Rapid preparation method of gamma-Al2O3 nano transparent ceramic in ultrahigh-pressure low-temperature environment

Abstract

The invention provides a rapid preparation method of gamma-Al2O3 nano transparent ceramic in an ultrahigh-pressure low-temperature environment. Specifically, the gamma-Al2O3 nano transparent ceramic with excellent performance is prepared by taking nano gamma-Al2O3 powder as an initial material through design of an undoped sintering aid system and carrying out optimization treatment through a stepped pressure increasing technology and a stepped pressure reducing technology in an ultrahigh-pressure low-temperature (20-250 DEG C) environment. Through assembly design and accurate stress and temperature regulation and control, high density, uniform nanostructure and high transparency of the block gamma-Al2O3 are realized, and the problem of deterioration of optical performance of a material caused by unclosed pores, stress accumulation and abnormal growth of crystal grains in the forming process of transparent ceramics is solved. The near-infrared light transmittance of the obtained gamma-Al2O3 nano transparent ceramic reaches up to 94.5%, the visible light transmittance is larger than 80%, the Vickers hardness is 20.60 GPa, the pressure maintaining time is shortened to the second level, the production efficiency is remarkably improved while the nano grain size is maintained, and the optical-mechanical synergistic performance of the gamma-Al2O3 nano transparent ceramic is far better than that of existing gamma-Al2O3 nano transparent ceramic.

Description

Technical Field

[0001] The present invention relates to the technical field of transparent ceramics, and particularly to a rapid preparation method of γ-Al2O3 nano transparent ceramics in an ultra-high pressure and low temperature environment. Background Art

[0002] Transparent ceramics have excellent optical transmittance, high mechanical strength and thermal stability, and have become key materials in high-end fields such as optical devices, lens elements, and armor protection, and have experienced significant development in the past few decades. The preparation of most transparent oxide ceramics is through conventional sintering techniques such as vacuum sintering, spark plasma sintering, hot pressing and hot isostatic pressing. However, these sintering techniques generally have problems such as high sintering temperature, and rapid growth of nano-scale powders into sub-micron or even micron-scale particles in a long-term high-temperature environment, which greatly affects the optical and mechanical properties of transparent ceramic materials. The visible light transmittance of alumina (Al2O3) ceramics prepared by traditional processes is generally lower than 70%, and the average grain size exceeds 200 nm, making it difficult to meet the performance requirements of high-precision optical devices. In addition, traditional Al2O3 ceramic materials have problems such as long preparation cycles and low yield rates, which will significantly increase energy consumption and costs, and are not conducive to large-scale industrial production applications.

[0003] Al2O3 ceramic materials are mainly composed of covalent bonds, which results in their high melting points. Due to the relatively high surface diffusion barriers of ions or atoms, it is more difficult to bond ceramic crystals in a low-temperature sintering environment. It has been found that applying pressure can reduce the thermodynamic and kinetic barriers required for nucleation and cause the phase transition to occur at a lower temperature. In addition, it can significantly enhance the density, transparency, and mechanical properties of ceramics. Al2O3 has multiple polymorphs, such as γ, θ, η, α, etc. Among them, the metastable γ-Al2O3 has a face-centered cubic structure, showing optical isotropy and having a relatively high phase stability temperature (~1200 °C). Mishra et al. studied the sintering behavior of nano γ-Al2O3 at 650 - 1100 °C. At 1 GPa, the phase transition temperature of γ-Al2O3 decreased from 1200 °C at 1 atm to 750 °C at 1 GPa. Kuskonmaz et al. studied the use of γ-Al2O3 to obtain translucent ceramic materials at 5 GPa and 600 °C in 2011. Chen et al. prepared transparent bulk materials using the prepared γ-Al2O3 powder at 5 GPa and 300 °C in 2024, and its Vickers hardness and compressive strength are close to those of single-crystal sapphire materials. In theory, this crystal structure can transmit light in a relatively long wavelength range and has the potential to be used as a material for devices such as infrared windows and optical lenses. However, there is little research on this ceramic compound as a transparent bulk material. There is no relevant research on γ-Al2O3 transparent ceramics that can be rapidly sintered and formed with high mechanical and optical properties within an extremely short pressure-holding time in an ultra-high pressure and low temperature (<300 °C) or even room temperature environment.

