WO2020122254A1

WO2020122254A1 - Mullite-base sintered compact and method for producing same

Info

Publication number: WO2020122254A1
Application number: PCT/JP2019/049076
Authority: WO
Inventors: Satoshi YAMAOKA
Original assignee: Showa Denko KK
Current assignee: Resonac Holdings Corp
Priority date: 2018-12-13
Filing date: 2019-12-06
Publication date: 2020-06-18
Anticipated expiration: 2021-06-13

Abstract

Disclosed is a mullite-base sintered compact containing an yttrium element in an yttrium oxide-equivalent amount of 0.01 to 0.25% by mass, wherein the average particle size of the mullite crystal particles 1 that constitutes the sintered compact is 1.00 to 3.00 μm and the average aspect ratio of the mullite crystal particles 1 is 1.65 or less.

Description

DESCRIPTION

Title of Invention

MULLITE-BASE SINTERED COMPACT AND METHOD FOR PRODUCING SAME

Technical Field

[0001]

The present invention relates to a mullite-base sintered compact that contains mullite as a main ingredient, and a method of producing the same.

Background Art

[0002]

Mullite is a compound of aluminum oxide and silicon oxide, and is generally represented by a compositional formula 3Al₂03-2Si0₂. A mullite-base sintered compact (ceramic) is excellent in heat resistance and mechanical strength at high temperature up to 130°C or so, and has a smaller thermal expansion coefficient than an alumina sintered compact. In addition, mullite is an oxide and is therefore more excellent in oxidation resistance than nitrides and carbides such as silicon nitride and silicon carbide that are excellent in heat resistance and mechanical strength at high temperatures.

Consequently, a mullite sintered compact is utilized for refractory materials, high-temperature structural materials, etc.

[0003]

Recently, use of ceramic materials has been diversified, and also the mullite-base sintered compact has become desired to be excellent in resistance to quickly heating and cooling, that is, excellent in thermal shock resistance so as to be usable even in higher temperature environments.

[0004]

Regarding this, for example, PTL 1 describes that, as a roller for roller hearth kilns, a roller formed of a mullite-base sintered compact having predetermined closed pores is excellent in thermal shock resistance and suffers from little deformation at high temperatures.

[0005]

PTL 2 describes, for obtaining a mullite-base sintered compact having high ductility and high strength, adding an yttrium oxide sol to an amorphous mullite and a crystalline mullite so as to be in a content of 0.25 to 1.5% by weigh therein thereby making the resultant mullite-base material contain a predetermined amount of columnar crystalline mullite particles having an aspect ratio of 3 or more.

Citation List

Patent Literature

[0006]

PTL 1: JP 2002-316869 A

PTL 2: JP 3-16958 A

Summary of Invention

Technical Problem

[0007]

However, the mullite-base sintered compact described in PTL 1 has a relative density of at most 95%, and could not be said to have thermal shock resistance to be up to a level recently required in the art.

[0008]

The mullite-base sintered compact described in PTL 2 has a large content of yttrium oxide that is an impurity except a mullite composition and further contains columnar crystalline particles having an extremely large aspect ratio, and therefore this may readily contain pores and could not be sufficiently densified, that is, also the sintered compact could not be said to have sufficient thermal shock resistance.

[0009]

Consequently, a mullite-base sintered compact not only having an excellent mechanical strength but also having a smaller content of any other component than aluminum oxide and silicon oxide that are mullite constituent component than before and also having an excellent thermal shock resistance is desired.

[0010]

The present invention has been made so as to solve the technical problems, and its object is to provide a mullite-base sintered compact having a small content of any other component than aluminum oxide and silicon oxide that are mullite constituent components, and excellent in thermal shock resistance and mechanical strength, and to provide a method for producing such a mullite-base sintered compact.

Solution to Problem

[0011]

The present invention is based on the finding that a mullite-base sintered compact containing a predetermined slight amount of an yttrium compound can have improved thermal shock resistance.

[0012]

Specifically, the present invention provides the following [1] to [14].

[1] A mullite-base sintered compact containing an yttrium element in an yttrium oxide-equivalent amount of 0.01 to 0.25% by mass, wherein the average particle size of the mullite crystal particles that constitute the sintered compact is from 1.00 to 3.00 pm and the average aspect ratio of the mullite crystal particles is 1.65 or less.

[2] The mullite-base sintered compact according to the above [1], having a relative density of 98.5% or more.

[3] The mullite-base sintered compact according to the above [1] or [2], wherein the standard deviation of the particle size of the mullite crystal particles is less than 1.00 pm.

[4] The mullite-base sintered compact according to any one of the above [1] to

[3], wherein the standard deviation of the aspect ratio of the mullite crystal particles is less than 0.70.

[5] The mullite-base sintered compact according to any one of the above [1] to

[4], wherein the average major diameter of the mullite crystal particles is from 1.50 to 4.00 pm.

[6] The mullite-base sintered compact according to any one of the above [1] to

[5], wherein the average minor diameter of the mullite crystal particles is from 1.00 to 2.00 pm.

[7] The mullite-base sintered compact according to any one of the above [1] to

[6], wherein the ratio by number of columnar crystal particles to spherical crystal particles among the mullite crystal particles is 1.00 or less.

