Thermal field device for growing ultraviolet low-absorption sapphire crystal and application of thermal field device

Abstract

The invention belongs to the technical field of crystal growth, and particularly discloses a thermal field device for growing ultraviolet low-absorption sapphire crystals and application of the thermal field device, and the device comprises a crucible, a heater, a melt area, a heat exchange support, an internal reflecting screen and an external thermal field. The internal reflecting screen is made of a multi-layer high-purity tungsten or molybdenum material, and the external thermal field is a graphite matrix of which the surface is coated with a vitrified layer or a silicon carbide coating. By optimizing the number of layers, the thickness and the spacing of the reflecting screens and combining a coating process, carbon volatilization and metal pollution are effectively inhibited, and the ultraviolet transmittance of the crystal in the wave band of 200-400 nm is remarkably improved. The device is suitable for growing sapphire crystals by a heat exchange method, has the characteristics of high thermal field stability, low energy consumption, excellent crystal quality and the like, has the ultraviolet linear transmittance of not less than 80%, and is suitable for high-end optical fields such as high-power lasers and gravitational wave detectors.

Description

Technical Field

[0001] This invention belongs to the field of crystal growth technology, specifically relating to a thermal field device for growing ultraviolet low-absorption sapphire crystals and its application, suitable for growing sapphire crystals by heat exchange method (HEM). Background Technology

[0002] Sapphire crystal (Al₂O₃) is widely used in LED substrates, optical windows, and laser media due to its excellent mechanical, optical, and thermal properties. Domestic and international research indicates that sapphire crystal growth methods mainly include Kyropoulos method, heat exchange method (HEM), temperature gradient method (TGT), and edge-defined thin film method (EFG). Among these, sapphire crystals grown using the HEM method, because the crystal is surrounded by the melt during growth, can be precisely controlled through heat exchange, resulting in high-quality crystals. This method can achieve crystal growth with diameters exceeding 50 cm. The high-quality crystals used in high-power lasers and gravitational wave detectors (LIGO project) require low absorption characteristics in the ultraviolet band (200-400 nm), with an absorption coefficient as low as 20 ppm / cm.

[0003] In existing technologies, graphite thermal fields possess significant advantages such as excellent high-temperature stability and thermal conductivity, long service life, and low cost. Graphite is frequently used as the primary thermal field component in sapphire crystal thermal field devices. However, it also has significant drawbacks: graphite readily releases ash at high temperatures, leading to metallic impurities (such as iron and nickel) contaminating the melt and causing absorption losses in the ultraviolet band. Furthermore, the volatile carbon elements in the graphite thermal field may react with alumina in the melt to form trace carbides, and carbon elements can easily cause sapphire to produce fluorine (F) and ferrous oxide (F₂). + Color center absorption greatly increases the ultraviolet absorption coefficient; sapphire crystals grown in a graphite thermal field have absorption coefficients exceeding 1 cm⁻¹ in the 200-300 nm wavelength range. -1 Some have an absorption coefficient greater than 10 cm⁻¹ -1 The transmittance dropped to below 70%. Although sapphire crystals grown with graphite thermal fields or graphite components can reduce ultraviolet absorption through subsequent high-temperature annealing and decarburization processes, this increases growth costs and has problems such as instability.

[0004] To address these issues, researchers both domestically and internationally have explored various improvement schemes. Internationally, research in countries like France and Japan has focused on the growth of low-absorption sapphire, achieving crystals with absorption coefficients below 56 ppm / cm by optimizing the thermal field to reduce impurities. Chinese domestic patents (such as CN105040093A) propose a hybrid thermal field structure, combining graphite heaters, tungsten-molybdenum materials, and alumina insulation to reduce high-temperature volatilization pollution, but carbon residue remains a problem. Other patents (such as CN215289036U) emphasize independent control of multiple heaters to optimize the temperature gradient, but do not specifically address low-absorption ultraviolet light. Overall, existing thermal field devices are still insufficient in reducing carbon volatilization and improving ultraviolet transmittance, failing to meet the demand for high-purity, large-size low-absorption ultraviolet sapphire crystals.

