A piezoresistive sensor and its preparation process

Abstract

The present invention relates to a piezoresistive sensor, comprising interdigitated electrodes, on which a pressure-sensitive layer is encapsulated, and the pressure-sensitive layer is a double-conductive layer micro-dome structure, which comprises, from the outside to the inside, an outermost conductive elastic film, an intermediate high-conductivity electrode, and a base superelastic material. The preparation process of the pressure-sensitive layer of the present invention is to place a pre-prepared double conductive thin layer on a through-hole template, use the pressure of the exhaust to form a reverse micro-dome structure, and then spin-coat an elastic polymer material, and in-situ solidify to obtain the micro-dome structure. The present invention can achieve that during the operation of the piezoresistive sensor, when the pressure-sensitive layer is subjected to high repeatability pressure, no material will fall off on the contact surface, thereby maintaining high reliability.

Description

Technical Field

[0001] The invention belongs to the field of flexible sensors, and in particular relates to a piezoresistive sensor and a preparation process thereof. Background Art

[0002] In recent years, flexible pressure sensors, as a flexible electronic device that can sense the magnitude of the force acting on the surface of an object, have been widely used in the fields of medical health, robotics, biomechanics, etc. due to their good adaptability and ability to be attached to various irregular surfaces. According to the working mechanism of pressure sensors, they can be roughly divided into the following four categories: piezoresistive, capacitive, piezoelectric, and triboelectric. Piezoresistive sensors have attracted widespread attention due to their simple manufacturing process, low cost, and simple signal processing. They have far-reaching significance in the application of electronic skin in biological motion detection, medical health monitoring, etc.

[0003] At present, most flexible piezoresistive sensors characterize the pressure magnitude through the relative current change of the pressure-sensitive layer. Now, microstructures are generally introduced on the surface of the pressure-sensitive layer to improve the sensitivity. At present, most methods of introducing microstructures are through template molding. The templates are mostly obtained by photolithography, which is not only costly, but also the surface quality of the molded microstructure is not high. Secondly, there are also molds made from the microstructures on the surface of plant leaves in nature. Although plant leaf templates are easy to obtain and have rich and diverse microstructures, their irregularity affects the consistency of the sensor, which is not conducive to the array application and mass preparation of sensors in the later stage.

[0004] Even if the quality of the microstructure after molding is good, there are still two problems. First, the surface of the elastomer substrate with microstructure needs to be coated with a conductive layer. The spraying or coating process causes the conductive layer on the substrate surface to fall off and peel off during long-cycle pressure, resulting in unstable output and low reliability. Second, while the sensitivity is improved, due to the small elastic modulus of the substrate elastomer, the effective contact area of ​​the microstructure is easily saturated under low pressure, so the current also tends to saturate, and the sensor eventually sacrifices its linearity over a wide pressure range. Therefore, preparing a sensor with high sensitivity and linearity over a wide pressure range is a difficulty in current research. Summary of the invention

[0005] The object of the present invention is to provide a piezoresistive sensor and a preparation process thereof, so that during the operation of the piezoresistive sensor, when the pressure sensitive layer is subjected to high repetitive pressure, no material will fall off on the contact surface, thereby maintaining high reliability.

[0006] To achieve the above-mentioned purpose of the invention, the technical solution adopted by the present invention is: a piezoresistive sensor, including interdigitated electrodes, on which a pressure-sensitive layer is encapsulated, and the pressure-sensitive layer is a double-conductive layer micro-dome structure, which, from the outside to the inside, comprises an outermost conductive elastomer film, a middle layer high-conductivity electrode and a base superelastic material.

[0007] In the above solution, the pressure-sensitive layer is packaged on the interdigital electrodes through PI tape.

[0008] In the above scheme, the outermost conductive elastomer film material is any one of PDMS (polydimethylsiloxane), Ecoflex silicone, and PU (polyurethane); the middle layer high conductivity electrode material is any one of AgNWs, PEDOT:PSS, and MXene; the base superelastic material is any one of PDMS and Ecoflex silicone.

[0009] In the above solution, the interdigitated electrodes are interdigitated electrodes with a finger width and a spacing of 200 μm.

[0010] The present invention also provides a preparation process of a piezoresistive sensor, comprising the following steps: a. measuring a certain amount of n-hexane in a container, adding a small amount of PDMS main agent, and mixing them thoroughly by magnetic stirring; b. weighing a certain proportion of conductive filler and adding it to the mixed solution, using an ultrasonic cell disruptor to evenly disperse the conductive filler in the solution, and then stirring for a period of time; c. adding a curing agent to the solution at a ratio of 10:1, stirring thoroughly for a period of time, spin coating on a glass sheet, heating and curing on a hot plate, and peeling off the film to prepare a low-conductive layer elastomer film; d. dripping or spin coating the AgNWs solution on the film in batches, heating on a hot plate to fully volatilize ethanol, so as to prepare a high-conductive layer; e. placing the film on a through-hole mold, and then placing them together on the suction cup of a glue spreader, evacuating, spin coating PDMS, maintaining the evacuation state, and curing with an infrared lamp to obtain a double-conductive layer micro-dome structure; f. The desired size was cut out, the micro-dome structure was encapsulated on the interdigital electrodes using PI tape, and the wires were fixed on both sides of the interdigital electrodes using conductive silver paste to obtain a piezoresistive sensor.

[0011] The present invention also provides a method for preparing a pressure sensitive layer for a piezoresistive sensor, which is characterized by comprising the following steps: a. Take 15 ml of n-hexane in a small flask, then add 1.5 g of a PDMS main agent, and stir magnetically at 500 rpm for 10 min to fully mix the main agent with the n-hexane; b. Weigh 0.9 g of carbon black and add it to the mixed solution, and use an ultrasonic cell disruptor to ultrasonicate the solution for 50 min at a power of 500 W; c. Then add 0.15 g of a curing agent, and stir magnetically at 800 rpm for 30 min; d. Spin coat the solution on a glass sheet, heat the glass sheet at 100° C. on a hot plate for 30 min, and peel off the film; e. Treat the C-PDMS film with oxygen plasma for 10 min, take it out and place it on a glass plate; f. Spin coat the AgNWs solution on the glass plate, and heat it for 20 min to volatilize the ethanol; the temperature is 60° C. g. Place the double conductive layer film on a 2cmx2cm, 500μm thick copper round through hole template; the through hole diameter is 500μm, and the hole center spacing is 800μm; h. Weigh 0.15g of curing agent and add 1.5g of PDMS main agent, stir evenly, vacuum to remove bubbles, and wait for standby; i. Place the above whole on the suction cup of KW-4A glue machine, turn on the vacuum, spin coat PDMS at 800rpm and keep it in the vacuum state, place the infrared lamp 20cm above the suction cup, turn on the infrared lamp to cure for 1h; j. After curing, turn off the vacuum, remove the double conductive layer micro dome structure, cut off the part without micro dome, and finally prepare a pressure sensitive layer.

[0012] Beneficial effects of the present invention: (1) The pressure-sensitive sensor provided by the present invention has a wider pressure detection range while maintaining high sensitivity. (2) The pressure-sensitive sensing unit is designed as a double conductive layer. The inner and outer conductive layers cooperate with each other under the pressure of increasing layer by layer to achieve good linearity under a larger pressure range; the process of the micro-dome structure is to place the prepared conductive layer film on the through-hole mold, use the pressure during the vacuum process to obtain the micro-dome film, and then spin-coat the elastomeric material to obtain a high-quality pressure-sensitive structure. The microstructure prepared by this process can adjust the structure of the sensitive unit by adjusting the size of the through-hole template and the pressure of the vacuum. Compared with the traditional method of spraying sensitive materials on the surface of the microstructure, the double conductive thin layer prepared by the present invention has good adhesion with the elastic polymer. The prepared structure shows good reliability in long-term cyclic loading, overcoming the problem of poor reliability caused by the conductive layer being easily peeled off under long-cycle loading and release. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a schematic diagram of the structure of the piezoresistive sensor of the present invention.

[0014] Figure 2 A cross-sectional view of a single microdome.

[0015] Figure 3 This is a diagram of the relative current change of the pressure-sensitive layer under four different specifications of interdigital electrodes.

[0016] Figure 4 This is the SEM image of the micro dome cross section.

[0017] Figure 5 This is a diagram showing the relative current changes of a single conductive layer pressure-sensitive layer and a double conductive layer pressure-sensitive layer under the interdigital electrodes.

[0018] Figure 6 For this purpose, the sensor was subjected to 10,000 stability tests at a pressure of 10 kPa.

[0019] Figure 7 This is the surface SEM image of the pressure-sensitive layer after 10,000 stability tests.

[0020] Figure 8 8000 times stability test of drop-coated double conductive layer samples.

[0021] Fig. 9 This is the surface SEM image of the drop-coated double conductive layer sample after 8000 stability tests.

[0022] In the figure, 1 is PI tape, 2 is pressure-sensitive layer, 3 is interdigital electrode, 4 is C-PDMS film, 5 is AgNWs, and 6 is PDMS. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is described in more detail below in conjunction with the accompanying drawings.

[0024] The present invention provides a flexible piezoresistive sensor with high sensitivity and wide range, the structure of which is as follows: Figure 1 As shown, the interdigital electrode 3 is encapsulated with a pressure-sensitive layer 2 by a PI tape 1. Figure 2 As shown, the pressure-sensitive layer 2 is a double-conductive layer micro-dome structure, which includes, from the outside to the inside, the outermost conductive elastomer film, the middle layer high conductivity electrode, and the base superelastic material. In the above technical method, the elastomer matrix material can be PDMS (polydimethylsiloxane), Ecoflex silicone, PU (polyurethane), etc. The middle layer high conductivity layer can be AgNWs (silver nanowires), PEDOT:PSS, MXene. The superelastic can be PDMS, Ecoflex silicone, etc.

[0025] The preparation method of the piezoresistive sensor in the above scheme comprises the following steps: a. Measure a certain amount of n-hexane into a container, add a small amount of PDMS main agent, and stir magnetically to mix thoroughly. b. Weigh a proportion of conductive filler and add it to the mixed solution. The conductive filler can be carbon black, carbon nanotubes, graphene, etc., and use an ultrasonic cell disruptor to evenly disperse the conductive filler in the solution, and then stir for a period of time. c. Add a curing agent to the solution at a ratio of 10:1 and stir thoroughly for a period of time. Spin coat it on a glass sheet, heat and cure it on a hot plate, and peel off the film to prepare a low-conductivity layer elastomer film. d. Drip or spin coat the AgNWs solution on the above film in batches, heat on a hot plate to fully volatilize the ethanol, and prepare a high-conductivity layer. e. Place the above film on a through-hole mold, and then place it together on the suction cup of a glue spreader, evacuate, spin coat PDMS, keep the evacuation state, and use an infrared lamp to cure, so as to obtain a double-conductive layer micro-dome structure. f. The desired size was cut out, the micro-dome structure was encapsulated on the interdigital electrodes using PI tape, and the wires were fixed on both sides of the interdigital electrodes using conductive silver paste to obtain a piezoresistive sensor.

[0026] Specific embodiment: PDMS (polydimethylsiloxane) is Sylgard 184 produced by Dow Corning, and the conductive carbon black is Ketjen Black ECP-600JD.

[0027] Example 1: Preparation of PDMS conductive elastomer film (C-PDMS) containing 6% carbon black.

[0028] The specific preparation steps are as follows: a. Take 15ml of n-hexane in a small flask, then add 1.5g of PDMS main agent, use magnetic stirring at 500rpm for 10min to fully mix the main agent and n-hexane. b. Weigh 0.9g of carbon black and add it to the mixed solution, use ultrasonic cell disruptor to ultrasonicate the solution for 50min, power 500w. c. Then add 0.15g of curing agent, magnetic stirring at 800rpm for 30min. d. Spin-coat the solution on a glass slide, heat the glass slide at 100℃ for 30min on a hot plate, and peel off the film.

[0029] Example 2: Preparation of double conductive layer micro-dome structure.

[0030] The specific preparation steps are as follows: a. Treat the C-PDMS film with oxygen plasma for 10 minutes. Take it out and place it on a glass plate. b. Spin-coat the AgNWs solution on the glass plate and heat it for 20 minutes to evaporate the ethanol; the temperature is 60°C. c. Place the double conductive layer film on a 2cmx2cm copper round through hole template with a thickness of 500μm. The through hole diameter is 500μm and the hole center spacing is 800μm. d. Weigh 0.15g of curing agent and add 1.5g of PDMS main agent, stir evenly, vacuum to remove bubbles, and wait for standby. e. Place the above whole on the suction cup of the KW-4A glue machine, turn on the vacuum, spin-coat PDMS at 800rpm and keep it in a vacuum state, place the infrared lamp 20cm above the suction cup, turn on the infrared lamp to cure for 1h. f. After curing, turn off the vacuum, remove the double conductive layer micro-dome structure, cut off the part without micro-dome, and finally prepare it into a pressure-sensitive layer.

[0031] The electrodes of this embodiment use interdigitated electrodes, which are coplanar electrode designs. The interdigitated line width and line spacing can be adjusted to adjust the output, so that the best sensitivity can be achieved in a wide pressure range. Secondly, the coplanar electrode design makes the contact resistance more dominant in the pressure change, and its sensitivity is improved. This is an advantage that the sandwich electrode design does not have. Figure 3 As shown, interdigital electrodes with finger widths and spacings of 50μm, 100μm, 200μm, and 350μm were selected and packaged with a pressure-sensitive layer, and their pressure-relative current was tested, showing that the sensitivity was 2.07kPa, respectively. -1 , 1.45kPa -1 , 5.75kPa -1 、0.08169kPa -1 It can be seen that the sensor with interdigital electrodes with a finger width and a spacing of 200 μm has the highest sensitivity.

[0032] Compared with the prior art, the breakthroughs of this embodiment are reflected in the following three aspects.

[0033] First: double conductive layer structure, the outer layer is a low conductivity film, the inner layer is drop-coated or spin-coated with a high conductivity material. Figure 4 This is a SEM image of the micro-dome cross section. When the sensor is in the low pressure range, a loop is first formed between the outer conductive layer and the electrode. As the external pressure gradually increases, the inner layer material also participates in the conductive path, the overall resistance of the pressure-sensitive sensing layer can continue to decrease, and the current can gradually increase, making the pressure detection range wider. Figure 5 As shown, the sensitivity of the single conductive layer microdome piezoresistive sensor is 2.06 kPa -1 The sensitivity of the double-conductive-layer microdome piezoresistive sensor is 5.07 kPa. -1It can be seen that the double conductive layer structure has a significant improvement in sensitivity and pressure detection range compared to the single conductive layer structure.

[0034] Second: The present invention uses a photocurable through-hole mold to prepare a micro-dome structure, which has the characteristics of low cost and easy adjustment of parameters compared with the usual silicon mold and metal mold inversion method. Secondly, this method of preparing a uniform specification of micro-dome pressure-sensitive layer provides convenience for future batch production.

[0035] Third: Compared with the previous spraying of conductive materials or sputtering coating on the microstructure, the preparation process of the pressure-sensitive layer of the present invention is to place a pre-prepared double conductive thin layer on a through-hole template, use the pressure of the vacuum to form a reverse micro-dome structure, and then spin-coat an elastic polymer material and in-situ cure to obtain the micro-dome structure.

[0036] Fourth: Compared with traditional thin film structures, the structural design of the micro-dome can improve the deformation behavior of the entire structure under the pressure range, thereby improving performance indicators such as the sensitivity and detection range of the device output.

[0037] Compared with spraying or sputtering on microstructures, the conductive elastic film contacts the electrode during operation of the sensor, so the pressure-sensitive layer will not produce material shedding on the contact surface during high repetitive pressure, maintaining high reliability and providing guarantee for the performance of the sensor. In addition, the micro-dome is made of the prepared film, and the dome surface is smooth and not easy to crack. Figure 6 The results of 10,000 cycles of testing of this sensor under 10 kPa show that the sensor prepared by this process has excellent stability. Figure 7 This is the surface SEM image of the pressure-sensitive layer after 10,000 stability tests, which also illustrates the reliability of the sensor; Figure 8 To test the stability of samples with double conductive layers drop-coated on the prepared micro-dome substrate, Fig. 9 This is the surface SEM image of the drop-coated double-conductive layer sample after 8000 stability tests. Obvious cracks can be seen on the surface of the sample, indicating that the surface coating has fallen off, causing the inner layer of high-conductivity material to directly contact the electrode. Therefore, the initial current in the cycle test is large, and then during the periodic loading and unloading process, the material falls off more seriously and the current gradually increases. Therefore, its reliability is not high.

Claims

1. A process for preparing a piezoresistive sensor, the piezoresistive sensor comprising an interdigitated electrode (3), characterized in that: The interdigital electrodes (3) are encapsulated with a pressure-sensitive layer (2), the pressure-sensitive layer (2) being a double-conductive layer micro-dome structure, which comprises, from the outside to the inside, a low-conductive layer elastic film, a high-conductive electrode and a base superelastic material, the pressure-sensitive layer (2) being encapsulated on the interdigital electrodes (3) by a PI tape (1), and characterized in that the method comprises the following steps: a. Measure a certain amount of n-hexane into a container, add a small amount of PDMS main agent, and mix thoroughly with magnetic stirring; b. Weigh a certain proportion of conductive filler and add it to the mixed solution, use an ultrasonic cell disruptor to evenly disperse the conductive filler in the solution, and then stir for a period of time; c. Add curing agent to the solution at a ratio of 10:1, stir thoroughly for a while, spin-coat on a glass sheet, heat and cure on a hot plate, and peel off the film to prepare a low-conductivity elastomer film; d. Drop-coat or spin-coat the AgNWs solution on the above film in batches, and heat on a hot plate to fully evaporate the ethanol to prepare a high-conductivity electrode; e. Place the above film on the through-hole mold, and then place them together on the suction cup of the glue machine, evacuate, spin-coat PDMS, keep the vacuum state, and use infrared lamp to cure, so as to obtain a double conductive layer micro dome structure; f. Cut out the required size, use PI tape to encapsulate the micro-dome structure on the interdigital electrodes, and use conductive silver paste to fix the wires on both sides of the interdigital electrodes to obtain a piezoresistive sensor.

2. The process for preparing a piezoresistive sensor according to claim 1, characterized in that: The interdigital electrodes (3) are interdigital electrodes with a finger width and a spacing of 200 μm.

3. A method for preparing a pressure sensitive layer for a piezoresistive sensor, characterized in that: The steps include: a. Take 15 ml of n-hexane in a small flask, then add 1.5 g of PDMS main agent, and stir with a magnetic stirrer at 500 rpm for 10 min to fully mix the main agent and n-hexane; b. Weigh 0.9g of carbon black and add it to the mixed solution. Ultrasonicate the solution with an ultrasonic cell disruptor for 50min at a power of 500w. c. Add 0.15g curing agent and stir magnetically at 800rpm for 30min; d. Spin-coat the solution onto a glass slide, heat the glass slide on a hot plate at 100°C for 30 minutes, and peel off the film; e. Treat the C-PDMS film with oxygen plasma for 10 min, take it out and place it on a glass plate; f. Spin-coat the AgNWs solution on the C-PDMS film and heat for 20 min to evaporate the ethanol at 60°C. g. Place the double conductive layer film on a 2cmx2cm, 500μm thick copper round through hole template; the through hole diameter is 500μm, and the hole center spacing is 800μm; h. Weigh 0.15g of curing agent and add 1.5g of PDMS main agent, stir evenly, vacuum to remove bubbles, and wait for use; i. Place the double conductive layer film on the round hole template in step g on the suction cup of the KW-4A coating machine, turn on the vacuum, spin-coat PDMS at 800 rpm and keep it in the vacuum state, place the infrared lamp 20 cm above the suction cup, turn on the infrared lamp to cure for 1 hour; j. After curing is completed, the vacuum is turned off, the double conductive layer micro-dome structure is removed, and the part without micro-dome is cut off to finally prepare a pressure-sensitive layer.

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