A PVDF-based composite membrane and its preparation method and application

Abstract

The present invention proposes a PVDF-based composite membrane and a preparation method and application thereof, and relates to the technical field of polyvinylidene fluoride membrane preparation. The preparation of the PVDF-based composite membrane comprises the following steps: S1, adding modified nano-silver particles to an organic solvent mixed solution of PVDF-modified nano-barium titanate to obtain a mixed dispersion; S2, applying the mixed dispersion of step S2 on a float glass substrate, drying and peeling off to obtain a PVDF-based composite membrane. The method for preparing a PVDF-based composite membrane of the present invention realizes the uniform blending of nano-barium titanate (BTPM) ceramic particles and nano-silver (Ag) particles in a PVDF-based composite membrane. The method further simplifies the preparation process, and overcomes the technical difficulty in the prior art that the two need to be chemically bonded to obtain good dielectric and energy storage performance, thereby significantly improving the overall performance of the material.

Description

Technical Field

[0001] The invention relates to the technical field of polyvinylidene fluoride film preparation, and in particular to a PVDF-based composite film and a preparation method and application thereof. Background Art

[0002] Dielectric capacitors play an irreplaceable role in high-power electrical applications with their excellent power density. The dielectric properties of the dielectric material in dielectric capacitors directly determine the discharge energy density (U e ) and charge and discharge efficiency (η). These dielectric properties mainly include the relative dielectric constant (ε r ), dielectric breakdown strength (E b ) and dielectric loss (tanδ). Traditional ceramic-based dielectric materials have been widely used in industry. Although they have extremely high dielectric constants, their ability to withstand electrical breakdown is poor, usually tens of kV / mm. In addition, the inherent high density and brittleness of ceramics make ceramic-based dielectric capacitors extremely heavy, difficult to process, and the risk of electrical breakdown causing overall damage to the capacitor is too high. Therefore, polymer dielectric materials have the flexibility, low density and high processability as well as extremely high intrinsic resistance to electrical breakdown (i.e., breakdown strength, E b , usually several hundred kV / mm) have shown great application potential compared to ceramic dielectrics. Among the existing dielectric polymers, polyvinylidene fluoride (PVDF) has attracted much attention due to its high breakdown strength and higher dielectric constant than other polymers. However, the biggest disadvantage of polymer dielectrics compared to ceramic dielectrics is their too low relative dielectric constant (ε r , about 10). In order to improve the discharge energy density (U e ) and charge-discharge efficiency (η), the technical strategy usually adopted by researchers is to prepare PVDF-based composite materials by filling nanofillers in the PVDF matrix to optimize the relative dielectric constant, dielectric loss and breakdown strength of PVDF-based composite materials. Large relative dielectric constant and high breakdown strength help to improve the polarization strength of dielectric materials, while low dielectric loss helps to improve the charge-discharge efficiency of materials. Studies have shown that the improvement of dielectric constant and breakdown strength and the reduction or suppression of dielectric loss are the main strategies for improving the dielectric discharge energy density and charge-discharge efficiency of PVDF-based composite materials.

[0003] However, PVDF is a nonlinear dielectric material. When the external electric field applied to the electrodes on the top and bottom surfaces of the PVDF membrane material gradually increases, the polarization intensity of the PVDF membrane material will tend to saturate before reaching the breakdown electric field of PVDF. The occurrence of this polarization intensity saturation phenomenon originates from the intrinsic dielectric constant of the material. The too low intrinsic relative dielectric constant of PVDF will cause the polarization intensity saturation phenomenon of the PVDF membrane in the polarization process (i.e., the charging process) to occur long before the external electric field reaches the breakdown electric field level. Therefore, improving the relative dielectric constant of PVDF is extremely important for improving the dielectric discharge energy density of PVDF. Generally, ceramic nanofillers with high dielectric constants are used to fill into the PVDF matrix to improve the relative dielectric constant of PVDF-based composite materials. However, a large number of studies have shown that the effective improvement of the relative dielectric constant of PVDF-based composite materials is severely limited by the selection of fillers and the structural design of fillers. Specifically, if a ceramic nanofiller with a high dielectric constant is used, studies have shown that when the filler filling amount is as high as about 50vol%, the improvement of the PVDF-based composite material is only about 10 times the relative dielectric constant of pure PVDF, and it will also be accompanied by a sharp drop in breakdown strength and a sharp increase in dielectric loss, which is very limited to the improvement of the final discharge energy density and charge-discharge efficiency of the PVDF-based composite material. In addition, studies have shown that directly using conductive nanofillers to fill the PVDF matrix can increase the dielectric constant of the obtained PVDF-based composite material by an order of magnitude. However, this composite percolation system composed of conductive fillers and polymer matrix will make it extremely easy for PVDF-based composite materials to form a conductive network between fillers under the action of an external electric field, generate leakage current, and cause great dielectric loss (up to hundreds, hundreds of times higher than the dielectric loss of PVDF), and the charge-discharge efficiency is extremely low. It is obviously unable to adapt to the application scenarios of PVDF-based composite materials in the field of dielectric energy storage. From the perspective of increasing the dielectric constant, the efficiency of conductive nanofillers is obviously much better than that of ceramic nanofillers. However, the huge dielectric loss severely limits the application of conductive nanofillers in the field of PVDF dielectric energy storage materials. Therefore, how to reasonably use conductive nanofillers to improve the dielectric constant of PVDF-based composites is both very attractive and very challenging. Summary of the invention

[0004] In view of this, the present invention proposes a PVDF-based composite membrane and a preparation method and application thereof. The method fills nano-barium titanate and nano-silver particles into a PVDF matrix by a simple physical blending method, thereby ensuring the uniform dispersion of the two fillers and reducing the dielectric loss of the PVDF-based composite membrane, thereby improving the charge and discharge efficiency and further improving the discharge energy density.

