Wide-range high-temperature-resistant piezoresistive sensor, preparation method and application thereof

Abstract

The application relates to the technical field of bolt loosening monitoring, and discloses a wide-range high-temperature-resistant piezoresistive sensor and a preparation method and application thereof. The piezoresistive sensor comprises a sensing layer and an FPC board, wherein the sensing layer comprises a polyimide matrix and conductive fillers, nano-aluminum oxide and nano-clay dispersed in the polyimide matrix. The application can improve the dispersion uniformity of the nano-clay and nano-aluminum oxide in the polyimide matrix, and can form some shallow energy level traps in the polyimide matrix to replace the original trap positions, so as to adjust the electron transmission path and relieve the space charge accumulation, thereby reducing the microstructure damage caused by high temperature, enhancing the thermal stability of the sensing layer material, and maximizing the high-temperature-resistant performance of the piezoresistive sensor. The piezoresistive sensor of the application can reach a range of 30 MPa and can resist a temperature of more than 300 DEG C in a high-temperature environment for bolt loosening monitoring.

Description

A wide-range, high-temperature resistant piezoresistive sensor, its fabrication method, and its application. Technical Field

[0001] This invention relates to the field of bolt loosening monitoring technology, and discloses a wide-range high-temperature resistant piezoresistive sensor, its preparation method, and its application. Background Technology

[0002] Bolted connections, as an important form of connection in industrial assembly, are widely used in major equipment such as marine ports, aerospace, bridge engineering, and wind turbines due to their advantages of convenient disassembly, low cost, high connection strength, and excellent environmental adaptability. Given the large size and weight of the components in these equipment systems, higher requirements are placed on the load-bearing capacity of the connection structure, typically employing large-size bolts to ensure the overall structural load-bearing capacity and operational stability. In practical applications, large-size bolts often operate in harsh environments such as high temperatures, highly corrosive media, and strong vibrations, making them prone to fatigue damage, fracture failure, and loosening / degradation. Bolt loosening failure, in particular, is difficult to detect in a timely manner due to its high degree of concealment and suddenness, and once it occurs, it seriously threatens structural safety.

[0003] The loosening process of bolted connections is essentially a dynamic process of gradual attenuation of preload. Under continuous cyclic loading of pressure and shear force, microscopic slippage occurs at the threaded contact surface, leading to a continuous decrease in clamping force. When the preload drops to a critical threshold, the connection structure will experience macroscopic slippage under dynamic load impact, resulting in catastrophic consequences such as component detachment and system instability. Statistics show that in critical sectors such as aviation and chemical engineering, equipment failures caused by bolt loosening account for over 30%, causing not only severe economic losses but also potential secondary disasters such as leaks and explosions, threatening personnel safety and damaging the ecological environment. Given the large number and complex distribution of bolted connections in major equipment, conventional periodic inspection methods are difficult to implement and costly, making real-time monitoring of bolt loosening status challenging.

[0004] Existing methods for detecting bolt loosening mainly include ultrasonic guided wave detection, fiber optic grating sensing, and machine vision analysis. While these methods have demonstrated some monitoring effectiveness under laboratory conditions, they still have significant limitations in engineering applications: ultrasonic guided wave detection is highly sensitive to boundary conditions and relies on complex signal analysis processes; fiber optic grating sensing technology is costly and has limited environmental adaptability; and machine vision systems are constrained by lighting conditions and insufficient algorithm robustness. In contrast, piezoresistive sensing technology has significant advantages such as simple principle, low cost, high integration, and strong environmental tolerance, showing important application potential in monitoring bolt loosening conditions under high-temperature environments.

[0005] Piezoresistive sensing technology is designed based on the piezoresistive effect. Its working mechanism involves the microscopic changes in the distribution and contact relationships of the conductive filler inside the sensor when external pressure or strain is applied, leading to a reconstruction of the conductive path and consequently a change in resistance. This change in resistance is further converted into a detectable and recordable electrical signal, reflecting the magnitude of the external pressure and enabling real-time pressure monitoring. Piezoresistive sensing technology features a simple structure, ease of manufacturing and integration, and intuitive and clear signal output. It can efficiently interface with various electronic systems and achieve data exchange, demonstrating good system compatibility and ease of application. Furthermore, this technology has low production costs, offering excellent cost-effectiveness and making it suitable for large-scale application. In particular, piezoresistive sensing technology combines high sensitivity with a wide detection range, enabling stable and accurate responses to minute pressure or strain changes, providing reliable technical support for various complex application scenarios.

