Premelting Effects Research Articles

Specific-heat anomaly during vitrification of hydrided Fe2Er single crystals.

A solid-state amorphization reaction is observed on hydrided, initially single-crystalline, ${\mathrm{Fe}}_{2}$Er powder samples (${\mathrm{Fe}}_{2}$${\mathrm{ErH}}_{\mathit{x}}$, 3.0x3.4). Specific-heat measurements at constant concentration (x=3.4) show pronounced premelting effects exhibiting a lambda-type transition at ${\mathit{T}}_{\mathit{c}}$=475.6 K and a logarithmic temperature dependence. This typical feature of an instability underlying the ``melting'' transition has been predicted if the vitrification occurs in the vicinity of the triple point between supersaturated crystal, undercooled liquid, and glass.

On theC p toC v conversion of solid linear macromolecules II

A modification is proposed for the Nernst-Lindemann equation that is used to convert calculated heat capacities at constant pressure (C p ) to heat capacities at constant volume (C v ) for solid, linear macromolecules. the constant A0 per mole of repeating unit in this equation is derived by taking into account the variable number of vibrators excited at different temperatures. With the new equation it is possible to calculateC p for solid polymers over a wider temperature range. The constant is calculated for solid polymers from experimental thermal expansivity, isothermal compressibility and heat capacity data obtained from the literature. An average value of (3.9±2.4)×10−3(K mol)/J was obtained for A0 (new) from data on 22 solid polymers. This average value may be used as a universal constant in case no experimental data on compressibility and expansivity are available for computation ofA 0. The remaining variation of A0 (new) with temperature is discussed and example calculations are shown for polyethylene. Effects of premelting and possibly large-amplitude motion are discovered for polyethylene in the temperature range 290 to 410 K.

Solid-liquid transition of metallic clusters. Occurrence of surface melting

Optical measurements have been performed to study the melting and solidification of distributions of small metallic particles embedded in a matrix. Dark field electron microscopy and microdiffraction have been used to show that the particles rotate in their host in a 20 K temperature interval just before melting. An interpretation related to premelting effect or surface melting is given.

ChemInform Abstract: Li2UCl6, Na2UCl6, and Cs2UCl6 Compounds: Enthalpies of Phase Transitions and Electrical Conductivity in the Solid and Liquid States

Using several experimental techniques (electrical conductivity measurement, differential thermal analysis and differential enthalpic analysis), the Li2UCl6, Na2UCl6 and Cs2UCl6 Compounds have been investigated. —The joint examination of thermograms and phase diagrams suggests that the fusion of Li2UCl6 and Na2UCl6 compounds is preceeded by small thermal effects which are not always reproducible. —Melting temperatures and enthalpies of transformation and fusion have been measured: These new results were compared with those previously found for similar compounds. The calorimetric and electrical experimental values obtained for Li2UBr6 and Li2UCl6 allowed a parallelity to be established: the very close values of entropy of fusion, 61 and 57 J · mol−1 · K−1, respectively, suggest that the melting mechanism is similar for both compounds. —On the other hand, in spite of similar entropies of fusion, Li2UCl6 and Na2UCl6 exhibit a strongly different behaviour of the electrical conductivity. The electrical properties of Li2UCl6, Na2UCl6 and Cs2UCl6 were also tentatively compared with those of Li2UBr6 and Na2UBr6 but only a qualitative interpretation can be put forward. —However the similar premelting effects and entropies of fusion (13.8 and 15.8 J · mol−1 · K−1 respectively) found for Na2UBr6 and Cs2UCl6 suggest a similar melting mechanism for both compounds. Therefore high temperature structural investigations would be essential and the clue to the electrical and thermodynamic behaviours of Li2UCl6, Na2UCl6 and Cs2UCl6.

