Abstract Body

Chemical solution deposition (CSD) of REBa₂Cu₃O_7-δ (REBCO, RE=Y or other rare earth) nanocomposites from colloidal precursor solutions containing preformed nanocrystals is a promising, cost-effective, and reproducible approach to producing superconducting films with high critical current density (J_c) and enhanced flux pinning. Although the preformed nanocrystals addition technology has achieved great success in controlling the introduction of nanophases, the size of BMO (BaMO₃, M=Hf, Zr) nanocrystals has significantly coarsened after sintering of the REBCO film, which significantly reduces the size control of the introduced nanocrystals. Here, the evolution of the size of BHO nanocrystals is studied. We changed the collection process of BHO nanocrystals after preparation, and successfully separated the BHO nanocrystals with an average size of 6 nm into two kinds of nanocrystals with an average size of 4.5 nm and 7.7 nm, respectively. The evolution of three different BHO sizes, i.e.,4.5 nm, 6 nm and 7 nm, in REBCO superconducting thin film with a thickness of 2.2 μm and 10 mol%-BHO addition was studied. We found that small-sized preformed nanocrystals cannot exist stably at high temperatures and will decompose, so that the other sizes nanocrystals to grow, resulting in the coarsening of preformed nanocrystals. After the nanocrystalline size optimization, it is found that nanocrystals containing a small amount of small size change the smallest, and the corresponding REBCO films have the best performance in the field, especially at low temperature. At 30 K, 3 T, its J_c increased by 21% compared to the unseparated one, which has a very important guiding value in guiding nanocrystal addition technology.

Acknowledgment

The work is supported by the National Natural Science Foundation of China (52172271), the National Key R&D Program of China (2022YFE03150200), Shanghai Science and Technology Innovation Program (22511100200), the National Funded Postdoctoral Researcher Program (GZC20231524).

Keywords: REBa₂Cu₃O_7-δ (REBCO, RE=Y or other rare earth) nanocomposites, BaHfO₃ nanocrystals, separated the BaHfO₃ nanocrystals, critical current density

Abstract Body

Ensuring that high-temperature superconducting (HTS) magnets meet their design specifications is critical, necessitating precise verification of the thickness of HTS Coated Conductors (CCs) prior to their integration into the magnets. Currently, there is no available device capable of continuously measuring the thickness of HTS CCs in a reel-to-reel process. Moreover, HTS CCs often exhibit non-uniform thickness, both along their length and across their width. Consequently, it is imperative to develop a reel-to-reel thickness measurement system capable of accurately evaluating both the thickness and cross-sectional profile of HTS CCs.

This study presents the development of a reel-to-reel thickness measurement system specifically designed for HTS CCs. By integrating a reel-to-reel transfer mechanism with a confocal laser sensor, the system enables continuous thickness measurement during the transfer of long HTS CCs. The presentation will cover the following key aspects: (1) the system's technical specifications, (2) the measurement methodology, and (3) the preliminary results from system trials.

Acknowledgment

This research was supported by National R&D Program through the National Research Foundation of Korea(NRF) funded by Ministry of Science and ICT(2022M3I9A1076881)

Abstract Body

Various thin film deposition methods such as RCE-DR, MOD, MOCVD, and PLD have been used to fabricate second-generation high-temperature superconducting (2G HTS) tapes. Especially, 2GHTS tapes fabricated by the pulsed laser deposition process show excellent electrical current conduction properties under high magnetic field and have recently become a popular research field worldwide.

We focused on identifying the optimal conditions for the continuous deposition of YBa₂Cu₃O₇-δ(YBCO) superconducting layers using the reel-to-reel PLD method on the bi-axially textured substrates made by Ion Beam Assisted Deposition (IBAD) process. Key parameters such as substrate heating temperature, oxygen partial pressure, and pulsed laser settings in a vacuum were examined to determine their impact on the quality of the HTS tapes.

The fabricated tapes were characterized for biaxial alignment using XRD, assessing the 2-theta, in-plane and out-of-plane texture. Additionally, surface and cross-sectional analyses were conducted using SEM, and critical current measurements were performed using the Hall-Ic measurement system.

