Glass substrate for display and method for producing same

Abstract

A glass substrate that achieves a high strain point while having a low devitrification temperature; and a method for producing the glass substrate. This glass substrate for a display is made of a glass comprising SiO.sub.2 and Al.sub.2O.sub.3, comprising 0% or more to less than 4% B.sub.2O.sub.3 in mass %, and substantially devoiding Sb.sub.2O.sub.3, wherein 3BaO/(MgO+CaO+SrO) is 5 or less, MgO/(CaO+SrO) is 0.36 or greater, the devitrification temperature is 1235 C. or lower, and the strain point is 700 C. or higher. The method comprises: melting, by using at least direct electrical heating, a glass material prepared to have a predetermined composition; forming, into a flat glass sheet, the molten glass that has been melted in the melting step; and annealing the flat glass sheet, wherein a condition for cooling the flat glass sheet is controlled so as to reduce the heat shrinkage rate of the flat glass sheet.

Claims

1. A glass substrate for a display composed of a glass, the glass comprising SiO.sub.2 and Al.sub.2O.sub.3; and the glass further comprising, in % by mass, B.sub.2O.sub.3 at 0.1% or more and less than 4%, K.sub.2O at 0.01% to 0.6%, and BaO; being substantially devoid of Sb.sub.2O.sub.3, La.sub.2O.sub.3, and As.sub.2O.sub.3; having MgO/(CaO+SrO) of 0.36 or more; having a devitrification temperature of 1235 C. or lower; and having a strain point of 700 C. or higher, wherein the value calculated in the formula 3BaO/(MgO+CaO+SrO) is in the range of 1.0 to 5.0, and the value calculated in the formula SrO/CaO is equal to or more than 0.32 and equal to or less than 1.0.

2. The glass substrate according to claim 1, wherein the glass contains, in % by mass, R.sub.2O (Li.sub.2O+Na.sub.2O+K.sub.2O) at 0.01% to 1.0%.

3. The glass substrate according to claim 1, wherein the glass substrate has a heat shrinkage rate of 15 ppm or less, the heat shrinkage rate being represented by the following formula when the glass substrate is kept at a temperature of 500 C. for 30 minutes followed by cooling to normal temperature:
Heat shrinkage rate (ppm)={Amount of shrinkage of glass before and after heat treatment/Length of glass before heat treatment}10.sup.6.

4. The glass substrate according to claim 1, wherein the glass fulfils, in % by mass, MgO/(RO+ZnO) of 0.01 to 0.8, wherein RO represents (MgO+CaO+SrO+BaO).

5. The glass substrate according to claim 2, wherein the glass substrate has a heat shrinkage rate of 15 ppm or less, the heat shrinkage rate being represented by the following formula when the glass substrate is kept at a temperature of 500 C. for 30 minutes followed by cooling to normal temperature:
Heat shrinkage rate (ppm)={Amount of shrinkage of glass before and after heat treatment/Length of glass before heat treatment}10.sup.6.

6. The glass substrate according to claim 2, wherein the glass fulfils, in % by mass, MgO/(RO+ZnO) of 0.01 to 0.8, wherein RO represents (MgO+CaO+SrO+BaO).

7. The glass substrate according to claim 3, wherein the glass fulfils, in % by mass, MgO/(RO+ZnO) of 0.01 to 0.8, wherein RO represents (MgO+CaO+SrO+BaO).

Description

DESCRIPTION OF EMBODIMENTS

(1) In the present specification, the composition of glass is, unless otherwise stated, expressed in % by mass and the content represented by % means % by mass. The ratio between components that constitute a glass composition is expressed in mass ratio.

(2) The glass substrate for a display (first embodiment) of the present invention is composed of a glass,

(3) the glass comprising SiO.sub.2 and Al.sub.2O.sub.3; and

(4) the glass further comprising, in % by mass, B.sub.2O.sub.3 at 0% or more and less than 4%;

(5) being substantially devoid of Sb.sub.2O.sub.3;

(6) having 3BaO/(MgO+CaO+SrO) of 5 or less;

(7) having MgO/(CaO+SrO) of 0.36 or more;

(8) having a devitrification temperature of 1235 C. or lower; and

(9) having a strain point of 700 C. or higher.

(10) The glass substrate for a display (second embodiment) of the present invention is composed of a glass,

(11) the glass comprising SiO.sub.2 and Al.sub.2O.sub.3; and

(12) the glass further comprising, in % by mass, B.sub.2O.sub.3 at 0% or more and less than 4%;

(13) MgO at 1.8% to 15%; and

(14) BaO at 6.5% to 14%;

(15) being substantially devoid of Sb.sub.2O.sub.3;

(16) having 3BaO/(MgO+CaO+SrO) of 5 or less;

(17) having a devitrification temperature of 1235 C. or lower; and

(18) having a strain point of 700 C. or higher.

(19) The glass substrate for a display of the present embodiments are hereinafter described. Unless otherwise stated, the following descriptions apply to both the first and second embodiments of the present invention.

(20) The glass constituting the glass substrate for a display of the present invention contains SiO.sub.2 and Al.sub.2O.sub.3.

(21) SiO.sub.2 is a skeletal component of glass, and is thus an essential component. If the content is low, there are tendencies that the strain point decreases and the coefficient of thermal expansion increases. If the SiO.sub.2 content is too low, it becomes difficult to reduce the density of the glass substrate. On the other hand, if the SiO.sub.2 content is too high, there are tendencies that the specific resistance of molten glass increases, the melting temperature significantly increases and melting becomes difficult. If the SiO.sub.2 content is too high, the etching rate becomes low. From such standpoints, the SiO.sub.2 content may be appropriately adjusted. The SiO.sub.2 content of the glass is, for example, in the range of preferably 40% to 80%. The content of SiO.sub.2 is more preferably in the range of 50% to 75% or 50% to 70%, still more preferably 52% to 68% and even more preferably 55% to 65%.

(22) Al.sub.2O.sub.3 is an essential component that increases the strain point. If the Al.sub.2O.sub.3 content is too low, the strain point decreases. Further, if the Al.sub.2O.sub.3 content is too low, there are tendencies that the Yong's modulus and the rate of etching using acid also decrease. On the other hand, if the Al.sub.2O.sub.3 content is too high, the devitrification temperature of glass increases and the devitrification resistance decreases, and thus formability tends to deteriorate. From such standpoints, the content may be appropriately adjusted. The Al.sub.2O.sub.3 content of the glass is in the range of, for example, 10% to 35%. The Al.sub.2O.sub.3 content is in the range of preferably 13% to 30%, more preferably 15% to 25%, still more preferably 15% to 23% and even more preferably 16% to 22%.

(23) B.sub.2O.sub.3 is a component that reduces the viscosity at high temperatures of glass and improves meltability. Namely, the component reduces the viscosity in the vicinity of the melting temperature, thereby improving meltability. The component also reduces the devitrification temperature. If the B.sub.2O.sub.3 content is too low, there are tendencies that the meltability and the devitrification resistance deteriorate. If the B.sub.2O.sub.3 content is too high, the strain point and the Young's modulus decrease. Due to the volatilization of B.sub.2O.sub.3 during glass formation, devitrification is prone to occur. Particularly, glass having a high strain point tends to have high formation temperature, and thus the volatilization is promoted and the occurrence of devitrification causes a significant problem. Due to volatilization of B.sub.2O.sub.3 during glass melting, the glass becomes significantly nonuniform and striae are prone to occur. From such standpoints, the B.sub.2O.sub.3 content is 0% or more and less than 4%. The B.sub.2O.sub.3 content is in the range of preferably 0% to 3.8%, more preferably 0% to 3.5%, still more preferably 0.1% to 3.4%, even more preferably 0.5% to 3.3% and yet more preferably 1% to 3.2%.

(24) MgO is a component that increases meltability and is an essential component in the second embodiment of the present invention. MgO is also a component that hardly increases the density among other alkaline earth metals, and thus if the content thereof is relatively increased, the density may be easily decreased. Inclusion thereof may decrease the specific resistance and melting temperature of molten glass. However, if the MgO content is too high, the devitrification temperature of glass sharply increases, thereby frequently causing devitrification particularly in the forming step. From such standpoints, in the second embodiment of the present invention, the MgO content is in the range of 1.8% to 15%, preferably 1.8% to 13%, more preferably 1.9% to 10% and still more preferably 1.9% to 7%. In the first embodiment of the present invention, the MgO content is in the range of preferably 0% to 15%, more preferably 0% to 13% and still more preferably 0% to 10%.

(25) CaO is a component that is not essential, but is efficient for improving meltability of glass without sharply increasing the devitrification temperature of the glass. CaO is also a component that hardly increases the density among other alkaline earth metal oxides, and thus if the content thereof is relatively increased, the density may be easily decreased. If the content is too low, there are tendencies that the specific resistance of molten glass increases and the devitrification resistance decreases. If the CaO content is too high, there are tendencies that the coefficient of thermal expansion increases and the density increases. From such standpoints, the CaO content is in the range of preferably 0% to 20%, more preferably 0% to 15% and still more preferably 0% to 10%.

