SILICON NITRIDE SINTERED BODY WITH HIGH THERMAL CONDUCTIVITY AND METHOD FOR MANUFACTURING SAME

Abstract

Embodiments relate to a method for manufacturing a silicon nitride sintered body with high thermal conductivity, which includes the steps of: a) obtaining a slurry by mixing a silicon nitride powder and a non-oxide based sintering aid; b) obtaining a mixed powder by drying the slurry; c) forming a compact by pressurizing the mixed powder; and d) sintering the compact.

Claims

1. A method for manufacturing a silicon nitride sintered body with high thermal conductivity, the method comprising the steps of: a) obtaining a slurry by mixing a silicon nitride powder and a non-oxide-based sintering aid; b) obtaining a mixed powder by drying the slurry; c) forming a compact by pressurizing the mixed powder; and d) sintering the compact.

2. The method of claim 1, wherein the silicon nitride powder is thermally pretreated at 1,400 to 1,600 C. under a nitrogen or argon atmosphere for 1 to 15 hours.

3. The method of claim 2, wherein the thermally pretreated silicon nitride powder has an oxygen content of less than 1 wt % and an average particle diameter of 0.5 to 0.8 m.

4. The method of claim 2, wherein the thermally pretreated silicon nitride powder has a carbon content of less than 1 wt %.

5. The method of claim 1, wherein the non-oxide-based sintering aid comprises a rare earth fluoride.

6. The method of claim 5, wherein the rare earth fluoride is one or more selected from the group consisting of YF.sub.3, YbF.sub.3, LaF.sub.3, NdF.sub.3, GdF.sub.3 and ErF.sub.3.

7. The method of claim 5, wherein the non-oxide-based sintering aid further comprises a magnesium silicide-based compound.

8. The method of claim 7, wherein the magnesium silicide-based compound is one or more selected from the group consisting of MgSiN.sub.2 and Mg.sub.2Si.

9. The method of claim 1, wherein the non-oxide-based sintering aid is included at a content of 0.1 to 10 wt % based on a total weight of the silicon nitride powder and the non-oxide-based sintering aid.

10. The method of claim 1, wherein the step of forming the compact is performed by cold isostatic pressing under a pressure condition of 100 to 400 MPa.

11. The method of claim 1, wherein the step of sintering the compact is performed by applying a pressure of 10 to 50 MPa under a nitrogen atmosphere at 1,700 to 1,800 C. for 1 to 20 hours.

12. The method of claim 1, further comprising the step of: e) thermally post-treating the sintered body at 1,800 to 1,850 C. for 3 to 10 hours.

13. A silicon nitride sintered body with high thermal conductivity manufactured by the method according to claim 1 and has a bending strength of 660 to 870 MPa and a high thermal conductivity of 70 W/mK or more.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0020] FIG. 1 is a flowchart of a process of manufacturing a silicon nitride sintered body with high thermal conductivity according to an embodiment.

[0021] FIG. 2 shows reference images for describing Experimental Example 1 of an embodiment.

[0022] FIG. 3 shows reference images for describing Experimental Example 3 of an embodiment.

[0023] FIG. 4 shows reference images for describing Experimental Example 5 of an embodiment.

[0024] FIG. 5 shows reference images for describing Experimental Example 6 of an embodiment.

DETAILED DESCRIPTION

[0025] Hereinafter, exemplary embodiments will be described with reference to the accompanying drawings. Embodiments disclosed are intended to illustrate and not limit the technical spirit of the invention. Since the illustrated drawings magnify only essential principal elements and omit auxiliary elements for the purpose of clarity of the invention, the embodiments should not be interpreted only based on the drawings.

[0026] In order to obtain a silicon nitride sintered body with high thermal conductivity, a non-oxide-based sintering aid capable of minimizing the concentration (content) of oxygen in a sintered body is used to manufacture the sintered body.

[0027] In addition, in order to obtain a silicon nitride sintered body with high thermal conductivity, a silicon nitride ingredient powder in which defects present therein are effectively controlled is used to manufacture the sintered body.

[0028] FIG. 1 is a flowchart of a manufacturing process according to an embodiment. According to at least one embodiment, a method for manufacturing a silicon nitride sintered body includes the steps of: obtaining a slurry by mixing a silicon nitride powder and a non-oxide-based sintering aid; obtaining a mixed powder by drying the slurry; forming a compact by pressurizing the mixed powder; and sintering the compact.

