| Summary: | 碩士 === 國立交通大學 === 應用化學研究所 === 86 ===
This research is attempted to investigate the effect of Pb, La, and K cation substitution on the structural and superconducting properties (i.e., Tc, electrical resistrvity and magnetic susceptbility) of (Tl1-yPby)(Ba21-xLax2)CaCu2O7-δ(0≦x≦1.0; 0≦y≦0.4; abbr. as Pb-and La-doped Tl-1212) and (Tl1-xKx)(Ba0.5Sr1.5)Ca2Cu3Oz(K-doped Tl-1212) high-Tc cuprate superconductors that were synthesized at 900℃ and 920℃, respectively, under ambient oxygen atmosphere. Pure Tl-1212 phases with Pb content of 0 and 0.2 can be prepared, whereas BaPbO3 and Tl-1212 were found to coexist when the substitution content of Pb is 0.4, as indicated by x-ray diffraction data. Furthermore, cell parameters and c of Pb-doped Tl-1212 phases were found to incease and decrease with increasing La3+ content, respectively.
On the other hand, Tc of Pb-doped Tl-1212 phases was found to first increase and then decrese when substituted by La3+ for phases with Pb content of 0, 0.2, 0.4, respectively. The optimal Tc was determined to be 87K, 88K, and 83K for phases with la3+ content of 0.3, 0.2, and 0.1, respectively. In the substitution range of higher La3+ content Tc of Tl-1212 was then found to decrease with increasing La3+ content and La-doped Tl-1212 eventually exhibit semiconducting and insulating behaviors. In addition, the oxygen composition of Tl-1212 did not exhibit systematics which was attributed to the uncontrollable deficiency of Tl, as indicated by oxygen content analysis.
The crystal structure and Tc of K-doped Tl(Ba0.5Sr1.5)Ca2Cu3Oz phases were found to be unchanged upon K+ doping, while superconductor volume fraction of Tl-1223 phases was found to decrease upon increasing K+ content. These observations may be rationalized by the proposal that added K+ cation was considered not entering the crystal latice and substituting for Tl3+ ion.
|
|---|
