A gradient multi-layer flexible piezoresistive sensor and its preparation method

Abstract

The present invention discloses a gradient multi-layer flexible piezoresistive sensor and a preparation method thereof, which includes coplanar metal electrodes and a composite material sensitive layer disposed thereon. The composite material sensitive layer includes multiple layers of composite material films stacked sequentially from top to bottom. The material of the composite material film is carbon nanotube polydimethylsiloxane, and the proportion of carbon nanotubes in each composite material film decreases sequentially from top to bottom. Each of the composite material films between the coplanar metal electrodes and the uppermost composite material film includes two spaced composite material sheets to form an arch structure. Two electrodes are formed on the coplanar metal electrodes, and the two composite material sheets of the lowermost composite material film are respectively located on the two electrodes of the coplanar metal electrodes. The multi-layer composite material film has a specific gravity gradient, and simultaneously realizes gradient changes in resistance, modulus and roughness. The piezoresistive sensor not only has high sensitivity, a wide response range and good linearity, but also has the characteristics of simple structure and easy preparation.

Description

Technical Field

[0001] The present invention relates to the technical field of sensors, and specifically to a gradient multi-layer flexible piezoresistive sensor and a preparation method thereof. Background Art

[0002] Flexible wearable pressure sensors have advantages such as biocompatibility, stretchability, transparency, and wearability, and have broad application prospects in the fields of electronic skin, intelligent robots, motion sensing, etc. According to different sensing mechanisms, flexible wearable pressure sensors are divided into piezoresistive sensors, capacitive sensors, piezoelectric sensors, and triboelectric sensors. Among them, piezoresistive sensors have received extensive attention and rapid development due to their simple structure and easy preparation.

[0003] Most existing flexible piezoresistive sensors can often exhibit high sensitivity within a small pressure range, but their pressure sensing range is small and the linearity is poor. Therefore, there is an urgent need to propose a design strategy for piezoresistive sensors that can combine high sensitivity, a wide sensing range, and good linearity. Summary of the Invention

[0004] Aiming at the problems existing in the prior art, the present invention provides a gradient multi-layer flexible piezoresistive sensor, including a multi-layer multi-gradient carbon nanotube polydimethylsiloxane composite material, a metal electrode, and a polymer protective layer. This piezoresistive sensor has good flexibility, a large range, high sensitivity, good linearity, good piezoresistive repeatability, and can be used durably.

[0005] The present invention is realized through the following technical solutions:

[0006] A gradient multi-layer flexible piezoresistive sensor includes a coplanar metal electrode and a composite material sensitive layer disposed thereon. The composite material sensitive layer includes multiple composite material films stacked in sequence from top to bottom. The material of the composite material film is carbon nanotube polydimethylsiloxane, and the proportion of carbon nanotubes in each composite material film decreases sequentially from top to bottom;

[0007] Each composite material film between the coplanar metal electrode 1 and the topmost composite material film includes two spaced composite material sheets. Two electrodes are formed on the coplanar metal electrode layer 1, and the two composite material sheets of the lowermost composite material film are respectively located on the two electrodes of the coplanar metal electrode 1.

[0008] Preferably, the carbon nanotube polydimethylsiloxane composite material layer is one or a mixture of single-walled carbon nanotubes and multi-walled carbon nanotubes, or a conductive polymer doped in polydimethylsiloxane after modification.

[0009] Preferably, the number of composite material films in the composite material sensitive layer is 3 - 10 layers.

[0010] Preferably, the thickness of the composite material film is 50 μm to 500 μm, and the thicknesses of the composite material films of each layer are the same.

[0011] Preferably, the coplanar metal electrode 1 includes a flexible substrate and two electrodes deposited on one side thereof.

[0012] Preferably, the thickness of the electrode is 1 μm to 5 μm.

[0013] Preferably, polymer protective layers are respectively provided at the bottom of the coplanar metal electrode 1 and the top of the uppermost composite material film.

[0014] Preferably, the proportion of carbon nanotubes in the composite material film is 0.5 wt% - 10 wt%.

