Preparation method of PdO / alpha-MoO3 hollow microspheres and gas sensitive element based on in-situ growth

Abstract

The invention discloses a preparation method of PdO / alpha-MoO3 hollow microspheres and a gas sensitive element based on in-situ growth. The preparation method comprises the following steps: growing multilevel-structure PdO / alpha-MoO3 hollow microspheres with different PdO composite molar ratios on a ceramic tube in situ by adopting a solvothermal method; when the PdO / alpha-MoO3 composite material with the molar ratio of PdO being 2.5 at% has the best sensing performance to triethylamine gas, the response value of the material to 50ppm triethylamine gas is 122.1 at the optimal working temperature of 217 DEG C, the triethylamine gas as low as 0.1 ppm can be detected, the trace gas monitoring requirement is met, and the composite material is suitable for early warning or trace pollutant detection in a severe environment; the response speed is accelerated, the response time to the triethylamine gas is only 28.6 seconds, the gas concentration change can be captured in time, the real-time monitoring efficiency is improved, and the mechanism of the influence of the formation of the p-n heterostructure on the gas-sensitive performance is disclosed; in addition, the anti-interference capability is strong, and compared with interference gases such as aniline, acetone, ammonia gas and the like, the sensor has remarkable selectivity on triethylamine.

Description

Technical Field

[0001] This invention relates to the field of metal oxide material preparation and gas sensor detection technology, specifically a method for preparing PdO / α-MoO3 hollow microspheres and gas-sensitive elements based on in-situ growth, and a method for preparing gas-sensitive elements based on this material for high-sensitivity detection of triethylamine gas. Background Art

[0002] Triethylamine is a volatile organic compound, a colorless, transparent liquid with a strong ammonia-like odor. It is widely used as a solvent, catalyst, and raw material in organic synthesis. It can also be secreted in dead fish and marine organisms; the concentration of triethylamine increases with the decay of dead fish and seafood. Given its volatility, high flammability, and toxicity, triethylamine can cause serious harm to human health and the environment, such as chemical burns to the eyes and skin upon contact; it can also cause pulmonary edema and even death upon inhalation. Although several methods have been employed to detect triethylamine, such as chromatography, electrochemical analysis, and colorimetry, the expensive reference electrodes and equipment limit their application. Therefore, there is a significant need for accurate and rapid detection of triethylamine in biomedicine, chemistry, the food industry, and our daily lives.

[0003] Based on the working principle of gas sensors, many types of gas sensors have been developed, including acoustic, electrochemical, optical, and resistive types. Semiconductor metal oxide gas sensors have advantages such as high sensitivity, ease of fabrication, portability, low cost, and simple detection methods, making them one of the current hot topics in gas sensor research. To date, metal oxide nanomaterials have been widely studied for monitoring triethylamine gas in the living environment, including MoO3, ZnO, WO3, Fe2O3, CO3O4, and In2O3.

[0004] MoO3 is an n-type metal oxide semiconductor nanomaterial that has been widely studied and applied in catalysts, field emission, electrochemistry, gas sensing, gas chromaticity, and photochromism. It possesses a unique layered structure with a band gap of 3.0 eV-3.8 eV. Due to its advantages of low cost, non-toxicity, and environmental friendliness, it has been widely used in gas sensors for monitoring gases such as O2, CO, NO2, SO2, H2, H2S, C2H5OH, NH3, and TEA. Research shows that the surface morphology and structure of gas sensing materials have a significant impact on their gas-sensing performance. Among various morphologies, 3D hierarchical nanomaterials assembled from 0D nanoparticles, 1D nanowires, nanorods, nanotubes, and 2D nanosheets are highly regarded for their large specific surface area and low aggregation configuration. This structure is noteworthy for its ability to achieve efficient gas diffusion and charge transport. To date, three-dimensional MoO3 nanostructures designed and applied in gas sensors include nanoflower structures, nanoarray structures, hollow sphere structures, and core-shell structures. However, methods for synthesizing hierarchical MoO3 structures are overly complex. For example, hierarchical spherical MoO3 nanoflowers were hydrothermally synthesized using polyvinylpyrrolidone (PVP) as a surfactant, and hierarchical rose-shaped MoO3 nanoflowers were solvothermally synthesized using sodium thiosulfate as a surfactant. Sea urchin-like MoO3 nanomaterials were successfully synthesized via a hydrothermal process assisted by hexadecyltrimethylammonium bromide (CTAB). Therefore, it is necessary to further explore simpler synthetic methods to construct novel hierarchical molybdenum oxide nanomaterials and conduct in-depth performance studies on their application in gas sensors.

