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 purification solvent.

*SiVARA® technology, developed by Degussa Novara Technology.

Sintering is performed in a horizontal furnace.

Process Innovations

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.

+Dynasil® A

Taking Shape

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 range.

For more information, contact Degussa Corp. at 2 Turner Place, Piscataway, NJ 08855-0365; (732) 981-5087; fax (732) 981-5275; e-mail robert_e.johnson@degussa.com; or visit www.novaratechnology.com.


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