Highly thermally conductive silicon nitride ceramic insulating board and its manufacturing method

Abstract

The present invention provides a high thermal conductivity silicon nitride ceramic insulating plate and its manufacturing method, which belongs to the technical field of ceramic substrate. The present invention uses magnesium oxide as the first sintering aid, which reacts with the raw silicon nitride powder to generate a liquid phase during sintering, thereby reducing the sintering temperature and saving production costs, and at the same time, can increase the density and bending strength of the silicon nitride ceramic insulating plate. The present invention uses hafnium oxide as the second sintering aid, which utilizes its relatively strong bonding ability with oxygen to bond with the impurity oxygen in the raw silicon nitride powder during sintering, remove the lattice oxygen content, and increase the thermal conductivity of the silicon nitride ceramic insulating plate. The third sintering aid of the present invention promotes the liquid phase reaction during sintering, reacts with the silicon nitride powder to form a solid solution, and reduces the amorphous phase at the grain boundary, ensuring high thermal conductivity and increasing the bending strength of the product. In addition, the present invention solves the difficult problem of the prior art that it is difficult to manufacture ultra-thin large-sized substrates.

Description

[Technical Field]

[0001] This application claims priority from a Chinese patent application filed with the China Patent Office on December 25, 2020, bearing application number 202011559771.6 and entitled "High thermal conductive silicon nitride ceramic insulating board and manufacturing method thereof," the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to the ceramic substrate technical field, and more particularly to a highly thermally conductive silicon nitride ceramic insulating board and a method for producing the same. [Background technology]

[0003] As chip feature sizes shrink and integration levels increase, chips have entered the era of ultra-large-scale integration. High integration levels mean higher device load power and increased heat generation, so highly integrated circuits pose significant heat dissipation problems. The development of substrate materials with high heat resistance and thermal conductivity is of great significance to the further development of integrated circuits. Currently, the most commonly used ceramic substrate materials are alumina (Al2O3) and aluminum nitride (AlN), but both have several drawbacks. The thermal conductivity of alumina substrates is too low (~20 W·m -1 ·k -1 ), the thermal conductivity of the aluminum nitride substrate is relatively high (~120 W m -1 ·k -1 ), but their bending strength is very poor (~300 MPa), and neither of these two materials can meet the heat dissipation and load requirements of highly integrated circuits.

[0004] Silicon nitride ceramics have attracted widespread attention due to their excellent properties, including high strength, high toughness, high-temperature performance, and high theoretical thermal conductivity, making them ideal substrate materials. However, silicon nitride ceramics have problems such as being susceptible to decomposition at high temperatures (around 1870°C), high production costs, and an inability to achieve both bending strength and thermal conductivity. Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a highly thermally conductive silicon nitride ceramic insulating board that has both high bending strength and high thermal conductivity and has a relatively low sintering temperature, and a method for producing the same.

[0006] In order to achieve the above object of the invention, the present invention provides the following technical means.

[0007] The present invention provides a high thermal conductive silicon nitride ceramic insulating board comprising manufacturing raw materials consisting of 86 to 96% silicon nitride powder, 1 to 5% of a first sintering aid, 2.5 to 8% of a second sintering aid, and 0.5 to 1% of a third sintering aid, wherein the first sintering aid is magnesium oxide, the second sintering aid is hafnium oxide, and the third sintering aid is one or more of boron oxide, diiron trioxide, calcium oxide, and titanium dioxide.

[0008] Preferably, the silicon nitride powder has a D 50<1 μm, an α-phase content of more than 90%, and a specific surface area<15 m 2 / g, oxygen content <1 wt.%, Fe <0.0005 wt.%, Al <0.001 wt.%, Ca <0.0001 wt.%.

[0009] Preferably, the D50 of the first sintering aid, the second sintering aid and the third sintering aid are independently Te1 It is less than μm.

[0010] The present invention provides a method for producing a mixed slurry comprising the steps of: mixing a silicon nitride powder, a first sintering aid, a second sintering aid, a third sintering aid, a solvent, and an adhesive; granulating the mixed slurry to obtain a granulated powder; a step of molding the granulated powder by a press to obtain a ceramic biscuit; cold isostatic pressing of the ceramic biscuit to obtain a dense biscuit; The dense biscuit is degreased and sintered in order, and after cooling, a high thermal conductive silicon nitride ceramic insulating board is obtained; The present invention provides a method for producing a highly thermally conductive silicon nitride ceramic insulating board according to the above technical solution, wherein the sintering temperature is 1800 to 1950°C, and the cooling rate is 30°C / min or less.