[0005] Traditional metal oxide semiconductor gas-sensitive materials are made by coating a slurry of metal oxide onto the surface of the gas-sensitive element. During the coating process, to ensure uniform coating, the sensitive film often has a thick, uneven thickness, and may even crack. Compared to traditional coated film sensors, the sensitive film grown in situ on a ceramic tube is relatively thin and uniformly distributed, thus possessing more effective active sites, which is beneficial for gas transport and electron transfer, effectively improving the gas-sensing characteristics of the gas sensor. For example, in-situ grown TiO2 nanorod arrays, NiO nanowall arrays, porous CuO nanosheet arrays, and α-Fe2O3 nanorod arrays exhibit significantly improved gas sensitivity compared to coated film devices prepared from the same synthetic materials. Furthermore, to enhance the sensing performance of metal oxide gas-sensitive materials, noble metals such as Pt, Pd, and Ag are loaded onto the surface of the metal oxide. Using noble metals such as Au is one of the simple and effective methods to improve the performance of gas-sensitive materials. Commonly used methods for loading metal oxides with noble metals include hydrothermal methods, electrospinning methods, surfactant modification methods, and sputtering deposition methods. For example, MoO3 nanoribbons were grown using chemical spray pyrolysis (CSP) deposition technology at an optimal glass substrate temperature of 400℃. XPS and EDS studies confirmed that the prepared material was a Pd-sensitized MoO3 film. SEM images showed the nanoribbon morphology of the MoO3 film. The Pd / MoO3 nanoribbon gas sensor exhibited ultra-high sensitivity and selectivity to NO2 gas. Before Pd loading, the MoO3 nanoribbons had a response rate of 68% to 100 ppm NO2 gas at an operating temperature of 200℃, with response and recovery times of 15 s and 150 s, respectively. After Pd loading, the MoO3 nanoribbons showed a higher response rate to 100 ppm NO2 gas. The response to NO2 gas was enhanced by 95.3%, with response and recovery times of 74 s and 297 s, respectively. The fabrication of Pd / MoO3 nanoribbons will provide a technological option for low-power, low-cost, high-sensitivity, and fast-response NO2 sensors.

[0006] Based on the above analysis, it can be found that the gas-sensing properties of sensors can be improved by constructing multi-level structures and changing the film formation method of gas-sensitive materials. However, there are very few reports on the application of in-situ grown molybdenum oxide spherical nanomaterials in the field of gas sensors, and their performance testing is still in its early stages. Therefore, it is necessary to develop a process for constructing multi-level structured molybdenum oxide nanomaterials on the surface of ceramic tubes using a simple in-situ growth method, and apply it to the field of gas sensors to further explore its gas-sensing mechanism. This invention provides a method for in-situ growth of multi-level structured PdO / α-MoO3 hollow microspheres with different PdO composite molar ratios on ceramic tubes using a solvothermal method with molybdenum acetylacetonate as the molybdenum source, palladium acetate as the palladium source, and n-butanol and glacial acetic acid as solvents. This method is then applied to the study of the gas-sensing performance and mechanism analysis of triethylamine. Summary of the Invention

[0007] In view of the problems existing in the prior art, the present invention discloses a method for preparing hollow PdO / α-MoO3 microspheres based on in-situ growth, the technical solution of which includes the following specific steps: Step 1: Weigh 0.17g of molybdenum acetylacetonate and 0.0029g of palladium acetate, then dissolve them in a mixed solution of 20mL n-butanol and 15mL glacial acetic acid. Sonicate the mixture until dissolved and stir magnetically at room temperature for 1 hour. Step 2: Transfer the mixed solution from Step 1 to a reactor with a 50 mL polytetrafluoroethylene liner, while suspending the ceramic tube inside the reactor. React at 180°C for 10 hours, then cool to room temperature. Step 3: Centrifuge to collect the dark blue precipitate obtained after cooling in Step 2, and rinse the ceramic tube several times with ultrapure water and anhydrous ethanol to obtain the α-MoO3 precursor; Step 4: Calcine the ceramic tube and the obtained α-MoO3 precursor powder in air at 400℃ for 1 hour. It can be observed that the color of the product powder gradually changes from grayish-white to light yellow, and finally PdO / α-MoO3 hollow microspheres are obtained.

