GLASS COMPOSITION FOR CHEMICALLY STRENGTHENED ALKALI-ALUMINOSILICATE GLASS AND METHOD FOR THE MANUFACTURE THEREOF

Abstract

A glass composition for producing chemically strengthened alkali-aluminosilicate glass and a method for manufacturing the chemically strengthened alkali-aluminosilicate glass. The chemically strengthened alkali-aluminosilicate glass is suitable for use as high-strength cover glass for touch displays, solar cell cover glass and laminated safety glass.

Claims

1. An ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass, comprising: from about 59.0 to about 65.0 wt % of SiO.sub.2, from about 11.9 to about 13.0 wt % of Al.sub.2O.sub.3, from about 16.0 to about 18.5 wt % of Na.sub.2O, from about 0 to about 1.0 wt % of B.sub.2O.sub.3, from about 2.8 to about 3.3 wt % of K.sub.2O, and from about 2.0 to about 6.0 wt % of MgO, wherein 1.5 is >(B.sub.2O.sub.3+Na.sub.2O+K.sub.2O)/Al.sub.2O.sub.3; and wherein 1.0 is >(Na.sub.2O+K.sub.2O)/(Al.sub.2O.sub.3+MgO).

2. A chemically strengthened alkali-alumino-silicate glass made from a glass composition comprising: from about 59.0 to about 65.0 wt % of SiO.sub.2, from about 11.9 to about 13.0 wt % of Al.sub.2O.sub.3, from about 16.0 to about 18.5 wt % of Na.sub.2O, from about 0 to about 1.0 wt % of B.sub.2O.sub.3, from about 2.8 to about 3.3 wt % of K.sub.2O, and from about 2.0 to about 6.0 wt % of MgO, wherein 1.5 is >(B.sub.2O.sub.3+Na.sub.2O+K.sub.2O)/Al.sub.2O.sub.3; wherein 1.0 is >(Na.sub.2O+K.sub.2O)/(Al.sub.2O.sub.3+MgO); wherein the glass composition is ion-exchanged and has a surface compressive stress layer and a central tension zone.

3. The chemically strengthened alkali-alumino-silicate glass according to claim 2, wherein the surface compressive stress layer has a compressive stress of at least about 650 MPa.

4. The chemically strengthened alkali-alumino-silicate glass according to claim 2, wherein the surface compressive stress layer has a compressive stress of at least about 700 MPa.

5. The chemically strengthened alkali-alumino-silicate glass according to claim 4, wherein the surface compressive stress layer has a compressive stress of from about 720 MPa to about 755 MPa.

6. The chemically strengthened alkali-alumino-silicate glass according to claim 2, wherein the depth of the surface compressive stress layer is at least about 30.0 m.

7. The chemically strengthened alkali-alumino-silicate glass according to claim 2, wherein the depth of the surface compressive stress layer is at least about 35.0 m.

8. The chemically strengthened alkali-alumino-silicate glass according to claim 2, wherein the depth of the surface compressive stress layer is from about 30.0 m to about 45.0 m.

9. The chemically strengthened alkali-alumino-silicate glass according to claim 8, wherein the depth of the surface compressive stress layer is from about 35.0 m to about 45.0 m.

10. The chemically strengthened alkali-alumino-silicate glass according to claim 2, wherein the central tension zone has a central tension of up to about 70 MPa.

11. The chemically strengthened alkali-alumino-silicate glass according to claim 2, wherein the central tension zone has a central tension of up to about 55 MPa.

12. The chemically strengthened alkali-alumino-silicate glass according to claim 2, wherein the central tension zone has a central tension of from about 40 MPa to about 55 MPa.

13. The chemically strengthened alkali-aluminosilicate glass according to claim 2, wherein the glass has a density of up to about 2.6 g/cm.sup.3.

14. The chemically strengthened alkali-aluminosilicate glass according to claim 2, wherein the glass has a linear coefficient of expansion (.sub.25-300 10.sup.7/ C.) of from about 97.0 to about 105.0.

15. The chemically strengthened alkali-aluminosilicate glass according to claim 2, wherein the glass has a transition temperature of less than 545 C.

16. The chemically strengthened alkali-aluminosilicate glass according to claim 2, wherein the glass has a softening temperature of less than 765 C.

