A flexible superpiezoelectric metal-organic framework crystal material, a preparation method therefor and an application thereof

Abstract

The present application relates to a kind of flexible superpiezoelectric metal organic framework crystal materials and its preparation method and application.The crystal structure of the flexible superpiezoelectric metal organic framework crystal material is the three-dimensional network structure formed by the coordination bond connection of four (2-fluorine p-phenylenediformic acid) four manganese (II) building unit.The method is: manganese source, 2-fluorine p-phenylenediformic acid, organic solvent, alcohol, water and acetic acid are uniformly mixed to obtain precursor solution;The precursor solution is reacted at 110~150 ℃ for 18~30 h, then cooled to 40~60 ℃ and kept for 6~10 h, then cooled and centrifuged, to obtain precipitate;The precipitate is washed and then sequentially freeze-dried and vacuum dried to obtain the flexible superpiezoelectric metal organic framework crystal material.The flexible superpiezoelectric metal organic framework crystal material in the present application has excellent piezoelectric properties, mechanical flexibility and elasticity, and is suitable for flexible, wearable and piezoelectric devices.

Description

Technical Field

[0001] This invention belongs to the field of functional materials technology, and particularly relates to a flexible superpiezoelectric metal-organic framework crystal material, its preparation method and application. Background Technology

[0002] With the rapid development of flexible, wearable, and self-powered electronic devices, the demand for piezoelectric materials that combine high voltage performance with excellent mechanical flexibility is becoming increasingly urgent. Currently, commonly used flexible piezoelectric materials mainly fall into two categories: one is a composite of brittle inorganic piezoelectric materials (such as PZT and BaTiO3) and flexible polymers; the other uses organic piezoelectric materials (such as PVDF and Nylon-11). However, the piezoelectric coefficient of these materials (…) d 33 The overall efficiency is generally low, making it difficult to meet the requirements of high-sensitivity sensing and efficient energy harvesting.

[0003] Chinese patent application CN112940274A discloses a metal-organic framework piezoelectric material composed of a halogen-substituent terephthalic acid ligand coordinated with a chromium salt, with the structure Cr(OH)·[O2C-C6H3X-CO2] (MIL-53 type). However, this metal-organic framework piezoelectric material has low piezoelectric performance and contains Cr. 3+ Toxic ions are insufficient to meet the multiple requirements of flexible wearable electronic devices, which demand high-voltage response, excellent flexibility, and non-toxicity.

[0004] In summary, it is essential to provide a flexible superpiezoelectric metal-organic framework crystal material, its preparation method, and its applications. Summary of the Invention

[0005] To address one or more technical problems existing in the prior art, this invention provides a flexible superpiezoelectric metal-organic framework crystal material, its preparation method, and its application.

[0006] In a first aspect, the present invention provides a flexible superpiezoelectric metal-organic framework crystal material, wherein the crystal structure of the flexible superpiezoelectric metal-organic framework crystal material is a three-dimensional network structure formed by the connection of tetra(2-fluoroterephthalic acid) tetramanganese(II) building units through coordination bonds.

[0007] Preferably, the flexible superpiezoelectric metal-organic framework crystal material is monoclinic with space group P2; the tetra(2-fluoroterephthalic acid)tetramanganese(II) building unit has two intrinsic dipoles with a dipole moment ranging from 30 to 40D, and the angle between the two dipoles can change with external stress within the range of 25° to 50°; and / or the piezoelectric coefficient of the flexible superpiezoelectric metal-organic framework crystal material. d 33The values ​​are 1450~2100 pm / V, Young's modulus is 5.1~13.6 GPa, and compressive strain is 20~30%.

[0008] In a second aspect, this invention provides a method for preparing a flexible superpiezoelectric metal-organic framework crystal material, the method comprising the following steps: (1) Mix manganese source, 2-fluoroterephthalic acid, organic solvent, alcohol, water and acetic acid evenly to obtain precursor solution; (2) The precursor solution is reacted at 110~150℃ for 18~30h, then cooled to 40~60℃ and kept at that temperature for 6~10h, and then cooled and centrifuged to obtain the precipitate; (3) The precipitate was washed and then freeze-dried and vacuum-dried in sequence to obtain a flexible superpiezoelectric metal-organic framework crystal material.

[0009] The manganese source is one or more of manganese chloride, manganese nitrate, manganese acetate, manganese sulfate, manganese carbonate, and manganese acid phosphate; the organic solvent is N,N-dimethylformamide and / or dimethyl sulfoxide; and / or the alcohol is anhydrous ethanol and / or methanol.

[0010] Preferably, the ratio of manganese source, 2-fluoroterephthalic acid, organic solvent, alcohol, water and acetic acid is (2.5~4) mmol: (0.75~1) mmol: (20~25) mL: (1.4~1.6) mL: (0.6~0.8) mL: (0.2~0.5) mL.

[0011] Preferably, the vacuum drying is performed at 50~80℃ for 4~12 hours.

[0012] In a third aspect, the present invention provides a flexible superpiezoelectric metal-organic framework crystal material prepared by the preparation method described in the second aspect of the present invention.

[0013] In a fourth aspect, the present invention provides the application of the flexible superpiezoelectric metal-organic framework crystal material described in the first aspect or the flexible superpiezoelectric metal-organic framework crystal material prepared by the preparation method described in the second aspect of the present invention in the preparation of flexible piezoelectric thin films; the flexible piezoelectric thin film comprises the flexible superpiezoelectric metal-organic framework crystal material, preferably, the flexible piezoelectric thin film is composed of the flexible superpiezoelectric metal-organic framework crystal material and a polymer matrix, preferably, the polymer matrix is ​​polyvinylidene fluoride, and the mass ratio of the flexible superpiezoelectric metal-organic framework crystal material to polyvinylidene fluoride is 1:(1~2).

