Glass for magnetic recording medium substrate, magnetic recording medium substrate, magnetic recording medium and glass spacer for magnetic recording and reproducing apparatus
A glass for magnetic recording medium substrate is an amorphous oxide glass. In terms of mol %, SiO 2 content ranges from 45 to 68%, Al 2 O 3 from 5 to 20%, total content of SiO 2 and Al 2 O 3 60 to 80%, B 2 O 3 from 0 to 5%, MgO from 3 to 28%, CaO from 0 to 18%, total content of BaO and SrO 0 to 2%, total content of alkali earth metal oxides from 12 to 30%, total content of alkali metal oxides from 3.5 to 15%, and at least one kind selected from the group made of Sn oxide and Ce oxide being included, a total content of Sn oxide and Ce oxide ranges from 0.05 to 2.00%, a glass transition temperature ≥625° C., a Young's modulus ≥83 GPa, a specific gravity ≤2.85, and an average linear expansion coefficient at 100 to 300° C.≥48×10 −7 /° C.
1 . A glass,
which is a glass for magnetic recording medium substrate,
which is an amorphous oxide glass,
wherein the glass has a glass transition temperature of equal to or higher than 680° C. and a Young's modulus of equal to or more than 89.5 GPa, and
wherein, in terms of mol %:
an SiO 2 content ranges from 52 to 68%;
an Al 2 O 3 content ranges from 12 to 18%;
a total content of SiO 2 and Al 2 O 3 (SiO 2 +Al 2 O 3 ) is equal to or less than 80%;
a B 2 O 3 content ranges from 0 to 5%;
an MgO content ranges from 3 to 28%;
a CaO content ranges from 0 to 18%;
a TiO 2 content ranges from 0 to 5%;
a ZrO 2 content ranges from 0 to 3%,
a total content of BaO and SrO (BaO+SrO) ranges from 0 to 2%;
a total content of alkali earth metal oxides (MgO+CaO+SrO+BaO) ranges from 12 to 30%;
a molar ratio of a total content of MgO and CaO relative to a total content of alkali earth metal oxides {(MgO+CaO)/(MgO+CaO+SrO+BaO)} ranges from 0.75 to 1.00;
a total content of alkali metal oxides (Li 2 O+Na 2 O+K 2 O) ranges from 3.5 to 13%;
a Li 2 O content is equal to or less than 8%;
a Na 2 O content is equal to or less than 6%;
a K 2 O content is equal to or less than 5%;
a total content of HfO 2 , Nb 2 O 5 , Ta 2 O 5 , La 2 O 3 and Y 2 O 3 (HfO 2 +Nb 2 O 5 +Ta 2 O 5 +La 2 O 3 +Y 2 O 3 ) ranges from 0 to 5%;
a ZnO content ranges from 0 to 3%; and
a P 2 O 5 content ranges from 0 to 2%.
2 . The glass according to claim 1 ,
wherein a specific elastic modulus is equal to or more than 31 MNm/kg.
3 . The glass according to claim 1 ,
wherein no crystals are confirmed by magnification observation, magnifying power of 40 to 100, with an optical microscope after the glass is left to stand for 16 hours with a temperature of 1250° C. kept, and then cooled to room temperature.
4 . The glass according to claim 1 ,
wherein a bubble density per unit mass is less than 50/kg, as a density of bubbles each having a diameter exceeding 0.03 mm when observation is performed with an optical microscope at magnifications from 40 to 100.
5 . A magnetic recording medium substrate, which is comprised of the glass according to claim 1 .
6 . A magnetic recording medium substrate, which is comprised of the glass according to claim 2 .
7 . A magnetic recording medium substrate, which is comprised of the glass according to claim 3 .
8 . A magnetic recording medium substrate, which is comprised of the glass according to claim 4 .
9 . A magnetic recording medium, which comprises a magnetic recording layer on the magnetic recording medium substrate according to claim 5 .
10 . A magnetic recording medium, which comprises a magnetic recording layer on the magnetic recording medium substrate according to claim 6 .
11 . A magnetic recording medium, which comprises a magnetic recording layer on the magnetic recording medium substrate according to claim 7 .
12 . A magnetic recording medium, which comprises a magnetic recording layer on the magnetic recording medium substrate according to claim 8 .
13 . A magnetic recording and reproducing apparatus, which comprises the magnetic recording medium according to claim 9 .
14 . A magnetic recording and reproducing apparatus, which comprises the magnetic recording medium according to claim 10 .
15 . A magnetic recording and reproducing apparatus, which comprises the magnetic recording medium according to claim 11 .
16 . A magnetic recording and reproducing apparatus, which comprises the magnetic recording medium according to claim 12 .