[8] The mullite-base sintered compact according to any one of the above [1] to

[7], wherein the mullite content is from 99.00 to 99.99% by mass. [9] The mullite-base sintered compact according to any one of the above [1] to

[8], wherein the yttrium element is, in at least a part of the surface of the mullite crystal particles, distributed in layers along the surface thereof.

[10] The mullite-base sintered compact according to any one of the above [1] to

[9], wherein the thermal shock-resistant temperature difference, as measured according to a thermal shock test method of a relative method stipulated in JIS R 1648:2002, is 300°C or higher.

[11] The mullite-base sintered compact according to the above [10], wherein L the thermal shock-resistant temperature difference is 320°C or higher.

[0013]

[12] A method for producing a mullite-base sintered compact of any one of the above [1] to [11], including: a mixing step of preparing a mixed raw material containing a mullite raw material powder and an yttrium compound, a molding step of molding the mixed raw material to produce a molded article, and a firing step of firing the molded article at 1,500 to 1,800°C to give a mullite-base sintered compact, in which the amount of the yttrium compound to be added is such that the content of the yttrium element contained in the mullite-base sintered compact is from 0.01 to 0.25% by mass as an yttrium oxide- equivalent content thereof.

[13] The method for producing a mullite-base sintered compact according to the above [12], wherein in the mixing step, the mixed raw material is prepared by wet mixing.

[14] The method for producing a mullite-base sintered compact according to the above [12] or [13], including a step of heat-treating the mullite-base sintered compact obtained in the firing step at 1,300 to 1,600°C.

Advantageous Effects of Invention

[0014]

According to the present invention, there can be provided a mullite-base sintered compact having a small content of any other component than aluminum oxide and silicon oxide that are mullite constituent components, and excellent in thermal shock resistance and mechanical strength.

Also according to the production method of the present invention, there can be obtained a mullite-base sintered compact excellent in thermal shock resistance and mechanical strength. Brief Description of Drawings

[0015]

Fig. 1 is a schematic cross-sectional view of a microstructure of a mullite-base sintered compact of the present invention.

Fig. 2 is a scanning electron microscope (hereinafter abbreviated as "SEM") image (magnification 2,500 times) of a cross-sectional surface of a mullite-base sintered compact of Example 2.

Fig. 3 is a SEM image (magnification 2,500 times) of a cross-sectional surface of a mullite-base sintered compact of Comparative Example 4.

Fig. 4 is a SEM image (magnification 2,500 times) of a cross-sectional surface of a mullite-base sintered compact of Example 9.

Fig. 5 is a SEM image (magnification 2,500 times) of a cross-sectional surface of a mullite-base sintered compact of Comparative Example 6.

Description of Embodiments

[0016]

The mullite-base sintered compact and the production method thereof of the present invention are hereunder described in detail.

[0017]

(Mullite-base Sintered Compact)

The mullite-base sintered compact of the present invention contains an yttrium element in an yttrium oxide-equivalent amount of 0.01 to 0.25% by mass. In this, the average particle size of the mullite crystal particles that constitute the sintered compact is 1.00 to 3.00 pm and the average aspect ratio of the mullite crystal particles is 1.65 or less.

As just described, the mullite-base sintered compact which contains a predetermined amount of an yttrium element and in which the mullite crystal particles have a predetermined average particle size and a predetermined average aspect ratio is excellent in thermal shock resistance and mechanical strength.

Accordingly, the mullite-base sintered compact of the present invention is, for example, favorable for setters for firing, refractory linings, refractory materials, structural materials, support substrates for electronic devices, and ceramic packages, and is also favorably used as other various ceramic members that are required to have good heat resistance and mechanical strength.

[0018] The mullite-base sintered compact as referred to in the present invention means a sintered compact that contains mullite as a component in a largest amount therein, and a sintered compact containing mullite as a main ingredient therein.

Mullite is a compound of aluminum oxide and silicon oxide, in which the compositional ratio of aluminum atom to silicon atom (Al/Si) can be between 3 and 4. In general, mullite is represented by a compositional formula 3Al203-2Si02, in which Al/Si is 3, and also in the present invention, this is referred to as mullite, but the ratio is not always required to be 3 and does not exclude any other compositional ratio. _ _

[0019]

Preferably, the mullite-base sintered compact does not contain any other impurity element than the above-mentioned yttrium element apart from mullite. However, this shall not apply to impurity elements derived from impurities inevitably contained in a mullite raw material for use in producing the mullite-base sintered compact.

The mullite content in the mullite-base sintered compact is preferably 99.00 to 99.99% by mass, more preferably 99.20 to 99.98% by mass, even more preferably 99.50 to 99.98% by mass.

The mullite-base sintered compact having such a high mullite purity can readily secure excellent thermal shock resistance and mechanical strength. In addition, from the viewpoint of preventing the substances that are in contact with the mullite-base sintered compact in using them in heating environments from being contaminated with impurities, the mullite purity is desired to be high.

[0020]

The mullite-base sintered compact contains an yttrium element. The yttrium element is generally contained as an oxide, yttrium oxide (Y2O3). The mullite-base sintered compact may contain an yttrium atom not constituting yttrium oxide, and including such an yttrium atom, the yttrium element content in the mullite-base sintered compact is 0.01 to 0.25% by mass in terms of yttrium oxide, preferably 0.01 to 0.20% by mass, more preferably 0.01 to 0.15% by mass, even more preferably 0.02% by mass or more and less than 0.10% by mass, still more preferably 0.02 to 0.05% by mass, and most preferably 0.02 to 0.04% by mass.

