From hot, to cold, to electricity

Materials World magazine
,
30 Apr 2017

A new thermoelectric material could allow you to check your heart rate and charge your phone from the stove, as Ellis Davies reports.

Thermoelectric materials have the potential to fulfil the electrical needs of portable devices and body sensors. However, the drawback of current thermoelectric materials is their toxicity, making them unsuitable for use in bio-friendly products. Engineers at the University of Utah, USA, have developed a possible solution to this issue by producing a thermoelectric material that combines calcium, cobalt and terbium.

The thermoelectric effect is the process through which temperature difference is used to generate electricity. For this to work, one end of the material must be hot, and the other cold. Charge carriers from the hot side travel through the material towards the cold side, generating electrical voltage. The process can be powered by a number of everyday activities, such as heating a saucepan of water or wearing jewellery.

To produce the material, the team mixed powdered calcium, cobalt and terbium oxides to create a homogenous powder. This was calcined twice at 700oC for 20 hours before being compacted into 1.25cm pellets. After further compacting for uniformity, the pellets were shaped into 10mm x 4mm x 2mm rectangular samples.

Professor Shrikant Saini, first author of the study, told Materials World, ‘We tested the thermoelectric response by measuring resistance, Seebeck voltage (voltage generation while maintaining temperature difference), thermal conductivity and many other structural characterisations.’ Testing revealed that overall electrical resistivity in the samples decreased with an increase in temperature, indicating semi-conductive behaviour. The dosing of terbium also had a positive effect on the material, providing
a combination of merit at high temperature and good chemical stability. 

‘The basic elements are abundant in nature and the fabrication process is simple,’ said Saini, highlighting the inexpensiveness of the material as an advantage over commonly used cadmium, telluride or mercury based materials. Its non-toxic nature also makes it suitable for applications that come into contact with the human body, such as biosensors and other medical devices such as blood-glucose and heart monitors. 

The material also performs well under intense heat, with the potential for use in power plants to harness waste energy. Up to 60% of energy can be wasted as heat. ‘This material can be used in such an environment, as it is an oxide. We can reuse some of this waste heat to generate electricity using this material,’ Saini explained. 

Saini also highlighted the material for use in developing countries where electricity is scarce. This application would take the form of a saucepan, with the base made of the thermoelectric material. The water or food would act as the cold side, and the fire the hot. The material only requires a 1oC difference in temperature to create a charge, which is enough energy to power small devices such as mobile phones or tablets via a charging cable. 

Moving forward, the team is looking to scale the manufacturing process for commercialisation. ‘We are looking forward to bringing this product into society, but before that we are planning to test it on a higher scale. It could be two years before the material is on the market,’ said Saini, highlighting that the material will be studied further to make improvements, and experiments with alternative oxide combinations will be carried out as it moves towards commercialisation. A promising candidate for the next generation of thermoelectric applications.

View the study at go.nature.com/2oBEhiY.