Apparent viscosity of sol-gel processed silica

198

Journal of Non-Crystalline Solids 82 (1986) 198-204 North-Holland, Amsterdam

APPARENT VISCOSITY OF S O L - G E L PROCESSED SILICA

T.A. GALLO and L.C. KLEIN Center for Ceramics Research, Rutgers- The State University of New Jersey, PO Box 909, Piscataway, NJ 08854, USA

The densification behavior of a sol-gel processed sihca was studied. Thermogravimetric weight loss analysis was used to study dehydration. Gas (He) pycnometry was used to measure skeletal density and to estimate an apparent fictive temperature. Isothermal shrinkage data were used to calculate apparent viscosities. Isothermal changes in apparent viscosity are due to dehydration and skeletal relaxation. Shrinkage data from step heat treatments were used to separate the effects of relaxation from those of dehydration.

1. Introduction

When a conventionally melted silica glass is heated, its viscosity is expected to decrease because viscous flow is an activated process. The viscosity of a silica gel cannot be described so easily [1]. Once a gel-derived silica has been heated to about 350°C, it can be described as a porous, hydrated glass [2]. When it is heated higher, condensation-polymerization reactions occur. Water is the product of these reactions and the gel is, therefore, being dehydrated. The water content of a gel-derived glass can be expected to change by 3-5 wt% during heating. This change in the composition of the glass affects the activation energy for viscous flow [3], and possibly the pre-exponential. Dehydration of course is expected to increase the viscosity of a glass [4]. The surface tension of a fully hydrated silica surface ( 4 . 5 0 H / n m z) is reported to be 130 erg/cm 2, whereas the surface tension of a fully dehydrated silica surface is reported to be 260 erg/cm 2 [5]. The surface hydroxyl concentration is largely a function of temperature and it decreases with increasing temperature. In gels, densification at higher temperatures occurs by viscous sintering [1]. The energy dissipated in viscous sintering is the reduction in surface energy [5]. This means that the surface tension which increases as the temperature is increased should enhance the sintering of a gel-derived glass. Another interesting observation made about gel-derived silica is that the skeletal density of a gel is usually less than the density of its corresponding melted glass. This lower density or excess free volume can be described as a very high fictive temperature (TF) [7]. Incorporating the effects of fictive temperature in the Arrhenius equation yields: ~ -- ~o exp ~

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T A. Gallo, L.C. Klein / Apparent viscosity of sol-gel processed silica

199

where 0 ~< X ~< 1 [8]. In a glass with a fictive temperature higher than that for the well annealed glass, the viscosity will increase with time until T v reaches T, and eq. (1) reduces to the Arrhenius equation. As a gel is heated, its skeletal density tends to increase and so its apparent fictive temperature decreases. The consequence of this effect is an increase in viscosity with temperature for a period of time. The net effect of dehydration and skeletal relaxation operating simultaneously is both a decrease in viscosity with temperature and an increase in viscosity with time.

2. Experimental techniques The gel chosen for study was 100% SiO 2. A two-to-one volume solution of tetraethyl orthosilicate (TEOS) to ethanol was prepared with 16 mol. of water to one mole TEOS. The solution was catalysed using H N O 3 at a total solution concentration of 0.01 M. After refluxing the solution for one hour, the solution was cooled, and then the flask was attached to a mechanical pump. As primarily ethanol was removed from the flask the density of the solution increased. At a density of 1.10 g / c m 3 samples were cast into 5 ml polystyrene pipettes with one end sealed. Samples were gelled in a bell jar heated to 70°C for 2 days, after which the samples were dried for about one month at 70°C. The resulting monoliths were slightly cloudy and had an axial and radial shrinkage of about 42%. Rods were cut and dry polished from the same section of the dried monoliths. Samples were either 1.27 cm or 0.25 cm long and 0.35 cm in diameter. Weight loss by thermogravimetric analysis (TGA) was measured using a Dupont 1090 Thermal Analyzer, and linear shrinkage by thermomechanical analysis (TMA) was measured using an Orton Model 1500 Automatic Recording Dilatometer. Surface area and pore size were measured using a Quantasorb T M surface area analyzer. Samples were fired to temperatures between 450°C and 850°C at 50°C intervals and held for 16 h or fired to multiple temperatures and held for 4 h at each temperature. All heat treatments were performed using a heating rate of 2 . 0 ° C / m i n to reach the soak temperature in oxygen at a flow rate of 30 cm3/min. The T M A pushrod pressure was not greater than 40 g. Weight loss above 350°C was assumed to be all water. At the same time rod shaped samples were prepared for T G A and TMA, disks, 2.7 cm in diameter and 0.4 cm thick, were prepared for skeletal and bulk density measurements. Four of these disks were fired together at temperatures from 200 to 700°C and soaked for 16 h. After each firing the skeletal and bulk densities were measured. The skeletal density was measured using a Micromeritics 1320 Autopycnometer with He gas. The bulk densities were calculated from measured weights and dimensions.

