A calcareous lithium aluminosilicate glass and a method of making the same

Abstract

This invention discloses a calcium-lithium aluminum silicate glass and its preparation method, wherein the calcium-lithium aluminum silicate glass comprises the following raw materials by mass percentage: SiO2 60-70wt%; Al2O3 3 16‑23wt%; Li2O 3‑7wt%; CaO 3‑8wt%; Y2O 3 0.1-3 wt%, also includes B2O 3 0.1‑3wt%; MgO 0‑3wt%; ZnO 0‑2wt%; La2O 3 0.1-2 wt%, ZrO 2 One or more of the following: 0-0.5 wt%. The preparation method involves melting and molding the raw material followed by two-step chemical strengthening. The calcium-lithium aluminum silicate glass of this invention exhibits excellent chemical stability, drop resistance, and scratch resistance, and can be widely used in electronic cover glass.

Description

Technical Field

[0001] This invention belongs to the field of lithium aluminum silicate glass and glass processing technology, specifically relating to a calcium lithium aluminum silicate glass and its preparation method. Background Technology

[0002] Lithium-aluminum silicate glass (LiASi) is a high-strength glass system primarily composed of alkali metal oxides R₂O, Al₂O₃, and SiO₂. It features a dense network structure, high elastic modulus, and suitability for two-step chemical tempering, and is considered a second-generation high-strength glass substrate. After chemical strengthening, LiASi glass exhibits high compressive stress and a deep ion exchange layer, making it widely used in protective cover glass for smartphones, portable terminals, and automotive displays. However, during cover glass processing, due to its poor chemical stability, numerous fine scratches appear on the glass surface after acid or alkali immersion cleaning, leading to problems such as bluing, haziness, and fogging. Furthermore, current chemically strengthened LiASi glass exhibits relatively low stress, thus its drop resistance is currently limited to no more than 1.2 meters. With the development of new display technologies and the increasing demands from end customers for the durability of electronic cover glass, there is a need for better impact resistance, scratch resistance, and drop resistance, with a drop height requirement of over 1.5m. Existing sodium lithium aluminum silicate glass, after being used by customers, still needs further improvement in its chemical stability, scratch resistance, and drop resistance.

[0003] Chinese patent CN113480169A discloses an acid and alkali resistant lithium aluminum silicon glass, comprising SiO2, Al2O3, Na2O, K2O, MgO, ZrO2, Li2O and a clarifying agent. The glass prepared by this formula has the advantages of strong chemical corrosion resistance, high hardness, high light transmittance, high strength and impact resistance, and is suitable for various protective glasses for displays.

[0004] Chinese patent CN110590156B discloses chemically strengthenable ultrathin glass and its preparation method. The composition, by weight percentage, is SiO2 50-58%, Al2O3 16-20%, Na2O 11-13%, MgO 6-10%, K2O 1.5-3%, CaO 0.5-1%, and TiO2 3-5%. After melting, the glass is cast or drawn into glass ingots, cut into glass sheets using diamond wire, and then chemically strengthened. This invention, by selecting a glass ingot cutting process, differs from the limitations of float glass and overflow glass methods that directly produce thin sheets, thus overcoming the limitations of process window on formula selection. The resulting glass is thinner than that produced by float glass and overflow methods, and exhibits excellent scratch and impact resistance after chemical strengthening. It can be widely used in front protective panels and rear protective covers for mobile devices.

[0005] Chinese patent CN112723736B discloses glass, tempered glass, its preparation method, and electronic products. The glass, by mass percentage, comprises: SiO2 50%–63%, Al2O3 23.1%–33%, Li2O 4%–7%, Na2O 1.5%–5.9%, K2O 0.01%–3%, B2O3 0.4%–6%, ZrO2 0.4%–3%, MgO 1%–5%, P2O 50%–4%, CaO 0%–3%, and ZnO 0%–2%. By adjusting the composition and proportions, the glass, after tempering, possesses a surface Vickers hardness exceeding 700 HV, giving it excellent scratch resistance, a four-point bending strength exceeding 740 MPa, and the ability to withstand drops from a height of over 170 cm using 180-grit sandpaper. It exhibits excellent scratch resistance and drop resistance.