17. An ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass, consisting essentially of: from about 59.0 to about 65.0 wt % of SiO.sub.2, from about 11.9 to about 13.0 wt % of Al.sub.2O.sub.3, from about 16.0 to about 18.5 wt % of Na.sub.2O, from about 0 to about 1.0 wt % of B.sub.2O.sub.3, from about 2.8 to about 3.3 wt % of K.sub.2O, and from about 2.0 to about 6.0 wt % of MgO, wherein 1.5 is >(B.sub.2O.sub.3+Na.sub.2O+K.sub.2O)/Al.sub.2O.sub.3; and wherein 1.0 is >(Na.sub.2O+K.sub.2O)/(Al.sub.2O.sub.3+MgO).

18. A method for producing a chemically strengthened alkali-aluminosilicate glass, comprising: mixing and melting glass raw material components to form a homogenous glass melt comprising: from about 59.0 to about 65.0 wt % of SiO.sub.2, from about 11.9 to about 13.0 wt % of Al.sub.2O.sub.3, from about 16.0 to about 18.5 wt % of Na.sub.2O, from about 0 to about 1.0 wt % of B.sub.2O.sub.3, from about 2.8 to about 3.3 wt % of K.sub.2O, and from about 2.0 to about 6.0 wt % of MgO, wherein 1.5 is >(B.sub.2O.sub.3+Na.sub.2O+K.sub.2O)/Al.sub.2O.sub.3; and wherein 1.0 is >(Na.sub.2O+K.sub.2O)/(Al.sub.2O.sub.3+MgO); shaping the glass using a method selected from the down-draw method, the floating method and combinations thereof; annealing the glass; and chemically strengthening the glass by ion exchange.

19. The method of claim 18, wherein the glass raw material components are melted for up to about 12 hours at a temperature of about 1650 C.

20. The method of claim 19, wherein the glass raw material components are melted for up to about 4 hours at a temperature of about 1650 C.

21. The method of claim 18, wherein the glass is annealed at a rate of about 1.0 C./hour.

22. The method of claim 18, wherein the glass is chemically strengthened by ion exchange in a molten salt bath.

23. The method of claim 22, wherein the molten salt is KNO.sub.3.

24. The method of claim 18, wherein the glass is chemically strengthened by ion exchange at a temperature of from about 380 C. to about 450 C.

25. The method of claim 24, wherein the glass is chemically strengthened by ion exchange at a temperature of about 420 C.

26. The method of claim 18, wherein the glass is chemically strengthened by ion exchange for up to about 8 hours.

27. The method of claim 26, wherein the glass is chemically strengthened by ion exchange for up to about 4 hours.

28. The method of claim 27, wherein the glass is chemically strengthened by ion exchange for up to about 2 hours.

Description

DETAILED DESCRIPTION

[0013] In several exemplary embodiments, the present invention provides an ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass having a surface compressive stress layer with high compressive stress (CS) and a low depth of layer (DOL). The high compressive stress (CS) together with the low depth of layer (DOL) is obtained through a chemical strengthening process in which sodium ions on the glass surface are replaced by larger potassium ions. A low DOL is beneficial for glass finishing since the yield of the scribing process is increased. Also, a glass surface with high compressive stress yields a stronger glass that can withstand increased external impaction forces and resist scratching.

[0014] In several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass includes:

[0015] from about 59.0 to about 65.0 weight percent (wt %) of silicon dioxide (SiO.sub.2),

[0016] from about 11.9 to about 13.0 wt % of aluminum oxide (Al.sub.2O.sub.3),

[0017] from about 16.0 to about 18.5 wt % of sodium oxide (Na.sub.2O),

[0018] from about 0 to about 1.0 wt % of boron trioxide (B.sub.2O.sub.3),

[0019] from about 2.8 to about 3.3 wt % of potassium oxide (K.sub.2O), and

[0020] from about 2.0 to about 6.0 wt % of magnesium oxide (MgO), [0021] wherein the weight ratio of the combined total of B.sub.2O.sub.3, Na.sub.2O and K.sub.2O to Al.sub.2O.sub.3 is greater than 1.5; and [0022] wherein the weight ratio of the combined total of Na.sub.2O and K.sub.2O to Al.sub.2O.sub.3 and MgO is greater than 1.0.

