A preparation method of a large-size optical mold material by hot pressing pre-sintering

Abstract

The application discloses a kind of preparation methods of large-size optical mold material heat-pressing pre-sintering, comprising the following steps: proportioning alpha-SiC powder, beta-SiC powder, yttrium oxide powder, aluminum oxide powder, boron carbide powder and nano carbon powder;Preparation of sintering aid dispersion liquid and SiC compound, wet ball milling, spray granulation, layering and cold isostatic pressing forming to obtain large-size ceramic preform;Again, segmented degassing, pulse exhaust, hot-pressing pre-sintering and SPS sintering, annealing cooling to obtain large-size optical mold material.The application can improve the uniformity of large-size ceramic preform by alpha-SiC powder and beta-SiC powder compounding, sintering aid dispersion adsorption and layering;Combined with pulse exhaust, three-stage hot-pressing pre-sintering and discharge plasma sintering, it can promote residual gas discharge and pore closure to obtain large-size optical mold material with dense uniformity and good mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of optical mold materials, and more particularly to a hot-pressing pre-firing method for preparing large-size optical mold materials. Background Technology

[0002] Large-size optical mold materials are the basic materials used to manufacture molds for large-scale optical components. They typically need to have high density, structural uniformity, and dimensional stability to meet the requirements of subsequent precision machining and high-temperature molding. Among them, silicon carbide ceramic materials have the characteristics of high hardness, good wear resistance, low coefficient of thermal expansion, and high thermal stability, and can be used as large-size optical mold materials.

[0003] In existing technologies, large-size silicon carbide ceramic materials are typically produced by forming ceramic powder into a green body and then densifying it through high-temperature sintering. However, as the size of the ceramic green body increases, it becomes more difficult for the decomposition products of the binder and the residual gas in the particle pores inside the green body to be fully expelled. During the heating and sintering process, the problem of residual internal gas is prone to occur. When the outer area of ​​the green body shrinks and densifies first, the internal exhaust channels are also prone to close prematurely, thus forming residual pores inside the sintered body. At the same time, during the high-temperature sintering process of large-size ceramic green bodies, the temperature and shrinkage of different areas are prone to be different, which can easily cause uneven densification inside the sintered body and produce problems such as local stress concentration, warping or cracking, thereby affecting the structural strength, dimensional stability and subsequent processing quality of large-size optical mold materials.

[0004] Therefore, existing technologies have shortcomings and need to be improved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a hot pressing and pre-firing method for preparing large-size optical mold materials with sufficient exhaust, high density and uniform structure.

[0006] To achieve this objective, the present invention employs the following technical solution: a method for preparing large-size optical mold materials by hot pressing and pre-firing, comprising the following steps: S1. Weigh 55-70 parts of α-SiC powder, 25-40 parts of β-SiC powder, 1.0-3.0 parts of yttrium oxide powder, 0.5-2.0 parts of alumina powder, 0.1-0.5 parts of boron carbide powder, and 0.1-0.4 parts of nano-carbon powder according to the mass ratio. Premix the α-SiC powder and β-SiC powder to form a SiC mixture. S2. Prepare a sintering aid dispersion by yttrium oxide powder and alumina powder, add the SiC mixture obtained in step S1 to the sintering aid dispersion for mixing, so that the yttrium oxide powder and alumina powder are adsorbed on the surface of SiC particles, and after drying, add boron carbide powder and nano carbon powder for secondary mixing to obtain composite ceramic powder. S3. The composite ceramic powder is subjected to wet ball milling. After ball milling, binder and plasticizer are added and mixed. Then, it is spray granulated and dried to obtain granulated powder. S4. The granulated powder is layered and loaded into the mold. Vibration leveling and pre-pressing are performed during the loading process. After loading, cold isostatic pressing is performed. After demolding, a large-size ceramic preform is obtained. S5. Place the large-size ceramic preform in a vacuum hot press furnace for segmented heating and glue removal, and perform pulse exhaust treatment with alternating vacuum and inert gas after glue removal. S6. After completing the exhaust treatment in step S5, the large-size ceramic preform is heated and axial pressure is gradually applied. When the temperature reaches 1400℃, the axial pressure is increased to 5MPa and hot-pressing is performed to form a pre-fired skeleton. S7. The pre-sintered skeleton is loaded into the discharge plasma sintering equipment and sintered by segmented heating and graded pressurization in a vacuum environment. When the sintering temperature reaches 1700℃, the axial load is increased to 35kN and heat preservation sintering is carried out. After sintering, graded depressurization and slow cooling are carried out. After annealing, it is cooled to room temperature to obtain large-size optical mold material.

[0007] Using the above technical solution, in step S1, the D10 particle size of the α-SiC powder is 0.8-1.5 μm, the D50 particle size is 2.0-3.0 μm, and the D90 particle size is 4.0-6.0 μm; the D50 particle size of the β-SiC powder is 0.25-0.50 μm; and the mass ratio of the α-SiC powder to the β-SiC powder is 1.8-2.8:1. Before premixing, α-SiC powder and β-SiC powder are dried in a vacuum environment at 120-160℃ for 2-5 hours to ensure that the moisture content of α-SiC powder and β-SiC powder is not higher than 0.15wt%. Then, they are cooled to below 40℃ at a cooling rate of 10-20℃ / min, and α-SiC powder and β-SiC powder are premixed in a closed environment to obtain the SiC mixture.

[0008] Using the above technical solution, in step S2, the preparation method of the sintering aid dispersion is as follows: yttrium oxide powder and alumina powder are added to a mixture of ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 3-6:1, and ammonium polyacrylate accounting for 0.3-0.8 wt% of the total mass of yttrium oxide powder and alumina powder is added. The mixture is sheared and dispersed at 1500-3000 rpm for 20-40 min to obtain the sintering aid dispersion.

[0009] Using the above technical solution, in step S3, the wet ball milling uses anhydrous ethanol as the ball milling medium, the liquid-solid mass ratio between the composite ceramic powder and anhydrous ethanol is 0.6-1.0:1, and silicon carbide balls are used as the grinding medium, the ball-to-material mass ratio is 3-6:1, and the wet ball milling is carried out at 100-180 rpm for 3-6 hours. After wet ball milling, the resulting slurry is vacuum degassed for 10-30 minutes. Then, 0.5-1.5 wt% polyvinyl alcohol as a binder and 0.2-0.6 wt% polyethylene glycol as a plasticizer are added. After mixing for another 20-40 minutes, spray granulation is performed to obtain granulated powder with a D50 particle size of 40-90 μm and a loose packing density of 0.85-1.20 g / cm³. The granulated powder is then dried in a vacuum environment at 80-110℃ for 4-8 hours.

