Lanthanum-barium copper oxide

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. 

The now-famous formula for the first superconducting ceramic lanthanum-barium-copper-oxide. 

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. 

Some positive steps were made in this direction through the use of niobium-based alloys. In 1973 the alloy Nb3Ge was found to have a Tc = 23 K, which remained the highest attainable critical temperature until 1986. In 1986 two IBM scientists in Zurich, the German, Johannes Georg Bednorz and the Swiss, Karl Alex Muller found that a ceramic oxide based on lanthanum, barium and copper (of stoichiometry... 

The discovery of a barium-doped lanthanum copper oxide which became superconducting at 35 K led to a flood of new high temperature superconductors some of which were superconducting above the boiling temperature of nitrogen, 77 K. Over 50 high temperature superconductors, almost all containing copper oxide layers, are now known. 

Research chemists found that they could modify the conducting properties of solids by doping them, a process commonly used to control the properties of semiconductors (see Section 3.13). In 1986, a record-high Ts of 35 K was observed, surprisingly not for a metal, but for a ceramic material (Section 14.24), a lanthanum-copper oxide doped with barium. Then early in 1987, a new record T, of 93 K was set with yttrium-barium-copper and a series of related oxides. In 1988, two more oxide series of bismuth-strontium-calcium-copper and thallium-barium-calcium-copper exhibited transition temperatures of 110 and 125 K, respectively. These temperatures can be reached by cooling the materials with liquid nitrogen, which costs only about 0.20 per liter. Suddenly, superconducting devices became economically viable. 

Since 1911, scientists have been searching for materials that superconduct at higher temperatures, and more than 6000 superconductors are now known. Until 1986, however, the record value of Tc was only 23.2 K (for the compound Nb3Ge). The situation changed dramatically in 1986 when K. Alex Muller and J. Georg Bednorz of the IBM Zurich Research Laboratory reported a Tc of 35 K for the non-stoichiometric barium lanthanum copper oxide BavLa2-.vCu04, where x has a... 

Second, the composition of these materials is difficult to determine. They usually contain barium, copper, lanthanum, yttrium, and oxygen. They often contain other elements. But they are not simple compounds, like copper oxide (CuO) or yttrium oxide (Y2O3). Instead, they are complex mixtures of the elements. 

Using the same technique, it is also possible to precipitate composite particulates. The latter can be homogeneous of exact stoichiometry, as exemplified by pure or doped barium titanates [9]. To achieve these conditions rapid mixing is required, such as by using the controlled double jet precipitation process. In contrast, slow precipitation results, as a rule, in internal inhomogeneity, that is, the composition changes from the center to the periphery, although the particles may still be perfectly spherical, as observed with mixed alumina/silica [10] or copper/ lanthanum and copper/yttrium oxides [11]. 

Barium oxide, barium salicylate and/or other organobarium salts 2) Organometallic salts of lanthanum, hafnium and tantalum and their oxides Replacements for lead and copper compounds currently employed for ballistic modification. Particularly attractive due to production of considerably less smoke during combustion of propellants [245]. 

In January 1986, Bednorz was working alone in his lab with his latest mix. Instead of just combining the powdery oxides, as the French team had done, he dissolved them in water, allowed the particles to settle, then fired them at around 1,800° F. What he came up with was a sandwich consisting of layers of lanthanum and barium atoms alternating with layers of copper and oxygen atoms. 

Bednorz and Muller showed that a cuprate, La2Cu04, becomes superconducting at Tc = 35 K if some lanthanum atoms are replaced by barium atoms. This doped compound may be written as La(2 x)BajCn04. The oxidation state for La is +3, for Ba +2, and for oxygen -2. The oxidation state of copper, y, is thus determined from... 

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