Fiber bragg grating humidity sensor based on polyimide and MXene composite coating and preparation method of fiber bragg grating humidity sensor

Abstract

The invention provides a preparation method of a fiber bragg grating humidity sensor based on a polyimide and MXene composite coating. The preparation method comprises the following steps: preparing a surface hydrophobic modification solution according to an MPTMS solution, a PAA aqueous solution and a nano SiO2 ethanol dispersion liquid; the method comprises the following steps: sequentially coating polybenzimidazole amic acid, MXene and polybenzimidazole amic acid on the surface of a gate region of the fiber bragg grating, so that a polybenzimidazole amic acid inner layer, an MXene middle layer and a polybenzimidazole amic acid outer layer are sequentially formed on the surface of the gate region from the surface to the outer layer; immersing the coated fiber bragg grating into a surface hydrophobic modification solution to carry out surface hydrophobic modification treatment on the outer layer of the polybenzimidazole amic acid; and carrying out annealing curing treatment on the fiber bragg grating subjected to surface hydrophobic modification treatment to obtain the fiber bragg grating humidity sensor based on the polyimide and MXene composite coating. According to the sandwich structure, through material function layering and interface optimization, collaborative improvement of sensitivity, response speed, stability and environmental adaptability is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensors, and particularly relates to an optical fiber grating humidity sensor based on a polyimide and MXene composite coating and a preparation method thereof. Background Art

[0002] During the operation of a proton exchange membrane fuel cell (PEMFC), the temperature field in the stack and the hydration control of the membrane electrode are crucial for maintaining the integrity of the membrane electrode and the proton conduction efficiency. However, the complex gas-liquid two-phase flow and dynamic operating conditions inside the fuel cell make it difficult to monitor the temperature and humidity.

[0003] In view of the medium requirements for relative humidity measurement, the current sensors used for on-line detection of temperature and humidity in fuel cells are mainly contact sensors such as polymer thin film capacitive sensors. Polymer thin film capacitive temperature and humidity sensors occupy a mainstream position in fuel cell comprehensive test benches due to their low cost, high responsiveness, and easy integration. However, their metal electrode structure is prone to electrochemical corrosion with the condensed liquid water in a high-humidity environment, resulting in a long-term detection drift of more than 5%RH. The high-humidity failure risk and the shortcoming of long-term reliability restrict their application under harsh test conditions.

[0004] Optical fiber grating humidity sensors have significant advantages in anti-electromagnetic interference and anti-chemical corrosion. However, traditional polymer-coated FBG (fiber Bragg grating) humidity sensors mostly use high polymers containing hydrophilic groups such as polyimide or polyvinyl alcohol as the sensitive layer. Such materials can significantly enhance the water molecule adsorption density due to their rich oxygen-containing functional groups. However, the sensing mechanism of such sensors based on physical adsorption requires water molecules to diffuse to the internal sites of the sensitive layer. The diffusion mechanism follows the free volume random diffusion model, and the diffusion time is greatly affected by the size of the free volume holes inside the polymer, resulting in a response / recovery time generally greater than 60 seconds, and the typical humidity sensitivity is only 0.3 - 1.0 pm / %RH, which cannot meet the real-time monitoring requirements for sudden changes in the intake humidity of fuel cells. In addition, in a high-humidity environment above 60°C, the thermal expansion coefficients (CTEs) of the conventional polymer coating and the optical fiber (silica) material are mismatched, usually resulting in the accumulation of swelling stress. The non-uniform deformation of the FBG grating period caused by the swelling effect leads to non-linear drift of the wavelength, increasing the sensing error. The swelling effect under long-term saturated humidity causes changes in the material structure, affecting its moisture absorption and desorption performance.

[0005] The above technical defects directly restrict the engineering applicability of existing humidity sensors in the monitoring of complex temperature and humidity coupling environments, and there is an urgent need to develop a new sensing solution with both high humidity sensitivity and fast response characteristics. Summary of the Invention

[0006] The object of the present invention is to provide a preparation method of an optical fiber grating humidity sensor based on a polyimide and MXene composite coating in view of the defects of the prior art, including the following steps: preparing a surface hydrophobic modification solution from an MPTMS solution, a PAA aqueous solution and a nano-SiO2 ethanol dispersion; sequentially coating polybenzimidazole amic acid, MXene, and polybenzimidazole amic acid on the surface of the grating region of the fiber Bragg grating, so that a polybenzimidazole amic acid inner layer, an MXene middle layer, and a polybenzimidazole amic acid outer layer are formed on the surface of the grating region from the surface to the outer layer in sequence; immersing the fiber Bragg grating after coating in the surface hydrophobic modification solution to perform surface hydrophobic modification treatment on the polybenzimidazole amic acid outer layer; annealing and curing the fiber Bragg grating after surface hydrophobic modification treatment to obtain an optical fiber grating humidity sensor based on a polyimide and MXene composite coating.

