Preparation method and application of flexible self-powered piezoelectric sensor

Abstract

This invention discloses a method for fabricating a flexible self-powered voltage sensor, comprising: S1, substrate treatment and film printing; S2, thermal annealing; S3, ion sputtering of a silver electrode; S4, high-voltage polarization; and S5, packaging. The flexible self-powered voltage sensor fabricated by this invention possesses both good structural uniformity and mechanical stability, and has application potential in wearable sensing, human motion monitoring, flexible energy harvesting devices, as well as in online monitoring and intelligent sensing of precision drilling and heterogeneous connection equipment systems for pods.

Description

Technical Field

[0001] This invention belongs to the field of composite piezoelectric sensor technology, and particularly relates to a method for preparing and applying a flexible self-powered piezoelectric sensor. Background Technology

[0002] Flexible piezoelectric materials, with their advantages of lightweight, bendability, and ease of integration, have shown significant application prospects in wearable electronics, biomedical sensing, and self-powered energy harvesting. Polyvinylidene fluoride (PVDF), as a semi-crystalline fluoropolymer, possesses excellent flexibility, biocompatibility, and intrinsic piezoelectric, ferroelectric, and dielectric properties, making it a suitable core matrix material for flexible piezoelectric functional devices. The highly electronegative F atoms and strongly polar C–F bonds in the PVDF molecular chain are the source of its electroactivity, while the macroscopic piezoelectric response is highly dependent on the crystal phase structure and the degree of dipole order. PVDF mainly contains… α, β, γ, δ, ε Five crystal phases, of which only the all-trans (TTTT) conformation. β The phase can achieve an ordered arrangement of C–F dipoles along the chain axis, thereby generating a stable piezoelectric output. However, pure PVDF has a significant performance bottleneck: thermodynamically... α compared to β The phase is more stable (Gibbs free energy is about 0.2 kJ / mol lower), and it is nonpolar during melting and cooling. α Preferential crystallization of the phase. Pure PVDF films prepared by conventional solution casting and hot annealing. β The phase content is usually less than 15%, the crystallinity is only 35%–40%, and the piezoelectric coefficient d 33 Only 5–10 pC / N, far below the theoretical limit of 30 pC / N. Furthermore, traditional boosting... β Methods such as mechanical stretching and high-voltage polarization of PVDF phases suffer from high energy consumption, easy breakdown, and easy depolarization, limiting the practical application of PVDF-based flexible devices. Therefore, achieving [the desired effect] without mechanical stretching and by reducing the polarization voltage is crucial. β Highly efficient, controllable, and uniform induction of phases are key scientific issues in the development of PVDF-based piezoelectric materials.

[0003] Heterogeneous nucleation has been proven to be an effective way to overcome the bottleneck of PVDF crystal phase control. Nanofillers can reduce [the impact of] interfacial interactions. β Nucleation barriers are formed, increasing their content and piezoelectric response. Commonly used fillers include oxide ceramics (BaTiO3, ZnO, WO3, etc.), carbon-based nanomaterials (graphene, carbon nanotubes, etc.), and two-dimensional layered materials (MXene, MoS2, etc.). Among them, carbon-based fillers, due to their high conductivity, large specific surface area, and strong interfacial interactions, play a significant role in inducing nucleation barriers. β Graphene and its derivatives exhibit significant advantages in phase crystallization and enhanced dielectric and piezoelectric properties. They can be used for efficient triggering.α arrive β Phase transformation increases the proportion of electroactive phases; multi-walled carbon nanotubes (MWCNTs) can promote... β Phase nucleation can also improve the mechanical and thermal stability of composite materials.

[0004] Direct-write 3D printing (DIW) offers a new pathway for the customized fabrication of PVDF-based flexible piezoelectric devices, enabling complex structural designs, internal stress optimization, and enhanced piezoelectric output. However, the coupling mechanism between the dimensionality and dispersion behavior of nanofillers and the crystalline phase orientation of PVDF under the shearing and spatial confinement effects of DIW remains unclear. Existing research largely focuses on single-dimensional fillers, lacking quantitative analysis of dimensional effects and nucleation mechanisms. 0D particles (such as carbon black) can provide isotropic nucleation sites but are prone to aggregation; 1D tubes / wires (such as carbon nanotubes) facilitate anisotropic alignment but affect ink rheology; 2D nanosheets (such as graphene and MoS2) have large specific surface areas and strong interfacial interactions, enabling directional epitaxial nucleation, but interlayer stacking and colloidal stability are difficult to control.

[0005] Therefore, it is necessary to propose a method for fabricating and applying a flexible self-powered voltage sensor to solve the above problems. Summary of the Invention

[0006] This invention constructs a composite ink system composed of carbon-based fillers of different dimensions (0D, 1D, 2D) and a PVDF matrix. Through the synergistic effect of the heterostructure, the nucleation density, dispersion uniformity, and crystal orientation are synergistically optimized. The dispersion behavior, interfacial interactions, and microstructure regulation mechanisms of the carbon-based fillers are systematically analyzed. Furthermore, rGO / PVDF composite films are prepared using direct-write printing (DIW) technology, and the influence of the rGO filler ratio on the crystallization behavior, surface morphology, and microstructure of PVDF is investigated in depth. Based on this, ionic liquids (ILs) are introduced to utilize the piezoelectric ion effect to synergistically enhance the orientation of PVDF molecular chains. β Phase content can be adjusted to improve the piezoelectric properties of composite systems from a material design perspective.

[0007] The method for fabricating a flexible self-powered piezoelectric sensor (rGO / IL / PVDF composite piezoelectric sensor) provided by the present invention includes the following steps: S1. Substrate preparation and film printing: During the research phase: a flexible PET film with a pre-embedded bottom electrode was selected as the substrate. After cleaning and drying, a composite ink with an rGO content of 0.5 wt% was uniformly deposited on the substrate surface using a DIW inkjet printer to form a single-layer wet film. In order to further improve the dispersion uniformity of the nanofiller in the polymer matrix and adjust the degree of freedom of movement of the polymer chain segments to optimize the polarization effect, an ionic liquid IL was introduced as a synergistic modifier into the rGO / PVDF system.

[0008] A mixed solution with a PVDF:rGO:IL ratio of 11wt%:0.5wt%:10wt% was prepared. A flexible PET film was selected as the substrate, and the mixed solution was uniformly deposited on the substrate surface using a DIW inkjet printer to form a single-layer rGO / IL / PVDF composite piezoelectric film sample. S2. Heat annealing treatment: Place the sample in a vacuum drying oven and anneal at 80°C for 4 hours to completely remove the solvent and promote the rearrangement and crystallization of PVDF crystal chains. S3, Ion sputtered silver electrode: After polarization, a dense nano-silver layer is deposited on the surface of the thin film using a high-purity silver Ag target material via an ion sputtering instrument to form an electrode, thus constructing a complete electrode-piezoelectric layer-electrode structure. S4. High-voltage polarization: Electrodeization is performed with silver electrodes deposited on the upper and lower surfaces of the thin film. A 16 kV DC high voltage is applied to the rGO / IL / PVDF layer for 0.5 h at room temperature using a high-voltage needle electrode. The electric field can effectively penetrate the polymer medium and cause the electric dipole moment to align in the direction of thickness. S5. Packaging: Conductive adhesive is used to lead out copper wires, and PI tape is used to cover them for insulation protection, resulting in a flexible rGO / IL / PVDF piezoelectric film sensor.

[0009] In a preferred embodiment of this scheme, IL in S1 is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt.

[0010] Furthermore, after the mixed solution in S1 is prepared, vacuum degassing is required: place the prepared mixed solution in a vacuum drying oven and let it stand for 30 minutes to completely remove the air bubbles introduced during the stirring process, so as to prevent printing interruption or the generation of pore defects inside the film.

[0011] In a preferred embodiment of this scheme, after the polarization treatment in S4 is completed, the two poles of the sample are short-circuited and left to stand for 24 hours to release the residual space charge on the surface and eliminate unstable signal interference, thereby obtaining a piezoelectric composite film with stable performance.

