Method for purifying metal indium based on oriented growth of single crystal

Abstract

The embodiment of the invention discloses a method for purifying metal indium based on single crystal oriented growth. The method comprises the following steps: filling a cavity in a graphite boat with a metal indium raw material with the purity of not less than 5N; wherein the end part of the cavity in the graphite boat is conical, and the inner wall of the graphite boat is covered with a pyrolytic carbon or pyrolytic boron nitride layer; the end of the conical cavity in the graphite boat is heated, so that the metal indium raw material is molten and filled in the conical cavity; cooling the end part of the conical cavity of the graphite boat at a set rate, and forming an indium single crystal seed crystal at the tip of the conical cavity of the graphite boat; the area behind the indium single crystal seed crystal is heated to form a metal indium melting area with the length not smaller than the set length, the metal indium melting area is moved in the indium single crystal growth direction at the set speed, the indium single crystal seed crystal grows on a solid-liquid interface in an epitaxial mode, impurities gradually move towards the metal indium melting area, and the solid-liquid interface gradually moves backwards till the metal indium melting area is finished; and sequentially and repeatedly performing single crystal generation and single crystal directional zone smelting for multiple times to obtain metal indium with the purity being greater than 7N.

Description

Technical Field

[0001] This invention belongs to the field of high-purity metal material preparation technology, specifically relating to a method for purifying metallic indium based on single-crystal directional growth. Background Technology

[0002] Indium (In) is an important rare and dispersed metal. Its oxide (ITO) possesses excellent electrical conductivity and light transmittance, making it a core material for flat panel displays. High-purity indium is an irreplaceable substrate or source material for the preparation of III-V compound semiconductors (such as indium phosphide (InP) and indium antimonide (InSb). These applications have extremely stringent requirements for the purity of indium. Any trace impurities such as Cu, Fe, Zn, Sn, Cd, Tl, and Pb will significantly degrade its electrical properties and crystal defect density, leading to decreased device performance and reduced yield.

[0003] Currently, the main method for industrial production of high-purity indium is electrolytic refining combined with zone melting. Electrolytic refining can purify indium to 4N-5N (99.99%-99.999%) purity, but it is very difficult to further improve it to 6N and above. This is because although traditional polycrystalline zone melting can utilize the segregation effect for further purification, there are obvious bottlenecks: (1) Grain boundary trap effect: Indium has a low melting point (156.6℃) and coarse grains, and impurities are very easy to agglomerate at the grain boundaries; these grain boundaries not only become "warehouses" for impurities, but also form short-circuit paths for impurity migration during zone melting, which seriously hinders the effective transport of impurities to the end of the ingot. (2) Impurities with a segregation coefficient close to 1 are difficult to remove: For impurities with a segregation coefficient k≈1 in indium (such as certain forms of cadmium and tin), the driving efficiency of traditional zone melting is extremely low. (3) Oxidation and container contamination: Molten indium is easily oxidized in air and wets and corrodes most crucible materials (such as quartz and graphite) to varying degrees, introducing new contamination. (3) In order to break through the purity limit of polycrystalline materials, grain boundaries must be eliminated. Although there are crystal growth technologies such as the vertical Bridgman process in the existing technology, their main goal is to grow single crystals, and the equipment is complex and energy-intensive, which is not suitable for large-scale production of high-purity indium. Summary of the Invention

[0004] In view of this, the present invention discloses a method for purifying metallic indium based on single-crystal directional growth, which can effectively eliminate grain boundaries, suppress contamination, and efficiently remove all types of impurities (especially k≈1 impurities), and has significant industrial implications.

[0005] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, including:

[0006] Raw material preparation steps: Fill the cavity inside the graphite boat with metallic indium raw material with a purity of not less than 5N; wherein, the end of the cavity inside the graphite boat is set as conical, and the inner wall of the graphite boat is covered with a layer of pyrolytic carbon or pyrolytic boron nitride.

[0007] Single crystal formation steps: Heating the end of the conical cavity inside the graphite boat to melt and fill the conical cavity with metallic indium; cooling the end of the conical cavity of the graphite boat at a set rate of 0.5~2℃ / min to form an indium single crystal seed at the tip of the conical cavity of the graphite boat.

