Less can be more. Holes in polymers lead to a new paradigm of piezoelectric materials for electret transducers

The review begins with a very brief history of electrets and of piezo and pyroelectricity in nonuniform space charge electrets. The development of cellular propylene as an electret-transducer material since the mid-eighties is described, and the accompanying research into the origins of (quasi-)ferro-, pyro- and piezoelectric effects in voided polymer electrets is discussed in some detail. In this context, the most relevant charging and measuring techniques for piezo and pyroelectric polymer-foam electrets are introduced. Porous fluoropolymer electrets and their investigation as single or multiple-layer piezoelectrics are considered in view of their better thermal and temporal stabilities. Finally, some proposed or demonstrated applications of voided polymer electrets are briefly mentioned.     

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     

Electrically active centers of capture of charge carriers and electret effect in polymeric dielectrics and mica

The results of an experimental investigation of the electret effect and electrically active centers of capture in polymeric dielectrics modified by radiation and subjected to deformation (X-ray radiation of electric gas barrier discharge using the EGBD technique), as well as mica after δCo⁶⁰ radiation and heat treatment, have been presented. It has been shown that electrically active centers of capture in polymeric electrets are polar C-H bonds and, in mica crystals, these centers are radiation defects of K⁺-OH⁻ ionic crystalline sublattice. It was shown that the process of the formation of polymeric and mica electrets is described by the wave-current model of polarization.     

Electrical measurements for the characterization of electret properties of cellular polymers

Cellular polymeric materials may show interesting quasi-piezoelectric properties, upon charging under specific conditions of electrical field, temperature, pressure and environment. Electrical charging is the trigger of electret properties. It has been proved that the occurrence of electrical avalanches in the cavities which are present in a material is associated with the appearance of electret behavior, but the precise nature of the phenomena involved in this transformation is still under investigation. This paper provides a contribution to the study of these materials focusing on the relation between the appearance of electret properties and the presence of charge injection and partial discharge phenomena. For this purpose, space charge, partial discharges and low-frequency polarization measurements were carried out on polypropylene (PP) cellular materials, at different fields and temperatures. The existence of a threshold field for the start up of electret behavior that is larger than the threshold for space charge injection and partial discharge inception was observed. Steady charge is accumulated in the material, thus generating electret behavior, only if partial discharge repetition rate becomes sufficiently high and space charge is injected from electrodes. Space charge measurements were performed through the pulsed electroacoustic technique, in spite of significant attenuation of acoustic waves due to the cellular nature of the materials. Partial discharge measurements were carried out by an advanced system recording each discharge pulse, thus allowing observation of single and multiple pulses, besides achieving statistics of discharge pulse amplitude and phase. By such approach, space charge and partial discharge measurements proved to constitute a valuable tool to investigate the phenomena leading to piezoelectric behavior and, also, to infer electret time stability and optimize charging methodologies.     

Piezoelectric and Pyroelectric Properties of Pb(Zr,Ti)O3 Films for Micro-Sensors and Actuators

The piezoelectric and the pyroelectric properties of PZT films are systematically investigated for tetragonal (Zr/Ti = 30/70), morphotropic (52/48), and rhombohedral (70/30) compositions. The magnitude of the effective longitudinal piezoelectric coefficient (d₃₃) and pyroelectric coefficient (p) of these films is measured by atomic force microscopy and Byer-Roundy method, respectively. All films are consistently highly textured (111) orientation and have dense microstructures. Slightly less degree of texture in higher Zr-rich composition is observed due to the lattice mismatch between PZT and Pt bottom electrode and higher activation energy for nucleation. Squareness of polarization hysteresis loops is optimized in tetragonal composition, which indicates the tetragonal PZT is closer to the ideal hysteresis behavior than other compositions. It is shown that the piezoelectric coefficient and the pyroelectric figure of merit are dependent on the dielectric properties of the films. The morphotropic PZT films with high dielectric constant and low pyroelectric figure of merit show the largest piezoelectric coefficient values, while the tetragonal PZT films with low dielectric constant and high remanent polarization values show the largest pyroelectric figure of merit compared to other compositions, which indicate the suitability for PIR sensor devices.     

Pyroelectric Properties of PVDF:MWCNT Nanocomposite Film for Uncooled Infrared Detectors and Medical Applications

The biocompatible and flexible polyvinylidene fluoride (PVDF) has a number of interesting pyroelectric and piezoelectric properties with fast, dynamic response for use in touch/tactile sensors, infrared detectors and thermal vidicon/imaging devices. Pyroelectric multi-walled carbon nano-tubes:polyvinylidene fluoride nano-composite films were fabricated via the solution casting technique. The dielectric, pyroelectric and piezoelectric characteristics of composite films were measured. Using foregoing parameters, figures-of-merit for infrared detectors, and thermal-vidicons were calculated. The results indicated figures-of-merit of the composite film that were higher than pristine polyvinylidene fluoride film. The potential applications of composite films in medical devices are also discussed.     

