Preparation method of high-strength ceramic insulator

Abstract

The invention relates to the technical field of ceramics, in particular to a preparation method of a high-strength ceramic insulator. The preparation method comprises the following steps: carrying out spark plasma ball milling on corundum, tantalum fluoride powder and niobium fluoride powder to prepare modified corundum powder; mixing and ball-milling the modified corundum powder with feldspar, titanium oxide, wollastonite, magnesian clay and the like to prepare pug; and carrying out filter pressing, aging, forming and sintering to prepare the high-strength ceramic insulator. The niobium fluoride powder and the tantalum fluoride powder are introduced, and the spark plasma ball milling technology is adopted, so that the compactness, the volume density and the microstructure uniformity of the ceramic are remarkably improved, the porosity and the water absorption rate are reduced, and the bending strength and the pollution flashover resistance are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ceramics, in particular to a preparation method of high-strength ceramic insulator. BACKGROUND

[0002] As a key component in power transmission lines and electrical equipment, ceramic insulators are widely used in high-voltage transmission, substations and electrical equipment. Its main function is to support the wire and isolate the electrical connection between the wire and the grounding structure. With the development of modern power systems towards high voltage and large capacity, ceramic insulators need to have higher mechanical strength, insulation performance and anti-flashover ability to meet the use requirements in complex working conditions and harsh environments. However, the traditional ceramic insulator still has certain limitations in performance, which is difficult to fully meet the use requirements in high-strength and high-pollution environments.

[0003] Traditional high-strength ceramic insulators are usually made of corundum as the main raw material, supplemented by other additives. Although corundum-based ceramics have high mechanical strength and excellent insulation performance, the compactness and uniformity of the internal structure still need to be improved. The pores and micro-cracks in the material not only reduce the bulk density and mechanical strength, but also cause the water absorption to increase, thereby affecting the insulation performance, especially in high humidity or rain and snow and other harsh environments, which is prone to insulation failure. In addition, the traditional ceramic insulator is easy to adsorb pollutants in the high-pollution environment, which causes the surface electric field concentration effect to intensify, thereby causing flashover accidents, which seriously threatens the safe operation of the power system. SUMMARY

[0004] Therefore, the purpose of the present application is to provide a preparation method of high-strength ceramic insulator, which significantly improves the comprehensive performance of the ceramic insulator by introducing functional additives and optimizing the preparation process, so as to meet the use requirements in high-strength and high-pollution environments.

[0005] In order to achieve the above purpose, the present application provides a preparation method of high-strength ceramic insulator, which comprises the following steps: (1) Raw material treatment: grinding corundum with a ball mill to pass through a 200 mesh sieve, and then performing discharge plasma ball milling with tantalum fluoride powder and niobium fluoride powder to obtain modified corundum powder; (2) Powder mixing: mixing the modified corundum powder, feldspar, titanium oxide, wollastonite and magnesia clay, and then putting them into a ball mill, adding deionized water, and ball milling to pass through an 80-120 mesh sieve to obtain a crude mud; (3) Pressure filtration and aging: putting the crude mud into a pressure filter, and then aging to obtain a refined mud; (4) Mud forming: putting the refined mud into a forming mold, shaping, and drying to obtain a green body; (5) sintering of the green body: the green body is put into a sintering furnace and sintered to obtain a high-strength ceramic insulator.

[0006] Preferably, the weight ratio of corundum, tantalum fluoride powder and niobium fluoride powder in step (1) is 1000:40-60:20-40.

[0007] Preferably, the ball-to-material ratio in the ball milling in step (1) is 15-30:1, the discharge frequency is 20-40 KHz, the rotating speed is 1500-2500 rpm, and the time is 4-6 h.

[0008] Preferably, the weight ratio of modified corundum powder, feldspar, titanium oxide, wollastonite and magnesia clay in step (2) is 700-800:100-150:40-75:35-60:80-120.

[0009] Preferably, the amount of deionized water added in step (2) is equal to the total weight of the modified corundum powder, feldspar, titanium oxide, wollastonite and magnesia clay.

[0010] Preferably, the silica content of the feldspar in step (2) is 67wt%-68wt%.

[0011] Preferably, the magnesium oxide content of the magnesia clay is 25wt%-27wt%.

