Conducting heat with polymers

Materials World magazine
,
27 Sep 2019

Polymers designed to capture waste heat may create a cheaper and more efficient alternative for harvesting energy. Shardell Joseph finds out more.

A new method has been created for generating energy for electronic applications by designing electrically and thermally conductive polymers. The research team from the University of Massachusetts, USA, claims that by using a common material for generating energy, such as polymers, they could create a much cheaper and efficient alternative to existing conductive materials.

‘The polymers are environmentally friendly and solution-processable so they can be produced at a lower cost than inorganic thermoelectrics (TEs),’ the university’s Electrical and Computer Engineering Assistant Professor, Zlatan Aksamija, told Materials World. ‘They are also flexible, which makes it possible to use them in wearables or other applications where they need to conform to a shape.’

By observing polymer characteristics during the doping process, the researchers used this discovery as the key variable for making polymer materials conductive.

Doping polymers

The low thermal conductivity of polymers has been a huge barrier
for widening their use in industrial applications, such as in heat exchangers and electronic packaging. But through the use of doping – a process that adds molecules, or can
also remove or add electrons to polymers – the materials capacity to balance thermal and electric conductivity is enhanced.

‘For a good thermoelectric material, it should have high conductivity. Normally, polymers have poor conductivity,’ the university’s Chemistry Professor, Dhandapani Venkataraman, told Materials World. ‘One way to increase a material’s conductivity is to oxidise or reduce the polymer using chemical reagents, aka dopants.’

According to Aksamija, doping affects both of the two key TE properties – the conductivity and the Seebeck coefficient. ‘By increasing carrier concentration, doping improves conductivity,’ he explained. ‘Having more electrons available to carry the current makes the material more conductive.

‘At the same time, the average energy per electron goes down. The so-called thermopower, or Seebeck coefficient, is a measure of
the electrical voltage produced by every degree of temperature difference applied across the polymer. The Seebeck coefficient is closely related to the average energy per electron so doping decreases the Seebeck coefficient.’

The researchers explained how doping usually involves balancing the conductivity and Seebeck properties with a process called trade-off. ‘Basically, we are not seeking to maximise conductivity or Seebeck alone, we want the best possible combination of the two,’ Aksamija said. ‘That way we get the best of both – a higher thermoelectric voltage from the Seebeck while simultaneously being highly conductive – meaning less resistive and less lossy – material.’

Doping testing

To test the conductive and TE properties of the polymer, the team conducted experiments and efficiency analysis. This consisted of using different levels of doping, from zero to maximum, in order to discover the correct balance for electrically conductive polymers.

‘First we dope the polymer with iodine vapour for 24 hours,’ said Aksamija. ‘Then we measure the TE properties over the next 24 hours. Venkataraman’s lab has a custom-build setup to measure TE properties where the polymer is sandwiched between two copper blocks – copper is highly thermally and electrically conductive.

‘One block is heated and the other is kept at room temperature in order to create a temperature difference across the polymer, inducing a thermoelectric voltage. Then we measure the voltage and conductivity and store it. After a few minutes, some of the iodine vapour used to dope the polymer slowly evaporates, decreasing the concentration of dopants left.

‘We repeat the measurements at time intervals, gathering the Seebeck voltage and conductivity as the dopants evaporate, which gives us the entire sweep of these TE properties as a function of
doping concentration.’

The team expects the conductivity of polymers may come as a surprise to other scientists in the field, and believes their research may be relevant for future industrial application, as polymers could prove to be a useful material for generating energy.

‘I can envision polymer TEs being used to scavenge body heat and covert it to power wearable and health monitoring devices,’ said Aksamija. ‘Beyond that, we could take any source of waste heat and coat it with polymer TEs to recover part of that waste heat into electricity. This would mean less energy being wasted. Currently, more than half of all energy we use ends up as waste heat before reaching the consumer.’