[0008] Furthermore, the molar ratio of palladium to molybdenum in the acetylacetone molybdenum and palladium acetate in step one is controlled to be Pd / Mo = 2.5 at%.

[0009] A method for preparing a gas-sensitive element includes the following specific steps: Step 1: Take out the ceramic tube containing 2.5 at% PdO / α-MoO3 material after calcination; Step 2: Insert a heated Ni-Cr wire into the ceramic tube and weld it to the gas-sensitive element base to make a deposited film type side-heated gas-sensitive element; Step 3: Place the gas-sensitive element prepared in Step 3 in an aging bench to enhance stability and reproducibility.

[0010] Furthermore, in step three, the aging temperature is 300℃ and the aging time is 2 days.

[0011] The beneficial effects of this invention: The multi-level PdO / α-MoO3 hollow microsphere sensor prepared by the solvothermal method in this invention exhibits significant technical advantages and practical value in the field of triethylamine gas detection. Its beneficial effects are mainly reflected in the following aspects: 1. Significantly improved sensitivity: At the optimal operating temperature of 217℃, the composite material with a PdO molar ratio of 2.5 at% showed a response value of 122.1 to 50 ppm triethylamine gas, indicating that the composite structure effectively enhanced the material's adsorption and reaction capacity for triethylamine. 2. The detection limit is lowered, and it can detect triethylamine gas as low as 0.1 ppm, which meets the needs of trace gas monitoring and is suitable for early warning or detection of trace pollutants in harsh environments. 3. Faster response speed: The response time to triethylamine gas is only 28.6 seconds, which can promptly capture changes in gas concentration and improve real-time monitoring efficiency; 4. Strong anti-interference ability: Compared with interfering gases such as aniline, acetone, and ammonia, the sensor has significant selectivity for triethylamine; the response value (122.1) to 50 ppm triethylamine is much higher than that of dimethylamine (41.259) and hydrogen sulfide (13.883) at the same concentration, making it suitable for target gas identification in complex environments. 5. In terms of material synthesis, it is prepared by a one-step solvothermal method without the need for template agents or additional catalysts. The morphology and performance can be optimized by adjusting the amount of PdO composite (optimal 2.5 at%), reaction temperature (180℃) and time (10h). The process is simple and easy to control, and is suitable for large-scale production. 6. Economical material cost: Pd is cheaper than precious metals such as Pt and Au, and has a low composite content (only 2.5 at%). It reduces material costs while maintaining high performance, combining technological advancement with industrial applicability. In addition, considering the high toxicity and volatility of triethylamine, this sensor can monitor trace leaks in the environment in real time, providing an effective tool for industrial safety, occupational health protection and environmental protection, reducing human exposure risks and ecological hazards. Attached Figure Description

[0012] Figure 1 XRD patterns of PdO / α-MoO3 with different PdO loading molar ratios (0.5, 1, 2.5, 5, 7.5 at%) after calcination in air at 400 °C for 1 h.

[0013] Figure 2 XPS full spectra of PdO / α-MoO3 powders with different PdO loading molar ratios (0.5, 1, 2.5, 5, 7.5 at%): (a) and (b) fine XPS spectra of Pd 3d.

[0014] Figure 3 SEM images of (a, b) precursors and (c) calcined product;

[0015] Figure 4 (a, b) TEM images, (c) HRTEM image and (d) SAED image of PdO / α-MoO3 hollow microsphere composite material grown in situ in ceramic tube;

[0016] Figure 5(a) Temperature-response curves of 50 ppm triethylamine at different operating temperatures for a multi-level α-MoO3 and 2.5 at% PdO / α-MoO3 hollow microsphere sensor and (b) Response of the sensor to 50 ppm of different gases at 217 °C.

[0017] Figure 6 (a) Response-recovery curves of a multi-level PdO / α-MoO3 hollow microsphere sensor to 0.1-50 ppm triethylamine and (b) concentration-response value curves of the gas.

