CN120329014A - A rapid preparation method of γ-Al2O3 nano-transparent ceramics under ultra-high pressure and low temperature environment - Google Patents
A rapid preparation method of γ-Al2O3 nano-transparent ceramics under ultra-high pressure and low temperature environmentInfo
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Abstract
The invention provides a rapid preparation method of gamma-Al 2O3 nanometer transparent ceramics under an ultra-high pressure low temperature environment. Specifically, the gamma-Al 2O3 nano transparent ceramic with excellent performance is prepared by adopting an undoped sintering aid system design, taking nano gamma-Al 2O3 powder as an initial material and performing optimization treatment by a stepped boosting and stepped depressurizing technology in an ultra-high pressure low-temperature (20-250 ℃) environment. Through the assembly design and accurate stress and temperature regulation, the high density, uniform nano structure and high transparency of the block gamma-Al 2O3 are realized, and the problem of material optical performance degradation caused by the fact that pores are not closed, stress accumulation and abnormal grain growth of transparent ceramics occur in the forming process is solved. The near infrared light transmittance of the gamma-Al 2O3 nano transparent ceramic obtained by the invention is up to 94.5%, the visible light transmittance is more than 80%, the Vickers hardness is 20.60GPa, the pressure maintaining time is shortened to the second level, the production efficiency is obviously improved while the nano crystal grain size is maintained, and the optical-mechanical synergistic performance is far better than that of the existing gamma-Al 2O3 nano transparent ceramic.
Description
Technical Field
The invention relates to the technical field of transparent ceramics, in particular to a rapid preparation method of gamma-Al 2O3 nanometer transparent ceramics under an ultra-high pressure low temperature environment.
Background
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, armor protection and the like, and have undergone significant development in the past decades. Most transparent oxide ceramics are prepared by the following conventional sintering technologies, such as vacuum sintering, spark plasma sintering, hot pressing and hot isostatic pressing, but the sintering technologies generally have the problems that the sintering temperature is high, nano-scale powder rapidly grows up to form submicron-scale or even micron-scale particles in a long-time high-temperature environment, and the optical and mechanical properties of the transparent ceramic materials are greatly influenced. The visible light transmittance of the alumina (Al 2O3) ceramic prepared by the traditional process is generally lower than 70%, and the average grain size exceeds 200nm, so that the performance requirement of a high-precision optical device is difficult to meet. In addition, the traditional Al 2O3 ceramic material has the problems of long preparation period, low yield and the like, can obviously increase energy consumption and cost, and is not beneficial to large-scale industrial production and application.
Al 2O3 ceramic materials are mainly composed of covalent bonds, which lead to their high melting point, while bonding ceramic crystals in low temperature sintering environments increases difficulty due to the higher surface diffusion barrier of ions or atoms, whereas research has found that applying pressure reduces the thermodynamic and kinetic energy barriers required for nucleation and leads to phase transition transfer to lower temperatures, and in addition, significantly enhances the density, transparency and mechanical properties of the ceramic. Al 2O3 has multiple crystal forms such as gamma, theta, eta, alpha and the like, wherein the metastable gamma-Al 2O3 has a face-centered cubic structure, presents optical isotropy and has higher phase stability temperature (1200 ℃). Mishra et Al studied the sintering behavior of nano gamma-Al 2O3 at 650-1100 ℃, at 1GPa, gamma-Al 2O3 phase transition temperature was reduced from 1200 ℃ at 1atm to 750 ℃ at 1 GPa. Kuskonmaz et Al, 2011 studied using gamma-Al 2O3 to obtain translucent ceramic materials at 5GPa and 600 ℃. The Chen et Al prepared transparent bulk materials in 2024 under 5GPa and 300 ℃ environment by using the prepared gamma-Al 2O3 powder, and the Vickers hardness and the compressive strength of the transparent bulk materials are close to those of the sapphire single crystal materials. In theory, the crystal structure is capable of transmitting light over a relatively long wavelength range, and has potential as a device material for infrared windows, optical lenses, and the like. However, little research has been conducted on this ceramic compound as a transparent bulk material, and there has been no related research on gamma-Al 2O3 transparent ceramics which are rapidly sintered and formed in a very short dwell time under ultra-high pressure and low temperature (< 300 ℃) and even room temperature environments and which have high mechanical and optical properties.
