Keshab Dahal

Studies on thermoelectric figure of merit of Na-doped p-type polycrystalline SnSe

SnSe has attracted some attention due to the recent report of record high thermoelectric figure-of-merit (ZT) of ∼2.6 at about 923 K in single crystals. Even though the ZT is high in single crystal SnSe, it was very low in its polycrystalline form due to the very low electrical conductivity (<103 S m−1) from 300 K to 700 K. In this work, we report studies on enhancement of electrical conductivity by Na doping to optimize the carrier concentration in polycrystalline SnSe prepared by melting and hot pressing. A room temperature carrier concentration of ∼2.7 × 1019 cm−3 was obtained in 2 atm% Na-doped SnSe samples with the highest power factor obtained in 1.5 atm% Na-doping. A peak ZT of ∼0.8 was achieved at 773 K along the hot pressing direction and the average ZT was improved.

NbFeSb-based p-type half-Heuslers for power generation applications

We report a peak dimensionless figure-of-merit (ZT) of ∼1 at 700 °C in a nanostructured p-type Nb0.6Ti0.4FeSb0.95Sn0.05 composition. Even though the power factor of the Nb0.6Ti0.4FeSb0.95Sn0.05 composition is improved by 25%, in comparison to the previously reported p-type Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2, the ZT value is not increased due to a higher thermal conductivity. However, the higher power factor of the Nb0.6Ti0.4FeSb0.95Sn0.05 composition led to a 15% increase in the power output of a thermoelectric device in comparison to a device made from the previous best material Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2. The n-type material used to make the unicouple device is the best reported nanostructured Hf0.25Zr0.75NiSn0.99Sb0.01 composition with the lowest hafnium (Hf) content. Both the p- and n-type nanostructured samples are prepared by ball milling the arc melted ingot and hot pressing the finely ground powders. Moreover, the raw material cost of the Nb0.6Ti0.4FeSb0.95Sn0.05 composition is more than six times lower compared to the cost of the previous best p-type Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2. This cost reduction is crucial for these materials to be used in large-scale quantities for vehicle and industrial waste heat recovery applications.

Orientation Control of Graphene Flakes by Magnetic Field: Broad Device Applications of Macroscopically Aligned Graphene

Owing to a large diamagnetism, graphene flakes can respond and be aligned to magnetic field like a ferromagnetic material. Aligned graphene flakes exhibit emergent properties approaching single-layer graphene. Anisotropic optical properties also give rise to a magnetic writing board using graphene suspension and a bar magnet as a pen. This simple alignment technique opens up enormous applications of graphene.

Thermoelectric and mechanical properties on misch metal filled p-type skutterudites Mm0.9Fe4−xCoxSb12

Most of the recent work focused on improving the dimensionless figure-of-merit, ZT, of p-type skutterudites uses one or two fillers to tune the electrical and thermal properties. Considering the fact that the different fillers with varying atomic mass and ionic radii can vibrate with different amplitudes to scatter phonons of different mean free paths, we synthesized misch metal filled p-type skutterudites Mm0.9Fe4−xCoxSb12 (where Mm is La0.25Ce0.5Pr0.05Nd0.15Fe0.03, called misch metal). The samples were synthesized by hot pressing nano-powder made by ball milling the annealed ingot of Mm0.9Fe4−xCoxSb12 with varying concentration of cobalt, x. By tuning the Fe/Co ratio, we achieved a thermal conductivity of ∼2 W m−1 K−1 at room temperature and ∼2.3 W m−1 K−1 at about 530 °C and a power factor of ∼30 μW cm−1 K−2 at about 425 °C in Mm0.9Fe3.1Co0.9Sb12, leading to a peak ZT ∼1.1 at about 425 °C. The nano-indentation experiment reveals that hardness and elastic modulus of the material is about 4.2 GPa and 116...

