Methane gas sensor with double-layer structure

Abstract

The invention discloses a methane gas sensor with a double-layer structure, and belongs to the technical field of gas sensors, the sensor comprises a six-pin tube seat, a tube cap and a ceramic tube substrate, the ceramic tube substrate is provided with two annular gold electrodes which are distributed in parallel; a platinum wire lead is led out from the annular gold electrode, and a heating wire is arranged in the center of the ceramic tube; the outer wall of the ceramic tube is coated with a sensitive inner layer and an anti-toxic outer layer, the sensitive inner layer is made of a tin dioxide material of which the surface is grafted with silicon dioxide, and the anti-toxic outer layer is made of a tin dioxide-loaded material. According to the scheme, the organic silicon poisoning resistance can be enhanced, and the methane detection sensitivity is improved.

Description

Technical Field

[0001] This application relates to the field of gas sensors, and particularly to a methane gas sensor with a double-layer structure. Background Art

[0002] Methane, as the main component of natural gas and coal gas, has been widely used in fields such as power generation, industrial production, chemical synthesis, and residential fuel due to its clean and efficient energy characteristics. With the rapid development of social economy and the continuous optimization of the energy structure, the usage of methane continues to climb, and its application fields are also expanding day by day. However, in sharp contrast, traditional gas monitoring technologies have gradually exposed many technical bottlenecks in the actual application process and are difficult to meet the growing safety monitoring requirements. Existing gas sensors generally have a series of technical defects such as low detection sensitivity, poor selectivity, narrow applicable range, and poor reliable stability. These problems seriously restrict the large-scale application of gas sensors in the field of safety monitoring and detection, making many potential safety hazards unable to be timely and effectively warned and controlled.

[0003] In the actual application environment, gas sensors are often interfered by various complex factors. Among them, the poisoning effects of substances such as halogens, organosilicon, hydrogen sulfide (H 2 S), nitrogen oxides (NO x ), and sulfur oxides (SO x ) on the catalytic materials of the sensors have been confirmed by a large number of studies. These substances have been listed as poisons in multiple sensor industry standards and seriously affect the performance and lifespan of the sensors. Particularly noteworthy is that organosilicon is recognized as one of the most serious toxic substances for semiconductor metal oxide gas sensors. Among them, hexamethyldisiloxane (HMDSO) has become a typical representative species of organosilicon poisoning due to its wide presence in industrial production and indoor household environments and its significant destructive effect on sensor performance. HMDSO can undergo irreversible chemical reactions with the sensitive materials of the sensor, resulting in the coverage or destruction of its active sites, thereby causing a significant decrease in sensor sensitivity or even complete failure. However, unfortunately, the research on how to enhance the anti-organosilicon poisoning ability of sensors through specific gas-sensitive material design in the existing technology is extremely limited, and there is a lack of effective solutions. This leads to the sensors being prone to poisoning and failure in the actual application environment, especially in environments containing organosilicon pollutants such as HMDSO, and unable to provide reliable safety guarantees.

[0004] Therefore, there is an urgent need for a technical solution that can enhance the anti-organosilicon poisoning ability and improve the detection sensitivity to methane. Summary of the Invention

[0005] To address the deficiencies of the prior art, an embodiment of the present application discloses a methane gas sensor with a double-layer structure. The present application solves the technical problems such as insufficient resistance to organosilicon poisoning in the prior art.

[0006] An embodiment of the present application discloses a methane gas sensor with a double-layer structure, including: a six-pin socket, a tube cap, and a ceramic tube substrate. Two parallel annular gold electrodes are provided on the ceramic tube substrate; platinum wire leads are led out from the annular gold electrodes, and a heating wire is provided at the center of the ceramic tube; the outer wall of the ceramic tube is coated with a sensitive inner layer and an anti-poisoning outer layer, where the sensitive inner layer is a tin dioxide material grafted with silica on the surface, and the anti-poisoning outer layer is a material loaded with tin dioxide.

[0007] In a possible implementation, the component weight ratio of the sensitive inner layer is: tin dioxide grafted with silica on the surface is the matrix material; the addition amount of alumina is 3% by mass fraction; the mass ratio of platinum oxide to palladium oxide is 1:1, and the total doping amount is 1% to 2% by mass fraction.

[0008] In a possible implementation, the component weight ratio of the anti-poisoning outer layer is: alumina is the matrix material, with a mass fraction of 48% to 52%; the loading amount of tin dioxide is 45% to 50% by mass fraction; the mass ratio of platinum oxide to palladium oxide is 1:1, and the total doping amount is 1% to 3% by mass fraction.

[0009] In a possible implementation, the component weight ratio of the anti-poisoning outer layer is: tin dioxide is the matrix material, with a mass fraction of 80% to 85%; the addition amount of lanthanum oxide is 10% to 15% by mass fraction; the mass ratio of chloroplatinic acid to ammonium chloropalladate is 1:1, and the total doping amount is 3% to 5% by mass fraction.

[0010] In a possible implementation, the preparation steps of the sensitive inner layer include: adding concentrated nitric acid to a three-necked flask, adding tin granules in batches, and magnetically stirring until no brown-red gas is generated; centrifuging and washing the obtained precipitate until it is neutral, drying it in a vacuum drying oven, grinding it, and then calcining it in a muffle furnace, and grinding it again to obtain nano-tin dioxide powder; mixing the nano-tin dioxide powder with hydrogen peroxide, ammonia water, and deionized water, and adjusting the temperature to 70 °C for ultrasonic oscillation; mixing the intermediate after ultrasonic treatment with aminopropyltriethoxysilane and toluene, magnetically stirring, centrifuging, and then drying and calcining to obtain a tin dioxide material grafted with silica on the surface.

