Controllable Preparation Of NdFeO₃ -Based Micro/Nanoscale Structures And Their Properties

NdFeO3, which has good chemical, thermal and mechanical stability, is an important candidate material for gas sensors and intermediate-temperature solid oxide fuel cells (IT-SOFC). Since the morphology and composition have an effect on the properties of materials, the controllable preparation help us to investigate the relationship between them and design new materials with the properties we need. In this dissertation, the current status of studies on typical rare earth orthoferrities was reviewed briefly at first. The research progress and main problems about structure and properties of this kind of material have been summarized, and the various morphology and synthesize method have also been summarized and analyzed. Based on this analysis, the controllable preparation of NdFeOh was prepared by hydrothermal method, and then the synthesis conditions was modulated by analyzing their grow mechanism. The Nd1-xCaxFeO3 (x=0,0.3) core-shell hollow spheres were prepared by solid template method and their sensing properties were investigated. Finally, the first-principles calculation could help us to improve our understanding of the colossally increased conductivity of Ca doped NdFeO3. The work includes the following.(1) NdFeO3 was successfully synthesized by hydrothermal method, and the influence of the temperature, time, the concentration of KOH solution and mineralizer on products were discussed. It was found the morphologies of the final products strongly depend on the concentration of KOH solution. When the concentration of the KOH solution was adjusted from 16 M to 19 M, cuboid, cross-shaped and bar-shaped NdFeO3 crystals were obtained respectively. The bar-shaped NdFeO3 crystals are preferentially oriented along the direction of [010]. The magnetic properties of NdFeO3 crystals with different shapes were also explored, weak ferromagnetism was observed in all the samples, and NdFeO3 crystals with small sizes have the largest coercive field Hc. The studies for time-dependent phase evolution showed that the NdFeO3 was crystalized by the dissolution and precipitation of Fe2O3 microplates and Nd(OH)3 nanorods, and the control step is the dissolution of Fe2O3 microplates. Based on this result, the synthesis condition was modulated, and the hydrothermal time could be optimized.(2) Nd1-xCaxFeO3(x=0,0.3) core-shell hollow spheres were prepared by using carbonaceous microspheres as templates. The gas sensing properties of Nd0.7Ca0.3FeO3 core-shell hollow spheres were systematically investigated. The sensitivity to 500 ppm ethanol was 17 at 140 ℃, which is much higher than that of Nd0.7Ca0.3FeO3 nanoparticles. Also, the rapid response and recovery characteristics were achieved. Meanwhile, after 10 testing days and 50 cyclic tests, the gas response of the Ndo.7Cao.3Fe03 core-shell hollow spheres to 500ppm ethanol at 140℃ was maintained well and nearly constant. The long-term stability, good sensitivity and rapid response and recovery times demonstrate its potential application as the sensing material.(3) Through the electronic structure analysis of NdxCa1-xFeO3-δ (x=0.00,0.25,0.50, 0.75,1.00; δ=0.00,0.25) based on the first-principles calculations, it is found that the hole states introduced by Ca substitution appear just above the Fermi level, which implies a high mobility of electrons/holes along the Fe-O-Fe bonding network. Specifically, it becomes easier to form O vacancies after Ca doping. Since the diffusion of O2- anions occurs through a vacancy hopping mechanism, the ion conductivity is also improved. These findings help us to improve our understanding of the colossally increased conductivity of Ca doped NdFe O3 and turn the electronic conduction for its practical application in gas sensors and IT-SOFC.