[0007] Further, the MXene is Ti3C2Tx.

[0008] Further, the preparation method of the Ti3C2Tx is as follows: mixing Ti3AlC2 with a 50% concentration hydrofluoric acid solution, or Ti2AlC with a 50% concentration hydrofluoric acid solution and reacting, after the reaction is completed, repeating the steps of centrifuging and removing the supernatant and adding deionized water again after centrifuging until the pH of the reaction solution is between 6 and 7, adding ethanol or isopropanol to the reaction solution with the adjusted pH and performing ultrasonic treatment, and drying in vacuum after the ultrasonic treatment is completed to obtain Ti3C2Tx.

[0009] Further, the mass ratio of the mixture of Ti3AlC2 and a 50% concentration hydrofluoric acid solution, or the mixture of Ti2AlC and a 50% concentration hydrofluoric acid solution is 1:2, and the reaction condition after mixing is stirring at room temperature.

[0010] Further, the method of sequentially coating polybenzimidazole amic acid, MXene, and polybenzimidazole amic acid on the surface of the grating region of the fiber Bragg grating is as follows: immersing the pretreated fiber Bragg grating in a polybenzimidazole amic acid slurry, and forming a polybenzimidazole amic acid inner layer on the surface of the grating region of the fiber Bragg grating by dip coating, then immersing the fiber Bragg grating coated with the polybenzimidazole amic acid inner layer in an anhydrous ethanol mixture of MXene, so that MXene powder is deposited on the surface of the polybenzimidazole amic acid inner layer to form an MXene middle layer, and finally immersing the fiber Bragg grating coated with the polybenzimidazole amic acid inner layer and the MXene middle layer in the polybenzimidazole amic acid slurry again, and forming a polybenzimidazole amic acid outer layer on the surface of the MXene middle layer by dip coating; the pretreatment is to perform hydroxylation treatment and silane coupling agent modification treatment on the surface of the grating region of the fiber Bragg grating.

[0011] Further, the hydroxylation treatment is to immerse the fiber Bragg grating grating region in an acidic piranha solution, and the acidic piranha solution is a mixed solution of concentrated sulfuric acid with a content of 98% and a volume ratio of 70% and hydrogen peroxide solution with a content of 30% and a volume ratio of 30%; the silane coupling agent modification treatment is to immerse the fiber Bragg grating grating region after hydroxylation treatment in a silane coupling agent solution, and the silane coupling agent solution is a mixed solution of anhydrous ethanol with a volume ratio of 77%, γ-aminopropyltriethoxysilane with a volume ratio of 2.6%, and deionized water with a volume ratio of 20.4%.

[0012] Further, the specific method for preparing the surface hydrophobic modification solution from the MPTMS solution, PAA aqueous solution and nano-SiO2 ethanol dispersion is as follows: Add MPTMS to the acetate buffer solution with a pH value of 5 according to a volume ratio of 1:1, and stir at room temperature to obtain a pre-hydrolyzed MPTMS solution; Mix the 10wt% PAA aqueous solution and EDC according to a volume ratio of 5:1 to 6:1 under ice bath conditions, and stir under ice bath conditions to obtain an activated PAA aqueous solution; Mix the nano-SiO2 powder and anhydrous ethanol according to a mass ratio of 1:8 to 1:9 under ice bath conditions, stir and ultrasonicate under ice bath conditions to obtain a nano-SiO2 ethanol dispersion; Under ice bath conditions, the pre-hydrolyzed MPTMS solution, nano-SiO2 ethanol dispersion, and activated PAA aqueous solution are in a mass ratio of 9:2:3. Add the pre-hydrolyzed MPTMS solution and nano-SiO2 ethanol dispersion to the activated PAA solution in sequence, stir and perform vacuum degassing treatment to obtain a surface hydrophobic modification solution.

[0013] Further, the specific method for the annealing and curing treatment is: Heat at a heating rate of 1°C / min to 3°C / min, and keep warm at temperatures of 80°C, 110°C, 150°C, 200°C and 250°C respectively, and keep warm for 40 - 60 minutes each time.

