An electret and a preparation process thereof

Abstract

The application provides a kind of electret and its preparation process, relate to electret technical field.The material of the electret of the application is PVDF, porosity is 1-4%, beta crystal phase content is not less than 17%, relative dielectric constant is not less than 22.The electret can be thin film, can be prepared by using 3D printing process.

Description

Technical Field

[0001] This invention belongs to the field of electret technology and relates to an electret and its preparation process. Background Technology

[0002] Polyvinylidene fluoride (PVDF) has five crystal forms: α, β, γ, δ, and ε. These different crystal forms can interconvert under certain conditions. Among them, the β crystal form is the most important for PVDF used in piezoelectric and pyroelectric applications. In existing technologies, to prepare PVDF films with good piezoelectric, pyroelectric, and micro-deformation properties, the method generally involves increasing the β crystal phase content. For example, Chinese patent CN111958999A discloses a process for preparing high-β crystal phase PVDF films, where the β crystal phase content is not less than 50%, and the dielectric constant at 1 kHz is not less than 10.

[0003] However, the extent to which the β-phase content of PVDF can be increased is limited, which means that the relative permittivity of PVDF often enters a physical bottleneck period after reaching about 12-15, making it difficult to achieve further substantial breakthroughs while maintaining the intrinsic properties of the polymer.

[0004] In traditional dielectric physics, pores are typically considered harmful "defects" that induce partial discharge and degrade electrical performance, and are required to be eliminated as much as possible during fabrication. This has led to a long-standing lack of proactive development of microporous networks as physical potential wells within the industry. The applicant has discovered that electrets with a certain porosity significantly improve the internal charge retention capacity and achieve a nonlinear multiplication of the relative permittivity. Based on this, the present invention is proposed. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides an electret and its preparation process.

[0006] The technical solution of the present invention is as follows: An electret material is provided, wherein the electret is made of PVDF, has a porosity of 1-4%, a β-phase content of not less than 17%, and a relative permittivity of not less than 22 measured at 25°C and 100Hz using the impedance dielectric method.

[0007] Preferably, the porosity is 2-4%.

[0008] Preferably, the β-phase content does not exceed 35%.

[0009] Preferably, the relative permittivity is not less than 25.

[0010] Preferably, the electret is in the form of a thin film.

[0011] A process for preparing the electret according to any of the above embodiments, wherein the PVDF solution is 3D printed into the electret; The 3D printing environment is 20-25℃, relative humidity not exceeding 50%, printing voltage is 4-8KV, and substrate temperature is 70-90℃.

[0012] Preferably, the concentration of the PVDF solution is 10-16 wt%; The weight-average molecular weight (Mw) of PVDF in the PVDF solution is 200,000-600,000 g / mol.

[0013] More preferably, the PVDF is composed of low Mw PVDF and high Mw PVDF in a weight ratio of 1:3 to 3:1; The Mw of the low MwPVDF is 200,000-300,000 g / mol; The Mw of the high MwPVDF is 450,000-600,000 g / mol.

[0014] Preferably, the printing voltage is 6-8KV.

[0015] Preferably, the substrate temperature is 75-90°C.

[0016] The beneficial effects of this invention are: (1) The PVDF electret of the present invention has a certain porosity, and a high relative permittivity can be obtained with a low β crystal phase content.

[0017] (2) The present invention uses 3D printing technology to prepare PVDF electrets. By adjusting the substrate temperature and printing voltage, electrets with a certain porosity and a high relative permittivity can be obtained.

[0018] (3) This invention reveals that the dominant mechanism for the rise in dielectric constant of polymer matrix is ​​not a single crystal phase contribution, but is driven by the additional polarization response (dipole contribution) generated by the resident charge in the pores under the alternating electric field, providing a new path for the design of flexible high dielectric materials. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the 3D printing process in Example 1.

[0020] Figure 2 This is a schematic diagram of the mechanism by which the microporous structure of Example 1 captures charge.

[0021] Figure 3 This is a photograph of the thin film obtained in Example 1.

[0022] Figure 4Comparison of Micro-CT three-dimensional reconstruction and cross-sectional pore distribution of PVDF films in Examples 5 and 6.

[0023] Figure 5 The images show SEM images of the evolution of typical pore structures of thin films obtained at different substrate temperatures as shown in Examples 1-6, and their corresponding binarized pore identification mapping images. Detailed Implementation

[0024] The technical solution of the present invention will be further explained and described below through specific embodiments.

[0025] On the one hand, the present invention proposes an electret made of PVDF with a porosity of 1-4%, a β-phase content of not less than 17%, and a relative permittivity of not less than 22 measured at 100 Hz using the impedance dielectric method at 25 ℃.

[0026] The micropores of electrets act as "physical traps" for space charge, stably capturing it at the pore boundaries and within the cavities. Due to the high electrical insulation of the polymer matrix (PVDF), the migration and dissipation of space charge to the electret surface are effectively prevented, thus forming stable resident charge centers at the microscopic level. This achieves a nonlinear multiplication of the relative permittivity at low β-phase content, with the relative permittivity of the PVDF electret jumping from around 10 to no less than 22 at 25℃.

[0027] For example, the porosity can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, etc.

[0028] In some embodiments, the porosity is 2-4%, which is within the range of electret porosity, and the electret has a higher relative permittivity, which can reach 25 or higher. Furthermore, considering the stability of the microporous structure, the porosity can be 2-3.5%.

[0029] In some embodiments, the β-phase content does not exceed 35%. For example, the β-phase content can be 17%, 19%, 20%, 22%, 23%, 25%, 27%, 29%, 30%, 32%, 33%, 35%, etc. Therefore, the PVDF electret of the present invention can achieve a high relative permittivity with a low β-phase content.

[0030] In some embodiments, the relative permittivity is not less than 25.

[0031] In some embodiments, the electret is in the form of a thin film. That is, the electret in this invention can be a PVDF thin film.

[0032] On the other hand, the present invention also proposes a preparation process for the electret described in any of the above embodiments, wherein the PVDF solution is 3D printed into an electret. The ambient temperature for 3D printing is 20-25℃, the relative humidity is no more than 50%, the printing voltage is 4-8KV, and the substrate temperature is 70-90℃.

[0033] 3D printing of PVDF electrets can create stable porous structures with uniformly distributed pores between the printing track and gaps. Furthermore, the stable process parameters facilitate the spatial distribution of charge. During printing, the injected space charge is stably trapped at the pore boundaries and within the cavities. Due to the high electrical insulation of the polymer matrix, the migration and dissipation of space charge to the surface of the PVDF electret (e.g., thin films) are effectively prevented, thus forming stable resident charge centers at the microscopic level. By controlling the ambient temperature and relative humidity, the crystallization kinetics caused by solvent evaporation and the electrostatic dissipation during charge injection can be balanced.

[0034] For example, the printing voltage can be 4KV, 5KV, 6KV, 7KV, 8KV, etc., and the substrate temperature can be 70℃, 75℃, 80℃, 85℃, 90℃, etc. Adjusting the substrate temperature and printing voltage can adjust the porosity of the PVDF electret. In particular, higher substrate temperatures (70-90℃) can drastically change the curing kinetics of the PVDF solution and induce the formation of a high-density porous network.

[0035] In some embodiments, the concentration of the PVDF solution is 10-16 wt%; The weight-average molecular weight (Mw) of PVDF in PVDF solution is 200,000-600,000 g / mol.

[0036] For example, the concentration of PVDF solution can be 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, etc.; the solvent of PVDF solution can be N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), triethyl phosphate (TEP), etc.

[0037] The weight-average molecular weight (Mw) of PVDF can be 200,000 g / mol, 250,000 g / mol, 300,000 g / mol, 350,000 g / mol, 400,000 g / mol, 450,000 g / mol, 500,000 g / mol, 550,000 g / mol, 600,000 g / mol, etc.

[0038] In some embodiments, PVDF is composed of low Mw PVDF and high Mw PVDF in a weight ratio of 1:3 to 3:1. The Mw of low MwPVDF is 200,000-300,000 g / mol; The Mw of high MwPVDF is 450,000-600,000 g / mol.

[0039] Low-MwPVDF exhibits better processing flowability, film uniformity, crystallinity, and phase structure, while high-MwPVDF possesses higher strength, toughness, and thermal stability. Combining low-MwPVDF and high-MwPVDF can synergistically produce electrets with superior performance, such as PVDF films.

[0040] For example, the weight ratio of low MwPVDF to high MwPVDF can be 1:3, 1:2, 1:1, 2:1, 3:1, etc.

[0041] In some embodiments, the printing voltage is 6-8KV.

[0042] In some embodiments, the substrate temperature is 75-90°C.

[0043] This invention achieves effective control of the internal pores of electrets by precisely adjusting the process parameters of 3D printing. The highly connected and diffusely distributed pore network can provide a more sufficient physical potential well to anchor the space charge generated by electrical injection, thereby driving a significant improvement in dielectric properties.

[0044] The technical solution of the present invention will be further described and illustrated below with reference to various embodiments. Unless otherwise specified, the parts mentioned in the following embodiments are parts by weight.

[0045] Example 1 The PVDF solution consisted of low-Mw PVDF and high-Mw PVDF dissolved in NMP at a weight ratio of 1:1, with a concentration of 10 wt%. The low-Mw PVDF was commercially available Kynar. ® 721, Mw is approximately 275000 g / mol; high Mw PVDF is commercially available Kynar. ® 761, Mw is approximately 534000 g / mol.

[0046] 3D printing is performed at an ambient temperature of 23-25℃ and a relative humidity of 40-45% to form a thin film. The printing voltage is 6KV and the substrate temperature is 75℃.

[0047] The schematic diagram of the 3D printing process in this embodiment is attached. Figure 1 As shown in the attached diagram, the mechanism by which the microporous structure traps charge is illustrated. Figure 2 As shown in the attached photograph. This embodiment yields a physical photograph of the PVDF film. Figure 3 As shown.

[0048] Example 2-11 Examples 2-11 are based on Example 1, with adjustments made to the printing process parameters, including printing voltage, substrate temperature, and PVDF solution concentration.

[0049] The results are shown in Table 1 below.

[0050] Table 1

[0051] As shown in Table 1 above, under the same printing voltage, the porosity, β-phase content, and relative permittivity adjust accordingly with the substrate temperature. However, when the printing voltage is 6KV and the substrate temperature is 90℃ (Example 5), compared to substrate temperatures of 70℃ (Example 2) and 75℃ (Example 1), the β-phase content decreases, but the porosity and relative permittivity significantly increase. The relative permittivity of Example 5 reaches 32.6, which is several times higher than the relative permittivity of the non-porous PVDF film (10-12). When the substrate temperature is 95℃ (Example 6), the porosity decreases significantly, resulting in a significant decrease in the relative permittivity.

[0052] Appendix Figure 4 The images show a comparison of the micro-CT three-dimensional reconstruction and cross-sectional porosity distribution of the PVDF films of Example 5 (sample 1) and Example 6 (sample 2). Figure AD is the image of sample 1 (Example 5), and Figure EH is the image of sample 2 (Example 6), showing a significant difference in porosity.

[0053] Appendix Figure 5 The images shown are SEM images (AC and GI) illustrating the evolution of typical pore structures of thin films obtained at different substrate temperatures in Examples 1-6, along with their corresponding binary pore identification maps (DF and JL). Examples 1 correspond to Figures B and E, Examples 2 to Figures A and D, Examples 3 to Figures C and F, Examples 4 to Figures G and J, Examples 5 to Figures H and K, and Examples 6 to Figures I and L. As described above, the basic principles, main features, and advantages of the present invention have been shown and described. Those skilled in the art should understand that the present invention is not limited to the above embodiments, which are merely preferred embodiments and should not be construed as limiting the scope of the invention. All equivalent changes and modifications made in accordance with the scope of the patent and the description should still fall within the scope of the present invention. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A process for preparing an electret, characterized in that, The PVDF solution was 3D printed into the electret. The 3D printing environment temperature is 20-25℃, the relative humidity does not exceed 50%, the printing voltage is 4-8KV, and the substrate temperature is 70-90℃. The electret is made of PVDF with a porosity of 1-4%, a β-phase content of not less than 17%, and a relative permittivity of not less than 22 measured at 25°C and 100Hz using the impedance dielectric method. The PVDF solution contains PVDF composed of low Mw PVDF and high Mw PVDF in a weight ratio of 1:

1. The Mw of the low MwPVDF is 200,000-300,000 g / mol; The Mw of the high MwPVDF is 450,000-600,000 g / mol.

2. The electret preparation process according to claim 1, characterized in that, The concentration of the PVDF solution is 10-16 wt%.

3. The electret preparation process according to claim 1, characterized in that, The printing voltage is 6-8KV.

4. The electret preparation process according to claim 1, characterized in that, The substrate temperature is 75-90℃.

5. The electret preparation process according to claim 1, characterized in that, The porosity is 2-4%.

6. The electret preparation process according to claim 1, characterized in that, The β-phase content does not exceed 35%.

7. The electret preparation process according to claim 1, characterized in that, The relative permittivity is not less than 25.

8. The preparation process of the electret according to claim 1, characterized in that, The electret is in the form of a thin film.

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