Li2UCl6, Na2UCl6 and Cs2UCl6 compounds: Enthalpies of phase transitions and electrical conductivity in the solid and liquid states

AbstractUsing several experimental techniques (electrical conductivity measurement, differential thermal analysis and differential enthalpic analysis), the Li2UCl6, Na2UCl6 and Cs2UCl6 Compounds have been investigated. —The joint examination of thermograms and phase diagrams suggests that the fusion of Li2UCl6 and Na2UCl6 compounds is preceeded by small thermal effects which are not always reproducible. —Melting temperatures and enthalpies of transformation and fusion have been measured: magnified image These new results were compared with those previously found for similar compounds. The calorimetric and electrical experimental values obtained for Li2UBr6 and Li2UCl6 allowed a parallelity to be established: the very close values of entropy of fusion, 61 and 57 J · mol−1 · K−1, respectively, suggest that the melting mechanism is similar for both compounds. —On the other hand, in spite of similar entropies of fusion, Li2UCl6 and Na2UCl6 exhibit a strongly different behaviour of the electrical conductivity. The electrical properties of Li2UCl6, Na2UCl6 and Cs2UCl6 were also tentatively compared with those of Li2UBr6 and Na2UBr6 but only a qualitative interpretation can be put forward. —However the similar premelting effects and entropies of fusion (13.8 and 15.8 J · mol−1 · K−1 respectively) found for Na2UBr6 and Cs2UCl6 suggest a similar melting mechanism for both compounds. Therefore high temperature structural investigations would be essential and the clue to the electrical and thermodynamic behaviours of Li2UCl6, Na2UCl6 and Cs2UCl6.

THE EFFECT OF PREMELTING STUDIED BY MOLECULAR DYNAMICS

The effect of premelting is investigated by four molecular-dynamics models. It is shown that premelting is initiated by a disorder (roughening) normal to the surface. Also the interlayer spacings strongly influence the structural and dynamical properties of the outermost surface layer.

Molecular dynamics investigation of the premelting effects of Lennard-Jones (111) surfaces

Molecular dynamics simulations have been performed to study the premelting effects of noble-gas surfaces (argon) close to but below the bulk melting temperature. In particular, the increase of disorder as a function of temperature at (111) surface has been considered. The truncated Lennard-Jones (6-12) potential is used to describe the interactions between particles. Surface premelting has been analyzed by means of total energies, trajectory plots, mean sequare displacement functions, diffusion coefficients, vacancy concentrations and two-dimensional order parameters. The (111) surface starts to disorder by vacancy formation, which leads to the premelting of the surface layer far below the bulk melting temperature. Melting proceeds via a layer-by-layer mechanism, when temperature is further increased. This study supports the earlier observations of a melting that proceeds in the direction of high packing density.

Numerical simulation of surface diffusion on a rigid substrate

Surface diffusion coefficients are calculated by numerical molecular dynamics simulation for a wide range of temperature and surface coverage. It is found that the temperature dependence of surface diffusion may exhibit a non-Arrhenius behavior because of surface pre-melting effects and that the coverage dependence of the diffusion coefficient can be expressed by D = D o(1/C) Xexp[−C/(C o-C)], where the constant C o does not depend on temperature. A discussion is given in light of the available experimental data.

Sodium ion and solvent nuclear relaxation results in aqueous solutions of DNA.

AbstractThe field‐dependent 23Na nuclear relaxation in aqueous DNA solutions has been obtained for a range of temperatures, including the DNA melting region. At least two correlation times are needed to characterize the spectral density function for the 23Na relaxation. For the slow process (with the largest correlation time), the temperature dependence of the coupling constant and the correlation time were determined, and important premelting effects were observed. Possible origins of the slow process are discussed. The last process is shown to be correlated with the properties of the hydration water of DNA as reflected by the 17O relaxation rates in these solutions. The influence of the polyelectrolyte and NaCl concentrations on the 23Na relaxation rate is compared with previous results from solutions of linear flexible polyelectrolytes.

Molecular-dynamics study of surface premelting effects.