[0011] In one possible implementation, adjusting the temperature to 70 °C for ultrasonic oscillation includes: setting a hyperbolic coupling power control formula according to a preset maximum power value, total processing time, oscillation coefficient, attenuation coefficient, and angular frequency; controlling the power output of the ultrasonic oscillation device according to the power control formula; and correcting the oscillation coefficient when the solution temperature exceeds a predetermined temperature range.

[0012] In one possible implementation, the method for coating the outer wall of the ceramic tube includes: preparing a slurry by mixing an anti-toxic outer layer material with a mixed solution of absolute ethanol and deionized water, and coating the slurry on the surface of the sensitive inner layer with a brush; after drying in air, calcining using an alumina ceramic heat conduction plate assisted focusing heating method.

[0013] In one possible implementation, preparing a slurry by mixing an anti-toxic outer layer material with a mixed solution of absolute ethanol and deionized water, and coating the slurry on the surface of the sensitive inner layer with a brush includes: mixing absolute ethanol and deionized water according to a predetermined volume ratio to obtain a mixed solution; dry-grinding the anti-toxic outer layer material until the powder is fine and has no granular feeling; adding the mixed solution in two portions for wet-grinding, adding a predetermined proportion for the first time and grinding for a first preset time, and adding the remaining solution for the second time and grinding for a second preset time; confirming that the slurry viscosity meets the preset downstream length requirement by conducting a fluidity test on a glass sheet inclined at a preset angle; using a soft nylon brush with a predetermined diameter to uniformly coat the first layer along the axial direction of the ceramic tube; after the surface of the first layer reaches a preset dryness degree, obliquely coating the second layer at a predetermined angle and controlling the width of the overlapping area between the two layers.

[0014] In one possible implementation, after drying in air, calcining using an alumina ceramic heat conduction plate assisted focusing heating method includes: placing a high-purity alumina ceramic plate with a preset size in the furnace chamber of a muffle furnace, and multiple V-shaped grooves with a target depth and inclination angle are pre-processed on the ceramic plate; arranging high-purity alumina baffles on both sides of the ceramic plate to form a heat reflection structure with a predetermined included angle between the baffles and the ceramic plate; raising the temperature to a first target temperature at a first preset heating rate and maintaining for a first preset time to preheat the ceramic plate; raising the temperature to a second target temperature at a second preset heating rate and maintaining for a second preset time; and performing a cooling process at a third preset cooling rate.

[0015] In a methane gas sensor with a double-layer structure disclosed above, in the embodiment of the present application, by designing SnO 2 outside the sensitive inner layer SnO 2 / α-Al 2 O 3 and La 2 O 3 / SnO 2Two different anti-poisoning outer structures have achieved a significant improvement in the anti-HMDSO poisoning performance of the sensor. Among them, SnO 2 / α-Al 2 O 3 The anti-poisoning outer layer utilizes the relatively large resistance characteristic of Al 2 O 3 not only can effectively prevent the sharp drop in resistance caused by poisoning, but also can act as a filtering layer to prevent the inward diffusion of HMDSO and its decomposition products. And La 2 O 3 / SnO 2 The anti-poisoning outer layer further improves the detection sensitivity of the element to CH 4 by enhancing the oxygen species adsorption ability of the outer layer material and increasing the adsorbed oxygen concentration on the surface of the sensitive inner layer. The design of this double-layer structure not only solves the problem that traditional single-layer structure sensors are vulnerable to pollutant poisoning, but also improves the overall performance of the sensor through the synergistic effect of materials. Especially by using the silane coupling agent 3-aminopropyltriethoxysilane (KH-550) to perform surface modification on SnO 2 , the thermal stability of nano-SnO 2 is significantly improved, solving the technical problem of poor stability of traditional SnO 2 -based sensors in high-temperature working environments. In addition, by doping noble metals such as Pt and Pd in the sensitive inner layer and anti-poisoning outer layer materials, the sensitivity of the sensor element is further optimized, making it more suitable for accurate methane gas detection in complex environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0017] Figure 1 FIG. is a schematic structural diagram and an optical microscope image of a methane sensor with a sensitive inner layer - anti-poisoning outer layer double-layer gas-sensitive element based on SnO 2 disclosed in the embodiments of the present application;

[0018] Figure 2 FIG. is a TEM image of SnO 2 surface graft-modified SiO 2 which is a sensitive inner layer material disclosed in the embodiments of the present application;

[0019] Figure 3 FIG. is a TEM image of SnO supported on an anti-poisoning outer layer material disclosed in the embodiments of the present application2 SEM image of alumina fiber;

[0020] Figure 4 A kind of SnO disclosed in the embodiment of the present application 2 For 100 ppm CH 3 COCH 3 , CO, NH 3 And 10000 ppm CH 4 Response curve graph;

[0021] Figure 5 Schematic diagram of the change in resistance of a sensor element during poisoning recovery disclosed in the embodiment of the present application;

[0022] Figure 6 A kind of sensor disclosed in the embodiment of the present application under 10000 ppm CH 4 Schematic diagram of the short-term repeatability test results repeated 6 times under pulses;

[0023] Figure 7 A kind of sensor element disclosed in the embodiment of the present application for acetone, CO, NH 3 Gas selectivity schematic diagram. Detailed implementation manners

[0024] Now, various exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. It should be noted that: unless otherwise specifically stated, the relative arrangements of components and steps, numerical expressions and values set forth in these embodiments do not limit the scope of the present disclosure.

