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Article

The Effect of the Pre-Infiltration Temperature on the Liquid-Phase Infiltration Characteristics and the Magnetic Properties of Single-Domain GdBCO Bulk Superconductors

by
Tingting Wu
1,2,
Wanmin Yang
2,* and
Li Chen
2
1
Basic Teaching Department, Shanxi Vocational University of Engineering Science and Technology, Tai’yuan 030013, China
2
College of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, China
*
Author to whom correspondence should be addressed.
Crystals 2024, 14(10), 842; https://doi.org/10.3390/cryst14100842
Submission received: 10 September 2024 / Revised: 23 September 2024 / Accepted: 24 September 2024 / Published: 27 September 2024
(This article belongs to the Section Inorganic Crystalline Materials)
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Abstract

:
In this study, the effect of the pre-infiltration temperature (Ti) on the liquid-phase infiltration characteristics and its effect on the magnetic properties of single-domain GdBCO bulks prepared by the top-seeded infiltration growth (TSIG) method are comprehensively investigated. The results reveal that (1) the liquid Ba-Cu-O phase (LP) did not uniformly infiltrate into the Gd2BaCuO5 (Gd-211) solid-phase pellet (SPP). (2) The initial melting and infiltration of the LP occurred at approximately 920 °C. The average infiltration depth and amount of LP that infiltrated the SPP increased with increasing Ti accompanied by the shrinkage and depletion of the LP pellet. (3) The LP penetrated up to the top surface of the SPP and uniformly infiltrated throughout the SPP when the Ti was approximately 960 °C and 1020 °C, respectively. (4) The mean Gd-211 particle size of the pre-infiltrated SPP increased from 1.94 µm at 920 °C to 2.52 µm at 1040 °C as the Ti rose. (5) The magnetic properties and microstructure of the single-domain GdBCO bulks were significantly influenced by the microstructure of the pre-infiltrated SPP. The largest levitation force of 35.64 N and trapped field of 0.23 T were obtained in the single-domain GdBCO bulks at an infiltration Ti of 960 °C.

