Patent Application
20210163349
The present disclosure relates to mitigation of the effects of lithium in molten salt baths. In particular, the present disclosure relates to mitigation of transmittance haze formed on glass-based articles due to the presence of lithium in molten salt baths during ion exchange of lithium containing glass-based substrates.
The mobile nature of portable devices, such as smart phones, tablets, portable media players, personal computers, and cameras, makes these devices particularly vulnerable to accidental dropping on hard surfaces, such as the ground. Cover glasses adhered to these devices often function as display covers, which may incorporate touch functionality. In the event that a device is dropped, the cover glass may become damaged upon impact with a hard surface, thereby potentially reducing functionality of the device.
Glass can be made more resistant to fracture through an ion exchange process, which involves inducing compressive stress in the glass surface in a molten salt bath. In particular, lithium containing glass substrates have been subjected to ion exchange treatments for cover glass production because inclusion of lithium in these glasses allows for production of ion exchanged glass articles with a greater depth of compression at a faster exchange rate than glass substrates having other compositions. Glass substrates strengthened using this process typically exhibit improved performance, such as fracture resistance when dropped, when included in consumer electronic devices.
The ion exchange of lithium containing glasses in molten salt baths results in the release of lithium ions from the glass into the bath. This phenomenon is commonly referred to as ion exchange bath poisoning. The lithium poisoning of the ion exchange bath reduces the effectiveness of the ion exchange process, and eventually prevents the ion exchanged articles from exhibiting the desired compressive stress characteristics.
In order to counteract lithium poisoning, carbonate salts may be dissolved into the molten salt baths. The carbonate anions bind lithium cations to form LiCO3−, which is soluble in a salt bath, but has a much larger ion diameter than the lithium cations. Accordingly, the lithium cations are passivated, and cannot participate in ion exchange because the LiCO3− cannot diffuse back into the glass substrate. Thus, the ion exchange process can continue with very little impact from lithium poisoning.
However, the addition of carbonate salts to the molten salt bath during the ion exchange process typically results in transmittance haze formation on glass articles when the salt baths are used for multiple cycles, meaning multiple glass substrates are ion exchanged in the same molten salt bath. Glass articles with transmittance haze have a layer of sub-micron or micron features engraved into the glass, reducing their optical transparencies.
Generally, transmittance haze is caused when an excess of lithium cations are released from lithium containing glass substrates into a molten salt bath such that the solubility limit of Li2CO3 is reached, resulting in formation of small Li2CO3 crystals within the bath. These crystals can be deposited on the surfaces of glass substrates being ion exchanged within the molten salt bath. If moisture is present in the atmosphere above the salt bath, the Li2CO3 may react with water to produce OH− ions. The presence of such OH− ions may elevate the pH of the salt bath at the interface of the Li2CO3 crystals and the glass substrate, which may induce localized etching of the glass. The etching process can leave permanent structures on the glass surface, and the increase in pH may lead to the corrosion of the glass, making the surface rough, i.e., produce transmittance haze.
There is a need to mitigate the formation of transmittance haze such that the same molten salt bath may be reused several times to ion exchange multiple glass substrates. By reusing the same molten salt bath, production costs are reduced and the yield of glass articles may be improved.
The present disclosure is directed to methods for mitigation of transmittance haze formed on glass articles due to the presence of lithium in molten salt baths during ion exchange of lithium containing glass substrates. The ion exchange process may induce compressive stresses on the surfaces of the glass-based substrates in order to enhance their resistance to fracture when impacted. In some embodiments, the glass-based articles may, for example, be used as cover glass for electronic devices. In some embodiments, the glass-based substrates may be lithium containing glass-based substrates. When lithium containing glass-based substrates are ion exchanged in a molten salt bath within a furnace, for example, a concentration of lithium in the bath may become too high, which may poison the bath and prevent any further ion exchange. In some embodiments, carbonate ions may be dissolved in the molten salt bath to bind with lithium ions in order to counteract the poisoning. In some embodiments, the carbonate ions may react with the lithium to form lithium carbonate, which may crystallize within the molten salt bath. Crystallization of lithium carbonate may, for example, cause etching of glass-based substrates that may be ion exchanged within the molten salt bath, leading to formation of transmittance haze on the surfaces of glass-based articles that may be produced through ion exchange according to some embodiments. In some embodiments, silicic acid may be added to the molten salt bath to mitigate the effects of the lithium carbonate. In some embodiments, anhydrous phosphate salts may be added to the molten salt bath to mitigate the effects of the lithium carbonate. In some embodiments, a CO2 atmosphere may be introduced into the furnace to reduce the amount of moisture in the furnace and to reduce the pH of the molten salt bath.
A first aspect (1) of the present application is directed to a chemical ion exchange process, the process including dissolving a carbonate salt in a molten salt bath disposed in a furnace and including a non-lithium non-carbonate alkali salt; immersing a lithium-containing glass-based substrate in the molten salt bath including the dissolved carbonate salt and the non-lithium non-carbonate alkali salt, where immersing the lithium-containing glass-based substrate in the molten salt bath results in an ion-exchange between the lithium-containing glass-based substrate and the molten salt bath and results in the formation of a lithium-containing carbonate salt in the molten salt bath; and reducing a concentration of the lithium-containing carbonate salt in the molten salt bath or increasing a solubility limit of the lithium-containing carbonate salt in the molten salt bath.
In a second aspect (2), the process according to the first aspect (1) is provided, and reducing the concentration of the lithium-containing carbonate salt in the molten salt bath or increasing a solubility limit of the lithium-containing carbonate salt in the molten salt bath according to embodiments of the preceding paragraph includes one or more of: (i) directing a gas including CO2 into the furnace such that the molten salt bath is in contact with the gas; (ii) dissolving silicic acid in the molten salt bath; or (iii) dissolving at least one of anhydrous sodium or a potassium phosphate salt in the molten salt bath.
In a third aspect (3), the process according to the second aspect (2) is provided and reducing the concentration of the lithium-containing carbonate salt in the molten salt bath includes at least one of (ii) or (iii).