[0014] The present invention provides, in a fifth aspect, the application of the flexible superpiezoelectric metal-organic framework crystal material described in the first aspect of the present invention or the flexible superpiezoelectric metal-organic framework crystal material prepared by the preparation method described in the second aspect of the present invention in the preparation of piezoelectric devices, wherein the piezoelectric device comprises a flexible piezoelectric thin film as a sensor or energy harvesting unit, the flexible piezoelectric thin film comprising the flexible superpiezoelectric metal-organic framework crystal material; preferably, the piezoelectric device is a flexible, wearable electrical device; preferably, the piezoelectric device includes, but is not limited to, a self-powered voltage electrical device.

[0015] Preferably, the piezoelectric device is selected from at least one of the following: (a) Energy harvester; (b) Biomedical sensors; (c) Self-powered human-computer interaction device; (d) Self-powered keyboard; (e) Pressure indicator light; (f) Real-time pulse signal monitoring wristband.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: (1) The flexible superpiezoelectric metal-organic framework crystal material provided by the present invention has the highest flexible piezoelectric coefficient to date. d 33 With a value as high as 1450~2100pm / V, far exceeding that of traditional piezoelectric ceramics (such as lead zirconate titanate PZT) and polymer piezoelectric materials, it also possesses excellent mechanical flexibility (high flexibility) and elasticity, with a Young's modulus of 5.1~13.6GPa and a compressive strain of up to 20~30%, making it suitable for flexible, wearable and piezoelectric devices and a variety of wearable devices.

[0017] (2) The flexible superpiezoelectric metal-organic framework crystal material in this invention is suitable for harvesting weak human motion energy and monitoring biological signals. The piezoelectric device based on the flexible superpiezoelectric metal-organic framework crystal material can achieve high-sensitivity signal output without external power supply, and is suitable for wearable medical and micromechanical systems. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1These are SEM and XRD images of the flexible superpiezoelectric metal-organic framework (MFMOF) crystal material prepared in Example 1 of this invention. In the figures, A is the SEM image, showing that the MFMOF is in the form of nanosheets; B is the XRD image, showing that the prepared material has good crystallinity. Combined with calculation simulation, it is proved that the crystal structure of the prepared material is Mn4(2-fluoroterephthalate)4, and the cell parameters are: a=18.834nm, b=13.192nm, c=8.16nm, α=90°, β=98°, γ=90°.

[0020] Figure 2 This is an amplitude-voltage butterfly curve of the flexible superpiezoelectric metal-organic framework crystal material (MFMOF) prepared in Example 1 of this invention; in the figure, the vertical axis Amplitude represents the amplitude in pm, and the horizontal axis Voltage represents the voltage (DC bias applied to the surface of the material) in V; Figure 3 This is a comparison of the piezoelectric coefficient performance of the flexible superpiezoelectric metal-organic framework crystal material (MFMOF) prepared in Example 1 of this invention with that of conventional piezoelectric materials in the prior art; in the figure, the vertical axis d 33 The term represents the piezoelectric coefficient, with units of pC / N. Polymers indicates polymer piezoelectric materials, Composites indicates composite piezoelectric materials, Ceramics indicates ceramic piezoelectric materials, and Materials indicates materials. Figure 4 The curves (load-displacement curves) are the mechanical property curves of the flexible superpiezoelectric metal-organic framework crystal material (MFMOF) prepared in Example 1 of this invention. The vertical axis Load represents the load, and the horizontal axis Displacement represents the displacement. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments thereof. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0022] In a first aspect, this invention provides a flexible superpiezoelectric metal-organic framework crystal material (abbreviated as MFMOF), wherein the crystal structure of the flexible superpiezoelectric metal-organic framework crystal material is a three-dimensional network structure formed by tetrakis(2-fluoroterephthalic acid) tetramanganese(II) building units connected by coordination bonds, that is, dsp is connected by 2-fluoroterephthalic acid (2-fluoroterephthalate) organic ligands. 2A three-dimensional network structure composed of hybrid manganese atoms; in this invention, the tetra(2-fluoroterephthalate)tetramanganese(II) building unit is also referred to as Mn4(2-fluoroterephthalate)4 building unit.

[0023] According to some preferred embodiments, the flexible superpiezoelectric metal-organic framework crystal material is monoclinic with space group P2; specifically, the flexible superpiezoelectric metal-organic framework crystal material has a unique crystal structure, the crystal belongs to the monoclinic P2 space group, and has dsp linked by 2-fluoroterephthalic acid organic ligands. 2 A three-dimensional network structure composed of hybrid manganese atoms.

[0024] According to some preferred embodiments, the tetra(2-fluoroterephthalic acid) tetramanganese(II) building unit has two intrinsic dipoles with a dipole moment ranging from 30 to 40D, and the angle between the two dipoles can change with external stress in the range of 25° to 50°. In this invention, the piezoelectric effect of the flexible superpiezoelectric metal-organic framework crystal material (MFMOF) originates from the reorientation of the two intrinsic dipoles under external pressure and the charge redistribution between manganese and the 2-fluoroterephthalic acid ligand.

[0025] According to some preferred embodiments, the flexible superpiezoelectric metal-organic framework crystal material possesses excellent mechanical flexibility, excellent mechanoelasticity, and superpiezoelectric properties, and the piezoelectric coefficient of the flexible superpiezoelectric metal-organic framework crystal material is... d 33 The Young's modulus is 5.1-13.6 GPa, and the compressive strain (reversible compressive strain) is 20-30%. In this invention, the flexible superpiezoelectric metal-organic framework crystal material has a moderate Young's modulus of 5.1-13.6 GPa, indicating excellent mechanical flexibility. The reversible compressive strain of the flexible superpiezoelectric metal-organic framework crystal material is 20-30%, preferably not less than 25%, and it can basically recover to its original state after stress unloading, indicating excellent mechanical elasticity.

[0026] The flexible superpiezoelectric metal-organic framework crystal material of this invention is suitable for flexible, wearable, and piezoelectric devices and various wearable devices. Specifically, it is suitable for harvesting energy from weak human motion and monitoring biosignals. Piezoelectric devices based on this flexible superpiezoelectric metal-organic framework crystal material can achieve high-sensitivity signal output without an external power source, making them suitable for wearable medical and micromechanical systems.