[0015] A preparation method of a gradient multi-layer flexible piezoresistive sensor includes the following steps:

[0016] Step 1, uniformly coat liquid carbon polydimethylsiloxane on a mold and cure to form a flexible substrate;

[0017] Step 2, deposit two electrodes on the flexible substrate to obtain a coplanar metal electrode;

[0018] Step 3, disperse carbon nanotube powder in an organic solvent to obtain a carbon nanotube dispersion;

[0019] Step 4, add a polydimethylsiloxane prepolymer to the carbon nanotube dispersion, fully stir and then remove the organic solvent to obtain a mixed solution of carbon nanotube polydimethylsiloxane prepolymer;

[0020] Step 5, after adding a curing agent to the mixed solution, coat the mixed solution on a mold and cure to form a composite material film;

[0021] Step 6, repeat steps 3 - 5 to prepare composite material films with different proportions of carbon nanotubes;

[0022] Step 7, stack and bond the composite material films in ascending order of the proportion of carbon nanotubes from bottom to top, cut the combined multi-layer CNT / PDMS composite material film into two composite material groups, and bond the CNT / PDMS composite material film with the largest proportion of carbon nanotubes on the top surfaces of the two composite material groups to obtain a composite material sensitive layer;

[0023] Step 8, package the composite material sensitive layer and the coplanar metal electrode to obtain a gradient multi-layer flexible piezoresistive sensor.

[0024] 10. The preparation method of a gradient multi-layer flexible piezoresistive sensor according to claim 9, wherein the mass ratio of the polydimethylsiloxane prepolymer to the curing agent in the mixed solution in step 5 is (5-10):1.

[0025] Compared with the prior art, the present invention has the following beneficial technical effects:

[0026] A gradient multi-layer flexible piezoresistive sensor provided by the present invention includes a flexible substrate, coplanar electrodes, and an arch structure multi-layer composite material film disposed thereon. By changing the content of carbon nanotubes in the composite material, the proportion of carbon nanotubes in the composite material film decreases sequentially from top to bottom, thereby changing the resistance, modulus, and roughness of the composite material, and simultaneously realizing the gradient changes of resistance, modulus, and roughness. By applying a voltage to the metal electrode and applying pressure to the flexible piezoresistive device, the CNT / PDMS composite material layers and the interior thereof deform at different pressure stages. The deformations respectively cause the carbon nanotubes between the layers and inside the composite material layers to come into contact with each other, creating more conductive paths, resulting in a decrease in resistance and an increase in current, thereby converting the pressure signal into a current signal to achieve the pressure sensing performance. The modulus gradient can effectively regulate the stress transfer. The soft layer deforms first, and the hard layer deforms later, which helps to improve the pressure sensing range and linearity. The conductivity gradient can effectively regulate the electron transfer during the deformation process, affecting the activation sequence of the conductive paths, which helps to improve the sensitivity and linearity. The rough microstructure between the layers in the multi-layer structure increases the interlayer contact resistance on the one hand, improving the initial resistance of the sensitive material, and on the other hand, helps to cause local stress concentration, resulting in a significant change in the interlayer contact area and improving the sensitivity. In addition, the multi-layer structure can relieve the overall stress concentration of the sensitive material, dispersing the stress in each layer, which helps to improve the linearity and sensing range. The multi-gradient structure and the multi-layer structure have a composite effect on improving the sensitivity, sensing range, and linearity of the sensor. Finally, the arch structure and the coplanar electrodes can effectively regulate the current direction, enabling the current to pass through the high-resistance layer, the low-resistance layer, and the high-resistance layer in sequence, further enhancing the performance advantages of the gradient multi-layer sensitive material. Description of the Drawings

[0027] Figure 1 is a cross-sectional view of the gradient multi-layer flexible piezoresistive sensor of the present invention;

[0028] Figure 2 is a schematic structural diagram of the gradient multi-layer flexible piezoresistive sensor of the present invention;

[0029] Figure 3 is a resistance curve graph of CNT / PDMS composite materials with different concentrations. As the CNT content increases, the resistance of the composite material decreases;

[0030] Figure 4The figure shows the modulus curves of CNT / PDMS composites with different concentrations. As the CNT content increases, the modulus of the composite material increases;

[0031] Figure 5 The figure shows the roughness curves of CNT / PDMS composites with different concentrations. As the CNT content increases, the roughness of the composite material increases;

[0032] Figure 6 The figure shows the relationship between the resistance change rate and stress of the gradient multi-layer flexible piezoresistive sensor of the present invention;

[0033] Figure 7 The figure shows the relationship between the resistance change rate and stress of the flexible CNT / PDMS single-layer piezoresistive sensor of the comparative example;

[0034] Figure 8 The figure shows the relationship between the resistance change rate and stress of the flexible CNT / PDMS multi-layer piezoresistive sensor of the comparative example.

