Attractive processing techniques for functional nanoparticle synthesis by thermal plasmas has been developed, because atmospheric pressure plasmas offer unique advantages; these advantages include high enthalpy to enhance the reaction kinetics, high chemical reactivity, oxidation and reduction atmospheres in accordance with required chemical reactions, and rapid quenching. These advantages bring the advances and demands in plasma processing for nanoparticle synthesis. In this paper, generation of plasmas under atmospheric pressure and its characteristics is reviewed. Furthermore, plasma processing for functional nanoparticle synthesis of oxides, nitrides, carbides, and intermetallic compounds is reviewed to clarify the generation mechanisms. Numerical analysis is reviewed for intermetallic compound nanoparticle synthesis using an induction thermal plasma including the material evaporation and the nanoparticle growth with nucleation and co-condensation.
The application of plasma processing technologies to the engineering and industrial fields is expanding. Fine particles suspended in plasma, often called plasma dust, are known to be a major factor of disturbing the manufacturing processes of electronic devices, etc. Therefore, studies on the formation and behavior of fine particles in plasma are important from the viewpoint of developing methods to control plasma dust. In this article, recent studies are introduced on the observation and discussion for dust in a plasma CVD reactor using a particle visualization technique based on laser light scattering, and on the methods for suppressing particulate contamination by plasma dust.
Recently, a VOCs/odor control technology using nonthermal plasma has been installed in waste-water treatment plants, food process industries, etc., and its share in Japan increases gradually according to the marketing research. However, its application field is very much limited because the removal performance for high concentration VOCs are insufficient. The plasma-catalyst systems have been studied by many researchers in order to overcome these problems. Although the effectiveness of catalysts in plasma systems has been reported, the reaction mechanisms and the role of solid surfaces are not well understood. In this review, I summarize the significant role of solid surfaces under plasma discharge from recent papers. The relationship between solid surfaces and active species derived by plasma discharge is also discussed based on our research results.
New atmospheric-pressure nonequilibrium plasma combined exhaust gas aftertreatment technologies are developed without using precious metal catalysts and harmful ammonia etc. Two types of innovative environmental protection systems (a dry system and a wet system) , which enable to produce ultra low CO2, PM (Particulate Matter) , NOx emissions, fuel consumption and low cost, are realized for diesel hybrid engine, marine engine and combustion boiler applications. This paper mainly reviews the principles of these systems and some recent experimental results. The effective or apparent required plasma energy can be decreased greatly using these system. It is confirmed for a 5.5 kW diesel engine exhaust that the NOx complete reduction and the DPF continuous regeneration are possible using the dry system. It is also confirmed for a cylindrical smoke tube boiler (flow rate of heavy oil fuel = 52 L/h) that more than 80 % NOx reduction is achieved using the wet system.
A surface-discharge microplasma device was used as a bipolar ion source for aerosol charging (Surface-discharge microplasma aerosol charger ; SMAC) . In this study, we developed and tested a practical version of the SMAC using a replaceable ring electrode. Characteristics of the SMAC as an aerosol charge neutralizer was investigated by a comparison with those of 241Am radioactive source. The equilibrium charge state could be obtained for polydisperse DOS and diesel exhaust aerosol with 10 to 200 nm in mobility diameters regardless of initial charge state. It is found that photoelectron is emitted from the surface of silver nanoparticles (<10 nm) when the particles suspended in nitrogen carrier gas are irradiated by ultraviolet light emitted from microplasma device, and it affects the unipolar charging of aerosol.
A new electrospray pyrolysis method (named ac electrospray pyrolysis) is developed where droplets with a diameter smaller than 0.3 μm are formed through the breakdown of droplets by an ac voltage. The effectiveness of electrospray pyrolysis is demonstrated through the ZnS particle synthesis from the zinc nitrate and thiourea/ethyl alcohol solution. The amount of particles generated by the ac electrospray pyrolysis were about 10 times that by the conventional dc electrospray method at a given condition (the gas flow rate : 2 L/min, the furnace temperature : 873 K, the feeding rate of solution : 45 ∼ 38.5 mL/h) . SEM photographs showed that the particles prepared by ac voltage were agglomerated and the average size of the primary particles was almost the same as that of particles produced with dc voltage. X-ray diffraction patterns showed that particles produced by ac electrospray were ZnS.
Hourly time scale variation of modified components in individual Asian dust (Kosa) particles was analyzed with double thin film methods combined with time-series samplings. Kosa particles were collected with an impactor at Yasaka, Tango, Kyoto. 3 ∼ 4 days' samplings were run during springtime in 2003 (Run.1) and 2004 (Run.2) , and each one Kosa event could be observed. Several time-series samplings were performed every 3 ∼ 6 hours from the beginning of the Kosa episode. Double thin film methods were utilized to determine the mixing state of sulfate and nitrate, and that of nitrate and chloride in individual aerosol particles. Particles containing SO42- was detected in 25 ∼ 45 % of the coarse particles (50 % cut-off aerodynamic diameter : D50 ≥ 1.2 μm) collected onto each impactor stage at the first Kosa sampling in Run.1. The percentage decreased drastically 6 hours later and particles containing NO3- became the dominant species in the modified components. The latter result held for the samples in Run.2 irrespective of the sampling time. Meteorological condition and the results of the simultaneously performed bulk sample analysis in Run.1 implied sulfate in the Kosa particles was formed by the absorption of gaseous SO2 and the subsequent oxidation in high humidity condition. Percentage of the modified particles roughly decreased with time and that of the particles containing both Cl- and NO3- indicated similar pattern. In each sample, particles containing both SO42- and NO3- were hardly detected, which suggests that formation of nitrate and that of sulfate is exclusive.
Environmental tobacco smoke consists of 85 % of sidestream cigarette smoke and 11 % of exhaled mainstream smoke. In order to prevent non-smokers from passive smoking, exposure to the exhaled mainstream smoke as well as to the sidestream smoke has to be taken into account. In the present work, in order to elucidate the washout characteristics of inhaled smoke from the lung, smoking patterns were measured for the subjects with various vital capacities by using newly devised anemometer, and the particle concentrations and size distributions in the exhaled air were measured with SMPS (Scanning Mobility Particle Sizer, TSI Model 3936) and ELPI (Electrical Low Pressure Impactor, DEKATI Model 24317). As a result, (1) vital capacities of smokers do not affect the inspiration pattern of cigarette smoke, (2) exhaled air from a subject who is breathing room air contains particles with a bimodal distribution, (3) residual particles in the lung are washed out after 20 times of deep breathing, (4) human lung constantly generates particles with a median size smaller than 0.01 μm at the concentration of about 1.6 × 104 cm-3, (5) smoke exhaled from the lung has a bimodal distribution with the concentration of about 2.5 × 107 cm-3, (6) after smoking, complete washout of residual smoke in the lung requires normal breathing for 20 min.