Preparation method of platinum-zirconia porous catalytic electrode slurry and catalytic electrode

Abstract

The application provides a platinum-zirconia porous catalytic electrode slurry and a preparation method of a catalytic electrode. The platinum-zirconia porous catalytic electrode slurry comprises the following components in a weight ratio: 50%-65% of platinum powder, 10%-20% of a pore-forming agent, 2%-5% of 8YSZ, 2%-5% of metal carbide powder and 15%-30% of an organic carrier. The particle size of the platinum powder is 1-3 microns, and the decomposition temperature of the metal carbide is 600-1300 DEG C. In the slurry, the carbide is added, and in the sintering process of the slurry, the carbide is decomposed to form oxides. These oxides can block between platinum particles. Meanwhile, under the synergistic action of the pore-forming agent, the growth of the platinum particles is effectively inhibited, so that the catalytic specific surface area is retained to a greater extent, and the catalytic activity of the electrode is enhanced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electronic slurry, in particular to a platinum-zirconia porous catalytic electrode slurry and a preparation method of the catalytic electrode. BACKGROUND

[0002] The oxygen sensor is mainly used to monitor the oxygen content in the exhaust gas after engine combustion, and convert the oxygen content into a voltage signal to the ECU (Engine Control Unit), and the ECU analyzes the concentration of the mixture according to the signal, and modifies the injection time as appropriate, so that the engine obtains the best concentration of the mixture.

[0003] The platinum nanomaterial is widely used in various fields due to its excellent catalytic activity, but also because of the large specific surface area and large surface energy caused by small size, it tends to agglomerate and grow during drying and sintering. Due to the above reasons, at present, the platinum nanostructure in the preparation process of the sensor, the catalytic activity is obviously unable to achieve the expected situation. Especially as the platinum-based electrode material applied to the oxygen sensor, in order to realize the co-firing with the zirconia base porcelain film at high temperature (above 1400℃), and still need to maintain a high surface area after sintering as much as possible, therefore, it is necessary to reduce the agglomeration and growth of platinum grains in this process as much as possible; At the same time, for the electrolyte type gas sensor, such as zirconia-based oxygen sensor, the catalytic electrode also needs to obtain more "zirconia-platinum-gas" porous three-phase interface, so as to have high catalytic activity.

[0004] Therefore, it is necessary to design a platinum-zirconia porous catalytic electrode slurry and a preparation method of the catalytic electrode to solve the above problems. SUMMARY

[0005] The purpose of the present application is to provide a platinum-zirconia porous catalytic electrode slurry which can enhance the interface bonding strength between the electrode and the substrate and inhibit the sintering and growth of platinum particles at high temperature, so as to balance the catalytic activity and the reliability of the electrode.

[0006] In order to achieve the above purpose, the present application adopts the following technical scheme: a platinum-zirconia porous catalytic electrode slurry, comprising the following components in weight ratio: 50%-65% platinum powder, 10%-20% pore-forming agent, 2%-5% 8YSZ, 2%-5% metal carbide powder and 15%-30% organic carrier, wherein the particle size of the platinum powder is 1-3μm, and the decomposition temperature of the metal carbide is 600-1300℃.

[0007] As a further improved technical scheme of the present application, the metal carbide powder is one or both of zirconium carbide and aluminum carbide, and the particle size of the metal carbide is 200nm-1μm.

[0008] As a further improved technical solution of the present application, the molar ratio of aluminum carbide to zirconium carbide in the metal carbide powder is 1:(1-5).

[0009] As a further improved technical solution of the present application, the particle size of the pore-forming agent is 20 nm-10 μm, and the temperature at which the pore-forming agent forms pores in the catalytic electrode is 600-1200℃.

[0010] As a further improved technical solution of the present application, the pore-forming agent comprises nano-carbon powder with a particle size of 20-100 nm and micron-carbon powder with a particle size of 3-10 μm, the weight of the nano-carbon powder accounts for 40-60% of the total weight of the pore-forming agent, and the weight of the micron-carbon powder accounts for 40-60% of the total weight of the pore-forming agent.

[0011] As a further improved technical solution of the present application, the organic carrier comprises ethyl cellulose in a weight ratio of 15-30% and a solvent in a weight ratio of 70-85%, the solvent being selected from one or more of terpineol, butyl carbitol acetate, diethylene glycol butyl ether, diethylene glycol ethyl ether, and diethylene glycol butyl ether acetate.

