KR101729650B1 - Mullite ceramic and method for producing same - Google Patents

Abstract

The mullite ceramics according to the present invention comprises mullite spherical particles having an aspect ratio of not less than 1 and not more than 2 and mullite needle particles having an aspect ratio of not less than 2 but not more than 10 in a polished cross section and the average long diameter of the needle- Is 2 to 10 times the average particle diameter, and the area ratio of needle-like particles / total particles is 0.03 to 0.3. It is preferable that the ratio of the area of the coarse needle-shaped particles having an aspect ratio of more than 10 to the total area of the particles in the polished cross section is 0.2 or less. It is also suitable that the apparent porosity is 5 to 27%.

Description

MULLITE CERAMIC AND METHOD FOR PRODUCING SAME

The present invention relates to mullite ceramics and a process for producing the same. The mullite ceramics of the present invention is particularly useful as a refractory (refractory) such as, for example, a firing jig and a member for building a furnace.

As a conventional technique related to mullite ceramics, there has been known a mullite-like porous body having improved creep resistance and spalling resistance (see Patent Document 1). This porous body is formed by bonding mullite crystals and agglomerates thereof via a bonding phase (silica phase) in which silica is a main component. This porous article is obtained by baking a mixture of alumina powder and silicon carbide powder in the range of 1550 ° C to 1700 ° C under an oxidizing atmosphere.

Patent Document 2 describes a reactionally sintered mullite-containing ceramics formed body obtained by heat-treating a base material molded from a powder mixture of a finely dispersing powder comprising aluminum, Al ₂ O ₃ and a Si-containing material in an oxygen-containing atmosphere. This mullite-containing ceramics formed article is described in that the firing shrinkage is small.

JP2003-137671A US5843859A

There is a demand for a kiln tool which is thin and light in weight and has high strength, creep resistance and thermal shock resistance to some extent for the purpose of energy saving and low cost in firing ceramics including electronic parts made of ceramics. However, although the mullite ceramics described in Patent Document 1 described above has high creep resistance and high heat shock resistance, it is not satisfactory in terms of strength. In addition, the mullite ceramics described in the above-mentioned Patent Document 2 has a low firing temperature of less than 1,700 DEG C, so that a single phase of mullite can not be obtained and the creep resistance is lowered when the thickness is reduced.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a mullite ceramics capable of overcoming various drawbacks of the above-described conventional techniques.

The present invention relates to a spherical particle of mullite having an aspect ratio of 1 or more and 2 or less and a needle-shaped particle of mullite having an aspect ratio of 2 or more and 10 or less in the polished cross- Wherein the long diameter is 2 to 10 times the average particle diameter of the spherical particles, and the area ratio of the needle particles / total particles is 0.03 to 0.3.

The present invention also provides a method for producing a mullite ceramics,

Characterized by reacting and sintering a raw material containing alumina, silica and an Si or Si-containing compound (excluding silica and silicate) having an average particle diameter of 0.1 to 10 μm under an oxygen-containing atmosphere at 1700 to 1800 ° C to produce mullite And a method for producing the mullite ceramics.

According to the present invention, there is provided a mullite ceramics having excellent thermal shock resistance, creep resistance and high strength. Conventionally known mullite ceramics are bricks, and it is difficult to make the mullite ceramics thin enough to withstand practical use due to lack of strength. The mullite ceramics of the present invention can cope with a room temperature strength of more than 50 MPa and a thickness of 1.5 mm or less, thereby obtaining a sintered body having a minimum thickness of 0.5 mm.

1 is a scanning electron micrograph of a polished section of the mullite ceramics obtained in Example 1. Fig.
2 is a scanning electron micrograph of the polished section of the mullite ceramics obtained in Comparative Example 2. Fig.

Hereinafter, the present invention will be described based on preferred embodiments thereof. The mullite ceramics of the present invention have one of the features in the shape of the particles constituting the ceramics. Specifically, when the polished section of the mullite ceramics is expanded by a microscope, spherical particles of mullite (3Al ₂ O ₃ .2SiO ₂ ) and needle-like particles of mullite are mixed. Spherical particles and needle-shaped particles are mixed uniformly. The inventors of the present invention have found that mullite ceramics in which spherical particles and needle-like particles are mixed together have high heat shock resistance and excellent creep resistance. Here, the spherical particles refer to particles having an aspect ratio of not less than 1 and not more than 2 when the polished cross-section of the mullite ceramics is observed (hereinafter referred to as " spherical " Same as this). On the other hand, the needle-like particles mean particles having an aspect ratio of not less than 2 but not more than 10 when the polished cross-section of the mullite ceramics is observed. The coarse needle-shaped particles to be described later refer to particles having an aspect ratio exceeding 10 when the polished cross-section of the mullite ceramics is observed.

