Deposition of diamond on graphite and carbon felt from graphite heated in hydrogen at low pressure

Abstract

The objectives of this research were to deposit diamond on graphite and carbon felt using our new closedchamber method [1], and to determine the influence of centrifugation on this deposition. It has long been known that hydrogen reacts with graphite at elevated temperatures to form a variety of hydrocarbon products. Balooch and Olander [2] published a definitive molecular beam study of surface reaction mechanisms. They were not able to detect a reaction with molecular hydrogen at temperatures up to 2200 K. Atomic hydrogen produced only methane from 400 to 800 K, molecular hydrogen between 800 and 1000 K, and acetylene from 1000 to 2200 K. The reaction rate was much higher on the prism plane than on the basal plane. They concluded that both surface adsorption and bulk diffusion of hydrogen atoms occurred. Angus and co-workers provided experimental and theoretical evidence that diamond can be caused to nucleate on the edge of the basal planes of graphite by adsorption of atomic hydrogen in the presence of hydrocarbon gases [3–6]. On the other hand, it was claimed that diamond grew on graphite by chemical vapor deposition only if the graphite had been pre-coated with chromium, nickel or titanium [7]. Details of the surface pre-treatment were not given for diamond grown on graphite using a H2+CH4 microwave plasma [8, 9]. In another study, the surface of carbon fibers containing embedded diamond particles were converted to diamond by exposure to a hydrogen microwave plasma [10]. At 50 torr hydrogen pressure severe etching occurred with little diamond formation, while a ∼1 μm diamond coating was formed with 100 torr H2. The same authors coated carbon fibers with diamond in a H2+CH4 plasma at a rate of ∼0.4 μm/h, although considerable etching of the underlying fiber preceded the growth [11]. The fibers had been pre-treated in an ultrasonic bath consisting of 0.25 μm diamond paste in a methanol slurry. There was little difference in the behavior of fibers prepared from pitch and from polyacrylonitrile. Recently, we described a simple new technique for deposition of diamond on a wide variety of surfaces, with and without centrifugation to accelerate deposition [1]. A graphite rod is heated electrically to approximately 2000 ◦C in the presence of ∼0.1 atm H2 in a closed chamber. The desired substrate is placed nearby, and is heated by the graphite rod. The present paper deals with deposition of diamond on graphite and carbon felt without any prior contact with diamond powder. For experiments aimed at deposition onto graphite, 1.5× 15× 20 mm slabs were cut from U-120 graphite (Fig. 1) from the Ultra Carbon Division of Carbone of America. These were cleaned in an ultrasonic bath, alternating between ethanol and acetone, and then dried in air. Six experiments were performed using a 25× 2× 2 mm graphite rod placed 2.5 mm above the substrate. The chamber was alternately evacuated to ∼1 torr and backfilled with∼1000 torr H2 several times. Then it was filled with hydrogen gas to a pressure ranging from 30 to 60 torr, and sealed. Electric current was passed through the graphite heating rod, raising the substrate temperature to ∼740 ◦C (measured by a K-type thermocouple touching its under side) for 1 h. Figs 2 and 3 show typical results, with faceted diamond crystals randomly distributed over the graphite. Tape adhesion tests removed neither the diamonds nor any graphite. Two of the six experiments were performed at an acceleration of twice earth’s gravity (2g) on the Clarkson materials processing centrifuge HIRB [12]. Without centrifugation, the diamond crystals were on about 50% of the 3 cm2 surface of the graphite substrate, while crystals were on about 90% with centrifugation. The size of the crystals was about 55% larger when deposition was carried out at 2g under the same conditions as without centrifugation. Longer deposition times should yield complete coverage of graphite by polycrystalline diamond.