[0004] High pressure plays a crucial role in the cold bonding of ceramic particles. Once the pressure promotes enough atoms to meet the bonding conditions, it will produce instantaneous consolidation at the boundaries and form a stable structure. The ultra-high pressure sintering method (pressure greater than 1 GPa) can sinter materials in ambient air because a closed space is formed during assembly under high pressure. It can significantly reduce the sintering temperature and time and inhibit the crystal growth rate, providing an idea for the preparation of nano transparent ceramics. Summary of the Invention

[0005] The purpose of the present invention is to provide a rapid preparation method for γ-Al2O3 nano transparent ceramics in an ultra-high pressure and low temperature environment, and to solve the technical problems such as high-temperature sintering, long preparation cycle, and poor optical properties that occur during the preparation of γ-Al2O3 transparent ceramics.

[0006] To achieve the above purpose, the present invention provides a rapid preparation method for γ-Al2O3 nano transparent ceramics in an ultra-high pressure and low temperature environment, and the specific steps are as follows:

[0007] S1. First, dry the nano γ-Al2O3 powder and sieve it to obtain the raw material γ-Al2O3. Then, pour the raw material γ-Al2O3 into a molybdenum cup and fasten the molybdenum cup.

[0008] S2. Then, transfer the molybdenum cup to a mold and apply pressure to form a green body.

[0009] S3. Finally, transfer the molybdenum cup to a ultra-high pressure assembly, place the whole in a cubic press, and carry out ultra-high pressure and low-temperature sintering treatment to obtain the γ-Al2O3 nano transparent ceramic. The ultra-high pressure and low-temperature treatment conditions are as follows: Gradually increase the pressure to 3.0 - 10 GPa, hold the pressure for 30 s - 60 min at 20 - 250 °C, and then gradually decrease the pressure to atmospheric pressure (101.325 kPa).

[0010] Preferably, in step S1, the process conditions for the drying treatment are: drying at 60 °C for 24 hours.

[0011] Preferably, in step S1, sieve through a 100-mesh sieve and a 200-mesh sieve in sequence.

[0012] Preferably, in step S2, the pressure application conditions are: a load of 5 - 10 MPa, and hold for 3 - 15 minutes.

[0013] Preferably, in step S3, the ultra-high pressure assembly is composed of pyrophyllite, dolomite ring, dolomite tube, conductive plug, molybdenum cup containing the sample, graphite sheet, graphite column, graphite tube, molybdenum sheet, magnesia sheet, magnesia tube, etc., and is assembled as shown. The ultra-high pressure assembly includes a heating cavity for placing the sample, a clamping part, and a hexahedral pyrophyllite block that sleeves the two together. The sample heating cavity is a cylinder composed of a graphite tube and a graphite sheet. The graphite tube contains a magnesia tube, and the magnesia tube contains a molybdenum cup with the sample and magnesia sheets filled above and below. The clamping part includes a dolomite tube II around the heating cavity and dolomite rings on the upper and lower sides. There are graphite columns in the dolomite rings. The dolomite rings are in contact with the molybdenum sheet, and on the other side of the molybdenum sheet is a dolomite tube I, and a conductive plug is contained in the dolomite tube I. Figure 7 shown.

[0014] Preferably, in step S3, the gradient pressure increase is achieved in two steps, and the gradient pressure decrease is achieved in three steps.

[0015] Preferably, in step S3, when the low-temperature sintering condition is higher than room temperature (20 °C), after the gradient pressure increase ends, first raise the temperature and then lower the temperature to room temperature, and then carry out the gradient pressure decrease treatment.