[0029] As the silicon nitride ingredient powder, a commercially available powder may be used as is or a commercially available powder may be thermally pretreated at 1,400 to 1,600 C. under a nitrogen or argon atmosphere for 1 to 15 hours for use. According to at least one embodiment, the silicon nitride powder may be thermally pretreated to reduce defects present in the silicon nitride powder, and a sintered body may be manufactured using the silicon nitride powder to obtain a sintered body with high thermal conductivity. An oxygen content of a silicon nitride powder which is not thermally pretreated is 1.27 wt %, and may be controlled to less than 1 wt % by thermally treating the silicon nitride power. When an oxygen content is controlled to less than 1 wt % as described above, a silicon vacancy in the silicon nitride powder is minimized, and thus thermal conductivity of a sintered body may be increased. In this case, a carbon content of the thermally pretreated silicon nitride powder may be less than 1 wt %. Also, such a silicon nitride powder may have an average particle diameter of 0.5 to 0.8 m.

[0030] According to at least one embodiment, the non-oxide-based sintering aid is used to minimize the concentration of oxygen in a sintered body. As the non-oxide-based sintering aid, a rare earth fluoride, a magnesium silicide-based compound, or a mixture thereof may be used. That is, in accordance with at least one embodiment, a rare earth fluoride or a magnesium silicide-based compound is used instead of a rare earth oxide or a magnesium oxide conventionally used in the manufacture of a silicon nitride sintered body.

[0031] According to at least one embodiment, the rare earth fluoride may be one or more selected from the group consisting of YF.sub.3, YbF.sub.3, LaF.sub.3, NdF.sub.3, GdF.sub.3 and ErF.sub.3, and the magnesium silicide-based compound may be one or more selected from the group consisting of MgSiN.sub.2 and Mg.sub.2Si. The rare earth fluoride and the magnesium silicide-based compound have high volatility, and thus may minimize the formation of a secondary phase when being sintered. The presence of the secondary phase may cause phonon scattering which results in the degradation of thermal conductivity. According to at least one embodiment, the formation of a secondary phase may be minimized to reduce phonon scattering which results in the degradation of thermal conductivity, and thus a sintered body with high thermal conductivity may be obtained.

[0032] When the silicon nitride powder and the non-oxide-based sintering aid are mixed, the silicon nitride powder may be included at 80 to 99.8 wt %, preferably 90 to 99 wt %, and the non-oxide-based sintering aid may be included at 0.1 to 10 wt %, preferably 0.5 to 5 wt % based on a total weight of the silicon nitride powder and the non-oxide-based sintering aid.

[0033] When the non-oxide-based sintering aid is included at less than 0.1 wt %, a sintering process may not be smoothly performed. On the other hand, when the non-oxide-based sintering aid is included at greater than 10 wt %, a liquid amount is increased, and thus mechanical strength, thermal conductivity and the like of a sintered body may be degraded.

[0034] According to at least one embodiment, the silicon nitride powder and the non-oxide-based sintering aid are mixed with and dispersed in an organic solvent (e.g., isopropanol) to prepare a slurry. The mixing and dispersing process may be performed at a speed of 125 to 350 rpm at room temperature for 2 to 48 hours.

[0035] While being stirred with a magnetic bar to prevent the slurry from precipitating, the slurry is primarily dried at 70 to 150 C. for 0.5 to 2 hours and secondarily dried in a vacuum oven at 80 to 150 C. for 2 to 24 hours to prepare a mixed powder.

[0036] Then, the mixed powder is pressurized to form a compact. The compact may be formed under a pressure condition of 100 to 400 MPa. Also, a pressurizing method such as cold isostatic pressing may be used to form a compact having a constant density.

[0037] Afterward, the compacts are stacked in layers in a graphite sleeve, the graphite sleeve containing the compacts is placed in a sintering reactor (e.g., graphite furnace), and then the compacts are sintered to manufacture sintered bodies. Through such a sintering process, a pore is minimized and thus a sintered body with a dense structure may be obtained.

[0038] According to at least one embodiment, the sintering process of the compact may be performed through hot-press sintering, pressureless sintering, gas pressure sintering, or the like. The hot-press sintering may be performed under a pressure condition of 10 to 50 MPa and a nitrogen atmosphere at 1,700 to 1,800 C. for 1 to 20 hours. A compact may be prevented from being oxidized in the sintering process by maintaining a nitrogen atmosphere during the hot-press sintering.

[0039] According to at least one embodiment, the sintered body thus manufactured may be further subjected to a thermally post-treating process at 1,800 to 1,850 C. for 3 to 10 hours. The thermally post-treating process is a purification process for removing defects (e.g., Yb-precipitates, dislocations, or the like) present in a grain or at a grain boundary in the sintered body. Through the thermally post-treating process, the growth of a grain in the sintered body may be suppressed or a formed secondary phase may be removed. Accordingly, phonon scattering which results in the degradation of thermal conductivity is decreased, and thus thermal conductivity of the sintered body may be improved.