Abstract Body

High temperature superconductor (HTS) tapes [1-3] have been applied to develop magnets and coils. To generate strong magnetic fields using such HTS magnets or coils, HTS tapes of several or several-tens of kilometers in length are required, whereas the piece length of commercial Bi₂Sr₂Ca₂Cu₃O_8+x (Bi2223) multifilamentary tapes is less than 1 km. Therefore, many researchers have been focused on the development of HTS joint techniques to connect the tapes for magnets [4-9], which are a fundamental technology for practical applications such as nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). Recently, Kanazawa (Jin) et al, reported a method for a Bi2223/Bi2223 joint with critical current (I_c) of 12.2 A and 177 A at 77 K and 4.2 K, respectively [6-8]. In the work, we characterized the nanostructures of the Bi2223/Bi2223 joining region using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Particular attention was paid to the composition and crystal orientation of the joining region.

Bi2223 tapes with a width of 4 mm produced by Sumitomo Electric company were used for preparing a joint sample. The Bi2223 tapes were peeled to prepare a joint surface on which Bi2223 filament and Ag were exposed. The surfaces of the peeled Bi2223 tapes were brought into contact with each other, and the region was rolled with Pt tapes. The contact region was heated up to 890°C in a furnace and cooled down to room temperature [6-8]. We prepared 12 sections of the joint sample were prepared in total using an Ar ion beam in a JEOL IB-09010CP system, and each cross-section were examined in a Hitachi SU8000 SEM system with an energy-dispersive X-ray spectroscopy (EDS). TEM specimen was prepared from a cross-section corresponding to distance of 13 mm from the sample edge of the joint sample using a focused ion beam (FIB)-microsampling technique in a Hitachi NB5000 SEM-FIB system. The specimen was examined in a Topcon EM-002BF operated at an accelerating voltage of 200 kV.

Figure 1(a) shows a SEM image of the center region of cross-section corresponding to distance of 13 mm from the edge of the joint sample. The Ag sheaths appear brightest, and a region indicated by an arrow is slightly brighter than those of the Bi2223 filaments in fig. 1(a). The results of EDS analysis indicated that such the region denoted by the arrow was Bi, Sr-rich and Ca, Cu-poor compared to corresponding regions of Bi2223 filaments. A cross-sectional TEM specimen was prepared from the square region in fig. 1 using FIB-microsampling technique.

Figures 1(b) and (c) show cross-sectional TEM images corresponding to the square region in fig.1(a), and insets (i)-(iv) show selected area diffraction patterns (SADPs) with plane indexes. These TEM images correspond to the same region, but the TEM specimen was tilted a few degrees relative to the images shown in fig. 1(b) to (c). Insets (i) and (ii) were taken from the upper and the lower grains in fig. 1 (b), respectively, and insets (iii) and (iv) were also taken from those grains in fig. 1(c), respectively. The upper grain is identified as Bi₂Sr₂CaCu₂O_8+x (Bi2212) and the lower grains as Bi2223 from the SADPs (i) and (iv), respectively. These Bi2212 and Bi2223 phases were detected using X-ray diffraction taken from the joining region in the previous paper [6-8]. These grains are connected without secondary phase or gaps. Furthermore, the c-axes of the Bi2212 and the Bi2223 grains shown in figs. 1(b) and (c) are well aligned. These results indicate that Bi2212 grains should be transformed from Bi2223 grains at the joining region without a change of the crystal orientation during the heating process, as predicted in the previous report [6-8]. However, the joining region with the B2212 grains is likely to be approximately 3 mm along the width direction of Bi2223 tape and 1 mm along the length direction of the Bi2223 tapes. To increase the critical joint current, such the formation region of Bi2212 grains between the Bi2223 filaments should be enlarged in those joint Bi2223 wires.

