Science and Technology

An electrifying discovery

M Ramesh | Updated on October 10, 2021

Seebeck effect, a 19th-century discovery, is at the heart of Prof Kanishka Biswas’ technology to convert waste heat to electricity

A ‘magic material’ that maintains a temperature differential, yet conducts electricity

 

The discovery of a material — cadmium-doped silver antimony telluride — that can conduct electricity but not heat has fetched the scientist responsible the prestigious Shanti Swarup Bhatnagar award.

The awardee, Prof Kanishka Biswas of the Jawaharlal Nehru Centre for Advanced Research, Bengaluru, an autonomous institution of the governmental Department of Science and Technology, believes the material has immense industrial applications. It can be employed wherever there is waste heat — from vehicle exhaust to chimneys of power plants.

Tata Steel has latched on to this technology and is working on a prototype to convert waste heat from blast furnaces into electricity. Biswas is also developing a UK-funded project to tap waste heat from chulas (cookstoves) in rural areas into electricity.

Typically, about 65 per cent of all energy embedded in a source (such as gasoline) is wasted as heat. If even a small part of this could be converted into (say) electricity, it is an enormous gain to the economy. That is the significance of Biswas’ discovery.

He told Quantum the material can convert 10-15 per cent of waste heat into electricity.

Past forward

At the heart of it is scientific knowledge picked up from nature back in 1822. German researcher Thomas Johann Seebeck discovered that if there is a temperature difference between two regions of two different electrical conductors, an electrical voltage develops, as the heated electrons move towards the cooler ones. For example, if you connect a copper strip and an aluminium strip and heat one of the ends, you can get a small current.

Ever since, researchers have been drooling over the prospect of using this ‘Seebeck effect’ to produce electricity from waste heat. For example, the exhaust gases from vehicles or power plants are hot; if the heat can be employed to raise the temperature of one end of a suitable material — to create a temperature differential between the two ends of the material — you are in business.

But the issue is that the temperature differential will not remain for long. You just can’t keep one end hot and the other cold for long, because the heat will be conducted throughout the material in no time and it will become uniformly hot.

On the other hand, if you use a poor conductor of heat, such as glass, you won’t have the electrical conductivity — so the electricity from the Seebeck effect cannot be tapped.

Material difference

The challenge has been to find a magic material that will keep the temperature differential and yet conduct electricity. The material has to behave like a metal (good conductor of electricity) and glass (bad conductor of heat) at the same time.

Temperature conduction in a material happens because of the migration of ‘phonons’ from the hotter to colder areas. Phonons are ‘particles of heat’, just as photons are ‘particles of light’. The atoms in a material are typically organised as lattice structures (like the scaffolding around buildings); when heat is applied, these lattices vibrate, shaking off phonons. If you stop the flow of phonons, you stop the flow of heat.

This is what cadmium does in Biswas’ material — it stops phonon migration (‘phonon suppression’) while creating a channel for the flow of electrons. Cadmium-doped silver antimony telluride is, therefore, a ‘high-performance thermoelectric material’ that can produce electricity without generating an ounce of carbon dioxide.

Published on October 10, 2021

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