| Current synthesis techniques for preparing nanomaterials including sol-gel and coprecipitation pose problems in attaining desired monophase composition, crystallinity or nanostructured powders in short time. Aqueous combustion synthesis (CS) offers a promising solution to this issue.; Aqueous CS is a novel route to prepare advanced materials including perovskites, ferrites and zirconia. One version of this process involves a self-sustained reaction between metal nitrate solutions and fuels (e.g. glycine, hydrazine, etc). Specifically, after preheating to moderate temperature (∼115°--150°C), the reaction medium, in the form of a viscous liquid, self-ignites.; An attempt to study the mechanistic details of aqueous CS has been made by using a simple iron oxide system. For this, synthesis of three major iron oxide phases, i.e. alpha- and gamma-Fe2O 3 and Fe3O4, using the combustion approach and a combination of simple precursors such as iron nitrate and oxalate, as well as different fuels is investigated.; In addition to the above studies, the magnetic properties of the synthesized gamma-Fe 2O3 and Fe3O4 are also measured. Using this approach, for the first time, spherical nanoscale (6--10mn) iron oxide particles with excellent ferrimagnetic properties are synthesized. While the samples had particle size <10nm, they exhibited ferrimagnetic behavior at room temperature, as opposed to super paramagnetism as reported previously by numerous workers. Further, particularly for Fe3O4, the coercivity values are exceptionally high (213 Oe), indicating stable magnetization.; In the second part of this work, the applications of aqueous CS in areas such as solid oxide and direct methanol fuel cells are investigated. Specifically, for LaxSr1-xCrO 3 system used in solid oxide fuel cell interconnects, it is shown that synthesis of perovskites under the self-propagating high-temperature mode produced powders with high specific surface area (∼40 m2/g) and well defined crystalline structure. As a result, ceramics sintered by using these powders are dense (∼96% of theoretical) and possess high electronic and low ionic conductivities, important for interconnect applications.; For direct methanol fuel cell anodes, using the above synthesis method and a high throughput screening approach, a variety of high surface area catalysts including perovskites and oxides are synthesized and tested as anode catalysts. It is found that the Sr-based perovskites showed performance comparable with the standard Pt-Ru catalyst. Further, it is observed that the method of doping SrRuO3 with Pt influenced the activity. Specifically, platinum added during aqueous CS yields better catalyst than when added externally at the ink preparation stage. Finally, it is also demonstrated that the presence of SrRuO3 significantly enhanced the catalytic activity of Pt, leading to superior performance even at lower noble metal loadings. (Abstract shortened by UMI.)... |