[0016] Preferably, in step S3, the ultra-high pressure and low temperature treatment conditions are as follows: First, increase the pressure to 6.0 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 8.0 GPa, hold the pressure for 30 s, at room temperature environment, then decrease the pressure to 7.0 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 5.0 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure; or

[0017] Preferably, first increase the pressure to 4.0 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 5.5 GPa, hold the pressure for 3 minutes, at room temperature environment, then decrease the pressure to 5.0 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 3.0 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure; or

[0018] Preferably, first increase the pressure to 2.9 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 4.0 GPa, hold the pressure for 20 minutes, at room temperature environment, then decrease the pressure to 3.6 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 2.2 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure; or

[0019] Preferably, first increase the pressure to 2.2 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 3.0 GPa, hold the pressure for 60 minutes, at room temperature environment, then decrease the pressure to 2.7 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 1.6 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure; or

[0020] Preferably, in step S3, the ultra-high pressure and low temperature treatment conditions are as follows: First, increase the pressure to 4.0 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 5.5 GPa, hold the pressure for 20 minutes, heat up to 250 °C at a rate of 100 °C / min, keep the temperature for 3 min, then cool down to room temperature at a rate of 20 °C / min, then decrease the pressure to 5.0 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 3.0 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure.

[0021] Preferably, in step S3, the ultra-high pressure and low temperature treatment conditions are as follows: First, increase the pressure to 4.0 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 5.5 GPa, hold the pressure for 20 minutes, heat up to 150 °C at a rate of 100 °C / min, keep the temperature for 3 min, then cool down to room temperature at a rate of 20 °C / min, then decrease the pressure to 5.0 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 3.0 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure.

[0022] Preferably, in step S3, the ultra-high pressure and low temperature treatment conditions are as follows: First, increase the pressure to 4.0 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 5.5 GPa, hold the pressure for 30 minutes, at room temperature environment, then decrease the pressure to 5.0 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 3.0 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure.

[0023] Preferably, in step S3, after the ultra-high pressure and low temperature treatment, the molybdenum cup is removed, and then it is cut and polished to obtain the γ-Al2O3 nano transparent ceramic with a grain size of 20 nm.

[0024] The present invention has the following beneficial effects:

[0025] In the present invention, under the environment of ultra-high pressure and low temperature (20-250 °C), an optimized treatment of stepwise pressure increase and stepwise pressure decrease is carried out to rapidly prepare a γ-Al2O3 nano transparent ceramic with excellent performance, solving the problems of optical property deterioration of the material caused by non-closure of pores, stress accumulation and abnormal grain growth during the forming process of the transparent ceramic, improving the yield of the product, and having the potential for mass production. The near-infrared transmittance of the γ-Al2O3 nano transparent ceramic obtained by the present invention is as high as 94.5%, the visible transmittance > 80%, the Vickers hardness is 20.60 GPa, and the pressure holding time is shortened to the second level, and the comprehensive performance is better than that of the existing γ-Al2O3 nano transparent ceramic.

[0026] In addition to the purposes, features and advantages described above, the present invention has other purposes, features and advantages. The present invention will be further described in detail below with reference to the drawings. Description of the Drawings

[0027] The drawings constituting a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0028] Figure 1 are the ultra-high pressure and low temperature sintering of γ-Al2O3 ceramics (a) traditional high pressure sintering process, (b) optimized high pressure sintering process;

[0029] Figure 2 is the XRD pattern of γ-Al2O3 powder in the normal pressure sintering state;

[0030] Figure 3 is the powder SEM image of γ-Al2O3 powder at room temperature and normal pressure;

[0031] Figure 4 is the SEM image of γ-Al2O3 powder sintered at 900 °C for 2 h;

[0032] Figure 5 is the SEM image of γ-Al2O3 powder sintered at 1200 °C for 2 h;

[0033] Figure 6 is the XRD of (a) commercial α-Al2O3 powder and its (b) SEM image;

[0034] Figure 7 is the schematic diagram of ultra-high pressure assembly;