(26) SrO is a component that may decrease the devitrification temperature of glass. Although SrO is not essential, inclusion thereof improves devitrification resistance and meltability. However, if the SrO content is too high, the density increases. From such standpoints, the SrO content is in the range of 0% to 15%, preferably 0% to 10%, more preferably 0% to 7%, still more preferably 0% to 5% and even more preferably 0% to 3%.

(27) BaO is a component that may effectively reduce the devitrification temperature of glass and specific resistance of molten glass and is an essential component in the second embodiment of the present invention. Inclusion of BaO improves devitrification resistance and meltability. However, if the BaO content is too high, the density increases. From the standpoints of environmental load and the tendency of increasing the coefficient of thermal expansion, the BaO content in the second embodiment of the present invention is 6.5% to 14%. In the second embodiment of the present invention, the BaO content is in the range of 7% to 14%, preferably 7% to 13% and more preferably 7% to 12%. In the first embodiment of the present invention, the BaO content is in the range of preferably 0% to 15%, more preferably 2% to 14% and still more preferably 4% to 12%.

(28) The mass ratio (3BaO)/(MgO+CaO+SrO) causes an increase in the devitrification temperature if the mass ratio is too high or too low. If the value of (3BaO)/(MgO+CaO+SrO) is too high, the Young's modulus decreases, the density increases and the specific resistance also increases. Therefore, the mass ratio (3BaO)/(MgO+CaO+SrO) is in the range of preferably 5.0 or less, preferably 0.5 to 5 and more preferably 1 to 5.

(29) The mass ratio BaO/(MgO+CaO+SrO+BaO) causes an increase in the devitrification temperature if the mass ratio is too high or too low. If the value of BaO/(MgO+CaO+SrO+BaO) is high, the Young's modulus decreases, the density increases and the specific resistance also increases. Therefore, the mass ratio BaO/(MgO+CaO+SrO+BaO) is in the range of preferably 0 to 0.9, preferably 0.1 to 0.85 and more preferably 0.2 to 0.8.

(30) If the value of the mass ratio (MgO/(CaO+SrO)) is low, there are tendencies that the devitrification temperature decreases and the Young's modulus decreases. Further, the density increases and the specific resistance also increases. On the other hand, if the value is high, the devitrification temperature increases and the Young's modulus decreases. Therefore, in the first embodiment of the present invention, the mass ratio (MgO/(CaO+SrO)) is 0.36 or more, preferably 0.36 to 2 and preferably 0.4 to 1.

(31) If the value of the mass ratio (MgO/(RO+ZnO)) is low, there are tendencies that the devitrification temperature decreases and the Young's modulus decreases. Further, the density increases and the specific resistance also increases. On the other hand, if the value is high, the devitrification temperature increases and the Young's modulus decreases. Therefore, the mass ratio (MgO/(RO+ZnO)) is in the range of preferably 0.01 to 0.8, preferably 0.05 to 0.8, 0.07 to 0.6 and 0.08 to 0.4.

(32) Li.sub.2O and Na.sub.2O are components that may increase the coefficient of thermal expansion of glass and damage the substrate during heat treatment. The components also reduce the strain point. Meanwhile, the components may reduce the specific resistance of molten glass, thus can inhibit erosion of a melting tank by including the same. From such standpoints, the Li.sub.2O content is preferably 0% to 0.5% and more preferably Li.sub.2O is not substantially included. The Na.sub.2O content is preferably 0% to 0.5% and more preferably 0% to 0.2%. As Na.sub.2O is a component that reduces the strain point less than Li.sub.2O does, it is preferable that Na.sub.2O>Li.sub.2O. From the standpoint of preventing elution from the glass substrate to cause deterioration of TFT properties, it is preferable that Li.sub.2O and Na.sub.2O are not substantially included.

(33) K.sub.2O is a component that increases the basicity of glass and promotes refinability. The component also reduces the specific resistance of molten glass. Inclusion thereof reduces the specific resistance of molten glass, thereby enabling prevention of electric current flowing through refractory materials forming a melting tank and erosion of the melting tank. If refractory materials forming a melting tank include zirconia, elution of zirconia may be inhibited from the melting tank to molten glass by erosion of the melting tank, and thus devitrification due to zirconia may also be inhibited. The component may also reduce the viscosity of glass in the vicinity of the melting temperature, thereby improving meltability and refinability. If the K.sub.2O content is too high, there are tendencies that the coefficient of thermal expansion increases and the strain point decreases. From such standpoints, the K.sub.2O content is in the range of preferably 0% to 0.8%, more preferably 0.01% and 0.6% and still more preferably 0.1% to 0.5%.

(34) Li.sub.2O, Na.sub.2O and K.sub.2O are components that increase the basicity of glass, facilitate the oxidation of refining agents and allow manifestation of refinability. The components also reduce the viscosity at the melting temperature and improve meltability. The components also reduce the specific resistance of molten glass. Inclusion of Li.sub.2O, Na.sub.2O and K.sub.2O reduces the specific resistance of molten glass and improves refinability and meltability. The components may particularly prevent excess electric current flowing through refractory materials forming a melting tank and inhibit erosion of the melting tank. If the melting tank is formed with zirconia, the components may inhibit elution of zirconia from a melting tank to glass, and thus may inhibit devitrification due to zirconia. The components also reduce the viscosity of molten glass and thus may improve meltability and refinability. However, if the total content of Li.sub.2O, Na.sub.2O and K.sub.2O is too high, the components may elute from the glass substrate to deteriorate TFT properties. There are also tendencies that the strain point decreases and the coefficient of thermal expansion increases. The total content of Li.sub.2O, Na.sub.2O and K.sub.2O (hereinafter referred to as R.sub.2O) is preferably 1.0% or less, more preferably 0% to 1.0%, still more preferably 0.01% to 1.0% or 0% to 0.8%, even more preferably 0.01% to 0.8%, still even more preferably 0.01% to 0.6% and yet more preferably 0.1% to 0.5%.

(35) ZrO.sub.2 and TiO.sub.2 are components that improve the strain point of glass. However, if the amount of ZrO.sub.2 and the amount of TiO.sub.2 are too high, the devitrification temperature significantly increases and devitrification resistance tends to decrease. Particularly, ZrO.sub.2 has a high melting point and is practically insoluble, and thus may cause a problem of deposition of some raw materials at the bottom of a melting tank. The non-melting components mixed in a glass body may become an inclusion and impair the quality of glass. TiO.sub.2 is a component that causes tinting of glass, and thus is not preferable to be included in substrates for displays. From such standpoints, the glass substrate of the present embodiments contains ZrO.sub.2 and TiO.sub.2 in the range of 0% to 10% and more preferably 0% to 5%, respectively, and still more preferably does not substantially include ZrO.sub.2 or TiO.sub.2.

(36) ZnO is a component that improves meltability. However, the component is not essential. If the ZnO content is too high, there are tendencies that the devitrification temperature increases, the strain point decreases and the density increases. From such standpoints, the ZnO content is in the range of preferably 0% to 5% and more preferably 0% to 2% and still more preferably the glass substrate does not substantially include ZnO.

(37) P.sub.2O.sub.5 is a component that reduces the high-temperature viscosity and improves meltability. However, the component is not essential. If the P.sub.2O.sub.5 content is too high, the strain point decreases. Also, due to the volatilization of P.sub.2O.sub.5 during glass melting, the glass becomes significantly nonuniform and striae are prone to occur. From such standpoints, the P.sub.2O.sub.5 content is in the range of preferably 0% to 3%, more preferably 0% to 1% and still more preferably 0% to 0.5%, and even more preferably, the glass substrate does not substantially contain P.sub.2O.sub.5.

(38) The glass substrate of the present embodiments may contain a refining agent. The refining agent is not particularly limited as far as the refining agent has a small environmental load and excellent refinability of glass. Examples thereof include at least one selected from the group of metal oxides of Sn, Fe, Ce, Tb, Mo and W. The glass substrate of the present embodiments does not substantially contain Sb.sub.2O.sub.3. By being substantially devoid of Sb.sub.2O.sub.3, the environmental load may be reduced. SnO.sub.2 is suitable as a refining agent. If the content of the refining agent is too low, the quality of bubbles deteriorates. If the content is too high, the component may cause devitrification or tinting. The content of the refining agent may also depend on the type of the refining agent and the glass composition. For example, the total content of SnO.sub.2 and Fe.sub.2O.sub.3 is preferably 0.05% to 0.50% and more preferably 0.05% to 0.40%.

(39) SnO.sub.2 is a refining agent that exhibits a refining effect even at 1600 C. or higher and is one of few refining agents that may be used for production of a glass substrate for a flat panel display which can contain only minute amounts of Li.sub.2O, Na.sub.2O and K.sub.2O (for example, the total content of Li.sub.2O, Na.sub.2O and K.sub.2O is 0.01% to 0.8%). However, SnO.sub.2 is a component that may easily cause devitrification spontaneously and promotes devitrification of other components. From the standpoint of inhibiting devitrification, it is not preferable to add a high level of SnO.sub.2.

