James R. Salvador

Multiple-Filled Skutterudites: High Thermoelectric Figure of Merit through Separately Optimizing Electrical and Thermal Transports

Skutterudites CoSb(3) with multiple cofillers Ba, La, and Yb were synthesized and very high thermoelectric figure of merit ZT = 1.7 at 850 K was realized. X-ray diffraction of the densified multiple-filled bulk samples reveals all samples are phase pure. High-resolution scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) analysis confirm that multiple guest fillers occupy the nanoscale-cages in the skutterudites. The fillers are further shown to be uniformly distributed and the Co-Sb skutterudite framework is virtually unperturbed from atomic scale to a few micrometers. Our results firmly show that high power factors can be realized by adjusting the total filling fraction of fillers with different charge states to reach the optimum carrier density, at the same time, lattice thermal conductivity can also be significantly reduced, to values near the glass limit of these materials, through combining filler species of different rattling frequencies to achieve broad-frequency phonon scattering. Therefore, partially filled skutterudites with multiple fillers of different chemical nature render unique structural characteristics for optimizing electrical and thermal transports in a relatively independent way, leading to continually enhanced ZT values from single- to double-, and finally to multiple-filled skutterudites. The idea of combining multiple fillers with different charge states and rattling frequencies for performance optimization is also expected to be valid for other caged TE compounds.

Low thermal conductivity and high thermoelectric figure of merit in n-type BaxYbyCo4Sb12 double-filled skutterudites

Filled skutterudites are one of the most promising thermoelectric materials for power generation applications. The choice and concentration of filler atoms are key aspects for achieving high thermoelectric figure of merit values. We report on the high temperature thermoelectric properties in the double-filled skutterudites BaxYbyCo4Sb12. The combination of Ba and Yb fillers inside the voids of the skutterudite structure provides a broad range of resonant phonon scattering and consequently a strong suppression in the lattice thermal conductivity is observed. A dimensionless thermoelectric figure of merit of 1.36 at 800K is achievable for n-type BaxYbyCo4Sb12.

Improving thermoelectric performance of caged compounds through light-element filling

Heavy elements filling have been considered the most effective way to improve the thermoelectric performance of caged compounds such as CoSb3 by reducing kL. Here, we show an opposite example of filling a light element, Na, into CoSb3 for obtaining high thermoelectric figure of merit ZT.ZT=1.25 at 850 K for Na0.48Co4Sb12 is one of the highest values among all reported single-element-filled CoSb3. The Na-filling scatters phonons less effectively but it results in relatively high mobility thus large power factor. This most likely comes from the extra electronic states near the Fermi level induced by Na.

Zero thermal expansion in YbGaGe due to an electronic valence transition

Most materials expand upon heating. Although rare, some materials expand on cooling, and are said to exhibit negative thermal expansion (NTE); but the property is exhibited in only one crystallographic direction. Such materials include silicon and germanium at very low temperature (<100 K) and, at room temperature, glasses in the titania–silica family, Kevlar, carbon fibres, anisotropic Invar Fe-Ni alloys, ZrW2O3 (ref. 4) and certain molecular networks. NTE materials can be combined with materials demonstrating a positive thermal expansion coefficient to fabricate composites exhibiting an overall zero thermal expansion (ZTE). ZTE materials are useful because they do not undergo thermal shock on rapid heating or cooling. The need for such composites could be avoided if ZTE materials were available in a pure form. Here we show that an electrically conductive intermetallic compound, YbGaGe, can exhibit nearly ZTE—that is, negligible volume change between 100 and 400 K. We suggest that this response is due to a temperature-induced valence transition in the Yb atoms. ZTE materials are desirable to prevent or reduce resulting strain or internal stresses in systems subject to large temperature fluctuations, such as in space applications and thermomechanical actuators.

Thermoelectric properties of Ag-doped Cu2Se and Cu2Te

Cu2Se, Cu2Te and Ag-overstoichiometric compounds Cu1.98Ag0.2Se and Cu1.98Ag0.2Te were prepared by melting, annealing, followed by spark plasma sintering compaction. Low and high temperature thermoelectric properties were investigated by measuring the electrical conductivity, Seebeck coefficient, thermal conductivity and Hall coefficient between 2 K and 900 K. Structural analyses were performed by PXRD and SEM-EDX analyses. The Hall and Seebeck coefficients show that holes are the dominant carrier in all compounds. High temperature α–β phase transition in Cu2Se and Cu1.98Ag0.2Se between 350 and 400 K and multiple phase transitions (α–β, β–γ, γ–δ, δ–∈) in Cu2Te and Cu1.98Ag0.2Te between 350 K and 900 K were observed in measurements of heat capacity, temperature dependent PXRD data, and transport coefficients. Low temperature transport measurements (Hall coefficient, electrical conductivity, carrier mobility) strongly suggest the presence of yet another phase transition in Cu2Se, Cu1.98Ag0.2Se, and Cu1.98Ag0.2Te compounds at temperatures between 85 K and 115 K, reported here for the first time. Based on the transport data and structural analysis we conclude that doping Cu2Se and Cu2Te by Ag reduces the density of holes and strongly suppresses the thermal conductivity not only due to a smaller electronic contribution but also due to enhanced point defect scattering of phonons that reduces the lattice portion of the thermal conductivity. Moreover, the phase transition temperature is shifted to lower temperatures upon doping with Ag. The presence of Ag enhances thermoelectric performance of Cu2Te at all temperatures and Cu2Se benefits from Ag doping over a broad range of temperatures up to 700 K. The maximum ZT value of 1.2 at 900 K; 0.52 at 650 K; 0.29 at 900 K; and 1.0 at 900 K were achieved for Cu2Se, Cu1.98Ag0.2Se, Cu2Te and Cu1.98Ag0.2Te, respectively, between 2 K and 900 K.

