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Strong influence of precursor powder on the critical current density of Fe-sheathed MgB2 tapes  [PDF]
Dongliang Wang,Yanwei Ma,Zhengguang Yu,Zhaoshun Gao,Xianping Zhang,K. Watanabe,E. Mossang
Physics , 2007, DOI: 10.1088/0953-2048/20/6/015
Abstract: The effect of the quality of starting powders on the microstructure and superconducting properties of in-situ processed Fe-sheathed MgB2 tapes has been investigated. Three different types of commercial atomized spherical magnesium powder and two different purities of amorphous boron powder were employed. When using the 10-micrometre magnesium as precursor powders, the Mg reacted with boron more uniformly and quickly, thus the uniformity of the fabricated MgB2 was improved and the grain size of the MgB2 was decreased, hence significant critical current density (Jc) enhancements were achieved for MgB2 tapes. Jc at 4.2 K for MgB2 tapes made from the 10 um Mg and high purity boron powders was at least a factor of ten higher than values measured for MgB2 samples made from all other starting powders. At 20 K, 5 T, the typical Jc values of the tapes were over 1.0x10^4 A/cm^2 and were much better than those of tape samples reported recently.
A novel process to produce amorphous nano-sized B useful for MgB2 phase synthesis  [PDF]
Maurizio Vignolo,Gennaro Romano,Cristina Bernini,Alberto Martinelli,Antonio Sergio Siri
Physics , 2014, DOI: 10.1109/TASC.2012.2190510
Abstract: In this paper we report a new synthesis route to produce boron powders characterized as being amorphous and having very fine particle size. This route has been developed to improve the performances of superconducting MgB2 powders, which can be directly synthesized from this nano-structured boron precursor by following the ex-situ or the in-situ P.I.T. method during the manufacturing of tapes, wires and cables. All the procedure steps are explained and the chemical-physical characterization of the boron powder, using x-ray diffraction (Xrd), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques, is reported. Furthermore, a comparison with commercial boron is given. Preliminary results of the magnetic and electrical characterization, such as the critical temperature (TC) and the transport critical current density (JC t), for the MgB2 tape are reported and compared with the tape prepared with commercial boron.
Influence of Strand Design, Boron Type, and Carbon Doping Method on the Transport Properties of Powder-in-Tube MgB2-XCX Strands  [PDF]
Y. Yang,M. Susner,M. D. Sumption,M. Rindfleisch,M. Tomsic,E. W. Collings
Physics , 2011, DOI: 10.1109/TASC.2012.2184539
Abstract: The transport properties of a number of MgB2 strands have been investigated in terms of their response to strand design, starting B powder choice, and the approach to C doping used. The strands had various designs, specifically; (i) several chemical barriers were introduced, viz: Fe and Nb, (ii) the strands were encased in various outer-sheath materials, viz.: monel, Cu+monel, monel+glidcop, Nb+monel, (iii) the filament counts were varied (1, 18, and 36), and (iv) the final strand diameter was varied. In addition, for a subset of the strand designs several B powder and C-dopant types were investigated. Specifically, two types of amorphous B powder were used: (i) Moissan based "Tangshan boron" (ii) "SMI-boron" which is produced in a plasma torch by the reduction-by-hydrogen of BCl3. Two approaches to C doping were taken: (i) "malic-acid treatment" in which C is introduced into the B powder precursor by the moderate temperature drying out a slurry of B mixed in with a malic-acid-toluene solution (during which the malic acid decomposes leaving C as the only solid residue) before the Mg powder is mixed in; (ii) direct C doping of the SMI-produced B by introducing a known percentage of CH4 into the plasma flame. Critical current densities, Jc, were measured on 1.5 m long samples at 4.2 K in fields of up to 14 T; of all the strands measured, that doped with SMI-C at a nominal 4 mol% C (in relation B) yielded the highest Jc values e.g 1.1x105 A/cm2 at 7 T, 4.5x104 at 10 T, and 2.2x104 A/cm2 at 12 T. The n-values are given for all strands at 5 and 10 T, and for a certain set of strands the magnetic field dependencies of the n-values and the influence of C-doping is presented. Finally we demonstrate that, over a wide range of B, log(Jc) decreases linearly with B with a slope -{\alpha} such that the Jc(B) of any strand can be parameterized in terms of {\alpha} and its zero-field intercept Jc(B=0).
