amorphous solid

▪ physics

Introduction

      any noncrystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Such solids include glass, plastic, and gel.

      Solids and liquids (liquid) are both forms of condensed matter; both are composed of atoms in close proximity to each other. But their properties are, of course, enormously different. While a solid material has both a well-defined volume and a well-defined shape, a liquid has a well-defined volume but a shape that depends on the shape of the container. Stated differently, a solid exhibits resistance to shear stress while a liquid does not. Externally applied forces can twist or bend or distort a solid's shape, but (provided the forces have not exceeded the solid's elastic limit) it “springs back” to its original shape when the forces are removed. A liquid flows under the action of an external force; it does not hold its shape. These macroscopic characteristics constitute the essential distinctions: a liquid flows, lacks a definite shape (though its volume is definite), and cannot withstand a shear stress; a solid does not flow, has a definite shape, and exhibits elastic stiffness against shear stress.

 On an atomic level, these macroscopic distinctions arise from a basic difference in the nature of the atomic motion. Figure 1—> contains schematic representations of atomic movements in a liquid and a solid. Atoms in a solid are not mobile. Each atom stays close to one point in space, although the atom is not stationary but instead oscillates rapidly about this fixed point (the higher the temperature, the faster it oscillates). The fixed point can be viewed as a time-averaged centre of gravity of the rapidly jiggling atom. The spatial arrangement of these fixed points constitutes the solid's durable atomic-scale structure. In contrast, a liquid possesses no enduring arrangement of atoms. Atoms in a liquid are mobile and continually wander throughout the material.

Distinction between crystalline (crystal) and amorphous solids

 There are two main classes of solids: crystalline and amorphous. What distinguishes them from one another is the nature of their atomic-scale structure. The essential differences are displayed in Figure 2—>. The salient features of the atomic arrangements in amorphous solids (also called glasses), as opposed to crystals, are illustrated in the figure for two-dimensional structures; the key points carry over to the actual three-dimensional structures of real materials. Also included in the figure, as a reference point, is a sketch of the atomic arrangement in a gas. For the sketches representing crystal (A) and glass (B) structures, the solid dots denote the fixed points about which the atoms oscillate; for the gas (C), the dots denote a snapshot of one configuration of instantaneous atomic positions.

   Atomic positions in a crystal exhibit a property called long-range order or translational periodicity; positions repeat in space in a regular array, as in Figure 2A—>. In an amorphous solid, translational periodicity is absent. As indicated in Figure 2B—>, there is no long-range order. The atoms are not randomly distributed in space, however, as they are in the gas in Figure 2C—>. In the glass example illustrated in the figure, each atom has three nearest-neighbour atoms at the same distance (called the chemical bond length) from it, just as in the corresponding crystal. All solids, both crystalline and amorphous, exhibit short-range (atomic-scale) order. (Thus, the term amorphous, literally “without form or structure,” is actually a misnomer in the context of the standard expression amorphous solid.) The well-defined short-range order is a consequence of the chemical bonding between atoms, which is responsible for holding the solid together.

 In addition to the terms amorphous solid and glass, other terms in use include noncrystalline solid and vitreous solid. Amorphous solid and noncrystalline solid are more general terms, while glass and vitreous solid have historically been reserved for an amorphous solid prepared by rapid cooling (quenching) of a melt—as in scenario 2 of Figure 3—>.

 Figure 3—>, which should be read from right to left, indicates the two types of scenarios that can occur when cooling causes a given number of atoms to condense from the gas phase into the liquid phase and then into the solid phase. Temperature is plotted horizontally, while the volume occupied by the material is plotted vertically. The temperature T_b is the boiling point, T_f is the freezing (freezing point) (or melting) point, and T_g is the glass transition temperature. In scenario 1 the liquid freezes at T_f into a crystalline solid, with an abrupt discontinuity in volume. When cooling occurs slowly, this is usually what happens. At sufficiently high cooling rates, however, most materials display a different behaviour and follow route 2 to the solid state. T_f is bypassed, and the liquid state persists until the lower temperature T_g is reached and the second solidification scenario is realized. In a narrow temperature range near T_g, the glass transition occurs: the liquid freezes into an amorphous solid with no abrupt discontinuity in volume.

    The glass transition temperature T_g is not as sharply defined as T_f; T_g shifts downward slightly when the cooling rate is reduced. The reason for this phenomenon is the steep temperature dependence of the molecular response time, which is crudely indicated by the order-of-magnitude values shown along the top scale of Figure 3—>. When the temperature is lowered below T_g, the response time for molecular rearrangement becomes much larger than experimentally accessible times, so that liquidlike mobility (Figure 1—>, right) disappears and the atomic configuration becomes frozen into a set of fixed positions to which the atoms are tied (Figures 1—>, left, and 2B—>).

  Some textbooks erroneously describe glasses as undercooled viscous liquids, but this is actually incorrect. Along the section of route 2 labeled liquid in Figure 3—>, it is the portion lying between T_f and T_g that is correctly associated with the description of the material as an undercooled liquid (undercooled meaning that its temperature is below T_f). But below T_g, in the glass phase, it is a bona fide solid (exhibiting such properties as elastic stiffness against shear). The low slopes of the crystal and glass line segments of Figure 3—> in comparison with the high slope of the liquid section reflect the fact that the coefficient of thermal expansion of a solid is small in comparison with that of the liquid.

Preparation of amorphous solids

       Bonding types and glass transition temperatures of representative amorphous solidsIt was once thought that relatively few materials could be prepared as amorphous solids, and such materials (notably, oxide glasses and organic polymers) were called glass-forming solids. It is now known that the amorphous solid state is almost a universal property of condensable matter. The Table (Bonding types and glass transition temperatures of representative amorphous solids) presents a list of amorphous solids in which every class of chemical bonding type is represented. The glass transition temperatures span a wide range.

      Glass formation is a matter of bypassing crystallization. The channel to the crystalline state is evaded by quickly crossing the temperature interval between T_f and T_g. Nearly all materials can, if cooled quickly enough, be prepared as amorphous solids. The definition of “quickly enough” varies enormously from material to material. Four techniques for preparing amorphous solids are illustrated in Figure 4. These techniques are not fundamentally different from those used for preparing crystalline solids; the key is simply to quench the sample quickly enough to form the glass, rather than slowly enough to form the crystal. The quench rate increases greatly from left to right in the figure.

Melt quenching

      Preparation of metallic glasses requires a quite rapid quench. The technique shown in Figure 4C, called splat quenching, can quench a droplet of a molten metal roughly 1,000° C in one millisecond, producing a thin film of metal that is an amorphous solid. In enormous contrast to this, the silicate glass that forms the rigid ribbed disk of the Hale Telescope of the Palomar Observatory near San Diego, Calif., was prepared by cooling (over a comparable temperature drop) during a time interval of eight months. The great difference in the quench rates needed for arriving at the amorphous solid state (the quench rates here differ by a factor of 3 × 10¹⁰) is a dramatic demonstration of the difference in the glass-forming tendency of silicate glasses (very high) and metallic glasses (very low).

      The required quench rate for glass formation can vary significantly within a family of related materials that differ from one another in chemical composition. Figure 5 illustrates a representative behaviour for a binary (two-component) system, gold-silicon. Here x specifies the fraction of atoms that are silicon atoms, and Au_{1 - x}Si_x denotes a particular material in this family of materials. (Au is the chemical symbol for gold, Si is the symbol for silicon, and, for example, Au_0.8Si_0.2 denotes a material containing 20 percent silicon atoms and 80 percent gold atoms.) The solid curve labeled T_f shows the composition dependence of the freezing point; above this line the liquid phase is the stable form. There is a deep cusp near the composition x = 0.2. Near this special composition, as at a in the figure, a liquid is much more readily quenched than is a liquid at a distant composition such as b. To reach the glass phase, the liquid must be cooled from above T_f to below T_g without crystallizing. Throughout the temperature interval from T_f down to the glass transition temperature T_g, the liquid is at risk vis-à-vis crystallization. Since this dangerous interval is much longer at b than at a, a faster quench rate is needed for glass formation at b than at a.

