Poly(vinylidene fluoride-trifluoroethylene)-ZnO Nanoparticle

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Cite This: ACS Appl. Nano Mater. 2019, 2, 4350−4357

Poly(vinylidene fluoride-trifluoroethylene)-ZnO Nanoparticle Composites on a Flexible Poly(dimethylsiloxane) Substrate for Energy Harvesting Subash Cherumannil Karumuthil,* Sreenidhi Prabha Rajeev, and Soney Varghese Nanomaterials and Devices Research Laboratory, School of Materials Science and Engineering, National Institute of Technology, Calicut 673601, India Downloaded via NOTTINGHAM TRENT UNIV on August 9, 2019 at 07:32:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: In this article, we report on an energy-harvesting device using a hybrid flexible polymer nanocomposite as a tribo−piezo layer for energy generation. Piezoelectric nano zinc oxide (ZnO) and exfoliated graphene oxide (EGO) are used with a piezoelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) polymer matrix to prepare the energygenerating layer. An enhancement of the β phase in P(VDFTrFE) is achieved by the method used for film preparation and the addition of ceramic nanoparticles to the matrix. The devices are tested under manual and mechanical tapping to generate electrical energy which is stored in a commercial capacitor. The fabricated hybrid polymer nanocomposite device generated 2.27 V (manual tapping), 11.6 V (qualitative analysis), and 28.3 V (quantitative analysis). Knock sensing action of the prototype device is also demonstrated. The hybrid polymer nanocomposite device with excellent touch sensitivity that is lightweight with distinguished energy-generating efficiency can be used in the self-powered energy-harvesting electronic devices of tomorrow. KEYWORDS: polymer nanocomposites, P(VDF-TrFE), ZnO, EGO, piezoelectric, energy harvesting

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INTRODUCTION In the present scenario of wireless sensors, energy sources are required to remotely power them. An alternative to batteries, as the source of input power, is needed because of their maintenance cost as well as their impact on ecology. Energy scavenging from various sources has attracted researchers from multiple disciplines with importance given to cost-effective renewable energy-harvesting techniques from freely available sources. Mechanical energy in the form of vibration is the most versatile energy available in the environment. The conversion from these forms of mechanical energy to usable electrical energy is achieved by physical and chemical process occurring inside either the active material used as the generating layer or by the device’s structural design. On the basis of the process by which conversion occurs in the material, the effects can be electromagnetic, magnetostriction, and piezoelectricity, and the alteration in the structure of the device can lead to a triboelectric nature as a result of the interface charge generation by the materials in contact.1−6 Various organic and inorganic materials have piezoelectric properties, and many attempts have been made to construct self-powered piezoelectric harvesters by using inorganic nanomaterials such as PZT, ZnSnO3, GaN, ZnO, and BaTiO3.6−14 These inorganic materials have an excellent © 2019 American Chemical Society

piezoelectric coefficient but are unable to withstand higher mechanical strain because of their highly brittle nature. Some ferroelectric polymer materials such as poly(vinylidene fluoride) (PVDF) and its copolymers are well known for their piezoelectric properties. Even though they have lower piezoelectric coefficients than their inorganic counterparts, they are flexible and can withstand the higher mechanical strain. Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] is one of the copolymers of PVDF with good piezoelectric properties among the polymer material. The copolymerization process provides an 89% enhancement in crystallinity for the polymer. The better tribo−piezoelectric property of P(VDFTrFE), when compared to that of PVDF is due to the greater number of fluorine atoms present in its carbon backbone. Depending on the orientation of fluorine atoms on the carbon backbone, this copolymer exhibits different crystalline phases such as α, β, γ, and δ, among which the β phase exhibits better polarization and piezoelectric properties than do other phases. ZnO is a well-studied inorganic ceramic material which exhibits ferroelectric and piezoelectric behavior. This nanomaReceived: April 29, 2019 Accepted: June 10, 2019 Published: June 10, 2019 4350

DOI: 10.1021/acsanm.8b02248 ACS Appl. Nano Mater. 2019, 2, 4350−4357

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ACS Applied Nano Materials

Figure 1. Schematic representations of (a) polymer hybrid nanocomposite film preparation, (b) energy-generating device fabrication and packaging, and (c) the mechanism involved in energy generation by PNEG.

