Measurement of space charges

This technique, usually referred to as the PEA method, was originally devised by Maeno, Takada and co-workers (Takada Sakai, 1983 and Maeno et al., 1989) in order to study the way in which space charge develops near electrodes in a solid polymer under the application of a high voltage and deposits during electron beam irradiation. 

The response of the detector/amplifier system distorts the signal from the acoustic pulse. This can be overcome by deconvolution of the received signal, using as reference the signal generated from an induced surface charge obtained by application of a known steady voltage across the sample. 

The second technique, usually referred to as the PWP method, is really complementary to the first one in the sense that an acoustic wave is now used as the primary stimulus to displace the space charge and thereby produce an electrical signal. In the previous PEA method the primary stimulus was the electrical pulse whose direct effect on the space charge produced an acoustic 

In the absence of space charges, but where an electric field is applied across the insulating sample, the current transient reflects the induced charges at the electrode/insulator interface. Since these charges occupy a negligible depth, the profile of the current peaks reflects the profile of the pressure pulse itself. This enables a calibration of the measurement system to be made. 

In a typical experimental measurement a Nd YAG laser fires a 5 ns pulse on to the target electrode. It is convenient to use a polymeric material for the electrode, made conductive by the addition of conductive carbon black, to achieve intimate contact with the specimen. Adding a thin layer of a volatile liquid to the electrode surface just prior to firing the laser greatly enhances the magnitude of the signal, by helping to increase the size of the pressure pulse 

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Support for the applicability of this model to an explanation of the Meyer-Neldel rule comes from measurements of space-charge limited currents in anthracene where a correlation (see Fig. 20) has been found between the total density of traps H and the distribution parameter Tc (Owen et al, 1974). It has been shown that this effect is not fortuitous as suggested by some workers... 

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