Other glasses
[edit]Network glasses
Some glasses that do not include silica as a major constituent may have physico-chemical properties useful for their application in fibre optics and other specialized technical applications. These include fluoride glasses, aluminosilicates, phosphate glasses, borate glasses, andchalcogenide glasses.
There are three classes of components for oxide glasses: network formers, intermediates, and modifiers. The network formers (silicon, boron, germanium) form a highly cross-linked network of chemical bonds. The intermediates (titanium, aluminium, zirconium, beryllium, magnesium, zinc) can act as both network formers and modifiers, according to the glass composition. The modifiers (calcium, lead, lithium, sodium, potassium) alter the network structure; they are usually present as ions, compensated by nearby non-bridging oxygen atoms, bound by one covalent bond to the glass network and holding one negative charge to compensate for the positive ion nearby. Some elements can play multiple roles; e.g. lead can act both as a network former (Pb4+ replacing Si4+), or as a modifier.
The presence of non-bridging oxygens lowers the relative number of strong bonds in the material and disrupts the network, decreasing theviscosity of the melt and lowering the melting temperature.
The alkaline metal ions are small and mobile; their presence in glass allows a degree of electrical conductivity, especially in molten state or at high temperature. Their mobility however decreases the chemical resistance of the glass, allowing leaching by water and facilitating corrosion. Alkaline earth ions, with their two positive charges and requirement for two non-bridging oxygen ions to compensate for their charge, are much less mobile themselves and also hinder diffusion of other ions, especially the alkalis. The most common commercial glasses contain both alkali and alkaline earth ions (usually sodium and calcium), for easier processing and satisfying corrosion resistance.[11] Corrosion resistance of glass can be achieved by dealkalization, removal of the alkali ions from the glass surface by reaction with e.g. sulfur or fluorine compounds. Presence of alkaline metal ions has also detrimental effect to the loss tangent of the glass, and to itselectrical resistance; glasses for electronics (sealing, vacuum tubes, lamps...) have to take this in account.
Addition of lead(II) oxide lowers melting point, lowers viscosity of the melt, and increases refractive index. Lead oxide also facilitates solubility of other metal oxides and therefore is used in colored glasses. The viscosity decrease of lead glass melt is very significant (roughly 100 times in comparison with soda glasses); this allows easier removal of bubbles and working at lower temperatures, hence its frequent use as an additive in vitreous enamels and glass solders. The high ionic radius of the Pb2+ ion renders it highly immobile in the matrix and hinders the movement of other ions; lead glasses therefore have high electrical resistance, about two orders of magnitude higher than soda-lime glass (108.5 vs 106.5 Ohm·cm, DC at 250 °C). For more details, see lead glass.[12]
Addition of fluorine lowers the dielectric constant of glass. Fluorine is highly electronegative and attracts the electrons in the lattice, lowering the polarizability of the material. Such silicon dioxide-fluoride is used in manufacture of integrated circuits as an insulator. High levels of fluorine doping lead to formation of volatile SiF2O and such glass is then thermally unstable. Stable layers were achieved with dielectric constant down to about 3.5–3.7.[13]
[edit]Amorphous metals
In the past, small batches of amorphous metals with high surface area configurations (ribbons, wires, films, etc.) have been produced through the implementation of extremely rapid rates of cooling. This was initially termed "splat cooling" by doctoral student W. Klement at Caltech, who showed that cooling rates on the order of millions of degrees per second is sufficient to impede the formation of crystals, and the metallic atoms become "locked into" a glassy state. Amorphous metal wires have been produced by sputtering molten metal onto a spinning metal disk. More recently a number of alloys have been produced in layers with thickness exceeding 1 millimeter. These are known as bulk metallic glasses (BMG). Liquidmetal Technologies sell a number of titanium-based BMGs. Batches of amorphous steel have also been produced that demonstrate mechanical properties far exceeding those found in conventional steel alloys.[14][15][16]
In 2004, NIST researchers presented evidence that an isotropic non-crystalline metallic phase (dubbed "q-glass") could be grown from the melt. This phase is the first phase, or "primary phase," to form in the Al-Fe-Si system during rapid cooling. Interestingly, experimental evidence indicates that this phase forms by a first-order transition. Transmission electron microscopy (TEM) images show that the q-glass nucleates from the melt as discrete particles, which grow spherically with a uniform growth rate in all directions. The diffraction pattern shows it to be an isotropic glassy phase. Yet there is anucleation barrier, which implies an interfacial discontinuity (or internal surface) between the glass and the melt.[17]
[edit]Electrolytes
Electrolytes or molten salts are mixtures of different ions. In a mixture of three or more ionic species of dissimilar size and shape, crystallization can be so difficult that the liquid can easily be supercooled into a glass. The best studied example is Ca0.4K0.6(NO3)1.4.