terial is widely used in sensors and energy-harvesting systems15 as an active material. Advanced carbon materials such as carbon nanotubes and graphite can act as stress reinforcing agents, dispersants, and also conducting functional materials along with the polymer. The volume percentage of this type of material is highly critical in energy-harvesting applications. Diverse combinations of materials are used with ferroelectric and piezoelectric polymers for their improved performance. The enhancement of the piezoelectric property of these polymers can be achieved by incorporating inorganic ceramic nanoparticles into its matrix. Enhancement in remnant polarization and the piezoelectric efficiency of P(VDF-TrFE) by adding ZnO nanomaterial has already been reported.16 βphase enhancement of P(VDF-TrFE) by adding an optimized quantity of ZnO was reported in our previous work.17 The influence of surface functionalities and the size of ZnO nanoparticles on the piezoelectric properties of the polymer has also been studied.18 In this work, flexible tribo−piezoelectric energy-harvesting films are fabricated and tested with piezoelectric polymer P(VDF-TrFE), ZnO nanofiller to enhance the piezoelectric effect, and exfoliated graphene oxide (EGO) as a conductive dispersant without the application of any external electrical polling on the prepared films.

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MATERIALS AND METHODS

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METHODS

Materials. P(VDF-TrFE) copolymer powder (70:30) mol % (Solvay Solaxis, Italy) and ceramic nanopowder ZnO (40−100 nm) (Alfa Aesar) were used as received. Methyl ethyl ketone (MEK) (Merck) is used as a solvent for dispersing the copolymer. Graphite oxide (GO) was synthesized in the laboratory by a modified Hummer’s method using graphite powder (Loba Chemicals), sodium nitrate (NaNO3, Fisher Scientific), potassium permanganate, sulfuric acid, hydrochloric acid (Merck), and hydrogen peroxide (Merck). Graphite powder (2 g) and NaNO3 (1 g) were mixed with concentrated H2SO4 (96 mL) in an ice bath using a magnetic stirrer. Potassium permanganate (6 g) was added gradually to the above mixture by maintaining the temperature of the ice bath below 20 °C. Then the mixture was continuously stirred for 18 h at a temperature below 35 °C. Water (150 mL) was added to this mixture by maintaining the temperature below 50 °C, and 90 mL of water was added again with 5 mL of hydrogen peroxide and stirred for another 2 h. The mixture was then filtered and washed with a 10% aqueous solution of hydrochloric acid (250 mL). Further washing was done with deionized water, and the product was air-dried. The resulting graphite oxide was thermally exfoliated at 200 °C to obtain exfoliated graphene oxide (EGO).19

Film Preparation. To obtain a pure polymer film, composite film, and hybrid composite films, three different solutions were prepared 4351

