Composite gas sensitive material, preparation method thereof and application thereof in detection of SO2, characteristic gas of SF6 insulated electrical equipment

Abstract

This invention relates to the field of gas-sensitive materials technology, and particularly to a composite gas-sensitive material, its preparation method, and its application in detecting SO2, a fault characteristic gas in SF6-insulated electrical equipment. Preparation method: S1, WO3 nanorods are ultrasonically dispersed in deionized water, then tin salt and dilute hydrochloric acid are added. Concentrated ammonia is added dropwise under low-speed stirring to stabilize the pH of the system at 7.0–8.0. Sn(OH)2 is precipitated and coated in situ at room temperature. S2, The reaction solution from step S1 is centrifuged, washed, dried, and then calcined to obtain WO3-SnO2 heterojunction powder. S3, The WO3-SnO2 heterojunction powder is ultrasonically dispersed in DMF, zinc salt is added, followed by a ligand solution and glacial acetic acid, and a hydrothermal reaction is carried out. After the reaction, the powder is centrifuged, washed, and dried. The composite gas-sensitive material of this invention enables accurate detection of SO2 at ppm and even sub-ppm levels under complex SF6 cracking mixed atmospheres.

Description

Technical Field

[0001] This invention relates to the field of gas-sensitive materials technology, and in particular to a composite gas-sensitive material, its preparation method, and its application in detecting SO2, a fault characteristic gas in SF6-insulated electrical equipment. Background Technology

[0002] SF6 gas, with its excellent insulation and arc-quenching properties, is widely used in primary power equipment such as high-voltage circuit breakers and GIS. Internal insulation degradation, partial discharge, and overheating defects in these devices can trigger SF6 decomposition, generating trace amounts of SO2, a characteristic gas. Accurate quantitative detection of trace SO2 is a core technological path for early warning of latent defects in power equipment. Existing commercially available SO2 gas sensors use single-component SO2 or WO3 metal oxides (MOX) as sensing substrates. However, single oxides are limited by insufficient intrinsic carrier migration efficiency and a lack of surface active sites, generally exhibiting shortcomings such as low detection sensitivity, poor selectivity in multi-component coexisting atmospheres, and trace detection limits that fail to meet engineering requirements.

[0003] Existing technologies have developed MOX@MOFs core-shell composite structures to improve sensing performance, leveraging the high specific surface area, tunable pore size, and abundant metal open sites of MOF materials to enhance gas adsorption performance. However, the cores of currently reported MOX@MOFs composites are all made of single-phase metal oxides such as ZnO and In2O3, which cannot achieve intrinsic amplification of sensing signals by utilizing the energy level bending at the heterojunction interface or the space depletion layer modulation effect. At the same time, traditional preparation processes involve one-step solvothermal synthesis of metal oxide substrates with metal salts and organic ligands, resulting in random nucleation of metal ions on the oxide surface. This easily leads to defects such as MOF free crystallization, uneven shell thickness, and local bare substrate, significantly weakening the selective enrichment effect. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned shortcomings of the prior art by proposing a composite gas-sensitive material, its preparation method, and its application in detecting SO2, a fault characteristic gas in SF6-insulated electrical equipment.

[0005] The first objective of this invention is to provide a method for preparing a composite gas-sensitive material, comprising the following steps: S1. Disperse WO3 nanorods ultrasonically in deionized water, then add tin salt and dilute hydrochloric acid. Add concentrated ammonia dropwise while stirring at low speed to adjust the pH of the system to be stable at 7.0-8.0. Stir at room temperature to complete the in-situ precipitation and coating of Sn(OH)2. S2. The WO3-SnO2 heterojunction powder obtained by centrifuging, washing, drying and calcining the reaction solution of step S1; S3. WO3-SnO2 heterojunction powder was ultrasonically dispersed in DMF, metal salt was added, followed by the addition of 2,5-dihydroxyterephthalic acid ligand solution and glacial acetic acid, and then hydrothermal reaction was carried out. After the reaction was completed, the powder was centrifuged, washed and dried to obtain WO3-SnO2@M-MOF-74 core-shell composite gas-sensitive material. M can be any one of zinc, cobalt, or Ni.