[0014] A fiber grating humidity sensor with a polyimide and MXene composite coating prepared by a preparation method of a fiber grating humidity sensor with a polyimide and MXene composite coating.

[0015] The beneficial effects of the present invention are as follows:

[0016] 1. The present invention selects polybenzimidazole imide as the hydrophilic matrix. The benzimidazole ring (-NH- and =N-) in the main chain of polybenzimidazole imide forms dense hydrogen bond sites and hierarchical pores with its unique double-nitrogen ring structure. This structure enables the FBG coating layer based on polyimide materials to follow the dual-mode Fickian diffusion model for water molecule transport. Specifically, water molecules are transported in the optical fiber coating layer through two independent and parallel mechanisms: fast diffusion dominated by mesopores (Fickian mode) and adsorption-diffusion dominated by micropores (Langmuir mode). In an environment with changing relative humidity, rapid diffusion of water molecules is initially dominated by mesopores (2 - 5 nm) and polar sites, and adsorption and storage are achieved by micropores (<2 nm) in the later stage. Compared with the free volume model diffusion of conventional polyimide coating layers, it has higher water absorption and humidity response characteristics.

[0017] 2. Ti3C2Tx MXene powder has abundant polar surface functional groups, and its surface is mainly covered with high-density -OH and -O groups after etching. This highly polar surface greatly improves the adsorption activation ability for water molecules. And Ti3C2Tx MXene has a unique interlayer water molecule transport channel, and the confinement effect between its two-dimensional sheets promotes the rapid intercalation of water molecules and the formation of hydrogen bond networks, enabling the prepared humidity sensor to exhibit higher sensitivity.

[0018] 3. Although MXene has excellent hydrophilicity and interlayer water molecule transport ability, its surface-rich -OH, -O and other functional groups are prone to oxidation or structural degradation in a high-humidity environment. By forming a "sandwich" structure, the outer coating of polybenzimidazole imide can effectively isolate the direct contact between MXene and the external environment, preventing the performance degradation of MXene due to oxidation or hydrolysis, thereby improving the long-term stability of the sensor.

[0019] 4. The inner layer of polybenzimidazole imide is modified by hydroxylation treatment and silane coupling agent to form strong chemical bonds with the optical fiber surface to ensure the interfacial bonding force; the middle layer of MXene serves as the core sensitive layer, and its two-dimensional interlayer confinement effect and polar functional groups are used to achieve rapid adsorption / desorption of water molecules; the outer layer of polybenzimidazole imide adjusts the water molecule diffusion kinetics through hierarchical pores (mesopores + micropores) to form a dual-mode transport mechanism of "rapid adsorption - slow release" (synergy of Fickian diffusion and Langmuir adsorption), avoiding instantaneous overload or desorption lag of water molecules, thereby improving the response speed.

[0020] 5. The thermal expansion coefficients of polybenzimidazole imide and MXene are significantly different. The sandwich structure wraps MXene with symmetric inner and outer layers of polybenzimidazole imide, which can balance the interlayer stress distribution during the annealing and curing process, reduce coating cracking or interfacial peeling caused by temperature changes, and ensure the reliability of the sensor in high-temperature and high-humidity environments.

[0021] 6. After the outer layer of polybenzimidazole amic acid is subjected to composite hydrophobic modification with MPTMS / PAA / SiO2 and then annealed and cured to form the outer layer of polybenzimidazole imide, the contact angle reaches 108°, forming a hydrophobic barrier. This design can not only prevent liquid water from directly penetrating into the MXene layer and causing swelling failure, but also allow gaseous water molecules to diffuse through micropores to the MXene layer, realizing the anti-condensation function in high-humidity environments (such as 95% RH), and avoiding measurement errors caused by surface condensation of traditional sensors.

[0022] 7. As a two-dimensional material, MXene has low mechanical strength when forming a film alone and is prone to cracking due to external forces or environmental vibrations. The sandwich structure significantly improves the shear resistance and cyclic service life of the coating through the physical support of the inner and outer polybenzimidazole imide layers, and is particularly suitable for long-term monitoring under the dynamic flow field inside fuel cells. Description of the Drawings

[0023] Figure 1 Schematic structural diagram of the fiber Bragg grating humidity sensor based on the composite coating of polyimide and MXene prepared in the embodiment of the present invention;

[0024] Figure 2 Humidity sensitivity curve of the fiber Bragg grating humidity sensor based on the composite coating of polyimide and MXene prepared in the embodiment of the present invention;

[0025] Figure 3 Humidity sensitivity curve of the fiber Bragg grating humidity sensor based on polyimide prepared in Comparative Example 1 of the present invention;

[0026] Figure 4 Humidity response curve of the fiber Bragg grating humidity sensor based on the composite coating of polyimide and MXene prepared in the embodiment of the present invention from 25% RH to 95%;

[0027] Figure 5 Test comparison diagrams of the water contact angles of the fiber Bragg grating humidity sensor (a) with surface hydrophobic treatment in the embodiment and the fiber Bragg grating humidity sensor (b) without surface hydrophobic treatment in Comparative Example 2, respectively.

