Low-temperature preparation method of LiAl5O8 transparent ceramic

Abstract

The invention discloses a low-temperature preparation method of LiAl5O8 transparent ceramic, and belongs to the field of preparation of transparent ceramic materials. According to the method, gamma-LiAlO2 and alpha-Al2O3 are used as main phases, mixed powder with the chemical component being LiAl5O8 is used as a precursor, and the LiAl5O8 transparent ceramic is obtained in the temperature range of 1150-1290 DEG C through hot pressing or spark plasma sintering. According to the method, the sintering temperature is greatly reduced by utilizing the driving force formed in the process of generating LiAl5O8 through solid-phase reaction of gamma-LiAlO2 and alpha-Al2O3, so that cracking of LiAl5O8 ceramic due to ordered phase change in the cooling process is avoided from the source, and an obtained sample is complete, good in optical permeability and high in hardness.

Description

Technical Field

[0001] The present invention relates to the field of transparent ceramic material preparation, in particular to a new method for preparing LiAl5O8 transparent ceramics at low temperature. Background Art

[0002] Protective equipment, optical systems, communications equipment, medical applications, and other fields have a significant demand for ceramic materials that combine high transparency with high hardness and strength. LiAl5O8 has a cubic crystal structure and a dense atomic arrangement in the lattice, so it can well meet these performance requirements and can also compensate for the poor oxidation resistance of oxynitride ceramics ("Non-isothermal and isothermal oxidation behaviors of AlON translucent ceramic in air", Materials and Corrosion, 66(4), 2015, 328-333).

[0003] However, LiAl5O8 undergoes a reversible order-disorder phase transition at a critical temperature of 1295±5°C, accompanied by a significant volume change ("Phase Transitions in LiAl5O8", Journal of the American Ceramics Society, 46(8), 1963, 388-390). This temperature is lower than the conventional sintering temperature of transparent ceramics, posing a great challenge to their preparation. The literature ("Hot-Pressing of LiAl5O8", Journal of the American Ceramics Society, 55(3), 1972, 172-173) is the only report on the preparation of LiAl5O8 transparent ceramics to date. The sintering temperature is 1550~1600°C, and the phase transition of the highly dense ceramic body during the cooling process will cause significant stress in the sample, leading to sample cracking, which seriously restricts practical applications.

[0004] Clearly, an effective solution to these problems is to control the sintering temperature below the critical phase transition temperature. Lowering the sintering temperature also promotes grain refinement, further improving the mechanical properties of LiAl5O8 transparent ceramics. However, simply changing the hot-pressing sintering process parameters is difficult to achieve. Summary of the Invention

[0005] The main purpose of the present invention is to provide a preparation route to achieve the preparation of transparent ceramics under sintering conditions below the critical temperature of the LiAl5O8 disordered to ordered phase transition.

[0006] To achieve the above-mentioned purpose of the invention, the inventors have optimized the following solid-phase reaction pathway through theoretical screening and experimental exploration: γ-LiAlO2+2α-Al2O3══LiAl5O8 The two reactants, γ-LiAlO2 and α-Al2O3, have completely different crystal structures from LiAl5O8. Therefore, a reconfiguration phase transition occurs during the reaction. This crystal reconfiguration phase transition provides a significant driving force for sintering, significantly reducing the sintering temperature and effectively promoting the densification of the ceramic material. This is the principle of the present invention.

[0007] The technical solutions adopted by the present invention are as follows.

[0008] The invention discloses a low-temperature preparation method for LiAl5O8 transparent ceramics. The preparation method uses a mixed powder with γ-LiAlO2 and α-Al2O3 as main phases and the chemical composition of LiAl5O8 as a precursor, and obtains the LiAl5O8 transparent ceramics through hot pressing sintering or spark plasma sintering.

[0009] According to the above scheme, the precursor is obtained by mixing γ-LiAlO2 and α-Al2O3 powders, or selecting suitable lithium sources and aluminum sources through a method including but not limited to solid phase reaction method, hydrothermal method, sol-gel method, solution combustion method and co-precipitation method to obtain a mixed powder with γ-LiAlO2 and α-Al2O3 as main phases and chemical composition of LiAl5O8.

[0010] According to the above scheme, the molar ratio of γ-LiAlO2 and α-Al2O3 in the precursor is 1:2.

[0011] According to the above scheme, the lithium source and aluminum source include but are not limited to lithium and aluminum elements, oxides, nitrides, nitrates, sulfates, carbonates, acetates and alkoxides.

[0012] According to the above scheme, the powder prepared by lithium source and aluminum source has γ-LiAlO2 and α-Al2O3 as the main phases, and the chemical composition of the mixed powder is LiAl5O8, which means that the content of impurity cations is not higher than 1at%, and the molar ratio of Li and Al elements is between 0.95:5 and 1.05:5.

[0013] According to the above scheme, the hot pressing sintering or spark plasma sintering is carried out in a vacuum or inert atmosphere, the sintering temperature is 1150-1290° C., the applied pressure is 20-200 MPa, and the holding time is 0-5 h.

[0014] According to the above scheme, the sample prepared by hot pressing sintering or spark plasma sintering in the preparation method is subjected to surface grinding and polishing to obtain LiAl5O8 transparent ceramics.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The method provided by the present invention has a low sintering temperature, which prevents the cracking of LiAl5O8 ceramics due to the ordered phase transition during the cooling process from the source, thereby obtaining a complete sample with good optical transmittance and high hardness, which has important application value.

