Piezoelectric poling

For polycr5rstalline piezoceramics to work, the electrically charged dipoles within the entire piezoelectric component must be aligned by placing the piezoceramic within a high electric field—a process known as poling. The ability of piezoceramics to almost instantaneously convert electrical... 

As we have mentioned, the piezoelectric properties of ceramics are generated by a poling process. Apparently, if a strong electric field in a direction other than the poling direction is applied, the piezoelectric property is altered or lost. The safe value of an ac field to avoid causing depoling, Ej, can be found in the product specifications. 

Fig. 9.8. Deflection of a bimorph. Two long, thin plates of piezoelectric material are glued together, with a metal film sandwiched in between. Two more metal films cover the outer surfaces. Both piezoelectric plates are poled along the same direction, perpendicular to the large surface, labeled z. (A) By applying a voltage, stress of opposite sign is developed in both plates, which generates a torque. (B) The bimorph flexes to generate a stress to compensate the torque. The neutral plane, where the stress is zero, lies at hi i from the central plane.

It must in any case be noted that the piezoelectricity of polymer films due to charges is not an intrinsic property of polymers it varies from sample to sample and can be remarkably enhanced by drawing and/or poling, as will be described in following sections. 

Kawai (1) and (2) (1969) found that polar polymer films such as PVDF, poly (vinyl fluoride), PVC, nylon 11, and polycarbonate exhibit a strong piezoelectricity when they are drawn and then polarized under a high cLc. field Ep at a high temperature Tp and cooled keeping the d.c. field. The piezoelectricity thus obtained depends on Ep, Tp, and poling period. An improved poling technique was reported by Edelman, Grisham, Roth, and Cohen (1970). 

Fig. 26. Correlation between increment of spontaneous polarization from 80° C to 15° C and piezoelectric strain constant at room temperature for /9-form polarized poly(vinylidene fluoride) films. Poling temperature = 90° C. Poling field = 700 kV/cm (Murayama, 1972)...

Fig. 27 indicates the apparent piezoelectric constant e of roll-drawn PVDF as a function of static bias field E0 (Oshiki and Fukada, 1972). The value of e at E0=0 represents the true piezoelectric constant e. The curve exhibits a hysteresis and the polarity of e changes according to the poling history. If the piezoelectricity in /)-form PVDF originates from the polarization charge due to spontaneous polarization, inversion of polarity of e would mean the inversion of the polarization by the external field and hence /S-form PVDF may be a ferroelectric material, as was first suggested by Nakamura and Wada (1971). 

Pr is reasonable because the x-axis of the film is taken as the direction of the poling field. On the other hand, the spontaneous polarization of the P-form crystal of PVDF is estimated from the crystal structure as Ps= 1.3 x 10 5 coul/cm2 = 4x 104 cgsesu. Considering the incompleteness in crystallinity ami orientation, the estimated value of PJN is reasonable compared with PF However, it would still be premature to conclude that the piezoelectricity of drawn and polarized PVDF film is due to the strain dependence of spontaneous polarization. 

Moreover, a large scatter of the piezoelectric constant among samples of different poling conditions, such as is indicated in Table 3, seems to suggest that the piezoelectricity of polarized PVDF does not have a single origin. Further work is required to settle this problem. 

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). 

Piezoelectric materials are materials that exhibit a linear relationship between electric and mechanical variables. Electric polarization is proportional to mechanical stress. The direct piezoelectric effect can be described as the ability of materials to convert mechanical stress into an electric field, and the reverse, to convert an electric field into a mechanical stress. The use of the piezoelectric effect in sensors is based on the latter property. For materials to exhibit the piezoelectric effect, the materials must be anisotropic and electrically poled ie, there must be a spontaneous electric field maintained in a particular direction throughout the material. A key feature of a piezoelectric material involves this spontaneous electric field and its disappearance above the Curie point. Only solids without a center of symmetry show this piezoelectric effect, a third-rank tensor property (14,15). 

The piezoelectric coefficients are third rank tensors, hence the piezoelectric response is anisotropic. A two subscript matrix notation is also widely used. The number of non-zero coefficients is governed by crystal symmetry, as described by Nye [2], In most single crystals, the piezoelectric coefficients are defined in terms of the crystallographic axes in polycrystalline ceramics, by convention the poling axis is referred to as the 3 axis. 

In summary, piezoelectric coefficients are complex numbers that depend on the measurement frequency, excitation field, temperature, and time (e. g. time after poling in samples that show finite aging rates). Consequently, in reporting piezoelectric data, it is important to specify how the property was measured. 

Figure 2.4 Strain-field curves for < 001 > oriented 0.91PbZn1/3Nb2/303-0.09PbTi03 single crystals. The sample in (a) was poled at room temperature, where the resulting domain state is unstable (due to induction of tetragonal material associated with the curved morphotropic phase boundary), yielding substantial hysteresis. In (b) the crystal was poled at low temperatures to keep it in the rhombohedral phase. When measured at room temperature, the piezoelectric response is much more linear and non-hysteretic, due to the improved stability of the ferroelectric domain state. Data courtesy of S. E. Park.

In many microelectromechanical systems (mems) based on piezoelectric thin films, flexure is deliberately used to amplify the available displacements (or alternatively to increase the sensitivity of a sensor). For simplicity (and to keep poling and actuation voltages low), films are often poled and driven by electrodes at the top and bottom surfaces. As a result, the critical piezoelectric coefficient is often e31 j, rather than d33j [24], For the direct effect, the effective film coefficient, e3ij can be defined by... 

Figure 15.13 Dielectric spectra of x and x" vs. / of unpoled (curves 1 and 2) and poled (curves 1 and 2 ) SBN Ce taken at T = 294 K. Solid lines are guides to the eye and the vertical dotted line separates different response regimes. A piezoelectric anomaly at f = 0.5 MHz is marked by a double arrow. The inset shows x" vs. x (from [55]).

Figure 19.11 Phase maps f(x, y) of a 6x4/um region of a fatigued FeCap (108 cycles) after negative (left) and positive (right) poling and evolution map of the piezoelectric phase signal f(x, E) (central picture) under varying (triangular shape) electric field E of the horizontal line indicated by the horizontal arrows. PI, P2, LI and L2 are discussed in detail later.

Figure 19.20 Piezoelectric vibration maps of phase ((a), (c)) and amplitude ((b), (d)) (3x3 /um2) of a fatigued Pt-PZT-Pt structure after positive ((a), (b)) and negative ((c), (d)) poling. Bright and dark phase areas correspond to bottom-to-top and top-to-bottom polarization orientations, respectively.

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