Abstract Body

For technological applications, there are intense efforts to boost the critical current density J_c to reach metrics requisite for numerous applications, e.g., magnets for fusion reactors, proton irradiation therapy, motors for aircraft. The ultimate upper limit for J_c is the depairing current density J_d — the current that imparts sufficient kinetic energy to break apart electron pairs constituting the supercurrent. However, the motion of vortices, magnetic flux lines that penetrate type-II superconductors, introduces dissipation that restricts J_c. Consequently, the typical approach to enhancing J_c is to add defects to immobilize (pin) the vortices. Yet, when optimized, this method is only capable of achieving a maximum J_c of ~30% J_d, though most studies struggle to achieve even a small fraction of this ceiling. Recently, we combined raising the J_d (ceiling) itself by a thermodynamic route with our previously developed methods of tailoring the size and density of pinning centers in REBa₂Cu₃O_y coated conductors (CCs). We obtained J_c∼150 MA/cm² at 4.2 K in self-field [2]. Moreover, we demonstrated a similar enhancement in the depairing current in three classes of iron-based superconductors (1111-type, 11-type, 122-type) [3]. Our combination approach also allows us to slow the rate of thermally activated vortex motion (creep) in several superconducting material systems by reducing the Ginzburg number, a parameter that correlates with vortex creep rates. The resulting high J_c, slow creep rate, and low anisotropy constitute a serious step towards making superconductors using this combined approach attractive for applications. Detailed microstructural and superconducting properties will be presented.

References

[1] M. Miura et al., NPG Asia Materials 9(2017) e447.
[2] M. Miura et al., NPG Asia Materials 14(2022) 85.
[3] M. Miura et al., Nature Materials (Published: 18 July 2024, DOI: https://doi.org/10.1038/s41563-024-01952-7).

Acknowledgment

The work at Seikei University was supported by JST-FOREST (Grant Number JPMJFR202G). A part of this work was supported by JSPS KAKENHI (23H01453 and 23KK0073).

Abstract Body

High-performance coated conductors are being developed for various applications such as fusion, NMR, high field magnet, and motor etc. Because the length of coated conductors is in the km scale, completely homogeneous structure over the km scale is not realistic in the actual manufacturing. If the inhomogeneities degrade the properties such as critical current density, the products cannot be shipped. If there is the undetected inhomogeneity in coated conductors, it may disturb the operation of coil and magnet. Various factors affect the coated conductor homogeneity. The control of coated conductor homogeneity should be discussed for the design of nanorod, grain boundary, and process.

For Jc homogeneity, the pinning center structure should be homogeneous. In the pinning study, because the pinning centers are observed at the scale of ~100 nm, the information on inhomogeneity is missing. In this study, we developed the evaluation method of nanoscale pinning center inhomogeneity over the mm scale. Focused ion beam scanning electron microscopy (FIB-SEM) was used to observe the nanorod inhomogeneity at the ~5 mm scale. Based on the result, the influence of the nanorod material on the nanorod homogeneity will be presented.

The Ion beam assisted deposition (IBAD) substrates with the km length scale may contain local orientation degradation, although their average orientation is as small as a few degrees. This means that the grain boundary weak link degrades the local Jc to dominate the overall Jc. To suppress the local grain boundary weak link induced by the substrate inhomogeneity, the Ca doping is effective. Therefore, the Ca-doped YBa2Cu3O7(YBCO) films were deposited on the IBAD substrates to investigate the Jcimprovement by Ca. The Ca doping on the IBAD substrate with Df=5° achieved five times larger Jcthan that in the undoped YBCO, indicating that the Ca doping is effective in achieving the homogeneous Jc to overcome the local weak link. To implement this scenario in the coated conductors, the “find and patch process” based on the Ca doping to improve the grain boundary Jcwill be discussed.

To understand and design the process for homogeneity, the mathematical modeling and machine learning are very effective. The overall Jc is determined by the networked summation of local Jc, and the percolative mathematical modeling is performed. Although the process contains the complicated phenomena that are difficult to describe by the simple equation, the machine learning and mathematical modeling can describe the relationship between the process and the property. The mathematical modeling of percolative nature and the machine learning based process-property modeling will be also discussed for future process design.

Acknowledgment

This work was partially supported by JSPS, Grant-in-Aid for scientific research 22K18812, CREST, and Research Foundation for the Electrotechnology of Chubu. The IBAD substrates were supplied from Dr. Izumi and Dr. Ibi in AIST.

