Shape memory flexible piezoresistive sensor and method of making the same

Abstract

The application discloses a shape memory flexible piezoresistive sensor and a preparation method thereof. The sensor is provided with a shape memory composite conductive pressure sensitive film carrying a microstructure as a pressure sensitive layer, a shape memory polyimide film carrying a microstructure as an electrode layer, and a shape memory polyimide film without carrying a microstructure as a packaging layer. In the electrode layer, a conductive slurry is coated on the surface of the microstructure, and an electrode is drawn out. After the pressure sensitive layer, the electrode layer and the packaging layer are heated and shaped according to the temporary shape of an applied interface, the surface of the microstructure of the pressure sensitive layer is pressed on the surface of the microstructure of the electrode layer, the packaging layer is covered on the pressure sensitive layer, and the edges of the packaging layer and the edges of the electrode layer are bonded to obtain a flexible piezoresistive sensor which is fitted to the applied interface. The deformation of each layer of the sensor does not affect the sensing signal, the sensor has high sensitivity, excellent signal stability and recovery, and has a wide application prospect in the field of piezoresistive sensing.

Description

Technical Field

[0001] This invention belongs to the field of flexible sensor fabrication, specifically relating to a shape memory flexible piezoresistive sensor and its fabrication method. The piezoresistive performance of the piezoresistive film is excellent and stable, and the sensor can be stably applied to various complex non-planar interfaces. Background Technology

[0002] With the development of science and technology, wearable electronic devices have attracted much attention due to their enormous application potential in health monitoring, human-machine interfaces, and soft robotics. Among them, soft robotics technology has been extensively studied in recent years, bringing new possibilities for solving problems that traditional rigid robots cannot address. Soft robotics features a flexible and adaptable structure, making it easier to adapt to various environments and situations. This necessitates corresponding sensors to assist in sensing the surrounding environment. Compared to traditional electronic devices, the unique flexibility and extensibility of flexible electronic devices enable them to adapt to more complex working environments. Flexible pressure sensors have advantages such as high flexibility and sensitivity, allowing for better fit with non-planar interfaces. They are an excellent choice for assisting soft robots in sensing pressure, capable of adapting to complex non-flat surfaces and withstanding large deformations such as stretching, bending, and even folding. However, they are prone to mismatch between the sensor and the working interface, and unstable interactions can easily occur between the layers within the sensor, leading to sensing errors or even failure. Fixing the non-planar shape of the sensor during operation can solve this problem to some extent. Introducing the shape memory effect (SME) into a flexible sensing substrate allows the sensor's working shape to change and be fixed according to requirements, thus solving the signal instability phenomenon when assembling irregularly shaped flexible sensors. Furthermore, compared to non-shape memory materials, its deformation process does not damage the sensor's microstructure, protecting the device's sensitivity. Polyimide, as a high-stiffness flexible substrate, is an excellent choice for industrial flexible sensors due to its stability, excellent mechanical properties, and high glass transition temperature, which provides excellent creep resistance. However, its insolubility limits its widespread application. Therefore, this invention provides a shape memory flexible piezoresistive sensor. By mixing polyimide with soluble and shape memory properties with carbon nanotubes modified using a specific method and obtaining a thin film carrying a specific microstructure through a casting process, it exhibits excellent piezoresistive response. This flexible piezoresistive sensor is assembled from all shape memory materials, and is expected to advance the development of shape memory flexible sensor devices. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of existing technologies by providing a shape memory flexible piezoresistive sensor and its fabrication method. This solution solves the problems of poor dispersion of composite conductive voltage-sensitive films and unstable signals and difficulty in repeated processing of polyimide-based flexible piezoresistive sensors with irregular assembly. This improves the long-term stability of the flexible piezoresistive sensor, facilitating industrial production. Furthermore, its high glass transition temperature provides superior creep resistance, resulting in a wide operating temperature range. The soluble polyimide substrate facilitates secondary processing. The shape memory characteristic ensures that the irregular shape of the flexible piezoresistive film is well fixed when installed on non-planar interfaces, and subsequent changes in shape can be achieved through temperature adjustments. This improves the signal stability and long-term reliability of the conductive voltage-sensitive film under pressure loading in an irregular state. Moreover, the deformation process of the shape memory film does not damage the microstructure.

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

[0005] A shape memory flexible piezoresistive sensor is disclosed. The sensor uses a shape memory composite conductive voltage-sensitive film carrying a microstructure as a pressure-sensitive layer, a shape memory polyimide film carrying a microstructure as an electrode layer, and a shape memory polyimide film without a microstructure as an encapsulation layer. The electrode layer is coated with a conductive paste on the surface of the microstructure and electrodes are led out. The pressure-sensitive layer, electrode layer, and encapsulation layer are temporarily shaped according to the shape of the interface to be applied and then heated and shaped. The microstructure surface of the pressure-sensitive layer is pressed onto the microstructure surface of the electrode layer. The encapsulation layer is then covered with the pressure-sensitive layer, and the edge of the encapsulation layer is bonded to the edge of the electrode layer to obtain a flexible piezoresistive sensor that fits the interface to be applied.

