Composite material, composite, preparation method therefor, use thereof, and piezoelectric sensor

Abstract

A composite material, a composite, a preparation method therefor, a use thereof, and a piezoelectric sensor. The composite material comprises polyvinylidene fluoride microspheres and a flexible matrix material, wherein the polyvinylidene fluoride microspheres are dispersed in the flexible matrix material.

Description

Composite material, composite material, preparation method and use thereof, and piezoelectric sensor

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application is based on the Chinese patent application with application number 202311267388.7 and application date September 28, 2023, and claims the priority of the Chinese patent application. The entire content of the Chinese patent application is hereby introduced into this application as a reference. Technical Field

[0003] The present disclosure relates to a composite material, a composite material, a preparation method and use thereof, and a piezoelectric sensor. Background Art

[0004] Flexible piezoelectric sensors based on polyvinylidene fluoride (PVDF) have broad application prospects in fields such as artificial intelligence and the Internet of Things. However, compared with inorganic piezoelectric materials, piezoelectric polymers exhibit low piezoelectric properties, and the prepared piezoelectric sensors have low sensitivity, which limits the application of flexible piezoelectric sensors.

[0005] CN114539601A discloses a method for preparing a three-dimensional porous polyvinylidene fluoride piezoelectric film. A homogeneous, transparent solution containing PVDF is allowed to stand to remove bubbles, thereby obtaining a casting solution. The casting solution is then cast into a mold to form a liquid film. A micron-sized water mist is then sprayed onto the surface of the liquid film to induce initial sedimentation of the PVDF molecules, forming a porous PVDF surface layer on the surface of the liquid film. The entire mold is immersed in deionized water, which enters the PVDF liquid film through the porous PVDF surface layer and exchanges with the organic solvent, forming a three-dimensional porous PVDF film. The three-dimensional porous PVDF film is then immersed in deionized water and allowed to air-dry to obtain a three-dimensional porous polyvinylidene fluoride piezoelectric film. The polyvinylidene fluoride obtained by this method exhibits poor piezoelectric properties.

[0006] CN114507365A discloses a method for preparing a piezoelectric three-phase composite material based on the synergy of piezoelectric particles and carbon nanotubes. PVDF polymer is poured into an N,N-dimethylformamide (DMF) solution to make the solution completely transparent. Carbon nanotubes (CNTs) are added to the PVDF solution, ultrasonically treated, and stirred to disperse the CNTs in the solution. Highly dispersible BCZT powder prepared by a molten salt method is added to the mixed solution at different pulp ratios, ultrasonically treated, and stirred to obtain a mixed solution. The mixed solution is poured into deionized water and stirred to produce a white flocculent substance, which is then dried. The dried sample is dry-pressed into a disc, placed in a parallel plate mold, and naturally stretched and hot-pressed to obtain a piezoelectric three-phase composite material. This piezoelectric material requires a large amount of CNTs, which is relatively expensive.

[0007] CN114539601A and CN114507365A report methods for preparing polyvinylidene fluoride piezoelectric films. In addition, there are some methods for preparing polyvinylidene fluoride particles in the related art, but there is no suggestion of using polyvinylidene fluoride particles to prepare piezoelectric materials.

[0008] Summary of the Invention

[0009] In view of the above, one object of the present disclosure is to provide a composite material and a composite material having a low modulus and excellent piezoelectric properties. Furthermore, the composite material and the composite material prepared from the composite material are low in cost. Another object of the present disclosure is to provide a method for preparing the composite material and the composite material, which is simple to prepare. Another object of the present disclosure is to provide uses of the composite material and the composite material. Yet another object of the present disclosure is to provide a piezoelectric sensor.

[0010] The above objectives are achieved through the following technical solutions.

[0011] In a first aspect, the present disclosure provides a composite material comprising polyvinylidene fluoride microspheres and a flexible matrix material; the polyvinylidene fluoride microspheres are dispersed in the flexible matrix material.

[0012] According to the composite material disclosed herein, optionally, based on 100 parts by weight of the flexible matrix material, the content of the polyvinylidene fluoride microspheres is 10 to 40 parts by weight.

[0013] According to the composite material disclosed herein, optionally, the flexible matrix material is selected from one or more of polydimethylsiloxane, styrene block copolymer, polyurethane thermoplastic elastomer and silicone rubber.

[0014] According to the composite material disclosed herein, optionally, the particle size of the polyvinylidene fluoride microspheres is micro-nano scale.

[0015] According to the composite material disclosed herein, optionally, the particle size of the polyvinylidene fluoride microspheres is 5 to 25 μm.

[0016] According to the composite material disclosed herein, optionally, the polyvinylidene fluoride microspheres are obtained by the following method: annealing a polyvinylidene fluoride microsphere blank at 80-200° C. to obtain the polyvinylidene fluoride microspheres.

[0017] According to the composite material disclosed herein, optionally, the polyvinylidene fluoride microsphere blank is obtained by a phase separation method.

[0018] According to the composite material disclosed herein, illustratively, the polyvinylidene fluoride microsphere blank is obtained by the following method: a first solution comprising polyvinyl alcohol and a poor solvent for polyvinylidene fluoride is dropwise added to a stirred second solution comprising polyvinyl alcohol, polyvinylidene fluoride and a good solvent for polyvinylidene fluoride, wherein the temperature of the second solution is 30 to 98°C.

[0019] According to the composite material disclosed herein, optionally, the poor solvent for polyvinylidene fluoride is water, and the good solvent for polyvinylidene fluoride is selected from N,N-dimethylformamide and / or N,N-dimethylacetamide.

[0020] According to the composite material disclosed herein, optionally, the first solution further contains an inorganic piezoelectric material and a conductive material.

[0021] According to the composite material disclosed herein, optionally, based on the total mass of the polyvinylidene fluoride microspheres being 100%, the content of polyvinylidene fluoride is greater than or equal to 50 wt %.

[0022] According to the composite material disclosed herein, optionally, based on the total mass of the polyvinylidene fluoride microspheres being 100%, the content of polyvinylidene fluoride is greater than or equal to 70 wt %.

[0023] According to the composite material disclosed herein, optionally, based on the total mass of the polyvinylidene fluoride microspheres being 100%, the content of polyvinylidene fluoride is greater than or equal to 95 wt %.

[0024] According to the composite material disclosed herein, optionally, the composite material further comprises a filler, and the filler is selected from inorganic piezoelectric materials and / or conductive materials.

[0025] According to the composite material disclosed herein, optionally, the amount of the filler is less than or equal to 30 wt % of the mass of the flexible matrix material.

[0026] According to the composite material disclosed herein, optionally, the amount of the filler is less than or equal to 20 wt % of the mass of the flexible matrix material.

[0027] According to the composite material disclosed herein, optionally, the amount of the filler is less than or equal to 10 wt % of the mass of the flexible matrix material.

[0028] In a second aspect, the present disclosure provides a method for preparing a composite material, comprising the following steps:

[0029] The raw materials including the flexible matrix material and the polyvinylidene fluoride microspheres are mixed to obtain a first mixture, which is the composite material described in the first aspect.

[0030] According to the preparation method of the composite material disclosed herein, optionally, the curing agent is selected from one or more of methylvinylcyclosiloxane, tetra(butenyl)palladium(0), a photosensitizer, acrylic acid and a photoinitiator.

[0031] According to the method for preparing the composite material disclosed herein, optionally, the raw material further includes the filler.

[0032] In a third aspect, the present disclosure provides a composite material obtained by reacting the composite material of the first aspect with a curing agent.

[0033] According to the composite of the present disclosure, optionally, the composite has a thickness of 400-1000 μm.

[0034] In a fourth aspect, the present disclosure provides a method for preparing a composite, comprising the following steps:

[0035] The raw materials including the flexible matrix material and the polyvinylidene fluoride microspheres are mixed to obtain a first mixture, which is the composite material; the first mixture is mixed with a curing agent to obtain a second mixture; the second mixture is cast and then cured to obtain the composite material of the third aspect.

[0036] According to the preparation method of the composite disclosed herein, optionally, the curing agent is selected from one or more of methylvinylcyclosiloxane, tetra(butenyl)palladium(0), a photosensitizer, acrylic acid, and a photoinitiator.

[0037] According to the preparation method of the composite disclosed herein, optionally, the mass ratio of the flexible matrix material to the curing agent is 10:(0.2-2).

[0038] According to the preparation method of the composite disclosed herein, optionally, the curing temperature is 60-95°C.

[0039] In a fifth aspect, the present disclosure provides a composite comprising polyvinylidene fluoride microspheres and a flexible matrix material; the polyvinylidene fluoride microspheres are dispersed in the flexible matrix material, and the flexible matrix material is a styrene block copolymer.

[0040] In certain embodiments, based on 100 parts by weight of the flexible matrix material, the content of the polyvinylidene fluoride microspheres is 10 to 40 parts by weight.

[0041] In certain embodiments, the styrenic block copolymer is selected from at least one of styrene-butadiene-styrene block copolymer, styrene-ethylene-butylene-styrene block copolymer, hydrogenated styrene-butadiene block copolymer, and styrene-isoprene block copolymer.

[0042] In certain embodiments, the particle size of the polyvinylidene fluoride microspheres is micro-nano scale.

