English

• Piezoelectric materials play a crucial role in harvesting mechanical energy from the environment, especially in the context of energy harvesting and sensor applications [1-6]. Traditional inorganic ceramics like lead zirconate titanate (PZT) and barium titanate (BTO) dominate the field due to their large piezoelectric coefficients, high electromechanical coupling coefficients, and high phase transition temperatures [7-11]. However, the stiffness and brittleness inherent in these ceramics pose challenges for their integration into flexible wearable devices. There is a need for alternative piezoelectric materials to overcome these challenges and facilitate their application in flexible wearables.

  Different from inorganic ceramic piezoelectric materials, poly(vinylidene fluoride) (PVDF) as piezoelectric polymers, famous for good flexibility and easy processibility, demonstrating advantages toward wearable piezoelectric devices [12-25]. Nevertheless, the piezoelectric properties of traditional PVDF are limited by the content of β-crystal phase. Different physical methods, such as stretching [26-28], thermal annealing [29-31], electrospinning [32-35], and high electric-field polarization [36, 37], could enhance the β-phase content. For instance, Zhu et al. [38] achieved pure β-phase (100%) PVDF through biaxial stretching and DC unidirectional polarization. Moreover, Yang et al. [39] incorporating MXene nanofillers, has induced the β-phase by leveraging spatial confinement, resulting the maximum proportion of β-phase was 89.71%. The V_oc and I_sc values of piezo-electronic devices are 3.15 V and 134 nA respectively at 200 kPa, frequency 1.5 Hz, and acceleration 0.5 ms^-2. Although the above works enhance the piezoelectric properties, actual piezoelectric output is still cannot meet practical application requirements. Other strategies like integrating additional high voltage coefficient fillers like molecular ferroelectrics and inorganic ceramics were reported attempts to increase the actual output power density [40-42]. However, the mechanical mismatch with the PVDF substrate can impede stress transfer and compromise interface continuity, leading to diminished output and stability which limited the output power density of PVDF based composite. Therefore, novel approaches are essential to further enhance the piezoelectric performance of PVDF.

  To address the above challenges, liquid metal (LM), specifically gallium-indium-tin eutectic alloy, was selected as filler to composite with PVDF for improving properties of actual piezoelectric output. Compared with solid nano-fillers, the introduction of LMs establishes a continuous solid-liquid interface, effectively resolving the mismatch problem between nano-fillers and the mechanical strength of the substrate [43-45]. The formed liquid-solid/electric-dielectric interface between the LMs and PVDF improved the β-phase content above 90%. Additionally, LM exhibits high electrical conductivity, easy plasticity, low biotoxicity, and adjustable size through ultrasonic crushing [46-48]. Thus, facilitating effective charge transfer and improving piezoelectric output properties. Under external stimulation, the best-performing LM/PVDF composites exhibited significantly improved piezoelectric output properties, with the output power density of 353 µW/cm². This is nearly 1000 times higher than that of pure PVDF materials [29]. Such a high value is much higher than other PVDF based composites. Importantly, these composites showed excellent stability, with no significant degradation of the piezoelectric output performance observed during nearly 20,000 cycles. Furthermore, the composite material was applied to the testing of Handwriting information detection and human pulse monitoring, showcasing its potential for practical applications.

  To synthesize the LM/PVDF composites, initially, 1 g of PVDF powder was mixed with 10 mL of DMF solvent. This mixture was then heated and stirred at 50 ℃ for 8 h until a transparent solution was achieved. Concurrently, LMs with different mass fractions (5%, 15%, 25%) were added into 3 mL of DMF solvent. An Φ3 amplitude rod was immersed in the solution, and power was set to 50% using intermittent mode (on/off for 2/2 s). Ultrasonic crushing was conducted in an ice bath for 10 min to obtain a homo disperse LM turbid liquid. To prepare LM/PVDF composites, the DMF solvent with LMs was added to the above PVDF solution and continue stirred at 1 h. The resulting mixture was then poured onto a glass plate and evenly spread with a spatula. The glass plate was placed in a 60 ℃ oven for 6 h. LM/PVDF composites were obtained by peeling them from the glass plate. Finally, the composite thin film was polarized at a field intensity of 50 kV/mm for 24 h. For comparison purposes, pure PVDF thin films without LMs were also prepared.

