Polyvinylidene fluoride piezoelectric film with ultrafine beta crystal and preparation method thereof

Abstract

The application discloses a polyvinylidene fluoride piezoelectric film with superfine beta crystals and a preparation method thereof. Polyvinylidene fluoride particles are crushed into polyvinylidene fluoride powder; the dried polyvinylidene fluoride powder is kept warm on a forming plate higher than the melting point of the polyvinylidene fluoride powder to obtain a PVDF melt, then the temperature is kept unchanged, and the PVDF melt is processed through multiple reciprocating cycle rolling by a roller under a pressure of 8-15 KPa; the processed product is cooled to form a polyvinylidene fluoride film with a thickness of 10-20 microns; the polyvinylidene fluoride film is processed through multiple reciprocating cycle secondary rolling by a roller under a pressure of 8-15 KPa to complete the reconstruction of beta crystals in the polyvinylidene fluoride piezoelectric film. The application can realize size control and greatly improve the beta crystal content at the same time, and can stably generate high piezoelectric output, and has wide application potential in the fields of stress sensing and environmental energy capture.

Description

Technical Field

[0001] This invention belongs to the field of novel piezoelectric materials technology, and relates to polymer-based piezoelectric materials, particularly to a polyvinylidene fluoride piezoelectric film with ultrafine β crystals and its preparation method. Background Technology

[0002] Piezoelectric materials, due to their unique non-centrosymmetric crystal structure, can achieve interconversion between deformation and electricity. Among them, polymer-based piezoelectric materials are widely used in flexible wearable devices, environmental energy harvesting devices, and implantable devices due to their lightweight, environmental tolerance, good processability, and deformation capabilities. Polyvinylidene fluoride (PVDF) is the most commonly used piezoelectric polymer material, and its piezoelectricity is mainly attributed to its polar β crystals. Although the β phase (TTT) of PVDF can generate strong dipoles due to the F / H opposite-side arrangement, its thermodynamic metastability limits its further practical applications. In conventional processing lacking a directional field, the α phase (TGTG') dominates due to its lower energy and ease of nucleation, resulting in products that exhibit almost no piezoelectricity on a macroscopic scale, making them difficult to apply in electromechanical coupling scenarios.

[0003] To enhance the piezoelectric properties of PVDF and broaden its applications, researchers have developed various methods, including filler-induced, electric field polarization, and mechanical stretching, to increase the β-crystal content of PVDF materials. Among these, force field polarization uses a strong stress field to force a conformational transformation in PVDF, thereby increasing the β-crystal content and reconstructing the β-crystal structure. For example, Huang et al. developed cyclic pulsed pressure molding, which solved the problem that β crystals of PVDF are difficult to generate and exist stably under hot pressing in a single pressure field. By applying a high pressure field to PVDF in cycles, the dipoles are forced to gradually align, and PVDF powder is directly molded into a piezoelectric film. It was also confirmed that the cyclic pulsed pressure field can reduce the crystallite size of β crystals and induce domain polarization (Huang, Y.-Z.; Liu, Z.; Li, L.-W.; He, H.-Z.; Wang, ZL; Qu, J.-P.; Chen, X.; Huang, Z.-X. Giant Piezoelectric Coefficient of Polyvinylidene Fluoride with Rationally Engineered Ultrafine Domains Achieved by Rapid Freezing Processing. Advanced Materials 2025, 37(3), 2412344.). Force field polarization couples melt forming with conformational transformation to achieve filler-free, low-energy β-phase construction, proving to be a green and efficient processing method for preparing high-voltage electrical output PVDF materials. However, due to the limitations of a single stress field and high-temperature processing above 175°C throughout the process, cyclic pulse forming requires applying pressures up to 15 MPa to the film, and the insufficient effect on PVDF crystal reconstruction results in only a small increase in β-crystal content, only from 30% in typical similar materials to 40-50%.

[0004] Therefore, there is an urgent need to develop a simpler processing method that can simultaneously achieve size control and significantly increase β crystal content, while also requiring a lower processing temperature. Summary of the Invention

[0005] To address the issues of high processing temperatures and limited improvement in β-crystal content required by existing technologies, this invention provides a polyvinylidene fluoride (PVDF) piezoelectric film with ultrafine β-crystals and its preparation method. By introducing a cyclic shear-compression synergistic force field, the PVDF molecular chain is efficiently reconstructed, achieving both size control and a significant increase in β-crystal content at a lower processing temperature. Furthermore, the piezoelectric film can be prepared without fillers and electric field polarization, enabling low-energy, low-cost, and environmentally friendly processing of piezoelectric products.