References

[1] Maeda H, Tanaka Y, Fukutomi M and Asano T 1988 Jpn. J. Appl. Phys., 27 L209-L210
[2] Hikata T, Sato K and Hitotsuyanagi H 1989 Jpn. J. Appl. Phys., 28 L82-L84
[3] Larbalestier D C, Gurevich A, Feldmann D M and Polyanskii A 2001 Nature, 414 368-377
[4] Park Y, Lee M, Ann H, Choi Y H and Lee H 2014 NPG Asia Mater., 6 e98
[5] Ohki K, Nagaishi T, Kato T, Yokoe D, Hirayama T, Ikuhara Y, Ueno T, Yamagishi K, Takao T, Piao R, Maeda H and Yanagisawa Y 2017 Supercond. Sci. Technol., 30 115017
[6] Jin X, Suetomi Y, Piao R, Matsutake Y, Yagai T, Mochida H, Yanagisawa Y and Maeda H 2019 Supercond. Sci. Technol., 32 035011
[7] Kanazawa S, Yanagisawa Y 2019 J. Alloys. Compd., 806 897-900
[8] Kanazawa S 2021 IEEE Trans. Appl. Supercond., 31 7000104
[9] Takeda Y, Motoki T, Kitaguchi H, Nakashima T, Kobayashi S, Kato T and Shimoyama J 2019 Appl. Phys. Express, 12 023003

Acknowledgment

This work was supported by the Japan Science and Technology Agency (JST)-MIRAI Program Grant Number JPMJMI17A2, Japan.

Fig. 1 (a) shows SEM image of center region of cross-section corresponding to distance of 13 mm from the edge of the joint sample. Square region was prepared to TEM specimen. Fig. 1(b) and (c) shows cross-sectional TEM images of interface between Bi2212 and Bi2223 grains under the [1-10] zone axis of Bi2212 condition, and the [010] zone axis of Bi2223 condition, respectively. (i) – (iv) selected area diffraction pattens with the plane indexes, (i) and (ii) are taken from the Bi2212 and the Bi2223 grains in (b) (iii) and (iv) from those grains in (c).

Keywords: superconducting joint, Bi2223, nanostructure, SEM, TEM

Abstract Body

To fabricate larger RE-Ba-Cu-O (RE: rare earth elements) bulk superconductor and/or suitable bulk for pulse magnetization, we have studied the effects of various joining conditions by local melting method on the improvement of the superconducting properties at the joint.   Although the local melting method enables high-quality superconducting joints, it is known that many defects like pores and segregation of the second phase remained at the joint part depending on the fabrication conditions, such as the joining direction and local melting conditions, resulting in a decrease in the superconducting properties.

In this study, we investigated the effects of oxygen atmosphere and local melting conditions (joining direction, melt temperature, time, and cooling rate) on microstructure and superconducting properties (critical temperature and critical current density) of Gd (and Y)-Ba-Cu-O bulk superconductors joined using the local melt growth method in Er-Ba-Cu-O bulk.   As a result, it was found that joining in a high oxygen atmosphere reduces pores, and that high-speed growth reduces segregation and improves the J_c-B properties of the joined parts.  This fabrication method has the potential to produce high-strength, high-performance superconducting joined bulk superconductors.

Keywords: RE-Ba-Cu-O bulk superconductor, superconducting joint, oxygen atmosphere, pore

Figure 1. J_c -B properties of the RE-Ba-Cu-O bulk superconductors with different joining conditions.

Abstract Body

Ⅰ. Introduction
Due to the high cost of rare-earth based superconducting coated conductors, there is a pressing need to reduce production costs. One potential solution is to simplify the architecture by eliminating the silver stabilization layer. This can be achieved by epitaxially growing a conductive oxide on an oriented nickel substrate, followed by the epitaxial growth of YBCO. By reducing the silver layer, a significant cost reduction is expected. In this study, we focused on LaNiO₃ (LNO) as a conductive intermediate layer and investigated the conditions for epitaxial growth of LNO [1, 2]. Furthermore, we explored the conditions for epitaxial growth of YBCO films on LNO films.

