RU2072004C1 - Method of growing monocrystals of lithium ferrospinel - Google Patents

Abstract

FIELD: inorganic synthesis. SUBSTANCE: invention relates to crystals applicable to devices based on magnetic excitements. 17.1-18.0 wt % boron oxide is melted in a crucible with 52.0-53.5 wt % bismuth(III) oxide, 17.2-18.0 wt % lithium carbonate, and ferric oxide (balance). Solution-melt is heated to 980-1000 C, then temperature gradient is established with vertical constituent 1-3 C (temperature lowers with distance from crucible bottom) and the solution-melt is maintained at this temperature for 20-24 h, 4-6 h of them under stirring. At a temperature exceeding saturation temperature by 5-10 C, melt is brought in contact with horizontally oriented surfaces of inoculations. Crystal carrier with inoculations is rotated with velocity 40-60 rpm. 20-30 min later temperature is lowered 5-7 C below saturation temperature and further according to program with increasing cooling velocity from 1 to 5 C a day. At 750-800 C crystal carrier is lifted and grown up crystals beyond the solution-melt are cooled to room temperature with velocity 40-50 C per 1 h. The thus prepared specimens for magnetoacoustic investigations have H = 1.8-2.0 e at frequency 9.1 Hz. EFFECT: improved procedure.

Description

The invention relates to a technology for growing single crystals from molten solutions and may find application in the preparation of single crystals of lithium ferrospinel LiFe ₅ O ₈ for devices based on magnetic excitations.

In known solution-melt methods of growing single crystals of lithium ferrospinel, iron oxide Fe ₂ O ₃ and lithium carbonate Li ₂ CO _{3 were} fused with boron oxide B ₂ O ₃ and lead oxide PbO. The melt was overheated for several hours at temperatures of 1025 ^o C 1060 ^o C, then cooled at a speed of 0.8 ^o C 2 deg / h to temperatures of 300 600 ^o C. The grown crystals were separated from the solidified melt solution by dissolving the latter in a hot solution of nitric acid.

 These methods with spontaneous nucleation do not allow reproducibly producing high-quality single crystals for applied research and technical applications.

The method described in [1] by which the largest crystals were obtained is selected for the prototype. According to this method, iron oxide Fe ₂ O ₃ and lithium carbonate Li ₂ CO _{3 were} fused with boron oxide B ₂ O ₃ and lead oxide PbO in the following ratios, mol.

In ₂ O ₃ 20.28 (8.9 wt.)
PbO 44.32 (62.2 wt.)
Fe ₂ O ₃ 23.09 (23.2 wt.)
Li ₂ CO ₃ 12.31 (5.7 wt.)
The melt solution prepared in a 500 ml platinum crucible was heated to 1060 ^o and, having established a temperature gradient with a vertical component of +0.7 ^o C / cm (the temperature in the melt solution increases with distance from the bottom of the crucible), was held for 12 hours. After that, the solution-melt was cooled at a speed of 0.8 ^o C / hour to 600 ^o C, and then with the oven turned off. From the hardened mass, 6 crystals up to 1 cm in size and one with a maximum size of up to 2 cm were separated by dissolving it in a hot aqueous solution of nitric acid.

 The main disadvantage of this method is that it does not provide reproducible growing of high-quality single crystals of lithium ferrospinel with the dimensions required for microwave devices for magnetoelastic and magnetostatic waves. Due to microdefects in the form of inclusions of the melt solution, only certain parts of the grown crystals were suitable for the manufacture of samples. The qualitative fraction in the largest block crystal was less. At the same time, with an increase in crystal sizes, an increasing role of "banding" was observed. Cracks were observed in the crystals due to cooling in the solidified melt solution. In this method with spontaneous nucleation, both the number and size of crystals and their quality vary significantly from experiment to experiment.

 The purpose of the invention is the increase in the size of single crystals and the reproducibility of their properties.

