Xihong Chen

Enhanced Thermoelectric Performance of Single-Walled Carbon Nanotubes/Polyaniline Hybrid Nanocomposites

Hybrid nanocomposites containing carbon nanotubes (CNTs) and ordered polyaniline (PANI) have been prepared through an in situ polymerization reaction using a single-walled nanotube (SWNT) as template and aniline as reactant. TEM, SEM, XRD, and Raman analyses show that the polyaniline grew along the surface of CNTs forming an ordered chain structure during the SWNT-directed polymerization process. The SWNT/PANI nanocomposites show both higher electrical conductivity and Seebeck coefficient as compared to pure PANI, which could be attributed to the enhanced carrier mobility in the ordered chain structures of the PANI. The maximum electrical conductivity and Seebeck coefficient of composites reach 1.25 x 10(4) S m(-1) and 40 microV K(-1), respectively, and the maximum power factor is up to 2 x 10(-5) W m(-1) K(-2), more than 2 orders of magnitude higher than the pure polyaniline. This study suggests that constructing highly ordered chain structure is a novel and effective way for improving the thermoelectric properties of conducting polymers.

Effect of antisite defects on band structure and thermoelectric performance of ZrNiSn half-Heusler alloys

Band structures for ZrNiSn with Zr/Sn antisite defects are calculated with ab initio methods. Antisite defects shrink the band gap and enhance the density of states slope near the Fermi level, which are favorable to electrical transport properties for intrinsic semiconductors. The degree of Zr/Sn antisite defects are controlled by annealing time experimentally, and measurements show low electrical resistivity and high Seebeck coefficient for unannealed ZrNiSn, which benefits from the modified electronic structure caused by antisite defects. The maximum ZT is 0.64 at 800 K for unannealed ZrNiSn, which is the highest value for ZrNiSn systems without exterior doping.

Low thermal conductivity and enhanced thermoelectric performance of Gd-filled skutterudites

With Fe compensation, the heavy rare earth element Gd-filled GdyFexCo4−xSb12 (x<2) skutterudites have been successfully synthesized by melting-annealing approach. Fe substitution on the Co site brings two contrary effects on Gd filling: charge compensation which enhances the filling fraction of Gd, and Lattice expansion which is deleterious for the stability of filled compounds that contain smaller atoms. When Fe content is less than 1.7, pure GdyFexCo4−xSb12 compounds are obtained and the Gd maximum filling fraction (ymax) increases with Fe content. The power factor (S2σ) of the GdyFexCo4−xSb12 increases with Fe content. The lattice thermal conductivity is significantly depressed by Gd filling. The sample Gd0.41Fe1.48Co2.52Sb12 has a lattice thermal conductivity as low as 1.1 W m−1 K−1 at room temperature, and its figure of merit (ZT) reaches a maximum value of 0.83 at 700 K. At high temperature, thermal conductivity shows significant increase due to bipolar diffusion, which obstructs obtaining higher ZT.

A general strategy to bismuth chalcogenide films by chemical vapor transport

Developing a low-cost, simple manufacturing process for highly efficient thermoelectric films is an intriguing topic for both chemists and materialists. In this work, a new strategy was introduced to synthesize bismuth chalcogenide films by chemical vapor transport on presynthesized Bi films, including Bi2Se3, Bi2Te3 and Bi2Te2.7Se0.3. Through delicate control of the chalcogen vapor pressure, highly qualified and (00l) oriented bismuth chalcogenide films, constructed by oriented nanoplates, were obtained. The (00l) orientation of the films will facilitate the transport of the carrier for bismuth chalcogenides. Thermoelectric properties of these films were also characterized. At room temperature, the power factors reach 3 μW cm−1 K−1, 18.7 μW cm−1 K−1, 24.6 μW cm−1 K−1 for Bi2Se3, Bi2Te3, Bi2Te2.7Se0.3 films, respectively. Besides, the average power factor of 31.2 μW cm−1 K−1 in the temperature range of 300–500 K for Bi2Te2.7Se0.3 films indicates the present chemical vapor transport process is a promising technique for thermoelectric films.

Enhanced Thermoelectric Properties of Bi0.5Sb1.5Te3 Films by Chemical Vapor Transport Process

Bi₀.₅Sb₁.₅Te₃ films were prepared by a novel chemical vapor transport process through delicate controlling the temperature of the substrate and vapor source. The power factor reaches 30 μW cm⁻¹ K⁻¹ at room temperature, which is much higher than the value of the Bi₀.₅Sb₁.₅Te₃ films prepared by other techniques. The enhancement of thermoelectric properties might be attributed to the higher carrier mobility (252 cm² V⁻¹ s⁻¹), coming from the effective interparticle contiguity of (00L) oriented nanoplates embedded in the present Bi₀.₅Sb₁.₅Te₃ films.

Enhancing thermoelectric performance of bismuth selenide films by constructing a double-layer nanostructure

A new nanostructure of double-layer thin films (DLTFs) has been introduced to Bi2Se3 as thermoelectric films through a facile one-step and low-temperature solution route. The Bi2Se3 DLTFs consist of a (001) orientational inner layer and a (110) orientational outer layer with controllable thicknesses. The controllable release of the precursor ion is critical to adjust the double-layer nanostructures. The maximum power factor of the Bi2Se3 DLTFs reaches 100 μW m−1 K−1, which is larger by 79% than the value for a single layer Bi2Se3 film.

Thermodynamic descriptions of the Ga-Li system

Abstract The thermodynamic behavior of the Ga–Li system is analyzed using the calculation of phase diagram technique. The liquid phase is modeled with the Redlich–Kister equation. The ordered intermetallic compound GaLi is thermodynamically described by a two sublattice model. The other compounds are treated as stoichiometric compounds. A set of self-consistent thermodynamic parameters is obtained.

Formation mechanism of Type 2 micropipe defects in 4H–SiC crystals

We report experimental studies on the formation of Type 2 micropipe defects in 4H–SiC crystals grown by a physical vapor transport method. Compared with Type 1 micropipes, Type 2 micropipes exhibit new features allowing them to lie at an oblique angle about 12° to the [0001] crystal axis and are smaller in size than Type 1 micropipes. By changing the growth conditions, we find that a smaller axial temperature gradient and a larger grain size in the SiC source are beneficial to eliminate the Type 2 micropipes. We think that the liquid silicon is responsible for the formation of Type 2 micropipes. A possible formation mechanism for Type 2 micropipes is put forward.