[0006] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a polyvinylidene fluoride piezoelectric thin film with ultrafine β crystals includes the following steps:

[0008] (1) Preprocessing

[0009] The polyvinylidene fluoride particles were crushed into polyvinylidene fluoride powder.

[0010] (2) Cyclic shearing-compression processing

[0011] The dried polyvinylidene fluoride powder is heated on a molding plate with a melting point above its melting point until it partially melts to obtain PVDF melt. Then, the temperature is kept constant, and the PVDF melt is subjected to multiple reciprocating cycles of rolling processing under a pressure of 8-15 kPa.

[0012] (3) Cooling and molding

[0013] The processed product is cooled to form a polyvinylidene fluoride film with a thickness of 10-20 micrometers;

[0014] (4) Constructing β crystals

[0015] The polyvinylidene fluoride (PVDF) film is subjected to multiple reciprocating rolling processes under a pressure of 8-15 kPa on a molding plate with a melting point below its melting point. This process reconstructs the β crystals in the PVDF piezoelectric film, resulting in a PVDF piezoelectric film with ultrafine β crystals.

[0016] In the above-mentioned film preparation method, in step (1), the method for pulverizing polyvinylidene fluoride (PVDF) particles can refer to existing pulverizing methods. A feasible pulverizing method is to process PVDF particles into powder using a solid-phase shear grinding device and grind them to a particle size of 50-80 mesh.

[0017] In the above film preparation method, in step (2), the cylindrical pressure roller is pressed against the PVDF melt, and the pressure roller is controlled to roll forward. After the PVDF melt is rolled to the front limit, the pressure roller is controlled to roll backward. After the PVDF melt reaches the rear limit, the pressure roller is controlled to roll forward again. This process is repeated to perform cyclic shear-compression (CSC) processing.

[0018] Further, the dried polyvinylidene fluoride powder is heated to 175-195℃ until partially melted to obtain PVDF melt. The PVDF melt is then subjected to cyclic rolling processing at the corresponding temperature. When processing above the melting point of PVDF, the higher the processing temperature, the stronger the molecular chain mobility, and the faster the β-crystals transform into α-crystals, thus causing the β-crystals to be unstable. In a preferred embodiment, the heating temperature is 175-180℃, and the holding time is 2-3 minutes. The preferred number of cycles in this step is 30-90.

[0019] Furthermore, the pressure is a force applied to the PVDF melt and perpendicular to the plane of the forming plate, which can be controlled by a jack placed below the forming plate to control the pressure applied to the PVDF melt by the pressure roller.

[0020] Furthermore, the pressure roller applies pressure to the PVDF melt in the following manner: an initial pressure is set in the range of 8-15 kPa, and 10-15 cycles are taken as one round. After each round, the pressure applied by the pressure roller to the PVDF melt is readjusted to the initial pressure to ensure that the pressure on the PVDF melt does not change as the thickness of the PVDF melt decreases during processing. After 3-12 rounds, the rolling processing of the PVDF melt is completed, and the PVDF film reaches the set thickness range.

[0021] Furthermore, the surface temperature of the pressure roller is kept consistent with the temperature of the forming plate to avoid the pressure roller temperature being too high and affecting the processing stability, and the temperature being too low and causing an excessively severe cooling effect on the PVDF melt.

[0022] In the above-mentioned film preparation method, in step (3), the processed product is cooled to form a polyvinylidene fluoride film; preferably, an ice-water mixture can be used to cool the molding plate and the PVDF melt on it to form a PVDF film; then the PVDF film is peeled off from the molding plate. The peeled film can continue to be cooled.

[0023] In the above-mentioned film preparation method, in step (4), the reason why the heating temperature of the polyvinylidene fluoride film needs to be lower than the melting point during further processing is that the β phase is in a thermodynamic metastable state and is difficult to exist stably above the melting point. Therefore, the highly crystalline film prepared in the previous step is subjected to cyclic shear compression processing below the melting point to ensure that the molecular chains have sufficient mobility without causing the β crystals to transform into α crystals as soon as they are formed. The heating temperature for further processing of the polyvinylidene fluoride film is 80-160℃, preferably 80-150℃; the surface temperature of the pressure roller is kept consistent with the temperature of the forming plate. The number of cyclic processing cycles in this step is preferably 60-100 times.