(40) Glass having a high strain point (for example, glass having a strain point of 700 C. or higher) tends to have a higher devitrification temperature than glass having a low strain point (for example, glass having a strain point of less than 700 C.). Therefore, in order to inhibit devitrification, the temperature of molten glass in the forming step may have to be increased compared to glass having a low strain point, in some cases. Meanwhile, it is preferable that a forming device used in the overflow down-draw processing is formed with refractory materials including zirconia from the standpoint of creep resistance and heat resistance. If the forming method employed is overflow down-draw, the temperature of the forming device is required to be increased in order to increase the temperature of molten glass in the forming step. However, if the forming device has an increased temperature, zirconia may elute from the forming device and may cause devitrification of zirconia. There are also tendencies that in glass containing particularly a high level of SnO.sub.2, devitrification of SnO.sub.2 due to zirconia and devitrification of zirconia due to SnO.sub.2 occur.

(41) Further, glass having a high strain point (for example, glass having a strain point of 700 C. or higher) tends to have a higher temperature during melting of glass raw materials than glass having a low strain point (for example, glass having a strain point of less than 700 C.). Meanwhile, a melting tank for performing a melting step is preferably formed with high-zirconia-content refractory materials containing zirconia from the standpoint of erosion resistance. Also from the standpoint of energy efficiency, glass raw materials are preferably melted by an electrical melting process or an electrical melting process combined with another heating means. However, when glass having a high strain point and containing only minute amounts of Li.sub.2O, Na.sub.2O and K.sub.2O as described in the present embodiments are melted, the molten glass has high specific resistance and thus may allow electric current to flow through the high-zirconia-content refractory material and elution of zirconia into the molten glass. If zirconia is eluted, there are tendencies that devitrification of zirconia and devitrification of SnO.sub.2 described above occur.

(42) Namely, from the standpoint of inhibiting devitrification of zirconia and SnO.sub.2, it is not preferable that the glass substrate of the present embodiments contains above 0.8% of SnO.sub.2. From such a standpoint, the SnO.sub.2 content is in the range of, for example, preferably 0.01% to 0.8% inclusive, more preferably 0.01% to 0.6%, still more preferably 0.02% to 0.6%, even more preferably 0.05% to 0.50% and yet more preferably 0.05% to 0.40%.

(43) Fe.sub.2O.sub.3 is a component that, in addition to serving as a refining agent, reduces the specific resistance of molten glass. Glass having a high high-temperature viscosity and having a difficulty in melting preferable to contain Fe.sub.2O.sub.3 in order to decrease the specific resistance of molten glass. However, if the Fe.sub.2O.sub.3 content is too high, glass is tinted and the transmittance decreases. Therefore, the Fe.sub.2O.sub.3 content is in the range of 0% to 0.1%, preferably 0% to 0.08%, more preferably 0.001% to 0.06%, still more preferably 0.001% to 0.05% and even more preferably 0.001% to 0.04%.

(44) In the present embodiments, it is preferable that the refining agent used is a combination of SnO.sub.2 and Fe.sub.2O.sub.3. From the standpoint of inhibition of devitrification, it is not preferable that the content of SnO.sub.2 is high, as described above. However, in order to have a sufficient refining effect, it is required to include a certain amount or more refining agent. By combining SnO.sub.2 and Fe.sub.2O.sub.3, glass substrate with less bubbles may be produced with sufficient refining effect without increasing the SnO.sub.2 content to the level that causes devitrification. The total content of SnO.sub.2 and Fe.sub.2O.sub.3 is in the range of preferably 0.05% to 0.50%, more preferably 0.05% to 0.20% and still more preferably 0.05% to 0.40%.

(45) If the mass ratio (SnO.sub.2/(SnO.sub.2+Fe.sub.2O.sub.3)) of the SnO.sub.2 content relative to the total content of SnO.sub.2 and Fe.sub.2O.sub.3 is too high, devitrification is prone to occur and if the mass ratio is too low, a sufficient refining effect may not be obtained and glass may be tinted. Therefore, the mass ratio is in the range of preferably 0.6 to 1.0 and more preferably 0.7 to 0.98.

(46) It is preferable that the glass substrate of the present embodiments does not substantially contain As.sub.2O.sub.3 from the standpoint of environmental load. The glass substrate of the present embodiments does not substantially contain Sb.sub.2O.sub.3 from the standpoint of environmental load.

(47) It is preferable that the glass substrate of the present embodiments does not substantially contain PbO or F from environmental reasons.

(48) As used herein, the phrase does not substantially contain or the like means that a substance that serves as a raw material of the component in question is not used as a glass raw material, and does not exclude contamination of the component in question as an impurity from a glass raw material for another component or of the component in question into glass from a production equipment such as a melting tank and a forming device.

(49) If the total content of SiO.sub.2 and Al.sub.2O.sub.3, SiO.sub.2+Al.sub.2O.sub.3, is too low, the strain point tends to decrease and if the total content is too high, devitrification resistance tends to deteriorate. Therefore, SiO.sub.2+Al.sub.2O.sub.3 is in the range of preferably 70% to 90%, preferably 73% to 88%, more preferably 75% to 85% and still more preferably 77% to 83%.

(50) If the value of the difference between the SiO.sub.2 content and a quarter of Al.sub.2O.sub.3, SiO.sub.2(Al.sub.2O.sub.3), is too high, the etching rate may decrease. From such a standpoint, SiO.sub.2(Al.sub.2O.sub.3) is preferably 65% or less and more preferably 60% or less. On the other hand, if the value of SiO.sub.2(Al.sub.2O.sub.3) is too low, devitrification resistance may decrease. From such a standpoint, SiO.sub.2(Al.sub.2O.sub.3) is preferably 40% to 65%, more preferably 50% to 60% and still more preferably 50% to 58%.

(51) If the value of the mass ratio SiO.sub.2/Al.sub.2O.sub.3 is too high, the etching rate may decrease and if the mass ratio is too low, devitrification resistance may decrease. From such standpoints, the mass ratio SiO.sub.2/Al.sub.2O.sub.3 is in the range of preferably 1.5 to 4.5, more preferably 2.0 to 4.0 and still more preferably 2.5 to 3.7. Between glasses having compositions in which values of SiO.sub.2+Al.sub.2O.sub.3 are similar, the etching rate more significantly depends on SiO.sub.2/Al.sub.2O.sub.3. From the standpoint of achieving all high strain point, devitrification resistance and etching rate, it is preferable that SiO.sub.2+Al.sub.2O.sub.3 is in the range of 70% to 90% and SiO.sub.2/Al.sub.2O.sub.3 is 1.5 to 4.5, and more preferably SiO.sub.2+Al.sub.2O.sub.3 is 73% to 88% and SiO.sub.2/Al.sub.2O.sub.3 is 2.0 to 4.0.

(52) The mass ratio B.sub.2O.sub.3/(SiO.sub.2+Al.sub.2O.sub.3) serves as indexes of mainly meltability, devitrification resistance and strain point. If B.sub.2O.sub.3/(SiO.sub.2+Al.sub.2O.sub.3) is too high, the strain point decreases. On the other hand, if B.sub.2O.sub.3/(SiO.sub.2+Al.sub.2O.sub.3) is too low, there are tendencies that meltability and devitrification resistance deteriorate. B.sub.2O.sub.3/(SiO.sub.2+Al.sub.2O.sub.3) is in the range of preferably 0 to 0.050, more preferably 0 to 0.045, still more preferably 0.001 to 0.040 and even more preferably 0.005 to 0.035.

(53) If the total content of B.sub.2O.sub.3 and P.sub.2O.sub.5, B.sub.2O.sub.3+P.sub.2O.sub.5, is too low, meltability tends to decrease and if the total content is too high, glass of B.sub.2O.sub.3+P.sub.2O.sub.5 becomes significantly nonuniform and striae are prone to occur and the strain point tends to decrease. Therefore, B.sub.2O.sub.3+P.sub.2O.sub.5 is in the range of preferably 0% to 10%, more preferably 0% to 7%, still more preferably 0% to 5%, even more preferably 0% to 3.5% and yet more preferably 0.5% to 3.5%.

(54) MgO, CaO, SrO and BaO are components that reduce the specific resistance and the melting temperature of molten glass and improve meltability. If the total content of MgO, CaO, SrO and BaO, MgO+CaO+SrO+BaO (hereinafter referred to as RO), is too low, meltability deteriorates. If RO is too high, the strain point and the Young's modulus decrease and the density and the coefficient of thermal expansion increase. From such standpoints, RO is in the range of preferably 5% to 35%, more preferably 9% to 30%, still more preferably 10% to 27% and even more preferably 12% to 25%.

(55) The mass ratio (SiO.sub.2+Al.sub.2O.sub.3)/(B.sub.2O.sub.3+RO) serves as indexes of mainly the strain point and devitrification resistance. If the value is too low, the strain point decreases. On the other hand, if the value is too high, meltability and devitrification resistance decrease. Therefore, the mass ratio (SiO.sub.2+Al.sub.2O.sub.3)/(B.sub.2O.sub.3+RO) is in the range of preferably 1 to 8, more preferably 2 to 7, still more preferably 2.5 to 6.5 and even more preferably 3 to 6.

(56) In order to efficiently reduce the devitrification temperature without extremely increasing the density, CaO/RO is in the range of preferably 0 to 0.8, more preferably 0.1 to 0.7, still more preferably 0.15 to 0.6 and even more preferably 0.2 to 0.5.

