Patent Application
20110294648
Chemically strengthened glasses are used in touch screen and applications. Currently, many glasses must be ion exchanged by immersion in a molten salt bath for 8 to 10 hours to achieve a compressive layer of more than 50 microns deep with at least 500 MPa compressive stress at the surface.
An ion exchangeable glass comprising 0.1-10 mol % P2O5 is provided. The presence of P2O5 enables the glass to be ion exchanged more quickly and to a greater depth than comparable glasses that do not contain P2O5.
Accordingly, one aspect of the disclosure is to provide an ion exchangeable aluminosilicate glass. The ion exchangeable aluminosilicate glass is free of lithium and comprises 0.1-10 mol % P2O5 and at least 5 mol % Al2O3. The glass has a liquidus viscosity of at least 100 kpoise.
Another aspect of the disclosure is to provide a method of strengthening an ion exchangeable aluminosilicate glass. The method comprises providing an ion exchangeable aluminosilicate glass comprising 0.1-10 mol % P2O5, at least 5 mol % Al2O3, and a plurality of first monovalent cations; and exchanging at least a portion of the first monovalent cations with second monovalent cations to a depth of at least 20 μm in the glass article, wherein the second monovalent cations are different from the first monovalent cations. The exchanging of the second actions for the first cations in the glass article creates a compressive stress in a region adjacent to a surface of the glass article.
These and other aspects, advantages, and salient features will become apparent from the following detailed description, the accompanying drawings, and the appended claims.
In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that, unless otherwise specified, terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. In addition, whenever a group is described as comprising at least one of a group of elements and combinations thereof, it is understood that the group may comprise, consist essentially of, or consist of any number of those elements recited, either individually or in combination with each other. Similarly, whenever a group is described as consisting of at least one of a group of elements or combinations thereof, it is understood that the group may consist of any number of those elements recited, either individually or in combination with each other. Unless otherwise specified, a range of values, when recited, includes both the upper and lower limits of the range. Unless otherwise specified, all concentrations of elements and compounds are expressed in mole percent (mol %).
Referring to the drawings in general and to
Chemically strengthened glasses are used in display applications such as touch screens and ruggedized displays in mobile consumer electronics, including cell phones, mobile internet devices, entertainment devices, laptop and notebook computers, and the like. Some glasses, such as aluminosilicate glasses and alkali aluminosilicate glasses, can be strengthened chemically by a process known as ion exchange. In this process, ions in the surface layer of the glass are replaced by—or exchanged with—larger ions having the same valence or oxidation state as the ions in the surface layer of the glass. In those embodiments in which the glass comprises, consists essentially of, or consists of an alkali aluminosilicate glass, ions in the surface layer of the glass and the larger ions are monovalent alkali metal cations, such as Li+ (when present in the glass), Na+, K+, Rb+, and Cs+. Alternatively, monovalent cations in the surface layer may be replaced with monovalent cations other than alkali metal cations, such as Ag+, Cu+, Tl+, or the like. In addition, such cations can be initially present in the glass itself.
Ion exchange processes typically include immersing the aluminosilicate glass in a molten salt bath containing the larger ions to be exchanged with the smaller ions in the glass. By way of example, ion exchange of alkali metal-containing glasses may be achieved by immersion in at least one molten salt bath containing a salt such as, but not limited to, nitrates, sulfates, and chlorides of the larger alkali metal ion. The temperature of the molten salt bath is typically in a range from about 380° C. up to about 450° C., and immersion times range from about 2 hours up to about 16 hours. Such ion exchange treatments typically result in strengthened alkali aluminosilicate glasses having an outer surface layer (also referred to herein as the “depth of layer” or “DOL”) that is under compressive stress (CS).
A cross-sectional view of a glass sheet strengthened by ion exchange is schematically shown in
In order to achieve a desired depth of compressive layer of more than 50 μm and/or a desired compressive stress of at least 500 MPa at the surface, alkali aluminosilicate glasses typically undergo chemical strengthening by ion exchange ion exchange for 8 to 10 hours.
Described herein is an ion exchangeable aluminosilicate glass and articles made therefrom that are capable of undergoing ion exchange at rates that are up to four times faster than those previously observed for such glasses. The aluminosilicate glasses are ion exchangeable with at least one of sodium, potassium, rubidium, cesium, copper, silver, thallium, and the like.
The glasses described herein is substantially free of lithium (i.e., lithium is not actively added to the glass during initial batching or subsequent ion exchange, but may be present as an impurity) and comprises from about 0.1 mol % up to about 10 mol % P2O5 and at least 5 mol % Al2O3. The glass has a liquidus viscosity of at least 100 kpoise and, in some embodiments, at least 135 kpoise, which allows the glass to be formed by down-draw methods (e.g., slot-draw or fusion-draw) methods known in the art. In selected embodiments, the glass has a liquidus viscosity of at least 2 megapoise (Mpoise).
