Microstructural analysis of the bismuth-2212 high critical transition temperature superconductor prepared from the melt quenched precursor by transmission electron microscopy

Zi-ping Zhang, Purdue University

Abstract

Three phases are known to be responsible for the superconductivity in the Bi-Sr-Ca-Cu-O high Tc superconductor. They are: the n = 1, n = 2, and n = 3 phases. The Bi$\sb2$Sr$\sb2$Ca$\sb1$Cu$\sb2$O$\sb{8+\delta}$ (Bi-2212) superconductor in the present study is made by the melt quenching method. Its superconducting property is a function of the subsequent annealing condition. The microstructural development in Bi-2212 system, under annealing temperature between 500$\sp\circ$C and 865$\sp\circ$C, is studied by transmission electron microscopy (TEM) and X-ray powder diffraction. Electrical resistivity of samples, which is the main indicator of the superconductivity, is measured as a function of temperature. From X-ray powder diffraction, one can obtain the average structural information of the material. The growth characteristics of the superconducting phases are investigated by TEM. It is found that at low annealing temperature (T $<$ 600$\sp\circ$C) the n = 1 phase and a newly discovered phase, the "cubic" phase, grow with annealing time. The growth of the n = 2 phase is suppressed. The majority of the n = 1 phase has extremely small grain size ($\sim$200A). A few large grain of the n = 1 phase is detected and the modulation found in these large grains is found to be incommensurate both in the b and c directions. In addition, the 180$\sp\circ$ twist boundary is also found to exist in these grains. The hypothesis that the "cubic" phase is a new superconducting phase is further explored. In the mid annealing temperature range (650$\sp\circ$C $<$ T $<$ 750$\sp\circ$C), the n = 1 and the "cubic" phases are no longer dominant in the material. The n = 2 phase grows very slowly and with many defects. In the high annealing temperature range (T $>$ 800$\sp\circ$C), the n = 2 phase grows exclusively. Its preferential growth direction is its basal plane. The n = 2 phase grown at high temperature has fewer defects. In our study, we found that the n = 1 phase nucleates first out of the amorphous precursor. The n = 2 phase grows out of the n = 1 phase. A number of impurity phases are also discovered in our study. Their crystal structures and compositions are determined by electron diffraction and energy dispersion spectrometry (EDS) respectively.

Degree

Ph.D.

Advisors

Sato, Purdue University.

Subject Area

Materials science

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