Further, needle-shaped particles and coarse needle-like particles to be described later may be perceived as spherical particles in spite of being actually needle-shaped particles depending on the method of preparing the polished section of mullite ceramics. The needle-shaped particles recognized as such apparent spherical particles are referred to as spherical particles for convenience in the present invention.

The relationship between spherical particles and needle-like particle size affects the performance of mullite ceramics. As a result of the studies conducted by the present inventors, it has been found that mullite ceramics having the above-described thermal shock resistance and creep resistance can be obtained when the average particle diameter of the spherical particles is r and the average long diameter of the needle-shaped particles is in the range of 2r to 10r. If the average long diameter of the needle-like particles is less than 2r, even if the aspect ratio of the needle-shaped particles is large, the "brace effect" in which the needle-shaped particles enter the spherical particles is not sufficiently developed and the creep resistance is not improved. On the other hand, when the long diameter of the needle-like particles exceeds 10 r, coarse defects easily occur between the particles in the mullite ceramics. This coarse defect is a cause of deterioration in thermal shock resistance. When the range of the long diameter of the needle-shaped particles is in particular 3 r to 6 r, the thermal shock resistance and creep resistance of the mullite ceramics are further improved.

As described above, the relative sizes of the spherical particles and the needle-like particles are preferably such that the spherical particles themselves have an average particle diameter of 5 to 10 탆, particularly 6 to 9 탆. On the other hand, it is preferable that the size of the needle-shaped particles themselves is 10 to 100 탆, particularly 12 to 90 탆, provided that the size itself satisfies the above-mentioned range of 2 to 10 r. On the other hand, it is preferable that the short diameter is 1 to 50 탆, particularly 1 to 10 탆, provided that the aspect ratio is the above-mentioned aspect ratio (2 or more and 10 or less).

As for needle-like particles, as described above, the aspect ratio is more than 2, and the upper limit thereof is required to be 10 or less. In other words, it is preferable that needle-shaped particles having an aspect ratio of more than 10 (hereinafter, such needle-like particles are referred to as "coarse needle-like particles") are not excessively contained. The presence of these coarse-grained particles causes coarse defects between the particles in the mullite ceramics. This coarse defect is one factor of the deterioration of the thermal shock resistance. The needle-shaped particles have an effect of improving the creep resistance, but the needle-shaped particles having an aspect ratio of more than 10 have no effect. From this viewpoint, it is preferable to set the ratio of the area of the coarse needle-like particles having an aspect ratio of more than 10 to the area of the whole particles in the polished cross-section of the mullite ceramics to 0.2 or less, especially 0.1 or less.

In addition to the relative sizes of spherical particles and needle-shaped particles, the presence of spherical particles and needle-like particles in the mullite ceramics also affects the performance of the mullite ceramics. When the needle-like particles enter the spherical particles, a "brace effect" occurs to improve the creep resistance. However, the strength of the needle-shaped particles decreases due to grain growth, and the thermal shock resistance tends to decrease. On the other hand, as a result of the examination, the ratio of the spherical particles to the needle-shaped particles was adjusted such that the area ratio of the needle particles / the whole particles in the polished cross section of the mullite ceramics was in the range of 0.03 to 0.3, Mullite ceramics was obtained. When the area ratio is particularly in the range of 0.05 to 0.25, the thermal shock resistance and creep resistance of the mullite ceramics are further improved.

The abrasive cross section of the mullite ceramics in the above description can be obtained, for example, by rotating a disk grindstone sprayed with a diamond slurry, and pressing the surface with mullite ceramics and polishing.

In order to observe the polished cross section of the mullite ceramics obtained as described above, enlarged observation is performed using, for example, a scanning electron microscope (SEM). The measurement of the average particle diameter of the spherical particles and the measurement of the average long diameter and the average short diameter of the needle-like particles and the coarse needle-shaped particles are performed as follows. The size of the polished section is at least 10 mm x 2 mm, and an SEM image of an arbitrary portion is photographed at several points in an observation field of 200 m x 200 m in the polished section. An arbitrary straight line is drawn on the photographed image, and 100 particles crossing the straight line are selected. If the number of particles does not reach 100, repeat this operation until the number of particles crossing 100 is reached. The aspect ratio is calculated by measuring the long diameter and the short diameter of each selected particle. Concretely, the object particle is approximated to an ellipse, and the length of the major axis of the ellipse is measured. The length of the major axis is set to be the long diameter, and the direction orthogonal to the major axis is shortened. Spherical particles having an aspect ratio of not less than 1 and not more than 2 are classified into acicular particles, particles having an aspect ratio of not less than 10 and not more than 10 are classified as coarse acicular particles based on the thus obtained long axis and short axis. In the case of spherical particles, the average value of the long diameter and the short diameter is taken as the average particle diameter. In the case of acicular particles and coarse acicular particles, the average long diameter and the average short diameter are obtained by averaging the long diameter and the short diameter separately.