[0012] As a further improved technical solution of the present application, the particle size of the 8YSZ is 200 nm-1 μm.

[0013] The present application also aims to provide a preparation method of a platinum-zirconia porous catalytic electrode that enhances the interface bonding strength of the electrode and the substrate and inhibits the sintering and growth of platinum particles at high temperatures, thereby balancing the catalytic activity and the reliability of the electrode.

[0014] To achieve the above-mentioned purposes, the present application adopts the following technical solution: a preparation method of a platinum-zirconia porous catalytic electrode, comprising the following steps:

[0015] Step 1: stirring the solvent and ethyl cellulose until they are uniformly mixed to obtain an organic carrier;

[0016] Step 2: mixing platinum powder with 8YSZ and performing ball milling, and then drying to obtain platinum-8YSZ premixed powder;

[0017] Step 3: placing the platinum-8YSZ premixed powder, pore-forming agent, metal carbide powder, and organic carrier into a homogenizer to obtain a slurry semi-product;

[0018] Step 4: transferring the slurry semi-product obtained in Step 3 into a three-roll mill to perform grinding and vacuum defoaming, thereby obtaining a slurry product;

[0019] Step 5: printing the slurry product on a zirconia green ceramic membrane and performing sintering.

[0020] As a further improved technical solution of the present application, in step 2, the ball milling condition is 200-300 rpm, and the ball milling time is 1-3 h; in step 4, the roller speed of the three-roll mill is 50-100 r / min, the pressure is 2 MPa, and the rolling is 3-10 times.

[0021] As a further improved technical solution of the present application, the sintering temperature and time in step 4 are set as follows: from room temperature to T1 and keeping for 30-60 min, heating to T2 and keeping for 1-2 h, and heating to T3 and keeping for 1-2 h; wherein 200℃≤T1≤300℃, 500℃≤T2≤700℃, 900℃≤T3≤1300℃, the heating rate from room temperature to T1 is 0.5-1℃ / min, the heating rate from T1 to T2 is 0.5-1℃ / min, and the heating rate from T2 to T3 is 2-3℃ / min.

[0022] From the above technical solution, it can be seen that the platinum-zirconia porous catalytic electrode slurry and the preparation method of the catalytic electrode have the following advantages:

[0023] The decomposition temperature of the metal carbide added in the slurry is 600-1300℃, which is lower than the densification temperature (about 1300℃) of 8YSZ, and the decomposition of the metal carbide presents a continuous gradient reaction characteristic. This temperature difference makes the oxides generated by the decomposition of the carbide act on the substrate and platinum particles in advance before the densification of 8YSZ, creating conditions for improving the interface bonding and inhibiting the growth of platinum particles. At the same time, under the synergistic action of the pore-forming agent, the growth of platinum particles is effectively inhibited, so that the catalytic specific surface area can be retained to a greater extent, the porosity and uniformity of the electrode are significantly improved, and the catalytic activity of the electrode is enhanced.

[0024] In addition, the decomposition temperature of the carbide is about 600℃, which is slightly earlier than the sintering starting point of platinum and earlier than the densification temperature of the substrate zirconia. On the one hand, it is beneficial to the combination and shrinkage of the electrode slurry with the zirconia porcelain film substrate through the generated aluminum oxide and zirconia, so that the interface bonding is more compact. On the other hand, it effectively reduces the sintering delamination problem, improves the qualification rate and reliability of the electrode, and is suitable for the manufacture of high-precision oxygen sensors. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The surface scanning electron microscope image of the sample obtained by printing and sintering the slurry of Example 1 of the present application.

[0026] Figure 2 The surface scanning electron microscope image of the sample obtained by printing and sintering the slurry of Example 2 of the present application.

[0027] Figure 3This is a scanning electron microscope image of the surface of the sample obtained by slurry printing and sintering in Example 3 of the present invention.

[0028] Figure 4 This is a scanning electron microscope image of the surface of the sample obtained by slurry printing and sintering in Example 4 of the present invention.

[0029] Figure 5 This is a scanning electron microscope image of the surface of the sample obtained by slurry printing and sintering in Comparative Example 1 of the present invention.

