Gas sensing properties of nanocrystalline metal oxide powders produced by thermal decomposition and mechanochemical processing
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Doctoral thesis (article-based)
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Report / Helsinki University of Technology, Department of Electrical and Communications Engineering, Laboratory of Electromechanics, 71
AbstractThe objective of this research, was the synthesis of LaFeO3 and SnO2 fine powders for the subsequent preparation of thick film gas sensors. On producing fine metal oxide powders, often it is not possible to ensure separation of the particles during the synthesis, resulting in the formation of highly agglomerated material. In addition, there are often high synthetic costs associated with the powders obtained by these methods. Thermal decomposition and mechanochemical processing methods were selected to produce fine metal oxide powders. Thermal decomposition of a heteronuclear complex is a simple and relatively cheap method. Heat treatment of La[Fe(CN)6] · 4H2O leads to single-phase perovskite-type LaFeO3 fine powders. Heating in the temperature range 600-750 °C causes fast crystallite growth of slightly agglomerated particles and X-ray diffraction analysis showed only the pattern of orthorhombic transition phase of LaFeO3 particles. A paste for the preparation of the LaFeO3 thick film coating was obtained by mixing of polyvinyl alcohol solution and decomposed powder in a ball mill for 1 h. It was determined that there are two factors important for gas sensing, concentration of surface metal ions [Fe3+], and the concentration of oxygen adsorptive sites [Vo(..)]. LaFeO3−δ thick film with small crystallites, promotes a more rapid NO2 gas reaction at the surface and allows an equilibrium state to be obtained at 350 °C. Mechanochemical processing (MCP) is selected as the second, low cost method of manufacturing of fine powders in a conventional ball mill. During milling, deformation, fracture, and welding of powder particles continuously occur. The chemical reactions are activated by the repeated ball-powder collisions. Most of the reports on MCP that have appeared to date, concern the use of high-energy mills. It is shown that it may be possible to produce fine powder particles using a centrifugal mill of the conventional type instead of high-energy one. Nanocrystalline SnO2 powder was produced by two different chemical reactions. The first reaction, initiated by ball milling, produces water and the second reaction does not produce water. It should be noted that water, produced by the chemical reaction during milling, has a considerable influence on the reactivity of surface. Milling of predetermined stoichiometric amounts of SnCl2 with Ca(OH)2 and K2CO3 in an excess of CaCl2 and KCl respectively, resulted in the formation of the desired mass of SnO. After heat treatment and removal of the salt, slightly agglomerated SnO2 particles were produced with a tetragonal phase, confirmed by X-ray diffraction pattern. A very narrow particle size distribution of the powder is observed. The response of the LaFeO3 thick film to NO2 gas is investigated in the temperature range 250-350 °C, where the surface reactions are moderately fast. On exposure to low concentrations of H2S gas in air in the range 20-50 ppm the SnO2 film, prepared from anhydrous powder has higher gas response than the film prepared from hydrated powder.
thermal decomposition, mechanochemical processing, fine metal oxide powders, semiconductor gas sensor
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