Zhen-Hua Ge

Enhanced mid-temperature thermoelectric performance of textured SnSe polycrystals made of solvothermally synthesized powders

This work revealed that the thermoelectric properties of textured SnSe polycrystals, which are made of solvothermally synthesized powders with a preferred orientation grown along the (400) plane, can be enhanced, especially in the range of mid-temperatures. The electrical conductivity for the sintered sample is greatly improved in the whole measured temperature range, attributed to the high carrier concentration and unique mobility change. This results in a large power factor combined with the moderate Seebeck coefficient, especially in the range of 300–650 K, which exceeds 5 μW cm−1 K−2 at 423 K and even larger than 6 μW cm−1 K−2 at 521 K. Benefiting from the enhanced electrical conductivity and the low total thermal conductivity (<1 W m−1 K−1 in the range of 300 to 773 K), a higher ZT value than that reported for undoped single crystals was achieved in a relatively wide mid-temperature range, which reached 0.44 at 522 K and 0.5 at 573 K, and then showed weak temperature dependence in the range from 573 to 700 K. Above 700 K, the ZT value increased with temperature and a maximum value of nearly 0.6 was obtained at the maximum measured temperature of 773 K for the SnSe sample without any deliberate doping.

Boosting the Thermoelectric Performance of (Na,K)-Codoped Polycrystalline SnSe by Synergistic Tailoring of the Band Structure and Atomic-Scale Defect Phonon Scattering

We report the high thermoelectric performance of p-type polycrystalline SnSe obtained by the synergistic tailoring of band structures and atomic-scale defect phonon scattering through (Na,K)-codoping. The energy offsets of multiple valence bands in SnSe are decreased after Na doping and further reduced by (Na,K)-codoping, resulting in an enhancement in the Seebeck coefficient and an increase in the power factor to 492 μW m-1 K-2. The lattice thermal conductivity of polycrystalline SnSe is decreased by the introduction of effective phonon scattering centers, such as point defects and antiphase boundaries. The lattice thermal conductivity of the material is reduced to values as low as 0.29 W m-1 K-1 at 773 K, whereas ZT is increased from 0.3 for 1% Na-doped SnSe to 1.2 for 1% (Na,K)-codoped SnSe.

Highly Enhanced Thermoelectric Properties of Bi/Bi2S3 Nanocomposites

Bismuth sulfide (Bi2S3) has been of high interest for thermoelectric applications due to the high abundance of sulfur on Earth. However, the low electrical conductivity of pristine Bi2S3 results in a low figure of merit (ZT). In this work, Bi2S3@Bi core-shell nanowires with different Bi shell thicknesses were prepared by a hydrothermal method. The core-shell nanowires were densified to Bi/Bi2S3 nanocomposite by spark plasma sintering (SPS), and the structure of the nanowire was maintained as the nanocomposite due to rapid SPS processing and low sintering temperature. The thermoelectric properties of bulk samples were investigated. The electrical conductivity of a bulk sample after sintering at 673 K for 5 min using Bi2S3@Bi nanowire powders prepared by treating Bi2S3 nanowires in a hydrazine solution for 3 h is 3 orders of magnitude greater than that of a pristine Bi2S3 sample. The nanocomposite possessed the highest ZT value of 0.36 at 623 K. This represents a new strategy for densifying core-shell powders to enhance their thermoelectric properties.

Enhanced thermoelectric properties of SnSe polycrystals via texture control

We present in this manuscript that enhanced thermoelectric performance can be achieved in polycrystalline SnSe prepared by hydrothermal reaction and spark plasma sintering (SPS). X-ray diffraction (XRD) patterns revealed strong orientation along the [l 0 0] direction in bulk samples, which was further confirmed by microstructural observation through transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). It was noticed that the texturing degree of bulk samples could be controlled by sintering temperature during the SPS process. The best electrical transport properties were found in the sample which sintered at 450 °C in the direction vertical to the pressing direction, where the highest texturing degree and mass density were achieved. Coupled with the relatively low thermal conductivity, an average ZT of ∼ 0.38, the highest ever reported in pristine polycrystalline SnSe was obtained. This work set up a forceful example that a texture-control approach can be utilized to enhance the thermoelectric performance effectively.

Improvements of thermoelectric properties for p-type Cu1.8S bulk materials via optimizing the mechanical alloying process

Polycrystalline bulk Cu1.8S materials were fabricated using mechanical alloying (MA) and spark plasma sintering (SPS). The microstructure and thermoelectric transport properties were investigated in detail with emphasis on the effect of the mechanical alloying ball-milling process. Cu1.8S single-phase powders were directly synthesized from the elemental powders. High-density (>95%) p-type Cu1.8S bulk samples were fabricated using the subsequent SPS process at 773 K for 5 min. A Cu1.96S second phase was detected in the bulk samples when the ball-milling time was increased to 3 h, and the Cu1.96S content increased with increasing ball-milling time. The electrical properties and Seebeck coefficient of the Cu1.8S bulk samples were dependent on the milling time of the starting powders. The Cu1.8S bulk that was sintered by SPS using the powder treated with a 3 h ball-milling obtained a record-high power factor of 1552 μW m−1 K−2 at 773 K for the Cu–S system, which is 40% higher than that of pristine Cu1.8S. The ZT value of the Cu1.8S bulk sample that was sintered by SPS using the 12 h ball-milling treated powder reached 0.71, which is 67% higher than the ZT value of the sample that was sintered using the powder with a 1 h ball-milling time.