[0010] Using the above technical solution, in step S4, the granulated powder is divided into at least five powder units according to the total amount of material, and each powder unit is loaded into the mold in order from bottom to top; After each powder unit is added to the mold, the added granulated powder is vibrated and leveled at a frequency of 20-50Hz for 30-90s. Then, a pre-compression pressure of 5-15MPa is applied to the granulated powder and held for 30-120s. The pre-compression pressure is then released and the powder is loaded into the next powder unit until all the granulated powder is loaded. After the material is loaded, cold isostatic pressing is performed. First, the pressure is increased to 80-120MPa at a rate of 2-5MPa / min and held for 2-5min. Then, the pressure is increased to 180-250MPa and held for 5-15min. Subsequently, the pressure is gradually reduced to normal pressure at a rate not exceeding 8MPa / min. After demolding, the large-size ceramic preform is obtained.

[0011] Using the above technical solution, in step S5, the specific process flow of the segmented heating and adhesive removal is as follows: Under conditions where no axial pressure is applied or a contact pressure not exceeding 0.5 MPa is applied, the large-size ceramic preform is heated to 250-350℃ at a heating rate of 0.5-1.5℃ / min and held for 60-120 min. Then, the temperature is further increased to 500-650℃ at a heating rate of 1-2℃ / min and held for 60-150 min, so that the binder and plasticizer in the large-size ceramic preform are gradually decomposed and discharged. After the adhesive is discharged, the temperature is further increased to 850-1050℃ for pulse venting treatment. The pulse exhaust process is as follows: the furnace cavity of the vacuum hot press furnace is evacuated to 5-50 Pa and maintained for 5-10 min, then high-purity argon gas is introduced into the furnace cavity to increase the furnace cavity pressure to 5-20 kPa and maintain for 2-5 min, and then the evacuation process is repeated again. The vacuum and high-purity argon gas pressure switching is repeated 2-5 times.

[0012] Using the above technical solution, a delayed pressurization method is adopted in the segmented heating and debinding and pulsed degassing process. When the temperature of the large-size ceramic preform is below 1050℃, the axial pressure is maintained at 0-0.5MPa. When the temperature rises to 1050-1200℃, the axial pressure is increased to 0.5-1.0MPa, so that the large-size ceramic preform produces 0.5-2.0% axial pre-shrinkage. During the pulse exhaust process, each time the furnace is re-evacuated in a high-purity argon environment, when the furnace pressure drops below 200Pa, the pressure drop rate inside the furnace is controlled to be 2-10Pa / s, so that the residual gas inside the large-sized ceramic preform can be discharged outward through the particle pores.

[0013] Using the above technical solution, in step S6, the hot-pressing preheating adopts a three-stage pressurization process: In the first stage, the large-size ceramic preform is heated to 1200-1300℃, and the axial pressure is maintained at 0.5-1.0MPa; In the second stage, the large-size ceramic preform is heated to 1380℃, and the axial pressure is gradually increased to 2-3MPa at a pressurization rate of 0.02-0.05MPa / min. In the third stage, the large-size ceramic preform is heated to 1400℃. When the furnace temperature reaches 1400℃, the axial pressure is increased to 5MPa at a pressurization rate of 0.05-0.10MPa / min, and the temperature is held at 1400℃ and 5MPa for 45-75 minutes to form a pre-fired skeleton for the large-size ceramic preform. 10-20 minutes before the end of the hot-pressing pre-firing and heat preservation, reduce the furnace cavity pressure to 10-30 Pa, so that the trace amount of volatile gas remaining inside the pre-firing skeleton can be discharged outward along the opening pores. After hot pressing and pre-firing, the relative density of the resulting pre-firing skeleton is 72-84%, and the open porosity is 7-16%.

[0014] Using the above technical solution, in step S7, the pre-sintered skeleton is loaded into the graphite mold of the discharge plasma sintering equipment, the sintering chamber is evacuated to below 20 Pa, and pulsed DC current is applied to the graphite mold for SPS sintering. The pulsed DC current adopts an intermittent energizing method, with a pulse energizing to de-energizing time ratio of 8:2-12:2, and the pulse current output is gradually increased according to the increase of sintering temperature, so as to rapidly heat up the graphite mold and pre-sintered skeleton. During the temperature rise from room temperature to 900℃, the temperature is increased at a rate of 80-120℃ / min, and the axial load is controlled at 2-5kN. In the 900-1450℃ range, the temperature is increased at a rate of 60-90℃ / min, and the pressure is gradually increased to raise the axial load to 10-18kN. In the 1450-1650℃ range, the temperature is increased at a rate of 30-60℃ / min, and the pressure is gradually increased to raise the axial load to 20-28kN. After the sintering temperature reaches 1650℃, continue to raise the temperature to 1700℃ and simultaneously apply pressure to increase the axial load to 35kN. Hold at 1700℃ and 35kN for 5-20 minutes to allow the residual pores in the pre-sintered skeleton to close quickly.

[0015] Using the above technical solution, in step S7, after SPS sintering at 1700℃ and 35kN is completed, the heating is stopped, and the axial load of 35kN is maintained for cooling. When the temperature drops to 1550-1600℃, the first pressure relief is carried out to reduce the axial load to 20-25kN, and the cooling process continues. When the temperature drops to 1400-1450℃, the pressure is released for the second time to reduce the axial load to 5-10kN, and the temperature is further reduced to 1300-1350℃. The first stage of annealing is carried out by holding the temperature at 1300-1350℃ for 20-40 minutes. Then, the temperature was lowered to 1150-1250℃ and the pressure was completely released. High-purity argon gas was introduced into the sintering chamber, and the second stage of annealing was carried out by holding the temperature at 1150-1250℃ for 30-60 minutes. After the second stage of annealing, the temperature is reduced to 800°C at a cooling rate of 1-2°C / min, and then reduced to 500°C at a cooling rate of 3-5°C / min. The material is then cooled to room temperature in the furnace to obtain the large-size optical mold material.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention employs a blend of α-SiC and β-SiC powders. Through the mutual filling of SiC powders with different particle sizes, the packing density of the composite ceramic powder is improved, and the sintering aid is more uniformly distributed among the SiC particles. Simultaneously, preparing a sintering aid dispersion from yttrium oxide and alumina powders before mixing it with the SiC mixture allows the sintering aid to be more uniformly adsorbed onto the surface of the SiC particles, which is beneficial for the bonding and densification of the particle interfaces during subsequent sintering. During the molding process, the granulated powder is divided into multiple powder units for layered loading, and combined with vibration leveling, layer-by-layer pre-pressing, and cold isostatic pressing, which reduces the powder packing differences between different areas of large-sized ceramic preforms, resulting in a more uniform internal structure and providing better conditions for subsequent heat treatment and sintering. The invention achieves a consistent initial state. Secondly, after the large-size ceramic preform is debonded, it employs a pulsed exhaust method alternating between vacuum and high-purity argon gas, combined with delayed pressurization, to gradually expel the binder decomposition products and residual gases inside the preform before the particle pores are significantly closed. Subsequently, a three-stage pressurization method is used for hot-pressing pre-firing, which forms a preliminary sintering connection between SiC particles to obtain a pre-fired skeleton with a certain structural strength and still retaining some open pores. Then, through spark plasma sintering, the pre-fired skeleton is gradually densified under the action of rapid pulsed DC heating and axial pressurization, and the residual pores are further closed. After sintering, the adverse effects of rapid pressure and temperature changes after high-temperature sintering on the large-size sintered body can be reduced by staged depressurization, slow cooling, and staged annealing. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Furthermore, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., described below refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the technical features involved in the various embodiments of the invention can be combined with each other as long as they do not conflict with each other.