[0005] Therefore, in order to improve the quality and growth efficiency of sapphire crystals, there is an urgent need for a thermal field device that can effectively grow sapphire crystals. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a thermal field device for growing ultraviolet low-absorption sapphire crystals and its application. This device utilizes a high-purity tungsten or molybdenum metal reflector in the high-temperature zone, and an external vitrified graphite or dense silicon carbide coating to reduce carbon volatilization and significantly improve the transmittance of sapphire crystals in the ultraviolet 200-400nm wavelength range.

[0007] This invention includes the following technical solutions:

[0008] A thermal field device for growing ultraviolet low absorption sapphire crystals includes: a crucible, a heater, a melt zone, a heat exchange support, an internal reflector screen, and an external thermal field.

[0009] The internal reflective screen is made of multiple layers of high-purity tungsten or molybdenum material;

[0010] The external thermal field is a graphite matrix;

[0011] The thermal field device has a symmetrical stacked layout. The crucible is located in the central area, the heater is arranged around the crucible, the internal reflective screen is enclosed to form a relatively independent hot cavity, and the external thermal field constitutes an outer protective and thermal control structure.

[0012] Furthermore, in the aforementioned thermal field device, the internal reflective screen is a three-layer high-purity tungsten or molybdenum reflective screen. "High purity" refers to a purity ≥ 99.99%; this layer count is based on optimization of heat reflection effect and thermal field uniformity, making the thermal field more suitable for the growth requirements of ultraviolet low-absorption sapphire crystals.

[0013] Furthermore, in the aforementioned thermal field device, the single-layer thickness of the internal reflective screen is ≥0.5mm, preferably 0.5-1mm. This thickness range ensures structural strength while achieving good thermal reflection performance, avoiding thermal field disorder or material waste due to improper thickness. The reflective screen is used for heat reflection in high-temperature areas, and the coating is used to reduce carbon volatilization and improve the ultraviolet transmittance of the crystal in the 200-400nm range.

[0014] Furthermore, in the aforementioned thermal field device, the distance between the internal reflective screen and the crucible is ≥50mm, preferably 70-100mm; the distance between the external thermal field and the internal reflective screen is ≥5mm, preferably 5-10mm. This spacing setting improves the heat reflection efficiency by 10-15% while avoiding thermal radiation interference between components.

[0015] Furthermore, in the aforementioned thermal field device, the external thermal field is a graphite substrate with a surface coated with a glassy layer or a dense silicon carbide coating.

[0016] Furthermore, in the aforementioned thermal field device, the coating thickness of the external thermal field is ≥0.1mm, preferably 0.1-0.5mm. The coating reduces the carbon volatilization of the graphite matrix and remains stable at an operating temperature of 1800-2200℃.

[0017] Furthermore, in the aforementioned thermal field device, the glass transition coating or silicon carbide coating of the external thermal field is prepared using one or more of the following processes: chemical vapor deposition (CVD), physical vapor deposition (PVD), impregnation-sintering, slurry coating-sintering, or coating carburizing / siliconizing processes.

[0018] The coating formed by the process has a density of ≥95%, a bonding strength with the graphite matrix of ≥10MPa, and a continuous surface without cracks.

[0019] Furthermore, in the aforementioned thermal field device, the heat exchange support is positioned in contact with the bottom of the crucible, and a gas heat conduction channel is provided inside, along with a gas flow rate regulating device. By adjusting the temperature control rate at a gradient of 0.1-2℃ / h, in conjunction with the bottom heat exchange support rod, the He gas flow rate can be regulated; this further stabilizes the temperature gradient within the crystal surrounded by the melt, meeting the thermal field requirements for sapphire crystal growth.

[0020] Furthermore, in the aforementioned thermal field device, the heat exchange support is made of tungsten or a material with good thermal conductivity, and its contact diameter with the bottom of the crucible is 15-30 mm. The gas flow rate can be controlled within the range of 80-200 LPM. By controlling the heat exchange He gas flow rate within the range of 80-200 LPM, and introducing flowing high-purity (>7N) He or Ar at a pressure adjusted to 1-1000 Pa, combined with heating by a heater, heat reflection by a reflector screen, and an outer coating to protect the graphite thermal field, precise temperature control of the melt zone can be achieved.