[0025] Those skilled in the art can understand that the terms "first", "second", etc. in the embodiments of the present disclosure are only used to distinguish different steps, devices or modules, etc., and neither represent any specific technical meaning nor indicate an inevitable logical order between them. It should also be understood that in the embodiments of the present disclosure, "a plurality" may refer to two or more, and "at least one" may refer to one, two or more. It should also be understood that for any component, data or structure mentioned in the embodiments of the present disclosure, without clear limitation or contrary indication in the context, it can generally be understood as one or more. In addition, the term "and / or" in the present disclosure is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in the present disclosure generally represents an "or" relationship between the associated objects before and after. It should also be understood that the present disclosure emphasizes the differences between various embodiments, and the same or similar parts can be referred to each other. For the sake of brevity, they will not be repeated one by one.

[0026] Meanwhile, it should be understood that, for the convenience of description, the dimensions of the various parts shown in the drawings are not drawn according to the actual proportional relationship. The following description of at least one exemplary embodiment is actually merely illustrative and in no way restricts the present disclosure, its application, or its use. Technologies, methods, and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, the said technologies, methods, and devices should be regarded as part of the specification. It should be noted that similar reference numerals and letters denote similar items in the following drawings, and thus, once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Apparently, the described embodiments are some but not all of the embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts fall within the scope of protection of the present application.

[0028] This application discloses a methane sensor with a sensitive inner layer - anti - poisoning outer layer double - layer gas - sensitive element based on SnO 2 The sensor includes: a six - pin socket, a tube cap, and a ceramic tube substrate. Two parallel - distributed ring - shaped gold electrodes are provided on the ceramic tube substrate; platinum wire leads are led out from the ring - shaped gold electrodes, and a heating wire is arranged at the center of the ceramic tube; the outer wall of the ceramic tube is coated with a sensitive inner layer and an anti - poisoning outer layer, wherein the sensitive inner layer is a tin dioxide material grafted with silica on the surface, and the anti - poisoning outer layer is a material loaded with tin dioxide.

[0029] In one embodiment, the sensor is a sintered type indirectly - heated sensor, and its element consists of three parts: a six - pin socket, a tube cap, and a sensor element. There are two parallel - distributed ring - shaped gold electrodes on the ceramic tube substrate of the sintered type indirectly - heated sensor. Two Pt wire leads are led out from each gold electrode, and the heating wire passes through the center of the ceramic tube to expose the required working temperature. The gas - sensitive material is evenly coated on the outer wall of the ceramic tube. Among them, the material of the sensitive inner layer is SiO 2 Surface - grafted and modified SnO 2 、PtO 2 and PdO, and the material of the anti - poisoning outer layer is alumina fiber loaded with SnO 2

[0030] In one embodiment, the component weight ratio of the sensitive inner layer is: tin dioxide grafted with silica on the surface is the matrix material; the addition amount of alumina is 3% by mass fraction; the mass ratio of platinum oxide and palladium oxide is 1:1, and the total doping amount is 1% - 2% by mass fraction.

[0031] In another embodiment, the component weight ratio of the anti-poisoning outer layer is as follows: alumina is the matrix material with a mass fraction of 48% to 52%; the loading amount of tin dioxide is 45% to 50% in mass fraction; the mass ratio of platinum oxide to palladium oxide is 1:1, and the total doping amount is 1% to 3% in mass fraction.

[0032] In yet another embodiment, the component weight ratio of the anti-poisoning outer layer is as follows: tin dioxide is the matrix material with a mass fraction of 80% to 85%; the addition amount of lanthanum oxide is 10% to 15% in mass fraction; the mass ratio of chloroplatinic acid to ammonium chloropalladate is 1:1, and the total doping amount is 3% to 5% in mass fraction.

[0033] It should be understood that traditional methane gas sensors mainly adopt a single-layer SnO 2 sensitive material structure, and their preparation process usually uses a simple sol-gel method or co-precipitation method to prepare SnO 2 nanopowder, then mix it with an organic carrier to form a slurry, coat it on a ceramic tube substrate, and finally form a gas-sensitive layer through high-temperature calcination. This traditional process has obvious technical limitations, mainly reflected in the poor stability of the SnO 2 material itself, weak anti-interference ability, and low gas-sensing selectivity. Especially in the actual application environment, the traditional single-layer structure cannot effectively cope with the pollution of toxic substances such as silicone, resulting in a rapid deterioration of the sensor performance.

[0034] The embodiment of the present application introduces a surface modification technology into the preparation of the SnO 2 substrate material. By using the nitric acid-tin granule oxidation method to prepare the initial nano-SnO 2 , then using hydrogen peroxide and ammonia water for surface activation, and finally using 3-aminopropyltriethoxysilane (KH-550) for surface grafting modification, this refined preparation process significantly improves the thermal stability and interfacial bonding property of the SnO 2 material. Compared with the traditional process, this modification treatment not only enhances the heat resistance of the material, but also exposes more active sites, providing a good material basis for subsequent gas adsorption and detection. Especially by introducing SiO 2 surface grafting, the controllable adjustment of the surface properties of the material is realized at the microscale.

[0035] Furthermore, by coating SnO 2 on the optimized sensitive inner layer of SnO 2 / α-Al 2 O 3 or La 2 O 3 / SnO 2The anti-poisoning outer layer forms a composite structure with complementary functions. During the preparation of the anti-poisoning outer layer material, by optimizing the loading rate of SnO 2 (45 - 50 wt%), the best balance between anti-poisoning performance and conductivity is achieved. In addition, noble metals such as Pt and Pd are introduced into the inner and outer layer materials respectively, realizing the three-dimensional distribution of catalytic active sites. This not only improves the sensitivity of the sensor but also enhances the anti-poisoning ability of the material.