1. Introduction

High-temperature REBCO superconductors [RE represents an abbreviation for a rare earth element, including Er, Y, Eu, Sm, Gd, and so on] are widely utilized in various applications, such as magnetic levitation bearings, maglev transportation systems, energy storage flywheels, superconducting generators, quasi-permanent magnets, and superconducting motors [1,2,3,4]. This is primarily owing to their higher critical temperature, excellent ability to trap magnetic flux, self-stabilizing levitation, and stronger current carrying capacity. Recent studies have reported the remarkable superconducting performance of REBCO bulk superconductors. A magnetic field larger than 14 T of a Ag-doped YBCO bulk superconductor pair fixed with a stainless steel bandage at 22.5 K was reported by Fuchs [5]. A remarkable magnetic field of 14.3 T was achieved by Namburi, which was measured in a YBCO bulk superconductor stack configuration under 26 K [6]. Tomita reported that a YBCO bulk superconductor with a 26 mm diameter after post-fabrication treatment trapped a 17.24 T magnetic field under 29 K [7]. In two stacked Ag-doped GdBCO bulks, a magnetic field of up 17.6 T was trapped at a temperature of 26 K by Durrell [8]. These excellent performances are reportedly based on high-quality REBCO bulk superconductors.
For the fabrication of REBCO bulk superconductors, top-seeded infiltration growth (TSIG) is an important technique [9,10,11,12,13,14]. During this process, the Ba3Cu5O8 liquid phase (LP) is first melted, which then permeates into the solid-phase pellet RE2BaCuO5 (RE-211) at an elevated temperature. Subsequently, the RE-211 phase reacts with the Ba3Cu5O8 liquid phase to generate the REBa2Cu3O7-δ (RE-123) phase during a slow cooling process, as indicated by the reaction equation RE-211 + Ba3Cu5O8→RE-123 [14,15,16].
However, REBCO bulk superconductors prepared via the conventional TSIG method have poor magnetic properties. For instance, Umakoshi et al. prepared a YBCO bulk superconductor (20 mm diameter) using the TSIG technique and achieved a 0.129 T trapped field at a temperature of 77 K [17]. Wang et al. observed a magnetic field of 0.195 T in a GdBCO bulk superconductor, which was fabricated using the TSIG method and had a diameter of 20 mm [18]. In a 20 mm YBCO bulk fabricated by TSIG, a magnetic field of 0.12 T was trapped by Miryala [19]. Namburi et al. found that the trapped field of a YBCO sample with a 25 mm diameter fabricated by TSIG was about 0.25 T, which exhibited a notable decrease compared to samples of the same size prepared through the top-seeded melt growth method (TSMG) [20].
Considerable research has been conducted to elucidate the cause behind the poor magnetic properties in REBCO samples prepared by TSIG. More specifically, a detailed analysis encompassing morphology, microscopic structure, and superconductivity characteristics was conducted on REBCO crystals [21,22,23]. It was concluded that the excessive RE-211 content and the poor growth in the final REBCO samples caused by insufficient LP infiltration may be the primary reasons for the poor magnetic properties. Therefore, it is necessary to optimize the LP in the RE-211 precursor pellet during heating to enhance the magnetic properties of the samples. Agarwal et al. found that different LP components, such as YBa2Cu3O7-δ + Ba3Cu5O8 and ErBa2Cu3O7-δ + Ba3Cu5O8, have significant effects on the superconducting and microstructure of ternary (Sm, Eu, Gd)Ba2Cu3O7-y superconductors [21]. Kamarudin et al. found that mixing ErBa2Cu3O7-δ into a Ba3Cu5O8 LP facilitated the infiltration of a Ba3Cu5O8 LP into the Y-211 solid pellet [24]. Naik et al. investigated the influence of the mass of a Ba3Cu5O8 LP on the growth, microstructure, and superconductivity characteristics of a (Gd, Dy) BCO superconductor, and they found that the mass ratio of 1:1.4 between (Gd, Dy)-211 and Ba3Cu5O8 LP was optimum for infiltration growth technology [25]. It has been concluded that an excessive RE-211 content and poor growth in the final REBCO samples caused by insufficient LP infiltration may be the primary reasons for the poor magnetic properties. Therefore, it is necessary to optimize the amount of LP infiltration into the RE-211 precursor pellet to enhance the magnetic properties of the samples.
Existing studies have primarily attempted to enhance the magnetic properties of REBCO superconductors by adding a new element to the liquid phase to assist the Ba-Cu-O LP in infiltrating RE solid pellets or by increasing the mass of the LP pellet to improve the LP infiltration [25,26,27,28,29]. But there are no systematic studies on the liquid-phase infiltration process, such as the melting temperature of the LP, the infiltration temperature of the LP into the solid pellets, how the infiltration of the liquid phase changes with an elevated temperature, how the RE-211 particles in the solid pellet change during the liquid-phase infiltration process, etc.
In this study, the infiltration process of the Ba-Cu-O liquid phase into the Gd2Ba1Cu1O5 (Gd-211) pellet from the liquid-phase pellet was observed in the macro-morphology and micro-morphology of samples, and the effect of the pre-infiltration temperature (Ti) on the magnetic properties of single-domain GdBCO bulk superconductors was investigated. This study reveals the infiltration mechanism of the liquid phase in the infiltration growth (IG) technique for the first time, which is crucial for the preparation of high-performance REBCO bulks.

2. Experimental Section

2.1. Preparation of Precursor Powders

In this study, the infiltration growth (IG) technique was employed to prepare all samples. Gd2BaCuO5 (Gd-211) powder, a solid-phase precursor powder used in this work, was synthesized through the solid-state reaction method using Gd2O3 powder (99.0%), BaCO3 powder (99.0%), and CuO powder (99.0%) with a molar ratio of 1:1:1, and pure Gd-211 precursor powder was obtained by sintering at 920 °C three times. The liquid-phase (LP) source was obtained by mixing Y2O3, BaCuO2, and CuO; the corresponding molar ratio was 1:10:6. BaCuO2 powder for the LP source was obtained by the solid-state reaction method with a 1:1 molar ratio of BaCO3 and CuO.

2.2. Assembly of Precursor Pellets

Cylindrical pellets with a diameter of 20 mm were pressed using Gd-211 powder (15 g), liquid source powder (20 g), and Yb2O3 powder (5 g). These cylindrical pellets were labelled as the Gd-211-phase precursor pellet (SPP), the liquid-phase precursor pellet (LPP), and the Yb support pellet, respectively. Specifically, the SPP is green, while the LPP is black. The same diameter was chosen for both the SPP and LPP to observe appearance changes more intuitively during heat treatment. The sample assembly method is shown in Figure 1. The SPP, LPP, and Yb support pellets were arranged in a vertical stack along their central axis on MgO single crystals and finally positioned onto the Al2O3 plate. It should be noted that a seed crystal was not used at this stage.

