PHOTOCHEMICAL TECHNOLOGY FOR PROCESSING GLASS PRODUCTS*)

In its physicochemical properties, glass is an indispensable material for an enormous number of highly varied products. However, the broad and continually growing introduction of glass into production is substantially hindered by the very laborious technology of its mechanical processing. It is well known that the mass production of glass articles suffers from many shortcomings (nonuniformity, surface defects). At the same time, any noticeable improvement in the quality of this production—namely, an increase in the precision of manufacture of articles—proves extremely expensive, and at times practically impossible.

From this point of view, the path proposed by the author of the article under review—replacing the mechanical processing of glass by its photochemical processing—is of considerable interest, despite the fact that, in its present form, this method is still very imperfect and applicable only to a very limited class of glasses that have no serious industrial importance, namely to one of the types of photosensitive glass (the so-called “photosensitive opal”).

The essence of the proposed process is as follows. Having been exposed to light and then subjected to thermal treatment (“development”), glass of this type acquires increased solubility as compared with unexposed glass. Owing to this, a volumetric photographic image obtained in glass can be dissolved, while the dissolution process practically does not affect the regions that were not exposed to irradiation.

Let us consider this process in more detail.

In glasses of the photosensitive-opal type, which include lithium silicate modified with oxides of potassium and aluminum and containing cerium and silver impurities as a photosensitive ingredient \((\mathrm{SiO_2} — 81.5\%, \ \mathrm{Li_2O} — 12.0\%, \ \mathrm{K_2O} — 3.5\%, \ \mathrm{Al_2O_3} — 3.0\%, \ \mathrm{CeO_2} — 0.03\%, \ \mathrm{Ag} — 0.02\%)\), irradiation with light produces metal particles that form a latent image. During thermal development, these metallic particles serve as centers of crystallization of lithium metasilicate, as a result of which the irradiated regions of the glass become milky (X-ray analysis showed that the developed image contains about 35 weight percent of crystalline lithium metasilicate). In essence, the development process is divided into four successive stages: 1) neutralization of metallic ions by photoelectrons formed upon irradiation, 2) growth of metallic particles to critical sizes at which they can serve as crystallization nuclei, 3) nucleation of lithium metasilicate crystallites, and 4) growth of these crystallites. The ratio of the rates of these processes, which depend on temperature, determines the development regime.

In particular, development of an irradiated specimen cannot begin by heating it immediately to a high temperature, since this entails rapid destruction of the image formed. The nucleation of crystallization, which accelerates exponentially with increasing temperature, ceases at \(T \gtrless 540^\circ\mathrm{C}\). Experiments have shown that the most rational procedure is heating at a temperature between \(458^\circ\) (the annealing temperature) and \(540^\circ\mathrm{C}\) for approximately 5 minutes. The specimen is then heated to a higher temperature (below the softening temperature, \(637^\circ\)) for crystallization of lithium metasilicate. Complete development corresponds to the end of the process of coalescence of the faces of neighboring crystallites. The time of pro—

*) S. D. Stookly, Jrd. a. Eng. Chemistry 45, No. 1, 115 (1953).

is proportional to the viscosity of the glass and increases exponentially with temperature (at \(600^\circ\text{C}\), about 1 hour is required for the complete development of the image).

Articles made of photosensitive glass, produced by a photochemical method.

The sizes of the crystallites formed depend on the exposure. On average they are close to 4 microns, which corresponds to \(10^{10}\) crystallites per cubic centimeter.

If dissolution is carried out at room temperature in a 2–10% aqueous solution of hydrofluoric acid, then, as measurements show, the linear rate of dissolution of the crystallites is approximately 50 times greater than that of the uncrystallized glassy mass (the volumetric or mass rate of dissolution of the crystallites is 100,000 times greater than that of the glass). In this case the rate of dissolution of the image produced is, in general, the greater the more strongly the glass had been exposed (a saturation effect occurs). As a result, after dissolution the specimen assumes a form reproducing the contours of the negative image, with the exception of internal regions inaccessible to the solvent. The accuracy with which the form is reproduced is determined by the exclusive ratio of the dissolution rates of crystallized and uncrystallized glass, and proves to be sufficiently high.

The author sees the principal applications of this method of glass processing in its replacement of mechanical processes of drilling, cutting, and engraving. In particular, the method described was used in the manufacture of aperture plates for color-television systems. These plates contain hundreds of thousands of openings in the shape of truncated cones, whose axes are directed toward a common focus and whose diameter at the narrow end is 0.225 mm; moreover, very high requirements are imposed on the shape and position of the openings, and on accuracy. It turned out that, in addition to advantages associated with replacing metallic aperture plates by glass ones (dielectric!), the latter possessed a higher degree of accuracy and reproducibility.

Likewise, the photoengraving process carried out by the method described above has a number of substantial advantages over ordinary methods of photoengraving (in particular, better image reproduction), which gives grounds to hope for its broad use in the printing industry. Finally, the author successfully used photochemical treatment of photosensitive glass to make bas-reliefs and to create glass objects of complex design. Examples of products made by this method are shown in the figures.

In conclusion, let us note that the photosensitivity of the glass is sufficiently great and the process of obtaining an image does not take much time. Thus, using a mercury arc lamp as a light source and contact printing, the author obtained normal images in times measured in seconds or minutes.

R. G.