JP2017095333A - Alumina sintered body excellent in high temperature and corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an alumina sintered body excellent in high temperature property and corrosion resistance.SOLUTION: There is provided an alumina sintered body satisfying requirements <1> to <7>. <1> alumina content is 98.3 wt.% or more. <2> magnesia content is 0.1 to 1.0 wt.%. <3> silica content is 0.10 wt.% or less. <4> weight ratio of magnesia and silica (magnesia/silica) is 2.0 or more. <5> bulk density is 3.85 g/cmor more. <6> average crystal particle diameter is 3 to 20 μm. <7> a ratio of MgAlOspinel and alumina (MgAlOspinel/alumina) is 0.01 to 0.07.SELECTED DRAWING: None

Description

  The present invention relates to an alumina sintered body excellent in high temperature characteristics and corrosion resistance.

Alumina sintered bodies are excellent in corrosion resistance and heat resistance, and are cheaper and easier to handle than other ceramics. For a long time, such as high temperature members, heat treatment containers, setters, furnace core tubes, temperature measuring protection tubes, etc. Used in the field.
Recently, it is also used for heat treatment of positive electrode materials for lithium secondary batteries, electronic materials, phosphor materials, etc., but it is often used under severe conditions such as rapid temperature rise and fall. In addition, many processed products are heat-treated at once for efficiency of heat treatment, and a heat treatment member larger than the conventional product is required, but the load on such a large heat treatment member is It is larger than the load applied to the small heat treatment member. Therefore, there is a demand for a member for heat treatment made of alumina that can be used even under severe conditions and has excellent high-temperature characteristics such as high-temperature strength, creep characteristics, and thermal shock resistance. In the case of an alumina heat treatment member, if the time for repeated use and the high temperature environment is increased, alumina particles grow, resulting in deterioration in characteristics and variations in characteristics. Therefore, there is a demand for an alumina heat treatment member that is resistant to grain growth even in a high temperature environment for a long time and has excellent thermal stability.
However, since the conventional alumina sintered body has the second phase and the glass phase at the crystal grain boundary due to impurities contained in the sintered body, not only the high-temperature strength and creep characteristics decrease with increasing temperature. , Has the problem of low corrosion resistance.

  As a prior art, Patent Document 1 discloses an alumina sintered body containing alumina as a main component, containing zirconia, magnesia, and calcia in an amount of 0.05 to 10% by weight and having a maximum crystal grain size of 30 μm or less. Yes. However, if zirconia is contained in the alumina sintered body, there is a problem that mechanical properties and corrosion resistance at high temperatures are lowered, and if calcia is contained, there is a problem that strength is lowered due to abnormal grain growth. is there. Silica is an impurity that greatly affects not only high temperature characteristics but also corrosion resistance, and is an important factor that affects the characteristics. However, the necessity of controlling the amount of silica is not described at all.

  Patent Document 2 discloses an alumina sintered body having an alumina content of 99.8% by weight or more, an average crystal grain size of 2 μm or more, and excellent creep resistance at high temperatures. However, since this alumina sintered body has high purity of alumina and the amount of the sintering aid to be added is very small, 0.2% by weight or less, the alumina particles are prone to abnormal growth during firing, and the crystal grain size is small. There is a problem that it becomes non-uniform and the high temperature characteristics and corrosion resistance are reduced and variations are increased. Further, when the alumina sintered body is used at a high temperature, the thermal shock resistance is not described at all even though the thermal shock resistance affects the life of the alumina sintered body.

Further, in Patent Document 3, in a sintered body composed of α-Al ₂ O ₃ and MgAl ₂ O ₄ spinel dispersed in the grain boundary, the particle size of the spinel is 0.1 μm or less, and its content Discloses a heat-resistant ceramic having a content of 1.5 to 3.0% by weight in terms of magnesia. However, MgAl ₂ O ₄ spinel has higher corrosion resistance than alumina but low thermal shock resistance. Therefore, if it contains a large amount of MgAl ₂ O ₄ spinel phase, the thermal shock resistance decreases, and it can be applied to various applications. It is not a material. In addition, even when the temperature difference is the same, as the size of the alumina sintered body increases, the resulting thermal shock increases. Therefore, ceramics containing a large amount of MgAl ₂ O ₄ spinel as described above and having low thermal shock resistance are large. There is a problem that it is difficult.

