WO2011055642A1

WO2011055642A1 - Mullite ceramic and method for producing same

Info

Publication number: WO2011055642A1
Application number: PCT/JP2010/068748
Authority: WO
Inventors: 梶野　仁; 高文上野
Original assignee: 三井金属鉱業株式会社
Priority date: 2009-11-06
Filing date: 2010-10-22
Publication date: 2011-05-12
Also published as: CN102596850B; KR20120115211A; JP5718239B2; CN102596850A; JPWO2011055642A1; KR101729650B1

Abstract

Disclosed is a mullite ceramic which is characterized by containing spherical mullite particles having an aspect ratio of 1-2 (inclusive) and needle-like mullite particles having an aspect ratio of more than 2 but 10 or less in a polishing cross-section. The mullite ceramic is also characterized in that the average length of the needle-like particles is 2-10 times the average diameter of the spherical particles and the area ratio of the needle-like particles to all the particles is 0.03-0.3. It is preferable that the area ratio of the coarse needle-like particles having an aspect ratio of more than 10 to all the particles in the polishing cross-section is 0.2 or less. It is also preferable that the mullite ceramic has an apparent porosity of 5-27%.

Description

Mullite ceramics and method for producing the same

The present invention relates to mullite ceramics and a method for producing the same. The mullite ceramic of the present invention is particularly useful as a refractory material such as a firing jig and a firing furnace construction member.

As a conventional technique related to mullite ceramics, a mullite porous body with improved creep resistance and spalling resistance is known (see Patent Document 1). This porous body is formed by bonding mullite crystals and aggregates thereof through a binder phase mainly composed of silica. This porous body can be obtained by molding a blend of alumina powder and silicon carbide powder and firing it in the range of 1550 ° C. to 1700 ° C. in an oxidizing atmosphere.

Patent Document 2 discloses a reaction-sintered mullite-containing ceramic molded body obtained by heat-treating a substrate formed from a finely dispersible powder mixture made of aluminum, Al ₂ O ₃ and Si-containing material in an oxygen-containing atmosphere. Is described. This literature describes that the mullite-containing ceramic molded body has a small shrinkage during firing.

JP2003-137671A US5843859A

There is a need for kiln tools that have high strength, creep resistance, and thermal shock resistance while being thin and lightweight for the purpose of energy saving and low cost in firing ceramics, including ceramic electronic parts. . However, the mullite ceramics described in Patent Document 1 described above has high creep resistance and high thermal shock resistance, but is not satisfactory in terms of strength. Further, the mullite ceramics described in Patent Document 2 described above has a problem that the calcite temperature is as low as less than 1700 ° C., so that a single phase of mullite cannot be obtained, and the creep resistance decreases when the thickness is reduced. .

An object of the present invention is to provide a mullite ceramic that can eliminate various drawbacks of the above-described conventional technology.

The present invention includes a mullite spherical particle having an aspect ratio of 1 or more and 2 or less and a mullite needle particle having an aspect ratio of more than 2 and 10 or less, and the average major axis of the acicular particle is a spherical particle. An object of the present invention is to provide a mullite ceramic characterized in that it has an average particle diameter of 2 to 10 times and an area ratio of acicular particles / total particles of 0.03 to 0.3.

The present invention also provides a suitable method for producing the mullite ceramics as described above.
A raw material containing alumina, silica, and Si having an average particle size of 0.1 to 10 μm or a Si-containing compound (excluding silica and silicate) is subjected to reaction sintering at 1700 to 1800 ° C. in an oxygen-containing atmosphere to obtain mullite. An object of the present invention is to provide a method for producing mullite ceramics characterized by producing

According to the present invention, there is provided a mullite ceramic having excellent thermal shock resistance, creep resistance and high strength performance. Conventionally known mullite ceramics are brick-like, and it has been difficult to make them thin enough to withstand practical use due to insufficient strength. The mullite ceramics of the present invention can cope with a normal temperature strength of more than 50 MPa and a thickness of 1.5 mm or less, and a fired body having a thickness of 0.5 mm can be obtained.

1 is a scanning electron microscope image of a polished cross section of mullite ceramics obtained in Example 1. FIG. FIG. 2 is a scanning electron microscope image of a polished cross section of the mullite ceramics obtained in Comparative Example 2.

