Tailoring Sol-Gel Transition Processes for the Design of Novel Shape Metal Oxide Materials
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Tailoring Sol-Gel Transition Processes for the Design of Novel Shape Metal Oxide Materials Martin Järvekülg1, Raul Välbe1, Kathriin Utt1, Martin Timusk1, Tanel Tätte1 1 University of Tartu, Institute of Physics, Riia 142, 51014 Tartu, Estonia.
ABSTRACT In present paper we describe some unconventional adaptions of sol-gel method. Controlled sol-gel transformation processes of metal alkoxide based systems can lead to various novel shapes of metal alkoxide materials. Formation of different structures like tubular microstructures by gel sheet rolling, nano- and microfibres by direct drawing, as well as microtubes of metal oxides and gel dispersed liquid crystal materials are described. Different aspects of sol-gel processes leading to the formation of all of these structures are thereby discussed. INTRODUCTION Sol-gel method, in contrast to conventional high temperature powder sintering technologies, offers a flexible alternative for the production of metal oxide ceramics with relatively low energy consumption. Glassy materials are obtained from liquid-like sols at room temperature, offering many possibilities for doping. As precursors, metal alkoxides that readily undergo hydrolysis and polymerization reactions are used most often. The chemistry has been extensively studied since the early 1980s [1]. Most authors nowadays agree that processes start with the formation of dense partially crystalline metal-oxo nanoparticles, which are sterically stabilized by a shell of alkoxy groups. In later stages, the particles can form different secondary structures like gel networks, clusters, linear chains etc. [2]. The processes can be directed by parameters as solution concentration, pH, temperature, hydrolysis rate, nature of alkoxy groups and metallic center of alkoxides etc. Although the method is widely considered as chemical one, it is usually combined with mechanical shaping of the substance before gelation. Such combination enables the preparation of structures with functional shape, for example thin films [3], fibers [4], patterns on substrates [5] and organically modified surfaces [6]. Structures are formed from a suitable sol precursor that undergoes gelation while preserving the given shape. Sol-gel transformation is thereby a critical step in material synthesis. Gelation is usually accompanied by volume loss that leads to mechanical stress and can result in material cracking if the stresses exceed the strength of the structure. Numerous papers have been published on avoiding shrinkage and cracking by doping, limiting the dimensions of the material or extended processing times. On the other hand, instead of holding back the aforementioned naturally occurring processes, novel forms of oxide material can be obtained by controlling and directing the same. We hereby report our recent progress in the direction of controlling and exploiting the sol-gel transformation for gel film cracking and subsequent rolling of the film segments into tubular microstructures. In addition, micro- and nanofibres can be obtained
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