[0012] Preferably, the ball-to-material ratio in the ball milling in step (2) is 20-40:1.

[0013] Preferably, the working pressure of the pressure filtration in step (3) is 0.5-0.7 MPa, the pressure filtration time is 1.5-2.5 h, and the aging time is 20-28 h.

[0014] Preferably, the forming pressure in step (4) is 20-30 MPa.

[0015] Preferably, the sintering in step (5) is: heating at 4-6 ℃ / min to 900-1100 ℃, then heating at 1-3 ℃ / min to 1350-1450 ℃, and keeping the temperature for 2-4 h.

[0016] The beneficial effects of the present application are: The application provides a preparation method of a high-strength ceramic insulator, and the ceramic insulator prepared by the method has significant advantages in mechanical properties and insulation properties. The material has a large bulk density and a very low open porosity, a water absorption of less than 0.1%, and a bending strength of more than 180 MPa, far exceeding the performance requirements of traditional high-strength aluminum porcelain insulators, and showing higher load-carrying capacity and mechanical impact resistance, which can meet the requirements of complex working conditions and high strength. At the same time, the material has good anti-pollution flashover capability, which can effectively reduce the occurrence of pollution flashover accidents and improve the reliability and service life of the insulator.

[0017] The application significantly improves the bulk density, bending strength and anti-pollution flashover capability of the ceramic insulator by introducing niobium fluoride and tantalum fluoride powders. The niobium fluoride and tantalum fluoride powders can enhance the surface hydrophobicity, optimize the microstructure, reduce surface defects and electric field concentration effect, thereby improving the anti-pollution flashover performance. The niobium fluoride and tantalum fluoride powders are refined to nanoscale and uniformly dispersed by using a spark plasma ball milling technology, and a strong bonding interface is formed with the corundum matrix, which further improves the density of the material and the stress transfer capability between particles, significantly improving the bending strength and anti-pollution flashover capability. In addition, the addition of magnesia clay can promote sintering densification, reduce the porosity of the material, and enhance the grain bonding force, thereby improving the bulk density and mechanical strength. The ceramic insulator prepared by the application has excellent performance in high-strength and high-pollution environments, and has wide application prospects. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical scheme and advantages of the application clearer, the application is further described in detail below with specific examples.

[0019] Example 1 (1) Raw material treatment: 1000g corundum is ground through a 200 mesh screen by a ball mill, and then is subjected to spark plasma ball milling with 40g tantalum fluoride powder and 20g niobium fluoride powder, the ball-to-material weight ratio is 15:1, the discharge frequency is 20KHz, the rotation speed is 1500rpm, and the time is 4h, to obtain modified corundum powder; (2) Powder mixing: 700g modified corundum powder, 100g feldspar, 40g titanium oxide, 35g wollastonite and 80g magnesia clay are mixed, and then are put into a ball mill, the ball-to-material ratio is 20:1, 955g deionized water is further added, and the mixture is ground through an 80 mesh screen to obtain a crude mud; (3) Pressure filtration and aging: the crude mud is put into a pressure filter, the working pressure is 0.5MPa, the pressure filtration time is 1.5h, and then the mud is aged for 20h to obtain refined mud; (4) Mud forming: the refined mud is put into a forming mold, the forming pressure is 20MPa, and then the mud is dried at 75℃ for 20h to obtain a green body; (5) Sintering of the green body: Put the green body into a sintering furnace, increase the temperature to 900℃ at a rate of 4℃ / min, then increase the temperature to 1350℃ at a rate of 1℃ / min, and keep the temperature for 2h, to obtain a high-strength ceramic insulator.

[0020] Example 2: (1) Raw material treatment: Grind 1000g of corundum through a 200-mesh sieve using a ball mill, and then perform electrical discharge plasma ball milling with 50g of tantalum fluoride powder and 30g of niobium fluoride powder, with a ball-to-material weight ratio of 20:1, an electrical discharge frequency of 30KHz, a rotation speed of 2000rpm, and a time of 5h, to obtain modified corundum powder; (2) Powder mixing: Mix 750g of the modified corundum powder, 120g of feldspar, 60g of titanium oxide, 50g of wollastonite, and 100g of magnesia clay, put them into a ball mill, with a ball-to-material ratio of 30:1, and then add 1080g of deionized water, and ball mill through a 100-mesh sieve, to obtain a crude body; (3) Pressure filtration and aging: Put the crude body into a pressure filter, with a working pressure of 0.6MPa and a pressure filtration time of 2h, and then perform aging for 24h, to obtain a refined body; (4) Body shaping: Put the refined body into a shaping mold, with a shaping pressure of 25MPa, and then dry it at 80℃ for 24h, to obtain a green body; (5) Sintering of the green body: Put the green body into a sintering furnace, increase the temperature to 1000℃ at a rate of 5℃ / min, then increase the temperature to 1400℃ at a rate of 2℃ / min, and keep the temperature for 3h, to obtain a high-strength ceramic insulator.

[0021] Example 3: (1) Raw material treatment: Grind 1000g of corundum through a 200-mesh sieve using a ball mill, and then perform electrical discharge plasma ball milling with 60g of tantalum fluoride powder and 40g of niobium fluoride powder, with a ball-to-material weight ratio of 30:1, an electrical discharge frequency of 40KHz, a rotation speed of 2500rpm, and a time of 6h, to obtain modified corundum powder; (2) Powder mixing: Mix 800g of the modified corundum powder, 150g of feldspar, 75g of titanium oxide, 60g of wollastonite, and 120g of magnesia clay, put them into a ball mill, with a ball-to-material ratio of 40:1, and then add 1205g of deionized water, and ball mill through a 120-mesh sieve, to obtain a crude body; (3) Pressure filtration and aging: Put the crude body into a pressure filter, with a working pressure of 0.7MPa and a pressure filtration time of 2.5h, and then perform aging for 28h, to obtain a refined body; (4) Body shaping: Put the refined body into a shaping mold, with a shaping pressure of 30MPa, and then dry it at 85℃ for 28h, to obtain a green body; (5) Sintering of the green body: Put the green body into a sintering furnace, increase the temperature to 1100°C at a rate of 6°C / min, then increase the temperature to 1450°C at a rate of 3°C / min, and keep the temperature for 4h to obtain a high-strength ceramic insulator.

[0022] Comparative Example 1 The difference between Comparative Example 1 and Example 2 is that the tantalum fluoride powder in step (1) is replaced by niobium fluoride powder. The specific steps are as follows: (1) Raw material treatment: grind 1000g corundum through a 200 mesh sieve using a ball mill, and then perform electrical discharge plasma ball milling with 80g tantalum fluoride powder, the ball-to-material weight ratio is 20:1, the discharge frequency is 30KHz, the rotating speed is 2000rpm, and the time is 5h to obtain modified corundum powder; (2) Powder mixing: mix 750g modified corundum powder, 120g feldspar, 60g titanium oxide, 50g wollastonite, and 100g magnesia clay, put them into a ball mill, the ball-to-material ratio is 30:1, then add 1080g deionized water, and ball mill through a 100 mesh sieve to obtain a crude body; (3) Pressure filtration and aging: put the crude body into a pressure filter, the working pressure is 0.6MPa, the pressure filtration time is 2h, then perform aging, the aging time is 24h to obtain a refined body; (4) Body forming: put the refined body into a forming mold, the forming pressure is 25MPa, then dry it at 80°C for 24h to obtain a green body; (5) Sintering of the green body: Put the green body into a sintering furnace, increase the temperature to 1000°C at a rate of 5°C / min, then increase the temperature to 1400°C at a rate of 2°C / min, and keep the temperature for 3h to obtain a ceramic insulator.

[0023] Comparative Example 2 The difference between Comparative Example 2 and Example 2 is that the tantalum fluoride powder in step (1) is replaced by niobium fluoride powder. The specific steps are as follows: (1) Raw material treatment: grind 1000g corundum through a 200 mesh sieve using a ball mill, and then perform electrical discharge plasma ball milling with 80g niobium fluoride powder, the ball-to-material weight ratio is 20:1, the discharge frequency is 30KHz, the rotating speed is 2000rpm, and the time is 5h to obtain modified corundum powder; (2) Powder mixing: mix 750g modified corundum powder, 120g feldspar, 60g titanium oxide, 50g wollastonite, and 100g magnesia clay, put them into a ball mill, the ball-to-material ratio is 30:1, then add 1080g deionized water, and ball mill through a 100 mesh sieve to obtain a crude body; (3) Pressure filtration and aging: put the crude body into a pressure filter, the working pressure is 0.6MPa, the pressure filtration time is 2h, then perform aging, the aging time is 24h to obtain a refined body; (4) Molding of the refined clay: the refined clay was put into a molding mold, the molding pressure was 25 MPa, and then dried at 80℃ for 24 h to obtain a green body; (5) Sintering of the green body: the green body was put into a sintering furnace, heated to 1000℃ at a rate of 5℃ / min, then heated to 1400℃ at a rate of 2℃ / min, and kept for 3 h to obtain a ceramic insulator.