[0008] Single-crystal directional melting steps: Heat the graphite boat behind the indium single crystal seed, forming a metallic indium molten zone of not less than a set length within the graphite boat, and forming a solid-liquid interface between the single crystal seed and the metallic indium molten zone; move the metallic indium molten zone towards the indium single crystal growth direction at a set rate, so that the indium single crystal seed grows epitaxially at the solid-liquid interface, impurities gradually move towards the metallic indium molten zone, and the solid-liquid interface gradually moves backward until the metallic indium molten zone within the graphite boat ends;

[0009] Multiple melting steps: Repeat the above single crystal formation steps and single crystal directional region melting steps multiple times until metallic indium with a purity greater than 7N is obtained.

[0010] Furthermore, some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, wherein the single-crystal directional growth is carried out in a high-purity inert gas environment.

[0011] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, wherein the high-purity inert gas environment is a slightly positive pressure environment with a pressure of 105–110 kPa.

[0012] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, wherein the inert gas flow rate in the high-purity inert gas environment is 20–50 sccm.

[0013] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, wherein in the single-crystal directional regional melting step, the solid-liquid interface is set in a temperature range with a temperature gradient of 1 to 5 K / cm.

[0014] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, in which a graphite boat is placed inside a quartz tube, which is placed inside a horizontal tube furnace with three temperature zones. The three temperature zones include a high-temperature zone, a medium-temperature zone, and a low-temperature zone arranged sequentially. The temperature of the high-temperature zone is set at 180–200°C, and the length of the high-temperature zone is greater than the total length of the graphite boat. The starting temperature of the medium-temperature zone is 150–155°C, and the ending temperature is 100–120°C. The temperature of the low-temperature zone is set at 80–100°C. The length ratio of the high-temperature zone, the medium-temperature zone, and the low-temperature zone is set to 55:20:25.

[0015] In the single crystal formation step, the conical cavity end of the graphite boat is heated in the middle of the high temperature zone. After the metallic indium raw material is melted and filled into the conical cavity, the horizontal tube furnace is moved towards the high temperature zone at a set speed to form indium single crystal seed crystals at the tip of the conical cavity of the graphite boat.

[0016] In the single-crystal directional melting step, the graphite boat behind the indium single crystal seed is heated in the high-temperature zone to form a metallic indium molten zone, and a solid-liquid interface is formed between the single crystal seed and the metallic indium molten zone. The horizontal tube furnace is moved towards the high-temperature zone at a set rate, so that the indium single crystal seed grows epitaxially at the solid-liquid interface, impurities gradually move towards the metallic indium molten zone, and the solid-liquid interface gradually moves backward until the metallic indium molten zone in the graphite boat ends.

[0017] In some embodiments of the method for purifying metallic indium based on single-crystal directional growth, the horizontal tube furnace moving speed in the single-crystal directional region melting step is set to 1.0–8.0 mm / h.

[0018] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, wherein the quartz tube is set to a slightly positive pressure high-purity inert gas environment, the inert gas flow rate is 20-50 sccm, and the slightly positive pressure is 105-110 kPa.

[0019] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, wherein in the single-crystal formation step, the moving speed of the horizontal tube furnace is set to 0.4–2 mm / h; and in the single-crystal directional zone melting step, the moving speed of the horizontal tube furnace is set to 1.0–3.0 mm / h.

[0020] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, in which a graphite boat is placed inside a quartz tube, and the quartz tube is placed inside a precisely movable single-temperature zone horizontal heating furnace.

[0021] In the single-crystal directional melting step, a graphite boat in the region behind the indium single crystal seed is heated, and a metallic indium molten zone of not less than a set length is formed in the graphite boat. A solid-liquid interface is formed between the single crystal seed and the metallic indium molten zone. The single-temperature zone horizontal heating furnace is moved in the direction of indium single crystal growth at a set rate of 1.0 to 3.0 mm / h.

