A capacitive flexible pressure sensor based on a multi-layer structure and its fabrication method

Abstract

This invention relates to a capacitive flexible pressure sensor based on a layered structure and its fabrication method, aiming to improve the sensitivity and wearability of flexible sensors. The main approach involves preparing calcium copper titanate (CCTO) nanoparticles with high dielectric constants and fabricating nanofiber thin films using electrospinning technology. These nanofiber thin films are combined with a polydimethylsiloxane (PDMS) matrix mixed with CCTO nanoparticles, and then the nanofibers are etched away in situ to form a layered compressible elastomer. Electrodes are deposited on the upper and lower surfaces of this elastomer and connected to conductive wires to create a flexible pressure sensor. This sensor exhibits high sensitivity and high linearity, is unaffected by ambient temperature and humidity, and can detect dynamic loads, making it suitable for flexible wearable applications.

Description

Technical Field

[0001] This invention relates to the field of flexible electronics technology, specifically to a capacitive flexible pressure sensor based on a multilayer structure and its fabrication method. Background Technology

[0002] Currently, flexible pressure sensors mainly consist of sensing materials and flexible elastomers. The sensing methods of these materials primarily include resistive, capacitive, triboelectric, and piezoelectric types. Resistive sensors primarily use conductive metals, inorganic non-metallic carbon materials, or conductive polymers as sensing materials. During sensing, these materials are susceptible to resistance changes due to stress and temperature variations, which can affect the detection results. Triboelectric sensors primarily output signals by generating ionized electrons through the contact and separation of two materials with different electronegativity. This results in a limitation where detection is only possible through continuous relative motion. Piezoelectric materials generate signals from instantaneous pulses produced by external stress acting on the sensor, leading to similar limitations. Furthermore, both triboelectric and piezoelectric detection methods are highly susceptible to air humidity. Capacitive sensors effectively avoid these limitations, being unaffected by ambient temperature and humidity and capable of continuously monitoring dynamic loads. However, traditional capacitive flexible sensors suffer from low detection sensitivity due to limitations in their sensing structure. Summary of the Invention

[0003] To overcome the low sensitivity of existing flexible capacitive sensors, this technical solution provides a capacitive flexible pressure sensor based on a multi-layer structure. This sensor is not only unaffected by ambient temperature and humidity, but also has relatively sensitive sensing characteristics for dynamic loads.

[0004] To achieve the above objectives, the present invention employs the following technical solution:

[0005] This invention provides a method for fabricating a capacitive flexible pressure sensor based on a layered structure, comprising the following steps:

[0006] Step 1: Dissolve 0.5108g of 4,4'-diaminodiphenyl ether in 7.6ml of N,N'-dimethylacetamide, then add pyromellitic dianhydride with a molar ratio of 4,4'-diaminodiphenyl ether of 1.02:1, and stir for 4-6h to obtain a pale yellow viscous polyamic acid (PAA) solution. Electrospin the polyamic acid (PAA) solution to obtain nanofiber films.

[0007] Step 2: Add ammonia water to titanium tetrachloride solution to adjust the pH value of the solution to 7.8-8.2 to obtain titanium dioxide gel. Then dissolve the titanium dioxide gel in oxalic acid solution. Next, add calcium carbonate, copper nitrate and acetone to the solution to obtain a blue precursor. Then, calcine the blue precursor to obtain calcium copper titanate (CCTO) nanoparticles.

[0008] Step 3: After incorporating CCTO into polydimethylsiloxane, the nanofiber film is uniformly coated and cured. The cured matrix is ​​then immersed in N,N'-dimethylacetamide solution to obtain a layered compressible elastomer. Electrodes are deposited on the upper and lower surfaces of the elastomer and wires are connected to form a flexible pressure sensor.

[0009] In the above scheme, the pyromellitic dianhydride in step 1 needs to be added in four separate additions.

[0010] In the above scheme, the high voltage power supply voltage in step 1 is 16KV, the electrospinning flow rate is 0.2mL / h, the distance of the needle collection device is 16cm, and the ambient humidity during the electrospinning process needs to be controlled below 50%.

[0011] In the above scheme, all chemical reactions in step 2 must be carried out in a fume hood.

[0012] In the above scheme, the calcination temperature in step 2 is 750℃ to reach its crystal phase transformation temperature and form calcium copper titanate polycrystalline.

[0013] In the above scheme, the nanofiber film size in step 3 is 1*1*0.1mm. 3 Size, to meet the size requirements as a flexible wearable electronic device.

[0014] In the above scheme, the temperature of the N,N'-dimethylacetamide solution in step 3 should be 60℃~80℃, which can accelerate the etching of nanofiber films.

[0015] In the above scheme, the stirring time in step 1 is 4 hours.

[0016] In the above scheme, step 2 involves adjusting the solution pH to 8.

[0017] The present invention also provides a capacitive flexible pressure sensor based on a layered structure obtained by the aforementioned method for fabricating a capacitive flexible pressure sensor based on a layered structure.

[0018] Because the present invention employs the above-mentioned technical means, it has the following beneficial effects:

[0019] Based on the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solution to be protected by this invention are as follows:

[0020] I. The preparation method of this invention is simple in design, ingenious in structure, and simple in process, utilizing chemical methods to synthesize a dielectric constant of 10. 4 The CCTO nanoparticles described above were used to obtain a polyamic acid film formed by stacking multiple nanofibers through electrospinning. Using a flexible elastomer mixed with CCTO nanoparticles as the matrix and the nanofiber film as the sacrificial layer, the polyamic acid film was etched away. The resulting fiber membrane positions on the flexible elastomer formed layered voids, resulting in a flexible pressure sensor with a multi-layered structure. Furthermore, the introduction of CCTO nanoparticles with their large dielectric constant further improved the overall dielectric constant of the elastic matrix, solving the technical problem of low sensitivity in traditional capacitive sensors. This enabled the precise sensing of minute deformations in wearable electronic devices using capacitive sensors. The raw materials required for preparation are abundant, the preparation is rapid, and it is easy to scale up industrially.

[0021] II. The technical solution of this invention fills a technological gap in the domestic and international industry: Currently, flexible pressure sensors utilize resistive, capacitive, piezoelectric, and triboelectric pressure sensors. However, resistive sensors are significantly affected by temperature due to the conductivity of their sensing materials; piezoelectric and triboelectric pressure sensors cannot detect dynamic loads and are easily affected by ambient humidity; while capacitive sensors suffer from low sensitivity due to limitations in their sensing structure. This invention introduces CCTO nanoparticles with high dielectric constants into a PDMS matrix, using a multilayered nanofiber film as a sacrificial layer. Layered voids are formed in situ by etching the nanofiber film. According to the capacitance formula: C = ε₀ε₀ r S / 4πkd, the output signal capacitance of the capacitive sensor is only related to the relative permittivity ε of the sensing material. r The detection results are related to the area s and spacing d of the upper and lower electrodes. Temperature and humidity do not affect the detection results. This makes the flexible pressure sensor not only highly sensitive, but also unaffected by ambient temperature and humidity. It has the ability to detect dynamic loads, which promotes the development of thin film sensing technology.

[0022] Third, the technical solution of this invention overcomes technical biases: For a long time, flexible strain sensors have struggled to simultaneously meet technical requirements such as high sensitivity, high linearity, and immunity to temperature and humidity. For example, although resistive pressure sensors have high sensitivity, their conductive materials are limited and easily affected by temperature during sensing; for sensors based on triboelectric and piezoelectricity, humidity has a decisive impact on signal values; and capacitive sensors have consistently low sensitivity and linearity due to limitations in their sensing structure. This solution designs a flexible pressure sensor based on a layered structure and introduces CCTO nanoparticles with a large dielectric constant into the sensing material, thus solving the problems of low sensitivity and poor linearity in terms of both intrinsic material properties and sensing structure. Furthermore, the capacitive sensing method is unaffected by ambient temperature and humidity. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a flowchart of the fabrication method of a capacitive flexible pressure sensor based on a multi-layer structure provided in an embodiment of the present invention;

[0025] Figure 2 This is the X-ray diffraction (XRD) pattern of the CCTO nanoparticles obtained in Example 1 of this invention;

[0026] Figure 3 This is a scanning electron microscope (SEM) image of the layered structure obtained in Embodiment 1 of the present invention;

[0027] Figure 4 This is a structural diagram of the flexible pressure sensor obtained in Embodiment 1 of the present invention;

[0028] Figure 5 This is the mechanical-electrical response diagram of the flexible pressure sensor obtained in Embodiment 1 of the present invention;

