Measurement of charge storage and leakage in polyimides

Resistivity and electric charge storage properties of LaRC-SI and Kapton polyimide samples were studied in a dark evacuated environment, both before and after 50 kGy cobalt-60 gamma irradiation. In different tests, slow electrons, and 4 keV electrons were used to charge surfaces of 50 μm polyimide to about 500 V while the opposite surfaces were copper metalized and grounded. The subsequent decay of surface voltage can be interpreted as a current through the resistivity of the dielectric. The same samples tested by classical ASTM and IEC methods provided resistivities of order 1E16 Ω cm, but when tested by our evacuated charge decay method the resistivity was 5E19−5E20 Ω cm or more. Charging with low energy electrons produced resistivity data reasonably similar to that produced by charging with 4 keV electron beams. The gamma irradiation, followed by one month of rest under no bias, reduced the resistivity by a factor of 2 in both materials. Raising temperature from the normal 20–50 °C reduced the resistivity in Kapton, but the reduction was not observable in the LaRC-SI material. Even after 20 days of “constant-bias” the rate of fractional charge loss continued to decline indicating that polarization current rather than ideal resistivity may be the cause of most voltage decay, and that the true resistivity is higher than the values determined here, and much higher by several orders of magnitude than the values determined in classical ASTM and IEC test procedures.