Method for removing impurities in calcium fluoride crystal growth

Abstract

The invention relates to a calcium fluoride crystal growth impurity removal method, and belongs to the technical field of crystal growth. According to the method, HF is adopted to dehydrate and dehydrogenate the raw material CaF2, a BN porous ceramic filter plate is adopted to filter, and the high-purity calcium fluoride crystal is prepared in a graphite crucible subjected to surface treatment. The preparation method comprises the following steps: generating a volatile HF.H2O complex by utilizing high reaction activity of HF and water in raw materials, converting key impurity hydroxyl in the raw materials into volatile water molecules through a fluorination substitution reaction to realize dehydration and dehydrogenation of the raw materials, and intercepting solid impurities in a melt through micron-sized apertures of a BN porous ceramic filter plate to improve the crystal purity; the bottom SiC-SiO2 composite coating is coated on the graphite crucible to prevent carbon from being dissolved out, the top fluoride is doped with the BN coating, the chemical stability of BN and the self-repairing characteristic of a B2O3 glass phase generated at high temperature are utilized, the erosion resistance is enhanced, and a reliable impurity removal guarantee is provided for growth of calcium fluoride crystals.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of crystal growth, in particular to a method for removing impurities in the growth of calcium fluoride crystals. BACKGROUND

[0002] As a functional crystal material with excellent deep ultraviolet transmittance, low optical loss and high chemical stability, calcium fluoride crystal has irreplaceable core application value in strategic high-tech fields such as deep ultraviolet lithography, laser nuclear fusion device, high-energy particle detection and high-end optical window. In particular, large-size calcium fluoride crystals are the core components of large-scale high-end optical equipment. With the development of large-scale and high-precision equipment in the above-mentioned fields, more stringent requirements are put forward for the large-size, high-integrity and optical performance of calcium fluoride crystals. However, during the crystal preparation process, a large number of light scattering centers and absorption defects are introduced due to the hydroxyl impurities in the raw material, the carbon impurities dissolved by graphite crucible at high temperature, and the secondary pollution during the melt filtration, which seriously deteriorate the optical uniformity, laser damage threshold and application reliability of the crystal.

[0003] Traditional melt purification methods mainly rely on centrifugal separation or bubble exclusion method. The former uses the density difference between the double crucibles to separate impurities by rotation, but the interception efficiency of small solid impurities is less than 30%, which cannot meet the stringent requirements of the crystal for trace impurities. The latter enhances the convection of the melt to exclude bubbles by adjusting the temperature gradient, but has no purification effect on the suspended solid impurities in the melt, resulting in the entry of these impurities into the crystal with the melt solidification. Even if some technologies attempt to use filter media for auxiliary purification, ceramic fiber felt or glass fiber felt is often selected. Such filter materials are easy to soften and fall off at a high temperature of 1400-1450℃ required by CaF2 melt, or react with fluoride melt to dissolve metal ions, causing secondary pollution. More importantly, existing technologies often only control impurities in a single link, which cannot meet the application requirements of deep ultraviolet lithography machines, high-end infrared detection devices and other high-purity and high-optical-performance crystals. SUMMARY

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a method for impurity removal during the growth of calcium fluoride crystals. This method involves dehydrating and dehydrogenating the raw material CaF2 using HF, filtering it with a BN porous ceramic filter plate, and preparing calcium fluoride crystals in a surface-treated graphite crucible. Utilizing the high reactivity of HF, it reacts with moisture in the raw material to form a volatile HF·H2O complex. Simultaneously, a fluorination substitution reaction converts the key impurity hydroxyl groups in the raw material into volatile water molecules, achieving dehydration and dehydrogenation. The micron-sized pores of the BN porous ceramic filter plate then intercept solid impurities in the melt, improving crystal purity. A bottom SiC-SiO2 composite coating is applied to the graphite crucible to prevent carbon leaching, while a top fluoride-doped BN coating enhances melt compatibility and reduces the driving force for fluoride ion erosion through the chemical stability, lubricity, and fluoride doping of BN. Furthermore, the B2O3 glass phase generated by the reaction of BN with oxygen can autonomously fill cracks, forming a dual barrier of physical and chemical protection, providing reliable impurity removal for the growth of calcium fluoride crystals.

[0005] To achieve the above objectives, the present invention provides a method for removing impurities during the growth of calcium fluoride crystals, comprising the following steps:

[0006] S1. Pre-treat the inner wall of the graphite crucible to obtain a pre-treated graphite crucible;

[0007] S2. SiC powder, silica sol, and anhydrous ethanol are mixed and ball-milled to obtain a slurry. The slurry is then coated onto the inner wall of a pretreated graphite crucible, cured, and sintered for the first time to obtain a graphite crucible with a SiC-SiO2 composite coating as the bottom layer. BN powder, CaF2 powder, polyvinyl alcohol, and anhydrous ethanol are mixed and stirred to obtain a slurry. The slurry is then sprayed onto the bottom layer and sintered for the second time to obtain a graphite crucible with a double-layer coating.

[0008] S3. Mix CaF2 with LiF, introduce HF and heat for the first time, stir, heat for the second time, switch HF to Ar, purge, cool, and obtain dehydrated and dehydrogenated CaF2.

[0009] S4. Mix BN powder, Y2O3, soluble starch, ammonium polyacrylate and deionized water, ball mill to obtain filter plate slurry, and after molding, drying, sintering and processing, obtain BN porous ceramic filter plate; install BN porous ceramic filter plate in double-coated graphite crucible to obtain crucible for installing filter plate.

[0010] S5. Add the dehydrated and dehydrogenated CaF2 to the crucible with the filter plate installed, place it in the crucible lowering furnace, introduce Ar, heat up, set the crucible lowering program, anneal, and obtain calcium fluoride crystals grown in the crucible. After separation, cutting, grinding, cleaning and drying, obtain calcium fluoride crystals.

[0011] In one feasible implementation, the pretreatment step in S1 is as follows: A straight-walled cylindrical graphite crucible with an inner diameter of 215-225 mm, an outer diameter of 230-250 mm, and a height of 430-470 mm is selected. Using the upper surface of the crucible bottom as a reference, an annular area with a distance of 145-155 mm from the bottom and a width of 7.5-8.5 mm is marked on the inner wall. A milling cutter with a rotation speed of 300-500 r / min and a feed rate of 5-8 mm / min is used to mill the area outside the annular marked area. The inner wall of the crucible is milled to a depth of 3-5 mm in two passes. The outer wall of the crucible is then wrapped with an annular water-cooling jacket with a cooling water flow rate of 5 L / min to obtain a graphite crucible with annular support steps. The inner wall of the crucible is then polished with 800-1000 grit and 1200-1500 grit metallographic sandpaper until the roughness Ra=1.0-1.5μm. The crucible is then placed in anhydrous ethanol and ultrasonically cleaned with a power of 280-320W for 25-35 min. Finally, it is vacuum dried at 75-85℃ for 1.5-2.5 h.

[0012] By machining annular support steps in specific areas of the graphite crucible, a stable mounting reference is provided for the subsequent BN filter plate. These annular support steps disperse the pressure generated by the CaF2 melt through a mechanical interlocking structure, preventing the filter plate from shifting or deforming under the weight of the melt. They also tightly fit the filter plate edges, sealing any gaps and ensuring the melt must pass through the filter plate, preventing it from bypassing the filter plate and directly entering the growth zone, causing filtration failure. Furthermore, the roughness of the crucible's inner wall is reduced through progressive grinding, minimizing voids between the coating and the crucible surface and improving coating adhesion. Simultaneously, ultrasonic cleaning with anhydrous ethanol removes residual impurities such as oil and dust, followed by vacuum drying to eliminate moisture. This prevents moisture evaporation under high-temperature conditions, which could lead to bubbles or cracks in the coating, further ensuring its integrity.

