A flexible sensor that simultaneously senses pressure and temperature and its fabrication method

Abstract

This invention belongs to the field of flexible sensor technology, specifically a flexible sensor that simultaneously senses pressure and temperature, and its fabrication method. The sensor comprises, from top to bottom, a polyimide film, a pressure sensing layer, a buffer layer, a temperature sensing layer, and another polyimide film. The pressure sensing layer is made of a carbon nanotube / graphene / polydimethylsiloxane composite conductive film, with polydimethylsiloxane as the substrate and graphene and carbon nanotubes as conductive fillers. The temperature sensing layer is made of carbon graphite and screen-printed onto the polyimide film. The buffer layer is made of polydimethylsiloxane elastomer. By combining carbon nanomaterials with a negative temperature coefficient of resistance with polydimethylsiloxane, which has a thermal expansion effect, pressure sensing with a near-zero temperature coefficient of resistance is achieved. The addition of graphene effectively separates the carbon nanotubes, improving their dispersion and enhancing the electrical properties of the composite conductive film, reducing the penetration threshold, and increasing conductivity.

Description

Technical Field

[0001] This invention belongs to the field of flexible sensor technology, specifically a flexible sensor that simultaneously senses pressure and temperature and its fabrication method. Background Technology

[0002] In recent years, flexible electronics technology has attracted widespread attention as an emerging technology. Flexible sensors manufactured based on this technology have been applied in fields such as electronic skin, health monitoring, and aerospace. Compared with traditional silicon-based sensors, flexible sensors have good flexibility and ductility, and can be used in a variety of complex environments.

[0003] Flexible sensors are generally used to measure physical quantities such as force, strain, temperature, humidity, flow velocity, and light intensity. However, most flexible sensors can only monitor one physical quantity, failing to meet the needs of monitoring multiple physical quantities. Therefore, developing flexible sensors capable of simultaneously monitoring multiple physical quantities without signal crosstalk has significant practical implications and application potential.

[0004] Carbon nanomaterials are frequently used in the fabrication of flexible sensors due to their advantages such as good electrical conductivity, high strength, high temperature resistance, corrosion resistance, and light weight. However, carbon nanomaterials are prone to aggregation due to their large van der Waals forces, leading to a decrease in electrical conductivity and an increase in the permeation threshold. Therefore, the dispersion of carbon nanomaterials in composite materials determines the conductivity of the composite material. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is to provide a flexible sensor that simultaneously senses pressure and temperature, as well as a method for its fabrication.

[0006] The present invention solves the aforementioned technical problem by adopting the following technical solution:

[0007] A flexible sensor that simultaneously senses pressure and temperature comprises, from top to bottom, a polyimide film, a pressure sensing layer, a buffer layer, a temperature sensing layer, and another polyimide film; characterized in that the pressure sensing layer is made of a carbon nanotube / graphene / polydimethylsiloxane composite conductive film.

[0008] The preparation of the pressure sensing layer includes the following steps:

[0009] Step 1: Add graphene and carbon nanotubes to a polar solvent at a mass ratio of 1:1 and disperse them by ultrasonication; the mass of graphene and carbon nanotubes is 0.05-0.1g, and the volume of polar solvent is 50-80ml; the ambient temperature for ultrasonic dispersion is 5-15℃, the power is 1200W, and the treatment is intermittent for 1-1.5h.

[0010] Step 2: Add 5-7g of polydimethylsiloxane main agent to the ultrasonically dispersed solution, and then heat and magnetically stir. When using a heating table, the heating temperature is 100-120℃; when using a water bath, the heating temperature is 70-90℃; the magnetic stirring speed is 150-250rpm, and the stirring time is 2-4h.

[0011] Step 3: Add a non-polar solvent and 0.5-0.7g of polydimethylsiloxane curing agent dropwise to the mixture obtained in step 2, and perform magnetic stirring at room temperature; the magnetic stirring speed is 100-200 rpm, and the stirring time is 10-20 min;

[0012] Step 4: Perform vacuum degassing on the slurry obtained in step 3 for 5-10 minutes;

[0013] Step 5: Pour the degassed slurry into a mold and place it in a drying oven for curing to obtain a carbon nanotube / graphene / polydimethylsiloxane composite conductive film; the curing temperature is 60-80℃ and the curing time is 2-4h.

