Horizontal directional crystallization growth method of Nd: LuAG crystal

Abstract

The invention discloses a horizontal directional crystallization growth method of an Nd: LuAG crystal, which comprises the following steps: in a single crystal growth process, adopting a zone melting mode of three-phase coexistence of a single crystal, a melt and a pre-crystallized polycrystalline raw material, controlling a melting zone in a specific size range, and enabling the pre-crystallized polycrystalline raw material to form a narrow melting zone so as to obtain a solute non-conservative system, so that components are controlled, and the growth rate of the Nd: LuAG crystal is improved. Realizing a segregation coefficient k0lt; nd < 3 + > (k0 is equal to 0.18-0.25) which is greater than 1 is uniformly doped in a LuAG matrix, so that the Nd: LuAG crystal uniformly doped with Nd < 3 + > is obtained. And the crystal prepared by the method is relatively high in quality and has no defects of core, side core and the like, and the problem that the high-quality Nd: LuAG crystal with uniform and controllable components is difficult to prepare by the existing crystal growth method is solved.

Description

Technical Field

[0001] The invention relates to the technical field of crystal growth, and in particular to a horizontal directional crystal growth method for a Nd:LuAG crystal. Background Art

[0002] Nd:LuAG (neodymium-doped lutetium aluminum garnet) crystals have shown great application potential in the field of laser technology due to their high thermal conductivity, long fluorescence lifetime and relatively low emission cross section. Especially at room temperature, Nd:LuAG crystals are undoubtedly ideal gain media for lasers with high repetition rate (≥10Hz) and high energy (>100J). Its unique physical and chemical properties make this crystal have irreplaceable application value in cutting-edge fields such as military technology, particle acceleration technology and industrial laser shock peening.

[0003] However, despite the broad application prospects of Nd:LuAG crystals, there are many challenges in their growth process. Although traditional crystal growth methods, such as the Czochralski method, the hydrothermal method and the micro-pull-down method, can achieve crystal growth to a certain extent, due to the intrinsic properties of Nd:LuAG crystals, it is very difficult to grow high-quality crystals with controllable composition using traditional methods, and there are also some other limitations.

[0004] The Czochralski method, as the most widely used crystal growth method at present, is relatively mature in technology, but it is difficult to achieve precise control of the concentration distribution of doping ions (Nd³⁺) when growing Nd:LuAG crystals. In addition, crystals grown by the Czochralski method often have defects such as cores and side cores, which not only reduces the utilization rate of the crystals, but also increases the growth cost. Due to the high melting point of Nd:LuAG crystals, the Czochralski method usually requires the use of expensive iridium crucibles, and it is easy to leak the pot, which further increases the cost. At the same time, the growth cycle of the Czochralski method is long, which is difficult to meet the needs of large-scale production.

[0005] The hydrothermal method needs to be carried out under high temperature and high pressure conditions, which makes the growth control factors more complicated. At present, the technology of growing Nd:LuAG crystals by hydrothermal method is not mature yet and still needs further research and optimization.

[0006] Although the micro-pull-down method can achieve precise control of crystal growth to a certain extent, the fluidity and temperature fluctuation of the melt during the growth process have a greater impact on the quality of the crystal. This makes it difficult for Nd:LuAG crystals grown by the micro-pull-down method to reach an ideal quality.

[0007] In summary, the current Nd:LuAG crystal growth methods all have their own shortcomings and are difficult to meet the production needs of high-quality, large-size crystals. Therefore, improving the existing crystal growth methods and optimizing the process parameters to achieve the growth of large-size, high-quality Nd:LuAG crystals with controllable composition has become a key issue to be solved in the current crystal growth field. Summary of the invention