[0035] In the figure, 1 is a coplanar metal electrode, 2 is the first composite material film, 3 is the second composite material film, and 4 is the third composite material film. Detailed implementation manners

[0036] The present invention will be further described in detail below with reference to the accompanying drawings. The following is an explanation of the present invention rather than a limitation.

[0037] Refer to Figure 1 and 2 A gradient multi-layer flexible piezoresistive sensor includes a coplanar metal electrode 1 and a composite material sensitive layer disposed thereon. The composite material sensitive layer includes multiple layers of composite material films stacked in sequence from top to bottom. The material of the composite material film is coplanar carbon nanotube polydimethylsiloxane, and the proportion of carbon nanotubes in each composite material film decreases sequentially from top to bottom;

[0038] Each composite material film between the coplanar metal electrode 1 and the uppermost composite material film includes two spaced composite material sheets. Two electrodes are formed on the coplanar metal electrode layer 1. The two composite material sheets of the lowermost composite material film are respectively located on the two electrodes of the coplanar metal electrode 1, forming an arch structure.

[0039] The coplanar metal electrode 1 includes a flexible substrate and two electrodes deposited on one side thereof. The flexible substrate is carbon polydimethylsiloxane. The metal electrode is gold, silver, copper, aluminum or platinum.

[0040] The carbon nanotubes in the carbon nanotube polydimethylsiloxane are one or a mixture of single-walled carbon nanotubes and multi-walled carbon nanotubes, or a conductive polymer doped in polydimethylsiloxane after modification.

[0041] The number of layers of the composite material film in the sensitive layer is 3 - 10 layers, preferably 3 layers. The thickness of the composite material film is 50μm - 500μm, and the thicknesses of the composite material films in each layer are the same.

[0042] Polymer protective layers are respectively provided at the bottom of the coplanar metal electrode 1 and the top of the uppermost composite material film to protect the gradient multi-layer flexible piezoresistive sensor. The polymer protective layer is composed of a rubber-like polymer, a polyolefin polymer, or a resin-like polymer.

[0043] A gradient multi-layer flexible piezoresistive sensor provided by the present invention includes a flexible substrate and a multi-layer composite material film disposed thereon. The proportion of carbon nanotubes in the composite material film decreases successively from top to bottom, realizing resistance gradient transformation and modulus gradient change. By applying a voltage to the metal electrode and applying pressure to the flexible piezoresistive device, the contact area between the multi-layer composite material films changes, thereby increasing the current path, and realizing the reflection of the external pressure change through the change of current or resistance. The sensitive layer converts the pressure signal into a resistance signal for output to achieve the pressure sensing characteristic.

[0044] The preparation method of the above-mentioned gradient multi-layer flexible piezoresistive sensor provided by the present invention is described in detail below, including the following steps:

[0045] Step 1: Uniformly coat liquid carbon polydimethylsiloxane (PDMS) on a mold and cure it to form a flexible substrate;

[0046] Step 2: Cover a template on the flexible substrate and deposit metal to form two electrodes to obtain a coplanar metal electrode;

[0047] Step 3: Disperse carbon nanotube powder in an organic solvent to obtain a stable carbon nanotube dispersion;

[0048] The organic solvent is at least one of chloroform, isopropanol, THF, and N-methylpyrrolidone.

[0049] The method for dispersing the carbon nanotube powder in the organic solvent is as follows:

[0050] Ultrasonically disperse the carbon nanotube powder and the organic solvent for 3 - 8h, and the concentration of the organic dispersion is 1 - 3mg / ml.

[0051] Step 4: Add a polydimethylsiloxane prepolymer to the carbon nanotube dispersion, stir well, and then remove the organic solvent to obtain a mixed solution of carbon nanotube / polydimethylsiloxane prepolymer;

[0052] Specifically, after stirring well with a magnetic stirrer for 3h, heat the mixed solution to 50 - 80°C by means of water bath heating, and continuously stir for 1 - 3h to evaporate and remove the organic solvent.