2.3. TG-DCS Analysis of Liquid-Phase Powder

To determine the appropriate temperature window for LP pre-infiltration, the thermal behavior of the liquid source powder (Y2O3 + 10BaCuO2 + 6CuO) was analyzed via thermogravimetric and differential scanning calorimetry technology (TG-DSC). The result is illustrated in Figure 2. The analysis was conducted under an air atmosphere in the temperature range of 0 °C to 1200 °C, which corresponds to the environmental conditions required for preparing GdBCO bulk superconductors using the IG method. And the liquid source powder was heated up at a speed of 30 degrees Celsius per hour in a corundum crucible, with the corundum serving as the standard sample.
According to Figure 2, it can be observed that the TG–temperature curve of the liquid source powder did not change until 900 °C. Therefore, the initial temperature for this study was set at 900 °C. Furthermore, two endothermal peaks can be observed on the DSC–temperature curve due to the existence of a minute quantity of Y2O3 in the LP source powder (Y2O3 + 10BaCuO2 + 6CuO). The Y2O3 in the liquid source powder enhances the stability of the LP pellet during the heating procedure. The first peak of the DSC–temperature curve corresponds to the growth of YBa2Cu3O7-δ (960 °C), and the second peak corresponds to the decomposition of YBa2Cu3O7-δ (1015 °C).

2.4. Pre-Infiltration Process

The sample was positioned within a cylindrical furnace that featured a reasonable temperature gradient. The axial temperature gradient within the furnace measures approximately 1 °C per centimeter. The samples were heated up to different pre-infiltration temperatures (Ti) at a speed of 30 degrees Celsius per hour and then held at those temperatures for a duration of 2 h. Subsequently, the samples were cooled to room temperature (RT). The Ti values were 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C, 1020 °C, and 1040 °C, selected according to the TG-DSC result of the LP precursor powder and the heat treatment of fabricating a single-domain GdBCO crystal [30]. As a comparison sample, a sample without a pre-infiltration process was prepared, which is represented by N-P-I. The temperature profile is illustrated in Figure 3. In this process, a slower heating rate (30 °C/h) was used to reduce the gas released in the heating procedure and thus decrease the deformation of the sample.

2.5. Preparation of Metallographic Samples

To observe the cross-sectional morphology, the sample was first solidified by epoxy resin and then cut along the central axis, and a clear cross-sectional appearance was obtained after polishing. A microstructural image of the central region A (located 5 mm from the top) in the Gd-211 SPP was captured by scanning electron micrograph (SEM), as displayed in Figure 3. A software named Image J (Pro-Plus5.0) was used to measure the mean Gd-211 particle size.

2.6. The Measurement of Magnetic Properties

The magnetic properties of single-domain GdBCO crystals were measured using a 3D measurement system designed by Yang [31]. The levitation force was measured at 77 K, with the sample in a zero-field-cooled (ZFC) state. For this measurement, the NdFeB permanent magnet approaches the sample directly from a distance of 35 mm at a rate of 0.4 mm/s until reaching a proximity of 0.5 mm and then moves away from the sample at the same rate. The diameter of the NdFeB permanent magnet is 20 mm, and the surface magnetic field is 0.5 T.
The measurement on a trapped field was carried out at a temperature of 77 K, with the sample being in a state of cooling under the magnetic field (FC). The magnetic field was provided by the electromagnet, and the magnetic field strength was about (Bapp) 0.5 T. During the test, a Hall probe was utilized at a height of 0.5 mm above the sample surface.

3. Discussion

3.1. Effect of Pre-Infiltration Temperature on Liquid-Phase Infiltration Characteristics

3.1.1. Surface Morphology of Pre-Infiltration Samples

Figure 4 illustrates that the surface morphologies of pre-infiltration samples are different at different infiltration temperatures (Ti), including the pre-infiltrated SPP and LPP. Figure 5 presents the average diameter and shrinkage of SPP and LPP pre-infiltrated at different temperatures (Ti). And the average depth and amount of liquid phase (LP) infiltrating into the SPP increases as the Ti increases, which is accompanied by the shrinkage and depletion of the LPP. This phenomenon reflects the fact that the pre-infiltration temperature is the most crucial factor affecting the infiltration of the LP into the SPP from the LPP. For instance, when the Ti = 900 °C, the LPP diameter shrank by 5.5%; however, the SPP remained in its original morphology, and there was no adhesion between the SPP and the LPP. This indicates that the shrinkage of the LPP is not caused by LP infiltration; instead, it may be attributed to the decrease in porosity and the increase in density of the LPP during the heating process. The LP did not melt at this temperature, which is consistent with the TG-DSC results of the Y-liquid precursor powder. When the Ti was elevated to 920 °C, the morphology of the SPP was still green; however, the LPP stuck to the SPP, and its diameter was decreased by 12.3%. In other words, the LP melted at 920 °C and began to infiltrate into the SPP. When the Ti was elevated to 960 °C, the bottom region of the side surface of the SPP turned from green to black at approximately 2.6 mm in depth, and the rest of the surface region became dark, including the top surface region (see Figure 4d,i). This result suggests that a large amount of the LP infiltrated the bottom of the sample, and a small amount of the LP infiltrated the top surface of the SPP. This phenomenon reveals that LP infiltration from the LPP to the SPP is not uniform. Unlike the above samples, the samples pre-infiltrated at 1000 °C and higher temperatures exhibited signs of deformation, most likely caused by the shrinkage and melting of the LPP during the heat treatment process. For the SPP infiltrated at 1000 °C, its side surface completely turned black from green; however, it remained green on the top surface, and the infiltration was worse than that of the sample at 980 °C (see Figure 4e,j). This can be attributed to the serious deformation in the SPP-LPP contact region, which prevented the LP from penetrating up to the top surface of the SPP. When the Ti was increased to 1020 °C and 1040 °C, the surface of the SPP turned black, indicating that the entire SPP was infiltrated by the LP. Moreover, the average diameters of the SPP and the LPP at 1020 °C and 1040 °C, respectively, are nearly identical, suggesting that the entire SPP was fully penetrated by the LP at 1020 °C. In addition, the change in the pre-infiltrated SPP diameter is extremely small, less than 0.5%, which means that the pre-infiltration process has minimal influence on the final sample size.

