Strontium Titanate-based Materials for Thermoelectric Applications

Abstract

Strontium titanate (SrTiO3)-based ceramics are promising oxide thermoelectrics as they are based on non-toxic, earth-abundant elements and exhibit high thermal and chemical stability. This work systematically investigated the effects of doping, grain boundary additions and entropy changes on the thermoelectric properties of SrTiO3-based ceramics via both modelling and experiments. The electronic band structure and electrical transport properties of pure SrTiO3 and donor-doped Sr(Ti0.875M0.125)O3 (M = Cr, Mo, W, V, Nb, Ta) were calculated via Density Functional Theory (DFT) and semi-classical Boltzmann transport theory. The carrier concentration required to optimise the thermoelectric power factor was found to be at the level of 10^21 cm^-3. Among the six dopants examined, the substitution of Nb at the Ti (B) site was predicted to give a balance between electrical conductivity and the Seebeck coefficient and yield the best power factor up to 2250 μW m^-1 K^-2 at 450 K and the best price/performance ratio. Two carbon-based grain boundary additives [graphene oxide (GO) and carbon black (CB)] were incorporated in SrTi0.85Nb0.15O3 and the composites were densified at 1700 K under a reducing atmosphere. Both carbon-based materials were eliminated during sintering. CB was found to stimulate the formation of voids and defects (which strongly scatter phonons) leading to an ultra-low thermal conductivity of 1.7 W m^-1 K^-1 at 873 K. In contrast, GO leads to charge reduction in the grain boundaries, thereby eliminating the detrimental ‘grain boundary effects’, and enhancing electrical conductivity by nearly one magnitude. The improvement in charge transport led to a six-fold increase in power factor to 1659 μW m^-1 K^-2 at 323 K. High-performance ammonium tetrathiomolybdate/Nb-doped SrTiO3 (SrTi0.85Nb0.15O3) ceramics were synthesised. Through use of a reducing atmosphere, the ammonium tetrathiomolybdate was converted into metallic Mo, mainly located at the grain boundaries. By high-resolution STEM-EDX and Kelvin Probe techniques, the grain boundary regions were found to be reduced, characterised by lowered grain boundary energy barriers, leading to significant enhancement of carrier mobility and electrical conductivity to 2950 S cm^-1. A high power factor of 1727 μW m^-1 K^-2 was achieved at 300 K, and a zT value of 0.24 was achieved at 823 K. The strongly enhanced power factor at low temperature provides the basis for an expanded operational window for strontium titanate-based materials. High-entropy strontium titanate-based [(Ca0.33Sr0.33Ba0.33)(Ti¬xZrxNb1-x)O3, x = 0, 0.1, 0.2 and 0.33] ceramics were fabricated at 1673 K. Strong lattice distortion and a large number of defects were detected in the products. As a result, phonon scattering was significantly enhanced, leading to a glass-like, constant, ultra-low thermal conductivity as low as 1.6 W m^-1 K^-1 from 300 K to 873 K. This is the lowest reported amongst the heavy- and toxic-element-free strontium titanate-based ceramics.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Freer (Supervisor) & David Lewis (Supervisor)