Sol-gel process using porous mold

A sol-gel process for producing dry porous gel monoliths, e.g., silica glass monoliths, in which the successive process steps of gelling, aging and drying all occur within a mold formed of a porous material, e.g., graphite. The mold is inert to the gel solution and it has sufficient strength to withstand the temperatures and pressures encountered during the process, yet it has sufficient porosity to facilitate the escape of liquid from the gel pores directly through the mold, itself. The mold and gel thereby can remain within a sealed autoclave during these process steps, and mechanical handling of the mold and the gel are minimized. This substantially enhances the process' efficiency. Alternatively, the mold can have a non-porous inner skin.

BACKGROUND OF THE INVENTION 
This invention relates generally to sol-gel processes for producing dry gel 
monoliths that subsequently can be sintered into glass articles and, more 
particularly, relates to sol-gel processes of this kind using molds 
specially configured to enhance the process' effectiveness. 
Substantial efforts have recently been expended in developing improved 
sol-gel processes for fabricating high-purity monolithic articles of 
glass. In such processes, a desired solution, i.e., a sol, containing 
glass-forming compounds, solvents, and catalysts, is poured into a mold 
and allowed to react. The solution typically includes tetraethyl 
orthosilicate, water, an alcohol, and an acid and/or base catalyst. 
Following hydrolysis and condensation reactions, the sol forms a porous 
matrix of solids, i.e., a gel. With aging, the gel shrinks in size by 
expelling fluids from the pores of the gel. The wet gel is then dried in a 
controlled environment, typically by removing the gel from the mold and 
placing it into an autoclave for subcritical or supercritical heating. The 
dried gel then is sintered into a solid monolith. 
Advantages of the sol-gel process include chemical purity and homogeneity, 
flexibility in the selection of compositions, the ability to process at 
relatively low temperatures, and the producing of monolithic articles 
close to their final desired shapes, thereby minimizing finishing costs. 
The efficiency of the process can be enhanced if the steps of gelling, 
aging and drying all are carried out within a single chamber and without 
the need to remove the gel from the mold. The need to remove the gel from 
the mold at an intermediate step of the process not only requires 
mechanical handling of the fragile gel and mold, but also lengthens the 
processing time. This is because removing the gel from the mold following 
the step of aging can be performed only after the gel has cooled to room 
temperature from its aging temperature, e.g., 60.degree. C. 
An important factor bearing on the ability to perform the entire sol-gel 
process without removing the gel from the mold is the nature of the 
material from which the mold is formed. The ideal mold material should 
have good release characteristics, such that the fragile monolithic gel 
can be removed from the mold without damage. 
The mold material also should be inert to attack from chemicals used in the 
sol-gel process, e.g., acid catalysts such as hydrochloric acid (HCl) and 
base catalysts such as ammonium hydroxide (NH.sub.4 OH). This requirement 
effectively precludes the use of molds formed of metal, because metal 
impurities could be leached from the mold and trapped in the gel, thus 
being retained in the glass monolith. Metal impurities retained within a 
glass monolith are particularly undesirable, because they can reduce the 
transmission of ultraviolet light. Such leaching also can reduce the 
mold's life span. 
If the gel is to be dried while still located within the mold, the mold 
material must be able to withstand typical drying temperatures, e.g., 
200.degree. C. and above. This means that the mold must not decompose at 
such temperatures and it should not deform when repeatedly cycled between 
room temperature and the maximum drying temperature. This requirement 
effectively excludes the use of molds formed of common polymeric materials 
such as polymethyl pentane and Teflon, which have softening temperatures 
substantially lower than 200.degree. C. 
Some refractory ceramics, e.g., silicon carbide, boron nitride and 
carborundum, can survive the required high temperatures and are reasonably 
inert, making them suitable for use as mold materials. However, such 
materials are difficult and expensive to machine into the required mold 
shapes. In addition, these materials do not generally have good release 
characteristics, and gels can sometimes adhere to molds made of these 
materials. 
It should therefore be appreciated that there is a need for a sol-gel 
process in which the steps of gelling, aging and drying all are carried 
out without removing the material from the mold. The present invention 
fulfills this need and provides further related advantages. 
SUMMARY OF THE INVENTION 
The present invention resides in an improved sol-gel process for producing 
a dry porous gel monolith, in which the process steps of gelling, aging 
and drying all are carried out while the gel remains within a mold, thus 
substantially reducing mechanical handling of the gel and mold and 
substantially enhancing the process' efficiency. More particularly, the 
process incorporates steps of 1) placing a solution into a mold formed of 
a porous material such as graphite, silicon carbide, titanium carbide, or 
tungsten carbide, 2) allowing the solution to gel within the mold, 3) 
drying the gel within the mold, and 4) removing the dried gel from the 
mold to obtain the gel monolith. 
