UA78462C2 - Method for receiving raw material of alumina for leucosapphire monocrystal growth - Google Patents

Abstract

The method for receiving raw material for leucosapphire monocrystal growth includes melting of alumina mixture with 0.4-0.6 wt. % of carbon in the form of fine graphitic power, receiving a melting operation product in the skull and its following breakage. The received broken aluminous material is filled up previously before growth into the melting pot with a fuse put inside, the melting pot is disposed into the furnace chamber with graphite refractory facing and heating elements placed underneath. The atmosphere is vacuum-pumped to the pressure of 0.1-0.2 mm hg, the material is heated to premelting temperature for 20-24 hours. Then the argon is put on to the pressure 800-850 mm hg increasing the heating temperature to 2100-2200°С for 1-2 hours, liquid melt is aged for 2-3 hours and after cooling the melting pot with raw cake is taken out. The method provides the decreasing of the impurities content in the received raw material.

Description

Description of the invention

This invention belongs to the field of obtaining raw materials for growing single crystals of leukosapphire 2 by the method of horizontal directional crystallization (HSC).

At this time, there is a great need for single crystals of leucosapphire of high optical quality - class "Nidy Oriis", with uniform characteristics, and first of all, in the UV range. It is possible to obtain such single crystals from raw materials in which the concentration of individual impurities (Ti, Mao, Ca) does not exceed 5 ppt (the total concentration of all impurities is less than 30-50 ppt). 70 Single crystals with (0001) orientation on the working surface and a diameter of 200 mm or more are required for the manufacture of optical windows of the "Nyd oriis" class and substrates for LEDs. The most promising method for obtaining such single crystals is the HSC method.

A number of methods of obtaining raw materials for growing leucosapphire single crystals are known. In particular, the method (pat. RF Mo 2140876, SOIE 7/44) of obtaining raw materials of alpha-aluminum oxide by preliminary obtaining 12 carbonized gamma-aluminum oxide and its subsequent processing with oxygen-containing gas at a temperature of 1100-12502С until complete combustion of carbon. During such heat treatment, a phase transition of gamma-aluminum oxide into alpha-aluminum oxide occurs. See also Pat. RF Mo 1477682, Mo 2092438, Mo 2167817, Mo 2223221, SOI 7/02 and others.

The disadvantages of the known methods are high retention of impurities in the final product (concentration of individual impurities exceeds 10-30 ppt), low reproducibility of physico-chemical properties, since its composition is completely determined by the content of impurities in the raw material, high cost of the target product Due to high energy costs, which complicates their use in large-scale production.

A known method (pat. RF Mo 2257346, СОР 7/02, С09 K 3/14) of obtaining fine-crystalline alpha-aluminum oxide, which includes heating water and aluminum oxide or hydroxide in an autoclave to the synthesis temperature of 250-4002С under elevated pressure and exposure at this temperature. Aluminum oxide or hydroxide and water (CO) are taken in a weight ratio of 1:0.7-1.2, respectively. Aging is carried out at a pressure of 300-600 atm. After holding at the synthesis temperature for at least 20 hours, heat treatment is carried out, or in the process of cooling from the synthesis temperature to ambient temperature for at least 24 hours without lowering the pressure by 20 m, or in the process of reducing the pressure at the synthesis temperature at a rate of 20-50 atm/ hour to atmospheric pressure followed by turning off the heating in the autoclave. (Se)

Compared to the above methods, this method provides a reduction of impurities as follows: potassium oxide from 500 to 70 ppm, iron oxide from 100 to 40 ppm. In general, the total content of impurities decreases from 1000 to 500 ppt.

However, the content of impurities at such a level does not allow the use of the obtained raw materials for growing single crystals of leucosapphire of the Nidp oris class. In addition, like the previous methods, this method has a high M cost price.

Monocrystals grown from such raw materials are used to produce products for standard optics that do not have high requirements for transparency and resistance to UV radiation, for the uniformity of these parameters (lenses, optical glasses, optical windows of technological equipment, scanners of banking machines, etc.). . "20 A known method (pat. RF Mo 2118612, СО1Е 7/44) of obtaining o-alumina powder by calcination, with at least one starting material selected from modifications of aluminum oxide and aluminum compounds.

Firing is carried out in a halogen-containing atmosphere, which includes hydrogen halide gas, halogen gas, or a mixture of halogen gas and steam. The following dehalogenation makes it possible to obtain a powder of alpha-aluminum oxide with a purity of 99.995. | see also E. Dobrovinskaya et al. Zncyclopedia of sapphire. Kharkiv,

NTK Institute of Single Crystals", 2004, p. 214). -I From raw materials obtained by such methods, single crystals of leucosapphire of the "Nidy oriis" class are grown.

