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- ItemUltrahigh Power Factor in Thermoelectric System Nb0.95M0.05FeSb (M = Hf, Zr, and Ti)(Chichester : John Wiley and Sons Ltd, 2018) Ren, W.; Zhu, H.; Zhu, Q.; Saparamadu, U.; He, R.; Liu, Z.; Mao, J.; Wang, C.; Nielsch, K.; Wang, Z.; Ren, Z.Conversion efficiency and output power are crucial parameters for thermoelectric power generation that highly rely on figure of merit ZT and power factor (PF), respectively. Therefore, the synergistic optimization of electrical and thermal properties is imperative instead of optimizing just ZT by thermal conductivity reduction or just PF by electron transport enhancement. Here, it is demonstrated that Nb0.95Hf0.05FeSb has not only ultrahigh PF over ≈100 µW cm−1 K−2 at room temperature but also the highest ZT in a material system Nb0.95M0.05FeSb (M = Hf, Zr, Ti). It is found that Hf dopant is capable to simultaneously supply carriers for mobility optimization and introduce atomic disorder for reducing lattice thermal conductivity. As a result, Nb0.95Hf0.05FeSb distinguishes itself from other outstanding NbFeSb-based materials in both the PF and ZT. Additionally, a large output power density of ≈21.6 W cm−2 is achieved based on a single-leg device under a temperature difference of ≈560 K, showing the realistic prospect of the ultrahigh PF for power generation.
- ItemDiscovery of TaFeSb-based half-Heuslers with high thermoelectric performance(London : Nature Publishing Group, 2019) Zhu, H.; Mao, J.; Li, Y.; Sun, J.; Wang, Y.; Zhu, Q.; Li, G.; Song, Q.; Zhou, J.; Fu, Y.; He, R.; Tong, T.; Liu, Z.; Ren, W.; You, L.; Wang, Z.; Luo, J.; Sotnikov, A.; Bao, J.; Nielsch, K.; Chen, G.; Singh, D.J.; Ren, Z.Discovery of thermoelectric materials has long been realized by the Edisonian trial and error approach. However, recent progress in theoretical calculations, including the ability to predict structures of unknown phases along with their thermodynamic stability and functional properties, has enabled the so-called inverse design approach. Compared to the traditional materials discovery, the inverse design approach has the potential to substantially reduce the experimental efforts needed to identify promising compounds with target functionalities. By adopting this approach, here we have discovered several unreported half-Heusler compounds. Among them, the p-type TaFeSb-based half-Heusler demonstrates a record high ZT of ~1.52 at 973 K. Additionally, an ultrahigh average ZT of ~0.93 between 300 and 973 K is achieved. Such an extraordinary thermoelectric performance is further verified by the heat-to-electricity conversion efficiency measurement and a high efficiency of ~11.4% is obtained. Our work demonstrates that the TaFeSb-based half-Heuslers are highly promising for thermoelectric power generation.
- ItemDiscovery of ZrCoBi based half Heuslers with high thermoelectric conversion efficiency(London : Nature Publishing Group, 2018) Zhu, H.; He, R.; Mao, J.; Zhu, Q.; Li, C.; Sun, J.; Ren, W.; Wang, Y.; Liu, Z.; Tang, Z.; Sotnikov, A.; Wang, Z.; Broido, D.; Singh, D.J.; Chen, G.; Nielsch, K.; Ren, Z.Thermoelectric materials are capable of converting waste heat into electricity. The dimensionless figure-of-merit (ZT), as the critical measure for the material's thermoelectric performance, plays a decisive role in the energy conversion efficiency. Half-Heusler materials, as one of the most promising candidates for thermoelectric power generation, have relatively low ZTs compared to other material systems. Here we report the discovery of p-type ZrCoBi-based half-Heuslers with a record-high ZT of ∼1.42 at 973 K and a high thermoelectric conversion efficiency of ∼9% at the temperature difference of ∼500 K. Such an outstanding thermoelectric performance originates from its unique band structure offering a high band degeneracy (N v) of 10 in conjunction with a low thermal conductivity benefiting from the low mean sound velocity (v m ∼2800 m s-1). Our work demonstrates that ZrCoBi-based half-Heuslers are promising candidates for high-temperature thermoelectric power generation.
- ItemLarge thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers(London : Nature Publishing Group, 2018) Zhou, J.; Zhu, H.; Liu, T.-H.; Song, Q.; He, R.; Mao, J.; Liu, Z.; Ren, W.; Liao, B.; Singh, D.J.; Ren, Z.; Chen, G.Modern society relies on high charge mobility for efficient energy production and fast information technologies. The power factor of a material-the combination of electrical conductivity and Seebeck coefficient-measures its ability to extract electrical power from temperature differences. Recent advancements in thermoelectric materials have achieved enhanced Seebeck coefficient by manipulating the electronic band structure. However, this approach generally applies at relatively low conductivities, preventing the realization of exceptionally high-power factors. In contrast, half-Heusler semiconductors have been shown to break through that barrier in a way that could not be explained. Here, we show that symmetry-protected orbital interactions can steer electron-acoustic phonon interactions towards high mobility. This high-mobility regime enables large power factors in half-Heuslers, well above the maximum measured values. We anticipate that our understanding will spark new routes to search for better thermoelectric materials, and to discover high electron mobility semiconductors for electronic and photonic applications.
