Composite Structure Sagger for Cathode Material of Lithium Battery and Its Preparation Method

Abstract

The present invention discloses a composite structure sagger for lithium battery cathode materials and a preparation method thereof, which relates to the technical field of sagger kiln furniture preparation. When preparing the composite structure sagger for lithium battery cathode materials in the present invention, mullite, cordierite, and corundum are respectively impregnated in a furfuryl alcohol aqueous solution, and oxalic acid is added for polymerization, and then carbon-coated mullite, carbon-coated cordierite, and carbon-coated corundum are obtained through high-temperature calcination; mullite, cordierite, corundum, leucite, yellow dextrin, and water are mixed as the outer layer of the sagger, and carbon-coated mullite, carbon-coated cordierite, carbon-coated corundum, leucite, yellow dextrin, and water are mixed as the inner layer of the sagger. After the inner and outer layer structures are compounded and pressed into shape, the composite structure sagger for lithium battery cathode materials is obtained through calcination. The composite structure sagger for lithium battery cathode materials prepared in the present invention has the advantages of high flexural strength, good thermal shock resistance, and good erosion resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of preparing saggers as kiln furniture, and particularly to a composite structure sagger for lithium battery cathode materials and a preparation method thereof. Background Art

[0002] With the rapid development of industries such as new energy vehicles and energy storage, lithium-ion batteries with the advantages of green high energy, long cycle life, and high safety have been widely studied. It mainly consists of a positive electrode, a negative electrode, an electrolyte, and a separator. Among them, the cathode material is the key core material of lithium-ion batteries and accounts for the largest cost proportion. Therefore, the cathode material is crucial for the performance of lithium-ion batteries.

[0003] According to the different lithium battery cathode materials, their preparation methods are also different. Due to the requirements of industrial production for process equipment and economy, the high-temperature solid-phase synthesis method is still widely used among many synthesis methods because of its simple preparation process, low production cost, high output, and short production cycle. During the preparation process of the high-temperature solid-phase method, first, an alkaline lithium source and a precursor are mixed, and then placed in a sagger and calcined in a kiln. The sagger is an important carrier during the sintering process of lithium battery cathode materials. Currently, cordierite-mullite / spinel-based materials are mainly used, but the acidic oxides in its components, such as silicon dioxide, are easy to react with the alkaline components in the cathode material to form lithium silicate, β-lithium eucryptite, etc., and cause volume expansion, resulting in the peeling and damage of the sagger. At the same time, it causes "lithium loss" of the cathode material. Thus, it can be seen that the sagger suffers from thermal stress damage during the repeated service process and also faces the strong alkaline erosion of the lithium battery cathode material. Therefore, further improving the service life of the sagger and achieving its anti-erosion and long life have positive significance for the development of lithium-ion batteries. Summary of the Invention

[0004] The purpose of the present invention is to provide a composite structure sagger for lithium battery cathode materials and a preparation method thereof to solve the problems existing in the prior art.

[0005] To solve the above technical problems, the present invention provides the following technical solutions:

[0006] A composite structure sagger for lithium battery cathode materials. The composite structure sagger for lithium battery cathode materials is prepared by first impregnating mullite, cordierite, and corundum in an aqueous furfuryl alcohol solution respectively, adding oxalic acid for polymerization, and then calcining at high temperature to obtain carbon-coated mullite, carbon-coated cordierite, and carbon-coated corundum; mixing mullite, cordierite, corundum, leucite, yellow dextrin, and water as the outer layer of the sagger, and mixing carbon-coated mullite, carbon-coated cordierite, carbon-coated corundum, leucite, yellow dextrin, and water as the inner layer of the sagger. After the inner and outer layer structures are compounded and pressed into shape, they are calcined to obtain the composite structure sagger for lithium battery cathode materials.

[0007] Preferably, the outer layer of the sagger comprises the following mass components: 25-30 parts of mullite, 35-45 parts of cordierite, 10-15 parts of corundum, and 8-10 parts of leucite.

[0008] Preferably, the inner layer of the sagger comprises the following mass components: 25-30 parts of carbon-coated mullite, 35-45 parts of carbon-coated cordierite, 10-15 parts of carbon-coated corundum, and 8-10 parts of leucite.