[0006] The method for fabricating the shape memory flexible piezoresistive sensor includes the following:

[0007] First, a polyamic acid solution, a precursor of soluble shape memory polyimide, is obtained.

[0008] Carboxylated carbon nanotubes were functionalized by using organic compounds carrying functional groups to obtain modified carbon nanotubes; a portion of the precursor polyamic acid solution was mixed with the modified carbon nanotubes to obtain a composite film-forming solution.

[0009] A mold carrying a microstructure is prefabricated;

[0010] The precursor polyamic acid solution is added to the mold, the composite film-forming solution is added to the mold, and the precursor polyamic acid solution is dropped onto a smooth glass slide to level it. After drying, the film is peeled off and subjected to gradient heating thermal imidization to obtain the shape memory polyimide film carrying the microstructure, the shape memory composite voltage-sensitive film carrying the microstructure, and the shape memory polyimide film without the microstructure in sequence.

[0011] In the above technical solution, the preparation method of the precursor polyamic acid solution includes: dissolving 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (HFBAPP) and 4,4'-(hexafluoroisopropene)phthalic anhydride (6FDA) in N-methylpyrrolidone (NMP), and then continuously stirring at 0-10°C to obtain the precursor polyamic acid solution.

[0012] Furthermore, the molar ratio of HFBAPP to 6FDA is 1:1, and the reaction solids content is 20wt%.

[0013] Furthermore, the organic compound carrying the functionalized group is at least one of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (AHHFP) and ethylenediamine (EDA).

[0014] Furthermore, the aforementioned functional modification process specifically includes:

[0015] Carboxylated carbon nanotubes and a condensing agent were dispersed in a solvent to activate the carboxyl groups, resulting in dispersion A. An organic compound carrying functionalized groups was weighed and dissolved in the solvent to obtain solution B. Solution B was added to dispersion A to carry out a condensation reaction. After the reaction was completed, the mixture was washed with ethanol, filtered, and dried.

[0016] Furthermore, the degree of carboxylation of the carboxylated carbon nanotubes is no more than 1 wt%, which is more conducive to obtaining excellent dispersibility, and more preferably 1 wt%. The solvent is at least one of N-methylpyrrolidone (NMP) and ethylenediamine (EDA). The condensing agent is dicyclohexylcarbodiimide (DCC).

[0017] Furthermore, the modified carbon nanotubes account for 5-9 wt% of the total mass of the precursor polyamic acid solution monomer and the modified carbon nanotubes.

[0018] After modifying carbon nanotubes using the method of the present invention, especially using carbon nanotubes with a carboxylation degree of 1 wt% and grafting modification with AHHFP, the resulting modified functionalized carbon nanotubes have significantly better dispersibility in the polyimide substrate and the conductivity of the composite material made from them than the carboxylated carbon nanotubes before grafting. Furthermore, it is possible to add less conductive phase to the pressure-sensitive film without sacrificing conductivity.

[0019] Furthermore, the method for preparing the mold carrying the microstructure is as follows: the glass fiber cloth is laid flat in a container, defoamed PDMS is poured in, leveled, cured, and demolded to obtain the surface microstructure mold.

[0020] The shape memory flexible piezoresistive sensor of the present invention has excellent pressure response capability and stability, and can be applied in many fields such as battery thermal runaway early warning.

[0021] The beneficial effects of this invention are as follows:

[0022] This invention employs a full-shape memory device. By introducing the shape memory effect (SME) into the flexible sensing substrate, the working shape of the sensor can be changed and fixed according to requirements while protecting the microstructure of the pressure-sensitive layer. This solves the problems of unstable signals and difficulty in repeated processing in irregularly shaped assembly of polyimide-based flexible piezoresistive sensors. Furthermore, by designing the structure and materials of each layer of the sensor, especially by selecting rigid materials containing flexible groups (ether bonds) such as polyimide, the polyimide substrate material possesses excellent shape memory properties. The introduction of flexible groups also improves the insolubility of polyimide, making it easier to perform secondary processing. Simultaneously, a conductive voltage-sensitive material is prepared using carbon nanotubes, a carbon material with better conductivity and thermal conductivity. In particular, the carbon nanotubes are chemically grafted with organic compounds with similar structures to the substrate and carrying active groups, resulting in a conductive composite material with better compatibility with the organic substrate and superior dispersibility, conductivity, and shape memory properties. This allows for the addition of as little conductive phase as possible without sacrificing the performance of the pressure-sensitive film. Through overall design, this flexible piezoresistive sensor exhibits excellent pressure response capability, along with very good signal stability and resilience, and has broad application prospects. Attached Figure Description

[0023] Figure 1 The structural design and preparation principle of (a) high Tg soluble shape memory polyimide and (b) carboxylated carbon nanotube functional chemical grafting in this invention;

[0024] Figure 2. (a) DSC test results of homemade polyimide (0%) and its composite material with functionalized carbon nanotubes and (b) commercial polyimide;

[0025] Figure 3 (a) DMA-shape memory performance test of polyimide and (b) polyimide-functional carbon nanotube composite conductive film;

[0026] Figure 4 SEM images of cross-sections of polyimide-functional carbon nanotube composite conductive films obtained by grafting carbon nanotubes with different degrees of carboxylation.

