Silicon coating on very dense tungsten particles by fluidized bed CVD for nuclear application

Abstract

Tungsten particles were coated by silicon from silane (SiH4) at 645 °C using fluidized bed chemical vapor deposition (FB‐CVD). The coated particles were made from a surrogate tungsten powder and showed a median diameter (75 μm) and density (19,300 kg/m3). We studied the behavior of these tungsten particles due to their similarity with U(Mo) particles which are being investigated for use as fuel in research reactors. The results show that the fluidized bed's thermal gradient and pressure drop both increase as soon as silane is injected into the reactor. These phenomena are linked to the reactivity of silane and the very high density of the tungsten powder, and they represent a risk of bed agglomeration. A low inlet molar fraction of silane (0.5%) makes it possible to limit this risk. An increase in the fluidizing gas velocity and a decrease in the reactor diameter from 3.8 to 3.0 cm, have little impact on these phenomena for the operating range studied. Characterizations (EDX, SEM, XRD, etc.) show that all the tungsten particles are uniformly coated by a continuous film of silicon, mainly crystallized and of nodular morphology. The thickness of this silicon layers varies between 0.1 and 1 μm, which corresponds to the target. As the feasibility of the coating has been demonstrated, new experiments are in progress with fissile particles.

Amorphous alumina thin films deposited on titanium: Interfacial chemistry and thermal oxidation barrier properties

“…In the present article, we investigate the interaction between titanium, the base material used in the medium temperature engine parts, and three types of amorphous alumina thin films prepared by ALD and chemical vapor deposition (CVD) as protection barriers against thermal oxidation. ALD and CVD are powerful techniques to uniformly cover topologically complex objects or substrates , as demonstrated by the widespread use of the preparation methods in the microelectronic and glass coating industries . Moreover, these preparation techniques can be controlled to yield amorphous alumina films that have optimal oxygen barrier and mechanical properties, as shown for CVD films obtained from evaporated aluminum tri‐isopropoxide (ATI) precursor .…”

Effects of reducing the reactor diameter on the dense gas–solid fluidization of very heavy particles: 3D numerical simulations

“…First CVD experiments have shown that the conversion rate of silane is lower than for conventional powders. This can be explained by the simulation results, indicating a lower particle mobility and a lower particle mixing for these very dense tungsten particles (Vanni et al, 2015a).…”

“…First experiments have demonstrated that the FB-CVD technology can uniformly coat 1,500 g of tungsten powder with silicon in a reactor of 3.8 cm in diameter. Other experiments have shown that this weight cannot be decreased in the reactor of 3.8 cm because the target deposition temperature (650 • C) cannot be reached since heat exchange between the reactor walls and the bed of particles becomes too low (Vanni et al, 2015a). An experimental study has been conducted in glass and steel columns to specifically analyze the impact of decreasing the reactor diameter from 5 to 2 cm on the fluidization hydrodynamics of the tungsten powder (Vanni et al, 2015b).…”

“…Silicon seems to be a good candidate (Zweifel et al, 2013). Among other technics, the Fluidized Bed Chemical Vapor Deposition process (FB-CVD) is under study to deposit uniform silicon layers on U(Mo) particles (Vanni et al, 2015a). FB-CVD is an efficient technology to uniformly coat powders by a great variety of materials (Vahlas et al, 2006).…”