The area of the spherical particles in the polished section is calculated by considering the average particle diameter obtained by the above-mentioned method as the circle equivalent diameter. The area of the needle-like particles is calculated from the area (? Ab) of the ellipse by considering the average long diameter (a) and the average short diameter (b) of the needle-shape particles obtained by the above-described method as the long and short diameters of the ellipse.

When observing the polished cross-section of the mullite ceramics, it is preferable that only the spherical particles and the needle-like particles described above are observed on the polished cross section, but particles having shapes other than these spherical particles may be observed. Examples of the particles having such a shape include particles having a sharp-pointed shape. Specific examples of the particles include molten (molten) mullite particles, which will be described later, which are particles used as one of raw materials. The molten mullite particles are produced by pulverizing a molten mullite mass. This grinding produces molten mullite particles having a sharp-pointed shape.

It is preferable that the mullite ceramics include certain pores. The apparent porosity is preferably in the range of 5 to 27%, particularly 9 to 20%. By setting the apparent porosity in this range, it is possible to effectively balance the creep resistance and the thermal shock resistance. The apparent porosity is measured by a vacuum method in accordance with JIS-R2205.

It is also preferred that mullite ceramics do not contain coarse pores. The coarse pores refer to pores having a long diameter of 5 times or more of the average diameter of the needle-shaped particles among the pores observed in the polished section. The presence of such coarse pores may cause deterioration of the strength, thermal shock resistance and creep resistance of the mullite ceramics. From this viewpoint, in the polished cross section of the mullite ceramics, the ratio of the sum of the areas of the coarse pores to the area of the observation field (that is, the sum of the areas of the coarse pores / the area of the viewing field) is preferably 0.07 or less Is not more than 0.05. In order to suppress the formation of coarse pores, the grain size and firing conditions of the raw material components may be adjusted in a method of producing mullite ceramics to be described later. The long diameter of the coarse pores refers to the average value of the long diameter and the short diameter when the aspect ratio of the coarse pores is not less than 1 and not more than 2, and when the aspect ratio of the coarse pores is more than two, it refers to the long diameter.

The area of the coarse pores is set to be at least 10 mm x 2 mm in terms of the polished cross section and the SEM image of an arbitrary portion is photographed in an observation field of 200 m x 200 m in the polished cross section, The area of all coarse pores is calculated by the same method as the calculation of the area of the needle-shaped particles and the area of the spherical particles described above.

The mullite ceramics may contain an oxide of an alkali metal, such as Na ₂ O, or an oxide of a net-like oxide such as an oxide of an alkaline earth metal, as an impurity. Since these impurities lower the viscosity of the glass in the grain boundaries and affect the creep resistance, it is preferable to use a high purity raw material so that the total amount thereof is 0.01 to 0.3% by weight, particularly 0.03 to 0.25% by weight, based on the mullite ceramics Do. The ratio of the network-modified oxide contained in the mullite ceramics can be measured by a fluorescent X-ray analyzer. Further, the intermediate oxides such as Fe ₂ O ₃ , TiO ₂ , ZrO ₂ , CoO, and NiO stabilize the framework of the glass mesh to suppress the decrease in viscosity of the glass and contribute to improvement in creep resistance. By weight based on the total weight of the composition.

Next, a suitable method of the mullite ceramics of the present invention will be described. The mullite ceramics can suitably be produced by reactively firing a raw material containing alumina and silica in an oxygen atmosphere to produce mullite. Particularly, in order to produce spherical particles and needle-shaped particles in the aimed mullite ceramics, it is effective to use Si or a Si-containing compound (except for silica and silicate) in addition to alumina and silica. (In the following description, Si or a Si-containing compound is simply referred to as a Si-containing compound for the sake of simplicity.) Specifically, it has been found that by substituting a part of silica with a Si-containing compound in the reaction and sintering of alumina and silica, it is possible to provide a difference in production rate of mullite, . Specifically, when a part of the silica is substituted with a Si-containing compound, needle-shaped particles of mullite grow locally at the time of reaction sintering, while spherical particles located around the needle-shaped particles have a low degree of grain growth. As a result, mullite ceramics having high creep resistance and high thermal shock resistance can be easily obtained. When the reaction and sintering are carried out using only alumina and silica without using the Si-containing compound, the coarsening of the spherical particles takes precedence over the growth of the needle-shaped particles. As a result, the resulting mullite ceramics cause a decrease in strength and deteriorate thermal shock resistance. Further, since the growth of the needle-like particles is small, the "brace effect" hardly occurs and the creep resistance deteriorates.