[0028] Reference numerals: 1 - optical fiber; 2 - fiber Bragg grating; 3 - polyimide coating layer; 4 - MXene coating layer. Detailed Embodiments

[0029] Unless otherwise specified, the drugs used in the following examples are commercially available products, and the methods used are conventional methods in the art. The present invention will be further described below through examples and comparative examples.

[0030] Example

[0031] Prepare titanium carbide (Ti3C2Tx) MXene powder, and the preparation steps are as follows:

[0032] Mix the MAX phase powder (Ti3AlC2 or Ti2AlC) with a 50% concentration hydrofluoric acid solution in a polytetrafluoroethylene (PTFE) container at a mass ratio of 1:2;

[0033] Control the reaction conditions at room temperature, ensure uniform contact of the reactants by magnetic stirring, the rotation speed is 200 - 500 rpm, and the etching time is 12 - 24 h;

[0034] Dilute the reaction solution with deionized water, put the reaction solution into a centrifuge, the rotation speed is 4000 - 5000 rpm, and the centrifugation time is 5 - 10 min;

[0035] After centrifugation, remove the supernatant, add deionized water to resuspend the precipitate, and repeat until pH≈6 - 7 (neutralize residual HF);

[0036] Add ethanol or isopropanol and ultrasonically treat for 10 - 30 min to strip the residual substances between the MXene layers;

[0037] Under vacuum conditions, set the temperature to 60℃ - 80℃ and dry for 12 - 24 hours to finally obtain Ti3C2Tx MXene powder.

[0038] Ti3C2Tx MXene (T represents surface functional groups –OH, –O, –F, etc.) powder has rich polar surface functional groups. After etching, its surface is mainly covered with high-density –OH and –O groups (accounting for >60%). However, for other MXenes (such as Nb2CTx, V2CTx), due to synthesis condition limitations, the surface contains Cl or S residual impurities, resulting in a decrease in the density of hydrophilic groups (such as the –OH content of Mo2CTx is only 40%). This highly polar surface greatly improves the adsorption and activation ability of water molecules. And Ti3C2Tx MXene has a unique interlayer water molecule transport channel, and the confinement effect between its two-dimensional sheets promotes the rapid intercalation of water molecules and the formation of a hydrogen bond network. In summary, the Ti3C2Tx-based humidity sensor can exhibit higher sensitivity.

[0039] Prepare a surface hydrophobic modification solution according to the MPTMS solution, PAA aqueous solution and nano-SiO2 ethanol dispersion liquid. The specific method is as follows:

[0040] MPTMS (3-mercaptopropyl-triethoxysilane) was added to an acetate buffer solution with a pH value of 5 at a volume ratio of 1:1, and stirred at 300 rpm for 30 min at room temperature to obtain a pre-hydrolyzed MPTMS solution;

[0041] In an ice bath, 10 wt % PAA (polyacrylic acid) aqueous solution and EDC (carboxyl activator) were mixed in a volume ratio of 6:1 and stirred thoroughly to obtain an activated PAA aqueous solution;

[0042] Aerosil@200 powder (nano-SiO2 powder) was mixed with anhydrous ethanol at a mass ratio of 1:9 under ice bath conditions, stirred at 500 rpm for 30 min, and ultrasonically treated for 30 min to obtain a nano-SiO2 ethanol dispersion.

[0043] Under ice bath conditions, the pre-hydrolyzed MPTMS solution, nano-SiO2 ethanol dispersion, and activated PAA aqueous solution were slowly added to the activated PAA solution in a mass ratio of 9:2:3, and after mixing, stirred at 800 rpm for 15 minutes. Vacuum degassing was performed for 10 minutes to eliminate microbubbles and obtain a surface hydrophobic modified solution.

