Superconducting materials copper oxides

The idea behind this solid solution is simple enough. Starting from BaBiOs, the substitution of Pb for Bi removes electrons from the system, as Pb is one element to the left of Bi in the periodic table. Obviously, electrons can also be removed from the system by substitution of K+1 for Ba2+. If we suppose that the key to the occurrence of superconductivity in BaPb 75-Bi 25Os is related to the special charge fluctuations in Bi, then, in analogy to the copper oxides, a material with solely the active component on the electronically active sites should be a better superconductor. For the Ba K BiOg solid solution, Bi is formally... 

The discovery of high Tq superconductivity in La2-xMxCu04 (M = Ba, Sr) (Bednorz and Muller 1987) based on perovskite and rock-salt structures has led to an international effort in superconductivity research over the last decade. The principles that govern superconducting copper-oxide-based materials have enormous significance in the application of these oxides as potential catalysts... 

A major scientific breakthrough in the area of material science involving copper occurred in late 1986 and early 1987 the discovery of the so-called high-temperature cuprate superconductors (see Superconductivity). These copper-oxide containing materials hold the record for the highest transition temperature (Tc, the temperature at which all resistance to electricity is lost), currently 133 K, which is sigiuficant... 

Yttrium—barium—copper oxide, YBa2Cu202 is a newly developed high T material which has been found to be fully superconductive at temperatures above 90 K, a temperature that can be maintained during practical operation. The foremost challenge is to be able to fabricate these materials into a flexible form to prepare wines, fibers, and bulk shapes. Ultrapure powders of yttrium—barium—copper oxide that are sinterable into single-phase superconducting... 

Bismuth trioxide forms numerous, complex, mixed oxides of varying composition when fused with CaO, SrO, BaO, and PbO. If high purity bismuth, lead, and copper oxides and strontium and calcium carbonates are mixed together with metal ratios Bi Pb Sn Ca Cu = 1.9 0.4 2 2 3 or 1.95 0.6 2 2 3 and calcined at 800—835°C, the resulting materials have the nominal composition Bi PbQ4Sr2Ca2Cu20 and Bi 25PbQgSr2Ca2Cu20 and become superconducting at about 110 K (25). 

The discovery of high-temperature superconductivity in mixed oxides, such as the lanthanum-barium-copper oxide complexes, has created a great deal of interest in these materials. Superconductivity, that is, the absence of any resistance to the flow of electric current, is now possible at temperatures above the temperature of liquid nitrogen (77K). Many problems remain in the development of practical processes for these materials and commercialization is not likely to occur until these problems are solved. Among the several processing techniques now used, CVD appears one of the most successful. 

One of the most exciting developments in materials science in recent years involves mixed oxides containing rare earth metals. Some of these compounds are superconductors, as described in our Chemistry and Technology Box. Below a certain temperature, a superconductor can carry an immense electrical current without losses from resistance. Before 1986, it was thought that this property was limited to a few metals at temperatures below 25 K. Then it was found that a mixed oxide of lanthanum, barium, and copper showed superconductivity at around 30 K, and since then the temperature threshold for superconductivity has been advanced to 135 K. 

Those readers not familiar with superconductivity in organic materials may find the Tc values rather low. However, they are comparable to values for inorgaiuc metallic elements. Here is a list of some selected examples FcCNb) = 9.25 K, rc(Pb) = 7.20 K, rc(a-Hg) = 4.15 K, rc(Sn) = 3.72 K, Tc(Al) = 1.17 K, Tc(ri) = 0.40 K, etc. It is interesting to note that copper does not exhibit a superconducting transition. The highest known Tc values of any material correspond to the copper-oxide series with Tc 138 K as the absolute record for the thallium-doped mercury-cuprate compound. 

These copper-oxide compounds crystallize in the perovskite structure and superconductivity is based on the (hole or electron) doping in the copper-oxide planes. This is the reason why these materials can be regarded as being 2D. The first compound of the family was La2 i Sr i Cu04 with Tc 38 K, which soon led to YBa2Cu307 5 with Tc — 92 K for 5 < 1 (Bums, 1993). The non-copper oxide electron-doped perovskite Bai-jcK cBiOa exhibits superconductivity near 30 K for 0.3 < X < 0.5 (Cavaeta/., 1988). 

Alkaline-earth substitution in LajCuC was also investigated (121)(128)(131) during the early 1970 s in the U.S.S.R., India, and Japan, but unfortunately, these materials were never investigated at sufficiently low temperatures for detection of their superconducting properties. This oversight resulted in a 16 year delay in the discovery of high temperature superconductivity in copper-oxide compounds. 

In early 1987, the composition and structure of the La-Ba-Cu -O superconductor was still unknown to the general public in the United States. By March of that year certain facts became known from Japanese publications. But at this point in time, a newer, higher Tc (> 90 K) material was announced. This new copper oxide superconductor was quite easy to prepare and, in addition to interested physicists, these new materials could be synthesized by ceramists, chemists, metallurgists, material scientists, or anyone with a knowledge of a chemical approach to solid-state materials. Even high school students developed simple methods for the synthesis of these compounds. The "high" transition temperature and the possible use of liquid nitrogen made research in superconductivity accessible to most scientists and laboratories. The media also capitalized on this worthy news report and published it in newspapers and also presented it on television as a news item. 

With the discovery and disclosure of these events in the area of "High Tc Superconductivity", hundreds, if not thousands, of scientists actively became involved in research on these new materials. Newer materials and higher Tc s soon followed. The competition was fierce and the progress through 1987 and 1988 was moving at a rapid pace with numerous important discoveries. To date, the highest Tc is in the range of 110-125 K, some five times that obtained in 1973 on the revolutionary (A-15) intermetallic materials. These new copper -oxide systems, many of which will be described in detail by other contributors to this book, are presented in Table 10. 

The lack of homogeneity pervades most of the systems where high Tc superconductivity has been observed. The n-type copper oxides, e.g., Nd2 xCexCu04 and Nd2Cu04 xFx are particularly plagued with this problem because superconductivity exists over such a small range of x. The inevitable variations of x on a microscopic level will inevitably lead to materials which are not electronically homogeneous. 

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