3.1.2. Cross-Section Morphology of Pre-Infiltrated Samples

Figure 6 illustrates the cross-section morphology of pre-infiltrated samples at different temperatures (Ti). It can be seen from the figure that the amount of LP in the SPP and the average infiltration depth increased with the increasing Ti, accompanied by the shrinkage and depletion of the LPP, which is consistent with surface morphology results. Moreover, the contact surfaces between the pre-infiltrated SPP and LPP are almost concave downward in the middle region and curved upward on both sides, which can be attributed to the infiltration of the LP.
Notably, the sample pre-infiltrated at Ti = 900 °C was not cut, primarily because the infiltration of the LP could not be observed in the SPP. As can be seen from Figure 6a, a small amount of black LP is distributed at the bottom of the cross-section of the SPP pre-infiltrated at 920 °C, which explains why no infiltration of the LP is observed on the surface (Figure 4b) but the SPP and LPP stick together. When the Ti was elevated to 960 °C, the whole cross-section of the pre-infiltrated SPP became dark, indicating that the LP penetrated up to the top surface of the SPP. It is evident from the cross-section morphology of the SPP at Ti = 1000 °C (Figure 6e) that the distribution of the LP looks worse than on the surface. A large amount of black LP can be observed in the lower half of the cross-section, which may be caused by the severe deformation in the contact region of the SPP-LPP and a large crack in the pre-infiltrated SPP, jointly preventing the LP from penetrating up to the top region of the SPP (see Figure 4f and Figure 6e). The cross-section of the samples pre-infiltrated at 1020 °C and 1040 °C illustrates the black LP distributed across the entire SPP, indicating that the LP infiltrated the entire SPP at these temperatures. The cross-section of the SPP pre-infiltrated at 1020 °C and 1040 °C exhibits the coexistence of the green region and the black region, and the distribution of the green and black regions in the SPP at different temperatures is significantly different. Moreover, the LP distribution of the outer surface and the interior profile in the pre-infiltrated SPP is different at the same temperature, which indicates that the infiltration of the liquid phase is not uniform.

3.1.3. Microstructure of Pre-Infiltrated SPP

Figure 7 illustrates the microstructure of central region A (as shown in Figure 3a) of the pre-infiltrated SPP with different Ti. The mean Gd-211 particle size in the pre-infiltrated SPP was calculated using Image J, and the result is illustrated in Figure 8 (black curve).
As observed in Figure 7a,b, the microstructure of the SPP that was not pre-infiltrated is similar to that of the pre-infiltrated SPP at 920 °C. This is because pure Gd-211 precursor powder was obtained by sintering three times at 920 °C. The Gd-211 particles in these samples have an irregular sharp edge morphology, and there are many nano-sized Gd-211 particles attached to the bigger Gd-211 particles. There is a noticeable reduction in the quantity of nano-sized Gd-211 particles observed within the microtopography of the SPP pre-infiltrated at 940 °C, and the bigger Gd-211 particles tend to be ellipsoidal or spherical rather than with irregular sharp edges. Importantly, the distribution of the LP is not observed in the microstructure of the SPP at 920 °C and 940 °C, although the infiltration of the LP can be observed at the bottom of the cross-section morphology. The mean Gd-211 particle size increased from 1.94 µm at 920 °C to 2.07 µm at 940 °C. When the Ti was elevated to 960 °C, the mean Gd-211 particle size increased to 2.17 µm, and nano-sized Gd-211 particles cannot be observed in this sample. Additionally, a small amount of LP distribution can be observed in the sample, suggesting that the LP infiltrated this region, which also indicates that the color darkening of the surface and cross-section morphology at this temperature is caused by the infiltration of the LP. When the Ti was elevated to 1000 °C, Figure 7e shows that the Gd-211 particles were mostly covered by the LP. There are two types of microstructures of the SPP pre-infiltrated at 1020 °C and 1040 °C. One type is the coexistence region of the Gd-211 phase and the LP, as illustrated in Figure 7(g-2,h-2), corresponding to the green region in the cross-sectional morphology. The other type is the Gd-123 phase, as shown in Figure 7(g-1,g-2), corresponding to the black region of the cross-section morphology. This phenomenon also indicates that the LP infiltrated the SPP at 1020 °C and 1040 °C. The different distribution of green and black regions may be related to different heat treatment processes caused by the Ti.
Figure 7 and Figure 8 show that the mean Gd-211 particle size in the pre-infiltrated SPP increased from 1.94 µm to 2.52 µm as the Ti increased from 920 °C to 1040 °C. This is because many nano-sized Gd-211 particles have irregular sharp edges in Gd-211 precursor powder (see Figure 7a). These Gd-211 particles are of greater surface activity and are easy to melt and decompose at higher temperatures, thus promoting the further growth of the larger Gd-211 particles. These results illustrate that the pre-infiltration temperature is not only a crucial factor affecting the infiltration of the LP into the SPP from the LPP but also significantly affects the Gd-211 particle size in the SPP.

