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Diamond deposition was attempted on polyacrylonitrile (PAN) fiber and vapor grown carbon fiber (VGCF). PAN fibers were severely etched in a microwave plasma, whereas diamond was successfully deposited on VGCF. A diamond growth rate of 0.1 yttm/h on VGCF was determined at a gas mixture of 99.9 seem H 2 /0.1 seem CH4 and a pressure of 30 Torr. It is proposed that diamond formation on VGCF occurs on not only the prism planes, but also the basal planes owing to the unique structure of VGCF. An explanation is proposed to explain diamond nucleation on the basal planes.

I. INTRODUCTION

II. EXPERIMENTAL

Low pressure synthesis of diamond involving the use of hydrocarbon and hydrogen mixtures occurs when the gaseous mixtures are activated by dc or rf arc discharge,1'2 microwave energy,3"5 dc glow discharge,6 or hot filaments.7'8 The synthesis can also be done by using combustion reactions between oxygen and some hydrocarbons.9 Depending on the plasma and substrate conditions and the technique, the rate of diamond growth can range from less than one micron per hour up to several hundreds of microns per hour. Although the mechanism of diamond synthesis is not well established, it is recognized that diamond can be deposited from the vapor phase on diverse substrate materials including Si, Mo, Nb, Zr, Cu, WC, SiC, and SiO 2 . Intensive research efforts on the subject of low pressure diamond synthesis have substantiated appropriate conditions for diamond deposition on these substrates. However, relatively little effort has been reported where carbon materials (exclusive of diamond) have been used as the substrate.

Two types of carbon fibers were used in this study. One type of fiber is commercially available PAN (polyacrylonitrile) fiber (Hysol Grafil). This fiber has a diameter of about 7 fim. The other type of fiber is called vapor grown carbon fiber (VGCF) and is prepared in our laboratory. Vapor grown carbon fiber was produced by the pyrolysis of H 2 /CH 4 mixtures in the presence of iron catalytic particles in a two-stage process.16 This fiber has a diameter of about 5 ± 1 fim. Prior to the deposition, fibers were ultrasonically polished in a solution consisting of diamond dust and water. After the polishing, specimens were rinsed with water and then dried in air. Diamond dust was rarely found on the fiber surface after the polishing, nor was scratching observed. Diamond deposition was carried out using a microwave plasma enhanced chemical vapor deposition (MPECVD) technique. The microwave system utilizes an ASTEX HS-1000 magnetron with an S-1000 control unit/power supply to produce up to 1 kW of 2.45 GHz microwave power. A water-cooled applicator permits insertion of a 1.5 in. o.d. quartz reaction tube through the waveguide. Specimens were placed on a ceramic holder, with a diameter and a thickness of 1 in. and 0.25 in., respectively, and heated by the microwave plasma. Fibers were deposited in a gas mixture of 99.9 seem H2 and 0.1 seem CH 4 , at a pressure of 30.0 ± 0.5 Torr, and at 200-W of microwave power. A few expe