Method for inhibiting continuous brittle compounds of ceramic metal brazed joint through oxygen plasma surface modification

Abstract

The invention discloses a method for inhibiting continuous brittle compounds of a ceramic metal brazed joint through oxygen plasma surface modification, and belongs to the field of SiC and Nb brazed connection. The problems that in the existing SiC and Nb connecting process, nickel in nickel-based brazing filler metal reacts with SiC to generate a large amount of continuous Ni-Si phases and graphite, and the products can cause brittleness and failure of a connector are solved. The method comprises the following steps: 1, oxygen plasma surface modification; and 2, brazing. The method is used for inhibiting continuous brittleness of the ceramic metal brazed joint through oxygen plasma surface modification.

Description

Technical Field

[0001] The invention belongs to the field of brazing connection of SiC and Nb. Background Art

[0002] In recent years, the world is facing an energy crisis. One of the results of the overuse of traditional fossil energy is the shortage of resources. This overuse has also led to serious environmental pollution and greenhouse effect, exacerbating the earth's climate change. In this context, people are in urgent need of finding alternative energy sources to reduce their dependence on fossil energy. Hydrogen isotope deuterium-tritium (DT) nuclear fusion energy is considered to be an ideal clean energy source. Its main fuel deuterium has abundant reserves, and the reaction product is mainly non-reactive helium (He). It also has inherent safety. This energy has great potential to alleviate global energy problems and reduce environmental impacts.

[0003] In a nuclear fusion device, the blanket system is one of the most critical components, which provides neutron and high heat load shielding for the entire device. The environment in which it is located is harsh, and it is always subject to high heat load, high-flux neutron irradiation, corrosion from various liquid tritium breeders and coolants, and even affected by impurity elements produced by synergistic transmutation. Therefore, the choice of materials is particularly important. In order to meet these challenges, SiC material is an ideal choice as a blanket structure. This material can be used to make control rods, tie rods, heat exchangers and coolant jackets in high-temperature reactors and low-activity molten salt reactors (AHTR) for fuel assemblies. However, most of the above applications require SiC materials to connect alloy materials.

[0004] The expected operating temperature of silicon carbide can reach 500-1000℃, and its deformation resistance and high strength remain unchanged under an irradiation dose of 70dpa. At the same time, metal Nb plays an important role in connecting with silicon carbide composite materials. Nb has excellent physical properties: it has a high melting point and can serve for a long time in a high temperature environment. At the same time, Nb has high strength, high toughness and radiation resistance. The chemical properties of Nb are also very stable, with excellent oxidation resistance and corrosion resistance. This makes Nb an ideal choice for combining with SiC ceramics to form high-temperature composite components. The composite components of SiC and Nb can not only meet the environmental requirements for high temperature resistance, radiation resistance and corrosion resistance of the structure, but also make the composite components lightweight, so that it can combine its advantages and complement its disadvantages.

[0005] In order to achieve a reliable connection between SiC and Nb, a reliable connection method must be adopted. Traditional bonding and bolting methods are not suitable for high-temperature structures. Among welding methods, arc welding, laser welding, electron beam welding and other methods are not suitable for the connection of high-melting-point ceramics. Diffusion welding and pressure welding cannot meet the connection requirements of SiC and Nb with complex shapes. At present, brazing is considered to be the most suitable welding method for connecting SiC and Nb. Nickel-based alloys are considered to be the best brazing materials for high-temperature structures due to their excellent high-temperature performance and mechanical properties. In addition, studies have shown that nickel has a strong tendency to react with SiC. Many studies have shown that the strong reaction between nickel and SiC will generate a large amount of Ni-Si phase and continuous graphite. Unfortunately, graphite in the reaction product has been proven to be the source of joint failure. Graphite has a weak bonding ability with the surrounding phase and is prone to cracks. Cracks expand rapidly when the coefficient of thermal expansion (CTE) does not match, eventually leading to joint failure. Therefore, regulating the strong reaction between nickel and SiC is the key to improving the mechanical properties of the joint. Summary of the invention

[0006] The present invention aims to solve the problem that in the existing SiC and Nb connection process, the nickel in the nickel-based brazing filler metal reacts with SiC to generate a large amount of continuous Ni-Si phase and graphite, and these products lead to the brittleness and failure of the joint, and further provides a method for oxygen plasma surface modification to inhibit continuous brittle compounds in ceramic metal brazing joints.

