J.Y. Yang

Characterization and thermoelectric properties of p-type 25%Bi2Te3–75%Sb2Te3 prepared via mechanical alloying and plasma activated sintering

In the present work, starting from elemental bismuth, antimony and tellurium powders, p-type 25%Bi2Te3–75%Sb2Te3 thermoelectric materials with high density were prepared by mechanical alloying (MA) and plasma activated sintering (PAS). The single phase 25%Bi2Te3–75%Sb2Te3 alloys were obtained after MA for 12 h. The effect of sintering temperatures on microstructure and thermoelectric properties of the as-PASed samples was researched. Highly compact samples with relative density over 99% could be obtained when sintering temperature was over 653 K. A preferentially orientated microstructure with the (1 1 0) plane parallel to and the basal planes (0 0 l) perpendicular to the pressing direction was formed, and the orientation factors of the (0 0 l) planes changed from 0.11 to 0.12 at different sintering temperatures. The maximum power factor and figures of merit (Z) at room temperature were 3.10 × 10−3 W m−1 K−2 and 2.85 × 10−3 K−1, respectively. The Vickers microhardness reached 112.7 Hv, which was twice that of the single crystal samples prepared by zone-melting.

Bi2Te3 hexagonal nanoplates and thermoelectric properties of n-type Bi2Te3 nanocomposites

Bi2Te3 plate-like crystals with homogeneous hexagonal morphology were rapidly synthesized using a microwave assisted wet chemical method in 30 min. These Bi2Te3 nanoplates possessed a fixed edge with a length of ~0.5–2 µm, and the thickness was less than ~100 nm. The n-type Bi2Te3 nanocomposites were prepared by consolidating mixtures of these nanoplates and mechanically alloyed powders using plasma activated sintering, and the effect of nanoplate addition on the thermoelectric properties of the nanocomposites was investigated. When the content of the doped nanoplates was 15 wt%, the lattice thermal conductivity of the Bi2Te3 nanocomposites decreased by 18% compared with that of the undoped compounds. A preliminary investigation showed that nanopowder addition was an effective way to decrease the thermal conductivity and increase the thermoelectric efficiency.

Effect of nominal Sb2Te3 content on thermoelectric properties of p-type (Bi2Te3)x(Sb2Te3)1−x alloys by MA–HP

p-type (Bi2Te3)x(Sb2Te3)1−x alloys with homogeneous and fine microstructure were prepared by the mechanical alloying–hot pressing (MA–HP) method in the present work. X-ray diffraction, energy dispersive x-ray spectroscopy, scanning electron microscope–backscattered electron imaging were performed to characterize the MA–HPed materials. The effect of nominal Sb2Te3 content on mechanical and thermoelectric properties of the (Bi2Te3)x(Sb2Te3)1−x was investigated. Thermoelectric properties were measured at 300 K. By increasing the nominal molar fraction of Sb2Te3 from 0.7 to 0.9, the carrier concentration (nc) increased obviously from 0.43 × 1019 to 3.08 × 1019 cm−3; however, carrier mobility (μ) fluctuated between 198.0 and 285.5 cm2 V−1 s−1, and the Seebeck coefficient (α) and electrical resistivity (ρ) decreased acutely. The thermal conductivity (κ) increased with increase in the nominal Sb2Te3 content, while the lattice thermal conductivity (κph) decreased abruptly from 0.820 to 0.297 W m−1 K−1. When the nominal molar fraction of Sb2Te3 was 0.8, the resultant maximum power factor (PF) and thermoelectric figure of merit (Z) of the p-type (Bi2Te3)x(Sb2Te3)1−x alloys reached 3.81 × 10−3 W m−1 K−2 and 3.23 × 10−3 K−1 at 300 K, respectively. The bending strength reached 64 MPa, which was 3–5 times larger than that of single crystal materials.

Preferential orientation and thermoelectric properties of n-type Bi2Te2.85Se0.15 alloys by mechanical alloying and equal channel angular extrusion

Starting from elemental bismuth, tellurium and selenium powders, n-type Bi2Te2.85Se0.15 solid solution with a fine microstructure was prepared by mechanical alloying and equal channel angular extrusion (ECAE) in the present work. The effect of extrusion temperature on the microstructure and thermoelectric properties of the as-ECAEed samples was investigated. A preferentially oriented microstructure with the basal planes (0 0 l) in the parallel direction to extrusion was formed, and the orientation factors F of the (0 0 l) planes of the 703 K and 753 K ECAEed Bi2Te2.85Se0.15 alloys were 0.26 and 0.28, respectively. The electrical resistivity and the Seebeck coefficient decreased, and the thermal conductivity increased with increasing extrusion temperature. The electrical and thermal transmission performances were strongly affected by the preferentially oriented microstructure and the preferential orientation improved the thermoelectric properties of the ECAEed Bi2Te2.85Se0.15 alloys in the parallel direction to extrusion. The maximum dimensionless figure of merit was obtained when extruded at 753 K at a testing temperature of 343 K, ZT = 0.66.

