Borosilicate glass with modified surface layer

Abstract

A glass substrate includes an alkali metal-containing host and an alkali metal depleted surface layer. The alkali metal depleted surface layer is amorphous and includes a substantially uniform composition. The alkali metal-containing host and the alkali metal depleted surface layer include B2O3 and SiO2. A method of forming a glass substrate having a modified surface layer includes providing a glass substrate having a concentration of an alkali metal, a glass transition temperature (Tg), and a surface layer. And the glass substrate further comprises B2O3 (boron trioxide) and SiO2 (silicon dioxide). The method further includes reducing the concentration of the alkali metal in the surface layer such that the surface layer with reduced alkali metal concentration includes a substantially uniform composition.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 428,767, filed on November 30, 2022, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to glass substrates having a modified surface layer, and more particularly, to glass substrates having an alkali - metal - containing bulk and an alkali - metal - depleted surface layer. Background Art

[0004] Glass (e.g., including glass prepared by melting) formed or treated using known surface treatments typically includes a surface layer that is at least partially crystalline or includes a crystalline portion, or may exhibit phase separation (i.e., non - uniform composition). In other known methods for modifying the surface layer (e.g., leaching or wet chemical treatment), the resulting surface layer contains hydrogen, which may be present in the form of H + , H3O + , H2O, or a combination thereof.

[0005] Thermal poling has been used to alter the properties of glass. Thermal poling generally involves applying a voltage to the glass. Known uses of thermal poling include forming a depletion layer that inhibits alkali - metal migration in photovoltaic glass, forming an interfacial barrier layer between a display (or alkali - metal - free) glass and silicon, and forming surface textures or performing selective - area ion exchange with patterned electrodes.

[0006] Thermal poling has also been used to induce second - order nonlinear properties, particularly for generating second - order nonlinear optical properties for optical switches and devices. The poling method is also very similar to so - called anodic bonding, which is used to bond alkali - metal - containing or alkali - metal - free glass to other materials, particularly semiconductors.

[0007] The present disclosure provides glass substrates having various compositions of the borosilicate and aluminoborosilicate families and a surface layer with an altered composition and atomic structure. In an embodiment, the alkali - metal concentration in the surface layer is reduced while the bulk of the glass substrate contains alkali metals. The surface layer includes an atomic structure in which substantially all boron is in a 3 - coordinate state, while the bulk has an atomic structure in which most boron is in a 4 - coordinate state. The composition and atomic structure of the surface layer enable the glass substrate to have various surface properties and performance attributes. For example, the surface layer can be used to improve the corrosion resistance, diffusion barrier, hardness, elastic modulus, fatigue resistance, and damage resistance (e.g., anomalous deformation) of the glass substrate. Summary of the Invention

[0008] According to aspect (1), a glass substrate is provided. The glass substrate includes: an alkali metal-containing body; and an alkali metal-depleted surface layer, wherein the alkali metal-depleted surface layer is amorphous and includes a substantially uniform composition, and wherein the alkali metal-containing body and the alkali metal-depleted surface layer include B2O3 and SiO2.

[0009] According to aspect (2), the glass substrate of aspect (1) is provided, wherein the alkali metal-depleted surface layer includes about 0.5 atomic % or less of alkali metal.

[0010] According to aspect (3), the glass substrate of aspect (1) or aspect (2) is provided, wherein the alkali metal-depleted surface layer includes an atomic structure containing boron substantially in a 3-coordinated state.

[0011] According to aspect (4), the glass substrate of aspect (3) is provided, wherein more than about 60% of the total amount of boron in the alkali metal-depleted surface layer is in a 3-coordinated state in terms of fraction.

[0012] According to aspect (5), the glass substrate of aspect (3) is provided, wherein more than about 70% of the total amount of boron in the alkali metal-depleted surface layer is in a 3-coordinated state in terms of fraction.

[0013] According to aspect (6), the glass substrate of aspect (3) is provided, wherein more than about 75% of the total amount of boron in the alkali metal-depleted surface layer is in a 3-coordinated state in terms of fraction.

[0014] According to aspect (7), the glass substrate of any one of aspects (1) to (6) is provided, wherein the alkali metal-containing body includes an atomic structure containing boron in a 3-coordinated state and boron in a 4-coordinated state.

[0015] According to aspect (8), the glass substrate of aspect (7) is provided, wherein more than about 51% of the total amount of boron in the alkali metal-containing body is in a 4-coordinated state in terms of fraction.

[0016] According to aspect (9), the glass substrate of any one of aspects (1) to (8) is provided, wherein the alkali metal-depleted surface layer is substantially free of non-bridging oxygen.

[0017] According to aspect (10), the glass substrate of aspect (9) is provided, wherein the alkali metal-containing body includes non-bridging oxygen and bridging oxygen.

[0018] According to aspect (11), the glass substrate of aspect (10) is provided, wherein the alkali metal-containing body is substantially free of non-bridging oxygen.

[0019] According to aspect (12), the glass substrate of any one of aspects (1) to (11) is provided, wherein the alkali metal-containing body includes an alkali metal oxide selected from Li2O, Na2O, K2O, Rb2O, and Cs2O.

[0020] According to aspect (13), there is provided a glass substrate of aspect (12), wherein the alkali metal-containing bulk comprises at least 1 mol% of Na2O, K2O or Li2O.

[0021] According to aspect (14), there is provided a glass substrate of aspect (12), wherein the alkali metal-containing bulk comprises at least 1 mol% of Na2O.

[0022] According to aspect (15), there is provided a glass substrate of any one of aspects (1) to (14), wherein the alkali metal-depleted surface layer comprises B2O3 in the range of about 10 mol% to about 90 mol%.

[0023] According to aspect (16), there is provided a glass substrate of any one of aspects (1) to (15), wherein the alkali metal-depleted surface layer comprises a binary B2O3-SiO2 composition.

[0024] According to aspect (17), there is provided a glass substrate of any one of aspects (1) to (16), wherein the atomic% of silicon is greater than the atomic% of boron.

[0025] According to aspect (18), there is provided a glass substrate. The glass substrate comprises: a substrate thickness; an alkali metal-containing bulk having a bulk refractive index; and an alkali metal-depleted surface layer having a layer thickness in the range of about 10 nm to about 3000 nm, wherein the alkali metal-depleted surface layer has a layer refractive index less than the bulk refractive index, and wherein the alkali metal-containing bulk and the alkali metal-depleted surface layer comprise B2O3 and SiO2.

