A review is presented on the recent development of plasma spraying. Thermal spraying is a surface modification technique to impart new functions absent or insufficient in original materials. The remarkable improvement in coating quality was achieved by using thermal plasmas for melting and accelerating sprayed particles. Especially, the introduction of reduced pressure plasmas as well as radio frequency induced plasmas has provided a way to form clean, dense and thick coatings with strong cohesion and adhesion. Coupled with the recent development, the inherent high rates of solidification of the sprayed particles can yield unique micro structure of deposits. Novel examples of high performance coatings and production of near-net-shape components by low pressure plasma spraying are discussed.

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The electron temperature and electron density axial profiles in nitrogen plasma jet have been measured using the electrostatic double probe method. The electron temperature (_??_1.5-0.5eV) once decreases along the flow axis and increases again in downstream region. The behavior of the vibrationally excited nitrogen molecules which may play important role in this electron re-heating process were also studied by the spectroscopic measurement and the theoretical calculations. All the results show that the electron re-heating phenomenon can be explained by the super-elastic collision of the electron with the vibrationally excited nitrogen molecules which are formed by the three-body conversion of the nitrogen free atoms supplied from the arc discharge plasma region.

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Total radiation power of SF₆ gas blast arcs are measured by a photodetector which is composed of four photodiodes with different spectral sensitivities and designed for responding to radiation at wavelength ranging from 250 to 1, 800nm. The SF₆ gas flows along the arc column at 100l/min in our experiments. The arc plasmas are sustained with currents up to 10kA at the waveform frequency of 60Hz and up to 20kA at 120Hz. The radiation powers of 4, 000 to 8, 000kW/m are observed in 10kA arc plasma for both current frequencies. At the same time, a spectroscopic diagnoses are performed to monitor the arc temperature and diameter. Average temperature reaches more than 20, 000K in 10kA-class arc.
Theoretical calculations are carried out for emission coefficients in high temperature SF₆ gas contaminated with copper vapor. It is found that the emission coefficients are independent on admixture of the copper vapor at the high temperature above 15, 000K. The estimated total radiation powers based on the calculated emission coefficients are in good agreement with directly observed ones.

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When transferred type arc plasma is used for waste treatment and metal melting, the arc voltage may be changed by contamination of fumes and vapors from the molten bath. If the arc voltage goes over the open circuit voltage of the power supply, the arc extinguishes. On the other hand, if the arc voltage goes down, heat power and process efficiency decrease. This paper describes that the effect of contamination of iron vapor and fine particles on stabilized argon arc plasma. The calculations using transport properties and the energy equilibrium equation show decrease of axial electric field of arc column by contamination of iron vapor. This effect is maximum at the mixing rate of 3 to 5%. The tendency is almost same at the pressure range of 1 to 5atm. The experimental results on the wall stabilized arc contaminated by fine iron powders show good agreement with the tendency of calculations.

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It is expected to establish the technology for treament of low level radioactive solid waste generated in nuclear power plants. The main problems are volume reduction and stabilization of radionuclides acceptable for final disposal. Thermal plasma technology offers advantages for such waste treatment because of the easy supply of heat at high temperature independent on chemical and physical properties of the waste.
New, non-contaminated waste, such as metallic, inorganic and combustible materials, have been melted in a crucible furnace. Transferred type of a DC plasma torch with a solid electrode was used as heat source and rated up to 50kW. The main results obtained are as follows:
(1) Metallic and inorganic materials can be melted and vitrified. Combustible materials can be pyrolyzed. Consequently high volume reduction rates can be attained.
(2) Compressive strength of solidified products is more than twice as great as that of cement composites used for solidification of the liquid waste. Leaching rate of main components of the slag layer is in the same order of that of the high level radioactive waste glass.
(3) In order to get guiding principle for design of off-gas treating system, compositions of the off-gas and properties of the dust entrained in the off-gas are clarified.
From these results, technical prospects so that plasma heating technology can be applied for treatment of low level radioactive solid waste have been obtained.

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A technique to generate a pulsed high-current ablation arc jet was developed for the purpose of establishing a method to spray materials in the velocity range exceeding the limit of conventional thermal spraying. Basic structure of the arc jet generator was similar to that of the electromagnetic railgun except that the arc discharge was taking place through array of spot electrodes exposing on an insulating bore wall. Preliminary experiments were conducted using tungsten spot electrodes and fiber reinforced plastic (FRP) bore. By supplying a pulsed high current of 120kA in peak, an arc jet of 2-5km/s in exit velocity and 20MPa in peak bore pressure was generated. During the jet generation, about 0.6g of tungsten was ablated from the spot electrodes and sprayed to a steel substrate. However, the coating did not contain metallic tungsten but mainly consisted of tungsten carbide (W₂C), of which the carbon was considered to be supplied by ablation of the FRP wall. This result indicated that the choice of the insulating materials would be essential for controlling the composition of the coating. Two unique features were observed in the coating-substrate boundary, that is, a diffusion of both tungsten and iron and an irregular ripples of the boundary layer. Those features suggested that the coating process involved both intensive heating and hydrodynamic deformation of the substrate surface due to either a high heat flux from the arc jet or high-velocity collision of the particles.

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Electric field strength of the arcs burning through plastic tubes (PE, PP, PMMA, POM, PA, ABS, PVC, PTFE) at atmospheric pressure are measured as a function of arc current (2-500A) for various tube inner diameters (2, 4, and 6mmφ). Tube lengths are 10-30mm. The result shows that, as the curent increases, the electric field strenght decreases in lower current range and increases in higher current range. In lower current range, the electric field strength depends on the material: the electric field strength of PTFE arc is the lowest, that of PVC arc is higher than that of PTFE arc and those of the other material arcs are higher than that of PVC arc. In higher current range, as the current increases, the difference becomes small.
The gas volume ablated from the tube wall by the arc is measured for various tube materials and the arc currents. The result shows that the relationship between the ablation gas volume and the electric field strength in lower current range is linear if the current is constant, whereas that in higher current range is linear regardless of the tube material or the arc current.

