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.Department of Applied PhysicsMaterials Research CentreAuthor name used in this publication: Y. T. OrAuthor name used in this publication: C. K. WongAuthor name used in this publication: B. PlossAuthor name used in this publication: F. G. Shi

Vinylidene fluoride-trifluoroethylene copolymer films of molar ratio 70/30 with thickness of about 1 [ ]m have been deposited from solution in ethyl methyl ketone to a glass substrate with an aluminum electrode by spin coating. The solution has been filtrated through a PTFE membrane filter with pore size 0.2 [ ]m directly before spin coating or it has been used as is (unfiltrated). After deposition of a top electrode, the samples have been polarized by hysteresis loops with an electric field amplitude of about 100 V/[ ]m. In samples, annealed at temperature 145[ ]C for 3 h, a high remanent polarization of about 7.5 [ ]C/cm²has been achieved, without significant differences between samples fabricated of filtrated or unfiltrated solution. Spherulitic lamella are growing in films fabricated of filtrated solution when they are heated above the melting temperature to 159[ ]C for 3 min before the further annealing process at 145[ ]C. These films show substantially lower remanent polarization below 4 [ ]C/cm². Pyroelectric images recorded with a pyroelectric laser scanning microscope show that the spherulites have very small pyroelectric activity, i.e., the spherulites consist of flat-on lamella. In contrast, no spherulitic lamella are growing in films fabricated of unfiltrated solution heated above the melting temperature, melted and annealed under the same conditions. An explanation for this observation is that filtrating changes the structure of the copolymer in solution from polymer coil to rod. Copolymer rods deposited on a substrate will crystallize in flat-on lamella when heated above the melting temperature, in contrast to copolymer coils which crystallize in edge-on lamella.

The influence of semiconductor particle concentration and photoexcitation on the electrical and ferroelectric properties of ferroelectric-semiconductor-composites was investigated. For this purpose, 32 µm thin films of poly(vinylidene fluoride-co-trifluoroethylene) with (Cd:Zn)S particle concentrations of between 0 and 20 vol % were fabricated and characterized by scanning electron microscopy, Fourier transformed infrared spectroscopy, X-ray diffraction, and optical spectroscopy. It was shown that the particle concentration has only a negligible influence on the molecular structure of the polymer but strongly determines the optical properties of the composite. For (Cd:Zn)S particle concentrations below 20 vol %, the I-V characteristics of the composites is only marginally affected by the particle concentration and the optical excitation of the composite material. On the contrary, a strong influence of both parameters on the ferro- and pyroelectric properties of the composite films was observed. For particle fractions that exhibit ferroelectric hysteresis, an increased remanent polarization and pyroelectric coefficient due to optical excitation was obtained. A theoretical approach that is based on a “three phase model” of the internal structure was developed to explain the observed results.

The influence of optical excitation intensity on the electrical, ferroelectric and pyroelectric properties of ferroelectric-semiconductor-composites was investigated. For this purpose, composite thin films consisting of poly(vinylidene fluoride-co-trifluoroethylene) and 10 vol % (Cd:Zn)S particles with a thickness of 34 µm were fabricated. The samples were used to measure the absolute pyrocoefficient and to determine the relative pyroelectric depth profile using Laser Intensity Modulated Method. It was shown that a polarization of the samples without an optical excitation at the utilized relatively small peak-to-peak voltages could not be verified by the Sawyer–Tower circuit and the measurement setup of the pyroelectric coefficient, respectively. Both remanent polarization and pyroelectric coefficients increased with increasing optical excitation intensity during poling as well as increasing peak-to-peak voltage. The pyrocoefficient shows a temporal decay in the first hours after poling. The specific heat and thermal conductivity or the thermal diffusivity are required for the calibration of the pyroelectric depth profile. Rule of mixture and photo-acoustic investigations proved that the thermal properties of the utilized composites do not differ significantly from those of the pristine polymer. Based on the pyroelectric depth profile which is proportional to the polarization profile, the existing “three phase model” has been extended to generate a replacement circuit diagram, explaining the local polarization due to the optical excitation dependency for both local resistivity and local field strength.

Polymer–semiconductor hybrid materials or composites have been investigated with respect to their microstructure, optical, photoconductive, and ferroelectric properties. For this purpose, either CdSe quantum dots or (Cd:Zn)S microparticles were dispersed in poly(vinylidenefluoride-trifluoroethylene) solution and hot pressed to films. In both material systems, the electrical conductivity and the polarization behavior could be controlled by the intensity of the optical excitation. The simultaneous high optical transparency of the CdSe quantum-dot-based hybrid materials makes them particularly interesting for applications in the field of flexible, high-resolution sensors.