[0013] In one feasible implementation, in step S2, the mass-to-volume ratio of the SiC powder, silica sol, and anhydrous ethanol is (53-57) g:(65-69) mL:(30-34) mL; the silica sol is acidic sodium type, with a SiO2 content of 30 wt.% and a particle size of 10-15 nm; the ball milling rate is 280-320 r / min, and the ball milling time is 2.5-3.5 h; the coating rate is 5-8 cm / s, and the single wet film thickness is 20-25 μm; the curing step is: air-drying at room temperature for 25-35 min, followed by curing at 55-65℃. Dry for 0.8-1.2 hours, repeating the coating-air-drying-drying process 4 times; the first sintering step is as follows: under nitrogen protection at a flow rate of 180-220 sccm, first raise the temperature to 180-220℃ at 4-6℃ / min and hold for 1.2-1.8 hours, then raise the temperature to 580-620℃ at 0.8-1.2℃ / min and hold for 0.7-1.3 hours, then raise the temperature to 780-820℃ at 0.6-1.0℃ / min and hold for 2.5-3.5 hours, then raise the temperature to 1180-1220℃ at 3-5℃ / min and hold for 2-3 hours, and finally cool naturally to room temperature.

[0014] SiC, acting as the coating framework, possesses high-temperature resistance exceeding 2000℃ and excellent mechanical strength, maintaining structural stability at high temperatures and preventing deformation due to molten weight or temperature changes. SiO2, acting as a filler, fills the gaps between SiC micron-sized particles. During high-temperature processing, SiO2 undergoes a sol-gel transition, gradually polymerizing to form a continuous phase, followed by further vitrification. It then chemically bonds with the SiC particle surface, forming Si-OC bonds for tight bonding, ultimately creating a dense structure. This lack of significant porosity completely blocks the path of molten CaF2 penetrating into the graphite matrix. Crucially, neither SiC nor SiO2 reacts chemically with CaF2 at 1440℃, effectively preventing the coating itself from reacting with the melt and generating impurities. Simultaneously, the coating physically isolates graphite from the melt, fundamentally preventing carbon from the graphite from dissolving into the melt at high temperatures, thus solving the carbon contamination problem of traditional graphite crucibles.

[0015] In one feasible implementation, in step S2, the mass-to-volume ratio of BN powder, CaF2 powder, polyvinyl alcohol, and anhydrous ethanol is (78-82) g:(9-11) g:(4.5-5.5) g:(30-34) mL; the stirring rate is 480-520 r / min, and the stirring time is 1.2-1.8 h; the spraying pressure is 0.2-0.4 MPa, the spraying rate is 8-10 cm / s, and the wet film thickness is 10-15 μm; the second sintering step is as follows: after air drying at room temperature for 25-35 min, under nitrogen protection at a flow rate of 150-200 sccm, the temperature is raised to 680-720℃ at a rate of 4-6℃ / min, held at the temperature for 1.8-2.2 h, and then naturally cooled to room temperature.

[0016] The top-layer fluoride-doped BN coating focuses on chemical protection and self-healing: BN material itself has excellent chemical stability and lubricity. Doping with CaF2 can improve the compatibility between the coating and the CaF2 melt, reducing the driving force of fluoride ion erosion on the coating from a thermodynamic perspective, and preventing the coating from falling off due to reaction with the melt. At the same time, BN reacts with trace amounts of oxygen at high temperatures to generate a B2O3 glass phase. This glass phase has fluidity and can autonomously fill the micro-cracks generated in the coating during sintering or use, which not only enhances the coating's resistance to fluoride ion erosion but also improves its oxidation resistance. Combined with the bottom SiC-SiO2 composite coating, a dual barrier of "bottom-layer physical barrier + top-layer chemical protection and self-healing" is ultimately formed, ensuring that the crucible does not release any impurities into the melt in the high-temperature growth environment.

[0017] In one feasible implementation, in step S3, the mass ratio of CaF2 to LiF is (22-25) kg:(22-25) g; the flow rate of HF is 50-80 sccm; the first heating step is: heating to 590-610℃ at 2.5-3.5℃ / min; the stirring rate is 48-52 r / min, and the stirring time is 17-19 h; the second heating step is: heating to 640-660℃ at 2.5-3.5℃ / min, and holding at that temperature for 1.8-2.2 h; the flow rate of Ar is 90-110 sccm, and the purging time is 25-35 min; the cooling step is: cooling to 400℃ at a rate of 5℃ / min, holding at that temperature for 1.8-2.2 h, and finally cooling to room temperature at a rate of 5℃ / min.

[0018] Moisture and hydroxyl impurities in the raw materials are key factors leading to decreased transmittance and poor optical uniformity of calcium fluoride crystals. These impurities form hydroxyl scattering centers in the crystal, interfering with light propagation. Therefore, deep dehydration and dehydrogenation are necessary using an HF atmosphere. On one hand, HF reacts with residual moisture in the raw materials to generate a volatile HF·H₂O complex, which can be directly volatilized from the raw material system at high temperatures, achieving deep dehydration. On the other hand, HF reacts with hydroxyl groups in the raw materials through a fluorination substitution reaction, converting hydroxyl groups into water molecules. These water molecules can also volatilize at high temperatures, thus completely removing the key impurity hydroxyl groups from the raw materials. This reduces the formation of hydroxyl scattering centers during subsequent crystal growth, laying the foundation for excellent optical performance of the crystal. In addition, multiple auxiliary methods are needed to ensure the impurity removal effect during raw material processing: adding anti-caking agents can break the agglomeration force between CaF2 particles, making the raw material particles evenly dispersed, ensuring that the HF atmosphere can fully contact each raw material particle, and avoiding the inability to remove internal impurities due to particle agglomeration; while Ar atmosphere replacement and purging play a protective role. Ar replacement in the early stage can remove air in the reactor, preventing oxygen in the air from reacting with HF to generate impurities, or moisture in the air from affecting the dehydration effect; Ar purging in the later stage can thoroughly remove residual HF gas, preventing HF residue from corroding equipment or contaminating raw materials in subsequent steps, and finally achieving high-purity purification of raw materials.

[0019] In one feasible implementation, in step S4, the mass-to-volume ratio of the BN powder, Y₂O₃, soluble starch, ammonium polyacrylate, and deionized water is (88-92) g:(2.8-3.2) g:(19-21) g:(0.8-1.0) g:(48-52) mL; the particle size of the soluble starch is 1-5 μm, and the gelatinization temperature is 60-70℃; the ball milling rate is 280-320 r / min, and the ball milling time is 5.5-6.5 h; the molding step is as follows: the slurry is injected into an annular alumina mold at a rate of 5-10 mL / min, and left horizontally at room temperature for 11-13 h to set; the drying temperature... The temperature is 55-65℃, and the drying time is 22-26h; the sintering step is as follows: under an air atmosphere with a flow rate of 140-160 sccm, the temperature is increased to 480-520℃ at 0.4-0.6℃ / min and held at the temperature for 1.8-2.2h, then switched to nitrogen gas with a flow rate of 190-210 sccm and the temperature is increased to 1730-1770℃ at 2.5-3.5℃ / min and held at the temperature for 3.5-4.5h, followed by natural cooling to room temperature; the processing step is as follows: the filter plate is processed into a circular porous plate with an outer diameter of 214.8-224.8mm and a thickness of 4.9-5.1mm; the pore size of the BN porous ceramic filter plate is 1-3μm.

[0020] Melt filtration is the final impurity removal step before crystal growth. It intercepts solid impurities that may remain after the raw material melts, such as incompletely purified microparticles and crucible coating debris, while preventing secondary contamination caused by the filter material reacting with the melt. BN was chosen because of its excellent chemical compatibility with CaF2 melt at 1450℃; it neither dissolves in the melt nor reacts chemically with it, fundamentally avoiding the secondary contamination problems caused by reactions with the melt in traditional filter materials. Furthermore, BN's excellent thermal shock stability and low oxidation rate ensure that its filtration performance does not decline within a 50-100 hour growth cycle. Adding Y2O3 as a sintering aid lowers the sintering temperature of BN, promotes the bonding between BN particles, and ensures that the prepared filter plate has sufficient mechanical strength at high temperatures, avoiding... The molten material is crushed by the weight of the melt; while the soluble starch, as a pore-forming agent, is completely oxidized and decomposed into a gas without residue at high temperature, forming uniform micron-sized pores with a porosity of 30-40% and a pore size of 1-5μm inside the filter plate. This can intercept incompletely purified 2-10μm agglomerated particles, LiF residue particles, and 1-3μm peeling debris from the crucible coating in the raw material. It can effectively prevent solid impurities in the melt from entering the growth zone, without hindering the smooth flow of low-viscosity CaF2 melt, and ultimately provide an absolutely pure melt for the crystal growth zone, ensuring the growth of calcium fluoride crystals.

