A method for growing large-size high-purity PMN single crystal by top-seed solution method

Abstract

This invention belongs to the field of crystal material preparation technology and discloses a method for growing large-size, high-purity PMN single crystals using a top-seeded crystal growth method. It employs a gradient molten salt single crystal furnace equipped with a hollow oxygen-bearing seed crystal rod and a bidirectional rotation mechanism. The seed crystal rod and crucible rotate in opposite directions. By optimizing the composite flux ratio and identifying the drawbacks of excessively high / low ratios, adopting an extremely low cooling rate adapted to the temperature sensitivity of PMN, and designing a staged speed control strategy that precisely correlates crystal outline size with rotation speed, combined with temperature field optimization due to the opposite rotation of the seed crystal and crucible, and nucleation control through oxygen-bearing at the seed crystal to create a cold end, the crystal outline size is observed at high temperature, and process parameters are adjusted in real time to achieve high-purity growth of large-size PMN single crystals. The final obtained PMN single crystals are verified by XRD, scanning electron microscopy, EDS energy dispersive spectroscopy, and dielectric temperature spectroscopy to be free of impurities and have a pure crystal form. The single crystal size breaks through the large-size level and has good formability, meeting the application requirements of devices in the fields of electronics, piezoelectrics, and optics.

Description

Technical Field

[0001] This invention belongs to the field of crystal material preparation technology, specifically relating to a method for growing large-size, high-purity PMN single crystals based on the top seed crystal bubbling method (TSSG). The chemical formula of the PMN single crystal is Pb(Mg1 / 3Nb2 / 3)O3. The single crystals grown by this method are free of impurities and have good formability, and can be widely used in medical imaging, electrostrictive actuators, electro-optic modulation and other fields. Background Technology

[0002] PMN single crystals are typical relaxor ferroelectric materials, exhibiting excellent dielectric and electrostrictive properties near the Curie temperature, along with high transparency. They are a core material for fabricating high-performance electronic and optical devices, and the industry's demand for large-size, high-purity PMN single crystals is increasingly urgent. However, PMN single crystals are non-uniformly melting compounds and are highly sensitive to temperature changes. Current top-seeded methods for growing PMN single crystals still face numerous technical challenges.

[0003] 1. The flux ratio lacks precise optimization. If the ratio is too high, the crystallization temperature is too low, and the PMN solution is prone to polycrystalline precipitation by cooling. Single nucleation cannot be controlled, and the PMN content in the solute is relatively reduced, becoming a bottleneck for the growth of large-size single crystals. If the ratio is too low, the crystallization temperature is higher than 1250℃ (close to the melting point of pure PMN component 1320℃). Although it is still in the liquid phase, it will lead to increased PbO volatilization (PbO is easy to volatilize at high temperatures) and platinum crucible corrosion (PbO corrodes platinum at >1300℃), causing the system composition to deviate (the proportion of PMN increases). Subsequent crystallization is prone to the formation of pyrochlore impurities, reducing the yield. Neither of these can achieve the growth of high-purity large-size single crystals.

[0004] 2. PMN has a high surface viscosity and poor surface stability, making it extremely sensitive to temperature changes. It is very easy to form polycrystalline or dendritic phenomena at saturation temperature. The cooling rate of the existing growth process is too high, which can easily cause thermal stress concentration and component segregation inside the crystal, leading to the precipitation of impurity phases, the formation of polycrystalline and dendritic phenomena, and the appearance of many internal cracks, which limits the improvement of single crystal size and quality.

[0005] 3. The rotation speed control of the seed crystal and crucible is singular and does not combine with the crystal's visible outline size stage for dynamic correlation control. Moreover, the conventional rotation mode is prone to uneven melt temperature field distribution, turbulent melt convection, poor solute transport, and crystal growth is prone to problems such as inclusions, crystal breakage, and outline distortion.

[0006] 4. The nucleation stage lacks a directional cold end, resulting in disordered crystal growth, low nucleation quality, and a high defect rate in subsequent single crystal growth. At the same time, the growth process is mostly controlled in a rough manner, without adjusting parameters in real time based on the actual outline size observed by the naked eye, which easily leads to poor crystal formability.

[0007] 5. Existing technologies do not fully utilize the temperature field optimization advantage of the reverse rotation of the seed crystal and the crucible. The problems of local thermal accumulation and solute segregation in the melt temperature field are prominent, making it difficult to achieve continuous and stable growth of large-size single crystals.

[0008] Currently, PMN single crystals grown using existing technologies generally suffer from small size, low purity, high defect rate, and irregular profiles. XRD and dielectric temperature spectroscopy tests easily reveal impurity peaks, failing to meet the industrial demand for large-size, high-purity PMN single crystals and the crystal quality requirements of optical devices. Therefore, developing a top-oxygen-seeded crystal growth method that adapts to the characteristics of PMN single crystal materials, optimizes temperature field and nucleation control, and precisely correlates rotation speed with profile size, to achieve stable growth of large-size, high-purity PMN single crystals, has significant practical implications and industrial application value. Summary of the Invention

[0009] To address the shortcomings of existing PMN single crystal growth technologies, this invention provides a method for growing large-size, high-purity PMN single crystals using a top-oxygen-passing seed crystal growth method. This method optimizes the composite flux dosage ratio and identifies the drawbacks of excessively high or low ratios. It employs an extremely low cooling rate adapted to the temperature sensitivity of PMN, designs a staged rotation speed control strategy precisely correlated with the visually observable crystal outline size, and combines this with temperature field optimization through the reverse rotation of the seed crystal rod and crucible, nucleation control through oxygen passage of the hollow seed crystal rod to create directional cold ends, and strategies to reduce oxygen vacancies. By periodically observing the crystal outline size through the single crystal furnace observation window and adjusting process parameters in real time, this method solves the technical problems of impurity phase generation, size limitation, low nucleation quality, uneven temperature field, and irregular outline in PMN single crystal growth. Ultimately, it yields large-size, high-purity PMN single crystals free of impurities, as verified by XRD and dielectric temperature spectroscopy.