The high pressure plays a critical role in the cold bonding of ceramic particles, which once the pressure promotes sufficient atoms to meet the bonding conditions, it produces instantaneous consolidation at the boundaries and forms a stable structure. The ultra-high pressure sintering method (the pressure is more than 1 GPa) forms a closed space by assembling in a high-pressure environment, so that the material can be sintered in ambient air, the sintering temperature and time can be obviously reduced, the crystal growth speed can be inhibited, and a thought is provided for preparing the nano transparent ceramic.
Disclosure of Invention
The invention aims to provide a rapid preparation method of gamma-Al 2O3 nanometer transparent ceramics under an ultra-high pressure low-temperature environment, which solves the technical problems of high-temperature sintering, long-time preparation period, poor optical performance and the like in the preparation process of gamma-Al 2O3 transparent ceramics.
In order to achieve the purpose, the invention provides a rapid preparation method of gamma-Al 2O3 nanometer transparent ceramics under the environment of ultra-high pressure and low temperature, which comprises the following specific steps:
S1, drying nano gamma-Al 2O3 powder, sieving to obtain raw material gamma-Al 2O3, pouring raw material gamma-Al 2O3 into a metal molybdenum cup, and buckling the metal molybdenum cup;
s2, transferring the metal molybdenum cup into a mold, and applying pressure to form a biscuit;
And S3, finally transferring the metal molybdenum cup into an ultrahigh pressure assembly, integrally placing the metal molybdenum cup into a hexahedral top press, and performing ultrahigh pressure low temperature sintering treatment to obtain the gamma-Al 2O3 nanometer transparent ceramic, wherein the ultrahigh pressure low temperature treatment condition is that the gradient is increased to 3.0-10 GPa, the pressure is maintained for 30 s-60 min at 20-250 ℃, and the gradient is reduced to normal pressure (101.325 kPa).
Preferably, in step S1, the drying treatment is carried out under the process condition of 60 ℃ for 24 hours.
Preferably, in step S1, sieving is performed by passing through a 100 mesh sieve and a 200 mesh sieve in order.
Preferably, in the step S2, the pressure is applied under the condition of 5-10 MPa load, and the pressure is kept for 3-15 minutes.
Preferably, in step S3, the ultra-high pressure assembly is made up of pyrophyllite, dolomite ring, dolomite tube, conductive plug, metal molybdenum cup containing sample, graphite sheet, graphite column, graphite tube, molybdenum sheet, magnesium oxide tube, etc., and is assembled according to fig. 7. The ultrahigh pressure assembly comprises a heating cavity for containing a sample, a clamping part and hexahedral pyrophyllite blocks sleeved with the heating cavity, wherein the sample heating cavity is a cylinder formed by a graphite tube and a graphite sheet, a magnesium oxide tube is contained in the graphite tube, a metal molybdenum cup containing the sample and magnesium oxide sheets filled up and down are contained in the magnesium oxide tube, the clamping part comprises a dolomite tube II around the heating cavity and dolomite rings on the upper side and the lower side, graphite columns are arranged in the dolomite rings, the dolomite rings are contacted with the molybdenum sheets, a dolomite tube I is arranged on the other side of the molybdenum sheet, and a conductive plug is contained in the dolomite tube I.
Preferably, in step S3, gradient boosting is implemented in two steps and gradient depressurization is implemented in three steps.
Preferably, in step S3, when the low-temperature sintering condition is higher than room temperature (20 ℃), the temperature is raised and then lowered to room temperature after the gradient pressure raising is completed, and then the gradient pressure lowering treatment is performed.
