Alumina-based sintered compact and method for producing alumina-based sintered compact

Abstract

To suppress lowering of bending strength while realizing cost reduction of an alumina-based sintered compact.SOLUTION: The alumina-based sintered compact according to the present invention comprises a main phase made of Al2O3 and a heterogeneous phase containing Ca, Si, Al and O. The content of Al in the alumina-based sintered compact in terms of Al2O3 is 99.5 mass% or more and 99.9 mass% or less. Therefore, the alumina-based sintered compact can be manufactured using an alumina material containing a relatively large amount of impurities (i.e., the alumina purity is relatively low). Therefore, the cost reduction of the alumina sintered compact can be realized. In the alumina sintered compact, the average major diameter of the different phases is 30 μm or less. Thus, by suppressing coarsening of different phases in the alumina sintered compact, it is possible to suppress lowering of the bending strength of the alumina sintered compact even when a material containing a relatively large amount of impurities is used.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an alumina sintered body and a method for manufacturing the alumina sintered body.

Background Art

[0002] Conventionally, alumina sintered bodies mainly composed of alumina (Al2O3) have been used in various applications because of their high mechanical strength, chemical stability, electrical insulation, etc. In recent years, there has been an increasing demand for cost reduction of alumina sintered bodies.

[0003] As a method for realizing cost reduction of alumina sintered bodies, for example, using inexpensive alumina materials containing relatively large amounts of impurities such as silicon (Si) and calcium (Ca) has been practiced. In Patent Document 1, further, in an alumina sintered body, an amorphous phase containing Si, Ca, aluminum (Al), and oxygen (O) is formed at the grain boundaries of crystal particles constituting the sintered body to promote grain growth of the crystal particles, and by increasing the crystal grain size of the alumina sintered body, it has also been proposed to reduce the number of processing steps (i.e., reduce the processing cost) during grinding of the alumina sintered body.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, when manufacturing an alumina sintered body using an alumina material containing a relatively large amount of impurities as described above, there is a risk that the bending strength of the alumina sintered body may decrease.

[0006] The present invention has been made in view of the above problems, and an object thereof is to suppress a decrease in bending strength while realizing cost reduction of an alumina sintered body.

Means for Solving the Problem

[0007] Aspect 1 of the present invention is an alumina-based sintered body comprising a main phase composed of Al2O3 and a different phase containing Ca, Si, Al, and O. The content of Al in terms of Al2O3 in the alumina-based sintered body is 99.5 mass% or more and 99.9 mass% or less. The average major axis length of the different phase is 30 μm or less.

[0008] Aspect 2 of the present invention is the alumina-based sintered body of Aspect 1, wherein the content of Ca in terms of CaO in the alumina-based sintered body is 200 mass ppm or more.

[0009] Aspect 3 of the present invention is the alumina-based sintered body of Aspect 2, wherein the content of Ca in terms of CaO in the alumina-based sintered body is 350 mass ppm or more. Let the content of Ca in terms of CaO in the alumina-based sintered body be R1, and the content of Si in terms of SiO2 in the alumina-based sintered body be R2. R1 is greater than 0.55 times (R1 + R2).

[0010] Aspect 4 of the present invention is the alumina-based sintered body according to any one of Aspects 1 to 3, wherein the content of Na in terms of Na2O in the alumina-based sintered body is 100 mass ppm or more and 250 mass ppm or less. The content of Si in terms of SiO2 in the alumina-based sintered body is 150 mass ppm or more and 500 mass ppm or less.

[0011] Aspect 5 of the present invention is the alumina-based sintered body according to any one of Aspects 1 to 3 (it may also be any one of Aspects 1 to 4), wherein the average particle size of the Al2O3 particles is 10 μm or less.

[0012] Aspect 6 of the present invention is the alumina-based sintered body according to any one of Aspects 1 to 3 (it may also be any one of Aspects 1 to 5), wherein the relative density of the alumina-based sintered body is 98% or more.

[0013] Aspect 7 of the present invention is an alumina sintered body according to any one of Aspects 1 to 3 (which may be any one of Aspects 1 to 6). The 3σ lower limit value of the flexural strength of the alumina sintered body is 400 MPa or more.

[0014] Aspect 8 of the present invention is a method for manufacturing an alumina sintered body, comprising: a) a step of molding a material powder to form a molded body; and b) a step of firing the molded body to manufacture an alumina sintered body. The content of Al2O3 in the material powder is 99.5% by mass or more and 99.9% by mass or less. The alumina sintered body includes a main phase composed of Al2O3 and a heterogeneous phase containing Ca, Si, Al, and O. The average major axis of the heterogeneous phase is 30 μm or less.

[0015] Aspect 9 of the present invention is a method for manufacturing an alumina sintered body according to Aspect 8, wherein the content of CaO in the material powder is 200 ppm by mass or more.

[0016] Aspect 10 of the present invention is a method for manufacturing an alumina sintered body according to Aspect 9, wherein the content of CaO in the material powder is 300 ppm by mass or more. The content of CaO in the material powder is greater than 0.57 times the total content of CaO and SiO2 in the material powder.

[0017] Aspect 11 of the present invention is a method for manufacturing an alumina sintered body according to any one of Aspects 8 to 10, wherein the content of Na2O in the material powder is 300 ppm by mass or more and 700 ppm by mass or less. The content of SiO2 in the material powder is 200 ppm by mass or more and 500 ppm by mass or less.

[0018] Aspect 12 of the present invention is a method for manufacturing an alumina sintered body according to any one of Aspects 8 to 10 (which may be any one of Aspects 8 to 11). The content of Na in terms of Na2O in the alumina sintered body is less than the content of Na2O in the material powder. The difference between the content of Na2O in the material powder and the content of Na in terms of Na2O in the alumina sintered body is 100 ppm by mass or more.

[0019] Aspect 13 of the present invention is a method for manufacturing an alumina sintered body according to any one of Aspects 8 to 10 (or any one of Aspects 8 to 12), wherein the firing temperature in the step (b) is 1580 °C or higher.

[0020] Aspect 14 of the present invention is a method for manufacturing an alumina sintered body according to Aspect 13, wherein in the step (b), the heating rate in the range of 1580 °C or higher is 50 °C / h or higher.

Advantages of the Invention

[0021] In the present invention, it is possible to suppress a decrease in bending strength while achieving cost reduction of the alumina sintered body.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0023] Hereinafter, an alumina sintered body according to one embodiment of the present invention will be described. The alumina sintered body is a ceramic sintered body mainly formed of alumina (Al2O3). The alumina sintered body has high mechanical strength, chemical stability, electrical insulation, etc., and is used, for example, as parts for fixing and insulating conductive materials.

