Ferroelectric flexible composite films based on morphotropic phase boundary for self-powered multisensors

https://doi.org/10.1016/j.cej.2021.128840Get rights and content

Highlights

  • The MPB based ferroelectric 0.3Ba0.7Ca0.3TiO3-0.7BaSn0.12Ti0.88O3 (BCST) prepared.

  • Systematic analysis of structural, electrical properties was studied for BCST.

  • The PBCST-CFNG gives 92 V, 440 nA with a power density of 44 mW/m2.

  • The PBCST-CFNG is used to fabricate the piezoelectric ball for self-powered sensors.

  • The piezoelectric ball is used to monitor distance, rotation, impact position/counts.

Abstract

The synthesized 0.3(Ba0.7Ca0.3TiO3)-0.7(BaSn0.12Ti0.88O3) nanoparticles (BCST NPs) and their morphotropic phase boundary (MPB) evolution characterized based on the structural, vibrational, dielectric, and ferroelectric properties of the individual Ba0.7Ca0.3TiO3 (BCT), BaSn0.12Ti0.88O3 (BST) NPs. The MPB system had a dielectric constant/loss of 710/0.088 at 1 kHz frequency, with remnant polarization (Pr) of 8.5 μC/cm2, under a coercive field of 15 kV/cm at 1 Hz and its fatigue property evaluated up to 108 cycles. The BCST has a piezoelectric coefficient of 65 pC/N. Flexible polyvinylidene fluoride/BCST NP composite films (PBCST-CFs) were fabricated and characterized. The optimal loading of 15 wt% provided a more electroactive phase (70.93%), a dielectric constant of 33 and loss of 0.0066, and a Pr of 0.8 µC/cm2. The PBCST-CF (15%)-based nanogenerator (PBCST-CFNG) generated electrical output of 92 V, 440nA upon applied 8 N force under acceleration of 2 m/s2, with power density of 44 mW/m2 at 160 MΩ. Electrical poling, durability, and switching polarity testing, charging of commercial capacitors, and the driving of low-power electronics were also performed. A PBCST-CFNG (15%)-based piezoelectric ball was designed and used to monitor the ball height/distance, rotations, and impact positions/counts.

Introduction

Harvesting renewable and eco-friendly energy provide much-needed relief to the environment and society that largely dependent on fossil fuels, such as coal, oil, and gas. Renewable energy sources include biomass, geothermal, solar, and hydroelectric. However, these sources require large spaces for installation, operation and specific atmospheric conditions to generate energy. To power the ever-increasing number of smart and portable devices, self-powered sources are needed that can harvest energy instantly. Currently, intelligent devices consume stored energy from capacitors and batteries, which increases their weight and size [1], [2], [3]. Although energy cannot be created or destroyed, it can be converted. For intelligent and simple self-powered applications, a simple and cost-effective method is needed for converting readily available resources into electricity. This is most frequently achieved by converting mechanical motion, which is the most abundant energy source in our daily life and the surrounding environment and can even be human-generated. Nanogenerators (NGs) can convert mechanical forces into useful electrical power; these devices are generally classified as piezoelectric and triboelectric NGs, converting even minimal mechanical strain into useful electrical energy. Triboelectric NGs (TENGs) have an advantage over piezoelectric NGs (PNGs), with almost all the materials in nature can be used to develop triboelectric NGs. However, most of the reported triboelectric NGs work under a solid–solid contact state, resulting in high friction and serious material damage during the device operation. As we know, most of the TENGs have polymer as one layer, which is prone to easy reduction in stiffness due to the heat originated from continuous contact and separation of TENG operation, which results in low durability and reliability of the generated outputs“. In contrast, the PNGs have been highly stable for a long time due to their complete packing. The piezoactive material's structural stability and high sensitivity to even a small amount of force made them a potential candidate for self-powered sensor applications [4], [5], [6], [7], [8].

The strain-induced dipole orientation leads to electrical energy generation in certain materials, known as piezoelectric materials, which are fundamental for PNGs. Piezoelectric materials are classified as “smart” materials due to their multifunctional properties, making them suitable for various applications, including ferroelectric random-access memories, piezocatalysts, and sensors. They can be made of crystals, semiconductors, ceramics, polymers, and two-dimensional materials [9]. Crystals are primarily used in sensors, and most are soluble in water and fragile. The most studied semiconductors for multifunctional applications are ZnO and GaN, and devices made with these materials produce structural dependent piezoelectric performance. Among polymers, polyvinylidene fluoride (PVDF), cellulose, and collagen are highly flexible and easy to fabricate, but they have relatively low piezoelectric coefficients. Ceramics have the highest piezoelectric coefficients among all types of piezoelectric materials. This group includes PbZrTiO3 (PZT), BaTiO3 (BT), (K,Na)NbO3 (KNN), (Bi,Na)TiO3 (BNT), and (Bi,K)TiO3 (BKT), but their brittleness prevents operation under high mechanical strain [3], [10], [11], [12], [13], [14]. Researchers have recently started to focus on lead-free ceramic materials due to the toxicity of lead and its ready volatilization at high temperatures.