When the yttrium oxide-equivalent content of the yttrium element is 0.01% by mass or more, the mullite-base sintered compact can be excellent in thermal shock resistance and mechanical strength. When the content is more than 0.25% by mass, it is difficult to obtain a mullite-base sintered compact having sufficient thermal shock resistance and mechanical strength.

[0021]

Preferably, the yttrium element is, in at least a part of the surface of the mullite crystal particles, distributed in layers along the surface thereof.

Fig. 1 shows a schematic view of a micro structure of the mullite-base sintered compact. As shown in Fig. 1, the mullite-base sintered compact is such that mullite crystal particles 1 are kept in close contact with each other, and a crystal grain boundary glass phase 2 partly exists in a part between the mullite crystal particles 1. At both the boundary face A between the mullite crystal particle 1 and the crystal grain boundary glass phase 2, and the boundary face B between the mullite crystal particles 1, yttrium elements segregate as mentioned above, and accordingly, the mullite-base sintered compact can have an improved grain boundary strength, and even though the yttrium element content in the mullite-base sintered compact is a predetermined slight amount as mentioned above, the mullite-base sintered compact can be excellent in thermal shock resistance and mechanical strength.

[0022]

The yttrium element distribution state in the surfaces of the mullite crystal particles can be confirmed through elementary analysis according to energy-dispersive X-ray spectrometry (EDS). The detected characteristic X-ray is mapped as a two-dimensional image, and it is known that, in the mullite-base sintered compact, yttrium elements are distributed in layers along the surface of the mullite crystal particle 1 at both the grain boundaries A and B. The crystal grain boundary glass phase 2 is mainly amorphous silicon oxide (S1O2).

The mullite-base sintered compact containing an yttrium element in a predetermined slight amount can exhibit the above-mentioned excellent characteristics, and from this viewpoint, the thickness of each layer of the yttrium element distributed as above is preferably 5 nm or less, more preferably 1 to 4 nm, even more preferably 1 to 3 nm.

[0023]

(Mullite Crystal Particle)

The mullite crystal particles constituting the mullite-base sintered compact of the present invention have an average particle size of 1.00 to 3.00 pm, preferably 1.00 to 2.19 pm, more preferably 1.00 to 2.15 pm.

When the average particle size is 1.00 pm or more, the mullite-base sintered compact can have sufficient thermal shock resistance and mechanical strength. When the average particle size is 3.00 pm or less, sintering performance is good and the mullite-base sintered compact can be excellent in thermal shock resistance and mechanical strength.

[0024]

The particle size as referred to herein is an areal circle -equivalent diameter to be determined through image analysis, in which the mullite-base sintered compact is mirror-polished, then thermally etched, and the thus-processed surface is observed through SEM, and randomly-selected about 200 (200 or more) mullite crystal particles on the SEM image are analyzed. The average particle size is an arithmetic average value of about 200 mullite crystal particles targeted for image analysis. Specifically, the particle size is determined according to the method described in the section of Examples given hereinunder.

For identifying the crystal phase of the mullite-base sintered compact to be mullite, the sintered compact is ground into powder using a ball mill or the like and the resultant powder is analyzed through powdery X-ray diffractometry (powdery XRD). The yttrium element in the sintered compact exists in an amorphous state in the crystal grain boundary glass phase, which can be confirmed through X-ray absorption fine structure (XAFS) analysis.

[0025]

The mullite crystal particles have an average aspect ratio of 1.65 or less, preferably 1.55 or less, more preferably 1.50 or less.

When the aspect ratio is 1.65 or less, the compactness of the sintered compact is good, and the mullite-base sintered compact can be therefore excellent in thermal shock resistance and mechanical strength.

[0026]

The aspect ratio as referred to in the present invention is a ratio of the major diameter to the minor diameter (major diameter/minor diameter) of each crystal particle in image analysis of the SEM image of the mullite-base sintered compact mentioned above, in which a minimum rectangle drawn by surrounding the outer shape of the crystal grain with a rectangle, that is, the major side of the circumscribed rectangle is referred to as a major diameter, and the minor side thereof is referred to as a minor diameter. However, the crystal grain includes also a case where the major diameter and the minor diameter are the same, and in this case, the aspect ratio is 1. The average aspect ratio is an arithmetic average value of the aspect ratios of about 200 mullite crystal particles targeted in image analysis.

[0027]

The standard deviation of the particle size and that of the aspect ratio of the mullite crystal particles are preferably small. Specifically, from the viewpoint of obtaining good thermal shock resistance and mechanical strength, fluctuation in the size and the shape of the mullite crystal particles is preferably small.

Preferably, the standard deviation of the particle size is less than 1.00 pm, more preferably less than 0.90 pm, even more preferably less than 0.85 pm. Also preferably, the standard deviation of the aspect ratio is less than 0.70, more preferably less than 0.50, even more preferably less than 0.45.

[0028]

From the viewpoint of good compactness of the mullite-base sintered compact, preferably, the proportion of long columnar crystal particles is small among the mullite crystal particles. Accordingly, the average major diameter of the mullite crystal particles is preferably 1.50 to 4.00 pm, more preferably 1.70 to 3.00 pm, even more preferably 2.00 to 2,70 pm. The average minor diameter of the mullite crystal particles is preferably 1.00 to 2.00 pm, more preferably 1.20 to 1.85 pm, even more preferably 1.50 to 1.80 pm.