[0027] In a second aspect, this invention provides a method for preparing a flexible superpiezoelectric metal-organic framework crystal material, the method comprising the following steps: (1) Mix manganese source, 2-fluoroterephthalic acid, organic solvent, alcohol, water and acetic acid evenly to obtain a precursor solution; In this invention, for example, manganese source, 2-fluoroterephthalic acid, organic solvent, alcohol, water and acetic acid are added to a reaction vessel with a polytetrafluoroethylene liner and stirred evenly to obtain a precursor solution. (2) The precursor solution is reacted at 110~150℃ for 18~30h, then cooled to 40~60℃ (preferably 40~50℃) and kept at that temperature for 6~10h, and then cooled (e.g., naturally cooled to room temperature) and centrifuged to obtain the precipitate; in this invention, room temperature is, for example, 15~35℃; in this invention, the reaction at 110~150℃ for 18~30h is carried out under stirring, and the holding at 40~60℃ for 6~10h is carried out under no stirring; this invention does not specifically limit the stirring speed, which can be conventionally selected by those skilled in the art, for example, 200~600rpm; in this invention, the centrifugation speed is, for example, 5000~10000rpm, and the centrifugation time is, for example, 20~40min; (3) The precipitate is washed and then freeze-dried and vacuum-dried in sequence to obtain a flexible superpiezoelectric metal-organic framework crystal material; in this invention, the washing is performed by washing with DMF (N,N-dimethylformamide) and methanol multiple times in sequence; in this invention, manganese source is used as metal source and 2-fluoroterephthalic acid is used as organic ligand. The crystal structure of the flexible superpiezoelectric metal-organic framework crystal material obtained by steps (1) to (3) is a three-dimensional network structure formed by the coordination of tetra(2-fluoroterephthalic acid) tetramanganese(II) building units through ionic bonds, that is, dsp is connected by 2-fluoroterephthalic acid organic ligand. 2 The flexible ultra-piezoelectric metal-organic framework crystal material is a three-dimensional network structure composed of hybrid manganese atoms. It is a monoclinic crystal with space group P2. Its tetra(2-fluoroterephthalic acid)tetramanganese(II) building units possess two intrinsic dipoles with a dipole moment ranging from 30 to 40D. The angle between the two dipoles can change with external stress within a range of 25° to 50°, thereby endowing the flexible ultra-piezoelectric metal-organic framework crystal material with ultra-high piezoelectric responsiveness (piezoelectric coefficient). d 33 It possesses a high voltage (1450~2100 pm / V) and excellent mechanical flexibility and elasticity (Young's modulus of 5.1~13.6 GPa, compressive strain of 20~30%). Furthermore, this invention has found that if the manganese source in this application is replaced with another metal source (e.g., chromium), even with the same operating steps, it is impossible to obtain a material with ultra-high voltage response and excellent mechanical flexibility. A possible reason is that the manganese atoms used in this invention have a dsp... 2The hybrid orbital configuration, with four degenerate hybrid orbitals in the same plane, can form stable manganese cluster secondary structural units with 2-fluoroterephthalic acid ligands. These cluster units exhibit a unique dual intrinsic dipole arrangement in the three-dimensional crystal network, with dipole moments reaching 30–40D. Furthermore, the angle between the two dipoles can reversibly change with external stress within the range of 25°–50°. This is the structural basis for the ultra-high voltage electric response and excellent mechanical flexibility and elasticity of the material of this invention. In contrast, chromium atoms typically employ d... 2 sp 3 Hybrid configurations tend to form octahedral nodes, making it difficult to construct double-dipole structures with the same plane, and thus impossible to obtain materials with high voltage coefficients and excellent flexibility.

[0028] In this invention, in step (1), when preparing the precursor solution, alcohol, water and acetic acid are further introduced as a mixed solvent system while adding organic solvent. This invention found that the addition of alcohol can adjust the polarity of the solution and promote the uniform dispersion of ligands, the appropriate amount of water helps the dissolution of manganese salt and the mild progress of coordination reaction, and acetic acid, as a regulator, can regulate the crystal nucleation and growth rate through competitive coordination, thus avoiding crystal aggregation. Although esterification reactions between alcohols and acids are theoretically possible, under the conditions of this invention, the coordination reaction of metal ions is absolutely dominant, effectively suppressing the occurrence of side reactions. This ensures the formation of tetra(2-fluoroterephthalic acid) tetramanganese(II) secondary structural units with high crystallinity, few defects, and ordered arrangement of two intrinsic dipoles. The mixed solvent system of alcohol, water, and acetic acid in this invention can control the contact reaction between metal ions and ligands in the precursor solution and self-assemble them according to a specific coordination mode, which is beneficial for the subsequent construction of a three-dimensional network structure with ultra-high voltage response, excellent mechanical flexibility, and excellent mechanical elasticity. However, if alcohol, water, and acetic acid are not added when preparing the precursor solution, it is impossible to effectively prepare flexible ultra-high voltage metal-organic framework crystal materials.

[0029] In this invention, step (2) after reacting the precursor solution at 110~150℃ for 18~30h, also includes a process of cooling to 40~60℃ and holding for 6~10h, which is one of the key improvements of this invention. This invention found that the low-temperature holding process helps to alleviate the thermal stress accumulated inside the crystal due to high-temperature growth, promotes perfect crystal growth, promotes the full assembly of building units and the repair of structural defects, thereby effectively obtaining a three-dimensional network structure with high crystal integrity and ordered arrangement of dual intrinsic dipoles. However, if this holding step is omitted, the yield of the precipitate will be significantly reduced, and the crystal will naturally cool to room temperature. The process of holding the material at the time of heat treatment can easily generate a large number of lattice defects and internal stresses, leading to disordered dipole orientation, significantly reducing the piezoelectric coefficient and mechanical flexibility of the material, and making it impossible to effectively prepare the flexible superpiezoelectric metal-organic framework crystal material. If the holding time is too short, it will not only lead to a decrease in product yield, but also result in insufficient crystal growth, poor structural regularity, and the inability to form a stable bi-dipole configuration crystal. On the other hand, if the holding time is too long, it will lead to a prolonged preparation cycle and may cause excessive grain growth or secondary nucleation, particle agglomeration, and destruction of the original three-dimensional network structure, resulting in an increase in Young's modulus and a decrease in compressive strain, which is not conducive to the simultaneous optimization of flexibility and piezoelectric properties.