[0053] Step 5, after adding a curing agent to the mixed solution, the mixed solution is coated on a mold and cured to form a CNT / PDMS composite film;

[0054] The curing agent is a mixture of a diluent and a polydimethylsiloxane curing agent, and the preparation method is as follows:

[0055] Weigh the diluent and polydimethylsiloxane curing agent according to a certain ratio, mix them, and stir them thoroughly with a magnetic stirrer for 3 hours;

[0056] The diluent is at least one of n-hexane and cyclohexane, and the volume ratio of the diluent to PDMS is 3:1.

[0057] The mass ratio of polydimethylsiloxane to polydimethylsiloxane curing agent in the mixed solution is (5-10):1.

[0058] The CNT / PDMS composite material has a curing temperature of 80-100° C. and a curing time of 1-3 hours.

[0059] The mold is flat glass or various types of sandpaper.

[0060] Step 6: Repeat steps 3-5 to prepare a variety of different CNT / PDMS composite membranes, and the specific gravity of carbon nanotubes in each CNT / PDMS composite material is different.

[0061] The proportion of carbon nanotubes in the CNT / PDMS composite film is 0.5wt%-10wt%.

[0062] Step 7. Stack the CNT / PDMS composite film other than the CNT / PDMS composite film with the largest carbon nanotube specific gravity from top to bottom in order according to the specific gravity of the carbon nanotubes and bond them through silicone, and divide the combined multi-layer CNT / PDMS composite film into two composite material groups, and bond the CNT / PDMS composite film with the largest carbon nanotube specific gravity to the top surfaces of the two composite material groups through silicone to obtain a composite material sensitive layer.

[0063] Step 8: encapsulate the composite material sensitive layer and the coplanar metal electrode, and respectively provide a polymer protective layer at the bottom of the coplanar metal electrode 1 and the top of the uppermost composite material film to obtain a gradient multilayer flexible piezoresistive sensor.

[0064] Specifically, silver glue is applied to the surface of the electrode, the wide side of the electrode is aligned with the wide side of the sensitive layer, the silver glue is used to tightly combine the electrode and the sensitive layer, and PU tape is used to fit the electrode and the sensitive layer up and down to achieve packaging.

[0065] The gradient multilayer flexible piezoresistive sensor prepared by the above method has a tunneling effect within a single layer of composite material. The carbon nanotubes in the composite material are not overlapped with each other, but the distance is small enough that electrons have a certain probability of crossing the potential barrier, thereby making the interlayer conductive, and its resistance is related to the distance between the carbon nanotubes; the tunneling effect between single layers of the composite material. If the interlayer spacing of the multilayer structure is too large, electrons cannot cross the potential barrier, but as the stress increases, the interlayer distance decreases, and electrons have a certain probability of tunneling, forming a tunneling current and a conductive path; the piezoresistance effect of the composite material itself. When the CNT / PDMS composite material is subjected to stress, the carbon nanotubes in the composite material overlap with each other, so that the resistance of the composite material itself changes.

[0066] Example 1

[0067] A method for preparing a gradient multilayer flexible piezoresistive sensor comprises the following steps:

[0068] Step 1: Scrape the prepared PDMS solution onto a smooth glass plate, evacuate the plate and place it on a flat heating machine for heating and curing to obtain a PDMS electrode substrate layer;

[0069] Step 2: A symmetrical rectangular pattern with a width of 10 mm, a length of 4 mm, and a spacing of 2 mm is engraved on the polyamide sheet, and covered on the surface of the formed PDMS sheet. A metal layer is deposited on the surface by magnetron sputtering, and the thickness of the metal layer is 1 μm to obtain a coplanar electrode layer;

[0070] Step 3: According to the predetermined specific gravity of the composite material, a certain amount of carbon nanotube powder is weighed, and the carbon nanotube powder is dispersed in an organic solvent to obtain a stable carbon nanotube dispersion;

[0071] Step 4: Add polydimethylsiloxane prepolymer to the carbon nanotube dispersion, stir thoroughly for 3 hours with a magnetic stirrer, heat the solution and continue stirring for 3 hours to remove the organic solvent;

[0072] Step 5: Weigh the diluent and the polydimethylsiloxane curing agent in a certain proportion and mix them, and stir them thoroughly with a magnetic stirrer for 3 hours;

[0073] Step 6: Add a diluent / curing agent solution to the carbon nanotube / polydimethylsiloxane prepolymer, apply it on the mold, and place it on a flat heater to cure to form a CNT / PDMS composite material;

[0074] The mass ratio of the polydimethylsiloxane prepolymer to the dimethylsiloxane curing agent is 5:1.