[0007] A method for oxygen plasma surface modification to suppress continuous brittle compounds in ceramic metal brazing joints is carried out in the following steps:

[0008] 1. Oxygen plasma surface modification:

[0009] The SiC ceramic was placed in a PECVD reaction chamber, and the pressure in the chamber was adjusted to 1×10 -2 Pa~5×10 -2 Pa, then the temperature is raised to 700°C to 850°C, after the temperature is raised, the radio frequency system is turned on to convert oxygen into oxygen plasma, and the oxygen plasma treatment is performed for 5min to 30min under the conditions of radio frequency power of 150W to 200W and temperature of 700°C to 850°C, and finally the radio frequency system is turned off and the temperature is lowered to room temperature to obtain SiC ceramics surface-modified by oxygen plasma;

[0010] 2. Brazing:

[0011] The SiC ceramic / Ni-based brazing sheet / Nb metal surface-modified by oxygen plasma are stacked in this order to obtain a workpiece to be welded, and then the workpiece to be welded is placed in a vacuum brazing furnace and maintained at a temperature of 900°C to 1100°C for 10min to 30min. Finally, after cooling to room temperature, the method of suppressing continuous brittle compounds in ceramic metal brazing joints by oxygen plasma surface modification is completed.

[0012] The beneficial effects of the present invention are:

[0013] 1. The present invention can achieve the formation of high-quality SiO2 film on SiC surface at a relatively low temperature (700℃~850℃) through oxygen plasma treatment technology. Oxygen plasma has high reactivity and energy, and can effectively react with the SiC surface to form a dense and uniform SiO2 film layer, and the SiO2 / SiC interface is stable, without defects such as oxygen vacancies and carbon. The film layer not only has excellent chemical stability and thermal stability, but also can deeply modify the SiC surface without causing material damage. Compared with traditional thermal oxidation or chemical vapor deposition methods, oxygen plasma technology can be processed at a lower temperature, significantly reducing the thermal stress and deformation risk of the material during the processing process.

[0014] 2. The formed SiO2 film provides an effective physical and chemical barrier on the SiC surface, significantly reducing the strong reaction between SiC and the brazing material (especially nickel-based brazing material). The reaction between nickel and SiC will generate a large amount of Ni-Si phase and graphite. These reaction products are the main reasons for the formation of brittle compounds in the joint. The graphite phase is particularly prone to cause thermal crack propagation of the joint because it has weak bonding strength and is prone to cracking when the coefficient of thermal expansion (CTE) does not match. The formation of SiO2 film can effectively inhibit the occurrence of these adverse reactions, slow down the reaction rate between nickel and SiC, and thus avoid the excessive generation of Ni-Si phase and graphite. This reaction inhibition effectively avoids the accumulation of brittle compounds and significantly improves the bonding strength between the brazing material and the SiC surface.

[0015] 3. By precisely controlling the temperature, heating rate and cooling rate of the brazing process, the uniform melting and wetting of the brazing material can be ensured, avoiding the introduction of thermal stress, thereby effectively reducing cracks and defects caused by thermal expansion mismatch. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a SEM image of the SiO2 film on the surface of SiC ceramics surface modified by oxygen plasma prepared in step 1 of Example 1;

[0017] Figure 2This is the O1s spectrum of the XPS analysis of the SiO2 film on the surface of the SiC ceramic surface modified by oxygen plasma prepared in step 1 of Example 1;

[0018] Figure 3 To compare the joint morphology of SiC and Nb brazed with nickel-based brazing filler metal without oxygen plasma surface treatment;

[0019] Figure 4 The morphology of the joint between SiC and Nb brazed with nickel-based brazing filler metal during oxygen plasma surface treatment in Example 1;

[0020] Figure 5 The shear properties of two SiC and Nb brazing joints treated with oxygen plasma surface modification in Example 1 and untreated SiC in a comparative experiment. DETAILED DESCRIPTION

[0021] Specific implementation method 1: This implementation method is a method for oxygen plasma surface modification to inhibit continuous brittle compounds in ceramic metal brazing joints, which is carried out in the following steps:

[0022] 1. Oxygen plasma surface modification:

[0023] The SiC ceramic was placed in a PECVD reaction chamber, and the pressure in the chamber was adjusted to 1×10 -2 Pa~5×10 -2 Pa, then the temperature is raised to 700°C to 850°C, after the temperature is raised, the radio frequency system is turned on to convert oxygen into oxygen plasma, and the oxygen plasma treatment is performed for 5min to 30min under the conditions of radio frequency power of 150W to 200W and temperature of 700°C to 850°C, and finally the radio frequency system is turned off and the temperature is lowered to room temperature to obtain SiC ceramics surface-modified by oxygen plasma;

[0024] 2. Brazing:

[0025] The SiC ceramic / Ni-based brazing sheet / Nb metal surface-modified by oxygen plasma are stacked in this order to obtain a workpiece to be welded, and then the workpiece to be welded is placed in a vacuum brazing furnace and maintained at a temperature of 900°C to 1100°C for 10min to 30min. Finally, after cooling to room temperature, the method of suppressing continuous brittle compounds in ceramic metal brazing joints by oxygen plasma surface modification is completed.

[0026] The beneficial effects of this embodiment are:

[0027] 1. This embodiment uses oxygen plasma treatment technology to achieve the generation of high-quality SiO2 film on the SiC surface at a relatively low temperature (700°C to 850°C). Oxygen plasma is highly reactive and energetic, and can effectively react with the SiC surface to form a dense and uniform SiO2 film layer, and the SiO2 / SiC interface is stable and free of defects such as oxygen vacancies. This film layer not only has excellent chemical and thermal stability, but also can deeply modify the SiC surface without causing material damage. Compared with traditional thermal oxidation or chemical vapor deposition methods, oxygen plasma technology can be processed at lower temperatures, significantly reducing the thermal stress and deformation risk of materials during the processing process.

[0028] 2. The formed SiO2 film provides an effective physical and chemical barrier on the SiC surface, significantly reducing the strong reaction between SiC and the brazing material (especially nickel-based brazing material). The reaction between nickel and SiC will generate a large amount of Ni-Si phase and graphite. These reaction products are the main reasons for the formation of brittle compounds in the joint. The graphite phase is particularly prone to cause thermal crack propagation of the joint because it has weak bonding strength and is prone to cracking when the coefficient of thermal expansion (CTE) does not match. The formation of SiO2 film can effectively inhibit the occurrence of these adverse reactions, slow down the reaction rate between nickel and SiC, and thus avoid the excessive generation of Ni-Si phase and graphite. This reaction inhibition effectively avoids the accumulation of brittle compounds and significantly improves the bonding strength between the brazing material and the SiC surface.

[0029] 3. By precisely controlling the temperature, heating rate and cooling rate of the brazing process, the uniform melting and wetting of the brazing material can be ensured, avoiding the introduction of thermal stress, thereby effectively reducing cracks and defects caused by thermal expansion mismatch.

[0030] Specific implementation method 2: This implementation method is different from specific implementation method 1 in that the SiC ceramic described in step 1 is a pre-treated SiC ceramic, and the pre-treatment is specifically carried out in the following steps: using 400# and 800# diamond sandpaper to grind the surface of the SiC ceramic to make the surface smooth and uniform, then immersing it in anhydrous ethanol, ultrasonically cleaning it for 5min to 15min under the condition of ultrasonic power of 50W to 150W, and finally taking it out to dry. The rest is the same as specific implementation method 1.

[0031] Through meticulous surface treatment steps (including grinding with diamond sandpaper of different grits and ultrasonic cleaning), the surface of the SiC ceramic is ensured to be uniform and smooth, and surface impurities, oil stains and dust are removed, ensuring the surface is clean and pollution-free to the greatest extent.

[0032] Specific implementation method three: This implementation method is different from specific implementation method one or two in that: the vacuuming and oxygen introduction described in step one are specifically carried out according to the following steps: ① first vacuumize to 1×10 -3 Pa~5×10 -3 Pa, then stop vacuuming and inject oxygen, and then re-vacuum to atmospheric pressure; ② Repeat step ① three times, and finally introduce oxygen again to adjust the pressure in the chamber to 1×10 -2 Pa~5×10 -2 Pa. The rest is the same as the first or second specific implementation mode.

