Effect of heat treatment on the superconducting properties of Ag-doped Sr0.6K0.4Fe2As2 compounds

The Key Laboratory of Applied Superconductivity, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, and China Research have collaborated to reveal the heat treatment effects on the superconducting properties of Ag-doped Sr0.6K0.4Fe2As2 compounds. Because of its significant research value, the study is reported in issue 7 of SCIENCE CHINA Physics, Mechanics & Astronomy.

Previously, our group studied the effect of adding Ag to polycrystalline Sr0.6K0.4Fe2As2 and found that the critical current density Jc and the irreversibility field increase markedly. More recently, Wang et al. published the first report of significant transport of critical currents in Sr0.6K0.4Fe2As2 wires and tapes when Ag is used as an additive and a sheath material.

However, to our knowledge, systematic reports on the relationship between sample preparation, microstructure, and the superconducting properties of polycrystalline pnictide superconductors are very rare. In fact, the critical properties of superconductors are strongly influenced by the heat-treatment conditions, so an investigation is desirable on the heat-treatment effects on Ag-doped Sr0.6K0.4Fe2As2 superconductors.

In this work, we investigate the effect of heat-treatment temperatures, which vary from 700 to 900 °C, on the microstructure, Jc, Hirr, and Hc2, of 20 wt% Ag-doped Sr0.6K0.4Fe2As2. We find that Hirr and Jc increase with increasing heat-treatment temperature. However, the heat treatment temperature does not influence Tc and Hc2. When compared with pure Sr0.6K0.4Fe2As2 as a reference sample, we find that Jc for the Ag-doped sample is much larger than Jc for the pure sample if the heat-treatment temperature is ~900 °C, but Jc deteriorates upon Ag doping when the sinter temperature is below 900 °C. However, adding Ag enhances Jc for samples at higher sinter temperatures (~900°C) because of the diffusion of Ag and the disappearance of the gathered impurity phase. The data clearly demonstrate that Ag can improve the superconducting properties of Sr0.6K0.4Fe2As2 samples when the heat treatment temperature is 900 °C.

This research was supported by the Beijing Municipal Science and Technology Commission (Grant No. Z09010300820907), the National Natural Science Foundation of China (Grant No. 50802093), and the National Basic Research Program of China (Grant No. 2006CB601004).

Reference: Wang L, Qi Y P, Gao Z S, et al. The role of silver addition on the structural and superconducting properties of polycrystalline Sr0.6K0.4Fe2As2. Supercond. Sci. Technol., 2010, 23: 025027

Wang L, Qi Y P, Wang D L, et al. Large transport critical currents of powder-in-tube Sr0.6K0.4Fe2As2/Ag superconducting wires and tapes. Physica C, 2010, 470: 183�

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