The inefficiency of machinery such as automobiles is continuing to lead researchers to identify materials that can convert waste heat into electricity. Automobiles are a prime example because 60% of fuel efficiency is lost due to waste heat (see Figure 2).


Figure 2. Theoretical studies are underway to develop a superior thermoelectric material that could potentially convert waste heat produced during internal combustion into useful electricity. (Figure courtesy of The University of Texas Permian Basin.)

Thermoelectric materials are known to convert heat into electricity by using a temperature difference such as observed at the tailpipe of an automobile where there is a significant difference in the temperature of the automotive exhaust and the ambient temperature. The challenge is to improve the efficiency of thermoelectric materials to enable the process to be commercially viable.

In a previous TLT article (1), researchers developed a new approach for producing a thermoelectric material than taking advantage of a temperature gradient. A new thermoelectric material containing randomly dispersed platinum nanoparticles in a nickel matrix utilized the inverse spin Hall effect to create an electric voltage. Electricity was generated by taking advantage of the difference in the magnetic properties between the ferromagnetic material, nickel and a normal metal, platinum. 

Dr. Anveeksh Koneru, senior lecturer in mechanical engineering at the University of Texas Permian Basin in Odessa, Texas, is focused on trying to determine how to increase the figure of merit (ZT) for a thermoelectric material. He says, “ZT is a unitless, non-dimensional parameter that is directly related to the efficiency of a thermoelectric material. A higher ZT directly correlates to a better performing thermoelectric material.”

ZT is directly proportional to the Seebeck coefficient and the electrical conductivity of the material while inversely proportional to the thermal conductivity. Koneru says, “Most materials encountered exhibit both strong electrical and thermal conductivity. Finding a material with strong electrical conductivity but weak thermal conductivity is very challenging. One other factor is that the temperature of the system can impact the efficiency of the thermoelectric material. In moving from a temperature of 300 K to 700 K, the efficiency of the thermoelectric device can exhibit a five-fold improvement in efficiency.”

Koneru believes that ZT can be increased further by incorporating the spin of electrons in polarized materials through the use of the inverse spin Hall effect. This is known as spin caloritronics. He says, “The spin Seebeck effect should allow the harnessing of extra voltage in addition to the conventional voltage generated by the thermoelectric material. A key factor is that any change in the Seebeck coefficient leads to a four-fold improvement in ZT.”

The spin Seebeck effect is derived from the difference in the number of electrons with their spins in the upward direction compared to the number with their spins in the downward direction. Koneru envisions a model for a thermoelectric material that combines the conventional thermoelectric effect with a contribution from the spin thermoelectric effect. 

Koneru is in the process of conducting theoretical studies using supercomputers to identify a superior thermoelectric material.

Oxides of cobalt, nickel and zinc
In a recent presentation (2) where their initial results were shared, Koneru and his colleagues discussed the use of spin caloritronics to develop an oxide based on cobalt, nickel and zinc that displays superior thermoelectric properties. Koneru says, “Cobalt oxide is an appealing material to work with because other transition metals such as nickel can be substituted without an issue. Our objective is to work with different percentages of cobalt, nickel and zinc in order to optimize the characteristics of the thermoelectric material.”

The three metals have different properties with respect to magnetism and electrical conductivity. Cobalt and nickel are both magnetic and conductive while zinc is neither magnetic nor conductive. Differences in the magnetic properties may enable the band gap between electron states to increase leading to better thermoelectric properties.

The researchers are devising between 1,000 and 1,500 configurations of cobalt, nickel zinc oxides that are evaluated theoretically for band gap, lattice parameter, the effective mass of conduction electrons and spin polarization. The challenge is to find a material that maximizes the difference in the population densities of electrons in the upward spin state compared to the lower spin state. Koneru says, “A perfectly symmetric system with equal number of electrons in both states is not desired. The more asymmetry between the two electron spin states will lead to a better spin thermoelectric effect.”

The researchers are substituting nickel and zinc atoms for one of the three cobalt atoms in the transition metal oxide. Koneru says, “We found that a combination of 25% zinc and 75% nickel in the tetrahedral configuration of the transition metal oxide produced optimum performance in our initial calculations.”

Koneru considers this to be a sweet spot but is looking at other configurations in the future, including those based on manganese and iron. He says, “We also are going to take a close look at how the thermal conductivity parameters can be minimized to also increase ZT.”

Additional information can be found in the recent presentation and by contacting Koneru at anveeksh.koneru@utpb.edu. 

REFERENCES
1. Canter, N. (2017), “Transverse Thermoelectric Device,” TLT, 73 (3), pp. 12-13.
2. Hines, N., Resende, G., Girondi, F., Boadi, S., Musho, T. and Koneru, A. (2019), “Substitutional Effects of Bivalent Zn and Ni Cations on Spin Thermoelectric Properties of Co₃O₄,” Presented at the Materials Research Society Meeting in Phoenix, Ariz., on April 22, 2019.