A Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film and preparation method thereof

Abstract

A Zr-W ion pair co-doped bismuth vanadate-based photovoltaic thin film and its preparation method, which belong to the field of energy conversion in power supply technology. The present invention aims to solve the problems that the photocurrent density of existing bismuth vanadate materials is far less than the theoretical value, and the preparation method is complex and not suitable for mass production. The chemical general formula of the Zr-W ion pair co-doped bismuth vanadate-based photovoltaic thin film is Bi(Zr x W x )V 1‑2x O4. Preparation method: First, pre-treat the conductive substrate; second, weigh; third, prepare the sol; fourth, age; fifth, spin-coat, pyrolyze and anneal. The present invention is used for the Zr-W ion pair co-doped bismuth vanadate-based photovoltaic thin film and its preparation.

Description

Technical Field

[0001] The invention belongs to the field of energy conversion of power supply technology. Background Art

[0002] With the rapid development of science and technology, energy issues have become the main problem restricting the economic development of various countries. With the continuous improvement of human environmental awareness, photovoltaic power generation technology has become an important way to develop a low-carbon economy. Through the use of solar energy, energy diversification has been achieved. Its advantage is that the preparation of photovoltaic thin films is simple and has almost no impact on the environment. The mechanism of the photovoltaic effect is that when a semiconductor absorbs particles with energy higher than the intrinsic energy gap, the electrons at the top of the valence band will be excited to transition to the conduction band, generating electron and hole carriers. These excited carriers will quickly decay to the ground state and maintain energy conservation by emitting photons or phonons. However, when the electrostatic potential in the semiconductor has an asymmetric structure (built-in electric field), it may separate the electron carriers and the hole carriers and move in opposite directions, resulting in electron flow or hole flow, that is, current is generated.

[0003] Among the many existing semiconductor materials, bismuth vanadate has attracted a lot of attention as a lead-free material. In the past, bismuth vanadate materials were mostly used in the field of photoelectrochemistry. The band gap width (2.4eV) of bismuth vanadate is moderate, and it has good visible light absorption performance. Its good photoelectric performance is considered to be a good photovoltaic thin film material, but its actual photocurrent density is much smaller than the theoretical value. And there is still a large gap in the research on the photovoltaic performance of bismuth vanadate, and in the existing preparation method of bismuth vanadate, the operation is difficult and the steps are complicated, which is not suitable for mass production and has great limitations. For example, bismuth vanadate powder is prepared by solid phase reaction method, and a very high temperature is required to synthesize bismuth vanadate powder; bismuth vanadate thin film material prepared by radio frequency magnetron sputtering needs to consider more parameters and the machine is expensive. Summary of the invention

[0004] The present invention aims to solve the problems that the photocurrent density of the existing bismuth vanadate material is far less than the theoretical value, the preparation method is complicated and not suitable for mass production, and provides a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film and a preparation method thereof.

[0005] A Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film, the chemical formula of which is Bi(Zr x W x )V 1-2x O 4 , where x = 0.02~0.08.

[0006] A method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film is carried out according to the following steps:

[0007] 1. ultrasonically cleaning and drying the conductive substrate to obtain a pretreated conductive substrate;

[0008] 2. According to the chemical formula, Bi(Zr x W x )V 1-2x O 4 Weigh the vanadium source, bismuth source, zirconium source and tungsten source in a stoichiometric ratio; wherein x = 0.02 to 0.08;

[0009] 3. Add the weighed vanadium source to the solvent, heat and stir evenly, add the zirconium source and tungsten source after cooling, continue to heat and stir evenly, cool again, add the bismuth source and stir evenly at room temperature, finally add the stabilizer and stir evenly at room temperature to obtain a sol;

[0010] 4. subjecting the sol to aging treatment to obtain a colloid;

[0011] 5. ① Add the colloid dropwise onto the surface of the pretreated conductive substrate, and then spin-coat to form a thin film, which is then pyrolyzed and annealed;

[0012] ② Repeat step 5① 6 to 9 times to obtain a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film.