[0018] Example 1 Embodiment 1 of the present invention provides a method for hot pressing and pre-firing preparation of large-size optical mold materials, comprising the following steps: S1. Weigh out 63 parts by weight of α-SiC powder, 34 parts by weight of β-SiC powder, 1.5 parts by weight of yttrium oxide powder, 1 part by weight of alumina powder, 0.3 parts by weight of boron carbide powder, and 0.2 parts by weight of nano-carbon powder, with a total mass of 100 parts; wherein the α-SiC powder has a D10 particle size of 1.2 μm, a D50 particle size of 2.5 μm, and a D90 particle size of 5.0 μm, and the β-SiC powder has a D50 particle size of 0.4 μm, and the mass ratio of α-SiC powder to β-SiC powder is 63:34; Before premixing, α-SiC powder and β-SiC powder were dried in a vacuum environment at 140℃ for 3 hours to control the moisture content of α-SiC powder and β-SiC powder to 0.10wt%. After drying, the temperature was reduced to 35℃ at a cooling rate of 15℃ / min. Then, α-SiC powder and β-SiC powder were premixed in a closed environment to form SiC mixture. S2. Add 1.5 parts of yttrium oxide powder and 1 part of alumina powder to a mixture of ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 4:1, and add 0.5 wt% of ammonium polyacrylate, which accounts for the total mass of yttrium oxide powder and alumina powder. Shear and disperse at 2200 rpm for 30 min to prepare a sintering aid dispersion. The SiC mixture obtained in step S1 is added to the sintering aid dispersion for mixing, so that the yttrium oxide powder and alumina powder are uniformly adsorbed on the surface of SiC particles. After drying, 0.3 parts of boron carbide powder and 0.2 parts of nano carbon powder are added for secondary mixing, so that the boron carbide powder and nano carbon powder are uniformly dispersed between SiC particles to obtain composite ceramic powder. S3. The composite ceramic powder obtained in step S2 is subjected to wet ball milling. The wet ball milling uses anhydrous ethanol as the ball milling medium. The liquid-solid mass ratio between the composite ceramic powder and anhydrous ethanol is 0.8:1. Silicon carbide balls are used as the grinding medium. The ball-to-material mass ratio is 4:1. The wet ball milling is carried out at 150 rpm for 4 hours. After wet ball milling, the resulting slurry is vacuum degassed for 20 minutes. Then, 1.0 wt% polyvinyl alcohol as a binder and 0.4 wt% polyethylene glycol as a plasticizer are added as a total mass of composite ceramic powder. The mixture is then mixed for another 30 minutes to ensure that the binder and plasticizer are evenly dispersed in the slurry. After mixing, spray granulation is performed to control the D50 particle size of the resulting granulated powder to 60 μm and the loose packing density to 1.00 g / cm³. After spray granulation, the granulated powder is dried in a vacuum environment at 95°C for 6 hours to obtain granulated powder for molding. S4. Divide the granulated powder obtained in step S3 into five powder units according to the total amount of material and the same mass, and load the five powder units into the mold in order from bottom to top. After each powder unit is added to the mold, the added granulating powder is vibrated and leveled at a frequency of 35Hz for 60s to ensure that the granulating powder is evenly spread in the mold. Then, a pre-compression pressure of 10MPa is applied to the granulating powder and held for 60s. After that, the pre-compression pressure is released and the next powder unit is loaded. The same loading, vibration leveling and pre-compression process is repeated until the loading of five powder units is completed. After the material is loaded, cold isostatic pressing is performed. First, the pressure is increased to 100MPa at a rate of 3MPa / min and held for 3min to initially compact the granulated powder in the mold. Then, the pressure is increased to 220MPa and held for 10min to further compact the granulated powder evenly. Subsequently, the pressure is gradually reduced to normal pressure at a rate of 5MPa / min. After demolding, a large-size ceramic preform is obtained. S5. The large-size ceramic preform obtained in step S4 is placed in a vacuum hot press furnace for segmented heating and adhesive removal. An axial contact pressure of 0.3 MPa is applied to the large-size ceramic preform, and the temperature is raised to 300°C at a heating rate of 1°C / min. The temperature is held at 300°C for 90 min to allow the binder and plasticizer components that can decompose or volatilize under low temperature conditions to be gradually removed. Then, the temperature is raised to 600°C at a heating rate of 1.5°C / min and held at 600°C for 120 min to allow the remaining polyvinyl alcohol and polyethylene glycol in the large-size ceramic preform to be further decomposed and removed. After the glue removal is completed, the temperature is raised to 950℃ for pulse exhaust treatment with alternating vacuum and high-purity argon. The pulse exhaust treatment is as follows: first, the furnace cavity of the vacuum hot press furnace is evacuated to 20Pa and held for 8 minutes, then high-purity argon is introduced into the furnace cavity to increase the furnace cavity pressure to 10kPa and held for 3 minutes. Then, the vacuum treatment is performed again, and the vacuum and high-purity argon pressure are switched three times in the above manner to promote the discharge of residual gas inside the large-size ceramic preform. During the segmented heating and debinding and pulsed degassing process, a delayed pressurization method is adopted. When the temperature of the large-size ceramic preform is below 1050℃, the axial pressure is maintained at 0.3MPa. When the temperature continues to rise to 1150℃, the axial pressure is increased to 0.8MPa, so that the large-size ceramic preform produces 1.0% axial pre-shrinkage. During the pulse exhaust process, each time the furnace is re-evacuated from a high-purity argon environment, when the furnace pressure drops below 200Pa, the rate of pressure drop in the furnace is controlled to 5Pa / s, so that the residual gas inside the large-size ceramic preform is gradually discharged out through the particle pores. S6. After completing the exhaust treatment in step S5, the large-size ceramic preform is heated further and hot-pressed and pre-fired using a three-stage pressurization process. In the first stage, the temperature of the large-size ceramic preform is raised from 1200℃ to 1300℃, and the axial pressure is maintained at 0.8MPa during this temperature stage, so that the large-size ceramic preform is gradually heated under low pressure conditions. In the second stage, the temperature of the large-size ceramic preform was increased from 1300℃ to 1380℃, and the axial pressure was gradually increased to 2.5MPa at a pressurization rate of 0.03MPa / min. In the third stage, the large-size ceramic preform is further heated to 1400℃. Once the furnace temperature reaches 1400℃, the axial pressure is increased to 5MPa at a pressurization rate of 0.08MPa / min, and hot-pressing pre-firing is performed at 1400℃ and 5MPa for 60 minutes. This allows for the formation of preliminary sintering connections between adjacent SiC particles, resulting in a pre-fired skeleton with a certain structural strength for the large-size ceramic preform. After holding the hot-pressing pre-firing at 1400℃ and 5MPa for 45 minutes, the furnace pressure is reduced to 20Pa, and the pre-firing is continued under this vacuum condition for 15 minutes, allowing the trace amounts of volatile gases remaining inside the pre-fired skeleton to escape through the open pores. After the hot-pressing pre-firing, the relative density of the resulting pre-fired skeleton is 78%, and the open porosity is 12%. S7. The pre-sintered skeleton obtained in step S6 is loaded into the graphite mold of the discharge plasma sintering equipment, the sintering chamber is evacuated to 10 Pa, and pulsed direct current is applied to the graphite mold to perform discharge plasma sintering (SPS). The pulsed DC power supply adopts an intermittent power-on method, with a pulse power-on to power-off time ratio of 10:2. The pulse current output is gradually increased according to the increase of sintering temperature, so as to rapidly heat up the graphite mold and pre-sintered skeleton. During the stage from room temperature to 900℃, the temperature was increased at a rate of 100℃ / min, and the axial load was controlled at 3kN. After the sintering temperature reached 900℃, the temperature was increased to 1450℃ at a rate of 75℃ / min, and the pressure was gradually increased during the heating process to increase the axial load to 14kN. After the sintering temperature reached 1450℃, the temperature was increased to 1650℃ at a rate of 45℃ / min, and the pressure was gradually increased to increase the axial load to 24kN. After the sintering temperature reaches 1650℃, the temperature is further increased to 1700℃, and pressure is applied simultaneously to increase the axial load to 35kN. The temperature is held at 1700℃ and 35kN for 10 minutes to cause the residual pores inside the pre-sintered skeleton to shrink and close rapidly, thereby completing the rapid densification of the pre-sintered skeleton. After SPS sintering at 1700℃ and 35kN, the heating was stopped and the axial load of 35kN was maintained for cooling. When the temperature dropped to 1575℃, the pressure was first released to reduce the axial load to 22kN, and the cooling continued under this axial load. When the temperature drops to 1425℃, a second depressurization is performed to reduce the axial load to 8kN, and the temperature continues to drop to 1325℃. The first stage of annealing is carried out by holding at 1325℃ for 30 minutes. After the first stage of annealing, the temperature continues to drop to 1200℃ and the pressure is completely depressurized. Then, high-purity argon gas is introduced into the sintering chamber, and the second stage of annealing is carried out by holding at 1200℃ for 45 minutes. After the second stage of annealing, the temperature is reduced to 800℃ at a rate of 1.5℃ / min, and then reduced to 500℃ at a rate of 4℃ / min. The temperature is then cooled to room temperature with the furnace to obtain a large-size optical mold material with a length of 150mm, a width of 100mm, and a thickness of 20mm.

