Single crystal superconducting material growth reaction device and centimeter-level BKBO crystal preparation method

Abstract

The invention discloses a single crystal superconducting material growth reaction device and a preparation method of centimeter-level BKBO crystals, the growth reaction device comprises a reaction kettle body, the middle part of the reaction kettle body is provided with an accommodating cavity, and the reaction kettle body is sleeved with a crucible; the kettle cover covers the top of the reaction kettle body, and the kettle cover is provided with an air inlet hole and an air outlet hole which are communicated with the accommodating cavity; the stirring rod penetrates through the kettle cover along the central axis of the kettle cover and is rotationally connected with the kettle cover, and the lower part of the stirring rod extends into the accommodating cavity; the cathode and the anode are fixed on the kettle cover, and the lower parts of the cathode and the anode extend into the accommodating cavity. Based on the surface stability theory of crystal growth, creatively referring to the core thought of Kyropoulos method crystal growth, and based on the reaction device, the large-size centimeter-level BKBO crystal is prepared by accurately controlling the stirring speed and temperature in the reaction process.

Description

Technical Field

[0001] This invention belongs to the field of materials preparation technology, specifically relating to a single-crystal superconducting material growth reaction device and a method for preparing centimeter-scale BKBO crystals. Background Technology

[0002] Ba 1-x K x BiO3 (BKBO) exhibits a superconducting transition temperature of approximately 33 K in its optimal doping region, with the crystal appearing predominantly bluish-black. This material originates from the parent material BaBiO3, a purple crystal with a nominal valence of +4, forming an alternating +3 and +5 charge sequence within the crystal, resulting in overall insulation. Doping with K alters the crystal color, and the superconducting transition temperature reaches 30 K around x = 0.33-0.5. BKBO possesses an ideal three-dimensional perovskite structure, isotropic, and its superconducting coherence length (approximately 3-7 nm) is significantly greater than that of copper-based superconductors. These properties make it not only valuable for comparative research in exploring high-temperature superconductivity mechanisms (especially distinguishing it from two-dimensional copper-based superconductors), but also demonstrate immense potential in superconducting electronics applications, such as the fabrication of tunnel junctions and superconducting quantum interference devices (SQUIDs). Subsequently, several research teams have developed KOH-based flux methods, achieving growth temperatures below 270 °C.

[0003] The introduction of the molten salt anodic deposition method (i.e., the electrochemical method) brought a turning point to the growth of BKBO single crystals. Pioneering by Norton et al., this method's core principle is to use KOH as a molten salt solvent and electrolyte within a relatively low temperature range of 200-300℃, applying direct current to achieve electrochemical crystallization of BKBO single crystals on the anode (platinum electrode). This method effectively suppresses potassium volatilization and high-temperature decomposition, and is considered the most effective way to prepare high-quality BKBO single crystals. In subsequent studies, Nagata et al., along with several research groups in Poland and Japan, optimized the parameters of this method, successfully growing single crystals with superconducting transition temperatures exceeding 30 K, and reporting the preparation of millimeter-sized crystals.

[0004] Currently, the raw materials, crucibles, and specific processes used in the electrochemical synthesis of BKBO single crystals vary, the steps are cumbersome, and previously reported experimental setups are merely assemblies of simple chemical experimental equipment, lacking complete sets of equipment. Common problems in experimental setups include insufficient sealing of the reaction vessel, leading to severe potassium hydroxide volatilization, corrosion of glassware, and consequently, glass breakage and atmosphere leakage. To achieve a complete reaction, most processes employ stirring methods commonly used in chemistry, with electrodes distributed on the sides and top of the device, resulting in significant disturbance to the platinum wire at the crystal growth site. Simultaneously, the overall heating device, due to its large wall volume inherited from chemical reaction apparatus, generally suffers from insufficient temperature control precision and excessive temperature differences between the upper and lower parts of the reaction vessel. These factors collectively contribute to excessive nucleation and twinning, resulting in smaller crystal sizes, a larger superconducting transition width, and a superconducting volume ratio that fails to reach 100%. Summary of the Invention

