Smart meta-superconductor MgB₂ constructed by the dopant phase of luminescent nanocomposite

Abstract

On the basis of the idea that the injecting energy will improve the conditions for the formation of Cooper pairs, a smart meta-superconductor (SMSC) was prepared by doping luminescent nanocomposite Y₂O₃:Eu³⁺/Ag in MgB₂. To improve the superconducting transition temperature (T_C) of the MgB₂-based superconductor, two types of Y₂O₃:Eu³⁺/Ag, which has the strong luminescence characteristic, with different sizes were prepared and marked as m-Y₂O₃:Eu³⁺/Ag and n-Y₂O₃:Eu³⁺/Ag. MgB₂ SMSC was prepared through an ex situ process. Results show that when the dopant content was fixed at 2.0 wt.%, the T_C of MgB₂ SMSC increased initially then decreased with the increase in the Ag content in the dopant. When the Ag content is 5%, the T_C of MgB₂ SMSC was 37.2–38.0 K, which was similar to that of pure MgB₂. Meanwhile, the T_C of MgB₂ SMSC doped with n-Y₂O₃:Eu³⁺/Ag increased initially then decreased basically with the increase in the content of n-Y₂O₃:Eu³⁺/Ag, in which the Ag content is fixed at 5%. The T_C of MgB₂ SMSC doped with 0.5 wt.% n-Y₂O₃:Eu³⁺/Ag was 37.6–38.4 K, which was 0.4 K higher than that of pure MgB₂. It is thought that the doping luminescent nanocomposite into the superconductor is a new means to improve the T_C of SMSC.

Introduction

Improving the superconducting critical transition temperature of materials is an important scientific and technical problem in condensed matter physics and materials science. Recently, Fausti et al.¹ used mid-infrared femtosecond pulses to transform non-superconducting La_1.675Eu_0.2Sr_0.125CuO₄ into a transient 3D superconductor. A similar method was also applied to investigate YBa₂Cu₃O_6.5² and K₃C₆₀^3,4 and good experimental results were achieved. Ye et al. have reported the observation of field-induced superconductivity of ZrNCl and MoS₂ by quasi-continuous electrostatic carrier doping achieved by combining liquid and solid gating^5,6. Drozdov et al.⁷ reported conventional superconductivity at 203 K under high pressure in a sulfur hydride system. Adu et al.⁸ increased the T_C of commercial “dirty” MgB₂ by conducting non-substitutional hole-doping of the MgB₂ structure using minute, single-wall carbon nanotube inclusions. In accordance with homogeneous system theory⁹, Smolyaninov et al.^10,11,12 stated that a superconducting metamaterial with an effective dielectric response function that is less and approximately equal to zero may exhibit high T_C, and they verified this theory in their subsequent experiments. Recently, Cao et al.^13,14 investigated correlated insulator behavior at half-filling in magic-angle graphene superlattices and reported the realization of intrinsic unconventional superconductivity in a 2D superlattice created by stacking two sheets of graphene that are twisted relative to each other at a small angle. Another important method for studying superconductivity is the topological superconductors^{15,16,17,18,19,20}, which have attracted great attention in condensed matter physics. However, obtaining a practical superconductor with high T_C remains difficult.

The superconductivity of MgB₂ was discovered in 2001²¹. MgB₂ is a promising material with large-scale applications because of its excellent superconducting properties and simple crystal structure^{22,23,24,25,26,27}. Considering that the T_C of MgB₂ is close to the McMillan temperature limit^28,29, developing an effective experimental method to improve the T_C of MgB₂ is beneficial to its practical application and to the understanding of the superconducting mechanism. Chemical doping is a simple, effective, commonly used method to change the T_C of superconducting materials. However, many experimental results have confirmed that conventional chemical doping decreases the T_C of MgB₂^{30,31,32,33,34,35,36}. To date, no effective method has been developed to improve the T_C of MgB₂. The use of metamaterial structures to achieve special properties is an important method developed in recent decades^{37,38,39,40,41}, and it provides a new approach to improve the T_C of superconducting materials.

