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Permanent magnet with MgB2 bulk superconductor  [PDF]
A. Yamamoto,A. Ishihara,M. Tomita,K. Kishio
Physics , 2014, DOI: 10.1063/1.4890724
Abstract: Superconductors with persistent zero-resistance currents serve as permanent magnets for high-field applications requiring a strong and stable magnetic field, such as magnetic resonance imaging (MRI). The recent global helium shortage has quickened research into high-temperature superconductors (HTSs) materials that can be used without conventional liquid-helium cooling to 4.2 K. Herein, we demonstrate that 40-K-class metallic HTS magnesium diboride (MgB2) makes an excellent permanent bulk magnet, maintaining 3 T at 20 K for 1 week with an extremely high stability (<0.1 ppm/h). The magnetic field trapped in this magnet is uniformly distributed, as for single-crystalline neodymium-iron-boron. Magnetic hysteresis loop of the MgB2 permanent bulk magnet was detrmined. Because MgB2 is a simple-binary-line compound that does not contain rare-earth metals, polycrystalline bulk material can be industrially fabricated at low cost and with high yield to serve as strong magnets that are compatible with conventional compact cryocoolers, making MgB2 bulks promising for the next generation of Tesla-class permanent-magnet applications.
Magnetic levitation force between a superconducting bulk magnet and a permanent magnet  [PDF]
J. J. Wang,C. Y. He,L. F. Meng,C. Li,R. S. Han,Z. X. Gao
Physics , 2002, DOI: 10.1088/0953-2048/16/4/318
Abstract: The current density in a disk-shaped superconducting bulk magnet and the magnetic levitation force exerted on the superconducting bulk magnet by a cylindrical permanent magnet are calculated from first principles. The effect of the superconducting parameters of the superconducting bulk is taken into account by assuming the voltage-current law and the material law. The magnetic levitation force is dominated by the remnant current density, which is induced by switching off the applied magnetizing field. High critical current density and flux creep exponent may increase the magnetic levitation force. Large volume and high aspect ratio of the superconducting bulk can enhance the magnetic levitation force further.
Superconducting Properties of Highly Dense MgB2 Bulk Materials  [PDF]
G. Giunchi,S. Ceresara,L. Martini,V. Ottoboni,S. Chiarelli,M. Spadoni
Physics , 2002,
Abstract: Highly dense MgB2 policrystalline bulk materials, obtained by reactive liquid infiltration, have been characterized in their superconducting transport and magnetic properties in magnetic field and in a temperature range of interest for the MgB2 compound. The products have a granular morphology of composite nature and their overall density affects their properties. The irreversibility line, Bc1(T) values, magnetically measured current densities and the trapped magnetic field are evaluated by susceptibility curves and hysteresis curves, both obtained by SQUID magnetometry for samples of different density, up to 35K and up to 5T applied magnetic field. The experimental results have been compared with the best values reported in the literature for other bulk sintering techniques.
Effect of Carbon-Doping in Bulk Superconducting MgB2 Samples  [PDF]
M. Paranthaman,J. R. Thompson,D. K. Christen
Physics , 2001, DOI: 10.1016/S0921-4534(01)00424-5
Abstract: Bulk superconducting samples of MgB2 were prepared by solid state reaction of stoichiometric quantities of Mg turnings and B in a sealed Ta cylinder at 890 C for 2 hours. The as-synthesized MgB2 samples had a Tc of 39 K, as defined as the onset of diamagnetism. The crystal symmetry was found to be hexagonal with lattice parameters, a=3.0856 A, and c=3.5199 A, similar to the literature values. To study the effect of carbon doping in MgB2, various C-containing samples of x varying from 0 to 1.00 in MgB2-xCx were prepared. Magnetic characterizations indicate that the Tc onset is same for pure and C-doped samples for x = 0.05, and 0.10. However, the shielding signal decreased monotonically with C content, apparently due to the presence of carbon on the grain boundaries that isolates grains and prevents flow of supercurrents on the perimeter.
Study of flow fractionation characteristics of magnetic chromatography utilizing high-temperature superconducting bulk magnet
Satoshi Fukui, Yoshihiro Shoji, Jun Ogawa, Tetsuo Oka, Mitsugi Yamaguchi, Takao Sato, Manabu Ooizumi, Hiroshi Imaizumi and Takeshi Ohara
Science and Technology of Advanced Materials , 2009,
Abstract: We present numerical simulation of separating magnetic particles with different magnetic susceptibilities by magnetic chromatography using a high-temperature superconducting bulk magnet. The transient transport is numerically simulated for two kinds of particles having different magnetic susceptibilities. The time evolutions were calculated for the particle concentration in the narrow channel of the spiral arrangement placed in the magnetic field. The field is produced by the highly magnetized high-temperature superconducting bulk magnet. The numerical results show the flow velocity difference of the particle transport corresponding to the difference in the magnetic susceptibility, as well as the possible separation of paramagnetic particles of 20 nm diameter.
Superconducting Microwave Cavity Made of Bulk MgB2  [PDF]
G. Giunchi,A. Agliolo Gallitto,G. Bonsignore,M. Bonura,M. Li Vigni
Physics , 2006, DOI: 10.1088/0953-2048/20/4/L03
Abstract: We report the successful manufacture and characterization of a microwave resonant cylindrical cavity made of bulk MgB2 superconductor (Tc = 38.5 K), which has been produced by the Reactive Liquid Mg Infiltration technique. The quality factor of the cavity for the TE011 mode, resonating at 9.79 GHz, has been measured as a function of the temperature. At T = 4.2 K, the unloaded quality factor is 2.2x10^5; it remains of the order of 10^5 up to T ~ 30 K. We discuss the potential performance improvements of microwave cavities built from bulk MgB2 materials produced by reactive liquid Mg infiltration.
