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Cobaltates
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Cobaltates

A thirsty superconductor

Besides their superconducting properties, these cobaltates have many significant properties.

They are very efficient in transforming heat into electricity:  

If two different temperatures are applied at the ends of a metal or a semiconductor, a tiny electric voltage can appear. In these “thermoelectric” materials, this effect is stronger than anything we had observed in metals. Even if it is moderate (about 1 millivolt for a 10°C difference), we can hope to use these materials to convert heat in electric energy, using the heat from cars’ exhaust pipes. We do not clearly understand the origin of this property. It might be because the presence of sodiums means that the cobalts are not all identical. Some can carry little magnets (spins) while other do not. These magnets can hence be facing either up or down. Many configurations are possible, hence a strong “entropy” (a property representing all the possible configurations in a system). This spin entropy could explain these strong thermoelectric powers.

They can be used as very efficient batteries:

Sodiums can move between cobalt layers and conduct current, which enables to use these materials as batteries. These materials are called ionic conductors. If sodium is replaced by lithium, the latter being a lighter and smaller atom, it can move even better and the battery will also be more powerful. These lithium and cobalt batteries are used in IPods, for instance.

They become actual magnets when they should not

In these materials, each cobalt can carry a small magnet, a “spin”. One might predict that in the part of the phase diagram with few sodium atoms, where there are hence many spins, the spins could form a line and transform the material in an actual magnet. Quite the contrary however: where you think you would find magnetism, there is none, and the spins do not react. On the contrary, when a lot of sodium is added, we can see the spins forming a line! Why? This subject of research is still unresolved.

 

CNRSSociété Française de PhysiqueTriangle de la physique
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CNRSSociété Française de PhysiqueTriangle de la physique