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Normally, it is not a smooth sailing for electrons travell- ing in a material. They en- counter resistance from the atoms of the material, which themselves continuously move back and forth. This vibratory motion of atoms .often scatters the electrons from their paths. Additional 9pposition to the movement of electrons comes from im- purities and dislocations of the normal atomic arrange- ment in the crystal. The net effect is the electrical resis- tance of the material.
It is heat which provides the energy to atoms to vibrate. Hence the vibra- tions of atoms become weaker and weaker as the
Molecules vibrate more vigorously ... C
on heating a material
materIal IS cooled. onse-
quently, as the motion of
atoms becomes restricted at lower temperatures, their inter-
ference with the movement of electrons also becomes lesser
Magnets repelled by superconductors float
and lesser. So electrons carrying electric charges face less hurdles.
This exp lains the fall in resistance or the increase in con- ductivity observed
by Onnes on
gradually lower- ing the tempera- ture of mercury.
The temperature below which su- perconductivity occurs is called the superconducting transation
temperature (Tc) which differs from material to material.
Another important property of superconductors is that they repel magnets. This property was discovered in 1933 by a German scientist, Walther MEISSNER (1882-1974).In fact, It is called Meissner effect and it is used to test whether a compound is a superconductor or not. The zero electrical resistance and Meissner effect promised many potential ap- plications of superconductors, but most of these remained dreams because the temperatures involved were so low. It is very costly to realise and maintain such low temperatures.
However, things changed suddenly with the discovery of ceramic superconductors by Bednorz and Mueller. Bednorz and Mueller found that a mixed oxide of lanthanum, barium, and ~opper prepared by them turned out to be superconduct- ing. That too at a superconducting transition temperature of 35 K a temperature far above that of any superconductor
Alex Mueller and Georg Bednorz
known till then. It had one atom of copper and slightly less than four atoms of oxygen for every two atoms of lanthanum (Laz-xBaxCu04-y)with some of the lanthanum replaced by barium. Bednorz and Mueller prepared a series of related compounds by changing the amount of barium continuousl y, keeping the concentration of other elements same. They ul- timately hit upon a ceramic containing the right combination of these atoms which was superconducting. Later many other similar ceramic compounds, which are superconducting at still higher temperatures, were prepared. The most famous of these, prepared by
c.w.
COO and his coworkers at the University of Houston, USA, is an oxide of yttrium (Y), barium and copper (YBazCu307_y).It shows superconduc- tivity at 93 K. These temperatures are still much lower than the room temperahlre, but they can be easily attained at reasonable costs, being higher than the temperature of liquid nitrogen (77K)which can be produced cheapl y.New Recipes
The traditional method of preparing ceramics was used in the production of these high temperature superconductors. The
ct Oxygen
• Copper
The crystal lattice of YBa2CU307-y
individual solid metal oxides were mixed in the proper ratio, ground well with a pestle and mortar and finally heated to around 1000°C for 10-12 hours. A reaction occurring in the solid state converted the in- dividual oxides into the mixed oxide compound.
The property of su- perconductivity in these compounds depends upon a characteristic ar- rangement of con- stituent atoms. For
example, in
YBa2Cu307-y suc- cessive horizontal layers of copper- oxygen sheets are separated by rib- bon-like chains of copper-oxygen.
The square copper- oxygen sheets are the pathways of su-
perconducting electrons in these compounds. Even slight
disruption of the arrangement of atoms destroys supercon-
ductivity_ The preparation of superconducting ceramic,
therefore, is an exacting piece of work
The traditional method of ceramic preparation usually produces a number of other non-superconducting com- pounds also which differ from the desired one only slightly in the ratio of different atoms present in them. This is due to incomplete and non-uniform mixing of the reacting metal oxides. For practical applications, we need purest samples of superconductors, but the separation of pure superconduct- ing compounds from related compounds is very tedious.
Secondly, the prod ucts obtained are either powders or tablets instead of directly usable wires or thin sheets. Both the problems have been partly solved by new techniques of preparation which depend upon the reaction in the solution state. In the solution, the mixing of reactants is thorough and uniform. In one such method, the starting materials themsel- ves are changed. Metal alkoxides - compounds of metals with alcohols - are used instead of metal oxides. The alkoxides are soluble in organic solvents, and hence a thorough and uniform mixing can be obtained by stirring the mixture. On adding water, the alkoxides change into oxides, which being insoluble, settle at the bottom. The uniform mixture of oxides thus obtained is heated at 700°C to get the superconducting ceramic.