Effect of different nanoscale RE 2 BaCuO 5 additions on the formation of compositional fluctuation in Sm-Ba-Cu-O superco
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ann Chen Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China
Yang-Chung Liao Department of Physics and Materials Science Center, National Tsing Hua University, Hsinchu, Taiwan, Republic of China
Maw-Kuen Wu Institute of Physics, Academia Sinica, Taipei 701, Taiwan, Republic of China (Received 23 August 2004; accepted 10 November 2004)
This study presents the effect of different nanoscale RE211 additions—Y2BaCuO5 (nmY211), Sm2BaCuO5 (nmSm211), and Nd4Ba2Cu2O10 (nmNd422)—on the nano-scale compositional fluctuation and associated pinning mechanism of the melt-textured growth (MTG) Sm–Ba–Cu–O [SmBCO, of which the composition is Sm123 (Sm1Ba2Cu3Oy) + 25 wt% Sm211 (Sm2BaCuO5)] superconducting bulk materials. The superconductivity and microstructure results indicated that in samples with the addition of these nano-sized particles, nanoscale compositional fluctuations form during the peritectic transformation of melt-growth process, which cause Tc variation on a nanoscale and result in the formation of ␦Tc pinning centers at high magnetic field. The compositional fluctuation regions (␦Tc pinning centers) are altered by the difference in peritectic temperature, the solubility in the liquid phase, and the ion radius. The direct current transport R-T properties elucidate the change of flux pinning behavior. In addition, the different influence on microstructure and superconductivity between the two methods: mixing rare-earth elements in nanoscale RE211 or in the homogeneous mixed precursor powders [e.g., (Nd, Eu, Gd)–Ba–Cu–O (NEG)] is also discussed.
I. INTRODUCTION
It is known that to enhance the Jc-H performance of superconductors, both the Tc and the number of effective pinning centers should be increased. The coherence length of the RE–Ba–Cu–O (REBCO; RE stands for rare earth elements) superconductors is on the nanoscale; thus the increment of defects or size of compositional fluctuation in the nanoscale is beneficial for pinning. Including the interface of nonsuperconducting phase (211particles),1 dislocations,2 twins, and stacking faults3 have been proved to be flux pinning centers. Much effort has been made to increase the amounts of the small-sized
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0055 482
J. Mater. Res., Vol. 20, No. 2, Feb 2005
nonsuperconducting phase and defects in the superconductors, such as the addition of Pt,4 CeO2,5 BaCeO3,6 and modification of processes, e.g., quench melt growth (QMG)7 and melt powder melt growth (MPMG).8 However, the increment on Jc by the above methods was mainly in low-field regions. To extend the application field, enhancement of Jc performance in high-field regions is important. To achieve this, pinning centers, which can act in this field region should be induced. REBCO superconductors possess the distinctive peak effect,9 and are thus candidates for this purpose. The source of flux pinning in high-field regions in the REBCO materials have been investi
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