D. Nattland

WETTING AND PREWETTING TRANSITION IN METALLIC FLUID K-KCL SOLUTIONS STUDIED BY SECOND HARMONIC GENERATION

We report a detailed optical investigation of the wetting phenomena in fluid metalrich KxKCl1−x solutions for temperatures up to 820 °C and x≥0.86. To this end the fluid sample–sapphire interface has been probed by second harmonic generation (SHG) with continuous variation of temperature T at various x. At coexistence a wetting transition near Tw≊500 °C is signaled by a clear change in the temperature dependence of the SH intensity. This observation is in agreement with previous ellipsometric measurements. The precise location of the transition in these systems is strongly influenced by the presence of oxygen impurities segregating at the interface. This is manifested in a strong enhancement of the SH signal in the vicinity of 450 °C. In the homogeneous metal‐rich phase the SH intensity shows a pronounced maximum as a function of temperature for T≳Tw. The occurrence of these maxima is explained consistently by assuming a salt‐rich microscopic wetting film in between the metallic bulk fluid and the sapphir...

Interfacial wetting in a liquid binary alloy

We report the first study of interfacial wetting at the liquid-vapour surface of metallic gallium-bismuth alloys approaching the miscibility gap. Ellipsometry has been used to probe the interface continuously with increasing temperature along the coexistence curve. Below the monotectic (eutectic) temperature Tmono a liquid Ga-rich phase is in equilibrium with solid bismuth and the real part epsilon 1 of the complex dielectric function measured at the liquid surface clearly reflects Ga-like behaviour. Above Tmono the second liquid Bi-rich phase becomes stable and epsilon 1 changes from Ga-like to Bi-like behaviour. We conclude that the Bi-rich phase, which has the higher mass density, now covers the low-density phase. This can be consistently interpreted as complete wetting of the interface between the vapour and the Ga-rich phase by a coexisting Bi-rich layer.

Electron localization in metal–molten salt solutions: An optical study with in situ variation of composition

Optical absorption spectra are reported for liquid KxKCl1−x (10−5≤x≤5×10−2) and NaxNaBr1−x (10−3≤x≤3×10−2) solutions at 800 °C in the energy range 0.5≤hν≤5 eV. The composition has been varied in situ by Coulometric titration of the metal. With this method of simultaneous measurement of optical spectra and metal activity relative changes in the spectra are detected with high precision. In this way it has been possible, for the first time, to separate distinct components in the optical spectra of metal solutions. A contribution which is quantitatively described by the Drude model is attributed to weakly localized electrons in conduction band tail states with low mobility. The determined electronic mobilities are of the order of 0.1 cm2 V−1 s−1. The concentration dependence of the corresponding electronic conductivity quantitatively agrees with independent measurements of the dc‐electronic conductivity. The main component in the absorption spectra is due to excitations of localized electrons which are differ...

Surface freezing in liquid Ga–Bi alloys: optical second harmonic and plasma generation study

The phenomenon of surface freezing has been studied by second harmonic and plasma generation measurements in the entire composition range of liquid Ga–Bi alloys. Surface freezing was observed for the first time for all alloys with compositions between the eutectic (xBi = 0.0022, Teut = 29.48 °C) and monotectic point (xBi = 0.085, Tmono = 222 °C). On cooling of such alloys a Bi-rich solid-like film forms on top of the bulk liquid phase at temperatures well above the liquidus line. On melting of these films a clear hysteresis behaviour is found, characterizing this type of transition as a first order phase transition. The maximum difference of 20 K between the liquidus temperature and the melting temperature of the surface freezing films was observed for the eutectic alloy. The line of surface freezing temperatures merges with the liquidus approaching the monotectic point. An estimate of the thickness of the Bi-rich surface freezing films from the interfacial free energies yields a value of the order of 10 nm. The correlation of the surface freezing and wetting transition occurring in the Ga–Bi system as well as the thermodynamics of surface freezing are qualitatively discussed.

Surface freezing in a liquid eutectic Ga–Bi alloy

Abstract Surface freezing in a liquid eutectic Ga–Bi alloy has been studied by second harmonic generation and plasma excitation in several cooling and heating cycles at different temperature rates. An ordered solid-like film is observed on top of the bulk liquid which forms at a temperature well above the eutectic temperature. The hysteresis behaviour of crystallisation and melting of this film indicates that this surface phase transition is of first order, which is reported for the first time.

