scholarly journals Premelting and the Machanisms of Melting in the Alkali Halides

2021 ◽  
Author(s):  
◽  
Jeffery Lewis Tallon
Keyword(s):  
Melting Point ◽  
Alkali Halides ◽  
Surface Melting ◽  
Atomic Scale ◽  
Vibrational Entropy ◽  
Shear Moduli ◽  
Isothermal Expansion ◽  
Theory Of Melting ◽  
Apparent Specific Heat ◽  
Experimental Values

<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>

scholarly journals Premelting and the Machanisms of Melting in the Alkali Halides

2021 ◽  
Author(s):  
◽  
Jeffery Lewis Tallon
Keyword(s):  
Melting Point ◽  
Alkali Halides ◽  
Surface Melting ◽  
Atomic Scale ◽  
Vibrational Entropy ◽  
Shear Moduli ◽  
Isothermal Expansion ◽  
Theory Of Melting ◽  
Apparent Specific Heat ◽  
Experimental Values

<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>

Studies on the freezing of pure liquids I. Critical supercooling in molten alkali halides

1960 ◽  
Vol 259 (1298) ◽  
pp. 325-340 ◽  
Keyword(s):  
High Temperature ◽  
Critical Temperature ◽  
Melting Point ◽  
Alkali Halide ◽  
Alkali Halides ◽  
Solid Particles ◽  
High Supersaturation ◽  
Chamber Temperature ◽  
Reduced Temperature ◽  
Heater Coil

A high-temperature cloud chamber is described in which a bead of alkali halide is supported on a heater coil mounted in the roof. By passing the current through the coil the temperature of the bead may be momentarily raised by several hundred degrees, producing salt vapour at high supersaturation. Condensation ensues in the presence of the inert supporting gas, and clouds of droplets or solid particles appear depending on the chamber temperature. Light scattered from the clouds under strong illumination is examined with a telescope, and the presence of crystalline particles is detected by their capacity to scintillate, or ‘twinkle’. It is found that twinkling in clouds of alkali halides appears sharply as the temperature is lowered below the melting point, defining a critical temperature of solidification for each salt. Reasons are given for regarding this temperature as the freezing threshold of molten salt droplets, for which supercoolings of about 150 °C are indicated. A reduced temperature, given by the ratio of the freezing threshold to the melting point, has the value of approximately 0.8 for all the alkali halides examined.

High-Resolution-Electron-Microscopy Investigation of Nanosize Inclusions

MRS Bulletin ◽  
1997 ◽  
Vol 22 (8) ◽  
pp. 49-52 ◽  
Author(s):  
U. Dahmen ◽  
E. Johnson ◽  
S.Q. Xiao ◽  
A. Johansen
Keyword(s):  
Electron Microscopy ◽  
Melting Point ◽  
Giant Magnetoresistance ◽  
Materials Science ◽  
Semiconductor Nanocrystals ◽  
High Resolution Electron Microscopy ◽  
Solid Particles ◽  
Atomic Scale ◽  
Size And Shape ◽  
New Frontier

The behavior of solids in the nanometer size regime, as their dimensions approach the atomic scale, is of increasing fundamental and applied interest in materials research. Electronic, optical, magnetic, mechanical, or thermodynamic properties all may depend on the size and shape of the solid. As a result, in the nanoscale regime, size and shape may be used as design variables to tailor a material's properties such as giant magnetoresistance in multilayer films, or the optical properties in semiconductor nanocrystals. In most cases, the size dependence of properties is not well-understood. Nanophase materials constitute a new frontier in materials science, and accurate nanoscale characterization is extremely important in exploring this new frontier. In this area, transmission electron microscopy (TEM) plays a key role. Because of its unique ability to provide information on the structure and composition of internal interfaces in solids, TEM is particularly important in cases of buried nanophase structures such as small solid inclusions—that is, solid particles embedded within another solid.Nanoscale inclusions have recently been shown to exhibit unusual melting behavior that depends strongly on their size and the embedding matrix. For example, small inclusions of Pb in SiO exhibit melting-point depressions of several hundred degrees, whereas similarsized Pb inclusions in aluminum have shown large increases in melting point. Although a full understanding of these effects is still lacking, it appears that they are related not just to inclusion size but also to their shape and interface structure.

The growth of crystals and the equilibrium structure of their surfaces

1951 ◽  
Vol 243 (866) ◽  
pp. 299-358 ◽  
Keyword(s):  
Activation Energy ◽  
Transition Temperature ◽  
Crystallographic Orientation ◽  
Crystal Surface ◽  
Equilibrium Structure ◽  
Surface Melting ◽  
Atomic Scale ◽  
Nearest Neighbour ◽  
Two Dimensional ◽  
Rate Of Growth

