Superconductor technology: apparatus – material – process – High temperature – per se – Thallium containing
Patent
1995-01-31
1996-08-27
Lieberman, Paul
Superconductor technology: apparatus, material, process
High temperature , per se
Thallium containing
505100, 505125, 505778, 505783, 505779, 252518, 252521, H01B 1200, H01L 3912
Patent
active
055501052
DESCRIPTION:
BRIEF SUMMARY
This invention relates to new superconducting compositions and more particularly to superconducting ceramic compositions of the kind in which superconductivity is associated with a structure characterised by alternate layers approximating perovskite structure and rocksalt structure and having nominal compositions represented by the abbreviated formula 1-2-1-2. Representative superconductors of this kind that are now well established are (nominally)
The compositions containing Tl have high critical temperatures but have major toxicity hazards while the Pb/Cu ones are difficult to prepare because they are susceptible to variations in oxygen content that have major effects on properties.
Beales et al (Supercond. Sci. Technol. 5 47 (1992)) have put forward proposals to overcome these problems by using Cd-containing compositions some of which may be represented by the formula superconducting temperature range and while they have promising critical temperature onset values (around 90K) they seem to have rather broad transitions with zero-resistance temperatures as low as 45K.
Putilin et al (Mat Res Bull 26 1299-1307) report the preparation of 1-2-1-2 compositions using the metals Hg, Ba, RE, Cu, but found that they were not superconducting at any useful temperature.
We have now discovered a new group of superconducting compositions which do not necessarily require the presence of Tl and at least some of which have sharp transitions between metallic and superconductive behaviour with zero-resistance temperatures up to about 80K.
The superconductor in accordance with the invention is characterized by the formula B.sub.1-c)Cu.sub.2 O.sub.7 if any, are selected from one or more of Zn, Zn, Cd, Tl, Sc, Mg, Sc, Mg, and Cu; B is selected from Y and the rare earth elements; a is from 0.3 to 0.7; b is from 0 to 1 (inclusive); and c is from about 0.2 to about 0.5. The oxygen content is, as usual, nominal only.
We prefer that the A atoms do not include any Tl, and more especially that they include only Hg or Hg and one or both of Zn and Cd. Preferably B is Y.
Preferably a is in the upper part of its range, namely from 0.4 to 0.7; more especially we prefer that a is about 0.5, so that the valency of the Cu (which diminishes as Hg(II) is substituted for Pb(IV)) is about 2.1.
Preferably b is 1, that is Sr is preferred to Ba or mixtures of Sr and Ba.
The upper limit on the value of c is determined by the loss of metallic properties above the transition temperature and its lower limit by the appearance of an "impurity" phase. Preferably c is from 0.25 to 0.4, more especially about 0.3.
It will be understood that although Hg is itself toxic (as Pb is), its hazards, and the precautions to be taken to avoid or at least limit them, are much better understood than those of Tl.
The invention will be further described, by way of example, with reference to the accompanying drawings in which:
FIG. 1 is a series of X-ray diffraction patterns for four compositions in accordance with the invention;
FIGS. 2 and 3 are graphs showing the variation of two crystallographic lattice constants of these example compositions with changes in Ca/Y proportions;
FIG. 4 is an energy-dispersive X-ray diffraction spectrum for one of the example compositions (shown with that of a known superconducting composition for comparison);
FIG. 5 is a graph showing the normalised resistance of the example compositions as a function of temperature; and
FIG. 6 is a graph of the temperature dependence of the low-field magnetization for a preferred one of the compositions.
High-purity oxide powders of the stoichiometry PbO, HgO, SrO.sub.2, CaO, Y.sub.2 O.sub.3 and CuO were weighed out in four batches in proportions corresponding to the atomic ratios Pb 0.5, Hg 0.5, Sr 2, Ca 0.5, 0.6, 0.7 and 0.8, Ca+Y 1, Cu 2, mixed using a pestle and mortar and pressed into pellets with nominal dimensions of 10 mm diameter by 3 mm thick under a pressure of 5 tonne/cm.sup.2. The pellets were wrapped in gold foil and enclosed in a quartz tube evacuated to about 10.sup.-4 torr. The tubes we
REFERENCES:
Gupta et al "Mercury-Based Cuprate High-Transition Temperature Grain-Boundary Junchons . . ." Science, vol. 265, 19 Aug. 1994 pp. 1075-1077.
Adachi et al "Annealing effects on the Meissner Signals from Hg--Ba--Ca--Cu--O . . ." Physical, 214, 1993 pp. 313-315.
Supercond. Sci. Technol., vol. 5 (1992) pp. 47-49, T. P. Beales et al., "Super-conductivity at 92 in the (Pb, Cd)-1212 phase (PbO.5CdO.5)Sr2(YO.7CaO.3)Cu207--s".
Mat. Res. Bull., vol. 26 (1991) pp. 1299-1307, S. N. Putilin et al., "New Complex Copper Oxides: HgBa2RCu207 (R=La, Nd, Eu, Gd, Dy, Y)".
Letters to Nature, vol. 362, Mar. 18, 1993, pp. 226-228, S. N. Putilin et al., "Superconductivity at 94 K in HgBa2CuO4+s".
Editorial comment on Letters to Nature, vol. 362 (1993); Pick Your Poison, R. J. Cava (2 pages).
Letters to Nature, vol. 363, May 6, 1993, pp. 56-58, A. Schilling et al., "Superconductivity above 130 K in the Hg--Ba--Ca--Cu--O systems".
Chemical Abstracts, vol. 115, No. 16, Zhao et al., 21 Oct., 1991, pp. 946, abstract No. 172741b.
Edwards Peter P.
Hu Shu-Fen
Jefferson David A.
Liu Ru-Shi
BICC Public Limited Comapany
Kopec M.
Lieberman Paul
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