Large diameter multiwall nanotubes of MgB2: structural aspects and stability of superconducting nanotubular magnesium boride  [PDF]
P. Banacky,P. Noga,V. Szocs,J. Noga
Physics , 2014, DOI: 10.1002/pssb.201552027
Abstract: Based on a theoretical study, we demonstrated that magnesium boride nanotubes can reach the same stability as bulk MgB2 structure. However, most stable nanotubular forms are not structurally derived from mixed triangular-hexagonal structural motifs of a single layer sheet, which is thought to be the 2D precursor form of all boron nanotubes. MgB2 multiwall nanotubular structures that are derived from multilayer MgB2 slabs with honeycomb B-networks in hexagonal lattices are more stable. The results of an ab initio study of multilayer slabs of MgB2 show that a 25-layer slab approaches the stability of bulk MgB2. The critical parameter of the corresponding multiwall nanotubes is the inner diameter; the calculated value is ~ 32.6nm, which is independent of the number of walls. The outer diameters of 25-wall nanotubes are ~ 51 nm, and terminal Mg atoms are located on the outer surfaces of the nanotubes. The electronic band structures of MgB2-multiwall nanotubes (MgB2MWNT) correspond to the band structure character and topology of superconducting bulk MgB2. The results confirm that the quasi-1D superconductor MgB2MWNT is a stable structure and can be synthesized.
Magnetic characterization of sintered MgB2 samples: effect of the substitution or doping with Li, Al and Si  [PDF]
M. R. Cimberle,M. Novak,P. Manfrinetti,A. Palenzona
Physics , 2001, DOI: 10.1088/0953-2048/15/1/308
Abstract: Powdered and sintered MgB2 samples have been characterized through magnetic measurements performed from T = 5 K up to few degrees above the transition temperature of about 39 K. We found that the sintered samples behave as well-connected bodies, showing no trace of granularity. In order to obtain the critical current density value Jc the Critical State Model has been therefore employed in a straightforward way. With the aim either to decrease the electron mean free path or to increase its Jc we have attempted to introduce defects in the MgB2 structure by different procedures: substitution of Lithium on the Magnesium site and doping of the precursor Boron powders with Aluminum and Silicon. The best result in terms of Jc has been achieved by Silicon doping that, moreover, does not significantly affect the transition temperature.
Large irreversibility field in nanoscale C-doped MgB2/Fe tape conductors  [PDF]
Yanwei Ma,Xianping Zhang,S. Awaji,Zhaoshun Gao,Dongliang Wang,Zhengguang Yu,G. Nishijima,K. Watanabe
Physics , 2006, DOI: 10.1088/0953-2048/20/3/L01
Abstract: We investigated the effect of nanoscale-C doping on the critical current density Jc and irreversibility field Birr of Fe-sheathed MgB2 tapes prepared by the in-situ powder-in-tube method. The tapes were heat treated at 600-950C for 1 h. Higher values of Jc and Birr were seen for 5 at.%C-doped MgB2 tapes at higher sintering temperatures, where substantial substitution of boron for carbon occurred. The C-doped samples sintered at 950C showed the highest Birr, for example, at 4.2 K, the Birr reached 22.9 T. In particular, at 20 K, Birr for the C-doped tape achieved 9 T, which is comparable to the upper critical field of the commercial NbTi at 4.2 K. This role of nano-sized C particles can be very beneficial in the fabrication of MgB2 tapes for magnetic resonance imaging applications at 20 K.