[0006] Due to the stringent structural connection strength requirements for bolt loosening monitoring, piezoresistive sensing technology that minimizes the impact on the structural integrity of bolted connections is typically chosen. Among these technologies, nanocomposite material piezoresistive sensing technology stands out for its lightweight and convenient design, allowing for tight integration with bolted connections without compromising structural strength. The sensing layer in nanocomposite material piezoresistive sensing technology usually consists of two core components: conductive filler and matrix material. Conductive fillers include metal nanomaterials, conductive polymers, and carbon-based materials. Carbon-based materials, in particular, hold a significant position in the field of nanocomposite materials and sensors due to their stable and excellent electrical and mechanical properties. The polymer matrix, serving as the supporting framework of the sensing layer, must possess both mechanical flexibility and temperature stability. Commonly used matrix materials include polyurethane (PU), polydimethylsiloxane (PDMS), and polyvinylpyrrolidone (PVP) to construct the basic architecture of the piezoresistive component. However, most nanocomposite material piezoresistive sensors are currently unsuitable for monitoring bolt loosening in critical equipment due to the following issues: they are prone to decomposition and volatilization at high temperatures, leading to damage to the matrix structure; the high preload of bolted connections places high demands on the range of nanocomposite piezoresistive sensors, while ordinary nanocomposite piezoresistive sensors are also prone to plastic deformation and even breakage under high pressure; conventional nanocomposite piezoresistive sensors achieve excellent flexibility and high sensitivity by increasing material thickness, but the strength requirements of bolted connections necessitate limiting sensor thickness to avoid affecting the transmission of preload. These factors limit the application potential of nanocomposite material piezoresistive sensing technology in the high-temperature environment of critical equipment. Therefore, existing nanocomposite material piezoresistive sensing technology still faces many technical bottlenecks in meeting the needs of monitoring bolt loosening in high-temperature service environments. Summary of the Invention

[0007] The purpose of this invention is to provide a wide-range, high-temperature resistant piezoresistive sensor, its preparation method, and its application. It can perform long-term stable monitoring of bolt loosening for applications requiring high-temperature environments and different bolt sizes, and realize real-time assessment of bolt loosening status. It has the characteristics of a large range and sensitive response to changes in preload when bolts loosen. In high-temperature environments for bolt loosening monitoring, the range can reach 30 MPa and can withstand temperatures exceeding 300°C.

[0008] To achieve the above-mentioned technical effects, the technical solution adopted by the present invention is as follows:

[0009] A wide-range, high-temperature resistant piezoresistive sensor, comprising:

[0010] The sensing layer includes a polyimide matrix and conductive fillers, nano-alumina, and nano-clay dispersed in the polyimide matrix;

[0011] The FPC board has conductive components for transmitting electrical signals generated by the sensing layer.

[0012] Furthermore, the conductive filler is carbon black, and the carbon black accounts for 1 to 20% of the total mass of the sensing layer.

[0013] Furthermore, the total mass of the nano-alumina and nano-clay accounts for 0.1 to 3% of the total mass of the sensing layer, and the mass ratio between the nano-alumina and nano-clay is (2 to 10): 1.

[0014] Furthermore, the piezoresistive sensor has a range of 30 MPa or greater and a maximum operating temperature of 300°C or greater.

[0015] To achieve the above-mentioned technical effects, the present invention also provides a method for fabricating a wide-range, high-temperature resistant piezoresistive sensor. This method is used to fabricate the aforementioned piezoresistive sensor and includes:

[0016] Conductive filler, nano-alumina, and nano-clay are dispersed in a matrix solution to prepare a conductive solution; the matrix solution is a polyamic acid solution, and the solvent of the polyamic acid solution is N-methylpyrrolidone.

[0017] The conductive solution is coated onto the sensing area of ​​the FPC board to form a conductive solution coating of fixed thickness.

[0018] The FPC board with a conductive solution coating is heated according to a preset temperature gradient, so that the conductive solution coating is formed into a sensing layer and bonded to the sensing area of ​​the FPC board.

[0019] Furthermore, during the process of dispersing conductive fillers, nano-alumina, and nano-clay in the matrix solution, mechanical stirring and ultrasonic treatment are used to achieve uniform dispersion of powder materials in the matrix solution; wherein mechanical stirring is performed for 0.5 to 2 hours at a speed of 1000 r / min, and ultrasonic dispersion is performed for 20 to 30 minutes.