[0024] Furthermore, in steps (2) and (4) above, the migration rate of the pressure roller in the plane parallel to the forming plate is 0.01-0.02 m / s. This can avoid the external force being applied too frequently, which would prevent the roller from being pressed thinner effectively, and it can also avoid the processing being too slow, which would affect the processing efficiency.

[0025] This invention also provides a polyvinylidene fluoride (PVDF) piezoelectric film with ultrafine β-crystals prepared by the above method. The PVDF piezoelectric film has a β-crystal content of up to 50-76% and a size of up to 16 micrometers. The open-circuit voltage generated by the PVDF piezoelectric film can reach 15-26V.

[0026] The principle behind the cyclic shear force field technology for preparing smaller PVDF films with higher β content provided by this invention is as follows: During the processing of PVDF films, the higher the processing temperature, the stronger the mobility of the molecular chains, and the stronger the interconversion rate between the α and β phases within the PVDF film. This invention forces dipole alignment through a cyclic shear-compression coupling force field, which can gradually transform α crystals into β crystals. Furthermore, at temperatures below the melting point of the PVDF film, the temperature can be adjusted to control the mobility of the molecular chains, allowing the β crystals to persist for a longer period after formation. Therefore, through the cyclic force field of this invention, β crystal formation can be continuously promoted, increasing the β crystal content. With enhanced chain mobility, the interconversion rate between the α and β phases increases, resulting in more β phase formation points. However, since this is a solid-state processing, the crystals do not melt; only conformational changes such as the transformation from α to β phase occur. Therefore, the average size of the β phase decreases throughout the entire crystalline region.

[0027] Compared with the prior art, the polyvinylidene fluoride piezoelectric thin film with ultrafine β crystals and its preparation method provided by the present invention have the following beneficial technical effects:

[0028] 1. This invention prepares PVDF films by simultaneously applying pressure perpendicular to the melt plane and shear force parallel to the melt plane to partially molten PVDF powder. This method has a low processing temperature; rapid thinning of the melt can be achieved at a molding temperature of 175°C. At a processing temperature below 150°C, applying a cyclic shear force field to the PVDF film can achieve increased β-crystal content and low grain size. Furthermore, piezoelectric films can be prepared without fillers and electric field polarization, enabling low-energy, low-cost, and environmentally friendly processing of piezoelectric products.

[0029] 2. The polyvinylidene fluoride piezoelectric film with ultrafine β crystals prepared by the present invention has a β crystal size of less than 5.5 nanometers and a β crystal content of more than 75%, while maintaining an ultrathin thickness of less than 20 micrometers. In addition, it can stably generate high voltage output, and has broad application potential in stress sensing and environmental energy harvesting. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a cyclic shear compression process.

[0031] Figure 2 The results are infrared and XRD test results of different samples; (a) is the FTIR spectrum of different polyvinylidene fluoride piezoelectric film samples prepared in Example 4, (b) is the FTIR spectrum of different polyvinylidene fluoride piezoelectric film samples prepared in Example 5, (c) is the content of each phase in different polyvinylidene fluoride piezoelectric film samples prepared in Example 4, (d) is the content of each phase in different polyvinylidene fluoride piezoelectric film samples prepared in Example 5, (e) is the XRD pattern of different polyvinylidene fluoride piezoelectric film samples prepared in Example 4, (f) is the XRD pattern of different polyvinylidene fluoride piezoelectric film samples prepared in Example 5, (g) is the half-width at half-maximum (WHM) and β crystal size of the β (110 / 200) diffraction peak in different polyvinylidene fluoride piezoelectric film samples prepared in Example 4, and (h) is the half-width at half-maximum (WHM) and β crystal size of the β (110 / 200) diffraction peak in different polyvinylidene fluoride piezoelectric film samples prepared in Example 5.

[0032] Figure 3 The graph shows the mechanical property test results for different samples (TMP, CMP180, CSCP140).

[0033] Figure 4 These are the electrical performance test results of different samples; among them, (a) is the open circuit voltage test result of different PVDF films, (b) is the open circuit voltage test result of the CSCP140 film prepared in Example 4 under 2500 cycles of hammering, (c) is the open circuit voltage test result of the CSCP140 film prepared in Example 4 when pressed with an index finger, and (d) is the open circuit voltage test result of the CSCP140 film prepared in Example 4 when poked with an index finger. Detailed Implementation

[0034] The following examples further illustrate the preparation method of a polyvinylidene fluoride piezoelectric thin film with ultrafine β crystals provided by the present invention. It should be noted that the following examples are only for further illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above description are still within the scope of protection of the present invention.