[0023] According to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass consists essentially of:

[0024] from about 59.0 to about 65.0 weight percent (wt %) of silicon dioxide (SiO.sub.2),

[0025] from about 11.9 to about 13.0 wt % of aluminum oxide (Al.sub.2O.sub.3),

[0026] from about 16.0 to about 18.5 wt % of sodium oxide (Na.sub.2O),

[0027] from about 0 to about 1.0 wt % of boron trioxide (B.sub.2O.sub.3),

[0028] from about 2.8 to about 3.3 wt % of potassium oxide (K.sub.2O), and

[0029] from about 2.0 to about 6.0 wt % of magnesium oxide (MgO), [0030] wherein the weight ratio of the combined total of B.sub.2O.sub.3, Na.sub.2O and K.sub.2O to Al.sub.2O.sub.3 is greater than 1.5; and [0031] wherein the weight ratio of the combined total of Na.sub.2O and K.sub.2O to Al.sub.2O.sub.3 and MgO is greater than 1.0.

[0032] According to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass includes from about 59.0 to about 65.0 wt % of silicon dioxide (SiO.sub.2). Silicon dioxide is the largest single component of the alkali-aluminosilicate glass composition and forms the matrix of the glass together with boron trioxide (B.sub.2O.sub.3). Silicon dioxide also serves as a structural coordinator of the glass and contributes formability, rigidity and chemical durability to the glass. According to several exemplary embodiments, the glass viscosity is enhanced when the SiO.sub.2 is present in the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass in a range of from about 59.0 wt % up to about 65.0 wt %. At concentrations above 65.0 wt %, silicon dioxide raises the melting temperature of the glass composition and may cause the liquidus temperature to increase substantially in glass compositions having relatively high concentrations of K.sub.2O or MgO due to glass stability.

[0033] According to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass includes from about 11.9 to about 13.0 wt % of aluminum oxide (Al.sub.2O.sub.3). At concentrations of about 11.9 to about 13.0 wt %, the aluminum oxide enhances the strength of the chemically strengthened alkali-aluminosilicate glass and facilitates the ion-exchange between sodium ions in the surface of the glass and potassium ions in the ion exchange solution. According to several exemplary embodiments, the glass viscosity is enhanced when the Al.sub.2O.sub.3 is present in the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass in a range of from about 11.9 wt % up to about 13.0 wt %. At concentrations of aluminum oxide above 13.0 wt %, the viscosity of the glass may become prohibitively high so that it tends to devitrify the glass and the liquidus temperature may become too high to perform a continuous sheet forming process. Therefore, the ratio of the total content of flux oxides (B.sub.2O.sub.3, Na.sub.2O and K.sub.2O) in the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass to the total content of Al.sub.2O.sub.3 is greater than 1.5.

[0034] According to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass includes from about 16.0 to about 18.5 wt % of sodium oxide (Na.sub.2O). Alkali metal oxides serve as aids in achieving low liquidus temperatures and low melting temperatures. In the case of sodium, Na.sub.2O is also used to enable successful ion exchange. In order to permit sufficient ion exchange to produce substantially enhanced glass strength, sodium oxide is included in the composition in the concentrations set forth above. Also, to increase the possibility of ion exchange between sodium ions and potassium ions, according to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass includes from about 2.8 to about 3.3 wt % of potassium oxide (K.sub.2O).

[0035] According to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass includes from about 0 to about 1.0 wt % of boron trioxide (B.sub.2O.sub.3). Together with silicon, trivalent boron is a network-forming element and also plays a role in increasing the glass formability. In addition, as noted above, boron trioxide serves as a flux. The BO bond usually occurs in oxide glasses with coordination numbers of 3 and 4, which is of high field strength and indicate that the BO bond is also very strong. However, the bonds between the groups of [BO.sub.n] structure units are very weak at high temperatures which are different from the behavior of the vitreous groups of [SiO.sub.n]. The viscosity of boron oxide at high temperatures is much lower than that of silica, so that according to several exemplary embodiments of the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass boron oxide acts as a very efficient flux.

[0036] According to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass includes from about 2.0 to about 8.0 wt % of magnesium oxide (MgO). Magnesium oxide is also believed to increase the strength of the glass and to decrease the specific weight of the glass as compared to other alkaline oxides such as calcium oxide (CaO), strontium oxide (SrO) and barium oxide (BaO).