Preferably, in step S3, the ultrahigh pressure and low temperature treatment conditions are that the pressure is increased to 6.0GPa, the pressure is maintained for 2 minutes, the pressure is increased to 8.0GPa continuously, the pressure is maintained for 30 seconds, the room temperature environment is reduced to 7.0GPa, the pressure is maintained for 2 minutes, the pressure is reduced to 5.0GPa continuously, the pressure is maintained for 2 minutes, and the pressure is reduced to normal pressure finally, or
Preferably, the pressure is increased to 4.0GPa, maintained for 2 minutes, further increased to 5.5GPa, maintained for 3 minutes, maintained in room temperature environment, then reduced to 5.0GPa, maintained for 2 minutes, further reduced to 3.0GPa, maintained for 2 minutes, finally reduced to normal pressure, or
Preferably, the pressure is increased to 2.9GPa, maintained for 2 minutes, continuously increased to 4.0GPa, maintained for 20 minutes, maintained in room temperature environment, then reduced to 3.6GPa, maintained for 2 minutes, continuously reduced to 2.2GPa, maintained for 2 minutes, finally reduced to normal pressure, or
Preferably, the pressure is increased to 2.2GPa, maintained for 2 min, further increased to 3.0GPa, maintained for 60 min, maintained in room temperature environment, then reduced to 2.7GPa, maintained for 2 min, further reduced to 1.6GPa, maintained for 2 min, finally reduced to normal pressure, or
Preferably, in the step S3, the ultrahigh pressure and low temperature treatment conditions are that the pressure is firstly increased to 4.0GPa, the pressure is maintained for 2 minutes, the pressure is continuously increased to 5.5GPa, the pressure is maintained for 20 minutes, the temperature is increased to 250 ℃ according to the speed of 100 ℃ per minute, the temperature is kept for 3 minutes, then the temperature is reduced to room temperature at 20 ℃ per minute, the pressure is reduced to 5.0GPa, the pressure is maintained for 2 minutes, the pressure is continuously reduced to 3.0GPa, the pressure is maintained for 2 minutes, and finally the pressure is reduced to normal pressure.
Preferably, in the step S3, the ultrahigh pressure and low temperature treatment conditions are that the pressure is firstly increased to 4.0GPa, the pressure is maintained for 2 minutes, the pressure is continuously increased to 5.5GPa, the pressure is maintained for 20 minutes, the temperature is increased to 150 ℃ according to the speed of 100 ℃ per minute, the temperature is kept for 3 minutes, then the temperature is reduced to room temperature at 20 ℃ per minute, the pressure is reduced to 5.0GPa, the pressure is maintained for 2 minutes, the pressure is continuously reduced to 3.0GPa, the pressure is maintained for 2 minutes, and finally the pressure is reduced to normal pressure.
Preferably, in the step S3, the ultrahigh pressure and low temperature treatment conditions are that the pressure is firstly increased to 4.0GPa, the pressure is maintained for 2 minutes, the pressure is continuously increased to 5.5GPa, the pressure is maintained for 30 minutes, the room temperature environment is then reduced to 5.0GPa, the pressure is maintained for 2 minutes, the pressure is continuously reduced to 3.0GPa, the pressure is maintained for 2 minutes, and finally the pressure is reduced to normal pressure.
Preferably, in the step S3, after the ultrahigh pressure low temperature treatment is completed, removing the metal molybdenum cup, cutting and polishing to obtain the gamma-Al 2O3 nanometer transparent ceramic with the grain size of 20 nm.
The invention has the following beneficial effects:
According to the invention, under the conditions of ultra-high pressure and low temperature (20-250 ℃), the steps are boosted and stepped-reduced, the gamma-Al 2O3 nanometer transparent ceramic with excellent performance is rapidly prepared, the problem of optical performance degradation of materials caused by non-closure of air holes, stress accumulation, abnormal growth of grains and the like in the forming process of the transparent ceramic is solved, the yield of products is improved, and the method has the potential of mass production. The near infrared transmittance of the gamma-Al 2O3 nanometer transparent ceramic obtained by the invention is up to 94.5%, the visible transmittance is more than 80%, the Vickers hardness is 20.60GPa, the pressure maintaining time is shortened to the second level, and the comprehensive performance is superior to that of the existing gamma-Al 2O3 nanometer transparent ceramic.
