A carbon nanotube resin-based strain sensor and a preparation method thereof

Abstract

The application discloses a kind of carbon nanotube resin-based strain sensor and preparation method thereof, including carbon nanotube, dispersing agent, epoxy resin and curing agent;The carbon nanotube includes carbon nanotube one, carbon nanotube two, carbon nanotube three;The epoxy resin includes epoxy resin one, epoxy resin two, epoxy resin three;The curing agent includes curing agent one, curing agent two;The carbon nanotube mass percentage is 3~4%, dispersing agent mass percentage is 0.6~0.8%, epoxy resin one mass percentage is 73.23~74.15%, and curing agent one mass percentage is 21.97~22.25%.The sensor monitoring system of the application is simple, and only resistance tester can be measured.Carbon nanotube resin-based composite material is prepared into different dimension and size sensor according to demand, and has the advantages of reliable performance and rapid response;The application obtains the component proportion of optimal carbon nanotube resin-based sensor.

Description

Technical Field

[0001] This invention belongs to the field of strain monitoring of infrastructure structures under stress, specifically involving the optimized design of components in a carbon nanotube resin-based conductive composite material. By selecting the optimal ratio of carbon nanotubes and epoxy resin, a highly sensitive strain sensor is prepared for damage monitoring of engineering structures such as steel and concrete structures. Background Technology

[0002] Structural health monitoring refers to the strategies and processes for identifying and characterizing damage in engineering structures. Its main purpose is to maintain and improve structural safety by analyzing the impact of external information on the structure, ensuring the reliability and safety of the engineering structure in use, while reducing maintenance costs and making monitoring faster and cheaper. Currently, commonly used sensors for structural strain monitoring include metal strain gauges and vibrating wire strain gauges. However, metal strain gauges have low sensitivity, weak output signals, poor anti-interference capabilities, and can only measure the average strain at a single point or within a strain gauge range. Vibrating wire strain gauges have a large gauge length, typically 100–150 mm, and are mainly used for strain measurement in large engineering structures. They are not suitable for measuring small components, have a slow response speed, a small measurement range (typically -1500–1500 με), and the cost of testing elements and instruments is high.

[0003] Over the past decade, carbon nanomaterials, particularly carbon nanotubes, graphene, and carbon nanofibers, have become the most promising functional materials for characterizing, quantifying, and diagnosing structural health. Carbon nanotubes have attracted widespread attention for decades due to their excellent electrical, mechanical, and thermal properties. Compared to other nanofillers such as metal particles or carbon black, carbon nanotubes have a very high aspect ratio (≈100-1000) and can form conductive pathways at relatively low concentrations, thus being considered an effective filler for the fabrication of highly sensitive strain sensors (“Self-sensing behavior and mechanical properties of carbon nanotubes / epoxy resin composite for asphaltpavement strain monitoring,” Xin et al., p. 119404, Construction and Building Materials, 2020). There is a great deal of research on carbon nanotube / polymer nanocomposites, including carbon nanotube / polyisoprene, carbon nanotube / polyurethane, carbon nanotube / polydimethylsiloxane, carbon nanotube / polymethyl methacrylate, carbon nanotube / polycarbonate, carbon nanotube / polysulfone, carbon nanotube / polyelectrolyte, carbon nanotube / polyvinylidene fluoride, and carbon nanotube / epoxy resin. Among the above polymers, epoxy resin was selected for this patent because of its excellent mechanical and weather resistance properties, dimensional stability, and good adhesion to the substrate.

[0004] The differences between this patent and patent number CN109817383A are as follows: 1) The carbon nanotube block resistance prepared by patent number CN109817383A is 1000-5000Ω, while the resistance of the carbon nanotube resin-based strain sensor prepared by this patent does not exceed 2000Ω. The lower the sensor resistance, the more stable and superior the sensor performance; 2) This patent selects the best basic components of the carbon nanotube resin-based strain sensor and designs the optimal ratio; 3) The carbon nanotube resin-based strain sensor developed according to this patent has higher sensitivity than the sensor in patent number CN109817383A; 4) Patent number CN109817383A mainly prepares carbon nanotube strain sensors with fixed shapes, while this patent can prepare sensors of different dimensions and sizes according to needs, with a wider range of applications.