[0010] To achieve the above objectives, this invention provides a method for growing large-size, high-purity PMN single crystals using a top-seeded crystal growth method. The method employs a gradient molten salt single crystal furnace equipped with a hollow oxygen-bearing seed crystal rod and a bidirectional rotation mechanism. The seed crystal rod and crucible rotate in opposite directions. PMN single crystal growth is achieved through optimized flux ratios, extremely low cooling rates, and staged rotation speed control related to crystal outline dimensions. The crystal outline dimensions are observed in real-time via high-temperature monitoring, and process parameters are adjusted accordingly. The growth method includes the following steps:

[0011] S1. Using PbO, MgO and Nb2O5 as PMN raw materials, select a composite flux of PbO-Li2O-B2O3 ternary system, mix and grind them, and put them into a crucible;

[0012] S2. Fix the square PMN seed crystal to the lower end of the hollow seed crystal rod, connect the oxygen supply device to the seed crystal rod, and make the molten salt single crystal furnace, crucible and seed crystal center collinear by fine-tuning the seed crystal rod to complete the pre-growth positioning and equipment debugging.

[0013] S3. Place the crucible in the heating zone of the single crystal furnace, heat it to the melting temperature of PMN raw material and keep it at a constant temperature, so that the original PMN material and the composite flux can be fully melted to form a uniform melt.

[0014] S4. After the heat preservation is completed, crystal growth is started at an extremely low cooling rate of 0.5~2℃ / d. At the same time, oxygen is continuously supplied to the seed crystal through the hollow seed crystal rod to create an oriented cold end at the seed crystal. During the growth process, the seed crystal rod and the crucible are kept rotating in opposite directions. Meanwhile, by observing the outline size of the crystal growth, the rotation speed of the seed crystal rod and the crucible is adjusted in stages according to the different outline size stages of the crystal, and the rotation speed is gradually reduced in each stage.

[0015] S5. After the crystal grows to the target large size and has a regular outline, slowly lift the crystal out of the melt, and take it out after gradient cooling annealing treatment, thus obtaining a large-size high-purity PMN single crystal.

[0016] The present invention employs an extremely low cooling rate, which can be adapted to the high temperature sensitivity of PMN single crystals and avoid thermal stress, component segregation and impurity phase precipitation caused by excessively fast cooling rate; the seed crystal rod and crucible rotate in opposite directions, which can optimize the melt temperature field distribution, make the melt temperature field more uniform, and promote directional convection of melt and uniform transport of solute.

[0017] As a further preferred technical solution of the present invention, in step S1, the composite flux is a PbO-Li2O-B2O3 ternary system (molar ratio 85:5~15:3~8), and the mass ratio of the composite flux to the PMN raw material is (0.8~1.2):1. When the flux ratio is too high, it is easy for the flux to remain in the crystal and form impurity phases, reducing the purity of the PMN single crystal. At the same time, excessive flux will dilute the solute concentration in the melt, causing the crystal growth rate to drop sharply and crystal breakage to occur easily. When the flux ratio is too low, the raw material is not fully melted, the melt uniformity is poor, there are unmelted particles of raw material, and the crystal growth is prone to defects such as inclusions and dislocations. Moreover, the melt viscosity is high, solute transport is hindered, and continuous growth of large-size single crystals cannot be achieved.

[0018] As a further preferred technical solution of the present invention, in step S1, the PMN raw materials are PbO, MgO, and Nb2O5 with a purity ≥99.99%, and the molar ratio of the raw materials is PbO:MgO:Nb2O5=1:(1 / 3±0.01):(1 / 2±0.01). After mixing, the powder is ball-milled for 20-30 hours until the particle size is ≤50μm. The crucible is a cylindrical crucible made of 99.9% high-purity platinum. In a specific example, the diameter of the crucible is 60mm and the height is 70mm.

[0019] As a further preferred technical solution of the present invention, in step S3, the melting temperature of the PMN raw material is 1200~1260℃, preferably 1230℃, and the constant temperature holding time is 12~24h, preferably 24h.

[0020] As a further preferred technical solution of the present invention, in step S4, the continuous oxygen supply is a continuous supply of dry pure O2, with an oxygen flow rate of 5~15mL / min, preferably 10mL / min, and an oxygen pressure of 0.01~0.03MPa, preferably 0.02MPa. By supplying oxygen, a slightly larger directional cold end is formed at the contact point between the seed crystal and the melt, thereby achieving high-quality directional nucleation. The hollow seed crystal rod is made of platinum and has an internal air-permeable channel, so that no impurities are precipitated during the oxygen supply process.

[0021] As a further preferred technical solution of the present invention, in step S4, the gradient cooling annealing process is as follows: first, the temperature is lowered to 950°C at a rate of 10~20°C / h and held for 3~4 days; then, the temperature is lowered to 400°C at a rate of 5~10°C / h and held for 2~3 days; finally, the temperature is lowered to 25~30°C at a rate of 2~5°C / h and the crystal is taken out; the rate at which the crystal is lifted from the melt is 0.5~1 mm / h.