[0008] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a method for horizontal directional crystal growth of Nd:LuAG crystals, so as to solve the problem that the existing crystal growth methods are difficult to prepare high-quality Nd:LuAG crystals with uniform and controllable components.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: A method for horizontal directional crystal growth of Nd:LuAG crystals, comprising the following steps: (1) Lu2O3 powder, Nd2O3 powder and Al2O3 polycrystalline material are mixed evenly, melted and cooled to obtain pre-crystallized material; (2) The pre-crystallized material is crushed into small pieces and then spread flat in a boat-shaped crucible. Then, a seed crystal is inserted into the end of the boat-shaped crucible near the shoulder corner. The boat-shaped crucible containing the pre-crystallized material and the seed crystal is placed in a working furnace, and the equal-width area of ​​the boat-shaped crucible is located in the heating area of ​​the working furnace. The furnace is closed. (3) Open the cooling system of the working furnace and maintain the temperature of the cooling system at 28-30°C. Then evacuate the working furnace. When the vacuum degree is less than 1×10 -3 When the temperature is 0.0407°C, the heating power of the working furnace is turned on to heat the pre-crystallized material in the boat-shaped crucible in the heating area. By controlling the heating power, the pre-crystallized material in the heating area is melted to form a narrow melting zone with a length of 10-20 mm. (4) Keeping the heating power constant, move the boat-shaped crucible horizontally so that the heating area is aligned with the seed crystal for heating. When the pre-crystallization material in contact with the seed crystal just melts and the seed crystal neither melts nor grows, move the boat-shaped crucible slowly toward the end of the boat-shaped crucible containing the seed crystal at a seeding rate of 1.5-2 mm / h, and go through seeding, shoulder release and equal-width growth in sequence until all the pre-crystallization materials are completely melted and crystallized. After cooling and annealing, Nd:LuAG crystals are obtained. During the movement, the melt in the boat-shaped crucible that is gradually away from the heating area produces supercooling, which serves as a driving force for crystallization and promotes the production of single crystals.

[0010] Furthermore, the Lu2O3 powder, Nd2O3 powder and Al2O3 polycrystalline material in step (1) are prepared according to the chemical formula Lu 3(1-x) Nd 3x Al5O 12The atomic ratio in is weighed, where x ranges from 0.01 to 0.05. The mass of the raw materials is accurately prepared according to the solid phase reaction equation: 5Al2O3+3(1-x)Lu2O3+3xNd2O3=2Lu 3(1-x) Nd 3x Al5O 12 , x = doping concentration. Under a certain total mass, x can be used to calculate the different masses of each component. The reason why x is 0.01~0.05 is that too low doping concentration will lead to small gain, and too high concentration will easily cause serious thermal effects inside the laser crystal, thereby limiting the increase in output laser energy. In addition, the total weight of the raw materials can be adjusted according to the thickness of the growing crystal. Within the range of the crucible, the thickness of the crystal can be adjusted by appropriately increasing or decreasing the total amount of raw materials.

[0011] Furthermore, the crystal orientation of the seed crystal in step (2) is

[111] . Generally, the crystal with LuAG as the matrix has the highest symmetry along the

[111] direction, and the defects generated during the growth process are relatively small. Therefore, when the seed crystal is grown, the crystal orientation of the seed crystal is

[111] , which makes it easier to obtain high-quality crystals.

[0012] Further, the procedure of the cooling annealing in step (4) is: first, the furnace temperature is lowered to 1700-1800°C at a cooling rate of 10-30°C / h, and the temperature is kept for 10-20h, and in-situ annealing is performed to reduce the internal thermal stress of the crystal, and then the temperature is cooled to room temperature at a cooling rate of 30-60°C / h. Although a cooling rate that is too fast or a holding time that is too short can reduce the crystal growth cycle, a cooling rate that is too fast or a holding time that is too short will weaken the degree of stress elimination during the annealing process and increase the risk of cracking. Therefore, the present invention adopts a segmented cooling method for annealing, first cooling at a smaller rate and keeping the temperature for 10-20h to achieve in-situ annealing, reduce the internal thermal stress of the crystal, and then cool to room temperature at a faster rate to improve the crystal preparation efficiency.

[0013] Furthermore, the shoulder release angle in the shoulder release stage is 80º~120º. In the shoulder release stage, the shoulder release angle should not be too large or too small. When the shoulder release angle is too small, the part of the crystal before the shoulder release cannot be reasonably utilized, and the crystal utilization rate is low; if it is too large, it will cause difficulty in shoulder collection. When the shoulder release angle is 90º. The entire crystal ingot can be used most reasonably, which solves the problem of crystal ingot utilization rate well, and shoulder collection is also easier. Therefore, the preferred shoulder release angle is 90º.

