Anovel sol-gel process for the manufacture of high-purity silica has won Frost & Sullivan’s European Innovation of the Year Award. Known as SiVARA and developed by Degussa Novara Technology – now part of Essen-based Evonik Industries – the process offers a low-cost alternative to existing production methods.
Earlier attempts to manufacture silica glass using sol-gel routes failed because the silica glass produced was subject to breakage. Degussa Novara Technology has been able to overcome this obstacle and achieve very high geometric reproducibility of its glass objects by leveraging the principle of moulding and gelling of liquids at room temperature (Fig.1).
“Using the SiVARA technology it is possible to predict the final dimensions of an object with a precision of 1 over 1000,” said Frost & Sullivan research analyst Archana Jayarajah. “This high accuracy meets the requirements of many high-end applications and represents a level of precision reachable by conventional techniques only after expensive after-treatments.”
Unlike conventional techniques, the objects produced via SiVARA do not need after- treatments such as cutting, polishing, or grinding. Further, current techniques for the manufacturing of quartz objects are based on the melting of the raw materials and the pouring of the melt (at temperatures above 1700°C) in moulds. Degussa Novara Technology’s know-how facilitates moulding to be conducted at room temperature. This provides significant freedom in terms of shape-design and material used for moulds (Fig.2).
“As Degussa Novara Technology’s process is based on formulating a water-based dispersion that can be poured into a mould at a later stage and then densified in an oven, the investment is a fraction of what would be required for quartz produced via conventional techniques,” notes Jayarajah. “Moreover, the SiVARA process can work by using pre-existing or commonly available equipment, minimising upfront/fixed costs even while the simplicity of the procedure ensures that operational costs remain low.”
Other key benefits provided by the SiVARA technology include the production of a silica glass that does not foam or produce bubbles when subject to temperatures exceeding 1700° C. The process also enables modification of the sol composition to produce glasses doped in a homogeneous manner for high-tech applications.
The SiVARA process is essentially different from the classical sol-gel process. While ordinary methods are based on the hydrolysis and condensation of silicon alkoxide, the SiVARA technology involves a colloidal solgel process in which dispersed nano-agglomerates of the fumed silica AEROSIL react with tetraethoxyorthosilicate Dynasylan A to form a gel. This is done by controlling the pH value.
The use of AEROSIL in this way creates a significantly higher particle density in the gel, which allows the manufacture of larger glass objects than can be achieved with conventional technology.
Because the milky dispersion can be filled into nearly any mold desired – whether plastic or metal – there are hardly any limitations on a designer’s imagination. The reaction mixture gels in the moulds in one to two hours, forming a mechanically stable aquagel. Its backbone is based on a silicon-dioxide network, whose pores are filled with water. In preparation of the drying phase, the water contained in the pores must be replaced by an organic solvent. The solvent of choice is acetone, because it is inexpensive, virtually non-toxic, and easy to recycle and keep in the circuit.
This solvent is exchanged through diffusion, which makes it the most time-consuming step of the process. While the exchange normally takes one day for small objects, it can stretch out over 20 or 30days with large objects. On the other hand, the step does not require expensive equipment.
The ‘acetone gel’ that results from the solvent exchange is released from the mould and then dried. The ‘hypercritical drying step’ takes place in an autoclave at a temperature of about 250°C and a pressure of about 60bar. Under these conditions, the solvent is expelled from the pores and replaced by air without disturbing the isotropy of the silicon dioxide network. The last intermediate product of this step is an aerogel or, borrowing from the terminology of ceramics production, ‘green body’.
The white porous aerogel is actually compressed into silica glass in the last stage of the process, the oven process. During sintering, in which the temperature increases through various set points within one to two days to a maximum value of 1400°C, the starting volume shrinks in all directions in space by 50percent.
This isotropy makes the SiVARA process unique, because it allows very 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 having reached precisions as high as ±0.1perthousand – values that rival those achieved by classical grinding techniques. Another unique aspect of the process is the ability to control the size of the ultimate glass body within defined limits by correcting the amount of AEROSIL added. So if a customer wants to vary the size of his product – a lens, for example – at a later date, the original mould can be reused. The possibility of later modification is also a result of isotropy, and is a characteristic intrinsic to the quality of the material.
Protected by over 100 patents, the SiVARA process is said by Evonik to stand conventional glass technologies on their heads. Traditionally, glass-mould bodies like optical lenses are manufactured by processing block glass and then grinding and polishing it to the desired surface quality. In contrast, the sol-gel process takes a wet-chemistry approach in which a dispersion is solidified to a jelly in a mould (Fig.3).
Potential licensees must be prepared for the fact that the process has nothing in common with traditional glass grinding, and that the user, as process engineer, follows a wet-chemical process.
Today the most important markets for high-purity silica glass are:
l Semiconductor components: Mountings for wafers in high-temperature ovens. Protective coatings made of high-purity silica glass for wafers, for the purpose of securing circuits.
l Optics: spherical and non-spherical lenses. The latter are hard to grind and can be manufactured much more easily using the sol-gel process. Additional fields of application in the area of specialty optics.
l Quartz applications: a variety of possible uses in the laboratory. The palette ranges from quartz cells, through immersion probes, all the way to applications in cryoscopes.
l Optical fibres: production of ultra-pure optical specialty fibres for data transfer.
l Light/illuminants: mostly for applications with high demands on purity, for example, or – as in the case of halogen or xenon lamps – on form and stability.
l Optoelectronics: optical chips. Interfaces for optoelectronics. A variety of microstructures in the field of telecommunications, such as components for connecting light in optical fibres, also as multiplexers and demultiplexers, especially within the ‘last mile’.