Systematic study of electron localization in saltrich Mx(MX)1−x melts (M=Na, K, Cs; X=Cl, I) by optical absorption spectroscopy

We report optical absorption spectra of Kx(KI)1−x, Csx(CsI)1−x, and Csx(CsCl)1−x solutions at temperatures around 800 °C and in the saltrich concentration range up to metal mole fractions of xM⩽0.04 for energies 0.5⩽ℏω⩽5 eV. Employing a high-temperature electrochemical cell, we were able to change and determine the alkalimetal activity and the alkalimetal mole fraction in situ simultaneously with the optical absorption spectra. The high quality of the absorption spectra allows to distinguish spectral contributions to localized electronic states and mobile electrons. Together with previously measured spectra of Nax(NaI)1−x melts a systematic investigation of the alkali iodide melts reveals differences in the nature of the strongly localized electronic states with varying cation. Interpretation of the spectroscopic results with the aid of a chemical defect model shows that the formation of localized dimeric electron states (bipolarons) is pronounced in melts of smaller cations. The analysis of the optical c...

Localized and mobile electrons in metal - molten-salt solutions

At elevated temperatures (about ) liquid alkali metal - alkali halide solutions transform continuously from the nonmetallic to the metallic state (NM - M transition) as a function of the metal mole fraction . In this study we present results of new experiments on spectroscopic ellipsometry and on absorption spectroscopy across the transition regime. The data indicate that on both sides of the NM - M transition localized and mobile electronic states may coexist

Wetting phenomena at the liquid-vapor interface of gallium-bismuth alloys studied by spectroscopic ellipsometry

Abstract In order to study phase transitions at interfaces of liquid metallic alloys, spectroscopic ellipsometry was employed in the energy range 0.7 eV ≤hv .2 eV. The complex dielectric function ɛ = ɛ1 +iɛ2 of binary gallium-bismuth alloys was measured at bismuth concentrations 0 ≤XBi .28 and up to 621 K above the critical demixing temperature, 535 K. The character of the ɛ1-spectra undergoes a pronounced change from Ga-like to Bi-like behavior when the bismuth concentration in the liquid phase exceeds xBi = 0.085 which corresponds to the monotectic triple point. This change is interpreted as interfacial wetting of the gallium rich phase by a liquid bismuth rich phase.

Microcalorimetric determination of the entropy change upon the electrochemically driven surface aggregation of dodecyl sulfate.

The aggregation of amphiphilic molecules (e.g., the formation of micelles or membranes) is usually entropy-driven. We use electrochemical microcalorimetry to directly determine the entropy change a dodecyl sulfate molecule experiences upon potential-induced adsorption from aqueous solution into a surface aggregate. From measurements of the heat, which is reversibly exchanged during the adsorption or desorption process, we determined a value of 37 ± 9 J/(mol K) for the aggregation entropy. This value is in accordance with entropies of micellization of dodecyl sulfate in solution. A comparison with estimates of the entropy of aggregation of dodecane in aqueous solutions reveals that the aggregation is driven by the entropic contribution of the hydrophobic hydrocarbon tail, in accordance with general models for the aggregation of amphiphilic molecules.

Raman spectroscopic study of mixed valence neodymium and cerium chloride solutions in eutectic LiCl–KCl melts

We have studied the Raman spectra of (NdCl2)–(NdCl3) and Ce–(CeCl3) mixtures in eutectic (LiCl)0.58(KCl)0.42 as solvent in the temperature range up to 600 °C. For the highly corrosive samples a windowless cell in connection with a Raman microscope was utilised. To our knowledge the Raman spectra of the solutions of NdCl2, CeCl3 and (most likely) CeCl2 in eut.–LiCl–KCl are shown here for the first time. In accordance with other rare earth halides described in the literature the Raman spectra of the pure trivalent systems are dominated by octahedral LnCl63− species which show a typical broad polarized band centred at 245 cm−1 for NdCl3 and 240 cm−1 for CeCl3, respectively. NdCl2 in (LiCl–KCl)eu shows a complex Raman spectrum consisting of depolarized bands. In mixtures of divalent and trivalent neodymium chlorides both spectra can be observed in parallel and no additional Raman bands appear. Solutions of cerium in CeCl3–(eut.–LiCl–KCl) show temporarily new Raman bands which are presumably due to divalent cerium chloride. These bands disappear in our samples after about 1 h since CeCl2 is not stable under the experimental conditions. Our findings are discussed in the light of the strongly different electronic transport properties in the neodymium and cerium systems.