[0033] Specific embodiment 4: This embodiment differs from one of specific embodiments 1 to 3 in that: in step 1, the heating rate is 5°C / min to 10°C / min, and the temperature is raised to 700°C to 850°C. Others are the same as specific embodiment 3.

[0034] Specific implementation mode 5: This implementation mode is different from specific implementation modes 1 to 4 in that: in step 1, the temperature is lowered to room temperature at a cooling rate of 5°C / min to 10°C / min. The rest is the same as specific implementation modes 1 to 4.

[0035] Specific embodiment 6: This embodiment is different from specific embodiments 1 to 5 in that the Ni-based solder sheet described in step 2 is a pretreated Ni-based solder sheet, and the pretreatment is specifically carried out in the following steps: use 600# sandpaper to polish the surface of the Ni-based solder sheet to remove oxides and surface impurities, then soak it in ethanol, ultrasonically clean it for 3min to 10min under the condition of ultrasonic power of 30W to 100W, and finally take it out to dry. Others are the same as specific embodiments 1 to 5.

[0036] Specific embodiment 7: This embodiment is different from specific embodiments 1 to 6 in that: the Nb metal described in step 2 is pretreated Nb metal, and the pretreatment is specifically carried out according to the following steps: use 400# and 800# sandpaper to polish the surface of the Nb metal to make the surface smooth and uniform, then soak it in ethanol, ultrasonically clean it for 3min to 10min under the condition of ultrasonic power of 30W to 100W, and finally take it out to dry. Others are the same as specific embodiments 1 to 6.

[0037] Through meticulous surface treatment steps (including grinding with diamond sandpaper of different grits and ultrasonic cleaning), the surface of the Nb metal is ensured to be uniform and smooth, and surface impurities, oil stains and dust are removed, ensuring the surface is clean and pollution-free to the greatest extent.

[0038] Specific embodiment 8: This embodiment differs from specific embodiments 1 to 7 in that: the thickness of the Ni-based solder sheet in step 2 is 0.02 mm to 0.1 mm; the Ni-based solder sheet in step 2 is a BNi series solder sheet or an AuNi series solder sheet. The rest is the same as specific embodiments 1 to 7.

[0039] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that: in step 2, the workpiece to be welded is placed in a vacuum brazing furnace and evacuated to 1×10 -3 Pa~5×10 -3 Pa. The rest is the same as the first to eighth embodiments.

[0040] Specific embodiment 10: This embodiment differs from specific embodiments 1 to 9 in that: in step 2, the heating rate is 5°C / min to 15°C / min, and the temperature is raised to 900°C to 1100°C; in step 2, the cooling rate is 5°C / min to 10°C / min, and the temperature is cooled to room temperature. Others are the same as specific embodiments 1 to 9.

[0041] The following examples are used to verify the beneficial effects of the present invention:

[0042] Embodiment 1:

[0043] A method for oxygen plasma surface modification to suppress continuous brittle compounds in ceramic metal brazing joints is carried out in the following steps:

[0044] 1. Oxygen plasma surface modification:

[0045] The SiC ceramic was placed in a PECVD reaction chamber, and the pressure in the chamber was adjusted to 5×10 - 2 Pa, then heated to 700°C at a heating rate of 5°C / min, turned on the RF system after heating to convert oxygen into oxygen plasma, and treated with oxygen plasma for 5 minutes at an RF power of 150W and a temperature of 700°C, and finally turned off the RF system and cooled to room temperature at a cooling rate of 5°C / min to obtain SiC ceramics surface-modified by oxygen plasma;

[0046] 2. Brazing:

[0047] The SiC ceramics surface modified by oxygen plasma / Ni-based brazing sheet / Nb metal were stacked in the order to obtain the parts to be welded, and then the parts to be welded were placed in a vacuum brazing furnace and evacuated to 5×10 -3Pa, heated to 900°C at a heating rate of 5°C / min, maintained at 900°C for 10min, and finally cooled to room temperature at a cooling rate of 5°C / min to obtain a ceramic metal brazing joint surface modified by oxygen plasma.

[0048] The SiC ceramic described in step 1 is a pretreated SiC ceramic, and the pretreatment is specifically carried out according to the following steps: use 400# and 800# diamond sandpaper to polish the surface of the SiC ceramic (size 5mm×5mm×5mm) to make the surface smooth and uniform, then soak it in anhydrous ethanol, ultrasonically clean it for 10 minutes under the condition of ultrasonic power of 100W, and finally take it out and dry it.

