Modeling and validation of chemical vapor deposition of tungsten for tungsten fiber reinforced tungsten composites

Abstract

Tungsten is the most promising first wall material for nuclear fusion reactors. One disadvantage, however, is its intrinsic brittleness. Therefore, tungsten fiber reinforced tungsten (Wf/W) is developed for extrinsic toughening. Wf/W can be produced by chemical vapor deposition (CVD), e.g. by reducing WF6 with H2 using heated W-fibers as substrate. However, it still needs to be optimized regarding relative density and fiber volume fraction. The decisive factor is the tungsten deposition rate, which depends on the temperature and the partial pressures. For this dependence, however, there are controversial results in the literature. In this article, a new rate equation is presented, in which different literature equations are partially adapted and combined. It adjusts the WF6 reaction order between one and zero, depending on the temperature and the H2 and WF6 partial pressure. For validation, a simplified experimental setup with a single fiber was designed, which provides very well defined boundary conditions while varying the CVD process parameters heating temperature, pressure, gas flow rate and gas inlet composition. The experimental runs were simulated with COMSOL Multiphysics. The model was successfully validated by measurements of the WF6 consumption rates (< 2 to 100 %), deposited tungsten masses and spatially high-resolved tungsten deposition rates.