Silicate glasses are historically the oldest types of glasses which were manufactured by human beings, and are still the most common glasses. They largely consist of silicon dioxide (silica, SiO2), but in contrast to pure silica glass (fused silica) they contain some additional substances like soda, alumina, phosphorus pentoxide, germania and potassium carbonate. Depending on the composition, one arrives at names like aluminosilicate, germanosilicate, aluminogermanosilicate, borosilicate, phosphosilicate glass, etc. They generally exhibit good transparency throughout the visible spectral range and somewhat beyond, i.e., into the near ultraviolet and near infrared. The refractive index for visible light is normally around 1.5.
Silicate glasses are part of the wider group of oxide glasses. Additional substances like alumina (Al2O3) and germania (GeO2) are also oxides, while others are carbonates, for example. Important non-oxide glasses are chalcogenide glasses, e.g. sulfide, selenide and tellurite glasses, as used e.g. for infrared optics, and fluoride glasses.
Although nearly all silicate glasses are made in inexpensive ways with moderate qualities for not particularly sensitive applications such as windows for buildings (flat glass) and glass bottles (container glass), there are also high-quality silicate glasses used in optics:
- They are used as optical glasses (specifically for crown glasses), e.g. for lenses, prisms, mirror substrates, optical filters and various other types of optical components.
- Some variants are used for making optical fibers; see the article on silica fibers.
- Silica glasses can also be doped with various types of rare earth ions, mostly for achieving laser amplification.
In each case, the chemical composition can be optimized for the specific application; a wide range of relevant properties can be modified by adding or avoiding certain substances.
Types of Silicate Glasses
Particularly common are soda–lime glasses, also called soda–lime-silica glasses, where soda (sodium carbonate, Na2CO3) is added, essentially in order to obtain a lower glass transition temperature, and also lime (calcium oxide, CaO, generally obtained from limestone) for improved durability (e.g. resistance to water). Further common added materials are magnesium oxide (MgO) and alumina (Al2O3), which also improve the durability.
Soda–lime glasses are very widely used, for example as window glass, for bottles and light bulbs. They are inexpensive and easy to recycle, although the melting requires considerable amounts of energy.
Soda–lime glasses can also be used as optical glasses when they are optimized for good optical homogeneity. Unfortunately, their thermal expansion coefficient is relatively large. Other kinds of silicate glasses are better for optical applications in various respects.
My adding boron trioxide (B2O3) to silica, one obtains borosilicate glass. Often, one uses additional substances like soda and alumina. For example, the common BK7 glass is a crown glass of that type.
An important advantage of borosilicate glasses is their relatively low coefficient of thermal expansion – much smaller than for soda–lime glass, although still substantially larger than for pure silica glass (fused silica). Compared with fused silica, borosilicates feature a much lower glass transition temperature and are thus easier to process.
Terbium-doped borosilicate glasses are used for Faraday isolators, since they exhibit relatively large Verdet constants.
Germanate glasses are obtained by adding germanium oxide (GeO2) and sometimes other constituents (e.g. alumina) to silica. Their infrared transmission extends to roughly 3 μm.
The addition of germania leads to an increased refractive index compared with pure silica – of course with the magnitude depending on the amount of germania. That effect is utilized for making optical fibers: the fiber core (within a pure silica cladding) obtains a suitable germania doping profile, e.g. such that single-mode guidance is obtained in some wavelength range, or for obtaining a graded-index fiber, e.g. with an approximately parabolic profile for low intermodal dispersion. The latter is useful for optical fiber communications over moderate distances with multimode fibers.
Refined processes have been developed with which the germanosilicate glass for the core of optical fibers can be fabricated with extremely high chemical purity. That results in very low propagation losses; in the 1.5-μm spectra region, which is relevant for optical fiber communications, it can be below 0.2 dB/km. Further in the infrared, the propagation losses are higher, but still low enough for transmitting fairly high optical powers.
Germanosilicate glasses can also be doped with laser-active rare earth ions. Such rare-earth-doped fibers are used for fiber lasers and amplifiers. Additional substances like alumina can help to better incorporate the rare earth ions, i.e., for avoiding clustering.
Note that there are also germanate glasses, largely consisting of GeO2. They have higher refractive indices, a transmission range extending substantially further into the infrared, and a low glass transition temperature.
A phosphosilicate glass is a glass containing both phosphates and silicates – obtained by adding phosphorus pentoxide (P2O5) to a silicate. In optics, it is mostly used for rare-earth-doped fibers and Raman fibers. A possible advantage is that phosphosilicate glass allows one to realize a very large Raman frequency shift of ≈40 THz – around 3 times larger than for silica fibers. On the other hand, a very small Raman frequency shift of ≈3 THz is also possible.
Silicate Filter Glasses
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