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Amorphous Materials Reference - BillDoll
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Amorphous Materials
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Content derived from Wikipedia article on Amorphous Solids
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 or morphous). 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. As the cooling is performed, the material changes from a supercooled liquid, with properties one would expect from a liquid state material, to a solid. The temperature at which this transition occurs is called the 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). 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.
Unsolved problems in physics: What is the nature of the transition between a fluid or regular solid and a glassy phase? What are the physical processes giving rise to the general properties of glasses? 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 sputter deposition 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.
Related topics @ Wikipedia
Glass Amorphous silicon Supercooling Vitrification
Wikipedia link: http://en.wikipedia.org/wiki/Amorphous_solid
Pictures
See a network structure of amorphous materials – from IBM See a crystal structure of amorphous materials – from UIUC Large flake of clear amorphous material – from Medscape
Content derived from Wikipedia article on Amorphous Metals
Amorphous metal
An amorphous metal is a metallic material with a disordered atomic-scale structure. In contrast to most metals, which are crystalline and therefore have a highly ordered arrangement of atoms, amorphous alloys are non-crystalline. Materials in which such a disordered structure is produced directly from the liquid state during cooling are called "glasses", and so amorphous metals are commonly referred to as "metallic glasses" or "glassy metals". However, there are several other ways in which amorphous metals can be produced, including physical vapor deposition, solid-state reaction, ion irradiation, and mechanical alloying. Amorphous metals produced by these techniques are, strictly speaking, not glasses, but materials scientists commonly consider amorphous alloys to be a single class of materials, regardless of how they are prepared.
Bulk metallic glasses (BMG) are amorphous metals with critical cooling rates low enough to allow formation of amorphous structure in thick layers (over 1 millimeter).
History
The first metallic glass was an alloy (Au80Si20), produced at Caltech by Pol Duwez in 1957. This and other early glass-forming alloys had to be cooled extremely rapidly (on the order of one megakelvin per second, 106 K·s-1) to avoid crystallization. An important consequence of this was that metallic glasses could only be produced in a limited number of forms (typically ribbons, foils, or wires) in which one dimension was small so that heat could be extracted quickly enough to achieve the necessary cooling rate. As a result, metallic glass specimens (with a few exceptions) were limited to thicknesses of less than one-tenth of a millimeter.
In 1969, an alloy of 77.5% palladium, 6% copper, and 16.5% silicon was found to have critical cooling rate between 100 K/s to 1000 K/s.
In 1970s, AlliedSignal developed a new method of manufacturing thin ribbons of amorphous metals, by casting molten alloy of iron, nickel, phosphorus and boron onto a supercooled fast-spinning wheel. The material, known as Metglas, was commercialized in early 1980's and used for low-loss power distribution transformers. Metglas-2605 is composed of 80% iron and 20% boron, has Curie temperature of 373 °C and a room temperature saturation magnetization of 125.7 milliteslas.
In the early 1980's, glassy ingots with 5mm diameter were produced from the alloy of 55% palladium, 22.5% lead, and 22.5% antimony, by surface etching followed with heating-cooling cycles. Using boron oxide flux, the achievable thickness was increased to a centimeter.
The research in Tohoku University and Caltech yielded multicomponent alloys based on lanthanum, magnesium, zirconium, palladium, iron, copper, and titanium, with critical cooling rate between 1 K/s to 100 K/s, comparable to oxide glasses.
In 1988, alloys of lanthanum, aluminium, and copper or were found to be highly glass-forming.
In the 1990s, however, new alloys were developed that form glasses at cooling rates as low as one kelvin per second. These cooling rates can be achieved by simple casting into metallic molds. These "bulk" amorphous alloys can be cast into parts of up to several centimeters in thickness (the maximum thickness depending on the alloy) while retaining an amorphous structure. The best glass-forming alloys are based on zirconium and palladium, but alloys based on iron, titanium, copper, magnesium, and other metals are also known. Many amorphous alloys are formed by exploiting a phenomenon called the "confusion" effect. Such alloys contain so many different elements (often a dozen or more) that upon cooling at sufficiently fast rates, the constituent atoms simply cannot coordinate themselves into the equilibrium crystalline state before their mobility is stopped. In this way, the random disordered state of the atoms is "locked in".
