Hiroshi Miyadera

The role of hydrogen in diamond synthesis using a microwave plasma in a CO/H2 system

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 A, 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...

Characterization of diamond films synthesized in the microwave plasmas of CO/H2 and CO/O2/H2 systems at low temperatures (403–1023 K)

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 polyac...

Worldwide status of low temperature growth of diamond

Abstract Recent trends in low temperature growth of diamond films (LTGD) are reviewed. LTGD can be classified into non-substrate-heating processes and substrate-heating processes. In non-substrate-heating processes, room temperature growth of diamond has been demonstrated by sputtering, r.f. plasma, and laser excitation methods. However, the film quality has still not been analyzed quantitatively. In substrate-heating processes, polycrystalline diamond growth has been confirmed at substrate temperatures between 80 and 135 °C by microwave plasma of both CO H 2 and CO O 2 H 2 systems, electron cyclotron resonance plasma of C 2 H 5 OH (H 2 , Ar, He) systems, and tantalum filament decomposition of the CH 4 H 2 system. Source gases providing oxygen and excess atomic hydrogen in the gas phase were found to be suitable for LTGD because these species suppress the inclusion of amorphous components and the degradation of crystallinity which are likely to occur at low substrate temperature. The lowest substrate temperature for synthesis of diamond films with the same crystallinity as natural diamond is about 400 °C. The film quality deteriorates near 130 °C, which may be due to the incorporation of H, O, and OH from the gas phase into the diamond films. Growth rates are between 0.01 and 0.2 μm h −1 near 400 °C, and 0.035 and 0.3 μm h −1 at 130 °C. Growth rates can be accelerated by the addition of Ar or He to the source gas, but inert gases may cause the film quality to deteriorate.

Low temperature (∼400 °C) growth of polycrystalline diamond films in the microwave plasma of CO/H2 and CO/H2/Ar systems

In order to elucidate the growth conditions for pure diamond of good crystallinity, the correlation of the properties of deposited films and gas phase species in microwave plasma of CO/H2 and CO/H2/Ar systems was investigated through film characterization and optical emission spectroscopic measurements. It was found that the increase of gas phase atomic hydrogen and the suppression of gas phase acetylene were effective for the growth of pure diamond of good crystallinity. Low temperature growth was proposed to realize these growth conditions. It was confirmed that diamond films with good crystallinity and optical transparency could be synthesized at 410 °C (diamond film growth was possible even at 365 °C). The growth rate was accelerated in a CO/H2/Ar system (0.33 μm/h at 410 °C), which was several times faster than that obtained in a CO/H2 system (0.14 μm/h at 450 °C).

SUITABLE GAS COMBINATIONS FOR PURE DIAMOND FILM DEPOSITION

Abstract In order to identify the appropriate gas combinations that realize diamond film growth in an excess atomic hydrogen environment, the relative atomic hydrogen concentrations C [H] of various source gas systems (CH 4 /H 2 , CO/H 2 , CO 2 /H 2 , CH 4 O 2 (or CO 2 )/H 2 and CO/O 2 (or CO 2 )/H 2 ) were compared using plasma emission spectroscopy. C [H] was ranked in the following order: CO/O 2 /H 2 > CO 2 /H 2 CH 4 /O 2 /H 2 > CO/CO 2 /H 2 , CH 4 /CO 2 /H 2 > CO/H 2 > CH 4 /H 2 When oxygen-containing molecules (CO, CO 2 and O 2 ) were present in the plasma, there was also an increase in the amounts of atomic oxygen, O 2 and OH. These species had the same effect of eliminating non-diamond components and reproducting diamond-growing sites as atomic hydrogen. Thus enhancement of diamond-selective growth can be expected in the above-ordered gas systems. Diamond films synthesis was attempted using the CH 4 /H 2 , CO 2 /H 2 plasmas of these systems were correlated with the properties of the deposited films. Polycrystalline films could be synthesized at a growth rate of 0.93–1.2 microm h −1 in the CO(7%–10%)/H 2 system. However, no deposits were confirmed within 2 h in the CH 4 (1%)H 2 system and only amorphous phases were deposited in the system is considered to be due to the larger amounts of atomic oxygen, O 2 , OH and atomic hydrogen than in the CH 4 /H 2 system, which was due to the high concentration of oxygen in the plasma removing both diamond and amorphous deposits faster than they grew. The CO/O 2 /H 2 system was found to be promising for pure diamond synthesis because inclusion of the amorphous components was greatly suppressed with the addition of O 2 (the optimized concentration was about 2%). Diamond film with good qualities was synthesized in the CO/O 2 (2.2%)/H 2 system. The full width at half-maximum of the diamond Raman peak was 4.1 cm − , which is extremely close to that of natural diamond.