[0035] The crystallinity of each sample is calculated according to the following formula (1):

[0036] (1);

[0037] in, Crystallinity (%) The enthalpy of fusion of the sample is (J / g). The recrystallization enthalpy (J / g) is the enthalpy of recrystallization during the sample melting process. The theoretical enthalpy of fusion (J / g) for the sample at 100% crystallization is typically 104.7 J / g for PVDF.

[0038] The β-phase content of the sample was calculated using FTIR spectroscopy and formula (2):

[0039] (2);

[0040] in, The content of β phase (%) and The infrared spectrum at 840 cm⁻¹ is respectively -1 and 766cm -1 absorbance at that point and They are respectively and The absorbance coefficient, The value is usually 1.26.

[0041] The β-crystal size of the CSC PVDF film was calculated using XRD data and formula (3):

[0042] (3);

[0043] in, Here is the Scherrer constant, taken as 0.89. For the X-ray wavelength, kα is taken as 0.15406 nm. θ is the half-width at half-maximum of the β (110 / 200) diffraction peak, and θ is the diffraction angle of the peak.

[0044] Example 1

[0045] This embodiment provides a method for preparing polyvinylidene fluoride (PVDF) film, the preparation process as follows: Figure 1 As shown in the figure. In this embodiment, five groups of polyvinylidene fluoride (PVDF) films were prepared by controlling different molding temperatures (175-195℃).

[0046] The preparation steps of the polyvinylidene fluoride (PVDF) film provided in this embodiment are as follows:

[0047] (1) Preprocessing

[0048] Polyvinylidene fluoride particles were crushed into polyvinylidene fluoride (PVDF) powder (particle size 50-80 mesh), and then the prepared PVDF powder was dried at 50°C for 10 hours.

[0049] (2) Cyclic shearing-compression processing

[0050] Take 2g of dried PVDF powder and place it on the forming plate of the cyclic shear compression processing equipment (see the structure of the film dynamic forming equipment disclosed in CN202510385261.8). Control the temperature of the forming plate to 175℃ and keep it at that temperature for 3 minutes to partially melt the PVDF powder. Then, at this temperature, press the PVDF melt against the pressure roller. While the pressure roller (the surface temperature is the same as the forming plate and the migration rate is 0.015m / s) applies pressure to the PVDF melt, it rolls back and forth along the surface of the forming plate to perform cyclic shear compression processing on the PVDF melt. Control the pressure on the pressure roller to 10KPa (adjust the pressure applied by the pressure roller to the PVDF melt to 10KPa every 10 cycles) and process 60 times.

[0051] (3) Cooling and molding

[0052] The molding plate and the PVDF melt on it were placed in a cooling tank containing an ice-water mixture to cool them. After cooling, a PVDF film was obtained. The film sample was designated CMP175.

[0053] Change the temperature of the molding plate to 180℃, 185℃, 190℃ and 195℃, and repeat the above steps (2) and (3) to obtain four groups of film samples, which are named CMP180, CMP185, CMP190 and CMP195 respectively.

[0054] Example 2

[0055] The preparation steps of the polyvinylidene fluoride (PVDF) film provided in this embodiment are as follows:

[0056] (1) Preprocessing

[0057] Polyvinylidene fluoride particles were crushed into polyvinylidene fluoride (PVDF) powder (particle size 50-80 mesh), and then the prepared PVDF powder was dried at 50°C for 10 hours.

[0058] (2) Cyclic shearing-compression processing

[0059] Take 2g of dried PVDF powder and place it on the forming plate of the cyclic shear compression processing equipment. Control the temperature of the forming plate to 175℃ and keep it at that temperature for 3 minutes to partially melt the PVDF powder. Then, at this temperature, press the PVDF melt against the pressure roller. While the pressure roller (surface temperature 175℃, migration rate 0.01m / s) applies pressure, it rolls back and forth along the surface of the forming plate to perform cyclic shear compression processing on the PVDF melt. Control the pressure on the pressure roller to 15KPa (adjust the pressure applied by the pressure roller to the PVDF melt to 15KPa every 10 cycles) and process for 30 cycles.

[0060] (3) Cooling and molding

[0061] The molding plate and the PVDF melt on it are placed in a cooling tank containing a mixture of ice and water to cool the molding plate and the PVDF melt. After cooling, the PVDF film is cut out.

