Performance of a Piezoelectric Energy Harvester with Rubber Compound Modification

Recently, there has been a significant increase in the research on Piezoelectric energy harvester. Piezoelectric energy harvester is capable of producing electrical charge when mechanically deformed. The application is to be used to power up mobile electronic. The common problem in the piezoelectric energy harvester is the material is brittle and causes fatigue failure at the same time the material is relatively costly. One of the method to address this problem is to add a rubber compound layer which can improve the flexibility and power output of the energy harvester. It can also improve the cost per output density of the piezoelectric energy harvester because the rubber compound has low cost. But, currently none of existing literatures explore into this type application. Thus, the objective of this paper is to investigate the voltage output and effectiveness of piezoelectric energy harvester with rubber compound modification. The test in the laboratory was set up to optimize PZT (Lead Zirconate Titanate) Energy Harvester cantilever beams using rubber compound layer as modification. The tests were conducted using frequency range of 30 Hz to 70 Hz with fixed RMS 0.25g acceleration. The result shows promising output for the PZT energy harvester with rubber compound, with an increase of between 16.6% to 61.2% open circuit voltage when compared to the standard PZT.


Introduction
Energy harvesting is a process by which energy is derived from ambient sources for example solar power, thermal energy, wind energy, salinity gradients, and kinetic energy.The small energy can be captured and used for wireless autonomous devices especially in wearable electronics and wireless sensor networks.Energy harvesters can provide a small amount of power for low energy electronics.Mechanical vibrations energy can be harvested by using different types of transductions namely piezoelectric, electromagnetic and electrostatic [1][2][3][4][5][6].This work focuses on modification of the piezoelectric material using rubber as an energy harvester.First flexible Nanowire-based piezoelectric generators that can harvest sufficient mechanical energy to power small devices were demonstrated [7].Print piezoelectric ribbons onto to make stretchable energy conversion device [8].Piezoelectric thin films deposited on PET flexible substrate were presented [9].Although the lower power density of flexible technology compared to the silicone one which still can be optimized where the advantages of using a lower density polymer material is that it allows lower resonant frequencies for the same volume, wider bandwidth and easier manufacturing process, since it does not require clean room instrumentation and infrastructure [10].
The other work was the sputtering process of ZnO layers on flexible substrate makes warping effect due to the thermal deformation [9].PZT were chosen as piezoelectric material throughout this work due to its high piezoelectric coefficient and its best conversion efficiency [11] .Piezoelectric material generate power only when it subjected to an external force, this force will apply stress force and deform the structure of the material which will generate high voltage in response to these forces and the generated power and voltage will increase when the stressincrease.In order to enhance the stress that is generated by any low external resonant frequency; a cantilever structure with a proof mass at the free end tip was chosen [12].PZT or Lead Zirconate Titanate is most used due to its relatively low price and good piezoelectric material.PZT is polycrystalline ceramic and exhibiting excellent piezoelectric coefficients is rather better [13].Tensile strength is lower compared with PVDF.Strain Coefficient, d for PZT give higher output.Therefore, the purpose of this work is to show the piezoelectric modification by rubber for energy harvester application.

Experimental set up 2.1. Modification of Experiment
The experiment was set up with the modification of the material with rubber compound.The design of this experiment used Latex as rubber compound.The piezoelectric bender used in this work was piezoelectric (Lead Zirconate Titanate) PSI-5H4E which was bought from Piezo Sys.Inc.The standard length of PSI-5H4E used throughout this work was 70 mm and 31.8 mm with same width of 12.7 mm and thickness of 0.51 mm but after modification this work used 3 sizes of piezoelectric include both of that.They were all bimorph-type of benders with a center shim made of brass material.The following steps were the standard sample preparation processes performed for each of the piezoelectric bender used for all studies.The labels and dimensions of the PZT before modification with rubber respectively were (a) Long PZT (70 mm x 12.7 mm x 0.51 mm), (b) Short PZT (31.8 mm x 12.7 mm x 0.51 mm) and (c) Thin PZT (31.8 mm x 6.35 mm x 0.51 mm).
The specimens (a), (b) and (c) were also labeled as standard piezoelectric.The specimens labeled as (A), (B) and (C) were those with rubber layer modification.It uses Latex or Natural Rubber obtained from the Rubber Research Institute of Malaysia.The specimen was dipped in the Latex and was air dried for a few days.This dipped in 2 layer because the experiment before give the best result compare the others layers.So the sizes of new specimen were changed.The specimen (A) Long PZT (70.125mm x 13.05 mm x 1.05 mm), (B) Short PZT (32.05 mm x 13.05 mm x 1.05 mm) and (C) Thin PZT (32.05 mm x 6.425 mm x 1.05 mm).