(57) Li.sub.2O, Na.sub.2O and K.sub.2O are components that increase the basicity of glass, facilitate the oxidation of refining agents and allow exhibition of refinability. The components also reduce the viscosity at the melting temperature and improve meltability. The components also reduce the specific resistance of molten glass. Inclusion of Li.sub.2O, Na.sub.2O and K.sub.2O decreases the specific resistance and improves refinability and meltability of molten glass. Particularly, excess electric current flowing through refractory materials forming a melting tank may be prevented and erosion of the melting tank may be inhibited. If a melting tank is formed with zirconia, elution of zirconia from the melting tank to glass may be inhibited, and thus devitrification due to zirconia may also be inhibited. The components also reduce the viscosity of molten glass, and thus improve meltability and refinability. However, if the total content of Li.sub.2O, Na.sub.2O and K.sub.2O is too high, the components may elute from the glass substrate to deteriorate TFT properties. Also, there are tendencies that the strain point decreases and the coefficient of thermal expansion increases. The total content of Li.sub.2O, Na.sub.2O and K.sub.2O (hereinafter referred to as R.sub.2O) is 0% to 1.0%, more preferably 0% to 0.8%, still more preferably 0.01% to 0.8%, even more preferably 0.01% to 0.6% and yet more preferably 0.1% to 0.5%.

(58) K.sub.2O has a larger molecular weight than Li.sub.2O and Na.sub.2O, and is thus less prone to elute from the glass substrate. Thus, it is preferable to include a higher amount of K.sub.2O than Li.sub.2O or Na.sub.2O. If the proportion of Li.sub.2O and Na.sub.2O Is high, there is a higher concern that elution from the glass substrate may deteriorate TFT properties. The mass ratio K.sub.2O/R.sub.2O is in the range of preferably 0.5 to 1, more preferably 0.6 to 1, still more preferably 0.65 to 1 and even more preferably 0.7 to 1.

(59) The mass ratio (SiO.sub.2+Al.sub.2O.sub.3)/(B.sub.2O.sub.3+RO+(10R.sub.2O)) serves as indexes of mainly the strain point and meltability. If the value is too low, the strain point decreases. Therefore, the mass ratio (SiO.sub.2+Al.sub.2O.sub.3)/(B.sub.2O.sub.3+RO+(10R.sub.2O)) is in the range of 1.0 or more and preferably 2.0 or more. On the other hand, if the value is too high, meltability and devitrification resistance decrease. Therefore, the mass ratio (SiO.sub.2+Al.sub.2O.sub.3)/(B.sub.2O.sub.3+RO+(10R.sub.2O)) is in the range of preferably 1.0 to 10 and more preferably 2.0 to 7. (SiO.sub.2+Al.sub.2O.sub.3)/(B.sub.2O.sub.3+RO+(10R.sub.2O)) is preferably 2.5 to 5.

(60) RE.sub.2O.sub.3 refers to the sum of rare-earth metal oxides. Examples of the rare-earth metal oxide include Sc.sub.2O.sub.3, Y.sub.2O.sub.3, La.sub.2O.sub.3, Pr.sub.2O.sub.3, Nd.sub.2O.sub.3, Sm.sub.2O.sub.3, Eu.sub.2O.sub.3, Gd.sub.2O.sub.3, Tb.sub.2O.sub.3, Dy.sub.2O.sub.3, Ho.sub.2O.sub.3, Er.sub.2O.sub.3, Tm.sub.2O.sub.3, Yb.sub.2O.sub.3 and Lu.sub.2O.sub.3. RE.sub.2O.sub.3 is a component that increases the density and the coefficient of thermal expansion. The component is also expensive. Therefore, RE.sub.2O.sub.3 is in the range of 0% or more and less than 1.0% (0% inclusive) and more preferably 0% to 0.5% (0% inclusive) and it is particularly preferable that RE.sub.2O.sub.3 is not substantially included.

(61) From the standpoints of preventing an increase in the density and coefficient of thermal expansion and reducing the cost, it is preferable that Y.sub.2O.sub.3 and La.sub.2O are not substantially included.

(62) The glass substrate of the present embodiments has a devitrification temperature of 1235 C. or lower, preferably 1230 C. or lower, more preferably 1225 C. or lower, still more preferably 1220 C. or lower and even more preferably 1210 C. or lower. If the devitrification temperature is low, glass plates can be formed with ease by overflow down-draw processing. By employing overflow down-draw processing, a polishing process of the surface of the glass substrate may be omitted, and thus the glass substrate may have an improved surface quality. Also, the production cost may be reduced. If the devitrification temperature is too high, devitrification is prone to occur, and thus there is a tendency that it may be difficult to employ overflow down-draw processing.

(63) The glass substrate of the present embodiments has an average coefficient of thermal expansion (100 C. to 300 C.) at 100 C. to 300 C. in the range of 50.010.sup.7 C..sup.1 or less, preferably 28.0 to 50.010.sup.7 C..sup.1, more preferably 33.0 to 47.010.sup.7 C..sup.1, still more preferably 33.0 to 46.010.sup.7 C..sup.1, even more preferably 35.0 to 44.010.sup.7 C..sup.1 and yet more preferably 38.0 to 43.010.sup.7 C..sup.1. If the coefficient of thermal expansion is high, thermal impact and the heat shrinkage rate tend to increase in the heat treatment step. If the coefficient of thermal expansion is high, it is difficult to reduce the heat shrinkage rate. If the coefficient of thermal expansion is high or low, the coefficient of thermal expansion may be less compatible with that of other peripheral materials such as metals and thin films formed on the glass substrate, and the peripheral parts may peel off.

(64) Generally, if a glass substrate has low strain point, heat shrinkage is prone to occur in the heat treatment step during production of displays. The glass substrate of the present embodiments has a strain point of 700 C. or higher, preferably 710 C. or higher, more preferably 720 C. or higher, still more preferably 735 C. or higher and even more preferably 730 C. or higher.

(65) The glass substrate of the present embodiments has a heat shrinkage rate of preferably 15 ppm or less and more preferably 13 ppm or less or 10 ppm or less. If the heat shrinkage rate is too high, the pixel pitch becomes significantly uneven, and it becomes impossible to achieve a high-definition display. In order to control the heat shrinkage rate to a predetermined range, it is preferable to configure the glass substrate to have a strain point of 700 C. or higher or 715 C. or higher. In order to control the heat shrinkage rate to be 0 ppm, it would be required to significantly extend the annealing step or perform a heat shrinkage reduction process (offline annealing) after annealing and cutting steps. In this case, the productivity is reduced and the costs are increased. In view of productivity and costs, the heat shrinkage rate is, for example, 0.1 ppm to 15 ppm or preferably 0.5 ppm to 15 ppm, more preferably 1 ppm to 15 ppm, still more preferably 1 ppm to 13 ppm and even more preferably 2 ppm to 10 ppm.

(66) The heat shrinkage rate is expressed by the following equation after performing a heat treatment in which the glass substrate is kept at a temperature of 500 C. for 30 minutes and cooled to room temperature.
Heat shrinkage rate (ppm)={Amount of shrinkage of glass before and after heat treatment/Length of glass before heat treatment}10.sup.6

(67) In the equation, Amount of shrinkage of glass before and after heat treatment means Length of glass before heat treatmentLength of glass after heat treatment.

(68) The glass substrate of the present embodiments has a density of, from standpoints of reducing the weight of the glass substrate and reducing the weight of a display, preferably 3.0 g/cm.sup.3 or less, more preferably 2.8 g/cm.sup.3 or less and still more preferably 2.65 g/cm.sup.3 or less. If the density is too high, it is difficult to reduce the weight of the glass substrate, and thus it is also difficult to reduce the weight of a display.

(69) If the glass transition point (hereinafter referred to as Tg) is low, heat shrinkage is prone to occur in the heat treatment step during production of displays. The glass substrate of the present embodiments has Tg of preferably 750 C. or higher, more preferably 760 C. or higher and still more preferably 765 C. or higher. In order to control Tg of the glass substrate to the above range, it is appropriate to increase the amounts of components such as SiO.sub.2 and Al.sub.2O.sub.3 or decrease the amounts of components such as B.sub.2O.sub.3, RO and R.sub.2O within the ranges of the composition of the glass substrate of the present embodiments.

(70) The glass of the present embodiments has a temperature at which the viscosity is 10.sup.2.5 [dPa.Math.s] (hereinafter referred to as melting temperature) of preferably 1680 C. or lower, more preferably in the range of 1500 C. to 1680 C., still more preferably 1520 C. to 1660 C. and even more preferably 1540 C. to 1640 C. Glass having a low melting temperature is prone to have a low strain point. In order to increase the strain point, it is required to also increase the melting temperature to some extent. However, a high melting temperature may put a burden on a melting tank. In addition, higher energy is used, resulting in an increase in costs. When glass is melted by electric melting, electric current flows through heat resistant bricks forming a melting tank rather than through glass, thus the melting tank may be damaged. In order to control the melting temperature of glass to the above range, it is appropriate to decrease the viscosity, for example, by including components such as B.sub.2O.sub.3 and RO in the ranges described above within the ranges of the composition of the glass substrate of the present embodiments.

