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Article

Reinforcing Increase of ΔTc in MgB2 Smart Meta-Superconductors by Adjusting the Concentration of Inhomogeneous Phases

by
Yongbo Li
,
Guangyu Han
,
Hongyan Zou
,
Li Tang
,
Honggang Chen
and
Xiaopeng Zhao
*
Smart Materials Laboratory, Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, China
*
Author to whom correspondence should be addressed.
Materials 2021, 14(11), 3066; https://doi.org/10.3390/ma14113066
Submission received: 1 May 2021 / Revised: 26 May 2021 / Accepted: 2 June 2021 / Published: 4 June 2021
(This article belongs to the Special Issue Advances in Superconducting Materials: Characterization, Properties and Applications)
Browse Figures
Versions Notes

Abstract

Incorporating with inhomogeneous phases with high electroluminescence (EL) intensity to prepare smart meta-superconductors (SMSCs) is an effective method for increasing the superconducting transition temperature (Tc) and has been confirmed in both MgB2 and Bi(Pb)SrCaCuO systems. However, the increase of ΔTc (ΔTc = Tc ‒ Tcpure) has been quite small because of the low optimal concentrations of inhomogeneous phases. In this work, three kinds of MgB2 raw materials, namely, aMgB2, bMgB2, and cMgB2, were prepared with particle sizes decreasing in order. Inhomogeneous phases, Y2O3:Eu3+ and Y2O3:Eu3+/Ag, were also prepared and doped into MgB2 to study the influence of doping concentration on the ΔTc of MgB2 with different particle sizes. Results show that reducing the MgB2 particle size increases the optimal doping concentration of inhomogeneous phases, thereby increasing ΔTc. The optimal doping concentrations for aMgB2, bMgB2, and cMgB2 are 0.5%, 0.8%, and 1.2%, respectively. The corresponding ΔTc values are 0.4, 0.9, and 1.2 K, respectively. This work open a new approach to reinforcing increase of ΔTc in MgB2 SMSCs.