[0018] Figure 7 Here are schematic diagrams of the band structure: (a) PdO and MoO3 before recombination, (b) PdO and MoO3 after recombination, (c, d) Reaction mechanism diagrams of PdO / α-MoO3 in air and triethylamine gas. DETAILED DESCRIPTION

[0019] Example 1

[0020] This invention relates to a method for preparing PdO / α-MoO3 hollow microspheres based on in-situ growth, comprising the following specific steps: Step 1: Weigh 0.17g of molybdenum acetylacetonate and 0.0029g of palladium acetate, then dissolve them in a mixed solution of 20mL n-butanol and 15mL glacial acetic acid. Sonicate the mixture until dissolved and stir magnetically at room temperature for 1h. Control the molar ratio of palladium to molybdenum in the molybdenum acetylacetonate and palladium acetate in Step 1 to be Pd / Mo = 2.5 at%. Step 2: Transfer the mixed solution from Step 1 to a reactor with a 50 mL polytetrafluoroethylene liner, while suspending the ceramic tube inside the reactor. React at 180°C for 10 hours, then cool to room temperature. Step 3: Centrifuge to collect the dark blue precipitate obtained after cooling in Step 2, and rinse the ceramic tube several times with ultrapure water and anhydrous ethanol to obtain the α-MoO3 precursor; Step 4: Calcine the ceramic tube and the obtained α-MoO3 precursor powder in air at 400℃ for 1 hour. It can be observed that the color of the product powder gradually changes from grayish-white to light yellow, and finally PdO / α-MoO3 hollow microspheres are obtained.

[0021] This invention also relates to a method for preparing a gas-sensitive element, comprising the following specific steps: Step 1: Take out the ceramic tube containing 2.5 at% PdO / α-MoO3 material after calcination; Step 2: Insert a heated Ni-Cr wire into the ceramic tube and weld it to the gas-sensitive element base to make a deposited film type side-heated gas-sensitive element; Step 3: Place the gas-sensitive element prepared in Step 3 in an aging bench to enhance stability and reproducibility. The aging temperature in Step 3 is 300℃ and the aging time is 2 days.

[0022] Comparative Experiment 1

[0023] The 2.5 at% PdO / α-MoO3 hollow microspheres prepared in Example 1 and their application in the preparation of gas-sensitive elements were tested, and the results were obtained:

[0024] like Figure 1 The X-ray diffraction patterns of PdO / α-MoO3 with different PdO loading molar ratios after calcination in air at 400℃ for 1 h are shown in the figure. It can be seen from the figure that the diffraction peaks appearing at 2θ angles of 12.76°, 23.33°, 25.70°, 27.33°, 29.70°, 33.73°, 34.50°, 38.98°, 45.74° and 49.24° correspond to the (020), (110), (040), (021), (130), (111), (041), (060), (200) and (002) crystal planes of orthorhombic α-MoO3 (No.05-0508). It can be seen from the XRD image that no other diffraction peaks (palladium or palladium oxide) appear. This may be because the palladium content is relatively low or the crystallinity is not high, and the XRD cannot detect its content.

[0025] like Figure 2 As shown, XPS full spectra of PdO / α-MoO3 powders with different PdO loading molar ratios are (a) and (b) fine XPS spectra of Pd 3d, from the full spectrum of PdO / α-MoO3 composite materials. Figure 2 a) It can be seen that the main elements contained in these five PdO / α-MoO3 composite materials are Mo, C, O, and Pd, where C is the standard carbon used for peak correction; in order to further determine the state of Pd on the surface of α-MoO3 microspheres, Figure 2 b presents fine XPS spectra of Pd in ​​PdO / α-MoO3 microspheres with composite molar ratios of 0.5 at%, 1.0 at%, 2.5 at%, 5.0 at%, and 7.5 at%. Due to spin-orbit interactions, Pd 3d splits into Pd 3d... 3 / 2 and Pd 3d 5 / 2 The peaks were located at 341.5 eV and 336.1 eV, respectively, but the peak intensities were very weak, indicating that the amount of recombination was relatively small. Comparison with the standard binding energy showed that Pd in ​​the sample was in the form of Pd... 2+ It exists in ionic form; furthermore, from Figure 2 b. It can be observed that when the theoretical composite molar ratio increases, the peak height of the Pd 3d orbital also increases slightly, indicating that the relative content of PdO also increases accordingly.