[0022] As a further preferred technical solution of the present invention, the PMN seed crystal is (100) or (001) oriented, with a size of 3mm×3mm×10~20mm, free of impurities and obvious defects; the large-size high-purity PMN single crystal obtained by growth is tested by XRD phase analysis and has no impurity peaks, a single crystal form, and the characteristic peaks of dielectric temperature spectrum test are consistent with the peaks of relaxor ferroelectric single crystals, with characteristics such as diffuse phase transition and frequency dispersion, single crystal size ≥35mm (1 inch level), and can be adjusted to 50mm level and above according to requirements, the crystal has no obvious inclusions, cracks, pore defects, and yield ≥90%.

[0023] As a further preferred technical solution of the present invention, in step S4, by observing the outline size of the crystal growth, the rotation speed of the seed crystal rod and the crucible is adjusted in stages according to different outline size stages of the crystal.

[0024] The phased control of the correlation between crystal profile size and rotation speed is based on the visually identifiable crystal profile size, and is adjusted in multiple stages, preferably 3 to 10 stages. The rotation speed in each stage starts and stops gradually, and the rotation direction is reversible. The range of seed crystal rod rotation speed is 1 to 12 r / min, and the range of crucible reverse rotation speed is 0.5 to 6 r / min. The crystal is cubic, and the profile in each stage is square / cubic prism.

[0025] In one specific embodiment, based on the growth characteristics of PMN single crystals, the present invention uses the crystal outline size that can be identified by the naked eye as the core criterion for speed control. Combined with the core logic that "as the side length of the cubic crystal increases, the speed of the seed crystal rod and the crucible gradually decreases," a five-stage speed control strategy is designed to adapt to the needs of each growth stage of PMN single crystals, as detailed below:

[0026] Stage 1 (from crystal nucleus formation to spherical primary crystal stage): In this stage, the crystals range from tiny crystal nuclei visible to the naked eye to square outlines with a side length L≤10mm. The seed crystal rod rotation speed is controlled at 8~12r / min, and the crucible reverse rotation speed is 4~6r / min. The low rotation speed ensures the directional growth of the crystal nuclei and avoids the melt convection disturbing the crystal nuclei, which would lead to disordered nucleation.

[0027] Stage 2 (Preliminary Crystal Growth Stage): In this stage, the crystal is a regular cubic prism with a side length of L=10~20mm. The seed crystal rod speed is controlled at 6~8r / min, and the crucible reverse speed is 3~5r / min. The speed is appropriately reduced to adapt to the radial uniform growth of the cubic prism and maintain the stability of melt convection while avoiding crystal form shift.

[0028] Stage 3 (Medium-size formation stage): In this stage, the crystal outline expands from a cubic prism shape to a large-size flat shape, with a side length of 20 mm to 60% of the target side length. The seed crystal rod rotation speed is controlled at 3~5 r / min, and the crucible reverse rotation speed is 2~3 r / min. The low rotation speed reduces melt disturbance, enhances solute directional transport, and achieves a smooth expansion of the crystal shoulder into a square outline, avoiding crystal corner distortion.

[0029] Stage 4 (Larger Size Growth Stage): In this stage, the crystal edge length reaches 80%~95% of the target edge length, forming a regular cube outline. The seed crystal rod rotation speed is controlled at 2~3 r / min, and the crucible reverse rotation speed is 1~2 r / min. The extremely low rotation speed maintains the crystal's equal edge length growth, ensuring that all faces and corners of the cube are uniform and reducing internal stress during crystal growth.

[0030] Phase 5 (Final Shaping Phase): In this phase, the crystal edge length reaches 95% of the target edge length to the final size, completing the rounding of the cube outline. The seed crystal rod rotation speed is controlled at 1~2 r / min, and the crucible reverse rotation speed is 0.5~1 r / min. The minimum rotation speed minimizes the surface stress of the crystal and avoids cracks and missing corners on the cube edges during the final phase.

[0031] As a further preferred technical solution of the present invention, in step S4, the crystal outline size can be observed through the high borosilicate observation window of the single crystal furnace in conjunction with a high-temperature observation mirror. The crystal outline size is observed every 4 to 6 hours. The rotation speed is finely adjusted in real time according to the change in outline size. The rotation speed control accuracy is ±1 r / min (that is, the actual rotation speed is stabilized near the target value, and the deviation does not exceed ±1 r / min). The start-up and stop rate of the rotation speed at each stage is 1 r / min·min.

[0032] The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble growth method provided by this invention has undergone comprehensive process optimization considering the material properties of PMN single crystals and the actual operation scenarios of industrial production. Compared with existing technologies, it has the following significant advantages:

[0033] 1. This invention precisely optimizes the composite flux ratio and clarifies the drawbacks of excessively high / low ratios, thus avoiding the problems of impurity phase generation and poor melt uniformity from the source. Combined with the high purity of raw materials and precise ratio, it ensures the high purity of PMN single crystals, and XRD and dielectric temperature spectroscopy tests show no impurity peaks.

[0034] 2. This invention employs an extremely low cooling rate of 0.5~2℃ / d, which is perfectly suited to the high temperature sensitivity of PMN single crystals. This effectively avoids thermal stress concentration, component segregation, and impurity phase precipitation within the crystal, ensuring the integrity of the single crystal and fundamentally solving the cracking problem caused by excessively high cooling rates.

[0035] 3. The reverse rotation mode of the seed crystal rod and crucible designed in this invention significantly optimizes the melt temperature field distribution and melt directional convection, and solves the problems of local heat accumulation and solute segregation. At the same time, combined with the crystal outline size that can be identified by the naked eye, a five-stage rotation speed control strategy is designed to achieve precise matching between rotation speed and crystal growth outline, ensuring that the outline of large-size cubic single crystal is regular and free from problems such as skewing and missing corners.

