We evaluated the additive effects of malic acid (C 4 H 6 O 5 ), from 0 to 30 wt% of the total MgB 2 , on the lattice parameters, lattice strain, amount of carbon (C) substitution, microstructures, weight fraction of MgO, critical temperature (T c ), critical current density (J c ), and irreversibility field (H irr ) of a MgB 2 superconductor. The calculated lattice parameters show a large decrease in the a-axis lattice parameter for MgB 2 + C 4 H 6 O 5 samples from 3.0861(6) to 3.0736(1) Å, with even a 10 wt% addition. This is an indication of C substitution into boron sites, with the C coming from C 4 H 6 O 5 , resulting in enhancement of J c and H irr . Specifically, the H irr of the MgB 2 + C 4 H 6 O 5 samples prepared by the chemical solution route reached around 7 T at 20 K, with a T c reduction of only 1.5 K. In addition, the self-field J c of the MgB 2 + C 4 H 6 O 5 samples was only slightly reduced at an additive level as high as 30 wt%. However, residual oxygen after evaporation processing contributed to a large amount of MgO in our MgB 2 + 30 wt% C 4 H 6 O 5 samples. These problems can be further controlled by the amount of C 4 H 6 O 5 additive or different evaporation temperatures.

We studied the effects of sintering temperature on the phase transformation, lattice parameters, full width at half-maximum (FWHM), strain, critical temperature (Tc), critical current density (Jc) and resistivity (ρ) in MgB2/Fe wires. All samples were fabricated by the in situ powder-in-tube method (PIT) and sintered within a temperature range of 650–900 °C. It was observed that wires sintered at low temperature, 650 °C, resulted in higher Jc up to 12 T and lower Tc. The best transport Jc value reached 4200 A cm−2 at 4.2 K and 10 T. This is related to the grain boundary pinning due to small grain size. On the other hand, wires sintered at 900 °C had a lower Jc in combination with a higher Tc.

We fabricated MgB2/Fe wire by a powder-in-tube (PIT) technique, using an in situ process. All wire samples were sintered for 30 min at different sintering temperatures ranging from 650 to 1000 °C. We found strong correlations among crystallinity, critical current density (Jc), irreversibility field (Hirr), upper critical field (Hc2), and microstructures for all MgB2/Fe wires. We observed that the sample with the lowest sintering temperature, ∼ 650 °C, had a larger lattice strain, Jc, change in resistivity Δρ(ρ300 K–ρ40 K), Hirr, and Hc2, but a lower density and residual resistivity ratio (RRR). Based on the relationships between all these superconducting and microstructure parameters, grain boundaries are likely to be acting as the predominant pinning centers for MgB2, so grain growth of MgB2 corresponds to a reduction of effective pinning. It should be noted that changes in the MgO fraction within the MgB2 matrix were almost independent of the sintering temperature. This indicates that most MgO may be coming from the starting material.

In this work, we report on significantly enhanced critical current density (Jc) in MgB2 superconductor that was easily obtained by doping with a hydrocarbon, highly active pyrene (C16H10), and using a sintering temperature as low as ∼600°C. The processing advantages of the C16H10 additive include production of a highly active carbon (C) source, an increased level of disorder, and the introduction of small grain size, resulting in enhancement of Jc.

We present a detailed analysis of the effects of excess Mg on the lattice parameters, the critical temperature (Tc), the critical current (Jc), the irreversibility field (Hirr), and the upper critical field (Hc2) of MgB2 bulks and wires produced by an in situ solid-state reaction method and the powder-in-tube method, respectively. It was found that Jc, Hirr, and Hc2 were significantly enhanced for Mg excess samples. All these properties were highly sensitive to the processing temperature for Mg excess samples, while there was only a weak dependence on processing temperature for normal ones. The Tc variation for the 10% Mg excess sample was 1.6 K (36.3–37.9 K) when the sintering temperature changed from 650 to 850 °C while it only varied by 0.5 K (37.2–37.7 K) for the normal sample. The low-field Jc for the 10% Mg excess samples sintered at 750 °C increased by a factor of 3, compared to that for the normal MgB2 samples, while the Hc2 for the 10% Mg excess samples sintered at 650 °C reached 8.7 T at 25 K, compared to 6.6 T for the normal sample. Rietveld refinement x-ray diffraction (XRD) analysis showed that the MgO content was reduced in the 10% excess Mg samples, leading to an increase in the effective cross section of the superconductor.