[0036] Specifically, the preparation steps of the sensor are as follows:

[0037] (1) Prepare the sensitive inner layer material SiO 2 with surface grafted and modified SnO 2 , which includes: adding concentrated nitric acid into a three-necked flask, adding tin grains in batches, and magnetically stirring until no brownish-red gas is generated; centrifuging and washing the obtained precipitate until it is neutral, drying it in a vacuum drying oven, calcining it in a muffle furnace after grinding, and grinding it again to obtain nano-tin dioxide powder; mixing the nano-tin dioxide powder with hydrogen peroxide, ammonia water, and deionized water, and adjusting the temperature to 70 °C for ultrasonic oscillation; mixing the intermediate after ultrasonic treatment with 3-aminopropyltriethoxysilane and toluene, magnetically stirring, centrifuging, and drying and calcining to obtain the tin dioxide material with surface grafted silica.

[0038] Among them, adjusting the temperature to 70 °C for ultrasonic oscillation includes: setting a hyperbolic coupling type power regulation formula according to the preset maximum power value, total processing time, oscillation coefficient, attenuation coefficient, and angular frequency; controlling the power output of the ultrasonic oscillation device according to the power regulation formula; when the solution temperature exceeds the predetermined temperature range, correcting the oscillation coefficient.

[0039] Specifically, for the sensitive inner layer, nano-SnO 2 can be prepared by the typical tin grain-nitric acid oxidation method. First, measure 100 ml of concentrated nitric acid into a three-necked flask with a tail gas treatment device, add 25 g of tin grains in batches, and magnetically stir for 3 h until no brownish-red gas is generated. Then, centrifuge and wash the obtained precipitate until it is neutral, dry it in a 95 °C vacuum drying oven for 24 h, calcine it in a muffle furnace at 450 °C for 2 h to obtain a light yellow solid, and grind it again to obtain nano-SnO 2 powder.

[0040] Weigh 10 g of nano-SnO 2 powder, 10 ml of hydrogen peroxide, 10 ml of ammonia water, and 10 ml of deionized water, perform ultrasonic oscillation at 70 °C for 30 min, separate and dry to obtain the SnO 2 hydroxide intermediate.

[0041] Preferably, during the ultrasonic oscillation process, a hyperbolic coupling power control method is used. Specifically, the relationship between the ultrasonic power P and the time t is set as: Where Pm is the maximum power value of 523.4W, T is the total processing time of 30min, ξ is the oscillation coefficient of 0.168, η is the attenuation coefficient of 0.232, and ω is the angular frequency of 0.314rad / min. Through this equation, the continuous dynamic adjustment of ultrasonic power is realized, so that the power shows a periodic fluctuation but an overall slow decrease trend. At the same time, the solution temperature is stabilized within the range of 70±1.5℃. When the temperature exceeds this range, it is corrected by adjusting the ξ value, and the correction coefficient is 0.85. This control method can effectively prevent SnO while maintaining the stability of the dispersed system. 2 The prepared intermediate was transferred to a 100 ml beaker, 0.92 g 3-aminopropyltriethoxysilane (KH-550) and 40 ml toluene were added, and magnetic stirring was performed for 3 h. The reaction product was centrifuged and dried, and calcined at 600 ° C for 2 h to obtain a modified surface grafted with SiO 2 SnO 2 .

[0042] It should be noted that in the SnO 2 When sensitive materials are subjected to ultrasonic dispersion treatment, a 35-45 kHz fixed frequency ultrasonic cleaner is generally used in this process, with a constant temperature of 70-80°C and a fixed power of 300-400W for ultrasonic treatment. This fixed parameter treatment method has obvious technical defects: in actual production, when SnO 2 After the powder is added to the solution, the dispersion effect is often unstable due to slight fluctuations in the degree of powder agglomeration, solution temperature and pH value. Especially when processing large batches, some powders will form hard agglomerates that are difficult to disperse at the bottom of the solution, while powders that are over-dispersed on the surface will agglomerate due to local overheating. In traditional processes, this problem is solved by extending the processing time or increasing the power, but this method not only increases energy consumption, but also causes more serious agglomeration. By introducing a hyperbolic coupling power control method and dynamically adjusting the ultrasonic power according to the actual state of the powder during the dispersion process, the blind processing in the traditional process can be effectively avoided, ensuring the directionality and controllability of the dispersion process.

[0043] Al in sensitive inner layer materials 2 O 3 The addition amount is 3wt%, and the surface is grafted with SiO 2 SnO 2 , the sensitive inner layer gas-sensitive material is sensitive to CH 4 It has excellent gas-sensitive properties. The silane coupling agent 3-aminopropyltriethoxysilane (KH-550) can be used to modify the surface of SnO2 to improve the2 Thermal stability. In addition, the sensitive inner layer material can be doped with noble metals, and the introduction of noble metals such as Pt and Pd can improve the sensitivity of the sensor element.

[0044] (2) Preparation of anti-poisoning outer layer material - loading SnO 2 of Al 2 O 3 , in order to improve the performance of the sensor element against HMDSO poisoning, in the method of coating the anti-poisoning outer layer on the optimized sensitive inner layer in the embodiments of the present application, the preparation processes of two kinds of anti-poisoning outer layers, namely SnO 2 / α-Al 2 O 3 and La 2 O 3 / SnO 2 are respectively disclosed.

[0045] The first anti-poisoning outer layer: Take 20.00 ml of concentrated nitric acid in a three-necked flask, add 5.00 g of tin granules in batches, and stir magnetically for 3 h. Then add 2.328, 3.622, 5.433, 6.640, 8.150 and 12.677 g of α-Al 2 O 3 to the three-necked flask, and continue to stir for 2 h. After cooling, adjust the pH of the solution to 8, filter by suction and wash the filter cake many times. Finally, dry the precipitate, grind it and place it in a muffle furnace for calcination at 600 °C for 2 h to obtain the anti-poisoning outer layer material. The formula of the anti-poisoning outer layer material is Al 2 loaded with 45 wt% SnO 2 O 3 , and the anti-poisoning ability is stronger when the loading rate of SnO 2 is 45 - 50 wt%.

