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Dielectric and piezoelectric properties of PVDF/PZT composites: a review.


Piezoelectricity (pressure electricity) was discovered in 1880 [1], while carrying out studies on effect of pressure on the generation of electrical charge by crystals such as quartz, tourmaline, and Rochelle salt. Subsequently in 1881, the converse piezoelectric effect [2] was proven, using fundamental thermodynamic principles. The constitutive equations of direct and converse piezoelectric effect are given below [3].

Direct effect : D = dT + [epsilon]E (1)

Converse effect : X = sT + dE (2)

where D = electrical displacement, d = piezoelectric coefficient, T = stress, [epsilon] = permittivity of the material, E = electric field, X = strain, s = mechanical compliance.

The discovery of PZT and BaTi[O.sub.3] and the family of these piezoelectric materials, in 1950s, is viewed as a major breakthrough in the field of sensors [4, 5]. These ceramics exhibit very high dielectric and piezoelectric properties and find wide applications as sensor and actuator devices [6-9]. A major environmental glitch in using these materials is that they are extremely toxic owing to the lead content; and their toxicity is only further enhanced due to the volatilization at high temperatures during calcination and sintering. The high densities of these materials lead to large characteristic acoustic impedances, necessitating the usage of matching layers. The large relative permittivities of these materials facilitate electrical tuning, and reduce their piezoelectric voltage coefficients. The large mechanical quality factor ([Q.sub.m]) requires the addition of damping layers to reduce ringing to an acceptable level. Finally, ceramics are brittle, inflexible, and nonconformable. Despite having high dielectric constants, ceramic capacitors have low inherent breakdown strength. This renders them unfit for applications like energy harvesting.

PVDF polymer offers unique advantages over ceramic such as they are flexible and therefore can be formed easily on to the curved surfaces. Further, PVDF is chemically inert, tough, creep resistant, and has good stability when exposed to sunlight [10-13]. In addition, it has a low density along with low dielectric permittivity resulting in a very high voltage coefficient. PVDF is a semicrystalline polymer which exists in four different phases viz. [alpha], [beta], [gamma], and [delta] [14]. Of the four distinct phases that PVDF can assume (Fig. 1), [beta]-phase is the only one that exhibits a spontaneous polarization, and thus piezoelectricity. PVDF also has several inherent disadvantages that make its use limited; such as low [d.sub.33] and [d.sub.h] values, low dielectric constant. Also, PVDF film requires high electric field for poling and makes the poling slightly difficult. Despite their high dielectric breakdown values, the low piezoelectric voltage constant makes them inapt for applications like energy harvesting.

Heterostructural materials, such as polymer ceramic composites (PVDF-PZT), have received a lot of attention since these materials can combine the excellent pyroelectric and piezoelectric properties of ceramics with the strength of polymers, flexibility, processing facility, levity, relatively high dielectric permittivity, and breakdown strength [15-20], which are not attainable in a single phase piezoelectric material. These properties make composites made of electro active ceramics and a ferroelectric polymer very attractive for applications as they exhibit low acoustic impedance matching with water and human skin and their properties can be tailored to various requirements [21-26].

The first attempts to produce such composites could be traced back to Kitayama and Sugawara [27], Pauer [28], and Harrison [29], who used PZT as a filler material and polyurethane as a matrix in (0-3) connectivity. In a 0-3 composite, the electric field that acts on an individual spherical piezoelectric grain is mostly controlled by the dielectric constant of the polymer phase. Since most polymers have a lower dielectric constant compared with piezoelectric ceramic materials, most of the applied electric field will pass through the lower dielectric constant phase. Table 1 illustrates the comparison of properties pertinent to transducer applications of piezoelectric ceramic, polymer and piezoelectric ceramic and polymer composites [4, 30].

Though it has been reported that the microgeometry of the inclusion and the interface between the filler and the matrix could be important [23-25], the mixing rules of a given property are controlled by the connectivity of the individual phases. Newnham [31, 32] has developed the concept of connectivity to describe the manner in which these individual phases are self-connected. In a diphasic system, there are 10 types of connectivity in which each phase is continuous in zero, one, two, or three dimensions (e.g. 0-3, 2-2) with the first digit referring to the piezoelectrically active phase and second digit is used for the electromechanically inactive polymer phase. Among the composites studied so far, the simplest types are those with 0-3 connectivity, which consists of a three-dimensionally connected polymer matrix loaded with piezoelectrically active ceramic particles [32-35]. This material is very attractive for several reasons [36]. 0-3 piezoelectric ceramic/polymer composite [37-40] has advantages over other composites, particularly in terms of cost, design of a composite, and ease of manufacture. Amongst, the most widely studied composites are those consisting of PVDF or its copolymers and PZT or BaTi[O.sub.3] [41-50]. PbTi[O.sub.3], Ca-modified PbTi[O.sub.3], Pb(Zr,Ti)[O.sub.3], Nb-modified (Pb, Ba)(Zr,Ti)[O.sub.3], and Pb([Mg.sub.1/3][Nb.sub.2/3])[O.sub.3]-Pb(Zr,Ti)[O.sub.3] ceramic powders of grain size of a few micrometer were used to prepare 0-3 type composites with polyvinylidene fluoride (PVDF) and copolymer of vinylidene fluoride and trifluoroelhylene [22, 30, 51-62] (Table 2).

From the practical application point of view, piezoelectricity was first explored in the year 1917, by P. Langevin et al. [64], with the use of quartz sandwiched between two steel plates, to serve as ultrasound transducers for detecting submarines and torpedoes. The detection process utilizes an electromechanical transducer operating in the pulse-echo mode to transmit ultrasonic pulses into the body and also to receive the faint echoes produced by reflections from internal structures. The ultrasonic beam transmission capability of a transducer material can be characterized by its piezoelectric longitudinal charge coefficient ([d.sub.33]) and the echo receiving sensitivity is directly related to its longitudinal piezoelectric voltage coefficient ([g.sub.33]). Large values for both of these coefficients are highly desirable and some researchers use the product of [d.sub.33] and [g.sub.33] as a figure of merit for pulse-echo transducers [46, 56]. However, of these two piezoelectric coefficients, [g.sub.33], may be considered to be slightly more critical since a larger [g.sub.33] value enables the intensity of the ultrasonic beam to be decreased [57]. The dielectric constant (K) of a material plays an important role for both low frequency hydrophone and high frequency medical imaging applications. A K ~ 100 permits a large voltage coefficient and eases the electrical impedance matching (tuning) between the transducer and the system instrumentation [65]. The dielectric loss factor (tan (5) should also be minimized so as to prevent the loss of signal energy [57]. The thickness mode electromechanical coupling coefficient ([k.sub.t]) [66] describes the energy conversion efficiency and potential sensitivity in better way than the piezoelectric coefficients previously discussed [56]. For maximum efficiency, a thickness mode transducer should have a minimal planar mode coupling coefficient ([k.sub.p]) so that the ratio [k.sub.t]/[k.sub.p] is as large as possible. A low mechanical quality factor ([Q.sub.m]) implies that a material has large mechanical losses and that signal energy is being wasted [65].

Today, the films of composite materials are optimized for special applications ranging from mechanical structures to electronic devices, from applications in automotive and aerospace to active vibration damping and structural health monitoring. Few prominent applications of these PVDF-PZT based transducers are found in high energy storage capacitors, acoustic emission detection, biomedical imaging with ultrasound, and as hydrophones to measure low frequency noises under water (the stress is considered to be effectively hydrostatic) and as pressure sensors [67].

Different procedures are developed and used in order to prepare the polymer-ceramic composites, e.g. compression molding [68, 69], solvent casting [70-72], tape casting [73], or spincoating [74], Compression molding and solvent casting lead to films with thicknesses in the range of approximately 30 to 500 pm while spin-coating usually leads to relatively thin films of only a few [micro]m. Tape casting technique makes use of multilayer ceramic processing developed for the capacitor industry with state of the art for this technology permitting ceramic layer thicknesses <5 [micro]m with separating electrode layers on the order of 1 [micro]m. If ferroelectric ceramics are embedded in a ferroelectric polymer, the piezoelectric and pyroelectric transducer properties can be well controlled and in particular, adjusted in such a way that either the piezoelectricity/pyroelectricity is compensated or pronounced due to the parallel or anti-parallel polarization of the ceramic and the polymer dipoles [69, 70].

This article presents an overview of current advancements in the development of PVDF-PZT composites as a discussion running parallel with the process timeline starting from the single phase homogeneous materials to the final heterogeneous composite under consideration. In what follows, the main focus is laid on the piezoelectric properties and dielectric properties. As a prelude to the main content, few of the salient processing methods for preparing the composite have been discussed. Certain areas have been put forth as a direction for future research and to fill in the gap in research. The whole article has been grouped under effect of grain size, %PZT content, influence of individual phases, methods of processing, solvents used, electrical conductivity, and other effect of dopants and effect of different treatment methods. The article evaluates the above mentioned sections under the purview of dielectric and piezoelectric properties, while evaluating the sensitivity of the material for transducer application.


Methods of Preparation

Different procedures employed to make piezoelectric composites are same as those employed in ceramic processing; they include align and fill [75-77], dice and fill [78, 79], injection molding [80-83], lost mold [84-92], tape lamination [93-97], dielectrophoresis [98, 99], relic processing [94-96], laser or ultrasonic cutting [100-102], jet machining [87-90], reticulation [103, 104], co-extrusion [105, 106], hot pressing [107], solvent casting [108-110], and spin coating method [111]. A brief description of each processing method has been given below. Detailed description with images can be found in Ref. 14.

Dice and Fill Method. To achieve the various connectivity patterns, the sintered piezoelectric block is cut and subsequently backfilled and finally polished. Composites with rods with widths of 50[micro]m, separated by keifs of 25 [micro]m are feasible to be produced by this approach [112]. Today, CAD-based wafer dicing systems are employed for dicing, to achieve mono-block transducers; especially, in making arrays for transducer applications [75-77, 113, 114].

Lost Mold Method. The pioneering work to develop this fabrication method was made by Rittenmyer et al. [115] and Siemens Inc. (Munich, Germany). LIGA (Lithography Galvano-forming and plastic molding) process [85] is employed to manufacture a plastic mold which is filled with a slurry; the desired structure is the negative of the plastic mold. After drying, the mold is burned out and the structure is sintered to more than 98% of the X-ray density. The feasibility of fabrication of hexagonal rods ~50 [micro]m in diameter, 400 pm in height, and spaced 50 pm apart have been successfully demonstrated using this process [87-90], Sintered honeycomb structures with ~10 [micro]m wall thickness can also be made using this method. Although the LIGA process is very accurate, it is also expensive and time consuming [84, 90].

