Polyvinylidene fluoride glue preparation system suitable for spin-coating process and method for improving piezoelectric property by using polyvinylidene fluoride glue preparation system

Abstract

The invention provides a polyvinylidene fluoride gel preparation system suitable for a spin coating process and a method for improving piezoelectric performance, the gel preparation system is composed of PVDF and a solvent, and the mass ratio of the solid content of PVDF is 10%-20%; the PVDF is a polyvinylidene fluoride-trifluoroethylene copolymer, the mass ratio of the polyvinylidene fluoride is 78 to 81 percent, and the mass ratio of the trifluoroethylene is 19 to 22 percent; the solvent is an MEK-DMF solvent system, the mass ratio of MEK butanone is 20-40%, the mass ratio of DMF nitrogen-nitrogen dimethyl formamide is 60-80%, the method is suitable for a spin-coating process, and the piezoelectric property of PVDF is improved, so that the requirement of customized selection of a PVDF processing procedure is met.

Description

Technical Field

[0001] This invention belongs to the field of piezoelectric materials technology, specifically relating to a polyvinylidene fluoride (PVDF) formulation suitable for spin-coating processes and a method for improving piezoelectric performance. By adjusting the PVDF formulation ratio, solvent selection, crystallization conditions, and polarization conditions, the PVDF coated on the wafer generates a high piezoelectric effect, improving piezoelectric performance to meet product requirements. Background Art

[0002] PVDF (Polyvinylidene fluoride) piezoelectric film is a new polymer piezoelectric material. Sensors made from it offer advantages such as high sensitivity, wide bandwidth, low acoustic impedance, high voltage output, and the ability to be processed into specific shapes. They are widely used in various fields. The piezoelectric effect of PVDF material is due to the asymmetric arrangement of its molecular chains and the piezoelectric properties of its crystal structure. When external pressure or tension is applied, the PVDF molecular chains distort, generating a polarization effect. This causes the positive and negative charges on the material's surface to separate, creating a potential difference and, consequently, generating a voltage. The generation of this piezoelectric effect is closely related to the application of reverse pressure or tension. To achieve piezoelectricity, PVDF undergoes a crystallization process to transform the polymer's α-phase into a β-phase. Then, through strong electric field polarization, the molecular dipoles are oriented along the direction of the electric field, generating a macroscopic piezoelectric effect. The β-phase is the crystalline phase with the greatest piezoelectric effect. Summary of the Invention

[0003] In order to solve the problems existing in the prior art, the present invention provides a polyvinylidene fluoride adhesive system suitable for a spin coating process and a method for improving piezoelectric performance thereof.

[0004] As one aspect of the present invention, the present invention provides a glue preparation system suitable for a spin coating process, which is composed of PVDF and a solvent, wherein the mass proportion of the PVDF solid content is 10% to 20%; the PVDF is a polyvinylidene fluoride-trifluoroethylene copolymer, wherein the mass proportion of vinylidene fluoride is 78 to 81%, and the mass proportion of trifluoroethylene is 19 to 22%; the solvent is an MEK-DMF solvent system, wherein the mass proportion of MEK butanone is 20 to 40%, and the mass proportion of DMF nitrogen dimethylformamide is 60 to 80%.

[0005] Preferably, a MEK-DMF solvent system is first poured into the reactor and stirred for half an hour. After half an hour, PVDF is added in small amounts several times and stirred for 4-6 hours. After filtering, a clear and transparent PVDF slurry is obtained.

[0006] As one aspect of the present invention, the present invention provides an application of a glue dispensing system suitable for a spin coating process, which comprises the following steps:

[0007] Spin coating: Spin coating the adhesive system described in claim 1 on the silicon wafer;

[0008] Crystallization: After spin coating, use a nitrogen oven at a crystallization temperature of 120-160°C for 1-3 hours.

[0009] Polarization: After crystallization is completed, polarization treatment is performed through a polarization machine.

[0010] Preferably, the spin coating is performed by selecting a suitable rotation speed to achieve a target adhesive thickness, and selecting a suitable pre-baking temperature to make the appearance transparent after spin coating without whitish or discolored.

[0011] Preferably, the target thickness is 9-11 um.

[0012] Preferably, the polarization has an upper voltage of 9 to 15 kV, a lower voltage of 4 to 10 kV, and a duration of 10 to 30 minutes.

[0013] Preferably, the crystallization temperature is 120-145°C.

[0014] Beneficial effects:

[0015] The present invention provides a polyvinylidene fluoride adhesive system suitable for spin coating technology and a method for improving the piezoelectric performance thereof. The piezoelectric performance of PVDF is improved by adjusting the adhesive system, thereby meeting the demand for customized selection of PVDF processing procedures. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 This is the experimental diagram of testing d33 offline after polarization is completed;

[0018] Figure 2 This is the effect of polarization voltage and time on d33 performance. DETAILED DESCRIPTION

[0019] The endpoints of the ranges and any values disclosed herein are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in the present invention.

[0020] The raw materials used in the present invention are all common raw materials in the field, and those skilled in the art can directly purchase them from the market or prepare the same / similar raw materials by themselves.

[0021] The following will be combined with specific embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0022] Example 1

[0023] The process of the present invention uses a PVDF-MEK-DMF system. The PVDF powder is a polyvinylidene fluoride-trifluoroethylene copolymer, with 78-81% vinylidene fluoride and 19-22% trifluoroethylene. The solvent system, MEK-DMF, contains 20-40% by weight of MEK (butanone) and 60-80% by weight of DMF (nitrogen-nitrogen dimethylformamide). The PVDF solid content is 10-20% by weight.

[0024] PVDF slurry preparation: The slurry is prepared by stirring in a reactor to dissolve the PVDF powder into a slurry that does not react after mixing. A mixed solution of MEK and DMF is first placed in the reactor and stirred at 300 rpm for half an hour. After half an hour, the powder is added in small portions at 700 rpm and stirred for 4-6 hours at a temperature of 50-70°C. Filter the mixture to obtain a clear, transparent PVDF slurry. The d33 value of this slurry indicates that it is suitable for spin coating. PVDF spin coating: Spin coat the PVDF slurry onto a silicon wafer. Select an appropriate rotational speed (650-750 rpm) to achieve the target slurry thickness (the effect of slurry thickness on d33 values is shown in Table 1). Select an appropriate pre-bake temperature (70-110°C) to ensure a transparent surface after spin coating, free of whitish discoloration. Whitish discoloration is primarily due to delayed evaporation of the solvent in the PVDF slurry. Any whitish discoloration will affect the final piezoelectric performance of the PVDF.

[0025] Table 1

[0026] Average PVDF adhesive thickness (um) average value Maximum Minimum 9.87um 24.66 25 23 10.10um 24.83 26 24 11.17um 24.72 26 23

[0027] PVDF crystallization: After spin coating, use a nitrogen oven to crystallize at a temperature of 120-145°C for 1-3 hours. The d33 value of PVDF films obtained without crystallization is between the Curie temperature and melting point of the copolymer. The effects of crystallization temperature and time on d33 performance are shown in Table 2 below.

[0028] Table 2

[0029] Annealing (crystallization) average value 120℃，2H 16.55 120℃,60Min 15.32 140℃，2H 22.34 140℃,60Min 21.85 155℃，2H 19.23 155℃,60Min 18.92

[0030] PVDF polarization: After the crystallization is completed, the polarization treatment is carried out by the polarization machine, wherein the upper voltage is 9-15kV, the lower voltage is 4-10kV, and the duration is 10-30min. After the polarization is completed, it can be left to stand for more than 24 hours through the offline method (see below Figure 1 ) Test d33≥24PC / N, thus achieving the required PVDF piezoelectric performance. The influence of polarization voltage and time on d33 performance is shown below Figure 2 And Table 3.

[0031] Table 1

[0032]

[0033] Example 2

[0034] In addition to the MEK-DMF system, the present invention also tried the MEK-PGMEA system, in which the PVDF powder is a polyvinylidene fluoride-trifluoroethylene copolymer, of which vinylidene fluoride is 78-81% and trifluoroethylene is 19-22%; the solvent system is MEK-PGMEA, in which MEK (butanone) accounts for 50% by mass and PGMEA (propylene glycol methyl ether acetate) accounts for 50% by mass; and the mass proportion of PVDF solid content is 12% to 18%.

[0035] PVDF slurry preparation: The purpose of preparing the slurry is to dissolve the PVDF powder into a slurry in a reactor with stirring. The purpose is to prevent reaction after mixing. A mixed solution of MEK and PGMEA is first poured into the reactor and stirred at 300 rpm for half an hour. After half an hour, the powder is added in small amounts and stirred at 700 rpm for 4-6 hours at a temperature of 30-40°C. Filter to obtain a clear, transparent PVDF slurry. The d33 data of this slurry indicates that it is suitable for spin coating.

[0036] PVDF Spin Coating: Spin-coat the PVDF slurry onto the silicon wafer. Select an appropriate spin speed (650-750 rpm) to achieve the target slurry thickness (the effect of slurry thickness on d33 values is shown in Table 1). Select an appropriate pre-bake temperature (70-110°C) to ensure a transparent appearance after spin coating, without any whitish discoloration. Whitish discoloration is primarily due to the solvent in the PVDF slurry not evaporating in time. Any whitish discoloration will affect the final PVDF piezoelectric performance.

[0037] The average d33 value of this system is 22-23 pC / N, which is 1-2 pC / N lower than that of the MEK-DMF system. Therefore, the MEK-DMF glue system is currently selected.

[0038] The piezoelectricity of PVDF stems primarily from the directional arrangement of dipoles in its polar crystalline phases (β and γ phases), while the α phase is a nonpolar crystalline phase with weaker piezoelectricity. The molecular chains in the α phase (nonpolar) exhibit a TGTG' configuration (alternating trans-parallel arrangement), with the dipole moments of adjacent chains in opposite directions, resulting in macroscopic polarization cancellation. This results in low piezoelectricity, but high thermodynamic stability and is easily formed through melt cooling. The molecular chains in the β phase (polar) exhibit a fully trans (TTT) configuration, with highly ordered dipoles along the molecular chain, resulting in spontaneous polarization and strong piezoelectricity. This requires induction through stretching, electric field polarization, or high-pressure treatment. The piezoelectric constant d33 of PVDF is positively correlated with its β phase content. The theoretical d33 of pure β-phase PVDF can reach ~30 pC / N (single crystal or highly oriented film), but in practice, this is only slightly less than half of that. Actual materials often contain α / β mixed phases. The presence of α phase can reduce the overall polarization efficiency, but an appropriate amount of α phase can improve stability by enhancing interfacial polarization or mechanical support. However, too much α phase will cause d33 to drop sharply.

[0039] The present invention provides a polyvinylidene fluoride adhesive system suitable for spin coating technology and a method for improving the piezoelectric performance thereof. The piezoelectric performance of PVDF is improved by adjusting the adhesive system, thereby meeting the demand for customized selection of PVDF processing procedures.

[0040] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A glue dispensing system suitable for spin coating process, characterized by: The invention is composed of PVDF and a solvent, wherein the weight ratio of the PVDF solid content is 10% to 20%; the PVDF is a polyvinylidene fluoride-trifluoroethylene copolymer, wherein the weight ratio of vinylidene fluoride is 78% to 81% and the weight ratio of trifluoroethylene is 19% to 22%; the solvent is a MEK-DMF solvent system, wherein the weight ratio of MEK butanone is 20% to 40% and the weight ratio of DMF nitrogen dimethylformamide is 60% to 80%.

2. The method for preparing the adhesive dispensing system suitable for spin coating process according to claim 1, characterized in that: Pour the MEK-DMF solvent system into the reactor and stir for half an hour. After half an hour, add PVDF in small amounts several times and stir for 4-6 hours. After filtering, a clear and transparent PVDF slurry is obtained.

3. The use of the adhesive dispensing system suitable for spin coating process according to claim 1, characterized in that: The following steps are included: Spin coating: Spin coating the adhesive system described in claim 1 on the silicon wafer; Crystallization: After spin coating, use a nitrogen oven at a crystallization temperature of 120-160°C for 1-3 hours. Polarization: After crystallization is completed, polarization treatment is performed through a polarization machine.

4. The use according to claim 3, characterized in that: The spin coating is to select a suitable rotation speed to achieve the target adhesive thickness, and to select a suitable pre-baking temperature to make the appearance transparent after spin coating without whitish or discolored.

5. The use according to claim 4, characterized in that: The target thickness is 9-11 um.

6. The use according to claim 3, characterized in that: The polarization has an upper voltage of 9 to 15 kV, a lower voltage of 4 to 10 kV, and a duration of 10 to 30 minutes.

7. The use according to claim 3, characterized in that: The crystallization temperature is 120-145°C.

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