Thermal conductivity of (VO2)1-xCux composites across the phase transition temperature

The thermal conductivity across the metal-insulator transition (MIT) of hot-pressed polycrystalline vanadium dioxide (VO2) samples is studied. The change in the total thermal conductivity (k) of hot-pressed VO2 is insignificant across the MIT temperature. By adding copper (Cu) to make (VO2)1-xCux composites with x from 0 to 0.5, we find an increase in the electrical conductivity from 4 × 104 S m−1 to 1 × 106 S m−1 at 120 °C, resulting in an electronic thermal conductivity increase from 0.38 W m−1 K−1 for x = 0 to 3.8 W m−1 K−1 for x = 0.3, which is a significant increase. However, the total thermal conductivity did not increase due to the decrease in the value of the Lorenz number by an order of magnitude than its standard value using the Wiedemann-Franz relationship. On the basis of our experimental result, an empirical model is proposed to explain the thermal conductivity behavior of all (VO2)1-xCux samples with different Cu concentrations.

Deep defect level engineering: a strategy of optimizing the carrier concentration for high thermoelectric performance

Thermoelectric properties are heavily dependent on the carrier concentration, and therefore the optimization of carrier concentration plays a central role in achieving high thermoelectric performance. The optimized carrier concentration is highly temperature-dependent and could even possibly vary within one order of magnitude in the temperature range of several hundreds of Kelvin. Practically, however, the traditional doping strategy will only lead to a constant carrier concentration, and thus the thermoelectric performance is only optimized within a limited temperature range. Here, we demonstrate that a temperature-dependent carrier concentration can be realized by simultaneously introducing shallow and deep defect levels. In this work, iodine (I) and indium (In) are co-doped in PbTe, where iodine acts as the shallow donor level that supplies sufficient electrons and indium builds up the localized half-filled deep defect state in the band gap. The indium deep defect state traps electrons at a lower temperature and the trapped electrons will be thermally activated back to the conduction band when the temperature rises. In this way, the carrier concentration can be engineered as temperature-dependent, which matches the theoretically predicted optimized carrier concentration over the whole temperature range. As a result, a room temperature ZT of ∼0.4 and a peak ZT of ∼1.4 at 773 K were obtained in the n-type In/I co-doped PbTe, leading to a record-high average ZT of ∼1.04 in the temperature range of 300 to 773 K. Importantly, since deep defect levels also exist in other materials, the strategy of deep defect level engineering should be widely applicable to a variety of materials for enhancing the thermoelectric performance across a broad temperature range.

Hierarchical CoP/Ni5P4/CoP microsheet arrays as a robust pH-universal electrocatalyst for efficient hydrogen generation

Highly active catalysts composed of earth-abundant materials, performing as efficiently as Pt catalysts, are crucial for sustainable hydrogen production through water splitting. However, most efficient catalysts consist of nanostructures made via complex synthetic methods, making scale-up quite challenging. Here we report an effective strategy for developing a very active and durable pH-universal electrocatalyst for the hydrogen evolution reaction (HER). This catalyst is constructed using a sandwich-like structure, where hierarchical cobalt phosphide (CoP) nanoparticles serve as thin skins covering both sides of nickel phosphide (Ni5P4) nanosheet arrays, forming self-supported sandwich-like CoP/Ni5P4/CoP microsheet arrays with lots of mesopores and macropores. The as-prepared electrocatalyst requires an overpotential of only 33 mV to achieve a benchmark of 10 mA cm−2, with a very large exchange current density and high turnover frequencies (TOFs) in acid media, superior to most electrocatalysts made of metal phosphides, well-known MoS2 and WS2 catalysts, and it performs comparably to state-of-the-art Pt catalysts. In particular, this electrocatalyst shows impressive operational stability at an extremely large current density of 1 A cm−2, indicating its possible application toward large-scale water electrolysis. Additionally, this electrocatalyst is very active in alkaline electrolyte (71 mV at 10 mA cm−2), which demonstrates its pH universality as a HER catalyst with outstanding catalytic activity. This simple strategy does not involve any solvothermal and hydrothermal processes, paving a new avenue toward the design of robust non-noble electrocatalysts for hydrogen production, aimed at commercial water electrolysis.