Heat and mass transfer during in-flight nitridation of molybdenum disilicide powder in an induction plasma reactor
Introduction
Molybdenum disilicide (MoSi2) is an excellent candidate for use as a high temperature structural material [1]. However, improvement of its properties, especially its low temperature toughness and its high temperature strength, must be achieved before its technical viability can be assured.
Over the past decade, a number of researchers [2], [3], [4] have studied various kinds of reinforcement for MoSi2, e.g. metals, carbides, oxides, silicides and nitrides. More recently, Fan and coworkers [5], [6], [7], [8], [9] demonstrated the possibility of in-flight nitridation and carburization of MoSi2, using a thermal plasma process. This work confirmed the plasma route as one of the most promising methods for creating an in situ second phase structure. The optimization of such a process necessitates a thorough understanding of the heat and mass transfer phenomena that are present under nitridation reaction conditions. Most of the prior studies concerning these matters have dealt with plasmas involving mono-atomic gases [10], [11], [12] and induction plasmas operated at relatively low frequency (400 kHz) [13], [14], while the nitridation process [5] involves poly-atomic gases and radio-frequency plasmas (2–3 MHz). These high frequencies are of great importance because they enhance the decomposition of molecular gases, such as nitrogen and ammonia, which are involved in the nitridation process.
The unique features of the in-flight plasma nitridation process should be highlighted; (1) the thermal plasma process provides a very high temperature environment (∼104 K) for nitrogen-bearing gases that can be broken down to dissociated atoms, ions, and radicals etc.; (2) powder particles used in the plasma process are commonly in the size range 10–100 μm and are highly dispersed in the plasma, so there is a very large surface area for reactions; and (3) the thermal plasma is a highly heterogeneous system with severe variations in the velocity and temperature fields, the powder particles can experience remarkable quenching conditions (up to 106 K s−1).
Here, we present an experimental investigation of the phenomena of mass and energy transfer influencing the in-flight nitridation of MoSi2 powders when conducted in thermal plasma reactors. The operating conditions adopted for the nitridation process were chosen to be as close as possible to those previously used in the experimental MoSi2 nitridation work by Fan et al. [5]. However, our study was conducted without powders injection because the enthalpy probe could not operate under such conditions. Consequently, the measured temperature profiles certainly exhibit higher values than the corresponding profiles with powders injection. Nevertheless, the presented plasma flow diagnostics results provide important information on the parameters that affects the nitridation process of the treated powders. The parameters that were found by Fan et al. [5] to most influence the nitridation process and the product powders structure are investigated in the present diagnostics study. Those parameters are, the plasma plate power; the reactor pressure; the composition and flow rate of the quench gases.
Section snippets
Equilibrium calculations
The thermodynamic equilibrium compositions for the reaction mixture (0.6 mol Ar+1.59 mol N2+0.03 mol MoSi2) were calculated using the ChemSage program [15], [16], [17], [18] for a pressure of 40 kPa and for the temperature range, 500–4000 K. From Fig. 1, the results of these calculations show that the nitride compounds can be obtained only in the temperature range 500–1700 K. The possible reaction products are Si3N4, MoN, Mo2N, Mo3Si and Mo5Si3 while MoSi2 itself is thermodynamically stable
Experimental setup and procedure
The experimental setup used in the present study, illustrated by the schematic diagram of Fig. 2, consisted of three major parts (1) an induction plasma torch (model PL-50, from Tekna Plasma System Inc. Canada) connected to a 50 kW radio frequency (r.f.) power supply; (2) a water cooled cylindrical reactor (0.25 m i.d. and 1 m long) attached to a filter assembly; and (3) an enthalpy probe system combined with a mass spectrometer. The apparatus used for plasma generation is similar to the one
Results and discussion
Effective in situ reactions induced by plasma means has proven to be a promising route towards producing stable microstructure systems. In the plasma nitridation process for MoSi2 powders, the powder particles have to cross large temperature and concentration gradients and these gradients affect the degree of spheroidization and the morphology of the nitrided powders. To understand how these gradient conditions influence the final product powder quality, heat and mass transfer measurements were
Summary
A heat and mass transfer study was carried out for the nitridation process of MoSi2 powders conducted in induction plasma reactors. The temperature, velocity and concentration profiles, established in the reactor in the absence of MoSi2 powders, have provided for a better interpretation of the results of experimental studies performed by Fan et al. [5] on MoSi2 powders nitriding treatment, in terms of the measured nitrogen contents in the processed powders.
The plate power is found to affect
Acknowledgements
The authors would like to thank Dr Peter Lanigan for the proof reading of this paper. The financial support of the Sciences and Technology Agency of Japan, the Conseil de Recherche en Sciences Naturelles et en Génie du Canada (CRSNG) and the Fonds pour la formation de chercheurs et à l'aide à la recherche (FCAR) is gratefully acknowledged.
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Present address: Dept. de physique, Faculté des Sciences et Techniques, Université Hassan II, Mahammadia 20650, MAROC.
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Present address: Tekna Plasma Systems, 3535 boul. industriel, Sherbrooke, Québec, Canada J1L 1X7.