Effect of heat treatments on superconducting properties and connectivity in K-doped BaFe2As2

Chiara Tarantini; Chongin Pak; Yi Feng Su; Eric E. Hellstrom; David C. Larbalestier; Fumitake Kametani

doi:10.1038/s41598-021-82325-x

Effect of heat treatments on superconducting properties and connectivity in K-doped BaFe₂As₂

Chiara Tarantini, Chongin Pak, Yi Feng Su, Eric E. Hellstrom, David C. Larbalestier, Fumitake Kametani

Corrosion Science & Technology

Research output: Contribution to journal › Article › peer-review

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Abstract

Fe-based superconductors and in particular K-doped BaFe₂As₂ (K-Ba122) are materials of interest for possible future high-field applications. However the critical current density (J_c) in polycrystalline Ba122 is still quite low and connectivity issues are suspected to be responsible. In this work we investigated the properties of high-purity, carefully processed, K-Ba122 samples synthesized with two separate heat treatments at various temperatures between 600 and 825 °C. We performed specific heat characterization and T_c-distribution analysis up to 16 T and we compared them with magnetic T_c and J_c characterizations, and transmission-electron-microscopy (TEM) microstructures. We found no direct correlation between the magnetic T_c and J_c, whereas the specific heat T_c-distributions did provide valuable insights. In fact the best J_c-performing sample, heat treated first at 750 °C and then at 600 °C, has the peak of the T_c-distributions at the highest temperatures and the least field sensitivity, thus maximizing H_c2. We also observed that the magnetic T_c onset was always significantly lower than the specific heat T_c: although we partially ascribe the lower magnetization T_c to the small grain size (< λ, the penetration depth) of the K-Ba122 phase, this behaviour also implies the presence of some grain-boundary barriers to current flow. Comparing the T_c-distribution with J_c, our systematic synthesis study reveals that increasing the first heat treatment above 750 °C or the second one above 600 °C significantly compromises the connectivity and suppresses the vortex pinning properties. We conclude that high-purity precursors and clean processing are not yet enough to overcome all J_c limitations. However, our study suggests that a higher temperature T_c-distribution, a larger H_c2 and a better connectivity could be achieved by lowering the second heat treatment temperature below 600 °C thus enhancing, as a consequence, J_c.

Original language	English
Article number	3143
Journal	Scientific Reports
Volume	11
Issue number	1
DOIs	https://doi.org/10.1038/s41598-021-82325-x
State	Published - Dec 2021

Funding

This work is supported by the US Department of Energy, Office of High Energy Physics under the Grant number DE-SC0018750 and it was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and by the State of Florida.

Funders	Funder number
State of Florida
National Science Foundation	DMR-1644779
U.S. Department of Energy	DE-SC0018750
High Energy Physics
Center for Selective C-H Functionalization, National Science Foundation
Center for Hierarchical Manufacturing, National Science Foundation
Savannah River Operations Office, U.S. Department of Energy
Florida Department of State
Idaho Operations Office, U.S. Department of Energy

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title = "Effect of heat treatments on superconducting properties and connectivity in K-doped BaFe2As2",

abstract = "Fe-based superconductors and in particular K-doped BaFe2As2 (K-Ba122) are materials of interest for possible future high-field applications. However the critical current density (Jc) in polycrystalline Ba122 is still quite low and connectivity issues are suspected to be responsible. In this work we investigated the properties of high-purity, carefully processed, K-Ba122 samples synthesized with two separate heat treatments at various temperatures between 600 and 825 °C. We performed specific heat characterization and Tc-distribution analysis up to 16 T and we compared them with magnetic Tc and Jc characterizations, and transmission-electron-microscopy (TEM) microstructures. We found no direct correlation between the magnetic Tc and Jc, whereas the specific heat Tc-distributions did provide valuable insights. In fact the best Jc-performing sample, heat treated first at 750 °C and then at 600 °C, has the peak of the Tc-distributions at the highest temperatures and the least field sensitivity, thus maximizing Hc2. We also observed that the magnetic Tc onset was always significantly lower than the specific heat Tc: although we partially ascribe the lower magnetization Tc to the small grain size (< λ, the penetration depth) of the K-Ba122 phase, this behaviour also implies the presence of some grain-boundary barriers to current flow. Comparing the Tc-distribution with Jc, our systematic synthesis study reveals that increasing the first heat treatment above 750 °C or the second one above 600 °C significantly compromises the connectivity and suppresses the vortex pinning properties. We conclude that high-purity precursors and clean processing are not yet enough to overcome all Jc limitations. However, our study suggests that a higher temperature Tc-distribution, a larger Hc2 and a better connectivity could be achieved by lowering the second heat treatment temperature below 600 °C thus enhancing, as a consequence, Jc.",