[0026] According to aspect (19), there is provided a glass substrate of aspect (18), wherein the atomic% of silicon is greater than the atomic% of boron.

[0027] According to aspect (20), there is provided a method of forming a glass substrate having a modified surface layer. The method comprises: providing a glass substrate comprising a certain concentration of alkali metal, a glass transition temperature (Tg) and a surface layer, the glass substrate comprising B2O3 and SiO2; and reducing the concentration of alkali metal in the surface layer, wherein the surface layer with reduced alkali metal concentration has a substantially uniform composition.

[0028] According to aspect (21), there is provided a method of aspect (20), wherein the atomic% of silicon is greater than the atomic% of boron.

[0029] According to aspect (22), there is provided a method of aspect (20) or aspect (21), wherein reducing the alkali metal concentration in the surface layer comprises contacting the surface of the glass substrate with an electrode; and thermally polarizing the glass substrate.

[0030] According to aspect (23), there is provided the method of aspect (22), wherein the electrode comprises an anode in contact with the anode surface of the glass substrate and a cathode in contact with the cathode surface of the glass substrate, and wherein the thermal poling comprises applying a voltage to the glass substrate such that the anode is at a positive bias relative to the glass substrate to induce alkali metal depletion at the anode surface of the glass.

[0031] According to aspect (24), there is provided the method of aspect (22), wherein the thermal poling comprises heating the glass substrate and the electrode to a temperature below Tg before applying a voltage to the glass substrate.

[0032] According to aspect (25), there is provided the method of aspect (22), wherein the thermal poling comprises applying a voltage in the range of about 100 volts to about 10,000 volts to the glass substrate for a duration in the range of about 1 minute to about 6 hours.

[0033] According to aspect (26), there is provided the method of aspect (22), wherein the glass substrate is thermally poled under vacuum, in an inert gas environment or a permeable gas environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a side view of a glass substrate according to an embodiment;

[0035] Figure 2A is a ternary diagram of the precursor glass of Example 1;

[0036] Figure 2B shows Figure 2A the ternary diagram of, wherein the network-former-only composition formed by thermal poling of the precursor glass of Example 1 is projected onto the B2O3 - SiO2 binary edge of the ternary diagram;

[0037] Figures 3 to 7 shows the secondary ion mass spectrometry (SIMS) depth profile of certain elements through the alkali metal depletion surface layer of the selected glass in Example 1 after thermal poling;

[0038] Figure 8 is a bar chart summarizing the near-edge X-ray absorption fine structure (NEXAFS) spectral results of the selected glass of Example 1 after thermal poling;

[0039] Figure 9 is a set of line graphs reporting the molecular simulation changes in boron coordination state after thermal poling of different alkali metal and alkaline earth metal borosilicate precursor glasses; and

[0040] Figure 10 is a set of line graphs reporting Figure 9 the molecular simulation changes in Young's modulus after thermal poling of different alkali metal and alkaline earth metal borosilicate precursor glasses of Detailed Embodiments

[0041] To facilitate understanding of the principles of the present disclosure, reference will now be made to the embodiments shown in the accompanying drawings and described in the following written specification. It is to be understood that this is not intended to limit the scope of the present disclosure. It is further understood that the present disclosure includes any changes and modifications to the illustrated embodiments and further applications of the principles disclosed herein that would typically occur to one of ordinary skill in the art to which this disclosure pertains.

[0042] As used herein, when used in a list of two or more items, the term "and / or" means that any one of the listed items can be taken alone, or any combination of two or more of the listed items can be taken. For example, if a composition is described as containing components A, B, and / or C, the composition can contain A alone; B alone; C alone; a combination of A and B; a combination of A and C; a combination of B and C; or a combination of A, B, and C.

[0043] In this document, relational terms such as first and second, top and bottom, etc. are used only to distinguish one entity or action from another entity or action, and do not necessarily require or imply any actual such relationship or order between such entities or actions.

[0044] As used herein, the term "about" means that a quantity, size, formulation, parameter, and other quantities and characteristics are not and need not be exact, but can be approximate and / or larger or smaller as needed, thereby reflecting tolerances, conversion factors, rounding, measurement errors, etc. and other factors known to those of ordinary skill in the art. When the term "about" is used to describe a value or range endpoint, the present disclosure should be understood to include the specific value or endpoint recited. Whether or not the numerical value or range endpoint in the specification is recited with "about", the numerical value or range endpoint is intended to include two embodiments: one modified by "about" and one not modified by "about". It is further understood that each range endpoint is meaningful whether related to another endpoint or independent of another endpoint.

[0045] Unless otherwise defined in connection with a specific term or phrase, the terms "substantially", "substantially" and their variants as used herein are intended to indicate that the described feature is equal to or approximately equal to a value or description. For example, a "substantially flat" surface is intended to mean a flat or approximately flat surface. Further, "substantially" is intended to mean that two values are equal or approximately equal. In some embodiments, "substantially" can represent values that differ from each other by within about 10%, such as within about 5% of each other, or within about 2% of each other.

[0046] As used herein, directional terms such as up, down, right, left, front, back, top, bottom, above, below, etc. refer only to the drawings as depicted and are not intended to imply absolute orientation.

[0047] As used herein, the terms "the", "a", or "an" mean "at least one" and, unless explicitly indicated to the contrary, should not be limited to "only one". Thus, for example, a reference to "a component" includes embodiments having two or more such components unless the context clearly indicates otherwise.

[0048] As used herein, the terms "atom%", "atomic%", or "atomic-% " refer to the proportion of the mole percentage of each element such as O, Si, Al, B, Na, Ca, etc. in the glass composition. By convention, oxide glasses can also be described by the proportion of component oxides, each having a certain assumed oxygen stoichiometry, such as SiO2, Al2O3, B2O3, Na2O, CaO, etc. As used herein, the terms "mol%", "mole%", or "molar-% " describe the glass composition in terms of the proportion of the mole percentage of component oxides. The term "wt% " or "weight-% " describes the glass composition in terms of the proportion of the mass percentage of component oxides. Elements may be referred to interchangeably by their names or symbols (e.g., carbon or C, oxygen or O, etc.).