[0044] The pretreatment of the fiber Bragg grating area mainly includes chemical cleaning, hydroxylation function enhancement of the grating area surface and directional modification of coupled polar molecules. The steps of grating area pretreatment are as follows:

[0045] Place the fiber Bragg grating wiped with alcohol in a cleaned and dried glass container, and fix the two ends of the fiber Bragg grating on the wall of the glass container;

[0046] Pour the prepared acidic piranha solution (70% by volume of 98% concentrated sulfuric acid and 30% by volume of 30% hydrogen peroxide solution) until the fiber Bragg grating is immersed, and after leaving it at room temperature for 15-25 minutes, use a pipette to absorb the waste liquid in the glass container, and repeatedly rinse the fiber Bragg grating with deionized water for more than three times, and then rinse the grating surface with nitrogen to remove the residual liquid on the surface to obtain a hydroxylated fiber Bragg grating.

[0047] On the one hand, the above treatment utilizes the strong oxidizing property of the acidic piranha solution to remove organic pollutants on the surface of the cladding, prevents organic residues from interfering with the interface bonding, and is more conducive to the uniformity of the coating; on the other hand, by controlling the treatment time, an etching reaction can occur on the surface of the fiber Bragg grating region to generate high-density silanol (-OH), significantly increasing the surface energy and enhancing the subsequent chemical bonding between the fiber Bragg grating region surface and the coating layer, thereby improving the interface cross-linking ability.

[0048] Immerse the fiber Bragg grating treated with acidic piranha solution into the prepared silane coupling agent solution (anhydrous ethanol with a volume ratio of 77%, γ-aminopropyltriethoxysilane with 2.6%, and deionized water with 20.4%), and take it out after maintaining at room temperature for 30 minutes;

[0049] After treating the fiber Bragg grating with the silane coupling agent solution, conduct a drying treatment: specifically, put the fiber Bragg grating into a drying oven, set the temperature to 50 °C, and the placement time to 20 minutes.

[0050] As an organosilicon compound, the silane coupling agent itself contains a large number of hydrophilic groups (such as amino groups), which can form chemical bonds with the surface of the fiber Bragg grating grating region, enhance the adsorption ability of the fiber Bragg grating grating region surface to water molecules, and increase the humidity sensitivity. Secondly, the silane coupling agent can undergo a coupling reaction with the humidity-sensitive material (polymer), enhance the bonding between the fiber Bragg grating grating region and the sensitive layer, and further improve the lifespan and humidity sensitivity of the humidity sensor.

[0051] First, coat the fiber Bragg grating with polybenzimidazole amic acid slurry, and specifically proceed according to the following steps:

[0052] (1) Vertically fix the pretreated fiber Bragg grating in a customized glass container (length L: 30 mm, inner diameter of the upper bottom r: 0.5 mm, inner diameter of the lower bottom 0.1 mm, outer diameter R: 1 mm). The tail end of the fiber Bragg grating is fixed on the clamping arm of the dip-coating and lifting coating instrument, and the container is filled with polybenzimidazole amic acid slurry;

[0053] (2) Immerse in the polybenzimidazole amic acid slurry for two minutes, and control the fiber Bragg grating to be lifted at a speed of 180 - 200 μm / s by a stepping motor;

[0054] (3) Adjust the stepping motor to make the fiber Bragg grating immerse into the polybenzimidazole amic acid slurry at a speed of 200 - 240 μm / s;

[0055] (4) Repeat the above steps 2 times to complete the coating of the first 3 layers of polybenzimidazole amic acid on the fiber Bragg grating grating region.

[0056] Conduct MXene coating on the surface of the polybenzimidazole amic acid coating, and specifically proceed according to the following steps:

[0057] (1) Mix the MXene powder prepared in the above steps with anhydrous ethanol in a beaker at a mass ratio of 1:20, with a magnetic stirring speed of 300 - 500 rpm and a time of 5 - 10 minutes;

[0058] (2) Immerse the fiber Bragg grating coated with polybenzimidazole amic acid into the MXene solution prepared in step (1) for 1-2 hours to deposit MXene powder on the fiber Bragg grating, forming an MXene humidity-sensitive layer.

[0059] Refer to the above polybenzimidazole amic acid coating steps to coat the fiber Bragg grating coated with the MXene coating with polybenzimidazole amic acid again, so that a polybenzimidazole amic acid inner layer, an MXene middle layer, and a polybenzimidazole amic acid outer layer are formed from the inside to the outside on the surface of the grating area, as Figure 1 shown.

