A self-assembled ultra-flexible piezoelectric sensor and its preparation method and application

Abstract

The present invention relates to a self-assembled ultra-flexible piezoelectric sensor, a preparation method thereof and an application. The preparation method comprises the following steps: (1) preparing a BaTiO3 precursor solution; (2) preparing a barium titanate nanocube dispersion: after subjecting the BaTiO3 precursor solution to a hydrothermal reaction, barium titanate nanocubes are obtained and dispersed in a dispersant to obtain a barium titanate nanocube dispersion; (3) fabricating a BaTiO3 / GFF composite material; (4) fabricating a BaTiO3 / GFF-PVDF composite film; (5) connecting and encapsulating the BaTiO3 / GFF-PVDF composite film by using a double-sided conductive copper tape, and then a self-assembled ultra-flexible piezoelectric sensor can be obtained. This flexible sensor is used for handwriting intelligent recognition or an intelligent keyboard. Compared with the prior art, due to the use of the self-assembly technology in the present invention, the device can achieve excellent piezoelectric performance without high-temperature sintering, and simultaneously has the advantages of excellent mechanical properties and the ability to adhere to any curved surface, etc.

Description

Technical Field

[0001] The present invention relates to the field of piezoelectric sensors, and in particular to a self-assembled ultra-flexible piezoelectric sensor and a preparation method and application thereof. Background Art

[0002] In recent years, self-powered piezoelectric sensors have attracted widespread attention in the implementation of various technologies in the Internet of Things, big data analysis, and smart cities.

[0003] However, existing piezoelectric sensors cannot combine excellent piezoelectric properties, mechanical properties, and ultra-high flexibility to attach to any curved surface. Piezoelectric ceramics that require complex high-temperature sintering processes are mostly rigid, which restricts their application in the field of flexible wearable piezoelectric sensors. Although some people have proposed that barium titanate ceramics with a certain degree of flexibility can be prepared by electrospinning or 3D interconnected piezoelectric ceramic foam, the brittleness of barium titanate ceramic fibers themselves cannot be avoided.

[0004] Organic piezoelectric materials, such as polyvinylidene fluoride (PVDF) and its copolymers, have excellent flexibility and biocompatibility, but their low piezoelectric coefficient limits the performance of piezoelectric sensors. Organic-inorganic piezoelectric composites have good piezoelectric properties and flexibility, but there is usually a serious elastic modulus mismatch problem at their organic-inorganic interface, and the inorganic piezoelectric phase is discontinuous in the matrix, making it difficult to improve performance.

[0005] Recently, the technology of transferring piezoelectric films to flexible organic substrates by laser lift-off, standard micromachining and soft lithography has also been used to prepare flexible piezoelectric devices. Obviously, the two-step substrate transfer process achieves flexibility at the cost of high energy consumption and complex process. Summary of the invention

[0006] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a self-assembled ultra-flexible piezoelectric sensor having excellent piezoelectric properties, mechanical properties and the ability to be attached to any curved surface, as well as a preparation method and application thereof.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] The inventors learned that self-assembly is a ubiquitous phenomenon in nature, widely present in polymer materials and biomaterials, and builds a bridge between the nano world and the macro world. Self-assembly refers to the tightly arranged two-dimensional ordered molecular layer formed by the spontaneous adsorption of nanoparticles on the solid surface in solution or gas phase. Therefore, it has the characteristics of spontaneous formation in situ, highly ordered arrangement of bonds, few defects, and strong binding force. The process of spontaneous organization of nanomaterials into highly ordered structures determines that the self-assembly process does not require a high-energy consumption and time-consuming high-temperature sintering stage, so it is very suitable for flexible substrates with poor temperature resistance. The self-assembly method has been applied to the preparation process of electronic devices such as micro supercapacitors, flexible non-volatile memories, fuel-sensitized solar cells, and friction nanogenerators, but its application in the field of pressure sensors is still relatively rare. Taking into account the above facts, the use of self-assembly technology to prepare flexible piezoelectric sensors is environmentally friendly because it avoids the high-temperature sintering process and has a very large market application potential, so the following specific scheme is proposed:

[0009] A method for preparing a self-assembled ultra-flexible piezoelectric sensor, the method comprising the following steps:

[0010] (1) preparing a BaTiO3 precursor solution: mixing a barium salt, a titanium source, and an alkali with a solvent to prepare a BaTiO3 precursor solution;

[0011] (2) preparing a barium titanate nanocube dispersion: subjecting a BaTiO3 precursor solution to a hydrothermal reaction to obtain barium titanate nanocubes, and dispersing the barium titanate nanocubes in a dispersant to obtain a barium titanate nanocube dispersion;

[0012] (3) Preparation of BaTiO3 / GFF composite material: glass fiber cloth (GFF) is immersed in a dispersion of barium titanate nanocubes, and after the solvent evaporates, annealing heat treatment is performed to obtain a BaTiO3 / GFF composite material;

[0013] (4) Preparation of BaTiO3 / GFF-PVDF composite film: The BaTiO3 / GFF composite material is sandwiched between PVDF layers to obtain a BTS-GFF / PVDF piezoelectric composite material;

[0014] (5) The BaTiO3 / GFF-PVDF composite film is packaged using double-sided conductive copper tape to obtain a self-assembled ultra-flexible piezoelectric sensor.

[0015] The glass fiber fabric used is a high temperature resistant electronic grade glass fiber fabric, which is used in the self-assembly process of 10nm barium titanate nanocubes. Glass fiber fabric (GFF, Honghe Electronic Materials Co., Ltd.) is composed of hundreds of glass fibers with a diameter of 4-10μm integrated into a glass fiber bundle, and then a large number of glass fiber bundles are cross-woven to form a mesh structure. There are three reasons for using GFF as a substrate: first, because the surface of GFF has abundant hydroxyl groups, it is an excellent substrate for the self-assembly of 10nm BaTiO3 nanocubes; second, the layered structure of GFF plays a good role in the transfer of stress in the piezoelectric composite film, and the expanded strain / stress transfer can effectively relieve stress concentration, thereby improving the robustness and durability of the equipment; third, after barium titanate grows into a continuous phase on GFF, the overall mechanical properties of the composite material can be improved with the help of the glass fiber network with excellent mechanical properties. The improved mechanical strength can effectively improve the sensitivity of the piezoelectric sensor composed of BaTiO3 / GFF-PVDF composite film to external forces.

[0016] Using PVDF to coat the material can fill the tiny gaps in the composite material, and can also protect BaTiO3 / GFF and enhance the fatigue resistance of BaTiO3 / GFF.

[0017] Furthermore, the specific steps of step (1) include:

[0018] (1-1) Under stirring, oleic acid and oleylamine are added to 1-butanol to obtain a clear solution A;

[0019] (1-2) Under stirring, barium nitrate is dissolved in deionized water to obtain a clear solution B, and then the solution B is added dropwise to the solution A under stirring, and after ultrasonication, a uniform colloidal solution C is obtained, and the colloidal solution C is slowly poured into the inner lining of the hydrothermal kettle;

[0020] (1-3) Under stirring, tetrabutyl titanate is added to 1-butanol to obtain solution D, and then solution D is slowly added into the lining of a water-heated autoclave;

[0021] (1-4) Under stirring, sodium hydroxide is dissolved in deionized water to obtain a clear solution E, and then the solution E is slowly added into the lining of a water-heated autoclave to obtain a BaTiO3 precursor solution, and the lining is sealed;

[0022] The specific steps of step (2) include:

[0023] (2-1) placing the hydrothermal kettle as a whole in a homogeneous reactor to carry out a hydrothermal reaction;

[0024] (2-2) After cooling to room temperature, the product between the 1-butanol phase and the water phase, i.e., barium titanate nanocubes, is collected with ethanol and dispersed in toluene to obtain a stable milky white colloidal solution F for later use, i.e., barium titanate nanocube dispersion.

[0025] Furthermore, the volume ratio of oleic acid, oleylamine and 1-butanol is (7-8):(4-5):20;

[0026] The mass ratio of the barium nitrate, tetrabutyl titanate and sodium hydroxide is (1.0-1.1):(1.9-2.0):1.

[0027] Furthermore, the temperature of the hydrothermal reaction is 140-160° C., and the time is 16-20 hours.