[0011] Preferably, the mixing includes first stirring and milling the silicon nitride powder, the first sintering aid, the second sintering aid, the third sintering aid and the solvent to obtain an intermediate slurry; The intermediate slurry is added with an adhesive and subjected to a second stirring mill to obtain a mixed slurry.

[0012] Preferably, the solvent includes ethanol, and the amount of ethanol used is based on the solid content of the intermediate slurry being 40 to 70%; the adhesive includes polyvinyl butyral, and the amount of adhesive used is 3 to 5 wt.% of the intermediate slurry.

[0013] Preferably, the pressure of the press molding is 10 to 15 T, and the pressure holding time is 15 to 20 seconds.

[0014] Preferably, the cold isostatic pressing is performed at a pressure of 300 to 350 MPa, and the pressure holding time is 20 minutes.

[0015] Preferably, the degreasing temperature is 400 to 600°C, and the temperature holding time is 6 to 12 hours.

[0016] Preferably, the temperature is maintained for 12 to 24 hours.

[0017] The present invention provides a high thermal conductive silicon nitride ceramic insulating board comprising manufacturing raw materials consisting of 86 to 96% silicon nitride powder, 1 to 5% of a first sintering aid, 2.5 to 8% of a second sintering aid, and 0.5 to 1% of a third sintering aid, wherein the first sintering aid is magnesium oxide, the second sintering aid is hafnium oxide, and the third sintering aid is one or more of boron oxide, diiron trioxide, calcium oxide, and titanium dioxide.

[0018] The present invention uses magnesium oxide as the first sintering aid, which reacts with the raw silicon nitride powder during the sintering process to form a liquid phase, thereby lowering the sintering temperature and saving production costs, while also increasing the density and bending strength of the silicon nitride ceramic insulating board. The present invention uses hafnium oxide as the second sintering aid, which utilizes its relatively strong bonding ability with oxygen to bond with impurity oxygen in the raw silicon nitride powder during sintering, eliminating the lattice oxygen content and improving the thermal conductivity of the silicon nitride ceramic insulating board. The third sintering aid of the present invention promotes the liquid phase reaction during sintering, reacts with the silicon nitride powder to form a solid solution, and reduces the amorphous phase at the grain boundaries, ensuring high thermal conductivity and increasing the bending strength of the product.

[0019] The present invention also enables the production of ultra-thin, large-scale ceramic substrates, thereby reducing production costs compared to conventional techniques. Conventional large-scale ceramic substrates are often produced by grinding a thick substrate, wasting a large amount of raw material, or by laser cutting thin plates, resulting in high production costs. Direct pressing of thin substrates is not possible. This is because bending cracks occur during the sintering process when the large substrate is too thin. Conventional techniques require increasing the bending strength of the ceramic substrate to prevent bending cracks. However, increasing bending strength sacrifices thermal conductivity, making it difficult to obtain ultra-thin, large-scale substrates with both high thermal conductivity and bending strength. The present invention achieves both high thermal conductivity and bending strength by adopting the above-described formulation. The improved bending strength prevents cracks and bending during sintering, thereby solving the difficulty of directly pressing ultra-thin, large-scale substrates. The high thermal conductivity silicon nitride ceramic insulating board produced by the present invention measures 190 mm x 138 mm x 0.25 mm.

[0020] The present invention also provides a method for manufacturing the high thermal conductivity silicon nitride ceramic insulating board according to the above-mentioned scheme, in which the sintering temperature and cooling rate are controlled to favor sufficient growth of silicon nitride ceramic crystal grains, reduce grain boundaries, and further increase thermal conductivity.

[0021] As a result of the examples, the high thermal conductive silicon nitride ceramic insulating board manufactured by the present invention has a bending strength of ≥ 600 MPa and a thermal conductivity of ≥ 100 W m -1 ·k -1 , and showed a dielectric constant of 7 to 9. [Means for solving the problem]

[0022] The invention will now be further explained with reference to examples and drawings.