[0005] In a first aspect, the present invention provides a method for preparing a PVDF-based composite membrane, comprising the following steps:

[0006] S1, adding modified nano silver particles (AgPM) to an organic solvent mixed solution of PVDF-modified nano barium titanate (BTPM) to prepare a mixed dispersion;

[0007] S2. The mixed dispersion of step S2 is coated on a float glass substrate, and then peeled off after drying to obtain a PVDF-based composite membrane.

[0008] On the basis of the above technical solution, preferably, in step S1, the preparation of the modified nano-barium titanate comprises the following steps:

[0009] S11, mixing tris(hydroxymethyl)aminomethane powder, dopamine hydrochloride powder and barium titanate powder in deionized water, washing by centrifugation, filtering and obtaining precipitate I;

[0010] S12, the precipitate I of step S11, dopamine hydrochloride powder and mPEG-NH 2 The powder is mixed with deionized water, stirred and kept warm, washed by centrifugation, and filtered to obtain precipitate II;

[0011] S13, vacuum drying the precipitate II in step S12 to obtain modified nano-barium titanate.

[0012] On the basis of the above technical solution, preferably, in step S1, the preparation of the organic solvent mixed solution of PVDF-modified nano-barium titanate comprises the following steps:

[0013] S14, dispersing the modified nano-barium titanate in an organic solvent to prepare a dispersion;

[0014] S15. Under stirring conditions, dissolving PVDF powder in the dispersion of step S14 to obtain an organic solvent mixed solution of PVDF-modified nano-barium titanate.

[0015] On the basis of the above technical solution, preferably, in step S14, the light absorption capacity of the dispersion and the mass concentration of the modified nano-barium titanate in the dispersion satisfy the relationship shown in formula (1):

[0016] y = 0.000469947+19.62568 × x (1);

[0017] In formula (1), y represents the absorbance, and x represents the mass concentration of modified nano-barium titanate in the dispersion.

[0018] In the present invention, the inventors first dispersed 0.1 mg of BTPM powder in 10 mL of an organic solvent by water bath ultrasound, and tested it by UV-visible spectrophotometer, and obtained that the characteristic absorption peak of the dispersion was 345.6 nm, as shown in FIG. Figure 1 shown.

[0019] On the basis of the above technical solution, the inventors uniformly dispersed BTPM powders of different masses in 100 mL of organic solvent by water bath ultrasound to prepare DMF dispersions of BTPM with mass concentrations of 0.0189 mg / mL, 0.0314 mg / mL, 0.0440 mg / mL, 0.0566 mg / mL, and 0.0691 mg / mL, respectively, and obtained the absorption curves of the above dispersions in the visible light range of 300 nm to 800 nm by UV-visible spectrophotometer, as shown in FIG. Figure 2 Then, by reading the absorbance of each absorption curve at its characteristic absorption peak of 345.6nm (ordinate), combined with the corresponding mass concentration of BTPM (abscissa), a straight line is fitted to obtain a standard straight line, as shown in Figure 2 As shown in (b), Figure 2 (b) It can be seen that the linear matching degree between the fitted straight line and the scattered points is extremely high, which can accurately reflect the relationship between absorbance and mass concentration of BTPM. The reference equation of the straight line is shown in formula (1).

[0020] On the basis of the above technical solution, preferably, according to the mass concentration of the modified nano-barium titanate in the dispersion, the 24-hour dispersion of the modified nano-barium titanate is obtained, which needs to meet the condition shown in formula (2):

[0021] d 24 =200× x (2);

[0022] In formula (2), d 24 is the 24-hour dispersion of modified nano-barium titanate, and x represents the mass concentration of modified nano-barium titanate in the dispersion;

[0023] Wherein, obtaining the mass concentration of modified nano-barium titanate in the dispersion comprises the following steps:

[0024] Excess modified nano-barium titanate powder was dispersed in an organic solvent and allowed to stand for 24 hours. The upper suspension (i.e., 24-hour saturated dispersion) was obtained by pouring and diluted 200 times. The absorbance of the diluted dispersion at 345.6 nm was 0.718. Substituting it into formula (1), the mass concentration of the modified nano-barium titanate powder in the 24-hour saturated dispersion diluted 200 times was calculated. Then, multiplying it by 200, the concentration of BTPM in DMF was obtained. 24 .

[0025] More preferably, the 24-hour dispersion degree d of the modified nano-barium titanate is 24 =7.31mg / mL.

[0026] On the basis of the above technical solution, preferably, step S14 specifically comprises: determining the addition amount of the modified nano-barium titanate according to the 24-hour dispersion of the modified nano-barium titanate, dispersing it in an organic solvent to prepare a dispersion.

[0027] More preferably, 73.1 mg of the modified nano-barium titanate is uniformly dispersed in 10 ml of an organic solvent to prepare a dispersion.

[0028] On the basis of the above technical solution, preferably, in step S1, the preparation of the modified nano silver particles comprises the following steps:

[0029] S16, mixing tris(hydroxymethylaminomethane) powder, dopamine hydrochloride powder and nano-silver particles in deionized water, washing by centrifugation, filtering and obtaining precipitate III;

[0030] S17, the precipitate III of step S16, dopamine hydrochloride powder and mPEG-NH 2 The powder is mixed with deionized water, stirred and kept warm, washed by centrifugation, and filtered to obtain precipitate IV;

[0031] S18, vacuum drying the precipitate IV from step S17 to obtain modified nano-silver particles.