[0028] Furthermore, the specific steps of step (3) include:

[0029] (3-1) Immerse the glass fiber cloth in the milky white colloidal solution F and place it under a fume hood to allow the solvent to evaporate naturally, and repeat the process several times;

[0030] (3-2) placing the treated GFF on a hot plate for annealing to obtain a BaTiO3 / GFF composite material;

[0031] The specific steps of step (4) include:

[0032] (4-1) The PVDF solution was spin-coated on a glass slide, and then the BaTiO3 / GFF composite material was pressed on it and annealed;

[0033] (4-2) After cooling, spin-coat a layer of PVDF solution and quickly cover it with the carbon film;

[0034] (4-3) After annealing, the semi-finished film is peeled off the glass plate and turned over. The edges are fixed with polyimide tape. A layer of PDMS is spin-coated on the reverse side of the film and the carbon film is quickly attached.

[0035] (4-4) Finally, the whole is placed in a vacuum state for annealing to obtain a BTS-GFF / PVDF piezoelectric composite material.

[0036] Furthermore, the annealing temperature in step (3-2) is 380-420° C. and the annealing time is 3.5-4.5 h.

[0037] This heat treatment step is mainly to eliminate the intercrystalline voids generated by the close assembly of nanocrystals and the oleic acid chains between nanoparticles, making the BaTiO3 / GFF composite material more dense and continuous.

[0038] Furthermore, the annealing temperature in step (4) is 120-140° C., and the annealing time is 10 min-4 h.

[0039] A self-assembled ultra-flexible piezoelectric sensor prepared by the method described above.

[0040] An application of the self-assembled ultra-flexible piezoelectric sensor as described above, wherein the flexible sensor is used in the fields of intelligent handwriting recognition, smart keyboards and flexible wearables.

[0041] Furthermore, the specific application method of the handwriting intelligent recognition is: writing with a finger on an ultra-flexible piezoelectric sensor without BOPP tape packaging on the surface, the sensor can detect different deformations and generate output signals specific to different characters;

[0042] The specific application method of the smart keyboard is: stick the ultra-flexible piezoelectric sensor to an ordinary keyboard. When the keys under the sensor are tapped one by one, the output current signal will have personalized authentication features due to different keyboard usage habits.

[0043] Compared with the prior art, the present invention has the following advantages:

[0044] (1) The preparation method of the ultra-flexible piezoelectric sensor provided by the present invention is simple, the raw materials are low-cost, and the high-temperature sintering process required for the preparation of traditional piezoelectric ceramics is avoided;

[0045] (2) The ultra-flexible sensor prepared by the present invention has a continuously distributed piezoelectric phase and enhanced mechanical properties, and has excellent piezoelectric sensing performance, with ultra-high sensitivity (84.24nA / N and 2.68V / N in the range of 0-10N), ultra-low minimum detection limit (<1N), extremely short response time (25ms) and excellent durability (greater than 3000 cycles);

[0046] (3) The ultra-flexible sensor prepared by the present invention can be used in, but is not limited to, the fields of handwriting intelligent recognition, smart keyboards, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is a transmission electron microscope image of a 10 nm BaTiO3 cube prepared in the example;

[0048] Figure 2 is the X-ray diffraction pattern of the BaTiO3 / GFF piezoelectric film prepared in the example;

[0049] Figure 3 is the SEM spectrum of BaTiO3 / GFF prepared in the example;

[0050] Figure 4are the stress-strain curves of the BaTiO3 / GFF-PVDF composite film and the pure PVDF film prepared in the examples;

[0051] Figure 5 is the sensitivity test of the super-flexible sensor prepared in the examples;

[0052] Figure 6 is the comprehensive sensing output performance of the super-flexible sensor prepared in the examples;

[0053] Figure 7 are the current output signals when the testers write the Chinese characters "Jiao", "Tong", "Da", "Xue" and the symbols "×", "√" on the sensor with their fingers in the examples;

[0054] Figure 8 are the current output signals when the testers write the Roman numerals "1-6" on the sensor with their fingers in the examples;

[0055] Fig. 9 are the output current signals when three testers tap the keys below the sensor in turn according to their own keyboard typing habits in the examples;

[0056] Fig.10 is the current output signal when the tester repeatedly writes the Chinese character "Jiao" on the sensor with their finger in the examples;

[0057] Fig.11 is the photo of the super-flexible piezoelectric sensor in the examples. Detailed implementation manners

[0058] The present invention will be described in detail below with reference to the accompanying drawings and specific examples. This embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0059] The present invention will be described in detail below with reference to the accompanying drawings and specific examples.