[0046] The second anti-poisoning outer layer: Weigh 1.00 g of SnO 2 in an agate mortar, fully grind it and then add an appropriate amount of La(NO 3 ) 3 , and dry grind for 5 min. Then drop 0.40 ml of each of H 2 PtCl 6 ·H 2 O and (NH 4 ) 2 PdCl 4 , and continue wet grinding for 5 min. Transfer the well-mixed La 2 O 3 / SnO 2 anti-poisoning outer layer material precursor to a crucible, place it in a muffle furnace for calcination at 600 °C for 2 h to obtain the anti-poisoning outer layer material.

[0047] (3) Preparation of double-layer structure SnO 2For the methane sensor, the method for coating the outer wall of the ceramic tube includes: preparing a slurry by mixing the anti-poisoning outer layer material with a mixed solution of absolute ethanol and deionized water, and coating the slurry on the surface of the sensitive inner layer with a brush; after drying in air, calcining by using an alumina ceramic heat conduction plate assisted focusing heating method.

[0048] Among them, preparing a slurry by mixing the anti-poisoning outer layer material with a mixed solution of absolute ethanol and deionized water, and coating the slurry on the surface of the sensitive inner layer with a brush includes: mixing absolute ethanol and deionized water according to a predetermined volume ratio to obtain a mixed solution; dry-grinding the anti-poisoning outer layer material until the powder is fine and has no granular feeling; adding the mixed solution in two portions for wet-grinding, adding a predetermined proportion for the first time and grinding for a first preset time, adding the remaining solution for the second time and grinding for a second preset time; confirming that the viscosity of the slurry meets the requirement of a preset downward flow length by conducting a fluidity test on a glass sheet at a preset inclined angle; using a soft nylon brush with a predetermined diameter to uniformly coat the first layer along the axial direction of the ceramic tube; after the surface of the first layer reaches a preset drying degree, obliquely coating the second layer at a predetermined angle and controlling the width of the overlapping area between the two layers.

[0049] Among them, after drying in air, calcining by using an alumina ceramic heat conduction plate assisted focusing heating method includes: placing a high-purity alumina ceramic plate with a preset size in the muffle furnace hearth, and multiple V-shaped grooves with a target depth and inclination angle are pre-processed on the ceramic plate; arranging high-purity alumina baffles on both sides of the ceramic plate to form a heat reflection structure with a predetermined included angle between the baffles and the ceramic plate; raising the temperature to a first target temperature at a first preset heating rate and maintaining for a first preset time to preheat the ceramic plate; raising the temperature to a second target temperature at a second preset heating rate and maintaining for a second preset time; performing a cooling treatment at a third preset cooling rate.

[0050] In an implementation scenario, weigh 1.00 g of the anti-poisoning outer layer material, add 0.4 ml of the mixed solution of absolute ethanol and deionized water dropwise after sufficient grinding to prepare a paste, and continue wet-grinding for 10 min. Coat the prepared slurry on the surface of the optimized sensitive inner layer element with a fine brush to obtain a double-layer structure sensor element.

[0051] Preferably, during the slurry preparation process, anhydrous ethanol and deionized water are first mixed at a volume ratio of 2.7:1. After adding the anti-toxic outer layer material, it is ground using an agate mortar. The grinding process is divided into three consecutive steps: First, dry grind until the powder is fine and has no granular feeling. Then, add 2 / 3 of the mixed solution and conduct the first wet grind for 4 minutes. Finally, add the remaining solution and continue wet grinding for 3 minutes. During grinding, maintain a uniform circular motion, and keep the angle between the mortar and the pestle at about 60°. The diameter of each circle is about 7 cm. To ensure moderate fluidity of the slurry, after the second wet grind, transfer the prepared slurry to a glass petri dish. Dip a glass rod with a diameter of 1.5 mm into a little slurry and draw a line on a glass slide to observe its flowing length. When the slurry slowly flows down 5.8 - 6.4 cm on a glass slide tilted at 45°, it indicates that the viscosity of the slurry is moderate. When coating, select a soft nylon brush with a tip diameter of 0.8 mm and adopt a two-way staggered coating method: First, evenly coat a layer along the axial direction of the ceramic tube. After its surface is slightly dry (about 1.5 minutes), then obliquely coat the second layer at a 45° angle. The width of the overlapping area between the two layers is controlled within 0.7 - 0.9 mm. When coating, keep the angle between the brush and the surface of the ceramic tube between 55 - 60°, use a gentle sliding method, control the length of each brush stroke within 6 - 7 mm, and the interval time between adjacent brush strokes is 8 - 10 s. Ensure that the surface of the previous brush stroke is slightly dry before applying the next brush stroke. Through this two-way staggered coating method, problems such as coating leakage points, accumulation, or uneven thickness can be effectively avoided.