3.2. Effect of Pre-Infiltration Temperature on Magnetic Properties of GdBCO Bulk Superconductor

3.2.1. Morphology of Single-Domain GdBCO Bulks

To investigate the impact of the LP pre-infiltration on the magnetic characteristics of single-domain GdBCO crystals, a series of GdBCO crystals with a 20 mm diameter were fabricated. These samples were pre-infiltrated at different values of Ti, according to the temperature profile displayed in Figure 9. Notably, a larger diameter LPP (30 mm) was used to prevent the deformation of the final sample [32,33,34]. And a well-textured NdBCO seed crystal was used in this process. Subsequently, the prepared GdBCO samples underwent a 200 h annealing process with a pure oxygen flow, where temperatures were varied from 430 °C down to 350 °C. After that, a set of single-domain GdBCO bulk superconductors with varying Ti were obtained and labeled as S1, S2, S3, S4, S5, S6, SR7, and S8, as shown in Figure 10. And S1, S2, S3, S4, S5, S6, S7, and S8 correspond to the sample that was not pre-infiltrated and pre-infiltrated samples at a Ti of 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C, 1020 °C, and 1040 °C, respectively. Figure 10 shows that all samples exhibit a classical single-domain GdBCO bulk superconductor morphology, i.e., four growth facets run from the seed crystal towards the sample circumference. Moreover, the diameters of all samples are in the range of 20 ± 0.1 mm, indicating that the pre-infiltration process of the LP does not affect the final sample morphology.

3.2.2. Magnetic Properties of GdBCO Bulks

Figure 11 illustrates the levitation force (F) between the single-domain GdBCO crystals with different Ti values and a permanent magnet. The surface magnetic field and diameter of the permanent magnet are 0.5 T and 20 mm, respectively. And the measurement was conducted under 77 K in a zero-field-cooled (ZFC) state. The inner illustration depicts the highest value of the levitation force of the samples with varying Ti values.
As indicated in Figure 11, the GdBCO bulk without a pre-infiltration process (S1) is of minimum levitation force, which is only 14.74 N. And the F of the GdBCO samples with a pre-infiltration process under any Ti is significantly larger than S1. Additionally, as the pre-infiltration temperature Ti increased from 920 °C to 960 °C, the maximum levitation force of the GdBCO samples showed an increasing trend. However, when the pre-infiltration temperature Ti continued to increase from 960 °C to 1040 °C, the maximum levitation force of the GdBCO samples showed a decreasing trend. And the GdBCO sample with a Ti of 960 °C exhibited the largest levitation force (35.64 N).
Figure 12 illustrates the distribution of the trapped field (Btr) in three-dimensional space of the single-domain GdBCO crystals with varying Ti values. And the measurement was conducted at a temperature of 77 K in a field-cooled (FC) state. The sample was magnetized in a 0.5 T magnetic field using an electromagnet (Bapp = 0.5 T). Figure 13a presents the radial trapped field distribution in single-domain GdBCO crystals under different Ti at 77 K. Additionally, Figure 13b presents the peak value of the trapped field of the single-domain GdBCO crystals with varying Ti values. Figure 12 shows that all samples are of a symmetrical single-peak profile, indicating that the GdBCO bulk superconductors with any Ti all have single-domain magnetic characteristics. The peak values of the Btr in all bulks with the pre-infiltration process at any Ti are all significantly larger than that of the sample that was not pre-infiltrated; the peak value of the Btr in the S1 sample is merely 0.08 T. The peak value of the Btr in the single-domain GdBCO crystals with varying Ti values exhibits a similar trend to the maximum levitation force, i.e., initially increasing and then decreasing as Ti increases. When Ti = 960 °C, the GdBCO samples have the highest Btr (0.23 T). These results suggest that the pre-infiltration of the liquid phase contributes to enhancing the magnetic properties of single-domain GdBCO crystals.

