High-temperature thermoelectric properties of W-substituted CaMnO 3
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High-temperature thermoelectric properties of W-substituted CaMnO3 Dimas S. Alfaruq a, James Eilertsen a, Philipp Thiel a, Myriam H Aguirre a, Eugenio Otal a, Sascha Populoh a, Songhak Yoon a, Anke Weidenkaff a,* a. Empa, Solid State Chemistry and Catalysis, Ueberlandstrasse. 129, CH-8600 Duebendorf, Switzerland * Corresponding Author Abstract The thermoelectric properties of W-substituted CaMn1-xWxO3- (x = 0.01, 0.03; 0.05) samples, prepared by soft chemistry, were investigated from 300 K to 1000 K and compared to Nbsubstituted CaMn0.98Nb0.02O3- . All compositions exhibit both an increase in absolute Seebeck coefficient and electrical resistivity with temperature. Moreover, compared to the Nbsubstituted sample, the thermal conductivity of the W-substituted samples was strongly reduced. This reduction is attributed to the nearly two times greater mass of tungsten. Consequently, a ZT of 0.19 was found in CaMn0.97W0.03O3- at 1000 K, which was larger than ZT exhibited by the 2% Nb-doped sample. 1. Introduction Thermoelectric materials convert heat into electricity directly. Their thermoelectric efficiency is nominally quantified by a dimensionless figure of merit (ZT). The ZT is a function of the interdependent electrical and thermal transport properties of the material; specifically, the Seebeck coefficient (S), electrical resistivity ( ), and total thermal conductivity ( ) – determined at a particular temperature (T) in Kelvin: ZT = S2T/ȡț, where ț = țl + țe. Consequently, in order to enhance the ZT of a thermoelectric material, a high Seebeck coefficient must be maintained while the electrical resistivity and the total thermal conductivity of the material are reduced1. This can be achieved, in part, by introducing additional charge carriers up to a suitable concentration. As a result, the electrical resistivity can be reduced without reducing the Seebeck coefficient appreciably. Aliovalent transition-metal cation substitution, in particular with comparatively heavy transition metals, can achieve both increased charge carrier concentration and decreased lattice thermal conductivity due to massdifference phonon scattering 1-3. The calcium manganate, CaMnO3, is a promising thermoelectric material as it exhibits high Seebeck coefficients, strong resistance to chemical degradation at elevated temperatures, and a perovskite-type structure susceptible to A- and B-site substitution 4. Therefore, this paper reports an investigation into the effect of aliovalent B-site substitution on the structure and thermoelectric properties of CaMnO3. W-substituted CaMn1-xWxO3± (x = 0.01; 0.03; 0.05) samples were prepared by soft chemistry, sintered, and characterized by x-ray diffraction and electron microscopy. Thermogravimetric analysis experiments were performed to determine the oxygen stoichiometry of each sample. The thermoelectric properties were measured from 300 K to 1000 K and compared to Nb-substituted CaMn0.98Nb0.02O3- 4 in order to evaluate the effectiveness of W-substitution. 2. Experimental Polycrystalline perovskite-type C
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