Investigation of Electron Emission from Metal/Insulator Interface in Vacuum and Eletric Charge on the Insulator

Surface flashover is one of the problems in improving insulation performance in gas, liquid and vacuum. It has been regarded that the emission of electrons from the cathode triple junction (CTJ) is a first step of initiating the surface flashover. Therefore, it is important to investigate field electron emission phenomenon from around the CTJ region with respect to achieving the higher performance of electrical insulation along the insulator in vacuum. In order to investigate the electron emission characteristics from the boundary region between the insulator and the metallized layer, an alumina insulator sample with a metallized layer formed by the Mo-Mn method was used. In addition, since the positive charge of alumina is possible to be related to the electron emission from CTJ, an experiment was also conducted on a sample with Cr2O3 coating on the surface to suppress the propagation of positive charge. In the experiment, the precursor current was measured and the discharge position was photographed.The experimental results showed that a several numbers of breakdown are observed around the boundary between the insulator and the metallized layer. After such experiments repeat several times, the pico-ampere order current was measured. The value of field emission 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. In addition, for the coated sample, the electron emission current decreased and the breakdown voltage improved. Furthermore, no current rise phenomenon was observed when the voltage was constant. Based on the above results, this phenomenon is probably due to the effect of the alumina being charged. Therefore, we conducted a charge measurement experiment in the next step.In the experiment for measurement of a charge distribution, a sample in which CTJ was formed by alumina ceramics located on the center of a stainless steel plate was used. This sample was also experimented with a sample additionally coated with Cr2O3. In the experiment, after confirming an increase of electrical current and applying the DC voltage is immediately terminated, the surface potential distribution was measured. As a result of the experiment, it was observed that the alumina was negatively charged on the almost whole area while a part was positively charged. On the other hand, for the coated sample, no positive charge was observed. From the above results, it was confirmed that the electric field was locally amplified by locally positive charging, leading to an increase of current and finally discharge.