DOI: 10.1021/acsanm.8b02248 ACS Appl. Nano Mater. 2019, 2, 4350−4357

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Figure 2. SEM images of prepared polymer and nanocomposite films: (a) P(VDF-TrFE) pure film, (b) P(VDF-TrFE)/1% ZnO, (c) PVDF-TrFE/ 1% ZnO and a 0.01% EGO hybrid, (d) ATR-IR spectrum, and (e) XRD of the P(VDF-TrFE) pure film, P(VDF-TrFE)/1% ZnO nanocomposite film, and PVDF-TrFE/1% ZnO and 0.01% EGO hybrid nanocomposite films. with P(VDF-TrFE) as the pure polymer solution, 1% ZnO (with respect to polymer) dispersed in P(VDF-TrFE) as the composite solution, and 1% ZnO as well as 0.01% EGO dispersed in P(VDFTrFE) as the hybrid composite solution. A probe (Sonics 150) and ultrasound bath sonication were used to uniformly disperse the nanoparticles as well as nanosheets into the polymers. The solvent casting was employed to prepare three distinct films in similar molds with the same amount of solution. To enhance β-phase formation in these films, they were kept in a vacuum oven at a temperature of 120 °C for 2 h, and slow cooling was initiated. Figure 1a shows the schematic representation of steps carried out for hybrid polymer nanocomposite film preparation. Film Characterization. The morphology of fabricated films was analyzed using a scanning electron microscope (Hitachi SU6600FESEM). The attenuated total reflection mode of the infrared spectrometer (ATR-IR) (PerkinElmer Frontier FTIR) and XRD (Rigaku Miniflex 600, Cu Kα radiation (λ = 0.15406 nm)) were employed for β-phase enhancement identification in the polymer and polymer composites. Polarization switching characteristics of the material were studied using DCEFM (Park XE 100) with a lock-in amplifier (Stanford Research Systems). Hysteresis curves based on EFM amplitude and the EFM phase of the films were obtained using a voltage sweep applied to the sample from −10 to 10 V. The phase change of the hybrid films was also performed by applying −5, 0, and +5 V as the sample bias and was scanned by marking a scan area of 2.5 μm × 2.5 μm with a scan rate of 0.8 Hz. Device Fabrication. Polymer nanocomposite energy generators (PNEG) were fabricated by sandwiching each film in between thin copper foils as electrodes with copper wire as the connecting leads. Entire device assembly was encapsulated inside a flexible transparent insulating PDMS circular tile. To prevent the flow of encapsulating material into the fabricated device, the device was securely sealed using nonconductive tape. Circular tile acts as a protective layer and flexible pressure distributor for the whole device. Encapsulation of the device assembly in the PDMS substrate was done by pouring the PDMS polymer onto the sealed device kept in a circular mold and kept inside vacuum desiccators, which prevented the trapping of air bubbles in the encapsulated polymer. Figure 1b explains the energygenerating device fabrication and packaging schematically. Device Characterization. The generated voltage and current characteristics of the fabricated device were analyzed using a digital

storage oscilloscope (Agilent 2000X series) and a source meter (Keithley 2400 series). The generation of electrical energy was verified by a finger-tapping process, and the generation was quantified by using a universal vibration apparatus as the mechanical input source. Capacitor charging was observed using a multimeter (Keysight). Knock sensing action of the prototype device was also monitored by using an LED light with an arduino board.

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RESULTS AND DISCUSSION The feasible mechanism for the generation of piezoelectric potential in the device is explained according to the schematic illustration shown in Figure 1c. The device with P(VDF-TrFE) alone as the piezoelectric active layer is discussed under two specific conditions of persistent contact and releasing operations. During the initial stage of no force being applied, the net dipole moment inside the P(VDF-TrFE) film is zero because the positive and negative centers coincide. During the initial contact, the internal reticular of charge centers is not affected, and deformation occurs when persistent contact is applied to the device. This generates a potential difference in the device and leads to positive current flow in the external circuit as indicated by Ipiezo1. Later during the releasing operation, the deformed internal reticular tries to regain the initial arrangement, and this leads to a lesser potential and a lesser current in the reverse direction indicated by −Ipiezo1.20 The operating mechanism behind the second variant of the device with ceramic nanoparticles and nanosheets incorporated into the piezoactive layer is explained on the basis of three features. They are the nucleation sites provided by ceramic nanoparticle to the PVDF-TrFE matrix and dipole alignment facilitation of the matrix by nanosheets as well as the formation of the microcapacitor model inside the composite. The polymer chain alignment is due to the carbonyl and carboxyl groups in the nanosheet basal plane which attracts the fluorine atoms to one side. The addition of a carbon-based nanosheet to the polymer with a ceramic composite aids the accumulation of the free charge carriers by the dipoles from the polymer−ceramic composite. Because 4352

DOI: 10.1021/acsanm.8b02248 ACS Appl. Nano Mater. 2019, 2, 4350−4357

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Figure 3. (a) DCEFM amplitude analysis of samples. (b) DCEFM phase analysis of sample DC-EFM phase images at three different voltages. (c) P(VDF-TrFE) pure film. (d) P(VDF-TrFE)/ZnO nanocomposite film. (e) PVDF-TrFE/ZnO and EGO hybrid nanocomposite films.

the surface area of the nanosheets is more than that of any other conductive fillers such as nanotubes, the number of newly formed dipoles is greater in that particular position as shown by the inset of Figure 1c as a microcapacitor model. The enhancement of the β phase increases as a result of the presence of the ceramic piezo material present in the composite by acting as a nucleation site. Thus, on persistent contact, more piezopotential will be generated than that of the polymer device alone (i.e., Ipiezo2 will be greater than Ipiezo1). FESEM is employed to visualize the morphology of prepared polymer films. Figure 2a−c shows the FESEM micrograph of the pure polymer film (PVDF-TrFE) (film 1), polymer nanocomposite (P(VDF-TrFE)/ZnO) (film 2), and hybrid nanocomposite (P(VDF-TrFE)/ZnO and EGO) (film 3). The FESEM image shows the characteristic spherulite structure of polymer P(VDF-TrFE). During polymer nanocomposite preparation, nanofillers are covered by the polymer matrix.