(71) The molten glass during production of the glass substrate of the present embodiments has a specific resistance (at 1550 C.) in the range of preferably 30 to 700 .Math.cm, more preferably 30 to 400 .Math.cm, still more preferably 30 to 300 .Math.cm and even more preferably 50 to 300 .Math.cm. If the specific resistance is too low, excessive electric current is required for melting, which may give rise to constraints in terms of facility. In addition, electrodes may be increasingly worn. If the molten glass has a specific resistance which is too high, electric current flows through heat resistant bricks forming a melting tank rather than through glass, thus the melting tank may get erosion/wear. The specific resistance of the molten glass may be controlled to be within the above range by controlling the contents of mainly RO, R.sub.2O and Fe.sub.2O.sub.3.

(72) The glass that forms the glass substrate of the present embodiments preferably has an etching rate of 50 m/h or more. If the etching rate is high, productivity is improved. Particularly, when etching of the glass substrate is performed after bonding the glass substrates of TFT and of color filter in order to reduce the weight, the etching rate influences the productivity. However, if the etching rate is too high, devitrification resistance of glass may decrease despite an increase in the productivity during production of displays. The heat shrinkage rate is also prone to increase. The etching rate is preferably 60 to 140 m/h, more preferably 65 to 120 m/h and still more preferably 70 to 120 m/h. In order to increase the etching rate of glass, the value of SiO.sub.2(Al.sub.2O.sub.3) or SiO.sub.2/Al.sub.2O.sub.3 may be decreased. In the present embodiments, the etching rate is defined to be one determined under the following conditions. The etching rate (m/h) as used herein is an amount (m) of thickness reduction in one surface of a glass substrate per unit time (1 hour) when the glass substrate is immersed in an etching solution adjusted to have an HF concentration of 1 mol/kg and an HCl concentration of 5 mol/kg at 40 C. for 1 hour.

(73) The glass substrate of the present embodiments may have a thickness in the range of, for example, 0.1 to 1.1 mm or 0.3 to 1.1 mm. However, the thickness is not limited to this range. The thickness may be, for example, in the range of 0.3 to 0.7 mm or 0.3 to 0.5 mm. If the glass plate is too thin, the strength of the glass substrate per se decreases. For example, breakage is prone to occur during production of flat panel displays. If the thickness is too high, it is not preferable for displays that call for thickness reduction. In addition, the weight of the glass substrate is increased, making it difficult to reduce the weight of flat panel displays. Further, when etching of the glass substrate is performed after formation of TFTs, an increased amount of etching is required, consuming costs and time.

(74) The glass substrate of the present embodiments is used for production of a flat panel display in which, for example, etching of the surface of the glass substrate is performed after array/color filter bonding. The glass substrate of the present embodiments is suitable as a glass substrate for a display (excluding CRT (Braun tube) displays). Particularly, the glass substrate of the present embodiments is suitable as a glass substrate for a flat panel display containing a LTPS-TFT or OS-TFT formed thereon. Specifically, the glass substrate is suitable as a glass substrate for a liquid crystal display and a glass substrate for an organic EL display. Particularly, the glass substrate is suitable as a glass substrate for a LTPS-TFT liquid crystal display and a glass substrate for a LTPS-TFT organic EL display. Among others, the glass substrate is suitable as a glass substrate for a display for, for instance, mobile devices for which a higher degree of definition is demanded.

(75) <Flat Panel Display>

(76) The present embodiments encompass a flat panel display including a glass substrate and a LTPS-TFT or OS-TFT formed on the substrate, wherein the flat panel display includes the glass substrate of the present embodiments. The flat panel display of the present embodiments may be, for example, a liquid crystal display or an organic EL display.

(77) <Method for Producing a Glass Substrate>

(78) The method for producing the glass substrate for a display of the present embodiments includes a melting step of melting glass raw materials blended according to a predetermined composition by employing, for example, at least direct electrical heating, a forming step of forming a molten glass melted in the melting step into a flat-plate glass, and an annealing step of annealing the flat-plate glass.

(79) Particularly, the annealing step is preferably a step in which cooling conditions of the flat-plate glass is controlled so as to reduce a heat shrinkage rate of the flat-plate glass.

(80) [Melting Step]

(81) In the melting step, glass raw materials blended according to a predetermined composition are melted by, for example, direct electrical heating and/or combustion heating. The glass raw materials may be appropriately selected among well-known materials. From the standpoint of energy efficiency, it is preferable that in the melting step, the glass raw materials are melted by at least direct electrical heating. A melting tank in which the melting step is performed is preferably formed with a high-zirconia-content refractory material. The predetermined composition described above may be appropriately adjusted, for example, within the ranges of the contents described above for the components of glass.

(82) [Forming Step]

(83) In the forming step, the molten glass melted in the melting step is formed into a flat-plate glass. The method for forming into a flat-plate glass is suitably, for example, down-draw processing, particularly overflow down-draw processing, and the flat-plate glass formed is a glass ribbon. Other methods, such as float processing, re-draw processing, and roll-out processing, can also be employed. By employing down-draw processing, the principal surfaces of the obtained glass substrate are free surfaces which have contacted to only the atmosphere and thus become extremely smooth compared to the substrates obtained by other forming methods such as float processing, and thus the step of polishing the glass substrate surfaces after forming becomes unnecessary. As a result, production costs may be reduced, and productivity may be improved. Furthermore, the principal surfaces of the glass substrate formed by employing down-draw processing have a uniform composition, and thus, etching may be performed uniformly during etching regardless of the top side or reverse side formed during forming.

(84) [Annealing Step]

(85) By appropriately adjusting the conditions employed at the time of annealing, the heat shrinkage rate of the glass substrate may be controlled. Particularly, it is preferable to control cooling conditions of the flat-plate glass so as to reduce the heat shrinkage rate of the flat-plate glass. The heat shrinkage rate of the glass substrate is, as described above, 15 ppm or less, preferably 13 ppm or less and more preferably 1 to 13 ppm. In order to produce the glass substrate having such heat shrinkage rate by, for example, down-draw processing, it is preferable to perform annealing so that the cooling rate of the glass ribbon as the flat-plate glass is 30 C./minute to 300 C./minute in the temperature range from Tg to (Tg100 C.). If the cooling rate is too high, the heat shrinkage rate may not be sufficiently reduced. On the other hand, if the cooling rate is too low, productivity is decreased and the device for production of glass (annealing furnace) may become huge. The cooling rate is preferably in the range of 30 C./minute to 300 C./minute, more preferably 50 C./minute to 200 C./minute and still more preferably 60 C./minute to 120 C./minute. If the cooling rate is 30 C./minute to 300 C./minute, production of the glass substrate of the present embodiments may be ensured. The heat shrinkage rate may be decreased by performing offline annealing after cutting the flat-plate glass in the downstream of the annealing step. However, in this case, facilities for performing offline annealing is required in addition to the facilities for performing the annealing step. Therefore, as described above, it is preferable to control the heat shrinkage rate so as to reduce the same in the annealing step and omit the offline annealing from the standpoints of productivity and costs. In the present specification, the cooling rate of a glass ribbon represents the cooling rate of the central part of a glass ribbon in the width direction.

EXAMPLES

(86) Hereinbelow, the present embodiments are more specifically described based on Examples which do not limit the present embodiments. The following physical properties were measured in the following Examples and Comparative Examples.

(87) (Strain Point)

(88) Measurement was carried out on a beam bending measurement device (produced by Tokyo Kogyo Co., Ltd.) and the strain point was determined by calculation according to the beam bending method (ASTM C-598).

(89) (Devitrification Temperature)

(90) Glass was pulverized, passed through a 2380-m sieve and glass particles retained on a 1000-m sieve were placed in a platinum boat. The platinum boat was kept in an electric furnace having a temperature gradient of 1050 C. to 1380 C. for 5 hours before removing from the furnace, and devitrification that occurred inside of the glass was observed on an optical microscope at 50-fold magnification. The devitrification temperature was obtained as a maximum temperature at which devitrification was observed.

(91) (Method for Measuring Average Coefficient of Thermal Expansion and Tg in the Range of 100 C. to 300 C.)

(92) The measurement was performed on a differential dilatometer (Thermo Plus2 TMA8310). The heating rate during the measurement was 5 C./minute. Based on the measurement results, the average coefficient of thermal expansion and Tg in the temperature range of 100 C. to 300 C. was determined.

(93) (Heat Shrinkage Rate)

(94) The heat shrinkage rate was determined according to the scribe line method on glass with the size of 90 to 200 mm15 to 30 mm0.3 to 1 mm. In the heat treatment for measurement of the heat shrinkage, glass was kept at a temperature of 500 C. for 30 minutes in a furnace with air circulation (produced by Nabertherm GmbH, N120/85HA) and cooled to room temperature.
Heat shrinkage rate (ppm)={Amount of shrinkage of glass due to heat treatment/Distance between scribe lines on glass before heat treatment}10.sup.6

(95) When the heat shrinkage of glass was measured which was obtained by melting glass raw materials in a platinum crucible and draining the same onto an iron plate for cooling and solidification, the glass was cut, ground and polished so as to have a thickness of 0.5 mm, kept at a temperature of Tg+15 C. for 30 minutes in an electric furnace and removed from the furnace at a cooling rate of 150 C./minute to 250 C./minute and then used for measurement.

(96) (Density)

(97) The density of the glass was measured according to the Archimedean method.

