Method for purifying and obtaining calcium fluoride crystals

Abstract

FIELD: inorganic chemistry.SUBSTANCE: invention can be used in the manufacture of transmission and focusing optics for the ultraviolet range, as well as excimer lasers. Purification and production of calcium fluoride crystals is carried out by means of multi-pass zone melting of raw materials in the form of calcium fluoride powder of pure grade. The speed of movement of the zone is no more than 2.3 mm / h with a number of passes that ensures an increase in the transmission of ultraviolet radiation by the material in the wavelength range of 0.2-0.3 mcm compared to a single pass.EFFECT: growing optical quality calcium fluoride crystals from readily available raw materials while simultaneously purifying the material during crystal growth.1 cl, 2 dwg, 3 ex

Description

[0001] The invention relates to the field of purification and growth of crystals of inorganic compounds.

[0002] Calcium fluoride crystals are used as a material for optics in the ultraviolet, visible, and near-infrared ranges, due to both the wide transparency range of CaF2 and its low refractive index, which ensures low reflection losses. Interest in CaF2 crystals for transmission and focusing optics in the ultraviolet range has grown significantly. This is due to the development of excimer lasers generating radiation in the 0.2-0.3 μm wavelength range. The use of CaF2 in such lasers requires additional purification of the material, as this range lies close to the fundamental absorption edge of calcium fluoride, and impurity blurring of the edge significantly degrades the crystal's performance.

[0003] A known method for growing calcium fluoride crystals by vertical directional melt crystallization is technically similar to the Bridgman method [Guo Zonghai. Preparation method for large-sized square calcium fluoride crystals. Patent CN 104294362 A]. This method has an analog in which pre-purified CaF2 is used as the starting material. The main disadvantages of this method are the need for preliminary purification of the raw material and the impossibility of further purification of the raw material directly during the growth process.

[0004] A known method for producing calcium fluoride crystals by horizontal zone melting [H. Guggenheim. Growth of single-crystal calcium fluoride with rare-earth impurities. Journal of Applied Physics, 1961, v. 32, N 7, pp. 1337-1338] is an analogue in which pre-purified CaF2 is used as the raw material. This method allows for the growth of optical-quality crystals. The main disadvantages of this method are the need for preliminary purification of the raw material and the impossibility of further purification of the raw material directly during the growth process, which is due, in particular, to the very high growth rate of 1-12 inches per hour (25.4-304.8 mm / hour).

[0005] The closest in technical essence to the proposed method is the method for producing calcium fluoride by multi-pass zone melting [Sh. A. Khalimov, D. D. Ikrami, A. S. Paramzin, M. B. Ikrami. Method for producing crystalline calcium fluoride. USSR Author's Certificate SU 1798394 A1] - prototype. The method includes zone melting in a vacuum at a crucible pulling speed of 8-80 mm / hour through at least two melting zones. The description of the invention also contains information on the use of a speed of 4 mm / hour and an increase in the number of melting zones to 6, which, according to the authors, does not lead to further positive effect. The method allows for growing crystals suitable for producing calcium fluoride films. The method also allows for purification from a number of impurities (the description provides data on the content of 9 impurities in the crystals). The main disadvantage of the prototype method is the impossibility of obtaining optical quality CaF2 crystals.The description of the prototype invention lacks information on the optical properties of the crystals. However, data is provided on the concentration of impurities affecting the transmission and absorption spectra of CaF2, in particular, the content of impurities that can produce broad intracenter absorption bands in the crystals (Fe, Ni, Cu, Cr, Co, Mn). For Fe and Mn impurities, residual concentrations in the crystals are indicated at a level of ≈10. -4 % (mass.), which indirectly indicates the impossibility of using crystals in modern optics.

[0006] The technical result of this invention is the production of optical quality calcium fluoride crystals with the purification of raw materials directly during the growth process.

[0007] This technical result is achieved in the proposed method due to the fact that crystals are grown by multi-pass zone melting with a zone movement speed of no more than 2.3 mm / hour, and the required number of zone passes is determined by achieving the specified optical characteristics of the crystals.

[0008] The proposed method was used to obtain calcium fluoride crystals that have high transmittance of ultraviolet radiation in the wavelength range of 0.2-0.3 μm, which is required for the optics of excimer lasers.