Ⅱ. Experimental Methods
In this study, sintered LNO and YBCO targets were prepared, and LNO intermediate layers were deposited on SrTiO₃(STO) (100) single-crystal substrates using pulsed laser deposition (PLD). LNO intermediate layers were deposited on the STOsubstrates at a fixed substrate heater temperature of 900°C and an oxygen pressure of 50 Pa, with varying deposition times from 15 to 60 minutes (film thickness: 130-430 nm). Subsequently, YBCO films were deposited on these LNO layers at a substrate heater temperature of 900°C, an oxygen pressure of 23 Pa, and a deposition time of 60 minutes (film thickness: 480 nm) to fabricate multilayer films. The prepared thin films were characterized by X-ray diffraction (XRD) and temperature-dependent resistivity (R-T) measurements. Additionally, the film thickness was evaluated by step height measurement using a surface profiler.

Ⅲ. Results and Discussion
The relative intensity of the YBCO 103 peak in the XRD patterns decreased as the thickness of the LNO intermediate layer decreased. The YBCO 103 peak is the strongest diffraction peak for polycrystalline YBCO, indicating the presence of polycrystalline YBCO grains in the samples. Two possible reasons for the formation of polycrystalline YBCO are: (1) when the LNO intermediate layer is thick, the heat energy from the STO substrate is less likely to be transferred to the YBCO film being deposited on the intermediate layer, leading to the growth of polycrystalline YBCO films, and (2) as the LNO intermediate layer becomes thicker, the LNO surface becomes rougher, hindering the epitaxial growth of YBCO. On the other hand, Fig. 2 shows that the resistivity of the LNO intermediate layer at 250 K did not change significantly with decreasing film thickness. From these results, it was found that a thinner LNO intermediate layer is more favorable for the epitaxial growth of YBCO and that even a thin layer exhibits sufficiently low resistivity. Furthermore, a superconducting transition temperature of 82 K was obtained for the YBCO/LNO(130nm)/STO sample. In this presentation, we will also report on our investigation of improving the superconducting properties of YBCO thin films on LNO.

References

[1] M. Satyalakshmi et al., Appl. Phys. Lett. 62 (1993) 1233.
[2] M. S. Hegde et al, J. Mater. Res. 9 (1994) 898.

Acknowledgment

The authors thank to Yoshida laboratory at Nagoya University for providing us with access to their X-ray diffraction equipment.

Figure 1. Relative XRD intensity of YBCO 103 peak to STO 200 vs. LNO layer
Figure 2. Resistivity at 250K vs. LNO layer thickness.

Keywords: YBCO, PLD method, LaNiO3, Epitaxial growth

Abstract Body

I. Introduction
Pulsed Laser Deposition (PLD) is an effective thin film deposition method for fabricating YBa₂Cu₃O_y (YBCO) epitaxial films. In the PLD method, columnar ablation plasma, called a plume, is observed when a laser is irradiated onto a target. The plume shape is influenced by not only quantitative parameters such as laser energy and oxygen pressure but also non-quantitative parameters such as laser energy fluctuations and shock waves generated by the plume compressing the surrounding gas [1]. This results in a somewhat low reproducibility of superconducting properties even when the oxygen pressure and laser energy are fixed during the deposition of YBCO thin films.

In this study, with the aim of achieving highly reproducible deposition of YBCO thin films by the PLD method, we constructed a system for real-time observation of the plume shape and the color, and then we investigated the effects of various conditions such as oxygen pressure and laser energy on the plume shape and the color.

II. Experimental method
YBCO thin films were fabricated using the PLD method, and the plume during the deposition process was captured using an Intel Realsense Depth Camera D405. For a plume observation system, a Python code was developed to sequentially measure and record the color information around the plume and the plume size. In preparations of YBCO thin films on SrTiO₃(STO) (100) substrates, the substrate temperature was set to 920 °C, and the energy of the Nd:YAG laser (wavelength 266 nm, repetition rate 10 Hz) was varied to 10, 15, and 20 mJ. The oxygen pressure was changed every 10 minutes to 10^-2, 10-1, 1, 13.3, 26.3, 53.2, and 100 Pa to observe the plume shape. The plume size was measured by determining the height and width of the plume based on the three-dimensional coordinate information obtained from the depth camera.

