CN108164278A - High-gas-permeability silicon carbide porous ceramic material and preparation method thereof - Google Patents

Abstract

The invention relates to a silicon carbide porous ceramic material with high gas permeability and a preparation method thereof, belonging to the field of porous ceramic preparation. The silicon carbide mixed powder is prepared by dry mixing, the binder polyvinyl alcohol aqueous solution is added to the mixed powder, and after high-speed stirring, vibrating sieving granulation, vibrating filler, cold isostatic pressing, drying and programmed temperature sintering, the obtained High gas permeability silicon carbide porous ceramic material. The mass percent composition of raw materials in the preparation process is carbon powder 5-15%, ceramic fiber 2-10%, polyvinyl alcohol aqueous solution 3-8%, and the rest is silicon carbide. The invention adds ceramic fiber and carbon powder to the silicon carbide powder, thereby preparing a silicon carbide porous ceramic material with high gas permeability, which is widely used in the fields of industrial kiln flue gas treatment, metallurgy, energy, energy conservation and environmental protection application prospects.

Description

A kind of silicon carbide porous ceramic material with high gas permeability and preparation method thereof

technical field

The invention relates to a silicon carbide porous ceramic material with high gas permeability and a preparation method thereof, belonging to the field of porous ceramic preparation.

Background technique

Silicon carbide porous ceramics have the advantages of high strength, high temperature resistance, corrosion resistance, high thermal shock resistance, good chemical stability and low thermal expansion coefficient. They are widely used in metallurgy, chemical industry, coal-fired power plants, waste incineration, catalytic cracking and cracking Wide application prospects, mainly used in flue gas dust removal, heavy metal removal and catalyst recovery at high temperature. Silicon carbide has strong covalency, small self-diffusion coefficient, and high Si-C bond energy. It is difficult to obtain high-strength porous ceramic materials by low-temperature sintering. Usually, the sintering temperature is higher than 2100°C, which results in high application costs of silicon carbide porous ceramic products. In the past ten years, scholars have proposed to prepare silicon carbide porous ceramics at low temperature by in-situ reaction sintering [JH She, et al, Oxidation bonding of porous silicon carbide ceramics[J]. J. Mater. Sci. 37 (2002) 3615- 3622], the in-situ reaction sintering technology is to form a particle-linked neck phase by adding sintering aids and cristobalite produced by high-temperature oxidation of silicon carbide, thereby obtaining high-strength silicon carbide porous ceramic materials [Ding Shuqiang et al., in-situ reaction combined with carbonization Preparation and properties of silicon porous ceramics [J]. Journal of Inorganic Materials, 21 (2006) 122-125]. This method has the advantages of simplicity, low cost, and easy scale-up. However, studies have found that the currently selected firing aids for the preparation of silicon carbide porous ceramics at low temperatures are usually clay containing alkali metals and SiO ₂ , because this part of the material is prone to liquid phase at high temperature, resulting in a large amount of cristobalite phase and amorphous The glass phase is conducive to the connection between silicon carbide particles. However, a large amount of liquid phase generated by high temperature tends to reduce the number and size of pores accumulated by silicon carbide, resulting in a decrease in both porosity and average pore size, which greatly affects the gas permeability of porous materials, which is not conducive to the gas permeability of materials. Large-scale application in the field of solid separation. Therefore, how to increase the pore size and porosity of the material and improve the gas permeability of the material on the basis of ensuring the mechanical strength is the key technology to promote the application of silicon carbide porous ceramics in the field of gas-solid separation. Ceramic fiber, also known as refractory fiber, has the characteristics of high temperature resistance, high strength, high toughness and low thermal expansion coefficient. Using ceramic fibers to "bridge" the neck of silicon carbide particles, increasing the size of the pores, increasing the pore size and porosity, will greatly improve the gas permeability of the material; in addition, the pulling out and bridging of fibers It also improves the mechanical properties of the material [Shi Guopu, etc., the influence of mullite fiber on the properties of alumina ceramics [J]. Rare Metal Materials and Engineering, 38(2009) 447-449], because ceramic fibers have the characteristics of low thermal expansion coefficient , the silicon carbide porous ceramics prepared by this technology can also have good thermal shock resistance, which will greatly expand the service life and application fields of silicon carbide porous ceramics. However, there are few papers and patents related to the preparation of silicon carbide porous ceramic materials with high gas permeability by doping with ceramic fibers.

