A method for preparing a porous dielectric layer structure for a flexible capacitive sensor

Abstract

The application belongs to the field of flexible sensors and relates to a preparation method of a porous dielectric layer structure for a flexible capacitive sensor. The core of preparing the dielectric layer structure of the capacitive sensor is divided into two parts. One is to select a commercial sandpaper with a suitable mesh number and a vacuum negative pressure value to ensure that the dielectric layer film has a rich porous structure under the condition. The other is to control the dielectric constant of the dielectric layer by doping different concentrations of dielectric enhancement materials CNT in PDMS (the mixed solution of PDMS and CNT is referred to as CPDMS), and by setting the rotation speed of the spin coater, the thickness of the dielectric layer film can be controlled under the action of the centrifugal force of the CPDMS on the sandpaper, and the sensitivity of the sensor is optimized.

Description

Technical Field

[0001] This invention belongs to the field of flexible sensors and relates to a method for preparing a porous dielectric layer structure for flexible capacitive sensors. Background Technology

[0002] In recent years, research on flexible pressure sensors has matured, leading to their application in various fields such as medical health, industrial manufacturing, robotics, and sports. However, with in-depth research in these fields, flexible pressure sensors are increasingly struggling to meet demands, especially for high sensitivity. Further improving the sensitivity of flexible pressure sensors has become a key research focus for researchers both domestically and internationally. Based on their working mechanisms, pressure sensors can be broadly classified into four categories: capacitive, piezoresistive, piezoelectric, and triboelectric. Capacitive pressure sensors are widely used due to their simple manufacturing process, low cost, simple signal processing, high sensitivity, low hysteresis, good dynamic performance, low sensitivity to humidity and temperature, and ability to detect both static and dynamic forces.

[0003] To further improve the performance of flexible capacitive sensors, researchers are currently focusing on the fabrication methods of the dielectric layer, simplifying the fabrication process and enhancing the compressibility of the dielectric layer to improve sensor sensitivity. Since the dielectric layer is typically made of elastomers with high Young's modulus, introducing a specific dielectric structure can reduce the compressive modulus and thus improve sensitivity. Commonly used methods for fabricating dielectric layer structures include molding, creating micro-pyramids, micro-convexities, micro-dots, and micro-pillars. This induces the formation of the dielectric layer structure; typically, these microstructures are located on the surface of the dielectric layer and are easily deformed under external pressure, thereby increasing sensitivity.

[0004] There are two problems with the casting method for preparing dielectric layer structures. First, it is extremely difficult to prepare dielectric layer structures with high aspect ratios. During the template making process, the swelling properties of materials and organic solvents (such as PDM S and acetone) can lead to dielectric layer structure breakage and residue in the hard template during demolding, affecting the surface quality of the dielectric layer and making subsequent template cleaning difficult. Second, the casting method requires custom-made templates for the dielectric layer structure. The templates are small, resulting in high manufacturing costs and making it inconvenient for mass production. Some methods use plant leaves as templates for preparing dielectric layer structures, but the easy deformation of leaves makes it difficult to guarantee the quality of the dielectric layer structure, which is also not conducive to mass production.

[0005] This invention addresses common problems in the fabrication of dielectric layer structures for the aforementioned sensors by proposing a novel method for preparing such structures. The core of fabricating the dielectric layer structure for capacitive sensors consists of two parts: first, selecting commercially available sandpaper of appropriate mesh size and a vacuum negative pressure value to ensure the dielectric film possesses a rich porous structure under these conditions; second, controlling the dielectric constant of the dielectric layer by doping PDMS with different concentrations of CNTs (a mixed solution of PDMS and CNTs, abbreviated as CPDMS). By setting the spin coating speed of the spin coater, the thickness of the dielectric film can be controlled under centrifugal force on the CPDMS on the sandpaper, thus optimizing the sensor's sensitivity. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing porous dielectric thin film structures, and to propose a simple and reliable method for preparing dielectric layer structures.

