We have previously shown that Tc for Bi2 (Sr, Ca)n+1CunO2n+4+δ (n=1, 2 and 3) varies with oxygen stoichiometry δ determined by annealing in a variety of oxygen partial pressures and temperatures. Annealing results are now also presented for several Tl-superconductors in the series Tlm(Ba, Sr)2Can−1CunO2n+m+2+δ, for both m=1 and 2, revealing two general results for this class of Bi or TI superconductors: (1) effects on Tc due to labile oxygen occur for all m=2 compounds but are absent in m = 1 compounds with Sr. m=1 compounds with Ba do have variable oxygen but the effects on the c-axis are the opposite to m=2 compounds. This suggests for the latter that the labile oxygen resides in interstitial sites within the Bi2O2 or Tl2O2 bi layers; (2) the hole concentration per Cu decreases in progressing from n = 1 to 2 to 3 in either class so that the parent n = 3 and n = 2 compounds lie respectively on the low- and high-hole concentration sides of the peak in Tc while the n = 1 compounds extend out into the non-superconducting domain at very high hole concentration.

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General trends in oxygen stoichiometry effects on Tc in Bi and Tl superconductors

The processing and properties of Ag-sheathed Bi-based superconducting tapes prepared using the powder-in-tube technique have been reviewed. Powder processing and heat treatment, among various other factors, were found to be critical to the Jc of Ag/Bi-based superconducting wires. Reactive, homogeneous, carbonate-free and incompletely reacted powders, produced by the solution route are desirable. Liquid phase sintering, which is essential to achieve intimate grain connectivity and high grain alignment, can be achieved using a short period of melt or by annealing at temperatures near the melting point of the composites. An Ag sheath has been proved to be sufficient for achieving the flexibility and stability of these tapes. The interfacial layer between Ag and oxides shows an improved strength and flexibility. The thermally activated flux creep determines the Jc at high temperature. Weak links have been largely eliminated in high-quality tapes at high temperatures but the weak-link behaviour becomes pronounced at low temperatures. The dispersed 2212 phase and defects in the Ag/2223 tapes may act as potential pinning sites. Multiprobe measurements show no accumulative grain boundary resistance for long tapes. Fabrication of long tapes shows a better reproducibility than that for the short tapes.

https://iopscience.iop.org/article/10.1088/0953-2048/6/5/001/pdf

Ag-sheathed Bi(Pb)SrCaCuO superconducting tapes

Hole-doped cuprate high temperature superconductors have ushered in the modern era of high temperature superconductivity (HTS) and have continued to be at center stage in the field. Extensive studies have been made, many compounds discovered, voluminous data compiled, numerous models proposed, many review articles written, and various prototype devices made and tested with better performance than their nonsuperconducting counterparts. The field is indeed vast. We have therefore decided to focus on the major cuprate materials systems that have laid the foundation of HTS science and technology and present several simple scaling laws that show the systematic and universal simplicity amid the complexity of these material systems, while referring readers interested in the HTS physics and devices to the review articles. Developments in the field are mostly presented in chronological order, sometimes with anecdotes, in an attempt to share some of the moments of excitement and despair in the history of HTS with readers, especially the younger ones.

Hole-doped cuprate high temperature superconductors

The sensitivity of Tc to oxygen sorption and the Ca to Sr ratio has been studied for the homologous series Bi2.1(CaχSr1−χ)n+1CunO2n+4+δ for n=1, 2 and 3. The n=0 and n=∞ members have been produced as single phase samples but are found to be semiconducting. While single phase n=2 material has been prepared, the n=1 member always displays a small n=2 contribution to the X-ray diffraction pattern. n=3 is only found in multiphase samples. Single phase superconducting samples were equilibrated under a fixed partial pressure of oxygen at temperatures between 300 and 850°C and quenched in liquid nitrogen. It is shown that Tc can be controlled in a reversible and systematic way by varying the oxygen partial pressure and the annealing temperature and consequently the oxygen stoichiometry. The site energy per oxygen sorbed for n=2 is 3.24 eV, only a little less than that for YBa2Cu3O7. However, the volume change on sorption is ten times smaller than that for YBa2Cu3O7, and is manifested as a changing c-axis with the a-axis invariant. This can be interpreted as implying that oxygen sorption occurs within the Bi-O layers. Tc is found to be maximised for a Ca to Sr ratio of 1 to 2 for both n=1 and 2. Thermal expansion data for n=2 material is also reported to illustrate that oxygen sorption does effect the physical properties of these materials.

The influence of oxygen on the physical properties of the superconducting series Bi2.1(CaχSr1−χ)n+1Cun+4+δ

The recently demonstrated correlation of T c with copper or oxygen bond valence sums for a wide range of superconducting cuprates is shown to contradict the behaviour observed for YBa 2 CU 3 O x as x is varied from 6 to 7. This conflict is removed by introducing two independent variables, namely, the sum of and difference of the copper and oxygen bond valence sums. The latter is a measure of the hole density on the CuO 2 plane and as this increases T c passes through a maximum T c (max). The former indicates the relative distribution of these holes on the oxygen or copper sites for a given compound. T c (max) correlates closely with the preference for hole distribution on the oxygen atoms. A general phase-diagram using these bond sum parameters is presented.

The relationship between bond-valence sums and Tc in cuprate superconductors

Following the report of Maeda et al.1 on high-Tc superconductivity in samples of nominal composition BiSrCaCu2Ox we have identified three distinct phases in the homologous series Bi2.1(Ca, Sr)n+lCunO2n+4+δ with n = 1,2 and 3. These have zero resistance Tcs of 80 K, 91 K and 105 K respectively. The structures in the homologous series appear to be based on alternating double bismuth Bi2O2 layers and perovskite (Sr, Ca)O–CuO2 layers, with higher members obtained by intercalating additional 3.1 A Ca-CuO2 bilayers. The structures can be indexed on a pseudo-tetragonal subcell with a = b = 5.4 A and c = 24.4 A (n = 1), 30.76 A (n = 2) or 36 A (n = 3). We were unable to prepare the n =4 member. In each structure a 2× superlattice structure in the c-direction arises as a natural consequence of a 19/4 incommensurate structure in the b direction, which accounts for 2.1 Bi atoms in the unit formula. The critical temperature, Tc, for these phases is sharply dependent on the Sr/Ca ratio and oxygen stoichiometry, as determined by heat treatment.

L. A. Christian

Papers

High- T c superconducting phases in the series Bi 2.1 (Ca, Sr) n+l Cu n O 2n+4+δ