High-optical-uniformity potassium tantalate niobate crystal grown by floating zone method and method

Abstract

The invention relates to the field of photoelectric function crystal material growth, in particular to a high-optical-uniformity potassium tantalate niobate crystal grown through a floating zone method and a method. The method comprises the following steps: (1) calculating a raw material ratio, and determining components of a flux charge bar and a supply bar; (2) performing two-step sintering and cold isostatic pressing to form a raw material rod; (3) carrying out crystal growth by adopting an optical floating zone furnace, regulating and controlling the melting rate of a feeding rod and melt convection, and ensuring stable components; and (4) carrying out thermal annealing treatment in an oxygen environment to improve the performance of the crystal and obtain the high-quality KTN crystal with the component uniformity of 10 <-5 > / mm. A precious metal crucible is not needed, the technological process is simplified, the equipment and material cost is reduced, the raw material utilization rate is remarkably increased, and the method is suitable for large-scale production.

Description

Technical Field

[0001] The present invention relates to the field of preparation of functional crystal materials, and particularly to a potassium tantalate niobate crystal with high optical uniformity grown by the floating zone method and a method therefor. Background Art

[0002] Potassium tantalate niobate crystal (KTa1-xNbxO 3 , abbreviated as KTN) is a new type of electro-optic crystal material developed in recent years. This crystal has the largest second-order electro-optic coefficient among currently known materials, and its electro-optic performance is more than 80 times that of the most widely used lithium niobate electro-optic crystal. KTN crystals are mainly used to make electro-optic modulation and electro-optic deflection devices, which have the advantages of low driving voltage, fast response speed, and small device size, and have great application potential in the fields of optical communication, lidar, and optical imaging.

[0003] KTN crystal is a continuous solid solution material of KTaO 3 and KNbO 3 . It is usually grown by the Czochralski method (CZ) (CN110230099B) or the top-seeded solution growth method (TSSG). However, due to the incongruent melting characteristics of the crystal, the components of the melt in the crucible and the crystal pulled out during the growth process are constantly changing, resulting in growth striations in the finally obtained crystal and poor optical uniformity (i.e., Ta / Nb component uniformity). The component fluctuation of Ta / Nb can usually only reach 10 -3 / mm, and the dielectric and electro-optic properties of the crystal itself are affected, which is likely to cause transmission spot distortion and increased optical loss in optical applications, reducing the performance of optical devices and equipment.

[0004] To solve the problem of compositional striations in crystals and improve the optical uniformity of crystals, current methods are mainly improvements based on the Czochralski method. For example, the double crucible real-time feeding technology (Wang X P, Liu B, Yang Y G, et al. Growth of KTN crystals by double crucible Czochralski method[J]. Materials Research Innovations, 2014, 18(5):334-339.) ensures the stability of the melt composition by replenishing raw materials in the inner crucible in real time, obtaining crystals with high optical uniformity. However, this double crucible system requires simultaneous melting and growth in two precious metal crucibles and needs to be equipped with a precise real-time feeding system, resulting in a significant increase in production costs. In addition, since it is necessary to precisely control the temperature, pressure, and composition in both crucibles simultaneously and accurately calculate the ratio of external feeding in real time, this poses higher requirements for the accuracy of the equipment and the skills of the operators. To ensure the stability of the composition in the inner growth crucible, the actual control difficulty is very high. These two reasons lead to the low practicality of this double crucible technology. In addition, some researchers have proposed the method of "growing small crystals in a large crucible" to reduce the compositional changes during crystal growth (Wang Xuping, Wang Jiyang, Liu Bing. Research on the compositional uniformity of KTa 1-x Nb x O 3 crystals [J]. Journal of Synthetic Crystals, 2010, 39(B06):48-53.). However, if this method is used to grow centimeter-sized "striation-free" KTN single crystals, the weight of the required raw materials usually needs to reach more than 5000 g, while the traditional Czochralski method only requires 1000 g of raw materials to grow crystals with a diameter greater than 3 cm. In addition, this method of "growing small crystals in a large crucible" can only improve the compositional striation problem of crystals to a certain extent theoretically, and the compositional uniformity of the actually grown crystals can only reach 10 -3 / cm. SUMMARY OF THE INVENTION

[0005] In view of the above deficiencies of the prior art, the present invention proposes a method for growing potassium tantalate niobate crystals with high optical uniformity by the floating zone method. By adjusting the raw material ratio of the feed rod and the flux rod, KTN crystals with a compositional uniformity of 10 -5 / mm are prepared by the floating zone method. Compared with the traditional Czochralski method, the compositional uniformity is improved by two orders of magnitude. Compared with other growth methods of high optical uniformity crystals based on the improvement of the Czochralski method (such as the double crucible method), the process operation is simple and the practicality is strong. Since the floating zone method does not require precious metal crucibles for growing KTN crystals, the raw material cost and equipment investment cost are lower.