[edit]Aqueous solutions
Some aqueous solutions can be supercooled into a glassy state, for instance LiCl:RH2O in the composition range 4<R<8.
[edit]Molecular liquids
A molecular liquid is composed of molecules that do not form a covalent network but interact only through weak van der Waals forces or through transient hydrogen bonds. Many molecular liquids can be supercooled into a glass; some are excellent glass formers that normally do not crystallize.
A widely known example is sugar glass.
Under extremes of pressure and temperature solids may exhibit large structural and physical changes which can lead to polyamorphic phase transitions.[18] In 2006 Italian scientists created an amorphous phase of carbon dioxide using extreme pressure. The substance was namedamorphous carbonia(a-CO2) and exhibits an atomic structure resembling that of silica.[19]
[edit]Polymers
[edit]Colloidal glasses
Concentrated colloidal suspensions may exhibit a distinct glass transition as function of particle concentration or density.[20]
[edit]Glass-ceramics
Glass-ceramic materials share many properties with both non-crystalline glass and crystallineceramics. They are formed as a glass, and then partially crystallized by heat treatment. For example, the microstructure of whiteware ceramics frequently contains both amorphous andcrystalline phases. Crystalline grains are often embedded within a non-crystalline intergranular phase of grain boundaries. When applied to whiteware ceramics, vitreous means the material has an extremely low permeability to liquids, often but not always water, when determined by a specified test regime.[21][22]
The term mainly refers to a mix of lithium and aluminosilicates which yields an array of materials with interesting thermomechanical properties. The most commercially important of these have the distinction of being impervious to thermal shock. Thus, glass-ceramics have become extremely useful for countertop cooking. The negative thermal expansion coefficient (TEC) of the crystalline ceramic phase can be balanced with the positive TEC of the glassy phase. At a certain point (~70% crystalline) the glass-ceramic has a net TEC near zero. This type of glass-ceramic exhibits excellent mechanical properties and can sustain repeated and quick temperature changes up to 1000 °C.[21][22]
[edit]Scratch resistant glass
A recent development is scratch resistant glass for touch screens.
[edit]Structure
Main article: Structure of liquids and glasses
As in other amorphous solids, the atomic structure of a glass lacks any long range translational periodicity. However, due to chemical bonding characteristics glasses do possess a high degree of short-range order with respect to local atomic polyhedra.[23]
[edit]Glass versus supercooled liquid
Main article: glass transition
| What is the nature of thetransition between a fluid or regular solid and a glassy phase? "The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition." P.W. Anderson[24] |  |
In physics, the standard definition of a glass (or vitreous solid) is a solid formed by rapid melt quenching.[25][26][27][28][29] However, the term glass is often used to describe any amorphous solid that exhibits a glass transition temperature Tg. If the cooling is sufficiently rapid (relative to the characteristic crystallization time) then crystallization is prevented and instead the disordered atomic configuration of the supercooledliquid is frozen into the solid state at Tg. Generally, the structure of a glass exists in a metastable state with respect to its crystalline form, although in certain circumstances, for example in atactic polymers, there is no crystalline analogue of the amorphous phase.[30]
Glass is an amorphous solid. It exhibits an atomic structure close to that observed in the supercooled liquid phase but displays all the mechanical properties of a solid.[31][32] The notion that glass flows to an appreciable extent over extended periods of time is not supported by empirical research or theoretical analysis (see viscosity of amorphous materials).
Some people consider glass to be a liquid due to its lack of a first-order phase transition[31][33] where certain thermodynamic variables such as volume, entropy and enthalpy are discontinuous through the glass transition range. However, the glass transition may be described as analogous to a second-order phase transition where the intensive thermodynamic variables such as the thermal expansivity and heat capacityare discontinuous.[34] Despite this, the equilibrium theory of phase transformations does not entirely hold for glass, and hence the glass transition cannot be classed as one of the classical equilibrium phase transformations in solids.[28][29]
Although the atomic structure of glass shares characteristics of the structure in a supercooled liquid, glass tends to behave as a solid below its glass transition temperature.[35] A supercooled liquid behaves as a liquid, but it is below the freezing point of the material, and in some cases will crystallize almost instantly if a crystal is added as a core. The change in heat capacity at a glass transition and a melting transition of comparable materials are typically of the same order of magnitude, indicating that the change in active degrees of freedom is comparable as well. Both in a glass and in a crystal it is mostly only the vibrational degrees of freedom that remain active, whereas rotationaland translational motion is arrested. This helps to explain why both crystalline and non-crystalline solids exhibit rigidity on most experimental time scales.