Contents of the invention

The invention aims to improve the gas permeability of the existing silicon carbide porous ceramic material. The invention provides a method for preparing silicon carbide porous ceramics with high gas permeability. The method is simple in process and low in cost. In addition to the excellent performance of general silicon carbide ceramics, the prepared silicon carbide porous ceramic material also has high strength. It has the characteristics of superior thermal shock resistance and other characteristics, and can meet the requirements of filtration with relatively large amount of high-temperature flue gas treatment.

Technical scheme of the present invention is as follows:

A silicon carbide porous ceramic material with high gas permeability and its preparation method, the preparation steps are as follows:

(1) Put the raw materials silicon carbide, ceramic fiber and carbon powder in a three-dimensional mixer and mix them thoroughly for 24 hours to obtain the mixed powder a;

(2) Put the mixed powder a into a high-speed mixer and add the binder polyvinyl alcohol aqueous solution, stir for 30 minutes, the mixer speed is 1000 r/min, and then discharge and vibrate and sieve to obtain granules with uniform binder content Powder b;

(3) Transfer the powder b into the mold through a vibrating filler, put it into a cold isostatic press to prepare a silicon carbide ceramic green body, and then dry it in an oven at 70ºC for 24 hours to obtain the dried body c;

(4) The green body c is placed in a high-temperature furnace for sintering at programmed temperature, and finally cooled naturally to obtain a silicon carbide porous ceramic material.

The mass percent of each component in the raw material of the present invention is carbon powder 5-15%, ceramic fiber 2-10%, polyvinyl alcohol aqueous solution 3-8%, and the rest is silicon carbide.

The average particle size of the silicon carbide in the present invention is 100-300 μm, and the average particle size of the carbon powder is 20-80 μm. The diameter of the ceramic fiber is 10-20 μm, and the length of the ceramic fiber is 40-60 μm. The mass concentration of the polyvinyl alcohol aqueous solution is 5-10%.

In the step (1) of the present invention, the ceramic fiber is one of mullite fiber, alumina fiber, silicon carbide fiber and aluminum silicate fiber.

In the step (3) of the present invention, the vibration filling frequency is 50 Hz, the filling amount of powder b is 5.0-5.5 Kg, and the powder density in the mold is uniform and not eccentric.

Step (3) The pressure of intercooled isostatic pressing is 80-120 MPa, and the holding time is 60-120 s.

In step (4), the programmed temperature sintering temperature is as follows: heating at a rate of 2 to 3 ºC/min to 500 ºC, holding the temperature for 2 hours, then raising the temperature to 1200 ºC at 2 to 4 ºC/min, and then heating at 1 to 2 ºC/min To 1350 ~ 1550 ºC, heat preservation time 2 ~ 6 hours, and then naturally cool down.

In the above preparation method, in the step (4), the sintering process mainly refers to sintering in air, and the natural cooling also refers to cooling in air.

The method of the present invention, the principle of its preparation is: use ceramic fiber "bridging" construction technology to realize the connection and welding between silicon carbide particles, increase the porosity between particles, and improve the mechanical strength of the material at the same time, so as to obtain high Gas Permeability of Silicon Carbide Porous Ceramic Filter Material.

In the present invention, the mensuration of each parameter adopts following method:

1. The test method of porosity adopts the Archimedes drainage method, and deionized water is used as the soaking medium. Specific steps

Refer to the national standard GB/T 1966-1996 (the national standard of the People's Republic of China, test methods for apparent porosity and capacity of porous ceramics).

2. The flexural strength test method uses a flexural strength tester to measure the average value of the flexural strength at three points, with a span of 40 mm and a loading speed of 0.5 mm/min. For specific test steps, refer to the national standard GB/T1965-1996 (National Standard of the People's Republic of China, Test Method for Bending Strength of Porous Ceramics).

3. The pore size test method adopts the gas bubble pressure method, using deionized water as the wetting agent, and the specific test steps refer to the national standard GB/T 1967-1996 (National Standard of the People's Republic of China, Porous Ceramic Pore Diameter Test Method).

4. The gas permeability test method is to test the gas flow rate on the permeation side under different pressures. The gas permeability test device made by the laboratory is used. The specific test steps refer to the national standard GB/T 1969-1996 (National Standard of the People's Republic of China, Porous Ceramics Permeability test method).