[0006] The technical solution of the present invention is as follows:

[0007] On the one hand, the present invention provides a method for growing potassium tantalate niobate crystals with high optical uniformity by the floating zone method, comprising the following steps:

[0008] (1) Determine the required raw material ratio

[0009] Since the dielectric and electro-optic properties of potassium tantalate niobate crystals (KTa x Nb 1-x O 3 ) conform to the Curie-Weiss law, that is, they reach the optimum near the Curie temperature, and its Curie temperature varies with the composition. Therefore, during the crystal growth process, it is necessary to prepare crystals with a specific Curie temperature (usually close to room temperature) by adjusting the raw material ratio according to the temperature of the application environment and the specific requirements for the crystal properties. Assuming that the Curie temperature of the target crystal to be prepared is T c , according to the empirical formula:

[0010]

[0011] The composition of the crystal (i.e., the solid-phase composition) can be calculated Then, using the solidus equation and liquidus equation fitted from the KTaO 3 -KNbO 3 binary system phase diagram:

[0012] Solidus equation:

[0013] Liquidus equation:

[0014] The corresponding liquid-phase composition (i.e., the composition of the raw materials) can be calculated To solve the problem of non-uniform crystal composition caused by the incongruent melting characteristic of potassium tantalate niobate crystal growth (i.e., the melt and crystal compositions are different), we introduce two-component feed rods, namely, a "feeding rod" and a "flux rod" on the basis of the traditional floating zone method, where:

[0015] The molar ratio of the raw materials required for the "flux rod" is: The flux rod is used as the initial raw material, and the excessive K 2 CO 3 acts as a flux. On the one hand, it can effectively reduce the melting point of the melt, making the growth process proceed under milder conditions and reducing the adverse effect of high temperature on the volatilization of K 2 CO 3 ; on the other hand, the presence of the flux can increase the diffusion rate of the solute, thereby promoting the uniform distribution of chemical components and helping to obtain high-quality single crystals.

[0016] The molar ratio of the raw materials required for the "feeding rod" is: Its function is to supplement the loss of raw materials during the crystal growth process. The Ta / Nb composition of the feeding rod should theoretically be the same as that of the grown single crystal. The excessive K here 2 CO 3 is only to compensate for the volatilization of K during the pre-sintering and melting of the raw materials.

[0017] (2) Sintering and preparation of the raw material rods

[0018] Use a mixer to mechanically mix the above two ratios of raw materials for 24 - 48 hours respectively, and place the two mixed raw materials in platinum crucibles respectively. Use a muffle furnace for the first pre-sintering, with the sintering temperature: 850 - 950 °C and the sintering time 6 - 12 hours; then take out the two raw materials after the first sintering from the platinum pots, grind them respectively using a ball mill, and then place them in the platinum pots again and use a muffle furnace for the second sintering, with the sintering temperature 1000 - 1100 °C and the sintering time 6 - 12 hours.

[0019] Take out the two polycrystalline raw materials after the second sintering, grind them into powders again respectively, then pour them into spherical molds, and use the cold isostatic pressing method to make flux rods and feeding rods respectively. The pressure of the cold isostatic pressing is 200 - 300 MPa, and the pressing time is 3 - 5 minutes. The length of the flux rod should be slightly less than the height of the heating zone, specifically with the flux rod not exceeding the height of the heating zone as the standard.