author = "Chiara Tarantini and Chongin Pak and Su, {Yi Feng} and Hellstrom, {Eric E.} and Larbalestier, {David C.} and Fumitake Kametani",

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AU - Larbalestier, David C.

AU - Kametani, Fumitake

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N2 - Fe-based superconductors and in particular K-doped BaFe2As2 (K-Ba122) are materials of interest for possible future high-field applications. However the critical current density (Jc) in polycrystalline Ba122 is still quite low and connectivity issues are suspected to be responsible. In this work we investigated the properties of high-purity, carefully processed, K-Ba122 samples synthesized with two separate heat treatments at various temperatures between 600 and 825 °C. We performed specific heat characterization and Tc-distribution analysis up to 16 T and we compared them with magnetic Tc and Jc characterizations, and transmission-electron-microscopy (TEM) microstructures. We found no direct correlation between the magnetic Tc and Jc, whereas the specific heat Tc-distributions did provide valuable insights. In fact the best Jc-performing sample, heat treated first at 750 °C and then at 600 °C, has the peak of the Tc-distributions at the highest temperatures and the least field sensitivity, thus maximizing Hc2. We also observed that the magnetic Tc onset was always significantly lower than the specific heat Tc: although we partially ascribe the lower magnetization Tc to the small grain size (< λ, the penetration depth) of the K-Ba122 phase, this behaviour also implies the presence of some grain-boundary barriers to current flow. Comparing the Tc-distribution with Jc, our systematic synthesis study reveals that increasing the first heat treatment above 750 °C or the second one above 600 °C significantly compromises the connectivity and suppresses the vortex pinning properties. We conclude that high-purity precursors and clean processing are not yet enough to overcome all Jc limitations. However, our study suggests that a higher temperature Tc-distribution, a larger Hc2 and a better connectivity could be achieved by lowering the second heat treatment temperature below 600 °C thus enhancing, as a consequence, Jc.

AB - Fe-based superconductors and in particular K-doped BaFe2As2 (K-Ba122) are materials of interest for possible future high-field applications. However the critical current density (Jc) in polycrystalline Ba122 is still quite low and connectivity issues are suspected to be responsible. In this work we investigated the properties of high-purity, carefully processed, K-Ba122 samples synthesized with two separate heat treatments at various temperatures between 600 and 825 °C. We performed specific heat characterization and Tc-distribution analysis up to 16 T and we compared them with magnetic Tc and Jc characterizations, and transmission-electron-microscopy (TEM) microstructures. We found no direct correlation between the magnetic Tc and Jc, whereas the specific heat Tc-distributions did provide valuable insights. In fact the best Jc-performing sample, heat treated first at 750 °C and then at 600 °C, has the peak of the Tc-distributions at the highest temperatures and the least field sensitivity, thus maximizing Hc2. We also observed that the magnetic Tc onset was always significantly lower than the specific heat Tc: although we partially ascribe the lower magnetization Tc to the small grain size (< λ, the penetration depth) of the K-Ba122 phase, this behaviour also implies the presence of some grain-boundary barriers to current flow. Comparing the Tc-distribution with Jc, our systematic synthesis study reveals that increasing the first heat treatment above 750 °C or the second one above 600 °C significantly compromises the connectivity and suppresses the vortex pinning properties. We conclude that high-purity precursors and clean processing are not yet enough to overcome all Jc limitations. However, our study suggests that a higher temperature Tc-distribution, a larger Hc2 and a better connectivity could be achieved by lowering the second heat treatment temperature below 600 °C thus enhancing, as a consequence, Jc.

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ER -

Effect of heat treatments on superconducting properties and connectivity in K-doped BaFe₂As₂

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