[0006] Furthermore, in step S1, the mass ratio of WO3 nanorods to tin salt is 45–55:12–18.

[0007] Furthermore, in step S2, calcination is carried out in an air atmosphere at 430–470°C for 1.5–2.5 h.

[0008] Furthermore, in step S3, the mass ratio of WO3-SnO2 heterojunction powder to metal salt is 18-22:10-14.

[0009] Furthermore, in step S3, the solvent for the ligand solution is a mixture of ethanol, deionized water, and DMF.

[0010] Furthermore, in step S3, the hydrothermal reaction is carried out at 100–120°C for 12–24 h.

[0011] Furthermore, in step S3, the product is dried in a vacuum environment at 140–160°C for 10–14 hours.

[0012] A second objective of this invention is to provide a composite gas-sensitive material prepared by the preparation method described above.

[0013] A third objective of this invention is to provide an application of the composite gas-sensitive material described above in detecting SO2, a fault characteristic gas in SF6-insulated electrical equipment.

[0014] Furthermore, it is used as a gas-sensitive sensing layer in a ceramic-based sintered gas sensor.

[0015] This invention effectively solves the problems of insufficient sensitivity and poor selectivity of existing sensors, and can quickly and accurately detect trace amounts of SO2 characteristic gas generated by faults such as local overheating and discharge in SF6 insulated equipment, providing a reliable technical means for the early diagnosis of latent defects in power equipment.

[0016] This invention successfully synthesizes a core-shell composite gas-sensitive material with a WO3-SnO2 heterojunction core and a Zn-MOF-74 shell using a three-step synthesis technique combining hydrothermal and in-situ solvothermal methods. In this material, the band bending and depletion layer modulation effect at the WO3-SnO2 heterojunction interface provide a high-response semiconductor sensing channel, while the Zn-MOF-74 shell utilizes its open metal sites (Zn...) 2+This technology achieves highly selective adsorption and enrichment of SO2, with the two working synergistically to form a cascade sensing mechanism of "adsorption enrichment—charge transfer—signal amplification," simultaneously addressing the two major industry pain points of low sensitivity and poor selectivity from the material's fundamental source. The gas sensor obtained by coating this gas-sensitive material onto a ceramic tube electrode to form a sensitive layer film, followed by calcination and assembly, achieves a response value of 3.4 for 30 ppm SO2 at the optimal operating temperature of 300℃, with a detection limit as low as 0.5 ppm. These response values ​​are 2.52 times and 2.64 times that of pure WO3 and pure SnO2, respectively.

[0017] This invention proposes for the first time a core-shell coupled structure with an n-type WO3-SnO2 heterojunction as the core and a Zn-MOF-74 molecular sieve as the outer shell, breaking away from the traditional MOX@MOFs single oxide core design concept. One-dimensional WO3 nanorods construct a high-speed electron transport framework, while SnO2 is loaded onto the surface to form a heterojunction interface. A space charge depletion layer is constructed through spontaneous band bending, and the interface carrier concentration changes drastically after gas adsorption, achieving intrinsic amplification of the sensing signal. The outer Zn-MOF-74 layer utilizes unsaturated open Zn²⁺... + The site-specific coordination adsorption of SO2 relies on the sieving effect of molecular sieve channels to isolate interfering impurities such as H2S, CO, and alcohols, thereby enriching and concentrating SO2 gas on the material surface. It innovatively couples the dual advantages of WO3-SnO2 heterojunction internal charge amplification and Zn-MOF-74 outer molecular sieve selective enrichment, constructing a new cascade sensing mechanism integrating shell-directed adsorption and enrichment, heterojunction interface charge separation, and semiconductor channel signal amplification. This enables precise detection of SO2 at ppm and even sub-ppm levels in complex SF6 cracking mixed atmospheres, filling the technological gap in device technology for monitoring trace characteristic gases in the early stages of latent faults in high-voltage electrical equipment. Attached Figure Description

