Lassi Karvonen

Influence of tungsten substitution and oxygen deficiency on the thermoelectric properties of CaMnO3−δ

Polycrystalline tungsten-substituted CaMn1−xWxO3−δ (0.00 ≤ x ≤ 0.05) powders were synthesized from a polymeric precursor, pressed and sintered to high density. The impact of tungsten substitution on the crystal structure, thermal stability, phase transition, electronic and thermal transport properties is assessed. Tungsten acts as an electron donator and strongly affects high-temperature oxygen stoichiometry. Oxygen vacancies form in the high figure-of-merit (ZT)-region starting from about T = 1000 K and dominate the carrier concentration and electronic transport far more than the tungsten substitution. The analysis of the transport properties yields that in the investigated regime the band filling is sufficiently high to overcome barriers of polaron transport. Therefore, the Cutler-Mott approach describes the electrical transport more accurately than the Mott approach for small polaron transport. The lattice thermal conductivity near room temperature is strongly suppressed with increasing tungsten concen...

Thermoelectric properties of [Ca2CoO3−δ][CoO2]1,62 as a function of Co/Ca defects and Co3O4 inclusions

The misfit-layered cobalt oxide [Ca2-wCoO3-δ][CoO2]1.62 is a promising material for high-temperature thermoelectric heat recovery. Here, we show that discrepancies in the numerously reported thermoelectric performances can result from Co3O4 impurities or a change of the defects, i.e., the relative Co content in the [Ca2-wCoO3-δ][CoO2]1.62 phase. We observe that increasing the relative Co content in the [Ca2-wCoO3-δ][CoO2]1.62 phase leads to a larger figure of merit Z T. We attribute this increase of Z T to additional p-type charge carriers introduced by Ca vacancies and the resulting reduction of the electrical resistivity. For Co/Ca ratios above the miscibility limit, the increase in thermal conductivity due to the formation of Co3O4 impurities leads to a reduction of ZT when the volume fraction of the Co3O4 phase is increased from 1% to 3%. Hence, the best figure of merit is expected close to the upper phase boundary of the [Ca2-wCoO3-δ][CoO2]1.62 phase.

Structure stability of Li x H y V 3 O 8 exposed to water-vapour

Chemically lithiated Li x H y V 3 O 8 was prepared as the starting material in this study owing to its interesting properties for lithium-ion intercalation and further application in the positive electrode of lithium-ion batteries. The hydrophilic character of the synthesised compound was evaluated because even ppm water level inside batteries containing electrolytes with fluorinated salts like LiPF 6 can dramatically influence the battery stability. The structural stability of the lithiated vanadate after exposure to water-vapour was evaluated ex situ by XRD, TGA and TEM. TGA showed that at least 4 wt.% of water can be incorporated into the material. Water intercalates in the vanadate lattice filling the particles from the outer shell towards the bulk. Prolonged exposure to water-vapour saturated atmosphere leads to irreversible exfoliation of the crystal structure. Weakly bonded intercalated water can be removed without any damage to the crystal structure by an annealing step at 200oC for 2 h in air.