[0020] Furthermore, the method for heat-treating the FPC board with a conductive solution coating according to a preset temperature gradient includes:

[0021] The FPC board with the conductive solution coating was placed in a hot oven. The temperature of the conductive solution was increased from room temperature to 80℃ and held for 20 min, then increased to 120℃ and held for 30 min, then increased to 180℃ and held for 30 min, then increased to 200℃ and held for 10 min, then increased to 220℃ and held for 20 min, then increased to 250℃ and held for 30 min, and then increased to 280℃ and held for 20 min. The heating rate was 5℃ / min for all of these conditions.

[0022] Furthermore, the sensing area of ​​the FPC board is a circular structure with a groove in the middle. The sensing layer is formed in the groove, and an electrode material electrically connected to the conductive component is disposed in the groove. The circular structure of the FPC board has a thickness of 100μm and a diameter of 8mm. The groove in the middle of the circular structure has a length and width of 2mm*5mm and a depth of 60μm.

[0023] To achieve the above-mentioned technical effects, the present invention also provides an application of a wide-range, high-temperature resistant piezoresistive sensor, which is used to monitor bolt loosening.

[0024] Furthermore, the sensing layer of the piezoresistive sensor is located between the bolt washer and the edge of the corresponding bolt hole, and is used to monitor the preload of the bolt.

[0025] Compared with the prior art, the beneficial effects of this invention are:

[0026] 1. Based on conductive fillers, this invention utilizes nano-alumina linked to the surface of nano-clay particles to reduce the aggregation tendency of clay particles, thereby improving the dispersion uniformity of nano-clay and nano-alumina in the polyimide matrix. In addition, the linkage structure formed by alumina and clay can form some shallow traps in the polyimide matrix to replace the original traps, thereby regulating the electron transport path and alleviating the accumulation of space charge. This reduces the microstructure damage caused by high temperature, enhances the thermal stability of the sensing layer material, and thus maximizes the high temperature resistance of the piezoresistive sensor.

[0027] 2. This invention employs piezoresistive sensing technology as a means of monitoring bolt loosening in high-temperature environments. It utilizes the characteristic that the resistance value of the piezoresistive sensor changes with the bolt preload to reflect the degree of bolt loosening. In particular, it can perform long-term stable monitoring of bolt loosening for applications requiring high-temperature environments and different bolt sizes, achieving real-time assessment of the bolt loosening state. Furthermore, it features a large range and sensitive response to changes in preload when bolts loosen.

[0028] 3. The piezoresistive sensor of the present invention has a range of up to 30MPa in high-temperature environments for monitoring bolt loosening and can withstand temperatures exceeding 300℃. Attached Figure Description

[0029] Figure 1 is a schematic diagram of the FPC board structure;

[0030] Figure 2 is a schematic diagram of the FPC board and the sensing layer;

[0031] Figure 3 shows the resistance variation curves of piezoresistive sensors with different conductive filler mass fractions under cyclic pressure.

[0032] Figure 4 shows the resistance change rate-time curves of the piezoresistive sensors in Examples 2 and 6 at 300°C;

[0033] Figure 5 shows the experimental setup for monitoring bolt loosening;

[0034] Figure 6 shows the resistance change rate-pressure curve of the piezoresistive sensor during the bolt loosening process;

[0035] The components include: 1. Sensing layer; 2. FPC board; 3. Circular structure; 4. Groove; 5. Electrode material; 6. Conductive component; and 7. Gasket. Detailed Implementation

[0036] The present invention will now be described in further detail with reference to the embodiments and accompanying drawings. However, this should not be construed as limiting the scope of the above-described subject matter of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention.

[0037] Example 1

[0038] Referring to Figures 1-2, a wide-range, high-temperature resistant piezoresistive sensor includes:

[0039] Sensing layer 1 includes a polyimide matrix and conductive fillers, nano-alumina, and nano-clay dispersed in the polyimide matrix;

[0040] FPC board 2, on which conductive components 6 are provided for transmitting electrical signals generated by sensing layer 1.

[0041] In this embodiment, when the sensing layer 1 of the piezoresistive sensor is subjected to external mechanical stress, the internal microstructure of the sensing layer 1 changes, resulting in a corresponding change in resistivity. By monitoring the change in resistance value of the piezoresistive sensor in real time, the loosening state of the bolts under high temperature environment can be effectively reflected, and dynamic monitoring and evaluation of the connection structure can be achieved.