The first perovskite ferroelectric material to be synthesized was BT; it has been extensively studied over the past 70 years due to its good dielectric, ferroelectric, and piezoelectric properties [15]. The piezoelectric properties of any ceramic can be improved by doping, which is a simple way to obtain tremendous improvements in their material properties. The morphotropic phase boundary (MPB) has been explored to improve the piezoelectric properties with the high electromechanical coupling coefficient. The MPB is a phase transition that occurs due to variation in composition. It can occur between two ferroelectric phases or between one non-ferroelectric and one ferroelectric phase. Extensive doping and composition-dependent analyses have been carried out to improve the piezoelectric properties of BT. In the BT system, the MPB has been obtained by doping Ca atoms in the A-sites and Zr/Hf/Sn atoms in the B-sites, similar to the (Bi,Na,K)TiO3 (BNKT) systems [16], [17]. Although MPB ceramics have good piezoelectric properties, their applications have been limited primarily to capacitors [11], [13]. The most studied MPB BT-based ceramic is the Ba(Ti1–xZrx)O3–(Ba1–xCax)TiO3 (BCZT) system, which has been examined in terms of its utility for energy storage and harvesting [18], [19], [20]. For this work, we selected the 0.3(Ba0.7Ca0.3TiO3)-0.7(BaSn0.12Ti0.88O3) (BCST) MPB system, having a piezoelectric constant (d33) of ca. 530 pC/N, as our active piezoelectric ceramic material. This system has not been extensively studied compared with other MPBs [21].

Composites of polymers and ceramics have been extensively used to prepare flexible and highly active piezoelectric materials for energy harvesting. Ferroelectric PVDF is the most well-studied piezoelectric energy harvesting polymer due to its good piezoelectric properties and ease of preparation. This polymer can contain five phases, i.e., α, β, γ, δ, and ε with trans (T) or gauche (G) conformation, e.g., TGTGʹ for the α and δ phases, T3GT3Gʹ for the γ and ε phases, and TTTT for the highly electroactive β-phase. Various approaches have been used to convert the non-electroactive phase into a useful electroactive phase, i.e., by introducing conductive materials (graphene, carbon nanotubes, Ag), ions, and nanoparticles (NPs), and electrospinning [10], [11], [12], [13]. One of the most cost-effective strategies is using ultrasonication to dissolve PVDF polymer to prepare films. Compared with the usual mechanical stirring process, ultrasonication aligns more dipoles and enables piezoelectric energy harvesting. Herein, ultrasonically treated PVDF membranes were used to make composite films (CFs) with BCST NPs for high-performance and flexible PNGs.

Various weight percentages of BCST NPs (0, 5, 10, 15, 20, and 25 wt%) were added to a PVDF solution obtained from a probe sonication process to make flexible PVDF-BCST composite films (PBCST-CFs). To avoid the thickness effect on energy harvesting, all the composite films were made in the same thickness using bar coater (PBCST-BCCFs). We also fabricated composite films based NGs to convert waste mechanical energy (from constant applied forces, random mechanical motions, and biomechanical forces) into useful electrical energy. The energy harvesting performance was analyzed before and after poling of each PBCST-CFNG. The PBCST-CFNG (15%) displayed a higher electrical response (92 V, 440 nA upon an applied force of 8 N (pressure:20 kPa) under acceleration of 2 m/s2) and instant power density (44 mW/m2 at matching resistance of ca. 0.160 GΩ). The durability, commercial capacitor charging and powering of light-emitting diodes (LEDs), watches, and calculators were also evaluated. Additionally, the PBCST-CFNG (15%) was demonstrated as a height, position, and water wave sensor for monitoring applications.

Section snippets

Synthesis of Ba0.7Ca0.3TiO3, BaSn0.12Ti0.88O3, 0.3(Ba0.7Ca0.3TiO3)-0.7(BaSn0.12Ti0.88O3) nanoparticles (BCST NPs)

The lead-free polycrystalline ferroelectric systems of Ba0.7Ca0.3TiO3 (BCT), BaSn0.12Ti0.88O3 (BST), and BCST NPs were synthesized via a conventional solid-state reaction. The stoichiometric ratio of the elements was obtained by weighing the commercially available oxide and carbonate powders of BaCO3 (99.95%; High Purity Chemicals), TiO2 (98%; Dae-Jung), SnO2 (99.99%; Sigma–Aldrich) and CaCO3 (99.99%; High Purity Chemicals). The raw materials were manually mixed in ethanol medium in a mortar

Results and discussion

Fig. 1a shows the X-ray diffraction (XRD) patterns of the synthesized ferroelectric BCT, BST, and BCST system. Pure BT has a tetragonal crystal structure at RT. When Ba2+ (1.61 Å) ions were replaced with Ca2+ (1.34 Å) ions in the A-sites Ba(1-x)Ca(x)TiO3 with x = 0.30, a secondary phase of orthorhombic system emerged along with a significant tetragonal phase [18], [22]. When Ti4+ (0.60 Å) ions were replaced with Sn4+ (0.69 Å) in the B-sites of BaCaTi(1-x)Sn(x)O3 with x = 0.12, the material

Conclusion

The MPB-based piezoelectric BCST NP system was synthesized via a cost-effective solid-state reaction. The phase evolution and high performance of the material were understood by characterizing the structural, vibrational, morphological, dielectric, and ferroelectric properties of BCT and BST. The selected MPB system was incorporated in ferroelectric PVDF to prepare flexible PBCST-CFs via the simple solution-casting approach. We confirmed that PBCST-CF (15%) had the best electroactive phase

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.

Acknowledgment

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (2019R1A2C3009747, 2021R1A4A2000934).

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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