[0014] Furthermore, when using 0.05g of graphene with a diameter of 10000nm and a thickness of 4nm, 0.05g of carbon nanotubes with a length of 12000nm and a diameter of 80nm, 6g of polydimethylsiloxane as the main agent and 0.6g of polydimethylsiloxane as the curing agent, the temperature coefficient of resistance of the pressure sensing layer is close to zero.

[0015] Furthermore, the polar solvent is selected from isopropanol, ethanol, propanol or water, and the non-polar solvent is selected from xylene, with a mass of 8-10g of xylene.

[0016] Furthermore, the fabrication of the temperature sensing layer includes the following steps:

[0017] Step 1: Mix 5-7g of carbon ink with 0.3-0.5g of screen cleaning water and stir magnetically at room temperature; the magnetic stirring speed is 200-300 rpm and the stirring time is 2-4 hours.

[0018] Step 2: The stirred solution is printed onto a polyimide film using a screen printing stencil, and then placed in a drying oven for curing to obtain a temperature sensing layer; the curing temperature is 60-80℃, and the curing time is 2-4 hours.

[0019] Furthermore, the buffer layer is made of polydimethylsiloxane elastomer with a thickness of 1-2 mm.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] The flexible sensor of this invention achieves pressure sensing with a near-zero temperature coefficient of resistance by combining carbon nanomaterials with a negative temperature coefficient of resistance (TCR) with polydimethylsiloxane, which exhibits thermal expansion. This is because the increased volume of polydimethylsiloxane due to thermal expansion reduces the conductive path, compensating for the negative TCR of the carbon nanomaterials themselves. Furthermore, carbon nanotubes, as one-dimensional nanomaterials, are prone to aggregation due to their elongated structure. Therefore, adding graphene can effectively separate the carbon nanotubes, improving their dispersion and thus enhancing the electrical properties of the composite conductive film, lowering the permeation threshold, and increasing conductivity.

[0022] First, carbon nanotubes are ultrasonically dispersed in a polar solvent. Then, polydimethylsiloxane (PDS) is added and heated and stirred until the polar solvent evaporates completely. Next, a non-polar solvent and a PDS curing agent are added. The reason for not directly adding the carbon nanotubes to the non-polar solvent for subsequent processing is that carbon nanotubes disperse better in polar solvents compared to non-polar solvents. The purpose of adding the PDS curing agent last is to ensure thorough mixing of the carbon nanotubes and the PDS. Adding the PDS curing agent too early would cause cross-linking of the PDS, resulting in uneven mixing of the carbon nanotubes and PDS. The non-polar solvent makes the resulting slurry more fluid, facilitating pouring into the mold.

[0023] This invention utilizes polydimethylsiloxane elastomer as a buffer layer to separate the pressure sensing layer and the temperature sensing layer, preventing signal crosstalk caused by the pressure sensing layer affecting the temperature sensing layer. A serpentine structure for the temperature sensing layer is fabricated using screen printing, resulting in a sufficiently thin layer to minimize direct pressure application. Simultaneously, the serpentine structure also provides some resistance to changes in resistance caused by sensor bending. Attached Figure Description

[0024] Figure 1 This is a side view of the overall structure of the present invention;

[0025] Figure 2 This is an exploded view of the present invention;

[0026] Figure 3 This is a structural diagram of the pressure sensing layer of the present invention;

[0027] Figure 4 This is a structural diagram of the temperature sensing layer of the present invention. Detailed Implementation

[0028] Specific embodiments are given below with reference to the accompanying drawings. These specific embodiments are only used to describe the technical solution of the present invention in detail, and are not intended to limit the scope of protection of this application.

[0029] This invention proposes a flexible sensor that simultaneously senses pressure and temperature. From top to bottom, it consists of: a polyimide (PI) film, a pressure sensing layer, a buffer layer, a temperature sensing layer, and another polyimide film. During encapsulation, flame-retardant adhesive is used to bond the layers together, and conductive silver paste and copper tape are selected as the electrode materials.