[0005] This patent designs a highly sensitive strain sensor by optimizing the carbon nanotube resin-based strain sensor material. This sensor is not only highly designable and can solve the problems of complex equipment, low level of intelligence, and poor timeliness of traditional monitoring methods, but it can also open up new applications of carbon nanotube sensors in steel structure monitoring. Summary of the Invention

[0006] The purpose of this invention is to address the problems of low sensitivity and small strain detection range of commonly used strain sensors (metal foil strain gauges), as well as the issues that while higher carbon nanotube content in existing carbon nanotube resin-based materials theoretically improves conductivity, in reality, higher carbon nanotube content makes them more prone to aggregation in the resin matrix, increasing sensor cost without achieving good sensing performance. This invention aims to design a novel nanosensor with high sensitivity, a wide strain monitoring range, and cost-effectiveness, and its preparation method, through the rational selection of raw materials (including carbon nanotubes, resin matrix, and dispersant) and optimized material ratios.

[0007] The present invention adopts the following technical solution: a carbon nanotube resin-based strain sensor, comprising carbon nanotubes, dispersant, epoxy resin and curing agent;

[0008] The carbon nanotubes include carbon nanotube one, carbon nanotube two, and carbon nanotube three; carbon nanotube one is an array type with a diameter of 8-15 nm, a length of 20-50 μm, a purity greater than 95%, and a specific surface area of ​​230-280 m² / g; carbon nanotube two is an array type with a diameter of 8-15 nm, a length of 20-40 μm, a purity greater than 98%, and a specific surface area of ​​250-300 m² / g; carbon nanotube three is a wound type with a diameter of 5-15 nm, a length of 10-30 μm, a purity greater than 98%, and a specific surface area of ​​260-300 m² / g.

[0009] The epoxy resin includes epoxy resin one, epoxy resin two, and epoxy resin three; the viscosity of epoxy resin one is 1400-1800 mPa·s; the viscosity of epoxy resin two is 10000-12000 mPa·s; and the viscosity of epoxy resin three is 12000-15000 mPa·s.

[0010] The curing agent includes curing agent one and curing agent two; the viscosity of curing agent one is 10-15 mPa·s; the viscosity of curing agent two is 120-250 mPa·s.

[0011] Furthermore, the carbon nanotubes have a mass percentage of 3% to 4%, the dispersant has a mass percentage of 0.6% to 0.8%, the epoxy resin has a mass percentage of 73.23% to 74.15%, and the curing agent has a mass percentage of 21.97% to 22.25%. The sum of the mass percentages of the carbon nanotubes, dispersant, epoxy resin, and curing agent is 100%.

[0012] Furthermore, the carbon nanotubes have a mass percentage of 3% to 4%, the dispersant has a mass percentage of 0.6% to 0.8%, the epoxy resin II has a mass percentage of 76.16% to 77.12%, and the curing agent I has a mass percentage of 19.04% to 19.28%. The sum of the mass percentages of the carbon nanotubes, dispersant, epoxy resin II, and curing agent I is 100%.

[0013] Furthermore, the carbon nanotubes have a mass percentage of 3% to 4%, the dispersant has a mass percentage of 0.6% to 0.8%, the epoxy resin has a mass percentage of 76.16% to 77.12%, and the curing agent has a mass percentage of 19.04% to 19.28%. The sum of the mass percentages of the carbon nanotubes, dispersant, epoxy resin, and curing agent is 100%.