[0029]

The average major diameter is an arithmetic average value of major diameters of about 200 mullite crystal particles targeted in image analysis in determining the aspect ratio as above. Similarly, the average minor diameter is an arithmetic average value of minor diameters of about 200 mullite crystal particles targeted in image analysis in determining the aspect ratio as above.

[0030]

Further, also from the viewpoint of good compactness of the mullite-base sintered compact, the proportion of columnar crystal particles is small among the mullite crystal particles. Accordingly, the ratio by number of columnar crystal particles to spherical crystal particles (number of columnar crystal particles/number of spherical crystal particles: hereinafter this may be referred to as columnar/spherical ratio) is preferably 1.00 or less, more preferably 0.80 or less, even more preferably 0.70 or less.

The columnar crystal particles as referred to in the present invention are to indicate mullite crystal particles having an aspect ratio of 1.5 or more; and the spherical crystal particles are to indicate mullite crystal particles having an aspect ratio of less than 1.5.

[0031]

(Relative Density)

Preferably, the relative density of the mullite-base sintered compact is 98.5% or more, more preferably 99.0% or more, even more preferably 99.2% or more. The relative density is a ratio of apparent density to true density, and the mullite-base sintered compact preferably has a relative density nearer to 100% from the viewpoint of securing high compactness and good thermal shock resistance and mechanical strength.

The true density as referred to in this description is a value determined according to a gas exchange method for a powdery sample prepared by grinding the mullite-base sintered compact. The apparent density as referred to in this description is a value determined according to the method stipulated in JIS R 1634:1988. Specifically, these can be determined according to the method described in the section of Examples given hereinunder.

[0032]

(Thermal Shock Resistance)

The mullite-base sintered compact is excellent in thermal shock resistance, and specifically, the thermal shock-resistant temperature difference determined according to a thermal shock test method of a relative method stipulated in JIS R 1648:2002 is preferably 300°C or higher, more preferably 310°C or higher, even more preferably 320°C or higher.

Having a thermal shock-resistant temperature difference as above, the mullite-base sintered compact can be favorably used as heat-resistant ceramic members.

Specifically, the thermal shock-resistant temperature difference can be measured according to the method described in the section of Examples given hereinunder.

[0033]

(Mechanical Strength) The mullite-base sintered compact is excellent in mechanical strength, and specifically, this can be indicated by a four-point bending strength at room temperature (25°C). Specifically, in the present invention, a four-point bending strength is used as one index for indicating the mechanical strength of the mullite-base sintered compact.

Regarding the four-point bending strength, a larger numerical value is better, and in order to say that the mullite-base sintered compact has a sufficient mechanical strength, the four-point bending strength thereof is preferably 200 MPa or more, more preferably 210 MPa or more, even more preferably 215 MPa or more, and further more preferably 300 MPa or more.

The four-point bending strength is a value measured according to the test method stipulated in JIS R 1601:2008. Specifically, it can be measured according to the method described in the section of Examples to be given hereinunder.

[0034]

[Production Method for Mullite-base Sintered Compact]

As a method for producing the mullite-base sintered compact, for example, the mullite-base sintered compact is preferably produced according to the production method of the present invention. Specifically, the production method includes a mixing step of preparing a mixed raw material containing a mullite raw material powder and an yttrium compound, a molding step of molding the mixed raw material to produce a molded article, and a firing step of firing the molded article at 1,500 to 1,800°C to give a mullite-base sintered compact. In the method, the amount of the yttrium compound to be added is such that the content of the yttrium element contained in the mullite-base sintered compact is 0.01 to 0.25% by mass as an yttrium oxide-equivalent content thereof.

According to the process, the mullite-base sintered compact can be obtained in a favorable manner.

Hereinunder the steps of the production method are described sequentially.

[0035]

(Mixing Step)

In the mixing step, first, a mixed raw material containing a mullite raw material powder and an yttrium compound is prepared.

The mullite raw material powder may be a mullite powder or a mixed powder of an alumina (aluminum oxide) powder and a silica (silicon oxide) powder. A mullite pre-calcined powder prepared by previously pre-calcining the mixed powder in air at 1,000 to 1,500°C or so can also be used.

[0036]

In the case where a mullite powder is used as the mullite raw material powder, the particle size of the mullite powder, as a volume distribution 50% cumulation value (hereinafter referred to as D50 particle size) thereof is, from the viewpoint of readily producing the mullite-base sintered compact having mullite crystal particles, preferably 0.01 to 2.50 pm, more preferably 0.01 to 2.00 pm, even more preferably 0.01 to 1.00 pm.

In the case where a mixed powder of an alumina powder and a silica powder is used, D50 particle size of each powder is preferably 0.01 to 2.50 pm, more preferably 0.01 to 1.00 pm, even more preferably 0.01 to 0.50 pm. In this case, the blending ratio of the alumina powder and the silica powder is preferably such that the theoretical molar compositional ratio in the mullite compositional formula, 3Al203-2Si02, is Al203/Si02=3/2.

The particle size D50 can be determined according to a laser diffraction scattering method.

[0037]

As described above, preferably, the mullite-base sintered compact does not contain any other impurity element than the yttrium element and the impurity elements derived from the impurities inevitably contained therein in production of the mullite raw material powder. Accordingly, as the mullite raw material powder, a high-purity one is preferably used. Specifically, the purity of the mullite raw material powder is preferably 99.00% by mass or more each, more preferably 99.20% by mass or more, even more preferably 99.50% by mass or more.