[0022] The method for purifying metallic indium based on single-crystal directional growth disclosed in this invention eliminates the impurity trap and fast channel of polycrystalline grain boundaries through the single-crystal growth mode. This allows all impurities to be driven and concentrated into the liquid metallic indium through the segregation effect at the solid-liquid interface, improving the purity of solid metallic indium. It can effectively remove impurities with k≈1. Even when the segregation coefficient is close to 1, multiple cycles of "melting-single-crystal solidification-single-crystal directional growth" can still exponentially reduce the concentration through repeated segregation, which is impossible to achieve with traditional methods. The entire process is protected by a high-purity inert atmosphere combined with a specially coated graphite boat, which greatly inhibits the oxidation of indium and the contamination of container materials, ensuring the purification effect. The final product is ultra-high purity indium with a purity ≥7N. By precisely controlling parameters such as temperature gradient and growth rate, the process is stable and easy to realize for industrial-scale mass production. Detailed Implementation

[0023] The term "embodiment" used herein, as an example, is not necessarily to be construed as superior to or better than other embodiments. Performance testing in the embodiments of this application, unless otherwise specified, employs conventional testing methods in the art. It should be understood that the terminology used in this application is merely for describing particular implementations and is not intended to limit the scope of this disclosure.

[0024] Unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; other experimental methods and technical means not specifically mentioned in this application refer to experimental methods and technical means commonly used by one of ordinary skill in the art.

[0025] The terms “basic” and “approximately” used in this document are to describe small fluctuations. For example, they can mean less than or equal to ±5%, such as less than or equal to ±2%, such as less than or equal to ±1%, such as less than or equal to ±0.5%, such as less than or equal to ±0.2%, such as less than or equal to ±0.1%, such as less than or equal to ±0.05%. Numerical data presented or expressed in range format in this document are used for convenience and brevity only, and should therefore be flexibly interpreted to include not only the explicitly listed values ​​that define the range, but also all independent values ​​or subranges contained within that range. For example, a numerical range of “1–5%” should be interpreted to include not only the explicitly listed values ​​from 1% to 5%, but also the independent values ​​and subranges within the indicated range. Thus, this numerical range includes independent values ​​such as 2%, 3.5%, and 4%, and subranges such as 1%–3%, 2%–4%, and 3%–5%, etc. This principle also applies to ranges that list only one value. Furthermore, this interpretation applies regardless of the width of the range or the characteristics described.

[0026] In this document, including in the claims, conjunctions such as "comprising," "including," "with," "having," "containing," "involving," and "accommodating" are understood to be open-ended, meaning "including but not limited to." Only the conjunctions "consisting of" and "composed of" are closed conjunctions.

[0027] To better illustrate the content of this application, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this application can be implemented even without certain specific details. In the embodiments, some methods, means, instruments, and devices well known to those skilled in the art are not described in detail in order to highlight the main points of this application.

[0028] Without conflict, the technical features disclosed in the embodiments of this application can be combined arbitrarily, and the resulting technical solution belongs to the content disclosed in the embodiments of this application.

[0029] In some embodiments, the method for purifying metallic indium based on single-crystal directional growth includes:

[0030] Raw material preparation steps: Fill the cavity inside the graphite boat with metallic indium raw material with a purity of not less than 5N; wherein, the end of the cavity inside the graphite boat is set as conical, and the inner wall of the graphite boat is covered with a layer of pyrolytic carbon or pyrolytic boron nitride.

[0031] Single crystal formation steps: Heating the end of the conical cavity inside the graphite boat to melt and fill the conical cavity with metallic indium; cooling the end of the conical cavity of the graphite boat at a set rate of 0.5~2℃ / min to form an indium single crystal seed at the tip of the conical cavity of the graphite boat.