[0043] In certain embodiments, the particle size of the polyvinylidene fluoride microspheres is 5 to 25 μm.

[0044] In certain embodiments, the polyvinylidene fluoride microspheres are obtained by the following method: annealing a polyvinylidene fluoride microsphere blank at 80-200° C. to obtain the polyvinylidene fluoride microspheres.

[0045] In certain embodiments, the polyvinylidene fluoride microspheres are obtained by a phase separation method.

[0046] In certain embodiments, the polyvinylidene fluoride microsphere blank is obtained by the following method:

[0047] The first solution including polyvinyl alcohol and polyvinylidene fluoride poor solvent is added dropwise to the second solution including polyvinyl alcohol, polyvinylidene fluoride and polyvinylidene fluoride good solvent which is being stirred, wherein the temperature of the second solution is 30-98°C.

[0048] In certain embodiments, the poor solvent for polyvinylidene fluoride is water, and the good solvent for polyvinylidene fluoride is selected from N,N-dimethylformamide and / or N,N-dimethylacetamide.

[0049] In certain embodiments, the first solution further contains an inorganic piezoelectric material and a conductive material.

[0050] In certain embodiments, based on the total mass of the polyvinylidene fluoride microspheres being 100%, the content of polyvinylidene fluoride is greater than or equal to 50 wt %.

[0051] In certain embodiments, based on the total mass of the polyvinylidene fluoride microspheres being 100%, the content of polyvinylidene fluoride is greater than or equal to 70 wt %.

[0052] In certain embodiments, based on the total mass of the polyvinylidene fluoride microspheres being 100%, the content of polyvinylidene fluoride is greater than or equal to 95 wt %.

[0053] In certain embodiments, the composite further comprises a filler selected from an inorganic piezoelectric material and / or a conductive material.

[0054] In certain embodiments, the amount of the filler is less than or equal to 30 wt % of the mass of the styrene block copolymer.

[0055] In certain embodiments, the amount of the filler is less than or equal to 20 wt % of the mass of the styrene block copolymer.

[0056] In certain embodiments, the amount of the filler is less than or equal to 10 wt % of the mass of the styrene block copolymer.

[0057] In certain embodiments, the composite has a thickness of 50 to 600 μm.

[0058] In a sixth aspect, the present disclosure provides a method for preparing a composite, the method comprising the following steps:

[0059] The styrene block copolymer is dissolved in an organic solvent to obtain a dispersed solution; the raw materials including the dispersed solution and the polyvinylidene fluoride microspheres are mixed to obtain a mixed solution; the mixed solution is cast and then dried to obtain the composite of the fifth aspect.

[0060] In certain embodiments, the organic solvent is selected from at least one of toluene, xylene, acetone and n-hexane.

[0061] In certain embodiments, the mass concentration of the styrene-based block copolymer in the dispersed solution is 100 to 160 mg / L.

[0062] In certain embodiments, the raw material further comprises the filler.

[0063] In a seventh aspect, the present disclosure provides use of the composite material of the first aspect, the composite material of the third aspect, or the composite material of the fifth aspect in preparing a piezoelectric sensor.

[0064] In an eighth aspect, the present disclosure provides a piezoelectric sensor comprising a first electrode, a second electrode, the composite of the third aspect or the composite of the fifth aspect, and a flexible packaging shell;

[0065] The composite has a first surface and a second surface disposed opposite to each other;

[0066] The first electrode is attached to the first surface of the composite, the second electrode is attached to the second surface of the composite, and the first electrode and the second electrode are respectively provided with a wire;

[0067] The first electrode, the second electrode and the composite are located in the flexible packaging shell, and the wire is led out from the flexible packaging shell.

[0068] Compared with the related art, the advantages and technical effects of the present disclosure are:

[0069] The present disclosure disperses the polyvinylidene fluoride microspheres in the flexible matrix material to form the composite material. The present disclosure disperses the polyvinylidene fluoride microspheres in the flexible matrix material, then introduces a curing agent to obtain a cured flexible matrix material, and the polyvinylidene fluoride microspheres are dispersed in the cured flexible matrix material to form the composite. The present disclosure disperses the polyvinylidene fluoride microspheres in a styrene block copolymer of a flexible matrix material to form the composite. This improves the piezoelectric properties of polyvinylidene fluoride, and the composite has a lower modulus and can be applied to various piezoelectric sensors, particularly flexible piezoelectric sensors. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] FIG1 is a graph showing the piezoelectric properties of the membrane product obtained in Example 1.

[0071] FIG2 is a graph showing the piezoelectric properties of the membrane product obtained in Example 2.

[0072] FIG3 is a graph showing the piezoelectric properties of the membrane product obtained in Example 3.

[0073] FIG4 is a graph showing the piezoelectric properties of the membrane product obtained in Example 4.

[0074] FIG5 is a graph showing the piezoelectric performance of the membrane product obtained in Example 5.

[0075] FIG6 is a graph showing the piezoelectric performance of the membrane product obtained in Example 6.

[0076] FIG7 is a graph showing the piezoelectric properties of the film products obtained in Examples 7 to 10.

[0077] FIG8 is a graph showing the piezoelectric properties of the film product obtained in Example 11.

[0078] FIG9 is a graph showing the piezoelectric properties of the film product obtained in Example 12.

[0079] FIG10 is a graph showing the piezoelectric properties of the film product obtained in Example 13.

[0080] FIG11 is a diagram showing the piezoelectric properties of the membrane product obtained in Example 14.

[0081] FIG12 is a graph showing the piezoelectric properties of the film product obtained in Example 15.

[0082] FIG13 is a graph showing the piezoelectric properties of the film product obtained in Example 16.

[0083] FIG14 is a graph showing the piezoelectric performance of the film product obtained in Comparative Example 1.

[0084] FIG15 is a graph showing the piezoelectric performance of the film product obtained in Comparative Example 2. DETAILED DESCRIPTION

[0085] The present disclosure is further described below with reference to specific embodiments, but the protection scope of the present disclosure is not limited thereto.

[0086] <Composite Materials>

[0087] In a first aspect, the present disclosure provides a composite material. The composite material of the present disclosure includes polyvinylidene fluoride microspheres and a flexible matrix material, and the polyvinylidene fluoride microspheres are dispersed in the flexible matrix material. The polyvinylidene fluoride microspheres serve as piezoelectric units, and the flexible matrix material serves as a medium for dispersing the piezoelectric units. The flexible matrix material used in the present disclosure is essentially a polymer prepolymer, so the flexible matrix material is in liquid form before being solidified, and a solidification operation is required to obtain the composite of the present disclosure for later applications. Polyvinylidene fluoride is dispersed in the flexible matrix material in the form of microspheres, and the composite prepared from the composite material formed in this way has higher piezoelectric properties and lower modulus.

[0088] If polyvinylidene fluoride film is directly used instead of the polyvinylidene fluoride microspheres used in this disclosure, the resulting composite will be less flexible and, due to the weakened stress concentration effect, will also lead to a decrease in piezoelectricity. If a non-flexible matrix material is used instead of the flexible matrix material used in this disclosure, the resulting composite will lose its flexibility, which is not conducive to the assembly of wearable devices.

[0089] The flexible matrix material can be a flexible polymer material. For example, the flexible matrix material can be selected from one or more of polydimethylsiloxane, styrene block copolymers, polyurethane thermoplastic elastomers, and silicone rubber. In certain embodiments, the flexible matrix material is polydimethylsiloxane. These flexible substrate materials offer advantages such as flexibility, low modulus, ease of processing, and shape adjustability, making them suitable for assembling flexible wearable piezoelectric devices.

[0090] In certain embodiments, the particle size of the polyvinylidene fluoride microspheres is micro-nanoscale. Optionally, the particle size of the polyvinylidene fluoride microspheres is 5 to 25 μm, for example, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, 22 μm, 25 μm, etc. The interfacial interaction between the polyvinylidene fluoride microspheres with excessively large particle sizes and the cured flexible matrix material is weakened, and stress transfer is reduced. At the same time, under the action of external forces, the stress concentration effect of the polyvinylidene fluoride microspheres with excessively large particle sizes in the cured flexible matrix material is weakened, which is not conducive to improving the piezoelectric output. However, the micro-nanoscale, especially the polyvinylidene fluoride microspheres with a particle size of 5 to 25 μm, have a strong interfacial interaction with the cured flexible matrix material, resulting in large stress transfer. At the same time, under the action of external forces, the stress concentration effect of the polyvinylidene fluoride microspheres with small particle sizes in the cured flexible matrix material is strong, resulting in high piezoelectric output.

[0091] In certain embodiments, in addition to polyvinylidene fluoride, the polyvinylidene fluoride microspheres may also contain a certain amount of inorganic piezoelectric materials and / or conductive materials. The inorganic piezoelectric materials may include piezoelectric ceramics, etc. Examples of piezoelectric ceramics may include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, etc. Examples of conductive materials may include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, etc. The purpose of introducing inorganic piezoelectric materials and / or conductive materials into polyvinylidene fluoride microspheres is to increase the piezoelectric output of the composite, wherein the inorganic piezoelectric material can increase the piezoelectric effect of the composite, and the conductive material can reduce the impedance of the composite.