JP 2004-107193 A Japanese Patent No. 3130987 Japanese Patent No. 2879169

  An object of this invention is to provide the alumina sintered compact excellent in the high temperature characteristic and corrosion resistance.

In view of the present situation as described above, the present inventors have conducted extensive research focusing on the chemical composition and microstructure of the alumina sintered body, and as a result, magnesia was used as a sintering aid, and the magnesia addition amount and high temperature characteristics. In addition, the amount of impurities that reduce corrosion resistance is set within a certain range, the average crystal grain size is within a specific range, and MgAl ₂ O ₄ spinel particles are generated at a specific ratio at the grain boundaries of alumina particles, thereby increasing the high temperature strength. The present inventors have found that an alumina sintered body excellent in high temperature characteristics such as creep resistance, thermal shock resistance, thermal stability, and corrosion resistance can be obtained, thereby completing the present invention.

That is, the above-mentioned problems are solved by the following inventions 1) to 3).
1) An alumina sintered body that satisfies the following requirements <1> to <7>.
<1> Alumina content is 98.3% by weight or more <2> Magnesia content is 0.1 to 1.0% by weight
<3> silica content of 0.10 wt% or less <4> weight ratio of magnesia and silica (magnesia / silica) of 2.0 or more <5> bulk density 3.85 g / cm ³ or more <6> mean crystal Particle size is 3-20μm
<7> Ratio of MgAl ₂ O ₄ spinel and alumina crystal phase amount (MgAl ₂ O ₄ spinel / alumina) is 0.01 to 0.07
2) The alumina sintered body according to 1), which satisfies the following requirement <8>.
<8> Variation coefficient of distribution of alumina crystal grain size is 20% or less 3) The alumina sintered body according to 1) or 2), wherein the following requirement <9> is satisfied.
<9> Zirconia content is 0.10 wt% or less, calcia content is 0.5 wt% or less

According to the present invention, an alumina sintered body excellent in high temperature characteristics and corrosion resistance can be provided.
Since this alumina sintered body is excellent in high-temperature characteristics and corrosion resistance, it is used as a material for firing lithium-ion battery positive electrode materials, phosphor materials, electronic parts such as piezoelectric bodies, dielectric bodies, magnetic bodies, and ceramic capacitors. Is preferred. Furthermore, it is also suitable as a crucible for melting metals and alloys. Furthermore, since it has excellent thermal shock resistance, it is suitable for various furnace core tubes for electric furnaces, high-temperature transfer rollers, support tubes, radians and tubes, gas blowing tubes, and gas sampling tubes.

Hereinafter, the present invention will be described in detail.
-About requirement <1> In this invention, alumina content shall be 98.3 weight% or more. Preferably it is 99.1 weight% or more. When the alumina content is less than 98.3% by weight, the second phase and the glass phase increase at the grain boundaries of the alumina particles to reduce the corrosion resistance, and the mechanical properties, particularly the strength and creep properties at high temperatures. Decreases. In consideration of the amount of impurities contained in the raw material used and the amount of impurities mixed during the raw material processing, the upper limit is about 99.8% by weight.

-About requirement <2> In this invention, 0.1-1.0 weight% of magnesia is contained as a sintering auxiliary agent. Preferably it is 0.2-0.7 weight%. When the magnesia content is within the above range, magnesia suppresses abnormal grain growth of alumina particles during firing, so that an alumina sintered body having a uniform crystal grain size can be obtained. Furthermore, MgAl ₂ O ₄ spinel particles are formed at the grain boundaries of the alumina particles to improve the corrosion resistance, strengthen the grain boundaries, and have high temperature characteristics such as high temperature strength, creep resistance, thermal shock resistance, and thermal stability. improves. When the magnesia content is less than 0.1% by weight, the amount of MgAl ₂ O ₄ spinel particles produced is small, the effect of improving high temperature characteristics and corrosion resistance is small, and magnesia grows abnormally as alumina particles during firing. Therefore, the crystal grain size is increased, the variation in the grain size distribution is increased, and the mechanical characteristics are deteriorated. If the magnesia content exceeds 1.0% by weight, a large amount of the second phase and glass phase are formed at the grain boundary of the alumina particles and the corrosion resistance is lowered, and the thermal shock resistance contained in the alumina sintered body is low. Since low MgAl ₂ O ₄ spinel particles increase or increase in size, the thermal shock resistance of the alumina sintered body as the base material is lowered.