      Diagrams similar to (though slightly more complicated than) Figure 5 exist for many binary systems. For example, in the oxide system CaO-Al₂O₃, in which the two end-member compositions (x = 0 and x = 1) correspond to pure calcium oxide (CaO) and pure aluminum oxide (alumina) (Al₂O₃), there is a deep minimum in the T_f-versus-x curve near the middle of the composition range. Although neither calcium oxide nor aluminum oxide readily forms a glass, glasses are easily formed from mixed compositions; for reasons related to this, many oxide glasses have complex chemical compositions.

Vapour condensation techniques

       Bonding types and glass transition temperatures of representative amorphous solidsIn the gold-silicon system of Figure 5, at compositions far from the cusp, glasses cannot be formed by melt quenching—even by the rapid splat-quench technique of Figure 4. (This is the reason that the T_g curve of Figure 5 spans only compositions near the cusp.) Amorphous solids can still be prepared by dispensing with the liquid phase completely and constructing a thin solid film in atom-by-atom fashion from the gas phase. Figure 4D shows the simplest of these vapour-condensation techniques. A vapour stream, formed within a vacuum chamber by thermal evaporation (vaporization) of a sample of the material to be deposited, impinges on the surface of a cold substrate. The atoms condense on the cold surface and, under a range of conditions (usually a high rate of deposition and a low substrate temperature), an amorphous solid is formed as a thin film. Pure silicon can be prepared as an amorphous solid in this manner. Variations of the method include using an electron beam to vapourize the source or using the plasma-induced decomposition of a molecular species. The latter technique is used to deposit amorphous silicon from gaseous silane (SiH₄). Among the amorphous solids listed in the Table (Bonding types and glass transition temperatures of representative amorphous solids), those that normally require vapour-condensation methods for their preparation are silicon (Si), germanium (Ge), water (H₂O), and the elemental metallic glasses iron (Fe), cobalt (Co), and bismuth (Bi).

Other preparation techniques

      Numerous other methods exist for preparing amorphous solids, and new methods are continually invented. In melt spinning, a jet of molten metal is propelled against the moving surface of a cold, rotating copper cylinder. A solid film of metallic glass is spun off as a continuous ribbon at a speed that can exceed a kilometre per minute. In laser glazing, a brief intense laser pulse melts a tiny spot, which is swiftly quenched by the surrounding material into a glass. In sol-gel synthesis, small molecules in a liquid solution chemically link up with each other, forming a disordered network. It is possible to take a crystalline solid and convert it into an amorphous solid by bombarding it with high-kinetic-energy ions (ion). Under certain conditions of composition and temperature, interdiffusion (mixing on an atomic scale) between crystalline layers can produce an amorphous phase. Pyrolysis and electrolysis are other methods that can be used.

Atomic-scale structure

The radial distribution function

      The absence of long-range order is the defining characteristic of the atomic arrangement in amorphous solids. However, because of the absence in glasses of long parallel rows and flat parallel planes of atoms, it is extremely difficult to determine details of the atomic arrangement with the structure-probing techniques (such as X-ray diffraction) that are so successful for crystals. For glasses the information obtained from such structure-probing experiments is contained in a curve called the radial distribution function (RDF).

