Piezoelectric and Dielectric Properties of PZT-Epoxy Composite Thick Films

Ferroelectric films have been of interest over the last 20 years because of the possibility of using them for non-volatile memory applications [1-3], MEMS [4-6], ultrasonic devices [4,7], battery separators [8-10], biomedical scaffolds [11] and energy harvesting devices where they are coupled with batteries [12]. The properties if piezoelectric films cannot be compared directly with those of bulk materials of the same composition because their processing and electrical properties differ. Previous work on the preparation of piezoelectric films includes RF planar magnetron sputtering [13,14], ion beam sputtering [15] or DC magnetron sputtering [16], where most recently, researchers have succeeded in fabricating films using various chemical methods of deposition such as; metal organic chemical vapor deposition (MOCVD) [1719], chemical solution deposition [20,21], metalorganic decomposition (MOD) [22,23], and also pulsed laser deposition (PLD) [24-26]. The sol-gel method has the benefits of compositional control, reduced temperature processing of highly uniform, dense, crack-free films and low cost of fabrication [27,28]. These methods of processing ferroelectric films have been used with notable success to produce piezoelectric ceramic films comprised of Lead Zirconate Titanate (PbZrxTi1-xO3 also known as, PZT). PZT has a high dielectric constant, ferroelectric, piezoelectric, and pyroelectric properties. The ideal properties of PZT have made its application to transducer, sensor and actuator devices ubiquitous. However, the poor mechanical strength of these devices has limited their life cycle and performance. Therefore, interest in polymer-ceramic [29-33] (and ceramicceramic [34,35]) composites has emerged as an area of interest because polymers are flexible, low cost, easily processed [36,37] and able to be polarized under the influence of an external electric field [38].


Introduction
Ferroelectric films have been of interest over the last 20 years because of the possibility of using them for non-volatile memory applications [1][2][3], MEMS [4][5][6], ultrasonic devices [4,7], battery separators [8][9][10], biomedical scaffolds [11] and energy harvesting devices where they are coupled with batteries [12]. The properties if piezoelectric films cannot be compared directly with those of bulk materials of the same composition because their processing and electrical properties differ. Previous work on the preparation of piezoelectric films includes RF planar magnetron sputtering [13,14], ion beam sputtering [15] or DC magnetron sputtering [16], where most recently, researchers have succeeded in fabricating films using various chemical methods of deposition such as; metal organic chemical vapor deposition (MOCVD) [17][18][19], chemical solution deposition [20,21], metalorganic decomposition (MOD) [22,23], and also pulsed laser deposition (PLD) [24][25][26]. The sol-gel method has the benefits of compositional control, reduced temperature processing of highly uniform, dense, crack-free films and low cost of fabrication [27,28]. These methods of processing ferroelectric films have been used with notable success to produce piezoelectric ceramic films comprised of Lead Zirconate Titanate (PbZr x Ti 1-x O 3 also known as, PZT). PZT has a high dielectric constant, ferroelectric, piezoelectric, and pyroelectric properties. The ideal properties of PZT have made its application to transducer, sensor and actuator devices ubiquitous. However, the poor mechanical strength of these devices has limited their life cycle and performance. Therefore, interest in polymer-ceramic [29][30][31][32][33] (and ceramicceramic [34,35]) composites has emerged as an area of interest because polymers are flexible, low cost, easily processed [36,37] and able to be polarized under the influence of an external electric field [38].
Banerjee et al., [34] studied the influence of aluminum inclusions on the dielectric properties of three-phase PZT cement aluminum composites, where the aluminum and PZT particles were dispersed in a Portland cement matrix. The fabricated samples were nominally 6mm in thickness and were poled at a low poling voltage of 0-6kV/mm at a temperature of 160 ͦ C in air. It was reported that three-phase composites possessed a higher piezoelectric strain coefficient (d 33 ) than two-phase composites. The highest value of d33 observed for the three-phase composite was 8.1 pC/N for volume fractions equal to 0.7 and 0.2 for PZT and Al, respectively. This d 33 value for the three-phase piezoelectric composite was 1.64 times the value measured for the two-phase composite. Banerjee et al., [53] also investigated the influence of micro and nano sized aluminum inclusions in a three-phase PZT-Epoxy-Al bulk composite. The fabricated composites were ~ 6mm thick and were comprised of micron sized aluminum powder and PZT. These devices had higher dielectric constants that the composites that incorporated nanosized aluminum particles (both sets of samples were polarized at the same value: 0.2 kV/mm in air) due to interfacial effects, contact resistance and increased agglomeration. Specifically, for composites with PZT volume fractions equal to 0.20, 0.30 and 0.40 and micron-sized aluminum particles, the dielectric constants were equal to 405.7, 661.4, and 727.8 (pC/N), respectively. On the other hand, the dielectric constants for composites that were comprised of nano sized aluminum particles the (the same PZT volume fractions) dielectric constants were equal to 233.28, 568.81, and 657.41 (pC/N), respectively. They concluded that samples that contained micron sized aluminum inclusion yielded higher dielectric constant values compared to the Nano sized aluminum inclusions, which resulted in higher dielectric values for samples with micron sized piezoelectric fillers.
In this work, PZT-aluminum-epoxy thick film composites were fabricated where the volume fractions of PZT and Al were varied from 0.20 to 0.40 (increments of 0.10) and 0.0 to 0.17, respectively. A minimum of three samples were fabricated for each of the studies. In addition, two different types of top electrodes (sputtered gold and silver paint) were prepared and compared in terms of piezoelectric and dielectric constants. The change in the piezoelectric and dielectric properties with respect to time was also observed over a five-day period.

Materials and Methodology
The materials used in all experiments were PZT powder (READE), DGEBA Epofix TM Cold-Setting embedding resin (Electron Microscopy Sciences) [54], and aluminum -99.97%, 200 mesh i.e. ~ 75µm (Acros Organics) and 0.0254mm stainless steel substrates (Alfa Aesar). The piezoelectric and physical properties of the materials are presented in Table 1 & 2, respectively. The properties for the aluminum were obtained from the manufacturer website [55,56].  PZT epoxy aluminum thick composite films were deposited onto stainless steel substrate using a spin coat and deposition technique. The stainless-steel squares were 25mm x 25mm and 0.0254mm thick. An overview of the fabrication process for the devices is provided in Figure 1. The volume fractions of the PZT and aluminum were varied from 0.20 to 0.40 and 0.01 to 0.17, respectively. As shown in Figure 1, the PZT, aluminum and epoxy were weighed to achieve the desired volume fraction and then combined in a beaker. The contents in the beaker were hand stirred for 5minutes and 5ml of ethanol was added to this mixture. The mixture was subsequently sonicated in an ultra sonicator for 1hour, while being hand stirred every 15minutes. The epoxy hardener was then added to the mixture. The final compound was then poured onto the stainless-steel substrate using a dropper while the spin coater was rotated from 100 -500rpm for 30seconds, and subsequently by 1000rpm for 60seconds. The initial low rpm enabled an even distribution of the compound over the substrate. Figure 1: A schematic diagram explaining the stages in the preparation of the composite film described by Sundar et al., [1]. The mixture was spin coated by a two-step process, 500rpm for 30 seconds and 1000rpm for 60 seconds, followed by curing at 75 ͦ C, and then contact poling in a silicone bath for 15 minutes at 65 ͦ C.
After the spin coating process, the samples were removed and cured on a hot plate in the presence of air for 8 hours at 75 ͦ C. The samples were then cooled for 24 hours and eventually coated with colloidal silver paint as the top electrode. Samples were then contact poled at a voltage of 2.2kV/mm at 65 ͦ C in a silicone oil bath for 15 minutes. An additional set of samples were prepared, where the volume fractions of PZT and aluminum were 0.3 and 0.05, respectively. Sputtered gold was used as the top electrode for these samples instead of colloidal silver paint. All samples were then wrapped in aluminum foil and stored at room temperature in the air for 24 hours to remove any remaining residual charges that may be on the samples.

Dielectric and piezoelectric characterization
The piezoelectric strain coefficients, d 33 and d 31 , and capacitance, were measured using a Piezo Meter System manufactured by Piezo Test, Piezoelectric Materials & Device Testing Company, while the resistance was obtained using an Impedance/Gain Phase Analyzer (HP 4194 A). All the measurements were taken after polarization of the sample. The dielectric constant was calculated using the expression, In Equation (1), is the capacitance in Farads, A is the area of the sample, is the permittivity of free space ≈ 8.854×10e-12Fm -1 ; and t is the thickness of the sample. The impedance analyzer was used to measure the real part of the resistance, R, of each sample, and the resistivity was determined from Equation (2),

Material morphology and surface characterization
The surface morphology of the individual phases and the composite was examined using scanning electron microscopy (SEM) micrographs. Two types of images were gathered for these composites, one was from the top surface of the sample, which provides information about the distribution of the individual phases within the polymer matrix, and the other was the crosssectional image along the thickness of the sample. The crosssectional image represents how the particles are distributed along the thickness and a macroscopic view of the composite substrate interface.

Surface morphology
The surface morphology of the composite films and the particle distribution within the composite were observed via scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) micrographs. PZT epoxy-aluminum samples that were comprised of 0.30 PZT and 0.09 aluminum (by volume fraction) were observed.
The cross-sections of the samples were obtained by cutting the sample using a blade and placing the samples onto the SEM stud to reveal the samples cross-sectioned thickness or surface morphology.
In Figure 2, a SEM micrograph of the cross-section of the film is presented at 467× magnification. There do not appear to be any microscopic pores or delamination points at the interface between the substrate and the film. In Figure 3, the surface morphology of the composite film is depicted at a magnification of 520 X. PZT and aluminum particle agglomeration are observed in this SEM micrograph. The agglomeration may be attributed to several mechanisms of agglomeration, e.g. Brownian, gravitational, turbulent, and electrostatic, where the gravitational form could be mitigated n theory by the spin coating process. The as received PZT and aluminum powders tend to naturally form clusters that vary between 5-20µm in the case of aluminum and 10-25µm in the case of PZT as shown in Figure 4A and Figure 4B, where these agglomerations are most likely attributed to electrostatic agglomeration. Though ethanol was used to change the surface charge of the powder particles, its addition did not effectively change the pH of the solution to negate the surface charge on the all of the particles, which diminished its effectiveness in counteracting the electrostatic forces between particles. The extent of the effective-ness of the ethanol on the pH of the solution was also a function of the number of particles within the colloidal sol gel.     [49,58] where the active particle size was <34m with a poling voltage equal to 10kV/mm at room temperature, which is consistent with the data presented here. The differences in the d31 and d 31 values for the 0-3 and the 0-3-0 composites are greater for lower volume fractions of PZT, but the advantage of Corona poling over Contact Parallel plate is evident at lower volume fractions of aluminum.   The capacitance and dielectric constants of the two-phase composites are presented in Figure 9 & 10. The capacitance and dielectric constants were found to increase with PZT content, where the maximum value was ~8 pF and ~412 for the capacitance and permittivity, respectively for a volume fraction of 0.60 for PZT. It was observed that as the PZT con-tent increased, the polarization of the composite enhanced due to PZT's inherent ferroelectric property. Hence, higher overall dielectric constants were observed. It is expected that as the polarization density increases, the ability to store charge in the materials is enhanced as a function of the polarization vector magnitude. The dielectric constants for the three-phase samples are presented ( Figure 12). The dielectric constants for all samples increase with volume fraction of PZT and aluminum as was similar to the observations of [59][60][61]. All samples containing the aluminum filler had higher dielectric constants than their two-phase counterparts. The three-phase system with aluminum particles closely resembled a percolative system [62][63][64][65][66][67][68]. The capacitance for the PZT-epoxy composite that are plotted as a function of PZT volume fraction. The maximum value is 8pF and occurs at 0.60 volume fraction of PZT, which is a low value. However, the low value is due to the poor dielectric and electrically insulative properties of the epoxy used. These composites were polarized using the plasma corona discharge method.   This interfacial polarization or the so-called Maxwell-Wagner effect is responsible for the enhancement of the dielectric constant [63,64]. This increase in dielectric constant corresponds to an increase in volume fraction of aluminum particles and culminates at a critical value, known as the percolation threshold. Beyond this critical value the system begins to conduct electricity due to either (i) electron tunneling or electron hopping effect and/or (ii) the presence of conductive pathways, which geometrically align. Beyond this percolation range, the composite thick films cease to be a dielectric and become electrically conductive [63,64]. The dielectric constant values increased near percolation threshold due to the presence of micro-capacitor networks. These networks are formed by neighboring conductive filler particles [65][66][67][68] that sandwich PZT particles that are insulated by the polymer matrix. Once the micro capacitor networks begin to accrue in number, they contribute to an overall increase in capacitance [69,70] of the overall sample. The creation of these networks results in an increase in the local electric field when the conductive fillers are close together near percolation which pro-motes migration and accumulation of charge carriers at the interfaces between the aluminum particles and the insulating polymer matrix. These generated charges are then accumulated at the interface and once the conductive particles appear too close to one another they are relaxed by tunneling or ohmic conduction [63,[65][66][67][68]. The maximum value of dielectric constant was observed for samples that contained 0.40 PZT and 0.17 aluminum.