[0023] The present invention includes manufacturing raw materials including 86-96% silicon nitride powder, 1-5% first sintering aid, 2.5-8% second sintering aid, and 0.5-1% third sintering aid; the first sintering aid is magnesium oxide; the second sintering aid is hafnium oxide; The third sintering aid is one or more of boron oxide, iron trioxide, calcium oxide, and titanium dioxide to provide a highly thermally conductive silicon nitride ceramic insulating board.

[0024] In the present invention, all raw materials used are commercially available products well known in the art unless otherwise specified.

[0025] The high thermal conductivity silicon nitride ceramic insulating board provided by the present invention contains, by weight percent, 86 to 96%, preferably 87 to 95%, of silicon nitride powder. In the present invention, the silicon nitride powder preferably has a D50 of <1 μm, an α-phase content of more than 90%, and a specific surface area of ​​15 m 2 / g, preferably the oxygen content is <1 wt.%, preferably the Fe content is <0.0005 wt.%, preferably the Al content is <0.001 wt.%, and preferably the Ca content is <0.0001 wt.%.

[0026] The high thermal conductivity silicon nitride ceramic insulating board provided by the present invention contains, by weight, 1-5%, more preferably 2-4%, of a first sintering aid. In the present invention, the first sintering aid is magnesium oxide, and it is preferable that the D50 of the magnesium oxide is less than 1 μm. In the present invention, the magnesium oxide reacts with the raw silicon nitride powder during sintering to form a liquid phase, which reduces the sintering temperature, saves production costs, and increases the density and bending strength of the silicon nitride ceramic insulating board.

[0027] The high thermal conductivity silicon nitride ceramic insulating board provided by the present invention contains, by weight, 2.5-8%, preferably 4-6%, of a second sintering aid. In the present invention, the second sintering aid is hafnium oxide, and the D50 of the second sintering aid is preferably less than 1 μm. The present invention utilizes the relatively strong oxygen bonding ability of hafnium oxide, which bonds with impurity oxygen in the raw silicon nitride powder during sintering, eliminating lattice oxygen and increasing the thermal conductivity of the silicon nitride ceramic insulating board. Using hafnium oxide in an amount exceeding the above range can adversely affect the sintering of silicon nitride, resulting in a decrease in the thermal conductivity of the ceramic insulating board.

[0028] The high thermal conductivity silicon nitride ceramic insulating board provided by the present invention contains, by weight, 0.5-1%, preferably 0.6-0.8%, of a third sintering aid. In the present invention, the third sintering aid is one or more of boron oxide, iron trioxide, calcium oxide, and titanium dioxide, more preferably one of these. When the third sintering aid is a multiple-type compound among the above substances, the present invention does not require a specific blend ratio for each additive; any blend ratio is acceptable. The third sintering aid of the present invention promotes a liquid-phase reaction during sintering, reacts with silicon nitride powder to form a solid solution, and reduces the amorphous phase at grain boundaries, ensuring high thermal conductivity while increasing the bending strength of the product. Using the third sintering aid in an amount exceeding the above range adversely affects the progress of sintering and reduces the thermal conductivity of the ceramic insulating board.

[0029] The present invention provides a method for producing a mixed slurry comprising the steps of: mixing a silicon nitride powder, a first sintering aid, a second sintering aid, a third sintering aid, a solvent, and an adhesive; granulating the mixed slurry to obtain a granulated powder; a step of molding the granulated powder by a press to obtain a ceramic biscuit; cold isostatic pressing of the ceramic biscuit to obtain a dense biscuit; The dense biscuit is degreased and sintered in order, and after cooling, a high thermal conductive silicon nitride ceramic insulating board is obtained; The present invention provides a method for producing a highly thermally conductive silicon nitride ceramic insulating board according to the above technical solution, wherein the sintering temperature is 1800 to 1950°C, and the cooling rate is 30°C / min or less.

[0030] In the present invention, a silicon nitride powder, a first sintering aid, a second sintering aid, a third sintering aid, a solvent, and an adhesive are mixed to obtain a mixed slurry.

[0031] In the present invention, the mixing is preferably performed by first stirring and grinding the silicon nitride powder, the first sintering aid, the second sintering aid, the third sintering aid and the solvent to obtain an intermediate slurry, and then adding an adhesive to the intermediate slurry and performing a second stirring and milling to obtain a mixed slurry.

