| 摘要 |
PROBLEM TO BE SOLVED: To obtain a glass having high fractional toughness by specifying the oxygen atom density of a glass for toroidal plate-like information recording medium substrate. SOLUTION: This toroidal plate-like glass having >65 mm outer diameter has >=7.9×10-2 mol/cm3 oxygen atom density. Preferably the glass comprises substantially 30-60 mol% SiO2, 4-40 mol% Al2O3, 0.5-20 mol% B2O3, 0-40 mol% MgO, 0-20 mol% CaO, 0-20 mol% TiO2, 0-12 mol% Li2O, 0-10 mol% ZrO2, 0-10 mol% Ta2O5, 0-10 mol% Nb2O5, 0-10 mol% L2O5 and 0-15 mol% Y2O3. The glass is preferably a glass which is not causing a phase separation phenomenon. The method for producing the glass is not particularly limited and each production method can be adopted. For example, raw materials are mixed so as to give a target composition. The mixed raw materials are heated and melted in a melting furnace. The glass is homogenized by bubbling, addition of clarifier, stirring, etc., and formed in specific plate thickness. The glass is annealed and processed by grinding, polishing, etc.
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