Structure and Properties of Vanadium Pentoxide Gels
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STRUCTURE AND PROPERTIES OF VANADIUM PENTOXIDE GELS
J. LIVAGE, Spectrochimie du Solide - Universitd PARIS VI - 4, place Jussieu 75230 Paris, France.
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ABSTRACT Transition metal oxide gels can be obtained, through a polycondensation process, by acidification of aqueous solutions. Thin layers can be easily deposited onto a substrate. Their electronic and ionic properties could lead to new developments of the sol-gel process. The semiconducting properties of V 0 gels can be used for antistatic coatings or electrical s~i~ching devices. These gels exhibit a lamellar structure and can be described as particle hydrates. They are fast proton conductors and could behave as a host lattice for intercalation. INTRODUCTION The sol-gel synthesis of glasses and ceramics has received significant attention during the last decade (1-5). Dip-coating appears to be one of the most promising application of the sol-gel process (6). Single oxide coatings already appeared on the market in 1953 and multicomponent oxide coatings in 1969 (7). Most of the layers deposited by dip-coating are based on SiO and TiO 2 . They can be used for automotive rear-view mirrors, antireflection coatings or sunshielding windows (8). Silicate glass layers have also been described (9). They exhibit laser damage threshold four times greater than those of multilayer antireflection films (10). Coating of metals with ceramics via colloidal intermediates provides greater corrosion resistance (11). Transparent electrically conductive layers (12) and superionic conducting Nasicon thick films (13) have also been made by the sol-gel process. Transition metal oxide gels have been known for almost a century (14). The first V 0 gel for instance was reported by A. Ditte in 1885. They have however almMs• never been used in the sol-gel technology. In this paper, we would like to describe some properties of transition metal oxide coatings derived from gels and show that they could lead to new developments of the sol-gel technology (15). Transition metal oxide are usually mixed valence compounds. Metal ions may exhibit several oxidation states so that electron transfer, from low to high valence states, can take place. This leads to specific electrical and optical properties : semiconducting V 0 layers (16), electrochromic WO 3 display devices (17). Magnetic intera~t~ons may also occur when the two valence states have non zero magnetic moments as in ferrofluids (18). Transition metal oxide gels can also be considered as hydrated oxides. Ionic properties arise from the ionization of water molecules, trapped in the gel, by the oxide surface. Some hydrous oxides show high proton conductivities at room temperature (19) and could behave as inorganic ion exchangers (20). This is the case for vanadium pentoxide gels that exhibit a layered structure and could be considered as a host lattice for intercalation of ionic species. Mat. Res. Soc.. Symp. Proc. Vol. 32 (1984) 0 Elsevier Science Publishing Co..
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SOL-GEL SYNTHESIS OF TRANSITION METAL OXIDES Transition metal oxide g
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