[0014] The preparation method of the above-mentioned carbon nanotube resin-based strain sensor includes the following steps:

[0015] 1) Weigh the carbon nanotube powder and dispersant according to a carbon nanotube to dispersant mass ratio of 5:1. First, pour the dispersant into the container of a sand mill (any model of sand mill is acceptable), and slowly add the carbon nanotube powder in multiple batches. The zirconium bead milling speed is 2300 rpm, and the machine is cooled with circulating water throughout the process. The total dispersion time is controlled at 2.5 hours. The carbon nanotubes are uniformly dispersed without damaging their integrity and conductivity to prepare a carbon nanotube dispersion.

[0016] 2) Mix the prepared carbon nanotube dispersion with epoxy resin, ultrasonically disperse for 30 min (no limit on the type of ultrasonic instrument), and magnetically stir at 400 rpm for 30 min.

[0017] 3) After adding the curing agent, stir at 400 rpm for 10 minutes, remove air bubbles from the composite material by vacuuming, and let stand for 10 minutes;

[0018] 4) When the viscosity is appropriate, deposit the uniformly mixed carbon nanotube resin-based composite material by screen printing (deposition area is 20mm×5mm, screen printing machine model is not limited) onto the cut polyimide film (the size of the polyimide can be determined according to actual needs, but should not be less than 20mm×5mm).

[0019] 5) Place in a vacuum drying oven (any model of vacuum drying oven is acceptable) and cure at 100℃ for 5 hours;

[0020] 6) After the composite material has fully cured, use conductive silver paste (any type of conductive silver paste can be used) to attach copper sheets (0.05 mm thick) as electrodes at both ends of the sensor. Place them in a vacuum drying oven at 70°C for 3 hours (below the glass transition temperature of epoxy resin, 75°C) to improve the conductivity and adhesion of the conductive paste. Finally, weld wires (using copper core wires with an outer diameter of 1.8 mm) onto the copper electrodes to complete the fabrication of the carbon nanotube sensor.

[0021] The key to the carbon nanotube resin-based sensor of this invention lies in the optimal design of the components and the dispersion of the carbon nanotubes. Experimental results for carbon nanotube type 1, carbon nanotube type 2, and carbon nanotube type 3 (CNT-21, CNT-22, and CNT-23) show that the carbon nanotube composite material prepared from CNT-21 dispersion at 100°C exhibits the highest electrical conductivity. The percolation thresholds of the three types of carbon nanotubes are similar, all falling between 3.25 wt.% and 3.75 wt.%. Figure 2 When the curing temperature increased from 80℃ to 120℃, the electrical conductivity of the resin-based composite material with a carbon nanotube content of 3.75wt.% continuously increased (electrical conductivity of 245S / m, 437S / m, and 457S / m, respectively), and tended to level off after exceeding 100℃. Figure 3 The reason is that the high temperature during the curing process can accelerate the flow and diffusion of carbon nanotubes in the epoxy resin, making it easier to form a macroscopic conductive network. As the temperature increases further, a relatively complete conductive network has already been formed within the system, and it is difficult to further reduce its resistance by increasing the curing temperature. Therefore, the preferred curing temperature is 100℃.

