KR101698378B1 - Silicon carbide ceramics and method for preparing thereof - Google Patents

Abstract

The present invention relates to silicon carbide ceramic obtained by using AlN-Y_2O_3-Sc_2O_3 as a sintering additive in silicon carbide (SiC) and a method for preparing the same. According to the present invention, the silicon carbide ceramic that has a dense structure is prepared by the novel AlN-Y_2O_3-Sc_2O_3 sintering additive being added to silicon carbide powder. The silicon carbide ceramic prepared from the sintering additive composition according to the present invention has a high level of heat conductivity and its mechanical properties such as fracture toughness, bending strength, and Vickers hardness are better than those of existing silicon carbide ceramic. The silicon carbide ceramic obtained in the present invention is higher in relative density than existing pressure-sintered silicon carbide ceramic even in the case of liquid sintering under a zero-pressure normal pressure condition.

Description

Technical Field [0001] The present invention relates to a silicon carbide ceramics and a method of manufacturing the same,

The present invention relates to a silicon carbide ceramics and a method of manufacturing the same, and more particularly, to a silicon carbide ceramics subjected to atmospheric pressure sintering which has a dense structure by using a novel sintering additive system and has improved density and a method for manufacturing the same.

Conventionally, SiC ceramics having a completely dense structure having high thermal conductivity were prepared by a hot-pressing method using a Y ₂ O ₃ -Sc ₂ O ₃ additive. The thermal conductivity of the SiC ceramics was 234 W / mK at room temperature (rt). That is, in order to have a high density and thermal conductivity, a press-sintering process was essentially required.

The microstructure of the SiC ceramic sintered with the Y ₂ O ₃ -Sc ₂ O ₃ additive has a very unique structure: (1) the SiC surface formed on (Sc, Y) ₂ Si ₂ O ₇ and / (2) the absence of solubility of Y and Sc in the SiC lattice, and (3) the formation of a clean or crystallized SiC-SiC interface.

On the other hand, in order to obtain thermally conductive SiC ceramics, the SiC ceramics were produced in a state in which the Y ₂ O ₃ -Sc ₂ O ₃ additive was not applied (atmospheric pressure) -sintering. However, the relative density of the SiC ceramics produced by the pressureless sintering under the unpressurized condition using the Y ₂ O ₃ -Sc ₂ O ₃ additive was 88%.

It is generally known that the relative density of ceramic materials should be at least 95%, and below this value high density is required because ceramic materials may experience sudden failure.

On the other hand, in the production of thermally conductive SiC ceramics, a thermally conductive SiC ceramic having a high relative density can be produced by carrying out the (pressurization) sintering with the addition of additives and pressure.

However, in the case of the above-mentioned pressure-sintering, there is a disadvantage that it is impossible to manufacture ceramics having various shapes and complicated shapes due to the high pressure of the ceramic due to the necessity of adding a certain space for applying pressure and a system such as high- Therefore, there is a need for a material composition and a method capable of producing a thermally conductive SiC ceramic having a high relative density while sintering under normal pressure without applying pressure.

On the other hand, to have a high thermal conductivity in SiC ceramics, the addition of Al should be avoided as much as possible for the following reasons:

(1) In the SiC lattice, since Al has solubility, Si vacancies are formed, which increases phonon scattering

Al ₂ O ₃ ? 2Al _Si + 3O _C + V _Si

(2) The phase transition of SiC from β to α is accelerated by Al. During this phase transition, the α-phase acts as nuclei in lamination defects present in the starting material, β-phase, and is grown by glides of partial dislocations. Therefore, α-SiC has a large stacking defect density, which increases phonon scattering.

 Y.-W. Kim, K.-Y. Lim, and W.-S. Seo, "Microstructure and Thermal Conductivity of Silicon Carbide with Yttria and Scandia ", J. Am. Ceram. Soc. 97 [3] 923-928 (2014)

In the present invention, in order to produce SiC ceramics having a high density by densifying the density of an atmospheric pressure-sintered SiC ceramic by adding Y ₂ O ₃ -Sc ₂ O ₃ additive without pressure,

Accordingly, an object of the present invention is to provide a thermally conductive SiC ceramic capable of atmospheric-pressure sintering and having a high relative density.

