Reinforcing increase of ΔTC in MgB2 smart meta-superconductors by adjusting the concentration of inhomogeneous phases
Yongbo Li, Guangyu Han, Hongyan Zou, Li Tang, Honggang Chen, Xiaopeng Zhao
RReinforcing increase of ΔT C in MgB smart meta-superconductors by adjusting the concentration of inhomogeneous phases 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; * Correspondence: [email protected]
Abstract
Incorporating with inhomogeneous phases with high electroluminescence (EL) intensity to prepare smart meta-superconductors (SMSCs) is an effective method of increasing the superconducting transition temperature ( T C ) and has been confirmed in both MgB and Bi (Pb)SrCaCuO systems. However, the increase of ΔT C ( ΔT C = T C – T Cpure ) has been quite small because of the low optimal concentrations of inhomogeneous phases. In this work, three kinds of MgB raw materials, namely, MgB , MgB , and MgB , were prepared with particle sizes decreasing in order. Inhomogeneous phases, Y O :Eu and Y O :Eu /Ag, were also prepared and doped into MgB to study the influence of doping concentration on the ΔT C of MgB with different particle sizes. Results show that reducing the MgB particle size increases the optimal doping concentration of inhomogeneous phases, thereby increasing ΔT C . The optimal doping concentrations for MgB , MgB , and MgB are 0.5%, 0.8%, and 1.2%, respectively. The corresponding ΔT C values are 0.4, 0.9, and 1.2 K, respectively. This work open a new approach to reinforcing increase of ΔT C in MgB SMSCs.
Keywords:
MgB ; inhomogeneous phase; SMSCs; ΔT C Introduction
According to BCS theory, McMillan theoretically calculated the upper limit of the critical temperature ( T C ) of conventional BCS superconductors to be 40 K, which is called the McMillan limit temperature. Although the T C of conventional superconductors has an upper limit, the search for high- T C superconducting materials has been continuous. High-temperature superconductors, iron-based superconductors, high-pressure superconductors, and photo-induced superconductors have been gradually studied and discovered. However, these new uperconducting materials are not simple conventional superconductors. Breaking the McMillan limit temperature remains a challenge for conventional BCS superconductors. In 2001, the superconductivity of MgB was discovered. The excellent superconductivity, simple preparation process, and especially high T C of MgB quickly aroused great interest in the scientific community and led scholars to believe that the McMillan limit temperature may finally be surpassed. Various methods have been applied to improve the superconductivity of MgB , which would not only improve the practical application of MgB 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 T C of MgB . Thus far, no useful strategy for improving the T C of MgB is yet available. Meta-method is often used to achieve some special properties and provides new ways of improving the T C of materials. In 2007, our group proposed a method based on the structural design of metamaterials for increasing the T C of superconductors. In this method, electroluminescence (EL) materials are directly doped into a superconductor to form a smart meta-superconductor (SMSC). The T C of superconducting materials may be increased by improving the conditions for the formation of Cooper pairs with the help of the energy injected by EL materials. Our group subsequently conducted a series of studies, mainly using MgB as the base superconducting material and Y O :Eu as the base EL material. The results obtained in these studies show that unlike conventional chemical doping, which consistently reduces the T C of MgB , the SMSC method of doping EL materials could help increase the T C of MgB . The same conclusions were drawn from substituting the inhomogeneous phase with Y VO :Eu or luminescent nanocomposite Y O :Eu /Ag and replacing MgB with Bi(Pb)SrCaCuO. The effectiveness of improving the T C of superconducting materials through the SMSC method by doping with EL inhomogeneous phases has been proven, but the ΔT C ( ΔT C = T C – T Cpure ) values obtained are generally small (0.2–0.4 K). Our previous results show that the SMSC method can only improve T C at low concentrations of inhomogeneous phases and leads to a small ΔT C , greatly hindering the further improvement of the T C of MgB . In this work, three types of MgB raw materials, namely, MgB , MgB , and MgB , were prepared with particle sizes decreasing in order. Two types of inhomogeneous phases, namely, Y O :Eu and Y O :Eu /Ag, were also prepared based on our previous preparation method. Two other types of non-EL dopants, namely, Y O and Y O :Sm , were also prepared for comparison. These four types of dopants were incorporated into MgB , and the change of T C was studied. The results show that the T C of MgB doped with non-EL Y O and Y O :Sm is lower than that of pure MgB ( ΔT C < 0). By contrast, EL inhomogeneous phases Y O :Eu and O :Eu /Ag increase the T C ( ΔT C > 0), and the optimal doping concentration of the inhomogeneous phases increased from 0.5% to 1.2% with the decrease of MgB ’s particle size. The optimal doping concentrations for MgB , MgB , and MgB are 0.5%, 0.8%, and 1.2%, respectively. The corresponding ΔT Cs are 0.4 K, 0.9 K, and 1.2 K, which exhibit significant improvements compared with the ΔT Cs (0.2–0.4 K) in our previous work. Model
FIG. 1.