[0030] Unlike conventional MOF material preparation which relies on a single drying method (mostly vacuum drying), this invention employs a freeze-drying followed by vacuum drying method, specifically designed for the crystal structure characteristics of flexible ultra-piezoelectric metal-organic framework (MOF) crystals. Freeze-drying removes pore solvents through sublimation, effectively preventing nanoparticle aggregation and maintaining good dispersibility. By avoiding capillary forces generated during the liquid-gas phase transition, it effectively prevents the three-dimensional network structure from shrinking or collapsing during drying, thus preserving the ordered channels and oriented arrangement of the two intrinsic dipoles within the crystal. Subsequent vacuum drying further removes residual solvent molecules strongly bound to the framework, facilitating complete material activation. Compared to vacuum drying alone, which is prone to aggregation, channel shrinkage, or collapse due to surface tension, or freeze-drying alone, which may leave solvent molecules within the channels, this invention's freeze-drying followed by vacuum drying ensures that the flexible MOF crystal possesses high dispersibility, a complete three-dimensional network structure, and highly open pore structure, thus facilitating the acquisition of ultra-high piezoelectric responses. d 33 The flexible superpiezoelectric metal-organic framework crystal material has the following properties: 1450~2100 pm / V, excellent mechanical flexibility (Young's modulus of 5.1~13.6 GPa), and excellent mechanical elasticity (compressive strain of 20~30%).

[0031] According to some preferred embodiments, the manganese source is one or more of manganese chloride, manganese nitrate, manganese acetate, manganese sulfate, manganese carbonate, and manganese phosphate; of course, in this invention, the manganese source includes, but is not limited to, these six manganese sources; the organic solvent is N,N-dimethylformamide (DMF) and / or dimethyl sulfoxide (DMSO); and / or the alcohol is anhydrous ethanol and / or methanol.

[0032] According to some preferred embodiments, the ratio of manganese source, 2-fluoroterephthalic acid, organic solvent, alcohol, water (e.g., deionized water) and acetic acid is (2.5~4) mmol : (0.75~1) mmol : (20~25) mL : (1.4~1.6) mL : (0.6~0.8) mL : (0.2~0.5) mL.

[0033] In this invention, the preferred ratio of manganese source, 2-fluoroterephthalic acid, organic solvent, alcohol, water, and acetic acid is (2.5~4) mmol : (0.75~1) mmol : (20~25) mL : (1.4~1.6) mL : (0.6~0.8) mL : (0.2~0.5) mL. This invention has found that within this ratio range, the molar ratio of manganese source to 2-fluoroterephthalic acid ensures sufficient coordination and assembly between the metal center and ligand to form a secondary structural unit (constructive unit) with a manganese cluster as its core; the amount of organic solvent provides a suitable reaction medium, ensuring that the reactants are fully dissolved and uniformly dispersed. The appropriate introduction of alcohol and water jointly regulates solution polarity and reaction kinetics, avoiding product heterogeneity or decreased crystallinity caused by improper solvent ratio. The controlled amount of acetic acid not only plays a role in competitive coordination to regulate crystal growth, but also avoids excessive acetic acid causing excessive protonation of ligands and inhibiting the coordination reaction. This optimized ratio ensures that the precursor solution has a moderate concentration and reactivity, so that the crystal maintains a stable self-assembly environment during nucleation and growth. This is conducive to obtaining flexible ultra-piezoelectric metal-organic framework crystal materials with high crystallinity, complete structure, and ordered arrangement of dual intrinsic dipoles, ensuring that they achieve ultra-high voltage response and excellent mechanical flexibility and elasticity.

[0034] According to some preferred embodiments, the freeze-drying process involves first pre-freezing at -20 to -30°C for 2 to 6 hours, then drying for 8 to 16 hours at a cold trap temperature of -70 to -90°C, and finally drying for 8 to 16 hours at 10 to 50°C. The first and second drying processes are carried out under an absolute pressure of 10 to 1000 Pa. The present invention does not impose specific limitations on the cooling rate and / or heating rate during the freeze-drying process, for example, it can be 0.5 to 1.5°C / min.

[0035] According to some preferred embodiments, in step (1), the ratio of manganese source, 2-fluoroterephthalic acid, organic solvent, alcohol, water and acetic acid is 2.5 mmol: 0.75 mmol: 23 mL: 1.5 mL: 0.7 mL: 0.35 mL; in step (2), the precursor solution is reacted at 135 °C for 24 h, then cooled to 50 °C and kept at that temperature for 8 h, then naturally cooled to room temperature, and centrifuged to obtain the precipitate; in step (3), the piezoelectric coefficient of the flexible superpiezoelectric metal-organic framework crystal material is obtained. d 33 It has a value of 2055 pm / V, a Young's modulus of 13.57 GPa, and a compressive strain of 29.2%.

[0036] According to some preferred embodiments, the vacuum drying is performed at 50~80°C for 4~12 hours.

[0037] In a third aspect, the present invention provides a flexible superpiezoelectric metal-organic framework crystal material prepared by the preparation method described in the second aspect of the present invention.

[0038] In a fourth aspect, the present invention provides the application of the flexible superpiezoelectric metal-organic framework crystal material described in the first aspect or the flexible superpiezoelectric metal-organic framework crystal material prepared by the preparation method described in the second aspect of the present invention in the preparation of flexible piezoelectric thin films; the flexible piezoelectric thin film comprises the flexible superpiezoelectric metal-organic framework crystal material, preferably, the flexible piezoelectric thin film is composed of the flexible superpiezoelectric metal-organic framework crystal material and a polymer matrix, preferably, the polymer matrix is ​​polyvinylidene fluoride, and the mass ratio of the flexible superpiezoelectric metal-organic framework crystal material to polyvinylidene fluoride is 1:(1~2).

[0039] The present invention provides, in a fifth aspect, the application of the flexible superpiezoelectric metal-organic framework crystal material described in the first aspect of the present invention, or the flexible superpiezoelectric metal-organic framework crystal material prepared by the preparation method described in the second aspect of the present invention, in the preparation of piezoelectric devices. The piezoelectric device includes a flexible piezoelectric film serving as a sensor or energy harvesting unit, the flexible piezoelectric film comprising the flexible superpiezoelectric metal-organic framework crystal material. Preferably, the piezoelectric device is a flexible, wearable piezoelectric device. Preferably, the piezoelectric device includes, but is not limited to, a self-powered piezoelectric device. In the present invention, for example, the flexible superpiezoelectric metal-organic framework crystal material can be mechanically exfoliated or ultrasonically treated before being used to prepare flexible piezoelectric films or piezoelectric devices.

[0040] According to some preferred embodiments, the voltage device is selected from at least one of the following: (a) Energy harvester; for example, with an output voltage of not less than 4.5V and a peak power density of not less than 10W / cm² under a pressure of 0.5N.3 ; (b) A biomedical sensor, specifically a flexible wristband sensor for monitoring arterial pulses, capable of responding to radial artery pulse signals and outputting a peak voltage of 0.3V to 0.5V pulse peak; (c) A self-powered human-computer interaction device, specifically a flexible keyboard, wherein the electrical energy generated by a single key when triggered is sufficient to drive a low-power wireless transmission module; (d) Self-powered keyboard; (e) Pressure indicator light; (f) Real-time pulse signal monitoring wristband.

[0041] The present invention will be further described below by way of examples, but the scope of protection of the present invention is not limited to these embodiments. The present invention may have many other embodiments, and those skilled in the art can make various corresponding changes and modifications based on the present invention without departing from its spirit and essence. However, all such corresponding changes and modifications should fall within the scope of protection of the appended claims. Unless otherwise specified, the experimental methods used in the following embodiments and comparative examples are conventional methods. Unless otherwise specified, the materials used in the following embodiments and comparative examples are commercially available. Example 1

[0042] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.35 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C for 24 h with a stirring speed of 400 rpm for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 50 °C, and maintain the temperature at 50 °C for 8 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried. Subsequently, it was dried in a vacuum drying oven at 60℃ for 6 h to obtain a flexible superpiezoelectric metal-organic framework crystal material (MFMOF). The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying once at a cold trap temperature of -70℃ for 12 h, and finally drying a second time at 20℃ for 8 h. The first and second drying were carried out under an absolute pressure of 100 Pa.

[0043] The flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment has a crystal structure consisting of a three-dimensional network structure formed by coordination bonds connecting tetra(2-fluoroterephthalic acid) tetramanganese(II) building units. The tetra(2-fluoroterephthalic acid) tetramanganese(II) building units have two intrinsic dipoles. According to the first principle, the dipole moment ranges from 30 to 40D, and the angle between the two dipoles can change with external stress within the range of 25° to 50°. The flexible superpiezoelectric metal-organic framework crystal material is monoclinic with space group P2. Its cell parameters, determined by X-ray diffraction (XRD), are: a = 18.834 nm, b = 13.192 nm, c = 8.16 nm, α = 90°, β = 98°, γ = 90°.

[0044] The SEM morphology and XRD patterns of the flexible superpiezoelectric metal-organic framework (MFMOF) crystal material prepared in this embodiment are shown below. Figure 1 As shown; the present invention uses atomic force microscopy to test the flexible superpiezoelectric metal-organic framework crystal material, and the amplitude-voltage butterfly curve is shown below. Figure 2 As shown, from Figure 2 As can be seen, the amplitude-voltage butterfly curve is symmetrical, indicating high reliability of the piezoelectric performance. The piezoelectric coefficient can be determined by measuring the slope of the amplitude-voltage curve. d 33 The average value is 2055 pm / V, and the DC bias applied to the material surface is -10V to 10V.

[0045] A comparison of the piezoelectric coefficient properties of the flexible superpiezoelectric metal-organic framework (MFMOF) prepared in Example 1 of this invention with those of conventional piezoelectric materials in the prior art is shown in the figure below. Figure 3 As shown, this demonstrates that the flexible superpiezoelectric metal-organic framework crystal material provided by this invention possesses the highest flexible piezoelectric coefficient to date. d 33 It can reach up to 2055 pm / V, far exceeding traditional piezoelectric ceramic materials, composite piezoelectric materials and polymer piezoelectric materials.

[0046] In this embodiment, the mechanical property curves (load-displacement curves) of the flexible superpiezoelectric metal-organic framework (MFMOF) were tested using a nanoindentation apparatus. The results are as follows: Figure 4As shown, the Young's modulus was measured to be 13.57 GPa. In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment was also subjected to nanocompression test by in-situ transmission electron microscopy. The morphological evolution of the material during the compression process was observed in real time. The results showed that the material could withstand up to 29.2% compressive strain (reversible compressive strain) under axial compression, and could recover to its original morphology after stress unloading. No obvious plastic deformation or crystal fracture was observed. This indicates that the recoverable compressive strain of the flexible superpiezoelectric metal-organic framework crystal material is up to 29.2%, which is significantly higher than that of traditional piezoelectric ceramic materials and most reported MOF crystals.

[0047] The flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment is free of organic solvents and water, has superpiezoelectric properties, and has an ultra-high flexible piezoelectric coefficient of 2055 pm / V (i.e., 2055 pC / N). It also has excellent mechanical flexibility and mechanical elasticity. Example 2

[0048] Weigh out 4 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.2 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C with a stirring speed of 400 rpm for 24 h for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 40 °C, and maintain the temperature at 40 °C for 8 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried. Subsequently, it was dried in a vacuum drying oven at 60℃ for 6 h to obtain a flexible superpiezoelectric metal-organic framework crystal material (MFMOF). The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying once at a cold trap temperature of -70℃ for 12 h, and finally drying a second time at 20℃ for 8 h. The first and second drying were carried out under an absolute pressure of 100 Pa.