Hereinafter, the present invention will be described based on preferred embodiments thereof. The mullite ceramic of the present invention has one of the characteristics in the shape of the particles constituting the ceramic. Specifically, when the polished cross section of mullite ceramics is magnified with a microscope, a state in which spherical particles of mullite (3Al ₂ O ₃ .2SiO ₂ ) and acicular particles of mullite are mixed is observed. Spherical particles and acicular particles are uniformly mixed. As a result of the examination by the present inventors, mullite ceramics in which spherical particles and acicular particles are mixed has high thermal shock resistance and excellent creep resistance. Here, the spherical particle means a particle having an aspect ratio of 1 or more and 2 or less when a polished cross section of mullite ceramics is observed, and does not need to be a true sphere (hereinafter referred to as “spherical”). The meaning is the same.) On the other hand, the acicular particles are particles having an aspect ratio of more than 2 and 10 or less when a polished cross section of mullite ceramics is observed. Moreover, the coarse needle-like particle | grains mentioned later mean the particle | grains whose aspect-ratio is more than 10 when observing the grinding | polishing cross section of mullite ceramics.

Note that acicular particles and coarse acicular particles, which will be described later, may be recognized as spherical particles even though they are actually acicular particles, depending on how the mullite ceramics are polished. . In the present invention, such an acicular particle that is recognized as a spherical particle is regarded as a spherical particle for the sake of convenience.

The relationship between the size of spherical particles and acicular particles affects the performance of mullite ceramics. As a result of the study by the present inventors, when the average particle diameter of the spherical particles is r, the average major axis of the acicular particles is in the range of 2r to 10r, so that the mullite having the above-described thermal shock resistance and creep resistance is obtained. It has been found that ceramics can be obtained. If the average major axis of the needle-like particles is less than 2r, even if the aspect ratio of the needle-like particles is large, the “strut effect” that the needle-like particles enter between the spherical particles does not sufficiently develop, and creep resistance Does not improve. On the other hand, when the major axis of the acicular particles exceeds 10r, coarse defects are likely to occur between the particles in the mullite ceramics. This coarse defect contributes to a decrease in thermal shock resistance. The range of the major axis of the acicular particles is particularly preferably 3r to 6r, since the thermal shock resistance and creep resistance of mullite ceramics are further improved.

The relative sizes of the spherical particles and the acicular particles are as described above, and it is preferable that the spherical particles themselves have an average particle size of 5 to 10 μm, particularly 6 to 9 μm. On the other hand, the size of the acicular particles is preferably 10 to 100 μm, and more preferably 12 to 90 μm, on condition that the range of 2r to 10r is satisfied. On the other hand, the minor axis is preferably 1 to 50 μm, more preferably 1 to 10 μm, provided that the aspect ratio (over 2 and 10 or less) is used.

Regarding the acicular particles, as described above, the aspect ratio is over 2, but the upper limit needs to be 10 or less. In other words, it is preferable that acicular particles having an aspect ratio of more than 10 (hereinafter, such acicular particles are referred to as “coarse acicular particles”) are not excessively contained. The presence of such coarse needle-like particles causes coarse defects between particles in mullite ceramics. This coarse defect contributes to a decrease in thermal shock resistance. Needle-like particles have the effect of improving creep resistance, but needle-like particles having an aspect ratio exceeding 10 have no effect. From this viewpoint, the ratio of the area of coarse needle-like particles having an aspect ratio of more than 10 to the area of all particles in the polished cross section of mullite ceramics is preferably 0.2 or less, particularly preferably 0.1 or less.

In addition to the relative sizes of spherical particles and acicular particles, the presence ratio of spherical particles and acicular particles in mullite ceramics also affects the performance of mullite ceramics. When the acicular particles enter between the spherical particles, the “strut effect” is generated and the creep resistance is improved, but the strength is lowered by the grain growth of the acicular particles, and the thermal shock resistance tends to be lowered. On the other hand, as a result of the study, the abundance ratio of the spherical particles and the acicular particles is reduced so that the area ratio of acicular particles / total particles in the polished cross section of mullite ceramics is in the range of 0.03 to 0.3. By adjusting, it was found that mullite ceramics having both thermal shock resistance and creep resistance can be obtained. When the area ratio is in this range of 0.05 to 0.25, the thermal shock resistance and creep resistance of mullite ceramics are further improved, which is preferable.

The polished cross section of mullite ceramics in the above description is obtained by rotating a disc grindstone sprayed with diamond slurry, for example, and pressing the mullite ceramics on the surface for polishing.