In a fourth aspect (4), the process according to the second aspect (2) is provided and reducing the concentration of the lithium-containing carbonate salt in the molten salt bath includes (i).
In a fifth aspect (5), the process according to the second aspect (2) or the third aspect (3) is provided and a concentration of the silicic acid within the molten salt bath is within a range of 0.1 wt % to 2 wt % after the silicic acid has been dissolved in the molten salt bath.
In a sixth aspect (6), the process according to any of aspects (2), (3), or (5) is provided and the concentration of the lithium-containing carbonate salt in the molten salt bath is reduced by up to 0.5 wt %.
In a seventh aspect (7), the process according to any of aspects (2), (3), (5), or (6) is provided and the concentration of the lithium-containing carbonate salt in the molten salt bath is reduced by at least 0.1 wt %.
In an eighth aspect (8), the process according to any of aspects (1)-(7) is provided and removing the lithium-containing glass-based substrate from the molten salt bath after a period of time sufficient to induce a target compressive stress on a surface of the lithium-containing glass-based substrate, wherein the lithium-containing glass-based substrate has a transmittance haze of less than 0.03% after being removed from the molten salt bath.
In a ninth aspect (9), the process according to any of aspects (1)-(8) is provided and the lithium-containing carbonate salt is Li2CO3.
In a tenth aspect (10), the process according to any of aspects (1)-(9) is provided and the concentration of the lithium-containing carbonate salt within the molten salt bath is within a range of 0.1 wt % to 0.3 wt % before reducing the concentration of the lithium-containing carbonate salt or increasing a solubility limit of the lithium-containing carbonate salt.
In an eleventh aspect (11), the process according to any of aspects (1)-(10) is provided and reducing the concentration of the lithium-containing carbonate salt in the molten salt bath or increasing a solubility limit of the lithium-containing carbonate salt in the molten salt bath is performed prior to immersing the lithium-containing glass-based substrate in the molten salt bath.
In a twelfth aspect (12), the process according to any of aspects (1)-(10) is provided and reducing the concentration of the lithium-containing carbonate salt in the molten salt bath or increasing a solubility limit of the lithium-containing carbonate salt in the molten salt bath is performed while the lithium-containing glass-based substrate is immersed in the molten salt bath.
In a thirteenth aspect (13), the process according to any of aspects (1)-(12) is provided and includes removing the lithium-containing glass-based substrate from the molten salt bath after a period of time sufficient to induce a target compressive stress on a surface of the lithium-containing glass-based substrate; immersing a second lithium-containing glass-based substrate in the molten salt bath; and removing the second lithium-containing glass-based substrate from the molten salt bath after a period of time sufficient to induce a target compressive stress on a surface of the second lithium-containing glass-based substrate.
In a fourteenth aspect (14), the process according to the thirteenth aspect (13) is provided and the second lithium-containing glass-based substrate has a transmittance haze of less than 0.03% after being removed from the molten salt bath.
In a fifteenth aspect (15), the process according to the second aspect (2) is provided and the gas including CO2 is configured to reduce an atmospheric moisture content of an interior space within the furnace to no more than 1%.
In a sixteenth aspect (16), the process according to the second aspect (2) or the fifteenth aspect (15) is provided and the gas including CO2 is configured to reduce the pH of the molten salt bath.
In a seventeenth aspect (17), the process according to any of aspects (2), (15), or (16) is provided and directing the gas including CO2 into the furnace includes flowing the gas into an interior space within the furnace.
In an eighteenth aspect (18), the process according to the seventeenth aspect (17) is provided and the gas including CO2 completely fills the interior space within the furnace.
In a nineteenth aspect (19), the process according to the seventeenth aspect (17) is provided and directing the gas including CO2 into the furnace includes bubbling the gas inside the salt bath.
In a twentieth aspect (20), the process according to the nineteenth aspect (19) is provided and the gas is bubbled inside the salt bath for one hour or more.
In a twenty-first aspect (21), the process according to any of aspects (1)-(20) is provided and includes monitoring a concentration of a non-carbonate lithium-containing salt in the molten salt bath.
In a twenty-second aspect (22), the process according to the twenty-first aspect (21) is provided and reducing a concentration of the lithium-containing carbonate salt in the molten salt bath or increasing the solubility limit of the lithium-containing carbonate salt in the molten salt bath is performed when a concentration of the non-carbonate lithium-containing salt within the molten salt bath reaches at least 0.3 wt %.
In a twenty-third aspect (23), the process according to any of aspects (1)-(22) is provided and the non-lithium non-carbonate alkali salt is NaNO3 or KNO3.
In a twenty-fourth aspect (24), the process according to any of aspects (1)-(23) is provided and the molten salt bath including the non-lithium non-carbonate alkali salt includes NaNO3 within a range of 5 wt % to 50 wt %, and KNO3 within a range of 50 wt % to 95 wt %.
In a twenty-fifth aspect (25) the process according to any of aspects (1)-(24) is provided and the carbonate salt is K2CO3 or Na2CO3.
In a twenty-sixth aspect (26), the process according to any of aspects (1)-(25) is provided and the molten salt bath including the non-lithium non-carbonate alkali salt includes carbonate salt within a range of 0.5% to 10 wt % of a total weight of the molten salt bath.
A twenty-seventh aspect (27) of the present application is directed to a chemical ion exchange process, the process including: dissolving a carbonate salt in a molten salt bath including a non-lithium non-carbonate alkali salt, the molten salt bath disposed in a furnace including a CO2 atmosphere above the molten salt bath; immersing a lithium-containing glass-based substrate in the molten salt bath including the dissolved carbonate salt and the non-lithium non-carbonate alkali salt, wherein immersing the lithium-containing glass-based substrate in the molten salt bath results in an ion-exchange between the lithium-containing glass-based substrate and the molten salt bath; and removing the lithium-containing glass-based substrate from the molten salt bath after a period of time sufficient to induce a target compressive stress on a surface of the lithium-containing glass-based substrate, wherein the lithium-containing glass-based substrate has a transmittance haze of less than 0.03% after being removed from the molten salt bath.
In a twenty-eighth aspect (28), the process according to the twenty-seventh aspect (27) is provided and the CO2 atmosphere reduces the moisture content within the furnace.
