Film Synthesis on Powders by Cathodic ARC Plasma Deposition
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IAN G. BROWN*,
Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 ** Charles Evans & Associates, 301 Chesapeake Drive, Redwood City, CA 94063 *
ABSTRACT Cathodic arc plasma deposition was used to coat A120 3 powder (mesh size 60) with platinum. The power particles were moved during deposition using a mechanical system operating at a resonance frequency of 20 Hz. Scanning electron microscopy and Auger electron microscopy show that all particles are completely coated with a platinum film having a thickness of about 100 nm. The actual deposition time was only 20 s, thus the deposition rate was very high (5 nm/s). INTRODUCTION Coating of powders is of interest for a number applications such as catalysis and powder metallurgy and ceramic powder sintering. In particular, coating of ceramic powder with platinum is of interest in catalysis due to the large platinum surface area per mass unit. Complete coverage of the particles is sometimes difficult to achieve, and adhesion of the film to powder grains can be a problem. Powder coatings are generally done by fluidized-bed chemical vapor deposition [1], rotarybed chemical vapor deposition [2], plasma spraying, rotary-bed physical vapor deposition 13] or in-situ precipitation in aqueous solvents [4] or non-aqueous solvents [5]. In the present paper we describe a new approach to film synthesis on power particles using cathodic arc plasma deposition. In section 11we outline the features and capabilities of cathodic arc plasma deposition in general, and focus in section III on the application of coatings on power particles. As an example we deposited platinum on alumina powder. CATHODIC ARC PLASMA DEPOSITION A high current discharge between a cold cathode and an anode in vacuum or in a gas is called a "cathodic arc". The term "vacuum arc" is frequently used if no gas is present. Cathodic arcs are characterized by plasma production at small, non-stationary cathode spots (see, for instance, [6-8]). The arc current of typically 100 A or more is concentrated in a few micron-sized spots which move quickly over the cathode surface. The number of simultaneously active cathode spots depends on current, cathode material and temperature, and cathode surface conditions. Since the current density can be as high as 1012 A/m 2 [9], the solid cathode material is transformed into a completely ionized plasma which leaves the cathode spot region with a velocity of 1-2 x 104 rn/s (this number is valid for all cathode materials). This velocity corresponds to kinetic ion energies in the range from 20 eV (light ions such as carbon) to 300 eV (heavy ions such as platinum). Thus the cathode material is eroded and forms an energetic plasma jet. A cul'ent-carrying part of the plasma streams to the anode, and another part of the plasma can be used to synthesize metal films on arbitrary substrates. Sometimes the term "cathodic arc evaporation" is used. This is term suggests misleadingly that neutral vapor is involved. However, it is fully ionized plasma that is actually used in de
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