The process has particular advantages when used to produce gel monoliths in 
the form of high-purity silica. In such applications, the solution 
consists essentially of tetraethyl orthosilicate, an alcohol, deionized 
water, and an acid catalyst and/or a base catalyst, in prescribed relative 
proportions. In addition, the process can further include a step of aging 
the gel within the mold, before the step of drying, and a further step of 
sintering the dried gel after the step of removing. 
In one configuration, the mold is configured to be substantially 
homogeneous, with sufficient porosity to allow liquid present in the pores 
of the gel to escape therethrough during the step of drying. In an 
alternative configuration, the mold is configured to have a porous body 
with a substantially non-porous inner skin. In that alternative 
configuration, the pore liquid escapes from the narrow annular space 
between the graphite mold and the gel. In both configurations, the mold 
preferably has a substantially uniform thickness in the range of 3 to 5 
mm. In the case of molds formed of graphite, the graphite preferably has a 
bulk density of about 1.75 gm/cm.sup.3 and a porosity in the range of 
about 10 to 15%. 
In other more detailed features of the invention, the steps of allowing the 
solution to gel, aging the gel, and drying the gel all occur while the 
solution and gel remain located within the mold. In addition, these steps 
all occur while the mold is located within an autoclave. The step of 
drying the gel in the autoclave can occur either under subcritical or 
supercritical conditions. 
Other features and advantages of the present invention should become 
apparent from the following description of the preferred process, taken in 
conjunction with the accompanying drawing, which illustrates, by way of 
example, the principles of the invention.

DESCRIPTION OF THE PREFERRED PROCESS 
With reference now to the illustrative FIGURE, there is shown a mold 11 
located within an autoclave 13, for use in a sol-gel process for producing 
crack-free silica monoliths. The mold has a size and shape substantially 
the same as that desired for the monolith to be produced, and it is formed 
of a porous graphite material, which enables the successive sol-gel 
process steps of gelling, aging and drying all to be carried out without 
the need to remove the gel from the mold. 
In an initial step of the process, a suitably hydrolyzed silicon alkoxide 
sol is poured into the mold 11 and allowed to gel at room temperature for 
about 16 hours. One suitable sol can comprise tetraethyl orthosilicate 
(TEOS), ethanol, deionized water, hydrochloric acid, and ammonia, in 
relative molar proportions of about 1:1.5:4:0.0001:0.0003, respectively. 
Alternatively, tetramethyl orthosilicate (TMOS) can be substituted for the 
TEOS. 
After the sol has gelled to form a gel 15, a suitable amount of fresh 
liquid 17 is added to the mold 11, to top off and fully immerse the gel. 
This helps to prevent the gel from cracking during the subsequent step of 
aging. It also eliminates the need to use a lid on the mold, thus 
simplifying the process. The composition of this added liquid preferably 
is the same as the pore liquid contained within the gel. 
The graphite mold 11 with the immersed gel 15 is then introduced into the 
autoclave 13, where it is elevated above the floor of the autoclave on a 
support 19, and the temperature within the autoclave is raised to about 
6.degree. C. over a span of about six hours, and maintained at that 
temperature for about 42 hours. During this aging step, a saturated 
ambient is maintained within the autoclave by providing an excess of pore 
liquid on the floor of the autoclave, as indicated by the reference 
numeral 21. The aging step effectively increases the gel's average pore 
size and strengthens the gel, so as to reduce the gel's susceptibility to 
cracking during the subsequent step of drying. 
After the aging step has been completed, a drying solvent, e.g., 
isopropanol, is introduced into the autoclave 13, and the temperature and 
pressure within the autoclave are raised according to prescribed profiles. 
Drying can be achieved using both subcritical and supercritical 
procedures. One suitable subcritical drying procedure is disclosed in U.S. 
Pat. No. 5,473,826, which is incorporated by reference. 
Significantly, the step of drying is performed without first removing the 
gel 15 from the mold 11. The mold's porosity facilitates this drying by 
allowing the liquid contained within the gel's pores to escape directly 
through the mold, itself. In the preferred process, the mold is 
homogeneous and formed of an isomolded, fine-grained graphite material 
having high thermal conductivity and high strength. One suitable graphite 
material is available from Le Carbone-Lorraine, under the name Graphite 
Grade 2191. It has a bulk density of about 1.74 gm/cm.sup.3, and it has a 
porosity of about 13%. 
Alternatively, the graphite mold 11 can incorporate a non-porous, 
mirror-like portion defining its inner skin or wall. In that alternative 
case, the liquid evaporates from the narrow annular space between the 
mold's inner wall and the wet gel's outer surface. In addition, the mold 
can be formed of porous carbide materials such as silicon carbide, 
titanium carbide, tungsten carbide, and mixtures thereof All of these 
materials are inert to the alkoxide solution and are able to withstand 
drying temperatures of up to 300.degree. C. 