[0060] Immerse the fiber Bragg grating into the surface hydrophobic modification solution prepared according to the above steps to perform surface hydrophobic modification treatment on the polybenzimidazole amic acid outer layer;

[0061] Perform annealing and curing treatment on the fiber Bragg grating after surface hydrophobic modification treatment to imidize the polybenzimidazole amic acid to form polybenzimidazole imide, forming a polybenzimidazole imide inner layer, an MXene middle layer, and a polybenzimidazole imide outer layer, and obtain a fiber grating humidity sensor based on a polyimide and MXene composite coating. Specifically, the imidization of polybenzimidazole amic acid is completed through a multi-stage heating process. The multi-stage heating and curing process includes 5 heat preservation stages (80 °C, 110 °C, 150 °C, 200 °C, and 250 °C), with each stage continuously heat-preserved for 40-60 minutes, and the heating rate between different stages is 1 °C / min; the stress influence brought by the previous curing can be released through the annealing temperature and time, and finally the influence brought by the hysteresis effect at high temperature can be reduced.

[0062] The present invention selects polybenzimidazole imide as the hydrophilic matrix. The benzimidazole ring (-NH- and =N-) in the main chain of polybenzimidazole imide forms dense hydrogen bond sites and hierarchical pores with its unique double-nitrogen ring structure. This structure enables the FBG coating layer based on polyimide materials to follow the dual-mode Fickian diffusion model in the water molecule transmission. Specifically, water molecules are transported in the fiber coating layer through two independent and parallel mechanisms: fast diffusion dominated by mesopores (Fickian mode) and adsorption-diffusion dominated by micropores (Langmuir mode). In an environment with changing relative humidity, initially, the fast diffusion of water molecules is dominated by mesopores (2-5 nm) and polar sites, and later, micropores (<2 nm) achieve adsorption and storage. Compared with the free volume model diffusion of the conventional polyimide coating layer, polybenzimidazole imide has higher water absorption and humidity response characteristics.

[0063] The structure of the fiber Bragg grating humidity sensor based on polyimide coating prepared in this example is as Figure 1 shown.

[0064] Comparative Example 1

[0065] The difference between Comparative Example 1 and the above-mentioned embodiment is that the fiber Bragg grating is not subjected to MXene coating treatment, and the other steps are the same.

[0066] Comparative Example 2

[0067] The difference between Comparative Example 2 and the above-mentioned embodiment is that the fiber Bragg grating is not subjected to surface hydrophobic treatment, and the other steps are the same.

[0068] Performance test

[0069] (1) Place the sensors prepared in the examples and comparative examples in a test chamber, and conduct an initial inspection on the test piece under normal atmospheric conditions in the laboratory;

[0070] (2) Adjust the temperature of the test chamber to 20 °C and the humidity B (the initial value of B is 15% RH), and keep it for 30 min to make the test piece and the chamber reach a temperature and humidity stable state. Record the temperature and humidity values in the chamber and the demodulated temperature and humidity values of the temperature and humidity integrated sensor every 2 min for 10 min, and save all the central wavelength values of all channels stored by the optical performance monitoring module during the stable state period;

[0071] (3) Keep the temperature unchanged, increase the humidity by 25% RH, and repeat step (2) and record the results;

[0072] (4) Keep the temperature unchanged, increase the humidity by 35% RH, and repeat step (2) and record the results;

[0073] (5) Keep the temperature unchanged until the humidity reaches 45% RH, and repeat step (2) and record the results;

[0074] (6) Keep the temperature unchanged until the humidity reaches 55% RH, and repeat step (2) and record the results;

[0075] (7) Keep the temperature unchanged until the humidity reaches 65% RH, and repeat step (2) and record the results;

[0076] (8) Keep the temperature unchanged until the humidity reaches 75% RH, and repeat step (2) and record the results;

[0077] (9) Keep the temperature unchanged until the humidity reaches 85% RH, and repeat step (2) and record the results;

[0078] (10) Adjust the temperature in the test chamber to return to normal temperature and humidity, keep it for 1 h, then open the chamber door to make the sensor return to a stable state under normal atmospheric conditions in the laboratory;

[0079] The test results are as Figure 2 、 Figure 3 shown. From Figure 2As can be seen, the sensitivity of the fiber Bragg grating humidity sensor with a "sandwich" composite structure of polyimide-MXene-polyimide prepared in the embodiment of the present invention is 9.46 pm / %RH, and the fitting degree reaches 0.9996. While the sensitivity of the fiber Bragg grating humidity sensor prepared in the comparative example of the present invention is 1.19 pm / %RH. In the case of the polyimide coating layer with the same thickness, the sensitivity coefficient of the fiber Bragg grating humidity sensor prepared in the embodiment of the present invention is about 8 times that of the conventional polyimide-coated humidity sensor (i.e., the humidity sensor prepared in the comparative example).