[0035] Figure 8 are XRD patterns of γ-Al2O3 nanometer powder and at ultra-high pressure and room temperature;

[0036] Figure 9 are state diagrams of γ-Al2O3 nanometer powder at (a) 5.5 GPa - room temperature, (b) 2.0 GPa - room temperature, (c) 5.5 GPa - 250 °C (stepwise pressure reduction and optimization process), (d) the state after polishing of sample (c), (e) the state at 5.5 GPa - 300 °C, (f) 5.5 GPa - room temperature (traditional high-pressure sintering process);

[0037] Figure 10 are light transmittances of γ-Al2O3 transparent ceramics under different processes;

[0038] Figure 11 are SEM images of sample γ-Al2O3 sintered at ultra-high pressure (a) room temperature, (b) 250 °C;

[0039] Figure 12 is a schematic structural diagram of a green body forming mold. Specific Embodiments

[0040] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but the present invention can be implemented in many different ways defined and covered by the claims.

[0041] Embodiment 1:

[0042] A preparation method of γ-Al2O3 nanometer transparent ceramics, the specific steps are as follows:

[0043] S1. Pretreatment of nanometer γ-Al2O3 powder

[0044] The nanometer γ-Al2O3 powder (20 nm - 100 nm) is dried in an oven at a constant temperature of 60 °C for 24 h, and then successively sieved through 100-mesh and 200-mesh sieves; an appropriate amount of γ-Al2O3 is poured into a molybdenum cup, and then the molybdenum cup (cylindrical, diameter 11 mm, height 5.0 mm) is buckled.

[0045] S2. Green body forming process

[0046] The molybdenum cup containing the sample is placed in a specific mold, a load of 5 MPa is applied, and it is maintained for 5 min, and then the molybdenum cup is taken out.

[0047] The structure of the specific mold is shown in Figure 12, the mold includes three parts: an outer tube, a base, and an upper column, and a demoulding ring. Among them, the outer tube is a hollow tube with an inner diameter of 11.10 mm, an outer diameter of 27.9 mm, and a height of 25 mm; the base is a cylinder I with a cylindrical protrusion I in the center. The diameter of the cylinder I is 20 mm, the height is 5 mm, the diameter of the cylindrical protrusion I is 20 mm, and the height is 5 mm; the upper column is a cylinder II with a cylindrical protrusion II in the center. The diameter of the cylinder II is 15 mm, the height is 5 mm, the diameter of the cylindrical protrusion II is 11.02 mm, and the height is 26 mm; the demoulding ring is a hollow tube with an inner diameter of 22 mm, an outer diameter of 27.9 mm, and a height of 20 mm.

[0048] During use, place the molybdenum cup in the center of the cylindrical protrusion I of the base. The outer tube is sleeved outside the molybdenum cup. The cylindrical protrusion II of the upper column is embedded inside the outer tube and tightly presses the top of the molybdenum cup, and then apply pressure and maintain it. After completion, put the demoulding ring on the cylinder II of the upper column from top to bottom, press it tightly against the top of the outer tube, pull out the upper column, and take out the molybdenum cup.

[0049] S3. Ultra-high pressure sintering and forming

[0050] Put the molybdenum cup into a specific high-pressure assembly, as Figure 7 shown, and finally put it into a six-sided press, and perform ultra-high pressure and low-temperature experiments according to the set process (such as Figure 1 b) in it). The specific process is as follows: The first pressure increase is 6.0 GPa, the pressure is maintained for 2 min, continue to increase the pressure to 8.0 GPa, the pressure is maintained for 30 s, at room temperature environment (20 °C), then reduce the pressure to 7.0 GPa, the pressure is maintained for 2 min, then reduce the pressure to 5.0 GPa, the pressure is maintained for 2 min, and finally reduce to normal pressure. After the experiment, take out the metal cup containing the sample, remove the metal cup, and perform processing such as cutting and polishing to finally obtain the sample.

