The highest critical transition temperature (
MgB2 is the first superconductor to be proved to have two distinct superconducting gaps in its superconducting state . Initially, an unconventional exotic superconducting mechanism was suggested for the material [5, 6]. Then, other researchers proposed hole superconductivity, which is similar to what occurs in high temperature superconductors (HTS), based on the fact that holes are the dominant charge carriers in the normal state [7, 8]. MgB2 has now been accepted as a phonon-mediated BCS type superconductor. The superconductivity is attributed to selective coupling between specific electronic states and specific phonons, such as the
MgB2 is easy to make into bulk, wire, tape, and thin film forms. However, the critical current density (
The depairing current density,
where is the flux quantum, the permeability of vacuum, the penetration depth, and the coherence length . However, it is not the theoretical maximum . With the help of optimized pinning, about 15% of
The grain boundaries in MgB2 do not show the weak link effect, and clean grain boundaries are not obstacle to supercurrents [19, 20]. On the other hand, dirty grain boundaries do potentially reduce the critical current . Insulating phases on the grain boundaries, such as MgO, boron oxides  or boron carbide , normal conducting phases , porosity, and cracks , can further reduce the cross-section effective of supercurrents. The high porosity in
The concept of connectivity,
The reaction kinetics between magnesium and boron can be modified by chemical or compound dopants , which influence the grain shape and size [67, 68], the secondary phases , MgB2 density , and the element stoichiometry . Carbon doping is one of the most promising methods to improve the superconducting performance of MgB2. The carbon sources include B4C [72, 73], carbon [52, 66, 67, 74], carbon nanotubes [75-78], nanodiamonds [78, 79], NbC , SiC [41, 51, 57, 66, 72, 81-89], and organic compounds [39, 47, 90]. SiC is one of the most promising dopants because it can react with magnesium and boron to form C doped MgB2 at quite low temperatures (600 oC), based on the dual reaction model . Higher processing temperatures are necessary for most of the other carbon sources, leading to grain growth and worse pinning. Comparable results to those with SiC have also obtained, however, with nanoscale carbon powder , stearic acid , and carbon nanotubes .
2. Thermal-strain-induced high
J c in high density SiC-MgB2 bulk
The connectivity is considered to be a critical issue for improving the
Crystalline B with 99.999% purity was pressed into pellets or mixed with 10wt% SiC particles and then pressed into pellets. The pellets were sealed in iron tubes and padded with 99.8% Mg powder. The Mg to B atomic ratio was 1.15:2.0. The diffusion process is time dependente. The sintering condition were 1123 K for 10 h under a flow of high purity argon gas to achieve fully reacted MgB2 bulks. Then the samples were cooled down to room temperature. X-ray diffraction (XRD) was employed to characterize the phases, and the results were refined to determine the
The density of the pure MgB2 sample is about 1.86 g/cm3, which is about 80% of the theoretical density. This value is much higher than those of the samples made by the
To explain the abnormal
The unreacted SiC buried in the MgB2 matrix is believed to be one of the most effective sources of strain, and the strongly connected interfaces of SiC and MgB2 are the most effective flux pinning centers. The micro morphologies can be detected using TEM to explore the defects and grain boundaries both in the pure MgB2 and in the SiC-MgB2. Figures 3(a) shows a bright field image of pure MgB2. A high density of defects, such as dislocations and lattice distortion, is observed in the MgB2 phase, and the grain size is about 100 nm,as estimated from the grain boundaries. In contrast to with the highly porous structure in the MgB2 samples , the samples made by the diffusion process are well connected with high density. The indexed selected area diffraction (SAD) image shows very pure polycrystalline MgB2. A high resolution grain boundary image is shown in Figure 3(b). The interface is very clean and well connected. The indexed fast Fourier transform (FFT) pattern indicates that the right part parallels the (1 1 0) plane. The micro structure of SiC-MgB2 is similar with that of pure MgB2 with high density of defects. Furthermore, nanosize SiC particle are detected in the MgB2 matrix as indicated in Figure 3(c). Figure 3(d) shows the interface of SiC and MgB2. Based on the FFT analysis, the interface is marked with a dashed line on the image. The left side is a SiC grain paralleling the (1 0 1) plane and the right side is a MgB2 grain paralleling the (0 0 1) plane. This kind of interface will impose tensile stress along the
