Space charge and polarization in crosslinked polyethylene

We present, for XLPE samples cross-linked by the silane process, polarization / space charge profiles obtained using the laser-intensity- modulation-method (LIMM), with an estimated spatial resolution of 2 μm. The samples, 100 μm thick with vacuumevaporated aluminium electrodes, were subjected to dc fields of 50 kV/mm at room temperature for 18 hr. The scale transformation technique was used to obtain approximate pyroelectric coefficient profiles without needing to apply tedious mathematical convolution procedures. Separation of the polarization and space charge contributions to the profiles was attempted by comparing profiles (a) at various times after the end of poling, (b) for opposite polarity poling, and (c) after prolonged heating in vacuum before poling. The space charge profiles showed mostly positive charge, concentrated near the electrodes with densities of order 10-20 C m^-3 in those regions. The heat treatment in vacuum produced small changes in the space charge profiles, but negligible change in the FTIR spectra.