A solid in which the atoms are in some disordered arrangement, lacking the perfect ordered structure of crystals, as in glass, rubber, and polymers. Many substances may form either amorphous or crystalline states having radically different properties.
An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass is an amorphous ceramic, many polymers (such as polystyrene) are amorphous, and even foods such as cotton candy are amorphous solids.
Amorphous materials are often prepared by rapidly cooling molten material. For example addition of soda to silicon dioxide results in window glass and the addition of glycols to water results in a vitrified solid.
Some materials, such as metals, are difficult to prepare in an amorphous state.
Amorphous solids can exist in two distinct states, the 'rubbery' state and the 'glassy' state. The temperature at which the transition between the glassy and rubbery states is called their glass transition temperature or Tg.
Glasses
In common parlance, the term glass refers to amorphous oxides, and especially silicates (compounds based on silicon and oxygen). Without these additives silicon dioxide will (with slow cooling) form quartz crystals, not glass.
To avoid confusion, other types of glass often are referred to with a modifier, such as the term metallic glass to refer to amorphous metallic alloys.
Metallic glass
Some amorphous metallic alloys can be prepared under special processing conditions (such as rapid solidification, thin-film deposition, or ion implantation), but the term "metallic glass" refers only to rapidly solidified materials.
Even with special equipment, such rapid cooling is required that, for most metals, only a thin wire or ribbon can be made amorphous. Such alloy systems tend to have the following inter-related properties:
Many different solid phases are present in the equilibrium solid, so that any potential crystal will find that most of the nearby atoms are of the wrong type to join in crystallization The composition is near a deep eutectic, so that low melting temperatures can be achieved without sacrificing the slow diffusion and high liquid viscosity seen in alloys with high-melting pure components Atoms with a wide variety of sizes are present, so that "wrong-sized" atoms interfere with the crystallization process by binding to atom clusters as they form.Other synthesis routes
Amorphous solids produced by other routes, such as ion implantation and thin-film deposition are, technically speaking, not glasses.
Damage
One way to produce a material without an ordered structure is to take a crystalline material and remove the order by damaging it. This technique is known as ion implantation, and only forms amorphous solids if the material is too cold for atoms to diffuse back to their original positions as the process continues.
Cold deposition
Techniques such as sputtering and chemical vapour deposition can be used to deposit a thin film of material onto a surface.
Toward a strict definition
It is difficult to make a distinction between truly amorphous solids and crystalline solids in which the size of the crystals is very small (less than two nanometres). Even amorphous materials have some short-range order among the atomic positions (over length scales of less than five nanometres).
The transition from the liquid state to the glass, at a temperature below the equilibrium melting point of the material, is called the glass transition. From a practical point of view, the glass transition temperature is defined empirically as the temperature at which the viscosity of the liquid exceeds a certain value (commonly 1013 pascal-seconds). The transition temperature depends on cooling rate, with the glass transition occurring at higher temperatures for faster cooling rates. While it is clear that the glass transition is not a first-order thermodynamic transition (such as melting), there is debate as to whether it is a higher-order transition, or merely a kinetic effect.
Glass is sometimes referred to as a supercooled liquid;
Some examples of amorphous solids are glass, polystyrene, and the silicon in many thin film solar cells.
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