A method for preparing tungsten-copper composite material based on calcium heat reduction and infiltration process

Abstract

The application relates to a method for preparing a tungsten-copper composite material based on a calcium hot reduction and infiltration process, and relates to the technical field of tungsten-copper composite material preparation. First, tungsten oxide is blended with calcium powder and subjected to calcium hot reduction treatment, then is subjected to screening to remove the CaO impurity phase with a large particle size, tungsten powder raw material with more pores is obtained, the tungsten powder is pressed into a blank and is pre-sintered to prepare a high-porosity tungsten skeleton, and then is sintered and infiltrated with high-sphericity copper powder prepared by gas atomization to obtain tungsten-copper composite material with high compactness and high thermal conductivity, and effective combination between tungsten and copper is realized. The tungsten oxide is reduced by the calcium hot reduction process with high reduction efficiency, tungsten powder with high porosity is obtained, the combination sites of copper in the tungsten skeleton can be effectively improved, and the pore morphology can be improved, meanwhile, the high-sphericity copper powder prepared by gas atomization can significantly improve the wettability of the copper liquid in the tungsten skeleton, the driving force for the liquid-state copper to infiltrate into the tungsten skeleton is increased, and the sintering and infiltration process can improve the capillary action of the liquid-state copper in the tungsten skeleton and improve the combination strength.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of tungsten-copper composite material preparation, in particular to a method for preparing tungsten-copper composite material based on calcium heat reduction and infiltration process. BACKGROUND

[0002] With the progress of microelectronic technology, integrated circuit devices are developing towards miniaturization and high density at a high speed, the power per unit area is continuously increased, and the heat generated per unit area is more and more, so the requirements for packaging materials are increasing. Single traditional electronic packaging materials such as ceramics and metals cannot meet the actual requirements of the microelectronic industry, and the research and design of new high-thermal-conductivity, low-expansion, excellent processing performance and mechanical performance packaging composite materials have become a research hotspot in the industry. Tungsten-copper material has the characteristics of high thermal conductivity and low expansion coefficient, and the thermal conductivity and thermal expansion coefficient can be adjusted by changing the W and Cu ratio in the composite material, so as to match the ceramic materials, gallium arsenide, silicon wafer and other semiconductor elements in electronic devices well, prevent thermal fatigue damage caused by interface thermal stress, and at the same time, the heat in the semiconductor can be efficiently conducted out. These excellent properties make it one of the preferred packaging materials for high-power integrated circuits.

[0003] Because W and Cu two elements cannot form solid solution and intermetallic compound, tungsten-copper composite material is a pseudo-alloy combined by W (body-centered cubic structure) and Cu (face-centered cubic structure) with different crystal structures, which has the characteristics of high thermal conductivity of Cu and low expansion coefficient of W, and the physical properties of the material can be adjusted by adjusting the proportion of the two phases. The tungsten-copper packaging shell parts for electronic packaging require materials with high thermal conductivity and air tightness. Since the thermal conductivity is proportional to the sintering degree, the sintering density is an important indicator to measure the performance of tungsten-copper composite material. At present, copper infiltration method is mainly used to prepare tungsten-copper plate, which has the disadvantages of low raw material utilization rate and high processing cost. The infiltration method is a common method for preparing tungsten-copper alloy at present, which is to press tungsten powder into a blank, then sinter it into a porous tungsten skeleton containing pores in hydrogen, and then infiltrate copper into the porous tungsten skeleton at a temperature higher than the melting point of copper. Under the action of capillary force, liquid copper fills the pores in the porous tungsten skeleton, so as to obtain dense tungsten-copper alloy. The prepared tungsten-copper alloy has the characteristics of less impurities, higher density and better performance, and can obtain better sintering density.

