Evolution of Experimental Techniques for the Study of the Electrical Properties of Insulating Materials

When an electric field is applied to an insulating material three basic processes can take place: the dipoles tend to rotate, ions migrate and space charge can be injected at the interfaces, depending on such parameters as temperature or applied electric field. For many years, a direct analysis was impossible since the experimentally observable variables gave only an average of what was taking place in the samples under study. Many methods were developed over the years combining available measuring techniques with theoretical hypotheses. They led to phenomenological descriptions but it appeared that a direct measurement of space charge or polarization distributions in the materials would be a way for understanding the physical processes involved. In this paper we will present the evolution of these methods and will describe on which at present seems the most promising. It uses the propagation in the sample of a pressure wave which acts as a virtual probe sensitive to charge, field or potential. A very elegant technique to generate this pressure wave involves the use of a short-duration laser pulse. It will be shown how this can find applications in a very large number of areas. Examples will be given in such different fields as electrets, HV insulation, and transducer materials.