[0012] In a preferred embodiment of this solution, the prepared flexible self-powered voltage sensor has both good structural uniformity and mechanical stability, and can be applied to wearable sensing, human motion monitoring and flexible energy harvesting equipment, as well as online monitoring and intelligent optimization equipment for precision drilling and heterogeneous connection equipment systems of pods.

[0013] Implementing this invention has the following beneficial effects: This paper describes the fabrication method of an rGO / PVDF composite piezoelectric sensor and explores the influence of carbon-based fillers of different dimensions on the crystal structure of PVDF. The effects of three fillers—particulate (carbon black), bundled (carbon nanotubes), and sheet-like (graphene)—were compared. Experiments revealed that sheet-like graphene (rGO) can be better dispersed within the material, providing good crystallization attachment sites for PVDF and promoting its piezoelectric properties. β The phase transformation results in better overall performance than the other two types of fillers.

[0014] The material formulation of the composite film was optimized, and an PVDF matrix solution concentration of 11 wt% was experimentally determined to be suitable. The effect of graphene filler content on piezoelectric and dielectric properties was also tested. Results showed that a relatively uniform microcapacitor network could be formed within the material when the graphene content was 0.5 wt%. Utilizing the shear force during the 3D printing process, the piezoelectric and dielectric properties of the film were improved, and the piezoelectric constant dp was increased. 33 It achieves 29.0 pC / N while maintaining low energy loss.

[0015] Adding an ionic liquid (IL) to rGO / PVDF materials makes them more flexible, which helps the internal charge rearrangement under stress. After high-voltage polarization treatment at a polarization electric field strength of 100 MV / m, the rGO / IL / PVDF composite film exhibits a significant giant dielectric response at 1 kHz (dielectric constant reaches 44011.7, quasi-static piezoelectric constant dp). 33 (Achieving a pC / N of 58.0). A piezoelectric sensor was further fabricated by organizing the rGO / IL / PVDF piezoelectric composite film and tested. The test results show that the sensor can generate stable and continuous voltage signals in response to actions such as keyboard typing, finger bending, and foot stomping, demonstrating its application potential in motion monitoring and interactive devices. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 A schematic diagram illustrating the fabrication process of the rGO / IL / PVDF composite thin-film piezoelectric sensor provided by this invention; Figure 2 This is a schematic diagram of the test platform; Figure 3 XRD images of PVDF films with concentrations of 10%, 11%, and 12%; Figure 4 Comparison of XRD patterns of four composite film samples: (a) CB / PVDF, CNTs / PVDF, CNTs-COOH / PVDF, and rGO / PVDF; (b) Comparison of FTIR patterns of four composite film samples: CNTs-COOH / PVDF, CNTs / PVDF, CB / PVDF, and rGO / PVDF; (c) XRD patterns of rGO / PVDF film samples with four different filler ratios; (d) FITR patterns of rGO / PVDF film samples with four different filler ratios. Figure 5 SEM images of (ac)CB / PVDF, (df)CNTs / PVDF, (gi)CNTs-COOH / PVDF, and (jl)rGO / PVDF composites at different magnifications; Figure 6 EDS energy distribution and elemental composition of (a) rGO / PVDF composite film; (b) CB / PVDF composite film; (c) CNTs / PVDF composite film; (d) CNTs-COOH / PVDF composite film. Figure 7(a) Dielectric properties of pure PVDF film; (b) Dielectric properties of 0.125 wt% rGO / PVDF composite film; (c) Dielectric properties of 0.25 wt% rGO / PVDF composite film; (d) Dielectric properties of 0.5 wt% rGO / PVDF composite film; (e) Dielectric properties of 0.75 wt% rGO / PVDF composite film; (f) Comparison of dielectric constant and dielectric loss values ​​of PVDF composite film samples with five different rGO filler contents; (g) Comparison of dielectric constant and (h) dielectric loss values ​​of PVDF composite films with different rGO filler contents; (i) Bar chart comparing dielectric constant and dielectric loss values ​​of rGO / PVDF composite films with different graphene contents at 1 kHz. Figure 8 Comparison of (a) XRD and (b) FTIR images of rGO / PVDF and rGO / IL / PVDF composite films; Figure 9 (ac) are SEM images of the rGO / PVDF composite film material at different magnifications (400 nm, 2 μm and 10 μm); (df) are SEM images of the rGO / IL / PVDF composite film material at different magnifications (400 nm, 2 μm and 10 μm). Figure 10 (a) dielectric constant; (b) dielectric loss; (c) quasi-static piezoelectric constant (d) for pure PVDF, rGO / PVDF, and rGO / IL / PVDF composite films. 33 Comparison chart; Figure 11 (a) Experimental platform test diagram; (b) Sensor physical diagram; (c) Sensor test connection diagram; (d) Synchronous response of excitation force and open circuit voltage of the rGO / IL / PVDF composite thin film sensor under 10Hz constant load, and synchronous response of excitation force and short circuit current; (e) Real-time response of PVDF / rGO / IL composite thin film sensor in different practical application scenarios. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] I. Preparation of PVDF composite films, including: Preparation of printing solution: a. Preparation of PVDF solutions with concentrations of 10%, 11%, and 12%: (1) Weighing and mixing: Weigh PVDF powder with mass fractions of 10wt%, 11wt%, and 12wt% respectively using a precision electronic balance. Thoroughly clean the three glass reagent bottles with anhydrous ethanol. After the residual ethanol on the inner wall of the bottle has completely evaporated and dried, use a dropper to add the corresponding mass of NMP solvent to the bottle.

[0020] (2) Ultrasonic dissolution and degassing: Place the reagent bottles of the above three concentration mixtures into an ultrasonic cleaner at the same time, and set the ultrasonic time to 30 min.

[0021] (3) Adding acetone: After the PVDF powder is completely dissolved, use a dropper to add an appropriate amount of acetone to the reagent bottle. At this time, the solution will exhibit a brief "layering phenomenon," with the upper layer being low-density acetone and the lower layer being high-viscosity PVDF solution. Place the reagent bottle in an ultrasonic machine again and vibrate for 10 minutes. Due to the excellent miscibility of acetone and NMP, the layering interface will quickly disappear. Finally, a pure PVDF printing solution with moderate viscosity and controllable evaporation rate is obtained.

[0022] b. Preparation of PVDF solutions containing four fillers: carbon black (CB), graphene (rGO), carboxylated carbon nanotubes (CNTs-COOH), and carbon nanotubes (CNTs).

[0023] Composite inks were prepared by adding four fillers, namely carbon black (CB), graphene (rGO), carboxylated carbon nanotubes (CNTs-COOH), and carbon nanotubes (CNTs), with a mass fraction of 0.75%, to the matrix at the selected optimal concentration of 11 wt%.

[0024] (1) Dispersion and mixing of fillers: Take four clean and dry glass reagent bottles and weigh out equal amounts of the four types of conductive fillers mentioned above, placing them at the bottom of each bottle. Then, use a dropper to add the measured amount of solvent to the bottle and perform ultrasonic pre-dispersion for 1 hour to allow the black filler powder to initially deagglomerate in the solvent, forming a uniform black suspension. Next, according to a mass ratio of 11 wt%, slowly pour PVDF powder into the four black suspensions mentioned above. At this time, the white PVDF powder floats on the surface of the black liquid, with a clear solid-liquid interface. When the reagent bottle is shaken vigorously by hand, the white polymer powder is coated by the black filler suspension, and the mixture instantly turns into a gray-black or dark black turbid slurry. Due to the different specific surface areas of the fillers, the viscosity of the mixture of carbon black and graphene is significantly higher than that of the carbon nanotube mixture, and black mixed agglomerates easily adhere to the bottle wall.

[0025] (2) Ultrasonic Dissolution and Degassing: Four reagent bottles were placed in a 60℃ constant temperature water bath and magnetically stirred. Initially, undispersed filler deposits were still visible at the bottom of the bottles, and black and white polymer clumps floated on the surface of the solution. As the stirring time increased (approximately 8 hours) and the temperature rose, PVDF gradually dissolved, the system viscosity increased, and shear force caused the filler to undergo secondary dispersion in the matrix. After stirring, an appropriate amount of acetone was added to the reagent bottles using a dropper, and the bottles were transferred to an ultrasonic cleaner for secondary agitation for 30 minutes.