  The design strategy employed in this study is depicted in Fig. 1. The LMs possess electrical/thermal conductivity and excellent fluidity. Through the ultrasonication process, LMs were incorporated into the PVDF matrix, resulting in the formation of LM/PVDF piezoelectric composites. This design strategy offers two significant advantages: (1) Within the composite, a polar interaction occurs between LMs and the -CF₂/-CH₂ dipoles of PVDF. This interfacial polarization promotes the formation of the β-phase in PVDF, thereby improving its piezoelectric properties. (2) The mobility of the LMs themselves, coupled with their excellent electrical conductivity, also enhances the flexibility and piezoelectric output capacity of the PVDF. The detailed fabrication of LM/PVDF composites were synthesized using the solution casting method (Fig. S1 in Supporting information), LM/PVDF composites achieve complete flexibility (Fig. S2 in Supporting information).

  Figure 1

  

  Figure 1.  Schematic diagram of the design strategy process of LM/PVDF composites.

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  As previously mentioned, PVDF, being a piezoelectric material, manifests different crystal phases, primarily the α and β-phases (Fig. 2a). In the α phase, the arrangement of adjacent -CF₂/-CH₂ dipoles adopts a reverse parallel configuration, leading to the nullification of dipole moments, rendering it non-polar. Conversely, the β-phase, wherein all -CF₂/-CH₂ dipoles align in the same direction, harbors the highest macroscopic dipole moment and substantially contributes to PVDF's piezoelectric properties [3]. To ascertain the phase composition of PVDF in the composites doped with varying LM mass fractions (5%, 15% and 25%), X-ray diffraction (XRD) characterization was conducted (Fig. 2b). The characteristic peaks around 20.4° (110 and 200) correspond to the polar β-phases [49]. The PVDF composite with LMs exhibited stronger peaks near 20.4° compared to pure PVDF, indicating an increased content of the β-phase. This observation was further supported by Fourier- transform infrared spectroscopy (FT-IR) characterization (Fig. 2c). The characteristic absorption peaks around at 610, 766, and 975 cm^-1 belong to the PVDF α-phase, while around at 840, 1279 cm^-1 belong to the PVDF β-phase and the characteristic absorption peaks for the γ-phase are around at 431, 482, 811 and 1234 cm^-1. The influence of different proportions of LMs on PVDF β-phase can be quantified following Lambert-Beer's law:

  $ F(\beta)=\frac{A_\beta}{\left(\frac{K_\beta}{K_\alpha}\right) A_\alpha+A_\beta} $

  (1)

  Figure 2

  

  Figure 2.  (a) Schematic diagram of different crystal phases of PVDF. (b) XRD patterns and (c) FTIR spectra of PVDF composites with different liquid metal content. (d) SEM images of surface and cross section. (e) Particle size statistics and (f) EDS mapping images of LMs droplet of PVDF composites with 15% LMs content. (g) Schematic diagram of β-phase formation on LMs. (h) COMSOL finite element simulates the charge density of LM on PVDF composites. COMSOL finite element simulates the stress distribution on (i) pure PVDF and (j) PVDF composites. (k) Enlarged view of (j).

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  where A_α and A_β represent the absorption intensity at 766 and 840 cm^-1, respectively, and K_α and K_β are the absorption coefficients of their respective wavenumber, 6.1 × 10⁴ and 7.7 × 10⁴ cm²/mol, respectively [1]. The β-phase content increased with the addition of LMs, reaching a maximum (90%) at 15% LMs content, However, as the content of LMs is further increased, the agglomeration phenomenon of LMs increases (Fig. S3 in Supporting information), which also leads to the decrease of the content of β-phase (Fig. S4 in Supporting information). Field emission scanning electron microscope (FE-SEM) characterization of the PVDF composite with 15% LMs content revealed an even distribution of LMs with an average size of approximately 1.1 µm, as observed on both the surface and cross-section (Figs. 2d and e). Additionally, energy-dispersive X-ray spectroscopy (EDS) confirmed the elemental composition of LMs, with Ga, In and Sn were uniformly distributed in the PVDF matrix (Fig. 2f). The above XRD and FT-IR characterizations both showed that the LM/PVDF composites polar β-phase content increased, we attribute the increase of the polar β-phase of PVDF to the interfacial interaction between LMs and PVDF molecular chains. According to previous research reports [45], LMs has electron-rich properties, while the -CH₂- dipoles (δ⁺) in the PVDF molecular chain is positively charged. Through the charge interaction, the -CH₂- dipoles in the PVDF molecular chain is aligned in the same direction around the LMs, thus increasing the polar β-phase content (Fig. 2g) [50]. To validate this conclusion, COMSOL finite element simulation was conducted as illustrated in Supporting information, with the results presented in Fig. 2h confirming a significant enrichment of charge around LMs. Therefore, the increase in β-phase content is attributed to the introduction of LMs. Furthermore, LMs, as a filler, can concentrate stress. Under the same load, the PVDF composites with LMs are subjected to greater stress than pure PVDF materials, as shown in Figs. 2i and j, with a more pronounced effect observed in the enlarged Fig. 2k. Based on the aforementioned research results, the introduction of LMs enhances the polar β-phase of PVDF and bears greater stress, thereby improving the piezoelectric performance of the composite from these two different aspects, as further demonstrated in the subsequent piezoelectric performance test.