[0049] The vacuuming and oxygen introduction described in step 1 are specifically carried out in the following steps: ① vacuuming to 1×10 - 3 Pa, then stop vacuuming and inject oxygen, and then re-evacuate to atmospheric pressure; ② Repeat step ① three times, and finally introduce oxygen again to adjust the pressure in the chamber to 5×10 -2 Pa.

[0050] The Ni-based solder sheet described in step 2 is a pretreated Ni-based solder sheet, and the pretreatment is specifically carried out according to the following steps: use 600# sandpaper to polish the surface of the Ni-based solder sheet to remove oxides and surface impurities, then immerse it in ethanol, ultrasonically clean it for 10 minutes under an ultrasonic power of 50W, and finally take it out and dry it.

[0051] The Nb metal described in step 2 is pretreated Nb metal, and the pretreatment is specifically carried out according to the following steps: use 400# and 800# sandpaper to polish the surface of Nb metal (size is 10mm×10mm×4mm) to make the surface smooth and uniform, then soak it in ethanol, ultrasonically clean it for 10 minutes under the condition of ultrasonic power of 50W, and finally take it out and dry it.

[0052] The thickness of the Ni-based solder sheet described in step 2 is 0.02 mm; the Ni-based solder sheet described in step 2 is an AuNi18 solder sheet.

[0053] Comparative experiment: The difference between this comparative experiment and Example 1 is that: Step 1 is eliminated; in Step 2, SiC ceramic / Ni-based brazing sheet / Nb metal are stacked in the order to obtain the welded parts. The rest is the same as Example 1.

[0054] Figure 1This is an SEM image of the SiO2 film on the surface of SiC ceramics modified by oxygen plasma prepared in step 1 of Example 1. It can be seen from the image that there are no obvious large pores or cracks on the surface, indicating that the generated SiO2 film has good density and can play an effective physical and chemical barrier role.

[0055] Figure 2 This is the XPS analysis O1s spectrum of the SiO2 film on the surface of the SiC ceramic surface modified by oxygen plasma prepared in step 1 of Example 1; it can be seen from the figure that 532eV is the typical Si-O bond binding energy position, corresponding to the chemical state of silicon oxide in SiO2. The peak shape is symmetrical and has high intensity, indicating that the main component of the surface is high-purity SiO2, without oxygen vacancies and obvious impurities. The peak shape shows a symmetrical Gaussian distribution, and no other obvious secondary peaks (such as defect states in the low binding energy region) are observed.

[0056] Figure 3 The morphology of the joint between SiC and Nb brazed with nickel-based brazing filler metal without oxygen plasma surface treatment is compared. It can be seen that the joint has more gray Ni-Si phase and continuous black graphite phase, and the bonding ability of graphite with the surrounding phase is weak, resulting in continuous cracks.

[0057] Figure 4 The morphology of the joint of SiC and Nb brazed with nickel-based brazing alloy during oxygen plasma surface treatment in Example 1. It can be seen that the SiO2 film can effectively prevent the strong reaction between SiC and the brazing alloy, inhibit the formation of Ni-Si phase and graphite between nickel and SiC, and thus reduce the formation of brittle compounds.

[0058] Figure 5 The shear properties of two SiC and Nb brazed joints treated with oxygen plasma surface modification and untreated in Example 1 are shown in the comparative experiment. According to the national standard GB / T 11363-2008, the maximum shear strength after oxygen plasma surface modification is about 40MPa. The maximum shear strength of the joint without treatment is only about 20MPa. Oxygen plasma generates a dense and uniform SiO2 film on the SiC surface, providing a physical and chemical barrier, inhibiting the excessive generation of Ni-Si phase and graphite phase, thereby effectively improving the mechanical properties of the joint.

