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The effect of hydrostatic pressure on the dielectric characteristics of polyethylene/aluminum oxide nanocomposites is reported in the present work. Nanocomposite specimens with a thickness of 0.2 mm were placed in a hydrostatic press at pressures ranging from 50 to 200 MPa. Then, ac and dc breakdown and dielectric spectroscopy measurements were carried out on the original and treated specimens. In comparison to the dielectric properties of the original specimens, the results of the treated specimens demonstrate some interesting dielectric behaviors. For example, the breakdown strengths of the treated specimens are enhanced, and the permittivity and dielectric loss are reduced after the hydrostatic pressure treatments. It is found that the movements of molecular chains and polar groups are restricted especially in the interfacial region, leading to the reduction in permittivity and dielectric loss of specimens after hydrostatic pressure treatment. The improvement in breakdown properties is due to the reduction in free volume or a more uniform size distribution of free volume in the nanocomposites after hydrostatic pressure treatments.
Li et al. (Tue,) studied this question.