  Building upon the performance characterization discussed earlier, the actual piezoelectric effect of the material was investigated (Fig. 3). The PVDF composites, through electrical polarization, the dipoles achieve a more orderly arrangement, and upon the application and release of stress, an opposite charge transfer occurs between the electrode surfaces, converting the applied mechanical energy into electrical energy (Fig. 3b). Figs. 3c and d illustrate the piezoelectric signal output of the PVDF composites in forward and reverse modes. The results indicate that the 15% PVDF composites could produce a maximum voltage and current signal of 73 V and 13 µA, respectively, under a 5 N and 10 Hz external stimulation (Figs. 3e and f, Fig. S5 in Supporting information). The piezoelectric performance test photograph, as shown in Fig. S12 (Supporting information). However, the piezoelectric output of the PVDF composites exhibited different trends with changes in LM content (Fig. 3g). The output voltage for 5% doping was the lowest (~15 V), reaching 73 V for 15% doping. However, at 25% doping, the output voltage decreased to 28 V. The piezoelectric voltage coefficient (d₃₃) also showed a similar trend, with d₃₃ being the maximum 41 pC/N at 15% doping (Fig. S6 in Supporting information), indicating that the aggregation of LMs reduced the piezoelectric voltage output.

  Figure 3

  

  Figure 3.  (a) Schematic illustration on the configuration of LM/PVDF composites. (b) Schematic diagram of electricity generation principle of PVDF composites. (c) Voltage test diagram and (d) current test diagram of PVDF composites. Detail diagram of (e) 10 Hz voltage test and (f) current test for PVDF composites. (g) Comparison of voltage output performance of PVDF composites with different LMs content. (h) Power density comparison diagram.

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  Moreover, the best performance PVDF composite (15% LMs doping) demonstrate long-term reliability with over 20,000 electricity-generating cycles (Fig. S7 in Supporting information). To further evaluate the piezoelectric output performance of PVDF composites, the output voltage of the 15% PVDF composite load resistance was tested, the power density is calculated according to the following equation [51]:

  $ P=\frac{V^2}{R \times S} $

  (2)

  where V, R, and S represent the output voltage, the corresponding load resistance, and the stimulation area respectively. The maximum power density reaching 353 µW/cm² (Fig. S8 in Supporting information), significantly surpassing other types of PVDF composites, including metal oxides (ZnSnO₃, ZnO), inorganic piezoelectric ceramics (PZT), and organic-inorganic hybrid (FaPbBr₃, FaPbBr₂I) as shown in Fig. 3h [45, 52-59].

  The 15% PVDF composites demonstrates remarkable voltage and current output capabilities when subjected to minute mechanical forces. Leveraging this property, the work aims to utilize the material as a sensor for converting force signals into electrical signals and further develop a handwriting information recognition system (Fig. 4). A sensor array consisting of 9 sensors with an area of 1.5 × 1.5 cm² arranged in 3 × 3 configuration was prepared (as described in Supporting information). Additionally, each sensor is numbered to facilitate the capture of spatial information of handwritten characters (Fig. 4a). The spatial information is analyzed by the Microcontroller unit (MCU-STM32) and transmitted to the computer terminal for display (Fig. 4b). The detailed working mechanism is shown in Fig. 4c. Firstly, the electrical signal generated by the sensor array after stimulation is transmitted to the analog-to-digital converter (ADC) channel of the MCU through the signal amplifier (Fig. S9 in Supporting information). Subsequently, the ADC converts the analog signal into a digital signal (Fig. S10 in Supporting information) so that the MCU can read and process the value of the ADC channel. After removing the abnormal voltage value by digital filtering algorithm, the voltage value obtained is stored in MCU register. Following this, a data transmission channel is established between the MCU serial communication module (UART) (Fig. S11 in Supporting information) and the computer to package the processed data and transmit it to the computer. Meanwhile, the computer program controls the display state of the corresponding area in the nine-palace grid according to the change of data, thereby enabling the detection of handwritten information. To test its feasibility, the letters "Z", "J", "N", and "U" were sequentially handwritten on the sensor array. The corresponding letters were successfully displayed on the computer screen, and the response voltage signal of the sensor array was recorded (Fig. 4d). These tests verified the feasibility of the system.