[0020] (3) Crystal growth

[0021] Use an optical floating zone furnace. Fix a rod-shaped KTN seed crystal on the lower rotating rod, then place a flux rod above the seed crystal, fix a feeding rod on the upper rotating rod of the floating zone furnace, move the feeding rod to make it 1 - 2 mm above the flux rod, seal the seed crystal, the flux rod and the feeding rod in a quartz tube with a quartz tube, and use a four-ellipsoid reflective halogen lamp to heat the flux rod area, so that the upper end of the seed crystal, the lower end of the feeding rod and the whole flux rod are melted. Move the feeding rod downward so that its bottom contacts the melted flux rod to form a melting zone. At the upper end of the melting zone, the feeding rod gradually dissolves, increasing the solute concentration in the melting zone (i.e., the solution layer). The excess solute will flow to the low-temperature zone at the bottom of the melting zone through convection and reach the supersaturated state, and the solute begins to crystallize on the seed crystal, reducing the solute concentration in the melting zone. The feeding rod will continue to melt in a part of the material and reach the seed crystal through diffusion, thus forming a continuous dynamic process of dissolution - diffusion - crystallization. During the crystallization process of the melting zone, due to the segregation effect, there is a difference in the composition between the crystal and the chemical composition of the melting zone. To maintain the stability of the melt and crystal components, we determined the crystal composition obtained by the initial crystallization through calculation in the previous step, and configured a feeding rod with the same composition to supplement the solute consumed by the solid phase precipitated in the melting zone in real time, so as to maintain the component stability of the melting zone and the crystal during the growth process.

[0022] During the overall growth process, the upper shaft pushing speed and the lower shaft pulling speed are coordinated to ensure that the melting speed of the feed rod (the weight of the newly melted raw material per unit time) and the crystallization speed of the upper end of the seed crystal rod are consistent (the weight of the crystallization per unit time), in order to ensure the stability of the melt zone composition. Since the density of the crystal is greater than the density of the feed rod, in order to ensure the stability of the melt zone composition, the descent speed of the feed rod usually needs to be greater than the descent speed of the crystal. The descent speed of the feed rod is usually set to 1-3 mm / h, and the descent speed of the seed crystal is 0.5-2 mm / h. In order to ensure that the solute in the melting area of ​​the material rod can be quickly and evenly diffused to the melt zone and maintain the stability of the interface between the crystal and the melt growth, the upper shaft is usually rotated faster and the lower shaft is rotated slower. Specifically, the rotation speed of the upper shaft is generally 15-35 rpm, and the rotation speed of the lower shaft is 10-30 rpm, and the two rotate in opposite directions to enhance the melt convection effect, improve the uniformity of the components, and optimize the stability and quality of crystal growth. Oxygen was passed through the entire crystal growth process to reduce the formation of oxygen vacancy defects.

[0023] When the crystal grows to the required length, the upper shaft is pulled upward to separate the feed rod from the crystal, and the rotation of the upper and lower shafts is stopped. The temperature is slowly lowered to room temperature over 6 to 12 hours.

[0024] (4) Thermal annealing

[0025] After growing KTN crystals by the floating zone method, thermal annealing is usually required to improve the quality of the crystal and optimize its optical and electrical properties. The annealing process and parameters are: annealing is carried out in a tubular furnace, the annealing atmosphere is oxygen, and in order to achieve the effect of repairing oxygen vacancies, the annealing pressure should be 0.2-0.5MPa, the annealing temperature is 900-1000℃, the constant temperature time is 12-24h, the heating rate is 2-5℃ / min, and the cooling rate is 10-20℃ / h. For KTN crystals, the reasons and functions of thermal annealing are mainly: (1) During the growth process of the floating zone method, due to the large temperature gradient in the melting zone, there is usually a large internal stress inside the crystal. Thermal annealing can slowly release these internal stresses, improve the mechanical stability of the crystal, and reduce the risk of crystal breakage; (2) During the crystal growth process, K 2 CO 3 The decomposition of 2 When crystals grow in such a low-oxygen environment, oxygen atoms can easily detach from the crystal lattice due to thermal excitation, forming a large number of oxygen vacancies. This defect can cause color centers in the crystals, resulting in color changes (usually blue or yellow), and reducing the transmittance, dielectric, and electro-optical properties of the crystals.

[0026] On the other hand, the present invention provides potassium tantalate niobate crystals obtained by the above method.

[0027] The beneficial effects of the present invention are as follows:

[0028] (1) Aiming at the component fluctuation problem caused by the incongruent melting characteristics of potassium tantalate niobate crystals during the growth process, based on the traditional floating zone method, by optimizing the raw material ratio, different component designs are adopted for the initial melt zone (the area formed by the flux rod flux) and the feed rod, and a dissolution-diffusion-crystallization dynamic equilibrium mechanism is adopted. Using the replenishment effect of the feed rod to ensure the stability of the melt zone composition, high-quality KTN crystals with a component uniformity of 10 -5 / mm are successfully prepared, which is significantly better than the traditional CZ method (the component uniformity is only 10 -3 / mm).

