Melting


Melting
Ice melting
"Molten" redirects here. For the Japanese company, see Molten Corporation; or see Molton or Moulton.

Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid ordering of molecular entities in the solid breaks down to a less-ordered state and the solid liquefies. An object that has melted completely is molten. Substances in the molten state generally have reduced viscosity with elevated temperature; an exception to this maxim is the element sulfur, whose viscosity increases with higher temperatures in its molten state.[1]

Some organic compounds melt through mesophases, states of partial order between solid and liquid.

Contents

Thermodynamics of melting

When a substance melts and the solid and liquid phases are in an equilibrium, it maintains a constant temperature, the melting point. The energy used for melting is a latent heat. This characterizes the process of melting as a first-order phase transition.

From a thermodynamics point of view, at the melting point the change in Gibbs free energy (ΔG) of the material is zero, but the enthalpy (H) and the entropy (S) of the material are increasing (ΔHS > 0). Melting occurs when the Gibbs free energy of the liquid becomes lower than the solid for that material. The temperature at which this occurs is dependent on the ambient pressure.

Melting criteria

Among the theoretical criteria for melting, the Lindemann [2] and Born [3] criteria are those most frequently used as a basis to analyse the melting conditions . The Lindemann criterion states that melting occurs because of vibrational instability, e.g. crystals melt when the average amplitude of thermal vibrations of atoms is relatively high compared with interatomic distances, e.g. <δu2>1/2 > δLRs, where δu is the atomic displacement, the Lindemann parameter δL≈0.20-0.25 and Rs is a half of the inter-atomic distance. The Lindemann melting criterion is supported by experimental data both for crystalline materials and for glass-liquid transitions in amorphous materials. The Born criterion is based on rigidity catastrophe caused by the vanishing elastic shear modulus, e.g. when the crystal no longer has sufficient rigidity to mechanically withstand load.


Supercooling

Under a standard set of conditions, the melting point of a substance is a characteristic property. The melting point is often equal to the freezing point. However, under carefully created conditions, supercooling or superheating past the melting or freezing point can occur. Water on a very clean glass surface will often supercool several degrees below the freezing point without freezing. Fine emulsions of pure water have been cooled to -38 degrees Celsius without nucleation to form ice.[citation needed]. Nucleation occurs due to fluctuations in the properties of the material. If the material is kept still there is often nothing (such a physical vibration) to trigger this change, and supercooling (or superheating) may occur. Thermodynamically, the supercooled liquid is in the metastable state with respect to the crystalline phase, and it is likely to crystallize suddenly.

Melting of amorphous solids (glasses)

Glasses are amorphous solids (e.g. amorphous materials that are at temperatures below the glass transition temperature) which are usually fabricated when the viscous molten material cools very rapidly to below its glass transition temperature, without sufficient time for a regular crystal lattice to form. Whether a material is liquid or solid depends primarily on the connectivity between its elementary building blocks so that solids are characterised by a high degree of connectivity whereas fluids occur at lower connectivity of the structural blocks. Melting of a solid material can also be considered as a percolation via broken connections between particles e.g. connecting bonds [4]. In this approach melting of an amorphous material occurs when the broken bonds form a percolation cluster with Tg dependent on quasi-equilibrium thermodynamic parameters of bonds e.g. on enthalpy (Hd) and entropy (Sd) of formation of bonds in a given system at given conditions [5]:

Tg=Hd/[Sd+Rln[(1-fc)/fc]],

where fc is the percolation threshold and R is the universal gas constant. Although Hd and Sd are not true equilibrium thermodynamic parameters and can depend on the cooling rate of a melt they can be found from available experimental data on viscosity of amorphous materials.

Premelting (surface melting)

Premelting (also: Surface melting) describes the fact that, even below its melting point Ts, quasi-liquid films can be observed on crystalline surfaces. The thickness of the film is temperature dependent. This effect is common for all crystalline materials. Premelting shows its effects in e.g. frost heave, the growth of snowflakes and, taking grain boundary interfaces into account, maybe even in the movement of glaciers.

Related concepts

In genetics, melting DNA means to separate the double-stranded DNA into two single strands by heating or the use of chemical agents, cf. Polymerase chain reaction.

See also

References

  1. ^ C.Michael Hogan (2011) Sulfur, Encyclopedia of Earth, eds. A.Jorgensen and C.J.Cleveland, National Council for Science and the environment, Washington DC
  2. ^ F.A. Lindemann, Z. Phys. 11 (1910) 609-615.
  3. ^ M. Born, J. Chem. Phys. 7 (1939) 591-601.
  4. ^ S.Y. Park and D. Stroud, Phys. Rev. B 67, 212202 (2003).
  5. ^ M.I. Ojovan, W.E. Lee. J. Non-Cryst. Solids, 356, 2534-2540 (2010).

References and further reading


To
From Solid Liquid Gas Plasma
Solid Solid-solid transformation Melting/fusion Sublimation N/A
Liquid Freezing N/A Boiling/evaporation N/A
Gas Deposition Condensation N/A Ionization
Plasma N/A N/A Recombination/deionization N/A

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  • Melting — Melt ing, n. Liquefaction; the act of causing (something) to melt, or the process of becoming melted. [1913 Webster] {Melting point} (Chem.), the degree of temperature at which a solid substance melts or fuses; as, the melting point of ice is… …   The Collaborative International Dictionary of English

  • Melting — Melt ing a. Causing to melt; becoming melted; used literally or figuratively; as, a melting heat; a melting appeal; a melting mood. {Melt ing*ly}, adv. [1913 Webster] …   The Collaborative International Dictionary of English

  • melting — melting; pre·melting; un·melting; …   English syllables

  • melting — melting. См. плавление. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • melting — 1. adjective /ˈmɛltɪŋ/ a) Which is melting, dissolving or liquefying. What guards the purities of melting maids, / In courtly balls, and midnight masquerades [...]? b) Given over to strong emotion; tender; aroused; emotional, tearful. 2. noun… …   Wiktionary

  • melting — melt|ing [ˈmeltıŋ] adj [usually before noun] written if someone gives you a melting look or speaks to you in a melting voice, it makes you feel pity or love for them >meltingly adv …   Dictionary of contemporary English

  • Melting — Melt Melt, v. t. [imp. & p. p. {Melted} (obs.) p. p. {Molten}; p. pr. & vb. n. {Melting}.] [AS. meltan; akin to Gr. me ldein, E. malt, and prob. to E. smelt, v. [root]108. Cf. {Smelt}, v., {Malt}, {Milt} the spleen.] 1. To reduce from a solid to… …   The Collaborative International Dictionary of English

  • melting — adjective Date: 1565 tender, delicate < a love song s melting lyric > • meltingly adverb …   New Collegiate Dictionary

  • melting — adj. Melting is used with these nouns: ↑ice, ↑point, ↑snow, ↑temperature …   Collocations dictionary

  • melting — adjective (usually before noun) a melting look, voice, or expression makes you feel strong feelings of pity, love, or sympathy meltingly adverb …   Longman dictionary of contemporary English


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