Schematic depictions of (a) MgB SMSC, (b) Y O , (c) Y O :Sm , (d) Y O :Eu , and (e) Y O :Eu /Ag. (f,g) Model of MgB SMSC with different particle sizes.
Fig. 1(a) shows a schematic of the MgB SMSC model. The brown hexahedron represents the MgB particle, and the gray flakes represent the inhomogeneous phase. The flakes of the inhomogeneous phase mainly gather on the surface of the MgB particles. Figs. 1(b-e) present the schematics of Y O , Y O :Sm , Y O :Eu , and Y O :Eu /Ag. The gray flake represents Y O . The yellow, white, and green points represent Sm, Eu, and Ag, respectively. Obviously, the introduction of these four dopants inevitably reduces the T C of MgB . This is mainly because the dopants are not superconductor, which is unfavorable for the superconductivity of MgB , like the impurity phase of MgO in MgB . For convenience, the reduction in T C caused by introducing the dopants is referred to as the impurity effect. Non-EL dopants Y O and Y O :Sm can only decrease T C for the introduction of the impurity effect. Unlike Y O and Y O :Sm , introducing EL Y O :Eu and Y O :Eu /Ag may increase the T C of MgB . In the experiment, a four-probe method is used to measure the T C . During the measurements, the applied external electric field forms local lectric fields in the superconductor, which could excite the inhomogeneous phase to produce EL. The generated EL excites the electrons in turn, which is favorable to the formation of Cooper pairs and enables the increase in T C . This process is collectively referred to as the EL exciting effect. A distinct competition exists between these two effects. T C would be improved ( ΔT C > 0) when EL exciting effect dominates; otherwise, introducing the inhomogeneous phase would decrease T C ( ΔT C < 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 T C . The resulting superconductor is called a SMSC, and the T C of which can be improved and adjusted by incorporating EL inhomogeneous phases. However, the ΔT Cs 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 T C . To further improve the ΔT C of MgB , 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 T C 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 T C at concentrations exceeding the optimal value. A simple strategy to solve this problem is to reduce the particle size of MgB . Figs. 1(f) and 1(g) show the cross-sectional view of the MgB SMSC model with different particle sizes. It can be seen that reducing the particle size would increase the region between the particles, thereby increasing 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 ΔT C . Experiment Y O , Y O :Sm , Y O :Eu , and Y O :Eu /Ag were prepared by a hydrothermal method, which is described in detail in Ref. . Three types of MgB raw materials marked with MgB , MgB , and MgB were prepared simultaneously. A 500-mesh sieve was used to sifted MgB powder (99%, 100 mesh, Alfa Aesar) to prepare MgB , indicating that the maximum particle size of MgB was less than 25 μm. MgB was prepared by sifting MgB powder through vacuum filtration with the pore size of about 4.5–9 μm. Meanwhile, a type of MgB powder was prepared by traditional sintering process using Mg and nano boron powder as raw materials, which was then sifted through vacuum filtration with the pore size of about 3–4 μm to prepare MgB . MgB -based superconductors were synthesized by an ex situ preparation process, which is described in etail in Ref. . The doping concentrations in this work all refer to the mass percentage. Results and Discussion
FIG. 2. (a) EL intensities of Y O , Y O :Sm , Y O :Eu , and Y O :Eu /Ag. (b-d) SEM images of MgB , MgB , and MgB , respectively. (e) XRD patterns of MgB , MgB +0.5% Y O :Eu /Ag, MgB +0.8% Y O :Eu /Ag, and MgB +1.2% Y O :Eu /Ag. Fig. 2(a) shows the EL spectra of Y O , Y O :Sm , Y O :Eu , and Y O :Eu /Ag, which confirm that Y O and Y O :Sm are non-EL materials, whereas Y O :Eu and Y O :Eu /Ag show a remarkable EL property. Among the four materials tested, Y O :Eu /Ag showed the highest EL intensity because of the composite luminescence. Figs. 2(b-d) present the SEM images of the pure MgB samples prepared using three different raw materials. Fig. 2(b) is the SEM image of MgB , which shows that most of the particle exceeded 1 μm. For MgB , only a few of the particles exceeded 1 μm as shown in Fig. 2(c). Fig. 2(d) presents the SEM image of MgB , which shows that most of particles are below 500 nm. The particle sizes of MgB , MgB , and MgB decrease in order. Fig. 2(e) reveals the XRD patterns of four samples. The black and red curves depict the XRD patterns of MgB and MgB +0.5% Y O :Eu /Ag, respectively. The blue and magenta curves correspond to the XRD patterns of MgB +0.8% Y O :Eu /Ag and MgB +1.2% Y O :Eu /Ag, respectively. The black vertical lines represent the standard XRD patterns of MgB . The main phase of all the samples was clearly MgB . The Y O phase was found in the doped samples. Small amounts of the unavoidable MgO phase were also detected in all the samples. The XRD patterns of the other samples show a similar feature.