[0024] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the modified corundum powder in step (2) is replaced by a mixture of corundum powder ball-milled through a 200-mesh sieve, tantalum fluoride powder, and niobium fluoride powder; The specific steps are as follows: (1) Treatment of raw materials: 1000 g of corundum was ground through a 200-mesh sieve using a ball mill, and then mixed with 50 g of tantalum fluoride powder and 30 g of niobium fluoride powder to obtain a mixed powder; (2) Mixing of powders: 750 g of the mixed powder, 120 g of feldspar, 60 g of titanium oxide, 50 g of wollastonite, and 100 g of magnesia clay were mixed, put into a ball mill, the ball-to-material ratio was 30:1, 1080 g of deionized water was added, and then ball-milled through a 100-mesh sieve to obtain a crude clay; (3) Pressure filtration and aging: the crude clay was put into a pressure filter, the working pressure was 0.6 MPa, the pressure filtration time was 2 h, and then aging was performed, the aging time was 24 h to obtain a refined clay; (4) Molding of the refined clay: the refined clay was put into a molding mold, the molding pressure was 25 MPa, and then dried at 80℃ for 24 h to obtain a green body; (5) Sintering of the green body: the green body was put into a sintering furnace, heated to 1000℃ at a rate of 5℃ / min, then heated to 1400℃ at a rate of 2℃ / min, and kept for 3 h to obtain a high-strength ceramic insulator.

[0025] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that the modified corundum powder in step (2) is replaced by corundum powder ball-milled through a 200-mesh sieve; The specific steps are as follows: (1) Mixing of powders: 750 g of corundum powder ball-milled through a 200-mesh sieve, 120 g of feldspar, 60 g of titanium oxide, 50 g of wollastonite, and 100 g of magnesia clay were mixed, put into a ball mill, the ball-to-material ratio was 30:1, 1080 g of deionized water was added, and then ball-milled through a 100-mesh sieve to obtain a crude clay; (2) Pressure filtration and aging: the crude clay was put into a pressure filter, the working pressure was 0.6 MPa, the pressure filtration time was 2 h, and then aging was performed, the aging time was 24 h to obtain a refined clay; (3) Molding of the refined clay: the refined clay was put into a molding mold, the molding pressure was 25 MPa, and then dried at 80 °C for 24 h to obtain a green body; (4) Sintering of the green body: the green body was put into a sintering furnace, heated to 1000 °C at a rate of 5 °C / min, then heated to 1400 °C at a rate of 2 °C / min, and kept for 3 h to obtain a ceramic insulator.

[0026] Comparative Example 5: Comparative Example 5 differs from Example 2 in that no magnesia clay was added in step (2); The specific steps are as follows: (1) Treatment of raw materials: 1000 g of corundum was ground through a 200 mesh sieve using a ball mill, and then ball-milled with 50 g of tantalum fluoride powder and 30 g of niobium fluoride powder at a ball-to-material weight ratio of 20:1, a discharge frequency of 30 KHz, a rotation speed of 2000 rpm, and a time of 5 h to obtain modified corundum powder; (2) Mixing of powders: 750 g of modified corundum powder, 120 g of feldspar, 60 g of titanium oxide, and 50 g of wollastonite were mixed and put into a ball mill at a ball-to-material ratio of 30:1, and then 1080 g of deionized water was added and ball-milled through a 100 mesh sieve to obtain a crude clay; (3) Pressure filtration and aging: the crude clay was put into a pressure filter at a working pressure of 0.6 MPa, and then pressure-filtered for 2 h and aged for 24 h to obtain a refined clay; (4) Molding of the refined clay: the refined clay was put into a molding mold, the molding pressure was 25 MPa, and then dried at 80 °C for 24 h to obtain a green body; (5) Sintering of the green body: the green body was put into a sintering furnace, heated to 1000 °C at a rate of 5 °C / min, then heated to 1400 °C at a rate of 2 °C / min, and kept for 3 h to obtain a ceramic insulator.