[0080] To test the humidity response ability of the fiber optic humidity sensor based on the polyimide-MXene-polyimide "sandwich" composite structure, the specific steps of the humidity response time test scheme are as follows:

[0081] The humidity response uses T 63 to define the response time, and T 63 is equal to the time constant in a first-order linear time-invariant system. Therefore, with the temperature remaining constant, when the sensor is switched from a humidity environment of 25%RH to a humidity environment of 95%RH, the time it takes for the measured value of the sensor to reach 63% of the final value within one time constant is the humidity response time.

[0082] Specific test steps:

[0083] (1) Place the fiber optic humidity sensor based on the polyimide-MXene-polyimide "sandwich" composite structure prepared in the embodiment into the humidity test chamber, and at the same time place a conical flask of saturated potassium sulfate solution with a humidity environment of 95%RH in the humidity calibration chamber;

[0084] (2) Adjust the humidity calibration chamber to 25°C and 25%RH;

[0085] (3) After the humidity environment in the calibration chamber is stable, the upper computer starts to record the sensor wavelength for 5 minutes;

[0086] (3) Quickly transfer the sensor probe to the 95%RH conical flask;

[0087] (4) Read the wavelength saving program of the upper computer, subtract the intermediate operation time from the time when transferred from the 25%RH environment to the 95%RH environment, and then read the response time value T 63 ;

[0088] Figure 4 is the humidity response curve of the fiber optic humidity sensor based on the polyimide-MXene-polyimide "sandwich" composite structure from 25%RH to 95%. As shown in the figure, its T 63The response time is 7 s, further demonstrating the fast steady-state response performance of the fiber optic humidity sensor based on the polyimide-MXene-polyimide "sandwich" composite structure.

[0089] The present invention also tests the hydrophobic properties of the copolymer network formed by polyacrylic acid and polyimide. Figure 5 (a) and (b) are respectively the water contact angle test diagrams of the fiber optic humidity sensor in the example, i.e., the sensor with surface hydrophobic treatment, and Comparative Example 2, i.e., the sensor without surface hydrophobic treatment. The water contact angles of the sensors in the example and Comparative Example 2 are 69° and 108° respectively. It can be seen that the amphoteric copolymer network formed by polyacrylic acid and polyimide can effectively improve the surface hydrophobic properties of the humidity-sensitive film of the sensor, thus ensuring that liquid water droplets cannot stay on the surface of the sensor, and greatly enhancing the anti-dew condensation ability of the sensor in a high-humidity environment.

[0090] The above-described embodiments are only used to illustrate the technical solutions of the present application, and are not intended to limit the same; although the present application has been described in detail with reference to the foregoing embodiments, 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 on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included within the protection scope of the present application.

Claims

1. A preparation method of a fiber Bragg grating humidity sensor based on a polyimide and MXene composite coating, characterized in that, It includes the following steps: Prepare a surface hydrophobic modification solution from MPTMS solution, PAA aqueous solution and nano-SiO2 ethanol dispersion; Coat polybenzimidazole amic acid, MXene, and polybenzimidazole amic acid on the surface of the fiber Bragg grating in the grating area in sequence, so that a polybenzimidazole amic acid inner layer, an MXene middle layer, and a polybenzimidazole amic acid outer layer are formed from the surface to the outer layer on the surface of the grating area; Immerse the fiber Bragg grating after coating in the surface hydrophobic modification solution to perform surface hydrophobic modification treatment on the polybenzimidazole amic acid outer layer; Perform annealing and curing treatment on the fiber Bragg grating after surface hydrophobic modification treatment to obtain a fiber grating humidity sensor based on a polyimide and MXene composite coating.

2. The preparation method of the fiber Bragg grating humidity sensor based on the polyimide and MXene composite coating according to claim 1, characterized in that: The MXene is Ti3C2Tx.

3. The preparation method of the fiber Bragg grating humidity sensor based on the polyimide and MXene composite coating according to claim 2, wherein The preparation method of the Ti3C2Tx is as follows: Mix Ti3AlC2 with a 50% concentration hydrofluoric acid solution, or Ti2AlC with a 50% concentration hydrofluoric acid solution and react. After the reaction is completed, centrifuge multiple times and remove the supernatant after centrifugation and re-add deionized water until the pH of the reaction solution is between 6 and 7. Add ethanol or isopropanol to the reaction solution with adjusted pH and perform ultrasonic treatment. After the ultrasonic treatment is completed, dry it under vacuum to obtain Ti3C2Tx.

