Xiaoying Qin

Simultaneous enhancement in thermoelectric power factor and phonon blocking in hierarchical nanostructured β-Zn4Sb3-Cu3SbSe4

In Pb and Te-free β-Zn4Sb3 based composites incorporated with nanophase Cu3SbSe4 (∼200 nm), we concurrently realize ∼30% increase in thermoelectric power factor (PF) through an energy filtering effect caused by carrier scattering at interface barriers, and around twofold reduction in lattice thermal conductivity due to interface scattering allowing the figure of merit (ZT) to reach 1.37 at 648 K in the composite system with 5 vol. % of Cu3SbSe4. Present results demonstrate that simultaneous enhancement of PF and phonon blocking can be achieved via proper design of a material-system and its microstructures, resulting in large increase in ZT of a material-system.

Enhanced thermoelectric performance through energy-filtering effects in nanocomposites dispersed with metallic particles

The thermoelectric properties of semiconductor materials with metallic nanoinclusions were investigated by using the Boltzmann transport equation under the relaxation time approximation. The results showed that the Seebeck coefficient can be significantly enhanced due to interface potential barrier induced by metallic nanoinclusions, leading to the optimized power factor at T < 700 K as barrier height is around kBT larger than the Fermi energy (here, kB is the Boltzmann constant). Additionally, it was found that high-concentration nanoinclusions with radius 1–2 nm can effectively enhance the thermoelectric performance of nanocomposites through energy-selective carrier scattering.

Electrode activation via vesiculation: improved reversible capacity of γ-Fe2O3@C/MWNT composite anodes for lithium-ion batteries

Capacity fading caused by pulverization is the basic issue for transition-metal-oxide anodes in lithium-ion batteries (LIBs). Here we report a simple and scalable fabrication of core–shell structured γ-Fe2O3@C nanoparticle based composites incorporated with multi-walled carbon nanotubes (MWNTs), through a vacuum-carbonization of the synthesized metal–organic complex and MWNT hybrids. In the constructed γ-Fe2O3@C/MWNT architecture, the carbon shell layers can not only buffer the volume change of γ-Fe2O3 nanoparticles but also improve their conductivity; while the flexible and conductive MWNT networks can maintain the structural and electrical integrity of the electrodes during the charge/discharge cycles. As a result, such γ-Fe2O3@C/MWNT electrodes, tested as anodes for LIBs, exhibit excellent cycling performance with monotonically increased reversible capacities along with cycles. For instance, the specific capacity rises at a rate of ∼6.8 mA h g−1 per cycle to 1139 mA h g−1 after 60 cycles at the current density of 100 mA g−1. Such electrode activation was revealed to be closely related to the increased active surface area of the electrode arising from the gradual vesiculation in Fe2O3@C nanoparticles during lithiation/delithiation in the as-prepared robust γ-Fe2O3@C/MWNT architecture.

Giant scattering parameter and enhanced thermoelectric properties originating from synergetic scattering of electrons in semiconductors with metal nanoinclusions

The scattering processes of electrons in semiconductors with metal nanoinclusions were theoretically investigated in the framework of relaxation time approximation, in which perturbation theory was used to calculate scattering probabilities analytically. Our results reveal that it is not the single scattering at interface potential barriers caused by the nanoinclusions but the synergetic scattering of electrons at ionized impurities and the interface potential barriers or wells that can produce giant scattering parameter λ (=3.3–4.7). Due to giant λ and enhanced thermopower, the estimated figure of merit ZT=3.4 at 800 K for PbTe with Pb nanoinclusions.

Enhanced thermoelectric performance of β-Zn4Sb3 based nanocomposites through combined effects of density of states resonance and carrier energy filtering

It is a major challenge to elevate the thermoelectric figure of merit ZT of materials through enhancing their power factor (PF) and reducing the thermal conductivity at the same time. Experience has shown that engineering of the electronic density of states (eDOS) and the energy filtering mechanism (EFM) are two different effective approaches to improve the PF. However, the successful combination of these two methods is elusive. Here we show that the PF of β-Zn4Sb3 can greatly benefit from both effects. Simultaneous resonant distortion in eDOS via Pb-doping and energy filtering via introduction of interface potentials result in a ~40% increase of PF and an approximately twofold reduction of the lattice thermal conductivity due to interface scattering. Accordingly, the ZT of β-Pb0.02Zn3.98Sb3 with 3 vol.% of Cu3SbSe4 nanoinclusions reaches a value of 1.4 at 648 K. The combination of eDOS engineering and EFM would potentially facilitate the development of high-performance thermoelectric materials.

