GLASS PLATE AND PROCESS FOR PRODUCING THE SAME

Abstract

A glass plate has a dielectric dissipation factor at 10 GHz of tan δA and a glass transition temperature of Tg° C. The glass plate satisfies (tan δ100−tan δA)≥0.0004, where tan δ100 is a dielectric dissipation factor of the glass plate at 10 GHz after having been heated to (Tg+50)° C. and then cooled to (Tg−150)° C. at 100° C./min.

Description

TECHNICAL FIELD

The present invention relates to a glass plate and a process for producing the glass plate.

BACKGROUND ART

It has become usual to use appliances utilizing radio waves (hereinafter referred to as “radio appliances”), such as radars and portable telephones, in vehicles including motor vehicles and in buildings. Especially in recent years, radio appliances utilizing radio waves having frequencies in a high-frequency band (microwaves to millimeter waves), more specifically a gigahertz frequency band, e.g., 3-300 GHz range, have come to be intensively developed.

Circuit boards used in such radio appliances for high-frequency applications (hereinafter referred to as “high-frequency devices) generally employ insulating substrates such as resin substrates, ceramic substrates, and glass substrates. Such insulating substrates for use in high-frequency devices are required to attain reductions in transmission loss due to dielectric loss, conductor loss, etc., in order to ensure the properties of high-frequency signals, such as the quality and intensity thereof.

Meanwhile, glass plates for use as window materials for vehicles, e.g., motor vehicles, and buildings have been required to have a high visible-light transmittance and the ability to highly shield ultraviolet light and solar radiation and be visually satisfactory. Patent Literature 1 discloses an ultraviolet- and infrared-absorbing glass constituted from a soda-lime silica glass having a specific composition.

CITATION LIST

Patent Literature

Patent Literature 1: JP-A-2002-348143

SUMMARY OF INVENTION

Technical Problem

However, in view of the facts that millimeter-wave radars are coming to be mounted in vehicles, e.g., motor vehicles, and that electronic appliances are used in buildings, glass plates for use as window materials for the vehicles and buildings are required to attain reductions in propagation loss and transmission loss like the insulating substrates of high-frequency devices.

Accordingly, an object of the present invention is to provide a novel glass plate which is low in propagation loss and transmission loss in a high-frequency band and is usable as the substrates of high-frequency devices or as window materials, and to provide a process for producing the glass plate.

Solution to Problem

The present invention relates to the following.

1. A glass plate having a dielectric dissipation factor at 10 GHz of tan δA and a glass transition temperature of Tg° C.,

wherein the glass plate satisfies (tan δ100−tan δA)≥0.0004, where tan δ100 is a dielectric dissipation factor of the glass plate at 10 GHz after having been heated to (Tg+50)° C. and then cooled to (Tg−150)° C. at 100° C./min.

2. The glass plate according to 1 above having a relative permittivity at 10 GHz of εrA, wherein the glass plate satisfies 0.95≤(εr100/εrA)≤1.05, where εr100 is a relative permittivity of the glass plate at 10 GHz after having been heated to (Tg+50)° C. and then cooled to (Tg−150)° C. at 100° C./min.

3. The glass plate according to 1 or 2 above which has a principal surface having an area of 350 cm² or larger.

4. The glass plate according any one of 1 to 3 above wherein the dielectric dissipation factor at 10 GHz is 0.009 or less.

5. The glass plate according to any one of 1 to 4 above which has a relative permittivity at 10 GHz of 6.8 or less.

6. The glass plate according to any one of 1 to 5 above wherein any two portions separated from each other by 40 mm or more have a difference in dielectric dissipation factor at 10 GHz of 0.0005 or less.

7. The glass plate according to any one of 1 to 6 above wherein any two portions separated from each other by 40 mm or more have a difference in relative permittivity at 10 GHz of 0.05 or less.

8. The glass plate according to any one of 1 to 7 above which includes from 30 to 85% of SiO₂ as represented by mol % based on oxides.

9. The glass plate according to any one of 1 to 8 above which includes, as represented by mol % based on oxides,

SiO₂: from 57 to 70%,

Al₂O₃: from 5 to 15%,

B₂O₃: from 15 to 24%,

provided that Al₂O₃+B₂O₃ is from 20 to 40%, and

Al₂O₃/(Al₂O₃+B₂O₃) is 0.1 to 0.45,

MgO: from 0 to 10%,

CaO: from 0 to 10%,

SrO: from 0 to 10%,

BaO: from 0 to 10%,

Li₂O: from 0 to 5%,

Na₂O: from 0 to 5%,

K₂O: from 0 to 5%, and

R₂O (R=alkali metal) is from 0 to 5%.

10. The glass plate according to any one of 1 to 8 above which includes, as represented by mol % based on oxides,

SiO₂: from 55 to 80%,

Al₂O₃: from 0 to 15%,

provided that SiO₂+Al₂O₃ is from 55 to 90%,

B₂O₃: from 0 to 15%,

MgO: from 0 to 20%,

CaO: from 0 to 20%,

SrO: from 0 to 15%,

BaO: from 0 to 15%,

provided that MgO+CaO is from 0 to 30%, and

MgO+CaO+SrO+BaO: from 0 to 30%,

Li₂O: from 0 to 20%,

Na₂O: from 0 to 20%, and

K₂O: from 0 to 20%,

provided that R₂O (R=alkali metal) is from 0 to 20%.

11. The glass plate according to any one of 1 to 9 above which is for use as a substrate for a high-frequency device in which high-frequency signals having a frequency of 3.0 GHz or higher are handled.

12. The glass plate according to any one of 1 to 8 and 10 above which is for use as a window material.

13. A process for producing the glass plate according to any one of 1 to 12 above which includes, in the following order,

a melting/forming step in which raw materials for glass are melted to obtain a molten glass and the molten glass is formed into a plate shape,

a cooling step in which the molten glass formed into the plate shape is cooled to a temperature of (Tg−300)° C. or lower with respect to the glass transition temperature Tg (° C.) to obtain a glass base plate, and

a heat treatment step in which the obtained glass base plate is heated from the temperature of (Tg−300)° C. or lower to a temperature in a range of from (Tg−100)° C. to (Tg+50)° C., without being heated to a temperature exceeding (Tg+50)° C., and is then cooled again to (Tg−300)° C. or lower,

wherein the heat treatment step is conducted one or two or more times,

each heat treatment step extends until the temperature of the glass base plate exceeds (Tg−300)° C., thereafter reaches a maximum temperature Temax (° C.) in the range of from (Tg−100)° C. to (Tg+50)° C., and then declines again to (Tg−300)° C. or lower,

a total time period in which the temperature of the glass base plate is in the range of from (Tg−100)° C. to (Tg+50)° C. in a whole heat treatment step(s) is K (minutes) or longer, the K being represented by the following formula (1) using the maximum temperature Tmax (° C.) of the glass base plate in the whole heat treatment step(s), and

each heat treatment step satisfies the following formula (2), where t1 (minutes) is a time period in each heat treatment from a time when the temperature of the glass base plate lastly begins to decline from the maximum temperature Temax (° C.) to a time when the temperature of the glass base plate lastly passes (Tg−110)° C.

K=[{(Tg+50)−Tmax}/10]+15  Formula (1)

{Temax−(Tg−110)}/t1≤10  Formula (2)

14. The process for glass plate production according to 13 above wherein in each heat treatment step, the temperature of the glass base plate which has first become lower than (Tg−110)° C. after having declined from the maximum temperature Temax (° C.) does not exceed (Tg−110)° C. again.

15. The process for glass plate production according to 13 or 14 above wherein in each heat treatment step, if any two times in the step which lie between the time when the temperature of the glass base plate lastly begins to decline from the maximum temperature Temax (° C.) and the time when the temperature of the glass base plate lastly passes (Tg−110)° C. are expressed by t2 (minutes) and t3 (minutes) and if t2<t3, then

t2 and the t3 have a difference in time therebetween of 1 minute or more, and

if the temperature of the glass base plate at t2 is expressed by Te2 and the temperature of the glass base plate at t3 is expressed by Te3, then Te2 and Te3 satisfy the following formula (3).

(Te2−Te3)/(t3−t2)≤10  Formula (3)

16. The process for glass plate production according to any one of 13 to 15 above wherein in the cooling step, the molten glass is cooled from (Tg+50)° C. to (Tg−100)° C. at an average cooling rate exceeding 10° C./min.

17. The process for glass plate production according to any one of 13 to 16 above wherein in the cooling step, the molten glass is cooled at an average cooling rate of from 10 to 1,000° C./min.

Advantageous Effects of Invention

The glass plate according to the present invention shows little absorption of electromagnetic waves in a high-frequency band and can attain a high transmittance. Furthermore, using this glass plate in circuit boards makes it possible to provide practical high-frequency devices, such as electronic devices, reduced in propagation loss and transmission loss. Moreover, this glass plate, when used as window materials for vehicles, e.g., motor vehicles, and buildings, can propagate electromagnetic waves without causing considerable attenuation, in the case where millimeter-wave radars have been mounted in the vehicles or electronic appliances are used in the buildings.

DESCRIPTION OF EMBODIMENTS

The present invention is described below in detail, but the present invention is not limited to the following embodiments and can be modified at will within the gist of the present invention. Symbol “-” indicating a numerical range is used in the sense of including the numerical values set force before and after the “-” as a lower limit value and an upper limit value. Furthermore, “%” indicating the composition of a glass plate shows a value as represented by mol % based on oxides unless otherwise indicated.

<Glass Plate>

The glass plate according to this embodiment has a dielectric dissipation factor at 10 GHz of tan δA and a glass transition temperature of Tg° C., and this glass plate, after having been heated to (Tg+50)° C. and then cooled to (Tg−150)° C. at 100° C./min, satisfies the relationship (tan δ100−tan δA)≥0.0004, where tan δ100 is a dielectric dissipation factor thereof.

The dielectric dissipation factor (hereinafter sometimes referred to simply as “tan δ”) of a glass plate is a value represented by ε″/ε′ using complex permittivity, where ε′ is relative permittivity and ε″ is dielectric loss. The smaller the value of tan δ, the lower the absorption of electromagnetic waves in the frequency band and the higher the attained transmittance.

In this description, dielectric dissipation factor and relative permittivity are values measured at a measuring frequency of 10 GHz by the method as provided for in IEC 61189-2-721 (2015).

In general, values of tan δ can be regulated by changing the glass composition. However, the present invention is based on a newly discovered method whereby values of tan δ can be regulated without changing a glass composition. This makes it possible to obtain a glass which has a smaller value of tan δ than conventional glasses even when having the same composition.

Glasses differing in density are obtained by using different cooling rates in glass production. Specifically, a high cooling rate results in a vitreous state having a low density (sparse), while a low cooling rate results in a vitreous state having a high density (dense). It has been discovered that the density of the vitreous state correlates with the value of tan δ in a high-frequency band.

That is, in the case where the vitreous state is dense and has a high density, the glass plate can have an increased transmittance for electromagnetic waves in a high-frequency band (can be reduced in the absorption of the electromagnetic waves), resulting in a smaller value of tan δ in the high-frequency band. The term “high-frequency band” in this description is intended to mean frequencies of usually not shorter than 3.0 GHz, in particular, not shorter than 3.5 GHz, and actual tests were performed at 10 GHz.

The glass plate according to this embodiment has a smaller value of tan δA in a high-frequency band than conventional glass plates having the same composition. This can be assessed in terms of the value of (tan δ100−tan δA) described above. Specifically, in the case where a glass plate having a dielectric dissipation factor at 10 GHz of tan δA is heated to (Tg+50)° C. and then cooled to (Tg−150)° C. at 100° C./min and when this glass plate thereafter has a value of tan δ100 which is larger than the value of tan δA [(tan 1100−tan δA)≥0], then the glass plate is deemed to be a glass plate obtained through cooling conducted at a cooling rate lower than 100° C./min and has a high density and high transparency.

Furthermore, in the case where the value of (tan δ100−tan δA) is 0.0004 or larger to satisfy the relationship (tan δ100−tan δA)≥0.0004, this glass plate can be regarded as having a considerably smaller value of tan δA than conventional glass plates having the same composition and as showing high transparency to electromagnetic waves in a high-frequency band.

Although the tan δA of the glass plate satisfies the relationship (tan δ100−tan δA)≥0.0004 as stated above, the tan δA preferably satisfies (tan δ100−tan δA)≥0.0005, more preferably satisfies (tan δ100−tan δA)≥0.0006, from the standpoint of making the glass plate show higher transparency.

There is no particular upper limit on (tan δ100−tan δA). However, from the standpoint of shortening the heat-treatment period to improve the production efficiency, the tan δA may satisfy (tan δ100−tan δA)≤0.001, or may satisfy (tan δ100−tan δA)≤0.0008, or may satisfy (tan δ100−tan δA)≤0.0007, or may satisfy (tan δ100−tan δA)≤0.0006.

