CS253137B1 - Method of high-melting oxides' monocrystals cultivation and equipment for its realization - Google Patents

Abstract

A method of growing high-crystal monocrystals oxides in high quality, eliminating adverse effects of impurities the protective atmosphere where the target is achieved by having a closed protective atmosphere It is in its composition during the course growing process optimized by forced controlled bypass system of control elements and devices consisting of it is connected from a closed cultivation oven the recirculation circuit in which it is included gas pump, hydrogen and humidity sensors, cleaning elements and other humidity sensor a mechanical microfilter of dust particles.

Description

The present invention relates to a process for the cultivation of high-melting monocrystals and mixed oxides in a protective atmosphere and to an apparatus for carrying out the process.

The cultivation of monocrystals of high-melting oxides and mixed oxides, in particular yttrium aluminum, rare earths and possibly other congruent melting oxides, is most often industrially carried out by embryo-drawing for the most demanding applications. The method of drawing through a forming member, referred to as the EGF or the Stepan method, also achieves industrial expansion.

The chemical properties of these oxides indicate the suitability of cultivation in an oxidizing culture atmosphere, but structural and economic considerations guide the selection of crucible material for molybdenum and tungsten metals, which, however, must be protected from oxidation by a perfectly inert or slightly reducing atmosphere at high temperatures. argon or argon with hydrogen addition. Otherwise, known reactions such as:

Mo + r> _2i ---------- mo0 ₂ (2)

MoO ₂ + 0 ₂ -> MoO ₃

The reaction of reaction (1) is reversed already at 2000 ° C, so that part of the MoO 2 formed decomposes immediately and the released molybdenum contaminates the melt. Likewise, the melt entrapped MoO 2 is reduced in the melt to molybdenum metal, for example, by the aluminum suboxides present in the melt due to thermal decomposition of the superheated melt Al ₂ O ₃ or as a result of the interaction

Al ₂ O ₃ + 2 H ₂

800 ° C 1500 ° C

Al ₂ + 2 H ₂ 0 (3) in a growing atmosphere, resulting in

H ₂ (4)

H ₂ (5)

Mo (6)

The presence of gaseous carbon compounds in the cultivation atmosphere, such as ^C n ^H 2n ^{of C} n ^H 2n + 2 'CO _f leads to the absorption of the gaseous phase in the melt and consequently in reactions to form carbides and oxycarbides, such as the Al ^ O ^ C, Al-OC , Al ^C ^ and others.

In the final consequences of the presence of impurities in the melt culture atmosphere, in addition to argon, there may be: nitrogen, carbon monoxide, carbon dioxide, molybdenum dioxide, molybdenum oxide, oxygen, aluminum, aluminum suboxide, molybdenum, tungsten, aluminum carbides and oxycarbides and others. Their adverse effect on crystal growth is also experimentally evident; the process of concentration hypothermia occurs, trapping of excreted bubbles of gaseous substances and molybdenum microparticles into the microstructure of the crystal and thus significantly deteriorates its optical quality.

In addition, in the cultivation of high-melting monocrystals by the Stepan method, at the crystallization temperature, solid particles are deposited in the capillary dispensing system as they progress to the functional surface of the forming member and thereby gradually ingrown and lose function. Some of the fumes of the above reactions decompose and deposit on the colder surface of the drawn single crystal, thereby worsening heat exchange with the environment and reducing the temperature gradient at the phase interface, which in turn adversely affects the single crystal growth course and at least worsens the economics of cultivation.

This implies considerable importance of the purity of the culture atmosphere and the need to remove its undesirable components, namely traces of moisture. This is done by known gas purifiers before the protective atmosphere enters the furnace, but their function can never be perfectly perfect. The growing atmosphere of the budto furnaces passes through and is discharged through a siphon trap into the

Similarly, the presence of water vapor shows the same consequences:

Al 2 O + 2 HoO.

MoO ^ +

Al-O, + air (flowing atmosphere), or the growing atmosphere is infused into the evacuated furnace, closed in the furnace and possibly still dried directly in the kiln of the inserted drying agent (Closed atmosphere). The first way, i. the use of a flowing atmosphere, despite its economic complexity, does not produce perfect results, nor is the second, ie closed, atmosphere free of disadvantages.

The clean-up of the culture atmosphere prior to the growing process and the prevention of any contamination from the environment should not result in the above-mentioned adverse reactions being maintained. Maintaining such ideal conditions is in practice very difficult and almost impossible. The penetration of the welds and the walls of the furnace chamber continuously introduces moisture into the system, which the surface of the inserted desiccant, usually phosphorus pentoxide, does not retain after a short time, a crust is formed on the surface and the surface is blocked. Additional moisture is brought to the furnace on its surface by the raw material, both at the beginning of the cultivation and, increasingly, by continuous dosing. Gradually, moisture retained by capillary condensation in surface pores is also released from the furnace construction materials. In the case of the application of graphite components, another of the contaminants mentioned is gradually formed. The result is a significantly deteriorated and unusable cultivation result for highly demanding applications.