Thermoelectric Generators for Automotive Waste Heat Recovery Systems Part I: Numerical Modeling and Baseline Model Analysis

A numerical model has been developed to simulate coupled thermal and electrical energy transfer processes in a thermoelectric generator (TEG) designed for automotive waste heat recovery systems. This model is capable of computing the overall heat transferred, the electrical power output, and the associated pressure drop for given inlet conditions of the exhaust gas and the available TEG volume. Multiple-filled skutterudites and conventional bismuth telluride are considered for thermoelectric modules (TEMs) for conversion of waste heat from exhaust into usable electrical power. Heat transfer between the hot exhaust gas and the hot side of the TEMs is enhanced with the use of a plate-fin heat exchanger integrated within the TEG and using liquid coolant on the cold side. The TEG is discretized along the exhaust flow direction using a finite-volume method. Each control volume is modeled as a thermal resistance network which consists of integrated submodels including a heat exchanger and a thermoelectric device. The pressure drop along the TEG is calculated using standard pressure loss correlations and viscous drag models. The model is validated to preserve global energy balances and is applied to analyze a prototype TEG with data provided by General Motors. Detailed results are provided for local and global heat transfer and electric power generation. In the companion paper, the model is then applied to consider various TEG topologies using skutterudite and bismuth telluride TEMs.

Transport and mechanical properties of Yb-filled skutterudites

A series of Yb-filled skutterudites were produced and powder samples consolidated using spark plasma sintering (SPS). The effect of different heating cycles on the resulting transport properties of the consolidated samples was explored. Specifically, the effect of sample uniformity on the electrical and thermal transport properties was explored. In addition to the optimal Yb-filling fraction, other factors, such as heating profiles and sintering conditions, were found to play a pivotal role in the performance of these materials. Large quantities of Yb-filled skutterudite material can be generated with high purity and uniformity. Resonant ultrasound spectroscopy was used to determine the elastic modulus and Poissons ratio. Fracture strength was measured for 12 specimens and, taken with information obtained from resonant ultrasound spectroscopy and from thermal expansion and thermal transport characteristics, the thermal shock resistance parameter was evaluated. These parameters will be important for the engineering of thermoelectric modules based on skutterudite materials.

Double-filled skutterudites of the type YbxCayCo4Sb12: Synthesis and properties

Filled skutterudites based on CoSb3 exhibit high ZT values due to the inclusion of filler atoms into voids that comprise the crystal structure of CoSb3. These atoms act as electron-donating species that dope the parent material, thereby decreasing the electrical resistivity. Additionally, the loosely bound nature of the filler species acts to scatter heat carrying phonons, which reduce the thermal conductivity. Recently it has been reported that filler atoms from different chemical groups could be cofilled into CoSb3, which further reduces the thermal conductivity, likely by scattering a wider spectrum of phonons. Presented here is the synthesis and transport property evaluation of a series of double-filled skutterudites of the type YbxCayCo4Sb12. Good power factors, S2/ρ comparable to Yb-filled skutterudites are obtained for samples with a high Yb to Ca filling ratio. Filling with Ca and Yb did not yield a significant reduction in the thermal conductivity and as a result the ZT values are not improved as...

Conversion efficiency of skutterudite-based thermoelectric modules

Presently, the only commercially available power generating thermoelectric (TE) modules are based on bismuth telluride (Bi2Te3) alloys and are limited to a hot side temperature of 250 °C due to the melting point of the solder interconnects and/or generally poor power generation performance above this point. For the purposes of demonstrating a TE generator or TEG with higher temperature capability, we selected skutterudite based materials to carry forward with module fabrication because these materials have adequate TE performance and are mechanically robust. We have previously reported the electrical power output for a 32 couple skutterudite TE module, a module that is type identical to ones used in a high temperature capable TEG prototype. The purpose of this previous work was to establish the expected power output of the modules as a function of varying hot and cold side temperatures. Recent upgrades to the TE module measurement system built at the Fraunhofer Institute for Physical Measurement Techniques allow for the assessment of not only the power output, as previously described, but also the thermal to electrical energy conversion efficiency. Here we report the power output and conversion efficiency of a 32 couple, high temperature skutterudite module at varying applied loading pressures and with different interface materials between the module and the heat source and sink of the test system. We demonstrate a 7% conversion efficiency at the module level when a temperature difference of 460 °C is established. Extrapolated values indicate that 7.5% is achievable when proper thermal interfaces and loading pressures are used.

Thermoelectric properties of polycrystalline In4Se3 and In4Te3

High thermoelectric performance of a single crystal layered compound In4Se3 was reported recently. We present here an electrical and thermal transport property study over a wide temperature range for polycrystalline samples of In4Se3 and In4Te3. Our data demonstrate that these materials are lightly doped semiconductors, leading to large thermopower and resistivity. Very low thermal conductivity, below 1 W/m K, is observed. The power factors for In4Se3 and In4Te3 are much smaller when compared with state-of-the-art thermoelectric materials. This combined with the very low thermal conductivity results in the maximum ZT value of less than 0.6 at 700 K for In4Se3.