Phase Transformation and Synthesis of Ni Substituted CoSb3 Skutterudite Synthesis during Solid State Reaction

Starting from elemental Co (99.9%), Ni (99.9%) and Sb (99.99%) powders, Co4-xNixSb12 powder mixture with different Ni concentration were subjected to mechanical alloying by using a planetary ball mill. Phase transformation during mechanical alloying process of Co-Ni-Sb system was studied in this paper. Ni substituted skutterudite could be synthesized by solid state reaction. With assistance of hot pressing for two hours, the as mechanically alloyed low Ni concentration powders could be completely transformed into skutterudite compound. The lattice parameter of Ni substituted skutterudite compound complies well with Vegard law when x is not more than 0.2. When Ni concentration is larger than the maximum solubility, NiSb2 compound forms and single phase skutterudite compound can not be obtained.

Effect of Nanopowders Addition on the Thermoelectric Properties of n-Type Bi2Te3 Nanocomposites

Bi2Te3 nanopowders with an average grain size of about 40 nm were synthesized by vacuum arc plasma evaporation. The n-type bulk Bi2Te3 nanocomposites were prepared by consolidating the mixtures of these nanopowders and mechanically alloyed powders using plasma activated sintering, and the effect of the addition of nanopowders on the thermoelectric properties of bulk Bi2Te3 nanocomposites was investigated. With increasing the content of the added nanopowders, the electrical resistivity and Seebeck coefficient of the nanocomposites increased and the thermal conductivity decreased. When the content of the added nanopowders was 20 wt%, the thermal conductivity decreased by 21.4% compared with that of the nanopowders-free compounds. A preliminary investigation showed that the addition of nanopowders was an effective way to decrease the thermal conductivity and increase the thermoelectric efficiency.

Synthesis of Ni-Al Intermetallic Nanoparticle Catalysts by Vacuum Arc Plasma Evaporation

In order to pursue high catalytic performance of Ni-Al intermetallic compounds for hydrogen production, we synthesized Ni-Al intermetallic nanoparticles from Ni-Al alloy ingots by vacuum arc plasma evaporation technique for the first time. The characterization of the synthesized nanoparticles was carried out using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy. The catalytic properties of the Ni-Al nanoparticles for methanol decomposition were evaluated. It is found that the nanoparticles had a large surface area above 70 m2/g, and showed very high catalytic activity for methanol decomposition.

Catalytic Properties of Ni-Al Intermetallic Nanoparticles Fabricated by Thermal Plasma Process

The catalytic activity of Ni-Al (Ni25Al) nanoparticles fabricated by thermal plasma evaporation was examined for methanol decomposition and CO oxidation. The nanoparticles exhibited high activity for both reactions. Characterization of the nanoparticles revealed that the fabricated nanoparticles were mainly comprised of Ni and Ni3Al phases. During CO oxidation, the Ni phase was oxidized to NiO, while the Ni3Al phase remained unchanged. The NiO phase is supposed to serve as the active sites for CO oxidation. In contrast, during methanol decomposition, no obvious oxidation was observed for both Ni and Ni3Al phases. The Ni and Ni3Al phases are supposed to contribute to the high activity for methanol decomposition.

The Effect of Cooling Process on the Microstructure and Electrical Transport Properties of AgSbTe2 Compound

AgSbTe2 compounds have been synthesized via melting and subsequent cooling processes. The effect of cooling process, from air-cooling, water quenching to liquid nitrogen-quenching, on the microstructure and the electrical transport properties of AgSbTe2 has been investigated by means of powder X-ray diffraction, electron microscope, electrical resistivity, and Hall coefficient measurements. It has been found that the cooling process has apparent influence on the microstructure and corresponding electrical properties. The phase components and morphology changed as the cooling process altered. The electrical resistivity and the Seebeck coefficient of the as-prepared samples increased from air-cooled to liquid nitrogen-quenched sample.