, a thermodynamic analysis of the equilibrium states of the WCl 6 – x H 2 system was performed under isobaric-isothermal conditions in the temperature range of 400 – 6000 K at a pressure of 0.1 MPa. The authors conducted an experimental study of the production of tungsten nanopowder by reducing tungsten hexachloride powder in hydrogen thermal plasma of an electric arc plasma torch in a reactor with limited jet flow, which is a prototype of a real industrial reactor. It follows from the calculation results that with twice or higher hydrogen excesses (H/Cl ≥ 2 mol/mol), the yield of reduced tungsten close to 100 % is provided at temperatures exceeding 500 K. During the reduction of tungsten hexachloride powder in a stream of hydrogen–containing thermal plasma, tungsten nanopowders with a specific surface area of 10 – 12 m 2 /g, consisting of highly aggregated nanoparticles with an average size of 30 nm, were obtained experimentally. The chlorine content in the tungsten nanopowders did not exceed 0.3 wt. %. The phase composition of the nanopowders is characterized by the α-W phase with the presence of about 30 % of β-W phase. The presence of different phases in nanopowders can be related both to different conditions of nanoparticles formation in the volume of a plasma chemical reactor, and to the influence of the size effect.