In addition to the objects, features and advantages described above, the present invention has other objects, features and advantages. The present invention will be described in further detail with reference to the drawings.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the application. In the drawings:
FIG. 1 shows an ultra-high pressure low temperature sintering gamma-Al 2O3 ceramic (a) a conventional high pressure sintering process and (b) an optimized high pressure sintering process;
FIG. 2 is an XRD pattern of gamma-Al 2O3 powder in a normal pressure sintered state;
FIG. 3 is a powder SEM image of gamma-Al 2O3 powder at room temperature and pressure;
FIG. 4 is an SEM image of gamma-Al 2O3 powder sintered at 900 ℃ for 2 h;
FIG. 5 is an SEM image of gamma-Al 2O3 powder sintered at 1200 deg.C for 2 h;
FIG. 6 is an XRD of (a) commercial alpha-Al 2O3 powder and its (b) SEM image;
FIG. 7 is a schematic view of an ultra-high pressure assembly;
FIG. 8 is an XRD pattern of gamma-Al 2O3 nanopowder and at ultra-high pressure room temperature;
FIG. 9 is a state diagram of gamma-Al 2O3 nanopowder at (a) 5.5 GPa-room temperature, (b) 2.0 GPa-room temperature, (C) 5.5GPa-250 ℃ (step-down optimization process), (d) sample (C) polished state, (e) 5.5GPa-300 ℃, (f) 5.5 GPa-room temperature (conventional high pressure sintering process);
FIG. 10 is the light transmittance of gamma-Al 2O3 transparent ceramics under different processes;
FIG. 11 is an SEM image of a sample gamma-Al 2O3 after sintering at (a) room temperature and (b) 250℃under ultra-high pressure;
Fig. 12 is a schematic diagram of a biscuit forming mold structure.
Detailed Description
Embodiments of the invention are described in detail below with reference to the attached drawings, but the invention can be implemented in a number of different ways, which are defined and covered by the claims.
Example 1:
the preparation process of transparent gamma-Al 2O3 nanometer ceramic includes the following steps:
S1, pretreatment of nano gamma-Al 2O3 powder
Drying nanometer gamma-Al 2O3 powder (20 nm-100 nm) at a constant temperature of 60 ℃ for 24 hours in a drying oven, sieving sequentially through a 100-mesh sieve and a 200-mesh sieve, pouring a proper amount of gamma-Al 2O3 into a metal molybdenum cup, and buckling the metal molybdenum cup (cylinder, diameter 11mm, height 5.0 mm).
S2, biscuit forming process
The metal molybdenum cup containing the sample was placed in a specific mold, a load of 5MPa was applied and held for 5 minutes, and then the metal molybdenum cup was removed.
The specific mold structure is shown in fig. 12, and the mold comprises an outer pipe, a base, an upper column and a demolding ring, wherein the outer pipe is a hollow pipe, the inner diameter is 11.10mm, the outer diameter is 27.9mm, the height is 25mm, the base is a cylinder I with a cylindrical bulge I at the center, the diameter of the cylinder I is 20mm, the height is 5mm, the diameter of the cylindrical bulge I is 20mm, the height is 5mm, the upper column is a cylinder II with a cylindrical bulge II at the center, the diameter of the cylinder II is 15mm, the height is 5mm, the diameter of the cylindrical bulge II is 11.02mm, the height is 26mm, the demolding ring is a hollow pipe, the inner diameter is 22mm, the outer diameter is 27.9mm, and the height is 20mm.
When the metal molybdenum cup is used, the metal molybdenum cup is placed in the center of the cylindrical boss I of the base, the outer tube is sleeved outside the metal molybdenum cup, the cylindrical boss II of the upper column is embedded into the outer tube, the top of the metal molybdenum cup is tightly pressed, and then pressure is applied and maintained. After the completion, the demolding ring is sleeved downwards from the cylinder II of the upper column, so that the demolding ring is tightly pressed on the top of the outer tube, the upper column is pulled out, and the metal molybdenum cup is taken out.
S3, ultra-high pressure sintering molding
The metal molybdenum cup is placed in a specific high-pressure assembly, as shown in fig. 7, and finally placed in a hexahedral top press, and the ultra-high pressure and low-temperature experiment (as shown in fig. 1 b) is carried out according to the set process molding, wherein the specific process is that the pressure is increased to 6.0GPa for 2min for the first time, the pressure is continuously increased to 8.0GPa, the pressure is maintained for 30s, the room temperature environment (20 ℃) is maintained, then the pressure is increased to 7.0GPa, the pressure is maintained for 2min, then the pressure is increased to 5.0GPa, the pressure is maintained for 2min, and finally the pressure is reduced to the normal pressure. And taking out the metal cup containing the sample after the experiment is finished, removing the metal cup, and performing treatments such as cutting and polishing to finally obtain the sample.