[0030] Figure 6 This is a scanning electron microscope image of the interface between the electrode layer and the substrate after co-firing zirconium oxide and platinum electrodes in Example 1. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0032] Example 1

[0033] Preparation of the organic carrier: Diethylene glycol butyl ether was heated to 60°C, and ethyl cellulose was slowly added while stirring. The mixture was stirred continuously for 2 hours until completely dissolved, and then cooled to room temperature for later use. The weight ratio of ethyl cellulose to diethylene glycol butyl ether was 1:3.

[0034] Preparation of platinum-8YSZ premixed powder: 65g of platinum powder (spherical, particle size 1-3μm) was mixed with 3g of 8YSZ (particle size 500nm), and 200g of zirconia balls (diameter 5mm) were added. The mixture was ball-milled at 250rpm for 2 hours. After ball milling, the mixture was vacuum-dried at 80℃ for 3 hours and then passed through a 300-mesh sieve.

[0035] Preparation of the slurry: Platinum-8YSZ premixed powder, 10g of pore-forming agent (9g of nano-carbon powder + 6g of micron-sized carbon powder), and 5g of zirconium carbide powder (particle size 500nm) were added to a homogenizer. 17g of organic carrier was slowly added, and homogenization was carried out at 1500rpm for 60 minutes. The semi-finished slurry was then passed through a three-roll mill at 80r / min and 2MPa pressure, and milled 5 times. Degassing was performed under a vacuum of 0.09MPa for 30 minutes to obtain the final slurry.

[0036] Electrode preparation: The paste was screen-printed onto a zirconia green ceramic film with a thickness of 100 μm. Sintering curves: room temperature → 250℃ (heating rate 0.8℃ / min, hold for 45 minutes); 250℃ → 600℃ (heating rate 0.8℃ / min, hold for 1.5 hours); 600℃ → 1100℃ (heating rate 2.5℃ / min, hold for 1.5 hours).

[0037] Example 2

[0038] Preparation of organic carrier: Same as in Example 1.

[0039] Preparation of platinum-8YSZ premixed powder: 60g of platinum powder (spherical, particle size 1-3μm) was mixed with 5g of 8YSZ (particle size 800nm), and 200g of zirconia balls (5mm in diameter) were added. The mixture was ball-milled at 200rpm for 3 hours. After ball milling, the mixture was vacuum-dried at 80℃ for 3 hours and then passed through a 300-mesh sieve.

[0040] Preparation of the slurry: Platinum-8YSZ premixed powder, 15g of pore-forming agent (12g of nano-carbon powder + 3g of micron-sized carbon powder), and 3g of zirconium carbide powder (particle size 500nm) were added to a homogenizer. 17g of organic carrier was slowly added, and homogenization was carried out at 1500rpm for 60 minutes. The semi-finished slurry was then passed through a three-roll mill at 80r / min and 2MPa pressure, and milled 5 times. Degassing was performed under a vacuum of 0.09MPa for 30 minutes to obtain the final slurry.

[0041] Electrode preparation: Same as in Example 1.

[0042] Example 3

[0043] Preparation of organic carrier: Same as in Example 1.

[0044] Preparation of platinum-8YSZ premixed powder: 60g of platinum powder (spherical, particle size 1-3μm) was mixed with 4g of 8YSZ (particle size 400nm), and 200g of zirconia balls (5mm in diameter) were added. The mixture was ball-milled at 250rpm for 2 hours. After ball milling, the mixture was vacuum-dried at 80℃ for 3 hours and then passed through a 300-mesh sieve.

[0045] Preparation of the slurry: Platinum-8YSZ premixed powder, 12g pore-forming agent (6g nano carbon powder + 6g micron carbon powder), 3.2g zirconium carbide powder, and 0.8g aluminum carbide powder (particle size 500nm) were added to a homogenizer. 20g organic carrier was slowly added, and homogenization was carried out at 1500rpm for 60 minutes. The semi-finished slurry was then passed through a three-roll mill at 80r / min and 2MPa pressure, and circulated for 5 cycles. Degassing was performed under a vacuum of 0.09MPa for 30 minutes to obtain the final slurry.

