KR101140353B1 - Porous sintered reaction-bonded silicon nitride body and manufacturing methods for the same - Google Patents

Abstract

The present invention relates to a porous reactive silicon nitride sintered compact and a method for producing the same, which can be applied to commercially available compression molding techniques of granules by realizing high granular yield strength through plastic sintering. The present invention comprises the steps of granulating a raw material comprising silicon and a sintering aid for producing a silicon nitride sintered body from the silicon; Presintering the granulated raw material in an inert atmosphere; Preparing a molded body by press molding the pre-sintered granule raw material; And it provides a method for producing a porous reaction silicon nitride sintered body comprising the step of post-sintering the molded body in a nitrogen atmosphere. According to the present invention, the pore channel size is controlled so that coarse pores and micropores coexist in the material, thereby providing a porous reactive silicon nitride sintered body which simultaneously increases aeration performance and collection efficiency.

Description

Porous sintered reaction-bonded silicon nitride body and manufacturing methods for the same

The present invention relates to a porous reactive silicon nitride sintered body and a method of manufacturing the same, and more particularly, to a porous reactive silicon nitride sintered body manufactured by a commercial pressure forming technique with high granular yield strength and controlled in pore structure, and a method of manufacturing the same. .

Silicon nitride-based materials have been widely used in fields requiring good thermal mechanical and chemical properties due to their light weight and excellent strength, toughness, impact resistance, heat resistance, and corrosion resistance.

Conventionally, silicon carbide-based porous materials have been mainly used in fields requiring thermal, mechanical, and chemical resistance properties.Since silicon carbide has low thermal shock resistance and high hardness, the mold wears during molding, resulting in very short lifespan. Since the sintered at a high temperature of 2000 ℃ or more there is a problem that leads to an increase in the manufacturing cost.

As described above, the porous silicon nitride-based material has excellent heat resistance, mechanical properties, corrosion resistance, etc., so that it can be used as a filtration filter, a catalyst carrier, a heat insulating material, a filter to filter out fine dust of a diesel vehicle instead of the silicon carbide-based material. It is a promising material to be used.

However, most studies on existing silicon nitride materials have been concentrated on improving the mechanical and thermal properties by densifying microstructures, and research on methods of manufacturing porous silicon nitride materials is relatively lacking.

As an example of a technique for preparing a porous body with silicon nitride ceramics, Korean Patent Laid-Open Publication No. 195-702510 discloses a compound of Si ₃ N ₄ and a rare earth element and / or a transition metal compound for use as a filter or catalyst carrier for removing foreign substances. There is provided a method for producing a silicon nitride ceramic porous body consisting of, according to this method to prepare a porous body having a porosity of 30% or more by heat-treating the molded body of the mixed powder at a temperature of 1500 ℃ or more.

In addition, Korean Patent No. 10-0311694 provides a method for manufacturing a porous silicon oxynitride sintered body for manufacturing a porous silicon oxynitride sintered body applied to a refractory tile of a space shuttle, and according to this method, Si3N4: 11- 16 mass%, AlN: 3-5 weight%, Al2O3; 35-45 weight%, Y2O3: 35-45 weight% Low melting point powder is mixed and this blocky low melting powder is Si3N4: 57- 100 wt%, Al2O30-9 wt%, AlN: 0-33 wt% added to β-sialon silicon oxynitride powder 10-25 wt%, and then molded, and then sintered at a temperature of 1600-1700 ℃ for 1-8 hours A method for producing a porous silicon oxynitride sintered compact is provided.

On the other hand, Japanese Patent Application Laid-open No. Hei 9-100179 discloses a method for producing a silicon nitride porous body which can be used as a filtration filter or a catalyst carrier. This method makes contact with a silicon nitride-based porous body with an acid and / or an alkali to make silicon nitride. Some or all of the other components are dissolved to produce a porous body.

However, since all of the above methods use expensive silicon nitride as a raw material, there is a fundamental limitation in practical use itself, and the method used for pore formation is also impractical. For example, in the case of Korean Patent No. 10-311694, in order to form pores in the sintered compact, the low melting point composition powder is formed into a mass and then the molded body is mixed with the high melting point composition powder to secure the pores depending on the size of the mass compact. However, it is difficult to maintain the shape of the molded body in the mixing process, if you want to maintain it can not ensure sufficient mixing, it is difficult to control a consistent process and accompanied by increased costs. In addition, the method of forming pores by chemically treating the porous body prepared as Japanese Patent Application Laid-Open No. 9-100179 also requires a separate process called chemical treatment, and when the components present between silicon nitride are dissolved, the skeleton of the silicon nitride skeleton There is no guarantee that it will remain.