[0024] The alumina sintered body is manufactured using an inexpensive material containing a relatively large amount of impurities. The impurities contained in the material include, for example, silicon (Si) and calcium (Ca). Therefore, the alumina sintered body includes a main phase composed of Al2O3 and a foreign phase that is a phase other than the main phase. Further, the content of Al in terms of Al2O3 in the alumina sintered body is 99.5% by mass or more and 99.9% by mass or less. The content of Al in terms of Al2O3 is the content of Al2O3 in the alumina sintered body assuming that all the Al elements contained in the alumina sintered body are Al2O3. The same applies to the content in terms of oxides of other elements described later.

[0025] By the way, as described above, in an alumina sintered body manufactured using a material containing a relatively large amount of impurities, the flexural strength generally tends to decrease. As a result of intensive research, the inventor of the present application has obtained the finding that the cause of the decrease in flexural strength in the alumina sintered body is the coarsening of the foreign phase caused by impurities. Specifically, it has been found that a coarse foreign phase containing Ca, Si, aluminum (Al), and oxygen (O) is generated in the alumina sintered body, and that the fracture of the sample in the flexural strength test occurs starting from the coarsened foreign phase. As a result of further intensive research, the inventor of the present application has reached the invention of an alumina sintered body capable of suppressing a decrease in flexural strength while achieving cost reduction by controlling the average major axis of the foreign phase containing Ca, Si, Al, and O to 30 μm or less.

[0026] In this embodiment, the lower limit value of 3σ of the flexural strength of the alumina sintered body is, for example, 400 MPa or more. The flexural strength of the alumina sintered body is measured by a four-point bending method in accordance with "JIS R 1601". The lower limit value of 3σ of the flexural strength is a value obtained by measuring the flexural strength of each of a plurality (for example, 20) of samples cut out from the alumina sintered body and subtracting three times the standard deviation (σ) from the arithmetic mean of the flexural strengths of the plurality of samples. The relative density of the alumina sintered body is, for example, 98% or more. The relative density is measured by the Archimedes method in accordance with "JIS K 7112". Further, in the alumina sintered body according to this embodiment, the average particle size of the Al2O3 particles is, for example, 10 μm or less. Thereby, the denseness of the alumina sintered body is improved.

[0027] The average particle size of the Al2O3 particles in the alumina sintered body is determined as follows. First, a sample in a substantially rectangular parallelepiped shape of 1.5 mm × 2 mm × 20 mm is cut out from the alumina sintered body, and the 2 mm × 20 mm surface of the sample is mirror-polished. Subsequently, after washing the sample, thermal etching is performed at 1350 °C for 15 minutes in the atmosphere to obtain an observation sample. Next, an image obtained by observing the mirror-polished surface of the observation material with a scanning electron microscope (SEM) at a magnification of 3000 times is acquired. Then, five arbitrary straight lines are randomly set on the image, and the number of Al2O3 particles located on (i.e., passed by) the five straight lines is counted. Thereafter, the value obtained by dividing the total length of the five lines by the number of the Al2O3 particles is taken as the average particle size of the Al2O3 particles. Note that, as the above SEM, the SEM function of an electron probe microanalyzer (EPMA) may be used.

[0028] In this embodiment, when the content of Ca in terms of calcium oxide (CaO) in the alumina sintered body is defined as "R1", the content R1 is, for example, 200 mass ppm or more. The content R1 may be 350 mass ppm or more. When the content R1 is 350 mass ppm or more, when the content of Si in terms of SiO2 in the alumina sintered body is defined as "R2", R1 is greater than 0.55 times (R1 + R2). In other words, the ratio of R1 to the sum of R1 and R2 (that is, R1 / (R1 + R2)) is greater than 0.55.

[0029] In this embodiment, the content of sodium (Na) in terms of sodium oxide (Na2O) in the alumina sintered body is, for example, 100 mass ppm or more and 250 mass ppm or less. Also, the content of Si in terms of silicon dioxide (SiO2) in the alumina sintered body is, for example, 150 mass ppm or more and 500 mass ppm or less.

[0030] Next, a method for manufacturing the alumina sintered body according to this embodiment will be described. As shown in FIG. 1, when manufacturing the alumina sintered body, first, a material powder (that is, a raw material) is molded to form a molded body (step S11). In step S11, for example, the material powder is filled into a mold and cold isostatic pressing (CIP) is performed to form a molded body. The main component of the material powder is Al2O3. The content of Al2O3 in the material powder is 99.5 mass% or more and 99.9 mass% or less.

[0031] The material powder contains a relatively large amount of impurities (that is, components other than Al2O3). The impurities are, for example, CaO, SiO2, and Na2O. The content of CaO in the material powder is, for example, 200 mass ppm or more. The content of CaO in the material powder may be 300 mass ppm or more. In this case, the content of CaO in the material powder is greater than 0.57 times the total content of CaO and SiO2 in the material powder (that is, the sum of the content of CaO and the content of SiO2).

[0032] The content rate of Na2O in the material powder is, for example, 300 mass ppm or more and 700 mass ppm or less. The content rate of SiO2 in the material powder is, for example, 200 mass ppm or more and 500 mass ppm or less.

[0033] Subsequently, the formed body formed in step S11 is fired to produce the above-described alumina sintered body (step S12). The firing temperature in step S12 (i.e., the maximum temperature in the furnace during firing) is, for example, 1550 °C or higher, preferably 1580 °C or higher. The firing temperature in step S12 is, for example, 1650 °C or lower, preferably 1630 °C or lower, more preferably 1600 °C or lower, and still more preferably 1595 °C or lower. The firing time in step S12 (i.e., the time maintained at the above firing temperature) is, for example, 1 hour or more and 10 hours or less.

[0034] During the firing in step S12, due to impurities in the material powder, a β-phase, which is a non-stoichiometric compound containing Si, Al, Na, and O, is formed. As the firing progresses, Na volatilizes from the β-phase, and Ca present around the β-phase enters the region after the volatilization of Na, thereby forming a heterogeneous phase mainly containing Ca, Si, Al, and O. In this heterogeneous phase, Ca accumulates and becomes concentrated from the surroundings. In the following description, this heterogeneous phase mainly containing Ca, Si, Al, and O is also referred to as the "Ca-concentrated heterogeneous phase". The Ca-concentrated heterogeneous phase may also contain magnesium (Mg) or the like as a main component, but does not contain Na as a main component.

[0035] In the present embodiment, as described above, by adjusting the composition of the material powder, the sintering conditions, etc., and suppressing the coarsening of the Ca-enriched heterogeneous phase as exemplified in FIG. 2, the average major axis of the Ca-enriched heterogeneous phase (i.e., the average major axis of the crystal grains of the Ca-enriched heterogeneous phase) is controlled to be 30 μm or less. FIG. 2 is an image obtained by observing the mirror-polished surface of the sample used in calculating the average particle size of the above-mentioned Al2O3 particles (i.e., the 2 mm × 20 mm mirror-polished surface of a substantially rectangular parallelepiped sample of 1.5 mm × 2 mm × 20 mm) at a magnification of 500 times using the SEM function of EPMA. Note that the above-mentioned thermal etching is not performed on the observation sample for EPMA. The white substantially rectangular portion existing in the region surrounded by the broken line in FIG. 2 is the Ca-enriched heterogeneous phase. Also, substantially the entire area other than the white portion is the main phase composed of Al2O3. The average major axis of the Ca-enriched heterogeneous phase is the arithmetic mean of the major axes of a predetermined number (for example, several tens) of Ca-enriched heterogeneous phases (i.e., the longest of the distances between any two points on the contour of the Ca-enriched heterogeneous phase) in the image obtained by observing the mirror-polished surface of the above-mentioned observation sample at a magnification of 500 times with SEM.

[0036] On the other hand, FIG. 3 is an image obtained in substantially the same manner as the image of FIG. 2 for the alumina sintered body of the comparative example for comparison with the alumina sintered body according to the present invention. In the alumina sintered body of the comparative example, the average major axis of the Ca-enriched heterogeneous phase (i.e., the white substantially rectangular portion existing in the region surrounded by the broken line) is larger than 30 μm. For this reason, the flexural strength of the alumina sintered body of the comparative example decreases, and for example, the 3σ lower limit value of the flexural strength becomes less than 400 MPa.

[0037] In step S12, as described above, Ca accumulates in the region where Na has volatilized to form a Ca-enriched heterogeneous phase. In particular, when the temperature in the furnace during firing becomes 1580°C or higher, the accumulation of Ca tends to be promoted. Also, in the range where the furnace temperature is 1580°C or higher, the accumulation of Ca tends to be further promoted as the heating rate decreases. Therefore, when the firing temperature is higher than 1580°C, in step S12, the heating rate in the range of 1580°C or higher is preferably 50°C / h or more. Thereby, the accumulation of Ca is suppressed, and the coarsening of the Ca-enriched heterogeneous phase is suppressed.

[0038] As described above, since volatilization of Na occurs in step S12, the content rate of Na in terms of Na2O in the alumina sintered body after firing decreases relatively greatly compared to the content rate of Na2O in the material powder before firing. The difference between the content rate of Na2O in the material powder and the content rate of Na in terms of Na2O in the alumina sintered body is, for example, 100 mass ppm or more.

[0039] Next, examples of the alumina sintered body according to the present invention and comparative examples for comparison with the examples will be described with reference to Tables 1 to 4.

[0040] [Table 1]

[0041] [Table 2]

[0042] [Table 3]

[0043] [Table 4]

[0044] The alumina sintered compacts of Examples 1 to 6 and the alumina sintered compacts of Comparative Examples 1 to 3 were manufactured by the manufacturing method of Steps S11 to S12 described above. The mold used in Step S11 was a substantially cylindrical shape with a diameter of 20 mm and a height of 5 mm. The pressure applied by CIP in Step S11 was 1 ton, and the pressurization time was 30 seconds. The firing temperature in Step S12 was changed in the range of 1580 °C to 1630 °C, and the heating rate was changed in the range of 10 °C / h to 200 °C / h. The firing time in Step S12 was 1.5 hours. After completion of Step S12, the obtained alumina sintered compact was subjected to cutting, and a sample in a substantially rectangular parallelepiped shape of 1.5 mm × 2 mm × 20 mm was obtained. Then, the surface of 2 mm × 20 mm of the sample was mirror-polished, and after washing the sample, it was used as an observation sample for EPMA. Also, for particle size observation, the washed sample was thermally etched at 1350 °C for 15 minutes in the atmosphere to obtain an observation sample.

[0045] The compositions of the material powders in Tables 1 to 4 (that is, the chemical compositions of the raw materials) were obtained by performing impurity analysis by EPMA on the material powders before molding in Step S11. Also, the composition of the alumina sintered compact (that is, the chemical composition of the sintered compact) was obtained by performing impurity analysis by EPMA on the above observation sample. In these impurity analyses, EPMA-8050G manufactured by Shimadzu Corporation was used as the EPMA. The average major axis length of the Ca-enriched heterogeneous phase was determined as the arithmetic mean of the major axis lengths of 10 Ca-enriched heterogeneous phases by the same method as above. The calculation method of the average particle size of the Al2O3 particles in the alumina sintered compact was also the same as that described above. The flexural strength of the alumina sintered compact was determined by a four-point bending method in accordance with "JIS R 1601" as described above. The average value of the flexural strength is the arithmetic mean of the flexural strengths measured for 20 samples. The 3σ lower limit value of the flexural strength was obtained by subtracting three times the standard deviation (σ) from the arithmetic mean of the flexural strengths measured for the 20 samples. The relative density of the alumina sintered compact was determined by the Archimedes method in accordance with "JIS K 7112" as described above.

[0046] In Example 1, commercially available relatively high-purity alumina was used as the material powder. The respective contents of Al2O3, Na2O, CaO, and SiO2 in the material powder were 99.9 mass%, 350 mass ppm, 225 mass ppm, and 280 mass ppm. The content of CaO with respect to the total content of CaO and SiO2 in the material powder (i.e., CaO / (CaO + SiO2)) was 0.45. The firing temperature was 1580 °C, and the heating rate was 100 °C / h. The heating time was 1.5 hours.

[0047] The main phase of the alumina sintered body of Example 1 was Al2O3 (corundum). In the alumina sintered body, there was a Ca-enriched heterogeneous phase containing Ca, Si, Al, Mg, and O as main components. Also, in the alumina sintered body, there was no heterogeneous phase containing Na as a main component (hereinafter also referred to as "Na-containing heterogeneous phase"). The average major axis length of the Ca-enriched heterogeneous phase in the alumina sintered body was 18 μm.

[0048] The content of Al in terms of Al2O3 in the alumina sintered body of Example 1 was 99.9 mass%. The content of Na in terms of Na2O in the alumina sintered body was 140 mass ppm. The content of Ca (R1) in terms of CaO in the alumina sintered body was 230 mass ppm. The content of Si (R2) in terms of SiO2 in the alumina sintered body was 310 mass ppm. R1 / (R1 + R2) was 0.43.

[0049] As described above, since Na volatilizes during the firing in step S12, the content of Na in terms of Na2O in the alumina sintered body is 210 mass ppm less than the content of Na2O in the material powder. Also, due to the volatilization of Na and the addition of a dispersion material or the like in the manufacturing process of the alumina sintered body, the content of Ca in terms of CaO in the alumina sintered body is higher than the content of CaO in the material powder, and the content of Si in terms of SiO2 in the alumina sintered body is higher than the content of SiO2 in the material powder.

[0050] In Example 1, the relative density of the alumina sintered body was 98%. The average particle size of the Al2O3 particles in the alumina sintered body was 5 μm. The average value of the flexural strength of the alumina sintered body was 530 MPa, and the 3σ lower limit value (i.e., average value - 3σ) was 420 MPa.

[0051] In Example 2, the respective contents of Na2O, CaO, and SiO2 in the material powder were 390 ppm by mass, 240 ppm by mass, and 230 ppm by mass. The content of CaO with respect to the total content of CaO and SiO2 in the material powder was 0.51. Other conditions (i.e., the content of Al2O3 in the material powder, firing temperature, heating rate, heating time, etc.) were the same as those in Example 1.

[0052] The main phase of the alumina sintered body of Example 2 was Al2O3 (corundum). In the alumina sintered body, Ca-enriched heterogeneous phases were present, and Na-containing heterogeneous phases were not present. The average major axis length of the Ca-enriched heterogeneous phases in the alumina sintered body was 20 μm.

[0053] The content of Al in terms of Al2O3 in the alumina sintered body of Example 2 was 99.9% by mass. The content of Na in terms of Na2O in the alumina sintered body was 150 ppm by mass. The content of Ca (R1) in terms of CaO in the alumina sintered body was 280 ppm by mass. The content of Si (R2) in terms of SiO2 in the alumina sintered body was 300 ppm by mass. R1 / (R1 + R2) was 0.48.

[0054] In Example 2, the content of Na in terms of Na2O in the alumina sintered body was 240 ppm by mass less than the content of Na2O in the material powder. Also, the content of Ca in terms of CaO in the alumina sintered body was greater than the content of CaO in the material powder, and the content of Si in terms of SiO2 in the alumina sintered body was greater than the content of SiO2 in the material powder.

[0055] In Example 2, the relative density of the alumina sintered body was 98%. The average particle size of the Al2O3 particles in the alumina sintered body was 5 μm. The average value of the flexural strength of the alumina sintered body was 540 MPa, and the 3σ lower limit value was 420 MPa.

[0056] In Example 3, the respective contents of Na2O, CaO, and SiO2 in the material powder were 460 mass ppm, 280 mass ppm, and 240 mass ppm. The content of CaO with respect to the total content of CaO and SiO2 in the material powder was 0.54. Other conditions were the same as in Example 1.

[0057] The main phase of the alumina sintered body of Example 3 was Al2O3 (corundum). In the alumina sintered body, a Ca-enriched heterogeneous phase was present, and a Na-containing heterogeneous phase was not present. The average major axis length of the Ca-enriched heterogeneous phase in the alumina sintered body was 25 μm.

[0058] The content of Al in terms of Al2O3 in the alumina sintered body of Example 3 was 99.9 mass%. The content of Na in terms of Na2O in the alumina sintered body was 200 mass ppm. The content of Ca (R1) in terms of CaO in the alumina sintered body was 300 mass ppm. The content of Si (R2) in terms of SiO2 in the alumina sintered body was 300 mass ppm. R1 / (R1 + R2) was 0.50.

[0059] In Example 3, the content of Na in terms of Na2O in the alumina sintered body was 260 mass ppm lower than the content of Na2O in the material powder. Also, the content of Ca in terms of CaO in the alumina sintered body was higher than the content of CaO in the material powder, and the content of Si in terms of SiO2 in the alumina sintered body was higher than the content of SiO2 in the material powder.

[0060] In Example 3, the relative density of the alumina sintered body was 98%. The average particle size of the Al2O3 particles in the alumina sintered body was 8 μm. The average value of the flexural strength of the alumina sintered body was 550 MPa, and the 3σ lower limit value was 470 MPa.

[0061] In Example 4, the respective contents of Na2O, CaO, and SiO2 in the material powder were 350 ppm by mass, 380 ppm by mass, and 220 ppm by mass. The content of CaO relative to the total content of CaO and SiO2 in the material powder was 0.63. Other conditions were the same as in Example 1.

[0062] The main phase of the alumina sintered body of Example 4 was Al2O3 (corundum). In the alumina sintered body, a Ca-enriched heterogeneous phase was present, and a Na-containing heterogeneous phase was not present. The average major axis length of the Ca-enriched heterogeneous phase in the alumina sintered body was 25 μm.

[0063] The content of Al in terms of Al2O3 in the alumina sintered body of Example 4 was 99.9% by mass. The content of Na in terms of Na2O in the alumina sintered body was 160 ppm by mass. The content of Ca (R1) in terms of CaO in the alumina sintered body was 380 ppm by mass. The content of Si (R2) in terms of SiO2 in the alumina sintered body was 300 ppm by mass. R1 / (R1 + R2) was 0.56.

[0064] In Example 4, the content of Na in terms of Na2O in the alumina sintered body was 190 ppm by mass less than the content of Na2O in the material powder. Also, the content of Ca in terms of CaO in the alumina sintered body was the same as the content of CaO in the material powder, and the content of Si in terms of SiO2 in the alumina sintered body was greater than the content of SiO2 in the material powder.

[0065] In Example 4, the relative density of the alumina sintered body was 98%. The average particle size of the Al2O3 particles in the alumina sintered body was 10 μm. The average value of the flexural strength of the alumina sintered body was 570 MPa, and the 3σ lower limit value was 450 MPa.

[0066] In Example 5, the respective contents of Na2O, CaO, and SiO2 in the material powder were 370 ppm by mass, 450 ppm by mass, and 220 ppm by mass. The content of CaO with respect to the total content of CaO and SiO2 in the material powder was 0.67. Other conditions were the same as in Example 1.

[0067] The main phase of the alumina sintered body of Example 5 was Al2O3 (corundum). In the alumina sintered body, Ca-enriched heterogeneous phases were present, and Na-containing heterogeneous phases were not present. The average major axis length of the Ca-enriched heterogeneous phases in the alumina sintered body was 25 μm.

[0068] The content of Al in terms of Al2O3 in the alumina sintered body of Example 5 was 99.9% by mass. The content of Na in terms of Na2O in the alumina sintered body was 210 ppm by mass. The content of Ca (R1) in terms of CaO in the alumina sintered body was 450 ppm by mass. The content of Si (R2) in terms of SiO2 in the alumina sintered body was 300 ppm by mass. R1 / (R1 + R2) was 0.60.

[0069] In Example 5, the content of Na in terms of Na2O in the alumina sintered body was 160 ppm by mass less than the content of Na2O in the material powder. Also, the content of Ca in terms of CaO in the alumina sintered body was the same as the content of CaO in the material powder, and the content of Si in terms of SiO2 in the alumina sintered body was greater than the content of SiO2 in the material powder.

[0070] In Example 5, the relative density of the alumina sintered body was 98%. The average particle size of the Al2O3 particles in the alumina sintered body was 10 μm. The average value of the flexural strength of the alumina sintered body was 570 MPa, and the 3σ lower limit value was 450 MPa.

[0071] In Example 6, the respective contents of Na2O, CaO, and SiO2 in the material powder were 380 ppm by mass, 260 ppm by mass, and 450 ppm by mass. The content of CaO with respect to the total content of CaO and SiO2 in the material powder was 0.37. The firing temperature was 1630 °C. Other conditions were the same as in Example 1.

[0072] The main phase of the alumina sintered body of Example 6 was Al2O3 (corundum). In the alumina sintered body, Ca-enriched heterogeneous phases were present, and Na-containing heterogeneous phases were not present. The average major axis length of the Ca-enriched heterogeneous phases in the alumina sintered body was 25 μm.

[0073] The content of Al in terms of Al2O3 in the alumina sintered body of Example 6 was 99.9% by mass. The content of Na in terms of Na2O in the alumina sintered body was 180 ppm by mass. The content of Ca (R1) in terms of CaO in the alumina sintered body was 280 ppm by mass. The content of Si (R2) in terms of SiO2 in the alumina sintered body was 450 ppm by mass. R1 / (R1 + R2) was 0.38.

[0074] In Example 6, the content of Na in terms of Na2O in the alumina sintered body was 200 ppm by mass less than the content of Na2O in the material powder. Also, the content of Ca in terms of CaO in the alumina sintered body was higher than the content of CaO in the material powder, and the content of Si in terms of SiO2 in the alumina sintered body was the same as the content of SiO2 in the material powder.

[0075] In Example 6, the relative density of the alumina sintered body was 98%. The average particle size of the Al2O3 particles in the alumina sintered body was 15 μm. The average value of the flexural strength of the alumina sintered body was 420 MPa, and the 3σ lower limit value was 380 MPa.

[0076] In Comparative Example 1, the respective contents of Na2O, CaO, and SiO2 in the material powder were 420 ppm by mass, 135 ppm by mass, and 215 ppm by mass. The content of CaO relative to the total content of CaO and SiO2 in the material powder was 0.39. Other conditions were the same as in Example 1. In Comparative Example 1, since an expensive material powder with a low CaO content (i.e., less than 200 ppm by mass) in the material powder was used, the manufacturing cost of the alumina sintered body increased.

[0077] The main phase of the alumina sintered body of Comparative Example 1 was Al2O3 (corundum). In the alumina sintered body, there was no Ca-enriched heterogeneous phase, and a Na-containing heterogeneous phase was present.

[0078] The content of Al in terms of Al2O3 in the alumina sintered body of Comparative Example 1 was 99.9% by mass. The content of Na in terms of Na2O in the alumina sintered body was 400 ppm by mass. The content of Ca (R1) in terms of CaO in the alumina sintered body was 140 ppm by mass. The content of Si (R2) in terms of SiO2 in the alumina sintered body was 220 ppm by mass. R1 / (R1 + R2) was 0.39.

[0079] In Comparative Example 1, the content of CaO in the material powder was low, and Na did not volatilize much during the firing in Step S12. For this reason, Ca-concentrated heterophases were not substantially formed in the alumina sintered body. The content of Na in terms of Na2O in the alumina sintered body was not much different from the content of Na2O in the material powder and was only 20 mass ppm (i.e., less than 100 mass ppm) lower. Also, the content of Ca in terms of CaO in the alumina sintered body was higher than the content of CaO in the material powder, and the content of Si in terms of SiO2 in the alumina sintered body was higher than the content of SiO2 in the material powder.

[0080] In Comparative Example 1, the relative density of the alumina sintered body was 98%. The average particle size of the Al2O3 particles in the alumina sintered body was 5 μm. The average value of the flexural strength of the alumina sintered body was 580 MPa, and the 3σ lower limit value was 460 MPa.

[0081] In Comparative Example 2, the contents of Na2O, CaO, and SiO2 in the material powder were 390 mass ppm, 360 mass ppm, and 290 mass ppm, respectively. The content of CaO relative to the total content of CaO and SiO2 in the material powder was 0.55. Other conditions were the same as in Example 1.

[0082] The main phase of the alumina sintered body of Comparative Example 2 was Al2O3 (corundum). In the alumina sintered body, Ca-concentrated heterophases were present, and Na-containing heterophases were not present. The average major axis length of the Ca-concentrated heterophases in the alumina sintered body was 50 μm.

[0083] The content of Al in terms of Al2O3 in the alumina sintered body of Comparative Example 2 was 99.9 mass%. The content of Na in terms of Na2O in the alumina sintered body was 110 mass ppm. The content of Ca in terms of CaO (R1) in the alumina sintered body was 370 mass ppm. The content of Si in terms of SiO2 (R2) in the alumina sintered body was 370 mass ppm. R1 / (R1 + R2) was 0.50.

[0084] In Comparative Example 2, the content of Na in terms of Na2O in the alumina sintered body was 280 mass ppm lower than the content of Na2O in the material powder. Also, the content of Ca in terms of CaO in the alumina sintered body was higher than the content of CaO in the material powder, and the content of Si in terms of SiO2 in the alumina sintered body was higher than the content of SiO2 in the material powder.