[0022] On the one hand, percolation theory can be used to explain the behavior of conductive composites obtained by adding conductive fillers to an insulating matrix. This is due to the formation of a continuous conductive network, which allows electrons to flow within the material, thus producing conductivity. The amount of conductive filler is a key aspect of the transition from insulator to conductor. Direct contact between conductive filler particles leads to the formation of a conductive network in the polymer matrix, thereby creating conductive pathways. Another major mechanism leading to charge flow in polymer nanocomposites is the "tunneling" effect. Unlike the contact mechanism, filler particles form a conductive network through actual contact, and if adjacent tunnels in the "tunneling" phenomenon are close enough, electrons can penetrate from one particle tunnel to another. Electron tunneling is usually a major conductivity phenomenon in nanocomposites, and a sufficient amount of conductive filler is typically required to ensure that the gaps or distances between adjacent particles are small enough to promote electron tunneling. On the other hand, carbon nanotubes are prone to agglomeration in resins due to intermolecular van der Waals forces, and the agglomeration problem becomes increasingly difficult to solve with increasing carbon nanotube content. Therefore, it is necessary to modify and disperse carbon nanotubes based on physical dispersion methods such as magnetic stirring and ultrasonic dispersion. Covalent modification disrupts the structure of carbon nanotubes, affecting their conductivity. Non-covalent modification, on the other hand, reduces intermolecular forces while maintaining the integrity of the original structure, achieving dispersion and stability. By using a dispersant for non-covalent modification and dispersion of carbon nanotubes, the final sensor's resistance can be stably maintained between 1500Ω and 2000Ω. Epoxy resins II and III, due to their high viscosity, not only cause severe agglomeration of carbon nanotubes but also hinder screen printing. Therefore, epoxy resin I was chosen. Carbon nanotube III agglomerates severely in epoxy resin I, while carbon nanotubes I and II are well dispersed. However, the lower dispersion viscosity of carbon nanotube II affects the screen printing effect, resulting in poor sensor formation and low sensitivity. Therefore, carbon nanotube I was ultimately selected.

[0023] The invented carbon nanotube resin-based strain sensor can be used as a strain sensor to monitor steel structures and can replace metal foil strain gauges in engineering applications.

[0024] The beneficial effects of this invention are:

[0025] 1) The equipment is easy to operate.

[0026] The sensor monitoring system is simple, requiring only a resistance tester for measurement. It solves the problems of complex equipment, low level of intelligence, and poor timeliness of traditional monitoring methods.

[0027] 2) High designability

[0028] Carbon nanotube resin-based composite materials can be fabricated into sensors of different dimensions and sizes according to requirements, and have advantages such as reliable performance and fast response.

[0029] 3) Performance optimization

[0030] This invention yields the optimal component ratio for a carbon nanotube resin-based sensor. It provides a novel carbon nanotube strain sensor with high sensitivity (GF = 3.37) and high stability for the engineering field. The optimal component ratio for the sensor was determined by studying the effects of different types of carbon nanotubes and epoxy resins, as well as the carbon nanotube content, on the conductivity and piezoresistive properties of the carbon nanotube composite material. Attached Figure Description

[0031] Figure 1 This is a structural diagram of a carbon nanotube resin-based strain sensor.

[0032] Figure 2 The relationship between the content of different types of carbon nanotubes and the conductivity of the sensor;

[0033] Figure 3 Relationship between curing temperature and electrical conductivity of CNT-21 resin-based composite material (3.75 wt.%);

[0034] Figure 4 TEM micrograph of a 3.5 wt.% CNT-21 carbon nanotube resin-based composite material;

[0035] Figure 5 TEM micrograph of a 3.5 wt.% CNT-22 carbon nanotube resin-based composite material;

[0036] Figure 6 Resistance response curves of strain sensors fabricated from different types of carbon nanotubes. Detailed Implementation

[0037] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0038] Figure 1 This is a structural diagram of a carbon nanotube resin-based strain sensor. The sensor includes a wire 1, a solder joint 2, a carbon nanotube resin-based conductive composite material 3, a polyimide 4, and an electrode 5. The carbon nanotube resin-based conductive composite material 3 is disposed on the polyimide 4, and electrodes 5 are attached to both ends of the carbon nanotube resin-based conductive composite material 3. The electrodes 5 are connected to the wire 1 through the solder joint 2.

[0039] Example 1: Using carbon nanotubes (CNT-21 type) and epoxy resin, the carbon nanotube resin-based sensor was prepared according to the above-described method:

[0040] 1) Mix the prepared carbon nanotube dispersion with epoxy resin in a predetermined ratio (carbon nanotube content is 3.5 wt.%), ultrasonically disperse for 30 min, and magnetically stir at 400 rpm for 30 min.