[0021] Preferably, the heat exchange support is made of high-purity tungsten or molybdenum tube with high thermal conductivity, and the contact diameter with the bottom of the crucible is about 15-30 mm. The heat exchange support exchanges heat through He gas circulation at a flow rate of 80±5 LPM, and the temperature is controlled within the range of 2050-1900℃ at a stable cooling growth rate of 0.1-0.3℃ / h for crystal growth.

[0022] This invention also discloses the application of the above-mentioned thermal field device, which is used to grow ultraviolet low absorption sapphire crystals by heat exchange method, and its working method includes the following steps:

[0023] S1. Load the high-purity alumina raw material into a crucible made of high-purity tungsten or molybdenum.

[0024] S2. Start the heater to completely melt the alumina raw material at 2050±50℃ to form a melt zone;

[0025] S3. High-purity He or Ar gas with a flow rate of 80–200 LPM is introduced into the bottom of the crucible through heat exchange support, and the furnace atmosphere pressure is controlled to be 1–1000 Pa.

[0026] S4. Slowly decrease the temperature at a rate of 0.1–2℃ / h, and remove heat through heat exchange support to form an axial temperature gradient, inducing sapphire crystals to crystallize and grow from the bottom of the melt.

[0027] S5. After crystal growth is complete, cool the crystal to room temperature at a rate of 50–100℃ / h and remove the crystal.

[0028] S6. The linear transmittance of the obtained sapphire crystal in the 200–400 nm wavelength band is not less than 80%.

[0029] Compared with the prior art, the present invention has the following outstanding advantages:

[0030] 1. By using three layers of high-purity tungsten or molybdenum reflective screens with specific purity and thickness (purity > 99.99%, thickness 0.5-1mm), and with a reasonable spacing design between the reflective screens and the crucible and external hot field, the distribution of thermal radiation and heat flow can be precisely controlled. Compared with traditional hot field devices, the hot field provided by this invention is more suitable for growing high-quality sapphire crystals with low ultraviolet absorption, achieving an ultraviolet linear transmittance of over 83%, which is 10-15% higher than that of traditional graphite hot fields (transmittance approximately 70%).

[0031] 2. Pollution control: The low volatility of high-purity tungsten / molybdenum screens significantly reduces the contamination of the melt by metallic impurities.

[0032] 3. Thermal stability: The coating is prepared through processes such as CVD and PVD, ensuring density and strong bonding, reducing the carbon volatilization of the graphite matrix by ≥30%. Under high-temperature thermal environments of 1800-2200℃, the coating and the substrate are stably bonded and not easily peeled off, extending the service life of the external thermal field. On the other hand, the enclosed thermal space constructed by the multi-layer reflector and various components ensures that the heat loss rate of the thermal field device is ≤20%, improving energy utilization efficiency while ensuring long-term stable operation of the thermal field, reducing the risk of crystal scrapping due to thermal field fluctuations during production, and saving production costs.

[0033] 4. The heaters are symmetrically arranged around the crucible, covering the main heat demand area, and work in conjunction with components such as the heat exchange support, reflector, and external hot field. The heat exchange support adjusts the heat dissipation rate, working in conjunction with the heater's heating and the reflector's heat reflection to achieve precise temperature control in the melt zone. The layered structure at the top and bottom enhances the sealing and stability of the hot field. All components work together to form an efficient and coordinated hot field control system. Compared with traditional hot field devices, this system has significant advantages in crystal growth efficiency and quality control, facilitating the large-scale, high-quality production of ultraviolet low-absorption sapphire crystals. Attached Figure Description

[0034] Figure 1 Schematic diagram of the thermal field structure for growing ultraviolet low-absorption sapphire crystals;

[0035] Figure 2 Transmittance diagram of sapphire crystals grown using the thermal field device of this invention at 200-400 nm.

[0036] In the diagram: crucible 1, heater 2, melt zone 3, heat exchange support 4, internal reflector 5, and external heat field 6. Detailed Implementation

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] like Figure 1 As shown, a thermal field device for growing ultraviolet low absorption sapphire crystals includes: a crucible 1, a heater 2, a melt zone 3, a heat exchange support 4, an internal reflector 5, and an external thermal field 6.

