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Register a new userEffect of Chemical Doping on the Thermoelectric Properties of FeGa3
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Added by
Neel Haldolaarachchige
Publication date
2011
fields
Physics
and research's language is
English
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Thermoelectric properties of the chemically-doped intermetallic narrow-band semiconductor FeGa3 are reported. The parent compound shows semiconductor-like behavior with a small band gap (Eg = 0.2 eV), a carrier density of ~ 10(18) cm-3 and, a large n-type Seebeck coefficient (S ~ -400 mu V/K) at room temperature. Hall effect measurements indicate that chemical doping significantly increases the carrier density, resulting in a metallic state, while the Seebeck coefficient still remains fairly large (~ -150 mu V/K). The largest power factor (S2/{rho} = 62 mu W/m K2) and corresponding figure of merit (ZT = 0.013) at 390 K were observed for Fe0.99Co0.01(Ga0.997Ge0.003)3.
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The effects of Cu-doping on the structural, magnetic, and transport properties of La0.7Sr0.3Mn1-xCuxO3 (0 < x < 0.20) have been studied using neutron diffraction, magnetization and magnetoresistance (MR) measurements. All samples show the rhombohedral structure with the R3c space-group from 10K to room temperature (RT). Neutron diffraction data suggest that some of the Cu ions have a Cu3+ state in these compounds. The substitution of Mn by Cu affects the Mn-O bond length and Mn-O-Mn bond angle resulting from the minimization of the distortion of the MnO6 octahedron. Resistivity measurements show that a metal to insulator transition occurs for the x more than 0.15 samples. The x = 0.15 sample shows the highest MR(_80%), which might result from the co-existence of Cu3+/Cu2+ and the dilution effect of Cu-doping on the double exchange interaction.
We present a study of the electronic properties of Tl5Te3, BiTl9Te6 and SbTl9Te6 compounds by means of density functional theory based calculations. The optimized lattice constants of the compounds are in good agreement with the experimental data. The band gap of BiTl9Te6 and SbTl9Te6 compounds are found to be equal to 0.589 eV and 0.538 eV, respectively and are in agreement with the available experimental data. To compare the thermoelectric properties of the different compounds we calculate their thermopower using Motts law and show, as expected experimentally, that the substituted tellurides have much better thermoelectric properties compared to the pure compound.
It is well known that the efficiency of a good thermoelectric material should be optimized with respect to doping concentration. However, much less attention has been paid to the optimization of the dopants energy level. Thermoelectric materials doped with shallow levels may experience a dramatic reduction in their figures of merit at high temperatures due to the excitation of minority carriers that reduces the Seebeck coefficient and increases bipolar heat conduction. Doping with deep level impurities can delay the excitation of minority carriers as it requires a higher temperature to ionize all dopants. We find through modeling that, depending on the material type and temperature range of operation, different impurity levels (shallow or deep) will be desired to optimize the efficiency of a thermoelectric material. For different materials, we further clarify where the most preferable position of the impurity level within the band gap falls. Our research provides insights in choosing the most appropriate dopants for a thermoelectric material in order to maximize the device efficiency.
In this work, the structural and transport properties of (Nd0.7-xLax)Sr0.3MnO3 manganites with x = 0, 0.1 and 0.2 prepared by solid state reaction route are studied. These compounds are found to be crystallized in orthorhombic structural form. The influence of La substitution in place of Nd at A-site shifts the metal to semiconductor/insulator transition temperature (TMI) peak towards room temperature with x = 0, 0.1 and 0.2. A composition prepared with the value of x = 0.2 in (Nd0.7-xLax)0.7Sr0.3MnO3 manganites (i.e. (Nd0.5La0.2)0.7Sr0.3MnO3), TMI was observed at 289 K which is close to room temperature. The maximum percentage of TCR values of compounds are increasing with average radius <r_A> but %TCR are slightly equal in x = 0.1 and 0.2 as compared to the parent compound. The maximum %TCR value is almost independent with A-site average radius <r_A> in x = 0.1 and 0.2. The electrical resistivity data are explored by different theoretical models and it has been concluded that at low temperature (ferromagnetic metallic region) conduction mechanism presumably due to the combined effect of electron-electron, electron-phonon and electron-magnon scattering, while in paramagnetic semiconducting regime, the variation of resistivity with temperature are explained by (1) Mott variable range hopping mechanism, (2) Adiabatic small polaron hopping and (3) Thermally activated hopping. The polaron hopping and thermal activation energies are decreasing with increase of an average A-site ionic radius (<rA>). An appropriate enlightenment for the observed behavior is discussed in detail.
We have investigated the effect of Sb-deficiency on the thermoelectric figure of merit (zT) of Zn4Sb3 prepared by solid state reaction route. At high temperatures, the Seebeck coefficient (S) and electrical conductivity ({sigma}) increase with increase in Sb deficiency whereas the thermal conductivity (k{appa}) decreases giving rise to an increase in the overall zT value. The observations suggest that creation of vacancies could be an effective route in improving the thermoelectric properties of Zn4Sb3 system. This coupled to nanostructuring strategy could lead to the ultimate maximum value of zT in this system for high temperature thermoelectric applications.
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