Magnetic field processing to enhance critical current densities of MgB2 superconductors

Abstract

Magnetic field of up to 12 T was applied during the sintering process of pure MgB 2 and carbon nanotube (CNT) doped MgB 2 wires. We have demonstrated that magnetic field processing results in grain refinement, homogeneity and significant enhancement in J c (H) and H irr . The J c of pure MgB 2 wire increased by up to a factor of 3 to 4 and CNT doped MgB 2 by up to an order of magnitude in high field region respectively, compared to that of the non-field processed samples. H irr for CNT doped sample reached 7.7 T at 20 K. Magnetic field processing reduces the resistivity in CNT doped MgB 2 , straightens the entangled CNT and improves the adherence between CNTs and MgB 2 matrix. No crystalline alignment of MgB 2 was observed. This method can be easily scalable for a continuous production and represents a new milestone in the development of MgB 2 superconductors and related systems.The new superconductor, MgB 2 , has made a significant impact on the research and development of superconductors since its discovery 1 . The special feature of the two-gap superconductivity 2 and lack of week links at the grain boundaries 3 makes MgB 2 highly tolerant for doping which has been successfully used to enhance the critical current density, J c and the upper critical field, H c2 4-8 . Carbon and silicon carbide doping resulted in a significant increase of in-field J c and H c2 , and these records still stand for MgB 2 [5][6][7][8][9][10][11] . To further advance the development of MgB 2 for applications we report a new method of combining the advantages of magnetic field processing and of doping for processing MgB 2 superconductors. Magnetic field processing technology has been proved to be a powerful tool to produce aligned CNT in composites and neat macroscopic membranes 12-14 and control the phase transformation and behavior of the melts during condensation processes, resulting in major improvements in material properties 15,16 . Magnetic field processing has also been used to achieve the desired texture and improved J c performance in HTS [17][18][19][20] . In processing of MgB 2 bulk and wires the reaction in-situ technique in combination with the powder-in-tube (PIT) method has been used to produce the wires with the best field performance [21][22][23] . Other advantages of this process include easy fabrication of coils and the ability to incorporate dopants and additives, which are important for improvement of flux pinning and H c2 . In the in-situ reaction process, Mg melts before the MgB 2 formation by solid state reaction, provided the heating rate is high enough. The presence of a liquid phase provides a window of opportunity for applying a magnetic field processing technique to achieve a crystalline refinement, homogeneous distribution of additives and inclusions and possible alignment of both matrix materials and additives.In this work, a standard powder-in-tube method was used for Fe clad MgB 2 wire 23 . Powders of magnesium (99%) and amorphous boron (99%) were well mixed with 0 and 10 wt% of multi-wall ...

Role of carbon in enhancing the performance of MgB2 superconductor

“…3), are much higher than that of the substituted MgB 2-x C x samples. In the later case, the enhancement of J c (H) is entirely due to the disorder in σ band [9,12] Worth mentioning is the fact that breaking of doped nano-Carbon derivatives and ensuing substitution at B site in MgB 2 depends upon both nature of dopant and the heat treatment [13,[19][20][21]. For example, the n-SiC is more susceptible to react than n-Diamond [13,[19][20][21] at common MgB 2 synthesis temperatures.…”

SiC and carbon nanotube distinctive effects on the superconducting properties of bulk MgB2

“…Earlier attempts succeeded in raising the intragrain J c through the incorporation of Mg͑B 1−x O x ͒ 2 , 1 SiC, 2,12,13 Al, 3 Dy 2 O 3 , 4 nano-C, 5 and carbon nanotubes, 6,12,[14][15][16] which produce a decrease in the critical temperature ͑T c ͒ by chemical substitution and probably lattice strain. 17 The improvement of intergrain J c in bulk samples is related to grain connectivity 18 and may be achieved by optimizing the processing parameters ͑i.e., ultrasonication, 16 HIPing, 18 applying magnetic field during the sintering process, 19 etc.͒. The best results for H c2 ͑0͒ and J c at high field ͑H Ͼ 5 T͒ and 4 K in polycrystalline samples were achieved using C and SiC, respectively.…”

“…The synthesis temperatures were the optimal ones for each kind of doping. 5,11 In both cases ͑SiC and CNT͒ part of carbon dissolves into the MgB 2 structure during the fabrication process, 13,19 and the shift in the a-lattice parameter, obtained from x-ray diffraction, can be used as a measure of the actual amount of C ͑x͒ in the Mg͑B 1−x C x ͒ 2 structure. 22 The synthesis temperature, lattice parameters, and x values obtained from fitting the single crystal data of Kazakov et al 25 and the neutron diffraction data of Avdeev et al 27 are listed in Table I.…”

Physical property characterization of bulk MgB2 superconductor

“…In fact in recent years, n-SiC/n-diamond/n-carbontubes/n-carbon doped MgB 2 superconductor had yielded high dividends [12][13][14][15][16][17][18][19][20][21][22][23][67][68][69][70]. It is known that substitution of C at B-site in MgB 2 creates disorder in Boron plane and thus improves the superconducting performance [67][68][69][70]. This when clubbed with effective available nano-pinning centers, gives rise to high superconducting performance in applied fields [67][68][69][70][71][72].…”