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Abstract:
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Thermoelectric materials are revisited for various applications including power generation. The direct conversion of temperature differences into electric voltage and vice versa is known as thermoelectric effect. Possible applications of thermoelectric materials are in eco-friendly refrigeration, electric power generation from waste heat, infrared sensors, temperature controlled-seats and portable picnic coolers. Thermoelectric materials are also extensively researched upon as an alternative to compression based refrigeration. This utilizes the principle of Peltier cooling. The performance characteristic of a thermoelectric material, termed as figure of merit (ZT) is a function of several transport coefficients such as electrical conductivity (σ), thermal conductivity (κ) and Seebeck coefficient of the material (S). ZT is expressed asκσTZTS2=, where T is the temperature in degree absolute. A large value of Seebeck coefficient, high electrical conductivity and low thermal conductivity are necessary to realize a high performance thermoelectric material. The best known thermoelectric materials are phonon-glass electron – crystal (PGEC) system where the phonons are scattered within the unit cell by the rattling structure and electrons are scattered less as in crystals to obtain a high electrical conductivity.
A survey of literature reveals that correlated semiconductors and Kondo insulators containing rare earth or transition metal ions are found to be potential thermoelectric materials. The structural magnetic and charge transport properties in manganese oxides having the general formula of RE1−xAExMnO3 (RE = rare earth, AE= Ca, Sr, Ba) are solely determined by the mixed valence (3+/4+) state of Mn ions. In strongly correlated electron
systems, magnetism and charge transport properties are strongly correlated.
Within the area of strongly correlated electron systems the study of
manganese oxides, widely known as manganites exhibit unique magneto
electric transport properties, is an active area of research.Strongly correlated systems like perovskite manganites,
characterized by their narrow localized band and hoping conduction, were
found to be good candidates for thermoelectric applications. Manganites
represent a highly correlated electron system and exhibit a variety of
phenomena such as charge, orbital and magnetic ordering, colossal magneto
resistance and Jahn-Teller effect. The strong inter-dependence between the
magnetic order parameters and the transport coefficients in manganites has
generated much research interest in the thermoelectric properties of
manganites. Here, large thermal motion or rattling of rare earth atoms with
localized magnetic moments is believed to be responsible for low thermal
conductivity of these compounds. The 4f levels in these compounds, lying
near the Fermi energy, create large density of states at the Fermi level and
hence they are likely to exhibit a fairly large value of Seebeck coefficient. |