Magnesium fluoride crucible descending type directional crystallization purification method for ultraviolet optical crystal manufacturing

Abstract

The invention belongs to the technical field of optical crystal preparation, and provides a magnesium fluoride crucible descending type directional crystallization purification method for ultraviolet optical crystal manufacturing. Magnesium fluoride raw materials are placed in a vacuum furnace to be subjected to desorption pretreatment, then the raw materials are fused, subjected to heat preservation and homogenized, first-process directional crystallization is achieved through descending displacement of a crucible relative to a hot area, a first ingot body is obtained, impurities are enriched towards the tail end along with the crystallization process, and only a local section of the enriched end is heated and remelted to form a controlled molten pool; the middle-section main body is kept in a solid state; the temperature gradient direction is reversed by switching the power ratio of a heating area, the molten pool is subjected to second-process directional crystallization in the opposite direction to form a second ingot body, the second ingot body is axially cut in a segmented mode, a middle purification section is reserved to serve as a high-purity raw material, and the middle purification section can be repeatedly executed in the same furnace to obtain a raw material section; according to the method, segregation enrichment and reverse ending are completed in a clean atmosphere, and the method is suitable for manufacturing magnesium fluoride crystals for ultraviolet optical elements.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical crystal preparation, and relates to a magnesium fluoride crucible descending directional crystallization purification method for ultraviolet optical crystal manufacturing. BACKGROUND

[0002] Magnesium fluoride crystals are widely used in the manufacturing of optical elements due to their good transmittance and environmental stability in the ultraviolet waveband. As ultraviolet optical systems develop towards shorter wavelengths, higher power densities, and longer lifetimes, the sensitivity of optical elements to internal absorption, scattering, and photo-induced attenuation of materials is significantly improved. Trace impurity ions, volatile pollutants in raw materials, and related defects introduced by water and oxygen can all become key factors affecting transmittance, uniformity, and use stability. Therefore, magnesium fluoride crystal manufacturing usually requires raw materials with higher purity, more stable consistency, and lower risk of external pollution introduction.

[0003] At present, high-purity magnesium fluoride raw materials available in the industry have relatively low impurity content, but there may still be a need to further reduce the impurity level when facing ultraviolet optical crystal manufacturing. Deep removal of trace ions using chemical synthesis or wet purification methods often introduces new impurity sources and requires higher equipment, reagent purity, and process control. Meanwhile, the wet route may also bring problems such as water and oxygen residues, particle inclusions, and subsequent removal difficulties, which are not conducive to stable consistency of raw materials meeting ultraviolet optical applications. On the other hand, physical purification ideas based on melt solidification can utilize the partitioning difference of impurities between solid and liquid phases to achieve segregation, but traditional crucible descending directional crystallization processes mainly aim to obtain single-direction segregation results. There is still room for improvement in interface morphology control at the tail end of crystallization, effective treatment of the enriched end, and compatibility of different impurity behavior differences. Meanwhile, if the atmosphere and gas path are not properly controlled during melting and solidification, secondary introduction of water and oxygen and external pollution may occur, making it difficult to stabilize and repeat the segregation purification effect. SUMMARY

[0004] In view of the deficiencies in the prior art, the purpose of the present application is to provide a magnesium fluoride crucible descending directional crystallization purification method for ultraviolet optical crystal manufacturing. Through vacuum desorption pretreatment, melting homogenization, and crucible descending directional crystallization, impurities are enriched at the end. A controlled molten pool is formed by locally remelting the enriched end in a sealed environment. Then, reverse directional crystallization is performed to obtain a second ingot. The intermediate purification section is retained as high-purity raw material by segmenting and cutting according to the intermediate purification section criterion. The same vacuum furnace can be repeatedly used to obtain stable raw material sections, thereby meeting the needs of actual production.

[0005] To achieve this purpose, the present application adopts the following technical solutions:

[0006] In a first aspect, the present application provides a method for the purification of magnesium fluoride by directional solidification using a downward moving crucible, comprising:

[0007] a) a loading and desorption pretreatment step, in which 4N grade magnesium fluoride feedstock is loaded into a graphite crucible and sealed, the crucible is placed into a vacuum furnace with zoned heating, the furnace chamber and gas circuit are evacuated and baked;

[0008] b) a melting and melt homogenization step, in which the magnesium fluoride feedstock is heated to complete melting under vacuum or inert atmosphere, and the melt is kept at a temperature to homogenize the composition of the melt, obtaining a homogenized melt;

[0009] c) a first pass directional solidification step, in which the homogenized melt forms a solid-liquid interface under a controlled radial temperature field in an axial temperature gradient field, and the homogenized melt is gradually solidified in a first direction by the downward displacement of the graphite crucible relative to the hot zone, obtaining a first ingot, the end of the first ingot forms an enriched end;

[0010] d) a local remelting step of the enriched end, in which only the axial section of the enriched end of the first ingot is locally heated without breaking the sealed environment in the furnace, the section is remelted to form a controlled length local melt pool, while the main body of the middle section of the first ingot remains solid;

[0011] e) a second pass reverse directional solidification and middle section extraction step, in which the axial temperature gradient direction is reversed by switching the power ratio of the upper and lower heating zones while maintaining the downward displacement of the crucible, the local melt pool is solidified in a second direction opposite to step c) to form a second ingot; the second ingot is axially segmented and cut, the enriched ends are removed and the middle purified section is retained as high purity magnesium fluoride feedstock.

[0012] In the loading and desorption pretreatment stage, the magnesium fluoride powder and the inner surface of the graphite crucible carry adsorbed water, adsorbed gas and trace organic residues from packaging and the environment, which are attached to the particle surface, pores and microscopic defects on the wall in the form of physical adsorption, hydrogen bonding or weak coordination. When vacuumizing and baking, low-boiling-point adsorbents are first desorbed and removed by the vacuum system; as the temperature rises, water molecules and organic residues that are more tightly bound undergo thermal desorption and thermal cracking, and small volatile molecules generated are continuously pumped out. This process reduces the sources of water and oxygen that can participate in interfacial reactions during subsequent melting, so that the oxygen and hydrogen-containing volatile components in the surface of the feedstock and the atmosphere in the furnace chamber are at a lower level, thereby reducing the opportunity to form oxygen-containing or hydroxyl-containing defect precursors at high temperatures.

[0013] In the melting and melt homogenization stage, magnesium fluoride is converted from solid phase to ionic melt, and the system mainly has ion coordination structure of magnesium and fluorine. The non-uniform composition in the solid phase, the interface between particles and local inclusions are converted into concentration fluctuations in the melt. The solute in the melt realizes macroscopic composition convergence through diffusion by heat preservation and homogenization, and the local concentration gradient is weakened. Under vacuum or purified inert atmosphere, the volatile residues continue to migrate to the gas phase. In the molten state, trace metal ion impurities are dissolved in the form of ions constrained by the chemical environment of the melt, and the effective activity and coordination environment determine their partition behavior in the subsequent solidification process. If there is water and oxygen residue, oxygen-related groups or oxyfluoride defects may be induced and transferred at the melt interface or inclusion phase. Therefore, the cleanliness of the atmosphere and the control of the steady-state thermal field in this stage directly affect the number and morphology of defect sources.