[0075] Step 7: Repeat steps 3 to 6 to obtain three CNT / PDMS composite material sheets with different carbon nanotube specific gravities, namely, a first layer composite film 2, a second layer composite film 3 and a third layer composite film 4.

[0076] Step 8: Cut the first-layer composite material film and the second-layer composite material film into two rectangular sheets with a width of 10 mm and a length of 4 mm respectively, and cut the third-layer composite material film into a sheet with a length and width of 10 mm. Align the wide sides of the two first-layer composite material sheets with the wide sides of the two second-layer sheets respectively, and then align the two wide sides of the third-layer composite material sheet with the above two first- and second-layer sheets respectively, stack them and bond them with silicone to ensure that the three-layer composite material layers can be closely fitted to form a sensitive layer, and the proportion of carbon nanotubes in the three-layer composite material decreases in turn.

[0077] Step 9: Apply silver paste on the surface of the electrode, align the wide side of the electrode with the wide side of the sensitive layer, use the silver paste to closely combine the electrode and the sensitive layer, and use PU tape to fit the electrode and the sensitive layer up and down to achieve encapsulation.

[0078] Example 2

[0079] The difference between this example and the previous example lies in steps 6-8, and the rest of the steps are the same. Specifically as follows:

[0080] Step 6: Add a diluent / curing agent solution to the carbon nanotube / polydimethylsiloxane prepolymer, coat it on a mold, and place it on a flat heater to cure to form a CNT / PDMS composite material;

[0081] The mass ratio of the polydimethylsiloxane prepolymer to the dimethylsiloxane curing agent is 8:1.

[0082] Step 7: Repeat steps 3-6 to obtain a total of 5 kinds of CNT / PDMS composite material sheets with different carbon nanotube proportions;

[0083] Step 8: Cut four layers of CNT / PDMS composite materials with increasing carbon nanotube proportions into two rectangular sheets with a width of 10 mm and a length of 4 mm respectively. Then divide the 8 CNT / PDMS composite materials into two identical groups. According to the principle of increasing carbon nanotube proportion, stack the four CNT / PDMS composite materials from bottom to top in turn and bond them with silicone to form a composite material group. Finally, bond the CNT / PDMS composite material with the largest carbon nanotube proportion on the top layer of the two composite material groups to form a composite material sensitive layer.

[0084] Example 3

[0085] The difference between this example and the previous example lies in steps 6-8, and the rest of the steps are the same. Specifically as follows:

[0086] Step 6: Add a diluent / curing agent solution to the carbon nanotube / polydimethylsiloxane prepolymer, coat it on a mold, and place it on a flat heater to cure to form a CNT / PDMS composite material;

[0087] The mass ratio of the polydimethylsiloxane prepolymer to the dimethylsiloxane curing agent is 10:1.

[0088] Step 7: Repeat Steps 3 - 6 to obtain a total of 10 sheets of CNT / PDMS composites with different specific gravities of carbon nanotubes;

[0089] Step 8: Cut 8 layers of CNT / PDMS composites with gradually increasing specific gravities of carbon nanotubes into two rectangular sheets with a width of 10 mm and a length of 4 mm each. Then divide the 16 sheets of CNT / PDMS composites into two identical groups. According to the principle of gradually increasing specific gravity of carbon nanotubes, stack 8 sheets of CNT / PDMS composites from bottom to top in sequence and bond them with silicone to form a composite group. Finally, bond the CNT / PDMS composite with the largest specific gravity of carbon nanotubes on the top layer of the two composite groups to form a composite sensitive layer.

[0090] Example 4

[0091] A preparation method of a gradient multi-layer flexible piezoresistive sensor includes the following steps:

[0092] S1. Preparation of coplanar metal electrodes

[0093] S1.1: Measure 1 g of PDMS prepolymer and 0.1 g of curing agent, stir evenly, place in a vacuum environment of 0.1 for 30 min to remove air bubbles, obtain a polydimethylsiloxane solution, invert it on a glass plate, let it stand for 5 min to level automatically, and then dry it at 80 °C for 3 h, and peel off to obtain a PDMS film;

[0094] S1.2: Place a mask on the PDMS film, and use magnetron sputtering to deposit a 5-μm-thick silver film to obtain a coplanar electrode.

