The discrepancies in different facets of MgB₂ bulk superconductors prepared under various sintering durations by spark plasma sintering

Abstract

Seven MgB2 bulk superconductors with good superconductivity and crystallinity were prepared under the pressure of 50 MPa at 950 °C and different sintering durations (15 ∼ 240 min) by spark plasma sintering (SPS), with the onset critical temperature (T c,onset) around 37.5 K. The superconducting properties and microstructure measured from the facets perpendicular (PeF) and parallel (PaF) to the compression direction of the SPSed MgB2 bulk samples were also analyzed to investigate the effect of the sintering duration on the MgB2 bulks and the discrepancy between the PeF and the PaF. The optimum performance was obtained from the sample prepared for 45 min for both facets, and the critical current density (J c) measured from the PeF and the PaF are 364 kA cm−2 and 344 kA cm−2 in self field at 20 K, respectively. The PeF exhibits better Jc performance than the PaF at low fields and the PaF exhibits better J c performance than the PeF at high fields. It can be concluded that the sintering duration affects the properties of the PeF mainly by changing their crystallite size. While for the PaF, the sintering duration mainly affects the microstructural defects, such as cracks, pores and secondary phase particles, thus further affecting their J c performances. In terms of the flux pinning mechanism of the SPSed MgB2 bulk samples, the PaF is less susceptible to sintering duration than the PeF, and the better J c performance of the PaF at high fields is due to the stronger grain boundary pinning. It can be also found that the point pinning being stronger at higher temperature is the result of the decreasing anisotropy of MgB2 bulk samples with the temperature.

Overview of spark plasma synthesis and sintering of MgB2 superconductor

“…However, the exponents p and q are always far from the theoretical values. Similar discrepancies have been mentioned by other authors in highly dense SPS bulks [30][31][32][33]. As a consequence, they have used another scaling procedure proposed by Eisterer [34], where the pinning centers can be identified by the reduced field peak position.…”

“…They can provide a large interaction force when the direction of the Lorentz force is parallel to their plane. On the other hand, volume pins typically result from large precipitates, all of which have dimensions greater than d. We have carried out this work using the method the most used in the literature [29][30][31], which consists in considering h = H/H irr , where H irr is often taken as the magnetic field for which J c is equal to 100 A cm −2 . Table 3 shows the normalized flux pinning force densities of the samples and the best reproduction of these quantities with the Dew-Hughes scaling functions.…”

Thickness Dependence of Trapped Magnetic Fields in Machined Bulk MgB2 Superconductors

“…Magnetised bulk superconductors can act as powerful trapped field magnets (TFMs) and offer significant performance advantages over conventional permanent magnets in a number of applications, including desktop high-field magnet systems and rotating machines [9]. In addition to being inexpensive and lightweight, MgB 2 exhibits a number of additional advantages: a long coherence length, lower anisotropy, strongly-linked supercurrent flow in untextured polycrystalline samples, and the relative ease of fabrication has enabled a number of different processing techniques to be developed [10]- [15]. In this work, the thickness dependence of the trapped magnetic field in bulk MgB 2 superconductors is investigated, experimentally and numerically, to understand the effect of and optimise the geometry in order to maximise their performance as permanent magnet analogues.…”

The Influence of Preparation Temperature on the Different Facets of Bulk MgB2 Superconductors

“…Since Akimitsu [ 1 ] discovered the superconductivity of MgB 2 , it had been studied extensively by scientists from all over the world [ 2 ], and researchers have processed MgB 2 into tapes, wires [ 3 ], films [ 2 , 4 ], and bulks based on in situ and ex situ methods [ 5 ]. Compared to low-temperature superconductors, MgB 2 has a higher critical temperature ( T C ) and a higher critical current density ( J C ) [ 6 , 7 , 8 ]. However, it has a lower coherence length, lower anisotropy, and a good grain boundary connection compared with high-temperature superconductors [ 9 ].…”

“…The SPS process is carried out under uniaxial pressure, so there are perpendicular (PeF) and parallel (PaF) facets to the compression direction of the uniaxial pressure. The effects on the different facets under various sintering durations [ 7 ] at 950 °C have been reported. In this study, two kinds of MgB 2 bulks are prepared at different temperatures—950 °C, marked by S1, and 975 °C marked by S2—under a uniaxial pressure of 50 MPa for 2 h using the SPS technique to analyze the properties of MgB 2 bulk samples.…”