[0092] In the case where the polyvinylidene fluoride microspheres also contain an inorganic piezoelectric material and / or a conductive material in addition to containing polyvinylidene fluoride, the content of polyvinylidene fluoride in the polyvinylidene fluoride microspheres can be greater than or equal to 50wt%. In certain embodiments, the content of polyvinylidene fluoride in the polyvinylidene fluoride microspheres can be greater than or equal to 70wt%. Alternatively, the content of polyvinylidene fluoride in the polyvinylidene fluoride microspheres can be greater than or equal to 95wt%. When the content of polyvinylidene fluoride in the polyvinylidene fluoride microspheres is too low, it is not conducive to improving the uniformity of the morphology and particle size of the polyvinylidene fluoride microspheres.

[0093] In certain embodiments, the polyvinylidene fluoride microspheres may not contain inorganic piezoelectric materials and / or conductive materials in addition to polyvinylidene fluoride, that is, the polyvinylidene fluoride microspheres may be composed of polyvinylidene fluoride.

[0094] In certain embodiments, the content of the polyvinylidene fluoride microspheres can be 10 to 40 parts by weight, such as 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 35 parts by weight, or 40 parts by weight, based on 100 parts by weight of the flexible matrix material. Alternatively, the content of the polyvinylidene fluoride microspheres is 15 to 35 parts by weight. For example, the content of the polyvinylidene fluoride microspheres can be 25 to 35 parts by weight. The content of the polyvinylidene fluoride microspheres has a certain influence on the piezoelectric properties of the composite. This may be because the polyvinylidene fluoride microspheres in the cured flexible matrix material can be considered as dispersed dipoles. When the polyvinylidene fluoride microspheres are below a certain doping amount, the modulus of the composite does not change significantly; with the increase of the polyvinylidene fluoride microsphere content, the dipole content increases, and under a certain external force, the low modulus leads to an increase in the additional polarization generated, resulting in a large voltage; the content of polyvinylidene fluoride microspheres continues to increase, the dipole content further increases, but the modulus of the composite increases, resulting in a decrease in the deformation of the composite under the same external force, resulting in a decrease in the additional polarization generated, and then leading to a low piezoelectric output.

[0095] In certain embodiments, the composite material may further include fillers in addition to polyvinylidene fluoride microspheres and a flexible matrix material. The filler may be selected from inorganic piezoelectric materials and / or conductive materials. The inorganic piezoelectric material may include piezoelectric ceramics, etc. Examples of piezoelectric ceramics include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, etc. Examples of conductive materials include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, etc. The purpose of introducing inorganic piezoelectric materials and / or conductive materials into the composite material is to increase the piezoelectric output of the composite, wherein the inorganic piezoelectric material can increase the piezoelectric effect of the composite, and the conductive material can reduce the impedance of the composite.

[0096] When the composite material contains fillers in addition to polyvinylidene fluoride microspheres and a flexible matrix material, the amount of filler can be less than or equal to 30% of the mass of the flexible matrix material. In some embodiments, the amount of filler is less than or equal to 20% of the mass of the flexible matrix material. In other embodiments, the amount of filler is less than or equal to 10% of the mass of the flexible matrix material. When the amount of filler is too high, on the one hand, it is not conducive to improving the flexibility of the composite; on the other hand, if the content of polyvinylidene fluoride microspheres in the composite is too low, the stress on the polyvinylidene fluoride microspheres will be reduced due to the damping effect of the flexible matrix material after curing, which is not conducive to improving the piezoelectric output of the composite.

[0097] In certain embodiments, the composite material may contain no filler in addition to the polyvinylidene fluoride microspheres and the flexible matrix material, i.e., the composite material is composed of the polyvinylidene fluoride microspheres and the flexible matrix material. This composite material can achieve excellent piezoelectric properties without the use of inorganic piezoelectric materials. This may be because the piezoelectric polymer has a low modulus. Under the action of an external force, the large deformation of the composite causes the polyvinylidene fluoride microspheres to generate more additional polarization, thereby producing excellent piezoelectric properties.

[0098] The polyvinylidene fluoride microspheres can be obtained by annealing the polyvinylidene fluoride microsphere blank at 80-200° C. to obtain the polyvinylidene fluoride microspheres. This can improve the crystallinity of the polyvinylidene fluoride microspheres and enhance the piezoelectric properties of the composite.

[0099] In certain embodiments, the annealing temperature may be 90-155°C; for example, 110-150°C.

[0100] In certain embodiments, the annealing time may be 1 to 8 hours; optionally 2 to 6 hours; for example, 3 to 5 hours.

[0101] The polyvinylidene fluoride microsphere blank can be obtained by a phase separation method. Specifically, the polyvinylidene fluoride microsphere blank is obtained by dropwise adding a first solution comprising polyvinyl alcohol and a poor solvent for polyvinylidene fluoride to a stirred second solution comprising polyvinyl alcohol, polyvinylidene fluoride, and a good solvent for polyvinylidene fluoride, wherein the temperature of the second solution is 30 to 98°C.

[0102] In certain embodiments, in addition to containing polyvinyl alcohol and polyvinylidene fluoride poor solvents, the first solution may also contain inorganic piezoelectric materials and / or conductive materials. The inorganic piezoelectric materials may include piezoelectric ceramics, etc. Examples of piezoelectric ceramics may include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, etc. Examples of conductive materials include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, etc. In this way, the prepared polyvinylidene fluoride microsphere blank may contain inorganic piezoelectric materials and / or conductive materials, thereby allowing the polyvinylidene fluoride microspheres to contain inorganic piezoelectric materials and / or conductive materials.

[0103] In certain embodiments, the poor solvent for polyvinylidene fluoride may be water.

[0104] In certain embodiments, the concentration of polyvinyl alcohol in the first solution may be 5-50 mg / mL, optionally 10-40 mg / mL, for example 15-30 mg / mL. The molecular weight of polyvinyl alcohol may be 500,000-1,000,000, optionally 600,000-900,000, for example 700,000-850,000.

[0105] In some embodiments, the first solution is composed of polyvinyl alcohol and a poor solvent for polyvinylidene fluoride. The polyvinylidene fluoride microsphere blank prepared in this way does not contain other components such as inorganic piezoelectric materials and / or conductive materials, and thus the polyvinylidene fluoride microspheres also do not contain other components such as inorganic piezoelectric materials and / or conductive materials.

[0106] In certain embodiments, the good solvent for polyvinylidene fluoride may be selected from N,N-dimethylformamide and / or N,N-dimethylacetamide.

[0107] In certain embodiments, the concentration of polyvinylidene fluoride in the second solution may be 20-60 mg / mL, optionally 30-50 mg / mL, for example 40-45 mg / mL. The molecular weight of polyvinylidene fluoride may be 350,000-700,000, optionally 400,000-600,000, for example 500,000-550,000.

[0108] In certain embodiments, the concentration of polyvinyl alcohol in the second solution may be 5-50 mg / mL, optionally 10-40 mg / mL, for example 15-30 mg / mL. The molecular weight of polyvinyl alcohol may be 500,000-1,000,000, optionally 600,000-900,000, for example 700,000-850,000.

[0109] In certain embodiments, the second solution consists of polyvinyl alcohol, polyvinylidene fluoride, and a good solvent for polyvinylidene fluoride.

[0110] In certain embodiments, the rotation speed of the stirring device for stirring the second solution may be 150 to 700 rpm; optionally 200 to 500 rpm; for example, 300 to 400 rpm.

[0111] In certain embodiments, the temperature of the second solution may be 30-98°C; optionally 50-98°C; for example 80-95°C.

[0112] In certain embodiments, the volume ratio of the first solution to the second solution may be (1.5-5):1; optionally (2-4):1; for example (2.5-3.5):1.

[0113] In certain embodiments, the method for preparing polyvinylidene fluoride microspheres may further include the following steps: continuing stirring for 7 to 18 hours after the first solution is added; optionally, stirring for 10 to 15 hours; for example, stirring for 12 to 13 hours to obtain solid polyvinylidene fluoride; then washing the solid polyvinylidene fluoride and then drying to obtain polyvinylidene fluoride microspheres.

[0114] In certain embodiments, the solid polyvinylidene fluoride may be washed with water first and then with alcohol, wherein the alcohol may be selected from one or more of methanol, ethanol, propanol, and isopropanol.

[0115] <Method for preparing composite material>

[0116] In a second aspect, the present disclosure provides a method for preparing a composite material, comprising the following steps:

[0117] Raw materials including a flexible matrix material and polyvinylidene fluoride microspheres are mixed to obtain a first mixture, which is the composite material described in the first aspect.

[0118] In certain embodiments, the method for preparing the composite material may further include a step of preparing polyvinylidene fluoride microspheres.

[0119] Step of forming a first mixture

[0120] Mixing raw materials including a flexible matrix material and polyvinylidene fluoride microspheres to obtain a first mixture. Specifically, stirring the flexible matrix material and polyvinylidene fluoride microspheres at 20-35° C. to obtain the first mixture.

[0121] In certain embodiments, the raw materials may further include a filler, that is, a raw material comprising a flexible matrix material, polyvinylidene fluoride microspheres, and a filler is mixed to obtain a first mixture. The filler may be selected from an inorganic piezoelectric material and / or a conductive material. The inorganic piezoelectric material may include a piezoelectric ceramic. Examples of piezoelectric ceramics may include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, etc. Examples of conductive materials may include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, etc.