[0032] Typically, industrial high-purity indium ingots with a purity of 5N (99.999%) or higher are ultrasonically cleaned using a mixture of organic solvents and inorganic acids to thoroughly remove surface oxides and organic contaminants. After rinsing with high-purity water and drying, they are placed in a high-purity graphite boat. The graphite boat undergoes high-temperature pyrolysis treatment, depositing a dense layer of pyrolytic carbon (PyC) or pyrolytic boron nitride (PBN) coating on its inner surface to prevent graphite contamination of the high-purity indium ingot raw material. One end of the graphite boat is machined into a wedge-shaped or conical crystal-leading end with a specific orientation, and a conical cavity is formed inside the crystal-leading end. The core purpose of machining one end of the graphite boat into a specific shape for crystal-leading or crystal selection is to achieve single crystal "leading" or "selection". Typically, multiple When indium ingots are melted and then resolidified, they naturally form a polycrystalline structure with countless randomly oriented grains. The role of the crystal-drawing end is to create an environment that favors the preferential growth of a single grain and eliminates other grains through its special geometry. The crystal-drawing end of the graphite boat is part of its own structure. During loading, the indium ingot raw material is directly loaded into and fills the conical cavity in this part, making it a whole with the material in the main chamber. After the metallic indium ingot melts, the region of the crystal-drawing end is also filled with melt. During the directional solidification process of the conical crystal-drawing end of the graphite boat during cooling, the melt in this region solidifies first and completes the crystal selection process through its special geometry, forming metallic indium in the conical cavity of the crystal-drawing end.

[0033] In some embodiments, the graphite boat is disposed inside a quartz tube, which is disposed inside a horizontal tube furnace with three temperature zones. The three temperature zones include a high-temperature zone, a medium-temperature zone, and a low-temperature zone arranged sequentially. The temperature of the high-temperature zone is set at 180–200°C, and the length of the high-temperature zone is greater than the total length of the graphite boat. The starting temperature of the medium-temperature zone is 150–155°C, and the ending temperature is 100–120°C. The temperature of the low-temperature zone is set at 80–100°C. The length ratio of the high-temperature zone, the medium-temperature zone, and the low-temperature zone is set at 55:20:25.

[0034] Typically, a stable, controllable, and movable axial temperature gradient is established within a three-temperature zone horizontal tube furnace. The high-temperature zone, with a temperature of 180–200°C, is slightly longer than the graphite boat, accounting for 55% of the total length. The medium-temperature zone begins immediately adjacent to the high-temperature zone, with a temperature set at 150–155°C, slightly below the melting point of indium, and ends at 100–120°C, well below the melting point, accounting for 20% of the total length. The low-temperature zone is set at 80–100°C, accounting for 25% of the total length. Precise movement of the horizontal tube furnace is achieved by using a servo motor and high-precision linear guides to move the entire furnace body while keeping the quartz tube and graphite boat stationary. After sealing the quartz tube, a high vacuum is first evacuated using a mechanical pump and a molecular pump assembly to an ultimate vacuum level better than 5 × 10⁻⁶. -4Pa; then, repeatedly fill and drain high-purity argon gas (Ar purity ≥ 99.9999%) at least three times to fully replace the residual active gas in the quartz tube; finally, carry out subsequent operations under the protection of dynamic argon gas flow rate (e.g. 20-50 sccm), and maintain the system pressure at a slightly positive pressure (approximately 105-110 kPa).

[0035] Typically, the lead end of the graphite boat is moved to the center of the high-temperature heating zone, and the heating program is precisely controlled to ensure that only the lead end portion (approximately 5-10% of the total length of the indium material) is completely melted. Then, the cooling rate is precisely controlled (0.5-2°C / min) or a tube furnace is moved to allow the molten indium to slowly solidify in a directional manner from the tip of the lead end toward the main body, usually at a solidification rate of 0.4-2 mm / h. Utilizing the supercooling effect of the wedge structure and the grain competition growth mechanism, a single, well-oriented grain is finally selected and formed at the lead end as a "seed" for subsequent growth.

[0036] Single-crystal directional melting steps: Heat the graphite boat behind the indium single crystal seed, forming a metallic indium molten zone of not less than a set length within the graphite boat, and form a solid-liquid interface between the single crystal seed and the metallic indium molten zone; move the metallic indium molten zone within the graphite boat in the direction of indium single crystal growth at a set rate, so that the indium single crystal seed grows epitaxially at the solid-liquid interface, impurities gradually move towards the metallic indium molten zone, and the solid-liquid interface gradually moves backward until the metallic indium molten zone within the graphite boat ends;

[0037] In some embodiments, a narrow molten zone is established behind the formed single-crystal "seed crystal," with a width approximately 1 / 2 to 2 / 3 of the indium ingot diameter. The tube furnace is moved at a constant low speed of 1.0 to 8.0 mm / h, allowing the molten zone to sweep across the entire indium ingot at a uniform speed, thus realizing the movement of the metallic indium molten zone. During this process, the molten indium undergoes epitaxial growth on the solid-liquid interface of the existing single-crystal "seed crystal," inheriting its lattice orientation, thereby ensuring that the entire indium ingot continues to grow in a single-crystal form. Impurities undergo segregation in front of the moving solid-liquid interface; impurities with k < 1 are pushed into the liquid phase, while impurities with k > 1 are captured into the solid phase and ultimately continuously driven to the tail end of the ingot.

[0038] Multiple melting steps: Repeat the above single crystal formation steps and single crystal directional region melting steps multiple times until metallic indium with a purity greater than 7N is obtained.

[0039] In some embodiments, after the first round of single-crystal directional regional melting is completed, the indium ingot is cooled to room temperature under a protective atmosphere and removed from the graphite boat. Using a high-purity ceramic knife in an ultra-clean environment, approximately 15-25% of the impurity-rich tail section is removed. The remaining high-purity main body is used as raw material and reloaded into a cleaned coated graphite boat. The single-crystal formation step and the single-crystal directional regional melting step are repeated multiple times. Typically, the regional melting process is performed at least four times, preferably five to seven times. As the purity increases, the melting zone movement rate can be increased to 3.0-8.0 mm / h in subsequent rounds of single-crystal directional regional melting.

[0040] Typically, after the final round of purification, 5% of the initial solidified portion and 20% of the tail section of the final indium ingot are removed, leaving the indium single crystal in the middle. The high-purity indium single crystal is then sampled in a cleanroom and impurity analysis is performed using glow discharge mass spectrometry (GD-MS) or high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS).

[0041] In a preferred embodiment, a high axial temperature gradient G of 1 to 5 K / cm is maintained near the solid-liquid interface, and the G / V ratio (where V is the growth rate) is controlled to ensure that the solid-liquid interface grows stably in a flat interface manner, avoiding the formation of cellular or dendritic crystals. This is the key to achieving efficient impurity segregation and high-quality single crystal growth.

[0042] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, wherein in the single-crystal formation step, a horizontal tube furnace is moved toward the high-temperature zone at a set rate of 0.4 to 2 mm / h; and in the single-crystal directional zone melting step, the horizontal tube furnace is moved toward the high-temperature zone at a set rate of 1.0 to 3.0 mm / h.

[0043] For single crystal growth, the smoothness and speed precision of the movement are crucial, as any tiny jitter or speed fluctuation can introduce defects (such as dislocations and grain boundaries) into the crystal; therefore, in some embodiments, the entire furnace body is moved while the quartz tube and graphite boat remain stationary.

[0044] Typically, a precision-movable horizontal tube furnace includes a furnace body system and a control system; the furnace body system includes:

[0045] Precision ball screws, as high-precision, low-friction transmission components, are used to convert the rotary motion of a motor into linear motion.

[0046] Servo motors or stepper motors are used as power sources to drive precision ball screws; servo motors offer superior performance, providing smoother motion, faster response, and higher torque; stepper motors are less expensive, but may produce micro-vibrations at extremely low speeds.

[0047] High-precision linear guides are designed to support the furnace body, ensuring its smooth movement along a single axis without wobbling.

[0048] An integrated mobile platform is used to set up the entire three-temperature zone furnace, with its base connected to the lead screw nut and linear guide slider;

[0049] The control system includes:

[0050] The motion controller is used to receive the pull rate command set by the user;

[0051] The driver amplifies the low-voltage signal from the controller to drive the motor. Typically, the servo driver receives encoder feedback from the motor in real time, forming a closed-loop control to ensure accurate speed.

[0052] Human-computer interaction components, typically touchscreens or computer software, are used to input and display parameters such as movement speed and displacement.

[0053] Some embodiments disclose a method for purifying metallic indium based on single-crystal directional growth, in which a graphite boat is placed inside a quartz tube, and the quartz tube is placed inside a precision-moving single-temperature zone horizontal heating furnace.

