The selection of piezoelectric material for a power harvesting application is the major influence creation on its functionality and performance. Although number of different piezoelectric materials have been developed, lead zirconate titanate (PZT) as a power harvesting material. Despite of its extreme brittle nature causes limitations in strain. Lee5 highlights that these piezoceramics are susceptible to fatigue crack growth when subjected to high frequency cyclic loading.
Another common piezoelectric material is poly vinylidene fluoride (PVDF). It exhibits considerable flexibility when compared to PZT. Lee5,6 developed a PVDF film coated with poly(3,4-ethylenedioxy-thiophene)/ poly(4-styrenesulfonate) PEDOT:PSS electrodes. They compared the PEDOT:PSS coated films to inorganic electrode materials, indium tin oxide (ITO) and platinum (Pt) coated films. It was found that Pt electrodes and ITO electrodes showed fatigue crack on surface at vibration frequency of 33 kHz and 213 Hz, respectively. However, PEDOT:PSS film does not damage even at vibration for 10 h at 1 MHz.
Mohammadi7 developed a PZT fiber-based flexible composite piezoelectric material of various diameters (15, 45, 120, 250 µm) and were aligned, laminated, molded in an epoxy8. The voltage output of sample was tested by dropping a 33.5 g (20 mm diameter) stainless steel ball on piezoelectric material from a height of 10 cm. The peak power was also calculated considering a 1 M? load resistance. A maximum voltage and power output of 350 V and 120 mW was obtained for the thickest transducer, 5.85 mm thick, with the smallest fiber diameter, 15 µm. Upon studying the relationship between voltage output of the harvester and its physical geometry, it was determined that thicker plates have the capability of larger fiber displacements, and that samples with smaller diameter fibers have the highest piezoelectric coefficient, d33 and lowest dielectric constant defined in this study as K3, both of which contribute towards higher power outputs and more efficient systems.
Piezofiber power harvesting materials have also been investigated by Churchill et al9 who tested a composite consisting of unidirectionally aligned PZT fibers of 250 µm diameter embedded in a resin matrix. It was found that when a 0.38 mm thick sample of 130 mm length and 13 mm width was subjected to a 180 Hz vibration that caused a strain of 300 µ? in the sample, the composite was able to harvest about 7.5 mW of power. The results of this study show that a relatively small fiber-based piezoelectric power harvester can supply usable amounts of power from cyclic strain vibration in the local environment.
Sodano10 presented a comparison of several piezoelectric composite devices for power harvesting that are normally used for sensing and actuation. The power harvesting ability of macro-fiber composite (MFC), quick pack IDE (model QP10ni), and the quick pack model (QP10n) actuators was tested. The MFC contains piezofibers embedded in an epoxy matrix which affords it extreme flexibility, and it utilizes interdigitated electrodes, which allow the electric field to be applied along the length of the fiber and act in the higher d33 coupling mode, as shown in Fig. 4. Thus interdigitated electrode pattern of the MFC and the quick pack IDE results in low-capacitance devices which limits the amount of power that can be harvested.
Flexible piezoelectric materials are attractive for power harvesting applications because of their ability to withstand large amounts of strain. Larger strains provide more mechanical energy available for conversion into electrical energy. A second method of increasing the amount of energy harvested from a piezoelectric is to utilize a more efficient coupling mode. Two practical coupling modes exist; ?31 mode and ?33 mode. In ?31 mode, a force is applied in the direction perpendicular to the poling direction, an example of which is a bending beam that is poled on its top and bottom surfaces. In the ?33 mode, a force is applied in the same direction as the poling direction, such as the compression of a piezoelectric block that is poled on its top and bottom surfaces. An illustration of each mode is presented in Fig. 5. Conventionally, ?31 mode has been the most commonly used coupling mode: however ?31 mode yields a lower coupling coefficient, k, than ?33 mode.
Sodano12 compared the efficiencies of piezoelectric materials viz., traditional PZT, quick pack (QP) actuator, and macro-fiber composite (MFC), subjected to 0–500 Hz chirp, and exposed to random vibrations recorded from an air compressor of a passenger vehicle. It was found that the efficiency of PZT for each vibration scheme was fairly consistent (4.5% at resonance, 3.0% for a chirp, and 6.8% for random vibrations) and was higher than the other two devices. A summary of various piezoelectric materials discussed above are depicted in Table 1.
Ng and Liao15, 16 presented two types of bimorphs (Fig. 6) along with a unimorph piezoelectric harvester. The first bimorph (series triple layer) consisted of two piezoelectrics with a metallic layer sandwiched between them. The second bimorph (parallel triple layer) similar to series triple layer and piezoelectrics were connected electrically in parallel mode. Findings showed that under low load resistances and excitation frequencies the unimorph generated the highest power, under medium load resistances and frequencies. The parallel triple layer had the highest power output, and under high load resistances and frequencies the series triple layer produced the greatest power. Thus, a series connection should a increased device impedance leading to more efficient operation at higher loads.