Example 1

[0040] Ingredient powders consisting of compositions listed in Table 1 below were added to 100 ml of an isopropanol solvent, and then the mixture was subjected to ball milling using a silicon nitride ball at room temperature at 300 rpm for 24 hours to prepare a slurry.

[0041] While being stirred with a magnetic bar to prevent the slurry thus prepared from precipitating, the slurry was primarily dried at 120 C. for 1.5 hours and secondarily dried in a vacuum oven at 60 C. for 1 hour to prepare a mixed powder.

[0042] 2.0 g of the mixed powder thus prepared was taken, added to a mold, and then subjected to cold isostatic pressing under a pressure condition of 200 MPa to form a discoid compact having a diameter of 20 mm and a thickness of 2.5 mm.

[0043] The compacts thus formed were stacked in layers in a graphite sleeve, and graphite spacers were introduced between the compacts to prevent the compacts from being brought into contact with one another. Afterward, the graphite sleeve containing the compacts was placed in a graphite furnace, and the compacts were sintered under a nitrogen atmosphere at 1,800 C. for 2 hours and uniaxially pressurized at a pressure of 25 MPa to manufacture a sintered body.

Example 2

[0044] A sintered body was manufactured in the same manner as Example 1 except that ingredient powders consisting of compositions listed in Table 1 below were used.

Comparative Example 1

[0045] A sintered body was manufactured in the same manner as Example 1 except that ingredient powders consisting of compositions listed in Table 1 below were used.

Comparative Example 2

[0046] A sintered body was manufactured in the same manner as Example 1 except that ingredient powders consisting of compositions listed in Table 1 below were used.

TABLE-US-00001 TABLE 1 Comparative Comparative Ingredient powder Example 1 Example 2 Example 1 Example 2 Si.sub.3N.sub.4 powder 9.2 g 9.2 g 9.2 g 9.2 g (oxygen content: 1.27 wt % and (92 wt %) (92 wt %) (92 wt %) (92 wt %) average particle diameter: 0.5 to 0.8 m) MgSiN.sub.2 powder 0.3 g 0.3 g (average particle diameter: 3.0 m) (3 wt %) (3 wt %) MgO powder 0.3 g 0.3 g (average particle diameter: 1.0 m) (3 wt %) (3 wt %) YF.sub.3 0.5 g (5 wt %) YbF.sub.3 0.5 g (5 wt %) Y.sub.2O.sub.3 0.5 g (5 wt %) Yb.sub.2O.sub.3 0.5 g (5 wt %)

Experimental Example 1

[0047] The surface of each sintered body according to Examples 1 and 2 and Comparative Examples 1 and 2 was polished and plasma-etched. Afterward, the microstructure of the sintered body was identified using an electron microscope, the result of which is shown in FIG. 2.

[0048] Referring to FIG. 2, it can be confirmed that the sintered bodies according to Examples 1 and 2 exhibited a smaller amount of a secondary phase part (white part) compared to the sintered bodies according to Comparative Examples 1 and 2.

Experimental Example 2

[0049] Thermal conductivity of each sintered body according to Examples 1 and 2 and Comparative Examples 1 and 2 was measured through a laser flash method, the result of which is shown in Table 2 below.

TABLE-US-00002 TABLE 2 Comparative Comparative Classification Example 1 Example 2 Example 1 Example 2 Thermal 58.8 62.9 55.1 53.6 conductivity (k)

[0050] Referring to Table 2, it can be confirmed that the sintered bodies according to Examples 1 and 2 exhibited higher thermal conductivities compared to the sintered bodies according to Comparative Examples 1 and 2. This is considered to be due to the fact that, as the sintered bodies according to various embodiments have a smaller amount of a secondary phase part, phonon scattering caused by the secondary phase is reduced, and thus thermal conductivity is improved.

Example 3

[0051] Ingredient powders consisting of compositions listed in Table 3 below were added to 100 ml of an isopropanol solvent, and then the mixture was subjected to ball milling using a silicon nitride ball at room temperature at 300 rpm for 24 hours to prepare a slurry.

[0052] While being stirred with a magnetic bar to prevent the slurry thus prepared from precipitating, the slurry was primarily dried at 120 C. for 1.5 hours and secondarily dried in a vacuum oven at 60 C. for 1 hour to prepare a mixed powder.

[0053] 2.0 g of the mixed powder thus prepared was taken, added to a mold, and then subjected to cold isostatic pressing under a pressure condition of 200 MPa to form a discoid compact having a diameter of 20 mm and a thickness of 2.5 mm.

