Generation of microwave plasma under high pressure and fabrication of ultrafine carbon particles
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Generation of microwave plasma under high pressure and fabrication of ultrafine carbon particles H. Yagi, T. Ide, and H. Toyota Department of Mechanical Engineering, Ehime University, Matsuyama 79077, Japan
Y. Mori Department of Precision Science and Technology, Osaka University, Suita 565, Japan (Received 3 June 1997; accepted 5 September 1997)
A microwave plasma generator, which functions under high pressure, has been developed and used in the fabrication of fine carbon particles. The plasma generator is a two-stage-type resonator, which consists of rectangular and semi-cylindrical-type resonators which are coupled in series for torching plasma and keeping it stable under high pressure. The plasma can be torched in helium gas at 3 3 106 Pa by tuning the dimensions of apparatus elements. Fine carbon particles of ,50 nm are obtained using a mixture of helium and methane gas. The particles are found to be crystalline from the results of transparent electron microscopy and diffraction analysis.
I. INTRODUCTION
In recent years there has been an interest in research on microwave plasma, which is one way to obtain clean plasma.1 The advantage of this process is that it is free from contamination from discharging electrodes and processing chamber. Material with a low level of contaminants can be fabricated by applying this plasma to chemical vapor deposition. The microwave plasma is usually torched under low pressure below 100 Torr. In our study, we attempted to torch the microwave plasma under higher pressures than those used for conventional chemical vapor deposition. Under high pressure, gas molecules are present at high density, and the mean free pass of gas molecules is very short as determined from the kinetic theory of gases. Since the specific energy required to keep torching the plasma is proportional to the concentration of gas molecules, the plasma under high pressure has a high density of energy. The plasma energy under high pressure decreases due to a successive collision of gas molecules in the short range. Thus the plasma exists locally according to the form the energy supplied. The following advantages are expected as a result of carrying out the chemical vapor deposition under high pressure. One is a high growth rate of material due to the high density of source gas molecules. Powder production is possible as successions of nuclear formation and growth in the space. A new production method for contaminant-free material is expected to be developed using microwave plasma. Many advantages are expected compared with using the conventional microwave plasma in chemical vapor deposition. However, there are many problems associated with torching the microwave plasma under high pressure. In order to apply a high electric field to obtain plasma under high pressure, it is necessary to design a resonator for 1724
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 6, Jun 1998
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stable torching of the plasma. There are some reports of t
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