[0032] On the basis of the above technical solution, preferably, the volume fraction of the modified nanosilver particles in the dispersion is greater than 0 and less than 0.1 vol.%.

[0033] On the basis of the above technical solution, preferably, the organic solvent is N,N-dimethylformamide (DMF).

[0034] In a second aspect, the present invention provides a PVDF-based composite membrane prepared by the above-mentioned preparation method.

[0035] In a third aspect, the present invention relates to the use of PVDF-based composite films in flexible dielectric capacitors.

[0036] The PVDF-based composite membrane and its preparation method and application provided by the present invention have the following beneficial effects compared with the prior art:

[0037] (1) The method for preparing a PVDF-based composite membrane of the present invention adopts an environmentally friendly filler surface modification strategy and related processes. The method does not use any toxic reagents or drugs, has mild operating conditions, does not require high temperature treatment, and is energy-saving and environmentally friendly. The entire process is simple and easy to operate, and is suitable for mass production and large-scale application.

[0038] (2) The method for preparing PVDF-based composite membranes of the present invention realizes the uniform blending of nano-barium titanate (BTPM) ceramic particles and nano-silver (Ag) particles in PVDF-based composite membranes. This method further simplifies the preparation process and overcomes the technical difficulty in the prior art that the two need to be chemically bonded to obtain good dielectric and energy storage properties, thereby significantly improving the overall performance of the material.

[0039] (3) When preparing the PVDF-based composite membrane, the present invention is based on the 24-hour dispersion (d 24 ) to design the filler filling. This method avoids blind exploration, ensures the rationality and basis of the design, greatly saves the cost of technical exploration, and can accurately meet the functional preset requirements of composite materials.

[0040] (4) In the PVDF-based composite membrane of the present invention, the d 24 As high as 7.31mg / mL, which effectively ensures the high specific gravity BaTiO 3 The uniform dispersion in the PVDF matrix allows enough space between adjacent BTPMs to accommodate the subsequently filled Ag nanoparticles, which facilitates the subsequent adjustment of the number of Ag nanoparticles contained between adjacent BTPMs by regulating the filling amount of Ag nanoparticles, thereby achieving a spatial confinement effect on the Ag nanoparticles. This effect prevents direct contact between adjacent Ag nanoparticles, reduces the generation of leakage current, and reduces the dielectric loss of the PVDF / BTPM / AgPM composite film, thereby improving the charge and discharge efficiency, and then increasing the discharge energy density.

[0041] (5) The PVDF-based composite membrane prepared by the present invention greatly improves the dispersion ability of the filler, and the total filling amount of the filler is less than 2 vol.%, which greatly retains the flexibility and lightweight characteristics of the PVDF-based composite material. At the same time, the interface amount between the filler and the PVDF matrix inside the PVDF / BTPM / AgPM composite membrane is suppressed, reducing the risk of interface defect formation, which is easier to achieve in engineering practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0043] Figure 1 The UV-visible characteristic absorption spectrum of the DMF dispersion of BTPM prepared in the present invention;

[0044] Figure 2 The UV-visible characteristic absorption spectra of the DMF dispersions of BTPM with different mass concentrations prepared by the present invention and the standard straight line obtained by fitting the absorbance of the dispersions with different mass concentrations at 345.6 nm with the corresponding BTPM mass concentration;

[0045] Figure 3 TEM images of AgPM nanoparticles and BTPM nanoparticles prepared in the present invention;

[0046] Figure 4 HAADF images and EDS element distribution maps of AgPM nanoparticles and BTPM nanoparticles prepared in the present invention;

[0047] Figure 5 The optical photograph of the Tyndall effect of the dispersion prepared from the barium titanate powder and nano silver particles before and after modification of the present invention;

[0048] Figure 6 The relationship between the dielectric constant of the film materials obtained in Examples 1 to 4 of the present invention and Comparative Examples 1 to 3 and the frequency and the dielectric constant comparison diagram at a frequency of 1 kHz are shown;

[0049] Figure 7 The cross-sectional SEM images of the membrane materials prepared in Comparative Examples 1 to 2 and Example 3 of the present invention;

[0050] Figure 8 This is a graph showing the relationship between the particle spacing between adjacent BTPM nanoparticles and between adjacent AgPM nanoparticles in the PVDF-based composite membrane prepared in Example 3 of the present invention and the filling volume fraction;

[0051] Fig. 9 This is a diagram showing the possible distribution states between BTPM and AgPM nanoparticles in the PVDF-based composite membrane prepared in Example 3 of the present invention due to the particle spacing;

[0052] Fig.10 The AC conductivity comparison diagram of the membrane materials obtained in Examples 1 to 4 of the present invention and Comparative Examples 1 to 3 at 1 kHz and the AC conductivity variation diagram at 1 kHz with the volume fraction of AgPM;

[0053] Fig.11 The graphs of the dielectric loss of the film materials obtained in Examples 1 to 4 of the present invention and Comparative Examples 1 to 3 as a function of frequency, the dielectric loss comparison graph at a frequency of 1 kHz, and the relationship between the dielectric loss at a frequency of 1 kHz and the volume fraction of AgPM and their numerical comparison graphs;

[0054] Fig.12This is a graph showing the relationship between the breakdown strength of the PVDF-based composite membranes prepared in Examples 1 to 4 of the present invention and the volume fraction of AgPM;

[0055] Fig.13 PE curves, discharge energy density and charge-discharge efficiency of the membrane materials obtained in Examples 1 to 4 of the present invention and Comparative Examples 1 to 3. DETAILED DESCRIPTION

[0056] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0057] In the prior art, the chemical modification strategy of fixing conductive nanoparticles on the surface of insulating fillers has extremely stringent process requirements and is difficult to achieve large-scale batch production. In addition, since the specific gravity of metal conductive nanoparticles and ceramic insulating fillers is much greater than that of polymers, it is also a major challenge to achieve uniform dispersion of these high-density fillers in the polymer matrix. In order to significantly improve the dielectric constant of composite materials, existing methods usually require a high proportion (close to or reaching 50 vol.%) of filler filling, which not only introduces a large number of internal interfaces and easily forms interface defects, resulting in a sharp increase in dielectric losses in engineering applications, but also deteriorates the flexibility characteristics of composite materials and increases the weight of the material, seriously weakening the original flexibility and lightweight advantages of polymers.

[0058] Based on the above problems, in order to significantly improve the dielectric constant of PVDF-based composite materials at low filling amounts and inhibit the increase in dielectric loss and the decrease in breakdown strength, thereby obtaining higher discharge energy density and charge-discharge efficiency, the inventors synthesized AgPM that can be stably dispersed in DMF. 24 The filling amount of BTPM was designed to prepare a PVDF / BTPM composite solution matrix, and AgPM was gradually filled in. At a filling amount of less than 0.04 vol.%, BTPM formed a spatial confinement effect on AgPM, thereby overcoming the problem of easy accumulation of heavy metal nanofillers in the prior art to produce direct contact, inhibiting the direct contact of AgPM, and also giving full play to the strong polarization effect of AgPM, greatly improving the dielectric constant of the composite material, and curbing the excessive deterioration of the dielectric loss and breakdown strength of the composite material, achieving extremely high discharge energy density and charge and discharge efficiency at ultra-low filling amount (optimum 0.04 vol.%).

[0059] The technical solution of the present invention is further described below in conjunction with specific examples. The nano-barium titanate raw materials involved in the examples and comparative examples are purchased from Shandong Guoci Functional Materials Co., Ltd., and the nano-silver particles are purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Other materials are commercially available conventional commodities unless otherwise specified.

[0060] The preparation steps of the modified nano-barium titanate (BTPM), modified nano-silver particles (AgPM) and the DMF mixed solution of PVDF / BTPM involved in the following examples and comparative examples are as follows.

[0061] The preparation of the BTPM comprises the following steps:

[0062] S11, pour 240 mg of tris (hydroxymethyl) aminomethane (Tris) powder, 100 mg of dopamine hydrochloride powder and 4 g of BT powder into 200 mL of deionized water, use a magnetic stirrer to fully stir at room temperature for 48 h, then terminate the reaction, wash the resulting suspension with pure water by centrifugation for 3 times, filter, and retain the bottom precipitate to obtain precipitate I;

[0063] S12, the precipitate I of step S11, 100 mg of dopamine hydrochloride powder and 0.2 g of mPEG-NH 2 The powder was put into 200 mL of deionized water and stirred continuously in a 60°C water bath for 12 hours. After the reaction was completed, the suspension was washed by centrifugation and filtered to obtain precipitate II.

[0064] S13. Dry the precipitate II of step S12 at 60° C. under vacuum conditions for 12 hours to obtain BTPM powder.

[0065] The preparation of the AgPM comprises the following steps:

[0066] S14, pour 240 mg of tris (hydroxymethyl) aminomethane (Tris) powder, 100 mg of dopamine hydrochloride powder and 4 g of nano silver particles into 200 mL of deionized water, use a magnetic stirrer to fully stir at room temperature for 48 h, then terminate the reaction, wash the resulting suspension with pure water by centrifugation for 3 times, filter, and retain the bottom precipitate to obtain precipitate III;

[0067] S15, the precipitate III of step S14, 100 mg of dopamine hydrochloride powder and 0.2 g of mPEG-NH 2 The powder was put into 200 mL of deionized water and stirred continuously in a 60°C water bath for 12 hours. After the reaction was completed, the suspension was washed by centrifugation and filtered to obtain precipitate IV.

[0068] S16. Dry the precipitate IV of step S15 at 60° C. under vacuum conditions for 12 hours to obtain AgPM nanoparticles.

[0069] The preparation of the DMF mixed solution of PVDF / BTPM comprises the following steps:

[0070] 73.1 mg of BTPM was uniformly dispersed in 10 mL of DMF solvent by water bath ultrasound, and then 1 g of PVDF powder was fully dissolved in the BTPM DMF dispersion by magnetic stirring for 24 hours to obtain a PVDF / BTPM DMF mixed solution;

[0071] The BTPM and AgPM nanoparticles prepared above were subjected to the following tests, including:

[0072] (1) The BTPM and AgPM nanoparticles prepared above were processed using a transmission electron microscope. The results are as follows Figure 3 shown.

[0073] Depend on Figure 3 It can be seen that the surfaces of the silver nanoparticles and the barium titanate nanoparticles in the present invention are both coated with a shell layer.

[0074] (2) The BTPM and AgPM nanoparticles prepared above were processed by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS). Figure 4 shown.

[0075] Depend on Figure 4 It can be seen that the shell layer coated on the surface of AgPM and BTPM nanoparticles contains only polydopamine and mPEG-NH 2 The C and N elements contained in the molecule indicate that the shell is a modified polymer shell.

[0076] In order to further verify the dispersibility of the modified barium titanate powder and nanosilver particles, the inventors tested the sedimentation process changes of DMF dispersions of BT, BTPM, Ag and AgPM nanoparticles within 24 hours and the Tyndall effect optical photos of the dispersions, such as Figure 5 shown.

[0077] Depend on Figure 5It can be seen that after standing for 5 minutes, pure barium titanate (BT) and silver (Ag) nanoparticles immediately showed significant bottoming phenomenon. After standing for another 24 hours, the Tyndall linearity in the DMF dispersions of BT and Ag almost completely disappeared, while the Tyndall phenomenon in the DMF dispersions of BTPM and AgPM was very obvious, and no obvious precipitation was formed at the bottom of the medicine bottle, indicating that the BTPM and AgPM nanoparticles coated with the modified polymer shell can show better dispersion stability in DMF than BT and Ag, overcoming the problem that high-density ceramics and metal fillers are very easy to settle in solvents; the reason for this extremely strong dispersion stability is that the modified polymer shell on the surface of BTPM and AgPM nanoparticles can promote a strong steric hindrance effect between adjacent nanoparticles, resulting in strong repulsion between each other, which blocks the unmodified nanofillers that are usually prone to agglomeration to form large agglomerates due to their large surface energy and the gravity sedimentation of high-density fillers due to gravity.

[0078] The following are specific embodiments of the present invention.

[0079] Example 1

[0080] The method for preparing the PVDF-based composite membrane of this embodiment comprises the following steps:

[0081] S1, adding 0.01 vol.% of modified nano silver particles to an organic solvent mixed solution of PVDF-modified nano barium titanate to prepare a mixed dispersion;

[0082] S2. The mixed dispersion of step S2 is uniformly coated on a clean float glass substrate, and then dried at 110° C. and peeled off to obtain a PVDF-based composite membrane.

[0083] Example 2

[0084] The method for preparing the PVDF-based composite membrane of this embodiment comprises the following steps:

[0085] S1, adding 0.03 vol.% of modified nano silver particles to an organic solvent mixed solution of PVDF-modified nano barium titanate to prepare a mixed dispersion;

[0086] S2. The mixed dispersion of step S2 is uniformly coated on a clean float glass substrate, and then dried at 110° C. and peeled off to obtain a PVDF-based composite membrane.

[0087] Example 3

[0088] The method for preparing the PVDF-based composite membrane of this embodiment comprises the following steps:

[0089] S1, adding 0.04 vol.% of modified nano silver particles to an organic solvent mixed solution of PVDF-modified nano barium titanate to prepare a mixed dispersion;

[0090] S2. The mixed dispersion of step S2 is uniformly coated on a clean float glass substrate, and then dried at 110° C. and peeled off to obtain a PVDF-based composite membrane.

[0091] Example 4

[0092] The method for preparing the PVDF-based composite membrane of this embodiment comprises the following steps:

[0093] S1, adding 0.06 vol.% of modified nano silver particles to an organic solvent mixed solution of PVDF-modified nano barium titanate to prepare a mixed dispersion;

[0094] S2. The mixed dispersion of step S2 is uniformly coated on a clean float glass substrate, and then dried at 110° C. and peeled off to obtain a PVDF-based composite membrane.

[0095] Comparative Example 1

[0096] 1 g of PVDF powder was fully dissolved in 10 mL of DMF solvent by stirring to obtain a DMF solution of PVDF. The solution was then coated on a clean float glass substrate and dried at 110° C. and then peeled off to obtain a pure PVDF membrane.

[0097] Comparative Example 2

[0098] The difference between this comparative example and Example 3 is that in step S1, modified nanosilver particles are not added, and the other steps remain unchanged, and a PVDF / BTPM composite membrane is prepared.

[0099] Comparative Example 3

[0100] The difference between this comparative example and Example 3 is that in step S1, modified nanosilver particles with a volume fraction of 0.10 vol.% are added, and the other steps remain unchanged.

[0101] The following performance tests were performed on the above Examples 1 to 4 and Comparative Examples 1 to 3, including:

[0102] (1) The dielectric constant and dielectric loss of the above materials were measured at room temperature at 100 Hz to 10 MHz using a precision impedance analyzer (4990A, Agilent, USA). The test results are shown in the attached Figure 6 .

[0103] (2) Using a breakdown tester (CS2674AX, Nanjing Changsheng Instrument Co., Ltd.), the breakdown electric field value of the film was measured by slowly and gradually increasing the DC voltage on the above material until electrical breakdown occurred, and Weibull failure probability analysis was performed based on the obtained breakdown electric field values ​​to obtain the statistical electrical breakdown strength (E b ).

[0104] (3) A ferroelectric test system (Premier II, Radiant Technologies, Inc., USA) equipped with a high-voltage amplifier (TREK MODEL 609B) was used to measure the polarization intensity-applied electric field (PE) curve of the film at room temperature and 10 Hz using the monopolar mode to obtain the PE curve of the above materials, and then the discharge energy density and charge-discharge efficiency of the PVDF / BTPM / AgPM composite film were calculated based on the integrated area of ​​the curve.

[0105] The above test results are recorded in Table 1 below.