Injection Molding. Composites can be manufactured with ease using this method. The production of prefonns with (2-2) sintered sheet composites as fine as 25 [micro]m and (1-3) PZT rods, as fine as 30 to 40 [micro]m in diameter, have been reported from this technique [82]. Some of the advantages in employing injection molding are rapid throughput, low material waste, flexibility with respect to the transducer design, and a low cost per part. On the contrary, the mold making is a time taking and costly process. To tackle this problem techniques such as lost mold method [87-90] are employed. This method has been adapted to manufacture net piece piezoceramics by Materials Systems Incorporated (MSI, Concord, MA), greatly facilitating the manufacturing of large volumes of complex ceramic parts for underwater transducer applications, among others [116].

Tape Casting Method. This method is extensively used for making stacked type (2-2 connectivity) composites such as stacked tapes of PZT and polymer [117]. By changing the type of powder even 2-0-2 composites can be manufactured [118]. Multilayer composites with (2-2) and (1-3) connectivity patterns also have been fabricated using piezoelectric and conductive tapes [119]. Better signal-to-noise ratios can be achieved in such multilayered ultrasonic transducers [120] by providing appropriate support structures during sintering.

Microfabrication by Co-Extrusion. Van Hoy et al. [121] and Halloran [122] developed microfabrication by co-extrusion (MFCX) method that involves forcing a thermoplastic ceramic extrusion compound through a die with a given reduction ratio. Objects with complex shapes are fabricated by assembling an extrusion feed rod from a shaped ceramic compound with space-filling fugitive compound. Extrudates are assembled into a feed rod and extruded again, reducing the size and multiplying the number of shaped objects. The process has the potential to be used for fabrication of objects in the size range of 10 pm [121, 122]. The features produced by MFCX is comparable to those produced using the lost-mold technique, synchrotron radiation lithography, and approaches the resolution realized by micro-molding using photolithography.

Solid Freeform Fabrication (SFF). The connectivity, shape, and spatial distribution of the ceramic phase in the composite have been the pivotal parameters in case of previously used methods. But apart from nondependence, this method also allows the fabrication of a composite possessing complex internal hierarchy and symmetry. Some of the SFF or rapid prototyping methods that have found commercial success are outlined in Refs. 123 to 132. Most of these techniques are designed to manufacture net shape polymer parts for form/fit applications and design verification. In any SFF technique, the CAD models are prepared first to obtain a surface file of the CAD model which in turn is converted into cross-sectional slices for defining the tool path and build strategy.

Hot Press Method. Hot press method [107] combines solution and melt processing techniques. The polymer is dissolved in a solvent such as ATV-dimethylformamide (DMF) and the solution is concentrated to get an optimum viscosity for the loading of ceramic powder. PZT powder is added and stirred well to get a uniform distribution of the filler. The prepared slurry is then coagulated by the addition of nonsolvent and dried. Then composite discs are made by thermolamination of the coagulated mass, under optimized conditions of temperature and pressure. The other alternate approach to the hot press method is using the pallets or chips or powder of the polymer as well as ceramic and pressing it into a film. This approach is very effective in manufacturing 2-2 composites structure. Tape cast composites are also subjected to uni-axial hot press sometimes to reduce the ceramic thickness further. Hot press method works well, when one want to produce bulk samples. Hot pressing method can be done with or without a mold with a cavity for making films. The thickness of the film is determined by the cavity provided in the mold. Preferably a stainless steel mold with a mold release, like silicone oil, is used. Alternately, a closed cavity with a ram to press the film can also be incorporated. This provides the advantage of adjustable thickness of the hot pressed film.

Solvent Casting. In solvent casting method [108-110] small as well large area films--thin or thick can be easily manufactured. In this method, PVDF as well as PZT are mixed together in a solvent, such as A'-dimethylacetamide (DMA), DMF, or dimethyl siloxane (DMSO). Heating as well as magnetic stirrers may aid the stirring process. The longer stirring time helps to achieve more uniform mixing of PVDF as well as PZT granules or powder in the solution. The effect of granule size as well as the percentage composition of the filler material has been discussed in detail, in the later sections. The solution is then poured over a glass mould and kept in the furnace for a particular duration. Once the solvent has completely evaporated in the furnace, PVDF-PZT composite film is obtained which crystallizes into polar phase. Sometimes PZT in the form of rods might be introduced and only PVDF solution may be prepared and casted in and around PZT rods. This approach is used to obtain PVDF-PZT composites with 3-1 connectivity.

Spin Coating. In spin coating [111] uniform thickness of the composite film is obtained by coating a substrate layer with the homogeneous casting suspension. Rotation is continued until the fluid spins off the edges of the substrate, and the desired thickness of the film has been achieved. The applied solvent is usually volatile, and simultaneously evaporates. Thickness of the film depends on the angular speed of spinning, concentration of the solvent, and the solution. For microfabrication applications, it can be used to create thin films with thicknesses below 10 nm. This method finds wide application in photolithography. The growing relevance of this method in CMOS and substrate wafer deposition has led to mathematical models being developed to study the spin coating.

Cold Isostatic Pressing. Piezoelectric composites suffer from porosity and air voids that affect its dielectric and piezoelectric properties. This porosity increases when increasing the weight percentages of the ceramic phase above 50%. It occurs during the curing and evaporation of the solvent in the composite. During evaporation, the solvent leaves pores in the polymer which then become filled with air. Applying high pressure to the film can densify the material and reduce the number and size of the air voids. Cold Isostatic Pressing (CIP) [133] is a technique that applies a homogenous and continuous (i.e. depending on the holding time) force across the surface of the material at room temperature.

Methods of Treatment

Other than above-mentioned preparation methods, many researchers have studied different processing techniques to improve the material properties. Different type of treatments can be useful in modifying the composite as per the requirements and thus provide ability to tailor composite properties. Some of these treatments methods are cold isostatic pressing [133], swelling the composite by dipping in solvent [72], heat treatments [134], hydroxylation [135], and microwave irradiation [136], etc. These are discussed further in this article.

Mathematical Models

One of the problems inherent to the composite systems has been to predict their macroscopic properties using the property of the constituents. This problem has been addressed by various researchers with varied levels of agreements between the theoretical and experimental values.

Dielectric Properties of Polymer-Ceramic Composites. The variation in accuracy to predict the dielectric properties by various models is greatly determined by the numerical formulation and the parameters considered during the formulation. The dielectric properties of the composites are influenced not only by the relative permittivity but also by other factors such as the morphology, dispersion, and the interactions between the two phases [137], Thus, it becomes a priority to predict the relative permittivity of the composite, using the relative permittivity of the components, apart from the volume fraction of the filler especially in sensor applications. The equations used to predict the dielectric constant ([epsilon]) value of the composites are as follows.

1. Maxwell-Garnett model [138]

[epsilon] = 1 + 3[v.sub.f][([[epsilon].sub.2] - [[epsilon].sub.1])/([[epsilon].sub.2] + 2[[epsilon].sub.1])]/ 1 - [v.sub.f][([[epsilon].sub.2] - [[epsilon].sub.1])/([[epsilon].sub.2] + 2[[epsilon].sub.1])] (3)

where [[epsilon].sub.1], [[epsilon].sub.2], and [v.sub.f] denote the dielectric constant of the polymer, dielectric constant of the filler and volume fraction of the filler, respectively. This was probably the first model developed (1904) to predict the dielectric properties of composites and continues to be widely used even today. The ceramic particles (spherical shape) are assumed to be randomly distributed in the polymer substrate with no interaction between the individual phases.

2. Furukawa model [139]

[epsilon] = 1 + 2[v.sub.f]/1 - [v.sub.f] [[epsilon].sub.1] (4)

where [[epsilon].sub.1] denote the dielectric constant of the polymer and [v.sub.f] volume fraction of the filler. Furukawa model also assumes spherical and uniformly dispersed ceramic in polymer matrix. The dielectrically homogeneous system is assumed to depend mainly on the dielectric behavior of the matrix.

3. Maxwell-Wagner equation [140]

[[epsilon].sub.eff] = [[epsilon].sub.m] 2[[epsilon].sub.m] + [[epsilon].sub.i] + 2[v.sub.f]([[epsilon].sub.i] - [[epsilon].sub.m]/2[[epsilon].sub.m] + [[epsilon].sub.i] - [v.sub.f]([[epsilon].sub.i] - [[epsilon].sub.m]) (5)

where [[epsilon].sub.eff], [[epsilon].sub.i] and [[epsilon].sub.m] are the permittivity of the composites, filler, and matrix, respectively, and [v.sub.f] is the volume fraction of the ceramic. The Maxwell-Wagner equation holds well only when the properties of the two phases in the composite are similar [140].

4. Rayleigh's model [141]

[epsilon] = [[epsilon].sub.1] 2[[epsilon].sub.1] + [[epsilon].sub.2] - 2[v.sub.f]([[epsilon].sub.2] - [[epsilon].sub.1])/2[[epsilon].sub.1] + [[epsilon].sub.2] + [v.sub.f]([[epsilon].sub.2] - [[epsilon].sub.1]) (6)

where [[epsilon].sub.1], [[epsilon].sub.2], and [v.sub.f] denote the dielectric constant of the polymer, dielectric constant of the filler and volume fraction of the filler, respectively. This model was developed by Rayleigh based on his theory deduced from inferences taken from both the Maxwell-Garnett and Furukawa models for biphasic composite materials containing minor spherical filler.

5. Bhimasankaram model (BSP model) [141, 142]


where [epsilon], [[epsilon].sub.1], and [[epsilon].sub.2] are the dielectric constant of the composites, polymer matrix and filler respectively, [v.sub.f] is the volume fraction of the ceramic. This model assumes that the spherical piezoelectric material is randomly dispersed in a continuous medium of polymer. The main difference from the previous models is that each dielectric sphere is polarized and the dipoles get aligned in the direction of the applied electric field. These dipoles locally modify the field in the neighboring region and become predominantly important in case of larger fraction of piezoelectric particles. This model is also referred to as Jayasundere-Smith equation.

6. Series mixing formula [143]

1/[[epsilon].sub.eff] = [v.sub.f]/[[epsilon].sub.i] + (1 - [v.sub.f])/[[epsilon].sub.m] (8)

where [[epsilon].sub.eff], [[epsilon].sub.i], and [[epsilon].sub.m] are the permittivity of the composites, filler, and matrix, respectively, and [v.sub.f] is the volume fraction of the ceramic.

A plot showing the variation of the above models is as shown in the Fig. 2. Furukawa model (1979) plotted in the graph is not applicable for [V.sub.f] = I. Another Furukawa model has also been proposed which considers the effect of both the phases. The equation is same as the Maxwell-Wagner equation. The Rayleigh's model, the Maxwell-Wagner equation as well as the Furukawa model considering the effect of both the phases are same. The Jaysundere-Smith equation is same as the BSP model presented by Bhimasankaram et al. [141], All the models are found to trace the same trajectory for lower volume fractions.

In general, the theoretical predictions are valid for low filler contents and deviations from predictions increase with increasing filler contents. This is mainly due to the imperfect dispersion of filler ceramic particles at higher filler contents and also due to porosity or air enclosed by the composite.

All these above mentioned models do not take into account the shape of the ceramic particle or particle size or the interaction between the particle and matrix. A general assumption is made that the particle is spherical and the interactional effect between the particles are overlooked. However, in subsequent models these effects are included.

7. Modified Lichtnecker equation [143]

log [[epsilon].sub.eff] = log [[epsilon].sub.m] + [v.sub.f](1 - n) log([[epsilon].sub.i]/[[epsilon].sub.m]) (9)

where, [[epsilon].sub.eff], [[epsilon].sub.i], and [[epsilon].sub.m] are the permittivity of the composites, filler, and matrix, respectively, and [v.sub.f] is the volume fraction of the ceramic. The Lichtnecker logarithmic rule considers the composites as a random mixture of nearly spherical inclusions. The theoretical predictions are valid for composites with low volume fraction of filler material and shows deviation at higher filler content. This is mainly due to the imperfect dispersion of the filler ceramic particles at higher filler content and also due to the porosity or air enclosed by the composite. They are valid for only those composites where the filler and matrix relative permittivity values are nearly same. The modified Lichtnecker equation includes a fitting factor n, which represents the interaction between the filler and the matrix.

8. Effective medium theory (EMT) [143]

[[epsilon].sub.eff] = [[epsilon].sub.m][1 + [v.sub.f]([[epsilon].sub.i] - [[epsilon].sub.m])/ [[epsilon].sub.m] + n(1 - [v.sub.f])([[epsilon].sub.i] - [[epsilon].sub.m])] (10)

where [[epsilon].sub.eff], [[epsilon].sub.i], and [[epsilon].sub.m] are the permittivity of the composites, filler, and matrix, respectively, and [v.sub.f] is the volume fraction of the ceramic, and n is the fitting parameter or the morphology factor. The value n is found to be somewhat sensitive to both the polymer and the ceramic, thus reducing the feasibility of the Lichtnecker equation for different materials [145]. In EMT model, the dielectric property of the composite is treated as an effective medium whose relative permittivity is obtained by averaging the permittivity values of the constituents. The EMT model is a self-consistent model that assumes a random unit cell consisting of each filler particle surrounded by a concentric matrix layer.

9. Yamada model [146]

[epsilon] = [[epsilon].sub.1][1 + n.[v.sub.f]([[epsilon].sub.2] - [[epsilon].sub.1])/n.[[epsilon].sub.1] + (1 - [v.sub.f])([[epsilon].sub.2] - [[epsilon].sub.1])] (11)

where [[epsilon].sub.1], [[epsilon].sub.2], and [v.sub.f] denote the dielectric constant of the polymer, dielectric constant of the filler, and volume fraction of the filler, respectively. Yamada et al. [146] made one of the most general attempts of describing the dielectric behavior of composites. It is based on the properties of the individual materials and also considers a factor (n = 4 [pi]/m) related with the shape and relative orientation of the filler, while others authors only work with spherical particles.

The shape factor or fitting factor varies from material to material as well as for various particle sizes. The graph of the theoretical prediction for PVDF-PZT composites in case of particle sizes of 0.84 mm, 1.5 mm, and 1.65 mm are shown in Fig. 4.

If we compare these trajectories with the experimental results, for higher volume fractions, the deviation becomes significant (as shown in Fig. 3). The Jayasundere-Smith equation is only valid for filler content up to 0.3 vol% as it only considers the interactions between the neighboring spheres. The Maxwell-Wagner rule holds good only when the properties of the two phases in the composite are similar [140].

The behavior of the complex dielectric constant was evaluated according to the theoretical models presented previously. The results obtained for [alpha]-PVDF/PZT and [beta]-PVDF/PZT composites are shown in Fig. 5 [ 147].

Piezoelectric Properties of Polymer--Ceramic Composites. Not many models are available which predict the piezoelectric properties of the composite. Two noted models available to predict the piezoelectric properties are

1. Yamada model [146]


where [alpha] is the poling ratio of the ceramic inclusions, [[epsilon].sub.33i], is the dielectric constant of the ceramic inclusions in the ith poling direction and n is a dimensionless parameter depending on the shape and orientation of the ceramic particles. By fitting experimental data for [[epsilon].sub.33] and [d.sub.33] values of composites and varying the structural parameter n, [d.sub.33] and [g.sub.33] values for volume fractions up to the theoretical maximum can be predicted. The values are found to be in good agreement up to a volume fraction of 0.7.

2. Furukawa model [ 139]

d = [v.sub.f][L.sub.T][L.sub.E][d.sub.2] (13)


g = [v.sub.f][L.sub.T][L.sub.D][d.sub.2] (14)

where d and g are piezoelectric coefficients of composites; [d.sub.2], [g.sub.2] are piezoelectric constants of the phase 2 i.e. filler material. [L.sub.T], [L.sub.D], and [L.sub.E] are local field coefficients with respect to T, D, and E, respectively and [v.sub.f] is volume fraction of filler.


The piezoelectric composite from preparation stage to the application stage follows a process flow as illustrated in the Fig. 6. The polymer matrix under consideration is polyvinylidine fluoride (PVDF), while the filler material was chosen to be the PZT ceramic. The first step in the process timeline is to define the desired properties in the resulting composite and selection of the single phase materials. The resultant properties are given by the X-Y method. The determination of the properties can be done at a theoretical level by substituting the material properties of the individual phases in the available mathematical models. The composite under consideration is the PVDF/PZT composite. The pros and cons of the individual phases which drive the need for developing this composite has been already discussed in the preceding section. The next stage is the choice of grain size and percentage composition of the filler material.

Effect of Grain Size

On Microstructure. The grain size, being the building block, has a profound impact on the microstructure of the PVDF-PZT composite. The grain size can be varied both at the crushing stage and the stirring stage. The smaller grain size would facilitate a more homogeneous composite. But the adverse effect of the smaller grain size is that it leads to a lower surface energy of the composite particles while polishing, leading to adverse effects on the properties of the composite. On using very small particles the film thickness should be chosen such that the ceramic filler material is not completely encapsulated by the polymer substrate. Otherwise most of the electric field acting on the individual spherical piezoelectric grain will pass through the polymer phase of low dielectric constant. But on limiting the thickness of the material, it allows even the ceramic phase to be poled. Alternatively, to resolve this problem, PVDF is poled at an extremely high electric field, which in turn limits the thickness of the material [148, 149].

Additionally, grain size of PZT can affect the piezoelectric and dielectric properties of PZT, which in turn might affect the composite properties. Such variations in PZT properties have been discussed in details by various researchers (Jin et al. [150], etc). A comparative account of those has been presented by Clive A. Randall et al. [151], However, detailed discussion on effect of grain size on PZT properties alone are out of scope of present work and hence have not been discussed here. Here, variation in overall composite properties with grain size of PZT is presented.

Rujijanagul et al. [152] investigated the microstructural level implication of varying the particle size ([PHI]) and concluded that a difference in the morphology was witnessed. Further investigations were carried out at the microstructural level by Firmino Mendes et al. [147], who investigated this variation with respect to the [alpha] and [beta] phases (Fig. 7) and concluded that, with increase in ceramic content, the large spherulitic characteristic of the [alpha]-phase materials disappear [153, 154].

Though, unoriented films exclusively in the [beta]-phase can be obtained by crystallization of PVDF solution with DMF or DMA at temperatures below 70[degrees]C, but they are not preferred due to their high degree of porosity [155], because high porosity reduces the dielectric constant and renders the films out of condition for poling, which is necessary for the applications involving the piezoelectric, pyroelectric, and ferroelectric effects [153, 155, 156], Also, porosity renders the material highly fragile [155] and the films cannot be oriented by stretching. In case of [beta] phase composite samples, the characteristic porosity [154] is reduced with respect to the [beta] phase PVDF films as the PZT particles occupy the pores in the polymer structure. The inclusion of ceramic particles in the PVDF polymer matrix increases the complex dielectric constant and dynamical mechanical response of the composites. The resulting improvement is found to be same for both [alpha] and [beta] phase, with the PZT content unchanged. Thus, the polymer phase is not the main contributing factor for this composite behavior. Also there was a more pronounced increase in the dielectric and piezoelectric properties when a larger grain size of PZT was considered. This can be attributed to the increased probability of ceramic particles getting poled or the increased magnitude of the surface energy.

On Dielectric Properties. Firmino Mendes et al. [147] investigated the effect of PZT grain size as well as its percentage composition in PVDF on the dielectric properties. Fie found that the inclusion of ceramic particles increases the complex dielectric constant and dynamic mechanical response and the value is independent of the phase of PVDF polymer matrix (Fig. 8). Rujijanagul et al. [152] also reported that the dielectric constants increased with the samples containing the bigger particles and higher volume of ceramic powder. The increasing nature of the dielectric constant of the composites with the ceramic powder having bigger particles agrees well with result reported by Enomoto and Yamaji [157], Chattopadhyay et al. [158], and Lee et al. [159].

On Piezoelectric Properties. Banno et al. [35, 160, 161] investigated the effect of particle size in order to explain the variations in pressure dependence. Larger sized particles seemed to produce higher [d.sub.h] values, yet seemed to show moderate pressure dependence attributed to the porosity formation associated with the usage of larger particles. This was confirmed by relative density measurements, which was of the order of 93 to 96%. Smaller sized particles produced lower [d.sub.h] values, but showed little or no pressure dependence, which was attributed to little or no porosity and was confirmed by relative densities approaching 100%. The composites with the bigger PZT particle size gave higher [d.sub.33] values. A significant increase of the surface energy of the powder and the piezoelectricity was found in the composites with the ceramic powder with bigger particle size. This may enhance the effectiveness of poling the composites, thus producing the higher [d.sub.33] value, as contacting surface area between the bigger ceramic powder particles and the polymer is probably larger. During the sample polishing procedure, the surface damage in the smaller particle PZT powder is more than that of the coarse ones. Therefore, this could result in further lowering the [d.sub.33] value.

Greeshma et al. [162] investigated the role of PZT particle in phase formation of PVDF. PZT particles ranging from 40 nm to 106 mm were chosen for the study. The studies revealed that the electrical and structural properties of ceramic-polymer composites are influenced by the size of ceramic particle. DSC studies confirmed the decrease in the crystallinity percentage with decreasing ceramic particle size. De-convolution of DSC peaks revealed the decrease in the amount of [beta]-phase with decrease in particle size and it was confirmed by FT-1R studies as well. It was found that the PZT particles acts as a source of field and converts the other polymer phases to [beta] phase during poling. The decrease in the amount of [beta] phase led to the reduction in piezoelectric coefficient and remnant polarization with decrease in ceramic particle size.

Effect of PZT% Content

On Microstructure. Figure 9 shows the SEM photograph of the pure [alpha] and [beta]-phase of the PVDF and for PVDF-PZT composite film [163]. The grain-like feature observed on the surface of a-phase films are aggregates of spherulites (shown by arrow in Fig. 9a) contain acicular crystallites emanating in radial direction from the center (Fig. 9a). After hot-stretching of PVDF, the [alpha]-phase film transforms to [beta]-phase, the spherulites disappear and surface shows oriented fibril-like structure (Fig. 9b).

Addition of PZT powder leads to uniform distribution of PZT particles in the PVDF solution and also results in disappearance of spherulites (Fig. 9c-f). Better microstructure is observed in PVDF-coated composite films due to filling up of pores and reduction of structural defects. At lower concentration (10%), PZT uniformity in the solution is more and decreases with increase in the weight fraction of PZT (50%). PZT being denser, the chances of settling of PZT powder at higher weight fraction are more. Also at same magnification, top surface (Fig. 9c and e) shows less concentration of PZT particles as compared with bottom surface (Fig. 9d and f). This can be explained on the basis that the PZT powder starts settling down during the time of solvent evaporation itself. This feature has also been observed by earlier investigators Seema et al. [107], Firmino Mendes et al. [147] and Suresh et al. [111],

On Dielectric and Piezoelectric Properties. Fries and Moulson [165], while fabricating 0-3 PZT/PVDF composite, investigated the variation of the dielectric and piezoelectric properties (Fig. 10) with the percentage of PZT content. They found that the value of both the parameters increased with the increase in PZT content. As reported by Firmino Mendes et al. [147] the percentage increase in the values is independent of the phase of PVDF and mainly proportional to the PZT content. The increase in the dielectric and piezoelectric properties is mainly due to the decreasing percentage of PVDF which has very low dielectric constants when compared with PZT.

The dielectric constant variation with frequency for PVDF and PVDF-PZT composite films measured at RT is shown in Fig. 11. The increase in dipole-dipole interaction with increase in PZT content leads to higher dielectric constant in higher PZT composites.

Wesley Hackenberger et al. [73], in his research to produce single element and array transducers with resonance frequencies ranging from 20 to 50 MFIz, investigated the maximum percentage of PZT% content that can be adopted with reference to tape casting method. The stack of tapes was isostatically laminated followed by slow heating to remove the binder and volatilize the carbon. Kerfs were back filled with epoxy under vacuum in the space formed by decomposition of fugitive phase. The remaining process of composite fabrication was same as previously reported [80]. The strength of the tape was compared for varying %composition of ceramic in the polymer matrix. It was found that the strength of the tape decreased with increased ceramic content and 60% ceramic content was found to be the maximum amount that can be incorporated when the tape strength is considered. On increasing the support structure for the tapes, the uniformity was observed to improve. As per recent investigation by Buyer and Roundy [166], nonporous composites are only achieved if the ceramic volume fraction is adjusted below approximately 0.5. Though, PVDF-PZT composites developed using pure tape-casting technique are not reported commonly however, scattered attempts on preparing PVDF-PZT can be found in order to prepare 1-3 composites. PZT pillars are initially prepared by using extrusion process. Later these pillars can be reinforced in a mold where-in PVDF can be filled by back Filling process. However, the above mentioned method would be very costly.

Influence of Individual Phases of PVDF and PZT on the Composite

It is found that certain properties are tied together with the presence of PZT while other properties depend on PVDF phase. Abdullah and Gupta [53] studied the effect of PVDF and PZT phases on the composite by studying the absorption currents. PZT ceramic powder (3 pm) were mixed up with PVDF at 443 K, using a hot roller machine and synthesized into a film by compression molding method. Samples were then thermally treated in an evacuated measurement chamber (<[10.sup.6] torr) at 373 K for 24 h, before current absorption measurements were performed. The dielectric dispersion measurements were made apart from the pyroelectric currents using a direct method [166], by applying a linear heating rate of approximately 1[degrees]C [min.sup.-][1] to the samples which have been poled appropriately. Absorption currents and dielectric measurements were also made with PZT discs and piezel. The current was found to decrease progressively with time; however, at longer time and higher fields, the rate of decrease was greatly reduced. A possible explanation for the same could be the tendency to reach a steady state level. The broad peak at low charging fields, which moves to shorter times at higher fields, may be due to space charges in PVDF [167], At low temperature, the difference in current is very small but as the temperature reaches 363 K, a difference is found in PZT/PVDF composites with the order of factor ~30 in 10 and 50 vol% PZT. This confirms that at higher temperatures PZT phase contributes significantly. Comparing the PZT/PVDF curves (Fig. 12) at 10 and 50 vol% PZT shows the addition of more PZT phase will increase the d-values (Fig. 13).

Theoretical values obtained from permittivity equation for composite system by Yamada et al. [146] was in agreement with the observed readings. The observed peaks which occur at ~360 K in PZT/PVDF and at ~350 K in piezel are due to relaxation which is associated with molecular motions in the crystalline region of the polymer. The observed values of e" also demonstrate the substantial contribution of the high PZT phase content of PZT/PVDF composites at high temperatures (Fig. 14). When the sample was short circuited, a linear heating rate of 1 K [min.sup.-1] was applied from ~290 to 373 K and then cooled to room temperature; during the first run a very high increase in current was reported with peak around 373 K. In subsequent runs, considerable reduction in increase of current was observed indicating reversible pyroelectric current has been established. A similar observation had been made by Shakhtakhtinski et al. [168] in which no peak had been observed below 373 K. These experiments bring out the conclusion that the electrical conduction behavior in the PZT/PVDF composite may originate from an ionic hopping mechanism with as significant contribution from the PZT phase. However, the dielectric loss process has been controlled by the polymer phase with some substantial contribution from the PZT phase at low frequency and high temperature regions.

Experimental studies [38, 169] reveal that hot press technique was found to be more effective for the preparation of 0-3 connected composites with better piezoelectric properties. Greeshma et al. [170] investigated the influence of the individual phases by using densified and calcinated reinforcement by hot press technique and the analysis of the depoling studies. PZT-PVDF composites were prepared using solvent casting method, with cyclohexanone as a solvent. Depoling tests were carried out and [d.sub.33] values were measured by heating the test specimen in a temperature range of 30 to 100[degrees]C in steps of 5[degrees]C. The temperature was maintained for 10 min and then cooled to room temperature. The piezoelectric coefficient for the composite with calcinated PZT was found to be 14 pC/N whereas the composite with densified PZT shows 27 pC/N which is reported for the first time. The calcinated composite when subjected to stress will get displaced leading to an ineffective energy conversion. But in case of densified particles we witness an effective conversion. In case of pure PVDF, the [d.sub.33] values remain constant till 70[degrees]C and thereafter decrease rapidly. Similar effect has been reported by Tim et al. [171], Figure 15 shows the variation of [d.sub.33] with de-poling temperature for the composites with calcinated and densified PZT particles.

The [d.sub.33] value decreases exponentially with temperature. This is due to the oriented dipoles in the composite attain equilibrium position with temperature. A similar trend is also observed for the pure PZT, where the [d.sub.33] value decreased exponentially. The plot for the composite with calcinated reinforcement shows that the [d.sub.33] value is independent of the temperature up to 80[degrees]C and then decreases rapidly. A similar variation of [d.sub.33] is observed for pure PVDF which implies that the [d.sub.33] value is contributed mostly by the polymer phase. The study indicates that for composite with calcinated particles the [d.sub.33] value is contributed mostly by the polymer phase whereas the PZT particles merely act as a source of electric field which in turn decreases the poling temperature.

The Furukawa model [139] provides a sense of clarity in how does the d-value vary with respect to the individual phases. This variation has been tabulated in Table 3.

With increase in PZT content (or decrease in PVDF content) the piezoelectric constant d increases. The summary of variation of various properties with respect to the influencing phase/content is enlisted in the Table 4.

Effect of Dielectric Relaxation in PVDFIPZT Composites

Hilczer et al. [172] reported that the dielectric relaxation related to the wide-angle oscillation of polar groups attached to the main polymer chain determines the dielectric behavior of the composites. The aim of the study was to obtain a composite made of PVDF with PZT nanoparticles with dielectric response in wide frequency range of 100Hz to 1 MHz and temperature range 100 K to 450 K.

Dielectric absorption behaviour apparent in the range 180 to 330 K fits to the Vogel-Fulcher equation:

[[tau].sup.v-f] = [[tau].sub.0.sup.v-f.] exp[B/T - [T.sub.0]] (15)

with [[tau].sub.0] = 1.6 X [10.sup.-9] s, B = 364 K and [T.sub.0] = 223 K.

[[tau].sub.0] denotes the relaxation time and T is temperature and [T.sub.0] is characteristic temperature of static dipolar freezing of dipolar motion in the absence of long-range correlation. At [T.sub.0] all relaxation times diverge and the distribution of the relaxation times becomes infinitely broad [172], (Here, v-f in superscript stands for Vogel-Fulcher).

