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What We Do

1. We strive to understand the fundamental physics underlying the transport effects, which means that we study the interactions between the different particles in the presence of gradients in electric field and temperature.  Fundamentally, we showed for example that phonon transport does respond to external magnetic fields

Figure 1: Particles (purple) and the extensive thermodynamic quantities (green) they carry. The relationship between the particles and the extensive thermodynamic quantities investigated here are thermoelectric and spin caloritronic effects.Figure 1: Particles (purple) and the extensive thermodynamic quantities (green) they carry. The relationship between the particles and the extensive thermodynamic quantities investigated here are thermoelectric and spin caloritronic effects.

2. The thermoelectric and spin-caloritronic effects are applied in thermoelectric energy conversion device.  We strive to optimize these energy conversion mechanisms in order to maximize the thermal efficiency of the conversions devices.  These are all-solid-state heat-to-electricity converters, without moving parts.  As a result, they don’t wear out or require maintenance; they are vibration free, and, when properly designed, have an essentially infinite lifetime.  Unfortunately, as of today, they are less efficient than conventional heat engines (gas turbines, automotive engines, stem power plants), at least at a large scale.  At a power level below about 1kW, solid-state energy converters are competitive, see Fig. 2.

Figure 2: Schematic scaling relation for the efficiency of heat engines.Figure 2: Schematic scaling relation for the efficiency of heat engines.

 

3. Thermoelectric generators (TEG’s) are based on the Seebeck effect or thermopower.  Here a temperature gradient results in a voltage that does useful electrical work.  TEG’s can be used to recover waste heat into useful electrical power.  In particular, we discovered how particular dopants, called resonant levels, boost the thermopower and efficiency of thermoelectric semiconductors for TEG’s. 

 

4. Thermoelectric coolers (TEC’s) are based on the Peltier effect, and are useful in local small-scale cooling applications, such as the cooling of electronic devices (CPU’s, IR sensors and imaging systems, scientific instrumentation), camping gear and car seats.  For a review of the materials most commonly used in TEC’s, see Heremans and Wiendlocha, 2016.  A recent 5-year Air Force program on cryogenic Peltier cooling has resulted in the development of potassium-doped BiSb alloys that have competitive efficiencies down to 50 K.

  • Heremans, J. P. and Wiendlocha, B., "The Tetradymites: Bi2Te3-Related Materials," Materials Aspects of Thermoelectricity, CRC Press, Taylor and Francis, Boca Raton, FL (2016).
  • Heremans, J. P. and Jin, H., “BiSb and spin-related thermoelectric phenomena,” Proc. SPIE 9821, 982117 (2016).

 

5. The spin-Seebeck effect (SSE) arises when a layer of normal metal (Pt) is deposited onto a ferromagnetic insulator (e.g. yttrium iron garnet), see Fig. 3.

  • Boona, S. R., Myers, R. C., and Heremans, J. P., “Spin Caloritronics,” Energy Environ. Sci. 7, 885-910 (2014) (review).   

Figure 3: Two forms of advective transport are the Spin Seebeck Effect and the Magnon-Drag Effect. In advective transport, the temperature gradient accelerates the magnons that interact with the electrons, which in turn give rise to an electric voltage.Figure 3: Two forms of advective transport are the Spin Seebeck Effect and the Magnon-Drag Effect. In advective transport, the temperature gradient accelerates the magnons that interact with the electrons, which in turn give rise to an electric voltage.

A temperature gradient excites a magnon flux in the ferromagnet.  The magnons accumulate at the interface and spin-polarize the conduction electrons in the normal metal.  Due to spin-orbit interactions in the metal, the spin-polarized electrons create a voltage in the metal. This effect is now commonly measured in the geometry in Fig. 3.  Historically, however, the effect was first observed (K. Uchida, S. Takahashi, K. Harii, J. Ieda, W. Koshibae, K. Ando, S. Maekawa & E. Saitoh, Nature 455, 778 (2008)) ​in a “transverse” geometry, but scientific questions remain about the transverse spin-Seebeck effect. 

Nevertheless, in that geometry, the SSE was observed in ferromagnetic metals (Uchida), ferromagnetic insulators (Uchida), and magnetic semiconductors (Jaworski, et al., 2010). The intensity of the SSE can even exceed that of conventional thermoelectric effects at low temperature (Jaworski et al., 2012).

 

6. Magnon-drag gives rise to a strong enhancement of the thermopower of ferromagnetic metals, and Fig. 3 also shows how it is closely related to the spin-Seebeck effect (M. E. Lucassen, C. H. Wong, R. A. Duine, and Y. Tserkovnyak, Appl. Phys. Lett. 99, 262506 (2011)).  Again, a temperature gradient creates a flux of magnons in a ferromagnetic metal in which these interact directly with the conduction electrons.  This can be viewed as a self-SSE, in which no interface is necessary.  Current work aims at developing and refining the theory for magnon drag and at harnessing and optimizing it to possibly develop thermoelectric metals.

The use of metals in thermoelectric applications would enable the integration of the thermoelectric materials directly with the heat exchangers and the use of thermoelectrics in hostile environments.  The challenge, however, is enormous, because metals have a very low thermopower. We investigate if magnon-drag could boost the thermopower of specially designed alloys of ferromagnetic metals.