III. Experimental results and discussion
Fig. 1 shows images of the plume formed under different oxygen pressures when the laser energy was 20 mJ. The green rectangle is the bounding box that the plume observation system recognizes as enclosing the plume. This bounding box is obtained by converting the color image of the plume into a grayscale image, binarizing it, and then finding the bounding box of the resulting black-and-white boundary. Fig. 2 shows the plume height for each oxygen pressure and laser energy. From these results, the plume height decreases with increasing oxygen pressure above 13.3 Pa, while no clear trend was observed for oxygen pressures below 1 Pa. Additionally, the plume height decreased with decreasing laser energy at all oxygen pressures. At the presentation, we will discuss the effects of oxygen pressure and laser energy on the aspect ratio (height-to-width ratio) and color information of the plume. We will also report on the construction of a system for feedback control of the plume shape.

References

[1] K.Fukushima, Int’l J. Mod. Phys. B, vol. 9, No. 28, 1995.

Acknowledgment

This research was partly supported by the JST-CREST (JPMJCR2336). We would like to express our sincere gratitude to the Yoshida Laboratory, Department of Electrical Engineering, Nagoya University, for their significant contributions to the construction of the observation system.

Fig. 1 Images of YBCO ablation plumes at different O2 pressure in which the laser energy is fixed to 20 mJ.
Fig. 2 Height of YBCO ablation plume depending on O2pressures and laser energy.

Keywords: Pulsed Laser Deposition, YBa₂Cu₃O_y thin film, ablation plume

Abstract Body

I. Introduction
We have previously developed a Monte Carlo simulation for the crystal growth of REBCO thin films and investigated the self-organization of BaMO₃ (BMO, M=Zr, Sn, Hf…) in REBCO thin films doped with BMO [1]. However, this simulation has the issue that the grain size of REBCO is smaller than that observed in actual experimental results. In this study, we aimed to improve the accuracy of the REBCO crystal growth simulation by using Bayesian optimization to compare experimental and simulation results and investigate for simulation parameters that can reproduce experimental results.

II. Simulation
The simulation in this study is a crystal growth simulation using the Monte Carlo method, and the simulation code developed in reference [1] was used. The number and size of the islands of REBCO thin film crystals vary depending on the binding energy between REBCO molecules (E_AA), the binding energy between REBCO and the substrate (E_AS), and the evaporation energy from the substrate (E_des). In this study, E_des was fixed at 10000 K, and Bayesian optimization was used to optimize E_AA and E_AS in the range of 0 to 10000 K. The objective variable in Bayesian optimization was the crystal island density (σ) of REBCO, and an evaluation function f that becomes maximize when the experimental results [2] and the simulation results coincide was defined as follows.

Here, x = log10 𝜎, and 𝑥_𝑒𝑥 corresponds to the experimental YBCO island density 𝜎_𝑒𝑥. 𝑓(𝑥) is a Gaussian distribution centered at 𝑥_𝑒𝑥, with a full width at half maximum of ｗ.

III. Results and discussion
Fig. 1 shows the results of Bayesian optimization when E_AA and E_AS are varied in the range of 0 to 10000 K. From the figure, the parameters with the maximum evaluation function at present are around E_AA = 500 K and E_AS = 4000 K. Fig. 2 shows the evolution of the evaluation function value over 40 iterations of the Bayesian optimization. The results indicate that the evaluation function increased until the 21st iteration but plateaued thereafter. This suggests that varying only E_AA and E_AS was insufficient to achieve a satisfactory fit to the experimental data, as the maximum attainable value of the evaluation function (f=0.091) was significantly lower than the objective value of 1.

To further explore the parameter space and potentially improve the fit to the experimental data, we will present results from a Bayesian optimization study where all three parameters, E_AA, E_AS, and E_des, are varied.

References

[1] Y. Ichino, et al., JJAP 56 (2017) 015601, IEEE TAS 27 (2017) 7500304, IEEE TAS 31 (2021) 7500204.
[2] B. Dam, J.H. Rector, J.M. Huijbregtse, R. Griessen, Physica C 305 (1998) 1-10.