[0122] In certain embodiments, the stirring time may be 7 to 18 hours; optionally 10 to 15 hours; for example, 12 to 13 hours.

[0123] Steps for preparing polyvinylidene fluoride microspheres

[0124] The polyvinylidene fluoride microsphere blank is annealed at 80-200° C. to obtain the polyvinylidene fluoride microsphere. The annealing temperature and annealing time are as described above and will not be repeated here.

[0125] The preparation method of the polyvinylidene fluoride microsphere blank is specifically described above and will not be repeated here.

[0126] <Compound>

[0127] In a third aspect, the present disclosure provides a composite. The composite is obtained by reacting the composite material of the first aspect with a curing agent. The composite comprises polyvinylidene fluoride microspheres and a cured flexible matrix material; the polyvinylidene fluoride microspheres are dispersed in the cured flexible matrix material.

[0128] The flexible matrix material used in the present disclosure is essentially a polymer prepolymer. Therefore, the flexible matrix material is in a liquid state before being cured and needs to undergo a curing operation to obtain the composite of the present disclosure for later application. Specifically, the flexible matrix material and a curing agent undergo a curing reaction to obtain a cured flexible matrix material. The curing agent can be selected from one or more of methylvinylcyclosiloxane, tetra(butylene)palladium(0), a photosensitizer, acrylic acid, and a photoinitiator. Examples of photosensitizers can include, but are not limited to, Irgacure 184 and the like.

[0129] In certain embodiments, in addition to the polyvinylidene fluoride microspheres and the cured flexible matrix material, the composite may also contain a filler selected from an inorganic piezoelectric material and a conductive material. The inorganic piezoelectric material may include piezoelectric ceramics, etc. Examples of piezoelectric ceramics may include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, etc. Examples of conductive materials include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, etc. This allows the composite to contain filler inorganic piezoelectric materials and / or conductive materials, thereby improving the piezoelectric output of the composite.

[0130] The composite material disclosed herein may be a composite film. The present disclosure does not particularly limit the shape of the composite film, as long as it can be applied to a piezoelectric sensor. The composite film may be, for example, circular, rectangular, hemispherical, or the like.

[0131] In certain embodiments, the thickness of the composite film may be 400 to 1000 μm, for example, 400 μm, 450 μm, 500 μm, 550 μm, 600 μm, 650 μm, 700 μm, 750 μm, 800 μm, 850 μm, 900 μm, 950 μm, 1000 μm, etc. The flexible matrix material after curing in the present disclosure is a viscoelastic polymer, which has nonlinear mechanical properties, that is, the flexible matrix material after curing has a damping effect. Therefore, when using the same content of polyvinylidene fluoride microspheres, when the thickness of the composite is too large, the mechanical dissipation of the flexible matrix material after curing is large, and the external force applied to the polyvinylidene fluoride microspheres is too small, which is not conducive to improving the piezoelectric output of the composite; and when the thickness of the composite is too small, the polyvinylidene fluoride microspheres are densely dispersed in the flexible matrix material after curing, resulting in stress dispersion when the polyvinylidene fluoride microspheres are subjected to force, which is also not conducive to improving the piezoelectric output. A thickness of 400 to 1000 μm can ensure that the composite has a high piezoelectric output.

[0132] The Young's modulus of the composite of the present disclosure may be 0.5 to 6 MPa; in certain embodiments, 1 to 4 MPa; optionally 1.5 to 3.5 MPa; for example, 1.7 to 2.5 MPa.

[0133] <Method for preparing the composite>

[0134] In a fourth aspect, the present disclosure provides a method for preparing a composite. The method comprises the following steps: mixing raw materials including a flexible matrix material and polyvinylidene fluoride microspheres to obtain a first mixture; mixing the first mixture with a curing agent to obtain a second mixture; and casting the second mixture and then curing it to obtain the composite described in the second aspect.

[0135] In certain embodiments, the method for preparing the composite may further include a step of preparing polyvinylidene fluoride microspheres.

[0136] Step of forming a first mixture

[0137] Mixing raw materials including a flexible matrix material and polyvinylidene fluoride microspheres to obtain a first mixture. Specifically, stirring the flexible matrix material and polyvinylidene fluoride microspheres at 20-35° C. to obtain the first mixture.

[0138] In certain embodiments, the raw materials may further include fillers. The fillers may be selected from inorganic piezoelectric materials and / or conductive materials. The inorganic piezoelectric materials may include piezoelectric ceramics. Examples of piezoelectric ceramics may include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, and the like. Examples of conductive materials may include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, and the like. It is understood that when the raw materials also contain fillers, the raw materials including the flexible matrix material, the polyvinylidene fluoride microspheres, and the fillers are mixed to obtain a first mixture.

[0139] In other embodiments, the raw materials may not include fillers, that is, the flexible matrix material and polyvinylidene fluoride microspheres are mixed to obtain a first mixture.

[0140] In certain embodiments, the stirring time may be 7 to 18 hours; optionally 10 to 15 hours; for example, 12 to 13 hours.

[0141] Step of forming a second mixture

[0142] The first mixture is mixed with a curing agent to obtain a second mixture.

[0143] In certain embodiments, the curing agent may be selected from one or more of methylvinylcyclosiloxane, tetra(butenyl)palladium(0), a photosensitizer, acrylic acid, and a photoinitiator. Examples of photosensitizers may include, but are not limited to, Irgacure 184.

[0144] In certain embodiments, the mass ratio of the flexible matrix material to the curing agent can be 10:(0.2-2); optionally, 10:(0.5-1.5); for example, 10:(0.8-1.2). When the ratio of the flexible matrix material to the curing agent is too low, the modulus of the material is too low, and the prepared composite exhibits viscoelastic characteristics, resulting in increased stress dissipation and reduced piezoelectric output. When the ratio of the flexible matrix material to the curing agent is too high, bubbles may be introduced into the composite during preparation, which may reduce the modulus to a certain extent and be detrimental to improving the composite's piezoelectric output.

[0145] In certain embodiments, the first mixture and the curing agent may be mixed by stirring for a period of 2 to 30 minutes, optionally 5 to 20 minutes, for example 10 to 15 minutes.

[0146] In certain embodiments, after stirring, the second mixture may be allowed to stand for degassing. The standing time may be 15 to 60 minutes, optionally 20 to 50 minutes, for example 30 to 40 minutes.

[0147] Steps for pouring and curing

[0148] The second mixture is poured and then cured to produce a composite.

[0149] In certain embodiments, the curing temperature may be 50-100°C. Alternatively, the curing temperature may be 60-90°C. For example, the curing temperature may be 70-80°C. The curing temperature may affect the structure of the polyvinylidene fluoride microspheres and, in turn, the piezoelectric output of the composite. Therefore, curing within the above range may help improve the piezoelectric output of the composite.

[0150] In certain embodiments, the curing time may be 2 to 8 hours; optionally 3 to 6 hours; for example, 4 to 5 hours.

[0151] Steps for preparing polyvinylidene fluoride microspheres

[0152] The polyvinylidene fluoride microsphere blank is annealed at 80-200° C. to obtain the polyvinylidene fluoride microsphere. The annealing temperature and annealing time are as described above and will not be repeated here.

[0153] The preparation method of the polyvinylidene fluoride microsphere blank is specifically described above and will not be repeated here.

[0154] <Compound>

[0155] In a fifth aspect, the present disclosure provides another composite. The composite comprises polyvinylidene fluoride microspheres and a flexible matrix material; the polyvinylidene fluoride microspheres are dispersed in the flexible matrix material, which is a styrene-based block copolymer. The polyvinylidene fluoride microspheres serve as piezoelectric elements, and the styrene-based block copolymer serves as a medium for dispersing the piezoelectric elements. The polyvinylidene fluoride microspheres dispersed in the flexible matrix material exhibit high piezoelectric performance and low modulus.

[0156] In certain embodiments, the styrenic block copolymer can be selected from at least one of styrene-butadiene-styrene block copolymer (SBS), styrene-ethylene-butylene-styrene block copolymer (SEBS), hydrogenated styrene-butadiene block copolymer (SEPS), and styrene-isoprene block copolymer (SIS). These styrenic block copolymers offer advantages such as flexibility, low modulus, ease of processing, and shape adjustability, and can be used in the assembly of flexible wearable piezoelectric devices.

[0157] In certain embodiments, the particle size of the polyvinylidene fluoride microspheres is micro-nanoscale. Optionally, the particle size of the polyvinylidene fluoride microspheres is 5 to 25 μm, for example, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, 22 μm, 25 μm, etc. The interface interaction between the polyvinylidene fluoride microspheres with too large a particle size and the flexible matrix material after curing is weakened, and the stress transfer is reduced. At the same time, under the action of external force, the stress concentration effect of the polyvinylidene fluoride microspheres with too large a particle size in the flexible matrix material after curing is weakened, which is not conducive to improving the piezoelectric output. However, the micro-nanoscale, especially the polyvinylidene fluoride microspheres with a particle size of 5 to 25 μm, have a strong interface interaction with the flexible matrix material after curing, and the stress transfer is large. At the same time, under the action of external force, the stress concentration effect of the polyvinylidene fluoride microspheres with small particle size in the flexible matrix material after curing is strong, and the piezoelectric output is high.

