Eutectic System
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A eutectic system or eutectic mixture ( ) is a
homogeneous mixture In chemistry, a mixture is a material made up of two or more different chemical substances which are not chemically bonded. A mixture is the physical combination of two or more substances in which the identities are retained and are mixed in the ...
that has a
melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...
lower than those of the constituents. The lowest possible melting point over all of the mixing ratios of the constituents is called the ''eutectic temperature''. On a
phase diagram A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions (pressure, temperature, volume, etc.) at which thermodynamically distinct phases (such as solid, liquid or gaseous ...
, the eutectic temperature is seen as the eutectic point (see plot on the right). Non-eutectic mixture ratios would have different melting temperatures for their different constituents, since one component's
lattice Lattice may refer to: Arts and design * Latticework, an ornamental criss-crossed framework, an arrangement of crossing laths or other thin strips of material * Lattice (music), an organized grid model of pitch ratios * Lattice (pastry), an orna ...
will melt at a lower temperature than the other's. Conversely, as a non-eutectic mixture cools down, each of its components would solidify (form a lattice) at a different temperature, until the entire mass is solid. Not all
binary alloy Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that ta ...
s have eutectic points, since the
valence electron In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair forms ...
s of the component species are not always compatible, in any mixing ratio, to form a new type of joint crystal lattice. For example, in the silver-gold system the melt temperature ( liquidus) and freeze temperature (
solidus Solidus (Latin for "solid") may refer to: * Solidus (coin), a Roman coin of nearly solid gold * Solidus (punctuation), or slash, a punctuation mark * Solidus (chemistry), the line on a phase diagram below which a substance is completely solid * S ...
) "meet at the pure element endpoints of the atomic ratio axis while slightly separating in the mixture region of this axis". The term was coined in 1884 by British physicist and chemist
Frederick Guthrie Frederick Guthrie FRS FRSE (15 October 1833 – 21 October 1886) was a British physicist and chemist and academic author. He was the son of Alexander Guthrie, a London tradesman, and the younger brother of mathematician Francis Guthrie. Alon ...
(1833–1886). The word originates from the Greek ( 'well') and ( 'melting').


Eutectic phase transition

The eutectic solidification is defined as follows:. :\text \xrightarrow
text Text may refer to: Written word * Text (literary theory), any object that can be read, including: **Religious text, a writing that a religious tradition considers to be sacred **Text, a verse or passage from scripture used in expository preachin ...
\alpha \,\, \text + \beta \,\, \text This type of reaction is an invariant reaction, because it is in
thermal equilibrium Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in ...
; another way to define this is the change in
Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and pr ...
equals zero. Tangibly, this means the liquid and two
solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word ...
s all coexist at the same time and are in
chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the sy ...
. There is also a
thermal arrest A cooling curve is a line graph that represents the change of phase of matter, typically from a gas to a solid or a liquid to a solid. The independent variable (X-axis) is time and the dependent variable (Y-axis) is temperature.Garland, Nibler ...
for the duration of the change of phase during which the temperature of the system does not change. The resulting solid macrostructure from a eutectic reaction depends on a few factors, with the most important factor being how the two solid solutions nucleate and grow. The most common structure is a
lamellar structure In materials science, lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae. They are often observed in cases where a phase transition front moves quickly, leaving behind ...
, but other possible structures include rodlike, globular, and acicular.


Non-eutectic compositions

Compositions of eutectic systems that are not at the eutectic point can be classified as ''hypoeutectic'' or ''hypereutectic''. Hypoeutectic compositions are those with a smaller percent composition of species β and a greater composition of species α than the eutectic composition (E) while hypereutectic solutions are characterized as those with a higher composition of species β and a lower composition of species α than the eutectic composition. As the temperature of a non-eutectic composition is lowered the liquid mixture will precipitate one component of the mixture before the other. In a hypereutectic solution, there will be a proeutectoid phase of species β whereas a hypoeutectic solution will have a proeutectic α phase.


