Environmental Effects in Gel Derived Silicates
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ENVIRONMENTAL EFFECTS IN GEL DERIVED SILICATES L.L. HENCH Ceramics Division, Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 INTRODUCTION Various methods have been developed for producing crystals, powders, coatings and monoliths from gels [il. The scientific basis for understanding compositional effects, gelation, aging, drying and densification is also advancing rapidly [21. However, there is as yet relatively little information on the durability, weathering, or corrosion resistance of glasses, glass-ceramics, ceramics, or composites made via the gel route. Data is also sparse on the effects of vacuum, thermal exposure, or mechanical stress on the stability of gel-derived solids. Relationships between sol-gel processing variables and environmental stability are especially lacking at the present time. Since many end-use applications of gel derived materials involve exposure to severe environments, it is essential that the durability of these materials be established during their development. Consequently, the goal of this paper is to review the current status of understanding: (1) the mechanisms of weathering, corrosion and atmospheric attack of silicate glasses and glass-ceramics; (2) the effects of atmosphere on pre-densified gels, the phases developed, and the consequences of those phases on densification; (3) the rates of aqueous corrosion of gel-derived vs. melt-derived glasses; and (4) the weathering resistance of gel-derived coatings. Corrosion Mechanisms Studies of the interaction of many glass compositions with a variety of environments have led to the identification of at least ten types of corrosion phenomena which are summarized in Table I. A glass or a glassceramic may encounter one or more of the corrosion processes simultaneously or sequentially. The durability of gel-derived materials is controlled by these ten processes. TABLE I.
Corrosion Processes
1. Ion Exchange or Selective Leaching Involves the exchange of mobile species from the glass with protons or hydronium ions from the solution. Results in surface film formation. 2. Network Dissolution (Congruent and Surface Film Dissolution) Involves breaking down of structural bonds in the glass or surface film. May occur uniformly or locally. 3. Pitting Localized network dissolution due to surface heterogeneities, stresses or defects. 4. Solution Concentration Involves the concentration of the solution with respect to species from the glass. May result in a reduction of the corrosion rate. 5. Precipitation Involves the formation of insoluble compounds on the glass surface by reaction of the dissolved constituents from the glass with species already present in the solution. May be influenced by solution pH.
Mat.
Res. Soc. Symp.
Proc.
Vol.
32 (1984)
Published by Elsevier Science Publishing Co., Inc.
102
Stable Film Formation Involves the alteration of the glass surface composition by either diffusion processes or interfacially controlled reactions. 7. Surface Layer Exfoliation Involves the f
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