[0036] 4. The directional cold end manufactured by oxygen-bearing a platinum hollow seed crystal rod in this invention achieves high-quality directional nucleation of PMN single crystals, reduces the nucleation defect rate, decreases the formation of oxygen vacancies and other defects, and enhances ferroelectricity. Simultaneously, the temperature field stabilization effect of reverse rotation further improves crystal quality and reduces defects such as inclusions and pores.

[0037] 5. This invention adopts a control method of visual observation + real-time parameter adjustment, which is suitable for the actual operation scenario of industrial production. It does not require complicated monitoring equipment. The outline size can be identified by using the observation window of the single crystal furnace in conjunction with the high-temperature observation mirror. It is easy to operate and highly controllable. Precise control can be achieved by observing once every 4 to 6 hours, which greatly reduces the cost of production equipment.

[0038] 6. The PMN single crystals grown by this invention have been verified by XRD and dielectric temperature spectroscopy to be free of impurities and have pure crystal form. The single crystal size can be stably reached 1 inch or larger. The crystals are regular cubic with no obvious inclusions, cracks, or pore defects. The yield is ≥90%. The process is simple and has good repeatability, making it suitable for industrial-scale production. It fills the gap in the existing technology for growing large-size, high-purity PMN single crystals. Attached Figure Description

[0039] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0040] Figure 1 This is the internal structure of the crystal growth furnace for the top seed crystal growth method (TSSG) of the present invention.

[0041] Figure 2 The images show (a) a large-size PMN crystal blank and (b) an enlarged view of the crystal blank of Example 1.

[0042] Figure 3 XRD orientation of the natural plane of the PMN crystal in Example 1.

[0043] Figure 4 The X-ray diffraction (XRD) pattern of the PMN relaxor ferroelectric single crystal from Example 1 after it was ground into polycrystalline powder is shown.

[0044] Figure 5 The images show the cross-sectional scanning electron microscope (SEM) image and EDS elemental analysis of the PMN crystal in Example 1, where (a) is a cross-sectional image of the crystal magnified 3000 times under the SEM; (b) is the elemental analysis of the cross-sectional image under energy-dispersive X-ray spectroscopy; and (c) is the intensity distribution map of the elements under energy-dispersive X-ray spectroscopy.

[0045] Figure 6 The dielectric temperature spectrum of the PMN crystal in Example 1 is shown.

[0046] Figure 7 XPS oxygen vacancy analysis of PMN crystals grown with and without oxygen in Example 1, where (a) is a PMN single crystal grown without oxygen and (b) is a single crystal grown with oxygen.

[0047] Figure 8 Variable-temperature ferroelectric testing of the PMN crystal in Example 1.

[0048] The objectives, features, and advantages of this invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0049] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0050] Unless otherwise defined, the technical terms used in the following embodiments have the same meanings as commonly understood by those skilled in the art to which this invention pertains. Unless otherwise specified, the experimental reagents used in the following embodiments are conventional biochemical reagents; and the experimental methods described are conventional methods.

[0051] The test methods used in the examples are as follows:

[0052] 1. XRD phase analysis: X-ray diffractometer was used with a scanning range of 10°~80° and a scanning rate of 5° / min to analyze the crystalline phases and the presence of impurity phases;

[0053] 2. Dielectric temperature spectrum test: A precision impedance analyzer was used to test the temperature range of -100℃ to 200℃, the heating rate was 2℃ / min, and the test frequencies were 100Hz, 1kHz, 10kHz, 100kHz, and 1MHz. The dielectric characteristic peaks and the presence of superimposed impurity peaks were analyzed.

[0054] 3. Crystal size and profile: The crystal size is measured using digital vernier calipers, and the regularity of the crystal profile and the type of defects are observed by the naked eye and metallographic microscope. The yield rate is calculated.

[0055] Example 1

[0056] The method for growing large-size, high-purity PMN single crystals (35mm-scale, 1-inch-scale) provided in this embodiment is as follows:

[0057] S1. Using PbO, MgO, and Nb2O5 (99.99% purity) with a molar ratio of 1:0.333:0.5 as PMN raw materials, and selecting a composite flux of PbO-Li2O-B2O3 ternary system with a molar ratio of 85:10:5, the composite flux and PMN raw materials are mixed and ground at a mass ratio of 1:1 and placed into a cylindrical crucible (60mm in diameter and 70mm in height) made of 99.9% high-purity platinum.

[0058] S2. Fix the (100) oriented, defect-free PMN seed crystal with dimensions of 3mm×3mm×15mm to the lower end of the hollow seed crystal rod, connect the oxygen supply device to the seed crystal rod, and make the molten salt single crystal furnace, crucible and seed crystal center collinear by fine-tuning the seed crystal rod to complete the pre-growth positioning and equipment debugging.

[0059] S3. Place the crucible in the heating zone of the single crystal furnace, heat it to the melting temperature of PMN raw material 1230℃, and keep it at a constant temperature for 5 days to allow the original PMN material and composite flux to fully melt and form a uniform melt.