[0005] To address the aforementioned bottlenecks, the purpose of this invention is to provide a single-crystal superconducting material growth reaction device and a method for preparing centimeter-sized BKBO crystals. Based on the core physical principle of crystal growth—interface stability theory—this invention creatively draws on the core idea of ​​the Czochralski method for crystal growth (i.e., obtaining large-size single crystals by precisely controlling the thermal field and mass transport at the growth interface). The traditional electrochemical device has been redesigned to obtain a growth reaction device capable of preparing centimeter-sized BKBO crystals, and large-size BKBO crystal materials have been prepared.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first aspect of the present invention is to provide a single-crystal superconducting material growth reaction apparatus, comprising: The reactor body has a cavity in the middle; a crucible is fitted around the outside of the reactor body, and the outer crucible can provide a heating environment for the reactor. The vessel lid covers the top of the reactor body and has an air inlet and an air outlet communicating with the receiving cavity; the air inlet is connected to an air inlet pipe and the air outlet is connected to an air outlet pipe. A stirring rod passes through the lid along the central axis of the lid and is rotatably connected to the lid. The lower part of the stirring rod extends into the receiving cavity. A stirring head is installed at the bottom of the stirring rod. The cathode and anode are inserted through the vessel lid and fixed to it, with their lower parts extending into the receiving cavity.

[0007] As a preferred technical solution, the single-crystal superconducting material growth reaction device also includes: A heating device is used to heat the reactor body. Several temperature sensors are installed at different locations on the reactor body to detect and control the reaction temperature. Furthermore, two temperature sensors are used: one is fixed to the bottom of the outer wall of the reactor body for temperature control, and the other is fixed to the middle of the outer wall of the reactor body as a thermometer. The combination of the two temperature sensors can also show the temperature gradient. The temperature sensors are K-type thermocouples.

[0008] A temperature controller is used to control the temperature of the reactor body.

[0009] As a preferred technical solution, the reactor body, reactor lid, stirring rod, and stirring head are all made of polytetrafluoroethylene (PTFE), which has excellent corrosion resistance and heat resistance.

[0010] This invention utilizes a polytetrafluoroethylene (PTFE) sealed reactor design, abandoning the traditional magnetic stir bar rotation. Instead, a rotating stirring rod stabilizes material transport and fixes the electrodes to eliminate mechanical disturbances, significantly reducing the reaction vessel volume and potassium hydroxide volatilization. This avoids problems associated with glass reactors, such as potassium hydroxide reacting with the glass, potassium hydroxide volatilization and deposition due to excessively low temperatures at the top of the glass reactor, and temperature fluctuations of 10-20 degrees Celsius in older reactors. The reactor design significantly reduces the temperature difference between the top and bottom of the reaction vessel, and a high-precision temperature controller successfully maintains the synthesis temperature within 1 degree Celsius. This allows for the preparation of centimeter-scale BKBO single crystals without the use of seed crystals, with superconducting temperature transition performance far exceeding previous methods. The seedless growth method in this invention already meets the needs of most materials research and leaves room for future upgrades by introducing seed crystal technology. Subsequent use of seed crystals can further achieve precise control of crystal orientation, ensure 100% single crystallinity, and improve growth efficiency, thus providing a clear and feasible technical path for preparing ideal single crystal materials that meet the requirements of superconducting electronic devices.

[0011] A second aspect of the present invention is to provide a method for preparing centimeter-scale BKBO crystals, which is carried out using the single-crystal superconducting material growth reactor as described above, and includes the following steps: Weigh out KOH, Ba(OH)₂·8H₂O, and Bi₂O₃ according to the specified ratio. Place KOH into the container and heat it until it melts. Then add Ba(OH)₂·8H₂O and Bi₂O₃ to form a clear molten liquid. Seal the container with a lid to form a sealed container. Control the stirring rod speed to be 5–20 rpm. The heating temperature is 230–250℃, specifically 230℃, 240℃, or 250℃. A protective gas containing saturated water vapor is introduced into the reaction chamber through the inlet. The cathode and anode are connected to a power source, and an electrochemical crystallization reaction is carried out under a constant current. Black, cubic BKBO single crystals are precipitated on the anode, while metallic bismuth is reduced at the cathode. The protective gas is nitrogen or an inert gas. This invention utilizes a stirring rod to slowly stir and introduces an inert protective gas saturated with a hot water bath at a low flow rate (5 sccm), effectively ensuring the uniformity and stability of the oxidation potential within the reaction chamber, effectively protecting Bi2O3 from excessive reduction, and preventing KOH from forming a crust on the surface. This provides a guarantee for growing crystals with accurate stoichiometry and excellent superconducting properties.

