To all intents and purposes, every device and machine we use emits heat and because of this a lot of energy is wasted. Imagine if we could convert this heat into some other form of energy, let’s say electricity – this could possibly provide green tech energy efficiency that is a must for a sustainable future. These new thermoelectric materials to be used in thermoelectric devices of the future can make this happen.

Schematic illustration of the multilayer configuration with layers of different porosity (graded porous material). Each layer contains a concentration of periodically distributed pores of the same size (only one set of such particles is shown). |CREDIT: APL Materials

A new study shows how porous substance can act as thermoelectric materials and how these materials can be integrated in the thermoelectric devices of the future. Scientists have been trying to engineer more efficient thermoelectric materials which is needed to capture the amount of energy wasted as heat.

About 70 percent of all the energy generated in the world is wasted as heat, said Dimitris Niarchos of the National Center for Scientific Research Demokritos in Athens, Greece.

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Of all the thermoelectric devices that have been made to capture the heat wasted, a thermoelectric material that’s filled with tiny holes that range in size from about a micron (10-6 meters) to about a nanometer (10-9 meters), could be more effective than ever as porous thermoelectrics can play a significant role in improving thermoelectrics as a viable alternative for harvesting wasted heat, said Niarchos .

Heat travels through a material at a specific rate via phonons, a quantum of sound or elastic vibrations that act as heat-carrying particles . The rate it travels depends on the material itself, some materials allow heat to move quickly through them, some materials allow heat to move very slowly through them. So, in case of this porous substance, when a phonon runs into a hole, it scatters and loses energy. Thus, phonons can’t carry heat across a porous material as efficiently, giving the material a low thermal conductivity; this maximizes the efficiency of heat-to-electricity conversion. The more porous the material is, the more efficiency it is.

Niarchos said the researchers have yet to systematically model how porous materials maintain low thermal conductivity, however they have known the fact that the smaller the pores and the closer they’re packed together, the lower the thermal conductivity. They also show that, in principle, micro-nano porous materials can be several times better at converting heat to electricity than if the material had no pores.

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He and Tarkhanyan, his co-author, studied the thermal properties of four simple model structures of micro-nano porous materials. Their analysis provides a rough blueprint for how to design such materials for thermoelectric devices.

The researchers made a series of observations of their models to find out the one with the least thermal conductivity. The results of each observation are:

  1. The first model describes a material filled with holes of random sizes, ranging from microns to nanometers in diameter.
  2. The second is one with multiple layers in which each layer contains pores of different size scales, which gives it a different porosity.
  3. The third is a material that’s composed of a three-dimensional cubic lattice of identical holes.
  4. The fourth is another multilayered system. But in this case, each layer contains a cubic lattice of identical holes. The size of the holes is different in each layer.

Compared to the second model, the first and fourth models have lower thermal conductivities. The third on also has a lower thermal conductivity than the fourth model.

CONCLUSION: The third one has the least thermal conductivity, so it happens to be the best.

Niarchos said, “Except for the first model, however, all the models aren’t practical because they represent idealized situations with a perfect arrangement of pores. It’s also practically impossible to create precisely equal-sized pores. The first model is thus the most realistic.”

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According to Niarchos, all the models are have their own distinctive feature and each one of them demonstrates the importance of porosity in thermoelectric materials. Built upon simple and general analytical formulas, the models allow for a very fast and accurate computation of the effective lattice thermal conductivity of a porous material and the systematic analysis of such materials.