[0014] In the first-stage crucible lowering directional crystallization stage, the axial temperature gradient and the controlled radial temperature field establish a stable solid-liquid interface. As the crucible slowly moves relative to the hot zone, the solid-liquid interface advances along the axial direction, and the melt gradually converts into a unidirectionally grown crystal ingot. The key to the partitioning process of impurity ions at the solid-liquid interface lies in the effective distribution coefficient corresponding to the chemical potential difference of impurities in the solid and liquid phases. For impurities with a distribution coefficient less than one, the interface growth tends to be expelled into the liquid phase, resulting in an enrichment boundary layer in the liquid phase in front of the interface. During the advancing process, the enriched impurities gradually concentrate in the rear solidification area as the remaining melt volume decreases, forming axial segregation. For impurities with a distribution coefficient close to one or greater than one, their tendency to enter the solid phase is stronger, and they may have a higher proportion in the primary solidification area or a weaker segregation gradient. By simultaneously controlling the radial temperature field and displacement speed, the disturbance of strong convection to the boundary layer can be weakened, and the segregation can be mainly controlled by diffusion and interface partitioning, thereby obtaining a repeatable axial impurity distribution.

[0015] The essence of the enrichment end local remelting stage is to truncate the segregation results of the first stage in space and redistribute them locally. The enrichment end contains higher concentrations of impurities and non-equilibrium structures and possible micro-defects related to the end solidification. The local remelting of the end section as a local melt pool makes the impurities in the enrichment end re-enter the liquid phase and realize local homogenization through diffusion in the melt pool, while the middle section remains solid, meaning that the middle section plays the role of a low-impurity solid matrix and does not participate in the remelting and mixing.

[0016] The second reverse directional crystallization and the middle section material taking stage form an interface advancing in the opposite direction to the first stage by reversing the axial temperature gradient. The local melt pool experiences solid-liquid partitioning again in the new temperature field, and the impurities previously homogenized in the enriched end of the melt pool will re-establish segregation according to their respective partition coefficients and the direction of interface advancement: for impurities with a partition coefficient less than one, they will still tend to be expelled into the liquid phase at the interface and migrate to the new late solidification end; for impurities that can have a partition coefficient greater than one, they are more likely to enter the solid phase at the new early solidification end. Since this process occurs within the limited length of the melt pool, the segregation effect is equivalent to a reverse sweep of the end enrichment zone, so that impurities with different partition behaviors are more likely to concentrate in the two end regions of the second ingot after reverse solidification, while the middle section corresponds to the relatively low interval of the impurities after two interface partitioning. When cutting along the axial section, the removal of the two end enrichment sections can avoid bringing the segregation accumulation zone into the subsequent crystal manufacturing raw materials, and the middle purification section corresponds to the low platform of the segregation curve. If the same thermal history link is repeatedly performed on the middle purification section, it is equivalent to increasing the number of interface partitioning and diffusion shaping, thereby improving the consistency of the section and reducing the overall level of migratory impurities.

[0017] As a preferred technical solution of the present application, in the loading and desorption pretreatment step: the ultimate vacuum degree of the vacuum furnace is 1×10 -3 -1×10 -1 Pa, vacuumizing and temperature baking are carried out synchronously, the baking temperature is 350-650℃, the baking time is 2-10h, and after the baking is completed, the furnace cavity is continuously vacuumized and cooled to below the melting starting temperature, for example: the ultimate vacuum degree of the vacuum furnace is (1×10 -3 , 1.1×10 -2 , 2.2×10 -2 , 3.3×10 -2 , 4.4×10 -2 , 5.5×10 -2 , 6.6×10 -2 , 7.7×10 -2 , 8.8×10 -2 , 9.9×10 -2 or 1×10 -1 Pa, vacuumizing and temperature baking are carried out synchronously, the baking temperature is (350, 380, 410, 440, 470, 500, 530, 560, 590, 620 or 650)℃, the baking time is (2, 2.8, 3.6, 4.4, 5.2, 6, 6.8, 7.6, 8.4, 9.2 or 10)h, and after the baking is completed, the furnace cavity is continuously vacuumized and cooled to below the melting starting temperature, but not limited to the listed values, other unlisted values within the range of values are also applicable.

[0018] In some alternative embodiments, the vacuum furnace is connected with high-purity inert gas to form a protective atmosphere after the desorption pretreatment is completed, and a gas path purification module is arranged on the gas supply side of the inert gas, which at least includes:

[0019] i) a particle filtration unit using a metal sintered filter element or a membrane filter with a filtration accuracy of no more than 0.1 μm;

[0020] ii) a water removal unit using a molecular sieve drying unit and / or a low-temperature cold trap to reduce the moisture content entering the furnace cavity to a dew point of no higher than -60°C;

[0021] iii) an oxygen removal unit using a metal getter unit and / or a high-temperature deoxidation catalyst unit to reduce the oxygen content entering the furnace cavity to no more than 1 ppm;

[0022] iv) a pipeline and valve cleaning unit, the wet end components of the gas path and the furnace cavity are sealed with metal or vacuum-grade sealing structure, and vacuum baking or inert gas replacement is performed before each process.

[0023] In some alternative embodiments, in the melting and melt homogenization step, the melting temperature is controlled to be 1281-1381°C, the holding homogenization time is 1-8h, and a steady field standing section is arranged before entering the directional crystallization, which is to adjust the heating power to a steady-state thermal field that suppresses strong convection under the condition of maintaining the melt completely molten and stand for 30-180 min, for example, the melting temperature can be controlled to be 1281°C, 1291°C, 1301°C, 1311°C, 1321°C, 1331°C, 1341°C, 1351°C, 1361°C, 1371°C or 1381°C, the holding homogenization time is (1, 1.7, 2.4, 3.1, 3.8, 4.5, 5.2, 5.9, 6.6, 7.3 or 8) h, and a steady field standing section is arranged before entering the directional crystallization, which is to adjust the heating power to a steady-state thermal field that suppresses strong convection under the condition of maintaining the melt completely molten and stand for (30, 45, 60, 75, 90, 105, 120, 135, 150, 165 or 180) min, but not limited to the listed values, other unlisted values within the range are also applicable.

