Production of Sharp-Edged and Surface-Damaged Y2BaCuO5 by Ultrasound: Significant Improvement of Superconducting Performance of Infiltration Growth-Processed YBa2Cu3O7-δ Bulk Superconductors.

Growth and physical properties of bulk REBa2Cu3O7-δ (REBCO) superconductors fabricated by the infiltration growth (IG) method strongly depend on the initial size and morphology of the RE2BaCuO5 (211) particles. The present work details the novel method we developed for producing sharp-edged and surface-damaged 211 particles to be added to the REBCO bulks. We employed high-energy ultrasonic irradiation for pretreating the 211 particles and fabricated high-performance bulk single-grain YBa2Cu3O7-δ (YBCO) superconductors via the top-seeded IG process. Increasing the ultrasound irradiation power and time duration mechanically damaged the surface of the 211 particles, producing more fine and sharp edges. Systematic investigations of the microstructural properties of the final YBCO bulks indicated that the size and content of the 211 particles gradually decreased without any additional chemical doping. The effective grain refinement and improved interfacial defect densities enhanced the critical current density by a factor of two at 77 K and self-field as compared to a YBCO sample fabricated without any pretreatment. A maximum trapped field of 0.48 T at 77 K was obtained for a sample (20 mm diameter) with 211 particles treated for 60 min and 300 W ultrasound radiation. The effectiveness of the novel method is demonstrated by the superior performance of the YBCO bulk samples prepared as compared to bulk samples fabricated with the addition of Pt and CeO2. This method is novel, cost effective, and very convenient, maintaining high sample homogeneity, and is free of chemical contaminants as compared to other methods which significantly affect the properties of all REBCO bulk products grown by sintering, melt growth, and IG methods.

Introduction

Bulk (Y/RE)Ba2Cu3O7−δ [(Y/RE)BCO, RE-123 or 123; RE = light rare-earth elements, Sm, Nd, Eu, and Gd] superconductors are prominent applied materials and have drawn much attention in the literature. This is mainly due to their relatively high superconducting transition temperature (Tc), critical current density (Jc), low flux creep, and low anisotropy. Many applications can be realized because Tc of REBCO materials is well above 77 K. Trapping of high magnetic fields by bulk (Y/RE)BCO superconductors enables the fabrication of superconducting quasi-permanent magnets for magnetic resonance imaging, NMR, friction-free flywheels for energy storage, motors, and others.[1−3] One of the important criteria in the evaluation of the quality of a high-Tc superconductor is the Jc and its temperature and field dependence. Therefore, improving the trapped fields (TFs) and Jc, which stands up to high fields and high temperatures, is the recent topic of research.

Jc is not an intrinsic property of a superconductor and depends strongly on the final microstructure achieved. Several parameters such as the homogenous distribution of non-superconducting phases, oxygen content, cracks, and the coupling at grain boundaries may affect the Jc of the bulk REBCO materials. One of the most attempted approaches for improving Jc in these materials is the introduction of secondary phase inclusions.[4,5] The defects generated at the interface of non-superconducting and insulating (Y/RE)2BaCuO5 (hereafter abbreviated as Y-211/RE-211, 211) particles within the 123 matrix and those associated with nano-inclusions, dislocations, twins, and stacking faults have been proven to act as efficient flux pinning centers. For optimum performance, the size of these flux pinning sites should be comparable to 2ξ, the superconducting coherence length. Several methods such as sintering, melt growth (MG), and infiltration growth (IG) techniques have been developed for synthesizing various REBCO bulks. The IG process is found to be superior because of its advantageous merits such as the near-net-shape formation, minute amount of macrodefects such as pores, high dense microstructure embedded with fine-sized RE-211 inclusions, and so forth. In our recent work, we compared all these methods comprehensively.[6] In the IG method, it is well known that the initial size of the RE-211 particles is considered to be dominant in the growth of REBCO.[7−9] Therefore, the production and utilization of smaller-sized RE-211 particles is essential for the fabrication of high-quality REBCO bulk materials which support high Jc and trap large magnetic fields.

It is also well known that during the peritectic reaction between RE-211 and liquid phases, the smaller RE-211 particles will be consumed first to form the continuous 123 matrix. Employment of finer particles thus results in a continuous matrix with finer RE-211 particles left back. Much effort has been put through to the embedding of large fractions of fine-sized, non-superconducting inclusions and their homogeneous dispersion, which improves the interfacial defect density in the superconducting bulk samples. Conventional solid-state reaction and chemical routes and the combination of these with ball milling, coprecipitation, and spray-drying produce spherical RE-211 phase particles in the powder form with grain sizes ranging from submicron to several microns. To refine the RE-211 particles in the final products, many methods such as the addition of grain refiners (Pt, CeO2, Ba–Ce–O, etc.) have been tried.[10−15] However, these additives are economically costly and with chemical addition, there always exist problems in maintaining the chemical composition throughout the bulk, and the achieved homogeneity is questionable. Ball milling is also limited to produce particles with ∼100 nm size, but milling for several hours results in the introduction of impurities, which is not desirable. Hence, it is necessary to look for new and advanced methods of producing nanometer-sized RE-211 phase particles within the 123 matrix without using any chemical dopants.