[0158] In certain embodiments, in addition to polyvinylidene fluoride, the polyvinylidene fluoride microspheres may also contain a certain amount of inorganic piezoelectric materials and / or conductive materials. The inorganic piezoelectric materials may include piezoelectric ceramics, etc. Examples of piezoelectric ceramics may include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, etc. Examples of conductive materials may include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, etc. The purpose of introducing inorganic piezoelectric materials and / or conductive materials into polyvinylidene fluoride microspheres is to increase the piezoelectric output of the composite, wherein the inorganic piezoelectric material can increase the piezoelectric effect of the composite, and the conductive material can reduce the impedance of the composite.

[0159] In the case where the polyvinylidene fluoride microspheres also contain an inorganic piezoelectric material and / or a conductive material in addition to containing polyvinylidene fluoride, the content of polyvinylidene fluoride in the polyvinylidene fluoride microspheres can be greater than or equal to 50wt%. In certain embodiments, the content of polyvinylidene fluoride in the polyvinylidene fluoride microspheres can be greater than or equal to 70wt%. Alternatively, the content of polyvinylidene fluoride in the polyvinylidene fluoride microspheres can be greater than or equal to 95wt%. When the content of polyvinylidene fluoride in the polyvinylidene fluoride microspheres is too low, it is not conducive to improving the uniformity of the morphology and particle size of the polyvinylidene fluoride microspheres.

[0160] In certain embodiments, the polyvinylidene fluoride microspheres may not contain inorganic piezoelectric materials and / or conductive materials in addition to polyvinylidene fluoride, that is, the polyvinylidene fluoride microspheres may be composed of polyvinylidene fluoride.

[0161] In certain embodiments, the content of the polyvinylidene fluoride microspheres can be 10 to 40 parts by weight, such as 10 parts by weight, 15 parts by weight, 20 parts by weight, 25 parts by weight, 30 parts by weight, 35 parts by weight, or 40 parts by weight, based on 100 parts by weight of the flexible matrix material, the styrene block copolymer. Alternatively, the content of the polyvinylidene fluoride microspheres can be 15 to 35 parts by weight. For example, the content of the polyvinylidene fluoride microspheres can be 25 to 35 parts by weight. The content of the polyvinylidene fluoride microspheres has a certain influence on the piezoelectric properties of the composite. This may be because the polyvinylidene fluoride microspheres in the flexible matrix material styrene block copolymer can be considered as dispersed dipoles. When the polyvinylidene fluoride microspheres are below a certain doping amount, the modulus of the composite does not change significantly; with the increase of the polyvinylidene fluoride microsphere content, the dipole content increases, and under a certain external force, the low modulus leads to an increase in the additional polarization generated, resulting in a large voltage; the content of polyvinylidene fluoride microspheres continues to increase, the dipole content further increases, but the modulus of the composite increases, resulting in a decrease in the deformation of the composite under the same external force, resulting in a decrease in the additional polarization generated, and then leading to a low piezoelectric output.

[0162] The polyvinylidene fluoride microspheres can be obtained by annealing the polyvinylidene fluoride microsphere blank at 80-200° C. to obtain the polyvinylidene fluoride microspheres. This can improve the crystallinity of the polyvinylidene fluoride microspheres and enhance the piezoelectric properties of the composite.

[0163] In certain embodiments, the annealing temperature may be 90-155°C; for example, 110-150°C.

[0164] In certain embodiments, the annealing time may be 1 to 8 hours; optionally 2 to 6 hours; for example, 3 to 5 hours.

[0165] The polyvinylidene fluoride microsphere blank can be obtained by a phase separation method. Specifically, the polyvinylidene fluoride microsphere blank is obtained by dropwise adding a first solution comprising polyvinyl alcohol and a poor solvent for polyvinylidene fluoride to a stirred second solution comprising polyvinyl alcohol, polyvinylidene fluoride, and a good solvent for polyvinylidene fluoride, wherein the temperature of the second solution is 30 to 98°C.

[0166] In certain embodiments, in addition to containing polyvinyl alcohol and polyvinylidene fluoride poor solvents, the first solution may also contain inorganic piezoelectric materials and / or conductive materials. The inorganic piezoelectric materials may include piezoelectric ceramics, etc. Examples of piezoelectric ceramics may include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, etc. Examples of conductive materials include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, etc. In this way, the prepared polyvinylidene fluoride microsphere blank may contain inorganic piezoelectric materials and / or conductive materials, thereby allowing the polyvinylidene fluoride microspheres to contain inorganic piezoelectric materials and / or conductive materials.

[0167] In certain embodiments, the poor solvent for polyvinylidene fluoride may be water.

[0168] In certain embodiments, the concentration of polyvinyl alcohol in the first solution may be 5-50 mg / mL, optionally 10-40 mg / mL, for example 15-30 mg / mL. The molecular weight of polyvinyl alcohol may be 500,000-1,000,000, optionally 600,000-900,000, for example 700,000-850,000.

[0169] In some embodiments, the first solution is composed of polyvinyl alcohol and a poor solvent for polyvinylidene fluoride. The polyvinylidene fluoride microsphere blank prepared in this way does not contain other components such as inorganic piezoelectric materials and / or conductive materials, and thus the polyvinylidene fluoride microspheres also do not contain other components such as inorganic piezoelectric materials and / or conductive materials.

[0170] In certain embodiments, the good solvent for polyvinylidene fluoride may be selected from N,N-dimethylformamide and / or N,N-dimethylacetamide.

[0171] In certain embodiments, the concentration of polyvinylidene fluoride in the second solution may be 20-60 mg / mL, optionally 30-50 mg / mL, for example 40-45 mg / mL. The molecular weight of polyvinylidene fluoride may be 350,000-700,000, optionally 400,000-600,000, for example 500,000-550,000.

[0172] In certain embodiments, the concentration of polyvinyl alcohol in the second solution may be 5-50 mg / mL, optionally 10-40 mg / mL, for example 15-30 mg / mL. The molecular weight of polyvinyl alcohol may be 500,000-1,000,000, optionally 600,000-900,000, for example 700,000-850,000.

[0173] In certain embodiments, the second solution consists of polyvinyl alcohol, polyvinylidene fluoride, and a good solvent for polyvinylidene fluoride.

[0174] In certain embodiments, the rotation speed of the stirring device for stirring the second solution may be 150 to 700 rpm; optionally 200 to 500 rpm; for example, 300 to 400 rpm.

[0175] In certain embodiments, the temperature of the second solution may be 30-98°C; optionally 50-98°C; for example 80-95°C.

[0176] In certain embodiments, the volume ratio of the first solution to the second solution may be (1.5-5):1; optionally (2-4):1; for example (2.5-3.5):1.

[0177] In certain embodiments, the method for preparing polyvinylidene fluoride microspheres may further include the following steps: continuing stirring for 7 to 18 hours after the first solution is added; optionally, stirring for 10 to 15 hours; for example, stirring for 12 to 13 hours to obtain solid polyvinylidene fluoride; then washing the solid polyvinylidene fluoride and then drying to obtain polyvinylidene fluoride microspheres.

[0178] In certain embodiments, the solid polyvinylidene fluoride may be washed with water first and then with alcohol, wherein the alcohol may be selected from one or more of methanol, ethanol, propanol, and isopropanol.

[0179] In certain embodiments, the composite may further include fillers in addition to polyvinylidene fluoride microspheres and a flexible matrix material styrene block copolymer. The filler may be selected from inorganic piezoelectric materials and / or conductive materials. Inorganic piezoelectric materials may include piezoelectric ceramics, etc. Examples of piezoelectric ceramics include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, etc. Examples of conductive materials include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, etc. The purpose of introducing inorganic piezoelectric materials and / or conductive materials into the composite is to increase the piezoelectric output of the composite, wherein the inorganic piezoelectric material can increase the piezoelectric effect of the composite, and the conductive material can reduce the impedance of the composite.

[0180] When the composite contains fillers in addition to polyvinylidene fluoride microspheres and a flexible matrix material, the amount of filler can be less than or equal to 30% of the mass of the flexible matrix material, the styrene block copolymer. In certain embodiments, the amount of filler is less than or equal to 20% of the mass of the flexible matrix material, the styrene block copolymer. In other embodiments, the amount of filler is less than or equal to 10% of the mass of the flexible matrix material, the styrene block copolymer. When the amount of filler is too high, on the one hand, it is not conducive to improving the flexibility of the composite; on the other hand, if the content of polyvinylidene fluoride microspheres in the composite is too low, the stress on the polyvinylidene fluoride microspheres is reduced due to the damping effect of the flexible matrix material, the styrene block copolymer, which is not conducive to improving the piezoelectric output of the composite.

[0181] In certain embodiments, the composite can be free of fillers in addition to the polyvinylidene fluoride microspheres and the flexible matrix material. Specifically, the composite material consists of the polyvinylidene fluoride microspheres and the flexible matrix material, a styrenic block copolymer. This composite material achieves excellent piezoelectric properties without the use of inorganic piezoelectric materials. This is likely because the piezoelectric polymer has a low modulus. Under external force, the large deformation of the composite causes the polyvinylidene fluoride microspheres to generate more additional polarization, resulting in excellent piezoelectric properties.

[0182] The composite material disclosed herein may be a composite film. The present disclosure does not particularly limit the shape of the composite film, as long as it can be applied to a piezoelectric sensor. The composite film may be, for example, circular, rectangular, hemispherical, or the like.