[0029] (2) Compared with the traditional Czochralski method and its improved double crucible real-time feeding technology, the floating zone method for growing KTN crystals (i.e., step 3) does not need to rely on precious metal crucibles, avoiding the loss of crucible materials, thus significantly reducing the equipment cost and raw material consumption. In addition, the floating zone method process is more concise, without the need for a complex real-time feeding system or high-precision composition control equipment, greatly reducing the production difficulty and making it more suitable for large-scale industrial production.

[0030] (3) Due to the residue of the melt at the bottom of the crucible in the traditional Czochralski method, the raw material utilization rate is usually only 50-70%. The "large crucible growing small crystals" method improved based on the Czochralski method, although aiming to reduce component fluctuations, has a lower raw material utilization rate because a large amount of melt reserve is required, resulting in more raw material waste. In contrast, the floating zone method adopts a crucible-free design, the feed rod directly supplies the melt, and the melt directly crystallizes, with almost no material loss, making the raw material utilization rate close to 100% and greatly improving the resource utilization efficiency. Description of the Drawings

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative efforts.

[0032] Figure 1 is the phase diagram of the KTaO 3 -KNbO 3 binary system;

[0033] Figure 2 is the schematic diagram of the device for growing crystals by the floating zone method in this patent;

[0034] Figure 3 is the schematic diagram of the process of growing crystals by the floating zone method in this patent;

[0035] Figure 4Photograph of the crystal grown by the floating zone method for Example 1;

[0036] Figure 5 Component uniformity of the crystal in Example 1 in the

[100] and

[001] directions;

[0037] Figure 6 Component uniformity of the crystal in Example 2 in the

[100] and

[001] directions;

[0038] Figure 7 Component uniformity of the crystal in Example 3 in the

[100] and

[001] directions;

[0039] Figure 8 Crystal grown by the Czochralski method in Comparative Example 1 and its component uniformity in the

[100] and

[001] directions;

[0040] Figure 9 Comparison of the dielectric properties of the crystal grown by the Czochralski method in Comparative Example 1 and the crystal grown by the floating zone method in Example 1;

[0041] Figure 10 Crystal grown by the method of growing small crystals in a large crucible in Comparative Example 2 and its component uniformity on the (100) and (001) planes.

[0042] In the figure, 1. upper shaft rod, 2. quartz tube, 3. feeding rod, 4. ellipsoidal mirror, 5. halogen lamp, 6. molten zone, 7. seed crystal, 8. lower shaft rod, 9. air inlet hole, 10. heating zone, 11. flux rod. Detailed implementation manners

[0043] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0044] Example 1: Growth of KTN crystal with Curie temperature Tc = 25°C

[0045] (1) Determine the raw material ratio: KTN crystals with Curie temperature near room temperature are the most widely used. In this example, we plan to grow crystals with a Curie point of 25°C. According to the empirical formula:

[0046]

[0047] The corresponding crystal composition (i.e., solid-phase composition) can be calculated to be Combined with the phase diagram ( Figure 1 ), the corresponding liquid-phase components can be calculated using the solidus equation (1) and the liquidus equation (2) as Based on this, it is determined that:

[0048] The molar ratio of the raw materials for the required flux rod is: K 2 CO 3 :Ta 2 O 5 :Nb 2 O 5 = 1.4:0.2908:0.7092;

[0049] The molar ratio of the raw materials for the required feed rod is: K 2 CO 3 :Ta 2 O 5 :Nb 2 O 5 = 1.15:0.6115:0.3885.

[0050] (2) Sintering and preparation of raw material rods: Use high-purity raw materials of 4N grade K 2 CO 3 , Ta 2 O 5 and Nb 2 O 5 to prepare the raw materials required for the flux rod and the feed rod according to the above ratios respectively. First, weigh the required raw materials according to the raw material ratio, then use a mixer to mechanically mix the raw materials of the above two ratios for 24 hours, and place the mixed raw materials of the two ratios in a platinum crucible respectively, and use a muffle furnace for the first pre-sintering, the sintering temperature is 900 °C, and the sintering time is 8 hours; take out the two raw materials of the first sintering from the platinum pot, grind them respectively using a ball mill, and then place them in a platinum pot and use a muffle furnace for the second sintering, the sintering temperature is 1050 °C, and the sintering time is 8 hours.