[0019] Example 2 Embodiment 2 of the present invention provides a method for hot pressing and pre-firing preparation of large-size optical mold materials, comprising the following steps: S1. Weigh out 66 parts by weight of α-SiC powder, 30 parts by weight of β-SiC powder, 2 parts by weight of yttrium oxide powder, 1.2 parts by weight of alumina powder, 0.5 parts by weight of boron carbide powder, and 0.3 parts by weight of nano-carbon powder, with a total mass of 100 parts; wherein the α-SiC powder has a D10 particle size of 0.9 μm, a D50 particle size of 2.2 μm, and a D90 particle size of 4.5 μm, and the β-SiC powder has a D50 particle size of 0.3 μm, and the mass ratio of α-SiC powder to β-SiC powder is 66:30, i.e., 2.2:1; Before premixing, α-SiC powder and β-SiC powder were dried in a vacuum environment at 130℃ for 4 hours to control the moisture content of α-SiC powder and β-SiC powder to 0.12wt%. After drying, the temperature was reduced to 38℃ at a cooling rate of 12℃ / min. Then, α-SiC powder and β-SiC powder were premixed in a closed environment to form SiC mixture. S2. Add 2 parts of yttrium oxide powder and 1.2 parts of alumina powder to a mixture of ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 5:1, and add 0.7 wt% of ammonium polyacrylate, which accounts for 0.7 wt% of the total mass of yttrium oxide powder and alumina powder. Shear and disperse at 2600 rpm for 25 min to prepare a sintering aid dispersion. The SiC mixture obtained in step S1 is added to the sintering aid dispersion for mixing, so that the yttrium oxide powder and alumina powder are uniformly adsorbed on the surface of SiC particles. After mixing, the mixture is dried, and then 0.5 parts of boron carbide powder and 0.3 parts of nano carbon powder are added for secondary mixing, so that the boron carbide powder and nano carbon powder are uniformly dispersed between the SiC particles to obtain composite ceramic powder. S3. The composite ceramic powder obtained in step S2 is subjected to wet ball milling. The wet ball milling uses anhydrous ethanol as the ball milling medium. The liquid-solid mass ratio between the composite ceramic powder and anhydrous ethanol is 1.0:1. Silicon carbide balls are used as the grinding medium. The ball-to-material mass ratio is 5:1. The wet ball milling is carried out at 120 rpm for 5 hours. After wet ball milling, the resulting slurry is vacuum degassed for 25 minutes. Then, 1.3 wt% polyvinyl alcohol as a binder and 0.5 wt% polyethylene glycol as a plasticizer are added as a total mass of composite ceramic powder. Mixing continues for 35 minutes to ensure that polyvinyl alcohol and polyethylene glycol are evenly dispersed in the slurry. After mixing, spray granulation is performed to control the D50 particle size of the resulting granulated powder to 75 μm and the loose packing density to 1.10 g / cm³. After spray granulation, the granulated powder is dried in a vacuum environment at 105°C for 5 hours to obtain granulated powder for molding. S4. Divide the granulated powder obtained in step S3 into six powder units according to the total amount of material and the same mass, and load the six powder units into the mold in order from bottom to top. After each powder unit is added to the mold, the added granulating powder is vibrated and leveled at a frequency of 45Hz for 45s to ensure that the granulating powder is evenly distributed in the mold. Then, a pre-compression pressure of 12MPa is applied to the granulating powder and held for 90s. The pre-compression pressure is then released and the next powder unit is added. The same loading, vibration leveling and pre-compression process is repeated until the loading of six powder units is completed. After the material is loaded, cold isostatic pressing is performed. First, the pressure is increased to 110MPa at a rate of 4MPa / min and held for 4min to initially compact the granulated powder in the mold. Then, the pressure is increased to 240MPa and held for 12min to further compress the powder units into an integral green body. Subsequently, the pressure is gradually reduced to normal pressure at a rate of 6MPa / min. After demolding, a large-size ceramic preform is obtained. S5. Place the large-size ceramic preform obtained in step S4 into a vacuum hot press furnace for segmented heating and adhesive removal. Apply an axial contact pressure of 0.4 MPa to the large-size ceramic preform and heat it to 330°C at a heating rate of 1.2°C / min. Hold it at 330°C for 100 min to allow the low-temperature volatile components in the binder and plasticizer to gradually decompose and be removed. The temperature was then increased to 620℃ at a rate of 1.8℃ / min and held at 620℃ for 130 min to further decompose and remove the remaining polyvinyl alcohol and polyethylene glycol in the large-size ceramic preform. After the glue removal is completed, the temperature is raised to 1000℃ for pulse exhaust treatment with alternating vacuum and high-purity argon. The pulse exhaust treatment is as follows: first, the furnace cavity of the vacuum hot press furnace is evacuated to 30Pa and held for 6 minutes, then high-purity argon is introduced into the furnace cavity to raise the furnace cavity pressure to 15kPa and hold for 4 minutes, and then