The thermodynamical and structural behavior of a (110) face of a fcc (12-6) Lennard-Jones solid has been investigated by molecular-dynamics simulation on the solid-gas coexistence line. The temperature dependence of the relevant structural and mass-transport properties shows the following. (a) Despite the high degree of disorder which gradually appears on surface layers when the temperature is increased, the surface retains its solidlike character up to temperatures (T\ensuremath{\approxeq}0.64\ensuremath{\varepsilon}/${k}_{B}$) very close to the triple point (${T}_{t}$=0.68\ensuremath{\varepsilon}/${k}_{B}$). This conclusion does not confirm the findings of previous theoretical work predicting the formation of a liquid surface layer well below the bulk melting point. (b) The large concentration of vacancy-adatom pairs, produced at the surface in the high-temperature range, accounts for the high values of the surface diffusivity. (c) The Arrhenius plot of defect concentration indicates a progressive decrease of their formation energy for temperatures ranging from ${T}_{r}$=0.8${T}_{m}$ to the melting point. Consistently, the order parameter decreases slowly with increasing temperature up to ${T}_{r}$ but from T=${T}_{r}$ to the melting point it decreases much more rapidly than predicted by the extrapolation of the low-temperature data. These results are qualitatively consistent with the onset of a surface-roughening transition, in agreement with recent experimental results obtained from helium scattering on (110) copper surfaces.

Structural and dynamical behavior of noble-gas surfaces.

Molecular-dynamics calculations have been performed for noble-gas (krypton) surfaces. As pair potential between the particles, we used the potential of Barker et al. This potential is consistent with a wide range of experimental data and should be more realistic than the more familiar Lennard-Jones (6-12) potential. The calculations have been performed for temperatures of 7, 70, and 102 K. We used the following model: The krypton atoms are arranged as a slab-shaped fcc crystal, the two free surfaces being (100) planes. The slab consists of 11 layers of 50 atoms, i.e., the total number of particles used in the calculations was 550. The structure of the layers has been studied by means of the single-particle distribution function and the pair correlation function. Whereas the results for the innermost layer of the model agree well with the corresponding data of the bulk, there are relatively large effects for the outermost layer. This is also the case for the mean-square displacements 〈${u}^{2}$〉, which is much larger in the outermost layer than in the bulk of the crystal. For T=102 K, we observe the effect of surface premelting: The outermost layer is disordered and the particles perform a diffusive motion parallel to the surface and the diffusion coefficient D is comparable to that in liquids. The effect of premelting in our system is obviously much more pronounced than in the case of Lennard-Jones systems.

Lithium sulfate: Calorimetric determination of the temperatures and enthalpies of high-temperature phase transitions

Using a high-temperature Calvet calorimeter as a differential enthalpic analyser Li 2SO 4 was investigated in the temperature range 800–1200 K. One transformation in the solid state was observed at 847 K with a corresponding enthalpy increment of 24.2 kJ mol −1. The solid-liquid transition was found to occur at 1131 K with an enthalpy of fusion of 7.74 kJ mol −1. Furthermore, the analysis of the corresponding thermograms supports the presence of the premelting effect which was evidenced by other techniques. A critical comparison with previous results in the literature is given.

Pre-melting effects in lead from resistance measurements

The resistance of a lead sample was measured with very high accuracy from 295K to the melting temperature 600.7K. Up to about 540K the resistance is determined by electron-phonon and electron-vacancy scattering with a vacancy formation enthalpy of 0.38 eV. Above 540K there is an additional term in the resistance, well described by a thermally created defect with a formation enthalpy of 1.42 eV. While the vacancy formation entropy is approximately 1.6kB the formation entropy of the new defect is approximately 16kB. This leads the authors to believe that the new defect has a liquid-like structure and consists approximately of a central atom or vacancy plus its nearest neighbours.

Microscopic behaviour of noble-gas atoms (krypton) at the surface

Molecular-dynamics calculations have been performed for noble-gas (krypton) surfaces. As pair interaction between the atoms we used the potential of Barker et al. This potential is consistent with a wide range of experimental data. The calculations have been performed for temperatures of 7 K, 70 K and 102 K. For T = 102 K we observe the effect of surface pre-melting, and this effect is obviously much more pronounced in our system than in the case of a Lennard-Jones system. Furthermore, we have investigated the interplanar spacing of the atom layers relative to each other. We found that there are not only large quantitative differences between the results for our system and a Lennard-Jones system but there are also qualitative differences.