Another object of the present invention is to investigate the sinterability of SiC ceramics in a pressureless state by using a novel additive system (AlN-Y ₂ O ₃ -Sc ₂ O ₃ ). For this purpose, it is an object of the present invention to provide a method for producing SiC ceramics in which SiC powder is used as a starting material and liquid-sintered at normal pressure without applying pressure.

A silicon carbide ceramic according to an embodiment of the present invention is characterized in that it is produced by adding AlN-Y ₂ O ₃ -Sc ₂ O ₃ to a silicon carbide (SiC) powder as a sintering additive.

The silicon carbide (SiC) powder is preferably? -SiC and? -SiC.

The silicon carbide ceramics may be composed of 90 to 96 vol% of α-SiC and β-SiC silicon carbide powder, 0.5 to 3.0 vol% of AlN, and 1 to 10 vol% of Y ₂ O ₃ -Sc ₂ O ₃ .

The Y ₂ O ₃ -Sc ₂ O ₃ is preferably mixed in a molar ratio of 1: 3 to 3: 1.

According to an embodiment of the present invention, the silicon carbide ceramics sintered using the? -SiC may be composed of 6H, which is a major phase, and 4H, which is a minor phase.

According to one embodiment of the present invention, the silicon carbide ceramics sintered using the? -SiC may be composed of 4H, which is a major phase, and 6H and 3C, which are minor phases.

According to an embodiment of the present invention, the relative density of the silicon carbide ceramics is 96% or more.

According to an embodiment of the present invention, the thermal conductivity of the silicon carbide ceramics is 90 W / m · K or more.

According to an embodiment of the present invention, the silicon carbide ceramics may have a core-rim structure.

According to another embodiment of the present invention, there is provided a method of manufacturing a silicon carbide ceramic comprising mixing silicon carbide, AlN-Y ₂ O ₃ -Sc ₂ O ₃ sintering additive under a solvent, drying and pressing the mixed slurry , Cold isotactic pressing (CIP) the pressed material, and subjecting the CIP material to atmospheric pressure liquid phase sintering.

The AlN-Y ₂ O ₃ -Sc ₂ O ₃ sintering additive reacts with the SiO ₂ layer of the silicon carbide (SiC) to form an Al-Y-Sc-Si-ON liquid phase, The Si-ON-based liquid phase forms an Al-Y-Sc-Si-OCN liquid phase by dissolving SiC during the sintering process. The liquid phase of the Al-Y- It is characterized by facilitating.

The liquid-phase sintering is preferably performed under a normal pressure (1 atm) condition without pressure.

According to the present invention, a novel silicon carbide ceramic having a densified structure is prepared by adding a novel AlN-Y ₂ O ₃ -Sc ₂ O ₃ additive to silicon carbide powder.

Therefore, the silicon carbide ceramics produced from the sintering additive composition according to the present invention have excellent thermal conductivity, and mechanical properties such as fracture toughness, flexural strength, Vickers hardness, and the like have improved values compared to conventional silicon carbide ceramics.

In addition, in the present invention, a silicon carbide ceramic having a higher relative density than silicon carbide ceramics which have been conventionally pressure-sintered can be obtained even under liquid pressure sintering under normal pressure conditions without applying pressure.

Fig. 1 shows the results of phase analysis of SiC ceramics (SCA, SCB) prepared according to Examples 1 and 2,
2 and 3 are photographs of the microstructure of SiC ceramics (SCA, SCB) prepared according to Examples 1 and 2, respectively, by scanning electron microscopy,
FIG. 4 shows the results of measurement of thermal conductivity of atmospheric-pressure sintered SiC ceramics of the present invention and prior art,
FIG. 5 shows the results of measurement of thermal conductivity, phonon mean free path, and oxygen content in a SiC lattice of SiC ceramics (SCA, SCB) prepared according to Examples 1 and 2.

Hereinafter, the present invention will be described in more detail.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a," "an," and "the" include singular forms unless the context clearly dictates otherwise. Also, " comprise "and / or" comprising "when used herein should be interpreted as specifying the presence of stated shapes, numbers, steps, operations, elements, elements, and / And does not preclude the presence or addition of one or more other features, integers, operations, elements, elements, and / or groups.

