Liquid Metal Surface Research Articles

A statistical approach to energy loss during gas–liquid collisions II: Neon–liquid metal collisions

The statistical treatment of energy exchange during gas–liquid collisions is extended to collisions with liquid metal surfaces. Strong bonding between metal atoms, as reflected in high values of surface tension, restricts the surface motion to sinusoidal waves with relatively small values of the wave-vector. Such waves propagate far from the point of impact. Our model predicts that a small, fast atom colliding with a liquid metal surface will lose on average 25% of its energy. The model also accounts for very small trapping probabilities for atom-liquid metal collisions, and for low-energy shoulders in the energy distributions of scattered atoms.

Experimental Investigation of Arc Plasma in GaInSn Liquid Metal Current-Limiting Device

This paper focuses on an experimental investigation of arc plasma in a liquid metal current-limiting device based on pinch effect. A specially designed experimental setup is employed to observe the dynamic process of arc initiation and arc evolution. From the frames obtained by high-speed photography, the mechanism of arc initiation is revealed. It indicates that the arc ignites due to the self-pinch effect, which starts from the free surface of liquid metal and develops in the form of gas cavities. According to the analysis of the subsequent stage after arc ignition, it can be concluded that the arc plasma should be a mixture of metal vapor and air. With the morphological changes of the arc, the arc evolution can be classified into four stages. The arc erosion on electrodes is observed during the arc process, and its causes are analyzed in detail. Using a liquid metal current-limiting device, the arc characteristics during the current limitation are investigated. It is found that the duration of the prearcing stage and the arc voltage are influenced by the pass-through current. As prospective current increases, the prearcing time reduces because of the acceleration of the pinch process and the arc voltage rise due to the adequate arc burning.

The Tolman parameter, self-absorption, and surface tension on flat and curved surfaces of liquid metals

Analytical expressions for Tolman parameter jg, self-absorption on a tension surface db, and surface tension σ on flat and curved surfaces were found using the thermodynamic Gibbs method. The jg, db, and σ values were determined for 50 liquid metals at the melting temperature. It was established that surface tension decreases with a decrease in the radius of an equimolecular dividing surface.

A model of oxygen adsorption at liquid copper surfaces

A model of O-adsorption at liquid metal surfaces has been constructed, using liquid Cu as an example. The modeling approach used is similar to the regular solution scheme previously used successfully for modeling the adsorption/segregation behavior of metal alloys, in that the internal energy of the system is evaluated by nearest neighbor bond energies. In the model, the adsorption of oxygen in the near-surface region is allowed to occur at both surface and sub-surface sites. The model predicts a variety of possible adsorption characteristics, including the possibility of first order adsorption transitions which involve the formation of a 2-dimensional surface oxide, and different sequences for the occupancy of surface and sub-surface adsorption sites. In particular, by fitting the model to the experimental dependence of Cu surface energy on O-partial pressure, it is possible to conclude that O-adsorption in that case most likely occurs by the occupancy of sub-surface sites.

Atomic layering and related postmelting effects in small liquid metal clusters

The specific heat of ${\text{Na}}_{25}$, as obtained by first-principles molecular dynamics simulations, shows interesting anomalies above the melting phase-change region: (a) It steadily decreases with increasing temperature, an effect observed also in bulk liquid metals. This trend is explained in terms of the gradual conversion of vibrational modes into diffusion modes; (b) on top of this decreasing trend, the specific heat shows broad undulations, induced by the temperature-dependent atomic layering in the liquid cluster. Extended liquid metal surfaces also show atomic layering, but it does not affect the bulk heat capacity. This effect is therefore genuine of finite atomic systems and should be expected to be quite general in metal clusters.

Computer simulation calculations of the free liquid surface of mercury

Oscillatory density profiles are a well-known feature occurring at the free surfaces of liquid metals. Recently, the thermophysical properties of expanded liquid mercury have been successfully investigated with an atypic isotropic effective interaction Bomont et al., J. Chem. Phys. 124, 054504 2006 . We use this model to investigate the liquid-vapor interface properties taking explicitly into account the density dependence of the potential related to the metal-nonmetal transition along the interface. Ionic and electronic density distributions along the normal of the interface display strong stratification and the calculated reflectivities compare well with experimental data.