Acknowledgment

This research was supported by JST-CREST (JPMJCR2336). The AFM measurements were performed using an instrument provided by the Yoshida Laboratory, Department of Electrical Engineering, Nagoya University.

Figure 1. Distribution of evaluation function values for 40 different combinations of EAAand EAS, ranging from 0 to 10000 K.
Figure 2. Evaluation function values as a function of the number of Bayesian optimization iterations.

Keywords: Bayesian optimization, Monte Carlo simulation, REBCO thin film, crystal growth

Abstract Body

To achieve high critical current densities in both self-field and in-field conditions for high-T_c cuprate superconductor REBa₂Cu₃O_y (RE123), the densification and orientation of a significant number of grains are required. In the study of the practical utilization of RE123 as a superconducting wire, our group is recently focusing the magnetic alignment methods to lead biaxial orientation of the RE123 grains. The strong points of magnetic alignment are that it does not require highly oriented template material, and it is a room temperature process. These features of magnetic alignment open new possibilities for fabricating RE123 thick (> 10 μm) films.

In the magnetic alignment method, the expectation is that the easy and hard axes align perpendicular to the static magnetic field and the rotating magnetic field, respectively. When the grain shape is approximately spherical, the following formula can be used to estimate the required magnetic alignment time τ [Ref. 1]: τ^-1 = χaB² / 6ημ₀, where χ_a is the dimensionless difference between the magnetic susceptibility along the easy axis and that perpendicular to the easy axis, B is the magnetic field strength, η is the medium viscosity, and μ₀ is the vacuum permeability. Roughly ten years ago, superconducting magnets were essential for achieving the biaxial magnetic alignment of several RE123s [Ref. 2]. Recently, our group developed an original apparatus that can create a linear drive type of modulated rotating magnetic fields (LDT-MRF) using the permanent magnet arrays [Ref. 3]. This apparatus is simple and has relatively low costs. As an important achievement to date, this LDT-MRF equipment had achieved a static magnetic field of 0.9 T and a rotating magnetic field of 0.8 T [Ref. 4]. Traditionally, such research has been conducted using batch processes. However, the application of magnetic alignment for the superconducting wire needs a continuous process. In this study, we integrated a sample transport system into the LDT-MRF apparatus and evaluated its performance in a continuous magnetic alignment process.

DyBa₂Cu₃O_y (Dy123, y ~ 7) powders (ave. particle size ~ 2－4 μm) were selected as the test sample of magnetic alignments by using the LDT-MRF apparatus, showing its relatively large magnetic anisotropy among the RE123 compounds [Ref.2]. The epoxy resin was used as the dispersion medium for Dy123 grains. The initial viscosity of the epoxy resin is η_int ~ 40 Pa⋅s, and less than five times η_int even one hour after mixing the base agent and hardener. The Dy123 powder and epoxy were rapidly mixed and transferred to the sample space with the acrylic mold. During the operation of the LDT-MRF apparatus, the sample tray moved (// linear driving motion) at a speed of approximately 0.7 mm/min. The time when the sample entered/escaped the magnetic field region of the LDT-MRF apparatus was defined as t_sta/t_end, respectively. Here the t_sta/t_end of Sample I was ~ 8 min./~ 25 min. The t_sta/t_end of Sample II was ~ 20 min./~ 37 min. The degree of orientation for the Dy123 powder sample after magnetic alignment was evaluated by using the (1 0 3) pole figures.

Figures 1(b-c) show the pole figures at the bottom surface of the sample I and Sample II after magnetic alignment by using the LDT-MRF apparatus. The four-fold rotationally symmetric spots in each pole figures indicate the biaxial orientation of Dy123 grains with twin microstructures. In both the results for samples I and Sample II, four spots and their centers of gravity were shifted downward in parallel on this paper. This parallel shift suggests that the orientation state of the Dy123 grains has been tilted. The inclination angle of the Dy123 grains in Sample I (~ 10°) was smaller than that of Sample II (~ 20°). Namly, the inclination angle of the grains was suppressed due to a higher viscosity of the sample when it is escaped from the magnetic field. In the future, a continuous process of magnetic alignment may become possible by tuning the magnetic anisotropy of a target material and medium viscosity for the dispersion to the state of the MRFs. In this presentation, we will report details of the methods and results.

