Sintesi e caratterizzazione di leghe Half-Heusler per Applicazioni termoelettriche

In this work we focused on production of n-type Ti0.25Zr0.75NiSn and p-type: ZrCoBi0.8Sn0.2 Half- Heusler alloys and discovery of new Double Half-Heusler Ta2FeNiSn2 and Nb2FeNiSn2 and V2FeNiSn2 Alloys. The n-type Ti0.25Zr0.75NiSn was synthesized by arc-melting adjusting the content of Sn to avoid the presence of residual elemental tin. Different characterization techniques confirmed that the samples are characterized by two phased separated HH compounds. The thermoelectric characterization of the stoichiometric alloy showed a negative Seebeck Coefficient confirming the n-type nature of the alloy. About electrical conductivity and magnification, in lower temperatures region, a discontinuity can be observed which jump at exactly Sn melting point, because of residual elemental Sn. All the sample produced directly by arc melting (both stoichiometric and Sn-poor) showed a brittle behavior, suggesting to use to powder sintering in order to obtain dense and more robust samples The p-type: ZrCoBi0.8Sn0.2 could not be synthesised by arc-melting method. because Bi and Co were insoluble and can not be pre-alloyed with Zr because the melting point of Zr is higher than the boiling point of Bi. So for future work, we decided to make a pre-alloy of Zr, Co, Sn with arc melting first, and we make sure it’s a stable and homogeneous composition, and then we could try to add Bi with induction melting or ball milling. The Ta2FeNiSn2 could not be synthesised by arc-melting method. We pre alloyed Ta with Fe and Ni with Sn respectively. And then we melted the pre alloyed Ta-Fe and Ni-Sn together. The melting point of Tantalum is so much higher and that might cause trouble in melting the alloy. The Nb2FeNiSn2 synthesized by arc-melting but characterization results did not show us the desired Double-Half Heusler phase that we expected. The V2FeNiSn2 Alloy showed a small amount of DHH phase, however homogenization could not be achieved by annealing of the as cast ingot. Therefore, in future work we should use some other synthesis methods like ball milling or melt-spinning.