Ashkan Motaman

A comprehensive study of the pinning mechanisms of MgB2 wires treated with malic acid and their relationships with lattice defects

The effects of sintering temperature on the lattice parameters, structural strain, critical temperature (Tc), critical current density (Jc), irreversibility field (Birr), upper critical field (Bc2), and resistivity (ρ) of MgB2 wires treated with 10 wt.% malic acid (C4H6O5) are investigated in this paper. The a-lattice parameter of the sample treated with malic acid was drastically reduced, to 3.0745 A, as compared to those for the undoped samples. Reduction in the a-lattice parameter is related to crystalline imperfections arising from carbon substitution—as confirmed by x-ray diffraction and Raman spectra—which play a vital role in enhancing Jc, Bc2 and Birr. We have also analyzed the pinning mechanisms, and concluded that flux pinning is dominated by point and correlated pinning at lower and higher magnetic fields, respectively, for the carbon-doped samples sintered at both 700 and 900 ° C. The degree of flux pinning enhancement and the ratio RHH (Bc2/Birr) have been estimated to guide us towards further enhancement of Jc. We argue that δl and δTc pinning mechanisms, based on variation of the mean free path (l) and the critical temperature, respectively, coexist in the MgB2 wires treated with malic acid, regardless of the sintering temperature. The δl pinning is predominant at lower operating temperatures, and δTc pinning starts close to Tc, which means that spatial variation in the charge carrier mean free path is the main mechanism responsible for the flux pinning in the MgB2 wires treated with malic acid that were sintered at 700 and 900 ° C.

The roles of CHPD: superior critical current density and n-value obtained in binary in situ MgB2 cables

A binary magnesium diboride (MgB2) cable has been assembled by braiding six Nb/Monel sheathed monofilament strands around a central copper stabilizer for improving the operational environment. The total critical current (Ic) of the braided cable is obtained by multiplying the Ic of six single wires, without any dissipation. In this work, various mechanical deformations, i.e., swaging, two-axial rolling, groove rolling, and cold high-pressure densification (CHPD) at 1.8 GPa have been applied to the 6-stranded cable to obtain additional densification. The highest critical current density at both 4.2 and 20 K has been achieved in this work through the CHPD treated cable due to higher filament mass density. The present results are promising in view of the cable, particularly in power applications at industrial lengths that pave the way to seeking an optimal protocol to meet a practical functionality.

Synergetic Combination of LIMD With CHPD for the Production of Economical and High Performance

We propose an economical fabrication concept, the localized internal magnesium diffusion (IMD) method. Instead of using a single magnesium (Mg) rod in the center of a metal sheath tube, we use large-sized Mg particles (20-50 mesh) mixed well with cheap 97% crystalline boron powder to fill the metal sheath tube. After a repeated drawing process, the coarse Mg is elongated along the core wire axis of the metal sheath tube. Textured MgB2 grains are then formed during the sintering process. In the localized IMD process, however, there is still a need to improve the overall density. In order to increase the density of the composite, a modified cold high pressure densification (CHPD) technique has been applied before the reaction. It is found that the critical current density (Jc) of the sample made from large-sized Mg with crystalline boron powder and treated by CHPD is increased significantly, so that it is quite comparable with the Jc values of samples made from expensive small magnesium and nanosized amorphous boron powder. At 4.2 K and 8 T, the Jc value of the wire in this work with the cheapest starting materials reaches 10 000 A/cm2 , which is similar to reported values for samples made by the powder-in-tube and IMD processes with expensive nanosized amorphous boron powder. A possible mechanism is proposed, and the microstructure is analyzed to explain this interesting feature. The main goal of this work is to develop a novel and cost-effective fabrication technique by combining the localized IMD process with CHPD and using cheap crystalline boron powder to manufacture MgB2 superconductor wires with electromagnetic performance superior to that of low-temperature Nb-Ti superconductors.

\hbox{MgB}_{2}

Undoped and carbon doped magnesium diboride (MgB2) cables have been assembled by braiding six Nb/Monel and Nb/Cu/stainless steel (SS) sheathed mono- and multifilament strands with a central copper stabilizer for improving the operational environment. This paper presents the fabrication and characterization of two types of in situ powder-in-tube processed mono (pure) and multifilament (carbon doped) MgB2 cables with different twist pitch lengths; thereby making them possible candidates for industrial AC applications. Critical current is not influenced by the cabling that results in various twist lengths. The total critical current of the braided cables is obtained by multiplying the critical current of six single wires without any dissipation. The critical current density (Jc) of pure (mono) and carbon doped (18-filament) six stranded cable reached 10000 A/cm2 at 5.5 and 10 T, respectively; without any observable deleterious effect caused by varying the twist pitch. The engineering current density (Je) of both cables reached the same value of 10000 A/cm2 at 3.5 and 6.5 T, respectively. Compared to the literature, this work reports some of the highest Jc and Je values for carbon doped multifilament cables that remain unaffected upon varying the twist pitch. The present results are promising in terms of scaling up these cables to industrial lengths for transformers, fault-current limiters-based applications and paves the way for the development of optimal protocols for practical functionality.