When observing the polished cross section of the mullite ceramics obtained as described above, magnified observation using, for example, a scanning electron microscope (SEM) is performed. The average particle diameter measurement of the spherical particles and the average major axis and average minor axis of the acicular particles and coarse acicular particles are performed as follows. The polished cross section has a size of at least 10 mm × 2 mm, and several SEM images of an arbitrary part are taken in an observation field of view of 200 μm × 200 μm in the polished cross section. An arbitrary straight line is drawn on each photographed image, and 100 particles crossing the straight line are selected. When the number is less than 100, this operation is repeated until 100 particles are traversed. The major and minor diameters of each selected particle are measured and the aspect ratio is calculated. Specifically, the target particle is approximated to an ellipse, the length of the major axis of the ellipse is measured, this is taken as the major axis, the direction perpendicular to the major axis is taken as the minor axis, and the length Is the minor axis. Based on the major and minor axes thus determined, particles with an aspect ratio of 1 or more and 2 or less are spherical particles, particles with an aspect ratio of 10 or more are acicular particles, and particles with an aspect ratio of 10 or more are coarse needles. Classify as particles. In the case of spherical particles, the average value of the major axis and the minor axis is defined as the average particle diameter. In the case of needle-like particles and coarse needle-like particles, the average major axis and the minor axis are obtained by averaging the major axis and the minor axis separately.

The area of the spherical particles in the polished cross section is calculated by regarding the average particle size obtained by the above method as the equivalent circle diameter. The area of the acicular particles is calculated from the area πab of the ellipse, assuming that the average major axis a and the average minor axis b of the acicular particles obtained by the above method are the major axis and minor axis of the ellipse.

When observing a polished cross section of mullite ceramics, it is preferable that only the above-mentioned spherical particles and needle-like particles are observed on the polished cross section, but particles having shapes other than these shapes may be observed. . Examples of such shaped particles include particles having a sharp shape. Specific examples of the particles include electrofused mullite particles described later, which are particles used as one of the raw materials. Electrofused mullite particles are produced by pulverizing an electromelted mullite lump. By this pulverization, electrofused mullite particles having a pointed shape are generated.

The mullite ceramics preferably contains certain pores. The apparent porosity is preferably 5 to 27%, particularly preferably 9 to 20%. By setting the apparent porosity within this range, both creep resistance and thermal shock resistance can be effectively balanced in a balanced manner. The apparent porosity is measured by a vacuum method according to JIS-R2205.

Further, it is preferable that the mullite ceramics do not include rough air holes. The coarse pores mean pores having a major axis that is 5 times or more the average major axis of the acicular particles among pores observed in the polished cross section. The presence of such rough air holes may cause a decrease in strength, thermal shock resistance and creep resistance of mullite ceramics. From this point of view, in the polished cross section of mullite ceramics, the ratio of the total area of the rough air holes to the area of the observation field (that is, the total area of the rough air holes / the area of the observation field) is preferably 0.07 or less, more preferably Is 0.05 or less. In order to suppress the formation of rough atmospheric pores, the particle size and firing conditions of the raw material components may be adjusted in the method for producing mullite ceramics described later. When the aspect ratio of the rough air hole is 1 or more and 2 or less, the long diameter of the rough air hole is an average value of the long diameter and the short diameter, and when the aspect ratio of the rough air hole is more than 2, This is the major axis.

The area of the rough air hole is such that the polished cross section has a size of at least 10 mm × 2 mm, and an SEM image of an arbitrary part is taken with an observation visual field of 200 μm × 200 μm in the polished cross section. The area of the rough air hole is obtained by calculating by the same method as the calculation of the area of the acicular particles and the area of the spherical particles described above.

Mullite ceramics may contain, as impurities, network modification oxides such as alkali metal oxides such as Na ₂ O and alkaline earth metal oxides. Since these impurities lower the viscosity of the glass at the grain boundary and affect the creep resistance, the total amount of these impurities is 0.01-0.3% with respect to mullite ceramics using high-purity raw materials. It is preferable that the content is 0.03 to 0.25% by weight. The ratio of the network modification oxide contained in mullite ceramics can be measured with a fluorescent X-ray analyzer. In addition, intermediate oxides such as Fe ₂ O ₃ , TiO ₂ , ZrO ₂ , CoO, and NiO stabilize the network skeleton of the glass, suppress the decrease in the viscosity of the glass, and contribute to the improvement of creep resistance. The total amount of is preferably 0.01 to 0.3% by weight with respect to mullite ceramics.

