Influence of antimony trioxide nanoparticle doping on superconductivity in MgB 2 bulk
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In this work, antimony trioxide (Sb2O3) has been doped into MgB2 samples to act as an additive. The doping level varies from 2.5 to 15 wt%. The effects of Sb2O3 addition on the lattice parameters, critical temperature (Tc), critical current density (Jc), and upper critical field (Hc2) have been investigated in detail. It has been found that Sb2O3 doping results in a small depression in Tc. The Jc value is 2.4 103 Acm2 for the 2.5% Sb2O3-doped sample at 5 K and 8 T, which is more than two times higher than for the undoped sample. The significant Jc improvement at high fields is attributed to the Hc2 enhancement caused by the increased disorder.
I. INTRODUCTION
As one of the most promising superconductor materials,1 MgB2 has been studied extensively.2–6 On the one hand, its high critical temperature (Tc), low material cost, freedom from weak links,5,7 and multiple energy gaps6,8,9 have attracted considerable attention for practical applications. On the other hand, several problems exist that urgently need to be solved, such as its low upper critical field (Hc2) and its rapidly decreasing critical current density (Jc) under increasing magnetic field, as compared to Nb-based superconductors. Currently, from a more practical and scalable point of view, addition and substitution of nanoparticles, which cause chemical and nanostructural changes in MgB2, seem to be an effective way to induce flux pinning centers in MgB2, thereby improving Jc and Hc2.10 However, from the point of view of substitution into the lattice, only substitution of Al on Mg sites and substitution of C on B sites have proven to be possible for higher doping levels. Other substitutions, such as those involving transition metal elements (Fe, Y, Ti, Zr) in the Mg sublattice are limited to very low concentrations. In addition, compound doping with SiC,3,11,12 the most popular choice, and B4C13 is effective for enhancement of the irreversibility field (Hirr) and Jc under higher magnetic fields. Several types of oxide doping, such as with Y2O3,14 SiO2,15 TiO2,16 CeO2,17 and Nd2O318 have also shown positive effects on Jc and Hirr. The authors’ group has noted that antimony trioxide (Sb2O3) has a relatively low melting point, 655 °C, which is quite close to the melting point of Mg, 650 °C. The Mg and B reaction with added Sb2O3 dopant could involve not only Mg + B / MgB2 but also Mg + Sb2O3 / Mg3Sb2 + O2 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.255 J. Mater. Res., Vol. 26, No. 21, Nov 14, 2011
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above 655 °C. In our recent study of the effects of atmosphere on Jc of MgB2, the particular amount of O2 might play a very important role. (Results will be published elsewhere.) This is another reason why metal oxides have been attracting more attention than other possible dopants. These results have particularly attracted our interest towards codoping with Sb2O3. So, in this article, we report on a systematic study of the effects at different doping levels of the a
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