1. Introduction

According to BCS theory, McMillan theoretically calculated the upper limit of the critical temperature (Tc) of conventional BCS superconductors to be 40 K, which is called the McMillan limit temperature [1,2]. Although the Tc of conventional superconductors has an upper limit, the search for high-Tc superconducting materials has been continuous. High-temperature superconductors [3,4], iron-based superconductors [5,6], high-pressure superconductors [7,8,9,10], and photo-induced superconductors [11,12] have been gradually studied and discovered. However, these new superconducting materials are not simple conventional superconductors. Breaking the McMillan limit temperature remains a challenge for conventional BCS superconductors. In 2001, the superconductivity of MgB2 was discovered [13]. The excellent superconductivity, simple preparation process, and especially high Tc of MgB2 quickly aroused great interest in the scientific community and led scholars to believe that the McMillan limit temperature may finally be surpassed [14,15,16,17,18,19]. Various methods have been applied to improve the superconductivity of MgB2 [20,21,22,23,24], which would not only improve the practical application of MgB2 but also help transcend the McMillan limit temperature and further elucidate the superconducting mechanism. Chemical doping is often used to study superconductivity. Unfortunately, many experimental results confirm that this method reduces the Tc of MgB2 [25,26,27,28,29,30]. Thus far, no useful strategy for improving the Tc of MgB2 is yet available.
Metamaterial mainly refers to materials made up of two or more media, which can produce new properties that are not found in a single medium. Meta-method is often used to achieve some special properties and provides new ways of improving the Tc of materials [31,32,33]. In 2007, our group proposed a method based on the structural design of metamaterials for increasing the Tc of superconductors [34,35]. In this method, electroluminescence (EL) materials are directly doped into a superconductor to form a smart meta-superconductor (SMSC). The external field added during the measurement of the Tc of SMSC with a four-probe method can excite the inhomogeneous phases to generate EL, achieving the purpose of strengthening the Cooper pairs, resulting the change of Tc in macroscopic. A SMSC is a material whose Tc can be adjusted and improved by the stimulus of external field, which is a new property and cannot be achieved by traditional doping with a second phase [36,37,38,39,40,41,42]. Our group subsequently conducted a series of studies, mainly using MgB2 as the base superconducting material and Y2O3:Eu3+ as the base EL material [36,37,38]. The results obtained in these studies show that unlike conventional chemical doping, which consistently reduces the Tc of MgB2, the SMSC method of doping EL materials could help increase the Tc of MgB2. The same conclusions were drawn from substituting the inhomogeneous phase with YVO4:Eu3+ or luminescent nanocomposite Y2O3:Eu3+/Ag [39,40] and replacing MgB2 with Bi(Pb)SrCaCuO [41,42]. The effectiveness of improving the Tc of superconducting materials through the SMSC method by doping with EL inhomogeneous phases has been proven, but the ΔTc (ΔTc = Tc − Tcpure) values obtained are generally small (0.2–0.4 K). Our previous results show that the SMSC method can only improve Tc at low concentrations of inhomogeneous phases and leads to a small ΔTc, greatly hindering the further improvement of the Tc of MgB2. Very recently, our group has increased the Tc of smart meta-superconductor Bi(Pb)SrCaCuO by adjusting the content of inhomogeneous phase [42], implying that the Tc of MgB2 SMSC can be further improved through the similar method.
In this work, three types of MgB2 raw materials, namely, aMgB2, bMgB2, and cMgB2, were prepared with particle sizes decreasing in order. Two types of inhomogeneous phases, namely, Y2O3:Eu3+ and Y2O3:Eu3+/Ag, were also prepared based on our previous preparation method [43,44]. Two other types of non-EL dopants, namely, Y2O3 and Y2O3:Sm3+, were also prepared for comparison. These four types of dopants were incorporated into MgB2, and the change of Tc was studied. The results show that the Tc of MgB2 doped with non-EL Y2O3 and Y2O3:Sm3+ was lower than that of pure MgB2 (ΔTc < 0). By contrast, EL inhomogeneous phases Y2O3:Eu3+ and Y2O3:Eu3+/Ag increased the Tc (ΔTc > 0), and the optimal doping concentration of the inhomogeneous phases increased from 0.5% to 1.2% with the decrease of MgB2′s particle size. The optimal doping concentrations for aMgB2, bMgB2, and cMgB2 were 0.5%, 0.8%, and 1.2%, respectively. The corresponding ΔTcs were 0.4 K, 0.9 K, and 1.2 K, which exhibit significant improvements compared with the ΔTcs (0.2–0.4 K) in previous work [36,37,38,39,40]. Such an improvement of Tc is a novel property given that all the experiments before our work confirmed that doping a second phase decreased the Tc of MgB2.