the vacuum treatment is performed again. The vacuum and high-purity argon pressure are switched four times in the above manner to promote the discharge of residual gas inside the large-size ceramic preform. During the segmented heating and debinding process and pulsed degassing, a delayed pressurization method is adopted. When the temperature of the large-size ceramic preform is below 1050℃, the axial pressure is maintained at 0.4MPa. When the temperature continues to rise to 1180℃, the axial pressure is increased to 0.9MPa, so that the large-size ceramic preform produces 1.5% axial pre-shrinkage. During the pulse exhaust process, each time the furnace chamber pressure is reduced to below 200Pa after the vacuum is restored in a high-purity argon environment, the pressure drop rate in the furnace chamber is controlled to 8Pa / s, so that the residual gas inside the large-size ceramic preform is gradually discharged out through the particle pores. S6. After completing the exhaust treatment in step S5, the large-size ceramic preform is heated further and hot-pressed and pre-fired using a three-stage pressurization process. In the first stage, the temperature of the large-size ceramic preform is gradually increased from 1200℃ to 1300℃, and the axial pressure is maintained at 0.9MPa during this temperature stage, so that the large-size ceramic preform continues to be heated under low pressure conditions and maintains the air exhaust capacity of the particle pores. In the second stage, the temperature of the large-size ceramic preform is increased from 1300℃ to 1380℃, and the axial pressure is gradually increased to 2.8MPa at a pressurization rate of 0.04MPa / min, so that the ceramic particles gradually form a close contact. In the third stage, the large-size ceramic preform is heated to 1400℃. When the furnace temperature reaches 1400℃, the axial pressure is increased to 5MPa at a pressurization rate of 0.06MPa / min. The preform is then held at 1400℃ and 5MPa for 70 minutes for hot pressing and pre-firing. This process forms sintering necks between SiC particles and gradually connects them, so that the large-size ceramic preform forms a pre-fired skeleton with a certain structural strength. After hot-pressing and pre-firing at 1400℃ and 5MPa for 60 minutes, the furnace pressure was reduced to 25Pa, and the furnace was held under this vacuum for another 10 minutes to allow the trace amounts of volatile gases remaining inside the pre-fired skeleton to escape through the open pores. After hot-pressing and pre-firing, the relative density of the obtained pre-fired skeleton was 82%, and the open porosity was 9%. S7. The pre-sintered skeleton obtained in step S6 is loaded into the graphite mold of the discharge plasma sintering equipment, the sintering chamber is evacuated to 15Pa, and pulsed DC current is applied to the graphite mold to perform discharge plasma sintering (SPS). The pulsed DC current adopts an intermittent energizing method, with a pulse energizing to de-energizing time ratio of 12:2. The pulse current output is gradually increased according to the increase of sintering temperature, so as to rapidly heat up the graphite mold and pre-sintered skeleton. During the temperature rise from room temperature to 900℃, the temperature was increased at a rate of 110℃ / min, and the axial load was controlled at 4kN. When the sintering temperature reaches 900℃, the temperature is continued to rise to 1450℃ at a heating rate of 85℃ / min, and the pressure is gradually increased during the heating process to increase the axial load to 16kN. When the sintering temperature reaches 1450℃, the temperature is increased to 1650℃ at a heating rate of 55℃ / min, and the pressure is gradually increased to increase the axial load to 27kN. After the sintering temperature reaches 1650℃, the temperature is further increased to 1700℃, and pressure is applied simultaneously to increase the axial load to 35kN. The temperature is held at 1700℃ and 35kN for 15 minutes to further shrink and close the residual pores inside the pre-sintered skeleton, thereby completing the rapid densification of the pre-sintered skeleton. After SPS sintering at 1700℃ and 35kN, the heating was stopped and the axial load of 35kN was maintained for cooling. When the temperature dropped to 1600℃, the pressure was first released to reduce the axial load to 25kN, and the cooling continued under this axial load. When the temperature drops to 1450℃, a second depressurization is performed to reduce the axial load to 10kN, and the temperature continues to drop to 1350℃. The first stage of annealing is carried out by holding at 1350℃ for 40 minutes. After the first stage of annealing, the temperature continues to drop to 1250℃ and the pressure is completely depressurized. Then, high-purity argon gas is introduced into the sintering chamber, and the second stage of annealing is carried out by holding at 1250℃ for 60 minutes. After the second stage of annealing, the temperature is reduced to 800℃ at a rate of 2℃ / min, and then reduced to 500℃ at a rate of 5℃ / min. The temperature is then cooled to room temperature with the furnace to obtain a large-size optical mold material with a length of 180mm, a width of 120mm, and a thickness of 25mm.

