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

Microstructure of Metallorganic Chemical Vapor Deposited Aluminum Coatings on Ti6242 Titanium Alloy

Al-based alloys are used in aeronautics as a protective layer against the excessive oxidation of turbines blades. In this work, Al coatings were deposited by metallorganic chemical vapor deposition on Ti6242, a commercial titanium alloy, with Pt as a sublayer or codeposited to ensure this protection, eventually forming platinum aluminides. Continuous Al coatings were deposited on bare Ti6242, using both triisobutylaluminum and dimethylethylamine alane as precursors. The same operating conditions applied on a platinum sublayer lead to discontinuous layers mainly containing whiskers. The introduction of a surfactant, namely, ethyl iodide, in the input gas improves the coating morphology by providing nearly whisker-free continuous films, suitable for oxidation protection. Codeposition of Pt and Al results in reduced growth rate and yields layers with lower Pt content and with specific morphology. This behavior is attributed to the competition between Al and Pt compounds on the growing surface

Al-Pt MOCVD coatings for the protection of Ti6242 alloy against oxidation at elevated temperature

Results on isothermal oxidation at 873K for 90 h of Al-Pt coatings on Ti6242 coupons are reported. These coatings were obtained by low temperature, low pressure metalorganic chemical vapor deposition using Me3(MeCp)Pt(VI) and dimethylethylamine alane. Three coating architectures were investigated, namely pure Al, Pt and Al sequential sublayers, and co-deposited Al and Pt. Oxidation kinetics revealed a strong transient oxidation regime followed by a diffusion driven parabolic one. Such coatings allow to decrease oxidation kinetics more than one order of magnitude compared with those of the bare Ti6242. Scanning electron microscopy, second ion mass spectrometry, X-ray diffraction and transmission electron microscopy revealed that these coatings present a rough surface morphology. They are dense, they develop scales composed of γ-Al2O3 and δ-Al2O3 and they prevent titanium diffusion from the alloy to the surface. It is concluded that coatings produced by this process show promise for use as effective protection against oxidation of Ti6242 alloys and consequently they may raise the maximum operating temperature tolerated by corresponding parts in helicopter turboengines

Chemical vapour deposition and atomic layer deposition of amorphous and nanocrystalline metallic coatings: towards deposition of multimetallic films

This paper provides a prospective insight on chemical vapour deposition (CVD) and atomic layer deposition (ALD) as dry techniques for the processing of amorphous and nanocrystalline metallic thin films. These techniques are part of major technologies in application fields such as microelectronics, energy, or protective coatings. From thermodynamic analysis, areas of investigation to generate a set of materials with the strongest propensity for amorphization as well as useful guidelines for the target phase material deposition are provided. Prospective to develop MOCVD (metalorganic chemical vapour deposition) and ALD of intermetallic films, in view of fabrication of metallic glass thin films is proposed. Examples from selected ALD and MOCVD single element metallic deposition processes will be described to illustrate the effect of deposition parameters on the physico-chemical properties of the films. This processing approach is particularly promising for metallic glass thin films

Microstructural characterization of Ru-doped NiCoCrAlYTa coupons treated by thermal oxidation

Isothermal oxidation of sintered Ru-doped and undoped Ni-alloy coupons in the range of 1173–1423 K was investigated. To assess the effect of Ruthenium doping, microstructural characterization was performed to compare the samples before and after oxidation treatment at 1173 K. Furthermore, cyclic oxidation tests on the Ru-doped and undoped coupons were carried out in a thermogravimetry apparatus at 1223 and 1323 K. The addition of 0.8 wt.% of Ru in NiCoCrAlYTa powders sintered coupons does not degrade the material’s resistance to oxidation conditions

Low Temperature MOCVD-Processed Alumina Coatings

We first present a Review about the preparation of alumina as thin films by the technique of MOCVD at low temperature (550°C and below). Then we present our results about thin films prepared by the low pressure MOCVD technique, using aluminium tri-isopropoxide as a source, and characterized by elemental analysis (EMPA, EDS, ERDA, RBS), FTIR, XRD and TGA. The films were grown in a horizontal, hot-wall reactor, with N2 as a carrier gas either pure or added with water vapour. The deposition temperature was varied in the range 350-550°C. The films are amorphous. Those prepared at 350°C without water added in the gas phase have a formula close to AlOOH. Those deposited above 415°C are made of pure alumina Al2O3. When water is added in the gas phase, the films are pure alumina whatever the deposition temperature

Principles and applications of CVD powder technology

Chemical vapor deposition (CVD) is an important technique for surface modification of powders through either grafting or deposition of films and coatings. The efficiency of this complex process primarily depends on appropriate contact between the reactive gas phase and the solid particles to be treated. Based on this requirement, the first part of this review focuses on the ways to ensure such contact and particularly on the formation of fluidized beds. Combination of constraints due to both fluidization and chemical vapor deposition leads to the definition of different types of reactors as an alternative to classical fluidized beds, such as spouted beds, circulating beds operating in turbulent and fast-transport regimes or vibro-fluidized beds. They operate under thermal but also plasma activation of the reactive gas and their design mainly depends on the type of powders to be treated. Modeling of both reactors and operating conditions is a valuable tool for understanding and optimizing these complex processes and materials. In the second part of the review, the state of the art on materials produced by fluidized bed chemical vapor deposition is presented. Beyond pioneering applications in the nuclear power industry, application domains, such as heterogeneous catalysis, microelectronics, photovoltaics and protection against wear, oxidation and heat are potentially concerned by processes involving chemical vapor deposition on powders. Moreover, simple and reduced cost FBCVD processes where the material to coat is immersed in the FB, allow the production of coatings for metals with different wear, oxidation and corrosion resistance. Finally, large-scale production of advanced nanomaterials is a promising area for the future extension and development of this technique

Solvent free method for intense vaporization of solid molecular and inorganic compounds

New tools have been developed for vaporization of solid precursors to meet the demands of high feed rate for CVD, ALD and other deposition processes

Mass spectrometry as a tool study CVD process

Mass spectrometry as a tool study CVD process. Application of two mass spectrometric (MS) techniques to study chemical vapour deposition from organometallic precursors is described. CpCuPEt3 (Cp = η5-C5H5, Et =C2H5) was used as a model precursor in this work

Complex Pt/Al2O3 materials for small catalytic systems

This paper reports on the preparation of catalytic materials composed of a porous metallic preform containing winding microchannels whose surface is coated by a complex catalytic film. Al2O3 is deposited inside the pores by means of wet impregnation in a first step, in order to increase the surface and to serve as an intermediate layer on which Pt nanoparticles are deposited in a second step. The latter deposition takes place by means of forced metalorganic chemical vapour infiltration (F-MOCVI). Pt(acac)2, is used in appropriate low pressure operating conditions, to allow for a process with low thermal budget, compatible with the geometrical and physical characteristics of the porous substrate. The results are evaluated by means of BET, SIMS, SEM/EDX and FEG/SEM. The catalytic material is finally being tested following the oxidation of carbon monoxide, a model reaction with high industrial interest