The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100 degrees C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250 degrees C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86 degrees , 106 degrees , and 119 degrees C with hot-side temperatures set at 50 degrees, 100 degrees, and 150 degrees C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials.

By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 degrees C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 degrees C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost.

In this work, phonon transport in two-dimensional (2D) porous silicon structures with aligned pores is investigated by Monte Carlo simulations considering the frequency-dependent phonon mean free paths (MFPs). A boundary condition based on the periodic heat flux with constant virtual wall temperature is developed for the studied periodic structures. Such periodic boundary conditions enable the simulation of the lattice thermal conductivities with a minimum computational domain. For the 2D case, it is found that phonon size effects caused by the periodically arranged pores can be remarkable even when the pore size and spacing are much larger than the averaged phonon MFPs. Our results show the importance of considering the frequency dependence of phonon MFPs in the analysis of micro- and nanostructured materials.

Na-ion batteries represent an effective energy storage technology with slightly lower energy and power densities but potentially lower material costs than Li-ion batteries. Here, we report a new polyanionic intercalation cathode material of an unusual chemical class: sidorenkite (Na3MnPO4CO3). This carbonophosphate compound shows a high discharge capacity (∼125 mAh/g) and specific energy (374 Wh/kg). In situ X-ray diffraction measurement suggests that sidorenkite undergoes a solid solution type reversible topotactic structural evolution upon electrochemical cycling. Ex situ solid state NMR investigation reveals that more than one Na per formula unit can be deintercalated from the structure, indicating a rarely observed two-electron intercalation reaction in which both Mn2+/Mn3+ and Mn3+/Mn4+ redox couples are electrochemically active.

We have assembled tin dioxide nanobelts with low-power microheaters for detecting dimethyl methylphosphonate (DMMP), a nerve agent simulant. The electrical conductance of a heated nanobelt increased for 5% upon exposure to 78 parts per billion DMMP in air. The nanobelt conductance recovered fully quickly after the DMMP was shut off, suggesting that the single-crystal nanobelt was not subject to poisoning often observed in polycrystalline metal oxide sensors. While the sensitivity can be improved via doping nanobelts with catalytic additives, directed assembly or growth of nanobelts on microsystems will potentially allow for the large-scale fabrication of nanosensor arrays.

We have measured the thermal conductivities of a 53-nm-thick and a 64-nm-thick tin dioxide (SnO2) nanobelt using a microfabricated device in the temperature range of 80–350 K. The thermal conductivities of the nanobelts were found to be significantly lower than the bulk values, and agree with our calculation results using a full dispersion transmission function approach. Comparison between measurements and calculation suggests that phonon–boundary scattering is the primary effect determining the thermal conductivities.

The solubility of Yb in Yb x Co 4 Sb 12 was reported to be 0.19 in bulk skutterudites made by melting and slow cooling method. Surprisingly we increased x close to 0.5 by a special sample preparation method: ball mill and hot press. We show that a higher Yb concentration not only increases the power factor due to a higher electron concentration but also reduces the thermal conductivity k because of stronger phonon scattering. In this way, we have achieved a dimensionless thermoelectric figure of merit ZT of about 1.2 at 550°C in Yb 0.35 Co 4 Sb 12 .

Vanadium diselenide (VSe2), a member of the transition metal dichalcogenides (TMDs) family, is emerging as a promising two-dimensional (2D) candidate for the electronic and spintronic device with exotic properties including charge/spin density wave and ferromagnetism. The bulk crystal VSe2 exists in a crystallographic form of 1T-phase with metallic behavior. In this paper, we report a structural phase transition of multilayer VSe2 from 1T to 2H through annealing at 650 K, accompanying a metal–insulator transition. We observe that the 2H-phase is more thermodynamically favorable than the 1T-phase at 2D.