[0049] As Figure 1 As shown, a first aspect of the present disclosure relates to a glass substrate 100 that includes an alkali metal-containing body 120 (alternatively referred to as "the body") and an alkali metal-depleted surface layer 140. The alkali metal-containing body may include one or more alkali metal oxides selected from Li2O, Na2O, K2O, Rb2O, and Cs2O. In an embodiment, the alkali metal-depleted surface layer may be substantially free or completely free of alkali metals. For example, the alkali metal-depleted surface layer may include about 0.5 atom% or less of alkali metals. The alkali metal-depleted surface layer may be described as a borosilicate (i.e., atom% of Si > atom% of B) surface layer or a borosilicate-aluminate surface layer (i.e., atom% of Si > [atom% of B + atom% of Al]). The alkali metal-depleted surface layer exhibits a different composition and atomic structure from the body while exhibiting uniformity of composition and / or atomic structure within and throughout the surface layer. The alkali metal-depleted surface layer is integral with the glass substrate and is not a coating or attachment to the body.

[0050] In an embodiment, the substrate thickness t of the glass substrate can be in the following ranges: about 0.1 mm to about 3.0 mm, about 0.3 mm to about 3 mm, about 0.4 mm to about 3 mm, about 0.5 mm to about 3 mm, about 0.55 mm to about 3 mm, about 0.7 mm to about 3 mm, about 1 mm to about 3 mm, about 0.1 mm to about 2 mm, about 0.1 mm to about 1.5 mm, about 0.1 mm to about 1 mm, about 0.1 mm to about 0.7 mm, about 0.1 mm to about 0.55 mm, about 0.1 mm to about 0.5 mm, about 0.1 mm to about 0.4 mm, about 0.3 mm to about 0.7 mm, or about 0.3 mm to about 0.55 mm, and also includes all sub - ranges and sub - values between the endpoints of these ranges.

[0051] In an embodiment, the layer thickness of the alkali - metal - depleted surface layer can be in the following ranges: about 10 nm to about 3000 nm, about 10 nm to about 2000 nm, about 10 nm to about 1000 nm, about 10 nm to about 900 nm, about 10 nm to about 800 nm, about 10 nm to about 700 nm, about 10 nm to about 600 nm, about 10 nm to about 500 nm, about 50 nm to about 1000 nm, about 100 nm to about 1000 nm, about 200 nm to about 1000 nm, about 250 nm to about 1000 nm, about 300 nm to about 1000 nm, about 400 nm to about 1000 nm, about 500 nm to about 1000 nm, about 500 nm to about 1500 nm, about 500 nm to about 2000 nm, about 500 nm to about 2500 nm, or about 500 nm to about 3000 nm.

[0052] In an embodiment, the alkali - metal - depleted surface layer has a substantially uniform composition. In an embodiment, the composition of the alkali - metal - depleted surface layer is substantially the same along the layer thickness of the surface layer. In an embodiment, the composition of the alkali - metal - depleted surface layer is substantially the same throughout its entire volume. As used herein, the phrase "uniform composition" refers to a composition in which no phase separation has occurred and / or which does not contain portions with a composition different from other portions.

[0053] In an embodiment, the alkali - metal - depleted surface layer can be substantially free of microcrystals and / or substantially amorphous. For example, the alkali - metal - depleted surface layer can contain less than about 1 volume % microcrystals.

[0054] In an embodiment, the alkali - metal - depleted surface layer is substantially free of hydrogen. Such hydrogen may be in the form of H + 、H3O +in the form of H2O or a combination thereof. In an embodiment, the alkali metal-depleted surface layer contains about 0.1 atomic % or less hydrogen (e.g., about 0.08 atomic % or less hydrogen, about 0.06 atomic % or less hydrogen, about 0.05 atomic % or less hydrogen, about 0.04 atomic % or less hydrogen, about 0.02 atomic % or less hydrogen, or about 0.01 atomic % or less hydrogen). In contrast, glass substrates processed by leaching or other wet chemical treatments typically have a surface layer that contains hydrogen.

[0055] In an alternative embodiment, the alkali metal-depleted surface layer can incorporate hydrogen into its composition under certain thermally poled conditions. For example, under conditions where the thermal poling occurs in air, the amount of hydrogen contained in the alkali metal-depleted surface layer may be greater than the amount indicated in the previous paragraph.

[0056] In an embodiment, the alkali metal-containing host and the alkali metal-depleted surface layer include B2O3 and SiO2. The alkali metal-containing host and the alkali metal-depleted surface layer each have an atomic structure that includes boron in one or more coordination states.

[0057] In an embodiment, the alkali metal-depleted surface layer contains boron in a 3 - coordinate state (interchangeably referred to as a triangular B[3] unit or simply B[3]) and boron in a 4 - coordinate state (interchangeably referred to as a tetrahedral B[4] unit or simply B[4]). In an embodiment, substantially all of the boron in the alkali metal-depleted surface layer is in a 3 - coordinate state. In an embodiment, most of the boron in the alkali metal-depleted surface layer is in a 3 - coordinate state. For example, about 55% to about 100% (e.g., about 60% to about 100%, about 65% to about 100%, about 70% to about 100%, about 75% to about 100%, about 75% to about 95%, about 80% to about 100%, and including all sub - ranges and sub - values between the endpoints of these ranges) of the total amount of boron in the alkali metal-depleted surface layer is in a 3 - coordinate state, expressed as a fraction. Conversely, less than about 45% (e.g., less than about 40%, less than about 35%, less than about 30%, less than about 25%, less than about 20%, less than about 15%, less than about 10%, less than about 5% or less than about 2.5%) of the total amount of boron in the alkali metal-depleted surface layer is in a 4 - coordinate state, expressed as a fraction.

[0058] In an embodiment, the alkali metal-containing host contains boron in a 3 - coordinate state and boron in a 4 - coordinate state. In an embodiment, most (i.e., an amount greater than half of the total amount) of the boron in the alkali metal-containing host is in a 4 - coordinate state. For example, about 51% to about 100% (e.g., about 55% to about 100%, about 60% to about 100%, about 65% to about 100%, about 70% to about 100%, about 70% to about 95%, about 80% to about 100%, and including all sub - ranges and sub - values between the endpoints of these ranges) of the total amount of boron in the alkali metal-containing host is in a 4 - coordinate state, expressed as a fraction.