[0041] 2) After adding the curing agent, stir at 400 rpm for 10 minutes, remove air bubbles from the composite material by vacuuming, and let stand for 10 minutes;

[0042] 3) Once the viscosity is suitable, the uniformly mixed carbon nanotube resin-based composite material is screen-printed onto the cut polyimide film.

[0043] 4) Place in a vacuum drying oven and cure at 100℃ for 5 hours;

[0044] Figure 4 The TEM image shows the carbon nanotube-based resin composite material, revealing that carbon nanotube type I (CNT-21) is well dispersed in epoxy resin I. The resistivity of this conductive composite material is 1500-2000 Ω.

[0045] Example 2: Using carbon nanotube type II (CNT-22) and epoxy resin type I, the carbon nanotube resin-based sensor was prepared according to the above-described method:

[0046] 1) Mix the prepared carbon nanotube dispersion with epoxy resin in a predetermined ratio (carbon nanotube content is 3.5 wt.%), ultrasonically disperse for 30 min, and magnetically stir at 400 rpm for 30 min.

[0047] 2) After adding the curing agent, stir at 400 rpm for 10 minutes, remove air bubbles from the composite material by vacuuming, and let stand for 10 minutes;

[0048] 3) Once the viscosity is suitable, the uniformly mixed carbon nanotube resin-based composite material is screen-printed onto the cut polyimide film.

[0049] 4) Place in a vacuum drying oven and cure at 100℃ for 5 hours;

[0050] Figure 5 The image shows a TEM image of a carbon nanotube resin-based composite material. It can be seen that carbon nanotube type II (CNT-22) is severely agglomerated in epoxy resin type I, making it unsuitable as a sensor element.

[0051] Example 3: In the preparation of the carbon nanotube resin-based sensor, 3.5 wt.% carbon nanotubes (CNT-21 type) and 96.5 wt.% epoxy resin and corresponding curing agent were used. The piezoresistive performance of the sensor is as follows: Figure 6 As shown, compared with traditional metal foil strain gauges (sensitivity of 2), the carbon nanotube resin-based sensor exhibits a 68.5% increase in sensitivity.

[0052] In the fabrication of the carbon nanotube resin-based sensor, 3.5 wt.% carbon nanotubes (CNT-22 type) and 96.5 wt.% epoxy resin and corresponding curing agent were used. The sensor's piezoresistive performance is as follows: Figure 6 As shown, compared to a traditional metal foil strain gauge (sensitivity of 2), the carbon nanotube resin-based sensor exhibits a 13% reduction in sensitivity.

[0053] In the fabrication of the carbon nanotube resin-based sensor, 3.5 wt.% carbon nanotubes (CNT-23 type) and 96.5 wt.% E01 type resin and corresponding curing agent were used. The sensor's piezoresistive performance is as follows: Figure 6 As shown, compared to a traditional metal foil strain gauge (sensitivity of 2), the carbon nanotube resin-based sensor exhibits a 10% reduction in sensitivity.

[0054] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations of these embodiments within the scope of the principles and technical concept of the present invention still fall within the protection scope of the present invention.

Claims

1. A carbon nanotube resin-based strain sensor, characterized in that: Includes carbon nanotubes, dispersants, epoxy resins, and curing agents; The carbon nanotubes are carbon nanotube type 1, carbon nanotube type 2, or carbon nanotube type 3; the carbon nanotube type 1 is: array type, with a diameter of 8-15 nm, a length of 20-50 μm, a purity greater than 95%, and a specific surface area of ​​230-280 m². 2 / g; The carbon nanotubes are: array type, with a diameter of 8-15nm, a length of 20-40μm, a purity greater than 98%, and a specific surface area of ​​250-300m². 2 / g; The carbon nanotubes are: wound type, with a diameter of 5-15nm, a length of 10-30μm, a purity greater than 98%, and a specific surface area of ​​260-300 m². 2 / g; The epoxy resin is epoxy resin one, epoxy resin two, or epoxy resin three; the viscosity of epoxy resin one is 1400-1800 mPa·s; the viscosity of epoxy resin two is 10000-12000 mPa·s; and the viscosity of epoxy resin three is 12000-15000 mPa·s. The curing agent is either curing agent one or curing agent two; the viscosity of curing agent one is 10-15 mPa·s; the viscosity of curing agent two is 120-250 mPa·s.

