A method for preparing a Mg3Bi2 single crystal material

Abstract

The application discloses a preparation method of Mg3Bi2 single crystal material and belongs to the field of semiconductor materials. The preparation method comprises the following steps: selecting high-purity single elements Mg and Bi as starting raw materials, taking excess Bi elements as fluxes, dosing according to the stoichiometric ratio of Mg and Bi, sequentially placing into high-purity boron nitride crucibles, then placing into quartz tubes which are larger than the inner diameters of the crucibles, vacuumizing, and adopting a hydrogen-oxygen flame gun to seal; placing into a vertical Bridgman growth furnace to make the raw materials fully melt to form a uniform melt; cooling, starting a crucible descending device after stabilization, descending at a rate of 0.2-0.5 mm / h, and when the internal solute reaches saturation, the crystal spontaneously nucleates and stably grows, and the Mg3Bi2 single crystal material with high quality and effectively expanded lateral size is obtained. The application combines the flux and the crucible descending method, and prepares the Mg3Bi2 single crystal material with high quality and effectively expanded lateral size based on the controlled crystallization technology in the Bi-rich flux environment.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor materials, specifically relating to a method for preparing Mg3Bi2 single crystal material. Background Technology

[0002] Mg3Bi2, as a semi-metallic material with topological band structure, has shown significant application value in the fields of flexible electronics and thermoelectric conversion. Ball milling combined with high-temperature sintering is a common solid-state synthesis method for Mg3Bi2 material preparation. This method typically involves first achieving thorough mixing and refinement of the raw material powder through high-energy ball milling, inducing solid-state reaction activation, followed by densification through hot pressing or conventional sintering. The final sample is usually a polycrystalline bulk rather than a single crystal. Temperature cooling, on the other hand, gradually saturates the melt during cooling, causing spontaneous precipitation of the solute and formation of single crystals. Mg3Bi2 crystals prepared using this method are typically plate-like, with a typical size of about 10 mm, and readily cleave along the (001) plane. Due to the lack of a clear temperature gradient direction in the system, polynucleation is easily generated during crystal growth. The crucible lowering method is a common crystal preparation technique. By moving the crucible, the melt is passed through a region with a large temperature gradient, thereby gradually achieving directional solidification at the solid-liquid interface. This method can more effectively control the crystal orientation and microstructure uniformity, and therefore it is increasingly widely used in the research of thermoelectric material crystal growth.

[0003] During the growth of Mg3Bi2 crystals, the high chemical reactivity of Mg significantly increases the difficulty of preparation. Mg can react with crucible materials such as common quartz, graphite, and Al2O3 at high temperatures. Furthermore, the evaporation loss of Mg at high temperatures causes the system to deviate from the ideal stoichiometry, affecting the phase stability and physical properties of the crystal. Its high vapor pressure also increases the safety risks of the experimental process. Therefore, developing a crystal preparation process that can reduce the growth temperature is of great significance. Summary of the Invention

[0004] Therefore, the purpose of this invention is to provide a method for preparing Mg3Bi2 single crystal material, which uses a flux combined with a crucible lowering method, uses a high-purity boron nitride crucible as a crystal growth container, and uses excess Bi as a flux to slowly grow Mg3Bi2 single crystal material, thereby solving the problem of low-temperature safe growth of Mg3Bi2 crystals in existing preparation methods.

[0005] The above-mentioned objectives of the present invention can be achieved through the following technical solutions:

[0006] This invention provides a method for preparing Mg3Bi2 single crystal material, comprising the following steps:

[0007] S1. Raw material preparation: Select high-purity elemental elements Mg and Bi with a purity of not less than 99.99% as starting materials, and use excess Bi as flux. Prepare the raw materials according to the stoichiometric ratio of Mg to Bi of 3:4-6.

[0008] S2. Loading and Packaging: First, put the weighed raw materials into a high-purity boron nitride crucible with an inner diameter of 30-40 mm in the order of Mg and Bi. Then, place the loaded crucible into a quartz tube with an inner diameter 2-4 mm larger than that of the high-purity boron nitride crucible, evacuate the vacuum, and seal it with an oxyhydrogen flame gun.

