Use of semi-empirical modelling to design and control the electronic properties of Half-Heusler thermoelectrics

Abstract

With increasing energy demand and a move away from non-renewable power generation, the development of more efficient renewable energy sources is important. Thermoelectric generators (TEGs) can convert heat directly into electricity, which can be used to increase the overall efficiency of heat-based processes or operate independently in extreme conditions. Half-Heusler materials are of interest due to their good electronic and structural properties. Chapters 3 and 4 examine the thermoelectric properties of n-type XNiCuySn (X = Ti, Zr and Hf) materials, using the Single Parabolic Band (SPB) and Callaway models to rationalise the change in properties with Cu doping and X-site alloying. An effective synthetic protocol using arc-melting is established to maximise the thermoelectric performance, which yielded zT = 0.83 for TiNiCu0.03Sn based material and zT = 1 for the alloyed composition Ti0.7Zr0.3NiCu0.025Sn. Chapter 5 covers the behaviour of interstitial Ni in the XNi1+xSn (X = Ti, Zr and Hf) materials, which is responsible for increased scattering of electrons and phonons. Neutron and X-ray powder diffraction reveal that interstitial Ni is trapped after arc-melting and a large concentration can remain trapped in ZrNi1+xSn and HfNi1+xSn if not annealed above 700 °C. This knowledge is used to prepare a HfNi1.1Sn sample with an out of equilibrium excess interstitial Ni concentration and measure its electronic properties. In Chapter 6, band engineering to increase S is attempted in [Ti1-xVx][FeyCo1-y]Sb materials. X-ray powder diffraction and SEM reveal the samples to be phase segregated with preferential formation of TiCoSb. p-type samples are insulating despite high nominal doping, while n-type samples are metallic, although neither show promising thermoelectric properties. p-type samples show positive Lorentz magnetoresistance below 100 K, which becomes negative at 2 K, potentially due to magnetic ordering.