-About requirement <3> In this invention, the silica which is an impurity shall be 0.10 weight% or less. Preferably it is 0.07 weight% or less, More preferably, it is 0.05 weight% or less. When silica exceeds 0.10% by weight, it becomes easy to form a glass phase or a second phase with impurities other than silica at the crystal grain boundary, such as a decrease in corrosion resistance, high temperature strength, creep resistance, thermal stability, etc. Degradation of high temperature characteristics occurs. In particular, since silica is highly reactive with lead used in piezoelectric materials and the like, it is desirable to reduce the silica content as much as possible and improve the corrosion resistance to lead in the field where lead is used.

-About requirement <4> In this invention, the weight ratio (magnesia / silica) of magnesia and silica shall be 2.0 or more. If the weight ratio is less than 2.0 even if the requirements of magnesia content of 0.1 to 1.0% by weight and silica content of 0.10% by weight or less are satisfied, the glass phase is composed of silica and other components. In addition to the effects of improving the high temperature properties such as high temperature strength, creep resistance, thermal stability and corrosion resistance due to the MgAl ₂ O ₄ spinel particles, the corrosion resistance and high temperature properties of the glass phase and the second phase are increased. The effect of decline becomes significant. Further, since silica is an impurity that lowers the high temperature characteristics, it is desirable that the content be as small as possible. Therefore, the weight ratio of magnesia / silica may approach infinity, but no matter how large the weight ratio does not adversely affect the characteristics of the resulting alumina sintered body, There is no.

In-requirement <5> present invention for, the bulk density of the alumina sintered body and 3.85 g / cm ³ or more. Preferably it is 3.90 g / cm ³ or more. When the bulk density is less than 3.85 g / cm ³ , many pores are present inside the sintered body, causing a decrease in strength and thermal shock resistance. In addition, since corrosion and reaction are likely to proceed starting from pores, corrosion resistance is reduced. The bulk density of the alumina sintered body is theoretically 3.99 g / cm ³ at the maximum, but the actual upper limit is about 3.98 g / cm ³ . The bulk density is measured according to JIS R 1634.

-About requirement <6> In this invention, the average crystal grain diameter of an alumina sintered compact shall be 3-20 micrometers. Preferably, it is 5-15 micrometers. When the average crystal grain size is less than 3 μm, the corrosion resistance is lowered, and particularly the creep resistance at high temperature is lowered. On the other hand, when it exceeds 20 μm, mechanical properties such as strength are deteriorated.
The average grain size was mirror-polished from an alumina sintered body, subjected to thermal etching, and then observed with a scanning electron microscope at a magnification set so that 100 or more alumina particles could fit in one field of view. Was obtained from the average of 10 points by the following formula.

D = 1.8 × L / n
[D: Average crystal grain size (μm), L: Measurement length (μm), n: Number of crystals per length L]