[0032] In the present invention, the solvent preferably contains ethanol. The amount of ethanol used is preferably based on the solid content of the intermediate slurry being 40 to 70%, more preferably 45 to 65%, and even more preferably 50 to 60%.

[0033] In the present invention, the adhesive preferably contains polyvinyl butyral, and the amount of the adhesive used is preferably 3 to 5 wt.%, more preferably 4 wt.%, of the intermediate slurry. The present invention is advantageous in that the adhesive is used to aggregate the raw materials for subsequent granulation.

[0034] In the present invention, the first stirring polishing and the second stirring polishing preferably use silicon nitride balls as the polishing medium, preferably with a mass ratio of the polishing medium to the material to be polished of 1:1.

[0035] In the present invention, the time for the first stirring mill is preferably 24 to 36 hours, and the time for the second stirring mill is preferably 24 to 48 hours. The rotation speeds of the first stirring mill and the second stirring mill are independently preferably 25 to 35 rpm, more preferably 30 rpm.

[0036] After obtaining the mixed slurry, the present invention granulates the mixed slurry to obtain granulated powder.

[0037] In the present invention, the granulation method is preferably spray granulation. In the present invention, the conditions for the spray granulation are preferably a feed pump pressure of 0.6 to 0.8 MPa, an inlet temperature of 150 to 180°C, an outlet temperature of 50 to 70°C, and a cyclone pressure difference of 0.8 to 1 KPa. In a more preferred embodiment, the conditions for the spray granulation are a feed pump pressure of 0.65 to 0.75 MPa, an inlet temperature of 160 to 170°C, an outlet temperature of 55 to 65°C, and a cyclone pressure difference of 0.85 to 0.95 KPa.

[0038] In the present invention, it is preferable to further pass the spray-dried mixture through a 100-mesh sieve and take the undersieve as granulated powder. The present invention is advantageous in that it utilizes a spray granulation method to obtain granulated powder with good flowability suitable for press molding. In the present invention, the performance of the granulated powder is such that the apparent density is 0.8 to 1 g / m 3 and the flow time is 55.68 to 58.07 s.

[0039] In the present invention, after obtaining granulated powder, the granulated powder is press-molded to obtain a ceramic biscuit. In the present invention, the granulated powder is preferably passed through a dry powder press die, followed by indenter press molding. In the present invention, the pressure of the press molding is preferably 10 to 15 T, more preferably 11 to 14 T, and the pressure holding time is preferably 15 to 20 seconds, more preferably 16 to 18 seconds. The present invention does not impose any particular restrictions on the dimensions of the dry powder press die, which are determined based on the dimensions of the target ceramic insulating board. In the present invention, when the dimensions of the target ceramic insulating board are 190 mm × 138 mm × 0.25 mm, the dimensions of the dry powder press die are preferably 272 mm × 198 mm, and the height of the dry powder press die is preferably greater than 0.5 mm.

[0040] The present invention employs press molding to obtain large-sized ceramic biscuits, and by compounding silicon nitride ceramic insulating plates, it is possible to obtain large-sized, highly thermally conductive silicon nitride ceramic insulating plates.

[0041] In the present invention, after the ceramic biscuit is obtained, the ceramic biscuit is cold isostatically pressed to obtain a dense biscuit.

[0042] In the present invention, it is preferable to place the ceramic biscuit in a vacuum bag, place a steel plate as a substrate in the vacuum bag, then evacuate the bag, place the bag in the cavity of a cold isostatic press, and perform cold isostatic pressing. In the present invention, the vacuum level during the evacuation is preferably -0.1 MPa. The dimensions of the steel plate are preferably the same as those of the ceramic biscuit.

[0043] By adopting steel plate as the substrate, the present invention can prevent deformation and fracture during cold isostatic pressing of the ceramic biscuit.

[0044] In the present invention, the cold isostatic pressing pressure is preferably 300 to 350 MPa, more preferably 310 to 340 MPa, and the pressure holding time is preferably 20 minutes. In the present invention, by controlling the cold isostatic pressing conditions within the above range, the density of the ceramic biscuit can be increased, which is useful for the subsequent sintering and molding.

[0045] After obtaining the dense biscuit, the present invention sequentially degreases and sinters the dense biscuit, and after cooling, obtains a high thermal conductivity silicon nitride ceramic insulating board.