[0018] Figure 1 is a schematic diagram of the preparation process of the WO3-SnO2@Zn-MOF-74 core-shell composite gas-sensitive material of the present invention; Figure 2 shows SEM images of different materials prepared in Example 1, where (a) pure SnO2, (b) pure WO3, (c) WO3-SnO2 heterojunction, and (d) WO3-SnO2@Zn-MOF-74. Figure 3 shows the XRD patterns of the different materials prepared in Example 1; Figure 4 shows a comparison of the responses of different gas-sensitive materials prepared in Example 1 to 30 ppm SO2 at different operating temperatures; Figure 5 shows the response curves of the WO3-SnO2@Zn-MOF-74 gas sensor prepared in Example 1 to different concentrations of SO2; Figure 6 shows the cyclic stability test results of the WO3-SnO2@Zn-MOF-74 gas sensor prepared in Example 1 at the optimal operating temperature. Detailed Implementation

[0019] The following are specific embodiments of the present invention, which are described in conjunction with the accompanying drawings to further illustrate the technical solutions of the present invention. However, the present invention is not limited to these embodiments.

[0020] The preparation method of the WO3-SnO2@Zn-MOF-74 core-shell composite gas-sensitive material of the present invention is as follows: Figure 1 As shown: Step 1: Controllable hydrothermal synthesis of one-dimensional rod-shaped WO3 framework precursor According to the mass ratio: 3.0-3.6 g of sodium tungstate dihydrate and 1.5-2.0 g of potassium sulfate are dispersed together in 50-70 mL of deionized water and mechanically stirred at room temperature until the inorganic salts are completely dissolved; Under continuous stirring, dilute hydrochloric acid with a concentration of 2.5–3.5 mol / L was slowly added dropwise, and the pH of the reaction system was precisely controlled to be 1.2–1.8. The mixture was stirred at a constant temperature for 25–35 min to obtain a clear precursor solution. The precursor solution was then transferred to a polytetrafluoroethylene-lined hydrothermal reactor and kept at a constant temperature of 180–220℃ in a closed hydrothermal environment for 32–40 h. After naturally cooling to room temperature, centrifuge to separate solid and liquid components. Wash with deionized water and anhydrous ethanol alternately 2 to 4 times to remove residual inorganic salts and organic impurities. The washing product was dried in a forced-air oven at 70–90°C for 10–14 h, and then calcined in an air-atmosphere muffle furnace at 430–470°C for 1.5–2.5 h to crystallize and form a one-dimensional WO3 nanorod matrix with controllable aspect ratio and regular morphology.

[0021] Step 2: Preparation of WO3-SnO2 heterojunction (nn) powder Take 0.45–0.55 g of the above WO3 nanorods, disperse them in 45–55 mL of deionized water, and sonicate for 25–35 min to achieve full dispersion and surface activation of the nanorods; Add 0.12–0.18 g of stannous chloride dihydrate and 1.5–2.5 mL of dilute hydrochloric acid, and continue stirring until the solid is completely dissolved; Concentrated ammonia was added dropwise under low-speed stirring to stabilize the pH of the system at 7.0–8.0. The mixture was then allowed to stand at room temperature for 1.5–2.5 h with stirring to complete the in-situ precipitation and coating of Sn(OH)2. Centrifuge to collect the solid product, and wash with deionized water and anhydrous ethanol 2 to 4 times each; After drying at 70–90℃ to remove solvent, the mixture is calcined in air at 430–470℃ for 1.5–2.5 h to obtain WO3-SnO2 heterojunction (nn) powder with uniformly anchored SnO2 nanoparticles and tightly bonded interfaces through in-situ pyrolysis transformation.

[0022] This step precisely controls the SnO2 loading by adjusting the tin salt feeding ratio and regulates the contact barrier at the heterojunction interface by relying on the calcination process, thus providing a basis for energy level regulation for the efficient migration of sensing charges in the future.

