KR20170073082A - Human bone mimetic porous titanium for biomedical application and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a method of producing porous titanium for biomedical medical use, comprising the steps of: preparing a first mixed powder by mixing titanium powder and space holder powder, and applying pressure to the first mixed powder to produce an inner green compact, A step of forming a powder layer of titanium powder on the outer side of the green compact and then fabricating a green compact having an internal low density and an external high density structure by spark plasma sintering (SPS) Followed by sintering after removing the powder.

Description

TECHNICAL FIELD [0001] The present invention relates to a biomedical porous titanium, which mimics the shape of a bone, and a method for manufacturing the same. [0002]

The present invention relates to a method of manufacturing a porous titanium biomedical structure having a cylindrical structure with an outer and an inner densities different from each other by mimicking the shape of a bone.

Titanium is superior to polymers or ceramics used in biomedical materials, and has excellent nasal strength, corrosion resistance and biocompatibility. Therefore, it can be used as a substitute for hard tissue such as dental implants, knee joints, artificial hip joints and elbow joints It is used as a metal material.

Currently, titanium, which is used as a biomedical material, is mostly produced in bulk materials limited to melting, casting and forging techniques. Titanium produced in this way has a modulus of elasticity of about 110 GPa, which is much higher than the modulus of elasticity of human bone, 10-30 GPa. Therefore, when titanium having such a high modulus of elasticity is implanted into a human body, a stress shielding phenomenon due to a difference in elastic modulus from the surrounding human bone causes a problem of degenerating the surrounding bone.

To solve this problem, various methods have been proposed to reduce the difference in elastic modulus between the biomedical material and the human bone. As an example of such a method, there is proposed a method of reducing the difference in elastic modulus by inserting a porous body into the inside of a titanium to form a specific structure by using a conventional material to reduce the elastic modulus while using titanium as a main component.

These porous titanium materials have lower strength compared to bulk materials, and since the pores formed in the inside help to adhere to the cells, biocompatibility is improved, thereby replacing the knee cartilage and spine.

As a technology related to such porous titanium, a method of producing a porous titanium using powder metallurgy is disclosed in Korean Patent No. 0565558. [ The patent document discloses a porous titanium implant having the same Young's modulus as that of a human bone prepared by pre-sintering and pre-sintering of a titanium powder having a particle size of 40 to 250 μm.

In addition, U.S. Patent No. 7883661 discloses a method for providing a porous metal implant as a technique for powder metallurgy using a space holder. In addition, in order to supplement the strength while maintaining osseointegration ability, the porous titanium was coated with powder metallurgy on the outside of the bulk titanium in the Materials Science Forum, CE Wen, Y. Yamada et al, Vol 539-543, pp 720-725 (2007) A technique for manufacturing a slanting material is proposed. Also, Korean Patent Registration No. 1031121 discloses a method for producing a bone in which a dense body and a porous body structure are combined in a ceramic system.

The present invention is to provide a method for manufacturing medical porous titanium having a porous low density inside and a high density external double layer structure by using powder metallurgy, thereby having a structure similar to a bone of a human body.

In the case of having density differences between the inside and the outside as in the present invention, fine pores are formed at the interface between the inside and the outside, and such fine pores cause cracks at the interface during the sintering process, Lt; RTI ID = 0.0 > imperfect < / RTI > Accordingly, it is an object of the present invention to provide a high-quality implant material of a double-layer structure having a density difference between a porous low-density interior and a high-density exterior without such interface defects.

The present invention provides a method for producing a biocompatible porous titanium material having a high external density and a low internal density, comprising: preparing a first mixed powder by mixing a titanium powder and a space holder powder; (1) forming a powder layer of titanium powder on the outer periphery of the inner green compact by applying a pressure to the mixed powder, and forming a powder layer of an inner low density and an outer high density by spark plasma sintering (SPS) And a sintering step of removing the space holder powder contained in the green compact and sintering the green compact.

The mixing ratio of the titanium powder and the space holder powder may be adjusted to a volume ratio of 20 to 80 sodium chloride per 100 titanium particles.

A pressure of 120 to 150 MPa may be applied to the inner green compact.

At this time, the powder layer of the titanium powder formed on the outer periphery of the inner green compact may be a mixture of the titanium powder alone or the mixture of the titanium powder and the space holder powder, and the mixing ratio of the space holder powder may be lower than that of the first mixed powder.