[0037] According to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass includes a ratio of the combined total content of boron trioxide (B.sub.2O.sub.3), sodium oxide (Na.sub.2O) and potassium oxide (K.sub.2O) to the content of aluminum oxide (Al.sub.2O.sub.3) of greater than 1.5.

[0038] According to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass includes a ratio of the combined total content of sodium oxide (Na.sub.2O) and potassium oxide (K.sub.2O) to the combined total content of aluminum oxide (Al.sub.2O.sub.3) and magnesium oxide (MgO) of greater than 1.0.

[0039] According to several exemplary embodiments, the present invention provides a method for manufacturing a chemically strengthened alkali-aluminosilicate glass. According to several exemplary embodiments, the method includes: [0040] mixing and melting the components to form a homogenous glass melt; [0041] shaping the glass using the down-draw method, the floating method and combinations thereof; [0042] annealing the glass; and [0043] chemically strengthening the glass by ion exchange.

[0044] According to several exemplary embodiments, the manufacture of the chemically strengthened alkali-aluminosilicate glass may be carried out using conventional down-draw methods which are well known to those of ordinary skill in the art and which customarily include a directly or indirectly heated precious metal system consisting of a homogenization device, a device to lower the bubble content by means of fining (refiner), a device for cooling and thermal homogenization, a distribution device and other devices. The floating method includes floating molten glass on a bed of molten metal, typically tin, resulting in glass that is very flat and has a uniform thickness.

[0045] According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion-exchangeable glass composition is melted for up to about 12 hours at about 1650 C. According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion-exchangeable glass composition is melted for up to about 6 hours at about 1650 C. According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion-exchangeable glass composition is melted for up to about 4 hours at about 1650 C.

[0046] According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion-exchangeable glass composition is annealed at a rate of about 1.0 C./hour until the glass reaches 500 C., after which the glass composition is allowed to cool to room temperature (or about 21 C.).

[0047] According to several exemplary embodiments, the ion-exchangeable glass composition for producing chemically strengthened alkali-aluminosilicate glass described above is chemically strengthened according to conventional ion exchange conditions. According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion exchange process occurs in a molten salt bath. In several exemplary embodiments, the molten salt is potassium nitrate (KNO.sub.3).

[0048] According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion exchange treatment takes place at a temperature range of from about 380 C. to about 450 C.

[0049] According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion exchange treatment is conducted for up to about 8 hours. According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion exchange treatment is conducted for up to about 4 hours. According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion exchange treatment is conducted for up to about 2 hours. According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion exchange treatment is conducted for about 2 hours to about 8 hours. According to several exemplary embodiments of the method for manufacturing a chemically strengthened alkali-aluminosilicate glass described above, the ion exchange treatment is conducted for about 4 hours at a temperature of about 420 C.

[0050] According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a surface compressive stress layer having a compressive stress of at least about 650 MPa. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a surface compressive stress layer having a compressive stress of at least about 700 MPa. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a surface compressive stress layer having a compressive stress of from about 720 MPa to about 755 MPa. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a surface compressive stress layer having a compressive stress of at least about 1100 MPa. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a surface compressive stress layer having a compressive stress of up to about 1350 MPa. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a surface compressive stress layer having a compressive stress of from about 650 MPa to about 1350 MPa.

[0051] According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a compressive stress layer having a depth of at least about 30.0 m. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a compressive stress layer having a depth of at least about 35.0 m. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a compressive stress layer having a depth of from about 30.0 m to about 45.0 m. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a compressive stress layer having a depth of from about 35.0 m to about 45.0 m.

[0052] According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a tensile stress (CT) of less than about 70 MPa. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a tensile stress (CT) of less than about 55 MPa. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a tensile stress (CT) of from about 40 MPa to about 55 MPa.

[0053] According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a thickness of from about 0.4 to about 2.0 mm.

[0054] According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass has a density of up to about 2.6 g/cm.sup.3 and a linear coefficient of expansion .sub.25-300 10.sup.7/ C. in a range of from about 97.0 to about 105.0.

[0055] According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass may be used as a protective glass in applications such as solar panels, refrigerator doors, and other household products. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass may be used as a protective glass for televisions, as safety glass for automated teller machines, and additional electronic products. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass may be used as cover glass for consumer mobile electronic devices such as smart phones, tablets and note pads. According to several exemplary embodiments of the chemically strengthened alkali-aluminosilicate glass described above, the glass may be used as a touch display using high strength cover glass, solar cell cover glass and laminated safety glass and more particularly may be used as a touch display using curved cover glass and 3D cover glass as well as vehicle glass.

