Citation: Xing, Y.; Bernstein, P.; Miryala, M.; Noudem, J.G. High Critical Current Density of Nanostructured MgB 2 Bulk Superconductor Densified by Spark Plasma Sintering. Nanomaterials 2022, 12, 2583. https://doi.org/10.3390/ nano12152583 Academic Editor: Yassine Slimani Received: 23 June 2022 Accepted: 26 July 2022 Published: 27 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article High Critical Current Density of Nanostructured MgB 2 Bulk Superconductor Densified by Spark Plasma Sintering Yiteng Xing 1,2, * , Pierre Bernstein 1 , Muralidhar Miryala 2 and Jacques G. Noudem 1 1 Normandie University, ENSICAEN, UNICAEN, CNRS, CRISMAT, 14000 Caen, France; pierre.bernstein@ensicaen.fr (P.B.); jacques.noudem@ensicaen.fr (J.G.N.) 2 Materials for Energy and Environmental Laboratory, Superconducting Materials Group, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan; miryala1@shibaura-it.ac.jp * Correspondence: yiteng.xing@ensicaen.fr Abstract: In situ MgB 2 superconducting samples were prepared by using the spark plasma sintering method. The density of the obtained bulks was up to 95% of the theoretical value predicted for the material. The structural and microstructural characterizations of the samples were investigated using X-ray diffraction and SEM and correlated to their superconducting properties, in particular their critical current densities, J c , which was measured at 20 K. Extremely high critical current densities of up to 6.75 × 10 5 A/cm 2 in the self-field and above 10 4 A/cm 2 at 4 T were measured at 20 K, indicating that vortex pinning is very strong. This property is mainly attributed to the sample density and MgB 2 nanograins in connection to the presence of MgO precipitates and areas rich in boron. Keywords: spark plasma sintering; superconductivity; MgB 2 ; nano grains; critical current density 1. Introduction MgB 2 is an intermetallic superconductor [1]. The superconductivity of this material was reported for the first time in 2001 [2]. This compound has excellent superconducting properties, especially its high critical current density [3,4] and trapped magnetic field [5], due to a strong pinning of the vortices. The voids, disorders and impurities present in the material can behave as pinning centers that hamper the vortex motion and minimize energy dissipation. Unlike cuprate superconductors [6,7], the J c values of MgB 2 can be strongly enhanced by increasing the number of grain boundaries while decreasing the grains’ size [8,9]. Although its transition temperature is relatively low for many applications (39 K), the processing conditions of MgB 2 -based materials are simpler and much cheaper than those of the cuprate superconductors. Otherwise, for applications, costs can be saved by using cryo-coolers apparatus above 4 K or developing technologies that use liquid hydrogen as a cooling fluid [10]. To prepare highly dense MgB 2 bulk superconductors, spark plasma sintering (SPS) is a very interesting technique [11,12]. It is a rapid consolidation method that results in a good understanding and control of the sintering kinetics, as reported elsewhere [11]. The heat source is not external but is an electric current (AC, DC or pulsed) that flows across the die containing the powder to sinter. Simultaneously, a uni-axial pressure is applied. The main difference between SPS and conventional or hot-pressing methods is that SPS allows for the preparation of highly dense samples with grain growth control and a decreased processing time [13–16]. Concerning MgB 2 , the samples processed by SPS additionally show excellent mechanical properties [17]. In this contribution, we report the remarkable superconducting properties of MgB 2 samples containing nanoparticles densified by SPS, especially their very high critical current density, J c . Nanomaterials 2022, 12, 2583. https://doi.org/10.3390/nano12152583 https://www.mdpi.com/journal/nanomaterials