Employing novel methods for producing the fine-sized RE-211 particles and the successful incorporation into final IG-processed REBCO bulks are still challenging.[16−18] To address the above-mentioned problems and to improve the properties of bulk REBCO, sharp-edged, surface-damaged, and nanometer-sized RE-211 particles were produced employing the method of high-energy ultrasonic irradiation. The manipulated RE-211 particles were then utilized for fabricating single-grain YBCO bulk superconductors via the top-seeded IG (TS-IG) method. Therefore, the present work reports the systematic development of a novel method for the production of surface-modified and nanometer-sized Y-211 particles, controlling the content and size in the final bulk microstructures of bulk YBCO samples, which significantly improves the field-dependence properties and TFs. For comparison, we additionally fabricated a bulk YBCO sample with the addition of Pt and CeO2 to demonstrate the effectiveness of the present method.

Results and Discussion

Working Mechanism

The strength of the superconductivity of REBCO materials largely depends on the crystal imperfections developed in the samples during the growth. According to various models available on the growth mechanism of REBCO superconductors, the peritectic reaction progresses by consuming the RE reservoirs, mainly the RE-211 phase, as given in eq . Recently, the utilization of nanotechnology in all scientific fields has been increased because of the fact that the properties of the material dramatically changes when the size changes to the nanometer scale. At the initial stages of the peritectic reaction, smaller-sized RE-211 particles (nanometer-sized) will be utilized for the peritectic 123 phase formations followed by larger-sized RE-211 grains.[19−22] However, the growth reaction/rate could be further improved by geometrical modifications of precursor RE-211 particles.[23−25] This can be achieved by employing RE-211 particles with very sharp edges and damaged surfaces. The nanometer-sized, sharp-edged, and surface-damaged RE-211 particles will be consumed during peritectic reaction and so contribute to improve the growth rate and allow fabricating larger-sized single domains in shorter times. By this technique, one can produce highly dense and superior quality final bulk products with embedded and homogenously distributed fine-sized (nanometer) RE-211 particles in the bulk matrix. The working mechanism of manipulating the initial RE-211 particle size and morphology, employing the high-energy ultrasonication method, is illustrated in Figure .

Figure 1

Schematic illustration of the irradiation effect of ultrasonic energy on RE-211 precursor powders and the final bulk sample microstructure.

Microstructural Properties of Precursor Y-211 Powders

The Y-211 particles prepared by the solid-state sintering method will be of nearly spherical shape and are agglomerated because of which their size is effectively larger. The micrographs recorded by field emission scanning electron microscopy (FE-SEM) on the Y-211 precursor powders are displayed in Figure a,b. The effective size range of the 211 particles was determined to be as large as ∼3–6 μm because of agglomeration, which is commonly observed in any sintered powder particles. For demonstrating the effectiveness of the proposed method of applying ultrasonic energy and producing sharp-edged and surface-damaged particles, we have chosen the YBCO system. We manipulated the initial Y-211 particle size and morphology, employing high-energy ultrasonication. In order to investigate the effect of power and time duration on the breaking of the Y-211 particles, different conditions such as variation of power, duration (Δt), amount of power and liquid mediums, and so forth were tested.

Figure 2

(a,b) FE-SEM micrographs of the as-synthesized Y-211 particles. (c,d) Similar micrographs of Y-211 particles after irradiating ultrasound with 300 W for 15 min. The agglomerated 211 particles become individual and submicron-sized on applying ultrasonication.

During the irradiation of low-energy ultrasonic waves and smaller Δt, sintering bonds between the 211 particles were broken and became individual. In Figure c,d, the FE-SEM images were recorded at magnifications of 5000× and 20,000×, respectively, illustrating the damage after irradiation of ultrasonic waves with 300 W power for 15 min. During this stage, the sintering bonds between the agglomerated Y-211 grains were broken and became individual ones with sizes in the submicron range. These fine-sized Y-211 particles are believed to participate and to improve the crystal growth, and remaining fine-sized Y-211 particles will be acting as superior flux pinning centers. A detailed microstructural analysis of the Y-211 precursor powders pretreated by the high-energy ultrasonic irradiation was discussed in our recent work.[25] When an ultrasonic power of 300 W was irradiated, the average size range of the Y-211 phase particle was reduced to about 700 nm. Controlling the size of the initial additives and the homogenous distribution are the most prominent issues to achieve flux pinning centers within the 123 matrix, which is the key for enhancing the Jc up to large applied magnetic fields.