[0009] Preferably, the content of each particle size component of the mullite, cordierite, corundum, leucite, carbon-coated mullite, carbon-coated cordierite, and carbon-coated corundum is as follows: above 10 mesh, 3-5%; 10-18 mesh, 20-25%; 18-50 mesh, 15-20%; 50-200 mesh, 20-25%; 200-325 mesh, 20-25%; below 325 mesh, 10-15%.

[0010] A preparation method of a composite structure sagger for a lithium battery cathode material comprises the following preparation steps:

[0011] (1) Carbon coating: By mass, soak mullite, cordierite, and corundum in a furfuryl alcohol aqueous solution respectively, at a material-liquid ratio of 1 g / 10 ml, stir at 200-300 r / min at 80-90 °C for 30-40 min, dropwise add an equal volume of 0.3 mol / L oxalic acid aqueous solution at 3-4 drops per second, after the addition, stir and react at 200-300 r / min at 80-90 °C for 6-8 h, perform suction filtration, wash with pure water 3-4 times, vacuum dry at 50-60 °C for 8-10 h, calcine at 800-900 °C for 2-3 h in an argon atmosphere, with a heating rate of 5 °C / min, and naturally cool to room temperature to obtain carbon-coated mullite, carbon-coated cordierite, and carbon-coated corundum;

[0012] (2) Compression molding: Weigh 25-30 parts of mullite, 35-45 parts of cordierite, 10-15 parts of corundum, 8-10 parts of leucite, then add 5-7 parts of yellow dextrin and 3-5 parts of water and mix evenly, let it age for 24 h, first put it into a mold according to a thickness of 12-15 mm, then weigh 6-7 parts of carbon-coated mullite, 9-11 parts of carbon-coated cordierite, 2.5-4 parts of carbon-coated corundum, 2-2.5 parts of leucite, 1-2 parts of yellow dextrin, 0.75-1.25 parts of water and mix evenly, let it age for 24 h, also put it into the mold according to a thickness of 3-4 mm, seal and cure at room temperature for 24 h, press and mold at a pressure of 150 MPa, keep the pressure for 10-14 h, dry at room temperature and 110 °C for 24 h respectively to obtain a green body of the sagger;

[0013] (3) Calcination: Calcinate the green body of the sagger according to the sintering temperature, and naturally cool to room temperature to obtain a composite structure sagger for a lithium battery cathode material.

[0014] Preferably, the furfuryl alcohol aqueous solution in step (1) is prepared by mixing furfuryl alcohol and pure water in a volume ratio of 1:40 and stirring evenly.

[0015] Preferably, the sintering temperature in step (3) is as follows: 0 - 100°C, with a heating rate of 1.25°C / min; 100 - 900°C, with a heating rate of 5°C / min, hold at 900°C for 30 min; 900 - 1200°C, with a heating rate of 4°C / min; 1200 - 1400°C, with a heating rate of 2.5°C / min, hold at 1400°C for 3 h.

[0016] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0017] When preparing the composite - structure crucible for lithium - battery positive electrode materials, the present invention first impregnates mullite, cordierite, and corundum in a furfuryl alcohol aqueous solution respectively, adds oxalic acid for polymerization, and then calcines at high temperature to obtain carbon - coated mullite, carbon - coated cordierite, and carbon - coated corundum; mixes mullite, cordierite, corundum, leucite, yellow dextrin, and water as the outer layer of the crucible, and mixes carbon - coated mullite, carbon - coated cordierite, carbon - coated corundum, leucite, yellow dextrin, and water as the inner layer of the crucible. After the inner and outer layer structures are compounded, pressed into shape, and calcined, the composite - structure crucible for lithium - battery positive electrode materials is obtained.

[0018] First, furfuryl alcohol is used to in - situ polymerize a layer of polyfurfuryl alcohol on the surfaces of mullite, cordierite, and corundum under the catalysis of oxalic acid, and then calcined at high temperature to form a carbon layer, obtaining carbon - coated mullite, carbon - coated cordierite, and carbon - coated corundum. The raw materials coated with the carbon layer have effective anti - erosion performance, the carbon layer has good performance in resisting the erosion of alkali metals, and at the same time leucite also has good anti - erosion performance. A part of the added leucite can be dispersed near the carbon - coated raw materials to protect the carbon layer from being oxidized, effectively improving the anti - erosion performance.