[0007] To achieve the above objectives, the technical solution of this invention is as follows:

[0008] This invention uses sandpaper as a substrate and vacuum negative pressure to spin-coat a CPDMS mixed solution onto the sandpaper. The interface between the rough surface of the sandpaper and the CPDMS mixed solution contains abundant air gaps, which play a crucial role in the formation of pores in the dielectric layer film. The dielectric layer film sample is transferred into a vacuum drying oven preheated to a certain temperature. Under appropriate air pressure, the air gaps are maintained within the dielectric layer film structure, successfully introducing the air gaps into the interior of the dielectric layer film. The dielectric layer film not only forms a sandpaper microstructure on its surface but also a porous dielectric layer structure inside, ensuring high compressibility of the dielectric layer. This provides a low-cost, easy-to-prepare, and highly reliable method for preparing porous dielectric layer structures.

[0009] This invention provides a highly sensitive flexible capacitive sensor, the overall structure of which is as follows: Figure 1 As shown, from top to bottom are the upper aluminum foil electrode plate, the porous dielectric layer, and the lower aluminum foil electrode plate. By changing the compressive modulus and dielectric constant of the dielectric layer, a wide range and high sensitivity of the sensor can be achieved.

[0010] In the above technical methods, the porous dielectric layer structure is prepared by using CNT (carbon nanotubes) as the material to enhance dielectric properties, and PDMS (polydimethylsiloxane) as the elastomer matrix material.

[0011] The above-mentioned method for preparing porous dielectric layer structures includes the following steps:

[0012] a. Measure 25-35 ml of n-hexane into a beaker, add 2-5 g of PDMS main agent, and stir magnetically for 10 min to obtain mixed solution 1.

[0013] b. Weigh out the CNTs containing the dielectric enhancement material according to a mass concentration of 0.3%, add them to the mixed solution 1, and sonicate the mixed solution 1 at 64W for 20 minutes to obtain a mixed solution 2 containing the CNTs containing the dielectric enhancement material evenly dispersed.

[0014] c. Place the beaker containing mixed solution 2 on the hot plate of the fume hood, heat and stir magnetically until the organic solvent n-hexane completely evaporates.

[0015] d. Add PDMS and curing agent at a mass ratio of 10:1, and stir magnetically for 10 minutes to obtain a mixed solution 3.

[0016] e. Select 100-grit sandpaper. First, clean the surface of the sandpaper with alcohol. Place the sandpaper containing alcohol on a hot plate to dry it. Then, spray a fluorosilane solution (the fluorosilane solution is composed of trichlorosilane and isopropanol mixed in a volume ratio of 1:100) onto the cleaned and dried sandpaper surface. Finally, place the sandpaper sprayed with fluorosilane on a hot plate to complete the pretreatment of the sandpaper surface.

[0017] f. Cut sandpaper to 4cm x 4cm size, and control the thickness of the sample film according to the different speeds of the spin coater.

[0018] g. Place the prepared sample film of a certain thickness into a vacuum drying oven preheated to 80℃, and use an air pump to provide a pressure of -70 to -100 kPa. Figure 2 (a) The spin-coated structure shown (named SPS): The sample film was placed in a preheated 80°C vacuum drying oven and kept at a constant pressure of -70 kPa until abundant bubbles appeared on the sample film, and then kept at the same temperature and pressure for 2 hours. Figure 2 (b) The unstructured sample film (named NPS) is placed in a vacuum drying oven preheated to 80°C and kept at a constant pressure of -100 kPa until the air bubbles at the interface between the sample film and the sandpaper disappear completely. Then, the constant temperature and pressure are maintained for 2 hours.

[0019] h. After the sample film has solidified, remove the sample film and cool it to room temperature. Then, remove the film and cut it to a size of 1cm×1cm to obtain the dielectric layer film. Use PI tape to encapsulate the SPS and NPS structure dielectric layer films between the upper and lower aluminum foil electrode plates. Use conductive silver paste to fix the wires to both sides of the upper and lower aluminum foil electrode plates to obtain the capacitive sensor.

[0020] This invention uses aluminum foil as the electrode and innovatively employs a vacuum negative pressure template-free method to prepare SPS thin films. Due to its rich microstructure on the surface and inside, high compressibility, and high porosity, SPS thin films are suitable as dielectric layers for capacitive sensors. (See below.) Figure 2 As shown in (a)(b). Figure 2(a) is an innovative method for preparing porous dielectric layer structures without templates under vacuum negative pressure. For SPS structured thin films, due to the air gap at the interface between CPDMS and sandpaper, a large number of bubbles are formed inside the film under vacuum conditions of -70 kPa. The film has abundant pores and surface sandpaper microstructures inside. Figure 2 (b) represents the traditional film-forming method. For NPS structured films, under a negative pressure of -100 kPa, the air gap at the CPDMS-sandpaper interface is completely eliminated. The film only contains sandpaper microstructures on its surface. Since CPDMS elastomer material has a high modulus, its compressibility is mainly improved by the deformation of the microstructures on the film surface. Therefore, the NPS structured film only has microstructures on its surface and no internal pores, resulting in poor compressibility. From the perspective of sensor performance, NPS structured films are difficult to use as dielectric layers in sensors. The characterization of two different methods for preparing dielectric layer film structures for CNT films without dielectric enhancement materials in PDMS is shown in the figure. Figure 3 (a) and (d) represent digital images of SPS and NPS structured thin films, respectively. Figure 3 (b) and (e) show the SEM cross-sectional images of the SPS and NPS structured thin films, respectively. Figure 3 (c) and (f) represent the SEM surface images of SPS and NPS structured films, respectively. In summary, traditional film-making methods using sandpaper as a template produce films with only surface microstructures and no internal pores. However, the new film-making method shows that the digital image (a) of the SPS structured film, compared to the digital image (b) of the NPS structured film, has abundant surface pores. The SEM cross-sectional image (b) of the SPS structured film, compared to image (e) of the NPS structured film, contains a large number of internal pores. The SEM surface images (c) and (f) of both SPS and NPS structured films contain abundant sandpaper surface microstructures. Because the SPS structured film contains abundant surface and internal porous structures, it exhibits good compressibility, while the compressibility of the NPS structured film is mainly provided by the structural deformation created by the sandpaper surface microstructure. (See below) Figure 4 As shown, stress-strain tests were performed on SPS and NPS thin films prepared by two different processes. The tests revealed that SPS thin films exhibited better compressibility under pressure than NPS thin films, indicating that the SPS thin film structure is more suitable as the dielectric layer for capacitive sensors. The dielectric constant was controlled by doping the dielectric layer with different CNT concentrations as follows: Figure 5 The image shows the encapsulation tests using thin films with 0% SPS, 0% NPS, 0.3% SPS, and 0.3% NPS structures as dielectric layers. Figure 5 (a) and (b) show the absolute capacitance and relative compatibility of the sensor under pressure, respectively. Data analysis shows that the sensitivities of 0% SPS, 0% NPS, 0.3% SPS, and 0.3% NPS in the 0-5 kPa range are 0.0368 kPa. -1 0.0151 kPa-1 0.6144 kPa -1 0.3641 kPa -1 The sensitivity in the 5-40 kPa range is 0.0129 kPa. -1 0.0039 kPa -1 0.2973 kPa -1 0.1832 kPa -1 In summary, the new film-forming method shows that using a 0.3% SPS film as a dielectric layer significantly improves the sensitivity of the sensor.

[0021] Compared with the prior art, the breakthrough of this invention is reflected in the following two aspects:

[0022] First: From a technological perspective

[0023] This invention uses sandpaper as a substrate to prepare dielectric thin films with rich porous structures without the need for other templates. Traditional processes for preparing dielectric structures require customized templates, mainly introducing microstructures to reduce the compressive modulus of the dielectric layer to improve sensitivity. Compared with traditional preparation processes, this invention only requires sandpaper as a substrate and vacuum negative pressure adsorption, offering advantages such as simplicity, ease of preparation, and stable porous dielectric layer structure.

[0024] Second: Structural perspective

[0025] For the fabrication of dielectric layer microstructures, we propose a method using sandpaper as a substrate to introduce air gaps into the microstructure. The resulting dielectric layer film, formed under vacuum negative pressure, possesses not only abundant porous microstructures internally but also sandpaper-like microstructures on its surface. Comparison of methods for fabricating dielectric layer microstructure films: (e.g.) Figure 2 (a) shows the vacuum negative pressure templateless method used in this invention, such as Figure 2 (b) shows a traditional method for preparing dielectric layer microstructure thin films, in which the films prepared by this method only have microstructures on the surface. (See below) Figure 4 The figure shows the stress-strain diagrams of the thin film prepared by the process of this invention and the thin film prepared by the conventional process, which demonstrate the excellent compressibility of the thin film structure.

[0026] Dielectric thin films were prepared using the vacuum negative pressure template-free method of this invention and the conventional preparation method, respectively. The prepared dielectric thin films were then subjected to encapsulation testing. Figure 5 The sensor shown exhibits superior performance with a lower initial capacitance and higher sensitivity compared to the 0.3% NPS dielectric layer in the 0-40 kPa range, thanks to the 0.3% SPS dielectric layer. Attached image description:

[0027] Figure 1 This is a schematic diagram of the capacitive pressure sensor structure of the present invention;

[0028] Figure 2 Schematic diagrams of thin film microstructures generated using different processes;

[0029] Figure 3 Figures (a) and (d) show digital images of SPS and NPS structured thin films, respectively; Figures (b) and (e) show SEM cross-sectional images of SPS and NPS structured thin films, respectively; and Figures (c) and (f) show SEM surface images of SPS and NPS structured thin films, respectively.

[0030] Figure 4 Stress-strain testing of SPS and NPS structured thin films prepared by different processes;

[0031] Figure 5 Dielectric layers with different CNT contents were prepared using different processing methods, and the absolute and relative capacitances of the sensor were tested. Detailed Implementation

[0032] PDMS (polydimethylsiloxane) (Sylgard 184 silicone elastomer kit) and curing agent (Sylgard 184 silicone elastomer curing agent) were supplied by Dow Corning, Inc., Midland, Michigan. Trichloro(1H,1H,2H,2H-decadecaoctadecyl)silane was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Isopropanol AR, ≥99.5%, was purchased from Shanghai Maclean Biochemical Industrial Park. CNT carbon nanotubes with an average diameter of 3μm-15μm were supplied by Nanjing Pioneer Nanomaterials Technology Co., Ltd. Commercial sandpaper was purchased from Taobao. All purchased chemicals were of analytical purity.

[0033] Example 1: Preparation of CPDMS, a dielectric layer precursor material with a CNT content of 0.3%

[0034] a. Measure 30 ml of n-hexane into a beaker, then add 2 g of PDMS main agent, place in a fume hood, and use a magnetic stirrer at 300 rpm for 10 min to fully mix PDMS and n-hexane to obtain mixed solution 1.

[0035] b. Weigh 0.0066g of CNT and add it to the mixed solution 1 from step a. Sonicate the mixed solution 1 at 64W for 20 minutes to obtain a mixed solution 2 containing CNTs with uniform dispersion of the dielectric enhancement material.

[0036] c. Place the mixed solution 2 from step b on a hot plate in a fume hood at 90°C and stir with a magnetic stirrer at 300 rpm until the organic solvent n-hexane is completely evaporated, to obtain a mixed solution that does not contain n-hexane.

[0037] d. Add the curing agent (PDMS to curing agent mass ratio 10:1) to the mixed solution without n-hexane in step c above, and stir magnetically for 10 min to mix the solution evenly to obtain CPDMS mixed solution 3.

[0038] Example 2: Fabrication of dielectric thin film and encapsulation of capacitor device

[0039] a. First, select 100-grit commercial sandpaper, 4cm x 4cm in size. First, clean the surface of the sandpaper with alcohol, and then dry the sandpaper on a hot plate. Next, spray a fluorosilane solution (the fluorosilane solution is composed of trichlorosilane and isopropanol mixed at a volume ratio of 1:100) onto the surface of the cleaned and dried sandpaper. Finally, dry the fluorosilane-coated sandpaper on a hot plate in a fume hood. This surface pretreatment of the sandpaper is to ensure the quality of the dielectric layer film surface.

[0040] b. Spin-coating the CPDMS mixed solution onto the treated sandpaper surface using a spin coater. By setting the spin coater speed to 1000 r for 40 seconds, a dielectric thin film sample with uniform thickness is prepared.

[0041] c. ① The vacuum negative pressure templateless method is adopted, such as Figure 2 (a) Spin-coated structure (0.3% SPS): The sample prepared in step b was placed in a vacuum drying oven preheated to 80°C and vacuumed to bring the internal air pressure of the vacuum drying oven to -70 kPa. Abundant bubbles were observed on the surface of the sample. Then, the temperature and pressure were kept constant for 2 hours.

[0042] ② Using traditional thin film preparation methods, such as Figure 2 (b) Structureless (0.3% NPS): The sample prepared in step b was placed in a vacuum drying oven preheated to 80°C and vacuumed to bring the internal pressure of the drying oven to -100 kPa until the gas at the interface between CPDMS and sandpaper was completely discharged. Then the temperature and pressure were kept constant for 2 hours.

[0043] d. After the sample film has cured, remove the sample and cool it to room temperature. Remove the film and cut out a dielectric layer film of 1mm×1mm size. Use PI tape to encapsulate the dielectric layer on the upper and lower aluminum foil electrode plates. Use conductive silver paste to fix the wires on both sides of the upper and lower aluminum foil electrode plates to obtain a capacitive sensor.

Claims

1. A method for fabricating a porous dielectric layer structure for flexible capacitive sensors, characterized in that, a. Measure n-hexane into a beaker, add PDMS main agent, and stir magnetically to obtain mixed solution 1; b. Weigh out the CNTs containing the dielectric enhancement material according to a mass concentration of 0.3%, add them to the mixed solution 1, and sonicate the mixed solution 1 to obtain a mixed solution 2 containing the CNTs containing the dielectric enhancement material evenly dispersed. c. Place the beaker containing mixed solution 2 on the hot plate of the fume hood, heat and stir magnetically until the organic solvent n-hexane has completely evaporated; d. Add PDMS and curing agent at a mass ratio of 10:1, and stir magnetically to mix evenly to obtain mixed solution 3; e. Select sandpaper. First, clean the surface of the sandpaper with alcohol. Place the sandpaper containing alcohol on a hot plate to dry it. Then, spray a fluorosilane solution onto the cleaned and dried sandpaper surface. Finally, place the sandpaper sprayed with fluorosilane on a hot plate to complete the pretreatment of the sandpaper surface. f. Cut the sandpaper and control the thickness of the sample film according to the different speeds of the spin coater; g. Place the prepared sample film in a vacuum drying oven preheated to 80°C, and use an air pump to provide a pressure of -70 kPa. Maintain a constant pressure of -70 kPa until abundant bubbles appear on the sample film. Then, maintain the temperature and pressure for 2 hours to obtain a dielectric layer with an SPS structure.

2. The method for fabricating a porous dielectric layer structure for flexible capacitive sensors as described in claim 1, characterized in that, In step a), the ratio of n-hexane to PDMS is 25-35 ml: 2-5 g, and the magnetic stirring time is 10 min.

3. The method for fabricating a porous dielectric layer structure for flexible capacitive sensors as described in claim 1, characterized in that, In step b), the mixed solution 1 is sonicated at 64W for 20 min.

4. The method for fabricating a porous dielectric layer structure for flexible capacitive sensors as described in claim 1, characterized in that, In step d), the magnetic stirring time is 10 minutes.

5. The method for fabricating a porous dielectric layer structure for flexible capacitive sensors as described in claim 1, characterized in that, In step e), the sandpaper is 100 mesh, and the fluorosilane solution is composed of trichlorosilane and isopropanol mixed in a volume ratio of 1:

100.

6. The method for fabricating a porous dielectric layer structure for flexible capacitive sensors as described in claim 1, characterized in that, In step f), the sandpaper is cut to a size of 4cm × 4cm.

7. A capacitive sensor fabricated using a porous dielectric layer structure prepared by the method described in claim 1, characterized in that, After the SPS dielectric layer is cured, it is removed, cooled to room temperature, demolded, and cut to size to obtain a dielectric film. The SPS dielectric film is then encapsulated between upper and lower aluminum foil electrode plates using PI tape. Conductive silver paste is used to fix the wires to both sides of the upper and lower aluminum foil electrode plates to obtain a capacitive sensor.

8. The capacitive sensor as described in claim 7, characterized in that, The cut dimensions are 1cm x 1cm.

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