2. Model

Figure 1a–c show the cross-sectional view of MgB2 SMSCs models prepared using aMgB2 (Φa < 30 μm), bMgB2 (Φb < 15 μm), and cMgB2 (Φc < 5 μm) as raw materials. Φa, Φb, and Φc refer to the particle sizes of aMgB2, bMgB2, and cMgB2 powders, which will be described in detail at the experiment section. The brown hexagons represent the MgB2 particles, and the gray dashed lines represent the flakes of inhomogeneous phase with the surface size of approximately 20 nm and thickness of approximately 2.5 nm [40,45]. The flakes of Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag mainly gather on the surfaces of the MgB2 particles as shown in Figure 1d. Figure 1e–h present the schematics of Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag, respectively. The gray flake represents Y2O3. The yellow, white, and green points represent Sm, Eu, and Ag. Obviously, the introduction of these four dopants inevitably reduces the Tc of MgB2. This is mainly because the dopants are not superconductors, which is unfavorable for the superconductivity of MgB2, like the impurity phase of MgO in MgB2. For convenience, the reduction in Tc caused by introducing the dopants is referred to as the impurity effect [36,37,38,39,40,41,42]. Non-EL dopants Y2O3 and Y2O3:Sm3+ can only decrease Tc for the introduction of the impurity effect. Unlike Y2O3 and Y2O3:Sm3+, introducing EL Y2O3:Eu3+ and Y2O3:Eu3+/Ag may increase the Tc, which is referred to as the EL exciting effect [36,37,38,39,40,41,42]. Incorporating with inhomogeneous phases has already been confirmed to be an effective method of increasing the Tc for both MgB2 and Bi(Pb)SrCaCuO systems. The variation of Tc is often associated with the change of electron density. However, in the experiments, the inhomogeneous phases do not react with MgB2 and the diffusion between the inhomogeneous phases and MgB2 particles is difficult under the current preparation process and conditions. As a result, the dopants only exist between the MgB2 particles as shown in Figure 1a–c and cannot change the electron density significantly. Therefore, in principle, the electron density is not the key tuning parameter for the variation of Tc. Although the mechanism for this method remains unclear, we intend to interpret this phenomenon in terms of EL of inhomogeneous phases based on the results of our experiments. During the measurements, the applied external electric field forms local electric fields in the superconductor, which could excite the inhomogeneous phase to produce EL. The generated EL excites the electrons to inject energy, which is favorable to strengthen the Cooper pairs and enables the increase in Tc. However, the completeness of this interpretation needs further demonstration given that the photons may disrupt Cooper pairs. Anyway, further study is required to build a relatively complete theory, especially for such a new experimental phenomenon.
A distinct competition exists between the impurity effect and EL exciting effect. Tc would be improved (ΔTc > 0) when EL exciting effect dominates; otherwise, introducing the inhomogeneous phase would decrease Tc (ΔTc < 0). During the preparation process, the impurity effect should be reduced as extensively as possible, and the EL exciting effect should be enhanced to obtain samples with a high Tc. The resulting superconductor is called a SMSC, and the Tc of which can be improved and adjusted by incorporating EL inhomogeneous phases [36,37,38,39,40,41,42], which is a new property and cannot be achieved by traditional doping with a second phase. However, the ΔTcs obtained in our previous work through the SMSC method are quite small. The low doping concentrations of inhomogeneous phases greatly hindered the further improvement of Tc. To further improve the ΔTc of MgB2, the doping concentration of the inhomogeneous phase must be increased to enhance the EL exciting effect. However, the impurity effect inevitably increases with the increasing doping concentration, as analyzed above. The results of our previous work show that the impurity effect tends to dominate at high concentrations, which is not conducive to the Tc of the sample. This phenomenon is principally caused by the agglomeration of excessive inhomogeneous phase flakes, which cannot disperse well in the sample to improve Tc at concentrations exceeding the optimal value. A simple strategy to solve this problem is to reduce the particle size of MgB2 as shown in Figure 1a–c. It can be seen that reducing the particle size would increase the optimal doping concentration of the inhomogeneous phase. The inhomogeneous phase flakes can disperse well in the sample with small particle size and fully exert the EL exciting effect to further increase ΔTc. Such a strategy has already been successfully applied to increase the Tc of smart meta-superconductor Bi(Pb)SrCaCuO [42].

3. Experiment

Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag were prepared by a hydrothermal method [40,44]. Briefly, a certain amount of Y2O3 and Eu2O3 were weighed and dissolved in HCl to make a precursor. The precursor was dissolved in benzyl alcohol and stirred with a magnetic stirrer. A certain amount of octylamine and AgNO3 was added dropwise into the beaker in turn. Then the mixture was transferred to a high-pressure reaction kettle, which was then placed in a drying oven and kept at 250 °C for 24 h. Thereafter, the reaction kettle was naturally cooled to room temperature. The precipitate was washed several times with absolute ethanol to remove impurities and then separated from the solution by centrifugation, precipitation, and drying. The obtained solids were placed in a high-temperature tube furnace and heated at 800 °C for 24 h to form a white powder. After illumination, Y2O3:Eu3+/Ag was obtained. The same procedure was carried out prepare Y2O3, Y2O3:Eu3+, and Y2O3:Sm3+ by controlling the addition of Eu2O3 and AgNO3 and replacing Eu2O3 with Sm2O3. The morphology of Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag is flaky with surface size of approximately 20 nm and thickness of approximately 2.5 nm [40,45].
Three types of MgB2 raw materials marked with aMgB2, bMgB2, and cMgB2 were prepared in this work. Φa, Φb, and Φc refer to the particle sizes of aMgB2, bMgB2, and cMgB2 powders. A 500-mesh sieve was used to sift MgB2 powder (99%, 100 mesh, Alfa Aesar) to prepare aMgB2, indicating that Φa < 30 μm. bMgB2 was prepared by sifting aMgB2 powder through vacuum filtration with a pore size of about 15 μm, indicating that Φb < 15 μm. Meanwhile, Mg and nano boron powder sifted through vacuum filtration with the pore size of about 5 μm were applied to prepare MgB2 powder by the traditional sintering process. The obtained MgB2 powder was then sifted through vacuum filtration with the pore size of about 5 μm to prepare cMgB2, indicating that Φc < 5 μm. MgB2-based superconductors were synthesized by an ex situ preparation process, which is described in detail in our previous work [37,40]. The doping concentrations in this work all refer to the mass percentage.