[0004] Powder modification refers to a method of changing the characteristics (such as morphology, particle size, specific surface area, surface friction, etc.) or interface characteristics of the powder by physical or chemical means, which is a key technology for optimizing the performance of the powder. The surface modified powder obtained by reducing the oxide powder through a reduction process has been used in the manufacturing process of various alloys. The oxide powder has a large surface roughness, and the large contact area can effectively provide contact pores. The reduction treatment also retains the rough interface of the surface, maintains a strong interface bonding energy, and calcium as a strong reducing agent usually has a reduction capacity superior to other reducing agents such as hydrogen or carbon, which can effectively reduce many metal oxides, including tungsten trioxide (WO3) which is difficult to reduce, and helps to ensure the complete reduction reaction and improve the reduction efficiency. The reduction process is relatively simple, mainly relying on high temperature and calcium as a reducing agent, without the need for complex equipment or catalysts. This can reduce production costs and operational complexity. At the same time, the calcium oxide impurities generated during the reduction process are easy to remove, and calcium vapor is generated at 900℃, which can remove the residual calcium during the sintering process, making the reaction matrix easy to purify.

[0005] The tungsten powder reduced by calcium heat can obtain a tungsten skeleton with high porosity after pre-sintering, which has surface roughness and bonding sites that traditional ultra-fine tungsten powder cannot provide, and improves the interface bonding energy. By using the calcium heat reduction process and infiltration process, surface modified tungsten powder and tungsten copper composite materials with high bonding strength can be effectively obtained in large quantities, which is simple to operate and has high efficiency, and has certain significance for the production and preparation of tungsten copper composite materials.

[0006] Therefore, the present application uses tungsten oxide powder, calcium powder and gas atomized copper powder as raw materials, and obtains a tungsten skeleton by pre-sintering the green body after calcium heat reduction treatment, and obtains a tungsten copper composite material prepared by calcium heat reduction and infiltration process by sintering and densifying the gas atomized copper powder. SUMMARY

[0007] The present application provides a method for preparing a tungsten copper composite material based on calcium heat reduction and infiltration process, which mainly solves the problem that the tungsten obtained by reducing tungsten oxide as raw material has a large roughness, which can effectively solve the significant difference in thermal expansion coefficient and the problem that W and Cu are inherently immiscible, which leads to the difficulty of effectively bonding W and Cu. At the same time, the reduction temperature of tungsten oxide powder in traditional hydrogen reduction process is high, the reaction rate is low, the reaction is incomplete, and there are many reaction products, which leads to the defects that the tungsten powder obtained by hydrogen reduction of tungsten oxide powder has poor performance. The present application reduces the tungsten oxide by the high reduction efficiency of calcium heat reduction process, obtains a tungsten powder with high porosity, which can effectively improve the bonding sites of copper in the tungsten skeleton and improve the pore morphology, and the high sphericity copper powder prepared by gas atomization can significantly improve the wettability of the copper liquid in the tungsten skeleton, increase the driving force of the liquid copper into the tungsten skeleton, and the infiltration process can improve the capillary action of the liquid copper in the tungsten skeleton to improve the bonding strength.

[0008] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0009] A method for preparing tungsten-copper composite material based on calcium reduction and infiltration process, tungsten oxide and calcium powder are blended and subjected to calcium reduction treatment, then screening is performed to remove CaO impurities with large particle size, thus obtaining tungsten powder raw material with more pores, the tungsten powder is pressed into a blank, pre-sintering is performed to prepare tungsten skeleton, thus obtaining tungsten skeleton with certain roughness and high porosity, the tungsten skeleton is subjected to spark plasma sintering and infiltration with high-sphericity copper powder prepared by gas atomization, thus obtaining tungsten-copper composite material with high density and high thermal conductivity, and effective combination between tungsten and copper is realized.

[0010] As a preferred technical scheme of the present application, the preparation method specifically comprises the following steps:

[0011] Step one, calcium reduction of tungsten oxide powder:

[0012] The tungsten oxide powder and calcium powder are blended through a blender, then the mixed powder is put into a tube furnace, heating is performed and the powder is kept at a temperature for a period of time, then the powder is taken out and screened through a screening machine, thus obtaining tungsten powder after calcium reduction;

[0013] Step two, preparation of porous tungsten skeleton:

[0014] The tungsten powder after calcium reduction in step one is pressed into a blank with a certain diameter using a hydraulic press, then the blank is pre-sintered in a hydrogen atmosphere, and after in-furnace cooling, a porous tungsten skeleton is obtained;

[0015] Step three, spark plasma sintering and infiltration:

[0016] The porous tungsten skeleton obtained in step two is subjected to spark plasma sintering with gas-atomized copper powder, thus finally obtaining tungsten-copper composite material prepared by calcium reduction and infiltration process.