[0013] The beneficial effects of the present invention are:

[0014] 1. Compared with the undoped bismuth vanadate film, the photocurrent density of the Zr-W ion co-doped bismuth vanadate film prepared by the present invention is 0.18 mA / cm 2 Increased to 0.84mA / cm 2 , the open circuit voltage increased from 0.099V to 0.44V.

[0015] 2. The present invention adopts a simple sol-gel method to prepare bismuth vanadate photoelectric thin film materials. This method is simple and easy, the equipment is low-cost, there is no pollution to the environment during the experiment, it conforms to the concept of green development, the process cycle is short, and a large number of samples can be prepared in a short time. Compared with other methods, it greatly saves construction time.

[0016] The invention is used for a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film and a preparation method thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The X-ray diffraction diagram of the bismuth vanadate-based photovoltaic film co-doped with Zr-W ion pairs, (a) is the full spectrum, (b) is the (112) peak magnification diagram, 1 is a comparative experiment, 2 is Example 1, 3 is Example 2, 4 is Example 3, and 5 is Example 4;

[0018] Figure 2 450nm blue light (160mW / cm 2) irradiation, the current density-voltage (JV) curve and photocurrent density J of Zr-W ion pair co-doped bismuth vanadate-based photovoltaic films SC and open circuit voltage V OC Change trend diagram, (a) is the current density-voltage (JV) curve, 1 is the comparative experiment, 2 is the embodiment 1, 3 is the embodiment 2, 4 is the embodiment 3, 5 is the embodiment 4, (b) is the photocurrent density J SC and open circuit voltage V OC Changing trends;

[0019] Figure 3 The Jt curves of the Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film prepared in Example 3, (a) is the Jt curve of continuous operation for 1350 s under 450 nm light, and (b) is the Jt curve under 450 nm light switching;

[0020] Figure 4 The Jt curves of the prepared Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to Example 3 with zero bias of switching light at different wavelengths, (a) is 365nm, (b) is 450nm, (c) is 500nm, (d) is a statistical bar graph of the rise time and fall time, 1 is the rise time, 2 is the fall time;

[0021] Figure 5 The JV curve and photocurrent density J during the heating process of the Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film prepared in Example 3 SC and open circuit voltage V OC The change trend with increasing temperature, (a) is the current density-voltage (JV) curve tested at different temperatures, (b) is the photocurrent density J SC and open circuit voltage V OC The changing trend with increasing temperature, 1 is the open circuit voltage, 2 is the photocurrent density. DETAILED DESCRIPTION

[0022] Specific implementation method 1: In this implementation method, a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film has a chemical formula of Bi(Zr x W x )V 1-2x O 4 , where x = 0.02~0.08.

[0023] This specific embodiment adopts the sol-gel method, and uses a spin-coating machine to evenly coat the colloid on the conductive substrate to form a dense bismuth vanadate film. After each spin coating, the film is placed on a flat furnace for pyrolysis. After a certain period of time, the film material is placed in an RTA rapid annealing furnace for crystallization.

[0024] During the spin coating process of this specific implementation method, the surrounding air should be kept dry as much as possible, that is, the spin coating is performed in an environment with suitable humidity; the film is in a wet film state after each spin coating, and the film is immediately pyrolyzed after the spin coating is completed, the purpose of which is to evaporate the solvent so that the solvent can fully evaporate from the film and achieve the effect of pre-crystallization.

[0025] In this embodiment, Zr and W having an average ion radius larger than V are doped into bismuth vanadate by a sol-gel method through ion pair doping, and ion pair co-doping is used to cause lattice distortion of bismuth vanadate and improve its photovoltaic performance.

[0026] The beneficial effects of this embodiment are:

[0027] 1. Compared with the undoped bismuth vanadate film, the photocurrent density of the Zr-W ion co-doped bismuth vanadate film prepared in this embodiment is 0.18 mA / cm 2 Increased to 0.84mA / cm 2 , the open circuit voltage increased from 0.099V to 0.44V.

