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Preparation of ZrW2 O8 Xerogel and Aerogels by Sol-Gel Chemistry

Xerogels and aerogels with the composition ZrW2O8 have been synthesized using sol-gel chemistry. For the xerogel, trigonal phase is formed at 650 °C and the cubic phase forms above 1100 °C in less than 30 seconds. Aerogels have been prepared with an average pore diameter of 110 Å and a surface area of 510 mg. Introduction ZrW2O8 exhibits large isotropic negative thermal expansion over its entire stability range (α = -9.07 x 10 K from 0.3 to 350 K)[1] and so has potential uses in many applications including actuators, electronics, optics, fuel cells, oxygen and temperature sensors, dental filling products, household ceramicware or any application requiring dimensional thermal stability. For most of these applications, it is envisioned that cubic ZrW2O8 is used as filler in a composite material. Recent theory has highlighted the correlation between the particle size of the filler material and the thermal expansion of the composite.[2, 3] For smaller filler particles, especially for nanoscale particles, there is a decrease in thermal expansion of the composite due to an increase in the interfacial region between the filler and matrix materials. Also, as demonstrated by gold nanoparticles, nanoscale materials may exhibit surprisingly unique thermal expansion properties when compared to their bulk counterparts.[4] Nanoparticles of cubic ZrW2O8 have not been reported due to difficulties in processing. Attempts to mechanically decrease the particle size of as synthesized cubic ZrW2O8 results in amorphization, and amorphization has been reported even for heavy grinding in an agate mortar.[5] Although cubic ZrW2O8 is thermodynamically stable only over a narrow temperature range (1105 – 1257 °C)[6], once formed it is kinetically stable up to ~ 770 °C. The cubic phase results from the solid state reaction between ZrO2 and 2 WO3, but the reaction does not reach completion even after several days at high temperatures (1230 °C) with intermittent grinding. This high temperature reaction is complicated by volatilization of WO3 at the reaction temperature and products tend to be multiphase.[7] Obviously the solid state synthetic route is problematical for this technologically important material. Several soft chemistry routes to cubic ZrW2O8 have been reported. Mixing aqueous solutions of zirconium oxynitrate and ammonium metatungstate yields a white precipitate that forms the desired single phase product when heated to 1150 °C.[8] Refluxing the precipitate in HCl produces ZrW2O7(OH)2(H2O)2, which can be dehydrated to form fairly pure cubic ZrW2O8 around 600 °C.[9] Cubic ZrW2O8 prepared at low temperatures from the hydrate was reported to have an average particle size close to 1 μm. Another preparatory route involves heating a mixture of WCl3 suspended in CHCl3 with a solution of zirconium isopropoxide isopropanol complex dissolved in THF to 110 °C for a week in a sealed tube.[10] This results in an amorphous precipitate which transforms to cubic ZrW2O8 when heated to 1200 °C for 15 minutes. None of these synthetic routes is suitable for preparation of transparent gels, aerogels or nanoscale materials. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 363-366 doi:10.4028/www.scientific.net/KEM.264-268.363 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,18:13:12) Here we report the preparation of both transparent gels as well as precipitate gels with the nominal composition ZrW2O8, aerogels and xerogels prepared from these gels, and phase formation as a function of temperature for the xerogels. Aerogels are inherently nanomaterials, as they are a combination of empty pores and a framework matrix on the order nanometers.[11, 12] Aerogels have high surface areas, thus a ZrW2O8 aerogel composite will have a high interfacial region.