CN1421551A - Combined crucible rotating and molten salt pulling method for growing RE3+: KGd (WO4)2 laser crystal - Google Patents

Abstract

The present invention is combined crystal growth technology combining alternate crucible rotating technology (ACRT) and molten salt pulling technology to produce large-size REKGW laser crystal. REKGW laser crystal is one kind of excellent laser crystal with low threshold and several working wavebands. The ACRT process is one crystal growth process, in which the crucible is rotated periodically in both forward and inverse directions to raise the mass and energy conveynig speed and inhibit multinuclear growth. Combining the ACRT process to molten salt pulling process of growing REKGW laser crystal can raise the crystal growth rate and quality greatly.

Description

Method for growing RE by combining rotary crucible with molten salt pulling method3+：KGd(WO4)2Laser crystal

The invention relates to the technical field of crystal growth, which explains the method of combining a rotary crucible with a molten salt pulling method to grow large-size RE³⁺：KGd(WO₄)₂Laser crystal (RE: rare earth Nd, Yb, etc.).

KGd(WO₄)₂KGW belongs to a monoclinic crystal structure, and the space group is C2/C, RE³⁺：KGd(WO₄)₂The (REKGW for short) is a low-threshold, excellent multi-wavelength laser crystal material.

We have determined Nd³⁺：KGd(WO₄)₂-K₂WO₄The Nd with the size of 50mm is grown according to the growth temperature curve of the growth system³⁺：KGd(WO₄)₂Cutting a high-quality laser device with the size of 5mm multiplied by 6mm, adopting a titanium gem to simulate LD pumping, obtaining laser output of 260mw at the position of 1.06 mu m, wherein the slope efficiency reaches 60 percent, and the laser experimental curve is shown in figure 1; the 135mw laser output is obtained at 1.03 μm, and the slope efficiency reaches 34.3%. Meanwhile, research on KGW crystals and devices has attracted great attention in the International laser academia, U.S. V.Kushawaha et al and K.A.Stankov et al in Germany have used Xe flash lamps as pumping sources and used KGW laser bars of phi 5mm x 80mm and phi 6.3mm x 75mm to perform laser experiments, so that laser output and efficiency higher than that of YAG crystals under the same conditions are obtained, and the wavelength is 1.067 mu m. At present, research on KGW crystals at home and abroad is also in a rapidly developing trend.

The KGW crystal is grown by a flux method, and at present, a large-size crystal can be rapidly grown by a flux Czochralski method. However, because a large amount of potassium tungstate flux exists in the melt, W — O can form a multi-W polymer, and a network structure is easily formed in the melt, so that the viscosity of the melt is high, the transport rate of ionic groups is reduced, the growth of crystals is hindered, and particularly, a high growth rate is required in the fused salt pulling method, however, the crystal quality is poor and the crystals are difficult to pull due to the low transport rate of the ionic groups. This is because in a high-viscosity melt, the melt mixing degree is low, and the diffusion of ion groups from the melt to the crystal surface and the diffusion of impurity-removing flux ions into the melt are slow, which becomes the main control step of crystal growth by the molten salt method. In the case of low melt mixing, the flux ions discharged from the crystal growth cannot diffuse from the crystal into the melt in time, and the flux cannot diffuse from the melt to the crystal surface in time, which will hinder the crystal growth and cause the crystal to starve, produce wrapping, and even cause local overheating at the crystal boundary, such as peritectic and overheating in our case of growing LBO crystals, which obviously hinders the crystal growth. Meanwhile, due to the non-uniformity of the melt, local supercooling can be generated on the bottom or the wall of the crucible along with the reduction of the temperature, so that multi-core growth is caused, for example, when NYAB series crystals grow, a plurality of small crystals with different sizes can be observed at the bottom of the crucible, and the maximum size can reach 15mm multiplied by 3mm multiplied by 5mm, so that the utilization rate of the melt is reduced, and the growth of the crystals is hindered.

However, the techniques commonly used to mix the melt are the top rotating stirring seed crystal method and the natural convection method caused by the temperature gradient, which has many limitations, such as (1) the seed crystal is only stirred in a limited area at the top of the melt, only the mixing in a limited area is caused, and at the same time, the seed crystal accelerates the exertion of the top melt and narrows the metastable zone; (2) the degree of mixing caused by natural convection due to temperature gradients is not efficient enough and causes large temperature fluctuations.

A more efficient stirring method must be employed. Therefore, the present invention is directed to applying the rotating crucible method to a large-sized RE³⁺：KGd(WO₄)₂During the growth of the laser crystal (RE, rare earth Nd, Yb, etc.) by a molten salt pulling method, the transport rate of ion groups is improved, and the growth rate and quality of the crystal are greatly improved.

