A method for preparing gem-grade beryllium aluminate crystals

Abstract

The invention relates to a method for preparing gem-grade beryllium aluminate crystals, comprising the following steps: (S1) an aluminum source and a beryllium source are added to a crucible, the temperature is raised to 1800-1850°C for reaction, beryllium aluminate is generated, the temperature is further raised to 50-100°C and kept for 1-2 hours to form a beryllium aluminate crystal phase, the temperature is further raised to 2100-2200°C to form a beryllium aluminate melt; (S2) a crystal growth and pulling device is evacuated and filled with argon gas, a beryllium aluminate seed crystal is moved down to above the liquid surface of the beryllium aluminate melt, the seed crystal is lowered below the liquid surface of the melt, the seed crystal is rotated and pulled upward, and a periodically alternating ultrasonic environment is applied to the environment, the crystal starts to grow, and the temperature is maintained until the crystal growth ends; (S3) after the crystal growth ends, the crystal is pulled away from the melt, and the temperature is slowly lowered to room temperature to obtain a gem-grade beryllium aluminate crystal. The beryllium aluminate crystal prepared by the invention has good optical properties, low weak light absorption coefficient, and good optical uniformity.

Description

Technical Field

[0001] The invention belongs to the technical field of optical crystal manufacturing, and in particular relates to a method for preparing gem-grade beryllium aluminate crystal. Background Art

[0002] Beryllium aluminate (BeAl2O4) is an excellent terminal phonon laser matrix crystal material with a tunable range of 650-800nm. It can be used for red lasers. Its physical and chemical properties exceed those of laser-quality crystals such as yttrium aluminum garnet, and it is widely used in laser precision processing, spectral analysis, pulsed lasers, etc. However, during the growth of beryllium aluminate crystals, due to the high viscosity of the beryllium aluminate melt and the fact that BeAl2O4 is an anisotropic crystal, the process conditions are harsh; and the components are easily segregated, resulting in melt aging. The resulting large-sized crystals are prone to cracking, cracks, micropores and other defects, resulting in a decrease in optical properties. Therefore, although it has very outstanding optical properties, its application has been limited.

[0003] CN101407402A discloses a beryllium aluminate crystal matrix color retention and a preparation method thereof, which is doped with Cr, Fe, Ti or Ni elements through a vertical static temperature gradient method. However, the crystal quality is not good enough, the temperature gradient is difficult to control, and defects are prone to occur during the crystal growth process. CN1824847A discloses a method for preparing Cr-doped beryllium aluminate, using BeO, Al2O3, and Cr2O3 as raw materials, and through the periodic variable speed rotation of the crucible, the speed gradually increases from 0 and then gradually decreases to 0. The periodic crucible speed change can reduce the phenomenon of component segregation, but it will produce defects such as stripes, color bands, and inclusions during the crystal growth process, which also affects the physicochemical properties of the beryllium aluminate crystal.

[0004] Therefore, there is an urgent need to develop a method for preparing gem-grade beryllium aluminate. The preparation method has a simple process, and the obtained beryllium aluminate crystal has good optical properties, no defects or few defects, reaching gem grade, and can be applied to high-precision laser processing, laser pulsers and other fields. Summary of the invention

[0005] In order to overcome the problem that the performance of products obtained by the existing beryllium aluminate crystal preparation process is not good enough, especially the laser performance, which cannot meet the high requirements of laser precision manufacturing, laser pulses and other fields. The present invention proposes a method for preparing gem-grade beryllium aluminate crystals, which is prepared by an upward pulling method. In the initial crystal growth stage of the seed crystal, a periodically changing external ultrasonic environment is applied, which can well suppress the defects caused by crystal growth, and obtain large-sized gem-grade beryllium aluminate crystals with few defects. Specifically, the present invention achieves the above purpose through the following technical solutions:

[0006] A method for preparing gem-grade beryllium aluminate crystals comprises the following steps:

[0007] (S1) adding an aluminum source and a beryllium source into a crucible, heating the crucible to 1800-1850° C. and reacting for 15-20 hours to generate beryllium aluminate, further heating the crucible to 50-100° C. and keeping the temperature for 1-2 hours to form a beryllium aluminate polycrystalline phase, and further heating the crucible to 2100-2200° C. to form a beryllium aluminate melt;

[0008] (S2) the crystal growth and pulling equipment is evacuated and filled with argon gas, the beryllium aluminate seed crystal is moved down to above the liquid surface of the beryllium aluminate melt in step (S1) and kept at a constant temperature for a period of time, the seed crystal is started to be lowered, the seed crystal is allowed to drop below the melt surface and maintained for a period of time, the seed crystal is started to be rotated and pulled upward, and at the same time, an ultrasonic environment with a periodic alternating change of 50-60kHz to 100-150kHz ultrasonic frequency is applied to the environment, each cycle is 10-20min, the crystal starts to grow, and when the crystal diameter reaches the requirement, the ultrasonic environment is removed and the temperature is maintained until the crystal growth is completed;

[0009] (S3) After the crystal growth is completed, the crystal is pulled out of the melt and slowly cooled to room temperature to obtain gem-grade beryllium aluminate crystals.

