Preparation method of high-purity calcium fluoride

Abstract

The invention relates to a preparation method of high-purity calcium fluoride. The preparation method comprises the following steps: 1) heat treatment: carrying out first-stage heat treatment on a calcium fluoride raw material under inert gas protection or a vacuum environment; 2) deep deoxidation: introducing fluorine-containing gas into the calcium fluoride material obtained in the step 1) to carry out a second-stage thermal reaction; and 3) cooling and post-processing: stopping heating after the reaction is finished, and cooling the material to room temperature under the protection of inert gas atmosphere to obtain the product. According to the method, the strong oxidizing property of the fluorine-containing gas and the synergistic effect under high temperature and high pressure are utilized, oxygen impurities in calcium fluoride can be deeply removed, the oxygen content of the finally obtained high-purity calcium fluoride product is reduced to 18 ppm or below from about 356 ppm, and the requirement of the high-end optical field can be completely met.

Description

Technical Field

[0001] This invention belongs to the field of inorganic material preparation technology, and specifically relates to a method for preparing high-purity calcium fluoride. Background Technology

[0002] High-purity calcium fluoride crystals, due to their excellent light transmittance and extremely high laser damage threshold across a very broad spectral range from infrared to ultraviolet, have become a key material for precision optical systems such as high-end lithography machine lenses and high-energy laser windows. However, oxygen impurities in the crystals (usually in the form of oxides, hydroxides, or O replacing F) can cause problems. 2- The presence of oxygen impurities (in their physical form) will severely degrade their optical performance. Especially under high-frequency irradiation such as X-rays and extreme ultraviolet rays, oxygen impurities will become color centers, leading to a significant increase in the light absorption coefficient, triggering the thermal lensing effect, reducing laser transmission efficiency, and ultimately causing permanent damage to optical components.

[0003] Currently, high-purity calcium fluoride is typically prepared using chemical precipitation or ammonium bifluoride fluorination, but these methods struggle to completely remove oxygen in various forms. Conventional high-temperature vacuum treatment can remove some adsorbed water and hydroxyl groups, but its effectiveness is limited in removing oxygen or stable metal oxide impurities within the crystal lattice.

[0004] Therefore, developing a calcium fluoride preparation process that can achieve deep and efficient deoxygenation is of great significance for improving the performance and reliability of high-end optical components in my country and breaking the international technological monopoly. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a simple and highly efficient method for producing high-purity calcium fluoride. This method utilizes the strong oxidizing properties of fluorine-containing gases and the synergistic effect under high temperature and pressure to deeply remove oxygen impurities from calcium fluoride, reducing the oxygen content of the final high-purity calcium fluoride product from approximately 356 ppm to below 18 ppm, fully meeting the requirements of high-end optical applications.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing high-purity calcium fluoride, comprising the following steps: 1) Heat treatment: Place the calcium fluoride raw material in a reactor and carry out the first stage of heat treatment under inert gas protection or vacuum environment; 2) Deep deoxidation: Fluorine-containing gas is introduced into the calcium fluoride material obtained in step 1) to carry out the second stage of thermal reaction; 3) Cooling and post-processing: After the reaction is complete, stop heating and cool the material to room temperature under an inert gas atmosphere to obtain high-purity calcium fluoride product.

[0007] Specifically, in step 1), the heat treatment temperature is 200℃~500℃, and the holding time is 2~5 hours, in order to remove most of the physically adsorbed water and some of the chemically bound water.

[0008] Specifically, in step 2), the reaction temperature is 500℃~800℃, the reaction pressure is 0.1~0.5 MPa, and the reaction time is 1~5 hours.

[0009] Furthermore, in step 1), the inert gas can be high-purity nitrogen or high-purity argon with a purity ≥ 99.999%; the vacuum degree of the vacuum environment is no greater than 5 × 10⁻⁶. -2 Pa.

[0010] Furthermore, in step 1), the calcium fluoride raw material is preferably calcium fluoride powder synthesized by chemical precipitation or wet process, and its purity is not less than 99.9%.

