RESUMO
Oxygen vacancy control has been one of the most efficient methods to tune the physicochemical properties of conventional oxide materials. A new conceptual multi-principal oxide (MPO) is still lacking a control approach to introduce oxygen vacancies for tuning its inherent properties. Taking multi-principal rare earth-transition metal (CeGdLa-Zr/Hf) oxides as model systems, here we report temperature induced oxygen vacancy generation (OVG) phenomenon in MPOs. It is found that the OVG is strongly dependent on the composition of the MPOs showing different degrees of oxygen loss in (CeGdLaZr)Ox and (CeGdLaHf)Ox under identical high temperature annealing conditions. The results revealed that (CeGdLaZr)Ox remained stable single phase with a marginal decrease in the band gap of about 0.08 eV, whereas (CeGdLaHf)Ox contained two phases with similar crystal structure but different oxygen vacancy concentrations causing semiconductor-to-metal like transition. Due to the intrinsic high entropy, the metallic atoms sublattice in (CeGdLaHf)Ox remains rather stable, regardless of the interstitial oxygen atoms ranging from almost fully occupied (61.84 at%) to almost fully empty (8.73 at%) state in the respective crystal phases. Such highly tunable oxygen vacancies in (CeGdLa-Zr/Hf) oxides show a possible path for band gap engineering in MPOs for the development of efficient photocatalysts.
RESUMO
Discovery of advanced soft-magnetic high entropy alloy (HEA) thin films are highly pursued to obtain unidentified functional materials. The figure of merit in current nanocrystalline HEA thin films relies in integration of a simple single-step electrochemical approach with a complex HEA system containing multiple elements with dissimilar crystal structures and large variation of melting points. A new family of Cobalt-Copper-Iron-Nickel-Zinc (Co-Cu-Fe-Ni-Zn) HEA thin films are prepared through pulse electrodeposition in aqueous medium, hosts nanocrystalline features in the range of ~ 5-20 nm having FCC and BCC dual phases. The fabricated Co-Cu-Fe-Ni-Zn HEA thin films exhibited high saturation magnetization value of ~ 82 emu/g, relatively low coercivity value of 19.5 Oe and remanent magnetization of 1.17%. Irrespective of the alloying of diamagnetic Zn and Cu with ferromagnetic Fe, Co, Ni elements, the HEA thin film has resulted in relatively high saturation magnetization which can provide useful insights for its potential unexplored applications.
RESUMO
The oxidation behaviour of tungsten disulphide (WS2) nanosheet powder with an average thickness of about 10 nm was studied in the temperature range of 25-700 °C. The samples were subjected to exposure in air in a short continuous mode as well as extended isothermal holding. It was observed that WS2 nanosheets were stable below 350 °C in air for short exposure times. A two-dimensional WS2/WO3 heterostructure evolved at 350 °C on short exposures to the oxidising atmosphere. Complete oxidation of WS2 nanosheets was observed at a temperature of about 450 °C in the continuous heating mode and at 350 °C under isothermal holding for an extended exposure time. During oxidation, WS2 nanosheets were initially transformed to 2D-WO3 nanosheets with an average thickness of about 10 nm. Significant distortion in the monoclinic structure of 2D-WO3 was observed. At higher temperatures, the WO3 nanosheets disintegrated initially to rod-shaped WO3 particles, which were subsequently transformed to thermodynamically stable spherical shaped WO3 nanoparticles.
RESUMO
Structural changes during the deformation-induced synthesis of nanocrystalline Fe-10Cr-3Al alloy powder via high-energy ball milling followed by annealing and rapid consolidation by spark plasma sintering were investigated. Reduction in crystallite size was observed during the synthesis, which was associated with the lattice expansion and rise in dislocation density, reflecting the generation of the excess grain boundary interfacial energy and the excess free volume. Subsequent annealing led to the exponential growth of the crystallites with a concomitant drop in the dislocation density. The rapid consolidation of the as-synthesized nanocrystalline alloy powder by the spark plasma sintering, on the other hand, showed only a limited grain growth due to the reduction of processing time for the consolidation by about 95% when compared to annealing at the same temperature.