[0020] Comparative Example 1 The difference between the hot-pressing pre-firing preparation method of the large-size optical mold material in Comparative Example 1 and Example 1 is that in step S1, instead of using a combination of α-SiC powder and β-SiC powder, a single α-SiC powder is used.

[0021] Comparative Example 2 The difference between the hot pressing and pre-sintering preparation method of the large-size optical mold material in Comparative Example 2 and Example 1 is that: in step S2, instead of preparing a sintering aid dispersion, yttrium oxide powder and alumina powder are directly mechanically mixed with SiC mixture.

[0022] Comparative Example 3 The difference between the hot pressing and pre-firing preparation method of the large-size optical mold material in Comparative Example 3 and Example 1 is that: in step S4, layered loading, layer-by-layer vibration leveling and pre-pressing are not used. Instead, all the granulated powder is loaded into the mold at one time and then cold isostatic pressing is performed.

[0023] Comparative Example 4 The difference between the hot pressing and pre-firing preparation method of the large-size optical mold material in Comparative Example 4 and Example 1 is that in step S5, after the glue is removed, the pulse exhaust process of alternating vacuum and high-purity argon is not performed, but continuous vacuum exhaust is used.

[0024] Comparative Example 5 The difference between the hot pressing and pre-firing preparation method of the large-size optical mold material in Comparative Example 5 and Example 1 is that: in step S6, the hot pressing and pre-firing treatment at 1400°C and 5MPa is not performed, but the subsequent SPS sintering is performed directly.

[0025] Comparative Example 6 The difference between the hot pressing and pre-sintering preparation method of the large-size optical mold material in Comparative Example 6 and Example 1 is that: in step S7, instead of using a graded pressurization method, the axial load is directly increased to 35kN at the beginning of SPS sintering and maintained until the end of sintering.

[0026] To verify the effect of the preparation method of the present invention on improving the density, structural uniformity and mechanical properties of large-size optical mold materials, performance tests were conducted on the large-size optical mold materials prepared in Examples 1-2 and Comparative Examples 1-6. The results are shown in Table 1. Before the test, the surface of each group of samples was cleaned and dried in an environment of 120°C for 2 hours, and then cooled to room temperature. Each group was sampled 3 times and the average value was calculated.

[0027] Relative density test: The actual bulk density of the sample was determined by Archimedes' displacement method. The dry mass m1, the suspended mass m2 and the saturated mass m3 of the sample were measured respectively. The actual bulk density of the sample was calculated according to ρ = m1 × ρwater / (m3-m2), where ρwater is the density of deionized water at the test temperature. The theoretical density ρ0 was calculated based on the mass fraction of different components in each group of materials and their corresponding true densities. The relative density of each group of samples was calculated according to relative density = ρ / ρ0 × 100%. Each group was tested 3 times and the average value was taken.

[0028] Open porosity test: The open porosity of the sample was determined by vacuum immersion method. The dry mass m1, the suspended mass m2 and the saturated mass m3 of the sample were measured respectively. The open porosity of each group of samples was calculated according to the formula: open porosity = (m3-m1) / (m3-m2)×100%. Each group was tested 3 times and the average value was taken.

[0029] Bending strength test: The bending strength of the specimen is determined by the three-point bending method. The specimen is placed between two supports and a load is applied from the middle until it breaks. The maximum load F at the time of fracture is recorded. The specimen width b, thickness h and support span L are measured. The bending strength of each group of specimens is calculated according to bending strength = 3FL / (2bh²). Each group is tested 3 times and the average value is taken.

[0030] Vickers hardness test: Samples are cut from the large-size optical mold materials of each group, and the test surface is ground and polished in sequence to make the test surface flat and smooth. Then, a diamond indenter is pressed into the sample surface using a Vickers hardness tester, held for a set time, and then unloaded. The lengths of the two diagonals of the indentation are measured, and the Vickers hardness of the sample is calculated based on the loaded load and the area of ​​the indentation. At least 5 different locations are selected for testing each sample, and the average value is taken as the Vickers hardness test result.

[0031] Center-edge relative density difference test: Samples of the same size were cut from the central region and the region near the edges of each group of large-size optical mold materials. The relative density of the central region sample and the edge region sample were measured using the same Archimedes displacement method. The maximum difference between the relative density of the central region and the relative density of each edge region was taken as the center-edge relative density difference, which was used to evaluate the density uniformity between different regions of the large-size optical mold material.

[0032] Maximum warpage test: Place each group of large-size optical mold materials on a horizontal reference platform, with one of the large-area end faces the reference platform. Select multiple detection positions along the length, width, and central area of ​​the material surface. Measure the height difference of each detection position relative to the reference plane using a height detection device. The difference between the maximum and minimum height values ​​among all detection positions is taken as the maximum warpage, which is used to evaluate the overall deformation degree of the large-size optical mold material after hot pressing, SPS sintering, and cooling.

[0033] Table 1. Comparison of material performance tests for large-size optical molds in various embodiments and comparative examples. As shown in Table 1 above, in the large-size optical mold materials prepared in Examples 1 and 2, the present invention uses α-SiC powder and β-SiC powder to compound each other. Through the mutual filling between SiC powders of different particle sizes, the packing density of the composite ceramic powder can be improved, and the sintering aid can be more uniformly distributed between the SiC particles. At the same time, the yttrium oxide powder and alumina powder are prepared into a sintering aid dispersion and then mixed with the SiC mixture, so that the sintering aid can be more uniformly adsorbed on the surface of the SiC particles, which is beneficial to the bonding and densification of the particle interface during the subsequent sintering process. During the molding process, the granulated powder is divided into multiple powder units for layered loading, and combined with vibration leveling, layer-by-layer pre-pressing and cold isostatic pressing, which can reduce the powder packing difference between different areas of the large-size ceramic preform, making the internal structure of the preform more uniform, and providing a more consistent initial state for subsequent heat treatment and sintering.

[0034] Secondly, after the large-size ceramic preform is debonded, the present invention employs a pulsed exhaust method alternating between vacuum and high-purity argon gas, combined with delayed pressurization, to gradually expel the binder decomposition products and residual gases inside the preform before the particle pores are significantly closed. Subsequently, a three-stage pressurization method is used for hot-pressing pre-firing, which forms a preliminary sintering connection between SiC particles to obtain a pre-fired skeleton with a certain structural strength and still retaining some open pores. Then, through spark plasma sintering, the pre-fired skeleton is gradually densified under the action of rapid pulsed direct current heating and axial pressurization, and the residual pores are further closed. After sintering, the adverse effects of rapid pressure and temperature changes after high-temperature sintering on the large-size sintered body can be reduced by staged depressurization, slow cooling and staged annealing.

[0035] Therefore, the large-size optical mold materials prepared in Examples 1 and 2 have high relative density, low open porosity, and good bending strength and hardness. At the same time, the relative density difference between the central region and the edge region and the maximum warpage are small, indicating that the obtained materials have good density and structural uniformity.

[0036] In Comparative Example 1, instead of using a blend of α-SiC powder and β-SiC powder, a single α-SiC powder was used. This weakened the filling effect between powders of different particle sizes, resulting in a decrease in the density of powder packing. The ability of particles to form a dense structure during sintering was affected, thus the relative density of the resulting material decreased and the open porosity increased. At the same time, the flexural strength and Vickers hardness were lower than those of Examples 1 and 2.

[0037] In Comparative Example 2, yttrium oxide powder and alumina powder were not prepared into a sintering aid dispersion, but were directly mechanically mixed with SiC mix. This reduced the uniformity of the distribution of the sintering aid among the SiC particles, making it easy for local differences in the content of the sintering aid to occur. Consequently, it affected the uniform bonding of the particle interface and the densification of the sintering, resulting in a decrease in the relative density, flexural strength and Vickers hardness of the obtained material. At the same time, the relative density difference between the central region and the edge region increased.