[0028] 2. This embodiment adopts a simple sol-gel method to prepare bismuth vanadate photoelectric thin film materials. This method is simple and easy, the equipment is low-cost, there is no pollution to the environment during the experiment, it conforms to the concept of green development, the process cycle is short, and a large number of samples can be prepared in a short time. Compared with other methods, it greatly saves construction time.

[0029] Specific implementation method 2: This implementation method is a method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film, which is carried out according to the following steps:

[0030] 1. ultrasonically cleaning and drying the conductive substrate to obtain a pretreated conductive substrate;

[0031] 2. According to the chemical formula, Bi(Zr x W x )V 1-2x O 4 Weigh the vanadium source, bismuth source, zirconium source and tungsten source in a stoichiometric ratio; wherein x = 0.02 to 0.08;

[0032] 3. Add the weighed vanadium source to the solvent, heat and stir evenly, add the zirconium source and tungsten source after cooling, continue to heat and stir evenly, cool again, add the bismuth source and stir evenly at room temperature, finally add the stabilizer and stir evenly at room temperature to obtain a sol;

[0033] 4. subjecting the sol to aging treatment to obtain a colloid;

[0034] 5. ① Add the colloid dropwise onto the surface of the pretreated conductive substrate, and then spin-coat to form a thin film, which is then pyrolyzed and annealed;

[0035] ② Repeat step 5① 6 to 9 times to obtain a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film.

[0036] Specific implementation method 3: This implementation method is different from specific implementation method 2 in that: the conductive substrate described in step 1 is a Pt sheet, ITO or FTO conductive glass; the ultrasonic cleaning described in step 1 is specifically carried out under the condition of a power of 90W to 100W, using propanol, deionized water and anhydrous ethanol for ultrasonic cleaning for 10min to 30min respectively. The rest is the same as specific implementation method 2.

[0037] Specific embodiment 4: This embodiment is different from specific embodiment 2 or 3 in that: the vanadium source in step 2 is vanadium acetylacetonate; the bismuth source in step 2 is bismuth nitrate pentahydrate, bismuth sulfate, bismuth acetate or bismuth citrate; the zirconium source in step 2 is zirconium n-propoxide, zirconium acetate, zirconium acetylacetonate or n-butyl zirconium; the tungsten source in step 2 is ammonium metatungstate. The rest is the same as specific embodiment 2 or 3.

[0038] Specific embodiment 5: This embodiment differs from Specific embodiments 2 to 4 in that: the solvent in step 3 is a mixed solution of ethylene glycol methyl ether and acetic acid in a volume ratio of (8-9):2; the stabilizer in step 3 is acetylacetone. The rest is the same as Specific embodiments 2 to 4.

[0039] Specific embodiment 6: This embodiment is different from specific embodiments 2 to 5 in that: the volume ratio of the total mole number of the vanadium source, bismuth source, zirconium source and tungsten source described in step 3 to the solvent is 0.002 mol: (10-15) mL; the volume ratio of the total mole number of the vanadium source, bismuth source, zirconium source and tungsten source described in step 3 to the stabilizer is 0.002 mol: (2-3) mL. The rest is the same as specific embodiments 2 to 5.

[0040] Specific embodiment 7: This embodiment is different from Specific embodiments 2 to 6 in that the aging treatment described in step 4 is to leave the mixture to stand in a dark place for 12 to 24 hours. The rest is the same as Specific embodiments 2 to 6.

[0041] Specific embodiment 8: This embodiment differs from specific embodiments 2 to 7 in that the spin coating described in step 5① is carried out for 20s to 40s under the conditions of an ambient humidity of 20% to 30% and a rotation speed of 2000rpm to 8000rpm. The rest is the same as specific embodiments 2 to 7.

[0042] Specific embodiment 9: This embodiment is different from specific embodiments 2 to 8 in that the pyrolysis in step 5 is carried out at a temperature of 300° C. to 350° C. for 3 to 5 minutes. The rest is the same as specific embodiments 2 to 8.

