Xiaojian Tan

Enhanced thermoelectric performance in p-type polycrystalline SnSe benefiting from texture modulation

Tin selenide (SnSe) compound has attracted much attention due to its unprecedented high ZT (∼2.6) in single crystals. The polycrystalline SnSe materials were then prepared to improve the mechanical performance for large-scaled application. However, the ZT values of 0.3–0.8 were much lower due to their poor electrical properties. In the present study, the zone melting method is employed to prepare the polycrystalline SnSe samples, which show highly textured structures and strong anisotropic thermoelectric performance. A maximum power factor (S2σ) of 9.8 μW cm−1 K−2 was obtained in the polycrystalline samples, which is comparable with that of SnSe single crystals, resulting in a peak ZT of 0.92 at 873 K. The zone-melted ingot was then pulverized into powders and the bulk material was prepared by the spark plasma sintering (SPS) technique. As a result, the ZT value was enhanced to be over 1.0, owing to the slight reduction of lattice thermal conductivity and maintenance of electrical performance. The present investigation indicates that the TE performance of the SnSe compound can be significantly improved by the texture modulation.

Valence band engineering and thermoelectric performance optimization in SnTe by Mn-alloying via a zone-melting method

Tin telluride (SnTe) has recently attracted lots of interest due to its potential thermoelectric application as a lead-free rock-salt analogue of PbTe. However, pristine SnTe samples have high hole concentration due to the presence of intrinsic Sn vacancies, and shows a low Seebeck coefficient and high electrical thermal conductivity, resulting in poor thermoelectric performance. In this report, we show that zone-melted SnTe systems with additional Mn (1–7 mol%) can control the hole concentration by reducing the Sn vacancies, and modulate the electronic band structure by increasing the band gap and decreasing the energy separation between the light and heavy hole valence bands. Therefore, alloying with additional Mn enhances the contribution of the heavy hole valence band and significantly improves the Seebeck coefficient in SnMnxTe with the highest value of ∼270 μV K−1. A record power factor of 31.9 μW cm−1 K−2 has been obtained at 820 K. The maximum thermoelectric figure of merit ZT of ∼1.25 is found at 920 K for the high quality crystalline ingot of p-type SnMn0.07Te.

Optimization of thermoelectric properties in n-type SnSe doped with BiCl3

N-type SnSe compound has been synthesized through melting with spark plasma sintering. By doping BiCl3, the carrier concentration of SnSe is significantly increased, leading to a large enhancement of electrical conductivity. Meanwhile, the SnSe0.95-BiCl3 samples also exhibit higher Seebeck coefficient and lower lattice thermal conductivity, compared with polycrystalline SnSe. Consequently, a high power factor of similar to 5 mu W cm(-1) K-2 and a ZT of 0.7 have been achieved at 793 K. The synergistic roles of BiCl3 doping in SnSe provide many opportunities in the optimization of n-type SnSe materials

High lithium electroactivity of boron-doped hierarchical rutile submicrosphere TiO2

We have reported a facile method to fabricate hierarchical boron-doped rutile submicrosphere TiO2 (SMT), whose primary particles are ∼20 nm in diameter. The as-synthesized boron-doped SMT shows excellent cycling performance and rate capability in comparison with undoped TiO2 as an anode material in Lithium-Ion Batteries (LIBs). It has a very stable capacity of ∼190 mA h g−1 for 500 cycles at 1C. In addition, the density functional theory (DFT) calculations are carried out to indicate that a low concentration (<1.0 at%) of boron doping could enhance the carrier mobility μ and electrical conductivity σ, and thus reveal the relationship between the electronic structure of boron-doped SMT and the performances of the boron-doped SMT anode in LIBs. Our results also clearly demonstrate the importance and advantage of the hierarchical submicrometer-sized spherical morphology of the TiO2 anode in LIBs.