Dielectric absorption in the crystalline phase of the polymer observed in temperature range from 330 to ~380 K is described by the Arrhenius law:

[[tau].sup.A] = [[tau].sub.0.sup.A] exp[[DELTA]H/kT] (16)

with [[tau].sub.0] = 8 X [10.sup.-20] s, k is Boltzmann's constant, and [DELTA]H = 108 kJ/mol and T denotes temperature (here, A in superscript stands for Arrhenius).

Figure 16 shows the Dielectric relaxation time of radially oriented PVDF as well as PVDF-PZT where the solid lines represent the fits to the Vogel-Fulcher equation and Arrhenius law.

Dipolar segments of the main chain and side groups of various length scales can rotate resulting in a variety of relaxation processes. The dielectric relaxation studies resolve only the frequency/time scale. It was reported that the melting temperature of PVDF depends strongly on the phase content: the melting temperature of TGTG conformation is close to 448 K, the all-trans conformation with folded chains melts at 458 K, whereas that with extended chain has [T.sub.m] [approximately equal to] 480 K [173, 174]. The knowledge of dynamic melting behavior is important in the interpretation of high temperature dielectric relaxation data in the polymers, as the melting point depends on both the heating rate and the probe frequency [175], Inference from the work was that in the low temperature range the dielectric response of the composites is determined by the anomaly characteristic of the glass transition of the polymer, whereas in the high temperature range the relaxation related to wide angle oscillation of polymer polar groups followed by their rotation with main chain co-operation is dominant.

A Comparative Account: PVDF, PVDF-PZT Composite and PVDF-PZT Composite Coated with PVDF: Spin Coating of a Film Initially Prepared Using Sol-Gel Process

Suresh et al. [111] used sol-gel process as well as spin coating technique to prepare polymer films and composite films of PVDF and PZT and coated it with PVDF. They dissolved PVDF in DMSO and allowed it to stir for 12 h till the complete dissolution of the polymer. The nanocomposites comprising of nano-sized PZT particle and PVDF polymer were prepared at 60[degrees]C for dissolution of PVDF and different amount of PZT (0 wt% (PVDF), 5 wt% (PVDF-PZT5), and 10 wt% (PVDF-PZT10)) in DMSO with full stirring to obtain uniform and homogeneous casting suspension. This solution was coated onto an AZO-coated PET substrate as well as PZT-PVDF substrate using spin coating; to get a uniform film thickness of 6 pm. PVDF-PZT film was dried at 100[degrees]C for 22 min and simultaneously annealed at 80[degrees]C for 1 h to improve the crystallinity and adhesion between the electrode and the composite.

The SEM results of PVDF and PVDF-PZT5 (Fig. 17) were compared and found that the morphologies reveal the homogeneity of the PZT grain distribution and predominant 0-3 connectivity. Needle type distributions indicate that the [alpha]-phase predominates. When the XRD test was carried out, PVDF films resulted in little amount of [beta]-phase and more a-phase as observed at 20 of 20[degrees] and 33.8[degrees], respectively [176]. As the weight fraction of PZT increases, uniformity will be reduced due to variation in the settling time of PZT. Better microstructure is observed in PVDF coated composite films due to filling up of pores and reduction of structural defects as shown in Fig. 18.

Figure 19 shows hysteresis loops of PVDF film and PVDF-PZT composite films recorded at room temperature (RT). The lower remnant polarization of pure PVDF film (1.99 [micro]C/[cm.sup.2]) might be affected by the random distribution of needle like grains and existence of higher amount of a phase. The higher remnant polarization in PVDF-PZT10 composite can be attributed to the enhanced polarization from the dipole-dipole interaction of closely packed powders as well as crack free and defect free composite formation in the polymer film.

The improvement in remnant polarization by coating the PVDF layer on composite film is observed in Fig. 20. The addition of PVDF layer on composite film has an effect in reducing the leakage current density from [10.sup.-7] to [10.sup.-9] A/[cm.sup.2]; the leakage current density is low enough for device application.

The permittivity reported for 5% and 10% PZT were 47 and 66. This higher value over other values reported for tape casting, hot press method et al. is due to the increase in packing density owing to sol-gel method of preparation. The dielectric constant of the composites at low frequency is higher than that at higher frequency. It is found that there is a sudden decrease in dielectric constant beyond a particular frequency; this might be attributed to the relaxation losses related to the PVDF polymer [177].

Effect of Hot and Cold Pressing

On the Structural Properties. The uniformity in layer thickness is affected by the packing density before the sintering or shaping stages. One approach, opted by Hackenberger et al. [73] to study the same, was to investigate the percentage composition of PZT in the polymer composite (discussed under Effect of PZT% content On Dielectric and Piezoelectric Properties section). Zhang et al. [169] investigated these variations in the structural and electrical properties by evaluating the impact of shaping process (hot and cold press approach). The PVDF/ PZT composites were prepared with the PZT nanopowders content (with 40, 50, 60, 70, and 80 vol%). One set was pressed at 150[degrees]C while the other was pressed at normal temperature with a pressure of 10 MPa. Figure 21 shows the schematic illustration of microstructure in PVDF-PZT composites by both the methods. Figure 22 shows the SEM photographs of these composites. In case of the cold press approach, the PVDF does not obtain the curls which still exist on the PZT. But during hot press, the PVDF has obtains the full curl in a particular orientation and quite a few of these PVDFs exist by the shaping of PZT. Thus, indicating the connection characteristic of wrap and curl leading to reduced number of crevices as the grains of the PZT and PVDF are much closer and smaller.

On Piezoelectric Properties. The [d.sub.33] value increases with increase in the PZT content in both hot and cold press method

(Fig. 23) as reported by Zhang et al. [169], However, in our laboratory, it was found that the [d.sub.33] values will be always higher in case of hot pressed composites. In cold-press process, PVDF and PZT are only piled up simply, and PVDF merely holds some positions of the PZT; and the electric performance of the PVDF is far lower than that of the PZT. This explains the increase in the electrical properties with increase in PZT content. The increase of [d.sub.33] values with increase in PZT content is consistent with the works of Son et al. [178]. XRD graphs of PZT/PVDF composites (Fig. 24) shows a peak PVDF between 17.3 and 20.7[degrees]. It is therefore presumed that the [beta]-type crystals have been formed. This leads to better electrical properties for hot pressed samples. Beyond 70% there is a dip in the values of [d.sub.33], as observed. This phenomenon can be explained in terms of dielectric breakdown of the material as discussed by Yuan et al. [179].

On Dielectric Properties. The dielectric constant value also increases in a nature similar to the [d.sub.33] values (Fig. 25). The drop in the [epsilon] value beyond 70% in case of hot press method depicts that the dielectric breakdown of the composite has occurred. As the content of PZT exceed 70%, the wrap and curl effect of the PVDF descends and the content of PVDF also reduces, which makes the decline of the values of electric properties ([epsilon] and [d.sub.33]) for the composites prepared by hot-press approach.

Effect of Cold Isostatic Pressing. Almusallam et al. [133] reported improvement in dielectric and piezoelectric properties of screen-printed PZT/polymer films for flexible electronics applications using Cold Isostatic Pressing (CIP). The investigation involved half/fully cured PZT/polymer composite pastes with weight ratio of 12:1 to investigate the effect of the CIP process on the piezoelectric and dielectric properties. It was observed that the highest dielectric and piezoelectric properties are achieved at pressures of 5 MPa and 10 MPa for half and fully cured films, respectively. The relative dielectric constants were 300 and 245, measured at 1 kHz for the half and fully cured samples. Using unoptimized poling conditions, the initial [d.sub.33] values were 30 and 35 pC/N for the half and fully cured films, respectively. The fully cured sample was then poled using optimized conditions. It gave a [d.sub.33] value approximately 44 pC/ N, i.e. an increase of 7% compared with non-CIP processed materials.

Effect on Density and Dielecric Properties of PVDF-PZT Composites Prepared by Hot Press and Tape Casting Techniques Seema et al. [107] carried out comparative study of PVDFPZT-5H Composites prepared by hot press and tape casting method, under the purview of loss on ignition, physical and dielectric properties. PZT concentration was varied between 20% and 60% volume. In Hot press method, the viscous solution of PVDF in DMF is loaded with the ceramic powder, coagulated by a nonsolvent, dried, and then pressed under optimized conditions of temperature and pressure. In case of tape casting, a well-dispersed and concentrated solution is degassed and spread on a flat moving carrier surface using double doctor blade process. The solvent is evaporated and a thin sheet of high uniformity is formed. The tapes are then cut into the required shapes, stacked and laminated similar to hot press technique. The increase in powder packing density increases the uniformity of thickness.

Theoretical value of density was calculated using equation

[rho] = [V.sub.f] [[rho].sub.f] + (1 - [V.sub.f]) [[rho].sub.m] (17)

where [rho], [V.sub.f], [[rho].sub.f], [[rho].sub.m] are the density of the composites, volume fraction of filler (ceramic), density of filler, and density of matrix (PVDF). Density of PZT was taken as 7.5 g/[cm.sup.3] and density of PVDF as 1.7 g/[cm.sup.3] for calculation. These theoretical values were calculated with the assumption that there are no voids or defects in the composites. However, during the preparation, minor voids or defects may be formed which results in lower density. Compared with tape casting technique, nonsolvent method gives excellent dispersion of ceramic particles in the polymer matrix and porosity will be less in non-solvent hot pressed samples. At low concentration of PZT, density of the composites prepared through hot press and tape techniques shows close values while in hot press density will be higher. At higher volume fraction (> 30%), it is difficult to ensure uniform filler distribution as the ceramic particles may form aggregates and may tend to settle down in case of tape casting leading to lower density (Table 5).

The dielectric constant variation is as shown in the Table 6. The dielectric constant of the composite increases with filler loading and dielectric loss is only marginally changed with filler concentration. The dielectric constant of the composites decreases as the frequency increases. The dielectric characteristics of the composites prepared through hot press technique showed better results compared with tape casting technique.