The deficiencies of the prior art are virtually eliminated by the method of growing high-melting monocrystals in a closed protective atmosphere, which is characterized in that the composition of the atmosphere is optimized during the growing process by forced controlled bypass of this atmosphere with an embedded set of control elements and devices for carrying out the method. A recirculating circuit is connected to a closed culture furnace which includes a gas pump, flow meter, humidity and hydrogen sensors, cleaning elements and other humidity sensor, and a mechanical microfilter of dust particles.

The device according to the invention is described in more detail below with reference to the attached schematic drawing. Detailed explanation of the method follows from the following examples.

In the attached drawing, a furnace chamber 2, crucible 2 drawn by a single crystal drawing rod 5, a vacuum pumping opening 5, an inlet 6 and an outlet 10 of a protective atmosphere, an inlet and outlet j of a recirculation circuit 10, in which a gas pump 11 is arranged. flowmeter 12, humidity sensor 22, hydrogen sensor 14, cleaning elements 15, 16 and 17 for capturing unwanted hydrocarbon content, reacting contaminating oxygen with hydrogen to water, capturing water and penetrating moisture, and plotted further humidity sensor 28 and mechanical microfilter 19 of dust particles.

By controlling the gas flow rate and its combination with the selected charge in the elements 15, Af. and 17, it is possible to more or less deepen, regulate the cleaning and, in particular, the drying effect of the recirculation circuit, thereby adjusting - optimizing the hydrogen-moisture ratio in the culture atmosphere. Activated charcoal, oxygen reduction with hydrogen on a palladium catalyst on an alumina support, and drying silica gel, molecular sieves, natural modified zeolites, phosphorus pentoxide, and the like can be advantageously used to capture hydrocarbons.

Example 1

The molybdenum crucible, filled with purified and remelted yttrium-aluminum garnet raw material Y ^ Alj-O-Ndd, was placed in a 130 L culture furnace chamber and after evacuation to 5 Pa was filled with purified 99.99% argon with a dew point at 20 ^U C corresponding to -40 ° C and having a hydrogen content of 4% vol. A vibration diaphragm pump with a maximum power of 4 l / min was first set to maximum power. The cleaning elements were pre-filled with activated charcoal 0.5 L with a palladium catalyst 0.25 L and a molecular sieve A5 0.5 L. The atmosphere was re-circulated, the dew point of the gas first began to rise to -5 ° C as moisture in the furnace was gradually released. The furnace temperature was gradually adjusted to a cultivation temperature of 1,950 ° C and cultivation was started. At the same time, after 10 hours from the start of the recirculation until the end of the cultivation process, the flow rate was maintained at 2.5 l / min and the argon atmosphere parameters were thus maintained at dew point (20 ° C) = -50 ° C, hydrogen content of 3% by volume.

The cultivated single crystal showed no defect due to the increased content of hydrocarbons, oxygen or water in the growing atmosphere and was suitable for the most demanding optical applications after tempering. If the process of the invention was not used in the cultivation, ie the enclosed atmosphere was not recirculated through the control circuit, there were already adverse defects in the tightness of the welds and joints, evidently there was an increase in moisture content and the cultivated monocrystal showed apparent light scattering solid phase, making it unusable for demanding applications.

Example 2

The molybdenum crucible unit and the feedstock as in Example 1 were flushed with hydrogen after argon evacuation and purging and backfilled with argon so that 25% by volume of hydrogen remained in the argon atmosphere and the dew point was -5 ° C. After 10 hours from the start of the recirculation, the dew point stabilized at -50 ° C and the hydrogen content dropped to 22% by volume. Full pump capacity was needed to offset these parameters, ie 4 L / min. The cultivated single crystal was of high quality as in Example 1. If cultivated without recirculation, a single crystal, the quality of which did not satisfy demanding applications, was obtained as in Example 1.

Claims (2)

SUBJECT OF THE INVENTION 1. A method for growing single-crystal high-melting oxides in a closed protective atmosphere, characterized in that the composition of the atmosphere is optimized during the growing process by forced controlled by-pass of this atmosphere with an inserted set of control elements. 2. An apparatus for carrying out the method according to claim 1, comprising a closed culture furnace, characterized in that a recirculation circuit is connected thereto, which comprises a gas pump, a flow meter, hydrogen and moisture sensors, cleaning elements and another moisture sensor and a mechanical microfilter. of dust particles.