In a twenty-ninth aspect (29), the process according to the twenty-seventh aspect (27) or the twenty-eighth aspect (28) is provided and the CO2 atmosphere changes the pH of the molten salt bath.
In a thirtieth aspect (30), the process according to any of aspects (27)-(29) is provided and the CO2 atmosphere causes an increase in the solubility limit of a lithium-containing carbonate salt in the molten salt bath.
In a thirty-first aspect (31), the process according to any of aspects (27)-(30) is provided and a concentration of the lithium-containing carbonate salt in the molten salt bath is within a range of 0.1 wt % to 0.3 wt %.
In a thirty-second aspect (32), the process according to any of aspects (27)-(31) is provided and the CO2 atmosphere is created by flowing a gas including CO2 into an interior space within the furnace.
In a thirty-third aspect (33), the process according to the thirty-second aspect (32) is provided and the CO2 atmosphere completely fills the interior space within the furnace.
In a thirty-fourth aspect (34), the process according to any of aspects (27)-(33) is provided and includes bubbling a gas including CO2 inside the salt bath.
In a thirty-fifth aspect (35), the process according to any of aspects (27)-(34) is provided and the target compressive stress on the surface of the lithium-containing glass-based substrate is 200 MPa or more.
In a thirty-sixth aspect (36), the process according to any of aspects (27)-(35) is provided and includes immersing a second lithium-containing glass-based substrate in the molten salt bath including the dissolved carbonate salt and the non-lithium non-carbonate alkali salt after removing the lithium-containing glass-based substrate from the molten salt bath, wherein immersing the second lithium-containing glass-based substrate in the molten salt bath results in an ion-exchange between the second lithium-containing glass-based substrate and the molten salt bath, and removing the second lithium-containing glass-based substrate from the molten salt bath after a period of time sufficient to induce a target compressive stress on a surface of the second lithium-containing glass-based substrate, where the second lithium-containing glass-based substrate has a transmittance haze of less than 0.03% after being removed from the molten salt bath.
In a thirty-seventh aspect (37), the process according to the thirty-sixth aspect (36) is provided and the target compressive stress on the surface of the lithium-containing glass-based substrate is 200 MPa or more, and wherein the target compressive stress on the surface of the second lithium-containing glass-based substrate is 200 MPa or more.
In a thirty-eighth aspect (38), the process according to any of aspects (27)-(37) is provided and includes dissolving silicic acid in the molten salt bath.
In a thirty-ninth aspect (39), the process according to any of aspects (27)-(38) is provided and includes dissolving at least one of anhydrous sodium or a potassium phosphate salt in the molten salt bath.
A fortieth aspect (40) of the present application is directed to a chemical ion exchange process, the process including: dissolving a carbonate salt in a molten salt bath disposed in a furnace and including a non-lithium non-carbonate alkali salt; immersing a first lithium-containing glass-based substrate in the molten salt bath for a period of time sufficient to induce a target compressive stress on a surface of the first lithium-containing glass-based substrate; immersing a second lithium-containing glass-based substrate in the molten salt bath for a period of time sufficient to induce a target compressive stress on a surface of the second lithium-containing glass-based substrate; and directing a gas including CO2 into the furnace such that the molten salt bath is in contact with the gas before immersing at least one of: the first lithium-containing glass-based substrate in the molten salt bath or the second lithium-containing glass-based substrate in the molten salt bath.
In a forty-first aspect (41), the process according to the fortieth aspect (40) is provided and the second lithium-continuing glass-based substrate is immersed in the molten salt bath after the first lithium-continuing glass-based substrate is removed from the molten salt bath.
In a forty-second aspect (42), the process according to either the fortieth (40) or the forty-first aspect (41) is provided and the first lithium-containing glass-based substrate has a transmittance haze of less than 0.03% after being removed from the molten salt bath, and where the second lithium-containing glass-based substrate has a transmittance haze of less than 0.03% after being removed from the molten salt bath.
In a forty-third aspect (43), a first lithium-containing glass-based article and a second lithium-containing glass-based article produced by the process according to aspect (40) is provided.
In a forty-fourth aspect (44), the first lithium-containing glass-based article and the second lithium-containing glass-based article according to the forty-third aspect (43) are provided and both the first lithium-containing glass-based article and the second lithium-containing glass-based article have a transmittance haze of less than 0.03%.
In a forty-fifth aspect (45), the first lithium-containing glass-based article and the second lithium-containing glass-based article according to the forty-third aspect (43) are provided and both the first lithium-containing glass-based article and the second lithium-containing glass-based article have a transmittance haze of less than 0.01%.
The accompanying figures, which are incorporated herein, form part of the specification and illustrate embodiments of the present disclosure. Together with the description, the figures further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the disclosed embodiments. These figures are intended to be illustrative, not limiting. Although the disclosure is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the disclosure to these particular embodiments. In the drawings, like reference numbers indicate identical or functionally similar elements.
The following examples are illustrative, but not limiting, of the present disclosure. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in the field, and which would be apparent to those skilled in the art, are within the spirit and scope of the disclosure.
Glass articles used in various applications, for example, cover glass for different types of electronic devices, needs to have increased strength in order to protect the devices from impact. One method of strengthening glass is through an ion exchange process, which involves inducing compressive stresses in the surface of a glass substrate. Sodium containing glass substrates may be subjected to ion exchange treatments; however, lithium containing glass substrates may alternatively be used in order to maximize strength because the inclusion of lithium in the glass may allow for a greater depth of compression at a faster ion exchange rate than with sodium containing glasses. The resulting strengthened glass articles may exhibit improved performance, such as resistance to fracture when dropped, when included in consumer electronic devices.
As used herein, the terms “glass-based article” and “glass-based substrates” are used in their broadest sense to include any object made wholly or partly of glass. Glass-based articles include laminates of glass and non-glass materials, laminates of glass and crystalline materials, and glass-ceramics (including an amorphous phase and a crystalline phase). While much of the description and many of the examples herein are directed to glass articles, the concepts and results are applicable to glass-based materials, including glass-ceramic materials.
