Research Article
Tuning of highly piezoelectric bismuth ferrite/PVDF-copolymer flexible films for efficient energy harvesting performance

https://doi.org/10.1016/j.jallcom.2022.167569Get rights and content

Highlights

  • BiFeO3 nanoparticles were synthesized by hydrothermal method at 180 °C.

  • The corona poled composite films showed better dielectric properties.

  • The best mechanical energy harvesting performance with the optimal content of BFO in PVDF-TrFE polymer was ∼18.5 V.

  • The device generated a maximum instantaneous power density of 0.92 µW/cm2

  • The higher β-phase content contributed to the better performance of the composite film.

Abstract

BiFeO3 (BFO) is a popular multiferroic material exhibiting robust antiferromagnetic and ferroelectric properties and is a promising candidate for use in sensors and transducers. In this work, BFO piezoceramics were synthesized by a simple hydrothermal method and were incorporated into the copolymers PVDF-TrFE and PVDF-HFP to fabricate flexible nanocomposite films. BFO nanoparticles (NPs) act as nucleating agent inside the polymer matrix, thus improving the overall performance of the piezoelectric film. The phase purity of the synthesized BFO NPs was confirmed by XRD and the β-phase content of the fabricated film was calculated from FTIR analysis. The nanocomposite film PVDF-TrFE/BFO with 6 wt% filler loading (PTB6) showed better piezoelectric performance among others. The intrinsic functional properties of the composite film were evaluated by P-E (polarization-electric field) hysteresis loop test. Further, the nanocomposite film, after corona poling, was used for fabrication of a flexible piezoelectric nanogenerator (PNG). The poled films exhibited better piezoelectric as well as dielectric properties. The fabricated PTB6 based PNG generated a maximum of 18.5 V under a biomechanical finger tapping force. This study suggests that the proposed flexible device is a potential candidate for driving low-power electronic devices.

Introduction

The recently growing research interest in the class of multiferroic nanomaterials is due to the different order parameters coexisting in a crystalline phase. Due to their wide applications in low-power consumption, multifunctional and environmentally friendly devices [1], it is required to couple these parameters at room temperature which is not the case in many multiferroic nanomaterials. However, BFO possess a Curie temperature of 830 °C [2] that is much higher than room temperature and also has large spontaneous polarization in single crystals [3], thin films [4] and bulk ceramics [5]. BFO ceramics have intrinsic piezoelectric properties but as compared to other piezoelectric ceramics they have a lower dielectric constant. Therefore, it is required to fabricate composite films (CFs) using BFO embedded into suitable polymers having comparable dielectric permittivity. These composites simultaneously exhibit electric and magnetic hysteresis properties at room temperature [6]. Moreover, BFO particles require a high coercive field for their poling due to their large leakage currents. Thus, incorporation of BFO into a non-conductive polymer matrix [7], [8], [9] reduces the leakage current thereby preventing leakage and short-circuiting [10].

PVDF and its copolymers are multifunctional soft materials having qualities like good electrical resistance, good dielectric property, easy process-ability, piezoelectric [11] and pyroelectric properties. These ferroelectric polymers [12], [13] are semi-crystalline in nature having good mechanical reliability and are therefore favorable candidates for flexible electronic devices [14]. PVDF-TrFE and PVDF-HFP are copolymers of PVDF that also exhibit a piezoelectric effect. Piezoelectricity depends on various factors like crystallinity degree, β-phase content, and dipole moment alignment in molecular chains. Therefore, in recent decades different methods have been reported for the enhancement of piezoelectricity. Among them, nanofiller addition is a widely chosen method to enhance the piezoelectric performance. There are few similar research works conducted on fabricating composites using one-dimensional BFO particles and PVDF-based polymers. Recently Hu et al. [15] reported a work on composites of PVDF-TrFE and BFO−BaTiO3 nanofibers that were fabricated via electrospinning method. The P-E loop analysis done for the composites showed a maximum remnant polarization of 0.6 C/m2. You et al.[6] also reported a similar work on BFO and PVDF-TrFE composite nanofibers prepared by a double step sol-gel based electrospinning process keeping the fiber composition contents of BFO at 0, 25 and 50 wt%. There exist many studies that suggest that the addition of nanofillers into the polymer matrix induces interfacial effects in the composites resulting in dielectric enhancement of the composites [16], [17].