(98) (Etching Rate)

(99) The etching rate (m/h) was calculated by measuring the amount (m) of thickness reduction after immersing glass (12.5 mm20 mm0.7 mm) in an etching solution (200 mL) adjusted to have an HF concentration of 1 mol/kg and an HCl concentration of 5 mol/kg at 40 C. for 1 hour and calculating the amount (m) of thickness reduction in one surface of the glass substrate per unit time (1 hour).

(100) The compositions and evaluations in Examples are hereinafter described.

(101) Glasses of Examples 1 to 51 were prepared according to the following procedures so as to have the glass compositions indicated in Table 1. The obtained glasses were analyzed for the strain point, the devitrification temperature, Tg, the average coefficient () of thermal expansion in the range from 100 C. to 300 C., the heat shrinkage rate, the density and the etching rate.

(102) TABLE-US-00001 TABLE 1 Example Example Example Example Example Example wt % 1 2 3 4 5 6 SiO.sub.2 59.5 61.1 60.9 60.0 63.3 61.0 Al.sub.2O.sub.3 19.9 18.5 20.0 18.5 17.9 19.3 B.sub.2O.sub.3 3.0 1.5 1.5 1.5 1.5 1.5 Na.sub.2O 0.0 1.0 0.0 0.0 0.0 0.0 K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 MgO 2.6 2.7 2.6 2.9 2.5 2.7 CaO 4.8 5.1 4.8 5.4 4.7 4.9 SrO 0.0 0.0 0.0 0.0 0.0 0.0 BaO 9.8 10.5 9.8 11.1 9.6 10.1 ZnO 0.0 0.0 0.0 0.0 0.0 0.0 ZrO.sub.2 0.0 0.0 0.0 0.0 0.0 0.0 Sb.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 SnO.sub.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe.sub.2O.sub.3 0.046 0.046 0.046 0.046 0.047 0.046 Total 100.0 100.0 100.0 100.0 100.0 100.0 LiO.sub.2 + Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 SiO.sub.2 Al.sub.2O.sub.3 49.5 51.8 50.9 50.8 54.3 51.4 SiO.sub.2 Al.sub.2O.sub.3 54.5 56.5 55.9 55.4 58.8 56.2 RO(MgO + OaO + SrO + BaO) 17.1 18.3 17.1 19.5 16.7 17.7 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO)) 3.9 4.0 4.3 3.7 4.5 4.2 B.sub.2O.sub.3 + RO + ZnO 20.9 19.9 18.6 21.0 18.3 19.3 SiO.sub.2 + Al.sub.2O.sub.3 79.4 79.6 80.9 78.5 81.2 80.2 B.sub.2O.sub.3/(SiO.sub.2 + Al.sub.2O.sub.3) 0.04 0.02 0.02 0.02 0.02 0.02 SiO.sub.2/Al.sub.2O.sub.3 2.98 3.29 3.05 3.25 3.53 3.17 B.sub.2O.sub.3/RO 0.18 0.08 0.09 0.08 0.09 0.09 BaO/RO 0.57 0.57 0.57 0.57 0.57 0.57 3 BaO/(MgO + CaO + SrO) 4.0 4.0 4.0 4.0 4.0 4.0 MgO/BaO 0.26 0.26 0.26 0.26 0.26 0.26 MgO/(CaO + SrO) 0.54 0.54 0.54 0.54 0.54 0.54 CaO/RO 0.28 0.28 0.28 0.28 0.28 0.28 MgO/(RO + ZnO) 0.15 0.15 0.15 0.15 0.15 0.15 SrO/CaO 0 0 0 0 0 0 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO + 10 R.sub.2O) 3.54 3.59 3.86 3.37 3.95 3.72 Devitrification Temperature 1186 1218 1229 1205 1195 1226 Tg( C.) 774 772 789 770 785 785 Strain Point ( C.) 723 722 739 720 733 735 ( 10.sup.7) (100-300 C.) 40 42 40 43 40 40 Density (g/cm.sup.3) 2.60 2.63 2.61 2.64 2.59 2.61 Heat Shrinkage Rate 8 8 6 9 7 7 Etching Rate (m/h) 80 75 77 78 69 76 Example Exampe Example Example Example Example wt % 7 8 8 10 11 12 SiO.sub.2 59.9 62.1 62.3 63.2 64.5 59.6 Al.sub.2O.sub.3 19.2 18.6 17.2 18.7 17.3 20.0 B.sub.2O.sub.3 1.5 1.5 1.5 1.5 1.5 2.8 Na.sub.2O 0.0 0.0 0.0 0.0 0.0 0.0 K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 MgO 2.8 2.6 2.8 2.4 2.4 3.0 CaO 5.3 4.8 5.1 4.5 4.5 3.9 SrO 0.0 0.0 0.0 0.0 0.0 1.7 BaO 10.8 9.8 10.6 9.2 9.3 8.6 ZnO 0.0 0.0 0.0 0.0 0.0 0.0 ZrO.sub.2 0.0 0.0 0.0 0.0 0.0 0.0 Sb.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 SnO.sub.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe.sub.2O.sub.3 0.046 0.047 0.047 0.047 0.047 0.000 Total 100.0 100.0 100.0 100.0 100.0 100.0 LiO.sub.2 + Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 SiO.sub.2 Al.sub.2O.sub.3 50.3 52.8 53.8 53.9 55.8 49.6 SiO.sub.2 Al.sub.2O.sub.3 55.1 57.5 58.1 58.5 60.2 54.6 RO(MgO + OaO + SrO + BaO) 18.8 17.2 18.5 16.1 16.2 17.1 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO)) 3.9 4.3 4.0 4.6 4.6 4.0 B.sub.2O.sub.3 + RO + ZnO 20.4 18.8 20.0 17.6 17.7 20.0 SiO.sub.2 + Al.sub.2O.sub.3 79.1 80.7 79.5 81.9 81.8 79.6 B.sub.2O.sub.3/(SiO.sub.2 + Al.sub.2O.sub.3) 0.02 0.02 0.02 0.02 0.02 0.04 SiO.sub.2/Al.sub.2O.sub.3 3.12 3.34 3.63 3.39 3.73 2.98 B.sub.2O.sub.3/RO 0.08 0.009 0.08 0.09 0.09 0.16 BaO/RO 0.57 0.57 0.57 0.57 0.57 0.50 3 BaO/(MgO + CaO + SrO) 4.0 4.0 4.0 4.0 4.0 3.0 MgO/BaO 0.26 0.26 0.26 0.26 0.26 0.35 MgO/(CaO + SrO) 0.54 0.54 0.54 0.54 0.54 0.53 CaO/RO 0.28 0.28 0.28 0.28 0.28 0.23 MgO/(RO + ZnO) 0.15 0.15 0.15 0.15 0.15 0.17 SrO/CaO 0 0 0 0 0 0.42 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO + 10 R.sub.2O) 3.49 3.83 3.56 4.10 4.07 3.57 Devitrification Temperature 1223 1222 1185 1193 1218 1188 Tg( C.) 777 783 773 788 782 773 Strain Point ( C.) 728 731 721 739 732 723 ( 10.sup.7) (100-300 C.) 43 41 42 38 39 40 Density (g/cm.sup.3) 2.64 2.60 2.61 2.58 2.57 2.61 Heat Shrinkage Rate 8 7 8 6 7 8 Etching Rate (m/h) 79 71 69 68 62 79 Example Example Example Example Example Example wt % 13 14 15 16 17 18 SiO.sub.2 63.2 62.3 63.1 64.4 60.1 60.1 Al.sub.2O.sub.3 17.9 17.1 18.6 17.2 20.1 20.1 B.sub.2O.sub.3 0.5 0.5 0.5 0.5 2.0 2.0 Na.sub.2O 0.0 0.0 0.0 0.0 0.0 0.0 K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 MgO 2.7 2.9 2.6 2.6 3.0 3.0 CaO 5.0 5.5 4.8 4.9 4.0 4.0 SrO 0.0 0.0 0.0 0.0 1.7 1.6 BaO 10.2 11.2 9.9 9.9 8.7 8.7 ZnO 0.0 0.0 0.0 0.0 0.0 0.0 ZrO.sub.2 0.0 0.0 0.0 0.0 0.0 0.0 Sb.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 SnO.sub.