[0085] In Comparative Example 2, the relative density of the alumina sintered body was 98%. The average particle size of the Al2O3 particles in the alumina sintered body was 8 μm. The average value of the flexural strength of the alumina sintered body was 470 MPa, and the 3σ lower limit value was 300 MPa. Based on Griffith's theory (also referred to as Griffith's condition), it is considered that the flexural strength decreases as the average major axis length of the Ca-enriched heterogeneous phase increases in the alumina sintered body. In Comparative Example 2, since the average major axis length of the Ca-enriched heterogeneous phase in the alumina sintered body was as large as 50 μm, it is considered that the 3σ lower limit value of the flexural strength of the alumina sintered body became as small as 300 MPa.

[0086] In Comparative Example 3, the respective contents of Na2O, CaO, and SiO2 in the material powder were 420 mass ppm, 160 mass ppm, and 360 mass ppm. The content of CaO with respect to the total content of CaO and SiO2 in the material powder was 0.31. The firing temperature was 1650°C. Other conditions were the same as in Example 1. In Comparative Example 3, since an expensive material powder with a low content of CaO in the material powder (i.e., less than 200 mass ppm) was used, the manufacturing cost of the alumina sintered body increased.

[0087] The main phase of the alumina sintered body of Comparative Example 3 was Al2O3 (cordierite). In the said alumina sintered body, there was no Ca-enriched heterogeneous phase, and a Na-containing heterogeneous phase was present.

[0088] In the alumina sintered body of Comparative Example 3, the content of Al in terms of Al2O3 was 99.9% by mass. The content of Na in terms of Na2O in the alumina sintered body was 410 ppm by mass. The content (R1) of Ca in terms of CaO in the alumina sintered body was 160 ppm by mass. The content (R2) of Si in terms of SiO2 in the alumina sintered body was 360 ppm by mass. R1 / (R1 + R2) was 0.31.

[0089] In Comparative Example 3, the content of CaO in the material powder was low, and Na did not volatilize much during the firing in Step S12. For this reason, Ca-enriched heterogeneous phases were not substantially formed in the alumina sintered body. The content of Na in terms of Na2O in the alumina sintered body was not much different from the content of Na2O in the material powder and was only 10 ppm by mass (i.e., less than 100 ppm by mass) smaller. Also, the content of Ca in terms of CaO in the alumina sintered body was the same as the content of CaO in the material powder, and the content of Si in terms of SiO2 in the alumina sintered body was also the same as the content of SiO2 in the material powder.

[0090] In Comparative Example 3, the relative density of the alumina sintered body was 98%. The average particle diameter of the Al2O3 particles in the alumina sintered body was 20 μm. The average value of the flexural strength of the alumina sintered body was 380 MPa, and the 3σ lower limit value was 310 MPa.

[0091] As a result of examining Examples 1 to 6 and Comparative Example 2 based on the above Griffith theory, it is considered that by setting the average major axis of the Ca-enriched heterogeneous phases to 30 μm or less, a decrease in the flexural strength of the alumina sintered body can be preferably suppressed. Specifically, in Comparative Example 2, the 3σ lower limit value of the flexural strength of the alumina sintered body was 300 MPa, whereas in Examples 1 to 6, the 3σ lower limit value of the flexural strength of the alumina sintered body was 380 MPa to 470 MPa.

[0092] Comparing Examples 1 to 5 with Example 6, the firing temperature of Example 6 is 1630 °C, which is higher than the firing temperatures (1580 °C) of Examples 1 to 5. Therefore, in Example 6, the average particle size of Al2O3 particles in the alumina sintered body is 15 μm (i.e., larger than 10 μm), which is larger than the average particle size (5 μm to 10 μm) of Al2O3 particles in the alumina sintered bodies of Examples 1 to 5. As a result, the 3σ lower limit value of the flexural strength of the alumina sintered body in Example 6 became smaller than that in Examples 1 to 5. Specifically, in Examples 1 to 5, the 3σ lower limit value of the flexural strength of the alumina sintered body is 420 MPa to 470 MPa (i.e., 400 MPa or more), while in Example 6, the 3σ lower limit value of the flexural strength of the alumina sintered body is 380 MPa (i.e., less than 400 MPa).

[0093] Comparing Example 6 (firing temperature: 1630 °C) with Comparative Example 3 (firing temperature: 1650 °C), where the firing temperatures are relatively close, the average particle size of Al2O3 particles in the alumina sintered body was larger than 10 μm in both cases. Specifically, the average particle size of Al2O3 particles in the alumina sintered body of Example 6 was 15 μm, and the average particle size of Al2O3 particles in the alumina sintered body of Comparative Example 3 was 20 μm. On the other hand, in Example 6, a Ca-enriched heterogeneous phase with an average major axis of 25 μm was present in the alumina sintered body, while in Comparative Example 3, no Ca-enriched heterogeneous phase was present in the alumina sintered body. For this reason, in Example 6, the 3σ lower limit value of the flexural strength of the alumina sintered body is 380 MPa (i.e., 350 MPa or more), while in Comparative Example 3, the 3σ lower limit value of the flexural strength of the alumina sintered body was as low as 310 MPa.

[0094] Comparing Examples 4 to 5 with Comparative Example 2, in Examples 4 to 5 and Comparative Example 2, the content rate of CaO in the material powder (360 mass ppm to 450 mass ppm) is relatively high at 300 mass ppm or more. Also, regarding the content rate of CaO with respect to the total content rate of CaO and SiO2 in the material powder, in Examples 4 to 5 it is 0.63 to 0.67 (i.e., 0.57 or more), while in Comparative Example 2 it is 0.55 (i.e., less than 0.57). For this reason, in Examples 4 to 5, the average major axis length of the Ca-enriched heterogeneous phase in the alumina sintered body is 25 μm (i.e., 30 μm or less), and the 3σ lower limit value of the flexural strength of the alumina sintered body is 450 MPa (i.e., 400 MPa or more). In contrast, in Comparative Example 2, the average major axis length of the Ca-enriched heterogeneous phase in the alumina sintered body is as large as 50 μm (i.e., larger than 30 μm), and the 3σ lower limit value of the flexural strength of the alumina sintered body is small at 300 MPa (i.e., less than 400 MPa).