Abstract Body

The REBa₂Cu₃O_y (RE: rare earth element) superconductors are one of the most promising materials for superconducting magnetic applications because of their high critical temperature (T_c) and critical current density (J_c). However, the J_c decreases significantly with increasing magnetic field due to the motion of vortices. Previously, we have shown a large enhancement in J_c at not only self-field, but also in-field, by introducing a high density of incoherent BaHfO₃ nanoparticles (BHO NPs) of a tailored size into (Y_0.77Gd_0.23)Ba₂Cu₃O_y ((Y,Gd)123) coated conductors (CCs) grown by the Trifluoroacetate Metal Organic Deposition (TFA-MOD) method, which leaves the matrix unaltered and with just slightly decreased superconducting properties [1,2]. By introducing an intermediate heat treatment (IHT) before the conversion process, we successfully controlled the size of BaMO₃ NPs compared to that without IHT [3].

In this work, in order to investigate the influence of the IHT temperature (T_IHT) on the in-field superconducting properties, we fabricated BHO doped (Y,Gd)BCO CCs using various T_IHT. All CCs show almost the same crystallinity and T_c. On the other hand, the BHO doped (Y,Gd)BCO CC with T_IHT=570^oC shows a higher self-field and in-field J_c and a less angular dependent J_c compared to those with other T_IHT. Our results demonstrate that controlling the IHT temperature is an important way for controlling the size and density of BHO NPs and improving the in-field J_c of TFA-MOD CCs. Detailed in-field J_c properties and the microstructures of BHO doped (Y,Gd)BCO CC with various T_IHT will be presented.

References

[1] M. Miura et. al., NPG Asia Materials 9 (2017) e447.
[2] M. Miura et. al., NPG Asia Materials 14 (2022) 85.
[3] K. Nakaoka et al., IEEE Trans. Appl. Supercond. 26 (2016) 800304.

Acknowledgment

This work was supported by JST-FOREST (Grant Number JPMJFR202G). A part of this work at Seikei University was supported by JSPS KAKENHI (23H01453 and 23KK0073).

Keywords: REBCO, YGdBCO, TFA-MOD, IHT, J_c, BHO, pinning center, nanoparticle

Abstract Body

The uniformity of long REBa₂Cu₃O_7-δ (REBCO, where RE is rare earth element) tapes is a pivotal factor for their practical applications. This uniformity along the length or over the width, affected by variations in growth conditions during the deposition process, such as temperature, plume direction, local oxygen pressure and so on. As the demand for REBCO tapes escalates, addressing this issue has become a pressing concern. This work focuses on a REBCO tape produced using a multi-plume, multi-turn pulsed laser deposition system. The microstructure and current carrying capacity of the tape were characterized to understand the causes of uniformity in the transverse direction of the REBCO layer. Following the measurement of the critical current(I_c) distribution along transverse dimension of the tape, a significant disparity in I_c was observed between the edges and the central region of the tape. Microstructure wise, several typical defects were identified at the edges: 1) REBCO layer on the surface decomposed into some flower-like formations with a radius of about 100 μm; 2) several buffer layers expanded, presenting an arch-like structure, which led to a varying orientation of REBCO phases on the top. The helical travelling of the tape causes uneven tension along the transverse dimension, which affects difference thermal contact. This would result in a considerable temperature difference between the edge and the center of the tape. This study offers valuable insights for enhancing the transverse uniformity of tapes deposited by reel-to-reel manner, paving the way for future improvements in the production of REBCO tapes.

Acknowledgment

This work was financially supported by the National Key R&D Program of China of National Natural Science Foundation of China (Grant No. 2022YFE03150201), the National Natural Science Foundation of China (Grant No.52277027). The authors would like to acknowledge Professor Satoshi Awaji, Associate Professor Yuji Tsuchiya and Assistant Professor Tatsunori Okada from Tohoku University for their invaluable assistance with I_c distribution measurement conducted at 77 K.

Figure 1. (a) the I_c distribution along transverse dimension (b) the cross-sectional TEM image at the edge of tape (c) the SEM image of REBCO layer at the edge

Keywords: High-temperature superconductor, Pulsed laser deposition, homogeneity along width, Structure

Abstract Body

Biaxial-oriented and densified microstructures are necessary to achieve self-field and in-field Jc in high-Tc cuprate superconductors. The typical technique for realization of the biaxial orientation and densification is epitaxial growth technology, such as melt-solidification and thin film growth on highly-oriented substrate. On the other hand, our group focuses on the biaxial magnetic alignment by modulated rotating magnetic field (MRF)[1,2] as a triaxial alignment technique of materials and is currently investigating a material production process based on MRF The advantageous points of the magnetic alignment using MRF are “room temperature process”, “no need to use highly oriented template” and “triaxial grain alignment”. These intriguing features of MRF leads to possibilities of production of biaxially oriented REBa₂Cu₃O_y (RE123) thick films (in tens and hundreds micron levels) with higher Ic.

