The heat goes up on thermoelectric materials There are huge sources of heat in the environment, which can be converted into electricity by thermoelectric materials. The NANOthermMA project aims to help researchers identify how materials can be engineered and nanostructured in order to improve thermoelectric performance, as Dr Neophytos Neophytou explains. The development of thermoelectric materials opens up the possibility of generating electricity from temperature differences, an exciting prospect in the context of energy sustainability concerns. However, thermoelectric materials have traditionally been quite inefficient, and even current nanostructured materials - which provide a boost in performance - are not yet efficient enough for wider implementation, a topic Dr Neophytos Neophytou and his colleagues are addressing in the NANOthermMA project. “The goal in the project is to create a new class of more efficient nanostructured materials, building on theory, simulations and experiments,” he outlines. The thermoelectric performance of a material is quantified by the ZT figureof-merit. “The numerator in the ZT includes the electrical conductivity multiplied by the square of the Seebeck coefficient. This is what we call the power factor,” explains Dr Neophytou. “The denominator is the thermal conductivity, which determines the heat flow, resembling the losses in the process.” A lot of attention has previously been focused on putting nanostructured features into a material to reduce thermal conductivity, so essentially reducing the denominator in the ZT figure-of-merit. While this has proved effective up to a point, there is only limited potential for further reductions, so Dr Neophytou and his colleagues are now taking a different approach. “The way to further improve
Nanostructured material geometry. Grain boundaries, nanoinclusions, and atomistic defects present obstacles for phonon transport (heat) as it propagates from the hot to the cold sides of a thermoelectric material.
efficiency is to work with the numerator, the power factor,” he outlines. There are some significant technical hurdles to overcome in this work however. “Nanostructuring reduces the electrical conductivity. We want to develop a nanostructure that doesn’t reduce the conductivity too much,” says Dr Neophytou. “There’s also an inherent relationship between the conductivity and the Seebeck coefficient – they are inversely proportional to each other.”
NANOthermMA project A material with a very high electrical conductivity will have a low Seebeck coefficient and vice-versa, and any effort to increase one
multi-scale geometries
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will reduce the other. The aim of the project is to investigate how to effectively decouple this interdependence, which could eventually lead to the development of more efficient thermoelectric materials. “The inverse relation between conductivity and the Seebeck coefficient is strong, but there are ways that you can tweak that,” explains Dr Neophytou. This research is built on a deep understanding of material structure and how electrons and phonons flow through it. “If you zoom into a 3-d bulk material you will often find that it has a lot of grain boundaries, dislocations and defects. It’s a bulk material, but if you zoom in there internally, you find that it’s not a perfect crystal, it has a lot of defects,”
Zooming in a nanostructured thermoelectric material, where the matrix material is described in a continuum way, but the smaller defects are treated in an atomistic manner.
atomistic scale EU Research