[0062] Example 3

[0063] The preparation steps of the polyvinylidene fluoride (PVDF) film provided in this embodiment are as follows:

[0064] (1) Preprocessing

[0065] Polyvinylidene fluoride particles were crushed into polyvinylidene fluoride (PVDF) powder (particle size 50-80 mesh), and then the prepared PVDF powder was dried at 50°C for 10 hours.

[0066] (2) Cyclic shearing-compression processing

[0067] Take 2g of dried PVDF powder and place it on the forming plate of the cyclic shear compression processing equipment. Control the temperature of the forming plate to 175℃ and keep it at that temperature for 2 minutes to partially melt the PVDF powder. Then, at this temperature, press the PVDF melt against the pressure roller. While the pressure roller (surface temperature 175℃, migration rate 0.02m / s) applies pressure, it rolls back and forth along the surface of the forming plate to perform cyclic shear compression processing on the PVDF melt. Control the pressure on the pressure roller to 8KPa (adjust the pressure applied by the pressure roller to the PVDF melt to 8KPa every 15 cycles) and process 90 times.

[0068] (3) Cooling and molding

[0069] The molding plate and the PVDF melt on it are placed in a cooling tank containing a mixture of ice and water to cool the molding plate and the PVDF melt. After cooling, the PVDF film is cut out.

[0070] Comparative Example 1

[0071] In this comparative example, polyvinylidene fluoride (PVDF) films were prepared using a single stress field, and the steps are as follows:

[0072] (1) Preprocessing

[0073] Polyvinylidene fluoride particles were crushed into polyvinylidene fluoride (PVDF) powder (particle size 50-80 mesh), and then the prepared PVDF powder was dried at 50°C for 10 hours.

[0074] (2) Compression processing

[0075] Take 2g of dried PVDF powder and place it between two PET (polyethylene terephthalate) plates. Control the pressure between the two PET plates to be 15MPa and the temperature to be 195℃, and keep it at the same temperature and pressure for 20 minutes.

[0076] (3) Cooling and molding

[0077] The molding plate and the PVDF melt on it were placed in a cooling tank containing an ice-water mixture to cool the molding plate and the PVDF melt. After cooling, a PVDF film was cut and the film sample was denoted as TMP.

[0078] Example 4

[0079] In this embodiment, the polyvinylidene fluoride film sample CMP180 prepared in Example 1 was subjected to a second cycle shear compression process at different temperatures to construct more β crystals, reduce the size of the β crystals, and improve the piezoelectric output capability of the film.

[0080] The polyvinylidene fluoride piezoelectric thin film preparation steps provided in this embodiment are as follows:

[0081] Take the PVDF film sample CMP180 prepared in Example 1 and place it on the forming plate of the cyclic shear compression processing equipment. Control the temperature of the forming plate to 80°C. Press the PVDF film sample against the pressure roller. While the pressure roller (the surface temperature is the same as the forming plate temperature and the migration rate is 0.015m / s) applies pressure to it, it rolls back and forth along the surface of the forming plate, thereby performing 60 cycles of cyclic shear compression processing on the PVDF film sample. Control the pressure on the pressure roller to 10KPa (adjust the pressure applied by the pressure roller to the PVDF melt to 10KPa every 10 cycles). The resulting polyvinylidene fluoride piezoelectric film sample is recorded as CSCP80.

[0082] By changing the temperature of the molding plate to 100℃, 120℃, 130℃, and 140℃, and repeating the above steps, four sets of film samples were obtained, which were named CSCP100, CSCP120, CSCP130, and CSCP140 respectively.

[0083] Example 5

[0084] In this embodiment, the polyvinylidene fluoride film sample CMP180 prepared in Example 1 was subjected to secondary cyclic shear compression processing at different cycles to construct more β crystals, reduce the size of the β crystals, and improve the piezoelectric output capability of the film.

[0085] The polyvinylidene fluoride piezoelectric thin film preparation steps provided in this embodiment are as follows:

[0086] Take the PVDF film sample CMP180 prepared in Example 1 and place it on the forming plate of the cyclic shear compression processing equipment. Control the temperature of the forming plate at 140°C. Press the PVDF film sample against the pressure roller. While the pressure roller (surface temperature 140°C, migration rate 0.015m / s) applies pressure to it, it rolls back and forth along the surface of the forming plate, thereby performing 60 cycles of cyclic shear compression processing on the PVDF film sample. Control the pressure on the pressure roller at 10KPa (adjust the pressure applied to the PVDF melt by the pressure roller to 10KPa every 10 cycles). The resulting polyvinylidene fluoride piezoelectric film sample is recorded as CSCP-60 (the same sample as CSCP140).

[0087] By changing the number of cyclic shear compression processes to 30, 80, and 100 times, and repeating the above steps, three groups of film samples were obtained, which were designated CSCP-30, CSCP-80, and CSCP-100, respectively. The PVDF film sample that did not undergo a second cyclic shear compression process was designated CSCP-0.

[0088] (I) Structural Analysis of PVDF Thin Films

[0089] The specific data on the thickness and crystallinity of different samples in Example 1 and Comparative Example 1 are shown in Table 1.

[0090] Table 1 compares the thickness and crystallinity of different samples in Example 1 and Comparative Example 1.

[0091]

[0092] Table 1 shows that as the molding temperature increases, the melt viscosity decreases, and the fluidity increases, resulting in a thinner CMP PVDF film. The presence of melt elasticity allows it to effectively resist thinning by the compressive shear field once a certain thickness is reached. After cyclic shear compression processing, the film thickness can quickly decrease to 16 micrometers. The CMP film thickness only further decreases to 11 micrometers when the molding temperature reaches 195℃. However, the efficiency of preparing films using a single stress field is too low; the TMP PVDF film obtained in Comparative Example 1 has a thickness as high as 101 micrometers. Therefore, the crystallinity results of samples prepared at different molding temperatures also indicate that the optimal temperatures are 175℃ and 180℃, at which the PVDF films prepared have high crystallinity and possess more potential β-crystals.

[0093] (II) Structural analysis of reprocessed PVDF films

[0094] The FTIR test results of different samples prepared in Examples 4 and 5 are as follows: Figure 2 As shown in (a) and (b), the phase content in the different samples prepared in Examples 4 and 5 is as follows. Figure 2As shown in (c) and (d), the XRD test results of the different samples prepared in Examples 4 and 5 are as follows. Figure 2 As shown in (e) and (f), the red curve corresponds to the β phase, and the blue curve corresponds to the α / γ phase. The β crystal sizes of the different samples prepared in Examples 4 and 5 are shown below. Figure 2 As shown in (g) and (h).

[0095] The β-crystal content and crystallite size of different samples prepared in Examples 4 and 5 were tested, and the results are shown in Tables 2 and 3.

[0096] Table 2 shows the β-crystal content and crystallite size of the PVDF piezoelectric films at different processing temperatures in Example 4.

[0097]

[0098] Table 3 shows the β-crystal content and crystallite size of the PVDF piezoelectric film under different processing cycles in Example 5.

[0099]

[0100] Depend on Figure 2 As shown in (a) and (b), the infrared absorption peaks of α, β, and γ crystals of PVDF are concentrated in the 700-1300 cm⁻¹ range. -1 The range. Among them, the area located at 766cm. -1 The TGTG' conformational absorption peak corresponds to the α phase and is located at 840 cm⁻¹. -1 The TTT / T3GT3G' absorption peak is shared by both the β and γ phases. TTT corresponds to the β phase, and T3GT3G' is an abbreviation of TTTGTTTG', corresponding to the γ phase. Additionally, at 1275 cm⁻¹... -1 Absorption peaks can also be observed in the TTT conformation. For example... Figure 2 (a) As the processing temperature increases, the transformation from the TGTG' conformation to the TTT conformation becomes more likely to occur under the influence of the cyclic compression-shear coupling force field due to the enhanced chain motion capability. At 80℃, 766 cm⁻¹ -1 and 840cm -1 The absorption peak intensities at these locations are almost uniform, indicating an increase in the β-phase content in the unprocessed PVDF film. As the processing temperature continues to rise, the β-phase gradually becomes dominant in all crystal forms. Figure 2 (b) The FTIR plots under different processing times more intuitively reflect that the β phase gradually becomes dominant in the PVDF piezoelectric film under the action of cyclic shear-compression force field.

[0101] Based on the infrared spectrum test results, the β phase content of different samples in Example 4 was calculated according to the formula (2) given above. The results are shown in Tables 2 and 3. Figure 2 As shown in (c) and (d) in Table 2. Figure 2 As shown in (c), with increasing temperature, the chain mobility of PVDF increases, and α crystals are more easily transformed into β crystals under the action of cyclic shear compression. When the processing temperature rises to around 140℃, the β phase content no longer increases. This is because the chain mobility is too strong, and after the β crystals are formed, they partially transform back into α crystals. From Table 3 and... Figure 2 As shown in (d), for the sample without secondary processing, the β crystal content is only about 32.4%. After 80 CSC processing cycles, the β crystal content increases to about 76%. At this point, the proportion of the TTT conformation in the molecular chain is too high, the Gibbs free energy is high, and the interconversion between the α and β phases reaches equilibrium. Therefore, further processing will not increase the β phase content further.

