Liangwei Fu

Multiple heteroatom induced carrier engineering and hierarchical nanostructures for high thermoelectric performance of polycrystalline In4Se2.5

In this paper, different atom combinations of Pb, I and Cu have been doped into the In4Se2.5 matrix and a systematic investigation has been carried out for the synergistic effect of multiple heteroatoms on the microstructure and thermoelectric properties of polycrystalline In4Se2.5. By this approach, the electron and phonon transport properties are rationally regulated and the electrical conductivity increases greatly due to the multiple doping, which result in a simultaneous increase of carrier concentration and mobility. The Seebeck coefficient also remains at a relatively high level in a high temperature range due to the energy-dependent electron scattering at the metal nanoparticle–matrix interfaces. In addition, the lattice thermal conductivity is also greatly reduced because of the wide frequency phonon scattering by the point defects and hierarchical metal nanoparticles combined with the phonon–phonon interactions. Consequently, an enhancement of the ZT with a maximum of 1.4 (723 K) has been achieved in the multiple doped In4Se2.5 sample.

Enhancement of thermoelectric properties of Yb-filled skutterudites by an Ni-Induced “core–shell” structure

Since the lattice thermal conductivity of n-type multi-filled skutterudites have been reduced below 1 W (mK−1), the development of new strategies that can further enhance the power factor while maintaining the low thermal conductivity is highly desired. In this paper, we conducted a pioneering work by introducing a “core–shell” microstructure into Yb single-filled skutterudite thermoelectric materials to realise this purpose. The “core–shell” structure formed by the thermal diffusion of well dispersed Ni nanoparticles in the Yb0.2Co4Sb12 powder during hot pressing is composed of the normal “core” grains surrounded by Ni-rich nanograin “shells”. The electrical resistivity is greatly reduced due to the increase in both carrier concentration and mobility. However, the Seebeck coefficient first increases due to the increased density of states at the Fermi energy and then decreases gradually. As a consequence, the power factor is remarkably increased for the samples with the addition of Ni nanoparticles. In addition, the lattice thermal conductivity is also reduced by the extra phonon scattering introduced by the “core–shell” microstructure. The concomitant effects enable a maximum ZT of 1.07 for the 0.2 wt% Ni sample at 723 K.

A study of Yb0.2Co4Sb12–AgSbTe2 nanocomposites: simultaneous enhancement of all three thermoelectric properties

The single-filled skutterudite Yb0.2Co4Sb12 has been long known as a promising bulk thermoelectric material. In this work, we adopted a melting–milling–hot pressing procedure to prepare nanocomposites that consist of a micrometer-grained Yb0.2Co4Sb12 matrix and well-dispersed AgSbTe2 nanoinclusions on the matrix grain boundaries. Different weight percentages of AgSbTe2 inclusions were added to optimize the thermoelectric performance. We found that the addition of AgSbTe2 nanoinclusions systematically and simultaneously optimized the otherwise adversely inter-dependent electrical conductivity, Seebeck coefficient and thermal conductivity. In particular, the significantly enhanced carrier mobility led to a ∼3-fold reduction of the electrical resistivity. Meanwhile the absolute value of Seebeck coefficient was enhanced via the energy filtering effect at the matrix–nanoinclusion interfaces. Moreover there is a topological crossover of the AgSbTe2 inclusions from isolated nanoparticles to a nano-plating or nano-coating between 6 wt% and 8 wt% of nanoinclusions. Above the crossover, further addition of nanoinclusions degraded the Seebeck coefficient and the electrical conductivity. Meanwhile, the addition of nanoinclusions generally reduced the lattice thermal conductivity. As a result, the power factor of the 6 wt% sample was ∼7 times larger than that of the nanoinclusion-free sample, yielding a room temperature figure of merit ZT ∼ 0.51.

Ternary CuSbSe2 chalcostibite: facile synthesis, electronic-structure and thermoelectric performance enhancement

A single phase CuSbSe2 polycrystalline chalcostibite compound has been facilely synthesized through mechanical alloying for the first time, and the phase evolution has been revealed in detail. The large Seebeck coefficient and poor electrical conductivity of this compound are ascribed to the existing Cu–Se ionic bonding and heavy band at the VBM. The active lone-pair s2 electrons in Sb3+ ions are likely to be responsible for the experimentally observed low thermal conductivity in the CuSbSe2 compound. By introducing narrow band-gap Cu3SbSe4 into the CuSbSe2 matrix, the carrier concentration and mobility can be tuned effectively; thus the power factor has been improved remarkably. As a consequence, the figure of merit (ZT) is increased by a factor of 1.6 at 623 K, from 0.25 (matrix) to 0.41.

A simultaneous increase in the ZT and the corresponding critical temperature of p-type Bi0.4Sb1.6Te3 by a combined strategy of dual nanoinclusions and carrier engineering

By means of β-Zn4Sb3 addition and thermal decomposition, dual nanoinclusions of Zn and ZnSb were introduced and Zn atoms were doped into p-type Bi0.4Sb1.6Te3 successfully. Due to the increase of hole concentration by Zn doping and band structure optimization, the bipolar conduction was suppressed and the intrinsic excitation shifts to higher temperature. The power factors of the samples were slightly improved, while the lattice thermal conductivities of the samples were greatly reduced due to the extra phonon scattering introduced by the dual nanoinclusions. As a result, the critical temperature corresponding to the maximum ZT value was greatly increased to 423 K, which is about 120 K higher when compared with the conventional Bi2Te3-based materials. A maximum ZT of 1.44 was achieved for the sample with 1.5 wt% β-Zn4Sb3 at 423 K, which is also the highest ZT value ever reported at such a high temperature for p-type Bi2Te3-based materials. This work is of importance to expand the application of Bi2Te3-based materials for low grade waste-heat recovery.