In order to solve the above-mentioned problems, the present inventors prepare a molded body after mixing rare earth oxide or rare earth oxide / alumina or rare earth oxide / magnesia as a sintering aid in silicon (Si), and the molded body is nitrogen atmosphere. It has been proposed a method of manufacturing a silicon nitride filter for automobiles by firing in the medium temperature region of the silicon nitride to produce silicon nitride and then sintering the silicon nitride in the high temperature region (Patent Application No. 10-2008-0040395).

According to this method, using cheap silicon as a starting material, it has excellent thermal shock resistance, mechanical properties such as strength, and stability at high temperature, so that it can be used as a soot filtration device and can be used as a particulate filter while optimizing its aspect ratio. The fine dust, which could not be filtered in the soot filtration device, was able to be filtered, and the sintering was possible at a lower temperature, thereby lowering the manufacturing cost.

However, the above-described invention has a problem in that pore channels of sufficient size can not be formed by limiting the size of the pores by the particle size of the gas phase-solid nitride reaction mechanism and the post sintered body.

In order to solve the above problems, it is an object of the present invention to provide a porous silicon nitride sintered body and a method for producing the same to ensure a pore channel of a sufficient size.

On the other hand, according to the preceding invention is formed a fine pore channel of a relatively uniform size, when using a porous body having a uniform pore size of the micropore channel in the soot filtration device has a high particle collection efficiency, This can lead to a lack of breathability, which creates a large back pressure during operation, which can contribute to the degradation of the system in which the filter is mounted.

Accordingly, an object of the present invention is to provide a reactive silicon nitride sintered compact and a method for producing the same, which control pore channel size so that coarse pores and micropores coexist in a material, thereby simultaneously improving air permeability and collection efficiency.

In order to achieve the above technical problem, the present invention comprises the steps of granulating a raw material comprising silicon and a sintering aid for producing a silicon nitride sintered body from the silicon; Presintering the granulated raw material in an inert atmosphere; Preparing a molded body by press molding the pre-sintered granule raw material; And it provides a method for producing a porous reaction silicon nitride sintered body comprising the step of post-sintering the molded body in a nitrogen atmosphere.

In the present invention, the sintering aid may include yttria and alumina, and may further include at least one alkaline earth metal oxide.

In the present invention, the post-sintering step is preferably performed at a temperature of 1700 ~ 1900 ℃.

In the present invention, the content of the sintering aid is preferably included 2 to 6% by weight based on the complete nitriding of the silicon.

In the present invention, the pressure forming step is characterized in that the molded body consisting of the granules by pressure molding at a pressure of 1 ~ 15MPa.

In order to achieve the above technical problem, the present invention provides a silicon nitride sintered body having coarse pore channels formed between the granular sintered regions and having an array of granular regions having fine pore channels therein, the composition of which is alkali. Provided is a porous reactive silicon nitride sintered body comprising an earth metal compound as a sintering aid.

In the present invention, the pore distribution of the silicon nitride sintered compact shows a bimodal distribution having a first peak and a second peak having a larger pore size than the first peak, wherein the first peak is due to the microporous channel. And the second peak is due to the coarse pore channel.

In the present invention, the first peak is present in the pore size range of less than 1 micron, and the second peak is present in the pore size of 1 micron or more.

In the present invention, it is preferable that the first peak is present in the pore size of less than 1 micron, and the second peak is present in the pore size of 5 to 15 microns.

In the present invention, the granular sintered region preferably has an average diameter in the range of 30 ~ 150 microns.

In addition, in the present invention, the maximum frequency diameter of the granular sintered zone is preferably present in the range of 50 to 150 microns.

According to the present invention, the pore channel size is controlled so that coarse pores and micropores coexist in a material, thereby providing a porous reactive silicon nitride sintered compact and a method of manufacturing the same, which simultaneously improves air permeability and collection efficiency.