Next, a preferred method for the mullite ceramics of the present invention will be described. Mullite ceramics can be suitably produced by reacting and firing raw materials containing alumina and silica in an oxygen atmosphere to produce mullite. In particular, it is effective to use Si or a Si-containing compound (excluding silica and silicate) in addition to alumina and silica in order to produce spherical particles and acicular particles in the target mullite ceramics. As a result of the study by the present inventors, in the following description, for the sake of simplicity, Si or Si-containing compounds are generically referred to simply as Si-containing compounds. Specifically, it has been found that, in the reactive sintering of alumina and silica, by replacing part of the silica with a Si-containing compound, a difference in the mullite generation rate can be provided, whereby needle-like particles can be easily generated. . Specifically, when a part of the silica is replaced with a Si-containing compound, the mullite needle-like particles grow locally during reaction sintering, while the spherical particles located around the needle-like particles have a degree of grain growth. Becomes lower. As a result, mullite ceramics with high creep resistance and high thermal shock resistance can be easily obtained. When reaction sintering is performed using only alumina and silica without using a Si-containing compound, the coarsening of spherical particles is given priority over the growth of acicular particles. As a result, the obtained mullite ceramics causes a decrease in strength and is inferior in thermal shock resistance. Further, since the growth of the acicular particles is small, the “bracing effect” is less likely to occur, and the creep resistance is reduced.

As described above, the Si-containing compound is used to generate mullite needle-like particles during reactive sintering in the production of mullite ceramics. As the Si-containing compound, those known as ceramic materials are used. Examples thereof include inorganic Si-containing compounds. Examples of inorganic Si-containing compounds 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 used. Sialon is one the Si ₃ N ₄ material obtained by solid solution of Al ₂ O ₃ and SiO ₂ in Si ₃ N _4. Since the Si-containing compound oxidizes and expands during firing in the production of mullite ceramics, it has an effect of complementing the firing shrinkage of alumina or silica. As a result, the coarsening of pores that occur during shrinkage and the development of cracks that may occur in ceramics are suppressed, and as a result, a decrease in thermal shock resistance is also suppressed.

The ratio of each component in the raw material is determined in consideration of the stoichiometric ratio of alumina and silica in the target mullite ceramics. Specifically, when each component in the raw material is classified into alumina, which is a compound containing Al, and silica and Si-containing compound, which are compounds containing Si, the ratio of alumina to silica and Si-containing compound Is preferably in the range of 3: 2 to 3.5: 1.5, particularly 3.1: 1.9 to 3.4: 1.6 in terms of the molar ratio of alumina to silica.

The ratio of the silica and the Si-containing compound in the raw material is calculated by converting the molar ratio of Si to silica: Si-containing compound = 0.1: 1.9 to 1.9: 0.1, particularly 0.5: 1. .5 to 1.5: 0.5 is preferable. By using the silica and the Si-containing compound in combination at this ratio, the acicular particles can be grown while preventing the coarsening of the spherical particles. Moreover, it is possible to effectively prevent the crystallinity of the structure from decreasing due to the heat resistance of the Si-containing compound, and the increase in pores and the decrease in creep resistance resulting therefrom.

Α-alumina or γ-alumina is preferably used as alumina as one of the raw materials. It is also possible to use a mixture of these. There are no particular limitations on the shape of the alumina particles, and various shapes known in the art can be used. The shape preferably used is spherical. Regardless of the shape, alumina preferably has an average particle size of 0.1 to 20 μm, particularly 1 to 10 μm. It is desirable that alumina does not contain alkali components such as Na and K as much as possible.

Silica is not particularly limited in the shape of the particles, and various shapes known in the technical field can be used. The shape preferably used is spherical. Regardless of the shape, silica preferably has an average particle size of 0.05 to 30 μm, particularly 0.1 to 20 μm.

The average particle diameters of alumina and silica are measured using, for example, a laser diffraction particle size distribution analyzer (the same applies to the Si-containing compounds described below and the mullite particles described below).

The particle size of the Si-containing compound particles affects the performance of the target mullite ceramics. Specifically, when the particle size of the Si-containing compound particles is too large, coarse defects are likely to occur in the resulting mullite ceramic structure. Moreover, it cannot fully oxidize, it is difficult to become a single mullite composition, and creep resistance may deteriorate. In addition, strength and thermal shock resistance are likely to decrease. On the other hand, if the particle size of the Si-containing compound particles is too small, the Si-containing compound tends to be oxidized at a low temperature range, and the generation of acicular mullite particles is difficult to be promoted. From these viewpoints, the average particle diameter of the Si-containing compound particles is 0.1 to 10 μm, preferably 1 to 10 μm. This advantageous effect is particularly remarkable when SiC is used as the Si-containing compound. Regarding the shape of the Si-containing compound particles, it is preferable to use a spherical one.