[0021] In one feasible implementation, in step S5, the amount of dehydrated and dehydrogenated CaF2 added is (22-25) kg; the flow rate of Ar is 130-150 sccm; the heating step is as follows: heating to 980-1020℃ at a rate of 9-11℃ / min, holding at that temperature for 0.8-1.2h, and then heating to 1420-1460℃ at a rate of 4.5-5.5℃ / min, holding at that temperature for 4-6h; the crucible descent program is set to descend continuously at a descent rate of 1-2 mm / h for 50-100h, during which the axial temperature gradient near the growth interface is maintained at 23℃ / mm and the radial temperature difference is <1℃ through the temperature control system.

[0022] During the crystal growth stage, the initial Ar atmosphere replacement not only isolates the furnace from air, preventing oxygen in the air from reacting with the 1440℃ CaF2 melt to form CaO impurities, but also prevents moisture in the air from reintroducing hydroxyl groups, thus preserving the purification effect of the preceding raw materials. The stepped heating and isothermal process first preheats the raw materials to ensure uniform heating and prevent excessive temperature differences that could cause them to crack. Then, high-temperature isothermal treatment completely melts and clarifies the raw materials, removing bubbles from the melt and preventing them from forming inclusion defects in the crystal. The subsequent crucible descent method is crucial for crystal growth; controlling the slow descent of the crucible... The crucible descends at a rate that allows the bottom, slightly below the melting point, to begin solidification first. The solidification interface moves upward at a uniform speed, giving atoms ample time to align in an orderly manner along the CaF2 lattice orientation, reducing the generation of line defects such as dislocations. The axial temperature gradient of 23℃ / mm near the growth interface maintains stable solidification dynamics, ensuring a smooth solidification interface and preventing crystal structure inhomogeneity caused by excessive gradient fluctuations. A radial temperature difference of <1℃ further avoids internal stress caused by differences in solidification rates at different radial positions of the crystal, preventing cracking or a decrease in optical uniformity, ultimately achieving stable axial growth of the crystal.

[0023] In one feasible implementation, the annealing step in S5 is as follows: cooling to 1135-1145℃ at a rate of 0.4-0.6℃ / h, holding at that temperature for 9-11h, then cooling to 1095-1105℃ at a rate of 1.6-2.4℃ / h, holding at that temperature for 7-9h, then cooling to 190-210℃ at a rate of 9-11℃ / h, and finally allowing the furnace to cool naturally to room temperature; the separation step is as follows: using a 70-90 mesh silicon carbide grinding wheel at a rotation speed of 900-1100 r / min to break the graphite crucible and separate the calcium fluoride crystals. The cutting steps are as follows: using a diamond tool to remove the impurity-rich areas at both ends of the crystal, removing 5-10 mm from each end; the polishing steps are as follows: successively polishing the crystal surface with 800-1000 grit sandpaper and 1200-1500 grit sandpaper at a speed of 280-320 r / min and a pressure of 5-10 N until smooth; the cleaning steps are as follows: immersing the crystal in anhydrous ethanol and ultrasonically cleaning it with a power of 280-320 W for 25-35 min; the drying temperature is 75-85℃, and the drying time is 1.5-2.5 h.

[0024] The annealing process achieves stress relaxation through an extremely slow cooling rate and multiple isothermal stages, providing time for the atoms inside the crystal to rearrange and releasing the internal stress generated by the temperature gradient during crystal growth. This prevents the crystal from cracking due to excessive internal stress during cooling. Removing the impurity-rich regions at both ends of the crystal is necessary because, in the initial stage of crystal growth, trace solid particles that are not completely filtered in the melt and tiny debris from the crucible coating are easily pushed towards the unsolidified melt by the solidification interface, eventually accumulating at the bottom of the crystal. Towards the end of growth, the impurity concentration in the melt at the top of the crucible increases, accumulating at the top of the crystal during crystallization. Removing these impurities significantly improves the purity of the crystal body. Subsequent polishing and ultrasonic cleaning remove residual impurities and scratches from the crystal surface, reducing the impact of surface scattering on optical properties. Finally, vacuum drying removes moisture, preventing hydroxyl groups in the moisture from re-adsorbing onto the crystal surface or seeping into micropores, ensuring stable optical properties and ultimately yielding high-performance calcium fluoride crystals.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] This method employs an HF atmosphere to dehydrate and dehydrogenate the raw material CaF2, followed by filtration using a BN porous ceramic filter plate. A calcium fluoride crystal is then prepared in a surface-treated graphite crucible, ensuring the purity of the calcium fluoride crystal from the source. Utilizing the high reactivity of the HF atmosphere, residual moisture in the raw material is reacted to form a volatile HF·H2O complex. Simultaneously, a fluorination substitution reaction converts the key impurity hydroxyl groups in the raw material into volatile water molecules for removal, achieving dehydration and dehydrogenation. Then, a BN porous ceramic filter plate with a 1-3 μm micron-sized pore structure is used to filter solid impurities larger than 1 μm in the molten CaF2 raw material, reducing impurity scattering centers in the crystal from the source, improving crystal purity, and thus enhancing the optical uniformity of the crystal. The operation is carried out in a surface-treated graphite crucible, employing a double-layer coating of SiC-SiO2 and a fluoride-doped BN coating to form a dual protective structure. The bottom layer is a dense SiC-SiO2 layer. The SiO2 composite coating blocks the contact between molten CaF2 and the graphite matrix, preventing the high-temperature dissolution of carbon. The top fluoride-doped BN coating, on the other hand, utilizes the excellent chemical stability and lubricity of BN itself. Furthermore, the fluoride doping enhances the compatibility between the coating and the melt, thermodynamically reducing the driving force of fluoride ion erosion. Simultaneously, BN can react with oxygen to generate the B2O3 glass phase, which flows autonomously and fills cracks, further enhancing the coating's resistance to fluoride ion erosion and its antioxidant properties. Together, they form a dual barrier of physical and chemical protection, ensuring that the crucible does not release impurities into the melt during the high-temperature growth environment, thus guaranteeing crystal purity and providing a reliable impurity removal guarantee for the growth of calcium fluoride crystals. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the preparation process of calcium fluoride crystals according to the present invention. Detailed Implementation

[0028] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the application will be further described in detail below with reference to embodiments. However, this should not be construed as limiting the scope of this application to the following examples. All other embodiments obtained by those skilled in the art without creative effort without departing from the above-described methodological spirit of this application are within the scope of protection of this application.

[0029] The singular forms “for,” “or,” “a,” “any,” and “described” used in this application are intended to include the plural forms unless the context clearly indicates otherwise. Furthermore, the terms “first” and “second” are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0030] Example 1

[0031] like Figure 1 As shown, a method for removing impurities during the growth of calcium fluoride crystals includes the following steps:

[0032] S1. A straight-walled cylindrical graphite crucible with an inner diameter of 220 mm, an outer diameter of 240 mm, and a total height of 450 mm was selected. First, using the bottom surface of the crucible as a reference, an annular area 150 mm from the bottom and 8 mm wide was marked on the inner wall. Using a milling cutter at 400 r / min and a feed rate of 7 mm / min, the inner wall outside the annular marked area was milled to a depth of 4 mm, completed in two passes (2 mm each). During milling, an annular water-cooling jacket was used to wrap the outer wall of the crucible, with a cooling water flow rate of 5 L / min to prevent graphite cracking, resulting in a graphite crucible with annular support steps. After milling, the inner wall of the crucible was polished sequentially with 900 and 1300 grit metallographic sandpaper until the roughness Ra = 1.2 μm. The crucible was then placed in anhydrous ethanol and ultrasonically cleaned at 300 W for 30 min, followed by vacuum drying at 80℃ for 2 h to obtain a pretreated graphite crucible.

