Abstract
Magnesium diboride (MgB2) is a superconductor characterized by interesting properties like rather high superconducting transition temperature Tc = 39 K, long coherence length and low anisotropy. In addition, it has a very simple crystal structure and low density. Those properties make the MgB2 an ideal candidate for a wide range of applications. To improve the electromagnetic properties of MgB2, magnetic nickel-cobalt-boron (NiCoB) nanoparticles (mean grain size 17 ± 3 nm) were added to Mg and B precursor powders and sintered at 650 °C, i.e. the temperature of MgB2 superconductor formation. The nearly spherical NiCoB nanoparticles, as-prepared by the chemical reduction of metallic salts, were amorphous according to previous study. The resulting MgB2 sample, formed after the sintering at 650 °C, was subjected to detailed microstructural analysis which included the application of various experimental methods: XRD, FE-SEM, EDS, elemental mapping, TEM and SAED. The methods confirmed the formation of new crystal CoNi phase (due to heat treatment at 650C), consisting of spherical nanoparticles (~ 6 nm) with tendency to spherical agglomerates formation. Those nanosized magnetic particles (characterized by the single domain magnetic structure and blocking temperature TB below room temperature), located at MgB2 grain boundaries, could serve as effective magnetic pinning centers in MgB2, thus improving its electromagnetic properties
Highlights
T HE MgB2 is characterized with the simple AlB2 type hexagonal structure
Water used in preparation procedure of asprepared sample with NiCoB nanoparticles[14] could act as a source of oxygen, due to incompletely removing from starting solutions through the argon bubbling
S., Ref. [22]), the small amount of the B2O3 present in the starting semicrystalline boron powder produced at low plasma power, observed as a barely visible maximum at 2Θ = 15 in the corresponding X-ray powder diffraction (XRD) pattern,[22] can act as a mediator in forming the MgO
Summary
T HE MgB2 is characterized with the simple AlB2 type hexagonal structure (space group P6/mmm). The structure consists of alternating triangular planes of Mg which are separated by honeycomb-net planes of boron, with each Mg atom located at the center of hexagon formed by boron (donating its electrons to the boron planes). The structure of MgB2 has been identified very early, in 1954,[2] the superconductivity in the MgB2 was not discovered until 2001.[3] Since the discovery, extensive studies have been made covering its fundamental aspects as well as practical applications.[4,5,6] Due to rather high critical temperature Tc (39 K), possibility of operation at temperatures ≥ 20 K in liquid hydrogen or with cryocoolers and relatively low costs of raw material, the MgB2 represents a promising superconductor for next-generation superconducting application in the temperature range of 20–30 K, where the conventional superconductors cannot operate.[7]
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