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The in-flight particle parameters of velocity, size, and temperature were measured for nickel powder injected into an Ar/H₂ DC plasma jet, by using the Light Pulse Analysis method, which has been recently developed and verified in the induction plasma mode. Measurements were carried out under several plasma power levels from 18 to 32kW, as well as at different axial positions of 75 to 125mm from the nozzle exit of plasma torch. Although the particle velocity shows almost constant value of 100-120m/s through axial distance, the particle surface temperature has a maximum value at around Z=125mm under the plasma power condition of 21 to 24kW. The heating property of nickel paricle with a low melting point 1, 728K is found to be affected significantly by the plasma power, as expected. The cross-correlation between the particle parameters and hardness of coating was investigated. The results showed that the abrasion weight of the coating formed at the lower power of 21kW is small compared to that formed at high power condition of 32kW. Thus, for the nickel powders with relatively lower melting point, an excessive plasma power is not necessary for forming the appropriate spray coating.

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In this paper, we report the magnetic properties of carbon fine particles prepared in He plasma.
The exprimental results obtained are summarized as follows: The magnetic properties of the carbon fine particle are effected by a He gas pressure. The saturation magnetization of the carbon fine particle are strikingly influenced by a degree of crystallinity. When the degree of crystallinity of the carbon fine particle is approximately 58%, the saturation magnetization is 8.5×10^-7Wb•m/kg and the coercive force is 6.13kA/m. The saturation magnetization of the carbon fine particle having the degree of crystallinity of 58% decreases gradually with increasing temperature and disappears at temperature of 334°C. From the electron spin resonance spectrum, we calculated to be gyromagnetic ratio (g-factor) of 2.002 and electron spin density of 1.6×1022kg^-1. From the value of g-factor, it seems that magnetism of the carbon fine particle in the room temperature is caused by spin of electron.

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Recently, AC arc plasma heating has been used for melting treatment of flyash and similar dusts and for temperature control of molten steel in a tundish of continuous casting process. But, in using AC arc plasma heating, there are some problems that the electrode erosion in plasma torch is severe and arc plasma is apt to be unstable at current zero.
This time, in order to catch hold of the electrode erosion and reduce it, the effects of electrode stem size and tip shape on the electrode erosion were clarified at a low current of 200 A by using an experimental plasma torch. The electrode was solid type and its material was Lanthanum-oxide-tungsten. The electrode tip was in touch with arc plasma and the electrode root was cooled with water. Main results are as folows. As the electrode was thickened radially and shortened axially and the electrode tip flat area was increased, the electrode erosion rate was decreased to 1ng/C of a commercial level. Then calculation of the temperature distribution along the electrode axis was carried out. As a result, there was a high correlation between the calculated temperature of the electrode tip and the measured electrode erosion rate. That is, as the tip temperature was decreased to 2, 700K, the electrode erosion rate was decreased to 1ng/C.

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Characteristics of the ablation plasma produced by the irradiation of an intense, pulsed, ion-beam on targets, which has been known to be very effective to prepare thin films named as ion-beam evaporation (IBE), have been investigated both experimentally and theoretically by measuring the ion-flux of the plasma. From the experimental data of ion-flux signal, the temperature (T₀) of the plasma is deduced by using an analytic solution, derived from one-dimensional hydrodynamic model assuming the initial ion-beam driven and the following adiabatic expansions in the IBE, and the dependence of T₀ on the ion-beam power density is evaluated. The dynamic pressure of the plasma is also evaluated from the data of ion-flux and the target mass loss. Furthermore, the impulse due to the plasma on the substrate is measured by using a copper plate.

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Thin films of hydrogenated amorphous silicon (a-Si: H) fabricated by plasma enhanced chemical vapor deposition (PECVD) are valuable for functional materials used in an electronic device such as solar cells. We have been applying molcular dynamics method with empirical (Tersoff potential) to analyze a-Si: H thin-film growth processes. As a first step toward film growth simulations, we confirmed that the potential is avalilable for molecular dynamics simulation of amorphous silicon (a-Si) formation in our previous work. Next, molecular dynamics simulation of a-Si: H formation with rapid quenching has been performed by defining new Tersoff-type interatomic potential between Si and H in this study. Visualization of the a-Si: H sample by computer graphics made it possible to see that there are some voids in the sample and that there are two kinds of hydrogen in the sample, (1) one hydrogen gather closely together and (2) the others lie scattered, which was predicted in IR absorption experimental results. As a result of the study of silicon hydrogen bonding form variety, it was observed that the number of SiH bonds decreases when total bonding hydrogen exceed about 17%, while that of SiH₂ Plus SiH₃ increases drastically, which is consistent with the obtained results by IR absorption or ¹HNMR experiment of other groups.

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In this paper temperature and frequency dependence of dielectric properties and their relation to dc electrical conduction current of polysiloxaneimide thin films in high temperature region were studied and their mechanisms were discussed.
As temperature rises, dielectric dissipation factor (tanδ) increased, especially tanδ in the low frequency region markedly increased even at relatively low temperature. Dielectric constant showed little dependence on temperature. Temperature dependence of dc conduction current almost obeyed Ahrenius-formula. Based on these results, the mechanism of ac conduction was discussed. The analysis revealed that the ac conduction loss due to the mobile carrier becomes prominent at higher temperature and at lower frequency, and the ac inherent loss such as dipole relaxation loss are conspicuous at lower temperature and at higher frequency.

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