Development in Zn4Sb-based thermoelectric materials

Research output: Book/anthology/dissertation/reportPh.D. thesisResearch

  • Department of Chemistry
  • iNano-School
Thermoelectric material, as a functional material which has the dual ability of electrical-thermal energy conversion, has attracted tremendous interests in the last decades, especially against the background of global energy shortage and surging of new materials. The present work focuses on the notable Zn4Sb3, with the effort to further the basic understanding of the compound, as well as improve the thermoelectric performance to meet the commercial use.
The maximum efficiency of a thermoelectric material is determined by its figure of merit, zT=TS2/ where S is the Seebeck coefficient or thermopower,  the electrical conductivity, the thermal conductivity and T the absolute temperature. The best thermoelectrics are heavily doped semiconductors with high thermoelectric power factors and low thermal conductivities, known as “Phonon Glasses Electrical Crystals”. Zn4Sb3 is one such material working in the temperature range of 200~400 ºC, where there is few other candidate available. Compared to other state-of-the-art thermoelectric materials, Zn4Sb3 only has moderate power factor (S2), it is the extremely low thermal conductivity that endows the over-unit zT value. The reason to the abnormity in thermal conductivity has been revealed to be the interstitial Zn sites in the structure. Meanwhile, enhancing the power factor is intensively attempted by doping and applying new synthesis routes. However, the instability of Zn4Sb3 in the expected working temperature range still limits the practical use of the compound.
In the first part of the present thesis, the thermal stability of Zn4Sb3 heated in inert argon atmosphere has been investigated by high resolution multi-temperature synchrotron powder diffraction. By comparing the decomposition behaviors of four different samples, a two-process decomposition mechanism is proposed. One process that produces ZnSb and Zn is more “intrinsic”, happens only to a degree of a few percent regardless of synthesis methods, doping or heating atmosphere. The other which generates Sb and Zn is greatly promoted if the generated Zn easily gets exposed to oxygen. The migration of Zn in compacted solid has also been observed and investigated. Zn4Sb3 powders are pressed by Spark Plasma Sintering (SPS) and Short Time Sintering (STS). A compositional discrepancy at the two ends of the pellet indicates that the compaction methods have significant effects on the homogeneity and purity of the Zn4Sb3 pellets. Zn4Sb3 decomposes during the process of the compaction; generated Zn diffuses driven by the direct current. This leads to a gradient in Zn content across the axes of the pellet. Thus, the Seebeck coefficient distribution on the cross-section of the pellet is inhomogeneous. X-ray diffraction and Electron backscatter diffraction are combined to identify the phases appear during the Zn lost. An X-ray diffraction scan on the cross-section of the SPS pressed pellet was carried out. Quantitative analysis of the different phases explains the Seebeck coefficient changes on the cross-section.
The following part reports the effect of nano-particles on the thermoelectric properties and thermal stability of Zn4Sb3. Though TiO2 nano particles have remarkably enhanced the stability, the thermoelectric performance of all the nano-composites deteriorates. Optimization of the content of the nano-inclusions is still under studying.
Finally, a method that combines the synthesis and pressing of Zn4Sb3 by SPS is explored. By optimizing the pressing parameters and compensating Zn lost during the process, homogeneous phase pure pellet can be produced. The procedure is much easier and faster than the traditional methods.
Original languageEnglish
PublisherFællestrykkeriet, Det Sundshedsvidenskabelige Fakultet, Aarhus Universitet
Number of pages99
Publication statusPublished - 15 Aug 2011

    Research areas

  • Thermoelectric Materials, Zn4Sb3, Synchrotron radiation, Thermal Stability, SPS, Nano-composites

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