ABSTRACT
The untiring endeavour towards green energy is a trending research among the research community. Thermoelectric materials are of vital importance here owing to their emission-free operation. As a righteous candidate, calcium manganate materials are being explored to increase its figure of merit. In this study, the structural, microstructural, electrical transport, and high-temperature thermoelectric measurements of LaxDyxCa1-2xMnO3 {x = 0.025 (L25D25), 0.05 (L50D50), 0.075 (L75D75), and 0.1 (L100D100)} were systematically performed. The structural confirmation of the synthesised sample was validated using X-ray diffraction, which also revealed the orthorhombic (space group: Pnma) crystallisation of co-doped samples with no traces of secondary peaks. A significant increase in the unit cell volume was observed with rare earth substitutions. The morphological studies revealed that the prepared samples were highly dense and the grain size was reduced with rare earth concentration. The substitution of La and Dy enhanced the conductivity values of pristine CMO by two orders of magnitude due to the high concentration of charge carriers and the presence of Mn3+ ions due to rare earth doping. The conductivity increased with rare earth concentrations but diminished for x = 0.1 due to the localization of charges. The Seebeck coefficient values were negative for all the prepared samples, indicating electrons as the predominant carriers over the entire operating range. A minimum κ of 1.8 W m-1 K-1 was achieved for La0.1Dy0.1Ca0.8MnO3 and the maximum value zT obtained was 0.122 at 1070 K for La0.075Dy0.075Ca0.85MnO3.
ABSTRACT
This work investigates and reports the effect of ZnO addition on the ferroelectric properties of (K0.5Na0.5)(Nb0.7Ta0.3)O3 (KNNT) ceramics prepared by a solid state reaction method. Though literature is abundant on the study of the effect of ZnO on the sinterability, microstructure and electrical properties of KNN based materials, the effect of ZnO on their ferroelectric properties has seldom been studied in detail, especially in KNNT. In the current study, 2, 4 and 6 wt% of ZnO was added to KNNT ceramics. The XRD results revealed ZnO addition has no effect on the crystal symmetry of KNNT. However, a ZnO secondary phase was found in KNNT ceramics with 4 and 6 wt% ZnO doping. An increase in grain size was observed with increases in the concentration of ZnO, indicating a direct dependence of grain size on the concentration of ZnO in the KNNT matrix. From ferroelectric studies it was observed that a lower electric field was sufficient to get maximum polarization for ZnO doped KNNT samples compared to that of pure KNNT ceramics. A high remnant polarization (P r = 14.0 µC cm-2) and lower coercive field (E c = 5.6 kV cm-1) was obtained for 2 wt% ZnO doped KNNT. These samples showed the least fatigue (0.8%) after 109 cycles in comparison to pure (5%), 4 wt% ZnO doped (24.9%) and 6 wt% ZnO doped (30%) KNNT ceramics. The diminution in P s, P r, and E c was only 26.0%, 26.2% and 18.5%, respectively, with an increase in measurement temperature, which indicates improved thermal stability in 2 wt% ZnO doped KNNT. From the present study the optimum concentration of ZnO in KNNT is identify to be 2.0 wt% and their improved properties in comparison to the pure KNNT ceramics are discussed in detail.
ABSTRACT
The aurivillius family of compounds SrBi4Ti4O15 (SBTi) and SrBi4Ti3.8Nb0.2O15 has been prepared using solid state reaction techniques. The niobium doping enhances the value of the dielectric constant, but decreases the phase transition temperature and grain size of SBTi. Grain conductivity evaluated from the impedance data reveals that Nb doping increases the resistance of grains which indicates the decrease in oxygen vacancies. The negative temperature coefficient of resistance shown by the grain boundary conductivity is explained using the Heywang-Jonker model. The variation of ac conductivity with frequency is found to obey Jonscher's universal power law. The frequency exponent (n), pre-exponential factor (A), and bulk dc conductivity (σ dc) are determined from the fitting curves of Jonscher's universal power law. From the frequency exponent (n) versus temperature curve, we conclude that the conduction mechanism of SBTi changes from large-polaron tunneling (300-475 °C) to small-polaron tunneling (475-550 °C), and in that of the niobium doped it is small-polaron tunneling (300-375 °C) to correlated band hopping (375-550 °C). Activation energies have been calculated from different functions such as loss tangent, relaxation time, grain and grain boundary conductivities, and ac and dc conductivity. The activation energies reveal that conductivity in the sample has contributions from migrations of oxygen vacancies, bismuth ion vacancies, electrons ionized from strontium vacancies, strontium ion vacancies and valence fluctuations of Ti4+/Ti3+ ions.