[0016] 2. The preparation method of the present invention is simple and the raw materials are easy to obtain. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is the X-ray diffraction pattern of the mixed powder in Example 3.

[0018] Figure 2 These are the appearance photos of the LiAl5O8 transparent ceramic samples prepared in Examples 1-4. DETAILED DESCRIPTION

[0019] The technical solutions and effects of the present invention are further described below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto. Example 1

[0020] γ-LiAlO2 and α-Al2O3 powders were mixed uniformly by ball milling in a molar ratio of 1:2, then passed through a 100-mesh sieve, loaded into a graphite mold, and hot-pressed and sintered under an external pressure of 22 MPa. The mixture was kept at 1285°C for 2 hours under vacuum, and LiAl5O8 transparent ceramics were obtained after grinding and polishing. Example 2

[0021] γ-LiAlO2 and α-Al2O3 powders were mixed uniformly by ball milling in a molar ratio of 1:2, then passed through a 100-mesh sieve, loaded into a graphite mold, and subjected to spark plasma sintering. An external pressure of 200 MPa was applied and the mixture was kept at 1150°C in an Ar atmosphere for 15 minutes. After grinding and polishing, LiAl5O8 transparent ceramics were obtained. Example 3

[0022] α-Al2O3 powder was added to a LiNO3 aqueous solution while stirring, achieving a molar ratio of LiNO3 to α-Al2O3 of 1:2.5. The resulting slurry was then freeze-dried and heat-treated at 900°C for 1 hour to produce a mixed powder composed primarily of γ-LiAlO2 and α-Al2O3. This powder was then passed through a 100-mesh sieve, placed in a graphite mold, and hot-pressed. The mixture was then heated at 1260°C for 1 hour under vacuum and ground and polished to yield a transparent LiAl5O8 ceramic.

[0023] The concentration ratio of Li and Al in the mixed powder of this embodiment was measured by inductively coupled plasma spectroscopy to be 5Li / Al=0.9962, and the X-ray diffraction pattern was as follows: Figure 1 As shown by Figure 1 It shows that γ-LiAlO2 and α-Al2O3 are the main phases. Example 4

[0024] LiNO₃, Al(NO₃)₃, and citric acid were dissolved in deionized water in a molar ratio of 1:5:6. Ammonia was then added dropwise with stirring to adjust the solution's pH to approximately 8.5. The resulting sol was heated to 80°C to transform it into a gel. The gel was dried at 110°C for 10 hours and then calcined at 900°C for 6 hours to obtain a mixed powder composed primarily of γ-LiAlO₂ and α-Al₂O₃ (the Li / Al ratio in the mixed powder was 5Li / Al = 1.0097, as measured by inductively coupled plasma spectroscopy). The powder was passed through a 100-mesh sieve, placed in a graphite mold, and subjected to spark plasma sintering. The mixture was heated at 1240°C in an Ar atmosphere for 15 minutes under an applied pressure of 90 MPa. After grinding and polishing, the resulting LiAl₅O₃ transparent ceramic was obtained.

[0025] The Vickers hardness values ​​HV1 of the LiAl5O8 transparent ceramics prepared in Examples 1-4 are 16.8 GPa, 19.0 GPa, 17.2 GPa, and 18.3 GPa, respectively. Figure 2 As shown, they have good optical transparency.

[0026] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A low-temperature preparation method for LiAl5O8 transparent ceramics, characterized in that: The preparation method is to use γ-LiAlO2 and α-Al2O3 as main phases, and a mixed powder with the chemical composition of LiAl5O8 as a precursor, and obtain LiAl5O8 transparent ceramics through hot pressing sintering or spark plasma sintering.

2. The low-temperature preparation method of LiAl5O8 transparent ceramics according to claim 1, characterized in that: The precursor is obtained by mixing γ-LiAlO2 and α-Al2O3 powders, or selecting suitable lithium sources and aluminum sources and preparing a mixed powder with γ-LiAlO2 and α-Al2O3 as main phases and a chemical composition of LiAl5O8 through a method including but not limited to solid phase reaction method, hydrothermal method, sol-gel method, solution combustion method and co-precipitation method.

3. The low-temperature preparation method of LiAl5O8 transparent ceramics according to claim 2, characterized in that: The molar ratio of γ-LiAlO2 to α-Al2O3 in the precursor is 1:

2.

4. The low-temperature preparation method of LiAl5O8 transparent ceramics according to claim 2, characterized in that: The lithium source and aluminum source include, but are not limited to, simple substances, oxides, nitrides, nitrates, sulfates, carbonates, acetates, and alkoxides of lithium and aluminum.

5. The low-temperature preparation method of LiAl5O8 transparent ceramics according to claim 2, characterized in that: The powder prepared by lithium source and aluminum source has a mixed powder with γ-LiAlO2 and α-Al2O3 as main phases and a chemical composition of LiAl5O8, which means that the content of impurity cations is not higher than 1at%, and the molar ratio of Li and Al elements is between 0.95:5 and 1.05:

5.

6. The low-temperature preparation method of LiAl5O8 transparent ceramics according to claim 1, characterized in that: The hot pressing sintering or spark plasma sintering is carried out in a vacuum or inert atmosphere, with a sintering temperature of 1150-1290° C., an applied pressure of 20-200 MPa, and a holding time of 0-5 hours.

7. The low-temperature preparation method of LiAl5O8 transparent ceramics according to claim 1, characterized in that: In the preparation method, a sample prepared by hot pressing sintering or spark plasma sintering is subjected to surface grinding and polishing to obtain LiAl5O8 transparent ceramic.

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