Moreover, the above-mentioned raw material may contain mullite particles as an aggregate. By including mullite particles in the raw material, the progress of cracks that may be caused by thermal shock can be delayed by the detour effect, so that an advantageous effect that mullite ceramics can be used for a longer time is exhibited. If the particle size of the mullite particles contained in the raw material is too large, rough air holes are likely to be generated in the mullite ceramics, which may reduce the strength and thermal shock resistance of the mullite ceramics. Therefore, the average particle size of the mullite particles contained in the raw material is preferably 20 to 100 μm, particularly preferably 20 to 50 μm. When the raw material contains mullite particles, the content is preferably 15% by weight or less, particularly 10% by weight or less in the raw material from the viewpoint of improvement in strength and thermal shock resistance.

The target mullite ceramics can be obtained by mixing the raw materials containing the above-mentioned components and reaction firing. For mixing each component, a mixing method known in the technical field such as wet mixing, semi-wet mixing, and dry mixing can be used. From the viewpoint of reliably generating 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 Si-containing compounds are wet mixed using a liquid medium to form a slurry. The obtained slurry is cast-molded, or granulated obtained by spray-drying the slurry is press-molded or CIP-molded and then subjected to reactive firing. As an apparatus used for wet mixing, a known kneading apparatus, for example, a media mill such as a ball mill can be used. The solid content concentration in the slurry is preferably about 35 to 45% by weight. A binder can also be added to the slurry. As a binder, what is normally used in the said technical field, such as polyvinyl alcohol (PVA) and carboxymethylcellulose (CMC), for example can be used without a restriction | limiting in particular. The molding pressure when performing press molding or CIP molding is preferably set to about 70 to 150 MPa.

When semi-wet mixing is performed, alumina, silica and Si-containing compound are made into a semi-fluid using a liquid medium, and kneaded to obtain a kneaded product. The solid content in the kneaded product can be preferably about 10 to 15% by weight. The kneaded product is formed into a desired shape by plastic molding such as extrusion molding.

In any of the above-described molding methods, the reaction firing atmosphere is an oxygen-containing atmosphere such as air. The temperature for the reaction firing is preferably set to 1700 to 1800 ° C., particularly 1730 to 1790 ° C. By firing at a temperature in this range, the time for maintaining this firing temperature is preferably 1 to 8 hours, particularly 2 to 7 hours. By performing the reaction firing under these conditions, it is possible to prevent the increase of pores in the ceramic and to successfully obtain a mullite ceramic having good creep resistance.

In the temperature range of 900 to 1700 ° C., the average temperature increase rate when the temperature is raised to the above-described firing temperature is set to 25 to 300 ° C./h, preferably 30 to 200 ° C./h. By setting the average temperature rising rate within this range, it is possible to prevent the increase of pores in the ceramic and to successfully obtain a mullite ceramic having good creep resistance. If the heating rate is less than 25 ° C./h, the oxidative expansion is completed before the sintering starts, the interparticle distance increases, the sinterability decreases, and the pores increase. On the other hand, if the rate of temperature rise exceeds 300 ° C / h, the sintering is completed before the oxidation is completed, and non-oxidized substances are likely to remain inside the sintered body, reducing the creep resistance of mullite ceramics. It will be a cause.

As described above, the firing atmosphere can be air, that is, an oxygen-containing atmosphere having an oxygen concentration of about 20%. However, when the temperature of the atmosphere is 900 ° C. or higher, the oxidation of Si and the Si-containing compound becomes significant. When the temperature of the atmosphere becomes 900 ° C. or more due to temperature, it is preferable to reduce the oxygen concentration in the oxygen-containing atmosphere to 3% or less. By reducing the oxygen concentration to 3% or less, it is possible to prevent the oxidative expansion from being completed before sintering starts, thereby effectively increasing the interparticle distance, lowering the sinterability, and increasing the pores. Can be prevented. On the other hand, the lower limit value of the oxygen concentration in the oxygen-containing atmosphere is preferably more than 0.5% regardless of the temperature of the atmosphere. By doing so, it is possible to prevent the sintering from being completed before the oxidation is completed, thereby leaving a non-oxidized substance inside the sintered body and reducing the creep resistance of the mullite ceramics. Can be effectively prevented.