Claims

1. A method for suppressing continuous brittle compounds in ceramic metal brazing joints by oxygen plasma surface modification, characterized in that It is carried out in the following steps:

1. Oxygen plasma surface modification: The SiC ceramic was placed in a PECVD reaction chamber, and the pressure in the chamber was adjusted to 1×10 -2 Pa~5×10 -2 Pa, then the temperature is raised to 700°C to 850°C, after the temperature is raised, the radio frequency system is turned on to convert oxygen into oxygen plasma, and the oxygen plasma treatment is performed for 5min to 30min under the conditions of radio frequency power of 150W to 200W and temperature of 700°C to 850°C, and finally the radio frequency system is turned off and the temperature is lowered to room temperature to obtain SiC ceramics surface-modified by oxygen plasma; 2. Brazing: The SiC ceramic / Ni-based brazing sheet / Nb metal surface-modified by oxygen plasma are stacked in this order to obtain a workpiece to be welded, and then the workpiece to be welded is placed in a vacuum brazing furnace and maintained at a temperature of 900°C to 1100°C for 10min to 30min. Finally, after cooling to room temperature, the method of suppressing continuous brittle compounds in ceramic metal brazing joints by oxygen plasma surface modification is completed.

2. The method for suppressing continuous brittle compounds in ceramic metal brazing joints by oxygen plasma surface modification according to claim 1, characterized in that The SiC ceramic described in step 1 is a pretreated SiC ceramic, and the pretreatment is specifically carried out according to the following steps: use 400# and 800# diamond sandpaper to polish the surface of the SiC ceramic to make the surface smooth and uniform, then soak it in anhydrous ethanol, and ultrasonically clean it for 5min to 15min under the condition of ultrasonic power of 50W to 150W, and finally take it out and dry it.

3. The method for suppressing continuous brittle compounds in ceramic metal brazing joints by oxygen plasma surface modification according to claim 1, characterized in that The vacuuming and oxygen introduction described in step 1 are specifically carried out in the following steps: ① vacuuming to 1×10 -3 Pa~5×10 -3 Pa, then stop vacuuming and inject oxygen, and then re-vacuum to atmospheric pressure; ② Repeat step ① three times, and finally introduce oxygen again to adjust the pressure in the chamber to 1×10 -2 Pa~5×10 -2 Pa.

4. The method of oxygen plasma surface modification to suppress continuous brittle compounds in ceramic metal brazing joints according to claim 1, characterized in that In step 1, the temperature is raised to 700°C to 850°C at a heating rate of 5°C / min to 10°C / min.

5. The method of oxygen plasma surface modification to suppress continuous brittle compounds in ceramic metal brazing joints according to claim 1, characterized in that In step 1, the temperature is lowered to room temperature at a cooling rate of 5°C / min to 10°C / min.

6. The method of oxygen plasma surface modification to suppress continuous brittle compounds in ceramic metal brazing joints according to claim 1, characterized in that The Ni-based solder sheet described in step 2 is a pretreated Ni-based solder sheet, and the pretreatment is specifically carried out according to the following steps: use 600# sandpaper to polish the surface of the Ni-based solder sheet to remove oxides and surface impurities, then immerse it in ethanol, and ultrasonically clean it for 3min to 10min under the condition of ultrasonic power of 30W to 100W, and finally take it out and dry it.

7. The method of claim 1 for suppressing continuous brittle compounds in ceramic metal brazing joints by oxygen plasma surface modification, characterized in that The Nb metal described in step 2 is pretreated Nb metal, and the pretreatment is specifically carried out according to the following steps: use 400# and 800# sandpaper to polish the surface of the Nb metal to make the surface smooth and uniform, then soak it in ethanol, and ultrasonically clean it for 3min~10min under the condition of ultrasonic power of 30W~100W, and finally take it out and dry it.

8. The method of oxygen plasma surface modification to suppress continuous brittle compounds in ceramic metal brazing joints according to claim 1, characterized in that The thickness of the Ni-based solder sheet described in step 2 is 0.02 mm to 0.1 mm; the Ni-based solder sheet described in step 2 is a BNi series solder sheet or an AuNi series solder sheet.

9. The method of oxygen plasma surface modification to suppress continuous brittle compounds in ceramic metal brazing joints according to claim 1, characterized in that In step 2, the workpiece to be welded is placed in a vacuum brazing furnace and evacuated to 1×10 -3 Pa~5×10 -3 Pa.

10. The method of oxygen plasma surface modification to suppress continuous brittle compounds in ceramic metal brazing joints according to claim 1, characterized in that In step 2, the temperature is raised to 900°C to 1100°C at a heating rate of 5°C / min to 15°C / min; in step 3, the temperature is cooled to room temperature at a cooling rate of 5°C / min to 10°C / min.

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