[0043] Specific embodiment ten: This embodiment is different from any of specific embodiments two to nine in that the annealing treatment described in step five is specifically carried out according to the following steps: in an air atmosphere, first raise the temperature from room temperature to 250℃~260℃, and keep it warm for 10s~20s at a temperature of 250℃~260℃, then raise the temperature from 250℃~260℃ to 300℃~320℃, and keep it warm for 25s~30s at a temperature of 300℃~320℃, then raise the temperature from 300℃~320℃ to 400℃~420℃, and keep it warm for 20s~25s at a temperature of 400℃~420℃, then raise the temperature from 400℃~420℃ to 500℃~510℃, and keep it warm for 300s~350s at a temperature of 500℃~510℃, and finally cool down with the furnace by 70℃ to below. The rest is the same as the second to ninth embodiments.

[0044] The following examples are used to verify the beneficial effects of the present invention:

[0045] Embodiment 1:

[0046] A Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film, the chemical formula of which is Bi(Zr x W x )V 1-2x O 4 , where x = 0.2.

[0047] The method for preparing the above-mentioned Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film is carried out according to the following steps:

[0048] 1. Under the condition of a power of 100 W, the conductive substrate was ultrasonically cleaned with propanol, deionized water and anhydrous ethanol for 15 minutes respectively, and then the anhydrous ethanol droplets on the surface were wiped off with lens cleaning paper. Finally, it was preheated on a flat furnace at a temperature of 300°C until the water vapor on the surface was completely evaporated, and then cooled at room temperature to obtain a pretreated conductive substrate;

[0049] 2. According to the chemical formula, Bi(Zr x W x )V 1-2x O 4 Weigh the vanadium source, bismuth source, zirconium source and tungsten source in a stoichiometric ratio; wherein x = 0.02;

[0050] 3. Add the weighed vanadium source to the solvent, heat and stir evenly at 60°C, add the zirconium source and tungsten source after cooling, continue to heat and stir evenly at 60°C, cool again, add the bismuth source and stir evenly at room temperature, finally add the stabilizer and stir evenly at room temperature to obtain a sol;

[0051] The volume ratio of the total mole number of the vanadium source, bismuth source, zirconium source and tungsten source to the solvent is 0.002 mol:12 mL; the volume ratio of the total mole number of the vanadium source, bismuth source, zirconium source and tungsten source to the stabilizer is 0.002 mol:2 mL;

[0052] 4. The sol is allowed to stand in a dark place for 12 hours to obtain a colloid;

[0053] 5. ① Place the pre-treated conductive substrate on the spin-gel machine and fix the conductive substrate on the rotating table by connecting a vacuum pump;

[0054] ② Use a pipette to absorb the colloid, drop it on the pretreated conductive substrate, spin coat for 20 seconds under the condition of ambient humidity of 20% and rotation speed of 6000rpm to obtain a wet film, place the wet film on a flat furnace at a temperature of 300℃, pyrolyze for 3min to obtain a dry film, place the dry film in an RTA rapid annealing furnace, and in an air atmosphere, first increase the temperature from room temperature to 250℃, and keep it at 250℃ for 10s, then increase the temperature from 250℃ to 300℃, and keep it at 300℃ for 30s, then increase the temperature from 300℃ to 400℃, and keep it at 400℃ for 20s, then increase the temperature from 400℃ to 500℃, and keep it at 500℃ for 300s, and finally cool down by 70℃ to below with the furnace;

[0055] ③ Repeat steps 5 and ② six times to obtain a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film, wherein the thickness of the Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film is 210 nm.

[0056] The conductive substrate described in step one is a Pt sheet; the vanadium source described in step two is vanadium acetylacetonate; the bismuth source described in step two is bismuth nitrate pentahydrate; the zirconium source described in step two is a 70wt% zirconium n-propoxide solution; and the tungsten source described in step two is ammonium metatungstate.

[0057] The solvent described in step three is a mixed solution of ethylene glycol methyl ether and acetic acid in a volume ratio of 4:1; the stabilizer described in step three is acetylacetone.

[0058] Embodiment 2: This embodiment is different from Embodiment 1 in that: x=0.04 in step 2. Other aspects are the same as Embodiment 1.

[0059] Embodiment 3: This embodiment is different from Embodiment 1 in that: x=0.06 in step 2. Other aspects are the same as Embodiment 1.