[0012] Compared with the prior art, the present invention has the following beneficial effects: The Czochralski method achieves single-crystal growth by precisely controlling the thermal field and mass transport at the crystal growth interface (solid-liquid interface) in a near-steady-state process, allowing the crystal to be slowly "pulled" out of the melt. This invention, based on the principles of Czochralski crystal growth, represents a successful interdisciplinary invention combining the physical concepts of the method with electrochemical growth mechanisms. Instead of using a traditional magnetic stir bar, it employs a rotating stirring rod to stabilize mass transport and fix electrodes to eliminate mechanical disturbances, creating a "quiet growth interface" for BKBO crystals similar to that required by the Czochralski method.

[0013] Compared to traditional open, large-cavity devices, this invention eliminates the cumbersome steps of previous experimental growth processes, integrating the materials into a single reaction vessel with a compact design. This effectively reduces the temperature gradient and improves temperature control accuracy. In terms of materials, it uses PTFE (polytetrafluoroethylene) instead of easily broken glassware that is susceptible to potassium hydroxide corrosion, avoiding the introduction of metal ions and exhibiting excellent corrosion resistance. Simultaneously, it achieves self-sealing of the gas path and stirring rod, preventing leakage and volatilization, balancing performance and cost. Regarding the stirring method, a clever design reduces the stirring rod speed to 5–20 rpm, significantly reducing interference with the growth interface compared to no stirring or using a magnetically attracted high-speed (60–180 rpm) stir bar, which causes severe disturbance and excessive nucleation. Furthermore, without adding seed crystals, the growth process is optimized to achieve Ba values ​​at the centimeter level and above. 1-x K x The purpose of BiO3 single crystal growth.

[0014] This invention decouples and optimizes the seemingly contradictory requirements of "stirring the melt" and "stabilizing the growth interface." The low-speed rotation of the PTFE stirring rod is sufficient to achieve gentle and uniform stirring of the molten salt, ensuring effective transport of reactant ions to the electrode region. Simultaneously, the anode platinum wire for crystal growth is almost stationary relative to the melt, forming an extremely stable solid-liquid growth interface. This minimizes mechanical disturbance, effectively suppresses stray nucleation, and provides a crucial physical environment for the stable, layer-by-layer epitaxial growth of crystals to centimeter-scale dimensions via a "step-growth mechanism." Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the overall structure of the reaction vessel body, the vessel lid, and the crucible in this invention; Figure 2 for Figure 1 Exploded view; Figure 3 A photograph of the product prepared in Example 2; Figure 4 Photographs of the products prepared for Comparative Example 1 and Comparative Example 2; Figure 5 The results of the superconducting transition temperature test of the product prepared in Example 2 are shown below; Figure 6 The superconducting transition temperature test results are for the products prepared in Comparative Example 1 and Comparative Example 2. Figure reference numerals: 1-Reaction vessel body, 2-Containing cavity, 3-Crucible, 4-Vessel lid, 5-Gas inlet, 6-Exhaust outlet, 7-Stirring rod, 8-Stirring head, 9-Cathode, 10-Anode. Detailed Implementation

[0016] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0017] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail; the parts in the following embodiments refer to parts by weight.

[0018] Some of the raw materials and operations in this invention are as follows: Raw materials: KOH, Guoyao Reagent, GR purity, must use KOH with Na ion ratio less than 1%; Ba(OH)2*8H2O, CR purity / 98%; Bi2O3, purity 99.99%.

[0019] The gas path in the reaction is as follows: nitrogen is selected as the protective gas. After passing through hot water, the nitrogen is saturated with hot water and then flows into the reactor body from the inlet under the control of a gas flow rate of 5ccm / min and flows out from the outlet.

