Fabrication of flexible piezoelectric PMN-PT based composite films for energy harvesting

Flexible piezoelectric composite films of lead magnesium niobate and lead titanate (PMN-PT) ceramic and multiwalled carbon nanotube (MW-CNT) in the matrix of polyvinyldene fluoride (PVDF) were fabricated for green energy harvesting and self-powered sensing. Compositions of PMN-PT was varied from 10-50 volume (vol. %) in PVDF matrix while a constant concentration of MW-CNT was maintained (1vol. %). Phase purity of the synthesized composites was confirm by X-ray diffraction (XRD) analysis. PMN-PT powder was found to be in single phase without the presence of any additional peak, which generally arises due to crystallization of another pyrochlore phase. Surface morphology study by scanning electron microscopy (SEM) revealed a well dispersed PMN-PT/CNT in PVDF matrix. The maximum measured output voltage and current during mechanical pressing and releasing conditions were found to be ∼ 3 volt and 30 nA, respectively in 30 vol. % PMN-PT composite.


Introduction
Extraction and utilization of ubiquitous energy from the environment have been a challenging issue in energy harvesting technology. The most common and accessible renewable energy sources such as solar, wind, thermal and wave can provide the large scale of power. However, for driving the microelectronics devices, wireless sensor networks in healthcare and implantable biomedical devices, etc., milliwatt-scale power should be scavenge from the environment. The harvesting of one such low power energy from mechanical vibrations viz., vibrations from machines, movements in human beings, flow of water, etc., can therefore be a simple and innovative approach. The most common way to generate electrical energy from mechanical vibrations is the use of ceramic piezoelectric actuators made from lead zirconate titanate (PZT), or barium titanate (BTO). However, those monolithic piezoelectric ceramic materials are brittle and hence difficult to integrate in devices. Polymer-ceramic composite materials can combine the flexibility of polymer and the high piezoelectric coupling coefficient of piezoelectric ceramic materials and hence can avoid the breakdown and cracking of the piezoelectric material under stress. Piezoelectric semiconductor like ZnO [1], GaN [2], CdS [3], InN [4], etc., based devices are extensively studied to achieve high performance by fabricating in different processes such as dense nanowire growth, electrospinning and transfer techniques. In order to increase the efficiency and piezo sensitivity, higher piezoelectric symmetry with high mechanical compliant are indeed for energy harvesting. Recently, perovskite based PbZrxTi1-xO3 (PZT) [5], BaTiO3 [6] and NaNbO3 [7] piezoelectric composites are fabricated on flexible substrates to generate high level of power density. Park et al. [5] have reported carbon nanotube based composites used in PZT-5H and have demonstrated its applications to energize the RGB (Red, Green, Blue) LED (Light emitting diode) arrays and the commercially available seven segment decoder.
Flexible Composites are mostly intend to study for energy harvesting and self-powered sensing due to its improvisation in energy production. Among the commonly used piezoelectric ceramic materials, the lead magnesium niobate and lead titanate (PbMg1/3Nb2/3O3 -PbTiO3), abbreviated as PMT-PT have drawn much attention due its highest piezoelectric coupling coefficient (d33) up to 2500 pm/V which is almost 4 times higher than conventionally used PZT bulk and 30 times higher than BaTiO 3 . It has other specific advantages such as negligible hysteresis, lower creep and lack of high voltage as compared to PZT [8]. One can also manipulate the piezoelectric properties by varying the compositional ratio of PMN and PT around the morphotropic phase boundary i.e., PMN-65% and PT-35%. Flexible piezoelectric PMN-PT nanowires based device has produced very large output voltage and current as compared to BaTiO3 nanoparticle and NaNbO3 nanowires based devices [9]. So by reviewing the different kind of fabrication techniques and procedures, we are focused on fabrication of flexible composite which is made up of PMN-PT ceramic particles and commercially available multiwalled carbon nanotube (MW-CNT) as organic nanofiller in a polymeric matrix such as Polyvinyldene fluoride (PVDF). MW-CNT organic filler is commonly used in polymeric nanocomposites in order to achieve good mechanical, thermal and electroactive properties because of its low mass density and large aspect ratio and hence chosen as an energy improviser in composites. The important role of the MW-CNTs in these composites is to enhance the stress transfer from the polymer to the PMN-PT particle.

Experimental a) Synthesis of PMN-PT
The PMN-PT powder was synthesized by solid state route via columbite method [10], Fig.1 c) Characterizations X-Ray diffraction (XRD) patterns of the synthesized ceramic powders as well as polymer composites were recorded using Bruker Advance D8 by employing CuK radiation. The microstructure was studied by Scanning electron microscope (Zeiss Gemini SEM) and energy dispersive X-ray (EDX) analysis. The composites were cut into 1.5 cm x 1.5 cm and were painted with silver paste on both sides that served as electrodes. The connecting wires were connected to the top and bottom electrodes and the whole assembly was sandwiched between laminate pouch and then hot rolled to measure the electrical outputs. The film was repeatedly pressed and released to measure the generated output voltage and current using Keitheley Source meter 2410.  Fig. 3 (c), particle-particle agglomeration takes place due to higher volume fraction of ceramics. Fig. 3(d) shows the crossectional view of 30 vol.% PMN-PT composite from which the average thickness is found to be ~70 µm.

Output Voltage and Output Current
The typical voltage generation graphs, shown in Fig. 4 are obtained on a 3 cm × 3 cm sample area, by manually finger tapping which depicts the real life energy harvesting. The 10 vol.% PMN-PT content composite generates an output voltage of around 1V, Fig.4 (b). With the increase in PMN-PT content to 20 vol.%, the composite generates an output voltage of around 2 V and is shown Fig. 4 (c). The 30 vol.% PMN-PT content composite, Fig.4 (d), shows the maximum output voltage of ~ 4 V, which is sufficient to light a commercial LED. The generated output voltages decrease with further increase in PMN-PT content and is found to be in the range of 1-2 V for 40 and 50 vol.% PMN-PT, Fig.4 e-f. This may be due to agglomeration, as revealed from the SEM micrographs, of ceramics in the polymer composite. The output current of all the synthesized composites are shown in Fig.5 which demonstrates an increases in output current with the increase in ceramic content upto 30 vol.% PMN-PT and then decreases with further addition of ceramics. The 10 vol.% PMN-PT composite generates an output current around 5 nA. The slight increase in magnitude of current is observed in 20 vol.% PMN-PT composite with a maximum magnitude of ~10 nA. However, an improvement in output current is clearly seen in 30 vol.% PMN-PT composite, Fig.4(d), with a maximum value of ~30 nA. With further increase in reinforcement of ceramic particles to 40 and 50 vol.% PMN-PT in composites, the maximum output current shows a decrement and the magnitudes are found to be around 15-20 nA as shown in Fig. 5.e-f. This is ascribed to the fact that agglomeration of ceramic particles, as revealed from SEM micrograph in Fig. 3(c), might have contributed to poor distribution of ceramic particles in the polymer matrix.

Conclusions
A pyrochlore free PMN-PT and CNT based piezoelectric flexible composite films for energy harvesting has been fabricated. The investigation of the output voltage and current reveals that the 30 vol.% PMN-PT/CNT/PVDF composite has the highest output voltage and current with a maximum magnitude ~4 V and 30 nA, respectively. This output voltage and current is comparable with other reported flexible piezoelectric composites. Hence, this flexible composite fabricated with easy technique can be a promising candidate for energy harvesting.