Liquid Metals

opaque liquids with characteristic luster, having high thermal conductivity and electrical conductivity, as well as other special properties characteristic of solid metals. Liquid metals include all molten metals and metallic alloys, as well as a number of intermetallic compounds. Some semimetals and semiconductors become typical metals in the liquid state: some take on metallic properties immediately upon melting (for example, Ge, Si, and GaSb); others, upon heating above the melting point (for example, Te-Se, PbTe, PbSe, and ZnSb). Some nonmetals (P, C, and B) become liquid metals at high pressures. Only mercury (melting point –38.9°C) is a liquid at room temperature and atmospheric pressure.

According to such properties as viscosity, surface tension, and diffusion, liquid metals are similar to other liquids, but at the same time they differ sharply from the other liquids in their higher thermal conductivity, electrical conductivity, and ability to reflect electromagnetic waves, as well as lower compressibility. Liquid metals are close to solid metals in terms of these special properties.

For liquid and solid metals, the current carriers are electronic. Upon melting, the electrical conductivity of pure metals decreases by a factor of 1.5–3.0, depending on the type of metal, and decreases linearly with temperature upon further heating. The divalent metals are an exception; their electrical conductivities pass through minima at elevated temperature. The coefficient of the thermoelectromotive force exhibits a jump upon melting and becomes a linear function of the temperature in the liquid state (it is proportional to the absolute temperature for many liquid metals). The Hall coefficient changes upon melting; it is negative for liquid metals and may be calculated using a free electron model from the equation R_H = (ne)^-l, where n is the electron density (calculated from the density and the valence) and e is the electron charge (there are exceptions to these general rules). The electrical properties of liquid metals may be understood only on the basis of a rigorous quantum-mechanical theory of electronic processes in liquids.

The melting of metals causes the thermal conductivity to change almost in the same way as the electrical conductivity. This is also true for Bi, whose thermal and electrical conductivities increase rather than decrease, as with other metals. The free electrons carry the largest part of the heat flux. Therefore, liquid metals have a higher thermal conductivity than liquid dielectrics. Some liquid metals combine a considerable thermal conductivity with a high heat capacity. This makes possible the use of liquid metals in heat technology as heat carriers. The monatomic liquid metals—sodium and potassium—have been studied in the greatest detail. These metals have sufficiently low melting points and are used either separately or in the form of alloys to draw off heat from nuclear reactors.

Like solid metals, liquid metals are poorly compressible (considerably less than other liquids), since the decrease in volume in both cases requires the concentration of electrons in a smaller volume. Therefore, the speed of sound in liquid metals is usually higher than in other liquids. Liquid metals, like other liquids, are incapable of resisting static shear, but ultrasonic waves of very high frequencies may be propagated in liquid metals as shear disturbances.