A phase diagram for a fictitious binary chemical mixture (with the two components denoted by A and B) used to depict the eutectic composition, temperature, and point. (L denotes the liquid state.)

A eutectic system or eutectic mixture (/juːˈtɛktɪk/ yoo-TEK-tik)^[1] is a homogeneous mixture that has a melting point lower than those of the constituents.^[2] The lowest possible melting point over all of the mixing ratios of the constituents is called the eutectic temperature. On a phase diagram, the eutectic temperature is seen as the eutectic point (see plot on the right).^[3]

Non-eutectic mixture ratios would have different melting temperatures for their different constituents, since one component's lattice 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 alloys have eutectic points, since the valence electrons 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) "meet at the pure element endpoints of the atomic ratio axis while slightly separating in the mixture region of this axis".^[4]

The term eutectic was coined in 1884 by British physicist and chemist Frederick Guthrie (1833–1886). The word originates from Greek εὐ- (eû) 'well', and τῆξῐς (têxis) 'melting'.^[2]

Eutectic phase transition

Four eutectic structures: A) lamellar B) rod-like C) globular D) acicular.

The eutectic solidification is defined as follows:^[5]

${\displaystyle {\text{Liquid}}\quad {\xrightarrow[{\text{cooling}}]{{\text{eutectic}} \atop {\text{temperature}}}}\quad \alpha {\text{ solid solution}}\ +\ \beta {\text{ solid solution}}}$

This type of reaction is an invariant reaction, because it is in thermal equilibrium; another way to define this is the change in 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 change of phase during which the temperature of the system does not change.^[5]

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, but other possible structures include rodlike, globular, and acicular.^[6]

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.^[5]

Types

Others

Solid–liquid phase change of ethanol–water mixtures

• Sodium chloride and water form a eutectic mixture whose eutectic point is −21.2 °C^[8] and 23.3% salt by mass.^[9] 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).
• 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.
• "Solar salt", 60% NaNO₃ and 40% KNO₃, forms a eutectic molten salt mixture which is used for thermal energy storage in concentrated solar power plants.^[10] To reduce the eutectic melting point in the solar molten salts, calcium nitrate is used in the following proportion: 42% Ca(NO₃)₂, 43% KNO_3, and 15% NaNO₃.
• Lidocaine and prilocaine—both are solids at room temperature—form a eutectic that is an oil with a 16 °C (61 °F) melting point that is used in eutectic mixture of local anesthetic (EMLA) preparations.
• Menthol 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.^[11]
• Minerals may form eutectic mixtures in igneous rocks, giving rise to characteristic intergrowth textures exhibited, for example, by granophyre.^[12]
• Some inks are eutectic mixtures, allowing inkjet printers to operate at lower temperatures.^[13]
• Choline chloride produces eutectic mixtures with many natural products such as citric acid, malic acid and sugars. These liquid mixtures can be used, for example, to obtain antioxidant and antidiabetic extracts from natural products.^[14]

Strengthening mechanisms

Other critical points

Iron–carbon phase diagram, showing the eutectoid transformation between austenite (γ) and pearlite.

Peritectic

Gold–aluminium phase diagram

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 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 at 1,495 °C (2,723 °F) 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 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.^[22]

The Gibbs free energy G depends on its own differential:

${\displaystyle G=H-TS\Rightarrow {\begin{cases}H=G+TS\\\left({\frac {\partial G}{\partial T}}\right)_{P}=-S\end{cases}}\Rightarrow H=G-T\left({\frac {\partial G}{\partial T}}\right)_{P}.}$

Thus, the G/T derivative at constant pressure is calculated by the following equation:

${\displaystyle \left({\frac {\partial G/T}{\partial T}}\right)_{P}={\frac {1}{T}}\left({\frac {\partial G}{\partial T}}\right)_{P}-{\frac {1}{T^{2}}}G=-{\frac {1}{T^{2}}}\left(G-T\left({\frac {\partial G}{\partial T}}\right)_{P}\right)=-{\frac {H}{T^{2}}}.}$

The chemical potential ${\displaystyle \mu _{i}}$ is calculated if we assume that the activity is equal to the concentration:

${\displaystyle \mu _{i}=\mu _{i}^{\circ }+RT\ln {\frac {a_{i}}{a}}\approx \mu _{i}^{\circ }+RT\ln x_{i}.}$

At the equilibrium, ${\displaystyle \mu _{i}=0}$, thus ${\displaystyle \mu _{i}^{\circ }}$ is obtained as

${\displaystyle \mu _{i}=\mu _{i}^{\circ }+RT\ln x_{i}=0\Rightarrow \mu _{i}^{\circ }=-RT\ln x_{i}.}$

Using and integrating gives

${\displaystyle \left({\frac {\partial \mu _{i}/T}{\partial T}}\right)_{P}={\frac {\partial }{\partial T}}\left(R\ln x_{i}\right)\Rightarrow R\ln x_{i}=-{\frac {H_{i}^{\circ }}{T}}+K.}$

