CN113465490B - Positive pressure-induced strain sensor and preparation method thereof - Google Patents

Abstract

The invention discloses a positive pressure conduction strain sensor and a preparation method thereof, which uses insulating nanocellulose (CNF) in CNF-C transparent hydrogel as a filling material, and Na ⁺ or K ⁺ dissolved in the transparent hydrogel , Cl ⁺ ions are the conductive phase, and after uniform dispersion, a viscous liquid conductive gel is obtained. The conductive gel is injected into the through hole of the elastic matrix for coating, and a stretchable strain sensor can be obtained, which solves the problem of In the field of strain sensing, it is difficult to measure the problem that the resistance signal is too large or the current signal is too small under large strain. From the perspective of signal measurement, the expansion of the strain sensing range is realized, and the required raw materials are cheap and easy to obtain, and the cost is greatly reduced. .

Description

A kind of positive pressure guide strain sensor and preparation method thereof

technical field

The invention relates to the field of strain sensing, more specifically, it relates to a positive pressure conduction strain sensor and a preparation method thereof.

Background technique

In recent years, stretchable strain sensors based on conductive elastic composites have been widely used in flexible electronics such as electronic skin, soft robots, and wearable devices. These composites are typically mixtures of conductive fillers filled within an elastomeric matrix, and the strain elongation is sensed by the change in electrical resistance produced by the filler during separation. Therefore, most elastic composites exhibit a negative pressure conduction effect, that is, the electrical resistance of the filler increases under tensile strain and the electrical conductivity of the composite decreases. In practical applications, the negative pressure conduction effect has strict requirements on the measuring equipment: when the strain reaches a large value, the resistance signal is likely to appear on the order of 10 ⁷ -10 ⁹ , which is beyond the range of common resistance testing instruments. Therefore, the piezoconductance properties (or strain response) of conductive elastic composites are crucial for the application of composites in the field of stretchable strain sensors.

However, most of the existing studies on the piezoconductance properties of conductive elastic composites focus on reducing the initial resistance of the composites, in order to detect the resistance signal under a larger strain as much as possible within the range of the resistance measurement equipment. The effect of this type of optimization is very limited, and it cannot fundamentally solve the problem of the limitation of the equipment range on the tensile upper limit of the composite material. And it has been reported in the literature that the sensitivity of conductive elastic composites to strain is related to the initial resistance, blindly reducing the initial resistance of composites is likely to weaken its sensitivity to tensile strain. Therefore, it is urgent to design a new material system or new process, which can obtain conductive elastic composite materials with positive pressure conductivity characteristics, and fundamentally solve the contradiction between the equipment range and the ultrahigh resistance signal of the composite material under large strain.

The core of the existing strain sensor is the conductive elastic composite material, which usually has negative pressure conduction characteristics, that is, the electrical conductivity decreases with the increase of tensile strain, and the measurable resistance (or current) range of the device is the upper limit of the sensing of the composite material. are extremely restrictive.

In recent years, several optimization methods have appeared for this problem. The research team of the Chinese Academy of Sciences increased the graphene content in the Ti ₃ C ₂ T _x /graphene/PDMS strain sensing structure (DOI: 10.1016/j.nanoen.2019.104134), which reduced the initial resistance of the composite material, and the upper limit of detectable strain Increased from 35% to 75%. However, its resistance signal is at least on the order of 10 ⁷ , which requires special scientific research equipment to measure. And after the initial resistance is reduced, the sensitivity of the device to tensile strain is significantly reduced. The research team of the University of Wollongong in Australia innovatively designed a positive pressure conductive material composed of liquid metal and magnetic nanoparticles (DOI: 10.1038/s41467-019-09325-4). Under the action of tensile strain, the conductivity of the material is enhanced, and the resistance (current) signal changes to a level that can be detected by an ordinary multimeter. However, its raw material is GaIn liquid alloy, which is very expensive and not biocompatible, so it is not suitable for use in the field of flexible electronics related to human contact; and the preparation process relies on strong magnetic fields, which requires high equipment.

Although some optimization methods have been disclosed in existing studies, either the optimization effect is limited, the important performance parameter of sensitivity needs to be sacrificed, or there are defects such as complex equipment, cumbersome process, and high cost of raw materials. Therefore, it is of great significance to develop a low-cost, green, conductive elastic composite material with positive pressure conductivity.