  Figure 4

  

  Figure 4.  (a) Schematic diagram of sensor array based on LM/PVDF composites. (b) Photograph of handwriting information recognition system based on sensor array. (c) The working mechanism of handwriting information recognition system. (d) Handwriting information recognition system.

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  The aforementioned applications have demonstrated the reliable sensing properties of LM/PVDF composites, which can further be employed in human pulse signal (heart frequency) detection (Fig. 5a). The pulse is converted into a voltage signal output and recorded, when this material is placed on the human wrist. By analyzing the voltage signal, the human heart rate can be monitored. As depicted in Figs. 5b and c, under normal conditions, the human heart frequency is 79 beats per minute. At this time, the voltage signal generated by LM/PVDF composites fitted on the wrist is illustrated in Fig. 5d. Under the action of pulse beating, the voltage generated is approximately 0.79 V, with 79 Vage peaks detected within 60 s. When the human body exercises, the heart frequency increases to 109 beats per minute. At this juncture, the output voltage signal of LM/PVDF composites also increases, reaching about 0.96 V, with 101 Vage peaks detected in 60 s. This closely matches the heart rate measured by the oxygen meter.

  Figure 5

  

  Figure 5.  (a) Schematic diagram of human pulse wave monitoring application. LM/PVDF composites under (b) normally and (c) exercise used for pulse monitoring; Voltage signal output diagram of LM/PVDF composites under (d) normal circumstances and (e) after exercising.

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  In summary, this work presents a novel composite material based on PVDF and LMs. The incorporation of LMs not only increases the β-phase content of LM/PVDF composites but also enhances its polarizability and charge transport capabilities. When the content of LMs reached 15%, there is higher β-crystal phase content (90%) and the best electricity generation performance. Under an external force of 5 N and 10 Hz, the maximum output voltage and current could reach 73 V and 13 µA. The output power density of LM/PVDF composites is enhanced to 353 µW/cm², which is nearly 1000 times higher than that of pure PVDF materials. Such a high value is much higher than other PVDF based composites so far. Furthermore, the composite sustained 20,000 cyclic piezoelectric tests without any observed attenuation in the voltage output signal. These remarkable piezoelectric properties make the composite material suitable for flexible applications, such as wearables and health monitoring.

  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.

  CRediT authorship contribution statement

  Qiang-Qiang Jia: Data curation. Jia-Qi Luo: Investigation. Zhi-Yu Xue: Resources. Jing-Song Tang: Formal analysis. Wen-Qiang Qiu: Resources. Chang-Feng Wang: Data curation. Zhi-Xu Zhang: Data curation. Hai-Feng Lu: Project administration. Yi Zhang: Writing – review & editing. Da-Wei Fu: Writing – review & editing.

  Acknowledgments

  This work was financial supported by National Natural Science Foundation of China (Nos. 52303256, 21991141, 22375182), Natural Science Foundation of Zhejiang Province (Nos. LQ23B040004, LZ24B010001) and Jinhua Industrial Major Project (No. 2022–1–043).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110471.

  
  
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• 

  Figure 1  Schematic diagram of the design strategy process of LM/PVDF composites.

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  Figure 2  (a) Schematic diagram of different crystal phases of PVDF. (b) XRD patterns and (c) FTIR spectra of PVDF composites with different liquid metal content. (d) SEM images of surface and cross section. (e) Particle size statistics and (f) EDS mapping images of LMs droplet of PVDF composites with 15% LMs content. (g) Schematic diagram of β-phase formation on LMs. (h) COMSOL finite element simulates the charge density of LM on PVDF composites. COMSOL finite element simulates the stress distribution on (i) pure PVDF and (j) PVDF composites. (k) Enlarged view of (j).

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  Figure 3  (a) Schematic illustration on the configuration of LM/PVDF composites. (b) Schematic diagram of electricity generation principle of PVDF composites. (c) Voltage test diagram and (d) current test diagram of PVDF composites. Detail diagram of (e) 10 Hz voltage test and (f) current test for PVDF composites. (g) Comparison of voltage output performance of PVDF composites with different LMs content. (h) Power density comparison diagram.

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  Figure 4  (a) Schematic diagram of sensor array based on LM/PVDF composites. (b) Photograph of handwriting information recognition system based on sensor array. (c) The working mechanism of handwriting information recognition system. (d) Handwriting information recognition system.

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  Figure 5  (a) Schematic diagram of human pulse wave monitoring application. LM/PVDF composites under (b) normally and (c) exercise used for pulse monitoring; Voltage signal output diagram of LM/PVDF composites under (d) normal circumstances and (e) after exercising.

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