[0038]

The yttrium compound is preferably yttrium oxide or a compound that forms yttrium oxide in a firing step but does not produce any other residual impurity element in the mullite-base sintered compact. Examples of such a compound include yttrium carbonate, yttrium nitrate, yttrium sulfate, yttrium oxalate, yttrium acetate, and hydrates thereof, as well as organic yttrium compounds such as yttrium stearate, yttrium isopropoxide, yttrium 2-ethylhexanoate and yttrium acetylacetonate. Among these, one alone or two or more can be used either singly or as combined. The other constitutive elements than the yttrium element in the compounds can evaporate away, for example, as carbon dioxide, steam or any other gaseous component in the later firing step, and are therefore considered not to be impurity elements in the mullite-base sintered compact.

[0039]

The amount of the yttrium compound to be added is such that the yttrium element content in the produced mullite-base sintered compact is 0.01 to 0.25% by mass as an yttrium oxide-equivalent amount thereof, preferably 0.01 to 0.15% by mass, more preferably 0.02% by mass or more and less than 0.10% by mass, even more preferably 0.02 to 0.05% by mass.

When the yttrium compound is added in an amount falling within the range, the particle size and the aspect ratio of the resultant mullite crystal particles can be prevented from fluctuating, and therefore coarse particles that may be fracture start points in receiving thermal shock could hardly form, and accordingly, the resultant mullite-base sintered compact could be given sufficient thermal shock resistance. When the yttrium compound is added excessively, columnar crystal particles of mullite may readily form, but when the amount thereof is a slight amount falling within the above-mentioned predetermined range, such columnar crystal particles are prevented from grow, and therefore density reduction owing to increase in pores in the mullite-base sintered compact can be prevented and the resultant mullite-base sintered compact can be given excellent mechanical strength.

In particular, regarding the mullite-base sintered compact that has been processed for annealing treatment as mentioned below, the thermal shock resistance and the mechanical strength characteristics of the resultant mullite-base sintered compact can be bettered more efficiently by adding thereto a slight amount of the yttrium compound as described above.

[0040]

In the mixed raw material, a dispersant for the purpose of improving the dispersibility of various constituent components, as well as other various additive components such as a molding auxiliary agent for the purpose of improving moldability in the later molding step can be added within a range not detracting from the advantageous effects of the mullite-base sintered compact of the present invention. As the additive components, those not forming residual impurity elements in the mullite-base sintered compact are used. Examples of the dispersant include acrylic acid, ammonium acrylate oligomer, carboxymethyl cellulose ammonium, ammonium polycarboxylate, and monoethylamine. In addition, wax emulsion, fatty acids, anionic surfactants and synthetic surfactants can also be sued as the dispersant.

Examples of the molding auxiliary agent include polyvinyl alcohol, methyl cellulose, hydroxypropylmethyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, starch, polyethylene oxide, sodium polyacrylate, polyacrylamide, polyvinyl butyral, polyethylene, polypropylene, polystyrene, glycerin, polyethylene glycol, dibutyl phthalate, acrylic resins, and polyamide resins.

[0041]

As the mixing means for preparing the mixed raw material, a known mixing method of, for example, a chamber rotation type, a mechanical stirring type, a fluid stirring type, or a high-speed shearing/impact type can be used.

The mixed raw material may be prepared by dry- mixing the above-mentioned mullite raw material and the above-mentioned yttrium compound alone, but from the viewpoint of attaining more uniform mixing performance, preferably, the mixed raw material is prepared by wet-mixing using a liquid dispersion medium.

In the case of wet mixing, preferably, the mixed raw material is mixed while preventing coagulation thereof. For example, in the case of a chamber rotation system, preferably, wet-grinding and mixing is carried out at a time in a ball mill or the like to prepare the mixed raw material.

[0042]

As the dispersion medium, preferred is one that can give a mixed raw material of the mullite raw material powder and the yttrium compound uniformly mixed therein, and can readily evaporate away so as to hardly remain in the produced mullite-base sintered compact; and examples thereof include water, methanol, ethanol, 1-propanol, 2-propanol, butanol, formic acid, acetic acid, N,N-dimethylformamide, acetonitrile, acetone, ethyl acetate, diethyl ether, toluene, methylene chloride, chloroform, carbon tetrachloride, benzene and hexane. One alone of these or two or more thereof may be used either singly or as combined. Among these, from the viewpoint of easy availability and easy handleability as well as cost thereof, water is preferably used.

[0043]

The amount of the dispersion medium in wet-mixing may be appropriately controlled in accordance with the blending amount of the mullite raw material powder, the yttrium compound and other components and in consideration of uniform mixing capability thereof, but from the viewpoint of removal efficiency in the later step, it is desirable that their amounts are not more than necessary.

The amount of the dispersant to be used depends on the kind of the dispersant, and in the case where water is used, the amount thereof is preferably 40 to 200 parts by mass relative to 100 parts by mass of the total of the mullite raw material powder and the yttrium compound, more preferably 40 to 150 parts by mass, even more preferably 40 to 100 parts by mass.

[0044]

(Molding Step)

In the molding step, the mixed raw material prepared in the mixing step is molded to give a molded article.