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe.sub.2O.sub.3 0.000 0.000 0.000 0.000 0.000 0.000 Total 100.0 100.0 100.0 100.0 100.0 100.0 LiO.sub.2 + Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 SiO.sub.2 Al.sub.2O.sub.3 54.3 53.7 53.8 55.8 50.0 50.0 SiO.sub.2 Al.sub.2O.sub.3 58.7 58.0 58.5 60.1 55.1 55.1 RO(MgO + OaO + SrO + BaO) 17.9 19.6 17.3 17.4 17.3 17.3 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO)) 4.4 3.9 4.6 4.6 4.2 4.2 B.sub.2O.sub.3 + RO + ZnO 18.4 20.1 17.8 17.9 19.3 19.3 SiO.sub.2 + Al.sub.2O.sub.3 81.1 79.4 81.8 81.6 80.2 80.3 B.sub.2O.sub.3/(SiO.sub.2 + Al.sub.2O.sub.3) 0.01 0.01 0.01 0.01 0.03 0.03 SiO.sub.2/Al.sub.2O.sub.3 3.53 3.63 3.39 3.73 2.98 2.98 B.sub.2O.sub.3/RO 0.03 0.03 0.03 0.03 0.12 0.12 BaO/RO 0.57 0.57 0.57 0.57 0.50 0.50 3 BaO/(MgO + CaO + SrO) 4.0 4.0 4.0 4.0 3.01 3.04 MgO/BaO 0.26 0.26 0.26 0.26 0.35 0.35 MgO/(CaO + SrO) 0.54 0.54 0.54 0.54 0.53 0.54 CaO/RO 0.28 0.28 0.28 0.28 0.23 0.23 MgO/(RO + ZnO) 0.15 0.15 0.15 0.15 0.17 0.17 SrO/CaO 0.00 0.00 0.00 0.00 0.42 0.40 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO + 10 R.sub.2O) 3.91 3.53 4.06 4.03 3.71 3.72 Devitrification Temperature 1195 1200 1217 1207 1180 1180 Tg( C.) 795 776 798 195 781 785 Strain Point ( C.) 745 726 747 746 730 734 ( 10.sup.7) (100-300 C.) 39 43 40 40 40 40 Density (g/cm.sup.3) 2.60 2.63 2.60 2.60 2.62 2.61 Heat Shrinkage Rate 6 8 5 6 7 7 Etching Rate (m/h) 67 70 68 63 80 79 Example Example Example Example Example Example wt % 19 20 21 22 23 24 SiO.sub.2 60.0 59.4 59.7 60.0 58.5 58.6 Al.sub.2O.sub.3 19.8 20.0 19.7 19.4 19.8 19.7 B.sub.2O.sub.3 2.0 2.4 2.4 2.4 2.8 2.8 Na.sub.2O 0.0 0.0 0.0 0.0 0.0 0.0 K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 MgO 3.1 3.1 3.1 3.1 3.0 3.0 CaO 3.8 4.1 4.1 3.8 4.0 4.0 SrO 2.6 1.6 1.6 2.6 1.8 1.8 BaO 8.2 8.9 8.9 8.3 8.7 8.7 ZnO 0.0 0.0 0.0 0.0 0.0 0.0 ZrO.sub.2 0.0 0.0 0.0 0.0 0.0 0.0 Sb.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 SnO.sub.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe.sub.2O.sub.3 0.000 0.000 0.000 0.000 0.000 0.000 Total 100.0 100.0 100.0 100.0 100.0 100.0 LiO.sub.2 + Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 SiO.sub.2 Al.sub.2O.sub.3 50.0 49.3 49.8 50.3 49.6 49.8 SiO.sub.2 Al.sub.2O.sub.3 55.0 54.3 54.7 55.1 54.5 54.7 RO(MgO + OaO + SrO + BaO) 17.7 17.7 17.8 17.8 17.4 17.4 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO)) 4.0 3.9 3.9 3.9 3.9 3.9 B.sub.2O.sub.3 + RO + ZnO 19.7 20.2 20.2 20.2 20.2 20.2 SiO.sub.2 + Al.sub.2O.sub.3 79.8 79.4 79.4 79.3 79.3 79.3 B.sub.2O.sub.3/(SiO.sub.2 + Al.sub.2O.sub.3) 0.03 0.03 0.03 0.03 0.04 0.04 SiO.sub.2/Al.sub.2O.sub.3 3.02 2.96 3.03 3.10 3.00 3.03 B.sub.2O.sub.3/RO 0.11 0.14 0.14 0.14 0.16 0.16 BaO/RO 0.46 0.50 0.50 0.46 0.50 0.50 3 BaO/(MgO + CaO + SrO) 2.61 3.04 3.04 2.61 2.97 2.97 MgO/BaO 0.38 0.35 0.35 0.38 0.35 0.35 MgO/(CaO + SrO) 0.48 0.54 0.54 0.48 0.52 0.52 CaO/RO 0.22 0.23 0.23 0.22 0.23 0.23 MgO/(RO + ZnO) 0.17 0.17 0.17 0.17 0.17 0.17 SrO/CaO 0.67 0.40 0.40 0.67 0.46 0.46 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO + 10 R.sub.2O) 3.62 3.54 3.53 3.52 3.52 3.52 Devitrification Temperature 1187 1207 1182 1209 1194 1187 Tg( C.) 780 773 774 777 778 776 Strain Point ( C.) 728 722 724 727 726 725 ( 10.sup.7) (100-300 C.) 41 41 42 41 40 40 Density (g/cm.sup.3) 2.62 2.62 2.61 2.61 2.62 2.61 Heat Shrinkage Rate 7 8 8 8 8 8 Etching Rate (m/h) 80 83 81 80 82 81 Example Example Example Example Example Example wt % 25 26 27 28 29 30 SiO.sub.2 60.8 59.9 60.5 60.3 62.3 59.9 Al.sub.2O.sub.3 19.7 20.2 18.8 19.7 17.5 20.5 B.sub.2O.sub.3 1.6 1.6 1.6 1.6 1.6 1.6 Na.sub.2O 0.0 0.0 0.0 0.0 0.0 0.0 K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 MgO 3.0 3.1 3.2 3.1 3.1 3.0 CaO 4.0 4.1 4.2 4.1 4.1 4.0 SrO 1.8 1.8 1.9 1.9 1.9 1.8 BaO 8.7 8.9 9.3 9.0 9.0 8.8 ZnO 0.0 0.0 0.0 0.0 0.0 0.0 ZrO.sub.2 0.0 0.0 0.0 0.0 0.0 0.0 Sb.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 SnO.sub.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe.sub.2O.sub.3 0.000 0.000 0.000 0.000 0.000 0.000 Total 100.0 100.0 100.0 100.0 100.0 100.0 LiO.sub.2 + Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 SiO.sub.2 Al.sub.2O.sub.3 51.0 49.8 51.1 50.4 53.5 49.6 SiO.sub.2 Al.sub.2O.sub.3 55.9 54.9 55.8 55.3 57.9 54.7 RO(MgO + OaO + SrO + BaO) 17.5 17.9 18.6 18.0 18.2 17.6 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO)) 4.2 4.1 3.9 4.1 4.0 4.2 B.sub.2O.sub.3 + RO + ZnO 19.1 19.5 20.3 19.6 19.8 19.2 SiO.sub.2 + Al.sub.2O.sub.3 80.5 80.1 79.3 79.9 79.7 80.3 B.sub.2O.sub.3/(SiO.sub.2 + Al.sub.2O.sub.3) 0.02 0.02 0.02 0.02 0.02 0.02 SiO.sub.2/Al.sub.2O.sub.3 3.09 2.97 3.22 3.06 3.56 2.93 B.sub.2O.sub.3/RO 0.09 0.09 0.09 0.09 0.09 0.09 BaO/RO 0.50 0.50 0.50 0.50 0.50 0.50 3 BaO/(MgO + CaO + SrO) 2.97 2.97 2.97 2.97 2.97 2.97 MgO/BaO 0.35 0.35 0.35 0.35 0.35 0.35 MgO/(CaO + SrO) 0.52 0.52 0.52 0.52 0.52 0.52 CaO/RO 0.23 0.23 0.23 0.23 0.23 0.23 MgO/(RO + ZnO) 0.17 0.17 0.17 0.17 0.17 0.17 SrO/CaO 0.45 0.45 0.45 0.45 0.45 0.45 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO + 10 R.sub.2O) 3.76 3.68 3.51 3.65 3.60 3.73 Devitrification Temperature 1205 1223 1199 1206 1192 1246 Tg( C.) 792 793 787 793 783 793 Strain Point ( C.) 740 742 736 741 732 741 ( 10.sup.7) (100-300 C.) 40 41 42 41 38 41 Density (g/cm.sup.3) 2.61 2.63 2.63 2.62 2.62 2.63 Heat Shrinkage Rate 6 6 7 6 7 6 Etching Rate (m/h) 77 81 77 79 71 81 Example Example Example Example Example Example 31 32 33 34 35 36 SiO.sub.2 60.4 60.8 60.4 60.3 60.6 60.8 Al.sub.2O.sub.3 19.9 20.1 19.9 19.9 20.0 18.5 B.sub.2O.sub.3 1.6 1.6 1.6 1.6 1.6 1.5 Na.sub.2O 0.0 0.0 0.0 0.0 0.0 0.0 K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 MgO 3.1 3.3 3.3 3.2 3.1 3.1 CaO 3.9 4.0 3.6 3.6 3.9 3.3 SrO 2.1 1.8 1.8 2.1 2.5 3.3 BaO 8.5 7.8 8.8 8.8 7.8 9.1 ZnO 0.0 0.0 0.0 0.0 0.0 0.0 ZrO.sub.2 0.0 0.0 0.0 0.0 0.0 0.0 Sb.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 SnO.sub.