[0095] Regarding Examples 4 to 5 and Comparative Example 2, when paying attention to the composition of the alumina sintered body, in Examples 4 to 5 and Comparative Example 2, the content rate of Ca in terms of CaO in the alumina sintered body (370 mass ppm to 450 mass ppm) is relatively high at 350 mass ppm or more. Also, regarding R1 / (R1 + R2) in the alumina sintered body, in Examples 4 to 5 it is 0.56 to 0.60 (i.e., 0.55 or more), while in Comparative Example 2 it is 0.50 (i.e., less than 0.55). For this reason, as described above, in Examples 4 to 5, the average major axis length of the Ca-enriched heterogeneous phase in the alumina sintered body is 25 μm (i.e., 30 μm or less), and the 3σ lower limit value of the flexural strength of the alumina sintered body is 450 MPa (i.e., 400 MPa or more). In contrast, in Comparative Example 2, the average major axis length of the Ca-enriched heterogeneous phase in the alumina sintered body is as large as 50 μm (i.e., larger than 30 μm), and the 3σ lower limit value of the flexural strength of the alumina sintered body is small at 300 MPa (i.e., less than 400 MPa).

[0096] As described above, the alumina sintered body includes a main phase composed of Al2O3 and a foreign phase containing Ca, Si, Al, and O. The content of Al in terms of Al2O3 in the alumina sintered body is 99.5% by mass or more and 99.9% by mass or less. Therefore, the alumina sintered body can be manufactured using an alumina material containing a relatively large amount of impurities (i.e., relatively low alumina purity). For this reason, cost reduction of the alumina sintered body can be achieved.

[0097] Further, in the alumina sintered body, the average major axis of the above-mentioned foreign phase is 30 μm or less. Thus, by suppressing the coarsening of the foreign phase in the alumina sintered body, even when a material containing a relatively large amount of impurities is used, a decrease in the bending strength of the alumina sintered body can be suppressed. As a result, the alumina sintered body can be suitably used for applications requiring high strength. From the viewpoint of further cost reduction of the alumina sintered body, the content of Al in terms of Al2O3 in the alumina sintered body is preferably less than 99.9% by mass.

[0098] As described above, the content of Ca in terms of CaO in the alumina sintered body is preferably 200 mass ppm or more. Thereby, cost reduction of the alumina sintered body can be suitably achieved. Further, even under conditions where a foreign phase containing Ca, Si, Al, and O (i.e., a Ca-enriched foreign phase) is likely to occur, by suppressing the coarsening of the foreign phase, a decrease in the bending strength of the alumina sintered body can be suppressed.

[0099] As described above, the content rate of Ca in terms of CaO in the alumina sintered body may be 350 mass ppm or more. Thereby, cost reduction of the alumina sintered body can be more suitably realized. In this case, when the content rate of Ca in terms of CaO in the alumina sintered body is R1 and the content rate of Si in terms of SiO2 in the alumina sintered body is R2, R1 is preferably larger than 0.55 times of (R1 + R2). Thereby, even under conditions where Ca-enriched heterogeneous phases are more likely to occur, coarsening of the heterogeneous phases can be suitably suppressed. As a result, a decrease in the flexural strength of the alumina sintered body can be suitably suppressed.

[0100] As described above, preferably, the content rate of Na in terms of Na2O in the alumina sintered body is 100 mass ppm or more and 250 mass ppm or less, and the content rate of Si in terms of SiO2 in the alumina sintered body is 150 mass ppm or more and 500 mass ppm or less. Thereby, cost reduction of the alumina sintered body can be suitably realized. Also, in this way, even under conditions where Ca-enriched heterogeneous phases are likely to occur, by suppressing coarsening of the heterogeneous phases, a decrease in the flexural strength of the alumina sintered body can be suppressed.

[0101] As described above, the average particle size of the Al2O3 particles in the alumina sintered body is preferably 10 μm or less. Thereby, the denseness of the alumina sintered body can be improved.

[0102] As described above, the relative density of the alumina sintered body is preferably 98% or more. Thereby, an alumina sintered body with high denseness can be provided.

[0103] As described above, the 3σ lower limit value of the flexural strength of the alumina sintered body is preferably 400 MPa or more. Thereby, a high-strength alumina sintered body can be provided.

[0104] The manufacturing method of the above alumina sintered body includes a step of molding a material powder to form a molded body (step S11) and a step of firing the molded body to manufacture an alumina sintered body (step S12). The content of Al2O3 in the material powder is 99.5% by mass or more and 99.9% by mass or less. Thus, by using a material powder containing a relatively large amount of impurities (i.e., relatively low alumina purity), the cost reduction of the alumina sintered body can be achieved. The alumina sintered body includes a main phase composed of Al2O3 and a heterogeneous phase containing Ca, Si, Al, and O. The average major axis of the heterogeneous phase is 30 μm or less. Thus, by suppressing the coarsening of the heterogeneous phase in the alumina sintered body, an alumina sintered body with suppressed decrease in bending strength can be manufactured even when using a material powder containing a relatively large amount of impurities. From the viewpoint of further reducing the cost of the alumina sintered body, the content of Al2O3 in the material powder is preferably less than 99.9% by mass.

[0105] As described above, the content of CaO in the material powder is preferably 200 ppm by mass or more. Thereby, the cost reduction of the alumina sintered body can be preferably achieved. Also, thus, even in manufacturing conditions where a heterogeneous phase containing Ca, Si, Al, and O (i.e., a Ca-enriched heterogeneous phase) is likely to occur in the alumina sintered body, by suppressing the coarsening of the heterogeneous phase, the decrease in the bending strength of the alumina sintered body can be suppressed.

[0106] As described above, the content of CaO in the material powder may be 300 ppm by mass or more. Thereby, the cost reduction of the alumina sintered body can be more preferably achieved. In this case, the content of CaO in the material powder is preferably greater than 0.57 times the total content of CaO and SiO2 in the material powder. Thereby, even in manufacturing conditions where a Ca-enriched heterogeneous phase is more likely to occur in the alumina sintered body, the coarsening of the heterogeneous phase can be preferably suppressed. As a result, the decrease in the bending strength of the alumina sintered body can be preferably suppressed.

[0107] As described above, preferably, the content rate of Na2O in the material powder is 300 mass ppm or more and 700 mass ppm or less, and the content rate of SiO2 in the material powder is 200 mass ppm or more and 500 mass ppm or less. Thereby, the cost reduction of the alumina sintered body can be suitably realized. Further, in this way, even in the manufacturing conditions where Ca-enriched heterogeneous phases are likely to occur in the alumina sintered body, by suppressing the coarsening of the heterogeneous phases, a decrease in the bending strength of the alumina sintered body can be suppressed.

[0108] As described above, preferably, the content rate of Na in terms of Na2O in the alumina sintered body is smaller than the content rate of Na2O in the material powder, and the difference between the content rate of Na2O in the material powder and the content rate of Na in terms of Na2O in the alumina sintered body is 100 mass ppm or more. In this way, even when the volatilization amount of Na during firing is relatively large and Ca-enriched heterogeneous phases are likely to occur in the alumina sintered body, by suppressing the coarsening of the heterogeneous phases, a decrease in the bending strength of the alumina sintered body can be suppressed.

