Microstructure of (Nd, Eu, Gd)-123 matrix with (Nd, Eu, Gd)-211 inclusions
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Microstructure of (Nd, Eu, Gd)Ba2Cu3O7−␦ (NEG-123) samples with (Nd, Eu, Gd)2BaCuO5 (NEG-211) particles were observed by transmission electron microscopy. High-resolution electron microscopy observation demonstrated that the density of microstructural defects was small around the NEG-211 secondary phase particles. Furthermore, the 123/211 interfaces were found to be very clean and sharp. Chemical compositional analysis of the submicron secondary phase particles revealed that these fine particles are not composed of NEG-211 but Eu2BaCuO5 (Eu-211) or Gd2BaCuO5 (Gd-211).
I. INTRODUCTION
The oxygen-controlled melt-growth (OCMG) process has received a great deal of attention in the field of bulk superconductors, because RE-123 (RE: Nd, Sm, Eu, Gd) materials fabricated with the OCMG process exhibit better flux pinning than YBa2Cu3Oy (Y-123) up to high magnetic fields.1–3 Flux pinning was further enhanced in newly developed ternary (Nd0.33Eu0.33Gd0.33)Ba2Cu3Oy−␦ (NEG-123) superconductors and the critical current density (Jc) values recorded in these materials are attractive for high field applications at 77 K.4–7 However, a further enhancement of Jc is still desirable for both better performance and safety margins. It is well known that the introduction of effective pinning centers are essential for the achievement of high Jc values in high-Tc superconductors. In the Y-123 system, twin planes, oxygen defects, nonsuperconducting particles, dislocations, and stacking faults have been found to work as pinning centers.8–14 The pinning provided by nonsuperconducting particles is mainly expected to be active at low fields and high temperatures (>77 K). However, our recent results demonstrated that NEG-211 addition to the NEG-123 matrix increases Jc even at higher fields.15 Furthermore, Jc at low fields increases continuously with increasing NEG-211 content up to 40 mol%. Transmission electron microscopy (TEM) investigations with energy dispersive x-ray (EDX) for the NEG-123/ NEG-211 composite system revealed that extremely fine 211 particles with the average size less than 0.1 m were
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 2, Feb 2001
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not NEG-211 but Gd-211. The size of these particles is approaching the value of the coherence length, which is believed to be the optimal size.16 In melt-textured Y–Ba– Cu– O systems, the Y123/ Y211 interfaces are found to provide flux pinning, and thus an increase in their effective surface area is important for Jc enhancement. Similar tendency has been observed in the OCMG-processed RE-123–RE-211 composites, although field-induced pinning is also active. To understand both the flux-pinning mechanism and the chemical processes involved in the crystallization, the nature of the interface between the 123 matrix and 211 phase is of primary interest for the NEG system. In this work, we focused on high-resolution transmission electron microscopy (HRTEM) observation of the 123/2
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