[0183] In certain embodiments, the thickness of the composite film can be 50 to 600 μm, for example, 50 μm, 80 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, 500 μm, 550 μm, 600 μm, etc. Alternatively, the thickness of the composite film can be 150 to 500 μm. The above thickness can ensure that the composite has a high piezoelectric output. The flexible matrix material styrene block copolymer in the present disclosure is a viscoelastic polymer having nonlinear mechanical properties, that is, the flexible matrix material styrene block copolymer has a damping effect. Therefore, when using the same content of polyvinylidene fluoride microspheres, when the thickness of the composite is too large, the mechanical dissipation of the flexible matrix material styrene block copolymer is large, and the external force exerted on the polyvinylidene fluoride microspheres is too small, which is not conducive to improving the piezoelectric output of the composite; and when the thickness of the composite is too small, the polyvinylidene fluoride microspheres are densely dispersed in the flexible matrix material styrene block copolymer, resulting in stress dispersion when the polyvinylidene fluoride microspheres are subjected to force, which is also not conducive to improving the piezoelectric output.

[0184] The Young's modulus of the composite of the present disclosure may be 0.5 to 6 MPa; in certain embodiments, 1 to 4 MPa; optionally 1.5 to 3.5 MPa; for example, 1.7 to 2.5 MPa.

[0185] <Method for preparing the composite>

[0186] In a sixth aspect, the present disclosure provides a method for preparing a composite. The method comprises the following steps: dissolving a flexible matrix material, a styrene block copolymer, in an organic solvent to obtain a dispersed solution; mixing raw materials including the dispersed solution and polyvinylidene fluoride microspheres to obtain a mixed solution; casting the mixed solution, and then drying to obtain the composite of the fifth aspect.

[0187] The styrene block copolymer used in the preparation method of the sixth aspect is a solid raw material, so it needs to be dissolved in an organic solvent to obtain a dispersed solution, and then the raw materials including the dispersed solution and polyvinylidene fluoride microspheres are mixed to obtain a mixed solution. Finally, after the organic solvent in the mixed solution evaporates, the composite of the fifth aspect can be obtained.

[0188] In certain embodiments, the method for preparing the composite may further include a step of preparing polyvinylidene fluoride microspheres.

[0189] Step of forming a dispersed solution

[0190] The flexible matrix material, a styrene block copolymer, is dissolved in an organic solvent to obtain a dispersion solution. Specifically, the flexible matrix material, a styrene block copolymer, and the organic solvent are mixed at 50 to 100° C., for example, 50° C., 60° C., 70° C., 80° C., 90° C., 100° C., etc., to obtain a dispersion solution.

[0191] In certain embodiments, the organic solvent may be at least one of toluene, xylene, acetone, and n-hexane, etc. The organic solvents listed above can well dissolve the flexible matrix material styrene block copolymer.

[0192] In certain embodiments, the mass concentration of the flexible matrix material, the styrene-based block copolymer, in the dispersion solution can be 100 to 160 mg / L, for example, 100 mg / L, 110 mg / L, 120 mg / L, 130 mg / L, 140 mg / L, 150 mg / L, 160 mg / L, etc. If this mass concentration is too low, the organic solvent takes too long to evaporate, which is not conducive to improving production efficiency. If this mass concentration is too high, the dispersion solution becomes too viscous, which is not conducive to uniform dispersion of the polyvinylidene fluoride microspheres.

[0193] Steps for forming a mixed solution

[0194] The raw materials including the dispersion solution and the polyvinylidene fluoride microspheres are mixed to obtain a mixed solution.

[0195] In certain embodiments, the raw materials may further include fillers. The fillers may be selected from inorganic piezoelectric materials and / or conductive materials. The inorganic piezoelectric materials may include piezoelectric ceramics. Examples of piezoelectric ceramics may include, but are not limited to, one or more of barium titanate, lead zirconate titanate, zinc oxide, and the like. Examples of conductive materials may include, but are not limited to, one or more of silver nanowires, carbon nanotubes, graphene, graphene oxide, and the like. It is understood that when the raw materials also contain fillers, the raw materials including the dispersed solution, polyvinylidene fluoride microspheres, and the fillers are mixed to obtain a mixed solution.

[0196] In other embodiments, the raw materials may not include fillers, that is, the dispersed solution and polyvinylidene fluoride microspheres are mixed to obtain a mixed solution.

[0197] Steps in complex formation

[0198] The mixed solution is cast and then dried to obtain the composite of the fifth aspect.

[0199] The drying temperature of the preparation method of the sixth aspect is not particularly limited, as long as the organic solvent can be evaporated to obtain the composite. In some embodiments, the drying temperature can be 60-100°C, for example, 60°C, 70°C, 80°C, 90°C, 100°C, etc.

[0200] Steps for preparing polyvinylidene fluoride microspheres

[0201] The polyvinylidene fluoride microsphere blank is annealed at 80-200° C. to obtain the polyvinylidene fluoride microsphere. The annealing temperature and annealing time are as described above and will not be repeated here.

[0202] The preparation method of the polyvinylidene fluoride microsphere blank is specifically described above and will not be repeated here.

[0203] <Application of the compound>

[0204] In a seventh aspect, the present disclosure provides use of the composite material of the first aspect, the composite material of the third aspect, or the composite material of the fifth aspect in the preparation of a piezoelectric sensor. The composite material of the present disclosure exhibits high piezoelectric performance and low modulus. Therefore, the composite material of the present disclosure can be used to prepare piezoelectric sensors, particularly those suitable for preparing flexible piezoelectric sensors.

[0205] According to different application scenarios, piezoelectric sensors can be divided into piezoelectric pressure sensors, piezoelectric strain sensors, piezoelectric vibration sensors, and piezoelectric acceleration sensors. The composite disclosed in the present invention can be applied to various piezoelectric sensors mentioned above.

[0206] The working principle of the piezoelectric pressure sensor is: applying external force in the vertical direction causes the piezoelectric sensor to 33 Piezoelectric output in the direction. Piezoelectric pressure sensors are mainly used for dynamic pressure monitoring, such as pulse, blood pressure, etc., and can also be used in smart home applications, such as anti-theft monitoring.

[0207] The working principle of the piezoelectric strain sensor is: applying a tensile force in the horizontal direction causes the piezoelectric sensor to 31 Piezoelectric output in the direction. Piezoelectric pressure sensors are mainly used in artificial intelligence, gesture recognition, monitoring of human joint movement and automobile bearings.

[0208] Piezoelectric vibration sensors operate on the principle that mechanical vibrations exert stress on the sensor, creating an uneven charge distribution in the piezoelectric material and generating an electric potential. These sensors can be used to monitor instrument vibrations, such as those used to monitor the health of bridges and highways, and can also be used for sound recognition.

[0209] The operating principle of a piezoelectric accelerometer: Changes in acceleration cause changes in the force applied to the piezoelectric sensor, which in turn produces changes in the piezoelectric potential. Piezoelectric accelerometers are used in industrial, aerospace, and medical fields, for example, to monitor vibration in industrial machinery, detect vibration in aircraft and satellites, and monitor recovery from sports injuries.

[0210] <Piezoelectric Sensor>

[0211] In an eighth aspect, the present disclosure provides a piezoelectric sensor, which includes the composite of the third aspect or the composite of the fifth aspect, a first electrode, a second electrode, and a flexible packaging case.

[0212] The composite has a first surface and a second surface disposed opposite to each other.

[0213] The first electrode is attached to the first surface of the composite. The first electrode can be selected from copper foil or aluminum foil. A wire is provided on the first electrode.

[0214] The second electrode is attached to the second surface of the composite. The second electrode can be selected from copper foil or aluminum foil. A wire is provided on the second electrode.

[0215] The first electrode, the second electrode, and the composite are located in a flexible encapsulation shell. The wires are led out of the flexible encapsulation shell. The flexible encapsulation shell can be formed of one or more materials such as thermoplastic polyurethane elastomer, polyimide tape, and polydimethylsiloxane.

[0216] The present disclosure is described in detail below with reference to embodiments.

[0217] The main raw materials used in the examples and comparative examples are as follows: polyvinylidene fluoride (PVDF) with a molecular weight of 530,000, polyvinyl alcohol (PVA) with a molecular weight of 820,000, and methylvinylcyclosiloxane as the curing agent.

[0218] Example 1

[0219] Polyvinyl alcohol and water were mixed to obtain a first solution with a polyvinyl alcohol concentration of 20 mg / mL.

[0220] Polyvinylidene fluoride and polyvinyl alcohol were added to the N,N-dimethylacetamide solution to obtain a second solution. In the second solution, the concentration of polyvinylidene fluoride was 40 mg / mL, and the concentration of polyvinyl alcohol was 20 mg / mL.

[0221] The first solution was added dropwise to the second solution, which was stirred at 95°C. The stirring device for the second solution was set at 300 rpm. The volume ratio of the first solution to the second solution was 3:1. After the first solution was added, stirring was continued for 12 hours to obtain solid polyvinylidene fluoride. The solid polyvinylidene fluoride was washed three times with deionized water and then three times with ethanol, and dried to obtain polyvinylidene fluoride microspheres.

