Tokyo Institute of Technology (TIT) scientists demonstrated that breaking open the trade-off problem between thermopower and conductivity improves thermoelectric performance.

Energy consumption in developed countries has been rather wasteful. Nearly two-thirds of the total energy is typically discarded as “waste heat,” which ends up simply dumped into the atmosphere. Lately, finding a way to productively use this heat has been for many material researcher’s a priority.

One of the possible ways to recover this waste heat as electricity is via “thermoelectric conversion” that uses temperature differences in semiconductors to convert waste heat into electric power. The thermopower (S) is a measure of the magnitude of an induced thermoelectric voltage in response to a temperature difference from the heated side to the cool side. The electric power is evaluated by power factor (PF), which is the product of thermopower (S)-squared and electronic conductivity (s). Therefore, the high electric power (PF) is capable of combining large S with high s in thermoelectric materials.

However, the PF is constrained by a trade-off between S and s. The S and s depend on carrier concentration, and thus the PF is usually maximized by tuning the carrier concentration with the addition of impurity elements – the s increases with increasing impurity concentration but the S decreases. This trade-off limits the PF.

Image Credit: Tokyo Institute of Technology. Click image for the largest view.

The TIT team suggested introducing lattice strain into Mott insulator oxide LaTiO3 converting the electronic state to metal, and increasing both thermopower and conductivity to induce a 100-fold increase in power factor, which in turn enables the conversion of waste heat to electricity more efficiently. Their findings could help ensure a more energy-efficient future.

In a recent study published in Advanced Science, a team of scientists led by Associate Professor Takayoshi Katase of the Tokyo Tech have discovered a way to break this trade-off. The scientists grew thin films of the Mott insulator oxide LaTiO3 on different substrates and found a way to introduce epitaxial strain, a strain that is born from a mismatch in the lattice structures of the substrate and the deposited (epitaxial) film. The artificial compressive strain was able to change LaTiO3 from the Mott insulator to metal. In the metallic state, increase in both S and s resulted in a hundred-fold increase in PF.

Dr. Katase said, “Different from the conventional way of impurity doping, the behaviors of s and S are apparently decoupled in lattice strained LaTiO3 films, which leads to a spectacular boost in power factor that defies conventional wisdom.”

Increasing the epitaxial compressive strain can induce a change in carrier polarity from p-type to n-type. Dr. Katase and the team found that while the absolute value of thermopower increased with increasing carrier concentration under compressive strain, electronic conductivity also increased with increases in carrier mobility due to electronic structure change from Mott insulator to metal. Density functional theory calculation clarified that the Ti 3d band splits to form energy gap in p-type LaTiO3 film, while the energy gap was closed in the n-type film, which led to an unusual simultaneous increase in conductivity and thermopower.

This discovery promises to advance the field of thermoelectric materials.

Dr. Katase explained, “Our experiments suggest that epitaxial strain will be a novel tool to harvest large power factors from thermoelectric oxides that are inconspicuous in their bulk by breaking the trade-off problem. Metal chalcogenides such as Bi2Te3 have been known as high performance thermoelectric materials, but the chalcogenides have problems with toxic elements, and low thermal and chemical stability, which restrict the large-scale use of thermoelectricity.”

Contrarily, since oxides are stable in air and even at high temperatures, they are ideal for maintenance-free thermoelectric conversion applications. The thermoelectric conversion efficiency is much lower than that of metal chalcogenides at this stage.” he said.

Dr. Katase concluded, “But, by greatly improving the thermoelectric performance of oxides beyond the trade-off relationship, thermoelectric conversion is expected to become widespread as a general energy source.”


Your humble writer admits this post is quite into its technical niche, with the obscure seeming to be the prime items of interest. One could have almost skipped to Dr. Katase’s conclusion, he and his team have greatly expanded the function of thermoelectrics and just might have gotten far enough that commercial scaling could take place.

The reality that more than half of the energy and fuels we use simply gets wasted is expensive. If thermoelectric materials haul back a good share of that, the heat would be put to work instead of being lost warming up the air.

This is an improvement and just might qualify as a breakthrough.


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