Ultra-high average figure of merit in synergistic band engineered SnxNa1−xSe0.9S0.1 single crystals

Thermal-electricity conversion is one of the most promising routes to harvest heat and convert it as easily storable and deliverable electric energy. Signi?cant progress has been made since the discovery of Seebeck effect in 1821, particularly, the figure of merit zT approached a record high value of 2.6 in 2014. However, for thermoelectric devices, high average zT values (zT_ave) over the operating temperature range is more important as it is directly related to the conversion efficiency ($\eta$). Approaching highly stable and repeatable ultra-high zT_ave for Te-free materials has been historically challenging over the past century though exciting progress with zT_ave well above 1.10 was made recently. Here, through synergistic band engineering strategy for single crystalline SnSe, we report a series of record high zT_ave over a wide temperature range, approaching ～1.60 in the range from 300?K to 923?K in Na-doped SnSe_0.9S_0.1 solid solution single crystals, with the maximum zT of 2.3 at 773?K. These ultra-high thermoelectric performance derive from the new multiple valence band extrema near the band edges in SnSe_0.9S_0.1 and the shift of Fermi level towards the multi-valley bands through Na doping which introduce additional carrier pockets to attend electrical transport. These effects result in an optimized ultrahigh power factor exceeding 4.0?mW?m^?1?K^?2 in Sn_0.97Na_0.03Se_0.9S_0.1 single crystals. Combined with the extremely lowered thermal conductivity attributed from the intrinsic anharmonicity and point defect phonon scattering, the series of ultra-high zT_ave and a record high calculated conversion efficiency of 21% over a wide temperature range are approached.     

Efficient Monte Carlo simulation of delayed activation analysis experiments

An approach is described for efficiently modelling delayed activation analysis experiments, where radioactivity is induced in a sample through irradiation with neutrons or gamma-rays and subsequently measured by a separate detection system. The continuous single-history method is used, in which a single computer code tracks both the initial irradiation quantum and the decay products of any induced radioactive nuclei in a single event using Monte Carlo methods. Key aspects of the approach include the forcing of both the irradiation and decay processes to improve efficiency and the ability to simulate complex radioactive decays producing multiple, coincident decay products. The method has been implemented in the general-purpose EGSnrc code. A comparison with experimental results for the photoactivation of a gold foil is presented.     

The thermal conductivity of fibre-reinforced concrete

The effect of copper and steel fibre inclusions on the thermal conductivity of mortar and concrete is investigated. The experimental technique is based on the conventional steady-state method using desiccated specimens. The results indicate that copper fibres significantly increase thermal conductivity while steel fibres have a lesser effect. Vibration of the fresh concrete, during specimen manufacture, produces some fibre alignment. As a result, it is found that theoretical methods based on the assumption of a random fibre distribution under-estimate the experimental values.     

High Thermoelectric Performance in Supersaturated Solid Solutions and Nanostructured n‐Type PbTe–GeTe

Sb‐doped and GeTe‐alloyed n‐type thermoelectric materials that show an excellent figure of merit ZT in the intermediate temperature range (400–800 K) are reported. The synergistic effect of favorable changes to the band structure resulting in high Seebeck coefficient and enhanced phonon scattering by point defects and nanoscale precipitates resulting in reduction of thermal conductivity are demonstrated. The samples can be tuned as single‐phase solid solution (SS) or two‐phase system with nanoscale precipitates (Nano) based on the annealing processes. The GeTe alloying results in band structure modification by widening the bandgap and increasing the density‐of‐states effective mass of PbTe, resulting in significantly enhanced Seebeck coefficients. The nanoscale precipitates can improve the power factor in the low temperature range and further reduce the lattice thermal conductivity (κ_lat). Specifically, the Seebeck coefficient of Pb_0.988Sb_0.012Te–13%GeTe–Nano approaches ?280 µV K^?1 at 673 K with a low κ_lat of 0.56 W m^?1 K^?1 at 573 K. Consequently, a peak ZT value of 1.38 is achieved at 623 K. Moreover, a high average ZT_avg value of ≈1.04 is obtained in the temperature range from 300 to 773 K for n‐type Pb_0.988Sb_0.012Te–13%GeTe–Nano.