In 1992, the first commercial amorphous alloy, Vitreloy 1 (41.2% Zr, 13.8% Ti, 12.5% Cu, 10% Ni, and 22.5% Be), was developed at Caltech, as a part of Department of Energy and NASA research of new aerospace materials. More variants followed.
In 2004, two groups succeeded in producing bulk amorphous steel, one at Oak Ridge National Laboratory, the other at University of Virginia. The Oak Ridge group refers to their product as "glassy steel". The product is non-magnetic at room temperature and significantly stronger than conventional steel, though a long research and development process remains before the introduction of the material into public or military use.
Properties
An amorphous metal is usually an alloy rather than a pure metal. The alloys contain atoms of significantly different sizes, leading to low free volume (and therefore up to orders of magnitude higher viscosity than other metals and alloys) in molten state. The viscosity prevents the atoms moving enough to form an ordered lattice. The material structure also results in low shrinkage during cooling, and resistance to plastic deformation. The absence of grain boundaries, the weak spots of crystalline materials, leads to better resistance to wear and corrosion. Amorphous metals, while technically glasses, are also much tougher and less brittle than oxide glasses and ceramics.
Thermal conductivity of amorphous materials is lower than of crystals. As formation of amorphous structure relies on fast cooling, this limits the maximum achievable thickness of amorphous structures.
To achieve formation of amorphous structure even during slower cooling, the alloy has to be made of three or more components, leading to complex crystal units with higher potential energy and lower chance of formation. The atomic radius of the components has to be significantly different (over 12%), to achieve high packing density and low free volume. The combination of components should have negative heat of mixing, inhibiting crystal nucleation and prolongs the time the molten metal stays in supercooled state.
The alloys of boron, silicon, phosphorus, and other glass formers with magnetic metals (iron, cobalt, nickel) are magnetic, with low coercivity and high electrical resistance. The high resistance leads to low losses by eddy currents when subjected to alternating magnetic fields, a property useful for eg. transformer magnetic cores.
Amorphous alloys have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition, and they can sustain larger reversible ("elastic") deformations than crystalline alloys. Amorphous metals derive their strength directly from their non-crystalline structure, which does not have any of the defects (such as dislocations) that limit the strength of crystalline alloys. One modern amorphous metal, known as Vitreloy, has a tensile strength that is almost twice that of high-grade titanium. However, metallic glasses at room temperature are not ductile and tend to fail suddenly when loaded in tension, which limits the material applicability in reliability-critical applications, as the impending failure is not evident. Therefore, there is considerable interest in producing metal matrix composite materials consisting of a metallic glass matrix containing dendritic particles or fibers of a ductile crystalline metal.
Perhaps the most useful property of bulk amorphous alloys is that they are true glasses, which means that they soften and flow upon heating. This allows for easy processing, such as by injection molding, in much the same way as polymers. As a result, amorphous alloys have been commercialized for use in sports equipment, medical devices, and as cases for electronic equipment.
Thin films of amorphous metals can be deposited via high velocity oxygen fuel technique as protective coatings.
References
^ (2005) "Glassy Steel". ORNL Review 38 (1). ^ V. Ponnambalam, S. Joseph Poon and Gary J. Shiflet (2004). "Fe-based bulk metallic glasses with diameter thickness larger than one centimeter". Journal of Materials Research 19 (5).
Companies
MetGlas LiquidMetal nettle_lynx Vacuumschmelze
Related topics in Wikipedia
Glass-ceramic-to-metal seals materials science Retrieved from http://en.wikipedia.org/wiki/Amorphous_metal
Credits & Copyright: This page is licensed under the GNU Free Documentation License. It uses material from the ||Wikipedia articles Amorphous metal & Amorphous solid
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