Fikret Kirkbir

Drying and sintering of sol-gel derived large SiO2 monoliths

This review article summarizes the development of drying and sintering techniques for the production of sol-gel derived, large silica glass components. Gels may be synthesized using particulate or metal alkoxide precursors, or both in combination. Rapid fracture-free drying has been achieved easily with particulate gels because of their large pore size (100–6000 Å). Alkoxide gels, which generally have small pores (<200 Å), were initially difficult to dry without cracking. However, recent studies have shown that large alkoxide gel monoliths can also be dried in reasonably short times (<10 days). During subsequent heat treatment, alkoxide gels tend to have high shrinkage rates, which may cause trapping of hydroxyl ions or organic groups remaining on the gel surface. Although the removal of these species is easier for particulate gels, their large pore size necessitates heating above 1400°C to achieve full consolidation. Sintering at such temperatures was observed to deteriorate glass quality, through crystallization, warping, and/or sagging. Extensive optimization of the entire process has shown that on a laboratory scale, high-optical-quality glass can be produced from both alkoxide and particulate gels. It remains to be seen whether sol-gel process will be feasible for the manufacture of high-quality glass products on a commercial scale.

Drying of aerogels in different solvents between atmospheric and supercritical pressures

Crack-free SiO2 aerogels were repeatedly dried with little shrinkage (<2%) at subcritical conditions by using various solvents as pore liquids. Gels dried in ethanol showed shrinkage below a threshold pressure of 4.7 MPa. When solvents such as iso-butanol, 2-pentanol and iso-octane were used in drying, this threshold pressure was not observed within the pressure range of this investigation. For example, in iso-butanol the gels showed no shrinkage at a drying pressure of 1.8 MPa which is significantly lower than the critical pressure of 4.85 MPa. Subcritical aerogel drying pressure was reduced by using a suitable drying solvent.

Parametric study of strength of silica gels

Abstract Cylindrical gel bodies were produced by using Si(OC2H5)4, C2H5OH and H2O with various catalysts. A systematic parametric study of gel strength and microstructure as a function of C2H5OH/H2O molar ratio and catalyst concentration in sol, catalyst type as well as aging time, was conducted. The strongest gels were obtained by using H2SO4, HNO3 or HCl as catalysts. Gels catalyzed with oxalic acid were weaker. The strength of the gels increased with a decrease of C2H5OH/H2O ratio. The xerogel microstructures of gels catalyzed with H2SO4, HNO3, HCl, or oxalic acid were very similar, indicating that they had similar permeabilities during the later stages of drying. However, gels catalyzed with HF showed significantly distinct properties. They were much weaker and had larger pores, larger porosity and smaller surface area. Although the strength of HF-catalyzed gels could be increased by increasing the catalyst concentration, aging weakened gels considerably because of the dissolution of the network by HF present in the pores.

DRYING OF LARGE MONOLITHIC AEROGELS BETWEEN ATMOSPHERIC AND SUPERCRITICAL PRESSURES

The objectives of this investigation are to show the feasibility of producing large monoliths with minimal shrinkage at subcritical conditions, and to understand the drying behavior. Crack-free SiO2 monoliths (5.6 cm in diameter and 25 cm in length) were repeatedly dried with little shrinkage (<2%). Some gels showed increasing shrinkage with decreasing pressure. However, this shrinkage was reduced to negligible levels at conditions considerably less than supercritical pressure by increasing both the pore size and the gel strength. This moderate pressure drying (MPD) process may make aerogel fabrication economically more feasible due to reduced pressure chamber costs.

Drying and Sintering of Bulk Silica Gels

Over the last few years, the feasibility of fabricating near net shape silica glass components, using a sub-critical drying process for pure alkoxide gels, has been demonstrated in our laboratories. Cracking during drying, due to capillary forces generated in the gel body, was overcome through two particular innovations. The first was the development and optimization of a dual-catalyzed high strength gel. The second was a controlled atmosphere drying process that allowed the gel to dry utilizing a newly observed phenomenology, postulated to be due to cavitation of the pore fluid. Contrary to conventional wisdom, in this drying approach the smallest pore size gels are the easiest to dry. Details of the types of gels and the drying process are reported. Gels of small size were sintered into crack-free glasses, utilizing conventional sintering approaches. However, large size gels always developed visible surface cracks that formed above 800°C. To successfully dry and sinter large monolithic gels then required re-optimization of the entire process. A great number of micron-range defects were initially detected in these sintered bodies. After analyzing the defects, further steps were taken to improve glass quality to the level of optical glass produced by vapor deposition processes. This included mixing and filtering of sols in a clean room, varying the pore size distribution, and optimizing the pre-sintering and sintering processes. Data of relevant glass quality parameters attained so far in the laboratory are reported.