On the basis of metamaterials, our group investigated the effects of ZnO electroluminescent (EL) material doping on the superconductivity of BSCCO in 2007 and attempted to change the T_C of this superconductor⁴². Meanwhile, it is proposed that the combination of chemical doping and EL excitement, that is, doping EL materials in superconducting materials to form a meta structure, may be an effective method to improve the T_C of superconductors⁴³. On the basis of these results, a smart meta-superconductor (SMSC) model for improving the T_C of materials has been proposed recently. In the model, the dopant phase is used to inject energy through its EL under the external field to strengthen the Cooper pairs, thereby achieving the purpose of changing the T_C. Zhang et al.⁴⁴ prepared MgB₂ doped with Y₂O₃:Eu³⁺ particles through an in situ process. Tao et al.⁴⁵ prepared MgB₂ doped with Y₂O₃:Eu³⁺ nanorods with different EL intensities through an ex situ process. Their results indicated that doping EL materials is favorable for the improvement of T_C compared with conventional doping, which always reduces the superconducting transition temperature of the sample. In addition, similar experimental results were obtained by replacing Y₂O₃:Eu³⁺ with Y₃VO₄:Eu³⁺ flakes⁴⁶. Meanwhlie, results also indicated that the T_C can be changed by adjusting the Y₂O₃:Eu³⁺ concentration and EL exciting current⁴⁷. However, there are some problems need to improve in the experiments. Y₂O₃:Eu³⁺ particles or flakes tended to agglomerate during the preparation process and its EL intensity was weak^44,45,46,47. The quality of commercial MgB₂ was poor and its superconducting transition width (ΔT) is 4.6 K^46,47, further experiment and improvement will be needed. Moreover, the dopants would decrease the T_C in the case of double dopants, i.e., simultaneous incorporation of Y₂O₃:Eu³⁺ and nano-Ag into MgB₂ ⁴⁷.

In this paper, a kind of new phase of luminescent nanocomposite Y₂O₃:Eu³⁺/Ag was prepared by compounding nano Ag into the Y₂O₃:Eu³⁺ matrix directly⁴⁸. The luminescent intensity of Y₂O₃:Eu³⁺/Ag is three times higher than that of Y₂O₃:Eu³⁺ due to the composite illumination of EL and photoluminescence (PL). Two kinds of nanocomposite illuminator Y₂O₃:Eu³⁺/Ag with different sizes, namely, micro Y₂O₃:Eu³⁺/Ag (m-Y₂O₃:Eu³⁺/Ag) and nano Y₂O₃:Eu³⁺/Ag flakes (n-Y₂O₃:Eu³⁺/Ag), are prepared. Meanwhile, a new kind of commercial MgB₂ with a small ΔT of 0.8 K was used. SMSC was prepared by doping Y₂O₃:Eu³⁺/Ag in MgB₂ through an ex situ process⁴⁹. The T_C of the MgB₂-based superconductor is investigated by changing the Ag content in the dopant phase, the sizes of the dopant phase, and the doping concentration. The results indicate that the T_C of MgB₂ doped with 0.5 wt.% n-Y₂O₃:Eu³⁺/Ag is 37.6–38.4 K, which is 0.4 K higher than that of pure MgB₂, which further confirmed that the SMSC is a new way to improve the critical transition temperatures.