[0106] Table 1 Performance test results

[0107]

[0108]

[0109] It can be seen from Table 1 that the filling amount of BTPM and AgPM has a significant effect on the dielectric constant (ε) and dielectric loss (tanδ) of the composite material. As the filling amount of AgPM increases, the dielectric constant shows an upward trend, from 23.78 in Example 1 to 37.11 in Example 4. This shows that the introduction of a small amount of AgPM can effectively improve the dielectric properties of PVDF-based composite materials. However, when the filling amount of AgPM is further increased to 0.1vol.% in Comparative Example 3, although the dielectric constant reaches the highest value of 46.97, the dielectric loss also increases sharply to 0.314, which is much higher than other embodiments. The reason for this is that because the sample prepared in Comparative Example 3 is filled with too much AgPM, BTPM cannot effectively produce a spatial confinement effect on AgPM, resulting in a high probability of direct contact between adjacent AgPMs, resulting in a certain degree of electrical conduction, resulting in excessive leakage current, and ultimately leading to a sharp increase in dielectric loss, reducing the growth efficiency of discharge energy density and charge and discharge efficiency, and resulting in a reduction in the utilization efficiency of fillers.

[0110] By comparing Examples 1 to 4, it can be seen that when the filling amount of AgPM nanoparticles is 0.04 vol.%, the comprehensive performance of the PVDF-based composite membrane is the best.

[0111] Combining Table 1 and Figure 6(a) It can be seen that as the amount of AgPM filling increases, the dielectric constant of the PVDF / BTPM / AgPM composite film gradually increases. In addition, the dielectric constant at a frequency of 1kHz is compared. Figure 6 (b) It can be seen that: after adding 1.72 vol.% BTPM, the dielectric constant of the PVDF / BTPPM-1.72 composite membrane increased by 136% compared with pure PVDF, and on the basis of the PVDF / BTPPM-1.72 composite membrane, after additionally adding 0.04 vol.% AgPM, the dielectric constant of the PVDF / BTPM / AgPM-0.04 composite membrane was further increased by 49% compared with PVDF / BTPPM-1.72, and it was increased by 252% compared with pure PVDF. This shows that the co-filling of AgPM and BTPM nanoparticles can fully tap the high dielectric constant characteristics of the filler and efficiently improve the dielectric constant of PVDF-based composite materials.

[0112] Taking Example 3 and Comparative Examples 1 to 3 as examples, the following results were obtained by scanning electron microscopy: Figure 7 The cross-sectional SEM image.

[0113] Depend on Figure 7 It can be seen that both BTPM and AgPM nanoparticles show extremely high uniform distribution in the PVDF matrix. The formation of this excellent filler uniform distribution state is due to the ultra-dispersed surface modification strategy for high-density nanofillers.

[0114] Taking Example 3 with the best overall performance as an example, the following performance analysis is performed, including:

[0115] (1) By analyzing the relationship between the particle spacing between adjacent BTPM nanoparticles and between adjacent AgPM nanoparticles in the prepared PVDF-based composite membrane and the filling volume fraction, and combining the report in the published work of Hou et al. (DOI: 10.1016 / j.ceramint.2022.03.177), the filler particle spacing (d inter ) can be calculated by the following formula: inter =r[(4π / 3f v ) 1 / 3 -2], where r is the radius of the spherical particle, f v is the volume fraction of filler in the composite material. Based on this formula and the raw material Ag and raw material BaTiO 3 The average diameter (D AgPM =50nm, D BTPM =100nm), we get Figure 8The relationship between the distance between adjacent filler particles (ordinate) and the filler filling amount (volume percentage) of AgPM and BTPM is shown.

[0116] Depend on Figure 8 It can be seen that: theoretically, in the PVDF / BTPM / AgPM-0.04 composite membrane, at a filling amount of 1.72 vol.%, the distance between adjacent BTPM particles dinter-BTPM is 263 nm, and at a filling amount of 0.04 vol.%, the distance between adjacent AgPM particles dinter-AgPM is 217 nm.

[0117] (2) By analyzing the possible distribution states between BTPM and AgPM nanoparticles in the prepared PVDF-based composite membrane due to the particle spacing, the results are as follows: Fig. 9 shown.

[0118] Depend on Fig. 9 It can be seen that when 2×D AgPM +d inter-AgPM >d inter-BTPM When the conductive fillers are spaced apart from each other, the adjacent AgPM nanoparticles can be isolated by the adjacent BTPM nanoparticles without direct contact, which is the spatial confinement effect of BTPM on AgPM. On the contrary, the adjacent BTPM nanoparticles cannot block the direct contact between the adjacent AgPM nanoparticles, which will most likely lead to electrical conduction between the conductive fillers, causing a sharp increase in dielectric loss, resulting in low charge and discharge efficiency and discharge energy density.

[0119] Further integration Figure 8 , it can also be obtained that: in the PVDF / BTPM / AgPM-0.04 composite membrane prepared in Example 3, at a BTPM filling amount of 1.72 vol.%, the distance between adjacent BTPM particles d inter-BTPM is 263nm; at a filling amount of 0.04vol% AgPM, the distance between adjacent AgPM particles d inter-AgPM is 217nm; in short, based on the filling amount of the above two fillers, BTPM can just produce an effective spatial confinement effect on AgPM. When the filling amount of AgPM continues to increase, the effectiveness of this spatial confinement effect will weaken.

[0120] (3) Taking the membrane materials obtained in Example 3 and Comparative Examples 1-2 as examples, the AC conductivity comparison at 1 kHz was analyzed.