The disadvantage of these methods is the high cost of raw materials, which ranges from 3 to 70 dollars. USA per kg. For example, the cost of one kg of aluminum oxide from KZA (France), obtained by the above method, is $55.00. USA.

It should also be noted that the raw material obtained by all the above methods is a finely dispersed powder with a bulk weight of 1.7-1.9 grams/cm'. When growing leucosapphire single crystals

The bulk density of the powdered raw material should not be less than 3.0 grams/cm". Therefore, prior to growing single crystals, preliminary sintering or fusion of the raw material and its subsequent crushing to the specified grain size is required. This preliminary operation additionally increases the cost of the final product - single crystals.

The cheapest starting product for obtaining raw materials for growing leukosapphire single crystals is metallurgical alumina - a finely dispersed powder that is used in the production of aluminum. The cost of 1 kg of alumina does not exceed 0.5 dollars. USA. However, due to its low chemical purity and low bulk weight, it cannot be used to grow single crystals of leucosapphire without deep purification and fusion. 60 A known method (patent of Ukraine Mo 50000, 330 U 29/20) for obtaining raw materials from alumina, which includes mixing alumina powder with 0.4-0.6 wt.o of carbon in the form of finely dispersed graphite powder, loading the mixture into a furnace, which pumping down to a pressure of 0.2-0.4 mm Hg, heating the mixture from above to a temperature of 1600-190092С7 and holding for 5-6 hours in a reducing environment based on carbon monoxide. Then argon is injected to a pressure of 800-850 mm Hg. and continue heating until a melt appears on the surface of the powder mass (in the garnish).

At the same time, the reducing medium based on carbon monoxide is installed in the furnace already at 1300 C thanks to the graphite lining of the furnace itself and the introduced carbon powder.

Then the furnace is cooled naturally. The resulting cake is taken out and crushed to a particle size of 1-5 mm (bulk weight of such crumbs reaches 3 grams/cm 7). The resulting crumb is poured into a crucible with an igniter and the crucible is placed in a growth furnace for growing single crystals of leukosapphire by the HSC method.

Compared to the above analogues, this method provides a more cost-effective production of large-sized leucosapphire single crystals with zero orientation. The method is simpler and less energy-intensive, and, most importantly, for its implementation, alumina is used, which is available in tonnage production, and therefore cheap. As a result, the cost of 1 kg of raw materials does not exceed 5.0 dollars. USA. 70 However, impurities such as CaO, Mas, Ti 5, BIO remain in the obtained raw materials in concentrations of more than 10 ppt. Additional purification of raw materials takes place at the stage of preliminary melting of raw materials (melting of crumb at a speed of movement of the crucible of 40-60 mm/hour) and, to a greater extent, in the process of direct growth of single crystals due to the pressing out of impurities by the boundary of the phase distribution and their subsequent evaporation. At the same time, the process of evaporation of impurities is limited by diffusion processes. This leads 75 to an inhomogeneous distribution of impurities along the length and thickness of the grown single crystal (a higher concentration of impurities is at the end and bottom of the grown single crystal). It is also worth paying attention to the fact that additional contamination of the received raw materials is not excluded during the crushing process.

Therefore, the raw material obtained in this way does not ensure the growth of single crystals of the "Nidy oriis" class.

Grown single crystals can be used only in standard optics.

We chose the last of the analogues as a prototype.

The basis of this invention is the task of creating an economical method of obtaining raw materials from alumina for growing leucosapphire single crystals of high optical quality with uniform optical characteristics, in particular, in the UV range, i.e., the "Nidy Oriis" class.

The solution to the task is provided by the fact that in the method of obtaining raw materials from alumina for the growth of single crystals of leucosapphire, which includes fusion of a mixture of alumina with 0.4-0.6 wt.bo of carbon in the form of finely dispersed graphite powder, obtaining the product of melting in the garnish and its subsequent crushing, and 9) according to the invention, crushed alumina raw materials are poured into a crucible with an igniter installed in it before growing, the crucible is placed in the furnace chamber that has a graphite lining with heating elements located below, the furnace is pumped down to a pressure of 0.1-0.2 mm mercury st., heat the SU raw material to the pre-melting temperature for 20-24 hours, then blow in argon to a pressure of 800-850 mm Hg. Art., raise the heating temperature to 2100-22002C for 1-2 hours, keep the melt for 2-3 hours and after cooling, remove the crucible with raw material-ingot. listen