[0026] (3) Solution standing: After removing the reagent bottle and allowing it to stand to remove bubbles, the four composite solutions all showed a uniform deep black color (carbon black / graphene group) or grayish black color (CNTs group). When the reagent bottle was tilted, the ink left a uniform black liquid film on the bottle wall. The backflow was smooth, without any particle feel or bubble residue, indicating that the filler was uniformly dispersed in the 11wt% PVDF matrix and had good printing rheological properties.

[0027] Modeling and printing: A square film model with dimensions of 20 mm x 20 mm and a thickness of 0.2 mm was constructed using SolidWorks 3D modeling software. The model was exported as an STL file and imported into the printer's slicing software. In the slicing parameter settings, the print layer height was set to 0.2 mm, the extrusion diameter (i.e., nozzle linewidth) to 0.6 mm, the flow rate to 1, and a zig-zag fill pattern was selected with a fill angle of 0° (parallel to the X-axis) to induce material orientation using shear force. Subsequently, a clean glass slide was selected as the printing substrate, wiped with anhydrous ethanol, and fixed on the printing platform. The initial distance between the print head and the substrate was calibrated to approximately 0.2 mm using the leveling system. In the experiment, the prepared composite ink was drawn into a syringe and fitted with a precision needle with an inner diameter of 0.6 mm, which was then fixed to the extrusion module of a direct-write 3D printer. A motor-driven mechanical extrusion mode was used, with the printing speed set to 120 mm / s. The program was started, allowing the ink to be stably extruded and uniformly deposited according to preset parameters under the precise push of a stepper motor, forming a wet film. After printing, the sample, along with the substrate, was placed in a vacuum drying oven and continuously dried at a constant temperature of 85°C for 10 hours to complete the solvent evaporation and thermal annealing crystallization of the material in one step. After naturally cooling to room temperature, the cured film was peeled off from the substrate, yielding a flexible composite piezoelectric film.

[0028] II. Preparation of PVDF films with four different graphene contents, including: Experimental material ratio: Keeping the PVDF matrix concentration constant at 11 wt%, graphene powder with mass fractions of 0.125%, 0.25%, 0.5%, and 0.75% was added, respectively, based on the total solvent mass. The specific formulations for each experimental group are shown in Table 1.

[0029] Table 1. Proportions of Four Printing Solutions

[0030] III. Sample preparation process: The specific steps of the composite film preparation process are as follows; please refer to [link / reference]. Figure 1 : (1) Preparation of printing solution: First, the measured graphene powder is added to the NMP / acetone mixed solvent and dispersed by high-power ultrasonication for 1 h. The ultrasonic cavitation effect is used to open the graphene sheets and form a uniform black suspension.

[0031] (2) Solution mixing: PVDF powder is slowly added to the above suspension and magnetically stirred for 8 hours in a constant temperature water bath at 60°C to allow the polymer molecular chains to fully extend and entangle between the graphene sheets, resulting in a uniform and stable composite printing ink.

[0032] (3) Vacuum degassing: Place the prepared ink in a vacuum drying oven and let it stand for 30 minutes to completely remove the air bubbles introduced during the stirring process, so as to prevent printing interruption or the formation of pores in the film.

[0033] (4) Printing film: Using optimized printing parameters (extrusion pressure, printing speed and number of layers), inks of different concentrations are printed onto a clean glass substrate.

[0034] (5) Thermal annealing: After printing, the wet film and the substrate are placed in an 85°C drying oven for 10 hours to completely remove the solvent and induce crystal perfection, and finally obtain a flexible piezoelectric composite film with uniform thickness.

[0035] (6) Polarization Treatment: To impart macroscopic piezoelectric activity to the PVDF / graphene composite film, the sample was treated using a room-temperature high-voltage DC polarization method. First, conductive silver paste was coated onto the upper and lower surfaces of the film as electrodes. Then, the film was placed in a high-voltage polarization device, and a DC voltage of 16 kV was applied at room temperature and maintained for 0.5 h. The strong electric field was then used to force the polarization process... β The electric dipoles in the phase align in a specific direction. After polarization, the two poles of the sample are short-circuited and left to stand for 24 hours to release the residual space charge on the surface and eliminate unstable signal interference, thereby obtaining a piezoelectric composite film with stable performance.

[0036] IV. Fabrication process of flexible piezoelectric thin film sensors: Based on the previous three steps, an ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, IL) was introduced into the rGO / PVDF system as a synergistic modifier to prepare a flexible piezoelectric thin film sensor. To avoid the risk of electrode edge breakdown and improve polarization efficiency, the experiment used direct-write (DIW) 3D printing technology to prepare independent single-layer piezoelectric thin film units.

[0037] Substrate preparation and film printing: A flexible PET film with a pre-embedded bottom electrode is selected as the substrate. After cleaning and drying, a composite ink (rGO content of 0.5 wt%) is uniformly deposited on the substrate surface using a DIW inkjet printer to form a single-layer wet film.

[0038] To further improve the dispersion uniformity of nanofillers in the polymer matrix and adjust the degrees of freedom of polymer chain movement to optimize polarization, an ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, IL) was introduced as a synergistic modifier into the rGO / PVDF system. A mixed solution with a PVDF:rGO:IL ratio of 11wt%:0.5wt%:10wt% was prepared, and a single-layer rGO / IL / PVDF composite piezoelectric film was formed by printing according to the aforementioned steps.

[0039] (2) Heat annealing: The sample was placed in a vacuum drying oven and annealed at 80°C for 4 h to completely remove the solvent and promote the rearrangement and crystallization of PVDF crystal chains.

[0040] (3) Ion sputtering silver electrode: After polarization, a dense nano-silver layer is deposited on the upper surface of the thin film using high-purity silver (Ag) as the target material to form a complete “electrode-piezoelectric layer-electrode” structure.

[0041] (4) High-voltage polarization: Polarization is performed with silver electrodes deposited on the upper and lower surfaces of the thin film. A 16 kV DC high voltage (room temperature, 0.5 h) is applied to the PVDF layer using a high-voltage needle electrode. The electric field can effectively penetrate the polymer medium and cause the electric dipole moment to align in the direction of thickness.

[0042] (5) Packaging: Copper wires are led out using conductive adhesive and covered with PI tape for insulation protection to obtain a flexible piezoelectric thin film sensor.

[0043] V. Test Platform Setup and Testing Methods: To quantitatively characterize the dynamic performance of the sensor, this solution establishes a simplified dynamic mechanical testing platform. This platform covers the entire chain from mechanical excitation and signal conditioning to data acquisition, as illustrated in the physical connection diagram below. Figure 2As shown. During the test, the prepared flexible piezoelectric thin film sensor sample was first fixed below the force-applying head of the self-made dynamic force-applying device (vibrator), ensuring good contact and uniform force. The vibrator, as a mechanical excitation source, has its output shaft acting perpendicularly to the sensor surface. By adjusting the frequency and amplitude of the drive motor, a periodic sinusoidal pressure F with controllable frequency (1 Hz-50 Hz) and amplitude is applied to the sample. t A piezoelectric thin-film laboratory amplifier (PVDF) was used to simulate vibration conditions in real-world applications. Since the sensor outputs a high-impedance charge signal, direct measurement is susceptible to interference and suffers from severe attenuation. Therefore, its output is connected to a PVDF laboratory amplifier (signal amplifier). This device converts the weak charge signal into a low-impedance, high-amplitude voltage signal and performs necessary filtering. To accommodate the high internal resistance of the PVDF and filter out environmental noise, the amplifier parameters were set as follows: voltage mode, input impedance 100 MΩ, bandpass filter frequency range 0.1 Hz-10 Hz, and gain set to zero (0 dB). The conditioned voltage signal is acquired and displayed in real-time using a digital oscilloscope and transmitted to a computer via a data cable for storage and subsequent spectrum analysis, thereby achieving a systematic characterization of the sensor's sensitivity, linearity, and operational stability.

[0044] VI. Research Results: 1. Results of the study on the influence of PVDF matrix concentration and carbon-based filler: Through experimental measurements, this scheme systematically studied the crystal phase structure and crystallization behavior of pure PVDF films at different concentrations (10wt%, 11wt%, 12wt%) to determine the optimal matrix concentration. Then, the inductive effects of carbon black (CB), carbon nanotubes (CNTs) and reduced graphene oxide (rGO) on the crystal phase transformation of PVDF were compared and analyzed. Combined with multi-scale characterization methods, fillers with better conductivity and interface control ability were selected. Finally, a composite material system and component ratio suitable for subsequent research were established.

[0045] Optimization of pure PVDF matrix concentration and study on crystallization behavior: In the preparation system of composite piezoelectric ink, the concentration of the PVDF matrix solution directly determines the printing rheological properties and the crystallization kinetics after film formation. To determine the optimal matrix concentration, this study first prepared pure PVDF films with mass fractions of 10 wt%, 11 wt%, and 12 wt%, and then used X-ray diffraction (XRD) technology to perform fine analysis of their crystal structures.

[0046] like Figure 3 As shown in the figure, in the XRD patterns of the three samples with different concentrations, obvious characteristic diffraction peaks are observed near 2θ of 18.4° and 26.5°, corresponding to PVDF, respectively. αThe (020) and (021) crystal planes of the monoclinic crystal system, and the (200) crystal plane diffraction peaks near 2θ = 20.6° are also present. β The diffraction peaks of the phase indicate that, even without the addition of a heterogeneous nucleating agent, under the action of the shear flow field in 3D printing, the PVDF molecular chains still tend to spontaneously coil through the TGTG' (trans-trans-trans-trans) conformation, forming a thermodynamically stable nonpolar structure. α Phase structure. By comparing the peak shape and intensity of the diffraction peaks, it can be found that the 10 wt% sample has weaker diffraction peaks and a larger half-width at half maximum (FWHM), indicating low crystallinity and refined grains. In contrast, the 11 wt% PVDF sample shows increased intensity of the aforementioned characteristic peaks and a decreased FWHM, while the diffraction peak corresponding to the (021) crystal plane disappears. However, when the concentration is increased to 12 wt%, α The characteristic diffraction peaks corresponding to the (021) crystal plane of the crystal phase reappear. This is because the excessive solvent content leads to rapid solvent evaporation during the drying process of the wet film, causing significant volume shrinkage and generating large internal stress. Simultaneously, the film is geometrically constrained by the substrate; this shrinkage is limited in the thickness direction but released in the planar direction, leading to anisotropic accumulation of internal stress. Secondly, although increasing the polymer content theoretically benefits crystallization, the increase in solution viscosity inhibits the diffusion and rearrangement of chain segments. According to the Doi-Edwards entanglement theory, polymer chains form a dense physical entanglement network in concentrated solutions. This topological constraint greatly hinders the movement of chain segments during the phase transition process, resulting in discontinuous crystal growth. In summary, although pure PVDF mainly exhibits [certain characteristics] at this stage... α While the 11wt% system offers the highest total crystallinity and densest molecular chain network among the three groups, this ordered structure constitutes the optimal precursor, facilitating the subsequent introduction of nanofillers and their efficient conversion into piezoelectric components through spatial confinement and chemical anchoring. β This laid a solid structural foundation.

[0047] 2. Crystal structure analysis (XRD, FTIR) The XRD patterns of composite films composed of four carbon-based fillers—CB / PVDF, CNTs / PVDF, CNTs-COOH / PVDF, and rGO / PVDF—with PVDF are shown below. Figure 4 As shown in (a). After adding carbon-based fillers to the PVDF polymer matrix, the electroactivity... β The characteristic peaks of the phases clearly appear at 20.6° ((020) and (110)). The characteristic peaks of the CNTs / PVDF and CNTs-COOH / PVDF thin film samples at 2θ≈20.6° and 26.6° respectively correspond to... β Phase (020) and αThe diffraction peaks of phase (021) all exhibit large half-widths (FWHMs), indicating that CNTs and CNTs-COOH, as one-dimensional nanomaterials, are prone to entanglement or network structures in polymer matrices due to their unique geometric morphology (high aspect ratio). However, their physical cross-linking structure and surface functional groups... β The inductive effect of the phase transformation is limited. In contrast, the CB / PVDF samples... α The (021) phase diffraction peak exhibits a small full width at half maximum (FWHM), which may be related to the inhomogeneous interfacial interactions caused by the disordered structure and low specific surface area of ​​CB. Although CB can affect the crystallization behavior of PVDF, its inductive ability on the β phase is not significant. Furthermore, this diffraction peak is not prominent in the rGO / PVDF composite film sample, remaining near 2θ≈18.4°. α The (020) crystal plane of the phase and the area around 20.6° β The diffraction peaks of phase (200) crystal plane are smoother and free of impurities, indicating that the introduction of two-dimensional graphene better induces PVDF. α Especially β The formation of the phase is mainly attributed to the large specific surface area and abundant sp in rGO. 2 The conjugated structure helps enhance the interaction between rGO and PVDF molecular chains, promoting the PVDF segments to adopt an extended (TTTGTG') conformation, thereby inducing the formation of rGO / PVDF composite films. β Nucleation and orientation growth of crystal phases.

[0048] FTIR spectroscopy further validated this conclusion at the molecular vibrational level and provided a basis for quantitative analysis. Each component at 840 cm⁻¹... -1 place β The intensity of the characteristic absorption peaks of the phases differs to some extent, such as Figure 4 As shown in (b).

[0049] FTIR spectra of four composite thin films: 840 cm⁻¹ -1 place β The intensity order of the phase characteristic absorption peaks is: rGO / PVDF > CB / PVDF > CNTs / PVDF > CNTs-COOH / PVDF.

[0050] calculate β relative content F ( β The relative content of the β-phase crystal form F in the sample is related to the total content and, according to Lambert-Beer's law, the relative content F( β The following formula is usually used for calculation: F( β )=K α A β / (K β Aα +K α A β (1); In the formula, A α and A β The 763 cm⁻¹ in the FTIR spectrum is shown below. -1 ( α (Phase characteristic peaks) and 840 cm⁻¹ -1 ( β The absorption intensity of the absorption peak at the phase characteristic peak; K α and K β α phase 763cm -1 and β phase 840cm -1 The absorption coefficients of the absorption bands are respectively K. α =6.1×10 4 cm 2 / mol, K β =7.7×10 4 cm 2 / mol.

[0051] This application combines the results of X-ray diffraction (XRD) and ATR-FTIR tests, as shown below. Figure 4 As shown in (c) and (d), the crystallinity of the PVDF piezoelectric film and β Relative content of phase crystal type F( β The product calculation of the thin film β The total phase content is shown in formula (2): β Total phase content = crystallinity × F( β (2); The results are shown in Table 2. The F(x) of the rGO / PVDF composite film... β The content was 63.97%, and the crystallinity was 39.15%. β The relative content was 25.04%, significantly higher than other fillers. This is mainly because graphene has a high specific surface area and a two-dimensional sheet structure, which adsorbs PVDF molecular chains through interfacial interactions and induces their transformation from the coiled TGTG' conformation to the extended TTTT all-trans conformation in the two-dimensional plane, promoting the nucleation process and thus significantly improving the crystallinity of the composite material. β Phase content.

[0052] Table 2. Comparison of crystal phase content parameters of different carbon-based fillers / PVDF composite films (0.75wt%) (total) β Phase content represents the total amount of crystals that actually have a piezoelectric effect per unit volume.

[0053] In summary, the geometric dimensions and surface properties of the filler are key factors in regulating the crystallization behavior and phase transformation of PVDF. Combined with XRD and FTIR analysis results, it is evident that compared to zero-dimensional carbon black (CB) and one-dimensional carbon nanotubes (CNTs, CNTs-COOH), two-dimensional sheet-like graphene exhibits superior β-phase conductivity.