This goal is achieved by the fact that in the known method, comprising fusing iron oxide Fe ₂ O ₃ and lithium carbonate Li ₂ CO ₃ with a double oxide mixture containing boron oxide B ₂ O ₃ , subsequent overheating and crystallization upon cooling, oxide is introduced as the second component of the mixture bismuth Bi ₂ O ₃ in the following general ratio in wt.

B ₂ O ₃ 17.1 18.0
Bi ₂ O ₃ 52.0 53.5
Li ₂ CO ₃ 17.2 18.0
Fe ₂ O _{3 the} rest,
the resulting molten solution is heated to 980 1000 ^o C and, having established a temperature gradient with a vertical component (1 3) ^o C / cm (the temperature decreases with distance from the bottom of the crucible), incubated for 20 24 hours, of which 4 6 hours with stirring, then at a temperature on May 10 ^o C above the saturation temperature T _we horizontally oriented surface (III) zatvorok brought into contact with a solution, melt and rotating the chip at a rate of 40 to 60 rev / min, 20 30 min, the temperature was lowered to July 5 ^o C lower T _us to continue the program with increasing speed Okhla Denia from 1 to 5 ^o C / day, and at a temperature of 750 800 ^o C chip carrier is lifted, and the grown crystals melt solution is cooled to room temperature at a rate of 40 to 50 ^o C per hour.

From publicly available sources of information at the filing date of the application is not known the claimed method of growing single crystals of lithium ferrospinel LiFe ₅ O ₈ , which implies that the invention is new.

In the inventive method, with the specified ratio of the components of the solution-melt, lithium ferrospinel is a high-temperature phase crystallizing in the temperature range from 900 950 ^o C to 740 - 750 ^o C. The density of the solution-melt is less than its density, which excludes the possibility of stable long-term development of free-floating "parasitic" crystals. Overheating of such a melt solution at 980-1000 ^° C (i.e., at temperatures of 50-80 ^° C higher than the saturation temperature) in an inhomogeneous field with a temperature decreasing with distance from the bottom of the crucible and stirring achieves complete homogeneity regardless of the initial state. By introducing seeds, the initial number of crystallization centers and the growth surface are fixed. The temperature field, hydrodynamic conditions, and the cooling regime after the introduction of the seeds are coordinated so that predominant crystallization occurs on the seeds rotating in the surface layer of the melt solution. The growth of crystals occurs under conditions close to equilibrium with intensive washing of their faces with a solution-melt, and the probability of the formation of macrodefects such as inclusions of a solution-melt of chalk. Cooling the grown crystals to room temperature outside the solidified melt solution eliminates the occurrence of stresses and cracks.

Deviations from the ratio of components, as well as from the thermophysical and hydrodynamic parameters specified in the claimed method, are accompanied by the replenishment of factors that ultimately lead either to a decrease in the size of the resulting crystals, or to a decrease in their quality and reproducibility. Thus, with an increase in the content of B ₂ O ₃ and Bi ₂ O ₃ (respectively, a decrease in Li ₂ CO ₃ ), the temperature range of crystallization of ferrospinel noticeably narrows beyond these limits, and with some excess, the hematite becomes a high-temperature phase. A narrowing of the crystallization interval of ferrospinel followed by a change in the high-temperature phase occurs with a decrease in the content of B ₂ O ₃ and Bi ₂ O ₃ (respectively, an increase in Li ₂ CO ₃ ) beyond these limits. However, in this case, the competing phase will be Li ₂ O containing compound. Along with this, it should be pointed out that depletion of the melt solution by low-density components (B ₂ O ₃ and Li ₂ CO ₃ ) is unacceptable because of the danger of the growth of “parasitic” floating crystals. When enriched with such components, the density difference between the ferrospinel and the melt solution increases, and to ensure predominant crystallization on the seeds, large temperature gradients will be required, and this will inevitably lead to an increase in temperature fluctuations and a narrowing of the metastable zone. The concentration of Fe ₂ O ₃ cannot be increased (with its increase by one mass%, the saturation temperature increases by 20 25 ^o С), otherwise the working temperatures will exceed 1000 ^o С, which will entail an uncontrolled change in crystallization conditions due to intensive evaporation. A decrease in the concentration of Fe ₂ O ₃ directly means a decrease in the size of the grown crystals.