[0051] Take out the two polycrystalline materials after the second sintering, grind them again using a ball mill to obtain refined powder, and then pour the powder into a mold and use the cold isostatic pressing method to make the flux rod and the feed rod. The pressure of the cold isostatic pressing is 260 MPa, and the pressing time is 4 minutes. The size of the finally formed feed rod is The size of the flux rod is The height of the flux rod is less than 10 times the heating zone (i.e., the melting zone height, about 15 mm) of the floating zone furnace used.

[0052] (3) Crystal growth: Use an optical floating zone furnace (model: FZ-T-12000-X-I-S-SU) from Crystal Systems. The device structure is as Figure 2, fix the rod-shaped KTN seed crystal 7 at the upper end of the lower shaft rod 8 of the floating zone furnace, place the flux rod on the upper part of the seed crystal, fix the feeding rod 3 at the lower end of the upper shaft rod 1 of the floating zone furnace, adjust the axes of the feeding rod and the flux rod 11 so that they are aligned vertically, and move the feeding rod downward so that it is 1-2 mm above the flux rod ( Figure 3 a), seal the seed crystal, the flux rod and the feeding rod in the quartz tube 2. Use a four-ellipsoid reflective halogen lamp (mainly including the ellipsoid reflector 4 and the halogen lamp 5) to heat the flux rod area to melt the upper end of the flux rod, the seed crystal and the lower end of the feeding rod ( Figure 3 b), move the feeding rod downward so that it contacts the already melted flux rod to form a molten zone 6 ( Figure 3 c), start crystal growth. Oxygen is introduced through the air inlet hole 9 during the whole crystal growth process to reduce the formation of oxygen vacancy defects.

[0053] During the whole growth process, the halogen lamp remains stationary, and both the feeding rod and the seed crystal rod move downward. The diameter and composition of the crystal are controlled by coordinating the downward feeding speed of the feeding rod and the downward pulling speed of the seed crystal rod. In the initial stage, it is usually necessary to reduce the ratio of the descent rate of the feeding rod to the seed crystal to narrow the crystal diameter to eliminate dislocations, inhibit the spread of defects and impurity diffusion; in the isodiametric stage, set the downward speed of the feeding rod to 2.2 mm / h, the downward speed of the seed crystal to 1.5 mm / h, the rotation speed of the upper shaft rod to 30 rpm in the forward direction, and the rotation speed of the lower shaft rod to 20 rpm in the reverse direction. During this process, on one hand, the raw materials in the feeding rod supplement the composition of the molten zone as the temperature rises, and on the other hand, the melt in the molten zone crystallizes on the seed crystal as the temperature drops. The two form a dynamic balance, thus maintaining the stability of the melt composition in the molten zone; when the crystal grows to about 75 mm, gradually reduce the ratio of the descent rate of the feeding rod to the seed crystal, appropriately neck down and then change the upward shaft rod to upward pulling to separate the feeding rod from the crystal, and then stop the rotation of the upper shaft rod and the lower shaft rod, and slowly cool to room temperature after 10 hours to obtain a crystal with a length of 70 mm and a maximum diameter of about 9 mm ( Figure 4 ).

[0054] (4) Thermal annealing treatment: Place the crystal prepared in the previous step in a muffle furnace for annealing. The annealing atmosphere is oxygen, the annealing pressure is 0.3 MPa, the heating rate is 4 °C / min, the annealing temperature is 950 °C, the holding time is 24 h, and then slowly cool to room temperature at a rate of 15 °C / h.

[0055] After cutting, grinding and polishing the crystal obtained by the above steps, measure its composition uniformity in the

[100] and

[001] directions. The results are as Figure 5 , the composition fluctuation of the crystal in the

[001] direction is slightly larger than that in the

[100] direction, but the composition uniformity has reached 10 -5 / mm level, which can meet the requirements of optical applications.

