Pyroelectric and Piezoelectric Ceramics

piezoelectric and pyroelectric behavior is possible only in ferroelectric ceramics, or in otherwise polar materials that are deposited as textured thin films. 

Other piezoelectric materials require either utilization as single crystal slices cut in fixed relation to the electrical axes, as in a-quartz (Bradaczek et al., 1990) and langasite (Bohm et al., 1999), or as epitaxially deposited layers on suitable substrates (Heywang et al, 2009). 

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The most interesting properties of polymers are their high mechanical and electrical strength and low electrical conductivity and acoustic impedance, whereas the ferroelectric ceramics exhibit good dielectric, pyroelectric, and piezoelectric properties (4,8,67]. 

The main categories of electrical/optical ceramics are as follows phosphors for TV, radar and oscilloscope screens voltage-dependent and thermally sensitive resistors dielectrics, including ferroelectrics piezoelectric materials, again including ferroelectrics pyroelectric ceramics electro-optic ceramics and magnetic ceramics. 

The bending piezoelectricity in drawn and polarized polymer films was studied in detail by Kawai (1) (1970). Kitayama and Nakayama (1971) reported a very high piezoelectricity in composite films of polymer (PVDF, nylon 11, PVC) and powdered ceramics (barium titanate, PZT) after poling. In the case of PVDF and nylon, the piezoelectric constant increase by a factor of 102 when the ceramics make up 50% of the volume. The pyroelectricity and optical nonlinearity of polarized PVDF films have been studied by Bergmann, McFee, and Crane (1971). 

Because a ceramic is composed of a large number of randomly oriented crystallites it would normally be expected to be isotropic in its properties. The possibility of altering the direction of the polarization in the crystallites of a ferroelectric ceramic (a process called poling ) makes it capable of piezoelectric, pyroelectric and electro-optic behaviour. The poling process - the application of a static electric field under appropriate conditions of temperature and time -aligns the polar axis as near to the field direction as the local environment and the crystal structure allow. 

Pyro- and Piezoelectric Properties The electric field application on a ferroelectric nanoceramic/polymer composite creates a macroscopic polarization in the sample, responsible for the piezo- and pyroelectricity of the composite. It is possible to induce ferroelectric behavior in an inert matrix [Huang et al., 2004] or to improve the piezo-and pyroelectricity of polymers. Lam and Chan [2005] studied the influence of lead magnesium niobate-lead titanate (PMN-PT) particles on the ferroelectric properties of a PVDF-TrFE matrix. The piezoelectric and pyroelectric coefficients were measured in the electrical field direction. The Curie point of PVDF-TrFE and PMN-PT is around 105 and 120°C, respectively. Different polarization procedures are possible. As the signs of piezoelectric coefficients of ceramic and copolymer are opposite, the poling conditions modify the piezoelectric properties of the sample. In all cases, the increase in the longitudinal piezoelectric strain coefficient, 33, with ceramic phase poled) at < / = 0.4, the piezoelectric coefficient increases up to 15 pC/N. The decrease in da for parallel polarization is due primarily to the increase in piezoelectric activity of the ceramic phase with the volume fraction of PMN-PT. The maximum piezoelectric coefficient was obtained for antiparallel polarization, and at < / = 0.4 of PMN-PT, it reached 30pC/N. 

Lam et al. [2005] also reported the evolution of the pyroelectric coefficient (pe) with the volume fraction of PMN-PT. The pyroelectric coefficients of ceramic and copolymer have the same sign, but not their 33 coefficients. The maximum increase was obtained for a parallel polarization procedure. In both cases, the increase was quasilinear as a function of filler content from 5 to 40% of PMN-PT to 40%, the pyroelectric coefficient, pe, increased by a factor of 3. A linear increase in the piezoelectric coefficients of composites has also been shown in a PA-11/BT system [Capsal et al., 2007]. It was found that BT particles increase the piezoelectricity of the composite up to 6pC/N for piezoelectric activity with decreasing filler size, due to the decrease in tetragonality (ferroelectric phase). 

