Vanadium Pentoxide Gels: Structural Development and Rheological Properties
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VANADIUM PENTOXIDE GELS: STRUCTURAL DEVELOPMENT AND RHEOLOGICAL PROPERTIES J.K. BAILEY, T. NAGASE, G.A. POZARNSKY, and M.L. MECARTNEY Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 ABSTRACT Cryogenic transmission electron Microscopy (cryo-TEM) and rheological characterization were conducted in order to understand structural development of vanadium pentoxide gels during processing. Sols were prepared by ion exchange from sodium metavanadate solutions. CryoTEM revealed that fine threads about 1.5nm wide initially form and grow into ribbons approximately 25nm wide and at least 1000nm long. The threads appear to self assemble into the ribbons. During this structural development, the dynamic viscosity increased. Upon steady shearing of the sols, the system exhibited thixotropy, i.e. the viscosity decreased with time under constant shear stress and subsequently rheopexy, the viscosity increased with time. Comparison of the structure before and after shearing indicated that during the rheological experiments aggregation of small particles or fragments was occurring. INTRODUCTION Vanadium pentoxide gels have several useful applications such as anti-static coatings and switching devices. The production of these materials by a sol-gel route is suitable for novel processing applications such as extrusion and thin film coatings. This study investigates the development of microstructure in vanadium pentoxide gels produced from ion exchange of sodium metavanadate solutions through cryogenic transmission electron microscopy and rheological characterization. Some of the earliest work on characterizing the structure of vanadium pentoxide sols was performed by Heller, Watson, and Wojetowicz [1]. The resulting gel structure, examined by viewing dried gels in the TEM was composed of V2 0 5 ribbons 6.5 nm wide by over 500nm long [2]. Recently Livage and coworkers [3,4] have performed extensive studies on the structure and electrical properties of vanadium pentoxide gels produced by ion exchange of sodium metavanadate solutions and have determined by X-ray and electron diffraction that dried gels (xerogels) form layered sheets comprised of V2 0 5 ribbons. While there have been studies which characterize the structure of the xerogels, there has been relatively little work on characterizing how the structure forms and how the processing affects the structure. We report
Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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here preliminary results on direct observation by cryo-TEM of the developing structure in the liquid state and the changes that the material undergoes during prolonged shearing. EXPERIMENTAL The vanadium pentoxide sols were made by first preparing a solution of sodium metavanadate 0.35M - 0.5M in deionized water. The solution was run through an ion-exchange column containing Dowex 50W-X2 resin which had been thoroughly washed with HCI and subsequently rinsed with deionized water [6]. The vanadium concentrations studied varied from 0.
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