Such a sintered compact of the present invention can be easily applied as a filter for hot gas, a filter of a soot filtration device, and a water treatment filter material.

1 is a photograph of the biogranules having various sizes by classifying the biogranules obtained according to an embodiment of the present invention.
Figure 2 is an enlarged photograph of the biogranules obtained in accordance with an embodiment of the present invention.
Figure 3 is a graph showing the weight distribution of the sifted and classified samples according to an embodiment of the present invention.
Figure 4 (a) is a raw granule shape before plasticization, (b) is a granular shape after pre-sintering and grinding, (c) is a hydrostatic pressure molding of the granules after pre-calcination at a pressure of 100 MPa to polish the cross section It is a photograph showing the result of observation.
5 is a load-displacement relationship graph of granules prepared according to the present invention.
Figure 6 is a graph calculating the molding density-forming pressure relationship of the granules prepared according to the present invention.
7 (a) and 7 (b) are photographs of cross-sections of samples uniaxially formed at pressures of 3.7 MPa and 18.6 MPa, respectively, of the fresh granules obtained in the present invention.
8 (a) to (d) are photographs taken of a cross section of a sample uniaxially formed at pressures of 3.7 MPa, 7.5 MPa, 18.6 MPa and 46.6 MPa of the pre-sintered granules of the present invention.
9 is a diagram schematically illustrating the pore structure of the molded article produced according to the embodiment of the present invention.
10 is an electron micrograph of the raw granules and plastic granules obtained according to an embodiment of the present invention.
Figure 11 is a graph measuring the porosity, shrinkage and weight loss by post-sintering temperature of uniaxial molded body using the m76.5 plastic granules of the present invention.
12 is a low magnification (x 300) photograph of the fracture surface of the specimen sintered at 1700 ^o C, 1800 ^o C and 1900 ^o C with m76.5 pre-sintered granules of the present invention.
FIG. 13 is a photograph of the inside of the granulated region of each specimen of FIG. 12 at high magnification (x 10k).
FIG. 14 is a graph showing pore distribution and pore specific surface area measured by mercury porosity analysis of the post-sintered porous body of FIG. 12.
FIG. 15 is a graph measuring aeration performance of specimens sintered at 1700 ° C. and 1800 ° C. in the post-sintered specimen of FIG. 12.
Figure 16 is a graph measuring the porosity and shrinkage (weight loss rate) for the sintered specimens after varying the granule size by classification according to the present invention.
FIG. 17 is a photograph of fracture surface according to granule size for the post sintered specimen of FIG. 16. FIG.
FIG. 18 is a photograph of the polished surface taken after the resin is impregnated into the post-sintered specimen of FIG. 16.
19 is a graph showing the results of analyzing the post-sintered specimen of FIG. 16 by the mercury porosity apparatus.
FIG. 20 is a graph illustrating the air permeability of the post-sintered specimen of FIG. 16.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

Granulation of Si + Sintering Agent Raw Powder

In the present invention, granulated powder was prepared by spray drying. The granular powder contains a sintering aid that promotes nitriding reaction and sintering of silicon and silicon. The sintering aid may be composed of a ternary or higher melting point sintering preparation further comprising an alkaline earth metal such as MgO, CaO, SrO, BaO, as well as a conventional binary high melting point sintering aid for sintering silicon nitride composed of yttria and alumina. have. In addition, there typically a Si film is formed on the surface of raw materials SiO _2, the SiO ₂ can aid the sintering process in the sintering after jalhwa with a sintering aid other film.

In the present invention, it is preferable that the amount of the sintering aid added to the sintering aid is 2 to 6 wt% based on Si ₃ N ₄ calculated on the assumption that Si is completely nitrided. As the amount of sintering aid added in the present invention increases, the strength of the granular powder produced by plastic sintering preferably increases.

In the present embodiment, as shown in Table 1, a high melting point YA system (Y ₂ O ₃ -Al ₂ O ₃ , T _eu = 1370 ° C) and a low melting point YAC system (Y ₂ O ₃ -Al ₂ O ₃ -CaO, Granules with varying atomizer rotational speeds (1,000, 10,000 rpm) were prepared with respect to the Si mixed powder in which T _eu = 1170 ° C.) was added as a sintering aid.