Contents of the invention

The object of the present invention is to provide a kind of positive pressure conduction strain sensor and preparation method thereof, take the nano-cellulose (CNF) of insulation in the CNF-C transparent hydrogel as filling material, dissolve in the Na ⁺ or K ⁺ and Cl ⁺ ions are the conductive phase, and after uniform dispersion, a viscous liquid conductive gel is obtained. The conductive gel is injected into the through hole of the elastic matrix for coating, and a stretchable strain sensor can be obtained. In the field of strain sensing, it solves the problem that the resistance signal is too large or the current signal is too small under large strain, which makes it difficult to measure. From the perspective of signal measurement, the expansion of the strain sensing range is realized, and the required raw materials are cheap and easy to obtain, and the cost is extremely low. The earth lowers.

The invention discloses a positive pressure conduction strain sensor, which comprises insulating CNF with a three-dimensional network structure, deionized water and water-soluble NaCl crystals. CNF, that is, nanocellulose, is an insulating material. It is made from the fibers of tree branches, cotton flowers, etc., and is refined into nanoscale cellulose after physical and chemical treatment. In the dry powder state, it is a curly filament, such as a ball of wool; when dispersed in water and other polar solvents, the fiber filaments unfold into a network structure.

Preferably, the CNF nanofiber has a length of 1-3 μm and a diameter of 4-10 nm.

Preferably, NaCl crystals can be replaced by KCl crystals.

The invention also discloses a preparation method of a positive pressure conduction strain sensor, comprising the following steps:

Step 1. Dissolve CNF powder in deionized water and stir to obtain CNF-C transparent hydrogel;

Step 2. Add NaCl crystals to the CNF-C transparent hydrogel and stir to obtain a uniform and stable NaCl/CNF-C conductive gel; after the stirring is completed, the conductive gel is transparent as a whole without obvious precipitation or layering, which is regarded as Uniform; after 48 hours of sealed storage, no white NaCl crystals are precipitated, and the test resistance has no obvious change, which is considered stable.

Step 3. Build an elastic matrix with through holes: separate the square area on the PET substrate; set the metal rod in the middle of the square area, fix the two ends so that the metal rod is suspended; put the two-component epoxy resin at room temperature and pressure Mix A:B=1:1 and pour it into a square area; after the elastic matrix is solidified, pull out the metal rod to form a through hole; the construction process of the elastic matrix must rely on a regular-shaped "container", and the container can be changed according to demand Shape, PET is cheap, flexible, easy to remove, very suitable as the base of the container;

Step 4. Inject the NaCl/CNF-C conductive gel into the through hole of the elastic matrix, and seal the two ends with wires to obtain a positive pressure conductive strain sensor.

Preferably, in step 1, use an ultrasonic cell breaker to stir at a speed of 15,000 rpm for 7-8 minutes, or use a high-pressure homogenizer to stir at a pressure of 40 MPa for 1-2 minutes.

Preferably, CNF-C is an insulating gel-like material. CNF thixomorphizes to a gel rather than a solution after dissolving >0.3% in water. To distinguish it from its dry powder state (CNF), the gel formed after dissolving in water is called CNF-C. The CNF dry powder needs to meet two conditions in the aqueous solution to thixomorphize into a gel state, otherwise it is a suspension: CNF mass fraction > 0.3%, and high-speed stirring.

Preferably, the mass fraction of the NaCl component in the NaCl/CNF-C conductive gel ranges from 10% to 40%. The strain response characteristics of the conductive gel can also be regulated by adjusting the mass fraction of NaCl.

Preferably, the NaCl/CNF-C conductive gel has positive pressure conductivity, and the conductivity is 0.7-1.2 S/m without tensile strain. Conductivity is better under tensile strain.

Beneficial effects of the present invention:

(1) The NaCl/CNF-C transparent hydrogel structure in the present invention basically retains the optical transparency of the NaCl aqueous solution, but has a strain response characteristic completely opposite to it, and the resistance signal of the NaCl aqueous solution increases with the increase of the tensile strain, After reaching a certain order of magnitude, it is difficult for ordinary electrical testing instruments to detect the resistance signal. The resistance signal of the NaCl/CNF-C hydrogel of the present invention decreases with the increase of strain, which is beneficial to the measurement of the resistance signal under large strain.

(2) The raw materials used in the present invention are nanocellulose and NaCl crystals or KCl crystals, all of which are cheap and abundant materials, which greatly reduces the preparation cost of the stretchable strain sensor.