[0059] In some cases, the alkali metal - depleted surface layer is substantially free of non - bridging oxygen, while in some embodiments, the alkali metal-containing host includes non - bridging oxygen and bridging oxygen. When the alkali metal-containing host is substantially free of non - bridging oxygen, an alkali metal - depleted surface layer may also be present or formed.

[0060] In an embodiment, the alkali metal - depleted surface layer includes B2O3 in the range of about 1 mol% to about 90 mol%. In an embodiment, the amount of B2O3 can be in the following ranges: about 1 mol% to about 80 mol%, about 1 mol% to about 70 mol%, about 1 mol% to about 60 mol%, about 1 mol% to about 50 mol%, about 5 mol% to about 90 mol%, about 10 mol% to about 90 mol%, about 20 mol% to about 90 mol%, about 30 mol% to about 90 mol%, about 1 mol% to about 55 mol%, 5 mol% to about 45 mol%, or about 3 mol% to about 35 mol%.

[0061] In an exemplary embodiment, the alkali metal - depleted surface layer includes a binary B2O3 - SiO2 composition, but may also contain other non - alkali metal components.

[0062] The glass substrate before the thermopolarization treatment and the alkali metal-containing host to be described herein can include a variety of glass compositions. Such glass compositions used in the glass substrate before the thermopolarization treatment and present in the alkali metal-containing host after the thermopolarization treatment can be referred to herein as "precursor" glasses or glass compositions. The range of precursor compositions can range from simple alkali metal or alkaline earth metal silicates, borosilicates, or borosilicate aluminates to more complex multi - component glasses capable of forming a modified surface layer through the thermopolarization process. In one embodiment, the alkali metal-containing host may show signs of nanoscale phase separation, but when these glasses are thermopolarized, the layer contains a single phase.

[0063] In an embodiment, the precursor glass composition is configured to form a homogeneous glass (i.e., no phase separation has occurred, no devitrification).

[0064] In an embodiment, the precursor glass composition includes a certain amount of alkali metal. For example, the precursor glass composition includes an alkali metal oxide selected from Li2O, Na2O, K2O, Rb2O, and Cs2O in an amount greater than or equal to 1 mol%. In an exemplary embodiment, the precursor glass composition includes Na2O in an amount greater than or equal to 1 mol%. For example, the precursor glass composition may include Na2O in an amount greater than or equal to 1 mol%, 1.5 mol%, 2 mol%, 3 mol%, 4 mol%, 5 mol%, 6 mol%, 7 mol%, 8 mol%, 9 mol%, or 10 mol%.

[0065] In an embodiment, the precursor glass composition includes a certain amount of boron. For example, the precursor glass composition includes B2O3 in an amount greater than or equal to 1 mol%. In an embodiment, the precursor glass composition may include B2O3 in an amount greater than or equal to 1 mol%, 1.5 mol%, 2 mol%, 3 mol%, 4 mol%, 5 mol%, 6 mol%, 7 mol%, 8 mol%, 9 mol%, 10 mol%, 15 mol%, or 20 mol%.

[0066] In an embodiment, the precursor glass composition is a simple sodium borosilicate, including Na2O, B2O3, and SiO2. In one example, the precursor glass composition includes from about 8.5 mol% to about 43 mol% Na2O, from about 9 mol% to about 87 mol% B2O3, and from about 9.5 mol% to about 88 mol% SiO2. In another example, the precursor glass composition includes from about 9.5 mol% to about 39 mol% Na2O, from about 10 mol% to about 79.5 mol% B2O3, and from about 10.5 mol% to about 80 mol% SiO2.

[0067] In an embodiment, there is the following relationship between the amounts of silicon and boron: atomic% of silicon > atomic% of boron. For example, the precursor glass composition includes from about 8.7 mol% to about 21.5 mol% Na2O, from about 9 mol% to about 33 mol% B2O3, and from about 54 mol% to about 88 mol% SiO2. In another example, the precursor glass composition includes from about 9.5 mol% to about 19.5 mol% Na2O, from about 10 mol% to about 30 mol% B2O3, and from about 60 mol% to about 80 mol% SiO2.

[0068] In an embodiment, the precursor glass composition is an alkali metal borosilicate, an alkali metal borosilicate-aluminate, or a combination thereof. In such embodiments, there is the following relationship: the amount of silicon is greater than the amount of boron alone, the amount of aluminum alone, or the combined amount of boron and aluminum (e.g., atomic% of silicon > [atomic% of aluminum + atomic% of boron]). In an embodiment, the amount of alkali metal can be limited such that there is no inverse glass in which the alkali metal is more than the network formers. In such embodiments, there is the following relationship between the amounts of alkali metal, aluminum, boron, and silicon: atomic% of alkali metal < (atomic% of silicon + atomic% of aluminum + atomic% of boron).

[0069] In an exemplary embodiment, the precursor glass composition comprises from about 46 mol% to about 80 mol% SiO2, from about 0 mol% to about 17 mol% Na2O, from about 8 mol% to about 25 mol% Al2O3, from about 2 mol% to about 15 mol% B2O3, from about 0 mol% to about 8 mol% MgO, from about 0 mol% to about 5 mol% K2O, from about 0 mol% to about 11 mol% CaO, and from about 0.05 mol% to about 0.5 mol% SnO2.

[0070] In another exemplary embodiment, the precursor glass composition comprises from about 57 mol% to about 67 mol% SiO2, from about 1 mol% to about 14 mol% Na2O, from about 11 mol% to about 21 mol% Al2O3, from about 3 mol% to about 10 mol% B2O3, from about 0 mol% to about 5 mol% MgO, from about 0 mol% to about 3 mol% K2O, from about 0 mol% to about 8 mol% CaO, and from about 0.05 mol% to about 0.25 mol% SnO2.

[0071] In an embodiment, the precursor glass composition can be substantially free of aluminum. For example, the precursor glass composition and / or the glass substrate after thermally poling can contain less than about 1 mol% or less than about 0.1 mol% Al2O3 or aluminum in any form.

[0072] In an embodiment, the alkali metal host can contain an amount of Na2O that is approximately equal to the amount of B2O3 present in the host.

[0073] The precursor glass compositions described herein can contain fining agents such as SnO2 and other known fining agents.

[0074] In an embodiment, the glass substrate after thermally poling exhibits a refractive index in the range of about 1.45 to about 1.55, wherein the refractive index of the alkali metal-depleted surface layer is lower than that of the alkali metal host.

[0075] In an embodiment, as measured using ball-on-ring testing with at least 5, at least 10, at least 15, or at least 20 samples, the glass substrate can exhibit an average strain-to-failure of 0.5% or greater, 0.6% or greater, 0.7% or greater, 0.8% or greater, 0.9% or greater, 1% or greater, 1.1% or greater, 1.2% or greater, 1.3% or greater, 1.4% or greater, 1.5% or greater, or even 2% or greater at the surface on one or more opposing major surfaces. In an embodiment, the glass substrate can exhibit an average strain-to-failure of about 1.2%, about 1.4%, about 1.6%, about 1.8%, about 2.2%, about 2.4%, about 2.6%, about 2.8%, or about 3% or greater at the surface on one or more opposing major surfaces.

[0076] After thermally poling, the glass substrates described herein can exhibit an elastic modulus (or Young's modulus) in the range of about 30 GPa to about 120 GPa. For example, the elastic modulus of the glass substrate can be in the following ranges: about 30 GPa to about 110 GPa, about 30 GPa to about 100 GPa, about 30 GPa to about 90 GPa, about 30 GPa to about 80 GPa, about 30 GPa to about 70 GPa, about 40 GPa to about 120 GPa, about 50 GPa to about 120 GPa, about 60 GPa to about 120 GPa, or about 70 GPa to about 120 GPa, and all ranges and sub-ranges therebetween.

[0077] In an embodiment, the glass substrate can be strengthened or unstrengthened. For example, a strengthened glass substrate can be thermally poled such that an alkali metal-depleted surface layer is formed on top of the compressive stress layer in the strengthened glass substrate.

[0078] The glass substrate can be generally flat or sheet-like, but other embodiments can utilize curved or other shaped or sculpted substrates. The glass substrate can be generally optically clear, transparent, and free of light scattering. In such embodiments, the average total transmittance exhibited by the glass substrate over a range of light wavelengths can be about 85% or higher, about 86% or higher, about 87% or higher, about 88% or higher, about 89% or higher, about 90% or higher, about 91% or higher, or about 92% or higher.

[0079] Additionally or alternatively, for aesthetic and / or functional reasons, the physical thickness of the glass substrate can vary along one or more of its dimensions. For example, the edges of the glass substrate may be thicker compared to more central regions of the glass substrate. The length, width, and physical thickness dimensions of the glass substrate can also vary depending on the application or use.

[0080] The glass substrate can be provided using various forming methods, which can include the float glass process and the down-draw process, such as fusion draw and slot draw.

[0081] The resulting glass substrate comprising the alkali metal-containing body and the alkali metal-depleted surface layer described herein exhibits improved corrosion resistance, improved diffusion barrier properties, higher hardness and / or elastic modulus values, greater fatigue resistance, and / or improved damage resistance (via so-called anomalous deformation).

[0082] The glass network has near-maximum connectivity, which is beneficial for obtaining high hardness and / or high modulus. The absence of mobile alkali metals or other network modifiers means that the pathways for ionic hopping conduction are very limited, thus translating into inhibited diffusion (also known as diffusion barrier) properties. Similarly, the absence of mobile alkali metals or other network modifiers will increase the corrosion resistance to chemicals that act mainly through an ion-exchange mechanism (e.g., ). The most typical example is acidic chemicals. Even in chemicals that do not act mainly through an ion-exchange mechanism, the absence of non-bridging oxygen and high network connectivity is expected to result in reduced network dissolution in, for example, neutral alkaline pH chemicals (considering the dissolution rate of silica at alkaline pH compared to other multicomponent glasses). For those skilled in the art, compared with materials such as silica, the fatigue resistance and crack initiation of alkali metal-containing glasses are significantly worse. Therefore, it is expected that the alkali metal-depleted surface layer shows lower fatigue parameters and a high crack initiation threshold. Finally, the indentation behavior in glass is structurally related to a well-connected network with a large amount of free volume. Since the layers described in the present disclosure are formed at temperatures far below Tg, it is expected that these layers are very different from the melt equilibrium structure that might otherwise be obtained by melting, and thus may contain a large amount of free volume, resulting in expected anomalous deformation behavior and so-called as-received damage resistance.

[0083] In an embodiment, the refractive index of the alkali metal-depleted surface layer is less than the bulk refractive index of the alkali metal-containing body. For example, the refractive index of the alkali metal-depleted surface layer at a wavelength of about 550 nm can be in the range of about 1.45 to about 1.55. This result generally has sufficient refractive index contrast and thickness to produce a visible anti-reflection effect in thermally poled glass. In addition, in the case of borosilicate glass, the boron structural units in the alkali metal-depleted surface layer change from tetrahedral B[4] units to triangular B[3] units. Compared with the corresponding B[4] units, the triangular B[3] units can correspond to a lower glass density. Compared with aluminosilicate glass, this can further increase the refractive index contrast between the poled material and the bulk material of the borosilicate.

[0084] In an embodiment, compared to an alkali metal-containing substrate (or a glass substrate before forming an alkali metal-depleted surface layer), a glass substrate having an alkali metal-containing substrate and an alkali metal-depleted surface layer can exhibit an increased elastic modulus. For example, the elastic modulus of the glass substrate can be about 10% greater than the elastic modulus of the alkali metal-containing substrate (or the glass substrate before forming the alkali metal-depleted surface layer). For example, when the alkali metal-containing substrate (or the glass substrate before forming the alkali metal-depleted surface layer) exhibits an elastic modulus of about 80 GPa, the glass substrate exhibits an elastic modulus of about 90 GPa.

[0085] In an embodiment, the hardness of the glass substrate described herein is also greater than the hardness of the alkali metal-containing substrate. For example, the hardness of the glass substrate can be about 10% or even 20% greater than the hardness of the alkali metal-containing substrate (or the glass substrate before forming the alkali metal-depleted surface layer). In one example, at an indentation depth of about 0 nm to about 200 nm, the hardness of the alkali metal-containing substrate (or the glass substrate before forming the alkali metal-depleted surface layer) can be about 6 GPa, while the glass substrate exhibits a hardness of about 7 GPa. Unless otherwise specified, the hardness values described herein refer to Vickers hardness.