[0054] The compacts thus formed were stacked in layers in a graphite sleeve, and graphite spacers were introduced between the compacts to prevent the compacts from being brought into contact with one another. Afterward, the graphite sleeve containing the compacts was placed in a graphite furnace, and the compacts were sintered under a nitrogen atmosphere at 1,750 C. for 2 hours and uniaxially pressurized at a pressure of 25 MPa to manufacture a sintered body.

[0055] Then, the sintered body thus manufactured was thermally post-treated at 1,850 C. for 4 hours.

Example 4

[0056] A sintered body was manufactured in the same manner as Example 3 except that ingredient powders consisting of compositions listed in Table 3 below were used.

Comparative Example 3

[0057] A sintered body was manufactured in the same manner as Example 3 except that ingredient powders consisting of compositions listed in Table 3 below were used.

Comparative Example 4

[0058] A sintered body was manufactured in the same manner as Example 3 except that ingredient powders consisting of compositions listed in Table 3 below were used.

TABLE-US-00003 TABLE 3 Comparative Comparative Ingredient powder Example 3 Example 4 Example 3 Example 4 Si.sub.3N.sub.4 powder 9.5 g 9.5 g 9.5 g 9.5 g (oxygen content: 1.27 wt % and (95 wt %) (95 wt %) (95 wt %) (95 wt %) average particle diameter: 0.5 to 0.8 m) MgSiN.sub.2 powder 0.2 g 0.2 g 0.2 g 0.2 g (average particle diameter: 3.0 m) (2 wt %) (2 wt %) (2 wt %) (2 wt %) YF.sub.3 0.3 g (3 wt %) YbF.sub.3 0.3 g (3 wt %) Y.sub.2O.sub.3 0.3 g (3 wt %) Yb.sub.2O.sub.3 0.3 g (3 wt %)

Experimental Example 3

[0059] The surface of each sintered body according to Examples 3 and 4 and Comparative Examples 3 and 4 was polished and plasma-etched. Afterward, the microstructure of the sintered body was identified using an electron microscope, the result of which is shown in FIG. 3.

[0060] Referring to FIG. 3, it can be confirmed that the sintered bodies according to Examples 3 and 4 exhibited a smaller amount of a secondary phase part (white part) compared to the sintered bodies according to Comparative Examples 3 and 4.

Experimental Example 4

[0061] Thermal conductivity of each sintered body according to Examples 3 and 4 and Comparative Examples 3 and 4 was measured through a laser flash method, the result of which is shown in Table 4 below.

TABLE-US-00004 TABLE 4 Comparative Comparative Classification Example 3 Example 4 Example 3 Example 4 Thermal 79.8 84.3 71.8 75.8 conductivity (k)

[0062] Referring to Table 4, it can be confirmed that the sintered bodies according to Examples 3 and 4 exhibited higher thermal conductivities compared to the sintered bodies according to Comparative Examples 3 and 4. This is considered to be due to the fact that, as a secondary phase formed during a sintering process is removed through thermal post-treatment, phonon scattering caused by the secondary phase is reduced, and thus thermal conductivity is improved.

Example 5

[0063] Ingredient powders consisting of compositions listed in Table 5 below were added to 100 ml of an isopropanol solvent, and then the mixture was subjected to ball milling using a silicon nitride ball at room temperature at 300 rpm for 24 hours to prepare a slurry.

[0064] While being stirred with a magnetic bar to prevent the slurry thus prepared from precipitating, the slurry was primarily dried at 120 C. for 1.5 hours and secondarily dried in a vacuum oven at 60 C. for 1 hour to prepare a mixed powder.

[0065] 2.0 g of the mixed powder thus prepared was taken, added to a mold, and then subjected to cold isostatic pressing under a pressure condition of 200 MPa to form a discoid compact having a diameter of 20 mm and a thickness of 2.5 mm.

[0066] The compacts thus formed were stacked in layers in a graphite sleeve, and graphite spacers were introduced between the compacts to prevent the compacts from being brought into contact with one another. Afterward, the graphite sleeve containing the compacts was placed in a graphite furnace, and the compacts were sintered under a nitrogen atmosphere at 1,750 C. for 2 hours and uniaxially pressurized at a pressure of 25 MPa to manufacture a sintered body.

Example 6

[0067] A sintered body was manufactured in the same manner as Example 5 except that ingredient powders consisting of compositions listed in Table 5 below were used, wherein a Si.sub.3N.sub.4 powder was thermally pretreated under a nitrogen atmosphere at 1,500 C. for 4 hours.