[0052] Naturally dry in air for 30 minutes, and then place it in a muffle furnace at 600 °C and calcine for 1 hour. In one embodiment, during the muffle furnace calcination process, an alumina ceramic heat conduction plate assisted focusing heating method can be adopted. Specifically, place a high-purity alumina ceramic plate (purity ≥ 99.6%) with a size of 15×12×1.8 cm in the muffle furnace hearth. Eight "V"-shaped grooves are pre-processed on the ceramic plate, the groove depth is 0.6 cm, the angle between the inclined surface and the horizontal plane is 50° - 55°, and the distance between adjacent grooves is 2.4 cm. The special geometric structure of the "V"-shaped groove can form a heat focusing effect during the heating process, enabling heat to be concentrated on the surface of the sensor element. At the same time, set high-purity alumina baffles (12×8×0.6 cm) on both sides of the ceramic plate, and the angle between the baffle and the ceramic plate is 85° - 90°, forming a heat reflection structure similar to a paraboloid. During the 1-hour calcination process, first heat up at a rate of 3 °C / min to 378 °C and hold for 12 minutes to fully preheat the ceramic heat conduction system; then heat up at a rate of 5 °C / min to 600 °C and keep the temperature for 28 minutes; finally, cool down at a rate of 3 °C / min. Through the heat focusing effect of the "V"-shaped groove and the heat reflection effect of the baffle, directional and uniform heating of the surface of the sensor element can be achieved. At the same time, the excellent high-temperature resistance performance and moderate heat capacity characteristics of the ceramic material can ensure the stability of the thermal field distribution.

[0053] It should be noted that the coating process of conventional methane sensors mainly uses the suspension impregnation method or the single - brush coating method. In the impregnation method, the ceramic tube substrate is immersed in a pre - prepared suspension of gas - sensitive materials, and a coating is formed by capillary action and surface tension. Although this method is simple to operate, it has serious technical limitations: it is difficult to precisely control the concentration of the suspension, resulting in uneven coating thickness; small differences in the surface tension of the substrate will cause uneven coating distribution; and flow marks and edge accumulation are likely to form during the lifting process. The single - brush coating method can improve the coating uniformity to a certain extent, but since the gas - sensitive materials are prone to orientational arrangement during the coating process, forming a preferred orientation, this microstructure will lead to significant differences in the gas - sensitive characteristics of the sensor in different directions. Traditional processes improve the coating quality by increasing the number of coating times or adjusting the slurry concentration, but these methods often bring new problems, such as poor inter - layer bonding force and low utilization rate of gas - sensitive materials. The two - way staggered coating method not only solves the problem of coating uniformity in traditional processes, but also realizes the multi - directional distribution of gas - sensitive material particles through specific coating angles and interval time control, fundamentally improving the microstructure of the coating.

[0054]

[0054]

[0055] During the heat treatment of the conventional process, the calcination temperature is usually set within the range of 580 - 620 °C, and a simple program of uniform heating and constant temperature holding is adopted. This temperature control method ignores the thermal response differences between the sensitive inner layer and the anti-poisoning outer layer in the double-layer structure sensor. Due to the differences in thermal conductivity and specific heat capacity of the two layers of materials, thermal stress is likely to be generated at the interlayer interface during the rapid heating process, resulting in local cracking or delamination. Especially in the initial stage of heating, due to the volatilization of organic solvents and the decomposition of precursors, a large amount of gas will be generated inside the coating. These gases are difficult to escape evenly under the traditional flat or stacked methods, and it is easy to form micropores or cracks in the coating. Some improved processes attempt to solve these problems by extending the heat treatment time or reducing the heating rate, but this method significantly reduces the production efficiency, and the long-term high-temperature treatment will cause obvious agglomeration of the gas-sensitive material particles, reducing its specific surface area and gas-sensitive response.

[0056] The alumina ceramic heat conduction plate-assisted focusing heating method of the embodiment of the present application not only solves the problems of component positioning and uneven heat conduction through a special "V"-shaped groove design, but also realizes the control of the thermal field distribution through the optimized design of the reflection structure, ensuring the consistency of product quality in mass production. Specifically, the inclination angle of the groove wall can make the reflected thermal radiation form a uniform heat flux distribution on the surface of the sensor element. The baffle forms a parabolic-like heat reflection structure through the angle setting, effectively suppressing the thermal convection disturbance in the furnace cavity and enhancing the directivity of the thermal radiation at the same time. The high-purity alumina material (purity ≥ 99.6%) has excellent thermal conductivity and thermal stability, ensuring good shape and performance stability in a high-temperature environment of 600 °C.

[0057] Finally, the chromium-nickel heating wire is passed through the ceramic tube and welded together with the Pt lead on the six-pin socket to obtain the gas sensor element. The obtained sensor element is aged for 72 h under the working voltage for subsequent performance testing.

[0058] Figure 1 It is a structural schematic diagram and an optical microscope image of a methane sensor based on a SnO2 double-layer gas-sensitive element with a sensitive inner layer - anti-poisoning outer layer disclosed in the embodiment of the present application. In the structural schematic diagram, the sensor adopts a sintered type indirect heating design and is composed of three main parts: a six-pin socket, a tube cap, and a sensor element. There are two parallel annular gold electrodes on the surface of the ceramic tube substrate, and each gold electrode is connected to two platinum wire leads for electrical signal conduction. A heating wire runs through the center of the ceramic tube to provide the working temperature required for the sensor. The gas-sensitive material layer is evenly coated on the outer wall surface of the ceramic tube, forming a complete double-layer structure.

[0059] The optical microscope image shows the microscopic morphological characteristics of the sensor element. It can be observed from the image that the gas-sensitive material layer has a uniform and dense surface morphology, without obvious crack or pore defects. A bilayer structure with a clear interface is formed between the inner layer and the outer layer, and the two layers of materials are tightly combined. The surface topography presents the typical characteristics of ceramic materials, with a certain surface roughness, and this microstructure is beneficial to the adsorption of gas molecules and the progress of the sensing process.