[0102] From Table 3, Figure 2 As shown in (e) and (f), at lower temperatures and fewer processing cycles, the diffraction peaks of the β phase are very small. As the processing temperature and the number of processing cycles increase, the red diffraction peaks (β phase) gradually "arch," indicating a gradual increase in the β phase content. The XRD results more intuitively demonstrate the conformational transformation during the cyclic shear-compression process and corroborate the results with those of FTIR.

[0103] Crystallite size is considered a crucial factor affecting the piezoelectric properties of PVDF. Smaller β-crystal sizes imply higher grain boundary density, leading to greater interfacial polarization and thus improving the material's response to external electric fields. Enhanced chain mobility increases the inter-phase transition rate between the α and β phases, while also creating more β-phase formation sites. Figure 2 As shown in (g) and (h), the interconversion rate between the α and β phases increases with enhanced chain mobility, and the number of β phase formation sites increases. Therefore, a decreasing trend in β phase size is observed as the processing temperature increases. The optimal temperature is 140℃, at which PVDF films with high β crystal size and low microcrystal size can be prepared.

[0104] The decrease in β-phase size with increasing processing cycles can be achieved through… Figure 2 In the study (h), it was observed that when the processing time was too long and the film was heated for too long, the β phase size would start to increase again, which explains the increase in β phase size after 100 CSC processing cycles.

[0105] As shown in Table 3, the β-crystal content increased rapidly after cyclic shear compression processing. Compared with the unprocessed sample, the β-crystal content of CSCP-80 increased by 114%, while the crystallite size decreased to 5.4 nm. This indicates the effect of the cyclic shear compression force field on the reconstruction of PVDF β-crystals.

[0106] Therefore, cyclic shear-compression processing can significantly increase the β phase content while reducing the size of β crystals and improving the efficiency of polarization, enabling PVDF films processed by CSC to have potential piezoelectric application capabilities.

[0107] (III) Analysis of Mechanical and Electrical Properties

[0108] 1. Mechanical property analysis

[0109] The mechanical properties of samples TMP, CMP180, and CSCP140 were tested using a universal testing machine. The samples were cut into strips of 3cm × 1cm, clamped by pneumatic clamps, and subjected to tensile testing at a tensile rate of 10mm / s. Stress-strain curves were recorded, and tensile strength and elastic modulus were calculated. Figure 3 As shown.

[0110] Under cyclic shear-compression force, the formation of an ordered PVDF structure results in higher modulus for CMP180 and CSCP140 PVDF films while maintaining sufficient tensile strength to withstand large one-dimensional tensile forces (37 MPa, approximately 6 N). Furthermore, the CSCP140 film also exhibits sufficient elongation at break to match the tensile strain (2.5%-5%) required for compressive energy harvesting or use in wearable devices.

[0111] 2. Electrical performance analysis

[0112] Electrical performance tests were performed on samples TMP, CMP180, and CSCP140. Copper foil electrodes with an effective area of ​​1.5cm × 1.5cm were attached to the upper and lower surfaces of a PVDF film. The devices were encapsulated using PI material to isolate them from environmental interference and ensure close contact between the copper foil electrodes and the film. The piezoelectric performance was evaluated by driving a periodic impact component using a linear motor (NTI AG HS01-37×166) as the impact source. A low-noise voltage preamplifier (Keithley-6514 system electrometer) was used to measure the component's output voltage signal, and the signal was acquired and analyzed using a data acquisition card (NI PCI-6221). The test results are as follows: Figure 4 As shown.

[0113] Open-circuit voltage test results for different PVDF films are as follows: Figure 4 As shown in (a), the open-circuit voltage test results of the CSCP140 thin film under 2500 cycles of hammer impact are as follows. Figure 4 As shown in (b), the open-circuit voltage test results of the CSCP140 film under index finger pressure and puncture are as follows. Figure 4 As shown in (c) and (d).

[0114] Due to its polar molecular structure, β-crystals exhibit superior conductivity compared to amorphous and α-crystals. The open-circuit voltages (e.g., β-crystals) of TMP, CMP180, and CSCP140 PVDF films were tested and compared here. Figure 4As shown in (a), the magnitude of the voltage output clearly demonstrates that, compared to the TMP sample obtained under a single stress field, the CM180P and CSCP140 samples obtained under cyclic shear-compression force fields can utilize the mechanical energy during processing to undergo crystal transformation with much higher efficiency and produce higher piezoelectric output. The open-circuit voltage of the CSCP140 PVDF film is as high as 26.1 volts.