4. Results and Discussion

Figure 2a shows the EL spectra of Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag, which confirm that Y2O3 and Y2O3:Sm3+ are non-EL materials, whereas Y2O3:Eu3+ and Y2O3:Eu3+/Ag show a remarkable EL property. Among the four materials tested, Y2O3:Eu3+/Ag showed the highest EL intensity because of the composite luminescence [44]. Figure 2b–d present the SEM images of the pure MgB2 samples prepared using three different raw materials. Figure 2b is the SEM image of aMgB2, which shows that most of the particle exceeded 1 μm. For bMgB2, only a few of the particles exceeded 1 μm as shown in Figure 2c. Figure 2d presents the SEM image of cMgB2, which shows that most of particles are below 500 nm. The particle sizes of aMgB2, bMgB2, and cMgB2 decrease in order. Figure 2e reveals the XRD patterns of four samples. The black and red curves depict the XRD patterns of aMgB2 and aMgB2 + 0.5% Y2O3:Eu3+/Ag, respectively. The blue and magenta curves correspond to the XRD patterns of bMgB2 + 0.8% Y2O3:Eu3+/Ag and cMgB2 + 1.2% Y2O3:Eu3+/Ag, respectively. The black vertical lines represent the standard XRD patterns of MgB2. The main phase of all the samples was clearly MgB2. The Y2O3 phase was found in the doped samples. Small amounts of the unavoidable MgO phase were also detected in all the samples [46,47,48,49]. The XRD patterns of the other samples show a similar feature.
Figure 3a illustrates the normalized resistivity-temperature (R–T) curves of aMgB2 doped with x% Y2O3 (x = 0, 0.2, 0.5, 0.8, 1.0, 1.2). The black curve corresponds to the aMgB2 sample, which shows that the Tc of the pure sample was 37.4–38.2 K. The other curves represent aMgB2 doped with Y2O3 with concentrations of 0.2%, 0.5%, 0.8%, 1.0%, and 1.2%, indicating that the corresponding Tcs are 37.0–37.8 K, 36.8–37.6 K, 36.5–37.3 K, 36.1–37.0 K, and 35.8–36.8 K. The results show that like conventional chemical doping, the introduction of non-EL Y2O3 decreases the Tc of MgB2 (ΔTc < 0) and tends to increase the superconducting transition width [50]. Meanwhile, the Tcs of the doped samples decrease with the increase of the doping concentration as shown in the inset figure. Figure 3b shows the normalized R–T curves of aMgB2 doped with 0.5% y (y = 0, Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag). The doping concentration was fixed at 0.5% base on our previous work [40]. The Tc values of MgB2 doped with Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag were 36.8–37.6 K, 36.9–37.7 K, 37.6–38.4 K, and 37.8–38.6 K. The results clearly show that non-EL Y2O3 and Y2O3:Sm3+ decreased the Tc of MgB2, while EL Y2O3:Eu3+ and Y2O3:Eu3+/Ag increased the Tc of MgB2, as shown in the inset. The Tc values of MgB2 doped with Y2O3:Eu3+ and Y2O3:Eu3+/Ag increased by 0.2 and 0.4 K, respectively, compared with that of aMgB2. This finding is similar to those of our previous studies.
Figure 4a illustrates the normalized R–T curves of bMgB2 doped with x% Y2O3:Eu3+ (x = 0, 0.5, 0.6, 0.7, 0.8, 1.0). The black curve corresponds to bMgB2, which shows that the Tc of the pure sample is 36.6–37.4 K. The other curves are the R–T curves of bMgB2 doped with Y2O3:Eu3+ with doping concentrations of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, and 1.0%, indicating that the corresponding Tcs are 36.8–37.6 K, 37–37.8 K, 37.2–38.0 K, 37.4–38.2 K, 37.0–37.9 K, and 36.7–37.7 K. The Tc of the doped samples first increased and then decreased with the increase of the doping concentration. The inset summarizes the evolution of ΔTc as a function of the doping concentration. The optimal doping concentration and the corresponding ΔTc increased to 0.8% and 0.8 K, respectively, compared with those of the samples prepared using aMgB2 as raw material. Figure 4b demonstrates the normalized R–T curves of bMgB2 doped with 0.8% y (y = 0, Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag). The Tcs of bMgB2 doped with Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag were 35.8–36.6 K, 36.0–36.8 K, 37.4–38.2 K, and 37.5–38.3 K, respectively. Among these samples, bMgB2 + 0.8% Y2O3:Eu3+/Ag obtained the highest ΔTc (0.9 K) because of the high EL intensity, as shown in Figure 2a.
