An ultradielectric molecular ferroelectric polymer composite film, a preparation method and application thereof

Abstract

This invention discloses a superdielectric molecular ferroelectric polymer composite film, its preparation method, and its applications, belonging to the technical field of ferroelectric materials and their preparation. This invention employs a microwave-assisted co-growing competitive growth control design to prepare a molecular ferroelectric polymer composite material with a nested structure, formed by the segmental co-growth of functional nanoscale MOF unit cells and PVDF molecules. This composite material exhibits both ultra-high dielectric constant and high breakdown strength.

Description

Technical Field

[0001] This invention relates to a superdielectric molecular ferroelectric polymer composite film, its preparation method and application, belonging to the field of ferroelectric materials and their preparation technology. Background Technology

[0002] The enormous demand for energy and the rapid advancements in microelectronics have created a need for high capacitance in energy storage materials. High capacitance typically means storing more charge per unit area or volume, which places extremely high demands on the dielectric constant or ion storage capacity of materials. Increasing the dielectric constant of dielectric materials is an effective and direct method to achieve high capacitance for practical applications. Giant dielectric constant (CP) materials, or materials with a real part exceeding 1000, are several orders of magnitude higher than traditional solid dielectrics, and can further expand applications in modern electronics, sensors, energy storage, and multifunctional devices, thus attracting extensive research.

[0003] Most existing materials with giant dielectric constants belong to ferroelectrics. However, in traditional ferroelectric materials, some ions in the crystal undergo thermodynamic shifts, resulting in spontaneous polarization. Limited by the crystal structure, it is difficult to further improve their dielectric properties. Molecular ferroelectrics combine the flexibility of organic materials with the polarization hysteresis of inorganic ferroelectrics. Their high spontaneous polarization, high phase transition temperature, and good flexibility are the preferred characteristics for modern ferroelectric materials. However, because molecular ferroelectrics composed of small organic molecules often have relatively high symmetry and uniaxiality, they exhibit a single axial polarization direction. Therefore, compared to multiaxial inorganic ferroelectric materials, their polarization intensity is limited.

[0004] Among existing materials, metal-organic frameworks (MOFs) are crystalline materials with high order and high porosity formed by the coordination of metal ions and organic ligands. Their crystal structure is similar to that of inorganic materials, exhibiting multiaxiality, making them promising candidates for ferroelectric materials. However, limited by space point group and structural symmetry, only UIO-66 among tens of thousands of MOFs has been found to possess ferroelectric properties at ultra-low temperatures, but its practical application is limited. Therefore, providing a molecular ferroelectric with superdielectric properties is essential. Summary of the Invention

[0005] In order to solve the problems existing in the prior art, the present invention provides a superdielectric molecular ferroelectric polymer composite film, its preparation method and application.

[0006] The technical solution of the present invention: One objective of this invention is to provide a method for preparing a superdielectric molecular ferroelectric polymer composite film, the method comprising the following steps: (1) Add PVDF to the MIL-53 precursor solution, stir evenly by ultrasonication, and then cast into a film to obtain a wet film; (2) The wet film was placed in a preheated microwave reactor for heating treatment. After heating, it was quickly cooled to room temperature and placed in a methanol aqueous solution. It was then immersed at 40°C and vacuum dried after immersion to obtain a molecular ferroelectric polymer composite film.

[0007] Further specifying, the preparation method of the MIL-53 precursor solution in (1) is as follows: Fe(NO3)3·9H2O was dissolved in DMF. After complete dissolution, terephthalic acid was added and mechanically stirred to obtain the MIL-53 precursor solution.

[0008] Furthermore, the molar ratio of Fe(NO3)3·9H2O to terephthalic acid is 0.95:1.05.

[0009] Further specifying, the PVDF added in (1) reacts with the Fe in the MIL-53 precursor solution. 3+ The molar ratio of the substances is 1:2.

[0010] Further, the ultrasonic stirring time in (1) is 30 min.

[0011] Further, in (2), the microwave reaction generator preheating temperature is 180°C and the time is 10 min; the heating treatment temperature is 120°C and the time is 2 h.

[0012] Further specifying, (2) the methanol aqueous solution is a mixture of methanol and water in a mass ratio of 5:1; the wet film after being heated by a microwave reactor is soaked in a methanol aqueous solution at 40°C for 3 days.

[0013] Further specified, in (2) the vacuum drying temperature is 70℃ and the time is 20h.

[0014] The second objective of this invention is to provide a superdielectric molecular ferroelectric polymer composite film obtained by the above preparation method.

[0015] The third objective of this invention is to provide an application of the above-mentioned superdielectric molecular ferroelectric polymer composite film, specifically for the fabrication of memory, piezoelectric sensors, and flexible energy storage devices.

[0016] Beneficial effects: This invention employs a microwave-assisted competitive growth design to prepare a molecular ferroelectric polymer composite material with a nested structure, formed by the segmental co-growth of functional nanoscale MOF unit cells and PVDF molecules. This composite material exhibits ultra-high dielectric constant while also achieving high breakdown strength and energy storage efficiency. Attached Figure Description

[0017] Figure 1 SEM images of the samples prepared for Example 1 and Comparative Examples 1-3, and the samples after PVDF removal treatment; Figure 2 SEM image of the cross section of the sample prepared in Example 1; Figure 3 TEM and elemental surface scan images of the sample prepared in Example 1; Figure 4 High-resolution TEM lattice image of the sample prepared in Example 1; Figure 5 The XRD patterns of the samples prepared in Example 1 and the comparative example are shown. Figure 6 FTIR spectra of the samples prepared in Example 1 and the comparative example; Figure 7 XPS spectrum of the sample prepared in Example 1; Figure 8 The TG-DSC spectrum of the sample prepared in Example 1; Figure 9 The graph shows the dielectric properties of the samples prepared in Example 1 and the comparative example. Figure 10 The image shows the Weibull distribution of the AC withstand voltage characteristics of the samples prepared in Example 1 and the comparative example. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0019] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0020] Example 1 The method for preparing the superdielectric molecular ferroelectric polymer composite thin film in this embodiment includes the following steps: Step (1): Dissolve 0.095 mol Fe(NO3)3·9H2O in 120 mL DMF. After complete dissolution, add 0.105 mol terephthalic acid and stir mechanically to obtain the MIL-53 precursor solution. Step (2): For each 2 mol Fe 3+1 mol of PVDF was added to the MIL-53 precursor solution, and the mixture was ultrasonically stirred for 30 min. The film was then cast and the thickness of the doctor blade was controlled to be 900 μm to obtain a wet film with a thickness of 900 μm. Step (3): Preheat the microwave reactor to 180°C for 10 min, then place the wet film in the microwave reactor and treat it at 120°C for 2 h. After heating, quickly cool it to room temperature and place it in a methanol-water solution (5:1 by mass) at 40°C for 3 days. After soaking, place it in a vacuum drying oven at 70°C for 20 h. The resulting molecular ferroelectric polymer composite film sample was named PVDF:MIL-53(Fe)=1:2.

[0021] Comparative Example 1 The difference between this comparative study and Example 1 is that the amount of PVDF added in step (2) is 0 mol, while the remaining process steps and parameter settings are the same as in Example 1. A molecular ferroelectric polymer composite film sample was obtained and named Pure-MIL-53(Fe).

[0022] Comparative Example 2 The difference between this comparative example and Example 1 is that in step (2), the Fe in the PVDF and MIL-53 precursor solution... 3+ The molar ratio of the substances was 1:1, and the remaining process steps and parameter settings were the same as in Example 1. A molecular ferroelectric polymer composite film sample was obtained and named PVDF:MIL-53(Fe)=1:1.

[0023] Comparative Example 3 The difference between this comparative example and Example 1 is that in step (2), the Fe in the PVDF and MIL-53 precursor solution... 3+ The molar ratio of the substances was 1:3, and the remaining process steps and parameter settings were the same as in Example 1. A molecular ferroelectric polymer composite film sample was obtained and named PVDF:MIL-53(Fe)=1:3.

[0024] Comparative Example 4 The method for preparing nanoparticle-doped composite thin films in this comparative example includes the following steps: (1) Dissolve 0.095 mol Fe(NO3)3·9H2O in 120 mL DMF. After complete dissolution, add 0.105 mol terephthalic acid and stir mechanically to obtain the MIL-53 precursor solution. (2) Preheat the microwave reactor at 180°C for 10 min, then place the MIL-53 precursor solution in the microwave reactor and treat it at 120°C for 2 h. After heating, quickly cool it to room temperature and place it at 40°C. Wash it three times with ethanol, then dry it at 50°C for 4 h to obtain MIL-53(Fe) particles for later use. (3) Dissolve MIL-53(Fe) particles in 120 ml of DMF solvent, sonicate for 1 h, and add PVDF powder to the above solution (according to the ratio of Fe in PVDF to MIL-53 precursor solution). 3+ The substance was added in a molar ratio of 1:2, and ultrasonic dispersion was continued for 3 hours. After ultrasonic dispersion, the obtained adhesive solution was vacuum-degassed to remove air bubbles, cast into a film, and the thickness of the doctor blade was controlled at 900 μm. The resulting wet film was kept at 70°C. o After drying in a forced-air environment at C for 3 hours, a composite film doped with nanoparticles was obtained and named PVDF@MIL-53(Fe)=1:2.

[0025] Comparative Example 5 The difference between this comparative example and Example 1 is that step (2) contains Fe. 3+ The amount of MIL-53 precursor solution added was 0 mol, and the remaining process steps and parameter settings were the same as in Example 1. A molecular ferroelectric polymer composite film sample was obtained and named Pure PVDF.

[0026] Example of effect The microstructure and properties of the samples prepared in Example 1 and Comparative Examples 1-3 were characterized, and the results are as follows: (1) The microstructure of the samples prepared in Example 1 and Comparative Examples 1-4, as well as the samples after PVDF removal treatment, was characterized. SEM images are shown below. Figure 1As shown, (a) and (e) are SEM images of MIL-53(Fe) prepared in Comparative Example 1 at different magnifications; (b) is a PVDF:MIL-53(Fe) = 1:1 prepared in Comparative Example 2; (c) is a PVDF:MIL-53(Fe) = 1:2 prepared in Example 1; (d) is a PVDF:MIL-53(Fe) = 1:3 prepared in Comparative Example 3; (f) is a SEM image of PVDF:MIL-53(Fe) = 1:1 after PVDF removal treatment; (g) is a SEM image of PVDF:MIL-53(Fe) = 1:2 after PVDF removal treatment; and (h) is a SEM image of PVDF:MIL-53(Fe) = 1:3 after PVDF removal treatment. The PVDF removal process involved washing the material with the organic solvent dmf to remove PVDF from the film that did not participate in the interpenetration reaction. As shown in the figures, the PVDF:MIL-53(Fe) = 1:2 sample prepared in Example 1 exhibits spherical porous structures with a diameter of 4 μm on its surface (as shown in Figure c). After PVDF removal treatment, the MOF structure inside the sample exhibits a coral-like porous structure (as shown in Figure g). The PVDF:MIL-53(Fe) = 1:3 sample prepared in Comparative Example 3 exhibits a dense overall structure on its surface (as shown in Figure d). After PVDF removal treatment, the sample shows a dense spherical structure with a diameter of 20 μm and uniform overall size; no dense porous structure was observed (as shown in Figure h). The MIL-53(Fe) prepared in Comparative Example 1 has a conventional single-crystal structure (as shown in Figures a and e). Overall, the comparison shows that the diameter of the spherical structure increases with the increase of PVDF content. This is because when the PVDF content is low, the distance between the organic ligands and metal ions gradually shortens during the MOF assembly process, increasing the number of MOFs generated and thus increasing the spherical size. Therefore, PVDF has a significant impact on the microstructure of the overall composite material. This indicates that during solvent evaporation, the sequencing and crystallization of PVDF molecular chains occur synchronously with the crystal growth of MIL-53, and the two exhibit a competitive growth relationship in the solvent. The porous structure on the surface is due to the rapid evaporation of the solvent inside the mixed solution under high temperature conditions. Many submicron-sized voids were also found on the surface of the generated MOF structure. Under thermodynamic action, the MOF intermediate structure, which is in an amorphous state with incomplete coordination, transforms into a crystalline MOF structure. Due to the long length and large number of PVDF molecular chains, some PVDF molecular chain segments are embedded into the crystal along with the MOF crystallization to form a new ferroelectric crystal. The exposed PVDF chains restrict the growth of MOF to form a dense spherical structure, resulting in an overall coral-like spherical structure.

[0027] The PVDF:MIL-53(Fe) = 1:2 sample prepared in Example 1 was subjected to brittle fracture under liquid nitrogen conditions, and the microstructure of the resulting fracture surface was characterized. SEM images are shown below. Figure 2 As shown in the figure, the cross-section of the film co-grown by MIL-53 (Fe) and PVDF shows a relatively uniform material composition under low magnification, with no obvious clusters or defects, proving that the co-grown system produces a uniform material film.

[0028] (2) The PVDF:MIL-53(Fe) = 1:2 sample prepared in Example 1 was characterized by TEM, and the results are as follows: Figure 3 As shown in Figure b, the material exhibits a partially disordered PVDF polymer structure under electron beam irradiation using a transmission electron microscope, but overall presents a bulk MOF crystalline state. Further elemental scanning analysis revealed that Fe and F elements are uniformly distributed within the material (as shown in Figures c and d). Further magnification revealed clear MOF crystal lattice fringes with a lattice spacing of 0.255 nm. Figure 4 As shown in Figure a, the lattice spacing measured in other areas of the same copper-clad wire mesh is 0.225 nm, indicating structural differences within different MOF domains. This difference is attributed to the partial embedding of PVDF chains into MOF pores during crystallization, thereby disrupting the lattice parameters. Figure 4 Further analysis in section b revealed a step-like ferroelectric domain structure within the MOF crystal. These structures originate from embedded PVDF chains: under thermal excitation or dipole-dipole interactions, fluorine atoms in the PVDF undergo slight displacement, inducing Fe atoms in the MOF lattice through dipole interaction forces. 3+ Ions undergo displacement. This disrupts the symmetry of the charge distribution and leads to spontaneous polarization (a characteristic feature of ferroelectric behavior).

[0029] (3) XRD patterns of the samples prepared in Example 1 and the comparative example. As shown in the figure, for the XRD image, the diffraction peaks of the MIL-53(Fe) sample prepared in Comparative Example 1 match the peak values ​​of the XRD image simulated by the MOF crystal constructed through modeling. The XRD of MIL-53(Fe) also matches well with the simulated ferroelectric crystal. For MIL-53(Fe), although there are diffraction peaks of MOF crystal planes in the image, some diffraction peaks are significantly different from those of MIL-53(Fe) and fit well with the ferroelectric structure. Compared with MIL-53(Fe), the diffraction peaks of the crystalline phase representing PVDF, α(100), (020), (110), and (021) crystal planes, are at 17.66. o 18.30 o 19.90 o and 26.56 oThe relative intensity of the diffraction peak at this point has weakened or even disappeared compared to MIL-53 (Fe), and the composite film at 19.90... o The nearby diffraction peaks do not exhibit the (021) crystal plane under different stoichiometric conditions. This is because, after competitive growth of MOF and PVDF, the molecular chains are embedded into the MOF voids, improving the dispersion effect and reducing the crystallinity of PVDF. Meanwhile, the free PVDF is subjected to forces from the MOF crystal surface, causing the PVDF crystal orientation to shift from the α phase to the β phase, further increasing the dielectric constant of the material. PVDF at 20.26... o The diffraction peaks representing the β phase (110) and (200) crystal planes of PVDF and the diffraction peak of the γ phase (110) crystal plane appear at 20.04 o The peaks overlapped at point 8, resulting in a broader organic diffraction peak. As the proportion of MOF in PVDF / MOF increases, the peaks at point 8... o The diffraction peak intensity first increased and then decreased, with the highest intensity observed in the PVDF:MIL-53(Fe) = 1:2 sample prepared in Example 1. This indicates that the MOF exhibits the highest degree of crystallinity and forms the most ferroelectric structures at this ratio. These results demonstrate that the introduction of polymers significantly influences the crystallinity of MOFs, and that a shift in the matrix phase distribution towards the β and γ phases is more beneficial for improving the dielectric properties of the composite film.

[0030] (4) To further analyze the influence of MOF crystals on the crystallization behavior of the PVDF matrix, infrared spectroscopy was performed on the samples prepared in Example 1 and the comparative example. The test results are as follows: Figure 6 As shown in the figure, all samples exhibit largely consistent absorption peaks. This is because infrared spectroscopy can detect the absorption peaks of organic functional groups. Although inorganic particles in MOF cannot cause infrared absorption peaks, they affect the crystallinity of the matrix, thus altering the infrared spectrum. Specifically, the Pure PVDF, crystalline co-grown MOF / PVDF (Examples 1, 2, and 3), and blended MOF / PVDF (Comparative Example 4) composite films all show similar absorption peaks at a wavenumber of 614 cm⁻¹. -1 766cm -1 795cm -1 855cm -1 And 97cm -1 An absorption peak representing the α-crystalline phase of PVDF was generated at 855 cm⁻¹, but the intensities of the absorption peaks varied slightly. Specifically, the composite film showed an absorption peak at 855 cm⁻¹. -1 976cm -1 The absorption peak intensity representing the α phase is lower than that of Pure PVDF. Furthermore, the FT-IR spectra of the blended MOF / PVDF composite films are in the 800-880 cm⁻¹ range. -1 Within the frequency band of 840cm-1 and 820cm -1 It exhibits a new characteristic absorption peak at 840 cm⁻¹ -1 The peak at 820 cm⁻¹ represents a mixed absorption peak of the β and γ phases of PVDF. -1 The peak value represents the para-substituted benzene ring in the MOF organic ligand terephthalic acid. This peak value indicates that as the contact between the MOF and PVDF increases, a phase transition occurs in the crystalline phase of the PVDF matrix towards a more polarizable phase. Therefore, the infrared spectroscopy analysis further demonstrates the successful synthesis of an amorphous MOF structure within the material. This amorphous nanostructure exhibits a globally fragmented, interconnected amorphous MOF framework, rather than a single-coordinate molecular microstructure. The synthesis of the crystalline MOF effectively promotes the formation of the β and γ phases in the PVDF matrix, consistent with X-ray diffraction results.

[0031] (5) Figure 7 XPS spectra of the PVDF:MIL-53(Fe) = 1:2 sample prepared in Example 1, analyzed by electron spectroscopy, such as... Figure 7 As shown in Figure a, the peak values ​​of the elements in the full spectrum are in good agreement with the experimental elements. Fine elemental spectral analysis revealed CF and CC bonds at 683-690 eV. For pure PVDF films, the CF bonds consist of both half-ionic and covalent bonds, with peak values ​​at 687.4 eV and 686.3 eV, respectively. This is further confirmed in the fine F-spectrum (as shown in Figure a). Figure 7 As shown in Figure b), compared to pure PVDF, the number of half-ionic bonds is significantly increased and the number of covalent bonds is reduced. This is due to the Fe in MOF. 3+ The F atoms in the ions and PVDF are subjected to interpolar interactions, which increases the bond length of the FC bond and decreases the bond energy of the CF bond, causing more covalent bonds to transform into half-ionic bonds, further confirming the influence of interpolar interactions on the internal microstructure of the material.

[0032] (6) Under N2 protection and at a test temperature of 20~600℃, the PVDF:MIL-53(Fe)=1:2 sample prepared in Example 1 was subjected to thermal analysis. The thermogravimetric-differential thermal analysis results are as follows: Figure 8As shown in the figure, the thermogravimetric analysis spectrum of the composite film exhibits multiple stepped curves, with a transition occurring at 435.35℃. This is due to the decomposition of PVDF at this point. Relatively speaking, the initial decomposition temperature of the composite film is slightly lower than that of pure PVDF material. This is because the amorphous structure dominates in the MOF of the composite film, while a MOF crystal transformation occurs at 200℃. This is because, under thermodynamic action, the PVDF molecular chains detach from the MOF crystal, and the process releases heat from the inside out, accelerating the decomposition of PVDF and making the decomposition temperature slightly lower than that of pure PVDF film. For DSC analysis, the temperature ranges from 20 to 600℃. After 50℃, the overall heat flux is negative. After 440℃, a small portion of the fully coordinated amorphous MOFs gradually transform into crystalline MOFs. This process is exothermic. At 500℃, the MOFs decompose and become endothermic. The DSC curve rises initially due to the endothermic process. After the decomposition reaction begins, the overall curve continues to decline as the material becomes exothermic. At 520℃, terephthalic acid begins to decompose and become endothermic, resulting in an overall heat flux value higher than the previous part of the curve, which is reflected as a higher peak in the DSC curve.

[0033] (7) The room temperature dielectric properties of the samples prepared in Example 1 and the comparative example were characterized, and the results are as follows: Figure 9 As shown in Figure a, (a) is the dielectric constant at room temperature, (b) is the dielectric loss at room temperature, (c) is the ferroelectric hysteresis loop test spectrum of Pure PVDF prepared in Comparative Example 5, (d) is the ferroelectric hysteresis loop test spectrum of PVDF:MIL-53(Fe)=1:1 prepared in Comparative Example 2, (e) is the ferroelectric hysteresis loop test spectrum of PVDF:MIL-53(Fe)=1:2 prepared in Example 1, and (f) is the ferroelectric hysteresis loop test spectrum of PVDF:MIL-53(Fe)=1:3 prepared in Comparative Example 3. As can be seen from Figure a, with the increase of MOF content, the dielectric constant of the crystal co-grown composite system first increases and then decreases. When the ratio of PVDF to MOF is 1:2 (Example 1, PVDF:MIL-53(Fe)=1:2), the peak dielectric constant of the film at 100Hz is 163,588. Example 1 and Comparative Example 5: Pure PVDF (at 10...) 2 The ε' at Hz ≈ 8.97 is 18,237 times higher than that of Comparative Example 4 (PVDF@MIL-53(Fe)=1:2) (ε'≈14.56). Example 1 is 11,235 times higher than that of Comparative Example 4 (PVDF@MIL-53(Fe)=1:2) (ε'≈14.56).

[0034] The significant increase in dielectric constant of the co-grown crystal system is attributed to the formation of a ferroelectric interpenetrating structure, in which PVDF molecular chains are embedded within the MOF crystal framework; this framework is composed of Fe... 3+The PVDF is composed of octahedral coordination units formed by ions and six oxygen atoms. Under the influence of an external electric field, the displacement of fluorine atoms in the PVDF induces Fe... 3+ The movement of ions leads to the formation of large-scale domain structures. This mechanism enhances spontaneous polarization and results in an exponential increase in the dielectric constant. As the MOF content further increases, the solvent reaction environment reduces the MOF grain size, decreasing the probability of successful molecular chain insertion, which in turn reduces the ferroelectric domain density and the overall dielectric response. Therefore, the overall dielectric constant of the PVDF:MIL-53(Fe)=1:3 prepared in Comparative Example 3 is lower than that in Example 1.

[0035] For ordinary nanoparticle doped systems (PVDF@MIL-53(Fe)=1:2 prepared in Comparative Example 4), there is no chemical bonding between MOF particles and the PVDF matrix. Since both are electrically neutral, they do not form significant electrostatic forces. The increase in the dielectric constant of the entire system mainly comes from the Maxwell-Wagner-Sillars interface polarization, which leads to the improvement in dielectric constant.

[0036] Depend on Figure 9 Analysis of b shows that the dielectric loss gradually increases with the addition of MOF. At 10... 3 At Hz frequencies, the loss of pure PVDF is 0.016, while for the crystal co-growth systems (Example 1, Comparative Example 2, and Comparative Example 3), the losses of high-ratio (1:1, 1:2, 1:3) samples are extremely high (tanδ>1, even exceeding 10). This indicates that while achieving a high dielectric constant, significant leakage current is generated within the material. Unlike traditional doped systems (Comparative Example 4), where dielectric loss typically increases at high frequencies, the dielectric loss of the MOF / PVDF co-grown composite material in the high-frequency region shows a trend of first rising to a peak and then decreasing.

[0037] Figure 10The figure shows the Weibull distribution of AC dielectric properties for the samples prepared in Example 1 and the comparative example. As can be seen from the figure, the breakdown field strength of the composite film decreases with different interpenetrating MOF precursor contents compared to Pure PVDF (1235 kV / cm). This is because a small number of uncoordinated ions are present during the synthesis of the interpenetrating MOF structure. Under high voltage, these ions undergo directional migration, increasing the carrier concentration. Simultaneously, the high conductivity interacts with these ions, thus reducing the material's electrical breakdown resistance. The breakdown strength of the composite film decreases with increasing MOF content. According to band structure theory, ferroelectric MOFs, due to their high polarization and the presence of band gaps, possess a low breakdown voltage. As the MOF precursor composition increases, the interaction strength between atoms in the MOF and F atoms in the PVDF increases, and the band gap width improves. However, continuously increasing the precursor concentration leads to excessive ion concentration, forming a conductive loop and significantly reducing the breakdown voltage. Furthermore, high polarization can cause the material to be unable to withstand excessively high field strengths, thus reducing the breakdown field strength.

[0038] Since the maximum electric displacement of the composite thin film is related to the material's own electrical breakdown resistance, further... Figure 4 As shown in c~f, the PVDF:MIL-53(Fe)=1:2 sample prepared in Example 1 exhibits a maximum electric displacement of 38 uC / cm under an electric field strength of 340 kV / mm. 3 Exceeding the maximum electric displacement of Pure PVDF at 211 Mv / m, 6.2 uC / cm. 3 (Depend on Figure 4 (As shown in c and e). Meanwhile, composite films with different MOF precursor solution contents exhibit different polarization intensities. The PVDF:MIL-53(Fe) = 1:1 film, under an electric field strength of 75 kV / mm, shows a maximum electric displacement of 9.8 uC / cm. 3 (Depend on Figure 4 As shown in d), the maximum electric displacement of PVDF:MIL-53(Fe) = 1:3 at an electric field strength of 390 kV / mm reaches 19 uC / cm. 3 (Depend on Figure 4 As shown in f), the electric displacement of the thin film generally increases and then decreases with increasing Fe content. This is because before the composite film is broken down, the electric displacement intensity of the material is closely related to the dielectric constant and depends on the effective electric dipole moment of the material. In the MOF crystal structure, Fe... 3+The octahedral coordination of six oxygen atoms, along with the displacement of electron centers in the metal active sites and F atoms in PVDF, leads to a breakdown of spatial inversion symmetry in the overall cell structure. The space group changes from IMMA to p1, forming a ferroelectric structure, resulting in spontaneous polarization of the material macroscopically and significantly increasing its electric displacement intensity. However, the electro-compression effect of the MOF crystal causes the distance between positive and negative charge centers along the Z-axis to decrease with increasing external field strength. In the X-axis direction, the MOF crystal can be considered stretched, increasing its X-axis polarization intensity. This achieves multiaxial spontaneous polarization, which can increase in a specific direction with varying external field strength. Compared to nanosheet composite films obtained through conventional doping, the spontaneous polarization of the nanosheets enhances the polarization ability of the hydrogen-fluorine bonds in the PVDF matrix. Simultaneously, electrons cause a greater displacement of dipoles under the influence of an electric field, resulting in a higher electric dipole moment. This leads to stronger polarization displacement in the MOF, making the MOF / PVDF composite film exhibit a higher polarization displacement than the PVDF matrix itself.

