C.J. Dias

Inorganic ceramic/polymer ferroelectric composite electrets

Ferroelectric composites are now an established alternative to conventional ferroelectric ceramic materials and to the more recently discovered ferroelectric polymers. These materials due to their unique blending of polymetric properties of mechanical flexibility, formability and low cost with high electro-active properties have been been suggested to be a viable alternative both in piezoelectric and pyroelectric transducer applications. This review is devoted to the piezoelectric and pyroelectric properties exhibited by these type of composites with a special reference to those made of ceramic particles embedded in a polymer matrix (i.e. 0-3 connectivity type composite). A review of models predicting the electro-active properties of 0-3 composites is presented together with a proposal for a new mixed connectivity cubes model to be applicable to the case of high ceramic loading and/or when the ceramic grain size incorporated in the polymer matrix is comparable to the thickness of the sample. A review of the experimental results of the piezo- and pyroelectric properties of various ferroelectric composite materials, reported by several workers, is also presented in this paper. Special reference is made to composites made from calcium modified lead titanate and poly(vinyldene fluoride-trifluorethylene) emphasizing their advantages in the poling process which is a critical phase in the process of obtaining successful electro-active 0-3 composite electrets.

Measurement of the pyroelectric coefficient in composites using a temperature-modulated excitation

Ferroelectric polymers such as PVDF and its copolymer P(VDF/TrFE) as well as ferroelectric ceramic-polymer composites have to be annealed at a high temperature for an extended period of time after polling, before the measurement of the pyroelectric coefficient by the direct method can be performed, in order to eliminate the high depolarization current which hinders the determination of the true pyroelectric current. Here it is suggested that a periodic temperature oscillation of a ceramic-polymer composite sample would permit measurement of its true pyroelectric coefficient without the need to anneal the sample. The temperature distribution inside the composite sample, the magnitude of the pyroelectric current and their variation with the frequency of temperature oscillation have been simulated with the present theory. Pyroelectric measurements using this technique have been made on a poled composite of calcium-modified lead titanate (PbTiO3) and a copolymer of vinylidene fluoride and trifluoroethylene P(VDF/TrFE) and compared with the results of the direct measurements before and after annealing, in order to test the validity of the method.

Recent advances in ceramic-polymer composite electrets

Recent work on ceramic-polymer composites for piezoelectric and pyroelectric applications is presented with special regard to the production and characterisation of new composite materials as well as two new applications of these composite materials. One of these composites is made using ceramic powders obtained using the sol-gel technique. This technique allows a better control of the stoichiometry as well as a lower temperature of crystallisation as compared with the conventional mixed oxides route. A better control of powder morphology also produces ceramic grains in the submicron range enabling the production of nanocomposites with electroactive properties. A second type of composite is reported using high temperature polymer PEEK, thus extending the temperature range of common electroactive composite materials. Finally the use of these materials is demonstrated in two piezoelectric applications, an angular acceleration accelerometer and an acoustic emission sensor.

Polymer/ceramic composites for piezoelectric sensors

Abstract Polymer/ceramic composites with 0–3 connectivity have been produced by introducing fine-grain modified lead titanate powder in the matrix of a copolymer of vinylidene fluoride and trifluorethylene (VDF/TrFE). The electromechanical coupling factor k t , the acoustic impedance Z 0 and the mechanical quality factor Q m of these films are determined. k t values in the range 30–35% have been achieved. Ultrasound transducers have been fabricated with these films. The impulse response of the transducer in the transmitting mode is determined. An attempt has been made to produce a theoretical model to match the experimental observations.

Hysteresis measurements on ferroelectric composites

Ceramic/polymer composites made of calcium‐modified lead titanate grains in the matrix of a copolymer of vinylidene fluoride and trifluorethylene, in a 65:35 volume fraction percentage, respectively, were produced in order to obtain mechanically strong ferroelectric films with improved piezo‐ and pyroelectric properties. Their poling behavior was studied using a modified ferroelectric hysteresis measurement setup that allowed for the measurement of the remanent polarization Pr and coercive field Ec, as a function of the peak of the sinusoidal applied field, the frequency of the excitation, and the poling temperature. The ranges of variation of the electric field, frequency, and temperature were from 5 to 25 MV/m, from 0.04 to 2 Hz, and from 50 to 120 °C, respectively. Estimates of the activation field and saturation polarization were calculated based on these measurements. The observed properties in the ferroelectric composites were then compared and correlated with the properties of the ceramic material.

Effects of a corona discharge on the charge stability of Teflon FEP negative electrets

Measurements of TSD together with surface potential were used to study negative corona discharge effects on the charge stability of Teflon FEP electrets. During charging, liquid formation over the sample surface (clearly visible for long-time corona irradiation) was concluded to be responsible for a decreasing in the thermal charge stability. The authors also discuss the charge stability at room temperature, while the sample is exposed to the neutral activated species from a corona discharge source. A decay in the potential was observed and this was attributed to a surface charge compensation by positive ions, and not to neutral activated molecules coming from the corona tip as suggested previously.