The ion exchange process may be conducted in a molten salt bath. The molten salt bath may include a first salt having a metal ion, which has a larger ionic radii than the alkali metal of an alkali metal oxide included in a glass substrate, and an anion, and a second salt that is dissolved in the molten salt bath and includes the same metal ion as the first salt and an anion different from the anion of the first salt. For example, in some embodiments, the molten salt bath includes a nitrate salt, such as KNO3, NaNO3, and their mixture. The molten salt bath may optionally include K2CO3, Na2CO3, K3PO4, Na3PO4, K2SO4, Na2SO4, K3BO3, Na3BO3, KCl, NaCl, KF, and NaF dissolved therein. The additional salt may be added to the molten salt bath as a dissolved liquid solute in order to facilitate ion exchange and enhance efficiency of the ion exchange process.
When the additional salt is a carbonate salt, and when a lithium containing glass substrate is ion exchanged within the molten salt bath, the lithium may react with the carbonate and Li2CO3 crystals may be formed. These crystals may be deposited on the surfaces of glass substrates being ion exchanged within the molten salt bath. If moisture is present in the atmosphere above the salt bath, the Li2CO3 may react with water to produce OH− ions. The presence of such OH− ions may elevate the pH of the salt bath at the interface of the Li2CO3 crystals and the glass substrate, which may induce localized etching of the glass. The etching process can leave permanent structures on the glass surface, or may lead to the corrosion of the glass, making the surface rough. These surface defects may be categorized as “transmittance haze,” which is undesirable not only because it decreases optical transparency of the glass articles, but also because the presence of transmittance haze reduces the compressive stresses induced by the ion exchange process. In addition, Li2CO3 crystals may have a direct solid-solid reaction with glass, which can make the glass surface rough and appear hazy.
Different techniques may be utilized, alone or in combination with one another, to mitigate the production of transmittance haze on the surfaces of glass articles. For example, in some embodiments, silicic acid may be added to the molten salt bath. The silicic acid may dissociate to release H+ ions, which may react with the OH− ions to neutralize the salt bath, thereby quenching the etching process. However, if too much silicic acid is added to the bath, sludge may be formed on the bottom of the tank.
Additionally, in some embodiments, anhydrous sodium or potassium phosphate salts are added into the salt bath. Such alkali phosphates can react with lithium to form lithium phosphate salts, for example Li3PO4, Li2NaPO4, and Li2KPO4. Lithium phosphate salts may have very low solubility in a molten salt bath. Therefore, the lithium phosphate salts may be easily precipitated, and the formation of Li2CO3 crystals may be avoided because not enough lithium cations are present within the salt bath to react with the carbonate to generate Li2CO3. Like with silicic acid, though, too much phosphate salt within the molten salt bath may result in the formation of sludge in the bottom of the tank.
Finally, in some embodiments, the ion exchange process is executed under CO2 atmosphere. The CO2 may react with the OH− ions in the salt bath, thereby reducing the OH− ions by up to 50 times, which may neutralize the pH of the salt bath and quench the glass etching process. Additionally, the CO2 atmosphere may replace the original atmosphere within the furnace and around the salt bath, which may contain moisture. Accordingly, the moisture contained within the original atmosphere may be prevented from entering the salt bath, which may increase the solubility limit of the Li2CO3, thereby helping to prevent formation of Li2CO3 crystals. In some embodiments, the CO2 atmosphere may reduce an atmospheric moisture content of the interior space of the furnace to less than 10%. In some embodiments, the CO2 atmosphere may reduce an atmospheric moisture content of the interior space of the furnace to less than 5%. In some embodiments, the CO2 atmosphere may reduce an atmospheric moisture content of the interior space of the furnace to less than 1%. Furthermore, a reversible reaction between Li2CO3 and silicate glass may be controlled such that transmittance haze is not formed on the surface of the glass.
Compressive or compressive stress is typically expressed as a negative (<0) stress and tension or tensile stress is typically expressed as a positive (>0) stress. Throughout this description, however, CS is expressed as a positive or absolute value—i.e., as recited herein, CS=|CS|. The compressive stress (CS) has a maximum at the surface of the glass, and the CS varies with distance d from the surface according to a function. Referring again to
As discussed above, the compressive stress layers in glass articles may be formed by exposing glass substrates to an ion exchange bath. In some embodiments, the ion exchange bath may be a molten salt bath, such as a molten non-carbonate salt bath, for example a molten nitrate salt bath, a molten nitrite salt bath, a molten phosphate salt bath, or a molten sulfate salt bath. In some embodiments, the ion exchange bath may contain one of the above molten salt baths where potassium and/or sodium is the cation of the salt. In some embodiments, the ion exchange bath may be molten KNO3, molten NaNO3, or combinations thereof. The following ion exchange bath embodiments may refer to the composition of the bath prior to poisoning. In certain embodiments, the ion exchange bath may comprise less than about 95% molten KNO3, such as less than about 90% molten KNO3, less than about 80% molten KNO3, less than about 70% molten KNO3, less than about 60% molten KNO3, or less than about 50% molten KNO3. In certain embodiments, the ion exchange bath may comprise about 5% or more molten NaNO3, such as about 10% or more molten NaNO3, about 20% or more molten NaNO3, about 30% or more molten NaNO3, about 40% or more molten NaNO3, or about 50% or more molten NaNO3. In other embodiments, the ion exchange bath may comprise about 95% molten KNO3 and about 5% molten NaNO3, about 94% molten KNO3 and about 6% molten NaNO3, about 93% molten KNO3 and about 7% molten NaNO3, about 80% molten KNO3 and about 20% molten NaNO3, about 75% molten KNO3 and about 25% molten NaNO3, about 70% molten KNO3 and about 30% molten NaNO3, about 65% molten KNO3 and about 35% molten NaNO3, or about 60% molten KNO3 and about 40% molten NaNO3, and all ranges and sub-ranges between the foregoing values. In some embodiments, the ion exchange bath may include about 100% molten KNO3 or about 100% molten NaNO3. The above KNO3 and NaNO3 percentages are merely exemplary and similar percentages can be used when other sodium and potassium salts are used in the ion exchange solution, such as, for example sodium or potassium nitrites, phosphates, or sulfates. It should be understood that while molten nitrate salt baths are discussed herein for the sake of simplicity, the processes described herein may also utilize molten salt baths including any appropriate non-lithium non-carbonate alkali salt.
