Xianglian Liu

Bandgap reduction responsible for the improved thermoelectric performance of bulk polycrystalline In2–xCuxSe3 (x = 0−0.2)

α-In2Se3 is of large bandgap (∼1.4 eV) semiconductor and its structure is based on two-layer hexagonally packed arrays of selenium atoms with 1/3 of the sites of indium atoms being empty. Here we report a bandgap Eg reduction due mainly to the formation of a Cu2Se slab in the host In2Se3, which is responsible for the remarkable improvement of thermoelectric performance of bulk polycrystalline In2−xCuxSe3 (xu2009=u20090.1–0.2). When xu2009=u20090.2 the dimensionless figure of merit ZT and power factor were increased by a factor of 2 and 3, respectively, at 846 K if compared to those of Cu-free In2Se3. Interestingly, an incorporation of Cu into the lattice of In2Se3 results in a change in morphology from amorphouslike structure represented by In2Se3 to a visible polycrystalline form attributed to partial crystallization of the structure. This change enhances lattice thermal conductivities κL over the very low values of In2Se3. However, the enhancement is only moderate because of the effective scattering of phonons in the p...

Thermoelectric properties in nanostructured homologous series alloys GamSbnTe1.5(m+n)

In this paper we reported the thermoelectric (TE) properties in nanostructured homologous series alloys GamSbnTe1.5(m+n) over the temperature range of 318–482 K and observed the maximum TE figure of merit (ZT) value of 0.98 for the alloy with m:n=1:10 at 482 K, which is approximately 0.24 higher than that of undoped Sb2Te3 at the corresponding temperature. This improvement is mainly attributed to the substantial reduction in lattice thermal conductivity due to the phonon scattering caused partly by the nanograins (<30u2002nm) and amorphous structure conceived in the matrix and partly by the lattice distortion resulted from an occupation of some Ga atoms in the Sb sites and a certain amount of Ga2Te3 precipitation. If in comparison with the TE properties for Ga directly doped Bi–Sb–Te solid solutions, we conclude that these Bi-free nanostructured homologous series alloys GamSbnTe1.5(m+n) with proper compositions are of great potentiality for the improvement of TE performance.

Effects of Cu5Zn3 addition on the thermoelectric properties of Zn4Sb3

The structures and thermoelectric properties of mCu5Zn3⋅nZn4Sb3 with multiphase coexistence are reported. Rietveld analysis reveals that at least 92.3% wtu2009% β-Zn4Sb3 phase can be obtained with only small quantities of ZnSb and Cu5Zn8 phases precipitated after proper Cu5Zn3 addition. Measurements indicate that although the β-Zn4Sb3 phase plays a determining role in controlling the transport properties involving the Seebeck coefficient, electrical conductivity, and thermal conductivity, the impurity phases Cu5Zn8 and ZnSb with a crooked riverlike and intertwined tree stump morphologies, respectively, are still of great significance to tune the thermoelectric performance. The highest ZT value of 0.84 can be obtained for the alloy mCu5Zn3⋅nZn4Sb3 (m/n=1/200) at 631 K, approximately 1.8 times that of undoped β-Zn4Sb3, proving that a good combination between the transports of carriers and phonons can be achieved if a proper dopant is introduced in the Zn4Sb3 matrix.

Improvement of the thermoelectric performance of InSe-based alloys doped with Sn

Here we present InSe-based alloys InSeSnx (x = 0–0.02) with improved thermoelectric performance upon Sns preferential occupation on In lattice sites. This improvement is attributed to the enhancement in carrier concentration (n) and reduction in lattice thermal conductivity (κL). However, the enhancement in n is limited due to the presence of the intermediate band in the middle of the bandgap, which acts as an annihilation center for electrons and holes. The reduction in κL is caused by increased phonon scattering on the newly-created defect SnIn+. As a result, we attain the highest ZT value of 0.23 at x = 0.01@830 K, which is about 2.9 times that of virgin InSe.

