Session H17: Focus Session: Thermoelectrics - Characterization/Molecular junctions/Magnetic and Low Temp

Strategies for developing optimal thermoelectric metrology protocols

The Seebeck coefficient is an essential physical property routinely measured to evaluate the potential performance of new thermoelectric materials. These materials facilitate the inter-conversion of thermal and electrical energy and are useful in power generation or solid-state refrigeration applications. However, the diversity in Seebeck coefficient measurement techniques, conditions, and probe arrangements has resulted in conflicting materials data, further complicating the inter-laboratory confirmation of reported higher efficiency thermoelectric materials. In an effort to identify optimal thermoelectric measurement protocols, we have developed a complimentary strategy to both evaluate and compare these different probe arrangements and measurement methodologies: first, through the design of an innovative experimental apparatus, and second, through error modeling of Seebeck coefficient measurements using finite element analysis. This talk will include a discussion of key measurement challenges, example diagnostics, and recommended practices to effectively manage uncertainty in Seebeck coefficient measurements.   

Experimental Determination of the Lorenz Number

In an effort to improve the dimensionless thermoelectric figure of merit (ZT), thermal conductivity reduction is imperative. Most efforts are made to reduce the lattice portion of the thermal conductivity through nanostructuring. However there is no direct way to measure the lattice contribution and typically the lattice thermal conductivity is approximated by various methods. By experimentally determining the Lorenz number, the lattice thermal conductivity can be directly calculated. A method for determining the Lorenz number of thermoelectric materials Bi$_{2}$Te$_{2.7}$Se$_{0.3}$ and Bi$_{0.88}$Sb$_{0.12}$ will be presented.   

Anisotropic Two-band Transverse Thermoelectrics in Zero Magnetic Field

Narrow gap materials with anisotropic electron and hole band conductance are shown to function as anisotropic two-band transverse (A2T) thermoelectrics, whereby longitudinal electrical currents generate transverse Peltier heat flow. Unlike the Ettingshausen effect which requires external magnetic field, a large transverse Seebeck coefficient in A2T thermoelectric results from the anisotropic electron and hole mass tensors without magnetic field. Compared to synthetic transverse thermoelectrics, A2T thermoelectric coolers can be scaled to nanoscale, and the intrinsic nature of this phenomenon is promising for cryogenic applications. With exponentially tapered coolers, arbitrary $\Delta$T can be reached with sufficiently thick layers and a small electric field. Equations for A2T thermoelectric transport from an electron-hole band model yield the optimal orientation to achieve maximum transverse figure of merit $Z_\perp T$. The InAs/GaSb type II superlattice is shown to have the appropriate anisotropic band structure, and bandgaps of order $kT$ are calculated to give a competitive $\Delta$T = 14~K at room temperature. Thermal conductivity of the superlattice is 4~W/m$\cdot$K at 300~K using 3$\omega$ method. Preliminary data on in-plane Seebeck coefficient will also be presented.   

Magnetotransport in thermoelectric materials.

The mechanisms behind the magnetic field and temperature dependent thermoelectric effects that have recently been found experimentally in the electrical resistivity, thermopower and thermal conductivity of nanocomposite samples of Bi2Te3 and related compounds is studied. Large deviations in the transport coefficients in the presence of an applied magnetic field, both when the magnetic field is parallel and perpendicular to the transport direction has been observed in Bi2Te3 compounds, despite the small electronic mean free paths in these materials. An accurate analysis of the experimentally measured data leads to the estimation of electron mean free path and extraction of lattice thermal conductivity contribution from the total thermal conductivity. We will apply a Boltzmann transport equation based formalism to analyze the experimental data and improve the accuracy of the estimation of electronic and lattice part of the thermal conductivity. A particular attention will be paid to the analysis of the anisotropic effects in these samples.   

Separation of Joule Heating and Peltier Cooling via Time-Resolved X-Ray Di?raction in Si/SiGe Superlattice

We present detailed measurements of the thermal pro?le in a pulsed current SiGe-based thermoelectric micro-cooler. The evolution of heat ?ow in thermoelectric materials has been previously studied using time-domain thermore?ectance imaging; however, such methods are typically only sensitive to the surface temperature of the device, and the heat ?ow into the material remains hidden. Using time-resolved x-ray di?raction, we probe the transient temperature change in both the surface gold electrode and the underlying Si/SiGe superlattice using the shift in diffraction pattern caused by thermal expansion. We are also able to resolve Joule heating vs. Peltier cooling taking place in the gold through separation of timescales made possible by the relatively short duration (100ps) of the Advanced Photon Source.   