[0056] Example 2: Growth of KTN Crystal with Curie Temperature Tc = 50 °C

[0057] The operation process of this example is the same as that of Example 1, and only some process parameters are modified. The specific modifications are as follows:

[0058] (1) Determine the raw material ratio: In this example, we plan to grow a crystal with a Curie point of 50 °C. According to the empirical formula, the corresponding crystal composition (i.e., solid-phase composition) should be Using the solidus equation and liquidus equation, the corresponding liquid-phase composition can be calculated as Since the melting point corresponding to this composition is higher than that in Example 1, to compensate for the loss of K 2 CO 3 volatilization, it is necessary to appropriately increase the proportion of K 2 CO 3 Accordingly, it is determined that:

[0059] The molar ratio of raw materials for the required flux rod is: K 2 CO 3 :Ta 2 O 5 :Nb 2 O 5 = 1.5:0.2598:0.7402;

[0060] The molar ratio of raw materials for the required feed rod is: K 2 CO 3 :Ta 2 O 5 :Nb 2 O 5 = 1.2:0.5749:0.4251.

[0061] (2) Sintering and preparation of raw material rods: Weigh the required raw materials according to the above raw material ratio, mix them mechanically and then conduct two sinterings: the first pre-sintering temperature is 950 °C and the sintering time is 6 hours; the second sintering temperature is 1100 °C and the sintering time is 6 hours. Then grind them again to obtain refined powder, pour the raw materials of the two ratios into the mold and press them into a flux rod and a feed rod respectively. The pressure of cold isostatic pressing is 300 MPa and the pressing time is 3 minutes. The size of the finally formed feed rod is The size of the flux rod is

[0062] (3) Crystal growth: Using an optical floating zone furnace, install the feed rod, flux rod, and seed crystal prepared in the previous step. After necking down, enter the isodiametric growth stage. In this stage, the descent speed of the feed rod is set to 3 mm / h, the descent speed of the seed crystal remains 2 mm / h, the rotation speed of the upper shaft is 15 rpm in the forward direction, and the rotation speed of the lower shaft is 10 rpm in the reverse direction. When the crystal grows to about 75 mm, after appropriate necking down, separate the feed rod from the crystal, and slowly cool it to room temperature after 12 hours to obtain a crystal with a length of 72 mm and a maximum diameter of about 7 mm.

[0063] (4) Thermal annealing treatment: Place the crystal prepared in the previous step in a muffle furnace for annealing. The annealing atmosphere is oxygen, the annealing pressure is 0.2 MPa, the heating rate is 5 °C / min, the annealing temperature is 1000 °C, the holding time is 20 h, and then slowly cool it to room temperature at a rate of 20 °C / h.

[0064] After cutting, grinding, and polishing the crystal obtained by the above steps, measure its compositional uniformity in the

[100] and

[001] directions. The results are as Figure 6 , and the compositional fluctuations of the crystal in the

[001] and

[100] directions both reach 10 -5 / mm level, which can meet the requirements of optical applications.

[0065] Example 3: Growth of KTN crystal with Curie temperature Tc = 0 °C

[0066] The operation process of this example is the same as that of Example 1, only some process parameters are modified. The specific modifications are as follows:

[0067] (1) Determine the raw material ratio: In this example, we plan to grow a crystal with a Curie point of 0 °C. According to the empirical formula, the corresponding crystal composition (i.e., solid-phase composition) should be Then, using the solidus equation and liquidus equation, the corresponding liquid-phase composition can be calculated as Since the melting point corresponding to this composition is slightly lower and the evaporation of K 2 CO 3 is slightly less, we appropriately reduce the proportion of K 2 CO 3 . Based on this, it is determined that:

[0068] The molar ratio of raw materials for the required flux rod is: K 2 CO 3 :Ta 2 O 5 :Nb 2 O 5 = 1.2:0.3252:0.6748;

[0069] The molar ratio of raw materials for the required feed rod is: K 2 CO3 :Ta 2 O 5 :Nb 2 O 5 = 1.1:0.6482:0.3518.

[0070] (2) Sintering and preparation of raw material rods: Weigh the required raw materials according to the above raw material ratio, mechanically mix them and then conduct two sinterings: The first pre-sintering temperature is 850 °C and the sintering time is 12 hours; the second sintering temperature is 1000 °C and the sintering time is 12 hours. Then grind again to obtain refined powder, and pour the raw materials of the two ratios into the mold to press into flux rods and feeding rods respectively. The pressure of cold isostatic pressing is 200 MPa and the pressing time is 5 minutes. The size of the finally formed feeding rod is The size of the flux rod is

[0071] (3) Crystal growth: Use an optical floating zone furnace, install the feeding rod, flux rod and seed crystal prepared in the previous step. After necking down, enter the equal-diameter growth stage. In this stage, the descending speed of the feeding rod is set at 1 mm / h, the descending speed of the seed crystal remains 0.5 mm / h, the rotation speed of the upper shaft rod is 35 rpm in the forward direction, and the rotation speed of the lower shaft rod is 30 rpm in the reverse direction. When the crystal grows to about 75 mm, after appropriate necking down, separate the feeding rod from the crystal, and slowly cool it to room temperature after 12 hours to obtain a crystal with a length of 64 mm and a maximum diameter of about 6 mm.