[0017] As a further preferred technical scheme of the present application, in the preparation method:

[0018] In step one, the temperature for keeping the tube furnace at a temperature is 800-1000℃, the keeping time is 3-5h, and the mass ratio of calcium powder to tungsten oxide powder is 4-8:1.

[0019] In step two, the pressure for pressing the blank is 30-50MPa, and the initial density of the blank is controlled to be 50%-60%.

[0020] In step two, the pre-sintering temperature is 1300-1500℃, and the sintering time is 3-4h.

[0021] The specific step of sintering densification in the step three is: the porous tungsten skeleton is loaded into a sintering mold, the gas atomized copper powder is added on the surface of the tungsten skeleton with rough porous structure, the carbon paper is used to insulate the mold wall and the powder, and the sintering furnace is placed in the discharge plasma sintering furnace, and the sintering furnace is vacuumized; the initial pressure is set to 10 MPa, the sintering is started, the temperature is increased from room temperature to 600 DEG C and is kept for 5 min; the temperature is increased to 1000 DEG C and is kept for 5 min, and the sintering pressure is increased to 50 MPa manually in the heating process; after the heat preservation is finished, the furnace is cooled down, the product is taken out, the carbon paper on the surface is polished by a grinding machine, and finally the tungsten-copper composite material prepared by the calcium thermal reduction and infiltration process is obtained.

[0022] Compared with the prior art, the beneficial effects of the present application are as follows:

[0023] 1. Calcium as a strong reducing agent, the reduction ability is usually better than other reducing agents, such as hydrogen or carbon, can effectively reduce many metal oxides, including difficult to reduce tungsten oxide (WO3), help to ensure the complete reduction reaction, improve the reduction efficiency, the reduction process is relatively simple, mainly relies on high temperature and calcium as a reducing agent, does not need complex equipment or catalyst, can reduce production cost and operation complexity.

[0024] 2. In the calcium thermal reduction process, calcium will produce calcium vapor volatilization at 900 DEG C, which helps the reaction to be complete, and avoids the pollution of impurity calcium to tungsten powder, the produced calcium oxide impurity has a large particle size and is easy to remove, the calcium oxide (CaO) can be reused or used as an industrial byproduct for other purposes, improving the economic value.

[0025] 3. The reduction of tungsten oxide as a tungsten powder raw material, the tungsten powder prepared has a large roughness on the surface, which helps to produce porous in the tungsten skeleton, improves the pore morphology, and can provide effective binding sites for the copper liquid in the infiltration process.

[0026] 4. The copper powder used is prepared by gas atomization, has high sphericity, good flowability, and stable and reliable performance, compared with electrolytic copper powder and flaky copper powder, it can significantly improve the wettability of the copper liquid on the tungsten skeleton, has smaller binding resistance with the tungsten skeleton, increases the driving force of the liquid copper into the tungsten skeleton, can fully combine with the rough pores on the inner surface of the tungsten skeleton, and reaches high density.

[0027] 5、The melting and infiltration process can melt the spherical copper powder with good fluidity at high temperature, and fill and wet the structure by the capillary action of liquid copper through the gap between the porous W frame particles. The technology can produce tungsten-copper materials with relatively high density, large bonding force and good comprehensive performance. Compared with the traditional liquid phase sintering method, the composition change caused by liquid phase overflow or evaporation in the liquid phase sintering process is avoided, which leads to uneven sample composition and unstable performance. By controlling the process parameters such as pressure and flow rate, the process stability of the infiltration process can be stably controlled. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is the process flow chart of the calcium thermal reduction and infiltration process for preparing tungsten-copper composite material of the present application.

[0029] Figure 2 is the SEM image of the porous tungsten skeleton of the calcium thermal reduction and infiltration process for preparing tungsten-copper composite material of the present application.

[0030] Figure 3 is the surface EDS image of the calcium thermal reduction and infiltration process for preparing tungsten-copper composite material of the present application.

[0031] Figure 4 is the fracture SEM image of the calcium thermal reduction and infiltration process for preparing tungsten-copper composite material of the present application.

[0032] Figure 5 is the XRD image of the calcium thermal reduction and infiltration process for preparing tungsten-copper composite material of the present application.