· Requirement in the present invention for <7>, you are necessary to contain the MgAl ₂ O ₄ spinel particles in the alumina sintered body. The presence of MgAl ₂ O ₄ spinel particles at the grain boundaries of alumina particles improves corrosion resistance, and the grain boundaries are strengthened to improve high temperature properties such as high temperature strength, creep resistance, thermal shock resistance, and thermal stability. . Further, in the present invention, the crystal phase amount ratio (MgAl ₂ O ₄ spinel / alumina) of MgAl ₂ O ₄ spinel and alumina present in the alumina sintered body needs to be 0.01 to 0.07, preferably It is the range of 0.02-0.05. Even when the requirement <2> of the magnesia content of 0.1 to 1.0% by weight is satisfied, when the ratio is less than 0.01, there are few MgAl ₂ O ₄ spinel particles present at the grain boundaries of the alumina particles. Therefore, the effect of improving the high temperature characteristics such as high temperature strength, creep resistance, thermal shock resistance, and corrosion resistance is small, and the thermal stability is significantly reduced. On the other hand, when the ratio exceeds 0.07, MgAl ₂ O ₄ spinel particles present at the grain boundaries of the alumina particles increase or increase. Since MgAl ₂ O ₄ spinel has lower thermal shock resistance than alumina, an alumina sintered body containing a large amount of MgAl ₂ O ₄ spinel particles has lower thermal shock resistance. The size of the MgAl ₂ O ₄ spinel particles is preferably 5 μm or less, and the alumina sintered body containing MgAl ₂ O ₄ spinel particles exceeding 5 μm has a reduced thermal shock resistance. The size of the MgAl ₂ O ₄ spinel particles was observed with a scanning electron microscope at a magnification set so that 100 or more alumina particles could be included in one field of view after mirror-polishing the alumina sintered body and performing thermal etching. and to EDX analysis, the particles magnesia and alumina coexist defined as MgAl ₂ O ₄ spinel particles were determined from the average of the maximum diameter of 10 MgAl ₂ O ₄ spinel particles.

In the present invention, when the alumina sintered body contains MgAl ₂ O ₄ spinel particles, the alumina sintered body is polished, and its mirror surface is X-ray source CuKα, tube voltage 40 kV, tube current 40 mA, diverging slit 0. .3 °, receiving side slit OPEN, scanning speed 3.0 ° / min, scanning axis 2θ / θ, scanning range 10 ° to 70 °, XRD (X-ray diffraction) measurement, MgAl in addition to alumina It means that a diffraction peak of ₂ O ₄ spinel is observed.
The ratio of the amount of MgAl ₂ O ₄ spinel crystal phase present in the alumina sintered body to the amount of crystal phase of alumina is the (311) plane peak of the MgAl ₂ O ₄ spinel crystal phase obtained by the above measurement and the alumina crystal It calculated | required from the intensity ratio (refer following formula) of the (113) plane peak of a phase.

Ratio of crystal phase amount = peak intensity of (311) plane of MgAl ₂ O ₄ spinel /
Peak intensity of (113) plane of alumina

-About requirement <8> In this invention, it is preferable that the variation coefficient of distribution of an alumina crystal grain diameter is 20% or less. The coefficient of variation is a value obtained by dividing the standard deviation of the crystal grain size by the average crystal grain size (see the following formula).

Coefficient of variation (%) = (standard deviation of alumina crystal grain size / average crystal grain size of alumina) × 100

When the coefficient of variation is larger than 20%, it means that the distribution of the crystal grain size is wide. There are a part where the crystal grain size is small and a part where the crystal grain size is small, and the part where the crystal grain size is small is a decrease in corrosion resistance, especially creep resistance. A portion having a large crystal grain size is not preferable because it causes a decrease in bending strength and thermal shock resistance.

-About requirement <9> Since impurities other than a silica also reduce the high temperature characteristic and corrosion resistance of an alumina sintered compact, it is preferable that there are few. Among them, zirconia is preferably 0.10% by weight or less, more preferably 0.05% by weight or less, and still more preferably 0.01% by weight or less. If it exceeds 0.10% by weight, it is not preferable because it may cause deterioration of mechanical properties and corrosion resistance at high temperatures.
Further, calcia is preferably 0.5% by weight or less, more preferably 0.3% by weight or less, and further preferably 0.1% by weight or less. If it exceeds 0.5% by weight, abnormal grain growth of alumina particles is promoted, the crystal grain size is increased, and further, the dispersion of the grain size distribution is increased, which may cause a decrease in mechanical properties. Absent.

The alumina sintered body of the present invention can be produced as follows.
The alumina raw material has a purity of 99.7% by weight or more, preferably 99.8% by weight or more, an average particle diameter of 3 μm or less, preferably 2 μm or less, a specific surface area of 3 to 10 m ² / g, preferably 4 to 8 m ² / g. Use one. A powder having an average particle size larger than 3 μm and a specific surface area smaller than 3 m ² / g is not preferable because it has low moldability, and therefore defects are easily generated inside the sintered body, and strength and corrosion resistance are lowered. In addition, a powder having a specific surface area larger than 10 m ² / g is not preferable because abnormal grain growth is liable to occur during firing, and the microstructure of the alumina sintered body becomes non-uniform and high temperature characteristics and corrosion resistance deteriorate.