[0060] The following specific embodiments illustrate the implementation manners of the present invention, and those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0061] Before further describing the specific implementation manners of the present invention, it should be understood that the protection scope of the present invention is not limited to the following specific implementation manners; it should also be understood that the terms used in the embodiments of the present invention are for the purpose of describing specific implementation manners, rather than limiting the protection scope of the present invention. The test methods without specific conditions noted in the following examples are usually carried out under conventional conditions or according to the conditions recommended by each manufacturer.

[0062] When the embodiments give numerical ranges, it should be understood that, unless otherwise specified in the present invention, both endpoints of each numerical range and any numerical value between the two endpoints can be selected. Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as those generally understood by those skilled in the art. In addition to the specific methods, equipment, and materials used in the embodiments, according to the grasp of the prior art by those skilled in the art and the record of the present invention, any methods, equipment, and materials of the prior art similar or equivalent to the methods, equipment, and materials described in the embodiments of the present invention can also be used to realize the present invention.

[0063] A method for preparing a self-assembled ultra-flexible piezoelectric sensor, the method comprising the following steps:

[0064] Step 1: Under stirring, add 7.2 mL of oleic acid and 3.6 mL of oleylamine to 20 mL of 1-butanol to obtain a clear solution A;

[0065] Step 2: Under stirring, 1.045 g of barium nitrate was dissolved in 10 mL of deionized water to obtain a clear solution B, and then solution B was added dropwise to solution A under stirring and ultrasonicated for more than 30 min to obtain a uniform colloidal solution C, and colloidal solution C was slowly poured into a Teflon liner of a hydrothermal autoclave with a volume of 100 mL;

[0066] Step 3: Under stirring, add 1.906 g of tetrabutyl titanate to 20 mL of 1-butanol to obtain solution D, and then slowly add solution D into the Teflon liner along the bottle wall;

[0067] Step 4: Under stirring, 1 g of sodium hydroxide was dissolved in 10 mL of deionized water to obtain a clear solution E, and then the solution E was slowly added into the Teflon liner along the bottle wall and the liner was sealed, and the hydrothermal autoclave was placed in a homogeneous reactor at 150° C. for 18 hours;

[0068] Step 5: After cooling to room temperature, the product between the 1-butanol phase and the water phase is collected with ethanol, namely, 10 nm barium titanate nanocubes, and dispersed in toluene to obtain a stable milky white colloidal solution F for later use;

[0069] Step 6: Immerse the glass fiber cloth (GFF) in 5 mL of milky white colloidal solution F and place it under a fume hood to allow the solvent to evaporate naturally. Repeat 10 times.

[0070] Step 7: Place the treated glass fiber on a hot plate and anneal at 400°C for 4 hours to obtain BaTiO3 / GFF piezoelectric fiber cloth;

[0071] Step 8: First, use the prepared PVDF solution to spin-coat on the glass sheet, then press BaTiO3 / GFF on it, anneal at 120℃ for 10min, then spin-coat a layer of PDMS, and quickly cover it with a carbon film. After annealing at 120℃ for 10min, tear the film off the glass plate, turn it over, and fix the edges with polyimide tape, spin-coat another layer of PDMS on the back of the film, and quickly stick the carbon film. Place the whole in a vacuum oven and anneal at 135℃ for 4h;

[0072] Step 9: After cooling to room temperature, use double-sided conductive copper tape for wiring and packaging to obtain an ultra-flexible piezoelectric sensor.

[0073] Figure 1 (a)-(c) are transmission electron microscope images of 10nm BaTiO3 nanocubes prepared in the embodiment. BaTiO3 nanocubes are almost monodisperse in each TEM image under different scales, with clear faces, single crystals and regular 10nm square morphology. This result shows that the present invention successfully prepared a 10nm almost monodisperse single-domain barium titanate nanoparticle with a regular square morphology by low-temperature hydrothermal method.

[0074] Figure 2 This is the X-ray diffraction pattern of the BaTiO3 / GFF piezoelectric film prepared in the example. Since GFF is a typical amorphous material, there is no obvious diffraction peak in the XRD image. By comparing with the XRD diffraction peak of tetragonal barium titanate, it is proved that 10nm barium titanate nanocubes have been successfully loaded onto GFF through the self-assembly process.

