Brigitte Caussat

Carbon nanotubes produced by fluidized bed catalytic CVD: first approach of the process

Abstract Multi-walled carbon nanotubes have been produced with high yield on an iron supported catalyst by catalytic chemical vapor deposition in a fluidized bed reactor. The choice of such a technique allows to reach high selectivity towards the desired material. A remarkable feature of this process is the huge bed expansion observed during the nanotubes growth that affects the fluidization regime due to the evolution of the apparent density of the composite powder. The catalytic powder, the composite material and the purified nanotubes have been analyzed by SEM, TEM and BET nitrogen adsorption.

Silicon deposition from silane or disilane in a fluidized bed—Part I: Experimental study

Abstract Fluidized-bed silicon deposition from silane constitutes an attractive alternative way to produce ultrapure silicon for solar-cell and microelectronic industries. Moreover, it allows the protection of bed particles from oxidation and corrosion. Studies available in the literature have proved the feasibility of this process, and pointed out its numerous advantages (high throughput, low-cost technology,…). However, two significant limiting problems exist: the parasitic formation of fines during experiments, and particles agglomeration when the initial concentration of silane exceeds a critical value. The first part of this article presents an experimental study about silicon deposition from monosilane and, for the first time, from disilane, in a fluidized-bed reactor. During experiments with monosilane, for inlet concentrations lower than 20% in nitrogen, silane conversion has always been quite complete, and fines formation limited. For higher concentrations, the fluidized bed has agglomerated systematically. Furthermore, for the first time, thermal perturbations of the fluidized bed have been put in evidence during all the runs, as soon as silane was introduced into the reactor. Such disturbances did not appear during disilane experiments, but fines were formed in larger amounts. Their zone of apparition was restricted to the coldest regions of the reactor. This has led us to think, in complete disagreement with literature opinions, that fines are formed from heterogeneous chemical reactions, on cold surfaces of the reactor. Moreover, thermal disturbances and agglomeration phenomena have been explained by an increase in particles cohesiveness, due to the presence of silane in the bed.

Fluidization, Spouting, and Metal–Organic CVD of Platinum Group Metals on Powders

In the first part of this paper, processes used for the fluidization of particles in a CVD reactor are reviewed, then the concept of the spouted bed is introduced, and the possibility of using it in association with metal-organic (MO)CVD is discussed. In the second part, the particulars of the MOCVD of platinum group metals are recalled. Finally, the MOCVD of these metals in fluidized bed and spouted bed reactors is illustrated with results obtained during two research projects, one of which concerns the preparation of finely dispersed rhodium, platinum, and palladium-based catalysts by fluidized bed MOCVD, the other, the doping of NiCrAlY powders (a raw material for thermal barriers) with ruthenium by spouted bed MOCVD. Results on the morphology and purity of the deposits are presented, and the applicability of such techniques, either for industrial uses or for initial screening of dopants (nature and level), are discussed.

Silicon deposition from silane or disilane in a fluidized bed—Part II: Theoretical analysis and modeling

Abstract To strengthen the results obtained during the experimental study presented in Part I of this article, a detailed theoretical analysis of silicon deposition from silanes in a fluidized bed has been performed. In a first step, four classical models of fluidized-bed reactors have been modified, to treat silicon deposition from monosilane, considering only reactions on surfaces. A convenient agreement has been obtained between the observed and the calculated data, particularly for the Kato and Wen model. But, this approach has been completely unable to treat the case of disilane. Consequently, for the first time, to our knowledge, chemical reactions in the gaseous phase have been included in the Kato and Wen model. This original approach, applied both to the cases of silane and disilane, has improved our understanding of deposition phenomena. More precisely, concerning fines formation, this work has confirmed the explanation given in Part I: it has shown the presence of sufficient active species at the top of the bed, so that these compounds could react on the cold walls of the reactor to give fines. It has also reinforced our rejection of the mechanism of gas nucleation inside the bed proposed by several authors. Secondly, this study has allowed us to strengthen our explanation of thermal disturbances and bed agglomeration, by the fact that the increase in particles cohesiveness seems to be due specifically to deposition mechanisms from monosilane, and not from silylene. However, investigations are still in progress on these subjects.

