Melting metal powder particles in an inductively coupled r.f. plasma torch
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I.
INTRODUCTION
PLASMAdeposition
is a process that combines particle melting, quenching, and consolidation in a single operation. The process involves injection of powder particles (metallic, ceramic, etc.) into the plasma jet which emanates either from the anode or nozzle of a d.c. plasma gun, or from an electrodeless r.f. discharge. Particle melting occurs and subsequently, the molten droplets are quenched upon impact at the substrate. The development of atmospheric pressure induction discharge can be traced back to the end of the nineteenth century. The major design concepts of induction plasma as we know it today are reported by Reed. N An inductively coupled plasma discharge can be maintained in an open tube in the presence of multiple gas streams. Upon leaving the discharge region, the partially ionized gas forms a low velocity plasma jet with an average temperature ranging from 8000 to 10,000 K. Considerable efforts have been made in understanding the characteristics of such a plasma generating device, its principal design, and operating parameters. Excellent reviews have been published in this area by Eckert [2] and Boulos. 131 The physical phenomena governing the operation of the induction plasma are the coupling between the applied high frequency alternating magnetic field and the eddy currents induced in the plasma. The current carrying plasma in the coil can be represented by an equivalent cylindrical work load with uniform temperature and electrical conductivity. Based on conventional induction heating theory, eddy currents are generated at the external cylindrical shell of the load by applying an oscillating magnetic field. The thickness of this shell (i.e., skin depth 8) which is dependent on the oscillator frequency (to) and the average electrical conductivity of the load (0-e), can be estimated in the following manner: t41
where ( is the magnetic permeability of the plasma gas. The coupling efficiency of induction plasma depends on the ratio of plasma radius to the radius of induction coil. A ratio as close as possible to one is most desirable; however, values as low as 0.7 to 0.8 are used due to physical configuration constraints. The components of an inductively coupled plasma torch are illustrated in Figure 1. The plasma torch consists of two concentric quartz tubes; the outer tube acts as the plasma confining tube and is surrounded by a short watercooled copper induction coil. The coil consists of three to five turns depending on the characteristics of the r.f. power supply. The inner surface of the confining tube is protected by a stream of cooling gas (sheath gas), introduced in the annular space between the external tube and the second concentric tube. The plasma gas is introduced in the cenSheath gas Plasma gas Carrier gas
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Plasma confinement tube Induction coil
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8 = 1/(~'~O'eto) 1/2 DANIEL Y. C. WEI and DIRAN APELIAN, Departmentof Materials Engineering, and BAKHTIERFAROUK, Departmentof Mechanicaland Mechanics Engineering, are with Drexel
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