P. Cheyssac

A Direct Observation of Low-Dimensional Effects on Melting of Small Lead Particles

The melting of small lead particles embedded in a silicon monoxide matrix has been studied by dark-field electron microscopy. Thomsons idea from 1888, that the melting temperature depends on the size of the particles and Faradays idea from 1860, that melting of the surface occurs below the bulk melting temperature, are demonstrated here straight-forwardly. The electron microscope pictures give the most direct observation on the existence of a molten surface layer in equilibrium with a solid core at temperatures below the melting point. The width of the molten layer is demonstrated to depend on the particle curvature as well as on temperature indicating a continuous transition. Quantitatively, it was found that for the small lead particles, the width of the molten layer is wider than the reported one for bulk lead.

Solid-liquid phase transitions optically investigated for distributions of metallic aggregates. absence of hysteresis for the smallest sizes

Melting and solidification temperatures for distributions of lead aggregates of very small sizes (mean radius = 42 A, the minimum detectable radius being r = 23 A) are reported. These new results show the existence of a limit temperature TL = 392K below which all the aggregates are solid. A discussion of the results for the melting point depression is given on the basis of a phenomenological approach. For the case of solidification, TL is compared to the stability limit temperature of the supercooled liquid predicted by a simple adiabatic nucleation model.

Anisotropic surface effects in Au and Cu monocrystals

Electroreflectance spectra at normal incidence of (100) and (110) faces of gold and copper monocrystals are given, in the spectral range from 0.22–0.7 μm. The fractional change in reflectance is different with (110) faces when light is polarized parallel to the [001] direction and parallel to the [110] direction while no anisotropy is seen on (100) faces. This shows that electroreflectance is a powerful tool to investigate metal surfaces where the optical electrons are sensitive to the distribution of the surface atoms.

Étude expérimentale et théorique de l'électroréflexion de l'or en incidence oblique

Abstract Electroreflectance spectra of gold have been investigated at oblique incidence, the light wave being polarized either perpendicular or parallel to the plane of incidence. The experimental procedure is described and the electrochemical behaviour of the metal-electrolyte junction is examined in great detail. A phenomenological interpretation of the electro-reflectance at the metal-electrolyte interface is given, in which this effect is due to a surface current related to a change in the electron density at the surface of the metal. The agreement between the theoretical and experimental results is quite good.

Metallic electroreflectance; anisotropic aspects

Abstract New metallic electroreflectance (MER) experiments have been done at normal incidence for the (100), (111) and (110) faces of gold single crystals in contact with an electrolyte. For every face, a mapping of the fractional change in reflectance has been drawn up in the photon energy range 1.5 to 5.0eV, and in the metallic surface charge interval −15 μC/cm 2 to +20 μC/cm 2 . These results, obtained by a non-traditional potentiodynamic method, are seen to be free of any drift during the experiments. Furthermore, the fractional change in reflectance is found to be different with (110) face when light is polarized parallel to the 〈001〉 direction and parallel to the 〈110〉 direction, while no anisotropy is seen for the (100) and (111) faces. A new theoretical calculation based upon a non-local analysis of the dielectric function of the metal is given, which allows to find the shape, the magnitude and the good sign of the experimental electroreflectance spectrum.

Optically monitored melting, supercooling and solidification of lead aggregates embedded in a dielectric matrix

The solid liquid transitions of lead aggregates embedded in a silicon matrix are investigated through optical measurements. A phenomenological model linking optics and thermodynamics of melting is proposed.

Optical investigation of the solid-liquid transition in gallium

Reflectance of thin gallium films in the spectral range 0.3-0.9 mu m is measured for several temperatures around the melting point. When melting occurs, the reflectance increases considerably in the whole spectral range (about 20% at 0.6 mu m). A free-electron model is assumed; this is found to be in good agreement with experiments on liquid-phase systems. A qualitative interpretation is proposed and is found to explain the considerable change occurring during melting.

Modulation spectroscopy of gold: Comparative studies

Abstract An apparatus has been constructed to investigate the modulated optical properties of thin films. We report here the piezo- (PR), thermo- (TR) and electroreflectance (ER) spectra for a thin gold film, 800 A thick. The experimental results show that the PR and TR spectra are similar to each other but quite different from the ER one. A new interpretation of ER is given in which this phenomenon is assumed to be due to a surface current related to a change in the electron density. The agreement between the theoretical and experimental results is encouraging.

Thermoreflectance as a practical tool for the extension of the optical constant data in a wide domain of temperature. Application to lead between 298 K and 538 K

Abstract Thermoreflectance measurements allow the extension of the optical constant data of a material, known for one or a few temperatures, in a wider domain of temperature. This extrapolation method is easier to use than the conventional produres on both the experimental and theoretical grounds.

Comment on “a Direct Observation of Low-Dimensional Effects on Melting of Small Lead Particles” by J. Bohr

The comment of J. Bohr sets the general problem of the interaction between an inclusion and its embedding matrix. We will put in evidence that the effects may be very different according to the kind of sample under study. In the case of our Pb/SiO experiment (Europhys. Lett., 12 (1990) 709), the SiO matrix is a thin amorphous evaporated film whose interaction with the Pb particle is minimum (wetting of SiO by liquid Pb is very low), the role of the matrix being essentially to maintain approximately a constant shape for the particle (and not a constant volume). It will be shown that arguments based on pressure effects lead to results inconsistent with our experiments. For the Pb/Al experiment of Bohr et al. (Phys. Rev. Lett., 64 (1990) 924) interaction between the matrix and the inclusion is responsible for the existence of superheating for the Pb particle as well as a reduction of its supercooling.