60th Annual Report on Research 2015

Publications

Rodriguez, B., Kalathur, H. and Lakes, R. S., A sensitive piezoelectric composite lattice: experiment, Physica Status Solidi, 251(2) 349-353 (2014).

Abstract

Lattice structures based on bimorph rib elements are fabricated and studied experimentally. The effective piezoelectric sensitivity d is observed to be much larger, by a factor of at least 10,000, in magnitude than that of material comprising the lattice ribs. Bending of the ribs in response to input voltage is responsible for the large sensitivity.

Faust, D. and Lakes, R. S., "Temperature and Substrate Dependence of Piezoelectric Sensitivity for PVDF films", Ferroelectrics, 481(1), 1-9 Sept. (2015).

Abstract

The piezoelectric sensitivity, via both the direct and converse effects, for commercial polyvinylidene fluoride (PVDF) films is measured as a function of temperature and frequency, for two substrates, nylon and aluminum. The average effective sensitivity for the PVDF on nylon was 29 pm/V for both direct and converse effect, independent of frequency over 0.5 to 200 Hz. Direct effect sensitivity on aluminum substrate was about a factor of five greater. Analysis of effects of substrate's thermal and elastic constraint disclosed insufficient effect to account for the observed increase of sensitivity. Flexoelectric effects were considered as the cause. The direct and converse sensitivity increased at approximately 2% per degree Celsius over the frequency range 0.5 to 200 Hz

Lakes, R. S., Third-rank piezoelectricity in isotropic chiral solids, Appl. Phys. Lett., 106, 212905, May (2015).

Abstract

The highest symmetry in which piezoelectricity was thought to occur is cubic. Here, it is shown that third rank piezoelectricity can occur in isotropic chiral solids. Polarization is coupled via an isotropic third rank tensor to the antisymmetric part of the stress. Asymmetric stress can occur if balanced by moments distributed over area or volume. Such moments occur in heterogeneous solids, in which there exists a characteristic length associated with the microstructure: the Cosserat or micropolar solids. Effects associated with nonzero structure size are predicted, including radial polarization in response to torsion. These effects do not occur in gradient type flexoelectric materials; they are governed by a different tensorial rank and symmetry.

Faust, D. and Lakes, R. S., "Reciprocity failure in piezoelectric polymer composite", Physica Scripta, 90 085807 (2015).

Abstract

Reciprocity principles, which entail equivalent outcome on exchange of cause and effect, are widely used and accepted. We present a piezoelectric composite system designed so that reciprocity does not hold; sensitivity is substantially enhanced. Reciprocity failure is observed in which the piezoelectric direct effect (stress causes polarization) sensitivity d is unequal to the converse effect (electric field causes deformation) sensitivity d. The piezoelectric polymer PVDF under isothermal conditions on a polymer substrate obeys reciprocity. Reciprocity failure occurs when a bumpy contact condition causes stress gradients. Reciprocity failure with strong frequency dependence occurs in the presence of thermal flux that is modulated by force: a non-equilibrium condition. Non-reciprocal effects give rise to a maximum enhancement of a factor of five in sensitivity.