Experimental investigation of electrostrictive polarization biased direct apparent piezoelectric properties in polyurethane elastomer under quasistatic conditions

Polyurethane elastomer was recently discovered to demonstrate a very high field induced electrostrictive response. In this work an experimental setup, consisting of an electric circuit and a mechanical system, was designed and constructed for the measurement of the electrostrictive polarization biased apparent piezoelectric response of polyurethane elastomers in a direct piezoelectric effect under quasistatic conditions. The electric circuit design allows the application of a direct current (dc) bias electric field to the sample and the possibility of picking up the generated quasistatic electrical signal separately. The mechanical system provides the function of a vibration source from which the stress and strain of the samples can be measured. Therefore, such effective piezoelectric properties as d₃₁ and k₃₁ can be measured. The electromechanical coupling coefficient was derived by two different methods. One was from the deduction based on the piezoelectric equations. The other was from the calculation based on the basic definition of the electromechanical coupling coefficient (i.e., through the exact measurement of input mechanical energy and output electric energy). In the latter case, the internal resistance of the sample and the dc bias blocking capacitor were found to be the critical factors for precision determination of the total electrical energy output. The different approaches led to close agreement. The effective d₃₁ can be 184 pC/N under a 25 MV/m bias electric field in a 30-μm thick sample, which is much higher than that of typical piezoelectric polymers.