Foam sensor with ultra-wide tensile strain sensing range and preparation and use thereof

Abstract

The application discloses a foam sensor with an ultra-wide tensile strain sensing range, a preparation method of the foam sensor and application of the foam sensor as a tensile strain sensor in human motion monitoring, human-computer interaction, health monitoring and intelligent robots. The preparation method comprises the following steps: adding micron-sized base particles and conductive fillers into a roller with steel balls for mechanical mixing, so that the conductive fillers are coated on the surfaces of the base particles to obtain composite particles; hot-compression molding the composite particles in a flat plate vulcanizing machine to obtain a composite material with an isolation structure; and heat-preserving and pressure-preserving the composite material in a physical foaming agent, releasing pressure to foam, and cooling and shaping to obtain the foam sensor with the ultra-wide tensile strain sensing range. The thickness of the foam sensor is 0.1-1 mm. The foam sensor has a porous structure, and the foam sensor presents a gradient structure with the cell size gradually increasing from the two end surfaces to the center in the thickness direction.

Description

Technical Field

[0001] This invention relates to the field of tensile strain foam sensors, specifically to a foam sensor with an ultra-wide tensile strain sensing range, its preparation method, and its applications. Background Technology

[0002] Flexible sensors have broad application prospects and are developing rapidly in wearable electronics and human-computer interaction. Strain sensors have attracted widespread attention due to their low cost and simple structure. Sensors based on metals and semiconductors typically have a low strain response range due to their poor tensile properties, limiting their application in many scenarios. Thanks to the excellent tensile properties and tensile recovery rate of elastomers, they are commonly used as the matrix for strain sensors (such as polyolefin elastomers (POE), polydimethylsiloxane (PDMS), and thermoplastic polyurethane (TPU)). Tensile strain sensors with wide response range and high sensitivity remain a major development direction.

[0003] Porous sensors have seen widespread development in recent years due to their lightweight and good permeability. They are mainly fabricated using methods such as freeze-drying, template methods, impregnation, etching, and chemical vapor deposition (CVD), but these methods increasingly face challenges related to complex processes and the use of organic solvents. However, realizing flexible porous electronic devices with a wide strain range remains a significant challenge. Jeong et al. fabricated a flexible porous sensor using CVD, but its elongation was low, only 77% strain. Wang et al. fabricated a TPU / MWCNTs@MXene foam sensor using a salt template method, achieving a sensitivity of 363, but its strain detection range was only 100%. Gong et al. and Yuan et al. reported detection ranges of 250% and 221% strain, respectively, for their porous sensors, but further improvements are needed. Although Zhang et al. developed a negative Poisson's ratio porous metamaterial with an ultrawide response range (1200% strain) through freeze-drying, its sensitivity was low, with a relative resistance change of only ~60 at 1200% strain. Therefore, there is an urgent need for flexible porous sensors with ultrawide detection ranges and high sensitivity.

[0004] Sensors with wide operating ranges typically require good tensile strength and high conductivity. However, in randomly distributed conductive elastomer composites (CECs), high conductivity requires a large amount of conductive filler, but this severely degrades tensile properties. While CECs with less conductive filler exhibit high tensile strain, they have low conductivity. Therefore, a trade-off must be struck between good tensile strength and high conductivity. Furthermore, the type of conductive filler plays a crucial role in the sensor's operating range. Generally, conductive fillers with low aspect ratios exhibit high percolation thresholds and low conductivity. Additionally, due to their high aspect ratio, 1D fillers typically have lower sensitivity than 2D and 0D fillers. Therefore, conductive fillers with high aspect ratios are beneficial for improving the sensor's operating range. Carbon nanostructures (CNSs) are a novel type of branched multi-walled carbon nanotubes with high aspect ratios. Supercritical CO2 (scCO2) foaming, using scCO2 as a foaming agent, is a simple and environmentally friendly method for preparing porous materials. While there is considerable research on compressive sensors prepared using scCO2 foaming, research on tensile strain sensors is limited.

[0005] In conclusion, the development of a porous tensile strain sensor with an ultra-wide operating range and high sensitivity is of great significance. Summary of the Invention

[0006] To address the aforementioned technical problems and shortcomings in this field, the present invention provides a foam sensor with an ultra-wide tensile strain sensing range, its preparation method, and its application.