Types


Alloys

Eutectic
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
s 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. Conversely, when a well-mixed, eutectic alloy melts, it does so at a single, sharp temperature. The various phase transformations that occur during the solidification of a particular alloy composition can be understood by drawing a vertical line from the liquid phase to the solid phase on the phase diagram for that alloy. Some uses include: *NEMA Eutectic Alloy Overload Relays for electrical protection of 3-phase motors for pumps, fans, conveyors, and other factory process equipment. *Eutectic alloys for
soldering Soldering (; ) is a process in which two or more items are joined by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involv ...
, both traditional alloys composed of
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
(Pb) and
tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
(Sn), sometimes with additional
silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
(Ag) or
gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
(Au) — especially SnPb and SnPbAg alloy formula for electronics - and newer lead-free soldering alloys, in particular ones composed of
tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
(Sn),
silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
(Ag), and
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
(Cu) such as SnAg. *Casting alloys, such as aluminium-silicon and
cast iron Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...
(at the composition of 4.3% carbon in iron producing an
austenite Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K ...
-
cementite Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, bri ...
eutectic) *
Silicon chip An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
s are bonded to gold-plated substrates through a silicon-gold eutectic by the application of
ultrasonic Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
energy to the chip. See
eutectic bonding Eutectic bonding, also referred to as eutectic soldering, describes a wafer bonding technique with an intermediate metal layer that can produce a eutectic system. Those eutectic metals are alloys that transform directly from solid to liquid state ...
. *
Brazing Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal. Brazing differs from we ...
, where diffusion can remove alloying elements from the joint, so that eutectic melting is only possible early in the brazing process *Temperature response, e.g.,
Wood's metal Wood's metal, also known as Lipowitz's alloy or by the commercial names Cerrobend, Bendalloy, Pewtalloy and MCP 158, is a metal alloy that is useful for soldering and making custom metal parts, but which is toxic to touch or breathe vapors from. ...
and Field's metal for
fire sprinkler A fire sprinkler or sprinkler head is the component of a fire sprinkler system that discharges water when the effects of a fire have been detected, such as when a predetermined temperature has been exceeded. Fire sprinklers are extensively use ...
s *Non-toxic
mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
replacements, such as galinstan *Experimental glassy metals, with extremely high strength and
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
resistance *Eutectic alloys of
sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
and
potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
(
NaK In data networking, telecommunications, and computer buses, an acknowledgment (ACK) is a signal that is passed between communicating processes, computers, or devices to signify acknowledgment, or receipt of message, as part of a communicatio ...
) that are liquid at room temperature and used as
coolant A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosio ...
in experimental fast neutron nuclear reactors.


Others

*
Sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...
and
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
form a eutectic mixture whose eutectic point is −21.2 °C and 23.3% salt by mass. The eutectic nature of salt and water is exploited when salt is spread on roads to aid
snow removal Snow removal or snow clearing is the job of removing snow after a snowfall to make travel easier and safer. This is done by both individual households and by governments and institutions. De-icing and anti-icing De-icing is defined as removal ...
, or mixed with ice to produce low temperatures (for example, in traditional
ice cream Ice cream is a sweetened frozen food typically eaten as a snack or dessert. It may be made from milk or cream and is flavoured with a sweetener, either sugar or an alternative, and a spice, such as cocoa or vanilla, or with fruit such as str ...
making). * Ethanol–water has an unusually biased eutectic point, i.e. it is close to pure ethanol, which sets the maximum proof obtainable by
fractional freezing Fractional freezing is a process used in process engineering and chemistry to separate substances with different melting points. It can be done by partial melting of a solid, for example in zone refining of silicon or metals, or by partial crys ...
. * "Solar salt", 60% NaNO3 and 40% KNO3, forms a eutectic molten salt mixture which is used for
thermal energy storage Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, ...
in
concentrated solar power Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight into a receiver. Electricity is generated when ...
plants. To reduce the eutectic melting point in the solar molten salts,
calcium nitrate Calcium nitrate, also called ''Norgessalpeter'' (Norwegian salpeter), is an inorganic compound with the formula Ca(NO3)2(H2O)x. The anhydrous compound, which is rarely encountered, absorbs moisture from the air to give the tetrahydrate. Both anhyd ...
is used in the following proportion: 42% Ca(NO3)2, 43% KNO3, and 15% NaNO3. *
Lidocaine Lidocaine, also known as lignocaine and sold under the brand name Xylocaine among others, is a local anesthetic of the amino amide type. It is also used to treat ventricular tachycardia. When used for local anaesthesia or in nerve blocks, lidoca ...
and
prilocaine Prilocaine () is a local anesthetic of the amino amide type first prepared by Claes Tegner and Nils Löfgren. In its injectable form (trade name Citanest), it is often used in dentistry. It is also often combined with lidocaine as a topical ...
—both are solids at room temperature—form a eutectic that is an oil with a melting point that is used in eutectic mixture of local anesthetic (EMLA) preparations. *
Menthol Menthol is an organic compound, more specifically a monoterpenoid, made synthetically or obtained from the oils of corn mint, peppermint, or other mints. It is a waxy, clear or white crystalline substance, which is solid at room temperature and ...
and camphor, both solids at room temperature, form a eutectic that is a liquid at room temperature in the following proportions: 8:2, 7:3, 6:4, and 5:5. Both substances are common ingredients in pharmacy extemporaneous preparations. *
Mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
s may form eutectic mixtures in
igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
rocks, giving rise to characteristic intergrowth textures exhibited, for example, by
granophyre Granophyre ( ; from ''granite'' and ''porphyry'') is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image. The texture is called granophyric. The texture can b ...
. * Some inks are eutectic mixtures, allowing
inkjet printer Inkjet printing is a type of computer printing that recreates a digital image by propelling droplets of ink onto paper and plastic substrates. Inkjet printers were the most commonly used type of printer in 2008, and range from small inexpensi ...
s to operate at lower temperatures. *
Choline Choline is an essential nutrient for humans and many other animals. Choline occurs as a cation that forms various salts (X− in the depicted formula is an undefined counteranion). Humans are capable of some ''de novo synthesis'' of choline but r ...
chloride produces eutectic mixtures with many natural products such as
citric acid Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...
,
malic acid Malic acid is an organic compound with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms ...
and
sugars Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...
. These liquid mixtures can be used, for example, to obtain antioxidant and antidiabetic extracts from
natural product A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical syn ...
s.