Figure 4c reveals the normalized R–T curves of cMgB2 doped with x% Y2O3:Eu3+ (x = 0, 0.8, 1.0, 1.2, 1.5). Similarly, the black curve corresponds to the pure sample, indicating that the Tc of cMgB2 is 36.0–36.8 K. The other curves correspond to cMgB2 doped with Y2O3:Eu3+ at different concentrations of 0.8%, 1.0%, 1.2%, and 1.5%, indicating that the corresponding Tcs are 36.2–37.0 K, 36.6–37.4 K, 37.0–37.8 K, and 36.4–37.2 K, respectively. It is same with the results in Figure 3a, that is, Tc first increases and then decreases with the increase of the doping concentration, as shown in the inset figure. The optimal doping concentration is 1.2%, and the corresponding ΔTc is 1.0 K. Figure 4d shows the normalized R–T curves of cMgB2 doped with 1.2% y (y = 0, Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag). The Tc values of cMgB2 doped with Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag are 34.7–35.7 K, 34.9–35.7 K, 37.0–37.8 K, and 37.2–38.0 K. Y2O3 and Y2O3:Sm3+ decrease Tc, whereas Y2O3:Eu3+ and Y2O3:Eu3+/Ag increase Tc. These results are consistent with those of the samples prepared using aMgB2 and bMgB2 as raw materials. The Tc of cMgB2 + 1.2% Y2O3:Eu3+/Ag was enhanced by 1.2 K compared with that of the pure sample, exhibiting the highest ΔTc among the samples.
Figure 5a shows the SEM image of aMgB2 + 0.5% Y2O3:Eu3+/Ag. Figure 5b–e are the EDS mapping for elements Mg, Y, Eu, and Ag listed in the lower right corner of each figure. Figure 5h shows the SEM image of cMgB2 + 1.2% Y2O3:Eu3+/Ag. Figure 5g–j are the EDS mapping for elements Mg, Y, Eu, and Ag. Given that the inhomogeneous phase did not react with MgB2, the mapping of elements Y, Eu, and Ag can reflect the distribution of the inhomogeneous phase in the sample. It can be seen that Y2O3:Eu3+/Ag is relatively evenly distributed in aMgB2. Similarly, the inhomogeneous phase did not generate significant agglomeration in cMgB2, even though the optimal concentration was enhanced to 1.2% as the particle size decreased, as shown in Figure 5g–j. Therefore, the inhomogeneous phase was able to fully exert the EL exciting effect to further increase ΔTc at high concentrations.
Table 1 shows the ΔTcs for aMgB2 + 0.5% x, bMgB2 + 0.8% x, and cMgB2 + 1.2% x (x = Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag). For the three kinds of MgB2 raw materials, non-EL dopants Y2O3 and Y2O3:Sm3+ can only decrease Tc (ΔTc < 0) and the higher the doping concentration, the lower the Tc. However, EL inhomogeneous phases can increase the Tc (ΔTc > 0). For the aMgB2 raw material, we prepared the MgB2 SMSCs doped with 0.5% inhomogeneous phase. The results show that ΔTc values for aMgB2 doped with Y2O3:Eu3+ and Y2O3:Eu3+/Ag are 0.2 K and 0.4 K. For the bMgB2 raw material with a smaller particle size than that of aMgB2, the optimal doping concentration was first explored by changing the concentration of Y2O3:Eu3+ from 0.5% to 1.0%. The results show that the optimal doping concentration is 0.8%. Subsequently, 0.8% Y2O3:Eu3+, and Y2O3:Eu3+/Ag were separately doped into bMgB2 and the corresponding ΔTc values were 0.8 K and 0.9 K, respectively. Similar results were obtained in the samples prepared using cMgB2 as the raw material. For cMgB2, which has the smallest particle size among the three raw materials, the optimal concentration was enhanced to 1.2%. The ΔTcs for cMgB2 doped with Y2O3:Eu3+ and Y2O3:Eu3+/Ag were 1.0 K and 1.2 K, respectively. These results indicate that reducing the particle size can effectively increase the optimal doping concentration of the inhomogeneous phase, thereby enhancing the ΔTc.
In this work, the ΔTc is improved by increasing the optimal doping concentration of inhomogeneous phases through reducing the particle size, however, the Tc values of MgB2 SMSCs are relatively low due to the low Tc of the pure MgB2 sample. As the particle size decreases, the grain boundaries in the sample increase and the connectivity decreases, which are disadvantages to the superconductivity [51,52,53]. One possible solution is to incorporate the inhomogeneous phase into the interior of the particles to overcome the disadvantages caused by the increasing grain boundaries with the doping concentration increasing.