[0030] The pressure sensing layer is made of a carbon nanotube / graphene / polydimethylsiloxane composite conductive film, with polydimethylsiloxane (PDMS) as the substrate and graphene and carbon nanotubes with negative temperature coefficient of resistance as conductive fillers. The pressure signal is monitored by means that the pressure direction is perpendicular to the conductive path. The mass ratio of graphene to carbon nanotube is 1:1. The graphene sheet diameter is 10000nm and the thickness is 3-4nm. The carbon nanotube has a length of 12000nm and a diameter of 80nm.

[0031] The temperature sensing layer is made of carbon ink and is screen-printed onto a polyimide film. In this embodiment, the temperature sensing layer has a serpentine structure.

[0032] The buffer layer is made of polydimethylsiloxane elastomer to ensure that the temperature sensing layer is not affected by the pressure sensing layer.

[0033] The preparation of the pressure sensing layer includes the following steps:

[0034] Step 1: Add graphene and carbon nanotubes to a polar solvent at a mass ratio of 1:1 and then perform ultrasonic treatment;

[0035] The graphene and carbon nanotubes each weigh 0.05–0.1 g, and the polar solvent volume is 50–80 ml. The polar solvent can be isopropanol, ethanol, propanol, water, etc. The graphene sheet diameter is 10000 nm and the thickness is 3–4 nm. The carbon nanotube length is 12000 nm and the diameter is 80 nm. The ultrasonic treatment environment temperature is 5–15℃, the ultrasonic power is 1200 W, and the treatment is carried out continuously for 1–1.5 h in the form of ultrasonic treatment for 1–2 seconds followed by a pause of 3–4 seconds.

[0036] Step 2: Add 5-7g of polydimethylsiloxane main agent to the ultrasonically treated solution, and heat and magnetically stir to evaporate the polar solvent; when using a heating table, the heating temperature is 100-120℃; when using a water bath, the heating temperature is 70-90℃; the magnetic stirring speed is 150-250rpm, and the stirring time is 2-4h.

[0037] When using 0.05g of graphene with a diameter of 10000nm and a thickness of 4nm, 0.05g of carbon nanotubes with a length of 12000nm and a diameter of 80nm, 6g of polydimethylsiloxane as the main agent and 0.6g of polydimethylsiloxane as the curing agent, the temperature coefficient of resistance of the pressure sensing layer is close to zero.

[0038] Step 3: After the polar solvent has completely evaporated, add the non-polar solvent and polydimethylsiloxane curing agent dropwise, and stir magnetically at room temperature;

[0039] Xylene is selected as the non-polar solvent, with a mass of 8-10g. The non-polar solvent is used to ensure the flowability of the polydimethylsiloxane main agent; the mass of the polydimethylsiloxane curing agent is 0.5-0.7g; the magnetic stirring speed is 100-200rpm, and the stirring time is 10-20min.

[0040] Step 4: Perform vacuum degassing on the slurry obtained in step 3; the vacuum degassing time is 5-10 minutes.

[0041] Step 5: Pour the degassed slurry into a polytetrafluoroethylene (PTFE) mold and place it in a drying oven for curing to obtain a carbon nanotube / graphene / polydimethylsiloxane composite conductive film; use a laser cutter to cut the carbon nanotube / graphene / polydimethylsiloxane composite conductive film; use conductive silver paste and copper tape to attach external electrodes, and the electrodes are adhered to the polyimide film;

[0042] The curing temperature is 60–80℃, the curing time is 2–4 h, and the thickness of the carbon nanotube / graphene / polydimethylsiloxane composite conductive film is 0.3–0.5 mm.

[0043] The preparation of the temperature sensing layer includes the following steps:

[0044] Step 1: Mix 5-7g of carbon ink with 0.3-0.5g of screen cleaning water and stir magnetically at room temperature;

[0045] The magnetic stirring speed is 200-300 rpm, and the stirring time is 2-4 hours.

[0046] Step 2: The stirred solution is printed onto a polyimide film using a screen printing stencil, and then placed in a drying oven for curing to obtain a temperature sensing layer.

[0047] The curing temperature is 60–80℃, and the curing time is 2–4 hours.

[0048] The preparation of the buffer layer includes the following steps:

[0049] Step 1: Mix the polydimethylsiloxane main agent and curing agent at a mass ratio of 10:1 and perform magnetic stirring; the magnetic stirring speed is 150-200 rpm and the stirring time is 5-15 min;

[0050] Step 2: Vacuum degassing treatment is performed on the stirred polydimethylsiloxane; the vacuum degassing time is 5-10 minutes.