      Figure 6 shows a comparison of the experimentally determined RDFs of the crystalline and amorphous forms of germanium, an elemental semiconductor similar to silicon. The heavy curve labeled a-Ge corresponds to amorphous germanium; the light curve labeled c-Ge corresponds to crystalline germanium. The significance of the RDF is that it gives the probability of neighbouring atoms being located at various distances from an average atom. The horizontal axis in the figure specifies the distance from a given atom; the vertical axis is proportional to the average number of atoms found at each distance. (The distance scale is expressed in angstrom units; one angstrom equals 10^-8 centimetre.) The curve for crystalline germanium displays sharp peaks over the full range shown, corresponding to well-defined shells of neighbouring atoms at specific distances, which arise from the long-range regularity of the crystal's atomic arrangement. Amorphous germanium exhibits a close-in sharp peak corresponding to the nearest-neighbour atoms (there are four nearest neighbours in both c-Ge and a-Ge), but at larger distances the undulations in the RDF curve become washed out owing to the absence of long-range order. The first, sharp, nearest-neighbour peak in a-Ge is identical to the corresponding peak in c-Ge, showing that the short-range order in the amorphous form of solid germanium is as well-defined as it is in the crystalline form.

      The detailed shape of the a-Ge RDF curve of Figure 6 is the input used in the difficult task of developing a model for the atomic arrangement in amorphous germanium. The normal procedure is to construct a model of the structure and then to calculate from the model's atomic positions a theoretical RDF curve. This calculated RDF is then compared to the experimental curve (which provides the definitive test of the validity of the model). Computer-assisted refinements are then made in the model in order to improve the agreement between the model-dependent theoretical RDF and the experimentally observed RDF. This program has been successfully carried out for many amorphous solids, so there is now much that is known about their atomic-scale structure. In contrast to the complete information available for crystals, however, the structural knowledge of glasses still contains gaps.

Models of atomic scale structures

 Amorphous solids, like crystalline solids, exhibit a wide variety of atomic-scale structures. Most of these can be recognized as falling within one or another of three broad classes of structure associated with the following models: (1) the continuous random-network model, applicable to covalently bonded glasses, such as amorphous silicon and the oxide glasses, (2) the random-coil model, applicable to the many polymer-chain organic glasses, such as polystyrene, and (3) the random close-packing model, applicable to metallic glasses, such as Au_0.8Si_0.2 gold-silicon. These are the names in conventional use for the models. Although each of them contains the word random, the well-defined short-range order means that they are not random in the sense that the gas structure of Figure 2C—> is random.

    An illustration of the continuous random-network model is shown in Figure 7A—> and of the random-coil model in Figure 7B—>. Figure 7A—> reproduces a famous diagram published by W.H. Zachariasen in 1932. It is for a hypothetical two-dimensional A₂B₃ glass in which every A atom is bonded to three B atoms and every B atom to two A atoms. This picture bears a reasonable resemblance to current models for the arsenic chalcogenide glasses As₂S₃ and As₂Se₃. (Sulfur, S, and selenium, Se, belong to the group of elements called chalcogens.) The model was introduced as a schematic analogue for the network structure of the oxide glasses. The prototypical oxide glass is amorphous SiO₂, or silica glass. (Quartz, which is present in sand, is a crystalline form of SiO₂.) In amorphous SiO₂ each silicon atom is bonded to four oxygen atoms, and each oxygen atom is bonded to two silicon atoms. This structure is difficult to represent in a two-dimensional picture, but Figure 7A—> is a useful analogue with the hollow circles representing oxygen atoms and the small solid dots representing silicon atoms. The fourth bond originating at each silicon can be imagined to be out of the plane of the diagram.

 The network structure shown in Figure 7A—> clearly demonstrates how short-range order (note the triangle of neighbours surrounding each solid dot) is compatible with the absence of long-range order. At the bridging oxygen atoms, the bond angles have some flexibility, so it is easy to continue the network. Common oxide glasses are chemically more complex than SiO₂, as discussed in the next section. Chemical species such as phosphorus and germanium, which (like silicon) enter into the structure of the network by forming strong chemical bonds with oxygen atoms, are called network formers. Chemical species such as sodium and calcium, which do not bond directly to the network but which simply sit (in ionic form) within its interstitial holes, are called network modifiers.