4. The preparation method of the fiber Bragg grating humidity sensor based on the polyimide and MXene composite coating according to claim 3, wherein: The mass ratio of the mixture of Ti3AlC2 and a 50% concentration hydrofluoric acid solution, or the mixture of Ti2AlC and a 50% concentration hydrofluoric acid solution is 1:2, and the reaction condition after mixing is stirring at room temperature.

5. The preparation method of the fiber Bragg grating humidity sensor based on the polyimide and MXene composite coating according to claim 1, characterized in that, The method of coating polybenzimidazole amic acid, MXene, and polybenzimidazole amic acid on the surface of the fiber Bragg grating in the grating area in sequence is as follows: Immerse the pretreated fiber Bragg grating in the polybenzimidazole amic acid slurry, and form a polybenzimidazole amic acid inner layer on the surface of the fiber Bragg grating grating area by dip coating. Then immerse the fiber Bragg grating coated with the polybenzimidazole amic acid inner layer in the anhydrous ethanol mixture of MXene, so that the MXene powder deposits on the surface of the polybenzimidazole amic acid inner layer to form an MXene middle layer. Finally, immerse the fiber Bragg grating coated with the polybenzimidazole amic acid inner layer and the MXene middle layer in the polybenzimidazole amic acid slurry again, and form a polybenzimidazole amic acid outer layer on the surface of the MXene middle layer by dip coating; The pretreatment is to perform hydroxylation treatment and silane coupling agent modification treatment on the surface of the fiber Bragg grating grating area.

6. The preparation method of the fiber Bragg grating humidity sensor based on the polyimide and MXene composite coating according to claim 5, characterized in that: The hydroxylation treatment is to immerse the fiber Bragg grating grating area in an acidic piranha solution, and the acidic piranha solution is a mixed solution of concentrated sulfuric acid with a content of 98% and a volume ratio of 70% and hydrogen peroxide solution with a content of 30% and a volume ratio of 30%; The silane coupling agent modification treatment is to immerse the fiber Bragg grating grating area after hydroxylation treatment in a silane coupling agent solution, and the silane coupling agent solution is a mixed solution of anhydrous ethanol with a volume ratio of 77%, γ-aminopropyltriethoxysilane with a content of 2.6%, and deionized water with a content of 20.4%.

7. The preparation method of the fiber Bragg grating humidity sensor based on the polyimide and MXene composite coating according to claim 1, characterized in that The specific method for preparing the surface hydrophobic modification solution from the MPTMS solution, PAA aqueous solution and nano-SiO2 ethanol dispersion is as follows: MPTMS was added to the acetic acid buffer solution with a pH value of 5 according to a volume ratio of 1:1, and stirred at room temperature to obtain a pre-hydrolyzed MPTMS solution; Under ice bath conditions, 10 wt% aqueous PAA solution and EDC were mixed according to a volume ratio of 5:1 - 6:1, and stirred under ice bath conditions to obtain an activated aqueous PAA solution; Under ice bath conditions, nano-SiO₂ powder and absolute ethanol were mixed according to a mass ratio of 1:8 - 1:9, and stirred and ultrasonicated under ice bath conditions to obtain a nano-SiO₂ ethanol dispersion; Under ice bath conditions, the pre-hydrolyzed MPTMS solution, nano-SiO₂ ethanol dispersion, and activated aqueous PAA solution were in a mass ratio of 9:2:

3. The pre-hydrolyzed MPTMS solution and nano-SiO₂ ethanol dispersion were successively added to the activated PAA solution, stirred and vacuum degassed to obtain a surface hydrophobic modified solution.

8. The preparation method of the fiber Bragg grating humidity sensor based on the polyimide and MXene composite coating according to claim 1, characterized in that, The specific method of the annealing and curing treatment is as follows: heating at a heating rate of 1 °C / min - 3 °C / min, and holding at temperatures of 80 °C, 110 °C, 150 °C, 200 °C, and 250 °C respectively, with each holding time being 40 - 60 min.

9. The preparation method of the fiber Bragg grating humidity sensor based on the polyimide and MXene composite coating according to claim 5, characterized in that, The preparation method of the anhydrous ethanol mixed solution of MXene is: mixing MXene and absolute ethanol according to a mass ratio of 1:

20.

10. A fiber Bragg grating humidity sensor with a polyimide and MXene composite coating prepared by the preparation method of the fiber Bragg grating humidity sensor based on a polyimide and MXene composite coating according to any one of claims 1 - 9.

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