[0060] S4. After the heat preservation period, crystal growth is initiated at an extremely low cooling rate of 1℃ / d. Simultaneously, 4N (99.99%) high-purity O2 is continuously supplied to the seed crystal via a hollow seed crystal rod at a flow rate of 10mL / min and a pressure of 0.02MPa to create an oriented cold end at the seed crystal. During the growth process, the seed crystal rod and crucible are kept rotating in opposite directions. The crystal growth profile is observed visually every 5 hours through a high-borosilicate observation window and a high-temperature observation microscope in the single crystal furnace. Based on the visually identifiable changes in the crystal profile, the rotation speed of both the seed crystal rod and the crucible is dynamically adjusted in five stages. Each stage uses a slow start-stop mode (acceleration / deceleration rate of 1r / min·min), and the rotation direction is reversible to avoid melt convection turbulence caused by sudden changes in rotation speed. After observation, the rotation speed is finely adjusted in real time according to the profile size, with a speed control accuracy of ±1r / min. The specific rotation speeds are as follows:

[0061] Phase 1 (L≤10mm): Seed rod 10r / min, crucible 5r / min;

[0062] Phase 2 (L=10~20mm): Seed rod 7r / min, crucible 4r / min;

[0063] Phase 3 (L=20~28mm): Seed rod 4r / min, crucible 2.5r / min;

[0064] Phase 4 (L=28~33.25mm): Seed rod 2.5r / min, crucible 1.5r / min;

[0065] Stage 5 (L=33.25~35mm): Seed rod 1.5r / min, crucible 0.8r / min;

[0066] After the crystal grows to the target large size and has a regular outline, the crystal is slowly lifted out of the melt and annealed according to the following process: 15℃ / h→950℃ (1.5d)→8℃ / h→400℃ (1d)→3℃ / h→28℃, and naturally held at the temperature for 20h. Gradient cooling annealing treatment is then carried out to finally obtain a high-purity PMN single crystal with a diameter of 35.1mm×34.8mm×34.7mm.

[0067] This embodiment successfully obtained a large-size PMN single crystal. The crystal is a regular cube with a rounded outline, no skew, missing corners, or cracks. XRD phase analysis showed that only the characteristic diffraction peaks of PMN single crystal appeared in the scanning range of 10°~80°, without any impurity peaks, indicating a single crystal form. Dielectric temperature spectroscopy at a frequency of 1kHz showed that the dielectric peak of the PMN single crystal was sharp and without superposition of impurity peaks, proving that the single crystal was free of impurity phases and had excellent purity. Metallurgical microscopy showed that the crystal had no inclusions or pore defects, with a yield of 95%.

[0068] Example 2: Growth of 36 mm - grade PMN single crystal

[0069] The process steps are as follows:

[0070] S1. Take PbO, MgO, and Nb2O5 (purity 99.99%) with a molar ratio of 1:0.33:0.505 as PMN raw materials, select a PbO - Li2O - B2O3 composite flux with a molar ratio of 85:10:5, mix and grind them in a mass ratio of 0.8:1 until the particle size is ≤50 μm, and load them into a 99.9% high - purity platinum cylindrical crucible ( );

[0071] S2. Select a defect - free PMN seed crystal with a (100) orientation and a size of 3 mm×3 mm×15 mm, fix it at the lower end of the platinum hollow seed rod, connect an oxygen - passing device, and level it to make the centers of the single - crystal furnace, crucible, and seed crystal collinear;

[0072] S3. Place the crucible in the heating zone, raise the temperature to 1200 °C (the lower limit of the PMN melting temperature), keep it at a constant temperature for 24 h to fully melt the raw materials and the flux to form a homogeneous melt;

[0073] S4. After the heat preservation ends, grow at an extremely low cooling rate of 0.5 °C / d, continuously pass dry 4N (99.99%) high - purity O2 through the seed rod at a flow rate of 5 mL / min and a pressure of 0.01 MPa to create a directional cold end; keep the seed rod and the crucible rotating in opposite directions, observe the crystal contour every 6 h, and adjust the rotation speed in five stages according to different contour stages of the crystal (the slow - start and slow - stop rate is 1 r / min·min, and the adjustment accuracy is ±1 r / min). The specific rotation speeds are as follows:

[0074] Stage 1 (L≤10 mm): Seed rod 8 r / min, crucible 4 r / min;

[0075] Stage 2 (10 mm < L≤20 mm): Seed rod 6 r / min, crucible 3 r / min;

[0076] Stage 3 (20 mm < L≤21.6 mm, 60% of the target side length): Seed rod 3 r / min, crucible 2 r / min;

[0077] Stage 4 (28.8 mm≤L≤34.2 mm, 80% - 95% of the target side length): Seed rod 2 r / min, crucible 1 r / min;

[0078] Stage 5 (34.2 mm < L≤36 mm): Seed rod 1 r / min, crucible 0.5 r / min;

[0079] After the crystal grows to more than 30 mm, the crystal is lifted from the melt at a rate of 0.5 mm / h, and gradient cooling annealing is adopted: it is cooled to 950 °C at 10 °C / h and held for 4 days → cooled to 400 °C at 5 °C / h and held for 3 days → cooled to 25 °C at 2 °C / h, and then the crystal is taken out after completion.

[0080] The growth results of this embodiment are as follows:

[0081] A regular cubic PMN single crystal of 36.2 mm × 35.9 mm × 35.7 mm is obtained; only PMN characteristic diffraction peaks appear in the XRD scan from 10° to 80°, without impurity peaks; the characteristic peak in the dielectric temperature spectrum at 1 kHz is sharp, without impurity peak superposition; no inclusions, cracks, or pores are observed by scanning electron microscopy, and the yield is 92%.