[0046] Electrode preparation: The paste was screen-printed onto a zirconia green ceramic film with a thickness of 100 μm. Sintering curves: room temperature → 220℃ (heating rate 1℃ / min, hold for 60 minutes); 250℃ → 700℃ (heating rate 1℃ / min, hold for 1 hour); 700℃ → 1200℃ (heating rate 3℃ / min, hold for 1 hour).

[0047] Example 4

[0048] Preparation of organic carrier: Same as in Example 1.

[0049] Preparation of platinum-8YSZ premixed powder: 58g of platinum powder (spherical, particle size 1-3μm) was mixed with 4g of 8YSZ (particle size 600nm), and 200g of zirconia balls (5mm in diameter) were added. The mixture was ball-milled at 250rpm for 2 hours. After ball milling, the mixture was vacuum-dried at 80℃ for 3 hours and then passed through a 300-mesh sieve.

[0050] Preparation of the slurry: Platinum-8YSZ premixed powder, 16g of pore-forming agent (8g nano carbon powder + 8g micron carbon powder), 2g of zirconium carbide powder, and 2g of aluminum carbide powder (particle size 500nm) were added to a homogenizer. 18g of organic carrier was slowly added, and homogenization was carried out at 1500rpm for 60 minutes. The semi-finished slurry was then passed through a three-roll mill at 80r / min and 2MPa pressure, and circulated for 5 cycles. Degassing was performed under a vacuum of 0.09MPa for 30 minutes to obtain the final slurry.

[0051] Electrode preparation: Same as in Example 3.

[0052] Example 5

[0053] Preparation of organic carrier: Same as in Example 1.

[0054] Preparation of platinum-8YSZ premixed powder: 57g of platinum powder (spherical, particle size 1-3μm) was mixed with 4g of 8YSZ (particle size 400nm), and 200g of zirconia balls (5mm in diameter) were added. The mixture was ball-milled at 250rpm for 2 hours. After ball milling, the mixture was vacuum-dried at 80℃ for 3 hours and then passed through a 300-mesh sieve.

[0055] Preparation of the slurry: Platinum-8YSZ premixed powder, 12g of pore-forming agent (6g nano carbon powder + 6g micron carbon powder), and 4g of aluminum carbide powder (particle size 500nm) were added to a homogenizer. 23g of organic carrier was slowly added, and homogenization was carried out at 1500rpm for 60 minutes. The semi-finished slurry was then passed through a three-roll mill at 80r / min and 2MPa pressure, and circulated for 5 cycles. Degassing was performed under a vacuum of 0.09MPa for 30 minutes to obtain the final slurry.

[0056] Electrode preparation: Same as in Example 1.

[0057] Comparative Example 1

[0058] Preparation of organic carrier: Same as in Example 1.

[0059] Preparation of platinum-8YSZ premixed powder: 65g of platinum powder (spherical, particle size 1-3μm) was mixed with 5g of 8YSZ (particle size 400nm), and 200g of zirconia balls (5mm in diameter) were added. The mixture was ball-milled at 250rpm for 2 hours. After ball milling, the mixture was vacuum-dried at 80℃ for 3 hours and then passed through a 300-mesh sieve.

[0060] Preparation of slurry: Platinum-8YSZ premixed powder was added to a homogenizer, and 30g of organic carrier was slowly added. Homogenization was carried out at 1500rpm for 60 minutes. The semi-finished slurry was then passed through a three-roll mill at a roller speed of 80r / min and a pressure of 2MPa, and circulated for 5 times. Degassing was carried out under a vacuum of 0.09MPa for 30 minutes to obtain the final slurry.

[0061] Electrode preparation: Same as in Example 1.

[0062] Comparative Example 2

[0063] Preparation of organic carrier: Same as in Example 1.

[0064] Preparation of platinum-8YSZ premixed powder: 65g of platinum powder (spherical, particle size 1-3μm) was mixed with 3g of 8YSZ (particle size 400nm), and 200g of zirconia balls (5mm in diameter) were added. The mixture was ball-milled at 250rpm for 2 hours. After ball milling, the mixture was vacuum-dried at 80℃ for 3 hours and then passed through a 300-mesh sieve.