Silicon Chemical Vapor Deposition (CVD) on microporous powders in a fluidized bed

The fluidized bed Chemical Vapor Deposition (CVD) process constitutes today one of the most efficient techniques to modify the surface properties of dense (nonporous) powders. The success encountered in this field is the reason why this process has been extended to the treatment of microporous powders, for which there are very important industrial interests. The article presents an experimental analysis of the influence of the main operating parameters on the spatial localisation and the nanostructure of silicon deposits from silane on microporous powders. The first results obtained show that uniform deposits appear on all the surfaces of the pores in the form of discontinuous rafts. These results, which will be coupled with a theoretical predictive model, prove that the control at the nanometric scale of the organization of CVD materials on microporous powders today has become possible.

A dimensionless study of the evaporation and drying stages in spray pyrolysis

An original dimensionless study of the pure evaporation and precipitation stages of a spray pyrolysis process has been performed. An estimation of the evaporation time is proposed and the influence of the main processing parameters has been investigated. For operating conditions corresponding to industrial requirements, the main limiting step of the evaporation stage is thermal transfer from the column walls to the gas, not mass or thermal transfer at the droplet surface. Therefore, gas and liquid temperatures remain equal and constitutive equations can be greatly simplified. Moreover, in these conditions, neither solute concentration nor temperature gradients exist inside micronic droplets. Some data from the literature have been modelled and show the large range of validity of the equations and explanations proposed. Finally, with the assumptions made here, the dimensionless study of the precipitation stage shows that the presence of a crust can increase the drying time four-fold. However, a filled particle can still be formed.

Extension du procédé de dépôt de silicium par CVD en lit fluidisé à des conditions opératoires peu conventionnelles: dépôts sur poudres poreuses et/ou sous pression réduite

Abstract The applications field of the fluidized bed chemical vapor deposition process has recently been extended towards an important industrial sector: the elaboration of supported catalysts. This necessitates non-conventionnal operating conditions, because depositions take place on porous powders, and under reduced pressure. With the aim to analyze the possible modifications induced by these new working conditions, some experiments, and then simulations have been done, using a well-known deposit, that of silicon from silane and disilane. The detailed analysis of results demonstrates the good applicability of this process to this kind of elaborations.

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.

Local modeling of mass transfer with chemical reactions in a gas–solid fluidized bed. Application to the chemical vapor deposition of silicon from silanes

Abstract For several years, new processes have been emerging in the field of gas solid fluidization, involving complex gaseous chemical reactions, more often at high temperatures. As an illustration of these new technologies, the particular case of silicon CVD (Chemical Vapor Deposition) from silanes will be described. Traditional models of fluidized beds are intrinsically unable to represent such complex operations. Hence, as a first attempt, a new model has been developed to locally calculate mass transfer inside and around an isolated bubble rising through the fluidized bed. Examples of results in the case of CVD are presented. They confirm the applicability of the approach, which however needs further improvements.

Mechanical and Surface Properties of Chemical Vapor Deposited Protective Aluminium Oxide Films on TA6V Alloy

Mechanical, barrier and surface properties of aluminium oxide films were investigated by nanoindentation, microscratch and micro tensile tests, by isothermal oxidation and voltammetry, and by contact angle measurement. The films were grown on TA6V substrates by a low pressure MOCVD process from aluminium tri-isopropoxide. Modelling of local gas flow, gas concentration and deposition rate profiles was performed using the CFD code Fluent on the basis of an apparent kinetic law. Films grown at 350 °C are amorphous AlO(OH), the one at 480 °C is amorphous Al2O3 and the one at 700 °C is nanocrystalline -Al2O3. Scratch tests and micro tensile tests resulted in adhesive failure on the two films grown at low temperature whereas cohesive failure was observed for the high temperature growth. Sample processed at 350 °C presents significantly lower oxidation kinetics in dry air than the bare substrate. Contact angle changes approximately from 100 to 50 degrees for films processed at 350-480 °C and 700 °C, respectively. Concerning the electrochemical behavior in NaCl environment, polarization curves revealed that amorphous alumina coatings improved the corrosion resistance by comparison with the others oxide films. These consolidated results reveal promising combination of properties for the films grown at different temperatures with regard to the targeted applications.