PC9-3-INV

CaKFe4As4 superconductors: Synthesis, post-annealing and Sn addition effects on the phase stability, structure, microstructure, and physical properties

Dec.3 16:30-17:00 (Tokyo Time)

*Pavan Kumar Naik S1,2, Ishida Shigeyuki1, Kamiya Yoshihisa1,3, Tsuchiya Yoshinori1, Kawashima Kenji3, Iyo Akira1, Eisaki Hiroshi1, Nishio Taichiro2, Ogino Hiraku1,2

Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Central 2, Umezono, Tsukuba, Ibaraki, 305-8568, Japan1

Department of Physics, Tokyo University of Science, 1 Chome-3 Kagurazaka, Shinjuku City, Tokyo 162-8601, Japan2

IMRA Material R&D Co., Ltd., 2-1 Asahi-machi, Kariya, Aichi 448-0032, Japan3

AeAFe4As4 (1144, where Ae = alkaline earth elements and A = alkaline metals) superconductors belong to Fe-based superconductors and were developed recently in our group with superconducting transition temperature (Tc) of 36 K [1]. While these compounds have several desirable properties for practical applications [2], chemical complexity and formation of impurity phases should be potential difficulties to be solved. We optimized the synthesis conditions which indicates that two times sintering at 900 oC/915 oC followed by quenching is required for producing high pure CaKFe4As4 (CaK-1144 or 1144) polycrystalline powders. The Tc of the CaK1144 samples was increased systematically from 32 K -35 K with post-annealing temperature, probably due to similar effect reported in CaFe2As2. To investigate reactivity and improve the flux pinning strength of CaK1144 several attempts were made by adding various low-melting point metals. Sn is found to be most stable among the metals, and sintering temperature as well as amount of Sn was optimized. The optimized sample exhibited twice higher Jc compared to Sn-free sample at 4.2 K and self-field. Interestingly, at the middle temperature range (500-700 oC), Sn-added samples show higher phase stability compared to Sn-free samples. A thorough microstructural and elemental investigations evidenced that the enhancement in the Jc performance is ascribed to the improved GB connectivity and reduced impurity formation at the GB. We will also discuss the effect of detailed conditions, microstructural modifications, and their influence on the superconducting properties by Sn addition and annealing. We investigated reactivity of CaK1144 and improved superconducting properties by annealing and Sn addition and supporting promising Jc properties that are required for future practical applications.

References
[1] A. Iyo, et al., J. Am. Chem. Soc. 138 (2016) 3410
[2] S. Ishida, et al., npj Quantum Materials 4 (2019) 27

Keywords: CaKFe4As4, Microstructure, Microstructure, Critical current density