Relative effects of Pb and Re doping in Hg-1223 thick films grown on Ag substrates
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P.V.P.S.S. Sastry Center for Advanced Power Systems, Tallahassee, Florida 32310
J. Schwartz National High Magnetic Field Laboratory, Tallahassee, Florida 32310; Department of Mechanical Engineering, Florida Agricultural and Mechanical University-Florida State University College of Engineering, Tallahassee, Florida 32310; and Center for Advanced Power Systems, Tallahassee, Florida 32310 (Received 12 December 2003; accepted 27 May 2004)
The relative effects of Pb and Re doping on microstructure, irreversibility field, and electronic anisotropy of HgBa2Ca2Cu3O8+␦ (Hg-1223) thick films grown on Ag substrates prepared by dip-coating were studied. Both Pb- and Re-doped films exhibit the dominant phase of Hg-1223, characterized by a superconducting transition temperature (Tc) of 133 K. Both dopants distribute homogeneously in the Hg-1223 grains and promote grain growth. Pb-doped films have larger colony size compared to the Re-doped. The irreversibility fields (Hirr) of (Hg,Re)-1223 are significantly higher than those of (Hg,Pb)-1223 at temperatures below 100 K. The logarithmic plots of Hirr versus (1 − T/Tc) show both Pb- and Re-doped have a crossover temperature reflecting a transition from two- to three-dimensional behavior with increasing temperature. Re doping significantly decreases the electronic anisotropy ␥, which would enhance flux pinning and consequently improve the critical current density. The differences between Pb and Re dopants in affecting ␥ are explained in terms of crystal structure. I. INTRODUCTION
The HgBa2Ca2Cu3O8+␦ (Hg-1223) superconductor exhibits exceptional superconducting properties, particularly above 77 K. This material has the highest critical temperature (Tc) of all known superconductors, 133 K,1 modest flux-pinning strength,2 and relatively low anisotropy.3 Nevertheless, this compound has serious disadvantages for practical applications because of its chemical instability and a delicate sintering process. It has been found that partial substitution of Hg by higher-valence cations, such as Pb and Re, significantly improves the ease of formation and enhances the stability of the superconducting phases without affecting the Tc.4,5 Re doping reduces the sensitivity of precursors and superconducting phases against CO2 to levels allowing lesscomplicated preparation and handling,6 promotes the formation and grain growth of Hg-1223 phase,7 and enhances flux pinning.8 It was found that Pb doping also significantly promotes the formation and grain growth of
II. EXPERIMENTAL
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0340 2658
http://journals.cambridge.org
Hg-1223 phase and improves the chemical stability and magnetic properties of Hg-1223 bulk superconductors.9,10 Given the advantages of superconducting properties, Hg-1223 has potential for tape conductors as well as electronic devices. Hg-1223 thin films on single crystal ceramic substrates with a transport Jc ∼106 A/cm2 at 77 K and self-field have been achieved.11–12 For large current ap
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