[0019] Second, the carbon layer has a low coefficient of thermal expansion. Coating on the surfaces of mullite, cordierite, and corundum can effectively improve the thermal shock resistance.

[0020] Finally, except for carbon - coating modification, the components of the inner layer and the outer layer are basically the same, and the two layers have similar properties, making it have good bonding properties and avoiding the inner layer falling off after being used for a period of time. Specific Embodiments

[0021] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0022] The content of each particle size component of mullite, cordierite, corundum, leucite, carbon-coated mullite, carbon-coated cordierite, and carbon-coated corundum used in all the following examples and comparative examples is as follows: above 10 mesh: 5%; 10 - 18 mesh: 20%; 18 - 50 mesh: 15%; 50 - 200 mesh: 25%; 200 - 325 mesh: 25%; below 325 mesh: 10%.

[0023] Example 1:

[0024] A preparation method of a composite structure crucible for a lithium battery cathode material, the preparation method of the composite structure crucible for a lithium battery cathode material includes the following preparation steps:

[0025] (1) Carbon coating: By mass, soak mullite, cordierite, and corundum in a furfuryl alcohol aqueous solution respectively. According to the material-liquid ratio of 1 g / 10 ml, stir at 200 r / min for 40 min at 80 °C, and dropwise add an equal volume of 0.3 mol / L oxalic acid aqueous solution at a rate of 3 drops per second. After the addition is completed, stir and react at 200 r / min for 8 h at 80 °C, perform suction filtration, wash 3 times with pure water, dry in vacuum at 50 °C for 10 h, calcine at 800 °C for 3 h in an argon atmosphere with a heating rate of 5 °C / min, and naturally cool to room temperature to obtain carbon-coated mullite, carbon-coated cordierite, and carbon-coated corundum;

[0026] (2) Compression molding: Weigh 25 parts of mullite, 35 parts of cordierite, 10 parts of corundum, and 8 parts of leucite, then add 5 parts of yellow dextrin and 3 parts of water and mix evenly. Let it age for 24 h, first put it into the mold according to a thickness of 12 mm, then weigh 6 parts of carbon-coated mullite, 9 parts of carbon-coated cordierite, 2.5 parts of carbon-coated corundum, 2 parts of leucite, 1 part of yellow dextrin, and 0.75 parts of water and mix evenly. Let it age for 24 h, and also put it into the mold according to a thickness of 3 mm. Seal and cure at room temperature for 24 h, press and mold at a pressure of 150 MPa, keep the pressure for 10 h, and dry at room temperature and 110 °C for 24 h respectively to obtain a green crucible blank;

[0027] (3) Calcination: Calcinate the green crucible blank at the sintering temperature and naturally cool to room temperature to obtain a composite structure crucible for a lithium battery cathode material.

[0028] Example 2:

[0029] A preparation method of a composite structure crucible for a lithium battery cathode material, the preparation method of the composite structure crucible for a lithium battery cathode material includes the following preparation steps:

[0030] (1) Carbon coating: By mass fraction, mullite, cordierite, and corundum are respectively immersed in a furfuryl alcohol aqueous solution. According to the material-liquid ratio of 1 g / 10 ml, at 85 °C, stir at 250 r / min for 35 min. Dropwise add an equal volume of 0.3 mol / L oxalic acid aqueous solution to the furfuryl alcohol aqueous solution at a rate of 3 drops per second. After the addition is completed, stir and react at 85 °C at 250 r / min for 7 h. Filter by suction, wash 3 times with pure water, dry in vacuum at 55 °C for 9 h, calcine at 850 °C for 2.5 h in an argon atmosphere, with a heating rate of 5 °C / min, and naturally cool to room temperature to obtain carbon-coated mullite, carbon-coated cordierite, and carbon-coated corundum;