[0054] In the single-crystal directional melting step, a graphite boat in the region behind the indium single crystal seed is heated, and a metallic indium molten zone of not less than a set length is formed in the graphite boat. A solid-liquid interface is formed between the single crystal seed and the metallic indium molten zone. The single-temperature zone horizontal heating furnace is moved in the direction of indium single crystal growth at a set rate of 1.0 to 3.0 mm / h.

[0055] In a preferred embodiment, the single-temperature zone horizontal heating furnace can use resistance heating or induction heating, and it must ensure that the temperature field is uniform and the control accuracy is high, for example, the temperature accuracy is controlled within ±0.5℃.

[0056] The technical details are further illustrated below with reference to the embodiments.

[0057] Example 1

[0058] In Example 1, the method for purifying metallic indium based on single-crystal directional growth includes:

[0059] (1) Raw materials and pretreatment

[0060] Raw materials: Electrolytically refined indium of 5N (99.999%) was used as the starting material.

[0061] Pretreatment: The indium ingot is placed in a solution diluted with high-purity aqua regia (HCl:HNO3 = 3:1) and ultrapure water at a ratio of 1:10, and ultrasonically cleaned for 3-5 minutes to thoroughly remove surface oxides and organic contaminants; then it is repeatedly rinsed with ultrapure water (resistivity >18.2MΩ·cm) and dried in a vacuum drying oven at 100℃ for 2 hours.

[0062] Crucible preparation: A horizontal boat-shaped crucible made of high-purity pyrolytic boron nitride (PBN) is used, with one end of the crucible machined into a wedge-shaped crystal-leading end with a specific crystal orientation.

[0063] Equipment: A horizontal three-zone tubular resistance furnace with high-purity quartz tubes. This equipment can independently and precisely control the temperature of the three heating zones, thereby creating a stable and adjustable axial temperature gradient within the furnace.

[0064] Atmosphere control: After the PBN crucible filled with material is pushed into the center of the furnace tube and sealed, the system is first evacuated to a high vacuum (3×10⁻⁶) using a mechanical pump and a molecular pump assembly. -4 The process involves repeatedly filling and purging 6N-grade (99.9999%) high-purity argon gas three times; finally, the entire process is carried out under dynamic argon flow (30 sccm) and slight positive pressure (approximately 105 kPa) to prevent the oxidation and volatilization of indium.

[0065] (2) Single crystal seeding process (seed preparation)

[0066] Move the crystal-driving end of the crucible to the center of the high-temperature zone.

[0067] The temperature is increased by program, and only the indium at the seed end (about 10% of the total length of the raw material) is completely melted. The temperature is then controlled by program to cool down at a very slow rate (0.5 sequence control, with) so that the molten indium can solidify directionally from the tip of the wedge. By utilizing the constraint effect of the wedge and the grain competition growth mechanism, a single grain with an orientation of (101) is finally selected and formed as the "seed" for subsequent growth.

[0068] (3) Single crystal directional zone melting

[0069] A narrow molten zone approximately 15 mm wide is established behind the formed "seed crystal". The temperatures of each zone in the furnace are set, and an axial temperature gradient (G) of 2 K / cm is established and maintained at the solid-liquid interface. The heater or crucible is moved at a constant low speed V = 2.0 mm / h to allow the molten zone to sweep uniformly across the entire indium ingot.

[0070] Molten indium undergoes epitaxial growth at the solid-liquid interface of the single-crystal "seed crystal," inheriting its lattice orientation, thereby maintaining the single-crystal morphology of the entire ingot. Impurities are continuously segregated and driven to the tail end of the ingot.

[0071] (4) Multiple rounds of purification and excision

[0072] After the first round of zone melting is completed, the indium ingot is cooled to room temperature under a protective atmosphere and then removed.

[0073] In a clean bench, approximately 20% of the impurity-rich section at the tail end is removed using a high-purity ceramic scalpel. The remaining high-purity main body is then used as new raw material and reloaded into a cleaned PBN crucible.

[0074] Repeat steps (2) to (3) for a total of six rounds of zone melting. Starting from the fourth round, due to the increased purity of the raw materials, the melting zone moving speed can be appropriately increased to 4.0 mm / h to improve efficiency.