[0006] However, existing glass formulations all contain Na₂O and / or K₂O. During subsequent chemical strengthening of the glass, Na₂O... + and K + Lithium aluminosilicate glass (LALS) exhibits a certain degree of resistance to ion exchange, resulting in high corrosivity and reduced chemical stability, which in turn leads to a high defect rate during glass processing, limiting its application. Furthermore, its poor chemical stability significantly restricts the use of glass-ceramics. While magnesium aluminum spinel or zinc aluminum spinel SALS ...

[0007] This invention improves the glass's chemical stability by adjusting its composition, reducing the alkali metal oxide content and increasing the alkaline earth metal oxide content, thereby reducing surface scratches during acid and alkali solution cleaning and increasing the stress value CS_30 at 30μm after chemical strengthening. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a calcium-lithium aluminum silicate glass and its preparation method, and the prepared glass exhibits good chemical stability, drop resistance, and scratch resistance.

[0009] To achieve the above objectives, the present invention provides a calcium-based lithium aluminum silicate glass, wherein the calcium-based lithium aluminum silicate glass is composed of the following components by mass percentage of oxides: SiO2 60-70wt%, Al2O3 16-23wt%, Li2O 3-7wt%; CaO 3-8wt%; B2O3 0.1-3wt%; Y2O3 0.1-3wt%; MgO 0-3wt%; ZnO 0-2wt%; La2O3 0.1-2wt%; ZrO2 0-0.5wt%; wherein 0.5 < (Li2O + B2O3 + CaO) / Al2O3 < 0.7; 0.5 < CaO / (CaO + MgO) ≤ 1.

[0010] Preferably, the calcium-based lithium aluminum silicate glass is composed of the following components by mass percentage of oxides: SiO2 62-66wt%, Al2O3 18-21wt%, Li2O 4-6wt%; CaO 3.5-5.5wt%; B2O3 0.1-3wt%; Y2O3 0.1-2wt%; MgO 0-3wt%; La2O3 0.1-1wt%; wherein 0.5 < (Li2O + B2O3 + CaO) / Al2O3 < 0.7; 0.55 < CaO / (CaO + MgO) ≤ 0.70.

[0011] More preferably, the calcium-lithium-aluminum-silicate glass, when etched with a 5wt% hydrochloric acid solution at 95°C for 24 hours, exhibits an etching amount not exceeding 0.1 mg / cm³. 2 At 95℃, etched with 5wt% NaOH solution for 6 hours, the etch amount was not high, only 0.6 mg / cm³. 2 .

[0012] In this invention, SiO2 is an important glass-forming oxide and an essential component. It exists in the glass network structure as silicon-oxygen tetrahedra and is necessary for forming the glass skeleton. SiO2 is beneficial for obtaining glasses with good chemical stability, intrinsic strength, and scratch resistance. If the mass percentage of SiO2 is less than 60 wt%, the glass network structure is weak, and its chemical stability and intrinsic strength deteriorate, failing to meet the performance requirements of cover glass. If the mass percentage of SiO2 exceeds 70 wt%, the glass viscosity will increase significantly, making it difficult to remove bubbles in the melt during glass refining, resulting in a lower product yield. Furthermore, excessively high SiO2 content is detrimental to chemical strengthening ion exchange. Therefore, in this invention, the mass percentage of SiO2 is 60-70 wt%, more preferably 62-66 wt%.

[0013] In this invention, Al₂O₃ is a key glass component, an essential element that reduces the glass's crystallization tendency and improves its chemical stability, scratch resistance, and surface hardness. Al₂O₃ significantly improves the glass's elastic modulus and, after ion exchange, forms high surface compressive stress on the glass surface. Furthermore, Al in the glass... 3+ The formation of an aluminum-oxygen tetrahedral network [AlO4] is preferred, which promotes the migration and replacement rate of ions in the glass network structure. The higher the Al2O3 content, the larger the gaps in the framework network, which is more conducive to ion exchange, thus forming a deeper ion exchange layer. Therefore, the content is not less than 18 wt%. When the Al2O3 content is higher, the high-temperature viscosity of the glass increases significantly, the melting temperature is too high, energy consumption increases, and defects such as bubbles and inclusions increase significantly. In this invention, the Al2O3 content is not higher than 23 wt%. Therefore, in this invention, the mass percentage of Al2O3 is 16-23 wt%, more preferably 18-21 wt%.

[0014] In this invention, B2O3 is a forming oxide and an essential component. It can lower the high-temperature melting temperature of glass, reduce the coefficient of thermal expansion, improve the chemical stability of glass, reduce the tendency of glass crystallization, and improve the scratch resistance of glass. However, when the B2O3 content is too high, the ion exchange capacity of the glass decreases, and it will also result in excessively low surface compressive stress. In this invention, the B2O3 content is 0.1-3 wt%.