Charge storage and its dynamics in porous polytetrafluoroethylene (PTFE) film electrets

The outstanding space charge storage stability of porous polytetrafluoroethylene (PTFE) film electrets is studied by isothermal surface potential decay measurements and open-circuit thermally stimulated discharge (TSD) experiments after corona charging at room and elevated temperatures, or corona charging at RT and then aging at different temperatures. Charge storage properties of porous PTFE, nonporous PTFE (Teflon/spl reg/ PTFE) and nonporous FEP (Teflon/spl reg/ FEP) electrets are compared. The results show that porous PTFE has the best charge storage stability of organic materials for both negative and positive charges, especially at high temperatures. The structure of porous PTFE, investigated by a scanning electron microscope (SEM), is important for understanding the electret properties of this material. Charge dynamics, including the influence of environmental humidity and temperature on charge stability and shift of mean charge depth, and the kinetics of detrapped charges for the porous PTFE film electrets were also investigated by means of isothermal surface potential decay measurements and analysis of the TSD current spectra in combination with the heat pulse technique. It is found that from about RT to 200/spl deg/C slow retrapping plays a dominant role; from about 200/spl deg/C to 300/spl deg/C fast retrapping controls the transport.     

Silicon-based inorganic electrets for application in micromachineddevices

Layers of SiO₂ and Si₃N₄ have good mechanical properties for application in micro-machined electret capacitor microphones. These materials are investigated as single or double layers in terms of chargeability and long-term charge stability. The main emphasis is put on the miniaturization of electret layers. The lateral dimensions of the electrets are reduced to 2 mm and the charge decay characteristics under different environmental conditions are described. According to the experimental data, multilayer samples of silicon dioxide and nitride possess good chargeability and higher charge stability compared to the well investigated single layers. Typically, the double layers loose ~10% of their surface potential when annealed for 200 min at 300°C and show a peak of the thermally stimulated current at 430°C. It can also be shown that the miniaturization of samples does not necessarily cause a faster charge decay     

Current model and fulfilling Ohm’s law during polarization of centers of capture in polymer cable electrets

Results of an experimental study of influence of mechanical deformations, radiation cross linking, and thermal oxidation on the electret effect in polyolefin cable dielectrics are given. It is shown that the polarization of polyolefin electrets in the electric gas discharge can be described in the context of the wave (current) model. That Ohm’s law is fulfilled during polarization of centers of capture in polymer cable electrets is demonstrated.     

Some specificities of the pyro- and piezoelectric phenomena in heterogeneous ferroelectric materials

A system of equations has been constructed in order to find effective pyroelectric, dielectric, piezoelectric, elastic and thermoelastic constants of heterogeneous materials using the method of self-consistency. By means of these equations the characteristics are simulated of the electrically depoled piezoceramics and of composites with antiparallel polarization of the components. Such a composite, being a mixture of BaTiO₃ and PZT-19 ceramic particles, manifests at a certain concentration of PZT-19 high pyroelectric properties but does not practically possess the piezoeffect. But at another concentration it is a strong piezoelectric without any pyroeffect.     

Space-charge electrets

A literature review on space-charge electrets, covering the time period 1987 to 1996, is presented. The review starts out with a discussion of new inorganic and organic electret materials which also includes silicon based single and double layers. This is followed by a review of recent work on methods for measuring charge distributions. New results obtained with these methods on a number of differently charged materials are surveyed, together with data for charge transport and charge decay. Of particular significance is the charge storage capability of silicon dioxide/silicon nitride double layers which was discovered only very recently. The review closes with a survey of new, or improved electret applications     

Photostimulated discharge in electret polymers: an alternative approach for investigating deep traps

The stability of space charge in electrets such as polytetrafluoroethylene (PTFE), polyethylene terephthalate (PETP) and polypropylene (PP) under ultraviolet irradiation has been investigated using photostimulated discharge spectroscopy. While only weak discharge currents were observed in PTFE coated with semitransparent gold electrodes, up to 15 pA/cm/sup 2/ were found in PETP around the UV absorption edge near 310 nm. Space charge profiles obtained with the piezoelectrically generated pressure step method indicate that near-surface charges were almost completely removed. In PP foam, recent findings of a UV-reduced d/sub 33/ coefficient were confirmed for exposure times of up to 3.5 h, and a discharge peak at 200 nm could be assigned to the charges stored on the surfaces of the voids. The unique morphology and the (quasi-)piezoelectric properties of cellular PP make it a role model for the future investigation of charge storage in electrets.     

Piezoelectric polymers

Polymers with piezoelectric and pyroelectric properties that make them useful for biomedical applications and devices are considered. The characteristics of such polymers are examined, and transducers made from them are discussed. Laboratory developments of devices using piezoelectric polymers are briefly reviewed, and the slowness of product development in this area is commented on.     