 A large fraction of the everyday materials called plastics (plastic) are amorphous solids composed of long-chain molecules known as polymers (polymer). Each polymer chain has a backbone consisting of a string of many (up to roughly 100,000) carbon atoms bonded to each other. These organic polymeric glasses are present in innumerable familiar molded products (e.g., pens, tires, toys, appliance bodies, building materials, and automobile and airplane parts). The random-coil model of Figure 7B—>, first proposed in 1949 by P.J. Flory (Flory, Paul J.) (who later received a Nobel Prize in Chemistry for his pioneering work on polymers), is the established structural model for this important class of amorphous solids. As schematically sketched in the figure, the structure consists of intermeshed, entangled polymer chains. The chain configurations are well-defined, statistically, by a mathematical trajectory called a three-dimensional random walk.

      The third important structural model, the random close-packing model for metallic glasses, is difficult to illustrate with a simple diagram. Roughly speaking, it is similar to the structure that arises when a bunch of marbles are swiftly scrunched together in a paper bag.

Richard Zallen

Properties of oxide glasses

      The wide range of the properties of glasses depends on their composition, and special effects result from the presence of various modifying agents in certain basic glass-forming materials (see above Atomic-scale structure (amorphous solid)).

      One of the most important glass formers is silica (SiO₂). Pure crystalline silica melts at 1,710° C. In pure form, silica glass exhibits such properties as low thermal expansion, high softening temperatures, and excellent chemical and electrical resistance. In pure form it is relatively transparent over a wide range of wavelengths to visible and ultraviolet light and to ultrasonic waves.

      The high viscosity (see below) and melting temperature of silica glass are affected by the presence or absence of other materials. For example, if certain materials called fluxes are added, the most important being soda (Na₂O), both viscosity and melting (melting point) temperature can be reduced. If too much soda is added, the resulting glass is readily attacked by water, but, if there are suitable amounts of stabilizing oxides, such as lime (CaO) and magnesia (MgO), the glass becomes more durable. Most commercial glass has a soda-lime-silica (soda-lime glass) composition and is produced in vast quantities for plate and sheet glass, containers, and lightbulbs.

      In soda-lime-silica glasses, if lime is replaced by lead oxide (PbO) and if potash (K₂O) is used as a partial replacement for soda, lead-alkali-silicate glasses result that have lower softening points than lime glasses. The refractive indices, dispersive powers, and electrical resistance of these glasses are generally much greater than those of soda-lime-silica glasses.

      Boric oxide (B₂O₃), itself a glass former, acts as a flux (i.e., lowers the working temperature) when present in silica and forms borosilicate glass (Pyrex), and the substitution of small percentages of alkali and alumina increases the chemical stability. It also exhibits low thermal expansion, high dielectric strength, and high softening temperature.

      Aluminosilicate glasses find applications similar to those of borosilicates, but the former can stand higher operating temperatures; glasses with relatively high alumina contents and no boric oxide are exceptionally resistant to alkalies.

       Characteristics of oxide glassesThe above glasses all have silica as the glass former. With other glass formers, glasses have special properties. For example, if boric oxide is present, X rays are transmitted and rare-earth glasses will exhibit low dispersion and a high refractive index. Phosphate glasses (used as optical glasses) based on phosphorus pentoxide (P₂O₅) are highly resistant to hydrofluoric acid and act as efficient heat absorbers when iron oxide is added. The Table (Characteristics of oxide glasses) gives the compositions and physical properties of some typical commercial oxide glasses of the types described.

Ronald Walter Douglas Richard Zallen

Properties and applications of amorphous solids

      The following sections discuss technological applications of amorphous solids in connection with the properties that make those applications possible. It is important to understand that, although differences do exist between the properties of amorphous and crystalline solids, it is nevertheless broadly true that amorphous solids exhibit essentially the full range of properties and phenomena exhibited by crystalline solids. There are amorphous-solid metals, semiconductors, and insulators; there are transparent glasses and opaque glasses; and there are superconducting amorphous solids and ferromagnetic amorphous solids.