The green compact can be formed by disposing the inner green compact at the center of the mold and filling the outside with titanium powder.

The inner green compact and the green compact may be formed using a mold having a predetermined shape, and the mold may be formed of a carbon material mold.

The space holder powder may be selected from the group consisting of salt, urea, and starch, and the space holder powder may have a size of 360 to 540 탆.

The removal of the space holder powder contained in the green compact may be carried out by applying at least one of a method of melting the space holder powder by melting to a temperature not lower than the melting point of the space holder powder and a method of dissolving it in a solvent, .

The method according to claim 1, wherein the spark plasma sintering can be performed by uniaxial compression at a temperature of 500 to 550 ° C under a high vacuum of 10 ^-1 to 10 ^-2 Pa, and the uniaxial compression is performed at a pressure of 40 to 50 MPa .

The sintering may be performed at a temperature of 1000 to 1100 ° C for 10 to 12 hours under a high vacuum of 0.01 to 0.001 Pa or less.

The porous titanium according to the present invention has a porous low-density interior and a high-density external double-layer structure, and provides a high-quality biomedical porous titanium having a structure similar to a bone of a human body and free from bonding defects at an interface having a density difference do.

According to one embodiment of the present invention, in realizing a low-density inner and a high-density outer-layer two-layer structure, it is easy to control density and mechanical characteristics, and a relatively high strength can be maintained while lowering the elastic modulus.

FIG. 1 is a conceptual view illustrating a method of manufacturing titanium, which is made of porous material only, according to an embodiment of the present invention.
FIGS. 2 to 5 are optical photographs of biomimetic porous titanium prepared by using porous Ti having pores according to the mixing ratio of Ti: NaCl as an internal green compact and using 100% Ti on the outside, wherein Ti: NaCl 2 is 80:20, FIG. 3 is 60:40, FIG. 4 is 40:60, and FIG. 5 is about 20:80, wherein (a) is an optical photograph of a cross section , and (b) is an optical photograph of a longitudinal section.
6 and 7 are graphs showing relative density versus pure Ti versus strain-stress curve according to the mixing ratio of Ti: NaCl in a single pore and biomimetic titanium, FIG. 6 is a graph of a single pneumatic powder, Graph for an amorphous green compact.
Fig. 8 is a photograph of two sections of the green compact obtained in Examples 1 and 2 for observing the presence or absence of defects such as cracks on the inner and outer boundary surfaces. Fig. 8 (a) (B) is a green compact section of Example 2. Fig.
Fig. 9 is a photograph of a longitudinal section of the green compact obtained in Comparative Examples 1 and 2, wherein (a) is Comparative Example 5, and (b) and (c)
10 is a photograph showing a longitudinal section of Comparative Example 6, showing the presence of cracks generated at the interface due to the presence of minute bubbles.
11 is an optical photograph of a cross section of a titanium green compact using SPS and a vacuum heat treatment, which is made of Ti alone.
12 to 14 are optical photographs showing the whole cross-section and partial enlargement of the porous titanium using SPS having a single pore according to the mixing ratio of Ti: NaCl and the vacuum heat treatment. FIG. 12 shows the mixing ratio of Ti: Fig. 13 is 60:40, Fig. 14 is about 40:60, (a) is an optical photograph of a cross section, and (b) is an optical photograph of a longitudinal section.

Hereinafter, a method of manufacturing the biomimetic porous titanium of the present invention will be described in order to facilitate those skilled in the art to which the present invention belongs.

The porous titanium to be provided in the present invention is intended to be used as a material capable of replacing bone of a human body. The actual bone consists of the outer part of the non-porous cortical bone and the inner part of the porous cancellous bone. Accordingly, the porous titanium provided in the present invention is intended to have a porous structure similar to the bone structure of the human body, except that the outer surface is non-porous and the inner surface is porous.

The porous titanium for biomedical treatment provided by the present invention can be manufactured by manufacturing an internal green compact, forming an external green compact outside the internal green compact, and then sintering it.

First, the biomedical porous titanium of the present invention forms an internal green compact. The inner green compact corresponds to the porous cancellous bone of the human bone, and is made of a titanium material, and has pores therein. Accordingly, the inner green compact, together with the titanium powder, includes a space holder powder for forming porous pores.

