Author: Wong, Chung-kwan
Title: Conductivity and interfacial charge induced phenomena in ferroelectric films and composites
Degree: Ph.D.
Year: 2005
Subject: Hong Kong Polytechnic University -- Dissertations
Ferroelectric thin films
Ferroelectric crystals
Department: Department of Applied Physics
Pages: xiii, 250 leaves : ill. ; 30 cm
Language: English
Abstract: Ferroelectric materials are polar dielectrics [Burfoot, 1970] which normally possess small but finite electrical conductivity. We believe that conductivity in ferroelectrics can induce new phenomena or modify known physical phenomena, which may be difficult to understand if the materials are regarded as perfectly insulating. In this thesis, some "anomalous" phenomena are investigated for which the origins are still under debate in literature, and we suggest that these may very well be manifestations of electrical conduction and electric charges. Ferroelectric systems of interest to this investigation include composites and films. Electrical conductivity in ferroelectric composites allows free charge to accumulate at the matrix-inclusion interfaces. We focus on the role of interfacial charge at such interfaces in ferroelectric 0-3 composites (normally, ferroelectric ceramic inclusions dispersed in polymer matrices) in the interpretation of their peculiar experimental results. In one of the experiments we considered, the initially polarized lead zirconate titanate (PZT)/polyurethane (PU) composites are subjected to an increasing electric field in the reversed direction [Wong et al., 2001b]. Estimations based on previous models [e.g., Furukawa et al., 1976] suggest that the local electric field in the PZT particles at switching is an order smaller than the coercive field of PZT. We use a simple dynamic model to include the effect of accumulated charge at the matrix-inclusion interfaces on the polarization reversal characteristics of this ferroelectric 0-3 composite. Assuming that initially the interfacial charge has a magnitude which balances the depolarization field in the polarized PZT inclusions, theoretical calculation shows that the phenomenon of PZT switching at low fields can be understood based on this model. The effect of interfacial charge on the piezoelectric properties of ferroelectric 0-3 composites and the effect of electrical conductivity on their dielectric and piezoelectric properties are also investigated. Our previously developed model [Wong et al., 2001a] has been extended to include the additional contribution from the deformation of the inclusion particles (for discussing the effect of interfacial charge) due to the applied stresses in piezoelectric measurements, and for discussing the effect of conductivity to include its contribution as well as the frequency of measurement. The interfacial charge effect in this model provides an explanation to the anomaly that almost no enhancement in the effective piezoelectric coefficient is obtained when higher ferroelectric ceramic content is put into the composite samples in which the constituents are polarized in opposite directions, whilst the consideration of the contribution from conductivities can be used to understand the surprisingly high piezoelectric d33 values reported by e.g. Chen et al. [Chen et al., 1998]. Explicit expressions in closed form have been derived for the effective permittivity, d33, d31 and dh coefficients. Theoretical predictions show good agreement with published experimental data. Phenomena induced by electrical conductivity in other ferroelectric systems have also been studied. We consider the effects of electrical conductivity on the dynamic polarization behavior of ferroelectric films. Using a parallelogram-like P-E hysteresis model for the film material, explicit expressions are obtained for describing the D-E loops of ferroelectric films as would be measured from a Sawyer-Tower circuit [Sawyer and Tower, 1930] which originally assumes the measured sample is insulating. The calculation shows that resistive losses inflate the ferroelectric loop to a varying extent. In addition, an asymmetric conductivity (conductivity is different for positive and negative fields) will result in polarization offsets with the magnitude and direction depending on the values of the asymmetric conductivities of the material. When a dead layer (i.e. degraded polarization) [Zhou and Newns, 1997] with asymmetric conductivity is included at the film-electrode interface, anomalous horizontal (voltage) shift of the hysteresis loop is obtained. The inflated loops and the offset phenomena, which have been observed experimentally [e.g., see Zheng et al., 1996c; Wu et al., 2004], will be discussed by means of our model. Simple exact formulas have been derived and the effects of electric, dielectric and ferroelectric parameters as well as the applied field on the "apparent" polarization are also examined. Theoretical simulations have also been performed to study the anomalously large polarization shift behavior in compositionally graded ferroelectric films and the bias field dependent dielectricity of ferroelectric thin films. The model is capable of modeling saturated and unsaturated hysteresis behavior under arbitrary fields. The spatial variation of properties across the graded film thickness and the effect of dead layer in a thin ferroelectric film are modeled by a multi-layer structure. Moreover, a time-dependent space-charge-limited conduction is adopted to describe the temporal variation of electrical conductivities in graded ferroelectrics. On the other hand, in order to model the e-E loops in thin films, the simulated minor hysteresis loops at different bias fields are used to calculate the variation of dielectric permittivity with field. Simulated D-E and e-E loops are compared with the experimental data on a lanthanum modified lead zirconate titanate (PLZT) graded ferroelectric [Boerasu et al., 2000b] and a barium zirconate titanate (BZT) thin film [Tohma et al., 2003]. Effects of charge mobility in graded ferroelectrics, asymmetric conductivity in thin films and the applied AC field amplitude on the simulated D-E and e-E loops are also examined.
Rights: All rights reserved
Access: open access

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