Thermocapillary and thermoelectric effects in liquid lithium plasma facing components

Recent experiments have renewed interest in the use of liquid lithium as a plasma facing component (PFC). The liquid metal surface will experience a number of effects which are considered in the present work with simple analytical and more complete computational models. These include the thermal response under fusion relevant heat loads causing thermocapillary and thermoelectric effects. Analytical solutions for simplified flow are given showing the surface velocity of these liquid metals due to thermocapillary effects in conjunction with magnetohydrodynamic drag. A parameter describing dominant conduction or convection is developed which shows that thin-films relevant to NSTX will be conduction dominated. Thermoelectric effects could propel liquid lithium with velocities of 10 s of cm/s providing a significant boost to energy transport.

On thermal radiation from fusion related metals

In this paper, we use Mie theory to investigate the temperature-dependent emissivity of small particles composed of a fusion device relevant metal, lithium. We also use classical electromagnetic theory to estimate the high temperature emissivity of flat surfaces of liquid lithium and several metals, such as beryllium, being considered as plasma facing components in fusion devices. We use a Drude theory model for the temperature dependence of the optical constants of the metals. The results may have application to modeling or diagnosing the temperature of various metallic components in fusion environments.

Predicting of bead undercut defects in high-speed gas metal arc welding (GMAW)

In the gas metal arc welding (GMAW) process, when the welding speed reaches a certain threshold, there will be an onset of weld bead undercut defects which limit the further increase of the welding speed. Establishing a mathematical model for high-speedGMAWto predict the tendency of bead undercuts is of great significance to prevent such defects. Under the action of various forces, the transferred metal from filler wire to the weld pool, and the geometry and dimension of the pool itself decide if the bead undercut occurs or not. The previous model simplified the pool shape too much. In this paper, based on the actual weld pool geometry and dimension calculated from a numerical model, a hydrostatic model for liquid metal surface is used to study the onset of bead undercut defects in the high-speed welding process and the effects of different welding parameters on the bead undercut tendency.

Peculiar properties of hot plasma formation under the influence of intense femtosecond pulses on the surface of molten metal

The peculiar properties of plasma formation on the surface of different liquid metals by femtosecond laser radiation have been studied. It is shown that plasma formation and generation of hard X-radiation on the surface of molten metal depends substantially on the contrast and weakly on the polarization of laser radiation, which clearly distinguishes the plasma produced in our experiments from the plasma generated on the surface of a solid target.

The effective oxidation pressure of indium–oxygen system

A theoretical model on oxygen transport at the surface of liquid metals has been validated by dynamic surface tension measurements performed on liquid Indium as test metal. The oxygen contamination conditions have been obtained at different oxygen partial pressures under both low total pressure (Knudsen regime) and inert atmospheric pressure (Fick regime) conditions, confirming that an oxide removal regime occurs under an oxygen partial pressure much higher than the equilibrium one (the “Effective Oxidation Pressure”). Experimental results are reported which give a further insight on the relative importance of the various processes due to the oxygen mass transport between the liquid metal and the gas phase. The critical aspects involved in surface tension measurements of liquid metals, related to the problem of liquid metal–oxygen interactions, are also underlined.

Free‐surface instabilities during electromagnetic shaping of liquid metals

AbstractElectromagnetic shaping of free surfaces of liquid metals is a well‐known EPM technology used in a couple of metallurgic processes like cold crucible melting, semi‐levitation, and electromagnetic slit sealing, among others. However, the stability of such free surfaces is the most important problem and stability control is crucial for success. Within this context we investigate experimentally the stability behavior of liquid metal free surfaces submitted to a high‐frequency magnetic field. In this case, the induced Lorentz forces act as an electromagnetic pressure directly on the free surface of the liquid met al. We consider three experimental model configurations: (i) Sessile liquid metal drop (ii) liquid metal ring, and (iii) liquid metal disc. In each model experiment, upon increasing the feeding current beyond a certain threshold value, IC, we observe that the initial surface contour becomes unstable resulting in (i) drop oscillations (ii) electromagnetic pinching and (iii) static disc deformations. In each configuration the threshold value depends in a similar manner on the frequency of the applied magnetic field. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In-plane structure and ordering at liquid sodium surfaces and interfaces from ab initio molecular dynamics

