Eutectic system
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A eutectic system is an mixture that has a single chemical composition that solidifies at a lower temperature than any other composition. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature. On a phase diagram the intersection of the eutectic temperature and the eutectic composition gives the eutectic point.[1] Not all binary alloys have a eutectic point: those that form a solid solution at all concentrations, such as the gold-silver system, have no eutectic.[citation needed]
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[edit] Eutectic reaction
The eutectic reaction is defined as follows:[2]
![\text{Liquid} \xrightarrow[\text{cooling}]{\text{eutectic temperature}} \alpha \,\, \text{solid solution} + \beta \,\, \text{solid solution}](http://upload.wikimedia.org/math/1/f/6/1f6e610faa6afd06bf7e16c7bb620167.png)
This type of reaction is a invariant reaction, because it is in thermal equilibrium; another way to define this is the Gibbs free energy equals zero. Tangibly, this means the liquid and two solid solutions all coexist at the same time and are in chemical equilibrium. There is also a thermal arrest for the duration of the reaction.[2]
The resulting solid macrostructure from a eutectic reaction depends on a few factors. The most important factor is how the two solid solutions nucleate and grow. The most common structure is a lamellar structure, but other possible structures include rodlike, globular, and acicular.[3]
[edit] Non-eutectic compositions
Compositions of a eutectic systems that are not the eutectic composition are commonly defined to be hypoeutectic or hypereutectic. Hypoeutectic composition are composition to the left of the eutectic composition and hypereutectic composition are compositions to the right.[2]
[edit] Types
[edit] Alloys
Eutectic alloys have two or more materials and have a eutectic composition. When a non-eutectic alloy solidifies, its components solidify at different temperatures, exhibiting a plastic melting range. A eutectic alloy solidifies at a single, sharp temperature. The phase transformations that occur while solidifying a given alloy can be understood by drawing a vertical line from the liquid phase to the solid phase on a phase diagram.
Some uses include:
- eutectic alloys for soldering, composed of tin (Sn), lead (Pb) and sometimes silver (Ag) or gold (Au)
- casting alloys, such as aluminium-silicon and cast iron (at the composition for an austenite-cementite eutectic in the iron-carbon system)
- one method used by the semiconductor industry to bond silicon chips to gold-plated substrates is to induce a silicon-gold eutectic through the application of ultrasonic energy to the chip
- brazing, where diffusion can remove alloying elements from the joint, so that eutectic melting is only possible early in the brazing process
- temperature response, i.e. Wood's metal and Field's metal for fire sprinklers
- non-toxic mercury replacements, such as galinstan
- experimental glassy metals, with extremely high strength and corrosion resistance
- eutectic alloys of sodium and potassium (NaK) that are liquid at room temperature and used as coolant in experimental fast neutron nuclear reactors.
[edit] Others
Sodium chloride and water form a eutectic mixture. It has a eutectic point of −21.2 C[4] and 23.3%[5] salt by mass. The eutectic nature of salt and water is exploited when salt is spread on roads to aid snow removal, or mixed with ice to produce low temperatures (for example, in traditional ice cream making).
Lidocaine and prilocaine, both solids at room temperature, form a eutectic that is an oil with a 16°C melting point, used in EMLA (Eutectic Mixture of Local Anesthetic) preparations.
Minerals may form eutectic mixtures in igneous rocks,[6] giving rise to characteristic intergrowth textures such as that of granophyre.
Some inks are eutectic mixtures, allowing inkjet printers to operate at lower temperatures.[7]
[edit] Other critical points
[edit] Eutectoid
When the solution above the transformation point is solid, rather than liquid, an analogous eutectoid transformation can occur. For instance, in the iron-carbon system, the austenite phase can undergo a eutectoid transformation to produce ferrite and cementite, often in lamellar structures such as pearlite and bainite. This eutectoid point occurs at 727 °C (1,341 °F) and about 0.83% carbon.[8]
[edit] Peritectic
Peritectic transformations are also similar to eutectic reactions. Here, a liquid and solid phase of fixed proportions react at a fixed temperature to yield a single solid phase. Since the solid product forms at the interface between the two reactants, it can form a diffusion barrier and generally causes such reactions to proceed much more slowly than eutectic or eutectoid transformations. Because of this, when a peritectic composition solidifies it does not show the lamellar structure that you find with eutectic solidification.
Such a transformation exists in the iron-carbon system, as seen near the upper-left corner of the figure. It resembles an inverted eutectic, with the δ phase combining with the liquid to produce pure austenite at 1,495 °C (2,723 °F) and 0.17% carbon.
[edit] See also
[edit] References
- ^ Smith & Hashemi 2006, pp. 326–327.
- ^ a b c Smith & Hashemi 2006, p. 327.
- ^ Smith & Hashemi 2006, pp. 332–333.
- ^ Muldrew, Ken; Locksley E. McGann (1997). "Phase Diagrams". Cryobiology—A Short Course. University of Calgary. http://www.ucalgary.ca/~kmuldrew/cryo_course/cryo_chap6_1.html. Retrieved 2006-04-29.
- ^ Senese, Fred (1999). "Does salt water expand as much as fresh water does when it freezes?". Solutions: Frequently asked questions. Department of Chemistry, Frostburg State University. http://antoine.frostburg.edu/chem/senese/101/solutions/faq/saltwater-ice-volume.shtml. Retrieved 2006-04-29.
- ^ Fichter, Lynn S. (2000). "Igneous Phase Diagrams". Igneous Rocks. James Madison University. http://csmres.jmu.edu/geollab/Fichter/IgnRx/Phasdgrm.html. Retrieved 2006-04-29.
- ^ Davies, Nicholas A.; Beatrice M. Nicholas (1992). "Eutectic compositions for hot melt jet inks". US Patent & Trademark Office, Patent Full Text and Image Database. United States Patent and Trademark Office. http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=/netahtml/search-bool.html&r=1&f=G&l=50&co1=AND&d=ptxt&s1=5298062.WKU.&OS=PN/5298062&RS=PN/5298062. Retrieved 2006-04-29.
- ^ Iron-Iron Carbide Phase Diagram Example
[edit] Bibliography
- Smith, William F.; Hashemi, Javad (2006), Foundations of Materials Science and Engineering (4th ed.), McGraw-Hill, ISBN 0-07-295358-6.
[edit] Further reading
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Look up eutectic in Wiktionary, the free dictionary. |
- Askeland, Donald R.; Pradeep P. Phule (2005). The Science and Engineering of Materials. Thomson-Engineering. ISBN 0-534-55396-6.
- Easterling, Edward (1992). Phase Transformations in Metals and Alloys. CRC. ISBN 0-7487-5741-4.
- Mortimer, Robert G. (2000). Physical Chemistry. Academic Press. ISBN 0-12-508345-9.
- Reed-Hill, R.E.; Reza Abbaschian (1992). Physical Metallurgy Principles. Thomson-Engineering. ISBN 0-534-92173-6.
- Sadoway, Donald (2004). "Phase Equilibria and Phase Diagrams" (pdf). 3.091 Introduction to Solid State Chemistry, Fall 2004. MIT Open Courseware. http://ocw.mit.edu/NR/rdonlyres/Materials-Science-and-Engineering/3-091Fall-2004/6ECFB930-9D59-4DD5-A872-FE48002587B0/0/notes_10.pdf. Retrieved 2006-04-12.