In the case where the mixed raw material is prepared by wet mixing, a sheet-like molded article can be produced according to a sheet molding method (doctor blade method).

After the mixed raw material is dried into a dry powder, the powder may be molded. Examples of the method of producing a dry powder from the mixed raw material include a method of grinding a solidified dry product obtained by spray drying the mixed raw material or drying it in a constant-temperature drying device or the like, with a ball mill or the like. For example, when the dispersion medium in wet mixing is water, the drying temperature is preferably 80 to 300°C.

The particle size of the dry powder for use for molding is not specifically limited, but from the viewpoint of easy handleability and molding workability, in general, the particle size is preferably classified through a sieve or the like to fall within a range of 0.1 to 50 pm.

[0045]

Regarding the molding method for producing a molded article using the powdery mixed raw material prepared by dry mixing, or using the dried powder, for example, a known molding method may be used, including a uniaxial pressing method with a mold, a cold isotactic press (CIP) molding method or an extrusion molding method. Among these, two or more kinds of molding methods may be combined, and for example, pre-molding according to a uniaxial pressing method may be combined with CIP molding to produce a molded article. The molded article may be appropriately worked into a desired shape or size in accordance with the use and the object of the mullite-base sintered compact to be produced.

[0046]

(Firing Step)

In the firing step, the molded article produced in the previous molding step is fired at 1,500 to 1,800°C to give the above-mentioned mullite-base sintered compact.

As the firing method for producing the mullite-base sintered compact, for example, employable is a known firing (sintering) method of a normal-pressure firing method, a hot pressing method, a gas pressing firing method, a microwave heating and firing method, or a spark plasma sintering method. Among these, a normal-pressure firing method is an advantageous firing method in point of limitations of cost and conformation. In the present invention, using the above-mentioned mixed raw material, a mullite-base sintered compact excellent in thermal shock resistance and mechanical strength can be produced in a simplified manner according to a normal-pressure firing method in air.

In the normal-pressure firing method, for example, the molded article is fired in air in an electric furnace at 1,400 to 1,800°C for 0.5 to 5 hours to give the mullite-base sintered compact.

The sintered compact may be appropriately worked into a desired shape and size depending on the use and the object thereof.

[0047]

The mullite-base sintered compact obtained in the manner as mentioned above may be further heated at 1,300 to 1,600°C.

Such re-heating treatment after the firing step is so-called annealing treatment. Via such a heat treatment step, the strain in the crystal grain boundary glass phase that contain an yttrium element in the sintered compact can be relaxed to improve thermal shock resistance and mechanical strength. Accordingly, preferably, the above-mentioned heat treatment step is carried out after the firing step.

The heat treatment step is carried out, for example, by heating the sintered compact in air in an electric furnace at a temperature lower than the firing temperature for the mullite-base sintered compact and falling within a range of 1300 to 1600°C, for 0.5 to 5 hours. Examples

[0048]

Hereinunder the present invention is described more specifically with reference to Examples, but the present invention should not be restricted by the following Examples.

[0049]

[Production of Mullite-base Sintered Compact]

Details of various blending raw material components used in the following Examples and Comparative Examples are as follows.

Alumina powder: alumina purity 99.99% by mass, particle size D50: 0.3 pm

Silica powder: silica purity 99.99% by mass, particle size D50: 0.3 pm

Yttrium compound (1): yttrium acetate tetrahydrate (purity 99.9% by mass) Yttrium compound (2): yttrium oxide ("RUP" available from Shin-Etsu Chemical Co., Ltd., purity 99.9% by mass or more, particle size D50: 1.1 pm)

[0050]

(Example 1)

As mullite raw material powders, 71.76 parts by mass of an alumina powder and 28.18 parts by mass of a silica powder (corresponding a theoretical molar compositional ratio Al₂0₃/Si0₂=3/2 in a mullite compositional formula 3Al₂0₃-2Si0₂) were put into a ball mill. Further, as an yttrium compound, 0.06 parts by mass of yttrium acetate tetrahydrate and 64 parts by mass of water were added and wet-mixed therein using alumina ball (diameter 5 mm) to prepare a mixed raw material.

The amount of the yttrium compound to be added was so controlled that, presuming that the constituent components of the mullite-base sintered compact to be produced could be aluminum oxide, silicon oxide and yttrium oxide alone, the yttrium oxide content in the total of these constituent components, 100% by mass could be 0.02% by mass. Specifically, this 0.02% by mass means an yttrium oxide-equivalent content of yttrium element (Y) in 100% by mass of the mullite-base sintered compact.

Next, the mixed raw material was dried with a spray drier (spray-drying device), and classified through a sieve to give a dry powder having a particle size of 10 to 50 pm. The dry powder was pre-molded according to a uniaxial pressing method, and then CIP-molded to give a disc-like molded article having a diameter of 50 mm and a thickness of 7 mm.

The molded article was fired in an electric furnace in air at 1,700°C for 3 hours to give a mullite-base sintered compact.

[0051]

(Examples 2 to 5 and Comparative Examples 1 to 4)

Various mullite-base sintered compacts were produced in the same manner as in Example 1, except that in Example 1, the yttrium compound was added in such a manner that the content of the yttrium element (Y) in 100% by mass of the mullite-base sintered compact to be produced could be as in Table 1 below.