Reassessment of the carrier concentration in GeTe-based thermoelectric materials by $^{125}$Te NMR

Ge$_{1-x}$Ag$_{x/2}$Sb$_{x/2}$Te $p$-type thermoelectric materials (``TAGS-$n$'') were studied extensively in the 1970s and then again recently. They exhibit an unusual combination of large thermopower, $S$, and high hole concentration, $p$, reported based on the Hall effect data, which has not been explained. To solve this puzzle, we have synthesized GeTe, GeTe:Bi, and TAGS-$n$ with $n $= 97, 94, 90, and 85 and studied XRD, thermopower, electrical resistivity, thermal conductivity, and $^{125}$Te NMR. Most importantly, we have determined the carrier concentrations using $^{125}$Te NMR spin-lattice relaxation and Knight shift. In GeTe and GeTe:Bi, we found that carrier concentrations generally agree with the values reported from Hall effect. In TAGS-$n$, they are much lower but agree better with the values expected from $S$ vs. $p$ for GeTe-based materials, solving the puzzle partially. The NMR vs. Hall effect discrepancy in TAGS-$n$ can be due to the presence not only of holes but also electrons generated by Sb atoms, which results in artificially high hole concentration from Hall effect. Even though the true hole concentration is lower than reported, the thermopower of TAGS-$n$ is still significantly larger than that of GeTe and GeTe:Bi at similar carrier concentration. This can be explained by energy filtering enhanced by potential barriers formed due to Ag-Sb pairs in the TAGS-$n$ lattice.   

Charge and Energy Transport in Molecular Junctions

Charge and energy transport in molecular junctions, created by trapping short organic molecules ($\sim $1nm) between inorganic electrodes, is expected to be fundamentally different from transport in bulk materials due to their discrete electronic structure. In fact, numerous computational studies have suggested that it may be possible to utilize the novel thermoelectric and thermal transport phenomenon in nanoscale molecular junctions to create efficient energy conversion devices (e.g. thermoelectric devices). However, a large number of these effects remain to be experimentally verified. We will describe our experimental studies where thermoelectric properties of junctions were studied at the single/few molecule level enabling novel insights into the relationship between molecular structure and the thermoelectric properties of junctions. We will also present our recent experimental efforts to probe thermal transport in nanoscale molecular junctions and point contacts. In order to accomplish this goal, it is necessary to accurately measure heat currents as small as 10 picowatts. Towards this goal, we will present a novel device developed by us that is capable of resolving heat currents as small as 4 picowatts.   

The impact of electron-vibration interactions on the thermoelectric efficiency of molecular junctions

From first--principles approaches, we investigate the thermoelectric efficiency of a molecular junction where a benzene molecule is connected directly to the platinum electrodes. We calculate thermoelectric figure of merits ZT in the presence of electron--vibration interactions with and without local heating under two scenarios: linear response (zero bias) and finite bias (non--zero bias) regimes. In the linear response regime, ZT saturates around the electrode temperature $T_{e}=25$~K in the elastic case, while in the inelastic case we observe a non--saturated and a much larger ZT beyond $T_{e}=25$~K attributed to the tail of the Fermi--Dirac distribution. In the finite bias regime, the inelastic effects reveal the signatures of the molecular vibrations in the low temperature regime. The normal modes exhibiting structures in the inelastic profile are characterized by large components of atomic vibrations along the current density direction on top of each individual atom. In all cases, the inclusion of local heating leads to a higher wire temperature $T_{w}$ and thus magnifies further the influence of the electron--vibration interactions due to the increased number of local phonons.   

Interfacial Interactions in Polymer-Nanocrystal Thermoelectric Composites Provide a Novel Route for Power Factor Enhancement

The highest performing thermoelectric materials currently available are fabricated via expensive high-temperature vacuum processing techniques. Recently, there has been an increasing interest in the thermoelectric properties of solution-processable materials, which have the potential to dramatically reduce module fabrication costs. These solution-processed materials however often exhibit poor transport properties, which undermines their competitive advantage over the more traditional expensive thermoelectric materials. Here, we present the thermoelectric transport properties of a new class of solution-processable conducting-polymer/inorganic composite materials as a function of nanocrystal loading. In the Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Tellurium nanowire composite devices fabricated for this study, the thermoelectric performance of the composite exceeds that of either pure organic or inorganic component alone. This result suggests an interface-driven mechanism for this enhanced performance and provides an exciting route for improving the power factors of organic-inorganic hybrid thermoelectrics.   