[0054] 3. The effect of graphene content on the microstructure and crystal phase of composite films: This section presents a comprehensive analysis of the crystal structures of composite films with different graphene additions (0.125 wt%, 0.25 wt%, 0.5 wt%, and 0.75 wt%) using XRD and FTIR measurements. Figure 4 In the XRD diffraction patterns of (c), it can be observed that all samples exhibit a significant characteristic diffraction peak near 2θ = 20.6°, which corresponds to the PVDF peak. β The (110) and (200) crystal planes of the phase indicate that, under the synergistic effect of printing shear force and graphene heterogeneous nucleation, piezoelectrically active phases have been successfully induced in the PVDF matrix. β Comparing the diffraction curves of different concentrations, it was found that as the graphene content increased from 0.125 wt% to 0.5 wt%, the intensity of the characteristic peak gradually increased and the peak shape became sharper, indicating that an appropriate amount of graphene sheets were uniformly dispersed in the matrix, effectively promoting the growth and orderly arrangement of grains. To quantify this effect, peak fitting calculations were performed on the spectra. The results showed that the total crystallinity (Xc) of the 0.5 wt% sample was the highest, reaching 48.6%. However, when the graphene content was further increased to 0.75 wt%, the diffraction peak did not continue to strengthen, but instead broadened slightly, and the crystallinity decreased to 46.8%. This is mainly because excessively high concentrations of graphene are prone to agglomeration in the matrix. The formed agglomerates not only reduce the effective nucleation sites, but also hinder the orderly movement of polymer chain segments, thus limiting further improvement in crystal quality.

[0055] To further analyze the specific composition of the crystal phase, we analyzed the FTIR infrared spectrum of the sample. Figure 4 (d)). 763cm in the spectrum. -1 and 840cm -1 The absorption peaks at these locations correspond to the inactive... α Phase and high activity β Phase. Calculations based on the Lambert-Beer law show that... β The relative conversion rate F(phase) βThe overall conversion rate increased with increasing graphene content. Specifically, the 0.5 wt% sample had a conversion rate of 70.7%, while the 0.75 wt% sample was slightly higher at 72.5%. The 0.5 wt% sample maintained optimal crystal quality while achieving a high crystalline phase conversion rate, making it the best ratio for preparing high-performance piezoelectric sensors.

[0056] However, evaluating the performance potential of piezoelectric materials cannot solely rely on relative conversion rates; the actual effective energy contained within the material must also be considered. β Total phase content. Therefore, we compare the total crystallinity (Xc) measured by XRD with the relative conversion F (Fc) measured by FTIR. β Multiply by , calculate β Total content of phase (X) β As shown in Table 3. The calculation results show that although the relative conversion rate of the 0.75wt% sample is slightly higher, it is limited by the decrease in its total crystallinity, resulting in a lower final conversion rate. β Total phase content X β Only 33.9%; in contrast, the 0.5wt% sample, with the highest crystallinity, β Total phase content X β The graphene content reached 34.4%, the highest in the group. Combined XRD and FTIR analyses show that a graphene addition of 0.5 wt% maintains optimal crystal quality while ensuring high phase conversion rate, making it the best ratio for preparing high-performance piezoelectric sensors.

[0057] Table 3. Crystallinity of Seed Proportions β relative content, β Total content of phases

[0058] 4. Dispersion behavior and microstructure characterization (SEM) of multidimensional carbon-based fillers in the matrix: The uniformity of filler dispersion and interfacial bonding in the polymer matrix are key prerequisites for determining whether composite materials can effectively exert heterogeneous nucleation and electromechanical conversion properties. Figure 5 (al) shows the surface morphology of four composite films formed by dispersing carbon black (CB), carbon nanotubes (CNTs), carboxylated carbon nanotubes (CNTs-COOH) and graphene (rGO) fillers in a PVDF matrix at different magnifications.

[0059] like Figure 5As shown in (ac), in the CB / PVDF composite film, the carbon black filler is uniformly dispersed in the matrix in a particulate form. The CB particles are tightly bonded to the PVDF matrix, forming a large number of irregularly shaped and interconnected spherical agglomerates or particles with a size of approximately 5-10 μm. A small number of pores or defects exist in the film. The uniformly dispersed CB can act as a heterogeneous nucleating agent, providing numerous nucleation sites, thereby promoting the crystallization of PVDF and increasing its crystallinity and crystallization rate. Figure 5 In the (df) image, CNT fillers are clearly observed to adhere to the PVDF matrix surface in the form of aggregates. The CNTs exhibit a tangled coil or bundle structure, accompanied by obvious "pull-out" marks and micropores, increasing the film surface roughness and porosity. This is likely due to their large specific surface area and strong van der Waals forces between the tubes, leading to CNT aggregation. When carbon nanotubes aggregate, their interfacial bonding with the PVDF matrix is ​​limited, easily forming physical entanglements, reducing the effective specific surface area and nucleation sites, thus inhibiting the PVDF crystallization process and causing a decrease in crystallinity. For PVDF composite film samples modified with carboxylated CNTs ( Figure 5 (gi)), the filler is uniformly dispersed in the matrix in a particulate form. The surface morphology of the particles is smaller than that of CNTs / PVDF films, and they are more tightly bonded to the matrix, resulting in a more uniform film morphology. Although carboxylation treatment improves the dispersibility of CNTs in NMP solvent to some extent, the introduction of carboxyl functional groups enhances the polarity of the carbon nanotube surface. When these polar groups form strong interactions (such as hydrogen bonds or dipole-dipole interactions) with the -CF2 dipoles in the PVDF molecular chain, this interaction may also restrict the free movement of the PVDF molecular chain or cause molecular chain folding, thereby reducing the overall crystallinity.

[0060] In contrast, graphene (rGO) composite films exhibit more desirable microstructures, such as... Figure 5 As shown in (jl), the film surface is dense and relatively smooth, with no obvious macroscopic phase separation or large-sized agglomerates, indicating that the unique two-dimensional layered structure of graphene achieves good exfoliation and layer-by-layer stacking in the PVDF matrix. This layered structure significantly increases the contact area between the rGO filler and the polymer molecular chains, enabling the formation of an effective charge transport network in the PVDF matrix. Furthermore, the defect sites and residual oxygen-containing functional groups on the surface of the rGO nanosheets can serve as effective nucleation sites for PVDF crystallization, regulating the crystallization kinetics of PVDF and promoting further crystallization. β The formation of phase nuclei also indicates that the two-dimensional sheets can be oriented in accordance with the shear flow direction during the printing process. rGO, with its high specific surface area, can act as a heterogeneous nucleating agent, providing abundant nucleation sites, thereby inducing the formation of phase nuclei in PVDF. β The formation of crystalline phases increases crystallinity and crystallization rate.

[0061] In summary, the surface morphology images of the composite films composed of four carbon-based fillers and PVDF at different magnifications are shown below. Figure 5 As shown in (al). The CB / PVDF composite film exhibits a rough, granular surface, with numerous spherical aggregates visible under high-magnification SEM. This is due to the high specific surface energy of carbon black, which easily leads to self-aggregation and makes it difficult to form a uniformly dispersed system. In the carbon nanotube / PVDF composite films (Fig. 5(e) and (f)), CNTs exhibit a bundled entanglement state, with limited interfacial bonding with the PVDF matrix, showing obvious "pull-out" marks and micropores. The dispersibility of the carboxylated carbon nanotube / PVDF composite film is slightly improved, and it bonds more tightly to the PVDF matrix (Fig. 5(h) and (i)). However, the one-dimensional aspect ratio characteristic causes physical entanglement of some PVDF molecular chains, thus inhibiting the formation of a uniformly dispersed system within the PVDF matrix. β Crystal phase formation. The graphene / PVDF composite film has a dense and uniform surface. Under high magnification, the two-dimensional graphene sheets are visible stacked layer by layer in the PVDF matrix, with tight interfacial bonding and no obvious debonding gaps. This layered structure increases the contact area between the filler and the molecular chains, which is beneficial for... β The improvement of the crystal phase plays a certain promoting role.

[0062] 5. Elemental analysis of the surface micro-area of ​​four composite thin films: Elemental composition analysis (EDS) To investigate the uniformity of distribution and surface chemical modification effects of carbon-based fillers of different dimensions, types, and proportions in PVDF matrix, this application employed energy-dispersive X-ray spectroscopy (EDS) to perform surface micro-area elemental analysis on four types of composite films. EDS not only qualitatively identifies the elemental composition of the material surface but also quantitatively calculates the atomic percentage (At%) of each element, providing direct chemical evidence for filler modification grafting and compatibility with the matrix.

