Aime S. DeReggi

Numerical evaluation of the dielectric polarization distribution from thermal‐pulse data

A method for numerically carrying out the Fourier analysis for the thermal‐pulse experiment is given. It is shown that it is possible to obtain the polarization distribution across the thickness of a thin film (25 μm) to within the limits set by the experimental data. For such films, resolution of the distribution to within 0.1 of the film thickness is possible. Results are given for the experiment by using a charge measurement rather than a voltage measurement. The effect of a finite‐width pulse is shown to cut off the Fourier coefficients in such a way as to smooth any distribution. Pulsing the sample alternately on both sides is shown to greatly increase the resolution of the experiment. Results for a PVF2 film and a P(VF2‐TFE) copolymer film show that interesting details can be found by the experiment.

Hysteresis measurements of remanent polarization and coercive field in polymers

An experimental method is described which allows estimation of remanent polarization and coercive field without assuming functional forms for the capacitive and electrical resistance terms. The method can be used to measure polarization in specimens with voltage‐dependent conductivity (often arising from the presence of ions in the specimens), voltage‐dependent capacitance, and significant amounts of space charge. It consists of: (1) performing bipolar current/voltage hysteresis loops to allow a steady state of remanent polarization and space charge to build up in the specimen, and (2) following a bipolar loop with two or more unipolar loops in which the polarization changes in the first unipolar loop. Both sinusoidal and linear time‐dependent applied voltages may be used. Automatic data processing of hysteresis loops is described for cases in which specimen behavior may be considered to be ideal.

Effects of space charge on the poling of ferroelectric polymers

Thermal pulse measurements of the polarization distribution in ferroelectric polymers and copolymers after poling give distributions concentrated to one side when the electrical conductivity under poling conditions is significantly different from zero. These results could be explained if net charge were present in the material during poling. The direct observation of distributed charge in nonpolar polymers after exposure to poling conditions support this explanation. However, high resolution laser thermal pulse measurements on nominally well poled samples of polyvinylidene fluoride reveal a sharp drop of polarization to near zero value at a depth of ∼1 μm from the surfaces which is not explained. The spatial resolution of the experiment is discussed.

Pyroelectricity and charge transport in a copolymer of vinylidene fluoride and tetrafluoroethylene

Previous investigations have shown that a copolymer of vinylidene fluoride and tetrafluoroethylene can be made piezoelectric and pyroelectric by the temporary application of an electric field.∗1, 2 For applied fields in excess of about 250 kV/cm, the electrical response became independent of applied field, independent of poling temperature between 0° and 60°C, and independent of poling time between 5 minutes and 7 hours.1, 2 Such behavior implied saturation of polarization, and values of polarization required to account for the piezoelectric response were well within the maximum to be expected from alignment of dipoles within the crystalline phase of the copolymer. Measurements on similar films now indicate that the polarization distribution within the films is non-uniform and varies with poling temperature. This paper presents these results and will propose a model of charge transfer within the film to reconcile the evidence for saturation polarization and nonuniform charge distribution.

Method of Evaluating the Thermal Stability of the Pyroelectric Properties of Polyvinylidene Fluoride Effects of Poling Temperature and Field

The ac pyroelectric response of a number of differently poled polyvinylidene fluoride films has been measured while the temperature was varied at a constant rate ∼5 °C/min from room temperature to near the melting temperature. The response first increases with increasing temperature, which is attributed to an increase of the thermal expansion coefficient and eventually decreases due to melting and/or loss of electric dipole orientation. The details of the temperature dependence are influenced in a reproducible manner by the poling variables, especially the poling temperature. The measurement is therefore proposed as a way of evaluating the effect of processing variables on the thermal stability of the piezoelectric and pyroelectric properties.

Study of poling behavior of biaxially stretched poly(vinylidene fluoride) films using the constant‐current corona triode

This article reports systematic results of corona poling measurements obtained on biaxially stretched polyvinylidene fluoride films charged in dry air. Charging was performed using the constant current corona triode. The dependence of the poling process on the sample thickness, charging current, and successive charging processes was investigated. Phase transitions from α to δ and to β phases were observed when virgin samples were corona charged. The thermal pulse technique showed that the polarization profiles during charging can be made consistently almost uniform and that the ferroelectric reorientation can be associated with the rising plateau region displayed on potential buildup curves.

Preexisting polarization and influence of electrode materials on PVF2 electrets as determined by thermal pulse and pyroelectric methods

A number of polyvinylidene fluoride (PVF2) electrets were prepared with different permutations of gold and aluminum electrodes and poled with dc fields up to 160 MV m−1 at room temperature. Polarization distributions were measured by the thermal pulse method and pyroelectric coefficients were determined. Quantitative measurements were made of a significant level of polarization in nominally unpoled PVF2 and a contact electrification mechanism was proposed. No consistent effects of electrode materials on polarization distribution were found. PVF2 poled at room temperature has its highest polarization near the center of the thickness in contrast to the results on samples poled at elevated temperatures and cooled inhomogeneously with the field applied.

Effects of poling field and time on pyroelectric coefficient and polarization uniformity in polyvinyl fluoride

Abstract Electroded films of polyvinyl fluoride (PVF) were polarized with DC electric fields as high as 315 MV m−1 for periods between 60 and 7200 seconds at room temperature. Thermal pulse data to determine the depth of polarization and pyroelectric coefficients were obtained on all samples. Large pyroelectric coefficients could be produced above a threshold mean polarizing field of 100 MV m−1; with mean fields greater than 200 MV m−1, all samples appeared to be uniformly poled with pyroelectric coefficients in the 12–16 μC m−2 K−1 range. The thermal pulse data definitely established that, in partially-poled samples, the polarization was strongest near the poling cathode. Mathematical techniques were developed for the computation of fraction poled from the thermal pulse data using a nonlinear least squares analysis.

Thermal stability of internal electric field and polarization distribution in blend of polyvinylidene fluoride and polymethylmethacrylate

It is confirmed that a melt‐quenched and annealed blend of 80 wt % polyvinylidene fluoride and 20 wt % polymethylmethacrylate has the β‐crystal form of PVDF with optical clarity and other properties that are desirable for a host material in a guest‐host system where the guest is an optically nonlinear dye that is orientationally stabilized by the strong internal electric field of a poled ferroelectric. Combined measurements, in such a blend, of the internal electric field Ei, the pyroelectric coefficient Cpyro, and the polarization distribution after electrically poling and subsequently thermally aging for 2 h intervals at temperatures up to 120 °C are reported. Ei and Cpyro increase with increasing poling field Ep. Ei was found to be as large as three times Ep. An unexpected thermal stability of Ei was observed at annealing temperatures above the glass transition temperature of 60 °C up to 100 °C when significant loss of Cpyro was measured. Thermal pulse results yield polarization distributions that may ...

Thermal pulse study of the polarization distributions produced in polyvinylidene fluoride by corona poling at constant current

A thermal pulse study of the polarization profiles in samples of 12‐μm‐thick, biaxially oriented polyvinylidene fluoride after corona poling under approximately constant‐current conditions, using a modified corona triode in atmospheric air, is reported. An electrical characterization of the corona triode is also reported to show how it may be operated in the constant‐current mode. Samples poled without electrode on the corona‐exposed surface show polarization distributions sensitive to the corona polarity, with polarization depletion on the corona side of the samples when the corona is positive. Polarization‐reversal experiments show switching inhomogeneities with a pronounced dependence on the initial corona polarity. The above observations are consistent with a simple model in which positive charges from the positive corona partially penetrate the sample during poling and cause an inhomogeneous reduction of the poling field.