FIG. 3.
Normalized resistivity-temperature curves of MgB doped with (a) x % Y O ( x = 0, 0.2, 0.5, 0.8, 1.0, 1.2) and (b) 0.5% y ( y = 0, Y O , Y O :Sm , Y O :Eu , Y O :Eu /Ag). Insets: the values of ΔT C . Fig. 3(a) illustrates the normalized resistivity-temperature ( R – T ) curves of MgB doped with x % Y O ( x = 0, 0.2, 0.5, 0.8, 1.0, 1.2). The black curve corresponds to the MgB sample, which shows that the T C of the pure sample was 37.4–38.2 K. The other curves represent MgB doped with Y O with concentrations of 0.2%, 0.5%, 0.8%, 1.0%, and 1.2%, indicating that the corresponding T Cs 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 Y O decreases the T C of MgB ( ΔT C < 0) and the T Cs of the doped samples decrease with the increase of the doping concentration as shown in the inset figure. Fig. 3(b) shows the normalized R – T curves of MgB doped with 0.5% y ( y = 0, Y O , Y O :Sm , Y O :Eu , Y O :Eu /Ag). The doping concentration was fixed at 0.5% base on our previous work. The T C values of MgB doped with Y O , Y O :Sm , Y O :Eu , and Y O :Eu /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 Y O and Y O :Sm decreased the T C of MgB , while EL Y O :Eu and Y O :Eu /Ag increased the T C of MgB , as shown in the inset. The T C values of MgB doped with Y O :Eu and Y O :Eu /Ag increased by 0.2 and 0.4 K, respectively, compared with that of MgB . This finding is similar to those of our previous studies. FIG. 4.
Normalized
R–T curves of MgB doped with (a) x % Y O :Eu ( x = 0, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) and (b) 0.8% y ( y = 0, Y O , Y O :Sm , Y O :Eu , Y O :Eu /Ag). Normalized R–T curves of MgB doped with (c) x % Y O :Eu ( x = 0, 0.8, 1.0, 1.2, 1.5) and (d) 1.2% y ( y = 0, Y O , Y O :Sm , Y O :Eu , Y O :Eu /Ag). Insets: the values of ΔT C . Fig. 4(a) illustrates the normalized R – T curves of MgB doped with x % Y O :Eu ( x = 0, 0.5, 0.6, 0.7, 0.8, 1.0). The black curve corresponds to MgB , which shows that the T C of the pure sample is 36.6–37.4 K. The other curves are the R – T curves of MgB doped with Y O :Eu with doping concentrations of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, and 1.0%, indicating that the corresponding T Cs 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 T C of the doped samples first increased and then decreased with the increase of the doping concentration. The inset summarizes the evolution of ΔT C as a function of the doping concentration. The optimal doping concentration and the corresponding ΔT C increased to 0.8% and 0.8 K, respectively, compared with those of the samples prepared using MgB as raw material. Fig. 4(b) demonstrates the normalized R – T curves of MgB doped with 0.8% y ( y = 0, Y O , Y O :Sm , Y O :Eu , Y O :Eu /Ag). The T Cs of MgB doped with Y O , Y O :Sm , Y O :Eu , and Y O :Eu /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, MgB +0.8% Y O :Eu /Ag obtained the highest ΔT C (0.9 K) because of the high EL intensity, as shown in Fig. 2(a). Fig. 4(c) reveals the normalized R – T curves of MgB doped with x % Y O :Eu ( x = 0, 0.8, .0, 1.2, 1.5). Similarly, the black curve corresponds to the pure sample, indicating that the T C of MgB is 36.0–36.8 K. The other curves correspond to MgB doped with Y O :Eu at different concentrations of 0.8%, 1.0%, 1.2%, and 1.5%, indicating that the corresponding T Cs 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 Fig. 3(a), that is, T C 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 ΔT C is 1.0 K. Fig. 4(d) shows the normalized R – T curves of MgB doped with 1.2% y ( y = 0, Y O , Y O :Sm , Y O :Eu , Y O :Eu /Ag). The T C values of MgB doped with Y O , Y O :Sm , Y O :Eu , Y O :Eu /Ag are 34.7–35.7 K, 34.9–35.7 K, 37.0–37.8 K, and 37.2–38.0 K. Y O and Y O :Sm decrease T C , whereas Y O :Eu and Y O :Eu /Ag increase T C . These