[0065] Preparation of the slurry: Platinum-8YSZ premixed powder, 3.2g zirconium carbide powder, and 0.8g aluminum carbide powder were added to a homogenizer, and 28g organic carrier was slowly added. The homogenizer was homogenized at 1500rpm for 60 minutes. The semi-finished slurry was then passed through a three-roll mill at 80r / min and 2MPa pressure, and milled 5 times. The slurry was then degassed under a vacuum of 0.09MPa for 30 minutes to obtain the final slurry.

[0066] Electrode preparation: Same as in Example 3.

[0067] Comparative Example 3

[0068] Preparation of organic carrier: Same as in Example 1.

[0069] Preparation of platinum-8YSZ premixed powder: 65g of platinum powder (spherical, particle size 1-3μm) was mixed with 3g of 8YSZ (particle size 400nm), and 200g of zirconia balls (5mm in diameter) were added. The mixture was ball-milled at 250rpm for 2 hours. After ball milling, the mixture was vacuum-dried at 80℃ for 3 hours and then passed through a 300-mesh sieve.

[0070] Preparation of the slurry: Platinum-8YSZ premixed powder and 12g of pore-forming agent (6g nano-carbon powder + 6g micron-sized carbon powder) were added to a homogenizer, and 20g of organic carrier was slowly added. The homogenizer was homogenized at 1500rpm for 60 minutes. The semi-finished slurry was then passed through a three-roll mill at 80r / min and 2MPa pressure, and milled 5 times. Degassing was performed under a vacuum of 0.09MPa for 30 minutes to obtain the final slurry.

[0071] Electrode preparation: Same as in Example 3.

[0072] The content of each component in Examples 1-5 and Comparative Examples 1-3 is summarized in Table 1.

[0073] Table 1. Components and weight percentages of the Examples and Comparative Examples

[0074]

[0075] Performance tests were conducted on the slurry samples after sintering from Examples 1-5 and Comparative Examples 1-3:

[0076] Porosity and pore uniformity: The porosity of the samples was tested and calculated according to the method in ISO 2738:2017 Sintered metallic materials (excluding cemented carbides) – Determination of density, oil content and open porosity of permeable sintered metallic materials. The results are shown in Table 2. The pore uniformity of the samples was determined by scanning electron microscopy (SEM) observation. The results are shown in Table 2. Figures 1-5 As shown.

[0077] Catalytic activity determination: The slurry was screen-printed onto a zirconia green ceramic film, dried, stacked, and isostatically pressed to prepare a 4-line switchable oxygen sensor. Its catalytic activity was evaluated by the response time to oxygen. The results are shown in Table 2.

[0078] Interface bonding reliability assessment: The reliability of interface bonding was assessed by observing the signal attenuation after cycling under normal chip operating conditions (specifically, measuring the catalytic activity after 1000 cycles). The results are shown in Table 2. If the interface bonding is unreliable, microcracks will appear on the interface under thermal shock stress after a period of time, leading to a decrease in current density or even failure.

[0079] Table 2. Test results of slurry samples after sintering in the examples and comparative examples.

[0080]

[0081] Considering the differences in slurry composition and content, and the roles of metal carbides, carbon powder, and other components in the sintering process, the electrode test results of the examples and comparative examples differ. Please refer to [the relevant documentation / reference]. Figures 1-6 And Table 2, specifically:

[0082] The effect of metal carbides on electrode performance: In Examples 1-5 and Comparative Example 2, the addition of metal carbides improved interfacial bonding and inhibited platinum particle growth during sintering. Therefore, the bonding effect between the electrode layer and the substrate was significantly better than in Comparative Examples 1 and 3. Please refer to... Figure 6As shown, the white stripe represents the electrode layer, and the remaining portion is the substrate. The stripe is continuous and uninterrupted, and the contact line with the zirconium oxide substrate is continuous and without black gaps, indicating good bonding between the two. During co-firing, atoms diffuse between the platinum layer and the substrate surface, forming a strong bonding interface that can withstand subsequent processing. Furthermore, the growth of platinum particles is effectively suppressed, and the catalytic specific surface area is preserved to a greater extent. Therefore, the catalytic activity of this example is higher than that of the comparative example, demonstrating that metal carbides not only improve bonding strength but also have a certain influence on catalytic activity. Although Comparative Example 2 contains metal carbides, its overall synergistic effect in the formulation is not as good as that of the example, resulting in lower catalytic activity.