[0060] Embodiment 4: This embodiment is different from Embodiment 1 in that: x=0.08 in step 2. Other aspects are the same as Embodiment 1.

[0061] Comparative experiment: The difference between this comparative experiment and the first embodiment is that x=0 in step 2. The rest is the same as the first embodiment.

[0062] Figure 1 The X-ray diffraction diagram of the bismuth vanadate-based photovoltaic film co-doped with Zr-W ion pairs, (a) is the full spectrum, (b) is the (112) peak magnification diagram, 1 is a comparative experiment, 2 is Example 1, 3 is Example 2, 4 is Example 3, and 5 is Example 4; by comparing the X-ray diffraction spectrum with the standard PDF card, it can be seen that the prepared material is a monoclinic bismuth vanadate film, and no impurity peaks appear. From the (112) peak magnification diagram, it can be seen that with the increase of the ion pair doping content, the (112) peak gradually shifts to a lower angle, so the ion pair Zr-W ion pair co-doping will have an effect on BiVO 4 The lattice produces chemical stress.

[0063] Gold electrodes were plated onto the films prepared in Examples 1 to 4 and the comparative experiment by pulsed magnetron deposition to form circular electrodes with a diameter of r = 0.02 mm, and the following tests were performed:

[0064] Figure 2 450nm blue light (160mW / cm 2 ) irradiation, the current density-voltage (JV) curve and photocurrent density J of Zr-W ion pair co-doped bismuth vanadate-based photovoltaic films SC and open circuit voltage V OC Change trend diagram, (a) is the current density-voltage (JV) curve, 1 is the comparative experiment, 2 is the embodiment 1, 3 is the embodiment 2, 4 is the embodiment 3, 5 is the embodiment 4, (b) is the photocurrent density J SC and open circuit voltage V OC It can be seen that when the Zr-W doping content reaches 6%, the photocurrent density of the bismuth vanadate photovoltaic film increases from 0.18 mA / cm 2 Increased to 0.84mA / cm 2 , the open circuit voltage increased from 0.099V to 0.44V.

[0065] Figure 3The Jt curve of the Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film prepared in Example 3, (a) is the Jt curve of continuous operation for 1350s under 450nm illumination, and (b) is the Jt curve under 450nm illumination switching; in Figure (a), the light source is turned off at 1150s, and then turned on after 40s to 50s. It can be seen from the figure that the prepared thin film material has time effectiveness; in Figure (b), the light source is turned on for 10s and turned off for 10s as one cycle, and 14 cycles of switching light tests are performed within 300s. It can be seen from the figure that instantaneous polarization has no effect on the photovoltaic response of the prepared bismuth vanadate film.

[0066] Figure 4 The Jt curves of the Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film prepared in Example 3 with zero bias of switching light at different wavelengths, (a) is 365nm, (b) is 450nm, (c) is 500nm, (d) is a statistical bar graph of the rise time and fall time, 1 is the rise time, 2 is the fall time; the maximum photocurrent density J SC The part between 10% and 90% is regarded as the rise time and fall time; it can be seen from the figure that under 450nm illumination, the Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film prepared in Example 3 has the shortest rise time of 6.85ms, because the energy of 450nm is closer to the band gap of BVO.

[0067] Figure 5 The JV curve and photocurrent density J during the heating process of the Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film prepared in Example 3 SC and open circuit voltage V OC The change trend with increasing temperature, (a) is the current density-voltage (JV) curve tested at different temperatures, (b) is the photocurrent density J SC and open circuit voltage V OC The change trend with increasing temperature, 1 is the open circuit voltage, 2 is the photocurrent density; it can be seen from the figure that the photocurrent density is 0.83mA / cm at 273K 2 As the temperature increases to 523K, it becomes 1.77mA / cm 2 When the temperature increases from 273K to 523K, the highest open circuit voltage is 0.44mV and the lowest is 0.375mV. The increase in photocurrent density is due to the rapid increase in carrier concentration when the temperature is relatively low, but the internal resistance of the material will decrease with the increase in temperature, resulting in a decrease in open circuit voltage.