Thermoelectric properties of polycrystalline SnSe1±x prepared by mechanical alloying and spark plasma sintering

SnSe based materials have attracted much attention as high performance thermoelectric materials recently. In this work, polycrystalline SnSe1±x bulk samples were prepared by mechanical alloying (MA) combined with spark plasma sintering (SPS). When the Se stoichiometric ratio is above 1, the samples are p-type, the thermal conductivity of the samples decreased with increasing Se contents. When the Se stoichiometric ratio is under 1, the turning point from p-type to n-type was observed for all the samples, the SnSe0.8 bulk sample showed a total negative Seebeck coefficient at the measured temperature ranges from 323 to 773 K. Increases in the content of Se, lead to a considerable improvement of the power factor and a significant decrease of the thermal conductivity. Maximum ZT of ∼0.65 at 773 K was achieved for the p-type SnSe1.05 sample. n-Type SnSe0.8 polycrystals were also obtained with a peak ZT of 0.05 at 773 K.

Enhanced thermoelectric properties of In2O3(ZnO)5 intrinsic superlattice ceramics by optimizing the sintering process

Thermoelectric (TE) materials have a promising application as they can interconvert thermal energy to electrical energy directly. Oxide TE materials have attracted more attention for this application due to their high temperature stability. In this work, In2O3(ZnO)5 intrinsic superlattice ceramics with a layered structure were synthesized by reaction sintering a mixed powder of In2O3 and ZnO at 1200xa0°C for different holding times (t = 6, 8, 10, 12 and 15 h) in air. Their thermoelectric properties including the electrical conductivity, Seebeck coefficient and thermal conductivity, were measured from 323 to 973 K. The thermoelectric properties depended on the holding time. The highest ZT value of 0.19 at 973 K was obtained for the sample with a 12 h holding time.

Hydrothermal synthesis of SnQ (Q = Te, Se, S) and their thermoelectric properties

Lead-free IV-VI semiconductors SnQ (Qxa0=xa0Te, Se, S) are deemed as promising thermoelectric (TE) materials. In this work, we designed a hydrothermal route to selectively synthesize single phase SnTe, SnSe and SnS nanopowders. For all three samples, the phase structure were characterized by x-ray diffraction, SnTe particles with octahedron structure and SnSe/SnS particles with plate-like shape were observed by field emission scanning electron microscopy and transmission electron microscopy, the formation mechanism was discussed in detail. Then, SnTe, SnSe and SnS nanopowders were densified by spark plasma sintering for investigating TE properties. It was noticed that SnSe and SnS exhibited remarkably anisotropy in both electrical and thermal properties attributed to the layered crystal structure. The highest ZT values 0.79 at 873 K, 0.21 at 773 K, and 0.13 at 773 K were achieved for SnTe, SnSe and SnS bulk samples, respectively.Lead-free IV-VI semiconductors SnQ (Q=Te, Se, S) are deemed as promising thermoelectric materials. In this work, we designed a hydrothermal route to selectively synthesize single phase SnTe, SnSe and SnS nanopowders. For all three samples, the phase structure were characterized by X-ray diffraction, SnTe particles with octahedron structure and SnSe/SnS particles with plate-like shape were observed by field emission scanning electron microscopy and transmission electron microscopy, the formation mechanism was discussed in detail. Then, SnTe, SnSe and SnS nanopowders were densified by spark plasma sintering for investigating thermoelectric properties. It was noticed that SnSe and SnS exhibited remarkably anisotropy in both electrical and thermal properties attributed to the layered crystal structure. The highest ZT values 0.79 at 873 K, 0.21 at 773 K, and 0.13 at 773 K were achieved for SnTe, SnSe and SnS bulk samples, respectively.

Achieving high thermoelectric performance of Cu1.8S composites with WSe2 nanoparticles

Polycrystalline p-type Cu1.8S composites with WSe2 nanoparticles were fabricated by the mechanical alloying method combined with the spark plasma sintering technique. The Seebeck coefficient was significantly enhanced by the optimized carrier concentration, while the thermal conductivity was simultaneously decreased due to the refined grain and WSe2 nanoparticles. An enhanced Seebeck coefficient of 110 μV K-1 and a reduced thermal conductivity of 0.68 W m-1 K-1 were obtained for the Cu1.8Sxa0+xa01 wt% WSe2 sample at 773 K, resulting in a remarkably enhanced peak ZT of 1.22 at 773 K, which is 2.5 times higher than that (0.49 at 773 K) of a pristine Cu1.8S sample. The cheap and environmentally friendly Cu1.8S-based materials with enhanced properties may find promising applications in thermoelectric devices.

Microstructure and thermoelectric properties of CuInSe2/In2Se3 compound

CuInSe2 powders were synthesized by solvothermal method, and then the CuInSe2/In2Se3 bulk samples were fabricated by spark plasma sintering (SPS) technique. To investigate the phase composition, th...