[0020] The rotation in this invention can be achieved in two ways: one is to rotate the stirring rod, which can achieve stirring of the reactants; the other is to keep the stirring rod stationary, i.e., fix the vessel lid while rotating the reaction vessel body, which can also achieve the purpose of stirring. This invention achieves the purpose of stirring the reactants by controlling the rotation of the stirring rod.

[0021] Example 1 refer to Figures 1 to 2 A single-crystal superconducting material growth reaction apparatus includes: a reaction vessel body 1, with a receiving cavity 2 in the middle of the reaction vessel body 1; a crucible 3 fitted around the outside of the reaction vessel body 1; a lid 4 covering the top of the reaction vessel body 1, the lid 4 having an inlet 5 and an outlet 6 communicating with the receiving cavity; to facilitate the inflow and outflow of gas during the experiment, an inlet pipe is connected to the inlet 5, and an outlet pipe is connected to the outlet 6; a stirring rod 7 is rotatably connected at the central axis of the lid, the lower part of the stirring rod 7 extending into the receiving cavity 2; a stirring head 8 is installed at the bottom of the stirring rod; a cathode 9 and an anode 10 are inserted and fixed on the lid 4, the lower parts of the cathode 9 and the anode 10 extending into the receiving cavity 2; specifically, holes corresponding to the size of the cathode and anode can be pre-reserved in the lid to facilitate the penetration and fixation of the cathode and anode; the reaction vessel body, lid, stirring rod, and stirring head are all made of polytetrafluoroethylene (PTFE), which has excellent corrosion resistance and heat resistance.

[0022] In a preferred embodiment, in order to facilitate the experiment and more accurately control the temperature of the reaction system, the reaction apparatus also includes a heating device, several temperature sensors and a temperature controller. The heating device, several temperature sensors and temperature controller are all conventionally used equipment in the art. Those skilled in the art can select the specific models according to actual needs, and no particular limitation is made here.

[0023] Example 2 A method for preparing centimeter-scale BKBO crystals, using a single-crystal superconducting material growth reactor as described in Example 1, includes the following steps: Weigh KOH, Ba(OH)2·8H2O, and Bi2O3 according to a mass ratio of 43:1.49:3.22; introduce nitrogen gas containing saturated water vapor through the gas inlet; place KOH into the container and heat it at 250°C; then add Ba(OH)2·8H2O and Bi2O3 in sequence and stir for several hours to form a uniform and clear melt. Subsequently, platinum electrodes, which had undergone rigorous acid washing and polishing, were fixed in place by a vessel lid and inserted into the molten liquid, maintaining an electrode spacing of approximately 20 mm. The vessel lid was then used to seal the containment cavity, forming a closed container. The stirring rod was rotated at 10 rpm. A precision constant current source was connected, and electrochemical crystallization was carried out continuously for 48 hours under constant current. During this process, the target product—black, cubic BKBO single crystals—was successfully precipitated on the anode, while metallic bismuth was reduced at the cathode. After the reaction was completed, the electrodes were removed and cooled in a nitrogen atmosphere. Finally, after cleaning, drying, and fine exfoliation, high-quality BKBO crystal material suitable for property measurement was obtained.

[0024] Comparative Example 1 BKBO crystal material was prepared using a conventional quartz chemical reactor (parameters: temperature 250 degrees Celsius, rotation speed 60 rpm, other processes were the same as in Example 2). Comparative Example 2 Compared with Example 2, the only difference in this comparative example is the rotation speed of the stirring rod, which is 60 rpm. The other processes are the same as in Example 2.

[0025] Performance testing and analysis Figure 3 The image shows a photograph of the product prepared in Example 2. As can be seen from the image, the size of the product is approximately 10 mm, which is at the centimeter level. Figure 4 Photographs of the products prepared for Comparative Example 1 and Comparative Example 2, with dimensions of 3.5 mm and 1 mm, respectively; from Figure 3 and Figure 4 The comparison shows that the product size of the present invention is significantly larger than that of the comparative product, indicating that the process in the present invention can significantly improve the product size and obtain large-sized BKBO crystal materials in the centimeter range.