[0007] The technical solution of this invention will be described in detail below:

[0008] [1] A method for fabricating a foam sensor with an ultra-wide tensile strain sensing range, comprising the following steps:

[0009] (1) Micron-sized matrix particles and conductive fillers are added to a roller with steel balls and mechanically mixed to make the conductive fillers uniformly coated on the surface of the matrix particles to obtain composite particles.

[0010] (2) The composite particles are hot-compressed in a flat vulcanizing machine to obtain a composite material with an isolation structure;

[0011] (3) The composite material is kept warm and pressure-controlled, depressurized and foamed, and cooled and shaped in a physical foaming agent to obtain the foam sensor with an ultra-wide tensile strain sensing range;

[0012] The thickness of the foam sensor is 0.1 to 1 mm, more specifically 100 to 700 micrometers, for example, any value among 0.1 mm, 0.3 mm, 0.5 mm, and 1 mm, or a range between any two of the above.

[0013] The foam sensor has a porous structure, and the foam sensor exhibits a gradient structure in its thickness direction where the pore size gradually increases from both end faces to the center.

[0014] The preparation method of the present invention requires the use of micron-sized matrix particles to obtain the foam sensor of the above thickness. Only at the above thickness can the foam sensor of the present invention exhibit a gradient structure in which the cell size gradually increases from the two end faces to the center in the thickness direction and has an ultra-wide tensile strain sensing range.

[0015] Due to their small size, the matrix particles of this invention are difficult to mix with the conductive filler, and simple dry mixing is insufficient to achieve a selective and uniform distribution of the conductive filler at the micron-sized interface between the matrix particles. To ensure thorough mixing of the micron-sized matrix particles with the conductive filler, the preparation method of this invention utilizes a roller with steel balls to achieve complete mixing. According to the preparation method of this invention, in the foam sensor, the conductive filler is selectively and uniformly distributed at the micron-sized interface between the matrix particles, forming a three-dimensional conductive network.

[0016] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, in step (1), the matrix particles may include at least one of thermoplastic polyurethane elastomer particles, thermoplastic polyamide elastomer particles, thermoplastic polyester elastomer particles, polyolefin elastomer (POE) particles, and natural rubber particles, preferably including polyolefin elastomer (POE) particles.

[0017] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, in step (1), the average particle size of the matrix particles is preferably 20-1000 micrometers, more preferably 100-1000 micrometers, such as 380 micrometers. If the average particle size is too small, the specific surface area increases, requiring the addition of more conductive filler; if the particle size is too large, it will cause a decrease in the interfacial strength of the particle bonding, seriously damaging the stretchability of the material.

[0018] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, in step (1), the conductive filler may include at least one of carbon black (CB), silver nanowires, carbon nanotubes, branched carbon nanotubes (CNS), graphene (G), reduced graphene oxide (rGO), and MXene, preferably including branched carbon nanotubes (CNS).

[0019] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, in step (1), with the total mass of the matrix particles and the conductive filler being 100%, the mass percentage of the conductive filler is preferably 0.1% to 10%, more preferably 0.5% to 10%, for example, any value among 0.1%, 0.3%, 0.5%, 1.5%, 3%, 6%, and 10%, or any value between any two of the above. When too little filler is added, the composite material has poor insulation or conductivity, and the material does not have piezoresistive sensing capability or a narrow sensing range; when too much filler is added, the thickness of the conductive layer on the particle surface increases, which hinders the interdiffusion of POE molecular chains between particles during the molding process, reduces the interfacial strength, thereby damaging the stretchability of the material, and thus reducing the detection range of the strain sensor.

[0020] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, the mechanical mixing time in step (1) can be 1 to 360 minutes.

[0021] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, in step (2), the temperature of the hot compression molding can be 50 to 90°C, and the pressure can be 9 to 11 MPa, such as 10 MPa.

[0022] [1] The method for preparing the foam sensor with an ultra-wide tensile strain sensing range described above, step (3) can be carried out by intermittent foaming.

[0023] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, step (3) may include at least one of carbon dioxide and nitrogen as the physical foaming agent.