The mullite ceramics thus obtained include, for example, furnace tools for high-temperature furnaces and atmosphere furnaces, side walls, arches, hearths; setters for firing electronic components such as lining bricks, mortars, base plates; gas generation furnaces Various chemical reactor linings; ceramic substrates; carbide furnace linings; carbon black furnace linings; glass melting furnace linings;

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 such examples. Unless otherwise specified, “%” means “% by weight”.

[Example 1]
Alumina (average particle size 8 μm, spherical), SiC (average particle size 4 μm, spherical) and silica (average particle size 0.5 μm, spherical) have a molar ratio after oxidation of 3: 0.5: 1.5. Weighed as follows. When the ratio of these components is expressed by weight ratio, it 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 the liquid medium for the wet mixing. This slurry was spray-dried to obtain granules having an average particle size of 50 μm. This granule was press-molded to obtain a plate-shaped molded body. The press pressure was 100 MPa. This molded body was fired by reaction at 1750 ° C. for 4 hours in an air atmosphere. The temperature rising rate at this time was 40 ° C./h. After firing, it was naturally cooled to obtain the intended mullite ceramics. FIG. 1 shows a scanning electron microscope image of the polished cross section of the obtained mullite ceramics. The obtained ceramic was a single phase of mullite, and the SiC charged as a raw material was completely oxidized to form mullite, and was a sintered body particularly excellent in high creep resistance. Moreover, it had the strength which can be satisfied by thinning, and the thermal shock resistance was also excellent.

[Evaluation]
About the obtained mullite ceramics, the XRD measurement was performed and the compound contained in it was identified. Also, a polished cross section of mullite ceramics was prepared by the method described above, and the cross section was observed by SEM. Furthermore, the apparent porosity was measured by the method described above. Further, the content of the network modification oxide was measured by the method described above. Furthermore, room temperature three-point bending strength S, creep resistance and thermal shock resistance were measured by the following methods, respectively. The results are shown in Table 1.

[Normal temperature three-point bending strength S]
It was measured by a three-point bending test according to JIS R1601.

(Creep resistance)
A specimen processed to 100 mm × 30 mm × 2 mm was placed on a support so that the span would be 90 mm, and a load of 300 g was applied to the center. The amount of deflection after heating at 1400 ° C. for 12 hours was measured with a depth gauge, and this value was used as an index of creep resistance.

[Thermal shock resistance]
□ Four test specimens processed to 90 mm × 2.5 mm were prepared. Separately, a support column having a length of 10 mm, a width of 5 mm, and a height of 5 mm was prepared, and the support column was arranged on a ceramic base plate. The column arrangement positions were the positions of four corners of □ 90 mm. The test specimens were stacked in a four-layer stack on the support. Similarly, struts were sandwiched between the test specimens at four corners. Next, the temperature of the electric furnace was raised to a predetermined temperature and held for 30 minutes, and then the test specimen was placed in the furnace together with the base plate. After holding at that temperature for 60 minutes, the specimen was removed from the furnace together with the base plate and allowed to cool. It was visually confirmed whether or not the specimen was cracked or broken. The above operation was performed by increasing the temperature from 500 ° C. to 50 ° C., the upper limit of the temperature at which no cracking occurred was measured, and the value was used 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 evaluation performed in the same manner as in Example 1. In Example 2, SiC and silica were mixed at a blending ratio different from that in Example 1 to grow acicular crystals, and high creep resistance similar to that in Example 1 was obtained. Also, the thermal shock resistance was high as in Example 1. In Example 3, the raw material particle size of SiC was made smaller than that in Example 1. Although Example 3 was inferior to Example 1 in creep resistance and thermal shock resistance, it was judged that the characteristics can withstand practical use. Moreover, compared with the comparative example 5 mentioned later, high creep resistance, high thermal shock resistance, and high intensity | strength were shown. In Example 4, the SiC raw material particle size was made larger than that in Example 1. Compared to Example 1, Example 4 was determined to have a thermal shock resistance that could withstand practical use, although the thermal shock resistance was inferior because the pore diameter was larger as in Comparative Example 4 described later.

Example 5
Using the raw materials shown in Table 1 below, a slurry (solid content concentration: 42%) was obtained by wet mixing. CMC aqueous solution was used for the liquid medium of the wet mixing. This slurry was cast to obtain a plate-like molded body. This molded body was fired by reaction at 1750 ° C. for 4 hours in an air atmosphere. The temperature rising rate at this time was 40 ° C./h. After the completion of firing, it was naturally cooled to obtain mullite ceramics having high creep resistance and high thermal shock resistance. Table 1 shows the results of evaluation performed in the same manner as in Example 1.

