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Microprobe EELS Study of Oxygen Non - Stoichiometry in High Tc YBa₂Cu₂₃O7-δ Grain Boundaries
신동혁(D. H. Shin),S. E. Babcock(S. E. Babcock) 한국진공학회(ASCT) 1995 Applied Science and Convergence Technology Vol.4 No.2
2~3 ㎚의 공간분해능을 갖는 전자에너지손실분광법(Electron Energy Loss Spectroscopy, EELS)을 이용하여, 전기적 특성조사된 YBa₂Cu₂₃O_(7-δ) 고온초전도체 쌍결정의 결정경계(grain boundary)에서 산소의 조성변화를 조사하였다. Misorientation angle이 14˚, 28˚, 30˚인 3개의 쌍결정 중에서 Josephson junction 특성을 보인 28˚ 및 30˚ 결정경계에서의 oxygen 1s absorption edge는 결정내부에서의 oxygen 1s absorption edge와 매우 다름을 알 수 있었다. 이는 결정경계에서의 산소조성이 결정내부에 비해 낮음을 의미하며, 그 영역이 결정경계 부근 20~40 ㎚로, coherence length에 비해 큼을 알 수 있다. 반면에, flux pinning 특성을 보인 14˚ 결정경계에서의 oxygen 1s absorption edge는 결정내부에서와 별 차이를 보이지 않았다. 따라서 일반적으로 관찰되어온, misorientation angle이 큰 결정경계에서의 Josephson junction 특성은 결정경계 부근에서 산소의 조성이 낮아지는데에 기인하며, 그 원인은 결정경계면을 통해 산소가 out-diffusion 되기 때문인 것으로 생각된다. The oxygen concentration at grain boundaries in electromagnetically-characterized, bulk-scale bicrystals of high T_c, YBa₂Cu₂₃O_(7-δ) has been investigated using high-spatial resolution (2~3 ㎚) electron energy loss spectroscopy (EELS) in a TEM. Flux-grown bicrystals containing grain boundaries with misorientation relationships of 14°, 28°, and 30° about [001] have been studied. The 14° bicrystal showed flux-pinning (FP) critical current density characteristics, whereas the 28° and 30° bicrystals behaved like Josephson Junctions (JJ). The oxygen 1s absorption edge observed at the grain boundaries of Josephson junction bicrystals by TEM-EELS has a markedly different structure than those of the intragranular regions. The difference indicates a depressed charge carrier density and oxygen concentration in the boundary region. The carrier deficient zone extends from 20 to 40 ㎚ into each crystal, a distance much larger than a coherence length in the direction perpendicular to the boundary. In striking contrast, the oxygen K-edges in spectra taken from the grain boundary with flux-pinning character are nearly identical to the intra granular spectra. The observed oxygen deficiency of the Josephson junction boundaries is consistent with a model in which the high-angle boundaries provide high-diffusivity paths for the oxygen diffusion out of the sample.
초기조직 및 이상역열처리가 저합금 고강도강의 저온기계적 성질에 미치는 영향
조현권,박광균,신동혁,맹선재 ( H . K . Cho,K . G . Park,D . H . Shin,S . C . Maeng ) 한국열처리공학회 1994 熱處理工學會誌 Vol.7 No.1
Austenite formation and Low temperatures mechanical properties of HSLA steel of different starting structures have been studied by intercritical annealing(IA). The different starting structures are : ferrite+pearlite(FPl), martensite(M1), cold worked ferrite+pearlite(FP2) and cold worked martensite(M2). In most cases tensile strength and elongation was increased by decreasing the testing temperatures regardless of the IA time. Tensile strength of the cold worked starting structures was higher than that of the non-cold worked starting structures. However not any noticeable difference in elongation was found between two cases. Low temperatures impact properties were affected by the starting structures. Charpy V-notch impact transition temperatures of the M-starting structures were around -40℃, and those of the FP starting structures were around -10℃. Impact energy was lower in the cold worked specimens than in the non-cold worked specimens at the same starting structures. DP structure obtained from the M-starting structure has shown superior low temperatures mechanical properties than the DP structure obtained from the FP-starting structure.