Synthesis and characterisation of oxychalcogenides as promising thermoelectric materials for waste heat recovery

Abstract

This dissertation describes the preparation and investigation of crystal structure and thermoelectric properties of solid-solutions within three families of layered bismuth oxychalcogenides: BiOCuCh (Ch = S, Se, Te), Bi2YO4Cu2Se2, and Bi2O2Ch (Ch = Se, Te). The crystal structures of all materials were investigated using powder X-ray and neutron diffraction (for BiOCuCh). BiOCuCh (Ch = S, Se, Te) compounds crystallise in the ZrCuSiAs structure type (P4/nmm space group), and are composed of fluorite-type [Bi2O2]2+ and anti-fluorite-type [Cu2Ch2]2- slabs, stacked alternatively along the c – axis. Results show that BiOCuCh (Ch = S, Se, Te) are p-type semiconductors. The electrical conductivity increases while thermal conductivity decreases systematically with changing from S to Te in these compounds. Analysis of neutron diffraction data shows that the rattling behaviour of copper in a rigid framework (BiOCh) is at the origin of their low thermal conductivity. The figure of merit increases with increasing atomic weight of the chalcogenide. BiOCuSe shows the larger potential for thermoelectric applications in terms of its combination of economic cost and properties. Therefore, the effect of doping with divalent cations (Pb2+, Cd2+, Zn2+) on BiOCuSe was studied. Results show that substitution of trivalent Bi3+ with a 4-5 at.% of divalent Pb2+ leads to an enhancement of the power factor and a high figure of merit (ZT ~ 0.62 at 673 K), whilst the substitution of monovalent Cu+ with divalent Cd2+ or Zn2+ leads to an increase in the magnitude of the electrical resistivity and the Seebeck coefficient. In addition, a reduction of the thermal conductivity (κ ~ 0.77 W m−1 K−1) is achieved in ball-milled Bi0.95Pb0.05OCuSe. Bi2YO4Cu2Se2 crystallises in the Sr2Mn3Sb2O2 structure type (I4/mmm space group), and consists of fluorite-type [Bi2MO4]+ and anti-fluorite-type [Cu2Ch2]- layers stacked alternatively along the c – axis. It possesses metallic behaviour, with hole charge carriers and a fairly low figure of merit (ZT ~ 3x10-2 at 673 K). This behaviour is related to the oxidation state of the copper (+1.5) in which more hole charge carriers have been produced. Bi2O2Ch (Ch = Se, Te) crystallises in the anti-ThCr2Si2 structure (I4/mmm space group) and comprises fluorite-type [Bi2O2]2+ and square net Ch2- stacked alternatively along the c – axis. Results show that Bi2O2Te1-xSex (0 ≤ x ≤ 0.25) are n-type semiconductors, and that Bi2O2Te shows the highest figure of merit (ZT ~ 1.3x10-1 at 573 K) while Bi2O2Te1- xSex (0.5 ≤ x ≤ 1) and Bi2O2Se1±δ(0.05 ≤ δ ≤ 0.15) solid solutions show insulating behaviour.