Measurements of Nonlinear Dielectricity in Ferroelectric Polymers

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Abstract
Experimental procedures and exemplifying results are described in relation to a nonlinear dielectric investigation of ferroelectric polymers. The apparatus developed here was used to apply a sinusoidal electric field and to detect fundamental and higher harmonic electric displacements with a 0.01% accuracy by means of digital sampling and Fourier transform techniques. Three types of experiments were made for PVDF. Measurements at relatively low fields determined the nonlinear dielectric constants in that the second-order constant was shown to reflect the polarity of the sample and the third order one was positive being consistent with cooperative dipoles. The ferroelectric hysteresis loops obtained at very high fields were analyzed in terms of the higher-order Fourier coefficients. The use of a double frequency wave determined the field-dependent linear dielectric constants which showed a maximum during polarization reversal.