Experiment

Preparation of m-Y₂O₃:Eu³⁺/Ag and n-Y₂O₃:Eu³⁺/Ag

Y₂O₃ and Eu₂O₃ were weighed (the atomic ratio of Y and Eu is 0.95:0.05) and dissolved in a beaker with excess concentrated hydrochloric acid and subsequently heated and dried at 70 °C for 2 h to obtain a white precursor. One of the precursor was dissolved in 4 mL of deionized water to form a solution, and ammonium oxalate was added to it dropwise. The solution was subsequently stirred vigorously at 2 °C in a temperature-controlled water bath. A certain amount of AgNO₃ was added to the solution after been stirred for 30 min. After another 30 min of stirring, the pH value of the solution was adjusted to 9–10 by adding NaOH. The final solution, designated as solution A, was obtained after another 30 min of stirring. Another precursor was also prepared into solution with 24 mL benzyl alcohol. Octylamine (4 mL) was added dropwise to the solution, which was subsequently stirred for 1 h. Afterward, a certain amount of AgNO₃ was added to the solution. After stirring for another hour, another solution was obtained and designated as solution B. Solutions A and B were then transferred to two reaction kettles, respectively. A hydrothermal reaction occurred at 160 °C for 24 h. The products were washed several times with deionized water and absolute ethanol and sintered at 800 °C for 2 h to form Y₂O₃:Eu³⁺/AgCl. After illumination, the Y₂O₃:Eu³⁺/AgCl transformed into two kinds of luminescent Y₂O₃:Eu³⁺/Ag nanocomposite with different sizes and a certain amount of Ag. The two luminescent nanocomposite materials were designated as m-Y₂O₃:Eu³⁺/Ag and n-Y₂O₃:Eu³⁺/Ag. Y₂O₃:Eu³⁺/Ag with different Ag contents was prepared by changing the AgNO₃ content. In this paper, Ag contents uniformly refers to the initial nominal atomic ratio of Ag and Y. For example, 5% Ag means that the initial nominal atomic ratio of Ag to Y is 0.05:0.95. Meanwhile, similar method was applied to synthesize Y₂O₃ and Y₂O₃:Sm³⁺.

Preparation of MgB₂–based SMSC

At a certain ratio, commercial MgB₂ powder (Alfa Aesar) and the luminescent nanocomposite Y₂O₃:Eu³⁺/Ag were weighed and prepared into an alcohol solution. The two suspensions were sonicated for 20 min, then the dopant was added dropwise to MgB₂. After sonication for more than 20 min, the mixed solution was transferred to a culture dish. Subsequently, the culture dish was placed in a vacuum oven at 60 °C for 4 h to yield a black powder. The powder was pressed into a pellet with a diameter of 11 mm and a thickness of 2 mm and placed in a small tantalum container, which was annealed at 800 °C for 2 h in high-purity argon atmosphere. The MgB₂-based superconductor doped with luminescent nanocomposite materials of different sizes and Ag contents was synthesized to investigative the T_C of SMSC.

Results and Discussion

Figure 1a shows the EL spectra of Y₂O₃, Y₂O₃:Eu³⁺, m-Y₂O₃:Eu³⁺/Ag, and n-Y₂O₃:Eu³⁺/Ag. The Ag content of the luminescent nanocomposite materials was 5.0%. It shows that Y₂O₃ is a non-EL material and becomes a kind of EL material after the addition of a small amount of Eu element. The results also indicate that the EL intensity of the luminescent m-Y₂O₃:Eu³⁺/Ag nanocomposite and n-Y₂O₃:Eu³⁺/Ag is remarkably improved primarily due to the composite luminescence of the electroluminescence of Eu³⁺ centric and the surface plasma-enhanced photoluminescence of Ag. Among the four dopants, n-Y₂O₃:Eu³⁺/Ag had the highest EL intensity. Figure 1b shows the SEM image of m-Y₂O₃:Eu³⁺/Ag. The surface size and thickness of the m-Y₂O₃:Eu³⁺/Ag flake are approximately 300 nm and 30 nm, respectively. Figure 1c,d show AFM images of n-Y₂O₃:Eu³⁺/Ag and Fig. 1d presents a cross section of the AFM image in Fig. 1c. Figure 1e,f show TEM images of n-Y₂O₃:Eu³⁺/Ag. It can be seen that the surface size of n-Y₂O₃:Eu³⁺/Ag was 20 nm, and its thickness was approximately 2.5 nm, which is much smaller than that of m-Y₂O₃:Eu³⁺/Ag.

Figure 1

(a) EL spectra of Y₂O₃, Y₂O₃:Eu³⁺, m-Y₂O₃:Eu³⁺/Ag, and n-Y₂O₃:Eu³⁺/Ag; (b) SEM image of m-Y₂O₃:Eu³⁺/Ag; (c,d) AFM images and (e,f) TEM images of n-Y₂O₃:Eu³⁺/Ag.