[0086] In an embodiment, the alkali metal-depleted surface layer also prevents the diffusion of ions into the glass substrate or from the alkali metal-containing substrate into the alkali metal-depleted surface layer.

[0087] The glass substrates described herein can exhibit increased chemical durability in terms of resistance to dissolution in acids, water, or alkalis. In some examples, the glass substrates exhibit a reduction in the dissolution rate in acids, water, or alkalis by about 1.5 times or more, or even about 10 times or more.

[0088] A second aspect of the present disclosure relates to a method of forming a glass substrate having a modified surface layer. The method includes providing a glass substrate including a certain concentration of alkali metal and a surface layer, and reducing the alkali metal concentration in the surface layer. In an embodiment, the resulting surface layer with a reduced alkali metal concentration includes a substantially uniform composition.

[0089] In an embodiment, reducing the alkali metal concentration in the surface layer includes contacting the surface of the glass substrate with an electrode and thermally polarizing the glass substrate.

[0090] Before the thermal polarization treatment, the surface of the glass substrate (and thus the surface layer) can be cleaned or treated to remove typical contaminants that may accumulate after forming, storing, and transporting. Alternatively, the glass substrate can be treated immediately after forming to eliminate the accumulation of contaminants.

[0091] The electrodes for thermal polarization can include an anode in contact with the anodic surface of the glass substrate and a cathode in contact with the cathodic surface of the glass substrate. The anodic surface is subjected to a positive direct current (DC) bias, while the cathodic surface is subjected to a negative DC bias.

[0092] In an embodiment, the conductivity of the electrode material at the polarization temperature is significantly higher than that of the glass to provide field uniformity over the modified surface region. It is also desirable for the anode electrode material to have relative oxidation resistance to minimize the formation of interfacial oxides that may cause the glass to adhere to the template. Exemplary anode electrode materials include noble metals (e.g., Au, Pt, Pd, etc.) or oxidation-resistant conductive films (e.g., TiN and TiAlN).

[0093] The cathode electrode material can also be conductive to achieve field uniformity over the modified region as well. Exemplary materials for the cathode electrode material include materials that can accept alkali metal ions from the glass, such as graphite. In an embodiment, due to surface discharge, physical contact with the cathode electrode may not always be required.

[0094] In an embodiment, the electrodes are separate components in contact with the glass and can thus be separated after processing without complex removal steps. The electrodes can generally include a bulk material or be in the form of a thin film, e.g., a conductive thin film or coating deposited on the glass to act as an electrode.

[0095] In an embodiment, the electrodes can generally cover the entire surface or only a portion thereof and can be discontinuous or patterned as needed. The patterning can be achieved by any of a variety of methods, such as lithographic techniques, machining, or other means.

[0096] The curvature and / or flatness of the glass and the electrodes should ideally match to achieve a fairly tight contact at the interface over the affected region. However, even if the initial contact is not tight, the static charge at the interface when a voltage is applied will tend to pull the two surfaces into tight contact, which is an inherent part of the method.

[0097] Thermal polarization can include applying a voltage to the glass substrate such that the anode is at a positive bias relative to the glass substrate, thereby inducing alkali metal depletion at the anodic surface of the glass. The voltage can be a DC voltage or an AC voltage with a DC bias. Before applying the voltage, the method can include heating the glass substrate and the electrodes (i.e., the stack comprising anode / glass / cathode) to a temperature below Tg before applying the voltage to the glass substrate. In an embodiment, the glass substrate and the electrodes can be heated to a process temperature in the range of about 25 °C to about Tg or about 100 °C to about 300 °C. In an embodiment, equilibration at the desired process temperature can be used for thermal polarization to ensure temperature uniformity.

[0098] In an embodiment, the thermal polarization process comprises applying a voltage in the range of about 100 volts to about 10,000 volts (e.g., about 100 volts to about 1000 volts) to the glass substrate for a duration in the range of about 1 minute to about 6 hours (e.g., about 5 minutes to about 60 minutes, or about 15 minutes to about 30 minutes). It should be understood that the time and voltage of the thermal polarization process may vary depending on the glass composition. In an embodiment, the glass substrate is thermally polarized in an inert gas environment (e.g., dry N2) or a permeable gas environment (e.g., He) under vacuum.

[0099] The voltage may be applied in one or more discrete steps to achieve the desired maximum value, or may be stepped up (or increased) in a controlled / current-limited manner until the process voltage is reached. The advantage of each approach is that thermal dielectric breakdown due to excessive current through the glass, especially low-resistivity glass, is potentially avoided, thereby allowing higher final polarization voltages and potentially thicker surface layers. Alternatively, since breakdown strength varies with glass composition, surface condition, and ambient temperature, an "instant-on" voltage application strategy may also be acceptable under certain conditions and may be desired for convenience.

[0100] After the thermal poling process, the glass substrate may be cooled to a temperature in the range of about 25° C. to about 80° C. for subsequent processing. The voltage may be removed before or after cooling.

[0101] In an embodiment, an apparatus suitable for performing a polarization process may include any system that can simultaneously maintain heat and voltage to the glass / electrode stack in a controlled manner while avoiding practical problems such as leakage current paths or arcing. In an embodiment, the apparatus also provides control of the process atmosphere (e.g., under vacuum, in an inert gas environment such as dry N2, or a permeable gas environment), which can minimize the effects of the atmosphere and / or occluded gases at the interface. An example apparatus suitable for performing a thermal polarization process is disclosed in U.S. Provisional Patent Application Serial No. 63 / 193,334 filed on March 21, 2022, the disclosure of which is incorporated herein by reference in its entirety.

[0102] Examples

[0103] Various embodiments of the present disclosure may be better understood by reference to the following examples provided by way of illustration.The present disclosure is not limited to the examples given herein.

[0104] Example 1

[0105] A series of sodium borosilicate glasses were melted and the composition was verified by inductively coupled plasma optical emission spectroscopy (ICP-OES). The bulk composition results are given in Table 1 below. Table 1 contains the results measured by ICP-OES (left) andFigure 2A and Figure 2B The mapped target values (right) used in the ternary diagram of Figure 2B .

[0106] Table 1: Bulk composition information for Examples 1 to 10.