[0049] In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material was tested using an atomic force microscope using the same method as in Example 1, and its piezoelectric coefficient was measured. d 33The average value was 1840 pm / V. In this embodiment, the Young's modulus of the flexible superpiezoelectric metal-organic framework crystal material was measured to be 11.2 GPa using a nanoindenter, the same method as in Example 1. In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment was also subjected to nanocompression test by in-situ transmission electron microscopy using the same method as in Example 1. The morphological evolution of the material during the compression process was observed in real time. The results showed that the material could withstand a compressive strain of up to 25.3% under axial compression and could recover to its original morphology after stress unloading. No obvious plastic deformation or crystal fracture was observed. Example 3

[0050] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 1.0 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.5 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C for 24 h with a stirring speed of 400 rpm for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 50 °C, and maintain the temperature at 50 °C for 10 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried. Subsequently, it was dried in a vacuum drying oven at 60℃ for 6 h to obtain a flexible superpiezoelectric metal-organic framework crystal material (MFMOF). The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying once at a cold trap temperature of -70℃ for 12 h, and finally drying a second time at 20℃ for 8 h. The first and second drying were carried out under an absolute pressure of 100 Pa.

[0051] In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material was tested using an atomic force microscope using the same method as in Example 1, and its piezoelectric coefficient was measured. d 33The average value was 1680 pm / V. In this embodiment, the Young's modulus of the flexible superpiezoelectric metal-organic framework crystal material was measured to be 5.1 GPa using a nanoindenter, the same method as in Example 1. In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment was also subjected to nanocompression testing using in-situ transmission electron microscopy, the same method as in Example 1, and the morphological evolution of the material during the compression process was observed in real time. The results showed that the material could withstand a compressive strain of up to 23.3% under axial compression, and could recover to its original morphology after stress unloading, without obvious plastic deformation or crystal fracture. Example 4

[0052] Weigh out 4 mmol of manganese chloride monohydrate (MnCl2·H2O), 1.0 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.35 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain it at 135 °C for 24 h with stirring at 400 rpm for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 40 °C, and maintain it at 40 °C for 10 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried. Subsequently, it was dried in a vacuum drying oven at 60℃ for 6 h to obtain a flexible superpiezoelectric metal-organic framework crystal material (MFMOF). The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying once at a cold trap temperature of -70℃ for 12 h, and finally drying a second time at 20℃ for 8 h. The first and second drying were carried out under an absolute pressure of 100 Pa.

[0053] In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material was tested using an atomic force microscope using the same method as in Example 1, and its piezoelectric coefficient was measured. d 33The average value was 1658 pm / V. In this embodiment, the Young's modulus of the flexible superpiezoelectric metal-organic framework crystal material was measured to be 9.6 GPa using a nanoindenter, the same method as in Example 1. In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment was also subjected to nanocompression testing using in-situ transmission electron microscopy, the same method as in Example 1, and the morphological evolution of the material during the compression process was observed in real time. The results showed that the material could withstand up to 21% compressive strain under axial compression and could recover to its original morphology after stress unloading, without obvious plastic deformation or crystal fracture. Example 5

[0054] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.35 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 150 °C and maintain it at 150 °C with a stirring speed of 400 rpm for 18 h for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 50 °C, and maintain it at 50 °C for 8 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried. Subsequently, it was dried in a vacuum drying oven at 60℃ for 6 h to obtain a flexible superpiezoelectric metal-organic framework crystal material (MFMOF). The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying once at a cold trap temperature of -70℃ for 12 h, and finally drying a second time at 20℃ for 8 h. The first and second drying were carried out under an absolute pressure of 100 Pa.

[0055] In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material was tested using an atomic force microscope using the same method as in Example 1, and its piezoelectric coefficient was measured. d 33The average value was 1921 pm / V. In this embodiment, the Young's modulus of the flexible superpiezoelectric metal-organic framework crystal material was measured to be 12.53 GPa using a nanoindenter, the same method as in Example 1. In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment was also subjected to nanocompression testing using in-situ transmission electron microscopy, the same method as in Example 1, and the morphological evolution of the material during the compression process was observed in real time. The results showed that the material could withstand a compressive strain of up to 24.4% under axial compression, and could recover to its original morphology after stress unloading, without obvious plastic deformation or crystal fracture. Example 6

[0056] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.35 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 110 °C and maintain it at 110 °C with stirring at 400 rpm for 30 h for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 50 °C, and maintain it at 50 °C for 7 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried. Subsequently, it was dried in a vacuum drying oven at 60℃ for 6 h to obtain a flexible superpiezoelectric metal-organic framework crystal material (MFMOF). The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying once at a cold trap temperature of -70℃ for 12 h, and finally drying a second time at 20℃ for 8 h. The first and second drying were carried out under an absolute pressure of 100 Pa.

[0057] In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material was tested using an atomic force microscope using the same method as in Example 1, and its piezoelectric coefficient was measured. d 33The average value was 1468 pm / V. In this embodiment, the Young's modulus of the flexible superpiezoelectric metal-organic framework crystal material was measured to be 10.83 GPa using a nanoindenter, the same method as in Example 1. In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment was also subjected to nanocompression test by in-situ transmission electron microscopy using the same method as in Example 1. The morphological evolution of the material during the compression process was observed in real time. The results showed that the material could withstand a compressive strain of up to 20.1% under axial compression and could recover to its original morphology after stress unloading. No obvious plastic deformation or crystal fracture was observed. Example 7

[0058] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.6 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C for 24 h with a stirring speed of 400 rpm for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 50 °C, and maintain the temperature at 50 °C for 8 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried. Subsequently, it was dried in a vacuum drying oven at 60℃ for 6 h to obtain a flexible superpiezoelectric metal-organic framework crystal material (MFMOF). The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying once at a cold trap temperature of -70℃ for 12 h, and finally drying a second time at 20℃ for 8 h. The first and second drying were carried out under an absolute pressure of 100 Pa.

[0059] The sample yield prepared in this embodiment was reduced by 45% compared to Example 1. The flexible superpiezoelectric metal-organic framework crystal material was tested using atomic force microscopy with the same method as in Example 1, and its piezoelectric coefficient was measured. d 33The average value is 1580 pm / V. In this embodiment, the Young's modulus of the flexible superpiezoelectric metal-organic framework crystal material was measured to be 5.0 GPa using a nanoindenter, the same method as in Example 1. In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment was also subjected to nanocompression test by in-situ transmission electron microscopy using the same method as in Example 1. The morphological evolution of the material during the compression process was observed in real time. The results show that the material can withstand a compressive strain of up to 21.3% under axial compression. Example 8

[0060] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.1 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C with a stirring speed of 400 rpm for 24 h for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 50 °C, and maintain the temperature at 50 °C for 8 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried. Subsequently, it was dried in a vacuum drying oven at 60℃ for 6 h to obtain a flexible superpiezoelectric metal-organic framework crystal material (MFMOF). The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying once at a cold trap temperature of -70℃ for 12 h, and finally drying a second time at 20℃ for 8 h. The first and second drying were carried out under an absolute pressure of 100 Pa.

[0061] Compared to Example 1, the material sample prepared in this embodiment shows significant agglomeration. The flexible superpiezoelectric metal-organic framework crystal material was tested using atomic force microscopy with the same method as in Example 1, and its piezoelectric coefficient was measured. d 33 The average value is 1320 pm / V. In this embodiment, the Young's modulus of the flexible superpiezoelectric metal-organic framework crystal material was measured to be 10.36 GPa using a nanoindenter, the same method as in Example 1. In this embodiment, the flexible superpiezoelectric metal-organic framework crystal material prepared in this embodiment was also subjected to nanocompression test by in-situ transmission electron microscopy using the same method as in Example 1. The morphological evolution of the material during the compression process was observed in real time. The results show that the material can withstand a compressive strain of up to 24.5% under axial compression. Comparative Example 1

[0062] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, and 25.55 mL of N,N-dimethylformamide (DMF), and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C for 24 h with stirring at 400 rpm for a solvothermal reaction. Then stop stirring and allow the mixture to cool naturally to 50 °C, where it is maintained for 8 h. After the reaction is complete, allow the reaction vessel to cool naturally to room temperature to obtain the reaction product. Centrifuge the obtained reaction product at 8000 rpm for 30 min.

[0063] Compared with Example 1, no precipitate was generated in the prepared sample of this comparative example, and flexible superpiezoelectric metal-organic framework crystal materials could not be prepared. Comparative Example 2

[0064] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.35 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C for 24 h at a stirring speed of 400 rpm for a solvothermal reaction, after which the reaction is complete. After the reaction, allow the reaction vessel to cool naturally to room temperature to obtain the reaction product. Centrifuge the obtained reaction product at 8000 rpm for 30 min to obtain a small amount of precipitate.

[0065] Compared with Example 1, this comparative example produced a small amount of precipitate and could not effectively prepare flexible superpiezoelectric metal-organic framework crystal materials. Comparative Example 3

[0066] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.35 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C and 400 rpm for 24 h for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 50 °C, and maintain the temperature at 50 °C for 2 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried. Subsequently, it was dried in a vacuum drying oven at 60℃ for 6 h to obtain a metal-organic framework crystal material. The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying once at a cold trap temperature of -70℃ for 12 h, and finally drying a second time at 20℃ for 8 h. The first and second drying were carried out under an absolute pressure of 100 Pa.

[0067] Compared with Example 1, the amount of metal-organic framework crystal material prepared in this comparative example is significantly reduced. Comparative Example 4

[0068] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.35 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C for 24 h with a stirring speed of 400 rpm for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 50 °C, and maintain the temperature at 50 °C for 8 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The reaction product was centrifuged at 8000 rpm for 30 min to obtain the precipitate. The precipitate was washed three times with DMF and methanol respectively and then dried in a vacuum drying oven at 60℃ for 30 h to obtain the metal-organic framework crystal material.

[0069] Compared with Example 1, the metal-organic framework crystal material prepared in this comparative example showed significant agglomeration, and its piezoelectric properties decreased significantly compared to Example 1. The metal-organic framework piezoelectric material prepared in this comparative example was tested using atomic force microscopy with the same method as in Example 1, and its piezoelectric coefficient was measured. d 33 The average value is less than 1200 pm / V. Comparative Example 5

[0070] Weigh out 2.5 mmol of manganese chloride monohydrate (MnCl2·H2O), 0.75 mmol of 2-fluoroterephthalic acid, 23 mL of N,N-dimethylformamide (DMF), 1.5 mL of anhydrous ethanol, 0.7 mL of deionized water, and 0.35 mL of acetic acid, and add them sequentially to a reaction vessel lined with polytetrafluoroethylene. Stir magnetically for 30 min (400 rpm) until homogeneous to obtain a precursor solution. Heat the precursor solution to 135 °C and maintain the temperature at 135 °C for 24 h with a stirring speed of 400 rpm for a solvothermal reaction. Then stop stirring and allow it to cool naturally to 50 °C, and maintain the temperature at 50 °C for 8 h for aging and crystallization. After the reaction was completed, the reactor was allowed to cool naturally to room temperature to obtain the reaction product. The obtained reaction product was centrifuged at 8000 rpm for 30 min to obtain a precipitate. The precipitate was washed three times with DMF and methanol respectively and then freeze-dried to obtain a metal-organic framework crystal material. The freeze-drying process involved first pre-freezing at -30℃ for 4 h, then drying at a cold trap temperature of -70℃ for 12 h, and finally drying at 20℃ for 14 h. The first and second drying processes were carried out under an absolute pressure of 100 Pa.