To create more sharp-edged grains and to further modify the surface morphology, we employed an energy of 300 W at different time intervals of 15, 30, 45, and 60 min. We found that allowing 300 W for 45 min was the optimum condition for breaking the Y-211 phase particles while maintaining high homogeneity. Figure shows the FE-SEM micrographs of (a) 30, (b) 45, and (c) 60 min ultrasonically pretreated Y-211 particles. All the sample microstructures reveal that the Y-211 particles are individual, in which the sintering bonds between the agglomerated Y-211 particles were successfully divided. However, the morphology of sample Y-211-30 is nearly spherical, and the broken bonds are marked by the blue box (see Figure a), whereas, the Y-211-45 particles can be found to be differently shaped, that is, partially eradicated and sharp-edged. It was observed that the Y-211 phase particles show sharp edges because of bombarding a high energy of 300 W. The sharp edges produced because of ultrasonic irradiation were marked with arrows in Figure b. The ultrasonic waves were eradicating some part of the surfaces of Y-211 grains. In some cases, the Y-211 particles could break up in the middle. The eradication could be achieved in shorter times if higher ultrasonic powers could be employed. More importantly, the nanometer Y-211 particles of different morphologies made by ultrasonic irradiation were expected to participate in a new type of growth as compared to conventional spherical shape and larger-sized Y-211 precursor particles. This method also can be employed for creating such sharp-edged and surface-damaged particles for other types of materials, by which their reaction or physical properties could be engineered. As evinced in Figure c, when an ultrasonication power of 300 W was employed for 60 min, the Y-211 particles were damaged immensely because of which very small-sized flakes can be observed, as marked by the circles.

Figure 3

FE-SEM images of the Y-211 particles exposed to an ultrasonic energy of 300 W for (a) 30, (b) 45, and (c) 60 min. The Y-211-45 sample contains Y-211 particles of different shapes with sharp edges and surface damage because of partial eradicating surfaces of the material.

As the irradiation time interval Δt increased, more Y-211 grain surfaces were damaged and the sizes reduced along with increasing homogeneity. The created sharp edge and surface damages are believed to improve the peritectic reaction which enhances the growth rate and distribute the fine-sized nanometer-sized Y-211 particles homogeneously throughout the bulks. This is supported by reports available in the literature on other composites.[24] The experiments related to investigating the influence of ultrasonically irradiated Y-211 particles on the growth of various REBCO bulks are under way.

Structural Properties of Ultrasonicated Y-211 Powders and YBCO Bulk Samples

The as-processed Y-211 particles are agglomerated and are effectively large-sized (∼6 μm) grains. In order to evince the reduction of the Y-211 particle sizes with ultrasound irradiation time, X-ray diffraction (XRD) patterns of each pretreated powder were recorded (see Figure a). The expanded version of the characteristic Bragg peak (2θ = 29.6–30°) of Y-211 for all powder samples is shown in Figure b. It can be observed that as Δt increases, the Bragg peak is widened and the corresponding fwhm value (determined by fitting with the peak function) increased. The details of the Y-211 size, fwhm, dispersion, and so forth are given in Table . This clearly evinces that the Y-211 particle size decreases with increasing Δt. This result is consistent and gets further support from the microstructural observations made on the pretreated Y-211 powders.[23]

Figure 4

(a) XRD patterns of the Y-211 powders exposed to an ultrasonic energy of 300 W irradiated for different time intervals are compared to each other. (b) Characteristic Bragg peak of the Y-211 of all powders.

Table 1

Effect of Ultrasonic Irradiation on the Y-211 Particle Size and Its Dispersion

Y-211-	Y-211 size (nm)	fwhm (°)	dispersion	surface-damage	sharp-edges
0	3000–6000	0.146	agglomerated	no	no
15	700–1000	0.151	agglomerated	no	no
30	∼700	0.159	in-homogeneous	slightly	in-homogeneous
45	<500	0.163	homogeneous	moderate	homogeneous
60	100–500	0.169	homogeneous	more	homogeneous and large fraction

The as-processed bulk sample photographs of Y–U-0 to Y–U-60 and Y–Pt–CeO2 are shown in Figure a,f respectively. The fourfold facets for the samples clearly indicate that they grew in single-grain nature.

Figure 5

Top surface photographs of the TS-IG-processed bulk YBCO samples: (a) Y–U-0, (b) Y–U-15, (c) Y–U-30, (d) Y–U-45, (e) Y–U-60, and (f) Y–Pt–CeO2.