[0038] In Comparative Example 3, instead of layered loading, layer-by-layer vibration leveling, and pre-pressing, all the granulated powder was loaded into the mold at once and then cold isostatically pressed. This resulted in significant differences in the initial packing state of the large-sized green body at different locations. Although the subsequent cold isostatic pressing and sintering processes could improve the overall density, it was difficult to completely eliminate the regional density differences caused by the initial loading. Therefore, the relative density difference between the center and the edge of the resulting material and the maximum warpage were significantly increased. This indicates that layered loading and layer-by-layer leveling and pre-pressing are beneficial to improving the structural uniformity of the large-sized green body.

[0039] In Comparative Example 4, after the glue removal was completed, pulsed exhaust treatment with alternating vacuum and high-purity argon was not performed. Instead, continuous vacuum exhaust was used. The residual gas inside the large-sized ceramic preform was difficult to be fully discharged before sintering and densification. As the particle pores gradually shrink, the residual gas is easily trapped inside the material, thereby increasing the pore defects in the sintered body. This leads to an increase in the open porosity, a decrease in relative density and flexural strength of the obtained material, and an adverse effect on the overall structural uniformity.

[0040] In Comparative Example 5, instead of hot-pressing pre-firing at 1400℃ and 5MPa, the large-sized ceramic preforms that had undergone debinding and pulse venting were directly subjected to SPS sintering. This caused the preforms to undergo rapid heating and high-load densification before a stable pre-firing skeleton had been formed. As a result, it was difficult to maintain uniformity in the internal particle shrinkage and pore closure process, and some residual gases were also easily trapped during the rapid densification process. Consequently, the relative density of the resulting material was significantly reduced, the open porosity was increased, the flexural strength and Vickers hardness decreased, and the maximum warpage was increased. This demonstrates that hot-pressing pre-firing plays an important role in forming a stable skeleton, promoting venting, and improving the densification effect of subsequent SPS sintering.

[0041] In Comparative Example 6, the SPS sintering process did not employ a staged pressurization method. Instead, the axial load was directly increased to 35 kN at the beginning of sintering and maintained until the end of sintering. This caused the pre-sintered skeleton to be subjected to a large axial load at a low temperature, which easily led to inconsistent compaction and shrinkage rates between different regions and premature closure of some pores. Consequently, the relative density difference between the center and the edge of the resulting material and the maximum warpage were significantly increased. At the same time, the relative density, flexural strength, and Vickers hardness were all lower than those of Examples 1 and 2. This indicates that gradually increasing the axial load according to the temperature rise during SPS sintering is beneficial to improving the density and uniformity of large-size sintered bodies.

[0042] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing large-size optical mold materials by hot pressing and pre-firing, characterized in that, Includes the following steps: S1. Weigh 55-70 parts of α-SiC powder, 25-40 parts of β-SiC powder, 1.0-3.0 parts of yttrium oxide powder, 0.5-2.0 parts of alumina powder, 0.1-0.5 parts of boron carbide powder, and 0.1-0.4 parts of nano-carbon powder according to the mass ratio. Premix the α-SiC powder and β-SiC powder to form a SiC mixture. S2. Prepare a sintering aid dispersion by yttrium oxide powder and alumina powder, add the SiC mixture obtained in step S1 to the sintering aid dispersion for mixing, so that the yttrium oxide powder and alumina powder are adsorbed on the surface of SiC particles, and after drying, add boron carbide powder and nano carbon powder for secondary mixing to obtain composite ceramic powder. S3. The composite ceramic powder is subjected to wet ball milling. After ball milling, binder and plasticizer are added and mixed. Then, it is spray granulated and dried to obtain granulated powder. S4. The granulated powder is layered and loaded into the mold. During the loading process, vibration leveling and pre-pressing are performed respectively. After loading is completed, cold isostatic pressing is performed. After demolding, a large-size ceramic preform is obtained. S5. Place the large-size ceramic preform in a vacuum hot press furnace for segmented heating and glue removal, and perform pulse exhaust treatment with alternating vacuum and inert gas after glue removal. S6. After completing the exhaust treatment in step S5, the large-size ceramic preform is heated and axial pressure is gradually applied. When the temperature reaches 1400℃, the axial pressure is increased to 5MPa and hot-pressing is performed to form a pre-fired skeleton. S7. The pre-sintered skeleton is loaded into the discharge plasma sintering equipment and sintered by segmented heating and graded pressurization in a vacuum environment. When the sintering temperature reaches 1700℃, the axial load is increased to 35kN and heat preservation sintering is carried out. After sintering, graded depressurization and slow cooling are carried out. After annealing, it is cooled to room temperature to obtain large-size optical mold material.

2. The hot-pressing and pre-firing preparation method for large-size optical mold materials according to claim 1, characterized in that, In step S1, the α-SiC powder has a D10 particle size of 0.8-1.5 μm, a D50 particle size of 2.0-3.0 μm, and a D90 particle size of 4.0-6.0 μm, the β-SiC powder has a D50 particle size of 0.25-0.50 μm, and the mass ratio of the α-SiC powder to the β-SiC powder is 1.8-2.8:

1. Before premixing, α-SiC powder and β-SiC powder are dried in a vacuum environment at 120-160℃ for 2-5 hours to ensure that the moisture content of α-SiC powder and β-SiC powder is not higher than 0.15wt%. Then, they are cooled to below 40℃ at a cooling rate of 10-20℃ / min, and α-SiC powder and β-SiC powder are premixed in a closed environment to obtain the SiC mixture.

3. The hot-pressing and pre-firing preparation method for large-size optical mold materials according to claim 1, characterized in that, In step S2, the sintering aid dispersion is prepared by adding yttrium oxide powder and alumina powder to a mixture of ethanol and deionized water, wherein the volume ratio of ethanol to deionized water is 3-6:1, and adding ammonium polyacrylate accounting for 0.3-0.8 wt% of the total mass of yttrium oxide powder and alumina powder. The mixture is sheared and dispersed at 1500-3000 rpm for 20-40 min to obtain the sintering aid dispersion.

4. The hot-pressing and pre-firing preparation method for large-size optical mold materials according to claim 1, characterized in that, In step S3, anhydrous ethanol is used as the ball milling medium in wet ball milling. The liquid-solid mass ratio between the composite ceramic powder and anhydrous ethanol is 0.6-1.0:

1. Silicon carbide balls are used as the grinding medium, and the ball-to-material mass ratio is 3-6:

1. Wet ball milling is carried out at 100-180 rpm for 3-6 hours. After wet ball milling, the resulting slurry is vacuum degassed for 10-30 minutes. Then, 0.5-1.5 wt% polyvinyl alcohol as a binder and 0.2-0.6 wt% polyethylene glycol as a plasticizer are added. After mixing for another 20-40 minutes, spray granulation is performed to obtain granulated powder with a D50 particle size of 40-90 μm and a loose packing density of 0.85-1.20 g / cm³. The granulated powder is then dried in a vacuum environment at 80-110℃ for 4-8 hours.