200

T.A. Gallo, L.C. Klein / Apparent viscosity of sol-gel processed silica

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202

T.A. Gallo, L. C. Klein / Apparent viscosity of sol-gel processed silica

3. Results

Changes in bulk density with time during isothermal heat treatments at 450, 700 and 850°C are shown in fig. 1. The amount of residual water, in the form of hydroxyls, versus time for isothermal heat treatments was calculated from T G A data and is shown in fig. 2. Application of a viscous sintering model [6] to isothermal shrinkage rate data yields apparent viscosities versus time as shown in fig. 3. Combining the residual water data (fig. 2) and apparent viscosity data (fig. 3), the effect of dehydration on viscosity can be seen in fig. 4. To differentiate effects of skeletal changes and dehydration on viscosity, in fig. 5 step heat treatment data have been added to the isothermal heat treatment data (fig. 4).

4. Discussion

As shown in fig. 3, the apparent viscosity is increasing even after 1000 min at 450 and 700°C. At 850°C, the viscosity increases for about the first 300 min, but then levels off. This time corresponds to the time after which there is no measurable water loss (fig. 2). It is often claimed that water only affects the activation energy for viscous flow and not the pre-exponential. If this is the case, a plot of log viscosity versus residual water should yield a straight line. This type of plot is shown in fig. 4. It is clear that water has an effect on viscosity but there is something misleading in this plot. The lines appear to be converging at a low water level. Does this mean that anhydrous silica has the same viscosity over the range 750 to 850°C? This does not make sense. Instead the lines would be expected to converge at a high water level. This must mean that the high apparent fictive temperature suppresses the viscosity to a large degree and this effect is not erased in a 16 h heat treatment. To remove the effect of apparent fictive temperature and isolate the effect of hydroxyls, step heat treatments were tried to anneal out some of the excess free volume. In fig. 5 the result of the step heat treatments is shown, with lines drawn through the points. The difference in viscosity for a given water content and temperature is a measure of the amount of free volume annealed out. The difference is small for the 750°C samples but increases for the 800 and 850°C samples. Samples which were heat treated in steps have a higher water content because they are less dense and have higher surface areas. In this plot the lines appear to be converging at a high water level. This makes more sense that high water levels lower the activation energy and lessen the temperature dependence of the viscosity. The density of conventionally melted silica is 2.202 g / c m 3. The skeletal density of the gel heated to 250°C is only about 1.85 g / c m 3. This corresponds to a fictive temperature of 2500°C. The skeletal density increases to about 1.99 g / c m 3 at 550°C and then levels off. This corresponds to a fictive temperature of 2200°C. At these low temperatures the skeleton has relaxed displacively as

T.A. Gallo, L C. Klein / Apparent viscosity of sol-gel processed silica

203

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m u c h as it can and will continue at much higher temperatures to undergo reconstructive relaxation. The skeletal density is k n o w n to increase again above 800°C. At the same time, the bulk density increases rapidly above 800°C until full density is reached at 1000°C [2].

5. C o n c l u s i o n s

A comparison of isothermal shrinkage data and shrinkage data during heating in steps reveals contributions to the viscosity of gels from both dehydration and structural relaxation. Skeletal densities of gels are remarkably low even at 500°C. The low skeletal density can be assigned to a glass with a high fictive temperature. Taking the high fictive temperature into account, it is not surprising that the viscosity of gels is suppressed and increases for a long period of time before only the effects of hydroxyl are left. By removing the effects of relaxation by successive temperature steps, the effect of dehydration can be isolated.

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T.A. Gallo, L. C Klein / Apparent viscosity of sol-gel processed sifica

References [1] C.J. Brinker and G.W. Scherer, in: Proc. Int. Conf. Ultrastructure Processing of Ceramics, Glasses and Composites (Wiley, New York, 1983) p. 43. [2] L.C. Klein, T.A. Gallo and G.J. Garvey, J. Non-Cryst. Solids 63 (1984) 23. [3] G. Hetherington, K.H. Jack and J.C. Kennedy, Phys. Chem. Glasses 5 (1964) 130. [4] T.A. Gallo, C.J. Brinker, L.C. Klein and G.W. Scherer, in: MRS Syrup. Proc. Better Ceramics through Chemistry, Vol. 32 (North-Holland, New York, 1984) p. 85. [5] R.K. Iler, The Chemistry of Silica (Wiley, New York, 1979) p. 544. [6] G.W. Scherer, J. Am. Ceram. Soc. 60 (1977) 236. [7] R. Bruckner, J. Non-Cryst. Solids 5 (1970) 177. [8] O.S. Narayanaswamy, J. Am. Ceram. Soc. 54 (1971) 491.