[0011] Specifically, in step 2), the fluorine-containing gas is a mixture of nitrogen trifluoride and carbon tetrafluoride, wherein the volume fraction of nitrogen trifluoride is 10% to 50% and the remainder is carbon tetrafluoride; or it is a mixture of fluorine and nitrogen, wherein the volume fraction of fluorine is 5% to 20% and the remainder is nitrogen.

[0012] More preferably, in step 2), the flow rate of the fluorine-containing gas can be 100-500 mL / min.

[0013] Specifically, in step 3), the inert gas can be high-purity nitrogen or high-purity argon with a purity of ≥99.999%.

[0014] In the method of this invention, step 1) effectively removes volatile oxygen-containing impurities, such as H2O, from the raw materials through heat treatment, creating the basic conditions for subsequent deep deoxygenation. Step 2) Under high temperature and high pressure conditions, F2 and NF3, as strong fluorinating agents and oxidizing agents, can efficiently react with stubborn oxygen-containing impurities (such as CaO, Ca(OH)2, etc.) as follows; while CF4 and N2, as dilution and protective gases, can optimize the reaction atmosphere and prevent local overheating; at the same time, CF4 can also provide a certain fluorine source under high temperature and high pressure, and produce a synergistic effect with NF3, promoting the complete deoxygenation reaction.

[0015] 6CaO + 4NF3 → 6CaF2 + 2N2↑+ 3O2↑ 2CaO + 2F2 → 2CaF2 + O2↑ Compared with the prior art, the advantages and beneficial effects of the method of the present invention are as follows: This invention utilizes the strong oxidizing properties of fluorine-containing gases and their synergistic effect under high temperature and pressure to deeply remove oxygen impurities from calcium fluoride. This reduces the oxygen content of the final high-purity calcium fluoride product from approximately 356 ppm to below 18 ppm, fully meeting the requirements of high-end optical applications. Furthermore, the process flow of this invention is simple, highly controllable, and suitable for large-scale industrial production. Detailed Implementation

[0016] The technical solution of the present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0017] In the following embodiments, all raw materials used are common commercially available products that can be purchased directly, or can be obtained using conventional techniques in the art. For example, the calcium fluoride raw material used is preferably calcium fluoride powder synthesized by chemical precipitation or wet process, with a purity of not less than 99.9%. The inert gas is high-purity nitrogen or high-purity argon with a purity ≥ 99.999%.

[0018] Room temperature refers to 25±5℃.

[0019] Example 1 A method for preparing high-purity calcium fluoride, comprising the following steps: 1) Take 100g of calcium fluoride raw material with a purity of 99.92% (oxygen content of 356ppm), place it in a vacuum sintering furnace, and evacuate it to 5×10 -2 Pa, high-purity nitrogen gas is introduced as a protective gas, the temperature is raised to 500℃ and held for 2 hours for the first stage of heat treatment.

[0020] 2) After the heat preservation is completed, a mixed gas consisting of 10% NF3 and 90% CF4 (volume fraction) is introduced into the furnace, and the gas flow rate is controlled at 500 mL / min, while the system pressure is maintained at 0.5 MPa. Then the temperature is raised to 800℃, and the reaction is carried out at this temperature for 1 hour to carry out the second stage of thermal reaction for deep deoxygenation.

[0021] 3) After the reaction is complete, stop heating and cool to room temperature under continuous high-purity argon gas. Take out the sample and test it. The oxygen content in the high-purity calcium fluoride product is 18 ppm.

[0022] Example 2 A method for preparing high-purity calcium fluoride, comprising the following steps: 1) Take 200g of calcium fluoride powder with a purity of 99.93% (oxygen content of 356ppm), place it in a tube furnace, and evacuate it to a vacuum of 1×10⁻⁶. -2 Pa, high-purity argon gas is introduced as a protective gas, the temperature is raised to 400℃ and held for 3.5 hours.

[0023] 2) After the heat preservation is completed, a mixed gas consisting of 10% F2 and 90% N2 (volume fraction) is introduced into the furnace at a flow rate of 100 mL / min and a pressure of 0.1 MPa. Then the temperature is raised to 500℃ and the reaction is carried out for 5 hours to carry out the second stage of thermal reaction for deep deoxygenation.

[0024] 3) After the reaction is complete, stop heating and allow the mixture to cool naturally to room temperature under the protection of high-purity argon gas. Remove the sample; testing shows that the oxygen content in the high-purity calcium fluoride product is 16 ppm.