[0024] [1] In the method for preparing the foam sensor with an ultra-wide tensile strain sensing range, the physical foaming agent preferably accounts for 0.5% to 10% of the mass percentage of the composite material.

[0025] In some embodiments, in the method for preparing a foam sensor with an ultra-wide tensile strain sensing range described in [1], in step (3), the physical foaming agent includes carbon dioxide and nitrogen. Further, in the physical foaming agent, the volume ratio of carbon dioxide to nitrogen can be 99:1 to 1:99.

[0026] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, in step (3), the heat preservation temperature is preferably 30-80℃, such as 50℃, the pressure is preferably 7-20MPa, and the heat preservation time is preferably 30-120 minutes, such as 60 minutes. This allows the sample to reach saturation in a short time by dissolving carbon dioxide.

[0027] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, in step (3), the depressurization foaming time is preferably no more than 10 seconds, for example, 2-3 seconds. Rapid depressurization can increase the driving force for cell nucleation, improve cell density, and reduce cell size.

[0028] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, in step (3), the foaming ratio of the composite material in the foaming process of step (3) is preferably 1.1 to 4 times, for example, 2 times. The foam sensor exhibits excellent stretchability while being porous.

[0029] [1] In the method for preparing a foam sensor with an ultra-wide tensile strain sensing range, in step (3), the average pore size of the foam sensor is preferably 10 to 300 micrometers, for example, 15 micrometers. Microporous foam materials have superior mechanical properties.

[0030] [2] A foam sensor with an ultra-wide tensile strain sensing range was prepared according to the preparation method described in [1].

[0031] The foam sensor of the present invention has an ultra-high strain detection range, reaching 762% tensile strain.

[0032] [3] The application of the foam sensor in tensile strain sensing equipment as described in [2].

[0033] [4] The foam sensor described in [2] is used as a tensile strain sensor in human motion monitoring, human-computer interaction, health monitoring and intelligent robots.

[0034] The foam sensor of this invention can be applied to a full range of motion detection, including facial expressions, running, and fitness activities, for excellent monitoring.

[0035] Compared with the prior art, the beneficial effects of this invention are as follows:

[0036] This invention develops a simple, environmentally friendly, and mass-producible strategy for fabricating foam sensors with an ultra-wide tensile strain response range by combining micron-sized flexible polyolefin elastomer (POE) matrix particles with conductive fillers such as conductive branched carbon nanotubes (CNS). A flexible and waterproof foam sensor, exemplified by POE / CNS, with an isolation structure and gradient pore structure, was fabricated using physical foaming techniques such as supercritical carbon dioxide (scCO2).

[0037] The foam sensor of this invention features an ultra-wide tensile strain response range (e.g., 0.5% to 763%), high sensitivity (for example, a normalization factor GF of 15230), fast response time (for example, 200 milliseconds), good resilience, and durability. Therefore, the porous foam sensor of this invention exhibits the ability to monitor all-around human movement, even underwater, and can be used in badminton and fitness activities.

[0038] This invention develops a simple, mass-producible, and environmentally friendly method for fabricating foam sensors with a wide operating range and high sensitivity, significantly promoting the development of sensors in practical applications. According to one aspect of the invention, a foam strain sensor is provided, comprising a conductive filler and a matrix, the conductive filler being coated on the surface of a thermoplastic elastomer particle matrix. Attached Figure Description

[0039] Figure 1 The image shows the cross-sectional morphology of the POE / CNS composite material with an isolation structure containing 3 wt% CNS in Example 1.

[0040] Figure 2 This is a photograph of the cross-sectional morphology of the central region of the foam with a CNS content of 3 wt% in Example 1, along the thickness direction.

[0041] Figure 3 This is a photograph of the gradient pore structure near the bottom of the foam with a CNS content of 3wt% in Example 1, taken along the thickness direction.

[0042] Figure 4 The stress-strain curve is shown for the isolation structure foam with a CNS content of 3 wt% in Example 1.

[0043] Figure 5 The image shows the tensile strain sensing curve of the insulating structure foam with a CNS content of 3 wt% in Example 1.

[0044] Figure 6 The diagram shows the pressure resistance curve of the isolation structure foam with a CNS content of 3 wt% in Example 1 under 800% tensile cycles.