[0014] Furthermore, the boat-shaped crucible is a boat-shaped molybdenum crucible. The molybdenum crucible used in the present invention has a melting point of 2630°C, is easy to grow high-melting-point crystals, and during the growth process, the crucible has high stability, is not easy to leak, and has low cost.

[0015] Furthermore, the height of the boat-shaped crucible is 1 / 9 to 1 / 8 of the length.

[0016] Furthermore, in step (2), before closing the furnace, heat insulation screens are placed on both sides of the boat-shaped crucible along the length direction, and the heat insulation screens and the boat-shaped crucible are placed together on the slide rail, and then the furnace is closed. The shape of the heat insulation screen is a rectangular block, and the height and width are close to the size of the crucible. Under vacuum conditions, the heat in the furnace will be lost through heat conduction and radiation. The heat insulation screen is added to reduce the radiation loss generated from the high-temperature area to the low-temperature area. Because the more radiation loss, the more it deviates from the steady-state temperature field, which is not conducive to the growth of crystals. It is generally hoped that the heat generated per unit time in the furnace is equal to the heat loss, so as to establish a steady-state temperature field; but this is almost difficult to achieve in actual operation, so it is generally hoped that the difference between the heat generated and the heat loss is as small as possible.

[0017] The present invention adopts a horizontal directional crystallization method when preparing Nd:LuAG crystals, controls the melting zone length to 10-20 mm, and controls the seeding rate to 1.5-2 mm / h to achieve the goal of Nd:LuAG crystals. 3+ The reason why the melting zone length is controlled at 10~20mm is to obtain a narrow melting zone, because for the zone melting method of the solute non-conservative system, the shorter the melting zone length, the more uniform the distribution of the grown crystals, so as to reduce the Nd 3+ The concentration of doping ions caused by the segregation effect gradually increases with the increase of crystal growth time, thereby achieving the purpose of controllable components. However, the length of the melting zone should not be too short, because if the melting zone is too short, it is easy to produce an undercooled melt or a partially undercooled melt, which is not conducive to the stability of the growth interface. If the interface is unstable, it is easy to produce dendrites or cellular structures, which is not conducive to obtaining high-quality crystals. At the same time, during the crystal growth process, a certain temperature is also required to maintain the fluidity of the melt. Too low a temperature (or too small a melting zone) will lead to poor melt fluidity, thereby affecting the growth of the crystal.

[0018] During the crystal growth process, the more the segregation coefficient of the doped ions deviates from 1 (too large or too small), the more likely it is that the components will be overcooled. From the perspective of avoiding component overcooling, increasing the temperature gradient and reducing the growth rate can both avoid component overcooling. However, an excessively large temperature gradient can easily increase thermal stress, thereby increasing the risk of crystal cracking. The inventors of the present invention have found through research that for Nd with a segregation coefficient of 0.18-0.25, 3+ For example, when the melting zone length is set at 10~20mm, the narrow melting zone can be obtained to the maximum extent, achieving Nd 3+The uniform doping of components can be avoided, which can affect the quality of the crystal and produce defects such as clouding, while ensuring that the melt has good fluidity. Controlling the seeding rate at 1.5-2mm / h can provide the melt with enough time to evenly release latent heat and reduce local temperature and concentration fluctuations caused by excessively fast growth rate, thereby reducing the risk of component supercooling. At the same time, it can avoid the decline in production efficiency and increase in cost due to too low a growth rate. Therefore, the present invention achieves Nd by reasonably controlling the length of the melting zone and the seeding rate. 3+ The uniform doping of Nd in LuAG matrix resulted in 3 + Uniformly doped high quality Nd:LuAG crystal.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. The horizontal directional crystallization growth method of Nd:LuAG crystal of the present invention adopts a zone melting method in which single crystal, melt and pre-crystallized polycrystalline raw material coexist in the single crystal growth process, and controls the melting zone within a specific size range, so that the pre-crystallized polycrystalline raw material forms a narrow melting zone to obtain a solute non-conservative system, thereby controlling the composition (doping ion distribution) and achieving Nd with a segregation coefficient k0<1. 3+ (k0=0.18-0.25) uniformly doped in LuAG matrix, and by properly controlling the seeding rate, Nd 3+ Uniformly doped high-quality Nd:LuAG crystals. The crystals produced by this method are of high quality and have no defects such as cores and side cores, which solves the problem that existing crystal growth methods are difficult to produce high-quality crystals with uniform and controllable components.