Example 2:
The specific flow of the ultra-high pressure and low temperature experiment is as follows, the first pressure rise is 4.0GPa, the pressure is maintained for 2min, the pressure is continuously increased to 5.5GPa, the pressure is maintained for 3min, the room temperature environment (20 ℃) is maintained, the pressure is reduced to 5.0GPa, the pressure is maintained for 2min, the pressure is reduced to 3.0GPa, the pressure is maintained for 2min, and finally the pressure is reduced to the normal pressure.
The procedure is as in example 1.
Example 3:
The specific flow of the ultrahigh pressure and low temperature experiment is as follows, the first pressure rise is 2.9GPa, the pressure is maintained for 2min, the pressure is continuously increased to 4.0GPa, the pressure is maintained for 20min, the room temperature environment (20 ℃) is maintained, the pressure is reduced to 3.6GPa, the pressure is maintained for 2min, the pressure is reduced to 2.2GPa, the pressure is maintained for 2min, and finally the pressure is reduced to normal pressure.
The procedure is as in example 1.
Example 4:
The specific flow of the ultrahigh pressure and low temperature experiment is as follows, the first pressure rise is 2.2GPa, the pressure is maintained for 2min, the pressure is continuously increased to 3.0GPa, the pressure is maintained for 60min, the room temperature environment (20 ℃) is maintained, the pressure is reduced to 2.7GPa, the pressure is maintained for 2min, the pressure is reduced to 1.6GPa, the pressure is maintained for 2min, and finally the pressure is reduced to normal pressure.
The procedure is as in example 1.
Example 5 (c, d in fig. 9):
The specific flow of the ultrahigh pressure and low temperature experiment is as follows, the first pressure rise is 4.0GPa, the pressure is maintained for 2min, the pressure is continuously raised to 5.5GPa, the pressure is maintained for 20min, the temperature is raised to 250 ℃ at the speed of 100 ℃ per min, the temperature is maintained for 3min, then the temperature is reduced to room temperature at the speed of 20 ℃ per min, the pressure is reduced to 5.0GPa, the pressure is maintained for 2min, then the pressure is reduced to 3.0GPa, the pressure is maintained for 2min, and finally the pressure is reduced to normal pressure.
The procedure is as in example 1.
Example 6:
The specific flow of the ultrahigh pressure and low temperature experiment is as follows, the first pressure rise is 4.0GPa, the pressure is maintained for 2min, the pressure is continuously raised to 5.5GPa, the pressure is maintained for 20min, the temperature is raised to 150 ℃ at the speed of 100 ℃ per min, the temperature is maintained for 3min, then the temperature is reduced to room temperature at 20 ℃ per min, the pressure is reduced to 5.0GPa, the pressure is maintained for 2min, then the pressure is reduced to 3.0GPa, the pressure is maintained for 2min, and finally the pressure is reduced to normal pressure.
The procedure is as in example 1.
Example 7 (a in fig. 9):
the specific flow of the ultra-high pressure and low temperature experiment is as follows, the first pressure rise is 4.0GPa, the pressure is maintained for 2min, the pressure is continuously increased to 5.5GPa, the pressure is maintained for 30min, the room temperature environment (20 ℃) is maintained, the pressure is reduced to 5.0GPa, the pressure is maintained for 2min, the pressure is reduced to 3.0GPa, the pressure is maintained for 2min, and finally the pressure is reduced to the normal pressure.
The procedure is as in example 1.
Comparative example 1 (b in fig. 9)
The specific flow of the ultrahigh pressure and low temperature experiment is as follows, the first pressure rise is 1.5GPa, the pressure is maintained for 2min, the pressure is continuously increased to 2.0GPa, the pressure is maintained for 3min, the room temperature environment (20 ℃) is maintained, the pressure is reduced to 1.8GPa, the pressure is maintained for 2min, the pressure is reduced to 1.1GPa, the pressure is maintained for 2min, and finally the pressure is reduced to normal pressure.
The procedure is as in example 1.
Comparative example 2 (a in FIG. 1 and f in FIG. 9)
The specific flow of the ultra-high pressure low temperature experiment is as follows, the pressure is directly increased to 5.5GPa, the pressure is maintained for 3min, the room temperature environment (20 ℃) is maintained, and then the pressure is directly reduced to normal pressure.