[0040] The internal reflective screen 5 is a multi-layered reflective screen made of high-purity tungsten or molybdenum material;

[0041] The external thermal field 6 is a graphite matrix;

[0042] The overall thermal field device has a symmetrical stacked layout. The crucible 1 is located in the central area, the heater 2 is arranged around the crucible 1, the internal reflector 5 is enclosed to form a relatively independent hot cavity, and the external thermal field 6 constitutes an outer protective and thermal control structure.

[0043] The aforementioned thermal field device is used to grow ultraviolet low-absorption sapphire crystals via heat exchange. Its operation includes the following steps:

[0044] S1. Load the high-purity alumina raw material into a crucible 1 made of high-purity tungsten or molybdenum.

[0045] S2. Start heater 2 to completely melt the alumina raw material at 2050±50℃ to form melt zone 3;

[0046] S3. High-purity He or Ar gas with a flow rate of 80–200 LPM is introduced into the bottom of crucible 1 through heat exchange support 4, and the furnace atmosphere pressure is controlled to be 1–1000 Pa.

[0047] S4. The temperature is slowly reduced at a rate of 0.1–2℃ / h, and heat is discharged through heat exchange support 4 to form an axial temperature gradient, which induces sapphire crystals to crystallize and grow from the bottom of the melt.

[0048] S5. After crystal growth is complete, cool the crystal to room temperature at a rate of 50–100℃ / h and remove the crystal.

[0049] S6. The linear transmittance of the obtained sapphire crystal in the 200–400 nm wavelength band is not less than 80%.

[0050] Example 2

[0051] A thermal field device for growing ultraviolet low absorption sapphire crystals includes: a crucible 1, a heater 2, a melt zone 3, a heat exchange support 4, an internal reflector 5, and an external thermal field 6.

[0052] The internal reflective screen 5 is a multi-layered reflective screen made of high-purity tungsten or molybdenum material;

[0053] The external thermal field 6 is a graphite matrix;

[0054] The overall thermal field device has a symmetrical stacked layout. The crucible 1 is located in the central area, the heater 2 is arranged around the crucible 1, the internal reflector 5 is enclosed to form a relatively independent hot cavity, and the external thermal field 6 constitutes an outer protective and thermal control structure.

[0055] Preferably, crucible 1 is a tungsten crucible with a purity >99.995% to provide a stable environment for raw material melting. The inner diameter of crucible 1 is 170mm and the height is 300mm. The raw material is alumina with a purity >99.999%.

[0056] Heater 2 is a high-purity tungsten wire mesh heater: it is a cylindrical heater with tungsten wire wound around it, with a power setting of 50-75kW (depending on the size of the crucible and the amount of alumina raw material), tightly surrounding the crucible 1 to ensure that the heat is evenly transferred to the crucible 1 and the internal melt 3.

[0057] The internal three-layer high-purity tungsten reflective screen 5 has a purity of 99.995% and a single-layer thickness of 0.6mm. Each reflective screen 5 is concentrically nested, and the distance between it and the crucible 1 is precisely adjusted to 6mm. The high purity and suitable thickness of the reflective screen 5 effectively reflect heat from the high-temperature zone, reduce heat loss, and improve the efficiency of the thermal field.

[0058] The graphite substrate surface of the external thermal field 6 is coated with a 0.2 mm thick dense silicon carbide coating using a chemical vapor deposition (CVD) process. The coating density was tested to reach 96%, and the bonding strength with the graphite substrate reached 12 MPa, with a smooth and crack-free surface. The distance between the external thermal field 6 and the reflective screen 5 is set to 12 mm, constructing a stable outer thermal protection and control structure.

[0059] The heat exchange support 4 is made of high-purity tungsten or molybdenum tubes with high thermal conductivity, and its contact diameter with the bottom of crucible 1 is approximately dia15-30 mm. Heat exchange support 4 utilizes He gas circulation for heat exchange at a flow rate of 80±5 LPM. Crystal growth is achieved by maintaining a stable cooling rate of 0.1-0.3 °C / h within the temperature range of 2050-1900 °C. The crystal growth atmosphere is maintained within the range of 1-10 Pa, using flowing inert gas He or Ar.

[0060] The heating and cooling rates after crystal growth were 150-200℃ / Hr and 50-100℃ / Hr, respectively. After crystal growth, the linear transmittance of the crystal at 200-400 nm exceeded 80%. Figure 2 As shown.

