Christopher M. Jaworski

Observation of the spin-Seebeck effect in a ferromagnetic semiconductor

Reducing the heat generated in traditional electronics is a chief motivation for the development of spin-based electronics, called spintronics. Spin-based transistors that do not strictly rely on the raising or lowering of electrostatic barriers can overcome scaling limits in charge-based transistors. Spin transport in semiconductors might also lead to dissipation-less information transfer with pure spin currents. Despite these thermodynamic advantages, little experimental literature exists on the thermal aspects of spin transport in solids. A recent and surprising exception was the discovery of the spin-Seebeck effect, reported as a measurement of a redistribution of spins along the length of a sample of permalloy (NiFe) induced by a temperature gradient. This macroscopic spatial distribution of spins is, surprisingly, many orders of magnitude larger than the spin diffusion length, which has generated strong interest in the thermal aspects of spin transport. Here, the spin-Seebeck effect is observed in a ferromagnetic semiconductor, GaMnAs, which allows flexible design of the magnetization directions, a larger spin polarization, and measurements across the magnetic phase transition. This effect is observed even in the absence of longitudinal charge transport. The spatial distribution of spin currents is maintained across electrical breaks, highlighting the local nature of this thermally driven effect.

High Performance Na-doped PbTe–PbS Thermoelectric Materials: Electronic Density of States Modification and Shape-Controlled Nanostructures

Thermoelectric heat-to-power generation is an attractive option for robust and environmentally friendly renewable energy production. Historically, the performance of thermoelectric materials has been limited by low efficiencies, related to the thermoelectric figure-of-merit ZT. Nanostructuring thermoelectric materials have shown to enhance ZT primarily via increasing phonon scattering, beneficially reducing lattice thermal conductivity. Conversely, density-of-states (DOS) engineering has also enhanced electronic transport properties. However, successfully joining the two approaches has proved elusive. Herein, we report a thermoelectric materials system whereby we can control both nanostructure formations to effectively reduce thermal conductivity, while concurrently modifying the electronic structure to significantly enhance thermoelectric power factor. We report that the thermoelectric system PbTe-PbS 12% doped with 2% Na produces shape-controlled cubic PbS nanostructures, which help reduce lattice thermal conductivity, while altering the solubility of PbS within the PbTe matrix beneficially modifies the DOS that allow for enhancements in thermoelectric power factor. These concomitant and synergistic effects result in a maximum ZT for 2% Na-doped PbTe-PbS 12% of 1.8 at 800 K.

Giant spin Seebeck effect in a non-magnetic material

The spin Seebeck effect is observed when a thermal gradient applied to a spin-polarized material leads to a spatially varying transverse spin current in an adjacent non-spin-polarized material, where it gets converted into a measurable voltage. It has been previously observed with a magnitude of microvolts per kelvin in magnetically ordered materials, ferromagnetic metals, semiconductors and insulators. Here we describe a signal in a non-magnetic semiconductor (InSb) that has the hallmarks of being produced by the spin Seebeck effect, but is three orders of magnitude larger (millivolts per kelvin). We refer to the phenomenon that produces it as the giant spin Seebeck effect. Quantizing magnetic fields spin-polarize conduction electrons in semiconductors by means of Zeeman splitting, which spin–orbit coupling amplifies by a factor of ∼25 in InSb. We propose that the giant spin Seebeck effect is mediated by phonon–electron drag, which changes the electrons’ momentum and directly modifies the spin-splitting energy through spin–orbit interactions. Owing to the simultaneously strong phonon–electron drag and spin–orbit coupling in InSb, the magnitude of the giant spin Seebeck voltage is comparable to the largest known classical thermopower values.

Spin-seebeck effect: a phonon driven spin distribution.

Here we report on measurements of the spin-Seebeck effect of GaMnAs over an extended temperature range alongside the thermal conductivity, specific heat, magnetization, and thermoelectric power. The amplitude of the spin-Seebeck effect in GaMnAs scales with the thermal conductivity of the GaAs substrate and the phonon-drag contribution to the thermoelectric power of the GaMnAs, demonstrating that phonons drive the spin redistribution. A phenomenological model involving phonon-magnon drag explains the spatial and temperature dependence of the measured spin distribution.

Mean free path limitation of thermoelectric properties of bismuth nanowire

A limiting mean free path was considered in order to better understand the temperature and wire diameter dependence of the resistivity and Seebeck coefficient of bismuth microwire and nanowire samples. The mean free path limited mobility was numerically calculated from experimentally measured mobility in a bulk bismuth sample, and the electron and hole mobilities were dramatically decreased to a 10 μm mean free path. Therefore, the temperature dependence of resistivity in very thin wire was quite different from that of a bulk sample, which had a positive temperature coefficient. The calculations showed that the temperature coefficient decreased gradually with decreasing mean free path, and the coefficient became negative for a mean free path of less than 1 μm at about 150 K. The Seebeck coefficient was also calculated, but showed only a weak dependence on mean free path compared with the resistivity. Experimental comparisons were made to previous measurements of bismuth microwire or nanowire samples, and ...