Atoms at liquid metal surfaces are known to form layers parallel to the surface. We analyze the two-dimensional arrangement of atoms within such layers at the surface of liquid sodium using ab initio molecular dynamics (MD) simulations based on a full version of density functional theory. Nearest neighbor distributions at the surface indicate mostly fivefold coordination, though there are noticeable fractions of fourfold and sixfold coordinated atoms. Bond angle distributions suggest a movement toward the angles corresponding to a sixfold coordinated hexagonal arrangement of the atoms as the temperature is decreased towards the solidification point. We rationalize these results with a distorted hexagonal model at the surface, showing a mixture of regions of five- and sixfold coordination. The liquid surface results are compared with classical MD simulations of the liquid surface, with similar effects appearing, and with ab initio MD simulations for a model solid-liquid interface, where a pronounced shift towards hexagonal ordering is observed as the temperature is lowered.

Micro-impedance inclinometer with wide-angle measuring capability and no damping effect

This paper reports a high-performance impedance-type inclinometer fabricated in a glass–SU-8–glass sandwich structure for wide-angle measurements with rapid sensing response. The output signal of the proposed-inclinometer is read from a moving-metal pendulum based on the electrical potential measurement scheme. Two types of pendulum mass made by Ga–In–Sn alloy and mercury are used for the inclinometer. The friction between the liquid metal and substrate surfaces is dramatically reduced by applying an organic-based surfactant of SDS. Experimental results show the developed inclinometer has a wide linear range (320°) and a high angle resolution (0.3°). Performance test shows no time lag for the sensor output under a rotation speed of 1/8 rps, and no damping effect for an instant stop from a rotation speed of 45°/s. The developed inclinometer provides a simple yet high-performance solution for tile angle measurement.

A molecular dynamics study of nanoindentation of amorphous silicon carbide

Through molecular dynamics simulation of nanoindentation of amorphous a-SiC, we have found a correlation between its atomic structure and the load-displacement (P-h) curve. We show that a density profile of a-SiC exhibits oscillations normal to the surface, analogous to liquid metal surfaces. Short-range P-h response of a-SiC is similar to that of crystalline 3C-SiC, e.g., it shows a series of load drops associated with local rearrangements of atoms. However, the load drops are less pronounced than in 3C-SiC due to lower critical stress required for rearrangement of local clusters of atoms. The nanoindentation damage is less localized than in 3C-SiC. The maximum pressure under the indenter is 60% lower than in 3C-SiC with the same system geometry. The onset of plastic deformation occurs at the depth of 0.5Å, which is ∼25% of the corresponding value in 3C-SiC. a-SiC exhibits lower damping as compared to 3C-SiC, which is reflected in the longer relaxation time of transient forces after each discrete indentation step.

Advancements in the Reactive Liquid ${\rm Mg}$ Infiltration Technique to Produce Long Superconducting ${\rm MgB}_{2}$ Tubular Wires

The possibility to manufacture superconducting MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires, applying the reactive liquid Mg infiltration (RLI) process, was demonstrated several years ago, obtaining wires characterized by hollow cores. The short samples of these wires gave transport critical currents in line with the best powder in tube (PIT) wires produced by "in situ" technique. Nevertheless the extension of the RLI technology to long superconducting wires initially was prevented owing to difficulties in controlling the liquid magnesium dynamics during the reaction. Now we succeed in controlling the reaction and in the manufacturing wires or tubular long manufacts. For tubular manufacts having large diameter, of several millimetres, useful for current leads or relatively short resistive current limiters, the liquid Mg is also supplied continuously during the reaction, to refill completely the core of the tube. For thin wires, with diameter of the order of a millimetre or less, useful for magnets and racetrack winding and other filamentary manufacts, the movement of liquid Mg is counteracted by the liquid metal surface tension. Concerning the superconducting characteristics of the wires, we have explored some key variables effective on the quality of the crystalline MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> : they include the choice of the original Boron powders and the addition of doping elements.