[0115] Under 2500 cycles of hammering, the CSCP140 thin film maintained an output voltage of 26V, as... Figure 4 (b) This indicates that it can maintain high voltage output even under long-term external force. The above test results prove that the high β crystal content and fine piezoelectric domains brought about by the CSC force field to the PVDF film can produce high and stable piezoelectric output.

[0116] The stability, output signal strength, and waveform differences of the CSCP140 PVDF film under different external forces were tested using various finger movements. Figure 4 As shown in (c) and (d), when pressing with the index finger, the force applied is controlled to be roughly the same. First, a light pressure is applied, then a heavier pressure. Under light pressure, an open-circuit voltage of approximately 5V is measured. Under greater force, the voltage increases to 10V, while the waveform maintains its characteristic downward-sloping peak. Under a "poking" motion, the measured open-circuit voltage reaches as high as 20V, and the waveform differs from the pressing action, exhibiting a sharp peak. Research indicates that the CSCP140 PVDF film has the potential to distinguish different external forces through different waveforms and output intensities, and can generate a very high output voltage even under force applied by a finger. This reliable high output can bring higher energy conversion efficiency to piezoelectric devices, as well as higher sensing accuracy and signal-to-noise ratio.

Claims

1. A method for preparing a polyvinylidene fluoride piezoelectric thin film with ultrafine β crystals, characterized in that, Includes the following steps: (1) Pre-processing The polyvinylidene fluoride particles were crushed into polyvinylidene fluoride powder. (2) Cyclic shearing-compression processing The dried polyvinylidene fluoride powder is heated on a molding plate with a melting point above its melting point until it partially melts to obtain PVDF melt. Then, the temperature is kept constant, and the PVDF melt is subjected to multiple reciprocating cycles of rolling processing under a pressure of 8-15 kPa. (3) Cooling and molding The processed product is cooled to form a polyvinylidene fluoride film with a thickness of 10-20 micrometers; (4) Constructing β crystals The polyvinylidene fluoride (PVDF) film is subjected to multiple reciprocating rolling processes under a pressure of 8-15 kPa on a molding plate with a melting point below its melting point. This process reconstructs the β crystals in the PVDF piezoelectric film, resulting in a PVDF piezoelectric film with ultrafine β crystals.

2. The method for preparing a polyvinylidene fluoride piezoelectric thin film with ultrafine β-crystals according to claim 1, characterized in that, In step (1), the particle size of polyvinylidene fluoride powder is 50-80 mesh.

3. The method for preparing a polyvinylidene fluoride piezoelectric thin film with ultrafine β-crystals according to claim 1, characterized in that, In step (2), the polyvinylidene fluoride powder is heated to 175-180℃ and held for 2-3 minutes.

4. The method for preparing a polyvinylidene fluoride piezoelectric thin film with ultrafine β-crystals according to claim 1, characterized in that, In step (4), the heating temperature for further processing of the polyvinylidene fluoride film is 80-150℃.

5. The method for preparing a polyvinylidene fluoride piezoelectric thin film with ultrafine β-crystals according to any one of claims 1-4, characterized in that, In step (4), the number of cycles of the cyclic shearing and compression process is 60-100.

6. The method for preparing a polyvinylidene fluoride piezoelectric thin film with ultrafine β crystals according to any one of claims 1-4, characterized in that, In steps (2) and (4), the pressure roller is in a heated state, and the surface temperature of the pressure roller is consistent with the temperature of the forming plate during processing.

7. The method for preparing a polyvinylidene fluoride piezoelectric thin film with ultrafine β-crystals according to any one of claims 1-4, characterized in that, The migration speed of the pressure roller in the plane parallel to the forming plate is 0.01-0.02 m / s.

8. A polyvinylidene fluoride piezoelectric film with ultrafine β crystals obtained by the preparation method according to any one of claims 1-7.

9. The polyvinylidene fluoride piezoelectric film with ultrafine β-crystals according to claim 8, characterized in that, The β-crystal content in the polyvinylidene fluoride piezoelectric film can reach 50-76%.

10. The polyvinylidene fluoride piezoelectric film with ultrafine β-crystals according to claim 8, characterized in that, The open-circuit voltage generated by the polyvinylidene fluoride piezoelectric film can reach 15-26V.

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