The crystal structure of all bulk YBCO samples produced by the pretreated Y-211 and Pt–CeO2 was analyzed from the XRD patterns and is shown in Figure . It can be seen that all the Bragg peaks of samples are highly oriented in the (00l) direction, which indicates that the bulks are grown in c-axis orientation. Strong (00l) peaks along with no other (hkl) peaks suggest a high degree of texturing, indicating that all bulks are entirely a single grain. The prominent reflections show that all the XRD patterns can be well indexed to a YBCO superconducting phase with an orthorhombic unit cell. Some of the additional Bragg lines found in the Y–U-0 sample represent Y-211 as the minor phase and are marked with the symbol *. No other phases such as Ba–Cu–O were observed in any of the samples.

Figure 6

X-ray diffractograms recorded for various YBCO bulk samples using Cu Kα radiation.

Critical Temperature

The in-phase component (real part) of DC magnetic susceptibility (χ) with a function of temperature for all oxygenated YBCO samples produced by the ultrasonically pretreated Y-211 and Pt–CeO2 is depicted in Figure . A small magnetic field of 1 mT was applied parallel to the c-axis. To clearly see the onset transition of each sample, the curves are magnified in the inset of Figure . The onset of the superconducting transition for Y–U-0, Y–U-15, Y–U-30, Y–U-45, Y–U-60, and Y–Pt–CeO2 samples occurs at 90.8, 90.8, 90.2, 91, 90.3, and 90.5 K, respectively. This clearly evidences that all the samples are exhibiting superconducting nature. The superconducting transitions in all samples are very sharp, and their transition widths “ΔTc” of ≤1.1 K indicate the high-quality nature of the samples without any secondary phases. This result supports the observation of pure phases without any low-Tc phases in the XRD studies, as shown in Figure . A minor amount of distribution in Tc mainly at the tail of the curves represents the presence of small amounts of low-Tc phases of oxygen-deficient phases in the samples which may aid flux pinning. The values of the Tc and ΔTc are given in Table .

Figure 7

Variation of the temperature dependence of real part of χ for all YBCO bulks samples.

Table 2

Effect of Ultrasonic Irradiation on the Y-211 Particle Size and the Superconducting Properties

Y–U-	0	15	30	45	60	Pt–CeO2
average 211 size (μm)	4	3	1.5	0.9	∼0.9	1.8
Vf211 (%)	41	39	34	32	21	34
onset Tc (K)	90.8	90.8	90.2	91	90.3	90.5
delta Tc (K)	0.3	0.4	0.5	0.3	0.8	1.1
Jc(0) (kA/cm2)	45.1	57.4	68.1	80.8	68.5	62.5
Jc at 2 T (kA/cm2)	13.6	14.1	21.3	40.4	11.4	23.5
TFsurface	0.25	0.28	0.30	0.31	0.48	0.33
TF1mm	0.21	0.23	0.24	0.24	0.41	0.27

Microstructural Properties of Bulk YBCO Samples

The fundamental criterion in achieving high Jc in REBCO superconducting materials is engineering of the bulk matrix with fine-sized non-superconducting inclusions. It was predicted that the effective surface area of these particles is proportional to the flux pinning of the bulk samples.[26] Among, non-superconducting and non-interacting inclusions such as the 211 phase, particles are considered as effective pinning centers. The defect density due to the left back 211/123 interfaces will be of coherence length size and effectively pins the fluxons and aids in improving the Jc performance of the single-grain REBCO bulk products.[27−30]

However, even in the IG process, great care needs to be taken in dispersing the non-superconducting particles homogeneously throughout the bulk matrix. The incorporation of fine-sized Y-211 into the bulk samples depends greatly on the initial Y-211 conditions. If two differently sized Y-211 phase particles are employed, it is well known that the smaller-sized particles participate in the peritectic reactions as the surface energy is large and form a 123 matrix. The FE-SEM images of Y–U-0, Y–U-15, Y–U-30, Y–U-45, Y–U-60, and Y–Pt–CeO2 are displayed in Figure a–f, respectively. One important observation that can be made immediately from the microstructural features of all samples is that the Y-211 particle boundaries are spherical, even though the initial particles have sharp edges and surface damages. Indeed, this clearly demonstrates that during peritectic reaction, the sharp-edged particles were consumed and the Y-211 particles become spherical of nanometer-size and homogenously distributed throughout the matrix.

Figure 8

FE-SEM monographs of Y–U-0, Y–U-15, Y–U-30, Y–U-45, Y–U-60, and Y–Pt–CeO2 samples are shown in (a–f), respectively. The Y-211 grain sizes are gradually reducing, and the Y–U-45 sample encompassed a large fraction of nanometer Y-211 phase inclusions dispersed homogeneously.