[0042] Traditional polymer composites contain sites where electrons easily accumulate, known as traps. Electrons accumulate in these traps during transport, and at room temperature, this accumulation has little impact on the overall structure of the composite. However, at high temperatures, electron mobility increases, leading to the accumulation of high-energy charge carriers at these traps. These high-energy electrons can damage the molecular chain structure, causing structural defects. In this embodiment, nano-alumina and nano-clay powder are further added to the polyimide matrix in addition to the conductive filler. On one hand, some of the nano-alumina dispersed in the polyimide matrix links along the clay surface, reducing the tendency of nano-clay particles to aggregate and exhibiting good compatibility. This improves the dispersion uniformity of nano-clay and nano-alumina in the polyimide matrix. On the other hand, the linking structure formed by alumina and clay can create shallower traps in the polyimide matrix, replacing the original traps and regulating electron transport paths. This alleviates space charge accumulation, reducing microstructural damage caused by high temperatures and enhancing the thermal stability of the sensing layer 1 material. Ultimately, this maximizes the high-temperature resistance of the piezoresistive sensor.

[0043] In this embodiment, the conductive filler is carbon black (CB), where CB serves as the conductive filler inside the sensing layer 1 of the piezoresistive sensor, responsible for electron transmission in the sensing layer 1. When the sensor is subjected to pressure, the distance between the conductive fillers in the conductive network within the sensing layer 1 decreases, the tunneling distance decreases, current transmission becomes simpler, and the resistance value decreases, thereby enabling the piezoresistive sensor to achieve high sensitivity. In this embodiment, the carbon black accounts for 1-20% of the total mass of the sensing layer 1; piezoresistive sensors prepared with different mass fractions of carbon black material have different sensitivities to force. In some other embodiments, the preferred mass fraction of CB in the sensing layer 1 is 3%. At this ratio, the mass fraction of the conductive filler in the sensing layer 1 is near the percolation threshold, at which point the piezoresistive sensor is most sensitive.

[0044] In this embodiment, the total mass of the nano-alumina and nano-clay accounts for 0.1% to 3% of the total mass of the sensing layer 1, and the mass ratio of the nano-alumina to the nano-clay is (2-10):1. In our experiments, the total mass of the nano-alumina and nano-clay accounts for about 1% of the total mass of the sensing layer 1, and when the mass ratio of alumina / clay is 5:1, the two substances exhibit the highest high-temperature resistance through synergistic effect. This configuration maximizes the high-temperature resistance performance of the piezoresistive sensor.

[0045] Based on the same inventive concept, this embodiment also provides a method for fabricating a wide-range, high-temperature resistant piezoresistive sensor, comprising:

[0046] Step 1: Disperse conductive filler, nano-alumina, and nano-clay in a matrix solution to prepare a conductive solution; the matrix solution is a polyamic acid solution, and the solvent of the polyamic acid solution is N-methylpyrrolidone.

[0047] In this embodiment, in order to improve the uniformity of powder material dispersion in matrix solution, mechanical stirring and ultrasonic treatment are used to achieve uniform dispersion of powder material in matrix solution; wherein mechanical stirring is performed for 0.5 to 2 hours at a speed of 1000 r / min, and ultrasonic dispersion treatment is performed for 20 to 30 minutes.

[0048] Step 2: Apply the conductive solution to the sensing area on the FPC board 2 to form a conductive solution coating of fixed thickness;

[0049] In this embodiment, the conductive solution obtained in step one is uniformly coated onto the sensing area of ​​the FPC board 2 using a scraper to form a conductive solution coating of fixed thickness.

[0050] Step 3: Heat the FPC board 2 with the conductive solution coating according to the preset temperature gradient, so that the conductive solution coating is formed into the sensing layer 1 and bonded to the sensing area of ​​the FPC board 2.

[0051] In this embodiment, the FPC board 2 and the conductive solution coating obtained in step two are placed in a hot oven and heated according to a preset temperature gradient to form the conductive solution coating into a conductive sensing layer 1. At the same time, the polymer crosslinking reaction is used to chemically bond the sensing layer 1 and the FPC board 2 together, so that the sensing layer 1 is tightly bonded to the FPC substrate and the overall structure is thin and light.

