Material phase change
Melting ice cubes illustrate the process of fusion.
Melting
, or
fusion
, is a physical process that results in the
phase transition
of a
substance
from a
solid
to a
liquid
. This occurs when the
internal energy
of the solid increases, typically by the application of
heat
or
pressure
, which increases the substance's
temperature
to the
melting point
. At the melting point, the ordering of
ions
or
molecules
in the solid breaks down to a less ordered state, and the solid melts to become a liquid.
Substances in the molten state generally have reduced
viscosity
as the temperature increases. An exception to this principle is
elemental sulfur
, whose viscosity increases in the range of 160 °C to 180 °C due to
polymerization
.
[1]
Some
organic compounds
melt through
mesophases
, states of partial order between solid and liquid.
First order phase transition
[
edit
]
From a
thermodynamics
point of view, at the melting point the change in
Gibbs free energy
?G
of the substances is zero, but there are non-zero changes in the
enthalpy
(
H
) and the
entropy
(
S
), known respectively as the
enthalpy of fusion
(or
latent heat
of fusion) and the
entropy of fusion
. Melting is therefore classified as a
first-order phase transition
. 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.
Low-temperature
helium
is the only known exception to the general rule.
[2]
Helium-3
has a negative enthalpy of fusion at temperatures below 0.3 K.
Helium-4
also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, heat must be
removed
from these substances in order to melt them.
[3]
Criteria
[
edit
]
Among the theoretical criteria for melting, the
Lindemann
[4]
and
Born
[5]
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. <
δu
2
>
1/2
>
δ
L
R
s
, where
δu
is the atomic displacement, the Lindemann parameter
δ
L
? 0.20...0.25 and
R
s
is one-half of the inter-atomic distance.
[6]
: 177
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 a rigidity catastrophe caused by the vanishing elastic shear modulus, i.e. when the crystal no longer has sufficient rigidity to mechanically withstand the load, it becomes liquid.
[7]
Supercooling
[
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]
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 °C without nucleation to form
ice
.
[
citation needed
]
Nucleation occurs due to fluctuations in the properties of the material.
[
citation needed
]
If the material is kept still there is often nothing (such as 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.
Glasses
[
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]
Glasses are
amorphous solids
, which are usually fabricated when the molten material cools very rapidly to below its glass transition temperature, without sufficient time for a regular crystal lattice to form. Solids are characterised by a high degree of connectivity between their molecules, and fluids have lower connectivity of their structural blocks. Melting of a solid material can also be considered as a percolation via broken connections between particles e.g. connecting bonds.
[8]
In this approach melting of an amorphous material occurs, when the broken bonds form a percolation cluster with
T
g
dependent on quasi-equilibrium thermodynamic parameters of bonds e.g. on enthalpy (
H
d
) and entropy (
S
d
) of formation of bonds in a given system at given conditions:
[9]
![{\displaystyle T_{g}={\frac {H_{d}}{S_{d}+R\ln({\frac {1-f_{c}}{f_{c}}})}},}](https://wikimedia.org/api/rest_v1/media/math/render/svg/7970811f63ae8ab18b6cd0a0df38e4004fc2e048)
where
f
c
is the percolation threshold and
R
is the universal gas constant.
Although
H
d
and
S
d
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
.
Even below its melting point, 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. This
pre-melting
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 concept
[
edit
]
In
ultrashort pulse
physics, a so-called nonthermal melting may take place. It occurs not because of the increase of the atomic kinetic energy, but because of changes of the interatomic potential due to excitation of electrons. Since electrons are acting like a glue sticking atoms together, heating electrons by a
femtosecond laser
alters the properties of this "glue", which may break the bonds between the atoms and melt a material even without an increase of the atomic temperature.
[10]
In
genetics
, melting
DNA
means to separate the double-stranded DNA into two single strands by heating or the use of chemical agents,
polymerase chain reaction
.
Table
[
edit
]
See also
[
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]
References
[
edit
]
- ^
Sofekun, Gabriel O.; Evoy, Erin; Lesage, Kevin L.; Chou, Nancy; Marriott, Robert A. (2018).
"The rheology of liquid elemental sulfur across the λ-transition"
.
Journal of Rheology
.
62
(2). Society of Rheology: 469?476.
Bibcode
:
2018JRheo..62..469S
.
doi
:
10.1122/1.5001523
.
ISSN
0148-6055
.
- ^
Atkins, Peter; Jones, Loretta (2008),
Chemical Principles: The Quest for Insight
(4th ed.), W. H. Freeman and Company, p. 236,
ISBN
978-0-7167-7355-9
- ^
Ott, J. Bevan; Boerio-Goates, Juliana (2000),
Chemical Thermodynamics: Advanced Applications
, Academic Press, pp. 92?93,
ISBN
978-0-12-530985-1
- ^
Lindemann, F.A.
(1910).
"Uber die Berechnung molekularer Eigenfrequenzen"
.
Physikalische Zeitschrift
(in German).
11
(14): 609?614.
- ^
Born, Max
(1939). "Thermodynamics of Crystals and Melting".
The Journal of Chemical Physics
.
7
(8). AIP Publishing: 591?603.
Bibcode
:
1939JChPh...7..591B
.
doi
:
10.1063/1.1750497
.
ISSN
0021-9606
.
- ^
Stuart A. Rice (15 February 2008).
Advances in Chemical Physics
. John Wiley & Sons.
ISBN
978-0-470-23807-3
.
- ^
Robert W. Cahn
(2001)
Materials science: Melting from Within
,
Nature
413 (#6856)
- ^
Park, Sung Yong; Stroud, D. (11 June 2003). "Theory of melting and the optical properties of gold/DNA nanocomposites".
Physical Review B
.
67
(21). American Physical Society (APS): 212202.
arXiv
:
cond-mat/0305230
.
Bibcode
:
2003PhRvB..67u2202P
.
doi
:
10.1103/physrevb.67.212202
.
ISSN
0163-1829
.
S2CID
14718724
.
- ^
Ojovan, Michael I; Lee, William (Bill) E (2010). "Connectivity and glass transition in disordered oxide systems".
Journal of Non-Crystalline Solids
.
356
(44?49). Elsevier BV: 2534?2540.
Bibcode
:
2010JNCS..356.2534O
.
doi
:
10.1016/j.jnoncrysol.2010.05.012
.
ISSN
0022-3093
.
- ^
Medvedev, Nikita; Li, Zheng; Ziaja, Beata (2015). "Thermal and nonthermal melting of silicon under femtosecond x-ray irradiation".
Physical Review B
.
91
(5): 054113.
arXiv
:
1504.05053
.
Bibcode
:
2015PhRvB..91e4113M
.
doi
:
10.1103/PhysRevB.91.054113
.
S2CID
49258288
.
External links
[
edit
]
The dictionary definition of
melting
at Wiktionary
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