Piezoelectricity links the fields of electricity and acoustics. Piezoelectric materials are key components in acoustic transducers such as microphones, loudspeakers, transmitters, burglar alarms and submarine detectors. The Curie brothers [7] in 1880 first observed the phenomenon in quartz crystals. Langevin [8] in 1916 first reported the application of piezoelectrics to acoustics. He used piezoelectric quartz crystals in an ultrasonic sending and detection system - a forerunner to present day sonar systems. Subsequently, other materials with piezoelectric properties were discovered. These included the crystal Rochelle salt [9], the ceramics lead barium titanate/zirconate (pzt) and barium titanate [10] and the polymer poly(vinylidene fluoride) [11]. Other polymers such as nylon 11 [12], poly(vinyl chloride) [13] and poly (vinyl fluoride) [14] exhibit piezoelectric behavior, but to a much smaller extent. Strain constants characterize the piezoelectric response. These relate a vector quantity, the electrical field, to a tensor quantity, the mechanical stress (or strain). In this convention, the film orientation direction is denoted by 1, the width by 2 and the thickness by 3. Thus, the piezoelectric strain constant dl3 refers to a polymer film held in the orientation direction with the electrical field applied parallel to the thickness or 3 direction. The requirements for observing piezoelectricity in materials are a non-symmetric unit cell and a net dipole movement in the structure. There are 32-point groups, but only 30 of these have non-symmetric unit cells and are therefore capable of exhibiting piezoelectricity. Further, only 10 out of these twenty point groups exhibit both piezoelectricity and pyroelectricity. The piezoelectric strain constant, d, is related to the piezoelectric stress coefficient, g, by... 

An important point now emerges the requirement that the piezoelectric, pyroelectric and ferroelectric effects are restricted to non-centrosymmetric crystals implies that these physical phenomena should not be observed in a polycrystalline solid. This is because the individual grains of a polycrystalline body will polarise in random directions that will cancel overall. This was changed by the discovery, in 1945, of a way to endow polycrystalline ceramic articles with ferroelectric properties. 

Ferroelectric crystals (especially oxides in the form of ceramics) are important basic materials for technological applications in capacitors and in piezoelectric, pyroelectric, and optical devices. In many cases their nonlinear characteristics turn out to be very useful, for example in optical second-harmonic generators and other nonlinear optical devices. In recent decades, ceramic thin-film ferroelectrics have been utilized intensively as parts of memory devices. Liquid crystal and polymer ferroelectrics are utilized in the broad field of fast displays in electronic equipment. 

Ferroelectric materials, especially polyciystalhne ceramics, are utihzed in various devices such as high-permittivity dielectrics, ferroelectric memories, pyroelectric sensors, piezoelectric transducers, electrooptic devices, and PTC (positive temperature coefficient of resistivity) components. 

Zinc oxide (ZnO) powders are technologically attractive materials due to their inherent characteristics such as dielectric, piezoelectric, pyroelectric and semiconducting properties. To improve the varistor effect and achieve high breakdown fields from ZnO ceramics it is required to have a microstructure with uniformly distributed smaller grains. The grain size for conventional ZnO varistors is larger than 2.0 /rm (Chu, 2000). Note, however, that... 

Wb will try to suggest the answer to the question, What causes the piezo- and pyroelectricity in electrets—dipoles (or spontaneous polarization), charges, or both " The exact answer to this question was not found during the Special Discussion Sesskm at ISE 7 in Berlin, 1991. The investigation of relaxation processes by means of the TSD method seem to lead to the answer to the rtrave question conceming polymer-ceramic compodtes. it will be shown by the example of the electret and piezoelectric properties of some multilayer systems [17-19]. 

Pyroelectrics. Pyroelectric ceramics are materials that possess a uoique polar axis and are spontaneously polarized ia the abseace of an electric field. Pyroelectrics are also a subset of piezoelectric materials. Ten of the 20 crystal classes of materials that display the piezoelectric effect also possess a unique polar axis, and thus exhibit pyroelectricity. In addition to the iaduced charge resultiag from the direct pyroelectric effect, a change ia temperature also iaduces a surface charge (polarizatioa) from the piezoelectric aature of the material, and the strain resultiag from thermal expansioa. 

Dielectric, piezoelectric and pyroelectric properties of LiTa03 derived ceramics containing additives of LiF and MgF2 were investigated and reported on in [407]. The materials were prepared at 900°C by means of two methods Reaction sintering, yielding powdered polycrystalline material ... 

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