The glass substrates may be exposed to the ion exchange bath by immersing the glass substrates in the ion exchange bath. Upon exposure to the glass substrate, the ion exchange bath may, according to some embodiments, be at a temperature from greater than or equal to 350° C. to less than or equal to 420° C., such as from greater than or equal to 360° C. to less than or equal to 410° C., from greater than or equal to 370° C. to less than or equal to 400° C., or from greater than or equal to 380° C. to less than or equal to 390° C., and all ranges and sub-ranges between the foregoing values. In some embodiments, the glass substrates may be exposed to the ion exchange bath for a duration from greater than or equal to 10 minutes to less than or equal to 48 hours, such as from greater than or equal to 30 minutes to less than or equal to 44 hours, from greater than or equal to 1 hour to less than or equal to 40 hours, from greater than or equal to 4 hours to less than or equal to 36 hours, from greater than or equal to 8 hours to less than or equal to 32 hours, or from greater than or equal to 12 hours to less than or equal to 28 hours, and all ranges and sub-ranges between the foregoing values.
After the glass substrates are exposed to the ion exchange bath to form ion exchanged glass articles, the ion exchange bath may include lithium ions, which may result from the exchange of lithium ions out of the glass substrate during the ion exchange process. Once the concentration of lithium ions reaches a certain level, the ion exchange bath may be “poisoned” by the lithium. For example, an excess of lithium ions within the ion exchange bath may prevent further ion exchange of additional glass substrates. As utilized herein, a lithium poisoned molten salt bath refers to a molten salt bath that includes 0.1 wt % or more of a non-carbonate lithium containing salt, for example LiNO3. In some embodiments, a lithium poisoned molten salt bath may include 0.3 wt % or more of a non-carbonate lithium containing salt.
In some embodiments, carbonate ions may be introduced to the ion exchange bath to counteract the effects of lithium poisoning. The carbonate ions may react with the lithium ions to form lithium carbonate or LiCO3—, effectively deactivating the lithium ions, and the lithium carbonate may precipitate from the bath.
The carbonate ions may be introduced to the ion exchange bath by any process typically used to introduce ions into an ion exchange bath. In some embodiments, the carbonate ions may be introduced to the ion exchange bath by dissolving a carbonate salt in the ion exchange bath. For example, a carbonate salt may be added to an ion exchange bath that is at a bath temperature that will result in the carbonate salt at least partially dissolving in the bath. The carbonate salt added may be in solid form.
The lithium content of the molten salt bath may be given in terms of lithium nitrate for embodiments where the molten salt bath is a molten nitrate salt bath. In other embodiments, where the molten salt bath is a different molten salt bath (for example nitrite, phosphate, or sulfate salt baths), the lithium content is given in terms of the lithium version of the salt that makes up the molten salt bath. The molten salt bath also includes at least one other non-lithium non-carbonate alkali salt, such as a non-lithium alkali nitrate, for example those described above as useful for ion exchanging glass-based substrates. In some embodiments, the poisoned molten salt bath may include LiNO3 in an amount greater than or equal to 0.1 wt % to less than or equal to 2.0 wt %, such as greater than or equal to 0.2 wt % to less than or equal to 1.8 wt %, greater than or equal to 0.3 wt % to less than or equal to 1.6 wt %, greater than or equal to 0.4 wt % to less than or equal to 1.4 wt %, greater than or equal to 0.5 wt % to less than or equal to 1.2 wt %, greater than or equal to 0.6 wt % to less than or equal to 1.0 wt %, greater than or equal to 0.7 wt % to less than or equal to 0.9 wt %, 0.8 wt %, and all ranges and sub-ranges between the foregoing values.
In some embodiments, the carbonate salt may be added in any appropriate amount and in any appropriate form. In some embodiments, the carbonate salt may be added to the bath in an amount of greater than or equal to 0.5 wt % to less than or equal to 10 wt % on the basis of the total bath weight, such as greater than or equal to 1 wt % to less than or equal to 9 wt %, greater than or equal to 2 wt % to less than or equal to 8 wt %, greater than or equal to 3 wt % to less than or equal to 7 wt %, greater than or equal to 4 wt % to less than or equal to 6 wt %, 5 wt %, and all sub-ranges and ranges between the foregoing values.
In some embodiments, the carbonate salt may be added in particulate form, without any particular limitation on the particle size of the carbonate salt. The performance of the carbonate salt in regenerating the bath is not directly dependent on the particle size, as the carbonate salt will be fully or at least partially dissolved in the bath. This allows the use of large particle sizes than other regeneration materials, such as phosphates, and the avoidance of the negative health effects and expense associated with very small particle sizes.
In some embodiments, the bath may subsequently be heated to facilitate the dissolution of the carbonate salt, as the solubility of the carbonate salt in the bath may increase as the temperature of the bath increases. In some embodiments, the molten salt bath may be heated to a temperature greater than or equal to 430° C. to less than or equal to 500° C., such as greater than or equal to 440° C. to less than or equal to 490° C., greater than or equal to 450° C. to less than or equal to 480° C., or greater than or equal to 460° C. to less than or equal to 470° C., and all ranges and sub-ranges between the foregoing values. In some embodiments, the molten salt bath may be heated before the addition of the carbonate salt.
The bath may then be cooled to form lithium carbonate precipitates in the bath. In some embodiments, the bath may be cooled to a temperature desired for the ion exchange process. The formation of the lithium carbonate precipitates may remove lithium ions from the bath, reducing the concentration of lithium ions in the bath. For example, the formation of lithium carbonate precipitates reduces the degree of lithium poisoning of the bath.
In some embodiments, the temperature at which the molten salt bath is maintained to carry out the ion exchange process is selected such that the added carbonate salt is soluble in the molten salt bath but lithium carbonate is not. Such a temperature allows for the added carbonate salt to be dissolved in the molten salt bath while lithium carbonate precipitates from the molten salt bath, removing lithium ions from the molten salt bath.