The spray slurry was prepared by planetary milling. The ratio of solids and water was 1: 1, and the amount of sintering aid added was 3wt% based on Si ₃ N ₄ based on the assumption that Si was completely nitrided. Si + sintering aid) based on 0.1 ~ 0.8wt%, binder was 2 ~ 5wt% based on solid content (Si + sintering aid). During spray drying, the rotation speed of the stirrer was maintained at 100 rpm, and the temperatures of the inlet and the outlet were 150 to 300 ° C and 80 to 120 ° C, respectively.

division Sintering aid T _eu Sintering Granular strength Molding method YA series Y ₂ O ₃ -Al ₂ O _3- (SiO ₂ ) 1370 ^o C x weakness Tapping, Pressureless Molding YAC system Y ₂ O ₃ -Al ₂ O ₃ -CaO- (SiO ₂ ) 1170 ^o C O Strong Uniaxial, Extrusion

As a result of spray drying, coarse granules (50-250 μm) were formed under low speed atomizer (1,000 rpm), and the yield of granules was lower than 1% based on Si 100g batch. The yield was more than 40% at 10,000 rpm, and the size and recovery of granules increased with increasing batch and binder addition amount. 2)

Subsequently, SD4 (Y ₂ O ₃ : Al ₂ O ₃ = 2: 1; PVA 2 wt%) granules were classified by sieving and then examined by appearance using a scanning electron microscope (SEM). Figure 1 (a) is 45-63 μm of the particle size (called 'm54' as the median particle size), (b) the particle size 90-125 μm (m107.5) (c) particle size 125-150 μm (m137. It is classified as 5). It is observed from the photograph that the small granules remain spherical with little damage (Figs. 1 (a) and (b)), but the large granules are partially broken by the sieving impact (see arrow (c) in Fig. 1).

Figure 2 is an enlarged observation of the surface of the granules, it can be seen that it is composed of particles smaller than this compared to the average particle diameter of 2 μm of the starting Si. This is because not only the particle size of the added sintering aid is smaller than that of Si, but also the Si particles, which are the main raw materials, are also pulverized during the milling process, thereby decreasing the average particle size.

As a result of measuring the weight distribution according to the size of the granules after sieving, similar size distributions were shown without depending on the composition of the sintering aid and the amount of binder added under the same conditions of atomizer rotation speed (FIG. 3). As a result of sieving, most of the particles for each sample have a size of between 30 and 150 microns, and it can be seen that about 50% of the granules are present in the particle size of 90 μm or more and 106 μm or less.

Preparation of Plasticized Granule Powder

In general, the strength of the granules produced by the spray drying method is weak, 0.5 MPa or less. Therefore, it is impossible to maintain the spherical shape by breaking the granules in the usual pressure forming pressure range.

In the present invention, the sintered granule powder obtained by presintering the Si granule powder obtained above to secure molding strength.

The following two methods can be considered as a sintering method for improving the strength of Si mixed powder granules. First, sintering in granules can be induced through liquid phase sintering by heating above the process liquid phase temperature in an inert atmosphere (excluding nitrogen) to prevent oxidation of the raw powder. Second, the heat treatment in a nitrogen atmosphere may induce nitriding reactions to increase the strength in the granules. In the two sintering treatments, unintentional intergranular sintering occurs in parallel with the progress of the sintering of the desired granules, which may result in a case where it is difficult to maintain the spherical granular shape after grinding, and thus, temperature, time and Process control such as atmosphere is necessary.

The granular powder used in this example has a median particle diameter of 38.5 μm (32 to 45 μm, m38.5), 54 μm (m54), 76.5 μm (m76.5), and 107.5 μm (m107.5). As sieve-classified granules and unsieved granules (as-SD) containing all sizes were used.

(1) Plasticizing by Nitriding Reaction

High melting point YA-based granules (SD4; Y2O3: Al2O3 = 2: 1) were sintered by nitriding at 1300 ° C-6h. Since the process liquid phase temperature of the same system is 1370 ^° C, it is necessary to heat to 1370 ^° C or more in order to induce plasticization by liquid phase sintering. However, since the above temperature is a high temperature close to the melting point (1412 ^o C) of the main raw material Si, it was determined that stable process control would be difficult, so the nitriding reaction mechanism was selected to perform below the process liquid phase temperature. The nitriding reaction conditions described above induce a medium nitriding reaction to facilitate granule separation by grinding after plastic sintering. As a result, a nitriding rate of 65.6% was obtained.