[0109] As described above, preferably, the firing temperature in step S12 is 1580 °C or higher. In this way, even in the manufacturing conditions where Ca-enriched heterogeneous phases are likely to occur in the alumina sintered body, by suppressing the coarsening of the heterogeneous phases, a decrease in the bending strength of the alumina sintered body can be suppressed.

[0110] As described above, in step S12, preferably, the heating rate in the range of 1580 °C or higher is 50 °C / h or higher. Thereby, the coarsening of the Ca-enriched heterogeneous phases can be suitably suppressed.

[0111] In the above alumina sintered body and the method for manufacturing the alumina sintered body, various modifications are possible.

[0112] For example, in step S12 described above, the heating rate in the range of 1580 °C or higher may be less than 50 °C / h. Also, in step S12, the firing temperature may be less than 1580 °C.

[0113] The value obtained by subtracting the Na content in terms of Na2O in the alumina sintered body from the Na2O content in the material powder may be less than 100 mass ppm. Alternatively, the Na content in terms of Na2O in the alumina sintered body may be equal to or greater than the Na2O content in the material powder.

[0114] The Na2O content in the material powder may be less than 300 mass ppm or may be greater than 700 mass ppm. The SiO2 content in the material powder may be less than 200 mass ppm or may be greater than 500 mass ppm.

[0115] The CaO content in the material powder may be 0.57 times or less of the total content of CaO and SiO2 in the material powder. The CaO content in the material powder may be less than 300 mass ppm or may be less than 200 mass ppm.

[0116] For example, the 3σ lower limit value of the flexural strength of the alumina sintered body may be less than 400 MPa. Also, the relative density of the alumina sintered body may be less than 98%.

[0117] The average particle size of the Al2O3 particles in the alumina sintered body may be greater than 10 μm.

[0118] The Na content in terms of Na2O in the alumina sintered body may be less than 100 mass ppm or may be greater than 250 mass ppm. The Si content in terms of SiO2 in the alumina sintered body may be less than 150 mass ppm or may be greater than 500 mass ppm.

[0119] In the alumina sintered body, the above-mentioned R1 may be 0.55 times or less of (R1 + R2). The Ca content in terms of CaO in the alumina sintered body may be less than 350 mass ppm or may be less than 200 mass ppm.

[0120] The above alumina sintered body may be manufactured by a manufacturing method different from the above-described manufacturing method.

[0121] The above alumina sintered body may be used, for example, as parts of a semiconductor manufacturing apparatus or a liquid crystal manufacturing apparatus.

[0122] The configurations in the above embodiments and each modification may be appropriately combined as long as they do not contradict each other.

Industrial Applicability

[0123] The alumina sintered body according to the present invention can be used as parts of a battery and various devices.

Explanation of Signs

[0124] Steps S11 to S12

Claims

1. An alumina sintered body comprising Al ２ O ３ comprising a main phase consisting of, a foreign phase containing Ca, Si, Al, and O, and The content of Al in terms of Al ２ ２ O ３ ３ in the alumina sintered body is 99.5% by mass or more and 99.9% by mass or less, having an average major axis of the foreign phase of 30 μm or less.

2. The alumina sintered body according to Claim 1, wherein the content of Ca in terms of CaO in the alumina sintered body is 200 mass ppm or more.

3. The alumina sintered body according to Claim 2, wherein the content of Ca in terms of CaO in the alumina sintered body is 350 mass ppm or more. Let the content rate of Ca in terms of CaO conversion in the alumina sintered body be R １ and let the content rate of Si in terms of SiO ２ conversion in the alumina sintered body be R ２ . Then, an alumina sintered body in which R １ is larger than 0.55 times (R １ +R ２ ).

4. The alumina sintered body according to any one of Claims 1 to 3. The content of Na in terms of Na2O conversion in the alumina sintered body ２ is 100 mass ppm or more and 250 mass ppm or less, The SiO in the alumina sintered body ２ An alumina sintered body having a Si content in terms of conversion of 150 mass ppm or more and 500 mass ppm or less.

5. The alumina sintered body according to any one of Claims 1 to 3. Al ２ O ３ An alumina sintered body in which the average particle size of the particles is 10 µm or less.

6. The alumina sintered body according to any one of Claims 1 to 3. having a relative density of 98% or more.

7. The alumina sintered body according to any one of Claims 1 to 3. having a 3σ lower limit value of flexural strength of 400 MPa or more.

8. A method for manufacturing an alumina sintered body, comprising: a) a step of forming a green body by molding a material powder; and b) a step of firing the green body to produce an alumina sintered body, wherein the alumina sintered body Al in the material powder ２ O ３ has a content of 99.5% by mass or more and 99.9% by mass or less, comprises Al ２ O ３ comprising a main phase consisting of, a foreign phase containing Ca, Si, Al, and O, and has an average major axis of the foreign phase of 30 μm or less.

9. The method for manufacturing an alumina sintered body according to Claim 8, wherein the content of CaO in the material powder is 200 mass ppm or more.

10. The method for manufacturing an alumina sintered body according to Claim 9, wherein the content of CaO in the material powder is 300 mass ppm or more. The content rate of CaO in the material powder is greater than 0.57 times the total content rate of CaO and SiO ２ in the material powder, and a method for manufacturing an alumina sintered body.

11. The method for manufacturing an alumina sintered body according to any one of Claims 8 to 10. The content rate of Na ２ O in the material powder is 300 mass ppm or more and 700 mass ppm or less, The content of SiO in the material powder ２ is a method for manufacturing an alumina sintered body that is 200 mass ppm or more and 500 mass ppm or less.

12. The method for manufacturing an alumina sintered body according to any one of Claims 8 to 10. The content of Na in terms of Na2O conversion in the alumina sintered body is less than the content of Na2O in the material powder, ２ and ２ is smaller than the content of Na2O in the material powder, The Na content in the material powder ２ The difference between the content rate of Na ２ in the alumina sintered body in terms of Na conversion and the content rate of Na in the alumina sintered body is 100 mass ppm or more. A method for producing an alumina sintered body

13. The method for manufacturing an alumina sintered body according to any one of Claims 8 to 10. wherein the firing temperature in the step b) is 1580°C or higher.

14. The method for manufacturing an alumina sintered body according to Claim 13, wherein in the step b), the heating rate in the range of 1580°C or higher is 50°C / h or higher.

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