Thermoelectric properties of CaMnO 3 films obtained by soft chemistry synthesis
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Polycrystalline randomly oriented CaMnO3 films were successfully deposited on sapphire substrates by soft chemistry methods. The precursor solutions were obtained from a mixture of metal acetates dissolved in acids. The Seebeck coefficient and the electrical resistivity were measured in the temperature range of 300 K , T , 1000 K. Modifications of thermal annealing procedures during the deposition of precursor layers resulted in different power factor values. Thermal annealing of CaMnO3 films at 900 °C for 48 h after four-layer depositions (route A) resulted in a pure perovskite phase with higher power factor and electrical resistivity than four-layer depositions of films annealed layer by layer at 900 °C for 48 h (route B). The studied films have negative Seebeck coefficients indicative of n-type conduction and electrical resistivities showing semiconducting behavior. I. INTRODUCTION 2
S The thermoelectric figure of merit ZT ¼ qj T of materials is a function of four parameters: Seebeck coefficient (S), electrical resistivity (q), absolute temperature (T), and thermal conductivity (j). Good thermoelectric materials should have a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity for an efficient conversion of heat into electricity. Transition metal oxides with perovskite-type structure are attractive materials for thermoelectric applications.1 The properties of these materials originate from strong interactions between the d-state electrons of the transition metal atoms.2 Perovskite-type manganates are well known for their high magnetoresistance which can be explained by the double-exchange model. Calcium manganate, CaMnO3, is an antiferromagnetic insulator that was intensively studied in recent years in terms of colossal magnetoresistance (CMR),3,4 thermoelectricity,5–8 and its suitability as oxygen sensor.9 The CMR effect of perovskite manganates results from charge and/or orbital ordering and the competition between different magnetic interactions.2 The mixed valence of Mn3+ and Mn4+ as function of the elemental composition strongly govern the physical properties (e.g., thermoelectric properties) of manganates.1 An alternative way to modify the physical properties is by changing the morphology of the samples, e.g., by creating low dimensional structures or nanostructures. The application of bulk thermoelectric materials in miniaturized electronic devices is rather limited up to
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.63 J. Mater. Res., Vol. 27, No. 7, Apr 14, 2012
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date10 due to the lacking scalability of production processes. The synthesis of small converters could be realized by using highly efficient thermoelectric materials in the form of films. The different possible procedures of scalable soft chemistry synthesis methods allow us to control the grain size of particles in films and coatings from 100 to 400 nm diameter. The influence of the grain size and final composition on
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