[0222] The polyvinylidene fluoride microsphere blank was annealed in air at 150° C. for 4 h to obtain polyvinylidene fluoride microspheres with a particle size of 5 μm.

[0223] 100 parts by weight of polydimethylsiloxane and 30 parts by weight of polyvinylidene fluoride microspheres were stirred at 25°C for 12 hours to produce a first mixture. The first mixture was then stirred with 10 parts by weight of a curing agent for 10 minutes and allowed to stand for 30 minutes to produce a second mixture. The second mixture was poured into a mold and then cured at 80°C for 4 hours to produce a composite film with a thickness of 900 μm.

[0224] Example 2

[0225] The method of this embodiment is the same as that of embodiment 1, except that 10 parts by weight of polyvinylidene fluoride microspheres are used.

[0226] Example 3

[0227] The method of this embodiment is the same as that of embodiment 1, except that 20 parts by weight of polyvinylidene fluoride microspheres are used.

[0228] Example 4

[0229] The method of this embodiment is the same as that of embodiment 1, except that 40 parts by weight of polyvinylidene fluoride microspheres are used.

[0230] Example 5

[0231] The method of this embodiment is the same as that of embodiment 2, except that 20 parts by weight of curing agent is used.

[0232] Example 6

[0233] This embodiment is consistent with embodiment 5, except that the particle size of the polyvinylidene fluoride microspheres is 25 μm.

[0234] Example 7

[0235] The preparation method of polyvinylidene fluoride microspheres is the same as that in Example 1.

[0236] Styrene-butadiene-styrene block copolymer (SBS) was dissolved in an organic solvent instead of polydimethylsiloxane to obtain a dispersion solution with a mass concentration of 150 mg / L. The dispersion solution was then mixed with polyvinylidene fluoride microspheres to obtain a mixed solution. The mixed solution was poured into a mold and then dried at 80°C to obtain a composite film with a thickness of 200 μm.

[0237] Example 8

[0238] The method of this embodiment is the same as that of embodiment 7, except that styrene-ethylene-butylene-styrene block copolymer (SEBS) is used instead of styrene-butadiene-styrene block copolymer (SBS).

[0239] Example 9

[0240] The method of this embodiment is the same as that of Example 7, except that hydrogenated styrene-butadiene block copolymer (SEPS) is used instead of styrene-butadiene-styrene block copolymer (SBS).

[0241] Example 10

[0242] The method of this embodiment is the same as that of Example 7, except that styrene-isoprene block copolymer (SIS) is used instead of styrene-butadiene-styrene block copolymer (SBS).

[0243] Example 11

[0244] The method of this embodiment is the same as that of Example 1, except that 60 parts by weight of polydimethylsiloxane (PDMS) and 40 parts by weight of silicone rubber (ecoflex) are used instead of 100 parts by weight of polydimethylsiloxane (PDMS) in Example 1.

[0245] Example 12

[0246] This embodiment is consistent with embodiment 2, except that the thickness of the composite film is 660 μm.

[0247] Example 13

[0248] This embodiment is consistent with embodiment 5, except that the thickness of the composite film is 450 μm.

[0249] Example 14

[0250] The method of this embodiment is the same as that of embodiment 1, except that 100 parts by weight of polydimethylsiloxane, 30 parts by weight of polyvinylidene fluoride microspheres, and 6 parts by weight of filler are stirred at 25° C. for 12 hours to obtain a first mixture, wherein the filler is barium titanate.

[0251] Example 15

[0252] The method of this embodiment is the same as that of embodiment 1, except that 100 parts by weight of polydimethylsiloxane, 30 parts by weight of polyvinylidene fluoride microspheres and 0.2 parts by weight of filler are stirred at 25° C. for 12 hours to obtain a first mixture, wherein the filler is carbon nanotubes.

[0253] Example 16

[0254] The method of this embodiment is the same as that of Example 1, except that 100 parts by weight of polydimethylsiloxane, 30 parts by weight of polyvinylidene fluoride microspheres, and 5 parts by weight of filler are stirred at 25° C. for 12 hours to obtain a first mixture, wherein the filler consists of 4.8 parts by weight of barium titanate and 0.2 parts by weight of carbon nanotubes.

[0255] Comparative Example 1

[0256] 100 parts by weight of polydimethylsiloxane and 10 parts by weight of a curing agent were stirred for 10 minutes and then allowed to stand for 30 minutes to obtain a mixture. The mixture was poured into a mold and then cured at 80°C for 4 hours to obtain a polymer material. The polymer material was a polymer film.

[0257] Comparative Example 2

[0258] The method of this embodiment is the same as that of embodiment 1, except that polyvinylidene fluoride purchased from Sigma Aldrich is dissolved in DMF solvent, and a polyvinylidene fluoride film with a thickness of 50 μm is prepared by solution casting instead of polyvinylidene fluoride microspheres.

[0259] Experimental example

[0260] 1. Test the modulus using the following method:

[0261] The film products prepared in each Example and Comparative Example were clamped in the upper chuck of a universal stretching machine. The lower chuck was then moved to the appropriate clamping position to secure the lower end of the film product. The universal stretching machine was started, and a tensile force was applied to the film product until the sample broke. The stress-strain data of the film product was then read. The linear portion of the stress-strain curve was used to calculate the Young's modulus of the film product. The results are shown in Table 1.

[0262] Table 1

[0263] 2. The piezoelectric properties of the membrane products of each embodiment and comparative example were tested using the following method:

[0264] The membranes prepared in Examples 1-6 and 12-13 were subjected to oil bath contact polarization for 1 hour under a 15 kV electric field. Conductive copper foil was then cut into 1.5 x 1.5 cm pieces and placed on the upper and lower surfaces of the membranes as electrodes. External connection electrodes were then prepared by connecting copper wires with silver paste. Finally, the piezoelectric nanogenerator was encapsulated in thermoplastic polyurethane. The piezoelectric performance of the membrane in Example 14 was tested using the same method as described above, except that a contact polarization voltage of 13 kV and a contact polarization time of 1.5 hours were used.

[0265] The piezoelectric properties of the membrane products of Examples 7 to 11 were tested using the same method as in Example 1, except that corona poling was used with a voltage of 13.5 kV and a time of 0.5 h. This is because the membrane products of these examples are thin, and contact polarization is prone to breakdown, so corona poling was used.

[0266] The piezoelectric properties of the membrane products of Examples 15-16 were tested using the same method as in Example 1, except that corona poling was used with a voltage of 13.5 kV and a duration of 0.5 h. This is because the membrane products of Examples 15-16 contain carbon nanotubes, which increase their electrical conductivity. Contact polarization results in a high leakage current, making contact polarization unsuitable, so corona poling was used.

[0267] The assembled piezoelectric nanogenerators from the membrane products of each Example and Comparative Example were placed on a test platform. A linear motor was used to strike the piezoelectric nanogenerator, setting the motor's output force to 20 N. Piezoelectric data were collected using a 6517 electrometer. Each piezoelectric nanogenerator was recorded for 10 seconds. The output voltages of the membrane products from each Example and Comparative Example are shown in Table 2.

[0268] Table 2

[0269] Figures 1 to 15 are graphs of piezoelectric data collected under the test conditions disclosed herein for piezoelectric nanogenerators prepared using different membrane products. Figure 1 is a graph showing the piezoelectric performance of the membrane product obtained in Example 1. Figure 2 is a graph showing the piezoelectric performance of the membrane product obtained in Example 2. Figure 3 is a graph showing the piezoelectric performance of the membrane product obtained in Example 3. Figure 4 is a graph showing the piezoelectric performance of the membrane product obtained in Example 4. Figure 5 is a graph showing the piezoelectric performance of the membrane product obtained in Example 5. Figure 6 is a graph showing the piezoelectric performance of the membrane product obtained in Example 6. Figure 7 is a graph showing the piezoelectric performance of the membrane products obtained in Examples 7 to 10. Figure 8 is a graph showing the piezoelectric performance of the membrane product obtained in Example 11. Figure 9 is a graph showing the piezoelectric performance of the membrane product obtained in Example 12. Figure 10 is a graph showing the piezoelectric performance of the membrane product obtained in Example 13. Figure 11 is a graph showing the piezoelectric performance of the membrane product obtained in Example 14. Figure 12 is a graph showing the piezoelectric performance of the membrane product obtained in Example 15. Figure 13 is a graph showing the piezoelectric performance of the membrane product obtained in Example 16. Figure 14 is a graph showing the piezoelectric performance of the membrane product obtained in Comparative Example 1. Figure 15 is a graph showing the piezoelectric performance of the membrane product obtained in Comparative Example 2.

[0270] As can be seen from Figures 1 and 15, under the same processing conditions, the membrane product of Example 1 exhibits a higher piezoelectric output response, while the membrane product of Comparative Example 1 has a low piezoelectric output. It is worth noting that the membrane product of Comparative Example 1 is a non-piezoelectric material. Therefore, the output voltage of the membrane product of Comparative Example 1 is not due to the piezoelectric effect, but rather to the electret effect caused by polarization, which will gradually dissipate over time. In contrast, the higher voltage output of the membrane product of Example 1 than that of the membrane product of Comparative Example 1 is due to the piezoelectric effect of the polyvinylidene fluoride microspheres. Similarly, the output voltage of the membrane product of Comparative Example 2 is higher than that of the membrane product of Comparative Example 1, and the higher voltage is due to the piezoelectric effect of the polyvinylidene fluoride film.