Enhancing the thermoelectric performance of Cu3SnS4-based solid solutions through coordination of the Seebeck coefficient and carrier concentration

Improving the thermoelectric (TE) performance of Cu3SnS4 is challenging because it exhibits a metallic behavior, therefore, a strategy should be envisaged to coordinate the carrier concentration (nH) and Seebeck coefficient (α). The coordination in this work has been realized through the Fermi level (Ef) unpinning and shifting towards the conduction band (CB) via addition of excess Sn in Cu3SnS4. As a result, the solid solution Cu3Sn1+xS4 (x = 0.2) has a moderate α (178.0 μV K−1) at 790 K and a high nH (1.54 × 1021 cm−3) value. Along with the lowest lattice thermal conductivity κL (0.39 W K−1 m−1) caused by the increased phonon scattering by carriers, the highest ZT value of 0.75 is attained at ∼790 K. This value is 2.8 times that of the stoichiometric Cu3SnS4, and stands among the highest for ternary Cu–Sn–S sulfide thermoelectrics at the corresponding temperatures. More importantly, this approach used in the case of ternary Cu3SnS4 provides a guidance or reference to improve the TE performance of other materials.

Significant improvement in the thermoelectric performance of Sb-incorporated chalcopyrite compounds Cu18Ga25SbxTe50−x (x = 0–3.125) through the coordination of energy band and crystal structures

A newly developed chalcopyrite semiconductor Cu18Ga25Te50 (Cu/Ga = 0.72) has an inherent deficiency in Cu. Therefore, it is taken as a thermoelectric candidate due to a high vacancy rate of copper. In this work, we have observed that after Sb substitution for Te in Cu18Ga25SbxTe50−x, the active Sb-5p orbital hybridizes with those of Cu-4s and Te-5p in the valence band, which makes the Fermi level (Ef) unpin and move toward the inner side of the valence band as Sb content increases. The alteration in the band structure, which is the determining factor, causes the Hall carrier concentration (nH) to rise by more than one order of magnitude compared with those in pristine Cu18Ga25Te50, thereby significantly increasing the power factor (PF). Combined with the relatively low thermal conductivity, caused by the increased lattice disorder and general diminution of the crystal structure distortion as the x value increases, we have attained the best TE performance, where ZT reaches the highest value of 1.2 for the Sb-incorporated Cu18Ga25SbxTe50−x (x = 2.5) at 854 K. This result suggests that the coordination of energy band and crystal structures is a good approach to achieving high TE performance via the appropriate distortion of the crystal structure of ternary chalcopyrite semiconductors.

Improved thermoelectric performance of solid solution Cu 4 Sn 7.5 S 16 through isoelectronic substitution of Se for S

Cu-Sn-S family of compounds have been considered as very competitive thermoelectric candidates in recent years due to their abundance and eco-friendliness. The first-principles calculation reveals that the density of states (DOS) increases in the vicinity of the Fermi level (Ef) upon an incorporation of Se in the Cu4Sn7.5S16−xSex (xu2009=u20090–2.0) system, which indicates the occurrence of resonant states. Besides, the formation of Cu(Sn)-Se network upon the occupation of Se in S site reduces the Debye temperature from 395u2009K for Cu4Sn7S16 (xu2009=u20090) to 180.8u2009K for Cu4Sn7.5S16−xSex (xu2009=u20091.0). Although the point defects mainly impact the phonon scattering, an electron-phonon interaction also bears significance in the increase in phonon scattering and the further reducion of lattice thermal conductivity at high temperatures. As a consequence, the resultant TE figure of merit (ZT) reaches 0.5 at 873u2009K, which is 25% higher compared to 0.4 for Cu4Sn7.5S16.

Unequal bonding in Ag–CuIn3Se5-based solid solutions responsible for reduction in lattice thermal conductivity and improvement in thermoelectric performance

Owing to their unique crystal and band structures, in thermoelectrics increasing attention has recently been paid to compounds of the ternary I–III–VI chalcopyrite family. In this work, unequal bonding between cation and anion pairs in Cu1−yAgyIn3Se4.9Te0.1 solid solutions, which can be effectively used to disturb phonon transport, has been proposed. The unequal bonding, which is represented by the difference of bond lengths Δd, Δd = d(Cu–Se) − d(In–Se) and anion position displacement from its equilibrium position Δu = u − 0.25, is created by the isoelectronic substitution of Ag for Cu. At y = 0.2 both the Δd and Δu values reach their maxima, resulting in a remarkable reduction in lattice thermal conductivity (κL) and an improvement in TE performance. However, as the y value increase to 0.3 both Δd and Δu values decrease, causing the κL value to increase and the ZT value to decrease from 0.5 to 0.24 at 930 K. Accordingly, unequal bonding might be an alternative way to improve the TE performance of ternary chalcopyrites.