Any two portions of the glass plate which are separated from each other by 40 mm or more have a difference in dielectric dissipation factor tan δ at 10 GHz of preferably 0.0005 or less, more preferably 0.0004 or less, still more preferably 0.0003 or less. In the case where the difference in dielectric dissipation factor tan δ is 0.0005 or less, this glass plate can be regarded as having a narrow in-plane distribution of dielectric dissipation factor and can be regarded as a glass plate which had evenness in cooling rate and is homogeneous. Such differences in tan δ are hence preferred. The term “any two portions separated from each other by 40 mm or more” means any two portions lying on the same plane and separated by 40 mm or more.

There is no particular lower limit on the difference in dielectric dissipation factor tan δ at 10 GHz between any two portions of the glass plate which are separated from each other by 40 mm or more, but the difference may be 0.0001 or more.

It is preferable that the glass plate has a relative permittivity εrA at 10 GHz which satisfies the relationship 0.95≤(εr100/εrA)≤1.05, where εr100 is a relative permittivity at 10 GHz of the glass plate which has been heated to (Tg+50)° C. and then cooled to (Tg−150)° C. at 100° C./min. The value represented by (εr100/εrA) is more preferably 0.98 or larger, still more preferably 0.99 or larger, and is more preferably 1.03 or smaller, still more preferably 1.02 or smaller, especially preferably 1.01 or smaller.

Unlike the tan δA, the relative permittivity Fr of the obtained glass plate has a substantially constant value even when the glass plate has been produced using different cooling rates. Because of this, a reduction in loss in high-frequency devices can be attained without considerably changing the design of the devices.

Any two portions of the glass plate which are separated from each other by 40 mm or more have a difference in relative permittivity εrA at 10 GHz of preferably 0.05 or less, more preferably 0.04 or less, still more preferably 0.03 or less. In the case where the difference in relative permittivity εrA is 0.05 or less, this glass plate is a homogeneous glass plate which has a narrow in-plane distribution of relative permittivity and had evenness in cooling rate. Such differences in εrA are hence preferred. Although there is no particular lower limit on the difference in relative permittivity εrA at 10 GHz between any two portions of the glass plate which are separated from each other by 40 mm or more, the difference may be 0.01 or more.

The glass plate having such properties can be advantageously used as the substrates of high-frequency devices and as window materials. The high-frequency devices are more preferably ones in which high-frequency signals having a frequency of 3.0 GHz or higher, in particular 3.5 GHz or higher, are handled.

The glass plate preferably includes SiO₂ in an amount of 30-85% as represented by mol % based on oxides. For use as a substrate for high-frequency devices, the glass plate is more preferably an alkali-free glass. For use as a window material, the glass plate is more preferably a soda-lime glass.

Specific preferred glass compositions for the respective applications are as follows.

In the case where the glass plate is for use as the substrate of a high-frequency device, this glass plate more preferably has the following composition as represented by mol % based on oxides.

SiO₂: from 57 to 70%,

Al₂O₃: from 5 to 15%,

B₂O₃: from 15 to 24%,

Al₂O₃+B₂O₃: from 20 to 40%,

Al₂O₃/(Al₂O₃+B₂O₃): from 0.1 to 0.45,

MgO: from 0 to 10%,

CaO: from 0 to 10%,

SrO: from 0 to 10%,

BaO: from 0 to 10%,

Li₂O: from 0 to 5%,

Na₂O: from 0 to 5%,

K₂O: from 0 to 5%, and

R₂O (R=alkali metal): from 0 to 5%.

Each component of the composition is explained below.

SiO₂ is a network-forming substance. In the case where the content thereof is 57% or higher, satisfactory glass-forming ability and satisfactory weatherability can be attained and devitrification can be inhibited. Such SiO₂ contents are hence preferred. The content of SiO₂ is more preferably 58% or higher, still more preferably 60% or higher, yet still more preferably 61% or higher. Meanwhile, in the case where the content of SiO₂ is 70% or less, satisfactory glass meltability can be attained; such SiO₂ contents are hence preferred. The content thereof is more preferably 68% or less, still more preferably 66% or less, yet still more preferably 65% or less, especially preferably 64% or less, most preferably 63% or less.

Al₂O₃ is a component effective in improving the weatherability, improving the Young's modulus, inhibiting the glass from suffering phase separation, reducing the coefficient of thermal expansion, and so on. In the case where the content of Al₂O₃ is 5% or higher, the effects of the inclusion of Al₂O₃ are sufficiently obtained; such Al₂O₃ contents are hence preferred. The content of Al₂O₃ is more preferably 6% or higher, still more preferably 7% or higher, yet still more preferably 8% or higher. Meanwhile, in the case where the content of Al₂O₃ is 15% or less, the glass has satisfactory properties including meltability; such Al₂O₃ contents are hence preferred. The content thereof is more preferably 14% or less, still more preferably 13% or less, yet still more preferably 12% or less.

B₂O₃ is a component which improves the meltability, and the content thereof is preferably 15% or higher. B₂O₃ is also a component capable of lowering the dielectric dissipation factor in a high-frequency range. Hence, the content thereof is more preferably 16% or higher, still more preferably 17% or higher, yet still more preferably 17.5% or higher. Meanwhile, from the standpoint of obtaining satisfactory chemical resistance, the content of B₂O₃ is preferably 24% or less, more preferably 23% or less, still more preferably 22% or less.

The total content of Al₂O₃ and B₂O₃(Al₂O₃+B₂O₃) is more preferably 20% or higher, especially preferably 25% or higher, from the standpoint of glass meltability. From the standpoint of heightening the low-dielectric-loss characteristics of the glass plate while maintaining the glass meltability, etc., the total content thereof is preferably 40% or less, more preferably 37% or less, still more preferably 35% or less, especially preferably 33% or less.

MgO is a component which increases the Young's modulus without increasing the specific gravity, and can thereby heighten the specific modulus. MgO hence is effective in mitigating the problem of deflection and can improve the fracture toughness to heighten the glass strength. Furthermore, MgO is a component which improves the meltability also and can inhibit the glass from having too low a coefficient of thermal expansion. Although MgO may not be contained, the content of MgO, if it is contained, is preferably 0.1% or higher, more preferably 0.2% or higher, still more preferably 1% or higher, yet still more preferably 2% or higher. Meanwhile, from the standpoint of inhibiting the glass from having an elevated devitrification temperature, the content of MgO is preferably 10% or less, more preferably 9% or less, still more preferably 8% or less, yet still more preferably 7% or less, particularly preferably 6% or less, in particular 5% or less, especially preferably 4% or less, most preferably 3% or less.

CaO is characterized by being next to MgO among the alkaline-earth metals in heightening the specific modulus and by not excessively lowering the strain point, and is a component which improves the meltability like MgO. Furthermore, CaO is a component characterized by being less prone to heighten the devitrification temperature than MgO. Although CaO may not be contained, the content of CaO, if it is contained, is preferably 0.1% or higher, more preferably 0.2% or higher, still more preferably 0.5% or higher, yet still more preferably 1% or higher, especially preferably 2% or higher. Meanwhile, from the standpoints of preventing the glass from having too high an average coefficient of thermal expansion and of inhibiting the devitrification temperature from increasing and thereby preventing the glass from devitrifying when produced, the content of CaO is preferably 10% or less, more preferably 8% or less, still more preferably 7% or less, yet still more preferably 6% or less, particularly preferably 5% or less, in particular 4% or less, especially preferably 3% or less.

SrO is a component which improves the meltability without heightening the devitrification temperature of the glass. Although SrO may not be contained, the content of SrO, if it is contained, is preferably 0.1% or higher, more preferably 0.2% or higher, still more preferably 0.5% or higher, yet still more preferably 1% or higher, especially preferably 2% or higher. Meanwhile, from the standpoint of inhibiting the average coefficient of thermal expansion from becoming too high without increasing the specific gravity, the content of SrO is desirably 10% or less, preferably 9% or less, more preferably 8% or less, still more preferably 7% or less, yet still more preferably 6% or less, particularly preferably 5% or less, in particular 4% or less, especially preferably 3% or less, most preferably 2.5% or less.

BaO is a component which improves the meltability without heightening the devitrification temperature of the glass. Although BaO may not be contained, the content of BaO, if it is contained, is preferably 0.1% or higher, more preferably 0.2% or higher, still more preferably 1% or higher, especially preferably 2% or higher. Meanwhile, from the standpoint that too high BaO contents result in an increased specific gravity, a reduced Young's modulus, an elevated relative permittivity, and too high an average coefficient of thermal expansion, the content of BaO is preferably 10% or less, more preferably 8% or less, still more preferably 5% or less, yet still more preferably 3% or less.

ZnO is a component which improves the chemical resistance, but is prone to separate out and may heighten the devitrification temperature. Because of this, the content of ZnO is preferably 0.1% or less, more preferably 0.05% or less, still more preferably 0.03% or less, yet still more preferably 0.01% or less. Especially preferably, the glass composition contains substantially no ZnO. The term “containing substantially no ZnO” means that the content thereof is, for example, less than 0.01%.

The molar ratio represented by {Al₂O₃/(Al₂O₃+B₂O₃)} is preferably 0.1 or higher from the standpoint of enabling the glass to have improved acid resistance and excellent evenness with inhibited phase separation. From the standpoint of imparting an improved Young's modulus, that molar ratio is more preferably 0.3 or higher, still more preferably 0.33 or higher, yet still more preferably 0.35 or higher, especially preferably 0.38 or higher. Meanwhile, from the standpoint of enabling the glass to attain a reduction in dielectric loss in a range of high frequencies not lower than 10 GHz, preferably frequencies exceeding 30 GHz, that molar ratio is preferably 0.45 or less, more preferably 0.4 or less, still more preferably 0.35 or less, yet still more preferably 0.3 or less.

If the contents of Al₂O₃, MgO, CaO, SrO, and BaO as represented by mol % based on oxides are respectively expressed by [Al₂O₃], [MgO], [CaO], [SrO], and [BaO], then the value represented by {[Al₂O₃]−([MgO]+[CaO]+[SrO]+[BaO])} is preferably larger than −3, more preferably −2 or larger, still more preferably −1 or larger, especially preferably −0.5 or larger, from the standpoint of acid resistance. Meanwhile, from the standpoint of inhibiting the glass from devitrifying, the value represented by {[Al₂O₃]−([MgO]+[CaO]+[SrO]+[BaO])} is preferably less than 2, more preferably 1.5 or less, still more preferably 1.0 or less, especially preferably 0.5 or less.

The content molar ratio represented by {(SrO+BaO)/RO} is preferably 0.64 or higher, more preferably 0.7 or higher, still more preferably 0.75 or higher, especially preferably 0.8 or higher, from the standpoints of lowering the surface devitrification temperature and improving the glass production efficiency. Meanwhile, from the standpoint of reducing the raw-material cost in view of the fact that raw materials for SrO and BaO are expensive, the molar ratio is preferably 0.85 or less, more preferably 0.8 or less. The RO represents the total content of MgO, CaO, SrO, and BaO.

R₂O represents the total content of alkali metal oxides. Examples of the alkali metal oxides include Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O. Since Rb₂O and Cs₂O, among alkali metal oxides, are rarely contained in glasses, R₂O usually means the total content of Li₂O, Na₂O, and K₂O (Li₂O+Na₂O+K₂O).

The glass composition may not contain alkali metal oxides. However, inclusion of alkali metal oxides eliminates the need of excessive raw-material purification and makes it possible to obtain practical glass meltability and glass plate production efficiency and to regulate the coefficient of thermal expansion of the glass plate. Because of this, in the case where alkali metal oxides are contained, the total content thereof (R₂O) is preferably 0.001% or higher, more preferably 0.002% or higher, still more preferably 0.003% or higher, especially preferably 0.005% or higher. From the standpoint of enhancing the low-dielectric-loss characteristics of the glass plate, the total content thereof is preferably 5% or less, more preferably 3% or less, still more preferably 1% or less, yet still more preferably 0.2% or less, particularly preferably 0.1% or less, especially preferably 0.05% or less.

The content of Li₂O, as one of the alkali metal oxides, is preferably from 0 to 5%, more preferably 0.1% or higher, still more preferably 0.2% or higher, and is more preferably 4% or less, still more preferably 3% or less. The content of Na₂O is preferably from 0 to 5%, more preferably 0.1% or higher, still more preferably 0.2% or higher, and is more preferably 4% or less, still more preferably 3% or less. The content of K₂O is preferably from 0 to 5%, more preferably 0.1% or higher, still more preferably 0.2% or higher, and is more preferably 4% or less, still more preferably 3% or less.