Transport properties and valence band feature of high-performance (GeTe)85(AgSbTe2)15 thermoelectric materials

This paper aims at elucidating the origin of the high thermoelectric power factor of p-type (AgxSbTe_(x/2+1.5)15)(GeTe)_(85) (TAGS) thermoelectric materials with 0.4 ≤ x ≤ 1.2. All samples exhibit good thermoelectric figures of merit (zT) which reach 1.5 at 700 K for x = 0.6. Thermoelectric and thermomagnetic transport properties (electrical resistivity, Seebeck, Hall and transverse Nernst–Ettinghausen coefficients) are measured and used to calculate the scattering factor, the Fermi energy, the density-of-states (DOS) effective mass and hole mean free path (mfp). The DOS effective mass is very high due to the large band mass of the primary valence band and the high degeneracy of pockets in the Fermi surface from the second valence band. The highly degenerate Fermi surface increased the total DOS without decreasing mobility, which is more desirable than the high DOS that comes from a single carrier pocket. The high-temperature hole mfp approaches the Ioffe–Regel limit for band-type conduction, which validates our discussion based on band transport and is also important for TAGS alloys having high zT with heavy bands. The present results show that multiple degenerate Fermi surface pockets provide an effective way of substantially increasing the power factor of thermoelectric materials with low thermal conductivity.

Experimental study of the thermoelectric power factor enhancement in composites

D. J. Bergman and L. J. Fel [J. Appl. Phys. 85, 8205 (1999)] calculated that in composites the thermoelectric power factor, the product of the square of the thermopower and the electrical conductivity, can be enhanced over that of the individual constituents, but the figure of merit cannot. This is demonstrated here experimentally in the elemental bismuth-silver system, and ascribed to the fact that inclusions of a highly electrically conducting material (Ag) in a matrix of a material with a high thermopower (Bi) enhance the conductivity of the composite more than they reduce its thermopower. The power factor is technologically important in transient thermoelectric cooling.

Thermoelectric transport in indium and aluminum-doped lead selenide

We present galvanomagnetic and thermomagnetic properties of bulk PbSe doped by substituting the donor elements In and Al for Pb. Although prominent resonant level effects are not seen, lightly doped samples display a high thermoelectric figure of merit (zT) in excess of 1.2 at 600 K, a temperature corresponding well to automotive waste heat recovery applications. This materials high zT is achieved without the use of nanostructuring or the relatively rare element Te. Phonon drag contributions to thermopower appear at temperatures below 30 K in Al-doped samples.

Enhancement in the figure of merit of p-type Bi100−xSbx alloys through multiple valence-band doping

N-type Bi100−xSbx alloys have the highest thermoelectric figure of merit (zT) of all materials below 200 K; here, we investigate how filling multiple valence band pockets at the T and Η-points of the Brillouin zone produces high zT’s in p-type Sn-doped material. This approach, theoretically predicted to potentially give zT > 1 in Bi, was used in PbTe. We report thermopower, electrical and thermal conductivity (2 to 400 K) measurements of single crystals with 12 ≤ x ≤ 37 and polycrystals (x = 50-90), higher Sb concentrations than previous studies. We obtain a 60% improvement in zT to 0.13.

Opportunities for Thermoelectric Energy Conversion in Hybrid Vehicles

Much analysis has been performed on the application of thermoelectrics in automobiles, but the low efficiency of the materials has so far limited their use. As a result, little has been done in the physical design of how to most efficiently utilize thermoelectrics in a vehicles energy system. However, much progress has been and continues to be made in the field of thermoelectric materials. Developments in the areas of nanostructured materials have produced materials with double the efficiency of current commercially available materials. This, coupled with a growing need for the reduced consumption of fossil fuels and production of greenhouse gases, has generated renewed interest in the application of thermoelectrics in automotive systems. Hybrid-electric vehicle (HEV) designs have provided significant improvements in fuel efficiency and continue to evolve. This modified energy management strategy introduces new components and energy distributions which force traditional designs to be reconsidered. For example, the temperature and quantity of thermal energy transferred through the exhaust and radiator are lowered. Also, the IC engine may not be run continuously, creating difficulties in maintaining temperature in the catalytic converter, powering belt-driven accessories, and regulating cabin temperature. This contributes to an increased demand for electrical energy. Finally, the power electronics are typically liquid cooled (order of 60-65 °C) and the high voltage battery packs must be kept cool (typically below 45 °C) to maximize their life. A detailed computer model which captures the details of the energy transfers in HEVs, including thermal loads will be used to assess the unique thermal requirements of hybrid vehicles under average engine loads. Based on these requirements, specific thermal energy management strategies will be proposed. These modified systems will be added to the computer model in order to evaluate their potential using currently available thermoelectrics materials. Finally, the preferred thermal energy management system will be selected as the basis for future design optimization.Copyright