Yemao Han

Thermoelectric performance of SnS and SnS–SnSe solid solution

Solid solution is a potential way to optimize thermoelectric performance for its low thermal conductivity compared to those of the constituent compounds because of the phonon scattering from disordered atoms. Tin(II) sulfide (SnS) shows analogous band structure and electrical properties with tin selenide (SnSe), which was the motivation for investigating the thermoelectric performance of SnS and SnS–SnSe solid solution system. SnS compound and SnS1−xSex (0 < x < 1) solid solution were fabricated using the melting method and they exhibited anisotropic thermoelectric performance along the parallel and perpendicular to the pressing directions. For the SnS compound, the maximum zT∥ value is 0.19 at 823 K along the parallel to pressing direction, which is higher than that along the perpendicular to the pressing direction (zT⊥ = 0.16). The zT values of SnS0.5Se0.5 and SnS0.2Se0.8 were higher than those of the SnS compound and a maximum zT value of 0.82 was obtained for SnS0.2Se0.8 at 823 K, which is more than four times higher than that of SnS.

The thermoelectric performance of anisotropic SnSe doped with Na

Lead-free polycrystalline SnSe is a promising thermoelectric compound consisting of earth-abundant elements. However, the poor electrical transport property for low intrinsic defect concentration (3 × 1017 cm−3) limits the usage of the stoichiometric SnSe compound. In this work, Na2Se as an acceptor was doped into SnSe in order to optimize the electrical transport properties, especially to increase the carrier concentration. As a result, the carrier concentrations increased and saturated at about 1.0 × 1019 cm−3 for Na0.01Sn0.99Se at 300 K, and a maximum power factor of 0.48 mW m−1 K−2 was obtained. A maximum zT value of 0.75 was obtained at 823 K for Na0.01Sn0.99Se along the direction perpendicular to the sintering pressure, which is 25% higher than that (0.6) of the undoped SnSe compound.

Thermoelectric performance of co-doped SnTe with resonant levels

Some group III elements such as Indium are known to produce the resonant impurity states in IV-VI compounds. The discovery of these impurity states has opened up new ways for engineering the thermoelectric properties of IV-VI compounds. In this work, resonant states in SnTe were studied by co-doping with both resonant (In) and extrinsic (Ag, I) dopants. A characteristic nonlinear relationship was observed between the Hall carrier concentration (n_H) and extrinsic dopant concentration (N_I, N_(Ag)) in the stabilization region, where a linear increase of dopant concentration does not lead to linear response in the measured n_H. Upon substituting extrinsic dopants beyond a certain amount, the nH changed proportionally with additional dopants (Ag, I) (the doping region). The Seebeck coefficients are enhanced as the resonant impurity is introduced, whereas the use of extrinsic doping only induces minor changes. Modest zT enhancements are observed at lower temperatures, which lead to an increase in the average zT values over a broad range of temperatures (300–773 K). The improved average zT obtained through co-doping indicates the promise of fine carrier density control in maximizing the favorable effect of resonant levels for thermoelectric materials.

Broad negative thermal expansion operation-temperature window in antiperovskite manganese nitride with small crystallites

Using spark plasma sintering (SPS), Mn3Cu0.6Ge0.4N crystallites have been fabricated with different crystallite sizes, and their magnetic properties and thermal behaviors were systemically investigated. With decreasing crystallite size, the magnetic transition becomes increasingly slow, accompanied by broadening of the negative thermal expansion (NTE) operation-temperature window. The NTE operation-temperature window for the 12-nm crystallite sample reaches at 140 K, which is about 75% larger than that of the 74-nm crystallite sample. The magnetic properties and NTE operation-temperature window can be tuned by varying the crystallite size. This discovery will promote an even wider range of practical applications in precision devices.

High thermoelectric performance of In-doped Cu2SnSe3 prepared by fast combustion synthesis

Bulk In-doped Cu2SnSe3 samples have been prepared by a fast, one-step method combining the synthesis and sintering process, named high-pressure combustion synthesis (HPCS), and they were also prepared by conventional spark plasma sintering (SPS) for comparison. The relative densities of the In-doped Cu2SnSe3 samples are above 98%, a little higher than the 96% by SPS. The thermodynamic parameters and reaction kinetics of the HPCS process are investigated, showing the maximum combustion temperature of 708 K and combustion wave propagation velocity of 2 mm s−1. The thermoelectric properties of the Cu2Sn1−xInxSe3 samples (HPCS) with x ranging from 0 to 0.20 have been measured in the temperature range of 323–773 K. The electrical conductivity at 323 K is greatly enhanced by almost 6 times from 2.2 × 104 S m−1 to 12.9 × 104 S m−1 by the substitution of Sn with In (x = 0.15). The maximum ZT reaches 0.56 at 773 K for the sample of x = 0.10, which is about 20% higher than that of the unadulterated sample. Compared with the samples prepared by HPCS-SPS, the maximum ZT reaches 1.28 at 823 K for the In doping content of x = 0.10, which is much higher than that for the HPCS samples, attributing to the much lower thermal conductivity caused by strong boundaries scattering. The combustion synthesis offers a fast and more efficient approach for the fabrication of Cu2SnSe3 materials with reduced time and energy consumption.

Electronic transport and magnetoresistance in ultrathin manganite-titanate junctions

We present a systematic study on the rectifying behaviors of the heterojunctions composed of a ultrathin La0.67Sr0.33MnO3 film (∼2nm in thickness) and a SrTiO3 substrate doped by 0.05 or 1wt% Nb. These junctions exhibit excellent rectifying behaviors and a remarkable bias-dependent magnetoresistance (up to 60% under a field of 5T). The transport behaviors are dominated by thermal process and tunneling process for the junctions with low and high Nb contents, respectively.

Thermoelectric properties of S and Te-doped Cu2SnSe3 prepared by combustion synthesis

ABSTRACT S and Te-doped Cu2SnSe3 samples with chemical formula of Cu2SnSe3-xSx and Cu2SnSe3-xTex (x = 0.05, 0.10, 0.15, 0.20) are prepared, and the effect of S and Te-doping on thermoelectric properties is investigated. In all the samples, the Cu2SnSe3 phase is synthesized as the major product, and its lattice parameters decrease with S-doping but increase with Te-doping. EDS analysis confirms the incorporation of S and Te dopants in the Cu2SnSe3 phase, and reveals an element segregation in the Te-doped samples. S- and Te-doping decreases the electrical conductivity, enhance the Seebeck coefficient, and reduce the thermal conductivity. S-doping is effective to improve the ZT of Cu2SnSe3, and the Cu2SnSe3-xSx sample with x = 0.05 shows a ZT of 0.66 at 773 K, which is improved by more than 50% compared with that of the undoped sample. On the contrary, Te-doping leads to decreased ZT values owing to the significant reduction in electrical conductivity.