[0015] In this invention, Li₂O is a network-external oxide that effectively reduces high-temperature viscosity and lowers the glass melting temperature. During the chemical strengthening process, Li in the glass... + With Na in molten salt + Ion exchange can form a high compressive stress layer depth, improving the glass's impact resistance, scratch resistance, and drop resistance. If the mass percentage of Li2O is less than 3%, it is difficult for the glass to achieve a higher stress layer depth; if the mass percentage of Li2O is greater than 7 wt%, the glass expansion coefficient increases, and the tendency for glass crystallization increases. Therefore, in this invention, the mass percentage of Li2O is 3-7 wt%, preferably 4-6 wt%.

[0016] Sodium oxide (Na₂O) and potassium oxide (K₂O) are oxides in the outer layer of the glass network, lowering the melting temperature of aluminosilicate glass. During chemical strengthening, when the glass contains high levels of Na₂O and K₂O, ion exchange occurs in the glass within a molten salt containing sodium nitrate. This leads to the presence of Na₂O in the glass. + and K + When ions are present, they will hinder Li + and Na + Ion exchange leads to Li + and Na +Since the depth of ion exchange is limited, Na2O and K2O are not introduced into the glass composition of the present invention, and Na2O is not higher than 0.5 wt%, more preferably not higher than 0.1 wt%.

[0017] In this invention, CaO, as an oxide component of the network exosome, can lower the high-temperature melting temperature of glass and promote its melting and refining. Experiments have shown that when CaO is introduced into glass, it provides additional cation vacancies within the glass structure, offering favorable channels for ion diffusion and exchange during chemical strengthening. CaO influences the charge distribution and chemical potential on the glass surface. 2+ The presence of ions increases the negative charge on the glass surface, attracting more sodium ions to migrate from the molten salt to the glass surface. This facilitates the penetration of sodium ions into the glass interior, increasing the depth of ion exchange and compressive stress, especially improving the CS_30 value. A higher CS_30 value significantly improves the glass's drop resistance, making it an essential component of this invention. However, excessively high CaO content leads to an overly porous glass structure, hindering K... + Ions and Na + The exchange. Therefore, in this invention, the mass percentage of CaO is 3-8 wt%, preferably 3.5-5.5 wt%.

[0018] In this invention, MgO is an external oxide of the network, which can reduce the high-temperature viscosity of glass and increase its elastic modulus, resulting in a higher stress value. However, excessive MgO may form [MgO4] in the glass network structure, hindering ion exchange and leading to an insufficient ion exchange depth. Therefore, in this invention, the MgO content is 0-5 wt%, preferably 0-2 wt%, and more preferably 0-1 wt%.

[0019] ZnO is a divalent metal oxide and functions similarly to alkaline earth metal oxides, effectively lowering the melting temperature of glass and improving its alkali resistance. However, excessive ZnO can hinder ion exchange. Therefore, the mass percentage of ZnO is 0–2 wt%. More preferably, the glass does not contain ZnO.

[0020] Y₂O₃, as a network modifier for glass, can significantly reduce the high-temperature viscosity of glass, which is beneficial for glass melting. The Y₂O₃ composition can improve the micro-Vickers hardness, elastic modulus, and other mechanical properties of glass, thereby enhancing its mechanical properties. However, when the Y₂O₃ content is high, it increases the tendency of glass crystallization. Therefore, the Y₂O₃ content in the glass should not exceed 3 wt%. Thus, in this invention, the Y₂O₃ content is 0.1-3 wt%, more preferably 0.1-2 wt%.

[0021] La2O3, as a network modifier for glass, can improve the elastic modulus and intrinsic strength of glass. However, when the content of La2O3 is high, the tendency of glass crystallization increases. Therefore, the content of La2O3 in the glass should not exceed 2 wt%. Thus, the content of La2O3 in the glass should not exceed 2 wt%, and therefore, in this invention, the content of La2O3 is 0.1-2 wt%, more preferably 0.1-1 wt%.

