A unique nitriding technique with the use of an atmospheric-pressure pulsed-arc plasma jet has been developed to offer a non-vacuum, easy-to-operate process of nitrogen doping to metal surfaces. This technique, however, suffered from a problem of excess nitrogen supply due to the high pressure results in undesirable formation of voids and iron nitrides in the treated metal surface. To overcome this problem, we have first established a method to control the nitrogen dose amount supplied to the steel surface in the relevant nitriding technique. When the hydrogen fraction in the operating gas of nitrogen/hydrogen gas mixture increased from 1% up to 5%, the nitrogen density of the treated steel surface drastically decreased. As a result, the formation of voids were suppressed successfully. The controllability of the nitrogen dose amount is likely attributable to the density of NH radicals existing in the plume of the pulsed-arc plasma jet.

Bright nitriding of a steel surface is demonstrated by an atmospheric-pressure pulsed-arc plasma jet, keeping the original metallic luster and low roughness. As the hydrogen fraction in the nitrogen–hydrogen gas mixture increases, the optical emission intensity of NH radicals decreases. Following this fact, we examine a method to suppress the formation of a compound layer, in which the hydrogen fraction increases from the conventional value of 1% to larger values for suppressing excess nitrogen supply. The hydrogen fraction of 4% provides a nitrided surface without a compound layer, where surface reflectance is largest and surface roughness is lowest. However, the nitriding treatment produces a dark spot several millimeters in diameter at the sample center due to local oxidization.

Water purification by non-equilibrium atmospheric pressure plasma has attracted much attention and is expected to be a next-generation method. However, general approaches to improve the energy efficiency of the water purification have not been revealed. Therefore, to investigate important factors for increasing its energy efficiency, we developed coaxial cylindrical plasma reactors where pulsed streamers were generated between a high-voltage electrode and running water film. To evaluate the performance of the plasma reactors, we measured hydroxyl (OH) radicals in solution based on a chemical probe method using disodium terephthalic acid (NaTA) and decolorized indigo carmine solution. Our experimental results showed that the production rate of the OH radicals was approximately 20 nmol/s and that the energy efficiency of the decolorization was on the order of 10 g/kWh. In addition, we found that controlling liquid flow based on the Coandă effect and introducing the intermittent operation of the streamer discharges to use post-discharge reactions increased the energy efficiency by a factor of approximately 3.5, which indicated that these approaches are effective to improve the performance of the water purification by plasma.

A unique nitriding technique using the atmospheric-pressure plasma jet has been upgraded by extending the treatable area. The previous manner to heat up a steel sample was spraying the thermal plasma jet onto the sample surface, which could maintain the treatment temperature only within a limited area of sample to restrict the treatable area to ca. 300 mm². Here, we first introduced an external heating system to the plasma-jet nitriding to control the treatment temperature of a larger sample. As a result, the treatable area is successfully extended to ca. 6300 mm². Moreover, the lifetime of NH radicals produced by the plasma chemical reaction is found to be long enough to cover the relevant area, implying the importance of NH for plasma-jet nitriding.