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>

Approximate Analytical Method to Stefan Problem for Spheres with Wide Temperature Range of Phase Transition

The two-dimensional differential transform method is applied to solve the one-dimensional phase change problem for a solid sphere with time-dependent boundary temperature. The problem assumes that the phase change occurs over a range of temperatures and the initial temperature of the sphere is an arbitrary constant. An approximate analytical (series) solution is derived for the temperature profile in the melting or solidifying sphere. The solution is based on the apparent specific heat method. Numerical results illustrate the effects of the Stefan number, which is the ratio of sensible heat to latent heat, on the transient temperature profile in the sphere.

Review on Phase Change Material Slurries

Phase change materials (PCM) have recently received considerable attention in the field of thermal energy storage, due to their intrinsic properties. Phase change material slurry is a novel medium of heat storage and transfer, its apparent specific heat and heat transfer capacity is better than water.PCM slurries are being investigated for active thermal energy storage or as alternatives to conventional single phase fluids because they are pumpable and have advanced heat transport performance with phase change. This review mainly presents the information on PCM emulsions and microencapsulated PCM slurries (mPCM slurries).

Thermal Processing of Biological Tissue at High Temperatures: Impact of Protein Denaturation and Water Loss on the Thermal Properties of Human and Porcine Liver in the Range 25–80 °C

Biothermal engineering applications impose thermal excursions on tissues with an ensuing biological outcome (i.e., life or death) that is tied to the molecular state of water and protein in the system. The accuracy of heat transfer models used to predict these important processes in turn depends on the kinetics and energy absorption of molecular transitions for both water and protein and the underlying temperature dependence of the tissue thermal properties. Unfortunately, a lack of tissue thermal property data in the literature results in an overreliance on property estimates. This work addresses these thermal property limitations in liver, a platform tissue upon which hyperthermic engineering applications are routinely performed and a test bed that will allow extension to further tissue property measurement in the future. Specifically, we report on the thermal properties of cadaveric human and porcine liver in the suprazero range between 25 °C to 80 °C for thermal conductivity and 25 °C to 85 °C for apparent specific heat. Denaturation and water vaporization are shown to reduce thermal conductivity and apparent specific heat within the samples by up to 20% during heating. These changes in thermal properties significantly altered thermal histories during heating compared to conditions when properties were assumed to remain constant. These differences are expected to alter the biological outcome from heating as well.