Shengqiang Bai

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.

Realizing high figure of merit in heavy-band p-type half-Heusler thermoelectric materials.

Solid-state thermoelectric technology offers a promising solution for converting waste heat to useful electrical power. Both high operating temperature and high figure of merit zT are desirable for high-efficiency thermoelectric power generation. Here we report a high zT of ∼1.5 at 1,200 K for the p-type FeNbSb heavy-band half-Heusler alloys. High content of heavier Hf dopant simultaneously optimizes the electrical power factor and suppresses thermal conductivity. Both the enhanced point-defect and electron–phonon scatterings contribute to a significant reduction in the lattice thermal conductivity. An eight couple prototype thermoelectric module exhibits a high conversion efficiency of 6.2% and a high power density of 2.2 W cm−2 at a temperature difference of 655 K. These findings highlight the optimization strategy for heavy-band thermoelectric materials and demonstrate a realistic prospect of high-temperature thermoelectric modules based on half-Heusler alloys with low cost, excellent mechanical robustness and stability.

Ultrahigh Thermoelectric Performance by Electron and Phonon Critical Scattering in Cu2Se1-xIx

Iodine-doped Cu2 Se shows a significantly improved thermoelectric performance during phase transitions by electron and phonon critical scattering, leading to a dramatic increase in zT by a factor of 3-7 times culminating in zT values of 2.3 at 400 K.

High efficiency Bi2Te3-based materials and devices for thermoelectric power generation between 100 and 300 °C

By suppressing intrinsic excitation in p-type Bi2Te3-based materials, we report maximum and average zT values of up to 1.4 and 1.2 between 100 and 300 °C, respectively. Thermoelectric modules based on these high performance materials show energy conversion efficiencies of up to 6.0% under a temperature gradient of 217 K, and are greatly superior to current Bi2Te3-based modules.

Enhanced thermoelectric performance in In(1-x)Ga(x)Sb originating from the scattering of point defects and nanoinclusion

InSb-based semiconductors have not been regarded as a promising thermoelectric candidates before due to their high thermal conductivity. In this paper, Ga substitution and nanoinclusions poor in Ga were introduced to serve as phonon scattering centres. The nanoinclusions, possessing a heterogeneous isomorphic crystal structure with the matrix, contribute to the weakly influenced electrical transport and greatly depressed lattice thermal conductivity (κL). For the sample In0.9Ga0.1Sb, the κL is decreased by 53% and the ZT value is increased by 62% compared with pure InSb at 650 K. The highest ZT achieved was 0.73, which shows application perspectives at intermediate temperatures.

Effect of Ge Doping on Thermoelectric Properties of SryCo4Sb12-xGex

Skutterudite compounds, SryCo4Sb12-xGex (y=0.3, x=0–0.3), are synthesized by a melting method. The effect of germanium doping on thermoelectric properties is investigated. Electrical conductivity, Seebeck coefficient, and thermal conductivity are measured from 300 to 850 K. The substitution of germanium is effective in reducing thermal conductivity while keeping excellent electrical transport properties owing to the high filling fraction of Sr and Ge doping as charge compensation. The dimensionless thermoelectric figure of merit ZT is observed to increase from 0.9 at 850 K for Sr0.28Co4Sb12 to 1.05 for Sr0.34Co4Sb11.9Ge0.1 at 850 K. These results suggest that doping with germanium is an attractive avenue to optimization of filled skutterudites.

Intrinsically High Thermoelectric Performance in AgInSe2 n‐Type Diamond‐Like Compounds

Abstract Diamond‐like compounds are a promising class of thermoelectric materials, very suitable for real applications. However, almost all high‐performance diamond‐like thermoelectric materials are p‐type semiconductors. The lack of high‐performance n‐type diamond‐like thermoelectric materials greatly restricts the fabrication of diamond‐like material‐based modules and their real applications. In this work, it is revealed that n‐type AgInSe2 diamond‐like compound has intrinsically high thermoelectric performance with a figure of merit (zT) of 1.1 at 900 K, comparable to the best p‐type diamond‐like thermoelectric materials reported before. Such high zT is mainly due to the ultralow lattice thermal conductivity, which is fundamentally limited by the low‐frequency Ag‐Se “cluster vibrations,” as confirmed by ab initio lattice dynamic calculations. Doping Cd at Ag sites significantly improves the thermoelectric performance in the low and medium temperature ranges. By using such high‐performance n‐type AgInSe2‐based compounds, the diamond‐like thermoelectric module has been fabricated for the first time. An output power of 0.06 W under a temperature difference of 520 K between the two ends of the module is obtained. This work opens a new window for the applications using the diamond‐like thermoelectric materials.

Interface characterization of Cu–Mo coating deposited on Ti–Al alloys by arc spraying

Cu–Mo pseudobinary alloys are promising candidates as electrode materials in CoSb3-based skutterudite thermoelectric (TE) devices for TE power generation. In this study, Cu–Mo coatings were deposited onto Ti–Al substrates by applying a dual-wire electric arc spraying coating technique. The microstructure of the surfaces, cross sections and coating interfaces were analyzed by scanning electron microscopy (SEM) and energy dispersion spectrometry (EDS). Cu–Mo coatings showed a typical banded splat with compact microstructures, and have no coarse pores nor micro-cracks. The thermal shock resistance of the Cu–Mo coating was also investigated to show good combinations with Ti–Al substrates. After 50 thermal shock cycles, there were no cracks observed at the interface. In contrast, the test of the thermal shock resistance of the Cu coating on the Ti–Al substrate was also investigated. Due to a large difference in the thermal expansion coefficients between Cu and Ti–Al alloys, the Cu coating flaked from the Ti–Al substrate completely after 10 thermal shock cycles. The contact resistivity of the Ti–Al/Cu–Mo interface was about 1.6 μΩ⋅cm2 and this value was unchanged after 50 thermal shock cycles, indicating the low electric resistance and high thermal stability of the Cu–Mo/Ti–Al interface.

International Round-Robin Testing of Bulk Thermoelectrics

Two international round-robin studies were conducted on transport properties measurements of bulk thermoelectric materials. The study discovered current measurement problems. In order to get ZT of a material four separate transport measurements must be taken. The round-robin study showed that among the four properties Seebeck coefficient is the one can be measured consistently. Electrical resistivity has +4-9% scatter. Thermal diffusivity has similar +5-10% scatter. The reliability of the above three properties can be improved by standardizing test procedures and enforcing system calibrations. The worst problem was found in specific heat measurements using DSC. The probability of making measurement error is great due to the fact three separate runs must be taken to determine Cp and the baseline shift is always an issue for commercial DSC. It is suggest the Dulong Petit limit be always used as a guide line for Cp. Procedures have been developed to eliminate operator and system errors. The IEA-AMT annex is developing standard procedures for transport properties testing.

Mo/Ti/CoSb/sub 3/ joining technology for CoSb/sub 3/ based materials

CoSb/sub 3/ based materials are promising for power generation in the intermediate temperature range. In the present work, we have developed a novel technology for the fabrication of CoSb/sub 3/ thermoelectric (TE) couple by spark plasma sintering (SPS). Molybdenum (Mo) was selected as the electrode material and a titanium (Ti) layer was inserted between Mo and CoSb/sub 3/ to realize the joining of Mo to CoSb/sub 3/ at a lower temperature. Shear tests showed that the joint possesses a shear strength ranging from 55 MPa to 70 MPa. The potential voltage measurement showed that the joint exhibits good electrical properties. All the results indicate that the Mo/Ti/CoSb/sub 3/ joining technology is suitable for fabricating TE couples using the CoSb/sub 3/ based materials.