The silicon carbide ceramics according to an embodiment of the present invention is characterized in that silicon carbide (SiC) particles are used and AlN-Y ₂ O ₃ -Sc ₂ O ₃ is added by adding a sintering additive.

The silicon carbide is preferably? -SiC and? -SiC.

The silicon carbide ceramics preferably comprises 90 to 96 vol% of silicon carbide, 0.5 to 3.0 vol% of AlN, and 1 to 10 vol% of Y ₂ O ₃ -Sc ₂ O ₃ .

When the content of the silicon carbide is less than 90 vol%, a liquid phase having a very low thermal conductivity is excessively formed, and the thermal conductivity of the silicon carbide subjected to atmospheric pressure sintering is lowered to less than 90 W / mK, and more than 96 vol% There is a problem that the liquid phase is insufficient and the sintered density is lowered to less than 96%, which is not preferable.

If the AlN content is less than 0.5 vol%, the viscosity of the liquid phase becomes too high, and the sintered density is lowered to less than 96%. When the AlN content is less than 0.5 vol% If it is added in an amount exceeding 3.0 vol%, Si vacancies which cause phonon scattering are formed in a large amount, and the thermal conductivity is lowered to less than 90 W / m · K, which is not preferable.

The Y ₂ O ₃ -Sc ₂ O ₃ of the present invention is preferably mixed in a molar ratio in the range of 1: 3 to 3: 1, more preferably 1 to 10 vol% Is preferable for achieving a sintered density of 96% or more and a thermal conductivity of 90 W / m · K or more at the same time. If the molar ratio of Y ₂ O ₃ -Sc ₂ O ₃ is out of the range of 1: 3 to 3: 1, the sintered density is lowered to less than 96%. If the content of Y ₂ O ₃ -Sc ₂ O ₃ is less than 1 vol% , The sintered density is lowered to less than 96%, and when it is added in an amount exceeding 10 vol%, the thermal conductivity is lowered to less than 90 W / m · K, which is not preferable.

According to an embodiment of the present invention, the silicon carbide ceramics sintered using the? -SiC may be composed of 6H, which is a major phase, and 4H, which is a minor phase.

According to one embodiment of the present invention, the silicon carbide ceramics sintered using the? -SiC may be composed of 4H, which is a major phase, and 6H and 3C, which are minor phases.

According to an embodiment of the present invention, the relative density of the silicon carbide ceramics is 96% or more.

According to an embodiment of the present invention, the thermal conductivity of the silicon carbide ceramic is 90 W / m? Or more.

According to an embodiment of the present invention, the silicon carbide ceramics may have a core-rim structure.

According to another embodiment of the present invention, there is provided a method of manufacturing a silicon carbide ceramic comprising mixing silicon carbide, AlN-Y ₂ O ₃ -Sc ₂ O ₃ sintering additive under a solvent, drying and pressing the mixed slurry , Cold isotactic pressing (CIP) the pressed material, and liquid-sintering the CIP material at normal pressure.

In the present invention, each component included in the production of a silicon carbide ceramic is mixed with a SiC ball and a polypropylene jar in a solvent to form a slurry state.

The slurry is then dried and sieved, and then subjected to a pressing process. The pressing process is a molding process performed to make a certain shape.

Further, the pressed material is subjected to a cold isotropic pressing process to improve the density.

Finally, silicon carbide ceramics can be obtained by subjecting the material obtained by cold isostatic pressing to liquid phase sintering.

In the present invention, it is possible to manufacture silicon carbide ceramics having excellent density even when sintering is performed under a condition of normal pressure (1 atm) without applying pressure.

This is an effect that can be achieved by using the novel sintering additive composition according to the present invention, which can be explained as follows.

In the present invention, the AlN-Y ₂ O ₃ -Sc ₂ O ₃ When using a sintering additive, AlN-Y ₂ O ₃ -Sc ₂ O ₃ sintering additive according to the invention is pressureless-sintering process for a silicon carbide (SiC) surface to form a SiO ₂ layer and a reaction by Al-Y-Sc-Si- oN -based liquid, SiC is dissolved during the Al-Y-Sc-Si- oN system of the sintering process the liquid phase by being Al-Y- Thereby forming a Sc-Si-OCN-based liquid phase.