5. The test method for high temperature thermal shock resistance of silicon carbide porous ceramic materials shall be tested according to GB/T16536. The specific test steps are as follows: use the air cooling method and the water cooling method to test the macroscopic and microscopic changes in the rapid cooling process of the material respectively. Put the sample into the electric furnace at one time, keep it warm for 30 minutes, then take it out quickly and put it in the cold air close to 0 ºC to cool it down, which is recorded as a thermal shock cycle. After 3 consecutive times, take out 3 samples to measure their flexural strength. After 60 times, compare the change of the flexural strength before and after; the test procedure of the water-cooling method is: same as the air-cooling method, heat the electric furnace to 800 ºC, then put the sample into the electric furnace at one time, keep it warm for 30 minutes, and then quickly take it out and put it in cold water Rapid cooling is recorded as a thermal shock cycle. After 3 consecutive operations, 3 samples are taken out to measure their flexural strength. After 60 cycles, the changes in flexural strength before and after are compared.

Beneficial effect

1. In addition to all the advantages of silicon carbide ceramics, the silicon carbide porous ceramic material prepared by the present invention also has the characteristics of high gas permeability, large pore size, high porosity, and excellent thermal shock resistance.

2. The silicon carbide porous ceramics prepared by the present invention can expand the field of use and application environment of the product, especially in the field of high-temperature waste gas filtration, such as garbage incineration, industrial kilns, etc.

3. The silicon carbide porous ceramic prepared by the present invention not only has high porosity and large pore size, but also can be used as a carrier material for loading catalysts, and can be used in fields such as integrated treatment of gas pollutants.

4. The preparation method of the present invention has the advantages of simple process, easy operation, low cost and convenient industrial production.

Description of drawings

Accompanying drawing 1 is the SEM figure of the silicon carbide porous ceramic prepared in embodiment 1.

Accompanying drawing 2 is the XRD graph of the silicon carbide porous ceramics prepared in Example 1.

Accompanying drawing 3 is the thermal shock resistance diagram of the silicon carbide porous ceramic prepared in Example 3.

Detailed ways

The present invention will be further explained below in conjunction with the examples, the following examples are only used to illustrate the present invention, but are not intended to limit the scope of the present invention.

The test methods described in the following examples, unless otherwise specified, are conventional methods; the reagents and materials described, unless otherwise specified, can be obtained from commercial sources.

Example 1

78% by mass of SiC with an average particle size of 100 μm, 15% carbon powder with an average particle size of 20 μm, and 2% mullite fiber (average diameter of 10 μm, length 60 μm) were weighed into a three-dimensional mixer, Mechanically mixed for 24 h, after taking it out, add polyvinyl alcohol aqueous solution with a mass percentage of 5% (that is, PVA, with a mass concentration of 5%) and stir in a high-speed mixer for 30 min at a relative speed of 1000 r/min. Granular powder, vibration filler frequency 50 Hz, 5.5Kg granulated powder was loaded into the mold by vibration filler, and then put into an isostatic press to obtain a silicon carbide ceramic body. The isostatic molding pressure was 120 MPa, and the Press for 60 s, then put the green body in an oven at 70 ºC to dry for 24 h, and finally put it into a muffle furnace with a heating rate of 2 ºC/min to 500 ºC, keep it warm for 2 hours, and then heat up at 2 ºC/min to 1200 ºC, then heated at 1 ºC/min to 1450 ºC for 4 h, and cooled naturally to obtain a silicon carbide porous ceramic sample. After testing, various properties of the obtained silicon carbide porous ceramic samples are shown in Table 1 below. Accompanying drawing 1 is the SiC porous ceramic SEM picture that embodiment 1 prepares. Accompanying drawing 2 is the SiC porous ceramic XRD pattern prepared in embodiment 1.

Example 2 ~ 3

Using silicon carbide with an average particle size of 200 μm and 300 μm, samples were obtained according to the same raw material ratio and preparation method as in Example 1. After testing, the properties of silicon carbide porous ceramic samples with different particle sizes are shown in Table 1.

Table 1 Comparison of properties of SiC porous ceramic samples with different particle sizes

Accompanying drawing 3 is the thermal shock resistance diagram of SiC porous ceramics prepared in Example 3.