[0023] Step 3: In-situ growth of WO3-SnO2@Zn-MOF-74 core-shell composite gas-sensitive powder regulated by glacial acetic acid coordination Weigh 0.18–0.22 g of WO3-SnO2 heterojunction powder and add 16–20 mL of DMF solvent. Disperse the powder ultrasonically for 12–18 min to expose oxygen vacancies and hydroxyl defect sites on the material surface. Add 0.10–0.14 g of zinc nitrate hexahydrate and continue sonication for 12–18 min. The Zn nitrate will be attracted through electrostatic adsorption and hydroxyl group coordination. 2+ Uniformly anchored to the entire surface of the heterojunction, completing the pre-laying of nucleation sites; Dissolve 0.035–0.045 g of 2,5-dihydroxyterephthalic acid organic ligand in a mixture of 0.8–1.2 mL anhydrous ethanol, 0.8–1.2 mL deionized water, and the remainder DMF, and stir until the ligand is completely dissolved. Under magnetic stirring, the ligand solution was added dropwise at a low speed of 0.8–1.2 mL / min to the pretreated suspension; a ternary mixed solvent with a DMF:anhydrous ethanol:H2O ratio of (16–20):(0.8–1.2):(0.8–1.2) was added, and the volume was adjusted to 35–45 mL; 0.5–1.0 mL of glacial acetic acid was added to the system as a coordination competition regulator. The mixture was transferred into a polytetrafluoroethylene-lined autoclave and sealed. It was then kept at a constant temperature of 100–120°C for 12–24 hours for solvothermal treatment. Cool naturally to room temperature, centrifuge at 8000±500 rpm for 8–12 min to collect the solid; wash with DMF 1–3 times to remove free ligands, and wash with anhydrous methanol 2–4 times to replace residual solvent in the pores; The powder was dried in a vacuum environment at 140–160℃ for 10–14 h to obtain a complete WO3-SnO2@Zn-MOF-74 core-shell composite gas-sensitive powder with a shell thickness of 10–100 nm, free MOF impurities, and full coating.

[0024] The ceramic substrate sintered gas sensor is assembled and fabricated as follows: The above-mentioned core-shell composite powder was mixed with deionized water to form a viscous slurry with a mass fraction of 4% to 6%. A brush dip coating process is used to evenly coat the outer wall of the alumina ceramic tube. Ceramic tube structural parameters: inner diameter 0.7~0.9 mm, outer diameter 1.1~1.3 mm, tube length 4.5~5.5 mm, sidewall gold interdigitated electrode width 0.4~0.6 mm; The coated ceramic element is placed in a muffle furnace and calcined at 180–220°C for 1.5–2.5 h to complete the curing and molding of the sensitive film; A nickel-chromium alloy heating wire is inserted into the inner hole of a ceramic tube, and the electrode leads and heating wire are soldered together to a hexagonal metal base. The finished gas sensor is then packaged.

[0025] Example 1 I. Preparation of WO3 nanorods Weigh 3.30 g Na₂WO₄·2H₂O and 1.74 g K₂SO₄, dissolve them in 60 mL of deionized water, and stir until completely dissolved. Under stirring, slowly add 3 mol / L dilute hydrochloric acid to adjust the pH of the solution to 1.5, and continue stirring for 30 min. Transfer the mixed solution to a 100 mL polytetrafluoroethylene high-pressure reactor, heat to 200 °C, and react for 36 h. After the reaction is complete, allow it to cool naturally to room temperature, collect the product by centrifugation, wash it three times each with deionized water and anhydrous ethanol, dry it in an oven at 80 °C for 12 h, and finally calcine it in a muffle furnace at 450 °C for 2 h to obtain WO₃ nanorods.

[0026] II. Preparation of WO3-SnO2 heterojunction substrate Weigh 0.5 g of the prepared WO3 nanorods and disperse them in 50 mL of deionized water. Sonicate the solution for 30 min. Add 0.15 g of SnCl2·2H2O and 2 mL of dilute hydrochloric acid, and stir to dissolve. Under stirring, slowly add ammonia to adjust the pH to 7.5, and continue stirring at room temperature for 2 h. Collect the product by centrifugation, wash three times each with deionized water and anhydrous ethanol, dry in an oven at 80 °C for 12 h, and calcine in a muffle furnace at 450 °C for 2 h to obtain the WO3-SnO2 heterojunction substrate.