Placing the crucible with the raw material in the furnace chamber with heating elements located below, which has a graphite lining, ensures heating and melting of the raw material-crumbs from below (that is, throughout the entire volume) and as a result of prolonged heating and holding at the specified temperatures, intensive mixing of the melt is ensured due to thermal convection, which is visually observed by the presence of convection waves on its surface. In the process of raising the temperature after argon has been injected to the specified pressure, an environment containing 2-3,956 reducing components of CO and H is spontaneously formed in the furnace (the result of the interaction of oxygen and water vapor with graphite at high temperatures).

Thanks to this, intensive interaction of impurity oxides on the surface of the melt with CO and H 5, their reduction and evaporation is achieved. In this way, a deep cleaning of the melt is achieved throughout the thickness. As experiments showed, vacuuming the furnace in the pressure range of 0.1-0.2 mm Hg. Art. and raising the temperature to "pre-melting temperature" under these conditions for 20-24 hours ensures effective degassing of raw materials and thermal screens, in addition, mass transfer of molybdenum (crucible material) and the formation of molybdenum films in raw materials are excluded. - I Argon pressure in the range of 800-850 mm Hg. Art., on the one hand, excludes the inflow of atmospheric air into the volume of the furnace and an undesirable change in its composition, and on the other hand - precisely at this pressure of argon in the furnace, a gaseous environment is formed containing 2-396 reducing components (СО-Н .o), thanks to which, first of all, the melt is purified.

Argon pressure is more than 850 mm Hg. Art. impractical due to possible damage to individual units of the furnace:

Ф measuring glass lamps, oil seals, etc.

What) For economic reasons, a gas environment with an argon inflow of up to 800-850 mm Hg was chosen for the implementation of the claimed method. Art.

As experiments have shown, raising the temperature to the pre-melting temperature (1900-195022) faster than 20 hours leads to the formation of molybdenum films in the raw material - the result of mass transfer of molybdenum from the surface to the crucible in the presence of desorbed oxygen and water vapor; increasing the duration of the ascent beyond 24 hours is impractical for economic reasons. име) The minimum temperature of holding the melt (210022) is justified by the fact that the melt at this temperature becomes sufficiently liquid for the occurrence of intense convection currents; increasing the temperature by more than 2200 ss 60 is technically difficult and impractical (the service life of the crucible and heaters decreases).

Increasing the temperature to 2100-22002С faster than in one hour can lead to gaseous boiling of the melt; a long rise in temperature for more than two hours is impractical due to the increase in the duration of the process as a whole.

Holding the melt at a temperature of 2100-2200 for more than three hours is impractical, because a decrease in the concentration of impurities is not observed, and with a holding of less than two hours, the purification of the melt does not reach the maximum possible degree.

After cooling, the crucible with the obtained ingot raw material is removed from the furnace and placed in the growth furnace.

The ingot has a low and homogeneous concentration of impurities throughout the entire volume and does not require additional preparation (crushing and overloading in another crucible) before crystallization.

The table shows the value of the concentration of impurities contained in the raw materials obtained by the proposed method in comparison with other methods.

Figures 1 and 2 show optical transmission spectra, respectively, at the beginning and at the end of single crystals grown from raw materials obtained by the proposed method (solid line) and raw materials from the K5A company (dashed 7/0. Line) for their upper and bottom parts.

The proposed method is implemented in the following way.

Pre-dried alumina powder of the G-00 brand (used in the aluminum industry) is mixed with 0.5 wt.9o of carbon in the form of finely dispersed graphite powder of the ГМ3 brand. The mixture is loaded into a furnace with a tungsten heater, made on the basis of an industrial growth plant SZVN. The furnace chamber 7/5 is pumped down to a pressure of 0.2 mm Hg. Art. and start heating the mixture from above to a temperature of 19002С, keep it at this temperature for b hours in a reducing environment based on carbon monoxide. This reducing medium is installed in the chamber thanks to the graphite lining of the chamber vault, as well as the graphite introduced into the alumina already starting at a temperature of 13002C. Then argon is injected to a pressure of 800 mm Hg. st. and continue heating until a melt appears on the surface of the powder mass (in the garnish), and keep the melt for 2 hours. After the furnace cools down, the melting product remains on the garnish in the form of a cake with a diameter of 500 mm, a thickness in the center of about 70 mm and a mass of about 30 kg.