The spherulite structure size decreases with the addition of nanofillers. The crystallization kinetics of the polymer is reflected in this size change which is determined by the nucleation ability of the nanofillers as well as by the growth rate of spherulites.21 Thus, the nanofillers acts as nucleating centers of a crystalline phase, hence benefiting the β phase formation of the polymer.22 The ZnO nanofiller acts as a crystalline nucleating agent as well as a piezoelectric ceramic aiding the overall increase in charge separation under applied pressure, and the presence of EGO further increases the density of crystalline nucleus formation. EGO helps in dispersing ZnO as well as interacting with the −CF2 group in the polymer and can form strong interactions with the polymer matrix. FESEM images show the decrease in the size of spherulites by the addition of nanofillers (ZnO and EGO) as nucleating agents, which results in the formation of more nucleating sites with a significant reduction in grain size. 4353

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Figure 4. (a) Digital photograph of the voltage measurement by an oscilloscope under finger tapping. (b) Single peak voltage output from finger tapping. (c) Comparative studies of open circuit voltage from different devices under finger tapping. (d) Short circuit current characteristics of various devices under finger tapping. (e) Rectification of output. (f) Switch polarity test. (g) Open circuit voltage under various RPMs. (h) Short circuit current under various RPMs.

enhancement in the β phase. The presence of nanofillers in polymers helps in the escalation of the β phase in it by acting as nucleating sites. Figure 2e shows the X-ray diffraction data of these films, and the β phases of PVDF-TrFE and its composites are identified by the diffraction peak in the XRD pattern corresponding to orientation planes (100) and (200) with a 2θ value close to 20°. The increase in intensity and a peak shift to 20° are observed while considering the films from a pure to a hybrid composite, which shows the increase in its β

The enhancement in crystalline piezoelectric β-phase formation in films 1−3 is studied with ATR-IR and XRD. Figure 2d shows the ATR-IR spectrum of three different films, with the characteristic β-phase vibrations analyzed in the wavenumber ranging from 400 to 4000 cm−1. The peaks at 1288 cm −1 (−CF 2 symmetric stretching), 850 (−CF 2 symmetric stretching), and 1400 cm−1 (−CH2wagging vibration) correspond to the β phase in these films, and an increase in the intensity of these peaks recognizes an 4354

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Figure 5. (a) Variation of voltage and current with load resistance. (b) Voltage storage accomplished on a 220 μF capacitor. (c) Mechanical stability analysis. (d) Energy harvester as a knock sensor under the OFF condition. (e) Energy harvester as a knock sensor under the ON condition.

compared to that of the other two films, which explains the enhanced piezoelectric nature of the hybrid polymer nanocomposite over that of the other two materials. The phase hysteresis loop of DCEFM illustrates the local ferroelectric effect of the polymer nanocomposite. Figure 3b shows the corresponding phase shift of each film and reveals polarization reversibility characteristics with the direction of polarization of ∼180° at ±10 V polarities of sample voltages. DCEFM phase images of films 1−3 (Figure 3c−e) at different voltages indicate the polarization reversal capability of the material under study. Three different devices with films 1−3 (devices 1−3) are fabricated without any electrical polling process. Piezoelectric activities and the energy-generating capability of different devices are studied by irregular mechanical force applied to the top electrode by a human finger. Figure 4a shows the digital photograph of the finger tapping experiment performed on the packaged device. The generated output voltage from device 1− 3 under finger-assisted tapping is as indicated in Supporting Information S.1 with the maximum output voltage from the hybrid nanocomposite device being 3.47 V (Supporting Information S.4, movie 1). During the application of tapping to the PDMS tile, the pressure is directly distributed on the

phase. The addition of ZnO along with dispersing agent EGO provides a more efficient crystalline β phase than does pure polymer. The presence of oxygen functional groups such as carboxyl and carbonyl groups in the EGO basal plane aids the alignment of the polymer chain by attracting the fluorine atoms to one side. This can be accounted for in the β-phase enhancement in the hybrid composite compared to the composite with ZnO alone. The characteristic butterfly curve of EFM amplitude hysteresis illustrates the ferro−piezoelectric nature of the material under study. The amplitude of the response signal is directly associated with the strain experienced by the cantilever. Figure 3a shows the amplitude variation of the EFM signal with the applied voltage. While applying a positive bias to the films, the EFM amplitude increases because of the converse piezoelectric effect of the sample, and while retracing, a different path is followed because of the remnant polarization in the film. By applying negative bias, the amplitude of the EFM signal is again increased as a result of the converse piezoelectric nature of the material. The variation in effective EFM amplitude explains the piezoelectric nature of three prepared films. The hybrid polymer nanocomposite film exhibits an increased amplitude variation in the signal 4355

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knock sensor, usually located in engines (cylinder head, intake manifold, etc.), is used to perceive the engine vibration. The knock sensing action is achieved with the fabricated hybrid PNEG device and is as shown in Figure 5d,e. The device is used in conjunction with Arduino and can light an LED above the threshold value to be sensed during the press cycle, indicating the knock action. The proposed hybrid polymer nanocomposite with a high energy-generating capability can be used as flexible energy harvesters in various fields such as self-powered biomedical health monitoring devices, self-powered pressure sensor systems, and biomedical energy harvesting systems. The hybrid polymer nanocomposite is highly sensitive to human touch and is very flexible in nature. These materials act as a potential candidate for touch-sensing applications in bridges and roads. This material can be employed to scavenge freely available human energy by using it in footwear, railway platforms, and even vehicle tires as a tire pressure monitoring system.

electrode and gets transferred into the active material which is sandwiched between electrodes. Because of the alignment of dipoles as well as charges developed on the surface of the active tribo−piezo electric film, an electrical potential difference develops between the two electrodes of the active material, resulting in an open circuit voltage with positive and negative amplitude cycles (Figure 4b). Comparative studies of the output voltage and current obtained under finger-assisted tapping with the error bar from each device are depicted in Figure 4c,d. (Current characteristics of the hybrid device under human-assisted finger tapping are shown in Supporting Information S.2.) The piezoelectric activity of the pure polymer is enhanced by the addition of piezoelectric nanoparticles and is observed as energy output from the device. ZnO plays three roles in the fabricated PNEG device: (a) It acts as a nucleating agent and increases the β phase of the polymer matrix. Slight variations in the morphology of ZnO also help to trigger the dipole alignment in P(VDFTrFE). (b) It increases the surface charge density by increasing the dielectric constant of the polymer matrix. This increase in dielectric properties is accounted for by the Maxwell− Wagner−Sillars polarization occurring at the interface between the ZnO and PVDF chains where the system is heterogenerous with nonidentical conductivity. When loading increases, the interparticle distance decreases, thus leading to a tunnelling of electrons between closer ZnO nanoparticles. (c) It enhances the self-polarization of the film. The increase in the β phase with the addition of ZnO in combination with its piezoelectric nature produces a dual piezoelectric effect in the films. Thus, they act as an enhancer of self-polarization in the films. The addition of carbon-based nanofiller EGO helps to distribute the nanoparticle throughout the polymer matrix, and it also acts as a conducting interlink between the piezoelectric materials. Hence, it provides better output when compared to pure polymer and polymer nanocomposite devices. The output voltage generated from the hybrid polymer nanocomposite energy harvester during an impact of 5 N is rectified and plotted over time (Figure 4e). The switch polarity test is employed to verify whether the voltage is generated from the nanogenerator itself and is shown in Figure 4f. The voltage-generating ability of a hybrid polymer nanocompositebased piezoelectric nanogenerator at different input forces is evaluated with the help of an input force monitoring forcesensitive resistor (FSR) in combination with Arduino. (Details of FSR and measurement details are shown in Supporting Information S.3.) To study the quantitative energy-generating competence of the hybrid PNEG, a universal vibration apparatus is used as a mechanical input source (Supporting Information S.4, movie 2). The input tapping force is varied by revolutions per minute (RPMs) of the mechanized vibration apparatus. The maximum output voltage and current of the hybrid energy generator with an increase in input force is enhanced from 3.90 V/12.17 nA to 37.5 V/26.13 nA corresponding to each RPM as depicted in Figure 4g,h. The load-carrying capacity of the hybrid nanogenerators was tested under various resistive loads and is as indicated in Figure 5a. The effective power of the device is 52.02 μW at 5 KΩ. The output voltage is rectified using a rectifier circuit, and the generated energy is stored in a 220 μF capacitor (Figure 5b; Supporting Information S.4, movie 3). The robustness of the device was tested for 10 000 cycles under a mechanized tapping system at a force of 5 N without any decrement in the output voltage generated (Figure 5c). A