5. The hot-pressing and pre-firing preparation method for large-size optical mold materials according to claim 1, characterized in that, In step S4, the granulated powder is divided into at least five powder units according to the total amount of material, and each powder unit is loaded into the mold in order from bottom to top. After each powder unit is added to the mold, the added granulated powder is vibrated and leveled at a frequency of 20-50Hz for 30-90s. Then, a pre-compression pressure of 5-15MPa is applied to the granulated powder and held for 30-120s. The pre-compression pressure is then released and the powder is loaded into the next powder unit until all the granulated powder is loaded. After the material is loaded, cold isostatic pressing is performed. First, the pressure is increased to 80-120MPa at a rate of 2-5MPa / min and held for 2-5min. Then, the pressure is increased to 180-250MPa and held for 5-15min. Subsequently, the pressure is gradually reduced to normal pressure at a rate not exceeding 8MPa / min. After demolding, the large-size ceramic preform is obtained.

6. The hot-pressing and pre-firing preparation method for large-size optical mold materials according to claim 1, characterized in that, In step S5, the specific process flow for the segmented heating and adhesive removal is as follows: Under conditions where no axial pressure is applied or a contact pressure not exceeding 0.5 MPa is applied, the large-size ceramic preform is heated to 250-350℃ at a heating rate of 0.5-1.5℃ / min and held for 60-120 min. Then, the temperature is further increased to 500-650℃ at a heating rate of 1-2℃ / min and held for 60-150 min, so that the binder and plasticizer in the large-size ceramic preform are gradually decomposed and discharged. After the adhesive is discharged, the temperature is further increased to 850-1050℃ for pulse venting treatment. The pulse exhaust process is as follows: the furnace cavity of the vacuum hot press furnace is evacuated to 5-50 Pa and maintained for 5-10 min, then high-purity argon gas is introduced into the furnace cavity to increase the furnace cavity pressure to 5-20 kPa and maintain for 2-5 min, and then the evacuation process is repeated again. The vacuum and high-purity argon gas pressure switching is repeated 2-5 times.

7. The hot-pressing and pre-firing preparation method for large-size optical mold materials according to claim 6, characterized in that, During the segmented heating and debinding process and pulsed degassing, a delayed pressurization method is adopted. When the temperature of the large-size ceramic preform is below 1050℃, the axial pressure is maintained at 0-0.5MPa. When the temperature rises to 1050-1200℃, the axial pressure is increased to 0.5-1.0MPa, so that the large-size ceramic preform produces 0.5-2.0% axial pre-shrinkage. During the pulse exhaust process, each time the furnace is re-evacuated in a high-purity argon environment, when the furnace pressure drops below 200Pa, the pressure drop rate inside the furnace is controlled to be 2-10Pa / s, so that the residual gas inside the large-sized ceramic preform can be discharged outward through the particle pores.

8. The hot-pressing and pre-firing preparation method for large-size optical mold materials according to claim 1, characterized in that, In step S6, the hot-pressing preheating adopts a three-stage pressurization process: In the first stage, the large-size ceramic preform is heated to 1200-1300℃, and the axial pressure is maintained at 0.5-1.0MPa; In the second stage, the large-size ceramic preform is heated to 1380℃, and the axial pressure is gradually increased to 2-3MPa at a pressurization rate of 0.02-0.05MPa / min. In the third stage, the large-size ceramic preform is heated to 1400℃. When the furnace temperature reaches 1400℃, the axial pressure is increased to 5MPa at a pressurization rate of 0.05-0.10MPa / min, and the temperature is held at 1400℃ and 5MPa for 45-75 minutes to form a pre-fired skeleton for the large-size ceramic preform. 10-20 minutes before the end of the hot-pressing pre-firing and heat preservation, reduce the furnace cavity pressure to 10-30 Pa, so that the trace amount of volatile gas remaining inside the pre-firing skeleton can be discharged outward along the opening pores. After hot pressing and pre-firing, the relative density of the resulting pre-firing skeleton is 72-84%, and the open porosity is 7-16%.

9. The hot-pressing and pre-firing preparation method for large-size optical mold materials according to claim 1, characterized in that, In step S7, the pre-sintered skeleton is loaded into the graphite mold of the discharge plasma sintering equipment, the sintering chamber is evacuated to below 20 Pa, and pulsed DC current is applied to the graphite mold for SPS sintering. The pulsed DC current adopts an intermittent energizing method, with a pulse energizing to de-energizing time ratio of 8:2-12:2, and the pulse current output is gradually increased according to the increase of sintering temperature, so as to rapidly heat up the graphite mold and pre-sintered skeleton. During the temperature rise from room temperature to 900℃, the temperature is increased at a rate of 80-120℃ / min, and the axial load is controlled at 2-5kN. In the 900-1450℃ range, the temperature is increased at a rate of 60-90℃ / min, and the pressure is gradually increased to raise the axial load to 10-18kN. In the 1450-1650℃ range, the temperature is increased at a rate of 30-60℃ / min, and the pressure is gradually increased to raise the axial load to 20-28kN. After the sintering temperature reaches 1650℃, continue to raise the temperature to 1700℃ and simultaneously apply pressure to increase the axial load to 35kN. Hold at 1700℃ and 35kN for 5-20 minutes to allow the residual pores in the pre-sintered skeleton to close quickly.

10. The method for preparing large-size optical mold materials by hot pressing and pre-firing according to claim 1, characterized in that, In step S7, after SPS sintering at 1700℃ and 35kN is completed, the heating is stopped, and the axial load of 35kN is maintained for cooling. When the temperature drops to 1550-1600℃, the first pressure relief is carried out to reduce the axial load to 20-25kN, and the cooling process continues. When the temperature drops to 1400-1450℃, the pressure is released for the second time to reduce the axial load to 5-10kN, and the temperature is further reduced to 1300-1350℃. The first stage of annealing is carried out by holding the temperature at 1300-1350℃ for 20-40 minutes. Then, the temperature was lowered to 1150-1250℃ and the pressure was completely released. High-purity argon gas was introduced into the sintering chamber, and the second stage of annealing was carried out by holding the temperature at 1150-1250℃ for 30-60 minutes. After the second stage of annealing, the temperature is reduced to 800°C at a cooling rate of 1-2°C / min, and then reduced to 500°C at a cooling rate of 3-5°C / min. The material is then cooled to room temperature in the furnace to obtain the large-size optical mold material.

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