Understanding the Ion Jelly Conductivity Mechanism

The properties of the light flexible device, ion jelly, which combines gelatin with an ionic liquid (IL) were recently reported being promising to develop safe and highly conductive electrolytes. This article aims for the understanding of the ion jelly conductive mechanism using dielectric relaxation spectroscopy (DRS) in the frequency range 10(-1)-10(6) Hz; the study was complemented with differential scanning calorimetry (DSC) and pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. The room temperature ionic liquid 1-butyl-3-methylimmidazolium dicyanamide (BMIMDCA) used as received (1.9% w/w water content) and with 6.6% (w/w) of water content and two ion jellies with two different ratios BMIMDCA/gelatin/water % (w/w), IJ1 (41.1/46.7/12.2) and IJ3 (67.8/25.6/6.6), have been characterized. A glass transition was detected by DSC for all materials allowing for classifying them as glass formers. For the ionic liquid, it was observed that the glass transition temperature decreases with the increase of water content. While in subsequent calorimetric runs crystallization was observed for BMIMDCA with negligible water content, no crystallization was detected for any of the ion jelly materials upon themal cycling. To the dielectric spectra of all tested materials, both dipolar relaxation and conductivity contribute; at the lowest frequencies, electrode and interfacial polarization highly dominate. Conductivity, which manifests much more intensity relative to dipolar reorientations, strongly evidences subdiffusive ion dynamics at high frequencies. From dielectric measures, transport properties as mobility and diffusion coefficients were extracted. Data treatment was carried out in order to deconvolute the average diffusion coefficients estimated from dielectric data in its individual contributions of cations (D(+)) and anions (D(-)). The D(+) values thus obtained for IJ3, the ion jelly with the highest IL/gelatin ratio, cover a large temperature range up to room temperature and revealed excellent agreement with direct measurements from PFG NMR, obeying to the same VFT equation. For BMIMDCA(6.6%water), which has the same water amount as IJ3, the diffusion coefficients were only estimated from DRS measurements over a limited temperature range; however, a single VFT equation describes both DRS and PFG NMR data. Moreover, it was found that the diffusion coefficients and mobility are similar for the ionic liquid and IJ3, which points to a role of both water and gelatin weakening the contact ion pair, facilitating the translational motion of ions and promoting its dissociation; nevertheless, it is conceivable the existence of a critical composition of gelatin that leads to those properties. The VFT temperature dependence observed for the conductivity was found to be determined by a similar dependence of the mobility. Both conductivity and segmental motion revealed to be correlated as inferred by the relatively low values of the decoupling indexes. The obtained results show that ion jelly could be in fact a very promising material to design novel electrolytes for different electrochemical devices, having a performance close to the IL but presenting an additional stability regarding electrical measurements and resistance against crystallization relative to the bulk ionic liquid.

Poling behaviour of ceramic/polymer ferroelectric composites

Abstract Composite materials made from Calcium modified Lead Titanate grains embedded in a copolymer matrix of poly(vinyledene fluoride trifluorethylene) have been produced in order to optimise their pyroelectric and piezoelectric properties. The resulting coefficients are dependent both on the electrical, mechanical and thermal properties of the constituents materials and the morphology of the composite (i.e. connectivity, grain size of the ceramic). However in order to predict the properties of these composites one must take into account the degree of poling of the ferroelectric ceramic grains. This is specially so in composites with 0-3 connectivity as opposed for instance to the 1-3 connectivity pattern in which the poling voltage will be appear both across the polymer and the ceramic phases making the estimation of the poling of the ceramic easier. The materials we fabricated can be thought of as a mixed 0-3 and 1-3 connectivity composite. This will depend on the grain size of the ceramic used (relat...

Space charge studies in LDPE using combined isothermal and nonisothermal current measurements

Using a recently developed procedure combining isothermal and nonisothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the nonisothermal measurements appeared a broad peak (40/spl deg/C to 50/spl deg/C) that was possible to decompose into two or three peaks (35, 45 and 65/spl deg/C). At even higher temperature another peak was sometimes present (85/spl deg/C) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of /spl ap/15 /spl mu/m). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the nonisothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.

Ferroelectric ceramic/polymer composites and their applications

Abstract Thin composite films of calcium modified lead titanate (PTCa)/copolymer of vinylidene fluoride-trifluoroethylene P(VDF-TrFE) and PTCa/Epoxy have been produced and their piezoelectric d33- and electromechanical coupling coefficient kt, have been measured. These composite sensors have been used to monitor strain induced in a glass/epoxy laminate produced by cyclic loading. PTCa/P(VDF-TrFE) was embedded in a laminate structure to detect acoustic emission (AE) signals generated on the surface of the laminate by a conventional lead-break technique. The results show that the composite sensor is able to detect AE signals over a wide bandwidth: