CN114199424A

CN114199424A - Piezoresistive sensor and preparation process thereof

Info

Publication number: CN114199424A
Application number: CN202111431650.8A
Authority: CN
Inventors: 仲艳; 王嘉琦; 程广贵; 丁建宁
Original assignee: Jiangsu University
Current assignee: Jiangsu University
Priority date: 2021-11-29
Filing date: 2021-11-29
Publication date: 2022-03-18

Abstract

The invention relates to a piezoresistive sensor, comprising interdigital electrodes, on which a pressure-sensitive responsive layer is encapsulated, the pressure-sensitive responsive layer is a double-conductive-layer micro-dome structure, and the outermost layer is the outermost layer from the outside to the inside. Conductive elastomer films, interlayer high-conductivity electrodes, and base superelastomer materials. The preparation process of the pressure-sensitive responsive layer of the present invention is to place the pre-prepared double-conductive thin layer on the through-hole template, form a reverse micro-dome structure by using the pressure of air extraction, and then spin-coat the elastic polymer material, in-situ Curing to obtain a micro-dome structure. The invention can realize that during the working process of the piezoresistive sensor and the pressure-sensitive responsive layer in the process of high repetitive pressure, the contact surface will not have material falling off and maintain high reliability.

Description

Piezoresistive sensor and preparation process thereof

Technical Field

The invention belongs to the field of flexible sensors, and particularly relates to a piezoresistive sensor and a manufacturing process thereof.

Background

In recent years, a flexible pressure sensor is used as a flexible electronic device capable of sensing the magnitude of an applied force on the surface of an object. The adhesive has good adaptability and can be attached to the surfaces of various irregular objects, so that the adhesive has wide application prospects in the fields of medical health, robots, biomechanics and the like. Pressure sensors can be broadly classified into the following four categories according to their operation mechanisms: piezoresistive, capacitive, piezoelectric, triboelectric. Piezoresistive sensors have been widely noticed due to their simple manufacturing process, low cost and simple signal processing, and have profound significance for their application as electronic skin in biological motion detection, medical health monitoring, etc.

Most flexible piezoresistive sensors currently use the relative current change of a pressure sensitive sensing layer to characterize the pressure. At present, microstructures are generally introduced to the surface of the pressure-sensitive sensing layer to improve sensitivity, most of the methods for introducing the microstructures at present are template inversion, and most of templates are obtained by a photoetching method, so that the cost is high, and the surface quality of the inverted microstructures is not high. And secondly, the microstructure on the surface of the natural plant leaf is used for reverse molding, and although the plant leaf template is easy to obtain and has abundant and various microstructures, the uniformity of the sensor is influenced by the irregularity of the plant leaf template, so that the array application and the batch preparation of the sensor at the later stage are not facilitated.

Even if the microstructure quality after the die-reversing is good, two problems are faced, one is that the surface of the elastic body substrate with the microstructure needs to be coated with a conducting layer, and the conducting layer on the surface of the substrate is peeled off in the long-cycle compression process by a spraying or coating process, so that the final output is unstable and the reliability is low. II, secondly: the sensitivity is improved, and at the same time, because the elastic modulus of the base elastomer is small, the effective contact area of the microstructure is easy to saturate under low pressure, so that the current also tends to saturate, and finally the linearity of the sensor in a wide pressure range is sacrificed. Therefore, it is a difficult point of current research to prepare a sensor with high sensitivity and linearity in a wide pressure range.

Disclosure of Invention

The invention aims to provide a piezoresistive sensor and a preparation process thereof, which can ensure that the contact surface can not generate material falling in the high-repeatability pressing process of a pressure-sensitive sensing layer in the working process of the piezoresistive sensor and keep high reliability.

In order to realize the purpose, the invention adopts the technical scheme that: a piezoresistive sensor comprises an interdigital electrode, wherein a pressure-sensitive sensing layer is packaged on the interdigital electrode, the pressure-sensitive sensing layer is of a double-conductive-layer micro-dome structure, and an outermost conductive elastomer film, a middle-layer high-conductivity electrode and a substrate hyperelastomer material are sequentially arranged from outside to inside.

In the above scheme, the pressure sensitive sensing layer is packaged on the interdigital electrode through a PI tape.

In the above scheme, the outermost layer conductive elastomer film material is any one of PDMS (polydimethylsiloxane), Ecoflex silica gel, and PU (polyurethane); the intermediate layer high-conductivity electrode material is any one of AgNWs, PEDOT, PSS and MXene; the substrate hyperelastomer material is any one of PDMS and Ecoflex silica gel.

In the above scheme, the interdigital electrodes are interdigital electrodes with finger width and 200 μm spacing.

The invention also provides a preparation process of the piezoresistive sensor, which comprises the following steps: a. measuring a certain amount of normal hexane in a container, adding a small amount of PDMS (polydimethylsiloxane) main agent, and stirring and mixing fully by magnetic force; b. weighing a certain proportion of conductive filler, adding the conductive filler into the mixed solution, uniformly dispersing the conductive filler in the solution by using an ultrasonic cell disruptor, and then stirring for a period of time; c. adding a curing agent into the solution according to the ratio of 10:1, fully stirring for a period of time, spin-coating on a glass sheet, heating and curing by a hot plate, and uncovering the film to prepare the low-conductive-layer elastomer film; d. dripping or spin-coating AgNWs solution on the film for several times, heating with a hot plate to volatilize ethanol completely to prepare a high conductive layer; e. placing the film on a through hole die, then placing the film and the die together at a sucker of a spin coater, vacuumizing, spin-coating PDMS, keeping an air exhaust state, and curing by using an infrared lamp to obtain a double-conductive-layer micro dome structure; f. cutting out the required size, packaging the micro dome structure on the interdigital electrode by using a PI adhesive tape, and fixing the conducting wires on two sides of the interdigital electrode by using conductive silver paste to obtain the piezoresistive sensor.

The invention also provides a preparation method of the pressure-sensitive sensing layer applied to the piezoresistive sensor, which is characterized by comprising the following steps: a. taking 15ml of normal hexane to put in a small flask, adding 1.5g of PDMS main agent, and performing magnetic stirring for 10min at 500rpm by using magnetic stirring to fully mix the main agent and the normal hexane; b. weighing 0.9g of carbon black, adding the carbon black into the mixed solution, and ultrasonically treating the solution for 50min by using an ultrasonic cell disruption instrument with the power of 500 w; c. then 0.15g of curing agent is added, and magnetic stirring is carried out at 800rpm for 30 min; d. spin-coating the solution on a glass sheet, heating the glass sheet with a hot plate at 100 deg.C for 30min, and uncovering the film; e. carrying out oxygen plasma treatment on the C-PDMS film for 10min, taking out and placing on a glass plate; f. spin-coating AgNWs solution on a glass plate, and heating for 20min to volatilize ethanol; the temperature is 60 ℃; g. placing the double conductive layer film on a circular through-hole template of copper with a thickness of 500 μm, 2cmx2 cm; the diameter of the through hole is 500 μm, and the center distance of the holes is 800 μm; h. weighing 0.15g of curing agent, adding 1.5g of PDMS as a main agent, uniformly stirring, vacuumizing to remove bubbles, and waiting for later use; i. putting the whole on a sucker of a KW-4A spin coater, vacuumizing, spin-coating PDMS at 800rpm, vacuumizing, placing an infrared lamp 20cm above the sucker, and curing for 1h by turning on the infrared lamp; j. and after the curing is finished, closing and vacuumizing, taking down the double-conductive-layer micro-dome structure, cutting off the part without the micro-dome structure, and finally preparing the pressure-sensitive sensing layer.

The invention has the beneficial effects that: (1) the pressure-sensitive sensor provided by the invention has the advantages that the pressure detection range is widened while high sensitivity is kept. (2) The pressure-sensitive sensing unit is designed into double conductive layers, and the inner conductive layer and the outer conductive layer cooperate with each other under the pressure increasing layer by layer to promote good linearity under a larger pressure range; the process of the micro dome structure comprises the steps of placing a prepared conducting layer film on a through hole die, obtaining the micro dome film by utilizing pressure in the air exhaust process, and spin-coating an elastomer material to obtain a high-quality pressure-sensitive structure. The microstructure prepared by the process can regulate and control the structure of the sensitive unit by adjusting the size of the through hole template and the pressure of air exhaust. Compared with the traditional method of spraying a sensitive material on the surface of the microstructure, the method has the advantages that the prepared double-conductive thin layer and the elastic polymer have good adhesion, the prepared structure has good reliability in long-period cyclic loading, and the problem of poor reliability caused by the fact that the conductive layer is easy to peel off and fall off under the release of long-period cyclic loading is solved.

Drawings

FIG. 1 is a schematic diagram of a piezoresistive sensor according to the present invention.

Fig. 2 is a cross-sectional view of a single micro-dome.

Fig. 3 is a graph of relative current change of the pressure-sensitive sensing layer under four interdigital electrodes with different specifications.

Fig. 4 is a cross-sectional SEM image of a micro-dome.

FIG. 5 is a diagram of relative current changes of a single-conductive-layer pressure-sensitive sensing layer and a double-conductive-layer pressure-sensitive sensing layer under an interdigital electrode.

FIG. 6 shows 10000 stability tests of the sensor under 10 kPa.

Fig. 7 is a surface SEM image of the pressure sensitive layer after 10000 times stability test.

Figure 8 is 8000 stability tests of drop coated samples of a dual conductive layer.

FIG. 9 is a surface SEM image of a drop coated dual conductive layer sample after 8000 stability tests.

In the figure, 1 is a PI adhesive tape, 2 is a pressure sensitive sensing layer, 3 is an interdigital electrode, 4 is a C-PDMS film, 5 is AgNWs, and 6 is PDMS.

Detailed Description

The technical solution of the present invention will be described in more detail with reference to the accompanying drawings.

The invention provides a high-sensitivity wide-range flexible piezoresistive sensor which is structurally shown in figure 1, wherein a pressure-sensitive sensing layer 2 is packaged on an interdigital electrode 3 through a PI (polyimide) adhesive tape 1, and as shown in figure 2, the pressure-sensitive sensing layer 2 is of a double-conductive-layer micro-dome structure and is sequentially provided with an outermost conductive elastomer film, a middle high-conductivity electrode and a substrate hyperelastomer material from outside to inside. In the above technical method, the elastomer matrix material may be PDMS (polydimethylsiloxane), Ecoflex silica gel, PU (polyurethane), etc. The intermediate layer with high conductivity can be AgNWs (silver nanowire), PEDOT (PSS) or MXene. The super elastomer can be PDMS, Ecoflex silica gel, etc.

The preparation method of the piezoresistive sensor in the scheme comprises the following steps: a. a certain amount of normal hexane is measured and put into a container, a small amount of PDMS main agent is added, and the mixture is stirred and mixed fully by magnetic force. b. Weighing a proportion of conductive filler, adding the conductive filler into the mixed solution, wherein the conductive filler can be carbon black, carbon nano tubes, graphene and the like, uniformly dispersing the conductive filler in the solution by using an ultrasonic cell disruptor, and then stirring for a period of time. c. Adding the curing agent into the solution according to the ratio of 10:1, and fully stirring for a period of time. Spin coating on glass sheet, heating and curing with hot plate, and stripping to obtain low-conductivity elastomer film. d. And dropwise coating or spin coating AgNWs solution on the film in multiple times, and heating on a hot plate to fully volatilize ethanol so as to prepare the high-conductivity layer. e. And placing the film on a through hole die, then placing the film and the die together at a sucker of a spin coater, vacuumizing, spin-coating PDMS (polydimethylsiloxane), keeping an air exhaust state, and curing by using an infrared lamp to obtain the double-conductive-layer micro-dome structure. f. Cutting out the required size, packaging the micro dome structure on the interdigital electrode by using a PI adhesive tape, and fixing the conducting wires on two sides of the interdigital electrode by using conductive silver paste to obtain the piezoresistive sensor.

The specific embodiment is as follows: PDMS (polydimethylsiloxane) is model No. Sylgard 184 available from Dow Corning, and the conductive carbon black is Ketjen black ECP-600 JD.

Example 1: carbon black was prepared as a 6% PDMS conductive elastomer film (C-PDMS).

The preparation method comprises the following steps: a. 15ml of n-hexane is taken out to be put in a small flask, 1.5g of PDMS main agent is added, and magnetic stirring is carried out for 10min at 500rpm by magnetic stirring, so that the main agent and the n-hexane are fully mixed. b. 0.9g of carbon black was weighed and added to the mixed solution, and the solution was sonicated using a sonicator for 50min at a power of 500 w. c. Then, 0.15g of a curing agent was added thereto, and the mixture was magnetically stirred at 800rpm for 30 minutes. d. The solution is coated on a glass sheet in a spinning mode, the glass sheet is heated for 30min at the temperature of 100 ℃ by a hot plate, and the film is uncovered.

Example 2: and preparing a double-conductive-layer micro-dome structure.

The preparation method comprises the following steps: a. the C-PDMS film was subjected to oxygen plasma treatment for 10 min. Taken out and placed on a glass plate. b. Spin-coating AgNWs solution on a glass plate, and heating for 20min to volatilize ethanol; the temperature was 60 ℃. c. The double conductive layer film was placed on a circular via template of copper with a thickness of 500 μm at 2cmx2 cm. The diameter of the through holes is 500 μm, and the center-to-center distance of the holes is 800 μm. d. 0.15g of curing agent is weighed and added into 1.5g of PDMS main agent, the mixture is stirred evenly, and the mixture is vacuumized to remove air bubbles and is ready for use. e. And putting the whole on a sucker of a KW-4A spin coater, vacuumizing, spin-coating PDMS at 800rpm, vacuumizing, placing an infrared lamp 20cm above the sucker, and curing for 1h by turning on the infrared lamp. f. And after the curing is finished, closing and vacuumizing, taking down the double-conductive-layer micro-dome structure, cutting off the part without the micro-dome structure, and finally preparing the pressure-sensitive sensing layer.

The electrode of the embodiment adopts an interdigital electrode, is designed as a coplanar electrode, and can regulate and control the line width and the line distance of the interdigital to regulate the output, so that the optimal sensitivity is achieved in a wide pressure range. Secondly, the design of the coplanar electrodes is used, so that the contact resistance is more dominant in pressure change, and the sensitivity is improved. This is an advantage not possessed by the sandwich electrode design. As shown in figure 3, the interdigital electrodes with finger width, spacing of 50 μm, 100 μm, 200 μm and 350 μm are sequentially selected to be packaged with the pressure-sensitive sensing layer, the pressure-relative current is tested, and the sensitivity is shown to be 2.07kPa^-1、1.45kPa^-1、5.75kPa^-1、0.08169kPa^-1. The highest sensitivity of the sensor with the interdigital electrodes having a finger width and a spacing of 200 μm can be seen.

Compared with the prior art, the embodiment has the following three breakthrough aspects.

Firstly, the method comprises the following steps: the structure of the double conductive layers comprises a low-conductivity film on the outer layer and a high-conductivity material on the inner layer by drop coating or spin coating, and a micro-dome cross-section SEM image is shown in figure 4. When the sensor is in a low-voltage range, a loop is formed between the outer conducting layer and the electrode, the inner layer material also participates in the conducting path along with the gradual increase of the external pressure, the overall resistance of the pressure-sensitive sensing layer can be continuously reduced, the current can be gradually increased, and the pressure detection range is widened. As shown in FIG. 5, the sensitivity of the single conductive layer micro-dome piezoresistive sensor is 2.06kPa^-1The sensitivity of the double-conductive-layer micro-dome piezoresistive sensor is 5.07kPa^-1It can be seen that the double conductive layer structure has a significant improvement in sensitivity and pressure detection range over the single conductive layer structure.

Secondly, the method comprises the following steps: the invention uses the light-cured through-hole die to prepare the micro-dome structure, and compared with a silicon die and a metal die back-molding method which are used at ordinary times, the method has the characteristics of low cost, easy parameter adjustment and the like. Secondly, the method for preparing the micro-dome pressure-sensitive sensing layer with uniform specification provides convenience for subsequent batch preparation.

Thirdly, the method comprises the following steps: compared with the prior art of spraying a conductive material or sputtering a film on the microstructure, the preparation process of the pressure-sensitive sensing layer disclosed by the invention is to place a pre-prepared double conductive thin layer on a through hole template, form a reverse micro-dome structure by using the pressure of air suction, spin-coat an elastic polymer material, and obtain the micro-dome structure by in-situ curing.

Fourthly: compare in traditional thin film structure, the structural design of little dome can promote whole structure deformation behavior under the pressure range, and then performance index such as sensitivity and the detection range of promotion device output.

Compared with spraying or sputtering coating on the microstructure, the conductive elastomer film contacts with the electrode in the working process of the sensor, so that the contact surface of the pressure-sensitive sensing layer can not generate material shedding in the high-repeatability pressure-bearing process, high reliability is maintained, and the performance of the sensor is guaranteed. And the prepared film is used for preparing the micro dome, the round top surface is smooth, and the micro dome is not easy to crack. FIG. 6 is a 10000 times cycle test result of the sensor under 10kPa, which shows that the sensor prepared under the process has excellent stability, and FIG. 7 is a surface SEM image of the pressure sensitive sensing layer after 10000 times stability test, which also illustrates the reliability of the sensor; fig. 8 is a stability test of a sample with a double conductive layer dripped on a prepared micro-dome substrate, and fig. 9 is a surface SEM image of the sample with the double conductive layer dripped after 8000 times of stability tests, and it can be seen that there is a significant crack on the surface of the sample, which indicates that the surface coating is peeled off, resulting in the inner layer of high-conductivity material directly contacting with the electrode, so the initial current is large in the cycle test, and then the material is peeled off more and more during the periodic loading and unloading process, and the current is gradually increased, so the reliability is not high.

Claims

1. A piezoresistive sensor, comprising an interdigital electrode (3), characterized in that, a pressure sensitive layer (2) is encapsulated on the interdigitated electrode (3), and the pressure sensitive layer (2) is a double The micro-dome structure of the conductive layer is, from the outside to the inside, the outermost conductive elastomer film, the middle layer of high-conductivity electrodes and the base super-elastic material.

2 . The piezoresistive sensor according to claim 1 , wherein the pressure-sensitive responsive layer 2 is packaged on the interdigital electrodes ( 3 ) through a PI tape ( 1 ). 3 .

3. A piezoresistive sensor according to claim 1 or 2, wherein the material of the outermost conductive elastomer film is PDMS (polydimethylsiloxane), Ecoflex silica gel, PU (polyurethane) Any one of the above; the intermediate layer high-conductivity electrode material is any one of AgNWs, PEDOT:PSS, and MXene; the base superelastic body material is any one of PDMS and Ecoflex silica gel.

4 . The piezoresistive sensor according to claim 1 or 2 , wherein the interdigital electrodes ( 3 ) are interdigital electrodes with finger width and a spacing of 200 μm. 5 .

5. a preparation technology of piezoresistive sensor, is characterized in that, comprises the steps:

a. Measure a certain amount of n-hexane into the container, add a small amount of PDMS main agent, and mix thoroughly with magnetic stirring;

b. Weigh a certain proportion of the conductive filler into the mixed solution, use an ultrasonic cell disruptor to disperse the conductive filler evenly in the solution, and then stir for a period of time;

c. Add the curing agent to the solution at a ratio of 10:1, fully stir for a period of time, spin-coat on a glass sheet, heat and cure on a hot plate, and peel off the film to prepare a low-conductive layer elastomer film;

d. The AgNWs solution was drop-coated or spin-coated on the above-mentioned film in stages, and the ethanol was fully volatilized by heating on a hot plate to prepare a highly conductive layer;

e. The above-mentioned film is placed on the through-hole mold, and then placed together at the suction cup of the glue dispenser, vacuumized, spin-coated PDMS, maintained in the pumping state, and cured with an infrared lamp 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 electrode, and use conductive silver paste to fix the wires on both sides of the interdigital electrode to obtain a piezoresistive sensor.

6. A preparation method of a pressure sensitive layer applied to a piezoresistive sensor, characterized in that, comprising the steps:

a. Take 15ml of n-hexane in a small flask, then add 1.5g of PDMS main agent, and 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, and use an ultrasonic cell disruptor to sonicate the solution for 50min with a power of 500w;

c. Then add 0.15g of curing agent, magnetic stirring at 800rpm for 30min;

d. The solution was spin-coated on a glass sheet, heated with a hot plate at 100°C for 30 min, and the film was peeled off;

e. The C-PDMS film was treated with oxygen plasma for 10 min, taken out and placed on a glass plate;

f. The AgNWs solution was spin-coated on the glass plate, heated for 20 min to evaporate the ethanol; the temperature was 60 °C;

g. The double conductive layer film is placed on a 2cmx2cm copper circular through hole template with a thickness of 500μm; the diameter of the through hole is 500μm, and the distance between the centers of the holes is 800μm;

h. Weigh 0.15g of curing agent and add 1.5g of PDMS main agent, stir evenly, vacuum to remove air bubbles, and wait for standby;

i. Put the above whole on the suction cup of KW-4A glue homogenizer, turn on the vacuum, after spin-coating PDMS at 800rpm, it is in a vacuum state, place the infrared lamp 20cm above the suction cup, turn on the infrared lamp to cure for 1h;

j. After the curing is completed, the vacuum is turned off, the double conductive layer micro-dome structure is removed, and the part where the micro-dome is not formed is cut off, and finally a pressure-sensitive sensitive layer is prepared.