The integration constant K may be determined for a pure component with a melting temperature ${\displaystyle T^{\circ }}$ and an enthalpy of fusion ${\displaystyle H^{\circ }}$:

${\displaystyle x_{i}=1\Rightarrow T=T_{i}^{\circ }\Rightarrow K={\frac {H_{i}^{\circ }}{T_{i}^{\circ }}}.}$

We obtain a relation that determines the molar fraction as a function of the temperature for each component:

${\displaystyle R\ln x_{i}=-{\frac {H_{i}^{\circ }}{T}}+{\frac {H_{i}^{\circ }}{T_{i}^{\circ }}}.}$

The mixture of n components is described by the system

${\displaystyle {\begin{cases}\ln x_{i}+{\frac {H_{i}^{\circ }}{RT}}-{\frac {H_{i}^{\circ }}{RT_{i}^{\circ }}}=0,\\\sum \limits _{i=1}^{n}x_{i}=1.\end{cases}}}$

${\displaystyle {\begin{cases}\forall i<n\Rightarrow \ln x_{i}+{\frac {H_{i}^{\circ }}{RT}}-{\frac {H_{i}^{\circ }}{RT_{i}^{\circ }}}=0,\\\ln \left(1-\sum \limits _{i=1}^{n-1}x_{i}\right)+{\frac {H_{n}^{\circ }}{RT}}-{\frac {H_{n}^{\circ }}{RT_{n}^{\circ }}}=0,\end{cases}}}$

which can be solved by

${\displaystyle {\begin{array}{c}\left[{\begin{array}{*{20}c}{\Delta x_{1}}\\{\Delta x_{2}}\\{\Delta x_{3}}\\\vdots \\{\Delta x_{n-1}}\\{\Delta T}\\\end{array}}\right]=\left[{\begin{array}{*{20}c}{1/x_{1}}&0&0&0&0&{-{\frac {H_{1}^{\circ }}{RT^{2}}}}\\0&{1/x_{2}}&0&0&0&{-{\frac {H_{2}^{\circ }}{RT^{2}}}}\\0&0&{1/x_{3}}&0&0&{-{\frac {H_{3}^{\circ }}{RT^{2}}}}\\\vdots &\ddots &\ddots &\ddots &\ddots &{\vdots }\\0&0&0&0&{1/x_{n-1}}&{-{\frac {H_{n-1}^{\circ }}{RT^{2}}}}\\{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{\frac {-1}{1-\sum \limits _{i=1}^{n-1}{x_{i}}}}&{-{\frac {H_{n}^{\circ }}{RT^{2}}}}\\\end{array}}\right]^{-1}.\left[{\begin{array}{*{20}c}{\ln x_{1}+{\frac {H_{1}^{\circ }}{RT}}-{\frac {H_{1}^{\circ }}{RT_{1}^{\circ }}}}\\{\ln x_{2}+{\frac {H_{2}^{\circ }}{RT}}-{\frac {H_{2}^{\circ }}{RT_{2}^{\circ }}}}\\{\ln x_{3}+{\frac {H_{3}^{\circ }}{RT}}-{\frac {H_{3}^{\circ }}{RT_{3}^{\circ }}}}\\\vdots \\{\ln x_{n-1}+{\frac {H_{n-1}^{\circ }}{RT}}-{\frac {H_{n-1}^{\circ }}{RT_{n-1}^{\circ }}}}\\{\ln \left({1-\sum \limits _{i=1}^{n-1}{x_{i}}}\right)+{\frac {H_{n}^{\circ }}{RT}}-{\frac {H_{n}^{\circ }}{RT_{n}^{\circ }}}}\\\end{array}}\right]\end{array}}}$

See also

• Azeotrope, or constant boiling mixture
• Freezing-point depression
• Low melting alloy

References

Further reading

Look up eutectic in Wiktionary, the free dictionary.

• Askeland, Donald R.; Pradeep P. Phule (2005). The Science and Engineering of Materials. Thomson-Engineering. ISBN 978-0-534-55396-8.
• Easterling, Edward (1992). Phase Transformations in Metals and Alloys. CRC. ISBN 978-0-7487-5741-1.
• Mortimer, Robert G. (2000). Physical Chemistry. Academic Press. ISBN 978-0-12-508345-4.
• Reed-Hill, R. E.; Reza Abbaschian (1992). Physical Metallurgy Principles. Thomson-Engineering. ISBN 978-0-534-92173-6.
• Sadoway, Donald (2004). "Phase Equilibria and Phase Diagrams" (PDF). 3.091 Introduction to Solid State Chemistry, Fall 2004. MIT Open Courseware. Archived from the original (PDF) on 2005-10-20. Retrieved 2006-04-12.