The ability to control chemical and physical structuring at the nanometre scale is important for developing high-performance thermoelectric materials. 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 or multiscale hierarchical architectures. 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 these effects can effectively manipulate electron and phonon transport at nanometre and mesoscopic length scales and thereby improve the thermoelectric performance of the resulting nanocomposites.
Synergistically regulating carrier and phonon transport on the nanoscale is extremely difficult for all thermoelectric (TE) materials without cage structures. Herein BaFe 12 O 19 /Bi 2 Te 2.5 Se 0.5 thermoelectromagnetic nanocomposites are designed and synthesized as a benchmarking example to simultaneously tailor the transport properties on the nanoscale. A magneto-trapped carrier effect induced by BaFe 12 O 19 hard-magnetic nanoparticles (NPs) is discovered, which can lower the carrier concentration of n-type Bi 2 Te 2.5 Se 0.5 matrix by 16%, and increase the Seebeck coefficient by 16%. Meanwhile, BaFe 12 O 19 NPs provide phonon scattering centers and reduce the thermal conductivity by 12%. As a result, the ZT value of the nanocomposites is enhanced by more than 25% in the range of 300-450 K, and the cooling temperature difference increases by 65% near room temperature. This work greatly broadens the commercial application potential of ntype Bi 2 Te 2.5 Se 0.5 , and demonstrates magneto-trapped carrier effect as a universal strategy to enhance the electro-thermal conversion performance of TE materials with high carrier concentration.
Thermoelectric (TE) materials have great potential for waste-energyrecycling and solid-state cooling. Their conversion efficiency has attracted huge attention to the development of TE devices, and largely depends on the thermal and electrical transport properties. Magnetically enhanced thermoelectrics open up the possibility of making thermoelectricity a future leader in sustainable energy development and offer an intriguing platform for both fundamental physics and prospective applications. In this review, state-of-the-art TE materials are summarized from the magnetism point of view, via diagrams of the charges, lattices, orbits and spin degrees of freedom. Our fundamental knowledge of magnetically induced TE effects is discussed. The underlying thermo-electro-magnetic merits are discussed in terms of superparamagnetism-and magnetic-transition-enhanced electron scattering, field-dependent magnetoelectric coupling, and the magnon-and phonon-drag Seebeck effects. After these topics, we finally review several thermal-electronic and spin current-induced TE materials, highlight future possible strategies for further improving ZT, and also give a brief outline of ongoing research challenges and open questions in this nascent field.
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