Nanomaterials are cool

Materials World magazine
6 Aug 2019

Harvesting waste thermal energy using a novel method of producing a thermoelectric nanomaterial. Ceri Jones finds out how.

An economical thermoelectric nanomaterial has been developed that can capture heat energy emitted from machines and vehicles, and convert it back into electricity.

The process uses colloidal quantum dots with low thermal conductivity, which can be created at lower temperatures than current thermoelectric materials, according to researchers at King Abdullah University of Science (KAUST) in Saudi Arabia, led by PhD researcher Mohamad Nugraha. Due to its in-solution preparation method, the thermoelectric can be used as a coating for simple application to surfaces such as inexpensive flexible plastics.

Waste heat energy is largely considered a problem to address rather than a resource to harness. However, this may change with the continued growth of high-performance technologies. For instance, Big Data, the Internet of Things and 5G all hinge upon vast data centres of densely packed computing equipment needing to be cooled 24/7. Effective thermal energy harvesting could reduce the need for expensive air-conditioning or the electricity generated could be used to help power the centre.

How it works

The team at KAUST used spin coating to first apply the lead-sulphide quantum dot solution to the substrate film, then added a different solution of short linker ligands (acetonite) that cross-link the quantum dots together to enhance the material’s electronic properties. This ligand solution stage was repeated 8-9 times to build up layers until the coating reached 200nm-thick. Finally, the material was dried by thermal annealing.

Thermoelectric nanomaterials function by building an electrical charge along the temperature gradient, and the voltage generated by this is called the Seebeck coefficient. Nugraha said the coating proved to have good thermoelectric properties with a 580μV K−1 Seebeck coefficient, and that the method itself had ‘some key factors leading to the enhanced Seebeck coefficient in our materials’. Further, the Seebeck coefficient was improved by the quantum confinement of the material.

Nugraha believes thermoelectrics have been stunted by a focus on very high-temperature materials that require processing at temperatures of up to 400°C, and which can be prohibitively expensive. Therefore, the team used cross-linking short ligands to enable strong inter-quantum dot coupling that requires annealing at a temperature of 175°C.

‘This lower processing temperature could cut production costs and means that thermoelectric devices could be formed on a broad range of surfaces, including cheap flexible plastics,’ according to the university. ‘The discovery is a step toward practical high-performance, low-temperature, solution-processed thermoelectric generators,’ Nugraha said.

Read more in the paper Low-temperature-processed colloidal quantum dots as building blocks for thermoelectrics, published in Advanced Energy Materials, here: