Kati Raju

Thermopower and resistivity studies of Nd-Na-Mn-O manganites

The electrical transport properties of the polycrystalline Nd1−xNaxMnO3 (0.05 ≤ x ≤ 0.25) were investigated. All the samples are found to exhibit insulating behavior down to the lowest measured temperature. However, on application of 4 T magnetic field, metal-insulator transitions along with a low temperature resistivity upturn are observed for the samples with x ≥ 0.15. Thermopower values of samples with x = 0.05, 0.10, and 0.20 are negative in the entire temperature range of investigation, while they changes from negative to positive with decreasing temperature in the case of samples with x = 0.15 and 0.25. The resistivity and thermopower data in the high temperature region are explained within the framework of variable range and small polaron hopping models, while the resistivity minima are explained using the combined effect of electron-electron and electron-phonon scattering is considered.

Interfacial microstructure and shear strength of reactive air brazed oxygen transport membrane ceramic–metal alloy joints

To fabricate a multi-layered structure for maximizing oxygen production, oxygen transport membrane (OTM) ceramics need to be joined or sealed hermetically metal supports for interfacing with the peripheral components of the system. Therefore, in this study, Ag–10 wt% CuO was evaluated as an effective filler material for the reactive air brazing of dense Ce0.9Gd0.1O2–δ–La0.7Sr0.3MnO3±δ (GDC–LSM) OTM ceramics. Thermal decomposition in air and wetting behavior of the braze filler was performed. Reactive air brazing was performed at 1050 °C for 30 min in air to join GDC–LSM with four different commercially available high temperature-resistant metal alloys, such as Crofer 22 APU, Inconel 600, Fecralloy, and AISI 310S. The microstructure and elemental distribution of the ceramic-ceramic and ceramic-metal interfaces were examined from polished cross-sections. The mechanical shear strength at room temperature for the as-brazed and isothermally aged (800 °C for 24 h) joints of all the samples was compared. The results showed that the strength of the ceramic-ceramic joints was decreased marginally by aging; however, in the case of metal-ceramic joints, different decreases in strengths were observed according to the metal alloy used, which was explained based on the formation of different oxide layers at the interfaces.