(3) The NaCl/CNF-C conductive gel of the present invention has the characteristics of infinite flow and extension of liquid materials, and can be reused at the same time; by adopting a split filling process, the elastic matrix can be retained after the device fails, and the conductive gel can be replaced separately. Realize the recyclable use of the device.

(4) The present invention relates to processes such as normal temperature and pressure curing, physical mixing, etc. The operation is simple and does not require any medium or large equipment.

Description of drawings

Figure 1 is a schematic diagram of the structure of a positive pressure-induced strain sensor.

Figure 2 is an optical photo of the positive piezoconductive strain sensor.

Fig. 3 is a graph comparing the conductivity of NaCl/CNF-C conductive gels prepared in Example 1, Example 2 and Example 3.

Fig. 4 is the tensile strain response diagram of the NaCl/CNF-C conductive gels prepared in Example 1, Example 2 and Example 3.

Detailed ways

The structures involved in the present invention or the technical terms used are further described below. These descriptions are only used as examples to illustrate how the present invention is implemented, and do not constitute any limitation to the present invention.

Example 1

Dissolve 1 gram of CNF powder with a length of 1-3 μm and a diameter of 4-10 nm in 99 grams of deionized water, and disperse it into a CNF-C transparent hydrogel after being stirred by an ultrasonic cell disruptor. Take 20 grams of CNF-C transparent hydrogel, add 2 grams of NaCl crystals, and stir to form a uniform and stable NaCl/CNF-C conductive gel. The NaCl/CNF-C conductive gel has positive pressure conductivity, and the conductivity is 0.7S/m without tensile strain.

A 20 mm x 5 mm square area was isolated on the PET substrate with 3M insulating tape with a thickness of 3 mm. Set a metal rod with an outer diameter of 1 mm and a circular cross section in the middle of the square area, and fix the two ends so that the metal rod is suspended in the air. Mix the two-component epoxy resin at room temperature and pressure at A:B=1:1 and pour it into the square area until it is flush with the 3M tape. A generally refers to the main agent, and B generally refers to the curing agent or hardener. In this embodiment, Ecoflex 00-30 epoxy resin produced by Smooth-On Company is used. After the elastic matrix is solidified, the metal rod is pulled out from one end to obtain an elastic matrix with a size of 20×5×3 mm ³ and a through hole diameter of 1 mm, see FIG. 2 .

Inject NaCl/CNF-C conductive gel into the through hole of the elastic matrix, and seal the two ends with wires. The length of the wire placed in the through hole is 3 mm, and a positive pressure conductive strain sensor is obtained, as shown in Figure 1.

Example 2

The difference between this example and Example 1 is that the addition ratios of NaCl and CNF-C are different.

Dissolve 1 gram of CNF powder with a length of 1-3 μm and a diameter of 4-10 nm in 99 grams of deionized water, and disperse it into a CNF-C transparent hydrogel after being stirred by an ultrasonic cell disruptor. Take 10 grams of CNF-C transparent hydrogel, add 2 grams of NaCl crystals, and stir to form a uniform and stable NaCl/CNF-C conductive gel. The NaCl/CNF-C conductive gel has positive pressure conductivity, and the conductivity is 1S/m without tensile strain.

A 20 mm x 5 mm square area was isolated on the PET substrate with 3M insulating tape with a thickness of 3 mm. Set a metal rod with an outer diameter of 1 mm and a circular cross section in the middle of the square area, and fix the two ends so that the metal rod is suspended in the air. Mix the two-component epoxy resin at room temperature and pressure at A:B=1:1 and pour it into the square area until it is flush with the 3M tape. A generally refers to the main agent, and B generally refers to the curing agent or hardener. In this embodiment, Ecoflex 00-30 epoxy resin produced by Smooth-On Company is used. After the elastic matrix is solidified, the metal rod is pulled out from one end to obtain an elastic matrix with a size of 20×5×3 mm ³ and a through hole diameter of 1 mm, see FIG. 2 .

Inject NaCl/CNF-C conductive gel into the through hole of the elastic matrix, and seal the two ends with wires. The length of the wire placed in the through hole is 3 mm, and a positive pressure conductive strain sensor is obtained, as shown in Figure 1.

Example 3

The difference between this example and Example 1 is that the addition ratios of NaCl and CNF-C are different.