In some embodiments, the P2O5 concentration is less than or equal to the difference between the total concentration of metal oxide modifiers Σ(R2O) and the Al2O3 concentration—i.e., P2O5≦[Σ(R2O)−Al2O3]. In one embodiment, the glass is an alkali aluminosilicate glass that includes at least one monovalent metal oxide modifier—e.g., Ag2O, Na2O, K2O, Rb2O, and/or Cs2O. In such glasses, the P2O5 concentration is less than or equal to the difference between the total concentration of alkali metal oxide modifiers and the Al2O3 concentration—i.e., P2O5≦[(Na2O+K2O+Rb2O+Cs2O)−Al2O3]. When the P2O5 content exceeds the excess amount of alkali modifiers, refractory batch stones and aluminum phosphate inclusions begin to form in the glass. Alkaline earth oxides like MgO, CaO, SrO, and BaO can cause phase separation and/or devitrification. Consequently, the total concentration of alkaline earth oxides should be limited to approximately no more than one half of the P2O5 concentration; i.e., ΣR′O(R=Mg, Ca, Ba, Sr)≦0.5 P2O5. Similarly, in those embodiments where the modifiers are other metal oxides such as Ag2O. Cu2O, or Tl2O, the P2O5 concentration is less than or equal to the difference between the total concentration of metal oxide modifiers and the Al2O3 concentration—i.e., P2O5≦[(Ag2O+Tl2O+Cu2O−Al2O3)].
In one embodiment, the ion exchangeable aluminosilicate glass is an alkali aluminosilicate glass that comprises, consists essentially of, or consists of: 56-72 mol % SiO2; 5-18 mol % Al2O3; 0-15 mol % B2O3; 0.1-10 mol % P2O5; 3-25 mol % Na2O; and 0-5 mol % K2O; and is free of lithium. The alkali aluminosilicate glass can, in some embodiments, further include at least one of: 0-4 mol % CaO; 0-1 mol % MgO; and up to 0.5 mol % SnO2. Exemplary compositions of the glasses described herein are listed in Table 1. Physical properties, including strain, anneal, and softening points, coefficient of thermal expansion (CTE), density, molar volume, stress optical coefficient (SOC), and liquidus temperature of these glasses are listed in Table 2. Table 3 lists compressive stresses (CS), depth of layer (DOL), and potassium diffusivity (D) for selected glasses after ion exchange for 8 hours in a KNO3 bath at either 410° C. or 370° C. In another embodiment, the glass is an aluminosilicate glass that comprises, consists essentially of, or consists of: 56-72 mol % SiO2; 5-18 mol % Al2O3; 0-15 mol % B2O3; 0.1-10 mol % P2O5; and 2-20 mol % Ag2O. This glass as well, in some embodiments, is free of lithium.
The presence of P2O5 in the aluminosilicate or alkali aluminosilicate glass accelerates the rate of ion exchange of larger cations for smaller cations present in the glass. In particular, the presence of P2O5 accelerates the rate of ion exchange of K+ ions for Na+ ions in alkali aluminosilicate glasses. The data listed in Tables 1-3 show the effect of P2O5 concentration on ion exchange and physical properties of alkali aluminosilicate glasses. Curves a-d in
The ion exchange data listed in Table 3 and plotted in Na+ diffusivity is increased by a factor of four. Thus, the addition of P2O5 to a glass can decrease the time needed to achieve a given depth of layer by a factor of four. The diffusivity D of K+ ions in a phosphorus-free annealed alkali aluminosilicate glass (sample 1 in Tables 1-3) is given by the equation
D=exp(−3.8965−13240/T),
where the diffusivity D is expressed in (cm2/sec) and the temperature T is expressed in degrees Kelvin (K). At 410° C. (683 K), which is a temperature at which the exchange of K+ ions for Na+ ions is typically carried out, the diffusivity of K+ ions in the glasses described herein is 7.8×10−11 cm2/sec. The equation provided above is derived for annealed glasses. As previously described herein, alkali aluminosilicate glasses can be formed by down-draw processes such as fusion-draw processes. The diffusivity of K+ ions in fusion-formed glasses can be taken to be about 1.4 times greater than the diffusivity of these ions in annealed glass. Thus, the diffusivity of K+ ions in fusion-formed alkali aluminosilicate glasses is estimated to be about 1.1×10−10 cm2/sec at 400° C. (673 K). The faster K+Na+ ion exchange rates that are enabled by the enhanced diffusivity of alkali metal ions in the phosphorus-containing glasses described herein have been previously achieved only with smaller ions, such as Na+
Li+, which produces a lower compressive stress than K+
Na+ ion exchange. Thus, the compositions described herein permit compressive stresses achievable with K+
Na+ ion exchange to be carried out at the speed or rate of Na+
Li+ ion exchange. In one embodiment, the alkali aluminosilicate glasses described herein, when immersed in a KNO3 molten salt bath at 410° C. for less than six hours, are capable of exchanging K+ for Na+ in the glass to a depth of at least 50 μm. In some embodiments, the alkali aluminosilicate glasses described herein, when ion exchanged, have a compressive layer with a depth of layer of at least 20 μm and a compressive stress of at least 400 MPa. In other embodiments, the glasses are ion exchanged to a depth of layer of up to 150 μm, and in still other embodiments, the glasses are ion exchanged to a depth of layer of up to 100 μm.