 Some of the general differences between the properties of crystals and glasses, in addition to the fundamental one of the glass transition (as discussed above in connection with Figure 3—> and also below with regard to its value in technological settings), are noted here. The atomic-scale disorder present in a metallic glass causes its electrical conductivity to be lower than the conductivity of the corresponding crystalline metal, because the structural disorder impedes the motion of the mobile electrons that make up the electrical current. (This lower electrical conductivity for the amorphous metal can be an advantage in some situations, as discussed below in the section Magnetic glasses (amorphous solid).) For a similar reason, the thermal conductivity of an insulating glass is lower than that of the corresponding crystalline insulator; glasses thus make good thermal insulators. Crystals and glasses also differ systematically in their optical spectra, which are the curves that describe the wavelength dependence of the degree to which the solid absorbs infrared, visible, or ultraviolet light. Although the overall spectra are often similar, crystal spectra typically exhibit sharp peaks and other features that specifically arise as a consequence of the long-range order of the crystal's atomic-scale structure. These sharp features are absent in the optical spectra of amorphous solids.

       Characteristics of oxide glassesThe continuous liquid-to-solid transition near T_g, the glass transition, has a profound significance in connection with classical applications of glasses. While crystallization abruptly transforms a mobile, low- viscosity liquid to a crystalline solid at T_f, near T_g the liquid viscosity increases continuously through a large range in the transformation to an amorphous solid. Viscosity, expressed in units of poise, is used in the Table (Characteristics of oxide glasses) to specify characteristic working temperatures in the processing of the liquid precursors of various oxide glasses. A poise is the centimetre-gram-second (cgs) unit of viscosity. It expresses the force needed to maintain a unit velocity difference between parallel plates separated by one centimetre of fluid: one poise equals one dyne-second per square centimetre. Molten glass may have a viscosity of 10¹³ poise (similar to honey on a cold day), and it quickly gets stiffer when cooled since the viscosity steeply increases with decreasing temperature. The ability to “tune” the viscosity of the melt (by changing temperature) allows glass to be conveniently processed and worked into desired shapes; glassblowing is a classic example of the usefulness of this widely exploited property.

  The Table—> lists some important technological uses of amorphous solids. In addition to the application, the general type of amorphous solid used, and the material properties that make the application possible, the table also includes information about the chemical compositions of typical materials employed in these techniques. While the first entry—namely, window glass—represents the present status of a centuries-old technology, the other entries correspond to technologies that have blossomed during the second half of the 20th century. A significant theme of the Table—> is the role of amorphous solids in applications calling for large-area sheets or films. Amorphous solids often have great advantages over crystalline solids in such applications, since their use avoids the functional problems associated with polycrystallinity or the expense of preparing large single crystals. Thus, while it would be prohibitively expensive to fabricate large windows out of crystalline SiO₂ (quartz), it is practical to do so using SiO₂-based silicate glasses.

Transparent glasses

       Characteristics of oxide glassesThe terms glass and window glass are often used interchangeably in everyday language, so familiar is this ancient architectural application of amorphous solids. Not only are oxide glasses, such as those characterized in the Table (Characteristics of oxide glasses), excellent for letting light in, they are also good for keeping cold out, because (as mentioned above) they are efficient thermal insulators.

 The second application in the Table—> represents a modern development that carries the property of optical transparency to a phenomenal level. The transparency of the extraordinarily pure glasses that have been developed for fibre-optic telecommunications is so great that, at certain wavelengths, light can pass through 1 kilometre (0.6 mile) of glass and still retain 95 percent of its original intensity.

      Glass fibres (transmitting optical signals) are now doing what copper wires (transmitting electrical signals) once did and are doing it more efficiently: carrying telephone messages around the planet. How this is done is schematically indicated in Figure 8. Digital electrical pulses produced by encoding of the voice-driven electrical signal are converted into light pulses by a semiconductor laser coupled to one end of the optical fibre. The signal is then transmitted over a long length of fibre as a stream of light pulses. At the far end it is converted back into electrical pulses and then into sound.