[0082] Example 3: Growth of 34 mm - sized PMN single crystal

[0083] The process steps are as follows:

[0084] S1. Using PbO, MgO, and Nb2O5 (purity 99.99%) with a molar ratio of 1:0.34:0.495 as PMN raw materials, selecting a PbO - Li2O - B2O3 composite flux with a molar ratio of 85:10:5, mixing and grinding them in a mass ratio of 1.2:1 to a particle size ≤ 50 μm, and loading them into a 99.9% high - purity platinum crucible ( )

[0085] S2. Selecting a defect - free PMN seed crystal with (001) orientation and dimensions of 3 mm × 3 mm × 20 mm, fixing it at the lower end of a platinum hollow seed crystal rod, connecting an oxygen - passing device and completing the central positioning;

[0086] S3. Placing the crucible in the heating zone, heating it to 1260 °C (the upper limit of PMN melting temperature), and holding it at a constant temperature for 18 h to make the melt mix evenly;

[0087] S4. After the heat preservation ends, grow at an extremely low cooling rate of 2 °C / d, continuously introduce dry 4N (99.99%) high - purity O2 with a flow rate of 15 mL / min and a pressure of 0.03 MPa; observe the crystal contour every 4 h, and according to different contour stages of the crystal, adjust the reverse rotation speed in five stages (slow start and slow stop, accuracy ±1 r / min), specifically as follows:

[0088] Stage 1 (L ≤ 10 mm): The seed crystal rod rotates at 12 r / min, and the crucible rotates at 6 r / min;

[0089] Stage 2 (10 mm < L ≤ 20 mm): The seed crystal rod rotates at 8 r / min, and the crucible rotates at 5 r / min;

[0090] Stage III (20 mm < L ≤ 22.8 mm, 60% of the target side length): The seed crystal rod rotates at 5 r / min and the crucible rotates at 3 r / min;

[0091] Stage IV (30.4 mm ≤ L ≤ 36.1 mm, 80% - 95% of the target side length): The seed crystal rod rotates at 3 r / min and the crucible rotates at 2 r / min;

[0092] Stage V (36.1 mm < L ≤ 38 mm): The seed crystal rod rotates at 2 r / min and the crucible rotates at 1 r / min;

[0093] S5. After the crystal grows to 38 mm, it is lifted at a rate of 1 mm / h, and gradient cooling annealing is adopted: it is cooled to 950 °C at 20 °C / h and held for 3 days → cooled to 400 °C at 10 °C / h and held for 2 days → cooled to 30 °C at 5 °C / h, and then the crystal is taken out after completion.

[0094] The growth results of this embodiment are as follows:

[0095] A regular cube PMN single crystal of 33.1 mm × 34.8 mm × 30.9 mm is obtained; the XRD phase analysis shows a single crystal form and no impurity phase; the dielectric temperature spectrum conforms to the typical characteristics of relaxor ferroelectric single crystals and there is no superimposed impurity peak; the crystal has no inclusions and pores, and only very slight polishing marks that can be removed exist at the edge, no cracks, and the yield rate is 93%.

[0096] Figure 1 Figure 1 is a schematic structural diagram of the high-purity PMN single crystal growth furnace in Example 1, and the top oxygen supply method is used to directionally guide the growth of large-size PMN single crystals.

[0097] Figure 2 shows the large-size pure PMN relaxor ferroelectric single crystal blank grown in Example 1, and at the same time, a small-size PMN crystal prepared by the flux method is used as a control sample; it can be clearly observed that the contour size of the bulk PMN single crystal prepared by the present invention reaches 35 mm × 35 mm × 30 mm, close to the 1-inch level, the crystal is pure inside without obvious impurities, the contour is regular and square, and it is a typical perovskite structure crystal. The above-mentioned small-size PMN crystal used for comparison is prepared by the conventional unoptimized flux method: a traditional single / non-precise ratio of flux is mixed with PMN raw materials, where PbO is used as the fluxing agent and the ratio to the raw materials is 1:6. After being melted at 1300 °C, it is cooled and crystallized naturally at 5 °C per hour. The optimized processes such as directional cold-end nucleation, stage-by-stage rotation speed control, extremely low cooling rate, and oxygen supply are not adopted, and only small-size crystals are obtained through a simple molten salt crystallization method. Its growth process has no precise temperature field, nucleation, and contour control. Therefore, the crystal size is limited and the formability is far inferior to the large-size single crystal prepared by the present invention.

[0098] Figure 3XRD orientation of the natural plane of the PMN crystal in Example 1, standard perovskite along <001> The material was grown in a specific direction. XRD was used to orient it to obtain two diffraction peaks, (100) and (200), which correspond to the (001) plane.

[0099] Figure 4 The X-ray diffraction (XRD) pattern of the PMN relaxor ferroelectric single crystal from Example 1, after being ground into polycrystalline powder, is shown. All diffraction peaks correspond to typical characteristic peaks of perovskite, with no PbO inclusions or other secondary phases present, indicating a pure ABO3-type perovskite structure. Furthermore, the (200) peak indicates that the PMN crystal has a cubic structure at room temperature, consistent with the subsequently measured dielectric temperature spectrum.

[0100] Figure 5 The images show the cross-sectional scanning electron microscope (SEM) image and EDS elemental analysis of the PMN crystal from Example 1. Detailed analysis data are shown in Table 1, which lists the percentage of each element. No cracks or other porous structures were observed in the crystal cross-section at 3000x magnification, indicating it is a single crystal rather than a polycrystalline crystal. The elements present in the cross-section are Pb, Mg, Nb, and O, with no other elements present, demonstrating the purity and impurity-free nature of the PMN crystal, ensuring its suitability for optical and other precision applications.

[0101] Table 1. Elemental content data of PMN crystals under energy-dispersive X-ray spectroscopy.