The rotating crucible method (ACRT) refers to a technique of periodically accelerating and decelerating a rotating crucible in forward and reverse directions. Under the action of ACRT, the melt in the center of the crucible generates movement relative to the melt at the edge of the crucible due to the viscosity of the melt, and the flowing state of the melt comprises two basic movement states: spiral Shear and Ekman flow. Under certain rotation conditions, the Spiral Shear flow can form thousands of Spiral arms in the melt, so that the melt can be homogenized. The Ekman flow is similar to the rotational motion of the bulk air in a cyclone, and as the crucible spins up, the opposite flow occurs. While the rotating crucible is constantly accelerated and decelerated, fluid is constantly pumped through the Ekman layer. Thus, the Ekman flow and the Spiral Shear flow will achieve continuous uniform mixing of the melt.

The strong mixing effect of the ACRT can not only accelerate the transportation of mass energy, reduce the nucleation probability at the bottom and on the wall of the crucible and accelerate the growth rate of crystals. Furthermore, according to the crystal boundary layer thickness formula: $\mathrm{\δ}=2^{2/3}{D}^{1/3}{V}^{1/6}{W}^{-1/2}$ and growth rate formula: $V=\frac{D}{\mathrm{\ρ}}\frac{(n_{\mathrm{sn}}-n_e)}{\mathrm{\δ}},$ (D: diffusion coefficient, W: rotation)Speed, ρ: specific gravity, n_e: equilibrium melt concentration, n_sn: melt concentration, V: crystal growth rate) that would effectively reduce the thickness of the boundary layer and also accelerate the crystal growth rate. At the same time, this strong and effective mixing effect will greatly suppress temperature fluctuations.

Therefore, the object of the present invention is to use ACRT in combination with the molten salt pulling method, which can inhibit the growth of multi-nuclei of crystals and allow the crystals to grow at an extremely stable growth rate.

We propose to combine the rotating crucible method with the fused salt pulling method, and adopt 55 mol% K₂WO₄Or K₂W₂O₄As a fluxing agent, the following chemical reaction formula is adopted for mixing:

or The raw materials are weighed according to molar ratio, ground and mixed evenly by an agate mortar and are filled into a platinum crucible with the diameter of 70mm multiplied by 70 mm. The growth temperature is 950 ℃, the temperature is kept for 2 days at 50 ℃ higher than the growth temperature, and then the temperature is slowly reduced at the rate of 5 ℃/d. And at the end of the growth, treating the crystal by using water, separating out the crystal, using the obtained crystal for orientation, and cutting out a b-direction seed crystal. Then, further growing large crystals by adopting a seed crystal method: after the raw materials are melted, measuring the saturation temperature of the melt by using a seed crystal trial method, lowering the seed crystal into the melt at about 30 ℃ (980 ℃) above the saturation temperature, reducing the temperature to the saturation temperature after half an hour, starting to reduce the temperature for growth, gradually adjusting the temperature reduction rate from 2 ℃/d to 5 ℃/d, controlling the average rotation rate ofthe seed crystal to be 6rpm, gradually adjusting the crystal pulling rate from 5mm/d to 3mm/d, controlling the forward and reverse rotation rate of a crucible to be-100 rpm to 100rpm, controlling the time interval to be 1.5-5 minutes, lifting the crystal from the liquid level after 15 days of growth, reducing the temperature to the room temperature at the rate of 30 ℃/h, and finally obtaining the high-quality transparent single crystal with the size of 50 mm.

Compared with a pure molten salt pulling method, the method has the beneficial effect that the growth rate and the quality of the crystal can be greatly improved. When a pure molten salt pulling method is used for growth, the pulling rate can only be controlled at 1mm/d, only 30mm crystals can be obtained after one month of growth, the lower half part of the crystals has poor quality, a large amount of fluxing agent is contained in the crystals, and the transparency is poor. After the new method is adopted, the pulling rate can be improved by 3-5 times, the crystal with the same size can be grown in about 10 days, the quality can be greatly improved, and no coating of fluxing agent is needed.

Referring now to the drawings in which: FIG. 1 is Nd³⁺: laser experimental curve of KGW crystal; FIG. 2The device is a crystal growth furnace device, wherein (1) an aluminum cover, (2) a furnace cover, (3) an inner furnace tube, (4) a heating wire, (5) a crucible, (6) a crucible rotating tray, (7) a heat preservation layer, (8) a platinum seed crystal rod, (9) a melt, (10) a seed crystal, (11) a temperature control thermocouple, and (12) a temperature measurement thermocouple; FIG. 3 is a plot of crucible rotation rate; FIG. 4 is a crucible rotation rate curve.