Drying of alkoxide gels?Observation of an alternate phenomenology: Code: H6

The process of drying of a porous material as per the current phenomenological theory can be divided into two stages. At first the body shrinks by an amount equal to the volume of liquid that evaporates, and the liquid-vapor interface remains at the exterior surface of the body. The second stage begins when the body becomes too stiff to shrink and the liquid recedes into the interior, leaving air filled pores near the surface. We shall refer to this phenomenology as the drying front model. In our investigation of drying of alkoxide silica gels of less than 50 Angstroms pore radius, we have observed a different drying pattern, in which even after the gel body stops shrinking, drying continues to occur by evaporation on the exterior surface of the gel body, causing spontaneous nucleation of partially or fully dried opaque clusters, randomly distributed in the interior parts of the gel. These clusters than increase in number and size till they coalesce to form an opaque body. Upon further drying, the gel returns to its transparent form. We postulate that this is possible only if the rate of fluid flow in the pores by diffusion is faster than that by Darcys flow, as well as the evaporation rate at the surface of the gel body. We shall refer to this as the cluster drying model. We shall present results of pin-hole drying experiments on cylindrical alkoxide gels showing that for identical gels the evaporation rate can be increased to change the phenomenology from cluster drying to one that exhibits both phenomenology simultaneously and finally to that of the drying front phenomenology. We shall also show the effect of gel pore size distribution on the phenomenology of drying under identical drying conditions. Finally, we will present evidence that for successful drying of large cylindrical alkoxide gels, drying conditions favoring cluster drying phenomenology is desirable.

Optical fibers from sol-gel-derived germania-silica glasses

Step index multimode optical fibers were successfully drawn from germania doped silica rods prepared by sol-gel process. The fiber, drawn using rod-in-tube technique, had a 100 micron core with a pure silica cladding of 140 micron. The numerical aperture of the fiber was 0.21. Initial experimental results indicate an attenuation of 20 dB/km at 850 nm wavelength. Precursors used for sol preparation were tetraethyl orthosilicate, Si(OC2H5)4 and tetraethyl orthogermanate, Ge(OC2H5)4. Clear wet gels were routinely produced without any problem of premature precipitation of germanium dioxide even at high dopant concentration levels. The gels were dried by supercritical drying technique. Dry gels were consolidated to clear glass samples routinely at a temperature of 1300 degree(s)C. Fiber was drawn from these rods at a temperature of 1800 degree(s)C. The sintering parameters, i.e., type of gas flow at different steps of the sintering operation, duration of such steps and temperature were optimized to eliminate reboil of the glass above 1800 degree(s)C, resulting in bubble-free glass fibers.

Accelerated subcritical drying of large alkoxide silica gels

Fracture during drying has been the key hurdle in fabrication of large monolithic silica glass from alkoxide gels. Although existing literature suggests pore enlargement, aging, chemical additives, supercritical drying and freeze drying as helpful in avoiding fracture during drying, successful accelerated sub-critical drying of large silica monoliths from alkoxide gels has not yet been reported. In the present approach, acid catalyzed sols of TEOS, ethanol and water (pH equals 2) were cast as cylindrical rods in plastic molds of 8.0 and 10.0 cm diameter with volumes of 2000 cc and 3000 cc respectively. The resultant gels were aged for about 7 days and dried in a specially designed chamber under sub-critical conditions of the pore field. We have obtained monolithic dry gels in drying times of 3 - 7 days for sizes of 2000 - 3000 cc. The dry gels have narrow unimodal pore size distributions, with average pore radius of about 20 angstroms as measured by BET. Although capillary stress during drying increases with reduction of pore size, it was found that in this approach it is easier to dry gels of smaller pore size.

Mechanical and microstructural properties of two-step acid-base catalyzed silica gels

The mechanical and microstructural properties of two-step acid-base catalyzed silica gels were examined as functions of aging time, catalyst concentration, and hydrolysis time. Cylindrical gels were prepared using Si(OC{sub 2}H{sub 5}){sub 4}, C{sub 2}H{sub 5}OH, and H{sub 2}O, with HCl followed by NH{sub 3} as catalysts. Mechanical properties were obtained from three-point bend tests, and the microstructures of dried gels were analyzed using nitrogen adsorption/desorption techniques. Gel strength initially increased with aging time at 70 C, then leveled off after about one week. When the sol was hydrolyzed for less than two hours, there were significant differences in the properties of gels catalyzed with relative molar amounts of 0.0001 and 0.0002 HCl. However, as the hydrolysis time was increased, the gels all had similar properties, independent of the amount of HCl. The amount of NH{sub 3} influenced gelation time and to a lesser extent, the strength, but had no observable effect on pore size. The two-step catalysis procedure produced gels with strength and pore size combinations intermediate to those of either single acid or base-catalyzed gels.

Drying and sintering of large SiO2 monoliths

Sol-gel derived SiO2 glass objects are of interest because of their high purity. However, cracking of wet gels during drying, or of dried gels during hydrocarbon removal and sintering, are the main obstacles preventing the commercialization of this process. These difficulties can be overcome using gels with high strength and large pores. In this study, gels were prepared using Si(OC2H5)4, C2H5OH, H2O and acidic catalysts. The strength, shear modulus, microstructure, drying and sintering behavior of these gels were investigated as functions of catalyst type and aging time. Experiments indicate that the modulus of rupture must be high enough to overcome drying stresses. In addition, the shear modulus must be high enough so that the gels are sufficiently stiff to prevent bending during drying. Repetitive and reliable drying of relatively large gels (D equals 8 - 10 cm, L equals 40 cm) was achieved within a week. However, to obtain crack-free sintered glasses the xerogels must have larger pores (pore radius > 40 angstrom). This is required for easy removal of organic groups at 200 - 500 degree(s)C and of chemically bonded hydroxyl or silanol groups at 800 - 1200 degree(s)C.