[0020] 2. The boat-shaped crucible used in the present invention has a height much smaller than its length, which creates favorable conditions for the evaporation of impurity components in the melt, effectively reduces the scattering of the crystal, and improves the transparency and optical properties of the crystal. At the same time, the lower height makes it almost non-existent to the convection movement in the melt, increases the stability of the growth interface front, and the grown crystal has no core or side core, making it easy to obtain high-quality crystals.

[0021] 3. Before the seeding growth, the present invention first heats the equal width area of ​​the boat-shaped crucible to melt the polycrystalline material to obtain a narrow melting zone, and then heats the seed crystal at the same power, and then slowly seeding the crystal. In this way, the risk of excessive melting of the seed crystal is avoided, the operation difficulty is lower, the risk is smaller, and it is easy to promote and apply. This is because the shape of the equal width area is a rectangular parallelepiped, which is convenient for observing the temperature and judging whether the solid-liquid interface is stable, while the front end of the crucible where the seed crystal is located is a triangle, and the solid-liquid interface is very short, making it difficult to judge whether the solid-liquid interface is stable, and it is more difficult to control the power, which easily increases the risk of excessive melting of the seed crystal. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the thermal field structure when preparing Nd:LuAG crystals by the horizontal directional crystallization method of the present invention; In the figure, there are pre-crystallization material 1, heater 2, boat-shaped molybdenum crucible 3, seed crystal 4, single crystal 5, and melt 6. DETAILED DESCRIPTION

[0023] The specific implementation modes of the present invention are further described in detail below in conjunction with specific examples.

[0024] Example 1 This embodiment provides a method for horizontal directional crystal growth of Nd:LuAG crystal, comprising the following steps: (1) Ingredients: Weigh 1041.68 g Lu2O3 powder, 8.88 g Nd2O3 powder and 449.43 g Al2O3 polycrystalline material as polycrystalline material raw materials, and the purity of each polycrystalline material raw material is 5N; (2) Preparing a pre-crystallization material: the polycrystalline raw materials in step (1) are mixed evenly, placed in a rectangular molybdenum crucible, and then melted at 1800° C. to obtain a polycrystalline state. The polycrystal obtained after cooling is used as a pre-crystallization material; (3) Furnace loading: After the pre-crystallization material is crushed into small pieces, it is spread flat in a molybdenum boat crucible (size 170 mm × 90 mm × 20 mm) cleaned with alcohol, and a pure LuAG seed crystal is inserted into the end of the molybdenum boat crucible near the shoulder corner, with the seed crystal orientation

[111] ; then the molybdenum boat crucible containing the pre-crystallization material and the seed crystal is placed in the single crystal furnace, and the equal width part of the molybdenum boat crucible is located in the heating area directly below the observation hole of the single crystal furnace, and then tungsten heat insulation shields are placed on both sides of the molybdenum boat crucible along the length direction, and the heat insulation shields and the molybdenum boat crucible are placed on the slide rail together, and then the furnace is closed; (4) Melting the pre-crystallized material: Turn on the water cooling circulation system and keep the water temperature at 28°C. Then turn on the vacuum pump to draw vacuum. When the vacuum degree reaches 9×10 -4 Pa, the heating power of the single crystal furnace is turned on to heat the heating area. When the power reaches 23.5 kw, the pre-crystallized material in the heating area is melted by heat, and the length of the melting zone is 20 mm. The schematic diagram of the thermal field structure when the horizontal directional crystallization method of the present invention is used to prepare Nd:LuAG crystals is shown in FIG. Figure 1 As shown; (5) Seeding, shouldering, and equal-width growth: Move the molybdenum boat crucible so that the seed crystal is located in the heating area directly below the observation hole of the single crystal furnace. After 20 minutes of heat preservation, the raw material and the seed crystal just melt at the contact point, and the seed crystal neither melts nor grows. Keep the power unchanged at 23.5 kW, turn on the automatic movement program of the crucible, and move the molybdenum boat crucible toward the end with the seed crystal at a seeding rate of 2 mm / h. During the movement, the melt in the molybdenum boat crucible that gradually moves away from the heating area produces supercooling, which serves as a driving force for crystallization and promotes the production of single crystals. After 85 hours of operation, all the raw materials have completed crystallization after seeding, shouldering, and equal-width growth in sequence; the shouldering angle is 90º.