Effect of Crystallinity on Dielectric and Piezoelectric Properties of PVDF-PZT Composites

Dong et al. [134] studied the effects of heat treatment methods on crystallinity of PVDF/PZT composites. They found that the crystallinity of PVDF in composites of PVDF/PZT can be controlled effectively by different heat treatment methods. They prepared three different samples with different heat treatments. With volume ratio of PZT and PVDF 70/30, PZT and PVDF powders were mixed in a mini type mixer. Composites of 12 mm in diameter and 1.2 mm in thickness were prepared from as-mixed PZT and PVDF powder using a conventional hot-press method at 180[degrees]C and 10 MPa for 20 min and then different heat treatment were employed. Sample 1 was prepared by immediate quenching to hot-pressed sample using ice water. Sample 2 was prepared by decreasing the temperature to 120[degrees]C and at constant pressure for 4 h, and finally quenched using ice water. Sample 3 was prepared by cooling to room temperature at 10 MPa. Crystallinity of these samples was studied by DSC and found to be 21%, 34%, and 27%, respectively.

The dependence of dielectric constant [epsilon] and tan [delta] of polarized PZT/PVDF composites on frequency is shown in Fig. 26. As shown in Fig. 26a, at a PVDF crystallinity of 21%, [epsilon] of the composites can be up to 165, which is higher than that at 27% and 34% crystallinity, i.e., low crystallinity of the PVDF matrix in the PZT/PVDF composites gives higher relative dielectric permittivity. This is because the amorphous region in the polymer matrix exhibits a stronger ability to capture the charge than the crystalline region of the polymer. As shown in Fig. 26b, the dielectric loss values for all composites are lower than 0.04 at 103 Hz and 0.6 at 107 Hz. Notably, at a crystallinity of 21%, the tan [delta] values are only 0.03 at 103 Hz and 0.15 at 107 Hz, which are lower than those of the other composites. Therefore, it can be concluded that the crystallinity of PVDF has an effect on the dielectric properties of the PZT/PVDF composites.

Figure 27 shows the variation of [d.sub.33] of the PVDF/PZT composites with PVDF content ([f.sub.PVDF]). The value of [d.sub.33] decreases with increasing [f.sub.PVDF]. Notably, for sample 2, where the crystallinity of PVDF is 34%, the piezoelectric constant of the composite increases sharply. In this case, for [f.sub.PVDF] of 0.2, [d.sub.33] reaches as high as nearly 100 pC [N.sup.-1]. Under this heat treatment condition, the PVDF retains higher crystallinity, and the PZT/ PVDF composite shows excellent piezoelectric characteristics. This indicates that the crystalline structures of PVDF in the composites play an important role in electromechanical coupling. It is suggested that ceramic/polymer composites with high [d.sub.33] can be prepared by controlling the optimum crystallization of the polymer matrix.

Effect of Microwave Irradiation on Dielectric Properties

Microwave irradiation (2.45 GHz) can markedly change the interface compatibility of the composite and decrease the dielectric loss. Up to now, studies on the structure changes in PVDF/ PZT composites on exposure to microwave radiation are not many. The effect of microwave irradiation on the crystalline structure, interface interaction, and dielectric property of PVDF/ PZT composites was demonstrated by He and Zhang [136]. They studied the crystalline structure change and dielectric performance of microwave-irradiated PVDF/PZT composites.

They made PVDF/PZT composites by the hot compression of a mixture of PZT powder and PVDF powder. The composites were annealed at 80[degrees]C for 24 h to dispel residual stress. Prepared samples were exposed to microwave radiations by a continuous microwave irradiation device. The changes in the crystal structure and dielectric performance were then, studied by various techniques such as Fourier Transform-Infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), dynamic mechanical temperature analyser (DMTA), and dielectric analyser (DEA).

Figure 28a shows the temperature dependence of [epsilon]' (real [epsilon]) at a constant frequency [10.sup.4] Hz for microwave irradiated PVDF/ PZT composites. As shown in Figure, the dielectric constant [epsilon]' of PVDF/PZT composite decreases during the microwave irradiation. The increase in crystallinity of PVDF, which was measured by DSC, may influence the dielectric constant. This change is helpful to improve the sensitivity of PVDF/PZT composites. Figure 28(b) shows the variation of the dielectric loss of acrelaxation as a function of temperature. The magnitude of AC-relaxation peak drops down with increasing irradiation time and the peaks shift towards lower temperature. It is because that the microwave irradiation reduces the interface interaction, and lets the resistivity of the composites improve. Hence, the peak value of the dielectric loss curves decreases.

The dielectric analysis suggested that the dielectric permittivity and loss reduce, which is useful for improving the sensitivity of composites used in passive transducers.

Also, the structural analysis showed that the microwave irradiation promotes crystalline transformation of PVDF from [alpha] to [beta]. With the irradiation power and time increasing, the transformation from [alpha] to [beta] of PVDF is enhanced. The crystallinity of PVDF in polymer composites increases and [DELTA]T decreases ([DELTA]T = [T.sub.end] - [T.sub.onset]) corresponds to phase transition in DSC curve). The DMTA measurements illustrate that the value of E' (storage modulus) and tan [delta] (ratio of loss to storage modulus) peak increases after irradiation. The increase is more with increasing irradiation time.

Effect of Treating the Film in Solvent

Chen et al. [72] investigated the effect of dipping the shaped 0-3 PZT-PVDF composite in certain solvents (e.g. DMA). The investigations main focus was to increase the [d.sub.33] value. PZT powder (3 [micro]m) dispersed in dielectric gel polymer (PVDF) were made into a film by flow extension method. After thermal poling at 120[degrees]C and 100 kV [cm.sup.-1] dc field for 1 h, the virgin film was dipped into the solvent. The [d.sub.33] and [[epsilon].sub.r] (Figs. 29 and 30) were measured as a function of exposure time to air.

The [d.sub.33] values were found to be one to five times larger than the virgin sample values. The chemical coupling-induced percolation was carried out to study influence of solvent. The time-dependent behavior of the dielectric constant [[epsilon].sub.r] of treated specimens was similar to that of the [d.sub.33] coefficient. But after several hours, the [d.sub.33] values begin to decrease back gradually to the virgin value, or a value very close to the virgin value in certain cases. In the swollen state, the solvent content in the composite determines the swollen state of the polymer and is presumed to determine the change in piezoelectric and dielectric properties of the piezoelectric composite. At the initial stages when treated specimens are exposed to air, the solvent content is high in the composite and the polymer is strongly swollen, so interactions among the polymer molecules are greatly weakened, and ceramic particles tend to form connected ceramic paths along the pressure direction (Fig. 31) (transformation from 0-3 to 1-3 type). The change is only after a few hours indicating a critical solvent content. When the solvent content is higher than the critical content, the polymer becomes saturated and the [d.sub.33] coefficient of 0-3 composite hardly changes.

The Effects of Electrical Conductivity of the Constituents on the Dielectric and Piezoelectric Properties of Ferroelectric 0-3 Composites

The use of different solvent treatments with varying concentration to increase the conductivity of polymer electrolytes has found quiet a lot of attention for research due to the potential application in solid-state batteries, etc. [180-184]. Wong and Shin [185] suggested that the above enhancements (discussed in Effect of Treating the film in Solvent section) could very well be related to the increment of electrical conductivity in the matrix material brought about by the solvent. They investigated the effect of electrical conductivity on the dielectric and piezoelectric properties (Fig. 32) of ferroelectric 0-3 composites. The sample is excited by an ac electric field (dielectric measurement) or ac stress (piezoelectric measurement) of small amplitude.

Wong and Shin suggested that the anomaly is not directly related to the structural (configuration) changes in the treated composite which may be induced by the diffusion of solvent. This was based on the observations that the elastic constant of the composite was not changed by the solvent treatment; only the dielectric constant (60 to 120) and the [d.sub.33] value (27 to 130 pC/N) showed change. Wong et al. employed equations from quantum physics to deduce certain relations which indicate towards the effect of the dramatic increment of electrical conductivity as a cause of the observed phenomenon. It was observed that, the [d.sub.33] value that was reported to increase after several hours follows bell shaped curve with reference to the duration for which the film should be dipped in the solvent. This is based on the inference that the film starts to disintegrate on prolonged exposure to the solvent. In case of very less duration of exposure to solvent the saturation solvent content would not be reached.

Since the conductivity of solvent combinations such as ethylene carbonate (EC), propylene carbonate (PC), dimethylacetamide (DMA), dimethylformamide (DMF), and dimethylsulphoxide (DMSO) are about [10.sup.-6] [V.sup.-1] [cm.sup.-1], the effective conductivity of PVDF is thought to be dramatically increased; especially when the composite sample is just taken out from the solvent environment. The piezoelectric and dielectric properties does not vary much with increase in conductivity till [[sigma].sub.m] = [10.sup.-8] [V.sup.-1] [cm.sup.-1]. The increase is greater with increase in volume fraction of ceramic content. The results are in agreement with the work reported by Chen et al. [72].

Effect of Dopant on PZT-PVDF Composites

PZT-PVDF Doped with Polyaniline (PANI) (Dielectric and Piezoelectric Properties). Renxin et al. [186] investigated the effect of doping the PVDF-PZT composite with PANI (Polyaniline). The PZT powder (3 pm) was made from fired ceramics by grinding and milling. The PANI with a conductivity of 0.1 S [m.sup.-1] was prepared by chemical oxidation polymerization method [187], The required amounts of PVDF and PANI powders were dissolved in N-methyl-2-pyrrolidone (NMP) and PZT powders were uniformly dispersed in the polymer solution using an agitator. The volume fraction of ceramics in all the samples was 50%. The PANI contents in different samples were ranged from 0 to 14 vol% while the corresponding PVDF loading ranged from 50 vol% to 36 vol%. Dielectric and piezoelectric properties were measured by means of the resonance-anti-resonance method. As per Radhakrishnan and Kar et al.'s [188] result the PANI forms large numbers of particles and disperses uniformly in the PVDF. These particles effectively increase the dielectric constant in polymer blends by forming numerous fine capacitors. The enhancement in dielectric loss is mainly related with the increase of the electrical conductivity of the composite due to the presence of PANI. This was also reported by Wang et al. [189]. The relative dielectric constant and dielectric loss of PZT/PVDF composites with PZT of 50vol% and PANI ranging from 0 to 14vol% are shown in Fig. 33.