Dissolve 1 gram of CNF powder with a length of 1-3 μm and a diameter of 4-10 nm in 99 grams of deionized water, and disperse it into a CNF-C transparent hydrogel after being stirred by an ultrasonic cell disruptor. Take 5 grams of CNF-C transparent hydrogel, add 2 grams of NaCl crystals, and stir to form a uniform and stable NaCl/CNF-C conductive gel. The NaCl/CNF-C conductive gel has positive pressure conductivity, and the conductivity is 1.2S/m without tensile strain.

A 20 mm x 5 mm square area was isolated on the PET substrate with 3M insulating tape with a thickness of 3 mm. Set a metal rod with an outer diameter of 1 mm and a circular cross section in the middle of the square area, and fix the two ends so that the metal rod is suspended in the air. Mix the two-component epoxy resin at room temperature and pressure at A:B=1:1 and pour it into the square area until it is flush with the 3M tape. A generally refers to the main agent, and B generally refers to the curing agent or hardener. In this embodiment, Ecoflex 00-30 epoxy resin produced by Smooth-On Company is used. After the elastic matrix is solidified, the metal rod is pulled out from one end to obtain an elastic matrix with a size of 20×5×3 mm ³ and a through hole diameter of 1 mm, see FIG. 2 .

Inject NaCl/CNF-C conductive gel into the through hole of the elastic matrix, and seal the two ends with wires. The length of the wire placed in the through hole is 3 mm, and a positive pressure conductive strain sensor is obtained, as shown in Figure 1.

Figure 3: Comparison of conductivity of NaCl/CNF-C conductive gels prepared in Example 1, Example 2 and Example 3, showing that the more NaCl content, the higher the conductivity of the conductive gel.

Figure 4: Positive pressure conductivity characteristics of NaCl/CNF-C conductive gels prepared in Example 1, Example 2 and Example 3. NaCl simply dissolves in water and exhibits negative pressure conductivity characteristics, and its resistance increases with tensile strain When the strain is 50%, the resistance increases by 2.4%. On the other hand, the response behavior of NaCl/CNF-C conductive gel to tensile strain was positive, and the electrical resistance decreased with the increase of tensile strain. When the tensile strain is 50%, the resistance drops of Example 1, Example 2, and Example 3 are 12.4%, 15%, and 15%, respectively. It shows that increasing the content of NaCl can not only optimize the conductivity of the conductive gel, but also appropriately increase its sensitivity to tensile strain.

Finally, it should be noted that the above embodiments are only used to help understand the method and core idea of the present invention, not to limit it. Those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from this The spirit and scope of the inventive device scheme. Therefore, the present invention will not be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (7)

1. A preparation method of a positive pressure-induced strain sensor is characterized by comprising an insulating CNF with a three-dimensional network structure, deionized water and a water-soluble NaCl crystal, and comprises the following steps:

step 1, dissolving CNF powder in deionized water, and stirring to obtain CNF-C transparent hydrogel;

step 2, adding NaCl crystals into the CNF-C transparent hydrogel, and stirring to obtain uniform and stable NaCl/CNF-C conductive gel;

step 3, constructing an elastic matrix with a through hole: separating a square area on a PET substrate; arranging a metal rod in the middle of the square area, and fixing two ends of the metal rod to suspend the metal rod in the air; mixing the two-component epoxy resin at normal temperature and pressure with A: B =1:1, and pouring into a square area; after the elastic matrix is solidified, drawing out the metal rod to form a through hole;

and 4, injecting NaCl/CNF-C conductive gel into the through hole of the elastic matrix, and plugging two ends of the NaCl/CNF-C conductive gel by using a conducting wire to obtain the positive pressure conductive strain sensor.

2. The method of claim 1, wherein in step 1, the cell is stirred by an ultrasonic cell disruptor at 15000 rpm for 7-8 minutes, or by a high pressure homogenizer at 40MPa for 1-2 minutes.

3. The method of claim 1, wherein the CNF-C is an insulating gel-like material.

4. The method of claim 1, wherein the mass fraction of the NaCl component in the NaCl/CNF-C conductive gel is in the range of 10% to 40%.

5. The method of claim 1, wherein the NaCl/CNF-C conductive gel has a positive pressure conductivity property, and the conductivity is 0.7 to 1.2S/m without tensile strain.

6. A positive pressure induced strain sensor prepared according to any of claims 1 to 5, wherein the CNF nanofibers are 1 to 3 μm in length and 4 to 10nm in diameter.

7. The positive pressure induced strain sensor of claim 6, wherein the NaCl crystals are replaced with KCl crystals.

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