Enhanced ionic conduction in dispersed solid electrolyte systems (DSES) and/or multiphase systems: AglAl 2O 3, AglSiO 2, AglFly ash, and AglAgBr

Alternating current electrical conductivity (σ) data are presented for AgIAl 2O 3, AgISiO 2, AgI-fly ash, and AgIAgBr as functions of composition, temperature, and frequency. Unlike the predictions of the classical theories of Rayleigh and Maxwell, conductivity enhancements by as much as two to three orders of magnitude have been obtained at 25°C without appreciable change in the electronic conductivities. The σ enhancement strongly depends on the particle size and the concentration of the dispersoids, as well as on the process variables, e.g., the heat treatment of the dispersoids, premelting of the electrolytes, etc., and also on cold and hot pressing. The Al 2O 3 is found somewhat unique in the sense that it leads to a maximum enhancement (by ∼ 10 3 times) when used as received and when the electrolyte in the mixture is premelted, while SiO 2 and fly ash (without premelting the electrolyte) lead to an enhancement by a factor of ∼50 and show no significant effect of premelting. SEM studies, coupled with the frequency-dependent σ data, suggest that the enhancement is due to bulk rather than grain-boundary or surface conduction. The enhanced σ is usually accompanied by a decrease in activation energy, suggesting that the dispersoids generate excess of lattice defects and thereby increase the conductivity, which is consistent with a recent theoretical model, as well as with the latest thermoelectric power measurements.

Ul4 compound : determination of high temperature phase transitions from electrical conductivity and calorimetric measurements

Two complementary techniques — electrical conductivity and calorimetric measurements — were used to evidence the high temperature phase transitions in the Ul4 compound. The existence of a “premelting effect” was clearly detected from these both series of observations, identical to that previously found for the similar compound UBr4 , two more transformations appearing in solid Ul4 . The temperatures and the enthalpies associated with these solid-solid and solid-liquid transformations were determined.

UBr 4 compound: Determination of high temperature phase transitions from electrical conductibility and calorimetric measurements in the temperature range 715–930 K

The electrical conductibility of the UBr 4 uranium compound was measured, for both solid and liquid state, on a wide temperature range. Results obtained for the solid near the melting point suggest the occurence of a pre-melting effect. The fusion thermograms obtained from classical differential thermal analysis, with the same material, exhibited too an anomaly suggesting that fusion joins with another phenomenon. A more sensitive thermal method—differential enthalpic analysis—allowed to precise this phenomenon, never mentioned as far as we know, and to determine the premelting and melting temperatures as well as the enthalpy associated with the global effect.

A molecular dynamics study of surface melting

The 1,0,0 face of a Lennard-Jones crystal has been examined for surface premelting effects along the crystal-vapour coexistence line by the method of molecular dynamics. The outermost crystal layer becomes quasi-fluid within the narrow temperature range 0.48-0.50 epsilon /k, as evidenced by a rapid increase in diffusivity in the surface plane, and by discontinuities in equilibrium properties characteristic of anomalous first-order transition behaviour. Similar melting is observed for the second and third layers at 0.60-0.62 epsilon /k and 0.67-0.69 epsilon /k respectively, the latter being close to the triple-point temperature. Several properties of the system are reported and discussed in the context of both melting models and available techniques, indicating how surface melting might be detected by LEED experiments. It is conjectured that surface melting should occur for all single-component crystal-vapour interfaces by a similar mechanism to that reported here. A simple empirical melting rule is suggested: Each layer melts when the ratio of kinetic to potential energy per atom in that layer becomes equal to the same ratio as for the bulk crystal at its melting point.

On the existence of pre-melting and after-melting effects A neutron scattering investigation

Abstract In order to investigate the melting transition, neutron scattering measurements have been made on aluminium and lead very near their melting temperatures. Phonon, Bragg peak, and liquid structure factor measurements are presented. The results reveal no signs of lattice instability in connection with melting, and this is discussed in the light of previous reports of premonitory effects near the melting temperature. The influence of a free surface is considered, and a new theory of surface melting is presented.

Detailed Refractive Index Measurement near Melting in a Plastic Crystal: Succinonitrile

Abstract An interferometric measurement of the temperature derivative of the refractive index in succinonitrile is reported over a temperature range (30[ddot]C → 66[ddot]C) that includes melting. In pure material the behavior is clearly that of a first-order transition both on heating and on cooling. In impure material premelting effects are seen below fusion. In all cases appreciable supercooling can be observed.