[0046] In the present invention, it is preferable to stack the dense biscuits and place them in a crucible, place a graphite plate on the top surface of the stacked dense biscuits, and then place them in a multi-functional atmospheric sintering furnace for debinding and sintering.

[0047] In the present invention, the weight of the graphite plate is preferably 1 to 2 times, more preferably 1.2 to 1.8 times, the weight of the dense biscuit. By placing the graphite plate in the present invention, it is possible to prevent cracks and deformation of the dense biscuit during the debinding and sintering processes.

[0048] In the present invention, the debinding temperature is preferably 400 to 600°C, more preferably 450 to 550°C, and the temperature holding time is preferably 6 to 12 hours, more preferably 8 to 10 hours. The rate of temperature rise to the debinding temperature of the present invention preferably does not exceed 10°C / min. After the rate of temperature rise exceeds 10°C / min, the dense biscuit becomes prone to cracking.

[0049] In the present invention, the degreasing is preferably carried out in an air atmosphere. In the degreasing of the present invention, organic matter in the dense biscuit is discharged.

[0050] After the debinding is completed, the present invention directly raises the temperature from the debinding temperature to the sintering temperature and then sinters the material.

[0051] In the present invention, it is preferable that the rate of temperature increase up to the sintering temperature does not exceed 10° C. / min. If the rate of temperature increase exceeds 10° C. / min, the blank becomes prone to cracking.

[0052] In the present invention, the sintering temperature is 1800 to 1950°C, preferably 1850 to 1900°C, and the temperature holding time is preferably 6 to 12 hours, more preferably 6 to 10 hours. In the present invention, sintering is preferably performed in an air atmosphere. In the present invention, in the early stages of the sintering process, the first and third sintering aids react with SixOy on the surface of the silicon nitride powder, resulting in a reaction product with a low melting point and a liquid phase. As the reaction progresses, the liquid phase flows, filling the voids between the particles, further promoting sintering and densifying the material and improving the bending strength of the product. The transition from α-phase silicon nitride to β-phase silicon nitride that occurs during this process further increases thermal conductivity.

[0053] In the present invention, after the sintering is completed, the sintered product is cooled at a cooling rate of 30°C / min or less, preferably 10 to 30°C / min. In the present invention, the cooling rate is controlled within the above range, which is advantageous for sufficient growth of silicon nitride ceramic crystal grains, reduces grain boundaries, and further increases thermal conductivity.

[0054] Preferably, the present invention further comprises post-processing the resulting ceramic blank after cooling to obtain a high thermal conductivity silicon nitride ceramic insulating plate. The present invention is not particularly limited to the method of post-processing; any method well known in the art can be used as long as it ensures that the high thermal conductivity silicon nitride ceramic insulating plate achieves the desired precision in terms of thickness and surface roughness, specifically polishing. In the present invention, the roughness of the high thermal conductivity silicon nitride ceramic insulating plate is preferably less than 5 μm.

[0055] The highly thermally conductive silicon nitride ceramic insulating board and the method for manufacturing the same provided by the present invention will be described in detail below with reference to examples, but these examples should not be construed as limiting the scope of the present invention.

[0056] The silicon nitride powder used in the following examples has a D50 of <1 μm, an α-phase content of more than 90%, and a specific surface area of ​​<15 m 2 / g, oxygen content <1 wt.%, Fe <0.0005 wt.%, Al <0.001 wt.%, Ca <0.0001 wt.%; and the D50 of the magnesium oxide, hafnium oxide, boron oxide, iron trioxide, calcium oxide, and titanium dioxide used is all <1 μm. Examples 1 to 16 and Comparative Examples 1 to 5

