Pyrolysis and ignition characteristics of pulverized coals were examined under similar burning conditions to those of industrial burners. In the early stage, fine particles (less than 37 μm) were mainly pyrolyzed by convective heat transfer from surrounding gas. The coals ignited when pyrolyzed volatile matter mixed with surrounding air and formed a combustible mixture. Pyrolysis of large particles was delayed, but accelerated after ignition by radiant heat transfer from coal flames. The effects of radiant heat transfer were strong for intermediate-size particles (37–74 μm). Ignition temperature was examined analytically by using a modified distributed activation energy model for pyrolysis. The calculated results agreed with experimental ones obtained from both laboratory-scale and semi-industrial-scale burners.

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In order to clarify the role of hydrogen in diamond synthesis using a microwave plasma in a CO/H2 system, carbon films were grown by varying hydrogen mole fractions in a CO/H2/He microwave plasma. The correlation between film properties and plasma species was investigated through film characterization and plasma emission spectroscopy. C and C2 were formed in the gas phase of the CO/He system and only sootlike carbon was deposited. Hydrogen additions to the CO/He system were found to enhance diamond growth by suppressing the formation of C and C2, which inhibited diamond growth by blocking the nucleation sites. The complicated structure of amorphous hydrogenated carbon, diamond microcrystallites having a diameter of 100 Å, and graphitic carbon was formed in the CO(5%)/H2 (30%)/He system, while columnar polycrystallites were grown in the CO(5%)/H2 system. Almost the same amount of atomic hydrogen in the ground state was found to exist in both systems, whereas a larger amount of electronically excited atomic hydrogen existed in the CO/H2/He system than the CO/H2 system. The atomic hydrogen in the ground state enhanced diamond growth by removing the amorphous carbon deposits, while the electronically excited atomic hydrogen exhibited no contribution to diamond growth and reacted with the amorphous carbon deposits to form the amorphous hydrogenated carbon. Finally, oxygen-containing species and hydrocarbons were suggested to be important precursors for diamond crystallization.

Diamond films grown in (A)CO/H2 and (B)CO/O2/H2 systems at substrate temperatures (Ts) between 403 and 1023 K were characterized by x-ray diffraction, Raman spectroscopy, cathodoluminescence, and scanning electron microscopy. A large amount of polyacetylene inclusion occurred in the (A)CO/H2 system on reducing Ts, resulting in worsening of the diamond crystallinity (FWHM of the diamond Raman peak broadened from 6.4 to 19.5 cm−1 when Ts was decreased from 1023 to 403 K). On the contrary, polyacetylene inclusion was significantly suppressed in the (B)CO/O2/H2 system, and high quality diamond films (FWHM=4.0–4.1 cm−1) close to natural diamond (FWHM=2.6–3.0 cm−1) were obtained between 684 and 1023 K. Though there was a little deterioration of crystallinity at 403 K, the obtained film still had good crystallinity (FWHM=10.2 cm−1) compatible with conventional chemical vapor deposition diamond films. The presence of a large amount of atomic hydrogen, atomic oxygen, O2, and OH contributed to suppression of polyacetylene formation on a growth surface and promoted cleaning of deposited amorphous phases. These species provided the best condition for selective growth of pure diamond of good crystallinity in the (B)CO/O2/H2 system even at low temperature (∼403 K), where impurities are likely to be involved. Films grown in the (B)CO/O2/H2 system were characterized as large and well-defined crystallites of octahedral forms emitting intensive blue CL at 440 nm. The actual activation energy (7.0 kcal/mol) for homoepitaxial diamond growth was obtained using the (B)CO/O2/H2 system, and was in good agreement with previous quantum chemical calculations (6.33 kcal/mol) based on the methyl precursor model. Finally, the (B)CO/O2/H2 system was suggested to be one of the most promising gas combinations for low temperature growth of high quality diamond.

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