[0045] Figure 7 The response time diagram is for the isolation structure foam with a CNS content of 3wt% in Example 1.

[0046] Figure 8 This is a photograph of the cross-sectional morphology of a randomly distributed structured foam with a CNS content of 3 wt% in Example 2.

[0047] Figure 9 The diagram shows the pressure resistance curve of the randomly distributed structured foam with a CNS content of 3 wt% in Example 2. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0049] Unless otherwise specified, the operating methods in the following examples are generally performed under conventional conditions or as recommended by the manufacturer.

[0050] Example 1

[0051] First, branched carbon nanotube (CNS) particles were placed in a mechanical mixer and mixed for 10 minutes to crush the particles with steel balls. Then, polyolefin elastomer (POE) particles with an average particle size of 380 micrometers were added to the mechanical mixer and mixed for another 30 minutes to obtain CNS@POE particles with an average particle size of approximately 400 micrometers. A certain amount of CNS@POE particles was placed in a mold and hot-pressed at 90°C in a flat vulcanizing machine at a molding pressure of 10 MPa to obtain a POE / CNS composite material with an isolation structure and a thickness of approximately 300 micrometers. The mass fraction of CNS was 3 wt%, based on the total mass of matrix particles and conductive filler being 100%. The conductive filler CNS was selectively and uniformly distributed at the interface between micrometer-sized POE matrix particles, forming a three-dimensional conductive network, such as... Figure 1 As shown. The sample can be designated as POE / CNS composite material or CNS3.

[0052] The prepared composite material was placed in an autoclave, and carbon dioxide was injected to purge the air from the autoclave. Carbon dioxide was then reintroduced to bring the pressure inside the autoclave to 20 MPa. The autoclave was saturated at 50°C for one hour, followed by rapid depressurization over approximately 2-3 seconds. The resulting foam sample was then cooled in ice water to stabilize the cell structure, ultimately yielding a foam sensor with an isolation structure and a gradient pore structure, which can be designated as POE / CNS composite foam material or FCNS3, with a thickness of 500 micrometers. The foam expansion ratio was approximately 2, and the average cell size was approximately 15 micrometers. Figure 2 and Figure 3 As shown, the foam has a uniformly distributed pore structure, and along the thickness direction, the pore size gradually increases from the cortex to the core, exhibiting a gradient pore structure.

[0053] The tensile properties of the POE / CNS composite foam were tested on a universal testing machine. The material dimensions were 35 mm long, 2 mm wide, and 0.5 mm thick, with a tensile rate of 10 mm / min. Cyclic tensile tests on the POE / CNS composite foam were also conducted on the universal testing machine. The foam specimen dimensions were 6 cm long, 1 cm wide, and approximately 0.5 mm thick, with a cyclic tensile rate of 50 mm / min. Figure 4As shown, the isolated POE / CNS foam exhibits excellent stretchability, with an elongation at break as high as 952%. Figure 5 As shown, during the tensile process, the relative resistance of the foam sensor gradually increases with the increase of tensile strain, exhibiting an ultra-high sensitivity in the strain range of 700%-762%, with a GF as high as 15230, while also displaying an ultra-wide strain detection range from 0.5% to 762% strain. Figure 6 As shown, during the tensile cycle, the relative resistance of the foam sensor first increases with the increase of tensile strain, and then gradually decreases during the return process. Due to the presence of residual strain, the resistance cannot return to the initial value. Figure 7 This indicates that the foam sensor has a very short response time, as low as 200ms.

[0054] The strain sensing performance of POE / CNS foam was tested using an electrochemical workstation with an output voltage of 1V, taking a sample every 0.1s. The foam sample used for testing was 6cm long, 1cm wide, and 0.5mm thick. When the foam sensor underwent cyclic tensile testing on a universal testing machine, a computer connected to the electrochemical workstation recorded the current changes in the sample.

[0055] Example 2

[0056] POE and CNS were mixed in an internal mixer at 90°C for 8 minutes with a rotor speed of 50 rpm to obtain a POE / CNS composite material with a total mass of 100% POE and CNS, of which the mass fraction of CNS was 3 wt%. The obtained POE / CNS composite material was then molded in a flat vulcanizing machine at 15 MPa and 90°C to finally obtain a sheet-like POE / CNS composite material.