[0031] (2) Compression molding: Weigh 28 parts of mullite, 40 parts of cordierite, 12 parts of corundum, and 9 parts of leucite. Then add 6 parts of yellow dextrin and 4 parts of water and mix evenly. Let it age for 24 h. First, put it into the mold according to a thickness of 13.5 mm. Then weigh 6.5 parts of carbon-coated mullite, 10 parts of carbon-coated cordierite, 3 parts of carbon-coated corundum, 2.2 parts of leucite, 1.5 parts of yellow dextrin, and 1 part of water and mix evenly. Let it age for 24 h. Also put it into the mold according to a thickness of 3.5 mm. Seal and cure at room temperature for 24 h, press and mold at a pressure of 150 MPa, keep the pressure for 12 h, and dry at room temperature and 110 °C for 24 h respectively to obtain the green body of the sagger;

[0032] (3) Calcination: Calcinate the green body of the sagger according to the sintering temperature and naturally cool to room temperature to obtain the composite structure sagger for the positive electrode material of the lithium battery.

[0033] Example 3:

[0034] A preparation method of a composite structure sagger for the positive electrode material of a lithium battery, the preparation method of the composite structure sagger for the positive electrode material of the lithium battery includes the following preparation steps:

[0035] (1) Carbon coating: By mass fraction, mullite, cordierite, and corundum are respectively immersed in a furfuryl alcohol aqueous solution. According to the material-liquid ratio of 1 g / 10 ml, at 90 °C, stir at 300 r / min for 30 min. Dropwise add an equal volume of 0.3 mol / L oxalic acid aqueous solution to the furfuryl alcohol aqueous solution at a rate of 4 drops per second. After the addition is completed, stir and react at 90 °C at 300 r / min for 6 h. Filter by suction, wash 4 times with pure water, dry in vacuum at 60 °C for 10 h, calcine at 900 °C for 3 h in an argon atmosphere, with a heating rate of 5 °C / min, and naturally cool to room temperature to obtain carbon-coated mullite, carbon-coated cordierite, and carbon-coated corundum;

[0036] (2) Compression molding: Weigh 30 parts of mullite, 45 parts of cordierite, 15 parts of corundum, and 10 parts of leucite. Then add 7 parts of yellow dextrin and 5 parts of water and mix evenly. Let it age for 24 h. First, put it into the mold according to a thickness of 15 mm. Then weigh 7 parts of carbon-coated mullite, 11 parts of carbon-coated cordierite, 4 parts of carbon-coated corundum, 2.5 parts of leucite, 2 parts of yellow dextrin, and 1.25 parts of water and mix evenly. Let it age for 24 h. Also put it into the mold according to a thickness of 4 mm. Seal and cure it at room temperature for 24 h, and then press and form it under a pressure of 150 MPa, keep the pressure for 14 h, and dry it at room temperature and 110 °C for 24 h respectively to obtain the green body of the sagger.

[0037] (3) Calcination: Calcinate the green body of the sagger according to the sintering temperature and cool it naturally to room temperature to obtain the composite structure sagger for the positive electrode material of the lithium battery.

[0038] Comparative Example 1:

[0039] The preparation method of the composite structure sagger for the positive electrode material of the lithium battery in Comparative Example 1 is different from that in Example 2 in that step (1) is not carried out, and step (2) is modified as follows: Compression molding: Weigh 28 parts of mullite, 40 parts of cordierite, 12 parts of corundum, and 9 parts of leucite. Then add 6 parts of yellow dextrin and 4 parts of water and mix evenly. Let it age for 24 h. Put it into the mold according to a thickness of 17 mm. Seal and cure it at room temperature for 24 h, and then press and form it under a pressure of 150 MPa, keep the pressure for 12 h, and dry it at room temperature and 110 °C for 24 h respectively to obtain the green body of the sagger. The remaining steps are the same as those in Example 2.

[0040] Comparative Example 2:

[0041] The preparation method of the composite structure sagger for the positive electrode material of the lithium battery in Comparative Example 2 is different from that in Example 2 in that step (2) is different. Step (2) is modified as follows: Compression molding: Weigh 28 parts of mullite, 40 parts of cordierite, and 12 parts of corundum. Then add 6 parts of yellow dextrin and 4 parts of water and mix evenly. Let it age for 24 h. First, put it into the mold according to a thickness of 13.5 mm. Then weigh 6.5 parts of carbon-coated mullite, 10 parts of carbon-coated cordierite, 3 parts of carbon-coated corundum, 1.5 parts of yellow dextrin, and 1 part of water and mix evenly. Let it age for 24 h. Also put it into the mold according to a thickness of 3.5 mm. Seal and cure it at room temperature for 24 h, and then press and form it under a pressure of 150 MPa, keep the pressure for 12 h, and dry it at room temperature and 110 °C for 24 h respectively to obtain the green body of the sagger. The remaining steps are the same as those in Example 2.