5. Conclusions

Although the effectiveness of improving the Tc of superconducting materials through the SMSC method by doping with EL inhomogeneous phases has been proven in previous works, the ΔTcs obtained are quite small. To further increase ΔTc, three types of MgB2 raw materials, namely, aMgB2, bMgB2, and cMgB2, were prepared with particle sizes decreasing in order. EL inhomogeneous phases were incorporated into these three raw materials with different concentrations to study the change of ΔTc. The results show that the optimal doping concentrations for aMgB2, bMgB2, and cMgB2 are 0.5%, 0.8%, and 1.2%, respectively. The corresponding ΔTcs are 0.4, 0.9, and 1.2 K, respectively. Meanwhile, increasing the EL intensity of the inhomogeneous phase can be considered to further increase ΔTc. This work not only proves the effectiveness of the SMSC method in improving Tc but also provides an alternative approach to improving the Tc of superconducting materials.

Author Contributions

Conceptualization, X.Z.; methodology, X.Z. and Y.L.; software, Y.L., G.H.; validation, X.Z., Y.L. and G.H.; formal analysis, X.Z., Y.L., G.H., H.Z., L.T. and H.C.; investigation, Y.L., G.H., H.Z., L.T. and H.C.; resources, X.Z.; data curation, Y.L., G.H., H.Z., L.T. and H.C.; writing—original draft preparation, Y.L.; writing—review and editing, X.Z. and Y.L.; visualization, X.Z. and Y.L.; supervision, X.Z.; project administration, X.Z.; funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China for Distinguished Young Scholar, grant number 50025207.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Materials 14 03066 g001
Figure 1. The models of MgB2 SMSCs prepared using (a) aMgB2 (Φa < 30 μm), (b) bMgB2 (Φb < 15 μm), and (c) cMgB2 (Φc < 5 μm) as raw materials. Schematic depictions of (d) a particle of MgB2 SMSC, (e) Y2O3, (f) Y2O3:Sm3+, (g) Y2O3:Eu3+, and (h) Y2O3:Eu3+/Ag. The morphology of Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag is flaky with surface size of approximately 20 nm and thickness of approximately 2.5 nm [40].
Figure 1. The models of MgB2 SMSCs prepared using (a) aMgB2 (Φa < 30 μm), (b) bMgB2 (Φb < 15 μm), and (c) cMgB2 (Φc < 5 μm) as raw materials. Schematic depictions of (d) a particle of MgB2 SMSC, (e) Y2O3, (f) Y2O3:Sm3+, (g) Y2O3:Eu3+, and (h) Y2O3:Eu3+/Ag. The morphology of Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag is flaky with surface size of approximately 20 nm and thickness of approximately 2.5 nm [40].
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Figure 2. (a) EL intensities of Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag. (b–d) SEM images of aMgB2, bMgB2, and cMgB2. (e) XRD patterns of aMgB2, aMgB2 + 0.5% Y2O3:Eu3+/Ag, bMgB2 + 0.8% Y2O3:Eu3+/Ag, and cMgB2 + 1.2% Y2O3:Eu3+/Ag.
Figure 2. (a) EL intensities of Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag. (b–d) SEM images of aMgB2, bMgB2, and cMgB2. (e) XRD patterns of aMgB2, aMgB2 + 0.5% Y2O3:Eu3+/Ag, bMgB2 + 0.8% Y2O3:Eu3+/Ag, and cMgB2 + 1.2% Y2O3:Eu3+/Ag.
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Figure 3. Normalized resistivity-temperature curves of aMgB2 doped with (a) x% Y2O3 (x = 0, 0.2, 0.5, 0.8, 1.0, 1.2) and (b) 0.5% y (y = 0, Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag). Insets: the values of ΔTc (ΔTc = Tc − Tcpure).
