THERMAL PULSE STUDY OF THE POLARIZATION DISTRIBUTIONS PRODUCED IN POLYVINYLIDENE FLUORIDE BY CORONA POLING AT CONSTANT-CURRENT

A thermal pulse study of the polarization profiles in samples of 12-mu m-thick, biaxially oriented polyvinylidene fluoride after corona polin g under approximately constant-current conditions, using a modified co rona triode in atmospheric air, is reported. An electrical characteriz ation of the corona triode is also reported to show how it may be oper ated in the constant-current mode. Samples poled without electrode on the corona-exposed surface show polarization distributions sensitive t o the corona polarity, with polarization depletion on the corona side of the samples when the corona is positive. Polarization-reversal expe riments show switching inhomogeneities with a pronounced dependence on the initial corona polarity. The above observations are consistent wi th a simple model in which positive charges from the positive corona p artially penetrate the sample during poling and cause an inhomogeneous reduction of the poling field.