The larger the size of the powder, the smaller the density of the powder during the sintering becomes, and the part which is not sintered partially is formed, and unintentional fine pores are formed at the part, which may cause a problem such as a decrease in the mechanical strength. In order to prevent such a problem, it is necessary to use a powder having a sufficiently small size, and it is preferable to use a size of 60 mu m or less. The smaller the particle size, the smaller is the pore distribution of the micropores. Therefore, the lower limit of the particle size is not particularly limited. However, the smaller the particle size, the higher the manufacturing cost. When the pore distribution is reduced to increase the mechanical strength, and in consideration of such an economical point, a powder of 20 mu m or more can be used.

It is preferable that the titanium powder has a minimum magnetic or contamination degree. Therefore, it is necessary to use a powder having a purity of 99.9% as much as possible.

The pores provide a space in which bone cells can grow, and can be provided by a space holder method. The titanium powder and the space holder powder such as sodium chloride powder may be mixed to form a green compact of a predetermined shape, and then the space holder powder may be melted and removed to form pores.

The space holder is made of a material mixed with a titanium powder. After the green compact is manufactured, the space holder is formed by dissolving with a solvent or melting by a heat treatment. In the dissolution or heat treatment, It means a material that can form pores in the space occupied by space holders by recording only powder.

At this time, the pores are formed according to the particle size of the space holder powder, and it is preferable that the pores have a size of 360 to 540 탆. Having a size in the above range can be provided as a suitable space for cell growth and is also suitable for compatibility with surrounding human tissues. Accordingly, it is preferable to use a powder having a size of 360 to 540 탆 so as to provide the above-described pores.

The space holder powder may be suitably used in the present invention as long as it has properties not harmful to the human body even if it remains in the biomedical material. Therefore, as a specific example of the space holder powder, sodium chloride powder may be most suitably used, and other materials which react with water or an alcohol to dissolve may be used. However, it is preferable that the content of the impurity is minimized in the space holder powder. For example, it is preferable to use a material having a purity of 99% or more.

In addition, the space holder powder can be made into a pore through a heat treatment process using a material having a low melting point such as urea, starch and the like. At this time, it is preferable to use a material having a melting point of 200 ° C or less, in which oxidation of titanium is hardly occurred, as a melting point of the materials dissolving through heat treatment. It can be melted safely through heat treatment process over melting point.

As described above, the inner green compact can be formed with an inner porous structure suitable as a human bone by mixing the space holder powder together with the titanium powder at a suitable volume ratio according to the desired density to be implemented. The titanium powder and the space holder powder are not particularly limited as long as they can form a structure similar to the bone cortex structure which is the internal structure of the human bone. However, the higher the mixing ratio of the space holder, the more pores are formed in the biomedical material, To reduce the density.

Therefore, the density can be diversified by controlling the mixing ratio of the titanium powder and the space holder. However, it is preferable to mix the space holder powder at a volume ratio of 20 to 80 per 100 titanium powder volume. When the space holder powder is contained in an amount of less than 20 parts by volume, there is no problem of mechanical properties, but there is a limit in forming the porous interior of the artificial bone. When the volume of the space holder powder is more than 80 parts by volume, the content of the titanium powder is too small, And can not form pores of a desired size.

The mixing of the titanium powder and the space holder powder is not particularly limited, but may be carried out using a roller, a blend, or a closed container. The mixing time is not particularly limited as far as the two powders can be uniformly mixed, and the mixture can be mixed for 6 hours or more, for example, 6 hours to 24 hours.

Subsequently, a green compact of a predetermined shape is prepared by filling a mold with a mixture of the titanium powder and the space holder powder and applying a pressure or the like. The shape of the mold used for the production of the green compact is not particularly limited, and a mold having a shape suitable for the use may be used. For example, as shown in FIG. 1, a mold having a cylindrical void space may be used, and cylindrical green compacts of various sizes may be manufactured according to the shape of the mold.

The green compact can be filled with the mixed powder, and then pressed using a punch to produce a green compact. At this time, the higher the compressive strength, the better the sinterability. However, if the compressive strength is too large, it may cause defects in the mold or the green compact. Therefore, it is preferable to apply the pressure of about 120 to 150 MPa to prepare the green compact.

Thereby, an internal compact capable of providing a porous structure can be obtained, and an outer high-density structure serving as a bone cortex is formed around the obtained internal compact.