[0024] In some optional embodiments, in the first-stage crucible lowering directional crystallization step, the axial temperature gradient is 3-10°C / cm, the radial temperature gradient is 2-5°C / cm, and the graphite crucible lowering speed is 0.1-0.5 mm / h, and the solid-liquid interface is kept advancing monotonously and dendrites or remelted zones are avoided by adjusting the heat shield structure, the power ratio of the upper and lower heating zones, and the crucible outer insulation structure. For example, the axial temperature gradient can be (3, 3.7, 4.4, 5.1, 5.8, 6.5, 7.2, 7.9, 8.6, 9.3, or 10) °C / cm, the radial temperature gradient can be (2.0, 2.3, 2.6, 2.9, 3.2, 3.5, 3.8, 4.1, 4.4, 4.7, or 5.0) °C / cm, and the graphite crucible lowering speed can be (0.1, 0.14, 0.18, 0.22, 0.26, 0.3, 0.34, 0.38, 0.42, 0.46, or 0.5) mm / h, and the solid-liquid interface is kept advancing monotonously and dendrites or remelted zones are avoided by adjusting the heat shield structure, the power ratio of the upper and lower heating zones, and the crucible outer insulation structure, but not only limited to the listed values, and other values not listed in the range are also applicable.

[0025] In some optional embodiments, in the enrichment end local remelting step, the length of the local remelting section along the axial direction is 5%-30% of the total length of the first ingot, the local remelting is achieved by setting a local enhanced heating zone or a ring-shaped compensation heater at the corresponding position of the enrichment end, and the solid-liquid interface of the local molten pool is controlled to only cover the enrichment end section without invading the middle section main body, so that the middle section main body remains in solid phase. For example, the length of the local remelting section along the axial direction can be 5%, 7.5%, 10%, 12.5%, 15%, 17.5%, 20%, 22.5%, 25%, 27.5%, or 30% of the total length of the first ingot, but not only limited to the listed values, and other values not listed in the range are also applicable.

[0026] In some optional embodiments, the second pass reverse directional crystallization is achieved by switching the power ratio of the upper and lower heating zones and the position of the hot screen opening while the crucible continues to move downward at a rate of 0.1-0.5 mm / h, so that the cold end and the hot end are swapped in axial position, the solidification direction of the local melt pool is opposite to that of the first pass directional crystallization, and the local melt pool is kept at a constant temperature for 10-60 min before the reverse directional crystallization begins. For example, the power ratio of the upper and lower heating zones and the position of the hot screen opening can be switched while the crucible continues to move downward at a rate of (0.1, 0.14, 0.18, 0.22, 0.26, 0.3, 0.34, 0.38, 0.42, 0.46, or 0.5) mm / h, so that the cold end and the hot end are swapped in axial position, the solidification direction of the local melt pool is opposite to that of the first pass directional crystallization, and the local melt pool is kept at a constant temperature for (10, 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60) min, but the application is not limited to the listed values, and other values not listed in the range are also applicable.

[0027] In some optional embodiments, the graphite crucible is a sealed structure with a cover, and a buffer of the same source magnesium fluoride is arranged in the gas phase space between the crucible cover and the melt surface, the buffer of the same source magnesium fluoride being a bed of magnesium fluoride particles, a sintered block, or a pressed block, and the buffer of the same source magnesium fluoride is not in direct contact with the melt.

[0028] In some optional embodiments, the segmented cutting determines the removal boundaries of the enriched segments at both ends by using a mid-domain purification segment criterion, which satisfies at least one of the following conditions:

[0029] i) Sampling every fixed interval along the second ingot body axis and determining the trace metal ion content profile by ICP-MS and / or GDMS, and taking the inflection point where the impurity content rises significantly as the boundary of the enriched segment;

[0030] ii) Sampling along the second ingot body axis to prepare polished test pieces and testing the absorption coefficient, internal transmittance, or equivalent optical loss in the 193 nm waveband, and taking the inflection point where the optical index deteriorates significantly as the boundary of the enriched segment;

[0031] and removing the enriched segments at both ends and retaining the intermediate purification segment under the condition that the above-mentioned criterion is satisfied, the length of the intermediate purification segment accounting for 30%-80% of the total length of the second ingot body, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80%, but the application is not limited to the listed values, and other values not listed in the range are also applicable.

[0032] In some alternative embodiments, the reserved intermediate purification section performs the above-mentioned steps a) to e) at least once again in the same vacuum furnace.

[0033] Compared with the prior art, the present application has the following beneficial effects: the present application completes the melt homogenization and crucible lowering directional crystallization of the magnesium fluoride raw material under the synergistic conditions of vacuum desorption pretreatment and inert protective atmosphere, so that the distributable impurities in the melt form axial segregation and concentrate at the end during the advancement of the solid-liquid interface, thereby reducing the introduction and residual risks that may be caused by the wet chemical deimpurification from the process path; by implementing local remelting at the enriched end and performing reverse directional crystallization by reversing the temperature gradient in a closed environment, the end segregation zone and the finishing thermal history can be separated from the main ingot and reorganized, so that the two ends form an enriched section that can be removed, and a purification zone that can be used as a raw material section is formed in the middle of the ingot, which facilitates stable material taking and batch control. In combination with the gas path purification module, the steady-state thermal field and temperature gradient control, and the section taking method for determining the cutting boundary according to the impurity profile or the ultraviolet optical index, the purification process has a repeatable process window and a verifiable material taking basis, so that a high-purity magnesium fluoride raw material suitable for the manufacture of ultraviolet optical crystals is obtained. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 A complete magnesium fluoride physical map is provided for Example 1 of the present application;

[0035] Figure 2 A broken magnesium fluoride physical map is provided for Example 1 of the present application. DETAILED DESCRIPTION

[0036] The technical solutions of the present application will be described in detail below in combination with specific examples. The examples described herein are specific embodiments of the present application, which are used to illustrate the concept of the present application; these descriptions are all explanatory and exemplary, and should not be understood as limiting the embodiments of the present application and the protection scope of the present application. In addition to the examples described herein, those skilled in the art can also employ other technical solutions that are obvious based on the disclosure of the claims and the specification of the present application, which include technical solutions that make any obvious substitutions and modifications to the examples described herein.

[0037] The chemical reagents used in the examples and comparative examples of the present application are all commercially available goods and are not subjected to any further purification treatment.