At low frequencies (< [10.sup.3] Hz), the [[epsilon].sub.r] and tan [delta] in the composites with more PANI reduce more quickly with frequency (Fig. 34). This can be explained on the basis of the underlying fact that the space charges cannot be neglected in this frequency range. The more PANI in the polymer blend induces more space charges which can increase the dielectric constant and dielectric loss at a low frequency [188]. At high frequencies the [[epsilon].sub.r] decreases while the tan [delta] increases with a frequency similar to those found in PVDF and its composite with PZT by Hilczer et al. [172] (discussed in Effect of Dielectric Relaxation in PVDF/PZT Composites section). These changes are mainly caused by the dielectric relaxation in PVDF. The faster enhancement in tan [delta] at high frequencies (> [10.sup.4] Hz) may be concerned with the rapid increase of conductivities in PANI blends with frequency [190]. The remnant polarization [P.sub.r] was altered from 2.91 [micro]C/[cm.sup.2] of sample without PANI to 5.04 [micro]C/[cm.sup.2] of PANI-doped one. This clearly indicates the addition of an appropriate amount of PANI can markedly affect the polarization property of the composite. The increased remnant polarization means the poling of the piezoelectric phase can be efficiently carried out. Another reason for the increase in the remnant polarization in composite may be that the higher dielectric constant of the polymer matrix can provide larger retention of the polarization of the ceramic particles [191], The increase in conductivities of polymer can enhance the polarization of ceramics in composite when the conductivity of polymer is less than that of PZT [192].

However, the higher conductivity of polymer, which exceeds the ceramics, can lead the samples to break easily in the poling electrical field because of a large leak current. Thus the ceramics in composite cannot be poled effectively. In the Fig. 35, the decrease in [d.sub.33] values beyond 10% PANI indicates that the dielectric breakdown has occurred. This phenomenon was also confirmed by Yuan et al. [179]. He used percolative theory and microcapacitor modeling to explain these results.

PZT-PVDF Doped with Lanthanum. Hysteresis phenomenon. Not many studies are available on hysteresis of composites. Tripathi et al. [193] investigated the hysteresis phenomenon of Lanthanum-doped PZT-PVDF (PLZT-PVDF) composite. He prepared the composite with PLZT-PVDF in a ratio of 90:10. Sawyer and Tower circuit was used for hysteresis studies (Table 7), while the loops where traced by the oscilloscope with the temperature controlled to an accuracy of [+ or -] 2[degrees]C.

They postulated that most of the applied electric field would drop across the polymer as it has high resistivity compared with ceramic. Best results of poling were obtained when electric field was applied with a temperature of 100[degrees]C [194], The applied electric field helps in domain growth and alignment of PLZT dipoles. Thermal agitation becomes predominant beyond this temperature and hinders the dipole alignment. Change of ferroelectric tetragonal PLZT into paraelectric cubic phase is observed when further heated, as the Curie temperature of PLZT is around 105[degrees]C [195]. The hydrostatic pressure exerted by the polymer phase on the ceramic leads to an increase in the ferroelectric Curie temperature and a lowering of [P.sub.s] [4]. The saturation polarization temperature was found to be constant or decreased slightly when temperature was varied up to the Curie temperature for the same electric field and frequency [196]. With temperature rise the increase in conductivity is more in case of polymer rather than the ceramic [44]. This causes an increase of [P.sub.t] and [P.sub.r] values due to an increase in the electric field on the ceramic grains. With increase in temperature the flexibility, alignment of dipoles and ferroelectric spontaneous polarization increased. As the properties of the PLZT ceramic are very much pronounced, the local fields are sufficiently strong to change the a-form of PVDF to the [beta]-form [197, 198]. Increase in opposite polarization of the polymer may however, reduce the net [P.sub.r] and [E.sub.c]. Above 100[degrees]C, beyond the Curie temperature, due to change in the phase of the polymer PVDF-PZT, there would be a sudden fall in the values of [P.sub.t], [P.sub.r], and [E.sub.c]. But in PLZT-PVDF samples no such decrease in these values was observed up to 140[degrees]C. This may be owing to the hydrostatic pressure exerted by the polymer on the ceramic grains leading to an increase in the Curie temperature. Electrical conductivity of PVDF films shows a saturation effect, although with a tendency towards a slight decrease, above 100[degrees]C, instead of increasing with temperature [193].

Extending this hypothesis, it may be possible that the decrease in spontaneous polarization with temperature weakens the local fields to such an extent that the polymer dipoles may start aligning with the external field. Thus, explaining the deviation. The dielectric constant variation with respect to temperature is shown in Fig. 36.

Hydroxylation Treatment and Effect on Dielectric Properties. Min et al. [135] prepared nanocomposites of PVDF filled with superficial hydroxylated PLZT through solution bending and presented a comparison between crude and hydroxylated PLZT/ PVDF composites. Here, 15 g of crude PLZT nanoparticles were refluxed in an aqueous solution of [H.sub.2] [O.sub.2] at 106[degrees]C and then centrifuged and baked in an oven at 80[degrees]C for 12 h. PVDF powder was ultrasonically dispersed in DMA for 0.5 h and then different volume ratios of crude and hydroxylated PLZT were added by magnetic stirring for 12 h at room temperature. Finally the mixture was casted on glass plate and dried at 80[degrees]C for 4 h. Thus, nanocomposites with various filler concentration and thickness were obtained.

They investigated the microstructure of the nanocomposites and the influence of different volume ratios on the dielectric and piezoelectric properties. The SEM characterization indicated that hydroxylated PLZT nanoparticles have better dispersion and compatibility with PVDF matrix than crude PLZT, indicating stronger interaction between hydroxylated PLZT and PVDF than crude one. Dielectric constant and piezoelectric coefficient of PLZT/PVDF nanocomposites increased continuously with the PLZT content. The piezoelectric coefficient for hydroxylated PLZT/PVDF was found higher than crude PLZT/PVDF at 50% content. But dielectric constant decreased and loss increased with increase in frequency for hydroxylated PLZT/PVDF. This frequency dependence and higher dielectric constant makes hydroxylated PLZT/PVDF nanocomposites more attractive in application.

PVDF/PZT Doped With SR: Low Field AC Study. Sara Aftab et al. [199] prepared Composites of nanocrystalline [Pb.sub.0.96] [Sr.sub.0.04] ([Zr.sub.0.53], [Ti.sub.0.47]) [O.sub.3] (Sr doped PZT) and [alpha]-phase PVDF using solution casting technique followed by hot pressing and carried out Broadband Impedance analysis to study the effect of the addition of PZT on the low field ac electrical properties of PVDF. Nanocrystalline Sr-doped PZT was prepared by sol gel method. The composite films obtained were ground finely and the powders spread evenly in a die mould for hot pressing. Composite samples of different volume fractions of ceramic (0.20, 0.30, 0.40, and 0.50) were prepared. The hot pressed disks were 200 to 300 microns thick. Square samples of 9 x 9 [mm.sup.2] were punched from disks for testing. Thermal study of the samples was carried out from -100 to 200[degrees]C under an Argon atmosphere using a heating/cooling rate of 10[degrees]C in Netschz STA 409 analyzer. DSC and TG (Thermogravimetry) was used to investigate phase changes, percentage crystallinity, glass transition ([T.sub.g]), and material stability.

Table 8 presents the room temperature dielectric constant [[epsilon]'.sub.r] (real part) for the neat polymer and the composite samples measured using the LCR meter at 1 kHz. The average [[epsilon]'.sub.r] of the samples measured under the same conditions using impedance analyzer is also listed alongside for comparison purposes.

Nomenclature used for the samples is A2, A3, A4, and A5 indicating the PZT volume fraction 0.2, 0.3, 0.4, and 0.5, respectively.

A plot of [[epsilon]'.sub.r] with respect to temperature at constant frequency (Fig. 37), shows that the [[epsilon]'.sub.r] of all samples increases with an increase in the temperature. Furthermore, the [[epsilon]'.sub.r] value of the polymer increases from 7.3 to 22.7 for A3 at 1 kHz (see impedance data in Table 8). Similar results are reported by Chanmal and Jog [200].

Development of Piezoelectric Polymer Composite as Arrays Transducers (Linear and Phased): Current State of Research in Cardiovascular Ultrasound Imaging

Medical ultrasonic imaging is a real-time technique that uses high-frequency sounds to image many different parts of the body. These frequencies can have a range from 0.5 MHz to 50 MHz [201]. The difficulties in machining PZT ceramics [202-204] have led to maximum range of operations of < 20 MHz for pure PZT; patterned ZnO [205] and polymer films [206] have been used to as high as 100 MHz; however, these materials do not possess the very high piezoelectric and/or favorable acoustic properties of PZT/polymer composites. In order to increase the maximum range of operations and obtain larger lateral resolution, linear and phased array transducers are used. The phased arrays have traditionally been made by dicing large conventional crystals with diamond saws. This is slow and expensive subsequently leading to expensive arrays. Dicing PVDF, on the other hand, can be done with a razor blade which is cheaper. The printed circuit board technique can be employed to place electrodes anywhere and of any size on the PVDF sheet; along with the use of the razor blade to remove the spaces between, to run wires and to prevent cross talk between each element of the array (which otherwise would greatly reduce the array capabilities).

A linear array transducer (Fig. 38) can have up to 512 elements spaced over 75 to 120 mm. Adjacent elements typically 8 to 16 (more in wide-aperture designs), are pulsed simultaneously to prevent poor lateral resolution due to beam divergence [207]. In the subgroup of X elements, pulsing of the inner elements is delayed with respect to the outer elements. A focused beam results from the interference of the X small divergent wavelets.

The time delay determines the depth of focus for the transmitted beam and can be changed during scanning. The same delay factors are also applied to the X elements during the receiving phase resulting in a dynamic focusing effect on return. In this manner, a single scan line in the real-time image is formed. To generate the next adjacent scan line, another group of X elements is formed by shifting one element position along the transducer array from the previous group. Linear array systems are capable of lateral resolution on the order of less than 0.5 mm. A "wide aperture" array design means that pulses from a large number (say 128) or all the elements are used to form each scan line. A phased array is basically a way of forming a sound beam electronically. It would use multiple transducers instead of multiple elements.

The prerequisites of an ultrasonic transducer element [201] to be used for medical imaging are: (1) it couples energy into its measurement medium efficiently; (2) it generates 10 to 110 mW/[cm.sup.2] of acoustic power from reasonable input voltages; (3) it provides low signal-to-noise ratio; (4) low cross-coupling between elements; (5) it generates a wide range of acoustic frequencies and pulse widths <<2 [micro]s in duration; (6) it is lightweight and robust. This necessitates for very high and wide frequency of operation. The lateral resolution also needs to be high.

