Effect of electric conductivity in ferroelectric structures

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Effect of electric conductivity in ferroelectric structures


Author: Zhou, Yan
Title: Effect of electric conductivity in ferroelectric structures
Degree: M.Phil.
Year: 2006
Subject: Hong Kong Polytechnic University -- Dissertations
Electric conductivity
Department: Dept. of Applied Physics
Pages: ix, 148p. : ill. 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1957952
URI: http://theses.lib.polyu.edu.hk/handle/200/971
Abstract: Although ferroelectric materials are usually regarded as insulators, their small but finite conductivities have recently been taken into account to satisfactorily explain a number of new phenomena, which otherwise may be difficult to understand. Chan et al. [Chan et al, 2004] have demonstrated that the time-dependent space-charge-limited (TDSCL) conduction could be a possible origin of the polarization offset in graded ferroelectrics. Wong et al. [Wong et al., 2002; Wong and Shin, 2005] have systematically investigated the effects of electrical conductivity of the constituents on the dielectric, pyroelectric and piezoelectric properties of ferroelectric particulate composites. In this thesis, we focus on the study of the effect of electric conductivity in two kinds of ferroelectric structures: ferroelectric thin films and ferroelectric 0-3 composites. We have investigated theoretically the dependence of the "polarization offset" of hysteresis loops (also known as vertical or charge offset) on various parameters in compositionally graded ferroelectric thin films. Our model adopts the Landau-Khalatnikov equation to describe hysteresis behavior and takes the time-dependent space-charge-limited conductivity into account to study the effects of polarization and permittivity gradients, thickness and charge mobilities in graded ferroelectric thin films. We have found that both polarization and permittivity gradients are requisite for the occurrence of the offset phenomena. It is also found that larger gradients of remanent polarization and permittivity, a smaller thickness and larger charge mobilities can generally enhance the vertical offsets. The qualitative agreement between simulation and experiment further supports our notion that the asymmetric conduction current arising as a result of the composition gradient is an important factor leading to the "polarization offset" phenomenon. Theoretical models have been developed to explain the horizontal shifting of the measured D-E hysteresis loops (imprint effect) of ferroelectric thin films. The time-dependent space-charge-limited conduction is adopted to describe the spatial and temporal variation of electrical conductivity in the films. It is found that the imprint phenomenon can be explained by considering three mechanisms or their multiple effects: (1) stress induced by film/electrode lattice mismatch or clamping, (2) domain pinning induced by e.g. oxygen vacancies, or (3) degradation of ferroelectric properties in film/electrode surface layers. For film type ferroelectrics, we also studied theoretically the enhancement of remanent polarization and dielectric permittivity of interfacial-coupled ferroelectric superlattices based on the Landau-Ginzburg theory. Our model adopts the Landau-Khalatnikov equation to describe hysteresis behavior and takes the time-dependent space-charge-limited conductivity into account to investigate the ferroelectric and dielectric properties of ferroelectric superlattices. The results are in good agreement with recent experimental observations on the enhancement of remanent polarization and permittivity of BaTiO3/SrTiO3 superlattices and heterolayered Pb(Zr, Ti)O3 thin films [Shimuta et al, 2002; Pontes et al, 2004]. The effect of electrical conductivity is also essential to understanding the behavior of ferroelectric composites. We use a simple dynamic model to include the effect of the electrical conductivity of both constituent phases on the electrostriction behavior of a 0-3 composite comprising ferroelectric ceramic particles (lead-zirconate-titanate) embedded in a polymer matrix (polyurethane). Ohmic conductivity has been demonstrated to be sufficient to account for the electrostriction strain characteristics of this ferroelectric 0-3 composite system. Good agreement up to 30% volume fraction of lead-zirconate-titanate (PZT) is obtained for the butterfly shaped strain-electric-field loops with those observed in experiment [Lam et al. 2005]. The critical switching fields decrease while the electrostriction strain magnitudes increase with increasing PZT volume fraction, in good agreement with the experimental results. On the other hand, a relatively simple model for studying the magnetostrictively induced deformation behavior of magnetostrictive composites is also investigated. We use a conceptually simple and convenient approach to investigate magnetostriction for particulate composites of magnetostrictive inclusions in elastically isotropic, nonmagnetostrictive matrices. We calculate the magnetostriction responses of composites containing Terfenol-D and nickel. The macroscopic longitudinal strains parallel to the applied magnetic field for Terfenol-D/glass composites and both longitudinal and transverse strains for nickel/epoxy composites are obtained. Comparison with published experimental data [Nersessian et al. 2004; Chen et al. 1999a] indicates that good agreement up to very high volume fraction of inclusion particles has been achieved. The magnetostriction model is then extended to study magnetoelectric (ME) effect of mildly conducting magnetostrictive/piezoelectric particulate composites by taking the constituents' Ohmic conductivity into account. For illustrative purpose, we calculate the magnetoelectric voltage coefficients of particulate composites of nickel ferrite and lead zirconate titanate (NFO/PZT). The effects of electrical conductivity of both phases of the composite material are also studied. Our calculation demonstrates that the ME effect is larger for composites with low conductivity constituents and that the range over which notable ME effect is observed becomes smaller when conductivity increases. These trends are observed in recent experiments with particulate composites of nickel zinc ferrite/lead zirconate titanate (NZFO/PZT) and nickel ferrite/barium lead zirconate titanate (NFO/BLZT) [Laletin et al. 2002; Srinivasan et al. 2004]. The calculated longitudinal magnetoelectric voltage coefficient gradually increases with increasing frequency f, which is also experimentally observed in particulate composites of NFO/PZT [Zeng et al. 2004].

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