[0038] Example 1

[0039] The present example provides a magnesium fluoride crucible lowering directional crystallization purification method for the manufacture of ultraviolet optical crystals, which specifically comprises the following steps:

[0040] a) loading and desorption pretreatment step, 4N grade magnesium fluoride raw material is loaded into a graphite crucible and sealed with a cover, and the crucible is placed in a vacuum furnace with zoned heating, the furnace chamber and gas circuit are vacuumed and baked, and in the loading and desorption pretreatment step: the limit vacuum degree of the vacuum furnace is 1×10 -3 Pa, vacuuming and baking are carried out synchronously, the baking temperature is 350-650℃, the baking time is 10h, and after baking is completed, the furnace chamber is continuously vacuumed and cooled to below the melting starting temperature, the graphite crucible is a sealed structure with a cover, and a homologous magnesium fluoride buffer is arranged in the gas phase space between the crucible cover and the melt liquid surface, the homologous magnesium fluoride buffer is a magnesium fluoride particle bed, and the homologous magnesium fluoride buffer is not in direct contact with the melt, the vacuum furnace is connected with high-purity inert gas to form a protective atmosphere after the desorption pretreatment is completed, and a gas circuit purification module is arranged on the gas supply side of the inert gas, the gas circuit purification module at least includes:

[0041] i) particle filtration unit, metal sintered filter element is used, and the filtration precision is not more than 0.1μm;

[0042] ii) water removal unit, molecular sieve drying unit and low-temperature cold trap are used to reduce the water content entering the furnace chamber to a dew point not higher than-60℃;

[0043] iii) oxygen removal unit, metal getter unit and high-temperature deoxidation catalytic unit are used to make the oxygen content entering the furnace chamber not higher than 1ppm;

[0044] iv) pipeline and valve cleaning unit, the wet end parts connected with the furnace chamber are made of metal sealing structure, and vacuum baking is carried out before each process;

[0045] b) melting and melt homogenization step, under the inert protective atmosphere, the magnesium fluoride raw material is heated to complete melting and is kept for a period of time to make the melt composition uniform, and a homogenized melt is obtained, in the melting and melt homogenization step, the melting temperature is controlled to be 1381℃, the homogenization time is 1h, and a steady field standing section is arranged before directional crystallization, the steady field standing section is to adjust the heating power to a steady heat field that inhibits strong convection under the condition of maintaining the melt in complete melting, and stand for 180min;

[0046] c) first pass crucible descending directional crystallization step, the radial temperature field is controlled in the axial temperature gradient field, the homogenized melt forms a solid-liquid interface, and the homogenized melt is gradually directionally crystallized in the first direction by the descending displacement of the graphite crucible relative to the hot zone, and a first ingot is obtained, and the end of the first ingot forms an enriched end;

[0047] d) enrichment end local remelting step, under the condition of not breaking the closed environment in the furnace, only the axial section of the enrichment end of the first ingot is locally heated to make the section remelt to form a length controlled local melt pool, while the middle section body of the first ingot remains solid, in the first pass crucible descending directional crystallization step, the axial temperature gradient is 3℃ / cm, the radial temperature gradient is 5℃ / cm, the graphite crucible descending speed is 0.1mm / h, and by adjusting the heat shield structure, the power ratio of the upper and lower heating zones and the crucible outer insulation structure, the solid-liquid interface is kept advancing monotonously and the formation of dendrites or remelted zone is avoided, in the enrichment end local remelting step, the length of the local remelted section along the axial direction is 30% of the total length of the first ingot, the local remelting is realized by setting a local enhanced heating zone or a ring-shaped compensating heater at the corresponding position of the enrichment end, and the solid-liquid interface of the local melt pool is controlled to only cover the enrichment end section without invading the middle section body, so that the middle section body remains solid;

[0048] e) second pass reverse directional crystallization and middle section material taking step, under the condition of keeping the crucible descending displacement mode, the axial temperature gradient direction is reversed by switching the power ratio of the upper and lower heating zones, so that the local melt pool directional crystallizes in the second direction opposite to step c) to form a second ingot; the second ingot is axially segmented cut to remove the enrichment sections at both ends and keep the middle purification section as high-purity magnesium fluoride raw material, the second pass reverse directional crystallization is realized by switching the power ratio of the upper and lower heating zones and the heat shield opening position while the crucible continues to descend at a speed of 0.5mm / h, so that the cold end and the hot end are transposed in the axial position, the solidification direction of the local melt pool is opposite to that of the first pass directional crystallization, and the local melt pool is kept at a constant temperature for 10min before the reverse directional crystallization starts, the segmented cutting uses the middle purification section criterion to determine the removal boundaries of the enrichment sections at both ends, the middle purification section criterion meets the following conditions: sampling every fixed interval along the axial direction of the second ingot and determining the trace metal ion content profile by ICP-MS, taking the inflection point where the impurity content rises obviously as the enrichment section boundary; and under the condition of meeting the above criterion, the enrichment sections at both ends are removed and the middle purification section is kept, the length of the middle purification section accounts for 30% of the total length of the second ingot, and the kept middle purification section is subjected to steps a) to e) again in the same vacuum furnace.

[0049] Figure 1 The complete magnesium fluoride physical map provided for example 1 of the present application; Figure 2 The broken magnesium fluoride physical map provided for example 1 of the present application.

[0050] Example 2

[0051] The present example provides a magnesium fluoride crucible descending directional crystallization purification method for ultraviolet optical crystal manufacturing, which specifically comprises the following steps:

[0052] a) a charging and desorption pretreatment step, in which 4N-grade magnesium fluoride raw material is loaded into a graphite crucible and sealed with a cover, and the crucible is placed in a vacuum furnace with zoned heating, the furnace chamber and gas circuit are vacuumed, and the temperature is raised for baking, in the charging and desorption pretreatment step, the vacuum furnace has a limit vacuum degree of 1x10 -1 Pa, the vacuuming and temperature raising are performed synchronously, the baking temperature is 650℃, the baking time is 2h, and after the baking is completed, the furnace chamber is continuously vacuumed and cooled to below the melting starting temperature, the graphite crucible has a sealed structure with a cover, and a homologous magnesium fluoride buffer is arranged in the gas phase space between the crucible cover and the melt surface, the homologous magnesium fluoride buffer is a magnesium fluoride sintered block, and the homologous magnesium fluoride buffer is not in direct contact with the melt, after the desorption pretreatment is completed, the vacuum furnace is connected with high-purity inert gas to form a protective atmosphere, and a gas circuit purification module is arranged on the gas supply side of the inert gas, the gas circuit purification module at least includes:

[0053] i) a particle filtration unit, a membrane filter is used, and the filtration precision is not more than 0.1μm;

[0054] ii) a water removal unit, a molecular sieve drying unit is used, so that the water content entering the furnace chamber is reduced to a dew point of not higher than-60℃;

[0055] iii) an oxygen removal unit, a metal getter unit is used, so that the oxygen content entering the furnace chamber is not higher than 1ppm;

[0056] iv) a pipeline and valve cleaning unit, the wet end components connected with the furnace chamber are sealed with a vacuum level, and the inert gas is replaced before each process;

[0057] b) a melting and melt homogenization step, in which the magnesium fluoride raw material is heated to complete melting under vacuum and is kept for homogenization, so that the melt composition is homogenized to obtain a homogenized melt, in the melting and melt homogenization step, the melting temperature is controlled to be 1281℃, the homogenization time is 8h, and a steady field standing section is arranged before directional crystallization, in which the heating power is adjusted to a steady heat field for suppressing strong convection under the condition of maintaining complete melting of the melt, and the melt is kept for 30min;