The Si-containing compound is used for producing needle-shaped particles of mullite during reaction sintering in the production of mullite ceramics, as described above. As the Si-containing compound, those known as ceramics materials are used. Examples thereof include inorganic Si-containing compounds. Examples of the inorganic Si-containing compound include Si-containing non-oxide compounds. Specifically, SiC, Si ₃ N ₄ -based materials such as Si ₃ N ₄ , Si ₂ ON _2, and SiAlON can be given. SiAlON is one of Si ₃ N ₄ based material obtained by employing an Al ₂ O ₃ and SiO ₂ to Si ₃ N _4. The Si-containing compound is oxidatively expanded during firing in the production of mullite ceramics, and thus has an effect of compensating firing shrinkage of alumina or silica. As a result, the coarsening of the pores occurring at the time of shrinking and the progress of the cracks which may occur in the ceramics are suppressed, and further the lowering of the thermal shock resistance is suppressed.

The ratio of each component in the raw material is determined in consideration of the amount ratio of alumina and silica in the objective mullite ceramics. Specifically, when each component in the raw material is classified into alumina, which is an Al-containing compound, silica which is a compound containing Si, and a Si-containing compound, the ratio of alumina to silica and the Si- Silica is 3: 2 to 3.5: 1.5, particularly 3.1: 1.9 to 3.4: 1.6 in terms of the molar ratio of silica.

The ratio of the silica and the Si-containing compound in the raw material is preferably in terms of the molar ratio of Si to the silica: Si-containing compound = 0.1: 1.9 to 1.9: 0.1, particularly 0.5: 1.5 to 1.5: 0.5. By using the silica and the Si-containing compound at this ratio in combination, the needle-shaped particles can be grown while preventing the coarsening of the spherical particles. In addition, it is possible to effectively prevent the crystallinity of the structure from deteriorating due to the heat resistance of the Si-containing compound, and to effectively increase the pore and deteriorate the creep resistance.

As alumina which is one of raw materials,? -Alumina and? -Alumina are suitably used. There is no problem in using these mixtures. The shape of the alumina particles is not particularly limited, and various shapes known in the art can be used. Particularly preferably used shape is spherical. Regardless of the shape, alumina preferably has an average particle diameter of 0.1 to 20 μm, particularly 1 to 10 μm. It is preferable that the alumina does not contain alkali components such as Na and K at the maximum.

The shape of the particles of the silica is not particularly limited, and various shapes known in the art can be used. Particularly preferably used shape is spherical. Regardless of the shape, the silica preferably has an average particle diameter of 0.05 to 30 탆, particularly 0.1 to 20 탆.

The average particle diameter of alumina and silica is measured using, for example, a laser diffraction particle size distribution measuring apparatus (the same applies to the Si-containing compound described below and the mullite particles described below).

The particle size of the particles of the Si-containing compound affects the performance of the desired mullite ceramics. In detail, when the particle diameter of the Si-containing compound is too large, a large defect tends to occur in the structure of the obtained mullite ceramics. It can not be sufficiently oxidized, so that a single mullite composition is difficult to be obtained and the creep resistance may deteriorate. Further, the strength is lowered and the thermal shock resistance is likely to be lowered. On the other hand, if the particle diameter of the Si-containing compound is too small, the oxidation of the Si-containing compound tends to start in the low-temperature region, and generation of acicular mullite particles is hardly promoted. From these viewpoints, the average particle diameter of the particles of the Si-containing compound is set to 0.1 to 10 탆, preferably 1 to 10 탆. This advantageous effect is remarkable when SiC is used as the Si-containing compound. As for the shape of the particles of the Si-containing compound, it is preferable to use a spherical one.