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe.sub.2O.sub.3 0.000 0.000 0.000 0.000 0.000 0.000 Total 100.0 100.0 100.0 100.0 100.0 100.0 LiO.sub.2 + Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 SiO.sub.2 Al.sub.2O.sub.3 50.5 50.8 50.4 50.4 50.6 51.6 SiO.sub.2 Al.sub.2O.sub.3 55.5 55.8 55.4 55.4 55.6 56.2 RO(MgO + OaO + SrO + BaO) 17.5 17.0 17.6 17.7 17.4 18.8 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO)) 4.2 4.3 4.2 4.2 4.2 3.9 B.sub.2O.sub.3 + RO + ZnO 19.2 18.7 19.2 19.3 19.0 20.3 SiO.sub.2 + Al.sub.2O.sub.3 80.4 80.9 80.4 80.2 80.6 79.3 B.sub.2O.sub.3/(SiO.sub.2 + Al.sub.2O.sub.3) 0.02 0.02 0.02 0.02 0.02 0.02 SiO.sub.2/Al.sub.2O.sub.3 3.03 3.03 3.03 3.03 3.03 3.29 B.sub.2O.sub.3/RO 0.09 0.10 0.09 0.09 0.09 0.08 BaO/RO 0.48 0.46 0.50 0.50 0.45 0.48 3 BaO/(MgO + CaO + SrO) 2.79 2.55 3.01 2.96 2.44 2.80 MgO/BaO 0.37 0.43 0.38 0.37 0.40 0.34 MgO/(CaO + SrO) 0.53 0.57 0.60 0.56 0.49 0.47 CaO/RO 0.22 0.24 0.21 0.21 0.22 0.18 MgO/(RO + ZnO) 0.18 0.20 0.19 0.18 0.18 0.16 SrO/CaO 0.53 0.45 0.50 0.56 0.65 1.00 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO + 10 R.sub.2O) 3.74 3.85 3.74 3.71 3.78 3.51 Devitrification Temperature 1204 1195 1184 1202 1234 1234 Tg( C.) 792 793 794 790 791 791 Strain Point ( C.) 742 742 742 740 739 740 ( 10.sup.7) (100-300 C.) 40 38 40 40 41 42 Density (g/cm.sup.3) 2.62 2.60 2.63 2.63 2.62 2.64 Heat Shrinkage Rate 6 5 6 6 6 6 Etching Rate (m/h) 80 79 79 80 70 76 Example 1 Example Exarimle Example Example Example wt % 37 38 39 40 41 42 SiO.sub.2 61.0 61.1 60.0 60.6 61.6 62.5 Al.sub.2O.sub.3 20.0 19.3 19.9 19.3 19.3 20.1 B.sub.2O.sub.3 1.5 1.5 1.5 1.5 1.5 1.2 Na.sub.2O 0.0 0.0 0.0 0.0 0.0 0.0 K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 MgO 2.9 3.0 3.1 3.1 2.9 3.6 CaO 3.8 4.0 4.1 4.1 3.8 5.1 SrO 1.8 1.8 1.9 1.9 1.8 0.0 BaO 8.5 8.8 9.1 9.1 8.5 7.0 ZnO 0.0 0.0 0.0 0.0 0.0 0.0 ZrO.sub.2 0.0 0.0 0.0 0.0 0.0 0.0 Sb.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 SnO.sub.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe.sub.2O.sub.3 0.000 0.000 0.000 0.000 0.000 0.000 Total 100.0 100.0 100.0 100.0 100.0 100.0 LiO.sub.2 + Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 SiO.sub.2 Al.sub.2O.sub.3 51.0 51.5 50.0 50.9 52.0 52.4 SiO.sub.2 Al.sub.2O.sub.3 56.0 56.3 55.0 55.8 56.8 57.5 RO(MgO + OaO + SrO + BaO) 17.0 17.6 18.1 18.2 17.1 15.7 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO)) 4.4 4.2 4.1 4.1 4.4 4.9 B.sub.2O.sub.3 + RO + ZnO 18.5 19.2 19.6 19.7 18.6 16.9 SiO.sub.2 + Al.sub.2O.sub.3 81.0 80.4 79.9 79.8 80.9 82.6 B.sub.2O.sub.3/(SiO.sub.2 + Al.sub.2O.sub.3) 0.02 0.02 0.02 0.02 0.02 0.02 SiO.sub.2/Al.sub.2O.sub.3 3.05 3.17 3.01 3.14 3.19 3.11 B.sub.2O.sub.3/RO 0.09 0.09 0.08 0.08 0.09 0.08 BaO/RO 0.50 0.50 0.50 0.50 0.50 0.44 3 BaO/(MgO + CaO + SrO) 3.00 3.00 3.00 3.00 3.00 2.39 MgO/BaO 0.34 0.34 0.34 0.34 0.34 0.52 MgO/(CaO + SrO) 0.52 0.52 0.52 0.521 0.52 0.71 CaO/RO 0.23 0.23 0.23 0.23 0.23 0.32 MgO/(RO + ZnO) 0.17 0.17 0.17 0.17 0.17 0.23 SrO/CaO 0.46 0.46 0.46 0.46 0.46 0.00 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO + 10 R.sub.2O) 3.88 3.74 3.64 3.63 3.87 4.28 Devitrification Temperature 1222 1180 1200 1200 1202 1223 Tg( C.) 792 792 786 787 790 792 Strain Point ( C.) 742 737 735 736 740 740 ( 10.sup.7) (100-300 C.) 39 42 41 41 39 40 Density (g/cm.sup.3) 2.62 2.62 2.64 2.63 2.61 2.57 Heat Shrinkage Rate 6 6 6 6 6 6 Etching Rate (m/h) 77 76 80 77 74 76 Example Example Example Example Example Example wt % 43 44 45 46 47 48 SiO.sub.2 62.2 60.9 61.1 61.6 61.7 60.6 Al.sub.2O.sub.3 20.0 19.6 20.0 20.2 20.2 19.9 B.sub.2O.sub.3 1.2 1.2 1.5 1.5 1.5 1.5 Na.sub.2O 0.0 0.0 0.0 0.0 0.0 0.0 K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 MgO 3.4 3.3 2.9 2.9 2.9 2.2 CaO 4.7 3.5 4.1 4.1 4.1 4.1 SrO 1.5 1.5 1.4 2.9 1.4 1.5 BaO 6.5 9.5 8.4 6.4 6.4 8.4 ZnO 0.0 0.0 0.0 0.0 1.3 1.3 ZrO.sub.2 0.0 0.0 0.0 0.0 0.0 0.0 Sb.sub.2O.sub.3 0.0 0.0 0.0 0.0 0.0 0.0 SnO.sub.2 0.2 0.2 0.2 0.2 0.2 0.2 Fe.sub.2O.sub.3 0.000 0.000 0.000 0.000 0.000 0.000 Total 100.0 100.0 100.0 100.0 100.0 100.0 LiO.sub.2 + Na.sub.2O + K.sub.2O 0.2 0.2 0.2 0.2 0.2 0.2 SiO.sub.2 Al.sub.2O.sub.3 52.1 51.1 51.1 51.5 51.6 50.7 SiO.sub.2 Al.sub.2O.sub.3 57.2 56.0 56.1 56.5 56.6 55.6 RO(MgO + OaO + SrO + BaO) 16.1 17.8 16.8 16.3 14.9 16.2 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO)) 4.7 4.2 4.4 4.6 5.0 4.5 B.sub.2O.sub.3 + RO + ZnO 17.4 19.0 18.4 17.8 17.7 19.1 SiO.sub.2 + Al.sub.2O.sub.3 82.2 80.5 81.2 81.7 81.9 80.5 B.sub.2O.sub.3/(SiO.sub.2 + Al.sub.2O.sub.3) 0.02 0.02 0.02 0.02 0.02 0.02 SiO.sub.2/Al.sub.2O.sub.3 3.11 3.11 3.05 3.05 3.05 3.05 B.sub.2O.sub.3/RO 0.08 0.07 0.09 0.09 0.10 0.09 BaO/RO 0.40 0.53 0.50 0.39 0.43 0.52 3 BaO/(MgO + CaO + SrO) 2.01 3.44 3.00 1.93 2.25 3.24 MgO/BaO 0.52 0.35 0.35 0.46 0.46 0.26 MgO/(CaO + SrO) 0.54 0.67 0.53 0.42 0.53 0.39 CaO/RO 0.29 0.20 0.24 0.25 0.28 0.25 MgO/(RO + ZnO) 0.21 0.19 0.17 0.18 0.18 0.13 SrO/CaO 0.32 0.43 0.35 0.70 0.35 0.36 (SiO.sub.2 + Al.sub.2O.sub.3)/(B.sub.2O.sub.3 + RO + 10 R.sub.2O) 4.17 3.77 3.92 4.06 4.37 4.01 Devitrification Temperature 1221 1214 1206 1231 1253 1237 Tg( C.) 793 793 791 789 788 787 Strain Point ( C.) 742 741 739 738 737 735 ( 10.sup.7) (100-300 C.) 40 39 41 41 42 41 Density (g/cm.sup.3) 2.58 2.62 2.60 2.59 2.59 2.63 Heat Shrinkage Rate 6 6 6 6 6 6 Etching Rate (m/h) 76 77 77 77 78 76

(103) Raw materials for respective components were blended so as to have the glass compositions indicated in Table 1 and melting, refining and forming were performed.

(104) Examples 1 to 48 among glasses obtained as above had a heat shrinkage rate of 15 ppm or less. The devitrification temperature was also 1235 C. or lower. Similar results were obtained when glass raw materials were melted by employing direct electrical heating and glass substrates were produced by overflow down-draw processing. Therefore, with the glasses, glass substrates may be produced by overflow down-draw processing, which may be used for displays to which LTPS-TFTs are applied. The glass substrates are also suitable as glass substrates for OS-TFTs.