The third objective, efficient design, involved sandwiching the nanostructured bismuth telluride between two copper plates and then enclosing the upper plate (the one coated with the light-absorbing oxides) and the bismuth telluride in a vacuum.

ECONOMIST: Solar power

The copper plates conducted heat rapidly to and from the bismuth telluride, thus maintaining the temperature difference.

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And the only workable alternative uses bismuth which has its own supply problems.

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The second desideratum, of low thermal and high electrical conductivity, was achieved by dividing the bismuth telluride into pellets a few nanometres across.

ECONOMIST: Solar power