Phase evolution and microstructure of highJ_cSiC doped MgB₂fabricated by hot pressing

Abstract

We report the phase evolution, microstructure, and critical current density (Jc) of the SiC doped MgB2 superconductors. In our study, all samples were fabricated by hot pressing with a heating rate of 200 °C min−1. The results show that the reaction of 2Mg+SiC = Mg2Si+C can occur at about 500 °C, about 50 °C lower than the formation temperature of MgB2. On the other hand, according to the experimental results and thermodynamic calculations, boron (B) does not react with SiC to form B4C and Si, neither does MgB2 react with SiC to form Mg2Si and B4C. The MgB2 phase was formed via both a solid–solid reaction (during the heating process between about 500 and 650 °C) and a liquid–solid reaction (>650 °C) after the melting of Mg, and the two reactions resulted in differences in the grain size of the MgB2. From scanning electron microscopy, the Mg2Si particles are homogeneously distributed within the MgB2 matrix, with particle sizes ranging from 35 to 230 nm. From the perspective of superconductivity, the C substitution results in strong electron scattering centers in the MgB2 structure. It reduces the electron mean free path and thus may significantly enhance the magnetic Jc. The peak Jc of the 5 wt% SiC doped MgB2 reaches above 106 A cm−2 at 5 K, and decreases slowly with increasing field, remaining high, above 105 A cm−2, at 7 T.

Effect of Heat Treatments under High Isostatic Pressure on the Transport Critical Current Density at 4.2 K and 20 K in Doped and Undoped MgB2 Wires

“…Doping, e.g., SiC and C, promotes middle and high pinning centers, which increase J c in middle and high magnetic fields [7,[9][10][11][12]. SiC doping is more effective for increasing J c in high magnetic fields; however, it decreases the critical temperature (T c ) [8,13,14] and increases the irreversibility magnetic fields (B irr ) and upper magnetic fields (B c2 ) [8,10,[15][16][17]. A 2002, an investigation of SiC showed that the addition of nanometer-scale SiC can effectively increase B c2 and transport the critical current density (J ct ) at high temperatures and fields Materials 2021, 14, 5152 2 of 21 and decrease the anisotropy [11,13,18,19].…”

“…Serrano et al suggested that J c is increased by strong pinning centers [23]. The measurements indicate that nano SiC increased J c at 20 K more than other types of doping (e.g., carbon nanotubes) [23] and yielded a high irreversibility magnetic field (B irr ) of 7.3 T [16]. An investigation conducted by Li et al showed that B irr is responsible for improved J c performance in high magnetic fields [20].…”

“…An investigation conducted by Li et al showed that B irr is responsible for improved J c performance in high magnetic fields [20]. Qu et al suggested that impurities can increase or decrease J c in MgB 2 materials [16]. Moreover, Liang et al indicated that the negative effects on J c could be attributed to the absence of significant effective pinning centers (Mg 2 Si) due to the high chemical stability of crystalline SiC particles [24].…”

“…Hot pressing increases the reaction rate between Mg and B [16]. Qu et al showed that the density of the sample increases as the hot pressing temperature increases [16].…”

“…Moreover, the hot pressing process can reduce the size of pores, produce small Mg 2 Si particles (from 35 nm to 230 nm) with a homogeneous distribution [14], and increase J c from 5 K to 30 K [16]. Moreover, Qu et al showed that Mg 2 Si is formed at a temperature of 500 • C (hot pressing process) [16]. However, Mg 2 Si and MgB 2 are created only at 550 • C [16].…”