The above-mentioned raw material may contain mullite particles as an aggregate. By containing the mullite particles in the raw material, it is possible to delay the progress of the cracks, which may be caused by thermal shock, by the detouring effect, so that an advantageous effect that the mullite ceramics can be used for a longer time is obtained. If the particle diameter of the mullite particles contained in the raw material is too large, coarse pores are likely to be formed in the mullite ceramics, which may lower the strength and thermal shock resistance of the mullite ceramics. Thus, it is preferable that the average particle diameter of the mullite particles contained in the raw material is 20 to 100 탆, particularly 20 to 50 탆. When the raw material contains mullite particles, the content thereof is preferably 15% by weight or less, more preferably 10% by weight or less, in the raw material, from the viewpoint of improvement of strength and thermal shock resistance.

The raw materials containing the respective components described above are mixed and subjected to reaction firing to obtain the intended mullite ceramics. Mixing of the respective components may be performed by a known mixing method in the related art such as wet mixing, semi-wet mixing, dry mixing and the like. From the viewpoint of ensuring the reaction sintering, it is advantageous to perform wet mixing or semi-wet mixing rather than dry mixing.

When wet mixing is performed, alumina, silica, and a Si-containing compound are wet-mixed using a liquid medium to form a slurry. The obtained slurry is subjected to casting or spray-drying the slurry, and the obtained granules are subjected to press molding or CIP molding, followed by reaction firing. As a device used for wet mixing, a known mixing kneader, for example, a media mill such as a ball mill may be used. The solid content concentration in the slurry may be preferably about 35 to 45% by weight. A binder may be added to the slurry. As the binder, for example, polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC), which are usually used in the technical field, can be used without particular limitation. The molding pressure for press molding or CIP molding is preferably set to about 70 to 150 MPa.

In the case of semi-wet mixing, alumina, silica and a Si-containing compound are formed into a semi-fluid by using a liquid medium, and the mixture is kneaded to obtain a mixed kneaded product. The solid content concentration in the mixed batter may be preferably about 10 to 15% by weight. The mixed dough is molded into a desired shape by plastic molding such as extrusion molding.

Even in the case of using any of the above-mentioned molding methods, the atmosphere of the reaction firing is an oxygen-containing atmosphere such as air. The temperature of the reaction firing is preferably set to 1700 to 1800 ° C, particularly 1730 to 1790 ° C. It is preferable that the firing temperature is maintained for 1 to 8 hours, particularly 2 to 7 hours by firing at this temperature range. By carrying out the reaction firing in this condition, it is possible to prevent the increase of pores in the ceramics and smoothly obtain mullite ceramics having good creep resistance.

The average rate of temperature rise when the temperature is raised to the above-mentioned firing temperature is set to 25 to 300 ° C / h, preferably 30 to 200 ° C / h, in the temperature range of 900 to 1700 ° C. By setting the average temperature raising rate in this range, it is possible to prevent the increase of pores in the ceramics and smoothly obtain mullite ceramics having good creep resistance. If the temperature raising rate does not reach 25 占 폚 / h, the oxidation expansion is completed before the sintering starts, the intergranular distance increases, the sinterability decreases, and the pore increases. On the other hand, if the heating rate is higher than 300 ° C / h, the sintering is completed before the oxidation is completed, and the nonoxidized material tends to remain in the sintered body, which is a factor for lowering the creep resistance of the mullite ceramics.

The atmosphere of the firing can be an atmospheric oxygen-containing atmosphere having an oxygen concentration of about 20% as described above. However, when the temperature of the atmosphere is 900 ° C or higher, the oxidation of the Si- and Si- It is preferable to lower the oxygen concentration in the oxygen-containing atmosphere to 3% or less. By lowering the oxygen concentration to 3% or less, it is possible to prevent the oxidation expansion from being completed before the sintering is started, thereby effectively preventing the increase of the intergranular distance, the sintering property and the increase of the pore. On the other hand, it is preferable that the lower limit value of the oxygen concentration in the oxygen-containing atmosphere is set to more than 0.5% irrespective of the temperature of the atmosphere. By doing so, the sintering can be prevented from being completed before the oxidation is completed, whereby it is possible to effectively prevent the non-oxidized material from remaining in the sintered body and the creep resistance of the mullite ceramics to deteriorate.

The mullite ceramics thus obtained can be used in a variety of applications such as, for example, kiln tools, sidewalls, arches, furnace hearths in a high-temperature kiln furnace and an atmospheric furnace; Setter for sintering of electronic components such as internal bricks, sagger, board; Various chemical reaction devices including gas generating furnaces; A ceramic substrate; A built-in oven for a carbide furnace; Carbon black furnace interior; Built-in glass melting furnace; Ceramics firing kiln tools and the like.

<Examples>

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the scope of the present invention is not limited to these embodiments. Unless otherwise stated, "%" means "wt% ".

