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用扫描电镜(SEM)、俄歇电子谱仪(SAM)和X射线衍射法研究了草酸盐共沉淀粉末和由共沉淀粉烧制的YBCO超导体的显微组织和成分分布。共沉淀粉具有颗粒细(<1μm)、成分和粒度分布均匀等特点,在粉末预烧结过程中,在850℃烧结,即能完成到123相转变,在915℃能得到杂相含量非常少的123单相粉。用这种粉末制备的YBCO块状样品,其杂相含量比混合粉烧制样品要少得多。SEM几乎在晶界处看不到第二相存在。共沉淀粉烧制样品的平均晶粒尺寸在900—950℃烧结区间随烧结温度变化趋势也不如混合粉烧制样品的大。俄歇电子谱发现,共沉淀粉烧结样品的晶粒边界附近存在着厚度为2—5nm的非123相化学计量成分层,层中成分相应于123相来说是富铜、贫氧、贫钇。这一层非123相化学计量成分层是共沉淀粉烧制样品中的主要弱连接区,它同样品中存在的孔洞、裂纹等缺陷导致样品低的临界电流密度(J_c=10~2—10~3A/cm~2,77K,0场),并使J_c随外场加入迅速降低。
The microstructure and compositional distribution of oxalate coprecipitated powder and YBCO superconductor fired by coprecipitated powder were investigated by scanning electron microscopy (SEM), Auger electron spectroscopy (SAM) and X-ray diffraction. The coprecipitated powder has the features of fine grain (<1μm), uniform composition and particle size distribution. Sintering at 850 ℃ during powder pre-sintering can be completed to 123 phase transformation, and very little miscellaneous phase can be obtained at 915 ℃ 123 single-phase powder. YBCO bulk samples prepared from this powder have much less heterophasic content than mixed powder fired samples. The SEM does not see the second phase at almost the grain boundaries. The average grain size of coprecipitated powder sintered samples in the range of 900-950 ℃ was not as good as that of the mixed powder samples. Auger electron spectroscopy found that there is a non-123 phase stoichiometric composition layer with a thickness of 2-5 nm near the grain boundaries of the coprecipitated sintered sample. The composition of the layer corresponding to phase 123 is copper-rich, oxygen- . This layer of non-123 phase stoichiometric composition layer is the main weak connection zone in the coprecipitated fired samples. The defects such as holes and cracks in the same sample lead to the low critical current density (J_c = 10 ~ 2-10 ~ 3A / cm ~ 2,77K, 0 field), and make J_c rapidly decrease with the addition of external field.