[0075] Figure 3 : are SEM images of the plane and cross section of the BaTiO3 / GFF composite material prepared in the example. Figure 3 The planar SEM images of (a) and (b) characterize the uniform and continuous BaTiO2 phase on the GFF with an interconnected structure similar to a fishing net. Figure 3 The cross-sectional SEM image of (c) characterizes the microstructure of barium titanate nanoparticles self-assembled onto glass fibers, with a single glass fiber coated with abundant 10 nm BaTiO3 nanocubes.

[0076] Figure 4It is the tensile stress-strain curve of the BaTiO3 / GFF-PVDF film prepared in the embodiment. Compared with the blank PVDF film, the tensile strength of the composite film increases from 10.5 MPa to 24 MPa. This is because the GFF with a hierarchical structure plays a good role in transferring the stress in the piezoelectric composite film. The expanded strain / stress transfer can effectively relieve stress concentration, thereby improving the robustness and durability of the device. In addition, after barium titanate grows into a continuous phase on the GFF, its own mechanical properties can be improved by means of the glass fiber network with excellent mechanical properties. In the preparation of the prior art, calcination at about 800 °C is often required, which will greatly reduce the mechanical properties of the glass fiber. However, in this application, the heat treatment is at most about 400 °C, which will not have too much influence on the mechanical properties.

[0077] Example 1

[0078] Place the sensor on the table and gently tap the sensor with the palm at a frequency of about 2 Hz. The sensor can stably output a short-circuit current of ∼10 μA and an open-circuit voltage of ∼75 V.

[0079] Figure 5 It is the sensitivity test of the super-flexible sensor prepared in the embodiment of the present invention. Specifically: Figure 5 (a) is the voltage sensitivity of the super-flexible piezoelectric sensor. Figure 5 (b) is the current sensitivity of the super-flexible piezoelectric sensor. Figure 5 (c) is the open-circuit voltage of the super-flexible piezoelectric sensor under different stresses. Figure 5 (d) is the short-circuit current of the super-flexible piezoelectric sensor under different stresses. From Figure 5 (a)-(b), it can be seen that when the external pressure F is in the low range (1-10 N), the current and voltage sensitivities of the super-flexible piezoelectric sensor are 84.24 nA / N and 2.68 V / N respectively. From Figure 5 (c)-(d), it can be seen that when the applied pressure is 10 N < F < 50 N, the sensitivities of the sensor drop to 9.11 nA / N and 0.37 V / N respectively. The sensitivity of the sensor is very high in the low force range of 0-10 N and drops significantly after exceeding 10 N. This is because in the low force range, the polarization caused by strain increases with the increase of mechanical stress. Under a larger pressure, the ferroelectric polarization will approach saturation.

[0080] Figure 6 It is the comprehensive sensing output performance of the super-flexible sensor prepared in the implementation. Specifically: Figure 6 (a) is the response time of the super-flexible piezoelectric sensor. Figure 6 (b) is the short-circuit current of the super-flexible piezoelectric sensor under 3000 cycles of 50 N pressing. Figure 6(c) is the short - circuit current when the ultra - flexible piezoelectric sensor is gently tapped by hand. Figure 6 (d) is the open - circuit voltage when the ultra - flexible piezoelectric sensor is gently tapped by hand. As Figure 6 (a) shows, the response speed of the sensor is only 25 ms. In addition, after up to 3000 linear motor compression and recovery cycles, the output signal of the sensor can still maintain stability, as Figure 6 (b) shows, indicating that all FPS devices have excellent durability and stability and can meet the requirements of general commercial sensors. Figure 6 (c) and (d) are the output signals when the sensor is tapped by hand, with a current output up to 10 - 15 μA and a voltage output up to 75 V.

[0081] Example 2

[0082] Step 1: Write Chinese characters "Jiao", "Tong", "Da", and "Xue", and simple symbols such as tick, cross, and Roman numerals 1 - 6 on the sensor without BOPP tape encapsulation on the surface with a finger.

