Dependence of Dielectric Breakdown of Liquids on Imolecular Sturcture

High field carrier transport, electron multiplication and breakdown in liquids of various molecular structure are discussed. Breakdown characteristics depend on the molecular structure of liquids. Breakdown strength and its polarity effect are determined by the combination of the energy input from electric field and energy loss which depend on electron mobility and internal vibrational mode respectively, therefore on molecular structure. In rare gas liquids with high electron mobility like liquid Ar, Kr and Xe the electron avalanche is the mechanism of the breakdown. However, in liquid He, electron injection from cathode is the determining process for breakdown and the polarity effect of breakdown in liquid He is completely different from other liquids. In hydrocarbon liquids of ultra-high purity, which have high electron mobility in the case of spherical molecules, and also even in the case of linear molecules at high fields, the electron avalanche (or positive streamer) is also important in the short time range (nsec). However, thermal effects are significant in the long time range. Change of breakdown mechanism from bubble process to positive streamer process with decreasing pulse width and increasing pressure is indicated clearly in liquid N2 as an example. Effect of impurities on breakdown is also dependent on the molecular structure of liquids and solutes. Laser induced breakdown is strongly dependent on the molecular structuire and also can be explained in terms of electron avalanche by the high-frequency breakdown theory.