Spark Plasma Sintered Highly-Dense MgB2 Bulks Have Improved Mechanical, Superconducting Properties

Since the discovery of the superconductivity phenomena in 1911, scientists have been on the lookout for feasible materials that reach their superconducting state at higher temperatures, enabling them to be operated with cheaper coolants or advanced cryocoolers.

Out of the many types of viable materials, MgB2 superconductors have an operating temperature of 39 K, which lies between conventional metallic superconductors and high-temperature cuprate superconductors. It occupies a unique position where its intermediate operating temperature is offset by its unique material and electric properties.

MgB2 superconductors are non-toxic, lightweight materials that can be fabricated by simple sintering processes and have current densities similar to that of conventional metallic superconductors. These properties make them suitable for applications that require cheap and lightweight superconductors such as space applications and electric machines. However, the sintering of MgB2 superconductors is performed at ambient pressures, and it results in a highly porous material that has less than optimal superconducting properties. More specifically, the porous nature of the superconductor limits the magnetic field it can trap and hold.

Now in a study published in Materials Science and Engineering: B, researchers from Shibaura Institute of Technology, Japan, enhanced the trapped magnetic field in bulk MgB2 superconductors by fabricating highly dense, non-porous MgB2 bulks. Using spark plasma sintering (SPS), the researchers heated commercially available MgB2 powder with pulsed electrical current at high pressures of 50 MPa. The results were fascinating: "It is the first study in which large, 40 mm diameter MgB2 bulks with 99.8% packing ratio were fabricated via SPS," explains Prof. Muralidhar Miryala, who led the study.

The prepared sample was of similar quality to those prepared by conventional sintering methods and had very little impurities. The high packing density was found to improve the current density and the strength of the material. On comparing their samples with dense bulks prepared via an alternative high pressure hot isostatic pressing (HIP) technique, the researchers found that the SPS bulks displayed superior strengths and had bending strengths eight times that of the HIP processed bulks.

A similar improvement in trapped field (TP) performance was noticed in the highly dense samples where their increased current density enhanced their ability to trap and hold magnetic fields. On examining the TP performance by magnetizing the samples and measuring the TP at temperatures of 14 and 20 K, the researchers found that the MgB2 bulk showed high TP for external magnetic fields up to 1.6 tesla. However, when large external magnetic fields were applied, the poor thermal conductivity of the material caused the material to overheat and the TP value to drop.

Despite its thermal limitations, the highly dense MgB2 superconductors are a significant improvement over the conventionally sintered counterparts. Commenting on the importance and novelty of their findings, Prof. Miryala explains: "The large SPS MgB2 bulks produced this way can have wide applications as superconductors and our study also paves the way for further enhancing and realizing the commercialization of these superconductors."

Indeed, space exploration and electric transport, which necessitate cheaper, lighter, and more affordable superconductors, may soon be made more efficient using these impressive MgB2 superconductors.

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