[0056] The following examples are illustrative of the compositions and methods discussed above.

Examples

[0057] An ion-exchangeable glass composition that included the components shown below in Table 2 was prepared as follows:

TABLE-US-00002 TABLE 2 Oxide Wt % SiO.sub.2 63.35 Al.sub.2O.sub.3 12.0 Na.sub.2O 16.5 B.sub.2O.sub.3 0.3 K.sub.2O 3.0 MgO 4.85

[0058] Batch materials, as shown in Table 3 were weighed and mixed before being added to a 2 liter plastic container. The batch materials used were of chemical reagent grade quality.

TABLE-US-00003 TABLE 3 Batch raw materials Batch weight (gm) Sand 316.75 Alumina hydroxide 91.8 Soda ash 142.70 Borax 2.17 Potassium carbonate 22.01 Magnesia 22.25

[0059] The particle size of the sand was between 0.045 and 0.25 mm. A tumbler was used for mixing the raw materials to make a homogenous batch as well as to break up soft agglomerates. The mixed batch was transferred from the plastic container to an 800 ml. platinum-rhodium alloy crucible for glass melting. The platinum-rhodium crucible was placed in an alumina backer and loaded in a high temperature furnace equipped with MoSi heating elements operating at a temperature of 900 C. The temperature of the furnace was gradually increased to 1650 C. and the platinum-rhodium crucible with its backer was held at this temperature for 4 hours. The glass sample was then formed by pouring the molten batch materials from the platinum-rhodium crucible onto a stainless steel plate to form a glass patty. While the glass patty was still hot, it was transferred to an annealer and held at a temperature of 550 C. for 2 hours and was then cooled at a rate of 1.0 C./min to a temperature of 500 C. After that, the sample was cooled naturally to room temperature (21 C.).

[0060] The glass sample was then chemically strengthened by placing it in a molten salt bath tank, in which the constituent sodium ions in the glass were exchanged with externally supplied potassium ions at a temperature of 420 C. which was less than the strain point of the glass for 4 hours. By this method, the glass sample was strengthened by ion exchange to produce a compressive stress layer at the treated surface.

[0061] The measurement of the compressive stress at the surface of the glass and the depth of the compressive stress layer (based on double refraction) were determined by using a polarization microscope (Berek compensator) on sections of the glass. The compressive stress of the surface of the glass was calculated from the measured dual refraction assuming a stress-optical constant of 0.26 (nm*cm/N) (Scholze, H., Nature, Structure and Properties, Springer-Verlag, 1988, p. 260).

[0062] The results for the composition shown in Table 2 above are shown below in Table 4 in the column designated as Ex. 1. Additional compositions shown in Table 4 and designated as Ex. 2 to Ex. 10 were prepared in a similar manner as described above for the composition designated as Ex. 1.