[0058] c) a first-stage crucible descending directional crystallization step, in which the radial temperature field is controlled in the axial temperature gradient field, so that the homogenized melt forms a solid-liquid interface, and the homogenized melt is gradually directionally crystallized in a first direction by the descending displacement of the graphite crucible relative to the hot zone, to obtain a first ingot, and the end of the first ingot forms an enriched end;

[0059] d) enrichment end local remelting step, under the condition of not breaking the closed environment in the furnace, only the axial section of the enrichment end of the first ingot is locally heated to make the section remelt to form a length controlled local melt pool, while the main body of the middle section of the first ingot remains solid, in the first pass crucible descending directional crystallization step, the axial temperature gradient is 10℃ / cm, the radial temperature gradient is 2℃ / cm, the graphite crucible descending speed is 0.5mm / h, and by adjusting the heat shield structure, the power ratio of the upper and lower heating zones and the crucible outer insulation structure, the solid-liquid interface is kept advancing monotonously and dendrites or remelted zones are avoided, in the enrichment end local remelting step, the length of the local remelted section along the axial direction is 5% of the total length of the first ingot, the local remelting is realized by setting a local enhanced heating zone or a ring-shaped compensating heater at the corresponding position of the enrichment end, and the solid-liquid interface of the local melt pool is controlled to only cover the enrichment end section without invading the main body of the middle section, so that the main body of the middle section remains solid;

[0060] e) second pass reverse directional crystallization and middle section material taking step, under the condition of keeping the crucible descending displacement mode, the axial temperature gradient direction is reversed by switching the power ratio of the upper and lower heating zones, so that the local melt pool directional crystallizes in the second direction opposite to step c) to form a second ingot; the second ingot is axially segmented cut to remove the enrichment sections at both ends and keep the middle purification section as high-purity magnesium fluoride raw material, the second pass reverse directional crystallization is realized by switching the power ratio of the upper and lower heating zones and the heat shield opening position while the crucible continues to descend at a speed of 0.1mm / h, so that the cold end and the hot end are transposed in axial position, the solidification direction of the local melt pool is opposite to that of the first pass directional crystallization, and the local melt pool is kept at constant temperature for 60min before the reverse directional crystallization starts, the segmented cutting uses the middle purification section criterion to determine the removal boundary of the enrichment sections at both ends, the middle purification section criterion meets the following conditions: polished test pieces are prepared by sampling along the axial direction of the second ingot and tested for absorption coefficient, internal transmittance or equivalent optical loss in the 193nm waveband, and the inflection point where the optical indicators obviously deteriorate is taken as the boundary of the enrichment section; and under the condition of meeting the above criterion, the enrichment sections at both ends are removed and the middle purification section is kept, the length of the middle purification section accounts for 80% of the total length of the second ingot, and the kept middle purification section is subjected to steps a) to e) twice again in the same vacuum furnace.

[0061] Example 3

[0062] The embodiment provides a magnesium fluoride crucible descending directional crystallization purification method for ultraviolet optical crystal manufacturing, and specifically comprises the following steps:

[0063] a) a charging and desorption pretreatment step, in which 4N-grade magnesium fluoride raw material is loaded into a graphite crucible and sealed with a cover, and the crucible is placed in a vacuum furnace with zoned heating, the furnace chamber and gas circuit are vacuumed and baked, and in the charging and desorption pretreatment step, the vacuum furnace has a limit vacuum degree of 5*10 -2 Pa, the vacuuming and baking are performed synchronously, the baking temperature is 500°C, the baking time is 6h, and after the baking is completed, the furnace chamber is continuously vacuumed and cooled to below the melting starting temperature, the graphite crucible has a sealed structure with a cover, and a homologous magnesium fluoride buffer is arranged in the gas phase space between the crucible cover and the melt surface, the homologous magnesium fluoride buffer is a magnesium fluoride briquette, and the homologous magnesium fluoride buffer does not directly contact the melt, the vacuum furnace is connected with a high-purity inert gas to form a protective atmosphere after the desorption pretreatment is completed, and a gas circuit purification module is arranged on the gas supply side of the inert gas, and the gas circuit purification module at least includes:

[0064] i) a particle filtration unit, which uses a metal sintered filter element with a filtration accuracy of not more than 0.1 μm;

[0065] ii) a water removal unit, which uses a low-temperature cold trap to reduce the water content entering the furnace chamber to a dew point of not higher than -60°C;

[0066] iii) an oxygen removal unit, which uses a high-temperature deoxidation catalytic unit to reduce the oxygen content entering the furnace chamber to not higher than 1 ppm;

[0067] iv) a pipeline and valve cleaning unit, the wet end components connected with the furnace chamber and the gas circuit use a metal sealing structure, and the inert gas is replaced before each process;

[0068] b) a melting and melt homogenization step, in which the magnesium fluoride raw material is heated to complete melting under vacuum and is kept for a certain time to homogenize the melt composition, so as to obtain a homogenized melt, in the melting and melt homogenization step, the melting temperature is controlled to be 1350°C, the homogenization time is 4h, and a stable field standing section is arranged before directional crystallization, in which the heating power is adjusted to a stable thermal field that inhibits strong convection under the condition of maintaining complete melting of the melt, and the melt is kept standing for 100 min;

[0069] c) a first-stage crucible descending directional crystallization step, in which the radial temperature field is controlled in the axial temperature gradient field, so that the homogenized melt forms a solid-liquid interface, and the homogenized melt is gradually directionally crystallized in a first direction by the descending displacement of the graphite crucible relative to the hot zone, so as to obtain a first ingot, and the end of the first ingot forms an enriched end;

[0070] d) enrichment end local remelting step, under the condition of not breaking the closed environment in the furnace, only the axial section of the enrichment end of the first ingot is locally heated to make the section remelt to form a length controlled local melt pool, while the main body of the middle section of the first ingot remains solid, in the first pass crucible descending directional crystallization step, the axial temperature gradient is 6℃ / cm, the radial temperature gradient is 3℃ / cm, the graphite crucible descending speed is 0.3mm / h, and by adjusting the heat shield structure, the power ratio of the upper and lower heating zones and the crucible outer insulation structure, the solid-liquid interface is kept advancing monotonously and the formation of dendrites or remelted zone is avoided, in the enrichment end local remelting step, the length of the local remelted section along the axial direction is 15% of the total length of the first ingot, the local remelting is realized by setting a local enhanced heating zone or a ring-shaped compensating heater at the corresponding position of the enrichment end, and the solid-liquid interface of the local melt pool is controlled to only cover the enrichment end section without invading the main body of the middle section, so that the main body of the middle section remains solid;