[0052] This invention employs piezoresistive sensing technology as a means of monitoring bolt loosening in high-temperature environments. By integrating a fabricated piezoresistive sensor between the gasket 7 and the bolt connection, and combining it with a monitoring system, the resistance signal of the piezoresistive sensor is acquired and processed in real time. Utilizing the characteristic that the resistance value of the piezoresistive sensor changes with the bolt preload, the degree of bolt loosening is reflected. In particular, it can provide long-term stable monitoring of bolt loosening for applications requiring high temperatures and different bolt sizes, achieving real-time assessment of the bolt loosening state. It also features a large range and sensitive response to changes in preload during bolt loosening. Testing shows that the piezoresistive sensor prepared using the method described in this embodiment can be as thin as 100 μm, has a range of up to 30 MPa in high-temperature environments for bolt loosening monitoring, and can withstand temperatures exceeding 300°C.

[0053] Example 2

[0054] Weigh 0.108g of CB powder and slowly add it to 20mL of PAA (polyamic acid) solution. The solvent of the polyamic acid solution used is NMP (N-methylpyrrolidone), and the mass fraction of the solute is 18%. At room temperature, mechanically stir at 1000r / min for 1 hour and then sonicate at 90W for 30 minutes to fully disperse the powder material. Subsequently, the resulting conductive solution is coated into the groove 4 of the FPC board 2 (in this embodiment, the sensing area of ​​the FPC board 2 is a circular structure 3, with a groove 4 in the center of the circular structure 3; the sensing layer 1 is formed within the groove 4; and the electrode material 5, electrically connected to the conductive component 6, is disposed within the groove 4; the circular structure 3 of the FPC board 2 has a thickness of 100μm and a diameter of 8mm; the length and width of the groove 4 in the center of the circular structure 3 are 2mm*5mm, and the depth of the groove 4 is 60μm). Excess solution extending beyond the groove 4 is scraped off using a scraper. After coating, the FPC board 2, along with the conductive solution, is placed in a hot oven for thermal imidization treatment according to the following heating process: the temperature is increased at a rate of 5℃ / min, then raised to 80℃ and held for 20 min, 120℃ and held for 30 min, 180℃ and held for 30 min, 200℃ and held for 10 min, 220℃ and held for 20 min, finally raised to 250℃ and held for 30 min, and then raised to 280℃ and held for 20 min. Through this step-by-step heat treatment process, the thermal imidization reaction of the conductive solution is achieved, forming the sensing layer 1, which is tightly bonded to the FPC board 2, thus completing the fabrication of the piezoresistive sensor for bolt loosening monitoring.

[0055] Example 3

[0056] Weigh 0.18g of CB powder and slowly add it to 20mL of PAA solution, otherwise the same as in Example 2.

[0057] Example 4

[0058] Weigh 0.252g of CB powder and slowly add it to 20mL of PAA solution, otherwise the same as in Example 2.

[0059] Example 5

[0060] Weigh 0.036g of CB powder and slowly add it to 20mL of PAA solution, otherwise the same as in Example 2.

[0061] Example 6

[0062] Weigh 0.108g of CB powder, 0.030g of nano alumina powder and 0.006g of nano clay powder, and slowly add them to 20ml of PAA solution. Other steps are the same as in Example 2.

[0063] Figure 3 illustrates the relationship between the rate of resistance change and the number of cycles for the piezoresistive sensors in Examples 2, 3, and 4 (with CB mass fractions of 3%, 5%, and 7% in sensing layer 1, respectively) under pressure cycling test conditions. The test results show that the bolt loosening monitoring piezoresistive sensor of Example 2 (i.e., CB mass fraction of 3% in sensing layer 1) has the highest sensitivity (resistance change rate / stress), at 0.005 / kN. In contrast, the piezoresistive sensor of Example 5 (conductive filler mass fraction of 1% in sensing layer 1) suffers from excessively low conductive filler content, preventing the formation of a stable conductive network within sensing layer 1, resulting in excessively high resistance and rendering the monitoring system unable to perform effective resistance measurement.

[0064] Figure 4 shows the resistance change trends of the piezoresistive sensors used for bolt loosening monitoring in Examples 2 and 6 at 300°C. The results show that the sensor without nano-alumina and nano-clay experienced a 0.06% decrease in resistance within 1000 s, while the sensor with 1% by mass of nano-alumina and nano-clay mixture in sensing layer 1 experienced a 0.10% decrease in resistance under the same conditions. In comparison, the piezoresistive sensor in Example 6 exhibited smaller resistance fluctuations at high temperatures, demonstrating better overall stability than the piezoresistive sensor in Example 2.