[0025] (6) Annealing: After crystallization is completed, the furnace temperature is lowered to 1700 °C at a cooling rate of 30 °C / h and kept at this temperature for 10 h for in-situ annealing to reduce the internal thermal stress of the crystal. Then, the crystal is cooled to room temperature at a cooling rate of 50 °C / h to prepare large-sized, high-quality Nd:LuAG crystals.

[0026] Example 2 This embodiment provides a method for horizontal directional crystal growth of Nd:LuAG crystal, comprising the following steps: (1) Ingredients: Weigh 1382.65 g Lu2O3 powder, 17.78 g Nd2O3 powder and 559.57 g Al2O3 polycrystalline material as polycrystalline material raw materials, and the purity of each polycrystalline material raw material is 5N; (2) Preparing a pre-crystallization material: the polycrystalline raw materials in step (1) are mixed evenly, placed in a rectangular molybdenum crucible, and then melted at 1800° C. to obtain a polycrystalline state. The polycrystal obtained after cooling is used as a pre-crystallization material; (3) Furnace loading: After the pre-crystallization material is crushed into small pieces, it is spread flat in a molybdenum boat crucible (size 170 mm × 90 mm × 20 mm) cleaned with alcohol, and a pure LuAG seed crystal is inserted into the end of the molybdenum boat crucible near the shoulder corner, with the seed crystal orientation

[111] ; then the molybdenum boat crucible containing the pre-crystallization material and the seed crystal is placed in the single crystal furnace, and the equal width part of the molybdenum boat crucible is located in the heating area directly below the observation hole of the single crystal furnace, and then tungsten heat insulation shields are placed on both sides of the molybdenum boat crucible along the length direction, and the heat insulation shields and the molybdenum boat crucible are placed on the slide rail together, and then the furnace is closed; (4) Melting the pre-crystallized material: Turn on the water cooling circulation system and keep the water temperature at 30°C. Then turn on the vacuum pump to draw vacuum. When the vacuum degree reaches 6×10 -4 Pa, the heating power of the single crystal furnace is turned on to increase the temperature of the heating area. When the power reaches 23.2 kW, the pre-crystallized material in the heating area is melted by the heat, and the length of the melting zone is 15 mm. (5) Seeding, shouldering, and equal-width growth: Move the molybdenum boat crucible so that the seed crystal is located in the heating area directly below the observation hole of the single crystal furnace. After 30 minutes of heat preservation, the raw material and the seed crystal just melt at the contact point, and the seed crystal neither melts nor grows. Keep the power of 23.2 kw unchanged, turn on the automatic movement program of the crucible, and move the molybdenum boat crucible toward the end with the seed crystal at a seeding rate of 1.5 mm / h. During the movement, the melt in the molybdenum boat crucible that gradually moves away from the heating area produces supercooling, which serves as a driving force for crystallization and promotes the production of single crystals. After running for 113.4 hours, after seeding, shouldering, and equal-width growth, all the raw materials have completed crystallization; the shouldering angle is 90º.

[0027] (6) Annealing: After crystallization is completed, the furnace temperature is lowered to 1700 °C at a cooling rate of 20 °C / h and kept at this temperature for 20 h for in-situ annealing to reduce the internal thermal stress of the crystal. Then, the crystal is cooled to room temperature at a cooling rate of 40 °C / h to prepare large-sized, high-quality Nd:LuAG crystals.