Figure 2a shows the SEM image of pure MgB₂. The size of the MgB₂ particle was approximately 0.1–1 μm. The T_C of the samples was determined based on the R–T curve, which was measured using a four-probe method in a liquid helium cryogenic system developed by the Advanced Research Systems Company. Figure 2b shows the normalized R–T curve of pure MgB₂ and indicates that the onset temperature (T_{on c}) and offset temperature (T_{off c})^50,51 of pure MgB₂ were 38.0 and 37.2 K, respectively. The ΔT of pure MgB₂ was 0.8 K. Figure 2c shows the XRD spectra of pure MgB₂ and partially doped samples, in which the standard card of MgB₂ (PDF#38-1369) is demonstrated using black vertical lines. The results showed that the XRD spectrum of pure MgB₂ (black curve) matched the standard card of MgB₂ well, except for the inevitable small amount of the MgO phase^52,53,54,55. The red and blue curves represent the XRD spectra of MgB₂ doped with 2.0 wt.% m-Y₂O₃:Eu³⁺/Ag and 2.0 wt.% n-Y₂O₃:Eu³⁺/Ag, respectively. The Ag content was 5.0%. The main phase of the doped samples was MgB₂. Moreover, apart from a small amount of the MgO phase, the Y₂O₃ phase was also found in the XRD spectra of the doped samples. The XRD spectra of the other doped samples were similar.

Figure 2

(a) SEM image and (b) normalized temperature-dependent resistivity (R–T) curve of pure MgB₂; (c) XRD spectra of pure MgB₂ and MgB₂ doped with 2.0 wt.% m-Y₂O₃:Eu³⁺/Ag and 2.0 wt.% n-Y₂O₃:Eu³⁺/Ag.

Figure 3 shows the normalized R–T curves of MgB₂ doped with m-Y₂O₃:Eu³⁺/Ag with different Ag contents. On the basis of the results of our previous study^45,46, the content of m-Y₂O₃:Eu³⁺/Ag in the four samples was fixed at 2.0 wt.%. The Ag contents of m-Y₂O₃:Eu³⁺/Ag in the four samples were 1.0%, 4.0%, 5.0%, and 8.0%, as shown in the figure, and their T_C values were 34.8–35.6, 36.0–36.8, 37.2–38.0, and 34.8–35.6 K, respectively. The T_C of the doped samples initially increased then decreased with the increase in Ag content. Meanwhile, the corresponding doped sample had the highest T_C when the concentration of m-Y₂O₃:Eu³⁺/Ag was fixed at 2.0 wt.% and the Ag content was 5.0%, which is equal to that of pure MgB₂. The experimental results are similar to those of our previous studies, that is, doping EL materials may improve T_C in several cases compared with conventional doping, which always reduces the T_C of the sample. As a dopant, m-Y₂O₃:Eu³⁺/Ag exerts an impurity effect that decreases T_C. Meanwhile, as an EL material, m-Y₂O₃:Eu³⁺/Ag exerts an EL exciting effect that increases T_C ^45,46. An obvious competitive relationship exists between the impurity effect and the EL exciting effect. The final T_C of the samples increased when the EL exciting effect was fully utilized and the impurity effect was minimized.

Figure 3

Normalized R–T curves of MgB₂ doped with 2.0 wt.% m-Y₂O₃:Eu³⁺/Ag with different Ag contents.

Figure 4 shows the normalized R–T curves of MgB₂ doped with 0.1–2.0 wt.% n-Y₂O₃:Eu³⁺/Ag. Ag concentration was fixed at 5.0%. It can be seen that T_C of MgB₂ doped with n-Y₂O₃:Eu³⁺/Ag initially decreased, increased, then decreased again with the increase in doping concentration. A too low or too high doping concentration reduces T_C, which is similar to the finding of our previous study. When the doping concentration was in a low range, T_C decreased with the increase in doping concentration due to the dominance of the impurity effect of the dopant, which is similar to the results of conventional doping. The EL exciting effect of the dopant dominated with the further increase in doping concentration, resulting in the increase in T_C. The samples doped with 0.5 wt.% n-Y₂O₃:Eu³⁺/Ag had the highest T_C of 37.6–38.4 K, which is 0.4 K higher than that of pure MgB₂. However, the impurity effect of the dopant dominated when the doping concentration increased to a high range, which led to a low T_C. These results indicate that doping luminescent nanocomposite materials effectively adjusts and improves T_C at an appropriate doping concentration.

Figure 4

Normalized R–T curves of MgB₂ doped with n-Y₂O₃:Eu³⁺/Ag.