[0057] Silicon nitride powder and sintering aid were added to a stirring mill according to the raw material blending ratios and types shown in Table 1. Alcohol was used as the solvent medium, and silicon nitride balls were used as the polishing medium (a 1:1 mass ratio of polishing medium to polished material). Polishing was continued for 30 hours to obtain an intermediate slurry (solid content 60%). 4 wt.% polyvinyl butyral (PVB) was then added to the intermediate slurry, which was then polished for 48 hours to obtain a mixed slurry. The mixed slurry was then sieved through a 100-mesh sieve using a spray granulation process (feed pump pressure 0.7 MPa, inlet temperature 170°C, outlet temperature 60°C, cyclone pressure difference 0.9 KPa) to obtain a granulated powder suitable for press molding (apparent density 0.8-1 g / m). 3The granulated powder was passed through a dry powder press die and pressed at a pressure of 10 T for a pressure holding time of 20 seconds to obtain ceramic biscuits. The ceramic biscuits and a steel plate were placed in a vacuum bag and evacuated, followed by cold isostatic pressing at an isostatic pressure of 300 MPa for a pressure holding time of 20 minutes to obtain dense biscuits. The dense biscuits were stacked on top of a crucible, a graphite plate was placed on the top surface, and the container was then placed in a multi-purpose atmospheric sintering furnace. The temperature was raised to 600°C at a rate of 10°C / min and held at that temperature for 12 hours to debinder the biscuits. The temperature was then raised to the sintering temperature and held at that temperature for 24 hours. After sintering was complete, the biscuits were cooled at a cooling rate of 30°C / min or less to obtain ceramic biscuits. The ceramic blank was then polished to obtain a high-thermal-conductivity silicon nitride ceramic insulating board with dimensions of 190 mm × 138 mm × 0.25 mm and a roughness of less than 5 μm. The specific sintering temperature, cooling rate, and performance index are shown in Table 1.

[0058] [Table 1]

[0059] From the results in Table 1, it can be seen that the high thermal conductivity silicon nitride ceramic insulating board manufactured by the present invention has a bending strength of ≥ 600 MPa and a thermal conductivity of ≥ 100 W m -1 ·k -1 It was found that in Comparative Example 1, the cooling rate was too high compared to the examples of the present invention, so the thermal conductivity of the product was not high, in Comparative Example 2, the sintering temperature was too low, so the thermal conductivity of the product was low, in Comparative Example 3, the hafnium oxide content of the second sintering aid was too high, which was unfavorable for sintering the product, and the thermal conductivity of the product was low, in Comparative Example 4, the calcium oxide content of the third sintering aid was too high, so the cooling rate was high and the product had high three-point bending strength but low thermal conductivity, and in Comparative Example 5, the calcium oxide content of the third sintering aid was too high, so the thermal conductivity of the product was low.

[0060] The above description of the examples is merely intended to aid in the understanding of the method of the present invention and its core idea. It should be noted that those skilled in the art can make some improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention. It is clear to those skilled in the art that various modifications of these embodiments can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown in this specification, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for producing a highly thermally conductive silicon nitride ceramic insulating board, comprising: Mixing 86 to 96 wt% of silicon nitride powder, 1 to 5 wt% of a first sintering aid made of magnesium oxide, 2.5 to 8 wt% of a second sintering aid made of hafnium oxide, 0.5 to 1 wt% of a third sintering aid made of boron oxide, a solvent, and an adhesive to obtain a mixed slurry; granulating the mixed slurry to obtain a granulated powder; a step of press-molding the granulated powder to obtain a ceramic base material; cold isostatically pressing the ceramic body to obtain a dense body; the dense body is degreased, sintered, and cooled to obtain a high thermal conductivity silicon nitride ceramic insulating board; The sintering temperature is 1800 to 1950°C, and the cooling rate is 30°C / min or less.

2. The mixing includes first stirring and milling the silicon nitride powder, the first sintering aid, the second sintering aid, the third sintering aid, and a solvent to obtain an intermediate slurry; 2. The method according to claim 1, further comprising the step of adding an adhesive to the intermediate slurry and subjecting the mixture to a second stirring milling process to obtain a mixed slurry.

3. 3. The manufacturing method according to claim 2, wherein the solvent includes ethanol, and the amount of ethanol used is determined so that the solid content of the intermediate slurry is 40 to 70 wt %. The manufacturing method according to claim 2, wherein the adhesive includes polyvinyl butyral, and the amount of the adhesive used is 3 to 5 wt % of the intermediate slurry.

4. 2. The method according to claim 1, wherein the pressure of the press molding is 10 to 15 T, and the pressure holding time is 15 to 20 seconds.

5. 2. The method according to claim 1, wherein the cold isostatic pressing is performed at a pressure of 300 to 350 MPa and for a pressure holding time of 20 minutes.

6. 2. The method according to claim 1, wherein the degreasing temperature is 400 to 600° C. and the temperature holding time is 6 to 12 hours.

7. 2. The method according to claim 1, wherein the sintering temperature is maintained for 12 to 24 hours.

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