Amorphous solid
An
amorphous solid is a
solid in which there is no
long-range order of the positions of the
atoms. (Solids in which there is long-range atomic order are called
crystalline solids.) 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. 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
Tg.
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.
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.
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 10
13 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.
*
Glass*
Supercooling*
Vitrification*
Vogel-Tammann-Fulcher Equation Parameters*
Fragility thy name is glass