[0065] Figure 6 shows the results of the bolt loosening test in Example 6, conducted on a bolt loosening test platform consisting of an M20 large-size bolt, a 20×40×1.6mm flat washer, and a 150×150×60mm steel block. The test device is shown in Figure 5. During the experiment, the bolt was tightened, and a preload of 100 N·m was applied to the bolt using a digital torque wrench. The result was obtained using the formula... (Where F is the bolt preload, T is the torque, k is the tightening force coefficient, taken as 16.67 here, and d is the nominal bolt diameter) the bolt preload can be calculated to be 30kN. Based on the area of ​​washer 7 (940mm²), 2The pressure exerted on the piezoresistive sensor below gasket 7 can be calculated to be 31.91 MPa. During the loosening process, the bolt torque is gradually adjusted using a digital wrench, causing the bolt to loosen continuously, and the pressure on the sensor gradually decreases accordingly. As the pressure decreases, the spacing between the conductive fillers in the sensing layer 1 of the piezoresistive sensor gradually increases, leading to a decrease in the conductivity of the sensing network and a corresponding increase in the resistance value. The rate of change of sensor resistance has an approximately linear relationship with the applied pressure (i.e., the degree of loosening). When the bolt is completely loosened (preload is 0), the sensor resistance value increases by 2.3%, indicating that this monitoring method can effectively detect the bolt loosening state and achieve real-time monitoring of the reliability of the bolt connection.

[0066] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A wide-range, high-temperature resistant piezoresistive sensor, characterized in that, include: The sensing layer includes a polyimide matrix and conductive fillers, nano-alumina, and nano-clay dispersed in the polyimide matrix; The total mass of the nano-alumina and nano-clay accounts for 0.1-3% of the total mass of the sensing layer, and the mass ratio of the nano-alumina to the nano-clay is (2-10):1; FPC board, the FPC board is provided with conductive components for transmitting electrical signals generated by the sensing layer.

2. The wide-range, high-temperature resistant piezoresistive sensor according to claim 1, characterized in that, The conductive filler is carbon black, and the carbon black accounts for 1 to 20% of the total mass of the sensing layer.

3. The wide-range, high-temperature resistant piezoresistive sensor according to any one of claims 1-2, characterized in that, The piezoresistive sensor has a range of ≥30MPa and a maximum operating temperature of ≥300℃.

4. A method for fabricating a wide-range, high-temperature resistant piezoresistive sensor, wherein the method is used to fabricate the piezoresistive sensor according to any one of claims 1-2, characterized in that, include: A conductive solution is prepared by dispersing conductive filler, nano-alumina, and nano-clay in a matrix solution. The substrate solution is a polyamic acid solution, and the solvent of the polyamic acid solution is N-methylpyrrolidone; the conductive solution is coated on the sensing area of ​​the FPC board to form a conductive solution coating of fixed thickness; The FPC board with a conductive solution coating is heated according to a preset temperature gradient, so that the conductive solution coating is formed into a sensing layer and bonded to the sensing area of ​​the FPC board.

5. The preparation method according to claim 4, characterized in that, During the process of dispersing conductive fillers, nano-alumina, and nano-clay in the matrix solution, mechanical stirring and ultrasonic treatment are used to achieve uniform dispersion of powder materials in the matrix solution. Mechanical stirring was performed for 0.5 to 2 hours at a speed of 1000 r / min, and ultrasonic dispersion was performed for 20 to 30 minutes.

6. The preparation method according to claim 4, characterized in that, The method for heating an FPC board with a conductive solution coating according to a preset temperature gradient includes: placing the FPC board with the conductive solution coating in a hot oven, heating the conductive solution from room temperature to 80°C and holding for 20 minutes, then heating to 120°C and holding for 30 minutes, then heating to 180°C and holding for 30 minutes, then heating to 200°C and holding for 10 minutes, then heating to 220°C and holding for 20 minutes, then heating to 250°C and holding for 30 minutes, and then heating to 280°C and holding for 20 minutes; wherein the heating rate is 5°C / min.

7. The preparation method according to claim 4, characterized in that, The sensing area of ​​the FPC board is a circular structure with a groove in the middle. The sensing layer is formed in the groove, and an electrode material electrically connected to the conductive component is disposed in the groove. The circular structure of the FPC board has a thickness of 100μm and a diameter of 8mm. The groove in the middle of the circular structure has a length and width of 2mm*5mm and a depth of 60μm.

8. An application of a wide-range, high-temperature resistant piezoresistive sensor, characterized in that, The piezoresistive sensor described in any one of claims 1-2 is used to monitor bolt loosening.

9. The application according to claim 8, characterized in that, The sensing layer of the piezoresistive sensor is located between the bolt washer and the edge of the corresponding bolt hole, and is used to monitor the preload of the bolt.

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