The Effect of Lead Substitution on Superconducting BISMUTH(2)STRONTIUM(2)CALCIUM(2)COPPER(3)OXYGEN(DELTA)
Abstract
At least three superconducting phases are known to exist within the Bi-Sr-Ca-Cu-O system. Their approximate compositions are Bi_2Sr _2Ca_{rm n-1} Cu_{n}O delta where n takes on the values 1, 2, and 3. The n = 3 phase, Bi_2Sr _2Ca_2Cu _3Odelta, has the highest superconducting transition temperature, T_ {C} ~ 110 K. However, this phase occurs essentially as intergrowths, or stacking faults, within a matrix of the n = 2 phase, Bi _2Sr_2CaCu _2Odelta, which has T_{C} ~ 80 K. Through careful processing, a very large fraction of the n = 2 material can be converted to the n = 3 form, far more than should be necessary to create a percolative path. However, regardless of how large the fraction of 110 K material, samples produced by standard solid state sintering techniques invariably demonstrate a residual resistance at temperatures between the n = 2 and n = 3 transition temperatures yielding a two-step transition with zero resistance generally attained at 75-85 K. When Bi is partially substituted by Pb, the n = 3 phase forms much more readily. Furthermore, Bi _{2-x}Pb_ {x}Sr_2Ca _2Cu_3O delta ceramics (x <=q 0.35 for single-phase material) reproducibly demonstrate a single resistive transition with zero resistance regularly attained above 105 K. Microstructural investigations reveal that the n = 1 or n = 2 phase is always found adjacent to the grain boundary in Pb-free samples while in Pb-doped samples, the n = 3 phase is often observed adjacent to the grain boundary. Thus, Pb allows a filamentary path to be established between regions of n = 3 stoichiometry. Preparation conditions strongly affect the n = 3 phase. It is shown that the material is sensitive to compositional variations indicating that there is a very narrow range of solid solution among Bi, Sr, and Ca. Furthermore, oxygen is shown to be a prominent player in the formation and stability of (Bi,Pb)_2Sr _2Ca_2Cu_3 Odelta. It is put forward as conjecture that Pb^{2+} replacing Bi^{3+} regulates the oxygen content thus stabilizing the n = 3 phase.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1989
- Bibcode:
- 1989PhDT.......118G
- Keywords:
-
- LEAD;
- BISMUTH;
- STRONTIUM;
- CALCIUM;
- COPPER;
- OXYGEN;
- Engineering: Materials Science; Physics: Condensed Matter