References

[1] T. Kimura, Polym. J. 35, 823 (2003).

[2] S. Horii, T. Nishioka, I. Arimoto, S. Fujioka, and T. Doi, Supercond. Sci. Technol. 29, 125007 (2016).

[3] S. Horii, I. Arimoto, and T. Doi, J. Ceram. Soc. Jpn 126, 885 (2018).

[4] W. B. Ali, S. Adachi, F. Kimura, and S. Horii, J. Phys. Conf. Ser., 012016, 2545 (2023).

Acknowledgment

We are grateful to Mr. Kotakebayashi and Mr. Hiratsuka (the members of KUAS Machine Shop) for their support with modifying the magnetic alignment system.

Figure 1. (a) Schematic diagram of the apparatus for magnetic alignment using modulated rotating magnetic fields. (b-c) (103) pole figures at the bottom surface of the sample after magnetic alignment by using the apparatus of Figure 1a.

Keywords: Cuprate superconductor, REBCO, Magnetic alignment, Pole figure

Abstract Body

Bi₂Sr₂Ca₂Cu₃O_y (Bi-2223) is a highly regarded material in the field of superconductors for its high critical temperature (Tc ~110K), which allows it to operate under liquid nitrogen cooling (~77K). This study investigates aerosol deposition (AD) as a potential technique for fabricating Bi-2223 thin films with properties comparable to those produced by traditional methods like pulsed laser deposition (PLD) and sputtering. The potential to create dense, high-quality superconducting films at room temperature with shorter processing times offers significant advantages for large-scale applications. Additionally, AD's flexibility allows for the direct deposition of superconducting films on various substrates, including heat-sensitive ones, broadening the potential applications of Bi-2223 films in advanced technologies.

Bi(Pb)-2223 bulk materials were synthesized with a composition of Bi_1.75Pb_0.35Sr_1.9Ca_2.1Cu₃O_z using powder oxides as starting materials. After mixing, the samples were uniaxially pressed into pellets and underwent a series of heat treatments. This process resulted in a highly pure Bi-2223 phase with minimal impurities. X-ray diffraction (XRD) patterns confirmed the successful formation of the Bi-2223 phase, with no secondary phases. The synthesized bulk material exhibited a Tc of approximately 107K.

To prepare the bulk material for AD, the bulk material was wet milled using a planetary ball mill to produce fine particles with a mean diameter of 0.49μm. The size of the particles was essential for effective aerosolization and facilitating the thin film formation. The milled powder was then dried under vacuum for two hours at 200oC to remove residual solvents and moisture. The chamber is then placed in the system and aerosolized using N₂ gas at a rate of 10L/min. The deposition of Bi(Pb)-2223 films was carried out on an annealed silver substrate with a substate temperature of 200°C at a scan rate of 105μm/s for 10 scans.

The resulting films, approximately 64μm thick, retained the crystalline structure of the Bi-2223 precursor but showed partial amorphization. Post-deposition annealing at 845°C for 8 hours, followed by 820°C for 12 hours, was crucial in re-establishing crystalline order, enhancing c-axis orientation, and improving the films' superconducting properties. XRD analysis confirmed a highly oriented Bi-2223 phase after annealing, while resistance-temperature (R-T) measurements showed a superconducting transition with a Tc of 103K, indicating the successful formation of superconducting pathways predominantly composed of the Bi-2223 phase.

The presentation will further discuss data on the effects of varying bulk compositional ratios and deposition parameters on subsequent thin film formation via AD.