[0010] The upward pulling method for growing crystals plays a vital role in the initial crystal growth stage after adding seed crystals. Improper handling will lead to defects such as white color bands, and re-seeding is required at this time. And even if the seeding is successful, similar defects will also appear during the crystal growth process, and crystal growth cannot proceed smoothly, affecting the preparation of large-sized crystals. Ultrasonic technology has been applied and studied in crystal growth. Ultrasonic physical effects such as pressure changes and laminar flow are used to control crystal growth and morphology. The inventor unexpectedly found that in the initial crystal growth stage after adding seed crystals, an ultrasonic environment with periodic changes in the environment is given, and a sinusoidal function curve changes regularly from 50kHz-120kHz within a time period of 10-20min, which can effectively suppress the generation of defects and is conducive to the preparation of large-sized gem-grade beryllium aluminate crystals with few defects.

[0011] Furthermore, in step (S1), the aluminum source is selected from at least one of aluminum oxide and aluminum hydroxide, the beryllium source is selected from at least one of beryllium oxide, beryllium hydroxide and beryllium carbonate, and the purity of the raw materials is ≥99.9%, preferably ≥99.99%. The present invention uses high-purity aluminum oxide and beryllium oxide as raw materials to reduce the presence of impurities, especially non-ferrous metals such as Fe, Cu, etc. The aluminum source and the beryllium source are fed according to the Al:Be molar ratio of 2-2.1:1, preferably the Al:Be molar ratio of 2:1; ensure that the raw materials are fully reacted.

[0012] Further, in step (S2), the crystal growth pulling equipment is evacuated and then filled with argon gas, and the evacuation pressure is 1×10 -3 Pa to 5×10 -3Pa, and argon gas is filled with 0.8-1atm argon gas.

[0013] Furthermore, in step (S2), the beryllium aluminate seed crystal is moved down to 5-15 mm, preferably 8-12 mm, above the liquid surface of the beryllium aluminate melt in step (S1); the constant temperature period is maintained at a constant temperature of 0.5-1 h at ambient temperature, the purpose of which is to preheat the seed crystal and prevent the temperature difference between the seed crystal and the melt from being too large, resulting in explosion, cracking, etc. when the seed crystal contacts the melt.

[0014] Further, in step (S2), the beryllium aluminate seed crystal is oriented <001> , <010> or <100> .

[0015] Furthermore, in step (S2), the seed crystal is lowered to 2-3 mm below the melt surface and maintained for 5-10 minutes to allow the contacting part of the seed crystal and the melt to melt.

[0016] Further, in step (S2), an ultrasonic environment with a periodic alternating change of ultrasonic frequency from 50-60kHz to 120-150kHz is applied to the environment, and each cycle is 10-20 minutes, which means that the ultrasonic frequency gradually rises from a low frequency segment (50-60kHz) to a high frequency segment (120-150kHz), and then drops from the high frequency segment to the low frequency segment, which is considered a complete cycle. Each cycle time is 10-20 minutes.

[0017] Preferably, the periodic variation of the ultrasonic frequency is performed according to a sinusoidal function curve.

[0018] Furthermore, in step (S2), the crystal diameter meeting the requirement means that the crystal diameter reaches 30-50 mm. The seed crystal is rotated and pulled upwards, with a pulling speed of 1-2 mm / h and a rotation speed of 10-20 rpm.

[0019] Furthermore, in step (S3), the slow cooling is programmed cooling, which is cooling to 1100-1300°C at a cooling rate of 10-15°C / h; then cooling to 700-900°C at a cooling rate of 20-35°C / h, and finally cooling to room temperature at a cooling rate of 40-50°C / h, to obtain gem-grade beryllium aluminate crystals with large size and few defects.

[0020] The slow cooling program cooling method can eliminate the color center defects and stress generated during the crystal growth process, so that the large-sized crystals have fewer defects and are more stable. At the beginning of the cooling, the cooling rate is low, 10-15℃ / h, and then gradually increased to 20-35℃ / h, which can effectively prevent the crystal from cracking. After that, the cooling rate can be increased to 40-50℃ / h after the crystal is stable, and the temperature is reduced to room temperature to obtain large-sized beryllium aluminate crystals.