3.2.3. Microstructure of GdBCO Bulks

To clearly explain the impact of the Ti on the microtopography of GdBCO bulk, the microstructures of the a-b plane of all samples were observed with an SEM, as depicted in Figure 14. The test samples were taken from the central region of the GdBCO sample, which is located in the c growth sector of the GdBCO crystal, 5 mm away from the NdBCO seed. The mean Gd-211 particle size in the GdBCO bulk was calculated, as shown in Figure 8 (red curve).
Many studies show that the RE-211 particle distribution of the final REBCO crystal is closely dependent on the precursor SPP. A smaller RE-211 particle size in the precursor SPP results in a smaller RE-211 particle size in the final REBCO sample [35,36,37,38]. It can be seen from Figure 8 and Figure 14 that the mean Gd-211 particle size in the GdBCO bulks first decreases and then increases as the Ti increases. However, the mean Gd-211 particle size in the pre-infiltrated SPP increases as the Ti increases. And the micro-morphologies of GdBCO and the SPP were observed at the same position, that is, in the inner central region of the sample. Why are the results inconsistent with the reported conclusions? The reason for the outcome must be investigated.
To explain the relationship between the Gd-211 particles in the pre-infiltrated SPP and the final GdBCO bulk, the corresponding microstructures of the pre-infiltrated SPP and the GdBCO bulk with the same Ti were carefully observed. It was found that the mean Gd-211 particle size in the S1 sample (without a pre-infiltration process) is approximately 3.19 µm. A non-uniform distribution of Gd-211 particle size is observed, with larger Gd-211 particles measuring over 10 µm and smaller Gd-211 particles measuring less than 2 µm in the matrix. However, the corresponding SPP does not exhibit a similar size distribution of Gd-211 particles (see Figure 7a). The Gd-211 particles in the SPP are of irregular angular morphology, and many nano-sized Gd-211 particles are attached, which cannot be observed in the S1 sample. The existence of nano-sized Gd-211 particles in the Gd-SPP may account for the disparity in the microstructure between the SPP and the GdBCO bulk. The nano-sized Gd-211 particles have greater surface activity due to their small size and irregular shape and are very easy to melt and decompose in the subsequent heating process. Therefore, the further growth of the original bigger Gd-211 particles is promoted, resulting in the appearance of larger-sized Gd-211 particles in the final GdBCO sample. For the S2 sample, the mean Gd-211 particle size is approximately 2.95 μm, and the largest Gd-211 particle size is approximately 8.4 µm. The distribution of Gd-211 particle size in the S2 sample exhibits a slight improvement compared to that of the S1 sample, which may be related with the decomposition of nano-sized Gd-211 particles in the pre-infiltration procedure. For the microstructure of the SPP with Ti = 940 °C, the number of irregular nano-sized Gd-211 particles decreases sharply, but there still exists a small amount of irregular nano-sized Gd-211 particles. Therefore, bigger Gd-211 particles of about 4.38 µm were still observed in the S3 sample, and the mean Gd-211 particle size was about 2.84 µm. When the Ti is increased to 960 °C, nano-sized Gd-211 particles cannot be observed in the pre-infiltrated SPP; therefore, the distribution of the Gd-211 particles in sample S4 is relatively uniform, and the mean Gd-211 particle size is smaller, approximately 2.53 µm. When the Ti is increased from 980 °C to 1040 °C, the mean Gd-211 particle size in the GdBCO bulk exhibits an increasing trend, which is caused by the increasing mean of the Gd-211 particle size in the pre-infiltrated SPP.
The microstructure of GdBCO bulk superconductors produced by the TSIG process with the pre-infiltration process at different Ti is different, resulting in different magnetic properties of GdBCO bulk superconductors. For the S1 sample without pre-infiltration, approximately 10.2 µm larger Gd-211 particles exist in the matrix, resulting in the S1 sample having poor magnetic properties (15 N, 0.08 T). Moreover, the S4 sample with Ti = 960 °C has an optimal microstructure; therefore, the best magnetic properties are obtained in this sample (35.64 N, 0.23 T). These results further reveal that the microstructure and the magnetic properties of single-domain GdBCO bulks closely depend on the microstructure of the pre-infiltrated SPP.