Even though we added differently sized Y-211 particles to the samples Y–U-15, Y–U-30, Y–U-45, and Y–U-60, we did not observe any traces of solidified liquid phases in the samples as evinced from the XRD and Tc analysis. Therefore, it can be stated here that the sharp-edged and surface-damaged Y-211 particles were completely utilized together with the supplied LPs for the peritectic growth of the Y-123 matrix.

The Y-211 particles which are of different sizes are employed for the IG process to enhance the superconducting properties of the final YBCO bulks. In our previous studies, it was observed that employing two differently sized Y-211 (micron and nanometer) particles in the IG process is not suitable.[31−33] This is mainly due to the fact that the nanometer-sized particles of spherical morphology were observed to close the open gaps (porosity) available between the Y-211 particles in the preformed pellets. The amount of open porosity in the preformed pellets is crucial in infiltration of the liquid phases which will be further utilized during the peritectic reaction. If the open porosity is abundant, a large amount of liquid will infiltrate; thus, the preformed pellet will collapse and an inhomogeneous reaction will occur with left back non-superconducting Ba–Cu–O-rich phases in the final samples.[34] These unwanted phases will be detrimental to the superconducting properties of the samples. If, in contrast, the open porosity is too less, then a sufficient amount of liquids cannot be homogeneously supplied to the preformed pellets. Therefore, the open porosity available in the preformed pellets is determined by the applied pressure, and the preformed particle size is crucial in the growth of the bulk REBCO superconductors.

Another important and different observation made from our previous work of adding nanometer-sized inclusions in Y-211 for IG processing is the absence of the enlargement of Y-211 particles in the final samples. In all our previous studies, we added different nanometer-sized and spherical-shaped RE sources in the preformed Y-211. The spherical inclusions were observed to sinter and fuse the gaps between the Y-211 particles in the preformed pellets and limited the infiltration of the liquid phases. In the present case, even though we added nanometer- and micron-sized Y-211 phases in the preformed pellets because of sharp-edged particles sufficient amount of liquid phases were infiltrated for effective peritectic reaction. Another factor which caused enlargement of the 211 phase particles in the mixed YBCO/REBCO bulk superconductors is the growth of mixed RE-211 during the peritectic reaction, which was grown up to several micron sizes. As discussed in our recent work of mixed REBCO bulk superconductors, these larger-sized mixed RE-211 particles will be consumed during the peritectic reaction and form mixed RE-123 unit cells in the final samples.[19] However, this case is not valid in the present case of pure YBCO samples where no other RE elements are involved. Therefore, the sharp-edged particles were consumed for continuous growth of the matrix leaving very fine-sized Y-211 particles within the bulk samples.

The histograms presented in Figure a–f give the distribution of the Y-211 particle size of samples Y–U-0, Y–U-15, Y–U-30, Y–U-45, Y–U-60, and Y–Pt–CeO2, respectively. The average sizes and volume fractions (Vf211) of the left back Y-211 particles within the Y-123 matrix for samples Y–U-0, Y–U-15, Y–U-30, Y–U-45, Y–U-60, and Y–Pt–CeO2 are estimated to be 4 μm (41%), 3 μm (39%), 1.6 μm (34%), 0.9 μm (32%), ∼0.9 μm (21%), and 1.8 μm (34%), respectively. The systematic difference in Vf211 and sizes of Y-211 particles within the different samples clearly shows the difference between the pretreated ones and the ones without any ultrasonication. Obviously, the Y–U-0 sample contains coarser Y-211 particles because of the larger-sized and agglomerated precursors. Because of pretreating by increasing the irradiation time of ultrasonication, the 211 sizes are becoming finer. This implies that the sharp-edged and surface-damaged Y-211 particles have participated and were consumed effectively in the peritectic reaction.

Figure 9

Y–U-0, Y–U-15, Y–U-30, Y–U-45, Y–U-60, and Y–Pt–CeO2 sample histograms of the 211 size distribution are presented in (a)-(f), respectively. As Δt increased, the peak position of the average size of the Y-211 particles is shifting toward lower values.

The Y–U-45 sample exhibits a higher density of uniformly dispersed and fine-sized 211 particles within the bulk 123 matrix. The Y-211 particles left back in Y–U-45 are found to be finer, and their average is centered on 0.9 μm, whereas those in Y–U-0 are larger, falling in the 4 μm range. The size distribution of Y-211 precipitates in the case of Y–U-0 is observed to be much wider in the range 1–8 μm. As the irradiation time increased, the average size of the Y-211 particles is decreased with increasing density of submicron-sized Y-211 particles. The density of smaller-sized particles ≤200 nm is observed to be more in sample Y–U-45 as compared to other samples, thus indicating the optimal condition. The Y-211 content is largely decreased in the case of the Y–U-60 bulk sample because of the increased Y-211 particle dissolving rate in the melt at the growth front. The preformed Y-211 particles were finer and sharp (see Figure c) in the case of sample Y–U-60, which was mostly utilized in forming the 123 matrix. Therefore, irradiating an ultrasonic energy of 300 W for 45 min was observed to be optimum to achieve high homogeneity in dispersion of fine-sized Y-211 particles. The decrease in the size of the flux pinning inclusions for maintaining the same optimal volume fraction of Y-211 will increase the flux pinning strength. The systematic decrease of the Y-211 particle sizes and content in the final bulks shows the efficiency of the proposed method in tuning the microstructural properties without any addition of chemical dopants. As argued from the microstructural features of pretreated Y-211,[25] the reason for fine-sized left back Y-211 particles in Y–U-45 is attributed to present a sufficient amount of sharp edges with surface-damaged Y-211 precursor powders.