[0083] The effect of carbon powder on electrode performance: Carbon powder was added in Examples 1-5. As a pore-forming agent, the carbon powder oxidizes and volatilizes during sintering, forming pores. Simultaneously, in synergy with the decomposition of metal carbides, it further inhibits platinum particle growth, ensuring good pore uniformity and providing sufficient active sites and good mass transfer channels for the catalytic reaction (see reference). Figures 1-4 As shown in the figure, this improved the catalytic activity and binding force. Although Comparative Example 3 contains carbon powder, its overall synergistic effect on the formulation is not as good as that of the examples, and therefore its catalytic activity is also lower.

[0084] The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. The understanding of this specification should be based on those skilled in the art. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still make modifications or equivalent substitutions to the present invention. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A platinum-zirconia porous catalytic electrode slurry, characterized in that: The composition comprises the following components in the indicated weight ratios: 50%-65% platinum powder, 10%-20% pore-forming agent, 2%-5% 8YSZ, 2%-5% metal carbide powder, and 15%-30% organic carrier. The platinum powder has a particle size of 1-3 μm, and the metal carbide has a decomposition temperature of 600-1300℃. The metal carbide powder is one or both of zirconium carbide and aluminum carbide, and the metal carbide has a particle size of 200 nm-1 μm. The pore-forming agent has a particle size of 20 nm-10 μm, and the pore-forming agent forms pores in the catalytic electrode at a temperature of 600-1200℃.

2. The platinum-zirconia porous catalytic electrode slurry as described in claim 1, characterized in that: The molar ratio of aluminum carbide to zirconium carbide in the metal carbide powder is 1:(1-5).

3. The platinum-zirconia porous catalytic electrode slurry as described in claim 1, characterized in that: The pore-forming agent comprises nano-carbon powder with a particle size of 20-100nm and micro-carbon powder with a particle size of 3-10μm. The weight of the nano-carbon powder accounts for 40-60% of the total weight of the pore-forming agent, and the weight of the micro-carbon powder accounts for 40-60% of the total weight of the pore-forming agent.

4. The platinum-zirconia porous catalytic electrode slurry as described in claim 1, characterized in that: The organic carrier comprises 15-30% by weight of ethyl cellulose and 70-85% by weight of solvent, wherein the solvent is selected from one or more of terpineol, butyl carbitol acetate, diethylene glycol butyl ether, diethylene glycol ethyl ether, and diethylene glycol butyl ether acetate.

5. The platinum-zirconia porous catalytic electrode slurry as described in claim 1, characterized in that: The particle size of the 8YSZ is 200nm-1μm.

6. A method for preparing a platinum-zirconia porous catalytic electrode using the platinum-zirconia porous catalytic electrode slurry according to claim 1, characterized in that: Includes the following steps: Step 1: Stir the solvent and ethyl cellulose until they are evenly mixed to obtain an organic carrier; Step 2: Mix platinum powder with 8YSZ, ball mill, and dry to obtain platinum-8YSZ premixed powder; Step 3: Place the platinum-8YSZ premixed powder, pore-forming agent, metal carbide powder and organic carrier into a homogenizer and homogenize to obtain a slurry semi-finished product; Step 4: Transfer the semi-finished slurry obtained in Step 3 into a three-roll mill for grinding and degassing under reduced pressure to obtain the finished slurry; Step 5: Print the finished paste onto a zirconia green ceramic film and sinter it.

7. The method for preparing the platinum-zirconia porous catalytic electrode as described in claim 6, characterized in that: In step 2, the ball milling conditions are 200-300 rpm and the ball milling time is 1-3 h; in step 4, the roller speed of the three-roll mill is 50-100 r / min, the pressure is 2 MPa, and the rolling is 3-10 times.

8. The method for preparing the platinum-zirconia porous catalytic electrode as described in claim 6, characterized in that: The sintering temperature and time in step 4 are set as follows: from room temperature to T1 and hold for 30-60 min, to T2 and hold for 1-2 h, and to T3 and hold for 1-2 h; wherein 200℃≤T1≤300℃, 500℃≤T2≤700℃, 900℃≤T3≤1300℃, the rate of heating from room temperature to T1 is 0.5-1℃ / min, the rate of heating from T1 to T2 is 0.5-1℃ / min, and the rate of heating from T2 to T3 is 2-3℃ / min.

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