In our group, the intermittent type MRF[3], which is a rotating magnetic field including the resting process at every 180 degree, has been used and magnetic alignment of RE123 powder samples has been examined in epoxy resin at room temperature. The MRF enables the simultaneous alignment of the first easy and hard axes parallel to the direction of static field component and perpendicular to the plane created by the rotating magnetic field, respectively, in principle. However, a “sample-rotation” system in which samples rotate in the horizontal plane should be installed in order to generate MRF by a superconducting solenoidal magnet (SC magnet) with 300 kg in weight. This magnetic alignment technique is a batch process and is disadvantageous from the practical viewpoint.

Recently, our group has developed the linear-drive type MRF (LDT-MRF) equipment with a permanent magnet array[4], which does not require rotational motion of samples and rotation of the solenoidal magnet. It enables the generation of 0.9 T as the static field component and 0.8 T as the rotating field component. The equipment can continuously provide MRF in air gap between top and bottom parts of the magnet array, which achieves triaxial alignment, for long and sheet-shaped sample without the rotation processes of sample and magnet. In practice, Dy123 powders were biaxially oriented in LDT-MRF[4]. In the present study, we estimate uniformities of MRF in magnet arrays of LDT-MRF by the finite element method and design new magnet arrays to improve the uniformity of MRF on the basis of their electromagnetic simulations.

Figure 1(a) shows a schematic (bird view) of the magnet arrays. Three different magnet arrays were developed in the present study. The magnet arrays A, B and C have different widths (W in Fig. 1(a)), and respective values of W are 18, 36 and 44 mm for the magnet arrays A, B and C. In order to generate MRF, LDT-MRF requires reciprocation motion of the magnet array and its stroke is set to be 50 mm in the present study. Figure 1(b) shows the orientation degrees (F) of the magnetically aligned Y123 powders with 10 mm and 4 mm in size. Incidentally, the magnet array A (W=18 mm) is used and F [%] is determined from a ratio of the summation of intensities of the 4-fold symmetric peaks to the summation of intensities in a whole measured region in (103) pole figure. For reference, the F value for the Dy123 powder sample which was oriented in 1T-MRF using a superconducting (SC) magnet. For reference, FWHM (ΔΦ) values in the rotational angle direction on the four peaks are also shown. Clearly, F values were almost constant, whereas for the 10 mm-sample was the worst in the three. This deterioration in ΔΦ is reasonably explained in terms of the decrease in the uniformity of MRF in a direction parallel to W. The simulation by FEM revealed that the vertical rotating field component was inclined outward and the inclination angle was approximately 5 deg at 5 mm apart from the center. In the case of the 10 mm-sample, due to non-uniform MRF, the worse ΔΦ was obtained.

In the present study, the electromagnetic simulation results for the three magnet arrays and the position dependent F and ΔΦ for the magnetically aligned Dy123 powder samples with the magnet arrays A, B and C will be shown.

References

[1] Kimura et al., Langmuir 21 (2005) 4805.  
[2] Fukushima, Horii et al., Appl. Phys. Express 1 (2008) 111701.
[3] Horii et al., Supercond. Sci. Technol. 29 (2016) 125007.
[4] Horii et al., J. Ceram. Soc. Jpn 126 (2018) 885.

Figure 1. (a)Schematic of the magnet arrays. (b) F and ΔΦ values for the three different Dy123 powder samples; 1T-MRF in SC magnet, 10 mm in size in Array A, and 4 mm in size in Array A.

Keywords: Magnet array, Magnetic alignment, Modulated rotation magnetic field, 3D simulation

Abstract Body

Abstract body The second-generation high-temperature superconducting (2G-HTS) tapes are widely utilized in energy transmission, high-field magnets and so on. We have developed a novel ex-situ process to fabricate REBCO superconducting coated. This process consists of two main steps. The first step is to obtain a multicomponent metal film using multi-channel, multi-target DC magnetron co-sputtering. In the second step, the tape deposited with multicomponent metal film was subjected to sintering heat treatment to obtain a superconducting coating. In this paper, GdBCO superconducting films were produced using this novel process. By controlling temperature and oxygen partial pressure during the ex-situ sintering process, the transformation of multicomponent metal films to GdBCO has been investigated, and the evolution paths of their phase transitions have been identified. Furthermore, optimal sputtering parameters and sintering temperatures as well as oxygen partial pressure ranges for the growth of high quality superconducting coatings were determined. This new process we have developed shows great potential and significant advantages, especially in terms of reducing the cost of superconducting tape production and increasing the rate of superconducting tape manufacture. This study provides practical guidance and a reference for the development of a new technique to industrially fabricate REBCO superconducting tapes.