Direct evaluation of thermal-pulse, thermal-step, and thermal-wave results for obtaining charge and polarization profiles

The most difficult problem with the thermal techniques for the investigation of charge and polarization profiles in electrets is the signal interpretation. Because of the diffusive character of heat transport, the useful information is distributed on a square root t scale for the thermal-pulse and thermal-step techniques and on a square root omega scale for the thermal-wave technique. The resolution of the techniques is high near the heated surface and becomes smaller inside the film. It is shown that properly scaled, measured signals already represent a good estimate of the electric-field or polarization distribution. Experimental results from thermal-pulse investigations of 25 mu m thick electron-beam charged Teflon films, and of nominally well poled 25 mu m thick PVDF films as well as thermal-wave investigations of a 1.3 mu m thick double-layer of nonlinear optical (NLO) polymers with two different glass-transition temperatures demonstrate the strengths of the concept.