Researchers have been intrigued by the promising
principles of sol-gel for decades. In contrast to traditional processes that
require the re-shaping of solid blocks of silica glass material, molds of
almost any shape can be reproduced through sol-gel processing at room
temperature into high-quality silica glass. Sol-gel also allows for
applications like optics, sensors and medical devices, since essentially only
the molds need to be changed. Numerous companies have tried to industrialize
sol-gel processes for bulk glass, with little success.
A new sol-gel process* was recently developed that
provides a major improvement in the production of special glass-molded bodies
for a variety of applications, including quartz accessories, optics, optical
fibers and semiconductor equipment. By gelling a liquid precursor at room
temperature in a mold, the process provides a low-cost alternative to most
current manufacturing processes for glass objects, which normally require one
or more cutting, grinding and polishing steps. Because the process is water
based, it is environmentally friendly; the only byproduct is alcohol, which is
pure enough to be recycled for external sales or for use as a plant
technology, developed by
Degussa Novara Technology.
Sintering is performed in a horizontal furnace.
The new process involves mixing water with high-purity
fumed silicon dioxide** (SiO2
) and tetraethoxysilan+
at room temperature, casting the mixture into a mold and letting it gel. The
gel is then dried and sintered to form high-performance glass. The new
colloidal process overcomes obstacles to traditional sol-gel methods, including
the breakage of relatively large objects.
Previous sol-gel processes have failed because they were
not able to control the shrinking of the glass during sintering, which led to
tensions and cracks when larger objects were gelled. Researchers have
discovered that maintaining the consistency of the achieved material isotropy
(the 3-D uniform SiO2
network within the precursors)
during the production process results in final object sizes, lack of tension,
and even surface reproduction that other processes have not been able to reach.
The gel lengths achieved using this new process can reach into the meter range.
Fumed silica, one of the major ingredients in the new
process, has been used for decades in an array of applications ranging from
rubber to microelectronics. It improves the free-flowing properties of a
variety of powders, hardens coatings against scratches, and can even be used as
a component in ink-jet paper or as a polishing agent for microchips. The use of
fumed silica for glass and ceramic applications, however, is new. The high
purity of the fumed silica's fine powders results in optical transparency over
a wide spectrum, from ultraviolet to infrared light.
Based on the hydrolysis and condensation of silicon
alkoxides, the new sol-gel technology involves a colloidal sol-gel process in
which dispersed nano-agglomerates of fumed silica with a controlled pH value
react with tetraethoxysilane to form a gel. Incorporating fumed silica creates
a significantly denser gel, which enables the manufacture of larger glass
objects. The milky dispersion can be filled into nearly any desired mold
(plastic or metal), so few design limitations exist.
The reaction mixture gels in the mold within one to two
hours, forming a mechanically stable aquagel. Its backbone is based on a SiO2
network, the pores of which are filled with water. In preparation of the drying
phase, the water in the pores must be replaced by an organic solvent, which is
typically acetone because it is inexpensive, virtually nontoxic, and easy to
recycle and keep in the production cycle. The solvent facilitates the
subsequent drying process, in which an aerogel is formed. The aerogel is then
dried in an autoclave at approximately 250°C with a pressure of about 60 bar,
and sintered in an oven. During processing, the gel becomes transparent and
contracts invariably by 50% in all three dimensions.
The maximum temperature required is 1400°C, which allows
the final contour of the formed glass body to be accurately determined (it is
only above 1500°C that the silica glass melts, losing its shape). This isotropy
makes the new sol-gel process unique, because it allows for an extremely
precise calculation of the radius, curvature and surface roughness of the
resulting body based on the dimensions of the mold. The deviations are in the
range of ±1 per thousand, with some tests reaching precisions as high as ±0.1
per thousand, values that rival those achieved by traditional grinding
techniques. Another unique aspect of the process is the ability to control the
size of the ultimate glass body within defined parameters.
A Sol-Gel Alternative
In traditional glass object manufacturing, glass blanks
are produced that must undergo cutting and polishing to achieve their required
final form. In contrast, the glass blocks, cylinders, rods and tubes produced
by the new sol-gel process already possess the desired form (or are near-net
shape) on their removal from the furnace. There has also been concern that
larger objects, such as lenses, mirrors and odd shapes, would break during
sol-gel processing. This new technology makes it possible to manufacture large
silica glass objects without the tensions that have led to breakage in other,
failed processes, and it has overcome perceptions that sol-gel technology is
valuable only for thin films or small objects in the millimeter to centimeter
For more information, contact Degussa Corp. at
2 Turner Place, Piscataway, NJ 08855-0365; (732) 981-5087; fax (732) 981-5275;
e-mail email@example.com; or visit www.novaratechnology.com.