Yumei Wang

Electrical and magnetic properties of La0.5Sr0.5CoO3 thin films

Highly conductive perovskite La0.5Sr0.5CoO3 thin films were grown on (012) LaAlO3 by pulsed-laser deposition at different substrate temperatures. Magnetoresistance (MR) measurements show only a negative effect for the films grown at a temperature of 800u200a°C. However, a positive MR effect was found for the films grown at lower temperatures of 400 and 500u200a°C. The sign of the magnetoresistance changed from positive to negative near 150 K under an applied magnetic field of 5.0 T, and the largest positive MR of 2.6% was obtained at a temperature of 4.2 K for the film grown at 500u200a°C. Low-temperature magnetic hysteresis of the films was also studied, and it is suggested that the scattering of the carrier at the domain boundary may lead to the positive-MR effect.

Highly active and durable self-standing WS2/graphene hybrid catalysts for the hydrogen evolution reaction

The development of three-dimensional (3D) efficient and earth-abundant electrocatalysts with good catalytic performance and long-term durability is desirable for the hydrogen evolution reaction (HER) in water splitting. Tungsten disulfide (WS2) has been long-pursued as a promising candidate, but the progress is slow, which is probably due to its poor electrical contact with the support and low density of active sites. Here we developed a highly active and stable WS2 catalyst for the HER by growing it into a 3D porous architecture on conducting graphene/Ni foam. The catalysts exhibit a ∼0.1 V overpotential, accompanied by a large cathode current density (10 mA cm−2 at −119 mV vs. reversible hydrogen electrode), a low Tafel slope of ∼43 mV per decade and good stability. The HER performance of WS2 catalysts is greatly improved, and is comparable to that of the well-known catalysts (MoS2, CoSe2, CoS2, etc.), which benefits from the improved electrical conductivity of the WS2 catalysts by graphene and porous structures of the support. This work paves a new way to develop active and efficient WS2-based catalysts for hydrogen generation.

Thermoelectric properties of Bi-based Zintl compounds Ca1−xYbxMg2Bi2

Bi-based Zintl compounds, Ca1−xYbxMg2Bi2 with the structure of CaAl2Si2, have been successfully prepared by mechanical alloying followed by hot pressing. We found that the electrical conductivity, Seebeck coefficient, carrier concentration, and thermal conductivity can be adjusted by changing the Yb concentration. All Ca1−xYbxMg2Bi2 samples have low carrier concentrations (∼2.4 to 7.2 × 1018 cm−3) and high Hall mobility (∼119 to 153 cm2 V−1 s−1) near room temperature. The partial substitution of Ca with Yb causes structural disorders, which lowers the thermal conductivity. The highest figure of merit of ∼1.0 is observed in Ca0.5Yb0.5Mg2Bi2, and ∼0.8 in the unsubstituted CaMg2Bi2 and YbMg2Bi2. A small amount of free Bi was found in all the samples except YbMg2Bi2. By reducing the initial Bi concentration, we succeeded in obtaining phase pure samples in all compositions, which resulted in a much better thermoelectric performance, especially much higher (ZT)eng and a conversion efficiency near 11%. Such a high efficiency makes this material competitive with half-Heuslers and skutterudites.

Ultra-low thermal conductivities of hot-pressed attapulgite and its potential as thermal insulation material

In the past, there have been very few reports on thermal properties of attapulgite which is a widely used clay mineral. In this work, we report on extremely low thermal conductivities in attapulgite samples synthesized by hot-pressing. Attapulgite powder was hot-pressed at different temperatures into bulk samples, and a systematic study was conducted on the microstructures and thermal properties. Differential scanning calorimetry analysis shows that hot-pressing induces a rapid dehydration of the attapulgite powders. X-ray diffraction data and scanning/transmission electron microscopy reveal that the hot-pressed attapulgite features high porosity and complex microstructures, including an amorphous phase. As a result, the hot-pressed attapulgite exhibits thermal conductivity less than 2.5u2009Wu2009m−1u2009K−1 up to 600u2009°C. For one sample with porosity of 45.7%, the thermal conductivity is as low as 0.34u2009Wu2009m−1u2009K−1 at 50u2009°C. This suggests the potential of hot-pressed attapulgite as a candidate for thermal barrier mater...