Abstract
The ability to control chemical and physical structuring at the nanometre scale is important for developing high-performance thermoelectric materials1. 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 scales2,3,4,5,6 or multiscale hierarchical architectures7,8. 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 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.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (numbers 51620105014, 11274248, 51572210 and 51521001) and the National Basic Research Program of China (973-program) (number 2013CB632505). XRD, FESEM, EPMA, HRTEM and Raman-scattering experiments were performed at the Center for Materials Research and Testing at Wuhan University of Technology. Hall measurements were performed at the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing at Wuhan University of Technology. XPS were recorded at the Key Laboratory of Catalysis and Materials Science at the State Ethnic Affair Commission and Ministry of Education of South-Central University for Nationalities. The measurements of static magnetic properties and UPS were performed at the School of Physics and Technology at Wuhan University. We thank S. B. Mu, W. Y. Chen, M. J. Yang, C. H. Shen, X. Q. Liu and Y. Y. Qi for help with structure characterization, J. R. Sun and T. Y. Zhao from the Institute of Physics of CAS for suggestions, and W. Wang from Dongguan University of Technology for preparing magnetic nanoparticles.
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W. Zhao and Q.Z. designed a magnetic nanocomposite thermoelectric material for this work. Z.L., C.L., S.M., D.H. and P.J. synthesized the samples. W. Zhao, Z.L., Z.S., X.M., H.Z., X.S., J.S. and X.T. carried out the thermoelectric-property and Hall measurements. W. Zhu, X.N., P.W. and Y.L. performed the electron microscopy analysis and XPS experiments. W. Zhao, P.W., Z.S., Y.L., C.L., J.S., B.S. and X.D. performed the magnetic measurements. W. Zhao, Q.Z., Z.L., Z.S., J.Y. and B.S. conceived the experiments, analysed the results and wrote and edited the manuscript. All authors read the paper and commented on the text.
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Extended data figures and tables
Extended Data Figure 1 Effect of Co nanoparticles on structure.
a, XRD patterns of Co nanoparticles. b, XRD patterns of the first batch of xCo/Ba0.3In0.3Co4Sb12 (x = 0, 0.1%, 0.2% and 0.3%; labelled MNC00, MNC01, MNC02 and MNC03). c, Raman spectra of the first batch of samples. d–g, BEIs of the first batch of samples.
Extended Data Figure 2 Effect of the Co nanoparticles on microstructure.
a, b, FESEM images of MNC00 (a) and MNC02 (b). c, HRTEM image of MNC02. d–g, FESEM images of MNC00R, MNC01R, MNC02R and MNC03R, respectively.
Extended Data Figure 3 Theoretical blocking temperature (TB) and Curie temperature (TC), and the Ohmic contact experiment.
a, c, e, Theoretical TB dependence of the critical diameter DC. b, d, f, Theoretical TC dependence of the critical diameter DC. The oval shaded areas delineated by dotted lines indicate the range of TB and TC for the Co and Ni nanoparticles diameters of 5–10 nm and the Fe nanoparticles diameters of 2–5 nm. g, Experimental sample for measuring the Ohmic contact between Co and filled skutterudite. Co was first evaporated onto the matrix and then Cu electrodes were connected with the Co using a tin solder. h, I–V plot: a straight line through the origin (0, 0), demonstrating a good and stable Ohmic contact between the Co and the matrix, consistent with the analytical result from the work functions of the Co nanoparticles and the matrix.
Extended Data Figure 4 Experimental data from Hall measurements and theoretical Lorenz number (L).
a, b, The first batch of samples. c, d, The second batch of samples. The measuring current remains unchanged at 100 mA. The magnetic field is varied in the range 0.1–1.2 T. The ΔVxy values measured in the range 0.5–1.2 T are used to linearly fit the Hall coefficient. e, f, Temperature dependence of L for the first (e; MNC00, MNC01, MNC02 and MNC03) and second (f; MNC00R, MNC01R, MNC02R, MNC03R and MNC04R) batches of MNC thermoelectric materials batch in the range 300–850 K.
Extended Data Figure 5 Effect of the Co nanoparticles on the electrical and thermal transport properties in the range 300–850 K.
a–f, The first batch of samples. g–l, The second batch samples. a–l, Temperature dependence of the electric conductivity σ (a, g), the Seebeck coefficient α (b, h), the power factor α2σ (c, i), the thermal conductivity κ (d, j), lattice thermal conductivity κL (e, k) and ZT (f, l). The insets in d and j show the temperature dependence of the carrier thermal conductivity κE. The error bars in f and l are set to 5%.
Extended Data Figure 6 Effect of the Fe, Co and Ni nanoparticles on the thermoelectric and magnetic properties.
a–d, Temperature dependence of the electric conductivity σ (a) Seebeck coefficient α (b), thermal conductivity κ (d) and ZT for MNC00R and xTM/Ba0.3In0.3Co4Sb12 (TM = Fe, Co or Ni) with x = 0.2%, labelled MNC02R_Fe, MNC02R_Co and MNC02R_Ni, respectively. The inset in c shows the temperature dependence of the lattice thermal conductivity κL. e–h, M–H curves at room temperature. The insets in e–h show close-ups near zero field.
Extended Data Figure 7 Experimental evidence of the superparamagnetism of the Co nanoparticles with diameters of 5–10 nm.
a, ZFC and FC M–T curves. b, M–H curves at 300 K and 450 K. c, Langevin fitting of the M–H curves at 450 K. d, TEM image of the Co nanoparticles. The inset in b shows the M–H curves near zero field. The inset in c shows the results of the Langevin fitting.
Extended Data Figure 8 Experimental evidence of the superparamagnetism of the Fe nanoparticles with diameters of 2–5 nm.
a, ZFC and FC M–T curves. b, M–H curves at 50 K and 350 K. c, Langevin fitting of the M–H curves at 350 K. d, TEM image of the Fe nanoparticles. The inset in b shows the M–H curves near zero field. The inset in c shows the results of the Langevin fitting.
Extended Data Figure 9 Experimental evidence of the superparamagnetism of the Ni nanoparticles.
a, ZFC and FC M–T curves. b, M–H curves at 50 K and 350 K. c, Langevin fitting of the M–H curves at 350 K. d, TEM image of the Ni nanoparticles with diameters of 5–10 nm. The inset in b shows the M–H curves near zero field. The inset in c shows the results of the Langevin fitting.
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Zhao, W., Liu, Z., Sun, Z. et al. Superparamagnetic enhancement of thermoelectric performance. Nature 549, 247–251 (2017). https://doi.org/10.1038/nature23667
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DOI: https://doi.org/10.1038/nature23667
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