Pyroelectric and piezoelectric properties of leadtitanate/polyvinylidene fluoride-trifluoroethylene 0-3 composites

Composites of calcium modified lead titanate powder dispersed in a polyvinylidene fluoride/trifluoroethylene copolymer matrix have been prepared using solvent casting followed by compression molding. Two groups of poled samples have been prepared: one with only the ceramic phase polarized and the other with both phases polarized. The observed permittivities of the unpoled composites are consistent with the predictions of the Bruggeman model. The changes in the pyroelectric and piezoelectric coefficients of the poled composites with increasing ceramic volume fraction can be described by modified linear mixture rules. When the ceramic and copolymer phases are poled in the same direction, the piezoelectric activities of the two phases partially cancel each other while the pyroelectric activities reinforce. Hence, at a certain ceramic volume fraction, the composite is pyroelectric but not piezoelectric     

Dielectric and Pyroelectric Properties of Lead Zirconate Titanate Composite Films

A new sol-gel approach has been established to prepare dense and crack-free lead zirconate titanate (PZT)/PZT composite films. This new process combines the modified sol-gel method for preparing ceramic/ceramic composite films and the infiltration technique. In the modified sol-gel method, sintered PZT powder is dispersed in a PZT precursor solution to form a slurry which is then spin-coated on a substrate. However, the resulting composite film usually contains a considerable amount of pores, and thus resulting in serious degradation of the mechanical, ferroelectric and piezoelectric properties of the films. In the present work, an additional step, infiltration, has been included in the method. A diluted PZT precursor solution is deposited on the composite film to infiltrate and fill-up the pores, and hence a dense composite film is obtained. The dielectric, ferroelectric and pyroelectric properties of the resulting PZT/PZT composite film are comparable to those of a sol-gel derived PZT film; showing the values of 1200, 0.04, 21 μC/cm² and 180 μC/m² K for relative permittivity, dielectric loss, remanent polarization and pyroelectric coefficient, respectively.     

Electret effect and charge-carrier mobility in polymeric and mica dielectrics

We measure charge-carrier mobility μ _e,p in polymeric and mica electrets by using a new procedure for forming radiation conductivity gradient through an electret’s thickness; the experimental findings are presented in the paper.     

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.     

Piezoelectricity in cellular electret films

Permanently charged films with a cellular or porous structure represent a new family of polymer electrets. These materials show piezoelectric properties with high piezoelectric constants. The electromechanical response equations of such films are derived for their operation as sensors and as actuators. Experimental results are also presented for cellular polypropylene (pp). In particular, measurements of the direct and inverse piezoelectric constants in the frequency range 0 to 10 kHz and of the variation of these constants across the surface of the films are discussed. These measurements, performed by direct application of stress or by the use of a profilometer, an accelerometer and an interferometer yield a frequency-independent piezoelectric d₃₃ constant of ≲ 220 pC/N. Assuming reasonable charge distributions and charge densities, the calculated piezoelectric constants are in good agreement with the measured values. The theoretical model shows the reciprocity of the piezoelectric constants     

Charged cellular polymers with "ferroelectretic" behavior

Cellular space-charge electrets have recently emerged as a new class of materials for electromechanical devices, offering chances for a wide range of applications and challenges for materials optimization. However, many fundamental and applied aspects of the physics of these novel materials are not yet explored. Here we summarize our present understanding of the (quasi)-piezo- and -pyroelectricity in such materials. In contrast to the dipole-density piezoelectricity in ferroelectric polymers, the piezoelectric-like response of cellular polymers is intrinsic, with positive d/sub 33/ and negative d/sub 31/ and d/sub 32/ piezoelectric-like coefficients. Similarities with ferroelectric materials are outlined, especially switching of "polarization" and (quasi)-piezoelectricity. First steps towards patterned charging of cellular polymers are reported, an immediate consequence of the possibility for "polarization"-switchingin cellular materials. The results on cellular space-charge electrets suggest that well-known electret devices like microphones may be seen in a new light. Examples include dielectric and electromechanical hysteresis loops obtained with a commercially available electret microphone. In view of the results, cellular polymers may henceforth be called "ferroelectrets" and their material behavior "ferroelectretic". From an applied point of view, the performance of a Fresnel zone plate for focussing ultrasound is shown. Such a device may pave the way for a simple tool in nondestructive materials inspection, and demonstrates the large potential of cellular polymers for applications.     

Piezoelectricity, pyroelectricity and ferroelectricity inbiomaterials: Speculation on their biological significance

Many thousands of papers have been published on the piezoelectric, pyroelectric and ferroelectric properties of single crystals, ceramics, polymers and liquid crystals. Yet fewer than 100 papers have appeared on these properties in biological materials. Piezoelectric and pyroelectric effects have been measured on a number of plant and animal tissues and the possible existence of ferroelectricity has been suggested also. This paper reviews some measurements and speculates on their significance. Specific examples include piezoelectricity in a crystalline material in the pineal gland of the brain, pyroelectricity in the heat sensing organs of certain varieties of snakes and beetles, and ferroelectricity in the conduction of action potentials in nerve and muscle cells