Other critical points


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 Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K ...
phase can undergo a eutectoid transformation to produce ferrite and
cementite Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, bri ...
, often in lamellar structures such as
pearlite Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of ferrite (87.5 wt%) and cementite (12.5 wt%) that occurs in some steels and cast irons. During slow cooling of an iron-carbon alloy, pearlite forms ...
and
bainite Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite (the face-c ...
. This eutectoid point occurs at and 0.76 wt% carbon.


Peritectoid

A ''peritectoid'' transformation is a type of
isothermal In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and a ...
reversible reaction A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. : \mathit aA + \mathit bB \mathit cC + \mathit dD A and B can react to form C and D or, in the ...
that has two solid
phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...
s reacting with each other upon cooling of a binary, ternary, ..., ''n''-ary
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
to create a completely different and single solid phase. The reaction plays a key role in the order and
decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...
of
quasicrystalline A quasiperiodic crystal, or quasicrystal, is a structure that is ordered but not periodic. A quasicrystalline pattern can continuously fill all available space, but it lacks translational symmetry. While crystals, according to the classical c ...
phases in several alloy types. A similar structural transition is also predicted for rotating columnar crystals.


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 In materials science, lamellar structures or microstructures are composed of fine, alternating layers of different materials in the form of lamellae. They are often observed in cases where a phase transition front moves quickly, leaving behind ...
that is found 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 Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K ...
at and 0.17% carbon. At the peritectic decomposition temperature the compound, rather than melting, decomposes into another solid compound and a liquid. The proportion of each is determined by the
lever rule In chemistry, the lever rule is a formula used to determine the mole fraction (''xi'') or the mass fraction (''wi'') of each phase of a binary equilibrium phase diagram. It can be used to determine the fraction of liquid and solid phases for a ...
. In the Al-Au phase diagram, for example, it can be seen that only two of the phases melt congruently, AuAl2 and Au2Al , while the rest peritectically decompose.


Eutectic calculation

The composition and temperature of a eutectic can be calculated from enthalpy and entropy of fusion of each components. The Gibbs free energy ''G'' depends on its own differential: : G = H - TS \Rightarrow \begin H = G + TS \\ \left(\frac\right)_P = -S \end \Rightarrow H = G - T \left(\frac\right)_P. Thus, the ''G''/''T'' derivative at constant pressure is calculated by the following equation: : \left(\frac\right)_P = \frac \left(\frac\right)_P - \fracG = -\frac \left(G - T\left(\frac\right)_P\right) = -\frac. The chemical potential \mu_i is calculated if we assume that the activity is equal to the concentration: : \mu_i = \mu_i^\circ + RT\ln \frac \approx \mu_i^\circ + RT\ln x_i. At the equilibrium, \mu_i = 0, thus \mu_i^\circ is obtained as : \mu _i = \mu _i^\circ + RT\ln x_i = 0 \Rightarrow \mu_i^\circ = -RT\ln x_i. Using and integrating gives : \left(\frac\right)_P = \frac\left(R\ln x_i\right) \Rightarrow R\ln x_i = -\frac + K. The integration constant ''K'' may be determined for a pure component with a melting temperature T^\circ and an enthalpy of fusion H^\circ: : x_i = 1 \Rightarrow T = T_i^\circ \Rightarrow K = \frac. We obtain a relation that determines the molar fraction as a function of the temperature for each component: : R\ln x_i = -\frac + \frac. The mixture of ''n'' components is described by the system : \begin \ln x_i + \frac - \frac = 0, \\ \sum\limits_^n x_i = 1. \end : \begin \forall i < n \Rightarrow \ln x_i + \frac - \frac = 0, \\ \ln \left(1 - \sum\limits_^ x_i\right) + \frac - \frac = 0, \end which can be solved by : \begin \left \right= \left \right .\left \right \end


See also

*
Azeotrope An azeotrope () or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.Moore, Walter J. ''Physical Chemistry'', 3rd e Prentice-Hall 1962, pp. 140–142 This ...
, or constant boiling mixture *
Freezing-point depression Freezing-point depression is a drop in the minimum temperature at which a substance freezes, caused when a smaller amount of another, non- volatile substance is added. Examples include adding salt into water (used in ice cream makers and for ...
* Low melting alloy


References


Bibliography

*


Further reading

* * * * * {{Chemical solutions Materials science Chemistry Phase transitions