Figure 3. Normalized resistivity-temperature curves of aMgB2 doped with (a) x% Y2O3 (x = 0, 0.2, 0.5, 0.8, 1.0, 1.2) and (b) 0.5% y (y = 0, Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag). Insets: the values of ΔTc (ΔTc = Tc − Tcpure).
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Figure 4. Normalized R–T curves of bMgB2 doped with (a) x% Y2O3:Eu3+ (x = 0, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) and (b) 0.8% y (y = 0, Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag). Normalized R–T curves of cMgB2 doped with (c) x% Y2O3:Eu3+ (x = 0, 0.8, 1.0, 1.2, 1.5) and (d) 1.2% y (y = 0, Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag). Insets: the values of ΔTc (ΔTc = Tc − Tcpure).
Figure 4. Normalized R–T curves of bMgB2 doped with (a) x% Y2O3:Eu3+ (x = 0, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) and (b) 0.8% y (y = 0, Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag). Normalized R–T curves of cMgB2 doped with (c) x% Y2O3:Eu3+ (x = 0, 0.8, 1.0, 1.2, 1.5) and (d) 1.2% y (y = 0, Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, Y2O3:Eu3+/Ag). Insets: the values of ΔTc (ΔTc = Tc − Tcpure).
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Figure 5. (a) SEM image and (b–e) EDS mapping of aMgB2 + 0.5% Y2O3:Eu3+/Ag. (f) SEM image and (g–j) EDS mapping of cMgB2 + 1.2% Y2O3:Eu3+/Ag.
Figure 5. (a) SEM image and (b–e) EDS mapping of aMgB2 + 0.5% Y2O3:Eu3+/Ag. (f) SEM image and (g–j) EDS mapping of cMgB2 + 1.2% Y2O3:Eu3+/Ag.
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Table 1. ΔTcs for aMgB2 + 0.5% x, bMgB2 + 0.8% x and cMgB2 + 1.2% x (x = Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag).
Table 1. ΔTcs for aMgB2 + 0.5% x, bMgB2 + 0.8% x and cMgB2 + 1.2% x (x = Y2O3, Y2O3:Sm3+, Y2O3:Eu3+, and Y2O3:Eu3+/Ag).
ΔTcsY2O3Y2O3:Sm3+Y2O3:Eu3+Y2O3:Eu3+/Ag
aMgB2 (0.5%)−0.6 K−0.5 K0.2 K0.4 K
bMgB2 (0.8%)−0.8 K−0.6 K0.8 K0.9 K
cMgB2 (1.2%)−1.1 K−1.1 K1.0 K1.2 K
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Li, Y.; Han, G.; Zou, H.; Tang, L.; Chen, H.; Zhao, X. Reinforcing Increase of ΔTc in MgB2 Smart Meta-Superconductors by Adjusting the Concentration of Inhomogeneous Phases. Materials 2021, 14, 3066. https://doi.org/10.3390/ma14113066

AMA Style

Li Y, Han G, Zou H, Tang L, Chen H, Zhao X. Reinforcing Increase of ΔTc in MgB2 Smart Meta-Superconductors by Adjusting the Concentration of Inhomogeneous Phases. Materials. 2021; 14(11):3066. https://doi.org/10.3390/ma14113066

Chicago/Turabian Style

Li, Yongbo, Guangyu Han, Hongyan Zou, Li Tang, Honggang Chen, and Xiaopeng Zhao. 2021. "Reinforcing Increase of ΔTc in MgB2 Smart Meta-Superconductors by Adjusting the Concentration of Inhomogeneous Phases" Materials 14, no. 11: 3066. https://doi.org/10.3390/ma14113066

APA Style

Li, Y., Han, G., Zou, H., Tang, L., Chen, H., & Zhao, X. (2021). Reinforcing Increase of ΔTc in MgB2 Smart Meta-Superconductors by Adjusting the Concentration of Inhomogeneous Phases. Materials, 14(11), 3066. https://doi.org/10.3390/ma14113066

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