As shown in Fig. 1 (b), the outer high-density structure is formed by inserting the inner green compact of the porous structure into the central portion of the cylindrical mold having an inner diameter larger than that of the inner green compact, After filling, pressure is applied to produce a green compact having different internal and external densities. At this time, the outer portion has a higher density than the inner portion, and has a green compact structure of a double-layer structure having a low density and an outer high density inside.

The outer high density layer is not particularly limited as long as it has a density higher than that of the low density layer inside the porous body. For example, by packing the pure titanium powder, a dense structure having a density higher than that of the inner porous structure is formed In addition, if it is possible to increase the density as compared with the internal porosity, it is possible to fill other mixtures in addition to pure titanium. For example, a powder mixture of titanium powder and sodium chloride powder may be used as the outer powder layer.

The mold is not particularly limited as long as it is made of a material which can be used at a high temperature, and a mold made of a carbon material can be used.

After forming the low-density internal and high-density external double layer green compacts of the porous structure, the double-layer green compact is subjected to Spark Plasma Sintering (SPS). Micropores are generated due to such a difference in density at the interface between the outer and inner densities of the two-layer structure, thereby causing a problem of degrading the mechanical properties of the green compact.

However, when performing sintering through SPS before sintering as in the present invention, since the molecules are electrically coupled, the outer walls are actually denser and the generation of micropores at the interface having the density difference can be minimized.

The spark plasma sintering can be performed by, for example, uniaxial compression at a temperature of about 500 to 550 ° C under a high vacuum of 10 ^-1 to 10 ^-2 Pa. When SPS is performed at a temperature exceeding 550 ° C, NaCl contained as a space holder may be partially melted, which may cause a localized large pore portion to adversely affect the mechanical properties of the green compact.

Since the SPS is performed at a high temperature rather than a normal temperature, the strength of the mold is also weakened. Therefore, it is preferable to perform the pressure at a low pressure in consideration of the electrical coupling using the high temperature environment and the SPS, for example, 40 to 50 MPa.

In the SPS treatment, it is preferable to use a carbon material having a high strength at a high temperature. The mold may vary depending on the structure and size of the biomedical porous titanium to be obtained, and is not particularly limited. For example, when the inner green compact is inserted and fixed at the lower end of one punch, It is preferable that a groove corresponding to the diameter of the inner green compact is formed.

Subsequently, a step of removing the space holder from the green compact of the SPS-treated double layer is performed. The removal of the space holder is not particularly limited, and the green compact may be immersed in a solvent to be dissolved and removed, or may be melted by heating to a temperature not lower than the melting point of the space holder powder.

For example, if the space holder is sodium chloride, it can be removed by treating the SPS and then dipping the green compact in water to dissolve the sodium chloride. At this time, the dipping of the green compact is not particularly limited as long as it is sufficient time to form pores by dissolving the green compact. For example, the green compact may be dipped in water for 24 hours to remove sodium chloride.

And then sintering the green compact in which the pores are formed by removing the space holder. The green compact may be sintered after the space holder is melted. At this time, sintering may be performed through a high temperature heat treatment, or sintering may be performed in parallel with high temperature heat treatment and pressurization.

For example, the green compact may be subjected to a heat treatment at a pressure of about 10 ^-3 torr, for example, a pressure of 0.001 to 0.01 Pa and a temperature of 1000 캜, for example, 1000 to 1100 캜 for 10 to 12 hours to sinter the titanium powder can do. If the temperature is low, the sintering may be partially failed. If the temperature is high, the overall density of the porous titanium having a different outer and inner densities And the desired density may not be obtained. Further, if the heat treatment time is made longer, the economic loss is large, and if the heat treatment is performed for less than that, it is difficult to obtain the desired sintering property.

Example

Hereinafter, the present invention will be described in more detail with reference to examples. The following examples are illustrative of the present invention and are not intended to limit the scope of the present invention.

Example  1 to 4

On the basis of the volume ratio, the titanium powder and the salt powder were mixed in a proportion of 80:20 (Example 1), 60:40 (Example 2), 40:60 (Example 3), 20:80 (Example 4) , And the roller was homogenized for 6 hours or more.

As a mold as shown in Fig. 1, a cylindrical mold having a diameter of 8 mm and a height of 15 mm was filled with the mixed powder, and a pressure of 120 MPa was applied to produce an internal green compact.

The prepared green compact was placed and fixed at the center of a mold having a diameter of 20 mm as shown in Fig. 1 (b), and the periphery thereof was filled with titanium powder.

Subsequently, a porous double-layered titanium green compact having a double-layer structure was prepared by applying a pressure of 40 MPa for 10 to 15 minutes under a high vacuum of 10 ^-1 to 10 ^-2 Pa at 500 ° C. using SPS equipment.

Try removing by dissolving the sodium chloride contained in the interior of the green compact to more than 24 hours immersion of the titanium green form of the double-layer structure in water at room temperature, in a vacuum of less than 10 ^-2 Pa 1000? Sintering was performed for 12 hours to prepare a porous titanium green compact having an outer high density inner porous low density cylindrical structure.

The cross-sectional and longitudinal profiles of the porous titanium green compact of the double-layer structure produced in each of the above Examples were photographed clearly, and the results are shown in Figs. 2 to 5. Table 1 shows the relative density , The compression yield strength and the elastic modulus measured by the uniaxial compression test were measured and shown in FIG. 6, and the results as shown in Table 1 and FIG. 6 were obtained.

Out
Volume ratio inside
Volume ratio Relative density Compressive yield strength
(MPa) Compressive modulus of elasticity
(GPa) Example 1 Ti 100%
NaCl 0% Ti 80% -NaCl 20% 0.84 412 76 Example 2 Ti 60% -NaCl 40% 0.77 367 66 Example 3 Ti 40% -NaCl 60% 0.71 353 60 Example 4 Ti 20% -NaCl 80% 0.60 303 55

As can be seen from the above Table 1, it can be seen that the elastic modulus decreases as the internal porosity increases. Therefore, it can be seen that the elastic modulus can be controlled by controlling the inner porosity as well as the thickness of the outer non-porous layer.

On the other hand, as can be seen from FIG. 7, the strength of the biomimetic titanium of the present invention is superior to that of titanium having a single porosity at the same relative density.

Further, the presence or absence of defects such as cracks on the inner and outer boundary surfaces was observed, and the interface between Examples 1 and 2 was photographed and shown in FIG. (a) shows the first embodiment, and (b) shows the second embodiment.

As can be seen from FIG. 8, the low-density inside and the high-density outside do not show a clear boundary, and it is not observed that the fine pores are concentrated at the interface, and therefore, the possibility of cracking at the interface is remarkably low.

Comparative Example  One

A porous double-layered green compact having an external high-density internal porous low-density cylindrical structure was produced in the same manner as in Example 1, except that the space holder was not included and an SPS was not performed.

The longitudinal section of the obtained green compact was photographed, and the results are shown in Fig. 9 (a). As can be seen from FIG. 9 (a), it can be seen that fine pores are densely packed at the outer and inner boundaries, even though the space holder is not included. This is because the pressure applied at the time of manufacturing the inner green compact, The difference in density caused by the difference in pressure applied at the time of manufacture indicates that the pores are densely formed at the inner and outer boundaries to form a boundary. It can be seen that cracks are likely to occur at the interface due to the formation of such pores.

Comparative Example  2

A porous double-layered green compact having an external high-density internal porous low-density cylindrical structure was produced in the same manner as in Example 2, except that SPS was not performed.

The longitudinal section of the obtained green compact was observed. The longitudinal section is shown in Figs. 9 (b) and 9 (c). As can be seen from FIGS. 9 (b) and 9 (c), it can be seen that fine pores are densely packed in the outer and inner boundaries, which indicates that there is a high possibility of cracks at the interface.

On the other hand, the specimen obtained in Comparative Example 2 was observed with an electron microscope, and the photograph thereof is shown in Fig. As can be seen from FIG. 10, it can be seen that a crack is generated at the interface. These cracks originate from the weak bonding force due to the presence of micropores concentrated at the interface due to thermal expansion and contraction during sintering.

Reference example  1 to 4

Titanium powders and salt powders were mixed in a volume ratio of 100: 0 (Reference Example 1), 80:20 (Reference Example 2), 60:40 (Reference Example 3) and 40:60 (Reference Example 4) , And the roller was homogenized for 6 hours or more. At this time, the density of titanium was 4.516 g / cm 3 and the density of salt was 1.32 g / cm 3.

Subsequently, the resultant was placed in a mold having a diameter of 20 mm, and a pressure of 40 MPa was applied for 10 to 15 minutes under a high vacuum of 10 ^-1 to 10 ^-2 Pa at 500 ° C. using SPS equipment to produce a porous double-layered green compact of double layer structure Respectively. The obtained green compact had a diameter of 20 mm and a height of 17 mm.

The green compact thus prepared was immersed in water for 24 hours to dissolve and remove the salt, thereby preparing a porous titanium green compact.

The porous titanium green compact was sintered at a temperature of 1000? Under a high vacuum of 10 ^-2 Pa for 12 hours.

As a result, a porous titanium green compact having a cylindrical structure having the same outer and inner densities was obtained. The cross section and the longitudinal section of the obtained porous titanium green compact were optically photographed, and the results are shown in Figs.

The density of the porous titanium green compact was then calculated through specific gravity in ethanol using the principle of Archimedes, and the results are shown in Table 2. At this time, the relative density for 4.516 g / cm < 3 > was calculated.

Furthermore, the compressive yield strength and elastic modulus of each porous titanium green compact were measured, and the results are also shown in Table 2.

Kinds Volume ratio Relative density Compressive yield strength
(MPa) Compressive modulus of elasticity
(GPa) Reference Example 1 Ti 100% -NaCl 0% 0.95 553 105 Reference Example 2 Ti 80% -NaCl 20% 0.67 252 37 Reference Example 3 Ti 60% -NaCl 40% 0.56 174 29 Reference Example 4 Ti 40% -NaCl 60% 0.46 111 16

As can be seen from the above Table 2 and Figs. 11 to 14, it was found that a porous titanium green compact having different densities and different porosities from each other was obtained according to the blending ratio of titanium and sodium chloride.

Further, as can be seen from Table 2, although the green compact is manufactured using only pure titanium powder, density close to that of the bulk material can be realized, which can be realized by applying the SPS method.

That is, in the production of the green compact using the titanium powder, micropores can be removed by performing the sintering by the SPS method, thereby realizing a density similar to that of the titanium bulk material. In the present invention including the SPS treatment process It can be seen that fine pores concentrated at the interface between the layers due to the density difference can be removed.

Claims (14)

Preparing a first mixed powder by mixing a titanium powder and a space holder powder, and applying pressure to the first mixed powder to produce an inner green compact;
Forming a powder layer of titanium powder on the outer periphery of the inner green compact, and then spark plasma sintering (SPS) to produce a green compact having an inner low density and an outer high density; And
Removing the space holder powder contained in the green compact and then sintering;
Wherein the porous titanium material is a porous titanium material.
The method of manufacturing a porous titanium biomedical device according to claim 1, wherein the inner green compact uses a mold having a predetermined shape.
The method of manufacturing a porous titanium biomedical device according to claim 2, wherein the mold is a carbon material mold.
The method of claim 1, wherein the pressure for producing the inner pressure-reducing powder is 120 to 150 MPa.
The method according to claim 1, wherein the powder layer of titanium powder formed on the outer periphery of the inner green compact is a second mixed powder including a space holder powder together with a titanium powder, Wherein the mixing ratio of the spacer holder powder is low.
The method of claim 1, wherein the green compact is formed by disposing the inner green compact at the center of the mold and filling the outer surface with titanium powder.
The method of claim 1, wherein the mixing ratio of the titanium powder to the space holder powder is from 20 to 80 parts by weight of sodium chloride per 100 parts by weight of titanium.
8. The method of any one of claims 1 to 7, wherein the space holder powder is selected from the group consisting of salt, urea, and starch.
The method of any one of claims 1 to 7, wherein the space holder powder has a size of 360 to 540 탆.
[7] The method according to any one of claims 1 to 7, wherein the removal of the space holder powder contained in the green compact is carried out by at least one of a method of melting by heating to a temperature not lower than a melting point of the space holder powder, And then removing the porous titanium.
11. The method of claim 10, wherein the solvent is water or ethanol.
The method according to claim 1, wherein the porous titanium is uniaxially compressed under a high vacuum of 10 ^-1 to 10 ^-2 Pa and a temperature of 500 to 550 ° C.
13. The method of claim 12, wherein the uniaxial compression is performed at a pressure of 40 to 50 MPa.
The method according to claim 1, wherein the sintering is performed at a temperature of 1000 to 1100 캜 for 10 to 12 hours under a high vacuum of 0.01 to 0.001 Pa.

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