The test results of the thermal shock resistance test show that the flexural strength of the sample is 18.6 MPa at the beginning, and after 60 cycles of cooling and heating, the strength tends to stabilize and reach a value above 15 MPa, showing very good thermal shock resistance.

Example 4

78% by mass of SiC with an average particle size of 300 μm, 15% carbon powder with an average particle size of 20 μm, and 2% alumina fibers (average diameter of 20 μm, average length of 60 μm) were weighed into a three-dimensional mixer, Mechanically mixed for 24 h, after taking it out, add polyvinyl alcohol aqueous solution with a mass percentage of 5% (that is, PVA, with a mass concentration of 5%) and stir in a high-speed mixer for 30 min at a relative speed of 1000 r/min. Granular powder, vibration filler frequency 50 Hz, 5.5Kg granulated powder was loaded into the mold by vibration filler, and then put into an isostatic press to obtain a silicon carbide ceramic body. The isostatic molding pressure was 120 MPa, and the Press for 60 s, then put the green body in an oven at 70 ºC to dry for 24 h, and finally put it into a muffle furnace with a heating rate of 2 ºC/min to 500 ºC, keep it warm for 2 hours, and then heat up at 2 ºC/min to 1200 ºC, then heated at 1 ºC/min to 1450 ºC for 4 h, and cooled naturally to obtain a silicon carbide porous ceramic sample. After testing, various properties of the obtained silicon carbide porous ceramic samples are shown in Table 2 below.

Example 5

Aluminum silicate fibers (average diameter 10 μm, average length 40 μm) were used instead of alumina fibers, and samples were obtained according to the same raw material ratio and preparation method as in Example 4. After testing, each of the silicon carbide porous ceramic samples of different fiber types Item properties are shown in Table 2.

Table 2 Comparison of properties of silicon carbide porous ceramic samples with different fiber types

Example 6

Weigh 78% SiC with an average particle size of 300 μm, 15% carbon powder with an average particle size of 80 μm, and 2% mullite fiber (average diameter 20 μm, average length 50 μm) into a three-dimensional mixer , mixed mechanically for 24 h, took it out, added 5% by mass polyvinyl alcohol aqueous solution (ie PVA, with a mass concentration of 5%), stirred in a high-speed mixer for 30min at a relative speed of 1000 r/min, and then obtained by vibrating sieving Granulated powder, 5.5 Kg of granulated powder was packed into the mold by vibration filler, and then put into an isostatic press to form a silicon carbide ceramic body. The vibration filler frequency was 50 Hz, and the isostatic molding pressure was 120 MPa. The holding time was 60 s, and then the green body was dried in an oven at 70 ºC for 24 h, and finally placed in a muffle furnace at a heating rate of 2 ºC/min to 500 ºC, kept for 2 hours, and then heated at 2 ºC/min The temperature was raised to 1200 ºC, then heated at 1 ºC/min to 1450 ºC and kept for 4 hours, and then a silicon carbide porous ceramic sample was obtained after natural cooling. After testing, the larger the particle size of the carbon powder, the lower the porosity and the higher the strength of the ceramic. The properties are shown in Table 3 below.

Table 3 Effect of carbon powder particle size on properties of SiC porous ceramic samples

Example 7

Weigh 88% SiC with an average particle size of 300 μm, 5% carbon powder with an average particle size of 20 μm, and 2% alumina fibers (average diameter 20 μm, average length 60 μm) into a three-dimensional mixer, Mechanically mixed for 24 h, after taking it out, add polyvinyl alcohol aqueous solution with a mass percentage of 5% (that is, PVA, with a mass concentration of 5%) and stir in a high-speed mixer for 30 min at a relative speed of 1000 r/min. Granular powder, vibration filler frequency 50 Hz, 5.5Kg granulated powder was loaded into the mold by vibration filler, and then put into an isostatic press to obtain a silicon carbide ceramic body. The isostatic molding pressure was 120 MPa, and the Press for 60 s, then put the green body in an oven at 70 ºC to dry for 24 h, and finally put it into a muffle furnace with a heating rate of 2 ºC/min to 500 ºC, keep it warm for 2 hours, and then heat up at 2 ºC/min to 1200 ºC, then heated at 1 ºC/min to 1450 ºC for 4 h, and cooled naturally to obtain a silicon carbide porous ceramic sample. It has been tested that the lower the content of carbon powder, the lower the porosity and the higher the strength of the ceramic. At this time, the gas permeability of the sample is 1030 m ³ /m ² ·h·kPa.