MgB₂ doped with non-EL materials Y₂O₃ and Y₂O₃:Sm³⁺ were synthesized to prove the conclusions above. Figure 5 shows the normalized R–T curves of MgB₂ doped with Y₂O₃, Y₂O₃:Sm³⁺, Y₂O₃:Eu³⁺, and n-Y₂O₃:Eu³⁺/Ag. The doping concentration was fixed at 0.6 wt.%, and the Ag content in n-Y₂O₃:Eu³⁺/Ag was 5.0%. Results indicated that T_C of MgB₂ doped with non-EL materials Y₂O₃ or Y₂O₃:Sm³⁺ was much lower than that of pure MgB₂, which is different from MgB₂ doped with EL materials at the same concentration. MgB₂ doped with Y₂O₃:Eu³⁺ increased to 36.6–37.4 K due to the EL exciting effect. Meanwhile, MgB₂ doped with the luminescent n-Y₂O₃:Eu³⁺/Ag nanocomposite further increased to 37.0–37.8 K. The results show that doping EL materials facilitates an increase in T_C in several cases compared with conventional doping, which always reduces the T_C of the sample. Meanwhile, luminescent Y₂O₃:Eu³⁺/Ag nanocomposite materials increase the T_C of MgB₂ due to the strong EL intensity.

Figure 5

Normalized R–T curves of MgB₂ doped with Y₂O₃, Y₂O₃:Sm³⁺, Y₂O₃:Eu³⁺, and n-Y₂O₃:Eu³⁺/Ag.

The results in Fig. 4 show that the optimum concentration of n-Y₂O₃:Eu³⁺/Ag is 0.5 wt.%, which is lower than the value in our previous study^44,45,46 due to the small size of n-Y₂O₃:Eu³⁺/Ag. The disadvantages caused by the impurity effect can be reduced if luminescent nanocomposite materials have a small size and are relatively evenly distributed in the sample. Moreover, the ΔT of commercial MgB₂ in our previous study⁴⁶ was too large to accurately determine the influence of the dopant phase on T_C. In the current study, a new kind of commercial MgB₂ with a small ΔT of 0.8 K was used, and we obtained a similar conclusion, which further proves the effectiveness of this method.

Conclusions

In this paper, two types of luminescent nanocomposite Y₂O₃:Eu³⁺/Ag with different sizes were prepared and marked as m-Y₂O₃:Eu³⁺/Ag and n-Y₂O₃:Eu³⁺/Ag. SEM and AFM images indicated that the surface size and thickness of m-Y₂O₃:Eu³⁺/Ag are approximately 300 nm and 30 nm, which are 20 nm and 2.5 nm for n-Y₂O₃:Eu³⁺/Ag. The EL spectra showed that the luminescent intensity of Y₂O₃:Eu³⁺/Ag is three times higher than that of Y₂O₃:Eu³⁺. MgB₂ of SMSC was prepared through an ex situ process to improve the T_C of the MgB₂-based superconductor on the basis of the idea that the injecting energy will improve the conditions for the formation of Cooper pairs. The T_C of MgB₂ SMSC was determined based on the measured R–T curve by using the four-probe method in a liquid helium cryogenic system. Results show that the T_C of MgB₂ SMSC initially increased then decreased with the increase in the Ag content when m-Y₂O₃:Eu³⁺/Ag content was fixed at 2.0 wt.%. When the Ag content was 5.0%, the T_C of MgB₂ SMSC doped with 2.0 wt.% m-Y₂O₃:Eu³⁺/Ag was 37.2–38.0 K, which was equal to that of pure MgB₂. Meanwhile, the T_C of MgB₂ SMSC doped with n-Y₂O₃:Eu³⁺/Ag increased initially then decreased basically with the increase in the content of n-Y₂O₃:Eu³⁺/Ag, in which the Ag content is fixed at 5%. The T_C of MgB₂ SMSC doped with 0.5 wt.% n-Y₂O₃:Eu³⁺/Ag was 37.6–38.4 K, which was 0.4 K higher than that of pure MgB₂. It is thought that the doping luminescent nanocomposite into the superconductor is a new means to improve the T_C of SMSC. However, the increase in the T_C remains insufficient. In future work, new dopant phases with improved characteristics will be prepared to increase the T_C of MgB₂. Meanwhile, we will attempt to apply this method to other superconductors.