Enhanced thermoelectric performance of CuGaTe2 based composites incorporated with nanophase Cu2Se

We investigate the thermoelectric properties of CuGaTe2/xCu2Se composites in a moderate temperature range. The results indicate that the introduction of Cu2Se remarkably lowers the thermal conductivity of the composite system. Especially, the thermal conductivity of CuGaTe2/3 vol% Cu2Se decreases to 0.54 W m−1 K−1 at 859 K, which allows its ZT to reach 1.2 at 834 K.

Effect of niobium doping on the microstructure and electrochemical properties of lithium-rich layered Li[Li0.2Ni0.2Mn0.6]O2 as cathode materials for lithium ion batteries

Niobium-doped lithium-rich layered cathode materials, Li[Li0.2Ni0.2Mn0.6−xNbx]O2 (x = 0, 0.02, 0.04, and 0.06), were prepared and the effects of Nb doping on the microstructure and electrochemical properties were investigated. Upon Nb doping, the layered α-NaFeO2 structure is maintained but with an expanded interlayer spacing and the electrochemical properties are significantly enhanced. In particular, the sample with x = 0.04 delivers a large reversible discharge capacity of 254 mA h g−1 at 0.1 C rate with a high capacity retention rate of 92.3% after 100 cycles. Furthermore, it delivers 198 mA h g−1 at 1 C rate, much larger than that of the undoped sample (125 mA h g−1). Capacity differential results reveal that strong Nb–O bond can stabilize the material structure and thus lead to a stable cycling performance. Electrochemical impedance spectroscopy (EIS) analysis shows that Nb doping can decrease the whole cell impedance and expand the Li+ diffusion path in the lithium-rich layered cathode materials, resulting in the excellent rate capability.

Resonant distortion of electronic density of states and enhancement of thermoelectric properties of β-Zn4Sb3 by Pr doping

The thermoelectric properties of Pr-doped compounds β-(Zn1−xPrx)4Sb3 (x = 0, 0.001, 0.002, 0.003) were investigated at the temperatures from 300 K to 615 K. The results indicate that Pr doping causes the resonant distortion of density of states of β-Zn4Sb3, as manifested by almost 2-fold increase of the density of state effective mass md* of β-Zn4Sb3, which results in ∼50 μV/K increase of the thermopower for the doped samples with x = 0.002 and 0.003. The thermal conductivity decreases substantially upon Pr doping. As a result, the figure of merit, ZT, of β-(Zn0.008Pr0.002)4Sb3 is ∼23% larger than that of the un-doped one and reaches 0.65 at 615 K, suggesting that Pr doping is an effective approach to raise ZT of β-Zn4Sb3.

Transport properties and enhanced thermoelectric performance of aluminum doped Cu3SbSe4

The electrical transport and thermoelectric properties of Cu3Sb1−xAlxSe4 (x = 0, 0.01, 0.02 and 0.03) compounds are investigated in the temperature range of 300–600 K. The results indicate that with increasing Al content from x = 0 to x = 0.03, hole concentration increases monotonically from 8.04 × 1017 to 1.19 × 1019 cm−3 due to the substitution of Al3+ for Sb5+, thus leading to a large decrease in the electrical resistivity of Cu3Sb1−xAlxSe4. Meanwhile, the increase in hole concentration leads to a transition from a non-degenerate (x = 0) to a partial degenerate (x = 0.01, 0.02) and then to a degenerate state (x = 0.03). The power factor (PF) of all the Al-doped Cu3Sb1−xAlxSe4 samples is remarkably improved due to the optimization of hole concentration. Lattice thermal conductivity κL of the heavily doped sample (x = 0.03) is reduced. As a result, a large thermoelectric figure of merit ZT = 0.58 is obtained for Cu3Sb0.97Al0.03Se4 at 600 K, which is around 1.9 times as large as that of the un-doped Cu3SbSe4.

Enhanced thermoelectric performance through carrier scattering at heterojunction potentials in BiSbTe based composites with Cu3SbSe4 nanoinclusions

Thermoelectric materials with the thermoelectric figure of merit, ZT, being much larger than unit at near room temperature are vital for power generation by using low-grade waste heat. Here we show that by incorporating very small proportion (1 vol%) of Cu3SbSe4 nanoparticles into the BiSbTe matrix to form nanocomposites, besides large (∼50%) reduction of lattice thermal conductivity, both enhanced thermopower through energy-dependent scattering and alleviated reduction of carrier mobility via carrier scattering at heterojunction potentials occur at elevated temperatures, which allow the thermoelectric power factor of the composite material to reach ∼37 μW cm−1 K−2 at 467 K. Consequently, a largest value of ZT = 1.6 is achieved at 476 K. Moreover, it has excellent performance in a broad temperature range (say, ZT = 1.0 at 300 K and ZT = 1.5 at 500 K), which makes this material attractive for cooling and power generation.