The mold 11 has a size and shape substantially the same as that desired for 
the monolith to be produced, and it preferably has a uniform thickness in 
the range of about 3 to 5 mm. A minimum thickness of 3 mm will ensure that 
the mold has adequate structural integrity, and a maximum thickness of 5 
mm will ensure that the mold will not unduly inhibit the escape of pore 
liquid during the step of drying. When a mold having a non-porous inner 
skin is used, the inner skin preferably has a uniform thickness less than 
about 1 mm. 
Presented below is a more detailed description of the preferred process for 
producing dry gel monoliths using a mold 11 formed of a porous graphite 
material. Although the process is described with particular reference to 
silica gel monoliths formed of silica, it will be appreciated that the use 
of a mold formed of a graphite material can enhance the efficient 
production of other aerogel or xerogel monoliths as well. 
Mold Cleaning--For the graphite mold 11 to have the desired release 
properties, it is important that it be thoroughly cleaned of particles 
remaining inside the pores of the mold from a previous casting. This can 
be achieved by first immersing the mold in a dilute 7% hydrofluoric acid 
(HF) solution for 30 minutes, followed by an ultrasonic HF bath for 20 
minutes. The mold then is immersed in deionized water for 30 minutes, 
followed by two successive ultrasonic baths in deionized water, for 20 
minutes each. Finally, the mold is placed in a clean drying oven to dry. 
Sol Preparation and Casting--The prescribed sol is mixed in a reactor 
vessel that has been appropriately cleaned with a dilute 7% HF solution. 
In the preferred process, for producing a silica gel monolith, this sol 
incorporates TEOS, ethanol, deionized water, HCl, and NH.sub.4 OH, in 
relative molar proportions of about 1:1.5:4:0.0001:0.0003, respectively. 
The sol is then transferred to the previously cleaned graphite mold 11, 
while located in a Class 100 laminar flow hood. 
Gelation--The graphite mold 11, with the cast sol, is then introduced into 
the autoclave 13, for gelation. After the autoclave has been sealed, the 
sol is allowed to gel at room temperature for 16 hours. At that time, an 
aging solution having a composition of about 88% ethanol and 12% deionized 
water is pumped into the autoclave, to substantially fill the autoclave, 
The autoclave then is drained, leaving the mold topped off with the liquid 
and further leaving sufficient liquid 21 remaining on the floor of the 
autoclave to maintain a saturation pressure of 9 psi at 60.degree. C. This 
topping liquid is added to inhibit cracking of the gel during the 
subsequent aging step and further to eliminate the need for a lid on the 
mold, thus simplifying the process. The FIGURE depicts the autoclave and 
mold at this stage of the process. 
Aging--After the graphite mold 11 has been topped off with the prescribed 
aging solution, the temperature of the autoclave is linearly ramped up to 
60.degree. C. over a time span of 6 hours, and the temperature is then 
maintained at that temperature for a further 42 hours. This completes an 
in-situ aging step, in which the average pore size in the gel 15 is 
increased to a point where the gel can properly avoid cracking during the 
subsequent drying step. 
Drying--After the step of aging has been completed, the aging solution 21 
that remains on the floor of the autoclave 13 is drained away and pure 
isopropanol is pumped into the autoclave at a pressure of 9 psi, while the 
temperature is maintained at 60.degree. C. About 1500 milliliters of 
isopropanol are added for an autoclave having a volume of 20 liters. The 
temperature of the autoclave is then linearly ramped up to 240.degree. C. 
and allowed to equilibrate at that temperature for one hour. This 
typically increases the pressure to about 620 psi. The pressure within the 
autoclave is released over a period of about five hours, while the 
240.degree. C. temperature is maintained. Finally, the autoclave is cooled 
to room temperature and the graphite mold 11 and gel 15 are removed. 
The dried, crack-free monolithic aero gel 15 then can be readily removed 
from the graphite mold 11. Following sintering, a pure silica monolith of 
optical quality is obtained. The mold then can be used again to produce 
further dry porous gel monoliths, if it is appropriately cleaned in the 
manner described above. 
The special use of a mold 11 formed of graphite substantially enhances the 
efficiency of the sol-gel process. The use of this material is 
particularly effective, because it allows the successive sol-gel process 
steps of gelling, aging and drying all to be carried out without the need 
to remove the gel from the mold. The porosity of the mold facilitates the 
desired solvent exchange and also the escape of the pore liquid directly 
through the mold, itself, during the drying step. 
Graphite also is a particularly advantageous material for the mold 11, 
because it can withstand temperatures greater than 300.degree. C., under 
the specified drying conditions, without deforming or decomposing. In 
addition, graphite does not chemically interact with the specified sol, 
and it exhibits good mold release properties under controlled conditions. 
If any graphitic carbon is incidentally introduced as an impurity into the 
gel monolith, it can be readily removed during the sintering operation. 
Finally, molds formed of graphite are substantially less expensive than 
are molds formed of other conventional materials. 
Although the invention has been described in detail with reference to the 
presently preferred process, those of ordinary skill in the art will 
appreciate that various modifications can be made without departing from 
the invention. Accordingly, the invention is defined only by the following 
claims.