Example 8

Weigh 75% SiC with an average particle size of 300 μm, 15% carbon powder with an average particle size of 20 μm, and 5% mullite fiber (average diameter 20 μm, average length 60 μm) into a three-dimensional mixer , mixed mechanically for 24 h, took it out, added 5% by mass polyvinyl alcohol aqueous solution (ie PVA, with a mass concentration of 5%), stirred in a high-speed mixer for 30min at a relative speed of 1000 r/min, and then obtained by vibrating sieving Granulated powder, the vibration filler frequency is 50 Hz, 5.5Kg granulated powder is packed into the mold by vibration filler, and then put into an isostatic press to form a silicon carbide ceramic body, and the isostatic molding pressure is 120 MPa. The holding time was 60 s, and then the green body was dried in an oven at 70 ºC for 24 h, and finally placed in a muffle furnace at a heating rate of 2 ºC/min to 500 ºC, kept for 2 hours, and then heated at 2 ºC/min The temperature was raised to 1200 ºC, then heated at 1 ºC/min to 1450 ºC and kept for 4 hours, and then a silicon carbide porous ceramic sample was obtained after natural cooling. After testing, various properties of silicon carbide porous ceramics are shown in Table 4 below.

Example 9

Weigh 70% SiC with an average particle size of 300 μm, 15% carbon powder with an average particle size of 20 μm, and 10% mullite fiber (average diameter 20 μm, average length 60 μm) into a three-dimensional mixer , mixed mechanically for 24 h, took it out, added 5% by mass polyvinyl alcohol aqueous solution (ie PVA, with a mass concentration of 5%), stirred in a high-speed mixer for 30min at a relative speed of 1000 r/min, and then obtained by vibrating sieving Granulated powder, the vibration filler frequency is 50 Hz, 5.5Kg granulated powder is packed into the mold by vibration filler, and then put into an isostatic press to form a silicon carbide ceramic body, and the isostatic molding pressure is 120 MPa. The holding time was 60 s, and then the green body was dried in an oven at 70 ºC for 24 h, and finally placed in a muffle furnace at a heating rate of 2 ºC/min to 500 ºC, kept for 2 hours, and then heated at 2 ºC/min The temperature was raised to 1200 ºC, then heated at 1 ºC/min to 1450 ºC and kept for 4 hours, and then a silicon carbide porous ceramic sample was obtained after natural cooling. After testing, various properties of silicon carbide porous ceramics are shown in Table 4 below.

Table 4 Effect of mullite fiber content on the properties of silicon carbide porous ceramic samples

Example 10

Weigh 75% SiC with an average particle size of 300 μm, 15% carbon powder with an average particle size of 20 μm, and 2% mullite fiber (average diameter 20 μm, average length 60 μm) into a three-dimensional mixer , mixed mechanically for 24 h, after taking it out, add 8% by mass polyvinyl alcohol aqueous solution (that is, PVA, with a mass concentration of 10%), stir in a high-speed mixer for 30 min at a relative speed of 1000 r/min, and then sieve through vibration The granulated powder was obtained, and the frequency of the vibration filler was 50 Hz. 5.0 Kg of the granulated powder was loaded into the mold by the vibration filler, and then put into an isostatic press to obtain a silicon carbide ceramic body, and the isostatic molding pressure was 120 MPa , the holding time was 120 s, and then the green body was dried in an oven at 70 ºC for 24 h, and finally placed in a muffle furnace at a heating rate of 3 ºC/min to 500 ºC, kept for 2 h, and then heated at 2 ºC/min The temperature was raised to 1200 ºC for 1 min, and then heated to 1350 ºC at 1 ºC/min for 6 h. After cooling, a silicon carbide porous ceramic sample was obtained.

After testing, the porosity of silicon carbide porous ceramics is 42.5%, the average pore diameter is 43 μm, the flexural strength is 17.8 MPa, and the gas permeability is 1300 m ³ /m ² ·h·kPa.