In some embodiments, as shown in
A lithium containing glass substrate 230 may be ion exchanged in molten salt bath 240 to form an ion exchange glass article. The temperature of bath 240 during the ion exchange may be any of the desired bath temperatures for ion exchange described above. Similarly, the ion exchange may extend for any of the time periods described above. The carbonate ions in molten salt 240 bath may interact with the lithium ions introduced to the molten salt bath from the glass-based substrate to form lithium carbonate precipitates, slowing the rate of the lithium poisoning of the bath. After the ion exchange, the molten salt bath may contain lithium ions. In some embodiments, the molten salt bath 240 after ion exchange contains less than 2.0 wt % LiNO3, such as less than 1.9 wt % LiNO3, less than 1.8 wt % LiNO3, less than 1.7 wt % LiNO3, less than 1.6 wt % LiNO3, less than 1.5 wt % LiNO3, less than 1.4 wt % LiNO3, less than 1.3 wt % LiNO3, less than 1.2 wt % LiNO3, less than 1.1 wt % LiNO3, less than 1.0 wt % LiNO3, less than 0.9 wt % LiNO3, less than 0.8 wt % LiNO3, less than 0.7 wt % LiNO3, less than 0.6 wt % LiNO3, less than 0.5 wt % LiNO3, less than 0.4 wt % LiNO3, less than 0.3 wt % LiNO3, less than 0.2 wt % LiNO3, or less than 0.1 wt % LiNO3, and all sub-ranges and ranges between the foregoing values.
Although the addition of carbonate ions to the molten salt bath may aid in preventing lithium poisoning, the Li2CO3 that is produced may negatively affect the glass articles produced through the ion exchange process. For example, Li2CO3 crystals within the bath may be deposited on the surfaces of glass substrates being ion exchanged within the molten salt bath. If moisture is present in the atmosphere above the salt bath, the Li2CO3 may react with water to produce OH— ions. The presence of such OH— ions may elevate the pH of the salt bath at the interface of the Li2CO3 crystals and the glass substrate, which may induce localized etching of the glass. The etching process can leave permanent structures on the glass surface, and the increase in pH may lead to the corrosion of the glass, making the surface rough. Both etching and corrosion result in formation of transmittance haze on the glass article. As used herein, “transmittance haze” means the amount of light that is subject to wide angle scattering at an angle greater than 2.5° from normal when passing through a material measured according to ASTM D1003 “Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics.”
Transmittance haze due to Li2CO3 is typically an issue in molten salt baths that have been used for ion exchange of glass substrates several times. For example, transmittance haze may occur when the Li2CO3 reaches its solubility limit, for example from 0.1 wt % to 0.2 wt %, or when the concentration of the Li2CO3 within a molten salt bath, composed of 86% KNO3, 5% K2CO3 and 9% NaNO3 at a temperature of 380° C., reaches approximately 0.3 wt %, although this concentration may vary depending on the salt composition, the temperature and moisture level within the tank.
In some embodiments, the concentration of Li2CO3 within the molten salt bath may reach 0.1 wt % to 1 wt % before reducing the concentration of the Li2CO3 and/or increasing the solubility limit of Li2CO3. For example, the concentration of Li2CO3 may be in a range of from greater than or equal to 0.1 wt % to less than or equal to 1 wt %, such as from greater than or equal to 0.2 wt % to less than or equal to 0.9 wt %, from greater than or equal to 0.3 wt % to less than or equal to 0.8 wt %, from greater than or equal to 0.4 wt % to less than or equal to 0.7 wt %, or from greater than or equal to 0.5 wt % to less than or equal to 0.6 wt %, and all ranges and sub-ranges between the foregoing values. In some embodiments, the concentration of Li2CO3 may be in a range of from greater than or equal to 0.1 wt % to less than or equal to 0.3 wt %.
However, the ability to reuse a molten salt bath to ion exchange as many glass substrates as possible is desirable in order to reduce cost and improve the yield of glass articles produces without transmittance haze.
There are a few different methods of preventing transmittance haze by either controlling the lithium concentration in the molten salt bath to avoid formation of Li2CO3 crystals, or by controlling the pH of the molten salt bath to ensure it remains low in order to avoid glass etching.
Alternatively, in some embodiments, anhydrous sodium or potassium phosphate salts may be added into the molten salt bath in order to react with lithium and form lithium phosphate salts, for example, as shown in reaction 308. In some embodiments, a concentration of anhydrous sodium salt added to the molten salt bath is within a range of 0.05 wt % to 3 wt %. For example, the concentration of anhydrous sodium salt added to the molten salt bath may be from greater than or equal to 0.05 wt % to less than or equal to 3 wt %, such as from greater than or equal to 0.1 wt % to less than or equal to 2.5 wt %, from greater than or equal to 0.2 wt % to less than or equal to 2.4 wt %, from greater than or equal to 0.3 wt % to less than or equal to 2.3 wt %, from greater than or equal to 0.4 wt % to less than or equal to 2.2 wt %, from greater than or equal to 0.5 wt % to less than or equal to 2.1 wt %, from greater than or equal to 0.6 wt % to less than or equal to 2 wt %, from greater than or equal to 0.7 wt % to less than or equal to 1.9 wt %, from greater than or equal to 0.8 wt % to less than or equal to 1.8 wt %, from greater than or equal to 0.9 wt % to less than or equal to 1.7 wt %, from greater than or equal to 1 wt % to less than or equal to 1.6 wt %, from greater than or equal to 1.1 wt % to less than or equal to 1.5 wt %, or from greater than or equal to 1.2 wt % to less than or equal to 1.4 wt %, and all ranges and sub-ranges between the foregoing values.