[0033] Figure 6 is the mechanical property comparison chart of example 1 and comparative example 1 of the calcium thermal reduction and infiltration process for preparing tungsten-copper composite material of the present application. DETAILED DESCRIPTION

[0034] Please refer to Figure 1 As shown in the figure, the present application proposes a method for preparing tungsten-copper composite material based on calcium thermal reduction and infiltration process, using tungsten oxide powder as raw material, blending tungsten oxide with calcium powder, carrying out calcium thermal reduction treatment, and then screening to remove impurity phase CaO with large particle size, obtaining tungsten powder raw material with more pores, pressing the tungsten powder into a blank, and pre-sintering to prepare tungsten skeleton with a certain roughness and high porosity, and then carrying out discharge plasma sintering and infiltration with high-spherical copper powder prepared by gas atomization, to obtain tungsten-copper composite material with high density and high thermal conductivity, realizing effective combination between tungsten and copper.

[0035] The present application will be further described in detail below in combination with examples and drawings.

[0036] Example 1

[0037] The method steps for preparing tungsten-copper composite material in this example are as follows:

[0038] Step one, calcium reduction of tungsten oxide powder:

[0039] The tungsten oxide powder (WO3) and calcium powder were blended by a mixer, the mass ratio of calcium powder and tungsten oxide powder (WO3) was 6:1, then the mixed powder was put into a tube furnace, heated to 900℃ and kept for 5h, the powder was taken out and sieved by a sieve machine to obtain the calcium reduced tungsten powder.

[0040] Step two, preparation of porous tungsten skeleton:

[0041] The calcium reduced tungsten powder of step one was pressed into a certain diameter embryo by a hydraulic press at a pressure of 50MPa, the initial density was kept at 60%, then the green body was sintered in a hydrogen atmosphere at 1400℃ for 4h. After in-furnace cooling, the porous tungsten skeleton was obtained.

[0042] Step three, spark plasma sintering infiltration:

[0043] The porous tungsten skeleton obtained in step two was subjected to spark plasma sintering with gas atomized copper powder, and finally the tungsten-copper composite material prepared by calcium reduction and infiltration process was obtained. The specific steps of sintering densification were as follows:

[0044] The porous tungsten skeleton was loaded into a sintering mold, gas atomized copper powder was added on the surface of the tungsten skeleton with rough porous structure, carbon paper was used to isolate the mold wall and the powder, and the sintering furnace was placed in a spark plasma sintering furnace and vacuum treated; the initial pressure was set to 10MPa, the sintering started, the temperature was raised from room temperature to 600℃ and kept for 5min; then the temperature was raised to 1000℃ and kept for 5min, the pressure was manually increased to 50MPa during the temperature rising process; after the end of the holding period, the furnace was cooled down, the product was taken out, the carbon paper on the surface was polished by a grinding machine, and finally the tungsten-copper composite material prepared by calcium reduction and infiltration process was obtained.

[0045] Example 2

[0046] The method steps for preparing the tungsten-copper composite material in this example were as follows:

[0047] Step one, calcium reduction of tungsten oxide powder:

[0048] The tungsten oxide powder (WO 2.9 ) and calcium powder were blended by a mixer, the mass ratio of calcium powder and tungsten oxide powder (WO 2.9 ) was 6:1, then the mixed powder was put into a tube furnace, heated to 900℃ and kept for 5h, the powder was taken out and sieved by a sieve machine to obtain the calcium reduced tungsten powder.

[0049] Step two, preparation of porous tungsten skeleton:

[0050] The tungsten powder after calcium reduction in step one is pressed into a certain diameter embryo by a hydraulic machine at a pressure of 50 MPa, and the initial density is kept at 60%, and then the compact is sintered at 1400°C in a hydrogen atmosphere for 4h. After in-furnace cooling, a porous tungsten skeleton is obtained.

[0051] Step three, spark plasma sintering infiltration:

[0052] The porous tungsten skeleton obtained in step two is subjected to spark plasma sintering with gas-atomized copper powder, and finally a tungsten-copper composite material prepared by calcium reduction and infiltration process is obtained. The specific steps of sintering densification are as follows:

[0053] The porous tungsten skeleton is loaded into a sintering mold, gas-atomized copper powder is added on the surface of the tungsten skeleton with rough porous structure, carbon paper is used to isolate the mold wall and the powder, and the sintering furnace is placed in a spark plasma sintering furnace and vacuum treated; the initial pressure is set to 10 MPa, the sintering starts, the temperature is raised from room temperature to 600°C and kept for 5 min; then the temperature is raised to 1000°C and kept for 5 min, and the pressure is manually increased to 50 MPa during the temperature rising process; after the end of the holding period, the furnace is cooled down and the product is taken out, the surface carbon paper is polished with a grinding machine, and finally a tungsten-copper composite material prepared by calcium reduction and infiltration process is obtained.