[0051] Step 3: Pour the degassed polydimethylsiloxane into a polytetrafluoroethylene mold and place it in a drying oven for curing to obtain polydimethylsiloxane elastomer; the curing temperature is 80℃, the curing time is 2h, and the thickness of the polydimethylsiloxane elastomer is 1~2mm.

[0052] Step 4: Use a laser cutting machine to cut the polydimethylsiloxane elastomer into squares with a side length of 1-2 cm to obtain the buffer layer.

[0053] A method for fabricating a flexible sensor that simultaneously senses pressure and temperature includes the following steps:

[0054] S1: Two square polyimide films with a side length of 50mm and a thickness of 0.2mm are cut out using a laser cutting machine. The polyimide films are then cleaned with anhydrous ethanol and deionized water.

[0055] S2: Attach two polyimide films to the underside of the screen printing stencil used for printing electrodes using adhesive tape, and apply conductive silver paste to the mesh openings; after the coating is completed, peel off the polyimide film printed with conductive silver paste and cure it at room temperature.

[0056] S3: Add 0.05g of graphene with a diameter of 10000nm and a thickness of 4nm, and 0.05g of carbon nanotubes with a length of 12000nm and a diameter of 80nm to 50ml of isopropanol, and perform ultrasonic treatment at an ambient temperature of 10℃. The ultrasonic disperser has a power of 1200W, and the treatment is carried out continuously for 1h in the form of ultrasonic treatment for 2s and pause for 3s.

[0057] S4: Add 6g of polydimethylsiloxane main agent to the ultrasonically treated solution, and perform water bath heating and magnetic stirring to evaporate isopropanol; the water bath heating temperature is 90℃, the magnetic stirring speed is 200rpm, and the magnetic stirring time is 2h.

[0058] S5: Add 0.6g of polydimethylsiloxane curing agent and 8g of xylene to a viscous slurry containing graphene, carbon nanotubes, polydimethylsiloxane main agent and a small amount of isopropanol, and perform magnetic stirring at room temperature; the magnetic stirring speed is 150 rpm and the stirring time is 10 min; place the stirred viscous slurry into a vacuum degassing machine for degassing for 10 min;

[0059] S6: Pour the degassed slurry into a cylindrical polytetrafluoroethylene mold with a diameter of 5cm and a depth of 5mm. The volume of the slurry is 40% of the total volume of the mold. Place the mold containing the slurry into a drying oven at 80℃ for curing for 2 hours to obtain a carbon nanotube / graphene / polydimethylsiloxane composite conductive film with a thickness of 0.3mm.

[0060] S7: Use a laser cutter to cut the carbon nanotube / graphene / polydimethylsiloxane composite conductive film into squares with a side length of 1cm, and clean them with anhydrous ethanol and deionized water; use flame retardant adhesive to attach the carbon nanotube / graphene / polydimethylsiloxane composite conductive film to one of the polyimide films printed with conductive silver paste, then drop a small amount of conductive silver paste onto both ends of the carbon nanotube / graphene / polydimethylsiloxane composite conductive film, then attach copper tape, and drop a small amount of conductive silver paste to connect the copper tape to the carbon nanotube / graphene / polydimethylsiloxane composite conductive film; place the polyimide film with the carbon nanotube / graphene / polydimethylsiloxane composite conductive film adhered to it in an 80℃ drying oven for 1 hour to cure.

[0061] S8: Mix 3g of polydimethylsiloxane main agent and 0.3g of curing agent, and magnetically stir at room temperature at a speed of 200 rpm for 10 minutes; place the stirred polydimethylsiloxane in a vacuum degassing machine for 10 minutes to degas; pour the degassed polydimethylsiloxane into a cylindrical polytetrafluoroethylene mold with a diameter of 5cm and a depth of 5mm, with the volume of polydimethylsiloxane being approximately 40% of the total volume of the mold; place the mold containing polydimethylsiloxane in a drying oven and cure at 80℃ for 2 hours to obtain a polydimethylsiloxane elastomer with a thickness of 2mm; use a laser cutting machine to cut the polydimethylsiloxane elastomer into squares with a side length of 1cm, and clean with anhydrous ethanol and deionized water to obtain a buffer layer;

[0062] S9: Mix 5g of carbon ink and 0.35g of screen cleaning water, and stir magnetically at room temperature at 200 rpm for 3 hours. Attach another polyimide film printed with conductive silver paste to the underside of the screen printing stencil with a serpentine pattern using tape. Apply a small amount of carbon ink to the mesh openings. Then peel off the polyimide film with the serpentine pattern and carbon ink. Attach copper tape to both ends of the polyimide film, and then add a small amount of conductive silver paste to connect the copper tape to the carbon ink. Place the polyimide film with the carbon ink adhered to it in a drying oven and cure at 80℃ for 2 hours.