[0009] Technically, the method can be implemented in a vertical zone melting installation equipped with heat shields made of carbon-graphite materials, a resistive graphite heater and a graphite crucible.

[0010] An experimental verification of the method's performance was conducted using calcium fluoride of "Ch" grade (CaF298% content) as raw material. Crystals were grown in MG-osch-7-3 graphite crucibles.

[0011] The photograph in Fig. 1 shows polished wafers cut perpendicular to the growth axis of CaF2 crystals, where 2 is a wafer made of a crystal grown by two-pass zone melting, and 3 is a wafer made of a crystal grown by five zone passes. Visually, they are similar, but the transmission spectra of these wafers are significantly different. Fig. 2 shows the transmission spectra of polished wafers with a thickness of 3 mm, where 1 is a sample made of a crystal grown by single-pass zone melting, 2 is a sample made of a crystal grown by two zone passes (sample 2 in Fig. 1), and 3 is a sample made of a crystal grown by five zone passes (sample 3 in Fig. 1). It can be seen that in the wavelength range of 0.2 – 0.3 µm, the ultraviolet radiation transmission of sample 2 is higher than that of sample 1, and the radiation transmission of sample 3 is significantly higher than that of sample 2.This means that the content of impurities causing impurity broadening of the fundamental absorption edge decreases significantly with an increase in the number of zone passes, which confirms the purification of the material directly during the growth process.

[0012] The number of zone passes to achieve the required optical characteristics is determined experimentally, based on the purity qualification of the raw materials and the requirements for the characteristics of the crystals for specific applications.

[0013] The maximum permissible value of the zone movement speed of 2.3 mm / hour was determined experimentally: at speeds above 2.3 mm / hour, the optical quality of the crystals decreases, which, in particular, is expressed in a drop in the transmission of ultraviolet radiation.

[0014] Thus, the proposed method allows for the growth of optical quality calcium fluoride crystals while simultaneously purifying the material during the growth process.

[0015] Example 1

[0016] The raw material, in the form of calcium fluoride powder of grade "Ch," is placed in a crucible made of MG-osch-7-3 graphite. The crucible is mounted on the growth rod so that it is above the heater. The MG-osch-7-3 graphite heater forms a heating zone 20 mm high so that the temperature in the zone is 1430 о C. The crucible is lowered vertically through the heater at a rate of 2 mm / hour. Once the process is complete, the crucible is cooled and the crystal is removed. In Fig. 2, the transmission spectrum of this sample is indicated by the number 1.

[0017] Example 2

[0018] The raw material, in the form of calcium fluoride powder of grade "Ch," is placed in a crucible made of MG-osch-7-3 graphite. The crucible is mounted on the growth rod so that it is above the heater. The MG-osch-7-3 graphite heater forms a heating zone 20 mm high so that the temperature in the zone is 1430 оC. The crucible is lowered vertically through the heater at a rate of 2.3 mm / hour. Upon completion of the process, the crucible is raised to its original position and the zone melting is repeated. After the crucible passes through the heater a second time, it is cooled and the crystal is removed. In Fig. 1, a sample of this crystal is designated by the number 2. In Fig. 2, the transmission spectrum of this sample is designated by the number 2.

[0019] Example 3

[0020] The raw material, in the form of calcium fluoride powder of grade "Ch," is placed in a crucible made of MG-osch-7-3 graphite. The crucible is mounted on the growth rod so that it is above the heater. The MG-osch-7-3 graphite heater forms a heating zone 20 mm high so that the temperature in the zone is 1430 оC. The crucible is lowered vertically through the heater at a rate of 1.8 mm / hour. Upon completion of the process, the crucible is raised to its original position and the zone melting is repeated, and then the process is repeated three more times. After the fifth pass of the crucible through the heater, the crucible is cooled and the crystal is removed. In Fig. 1, a sample of this crystal is marked with the number 3. In Fig. 2, the transmission spectrum of this sample is marked with the number 3.

Claims

A method for purifying and producing calcium fluoride crystals using multi-pass zone melting, characterized in that the process is carried out at a zone movement speed of no more than 2.3 mm / h with a number of passes that ensures an increase in the transmission of ultraviolet radiation in the wavelength range of 0.2-0.3 μm compared to a single pass.

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