Experimental Procedure
Firstly, the standard size of PSI-5H4E was marked with a line in which the 'Long PZT' length to be clamped by the plastic holder.This was to enable longer piezoelectric length to be used for the study since the standard piezoelectric materials available were 2 that sizes in length.Once measured, the piezoelectric strip was clamped onto the plastic clamp.Once the piezoelectric was installed onto plastic clamp, it was then measured again to make sure the piezoelectric length is within the required value.The almost complete standard 'Long PZT' Piezoelectric Energy Harvester bender was then placed onto soldering jig.The location of solder was placed as close to the fixed end of the 'Long PZT' Piezoelectric Energy Harvester bender.This was because the stress was highest at the location close to the fixed end of the cantilever 'Long PZT' Piezoelectric Energy Harvester and this soldering location were maintained the same for each of the specimens of Piezoelectric Energy Harvester benders prepared for all the study.
The wiring for the piezoelectric was performed following the specification set by the manufacturer.The PSI-5H4E used in this study was all being polled for the current to be working in series operation.Since there were 2 layers of piezoelectric material in this bimorph, wiring in-series by soldering at the top and bottom layer of piezoelectric will cause the output voltage to be added up.

Mathematical modelling
Piezoelectric material can be used as an energy harvester by converting mechanical strain into electrical charge using cantilever beam vibration energy harvesting.Piezoelectric materials typically exhibit anisotropic characteristics; thus, the properties of the material differ depending upon the direction of forces and orientation of the polarization and electrodes.The anisotropic piezoelectric properties of ceramic are defined by a system of symbols and notation [14].This is related to the orientation of the ceramic and the direction of measurements and applied stresses or forces.Inertial based generator second order by spring mass systems.Figure 3 shows a general system based on a seismic mass, m, on a spring of stiffness, k.Energy losses within the system (comprising parasitic losses,cp, and electrical energy extracted by the transduction mechanism,ce) are represented by the damping coefficient, cT.The inertial frame which is being excited by an external sinusoidal vibration of the form as: Figure 4 shows the data collected at 30 Hz to 70 Hz at fixed RMS 0.25g acceleration.Overall, the rubber compound modifications had a significant effect on the output voltage in the range of 16.6-61.2%.This is expected and inline with findings from related work [1].

Conclusions and prospects
In this paper, it was concluded that the performance of the piezoelectric energy harvesters with rubber layer modification show significant increase in performance.It was evident that the long PZT has an increase of 61.2% value of voltage, or an open circuit voltage from 1.233V to 1.845 V after the rubber layer modification.The short PZT showed an increase from 1.757 V to 1.923 V which was 16.6% increased.It was also found that there was an increased of 18.7% (from 1.358 V to 1.545 V) recorded for the thin PZT.This shows that the additional layer of rubber on the piezoelectric energy harvesters with rubber got increasing value and has good performances besides that it can improved flexibility.Thus, for future works, it is recommended to investigate the modifications with various elastic or similar compound and its relation to the electrical properties as well as the power density as the output performance.
The first data was taken for open circuit voltage for six specimens where the 'Long PZT' for specimen (a) standard and (A) dipped Latex, 'Short PZT' for specimen (b) standard and (B) dipped Latex and 'Thin PZT' for specimen (c) standard and (C) dipped Latex.The frequency was set up low frequency and the targets are around 50 Hz.Another data was taken by different frequency with range 30 Hz to 70 Hz at acceleration 0.25g and 1g in this experiment used 6 specimens either with or without latex.The experiment for low frequency vibration-based Energy Harvesting and suitable for device can be positioned on any vibrating machinery e.g. in power plant where at a critical vibration pattern it will generate power to active a wireless sensor to caution for maintenance.The result of open circuit voltage was measured by using multimeter.

Figure 1 .
Figure 1.Experimental setup Basic PZT and Modification PZT with rubber.

Figure 4 .
Figure 4. Comparison of open circuit voltage for standard PZT and rubber dipped PZT at various frequency range

TABLE 1 .
Maximum open circuit voltage and its increment before and after the rubber compound dipped treatment at fixed frequency, 50 Hz.