[0057] The prepared composite material was placed in an autoclave, and carbon dioxide was injected to purge the air from the autoclave. Carbon dioxide was then reintroduced to bring the pressure inside the autoclave to 20 MPa. The autoclave was saturated at 50°C for one hour, followed by rapid depressurization (approximately 2-3 seconds). The resulting foam sample was then cooled in ice water to stabilize the cell structure, ultimately yielding a randomly distributed POE / CNS foam sensor, r-FCNS3, with a sensor thickness of approximately 500 micrometers. Figure 8 As shown, the randomly distributed POE / CNS foam has a uniform pore structure, and the conductive filler is uniformly dispersed in the pore walls.

[0058] Cyclic tensile tests on randomly distributed POE / CNS composite foam materials were conducted on a universal testing machine. The foam specimens were 6 cm long, 1 cm wide, and approximately 0.5 mm thick, with a cyclic tensile rate of 50 mm / min. Figure 9As shown, with the increase of tensile strain, the relative resistance of the sensor first decreases and then increases, and there is a narrow strain sensing range, which is only about 150% of the strain, far lower than the strain sensing range of the isolated POE / CNS foam sensor.

[0059] Furthermore, it should be understood that after reading the above description of the present invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. The application of a foam sensor with an ultra-wide tensile strain sensing range in tensile strain sensing devices, characterized in that, The method for fabricating the foam sensor with an ultra-wide tensile strain sensing range includes the following steps: (1) Micron-sized matrix particles and conductive fillers are added to a roller with steel balls and mechanically mixed to make the conductive fillers uniformly coated on the surface of the matrix particles to obtain composite particles; the mass percentage of the conductive fillers is 0.1%~10% based on the total mass of the matrix particles and the conductive fillers as 100%; the matrix particles are polyolefin elastomer particles. (2) The composite particles are hot-compressed in a flat vulcanizing machine to obtain a composite material with an isolation structure; (3) The composite material is kept warm and pressure-controlled in a physical foaming agent, then depressurized and foamed, and cooled and shaped to obtain the foam sensor with an ultra-wide tensile strain sensing range; the tensile strain sensing range is 0.5%~763%; In step (3), the foaming ratio of the composite material is 1.1 to 4 times. The thickness of the foam sensor is 0.1~1 mm; The foam sensor has a porous structure, and the foam sensor exhibits a gradient structure in its thickness direction where the pore size gradually increases from both end faces to the center.

2. The application according to claim 1, characterized in that, The thickness of the foam sensor is 100~700 micrometers.

3. The application according to claim 1, characterized in that, In step (1): The average particle size of the matrix particles is 20~1000 micrometers; The conductive filler includes at least one of carbon black, silver nanowires, carbon nanotubes, branched carbon nanotubes, graphene, reduced graphene oxide, and MXene. With the total mass of the matrix particles and the conductive filler being 100%, the mass percentage of the conductive filler is 0.5% to 10%.

4. The application according to claim 3, characterized in that, In step (1), the average particle size of the matrix particles is 100~1000 micrometers.

5. The application according to claim 1, characterized in that, In step (1), the mechanical mixing time is 1 to 360 minutes.

6. The application according to claim 1, characterized in that, In step (2), the temperature of the hot compression molding is 50~90℃ and the pressure is 9~11 MPa.

7. The application according to claim 1, characterized in that, Step (3) employs an intermittent foaming method; In step (3): The physical foaming agent includes at least one of carbon dioxide and nitrogen. The physical foaming agent accounts for 0.5% to 10% of the mass percentage of the composite material.

8. The application according to claim 1 or 7, characterized in that, In step (3), the physical foaming agent includes carbon dioxide and nitrogen; In the physical foaming agent, the volume ratio of carbon dioxide to nitrogen is 99:1 to 1:

99.

9. The application according to claim 1 or 7, characterized in that, In step (3): The insulation and pressure holding temperature is 30~80℃, and the pressure holding pressure is 7~20 MPa; The heat preservation and pressure holding time is 30~120 minutes; The depressurization and foaming time shall not exceed 10 seconds; The average pore size of the foam sensor is 10~300 micrometers.

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