At the same time, technological regimes can vary within the following limits: the amount of graphite admixture from 0.4 to 0.6 weight. 9 o'clock; pressure from 0.2 to 0.4 mm Hg. art.; heating temperature from 1600 to 19002; exposure time 5-6 hours; argon pressure 800-900 mm Hg. Art. In this case, the rate of temperature rise to melting of the mixture to obtain He melting product is not important. at

The resulting melting product is crushed to obtain crumbs with a particle size of 1-5 mm (the bulk weight of such crumbs reaches 3 grams/cm").

Prior to growing, the resulting crumb is put into a crucible with a fuse placed in it and placed in a furnace chamber with a graphite lining and heating elements located below, while the EM excludes the possibility of fuse melting. After vacuuming to a pressure of 0.2 mm Hg. Art. increase the furnace heating temperature to 19002 for 20 hours. Then argon is injected to a pressure of 850 mm Hg. Art. and continue heating to 21002 for 1.5 hours, keep the melt for 2.5 hours. As a result of the "proposed additional heat treatment of the previously obtained melting product, intensive mixing of the melt throughout the entire volume of the crucible and its interaction with reducing components of the environment, reduction of oxides of impurities and vapors of volatile components is ensured. in.

After cooling the furnace (natural cooling of the furnace is about 12 hours), depressurize the furnace and take out the crucible with the raw ingot.

The crucible with the raw material-ingot and with the igniter installed in advance is placed in a growth furnace (industrial "growth plants of the SZVN, Horizont type and grow a single crystal of leucosapphire by the well-known GSK method. o) c The chemical composition of the obtained raw materials and grown single crystals was studied by the method " » of spectral analysis. Transmittance spectra of T(75) were recorded on the spectrophotometer "ZRESOKO-OM-MIV" in the spectral range of 200-275 nm - in the range most sensitive to the concentration of impurities in raw materials and crystals.

V. and 2

Phew 2

Co.)

As can be seen from the table, the proposed method provides deep purification of the original alumina raw material to the level of expensive types of raw materials, for example, KZA (France). At the same time, the cost of the received raw materials, taking into account the costs of carrying out the entire cycle of its production, does not exceed 10 dollars. USA per kg, in comparison

Gee! from $55.00 USA company K5A.

From Fig. 1, 2 it is clearly visible that with the same level of optical transparency, its homogeneity in the UV range in single crystals grown from alumina prepared according to the claimed method is even slightly higher than in single crystals obtained from raw materials produced by the K5A company (France). because This fact makes it possible to use the raw materials obtained by the proposed method for the production of large-sized optical elements of the "Nidy Oriis" class and eliminates the need to purchase expensive imported raw materials.

The introduction of an additional operation of preliminary melting of the fragmented melting product into the technological cycle also made it possible to increase the reproducibility of the conditions for the subsequent growth of a single crystal in a growth furnace, 65 because the evaporation of volatile impurities (in particular Ma), as well as intensive degassing of raw materials, occur at an intermediate stage outside the growth furnace (unlike from the prototype). Therefore, during the subsequent growth of a single crystal, there is a less intense overgrowth of the heat screens of the growth furnace in the melt zone, which, as a rule, changes the pumping conditions and leads to an uncontrolled change in the crystallization conditions.

Thus, the proposed method provides: obtaining cheap raw materials at a price of no more than 10 dollars. USA for kKG for growing monocrystals of the Nidp oriis class; reduction of requirements for raw alumina raw materials; increasing the service life of growth furnaces for growing leucosapphire single crystals by the HSC method; elimination of dependence on imported raw materials.

Claims (1)

The formula of the invention The method of obtaining raw materials from alumina for growing single crystals of leukosapphire, which includes /5 bmelt mixture of alumina with 0.4-0.6 wt. 95 carbon in the form of finely dispersed graphite powder, obtaining the melting product in the garnish and its subsequent crushing, which is characterized by the fact that the crushed alumina material is poured into a crucible with a seed installed in it before growing, the crucible is placed in the furnace chamber with a graphite lining and heating elements located below elements, the atmosphere is pumped to a pressure of 0.1-002 mm Hg. century, heat the material to the pre-melting temperature for 20-24 hours, then blow in argon to a pressure of 800-850 mm Hg. Art., raise the heating temperature to 2100-22002C for 1-2 hours, keep the melt for 2-3 hours and after cooling, remove the crucible with raw material-ingot. s schi 6) s to te « (ee) and - - and? -i (ee) sh» (o) Ko) ime) 60 b5