[0022] This invention also provides a method for preparing calcium lithium aluminum silicate glass, comprising the following steps:

[0023] (1) Weigh and mix all silicon-containing raw materials, aluminum raw materials, boron raw materials, lithium raw materials, calcium raw materials, etc. to obtain a mixture;

[0024] (2) The mixture is melted at high temperature, clarified and homogenized, shaped and annealed to obtain calcium lithium aluminum silicon glass;

[0025] (3) The calcium-lithium aluminum-silicon glass is placed in the first strengthening molten salt for a first strengthening process;

[0026] (4) The primary strengthened glass is placed in the second strengthening molten salt for secondary strengthening to obtain a two-step chemically strengthened calcium lithium aluminum silicate glass.

[0027] Preferably, the uniformity of the mixture in step (1) is ≥98%.

[0028] Preferably, the melting temperature in step (2) is 1520-1660℃; the molding process is any one of float method, overflow pull method, slot pull method or casting.

[0029] Preferably, the first strengthening molten salt in step (3) is 50%-100% NaNO3, with the balance being KNO3.

[0030] Preferably, the second strengthening molten salt in step (4) is 90%-100% KNO3, with the balance being K2CO3 and / or NaNO3.

[0031] More preferably, the second enhanced molten salt contains 0.01-3% K2CO3.

[0032] Preferably, the primary strengthening temperature in step (3) is 380-450℃ and the time is 2-5h; the secondary strengthening temperature in step (4) is 400-480℃ and the time is 2-6h.

[0033] Preferably, the two-part chemically strengthened calcium lithium aluminum silicate glass obtained in step (4) has a compressive stress on the glass surface greater than 800 MPa, a compressive stress at 30 μm greater than 160 MPa, a sandpaper drop height of more than 1.7 m, and a scratch resistance threshold greater than 7 N.

[0034] The calcium-based lithium aluminum silicate glass of this invention can undergo two-step chemical strengthening. The first strengthening step involves placing the glass in a molten salt solution containing 90%-100% NaNO3 at 380-450℃ for 2-5 hours, forming an ion exchange layer (DOL) with a depth greater than 120 μm. The second strengthening step involves placing the glass, treated in the first step, in a solution containing 90%-100% KNO3 at 400-480℃ for 2-6 hours, forming a surface compressive stress (CS) greater than 800 MPa and a compressive stress at 30 μm greater than 160 MPa. The glass can withstand a sandpaper drop height of over 1.7 m and has a scratch resistance threshold greater than 7 N, thus achieving high surface compressive stress and a deep ion exchange layer, significantly improving the mechanical properties of the glass. Compared with other lithium aluminum silicate glasses, calcium-based lithium aluminum silicate glass has significant advantages in drop resistance and impact resistance, thereby improving the durability of cover glass.

[0035] The beneficial effects of this invention are as follows:

[0036] This invention utilizes CaO to replace the Na₂O and / or K₂O composition of conventional silicate glass, introducing a higher CaO content. By controlling the ratio of 0.5 ≤ (B₂O₃ + CaO + Li₂O) / Al₂O₃ ≤ 0.7 and 0.5 < CaO / RO ≤ 1, the glass network connectivity is enhanced, improving the glass's chemical resistance and intrinsic strength. A two-step chemical strengthening process is employed, achieving a surface compressive stress ≥ 800 MPa and a compressive stress at 30 μm ≥ 160 MPa. This invention, by introducing a higher concentration of alkaline earth metal oxides and reducing the alkali metal oxide content, provides intrinsic strength to the glass, reduces microcracks caused by acid and alkali etching, and increases the stress value at 30 μm, thereby improving the glass's scratch and drop resistance. Attached Figure Description

[0037] Figure 1 The surface stress diagram of the calcium lithium aluminum silicate glass prepared in Example 9 was tested using an FSM-6000LEUV after chemical strengthening. Detailed Implementation

[0038] The technical solution of the present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention should be determined by the contents of the claims. Modifications and substitutions made by those skilled in the art to the technical solution of the present invention without creative effort all fall within the scope of protection of the present invention.

[0039] This invention provides calcium-lithium aluminum silicate glass, and the main preparation process is as follows:

[0040] (1) Weigh each silicon-containing raw material, aluminum raw material, boron raw material, lithium raw material, calcium raw material, etc. in proportion, and mix them evenly to obtain a mixture;

[0041] (2) Then transfer the batch to a platinum crucible of about 500 mL, place the platinum crucible into a silicon molybdenum rod high temperature furnace, gradually raise the temperature to 1520-1660℃, hold the temperature for 4-8 hours, and accelerate the removal of glass bubbles and homogenize the glass by stirring.

