Controlling dielectric and pyroelectric properties of compositionally graded ferroelectric rods by an applied pressure

Abstract

The polarization, charge offset, dielectric, and pyroelectric properties of a compositionally graded ferroelectric rod inside a high-pressure polyethylene tube are studied using a thermodynamic model based on the Landau-Ginzburg-Devonshire formulation. The calculated distribution of the polarization in the rod is nonuniform, and the corresponding charge offset, dielectric, and pyroelectric properties vary according to the applied pressure. This behavior may be used as a convenient means to control these properties for design optimization. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2736308͔The use of graded ferroelectrics is essential in many applications, such as tunable microwave devices, transpacitors, and other dielectric devices.1-9 Distribution of the polarization in these materials is asymmetric and graded, and the hysteresis loop is not centrosymmetric, but displaced along the polarization axis, giving rise to an attendant charge offset. Properties such as the effective pyroelectric response, temperature dependence of the dielectric behavior, etc.1,3vary with the degree of asymmetry. Graded ferroic material systems studied recently are not restricted to cases where the gradient is in the composition, but include also those in which gradients also exist in the temperature and/or stress. Ban et al. 4 used a generalized Landau-Ginzburg model to study the offset hysteresis of polarization-graded ferroelectric materials, and found that the internal elastic energy due to the spatial variation of the self-strain tends to flatten the polarization distribution. The nonuniform polarization thus produced may then yield an asymmetrical hysteresis with "up" or "down" charge offset. Zhong et al. 1,3 investigated the pyroelectric response and dielectric permittivity of compositionally graded ferroelectric material, and found that the effective pyroelectric coefficients were directly related to the compositional gradient of the system. The effects on the polarization and charge offset of a ferroelectric due to a compression gradient normal to the substrate have also been studied. 6 Imposing a temperature gradient across ferroelectric materials, Fellberg et al. 7 observed up and down hysteresis offsets, similar to those found in polarization-graded ferroelectrics.In this article, the polarization, charge offset, dielectric, and pyroelectric properties of a compositionally graded ferroelectric rod inside a high-pressure polyethylene tube are considered. The calculations follow a thermodynamic approach based on the Landau-Ginzburg-Devonshire ͑LGD͒ model. The results are used to speculate on the feasibility of design optimization by adjusting the applied pressure.We consider a rod of compositionally graded ferroelectric ͑CGFR͒ material with radius R and height h, sandwiched between electrodes at both ends, and placed inside a highpressure polyethylene ͑HPPE͒ tube.8 A stiff steel cylindrical housing ͑SH͒ contains the HPPE tube and CGF rod. Moreover, two thin electrodes were sputter-deposited onto the surfaces perpe...

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