[0009] S3. High-temperature melting: Place the packaged quartz tube into a vertical Bridgman growth furnace, set the furnace temperature to 800-950℃, the heating rate to 80-150℃, and the holding time to 24-48 h, so that the raw material is fully melted to form a uniform melt.

[0010] S4. Crystal growth: After the melt is fully melted, the temperature is reduced to 600-750℃. After the temperature stabilizes, the crucible lowering device is started and the crucible is lowered at a rate of 0.2-0.5 mm / h. When the internal solute reaches saturation, the crystal spontaneously nucleates and grows stably to obtain Mg3Bi2 single crystal material.

[0011] Preferably, in step S1, the total mass of the target crystal component and flux is 200-500 g.

[0012] Preferably, in step S1, the stoichiometric ratio of Mg to Bi is 3:4-5.

[0013] Preferably, in step S2, the vacuum degree of the quartz tube is 10. -3 -10 -1 Pa, preferably 10 Pa -1 Pa.

[0014] Preferably, in step S2, the purity of the high-purity boron nitride crucible is greater than 99.99%.

[0015] Preferably, in step S3, the temperature of the vertical Bridgeman growth furnace is 850-900℃, and the holding time is 24-36 h.

[0016] Preferably, in step S3, the heating rate of the vertical Bridgeman growth furnace is 100-120℃ / h.

[0017] Preferably, in step S4, the descent speed of the crucible lowering device is 0.2-0.3 mm / h.

[0018] Preferably, in step S4, the temperature gradient of the single crystal growth region is 15-30℃ / cm.

[0019] Preferably, in step S4, the single crystal growth temperature is 600-750℃.

[0020] Preferably, the vertical Bridgeman furnace consists of a heating furnace, a sample support frame, and an adjustable lifting device.

[0021] Preferably, the lateral dimension of the Mg3Bi2 single crystal material is ≥20 mm.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. This invention combines a fluxing agent with a crucible lowering method to prepare high-quality Mg3Bi2 single-crystal materials with effectively expanded lateral dimensions based on controlled crystallization technology in a Bi-rich fluxing agent environment, solving the problem of difficult control of component segregation and volatilization in the preparation of Mg3Bi2 single crystals. The single crystals grown by this invention have significant crystal orientation, and the high-quality, large-size Mg3Bi2 single-crystal materials obtained provide important material support for subsequent research on the electroacoustic physical properties of this material.

[0024] 2. This invention uses excess Bi element as a flux to grow single crystal materials, effectively solving the problems of high melting temperature of compounds, easy volatilization of magnesium element, and deviation of compounds from ideal stoichiometry.

[0025] 3. This invention solves the problem of easy instability of the Mg3Bi2 growth interface by precisely coordinating the crucible descent rate (0.2-0.5 mm / h) and temperature gradient (15-30 K / cm), so that the lateral dimension of the obtained single crystal can reach more than 20 mm.

[0026] 4. This invention employs a fluxing agent combined with crucible lowering technology, using high-purity boron nitride as an inert crucible. By introducing excess Bi as a fluxing agent, the growth temperature is lowered, thus avoiding the violent volatilization of magnesium components. By designing and controlling the temperature gradient, lowering rate, and initial molar ratio of Mg and Bi, stable growth of Mg3Bi2 crystals under controlled supercooling is achieved. The resulting crystals have a lateral dimension ≥20 mm and exhibit excellent crystal integrity and orientation consistency. Attached Figure Description

[0027] Figure 1 The diagram shows the structure of the processing equipment for Mg3Bi2 single crystal material in the embodiment, where: 1-growth container; 2-heating body; 3-insulating layer; 4-lifting device; 5-melt; 6-growth material; 7-thermocouple.

[0028] Figure 2 This is a temperature variation diagram of Mg3Bi2 single crystal material grown in a crystal growth furnace in the example.

[0029] Figure 3The image shows the cleavage plane of a Mg3Bi2 single crystal obtained in the example.

[0030] Figure 4 The image shown is of the ingot obtained during the growth process in this embodiment.

[0031] Figure 5 The X-ray diffraction (XRD) of the Mg3Bi2 single crystals obtained in the examples and the comparison results with the standard XRD pattern are shown.