Based on the collective pinning model, ,
To investigate whether the lattice strain is significant in SiC-MgB2 during low temperature measurements to obtain
It should be noted that the broadened
In summary, the thermal strain originating from the interface of SiC and MgB2 is one of the most effective sources of flux pinning centers to improve the supercurrent critical density. The weak temperature dependence of the thermal expansion coefficient of SiC stretches the MgB2 lattice as the temperature decreases. The thermal strain supplies much more effective flux pinning force than the interfaces and grain boundaries themselves. The low temperature effects on Raman spectra include very strong lattice stretching at the application temperature of MgB2, which has benefits from both the
3. High connectivity MgB2 wires fabricated by combined
in-situ/ ex-situ process
The self-field critical current density,
MgB2 wires were fabricated by the powder-in-tube (PIT) process using a ball-milled mixture of Mg (99%) and amorphous B (99%). The
The phases and microstructure were characterized by XRD (D/max-2200) and field emission gun scanning electron microscopy (FEG-SEM: JSM-6700F) at room temperature. The superconducting properties were detected from 5 K to 305 K using a Physical Properties Measurement System (PPMS: Quantum Design). The critical superconducting transition temperature,
According to the indexed XRD patterns, the samples contain a small amount of MgO. The MgO contents are high in 950in, 1050in, and 1050exin compared with the other samples. The broad transition widths from the normal state to the superconducting state of these samples confirm the high impurity contents, as shown in the insets of Figure 7. The 1050in transition width is ~7 K compared with the width of ~2 K for the other samples, which is attributed to the degraded connectivity of the magnetic flux due to the high MgO content. The
Typical SEM images of the
The crystal shapes for the
Figure 10 compares the
A practical quantity to evaluate the connectivity is the active area fraction,
is the resistivity of fully connected MgB2 without any disorder , and
Figure 11 compares the
In summary, both connectivity and disorder show strong influences on the
4. Nano-SiC doped MgB2 wires made by combined
In-Situ/ Ex-Situ process
The powder-in-tube (PIT) process was employed to make practical MgB2 wires from a ball-milled mixture of Mg (99%), B (99%, amorphous), and SiC (< 15 nm).]The sample fabrication and characterization are similar to the techniques mentioned for the pure samples in the last section.
Figure13 shows the XRD patterns of the two batches of samples. According to the indexed XRD patterns, all samples show quite high purity of MgB2, with only small amounts of MgO and un-reacted Mg and SiC. The un-reacted Mg can be detected because of the high content of SiC in the raw materials [104, 115, 143]. The most interesting phase change relates to the change in the Mg2Si content with sintering temperature. 750in shows very high Mg2Si content, which decreases with increasing sintering temperature and becomes a trace peak in 1050in. However, more than a trace of Mg2Si can only be found in samples sintered at lower temperature using the combined
The critical transition temperatures (
The strength of the pinning force can be reflected by the dependence of
In conclusion, high sintering temperature can improve the critical current density of small-particle-size SiC doped MgB2. The two-step reactions between Mg, SiC, and B release free C and Si to form strong flux pinning centers. The current carrier density and flux pinning force are important factors in the improvement of the
The diffusion method can greatly improve the critical current density compared with the normal technique, which indicates that the critical current density greatly depends on the connectivity of MgB2 grains. The combined process improves the connectivity of MgB2 grains and the compactness of the superconducting core in wires, which induces high critical current density in zero field. The flux pinning force can also be improved by dopants for magnetic field application. Further research could focus on parameter optimization of the combined process to fabricate high quality MgB2 wires.
The authors thank Dr. T. Silver for her useful discussions. This work is supported by the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Australian Research Council (project ID: DP0770205), and Hyper Tech Research Inc.
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