TABLE-US-00004 TABLE 4 Oxide (wt %) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 SiO.sub.2 63.35 64.0 63.5 64.0 63.9 63.4 63.4 64.2 64.9 63.35 Al.sub.2O.sub.3 12.0 13.0 12.5 11.9 12.8 12.2 13.0 11.9 11.9 12.0 Na.sub.2O 16.5 16.4 17.0 16.0 16.4 18.0 16.5 16.0 16.0 18.0 B.sub.2O.sub.3 0.3 0.3 0.3 0.8 0.5 0.2 0.4 0.8 0.9 0.2 K.sub.2O 3.0 3.2 2.8 3.3 3.0 2.9 3.0 2.8 2.8 3.1 MgO 4.85 3.1 3.9 4.0 3.4 3.3 3.7 4.3 3.5 3.35 (B.sub.2O.sub.3 + Na.sub.2O + K.sub.2O)/Al.sub.2O.sub.3 1.65 1.53 1.61 1.69 1.55 1.73 1.53 1.65 1.66 1.78 (Na.sub.2O + K.sub.2O)/(Al.sub.2O.sub.3 + 1.16 1.22 1.21 1.21 1.20 1.35 1.17 1.16 1.22 1.37 MgO) d (g/cm.sup.3) 2.46 2.44 2.45 2.44 2.44 2.44 2.45 2.44 2.43 2.44 n.sub.D (20 C.) 1.503 1.499 1.501 1.501 1.501 1.500 1.501 1.502 1.500 1.501 (25-300 C., 10.sup.7/ C.) 99.5 98.7 100.1 98.8 98.5 103.5 98.6 97.5 97.5 104.3 T.sub.m(10.sup.2.5P/ C.) 1420 1458 1434 1438 1453 1419 1448 1439 1452 1414 T.sub.w(10.sup.4.0P/ C.) 1108 1125 1097 1111 1116 1097 1121 1119 1124 1092 T.sub.liquidus( C.) 900 860 880 900 880 860 890 910 890 880 T.sub.s(10.sup.7.6P/ C.) 752 761 753 757 761 742 759 759 761 738 T.sub.a(10.sup.13.0P/ C.) 542 550 544 545 549 537 549 547 549 535 T.sub.st(10.sup.14.5P/ C.) 501 510 503 505 509 498 508 506 508 495 T.sub.g(10.sup.13.4P/ C.) 536 542 535 538 541 528 540 538 541 526 VH (kgf/mm.sup.2) 542 563 549 556 563 535 554 565 580 530 VH w/CS (kgf/mm.sup.2) 625 640 627 635 643 612 634 647 652 600 CS (MPa)(420 C., 4 hrs) 731 743 733 723 734 752 746 726 730 721 DOL (m)(420 C., 4 hrs) 42.0 43.0 40.0 40.0 43.0 41.0 44.0 38.0 37.0 41.0 CT (MPa)(420 C., 4 hrs) 50.0 52.0 47.0 47.0 51.0 50.0 54.0 44.0 43.0 48.0

[0063] The definitions of the symbols set forth in Table 4 are as follows: [0064] d: density (g/cm.sup.3), which is measured with the Archimedes method; [0065] n.sub.D: refractive index, which is measured by refractometry; [0066] : coefficient of thermal expansion (CTE) which is the amount of linear dimensional change from 25 to 300 C., as measured by dilatometry; [0067] T.sub.m (10.sup.2.5 P/ C.): the temperature at the viscosity of 10.sup.2.5 poise, as measured by high temperature cylindrical viscometry; [0068] T.sub.w (10.sup.4.0 P/ C.): glass working temperature at the viscosity of 10.sup.4.0 poise by high temperature cylindrical viscometry; [0069] T.sub.liquidus ( C.): liquidus temperature where the first crystal is observed in a boat within a gradient temperature furnace, generally test is 72 hours for crystallization; [0070] T.sub.s (10.sup.7.6 P/ C.): glass softening temperature at the viscosity of 10.sup.7.6 poise as measured by the fiber elongation method; [0071] T.sub.a (10.sup.13.0 P/ C.): glass annealing temperature at the viscosity of 10.sup.13 poise as measured by the fiber elongation method; [0072] T.sub.st (10.sup.14.5 P/ C.): glass strain temperature at the viscosity of 10.sup.14.5 poise and measured by the fiber elongation method; [0073] T.sub.g (10.sup.13.4 P/ C.): glass transition temperature at the viscosity of 10.sup.13.4 poise and measured by a push-rod dilatometer; [0074] VH: Vicker's Hardness; [0075] VH w/CS: Vicker's Hardness after chemical strengthening; [0076] CS (MPa) (420 C., 4 hrs): compressive stress (in-plane stress which tends to compact the atoms in the surface); [0077] DOL (m) (420 C., 4 hrs): depth of layer which represents the depth of the compressive stress layer below the surface to the nearest zero stress plane; [0078] CT (MPa) (420 C., 4 hrs): central tensile stress which is calculated by the formula: CT=CSDOL/(t2DOL) where t is the thickness of the glass.

[0079] While the present invention has been described in terms of certain embodiments, those of ordinary skill in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

[0080] Any spatial references such as, for example, upper, lower, above, below, between, bottom, vertical, horizontal, angular, upwards, downwards, side-to-side, left-to-right, left, right, right-to-left, top-to-bottom, bottom-to-top, top, bottom, bottom-up, top-down, etc., are for the purpose of illustration only and do not limit the specific orientation or location of the structure described above.

[0081] The present disclosure has been described relative to certain embodiments. Improvements or modifications that become apparent to persons of ordinary skill in the art only after reading this disclosure are deemed within the spirit and scope of the application. It is understood that several modifications, changes and substitutions are intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.