[0027] Figure 5 Resistivity versus carbon nanotube mass fraction of polyimide-functional carbon nanotube composite conductive materials obtained by grafting carbon nanotubes with different degrees of carboxylation.

[0028] Figure 6 SEM images of the microstructure of the membrane carrying the microstructure before and after heat deformation;

[0029] Figure 7 Fabrication process of flexible piezoresistive sensor based on shape memory polyimide-carbon nanotube composite voltage-sensitive film;

[0030] Figure 8 Current-pressure curves of flexible piezoresistive sensors based on shape memory and non-shape memory polyimide. Detailed Implementation

[0031] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0032] The present invention discloses a shape memory flexible piezoresistive sensor, which uses a shape memory composite conductive voltage-sensitive film carrying microstructures as the piezoresistive layer, a shape memory polyimide film carrying microstructures as the electrode layer, and a shape memory polyimide film without microstructures as the encapsulation layer. Its preparation is mainly based on obtaining a polyamic acid solution as a precursor of soluble shape memory polyimide, and a composite film-forming solution of the polyamic acid solution and a specific modified carbon nanotube, combined with a mold having microstructures.

[0033] In some embodiments of the present invention, the synthesis of the precursor polyamic acid solution involves dissolving all raw materials and continuously stirring at 0-10°C for at least 12 hours.

[0034] In some embodiments of the present invention, during the preparation of modified carbon nanotubes, to ensure that all carboxyl groups on the carboxylated carbon nanotubes are chemically grafted, the carboxylated carbon nanotubes are added to a solution containing an excess of condensing agent and fully dispersed to activate the carboxyl groups. An excess of the organic compound carrying the functionalized groups to be grafted is fully dissolved in a solvent. The activated carboxylated carbon nanotube suspension is mixed and stirred evenly with the solution containing the compound to be grafted, and then placed in a 40°C water bath and stirred for 24 hours. The reaction temperature can be appropriately increased to accelerate the reaction rate. After the reaction is complete, the solvent is filtered off, and the mixture is washed with ethanol and filtered repeatedly to remove excess reactants, condensing agent, and solvent. The mixture is then dried in a 60°C oven and stored in a sealed container.

[0035] In some embodiments of the present invention, the required amount of functionalized carbon nanotubes to be added is calculated by combining the solid content of the polyamic acid solution and the mass fraction of functionalized carbon nanotubes in the polyamic acid. The polyamic acid solution and functionalized carbon nanotubes are mixed in a certain proportion and fully dispersed by using an ultra-high speed mixer. N-methylpyrrolidone (NMP) is added to dilute to the required viscosity and stored in an environment of 0-10°C.

[0036] In some embodiments of the present invention, the precursor polyamic acid solution and the composite film-forming solution are separately dropped into the pre-prepared PDMS microstructure mold, the precursor polyamic acid solution is dropped onto a flat glass surface to level, dried in an oven at 60°C, peeled off, and placed on a hot stage for gradient heating to perform thermal imidization.

[0037] In some embodiments of the present invention, the heating and holding process for thermal imidization can be: 90℃ / 1h, 130℃ / 1h, 170℃ / 1h, 210℃ / 1h, 250℃ / 1h, 300℃ / 1h. After the imidization process is completed, the temperature is lowered to room temperature to obtain the corresponding film.

[0038] In some embodiments of the present invention, for a shape memory polyimide film carrying a microstructure, silver paste is applied to its microstructure and leads are drawn out. After drying, it is heated and shaped based on the interface to be applied as a temporary shape to serve as an electrode layer. For a shape memory composite conductive voltage-sensitive film carrying a microstructure, it serves as a pressure-sensitive film, and its microstructure is pressed onto the surface of the electrode layer microstructure. For a shape memory polyimide film without a microstructure, it is directly heated and shaped in the same way and then used as an encapsulation layer to cover the pressure-sensitive layer, with the edges bonded to the electrode layer to obtain the flexible piezoresistive sensor.

[0039] The shape memory composite voltage-sensitive membrane carrying microstructures in this invention, through the selection of matrix groups and the treatment and dispersion of chemically grafted carbon nanotubes, has superior anti-creep and anti-relaxation properties (glass transition temperature of 259-262℃), good shape memory properties, fast shape recovery speed, and good solubility, while also exhibiting excellent piezoresistive properties.

[0040] The flexible piezoresistive sensor produced by this invention protects the microstructure of the piezoresistive layer while fixing the macroscopic shape of each layer. This can solve the mismatch problem between the sensor and the working interface and avoid unstable interactions between the layers in the sensor, which could lead to sensing errors or even failure.