Example 11

Weigh 80% of SiC with an average particle size of 300 μm, 15% carbon powder with an average particle size of 20 μm, and 2% mullite fiber (average diameter of 20 μm, average length of 60 μm) into a three-dimensional mixer , mixed mechanically for 24 h, after taking it out, add 3% by mass polyvinyl alcohol aqueous solution (i.e. PVA, with a mass concentration of 10%), stir in a high-speed mixer for 30 min at a relative speed of 1000 r/min, and then sieve through vibration The granulated powder was obtained, and the frequency of the vibration filler was 50 Hz. 5.0 Kg of the granulated powder was loaded into the mold by means of vibration filler, and then put into an isostatic press to form a silicon carbide ceramic body, and the isostatic molding pressure was 80 MPa , the holding time was 120 s, and then the green body was dried in an oven at 70 ºC for 24 h, and finally placed in a muffle furnace at a heating rate of 2 ºC/min to 500 ºC, kept for 2 h, and then heated at 4 ºC/min The temperature was raised to 1200 ºC for 1 min, and then heated to 1550 ºC at 2 ºC/min for 2 h. After cooling, a silicon carbide porous ceramic sample was obtained.

After testing, the porosity of silicon carbide porous ceramics is 44.2%, the average pore diameter is 46.7 μm, the flexural strength is 23.8 MPa, and the gas permeability is 1500 m ³ /m ² ·h·kPa.

Claims (9)

1. a kind of preparation method of high gas permeability carborundum porous ceramics material, which is characterized in that preparation process is as follows：

（1）Raw material silicon carbide, ceramic fibre and carbon dust are placed in three-dimensional mixer and are sufficiently mixed 24 h, obtains mixed powder a；

（2）Mixed powder a is put into homogenizer and adds in adhesive polyvinyl alcohol water solution, stirs 30 min, stirring Then 1000 r/min of machine rotating speed discharges and carries out shaking-sieving acquisition and coheres the uniform granulation powder b of agent content；

（3）Powder b is transferred to mold by jolt-packing, cold isostatic press is put into and prepares silicon carbide ceramics green compact, then 70 24 h are dried in the baking oven of oC, green body c must be dried；

（4）Green body c is placed in the heating sintering of high temperature furnace Program, last natural cooling obtains carborundum porous ceramics material.

2. the preparation method of high gas permeability carborundum porous ceramics material as described in claim 1, which is characterized in that system The mass percent of raw material is respectively 5 ~ 15 % of carbon dust during standby, 2 ~ 10 % of ceramic fibre, and polyvinyl alcohol water solution 3 ~ 8 %, remaining is silicon carbide.

3. the preparation method of high gas permeability carborundum porous ceramics material as described in claim 1, which is characterized in that institute The silicon carbide average grain diameter stated is at 100 ~ 300 μm, and the carbon dust average grain diameter is at 20 ~ 80 μm.

4. the preparation method of high gas permeability carborundum porous ceramics material as described in claim 1, which is characterized in that institute The ceramic fibre diameter stated is at 10 ~ 20 μm, and ceramic fibre distribution of lengths is at 40 ~ 60 μm.

5. the preparation method of high gas permeability carborundum porous ceramics material as described in claim 1, which is characterized in that institute The ceramic fibre stated is mullite fiber, one kind in alumina fibre, silicon carbide fibre, alumina silicate fibre.

6. the preparation method of high gas permeability carborundum porous ceramics material as described in claim 1, which is characterized in that institute The mass concentration for the polyvinyl alcohol water solution stated is 5 ~ 10 %.

7. the preparation method of high gas permeability carborundum porous ceramics material as described in claim 1, which is characterized in that step Suddenly（3）Described in jolt-packing frequency 50 Hz, powder b the amount of inserting for 5.0 ~ 5.5 Kg, powder density is uniform in mold It is and not eccentric.

8. the preparation method of high gas permeability carborundum porous ceramics material as described in claim 1, which is characterized in that step Suddenly（3）Middle 80 ~ 120 MPa of cold isostatic press briquetting pressure, 60 ~ 120 s of dwell time.

9. the preparation method of high gas permeability carborundum porous ceramics material as described in claim 1, which is characterized in that step Suddenly（4）Program heating sintering temperature be：Heating rate is heated to 500 oC for 2 ~ 3 oC/min, keeps the temperature 2 h, then with 2 ~ 4 oC/min are warming up to 1200 oC, are then heated to 1350 ~ 1550 oC, 2 ~ 6 h of soaking time with 1 ~ 2 oC/min, Natural cooling cools down again.