MgB₂ materials have been developed in the last two decades for superconducting wires, bulks and electronic devices. This review article summarizes the characteristics of the MgB₂ superconductor, and the determining factors of critical current properties of polycrystalline MgB₂ materials prepared by various methods. Prospects of the MgB₂ materials are also, discussed including recent advancements.

We have studied the trapped field properties of the MgB₂ bulk superconductors fabricated by various methods. The MgB₂ bulks were magnetized by a filed-cooled magnetization (FCM) method using a superconductor coil and a pulsed field magnetization (PFM) method using a copper coil. We firstly obtained a trapped field of 1.5 T at 14 K by FCM for an MgB₂ disc bulk (30 mm diameter and 7 mm thickness) with a filling factor of about 50 %, which was fabricated by an in-situ capsule method using a home-made capsule. The trapped field of MgB₂ bulk by FCM was enhanced up to 2.9 T by the densification and the enlargement. Such dense MgB₂ bulks were fabricated by an in-situ hot isostatic pressing under the pressure of up to 980 MPa or an ex-situ spark plasma sintering by applying uniaxial stress of 50 MPa. An in-situ infiltration method without the physical pressure also produced the dense MgB₂ bulks, which trapped a 3 T-class magnetic field. The Ti-doping improved considerably the trapped field by FCM up to 5.6 T at 11.3 K for the triple-stacked Ti-doped MgB₂ bulks. The PFM method also produced a Tesla-class MgB₂ bulk magnet. The trapped field of 1.1 T at 13 K was obtained for the HIP-processed MgB₂ disc bulk (22 mm diameter and 20 mm thickness). A magnetic field of 1.6 T was trapped at 20 K in the MgB₂ bulk composite, consisting of two ring-shaped MgB₂ bulks sandwiched by four thin copper plates and an inserted soft-iron yoke cylinder, using double PFM using a split-type coil with softiron yokes. We have shown the potential of the MgB₂ bulk magnets for the practical superconducting applications.

Two important factors that strongly affect J_c of the MgB₂ wires are impurity addition and the density of the MgB₂ core. Carbon or carbon-containing impurity addition brings about carbon substitution for the boron site of MgB₂ and increases Bc2 of the MgB₂ wires and hence, the J_c values at high magnetic fields. The most popular additive, SiC, brings about the formation of impurities, such as Mg2Si, which act as barriers to the superconducting current. The additions of hydrocarbons, such as coronene (C₂₄H₁₂) and anthracene (C₁₄H₁₀), are more effective in increasing the J_cvalues of the MgB₂ wires. Another interesting hydrocarbon additives are ethyltoluene (C₉H₁₂) and xylene (C₈H₁₀). Additions of these impurities also increase the J_c values of the MgB₂ wires although the amount of carbon substitution for the boron site is much less than those of the coronene and anthracene additions. The densification of the MgB₂ core is realized by the application of high-pressure treatment, such as hot uniaxial pressing, hot isostatic pressing and cold pressing before a heat treatment. Mechanical alloying and mechanical milling of boron and magnesium powder mixture using high energy ball milling are also effective to increase the density of the MgB₂ core and the J_c values. The use of nano-meter size magnesium powder accelerates the reaction of magnesium and boron powder, improves the connectivity of the MgB₂ core and increases the J_c values. Internal magnesium diffusion (IMD) method is also effective in increasing the density of reacted the MgB₂ layer and the J_c values. A small MgB₂ coil was fabricated using a 60-mlong IMD processed 7-filamentary MgB₂/Cu wire and was successfully excited with a reasonably high coil Ic, i.e., about 60 % of Ic obtained in a short sample at 4.2 K in a backup field up to 10 T.

The MgB₂ wire has a great potential as a low-cost superconducting wire, which does not require cooling with liquid helium. We have been developing In situ MgB₂ wires, which have practical J_c and J_e at 10-20 K in a 5-T or lower magnetic field. A few prototype coils and magnets were made by the Wind&React method with km-class wires, which have a good homogeneity and a large quench energy margin. By using this margin, a magnet for klystron and a fast ramp-up MRI magnet were demonstrated successfully. An MgB₂ magnet for Finger MRI was operated in a persistent mode with a superconducting joint. For realizing MgB₂ coils made by the React&Wind method, the strain-tolerance of the reacted wire should be improved.

MgB₂ is a known superconductor material competing with the commercially popular low-temperature superconductor of Nb–Ti. The key advantages of MgB₂ include a relatively high transition temperature of 40 K, low material cost, and large upper critical field, with potential in various superconducting applications in a cryogen-free environment. This review will summarize the state-of-the-art MgB₂ various conductors fabricated by Sam Dong Co., Ltd., Republic of Korea and provide clear insights to practical users.

It is known that four kinds of MgB₂ composite wires have been commercialized. The present study aims to compare the difference of microstructure dependence of mechanical property of those wires. The volume fraction of MgB₂ filaments in the wire was designed in the range between 0.1 and 0.19. The voids existing in MgB₂ filaments distributed in the scale smaller than a few ten micron meter. The tensile test was carried out at room temperature. Young’s modulus, 0.2 % proof stress and strain were determined. The calculated Young’s modulus was larger than the observed one for all four different wires. This discrepancy was attributed to the poor sintering of MgB₂ filaments as the existence of voids. This situation was proved by the numerically calculated results. A way to realize the higher ductility MgB₂ wire was proposed. The proof stress and strain shall be enhanced by replacing to other constituent metallic element with high thermal expansion coefficient and high Young’s modulus. The further requirement is to densify MgB₂ polycrystalline filaments for reducing the void fraction.

This paper reports the AC breakdown characteristics of an ice-starch sugar mixed system under LN₂. We observed the AC breakdown voltage and solidification state of the ice-starch sugar mixed system. These results were shown as follows: (i) the AC breakdown voltage of a 15~20 wt% starch sugar sample under LN₂ is higher than that of 0 wt% (only ice) and 100 wt% (only starch sugar). (ii) the AC breakdown voltage of ice-starch sugar samples does not depend on the cooling speed and freezing time under 243 K (- 30 ℃) or 193 K (- 80 ℃) cooling. (iii) the 0 wt% starch sugar sample (only ice) has many visible voids, but the number of visible voids decreases with the increase of the starch-sugar concentration.