The Effect of Space Charge on DC Conduction and Light Emission in Dielectric Liquids under Non-Uniform Field

Conduction current characteristics in transformer oil and n-hexane have been studied under negative pointplane electrodes. Experiments were performed with needle electrodes of various materials (W, Cu, Mo, and Ni) with various tip radii (3, 6, 10, 25, and 50 μm) at gap lengths of 1 to 10 mm. From the experimental results, the conduction current was found to be caused by Schottky emission affected by space charge rather than by Fowler-Nordheim emission. Electric field distribution including space charge effect was calculated by the Charge Simulation Method using a digital computer. From the curve-fitting method the following electron mobilities and the effective work functions of W were estimated: 6.0xl0-6 m2/V.s and 1.05 eV for transformer oil, and 10x10-6 m2/V.s and 1.14 eV for n-hexane. The effective work functions of Cu, Mo, and Ni in transformer oil were also estimated at 1.07, 1.06, and 1.08 eV, respectively. Light emission was observed at the point tip in dielectric liquids. The light emission in transformer oil was divided into two distinct regions while in n-hexane there was only one. From the analysis of space-charge-affected field distribution, these light emissions were found to be caused by electronic excitation and photoluminescence.