Nernst-Ettingshausen Effect in Elemental Rare-Earth Single Crystals

The transverse Nernst-Ettingshausen (N-E) coefficient $N $measurements of the elemental rare-earth (R-E) single-crystal are for the first time presented from 80 to 420 K. Since they have mainly hexagonal symmetry at room temperature, measurements are given with the heat flux along the [100] and the [001] axes. Due to their complex band structure and Fermi surface, their small thermopower (S) and their multicarrier systems involving electron (e) and hole (h) pockets, their $N$ are expected to be large. Indeed, for such systems, both $S $and $N$ can be expressed as$^{1} \quad S=(S_{e}$\textit{$\sigma $}$_{e}+ S_{h}$\textit{$\sigma $}$_{h}$\textit{)/( $\sigma $}$_{e}$\textit{+$\sigma $}$_{h})$ while $N=[(N_{e}$\textit{$\sigma $}$_{e}+ N_{h}$\textit{$\sigma $}$_{h}$\textit{)( $\sigma $}$_{e}$\textit{+$\sigma $}$_{h})+(S_{h}-S_{e})(R_{Hh}$\textit{$\sigma $}$_{h}- R_{He}$\textit{$\sigma $}$_{e}$\textit{)$\sigma $}$_{e}$\textit{$\sigma $}$_{h}$\textit{]/( $\sigma $}$_{e}$\textit{+$\sigma $}$_{h})^{a}$, where \textit{$\sigma $} is the electrical conductivity and $R_{H}$ the Hall coefficient and the subscript correspond to either carriers. Since $S_{h}>$0 and$ S_{e}<$0, the resulting $S$ should be low thus leading to a large $N$ . These solids are useful in single-material thermoelectric N-E coolers. They create a large temperature differences using thermomagnetic effects, without having to be cascaded. This would resolve th problem of contact resistances of actual multi-stage Peltier coolers, especially in the cryogenic temperature range. The dimensionless figure of merit of N-E coolers is \textit{zT}$_{N}=B^{2}N^{2}$\textit{$\sigma $(B)T/$\kappa $(B),} with $B$ is the magnetic field, $T$ the absolute temperature and \textit{$\kappa $} the thermal conductivity. a.E.H. Putley, \textit{The Hall Effect and Semiconductor Physics} , New York: Dover publication, 1968.   

Giant Bipolar Nernst Effect in the Quasi-One-Dimensional Metal, Li$_{0.9}$Mo$_6$O$_{17}$

The Nernst coefficient for the quasi-one-dimensional metal, Li$_{0.9}$Mo$_6$O$_{17}$, is found to be among the largest known for metals ($\nu\simeq 500\ \mu$V/KT at $T\sim 20$~K), and is enhanced in a broad range of temperature by orders of magnitude over the value expected from Boltzmann theory for carrier diffusion. A comparatively small Seebeck coefficient implies that Li$_{0.9}$Mo$_6$O$_{17}$ is bipolar with large, partial Seebeck coefficients of opposite sign. A very large thermomagnetic figure of merit, $ZT\sim 0.5$, is found at high field in the range $T\approx 35-50$~K.   

Cryogenic Thermoelectric Properties of the Bismuth-Magnesium and Bismuth-Antimony-Magnesium Systems

There is a need to increase the Figure of Merit of thermoelectric materials used in low temperature cooling applications. Band structure calculations show that substitutional magnesium in bismuth can form sharp density of states peaks, suggesting the presence of a resonant level. Single crystal samples of (Bi$_{1-x}$Sb$_{x})_{1-y}$Mg$_{y}$ (0 $\le $ x $\le $ 12{\%} and 0 $\le $ y $\le $ 0.7{\%} nominally) were synthesized in evacuated ampoules. The composition of each ingot was analyzed using x-ray diffraction, and transport properties were measured using a Thermal Transport Option (TTO) in a Physical Properties Measurement System (PPMS) from 300K to 2K. It is apparent that the addition of magnesium strongly influences thermopower; the data for Bi$_{90}$Sb$_{10}$Mg$_{0.7}$ shows a second minimum in thermopower at 20K, in addition to the expected minimum at approximately 50-60K. This could be due to the resonant scattering at the cryogenic temperatures which arises from the excess density of states. The addition of magnesium also appears to decrease thermal conductivity below 30K. We present systematic experimental approaches and the results to elucidate the role of magnesium in bismuth and bismuth-antimony systems.   

Low Temperature Electronic and Magnetic Properties of CePd$_{3}M_{x}$

The intermediate valence compound CePd$_{3}$ is a strong candidate for low temperature thermoelectric applications due to its unusually large Seebeck coefficient which peaks at approximately 115 $\mu $V/K near 125K. This phenomenon results from a sharp peak in the density of states near the Fermi level due to the hybridization of conduction electrons with those in the partially occupied cerium f-shell, thus making the system highly sensitive to changes in the average cerium valence state. We have systematically studied the structural and thermoelectric properties of various CePd$_{3}M_{x}$ compounds, where $M$ is an s- or p-block element and 0 $<$ x $<$ 0.1, in order to explore the effects of such partial filling on the cerium valence. The results of X-ray diffraction, Seebeck coefficient, and magnetic susceptibility measurements are reported. We have found that incorporating $M$ elements of various valence configurations has similar effects on the electronic and magnetic properties as changing the $M$ concentration, thus establishing an effective new mechanism for tailoring the thermoelectric properties of the system. \newline