[Examples 6 to 9]
A mullite ceramic was 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 evaluation performed in the same manner as in Example 1. In Example 6, compared with Example 1, the firing temperature was lowered to 1700 ° C. Although Example 6 was inferior in thermal shock resistance as compared with Example 1, it was judged to be a characteristic that could withstand practical use. Moreover, compared with the comparative example 6, high creep resistance, high thermal shock resistance, and high strength are shown. In Example 7, compared with Example 1, the firing temperature was increased to 1800 ° C. Although Example 7 was inferior in thermal shock resistance as compared with Example 1, it was judged to be a characteristic that could withstand practical use. In Example 8, compared with Example 1, the temperature increase rate was made slower. Although the thermal shock resistance of Example 8 was lower than that of Example 1, it was judged that the characteristics can withstand practical use. Moreover, since the porosity became low compared with the comparative example 7 mentioned later, the high intensity | strength and the high creep resistance were shown, the pore diameter became small by the porosity reduction, and the high thermal shock resistance was shown. In Example 9, compared with Example 1, the temperature raising rate was increased. In all the examples, the results showed high characteristics in both thermal shock resistance and creep resistance.

Example 10
Using the raw materials shown in Table 1 below, a kneaded product having a solid content concentration of 13% was obtained with a CMC aqueous solution. A mixing stirrer was used to prepare the kneaded product. This kneaded product was extrusion molded to obtain a plate-shaped molded body. This molded body was fired by reaction at 1750 ° C. for 4 hours in an air atmosphere. The temperature rising rate at this time was 40 ° C./h. After firing, it was naturally cooled to obtain mullite ceramics having thermal shock resistance and creep resistance that could withstand practical use. Table 1 shows the results of evaluation performed in the same manner as in Example 1.

[Examples 11 to 14]
A mullite ceramic was obtained in the same manner as in Example 1 except that the conditions shown in Table 2 below were used. Specifically, in Example 11, the press molding pressure was reduced to 70 MPa. In Example 12, the press molding pressure was reduced to 30 MPa, and the firing temperature was increased to 1800 ° C. In Example 13, a raw material having the composition shown in Table 4, that is, a raw material containing 10% of 220 mesh electrofused mullite particles was used. In Example 14, a high-purity raw material having a low content of alkali metal oxides such as Na ₂ O was used as a raw material for mullite ceramics. Table 2 shows the results of evaluation performed on Examples 11 to 14 in the same manner as Example 1. As shown in the table, in Example 11, the apparent porosity was lower than that in Example 1, and thus the creep resistance and thermal shock resistance were lowered, but the creep resistance and thermal shock resistance can withstand practical use. Value. In Example 12, since the press molding pressure was lowered, the raw material granules were not easily crushed, and as a result, coarse atmospheric pores generated 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, although the strength decreased because electrofused mullite particles were added to the raw material, the thermal shock resistance and creep resistance showed the same values as in Example 1. In Example 14, since the content of the network modification oxide, which is an impurity, was reduced as compared with Example 1, the creep resistance was further improved. In Example 13, the measurement of the particle diameter aspect ratio of the spherical particles and the acicular particles shown in Table 2 was performed excluding electrofused mullite particles. In the observation of the polished cross section, the electrofused mullite particles are clearly distinguished from the spherical particles and the acicular particles in terms of shape.