As the sintering aid, magnesia powder having a purity of 98% by weight or more, preferably 99% by weight or more, an average particle diameter of 5 μm or less, and a specific surface area of 5 m ² / g or more can be used. From an average particle diameter of 8 μm or less, a specific surface area of 10 m ² / g or more, and a purity of 98% by weight or more when converted to an oxide, a salt of a carbonate or the like, or an average particle diameter of 3 μm or less, a specific surface area MgAl ₂ O ₄ spinel powder of 5 m ² / g or more is preferable.
If the average particle size or specific surface area of these powders is out of the above range, the mixing and dispersibility into the alumina powder is reduced during pulverization and dispersion mixing, and the amount of MgAl ₂ O ₄ spinel phase contained in the sintered body and MgAl _{Since the 2} O ₄ spinel particle size does not fall within the predetermined range, the corrosion resistance and the high temperature characteristics are deteriorated and the variation becomes large, it is not preferable.

A sintering aid is added to the alumina raw material so as to have a predetermined amount and composition, and the mixture is pulverized and dispersed using high-purity alumina balls in a wet pot mill, attrition mill or the like using water as a solvent. The average particle size of the powder after pulverization / dispersion mixing needs to be 0.4 to 1.5 μm, preferably 0.5 to 1.0 μm. If the average particle diameter is out of the above range, moldability and sinterability are deteriorated, defects such as pores increase in the sintered body, and mechanical properties and corrosion resistance are deteriorated. Moreover, since impurities from mills, balls, etc. are mixed by pulverization / dispersion mixing, the pulverization / dispersion mixing conditions are appropriately set so that the amount of impurities mixed is minimized, and the average particle diameter and impurities of the resulting powder Make sure the amount is within a certain range.
When adopting press molding, rubber press molding, etc. as a molding method, a known molding aid (for example, wax emulsion, PVA, acrylic resin, etc.) is added to the dispersed / mixed slurry as necessary, and a spray dryer, etc. The powder is dried by a known method to produce a molded powder, which is then molded.

In addition, when adopting the cast molding method, a known molding aid (wax emulsion, acrylic resin, etc.) is added to the dispersed / mixed slurry as necessary, and the mud is discharged using a gypsum mold or a resin mold. Molding is performed by casting, filling casting, or pressure casting.
In addition, when an extrusion molding method is adopted, the dispersed and mixed slurry is dried, sized, and mixed with water, a binder (for example, methylcellulose, etc.) using a mixer to produce a clay, and extruded. Mold.
By firing the molded body obtained as described above at 1500 ° C. to 1750 ° C., preferably 1550 ° C. to 1700 ° C., an alumina sintered body excellent in high temperature characteristics and corrosion resistance of the present invention can be obtained. When the firing temperature is out of the above range, the obtained alumina sintered body does not satisfy at least one of the requirements of the present invention, and thus the alumina sintered body excellent in high temperature characteristics and corrosion resistance of the present invention cannot be obtained.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated further more concretely, this invention is not limited at all by these Examples.

Examples 1-14, Comparative Examples 1-11
Table 1 shows magnesium hydroxide (specific surface area 32 m ² / g), which is a magnesia source of a sintering aid, on an alumina raw material having an average particle size of 0.8 μm, a specific surface area of 6 m ² / g, and an alumina purity of 99.8% by weight. These were added and blended so as to have a magnesia content as shown in each column of Examples and Comparative Examples, and pulverized and mixed using a 92 wt% alumina pot mill and balls. An acrylic resin was added as a binder to the resulting slurry, and spray-dried to prepare a molding powder. The obtained molding powder was CIP-molded at a molding pressure of 100 MPa and fired at 1480 to 1780 ° C. for 2 hours in the air. The obtained alumina sintered body is ground and processed, a test piece for bending test (3 mm × 4 mm × 50 mm), a test piece for creep test (2 mm × 5 mm × 140 mm), a test piece for thermal shock test (5 mm × 6 mm × 50 mm) ), A test piece for thermal stability test (20 mm × 20 mm × 5 mm) and a test piece for PbO resistance test (15 mm × 15 mm × 2 mm). The surface roughness of the test piece was 0.20 μmRa or less.
In Comparative Example 5, zirconia balls were used as balls used for pulverization / dispersion mixing.
In Comparative Example 9, since the alumina raw material having an average particle size of 3.5 μm was used, the average particle size of the powder after pulverization / dispersion mixing was 1.59 μm, which exceeded the predetermined range.

