Electrical resistivity rare earth

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. 

Rare earths have higher magnetic performances, service temperatures, electrical resistance and costs. 

An interesting series of bismuth cuprates in terms of the variation of the Tc as well as the hole concentration with composition is provided by Bi2Sr2Ca1 a.Lna.Cu208 where Ln = Y or rare earth (Rao et al. 19906). The electrical resistivity data show a metal-insulator transition in the normal state with change in x (figure 12). The Ta as well as the nb show a maximum at a composition of x = 0.25 (figure 13). Note that when Ca is fully substituted by Ln, the material becomes a non-superconducting insulator. Hole concentration in these bismuth cuprates is readily determined by Fen-Fem redox titrations. 

More than one boride phase can be formed with most metals, and in many cases a continuous series of solid solutions may be formed. Several methods have been used for the relatively large-scale preparation of metal borides. One that is commonly used is carbon reduction of boric oxide and the appropriate metal oxide at temperatures up to 2000 °C. Fused salt electrolysis of borax or boric oxide and a metal oxide at 700 1000 °C have also been used. Small-scale methods available include direct reaction of the elements at temperatures above 1000 °C and the reaction of elemental boron with metal oxides at temperatures approaching 2000 °C. One commercial use of borides is in titanium boride-aluminum nitride crucibles or boats for evaporation of aluminum by resistance heating in the aluminizing process, and for rare earth hexaborides as electronic cathodes. Borides have also been used in sliding electrical contacts and as cathodes in HaU cells for aluminum processing. 

The study of the electrical resistivity p may be the first property where some conclusions can be drawn. In ordinary transition or rare earth metals, the presence of the d and f bands is clearly seen in the analysis of the resistivity (for example (18)). 

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. ... 

Certain features of the study were emphasized such as the fact that although the PuOi.ei phase was assumed b.c.c., the powder patterns were not good enough to show the superstructure lines. Also the eutectoid in the proposed diagram of Chikalla et al. (22) was not observed in the x-ray studies. No satisfactory explanation of electrical resistivity and thermal expansion measurements which led to this earlier construction has emerged. Gardner et al. (26) also looked for but did not see any indication of ordered intermediate phases at low temperatures such as were described above for the rare earth oxides nor did they observe a failure of the miscibility gap to close as would have occurred if the end members, PuOi.oi and PuOi.gg, had different symmetries. They could... 

Rare earth metal RE-doped SrTiOsCRE = Nd, Pr, and La) in forms of Sr i.zRE Ti i.zTi Os showed good electrical conductivity after being reduced at high temperatures in the H2 atmosphere, but exhibited low corrosion resistance in a 50% H2SO4 solution at 353 K, due to the formation of SrS04. 

Rare earth Density Surface tension Viscosity Heat capacity Thermal conductivity Magnetic susceptibility x Electrical resistivity AV(l s) = 645 nm Temp. 

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