[0063] First, the graphene / PVDF composite film was tested, and its EDS energy spectrum and elemental distribution are as follows: Figure 6 As shown. From the spectrum Figure 6In (a), the main peaks of carbon (C) and fluorine (F) can be clearly observed, indicating the composite of PVDF matrix and graphene filler. Quantitative analysis shows that the oxygen (O) content of this sample is only 4.26 At%, which is lower than the other samples in the four groups. This indicates that the graphene used has high lattice integrity and few surface oxidation defects. This structural characteristic corresponds to the high β-phase (28.65%) exhibited by the graphene composite film in the previous XRD analysis. It indicates that the promoting effect of graphene on the crystal transformation of PVDF mainly comes from the heterogeneous nucleation sites provided by its two-dimensional high specific surface area, rather than relying on the chemical induction of oxygen-containing functional groups.

[0064] Subsequently, the carbon black (CB) reinforced composite film was analyzed. Figure 6 (b) The carbon black / PVDF composite film exhibited the highest carbon content (74.85 At%), consistent with the extremely high carbon purity of carbon black itself. Although carbon black has a large specific surface area, its interparticle aggregation effect is often strong; the ratio of fluorine (19.02 At%) to carbon in the energy dispersive spectroscopy reflects its physical filling state on the matrix surface. To verify the effect of the surface chemical properties of one-dimensional carbon nanotubes on… β Regarding the role of crystal phase formation, this application compares the elemental composition of pristine carbon nanotubes (CNTs) and carboxylated carbon nanotubes (CNTs-COOH) composite films. Figure 6 (c) shows the EDS spectrum of the original CNTs / PVDF film, with a surface oxygen atom content of 4.77 At%, mainly from environmental adsorption or trace surface defect oxidation.

[0065] In contrast, the acid-modified CNTs-COOH / PVDF composite film ( Figure 6 (d) Significant changes were observed in the energy dispersive spectroscopy (EDS) characteristics. The O content of the CNTs-COOH / PVDF composite film was 7.54 At%, a significant increase compared to the original CNTs / PVDF (4.77 At%), rising from 4.77 At% to 7.54 At%, an increase of nearly 60%. This indicates that strong acid treatment grafted a large number of oxygen-containing polar functional groups (such as carboxyl-COOH and hydroxyl-OH) onto the surface of the carbon nanotubes. These introduced polar groups act as "chemical bridges" in the composite material system, tightly binding with fluorine atoms in the PVDF molecular chain through hydrogen bonds or dipole interactions, thereby effectively improving the dispersibility of carbon nanotubes in the polymer matrix and reducing agglomeration.

[0066] The detailed elemental contents of each group of samples are summarized in Table 4. The atomic percentage of fluorine remained stable between 18% and 23%, which is consistent with the stoichiometric characteristics of the PVDF matrix (-(CH2-CF2)n-) as a continuous phase. The EDS analysis results show that all four multidimensional carbon-based fillers were successfully introduced into the PVDF matrix, and the surface chemical characteristics of each component (such as oxygen content) highly matched the expected filler modification state. This provides a solid materials science basis for explaining the differences in crystallization behavior and electrical properties of different composite films. Table 4. Summary of EDS elemental analysis results for different carbon-based fillers / PVDF composite films

[0067] 6. The effect of graphene content on the dielectric properties of composite films: To thoroughly evaluate the contribution of the graphene network structure to the electrical properties of the materials, this application analyzed the dielectric properties of composite films with different graphene contents one by one, and finally made a comprehensive comparison, such as... Figure 7 As shown in (af).

[0068] (1) Dielectric properties of pure PVDF film As a matrix control group for composite materials, Figure 7 (a) shows the dielectric spectrum of the pure PVDF film. As can be seen from the figure, pure PVDF exhibits typical polar polymer dielectric characteristics. At 1 kHz, its dielectric constant is approximately 17.8, and its dielectric loss is 0.40. With increasing frequency, the dielectric constant shows a slow decreasing trend. This is because the dipole reorientation of the polymer chain segments has a relaxation time under an alternating electric field, and at high frequencies, the dipoles cannot keep up with the changes in the electric field. The low dielectric loss confirms the good insulation properties of the matrix itself, but it also reflects the inherent limitations of a single polymer in terms of charge storage density, making it difficult to meet the requirements of high-performance sensors.

[0069] (2) Dielectric properties of 0.125wt% graphene composite film Figure 7 (b) Dielectric property curves of the composite film with 0.125 wt% graphene. At this low concentration, the dielectric constant of the material slightly increases to 18.2 at 1 kHz, and the dielectric loss slightly increases to 0.59. This result indicates that when the amount of conductive filler added is extremely low, the graphene sheets are in an isolated and dispersed state in the matrix, with large spacing, failing to form an effective interactive network. Although graphene introduces a small number of interfacial polarization centers, it is insufficient to produce a significant enhancement effect on the macroscopic electrical properties. At this stage, the material still exhibits electrical properties close to those of an insulator.

[0070] (3) Dielectric properties of 0.25wt% graphene composite film When the graphene content increases to 0.25 wt% ( Figure 7 (c) The dielectric response of the composite material begins to show a significant enhancement. At 1 kHz, its dielectric constant increases to 23.0, which is about 29% higher than that of pure PVDF, while the dielectric loss increases moderately to 0.75. This indicates that as the filler concentration increases, the average distance between graphene sheets shortens, and local "micro-capacitor" structures begin to form in the matrix. The conductive graphene acts as the electrode of the micro-capacitor, and the insulating PVDF layer acts as the dielectric. Under the action of an alternating electric field, charge accumulates at the interface between the two phases, leading to an enhanced MWS interface polarization effect, thereby effectively improving the energy storage capacity of the material.

[0071] (4) Dielectric properties of 0.5wt% graphene composite film Figure 7 (d) shows the dielectric properties of the 0.5 wt% sample, which exhibits the best overall performance among all samples. The test results show that the dielectric constant of this sample surges to 34.2 at 1 kHz, nearly twice that of pure PVDF. This significant nonlinear increase indicates that graphene has formed an extremely well-developed and uniformly distributed "microcapacitor network" within the PVDF matrix. Furthermore, the β-phase crystallinity of PVDF is highest at this concentration, with the highly polar crystal structure and interfacial polarization effect producing a synergistic enhancement. Simultaneously, although the dielectric constant is significantly increased, its dielectric loss (1.04) remains within an acceptable range. This indicates that the graphene sheets are effectively isolated by an extremely thin PVDF insulating layer, preventing the formation of macroscopic conductive pathways, thus achieving high polarizability while avoiding insulation failure.

[0072] (5) Dielectric properties of 0.75wt% graphene composite film For a high concentration sample of 0.75 wt% ( Figure 7(e) Its dielectric constant did not continue its upward trend, but instead fell back to 27.1. More seriously, its dielectric loss reached the highest in the group at 1.16 at 1 kHz, and showed a sharp increase in the low-frequency region. This anomaly reveals the deterioration of the internal microstructure of the material, with excessive graphene sheets undergoing severe physical aggregation. Aggregates disrupt the uniformly dispersed microcapacitor network, reduce the effective interfacial polarization area, and lead to a decline in energy storage capacity. Aggregation leads to a surge in losses: the overlapping graphene aggregates form local leakage current paths within the matrix. According to percolation theory, the material is now close to the "percolation threshold," and the significant increase in leakage current is converted into heat dissipation, leading to a sharp deterioration in dielectric loss. This high loss characteristic means that the material is extremely prone to breakdown during polarization and cannot be used as a qualified piezoelectric dielectric layer.