Parts I and II deal with the theory of crystal growth, parts III and IV with the form (on the atomic scale) of a crystal surface in equilibrium with the vapour. In part I we calculate the rate of advance of monomolecular steps (i.e. the edges of incomplete monomolecular layers of the crystal) as a function of supersaturation in the vapour and the mean concentration of kinks in the steps. We show that in most cases of growth from the vapour the rate of advance of monomolecular steps will be independent of their crystallographic orientation, so that a growing closed step will be circular. We also find the rate of advance for parallel sequences of steps. In part II we find the resulting rate of growth and the steepness of the growth cones or growth pyramids when the persistence of steps is due to the presence of dislocations. The cases in which several or many dislocations are involved are analysed in some detail; it is shown that they will commonly differ little from the case of a single dislocation. The rate of growth of a surface containing dislocations is shown to be proportional to the square of the supersaturation for low values and to the first power for high values of the latter. Volmer & Schultze’s (1931) observations on the rate of growth of iodine crystals from the vapour can be explained in this way. The application of the same ideas to growth of crystals from solution is briefly discussed. Part III deals with the equilibrium structure of steps, especially the statistics of kinks in steps, as dependent on temperature, binding energy parameters, and crystallographic orientation. The shape and size of a two-dimensional nucleus (i.e. an ‘island* of new monolayer of crystal on a completed layer) in unstable equilibrium with a given supersaturation at a given temperature is obtained, whence a corrected activation energy for two-dimensional nucleation is evaluated. At moderately low supersaturations this is so large that a crystal would have no observable growth rate. For a crystal face containing two screw dislocations of opposite sense, joined by a step, the activation energy is still very large when their distance apart is less than the diameter of the corresponding critical nucleus; but for any greater separation it is zero. Part IV treats as a ‘co-operative phenomenon’ the temperature dependence of the structure of the surface of a perfect crystal, free from steps at absolute zero. It is shown that such a surface remains practically flat (save for single adsorbed molecules and vacant surface sites) until a transition temperature is reached, at which the roughness of the surface increases very rapidly (‘ surface melting ’). Assuming that the molecules in the surface are all in one or other of two levels, the results of Onsager (1944) for two-dimensional ferromagnets can be applied with little change. The transition temperature is of the order of, or higher than, the melting-point for crystal faces with nearest neighbour interactions in both directions (e.g. (100) faces of simple cubic or (111) or (100) faces of face-centred cubic crystals). When the interactions are of second nearest neighbour type in one direction (e.g. (110) faces of s.c. or f.c.c. crystals), the transition temperature is lower and corresponds to a surface melting of second nearest neighbour bonds. The error introduced by the assumed restriction to two available levels is investigated by a generalization of Bethe’s method (1935) to larger numbers of levels. This method gives an anomalous result for the two-level problem. The calculated transition temperature decreases substantially on going from two to three levels, but remains practically the same for larger numbers.

An Investigation of the Squeeze Film Between Rotating Annuli

1970 ◽  
Vol 92 (3) ◽  
pp. 435-440 ◽  
Author(s):  
C. W. Allen ◽  
A. A. McKillop
Keyword(s):  
Experimental Investigation ◽  
Melting Point ◽  
Liquid Metal ◽  
Decay Rates ◽  
Squeeze Film ◽  
Centrifugal Effect ◽  
Experimental Values ◽  
Free Falling ◽  
Air Film ◽  
Good Agreement

The squeeze film between two plane annuli is examined theoretically and experimentally. The theoretical analysis considers the inertia due to the “centrifugal effect” but neglects all other inertia terms. The experimental investigation is by means of a free-falling spinning rotor which is decelerated by the squeeze film. Fluids examined are kerosene, SAE 10 oil, and a low melting point liquid metal. Good agreement between the predicted and actual decay rates is obtained for kerosene but that for the oil and liquid metal is only fair. The theoretical and experimental values of film thickness are in good agreement. The results for the liquid metal suggest the possibility of a thin air film between the rotor and the liquid metal.

scholarly journals Calculation of the melting point of alkali halides by means of computer simulations

2012 ◽  
Vol 137 (10) ◽  
pp. 104507 ◽  
Author(s):  
J. L. Aragones ◽  
E. Sanz ◽  
C. Valeriani ◽  
C. Vega
Keyword(s):  
Melting Point ◽  
Computer Simulations ◽  
Alkali Halides

A Simple Dislocation Theory of Melting

1977 ◽  
Vol 30 (6) ◽  
pp. 641 ◽  
Author(s):  
FD Stacey ◽  
RD Irvine
Keyword(s):  
Melting Point ◽  
Phase Boundary ◽  
Pressure Dependence ◽  
Equilibrium Phase ◽  
Perfect Crystal ◽  
Dislocation Theory ◽  
Free Energies ◽  
Clapeyron Equation ◽  
Volume Increment ◽  
Theory Of Melting

The ratio of volume increment to energy for the introduction of a simple dislocation to a crystal is used in the Clausius-Clapeyron equation to determine the pressure dependence of the equilibrium phase boundary between a perfect crystal and a completely dislocated crystal. It yields the Lindemann melting formula, which is thermodynamically valid for materials with central atomic forces in which melting involves no gross changes in coordination. It is concluded that melting is properly described as the free proliferation of dislocations and that melting point is the temperature at which the free energies of dislocations vanish.

Modeling of Temperature-Dependent Hardness for Pure FCC and HCP Metals

2020 ◽  
Vol 12 (02) ◽  
pp. 2050022
Author(s):  
Niandong Xu ◽  
Weiguo Li ◽  
Jianzuo Ma ◽  
Yong Deng ◽  
Haibo Kou ◽  
...  
Keyword(s):  
Theoretical Model ◽  
Melting Point ◽  
Present Model ◽  
Experimental Results ◽  
Effect Of Temperature ◽  
Temperature Dependent ◽  
Different Temperatures ◽  
Pure Metals ◽  
Experimental Values ◽  
Hardness Values

In this study, a theoretical model is developed to characterize the quantitative effect of temperature on the hardness of pure FCC and HCP metals. The model is verified by comparison with the available experimental results of Cu, Al, Zn, Mg, Be, Zr, Ni, Ir, Rh, and Ti at different temperatures. Compared with the widely quoted Westbrook model and Ito–Shishokin model which need piecewise fitting to describe experimental values, the present model merely needs two hardness values at different temperatures to predict the experimental results, reducing reliance on conducting lots of experiments. This work provides a convenient method to predict temperature-dependent hardness of pure metals, and it is worth noting that it can be applied to a wide temperature range from absolute zero to melting point.

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