Premelting and the Machanisms of Melting in the Alkali Halides

<p>An experimental and theoretical study of premelting behaviour and mechanisms of melting in the alkali-halides is presented. Theories of melting and previous premelting experiments are first reviewed, then an elastic strain theory of melting is developed, which includes dilatation and shear contributions to the elastic energy and to the vibrational entropy, as well as a communal entropy and an entropy due to the isothermal expansion on melting. By fitting experimental melting parameters, dislocation-like local strains are implicated. The bulk and shear moduli are shown to be continuous with respect to dilatation through the melting expansion and one of the shear moduli vanishes at the dilatation of the melt at the melting temperature. A modified Born instability theory of melting is thus valid. Premelting rises in the apparent specific heat and electrical conductivity within 6 K of the melting point are studied and are shown to occur at the surfaces only. The use of guard rings to eliminate surface conduction is essential at all temperatures above the extrinsic/intrinsic conductivity 'knee', and electrical fringing must be taken into account for typical specimen sizes. For various surface orientations, the rises in surface conductivity occur at lower temperatures the lower the surface packing density, and for deformed specimens, the greater the deformation. The results are interpreted in terms of an atomic-scale surface melting below the melting point, and a consequent rapid rise in vaporisation rate. A dislocation theory of surface melting, melting and the solid-liquid interface is developed which gives good agreement with experimental values for the melting temperatures and the interfacial energies.</p>

Premelting and the Machanisms of Melting in the Alkali Halides

<p>An experimental and theoretical study of premelting behaviour and mechanisms of melting in the alkali-halides is presented. Theories of melting and previous premelting experiments are first reviewed, then an elastic strain theory of melting is developed, which includes dilatation and shear contributions to the elastic energy and to the vibrational entropy, as well as a communal entropy and an entropy due to the isothermal expansion on melting. By fitting experimental melting parameters, dislocation-like local strains are implicated. The bulk and shear moduli are shown to be continuous with respect to dilatation through the melting expansion and one of the shear moduli vanishes at the dilatation of the melt at the melting temperature. A modified Born instability theory of melting is thus valid. Premelting rises in the apparent specific heat and electrical conductivity within 6 K of the melting point are studied and are shown to occur at the surfaces only. The use of guard rings to eliminate surface conduction is essential at all temperatures above the extrinsic/intrinsic conductivity 'knee', and electrical fringing must be taken into account for typical specimen sizes. For various surface orientations, the rises in surface conductivity occur at lower temperatures the lower the surface packing density, and for deformed specimens, the greater the deformation. The results are interpreted in terms of an atomic-scale surface melting below the melting point, and a consequent rapid rise in vaporisation rate. A dislocation theory of surface melting, melting and the solid-liquid interface is developed which gives good agreement with experimental values for the melting temperatures and the interfacial energies.</p>

Anelastic Phenomena Preceding the Melting of Pure Metals and Alloys

Precursor phenomena of melting in pure metals (In, Pb, Bi and Sn) and alloys of the systems Pb-Bi and In-Sn with different compositions have been investigated by means of Mechanical Spectroscopy (MS), i.e. dynamic modulus and damping measurements. MS tests evidenced that a sharp drop of dynamic modulus E takes place in a temperature range ΔT before the formation of the first liquid: the modulus variation ΔE and the corresponding temperature range ΔT depend on the specific metal or alloy. The modulus drop is consistent with a relevant increase of interstitial concentration (self-interstitials assuming the dumbbell configuration), as predicted by the Granato’s theory of melting. The increase of damping in the same temperature range of modulus drop supports this explanation. Owing to their dumbbell configuration self-interstitials interact with the flexural vibration of samples and the periodic re-orientation under the external applied stress leads to energy loss and damping increase. The increase of self-interstitials has the effect to weaken interatomic bonds (modulus drop) and favours the collapse of crystal lattice (melting).