[0102]

[0103] Figure 6 The dielectric temperature spectrum of the PMN crystal in Example 1 shows the typical dielectric characteristics of relaxor ferroelectric materials. The dielectric constant-temperature (ε'-T) curve of the relaxor ferroelectric single crystal exhibits a broadened dielectric peak, lacking the sharp Curie point TC of traditional ferroelectrics. The temperature corresponding to the maximum dielectric constant is called the characteristic temperature Tm. Frequency dispersion effect: as the measurement frequency (f) increases, Tm shifts towards higher temperatures, and the maximum dielectric constant ε'max decreases. Furthermore, the dielectric peak of the PMN crystal at 1 kHz is located near -13℃, similar to PMN ceramics, further confirming the authenticity of the PMN crystal.

[0104] Figure 7XPS oxygen vacancy analysis was performed on PMN crystals grown with and without oxygen in Example 1, where (a) is a PMN single crystal grown without oxygen and (b) is a single crystal grown with oxygen. Three XPS oxygen vacancies were obtained through fitting: the lattice oxygen peak (~529.0 eV) corresponds to oxygen atoms at the O1 / O2 positions in the perovskite structure, participating in the formation of Pb-O, Mg-O, Nb-O, and Ti-O bonds; the defect correlation peak (~531.0 eV) corresponds to oxygen atoms around the oxygen vacancy whose binding energy decreases due to increased electron cloud density (the oxygen vacancy provides two free electrons), or oxygen species chemically adsorbed on the oxygen vacancy; the surface adsorption peak (~532.0 eV) corresponds to the possible presence of a small amount of adsorbed OH groups. - Or H2O, the higher the oxygen vacancy concentration, the higher the surface adsorbed oxygen content. By comparing the oxygen vacancy peak near 531.0 eV, it was found that the oxygen vacancy in the PMN single crystal after oxygen-passing growth was effectively suppressed compared to the un-oxygen-passing crystal, which will enhance the ferroelectricity and dielectric properties of the crystal;

[0105] Figure 8 The temperature-dependent ferroelectric test of the PMN crystal in Example 1 shows that below the TC temperature, the hysteresis loop is expanded, exhibiting typical relaxor ferroelectric characteristics. As the crystal gradually undergoes a trigonal to cubic phase transition, the hysteresis loop gradually narrows and loses its ferroelectricity.

[0106] Comparative Example 1 (flux ratio was too high, with a mass ratio of 7:1 to PMN raw material)

[0107] As a control experiment, the remaining process parameters were the same as in Example 1, except that the mass ratio of composite flux to raw materials was adjusted to 7:1.

[0108] Results: The grown crystal size was 23.5mm×22.9mm×12.7mm, and the crystal outline was basically regular. However, XRD test showed impurity peaks due to flux residue. Dielectric temperature spectrum test showed impurity peaks superimposed next to the characteristic peaks. Flux inclusions were present inside the crystal, indicating that the nucleation temperature was too low and the supercooling was too large, which caused the crystal to grow arbitrarily. In addition, the amount of flux was too small and the amount of material was too large, which reduced the nutrients required by the crystal and prevented it from growing to the required size. The yield was only 50%.

[0109] Comparative Example 2 (higher cooling rate, 5℃ / d)

[0110] As a control experiment, the remaining process parameters were the same as in Example 1, except that the cooling rate was adjusted to 5℃ / d.

[0111] Results: Dendritic growth or spontaneous nucleation occurred during crystal growth, leading to the formation of polycrystalline structures and disrupting the integrity of single crystals. Simultaneously, melt inclusions or impurities were captured by the rapidly growing crystal, forming defects such as voids and inclusions. Furthermore, the internal stress of the crystal could not be effectively released, which weakened the crystal's relaxation ferroelectric properties, such as dielectric peak broadening, reduced frequency dispersion effects, and even deterioration of ferroelectric performance, resulting in a yield of 0%.

[0112] Comparative Example 3 (Spindle speed is not related to profile size, single spindle speed)

[0113] As a control experiment, the remaining process parameters were the same as in Example 1, except that the reverse rotation speed of the single seed crystal rod at 7 r / min and the crucible at 4 r / min was used during the growth process, and was not adjusted according to the contour size stage.

[0114] Results: The grown crystals were 34.5mm×31.3mm×30.8mm in size. The crystal outline was severely skewed, with local missing corners and polycrystalline tops. XRD tests after grinding the crystals into powder showed no impurity peaks, but a small number of inclusions were found inside the crystals. The crystal surface was polycrystalline while the interior was monocrystalline, resulting in a yield of only 75%.

[0115] Comparative Example 4 (seed rod and crucible rotate in the same direction)

[0116] As a control experiment, the remaining process parameters were the same as in Example 1, except that the seed crystal rod and the crucible were adjusted to rotate in the same direction.

[0117] Results: The grown crystal size was 20.3 mm × 17.8 mm × 15.6 mm. The melt temperature field distribution was uneven, the radial growth rate of the crystals varied greatly, the outline was irregular, and a small number of inclusions were present inside. XRD test showed impurity peaks of PbO and pyrochlore phases. The yield was only 70%.

[0118] While specific embodiments of the present invention have been described above, those skilled in the art should understand that these are merely illustrative examples, and various changes or modifications can be made to these embodiments without departing from the principles and essence of the present invention. The scope of protection of the present invention is defined only by the appended claims.

