Ying-ming Poon

Dielectric and pyroelectric properties of lead zirconate titanate/polyurethane composites

0-3 composite ranging between 0 and 3, of ferroelectric ceramic lead zirconate titanate (PZT) and thermoplastic elastomer polyurethane (PU) were fabricated. The pyroelectric and dielectric properties of the hot-pressed thin film samples of various PZT volume fractions were measured. The experimental dielectric permittivities and losses agreed reasonably well with the Bruggeman model. The room temperature pyroelectric coefficients of the composites were found to increase linearly with PZT volume fraction and substantially larger than expected. For example, for a composite with 30% PZT, its pyroelectric coefficient is about 90μC∕m2K at room temperature, which is more than tenfold of a PZT∕PVDF composite of the same ceramic volume fraction. We propose a model in which the electrical conductivity of the composite system is taken into consideration to explain the linear relationship and the extraordinarily large pyroelectric coefficients obtained.

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.

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...

Dielectric response of graded composites having general power-law-graded cylindrical inclusions

The dielectric response of graded composites having general power-law-graded cylindrical inclusions under a uniform applied electric field is investigated. The dielectric profile of the cylindrical inclusions is modeled by the equation epsilon(i)(r)=c(b+r)(k) (where r is the radius of the cylindrical inclusions and c, b and k are parameters). Analytical solutions for the local electrical potentials are derived in terms of hypergeometric functions and the effective dielectric response of the graded composites is predicted in the dilute limit. Moreover, for a simple power-law dielectric profile epsilon(i)(r) = cr(k) and a linear dielectric profile epsilon(i)(r) = c(b + r), analytical expressions of the electrical potentials and the effective dielectric response are derived exactly from our results by taking the limits b -> 0 and k -> 1, respectively. For a higher concentration of inclusions, the effective dielectric response is estimated by an effective-medium approximation. In addition, we have discussed the effective response of graded cylindrical composites with a more complex dielectric profile of inclusion, epsilon(i)(r)=c(b+r)(k)e(beta r). (c) 2005 American Institute of Physics.

Temperature dependence of the complex effective piezoelectric coefficient of ferroelectric 0-3 composites

Temperature dependence of the complex effective piezoelectric coefficient d31* for a ferroelectric 0-3 composite of small ceramic volume fraction has been studied. Theoretical predictions are based on our previously derived explicit expression of d31 for a dilute dispersion of spherical particles in a continuous matrix [C. K. Wong, Y. M. Poon, and F. G. Shin, Ferroelectrics 264, 39 (2001); J. Appl. Phys. 90, 4690 (2001)]. Comparison is made with the well-known Furukawa’s model and their experimental measurements on a lead zirconate titanate (PZT)/epoxy composite with 13 vol % PZT [T. Furukawa, K. Fujino, and E. Fukada, Jpn. J. Appl. Phys. 15, 2119 (1976)], covering a wide temperature range from −140 to +140 °C. The real part and the imaginary part of the effective piezoelectric coefficient for the composite are investigated separately. Predictions for the real part of d31* agree well with the observed values for temperatures larger than 60 °C, but are larger than the observed values for lower temperatures...

Effective conductivity of a composite of poly-dispered spherical particles in a linear continuum

Rayleighs method is used to find the electric potentials of a composite of poly-dispered spherical particles in a linear continuum in an external electric field. Based on the solutions of potentials, analytical formula for the effective electric conductivity is derived. Based on the formula, several factors, such as the number of spherical inclusions, the spatial distribution of the spheres, the contrast ratio σi/σh (where, σi and σh are the conductivities of the spherical inclusion and the host medium, respectively) and volume fraction of the inclusions, are discussed. Our results show that at high volume fraction, the effective conductivity is also affected by the spatial distribution of the inclusions.

Effective piezoelectric coefficients of ferroelectric 0–3 composites

Abstract We studied theoretically the effective piezoelectric strain coefficients of a 0–3 composite comprising spherical ferroelectric inclusions dispersed in a ferroelectric matrix. Tensile loading and hydrostatic loading conditions were considered. Explicit expressions for the effective d 31, d 33 and d h coefficients were derived for the case of low volume concentration of dispersed particles in terms of the electro-mechanical properties of the constituents. The theoretical predictions of the piezoelectric coefficients were compared with experimental data from previous works and good agreement was obtained.

The bending mechanism of thermoplastic polyurethane under an applied electric field

A thin strip of thermoplastic polyurethane (PU) was found to bend to the cathode side under the application of a DC electric field. As the field was reversed, the bending went to the anode side at first and then switched back to the cathode side. The mechanism responsible for this bending and switching phenomena was investigated by means of monitoring the charging and discharging current during the application of electric field. It was found that the bending direction and magnitude can be correlated to the direction and magnitude of current flow. A detailed analysis of the current response shows that it is most likely a space charge limited current. A model based on a charge accumulation at the sample surface will also be presented in this paper. It can explain the bending as well as switching effect of the PU film. In order to visualize the charge contents in the PU film, we apply the thermally stimulated discharge current (TSDC) technique for the measurement. Our preliminary result has confirmed the existence of charged layers at the surfaces of the PU film.

Dielectric response of spherically anisotropic graded piezoelectric composites

A graded piezoelectric composite consisting of a spherically anisotropic graded piezoelectric inclusion imbedded in an infinite nonpiezoelectric matrix, with the physical properties of the graded spherical inclusion having a power-law profile with respect to the radial variable r, is studied theoretically. Under an external uniform electric field, the electric displacement field and the elastic stress tensor field of this spherically anisotropic graded piezoelectric composite are derived exactly by means of displacement separation technique, based on the governing equations in the dilute limit. A piezoelectric response mechanism, in which the effective piezoelectric response vanishes along the z direction (or x,y directions), is revealed in this kind of graded piezoelectric composites. Furthermore, it is found that the effective dielectric constant decreases (or increases) with the volume fraction p of the inclusions if the exponent parameter k of the grading profile is larger (or smaller) than a critical value

Predicting the Effective Piezoelectric Coefficients of a 0-3 Piezoelectric Composite Material Using New Stress Field Factors

We have derived four new stress field factors by taking into account the interaction effects between the inclusions in a 0-3 composite material and then used them to calculate the two effective piezoelectric coefficients (d 31 and d 33 ) for some 0-3 piezoelectric composite materials. Predicted values are compared with published experimental data and it is found that they agree with each other reasonably well, even with a high volume fraction of the inclusions.