[0033] S2. SiC powder, acidic sodium silicate sol (particle size 12nm) with a SiO2 content of 30wt.%, and anhydrous ethanol are mixed in a mass-to-volume ratio of 55g:67mL:32mL. The mixture is placed in an agate ball mill jar with a diameter of 10mm and a ball-to-particle ratio of 3:1. The mixture is ball-milled at 300r / min for 3h to obtain a slurry. The slurry is then uniformly brushed onto the inner wall of the pretreated graphite crucible along its axial direction using a soft brush at a speed controlled at 7cm / s. The thickness of a single wet film is controlled at 22μm. The mixture is then left to air dry horizontally at room temperature for 30min, and then transferred to a 60℃ forced-air drying oven. After curing for 1 hour, the process of brushing, air drying, and drying was repeated 4 times. Then, the crucible was placed in an atmosphere sintering furnace and sintered for the first time under nitrogen protection at a flow rate of 200 sccm, following the procedure below: first, the temperature was increased to 200℃ at 5℃ / min and held for 1.5 hours; then, the temperature was increased to 600℃ at 1℃ / min and held for 1 hour; then, the temperature was increased to 800℃ at 0.8℃ / min and held for 3 hours; then, the temperature was increased to 1200℃ at 4℃ / min and held for 2.5 hours; finally, the temperature was allowed to cool naturally to room temperature, resulting in a graphite crucible with a SiC-SiO2 composite coating as the bottom layer. BN powder, CaF2 powder, polyvinyl alcohol, and anhydrous ethanol were mixed in a mass-to-volume ratio of 80g:10g:5g:32mL and magnetically stirred at 500r / min for 1.5h to obtain a slurry. The slurry was then uniformly sprayed onto the SiC substrate using a spray gun, with the spray gun 18cm away from the inner wall of the crucible and a spraying pressure of 0.3MPa. The spraying was carried out at a uniform speed of 9cm / s along the circumference, controlling the wet film thickness to 12μm. After a second sintering process, the substrate was dried at room temperature for 30min, then heated to 700℃ at a flow rate of 5℃ / min under nitrogen protection at a flow rate of 180sccm and held at that temperature for 2h. The substrate was then allowed to cool naturally to room temperature, resulting in a double-coated graphite crucible.

[0034] S3. Mix 24 kg of CaF2 raw material with 24 g of LiF at a speed of 30 r / min for 15 min to ensure uniform dispersion of LiF, obtaining a mixed raw material. Transfer the mixed raw material to a reactor, which is connected to a tail gas absorption system. First, purge the air in the reactor three times with Ar at a flow rate of 100 sccm, then purge with HF at a flow rate of 60 sccm, and raise the temperature to 600°C at a rate of 3°C / min. Start mechanical stirring at a rate of 50 r / min and maintain the temperature for 18 h. Continue to raise the temperature to 650°C at a rate of 3°C / min and maintain the temperature for 2 h. Stop the HF flow and keep the reactor temperature constant at 650°C. Switch to Ar at a flow rate of 100 sccm and purge for 30 min. Then, lower the temperature to 400°C at a rate of 5°C / min and maintain the temperature for 2 h. After the program is completed, lower the temperature to room temperature at a rate of 5°C / min to obtain the dehydrated and dehydrogenated raw material.

[0035] S4. Mix 90g BN powder, 3g Y2O3, 20g soluble starch (particle size 3μm, gelatinization temperature 65℃), 0.9g ammonium polyacrylate, and 50mL deionized water. Place the mixture in an agate ball mill jar (agate ball diameter 8mm, ball-to-material ratio 4:1) and ball mill at 300r / min for 6h to obtain filter plate slurry. Inject the slurry into an annular alumina mold at a rate of 8mL / min. Let it stand horizontally at room temperature for 12h for preliminary shaping, then transfer it to a 60℃ forced-air drying oven for 24h to dry. Place the dried green body in an atmosphere sintering furnace and heat it to 500℃ at a rate of 0.5℃ / min in air with a flow rate of 150sccm, and hold it at that temperature for 2h. To completely oxidize and decompose the starch, a nitrogen atmosphere with a flow rate of 200 sccm was switched, and the temperature was increased to 1750℃ at a rate of 3℃ / min and held at that temperature for 4 hours. Then, the temperature was naturally cooled to room temperature. The sintered filter plate was dimensionally processed into a BN porous ceramic filter plate with an outer diameter of 219.8 mm, a thickness of 5.0 mm, and a pore size of 3 μm. The BN porous ceramic filter plate was installed as a baffle on the annular support step on the inner wall of a double-coated graphite crucible. A BN slurry with a solid content of 60% was evenly applied to the edge of the filter plate in contact with the crucible with a thickness of 1 mm using a scraper. After drying at room temperature, the crucible with the filter plate installed was sealed.

[0036] S5. 24 kg of dehydrated and dehydrogenated CaF2 raw material is loaded into the crucible above the filter plate, placed in a crucible-lowering furnace, and the furnace body is fixed and sealed. Ar is introduced at a flow rate of 140 sccm to replace the air three times, maintaining an Ar atmosphere. The temperature is increased to 1000℃ at a rate of 10℃ / min and held for 1 hour. Then, the temperature is increased to 1440℃ at a rate of 5℃ / min and held for 5 hours. Under the influence of gravity, the melt penetrates the BN filter plate from top to bottom and enters the crystal growth zone below. After the melt is completely clear, the crucible-lowering rate is set to 1.5 mm / h, and the descent continues for 70 hours. The temperature control system precisely maintains an axial temperature gradient of 23℃ / mm near the growth interface and a radial temperature difference of <1℃. After crystal growth is complete, stop the crucible descent and proceed with annealing: cool to 1140℃ at a rate of 0.5℃ / h, hold for 10h, then cool to 1100℃ at a rate of 2℃ / h, hold for 8h, then cool to 200℃ at a rate of 10℃ / h, shut off the main heating system, continue to introduce Ar at a flow rate of 50 sccm, allow the furnace to cool naturally to room temperature, remove the crucible, carefully break the graphite crucible with an 80-mesh silicon carbide grinding wheel at a speed of 1000 r / min to separate the calcium fluoride crystal, use a diamond to remove the impurity-rich areas at both ends of the crystal, removing 7 mm from each end, and then polish the crystal surface with 900-mesh sandpaper and 1300-mesh sandpaper at a speed of 300 r / min and a pressure of 7 N until smooth, place the crystal in anhydrous ethanol, ultrasonically clean with 300W power for 30 min, and finally place it in an 80℃ vacuum drying oven to dry for 2 h to obtain calcium fluoride crystal.

[0037] Example 2

[0038] like Figure 1 As shown, a method for removing impurities during the growth of calcium fluoride crystals includes the following steps:

[0039] S1. A straight-walled cylindrical graphite crucible with an inner diameter of 215 mm, an outer diameter of 230 mm, and a total height of 430 mm was selected. First, using the bottom surface of the crucible as a reference, an annular area 145 mm from the bottom and 7.5 mm wide was marked on the inner wall. Using a milling cutter at 300 r / min and a feed rate of 5 mm / min, the inner wall outside the annular marked area was milled to a depth of 3 mm, completed in two passes (1.5 mm each). During milling, an annular water-cooling jacket was used to wrap the outer wall of the crucible, with a cooling water flow rate of 5 L / min to prevent graphite cracking, resulting in a graphite crucible with annular support steps. After milling, the inner wall of the crucible was polished sequentially with 800 and 1200 grit metallographic sandpaper until the roughness Ra = 1.5 μm. Then, the crucible was placed in anhydrous ethanol and ultrasonically cleaned at 280 W for 25 min, followed by vacuum drying at 75 °C for 1.5 h to obtain a pretreated graphite crucible.