The raw material characteristics of Examples and Comparative Examples are shown in Table 1, and the sintered body characteristics are shown in Table 2. The measuring method of each characteristic is as follows.

<Raw material characteristics>
The alumina content, magnesia content and impurity content were measured using an ICP emission analyzer ICPS-8100 manufactured by Shimadzu Corporation. The average particle size of the powder after pulverization and mixing was measured using a Microtrac 3300EX manufactured by Nikkiso Co., Ltd.

<Sintered body characteristics>
The bulk density was measured according to JIS R 1634 using an electronic balance XP205DR manufactured by METTLER TOLEDO.
The average crystal grain size was determined by an intercept method using a field emission scanning electron microscope SU8020 manufactured by Hitachi High-Technologies Corporation after mirror-polishing an alumina sintered body and performing thermal etching. Further, from the obtained average crystal grain size and standard deviation, the coefficient of variation of crystal grain size distribution (crystal grain size coefficient of variation) was determined.
The MgAl ₂ O ₄ spinel / alumina crystal phase ratio was measured using an X-ray diffractometer D8ADVANCE manufactured by Bruker AXS.

The following tests were performed using the test pieces for the tests of the above Examples and Comparative Examples.
The results are summarized in Table 2.

<Bending strength>
A bending test was conducted at room temperature in accordance with JIS R 1601, and at 1400 ° C. (high temperature) in accordance with JIS R 1604, and bending strength (MPa) was measured. A universal material testing machine 5965 manufactured by INSTRON was used for measuring the bending strength at room temperature, and a precision universal testing machine AG-500C manufactured by Shimadzu Corporation was used for measuring the bending strength at high temperature.

<Creep resistance>
In the center of the test piece fixed at a fulcrum distance of 100 mm, a wormwood made of an alumina sintered body with a length of 31 mm is placed so that the load stress is 2 MPa, and heated in the atmosphere at 1450 ° C. for 2 hours using an electric furnace. The deflection (mm) of the test piece after heating was measured. For measurement, LINEAR GAGE LG-150 manufactured by MITUTOYO was used.

<Thermal shock resistance>
Insert the test piece into an electric furnace maintained at 100 to 230 ° C. in the atmosphere, hold it for 30 minutes, and then put it into 10 ° C. water. After 30 minutes, take out the test piece from the water and perform a bending test at room temperature. The temperature difference [ΔT (° C.)] at which the bending strength decreases was measured. ΔT (° C.) was obtained by the following equation.

ΔT (° C.) = Heating temperature in electric furnace (° C.) − Water temperature (° C.)

<Thermal stability>
The test piece was heated in the atmosphere at 1500 ° C. for 100 hours using an electric furnace. Each test piece before and after heating is mirror-polished and subjected to thermal etching, and then the average crystal grain size of the alumina particles before and after heating is determined by an intercept method using a scanning electron microscope. The grain growth rate (% ) Was calculated. For observation of the microstructure, a field emission scanning electron microscope SU8020 manufactured by Hitachi High-Technologies Corporation was used.

Grain growth rate (%) = [(average grain size after heating−average grain size before heating) /
Average grain size before heating] × 100

<PbO resistance>
A PbO molded body having a diameter of 12 mm × 1 mm is placed on a test piece, and a wormwood made of a high-purity ZrO ₂ sintered body is placed thereon so that the load is 0.5 × 10 ² Pa. Then, using an electric furnace, heating was performed in the atmosphere at 870 ° C. for 20 hours for 3 cycles, the weight of the test piece before and after heating was measured, and the weight change rate was calculated by the following equation.
An electronic balance LC1201S manufactured by Sartorius was used for the measurement.