A decrease in the temperature in the melt solution when moving away from the bottom of the crucible at the stage of overheating promotes acceleration of homogenization, and when cooling is necessary for predominant crystallization on seeds rotating in the surface layer. When the magnitude of the vertical component of the gradient is less than 1 ^o C / cm, the fraction of the substance crystallizing on the seeds decreases markedly, while at gradients of more than 3 ^o C / cm, the quality of the crystals decreases due to an increase in temperature fluctuations.

20 30-minute exposure of the seeds at temperatures of 5 10 ^o C above the saturation temperature allows you to dissolve only a surface disturbed layer of seeds and thereby exclude the inheritance of crystals of the corresponding defects. When seeds are introduced into the molten solution at a saturation temperature or lower, parasitization is also possible.

The initial growth temperature T _beginning T _us (5 7) ^o С is chosen in the middle of the metastability zone, and the rate of further cooling corresponds to the growth of 1 4 crystals with speeds of 0.5 0.7 mm / day. The rotation of the crystals at a speed of 40-60 rpm virtually eliminates the possibility of the formation of inclusions of the solution-melt. The growth is completed at temperatures of 750 - 800 ^o C, since due to a decrease in the concentration of crystal-forming oxides and an increase in the viscosity of the melt solution, the development of crystals slows down, the likelihood of macrodefects such as inclusions of the solution-melt increases.

 Thus, in the proposed method of growing, the physicochemical properties of the melt solution, temperature and hydrodynamic regimes are coordinated so that the bulk of the substance crystallizes on the seeds under conditions close to equilibrium, and this can significantly increase the size of single crystals and the reproducibility of their properties.

Example 1. A solution-melt weighing 1900 g was prepared in a 800 ml platinum cylindrical glass (D = 100 mm) by fusion at a temperature of 980 ^o With iron oxide and lithium carbonate with a mixture of boron oxide and bismuth oxide in the ratio, wt.

B ₂ O ₃ 17.6
Bi ₂ O ₃ 53.3
Li ₂ CO ₃ 17.9
Fe ₂ O ₃ 11.2
Qualification of all reagents "OSH". The necessary temperature distribution in the melt solution with a vertical component (1 3) ^o C / cm was set by controlling the heat output given by the main vertically located and bottom horizontal silicon carbide-silicon heaters of the crystallization furnace. In such an inhomogeneous field at a temperature of 980 ^° C (hereinafter, readings are indicated for a control thermocouple located at the surface of the melt solution on the outside of the glass), the melt solution was held for 24 hours, with the stirring being the last 6 hours (ω = 60 r / min).

The equilibrium temperature of the lithium ferrospinel crystal with the molten solution in the center of its surface (T _us ), recorded with an accuracy of 2 ^o C, was 900 ^o C.

After overheating was completed, a crystal holder with two seed crystals was introduced into the furnace, each of which facets (III), oriented horizontally, were facing the surface of the melt solution and were in the same plane. These faces in the form of regular triangles had areas of 0.15 cm ² and 0.1 cm ^2, respectively. At T = T _us - 10 ^° C = 910 ^° C, the seeds were brought into contact with the molten solution and the rotation of the crystal holder was switched on (w = 50 rpm). After 30 minutes, the temperature was lowered to T _beg = T _us
6 ^o C = 894 ^o C, and then cooling was carried out according to the program given in table 1:
At T _KOH = 782 ^° C, the crystal holder was raised until the contact between the grown crystals and the melt solution broke, and the furnace was cooled to room temperature at 40 ^° -50 ^° C / hour.