[0042] Test Example 1:

[0043] Mechanical property test: Test the cold flexural strength, cold compressive strength, and high-temperature flexural strength to evaluate the mechanical properties of the composite structure crucible for lithium battery cathode materials. Refer to GB / T 3001-2017 to test the cold flexural strength, refer to GB / T 5072-2023 to test the cold compressive strength, and refer to GB / T 5072-2023 to test the high-temperature flexural strength at a temperature of 1100 °C for 30 min. Test 5 specimens in each group, record the average data, and the results are shown in Table 1.

[0044] Table 1 Test results of mechanical properties

[0045] Flexural strength at room temperature / MPa Compressive strength at room temperature / MPa Flexural strength at high temperature / MPa Example 1 23.24 86.35 18.48 Example 2 24.56 87.51 18.69 Example 3 23.87 85.93 18.21 Comparative example 1 18.64 75.34 13.45 Comparative example 2 15.32 59.26 11.27

[0046] From the comparison of the experimental data of Examples 1-3 and Comparative Examples 1-2 in Table 1, it can be found that the composite structure crucible for lithium battery cathode materials prepared by the present invention has good mechanical properties.

[0047] Through the data comparison in the table, the carbon coating of mullite, cordierite, and corundum improves the mechanical properties and enhances the durability. The addition of leucite also improves the flexural strength and compressive strength.

[0048] Test Example 2:

[0049] Thermal shock resistance test: Refer to GB / T 30873-2014 for the thermal shock resistance test, use the water-cooling method, after 5 thermal cycles, test the cold flexural strength, calculate and record the strength retention rate, take the average value of 5 specimens in each group for recording, and the results are shown in Table 2.

[0050] Table 2 Test results of thermal shock resistance

[0051] Strength retention rate Strength retention rate Example 1 79.6％ Comparative example 1 72.3％ Example 2 80.7％ Comparative example 2 68.5％ Example 3 79.4％

[0052] From the comparison of the experimental data of Examples 1-3 and Comparative Examples 1-2 in Table 2, it can be found that the composite structure crucible for lithium battery cathode materials prepared by the present invention has good thermal shock resistance.

[0053] Through the data comparison in the table, both the carbon coating modification and the addition of leucite improve the thermal shock resistance of the composite structure crucible for lithium battery cathode materials.

[0054] Test Example 3:

[0055] Erosion resistance test: Mix the prepared Li(Ni 0.5 Co 0.2 Mn 0.3 )O 2The ternary cathode material precursor is placed on the composite structure crucible for preparing the cathode material of the lithium battery, and the mass of the precursor material on each sample is 3 g on average. The sample is placed in a muffle furnace, kept at 1000 °C for 4 h, and taken out after natural cooling. The above operation process of loading, firing, cooling, and sampling is continued for other samples. When serious peeling and cracking phenomena occur in the crucible sample, the erosion resistance experiment is terminated, the number of repeated uses is recorded, the average value is recorded for 5 samples in each group, and the result is rounded down. The results are shown in Table 3.

[0056] Table 3 Test results of erosion resistance performance

[0057] Number of reuse times Number of reuse times Example 1 58 Comparative example 1 38 Example 2 60 Comparative example 2 23 Example 3 57

[0058] From the comparison of the experimental data of Examples 1-3 and Comparative Examples 1-3 in Table 3, it can be found that the composite structure crucible for the cathode material of the lithium battery prepared by the present invention has excellent erosion resistance performance.

[0059] Through the comparison of the data in the table, the carbon coating modification effectively improves the erosion resistance performance. The addition of leucite not only improves the erosion resistance performance but also protects the carbon layer and extends the service life.

