Piezoelectric properties 1-3 composites

Zirconate compounds exhibit several interesting properties. Lead zirconate—titanate [12626-81 -2] compositions display piezoelectric properties which are utilized in the production of EM-coupled mode filters, resonators in microprocessor clocks, photoflash actuators, phonograph cartridges, gas... 

Fig. 9.4. Dependence of piezoelectric properties of PbZrOj-PbTiOj on composition. The zirconate-rich phase is rhombohedral, whereas the titanate-rich phase is tetrahedral. The piezoelectric coefficients reach a maximum near the morphotropic phase boundary, approximately 45% PbZrOj and 55% PbTiOj. (After Jaffe et al., 1954.)...

The crystallographic and piezoelectric properties of the ceramics depend dramatically on composition. As shown in Fig. 9.4, the zirconate-rich phase is rhombohedral, and the titanate-rich phase is tetragonal. Near the morphotrophic phase boundary, the piezoelectric coefficient reaches its maximum. Various commercial PZT ceramics are made from a solid solution with a zirconate-titanate ratio near this point, plus a few percent of various additives to fine tune the properties for different applications. 

The isovalent A-site substituents barium, strontium and calcium lower the Curie point and have a small influence on the morphotropic composition. At the 5-10 mol.% level they enhance the permittivity and piezoelectric properties. Pb0 94Sr0 06Ti0 47Zr0 53 for instance has a relative permittivity of 1300, a kp of 0.58... 

Table 6.2 Piezoelectric properties of some relaxor based compositions close to the MPB(II) (data taken from [11])...

The substitution of lead by barium enhances the piezoelectric properties which peak when Pb/Ba I. At this and higher barium contents the structure changes to tetragonal with the polar axis parallel to the A-site tunnels. There is a morphotropic boundary, similar to that found in PZT compositions, and peak values of piezoelectric properties are found near the Pb1/2Ba1/2Nb206 composition. d33 rises to 220 pCN-1 and d31 rises to — 90pCN 1, while Qm increases to 300 and the Curie point falls to 250 °C thus most of the features peculiar to... 

Technically useful properties of such perovskite ceramics are their high permittivities (relative dielectric constants), the semiconductor properties of certain chemical compositions and their piezoelectric properties. 

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. 

PHAs have rapidly gained interest both in research and industry due to their structural versatility and characteristics such as biodegradability, insolubility in water, nontoxicity, biocompatibility, piezoelectric property, thermoplasticity and or elastomeric properties, which make them favourable to be used in the packaging industry, medicine, pharmacy, agriculture, food industry and in the paint industry. The chemical and physical properties of the polymer are dependent on the monomeric composition which is determined by the producing microorganism and their nutrition. So far scl-PHAs are being studied extensively due to their easier... 

Fig. 7.17 Piezoelectric properties of PZN-PT crystal as a function of chemical composition and crystal orientation (Park and Shrout 1997a)...

Certain ternary solid solutions containing BTO as one of their major phase components show excellent piezoelectric properties in the vicinity of MPB. They have been considered as a potential replacement for existing lead-based piezoelectric perovskite, PbZrOs-PbTiOs, PMT-PT, and so on. In fact, the extremely high piezoelectric response of PZT and PMN-PT has been partially attributed to the presence of a MPB between rhombohedral and tetragonal phases at that concentration [12]. Therefore, the presence of a MPB is considered as a necessary component of a lead-free replacement material due of the enhancement in properties observed for compositions at or near MPB. The basic approach to achieve high piezoelectricity is to choose the composition of the material at the proximity of a MPB of the phase diagram. The strong piezoelectric and dielectric responses at the vicinity of MPB have been ascribed to the low polarization anisotropy (rotation of FE polarization between two equivalent states) and elastic softness at MPB. 

The existenee of the two classes of ferroelectric and piezoelectric polymers, PVDF and its eopolymers and odd-numbered nylons, makes it possible to develop polymer-polymer eomposite systems. Ferroelectric and piezoelectric PVDF-nylon 11 bilaminates and nylon 11-PVDF blends were studied by Su (1992) and Gao et al. (1999), respectively. Significant enhancements in ferroelectric and piezoelectric properties were observed in both polymer-polymer composite systems. Interfacial polarization plays an important role for the enhancement. 

To summarize the ferroelectric and piezoelectric properties of the discussed polymers, some important ferroelectric and piezoelectric parameters are tabulated in Table 4. As discussed in the previous sections, the ferroelectric and piezoelectric properties of polymeric and polymeric composite systems depend on various factors, such as crystallinity, pohng conditions, glass transition temperature, and before and after electrical poling treatments (electrical, mechanical, and thermal treatments). In addition to the factors mentioned above, for composite systems, laminates or blends, fraction of constituents, and interfacial polarization are also important. Therefore, the... 

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