[0027] Performance test: Bulk density, open porosity, and water absorption: the drainage method in GB / T 2997-2000 was used for testing, and the results are shown in Table 1; Bending strength: a hydraulic universal testing machine was used for testing according to GB / T 6569-2006, and the results are shown in Table 1.

[0028] Pollution flashover test: a salt solution of 10 ml of NaCl with a concentration of 0.0036 mg / ml was prepared, 0.216 g of diatomite was added to the salt solution and stirred to obtain a pollution liquid, the solvent was deionized water, the surfaces of the insulators prepared in the examples and comparative examples were pasted with aluminum foil to obtain a size of 12 mm x 15 mm (the distance between the aluminum foil electrodes was 12 mm, and the width was 15 mm), the area was 1.8 cm 2The contaminated area was determined, and 1 ml of the contamination liquid was dropped into the area using a 1 ml dropper. The contamination liquid was evenly coated in the designated contaminated area using the dropper. Subsequently, the contaminated sample was transferred to a forced air drying oven for drying to evaporate the surface solvent. After drying, the three samples were fixed on an angle table with an inclination angle of 60°. The water mist generator was turned on, and the input rate of the water mist was controlled at 380 mL / h. The water mist was blown directly against the surface of the sample for 1 h. After the surface was completely wet, the sample was placed in the test device to test the pollution flashover voltage. The dry flashover voltage of the sample without contamination was also tested, and the voltage drop rate after pollution flashover was calculated. The results are shown in Table 1.

[0029] Table 1 Performance test results

[0030] Data analysis: As can be seen from the data of Examples 1-3 in Table 1, the high-strength ceramic insulator prepared by the present application has significant advantages in mechanical properties and insulation performance. The large bulk density and extremely low open porosity indicate that the internal structure of the material is dense, reducing the adverse effects of pores on strength and insulation performance, thereby significantly improving the mechanical strength and durability of the material. The water absorption rate of less than 0.1% further proves the denseness and excellent water immersion resistance of the material, which is crucial for the stable operation of the insulator in harsh environments such as high humidity or rain and snow. In addition, the bending strength reaches more than 180 MPa, far exceeding the performance requirements of C130 high-strength aluminum porcelain insulators, indicating that the ceramic insulator of the present application has higher load-carrying capacity and mechanical impact resistance, and can meet the use requirements of higher strength and more complex working conditions. At the same time, its certain anti-pollution flashover ability indicates that the material surface has good anti-pollution performance and insulation stability, which can effectively reduce the occurrence of pollution flashover accidents and improve the reliability and service life of the insulator.

[0031] As can be seen from the data of Example 2 and Comparative Examples 1-3 in Table 1, the introduction of niobium fluoride powder and tantalum fluoride powder into corundum by spark plasma ball milling can synergistically improve the bulk density of the ceramic insulator, reduce the porosity and water absorption, and significantly improve the bending strength, which is mainly due to the fact that the spark plasma ball milling can uniformly disperse and refine the niobium fluoride powder and tantalum fluoride powder into nano-sized particles, and the high energy introduced in the ball milling process can promote the activation of the particle surface, enabling it to be closely combined with the corundum particles and promoting the formation of a more dense microstructure with other raw materials, significantly improving the bulk density of the ceramic material, reducing the number and size of pores, thereby reducing the porosity and water absorption. In addition, the high energy generated during the spark plasma ball milling process can also promote the interfacial reaction between the niobium fluoride and tantalum fluoride particles and the corundum matrix, forming a strong interfacial bond. This interfacial bond not only improves the overall density of the material, but also enhances the stress transfer capability between particles, thereby significantly improving the bending strength of the ceramic insulator.

