RU2471896C1 - Method of growing bulk monocrystals of alexandrite - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method involves dissolving the starting mixture, homogenisation thereof, placing a revolving single-crystal seed into the solution and growing a crystal, wherein the starting mixture contains 40 wt % beryllium aluminate with a chromium oxide additive in amount of 1-60 wt % of a solvent which consists of 95-98 wt % lead oxide and 2-5 wt % boron oxide, and the crystal is grown at temperature of 1250°C, axial temperature gradient of 2-20°C, drawing rate of up to 5 mm/day and rotational speed of up to 10 rpm.

EFFECT: obtaining bulk monocrystals of alexandrite of optical quality with low dislocation density.

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Description

The invention relates to methods for producing crystals, and in particular to a method for producing bulk single crystals of alexandrite, and can be used to produce high-quality raw materials for the manufacture of optical elements of laser systems.

A known method of growing bulk single crystals of chrysoberyl and its varieties, including melting the initial charge from beryllium and aluminum oxides and growing the crystal at a constant temperature close to the phase transition temperature (1853 ° C), using a charge with a stoichiometric ratio to BeO Al ₂ O ₃ composition, and the cultivation of α-crystal modification produced by the method of horizontal directional solidification (Gomov VV, Tsvetkov EG "The high-temperature polymorphic transition Chrysoberyl in "Inorganic Materials, 1998, t.34, №7, str.864-866).

The use of a batch of stoichiometric composition with respect to BeO · Al ₂ O ₃ allows one to grow bulk chrysoberyl single crystals with a sufficiently high linear crystallization rate.

However, according to this method, it is very difficult to grow a bulk single crystal, which is explained by the following: equilibrium crystallization of a stoichiometric melt begins with the formation at 1870 ° C of a high-temperature modification with a density of about 2.7 g / cm ³ , and when the crystal temperature decreases to 1853 ° C, a high-temperature transition occurs β-modification of the crystal into a low-temperature α-modification having an orthorhombic structure and a density of 3.7 g / cm ³ , which leads to cracking of the crystal and the formation of a fine crystalline aggregate.

A known method of growing bulk single crystals of chrysoberyl and its varieties described in the patent of the Russian Federation for invention No. 2315134 "Method for growing bulk single crystals of chrysoberyl and its varieties", claimed 2006.06.05, published 2008.01.20 and selected as a prototype, including the dissolution of the original charge, its homogenization, introduction into the solution of a rotating single-crystal seed and crystal growth.

This method provides for the growth of bulk single crystals of chrysoberyl and its varieties under conditions of guaranteed seeding of a single bulk single crystal of a low-temperature phase and its subsequent growth in the stability region of this phase with a high mass growth rate.

This method of growing bulk single crystals of chrysoberyl and its varieties has the following disadvantages:

- to completely eliminate the spontaneous transition of the low-temperature phase to the high-temperature phase during the growth process, it is necessary to use high-precision expensive equipment for temperature control due to the proximity of the process temperature to the phase transition temperature;

- due to the high operating temperature, the process can only be carried out in an inert gas or vacuum, and it is impossible to use this technique to obtain crystals in air, which leads to the complexity of the growth furnace design, increasing its cost and complicating maintenance.

This is due to the fact that the heat released during crystallization of the crystal heats the melt layers near the crystallization front, which will create conditions for the transition of the unstable low-temperature phase to a stable high-temperature one at this temperature.

The claimed invention provides a solution to the technical problem of growing bulk single crystals of alexandrite of optical quality with guaranteed seeding of a single bulk single crystal of its low-temperature phase.

The claimed technical solution is based on the first established relationship between the composition of the solution and the mode of growing a bulk single crystal of alexandrite at a controlled temperature drop.

Such a dependence and the achieved technical result could not be predicted in advance based on the analysis of dependencies known from the prior art. Specific values of quantitative traits also do not follow from known knowledge and were first experimentally established by the authors.

The initial mixture contains the main components of BeO, Al ₂ O ₃ , as well as a solvent. The composition of the solvent is the essence of the invention.

A method of growing bulk single crystals of alexandrite is to prepare the original mixture. In this case, beryllium aluminate with the addition of chromium oxide (up to 1 wt.%), In an amount of 40 wt.%, Is placed in the crucible. 60 wt.% Solvent is also introduced into the crucible - a mixture of lead oxides (95-98 wt.%) And boron (2-5 wt.%). After that, the solution is heated to a temperature of 1250 ° C, kept at this temperature for 2-4 days, the seeds are lowered and the growing process is carried out at an axial temperature gradient of 2 to 20 ° C, drawing speed up to 5 mm / day, rotation speed up to 10 rpm sec With a smaller axial gradient, it is impossible to achieve high-quality growth due to the emergence of zones of autonomous growth and the cellular structure of growth.

