Pengfei Qiu

High-temperature electrical and thermal transport properties of fully filled skutterudites RFe4Sb12 (R = Ca, Sr, Ba, La, Ce, Pr, Nd, Eu, and Yb)

Fully filled skutterudites RFe_(4)Sb_(12) (R = Ca, Sr, Ba, La, Ce, Pr, Nd, Eu, and Yb) have been prepared and the high-temperature electrical and thermal transport properties are investigated systematically. Lattice constants of RFe_(4)Sb_(12) increase almost linearly with increasing the ionic radii of the fillers, while the lattice expansion in filled structure is weakly influenced by the filler valence charge states. Using simple charge counting, the hole concentration in RFe_(4)Sb_(12) with divalent fillers (R = Ca, Sr, Ba, Eu, and Yb) is much higher than that in RFe4Sb12 with trivalent fillers (R = La, Ce, Pr, and Nd), resulting in relatively high electrical conductivity and low Seebeck coefficient. It is also found that RFe_(4)Sb_(12) filled skutterudites having similar filler valence charge states exhibit comparable electrical conductivity and Seebeck coefficient, and the behavior of the temperature dependence, thereby leading to comparable power factor values in the temperature range from 300 to 800 K. All RFe_(4)Sb_(12) samples possess low lattice thermal conductivity. The correlation between the lattice thermal resistivity WL and ionic radii of the fillers is discussed and a good relationship of W_L ~ (r_(cage)−r_(ion))^3 is observed in lanthanide metal filled skutterudites. CeFe_(4)Sb_(12), PrFe_(4)Sb_(12), and NdFe_(4)Sb_(12) show the highest thermoelectric figure of merit around 0.87 at 750 K among all the filled skutterudites studied in this work.

Effect of antisite defects on band structure and thermoelectric performance of ZrNiSn half-Heusler alloys

Band structures for ZrNiSn with Zr/Sn antisite defects are calculated with ab initio methods. Antisite defects shrink the band gap and enhance the density of states slope near the Fermi level, which are favorable to electrical transport properties for intrinsic semiconductors. The degree of Zr/Sn antisite defects are controlled by annealing time experimentally, and measurements show low electrical resistivity and high Seebeck coefficient for unannealed ZrNiSn, which benefits from the modified electronic structure caused by antisite defects. The maximum ZT is 0.64 at 800 K for unannealed ZrNiSn, which is the highest value for ZrNiSn systems without exterior doping.

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 by the combination of alloying and doping in TiCoSb-based half-Heusler compounds

TiCoSb-based half-Heusler compounds have been prepared and their thermoelectric properties are studied. By isoelectronic alloying on the Ti site with Zr, although both the thermal conductivity and electrical conductivity are suppressed, the Seebeck coefficient is improved remarkably with a highest value of −420 μV/K for Ti0.5Zr0.5CoSb at 600 K, which provides a larger space to optimize the thermoelectric performance. To further improve the performance of the TiCoSb-based isoelectronic alloy, doping Ni on the Co site was explored. It is found that small amount of Ni doping results in a great increase in the electrical conductivity, still with a relative large Seebeck coefficient. Ti0.6Hf0.4Co0.87Ni0.13Sb sample exhibits a peak power factor of 23.4μW∕cmK2, which is the highest value for n-type TiCoSb-based half-Heusler compounds reported so far. As a result, a maximum dimensionless figure of merit of 0.70 has been achieved at 900 K for Ti0.6Hf0.4Co0.87Ni0.13Sb.

Low thermal conductivity and enhanced thermoelectric performance of Gd-filled skutterudites

With Fe compensation, the heavy rare earth element Gd-filled GdyFexCo4−xSb12 (x<2) skutterudites have been successfully synthesized by melting-annealing approach. Fe substitution on the Co site brings two contrary effects on Gd filling: charge compensation which enhances the filling fraction of Gd, and Lattice expansion which is deleterious for the stability of filled compounds that contain smaller atoms. When Fe content is less than 1.7, pure GdyFexCo4−xSb12 compounds are obtained and the Gd maximum filling fraction (ymax) increases with Fe content. The power factor (S2σ) of the GdyFexCo4−xSb12 increases with Fe content. The lattice thermal conductivity is significantly depressed by Gd filling. The sample Gd0.41Fe1.48Co2.52Sb12 has a lattice thermal conductivity as low as 1.1 W m−1 K−1 at room temperature, and its figure of merit (ZT) reaches a maximum value of 0.83 at 700 K. At high temperature, thermal conductivity shows significant increase due to bipolar diffusion, which obstructs obtaining higher ZT.

