Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 43

In addition to the influence of chemical composition, it is the homogeneity, isotropy, and absence of any microstructure that brings about the main features shared by most industrially manufactured glasses. This group of materials stands out from others by

 aesthetics; historically, the fact that glass was looking like gems has been the predominant driving force for the invention of glass as a material (Chapter 10.2). Aesthetic requirements today remain an important aspect, technically expressed as quality since, for instance, the presence of a single blister calls for the rejection of a 3 × 6 m sheet of flat glass.

 its suitability for large‐scale continuous primary forming as sheets, rods, tubes, and fibers.

 its extreme variability of shapes in discontinuous forming, comprising shapes with undercut.

 optical transparency. By virtue of their electronic and ionic properties, homogeneity, and absence of any microstructure, most glasses have an excellent transparency in the visible range. A standard float glass (4 mm thick) is transparent for light at wavelengths from 300 to 3500 nm, covering the entire visible (400–760 nm) and the near‐IR ranges, the reflection losses of a standard glass sheet remaining slightly lower than 8%.

 an extremely wide and continuous compositional variability. Glasses can also easily incorporate functional components such as colorants.

 an extremely smooth surface, which originally allowed glass not to “smell” the odors of the substances it was storing. Today an as‐received float glass possesses a roughness (root mean squared [RMS] value) of approximately 0.5 nm on the atmosphere side, and 1 nm on the tin‐bath contact side. Even after an extended exposure to water or humid air at room temperature, RMS remains well below 10 nm. This makes glasses ideal substrates for metal and other functional coatings.

 excellent dielectric properties. Lead silicate glasses used in the back part of cathode‐ray tubes reach dielectric constants of 20. It is true, this number does not match the extremely high values of functional ceramics (polycrystalline TiO2 ≈ 100, BaTiO3 ≈ 1000). When it comes to breakthrough voltages, however, the lack of any microstructure confers a clear advantage to glasses over polycrystalline materials: polycrystalline alumina withstands less than 8–9 kV/mm, whereas alkali‐free glasses reach 40 kV/mm, like natural mica; glass ceramics doped with BaTiO3 crystals may even reach values higher than 400 kV/mm.

 excellent chemical durability against most chemicals. This is especially the case for silicate glasses in the low‐pH range (i.e. with strong acids), making them excellent materials for chemical‐process plants.

 an extremely high stiffness and intrinsic strength, both again by virtue of the absence of any microstructure. With a tensile strength of up to 4000 MPa (glass fibers), glass ranges among the strongest materials available. Its proverbial fragility is not a matter of strength, but of vulnerability of its surface and of low fracture toughness (Chapter 3.11).

It is a combination of the above features which gives glass such a prominent and indispensable place in the world of materials.