|M.Sc Student||Graff Ayelet|
|Subject||Effects of Lattice Defects on Thermoelectric Properties of|
Calciun-Manganite Compounds for Energy Harvesting
|Department||Department of Materials Science and Engineering||Supervisor||Professor Yaron Amouyal|
|Full Thesis text|
One of the directions in the search for alternative energy sources is capturing waste heat and converting it into electricity using thermoelectric (TE) generators. Such generators are based on the Seebeck effect. In the Seebeck effect, a temperature gradient across a material initiates diffusion of charge carriers along the specimen, thus creating an open-circuit voltage between the hot and cold poles of the material. A TE device consists of several TE couples of n-type and p-type legs, which are connected electrically in series and thermally in parallel.
An efficient TE material is characterized by high TE figure of merit, ZT, which is manifested by large Seebeck coefficient (thermopower, i.e. the ratio of voltage to temperature difference), high electrical conductivity, and low thermal conductivity.
TE oxides offer many advantages, among them are structural and chemical stability at high temperatures, non-toxicity, and low-cost. Whereas the figures of merit of p-type cobaltates approach unity, which is the criterion of applicability, the ZT-values of their n-type counterparts are significantly lower and should, therefore, be improved.
The present study focuses on Ruddlesden-Popper (RP) CaO(CaMnO3)m n-type compounds, which are modulated structures formed of m perovskite CaMnO3-layers separated by single rock-salt CaO-layers. We aim at drawing the correlation between the thermal conductivity of CaO(CaMnO3)m-based compounds and the periodicity of the planar CaO defects, denoted by the m-values. We are, additionally, interested in studying the effects of the planar periodicity on the other TE properties, namely electrical conductivity and thermopower.
We prepared several pure, La-, and Nb-doped CaO(CaMnO3)m compounds having different periodicities by mixing oxide powders followed by sintering processes. We characterized the obtained microstructures employing x-ray diffraction (XRD) and high-resolution scanning electron microscopy (SEM). We measured the thermal conductivities applying the laser flash analysis (LFA) technique, as well as the electrical conductivity and thermopower. We observed a remarkable reduction in thermal conductivity as a result of increasing the CaO-planes density for the RP compounds. This apparent correlation is elucidated in terms of phonon scattering. This trend is, however, accompanied by a respective reduction in electrical conductivity, which yields an overall decrease of ZT. We conclude this study with directions for further research, to be focused on examining other synthesis methods, synthesizing composite materials based on these compounds, and applying advanced characterization techniques to study the atomistic nature of this CaO/CaMnO3 layering.