Since Al and N (nitrogen) have a slight solubility in the SiC lattice, Si, C, Al, and N co-precipitate in large SiC particles to form Al-N composite doped SiC particles .

Thus, the Al-Y-Sc-Si-OCN-based liquid phase promotes densification of SiC by liquid phase sintering and leads to N-doping in the rim region of SiC grains. In addition, the crystalline Y ₂ O ₃ and Sc ₂ O ₃ phases are precipitated at the junction when subjected to a cooling process.

Therefore, the silicon carbide ceramics produced from the sintering additive composition of the present invention have excellent thermal conductivity and mechanical properties such as fracture toughness, flexural strength, Vickers hardness and the like are improved as compared with the conventional silicon carbide ceramics.

Hereinafter, preferred embodiments of the present invention will be described in detail. The following examples are intended to illustrate the present invention, but the scope of the present invention should not be construed as being limited by these examples. In the following examples, specific compounds are exemplified. However, it is apparent to those skilled in the art that equivalents of these compounds can be used in similar amounts.

Example 1

(AlN, Grade P, Tokuyama Soda, Tokyo, Japan) , Y ₂ O ₃ (99.99%, manufactured by Kojundo Chemical Lab Co., Ltd.). , Ltd., Sakado-shi, Japan), Sc ₂ O ₃ (99.99%, Kojundo Chemical Lab Co., Ltd., Sakado-shi, Japan).

93.5 vol% of alpha-SiC, AlN (1.5 vol%), and Y ₂ O ₃ -Sc ₂ O ₃ 5vol% of each powder was mixed with SiC ball and polypropylene jar in ethanol for 24 hours. The milled slurry was dried, sieved and subjected to a pressing process at a pressure of 20 MPa. Then, SiC ceramics (hereinafter referred to as "SCA") was produced by sintering at a pressure of 270 MPa and a nitrogen atmosphere at 1950 ° C for 6 hours.

Example 2

SiC ceramics (hereinafter referred to as "SCB") was produced in the same manner as in Example 1, except that β-SiC (~0.5 μm, BF-17, HC Starck, Berlin, ).

Experimental Example 1: Relative density measurement

The bulk density and the weight of the SiC ceramics prepared according to Examples 1 and 2 were measured and the density was calculated. The theoretical density of SCA was 3.337 g / cm ³ and the theoretical density of SCB was 3.335 g / cm ³ Relative density.

The relative density of SCA according to Example 1 was 98.6%, and the relative density of SCB according to Example 2 was 96.7%.

Silicon carbide ceramics made from the additive composition (AlN-Y ₂ O ₃ -Sc ₂ O ₃ ) according to the present invention are significantly improved compared to the relative density of 88% of the conventional additive composition (Y ₂ O ₃ -Sc ₂ O ₃ ) Respectively. That is, it can be seen that the structure of the silicon carbide ceramics produced by using AlN as an additive for sintering can be made more denser in the present invention.

Experimental Example 2: Phase Analysis

The Rietveld Method was used to analyze the phases of the starting material and sintered SiC ceramics of each polytype of SiC powder, and the results are shown in the following Table 1.

Quantitative phase analysis (Rietveld Method) Sample SiC phase (wt%) 3C 6H 4H α-SiC (starting powder) - 90.5 9.5 SCA - 80.8 19.2 β-SiC (starting powder) 86.6 13.4 - SCB 25.8 26.9 47.3

Referring to Table 1, it can be seen that the SCB sintered using β-SiC has major phases of 4H and minor phases of 6H and 3C. Conversely, SCA sintered with α-SiC shows that 6H is the major phase and 4H is the minor phase.

In the case of SiC ceramics prepared from α-SiC and β-SiC powder, phase transition from 6H to 4H in SCA and phase transformation from 3C to 6H and 4H occurred in SCB.

Experimental Example 3: Microstructure

 The microstructure of the SiC ceramics was observed with a scanning electron microscope. The results are shown in FIG. 2 (SCA) and 3 (SCB).

Referring to FIGS. 2 and 3, when AlN-Y ₂ O ₃ -Sc ₂ O ₃ is added simultaneously, SiC can not effectively inhibit the phase transformation from β to α during sintering without pressure at 1950 ° C. The SiC particles clearly show the core-rim structure in both specimens, indicating that the grain growth has occurred due to the dissolution-re-precipitation mechanism during the sintering process.