A drawing speed of 5 mm / day is optimal, because at a higher speed, the feed solution and gas inclusions are captured, and at a lower rate, the mass growth rate of bulk single crystals decreases.

A rotational speed of 10 rpm is optimal, i.e. provides quality growth. Exceeding this value leads to a loss in the stability of the shape of the transverse crystal, while at a lower frequency there is a possibility of capture of inclusions of the feed solution due to insufficient mixing of the solution near the crystallization front.

The temperature is kept constant, and the crystallization process is due to the constant mass transfer of the alexandrite mixture from the hot bottom of the crucible to the crystallization zone - the upper part, where the crystal grows. It was found that the introduction of 2-5 wt.% Boron oxide in the form of boric acid into the solution accelerates the dissolution of alexandrite components, creates optimal hydrodynamic flows in the crucible, which contributes to the removal of dissolved gases released during crystallization and improves the quality of crystals, and boron oxide itself prevents the evaporation of lead oxide - the main component of the charge).

When conducting a search on the sources of patent and scientific and technical information, no solutions were found containing the totality of the proposed features, which allows us to conclude that the proposed method meets the criteria of "novelty" and "inventive step".

Example 1. 100 g of the original mixture, consisting of 40% alexandrite and 60 wt.% Solvent is loaded into a platinum crucible and kept at a temperature of 1250 ° C for 2-4 days to saturate the solution. The composition of alexandrite BeAl ₂ O ₄ : Cr - 25 wt.% BeO, 74.9 wt.% Al ₂ O ₃ and 0.1 wt.% Cr ₂ O ₃ . The composition of the solvent is 98 wt.% PbO and 2% B ₂ O ₃ . After that, a single-crystal seed, which has dimensions 15 × 2 × 2 mm with an angle of inclination of the tip of 45 °, is introduced into the surface of the solution at a temperature of 1250 ° C and a rotation frequency of 5 rpm. Subsequently, the speed of rotation of the seed does not change and the crystal is pulled at a speed of 3 mm / day. For three days, the crystal grows to a cross section of 8 × 10 mm and after 40 days, the crystal is separated from the surface of the solution, cooled at a rate of up to 300 ° C / hour. The resulting crystal is oriented along the [001] axis, without taking into account the growth cone, has a length of 120 mm (cross section 8 × 10 mm) and is suitable for the manufacture of a laser rod.

Example 2. 100 g of the original mixture with a composition of 25 wt.% BeO, 74.9 wt.% Al ₂ O ₃ and 0.1 wt.% Cr ₂ O ₃ and 60 wt.% Solvent - 98 wt.% PbO and 2 % B ₂ O _{3 is} loaded into a platinum crucible and kept at a temperature of 1250 ° C for 2-4 days to saturate the solution. Then a single-crystal seed, which has dimensions of 15 × 2 × 2 mm with an angle of inclination of the tip of 45 °, is introduced into the solution at a temperature of 1250 ° C and a rotation speed of 5 rpm. In the future, the speed of rotation of the seed does not change and the crystal is not stretched. In 30 days, the crystal reaches 70% of the crucible area. The crystal is separated from the remainder of the solution, cooled at a rate of up to 300 ° C / hour. The resulting crystal is oriented along the [001] axis, has a diameter of up to 40 mm, a length of up to 20 mm, and is suitable for the manufacture of laser elements.

The experiments described in the examples were carried out on a resistive growth furnace with a ceramic thermal unit in air from a platinum crucible with a controlled temperature gradient, which is achieved due to additional heating elements located in close proximity to the top and bottom of the crucible. A control thermocouple was placed directly below the crucible.

The crystals obtained in the examples have optical quality, low dislocation density, and are suitable for the manufacture of active elements of laser systems. All grown crystals have faceted growth forms, the crystallization front is flat in all examples.

Claims (1)

A method of growing bulk alexandrite single crystals, including dissolving the initial charge, homogenizing it, introducing a rotating single-crystal seed into the solution and growing the crystal, characterized in that the initial charge is used, which contains 40 wt.% Beryllium aluminate with the addition of chromium oxide up to 1 wt.%, 60 wt.% Solvent - a mixture of lead oxides 95-98 wt.% And boron 2-5 wt.%, And the cultivation is carried out at a temperature of 1250 ° C, with an axial temperature gradient of 2 to 20 ° C, drawing speed up to 5 mm / day, rotational speed up to 10 r / s.