Borrelli - Photosensitive glass and glass-ceramics

Glass is normally thought of as an inert material, one that is stable to the ambient conditions and will remain so for eons. This property of inertness plays a dominant role in glasss numerous applications ranging from a simple window to the modern complex optical waveguide that drives todays optical network. However, there is a special class of much less known glass that can be made to interact strongly with its environment, and most importantly, with light either in a transient or permanent way. The general classification is termed photosensitive, which actually pertains to a number of different manifestations of the phenomenon although they contain a common physical basis. The simplest case is solarization, where the optical effect is observed after exposure. An example of photosensitive glass is one that colors when exposed to long periods of sunlight, such as glasses often used in the desert, and hence the origin of the term solarization. This property derives, as we will see, from a number of different sources; for example, from small amounts of naturally occurring impurities retained as the glass was formed.

The second effect, is termed photosensitive, is where an optical effect is seen only after a subsequent thermal treatment to the exposure. The third category and likely the most familiar example of a transient response would be a glass that darkens in response to sunlight and fades in the shade. This property is termed somewhat inappropriately as photochromic since there is no color change involved, just a uniform darkening. In this special case the glass has additional impurities (dopants) added in the melting stage that ultimately accounts for the photochromic effect through the development of an Ag halide phase that is very similar to what is used in photography. This will be discussed extensively in .

The photosensitive version to be covered in this book is of the permanent variety where special glasses that are exposed to light in an initial state then subsequently heated or otherwise treated develop a permanent color (, when the physics and chemistry of the photosensitive mechanism are discussed.

Until now, the role of the glass composition, and even more to the point, the glass structure as it pertains to the propensity of the glass to exhibit photosensitive behavior, has not been mentioned. By glass structure, we mean the physical atomic structure of the glassy network of atoms. Because glass is often described as amorphous, this is somewhat misleading. Although glass has no regular long-range order as do crystals, it does have short-range (regular silica SiO4 tetrahedra) and quasi-ordered intermediate showing up as quasi-ordered N-membered rings. This is schematically shown for silica in . This could be viewed as a disordered crystal where the disorder is in the long-range distances.

However, there are other properties which are quite similar to crystals that are not significantly dependent on an ordered lattice of atoms. Perhaps surprisingly, the electronic band structure is similar. shows the measured deep UV spectrum comparing silica to its crystalline counterpart, quartz.

The spectral comparison essentially shows that the energy levels available to the electrons as expressed by the electronic band structure are very similar. In a way this is not really so surprising when one realizes that the electronic energy levels arise from the atomic orbitals, mainly p-orbitals of the oxygen (valence band) and sp3 orbitals of the silicon in silica that makes up the conduction band..

A schematic two-dimensional representation of the difference between a crystalline structure (a) and its analogous disordered or glass counterpart (b).

One now defines a mobility gap because electrons promoted to these states cannot contribute to electronic conduction because the states are localized. Electronic conduction occurs through electrons being able to move continuous bands formed by the overlap of the atomic orbitals. The electrons must occupy the higher extended

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Schematic representation of the density of electronic states for a disordered structure such as silica showing the existence of a localized state and extended states, the latter is able to carry electrons.

As we will see, this aspect of the electronic structure plays a large role in understanding the photosensitive process. We will often refer to defect or impurity states as the source of the photosensitive effect. It is clear that the understanding of the physics and chemistry of oxide glasses and the amorphous state in general is an ongoing quest and further aspects such as photosensitive effects just adds another layer of complexity to an already complex situation. Nonetheless, one still tries to be as quantitative as possible in the explanations of the various photosensitive effects that our present understanding of the glassy state allows. This lack of understanding particularly applies to the electronic structure of glass, which then plays a significant role in the photosensitive mechanism, as we will see in the next chapter. Surprisingly, there is a misunderstanding by many in the field about the electronic structure of disordered materials as we discussed above, which hampers the progress of many of the issues we cover in this book. Glass has an electronic band structure as do all solids it is just not readily described by the mathematical framework that has been built so well for crystalline materials, therefore making it more difficult to produce quantitative predictions. This all notwithstanding, we proceed on.

In the above discussion the phrase special glasses was used, which implies that not all glasses can be made to be photosensitive; that is, to be altered in some way by light. This is generally true in all but one or two of the topics covered in this book. However, most of the photosensitive glasses all are members of the general class of common glasses ranging from soda-lime to alkali-aluminoborosilicates. Moreover, the role of the glass composition differs significantly depending on the specific photosensitive effect in question (e.g., changes in color, refractive index, or photochromism). As we will see, the specific photosensitivity derives from the ability for the structure to include dopants such as halogens or silver ions or the presence of inherent structural features such as nonbridging oxygen. These latter features supply the defect sites that trap charged species that are created by the light. This will be made clear as we discuss each topic in detail.