Density, piezoelectric PVDF

The unique piezoelectric and pyroelectric properties of semicrystalline films of PVDF arise from changes in the polarization imparted to the overall film by the crystalline P-phase. The polar nature of the P-phase is, in turn, a direct result of the parallel alignment of the dipole moment of the repeat units in the unit cell (Figure 11.1). The crystal polarization is defined as the dipole moment density of the crystal ... 

It is instructive to compare the basic properties of the piezoelectric polymer, polyvinylidene fluoride (PYDF) with those of PZT . The flexibility and low density of the polymer contrasts with the stiffness, brittleness and high density of PZT . On the other hand the piezoelectric d coefficient for PYDF is relatively small ( — 30pCN the mechanisms by which the polarisation in PVDF... 

With its low acoustic impedance, extreme bad width, high piezoelectric coefficient, and low density (only one-quarter the density of ceramic materials), PVDF is ideally suited as a transducer for hroad hand rmdenvater receivers in hghtweight hydrophones. The softness and flexibiHty of PVDF give it a comphance 30 times greater than ceramic. PVDF can thus he utilized in a hydrophone structure using various device configurations, such as compliant tubes, rolled cylinders, discs, and planar stacks of laminated material. 

We have for a number of years developed polymer-based piezoelectric textile filaments and yarns [44]. Filaments are melt spun and bicomponents core-and-sheath type based on a blend of a normal bulk polymer like high-density polyethylene and the piezoelectric material (PVDF), surrounding a core of a conductive polyethylene-carbon black mixture (see Fig. 28.25). The core is electrically conductive and will act like an electrode. 

Another strategy has to be chosen. This is to let the piezofiber (ie, the conductive core with high-density polyethylene (Aspun 6835A, Dow, USA) and carbon black (Ketjenblack EC-600JD Akzo Nobel, the Netherlands) and the piezoelectric sheath with PVDE (Solef 1006, Solvay Solexis, Italy)) mecharucaUy come into close proximity with a conductive thread. Here we take a thread from Class El above, Shieldex (Statex, Germany, silver-coated polyamide). By this we form a textile variant of the triad with the conductive carbon black core as the inner electrode, PVDF sheath as the active layer, and the Shieldex as the outer electrode. 

Equation 7 includes the prediction that the ratio of the piezoelectric coefficient and the remanent polarization P3 = Pr should be approximately equal to the elastic compliance /Ym of the matrix phase for the dipole-density effect or to the elastic compliance HYd of the dipole phase for the dipole-moment effect, respectively (or inversely proportional to the respective elastic modulus). First results assembled from the literature and from our own experimental data on PVDF and on cellular-foam PP and tubular-channel FEP ferroelectrets (Altafim et al. 2009) are shown in Fig. 5 (Qiu et al. 2013, 2014). They provide experimental evidence that the direct piezoelectric thickness coefficient of polymer materials can indeed be roughly approximated by the product of the remanent polarization in the poled material and of its overall elastic compliance. Additional data from the literature on other... 

Liquid crystalline elastomers are produced by the introduction of cross linking into liquid crystal polymer systems. This cross linking results in materials with a number of unusual properties, for example, stress-induced phase transitions and spontaneous ekmgatioo of samples in the liquid crystelline phase. When they contain chiral units. Sr elastomers are formed. For such materials piezoelectricity was predicted by Brand 103]. The helical structure of those systems can be untwisted by applicatioo of a mechanical stress, generating an electrical signal. This possibility provi the basis for the developnient of materials with piezoelectric properties. Such materials are of comaderablc interest, since the basis for their piezoelec c properties is rather different from that in the best-known piezoelectric polymer, poly(vinylidenefluoride) (PVDF). There exists rather more scope for the modification of their properties for example, the nature of the chiral unit may be varied to alter the helkal superstructure, or differences in cross link density can change the mechanical properties of the sample. 

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