[0107]

[0108] Figure 2A A ternary diagram summarizing the precursor glass compositions and corresponding experimental strategies is shown. Model glasses in the Na2O-B2O3-SiO2 system represent the "precursor" or bulk glass compositions on which a depleted surface layer is synthesized by thermally polarizing the positively biased surface of the glass. After thermal polarization, the corresponding compositional effect is the generation of an alkali metal-depleted surface layer in which the modifier species are driven out of the surface layer. The compositions containing only network formers generated in the alkali metal-depleted surface layer are projected onto the B2O3-SiO2 binary edge of the ternary diagram, as Figure 2B shown. Unless otherwise stated herein, the bulk glass or alkali metal-containing bulk glass has the same composition, structure, and characteristics as the precursor glass prior to thermally polarizing.

[0109] Using simple ternary compositions as shown in Table 1 allows for a more definitive determination of the structure while still maintaining relevance to commercial and other useful compositions. Additionally, the compositions were selected to provide examples of synthesizing alkali metal-depleted surface layers where the final compositions overlap but are formed from precursor glasses with different initial compositions and structures (i.e., demonstrating that alkali metal-depleted surface layers with the same composition can be formed from various precursor glasses). In this way, the role of the precursor glass composition can also be explored through different concentrations and types of alkali metal charge-balancing substances in the structure.

[0110] The glass wafers were made from the precursor glass compositions shown in Table 1 and polished into flat specimens with dimensions of approximately 25 to 50 square millimeters. The specimens had a thickness of approximately 1.0 mm.

[0111] For thermally polarizing, a bulk high-purity platinum (Pt) ingot was obtained and polished to an optical finish. This element was placed in contact with the surface of each glass wafer to serve as the positive bias electrode. On the cathode side of each glass wafer, a piece of graphite foil was used (e.g., graphite foil provided by Graftech International, trademark ). The electrode size was controlled so that it did not cover the entire surface on both sides of the glass to reduce or eliminate leakage current.

[0112] After loosely stacking the electrodes and glass sheets, each such stack is introduced into a vacuum furnace, a dry nitrogen atmosphere is generated and heated to a temperature between approximately 200°C and 300°C. After equilibration for approximately 15 minutes at a temperature within this range, a voltage of approximately +300V is applied to the platinum electrodes, with the current limited to a maximum of 1mA. An initial increase in current is observed, followed by a slow decay as a surface layer depleted of alkali metal forms. The voltage is applied for a period of approximately 15 minutes, then the heater is turned off, and the stack of electrodes and glass sheets is allowed to cool under voltage. When the temperature of the stack is below approximately 100°C, the voltage is removed, the chamber is evacuated, and the stack is manually separated.

[0113] The heat-polarized glass sheets are compared with the same glass sheets that are unpolarized or not heat-polarized for various forms of analysis.

[0114] Secondary ion mass spectrometry (SIMS) is used to evaluate the presence, depth, and composition of various parts of the alkali-metal-depleted surface layer and the bulk. The results of these analyses for Examples 4 to 8 in Table 1 are summarized respectively in Figures 3 to 7 In Figures 3 to 7 the SIMS elemental depth profiles are presented as the variation of the elemental concentration (with respect to the legend marked on the positive-biased surface) with depth (nm).

[0115] Figures 3 to 7 The generation and presence of an alkali-metal-depleted surface layer with a thickness in the range of approximately 100 nm to approximately 500 nm in various glass compositions are shown.

[0116] X-ray photoelectron spectroscopy (XPS) analysis is performed to evaluate and quantify the composition of the alkali-metal-depleted surface layer, thus confirming the SIMS findings. The results are given in Table 2 and reported as the average of three measurement points for each sample.

[0117] Table 2. XPS data of the surface composition of the positive-biased surfaces for selected examples.

[0118]

[0119] Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was performed at a synchrotron facility to probe the structure of the polarized surface layer of sodium borosilicate for selected examples and to understand whether the structure is uniquely different from the precursor glass. Data were acquired in the partial electron yield (TEY) mode to ensure that the results represent the polarized layer structure (i.e., surface-sensitive mode, top 2 to 5 nm), and were mainly performed at the B K-edge. For the detailed procedure, see "Boron coordination structure at the surfaces of sodium borosilicate and aluminoborosilicate glasses by B K-edge NEXAFS", Journal of Non-Crystalline Solids, Volume 545, October 1, 2020, 120247. The main metric of the glass structure extracted from this analysis is the fraction of 4-fold coordinated boron relative to the total boron. This parameter is called N4 = %B[4] / (%B[3] + %B[4])

[0120] Figure 8 The results of the B K-edge NEXAFS analysis are summarized, comparing the polarized anode-side surface with the air-broken surface (representing the bulk glass structure) for Examples 4 to 7 after thermal polarization. The results show that boron is mainly converted to 3-fold coordination (N4 ≈ 0) in the polarized layer compared to the much higher N4 fraction in the parent glass (usually a mixture of B[3] and B[4], i.e., N4 > 0). Without being bound by theory, this change is understood in terms of the expected composition of the B2O3-SiO2 glass in the Na depletion layer. Thus, there is no alkali metal available for charge compensation of B in the 4-fold coordination state. This interpretation assumes (and actually supports) that the depletion layer structure is resynthesized in situ at temperatures far below the Tg range of the parent glass, forming a unique layer structure and satisfying the bonding requirements of the remaining network formers in the depletion layer, as discussed by Smith et al. in Journal of the American Ceramic Society, 2019.102(6): pp. 3037-3062

[0121] Molecular simulations were performed on a series of alkali metal and alkaline earth metal borosilicate glasses to compare their structures before and after polarization. The compositions studied are shown in Table 3 and include sodium borosilicate, potassium borosilicate, magnesium borosilicate, and calcium borosilicate. In all the simulated compositions, the molar fraction ratio of B2O3 / SiO2 was kept constant (i.e., approximately 0.51). In Table 3, M = Na, K, Mg, or Ca, as Figure 9 and Figure 10 shown

[0122] Table 3. Precursor compositions for molecular simulations

[0123] <![CDATA[SiO2 (mol%)]]> <![CDATA[B2O3 (mol%)]]> <![CDATA[B2O3 / SiO2 (ratio)]]> <![CDATA[M2O or MO (mol%)]]> 66.28 33.72 0.51 0 64.98 33.06 0.51 1.96 61.94 31.51 0.51 6.54 58.65 29.84 0.51 11.50 54.78 27.87 0.51 17.36 50.21 25.55 0.51 24.24

[0124] Figure 9 shows the fractions of 3 - coordinated boron (black) and 4 - coordinated boron (red) in a series of alkali and alkaline earth metal borosilicate compositions shown in Table 3. As Figure 9 shown, after thermal poling, it can be observed that the coordination number of boron changes from a 4 - coordinated state to a 3 - coordinated state. Figure 9 It also shows that the conversion ratio of B[4] to B[3] increases with the increase in the content of network modifiers, because the fraction of B[4] increases with the increase in concentration.