[0095] S2. Preparation of CNT / PDMS composite conductive film:

[0096] S2.1: Weigh 30 mg of multi-walled carbon nanotubes and mix them with 60 ml of isopropanol, stir magnetically for 1 h, and then ultrasonically disperse for 4 - 6 h to obtain a well-dispersed carbon nanotube dispersion. Then add 1 g of dimethylsiloxane prepolymer, stir magnetically for 3 h, and then stir and evaporate at 80 °C for 3 h to remove isopropanol to obtain a CNT / polydimethylsiloxane prepolymer solution;

[0097] S2.2: Weigh 0.1 g of polydimethylsiloxane curing agent and mix it with 5 ml of n-hexane, stir magnetically for 3 h;

[0098] S2.3. The CNT / polydimethylsiloxane prepolymer solution and the n - hexane / polydimethylsiloxane curing agent solution are mixed and stirred, then coated on a 100×100 mm glass template, and cured at 80 °C for 3 h to form a 3 wt% CNT / PDMS composite material;

[0099] S2.4. Weigh 50 mg of multi - walled carbon nanotubes and mix them with 100 ml of isopropanol, and repeat steps S2.1 - S2.3 to obtain a 5 wt% CNT / PDMS composite material;

[0100] S2.5. Weigh 70 mg of multi - walled carbon nanotubes and mix them with 140 ml of isopropanol, and repeat steps S2.1 - S2.3 to obtain a 7 wt% CNT / PDMS composite material;

[0101] S2.5. Cut the CNT / PDMS composite materials prepared in step S2.3 and step S2.4 into two rectangular sheets with a width of 10 mm and a length of 4 mm respectively, and cut the CNT / PDMS composite material prepared in step S2.5 into a sheet with a length and width of 10 mm. Align the wide sides of the two composite material sheets in step S2.3 with the wide sides of the two sheets in step S2.4 respectively, then align the two wide sides of the composite material sheet in step S2.5 with the two first - and second - layer sheets respectively and stack them, and bond them with silicone rubber to ensure that the three - layer composite material layers can be closely attached to form a sensitive layer.

[0102] S3. Preparation of flexible piezoresistive sensor devices:

[0103] Use silver paste to tightly bond copper electrodes on both sides of the multi - layer composite film to prepare a piezoresistive sensor with resistance, modulus, and roughness gradients, and encapsulate it with PU tape to obtain a gradient multi - layer structure - based composite flexible piezoresistive sensor.

[0104] S4. Connect wires to both ends of the electrodes, and test the fabricated sensor with a micro - force compression stage. The relationship between the change rate of its resistance and strain is as Figure 6 shown.

[0105] Comparative Example 1

[0106] A preparation method of a single - layer piezoresistive sensor includes the following steps:

[0107] S1. Preparation of coplanar metal electrodes

[0108] S1.1. Measure 1 g of PDMS prepolymer and 0.1 g of curing agent, stir them evenly, place them in a vacuum environment of 0.1 for 30 min to remove air bubbles to obtain a polydimethylsiloxane solution. Invert it on a glass plate, let it stand for 5 min to level automatically, and then dry it at 80 °C for 3 h and peel it off to obtain a PDMS film;

[0109] S1.2. Place a mask on the PDMS film and deposit a 5-μm-thick silver film by magnetron sputtering to obtain coplanar electrodes.

[0110] S2. Preparation of CNT / PDMS composite conductive film:

[0111] S2.1. Weigh 30 mg of multi-walled carbon nanotubes and mix them with 60 ml of isopropanol. Stir magnetically for 1 h, then ultrasonically disperse for 4 - 6 h to obtain a well-dispersed carbon nanotube dispersion. Then add 1 g of dimethylsiloxane prepolymer and stir magnetically for 3 h. Stir and evaporate at 80 °C for 3 h to remove isopropanol, obtaining a CNT / polydimethylsiloxane prepolymer solution;

[0112] S2.2. Weigh 0.1 g of polydimethylsiloxane curing agent and mix it with 5 ml of n-hexane. Stir magnetically for 3 h;

[0113] S2.3. Mix and stir the CNT / polydimethylsiloxane prepolymer solution and the n-hexane / polydimethylsiloxane curing agent solution, coat it on a 100×100 mm glass template, and cure at 80 °C for 3 h to form a 3 wt% CNT / PDMS composite material;

[0114] S2.4. Cut the CNT / PDMS composite material prepared in S2.3 into sheets with a length and width of 10 mm each.

