Influence of ZrO2 and TiO2 nano particles in P(VDF-TrFE) composite for energy harvesting application

The Zirconium and Titanium ceramic materials are used to make PZT (Lead zirconate titanate) piezoelectric ceramic composite. In this article, the Zirconium dioxide (ZrO2) and Titanium dioxide (TiO2) ceramic fillers with, ferroelectric polymer PolyVinyliDene fluoride-Tri Fluoro Ethylene (P(VDF-TrFE)) are utilized to make the ZrO2/P(VDF-TrFE) and TiO2 /P(VDF-TrFE) nano-composite thinfilm for flexible vibration energy harvester application. The scanning electron microscopes (SEM) with (Energy Dispersive Spectrometer spectrum) EDS examine the TiO2, ZrO2 fillers present in the composite. The ceramic fillers molecules Ti 2p and Zr 3d binding energy are confirm by X-Ray photoelectron spectroscopy (XPS). Each composite reaches their piezoelectric β- phases are confirm by Fourier Transform—Infrared Spectroscopy (FT-IR). The low surface roughness of the thin-film reaches more flexibility and deformation of a cantilever. The ZrO2/P(VDF-TrFE) composite is obtained low average surface value of 10 nm in the region of 50 µm is measured from Gwyddion software. The ceramic composite beam reaches their natural resonance frequency below 100 Hz is measure by Laser Doppler Vibrometer (LDV). The flexible cantilever beam nanogenerators produces peak-to-peak output voltage 8.2 V. The harvested output voltages are used at electronic devices and wireless sensor applications.


Introduction
The piezoelectric polymer plays an important role in vibration energy harvester applications. The piezoceramics have a high dielectric constant e r and high piezoelectric coefficient (d 33 ), but the ceramic material has low mechanical strength and low vibration sensing because of its low voltage constant g 33 . The piezo-polymer materials are mechanically strong.
The piezo-polymers has high piezoelectric voltage constant because of its high vibration sensing behavior [1,2]. Nowadays the trend is to be developing new piezo composites materials to overcome the disadvantage of piezo-ceramic materials and piezopolymers. The polymers, piezoelectric properties are enhanced by inserted inorganic fillers into the polymer chain. The PVDF(-(CH 2 /-CH2)n-) and its copolymer P(VDF-TrFE) are mostly used polymer materials in vibration energy harvesting applications. [3,4]. The inorganic material TiO 2 has high thermal stability. TiO 2 nanoparticles and their composites are used in solar cell, biomedical and energy harvesting applications, etc. The PVDF/TiO 2 nanogenerator is used for physical sensing like mouse clicking and wrist pulse detection. TiO 2 nanoparticles are also used to improve the mechanical and electrical properties of PVDF. In PVDF/TiO 2 composite thin film, the TiO 2 nano particle are used to enhance the bphase and piezoelectric properties [5][6][7].
In the P(VDF-TrFE)/BaTiO 3 nanocomposite materials, the BaTiO 3 micro array pillars helps to get constant electrical voltage at the energy harvester [8]. In the 0.85(K0.5Na0.5NbO 3 )-0.15SrTiO 3 ceramic composite, the SrTiO 3 (ST) filler improves the dielectric properties and this composite has high energy storage density applications [9]. Lead Zirconate Titanate (PZT) ceramic and their composite materials play important roles in piezoelectric vibration energy harvester applications [10]. In the PbZr 0.52 Ti 0.48 O 3 microcube with P(VDF-TrFE) composite, input force readily concentrated on edges of cubes to compare with spherical shape and its help to improves the harvested output [11]. Fluorine-coated rutile titanium dioxide nanoparticles with PVDF nanocomposite, effectively induce the piezoelectric effect [12]. ZrO 2 thinfilm is a compatible material, which is used for nanoscale sensors, ferroelectric field effect transistor, energy harvester and piezoelectric applications. Nano ZrO 2 /PMNZT nanocomposite film piezoelectric property, surface roughness and mechanical fracture properties are high, compare with pure PMNZT piezoelectric thin film. The piezoelectric coefficient d 33 of PMNZT/ ZrO 2 is increased 80% compared with pure PMNZT film [13].
This paper presents the Nano-ZrO 2 and Nano-TiO 2 ceramic materials are utilized to fabricate the flexible piezoelectric vibration energy harvester application. ZrO 2 /P(VDF-TrFE) and TiO 2 /P(VDF-TrFE) nanocomposite thin films are fabricated using solution casting method. The fabricated nano composite film, surface morphology, material elemental energy levels are examined by SEM with EDS (Energy Dispersive Spectrometer spectrum). The molecular elements presents confirmed by XPS. Piezoelectric behaviours are confirmed by FT-IR measurement and surface roughness is measured by AFM (Atomic Force Microscopy). Energy harvesting device resonance frequencies are measured by LDV. The external excitation given to cantilever beam from DC motor and the corresponding energy harvester output voltages are measured through Digital storage oscilloscope.
This paper is organized as follows. Sect. 2 nanocomposite film preparation is presented. Sect. 3 characterizations of nanocomposites films are explained. Sect. 4 discussed the experimental performance of harvester. Finally, the conclusion presented in Sect. 5.