[0027] III. Preparation of WO3-SnO2@Zn-MOF-74 core-shell composite gas-sensitive materials 0.2 g of WO3-SnO2 heterojunction powder was added to a beaker containing 18 mL of DMF and ultrasonically dispersed for 15 min. 0.12 g of Zn(NO3)2·6H2O was added, and ultrasonication continued for another 15 min. 0.040 g of 2,5-dihydroxyterephthalic acid was dissolved in a mixed solvent of 1 mL anhydrous ethanol, 1 mL deionized water, and an appropriate amount of DMF. Under magnetic stirring, the ligand solution was slowly added dropwise to the suspension at a rate of approximately 1 mL / min. The DMF / ethanol / water mixed solvent (volume ratio 18:1:1) was added to a total volume of approximately 40 mL, and 0.8 mL of glacial acetic acid was added. The mixture was transferred to a 50 mL polytetrafluoroethylene-lined autoclave, sealed, and placed in an oven at 110 °C for 18 h. After natural cooling, the product was collected by centrifugation (8000 rpm, 10 min), washed twice with DMF, and three times with anhydrous methanol. The WO3-SnO2@Zn-MOF-74 core-shell composite gas-sensitive material was obtained by vacuum drying at 150℃ for 12 h.

[0028] IV. Fabrication of Gas Sensors The gas-sensitive material was mixed with an appropriate amount of deionized water to form a uniform 5 wt% slurry. This slurry was then evenly coated onto a ceramic tube electrode (inner diameter: 0.8 mm, outer diameter: 1.2 mm, tube length: 5 mm, gold electrode width: 0.5 mm) using a brush, forming a uniform sensitive layer film on the electrode surface. The gas-sensitive ceramic tube electrode coated with the gas-sensitive material was then calcined in a muffle furnace at 200°C for 2 hours. A nickel-chromium alloy heating wire was then inserted into the gas-sensitive ceramic tube electrode. Finally, the electrode and heating wire were welded onto a hexagonal base to obtain the gas sensor.

[0029] Example 2 The difference between Example 2 and Example 1 lies in the preparation of the WO3-SnO2@Zn-MOF-74 core-shell composite gas-sensitive material. In Example 2, Zn(NO3)2·6H2O is replaced with an equimolar amount of Co(NO3)2·6H2O to prepare the WO3-SnO2@Co-MOF-74 gas-sensitive material. Everything else is identical to Example 1.

[0030] Example 3 The difference between Example 3 and Example 1 lies in the preparation of the WO3-SnO2@Zn-MOF-74 core-shell composite gas-sensitive material. In Example 3, Zn(NO3)2·6H2O is replaced with an equimolar amount of Ni(NO3)2·6H2O to prepare the WO3-SnO2@Ni-MOF-74 gas-sensitive material. Everything else is identical to Example 1.

[0031] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the preparation step of the WO3-SnO2 heterojunction substrate is omitted, and pure SnO2 is used directly as the gas-sensitive material.

[0032] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the preparation step of the WO3-SnO2 heterojunction substrate is omitted, and pure WO3 is used directly as the gas-sensitive material.

[0033] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the preparation step of the Zn-MOF-74 shell is removed, and WO3-SnO2 heterojunction is used directly as the gas-sensitive material.

[0034] Figure 2 shows SEM images of different materials prepared in Example 1, where (a) pure SnO2, (b) pure WO3, (c) WO3-SnO2 heterojunction, and (d) WO3-SnO2@Zn-MOF-74; (a) pure SnO2 exhibits an irregular particle agglomeration morphology; (b) the raw material WO3 is a well-formed one-dimensional nanorod structure; (c) SnO2 nanoparticles are uniformly attached to the surface of the WO3 rods, forming a tightly bound heterojunction interface; (d) the outer layer of the heterojunction is uniformly coated with a dense MOF shell, with no exposed substrate or free impurities, which confirms from the microscopic morphology that the present invention achieves complete core-shell coating.

[0035] Figure 3 shows the XRD patterns of the different materials prepared in Example 1. The spectra show characteristic diffraction peaks of WO3, SnO2 and Zn-MOF-74 in sequence, with no extra impurity peaks generated, proving that the composite product contains three phases at the same time. No impurities were introduced during the preparation process. From the crystal structure level, it is proved that the WO3-SnO2@Zn-MOF-74 composite material was successfully prepared.

