Cordierite-based ceramic(s) and member for a telescope

1 . A cordierite-based opaque ceramic, consisting of:

a main crystalline phase thereof is a cordierite crystalline phase;

a content of Mg is 13.2% by mass or more and 13.8% by mass or less in an MgO equivalent;

a content of Al is 26.0% by mass or more and 32.1% by mass or less in an Al 2 O 3 equivalent;

a content of Bi is 1.6% by mass or more and 4.6% by mass or less in a B 2 O 3 equivalent;

a content of B is 1.5% by mass or more and 6.8% by mass or less in a B 2 O 3 equivalent;

a content of Si is 49.4% by mass or more and 51.4% by mass or less in an SiO 2 equivalent; and

a pseudo thermal expansion rate of a cordierite-based opaque ceramic is between 0.06 X 10 −7 /K and 2.19×10 −7 /K.

2 . The cordierite-based opaque ceramic according to claim 1 , wherein:

a content of Mg is 13.5% by mass or more and 13.8% by mass or less in an MgO equivalent;

a content of Al is 28.5% by mass or more and 32.1% by mass or less in an Al 2 O 3 equivalent;

a content of Bi is 1.6% by mass or more and 4.0% by mass or less in a Bi 2 O 3 equivalent;

a content of B is 1.5% by mass or more and 4.0% by mass or less in a B 2 O 3 equivalent; and

a content of Si is 50.2% by mass or more and 51.4% by mass or less in an SiO 2 equivalent.

3 . The cordierite-based opaque ceramic according to claim 1 , wherein:

a content of Mg is 13.5% by mass or more and 13.8% by mass or less in an MgO equivalent;

a content of Al is 28.5% by mass or more and 30.2% by mass or less in an Al 2 O 3 equivalent;

a content of Bi is 2.2% by mass or more and 4.0% by mass or less in a Bi 2 O 3 equivalent;

a content of B is 2.1% by mass or more and 4.0% by mass or less in a B 2 O 3 equivalent; and

a content of Si is 50.2% by mass or more and 51.4% by mass or less in an SiO 2 equivalent.

4 . The cordierite-based opaque ceramic according to claim 1 , wherein:

a content of Mg is 13.6% by mass or more and 13.8% by mass or less in an MgO equivalent;

a content of Al is 28.5% by mass or more and 29.3% by mass or less in an Al 2 O 3 equivalent;

a content of Bi is 2.2% by mass or more and 3.2% by mass or less in a Bi 2 O 3 equivalent;

a content of B is 3.1% by mass or more and 4.0% by mass or less in a Bi 2 O 3 equivalent; and

a content of Si is 50.8% by mass or more and 51.4% by mass or less in an SiO 2 equivalent.

5 . The cordierite-based opaque ceramic according to claim 1 , wherein behavior of thermal expansion thereof in a temperature range of 0° C. to 50° C. has a temperature where an extension rate is minimum, indicates a negative slope on a lower temperature side that is lower than the temperature, and indicates a positive slope on a higher temperature side that is higher than the temperature.

6 . A member for a telescope that is composed of the cordierite-based opaque ceramic according to claim 1 .

7 . A cordierite-based opaque ceramic, consisting of:

a main crystalline phase thereof is a cordierite crystalline phase;

a content of Mg is 13.2% by mass or more and 13.8% by mass or less in an MgO equivalent;

a content of Al is 26.0% by mass or more and 32.1% by mass or less in an Al 2 O 3 equivalent;

a content of Bi is 1.6% by mass or more and 4.6% by mass or less in a Bi 2 O 3 equivalent;

a content of B is 1.5% by mass or more and 6.8% by mass or less in a B 2 O 3 equivalent;

a content of Si is 49.4% by mass or more and 51.4% by mass or less in an SiO 2 equivalent; and

a pseudo thermal expansion rate of a cordierite-based opaque ceramic is between 0.06 X 10 −7 /K and 2.00× 10 −7 /K.

8 . The cordierite-based opaque ceramic according to claim 7 , wherein:

a content of Mg is 13.5% by mass or more and 13.8% by mass or less in an MgO equivalent;

a content of Al is 28.5% by mass or more and 32.1% by mass or less in an Al 2 O 3 equivalent;

a content of Bi is 1.6% by mass or more and 4.0% by mass or less in a Bi 2 O 3 equivalent;

a content of B is 1.5% by mass or more and 4.0% by mass or less in a B 2 O 3 equivalent; and

a content of Si is 50.2% by mass or more and 51.4% by mass or less in an SiO 2 equivalent.

9 . The cordierite-based opaque ceramic according to claim 7 , wherein:

a content of Mg is 13.5% by mass or more and 13.8% by mass or less in an MgO equivalent;

a content of Al is 28.5% by mass or more and 30.2% by mass or less in an Al 2 O 3 equivalent;

a content of Bi is 2.2% by mass or more and 4.0% by mass or less in a Bi 2 O 3 equivalent;

a content of B is 2.1% by mass or more and 4.0% by mass or less in a B 2 O 3 equivalent; and

a content of Si is 50.2% by mass or more and 51.4% by mass or less in an SiO 2 equivalent.

10 . The cordierite-based opaque ceramic according to claim 7 , wherein:

a content of Mg is 13.6% by mass or more and 13.8% by mass or less in an MgO equivalent;

a content of Al is 28.5% by mass or more and 29.3% by mass or less in an Al 2 O 3 equivalent;

a content of Bi is 2.2% by mass or more and 3.2% by mass or less in a Bi 2 O 3 equivalent;

a content of B is 3.1% by mass or more and 4.0% by mass or less in a B 2 O 3 equivalent; and

a content of Si is 50.8% by mass or more and 51.4% by mass or less in an SiO 2 equivalent.

11 . The cordierite-based opaque ceramic according to claim 7 , wherein behavior of thermal expansion thereof in a temperature range of 0° C. to 50° C. has a temperature where an extension rate is minimum, indicates a negative slope on a lower temperature side that is lower than the temperature, and indicates a positive slope on a higher temperature side that is higher than the temperature.

12 . A member for a telescope that is composed of the cordierite-based opaque ceramic according to claim 7 .