Claims

1. A Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film, Features Its chemical formula is Bi(Zr x W x )V 1-2x O 4 , where x = 0.02~0.

08.

2. The method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to claim 1, Features It is carried out in the following steps:

1. ultrasonically cleaning and drying the conductive substrate to obtain a pretreated conductive substrate; 2. According to the chemical formula, Bi(Zr x W x )V 1-2x O 4 Weigh the vanadium source, bismuth source, zirconium source and tungsten source in a stoichiometric ratio; wherein x = 0.02 to 0.08; 3. Add the weighed vanadium source to the solvent, heat and stir evenly, add the zirconium source and tungsten source after cooling, continue to heat and stir evenly, cool again, add the bismuth source and stir evenly at room temperature, finally add the stabilizer and stir evenly at room temperature to obtain a sol; 4. subjecting the sol to aging treatment to obtain a colloid; 5. ① Add the colloid dropwise onto the surface of the pretreated conductive substrate, and then spin-coat to form a thin film, which is then pyrolyzed and annealed; ② Repeat step 5① 6 to 9 times to obtain a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film.

3. The method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to claim 2, Features The conductive substrate described in step one is a Pt sheet, ITO or FTO conductive glass; the ultrasonic cleaning described in step one is specifically carried out under the condition of a power of 90W to 100W, using propanol, deionized water and anhydrous ethanol for ultrasonic cleaning for 10min to 30min respectively.

4. The method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to claim 2, Features The vanadium source described in step 2 is vanadium acetylacetonate; the bismuth source described in step 2 is bismuth nitrate pentahydrate, bismuth sulfate, bismuth acetate or bismuth citrate; the zirconium source described in step 2 is zirconium n-propoxide, zirconium acetate, zirconium acetylacetonate or n-butyl zirconium; the tungsten source described in step 2 is ammonium metatungstate.

5. The method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to claim 2, Features The solvent described in step 3 is a mixed solution of ethylene glycol methyl ether and acetic acid in a volume ratio of (8-9):2; and the stabilizer described in step 3 is acetylacetone.

6. The method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to claim 2, Features The volume ratio of the total mole number of the vanadium source, bismuth source, zirconium source and tungsten source described in step three to the solvent is 0.002 mol: (10-15) mL; the volume ratio of the total mole number of the vanadium source, bismuth source, zirconium source and tungsten source described in step three to the stabilizer is 0.002 mol: (2-3) mL.

7. The method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to claim 2, Features The aging treatment described in step 4 is specifically to stand and age for 12h to 24h in a dark place.

8. The method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to claim 2, Features The spin coating described in step ① in step 5 is specifically carried out under the conditions of an ambient humidity of 20% to 30% and a rotation speed of 2000 rpm to 8000 rpm for 20s to 40s.

9. The method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to claim 2, Features The pyrolysis described in step 5 is specifically carried out at a temperature of 300° C. to 350° C. for 3 min to 5 min.

10. The method for preparing a Zr-W ion pair co-doped bismuth vanadate-based photovoltaic film according to claim 2, Features The annealing treatment described in the step 5 specifically proceeds according to the following steps: under air atmosphere, first the temperature is heated to 250 ℃ ~ 260 ℃ by room temperature, and under the condition of 250 ℃ ~ 260 ℃, be incubated for 10s ~ 20s, then the temperature is heated to 300 ℃ ~ 320 ℃ by 250 ℃ ~ 260 ℃, and under the condition of 300 ℃ ~ 320 ℃, be incubated for 25s ~ 30s, then the temperature is heated to 400 ℃ ~ 420 ℃ by 300 ℃ ~ 320 ℃, and under the condition of 400 ℃ ~ 420 ℃, be incubated for 20s ~ 25s, then the temperature is heated to 500 ℃ ~ 510 ℃ by 400 ℃ ~ 420 ℃, and under the condition of 500 ℃ ~ 510 ℃, be incubated for 300s ~ 350s, and finally cool down by furnace cooling by 70 ℃ to below.

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