Similarly, in some embodiments, a concentration of potassium phosphate salt added to the molten salt bath is within a range of 0.05 wt % to 3 wt %. For example, the concentration of potassium phosphate salt added to the molten salt bath may be from greater than or equal to 0.05 wt % to less than or equal to 3 wt %, such as from greater than or equal to 0.1 wt % to less than or equal to 2.5 wt %, from greater than or equal to 0.2 wt % to less than or equal to 2.4 wt %, from greater than or equal to 0.3 wt % to less than or equal to 2.3 wt %, from greater than or equal to 0.4 wt % to less than or equal to 2.2 wt %, from greater than or equal to 0.5 wt % to less than or equal to 2.1 wt %, from greater than or equal to 0.6 wt % to less than or equal to 2 wt %, from greater than or equal to 0.7 wt % to less than or equal to 1.9 wt %, from greater than or equal to 0.8 wt % to less than or equal to 1.8 wt %, from greater than or equal to 0.9 wt % to less than or equal to 1.7 wt %, from greater than or equal to 1 wt % to less than or equal to 1.6 wt %, from greater than or equal to 1.1 wt % to less than or equal to 1.5 wt %, or from greater than or equal to 1.2 wt % to less than or equal to 1.4 wt %, and all ranges and sub-ranges between the foregoing values.
Lithium phosphate salts, for example Li3PO4, Li2NaPO4, and Li2KPO4, have low solubility within the molten salt bath. Accordingly, the lithium can easily be precipitated and formation of Li2CO3 crystals may be avoided, as there is not enough Li+ present in the bath to react with CO32−. Like with silicic acid, alkali phosphate salts may be added to the molten salt bath either before the ion exchange process, or after a certain number of ion exchanges have occurred. Also similar to silicic acid, though, if too much alkali phosphate salt is added to the molten salt bath, sludge may form on the bottom of the tank.
In some embodiments, reducing the concentration of the Li2CO3 may be performed when the concentration of a non-carbonate lithium-containing salt (LiNO3) within the molten salt bath reaches a concentration of 0.3 wt %.
Or, in some embodiments, the molten salt bath may be exposed to a CO2 atmosphere during the ion exchange process. The addition of CO2 may quench the production of haze in a number of ways without producing sludge. The reaction mechanism fundamentals are as follows:
a. Chemical reaction: CO32−+H2O↔2OH−+CO2 (for example, reaction 312);
As shown in equation (a), as moisture in the atmosphere within the furnace, and over the molten salt bath, enters the bath, it may react with carbonate salts to generate OH− ions. The equilibrium OH− concentration is determined by equation (c), which suggests that CO2 may reduce the OH− concentration in the salt bath by increasing the partial pressure of CO2, and simultaneously decreasing the partial pressure of H2O. In some embodiments, the partial pressure of CO2 may be increased by more than 2500 times, for example from 0.3 mmHg to 760 mmHg, resulting in the reduction of OH− by about 50 times, or up to 100 times, thereby quenching the glass etching process. Additionally, the CO2 may replace the original air atmosphere within the furnace. Accordingly, the moisture in the original air atmosphere may be prevented from entering the salt bath, thereby further preventing formation of transmittance haze.
Furthermore, in some embodiments, the addition of CO2 may aid in controlling the reversible reaction between Li2CO3 and the silicate glass (Li2CO3+SiO2 (glass)↔Li2SiO3+CO2), for example reaction 310. For example, when the partial pressure of the CO2 in the atmosphere is increased to 760 mmHg, the reaction is likely to occur in the left direction (Li2SiO3+CO2→Li2CO3+SiO2 (glass)), thereby quenching corrosion of the glass by Li2SiO3.
Referencing
CO2 gas may be introduced into furnace 210 via gas outlet 250 to create a CO2 atmosphere 252 within furnace 210. In some embodiments, gas outlet 250 may be placed in tank 240, and CO2 gas may be bubbled directly into molten salt bath 240 for one hour or more. Gas outlet 250 may then be removed from tank 240, and placed beside it such that the CO2 gas may completely fill an interior space 212 within furnace 210, thereby creating a CO2 atmosphere 252 that remains in contact with molten salt bath 240. In some embodiments, the bubbling step may be omitted. In some embodiments, a concentration of CO2 within the air inside the furnace is within a range of 10 wt % to 20 wt % in order to create a CO2 atmosphere that is effective to increase a solubility limit of a lithium-containing carbonate salt within the molten salt bath. For example, the concentration of the CO2 within the air inside the furnace may be greater than or equal to 10 wt % to less than or equal to 20 wt %, such as from greater than or equal to 11 wt % to less than or equal to 19 wt %, from greater than or equal to 12 wt % to less than or equal to 18 wt %, from greater than or equal to 13 wt % to less than or equal to 17 wt %, or from greater than or equal to 14 wt % to less than or equal to 16 wt %, and all ranges and sub-ranges between the foregoing values. In some embodiments, the CO2 atmosphere may be effective to increase a solubility limit of a lithium-containing carbonate salt within the molten salt bath by 0.1 wt % or more.
In some embodiments, the CO2 may be introduced into furnace 210 prior to beginning the ion exchange process by immersing glass substrate 230 in molten salt bath 240, or it may be introduced during the ion exchange process.
In some embodiments, a CO2 atmosphere may be provided in conjunction with the introduction of silicic acid into the molten salt bath. In some embodiments, a CO2 atmosphere may be provided in conjunction with the introduction of anhydrous phosphate salts into the molten salt bath.
In some embodiments, the CO2 may be introduced into the furnace after the molten salt bath has been already been used to ion exchange multiple glass substrates. The introduction of carbonate salts may counteract lithium poisoning within the bath, and the CO2 may further improve the longevity of the bath by preventing formation of Li2CO3 crystals. In some embodiments, under CO2 atmosphere, a single molten salt bath may quench haze formation to at least 0.45 m2/kg salt, meaning that 0.45 square meters is the maximum glass substrate surface area that may be ion exchanged per 1 kg of salt before haze begins to form. In some embodiments, a single molten salt bath may be used to ion exchange a number of glass substrates, including from 2 to 5,000 glass substrates; from 5 to 5,000 glass substrates, from 10 to 5,000 substrates, from 10 to 2,000 substrates, from 10 to 1,000 substrates and from 10 to 500 substrates.
