Hongxing Xin

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.

Thermoelectric properties of (Zn0.98M0.02)4Sb3 (M = Al, Ga and In) at low temperatures

Thermoelectric properties of substitutional compounds (Zn0.98M0.02)4Sb3 (M = Al, Ga and In) were investigated at temperatures from 5 to 310 K. The results indicate that substitution of M for Zn in Zn4Sb3 leads to a decrease in electrical resistivity ρ, thermopower S and thermal conductivity λ. The decrease in ρ and S of (Zn0.98M0.02)4Sb3 would originate from increased electron contribution caused by introduction of donor levels due to substitution of M3+ for Zn2+. Nevertheless, our experiments revealed that as the dopant changed from Al to Ga and then to In, the resistivity of (Zn0.98M0.02)4Sb3 increased monotonically, which could be caused by reduced mobility due to the increase in the ionic radius from Al to In. Meanwhile, λ of (Zn0.98M0.02)4Sb3 at low temperatures (especially at T 250 K due to the large reduction in its resistivity, indicating that the thermoelectric properties of β-Zn4Sb3 can be enhanced by Al doping.

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.

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.

Enhanced thermoelectric performance of Cu2Se/Bi0.4Sb1.6Te3 nanocomposites at elevated temperatures

Bi2Te3-based thermoelectric materials with large thermoelectric figure of merit, ZT, at elevated temperatures are advantageous in power generation by using the low-grade waste heat. Here, we show that incorporation of small proportion (0.3 vol. %) of nanophase Cu2Se into BiSbTe matrix causes an enhanced high-temperature thermopower due to elevated energy filtering of carriers and inhibition of minority transport besides enhanced phonon blocking from scattering at interfaces, which concurrently result in an ∼20% increase in the power factor and an ∼60% reduction in the lattice thermal conductivity at 488 K. As a result, ZT = 1.6 is achieved at 488 K in the composite system with 0.3 vol. % of Cu2Se. Significantly, its ZT is larger than unit in broad high-temperature range (e.g., ZT = 1.3 at 400 K and ZT = 1.6 at 488 K), which makes this material to be attractive for applications in energy harvesting from the low-grade waste heat.

Effects of Bi doping on the thermoelectric properties of β-Zn4Sb3

The thermoelectric properties of Bi-doped compounds (Zn1−xBix)4Sb3 (x=0,0.0025,0.005,0.01) were studied experimentally as well as theoretically. The results indicate that low-temperature (T<300 K) thermal conductivity of moderately doped (Zn0.9975Bi0.0025)4Sb3 reduces remarkably as compared with that of Zn4Sb3 due to enhanced phonon scattering of impurity (dopant). Electrical resistivity and Seebeck coefficient increase monotonically with increase in the Bi content resulting mainly from decrease in carrier concentration. Moreover, first-principle calculations were performed on the occupation options of Bi atoms in β-Zn4Sb3, which show that Bi will preferentially occupy the Zn sites and not Sb sites and act as donors, being consistent with the experimental observations. In addition, the lightly doped compound (Zn0.9975Bi0.0025)4Sb3 exhibits the best thermoelectric performance due to the improvement in both its thermal conductivity and Seebeck coefficient, whose figure of merit, ZT, is about 1.5 times large...

Electrical and thermoelectric properties of nanocrystal substitutional semiconductor alloys Mg3(BixSb1−x)2 prepared by mechanical alloying

Nanocrystal substitutional semiconductor alloys Mg3(BixSb1−x)2 (nano-Mg3(BixSb1−x)2) with a mean grain size of ~30 nm were prepared by mechanical alloying plus hot-pressing, and their dc electrical and thermoelectric properties were investigated from room temperature down to 20 K. The results indicated that lattice parameters a and c of nano-Mg3(BixSb1−x)2 increased linearly with increasing Bi content x, in agreement with Vegards law. The dc resistivity ρ of nano-Mg3(BixSb1−x)2 decreased monotonically with increasing x, and a drop of over five orders of magnitude was reached at 300 K when x increased from 0 to 1. Moreover, the temperature behaviour of the resistivity of nano-Mg3(BixSb1−x)2 changed sensitively with x, and a transition from the semiconducting state (i.e. dρ/dT 0) occurred between x = 0.7 and 0.8. Meanwhile, this transition was verified by the measurements of the temperature behaviour of the Seebeck coefficient S of nano-Mg3(Bi1−xSbx)2 with different x. In addition, Motts ρ ∝ T−1/4 law was observed at lower temperature regimes for the nano-Mg3(BixSb1−x)2 (x ≠ 0), suggesting the occurrence of hopping conduction. Although experiments showed that the Seebeck coefficient of nano-Mg3(Bi1−xSbx)2 decreased monotonically with x, their thermoelectric power factors PF changed non-monotonically, and a maximum PF of 1.4 µW cm−1 K−2 was achieved at room temperature for x = ~0.8, which was more than three orders magnitude greater than that of monolithic Mg3Sb2.

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.