Besides the components shown above, Fe may be contained in order to reduce resistance values within a melting-temperature range, e.g., the resistance value at 1,500° C. In the case where Fe is contained, the content thereof in terms of Fe₂O₃ is preferably 0.01% or higher, more preferably 0.05% or higher. However, since too high Fe contents may result in a decrease in visible-region transmittance, the content of Fe in terms of Fe₂O₃ is preferably 1% or less, more preferably 0.5% or less, still more preferably 0.1% or less.

The β-OH value, which is an index to the water content of the glass, is preferably 0.05 mm⁻¹ or higher, more preferably 0.1 mm⁻¹ or higher, still more preferably 0.2 mm⁻¹ or higher, especially preferably 0.3 mm⁻¹ or higher, from the standpoint of attaining a reduced resistance value in a temperature range where raw materials for glass are melted, for example, at around 1,500° C., to make the glass suitable for melting by electric heating. Meanwhile, from the standpoint of diminishing bubble defects in the glass, the β-OH value is preferably 1.0 mm⁻¹ or less, more preferably 0.8 mm⁻¹ or less, still more preferably 0.6 mm⁻¹ or less, especially preferably 0.5 mm⁻¹ or less.

The β-OH value in this description is a value determined by examining a glass sample for absorbance for light having wavelengths of from 2.75 to 2.95 μm and dividing a maximum absorbance β_max by the thickness (mm) of the sample.

The glass composition may contain at least one component selected from the group consisting of SnO₂, Cl, and SO₃ for improving the refinability of the glass plate. The total content of these (SnO₂+Cl+SO₃) may be from 0.01 to 1.0 mass % with respect to the total content of SiO₂, Al₂O₃, RO, and R₂O (SiO₂+Al₂O₃+RO+R₂O) as represented by mass % based on oxides, which is taken as 100%. The total content thereof is preferably 0.80 mass % or less, more preferably 0.50 mass % or less, still more preferably 0.30 mass % or less. Meanwhile, the total content thereof is preferably 0.02 mass % or higher, more preferably 0.05 mass % or higher, still more preferably 0.10 mass % or higher.

The glass composition may contain at least one component (hereinafter referred to as “minor component”) selected from the group consisting of Sc₂O₃, TiO₂, ZnO₂, Ga₂O₃, GeO₂, Y₂O₃, ZrO₂, Nb₂O₅, In₂O₃, TeO₂, HfO₂, Ta₂O₅, WO₃, Bi₂O₃, La₂O₃, Gd₂O₃, Yb₂O₃, and Lu₂O₃ for improving the acid resistance of the glass. However, in case where the content of minor components is too high, the glass has reduced evenness and is prone to suffer phase separation. Consequently, the content of minor components, in terms of the total content thereof as represented by mol % based on oxides, is 1.0% or less. Only one of those minor components may be contained, or two or more thereof may be contained.

The glass composition may be made to contain F for the purposes of improving the meltability, lowering the strain point, lowering the glass transition temperature, lowering the annealing point, etc. However, from the standpoint of preventing the glass from having an increased number of bubble defects, the content of F is preferably 1 mass % or less with respect to the total content of SiO₂, Al₂O₃, RO, and R₂O (SiO₂+Al₂O₃+RO+R₂O) as represented by mass % based on oxides, which is taken as 100%.

In the case where the glass plate is for use as a window material, this glass plate more preferably has the following composition as represented by mol % based on oxides.

SiO₂: from 55 to 80%,

Al₂O₃: from 0 to 15%,

SiO₂+Al₂O₃: from 55 to 90%,

B₂O₃: from 0 to 15%,

MgO: from 0 to 20%,

CaO: from 0 to 20%,

SrO: from 0 to 15%,

BaO: from 0 to 15%,

MgO+CaO: from 0 to 30%,

MgO+CaO+SrO+BaO: from 0 to 30%,

Li₂O: from 0 to 20%,

Na₂O: from 0 to 20%,

K₂O: from 0 to 20%, and

R₂O (R=alkali metal): from 0 to 20%.

Each component of the composition is explained below.

SiO₂ and Al₂O₃ are components which contribute to an improvement in Young's modulus and thereby make it easy to ensure the strength required of window materials for use in building applications, motor vehicle applications, etc.

From the standpoints of ensuring weatherability and preventing the glass plate from suffering thermal cracking due to too high an average coefficient of linear expansion, the content of SiO₂ is preferably 55% or higher, more preferably 57% or higher, still more preferably 60% or higher, yet still more preferably 63% or higher, particularly preferably 65% or higher, especially preferably 68% or higher, most preferably 70% or higher. Meanwhile, from the standpoints of preventing the glass from having increased melt viscosity and of thereby facilitating the glass production, the content of SiO₂ is preferably 80% or less, more preferably 78% or less, still more preferably 75% or less, most preferably 74% or less.

Al₂O₃ is a component which is for ensuring weatherability and which prevents the glass plate from suffering thermal cracking due to too high an average coefficient of linear expansion. Although Al₂O₃ may not be contained, the content of Al₂O₃, if it is contained, is preferably 0.01% or higher, more preferably 0.05% or higher, still more preferably 0.1% or higher. Meanwhile, from the standpoints of preventing the glass from having increased melt viscosity so that the temperature (hereinafter referred to as T2) at which the glass has a viscosity of 10² dPa·s and the temperature (hereinafter referred to as T4) at which the glass has a viscosity of 10⁴ dPa·s are kept low to facilitate the glass production and of making the glass plate have satisfactory radio-wave transmission characteristics, the content of Al₂O₃ is preferably 15% or less, more preferably 10% or less, still more preferably 5% or less, yet still more preferably 1% or less, especially preferably 0.5% or less.

The total content of SiO₂ and Al₂O₃(SiO₂+Al₂O₃) is preferably from 55 to 90% from the standpoint of obtaining a satisfactory radio-wave transmittance. From the standpoints of ensuring weatherability and preventing the average coefficient of linear expansion from becoming too high, the total content thereof is more preferably 57% or higher, still more preferably 60% or higher, yet still more preferably 65% or higher, especially preferably 70% or higher, most preferably 72% or higher. Meanwhile, from the standpoint of keeping the T2 and the T4 low to render the glass easy to produce, the total content thereof is more preferably 85% or less, still more preferably 80% or less, yet still more preferably 78% or less, especially preferably 75% or less.

B₂O₃ is a component which improves the meltability and glass strength and heightens the radio-wave transmittance. Meanwhile, B₂O₃ is a component which makes alkali elements prone to volatilize during melting/forming, leading to a decrease in glass quality. In addition, too high contents thereof reduce the average coefficient of linear expansion to render the glass difficult to physically strengthen. Because of these, the content of B₂O₃ is preferably 15% or less, more preferably 10% or less, still more preferably 8% or less, yet still more preferably 5% or less, particularly preferably 3% or less, especially preferably 1% or less. Most preferably, the glass composition contains substantially no B₂O₃. The expression “containing substantially no B₂O₃” means that the glass composition does not contain B₂O₃ except for the case where B₂O₃ has come into the glass as an unavoidable impurity.

MgO is a component which accelerates the melting of the raw materials for glass and improves the weatherability. Meanwhile, from the standpoint of preventing devitrification to heighten the radio-wave transmittance, the content of MgO is preferably 20% or less, more preferably 15% or less, still more preferably 8% or less, yet still more preferably 4% or less, especially preferably 1% or less, most preferably 0.5% or less. MgO may not be contained.

CaO, SrO, and BaO are components which lower the dielectric dissipation factor of the glass and can improve the meltability of the glass. One or more of these may be contained.

CaO may not be contained. However, in the case where CaO is contained, the content thereof is preferably 3% or higher, more preferably 6% or higher, still more preferably 8% or higher, yet still more preferably 10% or higher, especially preferably 110% or higher, from the standpoints that CaO reduces the dielectric loss of the glass to thereby improve the radio-wave transmittance and that CaO can further bring about an improvement in meltability (decreases in T2 and T4). Meanwhile, from the standpoints of avoiding an increase in the specific gravity of the glass and enabling the glass to retain the strength and low brittleness, the content of CaO is preferably 20% or less. From the standpoint of lower brittleness, the content thereof is more preferably 15% or less, still more preferably 14% or less, yet still more preferably 13% or less, especially preferably 12% or less.

The content of SrO is preferably 15% or less, more preferably 8% or less, still more preferably 3% or less, yet still more preferably 1% or less, from the standpoints of avoiding an increase in the specific gravity of the glass and enabling the glass to retain the strength and low brittleness. Especially preferably, the glass composition contains substantially no SrO. The expression “containing substantially no SrO” means that the glass composition does not contain SrO except for the case where SrO has come into the glass as an unavoidable impurity.

The content of BaO is preferably 15% or less, more preferably 5% or less, still more preferably 3% or less, yet still more preferably 2% or less, especially preferably 1% or less, from the standpoints of avoiding an increase in the specific gravity of the glass and enabling the glass to retain the strength and low brittleness. Most preferably, the glass composition contains substantially no BaO. The expression “containing substantially no BaO” means that the glass composition does not contain BaO except for the case where BaO has come into the glass as an unavoidable impurity.

The total content of MgO, CaO, SrO, and BaO (MgO+CaO+SrO+BaO) may be 0% (none of these is contained). However, from the standpoint of lowering the glass viscosity during production to lower the T2 and T4 or from the standpoint of heightening the Young's modulus, the total content thereof is preferably higher than 0%, more preferably 0.5% or higher, still more preferably 5% or higher, yet still more preferably 8% or higher, especially preferably 10% or higher, most preferably 11% or higher. Meanwhile, from the standpoint of improving the weatherability, the total content thereof is preferably 30% or less, more preferably 17% or less, still more preferably 16% or less, yet still more preferably 15% or less, especially preferably 14% or less, most preferably 13% or less.

Furthermore, from the standpoint of avoiding a trouble in which devitrification occurs during glass melting or during forming and this results in a decrease in glass quality, the total content of MgO and CaO (MgO+CaO) is preferably 30% or less, more preferably 25% or less, still more preferably 20% or less, yet still more preferably 15% or less, especially preferably 13% or less. The total content thereof may be 0% (neither is contained). However, from the standpoint of preventing the glass from having too high viscosity during melting/forming and from thereby becoming difficult to produce, the total content thereof is preferably 1% or higher, more preferably 2% or higher, still more preferably 5% or higher, yet still more preferably 8% or higher, especially preferably 10% or higher.

Li₂O is a component which improves the meltability of the glass and is also a component which renders the glass apt to have an increased Young's modulus, thereby contributing to an improvement in glass strength. Although Li₂O may not be contained, the inclusion thereof makes chemical strengthening possible and is sometimes effective in heightening the radio-wave transmittance. Because of this, the content of Li₂O, if it is contained, is preferably 0.1% or higher, more preferably 1% or higher, still more preferably 2% or higher, yet still more preferably 3% or higher, especially preferably 4% or higher. Meanwhile, too high Li₂O contents might result in devitrification or phase separation during glass production to make the production difficult. Hence, the content thereof is preferably 20% or less, more preferably 16% or less, still more preferably 12% or less, yet still more preferably 8% or less, especially preferably 7% or less, most preferably 6.5% or less.

Na₂O and K₂O are components which improve the meltability of the glass, and incorporating at least either of the two in an amount of 0.1% or larger makes it easy to regulate the T2 to 1,750° C. or lower and the T4 to 1,350° C. or lower. Meanwhile, too low total contents of Na₂O and K₂O might make the glass unable to have an increased average coefficient of linear expansion and to be thermally strengthened. Furthermore, by the inclusion of both Na₂O and K₂O, the weatherability can be improved while maintaining the meltability. There are cases where the inclusion thereof is effective also in heightening the radio-wave transmittance.

Although Na₂O may not be contained, the inclusion thereof renders chemical strengthening possible, besides having the effects shown above. Because of this, the content thereof is preferably 0.1% or higher, more preferably 1% or higher, still more preferably 3% or higher, yet still more preferably 5% or higher, especially preferably 6% or higher. Meanwhile, from the standpoint of preventing the glass plate from having too high an average coefficient of thermal expansion to become prone to suffer thermal cracking, the content of Na₂O is preferably 20% or less, more preferably 16% or less, still more preferably 14% or less, yet still more preferably 12% or less, especially preferably 10% or less, most preferably 8% or less.

Although K₂O may not be contained, the inclusion thereof produces the effects shown above. Because of this, the content thereof is preferably 0.1% or higher, more preferably 0.9% or higher, still more preferably 2% or higher, yet still more preferably 3% or higher, especially preferably 4% or higher. Meanwhile, from the standpoint of preventing the glass plate from having too high an average coefficient of thermal expansion to become prone to suffer thermal cracking and from the standpoint of preventing the weatherability from decreasing, the content of K₂O is preferably 20% or less, more preferably 16% or less, still more preferably 14% or less, yet still more preferably 12% or less, especially preferably 10% or less, most preferably 8% or less. Also from the standpoint of radio-wave transmittance, regulating the content of K₂O so as to be within that range makes it possible to obtain a high radio-wave transmittance.