Claims

1. A method for growing large-size, high-purity PMN single crystals using a top-seeded crystal bubble-growth method, characterized in that, Includes the following steps: S1. Using PbO, MgO and Nb2O5 in a molar ratio of 1:(1 / 3±0.01):(1 / 2±0.01) as PMN raw materials, a composite flux of PbO-Li2O-B2O3 ternary system is selected. The composite flux and PMN raw materials are mixed and ground in a mass ratio of (0.8~1.2):1 and loaded into a crucible. S2. Fix the square PMN seed crystal to the lower end of the hollow seed crystal rod, connect the oxygen supply device to the seed crystal rod, and make the molten salt single crystal furnace, crucible and seed crystal center collinear by fine-tuning the seed crystal rod to complete the pre-growth positioning and equipment debugging. S3. Place the crucible in the heating zone of the single crystal furnace, heat it to the melting temperature of PMN raw material and keep it at a constant temperature, so that the original PMN material and the composite flux can be fully melted to form a uniform melt. S4. After the heat preservation is completed, crystal growth is started at an extremely low cooling rate of 0.5~2℃ / d. At the same time, oxygen is continuously supplied to the seed crystal through the hollow seed crystal rod to create an oriented cold end at the seed crystal. During the growth process, the seed crystal rod and the crucible are kept rotating in opposite directions. Meanwhile, by observing the outline size of the crystal growth, the rotation speed of the seed crystal rod and the crucible is adjusted in stages according to the different outline size stages of the crystal, and the rotation speed is gradually reduced in each stage. S5. After the crystal grows to the target large size and has a regular outline, slowly lift the crystal out of the melt, and take it out after gradient cooling annealing treatment, thus obtaining a large-size high-purity PMN single crystal.

2. The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble-growing method according to claim 1, characterized in that, The molar ratio of PbO, Li2O, and B2O3 in the composite flux is 85:5~15:3~8.

3. The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble-growing method according to claim 1, characterized in that, The purity of the PbO, MgO, and Nb2O5 is ≥99.99%; And / or, the melting temperature of the PMN raw material is 1200~1260℃.

4. The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble-growing method according to claim 1, characterized in that, In step S1, the mixture is ground until the particle size is ≤50μm, and the crucible is a cylindrical crucible made of 99.9% high-purity platinum.

5. The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble-growing method according to claim 1, characterized in that, The continuous oxygen supply refers to the continuous introduction of dry, high-purity O2 at a flow rate of 5-15 mL / min. And / or, the oxygen supply pressure is 0.01~0.03MPa; And / or, the hollow seed crystal rod is made of platinum and has an internal ventilation channel.

6. The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble-growing method according to claim 1, characterized in that, The gradient cooling annealing process is as follows: first, the temperature is lowered to 950℃ at a rate of 10~20℃ / h and held for 3~4 days; then, the temperature is lowered to 400℃ at a rate of 5~10℃ / h and held for 2~3 days; finally, the temperature is lowered to 25~30℃ at a rate of 2~5℃ / h and the crystal is removed; the rate at which the crystal is lifted from the melt is 0.5~1mm / h.

7. The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble-growing method according to claim 1, characterized in that, The PMN seed crystal is (100) or (001) oriented, with a size of 3mm×3mm×10~20mm, free of impurities and obvious defects; the large-size high-purity PMN single crystal obtained by growth has a size ≥35mm (1 inch).

8. The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble-growing method according to claim 1, characterized in that, The phased correlation control between the crystal profile size and the rotation speed is based on the crystal profile size that can be identified by the naked eye. It is divided into at least three stages for adjustment. The rotation speed in each stage starts and stops slowly, and the rotation direction is reversible. The range of the seed crystal rod rotation speed is 1~12 r / min, and the range of the crucible reverse rotation speed is 0.5~6 r / min.

9. The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble-growing method according to claim 1, characterized in that, Based on different crystal contour size stages, the rotation speed of the seed crystal rod and the crucible is adjusted in stages, and the adjustment is carried out in five stages, as follows: Stage 1 (from crystal nucleus formation to spherical primary crystal stage): In this stage, the crystals range from tiny crystal nuclei visible to the naked eye to square outlines with a side length L≤10mm. The seed crystal rod rotation speed is controlled at 8~12r / min, and the crucible reverse rotation speed is 4~6r / min. Stage 2 (Preliminary Crystal Growth Stage): In this stage, the crystal is a regular cubic prism with a side length of L=10~20mm. The seed crystal rod rotation speed is controlled at 6~8r / min, and the crucible reverse rotation speed is 3~5r / min. Stage 3 (Medium-size formation stage): In this stage, the crystal outline expands from cubic prism to large-size flattened shape, with a side length of 20 mm to 60% of the target side length. The seed crystal rod rotation speed is controlled at 3~5 r / min, and the crucible reverse rotation speed is 2~3 r / min. Stage 4 (Larger Size Growth Stage): In this stage, the crystal edge length reaches 80%~95% of the target edge length, forming a regular cubic outline. The seed crystal rod rotation speed is controlled at 2~3 r / min, and the crucible reverse rotation speed is 1~2 r / min. Phase 5 (Final Shaping Phase): In this phase, the crystal edge length reaches 95% of the target edge length to the final size, completing the rounding of the cube outline. The seed crystal rod rotation speed is controlled at 1~2 r / min, and the crucible reverse rotation speed is 0.5~1 r / min.

10. The method for growing large-size, high-purity PMN single crystals using the top-seeded crystal bubble-growing method according to claim 1, characterized in that, In step S4, the crystal profile size is observed every 4 to 6 hours, and the rotation speed of the seed crystal rod and the crucible is finely adjusted in real time according to the changes in the profile size. The rotation speed control accuracy is ±1 r / min.

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