[0125] Refer to Figure 10 , molecular dynamics simulations show that the change in the coordination state also leads to a change in Young's modulus. Figure 10 Depicts the Young's moduli of different - composition molten ternaries, molten binaries, and binary sodium borosilicates obtained by thermal poling. As shown, the Young's modulus of the alkali - metal - depleted surface layer is generally lower than that of the bulk.

[0126] The results of the molecular simulations help to further determine / validate the broad applicability of thermal poling, that is, starting from a parent glass with a variety of different modifier species (such as the glasses disclosed herein), to produce articles with altered layers.

[0127] Although the present disclosure has been shown and described in detail in the drawings and the foregoing description, the drawings and the description should be regarded as illustrative and non - limiting. It should be understood that only the preferred embodiments are presented, and all changes, modifications, and further applications within the spirit of the present disclosure are desired to be protected.

Claims

1. A glass substrate, comprising: An alkali metal-containing body; and An alkali metal-depleted surface layer, wherein the alkali metal-depleted surface layer is amorphous and has a substantially uniform composition, and wherein the alkali metal-containing body and the alkali metal-depleted surface layer comprise B2O3 and SiO2.

2. The glass substrate according to claim 1, wherein the alkali metal-depleted surface layer comprises 0.5 atomic % or less of alkali metal.

3. The glass substrate according to claim 1 or claim 2, wherein the alkali metal-depleted surface layer comprises an atomic structure containing boron substantially in a 3 - coordinate state.

4. The glass substrate according to claim 3, wherein more than about 60% of the total amount of boron in the alkali metal-depleted surface layer is in a 3 - coordinate state in terms of fraction.

5. The glass substrate according to claim 3, wherein more than about 70% of the total amount of boron in the alkali metal-depleted surface layer is in a 3 - coordinate state in terms of fraction.

6. The glass substrate according to claim 3, wherein more than about 75% of the total amount of boron in the alkali metal-depleted surface layer is in a 3 - coordinate state in terms of fraction.

7. The glass substrate according to any one of claims 1 to 6, wherein the alkali metal-containing body comprises an atomic structure containing boron in a 3 - coordinate state and boron in a 4 - coordinate state.

8. The glass substrate according to claim 7, wherein more than about 51% of the total amount of boron in the alkali metal-containing body is in a 4 - coordinate state in terms of fraction.

9. The glass substrate according to any one of claims 1 to 8, wherein the alkali metal-depleted surface layer is substantially free of non - bridging oxygen.

10. The glass substrate according to claim 9, wherein the alkali metal-containing body comprises non - bridging oxygen and bridging oxygen.

11. The glass substrate according to claim 10, wherein the alkali metal-containing body is substantially free of non - bridging oxygen.

12. The glass substrate according to any one of claims 1 to 11, wherein the alkali metal-containing body comprises an alkali metal oxide selected from Li2O, Na2O, K2O, Rb2O, and Cs2O.

13. The glass substrate according to claim 12, wherein the alkali metal-containing body comprises at least 1 mol% of Na2O, K2O, or Li2O.

14. The glass substrate according to claim 12, wherein the alkali metal-containing body comprises at least 1 mol% of Na2O.

15. The glass substrate according to any one of claims 1 to 14, wherein the alkali metal-depleted surface layer comprises B2O3 in the range of about 10 mol% to about 90 mol%.

16. The glass substrate according to any one of claims 1 to 15, wherein the alkali metal-depleted surface layer comprises a binary B2O3 - SiO2 composition.

17. The glass substrate according to any one of claims 1 to 16, wherein the atomic % of silicon is greater than the atomic % of boron.

18. A glass substrate, comprising: A substrate thickness; An alkali metal-containing body having a bulk refractive index; and An alkali metal-depleted surface layer having a layer thickness in the range of about 10 nm to about 3000 nm, wherein the alkali metal-depleted surface layer comprises a layer refractive index less than the refractive index of the bulk wherein the alkali metal-containing bulk and the alkali metal-depleted surface layer comprise B2O3 and SiO2 19. The glass substrate according to claim 18, wherein the atomic % of silicon is greater than the atomic % of boron.

20. A method of forming a glass substrate having a modified surface layer, comprising: providing a glass substrate comprising a certain concentration of alkali metal, a glass transition temperature (Tg), and a surface layer, the glass substrate comprising B2O3 and SiO2; and reducing the concentration of the alkali metal in the surface layer wherein the surface layer with reduced alkali metal concentration has a substantially uniform composition.

21. The method according to claim 20, wherein the atomic % of silicon is greater than the atomic % of boron.

22. The method according to claim 20 or claim 21, wherein reducing the concentration of the alkali metal in the surface layer comprises contacting the surface of the glass substrate with an electrode; and thermally polarizing the glass substrate.

23. The method according to claim 22, wherein the electrode comprises an anode in contact with the anode surface of the glass substrate and a cathode in contact with the cathode surface of the glass substrate, and wherein thermally polarizing comprises applying a voltage to the glass substrate such that the anode is at a positive bias with respect to the glass substrate to cause depletion of alkali metal at the anode surface of the glass.

24. The method according to claim 22, wherein thermally polarizing comprises heating the glass substrate and the electrode to a temperature below Tg before applying a voltage to the glass substrate.

25. The method according to claim 22, wherein thermally polarizing comprises applying a voltage in the range of about 100 volts to about 10,000 volts to the glass substrate for a duration in the range of about 1 minute to about 6 hours.

26. The method according to claim 22, wherein the glass substrate is thermally polarized under vacuum, in an inert gas environment, or a permeable gas environment.

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