One of the grown crystals had a weight of 38.2 g and a size of 32 mm, the second crystal had a weight of 29.8 g and a size of 30 mm. The crystal faces were specular and free from macrodefects, with a dislocation density of not more than 10 cm ^-2 . Crystals were used to make samples in the form of disks (D 5 7 mm, h 2 4 mm), cylinders (D 3 4 mm, l 10 12 mm) for magnetoacoustic studies and spheres for FMR. The minimum line width at a frequency of 9.1 GHz at room temperature (H 1.8 2 Oe) corresponded to the best values given in the literature.

Example 2. A molten solution weighing 1900 g was prepared in 800 ml of a platinum cylindrical glass (D = 100 ml) by fusion at 980 ^o With iron oxide and lithium carbonate with a mixture of boron oxide and bismuth oxide in the ratio, wt.

B ₂ O ₃ 17.2
Bi ₂ O ₃ 52.1
Li ₂ O ₃ 17.4
Fe ₂ O ₃ 13.3
Qualification of all reagents "OSH". The saturation temperature of this melt solution is T _us = 946 ^o C. The crystallization furnace described in Example 1 was used. Overheating was carried out at T = 1000 ^o C for 24 hours, of which the last 4 hours with stirring (w = 60 rpm) . After overheating was completed, a crystal holder was introduced into the furnace with four seed crystals, each of which horizontally oriented faces (III) were facing the surface of the melt solution and were in the same plane. These faces in the form of regular triangles had areas of 0.1 cm ² , 0.13 cm ² , 0.15 cm ² , 0.17 cm ² . At T = T _us +5 ^° C = 951 ^° C, the seeds were brought into contact with the molten solution and the rotation of the crystal holder was switched on (w = 40 rpm). After 20 minutes, the temperature was lowered to T _beg = T _us - 7 ^o С = 939 ^o С, and then cooling was carried out according to the program given in Table 2:
At T _KOH = 770 ^° C, the crystal holder was raised to break the contact of the crystals with the melt solution and the furnace was cooled to room temperature at 40 ^° -50 ^° C / hour. The total weight of the obtained crystals is 117 g, their maximum sizes (31 mm, 30.5 mm, 28 mm, 25 mm). The dislocation density on the mirror faces did not exceed 10 ² cm ^-2 , the crystal faces did not have macrodefects. Crystals were used to make samples for magnetoacoustic studies and spheres for FMR. The minimum line width (f = 9.1 GHz, T = 300K) was in the range of 1.8 2 Oe, which corresponds to the best values given for lithium ferrospinel in the literature.

 Data for other experiments conducted according to the same scheme, but with different ratios of components, are given in Table 3.

Claims (1)

A method of growing single crystals of lithium ferrospinel LiFe ₅ O ₈ , comprising fusing iron oxide Fe ₂ O ₃ and lithium carbonate Li ₂ CO ₃ with a double oxide mixture containing boron oxide B ₂ O ₃ , followed by overheating and crystallization upon cooling, characterized in that the second a component of the oxide mixture is introduced bismuth oxide Bi ₂ O ₃ in the following ratio, wt. B ₂ O ₃ 17.1 18.0
Bi ₂ O ₃ 52.0 53.5
Li ₂ CO ₃ 17.2 18.0
Fe ₂ O ₃ Else
the resulting molten solution is heated to 980 1000 ^o C, establish a vertical temperature gradient of 1 3 ^o C / cm with decreasing temperature in the direction from the bottom of the crucible, incubated for 20 24 hours, of which 4 6 hours with stirring, then at a temperature of 5 10 ^o C above the saturation temperature, they are brought into contact with the solution-melt horizontally oriented in the direction of the surface of the seeds placed on the crystal holder, and rotate it at a speed of 40-60 rpm and after 20 30 minutes reduce the temperature by 5 7 ^o C below the saturation temperature, after which chilled lead to 750 800 ^o C, gradually increasing the cooling rate from 1 to 5 ^o C / day, then the crystal holder is raised and the grown crystals are cooled outside the melt solution to room temperature at a speed of 40 - 50 ^o C / h.

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