However, the sintering in the granules as well as the sintering between the granules proceeded by nitriding and sintering, and it was observed that most of the spherical granules were broken in an angular form by grinding. Figure 4 (a) is a granules before plasticization, Figure 4 (b) is a photograph of the shape of the granules after sintering and grinding. The result of the grinding of the sintered granules was hydrostatically formed at a pressure of 100 MPa, and the cross section was polished and shown in FIG. 4C. Part of the granules retaining the spherical shape was found from FIG. 4C, and it was confirmed that the granules obtained by the sintering can maintain the shape of the granules even in subsequent high pressure molding.

(2) Plastic sintering in inert atmosphere

In the present invention, the sintering was performed in an inert atmosphere so that sintering between the granules does not occur by nitriding reaction during the sintering of the granulated powder. The sintering temperature was carried out below the melting point of Si.

The low melting point composition can be calcined by liquid phase sintering at a relatively low temperature, so that it is easy to separate the granules, and thus, it is determined that the calcined granule powder that maintains the spherical granular shape even after the sintering process can be obtained. For example, since the YAC-based process liquid temperature is 1170 ° C, the difference between the melting point of Si (1412 ^o C) and the heat treatment in an inert gas atmosphere such as Ar above the process liquid temperature excludes the nitriding reaction. Granules

The sintering of the liver is insignificant and the liquid phase sintering will proceed by the sintering aid in the granules.

The YAC-based sintering aid composition used in the present example was maintained in the weight ratio 'Y ₂ O ₃ : Al ₂ O ₃ = 2: 1' as shown in Table 4 below, and the amount of CaO added was Al ₂ O ₃ -SiO _2- The proportional relationship between Al ₂ O ₃ and CaO corresponding to the process liquid phase composition of the CaO ternary state diagram was maintained (Table 3).

Calcining the result of experiments to find as 1200 ° or higher temperature liquid phase process using a tube in an Ar atmosphere for conditions C, 10 min heat treatment at 1300 ^o C and 1350 ℃ were performed (each PG1, PG3, PG4). Also, in the 1200 ^o C 60 min conditions were also carried out (PG2).

10 is a result of observing the surface microstructure of the fresh granules and plastic granules. First, the plastic granules of 1200 ^o C ((b) is 10 min, (c) is 60 min) are observed to decrease the surface roughness by filling the pores between particles by the amorphous liquid phase generated between the particles. On the other hand, when the sintering temperature was raised to 1300 ^o C ((d)) and 1350 ^o C ((e)) it was confirmed that the particle coalescence is actively progressed by the progress of the liquid phase sintering, and the distinction between the particle aggregates becomes clear. As a result of phase analysis by XRD, h-phase (Y ₅ Si ₃ O ₁₂ N) was detected at the pre-sintering temperature of all conditions, and the interposition of the liquid phase was confirmed.

Powder flowability before and after sintering was performed according to JIS standard Z 2502-1979. Approximately 5 g of granule samples were kept in a 105 ^° C. dryer for 1 hour to remove moisture, cooled to room temperature in a desiccator, and then measured for flow through a 0.1 ”(2.54 mm) orifice in diameter to compare flow rates. It was. At this time, the flow rate of the calcined granules was measured without undergoing any grinding process after calcining.

The flow rates of fresh granules (as-SD), 1200 ^o C-10min calcined grains (PG1) and 1350 ℃ -10 min calcined grains (PG4) of m107.5 were 0.4136 g / sec and 0.4068, respectively. g / sec, 0.3180 g / sec. As can be seen from the measurement results, the flow rate of the calcined granules showed a level similar to that of the fresh granules, indicating that the sintering between granules was suppressed during the calcining process, thereby obtaining a powder easily separated from the granules. Able to know. The decrease in the flow rate of the PG4 plastic granules is believed to be due to the increase in surface roughness due to particle aggregation.

That is, according to the method of the present invention it can be seen that sintering between the granules is suppressed to obtain a powder that is easy to separate the granules.

The molding characteristics of the obtained plasticized granule powder are as follows.

Molding Behavior of Plastic Granule Powders

To determine the granular strength of raw granules and pre-sintered granules, after filling the granules by self-loading into a cylindrical mold with a diameter of 10 mm, the 'load-displacement' relationship at constant speed (0.5 mm / min) was measured ( 5) The 'molding density-molding pressure' relationship (FIG. 6) was calculated by substituting the mass of the granules used in the experiment.

Since the sudden increase in load observed at a certain displacement is due to the increase in molding density after fracture of the granules, it is known that the incremental load increase and the large displacement observed at the previous stage are due to the granular flow and granule deformation. In FIG. 5, it can be seen that the magnitude of the displacement of the biogranules observed at the weak load until the sudden increase of the load occurs is considerably larger than the displacement of the plastic granules, which is generally consistent with the known molding behavior. On the other hand, Figure 5, showing the 'load-displacement' curve diagram according to the sintering conditions, by the sintering performed for the same 10 min at 1200 ^o C, 1300 ^o C and 1350 ℃, the liquid sintering progress toward the latter Because of the large degree, a more steep 'load-displacement' curve was obtained. Similar behavior was observed when the sintering time was changed at 1200 ^o C, which means that the influence of the sintering temperature is greater than the sintering time.

In Fig. 6, the pressure of the inflection point means the yield strength at which the breakdown of the granules starts, and this inflection point becomes the starting point of the rapid increase in density. In the graph of FIG. 6, the yield strength of the raw granules is 0.2 MPa or less, the yield strength of 1200 ^o C calcined granules is about 5-6 MPa, and the yield strength of 1300 ^o C calcined granules is about 10 MPa, 1350 ° C. The yield strength of the granules was about 20MPa, which was increased by 25 ~ 100 times by plastic sintering. Although not shown separately in this example, the yield strength is expected to be obtained higher than the above values when using a sample having a sintering aid content of 3% by weight or more.

In order to compare the spherical stability of plastic granules after molding, scanning electron microscopy was performed on the surface of the granulated raw granules and plastic granules after uniaxial molding and then impregnated with resin. The granules (PG1) were calcined at 1200 ^o C-10 minutes, and the yield strength of the granules was measured at about 5-6 MPa in FIG. 6, and 3.7 MPa, which is expected to be stable in terms of uniaxial pressure, Four types were selected: 7.5 MPa, 18.6 MPa, and 46.6 MPa, which are expected to deform or break the granules.

As a result of the observation of the polished surface, the raw granules were completely broken in spherical shape under the conditions of 3.7 MPa (Fig. 7 (a)) and 18.6 MPa (Fig. 7 (b)). The spherical shape was completely maintained at (a) and (b) in FIG. 8, and it was found that deformation or fracture occurred at a molding pressure of 18.6 MPa or more (FIG. 8 (c) and (d)). Considering that the conventional uniaxial molding is performed at about 5 MPa and the extrusion is performed at about 8 MPa, the plastic granules developed in the present invention have sufficient strength to be applicable to the commercial molding process used in the industrial field. I think I will have.

Manufacture of molded articles for silicon nitride reaction sintering

As can be seen in the above embodiment, it is possible to produce a molded article having a pore structure in which coarse pores and fine pores coexist with the granular powder of the present invention.

9 is a view for schematically explaining the pore structure of the molded article according to the present invention. First, as shown in the left side, when the plasticized granulated powder produced by the present invention is uniaxially molded, extruded or injection molded, coarse pores of a predetermined size are formed between the plasticized powders. These pores are connected between the stacked powders to form pore channels.

When nitriding and post sintering the Si granulated powder compacts as described above, the sintering aid contained in the raw powder forms a liquid phase during the temperature raising process, and the generated liquid phase remains in the fine pores inside the powder in accordance with the principle of capillary tube Helps to sinter, but does not fill coarse pores. As a result, as shown in the figure on the right, it is possible to obtain a microstructure having coarse pores almost the same size as the shape of the molded body.

Eventually, the size of the coarse pores depends on the size of the plasticized granules. For example, assuming that the granules are composed of the same sized plasticized granules and the granules are stacked in the closest structure, the minimum size of the coarse pores is about 0.077 * D (D Is the powder diameter) and the pore diameter in terms of equivalent area is about 0.23 * D. In general, however, at low molding pressures, the size of the pores is larger, so that when the particle size of the granules is between 30 and 150 microns, it is possible to secure pore channels having coarse pores of 1 to 10 microns or more, whereas The silicon nitride is formed by the nitriding reaction, and as the sintering proceeds, microporous channels of less than 1 micron may be formed.