Analysis of piezoelectric effect as a function of top electrode
Gold and silver top electrodes were applied to PZT (0.3)-Epoxy (0.05) samples to evaluate its influence on the electromechanical Academic Journal of Polymer science properties of the composite. Colloidal silver solution was applied as the top electrode with 5µm thickness ( Figure 13) and 400nm gold was sputtered on the sample using a sputtering machine ( Figure 14).    The electron transport properties at the interface of the composite thick film surface and the device electrode are influenced by the surface characteristics, such as the surface morphology of the thick films, the contact resistance and the total number of contact points connecting the thick film surface to the surface of the device electrode [71][72][73]. To draw a comparison between different top electrodes, the thick films were deposited with gold (Au) and silver (Ag) electrodes. The electrical resistivity and conductivity values are different for both materials. The variation in the piezoelectric strain coefficients d 33 and d 31 of the thick film are shown in Figure 15 & 16. The values are enhanced by the addition of the metallic electrodes below the percolation threshold. This improvement in the d 33 and d 31 values is due to the decrease in the contact resistance and increase in the mobility of electrons at the interface between the film surface and the electrode due to the presence of the metallic layer [72]. We can see a maximum d 31 and d 33 values for the samples with silver electrode are 2pC/N and 2.5pC/N, respectively occur at ~13% Al. These values are higher than that of gold, d 31 and d 33 are ~1.8pC/N and ~1.2pC/N, respectively because of the decreased electrical resistivity of silver (Ag ~0.00955 10-8 Ωm and Au ~0.0502 10-8 Ωm) [74].
The capacitance values are enhanced by the addition of the top electrode. The top electrodes were applied to samples containing a PZT volume fraction equal to 0.3 and aluminum volume fraction equal to 0.05. The capacitance for of the samples with the gold electrodes were ~43pF, ~42.8pF for the silver electrode and ~3.5pF when no electrode was applied. The enhancement of the capacitance could be due to several factors such as electron tunneling due to an increase in the intensity of local electric field [63]. Also, the conductive layer reduces the losses due to metal insulator transition and other interface defects as compared to the compo-site without a conductive layer [72].

Academic Journal of Polymer science
The composite properties were also recorded as a function of time: 1, 5 and 1027 days of poling, to study the effect of aging on the electromechanical properties of the composite. The values measured after five days were slightly lower than the values recorded after one day as indicated in Figures

Conclusion
Three-phase piezoelectric PZT-epoxy-Al thick film composites were fabricated and poled using parallel-plate contact poling method. The volume fractions of PZT and Al were varied from 0.2 to 0.4 and 0.0 to 0.17 respectively. The piezoelectric strain coefficient, d 33 and d 31 and the dielectric constant increased with PZT and Al volume content. Furthermore, the piezo-electric strain coefficients, longitudinal and axial, increased with Al volume fraction culminating at 0.13, beyond which the values began to drop. Two different top electrodes were applied to the samples, gold and silver and their values were compared. Silver proved to result in better overall properties due to reduced electrical resistivity at the interface. An aging study was also carried out to measure the piezoelectric strain coefficients between 1, 5 and 1027 days. The results showed that the values gradually dropped over time, falling quickly between 1 and 5 days, and more gradually between 5 and 1027 days.