Surface flashover characteristics of modified polyimide under DC voltage in vacuum

In this paper, direct current (DC) surface flashover characteristics of modified polyimide (PI) are studied in vacuum (10^-4Pa). Two kinds of semi-conductive powders (nano scale, micro scale) are used as modifier of raw PI. Meanwhile, the surfaces of flashover experimented samples are observed using scanning electron microscopy (SEM), and the influence of the secondary electron emission (SEE) coefficient on DC surface flashover characteristics is discussed. Experimental results show that, (1) In vacuum, DC surface flashover voltage of the nano modified PI is higher than pure PI, however that of micro modified PI is lower than pure PI; (2) The surface SEE coefficient of nano modified PI is smaller than pure PI.(3) Gas overflow phenomenon appears in the moment of prebreakdown. In combination with the results of SEM, we infer that the gas desorption is the key factor which affects the DC surface flashover voltage in vacuum.