Ceramic composites pyroelectricity

The ways of preparing and general properties of the composites of various types will first be described. Then their dielectric and electrect properties will be discussed, since they determine the piezo- and pyroelectricity of polymer-ceramic composites. The last-mentioned properties will be described in the last part of this chapter. 

The pyroelectric coefficients of the polymer-ceramic composites are large oompared to polymers, and the relative permittivities are small compared to ceramics (64j. Therefore tte figures of merit are enhanced over conventional single-phase polymer materials. (In certain pyroelectric systems a useful figure of merit to p/c, where p to the pyroelectric coefficient and c is the electric permittivity [16]). 

Both the above-mentioned methods are used in investigations of the pyroelectric properties of polymer-ceramk composites. We will compare the pyroelectric properties of a few polymer-ceramic composites as determined by different methods. 

Fig. 95 shows the change in cell parameters, density and Curie temperature for ceramics with initial compositions of Li(Tai xMgx)03.3xF3x (where 0 < x < 0.2) versus x value. It should also be mentioned that the pyroelectric coefficient for x = 0.05 was found to be 4.0 nC cm 2 K l. 

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). 

Ferroelectric composites are alternatives to standard piezoelectric and pyroelectric ceramics such as lead zirconate titanate (PZT) and BaHOs (BT). They combine the strong ferroelectric and dielectric properties of ceramics with the easy processing and good mechanical properties of polymers. Dispersion of micrometer-sized ferroelectric particles in an electrically passive epoxy matrix was first published by Furukawa et al. [1976] and later extended to ferroelectric matrices such as poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-3-fluoroethylene) (PVDF-TrFE) [Hsiang et al., 2001 Hilczer et al., 2002 Gimenes et al., 2004 Lam et al., 2005 Beloti et al., 2006]. However, the necessity of miniaturization of electronic components and... 

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). 

Pyroelectric sensors utilize a wide range of material forms crystals, bulk ceramics, thick layers, and thin films. Noteworthy among new compositions with improved performance is the class of relaxor pyroelectrics. Many novel pyroelectric sensors utilize thin and thick films. Bulk pyroelectric ceramics for room temperature, and pyroelectric sensors, are widely available. New compositions with higher permittivity are available now. 

Kreher, W. and Rodel, J. 1998. Ferroelectric ceramics and composites statistical models for effective piezoelectric and pyroelectric properties. Applications of Ferroelectrics, 1998, lASF 98, Proceedings of the Eleventh IEEE International Symposium of Ferroelectrics, Edited by E. Colla, D. Damjanovic, and N. Setter, pp 455-458. 

Therefore a composite oon i of highly piezo- and pyroelectric ceramic material combined with a polymer would be the ideal rcpiacemem to obtain the properties of both... 

The expected components of the piezoelectric and pyroelectric tensors for composites of polymer-ceramic films are (122,118,119]. 

Recently, Dias el aL [48] suggested that a periodic temperature oscillation of a ceramic-polymer composite sample would permit measurement of its true pyroelectric coefficient without the n to aiuvcal the sample. 

Sakamoto, W. K., D. H. F. Kanda, and D. K. Das-Gupta, Dielectric and pyroelectric properties of a composite of ferroelectric ceramic and polyurethane. Materials Research Innovations, 5, 257-260, 2002. 

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