[0026] Comparison of superconducting transition temperature differences The superconducting transition temperature refers to the critical temperature at which a material transitions from its normal state (with electrical resistance) to its superconducting state (with zero electrical resistance). Magnetic susceptibility measurement can more accurately characterize the degree of superconductivity of a sample. In the normal state, the magnetic susceptibility is close to zero, while below the superconducting transition temperature, the magnetic susceptibility is negative. Furthermore, the larger the superconducting volume ratio, the greater the magnetic susceptibility value. Therefore, the temperature difference between the temperature at which the magnetic susceptibility approaches saturation and the temperature at which the susceptibility begins to decline is commonly used to characterize the quality of the superconducting transition.

[0027] Figure 5 The results of the superconducting transition temperature test of the product prepared in Example 2 are as follows. Figure 6 The superconducting transition temperature test results for the products prepared in Comparative Examples 1 and 2; from Figure 5It can be seen that the superconducting transition temperature difference (defined at the 90% transition point) of the product prepared in Example 2 of the present invention is ≈3K, which is much smaller than the superconducting transition temperatures of Comparative Example 1 and Comparative Example 2. The superconducting transition temperature difference of the product prepared in Comparative Example 1 using a chemical reactor is ≈15K, indicating that the BKBO crystals prepared using the present invention are of superior quality.

[0028] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A single-crystal superconducting material growth reaction apparatus, characterized in that, include: The reactor body has a receiving cavity in the middle and a crucible is fitted on the outside of the reactor body; A lid is fitted over the top of the reactor body, and the lid has an air inlet and an air outlet communicating with the receiving cavity; A stirring rod passes through the lid along the central axis of the lid and is rotatably connected to the lid; the lower part of the stirring rod extends into the receiving cavity. The cathode and anode are inserted through and fixed to the vessel lid, with their lower portions extending into the receiving cavity.

2. The single-crystal superconducting material growth reactor according to claim 1, characterized in that, The single-crystal superconducting material growth reactor also includes: A heating device is used to heat the reactor body. Several temperature sensors are installed at different locations on the reactor body to detect and control the reaction temperature; A temperature controller is used to control the temperature of the reactor body.

3. The single-crystal superconducting material growth reactor according to claim 2, characterized in that, The number of temperature sensors is two, one temperature sensor is fixed to the bottom of the outer wall of the reactor body, and the other temperature sensor is fixed to the middle of the outer wall of the reactor body.

4. The single-crystal superconducting material growth reactor according to claim 2, characterized in that, The temperature sensor is a type K thermocouple.

5. The single-crystal superconducting material growth reactor according to claim 1, characterized in that, The air inlet is connected to an air inlet pipe, and the exhaust outlet is connected to an exhaust outlet pipe.

6. The single-crystal superconducting material growth reactor according to claim 1, characterized in that, A stirring head is installed at the bottom of the stirring rod.

7. The single-crystal superconducting material growth reactor according to claim 6, characterized in that, The reactor body, lid, stirring rod, and stirring head are all made of polytetrafluoroethylene.

8. A method for preparing centimeter-sized BKBO crystals, characterized in that, It is carried out using the single-crystal superconducting material growth reactor as described in any one of claims 1 to 7, and includes the following steps: Weigh KOH, Ba(OH)2·8H2O and Bi2O3 according to the specified ratio. Place KOH into the container and heat it. After KOH melts, add Ba(OH)2·8H2O and Bi2O3 to form a clear molten liquid. Then seal the container with the lid to form a sealed container. Control the stirring rod speed to 5–20 rpm. A protective gas containing saturated water vapor is introduced into the containment cavity through the inlet. The cathode and anode are covered and fixed, and then connected to a power source. An electrochemical crystallization reaction is carried out under a constant current. Black, cubic BKBO single crystals are precipitated on the anode, while metallic bismuth is reduced at the cathode.

9. The preparation method according to claim 8, characterized in that, The heating temperature is 230-250℃.

10. The preparation method according to claim 8, characterized in that, The protective gas is nitrogen or an inert gas.

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