Thermoelectric transport of Se-rich Ag2Se in normal phases and phase transitions

Small amount of Se atoms are used to tune the carrier concentrations (nH) and electrical transport in Ag2Se. Significant enhancements in power factor and thermoelectric figure of merit (zT) are observed in the compositions of Ag2Se1.06 and Ag2Se1.08. The excessive Se atoms do not change the intrinsically electron-conducting character in Ag2Se. The detailed analysis reveals the experiment optimum carrier concentration in Ag2Se is around 5 × 1018 cm−3. We also investigate the temperature of maximum zT and the thermoelectric transport during the first order phase transitions using the recently developed measurement system.

Effects of Sn-doping on the electrical and thermal transport properties of p-type Cerium filled skutterudites

Abstract p-type Sn-doped CoSb 3 -based skutterudite compounds have been prepared using melting–quenching–annealing method and spark plasma sintering technique. Sn atoms in our samples are completely soluted on Sb-site with a fixed charge state and non-magnetic feature, providing a better choice to ascertain the effect of element doping at the [Co 4 Sb 12 ] framework on the electrical and thermal transport properties in p-type skutterudites. Doping Sn at the framework introduces additional ionized impurity scattering to affect the electron transport greatly. Similar electrical transport properties between Ce 0.2 Co 4 Sb 11.2 Sn 0.8 and Co 4 Sb 11 Sn 0.6 Te 0.4 suggest that Ce fillers contribute little to the valence band edge. Filling Ce into the voids and doping Sn at the framework introduce additional phonon resonant and point defect scattering mechanisms, thereby reducing lattice thermal conductivity remarkably. Moreover, our data suggest that combining these two effects is more effective to suppress lattice thermal conductivity through scattering broad range of phonons with different frequencies.

Ultrahigh thermoelectric performance in Cu2Se-based hybrid materials with highly dispersed molecular CNTs

Here, by utilizing the special interaction between metal Cu and multi-walled carbon nanotubes (CNTs), we have successfully realized the in situ growth of Cu2Se on the surface of CNTs and then fabricated a series of Cu2Se/CNT hybrid materials. Due to the high degree of homogeneously dispersed molecular CNTs inside the Cu2Se matrix, a record-high thermoelectric figure of merit zT of 2.4 at 1000 K has been achieved.

Compound defects and thermoelectric properties in ternary CuAgSe-based materials

CuAgSe is a narrow band gap semiconducting material with superior carrier mobility and low lattice thermal conductivity, which are important and useful for high thermoelectric performance. However, its electrical and thermal transport properties are greatly affected by ionic deficiencies or compositional non-stoichiometry, which lead to a low thermoelectric figure of merit near room temperature. In this work, we systematically studied the compound defects in CuAgSe by tuning its starting chemical composition. We found that its phase purity is very sensitive to nominal chemical compositions. Only a small amount of Ag deficiency is allowed in CuAgSe to maintain phase purity, while the other non-stoichiometric compositions lead to impurity phases. Thermoelectric properties are weakly affected by these compound defects or impurity phases at 300 K, but greatly change at high temperatures. A single type carrier conduction is observed in CuAgSe, but a noticeable two-type carrier conduction is observed in the non-stoichiometric samples. This leads to an evident n to p conduction transition. Consequently, zT values in CuAgSe continuously increase to 0.6 at 450 K while the non-stoichiometric samples display considerably low values due to the contribution from both electrons and holes. The high zT value in n-type CuAgSe suggests that it is a promising thermoelectric material near room temperature.

Optimized thermoelectric properties in pseudocubic diamond-like CuGaTe2 compounds

A pseudocubic structure approach has been proposed recently to screen and design good thermoelectric materials via realizing overlapped band edges for excellent electrical transport properties. A diamond-like compound is a typical example agreeing with the concept of the pseudocubic structure by tuning its lattice distortion parameter to unity. However, besides the band structure, optimized carrier concentration and reduced lattice thermal conductivity are also required for a high thermoelectric figure of merit (zT). In this work, taking CuGaTe2 as an example, we have successfully demonstrated that Cu-deficiency can effectively tune carrier concentrations and In-alloying at Ga sites can effectively lower lattice thermal conductivity. By combining these two strategies, the electrical and thermal transports can be separately optimized in CuGaTe2-based pseudocubic diamond-like compounds, leading to much enhanced zTs, about 24% improvement for Cu0.99In0.6Ga0.4Te2 at 800 K. Furthermore, the average zTs from 300 K to 800 K are improved by 87% compared with that of the CuGaTe2 matrix. This study provides a promising way to optimize the TE performance in pseudocubic diamond-like compounds by simultaneously tuning electrical and thermal transport.