In some embodiments, the concentration of a non-carbonate lithium-containing salt within the molten salt bath may be monitored such that when a certain concentration is reached, the CO2 gas may be introduced into the furnace to increase the solubility limit of Li2CO3. In some embodiments, when the concentration of the non-carbonate lithium-containing salt within the molten salt bath reaches a concentration of 0.3 wt %, the CO2 gas may be introduced into the furnace to increase the solubility limit of Li2CO3. In some embodiments, the concentration of the non-carbonate lithium-containing salt may be monitored by collecting a sample of salt from the molten salt bath, dissolving the sample in water, and quantifying the concentration of inorganic cations, for example Li+, K+, or Na+, using ion chromatography. The concentration of Li+ cations may be used to calculate the concentration of LiNO3.
Upon being removed from the molten salt bath, the glass article subjected to the ion exchange process under CO2 atmosphere may have little to no transmittance haze on its surface, even when ion exchanged in a molten salt bath that had previously been used for ion exchange of glass substrates more than 50 times. In some embodiments, all glass articles that are subjected to the ion exchange process in the same molten salt bath may have little to no transmittance haze on their surfaces. In some embodiments, glass substrates that are ion exchanged under CO2 atmosphere may have a transmittance haze of less than 0.05%, 0.04%, 0.03%, 0.02%, or 0.01%.
In some embodiments, up to 0.5 wt % of a concentration of the lithium-containing carbonate salt in the molten salt bath may be reduced as discussed herein. For example, the concentration of the lithium-containing carbonate salt in the molten salt bath may be reduced by greater than or equal to 0.1 wt % to less than or equal to 0.5 wt %, such as from greater than or equal to 0.15 wt % to less than or equal to 0.45 wt %, from greater than or equal to 0.2 wt % to less than or equal to 0.4 wt %, or from greater than or equal to 0.25 wt % to less than or equal to 0.35 wt %, and all ranges and sub-ranges between the foregoing values. These reductions may be achieved by dissolving silicic acid in the molten salt bath at a weight percentage as described herein and/or dissolving at least one of anhydrous sodium or a potassium phosphate salt in the molten salt bath at the weight percentages described herein.
The concentration of the lithium-containing carbonate salt within the molten salt bath may be measured using ion chromatography. A sample of the molten salt bath, weighing approximately 5 grams, is collected and placed into water to dissolve the salts completely. The volume of water used depends on the sensitivity of the ion chromatography instrument. For example, some ion chromatography instruments are capable of measuring ion concentrations when 5 grams sample of salts is dissolved in 250 mL of water. The ion chromatography instrument then measures the conductivities of different liquid zones within the sample, as each zone has differently charged ions. The conductivities of the zones are positively correlated with the concentrations of ions inside each zone. For example, when the concentration of ions is high, the conductivity is also high. Tests can be run for several minutes to several hours, depending on the diffusion speed of different ions within the sample, and are run at room temperature.
The results from the ion chromatography tests are then compared to a calibration sample. The calibration sample is prepared by testing a sample with a known ion concentration, and then determining the correlation constant between the measured conductivity and the known concentration. The measured conductivity is linearly related with the concentrations of ions, such that the results of a test sample with an unknown concentration can be plotted against the results of the calibration sample to determine ion concentration.
The concentration of non-carbonate lithium-containing salts may also be measured using the ion chromatography method described above.
As shown in
Because of their optical clarity and increased strength due to the induced surface compressive stresses, the glass articles disclosed herein may be incorporated into another article such as an article with a display (or display articles) (e.g., consumer electronics, including mobile phones, tablets, computers, navigation systems, and the like), architectural articles, transportation articles (e.g., automobiles, trains, aircraft, sea craft, etc.), appliance articles, or any article that requires some transparency, scratch-resistance, abrasion resistance or a combination thereof. An exemplary article incorporating any of the glass articles disclosed herein is shown in
While various embodiments have been described herein, they have been presented by way of example, and not limitation. It should be apparent that adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It therefore will be apparent to one skilled in the art that various changes in form and detail can be made to the embodiments disclosed herein without departing from the spirit and scope of the present disclosure. The elements of the embodiments presented herein are not necessarily mutually exclusive, but may be interchanged to meet various situations as would be appreciated by one of skill in the art.
Embodiments of the present disclosure are described in detail herein with reference to embodiments thereof as illustrated in the accompanying drawings, in which like reference numerals are used to indicate identical or functionally similar elements. References to “one embodiment,” “an embodiment,” “some embodiments,” “in certain embodiments,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.
The examples are illustrative, but not limiting, of the present disclosure. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in the field, and which would be apparent to those skilled in the art, are within the spirit and scope of the disclosure.
The indefinite articles “a” and “an” to describe an element or component means that one or at least one of these elements or components is present. Although these articles are conventionally employed to signify that the modified noun is a singular noun, as used herein the articles “a” and “an” also include the plural, unless otherwise stated in specific instances. Similarly, the definite article “the,” as used herein, also signifies that the modified noun may be singular or plural, again unless otherwise stated in specific instances.
As used in the claims, “comprising” is an open-ended transitional phrase. A list of elements following the transitional phrase “comprising” is a non-exclusive list, such that elements in addition to those specifically recited in the list may also be present. As used in the claims, “consisting essentially of” or “composed essentially of” limits the composition of a material to the specified materials and those that do not materially affect the basic and novel characteristic(s) of the material. As used in the claims, “consisting of” or “composed entirely of” limits the composition of a material to the specified materials and excludes any material not specified.
Where a range of numerical values is recited herein, comprising upper and lower values, unless otherwise stated in specific circumstances, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the claims be limited to the specific values recited when defining a range. Further, when an amount, concentration, or other value or parameter is given as a range, one or more preferred ranges or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether such pairs are separately disclosed. Finally, when the term “about” is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to. Whether or not a numerical value or end-point of a range recites “about,” the numerical value or end-point of a range is intended to include two embodiments: one modified by “about,” and one not modified by “about.”
As used herein, the term “about” means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art.
The present embodiment(s) have been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.
It is to be understood that the phraseology or terminology used herein is for the purpose of description and not of limitation. The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined in accordance with the following claims and their equivalents.
This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No. 62/942,425 filed on Dec. 2, 2019, the content of which is relied upon and incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62942425 | Dec 2019 | US |
