Hydrodynamic study of fine metallic powders in an original spouted bed contactor in view of chemical vapor deposition treatments

An original gas–solid contactor was developed so as to treat by chemical vapor deposition, fine (mean diameter 23 μm) and dense (bulk density 7700 kg/m3) NiCoCrAlYTa powders with large size distribution. In order to avoid the presence of a distributor in the reactive zone, a spouted bed configuration was selected, consisting in a glass cylindrical column associated through a 60° cone to an inlet tube, connected at its bottom to a grid so as to support the powders at rest. A hydrodynamic study was conducted at ambient temperature and pressure, combining pressure drop measurements and visual observations as a function of gas velocity and of the ratio H/D of the height of the bed at rest over the bed diameter. Using conventional alumina particles belonging to Geldart's group B, it was shown that this equipment is able to ensure conventional spouted bed behavior, especially for H/D ratio equal to 1. From numerous experiments conducted with the fine metallic powders of interest, it was shown that (i) conventional pressure drop curves for spouted beds are obtained for H/D ratios between 1 and 1.8, (ii) due to the large grain size distribution of particles, minimum spouted bed velocities occur in a range rather than at precise values. Visual observations reveal the presence of the spout and fountain at the minimum spouted bed velocity and for H/D equal to 1

Fluidized bed as a solid precursor delivery system in a chemical vapor deposition reactor

Chemical vapor deposition (CVD) using precursors that are solids at operating temperatures and pressures, presents challenges due to their relatively low vapor pressures. In addition, the sublimation rates of solid state precursors in fixed bed reactors vary with particle and bed morphology. In a recent patent application, the use of fluidized bed (FB) technology has been proposed to provide high, reliable, and reproducible flux of such precursors in CVD processes. In the present contribution, we first focus on the reactor design which must satisfy fluidization,sublimation and CVD reactor feeding constraints. Then, we report masstransport results on the sublimation of aluminium acetylacetonate, a common precursor for the CVD of alumina films. Finally, we discuss the efficiency of the precursor feeding rate, we address advantages and drawbacks of the invention and we propose design modifications in order to meet the process requirements

Liquid and Solid Precursor Delivery Systems in Gas Phase Processes

Due to attractive surface properties and to intrinsic brittleness of Complex Metallic Alloys (CMAs), most of their potential applications involve materials with high surface-to-volume ratios, including thin films and coatings. While physical vapor deposition techniques are efficient for the processing of CMA films on line-of-sight surfaces, chemical vapor deposition (CVD) is well positioned for their application on complex surfaces. However, for CVD process to be implemented efficiently in the processing of CMA films a number of challenges must be addressed. Because numerous CVD reagents, commonly called precursors, are low vapor pressure liquids or solids, one of these challenges is the production of vapors of such precursors, which are decomposed in the deposition chamber to provide the desired film. Such a production has to be ensured at high rate and must be reproducible and stable during the whole process. Actual solutions to this question involve (i) bubbling inert gas through thermally regulated liquid precursors, (ii) leaching the surface of fixed precursor powder beds, and (iii) in situ generating the precursor flow by passing a reactive gas through a thermally regulated bed of the metallic element to be transported. Such solutions neither may be satisfactory for actual R&D needs nor may be transferable to industrial environments. These reasons are in part responsible for the limited implementation of advanced materials (including CMA-based ones) in numerous industrial and hence societal needs. More recently, innovative solutions have been proposed to feed deposition systems based on vapor phase chemical techniques (CVD and Atomic Layer Deposition, ALD). Such solutions are Direct Liquid Injection (DLI) of dissolved solid precursors and also sublimation of the latter in fluidized beds or in elaborated fixed beds. Such technological responses show promise for industrial applications of CVD, especially for the deposition of metals and ceramic compounds for which the available molecular and inorganic precursors present low vapor pressures. This review provides an overview of the methods by which precursor vapors are transported to the deposition chamber. Early and recent patents dedicated to such technologies will be revisited and considered in the light of the deposition of multimetallic alloy coatings

High activity redox catalysts synthesized by chemical vapor impregnation

The use of precious metals in heterogeneous catalysis relies on the preparation of small nanoparticles that are stable under reaction conditions. To date, most conventional routes used to prepare noble metal nanoparticles have drawbacks related to surface contamination, particle agglomeration, and reproducibility restraints. We have prepared titania-supported palladium (Pd) and platinum (Pt) catalysts using a simplified vapor deposition technique termed chemical vapor impregnation (CVI) that can be performed in any standard chemical laboratory. These materials, composed of nanoparticles typically below 3 nm in size, show remarkable activity under mild conditions for oxidation and hydrogenation reactions of industrial importance. We demonstrate the preparation of bimetallic Pd–Pt homogeneous alloy nanoparticles by this new CVI method, which show synergistic effects in toluene oxidation. The versatility of our CVI methodology to be able to tailor the composition and morphology of supported nanoparticles in an easily accessible and scalable manner is further demonstrated by the synthesis of Pdshell–Aucore nanoparticles using CVI deposition of Pd onto preformed Au nanoparticles supported on titania (prepared by sol immobilization) in addition to the presence of monometallic Au and Pd nanoparticles

Fluid dynamic simulation of CrO2(OH)2 volatilization and gas phase evolution during the oxidation of a chromia forming alloy

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Kinetics of the initial stages of film formation during low pressure chemical vapour deposition of polysilicon by pyrolysis of silane

A kinetic model for description of the process of silicon film formation on silica by thermal decomposition of silane at reduced pressure has been proposed. The model is based on the concept of kinetic interdependence between heterogeneous catalytic chemical reaction and fundamental structure forming phenomena - nucleation and nuclei growth. A number of experimental data for deposition rates and polysilicon grains sizes have been mathematically processed in order to derive kinetic equations for the rates of nucleation and nuclei growth as functions of reactor operating conditions (pressure and temperature) as well as process duration. Furthermore, based on both the good correspondence achieved between the experimental results and the model, and the deductions of thermodynamic theory of nucleation, the kinetic equations derived were analysed in regard to the general description of silicon film structure evolution. The analysis of the model, by confïrming the general trends established between the arrival and the surface diffusion rates of silicon adspecies, contributes to clarify the mechanism of the initial stages of film microstructure formation. The results obtained show that kinetics of structure evolution can be successfully described by developing the existing CVD phenomenological kinetic models further to an atomistic level

Atmospheric pressure chemical vapour deposition of BPSG films from TEOS, 03_3, TMB, TMPI_I: Determination of a chemical mechanism

APCVD of boro-phospho silicate glass from mixtures of TEOS, TMB and TMP, has been analysed then modelled in a continuous reactor. A reduced chemical mechanism has been developed and the corresponding rate constants have been identified. The first results obtained are encouraging and suggest that the very simple gas phase chemistry selected could be precise enough to represent the main trends of this very complex deposition procedure