The procedure is as in example 1.
Comparative example 3 (e in fig. 9)
The specific flow of the ultrahigh pressure and low temperature experiment is as follows, the first pressure rise is 4.0GPa, the pressure is maintained for 2min, the pressure is continuously raised to 5.5GPa, the pressure is maintained for 20min, the temperature is raised to 300 ℃ at the speed of 100 ℃ per min, the temperature is maintained for 3min, then the temperature is reduced to room temperature at 20 ℃ per min, the pressure is reduced to 5.0GPa, the pressure is maintained for 2min, then the pressure is reduced to 3.0GPa, the pressure is maintained for 2min, and finally the pressure is reduced to normal pressure.
The procedure is as in example 1.
Test examples
The Vickers hardness of the gamma-Al 2O3 nm transparent ceramics obtained in the examples and comparative examples under different pressures and low temperature sintering (20-250 ℃) is shown in Table 1.
TABLE 1 basic parameters of gamma-Al 2O3 nm transparent ceramics obtained under different pressure and low temperature (20 ℃ to 250 ℃) sintering environments
Fig. 2 is an XRD pattern of gamma-Al 2O3 powder in an atmospheric sintering state, in which gamma-Al 2O3 powder is sintered at a temperature lower than 1000 ℃ and is not phase-changed, and phase-changed when the powder is heat-preserved for 2 hours at 1100 ℃, and SEM of the corresponding sintered gamma-Al 2O3 powder is shown in fig. 3 to 5, and is completely converted into alpha-Al 2O3 when heat-preserved for 2 hours at 1200 ℃, which is consistent with XRD and SEM of commercial alpha-Al 2O3, as shown in fig. 6.
An assembly schematic of the ultra-high pressure test is shown in fig. 7. The sample phase did not change after sintering at a height Wen Shiwen, as shown in fig. 8.
The sample is prepared into gamma-Al 2O3 nanometer transparent ceramic material at the ultra-high pressure and low temperature (20 ℃ to 250 ℃) as shown in fig. 9, the sample formed at the room temperature is poor in transparency, the sample prepared at the ultra-high pressure and low temperature is higher than 3GPa, the sample prepared at the ultra-high pressure and low temperature is good in transparency, the gamma-Al 2O3 nanometer transparent ceramic material prepared according to the optimization process in fig. 1b is good in light transmittance, the sample can obtain gamma-Al 2O3 nanometer transparent ceramic in a very short dwell time as shown in fig. 10, a rear image can be clearly seen through the ceramic, the light transmittance in a visible light area is more than 80%, the near infrared highest transmittance reaches 94.5%, and the Vickers hardness is 20.62GPa. SEM images (figure 11) of samples under the optimized process at room temperature (20 ℃) and low-temperature sintering (250 ℃) show that the gamma-Al 2O3 transparent ceramic under the action of high pressure has smaller pores, the density is improved along with the temperature rise, the density is further improved, the particle size of the gamma-Al 2O3 transparent ceramic is smaller than 100nm, and the gamma-Al 2O3 transparent ceramic belongs to the nano transparent ceramic material.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (9)
1. A rapid preparation method of gamma-Al 2O3 nanometer transparent ceramics under the environment of ultra-high pressure and low temperature is characterized by comprising the following specific steps:
S1, drying nano gamma-Al 2O3 powder, sieving to obtain raw material gamma-Al 2O3, pouring raw material gamma-Al 2O3 into a metal molybdenum cup, and buckling the metal molybdenum cup;
s2, transferring the metal molybdenum cup into a mold, and applying pressure to form a biscuit;
s3, finally transferring the metal molybdenum cup into an ultrahigh pressure assembly, integrally placing the metal molybdenum cup into a hexahedral top press, and performing ultrahigh pressure low temperature sintering treatment to obtain the gamma-Al 2O3 nanometer transparent ceramic, wherein the ultrahigh pressure low temperature treatment condition is that the gradient pressure is increased to 3.0-10.0 GPa, the pressure is maintained for 30 s-60 min at 20-250 ℃, and the gradient pressure is reduced to normal pressure.
2. The method according to claim 1, wherein the drying process is carried out at 60℃for 24 hours in step S1.
3. The preparation method according to claim 1, wherein in step S1, sieving is performed by passing through a 100 mesh sieve and a 200 mesh sieve in order.
