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Local structural origin of enhanced thermoelectricity in dually doped BiCuSeO
From:   PublishDate:2017-06-16  Hits:

Thermoelectric materials are promising in thermal to electrical (vice versa) energy conversion applications. Nowadays the thermoelectric materials are becoming a spotlight of energy-related materials. For one thing, the thermoelectric devices can be applied in efficient utilization of waste-heat to generate power, which is also environment friendly. For another, the thermoelectric devices, thanks to their non-volatility and compactness, can be also manufactured for refrigeration purposes playing essential role in in aerospace crafts, detectors, etc. The BiCuSeO based thermoelectric materials hold great promises in high temperature applications with superb Figure of merit. The thermoelectric Figure of merit (ZT) value has been optimized to exceed 1 at 900 K through defect chemistry, band engineering, and structural manipulations. Most importantly, the record of the Figure of merit is continuously been broken.


Recently, a team consists of Dr. Yong Liu from Beijing Institute of Aeronautical materials of AECC, Prof. Yuan-hua Lin from Tsinghua University and Dr. Wei Xu from Institute of High Energy Physics of CAS and their coworkers, has developed a dually doped BiCuSeO-based thermoelectrics. The Figure of merit was optimized up to record high~1.5 at 873K. However, the origin of the enhanced thermoelectricity was mysterious. To unravel the origin, they have employed respectively the 1W2B and 4B7A endstation of Beijing Synchrotron Radiation Facility. By combining the local structural probe-X-ray Absorption Near-edge Spectroscopy (XANES) and theoretical simulations using Full Multiple Scattering Theory (FMST), they have revealed the actual occupation site of the dopants in matrix materials. It is known that the occupation sites are influencing the charge carrier density as well as the transport of phonons, even the entire macroscopic thermoelectricity.


As shown in Figure.1, it is confirmed that Pb ions enter the Bi sites via XANES at Pb L3-edge hard-X-ray. The occupation of Bi sites providing pathways for charge carriers manipulations (J. Mater. Chem. A, 1, 12154-12158. (2013), Adv. Mater. , 25, 5086-5090. (2013)). By contrast, the occupation sites for second dopant-Calcium was not determined directly. The Ca ions, which were not occupying the Bi sites as shown in Figure.2, formed bonds with O ions in the matrix. Interestingly, the Ca-O bond formed a unique hexagonal symmetry CaO2 nanocluster instead of CaO. The nanoclusters are effective scatterers for heat-carriers-phonons generated by lattice collective vibrations. Consequently the thermal conductivity was suppressed. In synergy with electron microscopy, shown in Figure.3, it is revealed that the dual-doping approach can induce a mesoscale structural manipulations which is favorable for thermal stability in comparison with single doped species. It is also demonstrated that dual doped systems can optimize the transport of charge carriers and simultaneously manipulate the phonon heat carriers. This findings pave way for optimization of thermoelectrics. The work was published on Advanced Energy Materials, 6, 1502423-1502431. (2016)

Figure.1 Determination of Pb occupation sites by XANES

Figure.2 Determination of Ca doping site by XANES

Figure.3 Multi-scale structural optimization of thermoelectric properties
Article: Y. Liu, L.-D. Zhao, Y. Zhu, Y. Liu, F. Li, M. Yu, D.-B. Liu, W. Xu*, Y.-H. Lin and C.-W. Nan, Synergistically Optimizing Electrical and Thermal Transport Properties of BiCuSeO via a Dual-Doping Approach, Advanced Energy Materials, 6, 1502423-1502431. (2016)
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