Amorphous solid

File:Candle with burnt wick.jpg

Wax and paraffin are amorphous.

An amorphous solid is a solid in which there is no long-range order in the positions of its atoms. On the other hand, if there is long-range atomic order, the solid is described as crystalline or morphous. Most classes of solid materials can be found or prepared in an amorphous form. For instance, common window glass, many polymers (such as polystyrene), and even foods such as cotton candy are amorphous solids.

Amorphous materials are often prepared by rapidly cooling molten material. The cooling reduces the mobility of the material's molecules before they can pack into a more thermodynamically favorable crystalline state. Amorphous materials can also be produced by additives which interfere with the ability of the primary constituent to crystallize. 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. Unless a material has a high melting temperature (as ceramics do) or a low crystallization energy (as polymers tend to), cooling must be done extremely rapidly.

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 T_g.

Glasses

In common parlance, the term glass refers to amorphous oxides, and especially silicates (compounds based on silicon and oxygen). Ordinary soda-lime glass, used in windows and drinking containers, is created by the addition of soda and lime (calcium oxide) to silicon dioxide. 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. This is enough for many magnetic applications, but thicker sections are required for most structural applications such as scalpel blades, golf clubs, and cases for consumer electronics. Recent efforts have made it possible to increase the maximum thickness of glassy castings, by finding alloys where kinetic barriers to crystallization are greater. 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.

One such alloy is the commercial "Liquidmetal", which can be cast in amorphous sections up to an inch thick.

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. A practical, controllable way to do this is by firing ions into the material at high speed, so that collisions inside the material knock all atoms from their original positions. 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. If the surface is kept cold, the atoms being deposited will not, on average, gain enough energy to diffuse along the surface until they find a place in an ordered crystal. For every deposition technique, there is a substrate temperature below which the deposited film will be amorphous. However, surface diffusion requires much less energy than diffusion through the bulk, so that these temperatures are often lower than those required to make amorphous films by ion implantation.

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). Furthermore, in very small crystals a large fraction of the atoms are located at or near the surface of the crystal; relaxation of the surface and interfacial effects distort the atomic positions, decreasing the structural order. Even the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales.

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. The precise nature of the glass transition is the subject of ongoing research. 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; this amounts to an assertion that the glass transition is purely a kinetic, rather than a thermodynamic effect. One argument against speaking this way is the fact that supercooled liquids flow whereas glass does not. In standard usage, the term supercooled means that the fluid is still a liquid but is at a temperature below its freezing point. For example, freezing rain falls in liquid form and freezes on contact because it is already below the freezing point. See pitch drop experiment and a related section in glass.

Some examples of amorphous solids are glass, polystyrene, and the silicon in many thin film solar cells.

Unsolved problems

• What is the nature of the phase transition between a fluid or regular solid and a glassy phase?
• What are the physical processes giving rise to the general properties of glasses?

See also

• Ceramic
• Crystal
• Glass
• Plastic
• Polymer

External links

• Vogel-Tammann-Fulcher Equation Parameters
• Fragility thy name is glass

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