As described above, by regulating the contents of Na₂O and K₂O to values within those ranges, the average coefficient of thermal expansion can be regulated to a desired value to render the glass plate suitable for use as window materials which satisfactorily match with other members, e.g., black ceramics and interlayers.

R₂O represents the total content of alkali metal oxides. Since Rb₂O and Cs₂O, among alkali metal oxides, are rarely contained in glasses, R₂O usually means the total content of Li₂O, Na₂O, and K₂O (Li₂O+Na₂O+K₂O).

Alkali metal oxides, although the glass composition may not contain these, are components which lower the glass viscosity during glass production to lower the T2 and the T4. Because of this, the total content of alkali metal oxides, if these are contained, is preferably higher than 0%, more preferably 1% or higher, still more preferably 5% or higher, yet still more preferably 6% or higher, particularly preferably 8% or higher, in particular 10% or higher, especially preferably 11% or higher, most preferably 12% or higher. Meanwhile, from the standpoint of improving the weatherability, the total content thereof is preferably 20% or less, more preferably 19% or less, still more preferably 18.5% or less, yet still more preferably 18.0% or less, especially preferably 17.5% or less, most preferably 17.0% or less.

In the case where the glass composition contains one or more alkali metal oxides, it is preferable that Na₂O is contained, and the molar ratio represented by (Na₂O/R₂O) is more preferably 0.01 or higher and is more preferably 0.98 or less, from the standpoint of sufficiently obtaining the effect of lowering the dielectric dissipation factor. That molar ratio is still more preferably 0.05 or higher, yet still more preferably 0.1 or higher, particularly preferably 0.2 or higher, especially preferably 0.3 or higher, most preferably 0.4 or higher. Meanwhile, that molar ratio is still more preferably 0.8 or less, yet still more preferably 0.7 or less, especially preferably 0.6 or less, most preferably 0.55 or less.

In the case where the glass composition contains one or more alkali metal oxides, it is also preferable that K₂O is contained, and the molar ratio represented by (K₂O/R₂O) is more preferably 0.01 or higher and is more preferably 0.98 or less, from the standpoint of sufficiently obtaining the effect of heightening the radio-wave transmittance. That molar ratio is still more preferably 0.05 or higher, yet still more preferably 0.1 or higher, particularly preferably 0.2 or higher, especially preferably 0.3 or higher, most preferably 0.4 or higher. Meanwhile, that molar ratio is still more preferably 0.8 or less, yet still more preferably 0.6 or less, especially preferably 0.55 or less.

It is preferable, from the standpoint of heightening the radio-wave transmittance, that the product (R₂O×MgO, %²) of the total content of alkali metal oxides (R₂O, %) and the content of MgO (%) is made small. (R₂O×MgO) is preferably 100%² or less, more preferably 80%² or less, still more preferably 66%² or less, yet still more preferably 60%² or less, particularly preferably 50%² or less, especially preferably 40%² or less, most preferably 30%² or less. Meanwhile, from the standpoint of improving the efficiency of glass production, that product is preferably 1%² or larger, more preferably 3%² or larger, still more preferably 5%² or less.

ZrO₂ is a component which lowers the glass viscosity during melting to accelerate the melting and improves the heat resistance and chemical durability. Meanwhile, too high contents thereof may result in an increase in liquidus temperature. Because of this, the content of ZrO₂ is preferably 5% or less, more preferably 2.5% or less, still more preferably 2% or less, yet still more preferably 1% or less, especially preferably 0.5% or less. Most preferably, the glass composition contains substantially no ZrO₂. The expression “containing substantially no ZrO₂” means that the glass composition does not contain ZrO₂ except for the case where ZrO₂ has come into the glass as an unavoidable impurity.

The total content of some of the components described above which is represented by (SiO₂+Al₂O₃+MgO+CaO+SrO+BaO+Li₂O+Na₂O+K₂O) is preferably 85% or higher, more preferably 88% or higher, still more preferably 90% or higher, yet still more preferably 92% or higher, particularly preferably 95% or higher, especially preferably 98% or higher, most preferably 99.5% or higher, from the standpoint that such values of that total content not only make it possible to produce the glass plate from easily available raw materials for glass but also make it easy to ensure the weatherability of the glass plate. That total content may be 100%, and is more preferably 99.9% or less in view of cases where a colorant, a refining agent, etc. are added to the glass plate.

The glass composition may contain at least one component selected from the group consisting of SnO₂, Cl, and SO₃ for improving the refinability of the glass plate. The total content of these (SnO₂+Cl+SO₃) may be from 0.01 to 1.0 mass % with respect to the total content of the main components SiO₂, Al₂O₃, RO, and R₂O (SiO₂+Al₂O₃+RO+R₂O) as represented by mass % based on oxides, which is taken as 100%. The total content thereof is preferably 0.80 mass % or less, more preferably 0.50 mass % or less, still more preferably 0.30 mass % or less. Meanwhile, the total content thereof is preferably 0.02 mass % or higher, more preferably 0.05 mass % or higher, still more preferably 0.10 mass % or higher.

In the case where the glass plate is for use as substrates for high-frequency devices, preferred examples of the glass transition temperature Tg, T2, T4, devitrification temperature, Young's modulus, acid resistance, alkali resistance, coefficient of expansion (average coefficient of expansion), strain point, density, plate thickness, and principal-surface area of the glass plate are as follows.

The glass transition temperature Tg is preferably 580° C. or higher, more preferably 600° C. or higher, from the standpoint of preventing the substrate from deforming in high-frequency device production steps. Meanwhile, from the standpoint of easily producing the glass plate, the Tg is preferably 750° C. or lower, more preferably 720° C. or lower. Values of glass transition temperature Tg are measured in accordance with JIS R 3103-3:2001.

The T2 is preferably 1,950° C. or lower, more preferably 1,700° C. or lower, from the standpoint of easily producing the glass plate. Meanwhile, from the standpoint of reducing the convection of the molten glass to make the glass-melting apparatus less apt to be damaged, the T2 is preferably 1,500° C. or higher.

The T4 is preferably 1,350° C. or lower, more preferably 1,300° C. or lower, from the standpoint of protecting the production apparatus. Meanwhile, from the standpoint that a decrease in the quantity of heat carried into the forming apparatus by the glass results in the necessity of increasing the quantity of heat to be inputted to the forming apparatus, the T4 is preferably 1,100° C. or higher.

Values of T2 and T4 are measured with a rotary high-temperature viscometer.

The devitrification temperature is preferably 1,350° C. or lower, more preferably 1,300° C. or lower, from the standpoint that in glass-plate forming, such devitrification temperatures enable the members of the forming apparatus to have lowered temperatures and hence prolonged lives. Meanwhile, although there is no particular lower limit on devitrification temperature, the devitrification temperature may be 1,000° C. or higher, or may be 1,050° C. or higher. The devitrification temperature is determined by placing particles of a crushed glass on a dish made of platinum, heat-treating the glass particles for 17 hours in electric furnaces having constant temperatures, examining the heat-treated sample with an optical microscope to determine a highest temperature which has resulted in crystal precipitation in the surface and inside of the glass and a lowest temperature which has not resulted in crystal precipitation, and taking an average of the highest and the lowest temperatures as the devitrification temperature.

The Young's modulus is preferably 50 GPa or higher, more preferably 55 GPa or higher, from the standpoint that such values of Young's modulus are effective in reducing the amount in which the glass plate deflects when used in high-frequency device production steps. Although there is no particular upper limit on Young's modulus, the Young's modulus may be 100 GPa or less. Values of Young's modulus are measured with an ultrasonic-pulse Young's modulus meter.

The term “acid resistance” means the amount of glass components extracted per unit surface area when the glass plate is immersed in an aqueous acid solution (6 wt % HNO₃+5 wt % H₂SO₄; 45° C.) for 170 seconds. The extraction amount indicating the acid resistance is preferably 0.05 g/cm² or less, more preferably 0.03 g/cm² or less, from the standpoint of preventing the glass surfaces from being roughened when cleaned with an acid solution. Although there is no particular lower limit on extraction amount, the extraction amount may be 0.001 g/cm² or larger.

The term “alkali resistance” means the amount of glass components extracted per unit surface area when the glass plate is immersed in an aqueous alkali solution (1.2 wt % NaOH; 60° C.) for 30 minutes. The extraction amount indicating the alkali resistance is preferably 0.10 g/cm² or less, more preferably 0.08 g/cm² or less, from the standpoint of preventing the glass surfaces from being roughened when cleaned with an alkali solution. Although there is no particular lower limit on extraction amount, the extraction amount may be 0.001 g/cm² or larger.

As the coefficient of expansion, use is made of values of the average coefficient of thermal expansion measured with a thermodilatometer in the temperature range of from 50 to 350° C. The average coefficient of thermal expansion is preferably 20×10⁻⁷ (K⁻¹) or higher, more preferably 25×10⁻⁷ (K⁻¹) or higher, from the standpoint of more suitably regulating the difference in thermal expansion coefficient between the glass plate and each of other members in configuring, for example, a semiconductor package as a high-frequency device. Meanwhile, the average coefficient of thermal expansion is preferably 60×10⁻⁷ (K⁻¹) or less, more preferably 50×10⁷ (K¹) or less.

The strain point is preferably 500° C. or higher, more preferably 550° C. or higher, from the standpoint of heat resistance. Meanwhile, from the standpoint of facilitating relaxation, the strain point is preferably 800° C. or lower. Values of strain point are measured in accordance with JIS R 3103-2 (2001).

The density is preferably 2.8 g/cm³ or less from the standpoint of making the glass plate lightweight. Although there is no particular lower limit on density, the density may be 2.0 g/cm³ or higher. Values of density are determined by the Archimedes method.

The plate thickness is preferably 0.05 mm or larger, more preferably 0.1 mm or larger, still more preferably 0.3 mm or larger, from the standpoint of ensuring the strength required of substrates. Meanwhile, from the standpoints of thickness reduction, size reduction, improvement in production efficiency, etc., the plate thickness is preferably 2.0 mm or less, more preferably 1.5 mm or less, still more preferably 1.0 mm or less, yet still more preferably 0.7 mm or less, especially preferably 0.5 mm or less.

In the case where the glass plate is for use as the substrate of a high-frequency device, the area of each principal surface of the glass plate is preferably 80 cm² or larger, more preferably 350 cm² or larger, still more preferably 500 cm² or larger, yet still more preferably 1,000 cm² or larger, even still more preferably 1,500 cm² or larger, and especially preferably 2,000 cm² or larger, 2,500 cm² or larger, 3,000 cm² or larger, 4,000 cm² or larger, 6,000 cm² or larger, 8,000 cm² or larger, 12,000 cm² or larger, 16,000 cm² or larger, 20,000 cm² or larger, and 25,000 cm² or larger in order of increasing preference. Meanwhile, the area of the principal surface of the substrate is usually preferably 5,000,000 cm² or less. Even when having such a large area, this glass plate has a narrow in-plane distribution of dielectric dissipation factor and is homogeneous. Because of this, the glass plate is suitable for use in producing large-area high-frequency devices and in windows for transmitting high-frequency waves, etc., unlike conventional glass plates. The area of the principal surface of the substrate is more preferably 100,000 cm² or less, still more preferably 80,000 cm² or less, yet still more preferably 60,000 cm² or less, especially preferably 50,000 cm² or less, particularly preferably 40,000 cm² or less, most preferably 30,000 cm² or less.

In the case where the glass plate is for use as window materials, preferred examples of the glass transition temperature Tg, T2, T4, devitrification temperature, Young's modulus, acid resistance, alkali resistance, coefficient of expansion (average coefficient of expansion), strain point, density, plate thickness, and principal-surface area of the glass plate are as follows. Methods for determining these properties are the same as those used for determining the properties in the case of the use as substrates for high-frequency devices.

The glass transition temperature Tg is preferably 500° C. or higher, more preferably 520° C. or higher, from the standpoint of performing glass bending. Meanwhile, the Tg is preferably 620° C. or lower, more preferably 600° C. or lower, from the standpoint of performing strengthening by air chilling.

The T2 is preferably 1,550° C. or lower, more preferably 1,480° C. or lower, from the standpoint of easily producing the glass plate. Meanwhile, from the standpoint of reducing the convection of the molten glass to make the glass-melting apparatus less apt to be damaged, the T2 is preferably 1,250° C. or higher.

The T4 is preferably 1,200° C. or lower, more preferably 1,100° C. or lower, from the standpoint of protecting the production apparatus. Meanwhile, from the standpoint that a decrease in the quantity of heat carried into the forming apparatus by the glass results in the necessity of increasing the quantity of heat to be inputted to the forming apparatus, the T4 is preferably 900° C. or higher.