The first embodiment is as follows: with 55 mol% K₂W₂O₄As a fluxing agent, the following chemical reaction formula is adopted for mixing:

the raw materials are weighed according to molar ratio, ground and mixed evenly by an agate mortar and are filled into a platinum crucible with the diameter of 70mm multiplied by 70 mm. As shown in FIG. 2, the experimental apparatus was maintained at a temperature 50 ℃ higher than the growth temperature (950 ℃) for 2 days by a top-seeded cooling method, and then slowly cooled at a cooling rate of 5 ℃/d. And at the end of the growth, treating the crystal by using water, separating out the crystal, using the obtained crystal for orientation, and cutting out a b-direction seed crystal. Then, further growing large crystals by adopting a seed crystal method: after the raw materials are melted, the saturation temperature of the melt is measured by a trial seed method, the seed crystal is put into the melt at about 30 ℃ (980 ℃) above the saturation temperature, and is reduced after half an hourAnd (3) starting cooling growth when the temperature is reached to the saturation temperature, gradually adjusting the cooling rate from 2 ℃/d to 5 ℃/d, setting the average rotation rate of the seed crystal to be 6rpm, gradually adjusting the crystal pulling rate from 5mm/d to 3mm/d, setting the forward and reverse maximum rotation rate of the crucible to be-70 rpm to 70rpm, setting the crucible rotation rate program as shown by a curve A in figure 3 and setting the period to be 2 minutes. After 15 days of growth, the crystal is lifted off the liquid surface, then the temperature is reduced to room temperature at the speed of 30 ℃/h, and finally, the high-quality transparent single crystal with the size of 50mm can be obtained.

Example two: with 55 mol% K₂WO₄As a fluxing agent, the following chemical reaction formula is adopted for mixing:

the raw materials are weighed according to molar ratio, ground and mixed evenly by an agate mortar and are filled into a platinum crucible with the diameter of 70mm multiplied by 70 mm. The experimental apparatus is shown in FIG. 2, and is prepared by maintaining the temperature at 50 deg.C higher than the growth temperature (950 deg.C) for 2 days by top-seeded cooling method, and slowly cooling at a rate of 5 deg.C/dAnd (5) cooling. And at the end of the growth, treating the crystal by using water, separating out the crystal, using the obtained crystal for orientation, and cutting out a b-direction seed crystal. Then, further growing large crystals by adopting a seed crystal method: after the raw materials are melted, measuring the saturation temperature of the melt by a trial seed crystal method, lowering the seed crystal into the melt at about 30 ℃ above the saturation temperature (namely 980 ℃), reducing the temperature to the saturation temperature after half an hour, starting cooling and growing, gradually adjusting the cooling rate from 2 ℃/d to 5 ℃/d, setting the average rotation rate of the seed crystal to be 6rpm, gradually setting the crystal pulling rate from 5mm/d to 3mm/d, setting the forward and reverse maximum rotation rate of the crucible to be-100 rpm-100rpm, and setting the rotation rate program of the crucible to be 3.5 minutes as shown by a curve B in figure 4. After 15 days of growth, the crystal is lifted off the liquid surface, then the temperature is reduced to room temperature at the speed of 30 ℃/h, and finally, the high-quality transparent single crystal with the size of 50mm can be obtained.

Claims (2)

1. Growth by combining rotary crucible and fused salt pulling methodRE³⁺：KGd(WO₄)₂Laser crystal (REKGW) with RE elements Nd, Yb, etc. is prepared through combining rotating crucible process with fused salt pulling process and adopting analytically pure WO₃And K₂CO₃Spectrally pure RE₂O₃、Gd₂O₃As raw material, with K₂WO₄A method for growing a REKGW laser crystal as a flux, characterized in that:

(1) the positive and negative rotation speed of the crucible is-100 rpm, and the period is 1.5-5 minutes;

(2) the rotation speed of the seed crystal is 4.5-15 rpm;

(3) the pulling rate of the crystal is 3-5 mm/d;

(4) the seed crystal orientation is b;

(5) and (3) inoculating the seed crystals at the melt temperature of about 980 ℃, and after half an hour, beginning to cool at the speed of 2-5 ℃/d when the melt temperature is reduced to 950 ℃ which is the saturation temperature.

2. The method of claim 1 for growing RE by combining a rotating crucible with a molten salt pulling method³⁺：KGd(WO₄)₂The laser crystal is characterized in that the fluxing agent can adopt K₂W₂O₄Instead.

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