Investigation of prebreakdown phenomenon and discharge locations in the insulator with the metallized layer in vacuum
In order to investigate the electron emission characteristics from the region of boundary between the insulator and the metallized layer, we used the alumina insulator sample on which the layer of Molybdenum-Manganese material is metallized. The shape of alumina insulator is the cylindrical disk-type insulator with the diameter of 10 mm and the thickness of 5 mm. The metallized layer is attached to the front face partially. The sample is fixed on the sample holder and faced to the copper meshed electrode in vacuum. The gap between the sample and the meshed electrode is adjusted and controlled parallel with face to face, which is 0.6 mm in vacuum. Under such a condition, the positive dc voltage is applied to the meshed electrode, the sample holder electrode and the metallized layer is ground. We measured the prebreakdown current and the location of discharge points through the meshed electrode by video camera simultaneously.
The experimental results showed that a several number of breakdowns are observed around the boundary between the insulator and the metallized layer when the applied voltage increased up to several kilo-voltages. After such experiments repeat several times, the unstable prebreakdown current of several tens ordered pico-ampere was measured under 3-5 kV voltage application. The value of prebreakdown current increased slowly with the elapsed time even if the applied voltage was kept constant. It increased more than two orders of magnitude during about 3-5 minutes, finally lead to a breakdown event or saturated to a certain value. When the Fowler-Nordheim (F-N) plot was applied to the results, the field enhancement factor ß was a very high value in the range of approximately 2000-3000. In the observation of the surface by SEM, some of discharge traces were confirmed at the edge of the metallized boundary. In addition, small gap at the boundary existed between the metallized layer and the alumina surface. Therefore, it is suggested that the discharges are caused by electron emission from this region. Based on experimental results, it is considered that this phenomenon such as slowly increasing current is probably due to the influence of alumina being charged by electrons emitted from CTJ and the electron emission is promoted by the charging of the alumina surface.
File Size: 711,2 KB
Pages NB: 6