There exist several other reported works on the dielectric, ferroelectric and magnetic properties of PVDF/BFO CFs [18], [19], [20], [21] and mainly deal with their energy storage applications. These research works are not focused on the effect of poling on PVDF/BFO CFs and their energy harvesting studies. Till date, there is also no significant work that investigates and compares the piezoelectric performance of a piezoceramic filler incorporated into the PVDF-based copolymers, i.e., PVDF-TrFE and PVDF-HFP. Recently Ichangi et al. [22] reported fabrication of BFO-PVDF nanofibers based flexible PNG fabricated by electrospinning method. The PNG generated a maximum peak-to-peak output voltage of 7.6 V under a finger knocking force. The present research work not only reports the fabrication of CF based on BFO embedded in the copolymers PVDF-TrFE and PVDF-HFP through a simple solution casting method but also studies their ferroelectric properties and the effect of poling on the dielectric and piezoelectric performance of the PNG. The external electrical poling also improves the piezoelectric performance of PVDF based CFs [23], [24]. This poling process helps the dipoles of the polymer composites to reorient along the field direction thereby improving the polarity and hence enhancing their piezoelectric property [23], [24], [25]. The fabricated PVDF-TrFE/BFO (6 wt%) based PNG generated a maximum output voltage of 18.5 V and a maximum current of 300 nA which is better than the previously reported works. Thus, the experimental results of the work suggest that the addition of an optimum amount of BFO nanofillers into the copolymers P(VDF-TrFE) and PVDF-HFP enhance the ferroelectric, dielectric and piezoelectric properties of the nanocomposite films. Moreover, it proves that PVDF-TrFE is the best choice for composite with BFO in terms of dielectric, ferroelectric and piezoelectric performance.

Section snippets

Materials

The materials used in synthesis of BFO include Bi (NO3)2·6H2O (Sigma Aldrich Pvt. Ltd.), Fe (NO3)3·9H2O (Himedia Pvt. Ltd.) and KOH (Himedia Pvt. Ltd.). For CF fabrication PVDF-TrFE (75:25 mol%) powder was procured from Solvay, India and PVDF-HFP powders from M/s Sigma Aldrich, India. The solvent N, N-dimethyl formamide (DMF) used for CF fabrication was procured from SRL Chem, India. PDMS used for PNG fabrication was obtained from KEVIN ELECTROCHEM, India.

Synthesis of BFO nanoparticles

Dilute nitric acid solution was first

Results and discussion

Bismuth ferrite NPs were first synthesized using hydrothermal method. Then the CFs were fabricated via solution casting method with different weight ratios (2, 4, 6, 8 and 10 wt%) of as-synthesized BFO NPs incorporated into the copolymers PVDF-TrFE and PVDF-HFP matrices. The measured average thickness of the piezoelectric CFs PTB and PHB were 42 µm and 48 µm, respectively.

Conclusion

In summary, this work successfully demonstrates the preparation of BFO NPs by hydrothermal method and fabrication of CFs PVDF-TrFE/BFO and PVDF-HFP/BFO by solution casting method. The structural characterization of the as-fabricated composites explored by different techniques like XRD and FTIR showed that the composition PVDF-TrFE/BFO (6 wt%) exhibited superior performance as compared to others. The incorporation of BFO NPs enhanced the nucleation and improves the piezoelectric polar β-phase.

CRediT authorship contribution statement

Alekhika Tripathy: Methodology, Investigation, Conceptualization, Data curation, Validation, Writing – original draft; Nirmal Prashanth Maria Joseph Raj: Resources, Formal analysis; B. Saravanakumar: Formal analysis, Writing – review & editing; Sang Jae Kim: Resources, Formal analysis; Ananthakumar Ramadoss: Conceptualization, Methodology, Writing – review & editing, Funding acquisition, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Scheme for Young Scientists and Technologists (SYST, Ref. no: SP/YO/2019/1432), Science for Equity, Empowerment and Development (SEED), Department of Science and Technology (DST), New Delhi. The authors also thank, CIPET: SARP-LARPM for providing the research facilities to carry out this research work.

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