[0051] Example 2:

[0052] The specific process of the ultra-high pressure and low-temperature experiment is as follows: The first pressure increase is 4.0 GPa, the pressure is maintained for 2 min, continue to increase the pressure to 5.5 GPa, the pressure is maintained for 3 min, at room temperature environment (20 °C), then reduce the pressure to 5.0 GPa, the pressure is maintained for 2 min, then reduce the pressure to 3.0 GPa, the pressure is maintained for 2 min, and finally reduce to normal pressure.

[0053] The rest is the same as in Example 1.

[0054] Example 3:

[0055] The specific process of the ultra-high pressure and low-temperature experiment is as follows: The first pressure increase is 2.9 GPa, the pressure is maintained for 2 min, continue to increase the pressure to 4.0 GPa, the pressure is maintained for 20 min, at room temperature environment (20 °C), then reduce the pressure to 3.6 GPa, the pressure is maintained for 2 min, then reduce the pressure to 2.2 GPa, the pressure is maintained for 2 min, and finally reduce to normal pressure.

[0056] The rest is the same as in Example 1.

[0057] Example 4:

[0058] The specific process of the ultra-high pressure and low temperature experiment is as follows: The first pressure increase is to 2.2 GPa, hold the pressure for 2 min, continue to increase the pressure to 3.0 GPa, hold the pressure for 60 min, at room temperature environment (20 °C), then reduce the pressure to 2.7 GPa, hold the pressure for 2 min, then reduce the pressure by 1.6 GPa, hold the pressure for 2 min, and finally reduce to atmospheric pressure.

[0059] The rest is the same as in Example 1.

[0060] Example 5( Figure 9 in c, d):

[0061] The specific process of the ultra-high pressure and low temperature experiment is as follows: The first pressure increase is to 4.0 GPa, hold the pressure for 2 min, continue to increase the pressure to 5.5 GPa, hold the pressure for 20 min, heat up to 250 °C at a rate of 100 °C / min, keep the temperature for 3 min, and then cool down to room temperature at a rate of 20 °C / min; reduce the pressure to 5.0 GPa, hold the pressure for 2 min, then reduce the pressure by 3.0 GPa, hold the pressure for 2 min, and finally reduce to atmospheric pressure.

[0062] The rest is the same as in Example 1.

[0063] Example 6:

[0064] The specific process of the ultra-high pressure and low temperature experiment is as follows: The first pressure increase is to 4.0 GPa, hold the pressure for 2 min, continue to increase the pressure to 5.5 GPa, hold the pressure for 20 min, heat up to 150 °C at a rate of 100 °C / min, keep the temperature for 3 min, and then cool down to room temperature at a rate of 20 °C / min; reduce the pressure to 5.0 GPa, hold the pressure for 2 min, then reduce the pressure by 3.0 GPa, hold the pressure for 2 min, and finally reduce to atmospheric pressure.

[0065] The rest is the same as in Example 1.

[0066] Example 7( Figure 9 in a):

[0067] The specific process of the ultra-high pressure and low temperature experiment is as follows: The first pressure increase is to 4.0 GPa, hold the pressure for 2 min, continue to increase the pressure to 5.5 GPa, hold the pressure for 30 min, at room temperature environment (20 °C), then reduce the pressure to 5.0 GPa, hold the pressure for 2 min, then reduce the pressure by 3.0 GPa, hold the pressure for 2 min, and finally reduce to atmospheric pressure.

[0068] The rest is the same as in Example 1.

[0069] Comparative Example 1( Figure 9 in b)

[0070] The specific process of the ultra-high pressure and low temperature experiment is as follows: The first pressure increase is to 1.5 GPa, hold the pressure for 2 min, continue to increase the pressure to 2.0 GPa, hold the pressure for 3 min, at room temperature environment (20 °C), then reduce the pressure to 1.8 GPa, hold the pressure for 2 min, then reduce the pressure to 1.1 GPa, hold the pressure for 2 min, and finally reduce to atmospheric pressure.

[0071] The rest is the same as in Example 1.