[0042] (3) After melting, the glass liquid is poured into a stainless steel mold for shaping, and then the glass block is taken out and transferred to a box annealing furnace for 2 hours of heat treatment at 600-650℃. Then, the temperature is reduced to 450℃ at a rate of 1℃ / min and then brought to room temperature with the furnace to obtain the glass block.

[0043] (4) The glass block will be cut using a diamond wire cutter to prepare a sample that meets the requirements for melting temperature, softening point temperature, annealing point temperature, strain point temperature, expansion coefficient and elastic modulus test.

[0044] (5) The glass surface is polished by automatic precision polishing to prepare samples that meet the requirements of acid and alkali resistance and chemical strengthening.

[0045] Some embodiments of the present invention and the physical properties of the glass samples are shown in Table 1, and their definitions and explanations are as follows:

[0046] A. Melting temperature (10 2.0 P) was tested using a rotational viscometer in accordance with SJT 11040-1996 "Test Method for High Temperature Viscosity of Electronic Glass".

[0047] B. Softening point temperature (10 7.6 P): The softening point was tested using a softening point apparatus in accordance with SJ / T 11038-1996 "Test Method for Softening Point of Electronic Glass".

[0048] C. Annealing temperature (10) 13.0 P): The expansion coefficient was measured using a dilatometer in accordance with ASTM E-228, "Standard for Testing the Linear Thermal Expansion Coefficient of Solid Materials", by measuring the expansion coefficient curve.

[0049] D. Strain point temperature (10) 14.5 P): The expansion coefficient was measured using an expansion meter in accordance with ASTM E-228 "Standard for Testing the Linear Thermal Expansion Coefficient of Solid Materials".

[0050] E. Coefficient of thermal expansion: Tested using a thermal expansion meter in accordance with GB / T 16920-2015 "Determination of the average linear thermal expansion coefficient of glass".

[0051] F. Elastic modulus: The elastic modulus was tested using an elastic modulus meter in accordance with JC / T 687-1997(2007) "Test Methods for Elastic Modulus, Shear Modulus and Poisson's Ratio of Glass Materials".

[0052] G. Chemical stability test: Immerse in 5% HCl solution in a 95℃ water bath for 24 hours, and then calculate the weight loss rate before and after immersion; immerse in 5% NaOH solution in a 95℃ water bath for 6 hours, and then calculate the weight loss rate before and after immersion.

[0053] H. Surface compressive stress CS_K, i.e., compressive stress formed by K ion exchange: tested using FSM-6000LEUV according to ASTM1422C / C1422-20 "Standard Specifications for Chemically Reinforced Flat Glass".

[0054] I.DOL_K, i.e., the depth of the compressive stress layer formed by K ion exchange: tested using FSM-6000LEUV according to ASTM 1422C / C1422-20 "Standard Specifications for Chemically Reinforced Flat Glass".

[0055] J.CS_30, i.e., the compressive stress formed by Na ion exchange at 30μm on the glass surface: tested using an SLP-2000 scattered light photoelastic stress meter according to ASTM 1422C / C1422-20 "Standard Specifications for Chemically Reinforced Flat Glass".

[0056] K.DOL_0, i.e., the maximum stress layer depth formed by Na ion exchange, was tested using an SLP-2000 scattered light photoelastic stress meter in accordance with ASTM 1422C / C1422-20, "Standard Specifications for Chemically Reinforced Flat Glass".

[0057] L. Scratch resistance threshold: The scratch resistance threshold was tested using a flat glass scratch resistance tester in accordance with GB / T39815-2021 "Test Method for Scratch Resistance of Ultra-thin Glass".

[0058] M. Vickers hardness value: The Vickers hardness tester was used in accordance with GB / T 16534~2009 "Test method for room temperature hardness of fine ceramics".

[0059] Drop test method: A controlled drop tester is used. The test conditions are: 180-grit sandpaper, 170g total weight, starting from a height of 0.8m, and testing is performed once for each height, increasing by 0.1m each time, until the object breaks.

[0060] Examples 1-8

[0061] The glass formulation and preparation process parameters are shown in Table 1-2.

[0062] Table 1. Calcium-lithium aluminum silicate glass and its properties

[0063]

[0064] Note: RO stands for CaO + MgO.

[0065] As shown in Table 1, the calcium lithium aluminum silicate glass prepared under the formulation of Example 5, without chemical strengthening, exhibits the least amount of etching in hydrochloric acid solution and NaOH solution.