[Example 1]

Alumina (average particle diameter 8 μm, spherical shape), SiC (average particle diameter 4 μm, spherical shape) and silica (average particle diameter 0.5 μm, spherical shape) were weighed so that the molar ratio after oxidation was 3: 0.5: 1.5. The ratio of these components in terms of weight ratio is as shown in Table 4 below. These components were wet mixed to obtain a slurry (solid content concentration: 40%). A PVA aqueous solution was used as a liquid medium for wet mixing. This slurry was spray-dried to obtain granules having an average particle diameter of 50 mu m. The granules were press-molded to obtain a plate-shaped molded article. The press pressure was set at 100 MPa. The formed body was subjected to reaction firing at 1750 ° C for 4 hours in an atmospheric environment. The rate of temperature rise was 40 ° C / h. After completion of the firing, the mixture was naturally cooled to obtain the objective mullite ceramics. The scanning electron micrograph of the polished section of the obtained mullite ceramics is shown in Fig. The obtained ceramics was mullite single phase, and the SiC charged as a raw material was completely oxidized to mullite, and in particular, was a sintered body excellent in creep resistance. It has a strength that can be satisfied even when the thickness is reduced, and the heat shock resistance is excellent.

[evaluation]

The obtained mullite ceramics were subjected to XRD measurement to identify the compounds contained therein. Further, the polished section of the mullite ceramics was prepared by the above-mentioned method, and the cross section was observed by SEM. The apparent porosity was also measured by the method described above. Further, the content of the mesh oxide oxide was measured by the above-mentioned method. Three-point bending strength (S) at room temperature, creep resistance and thermal shock resistance were measured by the following methods. The results are shown in Table 1.

[Bending strength at room temperature three points (S)]

And measured by a three-point bending test according to JIS R1601.

[Creep resistance]

A specimen processed to have a size of 100 mm x 30 mm x 2 mm was placed on a support so as to have a span of 90 mm and a load of 300 g was applied to the center. The amount of warping after heating at 1400 캜 for 12 hours was measured by a depth gauge, and this value was regarded as an index of creep resistance.

[Thermal shock resistance]

□ Four specimens processed to 90 mm × 2.5 mm were produced. Separately, a strut having a length of 10 mm, a width of 5 mm and a height of 5 mm was prepared, and the struts were placed on a ceramic base plate. The position of the supporting post was set to a position of four corners of □ 90 mm. The above test specimens were piled up in four stages on the support. Each of the specimens was placed in the same four corners of the specimen. Next, the electric furnace was heated to a predetermined temperature and maintained for 30 minutes, and then the above-mentioned test specimen was placed in a furnace as a main plate. After maintaining the temperature at that temperature for 60 minutes, the test piece was taken out of the furnace as a main plate and allowed to cool. It was visually confirmed whether or not the specimen was cracked or cut (torn). The above operation was performed by raising the temperature by 500 deg. C at 50 deg. C, and the upper limit of the temperature at which cracking did not occur was measured, and the value was taken as an index of thermal shock resistance.

[Examples 2 to 4]

In Examples 2 to 4, mullite ceramics were obtained in the same manner as in Example 1 except that the conditions shown in Table 1 below were used. Table 1 shows the results of the evaluation conducted in the same manner as in Example 1. [ In Example 2, SiC and silica were mixed at a mixing ratio different from that in Example 1 to grow needle-like crystals, and the same high creep resistance as in Example 1 was obtained. The thermal shock resistance was also increased as in Example 1. In Example 3, the raw material particle diameter of SiC was made smaller than that in Example 1. [ The creep resistance and the thermal shock resistance of Example 3 were lower than those of Example 1, but the characteristics were determined to be endurable to practical use. Compared with Comparative Example 5 described later, it exhibited high creep resistance, high thermal shock resistance and high strength. In Example 4, the raw material particle size of SiC was larger than that in Example 1. Compared with Example 1, Example 4 has a pore diameter larger than that of Example 1, which is similar to Comparative Example 4 to be described later, so that it is judged that the product has thermal shock resistance that can withstand practical use although its thermal shock resistance is lowered.

