Electrical resistivity lanthanum

Laboratory reagents, preparation of, 8-1 to 4 Laboratory Solvents and Other Liquid Reagents, 15-13 to 22 Laguerre polynomials, A-83 to 85 Lanthanum see also Elements electrical resistivity, 12-39 to 40 electron configuration, 1-18 to 19 heat capacity, 4-135 history, occurrence, uses, 4-1 to 42 ionization energy, 10-203 to 205 isotopes and their properties, 11-56 to 253 magnetic susceptibility, 4-142 to 147 molten, density, 4-139 to 141 physical properties, 4-133 to 134 thermal properties, 12-201 to 202 vapor pressure, 6-61 to 90 vapor pressure, high temperature, 4-136 to 137... 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

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. 

Chu kept it up, trying one recipe after another like a chef in pursuit of the perfect sauce. Finally, he and his team, along with Maw-Kuen Wu at the team s University of Alabama unit (Wu was a former graduate student of Chu s), replaced the lanthanum with the rare earth yttrium. They heated the mixture for hours at 1,652° F, ground the solid mass produced, and sintered it at 2,192° F. Wu drenched it with coolant, but this time he used liquid nitrogen. When Wu passed an electric current through the new ceramic, its resistance dropped sharply—at what physicists would later call a balmy 93° K (-292° F). We were so excited and so nervous, recalled Wu, that our hands were shaking. At first we were suspicious that it was an error. ... 

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