RU2040599C1 - Method for production of single crystals of rare-earth metal borides - Google Patents

Abstract

FIELD: growing of single and large-block crystals. SUBSTANCE: in course of crucibleless zone melting, crystallized part of remelting rod is cooled down to temperature of 1200-1300 C. Recrystallization is accomplished directly in the process of crucibleless zone melting when heating up to 1500-1600 C held for 3-5 h with subsequent slow cooling at the rate of 5-15 C per minute. EFFECT: higher efficiency. 1 dwg

Description

 The invention relates to crystal growth and can be used in the growth of mono- and large-block crystals of boron and its compounds by the crucible-free zone melting method.

At present, crystals of boron and its compounds of a high degree of purity are obtained by the method of zoneless melting. Boride crystals obtained by this method usually contain microcracks and structural defects due to the low value of thermal conductivity. The larger the diameter of the grown crystal, the greater the specified defects. In many cases, this is an obstacle to obtaining single crystal rods, i.e. the central, inner part is single-crystal, the peripheral part is polycrystalline [1]
A known method [2] the use of heat shields and additional heating furnaces, as well as passing a current with a high density through the sample.

 However, in these furnaces, the heating temperature is chosen above the brittle fracture temperature, i.e. deformations arising during crystal growth are elastic, or they carry out the annealing process at relatively low temperatures only to relieve thermal stresses. This is necessary to reduce defects.

 There is also known a method of producing a single crystal of yttrium boride by the method of crucible-free zone melting [3] In this case, a heating ring of a heat-resistant material with good thermal conductivity and low vapor pressure is located around the polycrystalline rod. The ring heats with a high-frequency current and a melt zone is obtained.

 The disadvantages of this method include some technical difficulties. Insignificant deviations of thermal processes can change the technological mode in such a way that during the growing process a polycrystalline rod grows.

 The purpose of the invention is to improve the quality of the single crystal and simplify the process.

The object is achieved by the fact that in the known method for producing single crystals of borides of rare earth metals, including smelting floating-band by moving the molten zone along the starting rod and cooled immediately after passing the molten zone remelting of part of the rod was cooled to 1200-1300 ^{° C,} subjected to recrystallization during 1500-1600 ^about C for 3-5 hours, and cooling is carried out at a speed of 5-15 deg / min.

Unlike the prior art in the present process in the process of floating zone melting crystallizable portion of the rod was cooled to 1200-1300 ^{° C,} at which there are plastic microstrain, and subjected to recrystallization.

Another difference is that the recrystallization is carried out in the process of floating zone melting at 1500-1600 ^{° C} for 3-5 hours in additional resistance furnace. In addition, to prevent the occurrence of microcracks during cooling after recrystallization, cooling is carried out with a decrease in temperature to room temperature of 5-15 ^about C in min

The fundamental difference of the proposed method of the presently known is that in the process of floating zone melting due to a low thermal conductivity and thereby large temperature gradients in the grown crystal, upon cooling the crystallizing part of the rod to 1200-1300 ^{° C} having plastic microstrain, and also the heterogeneity of defects in the microvolume, which create additional driving forces of recrystallization. As a result, recrystallization after passing the molten zone at 1500-1600 ^{° C} krupnoblochnogo crystal growth occurs from a solid crystallized phase. Crystal growth begins with a thin initial portion, which after a crucible-free zone melting has a single-crystal structure. Conducting recrystallization for 3-5 hours leads to the growth of a single crystal due to the additional force of recrystallization. In this case, annealing of residual thermal stresses and healing of defects are simultaneously carried out.

The quality of a single crystal substantially depends on the cooling mode, since boron compounds of the type MV ₆₆ have very low thermal conductivity. Stresses during cooling can cause microcracks in the rod. To prevent microcracking during cooling of the single crystal cools the temperature is decreased to 5-15 ^{° C} per minute.

Thus, it is possible to obtain single-crystal rods of rare-earth metal borides of the type MV ₆₆ without microcracks and with the smallest possible structural defects.

 The drawing schematically shows the proposed method for producing a single crystal of boron compounds.

Example 1. Crucibleless zone melting of GdB ₆₆ rods was carried out. The initial composition, wt. At 81.5 (the purity of the powder used 99.532) Gd 17.9 brand GDM-1 (TU-48-4-210-12). The main impurities, wt. Al 0.028; Si 0.07; Fe 0.27; C 0.095. The rods prepared by cold pressing of powders synthesized in the press CSP-10 at a pressure of 205 MPa (2087 kg / cm ^2), followed by sintering at 1600 ^C for 2 hours in an industrial furnace resistance SSHVL 0.62 / 25 at excess pressure high purity helium (99.985 TU-51-689-75) 118 KPa (1.2 atm). Zone melting is carried out on a high-frequency installation "Crystal DM" (operating frequency -5.28 MHz, vibrational power 6.3 kW) in a medium of very pure helium at a pressure of 127.4 KPa. Melt temperature 2100 ° ^C. In the process of floating zone melting remelt rod is moved at a speed of 0.8 mm / min to form the initial thin portions 1. As the motion of the rod downwardly from the high-frequency inductor 2, 3-crystallizable portion is cooled to 1300 ^C. and then the rod It begins to enter inside a resistance furnace 4, wherein heating is carried out up to 1600 ^{° C} (the temperature is measured with an optical pyrometer OPPIR-017 within 50 ° ^C). After entering the bar over the entire length in the oven, it was kept for 3 hours at 1600 ° ^C. Then it was cooled to room temperature, the furnace temperature is lowered 5 ^{° C} per minute. The chemical composition of the crystal, wt. B 81.75; Gd 18.15. The total impurity content of 0.1 wt. The dislocation density of 6.10 ⁴ 3.10 ^-5 cm ^{-2 was} determined by metallographic analysis on a MIM-8M microscope. Monocrystallinity was determined by x-ray according to the Laeu method. Shaft dimensions d 4-5 mm, l 70 mm.

EXAMPLE Example 2 The process is carried out as in Example 1 except that unlike Example 1, remelt rod is moved at a speed of 2 mm / min, crystallizable portion is cooled to 1200 ^C. and heated in a resistance furnace is carried out at 1500 ^о С with holding for 5 hours followed by cooling to room temperature with a decrease in temperature to 15 ^о С per min. The chemical composition of the crystal, wt. B 81.77; Gd 18.2, impurity content 0.1% dislocation density 8˙10 ⁴ 2˙10 ⁵ cm ^-2 .

The proposed method allows to obtain single crystals of semiconductor boron compounds from a rare-earth metal of the type MV ₆₆ (M-rare-earth metal), in particular GdB ₆₆ with lower values of the dislocation density.

 The method also allows to simplify the process, since it does not have a quick return of the power concentrator to the lower part of the rod, as well as the second passage of a narrow heated zone for recrystallization.

Claims (1)

METHOD FOR PRODUCING SINGLE CRYSTAL borides of rare earth metals, comprising floating-band melt by moving the molten zone along the starting rod and cooling, characterized in that, to improve the quality of single crystals and simplify the process, immediately after passing the molten zone remelting of part of the bar is cooled to 1200 1300 ^o C, subjected to recrystallization at 1500 1600 ^o C for 3-5 hours and cooling is carried out at a speed of 5 to 15 deg / min.

Images