[0075] (5) Final Products and Analysis

[0076] After completing the sixth round of purification, 5% of the head (the initially solidified part) and 20% of the tail section of the final indium ingot are removed, leaving the indium in the middle section.

[0077] The high-purity indium was analyzed by glow discharge mass spectrometry (GD-MS). Results: The analysis report showed that the total content of all 31 key metallic impurity elements (Cu, Fe, Ni, Zn, Cd, Sn, Tl, Pb, etc.) was less than 0.05 ppm, and the purity reached the 7N level.

[0078] Comparative Example 1

[0079] In Comparative Example 1, 7N grade metallic indium was prepared using a conventional zone melting method, the method including:

[0080] Pretreatment: Carburizing treatment is performed on quartz or graphite boats using 5N electrolytic indium raw material to reduce adhesion and contamination between molten indium and the vessel wall.

[0081] Charging and Atmosphere Protection: The indium raw material is loaded into the treated boat and placed in the zone furnace; a vacuum (1×10⁻⁶) is then applied. -4 After Pa), high-purity hydrogen and argon are introduced as protective and reducing gases, respectively, with a gas flow rate of 300 mL / min.

[0082] Establishing a molten zone and multiple zone melting: Use an induction heater to create a 20mm wide molten zone; move the heater at a speed of 15mm / h; repeat this process 18 times to gradually drive impurities to both ends.

[0083] Removal and reprocessing: After each zone melting cycle, 10% of the impurity-rich beginning and end portions are removed. The remaining high-purity portion can be used as raw material for the next round of purification, and the process can be repeated.

[0084] The purity of 5N grade metallic indium reaches 7N grade after 18 zone melting processes.

[0085] The method for purifying metallic indium based on single-crystal directional growth disclosed in this invention eliminates the impurity trap and fast channel of polycrystalline grain boundaries through the single-crystal growth mode. This allows all impurities to be driven and concentrated into the liquid metallic indium through the segregation effect at the solid-liquid interface, improving the purity of the solid metallic indium. It effectively removes impurities with a segregation coefficient close to 1. Even with a segregation coefficient close to 1, multiple cycles of "melting-single-crystal solidification-single-crystal directional growth" can still exponentially reduce the concentration through repeated segregation, which is impossible with traditional methods. The entire process is protected by a high-purity inert atmosphere combined with a specially coated graphite boat, which greatly inhibits indium oxidation and container material contamination, ensuring the purification effect. The final product is ultra-high purity indium (≥7N) and a structurally complete single crystal, with a value far exceeding that of ordinary polycrystalline high-purity indium, and can be directly used as a semiconductor epitaxial substrate. By precisely controlling parameters such as temperature gradient and growth rate, the process is stable and easily scalable for industrial-scale mass production.

[0086] The technical solutions and technical details disclosed in the embodiments of this application are merely illustrative of the inventive concept of this application and do not constitute a limitation on the technical solutions of this application. Any conventional changes, substitutions or combinations made to the technical details disclosed in this application have the same inventive concept as this application and are within the protection scope of the claims of this application.

Claims

1. A method for purifying metallic indium based on directional growth of single crystals, characterized in that, The application relates to a method for preparing high-purity indium single crystals. The raw material preparation step comprises the following steps: filling metal indium raw material with a purity of not less than 5N into a cavity in a graphite boat; wherein the end of the cavity in the graphite boat is in a conical shape, and the inner wall of the graphite boat is covered with a pyrolytic carbon or pyrolytic boron nitride layer; The single crystal forming step comprises the following steps: heating the conical cavity end in the graphite boat to melt the metal indium raw material and fill the conical cavity; the conical cavity end of the graphite boat is cooled at a set rate of 0.5-2 DEG C / min to form an indium single crystal seed at the tip of the conical cavity of the graphite boat; The single crystal directional zone melting step comprises the following steps: heating the graphite boat at the rear area of the indium single crystal seed to form a metal indium melting zone with a length of not less than a set length in the graphite boat, and forming a solid-liquid interface between the indium single crystal seed and the metal indium melting zone; moving the metal indium melting zone to the growth direction of the indium single crystal at a set rate to make the indium single crystal seed grow in an epitaxial mode at the solid-liquid interface, impurities gradually move to the metal indium melting zone, and the solid-liquid interface gradually moves backward until the metal indium melting zone in the graphite boat ends; The multiple melting step comprises the following steps: repeatedly performing the single crystal forming step and the single crystal directional zone melting step for multiple times to obtain metal indium with a purity of more than 7N.