[0041] The shape memory testing method for the conductive composite material prepared in this invention involves measuring the shape memory retention and recovery properties of the thin film sample using a dynamic thermomechanical analyzer (DMA). The measured shape fixation rate of the composite conductive voltage-sensitive film is above 99.8%, and the shape recovery rate is 95-97%. The shape memory performance of the thin film sample can be measured using the bending method, achieving a shape fixation rate of up to 100% and a shape recovery rate of up to 99%.

[0042] The resistivity of the composite conductive voltage-sensitive film is tested using a four-probe resistivity meter. The measured resistivity of the composite conductive voltage-sensitive film is adjustable within the range of 2-10 kΩ·cm.

[0043] The pressure sensing test method is as follows:

[0044] An electrode layer is prepared by coating a shape memory polyimide film carrying a microstructure with silver paste. Electrode leads are connected to a digital source meter to read the electrical signal. The microstructured surface of the piezoresistive membrane is placed on the electrode layer in contact with it. Finally, a shape memory polyimide film without microstructures is covered, and the electrode layer and encapsulation layer edges are connected with a high-temperature resistant adhesive to complete the encapsulation, resulting in a flexible piezoresistive sensor. Pressure is applied to the sensor using a push-pull pressure machine, and the change in current is recorded while the loading pressure is changed. By establishing the relationship curve between the electrical signal and pressure, the performance of a series of piezoresistive sensors can be measured.

[0045] Example 1

[0046] (1) Dissolve 5.185g HFBAPP and 4.442g 6FDA (i.e., the molar ratio of HFBAPP to 6FDA is 1:1) in 38.5g N-methylpyrrolidone (NMP) (solid content is 20wt%) and stir continuously at 0-10℃ for 12h to obtain a polyamic acid solution, which is then stored at 0-10℃.

[0047] (2) Take the polyamic acid solution prepared in (1) and drop it onto a glass slide. Let it stand on a flat surface until the liquid is homogenized. Then place it on a hot stage for solvent evaporation and thermal imidization. The imidization program is as follows: 90℃ / 1h, 120℃ / 1h, 150℃ / 1h, 180℃ / 1h, 210℃ / 1h, 240℃ / 1h, 270℃ / 1h, 300℃ / 1h. After the imidization program is completed, cool it to room temperature to obtain a planar polyimide film without microstructures. The film thickness is about 0.1 mm.

[0048] (3) The glass transition temperature of the polyimide film was measured to be 259℃ using differential scanning calorimetry (DSC).

[0049] (4) The shape memory retention and recovery properties of the thin film samples were measured using a dynamic thermomechanical analyzer (DMA). The shape retention rate of the obtained polyimide film was found to be above 99.8%, and the shape recovery rate was above 95.6%. The test curves are shown below. Figure 3 As shown in (a).

[0050] (5) The polyimide film was dissolved in N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) respectively, and stirred at 40°C to test its solubility. It could be completely dissolved in several common solvents within 1 hour.

[0051] (6) The interface on which the sensor is to be installed is molded, the polyimide film is bound in the mold, and placed in a muffle furnace and heated to the transition temperature (269°C). After holding at the temperature for 5 minutes, it is cooled to room temperature to obtain a polyimide film that fits well with the sensing interface, which serves as the encapsulation layer for the flexible piezoresistive sensor.

[0052] Example 2

[0053] (1) Dissolve 5.185g HFBAPP and 4.442g 6FDA (i.e., the molar ratio of HFBAPP to 6FDA is 1:1) in 38.5g N-methylpyrrolidone (NMP) (solid content is 20wt%) and stir continuously at 0-10℃ for 12h to obtain a polyamic acid solution, which is then stored at 0-10℃.

[0054] (2) Lay the clean glass fiber cloth flat and adhere it to the culture dish. After the PDMS is allowed to stand and defoam, pour it into the culture dish with the glass fiber cloth. Place it in a vacuum oven at 60℃ for 24h until it is completely cured. Remove it and demold it to obtain a PDMS mold with the glass fiber cloth microstructure.

[0055] (3) Take the polyamic acid solution obtained in step (1) and drop it into the PDMS mold prepared in step (2). Let it stand on a flat surface until the liquid flows evenly. Then place it on a hot plate for solvent evaporation and thermal imidization. The imidization program is: 90℃ / 1h, 130℃ / 1h, 170℃ / 1h, 210℃ / 1h, 250℃ / 1h, 300℃ / 1h. After the imidization program is completed, cool it to room temperature to obtain a planar polyimide film with microstructures and a film thickness of about 0.1 mm.

[0056] (4) The glass transition temperature of the polyimide film was measured to be 259℃ using differential scanning calorimetry (DSC).

[0057] (5) The shape memory retention and recovery properties of the thin film samples were measured using a dynamic thermomechanical analyzer (DMA). The shape retention rate of the obtained polyimide film was found to be above 99.8%, and the shape recovery rate was above 95.6%. The test curves are shown below. Figure 3 As shown in (a).

[0058] (6) The polyimide film obtained in step (3) was dissolved in N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF) and tetrahydrofuran (THF), respectively, and stirred at 40°C to test its solubility. It could be completely dissolved in several common solvents within 1 hour.