The devitrification temperature is preferably 1,100° C. or lower, more preferably 1,000° C. or lower, from the standpoint that in glass-plate forming, such devitrification temperatures enable the members of the forming apparatus to have lowered temperatures and hence prolonged lives. Meanwhile, although there is no particular lower limit on devitrification temperature, the devitrification temperature may be 900° C. or higher.

The Young's modulus is preferably 50 GPa or higher, more preferably 55 GPa or higher, from the standpoint of attaining a reduced deflection amount. Although there is no particular upper limit on Young's modulus, the Young's modulus may be 100 GPa or less.

The acid resistance is such that the extraction amount shown above is preferably 0.1 g/cm² or less, more preferably 0.05 g/cm² or less, from the standpoint of preventing the glass surfaces from being roughened when cleaned with an acid solution. Although there is no particular lower limit on extraction amount, the extraction amount may be 0.001 g/cm² or larger.

The alkali resistance is such that the extraction amount shown above is preferably 0.20 g/cm² or less, more preferably 0.10 g/cm² or less, from the standpoint of preventing the glass surfaces from being roughened when cleaned with an alkali solution. Although there is no particular lower limit on extraction amount, the extraction amount may be 0.001 g/cm² or larger.

As the coefficient of thermal expansion, use is made of values of the average coefficient of thermal expansion within the temperature range of from 50 to 350° C. The average coefficient of thermal expansion is preferably 60×10⁻⁷ (K⁻¹) or higher, more preferably 70×10⁻⁷ (K⁻¹) or higher, from the standpoint of facilitating strengthening by air chilling. Meanwhile, from the standpoint that too high average coefficients of thermal expansion result in poor thermal shock resistance, the average coefficient of thermal expansion is desirably 130×10⁻⁷ (K⁻¹) or less, preferably 110×10⁻⁷ (K⁻¹) or less.

The strain point is preferably 450° C. or higher, more preferably 500° C. or higher, from the standpoint of heat resistance. Meanwhile, from the standpoint of facilitating relaxation, the strain point is preferably 700° C. or lower. Values of strain point are measured in accordance with JIS R 3103-2 (2001).

The density is preferably 2.8 g/cm³ or less from the standpoint that too high densities make the glass plate too heavy and difficult to handle in conveyance. Although there is no particular lower limit on density, the density may be 2.0 g/cm³ or higher.

The plate thickness is preferably 1.0 mm or larger, more preferably 1.5 mm or larger, from the standpoint of ensuring the rigidity required of window materials. Meanwhile, from the standpoint of weight reduction, the plate thickness is preferably 6.0 mm or less, more preferably 5.0 mm or less.

In the case where the glass plate is for use as a window material, the area of each principal surface of the glass plate is preferably 350 cm² or larger, more preferably 500 cm² or larger, still more preferably 1,000 cm² or larger, even still more preferably 1,500 cm² or larger, and especially preferably 2,000 cm² or larger, 2,500 cm² or larger, 3,000 cm² or larger, 4,000 cm² or larger, 6,000 cm² or larger, 8,000 cm² or larger, 12,000 cm² or larger, 16,000 cm² or larger, 20,000 cm² or larger, and 25,000 cm² or larger in order of increasing preference.

Meanwhile, the area of the principal surface of the window material is usually 6,000,000 cm² or less. Even when having such a large area, this glass plate has a narrow in-plane distribution of dielectric dissipation factor and is homogeneous. Because of this, the glass plate is suitable for use in producing large-area high-frequency devices and in windows for transmitting high-frequency waves, etc., unlike conventional glass plates. Since too large principal-surface areas are undesirable from the standpoint of ensuring the homogeneity of the glass plate, the area of the principal surface of the window material is more preferably 100,000 cm² or less, still more preferably 80,000 cm² or less, yet still more preferably 60,000 cm² or less, especially preferably 50,000 cm² or less, particularly preferably 40,000 cm² or less, most preferably 30,000 cm² or less.

It is preferable that both the tan δA and the εrA are small. By configuring the glass plate so as to have such values of tan δA and εrA, the glass plate is rendered applicable to large-area high-frequency devices, windows for transmitting high-frequency waves, etc. unlike conventional glass plates. The tan δA is preferably 0.009 or less, more preferably 0.008 or less, 0.007 or less, 0.006 or less, and 0.005 or less in order of increasing preference, still more preferably 0.004 or less, especially preferably 0.0035 or less, particularly preferably 0.003 or less, most preferably 0.0025 or less.

There is no particular lower limit on tan δA. However, from the standpoint of actually producing the glass plate, the tan δA is 0.0001 or larger, more preferably 0.0004 or larger, still more preferably 0.0006 or larger, yet still more preferably 0.0008 or larger, most preferably 0.001 or larger. The εrA is preferably 6.8 or less, more preferably 6.5 or less, 6.0 or less, 5.5 or less, 5.2 or less, and 4.9 or less in order of increasing preference, still more preferably 4.7 or less, especially preferably 4.5 or less, particularly preferably 4.4 or less, most preferably 4.3 or less.

There is no particular lower limit on εrA. However, from the standpoint of actually producing the glass plate, the εrA is 3.5 or higher, more preferably 3.6 or higher, still more preferably 3.7 or higher, especially preferably 3.8 or higher, particularly preferably 3.9 or higher, most preferably 4.0 or higher.

<Process for Producing Glass Plate>

The process of glass plate production according to this embodiment includes, in the following order: a melting/forming step in which raw materials for glass are melted to obtain a molten glass and the molten glass is formed into a plate shape; a cooling step in which the molten glass formed into the plate shape is cooled to a temperature of (Tg−300)° C. or lower with respect to the glass transition temperature Tg (° C.) to obtain a glass base plate; and a heat treatment step in which the obtained glass base plate is heated from the temperature of (Tg−300)° C. or lower to a temperature in a range of from (Tg−100)° C. to (Tg+50)° C., without being heated to a temperature exceeding (Tg+50)° C., and is then cooled again to (Tg−300)° C. or lower.

The heat treatment step is conducted one, two, or three or more times.

Each heat treatment step extends until the temperature of the glass base plate exceeds (Tg−300)° C., thereafter reaches a maximum temperature Temax (° C.) in a range of from (Tg−100)° C. to (Tg+50)° C., and then declines again to (Tg−300)° C. or lower.

A total time period in which the temperature of the glass base plate is in the range of from (Tg−100)° C. to (Tg+50)° C. in the whole heat treatment step(s) is K (minutes) or longer, the K being represented by the following formula (1) using the maximum temperature Tmax (° C.) of the glass base plate in the whole heat treatment step(s).

Each heat treatment step satisfies the following formula (2), where t1 (minutes) is a time period in the step from a time when the temperature of the glass base plate lastly begins to decline from the maximum temperature Temax (° C.) to a time when the temperature of the glass base plate lastly passes (Tg−110)° C.

K=[{(Tg+50)−Tmax}/10]+15  Formula (1)

{Temax−(Tg−110)}/t1≤10  Formula (2)

Thus, the glass plate described above under <Glass Plate> is obtained.

(Melting/forming Step)

In the melting/forming step, which is a step for melting raw materials for glass to obtain a molten glass and forming the molten glass into a plate shape, conventionally known methods can be used without particular limitations. One example of the step is shown below.

Raw materials for glass are prepared so as to result in a desired composition of the glass plate. The raw materials are continuously introduced into a melting furnace and heated to preferably about 1,450 to 1,750° C. to obtain a molten glass.

Usable as the raw materials are oxides, carbonates, nitrates, sulfates, hydroxides, halides such as chlorides, etc. In the case where the melting and refining steps include a step in which the molten glass comes into contact with platinum, fine platinum particles are sometimes released into the molten glass and undesirably come as a foreign substance into the glass plate being obtained. Use of raw-material nitrates has the effect of inhibiting the inclusion of platinum as a foreign substance.

Usable as the nitrates are strontium nitrate, barium nitrate, magnesium nitrate, calcium nitrate, etc. It is more preferred to use strontium nitrate. With respect to the particle size of the raw materials, use can suitably be made of raw materials ranging from a raw material composed of particles which have a large particle diameter of several hundred micrometers but do not remain undissolved to a raw material composed of particles which have a small particle diameter of about several micrometers and which neither fly off when conveyed nor aggregate to form secondary particles. It is also possible to use granules. The water content of each raw material can be suitably regulated in order to prevent the raw material from flying off. The melting conditions regarding β-OH value and the degree of oxidation-reduction of Fe (redox [Fe²⁺/(Fe²⁺+Fe³⁺)]) can be suitably regulated.

Next, a refining step for removing bubbles from the obtained molten glass may be conducted. In the refining step, a method of degassing by depressurization may be used, or degassing may be conducted by heating the molten glass to a temperature higher than the temperature used for melting the raw materials. SO₃ or SnO₂ may be used as a refining agent.

Preferred SO₃ sources are sulfates of at least one element selected from Al, Li, Na, K, Mg, Ca, Sr, and Ba. More preferred are sulfates of alkali metals. Of these, Na₂SO₄ is especially preferred because this sulfate is highly effective in enlarging bubbles and shows satisfactory initial solubility. Also sulfates of alkaline-earth metals may be used. Of these, CaSO₄.2H₂O, SrSO₄, and BaSO₄ are more preferred because these sulfates are highly effective in enlarging bubbles.

As a refining agent in the method of degassing by depressurization, it is preferred to use a halogen such as Cl or F.

Preferred Cl sources are chlorides of at least one element selected from Al, Mg, Ca, Sr, and Ba. More preferred are chlorides of alkaline-earth metals. Of these, SrCl₂.6H₂O and BaCl₂.2H₂O are especially preferred because these chlorides are highly effective in enlarging bubbles and have low deliquescence.

Preferred F sources are fluorides of at least one element selected from Al, Na, K, Mg, Ca, Sr, and Ba. More preferred are fluorides of alkaline-earth metals. Of these, CaF₂ is still more preferred because this fluoride is highly effective in enhancing the meltability of raw materials for glass.

Tin compounds represented by SnO₂ evolve O₂ gas in glass melts. In glass melts, SnO₂ is reduced to SnO at temperatures not lower than 1,450° C. to evolve O₂ gas and thereby function to grow the bubbles. In producing glass plates, raw materials for glass are melted by heating to about 1,450 to 1,750° C. and, hence, the bubbles in the glass melt are more effectively enlarged.

Next, a forming step is conducted in which the molten glass, preferably the molten glass from which bubbles have been removed in the refining step, is formed into a plate shape to obtain a glass ribbon.

In the forming step, use can be made of a known method for forming a glass into a plate shape, such as a float process in which a molten glass is poured onto a molten metal, e.g., tin, and thereby formed into a plate shape to obtain a glass ribbon, an overflow downdraw process (fusion process) in which a molten glass is caused to flow downward from a trough member, or a slit downdraw process in which a molten glass is caused to flow down through a slit.

The molten glass may be subjected as such to the subsequent cooling step without being formed into a plate shape.

(Cooling Step)

The molten glass obtained in the forming step is cooled to a temperature of (Tg−300)° C. or lower with respect to the glass transition temperature Tg (° C.) to obtain a glass base plate. For this cooling, any desired average cooling rate can be employed without particular limitations thereon. For example, from the standpoint of preventing the glass from devitrifying, the average cooling rate is preferably 10° C./min or higher, more preferably 40° C./min or higher. Meanwhile, from the standpoint of preventing the glass from coming to have a strain therein in the cooling step, the average cooling rate is preferably 1,000° C./min or less, more preferably 100° C./min or less. An average cooling rate from (Tg+50)° C. to (Tg−100)° C. is preferably higher than 10° C./min, more preferably 15° C./min or higher.

The term “average cooling rate”, in the case where the center and edge portions of the glass plate differ in cooling rate, means an average value determined from refractive indexes. The cooling rates of the center and the edge portions can be determined by measuring the respective refractive indexes.

(Heat Treatment Step)

After the cooling step, a heat treatment step is conducted in which the obtained glass base plate is heated from the temperature of (Tg−300) ° C. or lower to a temperature in the range of from (Tg−100)° C. to (Tg+50)° C., without being heated to a temperature exceeding (Tg+50)° C., and is then cooled again to (Tg−300)° C. or lower. This heat treatment step may be conducted only once or may be conducted two or three or more times.

Each heat treatment step extends until the temperature of the glass base plate exceeds (Tg−300)° C., thereafter reaches a maximum temperature Temax (° C.) in a range of from (Tg−100)° C. to (Tg+50)° C., and then declines again to (Tg−300)° C. or lower.

In each heat treatment step, the heating rate from above the (Tg−300)° C. to the range of from (Tg−100)° C. to (Tg+50)° C. is not particularly limited. Before the temperature of the glass base plate reaches the range of from (Tg−100)° C. to (Tg+50)° C., the glass base plate may be repeatedly heated and cooled or may be held at a certain temperature.

