An important property of superconductors is that they can carry a large current without loss, so the use of superconducting magnets can generate a super strong magnetic field. This property can be applied in defense, medical, controlled nuclear fusion, high-energy accelerators, and next-generation magnetic levitation rail transit. On the magnetic field and temperature phase diagrams of superconductors, there is a boundary line identifying lossless transmission current called the irreversible line Hirr (T). Only at temperatures and magnetic fields below the irreversible line, superconductors can carry a certain superconducting current. Generally, the higher the irreversible magnetic field, the stronger the superconductor's ability to carry lossless current under strong magnetic fields, and a better application is expected.
Superconductors can enter the superconducting state only below their critical temperature Tc. Therefore, the application of superconductivity requires superconductors with higher critical temperatures. Liquid nitrogen has a boiling point temperature of 77.3K, making it an easy-to-manufacture and low-cost refrigerant. Therefore, it was found that superconductors with critical temperature above liquid nitrogen temperature and high irreversible magnetic field are very important for large-scale applications of superconductivity. Many of the copper-oxygen superconductor families have a critical temperature that exceeds the liquid nitrogen temperature, such as the Y-series (123, T c ≈ 90K), Bi-series (2223, T c ≈110K), and Tl-series (2223, T c ≈ 125K) and Hg-series (1234, T c ≈ 124K) and so on. But the latter two contain the toxic elements thorium and mercury, and the irreversible magnetic field in the temperature region of liquid nitrogen does not seem high. Although the Bi-system is not toxic, because it has too strong two-dimensionality, the superconducting critical current under the magnetic field decreases rapidly, and the irreversible magnetic field is very low, so it cannot be used for strong electric applications at liquid nitrogen temperature. The critical temperature of Y-series superconductor yttrium barium copper oxide (YBa 2 Cu 3 O 7-d, referred to as YBCO) exceeds the temperature of liquid nitrogen, and the irreversible magnetic field is also high. YBCO; but because of its short coherence length, it is extremely difficult to prepare long wires, so far no large-scale application has been achieved. The superconducting academic community urgently needs to find non-toxic superconductors with a superconducting critical temperature above the liquid nitrogen temperature and a higher irreversible magnetic field in order to achieve better applications.
Professor Wenhai Hu's team from the School of Physics of Nanjing University prepared a non-toxic copper-oxygen superconductor (Cu, C) Ba 2 Ca 3 Cu 4 O 11 d using high-temperature and high-pressure synthesis technology, with a critical temperature of about 116 K . Careful measurement of resistance and magnetization properties shows that the superconductor has the highest irreversible magnetic field in the temperature range of liquid nitrogen temperature and above, so it may bring better applications. The work was published online September 28, 2018 in Science Advances 4, eaau0192 (2018).
The origin of this work is based on their in-depth research and understanding of the irreversible magnetic field and critical current problems of copper oxide superconductors for many years. As mentioned earlier, many superconductors with a critical temperature exceeding 100K have toxic elements. Therefore, how to replace these toxic elements with non-toxic elements while keeping the irreversible magnetic field still high is a direction that they have paid special attention to in recent years. This led them to go back to around 1995. There was a preliminary exploration in this area in the international academic community, but at that time there was no unified understanding of the component expressions of superconductors, and no systematic research on irreversible lines. The team successfully synthesized (Cu, C) Ba 2 Ca 3 Cu 4 O 11 d superconductors with a volume content of more than 90% by using high temperature and high pressure synthesis methods, and confirmed the molecular formula and structure. They carefully measured the resistance and magnetization of the superconductor under a magnetic field and determined its irreversible line. Figure 1 shows the resistance of the superconductor as a function of temperature under different magnetic fields. It can be seen that even under the strong magnetic field of 15 Tesla, the resistance at 80K is still very small (reduced to the noise range of the instrument). They found that some samples had higher irreversible magnetic fields. Because the resistance disappearing behavior is mainly determined by the superconducting weak connection between the polycrystals, the weak connection is further improved, and the irreversible magnetic field of this type of superconductor will continue to increase.
Figure 1. Temperature dependence of resistivity of (Cu, C) Ba 2 Ca 3 Cu 4 O 11 d superconductor under different magnetic fields. A. Variation of resistivity with temperature. The illustration shows the variation of magnetization with temperature measured at an external magnetic field of 10 Oe. ZFC represents the magnetization data measured by cooling the sample to a low temperature under a zero magnetic field and then applying a magnetic field. Reflects full diamagnetism; FC stands for Field Cooling Measurement Mode. B. The same data shown in the main image of A is shown, and the resistivity is plotted in logarithmic form. It can be seen that both the magnetization and the resistance decrease sharply around 116K, and the sample begins to enter the superconducting state.
Using the criterion of 1% of the normal state resistance near the superconducting transition, they determined the irreversible magnetic field of the superconductor. The data is plotted in Figure 2. See the black square and red diamond data points. Also plotted in the figure are data from other polycrystalline samples or thin film / single-crystal samples (magnetic fields parallel to the c-axis) of other copper oxide superconductors. It can be seen that (Cu, C) Ba 2 Ca 3 Cu 4 O 11 d superconductor has the highest irreversible magnetic field in the liquid nitrogen temperature range up to 116K. The area marked by the blue diagonal line in the figure is a further improvement based on the irreversible line of yttrium barium copper oxide (YBCO) superconductor. It can be seen that both the superconducting critical temperature and the irreversible magnetic field have improved significantly. Once the work was posted on an international academic website, it attracted widespread attention. The three reviewers of Science Advances have given high evaluations, thinking that this is a huge result and a significant advance in the field, which may stimulate a new upsurge in the application research of copper oxide superconductors and Eventually lead to large-scale applications.
Figure 2. Comparison of irreversible magnetic field of (Cu, C) Ba 2 Ca 3 Cu 4 O 11 d superconductor with other copper oxide superconductors. The black square and red diamond data points in the figure are the irreversible magnetic field curves of the two samples. At present, the data of the most promising non-toxic superconductor YBCO in the field of strong current applications is indicated by red solid dots and green triangle data points. The area marked by the blue slash is an improvement over YBCO.
Prof. Wen Haihu's team, with the support of the Nanjing University School's double first-class project, quickly established the sample preparation conditions under high temperature and pressure, and put them into use to make this new result. It must be pointed out that the current materials are synthesized under high pressure, and the resistance and magnetization results are measured under normal pressure. Its properties indicate that the superconductor is extremely stable under normal pressure, which provides the application of the superconductor. may. But there is still a long way to go to achieve true large-scale applications. First, it is best to synthesize the sample under low or normal pressure. In addition, the group is also trying to make this superconductor into a thin film. The group is still conducting in-depth research on non-toxic superconductors with critical temperatures above 100K, hoping to make new progress in promoting the application of superconductors in the liquid nitrogen temperature region.