Therefore, the Si granule powder of the present invention can provide a porous silicon nitride sintered body having a fine structure in which coarse pores and fine pores coexist by reaction sintering and post sintering processes.

Post-sintering of the molded body

After applying appropriate amount of 5% PVA solution with PG1 (1200 ^o C-10min sintering) of YAC-based SD6 plastic granules, uniaxial molding was performed at a pressure of 3.7 MPa below the yield strength of the granules. After molding, it was dried in a drier at 105 ^° C. for 24 hours.

The dried specimen was subjected to nitriding for 2 hours in a fluidized nitrogen atmosphere mixed with hydrogen and post-sintered in a 0.1 to 0.9 MPa nitrogen atmosphere to prepare a porous body.

In order to determine the optimum post-sintering temperature, the porosity, shrinkage rate and weight loss of each sintered body of the uniaxial molded body using m76.5 plastic granules were measured and shown in FIG. 11. The porosity decreased due to the increase of shrinkage with increasing sintering temperature. In spite of the pressurized atmosphere, the weight loss of around 7% was measured at sintering above 1800 ^o C.

Figure 12 (a), (b) and (c) is a low magnification (x 300) of the fracture surface of the post-sintered specimen at 1700 ^o C, 1800 ^o C and 1900 ^o C as m76.5 plasticized granules, respectively It is a photograph. In the case of sintering at a temperature of 1800 ^o C or more ((b) and (C)), silicon nitride particles grown in a whisker phase in the void space between the granules are clearly observed as compared with the case of sintering at 1700 ^o C ((a)). .

(A), (b) and (c) of FIG. 13 are photographs observed at high magnification (x 10k) inside the granulated area of each specimen of FIG. In all specimens, microstructures with highly developed needle-like particles inherent in silicon nitride can be seen.

(A) and (b) of FIG. 14 are graphs showing pore distribution and pore specific surface area measured by mercury porosity analysis of the post-sintered porous bodies, respectively. It can be seen that as the sintering temperature increases, the frequency of the micropores decreases and the size of the coarse pores remains almost the same. In general, liquid phase sintering is known to promote densification and grain growth because the amount of liquid phase increases and viscosity decreases at higher temperatures. In the present invention, micropores are also produced because pore growth occurs simultaneously with grain coalescence and grain growth in the granule region. The size of is estimated to have increased.

On the other hand, as shown in Figure 14 (b), the pore specific surface area measurement results show a tendency to match with Figure 14 (a). That is, although the specific surface area due to coarse pores between granules is similar regardless of the sintering temperature, FIG. Is showing.

On the other hand, in the case of post-sintered specimen at 1900 ^o C, the specific surface area decreases rapidly due to excessive shrinkage.

15 is a graph measuring the aeration performance of the above specimen sintered at 1700 ℃ and 1800 ℃. Porous Materials Inc. model name CFP-1200-AEL was used as the measurement equipment. As a result, similar permeability constants (κ = 0.28 ~ 0.30x10 ¹² m ² ) were measured in two specimens with different sintering temperatures, which means that the size of the coarse pore channels dominating the air permeability is constant regardless of the sintering temperature. It is because of this.

Figure 16 (a) and (b) is a graph measuring the porosity and shrinkage (weight loss rate) for the sintered test pieces after varying the granule size by classification, respectively. Where as-SD is the post-sintered specimens of non-classified calcined granules, m38.5, m54, m76.5 and m107.5, respectively, refer to the median particle diameters of the classified granules. Moreover, the sintering temperature of these test pieces was 1700 degreeC. As a result, similar porosity was measured regardless of the granule size, and the porosity was about 50% and slightly decreased compared with the reaction sintered body, which is presumably due to the shrinkage exceeding the weight reduction effect.

17 is a photograph of fracture surface by granule size for each post sintered specimen of FIG. 16. In the case of small m38.5 and m54 specimens, the pore channel between the granules tends to be blocked by acicular silicon nitride particles. Therefore, it can be expected that the air permeability of the uniaxial porous body having a small granule size will decrease.

