LiF addition to (Cu,C) Ba2Ca3Cu4Oy superconductor

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Abstract

Samples of (Cu0.6C0.4)Ba2Ca3Cu4Oy ((Cu,C)-1234) with addition of 0.1 mol LiF have been synthesized by high pressure technique. It was found that LiF promotes the formation of superconducting phases with n higher than 4 of the (Cu,C)Ba2Can−1CunOy series. In order to obtain single-phase samples, for the same synthesis conditions, the oxygen content in the starting mixture (supplied by AgO) should be increased from 0.45 to 0.73 mol AgO, when using LiF. Samples with LiF have shown a certain morphology suggesting an enhanced 2D and suppressed 3D type of growth assisted by a liquid phase. Tc decreased from 116.5 to 113 K for the sample with LiF, but at the same time the transition width also decreased and Jc has shown a fish-tail effect.

Introduction

Several features of (Cu1−xCx)Ba2Ca3Cu4Oy ((Cu,C)-1234) [1] make this material interesting for applications; Tc is close to 117 K even in the over-doped region [2], Hirr versus (1−T/Tc) is relatively high [3], [4] and (Cu,C)-1234 does not contain any toxic or volatile element. These advantages are shadowed by the fact that synthesis is difficult (usually under high pressure) and the absolute Jc values are not superior to other HTS (Tl-, Hg-, Y-, Bi- based systems) suggesting that in (Cu,C)-1234 there are less pinning centers [4].

Therefore, search for additions or substitutions enhancing synthesis properties and/or Jc is of interest. In this paper we present our data on synthesis and characterization of (Cu,C)-1234 with addition of LiF.

Section snippets

Experimental

Samples with starting composition (Cu0.6C0.4)Ba2Ca3Cu4Oy with and without addition of LiF have been prepared by high pressure method. We have used a precursor powder with composition Ba2Ca2.7Cu4.6CxOy prepared by solid-state route (890 °C/24 h/O2-flow) from Ba and Ca carbonates and CuO. The carbon content x in the precursor powder determined by FT-IR was 0.1. One mol of precursor was mixed with 0.3 mol CaCO3. For the samples without LiF, addition of 0.45 mol AgO with the role of oxidizing agent

Results and discussion

For the same oxygen content in the starting mixture and the same synthesis conditions, addition of 0.1 mol LiF promoted the formation of the phases with n higher than 4 of the (Cu,C)Ba2Can−1CunOy series. This is probably due to the decrease in the local oxygen partial pressure when other elements (i.e. F and/or Li) in the vapor state are present in the system and also due to the formation of a higher amount of liquid phase (see next paragraph). As revealed by XRD and EDS, nearly single phase

Acknowledgements

P.B. and A.C. acknowledge STA fellowships.

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