4. Conclusions

In this work, the infiltration of the Ba-Cu-O liquid phase (LP) into the Gd-211 solid pellet (SPP) from the liquid-phase pellet (LPP) and the influence of pre-infiltration on the properties of single-domain GdBCO bulk were studied for the first time.
The results show that the LP did not uniformly infiltrate into the SPP during the infiltration process, which primarily depends on the microstructure of the SPP and the contact state between the SPP and the LPP. The contact surfaces of the SPP and the LPP of the samples are concave downward in the middle region and curved upward on both sides, which may be caused by the infiltration of the LP. The initial melting and infiltration process of the LP occurred at approximately 920 °C. The average infiltration depth and amount of LP that infiltrated the SPP increased with the increasing Ti, accompanied by the shrinkage and depletion of the LPP. The LP penetrated up to the top surface of the SPP and uniformly infiltrated throughout the SPP when the Ti was increased to approximately 960 °C and 1020 °C, respectively. The mean Gd-211 particle size of the pre-infiltrated SPP increased from 1.94 µm to 2.52 µm as the Ti increased from 920 °C to 1040 °C. Single-domain GdBCO bulk superconductors can be fabricated by the TSIG technology with the pre-infiltration process no matter what the pre-infiltration temperature is. However, the microstructure, levitation force, and trapped field of the single-domain GdBCO bulk are significantly influenced by the pre-infiltrated SPP microstructure. The largest levitation force of 35.64 N and trapped field of 0.23 T were obtained in the single-domain GdBCO bulk with pre-infiltration at 960 °C, which is an optimal microstructure in the final samples compared to the others. This study helps readers further understand the infiltration mechanism of the LP using the IG method and the fabrication of high-quality REBCO bulk superconductors.

Author Contributions

The author contributions of this paper are as follows: T.W. and W.Y. proposed the conceptualization and methodology; T.W. conducted the tests; T.W. and L.C. analyzed the data; T.W. wrote this paper; W.Y. reviewed and edited this paper. All authors have read and agreed to the published version of this manuscript.

Funding

This work was supported by the National Nature Science Foundation of China (No. 52072229, No. 51572164), the Fundamental Research Program of Shanxi Province (No. 202203021222326), and the Shanxi vocational University of Engineering Science and Technology research fund project (No. KJ202304).

