Direct current plasma spraying of mechanofused alumina-steel particles

Stainless steel particles (60 $\mu$m in mean diameter) cladded with an alumina shell (2 $\mu$m thick and manufactured by mechanofusion) were sprayed with an Ar-H2 (53-7 slm) d.c. plasma jet (I = 500 A, P = 28 kW, \rho_th = 56 %). Two main types of particles were collected in flight, as close as 50 mm downstream of the nozzle exit: particles with a steel core with pieces of alumina unevenly distributed at their surface and those consisting of a spherical stainless steel particle with an alumina cap. The plasma flow was modeled by a 2D steady parabolic model and a single particle trajectory by using the 3D Boussinesq-Oseen-Basset equation. The heat transfer, within the two-layer, stainless steel cladded by alumina, particle, considered the heat propagation phenomena including phase changes. The models allowed determining the positions, along the particle trajectory, where the convective movement could occur as well as the entrainment of the liquid oxide to the leading edge of the in-flight particles. The heat transfer calculations showed the importance of the thermal contact resistance TCR between alumina and steel

Influence des paramètres d'injection de poudres sur le traitement thermique des particules

International audienceEn projection plasma, la dispersion, en taille et en vitesse des particules à la sortie de l'injecteur, détermine les trajectoires des particules dans le jet plasma, et donne leurs histoires dynamiques et thermiques (vitesses, état de fusion, taux de vaporisation ...) qui sont des paramètres déterminants de la qualité de l'impact et du dépôt. Dans ce qui suit on décrit un modèle qui caractérise cette dispersion et qui tient compte de la distribution granulométrique de la poudre, des collisions inter-particulaires et des collisions avec les parois, il permet d'obtenir la distribution en vitesse et en taille des particules en sortie de l'injecteur

Etude de la solidification rapide d'une lamelle métallique en contact imparfait avec un substrat céramique

International audienceCe travail s'intéresse au problème de solidification de matériaux bicouche en régime transitoire, dont les propriétés thermophysiques dépendent de la température. Le modèle numérique utilisé est basé sur une approche enthalpique pour résoudre le problème de changement de phase dans chacun des matériaux en présence. Les résultats sont présentés pour plusieurs paramètres tel que l'épaisseur de la lamelle, la résistance thermique de contact, la nature des matériaux et leurs températures à l'impact

The Influence of Dry Particle Coating Parameters on Thermal Coatings Properties

International audienceThe physical properties of coatings elaborated by plasma spraying, especially the mechanical properties are strongly influenced by some fifty operating parameters of the spraying process. Several studies have been conducted to correlate these operating parameters with the coating microstructure, via the behavior of molten particles in flight to be impacted against the surface substrate, well known as splats. Then, it is expected to build coatings with tailored properties for mechanical and even thermal applications (Fauchais & Vardelle, 2000). Simultaneously to the operating parameters of plasma spraying, characteristics of raw powder play an important role in the coating elaboration (Vaidya et al, 2001). Depending on the production process, particles feature different characteristics concerning shape, size, specific density, purity, etc. This has a significant influence on the resulting coating properties (Sampath et al, 1996). Consequently, it becomes mandatory to have an intensive knowledge about the powder characteristics in order to better control the behavior of inflight particles and, thus obtaining coatings with the expected performance. For the elaboration of composite coatings, it is commonly to use composite powders. However, different characteristics of powders are obtained from the variety of processes nowadays available for powder production, even for powders with the same chemical composition! (Kubel, 2000) Kubel has compared powders produced from different techniques for plasma spraying (atomization, agglomeration by spray-drying, melting and grinding, wet particle coating; sintering). A variety of powder characteristics is found for which the operating parameters for plasma spraying must be adapted to obtain deposits featuring the desired properties. From this, certain components or materials are fabricated by some of these methods or exclusively just one

Simple modeling of the thermal history of d.c. plasma sprayed agglomerated nanosized zirconia particles

International audienceIn this work, are presented the results of a model coupling both dynamic and thermal histories of a single zirconia particle injected into a d.c plasma jet. The model developed calculates the heat transfer and phase changes within the particle along its trajectory. It is based on the Stefan problem with an explicit determination of the position of the interface solid/liquid. The evaporation is described according to the approach “Back pressure” The model is adapted to the calculation of thermal and dynamic behaviors of agglomerated particles

NTSR1 (neurotensin receptor 1 (high affinity))

Review on NTSR1, with data on DNA/RNA, on the protein encoded and where the gene is implicated

Calculation of combined diffusion coefficients from the simplified theory of transport properties

The aim of this study is to check if it is possible to use the combined diffusion coefficients introduced by Murphy at equilibrium in a two-temperature model (electron temperature Te different from that of heavy species Th) such as that denned by Devoto or Bonnefoi for transport properties. Murphy's coefficients describe the diffusive mixing of two non reactive ionized gases while the Devoto's or Bonnefoi's simplified theories allow the calculation of transport coefficients (except diffusion) out of thermal equilibrium. It has to be noticed that in the latter case when Te tends towards Th, the results are those obtained with an equilibrium calculation. The two-temperature (2-T) theory of transport properties was established by separating electrons and other species because of their mass difference. First, the exact combined diffusion coefficients of Murphy are calculated for an Ar-N2 (50wt%) mixture at atmospheric pressure. Then, expressions of combined diffusion coefficients are obtained using the simplified theory of Bonnefoi. The results of the calculation of combined diffusion coefficients from the simplified theory of transport properties, assuming equilibrium is achieved (Te=Th), is compared with those of Murphy at equilibrium. It is shown that large discrepancies occur when ionization is important. These results prove that the simplified 2-T theory cannot be used for the treatment of diffusion. Thus, a new theory of transport coefficients has to be developed taking into account the coupling of electrons and heavy species and work is in progress

Comparative Study between kinetic and thermodynamic calculation of composition in SF6 Plasmas

For an SF6 plasma at atmospheric pressure the compositions calculated in chemical equilibrium, in the multi-temperature model and using kinetics (33 reactions involving 18 species) are compared. The ratio of the electron temperature Te to that of heavy species Th was calculated as a function of electron density and an intermediate temperature T* between Te and Th was introduced to cope with the relationship kf(T*) / kr(T*) = Kx(T*) where kf and kr are the reaction rate coefficients (forward and reverse) and Kx the equilibrium constant. The main difference between equilibrium and kinetic calculations lies in the densities of S+, S2+, SF+, F- and S- species between 2000 and 6000 K. The multi-temperature method gives results very different from those of the kinetic calculations even when neglecting the charge loss reactions. This is in contrast to what happens with H2 or N2

New method to derive transport properties including diffusion in a two-temperature plasma

A new derivation of transport properties in a two-temperature plasma has been performed. The electron kinetic temperature Te is supposed to be different from that of heavy species Th. The resolution of the Boltzmann's equation, thanks to the Chapman-Enskog method, is used to calculate transport coefficients and it allows to the generalisation ofbracket integrals out of thermal equilibrium. Two-temperature diffusion coefficients are defined and the obtained results are presented for an atmospheric two-temperature argon plasma