Superconducting Properties

Critical Current Density

To evaluate the effect of irradiation of ultrasonic energy for different time intervals on the field dependence of Jc [i.e., Jc(H)] performance at 77 K, M–H loops were recorded up to 5 T field applied normal to the c-axis of each bulk specimen. The Jc values of all bulk samples were determined using the extended Bean critical state model.[35,36]Figure shows the field dependence of Jc for all the bulk, YBCO single-grain samples.

Figure 10

Field dependence of Jc curves of all YBCO samples determined at 77 K.

The zero field Jc [i.e., Jc(0)] for Y–U-0 is found to be 45.1 kA/cm2. As the irradiation time of ultrasonic waves is increased, the Jc values are increased. The Y–U-45 sample is exhibiting superior Jc performance with 80.8 kA/cm2 at the self-field. The details of Jc(0) and Jc at 2 T field for all samples are given in Table . If the Jc performance is compared with Y–Pt–CeO2 (62.5 kA/cm2 at 77 K and self-field) also, the Y–U-45 sample is superior, indicating the effectiveness of the present method. The analysis of the Jc(H) curves at a lower field may shed light on the effect of different grain refiners on the strength of the pinning performances.[37,38] Therefore, having a larger number of non-superconducting particles with fine size is suitable for supporting larger currents. Among all samples studied here, the density of fine-sized Y-211 particles (≤200 nm) was higher in sample Y–U-45 and supported larger currents as compared to the other samples. The magnitude of the Jc in the REBCO is determined by the ability of the microstructure which pins the magnetic flux. This result gets support from the fine-sized 211 phase particles as observed in the superior microstructure of the Y–U-45 sample.

Trapped Field

The oxygenated bulk samples are field-cooled by a permanent magnet for 15 min. The 3-D TF profiles recorded at 1 mm distance (sample surface—Hall probe) for Y–U-0, Y–U-15, Y–U-30, Y–U-45, Y–U-60, and Y–Pt–CeO2 single-grain samples are shown in Figure a–f, respectively. The single conical curves without any distortion for all the samples indicate that all the samples are grown as single grains and the complete bulk is oxygenated. The observed TF values at 0.3 mm (TFsurface) and 1 mm (TF1mm) for all the samples are given in Table . It is obvious from Table that the TF performance is gradually improved with irradiation of ultrasound energy duration, and a maximum TFsurface value of 0.48 T was recorded for the sample Y–U-60. This value is nearly two times that of sample Y–U-0. The gradual increase in the TF performance can be attributed to the improved pinning performance because of the employment of fine-sized 211 phase particles.

Figure 11

TF profiles of Y–U-0, Y–U-15, Y–U-30, Y–U-45, Y–U-60, and Y–Pt–CeO2 single grains containing different Y-211 particle sizes are shown in panels (a–f), respectively. All TF data are recorded at 77 K by cooling in the field of a permanent magnet (surface field of 0.5 T).

The Jc performance of sample Y–U-45 is superior compared to sample Y–U-60. As demonstrated from microstructural features of both samples, it relates to the density and the size of the Y-211 particles left back in the samples. The density of nanometer-sized Y-211 particles (≤200–500 nm) is high in the case of the Y–U-45 sample, and the dispersion of the Y-211 grains is also uniform which supports the higher magnitude of Jc values (Figure ). From microstructural observations, it is evident that even though the Y-211 particle sizes are smaller for Y–U-60, the lower Vf211 limits the Jc performance. However, here, we point out that the TF reflects the global Jc of the complete bulk sample. However, the Jc determined on a small subspecimen with a limited microstructure is sensitive to many parameters such as defects or fluctuations. Therefore, the Jc of Y–U-60 might be inferior to that of Y–U-45. To make a conclusive statement, however, we need a detailed growth and microstructure-related investigation which we will be carried out subsequently. The present results may be even improved by employing higher ultrasonic powers, which will also lead to reduction of the time interval, thus efficiently producing the sharp-edged 211 preformed particles and without chemically doping.