[0083] Step 2: The current signal output by the sensor will change with the change of the written characters. Repeat writing the character three times. The shapes (half - peak width, spacing, etc.) of the latter two signals are almost the same as that of the previous signal, but the signal peak may change slightly with the change of the finger force during each writing.

[0084] Figure 7 (a) - (f) are the current output signals when the tester writes Chinese characters "Jiao", "Tong", "Da", "Xue" and symbols "×", "√" on the sensor with a finger in the example. The output current signal will change with the change of the written characters. Repeat writing the character three times. The shapes, half - peak widths, and peak - to - peak spacings of the latter two signals are almost the same as that of the previous signal, but the signal peak may change slightly with the change of the finger force during each writing.

[0085] Figure 8 (a) - (f) are the current output signals when the tester writes Roman numerals "1 - 6" on the sensor with a finger in the example. Figure 7 Similar to the results, different Roman numerals have their own specific current signatures, which means that we can achieve handwritten input recognition by constructing a relevant recognition system.

[0086] Example 3

[0087] Step 1: Attach the ultra - flexible piezoelectric sensor above the keyboard. Three students tap the keys in the area below the sensor in turn. Since the keyboard usage habits of each student are different, the output current signals are different.

[0088] Step 2: The signal output by Student 1 is wide and has a small signal peak value, and the output current of the sensor is about 200 nA.

[0089] Step 3: The signal output by Student 2 is relatively wide, and the output current of the sensor is about 300 nA.

[0090] Step 4: The signal output by Student 3 is sharp and has a large peak value, and the output current of the sensor is about 400 nA.

[0091] Fig. 9 It is the output current signal when three testers in the embodiment tap the key under the sensor in accordance with their own keyboard typing habits in sequence. Since the keyboard usage habits of each student are different, the output current signals are also different. The signal output by Student 1 is wide and has a small signal peak value, the signal output by Student 3 is sharp and has a large peak value, while the signal output by Student 2 is between those of the two students. Therefore, we reasonably believe that the signal patterns collected by the intelligent keyboard can provide various bioengineering information from the testers. For example, the typing force and the pressing area can be judged by the signal amplitude and the full width at half maximum, and the key pressing frequency can be judged by the peak-to-peak interval, etc. All these additional obtained information can be used as effective identifiers for the personalized authentication system, demonstrating its advantages over traditional keyboards.

[0092] Fig.10 It is the current output signal when the tester repeatedly writes the Chinese character "Jiao" on the sensor with a finger. Inevitably, there are slight variations in the pressing force of each finger, resulting in slightly different peak values for each signal. However, it can be carefully observed that the shapes of each output current signal are almost the same, proving the stability of its working performance.

[0093] It can be seen that due to the use of the self-assembly technology in the present invention, the device can achieve excellent piezoelectric performance without high-temperature sintering, and at the same time has the advantages of excellent mechanical properties and the ability to adhere to any curved surface.

[0094] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention in other forms. Any person skilled in the art may use the technical content disclosed above to make changes or modifications into equivalent embodiments with equivalent changes. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the technical solution content of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a self-assembled ultra-flexible piezoelectric sensor, characterized in that: The method comprises the following steps: (1) Preparing a BaTiO3 precursor solution: mixing a barium salt, a titanium source and an alkali with a solvent to prepare a BaTiO3 precursor solution; (2) preparing a barium titanate nanocube dispersion: subjecting a BaTiO3 precursor solution to a hydrothermal reaction to obtain barium titanate nanocubes, and dispersing the barium titanate nanocubes in a dispersant to obtain a barium titanate nanocube dispersion; (3) Preparation of BaTiO3 / GFF composite material: impregnating glass fiber cloth into a dispersion of barium titanate nanocubes, and after the solvent evaporates, performing annealing heat treatment to obtain a BaTiO3 / GFF composite material; (4) Preparation of BaTiO3 / GFF-PVDF composite membrane: The BaTiO3 / GFF composite material is sandwiched between PVDF layers to obtain a BaTiO3-GFF / PVDF piezoelectric composite material, i.e., a BaTiO3 / GFF-PVDF composite membrane; (5) The BaTiO3 / GFF-PVDF composite film is packaged with double-sided conductive copper tape to obtain a self-assembled ultra-flexible piezoelectric sensor; The BaTiO3 nanocubes have well-defined faces, single crystals, and regular 10 nm square morphology.