[Example 5]

Using the raw materials shown in the following Table 1, a slurry (solid content concentration: 42%) was obtained by wet mixing. A CMC aqueous solution was used as a liquid medium for wet mixing. This slurry was subjected to cast molding to obtain a plate-shaped molded article. The formed body was subjected to reaction firing at 1750 ° C for 4 hours in an atmospheric environment. The rate of temperature rise was 40 ° C / h. After completion of the firing, the mixture was naturally cooled to obtain mullite ceramics having high creep resistance and high heat shock resistance. Table 1 shows the results of the evaluation conducted in the same manner as in Example 1. [

[Examples 6 to 9]

Mullite ceramics were obtained in the same manner as in Example 1 except that the conditions shown in the following Table 1 were used. Table 1 shows the results of the evaluation conducted in the same manner as in Example 1. [ In Example 6, the firing temperature was lowered to 1700 占 폚 as compared with Example 1. Example 6 was judged to have practically acceptable properties although the thermal shock resistance was lower than that of Example 1. And exhibits high creep resistance, high thermal shock resistance, and high strength as compared with Comparative Example 6. In Example 7, the sintering temperature was increased to 1800 占 폚 as compared with Example 1. Example 7 was judged to have practically acceptable characteristics although the thermal shock resistance was lower than that of Example 1. In Example 8, the rate of temperature rise was slower than that in Example 1. Example 8 was judged to be a property that could withstand practical use although the thermal shock resistance was lower than that of Example 1. In addition, compared with Comparative Example 7 described later, since the porosity was lowered, high strength and high creep resistance were exhibited, and porosity was reduced, resulting in reduced pore diameter and high impact resistance. In Example 9, the heating rate was faster than that in Example 1. Among all the examples, the results showed high properties in both heat shock resistance and creep resistance.

[Example 10]

Using the raw materials shown in Table 1 below, a mixed kneaded product having a solid content concentration of 13% was obtained with an aqueous CMC solution. A mixing agitator was used to prepare the mixed batter. The mixed kneaded product was extruded and molded to obtain a plate-shaped molded body. The formed body was subjected to reaction firing at 1750 ° C for 4 hours in an atmospheric environment. The rate of temperature rise was 40 ° C / h. After completion of the firing, the product was naturally cooled to obtain a mullite ceramics having thermal shock resistance and creep resistance capable of withstanding practical use. Table 1 shows the results of the evaluation conducted in the same manner as in Example 1. [

[Examples 11 to 14]

Mullite ceramics were obtained in the same manner as in Example 1 except that the conditions shown in the following Table 2 were used. Specifically, in Example 11, the press forming pressure was lowered to 70 MPa. In Example 12, the firing temperature was raised to 1800 占 폚 while the press forming pressure was reduced to 30 MPa. In Example 13, a raw material having a composition shown in Table 4, that is, a raw material containing 10% of molten mullite particles of 220 mesh was used. In Example 14, a high-purity raw material having a low content of an alkali metal oxide such as Na ₂ O was used as a raw material for mullite ceramics. Table 2 shows the results of evaluations carried out in the same manner as in Example 1 for Examples 11 to 14. As shown in the table, in Example 11, since the apparent porosity was lower than that in Example 1, the creep resistance and the thermal shock resistance were lowered. However, the creep resistance and the thermal shock resistance were values that can withstand practical use. In Example 12, since the press forming pressure was made low, the raw material granules were hardly crushed, and as a result, coarse pores occurring between the granules remained. Therefore, although the thermal shock resistance is lowered, the thermal shock resistance is a value that can withstand practical use. In Example 13, the strength was lowered because the molten mullite particles were added to the raw material, but the thermal shock resistance and the creep resistance were the same as those in Example 1. In Example 14, since the content of the oxide of the mesh, which is an impurity, was reduced in comparison with Example 1, the creep resistance was further improved. In Example 13, the particle size aspect ratios of the spherical particles and the needle-shaped particles shown in Table 2 were measured except for the molten mullite particles. In the observation of the polished section, the molten mullite particles are clearly distinguished from the spherical particles and the needle-shaped particles in terms of their shape.

[Comparative Examples 1 to 9]