The total time period in which the temperature of the glass base plate is in the range of from (Tg−100)° C. to (Tg+50)° C. in the whole heat treatment step(s) is K (minutes) or longer, the K being represented by the following formula (1) using the maximum temperature Tmax (° C.) of the glass base plate in the whole heat treatment step(s).

K=[{(Tg+50)−Tmax}/10]+15  Formula (1)

In the case where the heat treatment step is conducted two or more times, the total time period in which the temperature of the glass base plate is in the range of from (Tg−100)° C. to (Tg+50)° C. is the sum of the time period in the first heat treatment step when the temperature of the glass base plate is in the range of from (Tg−100)° C. to (Tg+50)° C. and the time period in the second and any succeeding heat treatment steps when the temperature of the glass base plate is in the range. In the case where the total time period in which the temperature of the glass base plate is in the range of from (Tg−100)° C. to (Tg+50)° C. is K (minutes) or longer, an improvement in radio-wave transparency is attained.

The total time period in which the temperature of the glass base plate is in the range of from (Tg−100)° C. to (Tg+50)° C. is preferably (K+5) minutes or longer, more preferably (K+10) minutes or longer. Although there is no particular upper limit thereon, that total time period is preferably (K+60) minutes or shorter, more preferably (K+45) minutes or shorter, still more preferably (K+30) minutes or shorter, from the standpoint of heightening the production efficiency.

So long as the total time period in which the temperature of the glass base plate is in the range of from (Tg−100)° C. to (Tg+50)° C. is K (minutes) or longer, the glass base plate is not particularly limited in temperature profile. That is, the glass base plate may be repeatedly heated and cooled in one heat treatment step so that, for example, the glass base plate is heated to a temperature in the range of from (Tg−100)° C. to (Tg+50)° C., subsequently cooled to a temperature higher than (Tg−300)° C. but lower than (Tg+50)° C., and then heated again to a temperature in the range of from (Tg−100)° C. to (Tg+50)° C. Moreover, the glass base plate may be held at a certain temperature.

Although the lower limit of that temperature range is (Tg−100)° C. from the standpoint of improving the radio-wave transparency, the lower limit is preferably (Tg−90)° C. or higher, more preferably (Tg−80)° C. or higher. Furthermore, although the upper limit of that temperature range is (Tg+50)° C. from the standpoint of preventing the glass from deforming, the upper limit is preferably (Tg+40)° C. or lower, more preferably (Tg+35)° C. or lower.

Each heat treatment step satisfies the following formula (2), where t1 (minutes) is a time period in each heat treatment step from a time when the temperature of the glass base plate lastly begins to decline from the maximum temperature Temax (° C.) to a time when the temperature of the glass base plate lastly passes (Tg−110)° C. In the case where the heat treatment step is conducted only once, the Temax (° C.) is the same as the Tmax (° C.). In the case where the heat treatment step is conducted two or more times, the highest temperature of the two or more values of Temax (° C.) is the Tmax (° C.).

{Temax−(Tg−110)}/t1≤10  Formula (2)

The glass base plate can have various temperature profiles, examples of which include: the aforementioned case where the glass base plate is heated to a temperature in the range of from (Tg−100)° C. to (Tg+50)° C., subsequently cooled to a temperature higher than (Tg−300)° C. but lower than (Tg+50)° C., and then heated again to a temperature in the range of from (Tg−100)° C. to (Tg+50)° C.; the case where the glass base plate is heated and cooled in the temperature range of from (Tg−100)° C. to (Tg+50)° C. and thereby undergoes temperature changes; and the case where the glass base plate is held at a certain temperature in the temperature range of from (Tg−100)° C. to (Tg+50)° C.

In such various temperature profiles, the expression “the time when the temperature of the glass base plate lastly begins to decline from the maximum temperature Temax (° C.)”, in the case where the temperature of the glass base plate reaches the maximum temperature Temax (° C.) only once, means this time. In the case where the temperature of the glass base plate reaches the maximum temperature Temax (° C.) two or more times, that expression means the last of the times of reaching the Temax (° C.). Furthermore, in the case where the glass base plate is held for a certain time period at the maximum temperature Temax (C), that expression means the last time when the holding ends.

The same applies also to the time when the temperature of the glass base plate lastly passes (Tg−110)° C. That is, after the time when the temperature of the glass base plate has lastly begun to decline from the maximum temperature Temax (° C.) and before the temperature thereof declines to (Tg−300)° C. or lower, the glass base plate may be repeatedly heated and cooled or may be held at a certain temperature. In the case where the temperature of the glass base plate passes (Tg−110)° C. only once before declining to (Tg−300)° C. or lower, the expression “the time when the temperature of the glass base plate lastly passes (Tg−110)° C.” means that time when the temperature of the glass base plate passes (Tg−110)° C. In the case where the temperature of the glass base plate passes (Tg−110)° C. at one time, thereafter rises again to above (Tg−110)° C., and then declines again, that expression means the time when the temperature of the glass base plate passes (Tg−110)° C. during the final cooling. Moreover, in the case where the glass base plate is held at (Tg−110)° C. for a certain time period, that expression means the last time when the holding ends.

Temperature profiles in the heat treatment step other than those described above are not particularly limited.

In each heat treatment step, it is preferable from the standpoint of shortening the production steps that the temperature of the glass base plate which has first become lower than (Tg−110)° C. after having declined from the maximum temperature Temax (° C.) does not exceed (Tg−110)° C. again.

It is also preferable that in each heat treatment step, if any two times in the step which lie between the time when the temperature of the glass base plate lastly begins to decline form the maximum temperature Temax (° C.) and the time when the temperature of the glass base plate lastly passes (Tg−110)° C. are expressed by t2 (minutes) and t3 (minutes) and if t2<t3, then the t2 and the t3 have a difference in time therebetween of 1 minute or more, and if the temperature of the glass base plate at the t2 is expressed by Te2 and the temperature of the glass base plate at the t3 is expressed by Te3, then the Te2 and the Te3 satisfy the following formula (3).

(Te2−Te3)/(t3−t2)≤10  Formula (3)

Formula (3) indicates that the cooling rate at the time t1 is not too high. In the case where the relationship of formula (3) is satisfied, a sufficient heat treatment period can be ensured. Such heat treatment step is hence preferred.

The value represented by (Te2−Te3)/(t3−t2) is more preferably 9 or smaller, still more preferably 8 or smaller. Although there is no particular lower limit thereon, that value may be 0.1 or larger from the standpoint of preventing the glass plate production from requiring too long a period.

In a final heat treatment step, if an average cooling rate of the center of the glass base plate and an average cooling rate of an edge portion of the glass base plate in cooling from the maximum temperature Temax (° C.) to (Tg−300)° C. or lower are respectively expressed by VC (° C./min) and VE (° C./min), then the ratio represented by VC/VE is preferably as close to 1 as possible, because such values of the ratio make it possible to obtain a homogeneous glass plate. Specifically, that ratio is preferably 0.8 or larger, more preferably 0.9 or larger, and is preferably 1.2 or less, more preferably 1.1 or less, most preferably 1.

The glass base plate which has been cooled to (Tg−300)° C. or lower in the heat treatment step is successively cooled to room temperature (e.g., 50° C. or lower), thereby obtaining the glass plate according to this embodiment.

Conditions for the cooling to room temperature are not particularly limited. For example, the average cooling rate from (Tg−300)° C. to 50° C. is preferably 0.5° C./min or higher and is preferably 50° C./min or less. Meanwhile, natural cooling may be conducted without performing temperature control.

The process for glass plate production is not limited to the embodiment described above, and modifications, improvements, etc. within ranges where the object of the present invention is achievable are included in the present invention.

For example, in producing a glass plate, a molten glass may be formed into a plate shape using a press forming method in which the glass is directly formed into a plate shape. After the glass plate has been obtained, this glass plate can be subjected to any desired treatments, processing, etc., such as, for example, strengthening by air chilling, chemical strengthening, and polishing.

In the melting of raw materials for glass and in refining, use may be made of not only a melting tank made of a refractory but also a crucible (hereinafter referred to as “platinum crucible”) made of platinum or an alloy including platinum as a main component, as a melting tank and/or a refining tank.

In a melting step in the case of using a platinum crucible, raw materials are prepared so as to result in the composition of a glass plate to be obtained, and the platinum crucible containing the raw materials is heated in an electric furnace preferably to about 1,450° C. to 1,700° C. A platinum stirrer is inserted thereinto to stir the contents for from 1 to 3 hours, thereby obtaining a molten glass.

In a forming step in steps for glass plate production using the platinum crucible, the molten glass may be poured, for example, onto a carbon plate or into a mold to form the molten glass into a plate shape or a block shape.

By using the thus-obtained glass plate as the substrate of a high-frequency device, the propagation loss of high-frequency signals can be reduced to improve the properties, e.g., quality and intensity, of the high-frequency signals. Because of this, substrates constituted of this glass plate are suitable for use in high-frequency devices in which high-frequency signals of 3.0 GHz or higher are handled. The substrates are suitable also for use in high-frequency devices in which signals in various high-frequency bands are handled, such as signals having frequencies of 3.5 GHz or higher, 10 GHz or higher, 30 GHz or higher, 35 GHz or higher, etc.

The high-frequency devices are not particularly limited. Examples thereof include high-frequency devices (electronic devices) such as semiconductor devices for use in communication appliances such as portable telephones, smartphones, portable digital assistants, and Wi-Fi appliances, surface acoustic wave (SAW) devices, radar components such as radar transceivers, and antenna components such as liquid-crystal antennas.

Besides being suitable for those applications, this glass plate is suitable also for use as window materials for vehicles, e.g., motor vehicles, and buildings. This is because there are cases where those high-frequency devices in which high-frequency signals are handled are disposed in vehicles like millimeter-wave radars. Furthermore, there are frequently cases where high-frequency devices are used in buildings like communication appliances and base stations. Because of this, the glass plate is exceedingly useful in reducing the propagation loss of high-frequency signals in the window materials.

In the case where the glass plate is to be used as a window material, this glass plate, besides being a glass plate formed into a flat-plate shape by, for example, a float process or a fusion process, may be a bent glass plate obtained by forming the flat-plate-shaped glass plate into a curved shape by gravitational forming, press forming, etc. The glass plate can be deformed at will before use in accordance with where the glass plate is to be disposed.

The glass constituting the glass plate is not particularly limited to soda-lime glass, aluminosilicate glass, alkali-free glass, etc., and an appropriate glass can be selected in accordance with applications. Furthermore, the glass may be a strengthened glass having a compression stress layer in the glass surfaces and a tensile stress layer in an inner portion of the glass. As the strengthened glass, use can be made of either a chemically strengthened glass or a glass strengthened by air chilling (physically strengthened glass).

EXAMPLES

The present invention is described in greater detail below by referring to Examples, but the present invention is not limited to these Examples.

Examples 1 to 4

Raw material for glass were put in a platinum crucible so as to result in the composition which is Composition 1 shown in Table 1, and were melted by heating at 1,650° C. for 3 hours in an electric furnace, thereby obtaining a molten glass. In the melting, a platinum stirrer was inserted into the platinum crucible and the contents were stirred therewith for 1 hour to homogenize the glass. The molten glass was poured onto a carbon plate and thereby formed into a plate shape (melting/forming step).

Thereafter, the plate-shaped molten glass was introduced into an electric furnace having a temperature of about (Tg+50)° C. and the temperature was maintained for 1 hour. Thereafter, the glass was cooled to room temperature ° C. at an average cooling rate of 1° C./min to obtain glass base plates (cooling step).

Subsequently, the glass base plates were heated to 630° C. at 10° C./min and held at 630° C. for the time periods shown in Table 2 in the row “Holding period (min)”. Thereafter, the glass base plates were cooled in the electric furnace from 630° C. to (Tg−300)° C. at the average cooling rates shown in Table 2 and then allowed to cool naturally to room temperature, thereby obtaining glass plates (heat treatment step). Examples 1 to 3 are Examples according to the present invention, and Example 4 is Comparative Example.

The obtained glass plates were examined for the following properties by the methods shown below.

The results thereof are shown in Tables 1 and 2 together with the composition. In Table 1, each blank in the composition indicates that the raw material had not been added on purpose, and each of the blanks regarding the properties indicates that the property was not determined.

[Glass Transition Temperature Tg (° C.)]

Glass transition temperature was measured with a thermodilatometer (Model TD5000SA, manufactured by MAC Corp.) in accordance with JIS R 3103-3:2001.

[T2, T4 (° C.)]

Each glass was examined for viscosity to calculate T2 and T4. Specifically, a rotary high-temperature viscometer (RVM-550, manufactured by OPT Corp.) was used to determine the viscosity of the glass in accordance with ASTM C965-96 (2002). MIST717a was used as a reference to correct the viscosity of the glass, and the T2 and the T4 were calculated therefrom.