[0063] S10: Use flame-retardant adhesive to attach the buffer layer to the center of the temperature sensing layer with a serpentine structure, place it in a drying oven and cure it at 80°C for 30 minutes, then take it out. Use flame-retardant adhesive to attach the pressure sensing layer to the buffer layer, place it in a drying oven and cure it at 80°C for 30 minutes, then take it out to obtain a flexible sensor that can simultaneously sense pressure and temperature.

[0064] Any aspects not covered in this invention are applicable to existing technologies.

Claims

1. A flexible sensor for simultaneously sensing pressure and temperature, comprising, from top to bottom, a polyimide film, a pressure sensing layer, a buffer layer, a temperature sensing layer, and a polyimide film; characterized in that, The pressure sensing layer is made of a carbon nanotube / graphene / polydimethylsiloxane composite conductive film. The preparation of the pressure sensing layer includes the following steps: Step 1: Add graphene and carbon nanotubes to a polar solvent at a mass ratio of 1:1 and disperse them by ultrasonication; the mass of graphene and carbon nanotubes is 0.05-0.1g, and the volume of polar solvent is 50-80ml; the ambient temperature for ultrasonic dispersion is 5-15℃, the power is 1200W, and the treatment is intermittent for 1-1.5h. Step 2: Add 5-7g of polydimethylsiloxane main agent to the ultrasonically dispersed solution, and then heat and magnetically stir. When using a heating table, the heating temperature is 100-120℃; when using a water bath, the heating temperature is 70-90℃; the magnetic stirring speed is 150-250rpm, and the stirring time is 2-4h. Step 3: Add a non-polar solvent and 0.5-0.7g of polydimethylsiloxane curing agent dropwise to the mixture obtained in step 2, and perform magnetic stirring at room temperature; the magnetic stirring speed is 100-200 rpm, and the stirring time is 10-20 min; Step 4: Perform vacuum degassing on the slurry obtained in step 3 for 5-10 minutes; Step 5: Pour the degassed slurry into a mold and place it in a drying oven for curing to obtain a carbon nanotube / graphene / polydimethylsiloxane composite conductive film; the curing temperature is 60-80℃ and the curing time is 2-4h. 2.The flexible sensor for simultaneously sensing pressure and temperature according to claim 1, wherein, When using 0.05g of graphene with a diameter of 10000nm and a thickness of 4nm, 0.05g of carbon nanotubes with a length of 12000nm and a diameter of 80nm, 6g of polydimethylsiloxane as the main agent, and 0.6g of polydimethylsiloxane as the curing agent, the temperature coefficient of resistance of the pressure sensing layer is close to zero.

3. The flexible sensor for simultaneously sensing pressure and temperature according to claim 1 or 2, characterized in that, The polar solvent is selected from isopropanol, ethanol, propanol or water, and the non-polar solvent is selected from xylene, with a mass of 8-10g of xylene.

4. The flexible sensor for simultaneously sensing pressure and temperature according to claim 1, characterized in that, The preparation of the temperature sensing layer includes the following steps: Step 1: Mix 5-7g of carbon ink with 0.3-0.5g of screen cleaning water and stir magnetically at room temperature; the magnetic stirring speed is 200-300 rpm and the stirring time is 2-4 hours. Step 2: The stirred solution is printed onto a polyimide film using a screen printing stencil, and then placed in a drying oven for curing to obtain a temperature sensing layer; the curing temperature is 60-80℃, and the curing time is 2-4 hours.

5. The flexible sensor for simultaneously sensing pressure and temperature according to claim 1, characterized in that, The buffer layer is made of polydimethylsiloxane elastomer with a thickness of 1-2 mm.

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