Data Availability Statement

All data generated or analyzed during this study are included in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Assembly diagram of precursor pellets.
Figure 1. Assembly diagram of precursor pellets.
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Figure 2. The TG-−DSC analysis of Y-−liquid precursor powder.
Figure 2. The TG-−DSC analysis of Y-−liquid precursor powder.
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Figure 3. Temperature profile for fabrication of pre-infiltrated sample. (a) The cross-section morphology of the sample after being solidified by epoxy resin and polished. The pink lines in the figure indicate the pre-infiltraion temperatures Ti, which are 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C, 1020 °C, and 1040 °C. The A region in the figure is used to obtain the microstructural image.
Figure 3. Temperature profile for fabrication of pre-infiltrated sample. (a) The cross-section morphology of the sample after being solidified by epoxy resin and polished. The pink lines in the figure indicate the pre-infiltraion temperatures Ti, which are 900 °C, 920 °C, 940 °C, 960 °C, 980 °C, 1000 °C, 1020 °C, and 1040 °C. The A region in the figure is used to obtain the microstructural image.
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Figure 4. The surface morphology of samples pre-infiltrated at different temperatures (Ti). (a) Ti = 900 °C, (b) Ti = 920 °C, (c) Ti = 940 °C, (d,i) Ti = 960 °C, (e,j) Ti = 980 °C, (f) Ti = 1000 °C, (g) Ti = 1020 °C, (h) Ti = 1040 °C.
Figure 4. The surface morphology of samples pre-infiltrated at different temperatures (Ti). (a) Ti = 900 °C, (b) Ti = 920 °C, (c) Ti = 940 °C, (d,i) Ti = 960 °C, (e,j) Ti = 980 °C, (f) Ti = 1000 °C, (g) Ti = 1020 °C, (h) Ti = 1040 °C.
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Figure 5. Average diameter of SPP and LPP pre−nfiltrated at different temperatures (Ti). The inset graph illustrates the average diameter shrinkage of pre−infiltrated SPP and LPP.
Figure 5. Average diameter of SPP and LPP pre−nfiltrated at different temperatures (Ti). The inset graph illustrates the average diameter shrinkage of pre−infiltrated SPP and LPP.
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Figure 6. Cross-section morphology of samples pre-infiltrated at different pre-infiltration temperatures (Ti). (a) Ti = 920 °C, (b) Ti = 940 °C, (c) Ti = 960 °C, (d) Ti = 980 °C, (e) Ti = 1000 °C, (f) Ti = 1020 °C, (g) Ti = 1040 °C.
Figure 6. Cross-section morphology of samples pre-infiltrated at different pre-infiltration temperatures (Ti). (a) Ti = 920 °C, (b) Ti = 940 °C, (c) Ti = 960 °C, (d) Ti = 980 °C, (e) Ti = 1000 °C, (f) Ti = 1020 °C, (g) Ti = 1040 °C.
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Figure 7. Microstructure of central region A of SPP pre-infiltrated at different temperatures (Ti). (a) No pre-infiltrated SPP (N-P-I), (b) Ti = 920 °C, (c) Ti = 940 °C, (d) Ti = 960 °C, (e) Ti = 980 °C, (f) Ti = 1000 °C, (g) Ti = 1020 °C, (h) Ti = 1040 °C. (g-1,g-2) Enlarged image of the white rectangle in figure (g). (h-1,h-2) Enlarged image of the white rectangle in figure (h).
Figure 7. Microstructure of central region A of SPP pre-infiltrated at different temperatures (Ti). (a) No pre-infiltrated SPP (N-P-I), (b) Ti = 920 °C, (c) Ti = 940 °C, (d) Ti = 960 °C, (e) Ti = 980 °C, (f) Ti = 1000 °C, (g) Ti = 1020 °C, (h) Ti = 1040 °C. (g-1,g-2) Enlarged image of the white rectangle in figure (g). (h-1,h-2) Enlarged image of the white rectangle in figure (h).
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Figure 8. Mean Gd-211 particle size in the pre-infiltrated SPP and single-domain GdBCO bulk superconductor at different pre-infiltration temperatures (Ti).
Figure 8. Mean Gd-211 particle size in the pre-infiltrated SPP and single-domain GdBCO bulk superconductor at different pre-infiltration temperatures (Ti).
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Figure 9. Temperature profile for the preparation of single-domain GdBCO bulk superconductors with the pre-infiltration process at different Ti values using the TSIG method.
Figure 9. Temperature profile for the preparation of single-domain GdBCO bulk superconductors with the pre-infiltration process at different Ti values using the TSIG method.
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Figure 10. Surface morphology of single-domain GdBCO bulk superconductors with the pre-infiltration process at different Ti values. (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, (g) S7, (h) S8.
Figure 10. Surface morphology of single-domain GdBCO bulk superconductors with the pre-infiltration process at different Ti values. (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, (g) S7, (h) S8.
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Figure 11. Levitation force between the single−domain GdBCO bulks with different Ti values and the permanent magnet (77 K, ZFC state).
Figure 11. Levitation force between the single−domain GdBCO bulks with different Ti values and the permanent magnet (77 K, ZFC state).
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Figure 12. The distribution of the trapped field in three−-dimensional space for the single-domain GdBCO bulks with different pre-infiltration temperatures (Ti) at 77 K. (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, (g) S7, and (h) S8.
Figure 12. The distribution of the trapped field in three−-dimensional space for the single-domain GdBCO bulks with different pre-infiltration temperatures (Ti) at 77 K. (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, (g) S7, and (h) S8.
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Figure 13. (a) The radial distribution of the trapped field in single-−domain GdBCO bulks with different pre-infiltration temperatures (Ti) at 77 K. (b) The peak values of the trapped field of GdBCO bulks with different Ti values at 77 K.
Figure 13. (a) The radial distribution of the trapped field in single-−domain GdBCO bulks with different pre-infiltration temperatures (Ti) at 77 K. (b) The peak values of the trapped field of GdBCO bulks with different Ti values at 77 K.
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Figure 14. Microstructure of the GdBCO bulks at a different Ti. (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, (g) S7, and (h) S8.
Figure 14. Microstructure of the GdBCO bulks at a different Ti. (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, (f) S6, (g) S7, and (h) S8.
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Wu, T.; Yang, W.; Chen, L. The Effect of the Pre-Infiltration Temperature on the Liquid-Phase Infiltration Characteristics and the Magnetic Properties of Single-Domain GdBCO Bulk Superconductors. Crystals 2024, 14, 842. https://doi.org/10.3390/cryst14100842

AMA Style

Wu T, Yang W, Chen L. The Effect of the Pre-Infiltration Temperature on the Liquid-Phase Infiltration Characteristics and the Magnetic Properties of Single-Domain GdBCO Bulk Superconductors. Crystals. 2024; 14(10):842. https://doi.org/10.3390/cryst14100842

Chicago/Turabian Style

Wu, Tingting, Wanmin Yang, and Li Chen. 2024. "The Effect of the Pre-Infiltration Temperature on the Liquid-Phase Infiltration Characteristics and the Magnetic Properties of Single-Domain GdBCO Bulk Superconductors" Crystals 14, no. 10: 842. https://doi.org/10.3390/cryst14100842

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