2. The carbon nanotube resin-based strain sensor according to claim 1, characterized in that: The carbon nanotubes have a mass percentage of 3% to 4%, the dispersant has a mass percentage of 0.6% to 0.8%, the epoxy resin has a mass percentage of 73.23% to 74.15%, and the curing agent has a mass percentage of 21.97% to 22.25%. The sum of the mass percentages of the carbon nanotubes, dispersant, epoxy resin, and curing agent is 100%.

3. The carbon nanotube resin-based strain sensor according to claim 1, characterized in that: The carbon nanotubes have a mass percentage of 3% to 4%, the dispersant has a mass percentage of 0.6% to 0.8%, the epoxy resin II has a mass percentage of 76.16% to 77.12%, and the curing agent I has a mass percentage of 19.04% to 19.28%. The sum of the mass percentages of the carbon nanotubes, dispersant, epoxy resin II, and curing agent I is 100%.

4. The carbon nanotube resin-based strain sensor according to claim 1, characterized in that: The carbon nanotubes have a mass percentage of 3% to 4%, the dispersant has a mass percentage of 0.6% to 0.8%, the epoxy resin has a mass percentage of 76.16% to 77.12%, and the curing agent has a mass percentage of 19.04% to 19.28%. The sum of the mass percentages of the carbon nanotubes, dispersant, epoxy resin and curing agent is 100%.

5. The method for preparing the carbon nanotube resin-based strain sensor according to claim 1, 2, 3 or 4, characterized in that: Includes the following steps: 1) Weigh carbon nanotube powder and dispersant according to a carbon nanotube to dispersant mass ratio of 5:1; first pour the dispersant into the sand mill container, then slowly add the carbon nanotube powder in multiple batches. The zirconium bead milling speed is 2300 rpm, and the machine is cooled with circulating water throughout the process. The total dispersion time is controlled at 2.5 h. The carbon nanotubes are uniformly dispersed without damaging their integrity and conductivity to prepare a carbon nanotube dispersion. 2) Mix the prepared carbon nanotube dispersion with epoxy resin, ultrasonically disperse for 30 min, and magnetically stir at 400 rpm for 30 min; 3) After adding the curing agent, stir at 400 rpm for 10 minutes, remove air bubbles from the composite material by vacuuming, and let stand for 10 minutes; 4) Once the viscosity is suitable, the uniformly mixed carbon nanotube resin-based composite material is screen-printed onto the cut polyimide film. 5) Place in a vacuum drying oven and cure at 100℃ for 5 hours; 6) After the composite material has fully cured, copper sheets are attached to both ends of the sensor as electrodes using conductive silver paste. The electrodes are then placed in a vacuum drying oven at 70°C for 3 hours to improve the conductivity and adhesion of the conductive paste. Finally, wires are welded onto the copper electrodes to complete the fabrication of the carbon nanotube sensor.

6. The method for preparing the carbon nanotube resin-based strain sensor according to claim 5, characterized in that: In step 4), the deposition area is 20mm × 5mm, and the size of the polyimide film must not be less than 20mm × 5mm.

7. The method for preparing the carbon nanotube resin-based strain sensor according to claim 5, characterized in that: In step 6), the thickness of the copper sheet is 0.05 mm, and the conductor is a copper core conductor with an outer diameter of 1.8 mm.

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