[0061] It is worth noting that the above description of the embodiments focuses on illustrating the technical solution of the present invention, rather than precisely defining its scope of protection. Those skilled in the art should understand that appropriate adjustments and optimizations can be made based on the technical details disclosed in the embodiments of the present invention, or equivalent substitutions can be implemented for individual or even all technical elements. Such adjustments and substitutions will not deviate from the core essence of the technical solution of the present invention and should be included within the technical protection scope of the embodiments of the present invention. In short, the protection of the present invention should not be limited to the concrete presentation of the above embodiments, but broadly covers all equivalent changes and improvements that do not depart from its basic concept. In summary, the protection definition of the present invention should be based on the statement of the claims, and the above embodiments are only used as a reference guide for understanding the present invention.

Claims

1. A thermal field device for growing ultraviolet low-absorption sapphire crystals, characterized in that, include: The crucible (1), heater (2), melt zone (3), heat exchange support (4), internal reflector (5), and external heat field (6); The internal reflective screen (5) is a multi-layered reflective screen made of high-purity tungsten or molybdenum material; The external thermal field (6) is a graphite matrix; The overall thermal field device is arranged in a symmetrical stacked layout. The crucible (1) is located in the central area, the heater (2) is arranged around the crucible (1), the internal reflector (5) surrounds and forms a relatively independent hot cavity, and the external thermal field (6) constitutes an outer protective and thermal control structure.

2. The thermal field device according to claim 1, characterized in that, The internal reflective screen (5) is a three-layer high-purity tungsten or molybdenum reflective screen.

3. The thermal field device according to claim 1, characterized in that, The thickness of a single layer of the internal reflective screen (5) is ≥0.5mm, preferably 0.5-1mm.

4. The thermal field device according to claim 1, characterized in that, The distance between the internal reflective screen (5) and the crucible (1) is ≥50mm, preferably 70-100mm; the distance between the external thermal field (6) and the internal reflective screen (6) is ≥5mm, preferably 5-10mm.

5. The thermal field device according to claim 1, characterized in that, The external thermal field (6) is a graphite substrate with a glassy layer or a dense silicon carbide coating on its surface.

6. The thermal field device according to claim 5, characterized in that, The coating thickness of the external thermal field (6) is ≥0.1mm, preferably 0.1-0.5mm.

7. The thermal field device according to claim 5, characterized in that, The glass transition coating or silicon carbide coating of the external thermal field (6) is prepared by one or more of the following processes: chemical vapor deposition (CVD), physical vapor deposition (PVD), impregnation-sintering, slurry coating-sintering, or coating carburizing / siliconizing. The coating formed by the process has a density of ≥95%, a bonding strength with the graphite matrix of ≥10MPa, and a continuous surface without cracks.

8. The thermal field device according to claim 1, characterized in that, The heat exchange support (4) is arranged in contact with the bottom of the crucible (1). The heat exchange support (4) is provided with a gas heat conduction channel and a gas flow rate regulating device.

9. The thermal field device according to claim 8, characterized in that, The heat exchange support (4) is made of tungsten or other materials with good thermal conductivity. Its contact diameter with the bottom of the crucible is 15-30 mm, and the gas flow rate can be controlled within the range of 80-200 LPM.

10. The application of the thermal field device as described in any one of claims 1-9, characterized in that, This thermal field device is used to grow ultraviolet low-absorption sapphire crystals via heat exchange. Its operation includes the following steps: S1. Load the high-purity alumina raw material into a crucible (1) made of high-purity tungsten or molybdenum. S2. Start the heater (2) to completely melt the alumina raw material at 2050±50℃ to form a melt zone (3). S3. High-purity He or Ar gas with a flow rate of 80–200 LPM is introduced into the bottom of the crucible (1) through the heat exchange support (4) to control the atmosphere pressure inside the furnace to 1–1000 Pa. S4. The temperature is slowly reduced at a rate of 0.1–2℃ / h, and heat is discharged through heat exchange support (4) to form an axial temperature gradient, which induces sapphire crystal to crystallize and grow from the bottom of the melt. S5. After crystal growth is complete, cool the crystal to room temperature at a rate of 50–100℃ / h and remove the crystal. S6. The linear transmittance of the obtained sapphire crystal in the 200–400 nm wavelength band is not less than 80%.

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