Novel Material Properties Based on Flame-synthesized Nanomaterials

Abstract

The principles of high-temperature reactive particle formation in flames are characterized by a sequence of partly interacting rate processes in the gas flow, while the necessary energy is delivered by the exothermic combustion reaction heating the flow to high temperatures. A complete description of the precursor decomposition kinetics and the subsequent oxidation/hydrolysis reactions is rarely obtained, while the properties of the products manufactured such as size, morphology, phase composition, and crystallography are decisively influenced by these parameters. A precise understanding and control of the initial steps is therefore required to open up the possibility of tuning particle properties.
In the present study, the formation of oxidic particles in flame reactors is presented. It will be shown that the stoichiometry and crystallography of oxides such as ZnO, SnO₂ and TiO₂, and therefore their physical and chemical properties, can be adjusted depending on the reaction conditions. In addition to the synthesis of pure materials, coated particles as well as nanocomposites are accessible when a few requirements are fulfilled. In the case of immiscible oxides such as TiO₂ and SiO₂, composites consisting of separate phases are produced, while the formation of composites from miscible compounds usually requires a two-step process that tends to produce poorly mixed materials. Nevertheless, in the case of kinetically controlled synthesis, a one-step formation of nanocomposites from miscible oxides can be realized when the kinetics of precursor decomposition and particle formation of the participating oxides are quite different. This results in materials that exhibit new properties according to the used oxides. As an example, the one-step formation of homogeneously dispersed superparamagnetic Fe₂O₃ in fumed silica will be shown. Chemically, this material behaves like common silica but due to the superparamagnetic characteristics of the embedded iron oxide, it can be heated in a contactless manner by means of an alternating magnetic field. Applications focusing on contactless hardening and bonding become apparent.