The frequency dependence of the impedance of a liquid crystal cell was measured for 4-n-pentyl-4 ′ -cyanobiphenyl. The resistivity of a liquid crystal layer decreases due to two relaxation processes. The relaxation time for the slow process is about 4000 h, and that for the fast process is of the order of 100 h. This implies that the carrier ion concentration increases over a long period of time. A surface double layer was detected at the polyimide in the low frequency region, the thickness of which tends to decrease with time.
Influence of a crack in an insulator with a backside electrode upon AC breakdown voltage (BDV) was investigated in air. In the test insulator, the crack runs parallel with the plane backside electrode. The breakdown path from the HV electrode propagates through the crack to the counter electrode. The experimental results show that BDV depends upon the width of the crack in the area of the electrode distances longer than 50mm. In this area of the electrode distances, BDV decreases with a decrease in the width of the creak when the width is smaller than 500μm. For example, in the electrode distance of 120mm, BDV at 200μm of the crack width is about 0.85 times lower than that at 600μm of the width. The decrease in BDV for the width _??_500μm may result from the propagation of a positive surface leader. The positive leader propagates to the counter electrode before the breakdown only in the range of the width _??_500μm. The appearance of the positive leader may be due to the electric field enhancement by charge accumulated on the insulator surface in the crack. In the width _??_500μm, on the other hand, BDVs are almost independent of the width. The positive leader does not appear, but the negative leader appears significantly in the range of the width _??_500μm. It was observed in the range of the width _??_500μm that the propagation of the positive leader is longer than that of the negative leader. The decrease in BDV for the range of the width _??_500μm may result from the longer propagation of the positive leader through the crack.
This paper describes the decrease in partial discharges in the void by the use of additives to the PE material for the insulator. Azo-compounds were used to control the affinity between the wall in the PE insulator and surface charge.