[0054] Example 3

[0055] The method steps for preparing the tungsten-copper composite material in this example are as follows:

[0056] Step one, calcium reduction of tungsten oxide powder:

[0057] The tungsten oxide powder (WO2) is blended with calcium powder in a blender, and the mass ratio of calcium powder to tungsten oxide powder (WO2) is 6:1, then the mixed powder is placed in a tube furnace, heated to 900°C and kept for 5h, taken out and sieved in a sieve machine to obtain the tungsten powder after calcium reduction.

[0058] Step two, preparation of porous tungsten skeleton:

[0059] The tungsten powder after calcium reduction in step one is pressed into a certain diameter embryo by a hydraulic machine at a pressure of 50 MPa, and the initial density is kept at 60%, and then the compact is sintered at 1400°C in a hydrogen atmosphere for 4h. After in-furnace cooling, a porous tungsten skeleton is obtained.

[0060] Step three, spark plasma sintering infiltration:

[0061] The porous tungsten skeleton obtained in step two is subjected to spark plasma sintering with gas-atomized copper powder, and finally a tungsten-copper composite material prepared by calcium reduction and infiltration process is obtained. The specific steps of sintering densification are as follows:

[0062] The porous tungsten framework is loaded into a sintering mold, the gas-atomized copper powder is added on the surface of the tungsten framework with a rough porous structure, the carbon paper is used to insulate the mold wall and the powder, and the sintering furnace is placed in a discharge plasma sintering furnace and vacuumized; the initial pressure is set to 10 MPa, the sintering is started, the temperature is increased from room temperature to 600 DEG C and is kept for 5 min; then the temperature is increased to 1000 DEG C and is kept for 5 min, and the sintering pressure is manually increased to 50 MPa during the temperature increasing process; after the keeping process is finished, the furnace is cooled down, the product is taken out, the carbon paper on the surface is polished by a grinding machine, and finally the tungsten-copper composite material prepared by the calcium thermal reduction and infiltration process is obtained.

[0063] Comparative Example 1

[0064] The method steps for preparing the tungsten-copper composite material in the present comparative example are as follows:

[0065] Step one, hydrogen reduction of tungsten oxide powder:

[0066] The tungsten oxide powder (WO3) is placed in a tube furnace, the temperature is increased to 900 DEG C under hydrogen atmosphere and is kept for 5 h, the powder is taken out and is sieved in a sieving machine, and the tungsten powder after hydrogen reduction is obtained.

[0067] Step two, preparation of tungsten framework:

[0068] The tungsten powder after hydrogen reduction in step one is pressed into an embryo with a certain diameter by using a hydraulic machine at a pressure of 50 MPa, the initial density is kept at 60%, and then the pressed blank is sintered at 1400 DEG C under hydrogen atmosphere for 4 h. After in-furnace cooling, the porous tungsten framework is obtained.

[0069] Step three, discharge plasma sintering infiltration:

[0070] The porous tungsten framework obtained in step two is subjected to discharge plasma sintering with the gas-atomized copper powder, and finally the tungsten-copper composite material prepared by the hydrogen reduction and infiltration process is obtained. The specific steps of sintering densification are as follows:

[0071] The tungsten framework is loaded into a sintering mold, the gas-atomized copper powder is added on the surface of the tungsten framework with a rough porous structure, the carbon paper is used to insulate the mold wall and the powder, and the sintering furnace is placed in a discharge plasma sintering furnace and vacuumized; the initial pressure is set to 10 MPa, the sintering is started, the temperature is increased from room temperature to 600 DEG C and is kept for 5 min; then the temperature is increased to 1000 DEG C and is kept for 5 min, and the sintering pressure is manually increased to 50 MPa during the temperature increasing process; after the keeping process is finished, the furnace is cooled down, the product is taken out, the carbon paper on the surface is polished by a grinding machine, and finally the tungsten-copper composite material prepared by the calcium thermal reduction and infiltration process is obtained.