[0072] Comparative Example 2 ( Figure 1 a in Figure 9 and f in

[0073] The specific process of the ultra-high pressure and low temperature experiment is as follows: Directly increase the pressure to 5.5 GPa, hold the pressure for 3 min, at room temperature environment (20 °C), and then directly reduce to atmospheric pressure.

[0074] The rest is the same as in Example 1.

[0075] Comparative Example 3 ( Figure 9 e in

[0076] The specific process of the ultra-high pressure and low temperature experiment is as follows: The first pressure increase is to 4.0 GPa, hold the pressure for 2 min, continue to increase the pressure to 5.5 GPa, hold the pressure for 20 min, heat up to 300 °C at a rate of 100 °C / min, keep the temperature for 3 min, and then cool down to room temperature at a rate of 20 °C / min; reduce the pressure to 5.0 GPa, hold the pressure for 2 min, then reduce the pressure to 3.0 GPa, hold the pressure for 2 min, and finally reduce to atmospheric pressure.

[0077] The rest is the same as in Example 1.

[0078] Test Example

[0079] The Vickers hardness of the γ-Al2O3 nano transparent ceramics obtained from the examples and comparative examples under different pressures and low temperature sintering (20 °C to 250 °C) is shown in Table 1.

[0080] Table 1. Basic parameters of the γ-Al2O3 nano transparent ceramics obtained under different pressures and low temperature (20 °C to 250 °C) sintering environments

[0081]

[0082] Figure 2 is the XRD pattern of the γ-Al2O3 powder in the atmospheric pressure sintering state. During the atmospheric pressure sintering process, the γ-Al2O3 powder did not undergo a phase change when sintered below 1000 °C, and a phase change occurred when holding the temperature at 1100 °C for 2 h. The corresponding SEM of the sintered γ-Al2O3 powder is as Figures 3 - 5 shown. It was completely transformed into α-Al2O3 when holding the temperature at 1200 °C for 2 h, which is consistent with the XRD and SEM of commercial α-Al2O3, as Figure 6 shown.

[0083] The assembly schematic diagram of the ultra-high pressure experiment is as Figure 7 shown. After the sample was sintered at high temperature and room temperature, no phase transformation occurred in the sample phase, as Figure 8 shown.

[0084] The preparation of γ-Al2O3 nano transparent ceramic materials at ultra-high pressure and low temperature (20 °C to 250 °C) is as Figure 9 shown. The transparency of the samples formed at room temperature below 2.0 GPa is very poor. The samples prepared at ultra-high pressure room temperature and ultra-high pressure low temperature above 3 GPa have better transparency. Among them, the γ-Al2O3 nano transparent ceramic materials prepared according to the optimized process in Figure 1 b have the best light transmittance, as Figure 10 shown. The γ-Al2O3 nano transparent ceramic was obtained in a very short pressure holding time for the sample. The image behind can be clearly seen through the ceramic. The light transmittance in the visible light region is > 80%, and the highest transmittance in the near infrared reaches 94.5%. The Vickers hardness is 20.62 GPa. SEM images of the samples under the optimized process at room temperature (20 °C) and low temperature sintering (250 °C) Figure 11 ). It can be found that: the pores of the γ-Al2O3 transparent ceramic under high pressure are smaller, and the density increases with the increase of temperature, and the density is further improved. The particle sizes of both are less than 100 nm, belonging to nano transparent ceramic materials.

[0085] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, various changes and modifications can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A rapid preparation method of γ-Al2O3 nano transparent ceramics under ultra-high pressure and low temperature environment, characterized in that, The specific steps are as follows: S1. First, dry the nano γ-Al2O3 powder and sieve it to obtain the raw material γ-Al2O3. Then pour the raw material γ-Al2O3 into a molybdenum cup and fasten the molybdenum cup. S2. Then transfer the molybdenum cup to a mold and apply pressure to achieve green body forming. S3. Finally, transfer the molybdenum cup to a ultra-high pressure assembly, place the whole in a six-sided press, and carry out ultra-high pressure and low temperature sintering treatment to obtain the γ-Al2O3 nano transparent ceramic. The ultra-high pressure and low temperature treatment conditions are: gradually increase the pressure to 3.0 - 10.0 GPa, hold the pressure for 30 s - 60 min at 20 - 250 °C, and gradually decrease the pressure to atmospheric pressure.