Liquid metal/ceramic interactions in the (Cu, Ag, Au)/ZrB 2 systems

Wetting and spreading experiments on ZrB 2 in contact with liquid Cu, Ag and Au have been performed by the sessile drop technique under a vacuum. The wetting and spreading characteristics and the interfacial reactions are discussed as a function of time and of the metal involved. The interfacial morphologies, analysed by optical microscopy, SEM and EDS show the presence of regular interfaces without macroscopic reaction layers. Gold, to a very large extent and copper are shown to give rise to extensive penetration along grain-boundaries, whereas silver neither wets nor penetrates. Interfacial diffusion/dissolution is taken into account and the consequent changes in liquid metal surface tension and wetting behaviours have been evaluated by means of thermodynamic calculations. Moreover, interfacial energetics at the atomistic level has been investigated by means of pseudopotential-based Density Functional Theory (DFT) technique. It is shown how the calculation of the ideal work of separation on the specific transition metal borides-molten metal systems can be used to interpret the wetting behaviour. Moreover, the dependence of the adhesion behaviour on the electronic structure at the interface and on the interface epitaxy and composition is also briefly discussed.

Ab initio study of the atomic motion in liquid metal surfaces: comparison withLennard-Jones systems

It is established that liquid metals exhibit surface layering at the liquid–vapour interface,while dielectric simple systems, like those interacting through Lennard-Jonespotentials, show a monotonic decay from the liquid density to that of the vapour.First principles molecular dynamics simulations of the free liquid surface ofseveral liquid metals (Li, Na, K, Rb, Cs, Mg, Ba, Al, Tl and Si), and theNa3K7 alloy near their triple points have been performed in order to study the atomic motion atthe interface, mainly at the outer layer. Comparison with the results of classical moleculardynamics simulations of a Lennard-Jones system shows interesting differences andsimilarities. The probability distribution function of the time of residence in a layer shows apeak at very short times and a long-lasting tail. The mean residence time in a layerincreases when approaching the interfacial region, slightly in the Lennard-Jones system butstrongly in the metallic systems. The motion within the layers, parallel to the interface, canbe described as diffusion enhanced (strongly in the case of the outermost layer) withrespect to the bulk, for both types of systems, despite its reduced dimensionality in metals.

Observation of Surface Layering in a Nonmetallic Liquid

Oscillatory density profiles (layers) have previously been observed at the free surfaces of liquid metals but not in other isotropic liquids. We have used x-ray reflectivity to study a molecular liquid, tetrakis(2-ethylhexoxy)silane. When cooled to T/Tc approximately 0.25 (well above the freezing point for this liquid), density oscillations appear at the surface. Lateral order within the layers is liquidlike. Our results confirm theoretical predictions that a surface-layered state will appear even in dielectric liquids at sufficiently low temperatures, if not preempted by freezing.

Nano-patterned liquid metal electrode for the synthesis of novel Prussian blue nanotubesand nanowires

We report the fabrication of Prussian blue nanotubes and nanowires via a novel techniquewherein a liquid metal surface is nano-patterned with a porous polycarbonate membraneused as a hard mask. Prussian blue as 1D nanowires and nanotubes is an excellentcandidate for ultra-low level sensing, optical waveguides and model systems to test theone-dimensional Ising model. One of the most feasible techniques for nanowire fabricationis to use a porous template. However, Prussian blue dissolves in strong acids anddecomposes in strong bases. Conventional procedures which use porous alumina blockedwith a back side metal contact require strong acid and base treatment in the varioussteps of the nanowire fabrication. We report a novel technique which will pave anew route for the facile synthesis of nanowires and nanotubes of materials thatare sensitive to strong acidic or basic treatment. Hence, nano-patterning of aliquid metal (Hg, which functions as a top-side metal contact) with polycarbonatemembrane instead of porous alumina membrane excludes the necessity of both strongacid and base during the fabrication of nanowires. By attempting this innovativetechnique, we report the synthesis of novel organic Prussian blue nanotubes andnanowires.