- ItemRelationships between submicrometer particulate air pollution and air mass history in Beijing, China, 2004–2006(München : European Geopyhsical Union, 2008) Wehner, B.; Birmili, W.; Ditas, F.; Wu, Z.; Hu, M.; Liu, X.; Mao, J.; Sugimoto, N.; Wiedensohler, A.The Chinese capital Beijing is one of the global megacities where the effects of rapid economic growth have led to complex air pollution problems that are not well understood. In this study, ambient particle number size distributions in Beijing between 2004 and 2006 are analysed as a function of regional meteorological transport. An essential result is that the particle size distribution in Beijing depends to large extent on the history of the synoptic scale air masses. A first approach based on manual back trajectory classification yielded differences in particulate matter mass concentration by a factor of two between four different air mass categories, including three main wind directions plus the case of stagnant air masses. A back trajectory cluster analysis refined these results, yielding a total of six trajectory clusters. Besides the large scale wind direction, the transportation speed of an air mass was found to play an essential role on the PM concentrations in Beijing. Slow-moving air masses were shown to be associated with an effective accumulation of surface-based anthropogenic emissions due to both, an increased residence time over densely populated land, and their higher degree of vertical stability. For the six back trajectory clusters, differences in PM1 mass concentrations by a factor of 3.5, in the mean air mass speed by a factor of 6, and in atmospheric visibility by a factor of 4 were found. The main conclusion is that the air quality in Beijing is not only degraded by anthropogenic aerosol sources from within the megacity, but also by sources across the entire Northwest China plain depending on the meteorological situation.
- ItemInstrumentation and Measurement Strategy for the NOAA SENEX Aircraft Campaign as Part of the Southeast Atmosphere Study 2013(Katlenburg-Lindau : Copernicus, 2016) Warneke, C.; Trainer, M.; de Gouw, J.A.; Parrish, D.D.; Fahey, D.W.; Ravishankara, A.R.; Middlebrook, A.M.; Brock, C.A.; Roberts, J.M.; Brown, S.S.; Neuman, J.A.; Lerner, B.M.; Lack, D.; Law, D.; Hübler, G.; Pollack, I.; Sjostedt, S.; Ryerson, T.B.; Gilman, J B; Liao, J.; Holloway, J.; Peischl, J.; Nowak, J.B.; Aikin, K.; Min, K.-E.; Washenfelder, R.A.; Graus, M.G.; Richardson, M.; Markovic, M.Z.; Wagner, N.L.; Welti, A.; Veres, P.R.; Edwards, P.; Schwarz, J.P.; Gordon, T.; Dube, W.P.; McKeen, S.; Brioude, J.; Ahmadov, R.; Bougiatioti, A.; Lin, J.J.; Nenes, A.; Wolfe, G.M.; Hanisco, T.F.; Lee, B.H.; Lopez-Hilfiker, F.D.; Thornton, J.A.; Keutsch, F.N.; Kaiser, J.; Mao, J.; Hatch, C.Natural emissions of ozone-and-aerosol-precursor gases such as isoprene and monoterpenes are high in the southeast of the US. In addition, anthropogenic emissions are significant in the Southeast US and summertime photochemistry is rapid. The NOAA-led SENEX (Southeast Nexus) aircraft campaign was one of the major components of the Southeast Atmosphere Study (SAS) and was focused on studying the interactions between biogenic and anthropogenic emissions to form secondary pollutants. During SENEX, the NOAA WP-3D aircraft conducted 20 research flights between 27 May and 10 July 2013 based out of Smyrna, TN. Here we describe the experimental approach, the science goals and early results of the NOAA SENEX campaign. The aircraft, its capabilities and standard measurements are described. The instrument payload is summarized including detection limits, accuracy, precision and time resolutions for all gas-and-aerosol phase instruments. The inter-comparisons of compounds measured with multiple instruments on the NOAA WP-3D are presented and were all within the stated uncertainties, except two of the three NO2 measurements. The SENEX flights included day- and nighttime flights in the Southeast as well as flights over areas with intense shale gas extraction (Marcellus, Fayetteville and Haynesville shale). We present one example flight on 16 June 2013, which was a daytime flight over the Atlanta region, where several crosswind transects of plumes from the city and nearby point sources, such as power plants, paper mills and landfills, were flown. The area around Atlanta has large biogenic isoprene emissions, which provided an excellent case for studying the interactions between biogenic and anthropogenic emissions. In this example flight, chemistry in and outside the Atlanta plumes was observed for several hours after emission. The analysis of this flight showcases the strategies implemented to answer some of the main SENEX science questions.