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CONCLUSIONS The work discusses the fabrication and characterization of flexible tribo−piezo electric-energy-generating devices using the piezoelectric property of polymer matrix P(VDF-TrFE) along with well-known piezoelectric ceramic nanomaterial ZnO and advanced carbon-based nanomaterial EGO. The piezoelectric property of the material is enhanced by introducing nanofillers into polymeric piezoelectric matrix P(VDF-TrFE). The β phase of the material also plays an important role in the piezoelectric properties. ATR-IR and XRD results show the presence and enhancement of the crystalline β phase in the pure polymer, polymer nanocomposite with ZnO, and hybrid polymer nanocomposite with ZnO and EGO. The ATR-IR measurement of the hybrid polymer nanocomposite shows a 233% enhancement in βphase content compared to that of a pure polymer film, which shows the better piezoelectric efficiency of hybrid polymer nanocomposite films compared to that of the pure polymer. The XRD results confirm the above FTIR results with an increase in intensity as well as a shift in the 2θ peak to 20°. FESEM images demonstrate the morphology of the polymer films with spherulite formations. Energy generators are fabricated and packaged inside a flexible, transparent PDMS tile. Without any electrical poling, the hybrid polymer nanocomposite film (P(VDF-TrFE)/ZnO and EGO) can produce high-output voltage pointing to the promising largescale production of a flexible, thin, lightweight, portable, and easily processable energy generator. A hybrid polymer nanocomposite film having an area of 1 cm2 exhibited a maximum voltage and current of 2.27 V and 9 nA, respectively, by irregular finger tapping. Qualitative force measurements by applying a different level of pressure produced a higher amount of voltage; the maximum open circuit voltage is extracted from the hybrid polymer nanocomposite by applying higher pressure. A mechanical tapping source is employed to quantify the maximum output from the energy harvester, and a maximum voltage of 37.5 V is produced with higher-rpm mechanized tapping. Commercial capacitor charging is demonstrated with mechanized tapping on hybrid polymer nanocomposite. The results demonstrate superior touch sensitivity as well as higher-output energy generation of the hybrid polymer nanocomposite. A smart hybrid polymer nanocomposite can be employed for lightweight, flexible, easily processable, portable, highly efficient, self-powered 4356

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electronic devices. These promising tribo−piezoelectric polymer nanocomposite materials can provide a powerful link for scavenging freely available energy from the environment.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b02248. Voltage output comparison of energy generating devices under irregular finger assisted tapping, current characterization of devices under finger assisted tapping, details regarding the force sensitive resistor (FSR), and calibration of force using FSR with respect to various RPMs of the universal vibration apparatus (PDF) Output voltage of PNEG through irregular mechanical stress applied to the top electrode by human finger tapping (AVI) Video showing voltage generated from PNEG by mechanized tapping (AVI) Video on the storage of voltage generated from PNEG in a 220 μF capacitor (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +919645092472. ORCID

Subash Cherumannil Karumuthil: 0000-0002-6080-6028 Sreenidhi Prabha Rajeev: 0000-0001-5756-477X Soney Varghese: 0000-0002-1970-1566 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the support from NIT Calicut and thank Dr. V. Sajith and Dr. Thulasi, School of Materials Science and Engineering, Dr. Sabarinath S, Mechanical Engineering Department and Mr Anandan, Electronics and Communication Engineering Department, NIT Calicut, for help with characterization.

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REFERENCES

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