[0060] Figure 2 A sensitive inner layer material SiO disclosed in the embodiment of the present application 2 Surface graft-modified SnO 2 TEM image. It can be observed from the image the morphology and distribution characteristics of nanoscale SnO 2 particles. These nanoparticles present a spherical or nearly spherical morphology, and the particle size distribution is relatively uniform, and the particle size is in the nanometer range. A thin and uniform coating layer can be seen on the particle surface, which is a surface modification layer formed by the graft modification of SiO 2

[0061] The TEM image further shows that there is a certain degree of agglomeration between the modified SnO 2 nanoparticles, forming a loose network structure. This network structure contains a large number of nanoscale pores, and these pores constitute the channels for gas molecule diffusion and transmission. The SiO 2 modified layer on the particle surface presents a uniform and continuous characteristic, indicating that the surface modification process has achieved good results.

[0062] Figure 3 An anti-poison outer layer material loaded with SnO disclosed in the embodiment of the present application 2 SEM image of alumina fibers. It can be observed from the image the three-dimensional network structure characteristics of the alumina fibers. The fibers present a relatively high aspect ratio and have obvious one-dimensional structure characteristics. A large number of SnO 2 nanoparticles are loaded on the fiber surface, and these particles are evenly distributed and have a high coverage. A rich three-dimensional cross-linked network is formed between the fibers, constructing an open porous structure.

[0063] The SEM image shows that the SnO 2 nanoparticles loaded on the surface of the alumina fibers have good dispersibility and no large-scale agglomeration phenomenon. A large number of micropores and mesopores exist in the fiber network structure, and this hierarchical pore structure is beneficial to the diffusion and transmission of gas molecules. The SnO 2 loading layer on the fiber surface is tightly combined with the matrix, and no obvious peeling or falling-off phenomenon occurs. The entire material system exhibits a multi-scale structural characteristic with distinct levels.

[0064] Figure 4A kind of SnO disclosed in the embodiments of the present application 2 For 100 ppm CH 3 COCH 3 , CO, NH 3 And 10,000 ppm CH 4 Response curve. In the left sub - graph, the abscissa is the time axis with a range of 0 - 300 seconds, and the ordinate is the gas - sensitive response value Ra / Rg with a range of 0 - 10. The dynamic response curve records the real - time response process of the gas - sensitive element to CH 4 , CH 3 COCH 3 , CO and NH 3 Four kinds of gases. Among them, the response curve of CH 3 COCH 3 Starts to respond at 100 seconds. The response value rapidly rises to a steady - state value of Ra / Rg = 9.2 within 50 seconds and maintains at this response platform for about 50 seconds. When the test atmosphere is switched to air, the response value rapidly returns to the initial baseline level within 40 seconds. The response curves of CH 4 , CO and NH 3 Show relatively small response amplitudes, and their Ra / Rg values do not exceed 2.0. The response - recovery processes of these three gases are similar to those of CH 3 COCH 3 In terms of time characteristics, showing typical first - order kinetic response characteristics during both the gas introduction and discharge phases.

[0065] In the right sub - graph, the steady - state response values of the sensor to the above four gases are compared in the form of a bar chart. The abscissa identifies the types of gases to be measured, and the ordinate uses the Ra / Rg dimension to represent the response intensity. The response column height of CH 3 COCH 3 Is 9.2, the response value of CH 4 Is 1.8, the response value of CO is 1.5, and the response value of NH 3 Is 0.9. The quantitative data of the response values correspond one - to - one with the steady - state response values in the dynamic curve. There is a difference of more than one order of magnitude in the numerical values of the response intensities of each gas, and the response values show a non - linear distribution characteristic with the change of gas types. The response intensities of the gas - sensitive element to these four gas molecules conform to the order relationship of CH 3 COCH 3 >CH 4 >CO>NH 3 . The whole set of data uses a unified response - value scale to horizontally compare the interaction strengths between different gas molecules and the sensing material. It reflects the interaction selectivity between the surface active sites of the gas - sensitive material and different gas molecules at the microscopic mechanism level, as well as the resulting macroscopic response signal differences.

[0066] Figure 5 This is a schematic diagram of the change in the resistance of a sensor element during the poisoning recovery process disclosed in the embodiments of the present application. The abscissa represents the time course, and the ordinate represents the resistance value of the sensor. The curve records the dynamic change process of the resistance value of the sensor when exposed to a toxic substance environment and its subsequent recovery behavior in a clean environment. The change trend of the resistance value reflects the interaction process between the sensor material and the toxic molecules.

[0067] The resistance change curve shows obvious stage characteristics. During the poisoning stage, the resistance value changes significantly; during the recovery stage, the resistance value gradually returns to the initial state. The curve contains multiple cycles of poisoning-recovery cycles, and each cycle shows a similar change pattern. The change amplitude and recovery degree of the resistance value reflect the anti-poisoning performance and recoverability of the sensor material. The resistance change during the whole process shows a certain regularity and repeatability.

[0068] Figure 6 This is a schematic diagram of the short-term repeatability test results of a sensor disclosed in the embodiments of the present application under 10000 ppm CH 4 pulses repeated 6 times. The abscissa represents the time series, and the ordinate represents the response value of the sensor. The curve records the response behavior of the sensor under continuous 6 times of CH 4 gas pulse stimulation. Each response cycle includes two stages of gas introduction and discharge, forming a complete response-recovery cycle. The periodic change of the curve reflects the repeated detection ability of the sensor.

[0069] The multi-cycle characteristics of the response curve show that the sensor has stable repeatability in detecting CH 4 gas. The signal change pattern in each response cycle is similar, and the response amplitude remains at a similar level. The baseline remains stable during multiple tests, and there is no obvious zero drift phenomenon. The dynamic characteristics of the response and recovery processes remain consistent during repeated tests, indicating that the sensor performance has good stability.

[0070] Figure 7 This is a schematic diagram of the selectivity of a sensor element to acetone, CO, NH 3 gas. The abscissa represents different types of gases, and the ordinate represents the response value of the sensor. The bar chart shows the response intensity of the sensor to different gas molecules, reflecting the gas selectivity characteristics of the sensor. The test conditions for each gas are kept consistent to ensure the comparability of the data.