Nano-composite film preparation
The flexible vibration energy harvester devices are fabricated from ZrO 2 /P(VDFTrFE) and TiO 2 / P(VDF-TrFE) polymer composite materials. While making polymer nanocomposite, the first process is to make a P(VDF-TrFE) (Sigma Alrich USA)/ MEK (Methyl ethyl ketone) polymer solution. The second process is, individually taken 2wt% ZrO 2 (Sigma Alrich USA), TiO 2 (Sigma Alrich USA) nanoparticles are mixed with P(VDF-TrFE) polymer solution. Then, the two different nanofiller polymer solutions are kept under 40minutes ultrasonication process at room temperature. The ultrasonication process is used for the purpose of uniform nanoparticle dispersion in polymer solution. After that, the well-dispersed nanoparticle solutions are kept on hot plate (REMI-1ML) for 6 h mechanically stirred at 60Celsius. The well-mixed polymer composite solutions are casted onto glass plate followed by slow evaporation of MEK solvent along with 70˚Celsius hot plate curing. After drying, the nanocomposite thinfilms are placed on a hot air oven at 140C for 1 h and then peeled off from glass substrate to achieve a uniform and flat thinfilm of ZrO 2 /P(VDF-TrFE) and TiO 2 / P(VDF-TrFE) composite [14]. Each thinfilm thickness of nearly 30 lm is measured through the SEM. Finally, the silver electrode placed on top and bottom of the thermally annealed active nanocomposite piezopolymer layer using thermal evaporation method. For energy harvesting device fabrication, the freestanding thinfilm are placed on the Indium Tin Oxide (ITO) coated Polyethylene terephthalate (PET) substrate.

Surface morphology
The ZrO 2 /P(VDFTrFE) and TiO 2 / P(VDF-TrFE) nano composites thin-film surface morphology and nano particles allocation are examined by SEM (GEMINI Ultra FE-SEM, carl Zeiss). The well dispersed nano particles ZrO 2 and TiO 2 presents in composites at 1 lm region are shown in Fig. 1a-b. EDS (GEMINI Ultra FE-SEM, carl Zeiss) confirm the presents of ZrO 2 , TiO 2 in polymer matrix and shown in Fig. 1c-d. In the ZrO 2 /P(VDF-TrFE) film, Zr molecule is obtained at the energy level of 2Kev and the Ti molecule presences at 4.4Kev energy level in TiO 2 /P(VDF-TrFE) thin film. The polymer elements Fluorine, Oxygen and Carbon energy signal emissions are present at their standard energy levels 0.5 keV, 0.6 keV and 0.24 keV respectively [15,16].

LDV
The velocity of cantilever beam is measured by using noncontact Laser Doppler Vibrometer. By using Fast  4 Experimental setup The model of novel ceramic filler filled polymer composite flexible energy harvester is shown in Fig. 8. The diamention of cantilever beam is listed in Table 2. Each nanocomposite cantilever beam is placed over the corrner of wooden table. The plastic tag was connected to 5 V DC motor and gives continuous oxidation to cantilever beam. The deformation of the beam produces an alternating voltage because of direct piezoelectric effect. Digital storage osiloscope is used to measure the harvested output voltage and shown in Fig. 9.

Output voltage analysis
The TiO 2 /P(VDF-TrFE) and ZrO 2 /P(VDF-TrFE) nanocomposite flexible cantilever type vibration    filled polymer nanocomposite material energy harvester produces more voltage compare with TiO 2 filled polymer. The proposed novel ZrO 2 /P(VDF-TrFE) nanogenerator gives best output performance compared with few published nanogenerator reports based on pure polymer and other oxide composite strictures as shown in Table 3.

Conclusion
The flexible piezoelectric vibration energy harvester devices piezoelectric performances are enhanced by separately embedding ZrO 2 and TiO 2 ceramic nanoparticles into high crystalline ferroelectric polymer P(VDF-TrFE). The ceramic nanoparticles present in composite and molecular elements Zr2p, Ti2p, C1s, F1s and O1s are confirmed by SEM with EDS and XPS. The FTIR analysis of 140C cured nano composite thin-film confirms the ferroelectric to piezoelectric transformation. The TiO 2 nanofillers in polymer composite, increases the surface roughness of the thin-film and this high surface roughness affect the energy harvesting performance. Both composite thin film cantilevers obtain the low natural resonance frequency value of less than 100 Hz. To compare with ZrO 2 /P(VDF-TrFE) and TiO 2 /P(VDF-TrFE) nano composite thinfilm nanogenerator, the ZrO 2 /P(VDF-TrFE) piezoelectric device harvested 8.2 V voltage from vibrations. Because of low surface roughness and high b-phase intensity, the ZrO2/P(VDF-TrFE) harvester device exhibited excellent energy harvesting performance.

Declarations
Conflict of interest Authors declare that they have no conflict of interest.