[0032] As can be seen from the data of Example 2 and Comparative Examples 3-4 in Table 1, directly mixing niobium fluoride powder and tantalum fluoride powder into the raw materials of the ceramic insulator can improve the anti-flashover ability of the ceramic insulator, which is mainly due to the fact that the niobium fluoride powder and tantalum fluoride powder can enhance the surface hydrophobicity, optimize the microstructure, reduce surface defects and electric field concentration effect, thereby improving the anti-flashover ability. And introducing the niobium fluoride powder and tantalum fluoride powder into corundum by spark plasma ball milling and then mixing with the remaining raw materials can further improve the anti-flashover ability, which is likely due to the fact that the high energy generated during the ball milling process promotes the interfacial reaction between the niobium fluoride, tantalum fluoride particles and the corundum matrix, forming a firmly bonded interface. This interfacial bond reduces the microcracks and pores inside the material, further reducing surface defects and electric field concentration effect.

[0033] As can be seen from the data of Example 2 and Comparative Example 5 in Table 1, adding magnesia clay to the raw materials of the ceramic insulator can effectively improve the bulk density and bending strength of the ceramic insulator, which is mainly due to the fact that the magnesia clay can promote the rearrangement and densification of ceramic particles during high-temperature sintering, reducing the porosity inside the material, thereby improving the bulk density and mechanical strength.

[0034] It should be understood by those of ordinary skill in the art that the above discussion of any of the embodiments is merely exemplary and is not intended to suggest that the scope of the application is limited to these examples; under the concept of the present application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the present application as described above. In order to be brief, they are not provided in detail.

Claims

1. A method for preparing a high-strength ceramic insulator, characterized in that, Includes the following steps: (1) Raw material processing: Corundum is ground through a 200-mesh sieve using a ball mill, and then subjected to spark plasma ball milling with tantalum fluoride powder and niobium fluoride powder to obtain modified corundum powder. (2) Powder mixing: Mix modified corundum powder, feldspar, titanium dioxide, wollastonite and magnesia clay, put them into a ball mill, add deionized water, and ball mill through an 80-120 mesh sieve to obtain coarse mud. (3) Filter press aging: Put the coarse mud into a filter press and then age it to obtain refined mud; (4) Clay molding: The refined clay is placed into the molding mold, shaped, and dried to obtain a clay blank; (5) Sintering of clay blanks: The clay blanks are placed in a firing furnace and sintered to obtain high-strength ceramic insulators; In step (1), the weight ratio of corundum, tantalum fluoride powder and niobium fluoride powder is 1000:40-60:20-40.

2. The method for preparing a high-strength ceramic insulator according to claim 1, characterized in that, In step (1), the ball-to-material weight ratio of the discharge plasma ball mill is 15-30:1, the discharge frequency is 20-40KHz, the rotation speed is 1500-2500rpm, and the time is 4-6h.

3. The method for preparing a high-strength ceramic insulator according to claim 1, characterized in that, In step (2), the weight ratio of modified corundum powder, feldspar, titanium dioxide, wollastonite and magnesian clay is 700-800:100-150:40-75:35-60:80-120.

4. The method for preparing a high-strength ceramic insulator according to claim 1, characterized in that, In step (2), the amount of deionized water added is equal to the total weight of modified corundum powder, feldspar, titanium dioxide, wollastonite and magnesian clay.

5. The method for preparing a high-strength ceramic insulator according to claim 1, characterized in that, In step (2), the silica content of feldspar is 67wt%-68wt%.

6. The method for preparing a high-strength ceramic insulator according to claim 1, characterized in that, The magnesium oxide content in the magnesian clay is 25wt%-27wt%.

7. The method for preparing a high-strength ceramic insulator according to claim 1, characterized in that, In step (2), the ball-to-material ratio in ball milling is 20-40:

1.

8. The method for preparing a high-strength ceramic insulator according to claim 1, characterized in that, In step (3), the working pressure of the filter press is 0.5-0.7 MPa, the filter press time is 1.5-2.5 h, and the aging time is 20-28 h.

9. The method for preparing a high-strength ceramic insulator according to claim 1, characterized in that, The molding pressure in step (4) is 20-30 MPa.

10. The method for preparing a high-strength ceramic insulator according to claim 1, characterized in that, The sintering in step (5) is as follows: the temperature is increased to 900-1100℃ at 4-6℃ / min, and then increased to 1350-1450℃ at 1-3℃ / min, and held for 2-4 hours.

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