[0052]

(Examples 6 to 7)

Various mullite-base sintered compacts were produced in the same manner as in Example 1, except that in Example 1, yttrium oxide was used as an yttrium compound, and the yttrium compound was added in such a manner that the content of the yttrium element (Y) in 100% by mass of the mullite-base sintered compact to be produced could be as in Table 1 below.

[0053]

(Examples 8 to 14, Comparative Examples 5 and 6)

Each mullite-base sintered compact obtained in Examples 1 to 7, Comparative Examples 2 and 4 was re-heated in an electric furnace in air at 1,500°C for about 3 hours.

[0054]

[Evaluation and Measurement of Mullite-base Sintered Compact]

Each mullite-base sintered compact obtained in the above Examples and Comparative Examples was evaluated and measured in point of the items mentioned below. The evaluation measurement results are collectively shown in Table 1.

[0055]

(Relative Density)

The apparent density was determined according to the method stipulated in JIS R 1634:1988.

The mullite-base sintered compact was powdered in a ball mill using alumina ball (diameter 5 mm), and the resultant powder was analyzed according to a gas exchange method (applied gas: helium) to determine the true density thereof.

The relative density was determined by calculating a ratio of the apparent density to the true density.

[0056]

(X-ray Analysis)

The same powder as that used in measurement of the true density was used as a sample, and using an X-ray diffraction measurement apparatus ("C' Pert PRO" available from Malvern Panalytical Ltd, target: copper, Cu-KaI ray), the sample was analyzed through powdery XRD within a measurement range for the diffraction angle 2Q of 10 to 80°. From comparison between the resultant measurement spectrum peak and a reference code, the crystal phase was identified to be mullite (3Al203-2Si02) alone.

In XAFS analysis, the yttrium element in the mullite-base sintered compact was identified to exist in an amorphous state in the crystal grain boundary glass phase.

[0057]

(SEM Observation of Mullite Crystal Particles)

The resultant mullite-base sintered compact was cut with a diamond grinding stone, and the cross-section surface thereof was mirror-polished with a diamond slurry (D50 particle size, 3 pm and 1 pm), and further thermally etched by heating in an electric furnace at 1,575°C for 10 minutes. The sample surface of the thermally-etched mullite-base sintered compact was observed with SEM ("JSM-6510V" available from JEOL Ltd.).

Figs. 2 to 5 show SEM images (magnification, 2500 times) in Example 2, Comparative Example 4, Example 9 and Comparative Example 6, respectively, as typical examples.

Randomly selected about 200 (at least 200) mullite crystal particles on the SEM image of each sample were analyzed using image analysis software (Mac- View ver. 4" available from Mountech Co., Ltd.) to determine the particle size, the major diameter, the minor diameter and the aspect ratio thereof.

[0058]

In the image analysis, an areal circle-equivalent diameter of each crystal particle was considered to be a particle diameter thereof. A minimum rectangle drawn by surrounding the outer shape of the crystal grain with a rectangle, that is, the major side of the circumscribed rectangle was considered to be a major diameter, and the minor side thereof was considered to be a minor diameter (including a case where a major diameter and a minor diameter are the same). With that, a ratio of the major diameter to the minor diameter (major diameter/minor diameter) was referred to as an aspect ratio.

From these measured values, an average particle size, a standard deviation of the particle size, an average aspect ratio, a standard deviation of the aspect ratio, an average major diameter, an average minor diameter, and a ratio by number of columnar crystal particles to spherical crystal particles (columnar/spherical ratio) were determined. The columnar/spherical ratio was calculated by dividing the number of columnar crystal particles by the number of spherical crystal particles with a presumption that crystal particles having an aspect ratio of 1.5 or more are considered to be columnar crystal particles and crystal particles having an aspect ratio of less than 1.5 are considered to be spherical crystal particles.

[0059]

(EDS Elemental Mapping)

As a typical example, the mullite-base sintered compact obtained in Example 2 was analyzed through EDS elemental mapping in point of the interface A between the mullite crystal particle 1 and the crystal grain boundary glass phase 2, and the interface B between the mullite crystal particles 1, using an atomic resolution analysis electron microscope ("JEM-ARM200F", available from JEOL Ltd., acceleration voltage 200 kV, STEM (scanning transmission) mode), and using as an EDS detector, Dual SDD (silicon drift detector "JED-2300T", available from JEOL Ltd., detection area 100 mm²), thereby determining the distribution condition of the yttrium element in the sample (see Fig. 1).

An aluminum element and a silicon element were also checked through EDS elemental mapping, and in the crystal grain boundary glass phase 2, silicon element distribution was confirmed but aluminum element distribution could not be confirmed.

[0060]

(Thermal Shock Resistance)

According to a thermal shock test method of a relative method stipulated in JIS R 1648:2002, a thermal shock-resistant temperature difference was measured. From the resultant mullite-base sintered compact, a test piece (40 mm x 4.0 mm x 3.0 mm) corresponding to a standard test piece I stipulated in JIS R 1601:2008 was cut out, heated at a test temperature in a box-type electric furnace and then freely dropped down into water to be given thermal shock. The test piece was tested according to a four-point bending test in JIS R 1601:2008 to measure the remaining bending strength thereof after the thermal shock test. For the other measurement conditions and the test process, reference was made to the method described in JIS R 1648:2002 to determine the thermal shock-resistant temperature difference.