Referring now to FIG. 2, the SCA specimen is composed of equiaxed SiC particles, whereas, referring to FIG. 3, the SCB specimen is composed of elongated particles and isotropic particles.

Experimental Example 4: Thermal Conductivity [

The thermal conductivity (K) was calculated by the following equation 1:

(Equation 1)

χ = αρCp

Where? Is the thermal diffusivity,? Is the density of the sample, and Cp is the specific heat of the sample.

Next, FIG. 4 is a result of measuring the thermal conductivity of liquid phase sintered SiC ceramics of the present invention and prior art.

As a result, SiC ceramics sintered with AlN-Y ₂ O ₃ -Sc ₂ O ₃ additive showed the highest thermal conductivity value (110 W / m · K) among conventional SiC ceramics without liquid pressure sintering. The AlN-Y ₂ O ₃ -Sc ₂ O ₃ This is because the additive effectively picked up oxygen in the SiC lattice.

Referring to FIG. 5, it can be seen that the thermal conductivity of the SiC ceramics is 91.9 W / m · K for SCA and 110.3 W / m · K for SCB.

Although the large phase transition from 3C to 6H and 4H occurs in SCB, it can be seen that the thermal conductivity of the SCB is higher than that of SCA. This phase transition usually causes lamination defects in the SiC lattice causing phonon scattering. The results of the present invention show that the grain boundary causes phonon scattering along with lamination defects.

That is, comparing the microstructure of FIGS. 2 and 3, it can be seen that the particle size of the SCA of FIG. 2 is smaller than the particle size of the SCB, which means that the SCA contains a larger area of grain boundaries per unit volume, Therefore, the phonon scattering occurs more in the grain boundaries, and the thermal conductivity of SCA is lower than that of SCB.

It can also be seen that the lattice oxygen content of the SCB is 0.25 wt.%, Which is smaller than that of SCA of 0.27 wt.%, Indicating that there is more phonon-Si vacancy scattering in the SCA.

The phonon average free distance (t) was calculated by the following equation 2:

(Equation 2)

t = 3K / VρCp

Where K is the thermal conductivity, V is the average sound velocity, p is the density of the sample, and Cp is the specific heat of the sample.

Phonon mean free paths of SCA and SCB were 11 nm and 12.7 nm at room temperature (r.t.), respectively.

The thermal conductivity of the SCB is 110 W / m · K, which is larger than that of SCA of 92 W / m · K. The higher thermal conductivity of the SCB means that the particle size of the SCB is larger than that of SCA and the oxygen content of the SCB is 0.27 wt %, SCB has a lower value of 0.25 wt%, which means that phonon-interfacial scattering and phonon-Si pervaporation are reduced in SCB, so that phonon mean free distance is larger than SCA, Therefore, higher thermal conductivity was shown.

Experimental Example 5: Mechanical Property -1

The mechanical properties of the SiC ceramics were measured and the results are shown in Table 2 below.

SiC
Starting powder Sintering additive Flexural strength
(MPa) Fracture toughness
(MPa.m1 / ² ) References beta _{AlN-Sc 2 O 3 -Y 2} O 3 520 5.1 Invention alpha _{AlN-Sc 2 O 3 -Y 2} O 3 509 4.1 Invention β, α
(Seed particle) Al ₂ O ₃ - Y ₂ O ₃ - 8 N. Padture, J. Am. Ceram. Soc., 77 , 519-23 (1994) β, α
(Seed particle) Al ₂ O ₃ -AlN-Y ₂ O ₃ 433 6.6 J.-H. Eom, Y.-K. Seo, Y.-W. Kim, and S.-J. Lee, Met. Mater. Int., 21 , 525-30 (2015) alpha Al ₂ O ₃ -CeO ₂ 437 4.6 H. Liang, X. Yao, J. Zhang, X. Liu, and Z. Huang, J. Eur. Ceram. Soc., 34 , 831-35 (2014) alpha Al ₂ O ₃ - Y ₂ O ₃ 498 5.6 A. Gubernat, L. Stobierski, and P. Labaj, J. Eur. Ceram. Soc., 27 , 781-89 (2007)

Referring to Table 2, the fracture toughness of SCA and SCB is 4.1 MPa.m ^1/2 and 5.1 MPa.m ^1/2 , respectively, and the flexural strengths of SCA and SCB are 509 MPa And 520 MPa, respectively.