Diamond synthesis from CO-H2 mixed gas plasma

Particulate or film-like diamond was prepared on silicon substrates from CO-H2 mixed gas using a microwave plasma technique. The growth rate of diamond without graphite and amorphous carbon, as measured by Raman spectroscopy, was 9Μm h−1 for particles and 4Μmh−1 for flims. These values were larger than those in other source gas systems, such as CH4-H2, CH4-H2-H2O and CH3OH-H2. The good formation rate and high quality of diamond in the CO-H2 system was attributed to acceleration of methyl radical formation by the reaction of excited CO and H2 molecules and removal of by-product graphite by OH radicals in the plasma.

Removal of unpleasant odor gases using an Ag—Mn catalyst

Abstract Conversions of acetaldehyde and trimethylamine as model unpleasant odor gases were measured over several catalysts. The activities of Ag and Mn2O3 were the highest of the catalysts that consisted of a single component. The addition of Ag to manganese oxide enhanced the activity and the durability of the catalysts. The activity of Mn catalyst was improved by the addition of Ag up to 40 atom-%, and the maximum activity was observed at 10 atom-%. Acetaldehyde was considered to be decomposed over the Ag Mn catalyst by oxidation according to the measurement of reaction products. The oxidation state of silver was maintained on the surface of the Ag Mn catalyst calcined at 773 K. The amount of oxygen adsorbed on the surface of the Ag Mn catalyst was about 2.9 times as much as that on Mn2O3. These experimental data suggested that manganese oxide supplied oxygen to silver and that the oxidation state of silver was maintained on the surface of the Ag Mn catalyst.

Diamond-like carbon films prepared from CH4-H2-H2O mixed gas using a microwave plasma

Diamond-like carbon films were synthesized on polished silicon substrates from CH4-H2-H2O mixed gas using a microwave plasma technique. The film properties were studied. Their growth rate was several times as fast as that for CH4-H2 mixed gas under the same experimental conditions. The films have a large Vickers hardness (5000 to 6000 kg mm−2), high electrical resistivity (1012 to 1013 Ω cm) and good optical transparency, especially in the infrared region. Low hydrogen and oxygen contents in the films were detected by secondary ion mass spectroscopy.

Pyrolysis and Ignition Characteristics of Pulverized Coal Particles

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.

Synthesis and purification of diamond films using the microwave plasma of a CO-H2 system

A variety of diamond films were deposited using the microwave plasma of a CO-H2 system. Qualities of the synthesized films were correlated with the gas phase atomic hydrogen concentration monitored using optical emission spectroscopy. The amorphous components contained in the synthesized films were of a polyacetylene structure, which was possibly formed by the successive polymerization of C2H2 in the gas phase.Excess atomic hydrogen allowed highly crystallized diamond films to be deposited at high growth rates which included only a small amount of polyacetylene components. Two possible explanations for these results were proposed: the suppression of polyacetylene formation and the production of appropriate precursor (CH3) for diamond synthesis under the excess atomic hydrogen condition.Finally, the ratioIH/IAr (whereI is the optical emission intensity) was suggested as a decisive parameter indicating the suitability of the plasma conditions for the growth of pure diamond with good crystallinity.