[0071] Compared with Example 1, the metal-organic framework crystal material prepared in this comparative example contains some solvent and water, resulting in a significant decrease in piezoelectric performance. The metal-organic framework piezoelectric material prepared in this comparative example was tested using atomic force microscopy with the same method as in Example 1, and its piezoelectric coefficient was measured. d 33 The average value is less than 1200 pm / V. Comparative Example 6

[0072] In a reaction vessel, 0.66 g of chromium trichloride hexahydrate and 0.23 g of 2-fluoroterephthalic acid were added, followed by 15 mL of N,N-dimethylformamide organic solvent. The reaction system was ultrasonically treated for 15 min to obtain a reaction solution. The reaction solution was heated to 150 °C and magnetically stirred at 150 °C (400 rpm) for 24 hours to obtain the first reaction product. The first reaction product was cooled to room temperature and centrifuged at 800 rpm for 30 minutes. The supernatant was discarded to obtain the first solid. The first solid was added to 10 mL of distilled water and mixed thoroughly. The mixture was then centrifuged at 800 rpm for 30 minutes, and the supernatant was discarded to obtain an intermediate. 20 mL of N,N-dimethylformamide organic solvent was added to the intermediate, and the mixture was stirred at room temperature (400 rpm) for 48 hours. After centrifugation at 800 rpm for 30 minutes, the supernatant was discarded to obtain the second solid. The obtained second solid was added to 20 mL of anhydrous ethanol and mixed evenly. The mixture was then centrifuged at 800 rpm for 30 minutes, and the supernatant was discarded. The mixture was then vacuum dried at 60 °C for 30 h to obtain the metal-organic framework piezoelectric material.

[0073] In this comparative example, the metal-organic framework piezoelectric material prepared in this comparative example was tested using atomic force microscopy using the same method as in Example 1, and its piezoelectric coefficient was measured. d 33 The average value is less than 100 pm / V.

[0074] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A flexible superpiezoelectric metal-organic framework crystal material, characterized in that, The crystal structure of the flexible superpiezoelectric metal-organic framework crystal material is a three-dimensional network structure formed by the connection of tetra(2-fluoroterephthalic acid) tetramanganese(II) building units through coordination bonds.

2. The flexible superpiezoelectric metal-organic framework crystal material according to claim 1, characterized in that: The flexible superpiezoelectric metal-organic framework crystal material is monoclinic and has a space group of P2. The tetra(2-fluoroterephthalic acid)tetramanganese(II) building unit has two intrinsic dipoles with a dipole moment ranging from 30 to 40D, and the angle between the two dipoles can change with external stress within the range of 25° to 50°; and / or the piezoelectric coefficient of the flexible superpiezoelectric metal-organic framework crystal material. d 33 The values ​​are 1450~2100 pm / V, Young's modulus is 5.1~13.6 GPa, and compressive strain is 20~30%.

3. A method for preparing a flexible superpiezoelectric metal-organic framework crystal material, characterized in that, The method includes the following steps: (1) Mix manganese source, 2-fluoroterephthalic acid, organic solvent, alcohol, water and acetic acid evenly to obtain precursor solution; (2) The precursor solution is reacted at 110~150℃ for 18~30h, then cooled to 40~60℃ and kept at that temperature for 6~10h, and then cooled and centrifuged to obtain the precipitate; (3) The precipitate was washed and then freeze-dried and vacuum-dried in sequence to obtain a flexible superpiezoelectric metal-organic framework crystal material.

4. The preparation method according to claim 3, characterized in that: The manganese source is one or more of manganese chloride, manganese nitrate, manganese acetate, manganese sulfate, manganese carbonate, and manganese acid phosphate. The organic solvent is N,N-dimethylformamide and / or dimethyl sulfoxide; and / or the alcohol is anhydrous ethanol and / or methanol.

5. The preparation method according to claim 3, characterized in that: The ratio of manganese source, 2-fluoroterephthalic acid, organic solvent, alcohol, water and acetic acid is (2.5~4) mmol: (0.75~1) mmol: (20~25) mL: (1.4~1.6) mL: (0.6~0.8) mL: (0.2~0.5) mL.

6. The preparation method according to claim 3, characterized in that: The vacuum drying is performed at 50~80℃ for 4~12 hours.

7. A flexible superpiezoelectric metal-organic framework crystal material prepared by any one of claims 3 to 6.

8. The application of the flexible superpiezoelectric metal-organic framework crystal material according to claim 1 or 2, or the flexible superpiezoelectric metal-organic framework crystal material prepared by the preparation method according to any one of claims 3 to 6, in the preparation of flexible piezoelectric thin films, characterized in that, The flexible piezoelectric film comprises the flexible superpiezoelectric metal-organic framework crystal material. Preferably, the flexible piezoelectric film is composed of the flexible superpiezoelectric metal-organic framework crystal material and a polymer matrix. Preferably, the polymer matrix is ​​polyvinylidene fluoride, and the mass ratio of the flexible superpiezoelectric metal-organic framework crystal material to polyvinylidene fluoride is 1:(1~2).

9. The application of the flexible superpiezoelectric metal-organic framework crystal material according to claim 1 or 2, or the flexible superpiezoelectric metal-organic framework crystal material prepared by any one of claims 3 to 6, in the preparation of piezoelectric devices, characterized in that: The piezoelectric device includes a flexible piezoelectric film serving as a sensor or energy harvesting unit, the flexible piezoelectric film comprising the flexible superpiezoelectric metal-organic framework crystal material; preferably, the piezoelectric device is a flexible, wearable piezoelectric device; preferably, the piezoelectric device includes, but is not limited to, a self-powered piezoelectric device.

10. The application according to claim 9, characterized in that, The piezoelectric device is selected from at least one of the following: (a) Energy harvester; (b) Biomedical sensors; (c) Self-powered human-computer interaction device; (d) Self-powered keyboard; (e) Pressure indicator light; (f) Real-time pulse signal monitoring wristband.

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