FIG. 18 is a photograph of the polished surface photographed after impregnating resin in each post-sintered specimen of FIG. 16. In m38.5 specimens, the intergranular boundary is unclear, which is presumed to block the channel between the granules due to silicon nitride particles generated between the granules and the deformation of the granules by uniaxial pressure, m76.5 and m107.5 It can be seen that the increase in the granule size, such as the specimen, is not observed.

19 is a graph showing the results of analyzing each post-sintered specimen of FIG. 16 by the mercury porosity apparatus. The graph depicted shows a bimodal distribution in which micropore peaks of less than 1 μm and coarse pore peaks of 1 μm or more coexist. However, in the case of the smallest m38.5 specimen, the peak corresponding to the coarse pore channel was not clearly observed, which indicates that the channel was blocked by the reaction product and the granule deformation. In addition, the largest coarse pore channel size and volume fraction were measured in m76.5 specimens rather than m107.5 specimens. This is presumed to cause partial coagulation of intergranular pores due to the breakage of some of the coarse granules during uniaxial molding of the m107.5 specimen.

20 is a graph showing the measurement of the ventilation performance of each specimen. The m76.5 specimen showed the best breathability, and the smaller the granule size, the lower the breathability.

The reaction-sintered silicon nitride sintered body post-sintered by the present invention as described above has the same fine structural features as described in FIG. 9. This is because the plasticized granule powder produced by the present invention has sufficient yield strength despite the press molding. As a result, coarse pores of a predetermined size are formed and maintained between the granule regions constituting the sintered body, and the coarse pores can maintain their shape despite the application of the post-sintering process. The pore channel size varies depending on the molding pressure and the particle size distribution of the granules. In the present invention, a sintered body having coarse pore channels of 10 microns or more can be produced.

On the other hand, the fine pore channel is formed in the granulated region of the sintered body of the present invention, it is possible to provide a porous silicon nitride sintered body having a microstructure in which coarse pores and fine pores coexist.

Claims (12)

Granulating a raw material comprising silicon and a sintering aid for producing a silicon nitride sintered body from the silicon;
Presintering the granulated raw material in an inert atmosphere;
Preparing a molded body by press molding the pre-sintered granule raw material; And
Pre-sintering the molded body in a mixed gas atmosphere containing hydrogen and nitrogen and post-sintering under a nitrogen atmosphere,
The sintering aid is a method for producing a porous reactive silicon nitride sintered body comprising at least one alkaline earth metal oxide. The method of claim 1,
Said sintering aid comprises yttria and alumina, The method for producing a porous reactive silicon nitride sintered body. delete The method of claim 1,
The post-sintering step is a method for producing a porous reaction silicon nitride sintered body, characterized in that carried out at a temperature of 1700 ~ 1900 ℃. The method of claim 1,
The content of the sintering aid is a method for producing a porous reactive silicon nitride sintered body, characterized in that 2 to 6% by weight based on the complete nitriding of the silicon. The method of claim 1,
Method for producing a porous reaction silicon nitride sintered compact characterized in that the molded by the granules in the pressure molding step under pressure of 1 ~ 15MPa. A silicon nitride sintered body having an array of granular regions having fine pore channels therein and having coarse pore channels formed between the granular regions, the composition of which comprises an alkaline earth metal compound as a sintering aid Porous reaction sintered silicon nitride sintered body. The method of claim 7, wherein
The pore distribution of the reactive sintered silicon nitride sintered compact exhibits a bimodal distribution having a first peak and a second peak having a larger pore size than the first peak, wherein the first peak is attributable to the fine pore channel. 2 peak is due to the coarse pore channel porous porous silicon nitride sintered body characterized in that. The method of claim 8,
The first peak is at a pore size in the range of less than 1 micron, and the second peak is at a pore size of 1 micron or more.
Porous reaction sintered silicon nitride sintered body, characterized in that. The method of claim 8,
The first peak is at a pore size of less than 1 micron, and the second peak is at a pore size of 5-15 microns.
Porous reaction sintered silicon nitride sintered body, characterized in that. The method of claim 7, wherein
The granular sintered zone is porous reactive silicon nitride sintered body characterized in that the average diameter is in the range of 30 ~ 150 microns. The method of claim 7, wherein
Porous silicon sintered porous reaction, characterized in that the maximum frequency of the granular sintered zone is present in the range of 50 ~ 150 microns.

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