2. The method for purifying metallic indium based on single crystal directional growth according to claim 1, characterized by, The directional growth is performed in a high-purity inert gas environment.

3. The method for purifying metallic indium based on single crystal directional growth according to claim 2, characterized by, The high-purity inert gas environment is a micro-positive pressure environment with a micro-positive pressure of 105-110 kPa.

4. The method for purifying metallic indium based on single crystal directional growth according to claim 2, characterized by, The inert gas flow of the high-purity inert gas environment is 20-50 sccm.

5. The method for purifying metallic indium based on single crystal directional growth according to claim 1, characterized by, In the single crystal directional zone melting step, the solid-liquid interface is arranged in a temperature gradient zone with a temperature gradient of 1-5 K / cm.

6. The method for purifying metallic indium based on single crystal directional growth according to claim 1, characterized by, The graphite boat is arranged in a quartz tube, the quartz tube is arranged in a horizontal tube furnace with a three-section temperature zone, the three-section temperature zone comprises high-temperature, medium-temperature and low-temperature zones arranged in sequence; wherein the temperature of the high-temperature zone is set to be 180-200 DEG C, the length of the high-temperature zone is greater than the total length of the graphite boat; the starting end temperature of the medium-temperature zone is 150-155 DEG C, and the end temperature is 100-120 DEG C; the temperature of the low-temperature zone is set to be 80-100 DEG C; the length ratio of the high-temperature zone, the medium-temperature zone and the low-temperature zone is 55:20:25; In the single crystal forming step, the conical cavity end of the graphite boat is arranged in the middle of the high-temperature zone for heating, and after the metal indium raw material is filled in the conical cavity, the horizontal tube furnace is moved to the high-temperature zone at a set rate to form an indium single crystal seed at the tip of the conical cavity of the graphite boat; In the single crystal directional zone melting step, the graphite boat at the rear area of the indium single crystal seed is heated in the high-temperature zone to form a metal indium melting zone, and a solid-liquid interface is formed between the indium single crystal seed and the metal indium melting zone; the horizontal tube furnace is moved to the high-temperature zone at a set rate of mm / h to make the indium single crystal seed grow in an epitaxial mode at the solid-liquid interface, impurities gradually move to the metal indium melting zone, and the solid-liquid interface gradually moves backward until the metal indium melting zone in the graphite boat ends.

7. The method for purifying metallic indium based on single crystal directional growth according to claim 6, characterized by, In the multiple melting step, the horizontal tube furnace moving rate in the single crystal directional zone melting step is set to be 1.0-8.0 mm / h.

8. The method for purifying metallic indium based on single crystal directional growth according to claim 6, characterized by, The quartz tube is arranged in a micro-positive pressure high-purity inert gas environment, the inert gas flow is 20-50 sccm, and the micro-positive pressure is 105-110 kPa.

9. The method for purifying metallic indium based on single crystal directional growth according to claim 6, wherein In the single crystal forming step, the moving speed of the horizontal tube furnace is set to 0.4-2 mm / h; in the single crystal directional zone melting step, the moving speed of the horizontal tube furnace is set to 1.0-3.0 mm / h.

10. The method for purifying metallic indium based on single crystal directional growth according to claim 1, characterized by, The graphite boat is arranged in a quartz tube, and the quartz tube is arranged in a single-temperature-zone horizontal heating furnace which can be precisely moved; In the single crystal directional zone melting step, the graphite boat at the rear of the indium single crystal seed is heated to form a metal indium melting zone with a length not less than a set length in the graphite boat, and a solid-liquid interface is formed between the single crystal seed and the metal indium melting zone; the single-temperature-zone horizontal heating furnace is moved to the indium single crystal growth direction at a set speed of 1.0-3.0 mm / h.