[0026] like Figure 3 As shown, SEM images of the PdO / α-MoO3 composite precursor grown in situ on a ceramic tube and the product after calcination at 400℃ are presented; Figure 3 As shown in Figure a, the precursor is a uniformly sized and well-dispersed spherical structure, with individual spheres having a diameter of approximately 600 nm-900 nm, as determined by high-magnification SEM. Figure 3 b discovered that the precursor microspheres were constructed from nanoparticles with rough surfaces; Figure 3 c is a SEM image of the α-MoO3 microspheres obtained after calcination. It can be seen that the microsphere size increases and the building unit changes from nanoparticles to nanoplates.

[0027] like Figure 4 As shown, (a, b) TEM images, (c) HRTEM image, and (d) SAED image of the PdO / α-MoO3 hollow microsphere composite material grown in situ in ceramic tubes are presented. Figure 4 As can be seen from a, the 2.5 at% PdO / α-MoO3 microspheres have a hollow structure, and their building blocks are composed of nanoplates with a thickness of approximately 30 nm. Figure 4 b); Figure 4 c is a high-resolution transmission electron microscope (HRTEM) image showing clear lattice fringes with a plane spacing of 0.326 nm, corresponding to the (021) crystal plane of α-MoO3. There are also lattice fringes with a plane spacing of 0.253 nm, corresponding to the (101) crystal plane of PdO, indicating that PdO has been successfully composited in MoO3; the sample's SAED... Figure 4 d indicates that PdO / α-MoO3 has a polycrystalline structure;

[0028] like Figure 5 As shown, the temperature-response curves of the multi-level α-MoO3 and 2.5 at% PdO / α-MoO3 hollow microsphere sensors at different operating temperatures to 50 ppm triethylamine are plotted (a), and the response curve to 50 ppm of different gases at 217 °C is plotted (b). Figure 5Figure a shows the response of the sensor to 50 ppm triethylamine gas within the operating temperature range of 90℃-250℃. It can be seen that the response value of the gas sensor increases with increasing operating temperature, reaching a maximum at 217℃. Then, with further increases in operating temperature, the response value of the gas-sensitive element decreases. At the same operating temperature, the 2.5 at% PdO / α-MoO3 sensor exhibits a higher response value compared to the α-MoO3 sensor. At the optimal operating temperature of 217℃, the response value of the 2.5 at% PdO / α-MoO3 sensor reaches 122.1 for 50 ppm triethylamine gas. This is because the PdO composite... The sensing performance of the α-MoO3 sensor was improved. Selectivity is also an important parameter for the sensor in practical complex environments. At the optimal operating temperature of 217℃, the response graphs of 2.5at% PdO / α-MoO3 and α-MoO3 sensors to 50ppm triethylamine and other toxic gases, such as aniline (C6H7N), acetone (C3H6O), benzene (C6H6), ammonia (NH3), ethanol (C2H5OH), toluene (C7H8), hydrogen sulfide (H2S), trimethylamine (C3H9N) and dimethylamine (C2H7N) are shown in Figure 5b, indicating that the sensor has good selectivity for triethylamine.

[0029] like Figure 6 As shown, the response-recovery curves (a) and concentration-response value curves (b) of the multi-level structure PdO / α-MoO3 hollow microsphere sensor to 0.1-50 ppm triethylamine are presented. At the optimal operating temperature of 217℃, the response-recovery curves of the 2.5 at% PdO / α-MoO3 microsphere sensor to different concentrations of triethylamine gas are also shown. Figure 6 As shown in Figure a, the concentration range is 0.1ppm-50ppm. When the PdO / α-MoO3 composite material is exposed to the reducing gas triethylamine, its resistance immediately decreases and reaches a minimum value. When the PdO / α-MoO3 composite material is placed in air, it recovers to the initial resistance value in air within a specific time. The gas response process exhibits rapid response and good reversibility, which corresponds to the gas-sensitive characteristics of n-type semiconductor oxides. Both sensors can reach the initial resistance value in air after each cycle, indicating that the sensor has good reversibility. The response time of the 2.5at% PdO / α-MoO3 microsphere sensor is 28.6s. Compared with the response value of the pure phase α-MoO3 microsphere sensor to the same concentration of triethylamine gas, the element with PdO composite shows improvements in both response value and sensitivity. Figure 6 b demonstrates the response of the 2.5 at% PdO / α-MoO3 sensor to different concentrations of triethylamine at an operating temperature of 217 °C. The sensor's response value increases with increasing triethylamine gas concentration, exhibiting a good linear relationship between 0.1 ppm and 50 ppm concentrations, with a linear correlation coefficient R0.2 =0.972.

