Fluidization and coating of very dense powders by Fluidized Bed Chemical Vapour Deposition

► The gas–solid fluidization of a very dense tungsten powder is studied. ► Its ability to fluidize at ambient temperature and at 400°C is demonstrated. ► Results on Fluidized Bed Chemical Vapour Deposition (FBCVD) of alumina are detailed. ► FBCVD appears as a suitable process to uniformly coat so dense particles. The hydrodynamic behaviour of a very dense tungsten powder, 75μm in median diameter and 19,300kg/m3 in grain density, has been studied in a fluidized bed at room temperature using nitrogen and argon as carrier gas. Even if fluidization was achieved, the small bed expansion indicated that it was imperfect. Then, the fluidization was studied at 400°C in order to investigate the feasibility of coating this powder by Fluidized Bed Chemical Vapour Deposition (FBCVD). In particular, the influence of the H0/D ratio (initial fixed bed height to reactor diameter) on the bed thermal behaviour was analyzed. It appeared that at least 1.5kg of powder (corresponding to a H0/D ratio of 1.8) was necessary to obtain an isothermal bed at 400°C. Finally, first results about alumina coatings on the tungsten powder by FBCVD from aluminium acetylacetonate are detailed. They show that for the quite low temperatures tested, the coatings are uniform on all bed particles and are formed of amorphous carbon containing alumina. This study demonstrates the efficiency to combine fluidization (instead of spouted bed) and CVD to coat such very dense powders.