Piezoelectric activator

A piezo-composite consists of a piezoelectric active phase and a passive plastic phase [2]. In the 1-3-configuration adopted in our case, piezoelectric rods parallely aligned in thickness direction are imbedded in a three-dimensional plastic matrix (Fig. 1). The distance between the rods has to be chosen inferior to the half wave length of the shear wave in the matrix material ensuring that the whole compound is vibrating as a quasi-homogeneous material. 

At the temperatures of the phase transitions, maxima of the dielectric constant up to 10,000 are found. Moreover, ia the ferroelectric state below T the material becomes pyroelectric and shows high piezoelectric activity. 

Of the 32 crystal classes, 20 lack a center of symmetry. Materials built up by these crystals have, therefore, the potential for piezoelectric activity. 

There are two principal types of materials that can function as piezoelectrics the ceramics and polymers. The piezoelectric materials most widely used are the piezoceramics based upon the lead zirconate titanate, FZT. The advantages of these piczoccramics arc that they have a high piezoelectric activity and they can be fabricated in many different shapes. 

There are two main approaches or configurations for the AFM-assisted detection of the local piezoelectric activity (pfm) in ferroelectric thin films for ferroelectric memory applications (FeRAM). The most used one was introduced in the early 90s and uses a conductive afm-tip as both top electrode and sensor for the induced vibration [2-4]. The second and more recent one [15,16], uses a normal metallic thin top electrode to apply the electric field and the vibration signal is detected by the AFM-tip above the top electrode. Both approaches present numerous advantages and disadvantages, as widely discussed in the literature, and are quite complementary. 

AFM-assisted detection of the local piezoelectric activity with a conductive AFM-tip is obviously the ideal tool to directly verify the existence and the expected properties of such a... 

The current understanding is that the MPB is not a sharp boundary but rather a temperature-dependent compositional range over which there is a mixture of tetragonal and monoclinic phases. At room temperature (300 K) the two phases coexist over the range 0.455 x 0.48 [7]. The enhanced piezoelectric activity of... 

PZT it has not been found possible to obtain a PZT ceramic as resistant to high pressures as BaTi03. For this reason the use of cobalt-doped BaTi03 for producing high acoustic powers has continued in some applications, despite its inferior piezoelectric activity. 

The replacement of zirconium by isovalent hafnium has little effect apart from shifting the morphotropic boundary (see Fig. 6.7) to 52mol.% PbTi03. Replacement by tin causes a slight loss in piezoelectric activity, shifts the morphotropic boundary to 42 mol. % PbTi03 and lowers the Curie point from 370 to 250 °C. 

Some applications require low mechanical as well as low dielectric losses combined with high piezoelectric activity. This seems to be best achieved by substituting a mixture of B-site donors and acceptors such as Nb and Mg in 2 1 atomic proportions. 

Fig. 18.8. Production of piezoelectrically active (polar) films of PVDF by stretching and poling. [Figure reproduced with permission from Ref. 40.]...

Giurgiutiu V, Zagrai AN. Embedded self-sensing piezoelectric active sensors for on-line structural identification. J Vib Acoust Trans ASME 2002 124(1) 116—25. http //dx.doi.org/10.1115/1.1421056. 

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 filler size, due to the decrease in tetragonality (ferroelectric phase). 

The QCM crystals used in this study are 1 inch diameter, AT-cut quartz crystals with thicknesses adjusted so drat di have resonant frequencies near 5 MHz (Valpey-Fisher). The central region of die crystal is sandwiched between two vapor deposited gold electrodes of ca. 300 nm thkkness widi a ca. 30 nm layer of Or applied prior to the gold to promote adhesion to die quartz substrate. A diagomiatical representation of die crystal is shown in Rgure 2. The area of die central, circular pad (i.e. the area of the crystal which is sandwiched between die gold electrodes) is 0.28 cm. This is die piezoelectrically active area of the crystal, a only mass changes which occur there are sensed by the QCM. 

Spun-cast polymers, such as polyvinylidenefluoride and its copolymers, show good values of pyroelectric coefficient, and favourable values of the figure of merit p/er. The major disadvantages of such materials are their large piezoelectric activity, and the necessity for electrical poling. In practical devices, piezoelectricity is a problem,because it can cause problems associated with microphony. 

PVDF is known to exhibit a strong piezoelectric effect [136] with the Pha.se I ( p form) being the most effective crystalline form for piezoelectric activity. Since molecular relaxation modes also contribute to overall piezoelectricity, high energy irradiatirm will affect the piezoelectric activity. This is due mainly to the effect of crosslinking which will increase the mechanical strength and change the molecular mobility of the polymer chains. A restriction in chain mobility will reduce reorientation of the molecular electric... 

While thermal budget reduction is the most promising approach, other approached using thermally and mechanically robust materials can be used to create MEMS devices. This approach affords a smaller range of material choices yet may result in more piezoelectrically active material due to the higher processing temperatures used. The question as to the most appropriate choice depends on the final requirements of the device in terms of performance, design and deployment. 

STRONG PIEZOELECTRIC ACTIVITY IN SIMULTANEOUSLY STRETCHED AND POLED POLY(VINYLIDENE FLUORIDE)... 

According to the theory developed by Wada and Hayakawa the piezoelectric activity in PVDF depends strongly on the magnitude of persistent polarization Ps which originates from a dipole orientation. The piezoelectric eai constant increases proportionally with the Ps. In order to orient the dipoles in form I crystals, it is necessary that the dipoles should be rotated in a main chain by the applied electric field. From this view point, it may be easily understandable that the internal rotation of dipoles more preferentially occurs in the roughly packed crystal lattice than in the closely and thermodynamically stable lattice. This is confirmed by the effect of poling on the intensity of X-ray diffraction. 

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