[edit]Behavior of antique glass
The observation that old windows are sometimes found to be thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a timescale of centuries. The assumption being that the glass was once uniform, but has flowed to its new shape, which is a property of liquid.[36] However, this assumption is incorrect; glass does not flow. The reason for the observation is that in the past, when panes of glass were commonly made by glassblowers, the technique used was to spin molten glass so as to create a round, mostly flat and even plate (the crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk became thicker as the glass spun. When installed in a window frame, the glass would be placed thicker side down both for the sake of stability and to prevent water accumulating in the lead cames at the bottom of the window.[37] Occasionally such glass has been found thinner side down or thicker on either side of the window's edge, the result of carelessness during installation.[38]
Mass production of glass window panes in the early twentieth century caused a similar effect. In glass factories, molten glass was poured onto a large cooling table and allowed to spread. The resulting glass is thicker at the location of the pour, located at the center of the large sheet. These sheets were cut into smaller window panes with nonuniform thickness, typically with the location of the pour centred in one of the panes (known as "bull's-eyes") for decorative effect. Modern glass intended for windows is produced as float glass and is very uniform in thickness.
Several other points can be considered which contradict the "cathedral glass flow" theory:
- Writing in the American Journal of Physics, physicist Edgar D. Zanotto states "...the predicted relaxation time for GeO2 at room temperature is 1032 years. Hence, the relaxation period (characteristic flow time) of cathedral glasses would be even longer."[39] (1032years is many times longer than the estimated age of the Universe.)
- If medieval glass has flowed perceptibly, then ancient Roman and Egyptian objects should have flowed proportionately more — but this is not observed. Similarly, prehistoric obsidian blades should have lost their edge; this is not observed either (although obsidian may have a different viscosity from window glass).[31]
- If glass flows at a rate that allows changes to be seen with the naked eye after centuries, then the effect should be noticeable in antique telescopes. Any slight deformation in the antique telescopic lenses would lead to a dramatic decrease in optical performance, a phenomenon that is not observed.[31]
- There are many examples of centuries-old glass shelving which has not bent, even though it is under much higher stress from gravitational loads than vertical window glass.
The above does not apply to materials that have a glass transition temperature close to room temperature, such as certain plastics used in daily life like polystyrene and polypropylene. Over time, they may well show viscoelastic behaviour, and this is a serious concern when applying these materials in construction.
[edit]Physical properties
See also: List of physical properties of glass
[edit]Optical properties
Glass is in widespread use largely due to the production of glass compositions that are transparent to visible wavelengths of light. In contrast,polycrystalline materials do not in general transmit visible light.[40] The individual crystallites may be transparent, but their facets (grain boundaries) reflect or scatter light resulting in diffuse reflection. Glass does not contain the internal subdivisions associated with grain boundaries in polycrystals and hence does not scatter light in the same manner as a polycrystalline material. The surface of a glass is often smooth since during glass formation the molecules of the supercooled liquid are not forced to dispose in rigid crystal geometries and can follow surface tension, which imposes a microscopically smooth surface. These properties, which give glass its clearness, can be retained even if glass is partially light-absorbing i.e. colored.[41]
Glass has the ability to refract, reflect and transmit light following geometrical optics, without scattering it, and it is used in the manufacture of lenses and windows. Common glass has a refraction index around 1.5. According to Fresnel equations, the reflectivity of a sheet of glass is about 4% per surface (at normal incidence), and its transmissivity about 92%. Glass also finds application in optoelectronics e.g. for light transmitting optical fibres.
[edit]Color
Main article: Glass coloring and color marking
Color in glass may be obtained by addition of electrically charged ions (or color centers) that are homogeneously distributed, and by precipitation of finely dispersed particles (such as inphotochromic glasses).[42] Ordinary soda-lime glass appears colorless to the naked eye when it is thin, although iron(II) oxide (FeO) impurities of up to 0.1 wt%[43] produce a green tint which can be viewed in thick pieces or with the aid of scientific instruments. Further FeO and Cr2O3 additions may be used for the production of green bottles. Sulfur, together with carbon and iron salts, is used to form iron polysulfides and produce amber glass ranging from yellowish to almost black.[44]A glass melt can also acquire an amber color from a reducing combustion atmosphere.Manganese dioxide can be added in small amounts to remove the green tint given by iron(II) oxide.
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