[0071] e) second pass reverse directional crystallization and middle section material taking step, under the condition of keeping the crucible descending displacement mode, the axial temperature gradient direction is reversed by switching the power ratio of the upper and lower heating zones, so that the local melt pool directional crystallizes in the second direction opposite to step c) to form a second ingot; the second ingot is axially segmented cut to remove the enrichment sections at both ends and keep the middle purification section as high-purity magnesium fluoride raw material, the second pass reverse directional crystallization is realized by switching the power ratio of the upper and lower heating zones and the heat shield opening position while the crucible continues to descend at 0.3mm / h, so that the cold end and the hot end are transposed in axial position, the solidification direction of the local melt pool is opposite to that of the first pass directional crystallization, and the local melt pool is kept at constant temperature and homogenized for 30min before the reverse directional crystallization starts, the segmented cutting uses the middle purification section criterion to determine the removal boundaries of the enrichment sections at both ends, the middle purification section criterion meets the following conditions at the same time: i) sampling every fixed interval along the axial direction of the second ingot and determining the trace metal ion content profile by ICP-MS, taking the inflection point of the obvious rise of impurity content as the enrichment section boundary; ii) sampling along the axial direction of the second ingot to prepare polished test pieces and testing the absorption coefficient, internal transmittance or equivalent optical loss at 193nm waveband, taking the inflection point of the obvious deterioration of optical index as the enrichment section boundary; and under the condition of meeting the above criterion, the enrichment sections at both ends are removed and the middle purification section is kept, the length of the middle purification section accounts for 60% of the total length of the second ingot, and the kept middle purification section is executed again in the same vacuum furnace once.

[0072] Example 4

[0073] The present embodiment provides a magnesium fluoride crucible descending directional crystallization purification method for ultraviolet optical crystal manufacturing, which specifically comprises the following steps:

[0074] a) loading and desorption pretreatment step, 4N grade magnesium fluoride raw material is loaded into a graphite crucible and sealed with a cover, and the crucible is placed in a vacuum furnace with zoned heating, the furnace chamber and gas circuit are vacuumed and baked, and in the loading and desorption pretreatment step: the limit vacuum degree of the vacuum furnace is 2x10 -2 Pa, vacuuming and baking are carried out synchronously, the baking temperature is 450℃, the baking time is 8h, and after baking is completed, the furnace chamber is continuously vacuumed and cooled to below the melting starting temperature, the graphite crucible is a sealed structure with a cover, and a homologous magnesium fluoride buffer is arranged in the gas phase space between the crucible cover and the melt liquid surface, the homologous magnesium fluoride buffer is a magnesium fluoride particle bed, and the homologous magnesium fluoride buffer is not in direct contact with the melt, the vacuum furnace is connected with high-purity inert gas to form a protective atmosphere after the desorption pretreatment is completed, and a gas circuit purification module is arranged on the gas supply side of the inert gas, the gas circuit purification module at least includes:

[0075] i) particle filtration unit, using a membrane filter with a filtration accuracy of not more than 0.1μm;

[0076] ii) water removal unit, using a molecular sieve drying unit to reduce the moisture content entering the furnace chamber to a dew point of not higher than -60℃;

[0077] iii) oxygen removal unit, using a high-temperature deoxidation catalytic unit to reduce the oxygen content entering the furnace chamber to not higher than 1ppm;

[0078] iv) pipeline and valve cleaning unit, the wet end components connected with the furnace chamber are sealed with a vacuum level, and vacuum baking is carried out before each process;

[0079] b) melting and melt homogenization step, under the inert protective atmosphere, the magnesium fluoride raw material is heated to complete melting and is kept for a certain time to homogenize the melt composition, and a homogenized melt is obtained, in the melting and melt homogenization step, the melting temperature is controlled to be 1300℃, the homogenization time is 6h, and a steady field standing section is arranged before directional crystallization, the steady field standing section is to adjust the heating power to a steady heat field that inhibits strong convection under the condition of maintaining the melt in complete melting and stand for 60min;

[0080] c) first pass crucible descending directional crystallization step, the radial temperature field is controlled in the axial temperature gradient field, so that the homogenized melt forms a solid-liquid interface, and the homogenized melt is gradually directionally crystallized in the first direction by the descending displacement of the graphite crucible relative to the hot zone, and a first ingot is obtained, and the end of the first ingot forms an enriched end;

[0081] d) enrichment end local remelting step, under the condition of not breaking the closed environment in the furnace, only the axial section of the enrichment end of the first ingot is locally heated to make the section remelt to form a length controlled local melt pool, while the main body of the middle section of the first ingot remains solid, in the first pass crucible descending directional crystallization step, the axial temperature gradient is 8℃ / cm, the radial temperature gradient is 4℃ / cm, the graphite crucible descending speed is 0.4mm / h, and by adjusting the heat shield structure, the power ratio of the upper and lower heating zones and the crucible outer insulation structure, the solid-liquid interface is kept advancing monotonously and the formation of dendrites or remelted zone is avoided, in the enrichment end local remelting step, the length of the local remelted section along the axial direction is 20% of the total length of the first ingot, the local remelting is realized by setting a local enhanced heating zone or a ring-shaped compensating heater at the corresponding position of the enrichment end, and the solid-liquid interface of the local melt pool is controlled to only cover the enrichment end section without invading the main body of the middle section, so that the main body of the middle section remains solid;

[0082] e) second pass reverse directional crystallization and middle section material taking step, under the condition of keeping the crucible descending displacement mode, the axial temperature gradient direction is reversed by switching the power ratio of the upper and lower heating zones, so that the local melt pool directional crystallizes in the second direction opposite to step c) to form a second ingot; the second ingot is axially segmented cut to remove the enrichment sections at both ends and keep the middle purification section as high-purity magnesium fluoride raw material, the second pass reverse directional crystallization is realized by switching the power ratio of the upper and lower heating zones and the heat shield opening position while the crucible continues to descend at a speed of 0.2mm / h, so that the cold end and the hot end are transposed in axial position, the solidification direction of the local melt pool is opposite to that of the first pass directional crystallization, and the local melt pool is kept at constant temperature and homogenized for 45min before the reverse directional crystallization starts, the segmented cutting uses the middle purification section criterion to determine the removal boundaries of the enrichment sections at both ends, the middle purification section criterion meets the following conditions: polished test pieces are prepared by sampling along the axial direction of the second ingot and tested for absorption coefficient, internal transmittance or equivalent optical loss in the 193nm wave band, the inflection point of the obvious deterioration of the optical index is taken as the boundary of the enrichment section, and under the condition of meeting the above criterion, the enrichment sections at both ends are removed and the middle purification section is kept, the length of the middle purification section accounts for 50% of the total length of the second ingot, and the kept middle purification section is subjected to steps a) to e) again in the same vacuum furnace.

[0083] Comparative Example 1

[0084] This comparative example provides a magnesium fluoride crucible descending directional crystallization purification method for ultraviolet optical crystal manufacturing, which is different from example 1 in that after the first pass crucible descending directional crystallization is completed to obtain a first ingot, no enrichment end local remelting and second pass reverse directional crystallization is performed, and the first ingot is directly axially segmented cut to remove the end enrichment section and keep the middle section as raw material, and other process parameters and operating conditions are exactly the same as those of example 1.