Example 7

[0068] A sintered body was manufactured in the same manner as Example 5 except that ingredient powders consisting of compositions listed in Table 5 below were used, wherein a Si.sub.3N.sub.4 powder was thermally pretreated under a nitrogen atmosphere at 1,500 C. for 10 hours.

TABLE-US-00005 TABLE 5 Ingredient powder Example 5 Example 6 Example 7 Si.sub.3N.sub.4 powder 9.5 g (average particle diameter: 0.5 to (95 wt %) 0.8 m) Si.sub.3N.sub.4 powder thermally pretreated 9.5 g for 4 hours (95 wt %) (average particle diameter: 0.5 to 0.8 m) Si.sub.3N.sub.4 powder thermally pretreated 9.5 g for 10 hours (95 wt %) (average particle diameter: 0.5 to 0.8 m) MgSiN.sub.2 powder 0.2 g 0.2 g 0.2 g (average particle diameter: 3.0 m) (2 wt %) (2 wt %) (2 wt %) YbF.sub.3 0.3 g 0.3 g 0.3 g (3 wt %) (3 wt %) (3 wt %)

Experimental Example 5

[0069] The contents of oxygen and carbon contained in each Si.sub.3N.sub.4 powder used in Examples 5, 6, and 7 were measured through a hot-gas extraction method using inorganic CS/ONH analysis (CS800/ONH-2000, Eltra, Haan, Germany), the result of which is shown in Table 6 below. Also, the microstructure of each Si.sub.3N.sub.4 powder was identified using an electron microscope, the result of which is shown in FIG. 4.

TABLE-US-00006 TABLE 5 Classification Oxygen content (%) Carbon content (%) Si.sub.3N.sub.4 powder in Example 5 1.273 0.116 Si.sub.3N.sub.4 powder in Example 6 0.979 0.106 Si.sub.3N.sub.4 powder in Example 7 0.969 0.100

[0070] Referring to Table 6, it can be confirmed that, as a Si.sub.3N.sub.4 powder was thermally pretreated, the contents of oxygen and carbon which are impurities were decreased.

[0071] In addition, referring to FIG. 4, it can be confirmed that a grain did not grow even when a Si.sub.3N.sub.4 powder was thermally pretreated for 10 hours to remove oxygen and carbon which are impurities.

Example 8

[0072] The sintered body according to Example 5 was thermally post-treated at 1,830 C. for 3 hours.

Example 9

[0073] The sintered body according to Example 5 was thermally post-treated at 1,830 C. for 7 hours.

Example 10

[0074] The sintered body according to Example 5 was thermally post-treated at 1,830 C. for 10 hours.

Example 11

[0075] The sintered body according to Example 6 was thermally post-treated at 1,830 C. for 3 hours.

Example 12

[0076] The sintered body according to Example 6 was thermally post-treated at 1,830 C. for 7 hours.

Example 13

[0077] The sintered body according to Example 6 was thermally post-treated at 1,830 C. for 10 hours.

Example 14

[0078] The sintered body according to Example 7 was thermally post-treated at 1,830 C. for 3 hours.

Example 15

[0079] The sintered body according to Example 7 was thermally post-treated at 1,830 C. for 7 hours.

Example 16

[0080] The sintered body according to Example 7 was thermally post-treated at 1,830 C. for 10 hours.

Experimental Example 6

[0081] Properties of each sintered body according to Examples 8 to 16 were evaluated, the results of which are shown in Table 7 below. Also, the microstructure of each sintered body was identified using an electron microscope, the result of which is shown in FIG. 5. Here, bending strength was measured using a sample with a size of 3.04.035 mm through a three-point bending test (crosshead speed: 0.5 mm/min).

TABLE-US-00007 TABLE 7 Thermal post- Thermal treatment Density conductivity Bending strength Classification time (g/cm.sup.3) (k) (MPa) Example 8 3 hours 3.22 75.5 916.1 Example 9 7 hours 3.21 84.4 862.1 Example 10 10 hours 3.22 96.9 792.1 Example 11 3 hours 3.21 79.7 890.8 Example 12 7 hours Example 13 10 hours Example 14 3 hours 3.19 101.2 871.4 Example 15 7 hours 3.2 109.9 727.8 Example 16 10 hours 3.21 120.1 699.6

[0082] Referring to Table 7, it can be confirmed that thermal conductivity of a sintered body was increased to a maximum of 120 W/mK by using a thermally pretreated Si.sub.3N.sub.4 powder and thermally post-treating a sintered body according to various embodiments of the invention. Also, it can be confirmed that a sintered body having a high bending strength ranging from 660 to 870 MPa was manufactured according to the manufacturing method of various embodiments of the invention.