[0021] The preparation method of the present invention is simple and can be completed by simply matching a conventional ultrasonic device in an existing Czochralski crystal growth device, without the need for complex and expensive equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of a crystal growing device using the Czochralski method of the present invention;

[0023] In the figure: 1-furnace wall, 2-water cooling inlet, 3-water cooling outlet, 4-gasket, 5-corundum cup, 6-insulating sand, 7-crucible, 8-growing raw material, 9-insulating cover, 10-intermediate frequency coil, 11-ultrasonic generating end, 12-lifting rod, 13-electronic scale, 14-seed crystal, 15-growing crystal.

[0024] Figure 2 The relationship between the ultrasonic frequency and the time period in the ultrasonic environment applied in Example 1;

[0025] Figure 3 This is a photo of the large-sized gem-grade beryllium aluminate crystal obtained in Example 1. DETAILED DESCRIPTION

[0026] The raw materials and equipment used in the examples of the present invention are all purchased from conventional commercial sources.

[0027] Al2O3 was purchased from Zhongtianli New Materials Co., Ltd. with a purity of 99.99%; BeO was purchased from Shanghai Taiyang Technology Co., Ltd. with a purity of 99.99%.

[0028] Example 1

[0029] (S1) Al2O3 and BeO are added to a corundum crucible in a molar ratio of 1:1, and the temperature is raised to 1800°C for reaction for 15 hours to generate beryllium aluminate, and the temperature is further raised to 1850°C and kept for 2 hours to form a beryllium aluminate polycrystalline phase, and the temperature is further raised to 2100°C to form a beryllium aluminate melt;

[0030] (S2) will be oriented <001> The beryllium aluminate seed crystal is moved down to 8mm above the liquid surface of the beryllium aluminate melt in step (S1), and the temperature is maintained at a constant temperature for 1h to preheat the seed crystal to prevent cracking caused by excessive temperature difference between the seed crystal and the melt, which is not conducive to crystal growth. After preheating, the seed crystal begins to be lowered to 2mm below the liquid surface of the melt, so that the contact part of the seed crystal and the melt is melted, and an ultrasonic environment with an ultrasonic frequency of 60kHz to 120kHz and a period of 10min is applied to the environment (that is, the ultrasonic frequency is increased from 60kHz to 120kHz and then reduced to 60kHz for 10min). The relationship between the ultrasonic frequency and the time period is as follows: Figure 2As shown, the change of ultrasonic frequency presents a sinusoidal function curve. At the same time, the seed crystal is pulled upward, the pulling speed is 2mm / h, the rotation speed is 15rpm, the crystal begins to grow, and when the crystal diameter reaches 35mm, the ultrasonic environment is removed and the temperature is maintained until the crystal growth ends;

[0031] (S3) After the crystal growth is completed, the beryllium aluminate crystal is pulled out of the melt and programmed to cool down, specifically, the temperature is cooled down to 1300°C at a cooling rate of 10°C / h; then the temperature is cooled down to 900°C at a cooling rate of 20°C / h, and finally the temperature is cooled down to room temperature at a cooling rate of 40°C / h, to obtain a large-sized gem-grade beryllium aluminate crystal with a length of 520 mm and a diameter of 35 mm. A 20mW He-Ne laser is used to irradiate the interior of the crystal blank, and no scattered particles are visible to the naked eye.

[0032] Figure 3 This is a photo of the large-sized gem-grade beryllium aluminate crystal prepared in this example.

[0033] Example 2

[0034] Other conditions are the same as those in Example 1, except that in step (S2), the ultrasonic environment applied is an ultrasonic frequency of 60 kHz to 150 kHz and changes in a period of 15 minutes.

[0035] Example 3

[0036] The other conditions are the same as those in Example 1, except that in step (S2), the ultrasonic environment applied is an ultrasonic frequency of 50 kHz to 100 kHz and changes in a period of 20 minutes.

[0037] Example 4

[0038] The other conditions are the same as those in Example 1, except that in step (S3), the temperature is lowered to room temperature at a cooling rate of 20°C / h, i.e., no programmed cooling is set.

[0039] Comparative Example 1

[0040] The other conditions are the same as those in Example 1, except that in step (S2), no ultrasonic environment is applied.

[0041] Comparative Example 2

[0042] The other conditions are the same as those in Example 1, except that in step (S2), the applied ultrasonic frequency is kept constant at 60 kHz.