[0115] S3. Preparation of flexible piezoresistive sensor device: Use silver paste to tightly bond copper electrodes on both sides of the composite film to prepare a 3 wt% single-layer piezoresistive sensor, and encapsulate it with PU tape to obtain a single-layer flexible piezoresistive sensor.

[0116] S4. Connect wires to both ends of the electrodes, and test the fabricated sensor with a micro-force compression stage. The relationship between the change rate of its resistance and strain is as Figure 7 shown.

[0117] Comparative Example 2

[0118] A preparation method of a non-gradient multi-layer piezoresistive sensor, comprising the following steps:

[0119] S1. Preparation of coplanar metal electrodes

[0120] S1.1. Measure 1 g of PDMS prepolymer and 0.1 g of curing agent, stir evenly, place in a vacuum environment of 0.1 for 30 min to remove air bubbles, obtain a polydimethylsiloxane solution, invert it on a glass plate, let it stand for 5 min to level automatically, and then dry at 80 °C for 3 h and peel off to obtain a PDMS film;

[0121] S1.2. Place a mask on the PDMS film and deposit a 5-μm-thick silver film by magnetron sputtering to obtain coplanar electrodes.

[0122] S2. Preparation of CNT / PDMS Composite Conductive Film:

[0123] S2.1. Weigh 30 mg of multi-walled carbon nanotubes and mix them with 60 ml of isopropanol. Stir magnetically for 1 h, then ultrasonically disperse for 4 - 6 h to obtain a well-dispersed carbon nanotube dispersion. Then add 1 g of dimethylsiloxane prepolymer and stir magnetically for 3 h. After that, stir and evaporate at 80 °C for 3 h to remove isopropanol, obtaining a CNT / polydimethylsiloxane prepolymer solution;

[0124] S2.2. Weigh 0.1 g of polydimethylsiloxane curing agent and mix it with 5 ml of n-hexane. Stir magnetically for 3 h;

[0125] S2.3. Mix and stir the CNT / polydimethylsiloxane prepolymer solution and the n-hexane / polydimethylsiloxane curing agent solution, coat it on a 100×100 mm glass template, and cure at 80 °C for 3 h to form a 3 wt% CNT / PDMS composite material;

[0126] S2.4. Cut the CNT / PDMS composite material prepared in S2.3 into four rectangular sheets with a width of 10 mm and a length of 4 mm;

[0127] S2.5. Then cut the CNT / PDMS composite material prepared in step S2.3 into sheets with a length and width of 10 mm;

[0128] S2.6. Take two of the composite material sheets prepared in step S2.4 as the first layer, and the other two sheets as the second layer. Align the wide sides of the first-layer sheets with the wide sides of the two second-layer sheets respectively. Then align the two wide sides of the composite material sheet in step S2.5 with the above two first- and second-layer sheets respectively and stack them, and bond them with silicone glue to ensure that the three composite material layers can be closely fitted to form a sensitive layer.

[0129] S3. Preparation of Flexible Piezoresistive Sensor Device: Use silver paste to tightly bond copper electrodes on both sides of the composite film to prepare a piezoresistive sensor with a resistance and modulus gradient multi-layer structure, and encapsulate it with PU tape to obtain a composite flexible piezoresistive sensor based on a non-gradient multi-layer structure.

[0130] S4. Connect wires to both ends of the electrodes, and test the fabricated sensor with a micro-force compression stage. The relationship between the change rate of its resistance and strain is as Figure 8 shown.

