(also fusion), the transition of a crystalline solid into the liquid state, accompanied by the absorption of heat. Melting is a phase transition of the first order. The relevant parameters in the melting of pure compounds are the melting point Tm and the heat required to accomplish the melting process—the heat of melting Qm.
Figure 1. Phase diagram for a pure substance: (A) triple point, (B) critical point, (p) pressure, (pcr) critical pressure, (Tcr) critical temperature, (AD) and (AD’) melting curves. AD’ shows the behavior of a substance that exhibits anomalous changes in volume during melting.
Figure 1 is a phase diagram for a pure substance. The melting point depends on the external pressure p. The phase diagram represents this relationship for a pure substance as the melting curve, along which the solid and liquid phases coexist (AD or AD’ in Figure 1). The melting of alloys and solid solutions usually occurs over a certain temperature range; eutectics, which melt at a fixed point, are an exception. With an alloy, the dependence of temperature on the alloy’s composition at the start and at the end of melting at a given pressure is represented on the phase diagram by special lines, called the liquidus and solidus. In certain macromolecular compounds, for example, those that can form liquid crystals, the transition from the solid, crystalline state to the isotropic, liquid state is a stepwise process that occurs over a certain temperature range; each step corresponds to a discrete stage in the degradation of the crystalline structure.
The existence of a precise melting point is an important criterion that reflects the regularity of a solid’s crystalline structure. This criterion may be used for readily distinguishing crystalline materials from amorphous solids, which do not have a precise melting point. Amorphous solids enter the liquid state gradually, becoming softer as the temperature rises.
Figure 2. Dependence of temperature rise of a crystalline solid on melting time: (T) temperature, (Tm) melting point. The time τ is proportional to the quantity of heat that is uniformly supplied to the body.
Tungsten, with a melting point of 3410°C, is the highest melting pure metal; mercury is the lowest, melting at — 38.9°C. High-melting refractories include TiN (3200°C), HfN (3580°C), ZrC (3805°C), TaC (4070°C), and HfC (4160°C). Materials with a high melting point usually require a high heat of melting. Impurities lower the melting point of crystalline solids; a practical application of this phenomenon is the preparation of freezing mixtures and low-melting alloys, for example, Wood’s metal, which melts at 68°C.
Melting begins when the crystalline material reaches the melting point. The temperature of the material remains at the melting point from beginning to completion of melting, regardless of the heat supplied to the material. This behavior is graphically shown in Figure 2. Under normal conditions, a crystal cannot retain its structure above the melting point. In contrast, a melt is relatively easily supercooled during crystallization.
The nature of the dependence of melting point on pressure is determined by the sign of the volume change, Δ Vm, during melting. Melting of materials is, in most cases, accompanied by an increase in volume; the increase is usually on the order of several percent. Under such conditions, an increase in pressure raises the melting point (see Figure 3). However, some materials, for example, water and certain metals and metalloids, exhibit a decrease in volume during melting (see Figure 1); the melting point of such materials decreases with increasing pressure.
Figure 3. Dependence of melting point Tm on pressure in the alkali metals. The melting curve of Cs reflects the two polymorphic transformations, a and b, of this metal.
Melting is accompanied by changes in the physical properties of a substance. The entropy increases, which reflects the disordering of the crystalline structure. Heat capacity and electrical resistance usually increase, although certain metalloids, for example, Bi and Sb, and semiconductors, such as Ge, possess a higher electrical conductivity in the liquid state than in the solid. Shear strength decreases practically to zero on melting, since transverse elastic waves cannot be propagated in a melt, and the speed of sound propagation decreases, since the speed of propagation of longitidunal waves decreases.
In terms of molecular kinetic theory, melting represents an increase in the amplitude of the vibrational energy of the atoms in a crystal when the heat is supplied to a solid. As the amplitude increases, the temperature of the solid rises and causes various defects within the crystal: unfilled sites, called vacancies, in the crystal lattice can arise, and the periodicity of the lattice can be disturbed by atoms that become inserted intersticially.
If the molecules are not spherical, molecular crystals may exhibit a partial disordering in the mutual orientation of the molecular axes. A gradual increase in and combination of defects characterizes the stage of premelting. Melting begins when the crystal reaches the melting point and a critical concentration of defects in the lattice is reached; at this point, the lattice disintegrates into mobile submicroscopic regions. The heat supplied during melting does not heat the solid body but rather cleaves interatomic bonds and destroys the long-range order of the crystal. The short-range order, however, in the submicroscopic regions does not change substantially during melting; that is, the coordination number in the melt at the melting point is in the majority of cases the same as in the crystal (see COORDINATION NUMBER). The preservation of short-range order in a melt explains why the heat of melting is lower than the heat of vaporization; it also accounts for the relatively small changes in various physical properties of materials during melting.
Important natural melting processes include the melting of snow and ice on the earth’s surface and the melting of minerals within the earth. Melting finds important applications in industry in the production of metals and alloys and in mold casting.
REFERENCES
Frenkel’, la. I.“Kineticheskaia teoriia zhidkostei.” Sobr. izb. trudov, vol. 3. Moscow-Leningrad, 1959.
Danilov, V. I. Stroenie i kristallizatsiia zhidkosti. Kiev, 1956.
Glazov, V. M., S. N. Chizhevskaia, and N. N. Glagoleva. Zhidkie poluprovodniki. Moscow, 1967.
Ubbelohde, A. Plavlenie i kristallicheskaia struktura. Moscow, 1969. (Translated from English.)
Liubov, B. Ia. Teoriia kristallizatsii v bol’shikh ob”emakh. Moscow (in press).B. IA. LIUBOV
melting - the process whereby heat changes something from a solid to a liquid; "the power failure caused a refrigerator melt that was a disaster"; "the thawing of a frozen turkey takes several hours"