4. The preparation method according to claim 1, wherein in the step S2, the pressure is applied under a load of 5-10 MPa for 3-15 minutes.
5. The method according to claim 1, wherein in step S3, the ultra-high pressure assembly is composed of pyrophyllite, dolomite ring, dolomite tube, conductive plug, metal molybdenum cup containing sample, graphite sheet, graphite column, graphite tube, molybdenum sheet, magnesium oxide tube.
6. The preparation method according to claim 1, wherein in step S3, gradient pressure increase is achieved in two steps and gradient pressure decrease is achieved in three steps.
7. The method according to claim 1, wherein in step S3, when the low temperature sintering condition is higher than room temperature, the temperature is raised and then lowered to room temperature after the gradient pressure raising is completed, and then the gradient pressure lowering treatment is performed.
8. The method according to claim 1, wherein the ultra-high pressure and low temperature treatment conditions in step S3 are that the pressure is raised to 6.0GPa, maintained for 2 minutes, the pressure is raised to 8.0GPa continuously, the pressure is maintained for 30 seconds, the room temperature environment is maintained, the pressure is lowered to 7.0GPa, the pressure is maintained for 2 minutes, the pressure is raised to 5.0GPa continuously, the pressure is maintained for 2 minutes, and the pressure is lowered to normal pressure finally, or
Firstly, the pressure is increased to 4.0GPa, the pressure is maintained for 2 minutes, the pressure is increased to 5.5GPa, the pressure is maintained for 3 minutes, the room temperature environment is maintained, then the pressure is reduced to 5.0GPa, the pressure is maintained for 2 minutes, the pressure is reduced to 3.0GPa, the pressure is maintained for 2 minutes, and finally the pressure is reduced to normal pressure, or
Firstly, raising pressure to 2.9GPa, maintaining pressure for 2 min, continuously raising pressure to 4.0GPa, maintaining pressure for 20 min, maintaining room temperature environment, then lowering pressure to 3.6GPa, maintaining pressure for 2 min, continuously lowering pressure to 2.2GPa, maintaining pressure for 2 min, finally lowering pressure to normal pressure, or
Firstly, raising pressure to 2.2GPa, maintaining pressure for 2 min, continuously raising pressure to 3.0GPa, maintaining pressure for 60 min, maintaining room temperature environment, then lowering pressure to 2.7GPa, maintaining pressure for 2 min, continuously lowering pressure to 1.6GPa, maintaining pressure for 2 min, finally lowering pressure to normal pressure, or
Firstly, raising pressure to 4.0GPa, maintaining pressure for 2 min, continuously raising pressure to 5.5GPa, maintaining pressure for 20 min, raising temperature to 250 ℃ according to the speed of 100 ℃ per min, preserving heat for 3min, then reducing temperature to room temperature at 20 ℃ per min, reducing pressure to 5.0GPa, maintaining pressure for 2 min, continuously reducing pressure to 3.0GPa, maintaining pressure for 2 min, and finally reducing pressure to normal pressure, or
Firstly, raising pressure to 4.0GPa, maintaining pressure for 2 min, continuously raising pressure to 5.5GPa, maintaining pressure for 20 min, raising temperature to 150 ℃ according to the speed of 100 ℃ per min, preserving heat for 3min, then reducing temperature to room temperature at 20 ℃ per min, reducing pressure to 5.0GPa, maintaining pressure for 2 min, continuously reducing pressure to 3.0GPa, maintaining pressure for 2 min, and finally reducing pressure to normal pressure, or
Firstly, the pressure is increased to 4.0GPa, the pressure is maintained for 2 minutes, the pressure is increased to 5.5GPa continuously, the pressure is maintained for 30 minutes, the room temperature environment is maintained, then the pressure is reduced to 5.0GPa, the pressure is maintained for 2 minutes, the pressure is reduced to 3.0GPa continuously, the pressure is maintained for 2 minutes, and finally the pressure is reduced to normal pressure.
9. The preparation method of claim 1, wherein in step S3, after the ultra-high pressure low temperature treatment is completed, the metal molybdenum cup is removed, and the gamma-Al 2O3 nm transparent ceramic is obtained by cutting and polishing.
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