      The glass fibre is somewhat thinner than a human hair. The simplest type, as sketched in the upper left of the figure, has a central core of ultratransparent glass surrounded by a coaxial cladding of a glass having a lower refractive index, n. This ensures that light rays propagating within the core, at small angles relative to the fibre axis, do not leak out but instead are 100 percent reflected at the core-cladding interface by the optical effect known as total internal reflection.

      The great advantage provided by the substitution of light-transmitting fibres of ultratransparent oxide glass for electricity-transmitting wires of crystalline copper is that a single optical fibre can carry many more simultaneous conversations than can a thick cable packed with copper wires. This is the case because light waves oscillate at enormously high frequencies (about 2 × 10¹⁴ cycles per second for the infrared light generally used for fibre-optic telecommunications). This allows the light-wave signal carrier to be modulated at very high frequencies and to transmit a high volume of information traffic. Fibre-optic communications have greatly expanded the information-transmitting capacity of the world's telecommunications networks.

Polymeric structural materials

   polystyrene, the organic polymer listed in the Table—, is a prototypical example of a polymeric glass. These glasses, whose atomic-scale structure has been discussed in connection with Figure 7B—>, make up a broad class of lightweight structural materials important in the automotive, aerospace, and construction industries. These materials are also ubiquitous in everyday experience as plastic molded objects. The quantity of polymer materials produced each year, measured in terms of volume, exceeds the quantity of steel produced.

      Polystyrene is among the most important of the thermoplastic materials that, when heated (to the vicinity of the glass transition temperature), soften and flow controllably, enabling them to be processed at high speeds and on a large scale in the manufacture of molded products. The chemical formula of a polystyrene chain may be written as (CH₂CHC₆H₅)_N. The building block (inside the parentheses) consists of two backbone carbon atoms to which three hydrogen atoms and one phenyl (C₆H₅) ring are bonded as side groups. The polymerization index N reaches values above 10⁵. Polystyrene is a purely hydrocarbon polymer (i.e., it contains only hydrogen and carbon); most organic polymers contain additional chemical components.

Amorphous semiconductors (semiconductor) in electronics

      Amorphous semiconductors, in the form of thin films prepared by methods such as that shown in Figure 4D, are important in applications requiring large areas of electronically active material. The first electronic application of amorphous semiconductors to occur on a large scale was in xerography (electrophotography) (or electrostatic imaging), the process that provides the basis of plain-paper copiers. Amorphous selenium (Se) and, later, amorphous arsenic selenide (As₂Se₃) were used to form the thin-film, large-area photoconducting element that lies at the heart of the xerographic process. The photoconductor, which is an electrical insulator in the absence of light but which conducts electricity when illuminated, is exposed to an image of the document to be copied. Throughout the world—in offices, libraries, schools, and so forth—the xerographic process makes more than five billion copies every day. This process is also widely used in laser printers, in which the photoconductor is exposed to a digitally controlled on-and-off laser beam that is raster scanned (like the electron beam in a television tube) over the photoconductor surface.

      Although still in use, selenium and arsenic selenide have been joined by other amorphous materials in this important technology. Polymeric organic glasses, in the form of thin films, are now used in multilayer photoconductor configurations in which the light is absorbed in one layer and electrical charge is transported through an adjacent layer. Both layers are formed of amorphous polymer films, and these photoreceptors can be made in the form of flexible belts.

 Amorphous silicon thin films are used in solar cells (solar cell) that power handheld calculators. This important amorphous semiconductor is also used as the image sensor in facsimile (fax) (“fax”) machines, and it serves as the photoreceptor in some xerographic copiers. All these applications exploit the ability of amorphous silicon to be vapour-deposited in the form of large-area thin films. As shown in the Table—>, the practical form of this amorphous semiconductor is not pure silicon but a silicon-hydrogen alloy containing 10 percent hydrogen. The key role played by hydrogen, in what is now called hydrogenated amorphous silicon, emerged in a scientific puzzle that took years to solve. Stated briefly, hydrogen eliminates the electronic defects that are intrinsic to pure amorphous silicon.