[0030] Comparative Experiment 2

[0031] The triethylamine sensitivity mechanism of the 2.5 at% PdO / α-MoO3 hollow microspheres prepared in Example 1 was tested, and the results were obtained:

[0032] like Figure 7 As shown, the band structure diagram is as follows: (a) before PdO and MoO3 recombination, (b) after PdO and MoO3 recombination, (c, d) reaction mechanism diagram of PdO / α-MoO3 in air and triethylamine gas. PdO is a p-type semiconductor. After recombination with α-MoO3, a pn heterojunction is formed. Due to the difference in Fermi levels between the two materials, electrons will transfer from the material with a higher Fermi level to the material with a lower Fermi level until the Fermi levels of the two materials reach equilibrium. During this process, the band will bend, such as... Figure 7 As shown in a, b; Figure 7 In options c and d, at an operating temperature of 217℃, because the work function of PdO (7.9 eV) is higher than that of α-MoO3 (6.9 eV), and the Fermi level of α-MoO3 is higher than that of PdO, electrons transfer from α-MoO3 to PdO, leading to an increase in the electron depletion layer thickness and simultaneously increasing the resistance Ra of PdO / α-MoO3 in air. When the PdO / α-MoO3 sensor is placed in triethylamine gas, electrons are released into the conduction band of the material, resulting in a decrease in the electron depletion layer thickness. This reduces the resistance Rg of the PdO / α-MoO3 sensor in triethylamine gas, thus improving the response of the α-MoO3 bulk material to triethylamine. Based on this, the sensing performance for triethylamine can be improved by complexing an appropriate amount of PdO onto the α-MoO3 sensor.

[0033] Components not described in detail in this article are existing technologies.

[0034] While the specific embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention, and modifications or variations without creative effort are still within the protection scope of the present invention.

Claims

1. A method for preparing hollow PdO / α-MoO3 microspheres based on in-situ growth, characterized in that, The specific steps include the following: Step 1: Weigh 0.17g of molybdenum acetylacetonate and 0.0029g of palladium acetate, then dissolve them in a mixed solution of 20mL n-butanol and 15mL glacial acetic acid. Sonicate the mixture until dissolved and stir magnetically at room temperature for 1 hour. Step 2: Transfer the mixed solution from Step 1 to a reactor with a 50 mL polytetrafluoroethylene liner, while suspending the ceramic tube inside the reactor. React at 180°C for 10 hours, then cool to room temperature. Step 3: Centrifuge to collect the dark blue precipitate obtained after cooling in Step 2, and rinse the ceramic tube several times with ultrapure water and anhydrous ethanol to obtain the α-MoO3 precursor; Step 4: Calcine the ceramic tube and the obtained α-MoO3 precursor powder in air at 400℃ for 1 hour. It can be observed that the color of the product powder gradually changes from grayish-white to light yellow, and finally PdO / α-MoO3 hollow microspheres are obtained.

2. The preparation method based on in-situ grown PdO / α-MoO3 hollow microspheres according to claim 1, characterized in that: The molar ratio of palladium to molybdenum in the acetylacetone molybdenum and palladium acetate mixtures in step one is controlled to be Pd / Mo = 2.5 at%.

3. A method for preparing a gas-sensitive element, characterized in that: The 2.5 at% PdO / α-MoO3 composite material prepared by the method described in claim 1 or 2 includes the following specific steps: Step 1: Take out the ceramic tube containing 2.5 at% PdO / α-MoO3 material after calcination; Step 2: Insert a heated Ni-Cr wire into the ceramic tube and weld it to the gas-sensitive element base to make a deposited film type side-heated gas-sensitive element; Step 3: Place the gas-sensitive element prepared in Step 3 in an aging bench to enhance stability and reproducibility.

4. The method for preparing a gas-sensitive element according to claim 3, characterized in that: In step three, the aging temperature is 300℃ and the aging time is 2 days.

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