Chung-kwan Wong

Electrical conductivity enhanced dielectric and piezoelectric properties of ferroelectric 0-3 composites

We have investigated the effects of electrical conductivity of the constituents on the dielectric and piezoelectric properties of ferroelectric 0-3 composites. The time-dependent internal electric fields are first derived, which can be induced by an applied ac field in dielectric measurement or stress in piezoelectric measurement. Our previously developed model [C. K. Wong, Y. M. Poon, and F. G. Shin, J. Appl. Phys. 90, 4690 (2001)] has been extended to include the additional contribution from the electrical conductivities and the frequency of measurement, which can be significant for ceramic/polymer composites possessing high conductivity in the matrix phase. The model provides an explanation to the surprisingly high piezoelectric d33 values reported by, e.g., Chen et al. [Sens. Actuators, A 65, 194 (1998)]. Explicit expressions for the transient and steady-state responses are given and the effective permittivity, d33, d31, and dh coefficients have been derived.

Polarization behavior of ferroelectric multilayered composite structures

We model the polarization behavior of ferroelectric multilayered composite structures including the double layer and the triple layer under the action of variable fields. The model takes into consideration the ceramic/polymer interfacial charge and the nonlinear hysteresis nature of the ferroelectric constituent materials to study the poling process of multilayered structures. The results obtained are compared with available experimental results on double-layer lead zirconate titanate/polyvinylidene fluoride-trifluoroethylene and triple-layered triglycine sulphate/polyvinylidene fluoride-trifluoroethylene/triglycine sulphate systems. In general the broad features of the experimental results were reproduced, and fairly good agreement between the predictions and the experimental results was found. The work also demonstrates that electrical conductivity in the ferroelectric materials is an important factor controlling the poling process.

Explicit formulas for effective piezoelectric coefficients of ferroelectric 0-3 composites

Explicit formulas have been found for the effective piezoelectric coefficients of a 0-3 composite of ferroelectric spherical particles in a ferroelectric matrix. Tensile loading and hydrostatic loading conditions were studied. Assuming that both phases are dielectrically and elastically isotropic, explicit expressions in simple closed form for the effective d33, d31 and dh coefficients were derived in terms of the constituents’ piezoelectric coefficients and the dielectric and elastic properties of the composite and constituents. Prediction of the piezoelectric coefficients for specific composite systems was compared with experimental values from published works, and good agreement with data was obtained. Goodness of fit is not limited to low volume fraction of inclusions.

A possible mechanism of anomalous shift and asymmetric hysteresis behavior of ferroelectric thin films

We studied theoretically the hysteresis behavior of ferroelectric thin films. The anomalous ferroelectric response is discussed by use of a bilayer model. Electrical conductivities of the films have been taken into account. To model the effects of the inhomogeneity of polarization and permittivity across the interface, the film is assumed to possess a secondary dielectric∕ferroelectric phase (a dead or passive layer) with asymmetric conductivity. This configuration is found to produce large shifting (along the field axis) and deformation of the measured hysteresis loop. This is a manifestation of the asymmetric conductivity of the material. Theoretical calculation based on this model shows that the observed phenomena of shifted and skewed hysteresis loop in ferroelectric thin films can be explained in this way.

Effect of interfacial charge on polarization switching of lead zirconate titanate particles in lead zirconate titanate/polyurethane composites

A simple model has been developed to include the effect of accumulated charge at the matrix–inclusion interface in ferroelectric 0–3 composites on the polarization reversal characteristics. This is used to illustrate a recent set of experimental results on initially polarized lead zirconate titanate (PZT)/polyurethane (PU) composites subjected to an increasing electric field in the reversed direction. Estimations based on previous models [e.g., T. Furukawa, K. Fujino, and E. Fukada, Jpn. J. Appl. Phys. 15, 2119 (1976)] suggest that the local electric field in the PZT particles at switching was an order smaller than the coercive field of PZT. By introducing the effect of interfacial charge and assuming that initially the charge has a magnitude which balances the depolarization field in the polarized PZT inclusions, theoretical calculation based on this image shows that the phenomenon of PZT switching at low fields can be explained as a manifestation of the depletion of interfacial charge which can no longe...