[0029] Figure 6 This is a sensitivity diagram of the flexible pressure sensor obtained in Embodiment 1 of the present invention;

[0030] Figure 7 This is a response time diagram of the flexible pressure sensor obtained in Embodiment 1 of the present invention;

[0031] Figure 8 This is a stability diagram of the flexible pressure sensor obtained in Embodiment 1 of the present invention. Detailed Implementation

[0032] The embodiments of the present invention will be described in detail below. Although the present invention will be described and illustrated in conjunction with some specific embodiments, it should be noted that the present invention is not limited to these embodiments. On the contrary, any modifications or equivalent substitutions made to the present invention should be covered within the scope of the claims of the present invention.

[0033] Furthermore, to better illustrate the present invention, numerous specific details are set forth in the following detailed embodiments. Those skilled in the art will understand that the present invention can be practiced without these specific details.

[0034] To demonstrate the inventiveness and technical value of the technical solution of this invention, this section provides specific product or related technology application examples of the technical solution claimed.

[0035] This invention provides a method for fabricating a capacitive flexible pressure sensor based on a layered structure, comprising the following steps:

[0036] Step 1: Dissolve 0.5108g of 4,4'-diaminodiphenyl ether in 7.6ml of N,N'-dimethylacetamide, then add pyromellitic dianhydride with a molar ratio of 1.02:1 to 4,4'-diaminodiphenyl ether, and stir for 4-6h to obtain a pale yellow viscous polyamic acid solution. Electrospin the polyamic acid solution to obtain a fluffy film composed of stacked polyamic acid nanofibers.

[0037] Step 2: Add ammonia water to titanium tetrachloride solution to adjust the pH value of the solution to 7.8-8.2 to obtain titanium dioxide gel. Then dissolve the titanium dioxide gel in oxalic acid solution. Next, add calcium carbonate, copper nitrate and acetone to the solution to obtain a blue precursor. Then, calcine the blue precursor to obtain calcium copper titanate nanoparticles.

[0038] Step 3: After incorporating calcium copper titanate into polydimethylsiloxane, the nanofiber film is uniformly coated and cured. The cured substrate is then immersed in N,N'-dimethylacetamide solution to etch away the loose, multi-layered nanofiber film, resulting in a multi-layered compressible elastomer. Electrodes are deposited on the upper and lower surfaces of the elastomer and wires are connected to form a flexible pressure sensor.

[0039] Example 1

[0040] This invention provides a method for fabricating a capacitive flexible pressure sensor based on a layered structure, comprising the following steps:

[0041] Step 1: Dissolve 0.5108g of 4,4'-diaminodiphenyl ether in 7.6ml of N,N'-dimethylacetamide, then add pyromellitic dianhydride with a molar ratio of 1.02:1 to 4,4'-diaminodiphenyl ether, and stir for 4h to obtain a pale yellow viscous polyamic acid (PAA) solution. Electrospin the PAA solution to obtain nanofiber films.

[0042] Step 2: Ammonia water is added dropwise to titanium tetrachloride solution to adjust the pH of the solution to 8.0, resulting in titanium dioxide gel. The titanium dioxide gel is then dissolved in oxalic acid solution, followed by the addition of calcium carbonate, copper nitrate, and acetone to the solution to obtain a blue precursor. The blue precursor is then calcined to obtain calcium copper titanate (CCTO) nanoparticles.

[0043] Step 3: CCTO is incorporated into polydimethylsiloxane (PDMS), uniformly coated onto a nanofiber film, and then cured. The cured matrix is ​​then immersed in an N,N'-dimethylacetamide solution to obtain a layered compressible elastomer. Electrodes are deposited on the upper and lower surfaces of the elastomer and wires are connected to form a flexible pressure sensor.

[0044] In the above technical solution, the pyromellitic dianhydride in step 1 needs to be added in four separate additions, which achieves the following effect:

[0045] Uniformity of reaction control: Batch addition of pyromellitic dianhydride helps to control the reaction process more evenly, avoiding problems such as localized supersaturation or excessively rapid reaction rates caused by adding it all at once. This ensures a more uniform temperature and concentration distribution in the reaction system, resulting in a more stable polyamic acid solution.

[0046] To prevent excessive cross-linking in the solution: Pyromellitic dianhydride is a multifunctional compound that readily undergoes cross-linking reactions. Adding it in stages can slow down the cross-linking rate and prevent excessive cross-linking in the solution, which could affect the subsequent electrospinning process and the performance of the final product.

[0047] Increasing molecular weight: Batch addition of pyromellitic dianhydride may promote polymer chain growth, thereby increasing the polymer's molecular weight. Higher molecular weight polymers generally exhibit better mechanical properties and thermal stability.

[0048] Controlling reaction conditions: By adding the polymer in batches, the temperature, time, and stirring speed of the reaction can be controlled more precisely to ensure that the desired polymer structure and properties are obtained.

[0049] Avoid gelation: In the polymerization reaction, if a large amount of pyromellitic dianhydride is added at once, the solution may gel rapidly, which is not conducive to subsequent processing and handling.

[0050] In the above technical solution, the high-voltage power supply voltage in step 1 is 16KV, the electrospinning flow rate is 0.2mL / h, the distance of the needle collection device is 16cm, and the ambient humidity during the electrospinning process needs to be controlled below 50%, which achieves the following effects:

[0051] The high-voltage power supply is 16KV: High voltage is a key factor in the electrospinning process because it provides the force needed to stretch the polymer solution. The voltage level directly affects the formation of the jet and the final fiber diameter. Higher voltage helps to obtain finer fibers, but excessively high voltage may lead to jet instability. 16KV is likely the optimal voltage determined after optimization experiments, producing both a stable jet and the desired fiber diameter.

[0052] Electrospinning flow rate is 0.2 mL / h: Flow rate controls the rate at which the solution is supplied, thus affecting the fiber production rate and diameter. Slower flow rates help form finer fibers because the solution has more time to be stretched in the electric field. Flow rates that are too high may result in coarser fiber diameters or the formation of unstable jets. A flow rate of 0.2 mL / h is likely intended to ensure that the solution is stretched uniformly and forms uniform fibers.

[0053] The needle collection device distance is 16cm: The distance between the needle and the collector (i.e., the electrospinning distance) affects the fiber stretch and deposition pattern. A longer distance helps the fiber gain more stretch before reaching the collector, resulting in finer fibers. The 16cm distance is likely to ensure that the fiber is sufficiently stretched before deposition while maintaining fiber uniformity.

[0054] The electrospinning process requires controlling the ambient humidity below 50%: Ambient humidity has a significant impact on the electrospinning process. High humidity can cause the solvent in the solution to evaporate rapidly, resulting in solidification at the needle tip, clogging the needle and affecting fiber formation. Furthermore, high humidity can also cause fibers to stick together during flight, affecting the quality of the final product. Therefore, controlling the ambient humidity below 50% is crucial to ensure the stable operation of the electrospinning process and the acquisition of high-quality nanofibers.

[0055] In the above technical solution, all chemical reactions in step 2 must be carried out in a fume hood.

[0056] In the above technical solution, the calcination temperature in step 2 is 750℃.

[0057] In the above technical solution, the nanofiber film size in step 3 is 1*1*0.1mm. 3 size.

[0058] In the above technical solution, the temperature of the N,N'-dimethylacetamide solution in step 3 should be 60°C.

[0059] The flexible pressure sensor fabricated using the method for preparing a layered capacitive flexible pressure sensor provided in this invention can be applied to fields such as intracranial pressure monitoring, electronic skin, and motion recognition. This invention has achieved positive results during research and development and application, and indeed possesses significant advantages compared to existing technologies. The following description, in conjunction with experimental data and graphs, illustrates these advantages.

[0060] The flexible pressure sensor prepared according to Example 1 is applied in the field of flexible electronics. Figure 2 The image shows a comparison between the XRD pattern of CCTO and the standard card. The diffraction peaks of the (200), (400), (422), and (440) crystal planes correspond one-to-one with those of the standard card, proving that the obtained product is CCTO. The crystal plane parameters of CCTO are calculated as follows: The high dielectric constant of CCTO ceramics is due to the boundary layer capacitance effect formed by the high-resistivity grain boundaries and the semiconductor-like grains, which allows CCTO to achieve a dielectric constant of 10. 4 That's all. From Figure 3 As can be observed, by etching away the polyamic acid film formed by stacked nanofibers in the composite elastomer, a fluffy layered structure can be obtained. This structure gives the sensing material good compressibility, which is also the guarantee of the sensor's high sensitivity. After depositing metal electrodes on the surface of the composite elastomer and leading out wires, it can be fabricated into a capacitive flexible pressure sensor with a layered structure. The specific structural diagram is shown below. Figure 4 As shown. Furthermore, different pressures were applied to the surface of the capacitive pressure sensor to study its pressure sensing characteristics, such as... Figure 5 As shown, a flexible pressure sensor with a multi-layered structure can sensitively detect pressure changes from 0.1 kPa to 20 kPa. Meanwhile, in Figure 6 In our study, we found that this dry-layer structure gives the sensor excellent linearity; the sensor's capacitance change increases with increasing pressure, and the sensitivity reaches as high as 3.3 kPa in the range of 0.1 kPa to 20 kPa. -1 The study of its response time revealed that this sensor can respond rapidly to external pressure stimuli, with response and recovery times as follows: Figure 7 As shown, both the response time and recovery time are approximately 0.5 seconds. Furthermore, this multi-layered flexible pressure sensor also exhibits good cyclic stability, such as... Figure 8 As shown, after more than 300 cycles, the response of the layered sensor to capacitance remained basically unchanged, indicating that the flexible pressure sensor designed with this layered structure not only has high sensitivity and linearity, but also excellent stability, and can be widely used in the field of flexible electronics.

Claims

1. A method for fabricating a capacitive flexible pressure sensor based on a layered structure, characterized in that, Includes the following steps: Step 1: Dissolve 0.5108g of 4,4'-diaminodiphenyl ether in 7.6ml of N,N'-dimethylacetamide, then add pyromellitic dianhydride with a molar ratio of 1.02:1 to 4,4'-diaminodiphenyl ether, and stir for 4-6h to obtain a pale yellow viscous polyamic acid solution. Electrospin the polyamic acid solution to obtain a fluffy film composed of stacked polyamic acid nanofibers. Step 2: Add ammonia water to titanium tetrachloride solution to adjust the pH value of the solution to 7.8-8.2 to obtain titanium dioxide gel. Then dissolve the titanium dioxide gel in oxalic acid solution. Next, add calcium carbonate, copper nitrate and acetone to the solution to obtain a blue precursor. Then, calcine the blue precursor to obtain calcium copper titanate nanoparticles. Step 3: After incorporating calcium copper titanate into polydimethylsiloxane, the nanofiber film is uniformly coated and cured. The cured substrate is then immersed in N,N'-dimethylacetamide solution to etch away the loose, multi-layered nanofiber film, resulting in a multi-layered compressible elastomer. Electrodes are deposited on the upper and lower surfaces of the elastomer and wires are connected to form a flexible pressure sensor.

2. The method for fabricating a capacitive flexible pressure sensor based on a layered structure according to claim 1, characterized in that, The pyromellitic dianhydride in step 1 needs to be added in four separate additions.

3. The method for fabricating a capacitive flexible pressure sensor based on a layered structure according to claim 1, characterized in that, In step 1, the high-voltage power supply voltage is 16KV, the electrospinning flow rate is 0.2mL / h, the distance of the needle collection device is 16cm, and the ambient humidity during the electrospinning process needs to be controlled below 50%.

4. The method for fabricating a capacitive flexible pressure sensor based on a layered structure according to claim 1, characterized in that, All chemical reactions in step 2 must be carried out in a fume hood.

5. The method for fabricating a capacitive flexible pressure sensor based on a layered structure according to claim 1, characterized in that, The calcination temperature in step 2 is 750℃.

6. The method for fabricating a capacitive flexible pressure sensor based on a layered structure according to claim 1, characterized in that, In step 3, the nanofiber film has a size of 1*1*0.1mm. 3 size.

7. The method for fabricating a capacitive flexible pressure sensor based on a layered structure according to claim 1, characterized in that, In step 3, the temperature of the N,N'-dimethylacetamide solution should be 60℃~80℃.

8. The method for fabricating a capacitive flexible pressure sensor based on a layered structure according to claim 1, characterized in that, The stirring time in step 1 is 4 hours.

9. The method for fabricating a capacitive flexible pressure sensor based on a layered structure according to claim 1, characterized in that, In step 2, adjust the pH of the solution to 8.

10. A capacitive flexible pressure sensor based on a multi-layered structure, characterized in that, It is prepared using the method for preparing a capacitive flexible pressure sensor based on a layered structure as described in any one of claims 1 to 7.

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