[0040] S2. SiC powder, acidic sodium silicate sol (particle size 10nm) with a SiO2 content of 30wt.%, and anhydrous ethanol are mixed at a mass-to-volume ratio of 53g:65mL:30mL. The mixture is placed in an agate ball mill jar with a diameter of 10mm and a ball-to-particle ratio of 3:1. The mixture is ball-milled at 280r / min for 2.5h to obtain a slurry. The slurry is then uniformly brushed onto the inner wall of the pretreated graphite crucible along its axial direction using a soft brush at a speed controlled at 5cm / s. The thickness of a single wet film is controlled at 20μm. The mixture is then left to air dry horizontally at room temperature for 25min, and then transferred to a 55℃ forced-air drying oven for curing. The coating-drying-air-drying process was repeated 4 times over 8 hours. Then, the crucible was placed in an atmosphere sintering furnace and sintered for the first time under nitrogen protection at a flow rate of 180 sccm, following the procedure below: first, the temperature was increased to 180℃ at 4℃ / min and held for 1.2 hours; then, the temperature was increased to 580℃ at 0.8℃ / min and held for 0.7 hours; then, the temperature was increased to 780℃ at 0.6℃ / min and held for 2.5 hours; then, the temperature was increased to 1180℃ at 3℃ / min and held for 2.5 hours; finally, the temperature was allowed to cool naturally to room temperature, resulting in a graphite crucible with a SiC-SiO2 composite coating as the bottom layer. BN powder, CaF2 powder, polyvinyl alcohol, and anhydrous ethanol were mixed in a mass-to-volume ratio of 78 g: 9 g: 4.5 g: 30 mL and magnetically stirred at 480 r / min for 1.2 h to obtain a slurry. The slurry was then uniformly sprayed onto a SiC substrate using a spray gun, with the spray gun 18 cm away from the inner wall of the crucible and a spraying pressure of 0.2 MPa. The spraying was performed at a uniform speed of 8 cm / s along the circumference, controlling the wet film thickness to 10 μm. After a second sintering process, the substrate was air-dried at room temperature for 25 min, then heated to 680 °C at a flow rate of 4 °C / min under nitrogen protection at a flow rate of 150 sccm and held at that temperature for 1.8 h. Finally, the substrate was allowed to cool naturally to room temperature to obtain a double-coated graphite crucible.

[0041] S3. Mix 22 kg of CaF2 raw material with 22 g of LiF at a speed of 30 r / min for 15 min to ensure uniform dispersion of LiF, obtaining a mixed raw material. Transfer the mixed raw material to a reactor, which is connected to a tail gas absorption system. First, purge the air in the reactor three times with Ar at a flow rate of 100 sccm, then purge with HF at a flow rate of 50 sccm, and raise the temperature to 590℃ at a rate of 2.5℃ / min. Start mechanical stirring at a rate of 50 r / min and maintain the temperature for 18 h. Continue to raise the temperature to 640℃ at a rate of 2.5℃ / min and maintain the temperature for 2.0 h. Stop the HF flow and maintain the reactor temperature at 650℃. Switch to Ar at a flow rate of 110 sccm and purge for 25 min. Then, lower the temperature to 400℃ at a rate of 5℃ / min and maintain the temperature for 2.0 h. After the program ends, lower the temperature to room temperature at a rate of 5℃ / min to obtain the dehydrated and dehydrogenated raw material.

[0042] S4. Mix 88g BN powder, 2.8g Y2O3, 19g soluble starch (particle size 1μm, gelatinization temperature 60℃), 0.8g ammonium polyacrylate, and 48mL deionized water. Place the mixture in an agate ball mill jar (agate ball diameter 8mm, ball-to-material ratio 4:1) and ball mill at 280r / min for 6h to obtain filter plate slurry. Inject the slurry into an annular alumina mold at a rate of 5mL / min. Let it stand horizontally at room temperature for 11h for preliminary shaping, then transfer it to a 55℃ forced-air drying oven for 22h. Place the dried green body in an atmosphere sintering furnace and heat it to 480℃ at a flow rate of 140sccm at a rate of 0.4℃ / min, then hold it at that temperature for 1.8h. To completely oxidize and decompose the starch, a nitrogen atmosphere with a flow rate of 190 sccm was switched, and the temperature was increased to 1750℃ at a rate of 3℃ / min and held at that temperature for 3.5 hours. Then, the temperature was naturally cooled to room temperature. The sintered filter plate was dimensionally processed into a BN porous ceramic filter plate with an outer diameter of 214.8 mm, a thickness of 4.9 mm, and a pore size of 2 μm. The BN porous ceramic filter plate was installed as a baffle on the annular support step on the inner wall of a double-coated graphite crucible. A BN slurry with a solid content of 60% was evenly applied to the edge of the filter plate in contact with the crucible with a thickness of 1 mm using a scraper. After drying at room temperature, the crucible with the filter plate installed was sealed.

[0043] S5. 22 kg of dehydrated and dehydrogenated CaF2 raw material was loaded into the crucible above the filter plate, placed in a crucible-lowering furnace, and the furnace was fixed and sealed. Ar was introduced at a flow rate of 130 sccm to replace the air three times, maintaining an Ar atmosphere. The temperature was increased to 980℃ at a rate of 9℃ / min and held for 0.8 h. Then, the temperature was increased to 1420℃ at a rate of 4.5℃ / min and held for 4 h. Under gravity, the melt penetrated the BN filter plate from top to bottom and entered the crystal growth zone below. After the melt was completely clarified, the crucible-lowering rate was set to 1 mm / h, and the descent continued for 100 h. The temperature control system precisely maintained an axial temperature gradient of 23℃ / mm and a radial temperature difference of <1℃ near the growth interface. After crystal growth was completed, the crucible descent was stopped, and annealing was performed: the temperature was lowered to 1135℃ at a rate of 0.4℃ / h and held for 9 hours, then lowered to 1095℃ at a rate of 2℃ / h and held for 7 hours, and then lowered to 190℃ at a rate of 9℃ / h. The main heating system was turned off, and Ar was continuously introduced at a flow rate of 50 sccm. The furnace was allowed to cool naturally to room temperature. The crucible was removed, and the graphite crucible was carefully broken with a 70-mesh silicon carbide grinding wheel at a speed of 900 r / min to separate the calcium fluoride crystal. The impurity-rich areas at both ends of the crystal were removed with a diamond tool, with 5 mm removed from each end. The crystal surface was then polished with 800-mesh sandpaper and 1200-mesh sandpaper at a speed of 280 r / min and a pressure of 5 N until smooth. The crystal was placed in anhydrous ethanol and ultrasonically cleaned with a power of 280 W for 30 minutes. Finally, it was placed in an 80℃ vacuum drying oven and dried for 2 hours to obtain calcium fluoride crystal.

[0044] Example 3

[0045] like Figure 1 As shown, a method for removing impurities during the growth of calcium fluoride crystals includes the following steps:

[0046] S1. A straight-walled cylindrical graphite crucible with an inner diameter of 225 mm, an outer diameter of 250 mm, and a total height of 470 mm was selected. Using the bottom surface of the crucible as a reference, an annular area 155 mm from the bottom and 8.5 mm wide was marked on the inner wall. Using a milling cutter at 500 r / min and a feed rate of 8 mm / min, the inner wall outside the annular marked area was milled to a depth of 5 mm, completed in two passes (2.5 mm each). During milling, an annular water-cooling jacket was used to wrap the outer wall of the crucible, with a cooling water flow rate of 5 L / min to prevent graphite cracking, resulting in a graphite crucible with annular support steps. After milling, the inner wall of the crucible was polished sequentially with 1000 and 1500 grit metallographic sandpaper until the roughness Ra = 1.0 μm. The crucible was then placed in anhydrous ethanol and ultrasonically cleaned at 320 W for 30 min, followed by vacuum drying at 85 °C for 2.5 h to obtain a pretreated graphite crucible.

[0047] S2. SiC powder, acidic sodium silicate sol (particle size 15nm) with a SiO2 content of 30wt.%, and anhydrous ethanol are mixed in a mass-to-volume ratio of 57g:69mL:34mL. The mixture is placed in an agate ball mill jar with a diameter of 10mm and a ball-to-particle ratio of 3:1. The mixture is ball-milled at 320r / min for 3.5h to obtain a slurry. The slurry is then uniformly brushed onto the inner wall of the pretreated graphite crucible along its axial direction using a soft brush at a speed controlled at 8cm / s. The thickness of a single wet film is controlled at 25μm. The mixture is then left to air dry horizontally at room temperature for 35min, and then transferred to a 65℃ forced-air drying oven for curing. The coating-drying-airing process was repeated 4 times over 1.2 hours. Then, the crucible was placed in an atmosphere sintering furnace and sintered for the first time under nitrogen protection at a flow rate of 220 sccm, following the procedure below: first, the temperature was increased to 220℃ at 6℃ / min and held for 1.2 hours; then, the temperature was increased to 620℃ at 1.2℃ / min and held for 1.3 hours; then, the temperature was increased to 820℃ at 1.0℃ / min and held for 3.5 hours; then, the temperature was increased to 1220℃ at 5℃ / min and held for 3 hours; finally, the temperature was allowed to cool naturally to room temperature, resulting in a graphite crucible with a SiC-SiO2 composite coating as the bottom layer. BN powder, CaF2 powder, polyvinyl alcohol, and anhydrous ethanol were mixed in a mass-to-volume ratio of 82g:11g:5.5g:34mL and magnetically stirred at 520r / min for 1.8h to obtain a slurry. The slurry was then uniformly sprayed onto the SiC substrate using a spray gun, with the spray gun 18cm away from the inner wall of the crucible and a spraying pressure of 0.4MPa. The spraying was carried out at a uniform speed of 10cm / s along the circumference, controlling the wet film thickness to 15μm. After a second sintering process, the substrate was air-dried at room temperature for 35min, then heated to 720℃ at a flow rate of 6℃ / min under nitrogen protection at a flow rate of 200sccm and held at that temperature for 2.2h. Finally, it was allowed to cool naturally to room temperature to obtain a double-coated graphite crucible.