[0121] (4) Taking Examples 1 to 4 and Comparative Example 3 as examples, the AC conductivity of PVDF / BTPM / AgPM composite materials obtained with different filling amounts of AgPM at 1 kHz was analyzed as a function of the volume fraction of AgPM.

[0122] The test results of performance tests (3) and (4) are as follows: Fig.10 shown.

[0123] Depend on Fig.10 (a) It can be seen that whether BTPM or AgPM is filled into the PVDF matrix, the AC conductivity of the corresponding PVDF-based composite membrane will increase, especially in the PVDF / BTPM / AgPM composite material, the filling of AgPM will significantly increase the AC conductivity of the composite material to some extent. This increase in AC conductivity reflects the improvement of the material's electrical conductivity, which will increase the leakage current density of the composite material under the action of an external electric field (charging process), resulting in a sharp increase in dielectric loss.

[0124] Depend on Fig.10 (b) It can be seen that when the filling amount of AgPM reaches 0.06 vol.%, the AC conductivity of PVDF / BTPM / AgPM-0.06 increases by 560.66% compared with pure PVDF. When it is further increased to 0.10 vol.%, the AC conductivity of PVDF / BTPM / AgPM-0.10 increases by 2569.26% compared with pure PVDF.

[0125] (5) Taking Comparative Example 1 and Examples 1 to 4 as examples, the variation of the dielectric loss of the film material with frequency was analyzed.

[0126] (6) Taking Comparative Example 1, Comparative Example 2 and Example 3 as examples, the dielectric loss of the film materials at a frequency of 1 kHz was analyzed.

[0127] (7) Taking Examples 1 to 4 as examples, the relationship between the dielectric loss of the PVDF-based composite membrane at a frequency of 1 kHz and the volume fraction of AgPM and their numerical comparison are analyzed.

[0128] The test results of performance tests (5), (6) and (7) are as follows: Fig.11 shown.

[0129] Depend on Fig.11 (a) It can be seen that: as the filling amount of AgPM gradually increases, the dielectric loss of PVDF / BTPM / AgPM composite materials increases more and more obviously.

[0130] Depend on Fig.11 (b) It can be seen that compared with pure PVDF, the dielectric loss of PVDF / BTPM increased by 90%, and when the filling amount of AgPM reached 0.04 vol.%, the dielectric loss of PVDF / BTPM / AgPM-0.04 increased by 100%.

[0131] Depend on Fig.11(c) It can be seen that: when the filling amount of AgPM is further increased to 0.06vol.%, the dielectric loss of PVDF / BTPM / AgPM-0.06 is increased by 276.67% compared with pure PVDF; the dielectric loss of PVDF / BTPM / AgPM-0.10 is increased by 946.67% compared with pure PVDF. This shows that in the PVDF / BTPM / AgPM composite system, the filling of AgPM causes the system to gradually transform from an insulating system to a conductive system, that is, a percolation system. This transformation is obviously not conducive to dielectric energy storage, and will cause most of the stored electrostatic energy to undergo conductivity loss during the discharge process and transform into Joule heat.

[0132] (8) Taking Examples 1 to 4 as examples, the breakdown strength (E) of the PVDF / BTPM / AgPM composite films of AgPM with different filling amounts was analyzed. b ) changes with the volume fraction of AgPM. The test results are as follows Fig.12 shown.

[0133] Depend on Fig.12 It can be seen that with the gradual increase of the AgPM filling amount, the E b When the volume fraction of AgPM exceeds 0.06 vol. % and gradually approaches 0.10 vol. %, the E b Will gradually decrease to the E of pure PVDF b (345kV / mm). This means that excessive addition of AgPM will gradually deteriorate the breakdown strength of the PVDF / BTPM / AgPM composite film, even lower than the breakdown strength of pure PVDF, reducing the electrical breakdown safety of the composite film, which will also lead to a significant decrease in the charge and discharge efficiency and discharge energy density of the composite film.

[0134] (9) Taking Comparative Example 1, Comparative Example 2 and Example 3 as examples, their PE curves, discharge energy density and charge-discharge efficiency were obtained.

[0135] (10) Taking Examples 1 to 4 as examples, the PE curves, discharge energy density and charge-discharge efficiency of PVDF / BTPM / AgPM composite films with different filling amounts of AgPM were obtained.

[0136] The test results of performance tests (9) and (10) are as follows: Fig.13 shown.

[0137] Depend on Fig.13 (a) and Fig.13 (b) It can be seen that the filling of AgPM makes the polarization intensity of PVDF / BTPM / AgPM composite film ( Fig.13The vertical axis of (a) is significantly higher than that of pure PVDF and PVDF / BTPM, making the discharge energy density of PVDF / BTPM / AgPM-0.04 as high as 30.58 J / cm 3 , the charge and discharge efficiency is still as high as 75%. This is because the filling of AgPM greatly improves the dielectric constant of the PVDF / BTPM / AgPM-0.04 composite film, and the effective spatial confinement effect of BTPM on AgPM significantly inhibits the increase of dielectric loss and the decrease of breakdown strength of the PVDF / BTPM / AgPM-0.04 composite film. However, with the further filling of AgPM, although the polarization strength ( Fig.13 (c) ordinate) will be improved, but its breakdown strength (saturation charge electric field ( Fig.13 (a) horizontal axis) will also decrease significantly, which will offset part of the advantage of the increased polarization strength, and the dielectric loss will also increase sharply, resulting in a negative growth behavior of the charge and discharge efficiency and discharge energy density, such as Fig.13 As shown in (d), the discharge energy density can reach as high as 31.6 J / cm when the filling amount of AgPM reaches 0.06 vol.%. 3 However, its charge and discharge efficiency dropped to 69% compared with 75% of PVDF / BTPM / AgPM-0.04 composite membrane. Furthermore, when the filling amount of AgPM was increased to 0.10 vol%, the discharge energy density of PVDF / BTPM / AgPM-0.10 composite membrane was 30.58 J / cm2 compared with 30.6 J / cm3 of PVDF / BTPM / AgPM-0.04 composite membrane. 3 It dropped to 28.64 J / cm 3 , its charge and discharge efficiency dropped to 63%.

