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Nanoscale Advanced Materials Using Aqueous Combustion Synthesis

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posted on 2005-03-28, 00:00 authored by Kishori T Deshpande
Nanoscale materials are of importance in a number of areas, including fuel cells, catalysis and ferrofluids. In particular, for direct methanol fuel cells, Pt-based catalysts are extensively used for anode and cathode reactions. However, due to problems such as catalyst poisoning and cost, researchers are actively seeking partial/ complete replacement for noble metal electrode catalysts. One possible solution is to replace them with high surface area non-noble metal perovskites. Similarly, for solid oxide fuel cells, interconnects and electrolytes should be fully dense to prevent mixing of gases during operation and possess high electronic and ionic conductivity, respectively. Specifically, lanthanum strontium chromite (LSC) is currently used for interconnect applications. However, due to its low sinterability, researchers are working on enhancing this property either by addition of sintering aids or increasing its surface area. Since the former may deteriorate the electrical conductivity, the latter approach appears promising. 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 /oC), the reaction medium, in the form of a viscous liquid, self-ignites. Further, owing to high exothermicity of the system, combustion temperature rapidly reaches ~1200oC and converts the precursor materials to fine crystalline powders of the desired complex oxide within short time (~ few seconds). It is important to understand the reaction mechanism to predict the final properties such as phase composition and surface area, which would facilitate tailor-made materials. However, it is noteworthy that while a number of publications exist on the use of this technique for synthesis of numerous compositions such as ferrites and perovskites, little work has been done in this direction. Hence in this work, 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. a®Õnand gFe2O3 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. Based on the obtained fundamental knowledge, for the first time in the literature, above powders with well-crystalline structures and surface areas in the range 50-175 m2/g are produced using a single one-step approach, thus avoiding additional calcination procedures. In addition to the above studies, the magnetic properties of the synthesized a-Fe2O3 and Fe3O4 are also measured. Using this approach, for the first time, spherical nanoscale (6-10nm) 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-xCrO3 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. In summary, this research work focuses primarily on understanding the reaction mechanism of aqueous CS for synthesizing nanoscale materials with tailor-made properties. Using the mechanistic details obtained for the binary iron oxide system, after optimizing the processing conditions, different novel complex compounds were synthesized for possible applications in the areas of solid oxide and direct methanol fuel cells.

History

Date Modified

2017-06-05

Defense Date

2005-01-14

Research Director(s)

Prof. Jesus Izaguirre

Committee Members

Prof. Alexander Mukasyan Prof. Paul McGInn Prof. Arvind Varma Prof. Agnes Ostafin Prof. William Strieder

Degree

  • Doctor of Philosophy

Degree Level

  • Doctoral Dissertation

Language

  • English

Alternate Identifier

etd-03282005-094958

Publisher

University of Notre Dame

Program Name

  • Chemical Engineering

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