[0060] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above description is only the specific embodiments of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A composite structure sagger for lithium battery positive electrode material, characterized in that: The composite structure sagger for lithium battery positive electrode material is prepared by firstly impregnating mullite, cordierite and corundum in a furfuryl alcohol aqueous solution respectively, adding oxalic acid for polymerization and then calcining at high temperature to obtain carbon-coated mullite, carbon-coated cordierite and carbon-coated corundum; mixing mullite, cordierite, corundum, leucite, yellow dextrin and water as the outer layer of the sagger, mixing carbon-coated mullite, carbon-coated cordierite, carbon-coated corundum, leucite, yellow dextrin and water as the inner layer of the sagger, and calcining the composite structure sagger for lithium battery positive electrode material after composite compression molding of the inner and outer layer structures; The outer layer of the sagger includes the following mass components: 25-30 parts of mullite, 35-45 parts of cordierite, 10-15 parts of corundum, and 8-10 parts of leucite; The inner layer of the sagger comprises the following components by mass: 25-30 parts of carbon-coated mullite, 35-45 parts of carbon-coated cordierite, 10-15 parts of carbon-coated corundum, and 8-10 parts of leucite.

2. The composite structure sagger for lithium battery positive electrode material according to claim 1, characterized in that: The contents of each grade of particle size components of the mullite, cordierite, corundum, leucite, carbon-coated mullite, carbon-coated cordierite, and carbon-coated corundum are as follows: 3-5% for particles above 10 mesh; 20-25% for particles between 10 and 18 mesh; 15-20% for particles between 18 and 50 mesh; 20-25% for particles between 50 and 200 mesh; 20-25% for particles between 200 and 325 mesh; and 10-15% for particles below 325 mesh.

3. A method for preparing a composite structure sagger for lithium battery positive electrode material, characterized in that: The method comprises the following preparation steps: (1) Carbon coating: Mullite, cordierite and corundum were soaked in furfuryl alcohol aqueous solution by weight, respectively, and stirred at 200-300 r / min for 30-40 min at a material-liquid ratio of 1 g / 10 ml at 80-90 °C, and an equal volume of 0.3 mol / L oxalic acid aqueous solution was added dropwise at a rate of 3-4 drops per second. After the addition was completed, the mixture was stirred at 80-90 °C and 200-300 r / min for 6-8 h, filtered, washed with pure water 3-4 times, vacuum dried at 50-60 °C for 8-10 h, calcined at 800-900 °C for 2-3 h in an argon atmosphere, with a heating rate of 5 °C / min, and naturally cooled to room temperature to obtain carbon-coated mullite, carbon-coated cordierite and carbon-coated corundum; (2) Compression molding: Weigh 25-30 parts of mullite, 35-45 parts of cordierite, 10-15 parts of corundum, 8-10 parts of leucite, then add 5-7 parts of yellow dextrin and 3-5 parts of water, mix evenly, age for 24 hours, put into a mold with a thickness of 12-15 mm, then weigh 6-7 parts of carbon-coated mullite, 9-11 parts of carbon-coated cordierite, 2.5-4 parts of carbon-coated corundum, 2-2.5 parts of leucite, 1-2 parts of yellow dextrin, 0.75-1.25 parts of water, mix evenly, age for 24 hours, put into a mold with a thickness of 3-4 mm, seal and cure at room temperature for 24 hours, press and mold at a pressure of 150 MPa, hold the pressure for 10-14 hours, and dry at room temperature and 110°C for 24 hours to obtain a sagger green body; (3) Calcination: The sagger green body is calcined at the sintering temperature and naturally cooled to room temperature to obtain a composite structure sagger for lithium battery positive electrode materials.

4. The method for preparing a composite structure sagger for lithium battery positive electrode material according to claim 3, characterized in that: The furfuryl alcohol aqueous solution in step (1) is prepared by uniformly mixing furfuryl alcohol and pure water in a volume ratio of 1:

40.

5. The method for preparing a composite structure sagger for lithium battery positive electrode material according to claim 3, characterized in that: The sintering temperature of step (3) is: 0~100°C, heating rate 1.25°C / min; 100~900°C, heating rate 5°C / min, keeping at 900°C for 30min; 900~1200°C, heating rate 4°C / min; 1200~1400°C, heating rate 2.5°C / min, keeping at 1400°C for 3h.