[0072] (4) Thermal annealing treatment: Place the crystal prepared in the previous step in a muffle furnace for annealing. The annealing atmosphere is oxygen, the annealing pressure is 0.5 MPa, the heating rate is 2 °C / min, the annealing temperature is 900 °C, and the constant temperature time is 12 h. Then slowly cool it to room temperature at a speed of 10 °C / h.

[0073] After cutting, grinding and polishing the crystal obtained by the above steps, measure its compositional uniformity in the

[100] and

[001] directions. The results are as Figure 7 , and the compositional fluctuations of the crystal in the

[001] and

[100] directions both reach 10 -5 / mm level, which can meet the requirements of optical applications.

[0074] Performance comparison between the KTN crystal grown by the traditional Czochralski method in Comparative Example 1 and the crystal prepared in Example 1

[0075] Using the SKJ-50 type single crystal furnace produced by Shenyang Kejing, according to K 2 CO 3 :Ta 2 O 5 :Nb 2 O 5Ingredients are proportioned at a ratio of 1.2:0.3:0.7, with a total raw material weight of 1000 g. After two pre-sintered materials, crystal growth is carried out by the Czochralski method. The size of the platinum crucible is φ73 mm × 51.5 mm, and the obtained crystal size is 9 mm × 9 mm × 27 mm( Figure 8 a), and its component uniformity is measured( Figure 8 b), and it is found that the components in the

[001] direction are approximately in a gradient distribution, and the uniformity is only 10 -3 / mm.

[0076] As an important electro-optic crystal, the electro-optic performance of KTN crystal is usually evaluated by measuring the relative permittivity near room temperature. A higher permittivity usually means a more excellent electro-optic coefficient. Figure 9 Shows the comparison of the dielectric properties of the KTN crystal grown by the traditional Czochralski method in this example and the crystal grown by the floating zone method in Example 1. The results show that near room temperature, the permittivity of the crystal grown by the floating zone method is increased by about 1.5 to 3.5 times compared with the crystal grown by the traditional Czochralski method, significantly improving its electro-optic performance.

[0077] Comparison of the component uniformity of the crystal grown by the method of "large crucible and small crystal" in Comparative Example 2

[0078] Using the TDL-H50A single crystal furnace produced by Xi'an University of Technology, according to K 2 CO 3 :Ta 2 O 5 :Nb 2 O 5 =1.2:0.3:0.7 for proportioning ingredients, the size of the crucible used is Ф120 mm × 60 mm, the melt mass is about 3000 g, and the size of the obtained KTN crystal is a × b × c = 10 mm × 12 mm × 13 mm( Figure 10 a), and its component uniformity is measured( Figure 10 b), and the specific data are as follows:

[0079]

[0080]

[0081] The results show that the component fluctuations on the (100) plane (corresponding to the

[001] direction) are relatively large, with a standard deviation of 0.000592, and the range (maximum value - minimum value) within 10 mm is 0.001884; while the component fluctuations on the (001) plane (corresponding to the

[100] direction) are relatively small, with a standard deviation of 0.000241 and a range of 0.000570.

[0082] Overall, the component uniformity of this method can only reach 10 -4 / mm level. Although the "large crucible with small crystals" method has improved compared to the traditional Czochralski method, there are still limitations. In contrast, the crystals grown by the floating zone method in this patent have a component uniformity of 10 -5 / mm level, showing higher stability and consistency.