Strongly connected ex-situ MgB2 polycrystalline bulks fabricated by solid-state self-sintering  [PDF]
Hiroya Tanaka,Akiyasu Yamamoto,Jun-ichi Shimoyama,Hiraku Ogino,Kohji Kishio
Physics , 2012, DOI: 10.1088/0953-2048/25/11/115022
Abstract: We have investigated the microstructure, normal-state electrical connectivity, and critical current density of ex-situ MgB2 polycrystalline bulks prepared by systematically varying the sintering conditions under low pressure. Samples heated at a high temperature of ~900{\deg}C for a long period showed an increased packing factor, a larger intergrain contact area, and a significantly enhanced electrical connectivity, all of which indicate solid-state self-sintering of MgB2. Sintered ex-situ MgB2 bulks from a laboratory-made ball-milled powder exhibited a greatly enhanced connectivity of 28%, which is the highest connectivity of pressureless ex-situ MgB2 bulks, wires, and tapes. Surprisingly, grain growth did not occur during long-duration (~100 h) sintering in the sintered ex-situ MgB2 bulks. This is in marked contrast to in-situ processed MgB2 samples for which significant grain growth occurred during heat treatment at ~900{\deg}C, producing grains that are several tens of times larger than the initial boron grains. Consequently, the critical current density as a function of the external magnetic field at 20 K progressively improved with sintering due to the relatively small grain size and good intergrain connectivity. We thus conclude that solid-state self-sintering is an effective approach for producing strongly connected, dense ex-situ MgB2 polycrystals without grain growth.
Transport properties of c-oriented MgB2 thin films grown by Pulsed Laser Deposition  [PDF]
C. Ferdeghini,V. Ferrando,G. Grassano,W. Ramadan,V. Braccini,M. Putti,P. Manfrinetti,A. Palenzona
Physics , 2001, DOI: 10.1016/S0921-4534(02)00990-5
Abstract: The electronic anisotropy in MgB2, which arises from its layered crystal structure is not completely clear until now. High quality c-oriented films offer the opportunity of studying such property. MgB2 thin films were deposited by using two methods both based on room temperature precursor deposition (by Pulsed Laser Ablation) and ex-situ annealing in Mg atmosphere. The two methods differ for the starting targets: stoichiometric MgB2 in one case and Boron in the other. The two films presented in this paper are grown by means of the two techniques on MgO substrates and are both c-oriented; they present TC values of 31.5 and 37.4 K respectively. Upper critical field measurements, up to 9T, with the magnetic field in perpendicular and parallel directions in respect to the film surface evidenced anisotropy ratios of 1.8 and 1.4 respectively. In this paper we will discuss this remarkable and surprising difference also in comparison with the literature data.
Growth methods of c-axis oriented MgB2 thin films by pulsed laser deposition  [PDF]
V. Ferrando,S. Amoruso,E. Bellingeri,R. Bruzzese,P. Manfrinetti,D. Marre',R. Velotta,X. Wang,C. Ferdeghini
Physics , 2002, DOI: 10.1088/0953-2048/16/2/320
Abstract: High quality MgB2 thin films have been obtained by pulsed laser deposition both on MgO and on Al2O3 substrates using different methods. In the standard two-step procedure, an amorphous precursor layer is deposited at room temperature starting both from stoichiometric target and from boron target: after this first step, it is annealed in magnesium atmosphere in order to crystallize the superconducting phase. The so obtained films show a strong c-axis orientation, evidenced by XRD analysis, a critical temperature up to 38 K and very high critical fields along the basal planes, up to 22T at 15K. Also an in situ one step technique for the realization of superconducting MgB2 thin films has been developed. In this case, the presence of an argon buffer gas during deposition is crucial and we observe a strong dependence of the quality of the deposited film on the background gas pressure. The influence of the Ar atmosphere has been confirmed by time and space-resolved spectroscopy measurements on the emission spectrum of the plume. The Ar pressure modifies strongly the plasma kinetics by promoting excitation and ionization of the plume species, especially of the most volatile Mg atoms, increasing their internal energy.
High critical current density in powder-in-tube processed MgB2/Ta/Cu wire
Yong Feng,Yong Zhao,A. K. Dradhan,Lian Zhou,Xianghong Liu,Ping Ji,Pingxiang Zhang,Yuping Sun
Chinese Science Bulletin , 2002, DOI: 10.1360/02tb9060
Abstract: The magnetization of dense MgB2/Ta/Cu wires prepared by the powder-in-tube method is measured by a SQUID magnetometer. The results indicate that the critical temperature of MgB2/Ta/Cu is around 38.4 K with a sharp transition width of 0.6 K. The MgB2/Ta/Cu wire shows a strong flux pinning and the critical current density is higher than 105 A/cm2 (5 K, self-field) and 104 A/cm2 (20 K, 1 T). Also, the irreversibility field of the sample reaches 6.6 T at 5 K.
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