      Hydrogenated amorphous silicon also is used in high-resolution flat-panel displays for computer monitors and for television screens. In such applications the large-area amorphous-semiconductor thin film is etched into an array of many tiny units, each of which forms the active element of a transistor that electronically turns on or off a small pixel (picture element) of a liquid-crystal display.

Magnetic glasses

 The last entry in the Table—> is an application of metallic glasses having magnetic properties. These are typically iron-rich (iron) amorphous solids with compositions such as Fe_0.8B_0.2 iron-boron and Fe_0.8B_0.1Si_0.1 iron-boron-silicon. They are readily formed as long metallic glass ribbons by melt spinning or as wide sheets by planar flow casting. Ferromagnetic glasses are mechanically hard materials, but they are magnetically soft, meaning that they are easily magnetized by small magnetic fields. Also, because of their disordered atomic-scale structure, they have higher electrical resistance than conventional (crystalline) magnetic materials. The three attributes of ease of manufacture, magnetic softness, and high electrical resistance make magnetic glasses extremely suitable for use in the magnetic cores of electrical power transformers. High electrical resistance (which arises here as a direct consequence of amorphicity) is a crucial property in this application, because it minimizes unwanted electrical eddy currents and cuts down on power losses. For these reasons, sheets of iron-based magnetic glasses (glass) are used as transformer-core (transformer) laminations in electrical power applications.

      Thin films of magnetic glass are finding use in many other applications. These include magnetic recording media for audio and video digital recording, as well as recording heads used with magnetic disks.

Richard Zallen

Additional Reading

Works on solids in general include Lawrence H. Van Vlack, Elements of Materials Science and Engineering, 6th ed. (1989), an elementary textbook; Charles A. Wert and Robb M. Thomson, Physics of Solids, 2nd ed. (1970), an intermediate-level text; Charles Kittel, Introduction to Solid State Physics, 6th ed. (1986), the standard college textbook; Neil W. Ashcroft and N. David Mermin, Solid State Physics (1976), an advanced textbook; George E. Bacon, The Architecture of Solids (1981), an introduction to bonding and structure; and Linus Pauling, The Nature of the Chemical Bond and the Structure of Molecules and Crystals, 3rd ed. (1960, reissued 1989), the classic reference work on chemical bonding.Gerald D. Mahan On amorphous solids in particular, a lucid introductory text accessible to a nontechnical reader is Richard Zallen, The Physics of Amorphous Solids (1983), with coverage of structural models for the various classes of amorphous solids as well as percolation theory, a modern paradigm for disordered systems. A classic advanced work is N.F. Mott and E.A. Davis, Electronic Processes in Non-crystalline Materials, 2nd ed. (1979), which features many of the theoretical contributions of Nobel Laureate coauthor Mott. A text providing a thorough treatment of oxide glasses is J. Zarzycki, Glasses and the Vitreous State (1991; originally published in French, 1982). A reference work with wide coverage of recent research topics, including detailed treatment of chalcogenide glasses, is S.R. Elliott, Physics of Amorphous Materials, 2nd ed. (1990). A comprehensive collection of detailed reviews is contained in R.W. Cahn, P. Haassen, and E.J. Kramer (eds.), Materials Science and Technology, vol. 6, Glasses and Amorphous Materials, ed. by J. Zarzycki (1991), including coverage of glass technology, formation, and structure, oxide glasses, chalcogenide glasses, metallic glasses, polymeric glasses, and the optical, electric, and mechanical properties of glasses. Amorphous silicon is treated in detail in another work, R.A. Street, Hydrogenated Amorphous Silicon (1991).Richard Zallen

* * *