[Devitrification Temperature]

Devitrification temperature was determined by placing particles of a crushed glass on a dish made of platinum, heat-treating the glass particles for 17 hours in electric furnaces having constant temperatures, examining the heat-treated sample with an optical microscope (Model ME600, manufactured by Nikon Corp.) to determine a highest temperature which had resulted in crystal precipitation in the surface and inside of the glass and a lowest temperature which had not resulted in crystal precipitation, and taking an average of the highest and the lowest temperatures as the devitrification temperature.

[Young's Modulus]

Young's modulus was measured with an ultrasonic-pulse Young's modulus meter (38DL-PAUS, manufactured by Olympus Co., Ltd.) in accordance with JIS R 1602 (1995).

[Acid Resistance]

Acid resistance was determined by immersing a glass sample in an aqueous acid solution (6 wt % HNO₃+5 wt % H₂SO₄; 45° C.) for 170 seconds and evaluating the amount of glass components extracted per unit surface area (mg/cm²).

[Alkali Resistance]

Alkali resistance was determined by immersing a glass sample in an aqueous alkali solution (1.2 wt % NaOH; 60° C.) for 30 minutes and evaluating the amount of glass components extracted per unit surface area (mg/cm²).

[Average Coefficient of Thermal Expansion]

The coefficient of thermal expansion within the temperature range of from 50 to 350° C. was measured with a TMA (Model TD5000SA, manufactured by MAC Corp.) in accordance with JIS R 3102 (1995). An average of the resultant coefficients of linear expansion at 50-350° C. was determined as the average coefficient of thermal expansion.

[Strain Point]

Strain point was measured in accordance with JIS R 3103-2 (2001).

[Density]

Density was determined in accordance with JIS Z 8807 (2012).

[Dielectric Dissipation Factor Tan δ]

The dielectric dissipation factor tan δA at 10 GHz of an obtained glass plate was measured with a resonator for 10 GHz (resonator for 10 GHz manufactured by OWED Company) by an SPDR method in accordance with IEC 61189-2-721 (2015).

Furthermore, the glass plate was heated to (Tg+50)° C. and cooled to (Tg−150)° C. at 100° C./min, and was then examined for dielectric dissipation factor tan δ100 at 10 GHz in the same manner.

In Table 2, “Δ tan δ” indicates the value of (tan δ100−tan δA).

[Relative Permittivity εr]

The relative permittivity εrA at 10 GHz of an obtained glass plate was measured with a resonator for 10 GHz (resonator for 10 GHz manufactured by OWED Company) by an SPDR method in accordance with IEC 61189-2-721 (2015).

Furthermore, the glass plate was heated to (Tg+50)° C. and cooled to (Tg−50)° C. at 100° C./min, and was then examined for relative permittivity εr100 at 10 GHz in the same manner.

TABLE 1
		Compo-	Compo-	Compo-	Compo-	Compo-	Compo-	Compo-
		sition	sition	sition	sition	sition	sition	sition
		1	2	3	4	5	6	7
Composition	SiO2	83.3	73.9	62.0	61.9	66.1	67.0	67.2
(mol %)	Al2O3	1.4		8.0	8.5	11.3	11.0	6.5
	B2O3	11.3		23.0	21.1	7.8	10.0	19.5
	MgO		0.1	4.0	0.2	5.1	2.0	0.5
	CaO		11.6	2.0	0.2	4.5	9.0	5
	SrO			0.8	7.0	5.2	1.0	0.5
	BaO			0.2	1.1
	Li2O
	Na2O	4.0	7.2					0.8
	K2O	0.05	7.2
Tg (° C.)	580	547	633	625	710	705	650
T2 (° C.)		1437	1626	1656	1645		1745
T4 (° C.)		1030	1231	1243	1275		1296
Devitrification		890	1270	1280	1266
temperature (° C.)
Young's modulus	63		59	59	76	73	58
(GPa)
Acid resistance			0.0027	0.013			0.017
(g/cm−2)
Alkali resistance			0.06	0.031			0.026
(g/cm−2)
Average coefficient	38	101	31	35	38	34	31
of thermal expansion
(10−7/° C.)
Strain point (° C.)	520	513	563	563	670	667
Density (g/cm3)	2.5	2.49	2.3	2.41	2.5	2.4	2.2

TABLE 2
	Composition 1
	Example 1	Example 2	Example 3	Example 4
Tmax (° C.)	630	630	630	630
Temax (° C.)	630	630	630	630
Holding period (min)	1550	210	75	30
Average cooling rate (° C./min)	0.1	1	10	500
εr100	4.49	4.49	4.49	4.49
tanδ100	0.0062	0.0062	0.0062	0.0062
εrA	4.51	4.47	4.46	4.42
tanδA	0.0044	0.0050	0.0056	0.0066
Δtanδ	0.0018	0.0012	0.0006	−0.0004
εr100/εrA	1.00	1.00	1.01	1.02
t1 (min)	1600	160	16	0.32
Value of K in formula (1)	15	15	15	15
Value of left side of formula (2)	0.10	1.0	10.0	500.0

Examples 5 to 8

Glass plates were obtained in the same manner as in Example 1, except that raw materials for glass were used so as to result in the composition shown as Composition 2 in Table 1, that in the subsequent heat treatment step, the glass base plates were heated to 597° C. at 10° C./min and held at 597° C. for the time periods shown in Table 3 in the row “Holding period (min)”, and that the glass base plates were cooled in the electric furnace to (Tg−300)° C. at the average cooling rates shown in Table 3. Examples 5 to 7 are Examples according to the present invention, and Example 8 is Comparative Example.

The obtained glass plates were examined for the properties under the same conditions as in Example 1. The results thereof are shown in Tables 1 and 3 together with the composition.

TABLE 3
	Composition 2
	Example 5	Example 6	Example 7	Example 8
Tmax (° C.)	597	597	597	597
Temax (° C.)	597	597	597	597
Holding period (min)	1550	210	75	30
Average cooling rate (° C./min)	0.1	1	10	500
εr100	6.74	6.74	6.74	6.74
tanδ100	0.0093	0.0093	0.0093	0.0093
εrA	6.63	6.67	6.72	6.76
tanδA	0.0078	0.0083	0.0088	0.0096
Δtanδ	0.0015	0.001	0.0005	−0.0003
εr100/εrA	1.02	1.01	1.00	1.00
t1 (min)	1600	160	16	0.32
Value of K in formula (1)	15	15	15	15
Value of left side of formula (2)	0.10	1.0	10.0	500.0

Examples 9 to 12

Glass plates were obtained in the same manner as in Example 1, except that raw materials for glass were used so as to result in the composition shown as Composition 3 in Table 1, that in the subsequent heat treatment step, the glass base plates were heated to 653° C. at 10° C./min and held at 653° C. for the time periods shown in Table 4 in the row “Holding period (min)”, and that the glass base plates were cooled in the electric furnace to (Tg−300)° C. at the average cooling rates shown in Table 4. Examples 9 to 11 are Examples according to the present invention, and Example 12 is Comparative Example.

The obtained glass plates were examined for the properties under the same conditions as in Example 1. The results thereof are shown in Tables 1 and 4 together with the composition.

TABLE 4
	Composition 3
	Example 9	Example 10	Example 11	Example 12
Tmax (° C.)	653	653	653	653
Temax (° C.)	653	653	653	653
Holding period (min)	1550	210	75	30
Average cooling rate (° C./min)	0.1	1	10	500
εr100	4.36	4.36	4.36	4.36
tanδ100	0.0029	0.0029	0.0029	0.0029
εrA	4.34	4.35	4.36	4.37
tanδA	0.0018	0.0022	0.0025	0.0032
Δtanδ	0.0011	0.0007	0.0004	−0.0003
εr100/εrA	1.00	1.00	1.00	1.00
t1 (min)	1300	130	13	0.26
Value of K in formula (1)	15	15	15	15
Value of left side of formula (2)	0.10	1.0	10.0	500.0

Examples 13 to 16

Glass plates were obtained in the same manner as in Example 1, except that raw materials for glass were used so as to result in the composition shown as Composition 4 in Table 1, that in the subsequent heat treatment step, the glass base plates were heated to 675° C. at 10° C./min and held at 675° C. for the time periods shown in Table 5 in the row “Holding period (min)”, and that the glass base plates were cooled in the electric furnace to (Tg−300)° C. at the average cooling rates shown in Table 5. Examples 13 to 15 are Examples according to the present invention, and Example 16 is Comparative Example.

The obtained glass plates were examined for the properties under the same conditions as in Example 1. The results thereof are shown in Tables 1 and 5 together with the composition.

TABLE 5
	Composition 4
	Example 13	Example 14	Example 15	Example 16
Tmax (° C.)	675	675	675	675
Temax (° C.)	675	675	675	675
Holding period (min)	1550	210	75	30
Average cooling rate (° C./min)	0.1	1	10	500
εr100	4.86	4.86	4.86	4.86
tanδ100	0.0032	0.0032	0.0032	0.0032
εrA	4.83	4.84	4.86	4.88
tanδA	0.0022	0.0025	0.0028	0.0034
Δtanδ	0.0010	0.0007	0.0004	−0.0002
εr100/εrA	1.01	1.00	1.00	1.00
t1 (min)	1600	160	16	0.32
Value of K in formula (1)	15	15	15	15
Value of left side of formula (2)	0.10	1.0	10.0	500.0

Examples 17 to 20

Glass plates were obtained in the same manner as in Example 1, except that raw materials for glass were used so as to result in the composition shown as Composition 5 in Table 1, that in the subsequent heat treatment step, the glass base plates were heated to 760° C. at 10° C./min and held at 760° C. for the time periods shown in Table 6 in the row “Holding period (min)”, and that the glass base plates were cooled in the electric furnace to (Tg−300)° C. at the average cooling rates shown in Table 6. Examples 17 to 19 are Examples according to the present invention, and Example 20 is Comparative Example.

The obtained glass plates were examined for the properties under the same conditions as in Example 1. The results thereof are shown in Tables 1 and 6 together with the composition.

TABLE 6
	Composition 5
	Example 17	Example 18	Example 19	Example 20
Tmax (° C.)	760	760	760	760
Temax (° C.)	760	760	760	760
Holding period (min)	1550	210	75	30
Average cooling rate (° C./min)	0.1	1	10	500
εr100	5.45	5.45	5.45	5.45
tanδ100	0.0063	0.0063	0.0063	0.0063
εrA	5.42	5.44	5.44	5.46
tanδA	0.0044	0.005	0.0056	0.0067
Δtanδ	0.0019	0.0013	0.0007	−0.0004
εr100/εrA	1.01	1.00	1.00	1.00
t1 (min)	1600	160	16	0.32
Value of K in formula (1)	15	15	15	15
Value of left side of formula (2)	0.10	1.0	10.0	500.0

Examples 21 to 24

Glass plates were obtained in the same manner as in Example 1, except that raw materials for glass were used so as to result in the composition shown as Composition 6 in Table 1, that in the subsequent heat treatment step, the glass base plates were heated to 760° C. at 10° C./min and held at 760° C. for the time periods shown in Table 7 in the row “Holding period (min)”, and that the glass base plates were cooled in the electric furnace to (Tg−300)° C. at the average cooling rates shown in Table 7. Examples 21 to 23 are Examples according to the present invention, and Example 24 is Comparative Example.

The obtained glass plates were examined for the properties under the same conditions as in Example 1. The results thereof are shown in Tables 1 and 7 together with the composition.

TABLE 7
	Composition 6
	Example 21	Example 22	Example 23	Example 24
Tmax (° C.)	755	755	755	755
Temax (° C.)	755	755	755	755
Holding period (min)	1550	210	75	30
Average cooling rate (° C./min)	0.1	1	10	500
εr100	5.14	5.14	5.14	5.14
tanδ100	0.0053	0.0053	0.0053	0.0053
εrA	5.18	5.17	5.16	5.12
tanδA	0.0033	0.0040	0.0047	0.0058
Δtanδ	0.0020	0.0013	0.0006	−0.0005
εr100/εrA	0.99	0.99	1.00	1.00
t1 (min)	8650	865	86.5	1.73
Value of K in formula (1)	15	15	15	15
Value of left side of formula (2)	0.10	1.00	10.00	500.00

Examples 25 to 28

Glass plates were obtained in the same manner as in Example 1, except that raw materials for glass were used so as to result in the composition shown as Composition 7 in Table 1, that in the subsequent heat treatment step, the glass base plates were heated to 700° C. at 10° C./min and held at 700° C. for the time periods shown in Table 8 in the row “Holding period (min)”, and that the glass base plates were cooled in the electric furnace to (Tg−300)° C. at the average cooling rates shown in Table 8. Examples 25 to 27 are Examples according to the present invention, and Example 28 is Comparative Example.

The obtained glass plates were examined for the properties under the same conditions as in Example 1. The results thereof are shown in Tables 1 and 8 together with the composition.