[0073] To determine the optimal modification formulation for rGO / PVDF composite films, this application selected a representative 1kHz frequency point to conduct a comprehensive quantitative analysis of the dielectric constant and dielectric loss of five groups of PVDF composite film samples, including pure PVDF and those with different rGO filler contents (0.125 wt%, 0.25 wt%, 0.5 wt%, and 0.75 wt%). The results are as follows: Figure 7 As shown in (g) and (h), the dielectric constant and loss of rGO / PVDF composite films with different graphene contents are compared at a test frequency of 1 kHz. Figure 7 As shown in (i), with the increase of the content of conductive filler graphene, the dielectric constant of the composite material does not show a simple linear increase, but follows the typical permeation theory law, showing a trend of first rising and then falling.

[0074] Specifically, the dielectric constant of pure PVDF is only 17.8, while as the rGO content gradually increases to 0.25wt% and 0.5wt%, the dielectric constant experiences a significant leap. Particularly at 0.5wt%, the dielectric constant reaches a peak of 34.2, an improvement of nearly 92% compared to the pure matrix. This significant enhancement effect is mainly attributed to the construction of a "microcapacitor" network: near the percolation threshold, countless tiny conductive graphene sheets are effectively isolated by the insulating PVDF matrix, forming a massive number of microcapacitors. Under the action of an alternating electric field, strong interfacial polarization and accumulation of charge occur at the two-phase interface; simultaneously, XRD and FTIR analyses show that the 0.5wt% sample has high β-phase crystallinity and total content, and high polarity. β This further enhances the contribution of dipole-turning polarization, thus macroscopically manifesting as the maximization of dielectric energy storage capacity.

[0075] However, when the graphene content crosses this critical point to 0.75 wt%, the dielectric properties of the material show a significant decrease. Its dielectric constant drops to 27.1 instead of increasing, while the dielectric loss rises to 1.16. This phenomenon reveals that the "agglomeration effect" of the material's internal microstructure has become dominant. Excessive graphene sheets cannot maintain uniform dispersion in the matrix; instead, they overlap to form physical agglomerates. These agglomerates not only disrupt the original microcapacitor network structure, leading to a reduction in the effective polarization interface area, but more seriously, they construct local leakage current paths within the matrix. According to percolation theory, this means the material is on the verge of transitioning from a "dielectric" to a "conductor," and the rapidly increasing leakage current converts electrical energy into heat dissipation, causing a surge in the loss tangent. This high-loss characteristic not only reduces the signal-to-noise ratio of the piezoelectric sensor but also increases the risk of breakdown during high-voltage polarization.

[0076] Performance Study of Flexible Self-Powered Voltage Sensor: XRD and FTIR tests: By comparison Figure 8 In (a), the XRD diffraction patterns of the two groups of samples show that the rGO / PVDF composite film exhibits a distinct characteristic diffraction peak near 2θ = 20.6°, corresponding to the PVDF... β Phase (110) / (200) crystal plane. This indicates that the two-dimensional graphene sheets have already played a role in heterogeneous nucleation in the early stage, inducing the formation of some polar phases. However, in the system without IL, the orientation of the molecular chains is still subject to certain spatial constraints. After introducing ionic liquid (EMIM-TFSI) to construct the rGO / IL / PVDF ternary system, the orientation at 20.6° β The intensity of the phase characteristic peaks was significantly enhanced, and the peak shapes became sharper. This phenomenon is mainly attributed to the highly polar anions (TFSI) in the ionic liquid. - A strong electrostatic interaction is generated between the CF2 group and the PVDF molecular chain. This localized charge interaction effectively stabilizes the all-trans (TTTT) conformation of the PVDF molecular chain, thereby further accelerating the residual nonpolarity based on rGO heterogeneous nucleation. α Reverse piezoelectric activity β Deep phase transformation.

[0077] and Figure 8 (b) The FTIR spectrum further confirms the XRD analysis results at the molecular vibrational level. In the infrared spectrum, 840 cm⁻¹ -1 The corresponding typical β Phase characteristic absorption peak, while 763 cm⁻¹ -1 The location represents nonpolarity.α Phase absorption peak. Comparing the infrared spectra of the two groups of samples clearly reveals that, compared to the binary rGO / PVDF film, the rGO / IL / PVDF film with added IL shows a higher absorption peak at 840 cm⁻¹. -1 The characteristic peak intensity at 763 cm⁻¹ showed a step-like increase, while at 763 cm⁻¹... -1 The peak value at that point was further suppressed. This indicates that IL not only acts as a plasticizer in the system, granting higher degrees of freedom of movement to polymer chain segments, but also significantly enhances the internal properties of the system through ion-dipole interactions. β The relative conversion rate of the phases. This high-purity, highly oriented polar crystalline phase structure effectively amplifies the electric dipole moment response inside the material.

[0078] SEM test: SEM of rGO / PVDF composites Figure 9 Compared to (ac), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt (EMIM-TFSI) ionic liquid (IL) can act as a dispersant for rGO, further reducing its layered stacking and agglomeration in the PVDF matrix, ensuring uniform distribution of the filler, such as Figure 9 As shown in (d), after drying and film formation, IL is mainly enriched in the amorphous regions of PVDF and the interlayer gaps of rGO. However, due to its volatility, electron beam sensitivity, and lack of significant morphological characteristics, it is difficult to distinguish directly under SEM and may appear as localized surface smoothing or slight wetting marks (see...). Figure 9 (d) Furthermore, the introduction of IL reduces the size of the spherulites in the PVDF matrix, and the rGO sheets are dispersed in the PVDF matrix in a wrinkled or locally aggregated form, with wrinkles and curls appearing around the PVDF spherulites. Figure 9 (e) and (f)), this may be due to the highly polar bis(trifluoromethanesulfonyl)imide anion (TFSI) in the ionic liquid. - There is an electrostatic interaction between the PVDF molecule and the -CF2 group on the PVDF molecular chain. This interaction can effectively stabilize the all-trans (TTTT) conformation of the PVDF molecular chain, thereby promoting nonpolarity. α Phase (TGTḠ conformation) piezoelectrically active β A phase transition occurs. Therefore, the rGO / PVDF / IL composite material simultaneously constructs electronic conduction channels provided by rGO and ionic conduction channels provided by the ionic liquid. This dual-channel structure contributes to the charge dynamics within the material, improving the efficiency and stability of the polarization process, and also enhancing the charge collection and output capabilities of the piezoelectric film under dynamic mechanical loading.

[0079] 8. Electrical performance testing: Based on the above crystal structure characterization, it has been confirmed that the rGO / IL / PVDF composite film exhibits high polarity. β Phase crystallinity. To further quantify the macroscopic electrical energy storage and electromechanical conversion characteristics of the material, this application investigates its dielectric constant, dielectric loss, and quasi-static piezoelectric constant (d). 33 A systematic comparative test was conducted.

[0080] At room temperature, the dielectric properties of three groups of monolayer thin film samples at a frequency of 1 kHz were measured using a broadband dielectric relaxation spectrometer. The test results are as follows: Figure 10 As shown in (a) and (b), the pure PVDF film exhibits the insulating properties of conventional polymers, with a dielectric constant of approximately 17.8 and a low loss of 0.40. When an appropriate amount of two-dimensional graphene (rGO) is doped into the matrix, the nanosheets construct a rich microcapacitor network within the polymer, and the enhanced interfacial polarization effect increases the dielectric constant of the composite film to 34.2. Further addition of ionic liquid (IL) as a synergistic modifier results in a strong giant dielectric response in the rGO / IL / PVDF system. Its dielectric constant increases dramatically to 44011.7, several orders of magnitude higher than the previous two groups. This phenomenon indicates that the mobile ions abundant in the ionic liquid undergo long-range migration under the influence of an alternating electric field and accumulate polarization at the electrode-filler interface, thus significantly increasing the material's ability to accommodate charge. Although the presence of a large number of ions increases the dielectric loss to 14.48, this high dielectric property provides ample potential energy reserves for efficient charge transfer during piezoelectric output processes.

