Superparamagnetic enhancement of thermoelectric performance

By embedding superparamagnetic nanoparticles in a thermoelectric matrix, phonon and electron transport within the material can be controlled simultaneously at nanometre and mesoscopic length scales, thereby improving the thermoelectric performance of the material. Magnetic performance enhancers Increasing the efficiency of thermal-to-electric energy conversion using thermoelectric materials typically focuses on nanostructuring—in particular, varying the chemical and physical properties at the nanoscale to modify the thermal conductivity of the whole material. Electrical conductivity is also a key parameter and now Wenyu Zhao et al . report a nanostructuring strategy that offers simultaneous control over thermal and electrical transport, with a net gain in overall thermoelectric performance of the material. They embed nanoparticles of a soft magnetic material in a thermoelectric matrix. The nanoparticles themselves provide conventional scattering sites for thermal vibrations (phonons) in the matrix while superparamagnetic fluctuations of the magnetization state of the nanoparticles interact with the matrix electrons, separately modifying their transport properties. The ability to control chemical and physical structuring at the nanometre scale is important for developing high-performance thermoelectric materials 1 . Progress in this area has been achieved mainly by enhancing phonon scattering and consequently decreasing the thermal conductivity of the lattice through the design of either interface structures at nanometre or mesoscopic length scales 2 , 3 , 4 , 5 , 6 or multiscale hierarchical architectures 7 , 8 . A nanostructuring approach that enables electron transport as well as phonon transport to be manipulated could potentially lead to further enhancements in thermoelectric performance. Here we show that by embedding nanoparticles of a soft magnetic material in a thermoelectric matrix we achieve dual control of phonon- and electron-transport properties. The properties of the nanoparticles—in particular, their superparamagnetic behaviour (in which the nanoparticles can be magnetized similarly to a paramagnet under an external magnetic field)—lead to three kinds of thermoelectromagnetic effect: charge transfer from the magnetic inclusions to the matrix; multiple scattering of electrons by superparamagnetic fluctuations; and enhanced phonon scattering as a result of both the magnetic fluctuations and the nanostructures themselves. We show that together