Study on lattice dynamics of filled skutterudites InxYbyCo4Sb12

As a promising class of thermoelectric materials, skutterudites are featured by the naturally formed oversize cages in its crystal lattice. Cage-filling by guest atoms has thus become an important approach to reducing the lattice thermal conductivity and optimizing the thermoelectric performance. To probe the impact of filler atoms on lattice dynamics, we herein reported specific heatmeasurements in two single-filled skutterudite samples In0.2Co4Sb12 and Yb0.2Co4Sb12, and a double-filled skutterudite sample In0.2Yb0.2Co4Sb12. The low temperature specific heat data was analyzed in the context of combined electronic specific heat (the Sommerfeld term), the Debye mode (long wavelength acoustic phonon modes), and the Einstein modes (localized vibration modes). We found that the specific heat difference between the single and double-filled samples can be well accounted by one extra Einstein mode, as expected from the extra filler atom and confirmed by the results of inelastic neutron scatteringmeasurements. Interestingly, two Einstein modes, in addition to the Sommerfeld term and the Debye term, are needed to satisfactorily account for the specific heat of the single-filled sample. The Einstein mode with lower frequency has the frequency close to the low-lying mode reported in La, Ce, Tl single-filled skutterudites, this mode is largely unaffected when the second type of filler atoms is introduced. The frequency of this mode has been verified by inelastic neutron scatteringmeasurement. The other Einstein mode with higher frequency may be originated from the motion of Sb atoms.

Synergistic tuning of carrier and phonon scattering for high performance of n-type Bi2Te2.5Se0.5 thermoelectric material

Polycrystalline Bi2Te2.5Se0.5 materials with different amounts of added MnTe2 have been fabricated by plasma activated sintering. XRD, SEM, EDS and HRTEM techniques and measurements of the electrical and thermal transport properties have been employed to study the effect of MnTe2 addition on the microstructure and the thermoelectric performance of Bi2Te2.5Se0.5 materials. Due to the substitution of Mn at the Bi sites, the XRD peaks shift towards a higher angle with an increase in the content of MnTe2, and the electron density increases; thus, the bipolar conduction is suppressed and accordingly shifts to a higher temperature . When the content of MnTe2 is over 3.0 at%, the dissolution of MnTe2 gets saturated, and some nanoinclusions with sizes of about 40–70 nm were observed and were identified as MnTe2 by EDS and HRTEM characterization. Due to the extra phonon scattering by the MnBi′ point defects and MnTe2 nanoinclusions, the phonon thermal conductivity decreased significantly. As a result, a maximum ZT of 1.0 at 523 K was achieved in the sample with 3.0 at% MnTe2, which is about a 150 K degree shift and a 100% improvement at this temperature compared with the conventional n-type Bi2Te3-based materials.

Enhancement of thermoelectric properties of Ce0.9Fe3.75Ni0.25Sb12 p-type skutterudite by tellurium addition

Recently, research interests in p-type skutterudites are focused on multi-filling in the intrinsic void sites of Fe4−xMxSb12. The four-membered antimony rings, another important structural feature of p-type skutterudites, seem to be overlooked. In this study, Te has been employed to substitute Sb in single-filled p-type Ce0.9Fe3.75Ni0.25Sb12 skutterudite and a systematic investigation has been carried out into the doping effect of Te on the microstructure and thermoelectric properties of this material. With an increase of Te doping, the electrical resistivity decreases due to the increase of hole concentration and the gradual decrease of mobility; while the density of state effective mass increases rapidly with the increase of hole concentration due to the intrinsic multiple bands effects, thus the Seebeck coefficient increases slightly. In addition, Te doping on Sb sites changes the valance electron balance and results in the formation of microscale γ-Ce and nanoscale CeTe2 compound. These microscale and nanoscale precipitates work together with the atomic scale distortion of Sb4 rings by the substitution of Te for Sb, could scatter phonons with a wide range of frequency thus result in a significant reduction of the lattice thermal conductivity. Therefore the thermoelectric performance has been enhanced greatly, and a maximum ZT value of 1.0 at 773 K was obtained for the Ce0.9Fe3.75Ni0.25Sb11.9Te0.1 sample.

Lattice thermal transport in double-filled skutterudites In0.1YbyCo4Sb12

In order to investigate the phonon scattering mechanisms in double-filled skutterudites, the low-temperature lattice thermal conductivities of In0.1YbyCo4Sb12 were measured and analyzed based on the Debye model. The eingenmode frequencies of In and Yb were obtained from low-temperature specific heat capacity analysis. It is found that filling these two types of guest atoms with different eingenmode frequencies into the voids in skutterudites could introduce strong point defect and resonant scattering to lattice phonons, thus lead to significant decrease in the lattice thermal conductivity.