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12.5: Materials for Optics

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    can be widely defined as an . An can be considered to have a random arrangement of , such as observed in a gas, but more realistically can considered to only lack long-range order such as those found in crystalline solids. is clear (optically transparent) silica which is composed largely of silicon dioxide (SiO). The definition of does not restrict either the composition or the optical properties of the material, implying a wide variety of different materials that are considered . In fact, theoretically, any crystalline solid that can be brought to a liquid state, can be forced into an state through rapid solidification via extraordinary cooling rates. This is easily observable by recognizing that quartz, a very common crystalline solid, has the same composition as silica (SiO) but was cooled slowly enough to form long-range order. Non-silica glasses, in particular metallic glasses, can obtain unique electric, optical, or thermal properties from their crystalline counterparts through glassification. Non-metallic glasses can obtain similarly unique properties by adjusting elemental compositions and introducing dopants. has been desirable for its optical properties since silica is transparent in the visible spectrum. Although there are crystalline materials that are similarly transparent (quartz), they have several properties which make them undesirable as optical media in many cases; though each grain may be transparent, grain boundaries reflect and/or scatter light in poly crystalline materials; unless cut along specific planes, the faces of crystals are forced to conform to a rigid geometric order which may also scatter light. Silica is not restricted by these constraints and has properties making it even more desirable both in terms of clarity and malleability; being unrestricted by a defined internal structure, the surface of is molecularly smooth, bound only by , even along curved faces. This is particularly important since most optical instruments (microscopes, telescopes, and eyeglasses) require smooth curved surfaces. isn’t the only the fits the profile for optical functions. Acrylic , poly(methyl methacrylate), is a polymer, fitting the description of by being an . Acrylic has a very similar refractive index to silica (~1.5) and is physically lighter, softer, and more shatter-resistant than . Polycarbonates is another class of optically transparent polymers. It has even more desirable physical properties than acrylic in terms of strength and impact resistance. , in particular the index of refraction, can be modified by doping the . Doping the with low density materials such as boron can lower the index of refraction. Similarly, doping the material with higher density dopants, such as oxides of lead, titanium, barium or zirconium, can drastically increase the index of refraction. Glasses made or doped with germanium or phosphates are vitally important in the field of. filters are often made of chalcogenide , composed of two non-oxygen group 16 elements (sulfur, selenium, tellurium) and one group 14 element (silicon, tin, lead) or group 15 element (phosphorus, arsenic, antimony, bismuth) in its most simple form. Often the size of the allow for leniency to create more complex amorphous chalcogenide glasses involving several different elements in compositions that only roughly match the two to one ratio of silica. These materials transmit only in the IR or near IR range, appearing black or faintly blue, while still having similar malleability of silica . Example IR filter chalcogenide glasses include AsSe, GeAsSe, and GeAsSeTe. is a well-known insulator, having a resistivity on the order of 10 ohm m. is especially desirable in the field of for its insulating properties; in device fabrication, is deposited between metals or as very thin insulators. Doped , such as phosphosilicate (phosphorus doped) or borophosphosilicate (boron and phosphorus doped) are often used instead of pure silica for their lower melting temperatures and increased planarization (forming smooth flat surfaces). The addition of fluorine, a highly electronegative element, into the can lower the and thus the dielectric constant of the , making it more desirable for integrated circuits. The addition of large , such as lead (II) oxide, can reduce the mobility of other significantly increasing the resistivity of the material. It’s possible to give silica a level of electrical conductivity by dissolving small alkali metal into the , which have high mobility’s at increasing temperatures. is commonly considered to be very susceptible to thermal shock and breaks or cracks easily when suddenly changing temperatures. This is true for the cheapest and most common form of which is soda-lime-silica , which contains roughly 30% sodium oxide (NaO), lime (CaO), magnesia (MgO) and alumina (AlO). Soda-lime has a of thermal expansion of 93.5E-7 cm/cm.°C, which describes the relative increase in size per degree Celsius change in temperature. Pure or nearly pure (~96%) silica has a small of thermal expansion of 7.5E-7 cm/cm.°C due to the homogeneity of the solid. Silica also has a significantly higher melting temperature, combined with the purity requirements, this makes it more expensive to produce. Borosilicate (~13% BO) is a very common form of used in cookware for its thermal shock resistance, having a of 35E-7 cm/cm.°C. Unlike pure silica , borosilicate is cheaper to produce, having a lower melting temperature and having less stringent purity requirements. Corning has developed glasses with ultra-low thermal expansion coefficients with values on the order of 10 cm/cm.°C. does play a huge role in thermal insulation in the form of fiber and wool. wool involves the production of very thin strands on soda-lime to form a low density packing material. wool can achieve higher specific heats than either or water on the order of 7 J/g.K. requires massive cooling rates, on the order of 10 K/s. These cooling rates can be achieved by a variety of methods including:

    12.5: Materials for Optics is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by LibreTexts.

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