2. The preparation method according to claim 1, characterized in that, In step S1, the process conditions for the drying treatment are: drying at 60 °C for 24 hours.

3. The preparation method according to claim 1, wherein, In step S1, sieve through a 100-mesh sieve and a 200-mesh sieve in sequence.

4. The preparation method according to claim 1, characterized in that, In step S2, the pressure application conditions are: a load of 5 - 10 MPa, and hold for 3 - 15 minutes.

5. The preparation method according to claim 1, characterized in that, In step S3, the ultra-high pressure assembly is composed of pyrophyllite, dolomite ring, dolomite tube, conductive plug, molybdenum cup containing the sample, graphite sheet, graphite column, graphite tube, molybdenum sheet, magnesia sheet, and magnesia tube.

6. The preparation method according to claim 1, characterized in that, In step S3, the gradient pressure increase is achieved in two steps, and the gradient pressure decrease is achieved in three steps.

7. The preparation method according to claim 1, wherein In step S3, when the low temperature sintering condition is higher than room temperature, after the gradient pressure increase ends, first raise the temperature and then lower it to room temperature, and then carry out the gradient pressure decrease treatment.

8. The preparation method according to claim 1, characterized in that, In step S3, the ultra-high pressure and low temperature treatment conditions are: first increase the pressure to 6.0 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 8.0 GPa, hold the pressure for 30 s, at room temperature environment, then decrease the pressure to 7.0 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 5.0 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure; or first increase the pressure to 4.0 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 5.5 GPa, hold the pressure for 3 minutes, at room temperature environment, then decrease the pressure to 5.0 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 3.0 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure; or first increase the pressure to 2.9 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 4.0 GPa, hold the pressure for 20 minutes, at room temperature environment, then decrease the pressure to 3.6 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 2.2 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure; or first increase the pressure to 2.2 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 3.0 GPa, hold the pressure for 60 minutes, at room temperature environment, then decrease the pressure to 2.7 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 1.6 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure; or first increase the pressure to 4.0 GPa, hold the pressure for 2 minutes, continue to increase the pressure to 5.5 GPa, hold the pressure for 20 minutes, heat up to 250 °C at a rate of 100 °C / min, keep the temperature for 3 min, then cool down to room temperature at a rate of 20 °C / min, then decrease the pressure to 5.0 GPa, hold the pressure for 2 minutes, continue to decrease the pressure to 3.0 GPa, hold the pressure for 2 minutes, and finally decrease the pressure to atmospheric pressure; or First, pressurize to 4.0 GPa, hold the pressure for 2 minutes, then continue to pressurize to 5.5 GPa and hold the pressure for 20 minutes. Heat up to 150 °C at a rate of 100 °C / min, keep the temperature for 3 min, then cool down to room temperature at a rate of 20 °C / min, and then reduce the pressure to 5.0 GPa, hold the pressure for 2 minutes, continue to reduce the pressure to 3.0 GPa, hold the pressure for 2 minutes, and finally reduce the pressure to atmospheric pressure; or First, pressurize to 4.0 GPa, hold the pressure for 2 minutes, then continue to pressurize to 5.5 GPa and hold the pressure for 30 minutes at room temperature environment, and then reduce the pressure to 5.0 GPa, hold the pressure for 2 minutes, continue to reduce the pressure to 3.0 GPa, hold the pressure for 2 minutes, and finally reduce the pressure to atmospheric pressure.

9. The preparation method according to claim 1, wherein, In step S3, after the ultra-high pressure and low temperature treatment is completed, remove the molybdenum cup, cut and polish it to obtain the γ-Al2O3 nanometer transparent ceramic.

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