Mullite ceramics were obtained in the same manner as in Example 1 except that the conditions shown in the following Table 3 were used. Table 3 shows the results of the evaluation conducted in the same manner as in Example 1. [ Fig. 1 shows a scanning electron micrograph of the polished cross section of the mullite ceramics obtained in Comparative Example 2. Fig. In Comparative Example 1, baking was performed at a baking temperature as high as 1810 캜 as compared with Example 1. [ As a result, the needle-shaped particles are abnormally ingrown and the coarse needle-like particles are increased to 30% of the total particles by the ratio of the cross-sectional areas of the abrasive grains. Further, since the calcination was performed at a high calcination temperature, the crystal state became unstable and the creep resistance decreased. In Comparative Example 2, unlike in Examples 1 and 2, because no Si-containing compound was contained in the raw material, no difference was generated in the rate of formation of mullite, so that needle-shaped particles did not grow and creep resistance decreased. In Comparative Example 3, unlike Examples 1 and 2, since silica was not contained in the raw material, similarly to Comparative Example 2, there was no difference in the production rate of mullite, so that needle-shaped particles did not grow and the creep resistance deteriorated. In Comparative Example 4, since coarse SiC was used in comparison with Example 1, the oxidation of SiC was difficult to proceed. As a result, the oxidation of SiC and the reaction and sintering of the mullite can proceed simultaneously, and the sintering can proceed without causing a decrease in sinterability due to the oxidation expansion. On the other hand, due to the oxidation expansion of SiC and the reaction and sintering of mullite, the place where SiC was present becomes pores after sintering. Due to this, coarse pores were formed in this comparative example using coarse SiC, so that the strength was lowered, the thermal shock resistance was lowered, and the creep resistance was lowered. In Comparative Example 5, contrary to Comparative Example 4, raw material particles of SiC were minimized. In this case, since the oxidation of SiC is easy to proceed, the oxidation of SiC is completed before the reaction and sintering of mullite proceeds. As a result, the decrease of the sinterability due to the increase of the distance between the particles causes the increase of the porosity and the decrease of the strength. As a result, the creep resistance deteriorates and the thermal shock resistance is lowered as compared with Example 1. In Comparative Example 6, firing was carried out at a lower firing temperature than in Example 1. As a result, mullite did not become single phase and creep resistance decreased. In Comparative Example 7, firing was carried out at a rate lower than that in Example 1. In this case, mullite reaction sintering proceeds after the completion of the oxidation of SiC, so that the decrease in sinterability due to the increase in the intergranular distance causes the increase in the porosity and the decrease in strength. As a result, the creep resistance and the thermal shock resistance were lowered as compared with Example 1. In Comparative Example 8, the heating rate was higher than that in Example 1, and firing was performed. In this case, since the sintering is completed before the oxidation of SiC is completed, SiC remains inside. As a result, the structure in which alumina, SiC, and mullite coexist, and creep resistance deteriorates. In Comparative Example 9, the press forming pressure was lowered to 30 MPa as compared with Example 14. In this Comparative Example, the creep resistance was lower than that of Example 14 even though the mesh oxide was the same as that of Example 14. This is considered to be because the apparent porosity was increased due to lowering of the press forming pressure, and thus the connectivity of each particle was lowered. Also, since the apparent porosity was increased, the thermal shock resistance was also lower than that of Example 14.

Claims (8)

Spherical particles of mullite having an aspect ratio of not less than 1 and not more than 2 and needle-like particles of mullite having an aspect ratio of not less than 2 but not more than 10 in the polished cross section, The average diameter of the needle-shaped particles is 2 to 10 times the average diameter of the spherical particles, the diameter of the needle-shaped particles is 12 to 90 μm, and the area ratio of the needle-shaped particles to the total particles is 0.03 to 0.3 Mullite ceramics. The method according to claim 1,
Wherein the ratio of the area of the coarse needle-shaped particles having an aspect ratio of more than 10 to the area of the total particles in the polished cross-section is 0.2 or less. The method according to claim 1,
And an apparent porosity of 5 to 27%. 4. The method according to any one of claims 1 to 3,
Wherein the ratio of the total area of coarse pores having a long diameter of not less than 5 times the average long diameter of the needle-shaped particles is 0.07 or less with respect to the area of the observation field of view. A method for producing the mullite ceramics according to claim 1,
A step of subjecting a raw material containing alumina, silica and an Si or Si-containing compound having an average particle diameter of 0.1 to 10 μm (excluding silica and silicate) to reaction firing at 1700 to 1800 ° C in an oxygen-containing atmosphere to produce mullite / RTI &gt;
Wherein the average heating rate between 900 and 1700 ° C in the sintering is set to 25 to 300 ° C / h. 6. The method of claim 5,
Wherein the Si-containing non-oxide compound is SiC, Si ₃ N ₄ , Si ₂ ON ₂ or SiAlON. The method according to claim 5 or 6,
Alumina, silica, and Si or Si-containing compounds having an average particle size of 0.1 to 10 占 퐉 (excluding silica and silicate) by wet mixing, casting the obtained slurry, or spray-drying the slurry Wherein the granules are subjected to press molding or CIP (Cold Isostatic Press) molding, followed by reaction firing. The method according to claim 5 or 6,
Alumina, silica, and Si or Si-containing compounds having an average particle diameter of 0.1 to 10 占 퐉 (excluding silica and silicate) are mixed and kneaded in a semi-wet process, the obtained kneaded mixture is subjected to plastic molding, .

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