Enhanced thermopower in rock-salt SnTe–CdTe from band convergence

SnCdxTe materials were synthesized by the zone-melting method for this thermoelectric performance study. The X-ray diffraction results show that the lattice parameter decreases with increasing x, following Vegards law of rock-salt structure SnTe and CdTe. Besides, the room temperature Seebeck coefficients of the SnCdxTe system are enhanced to >60 μV K−1, larger than those of Cd-doped SnTe synthesized by spark plasma sintering. A large power factor of ∼25 μW cm−1 K−1 is achieved in SnCd0.12Te at 820 K, which rivals those of high performance PbTe-based materials. As a result, the highest ZT of ∼1.03 at 820 K was achieved for SnCd0.12Te.

Theoretical understanding on band engineering of Mn-doped lead chalcogenides PbX (X = Te, Se, S)

Electronic structures of Mn-doped PbX (X = Te, Se, S) are investigated by first-principles calculations. It is found that the Mn-doping in PbTe enlarges the band gap and increases the valence bands degeneracy, showing good agreement with experimental measurements. This band adjustment is demonstrated to be from the anti-bonding of Te-p and Mn-d orbitals. Along the series of PbTe-PbSe-PbS, the band modification of Mn-doping undergoes a gradual transition from multiple valence bands type to resonant states type, owing to the downwards shifted anion-p orbitals. This work provides essential understandings on the band engineering of Mn-doped lead chalcogenides thermoelectric materials.

Enhanced power factor in the promising thermoelectric material SnPbxTe prepared via zone-melting

Tin telluride (SnTe) has recently attracted much attention as a promising thermoelectric material. In this work, SnTe is alloyed with additional Pb, and the high density crystalline ingots of SnPbxTe (x = 0, 0.02, 0.04, and 0.06) have been synthesized by a zone-melting method. Through this method, SnPbxTe samples show larger power factors than those prepared by other methods, and a maximum value of 30.5 μW cm−1 K−2 at 823 K has been reached in p-type SnPb0.02Te, which is the highest value reported so far. As a result, a promising figure of merit ZT of ∼0.81 has been obtained at 823 K.

Enhanced thermoelectric performance in n-type polycrystalline SnSe by PbBr2 doping

A series of PbBr2-doped polycrystalline SnSe samples were synthesized by melting and hot pressing. By PbBr2 doping, the carrier concentration of SnSe was increased to 1.86 × 1019 cm−3 from 2.41 × 1017 cm−3, resulting in an increased electrical conductivity of 40 ± 2 S cm−1 at 713 K while the undoped SnSe was only 5.1 ± 0.3 S cm−1. Meanwhile, the PbBr2-doped samples also exhibit a larger density of state effective mass (0.812m0). Therefore, a high power factor of 4.8 ± 0.5 μW cm−1 K−2 and a peak ZT of 0.54 ± 0.1 were achieved at 793 K perpendicular to the hot pressing direction.

First-principles study on the elastic properties of Cu2GeSe3

The elastic properties of Cu2GeSe3, including bulk modulus, shear modulus, Youngs modulus, Possions ratio, and their anisotropic properties, have been investigated by using first-principles calculations. The calculated lattice parameters are in good agreement with previous calculations and experimental measurements. The result of bulk modulus by fitting the Birch-Murnaghan 3rd-order equation of state is well consistent with that calculated from the elastic constants. The ductile nature of Cu2GeSe3 is characterized according to Pughs rule. The Debye temperature calculated from fitting heat capacity data is consistent with that obtained from sound velocity. Additionally, the elastic anisotropy is depicted in detail by plotting the directional dependence of the bulk and Youngs moduli. Copyright (C) EPLA, 2016

First-principles study on the lattice dynamics and thermodynamic properties of Cu2GeSe3

The lattice dynamics and thermodynamic properties of Cu2GeSe3 are investigated by first-principles calculations. The obtained phonon frequencies agree well with the measurements of Raman scattering. The thermodynamic properties are calculated within quasi-harmonic approximation, and the measured lattice thermal conductivity is well reproduced. The calculated Gruneisen parameter is found to be much smaller than previous prediction, indicating that the bonding anharmonicity is insufficient to explain the low thermal conductivity in Cu2GeSe3. Our study shows that the thermodynamic properties of Cu2GeSe3 are inherently related to its weak covalent Cu-Se bonding.