[0131] The comparison results of various data of the devices in the examples and comparative examples are as follows:

[0132] Linear detection range / KPa <![CDATA[S / KPa -1 > Example 4 0.003-120 14.93 Comparative Example 1 0.5-18 2.03 Comparative Example 2 0.04-80 5.16

[0133] In the table: S represents sensitivity, and the sensitivity calculation formula is as follows:

[0134]

[0135] In summary, by comparing Comparative Example 1 and Comparative Example 2, it can be found that the sensitivity, measurement range, and linearity values of the multi-layer structure are all higher than those of the single-layer structure;

[0136] By comparing Example 4 and Comparative Example 2, it can be found that the sensitivity, measurement range, and linearity values of the gradient multi-layer structure are all higher than those of the non-gradient multi-layer structure, and the comprehensive performance of this solution is better.

[0137] The above content is only to illustrate the technical idea of the present invention, and the protection scope of the present invention cannot be limited thereby. Any changes made on the basis of the technical solution according to the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A gradient multi-layer flexible piezoresistive sensor, characterized in that, it includes coplanar metal electrodes and a composite material sensitive layer disposed thereon. The composite material sensitive layer includes multiple layers of composite material films stacked sequentially from top to bottom. The material of the composite material film is carbon nanotube-polydimethylsiloxane, and the proportion of carbon nanotubes in each composite material film decreases sequentially from top to bottom; each of the composite material films between the coplanar metal electrode and the uppermost composite material film includes two spaced composite material sheets. Two electrodes are formed on the coplanar metal electrode layer, and the two composite material sheets of the lowermost composite material film are respectively located on the two electrodes of the coplanar metal electrode.

2. The gradient multi-layer flexible piezoresistive sensor according to claim 1, characterized in that, the carbon nanotube-polydimethylsiloxane composite material layer is one or a mixture of single-walled carbon nanotubes and multi-walled carbon nanotubes, or a conductive polymer doped in polydimethylsiloxane after modification.

3. The gradient multi-layer flexible piezoresistive sensor according to claim 1, characterized in that, the number of composite material films in the composite material sensitive layer is 3 - 10 layers.

4. The gradient multi-layer flexible piezoresistive sensor according to claim 1, characterized in that, the thickness of the composite material film is 50μm - 500μm, and the thicknesses of each layer of the composite material film are the same.

5. The gradient multi-layer flexible piezoresistive sensor according to claim 1, characterized in that, the coplanar metal electrode includes a flexible substrate and two electrodes deposited on one side of the flexible substrate.

6. The gradient multi-layer flexible piezoresistive sensor according to claim 1, characterized in that, the thickness of the electrode is 1μm - 5μm.

7. The gradient multi-layer flexible piezoresistive sensor according to claim 1, characterized in that, polymer protective layers are respectively provided at the bottom of the coplanar metal electrode and the top of the uppermost composite material film.

8. The gradient multi-layer flexible piezoresistive sensor according to claim 1, characterized in that, the proportion of carbon nanotubes in the composite material film is 0.5wt% - 10wt%.

9. A preparation method of the gradient multi-layer flexible piezoresistive sensor according to any one of claims 1 - 8, characterized in that, it includes the following steps: Step 1: Uniformly coat liquid carbon-polydimethylsiloxane on a mold and cure to form a flexible substrate; Step 2: Deposit and form two electrodes on the flexible substrate to obtain a coplanar metal electrode; Step 3: Disperse carbon nanotube powder in an organic solvent to obtain a carbon nanotube dispersion; Step 4: Add polydimethylsiloxane prepolymer to the carbon nanotube dispersion, fully stir and then remove the organic solvent to obtain a mixed solution of carbon nanotube-polydimethylsiloxane prepolymer; Step 5: Add a curing agent to the mixed solution, then coat the mixed solution on a mold and cure to form a composite material film; Step 6: Repeat steps 3 - 5 to prepare composite material films with different proportions of carbon nanotubes; Step 7: Stack and bond the composite material membranes in ascending order of the specific gravity of carbon nanotubes from bottom to top, and cut the combined multi-layer CNT / PDMS composite material membrane into two composite material groups. Bond the CNT / PDMS composite material membrane with the largest specific gravity of carbon nanotubes to the top surfaces of the two composite material groups to obtain a composite material sensitive layer. Step 8: Package the composite material sensitive layer with a coplanar metal electrode to obtain a gradient multi-layer flexible piezoresistive sensor.

10. The preparation method of a gradient multi-layer flexible piezoresistive sensor according to claim 9, characterized in that, in the mixed solution of step 5, the mass ratio of the polydimethylsiloxane prepolymer to the curing agent is (5-10):1.

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