[0025] Example 3 A method for preparing high-purity calcium fluoride, comprising the following steps: 1) Take 150g of calcium fluoride raw material with a purity of 99.95% (oxygen content of 356ppm), place it in a vacuum sintering furnace, and evacuate it to a vacuum of 2×10⁻⁶. -2 Pa, high-purity argon gas is introduced as a protective gas, the temperature is raised to 200℃ and held for 5 hours for the first stage of heat treatment.

[0026] 2) After the heat preservation is completed, a mixed gas consisting of 50% NF3 and 50% CF4 (volume fraction) is introduced into the furnace, and the gas flow rate is controlled at 200 mL / min, while the system pressure is maintained at 0.3 MPa. Then the temperature is raised to 600℃, and the reaction is carried out for 3.5 hours to carry out the second stage of thermal reaction for deep deoxygenation.

[0027] 3) After the reaction is complete, stop heating and cool to room temperature under continuous high-purity argon gas. Take out the sample and test it. The oxygen content in the high-purity calcium fluoride product is 11 ppm.

[0028] Comparative Example 1 100g of calcium fluoride raw material with a purity of 99.92% (oxygen content of 356ppm) from the same batch was placed in a vacuum sintering furnace and evacuated to a vacuum level of 5×10⁻⁶. -2 The sample was treated with high-purity nitrogen as a protective gas, heated to 500℃ and held at that temperature for 5 hours, without undergoing NF3 / CF4 mixed gas treatment. After treatment, the oxygen content was measured to be 132 ppm.

[0029] In summary, the method of this invention utilizes the strong oxidizing properties of fluorine-containing gas and the synergistic effect under high temperature and high pressure to deeply remove oxygen impurities from calcium fluoride, reducing the oxygen content of the final high-purity calcium fluoride product from about 356 ppm to below 18 ppm, which can fully meet the needs of high-end optical fields.

Claims

1. A method for producing high-purity calcium fluoride, characterized by, The method comprises the following steps: 1) heat treatment: the calcium fluoride raw material is subjected to first-stage heat treatment under inert gas protection or vacuum environment; 2) deep deoxidization: the calcium fluoride material obtained in step 1) is subjected to second-stage heat reaction by passing fluorine-containing gas; 3) cooling and post-treatment: after the reaction is completed, the heating is stopped, and the material is cooled to room temperature under the protection of inert gas atmosphere.

2. The method for preparing high-purity calcium fluoride as described in claim 1, characterized in that, In step 1), the heat treatment temperature is 200-500 DEG C, and the holding time is 2-5 hours.

3. The method of claim 1, wherein the high purity calcium fluoride is prepared by the steps of: In step 2), the reaction temperature is 500-800 DEG C, the reaction pressure is 0.1-0.5 MPa, and the reaction time is 1-5 hours. ​ 4. The method for preparing high-purity calcium fluoride as described in claim 1, characterized in that, In step 1), the inert gas is high-purity nitrogen or high-purity argon with purity ≥ 99.999%; the vacuum degree of the vacuum environment is not greater than 5 x 10 -2 Pa.

5. The method for preparing high-purity calcium fluoride as described in claim 1, characterized in that, In step 1), the calcium fluoride raw material is calcium fluoride powder synthesized by chemical precipitation method or wet method, and the purity is not less than 99.9%.

6. The method for preparing high-purity calcium fluoride as described in claim 1, characterized in that, In step 2), the fluorine-containing gas is a mixed gas composed of nitrogen trifluoride and carbon tetrafluoride, wherein the volume fraction of nitrogen trifluoride is 10-50%, and the rest is carbon tetrafluoride; or a mixed gas composed of fluorine gas and nitrogen gas, wherein the volume fraction of fluorine gas is 5-20%, and the rest is nitrogen.

7. The method for preparing high-purity calcium fluoride as described in claim 6, characterized in that, In step 2), the flow rate of the fluorine-containing gas is 100-500 mL / min.

8. The method for preparing high-purity calcium fluoride as described in claim 1, characterized in that, In step 3), the inert gas is high-purity nitrogen gas or high-purity argon gas with a purity of not less than 99.999%.