[0039] In summary, this invention synthesizes a nested composite material using the co-growth of Fe-MOF and PVDF. The high regularity of the MOF host crystal, being an inorganic crystal structure, results in a stepped domain structure, which enhances the high regularity of the molecular segments in the composite material, leading to this stepped domain structure (observed by TEM in the Fe-MOF / PVDF nested composite material). This domain structure contributes to increased molecular polarization, resulting in an ultra-high dielectric constant and improved ferroelectric properties. The final product is a molecular ferroelectric polymer composite film with a dielectric constant of 163588 and a dielectric loss of 1.47. Furthermore, the ferroelectric crystals are randomly dispersed throughout the matrix, giving the composite material both the high polarization of inorganic ferroelectrics and the flexibility of polymer materials. Further molecular dynamics simulations confirmed that the nested structure can break the spatial inversion symmetry of the overall unit cell structure, promoting multiaxial spontaneous polarization and thus achieving the ferroelectric phase transition. Therefore, the synthesis method provided by this invention can be extended to more flexible materials, opening a unique path for the synthesis of high-performance flexible ferroelectric materials. Furthermore, this invention can generate ferroelectric properties through high-temperature sintering using an auxiliary microwave method, reducing the high energy consumption of high-temperature sintering in the synthesis of ceramic ferroelectric materials. Based on the synthesis method provided by this invention, it can be extended to the growth and synthesis processes of high-performance nanoscale dielectrics and other composite functional materials, providing research ideas for the development of mobile and wearable technologies, promoting the rapid development of flexible ferroelectrics, and enabling their large-scale application in various devices.

[0040] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for preparing a superdielectric molecular ferroelectric polymer composite thin film, characterized in that, include: (1) Add PVDF to the MIL-53 precursor solution, stir evenly by ultrasonication, and then cast into a film to obtain a wet film; (2) The wet film was placed in a preheated microwave reactor for heating treatment. After heating, it was quickly cooled to room temperature and placed in a methanol aqueous solution. It was then immersed at 40°C and vacuum dried after immersion to obtain a molecular ferroelectric polymer composite film.

2. The preparation method according to claim 1, characterized in that, (1) The preparation method of the MIL-53 precursor solution is as follows: Fe(NO3)3·9H2O was dissolved in DMF. After complete dissolution, terephthalic acid was added and mechanically stirred to obtain the MIL-53 precursor solution.

3. The preparation method according to claim 2, characterized in that, The molar ratio of Fe(NO3)3·9H2O to terephthalic acid is 0.95:1.

05.

4. The preparation method according to claim 1, characterized in that, (1) The PVDF added to the solution reacts with the Fe in the MIL-53 precursor solution. 3+ The molar ratio of the substances is 1:

2.

5. The preparation method according to claim 1, characterized in that, (1) The ultrasonic stirring time is 30 min.

6. The preparation method according to claim 1, characterized in that, (2) The microwave reaction generator is preheated at 180°C for 10 min; the heating treatment temperature is 120°C for 2 h.

7. The preparation method according to claim 1, characterized in that, (2) The methanol-water solution is a mixture of methanol and water in a mass ratio of 5:1; the wet film after being heated by a microwave reactor is soaked in a methanol-water solution at 40°C for 3 days.

8. The preparation method according to claim 1, characterized in that, (2) The vacuum drying temperature is 70℃ and the time is 20h.

9. A superdielectric molecular ferroelectric polymer composite film obtained by the preparation method according to any one of claims 1 to 8.

10. An application of the superdielectric molecular ferroelectric polymer composite thin film according to claim 9, characterized in that, Used in the fabrication of memory, piezoelectric sensors, and flexible energy storage devices.

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