Figure 12

Jc (0 T) and TFsurface values at 77 K are compared as a function of ultrasonic irradiation time.

In order to further validate the results obtained from sample Y–U-60, we synthesized a new single-grain YBCO bulk sample (Y–U-60-Rpt) with 60 min ultrasonicated Y-211 powders by the TS-IG method. The microstructures of Y–U-45, Y–U-60, and Y–U-60-Rpt samples are displayed in Figure a–c, respectively. The comparison of the microstructures of samples Y–U-60 and Y–U-60-Rpt clearly depicts that both the samples show similar features with the low 211 content and dispersion. The microstructural features indicating that a large amount of fine-sized and sharp-edged Y-211 powders produced by ultrasonication for 60 min were utilized for fabricating the bulk single-grain YBCO sample. Because of this, the field dependence Jc curves determined at 77 K of Y–U-60 and Y–U-60-Rpt, as shown in Figure , exhibit a similar trend. The TF at 77 K for Y–U-60-Rpt was measured to be 0.38 and 0.45 T at the 1 and 0.3 mm distance from the sample surface, respectively, whereas these values for the Y–U-60 sample are 0.41 and 0.48 T, respectively. The 3-D TF profiles of Y–U-60 and Y–U-60-Rpt are given in (a,b), respectively, of Figure as insets. These results clearly imply that the produced samples have similar properties which strongly support the novel approach as the samples could be successfully reproduced. Also, this further supports the different reactivity of the ultrasonicated Y-211 in the bulk REBCO growth, which needs to be studied further.

Figure 13

Microstructures of (a) Y–U-45, (b) Y–U-60, and (c) Y–U-60-Rpt. The YBCO samples fabricated by the Y-211 particles pretreated by ultrasonication for 60 min are analogous.

Figure 14

Jc curves determined at 77 K for Y–U-60 and Y–U-60-Rpt superconductors fabricated via the IG process. The insets of (a,b) represent the 3-D TF profiles of Y–U-60 and Y–U-60-Rpt samples, respectively, recorded at 77 K and 1 mm distance from the sample surface. Both the Jc and TF measurements are nearly similar for both the samples.

Future Perspectives

As demonstrated, the proposed novel method is advantageous and superior to any of the available methods. More importantly, this method is general by which the sharp-edged rough surface 211 particles could be employed for any methods such as MG and IG. The effectivity of the present method can be further improved to develop the REBCO material as a candidate for various practical applications. The future perspectives for further investigation of the effect of ultrasonication on the REBCO superconductors are discussed below.

Precursor powders of different 123 and 211 phase powders synthesized via different methods such as solid state and chemical routes and test the ultrasonic effect. Utilizing the 123 and 211 phase particles with sharp-edged and damaged surfaces, synthesize the bulk samples via MG and IG processes. Ultrasonically pretreated 123/211 powders could be employed in fabricating other forms of superconducting materials such as thin films, tapes, thick films, cables, foams,[39] and so forth.

Testing of high ultrasonic powers (≥300 W) for quickly producing the pretreated various 211/123 phases for constructing high-performance REBCO bulk products.

Producing different sized 211/123 phases and mixing them as micron-sized particles, and studying the effects on the growth of the bulks and their final physical properties.

Using the pretreated initial 211 particles to study the growth and reaction in IG/MG processing of REBCO superconductors. This study may help in growing larger-sized bulk single grains in shorter durations.

Pretreat different REBCO phases and construct mixed REBCO bulk superconductors.

Batch processing of various YBCO/REBCO superconductors with ultrasonicated 211/123 phase particles.

Conclusions

We propose a novel, reliable, and cost-effective method, which controls and reduces the 211 particles size in precursor powder and then subsequently produces nanometer-sized 211 precipitates. The initial Y-211 particle size and morphology were manipulated employing high-energy ultrasonication for producing sharp-edged and surface-damaged Y-211 preformed powders. To demonstrate the effectivity of the method, bulk single-grain YBCO superconductors of high quality were fabricated by the TS-IG method and compared with the addition of grain refining agents of Pt–CeO2. As the time duration was increased, the Y-211 size was decreased and sharp-edged and different-shaped Y-211 particles were created. The effectiveness of the present method lies in the tuning of the initial Y-211 particle size, morphologies, and the grain boundary volume fraction in the final bulk YBCO samples. With increasing irradiation time, the 211 particle size and Vf211 in the final bulk samples were decreased. The TF performance of bulk single-grain YBCO samples exhibited a systematic improvement with the increase of irradiation time. The YBCO sample fabricated using Y-211 particles with ultrasonic irradiation for 60 min was able to trap a magnetic field of ∼0.48 T at 77 K. Jc (at 77 K) was increased monotonically for ultrasonic irradiation time of up to 45 min. A further increase of Δt led to a decrease of Jc because of lower interfacial defect density. The method presented will enable even in MG-processed composites to homogeneously disperse the 211 secondary phase particles in the bulk REBCO body.