Potassium diffusivity is plotted as a function of P2O5 concentration in two alkali aluminosilicate glasses in
The addition of P2O5 to alkali aluminosilicate glasses also can be used to obtain low liquidus temperatures. All the glasses listed in Table 1 have liquidus temperatures of less than about 700° C. Glass #2 has a liquidus viscosity over 248 million Poise (MP), and is therefore formable by down-draw methods, such as slot-draw and fusion draw methods that are known in the art. Alternatively, the glasses described herein are also formable by other methods, such as float, molding, and casting methods that are known in the art. The presence of P2O5 also decreases the viscosity of the glass at high temperatures. An addition of 2 mole % P2O5 is capable of lowering the 200P temperature of the alkali aluminosilicate glass by 50° C., which facilitates melting and fining of the glass.
A method of strengthening a glass article is also provided. A glass article comprising an aluminosilicate glass comprising 0.1-10 mol % P2O5 and at least 5 mol % Al2O3, such as those glasses described herein, is first provided. The glass also includes a plurality of first monovalent cations such as, for example, an alkali metal cation or a monovalent cation such as Ag+, Cu+, Tl+, or the like. In some embodiments, the aluminosilicate glass is an alkali aluminosilicate glass is lithium-free and comprises, consists essentially of, or consists of: 56-72 mol % SiO2; 5-18 mol % Al2O3; 0-15 mol % B2O3; 0.1-10 mol % P2O5; 3-25 mol % Na2O; and 0-5 mol % K2O. The alkali aluminosilicate glass can, in some embodiments, further include at least one of 0-4 mol % CaO; 0-1 mol % MgO; and up to 0.5 mol % SnO2. The glass has a liquidus viscosity of at least 100 kP and, in some embodiments, at least 135 kP, and can be made by those down-draw methods (e.g., slot-draw, fusion-draw, or the like). In addition, the alkali aluminosilicate glass possesses those properties (strain, anneal, and softening points, coefficient of thermal expansion (CTE), density, molar volume, stress optical coefficient (SOC), and liquidus temperature) previously described herein and reported in Table 1b. In other embodiments, the glass is an aluminosilicate glass comprising, consisting essentially of, or consisting of: 56-72 mol % SiO2; 5-18 mol % Al2O3; 0-15 mol % B2O3; 0.1-10 mol % P2O5; and 2-20 mol % Ag2O.
In the next step, second monovalent cations are exchanged for—or replaces—at least a portion the first monovalent cations in a region adjacent to the surface of the glass. The second monovalent cation is different from the first cation and, in some embodiments, larger than the first monovalent cation. In those instances where the second cation is larger than the first cation, the replacement of first cations with second cations in the region adjacent to the surface of the glass creates a compressive stress in that region. In those instances where the glass is an alkali aluminosilicate glass, for example, K+ ions are exchanged for Na+ ions in the alkali aluminosilicate glass by ion exchange, using those methods previously described herein. Potassium ions are ion exchanged for Na+ ions to a depth of up to up to 100 μm in the glass article and, in some embodiments, up to 150 μm. In some embodiments, K+ ions are exchanged for Na+ ions to a depth of at least 20 μm in the glass article, in other embodiments, at least 50 μm, and in still other embodiments, to a depth of 150 μm.
The glasses described herein can be formed into planar sheets for use as display windows, cover plates, screens, structural features, and the like, for applications such as, but not limited to, touch screens and mobile electronic devices, including telephones and other communication devices, entertainment devices, and hand-held, laptop and notebook computers. In other embodiments, the alkali aluminosilicate glass can be formed into three dimensional, non-planar shapes, such as curved sheets or the like.
While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the disclosure or appended claims. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present disclosure or appended claims.