[0032] Figure 6 The image shown is a scanning electron microscope (SEM) image of the Mg3Bi2 single crystal material obtained in the examples. Detailed Implementation

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the described embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] like Figure 1 The processing equipment shown includes a growth container 1, a heating element 2, an insulating layer 3, a lifting device 4, a melt 5, a growth material 6, and a thermocouple 7. A Mg3Bi2 single crystal material is prepared using this processing equipment, with the following steps:

[0035] S1. Raw material preparation: Select high-purity elemental elements Mg and Bi with a purity of not less than 99.99% as starting materials, and use excess Bi as flux. Prepare the raw materials according to the stoichiometric ratio of Mg to Bi of 3:4-6. The total mass of the target crystal components and flux is 200-500 g.

[0036] S2. Loading and Packaging: First, place the weighed raw materials into a high-purity boron nitride crucible with an inner diameter of 30-40 mm in the order of Mg and Bi. Then, place the loaded crucible into a quartz tube with an inner diameter 2-4 mm larger than that of the high-purity boron nitride crucible, and evacuate to 10°C. -3 -10 -1 Pa, was sealed using an oxyhydrogen flame gun;

[0037] S3. High-temperature melting: Place the packaged quartz tube into a vertical Bridgman growth furnace, set the furnace temperature to 800-950℃, the heating rate to 80-150℃, and the holding time to 24-48 h, so that the raw material is fully melted to form a uniform melt.

[0038] S4. Crystal growth: After the melt is fully melted, the temperature is reduced to 600-750℃. After the temperature stabilizes, the crucible lowering device is started and the crucible is lowered at a rate of 0.2-0.5 mm / h. When the internal solute reaches saturation, the crystal spontaneously nucleates and grows stably to obtain Mg3Bi2 single crystal material.

[0039] In some embodiments, in step S1, the stoichiometric ratio of Mg to Bi is 3:4-5.

[0040] In some embodiments, in step S2, the vacuum level of the quartz tube is 10. -1 Pa.

[0041] In some embodiments, in step S2, the purity of the high-purity boron nitride crucible is greater than 99.99%.

[0042] In some embodiments, in step S3, the temperature of the vertical Bridgeman growth furnace is 850-900°C, the holding time is 24-36 h, and the heating rate is 100-120°C / h.

[0043] In some embodiments, in step S4, the descent speed of the crucible lowering device is 0.2-0.3 mm / h.

[0044] In some embodiments, in step S4, the temperature gradient of the single crystal growth region is 15-30℃ / cm, and the single crystal growth temperature is 600-750℃.

[0045] In some embodiments, the vertical Bridgeman furnace consists of a heating furnace, a sample support frame, and an adjustable lifting device.

[0046] In some embodiments, the lateral dimension of the Mg3Bi2 single crystal material is ≥20 mm.

[0047] Example 1

[0048] High-quality, large-size Mg3Bi2 single crystals were prepared using the following method, with a Mg to Bi molar ratio of 3:5 as the starting design composition. The steps are as follows:

[0049] (1) Raw material preparation: Select high-purity Mg and Bi elements with a purity of 99.99%, and mix them according to the stoichiometric ratio of the chemical formula Mg3Bi5. The total mass of the crystal components and the co-solvent is 330g.

[0050] (2) Loading and Packaging: The weighed elemental raw materials are placed in a high-purity boron nitride crucible with a diameter of 33 mm and a purity greater than 99.99% in the order of Mg and Bi. The crucible is then placed in a quartz tube with a diameter of 36 mm. A vacuum pump is used to evacuate the quartz tube to a vacuum level of 10. -1Pa, and then, the quartz tube is sealed and encapsulated using an oxyhydrogen flame gun.

[0051] (3) High-temperature melting: The packaged quartz tube is placed in a vertical Bridgman growth furnace, the furnace temperature is set to 800℃, the heating rate is 100℃, and the temperature is maintained for 24 hours to allow the raw materials to fully melt and form a uniform melt.

[0052] (4) Crystal growth: After the melt is fully melted, the temperature is reduced to 680℃. After the melt temperature stabilizes, the crucible lowering device is started and lowered at a rate of 0.2 mm / h. When the internal solute reaches saturation, the crystal spontaneously nucleates and grows stably to obtain Mg3Bi2 single crystal material.