TABLE 8
	Composition 7
	Example 25	Example 26	Example 27	Example 28
Tmax (° C.)	700	700	700	700
Temax (° C.)	700	700	700	700
Holding period (min)	1550	210	75	30
Average cooling rate (° C./min)	0.1	1	10	500
εr100	4.54	4.54	4.54	4.54
tanδ100	0.0051	0.0051	0.0051	0.0051
εrA	4.51	4.52	4.53	4.55
tanδA	0.0032	0.0038	0.0044	0.0055
Δtanδ	0.0019	0.0013	0.0007	−0.0004
εr100/εrA	1.01	1.00	1.00	1.00
t1 (min)	8100	810	81	1.62
Value of K in formula (1)	15	15	15	15
Value of left side of formula (2)	0.10	1.00	10.00	500.00

Examples 29 to 31

Raw materials for glass were used so as to result in the composition shown as Composition 4 in Table 1, and a plate-shaped molten glass was obtained using a glass melting tank and a forming apparatus (melting/forming step).

Thereafter, the plate-shaped molten glass which had been 700° C. was cooled to room temperature with an annealing apparatus at an average cooling rate of 50° C./min to obtain glass base plates having a size of 37 cm×47 cm and a thickness of 1.1 mm (cooling step).

Subsequently, glass plates were obtained in the same manner as in Example 1, except that the glass base plates were heated to the temperatures Tmax (° C.) shown in Table 9 at 10° C./min, held at the temperatures Tmax (° C.) for the time periods shown in Table 9 in the row “Holding period (min)”, and cooled in the electric furnace to (Tg−300)° C. at the average cooling rates shown in Table 9. Examples 29 to 31 are Examples according to the present invention.

The temperature of the electric furnace was regulated so that if the average cooling rate of the center of each glass base plate and the average cooling rate of an edge portion thereof were respectively expressed by VC (° C./min) and VE (° C./min), then the ratio represented by VC/VE was 1.1 or less. The term “edge portion of the glass base plate” means a position which is at a distance of 10 cm from an edge of the glass base plate.

The obtained glass plates were examined for relative permittivity and dielectric dissipation factor under the same conditions as in Example 1. An approximately central portion of each plate and four portions near the corners were thus examined and maximum values and minimum values of those properties were recorded. The results thereof are shown in Table 9.

Furthermore, minimum values of Δ tan δ and maximum values of εr100/εrA are shown in Table 9. Table 9 further shows: the difference in tan δA between two portions which differed most in tan δA among the approximately central portion of the plate and the four portions near the corners; and the difference in εrA between two portions which differed most in εrA among those five portions.

TABLE 9
	Composition 4
	Example 29	Example 30	Example 31
Tmax (° C.)	675	675	625
Temax (° C.)	675	675	625
Holding period (min)	15	10	30
Average cooling rate (° C./min)	1	10	10
Maximum vallue of εr100	4.86	4.87	4.87
Minimum value of εr100	4.85	4.85	4.85
Maximum value of tanδ100	0.0033	0.0033	0.0033
Minimum value of tanδ100	0.0032	0.0032	0.0032
Maximum value of εrA	4.85	4.86	4.86
Minimum value of εrA	4.84	4.85	4.85
Maximum value of tanδA	0.0026	0.0028	0.0028
Minimum value of tanδA	0.0024	0.0027	0.0026
Minimum value of Δtanδ	0.0006	0.0004	0.0004
Maximum value of εr100/εrA	1.00	1.00	1.00
Maximum value of difference in	0.0002	0.0001	0.0002
tanδA between any two portions
Maximum value of difference in	0.01	0.01	0.01
εrA between any two portions
t1 (min)	785	78.5	73.5
Value of K in formula (1)	15	15	20
Value of left side of formula (2)	1.0	10.0	10.0

Examples 32 to 34

Raw materials for glass were used so as to result in the composition shown as Composition 6 in Table 1, and a plate-shaped molten glass was obtained using a glass melting tank and a forming apparatus (melting/forming step).

Thereafter, the plate-shaped molten glass which had been 800° C. was cooled to room temperature with an annealing apparatus at an average cooling rate of 800° C./min to obtain glass base plates having a size of 37 cm×47 cm and a thickness of 1.1 mm (cooling step).

Subsequently, glass plates were obtained in the same manner as in Example 1, except that the glass base plates were heated to the temperatures Tmax (° C.) shown in Table 10 at 10° C./min, held at the temperatures Tmax (° C.) for the time periods shown in Table 10 in the row “Holding period (min)”, and cooled in the electric furnace to (Tg−300)° C. at the average cooling rates shown in Table 10. Examples 32 to 34 are Examples according to the present invention.

The temperature of the electric furnace was regulated so that if the average cooling rate of the center of each glass base plate and the average cooling rate of an edge portion thereof were respectively expressed by VC (° C./min) and VE (° C./min), then the ratio represented by VC/VE was 1.1 or less. The term “edge portion of the glass base plate” means a position which is at a distance of 10 cm from an edge of the glass base plate.

The obtained glass plates were examined for relative permittivity and dielectric dissipation factor under the same conditions as in Example 1. An approximately central portion of each plate and four portions near the corners were thus examined and maximum values and minimum values of those properties were recorded. The results thereof are shown in Table 10.

Furthermore, minimum values of Δ tan δ and maximum values of εr100/εrA are shown in Table 10. Table 10 further shows: the difference in tan δA between two portions which differed most in tan δA among the approximately central portion of the plate and the four portions near the corners; and the difference in εrA between two portions which differed most in εrA among those five portions.

TABLE 10
	Composition 6
	Example 32	Example 33	Example 34
Tmax (° C.)	625	725	695
Temax (° C.)	625	725	695
Holding period (min)	20	10	30
Average cooling rate (° C./min)	1	10	10
Maximum value of εr100	5.13	5.13	5.13
Minimum value of εr100	5.12	5.12	5.12
Maximum value of tanδ100	0.0057	0.0057	0.0057
Minimum value of tanδ100	0.0056	0.0056	0.0056
Maximum value of εrA	5.16	5.17	5.16
Minimum value of εrA	5.15	5.15	5.15
Maximum value of tanδA	0.0048	0.0050	0.0051
Minimum value of tanδA	0.0047	0.0048	0.0050
Minimum value of Δtanδ	0.0008	0.0006	0.0005
Maximum value of εr100/εrA	1.00	1.00	1.00
Maximum value of difference in	0.0001	0.0002	0.0001
tanδA between any two portions
Maximum value of difference in	0.01	0.02	0.01
εrA between any two portions
t1 (min)	735.0	83.5	80.5
Value of K in formula (1)	28	18	21
Value of left side of formula (2)	1.00	10.00	10.00

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. This application is based on a Japanese patent application filed on Apr. 12, 2019 (Application No. 2019-76423), a Japanese patent application filed on Jun. 28, 2019 (Application No. 2019-120828), and a Japanese patent application filed on Nov. 27, 2019 (Application No. 2019-214690), the entire contents thereof being incorporated herein by reference. All the references cited here are incorporated herein as a whole.

Claims

1. A glass plate having a dielectric dissipation factor at 10 GHz of tan δA and a glass transition temperature of Tg° C.,

wherein the glass plate satisfies (tan δ100−tan δA)≥0.0004, where tan δ100 is a dielectric dissipation factor of the glass plate at 10 GHz after having been heated to (Tg+50)° C. and then cooled to (Tg−150)° C. at 100° C./min.

2. The glass plate according to claim 1, having a relative permittivity at 10 GHz of εrA,

wherein the glass plate satisfies 0.95≤(εr100/εrA)≤1.05, where εr100 is a relative permittivity of the glass plate at 10 GHz after having been heated to (Tg+50)° C. and then cooled to (Tg−150)° C. at 100° C./min.

3. The glass plate according to claim 1, comprising a principal surface with an area of 350 cm2 or larger.

4. The glass plate according to claim 1, having the dielectric dissipation factor at 10 GHz of 0.009 or less.

5. The glass plate according to claim 1, having a relative permittivity at 10 GHz of 6.8 or less.

6. The glass plate according to claim 1, wherein any two portions separated from each other by 40 mm or more have a difference in dielectric dissipation factor at 10 GHz of 0.0005 or less.

7. The glass plate according to claim 1, wherein any two portions separated from each other by 40 mm or more have a difference in relative permittivity at 10 GHz of 0.05 or less.

8. The glass plate according to claim 1, comprising from 30 to 85% of SiO2 as represented by mol % based on oxides.

9. The glass plate according to claim 1, comprising, as represented by mol % based on oxides,

SiO2: from 57 to 70%,

Al2O3: from 5 to 15%,

B2O3: from 15 to 24%,

provided that Al2O3+B2O3 is from 20 to 40%, and

Al2O3/(Al2O3+B2O3) is from 0.1 to 0.45,

MgO: from 0 to 10%,

CaO: from 0 to 10%,

SrO: from 0 to 10%,

BaO: from 0 to 10%,

Li2O: from 0 to 5%,

Na2O: from 0 to 5%, and

K2O: from 0 to 5%,

provided that R2O (R is alkali metal): from 0 to 5%.

10. The glass plate according to claim 1, comprising, as represented by mol % based on oxides,

SiO2: from 55 to 80%,

Al2O3: from 0 to 15%,

provided that SiO2+Al2O3 is from 55 to 90%,

B2O3: from 0 to 15%,

MgO: from 0 to 20%,

CaO: from 0 to 20%,

SrO: from 0 to 15%,

BaO: from 0 to 15%,

provided that MgO+CaO is from 0 to 30%, and

MgO+CaO+SrO+BaO: from 0 to 30%,

Li2O: from 0 to 20%,

Na2O: from 0 to 20%, and

K2O: from 0 to 20%,

provided that R2O (R is alkali metal) is from 0 to 20%.

11. The glass plate according to claim 1 which is for use as a substrate for a high-frequency device in which high-frequency signals having a frequency of 3.0 GHz or higher are handled.

12. The glass plate according to claim 1 which is for use as a window material.

13. A process for producing the glass plate according to claim 1, comprising, in the following order,

a melting/forming step of melting raw materials for glass to obtain a molten glass and forming the molten glass into a plate shape,

a cooling step of cooling the molten glass formed into the plate shape to a temperature of (Tg−300)° C. or lower with respect to the glass transition temperature Tg (° C.) to obtain a glass base plate, and

a heat treatment step of heating the obtained glass base plate from the temperature of (Tg−300)° C. or lower to a temperature in a range of from (Tg−100)° C. to (Tg+50)° C., without being heated to a temperature exceeding (Tg+50)° C., and then cooling the glass base plate again to (Tg−300)° C. or lower,

wherein the heat treatment step is conducted one or two or more times,

each heat treatment step extends until the temperature of the glass base plate exceeds (Tg−300)° C., thereafter reaches a maximum temperature Temax (° C.) in the range of from (Tg−100)° C. to (Tg+50)° C., and then declines again to (Tg−300)° C. or lower,

a total time period in which the temperature of the glass base plate is in the range of from (Tg−100)° C. to (Tg+50)° C. in a whole heat treatment step(s) is K (minutes) or longer, the K being represented by the following formula (1) using the maximum temperature Tmax (° C.) of the glass base plate in the whole heat treatment step(s), and

each heat treatment step satisfies the following formula (2), where t1 (minutes) is a time period in each heat treatment step from a time when the temperature of the glass base plate lastly begins to decline from the maximum temperature Temax (° C.) to a time when the temperature of the glass base plate lastly passes (Tg−110)° C.: K=[{(Tg+50)−Tmax}/10]+15  Formula (1) {Temax−(Tg−110)}/t1≤10  Formula (2).

14. The process for glass plate production according to claim 13, wherein in each heat treatment step, the temperature of the glass base plate which has first become lower than (Tg−110)° C. after having declined from the maximum temperature Temax (° C.) does not exceed (Tg−110)° C. again.

15. The process for glass plate production according to claim 13, wherein in each heat treatment step, if any two times in the step which lie between the time when the temperature of the glass base plate lastly begins to decline from the maximum temperature Temax (° C.) and the time when the temperature of the glass base plate lastly passes (Tg−110)° C. are expressed by t2 (minutes) and t3 (minutes) and if t2<t3, then

t2 and the t3 have a difference in time therebetween of 1 minute or more, and

if the temperature of the glass base plate at t2 is expressed by Te2 and the temperature of the glass base plate at t3 is expressed by Te3, then Te2 and Te3 satisfy the following formula (3): (Te2−Te3)/(t3−t2)≤10  Formula (3).

16. The process for glass plate production according to claim 13, wherein in the cooling step, the molten glass is cooled from (Tg+50)° C. to (Tg−100)° C. at an average cooling rate exceeding 10° C./min.

17. The process for glass plate production according to claim 13, wherein in the cooling step, the molten glass is cooled at an average cooling rate of from 10 to 1,000° C./min.