[0036] Gas Sensing Performance Test Optimal operating temperature test A static gas mixing method was used to construct a gas-sensitive performance testing platform. The sensor under test was fixed in a sealed gas-sensitive testing chamber. The operating temperature of the device was controlled in stages by adjusting the output power of the sensor heating wire to 210 ℃, 240 ℃, 270 ℃, 300 ℃, and 330 ℃. The test atmosphere was uniformly configured as 30 ppm SO2 (dry air as the substrate carrier gas). After the gas concentration inside the chamber was uniform and the device resistance stabilized, the resistance change was recorded. The resistance ratio of the sensor in clean air / test gas was used as the gas-sensitive response value. Each group of samples was tested in triplicate, and the average value was taken. The data are summarized in Table 1 and [Table data would be inserted here]. Figure 4 .

[0037] Table 1. Response values ​​of the gas sensor to 30 ppm SO2 at different temperatures.

[0038] As can be seen from the temperature gradient test results in the table above, the response values ​​of all samples show a pattern of first increasing and then decreasing with the increase of operating temperature, and the optimal operating temperature of all devices is concentrated at 300℃. The gas-sensing response of the three MOF-74 coated modified samples (Examples 1-3) is significantly better than that of single metal oxides (Comparative Examples 1 and 2) and the WO3-SnO2 heterojunction without MOF coating (Comparative Example 3) across the entire temperature range, confirming that the synergistic adsorption-signal amplification mechanism of the WO3-SnO2 heterojunction core and the MOF-74 shell can stably improve the SO2 sensing sensitivity. Among them, the Zn-MOF-74 modified sample (Example 1) has the best response performance, followed by the Co-MOF-74 and Ni-MOF-74 modified samples. This indicates that by replacing the central coordination metal of MOF-74, the gas-sensing response capability of the material can be flexibly controlled, and the appropriate model can be selected according to the complex atmosphere conditions on site, further verifying the versatility of the preparation process and the rationality of the structural design of this invention.

[0039] Figure 4 The curves show that the response of all samples first increases and then decreases with increasing temperature, and the optimal operating temperature is uniformly 300℃. Example 1 shows that the response across the entire temperature range is significantly better than that of pure SnO2, pure WO3 and WO3-SnO2 heterojunction without MOF coating, which quantitatively proves the synergistic effect of heterojunction and MOF shell.

[0040] Concentration gradient test Clean, dry air was used as the dilution carrier gas. SO2 standard gas with a gradient concentration of 0.5 ppm to 30 ppm was prepared stepwise using a dynamic gas mixing method. The sensor was fixed at the optimal operating temperature of 300℃. Different concentrations of the gas to be tested were introduced sequentially. After the sensor resistance signal stabilized, the response data was recorded. After a single concentration test was completed, fresh air was introduced to restore the device baseline before the next concentration test was carried out. Each concentration was repeated 3 times in parallel. The test results are shown in Table 2.

[0041] Table 2

[0042] The response values ​​of the three modified sensors decreased synchronously with the decrease of SO2 concentration. In the range of 0.5 to 30 ppm, the response showed a good linear relationship with the logarithm of the gas concentration, and the lowest detection limit could reach 0.5 ppm, which can meet the early detection requirements of trace SO2 in SF6 electrical equipment faults. Among them, Example 1 had the best sensing performance, which further verified that Zn-MOF-74 has a better selective enrichment effect on SO2 than Co-MOF-74 and Ni-MOF-74.

[0043] Figure 5 shows the response curves of the WO3-SnO2@Zn-MOF-74 gas sensor prepared in Example 1 to different concentrations of SO2. Under the optimal condition of 300 °C, as the SO2 concentration gradually increases from 0.5 ppm to 30 ppm, the sensor response increases step by step. The resistance changes rapidly after the gas is introduced and the baseline recovers smoothly after air purging, which intuitively demonstrates the excellent response recovery characteristics of the material and its effective detection over a wide concentration range.

[0044] Cyclic stability test The sensor prepared in Example 1 was kept at its optimal operating temperature of 300 °C. A complete test cycle consisted of "air purging → introducing 30 ppm SO2 → air desorption". After each gas adsorption and desorption process, the resistance was allowed to fully recover to baseline before starting the next test. Three sets of repeatable cycles were continuously completed, and the saturation response values ​​for each cycle were collected. The test data are shown in Table 3 and [Table data would be inserted here]. Figure 6 .

