Electronic and thermal transport decoupling

A new study from the University of Wollongong overcomes a major challenge of thermoelectric materials, which can convert heat to electricity and vice versa, improving conversion efficiency by more than 60%.

Current and potential future applications range from low-maintenance solid-state refrigeration to compact zero-carbon power generation, which could include small personal devices powered by body heat.

“The decoupling of electronic (electron-based) and thermal (phonon-based) transport will be a breakthrough in this area,” says UOW Prof Xiaolin Wang.

Thermoelectric applications and challenges

Bismuth telluride-based materials (Bi2Te3, Sb2Te3 and their alloys) are the most successful thermoelectric materials commercially available, with current and future applications falling into two categories: converting electricity to heat and vice versa:

• Converting electricity to heat: Reliable, low-maintenance solid-state refrigeration (heat pump) with no moving parts, no noise and no vibration.
• Conversion of heat to electricity, including fossil-free power generation from a wide range of heat sources or free powering of micro devices, using room or body temperature.

Heat harvesting takes advantage of the abundant free heat sources provided by body heat, automobiles, everyday life and industrial processes. Without the need for batteries or power, thermoelectric materials could be used to power smart sensors in remote and inaccessible locations.

An ongoing challenge of thermoelectric materials is the balance of electrical and thermal properties: in most cases, an improvement in the electrical properties of a material (higher electrical conductivity) means a worsening of thermal properties (higher thermal conductivity) and vice versa.

“The key is to separate thermal transport and electrical transport,” says lead author, Ph.D. student Guangsai Yang.

Improved efficiency through decoupling

The three-year project of the UOW Institute of Superconductivity and Electronic Materials (ISEM) has found a way to simultaneously decouple and improve both thermal and electronic properties.

The team added a small amount of amorphous nano-boron particles to thermoelectric materials based on bismuth telluride, using nanodefect engineering and structural design.

The amorphous nano boron particles were introduced using the spark plasma sintering (SPS) method.

“This reduces the thermal conductivity of the material and at the same time increases its electron transmission,” explains corresponding author, Prof Xiaolin Wang.

‘The secret of thermoelectric materials engineering is to manipulate the transport of phonons and electrons,’ explains Professor Wang.

Since electrons transport heat and conduct electricity, materials engineering based on electron transport alone is subject to the perennial trade-off between thermal and electrical properties.

Phonons, on the other hand, only carry heat. Therefore, blocking the transport of phonons reduces the thermal conductivity induced by the vibrations of the lattice, without affecting the electronic properties.

“The key to improving thermoelectric efficiency is to minimize heat flux via phonon blocking and maximize electron flux via (electron transmission),” says Guangsai Yang. “This is the origin of the record thermoelectric efficiency in our materials.”

The result is a record conversion efficiency of 11.3%, which is 60% better than commercially available materials prepared with the zone casting method.

In addition to being the most successful thermoelectric materials commercially available, bismuth telluride-based materials are also typical topological insulators.

Ultra high thermoelectric performance in BiSbTe / amorphous boron bulk composites with Nano-defective architectures was published in Advanced energy materials in September 2020.