[0059] (7) Apply silver paste to the microstructure side of the polyimide film in step (3) and dry to obtain a planar electrode layer.

[0060] (8) The interface on which the sensor is to be installed is molded, the polyimide film obtained in step (7) is bound in the mold, placed in a muffle furnace and heated to the transition temperature (269°C), kept at the temperature for 5 minutes and then cooled to room temperature to obtain a polyimide film that fits well with the sensing interface, which serves as the electrode layer of the flexible piezoresistive sensor.

[0061] (9) The surface morphology of the thin film sample can be observed by field emission scanning electron microscopy (SEM). The microstructure of the thin film and its microstructure after multiple deformations can be observed. The microstructure after multiple shape fixation-recovery processes is still well preserved.

[0062] Example 3

[0063] (1) Dissolve 5.185g HFBAPP and 4.442g 6FDA (i.e., the molar ratio of HFBAPP to 6FDA is 1:1) in 38.5g N-methylpyrrolidone (NMP) (solid content is 20wt%) and stir continuously at 0-10℃ for 12h to obtain a polyamic acid solution, which is then stored at 0-10℃.

[0064] (2) Carbon nanotubes with a carboxylation degree of 1 wt% and condensing agent DCC were dispersed in N-methylpyrrolidone (NMP) and stirred thoroughly (the molar ratio of carboxyl groups in carbon nanotubes to DCC was 1:10) to obtain a carbon nanotube dispersion; AHHFP (the molar ratio of carboxyl groups in carbon nanotubes to AHHFP was 1:10) was weighed and dissolved in N-methylpyrrolidone (NMP) and stirred until completely dissolved to obtain a diamine solution; the diamine solution was added to the activated carboxylated carbon nanotube dispersion and stirred at 40°C for 12 h to carry out a condensation reaction to obtain chemically grafted functionalized carbon nanotubes; after the reaction was completed, the nanotubes were washed with anhydrous ethanol and filtered, and the process was repeated three times. The nanotubes were then dried at 60°C and stored in a sealed container.

[0065] (3) Take 1.75g ​​of polyamic acid solution, add 0.022g of functionalized carbon nanotubes (functionalized carbon nanotubes account for 6wt% of polyamic acid solid), mix evenly with an ultra-high speed mixer at 2500r / min for 5min, and then add an appropriate amount of N-methylpyrrolidone (NMP) to adjust the viscosity of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion so as to control the amount of liquid for subsequent film preparation by dropper. Stir evenly and store in an environment of 0-10℃.

[0066] (4) Lay the clean glass fiber cloth flat and adhere it to the culture dish. After the PDMS is allowed to stand and defoam, pour it into the culture dish with the glass fiber cloth. Place it in a vacuum oven at 60℃ for 24h until it is completely cured. Remove it and demold it to obtain a PDMS mold with the glass fiber cloth microstructure.

[0067] (5) Take the polyamic acid-functionalized carbon nanotube conductive composite dispersion obtained in step (3) and drop it into the PDMS microstructure mold prepared in step (4). Let it stand on a flat surface until the liquid flows evenly. Then place it on a hot plate for solvent evaporation and thermal imidization. The imidization program is: 90℃ / 1h, 130℃ / 1h, 170℃ / 1h, 210℃ / 1h, 250℃ / 1h, 300℃ / 1h. After the imidization program is completed, cool it to room temperature to obtain a soluble shape memory polyimide-carbon nanotube composite voltage-sensitive film with a film thickness of about 0.1 mm.

[0068] (6) The glass transition temperature of the polyimide film was measured to be 261℃ using a differential scanning calorimeter (DSC).

[0069] (7) The shape memory retention and recovery properties of the thin film samples were measured using a dynamic thermomechanical analyzer (DMA). The shape retention rate of the polyimide-functionalized carbon nanotube composite conductive film was above 99.9%, and the shape recovery rate was 96.8%. The test curves are shown below. Figure 3 (b) shows that the shape memory performance of the thin film sample was measured by the bending method, and the shape fixation rate can reach 100% and the shape recovery rate can reach 98.9%.

[0070] (8) Its resistivity was measured to be approximately 9.5 kΩ·cm using the four-probe method.

[0071] (9) The interface on which the sensor is to be installed is molded, the pressure-sensitive film obtained in step (5) is bound in the mold, placed in a muffle furnace and heated to the transition temperature (271°C), kept at the temperature for 5 minutes and then cooled to room temperature to obtain a pressure-sensitive film that fits well with the sensing interface.

[0072] (10) The surface morphology of the thin film sample was observed using a field emission scanning electron microscope (SEM). The microstructure of the glass fiber cloth molded film was observed as follows: Figure 6 As shown, the microstructure remains well-preserved after multiple shape fixation-recovery processes. Field emission scanning electron microscopy (SEM) was used to observe the cross-sectional morphology of the thin film sample, revealing the dispersion of functionalized carbon nanotubes as shown... Figure 4 (f) Carboxylated carbon nanotubes before chemical grafting Figure 4 When compared with (d), it can be seen that its dispersion is significantly improved and there is no obvious aggregation.

[0073] (11) such as Figure 7 The process involves assembling the pressure-sensitive film obtained in step (9) with the curved encapsulation layer in Example 1 and the curved electrode layer in Example 2 to fabricate a fully shape-memory assembled curved flexible pressure sensor. This sensor is then installed on the interface for testing, and its pressure-current response is as follows: Figure 8 As shown in (b).

[0074] Example 4

[0075] Same as in Example 3, except that step (2) is replaced as follows:

[0076] (2) Carbon nanotubes with a carboxylation degree of 1 wt% and condensing agent DCC were dispersed in ethylenediamine (EDA) and stirred thoroughly (the molar ratio of carboxyl groups in carbon nanotubes to DCC was 1:10; the ratio of carboxyl groups in carbon nanotubes to EDA was 100 mg: 100 ml). The mixture was stirred at 40 °C for 12 h to carry out the condensation reaction and obtain chemically grafted functionalized carbon nanotubes. After the reaction was completed, the mixture was washed with anhydrous ethanol and filtered. The process was repeated three times. The mixture was then dried at 60 °C and stored in a sealed container.

[0077] The cross-sectional morphology of the thin film sample obtained in this example was observed using field emission scanning electron microscopy (SEM), revealing the dispersion of functionalized carbon nanotubes. Figure 4 (e) Carboxylated carbon nanotubes before chemical grafting Figure 4 Comparing with (d), it can be seen that its dispersion has been improved to some extent, but there is still a significant agglomeration phenomenon. Its resistivity measured by the four-probe method is about 10 kΩ·cm.

[0078] Example 5

[0079] Same as Example 3, except that:

[0080] In step (2), the degree of carboxylation of carbon nanotubes is 0.6 wt%, and in step (3), 0.035 g of functionalized carbon nanotubes are added (functionalized carbon nanotubes account for 9 wt% of polyamic acid solids).

[0081] The cross-sectional morphology of the thin film sample obtained in this example was observed using field emission scanning electron microscopy (SEM), revealing the dispersion of functionalized carbon nanotubes. Figure 4 (c) Carboxylated carbon nanotubes before chemical grafting Figure 4 Compared with (a), its dispersion is significantly improved, but a small amount of agglomeration still exists. Its resistivity, measured by the four-probe method, is approximately 3.2 kΩ·cm.

[0082] Example 6

[0083] Same as Example 4, except that:

[0084] In step (2), the degree of carboxylation of carbon nanotubes is 0.6 wt%, and in step (3), 0.035 g of functionalized carbon nanotubes are added (functionalized carbon nanotubes account for 9 wt% of polyamic acid solids).

[0085] The cross-sectional morphology of the thin film sample obtained in this example was observed using field emission scanning electron microscopy (SEM), revealing the dispersion of functionalized carbon nanotubes. Figure 4(b) Carboxylated carbon nanotubes before chemical grafting Figure 4 Comparing the two images (a and b), it can be seen that the dispersion has been improved to some extent, but there is still a relatively obvious aggregation phenomenon.

[0086] Example 7

[0087] (1) Commercial polyimide was heated and dissolved in N-methylpyrrolidone (NMP) (solid content of 20wt%) and stirred continuously until homogeneous to obtain a polyimide solution.

[0088] (2) Carbon nanotubes with a carboxylation degree of 1 wt% and condensing agent DCC were dispersed in N-methylpyrrolidone (NMP) and stirred thoroughly (the molar ratio of carboxyl groups in carbon nanotubes to DCC was 1:10) to obtain a carbon nanotube dispersion; AHHFP (the molar ratio of carboxyl groups in carbon nanotubes to AHHFP was 1:10) was weighed and dissolved in N-methylpyrrolidone (NMP) and stirred until completely dissolved to obtain a diamine solution; the diamine solution was added to the activated carboxylated carbon nanotube dispersion and stirred at 40°C for 12 h to carry out a condensation reaction to obtain chemically grafted functionalized carbon nanotubes; after the reaction was completed, the nanotubes were washed with anhydrous ethanol and filtered, and the process was repeated three times. The nanotubes were then dried at 60°C and stored in a sealed container.

[0089] (3) Take 1.75g ​​of polyimide solution, add 0.022g of functionalized carbon nanotubes (functionalized carbon nanotubes account for 6wt% of polyimide solid), mix evenly with an ultra-high speed mixer at 2500r / min for 5min, and then add an appropriate amount of N-methylpyrrolidone (NMP) to adjust the viscosity of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion so as to control the liquid volume of subsequent film preparation by dropper. Stir evenly and store in an environment of 0-10℃.

[0090] (4) Lay the clean glass fiber cloth flat and adhere it to the culture dish. After the PDMS is allowed to stand and defoam, pour it into the culture dish with the glass fiber cloth. Place it in a vacuum oven at 60℃ for 24h until it is completely cured. Remove it and demold it to obtain a PDMS mold with the glass fiber cloth microstructure.