Recently Ritter et al. [208, 209] have described a technique for making 30 MHz linear arrays where ceramic plates are polished to 33.5 [micro]m thick, coated with epoxy containing polystyrene microspheres (6.2 [+ or -] 0.9 [micro]m), and laminated to form a 2-2 composite. This represents a significant improvement over what can be achieved with dicing saw technology. The laminated plate technique is sill limited by the difficulty in handling very thin ceramic. Hackenberger et al. [73] have used tape casting technology to fabricate finely featured PZT composite preforms and extended it by use of multilayer ceramic processing developed for the capacitor industry. PZT dispersed in polymer matrix in the form of green tape (25-50 [micro]m) (Type II--TRS 200FG), polymer binders (polyvinyl butyral and polyethylene glycol), epoxy (epotek 301) was used for making these films. The stack of tapes is isostatically laminated followed by slow heating to remove the binder and volatilize the carbon. Kerfs are back filled with epoxy under vacuum in the space formed by decomposition of fugitive phase. The remaining process of composite fabrication is same as previously reported [209].They concluded that on reducing the thickness of the ceramic beam width and kerf spacing, the composites could be fabricated for 20 and even 30 MHz transducers.

Based on the investigation of composite uniformity, they concluded that increasing the support structure would increase the uniformity; which otherwise lead to excessive bending of elements, cross talk and poor signal to noise ratios. The other factor assisting in uniformity of thickness was powder packing density. To avoid lateral coupling in both single element an array transducers, ceramic beam spacing must be kept below [lambda]/2 (or [lambda]/4 for phased array transducers). Impedance spectra showed that no spurious modes are found (Fig. 39).

As a means of optimizing an imaging array architecture for medical use, a number of different array structures were been fabricated and their characteristic acoustic performance evaluated by Schlaberg and Duffy [201]. The constructed array is shown in the Figs. 40 and 41.

The crosstalk between elements (Fig. 42) of the monolithic transducer has been examined by activating one central element and observing the response in the elements to either side of the main emitter. The relatively low [d.sub.31] constant of the composites indicates that only a small cross-coupling effect in neighboring elements should be observed. The PCB-based construction of a transducer array provides a cleaner signal than the diced arrays due to the simple manufacturing process involved. However, the impedance mismatch at the air-PCB interface on the back of the transducer creates a second echo that makes this kind of array unsuitable in its present form.

Future Directions

PZT-PVDF composites are of interest as new candidates in the areas of

i. High energy density storage devices to reduce the power transmission losses

ii. Array transducers for biomedical imaging with a lateral resolution of less than 0.5 mm.

iii. Energy harvesting devices

iv. Direct wafer bonding for production of compliant substrates

For making these feasible, researchers working on PZT-PVDF composites must address the following gaps in research

i. Better impedance-matching between polymers and ceramic phase

ii. Greater operational range of frequency

iii. Further reduction in ceramic and kerf thickness

iv. Improving the workability of the ceramic phase material

v. Further improvisation and advancements in processing technology is required in order to explore the advantages of more complex connectivities and in order to reduce the lead time and cost.

vi. While PVDF has a high receiving constant (PVDF [g.sub.33] = -310 X [10.sup.-3] V m/N; PZT (PC5) [g.sub.33] = 25 X [10.sup.-3] V m/N), it has a very low thickness mode coupling coefficient (PVDF [K.sub.33] = 0.12; PZT (PC5) [K.sub.33] = 70). The thickness mode coupling coefficient of the composite has to be improved so that it can operate in pulse-echo mode (as a transceiver).

vii. Further investigation on effect of dopants and dielectric relaxation needs to be carried out so that the transducers could operate at higher temperatures.

viii. Compiling the microfluidics research in order to better control the individual phase movements at micro level.


Various models to predict the properties of the resultant composites have been developed. However, only EMT and Yamada model take into account the morphology factor. The other models out-rightly assume a spherical particle shape. For this reason, the Yamada model is the most extensively used of the available models. However, these models show a fair level of deviation beyond 40 to 50% filler content. This is because of the non-uniform distribution of ceramic particles in the resultant composite.

The flexibility to tailor the properties of the composite suited for transducer applications is an indispensable advantage which cannot be overlooked, when comparing with PVDF, PZT, and PVDF copolymers. If ferroelectric ceramics are embedded in a ferroelectric polymer, the piezoelectric and pyroelectric transducer properties can be well controlled. The first step in the process timeline is to define the desired properties in the resulting composite and selection of the single phase materials. Having a high FOM (figure of Merit, d X g) serves as a good comparison for general comparison of sensitivity during sensor characterization, across materials with different contributing phases. It is also important to focus individually on the parameters (d or g) when looking at specific applications. A good mechanical quality factor is required as low mechanical quality factor ([Q.sub.m]) implies that a material has large mechanical losses and that signal energy is being wasted.

In the current scenario, of need for better handling of the energy produced, it is extremely relevant to develop composites which can act as excellent high density charge storage capacitors. The thickness of the film needs due attention as the grain size and surface energy greatly impact the dielectric and piezoelectric properties of the poled films.


Authors thank Dr. Shyam Chetty, Director, NAL and Head, Material Sciences Division, NAL for their support and encouragement. Authors thank NPMASS and CSIR for providing all the necessary facilities and financial help, while completion of this article. Authors also thank Mrs. A. Gayathri and S. Jayanth Kumar for all their help and support throughout the completion of this article.


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Anjana Jain, (1) Prashanth K. J., (1) Asheesh Kr. Sharma, (1) Arpit Jain, (2) Rashmi P.N (1)

(1) Materials Science Division, National Aerospace Laboratories-CSIR, Bangalore, India

(2) NITK, Surathkal, Karnataka, India

Correspondence to: A. Jain; e-mail: Contract grant sponsor: NPMASS and CSIR.

DOI 10.1002/pen.24088

Published online in Wiley Online Library (

TABLE 1. Comparison of properties pertinent to transducer applications
of piezoelectric ceramic, polymer, and piezoelectric ceramic/polymer
composites [30].

Parameter             Ceramic     Polymer         Composite

Acoustic impedance    High (-)    Low (+)         Low (+)
Coupling factor       High (+)    Low (-)         High (+)
Spurious modes        Many (-)    Few (+)         Few (+)
Dielectric constant   High (+)    Low (-)         Medium (+)
Flexibility           Stiff (-)   Flexible (+)    Flexible (+)
Cost                  Cheap (+)   Expensive (-)   Medium (+)

(+) = Favorable, (-) = Unfavourable [4].

TABLE 2. Comparison of physical properties between ceramic, polymer,
and composite materials [63].

Properly                      PZT (PC5)   PVDF   Piezo flex 1   Piezel

Density (kg/[m.sup.3])          7750      1800       4500        5600
Sonic velocity (m/s)            2830      1400       2100        1687
Acoustic impedance (MRayls)      22        3          10         9.8
Compliance (x [10.sup.-9]       0.02      0.1         13         0.25
Relative permittivity           1800       10         32          70
Tan [delta] (at 1 KHz)          0.02      0.05       0.08       0.047
[d.sub.33] (pC/N)                410       30         25          40
[d.sub.31] (pC/N)               -175      -18        -4.6        -24
[g.sub.33] (mV m/N)              26       340         88          --

TABLE 3. Apparent piezoelectric constants calculated as a function of
individual phase (vf indicates PZT volume fraction).

            Apparent piezoelectric constants
                for a two-phase system

            Due to phase 1   Due to phase 2
[v.sub.f]     [d.sub.33]       [d.sub.33]

0                 33               0
0.1             28.69             2.80
0.2             25.34             5.57
0.3             22.64             8.53
0.4             20.41            11.96
0.5             18.50            16.26
0.6             16.82            22.17
0.7             15.25            31.26
0.8             13.59            47.78
0.9             11.26            89.01
1                 0               400

TABLE 4. Influence of individual phases on the composite.

Property      phase      Remarks

Dielectric    PZT        Increase in PZT increases the
property                 d-values.

Electrical    PZT        May have originated from an ionic
conduction               hopping mechanism

Dielectric    PVDF       PZT may have some contribution at
loss behavior            low frequencies and high

Disappearance PZT        The spherulites structures disappear
of spherulites           with increase in PZT content
features in

Porosity of   PZT        As the PZT molecule occupies the
li-phase PVDF            porous cavities, the porosity

Surface energyPZT        The surface energy of the composite
                         improves with increase in size
                         of PZT particles

Dielectric    PVDF       At low temperature, glass transition
relaxation               temperature governs the dielectric
                         relaxation while at high
                         temperature, wide angle
                         oscillations of the
                         conformation governs the
                         dielectric relaxation.

TABLE 5. Density of the PVDF-PZT-5H Composites [107].

Volume %                       Hot press               Tape
of PZT     Theoretical         technique         casting technique

20            2.96               2.74                  2.74
30            3.65               3.41                  3.51
40            4.21               3.99                  4.13
50            4.85               4.50                  4.43
60            5.47               5.13                  4.59

TABLE 6. Dielectric constant of PVDF-PZT-5H composites [107].

                             Dielectric constant at 1 MHz
Volume percentage
     of PZT         Hot press technique   Tape casting technique

20                         16.74                  18.22
30                         26.30                  21.51
40                         41.85                  32.01
50                         49.27                  42.73
60                         98.48                  57.82

TABLE 7. Observed values of P,, P, and Ec for 90:10 PLZT-PVDF
composite [193].

                [P.sub.t]      [P.sub.r]
Temperature     ([micro]C      ([micro]C        [E.sub.c]
([degrees]C)   [cm.sup.-2])   [cm.sup.-2])   (kV [cm.sup.-1])

20                 4.92           3.0             30.20
40                 4.77           2.30            30.20
60                 4.60           2.12            28.50
80                 3.70           1.50            28.50
100                3.36           1.32            26.85
120                3.36           1.32            26.85
140                3.36           1.32            26.85

TABLE 8. Comparison of [epsilon]'r of PVDF and composite samples
measured with an LCR meter and impedance analyzer (A2, A3, A4, and A5
correspond to PZT volume fraction 0,2, 0.3, 0.4, and 0.5,

Sample   LCR    Impedance analyzer

PVDF     9.2           7.3
A2       18.1          17.2
A3       23.0          22.7
A4       30.7          27.2
A5       40.3          37.4
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Title Annotation:polyvinylidene fluoride/lead zirconate titanate
Author:Jain, Anjana; Prashanth, K.J.; Sharma, Asheesh Kr.; Jain, Arpit; Rashmi P.N.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jul 1, 2015
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