[0066] Table 2. Strengthening and Properties of Calcium Lithium Aluminosilicate Glass

[0067]

[0068] Table 2 shows that after two-step chemical strengthening, the drop height of the calcium-lithium aluminum silicate glass can reach over 1.7m, the scratch resistance threshold is greater than 7N, and it exhibits high surface compressive stress and a high ion exchange layer depth, significantly improving the mechanical properties of the glass. Examples 9-14

[0069] The glass formulation and preparation process parameters are shown in Table 3;

[0070] Table 3 Composition and properties of calcium-lithium aluminum silicate glass

[0071]

[0072] Note: RO stands for CaO + MgO.

[0073] Table 4. Strengthening and Properties of Lithium Aluminosilicate Glass

[0074]

[0075] As shown in Table 3-4, the hydrochloric acid etching amount of the calcium lithium aluminum silicate glass prepared through two-step chemical strengthening is less than 0.1 mg / cm². 2 The sodium hydroxide corrosion amount is less than 0.6 mg / cm². 2 It has good chemical stability; the scratch resistance threshold can be increased to over 8N, and the drop height is ≥1.7m, exhibiting good drop resistance and scratch resistance.

[0076] Comparative Examples 1-5

[0077] The effect of varying CaO content on glass properties was investigated. Specific formulations are shown in Table 5. The chemical strengthening method was the same as in Example 2.

[0078] Table 5 Comparative glass composition and properties

[0079]

[0080] Note: RO stands for CaO + MgO.

[0081] As shown in Table 5, compared with Example 2, the lithium aluminum silicate glass prepared by replacing CaO with Na2O in Comparative Examples 1, 2 and 3 has poor acid and alkali resistance, and after strengthening, it has lower resistance to CS_30, CS_K, scratch threshold and drop height, resulting in poor mechanical properties of the glass.

[0082] As shown in Table 5, compared with Example 2, comparative examples 1, 2 and 3 used CaO to replace Al2O3 to prepare lithium aluminum silicate glass. The glass had good acid and alkali resistance, but the strengthening efficiency decreased significantly when CaO exceeded 8wt%, resulting in lower CS_30, DOL_0, scratch threshold and drop height, leading to poor mechanical properties of the glass.

Claims

1. A calcium-lithium aluminum silicate glass, characterized in that: The calcium-based lithium aluminum silicate glass is composed of the following components by mass percentage of oxides: SiO2 62-66wt%, Al2O3 18-21wt%, Li2O 4-6wt%; CaO 3.5-5.5wt%; B2O3 0.1-3wt%; Y2O3 0.1-2wt%; MgO 0-3wt%; La2O3 0.1-1wt%; wherein 0.5 < (Li2O + B2O3 + CaO) / Al2O3 < 0.7; 0.55 < CaO / (CaO + MgO) ≤ 0.

70.

2. A method for preparing calcium lithium aluminum silicate glass as described in claim 1, characterized in that: Includes the following steps: (1) Weigh and mix all raw materials to obtain a mixture; (2) The mixture is melted, clarified and homogenized, shaped and annealed to obtain calcium lithium aluminum silicon glass; (3) The calcium-lithium aluminum-silicon glass is placed in the first strengthening molten salt for a first strengthening process; (4) The primary strengthened glass is placed in the second strengthening molten salt for secondary strengthening to obtain a two-step chemically strengthened calcium lithium aluminum silicate glass.

3. The preparation method according to claim 2, characterized in that: The uniformity of the mixture in step (1) is ≥98%.

4. The preparation method according to claim 2, characterized in that: The melting temperature in step (2) is 1520-1660℃; the molding process is any one of float method, overflow pull method, slit pull method or casting.

5. The preparation method according to claim 2, characterized in that: The first strengthening molten salt mentioned in step (3) is 50%-100% NaNO3, with the balance being KNO3.

6. The preparation method according to claim 2, characterized in that: The second strengthening molten salt described in step (4) is 90%-100% KNO3, with the balance being K2CO3 and / or NaNO3.

7. The preparation method according to claim 6, characterized in that: The second enhanced molten salt contains 0.01-3% K2CO3.

8. The preparation method according to claim 2, characterized in that: The first strengthening temperature in step (3) is 380-450℃, and the time is 2-5h.

9. The preparation method according to claim 2, characterized in that: The secondary strengthening temperature in step (4) is 400-480℃, and the time is 2-6h.

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