Claims

1. A method for growing potassium tantalate niobate crystals with high optical uniformity by a floating zone method, characterized in that: The method comprises the following steps: (1) According to the Curie temperature Tc of the target crystal to be prepared, the solid phase composition is calculated by empirical formula (1) Then use the solid phase equation (2) and the liquid phase equation (3) to calculate the liquid phase composition in: Empirical formula: Solidus equation: Liquidus equation: From this, the raw material ratio of the material rod required for growing potassium tantalate niobate crystals by the floating zone method can be further determined: Molar ratio of raw materials for flux rod: Molar ratio of raw materials for feeding rod: (2) According to the raw material ratio determined in step (1), raw materials for flux rods and feed rods are prepared respectively, the two raw materials are mixed and pre-sintered for the first time, and after the pre-sintering is completed, the raw materials for the first sintering are ground, and then a second sintering is performed. After the pre-sintering is completed, the two raw materials are ground to form flux rods and feed rods; (3) Fix the feed rod on the upper shaft of the floating zone furnace, install the potassium tantalate niobate seed crystal on the lower shaft, then place the flux rod on the seed crystal, heat the flux rod area to melt the upper end of the seed crystal, the lower end of the feed rod and the entire flux rod, move the feed rod downward so that its bottom contacts the melted flux rod to form a melting zone, and start crystal growth; (4) Annealing the potassium tantalate niobate crystal obtained in the previous step to obtain the final crystal.

2. The method for growing potassium tantalate niobate crystals with high optical uniformity by the floating zone method according to claim 1, characterized in that: The step (2) is to prepare the raw materials of the flux rod and the feed rod respectively according to the raw material ratio determined in the step (1), mechanically mix the two raw materials respectively and put them into a platinum crucible, use a muffle furnace to carry out the first pre-sintering, and then grind the raw materials of the first sintering with a ball mill, and then carry out the second sintering. After the completion, grind the two raw materials into powder, pour them into spherical molds respectively, and use cold isostatic pressing to make flux rods and feed rods. The length of the flux rod should be slightly less than the height of the heating zone of the floating zone furnace.

3. The method for growing potassium tantalate niobate crystals with high optical uniformity by the floating zone method according to claim 1, characterized in that: The step (3) is to fix the feed rod on the upper shaft of the floating zone furnace, install the potassium tantalate niobate seed crystal on the lower shaft, then place the flux rod on the seed crystal, move the feed rod downward so that it is 1 to 2 mm above the flux rod, heat the flux rod area to melt the upper end of the seed crystal, the lower end of the feed rod and the entire flux rod, move the feed rod downward so that its bottom contacts the melted flux rod to form a melting zone, and start crystal growth. During the growth process, the melting rate of the feed rod and the crystallization rate of the upper end of the seed rod are kept equal by adjusting the downward speed of the feed rod and the downward speed of the seed crystal rod.

4. The method for growing potassium tantalate niobate crystals with high optical uniformity by a floating zone method according to claim 1, characterized in that: The annealing process and parameters of step (4) are as follows: annealing is performed in a tubular furnace, the annealing atmosphere is oxygen, the annealing pressure should be 0.2-0.5 MPa, the annealing temperature is 900-1000°C, the constant temperature time is 12-24h, the heating rate is 2-5°C / min, and the cooling rate is 10-20°C / h.

5. The method for growing potassium tantalate niobate crystals with high optical uniformity by a floating zone method according to claim 1, characterized in that: In the step (2), the temperature of the first pre-sintering is 850-950° C., and the sintering time is 6-12 hours.

6. The method for growing potassium tantalate niobate crystals with high optical uniformity by a floating zone method according to claim 1, characterized in that: In the step (2), the temperature of the secondary pre-sintering is 1000-1100° C., and the sintering time is 6-12 hours.

7. The method for growing potassium tantalate niobate crystals with high optical uniformity by a floating zone method according to claim 2, characterized in that: In the step (2), the cold isostatic pressing pressure is 200-300 MPa, and the pressing time is 3-5 minutes.

8. The method for growing potassium tantalate niobate crystals with high optical uniformity by a floating zone method according to claim 1, characterized in that: In the step (3), oxygen is passed through the entire crystal growth process.

9. The method for growing potassium tantalate niobate crystals with high optical uniformity by a floating zone method according to claim 3, characterized in that: In the step (3), the descending speed of the feed rod is 1-3 mm / h, the descending speed of the seed crystal is 0.5-2 mm / h, the rotation speed of the upper shaft is 15-35 rpm, the rotation speed of the lower shaft is 10-30 rpm, and the two rotate in opposite directions.

10. Potassium tantalate niobate crystal obtained according to the method described in any one of claims 1 to 9.

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