[0053] use Figure 1 The device shown, in Figure 2 Under the crystal growth temperature gradient of 30℃ / cm shown, the grown crystal is as follows: Figure 3 As shown.

[0054] Figure 4 For a complete ingot that has been grown, Mg3Bi2 single crystal material with obvious dissociation surface can be extracted, and the lateral dimension of the single crystal is greater than 20 mm.

[0055] Figure 5 The results are X-ray diffraction spectra of Mg3Bi2 single crystal material. By comparing with the standard X-ray diffraction spectrum of Mg3Bi2, it can be found that the material obtained is Mg3Bi2, and the diffraction spectrum of the cleavage plane represents the crystal plane orientation in the (001) direction, indicating that the material is a high-quality single crystal material.

[0056] Figure 6 This is a scanning electron microscope (SEM) image of the Mg3Bi2 single crystal material. Energy dispersive spectroscopy (EDS) analysis of random points revealed that the stoichiometric ratio of the main elements Mg and Bi in the obtained material is consistent with 3:2, further confirming the composition of the Mg3Bi2 single crystal material.

[0057] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing Mg3Bi2 single crystal material, characterized in that, Includes the following steps: S1. Raw material preparation: Select high-purity elemental elements Mg and Bi with a purity of not less than 99.99% as starting materials, and use excess Bi as flux. Prepare the raw materials according to the stoichiometric ratio of Mg to Bi of 3:4-6. S2. Loading and Packaging: First, place the weighed raw materials into a high-purity boron nitride crucible with an inner diameter of 30-40 mm in the order of Mg and Bi. Then, place the loaded crucible into a quartz tube with an inner diameter 2-4 mm larger than that of the high-purity boron nitride crucible, and evacuate to 10°C. -3 -10 -1 Pa, was sealed using an oxyhydrogen flame gun; S3. High-temperature melting: Place the packaged quartz tube into a vertical Bridgman growth furnace, set the furnace temperature to 800-950℃, the heating rate to 80-150℃, and the holding time to 24-48 h, so that the raw material is fully melted to form a uniform melt. S4. Crystal growth: After the melt is fully melted, the temperature is reduced to 600-750℃. After the temperature stabilizes, the crucible lowering device is started and the crucible is lowered at a rate of 0.2-0.5 mm / h. When the internal solute reaches saturation, the crystal spontaneously nucleates and grows stably to obtain Mg3Bi2 single crystal material.

2. The method for preparing Mg3Bi2 single crystal material according to claim 1, characterized in that, In step S1, the total mass of the target crystal component and flux is 200-500 g; And / or the stoichiometric ratio of Mg and Bi is 3:4-5.

3. The method for preparing Mg3Bi2 single crystal material according to claim 1, characterized in that, In step S2, the vacuum degree of the quartz tube is 10. -1 Pa; And / or the purity of the high-purity boron nitride crucible is greater than 99.99%.

4. The method for preparing Mg3Bi2 single crystal material according to claim 1, characterized in that, In step S3, the temperature of the vertical Bridgeman growth furnace is 850-900 ℃, and the holding time is 24-36 h.

5. The method for preparing Mg3Bi2 single crystal material according to claim 1, characterized in that, In step S3, the heating rate of the vertical Bridgeman growth furnace is 100-120 ℃ / h.

6. The method for preparing Mg3Bi2 single crystal material according to claim 1, characterized in that, In step S4, the descent speed of the crucible lowering device is 0.2-0.3 mm / h.

7. The method for preparing Mg3Bi2 single crystal material according to claim 1, characterized in that, In step S4, the temperature gradient in the single crystal growth region is 15-30 ℃ / cm, and the single crystal growth temperature is 600-750℃.

8. The method for preparing Mg3Bi2 single crystal material according to claim 1, characterized in that, The vertical Bridgeman furnace consists of a heating furnace, a sample support frame, and an adjustable lifting device.

9. The method for preparing Mg3Bi2 single crystal material according to claim 1, characterized in that, The lateral dimension of the Mg3Bi2 single crystal material is ≥20 mm.

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