The excellent fracture toughness and flexural strength of the SiC ceramics prepared from the β-SiC powder are as follows: the aspect ratio of the SCA obtained from the α-SiC powder is 1.38, whereas the aspect ratio of the SCB produced from the β-SiC powder is 2.04 Due to having larger particle size.

From these results, it can be seen that the SiC ceramics produced according to the present invention have superior mechanical properties to other SiC ceramic groups sintered under pressureless conditions, as can be seen from Table 2 above. This shows that AlN-Y ₂ O _3- Sc ₂ O ₃ It has been proved that the additive system is effective for improving the mechanical properties of SiC ceramics.

Experimental Example  6: Mechanical properties ( Mechanical Property )-2

The hardness of the prepared SiC ceramics was measured and the results are shown in Table 3 below.

Sintering additive Particle size
(탆) Hardness
(GPa) References _{AlN-Sc 2 O 3 -Y 2} O 3 1.4 27.2 The present invention SCA _{AlN-Sc 2 O 3 -Y 2} O 3 1.9 25.0 Invention SCB B ₄ C-AlN-C 1-2 29.2 A. Malinge, A. Coupe, S. Jouannigot, YL Petitcorps, R. Pailler, and P. Weisbecker
J. Eur. Ceram. Soc., 32 , 4419-26 (2012) Al ₂ O ₃ -Lu ₂ O ₃ 1.1 27.6 H. Liang, X. Yao, J. Zhang, X. Liu, and Z, Huang,
J. Eur. Ceram. Soc., 34 , 2865-74 (2014) Al ₂ O ₃ - Y ₂ O ₃ --TiO ₂ 1-2 27.4 H. Liang, X. Yao, H. Zhang, X. Liu, and Z, Huang, Ceram. Int., 40 , 10699-704 (2014) B-C 30 24.4 G. Magnani, A. Brentari, E. Burresi, and G. Raiteri, Ceram. Int., 40 , 1759-63 (2014)

Referring to Table 3, the hardness values of SCA and SCB were measured to be 27.2 GPa and 25.0 GPa, respectively.

It is generally known that monolithic SiC has higher hardness values under two conditions with smaller oxygen additive content and smaller particle size. Although SCA and SCB contain the same content, we can see that SCA has a higher hardness value than SCB because the particle size of SCA (1.4 ㎛) is smaller than SCB (1.9 ㎛).

From these results, it was confirmed that it is important to select an appropriate sintering additive and an additive having a finer structure in order to improve the hardness in the production of SiC ceramics without sintering.

Claims (12)

delete delete delete delete delete delete delete delete delete Silicon carbide powder, AlN-Y ₂ O ₃ -Sc ₂ O ₃ sintering additive in an ethanol solvent;
Drying the mixed slurry and pressing it at a pressure of 20 MPa;
Increasing the density of the pressed material by cold isotactic pressing (CIP) at a pressure of 270 MPa; And
Sintering the material subjected to the cold isostatic pressing at a normal pressure (1 atm)
Wherein the silicon carbide (SiC) powder is 93.5 vol%, the AlN is 1.5 vol%, the Y ₂ O ₃ -Sc ₂ O ₃ is 5 vol%, Y ₂ O ₃ and Sc ₂ O ₃ are 50:50 Wherein the silicon carbide ceramics are produced by a pressurized liquid sintering process.
11. The method of claim 10,
The AlN-Y ₂ O ₃ -Sc ₂ O ₃ sintering additive reacts with the SiO ₂ layer of the silicon carbide (SiC) to form an Al-Y-Sc-Si-ON liquid phase,
The Al-Y-Sc-Si-OC-based liquid phase forms an Al-Y-Sc-Si-OCN-based liquid phase by dissolving SiC in a sintering process,
Wherein the Al-Y-Sc-Si-OCN-based liquid phase promotes densification of SiC by liquid phase sintering. delete

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