Weight change rate (%) =
[(Test piece weight after heating−Test piece weight before heating) / Test piece weight before heating]
× 100

As can be seen from Tables 1 and 2, the alumina sintered body of the present invention produces MgAl ₂ O ₄ spinel particles at the grain boundaries of the alumina particles by controlling the amount of magnesia source added, thereby strengthening the grain boundaries. In addition, high temperature characteristics such as high temperature strength, creep resistance, and thermal shock resistance are improved, and MgAl ₂ O ₄ spinel particles are present at the grain boundaries and there are few impurities at the grain boundaries, resulting in high corrosion resistance.
Therefore, it is useful as a heat and corrosion resistant material, and further has a long life as a member for heat treatment because crystal grain growth at high temperature is very small and excellent in thermal stability.
On the other hand, an alumina sintered body that does not satisfy one or more of the requirements <1> to <7> of the present invention is inferior in high temperature characteristics and corrosion resistance.

That is, Comparative Examples 1 and 2 are examples in which the magnesia content is outside the range of the requirement <2>.
In Comparative Example 3, since the magnesia content was close to the lower limit of the requirement <2> and the firing temperature was lower than 1500 ° C., the amount of MgAl ₂ O ₄ spinel particles generated was reduced, and MgAl ₂ O ₄ spinel / alumina crystals This is an example in which the phase ratio is smaller than the range of requirement <7>.
In Comparative Example 4, since the magnesia content was close to the upper limit of the requirement <2> and the firing temperature was higher than 1750 ° C., the amount of MgAl ₂ O ₄ spinel particles generated increased, and MgAl ₂ O ₄ spinel / alumina crystals This is an example in which the phase ratio is larger than the requirement <7>.
In Comparative Example 5, zirconia balls were used instead of alumina balls when the alumina raw material and magnesium hydroxide were pulverized / dispersed and mixed, so that the content of silica and zirconia increased from the mill container and pulverized balls. is there.
In Comparative Example 6, when the alumina raw material and magnesium hydroxide are pulverized and dispersed and mixed, silica is added so that the silica content exceeds the range of requirement <3>.
In Comparative Example 7, when the alumina raw material and magnesium hydroxide were pulverized and dispersed and mixed, calcia was added so that the calcia content was more than 0.5% by weight, so the alumina content was more than the range of requirement <1>. This is an example of less.
Comparative Example 8 is an example in which the magnesia and silica contents are within the range of requirement <2><3>, but the weight ratio of magnesia / silica is smaller than the range of requirement <4>.
In Comparative Example 9, since an alumina raw material having a large average particle diameter was used, the powder average particle diameter after pulverization / dispersion mixing was large, and the firing temperature was lower than 1500 ° C., so the bulk density was in the range of requirement <5>. This is a smaller example.
In Comparative Example 10, since the firing temperature was lower than 1500 ° C., the average crystal grain size of the alumina crystal particles was smaller than the requirement <6> range.
In Comparative Example 11, since the firing temperature was higher than 1750 ° C., the average crystal grain size of the alumina crystal particles was larger than the range of requirement <6>.

Claims (3)

An alumina sintered body that satisfies the following requirements <1> to <7>.
<1> Alumina content is 98.3% by weight or more <2> Magnesia content is 0.1 to 1.0% by weight
<3> silica content of 0.10 wt% or less <4> weight ratio of magnesia and silica (magnesia / silica) of 2.0 or more <5> bulk density 3.85 g / cm ³ or more <6> mean crystal Particle size is 3-20μm
<7> Ratio of MgAl ₂ O ₄ spinel and alumina crystal phase amount (MgAl ₂ O ₄ spinel / alumina) is 0.01 to 0.07 The alumina sintered body according to claim 1, wherein the following requirement <8> is satisfied.
<8> Coefficient of variation of alumina crystal grain size distribution is 20% or less The alumina sintered body according to claim 1 or 2, wherein the following requirement <9> is satisfied.
<9> Zirconia content is 0.10 wt% or less, calcia content is 0.5 wt% or less