[Comparative Examples 1 to 9]
A mullite ceramic was obtained in the same manner as in Example 1 except that the conditions shown in Table 3 below were used. Table 3 shows the results of evaluation performed in the same manner as in Example 1. Moreover, the scanning electron microscope image of the grinding | polishing cross section of the mullite ceramics obtained by the comparative example 2 is shown in FIG. In Comparative Example 1, firing was performed at a firing temperature as high as 1810 ° C. as compared with Example 1. Therefore, the acicular particles grew abnormally, and the coarse acicular particles increased in the polishing cross-sectional area ratio to 30% of the total particles, resulting in coarse defects between the particles and a decrease in thermal shock resistance. Moreover, since it baked at the high calcination temperature, the crystal state became unstable and creep resistance fell. In Comparative Example 2, unlike Examples 1 and 2, since the raw material does not contain a Si-containing compound, there is no difference in the mullite generation rate, so that acicular particles do not grow and creep resistance is improved. Declined. In Comparative Example 3, unlike Examples 1 and 2, since the raw material does not contain silica, as in Comparative Example 2, there is no difference in the rate of mullite generation, so acicular particles do not grow and creep resistance is increased. Decreased. In Comparative Example 4, since SiC that is coarser than that in Example 1 is used, it is difficult for the oxidation of SiC to proceed. As a result, the oxidation of SiC and the reactive sintering of mullite proceed simultaneously, and the sintering can proceed without causing a decrease in sinterability due to oxidative expansion. On the other hand, the location where SiC was present becomes pores after sintering due to the oxidative expansion of SiC and the reactive sintering of mullite. Due to this, in this comparative example using coarse SiC, coarse air holes were formed, and a decrease in strength, a decrease in thermal shock resistance, and a decrease in creep resistance were observed. In Comparative Example 5, contrary to Comparative Example 4, the 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 reactive sintering of mullite proceeds. As a result, a decrease in sinterability due to an increase in the distance between particles causes an increase in porosity and a decrease in strength. As a result, the creep resistance and thermal shock resistance are reduced compared to Example 1. In Comparative Example 6, firing was performed at a lower firing temperature than in Example 1. As a result, the mullite single phase was not obtained, and the creep resistance was lowered. In Comparative Example 7, firing was performed at a lower temperature increase rate than in Example 1. In this case, since mullite reaction sintering proceeds after the oxidation of SiC is completed, a decrease in sinterability due to an increase in interparticle distance causes an increase in porosity and a decrease in strength. As a result, the creep resistance and the thermal shock resistance decreased as compared with Example 1. In Comparative Example 8, firing was performed at a higher temperature rising rate than in Example 1. In this case, since sintering is completed before the oxidation of SiC is completed, SiC remains inside. As a result, it becomes a structure in which alumina, SiC and mullite are mixed, and the creep resistance is lowered. In Comparative Example 9, the press molding pressure was lowered to 30 MPa as compared with Example 14. In this comparative example, although the network modification oxide was the same as that of Example 14, the creep resistance was lower than that of Example 14. The reason for this is thought to be that the apparent porosity has increased due to the lowering of the press molding pressure, so that the connectivity of each particle is lowered. Furthermore, since the apparent porosity increased, the thermal shock resistance was also lower than in Example 14.

Claims

In the polished cross section, mullite spherical particles having an aspect ratio of 1 or more and 2 or less, and mullite needle particles having an aspect ratio of more than 2 and 10 or less, and the average major axis of the acicular particles is the average particle diameter of the spherical particles. A mullite ceramic characterized by being 2 to 10 times and having an area ratio of acicular particles / total particles of 0.03 to 0.3.
The mullite ceramics according to claim 1, wherein the ratio of the area of coarse needle particles having an aspect ratio of more than 10 to the area of all particles in the polished cross section is 0.2 or less.
3. The mullite ceramic according to claim 1 or 2, wherein the apparent porosity is 5 to 27%.
4. The ratio of the total area of coarse atmospheric pores having a major axis that is 5 times or more of the average major axis of the acicular particles in the polished cross section to the area of the observation field is 0.07 or less. The mullite ceramics described in 1.
A method for producing mullite ceramics according to claim 1,
A raw material containing alumina, silica, and Si having an average particle size of 0.1 to 10 μm or a Si-containing compound (excluding silica and silicate) is subjected to reaction sintering at 1700 to 1800 ° C. in an oxygen-containing atmosphere to obtain mullite. Including the step of generating
A method for producing mullite ceramics, characterized in that an average heating rate between 900 and 1700 ° C. in firing is set at 25 to 300 ° C./h.
The manufacturing method according to claim 5, wherein the Si-containing non-oxide compound is SiC, Si 3 N 4 , Si 2 ON 2 or sialon.
Alumina, silica, and Si or Si-containing compound having an average particle size of 0.1 to 10 μm (excluding silica and silicate) are slurried by wet mixing, and the resulting slurry is cast or molded. The production method according to claim 5 or 6, wherein the granules obtained by spray drying are subjected to reaction firing after press molding or CIP molding.
Claims: Alumina, silica, and Si or Si-containing compound having an average particle size of 0.1 to 10 µm (excluding silica and silicate) are kneaded semi-wet, and the resulting kneaded product is plastic-molded and then subjected to reaction firing. 5. The production method according to 5 or 6.