[0081] The macroscopic piezoelectric activity of a material directly reflects its internal dipole moment's ability to rotate and release charge under mechanical stress. Figure 10 (c) shows the piezoelectric coefficients of three thin films after quasi-static polarization treatment (d) 33 Test results. d of pure PVDF film. 33 At a basic level, approximately 11.5 pC / N, consistent with the intrinsic piezoelectric properties of this material under normal conditions. The addition of two-dimensional graphene increases the dp of rGO / PVDF. 33 The ratio increased to 29.0 pC / N. This data verifies that the heterogeneous nucleation of graphene effectively induces polarity. β The formation of this phase lays the structural foundation for the macroscopic piezoelectric response. Based on this, the rGO / IL / PVDF ternary synergistic modification system exhibits even more prominent piezoelectric activity, with its d... 33The piezoelectric coefficient reached 58.0 pC / N. The addition of ionic liquid not only further improved the dispersion of graphene nanosheets in the polymer matrix but also acted as a plasticizer for macromolecular chains, effectively lowering the energy barrier for molecular chain inversion. During polarization, this synergistic plasticizing effect promoted a more thorough orientation of the electric dipole moments on the PVDF molecular chains along the thickness direction, ultimately amplifying the macroscopic piezoelectric coefficient. The above test data strongly confirms the feasibility of the interface synergistic modification strategy in improving electromechanical coupling efficiency.

[0082] 9. Dynamic electromechanical response test: To systematically characterize the electromechanical conversion efficiency of the PVDF / rGO / IL composite piezoelectric film, this application constructed a dynamic testing platform consisting of a computer-controlled exciter, a charge amplifier, and a digital oscilloscope, such as... Figure 11 As shown in (ac). The experiment was conducted at room temperature and under electromagnetic shielding. A single-layer piezoelectric film prepared with a PVDF:rGO:IL composition ratio of 11 wt%:0.5 wt%:10 wt% was selected as the research object, and its open-circuit voltage response and short-circuit current output capability under dynamic mechanical excitation were investigated.

[0083] Under periodic sinusoidal mechanical excitation with a frequency of 10 Hz and a peak amplitude of 10 N, the time-domain responses of the sensor's open-circuit voltage and short-circuit current are as follows: Figure 11 As shown in (d). In order to objectively and truthfully reflect the intrinsic sensing characteristics of the device, this section uses the raw electrical test data without any filtering or modification, and compares and displays it with the synchronously input mechanical excitation force signal on the same time axis.

[0084] Depend on Figure 11 (d) It can be seen that the single-layer sensor exhibits a stable continuous AC signal output with a waveform that maintains good sinusoidal characteristics, indicating that the polar β-phase electric dipole moment generated inside the material under the synergistic induction of IL and rGO has formed an effective polarization orientation. The measured peak-to-peak value of the open-circuit voltage is approximately 0.32 V; after conditioning the short-circuit current signal by a charge amplifier (gain 1 μA / V), its peak-to-peak value is measured to be approximately 0.064 μA.

[0085] By comparing the input mechanical excitation force with the electrical output signal, it can be found that the sensor's electrical signal exhibits good synchronous tracking characteristics to the mechanical excitation. The peaks and troughs of the open-circuit voltage and short-circuit current basically coincide with the application and removal times of the mechanical force, and no obvious phase delay was observed. This result is attributed to the synergistic plasticizing effect of the two-dimensional graphene microcapacitor network and the ionic liquid, which enhances the effective transmission of mechanical stress in the thickness direction and maximizes the collection efficiency of internal charge, verifying that this composite thin-film sensor has reliable stability and good practical application potential in the field of low-frequency dynamic mechanical monitoring.

[0086] 10. Practical application testing of the sensor: Based on the characteristics exhibited by the rGO / IL / PVDF composite piezoelectric sensor in dynamic electromechanical response testing, this paper further evaluates its applicability in typical human-computer interaction and motion monitoring scenarios.

[0087] The experiment selected three typical mechanical excitation scenarios for monitoring: (1) keyboard typing (low-amplitude impact force); (2) finger bending (continuous joint stretching deformation); and (3) foot stomping (large impact load). During the test, a single-layer sensor was attached to the target force area, and its open-circuit voltage response signal was recorded using an oscilloscope. The sensor attachment to the actual object and the corresponding time-domain output waveforms under each scenario are shown below. Figure 11 As shown in (e). Test results show that this single-layer sensor can convert various complex dynamic mechanical stresses into electrical signal waveforms with identifiable characteristics.

[0088] (1) Keyboard tapping monitoring: The sensor can capture the transient impact force generated by fingers tapping the keyboard and output a regular pulse waveform. The real-time voltage peak is about 1.66 V. This result shows that the device can effectively respond to low-amplitude dynamic contact. (2) Joint bending monitoring: The sensor is attached to the second joint of the index finger. As the finger joint bends and returns, the piezoelectric functional layer produces regular deformation. The output voltage signal presents a smooth continuous AC waveform, and the real-time voltage peak is about 1.66 V. This phenomenon shows that it has the ability to track continuous limb movements. (3) Foot stepping movement monitoring: After the sensor is placed at the front of the sole of the shoe, during normal walking, the sensor produces a stronger piezoelectric response due to the large human body load. The real-time voltage peak can reach 2.64 V. This test result verifies that the piezoelectric sensor has timely signal response and structural stability under large stress conditions.

[0089] Studies have shown that ionic liquids can serve as functional modifiers, effectively promoting the formation and stabilization of polar phases through interfacial coupling with PVDF molecular chains and carbon-based fillers. The shear force generated during DIW extrusion can induce the directional alignment of PVDF molecular chains, achieving a self-polarization effect and significantly improving... β Phase content and piezoelectric output performance.

[0090] In summary, the rGO / IL / PVDF composite film sensor exhibits excellent response capabilities. Its stability under different mechanical excitations provides a reference for the subsequent development of smart terminals, human-computer interaction systems, and self-driven sensor networks.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of preparing a flexible self-powered piezoelectric sensor, characterized in that, Includes the following steps: S1. Substrate treatment and film printing: A mixed solution with a PVDF:rGO:IL component ratio of 11wt%:0.5wt%:10wt% was prepared. A flexible PET film was selected as the substrate. The mixed solution was uniformly deposited on the substrate surface using a DIW inkjet printer to form a single-layer composite piezoelectric film sample of rGO / IL / PVDF. S2. Heat annealing treatment: Place the sample in a vacuum drying oven and anneal at 80°C for 4 hours to completely remove the solvent and promote the rearrangement and crystallization of PVDF crystal chains. S3, Ion sputtered silver electrode: After polarization, a dense nano-silver layer is deposited on the surface of the thin film using a high-purity silver Ag target material via an ion sputtering instrument to form an electrode, thus constructing a complete electrode-piezoelectric layer-electrode structure. S4. High-voltage polarization: Electrodeization is performed with silver electrodes deposited on the upper and lower surfaces of the thin film. A 16 kV DC high voltage is applied to the rGO / IL / PVDF layer for 0.5 h at room temperature using a high-voltage needle electrode. The electric field can effectively penetrate the polymer medium and cause the electric dipole moment to align in the direction of thickness. S5. Packaging: Conductive adhesive is used to lead out copper wires, and PI tape is used to cover them for insulation protection, resulting in a flexible rGO / IL / PVDF piezoelectric film sensor.

2. The method of claim 1, wherein the piezoelectric sensor is a flexible self-powered piezoelectric sensor. In S1, IL is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt.

3. The method of claim 2, wherein the piezoelectric sensor is flexible and self-powered. The mixed solution in S1 needs to be degassed under vacuum after preparation: place the prepared mixed solution in a vacuum drying oven and let it stand for 30 minutes to completely remove the air bubbles introduced during stirring, so as to prevent printing interruption or the formation of pores or defects inside the film.

4. The method of claim 1, wherein the piezoelectric sensor is flexible and self-powered. After the polarization treatment in step S4 is completed, the two poles of the sample are short-circuited and left to stand for 24 hours to release the residual space charge on the surface and eliminate unstable signal interference, thereby obtaining a piezoelectric composite film with stable performance.

5. The method of claim 1, wherein the piezoelectric sensor is flexible and self-powered. The prepared flexible self-powered voltage sensor has both good structural uniformity and mechanical stability. It can be applied to wearable sensing, human motion monitoring and flexible energy harvesting equipment, as well as online monitoring and intelligent optimization equipment for precision drilling and heterogeneous connection equipment systems of pods.

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