[0085] Comparative Example 2

[0086] The present comparative example provides a magnesium fluoride crucible descending directional crystallization purification method for ultraviolet optical crystal manufacturing, which is different from Example 1 in that after completing the first-stage crucible descending directional crystallization and obtaining the first ingot, no enrichment end local remelting step is performed, but the axial temperature gradient direction is directly reversed by switching the power ratio of the upper and lower heating zones, and the second-stage reverse directional crystallization is performed to form a second ingot under the condition of maintaining the crucible descending displacement mode, and then the second ingot is axially segmented and cut, the enrichment segments at both ends are removed, and the middle purification segment is retained as a raw material, and other process parameters and operating conditions are exactly the same as in Example 1.

[0087] Comparative Example 3

[0088] The present comparative example provides a magnesium fluoride crucible descending directional crystallization purification method for ultraviolet optical crystal manufacturing, which is different from Example 1 in that when the inert gas is introduced to form a protective atmosphere after the desorption pretreatment is completed, no gas path purification module is arranged on the inert gas supply side, and other process parameters and operating conditions are exactly the same as in Example 1.

[0089] Comparative Example 4

[0090] The present comparative example provides a magnesium fluoride crucible descending directional crystallization purification method for ultraviolet optical crystal manufacturing, which is different from Example 1 in that the graphite crucible has a sealed structure with a cover, but no isomeric magnesium fluoride buffer is arranged in the gas phase space between the crucible cover and the melt surface, and other process parameters and operating conditions are exactly the same as in Example 1.

[0091] Metal impurity total amount test method: sampling at end A, middle segment M, and end B along the axial direction of the ingot; after cutting the block with a clean diamond knife, remove the cutting surface layer and take the internal material as the sample to be tested. The sample is transferred using clean tweezers and clean containers throughout the process to avoid contact with glass, ordinary steel, and sweat, and the metal element content in the sample is determined by GDMS and / or ICP-MS. When ICP-MS is used, high-purity acid system is used for closed digestion / dissolution of the sample, reagent blank and method blank are set, and internal standard correction is performed; when GDMS is used, the sample is processed into a block or wafer that meets the test requirements and is subjected to surface cleaning treatment before direct testing, a set of metal elements (Fe, Ce, Sr, Pb, Na, K, Ba, Sr, Si, V, Co, Cu) is selected to obtain their mass concentrations Ci, the total amount of metal impurities is calculated as ΣM=ΣCi, and the lower limit is treated as a unified standard (counted as half of the detection limit), and expressed in ppm.

[0092] 193 nm absorption test method: sample processing from end A, middle section M, end B into the same thickness of double-sided polished sheet, after polishing, clean with anhydrous organic solvent and dry, avoid fingerprints, particles and scratches on the surface of the sample sheet, seal and store for testing, use ultraviolet / vacuum ultraviolet spectrophotometer to measure the transmittance T(193 nm) of the sample sheet at 193 nm (or the waveband range containing 193 nm). To reduce the influence of thickness difference, convert the absorption coefficient a(193 nm) according to the same thickness d, the conversion method is a(193 nm) = -ln[T(193 nm)] / d; if the internal transmittance (Tint) is used (excluding surface reflection loss), the calculation method used should be noted in the specification and kept consistent for all samples. Perform instrument baseline correction during testing, and repeat the measurement at the same spot position and direction to obtain the average value.

[0093] Haze test method: sample from the middle section M, process into double-sided polished sheet consistent with optical testing, thickness and surface state consistent with other samples, clean and dry according to the unified process before testing, ensure that the sample sheet surface is free of visible particles and scratches. Use a haze meter (or an integrating sphere type transmission and scattering test system) to measure the transmission haze of the sample sheet. When using the integrating sphere method, measure the total transmission and scattered transmission respectively and calculate the haze Haze = (scattered transmission / total transmission) x 100%; when using the haze meter method, measure the haze value according to the standard mode of the instrument. For comparison of material body scattering, the same sample thickness and the same measurement aperture / spot conditions should be used, and the average value of multiple measurements at different positions of the same sample should be taken.

[0094] The test results are shown in Table 1.

[0095] Table 1 Test results of magnesium fluoride crystal of Example 1-Example 4 and Comparative Example 1-Comparative Example 4

[0096] Total amount of metal impurities (end A / middle M / end B, ppm) 193 nm absorption (end A / middle M / end B, cm -1 ])]] Haze (middle M, %) Example 1 2.5 / 0.6 / 5.0 0.018 / 0.006 / 0.038 0.06 Example 2 2.0 / 0.4 / 4.5 0.015 / 0.004 / 0.032 0.05 Example 3 3.0 / 0.8 / 6.0 0.024 / 0.009 / 0.048 0.09 Example 4 1.5 / 0.25 / 3.8 0.012 / 0.003 / 0.028 0.04 Comparative Example 1 1.0 / 2.0 / 15.0 0.010 / 0.020 / 0.080 0.12 Comparative Example 2 2.0 / 1.2 / 8.5 0.020 / 0.012 / 0.060 0.10 Comparative Example 3 3.5 / 1.8 / 10.0 0.030 / 0.018 / 0.070 0.15 Comparative Example 4 2.3 / 0.9 / 8.0 0.025 / 0.012 / 0.055 0.11

[0097] As can be seen from Table 1, compared with Example 1, the total amount of metal impurities of Comparative Example 1 increases, the 193 nm absorption increases, and the haze increases; the total amount of metal impurities of Comparative Example 2 increases, the 193 nm absorption increases, and the haze increases; the total amount of metal impurities of Comparative Example 3 increases, the 193 nm absorption increases, and the haze increases; the total amount of metal impurities of Comparative Example 4 increases, the 193 nm absorption increases, and the haze increases. This is because, in Comparative Example 1, the enrichment end local remelting and reverse directional crystallization are cancelled, and only the first-stage segregation is relied on, the impurities are strongly enriched to the end with solidification, the low platform in the middle section is unstable, the total amount of metal impurities in the middle section is high, the 193 nm absorption and the haze increase. In Comparative Example 2, the local remelting is cancelled, the high impurity area at the enrichment end and the end structure are not cut off and homogenized again, but enter the reverse directional crystallization, the composition of the starting molten pool and the interface state fluctuate, the enrichment at both ends is widened, the purification in the middle section is limited, the total amount of metal impurities increases, the 193 nm absorption increases, and the haze increases. In Comparative Example 3, the gas purification module is cancelled, the inert gas carries water and oxygen and particles into the furnace chamber, forms oxygen / defect precursor containing hydroxyl at high temperature, and introduces Na / K plasma pollution, so that the total amount of metal impurities in the middle section increases, the 193 nm absorption and the haze increase. In Comparative Example 4, the homologous magnesium fluoride buffer is cancelled, the MgF2 volatilization and redeposition partial pressure buffer in the crucible gas phase space is insufficient, the composition near the liquid surface and the volatilization equilibrium fluctuate, recondensation micro-inclusions are easily produced and the interface stability is disturbed.