[0271] The present disclosure is not limited to the above-mentioned embodiments. Without departing from the essential content of the present disclosure, any deformation, improvement, and substitution that can be conceived by those skilled in the art shall fall within the scope of the present disclosure.

Claims

1. A composite material, comprising polyvinylidene fluoride microspheres and a flexible matrix material; the polyvinylidene fluoride microspheres are dispersed in the flexible matrix material.

2. The composite material according to claim 1, wherein Based on 100 parts by weight of the flexible matrix material, the content of the polyvinylidene fluoride microspheres is 10 to 40 parts by weight.

3. The composite material according to claim 1 or 2, wherein: The flexible matrix material is selected from one or more of polydimethylsiloxane, styrene block copolymer, polyurethane thermoplastic elastomer and silicone rubber.

4. The composite material according to any one of claims 1 to 3, wherein The particle size of the polyvinylidene fluoride microspheres is micro-nano level.

5. The composite material according to claim 4, wherein The particle size of the polyvinylidene fluoride microspheres is 5 to 25 μm.

6. The composite material according to any one of claims 1 to 5, wherein: The polyvinylidene fluoride microspheres are obtained by the following method: annealing the polyvinylidene fluoride microsphere blank at 80-200° C. to obtain the polyvinylidene fluoride microspheres.

7. The composite material according to claim 6, wherein The polyvinylidene fluoride microsphere blank is obtained by a phase separation method.

8. The composite material according to claim 7, wherein The polyvinylidene fluoride microsphere blank is obtained by the following method: A first solution including polyvinyl alcohol and a poor solvent of polyvinylidene fluoride is added dropwise to a second solution including polyvinyl alcohol, polyvinylidene fluoride and a good solvent of polyvinylidene fluoride which is being stirred, wherein the temperature of the second solution is 30 to 98°C.

9. The composite material according to claim 8, wherein The poor solvent for polyvinylidene fluoride is water, and the good solvent for polyvinylidene fluoride is selected from N,N-dimethylformamide and / or N,N-dimethylacetamide.

10. The composite material according to claim 8, wherein The first solution also contains an inorganic piezoelectric material and a conductive material.

11. The composite material according to claim 10, wherein Taking the total mass of the polyvinylidene fluoride microspheres as 100%, the content of polyvinylidene fluoride is greater than or equal to 50wt%.

12. The composite material according to claim 11, wherein Taking the total mass of the polyvinylidene fluoride microspheres as 100%, the content of polyvinylidene fluoride is greater than or equal to 70wt%.

13. The composite material according to claim 12, wherein: Taking the total mass of the polyvinylidene fluoride microspheres as 100%, the content of polyvinylidene fluoride is greater than or equal to 95wt%.

14. The composite material according to any one of claims 1 to 13, wherein: The composite material further comprises a filler, and the filler is selected from an inorganic piezoelectric material and / or a conductive material.

15. The composite material according to claim 14, wherein The amount of the filler is less than or equal to 30wt% of the mass of the flexible matrix material.

16. The composite material according to claim 15, wherein The amount of the filler is less than or equal to 20wt% of the mass of the flexible matrix material.

17. The composite material according to claim 16, wherein The amount of the filler is less than or equal to 10 wt % of the mass of the flexible matrix material.

18. The method for preparing a composite material according to any one of claims 1 to 17, comprising the following steps: The raw materials including the flexible matrix material and the polyvinylidene fluoride microspheres are mixed to obtain a first mixture, wherein the first mixture is the composite material.

19. The preparation method according to claim 18, wherein: The curing agent is selected from one or more of methylvinyl cyclosiloxane, tetra(butenyl)palladium(0), photosensitizer, acrylic acid and photoinitiator.

20. The preparation method according to claim 18 or 19, wherein: The raw material also includes the filler.

21. A composite material obtained by reacting the composite material according to any one of claims 1 to 17 with a curing agent.

22. The composite according to claim 21, wherein The thickness of the composite is 400-1000 μm.

23. The method for preparing the composite according to claim 21, comprising the following steps: Mixing raw materials including the flexible matrix material and the polyvinylidene fluoride microspheres to obtain a first mixture; The first mixture is mixed with the curing agent to obtain a second mixture; the second mixture is poured and then cured to obtain the composite.

24. The preparation method according to claim 23, wherein: The curing agent is selected from one or more of methylvinyl cyclosiloxane, tetra(butenyl)palladium(0), photosensitizer, acrylic acid and photoinitiator.

25. The preparation method according to claim 23 or 24, wherein: The mass ratio of the flexible matrix material to the curing agent is 10:(0.2-2).

26. The preparation method according to any one of claims 23 to 25, wherein: The curing temperature is 60-95°C.

27. A composite material, comprising polyvinylidene fluoride microspheres and a flexible matrix material; the polyvinylidene fluoride microspheres are dispersed in the flexible matrix material, and the flexible matrix material is a styrene block copolymer.

28. The composite according to claim 27, wherein Based on 100 parts by weight of the flexible matrix material, the content of the polyvinylidene fluoride microspheres is 10 to 40 parts by weight.

29. The complex according to claim 27 or 28, wherein The styrene block copolymer is at least one selected from styrene-butadiene-styrene block copolymer, styrene-ethylene-butylene-styrene block copolymer, hydrogenated styrene-butadiene block copolymer and styrene-isoprene block copolymer.

30. The composite according to any one of claims 27 to 29, wherein The particle size of the polyvinylidene fluoride microspheres is micro-nano level.

31. The composite according to claim 30, wherein The particle size of the polyvinylidene fluoride microspheres is 5 to 25 μm.

32. The composite according to any one of claims 27 to 31, wherein The polyvinylidene fluoride microspheres are obtained by the following method: annealing the polyvinylidene fluoride microsphere blank at 80-200° C. to obtain the polyvinylidene fluoride microspheres.

33. The composite according to claim 32, wherein The polyvinylidene fluoride microsphere blank is obtained by a phase separation method.

34. The composite according to claim 33, wherein The polyvinylidene fluoride microsphere blank is obtained by the following method: The first solution including polyvinyl alcohol and polyvinylidene fluoride poor solvent is added dropwise to the stirring solution including polyvinyl alcohol, polyvinylidene fluoride, In the second solution of vinylidene fluoride and polyvinylidene fluoride good solvent, the temperature of the second solution is 30-98°C.

35. The composite according to claim 34, wherein The poor solvent for polyvinylidene fluoride is water, and the good solvent for polyvinylidene fluoride is selected from N,N-dimethylformamide and / or N,N-dimethylacetamide.

36. The composite according to claim 34, wherein The first solution also contains an inorganic piezoelectric material and a conductive material.

37. The composite according to claim 36, wherein Taking the total mass of the polyvinylidene fluoride microspheres as 100%, the content of polyvinylidene fluoride is greater than or equal to 50wt%.

38. The composite according to claim 37, wherein Taking the total mass of the polyvinylidene fluoride microspheres as 100%, the content of polyvinylidene fluoride is greater than or equal to 70wt%.

39. The composite according to claim 38, wherein Taking the total mass of the polyvinylidene fluoride microspheres as 100%, the content of polyvinylidene fluoride is greater than or equal to 95wt%.

40. The composite according to any one of claims 27 to 39, wherein The composite further comprises a filler selected from an inorganic piezoelectric material and / or a conductive material.

41. The composite according to claim 40, wherein The amount of the filler is less than or equal to 30 wt % of the mass of the styrene block copolymer.

42. The composite according to claim 41, wherein The amount of the filler is less than or equal to 20 wt % of the mass of the styrene block copolymer.

43. The composite according to claim 42, wherein The amount of the filler is less than or equal to 10 wt % of the mass of the styrene block copolymer.

44. The composite according to any one of claims 27 to 43, wherein The thickness of the composite is 50-600 μm.

45. A method for preparing a composite according to any one of claims 25 to 44, comprising the following steps: The styrene block copolymer is dissolved in an organic solvent to obtain a dispersed solution; raw materials including the dispersed solution and the polyvinylidene fluoride microspheres are mixed to obtain a mixed solution; the mixed solution is cast and then dried to obtain the composite.

46. ​​The preparation method according to claim 45, wherein The organic solvent is selected from at least one of toluene, xylene, acetone and n-hexane.

47. The preparation method according to claim 45 or 46, wherein: The mass concentration of the styrene block copolymer in the dispersed solution is 100 to 160 mg / L.

48. The preparation method according to any one of claims 45 to 47, wherein: The raw material also includes the filler.

49. Use of the composite material according to any one of claims 1 to 17, the composite according to claim 21 or 22, or the composite according to any one of claims 27 to 44 in the preparation of a piezoelectric sensor.

50. A piezoelectric sensor, comprising a first electrode, a second electrode, the composite of claim 21 or 22 or the composite of any one of claims 27 to 44, and a flexible encapsulating shell; The composite has a first surface and a second surface disposed opposite to each other; The first electrode is attached to the first surface of the composite, the second electrode is attached to the second surface of the composite, and the first electrode and the second electrode are respectively provided with wires; The first electrode, the second electrode and the composite are located in the flexible packaging shell, and the wire is led out from the flexible packaging shell.

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