[0071] Selective test data shows the ability of the sensor to distinguish different gases. The differences in response values reflect the different interaction strengths between the sensor and different gas molecules. The data distribution indicates that the sensor has different degrees of response sensitivity to the test gases. The test results include multiple sets of parallel experimental data, and each set of data shows good repeatability. The error range in the bar chart reflects the stability and reliability of the measurement process.

[0072] It should be noted that the various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same or similar parts among the various embodiments, reference can be made to each other. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple. For the relevant parts, reference can be made to the description in the method part.

[0073] It should also be noted that in this text, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of another identical element in the process, method, article or device including the said element.

[0074] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A methane gas sensor with a double-layer structure, characterized in that: include: A six-pin tube seat, a tube cap, and a ceramic tube base, wherein the ceramic tube base is provided with two parallel annular gold electrodes; platinum wire leads are led out from the annular gold electrodes, and a heating wire is provided in the center of the ceramic tube; the outer wall of the ceramic tube is coated with a sensitive inner layer and a poison-resistant outer layer, wherein the sensitive inner layer is a tin dioxide material with silicon dioxide grafted on the surface, and the poison-resistant outer layer is a material loaded with tin dioxide.

2. The methane gas sensor according to claim 1, characterized in that: The weight ratio of the components of the sensitive inner layer is: Tin dioxide with silicon dioxide grafted on the surface is the base material; The amount of aluminum oxide added is 3% by mass; The mass ratio of platinum oxide to palladium oxide is one to one, and the total doping amount is one to two percent by mass.

3. The methane gas sensor according to claim 1, characterized in that: The weight ratio of the components of the anti-toxic outer layer is: Alumina is the matrix material, with a mass fraction of 48 to 52 percent; The loading amount of tin dioxide is 45 to 50 percent by mass; The mass ratio of platinum oxide to palladium oxide is one to one, and the total doping amount is one to three percent by mass.

4. The methane gas sensor according to claim 1, characterized in that: The weight ratio of the components of the anti-toxic outer layer is: Tin dioxide is the matrix material, with a mass fraction of 80 to 85 percent; The amount of lanthanum oxide added is 10 to 15 percent by mass; The mass ratio of chloroplatinic acid to ammonium chloropalladate is one to one, and the total doping amount is three to five percent by mass.

5. The methane gas sensor according to claim 2, characterized in that: The steps of preparing the sensitive inner layer include: Add concentrated nitric acid into a three-necked flask, add tin particles in batches, and stir magnetically until no brown-red gas is generated; The obtained precipitate is centrifugally washed to neutrality, dried in a vacuum drying oven, ground, calcined in a muffle furnace, and ground again to obtain nano-tin dioxide powder; Nano-tin dioxide powder was mixed with hydrogen peroxide, ammonia and deionized water, and the temperature was adjusted to 70°C for ultrasonic vibration; The intermediate after ultrasonic treatment is mixed with aminopropyltriethoxysilane and toluene, stirred by magnetic force, dried and calcined after centrifugation to obtain a tin dioxide material with silicon dioxide grafted on the surface.

6. The methane gas sensor according to claim 5, characterized in that: in, Adjust the temperature to 70°C for ultrasonic shaking, including: According to the preset maximum power value, total processing time, oscillation coefficient, attenuation coefficient and angular frequency, a hyperbolic coupling power control formula is set; Control the power output of the ultrasonic oscillation equipment according to the power control formula; When the solution temperature exceeds a predetermined temperature range, the oscillation coefficient is corrected.

7. The methane gas sensor according to claim 1, characterized in that: The outer wall coating method of the ceramic tube comprises: The anti-toxic outer layer material is mixed with a mixed solution of anhydrous ethanol and deionized water to form a slurry, and the slurry is applied to the surface of the sensitive inner layer with a brush; After drying in air, the calcination is carried out by using an alumina ceramic heat conducting plate assisted focused heating method.

8. The methane gas sensor according to claim 7, characterized in that: in, The anti-toxic outer layer material is mixed with anhydrous ethanol and deionized water to form a slurry, and the slurry is applied to the surface of the sensitive inner layer with a brush, including: Mixing anhydrous ethanol and deionized water in a predetermined volume ratio to prepare a mixed solution; Dry grind the anti-toxic outer layer material until the powder is fine and has no graininess; Adding the mixed solution twice for wet grinding, the first time adding a predetermined proportion and grinding for a first preset time, and the second time adding the remaining solution and grinding for a second preset time; By conducting a fluidity test on a glass sheet tilted at a preset angle, confirm that the slurry viscosity meets the preset downstream length requirements; Use a soft nylon brush of a predetermined diameter to evenly apply the first layer along the axial direction of the ceramic tube; After the surface of the first layer reaches a preset dryness, the second layer is applied obliquely at a predetermined angle, and the width of the overlapping area between the two layers is controlled.

9. The methane gas sensor according to claim 7, characterized in that: in, After drying in air, calcination is carried out using an alumina ceramic heat-conducting plate assisted focused heating method, including: A high-purity alumina ceramic plate of preset size is placed in the furnace of a muffle furnace, on which a plurality of V-shaped grooves of target depth and inclination are pre-processed; High-purity alumina baffles are arranged on both sides of the ceramic plate to form a heat reflection structure with a predetermined angle between the baffles and the ceramic plate; Raising the temperature to a first target temperature at a first preset heating rate and maintaining the temperature for a first preset time to preheat the ceramic plate; Raising the temperature to a second target temperature at a second preset heating rate and maintaining the temperature for a second preset time; The temperature reduction process is performed at a third preset temperature reduction rate.

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