Elucidation of Ignition-Area Extension of Barrier Discharge under High Temperature and its Application to Precise Control of Nitridable Area*

A planer DBD has recently been discovered to extend its ignition area gradually with increasing the ambient temperature. In this study, we investigated the physical mechanism of the DBD extension phenomenon under high temperature. Our numerical analysis of the reduced electric field showed that the increases in temperature (corresponding to the decrease in molecular density) and in applied voltage both increase the area of reduced electric field over 120 Td, which is the breakdown value. That is, the DBD extension phenomenon proved to result from extension of the reduced electric field. From this fact, we predicted that decreasing the applied voltage will suppress the DBD extension even under high temperature. Second, we conducted nitriding experiments to confirm if the nitridable area can be controlled following the controlled DBD ignition area. Here, N₂-H₂ mixture with H₂ partial pressure of 10% is used. 20x20 mm² steel samples were used as the ground electrode, the treatment temperature was 800 K, and the treatment time was 2 h. As a result, we succeeded in controlling the ignition area of DBD and the nitridable area even at 800 K by adjusting the applied voltage. The use of a point electrode with a diameter of 0.7 mm as the opposite electrode minimized the plasma ignition area and enabled submillimeter local nitriding without masking.