[0048] S3. Mix 25 kg of CaF2 raw material with 25 g of LiF at a speed of 30 r / min for 15 min to ensure uniform dispersion of LiF and obtain a mixed raw material. Transfer the mixed raw material to a reactor, which is connected to a tail gas absorption system. First, purge the air in the reactor three times with Ar at a flow rate of 100 sccm. Then, purge with HF at a flow rate of 80 sccm and heat to 610°C at a rate of 3.5°C / min. Start mechanical stirring at a rate of 52 r / min and stir at a constant temperature for 19 h. Continue to heat to 660°C at a rate of 3.5°C / min and stir at a constant temperature for 2.2 h. Stop the HF flow and keep the reactor temperature constant at 650°C. Switch to Ar at a flow rate of 110 sccm and purge for 35 min. Then, cool to 400°C at a rate of 5°C / min and stir at a constant temperature for 2.2 h. After the program ends, cool to room temperature at a rate of 5°C / min to obtain the dehydrated and dehydrogenated raw material.

[0049] S4. Mix 92g BN powder, 3.2g Y2O3, 21g soluble starch (particle size 5μm, gelatinization temperature 70℃), 1.0g ammonium polyacrylate, and 52mL deionized water. Place the mixture in an agate ball mill jar (agate ball diameter 8mm, ball-to-material ratio 4:1) and ball mill at 320r / min for 6.5h to obtain filter plate slurry. Inject the slurry into an annular alumina mold at a rate of 10mL / min. Let it stand horizontally at room temperature for 13h for preliminary shaping, then transfer it to a 65℃ forced-air drying oven for 26h. Place the dried green body in an atmosphere sintering furnace and heat it to 520℃ at a flow rate of 160sccm in air, holding the temperature for 2.2h. To completely oxidize and decompose the starch, a nitrogen atmosphere with a flow rate of 210 sccm was switched, and the temperature was increased to 1770℃ at a rate of 3.5℃ / min and held at that temperature for 4.5 hours. Then, the temperature was allowed to cool to room temperature by autothermal cooling. The sintered filter plate was then machined into a BN porous ceramic filter plate with an outer diameter of 224.8 mm, a thickness of 5.1 mm, and a pore size of 1 μm. The BN porous ceramic filter plate was installed as a baffle on the annular support step on the inner wall of a double-coated graphite crucible. A BN slurry with a solid content of 60% was evenly applied to the edge of the filter plate in contact with the crucible with a thickness of 1 mm using a scraper. After drying at room temperature, the crucible with the filter plate installed was sealed.

[0050] S5. 25 kg of dehydrated and dehydrogenated CaF2 raw material is loaded into the crucible above the filter plate, placed in a crucible-lowering furnace, and the furnace body is fixed and sealed. Ar is introduced at a flow rate of 150 sccm to replace the air three times, maintaining an Ar atmosphere. The temperature is increased to 1020℃ at a rate of 11℃ / min and held for 1.2 h. Then, the temperature is increased to 1460℃ at a rate of 5.5℃ / min and held for 6 h. Under gravity, the melt penetrates the BN filter plate from top to bottom and enters the crystal growth zone below. After the melt is completely clear, the crucible-lowering rate is set to 2 mm / h, and the descent continues for 50 h. The temperature control system precisely maintains an axial temperature gradient of 23℃ / mm near the growth interface and a radial temperature difference of <1℃. After crystal growth was completed, the crucible descent was stopped, and annealing was performed: the temperature was lowered to 1145℃ at a rate of 0.6℃ / h and held for 11h, then lowered to 1105℃ at a rate of 2.4℃ / h and held for 9h, then lowered to 210℃ at a rate of 11℃ / h. The main heating system was turned off, and Ar was continuously introduced at a flow rate of 50 sccm. The furnace was allowed to cool naturally to room temperature. The crucible was removed, and the graphite crucible was carefully broken with a 90-mesh silicon carbide grinding wheel at a speed of 1000 r / min to separate the calcium fluoride crystal. The impurity-rich areas at both ends of the crystal were removed with a diamond tool, with 10 mm removed from each end. The crystal surface was then polished with 1000-mesh sandpaper and 1500-mesh sandpaper at a speed of 320 r / min and a pressure of 10 N until smooth. The crystal was placed in anhydrous ethanol and ultrasonically cleaned with a power of 300W for 30 min. Finally, it was placed in an 85℃ vacuum drying oven and dried for 2.5 h to obtain the calcium fluoride crystal.

[0051] Comparative Example 1

[0052] A method for removing impurities during the growth of calcium fluoride crystals differs from Example 1 in that step S3 does not involve treating the raw materials with an HF atmosphere; instead, CaF2 and LiF are directly mixed and loaded into a crucible. The remaining steps and parameters are the same as in Example 1.

[0053] Comparative Example 2

[0054] A method for removing impurities during the growth of calcium fluoride crystals, which differs from Example 1 in that in step S2, the graphite crucible is only coated with a bottom SiC-SiO2 composite coating, and no fluoride-doped BN coating is prepared or coated. The remaining steps and parameters are the same as in Example 1.

[0055] Comparative Example 3

[0056] A method for removing impurities during the growth of calcium fluoride crystals, which differs from Example 1 in that a BN porous ceramic filter plate is not prepared in step S4, while the remaining steps and parameters are the same as in Example 1.

[0057] Comparative Example 4

[0058] A method for removing impurities during the growth of calcium fluoride crystals, which differs from Example 1 in that in step S5, an Al2O3 porous filter plate with an average pore size of 1 μm is used instead of a BN porous ceramic filter plate, while the remaining steps and parameters are the same as in Example 1.

[0059] Performance testing:

[0060] Infrared transmittance test: First, the calcium fluoride crystals prepared in Examples 1-3 and Comparative Examples 1-4 were processed into transparent thin films of 20mm×20mm×5mm, respectively. Then, a step-by-step polishing process was performed: first, both sides of the thin film were wet-polished with 800-grit, 1200-grit, and 2000-grit silicon carbide sandpaper, respectively; then, fine polishing was performed on a polishing machine using 5μm, 1μm, and 0.5μm diamond polishing slurries until the surface roughness Ra of the thin film was <0.2μm. The thin film was then immersed in anhydrous ethanol and ultrasonically cleaned at 300W for 15 minutes, and dried in a vacuum drying oven at 80℃ for 2 hours. Fourier transform infrared spectroscopy was used, with a test wavelength range of 2-12μm and a scanning resolution of 4cm. -1 The pretreated thin film was fixed on the sample holder, and tests were conducted at five different positions: the center, upper left corner, upper right corner, lower left corner, and lower right corner of the thin film. The infrared transmittance curve at each position was recorded. The average transmittance at 5 μm wavelength at the five positions was calculated as the peak infrared transmittance of the crystal.