Figure 1. XRD pattern of aerosol-deposited Bi(Pb)-2223 films before and after annealing
Figure 2. R-T graph of annealed Bi(Pb)-2223 films

Keywords: Bi-2223, aerosol deposition, thin film

Abstract Body

[Introduction]
To improve the critical current density (J_c) of REBa₂Cu₃O_y(RE123) superconductors (SCs) for practical use, it is important to achieve densification and bi- or tri-axial grain alignment. The thin film epitaxial growth technology is used for REBa₂Cu₃O_y [RE123] coated conductors with high critical current density (J_c). However, the thickness of the RE123 layer has the order of microns and material cost of the coated conductors is very high. Our group focuses on magnetic alignment based on the modulated rotating magnetic field (MRF) as a biaxial/triaxial alignment process. Biaxial alignment of RE123 grains with twinned microstructure has been achieved under the MRF of solenoidal SC magnet (SC-MRF) in an epoxy resin [1, 2]. One of the current issues in SC-MRF is development of continuous production process, and the linear drive type MRF (LDT-MRF) has been developed as a novel MRF which is appropriate for a continuous production. This equipment generates an MRF of ~ 0.8 T by reciprocating motion of a permanent magnet array [3]. Another issue is development of the biaxial aligned RE123 ceramics based on the colloidal process. Generally, viscosity levels of colloidal solutions are 2-4 orders of magnitude lower than that of epoxy resin used in our previous studies[1, 2]. We need to develop RE123 compounds appropriate for the biaxial magnetic alignment using the colloidal solution with the lower viscosities under the permanent magnet level. Y123 is relatively inexpensive in RE123, and appropriate for practical use. However, Y123 shows the smallest magnetic anisotropy in RE123 and its biaxial orientation was not achieved within the previous work[1]. In this study, we clarified the degrees of orientation of YBa₂Cu₃O_y [Y123] particles under the SC-MRF and LDT-MRF in different types of epoxy resin with various viscosities.

[Experimental detail]
Y123 polycrystals were synthesized by the standard solid-state reaction. Incidentally, the pelletized powders were sintered at 920 ℃ in air for 24 h as the final sintering process. The obtained Y123 pellets were annealed at 300℃ in flowing oxygen gas to achieve y ~ 7, and were pulverized in an agate mortar to obtain powders with ~10 μm in averaged grain size. Y123 powders were mixed with epoxy resins at weight ratio of 1:10 and aligned under SC-MRF of 1 T and LDT-MRF. Here, we used two different types of epoxy resins. Resin A shows a higher initial viscosity (η_init ~ 40 Pa･s). Resins B and C show lower initial viscosities (η_init ~ 0.5 Pa･s) and different curing times (17 h for Resin B, 37 h for Resin C). The orientation axes and degrees of orientation of the magnetically aligned powder samples of Y123 were determined from (103) pole figure measurements.

[Results and Discussion]
Figs. 1(a), (b), and (c) show (103) pole figures of the Y123 powder samples aligned under SC-MRF of 1 T in Resin A, Resin B, and Resin C, respectively. Note that the measurement plane for the (103) pole figure is a plane perpendicular to the direction of the static magnetic field component. The biaxial orientation degree (F_s) was evaluated as a ratio of summation of intensities of the four-symmetric peaks related to bi-axial alignment to summation of whole peak intensities. Within our experiment of the uni-axial alignment for RE123, F_s is equivalent to ~ 9 %. F_s for Resin A showed 13.2 %, which is close to the uni-axial alignment. F_s for Resin B and Resin C showed 20.8 % and 31.2 %, respectively indicating that Y123 powders were partially bi-axial aligned in Resin B and Resin C. It was suggested that the orientation degrees were improved by using epoxy resin with the lower viscosities. In this study, we will report the change in the orientation degrees for the magnetically aligned Y123 powder samples as functions of viscosity and magnetic field strength of MRF.

References

[1] Horii et al., SuST 29 (2016) 125007.

[2] Ali et al., J. Appl. Phys. 134 (2023) 163901.

[3] Horii et al., J. Ceram. Soc. Jpn. 126 (2018) 885.

Acknowledgment

Fig. 1 (103) pole figures of the magnetically aligned powder samples of Y123 under 1 T of SC-MRF in (a) Resin A, (b) Resin B, and (c) Resin C, respectively.

Keywords: Cuprate superconductor, Magnetic alignment, RE123