Chemically strengthened glass, and method for manufacturing the same

1. A chemically strengthened glass having a thickness of t [mm], and having a profile of a stress value CS x [MPa] at a depth x [μm] from a surface of the glass, the stress value being measured by a scattered-light photoelastic stress meter,

wherein a second-order differential value CS x ″ of the stress value CS x in the profile satisfies the following expression within a range of CS x ≥0:

0 <CS x ″≤0.050.

2. The chemically strengthened glass according to claim 1 , wherein a tensile stress value CT 2 [MPa] satisfies the following expression using the thickness t [mm]:

CT 2 ≤−120 t+ 164.

3. The chemically strengthened glass according to claim 1 , wherein a tensile stress value CT 2 [MPa] satisfies the following expression using the thickness t [mm]:

−120 t+ 150 ≤CT 2 .

4. The chemically strengthened glass according to claim 1 , wherein a stress value CS 50 at a depth of 50 [μm] from the surface of the glass satisfies the following expression using the thickness t [mm], the tensile stress CT 2 [MPa] and a depth DOL [mm] of a compressive stress layer:

CS 50 /( CT 2 ×( t− 2×DOL))/ t≥ 4.90.

5. The chemically strengthened glass according to claim 1 , wherein a first-order differential value CS x ′ of the stress value CS x [MPa] in the profile is −5.3 or more.

6. The chemically strengthened glass according to claim 1 , wherein the chemically strengthened glass is a chemically strengthened glass that has been chemically strengthened at two or more stages, and

the tensile stress value CT 2 [MPa] of the chemically strengthened glass is a value as large as 50% to 93% of a tensile stress value CT 1 [MPa] of the chemically strengthened glass that has been chemically strengthened at the first stage of the two or more stages.

7. The chemically strengthened glass according to claim 1 , wherein a base composition of the chemically strengthened glass comprises, by mol % in terms of oxides:

52 to 75% of SiO 2 ;

8 to 20% of Al 2 O 3 ;

5 to 16% of LiO 2 ; and

8% or lower of NaO 2 .

8. A method of manufacturing the chemically strengthened glass of claim 1 , the method comprising:

immersing a lithium-containing glass into a first molten salt composition comprising sodium ions and potassium ions to thereby perform first ion exchange; and

immersing the lithium-containing glass into a second molten salt composition comprising potassium ions to thereby perform second ion exchange,

wherein in the first molten salt composition, a concentration of potassium nitrate is higher than a concentration of sodium nitrate, and

in the second molten salt composition, a concentration of potassium nitrate is 85 mass % or higher, and a mass ratio of sodium ions to lithium ions is 0 or higher and 15 or lower.

9. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein the concentration of potassium nitrate in the first molten salt composition is higher than 50 mass %.

10. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein the second molten salt composition comprises sodium nitrate whose concentration is higher than 0 mass % and 5 mass % or lower.

11. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein the second molten salt composition comprises lithium ions whose concentration is 0.1 mass % or higher and 10 mass % or lower.

12. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein in the first ion exchange, the first molten salt composition has a temperature of 380° C. or higher and 450° C. or lower.

13. The method of manufacturing a chemically strengthened glass according to claim 12 , wherein in the first ion exchange, the lithium-containing glass is immersed in the first molten salt composition for 0.5 hours or more and 8 hours or less.

14. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein in the second ion exchange, the second molten salt composition has a temperature of 380° C. or higher and 450° C. or lower.

15. The method of manufacturing a chemically strengthened glass according to claim 14 , wherein in the second ion exchange, the lithium-containing glass is immersed in the second molten salt composition for an immersion time t2 [min] satisfying the following expression using a temperature T [° C.] of the second molten salt composition:

−0.38 T+ 173 <t 2<−1.4 T+ 650.

16. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein the chemical strengthening is performed so that a tensile stress value CT 2 [MPa] of the chemically strengthened glass after the second ion exchange is a value corresponding to 50% to 93% of a tensile stress value CT 1 [MPa] of the chemically strengthened glass after the first ion exchange.

17. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein a tensile stress value CT 1 [MPa] of the chemically strengthened glass after the first ion exchange satisfies the following expression using a thickness t [mm] of the chemically strengthened glass:

CT 1 >−120 t+ 164.

18. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein the lithium-containing glass comprises, by mol % in terms of oxides:

52 to 75% of SiO 2 ;

8 to 20% of Al 2 O 3 ;

5 to 16% of LiO 2 ; and

8% or lower of NaO 2 .

19. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein the second molten salt composition comprises silicic acid.

20. The method of manufacturing a chemically strengthened glass according to claim 8 , wherein the second molten salt composition comprises carbonate.