results are consistent with those of the samples prepared using MgB and MgB as raw materials. The T C of MgB +1.2% Y O :Eu /Ag was enhanced by 1.2 K compared with that of the pure sample, exhibiting the highest ΔT C among the samples. FIG. 5. (a) SEM image and (b-e) EDS mapping of MgB +0.5% Y O :Eu /Ag. (f) SEM image and (g-j) EDS mapping of MgB +1.2% Y O :Eu /Ag. Fig. 5(a) shows the SEM image of MgB +0.5% Y O :Eu /Ag. Figs. 5(b-e) are the EDS mapping for elements Mg, Y, Eu, and Ag listed in the lower right corner of each figure. Fig. 6(h) shows the SEM image of MgB +1.2% Y O :Eu /Ag. Figs. 6(g-j) are the EDS mapping for elements Mg, Y, Eu, and Ag. Given that the inhomogeneous phase did not react with MgB , the mapping of elements Y, Eu, and Ag can reflect the distribution of the inhomogeneous phase in the sample. It can be seen that Y O :Eu /Ag is relatively evenly distributed in MgB . Similarly, the inhomogeneous phase did not generate significant agglomeration in MgB , even though the optimal concentration was enhanced to 1.2% as the particle size decreased, as shown in Figs. 6(g-j). herefore, the inhomogeneous phase was able to fully exert the EL exciting effect to further increase ΔT C at high concentrations. For the MgB raw material, we prepared the MgB SMSCs doped with 0.5% inhomogeneous phase. The results show that ΔT C values for MgB doped with Y O :Eu and Y O :Eu /Ag are 0.2 K and 0.4 K. For the MgB raw material with a smaller particle size than that of MgB , the optimal doping concentration was first explored by changing the concentration of Y O :Eu from 0.5% to 1.0%. The results show that the optimal doping concentration is 0.8%. Subsequently, 0.8% Y O , Y O :Sm , Y O :Eu , and Y O :Eu /Ag were separately doped into MgB to study the change of T C . The results clearly show that Y O and Y O :Sm reduced T C , whereas Y O :Eu and Y O :Eu /Ag enhanced T C , and the corresponding ΔT C values were 0.8 K and 0.9 K, respectively. Similar results were obtained in the samples prepared using MgB as the raw material. For MgB , which has the smallest particle size among the three raw materials, the optimal concentration was enhanced to 1.2%. The ΔT Cs for MgB doped with Y O :Eu and Y O :Eu /Ag were 1.0 K and 1.2 K, respectively. These results indicate that reducing the particle to increase the region between the particles can effectively enhance the optimal doping concentration, thereby enhancing the ΔT C . Although the ΔT C is improved by increasing the optimal doping concentration of inhomogeneous phases through reducing the particle size, the T C values of MgB SMSCs are relatively low due to the low T C of the pure MgB sample. As the particle size decreases, the grain boundaries in the sample increase and the connectivity decreases, which are disadvantages to the superconductivity. 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.
Conclusion
Although the effectiveness of improving the T C of superconducting materials through the SMSC method by doping with EL inhomogeneous phases has been proven in previous works, the ΔT Cs obtained are quite small. To further increase ΔT C , three types of MgB raw materials, namely, MgB , MgB , and MgB , 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 ΔT C . The results show that the optimal doping concentrations for MgB , MgB , and MgB are 0.5%, 08%, and 1.2%, respectively. The corresponding ΔT Cs 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 ΔT C . This work not only proves the effectiveness of the SMSC method in improving T C but also provides an alternative approach to improving the T C of superconducting materials. cknowledgements This work was supported by the National Natural Science Foundation of China for Distinguished Young Scholar under Grant No.50025207.
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