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EFFECTS OF TRIOXANE ON SILICA GEL XIAOMING LI and P.F. JOHNSON New York State College of Ceramics at Alfred University, Alfred, NY 14802 ABSTRACT Hydrolysis, condensation and thermal reactions of silica gels derived from TEOS with and without 1,3,5-trioxane (C 3 H60 3 ) were studied. Effects of trioxane on viscosity and gelation time of solutions were determined. Specific surface, density, porosity and microhardness of gels were measured. The behavior of gels at elevated temperature was clarified using DTA, TGA, and dilatometry. The properties and structure of trioxane containing sol/gels may be explained by the role trioxane played in the sol-gel transition. The results of this investigation concerning sol-gel/gel-glass transitions, structure and properties of gels are presented. INTRODUCTION Trioxane (C3 HeO 3 ) has been used as an additive to improve the optical and physical properties of porous silica glasses derived from alkoxide precursors through the sol-gel route [1]. The effects of trioxane on sol-gel reactions in the HF-catalyzed TEOSC 2 HsOH-H 2 0 system have been studied by time-dependent Raman, FTIR and NMR spectroscopic techniques [2]. While previous studies concentrated on the effects of trioxane on hydrolysis kinetics of sols and on the structure and physical properties of gel derived glasses, the question of the extent to which trioxane can affect the thermal reaction, gel-glass transition and high temperature pyrolysis of gels has not been addressed. A knowledge of the thermal evolution of gels containing trioxane is very important for better understanding the effects of trioxane on sol-gel and gel-glass transition processes and for the selection of suitable thermal treatment schedules to remove gases and hydroxyl groups from the gels without breaking or bloating. The objectives of this work were to investigate reactions and transitions of silica gels containing trioxane at elevated temperatures and to clarify the role of trioxane in sol-gel and gel-glass transition. EXPERIMENTAL A standardized composition with the molar ratio of TEOS:H 20:C 2H6 OH:additives The codes, additives and some sol-gel transition properties for different solutions are reported in Table I. The sols were prepared, typically, by mixing 42.1 ml of TEOS, 13.62 ml of water, 44.06 ml of ethanol and additives with vigorously stirring at room temperature until the solution was homogeneous. The resulting solutions were then poured in plastic containers with or without covers and allowed to gel at room temperature. = 1:4:4:0.1 was selected for all the solutions studied.

Table I. Codes, additives and gelation data for the test solutions* Code - Additive

A B T N

NH 4 OH

Trioxane None

pH(to)

GT(O)

i.5 105

GT(C)

116

WL(T)

8-i.5

WL(G)

1i6-.4

8.0

125

292

87.2

42.6

5.45 5.5

136 185

420 768

59.3 42.8

50.0 35.0

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Gel Appearance

Trainsparent, cracksTransparent, cracks Translucent, fewer crack |Translucent, more crack

* pH(to), initial pH value; GT(O), gelation time (hrs.) in open system by viscometer measurement;