Modeling of poling, piezoelectric, and pyroelectric properties of ferroelectric 0–3 composites

A model for the piezoelectric and pyroelectric activities of 0–3 composites of ferroelectric particles in a linear matrix has been developed based on the polarization behavior of the inclusions. The model is applied to simulate the piezoelectric activity of a lead zirconate titanate/epoxy system polarized under different fields and the piezo- and the pyroelectric properties of lead titanate/polyvinylidene fluoride–trifluoroethylene 70/30 mol %] composites polarized with different poling times. The model predictions show reasonably good agreement with experimental data. The ability to predict the gross properties of 0–3 composites in terms of the poling field and poling time may be especially helpful in the making of practical composites with particular properties.

Modeling saturated and unsaturated ferroelectric hysteresis loops: An analytical approach

In ferroelectric materials, hysteresis behavior is very difficult to model due to its nonlinear and history-dependent characteristics. Among approaches that are able to describe unsaturated loops, many of them are either very complicated (numerical procedures must be employed) or the resulting loops contain some undesirable or defective features. In this work, a simple hysteresis model based on a special construction of the Preisach function is proposed. Explicit expressions for the polarization-field (P-E) responses under increasing and decreasing applied fields have been derived. The saturated and unsaturated P-E loops can be conveniently calculated by piecing together such responses. The technique is widely applicable to the modeling of ferroelectric hysteresis behavior of ceramics and polymers. As examples we study the applied field dependence of dielectric permittivity of a ferroelectric film and the remanent polarization of ferroelectric composites after ac poling. We find that the model predictions...

Effect of inclusion deformation on the magnetoelectric effect of particulate magnetostrictive/piezoelectric composites

We have investigated the magnetoelectric (ME) properties of particulate composites with magnetostrictive inclusions dispersed in a piezoelectric matrix. A simple model is proposed which includes a rarely discussed contribution from the shape deformation of the inclusion particles due to the magnetostrictive stresses generated under the action of an external magnetic field. The relative significance of this contribution is mainly determined by the ratio of the transverse and longitudinal magnetostrictive responses of the inclusion as well as the flexibility of the piezoelectric matrix phase. This model gives further insight on material selections for designing particulate ME composites.

Modeling of anomalous hysteresis behavior of compositionally graded ferroelectric films at low fields

We study the hysteresis behavior of compositionally graded ferroelectric films by theoretical simulations. Anomalous vertical (polarization) shift behavior of hysteresis loops measured by a Sawyer-Tower circuit at low/medium applied fields is investigated. The anomalous ferroelectric response is discussed by the use of a multilayer model to account for the variation of properties across the film thickness. Electrical conductivities of the ferroelectric layers have been taken into account and time-dependent space-charge-limited conduction has been adopted. The effects of charge mobility and the amplitude of applied field on the D-E loop shift were examined. Theoretical calculations are discussed in relation to the experimental data from previous works.

Explicit formulas for effective piezoelectric coefficients of ferroelectric 0–3 composites based on effective medium theory

Explicit formulas were derived for the effective piezoelectric stress coefficients of a 0–3 composite of ferroelectric spherical particles in a ferroelectric matrix which were then combined to give the more commonly used strain coefficients. Assuming that the elastic stiffness of the inclusion phase is sufficiently larger than that of the matrix phase, the previously derived explicit expressions for the case of a low volume concentration of inclusion particles [C. K. Wong, Y. M. Poon, and F. G. Shin, Ferroelectrics 264, 39 (2001); J. Appl. Phys. 90, 4690 (2001)] were “transformed” analytically by an effective medium theory (EMT) with appropriate approximations, to suit the case of a more concentrated suspension. Predictions of the EMT expressions were compared with the experimental values of composites of lead zirconate titanate ceramic particles dispersed in polyvinylidene fluoride and polyvinylidene fluoride-trifluoroethylene copolymer, reported by Furukawa [IEEE Trans. Electr. Insul. 24, 375 (1989)] an...