[0072] Table 1 Related properties of tungsten-copper composite materials prepared in the examples and comparative examples of the present application

[0073] Density (%) Hardness (Hv) Conductivity (%) Example 1 85.83％ 188 48.7％ Example 2 88.69％ 175 48.3％ Example 3 90.12％ 191 51.6％ Comparative Example 1 83.92％ 164 38.4％

[0074] As shown in Table 1, the calcium thermal reduction exhibits a better degree of reduction, resulting in higher conductivity, reduced impurities, and improved performance and density. A comparison of the examples prepared with different oxides reveals that the calcium thermally reduced WO2 (prepared in Example 3) has a higher degree of reduction, leading to the best performance.

[0075] Depend on Figure 2 It can be seen that the tungsten particles in the porous tungsten framework of calcium thermal reduction are nearly spherical and have a large roughness, which can provide effective bonding sites for subsequent copper infiltration.

[0076] Depend on Figure 3 It can be seen that tungsten and copper are tightly bonded and effectively dispersed on the surface of the calcium-thermally reduced tungsten-copper material, resulting in a good bonding effect.

[0077] Depend on Figure 4 It can be seen that the copper at the fracture surface is wrapped with tungsten particles on a rough surface, and has good dispersion, a certain number of burr holes, and a certain degree of plasticity.

[0078] Depend on Figure 5 It can be seen that the tungsten-copper material has obvious diffraction peaks of tungsten and copper, and no obvious impurity peaks, indicating that the material is relatively pure and has a high degree of calcium thermal reduction.

[0079] Depend on Figure 6 It can be seen that the mechanical properties of Example 1, which was subjected to calcium thermal reduction, are better than those of Comparative Example 1, which was subjected to hydrogen reduction. This indicates that the raw materials produced by calcium thermal reduction have better binding properties, which in turn lead to better mechanical properties.

[0080] The above description is merely an example and illustration of the concept of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the concept of the invention or exceed the scope defined in the claims, they should all fall within the protection scope of the present invention.

Claims

1. A method for preparing tungsten-copper composite materials based on calcium thermal reduction and melt infiltration processes, characterized in that, Specifically, the steps include the following: Step 1: Calcothermic reduction of tungsten oxide powder: Tungsten oxide powder and calcium powder are mixed in a mixer, and then the mixed powder is put into a tube furnace, heated and kept at a certain temperature for a period of time. After the powder is taken out, it is put into a sieve to remove the large-particle-size impurity phase CaO, and calcified tungsten powder is obtained. Step 2: Preparation of the porous tungsten framework: The tungsten powder after calcium thermal reduction in step one is pressed into a blank of a certain diameter using a hydraulic press. Then, the blank is pre-sintered in a hydrogen atmosphere and cooled in a furnace to obtain a porous tungsten skeleton with a rough surface. Step 3: Spark Plasma Sintering and Melting A porous tungsten skeleton was placed into a sintering mold, and gas-atomized copper powder was added to the surface of the tungsten skeleton. The mold wall and the powder were isolated with carbon paper, and the mold was placed in a discharge plasma sintering furnace. The sintering furnace was evacuated. The initial pressure was set to 10 MPa, and sintering was started. The temperature was raised from room temperature to 600 ℃ and held for 5 min. The temperature was then raised to 1000 ℃ and held for 5 min. During the heating process, the pressure was manually increased to 50 MPa. After the holding period, the furnace was cooled, the product was removed, and the carbon paper on the surface was polished with a grinding wheel to finally obtain the tungsten-copper composite material.

2. The method as described in claim 1, characterized in that, In step one, the heat preservation temperature inside the tubular furnace is 800~1000 ℃, the heat preservation time is 3~5 h, and the mass ratio of calcium powder to tungsten oxide powder is 4~8:

1.

3. The method as described in claim 1, characterized in that, In step two, the pressure of pressing the billet is 30~50 MPa, and the initial density of the billet is controlled to be 50%~60%.

4. The method as described in claim 1, characterized in that, In step two, the pre-sintering temperature is 1300~1500 ℃ and the sintering time is 3~4 h.

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