High-concentration niobium (V)-doped titanium dioxide (TiO2) nanoparticles of the nonequilibrium chemical composition have been synthesized via Ar/O2 radio-frequency thermal plasma oxidation of mist precursor solutions with various Nb5+ concentrations (Nb/(Ti + Nb) = 0–25.0 at.%). The solubility as high as ∼25.0 at.% has not been achieved before by wet-chemical techniques. The preferable anatase formation was attained in the plasma-synthesized powders and was enhanced by the niobium doping. All the powders were heated at high temperatures (600–800 °C) to investigate their phase transformation, band gap variation, inter-particulate binding behavior, and photocatalytic stability. The transformation from anatase to rutile was effectively inhibited by increasing the Nb5+ content. The Nb5+ doping prevented the band gap of TiO2 from narrowing after the heating. At high temperatures, Nb5+ doping could not only preserve particle size but also prevent inter-particulate binding. High concentration (25.0 at.%) Nb5+ doping retained the photocatalytic performance almost invariably irrespective of being thermally treated.

Titanium dioxide nanopowders were synthesized by in-flight oxidation of titanium nitride (TiN) in radio-frequency (rf) induction thermal plasma. The powders were characterized by x-ray diffraction, high-resolution transmission electron microscopy, field emission scanning electron microscopy, Raman spectroscopy, and optical microscopy to reveal the mechanisms of phase selection and particle morphology evolution. The reaction began with surface oxidation of TiN particles, leading to the formation of core-shell composites with oxidized shells and TiN cores, followed by gas-phase condensation of TiO2 nanoparticles. Phase selection of the resultant TiO2 powders was found to largely depend on the oxidation potential of the thermal plasma rather than on the heat transfer itself. Anatase content of the products increased steadily with increasing the O2 input, and TiO2 nanoparticles (∼50 nm) containing ∼90% of anatase were obtained through O2/Ar plasma treatment. Phase-pure rutile nanoparticles (∼50 nm, on average) were also synthesized in H2/Ar plasma injected with O2 as the powder carrier gas.

Hydrogen doping effects on luminescence properties of bulk ZnO samples were examined by using a pulse-modulated plasma irradiation technique. Three kinds of ZnO samples, including both single- and poly-crystals, were employed as specimens. Secondary-ion-mass-spectroscopy analysis revealed that the surface layer to 100 nm was doped with hydrogen after the irradiation and its concentration was in the order of 1017 cm-3. The efficiency of band edge emission was increased by the hydrogenation. However, the degree of the improvements depended on impurity and defect concentration in the original samples.

In the manufacture of solid-oxide fuel cells (SOFCs), thermal-plasma technology, especially plasma spraying, offers several advantages, the most important of which is the ability to achieve considerably higher deposition rates than those that can be obtained with such approaches as physical or chemical vapor deposition (PVD or CVD). This article describes an international collaborative research project partially funded by NEDO (New Energy and Industrial Technology Development Organization, Japan) and carried out at five research laboratories around the world: at the University of Tokyo and National Institute for Research in Inorganic Materials (NIRIM) in Japan, the University of Minnesota in the United States, the University of Sherbrooke in Canada, and DLR—Stuttgart in Germany. The objective of the project was to conduct a comparative study on the possible use of four thermal-plasma spraying techniques for the integrated fabrication of SOFCs.

In-flight modification of MoSi2 powders has been carried out by using an Ar–H2 induction plasma. Reactor pressure, powder feed rate, and plate power level were taken as the experimental parameters to alter the thermal history of the injected powder particles. Metastable hexagonal structure of β–MoSi2 is the major phase observed in the Ar–H2 induction plasma-treated molybdenum disilicide powders, while the stable phase of tetragonal structure of α–MoSi2 usually retains no less than 30 wt. %. Depending on the experimental condition and the deviation from stoichiometry in raw materials, low silicides, Mo5Si3 and Mo3Si, and free Si were observed,

The in-flight modification of titanium carbide powders was carried out in radio-frequency (rf) inductively coupled plasmas. The powders were partially melted and evaporated, and then subjected to modifications in morphology, size, and chemical composition. Both the Ar–H2 and Ar–N2 plasma treatments induced the formation of carbon-site vacancies in titanium carbide. The mixing of NH3 to Ar–H2 plasma at the plasma tail, and the Ar–N2 plasma treatment resulted in the partial substitution of carbon by nitrogen. The variation in physical and chemical modification was discussed compared with the predictions by the thermochemical analysis, and the numerically obtained heat transfer of our preceding paper.

Modeling of TiC powder behavior in Ar–H2 and Ar–N2 RF thermal plasmas has been performed as well as the numerical analysis of the plasma fields to investigate the plasma/powders interaction. The heat transfer rate from the plasma to the powders decreases with increasing the powder feed rate and with decreasing the plasma pressure, because of the significant local cooling owing to the presence of the injected powders. These results agree fairly well with the experimental results of our following paper. The modeling herein gives guidance for the rational design of new material processing using thermal plasmas.

A 13.56 MHz inductively coupled plasma(ICP) system has been applied to fabricate diamond and diamond-like carbon from a CH4/H2/Ar plasma. The characterizations of the obtained deposits by transmission electron diffraction, reflection high energy electron diffraction, and Raman scattering revealed that the deposits are diamond crystallites, with crystal sizes ∼1μm, and that they partially include disordered microcrystalline graphite. Although contamination due to etching of the quartz tube exists, this was drastically suppressed by the electrostatic shield(Faraday shield).