[0091] (5) Take the polyimide-functionalized carbon nanotube conductive composite dispersion obtained in step (3) and drop it into the PDMS microstructure mold prepared in step (4). Let it stand on a flat surface until the liquid flows evenly. Then place it on a hot plate to evaporate the solvent. The temperature rise program is: 90℃ / 2h, 150℃ / 1h, 200℃ / 1h. After the solvent has completely evaporated, cool it to room temperature to obtain a polyimide-carbon nanotube composite voltage-sensitive film with a thickness of about 0.1mm.

[0092] (6) The glass transition temperature of the polyimide film was measured to be 267℃ using a differential scanning calorimeter (DSC).

[0093] (7) The bending method is used to measure whether the thin film sample has shape memory properties. It cannot achieve good shape fixation and automatic shape recovery, and does not have shape memory properties.

[0094] (8) Its resistivity was measured to be approximately 9.5 kΩ·cm using the four-probe method.

[0095] (9) The interface on which the sensor is to be installed is molded, the pressure-sensitive film obtained in step (5) is bound in the mold, and placed in a muffle furnace and heated to above the glass transition temperature (311°C). After holding at the temperature for 5 minutes, it is cooled to room temperature. Its shape cannot fit the sensing interface.

[0096] (10) The surface morphology of the thin film sample was observed using a field emission scanning electron microscope (SEM). The microstructure of the glass fiber cloth molded film was observed as follows: Figure 6 As shown, the microstructure deformation or even disappearance after undergoing multiple bending-flattening processes with the assistance of external force.

[0097] Example 8

[0098] Same as Example 3, except for the device assembly:

[0099] In this example, the deformation process of the pressure-sensitive film in step (9) of Example 3 is not included, and step (11) is replaced as follows:

[0100] (11) such as Figure 7 The process involves using the planar composite film obtained above as the pressure-sensitive layer, and assembling it with the planar encapsulation layer in Example 1 (which did not undergo the deformation process in step (6)) and the planar electrode layer in Example 2 (which did not undergo the deformation process in step (8)). This produces a planar flexible pressure sensor that does not utilize shape memory properties. This sensor is then installed on the interface and tested, and its pressure-current response is as follows: Figure 8 As shown in (a).

[0101] This invention improves the rigidity of polyimide by selecting a rigid material containing flexible groups (ether bonds), giving the polyimide substrate excellent shape memory properties. This allows the sensor's working shape to be changed and fixed according to requirements, solving the problem of irregular assembly of flexible sensors while protecting the microstructure of the pressure-sensitive layer. It also overcomes the disadvantage of polyimide's poor solubility, facilitating secondary processing. The high glass transition temperature endows it with excellent creep and relaxation resistance. Chemical grafting of carbon nanotubes with excellent conductivity and thermal conductivity with diamines carrying active groups similar to the substrate structure yields a conductive composite material with better compatibility with organic substrates and superior dispersibility, conductivity, and shape memory properties. The use of finely structured, uniformly ordered glass fiber cloth for molding enhances device sensitivity. The flexible piezoresistive sensor based on shape memory polyimide-carbon nanotube composite conductive voltage-sensitive film protects the microstructure of the piezoresistive layer while fixing the macroscopic shape of each layer. This solves the mismatch problem between the sensor and the working interface, avoids sensing errors or even failures caused by unstable interactions between the layers within the sensor, and exhibits high sensitivity, excellent stability, and good resilience.

[0102] like Figure 5 The figure shows the resistivity versus carbon nanotube mass fraction of polyimide-functional carbon nanotube composite conductive materials with different carboxylation degrees (1 wt% and 0.6 wt%) and different diamine grafts (EDA and AHHFP), based on extensive experimental research. As the mass fraction increases, the resistivity of the composite material gradually stabilizes, meeting the requirements for preparing a piezoresistive sensor. Therefore, the mass fraction of carbon nanotubes in the composite material in the examples was selected based on this data. Figure 5 The middle are respectively Figure 4 The resistivity of composite materials with relatively uniformly dispersed fillers (1wt%-COOH-EDA, 0.6wt%-COOH-AHHFP, and 1wt%-COOH-AHHFP) was calculated from the mass fraction. The densities of the various functional carbon nanotubes were: 1wt%-COOH-AHHFP > 0.6wt%-COOH-AHHFP > 1wt%-COOH-EDA. Figure 5 As a result, it can be seen that 1 wt% -COOH-AHHFP can achieve a conductive composite varistor with the target resistivity with minimal carbon control. For composite materials, fewer inorganic conductive fillers to some extent avoid damage to the properties of the substrate material itself, and uniform dispersion can contribute to uniform reinforcement. In Example 3, using 1 wt% -COOH-AHHFP as a conductive filler enhances the shape memory properties of the material.

[0103] like Figure 7 , Figure 8As shown, Example 3, which utilizes shape memory properties, and Example 8, which does not utilize shape memory properties, both fabricated flexible pressure sensor devices using the same polyimide-based pressure-sensitive film. It can be seen that the bending device without shape memory properties exhibits significant unstable interactions between its layers, and the wrinkles generated by the deformation of each layer affect the stability of the device's performance. The difference between the two is clearly evident from the current-pressure curves. The flexible piezoresistive sensor based on the shape memory polyimide-carbon nanotube composite conductive voltage-sensitive film possesses excellent pressure response capability and stable signal transmission.