Experimental Details

Precursor powders of the Y-211 and liquid phase (LP = Er-123 and Ba3Cu5O8) were synthesized employing highly pure Y2O3, BaO2, and CuO raw materials via the solid-state sintering method. To assure single-phase formation of Y-211 and Er-123, the sintering process was carried out at 860, 880, and 900 °C for 4 h at each temperature. After each sintering, the powder was mechanically ground for 2 h for mixing the compounds employing an auto mortar grinder (Nitto Kagaku, model ANM-1000). For reducing the size and damaging the surface of the Y-211 particles, we employed a high-energy ultrasonic processor (MITSUI, model UX-300). This ultrasonic processor allows operation in two modes: (i) continuous and (ii) pulse. We employed both modes to compare the effect on reducing the size of the Y-211 particles. We observed that the Y-211 particles were effectively reducing their size operating the “pulse” mode. For this reason, we experimented by changing the energy of the ultrasonic probe from 100 to 300 W. It was observed that applying the ultrasound radiation with 300 W was effective. Then, to further optimize and control the Y-211 particle size and morphology, the irradiation time intervals (Δt) of 15, 30, 45, and 60 min were employed. Ethanol was used as an aqueous medium for all present ultrasonication experiments. Ultrasonically treated Y-211 powders were heat-treated at 400–600 °C for 12 h to eliminate the carbon-related compounds in the Y-211 powders. Utilizing the 15, 30, 45, and 60 min ultrasonically treated Y-211 precursor powders, we successfully grew YBCO bulk single grains via the TS-IG method. The Y-211 powders pretreated with 0, 15, 30, 45, and 60 min of ultrasonication are referred to as Y-211-0, Y-211-15, Y-211-30, Y-211-45, and Y-211-60, respectively.

The preformed pellets of Y-211 and LP sources of 20 mm diameter were made by applying a uniaxial pressure of 420 MPa.[19] A box furnace calibrated over a wide range of temperatures was employed for the YBCO bulk sample growth. The temperature profile recorded over the calibrated temperature range indicated a uniform thermal gradient. To fabricate a well-defined orientation of the single-grain YBCO bulk superconductors, a single crystalline NdBCO bulk was used as a seed. The time–temperature schedule followed for fabricating the YBCO bulks was designed as follows: The sample assembly was heated to 820 °C in 5 h and for improving the strength of the Y-211, a 1 h dwell was allowed. Then, the temperature was increased to 1060 °C (T) with a heating rate of 4 °C/min, and for homogeneous infiltration of liquid phases, a 1 h dwell was given. The temperature from Ti was cooled to 1005 °C (peritectic temperature “Tp”) in 30 min. To initiate the crystal growth and for growing the single grain of YBCO, a slow cooling of 100 h (0.25 °C/h) was facilitated from the Tp to 980 °C. Then, the furnace was allowed to cool naturally.

The YBCO bulk single grains fabricated with different time intervals (Δt) of the Y-211 ultrasound treatment of 0, 15, 30, 45, and 60 min are referred to as Y–U-0, Y–U-15, Y–U-30, Y–U-45, and Y–U-60, respectively. To corroborate the developed ultrasonic method, one more YBCO bulk single grain was fabricated with the addition of 0.25 wt % of Pt and 0.25 wt % of CeO2 and is referred to as Y–Pt–CeO2. Finally, all the samples were oxygenated at 450–400 °C for 250 h with a constant oxygen flow of 0.3 L/min and an additional 100 h for M–H measurements. It is also interesting to mention here that recently, Takanori Motoki and Jun-ichi Shimoyama group have proposed a novel method for reducing the prolonged oxygenation annealing process employing the oxygen-containing water vapor.[40] The YBCO bulks exhibit the typical spatial inhomogeneities and hence, their physical properties vary with position in the sample.[41,42] Keeping this in mind, in order to overcome the spatial dependence, the specimen (∼0.5 × 1.5 × 2 mm3) intended for the magnetic measurements from each sample was collected ∼2 mm below the seed.

Investigation of the microstructural properties was carried out by FE-SEM, JEOL model JSM-7100F. The structural properties of the pretreated Y-211 particles and of the final bulk YBCO samples were examined by the XRD technique. TF measurements were performed in liquid nitrogen (77 K) with a Hall probe and field cooling in a 0.5 T field provided by a permanent magnet. The superconducting transition temperature and field-dependence critical current densities of the samples were characterized with the superconducting quantum interference device (SQUID, Quantum design, model MPMS-XL5). Characterization details of various physical properties of the samples can be found in our previous studies.[43−45]