Synthesis, structure and properties of Zintl-type thermoelectric materials
Downie, Ruth Amy
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Zintl-type materials are of great interest in the field of thermoelectrics as their structures lend themselves to independent optimization of properties. For this reason, three distinct series, that can all be described using a Zintl-type model, were prepared using standard solid state reactions. A detailed investigation of X1-xX’xNiSn (X = Ti, Zr, Hf) half-Heusler compositions was undertaken, results of which are presented in Chapters 3-5. These materials are well established as promising thermoelectrics but progress in increasing efficiencies has been hindered by a lack of understanding of the structure and irreproducibility of properties. The work presented herein therefore aimed to provide a detailed analysis of the structure of these compositions. This was achieved primarily by neutron powder diffraction which has not previously been used in the study of these materials. Complimentary electron microscopy analysis was also carried out to gain information on the structure over various length-scales and thermoelectric property measurements were performed. Significant multiphase behaviour was discovered in all compositions where x ≠ 0, 1. Our data indicates that the compositional variations occur over long length-scales and has no significant impact on the thermoelectric properties. In addition, 0-3% excess Ni, located on interstitial sites in the half-Heusler structure was found in all samples, regardless of the synthesis method used. This has not previously been acknowledged in the literature and is difficult to identify without neutron diffraction. Three TiNiMySn series (M = Ni, Co, Cu) were subsequently prepared. Up to 8% interstitial metal was successfully introduced to the TiNiSn structure and had a doping effect on the thermoelectric properties. The introduction of interstitial metals was therefore found to be a new route to controlling the electronic properties of these promising thermoelectric materials. TiNiX (X = Si, Ge) and RMnSbO (R = Nd, La) were considered as new materials to the field of thermoelectrics as described in Chapters 6 and 7, respectively. Both series showed some promise for thermoelectric applications, with a large Seebeck coefficient found in NdMnSbO and low resistivity values displayed by the TiNiX compositions. In addition, detailed X-ray and neutron powder diffraction experiments and measurements of their magnetic properties were undertaken. Large magnetoresistances were found in TiNiX and incommensurate magnetic ordering was uncovered in NdMnSbO.