A larger value of the thermal shock-resistant temperature difference indicates more excellent thermal shock resistance. In this evaluation, samples having a thermal shock-resistant temperature difference of 300°C or higher can be said to be excellent in thermal shock resistance. Further, a value of 320°C or higher is preferred since the samples of the type can be said to be more excellent in thermal shock resistance.

[0061]

(Four-Point Bending Strength)

As one index to indicate a mechanical strength of a mullite-base sintered compact, a four-point bending strength of the mullite-base sintered compact was measured. Measurement for the four-point bending strength was carried out according to a four-point bending strength test method stipulated in JIS R 1601:2008, in which a standard sample piece I (40 mm x 4.0 mm x 3.0 mm) cut out from the resultant mullite-base sintered compact was tested at room temperature (25°C).

In this evaluation, the four-point bending strength of 200 MPa or more means that the tested sample is excellent in mechanical strength. Further, 210 MPa or more can be said to be more excellent in mechanical strength, and is therefore preferred. 300 MPa or more is more preferred.

[0063]

As known from the evaluation results shown in Table 1, it is recognized that the mullite-base sintered compact containing a predetermined amount of an yttrium element, in which the mullite crystal particles have a predetermined average particle size and a predetermined aspect ratio (Examples 1 to 14) are excellent in thermal shock resistance and mechanical strength.

In particular, the mullite-base sintered compacts containing an yttrium element in an yttrium oxide-equivalent amount of 0.02 to 0.05% by mass and having been processed for heat treatment (for re-heating treatment) (Examples 8 to 14) are recognized to have more excellent thermal shock resistance and mechanical strength.

[0064]

From the SEM images of Figs. 2 to 5, it is confirmed that the mullite-base sintered compacts of Examples 2 and 9 (Figs. 2 and 4) having a large relative density of 99.7% by mass have fewer voids and are excellent in compactness and have a lower ratio of columnar crystal particles, as compared with the mullite-base sintered compacts of Comparative Examples 4 and 6 (Figs. 3 and 5) having a large yttrium element content and having a smaller relative density of 99.7% by mass or less.

[0065]

In the EMS elemental mapping of the mullite-base sintered compact of Example 2, it is confirmed that yttrium elements are distributed in layers in a thickness of about 2 nm along the surface of the mullite crystal particle 1 both in the interfaces A and B.

Reference Signs List

[0066]

1 Mullite Crystal Particle

2 Crystal Grain Boundary Glass Phase

A Interface between Mullite Crystal Particle and Crystal Grain Boundary

Glass Phase

B Interface between Mullite Crystal Particles

Claims

[Claim 1]

A mullite-base sintered compact comprising an yttrium element in an yttrium oxide-equivalent amount of 0.01 to 0.25% by mass, wherein:

the average particle size of the mullite crystal particles that constitute the sintered compact is from 1.00 to 3.00 pm and the average aspect ratio of the mullite crystal particles is 1.65 or less.

[Claim 2]

The mullite-base sintered compact according to claim 1, having a relative density of 98.5% or more.

[Claim 3]

The mullite-base sintered compact according to claim 1 or 2, wherein the standard deviation of the particle size of the mullite crystal particles is less than 1.00 pm.

[Claim 4]

The mullite-base sintered compact according to any one of claims 1 to 3, wherein the standard deviation of the aspect ratio of the mullite crystal particles is less than 0.70.

[Claim 5]

The mullite-base sintered compact according to any one of claims 1 to 4, wherein the average major diameter of the mullite crystal particles is from 1.50 to 4.00 pm.

[Claim 6]

The mullite-base sintered compact according to any one of claims 1 to 5, wherein the average minor diameter of the mullite crystal particles is from 1.00 to 2.00 pm.

[Claim 7]

The mullite-base sintered compact according to any one of claims 1 o 6, wherein the ratio by number of columnar crystal particles to spherical crystal particles among the mullite crystal particles is 1.00 or less.

[Claim 8]

The mullite-base sintered compact according to any one of claims 1 to 7, wherein the mullite content is from 99.00 to 99.99% by mass.

[Claim 9] The mullite-base sintered compact according to any one of claims 1 to 8, wherein the yttrium element is, in at least a part of the surface of the mullite crystal particles, distributed in layers along the surface thereof.

[Claim 10]

The mullite-base sintered compact according to any one of claims 1 to 9, wherein the thermal shock-resistant temperature difference, as measured according to a thermal shock test method of a relative method stipulated in JIS R 1648:2002, is 300°C or higher.

[Claim 11]

The mullite-base sintered compact according to claim 10, wherein the thermal shock-resistant temperature difference is 320°C or higher.

[Claim 12]

A method for producing a mullite-base sintered compact of any one of claims 1 to 11, comprising:

a mixing step of preparing a mixed raw material comprising a mullite raw material powder and an yttrium compound,

a molding step of molding the mixed raw material to produce a molded article, and

a firing step of firing the molded article at 1,500 to 1,800°C to give a mullite-base sintered compact, wherein:

the amount of the yttrium compound to be added is such that the content of the yttrium element contained in the mullite-base sintered compact is from 0.01 to 0.25% by mass as an yttrium oxide-equivalent content thereof.

[Claim 13]

The method for producing a mullite-base sintered compact according to claim 12, wherein in the mixing step, the mixed raw material is prepared by wet mixing.

[Claim 14]

The method for producing a mullite-base sintered compact according to claim 12 or 13, comprising a step of heat-treating the mullite-base sintered compact obtained in the firing step at 1,300 to 1,600°C.