Improved Superconducting Properties in Bulk MgB2 Prepared by High Energy Milling of Mg and B Powder  [PDF]
Y. F. Wu,Y. F. Lu,G. Yan,J. S. Li,Y. Feng,H. P. Tang,S. K. Chen,H. L. Xu,C. S. Li,P. X. Zhang
Physics , 2006, DOI: 10.1088/0953-2048/19/11/021
Abstract: The MgB2 bulks were prepared by high energy milling of Mg and B powder. The correlations among milling times, microstructure and superconducting properties were investigated in MgB2 bulks. Samples were characterized by x-ray diffraction (XRD), energy dispersive spectrometry (EDX) and scanning electron microscope (SEM), and the magnetization properties were examined by a Superconducting quantum interfere device (SQUID) magnetometer. It showed that the high energy milling is an effective approach to get fine crystalline (40-100nm) bulk MgB2 with good grain connectivity and high Jc performance. The critical current density reaches to 2.0X10E6A/cm2 at 15K and 0.59T, 5.7X10E5A/cm2 at 2T and 3.0X10E4A/cm2 at 5T.
Effect of synthesis temperature on superconducting properties of n-SiC added bulk MgB2 superconductor  [PDF]
Arpita Vajpayee,R. Jha,A. K. Srivastava,H. Kishan,M. Tropeano,C. Ferdeghini,V. P. S. Awana
Physics , 2011, DOI: 10.1088/0953-2048/24/4/045013
Abstract: We study the effect of synthesis temperature on the phase formation in nano(n)-SiC added bulk MgB2 superconductor. In particular we study: lattice parameters, amount of carbon (C) substitution, microstructure, critical temperature (Tc), irreversibility field (Hirr), critical current density (Jc), upper critical field (Hc2) and flux pinning. Samples of MgB2+(n-SiC)x with x=0.0, 0.05 & 0.10 were prepared at four different synthesis temperatures i.e. 850, 800, 750, and 700oC with the same heating rate as 10oC/min. We found 750oC as the optimal synthesis temperature for n-SiC doping in bulk MgB2 in order to get the best superconducting performance in terms of Jc, Hc2 and Hirr. Carbon (C) substitution enhances the Hc2 while the low temperature synthesis is responsible for the improvement in Jc due to the smaller grain size, defects and nano-inclusion induced by C incorporation into MgB2 matrix, which is corroborated by elaborative HRTEM (high-resolution transmission electron microscopy) results. We optimized the the Tc(R=0) of above 15K for the studied n-SiC doped and 750 0C synthesized MgB2 under 140 KOe field, which is one of the highest values yet obtained for variously processed and nano-particle added MgB2 in literature to our knowledge.
Superconducting Properties of adipic acid doped Bulk MgB2 Superconductor  [PDF]
Arpita Vajpayee,V. P. S. Awana,G. L. Bhalla,A. K. Nigam,H. Kishan
Physics , 2008, DOI: 10.1088/0953-2048/22/1/015016
Abstract: We report the effect of adipic acid (C6H10O4) doping on lattice parameters, microstructure, critical temperature (Tc), current density (Jc), and irreversibility field (Hirr) for MgB2 superconductor. Actual carbon (C) substitution level for boron (B) is estimated to be from 0.40 percent to 2.95 percent for different doping levels. A reduction in Tc from 38.43 to 34.93 K and in lattice parameter a from 3.084(3) A to 3.075(6) Ais observed for the10 wt percent C6H10O4 doped sample in comparison to pristine MgB2. This is an indication of C substitution at boron sites, with the C coming from the decomposition of C6H10O4 at the time of reaction. Interestingly the doped samples have resulted in significant enhancement of Jc and Hirr. All the doped samples exhibit the Jc value of the order of 10^4 A/cm2 at 5 K and 8 T, which is higher by an order of magnitude as compared to undoped sample. This result indicates that C6H10O4 is a promising material for MgB2 for obtaining the excellent Jc values under higher magnetic fields.
Superconducting screening on different length scales in high-quality bulk MgB2 superconductor  [PDF]
J. Horvat,S. Soltanian,A. V. Pan,X. L. Wang
Physics , 2004, DOI: 10.1063/1.1790069
Abstract: High quality bulk MgB2 exhibit a structure of voids and agglomeration of crystals on different length-scales. Because of this, the superconducting currents percolate between the voids in the ensuing structure. Magnetic measurements reveal that the superconducting currents circulate on at least three different length-scales, of ~1 micrometre, ~10 micrometre and whole of the sample (~millimetre). Each of these screenings contributes to the measured irreversible magnetic moment (Dm). The analysis of the field dependence of Dm for samples of subsequently decreasing size showed that the critical current obtained using the simple critical state model is erroneous. This leads to the artefact of the sample size-dependent critical current and irreversibility field. Our data analysis enables the separation of the contribution of each of the screening currents to Dm. The field dependence of each of the currents follows a stretched exponential form. The currents flowing around whole of the sample give a dominant contribution to Dm in the intermediate fields (1T < H < 4T at 20K) and they can be used to obtain the value of Jc from critical state model, which corresponds to the transport Jc.
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