Unravelling the composition space of Zr2FeNiSb2 double half-Heusler thermoelectric material

Abstract

More than half of annual energy consumption in thermal power plants are released in the form of waste heat. However, this waste heat could be harnessed with the help of thermoelectric (TE) devices which can convert this waste heat into valuable electricity. In this doctoral work, the central focus was to comprehensively explore systems related to Zr2FeNiSb2 (ZNFS) double half-Heusler (DhH) material. The first aim was to obtain TE couples comprising n-type and p-type semiconductor legs through aliovalent substitution or alloying of Sn at the Sb (Zr2FeNiSb1.9Sn0.1) and Nb at the Zr Wyckoff positions (Zr1.9Nb0.1FeNiSb2). The study has analyzed these alloy systems' microstructural and thermoelectric properties, predominantly showing the presence of half-Heusler structure along with minor Fe-rich phases. The figure of merit (zT) achieved for Zr1.9Nb0.1FeNiSb2 and Zr2FeNiSb1.9Sn0.1 are ~ 0.006 and ~ 0.016 at 673 K, respectively. Efforts were also made to achieve single-phase systems from these Sb- and Nb-substituted systems by reducing Fe content. The maximum power factor achieved for these reduced Fe-containing Nb-ZNFS and Sn-ZNFS alloys (Zr1.9Nb0.1Fe0.9Ni1.1Sb2 and Zr2Fe0.9Ni1.1Sb1.9Sn0.1) were 85.43 µW m-1 K-2 and 9.03 µW m-1 K-2 at 973 K, respectively. After obtaining p-type and n-type materials, the next segment of the thesis delved into introducing disorder by varying Fe (ZrFe0.60Ni0.40Sb i.e. Ni-40) and Ni content (ZrFe0.45Ni0.55Sb; Ni-55, ZrFe0.40Ni0.60Sb; Ni-60, ZrFe0.35Ni0.65Sb; Ni-65, ZrFe0.30Ni0.70Sb; Ni-70 and ZrFe0.20Ni0.80Sb; Ni-80) in the ZNFS crystal structure. A detailed microstructural examination revealed that Ni-rich side comprise of composite microstructure with plate-like phase morphology along with demonstrating superior thermoelectric properties over the Fe-rich side of ZNFS alloy. Ni-60 emerged as the highest-performing alloy with the highest power factor; Pmax: ~ 1025 µW m-1 K-2 at 973 K. All these Ni rich alloys are n-type semiconductors exhibiting a distinctive plate-like morphology (PLM) in their microstructures which became the focus of exploration in the latter part of the thesis, particularly concentrating on Ni-60 as the main alloy. With the help of transmission electron microscopy (TEM), this PLM was identified as hH cubic phase which shares an orientation relationship (OR) with the matrix phase (orthorhombic ZrNiSb-type phase) with common planes shared between the two phases at the interface. Further, the evolution of this PLM in the orthorhombic matrix phase was studied in detail under two different conditions (i.e. quenched and furnace-cooled samples) while undergoing all alloys through isothermal heat treatment at 1173 K. The composition space of stable hH in the phase field of the quaternary elemental system is extended from the room temperature to 1173 K with Fe and Ni varying around (13-16 at%) and (19-21 at%), respectively. The maximum phase fraction of hH achieved after 2 days of heat treatment at 1173 K in quenched sample is ~ 97 % while in case of furnace cooled sample is ~ 81 %.