[0028] In specific implementation, the size of the crystal obtained depends on the size of the crucible. The size of the crucible used in the embodiment of the present invention is 170mm×90mm×20mm, and the total length of the crucible is 170mm (including the length of the shoulder area and the equal width area). Therefore, a large-sized plate-shaped Nd:LuAG crystal of 170mm×90mm×20mm can be grown. Among them, the size of the equal width area can be calculated according to the shoulder angle.

[0029] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A method for horizontal directional crystal growth of Nd:LuAG crystals, characterized in that: The following steps are involved: (1) Lu2O3 powder, Nd2O3 powder and Al2O3 polycrystalline material are mixed evenly, melted and cooled to obtain pre-crystallized material; (2) The pre-crystallized material is crushed into small pieces and then spread flat in a boat-shaped crucible. Then, a seed crystal is inserted into the end of the boat-shaped crucible near the shoulder corner. The boat-shaped crucible containing the pre-crystallized material and the seed crystal is placed in a working furnace, and the equal-width area of ​​the boat-shaped crucible is located in the heating area of ​​the working furnace. The furnace is closed. (3) Open the cooling system of the working furnace and maintain the temperature of the cooling system at 28-30°C. Then evacuate the working furnace. When the vacuum degree is less than 1×10 -3 When the temperature is 0.0407°C, the heating power of the working furnace is turned on to heat the pre-crystallized material in the boat-shaped crucible in the heating area. By controlling the heating power, the pre-crystallized material in the heating area is melted to form a narrow melting zone with a length of 10-20 mm. (4) Keeping the heating power constant, move the boat-shaped crucible horizontally so that the heating area is aligned with the seed crystal for heating. When the pre-crystallization material in contact with the seed crystal just melts and the seed crystal neither melts nor grows, move the boat-shaped crucible slowly toward the end of the boat-shaped crucible containing the seed crystal at a seeding rate of 1.5-2 mm / h, and go through seeding, shoulder release and equal-width growth in sequence until all the pre-crystallization materials are completely melted and crystallized. After cooling and annealing, Nd:LuAG crystals are obtained. During the movement, the melt in the boat-shaped crucible that is gradually away from the heating area produces supercooling, which serves as a driving force for crystallization and promotes the production of single crystals.

2. The method for horizontal directional crystal growth of Nd:LuAG crystal according to claim 1, characterized in that: The Lu2O3 powder, Nd2O3 powder and Al2O3 polycrystalline material in step (1) are prepared according to the chemical formula Lu 3(1-x) Nd 3x Al5O 12 The atomic ratio in is weighed, where the value of x ranges from 0.01 to 0.

05.

3. The method for horizontal directional crystal growth of Nd:LuAG crystal according to claim 1, characterized in that: The crystal orientation of the seed crystal in step (2) is [111].

4. The method for horizontal directional crystal growth of Nd:LuAG crystal according to claim 1, characterized in that: The procedure of the cooling annealing in step (4) is as follows: first, the furnace temperature is lowered to 1700-1800°C at a cooling rate of 10-30°C / h, kept at that temperature for 10-20h, and in-situ annealing is performed to reduce the internal thermal stress of the crystal, and then cooled to room temperature at a cooling rate of 30-60°C / h.

5. The method for horizontal directional crystal growth of Nd:LuAG crystal according to claim 1, characterized in that: The shoulder release angle during the shoulder release phase is 80º~120º.

6. The method for horizontal directional crystal growth of Nd:LuAG crystal according to claim 1, characterized in that: The boat-shaped crucible is a boat-shaped molybdenum crucible.

7. The method for horizontal directional crystal growth of Nd:LuAG crystal according to claim 1, characterized in that: The height of the boat-shaped crucible is 1 / 9 to 1 / 8 of the length.

8. The method for horizontal directional crystal growth of Nd:LuAG crystal according to claim 1, characterized in that: In the step (2), before closing the furnace, heat insulation screens are placed on both sides of the boat-shaped crucible along the length direction, and the heat insulation screens and the boat-shaped crucible are placed together on the slide rails, and then the furnace is closed.

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