[0045] Table 3

[0046] This indicates that the sensor has excellent repeatability and stability.

[0047] Figure 6 shows the cyclic stability test of the WO3-SnO2@Zn-MOF-74 gas sensor prepared in Example 1 at the optimal operating temperature. The device response amplitude fluctuation was minimal and the baseline repeatability was good during three consecutive adsorption-desorption cycles, confirming that the outer MOF protective layer can effectively suppress the core grain degradation at high temperature, and that the sensor has the structural and performance stability for long-term online monitoring.

[0048] Selective testing The sensor prepared in Example 1 operates at an optimal temperature of 300 °C. Various common SF6 cracking byproducts and interfering gases, including SO2, H2S, CO, CF4, and CH3OH, with a concentration of 30 ppm, were prepared and sequentially introduced into the test chamber using a dynamic gas mixing method. After each gas test, clean air was introduced to completely reset the device baseline, and then the test gas was replaced. Each gas group was tested in parallel three times, and the average response value was taken. The test results are shown in Table 4.

[0049] Table 4. Sensor response data to various interfering gases at 30 ppm

[0050] As shown in the data table, the sensor's response to the target gas SO2 is significantly higher than that of the other four interfering gases, while it shows almost no significant response to impurity gases. This is due to the high-density unsaturated Zn on the surface of the Zn-MOF-74 outer shell. 2+ The open sites can specifically capture SO2 molecules through coordination, while the MOF channels act as molecular sieves, preventing other impurity gases from contacting the sensitive interface of the WO3-SnO2 heterojunction core. This gives the sensor of this invention outstanding anti-interference capability in the complex SF6 cracking and mixing atmosphere, meeting the requirements for accurate SO2 identification and detection under the condition of multiple impurities coexisting on site.

[0051] For any points not covered above, existing technologies shall apply.

[0052] Although specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the direction of the invention or exceeding the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent substitutions, improvements, etc., made to the above embodiments based on the technical essence of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a composite gas-sensitive material, characterized in that, Includes the following steps: S1. Disperse WO3 nanorods ultrasonically in deionized water, then add tin salt and dilute hydrochloric acid. Add concentrated ammonia dropwise while stirring at low speed to adjust the pH of the system to be stable at 7.0-8.

0. Stir at room temperature to complete the in-situ precipitation and coating of Sn(OH)2. S2. The WO3-SnO2 heterojunction powder obtained by centrifuging, washing, drying and calcining the reaction solution of step S1; S3. WO3-SnO2 heterojunction powder was ultrasonically dispersed in DMF, metal M salt was added, and then 2,5-dihydroxyterephthalic acid ligand solution and glacial acetic acid were added and hydrothermal reaction was carried out. After the reaction was completed, the powder was centrifuged, washed and dried to obtain WO3-SnO2@M-MOF-74 core-shell composite gas-sensitive material. M can be any one of zinc, cobalt, or Ni.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of WO3 nanorods to tin salt is 45–55:12–18.

3. The preparation method according to claim 1, characterized in that, In step S2, calcination is carried out in air at 430–470°C for 1.5–2.5 h.

4. The preparation method according to claim 1, characterized in that, In step S3, the mass ratio of WO3-SnO2 heterojunction powder to metal salt is 18-22:10-14.

5. The preparation method according to claim 1, characterized in that, In step S3, the solvent for the ligand solution is a mixture of ethanol, deionized water and DMF.

6. The preparation method according to claim 1, characterized in that, In step S3, the hydrothermal reaction is carried out at 100–120°C for 12–24 hours.

7. The preparation method according to claim 1, characterized in that, In step S3, the product is dried in a vacuum environment at 140–160°C for 10–14 hours.

8. A composite gas-sensitive material prepared by the preparation method according to any one of claims 1-7.

9. The application of the composite gas-sensitive material as described in claim 8 in detecting SO2, a fault characteristic gas in SF6-insulated electrical equipment.

10. The application as described in claim 9, characterized in that, Used as a gas-sensitive sensing layer for sintered gas sensors on ceramic substrates.

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