[0138] In summary, when the total filling amount of BTPM and AgPM in the PVDF / BTPM / AgPM composite membrane prepared by the present invention is up to 1.78 vol.% within the protection scope of the present invention, the filling amount is extremely low. At the optimal filling amount (AgPM, 0.04 vol.%), its optimal dielectric constant is as high as 31, and the dielectric loss at this time is only 0.044. The breakdown strength is increased by 13% compared with pure PVDF. At different filling amounts of AgPM, the dielectric loss of the PVDF / BTPM / AgPM composite membrane is suppressed to below 0.2.

[0139] When the PVDF / BTPM / AgPM composite film prepared by the present invention has a BTPM filling amount of 1.72 vol.% and an AgPM filling amount of only 0.04 vol.%, the dielectric constant of the composite film is increased by 2.29 times compared with pure PVDF, and the discharge energy density is increased to 30.58 J / cm3 , which is 389.28% higher than that of pure PVDF. In addition, the charge and discharge efficiency of the composite membrane is as high as 97%.

[0140] 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 in the protection scope of the present invention.

Claims

1. A method for preparing a PVDF-based composite membrane, characterized in that: The following steps are involved: S1, adding modified nano silver particles to an organic solvent mixed solution of PVDF-modified nano barium titanate to prepare a mixed dispersion; S2, coating the mixed dispersion of step S2 on a float glass substrate, drying and peeling off to obtain a PVDF-based composite membrane; in, The preparation of the modified nano-barium titanate comprises the following steps: S11, mixing tris(hydroxymethyl)aminomethane powder, dopamine hydrochloride powder and barium titanate powder in deionized water, washing by centrifugation, filtering and obtaining precipitate I; S12, mixing the precipitate I of step S11, dopamine hydrochloride powder and mPEG-NH2 powder in deionized water, stirring and keeping warm, centrifuging and washing, filtering, and obtaining a precipitate II; S13, vacuum drying the precipitate II in step S12 to obtain modified nano-barium titanate; The preparation of the organic solvent mixed solution of PVDF-modified nano-barium titanate comprises the following steps: S14, dispersing the modified nano-barium titanate in an organic solvent to prepare a dispersion; S15, dissolving PVDF powder in the dispersion of step S14 under stirring to obtain an organic solvent mixed solution of PVDF-modified nano-barium titanate; The preparation of the modified nano silver particles comprises the following steps: S16, mixing tris(hydroxymethylaminomethane) powder, dopamine hydrochloride powder and nano-silver particles in deionized water, washing by centrifugation, and filtering to obtain precipitate III; S17, mixing the precipitate III of step S16, dopamine hydrochloride powder and mPEG-NH2 powder in deionized water, keeping warm and stirring, centrifuging and washing, filtering, and obtaining a precipitate IV; S18, vacuum drying the precipitate IV from step S17 to obtain modified nano-silver particles.

2. The method for preparing a PVDF-based composite membrane according to claim 1, wherein: In step S14, the light absorption capacity of the dispersion and the mass concentration of the modified nano-barium titanate in the dispersion satisfy the relationship shown in formula (1): y = 0.000469947+19.62568 × x (1); In formula (1), y represents the absorbance, and x represents the mass concentration of modified nano-barium titanate in the dispersion.

3. The method for preparing a PVDF-based composite membrane according to claim 2, characterized in that: According to the mass concentration of the modified nano-barium titanate in the dispersion, the 24-hour dispersion of the modified nano-barium titanate is obtained, which needs to meet the condition shown in formula (2): d 24 =200× x （2）； In formula (2), d 24 is the 24-hour dispersion of modified nano-barium titanate, and x represents the mass concentration of modified nano-barium titanate in the dispersion; Wherein, obtaining the mass concentration of modified nano-barium titanate in the dispersion comprises the following steps: An excess of modified nano-barium titanate powder was dispersed in an organic solvent and allowed to stand for 24 hours. The upper suspension was then decanted and diluted. The absorbance of the diluted dispersion at 345.6 nm was tested and substituted into formula (1) to calculate the mass concentration of the modified nano-barium titanate powder in the 24 h saturated dispersion diluted 200 times.

4. The method for preparing a PVDF-based composite membrane according to claim 1, wherein: Step S14 specifically includes: determining the addition amount of the modified nano-barium titanate according to the 24-hour dispersion of the modified nano-barium titanate, dispersing it in an organic solvent to prepare a dispersion.

5. The method for preparing a PVDF-based composite membrane according to claim 1, characterized in that: In step S1, the volume fraction of the modified nano-silver particles in the mixed dispersion is greater than 0 and less than 0.1 vol.%.

6. A PVDF-based composite membrane, characterized in that: The PVDF-based composite membrane is prepared by the preparation method according to any one of claims 1 to 5.

7. Use of the PVDF-based composite film as claimed in claim 6 in a flexible dielectric capacitor.

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