[0061] Optical uniformity test: Circular slices with a diameter of 100 mm and a thickness of 10 mm were cut from the calcium fluoride crystals prepared in Examples 1-3 and Comparative Examples 1-4. The slices were first polished with 2000-grit sandpaper, and then finely polished with 0.1 μm colloidal silica polishing liquid until the surface roughness Ra < 0.1 μm. After polishing, the surface of the slices was gently wiped with anhydrous ethanol and placed in a clean desiccator to stand for 12 h. A laser interferometer was used with a 632.8nm helium-neon laser as the light source. The Fizeau interferometry was employed. The thin film was placed on the sample stage of the interferometer, ensuring that the surface of the film was perpendicular to the optical path. The interference fringe pattern of the film was acquired, and the interference fringes were analyzed using the accompanying software to calculate the refractive index inhomogeneity Δn of the crystal (Δn = the refractive index difference corresponding to the maximum deformation of the fringes). The Δn values ​​of three different cross sections of the film were measured in the axial (thickness direction), radial (diameter direction), and diagonal directions, and the maximum value among the three cross sections was recorded as the final optical homogeneity index of the crystal (the smaller the Δn, the better the optical homogeneity).

[0062] Impurity content testing (hydroxyl, carbon, metallic impurities): 20mm × 20mm × 5mm thin sections from Examples 1-3 and Comparative Examples 1-4, pretreated for infrared transmittance testing, were reused to ensure surface cleanliness and absence of moisture adsorption. Infrared spectroscopy was employed, using pure calcium fluoride single crystals as a reference, and the transmittance was measured through a 3400cm² filter. -1The integral area of ​​the hydroxyl characteristic peak was compared with the calibration curve of the reference sample to calculate the hydroxyl content (ppm) in the sample. Three parallel tests were performed, and the average value was taken. Using a high-frequency infrared carbon-sulfur analyzer, approximately 1g of sample was taken from different regions (center, edge, top, and bottom) of the crystals from Examples 1-3 and Comparative Examples 1-4, pulverized into 100-mesh powder, and 50mg of sample was placed in a graphite crucible for high-frequency combustion in an oxygen stream. The high-frequency combustion power was set to 3.5kW, and the combustion time to 60s. The generated CO2 was detected by an infrared detector, and the carbon content (ppm) was calculated based on the peak area. Approximately 1g of sample was taken from different regions (center, edge, top, and bottom) of the crystals from Examples 1-3 and Comparative Examples 1-4, pulverized into 100-mesh powder, and 10mL of [unspecified substance] was added to the powder. A nitric acid-hydrofluoric acid mixed solution with a product ratio of 1:1 was microwave digested: the temperature was increased to 120℃ at 5℃ / min and held for 10 min, then increased to 180℃ at 5℃ / min and held for 20 min. After digestion, the digest was diluted to 100 mL with ultrapure water to obtain the test solution. Inductively coupled plasma mass spectrometry (ICP-MS) was used, with ultrapure water as a blank control, and standard curves were plotted using elemental concentration gradients of 0.1 ppb, 1 ppb, 10 ppb, and 100 ppb. The diluted test solution was injected into the ICP-MS, and the radio frequency power was set to 1550 W, the nebulizer gas flow rate to 1.05 L / min, and the sampling depth to 8 mm. The signal intensity of each metal element was detected, and the concentration (ppb) of the metal element in the test solution was calculated according to the standard curve. The test was performed in triplicate, and the average value was taken.

[0063] Table 1. Performance test results of calcium fluoride crystals prepared in Examples 1-3 and Comparative Examples 1-4

[0064]

[0065] As shown in Table 1, the infrared transmittance and optical uniformity of the crystals in Examples 1-3 are better than those in Comparative Examples 1-4, and the impurity content of the crystals is significantly lower than that in Comparative Examples 1-4. This indicates that the infrared transmittance, optical uniformity, and impurity removal performance of the calcium fluoride crystals prepared in Examples 1-3 are all better than those prepared in Comparative Examples 1-4.

[0066] Comparative Example 1, which did not employ HF atmosphere treatment on the CaF2 raw material, suffers from performance defects primarily due to the residual hydroxyl groups and moisture in the raw material. HF's core function is to react with the moisture in the raw material through high reactivity to form volatile HF·H2O complexes and to convert hydroxyl groups into water molecules through fluorination substitution reactions. Without HF treatment, the residual hydroxyl groups (-OH) and moisture in the raw material cannot be effectively removed. These impurities exist as hydroxyl scattering centers during crystal growth, resulting in a significant enhancement of the characteristic absorption peak of hydroxyl groups at 3400 cm⁻¹ in infrared testing, leading to a significant decrease in infrared transmittance in the 2-12 μm band. Simultaneously, the uneven distribution of hydroxyl groups in the crystal causes local refractive index differences, disrupting the overall refractive index uniformity of the crystal and increasing optical uniformity Δn. The content of carbon and metal impurities does not change significantly because HF treatment only targets hydroxyl groups and moisture, without involving carbon element control or metal contamination.

[0067] Comparative Example 2 suffers from insufficient resistance to fluorine corrosion and carbon impurity leaching due to the absence of a top-layer fluoride-doped BN coating: Although the bottom SiC-SiO2 composite coating can form a physical barrier, fluoride ions in molten CaF2 are still highly corrosive at high temperatures. Without the top-layer BN coating, fluoride ions gradually penetrate and corrode the bottom coating, causing localized peeling. The peeled coating debris becomes solid impurities in the melt, leading to an increase in impurities. At the same time, the graphite matrix is ​​directly exposed after the coating peels off. At high temperatures, graphite reacts with CaF2, resulting in a significant increase in carbon impurities. Carbon impurities and coating debris will cause light scattering, reducing infrared transmittance and increasing optical uniformity Δn, ultimately affecting the stability of the crystal structure.

[0068] Comparative Example 3, due to the removal of the BN porous ceramic filter plate, suffers from a core performance defect: the inability to intercept solid impurities in the melt. The core function of the BN filter plate is to intercept raw material agglomerates, such as undispersed CaF2 particles and tiny crucible coating debris, through micron-level pores. Without the filter plate, these impurities directly enter the growth zone with the melt. During crystallization, these impurities cannot integrate into the CaF2 lattice and remain inside the crystal as inclusions, significantly increasing the number of solid impurities and severely scattering infrared light, resulting in a significant decrease in transmittance. At the same time, the local refractive index abrupt change caused by the inclusions increases the optical uniformity Δn, making the optical uniformity significantly worse than that of the example.

[0069] Comparative Example 4, which uses an Al2O3 filter plate instead of a BN filter plate, suffers from performance problems stemming from secondary contamination caused by the chemical reaction between the filter plate and the melt. The key advantage of the BN filter plate is its thermodynamic stability with the CaF2 melt at 1450℃, meaning it does not react or dissolve. However, Al2O3 reacts chemically with CaF2 at high temperatures, producing CaO that dissolves in the melt, while Al remains as an impurity, increasing the Al content. The reaction product CaO and the residual Al alter the compositional uniformity of the melt, leading to uneven refractive index distribution during crystal growth and an increase in optical uniformity Δn. Simultaneously, the CaO and Al impurities enhance light scattering, significantly reducing infrared transmittance and affecting crystal purity and optical performance.

[0070] Comparative Examples 1-4, due to the absence of the HF atmosphere raw material dehydration and dehydrogenation process, the fluoride-doped BN coating on the top layer of the graphite crucible, and the BN porous ceramic filter plate, respectively, directly led to different types of impurity problems, such as residual hydroxyl groups in the raw material, dissolution of carbon impurities, residual solid impurities in the melt, and generation of metallic impurities. These problems collectively manifested as a significant deterioration in the two core performance characteristics of infrared transmittance and optical uniformity, fully demonstrating the necessity of each impurity removal process in the original scheme for maintaining the excellent optical performance of the crystal.

[0071] The above results demonstrate and describe the basic principles and main features of this application, as well as its advantages.

[0072] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A method for removing impurities during the growth of calcium fluoride crystals, characterized in that, Includes the following steps: S1. Pre-treat the inner wall of the graphite crucible to obtain a pre-treated graphite crucible; S2. SiC powder, silica sol, and anhydrous ethanol are mixed and ball-milled to obtain a slurry. The slurry is then coated onto the inner wall of a pretreated graphite crucible, cured, and sintered for the first time to obtain a graphite crucible with a SiC-SiO2 composite coating as the bottom layer. BN powder, CaF2 powder, polyvinyl alcohol, and anhydrous ethanol are mixed and stirred to obtain a slurry. The slurry is then sprayed onto the bottom layer and sintered for the second time to obtain a graphite crucible with a double-layer coating. S3. Mix CaF2 with LiF, introduce HF and heat for the first time, stir, heat for the second time, switch HF to Ar, purge, cool, and obtain dehydrated and dehydrogenated CaF2. S4. Mix BN powder, Y2O3, soluble starch, ammonium polyacrylate and deionized water, ball mill to obtain filter plate slurry, and after molding, drying, sintering and processing, obtain BN porous ceramic filter plate; install BN porous ceramic filter plate in double-coated graphite crucible to obtain crucible for installing filter plate. S5. Add the dehydrated and dehydrogenated CaF2 to the crucible with the filter plate installed, place it in the crucible lowering furnace, introduce Ar, heat up, set the crucible lowering program, anneal, and obtain calcium fluoride crystals grown in the crucible. After separation, cutting, grinding, cleaning and drying, obtain calcium fluoride crystals.

2. The method for removing impurities during calcium fluoride crystal growth according to claim 1, characterized in that, In step S1, the pretreatment step is as follows: Select a straight-walled cylindrical graphite crucible with an inner wall diameter of 215-225 mm, an outer wall diameter of 230-250 mm, and a height of 430-470 mm. Using the upper surface of the crucible bottom as a reference, mark an annular area 145-155 mm from the bottom and 7.5-8.5 mm wide on the inner wall. Use a rotation speed of 300-500 r / min and a feed rate of 5-8 mm / min. The inner wall outside the annular marking area is milled with a milling cutter to a depth of 3-5 mm to obtain a graphite crucible with annular support steps. The inner wall of the crucible is then polished with 800-1000 grit metallographic sandpaper and 1200-1500 grit metallographic sandpaper until the roughness Ra=1.0-1.5μm. The crucible is then placed in anhydrous ethanol and ultrasonically cleaned with a power of 280-320W for 25-35 min. Finally, it is vacuum dried at 75-85℃ for 1.5-2.5 h.

3. The method for removing impurities during calcium fluoride crystal growth according to claim 1, characterized in that, In step S2, the mass-to-volume ratio of SiC powder, silica sol, and anhydrous ethanol is (53-57) g:(65-69) mL:(30-34) mL; the silica sol is acidic sodium type, with a SiO2 content of 30 wt.% and a particle size of 10-15 nm; the ball milling rate is 280-320 r / min, and the ball milling time is 2.5-3.5 h; the coating rate is 5-8 cm / s, and the single wet film thickness is 20-25 μm; the curing step is: air-drying at room temperature for 25-35 min, followed by drying at 55-65℃ for 0.8- 1.2h, repeat the coating-air-drying-drying process 4 times; the first sintering steps are as follows: under nitrogen protection at a flow rate of 180-220 sccm, first raise the temperature to 180-220℃ at 4-6℃ / min and hold for 1.2-1.8h, then raise the temperature to 580-620℃ at 0.8-1.2℃ / min and hold for 0.7-1.3h, then raise the temperature to 780-820℃ at 0.6-1.0℃ / min and hold for 2.5-3.5h, then raise the temperature to 1180-1220℃ at 3-5℃ / min and hold for 2-3h, and finally cool naturally to room temperature.

4. The method for removing impurities during calcium fluoride crystal growth according to claim 1, characterized in that, In step S2, the mass-to-volume ratio of BN powder, CaF2 powder, polyvinyl alcohol, and anhydrous ethanol is (78-82) g:(9-11) g:(4.5-5.5) g:(30-34) mL; the stirring rate is 480-520 r / min, and the stirring time is 1.2-1.8 h; the spraying pressure is 0.2-0.4 MPa, the spraying rate is 8-10 cm / s, and the wet film thickness is 10-15 μm; the second sintering step is as follows: after air drying at room temperature for 25-35 min, under nitrogen protection at a flow rate of 150-200 sccm, the temperature is raised to 680-720℃ at a rate of 4-6℃ / min, held at a constant temperature for 1.8-2.2 h, and then naturally cooled to room temperature.

5. The method for removing impurities during calcium fluoride crystal growth according to claim 1, characterized in that, In step S3, the mass ratio of CaF2 to LiF is (22-25) kg:(22-25) g; the flow rate of HF is 50-80 sccm; the first heating step is to heat to 590-610℃ at a rate of 2.5-3.5℃ / min; the stirring rate is 48-52 r / min, and the stirring time is 17-19 h; the second heating step is to heat to 640-660℃ at a rate of 2.5-3.5℃ / min and hold the temperature for 1.8-2.2 h; the flow rate of Ar is 90-110 sccm, and the purging time is 25-35 min; the cooling step is to cool to 400℃ at a rate of 5℃ / min, hold the temperature for 1.8-2.2 h, and finally cool to room temperature at a rate of 5℃ / min.

6. The method for removing impurities during calcium fluoride crystal growth according to claim 1, characterized in that, In step S4, the mass-to-volume ratio of BN powder, Y2O3, soluble starch, ammonium polyacrylate, and deionized water is (88-92) g:(2.8-3.2) g:(19-21) g:(0.8-1.0) g:(48-52) mL; the particle size of the soluble starch is 1-5 μm, and the gelatinization temperature is 60-70℃; the ball milling rate is 280-320 r / min, and the ball milling time is 5.5-6.5 h; the molding step is as follows: the slurry is injected into an annular alumina mold at a rate of 5-10 mL / min, and placed horizontally at room temperature for 11-13 h for shaping; the drying temperature is 55-65℃, and the drying time is 22-26 h.

7. The method for removing impurities during calcium fluoride crystal growth according to claim 1, characterized in that, In step S4, the sintering step is as follows: under an air atmosphere with a flow rate of 140-160 sccm, the temperature is increased to 480-520℃ at a rate of 0.4-0.6℃ / min and held at that temperature for 1.8-2.2 hours. Then, the temperature is switched to nitrogen gas with a flow rate of 190-210 sccm and increased to 1730-1770℃ at a rate of 2.5-3.5℃ / min, held at that temperature for 3.5-4.5 hours, and then allowed to cool naturally to room temperature. The processing step is as follows: the filter plate is processed into a circular porous plate with an outer diameter of 214.8-224.8 mm and a thickness of 4.9-5.1 mm. The pore size of the BN porous ceramic filter plate is 1-3 μm.

8. The method for removing impurities during calcium fluoride crystal growth according to claim 1, characterized in that, In step S5, the amount of dehydrated and dehydrogenated CaF2 added is (22-25) kg; the flow rate of Ar is 130-150 sccm; the heating step is as follows: heating to 980-1020℃ at a rate of 9-11℃ / min, holding at that temperature for 0.8-1.2h, then heating to 1420-1460℃ at a rate of 4.5-5.5℃ / min, holding at that temperature for 4-6h; the crucible descent program is set to descend continuously at a rate of 1-2 mm / h for 50-100h, during which the axial temperature gradient of the growth interface is 23℃ / mm, and the radial temperature difference is <1℃.

9. The method for removing impurities during calcium fluoride crystal growth according to claim 1, characterized in that, In step S5, the annealing process is as follows: the temperature is lowered to 1135-1145℃ at a rate of 0.4-0.6℃ / h, held at that temperature for 9-11h, then lowered to 1095-1105℃ at a rate of 1.6-2.4℃ / h, held at that temperature for 7-9h, then lowered to 190-210℃ at a rate of 9-11℃ / h, and finally allowed to cool naturally to room temperature.

10. The method for removing impurities during the growth of calcium fluoride crystals according to claim 1, characterized in that, In step S5, the separation step is as follows: using a 70-90 mesh silicon carbide grinding wheel at a rotation speed of 900-1100 r / min to break the graphite crucible and separate the calcium fluoride crystal; the cutting step is as follows: using a diamond tool to remove the impurity-rich areas at both ends of the crystal, removing 5-10 mm from each end; the polishing step is as follows: successively polishing the crystal surface with 800-1000 mesh sandpaper and 1200-1500 mesh sandpaper at a rotation speed of 280-320 r / min and a pressure of 5-10 N until smooth; the cleaning step is as follows: immersing the crystal in anhydrous ethanol and ultrasonically cleaning it with a power of 280-320 W for 25-35 min; the drying temperature is 75-85℃, and the drying time is 1.5-2.5 h.

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