Six new AA′BB′O6 perovskites KLaMnWO6, NaLaMnWO6, NaNdMnWO6, NaTbMnWO6, NaNdCoWO6, and NaNdMgWO6 have been prepared. Each possesses the unusual combination of layered ordering of the A-site cations and rock-salt ordering of the B-site cations. The structure and properties of these samples have been characterized using monochromatic X-ray and neutron powder diffraction, UV–vis diffuse reflectance spectroscopy, and SQUID magnetometry. NaLaMnWO6, NaNdMnWO6, and NaTbMnWO6 adopt a structure with monoclinic P21 symmetry arising from the combination of cation ordering and a–a–c+ octahedral tilting. The structures of the other three compounds are similar, but the presence of satellite reflections in the neutron diffraction data suggests a more complicated superstructure. Each of the four AA′MnWO6 samples shows a paramagnetic to antiferromagnetic transition with Néel temperatures ranging from 6 to 15 K. The NaTbMnWO6 compound shows a second magnetic transition at ∼9 K. The origin of two magnetic phase transitions appears to arise from coupling between the Mn2+ sublattice and the Tb3+ sublattice.
Although SrTiO3-based perovskites showed a lot of promise as n-type thermoelectric (TE) materials, they demonstrated a low figure of merit value primarily because of their high lattice thermal conductivity (k l). Researchers found it difficult to reduce k l, as a popular route like nanostructuring did not work well with these perovskites possessing low phonon mean free paths. Here, we put forward a novel strategy of designing high-entropy perovskite (HEP) oxides having five transition metals in the B site to induce more anharmonicity causing enhanced multiphonon scattering in order to decrease k l. Using detailed thermodynamic calculations, we designed and synthesized a highly dense Sr(Ti0.2Fe0.2Mo0.2Nb0.2Cr0.2)O3 HEP ceramic. An ultralow thermal conductivity of 0.7 W/mK at 1100 K was achieved in this n-type rare-earth-free HEP oxide TE material. The concept of designing HEPs to achieve ultralow thermal conductivity potentially opens up a new avenue for enhancing TE performance of environmentally benign bulk oxides for high-temperature TE power generation.
Among the various oxide thermoelectric materials, double perovskites provide more flexibility to maneuver interdependent thermoelectric parameters to achieve enhanced thermoelectric figure of merit (ZT), as octahedral ordering, i.e., arrangement of B O 6 and B O 6 octahedra, present in the A 2 B B O 6 structure is impacted by cation doping. In this work, we investigated the role of octahedral distortion on thermoelectric properties of La 2-x Sr x CoFeO 6 (LSCF) double perovskites with 0.0 x 1.0, synthesized by the autocombustion route. Rietveld refinement of x-ray diffraction data revealed the phase transition with increasing Sr concentration (x) in LSCF from rhombohedral crystal structure with R-3c space group (x 0.6) to monoclinic P2 1 /n (0.8 x 1.0) space group. X-ray photoemission spectroscopy analysis further confirmed the presence of multiple oxidation states of Co and Fe, and shifts in oxidation states population driven by Sr content. These multivalent cations participated in the charge transport mechanism, which was explained by the small polaron hopping conduction model in these double perovskites. The electrical conductivity at room temperature was found to be increased by more than 10 7 times in LSCF due to Sr doping, causing a large enhancement in the thermoelectric power factor. Gradual decrease in the octahedral tilt angle with increasing Sr content in LSCF, leading toward the change of crystal structure from disordered (R-3c) to rock-salt-ordered (P2 1 /n) double perovskites, was found to be responsible for the large decrease in activation energy barrier for small polaron hopping conduction in the LSCF system, resulting in the phenomenal increase in electrical conductivity. Maximum thermoelectric figure of merit, ZT = 0.11 was obtained at 723 K for La 2-x Sr x CoFeO 6 with x = 0.2 composition.
Sodium niobate (NaNbO3) powders and nanowires were synthesized and subsequently used for preparing composites with polyvinylidene-fluoride (PVDF) using the cold sintering technique in the weight ratio of 80:20 (ceramic:polymer). Phase purity of ceramic powders and nanowires was confirmed by X-ray diffraction analysis. Different phases of PVDF formed under various process parameters were also identified. Thermo-gravimetric analysis was performed in order to check the thermal stability of the composites. SEM was used to study the surface morphology of these samples, and energy dispersive x-ray analysis was also carried out in order to view the distribution of polymers in the composites. Furthermore, the formation of the ceramic matrix in the composites was verified and distribution of polymers in the ceramic matrix was examined by micro-computed tomographic analysis. Dielectric constant and loss tangent were measured and compared with the theoretically calculated values. Theoretical breakdown strength and energy density were calculated and were found to be as high as 1345 kV/cm and 6.1 J/cm3, respectively, at room temperature. Discharge efficiency of 64% was obtained for annealed nanocomposites.
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