[0098] The above merely provides a specific implementation of the present application, but the protection scope of the present application is not limited thereto. It should be understood by those skilled in the art that any changes or replacements within the technical scope disclosed by the present application can be easily conceived by those skilled in the art, and all such changes and replacements fall within the protection scope and disclosure scope of the present application.

Claims

1. A method for the purification of magnesium fluoride crucible down directional crystallization for the production of UV optical crystals, characterized in that, The method comprises: a) loading and desorption pretreatment step, 4N grade magnesium fluoride raw material is loaded into graphite crucible and sealed, together with the crucible, into a vacuum furnace with zoned heating, vacuumizing the furnace cavity and gas circuit and baking at elevated temperature; b) melting and melt homogenization step, magnesium fluoride raw material is heated to complete melting under vacuum or inert protective atmosphere, and the melt composition is homogenized to obtain a homogenized melt; c) first pass crucible lowering directional crystallization step, the radial temperature field is controlled in the axial temperature gradient field, the homogenized melt forms a solid-liquid interface, and the homogenized melt is gradually directionally crystallized in a first direction by lowering displacement of the graphite crucible relative to the hot zone to obtain a first ingot, and the end of the first ingot forms an enriched end; d) enriched end local remelting step, only the axial section of the enriched end of the first ingot is locally heated without breaking the closed environment in the furnace, so that the section is remelted to form a length-controlled local melt pool, while the middle section of the first ingot remains solid; e) second pass reverse directional crystallization and middle section material taking step, the axial temperature gradient direction is reversed by switching the power ratio of the upper and lower heating zones while maintaining the crucible lowering displacement mode, so that the local melt pool directionally crystallizes in a second direction opposite to step c) to form a second ingot; the second ingot is axially segmented and cut, the enriched ends are removed, and the middle purified section is retained as high-purity magnesium fluoride raw material.

2. The magnesium fluoride crucible lowering directional crystallization purification method for ultraviolet optical crystal manufacturing according to claim 1, characterized in that: The vacuum furnace limit vacuum degree is 1×10 -3 -1×10 -1 Pa, the vacuumizing and temperature baking are simultaneously carried out, the baking temperature is 350-650℃, the baking time is 2-10h, and the furnace cavity is continuously vacuumized and cooled to below the melting starting temperature after the baking is completed.

3. The magnesium fluoride crucible lowering directional crystallization purification method for ultraviolet optical crystal manufacturing according to claim 1, characterized in that: The vacuum furnace is connected to high-purity inert gas to form a protective atmosphere after the desorption pretreatment is completed, and a gas circuit purification module is arranged on the gas supply side of the inert gas, and the gas circuit purification module at least comprises: i) a particle filtration unit using a metal sintered filter element or a membrane filter with a filtration accuracy of not greater than 0.1 μm; ii) a water removal unit using a molecular sieve drying unit and / or a low-temperature cold trap to reduce the moisture content entering the furnace cavity to a dew point of not higher than -60 ℃; iii) an oxygen removal unit using a metal getter unit and / or a high-temperature deoxidation catalyst unit to reduce the oxygen content entering the furnace cavity to not higher than 1 ppm; iv) a pipeline and valve cleaning unit, the wet end components connected to the furnace cavity are sealed with metal or vacuum grade sealing structure, and vacuum baking or inert gas replacement is performed before each process.

4. The magnesium fluoride crucible lowering directional crystallization purification method for ultraviolet optical crystal manufacturing according to claim 1, characterized in that: In the melting and melt homogenization step, the melting temperature is controlled to be 1281-1381 ℃, the homogenization time is 1-8 h, and a stable field standing section is arranged before directional crystallization, in which the heating power is adjusted to a stable thermal field that suppresses strong convection under the condition of maintaining complete melting of the melt, and the stable field standing section is statically placed for 30-180 min.

5. The method of claim 1, wherein the axial temperature gradient is 3-10 °C / cm and the radial temperature gradient is 2-5 °C / cm in the first-stage crucible lowering directional crystallization step, and the graphite crucible is lowered at a speed of 0.1-0.5 mm / h, and the solid-liquid interface is kept advancing monotonously and dendrites or remelted zone are avoided by adjusting the heat shield structure, the power ratio of the upper and lower heating zones, and the crucible outer insulation structure.

6. The method of claim 1, wherein the length of the local remelted zone along the axial direction is 5%-30% of the total length of the first ingot in the local remelted step, and the local remelt is achieved by setting a local enhanced heating zone or a ring-shaped compensating heater at the corresponding position of the enriched end, and the solid-liquid interface of the local melt pool only covers the enriched end section without invading the main body of the middle section, so that the main body of the middle section remains in solid phase.

7. The method of claim 1, wherein the second-stage reverse directional crystallization is achieved by switching the power ratio of the upper and lower heating zones and the heat shield opening position while the crucible continues to be lowered at a speed of 0.1-0.5 mm / h, so that the cold end and the hot end are transposed in the axial position, the solidification direction of the local melt pool is opposite to that of the first-stage directional crystallization, and the local melt pool is kept at a constant temperature for 10-60 min before the reverse directional crystallization starts.

8. The method of claim 1, wherein the graphite crucible has a sealed structure with a cover, and a same-source magnesium fluoride buffer is arranged in the gas phase space between the crucible cover and the melt surface, the same-source magnesium fluoride buffer being a magnesium fluoride particle bed, a sintered block, or a pressed block, and the same-source magnesium fluoride buffer is not in direct contact with the melt.

9. The method of claim 1, wherein the segmented cutting uses a middle domain purification section criterion to determine the rejection boundaries of the enriched sections at both ends, and the middle domain purification section criterion satisfies at least one of the following: i) sampling every fixed interval along the axial direction of the second ingot and determining the trace metal ion content profile by ICP-MS and / or GDMS, and taking the inflection point where the impurity content rises obviously as the enriched section boundary; ii) sampling along the axial direction of the second ingot to prepare polished test pieces and testing the absorption coefficient, internal transmittance, or equivalent optical loss in the 193 nm waveband, and taking the inflection point where the optical index deteriorates obviously as the enriched section boundary; and under the condition of satisfying the above criterion, the enriched sections at both ends are rejected and the middle purification section is retained, and the length of the middle purification section accounts for 30%-80% of the total length of the second ingot.

10. The method of claim 1, wherein ​ ​ ​ ​ ​ The retained intermediate purification section performs again steps a) to e) of claim 1 at least once in the same vacuum furnace.

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