[0043] Comparative Example 3

[0044] The other conditions are the same as those in Example 1, except that in step (S2), the applied ultrasonic frequency is constant at 120 kHz.

[0045] The optical properties of the beryllium aluminate crystals prepared in the above examples and comparative examples were tested, and the results are shown in Table 1 below.

[0046] The weak light absorption coefficient test wavelength is 1064nm.

[0047] Optical homogeneity is performed on a ZYGO laser digital interferometer.

[0048] Table 1 Optical performance test

[0049] .

[0050] It can be seen from the data in Table 1 that the large-sized beryllium aluminate crystal prepared by the upward pulling method under the periodically changing ultrasonic environment of the present invention has few defects, and the weak light absorption coefficient and optical uniformity are very excellent, which can meet the application requirements of high-end laser equipment. Although the application of ultrasonic environment can also improve the optical uniformity of the crystal to a certain extent, it still cannot reach a satisfactory level. The inventor unexpectedly found that the periodically changing ultrasonic frequency environment can greatly improve the optical uniformity of the crystal.

Claims

1. A method for preparing gem-grade beryllium aluminate crystals, characterized in that: The following steps are involved: (S1) adding an aluminum source and a beryllium source into a crucible, heating the crucible to 1800-1850° C. to react and generate beryllium aluminate, then heating the crucible to 50-100° C. and keeping the temperature for 1-2 hours to form a beryllium aluminate crystal phase, and then heating the crucible to 2100-2200° C. to form a beryllium aluminate melt; (S2) the crystal growth and pulling equipment is evacuated and filled with argon gas, the beryllium aluminate seed crystal is moved down to above the liquid surface of the beryllium aluminate melt in step (S1) and kept at a constant temperature for a period of time, the seed crystal is started to be lowered, the seed crystal is allowed to drop below the melt surface and maintained for a period of time, the seed crystal is started to be rotated and pulled upward, and at the same time, an ultrasonic environment with a periodic alternating change of 50-60kHz to 100-150kHz ultrasonic frequency is applied to the environment, each cycle is 10-20min, the crystal begins to grow, and grows until the crystal diameter reaches the requirement, the ultrasonic environment is removed, and the temperature is maintained until the crystal growth is completed; the periodic alternating change of ultrasonic environment refers to the ultrasonic frequency gradually rising from the low frequency segment to the high frequency segment, and then falling from the high frequency segment to the low frequency segment, which is considered a complete cycle, and the periodic change of the ultrasonic frequency is carried out according to the sine function curve; (S3) After the crystal growth is completed, the crystal is pulled out of the melt and slowly cooled to room temperature to obtain gem-grade beryllium aluminate crystals.

2. The preparation method according to claim 1, characterized in that: In step (S1), the aluminum source is selected from at least one of aluminum oxide and aluminum hydroxide, the beryllium source is selected from at least one of beryllium oxide, beryllium hydroxide and beryllium carbonate, and the purity of the raw materials is ≥99.9%.

3. The preparation method according to claim 1, characterized in that: In step (S1), aluminum source and beryllium source are added according to the Al:Be molar ratio of 2-2.1:1; the reaction time is 15-20h.

4. The preparation method according to claim 1, characterized in that: In step (S2), the crystal growth pulling equipment is evacuated and then filled with argon gas. The evacuation pressure is 1×10 -3 Pa to 5×10 -3 Pa, and argon gas is filled with 0.8-1atm argon gas.

5. The preparation method according to claim 1, characterized in that: In step (S2), the beryllium aluminate seed crystal is moved down to 5-15 mm above the liquid surface of the beryllium aluminate melt in step (S1); the constant temperature for a period of time is maintained at ambient temperature for 0.5-1 h.

6. The preparation method according to claim 1, characterized in that: In step (S2), the beryllium aluminate seed crystal is oriented <001> , <010> or <100> .

7. The preparation method according to claim 1, characterized in that: In step (S2), the seed crystal is lowered to 2-3 mm below the melt surface and maintained for 5-10 minutes to allow the contacting part of the seed crystal and the melt to melt.

8. The preparation method according to claim 1, characterized in that: In step (S2), the crystal diameter meeting the requirement means that the crystal diameter reaches 30-50 mm; the seed crystal is rotated and pulled upward, the pulling speed is 1-2 mm / h, and the rotation speed is 10-20 rpm.

9. The preparation method according to claim 1, characterized in that: In step (S3), the slow cooling is programmed cooling, cooling to 1100-1300°C at a cooling rate of 10-15°C / h; then cooling to 700-900°C at a cooling rate of 20-35°C / h, and finally cooling to room temperature at a cooling rate of 40-50°C / h.

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