2. The method for preparing a self-assembled ultra-flexible piezoelectric sensor according to claim 1, characterized in that: The specific steps of step (1) include: (1-1) Under stirring, oleic acid and oleylamine are added to 1-butanol to obtain a clear solution A; (1-2) Under stirring, barium nitrate is dissolved in deionized water to obtain a clear solution B, and then the solution B is added dropwise into the solution A under stirring, and after ultrasonic treatment, a uniform colloidal solution C is obtained, and the colloidal solution C is slowly poured into the inner lining of the hydrothermal autoclave; (1-3) Under stirring, tetrabutyl titanate is added to 1-butanol to obtain solution D, and then solution D is slowly added into the lining of a water-heated autoclave; (1-4) Under stirring, sodium hydroxide is dissolved in deionized water to obtain a clear solution E, and then the solution E is slowly added into the lining of the water hot autoclave to obtain a BaTiO3 precursor solution, and the lining is sealed; The specific steps of step (2) include: (2-1) placing the hydrothermal kettle as a whole in a homogeneous reactor to carry out a hydrothermal reaction; (2-2) After cooling to room temperature, the product between the 1-butanol phase and the water phase, i.e., barium titanate nanocubes, is collected with ethanol and dispersed in toluene to obtain a stable milky white colloidal solution F for later use, i.e., barium titanate nanocube dispersion.

3. The method for preparing a self-assembled ultra-flexible piezoelectric sensor according to claim 2, characterized in that: The volume ratio of oleic acid, oleylamine and 1-butanol is (7-8): (4-5): 20; The mass ratio of the barium nitrate, tetrabutyl titanate and sodium hydroxide is (1.0-1.1): (1.9-2.0):

1.

4. The method for preparing a self-assembled ultra-flexible piezoelectric sensor according to claim 2, characterized in that: The temperature of the hydrothermal reaction is 140-160° C. and the time is 16-20 h.

5. The method for preparing a self-assembled ultra-flexible piezoelectric sensor according to claim 1, characterized in that: The specific steps of step (3) include: (3-1) Immerse the glass fiber cloth in the milky white colloidal solution F and place it under a fume hood to allow the solvent to evaporate naturally, and repeat this process several times; (3-2) placing the treated GFF on a hot plate for annealing to obtain a BaTiO3 / GFF composite material; The specific steps of step (4) include: (4-1) Spin-coat the PVDF solution on a glass slide, then press the BaTiO3 / GFF composite material on top and anneal; (4-2) After cooling, spin-coat a layer of PVDF solution and quickly cover it with the carbon film; (4-3) After annealing, the semi-finished film is peeled off the glass plate and turned over. The edges are fixed with polyimide tape. A layer of PDMS is spin-coated on the reverse side of the film and the carbon film is quickly attached. (4-4) Finally, the whole is placed in a vacuum state for annealing to obtain a BaTiO3 / GFF / PVDF piezoelectric composite material, namely a BaTiO3 / GFF-PVDF composite film.

6. The method for preparing a self-assembled ultra-flexible piezoelectric sensor according to claim 5, characterized in that: The annealing temperature in step (3-2) is 380-420°C and the annealing time is 3.5-4.5 h.

7. The method for preparing a self-assembled ultra-flexible piezoelectric sensor according to claim 5, characterized in that: The annealing temperature in step (4) is 120-140°C and the annealing time is 10 min-4 h.

8. A self-assembled ultra-flexible piezoelectric sensor prepared by the method according to any one of claims 1 to 7.

9. An application of the self-assembled ultra-flexible piezoelectric sensor as claimed in claim 8, characterized in that: The self-assembled ultra-flexible piezoelectric sensor is used in the fields of handwriting intelligent recognition, smart keyboards and flexible wearables.

10. The use of a self-assembled ultra-flexible piezoelectric sensor according to claim 9, characterized in that: The specific application method of the handwriting intelligent recognition is: write with your finger on the ultra-flexible piezoelectric sensor without BOPP tape packaging on the surface, the sensor can detect different deformations and generate output signals specific to different characters; The specific application method of the smart keyboard is: stick the ultra-flexible piezoelectric sensor to an ordinary keyboard. When the keys under the sensor are tapped one by one, the output current signal will have personalized authentication features due to different keyboard usage habits.

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