[0104] The present invention has been illustrated with the above embodiments to explain the detailed method of the present invention. However, the present invention is not limited to the detailed method described above, that is, it does not mean that the present invention must rely on the detailed method described above to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, and selection of specific methods and conditions, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A shape memory flexible piezoresistive sensor, characterized in that, The sensor uses a shape memory composite conductive voltage-sensitive film carrying microstructures as the pressure-sensitive layer, a shape memory polyimide film carrying microstructures as the electrode layer, and a shape memory polyimide film without microstructures as the encapsulation layer. The electrode layer is coated with a conductive paste on the surface of the microstructure and electrodes are led out. The pressure-sensitive layer, electrode layer and encapsulation layer are heated and shaped according to the shape of the interface to be applied. The microstructure surface of the pressure-sensitive layer is pressed onto the microstructure surface of the electrode layer. The encapsulation layer is then covered with the pressure-sensitive layer, and the edge of the encapsulation layer is bonded to the edge of the electrode layer to obtain a flexible piezoresistive sensor that fits the interface to be applied. The shape memory polyimide film is prepared from a precursor polyamic acid solution, which is obtained by dissolving 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane HFBAPP and 4,4'-(hexafluoroisopropene)phthalic anhydride 6FDA in N-methylpyrrolidone NMP and then continuously stirring at 0-10°C. The shape memory composite voltage-sensitive film is prepared by mixing a precursor polyamic acid solution with modified carbon nanotubes to obtain a composite film-forming solution. The modified carbon nanotubes are obtained by functionalizing carboxylated carbon nanotubes with an organic compound carrying functional groups. The organic compound carrying functional groups is at least one of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (AHHFP) and ethylenediamine (EDA).

2. The method for fabricating the shape memory flexible piezoresistive sensor as described in claim 1, characterized in that, Including the following: First, a polyamic acid solution, a precursor of soluble shape memory polyimide, is obtained. Carboxylated carbon nanotubes were functionalized by using organic compounds carrying functional groups to obtain modified carbon nanotubes; a portion of the precursor polyamic acid solution was mixed with the modified carbon nanotubes to obtain a composite film-forming solution. A mold carrying a microstructure is prefabricated; The precursor polyamic acid solution is added to the mold, the composite film-forming solution is added to the mold, and the precursor polyamic acid solution is dropped onto a smooth glass slide to level it. After drying, the film is peeled off and subjected to gradient heating thermal imidization to obtain the shape memory polyimide film carrying the microstructure, the shape memory composite voltage-sensitive film carrying the microstructure, and the shape memory polyimide film without the microstructure in sequence.

3. The method for fabricating the shape memory flexible piezoresistive sensor according to claim 2, characterized in that, The method for preparing the precursor polyamic acid solution includes: 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane HFBAPP and 4,4'-(hexafluoroisopropene)phthalic anhydride 6FDA were dissolved in N-methylpyrrolidone NMP and stirred continuously at 0-10°C to obtain a polyamic acid precursor solution.

4. The method for fabricating the shape memory flexible piezoresistive sensor according to claim 3, characterized in that, The molar ratio of HFBAPP to 6FDA is 1:1, and the reaction solids content is 20wt%.

5. The method for fabricating the shape memory flexible piezoresistive sensor according to claim 2, characterized in that, The organic compound carrying the functionalized group is at least one of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (AHHFP) and ethylenediamine (EDA).

6. The method for fabricating the shape memory flexible piezoresistive sensor according to claim 2, characterized in that, The aforementioned functional modification process specifically includes: Carboxylated carbon nanotubes and a condensing agent were dispersed in a solvent to activate the carboxyl groups, resulting in dispersion A. The organic compound carrying the functionalized groups was weighed and dissolved in a solvent to obtain solution B. Solution B was added to dispersion A to carry out a condensation reaction. After the reaction was completed, the mixture was washed with ethanol, filtered, and dried.

7. The method for fabricating the shape memory flexible piezoresistive sensor according to claim 6, characterized in that, The degree of carboxylation of the carboxylated carbon nanotubes does not exceed 1 wt%, and the solvent is at least one of N-methylpyrrolidone (NMP) and ethylenediamine (EDA).

8. The method for fabricating a shape memory flexible piezoresistive sensor according to claim 2, characterized in that, The modified carbon nanotubes account for 5-9 wt% of the total mass of the precursor polyamic acid solution monomer and the modified carbon nanotubes.

9. The method for fabricating a shape memory flexible piezoresistive sensor according to claim 2, characterized in that, The method for preparing the mold carrying the microstructure is as follows: the glass fiber cloth is laid flat in the container, defoamed PDMS is poured in, leveled, cured, and demolded to obtain the mold carrying the microstructure.

10. The application of the shape memory flexible piezoresistive sensor as described in claim 1 in battery thermal runaway early warning.

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