Liquid Metal Surface Research Articles

Interplay between the ionic and electronic density profiles in liquid metal surfaces

First-principles molecular-dynamics simulations have been performed for the liquid-vapor interfaces of liquid Li, Mg, Al, and Si. We analyze the oscillatory ionic and valence electronic density profiles obtained, their wavelengths, and the mechanisms behind their relative phase shift.

Composition transients and saturation phenomena at a liquid metal–vapour interface: An advanced theoretical approach and an application to the oxidation of tin in a vacuum

This work is aimed to advance the understanding of interfacial saturation phenomena for liquid metal surfaces. The interactions between a liquid metal drop and an oxidising atmosphere can be described by a physical–mathematical description of the liquid metal interface/surrounding environment interface evolution. The closed system and equilibrium assumption were rejected and reference was made to a liquid metal drop as an open system and its related time variation conditions (stationary drop assumption). A better demonstration of the experimental relevance of evolution times towards the stationary condition, in particular for liquid tin is done.

Dynamic behavior of a liquid metal interface under the influence of a high-frequency magnetic field

We present experimental results on the stability of a free liquid metal surface influenced by an alternating magnetic field. The field is generated by an inductor fed by a high-frequency electrical current. The experimental setup consists of an annulus filled with the liquid metal galinstan. We observe three kinds of instabilities: exciting of capillary waves, a static high-amplitude surface deformation, and eventually an electromagnetic pinch at a critical inductor current I(cp). The data show that I(cp) correlates directly with the current frequency and the geometric parameters.

Overview of the ALPS Program

The US Advanced Limiter-divertor Plasma-facing Systems (ALPS) program is developing the science of liquid metal surface divertors for near and long term tokamaks. These systems may help solve the demanding heat removal, particle removal, and erosion issues of fusion plasma/surface interactions. ALPS combines tokamak experiments, lab experiments, and modeling. We are designing both static and flowing liquid lithium divertors for the National Spherical Torus Experiment (NSTX) at Princeton. We are also studying tin, gallium, and tin-lithium systems. Results to date are extensive and generally encouraging, e.g., showing: 1) good tokamak performance with a liquid Li limiter, 2) high D pumping in Li and non-zero He/Li pumping, 3) well-characterized temperature-dependent liquid metal surface composition and sputter yield data, 4) predicted stable low-recycle improved-plasma NSTX-Li performance, 5) high temperature capability Sn or Ga potential with reduced ELM & disruption response concerns. In the MHD area, analysis predicts good NSTX static Li performance, with dynamic systems being evaluated.

Numerical modelling of entrainment of oxide film defects in filling of aluminium alloy castings

Recent advances in understanding the filling process during casting have indicated that metal flow conditions play a key role in controlling the quality of the casting. Inappropriate filling of castings usually causes surface oxide films to be folded into the bulk liquid or broken as a result of a higher liquid metal kinematic energy, resulting in so-called 'entrainment damage'. These folded oxide film defects are entrained in aluminium alloy casting and significantly affect both static and fatigue reliabilities. In the present study, an algorithm, Oxide Film Entrainment Tracking (OFET, 2-D), has been developed, tested and validated for numerical modelling of the liquid aluminium flow and movement, fold over and entrapment of oxide films during the mould filling process of aluminium castings. The Volume of Fluid (VOF) method is employed to track the free surface of liquid metal flow. A kinematic approach has been utilised for modelling the movement and entrainment of the oxide films on the liquid metal surface. Because the oxide films on the free surface are usually very thin, the inertial forces of the films can be disregarded in comparison to other forces like gravity and pressure. The computer program based on the proposed algorithm is able to model the metal flow behaviour and oxide film defect entrainment in filling of aluminium castings.

Surface oxidation of liquid Sn

We report the results of an X-ray scattering study that reveals oxidation kinetics and formation of a previously unreported crystalline phase of SnO at the liquid–vapour interface of Sn. Our experiments reveal that the pure liquid Sn surface does not react with molecular oxygen below an activation pressure of ∼5.0 × 10 −6 Torr. Above that pressure a rough solid Sn oxide grows over the liquid metal surface. Once the activation pressure has been exceeded the oxidation proceeds at pressures below the oxidation pressure threshold. The observed diffraction pattern associated with the surface oxidation does not match any of the known Sn oxide phases. The data have an explicit signature of the face-centred cubic structure, however it requires lattice parameters that are about 9% smaller than those reported for cubic structures of high-pressure phases of Sn oxides.

Liquid metal systems for gasification-based power generation

A closed-loop liquid metal system is proposed for the simultaneous quenching and desulphurization at elevated temperature of raw fuel gas from commercial gasifiers. Besides its origins in a recently conceived liquid metal transport gasifier, application to the oxygen-blown Shell gasifier and the air-blown Mitsui-Babcock gasifier is discussed. The quenched fuel gas is filtered at relatively low temperature before being reheated to 750 °C or above by direct contact heat exchange with liquid metal in advance of partial precombustion to raise the temperature (1735 and 1194 °C respectively). Ammonia is reduced and equilibrium established prior to massive superheated steam injection, utilizing all available steam, and then rapid transmission to the can-annular combustors of a 1500 °C Class gas turbine at 870 °C, where autoignition of the highly preheated fuel gas is predicted. NOx emissions as low as 2 ppmv are anticipated. Mercury is projected to be recovered in elemental form, while other air toxics are prevented from discharging into the environment. Water recovery at 99 per cent appears feasible in a low-pressure drop system, and visible plume discharge is avoided by reheating the condenser off-gas in direct contact with a passivated liquid metal surface. Parallels are drawn with the molten tin based float glass process.

Surface layering of liquids: The role of surface tension

Recent measurements show that the free surfaces of liquid metals and alloys are always layered, regardless of composition and surface tension; a result supported by three decades of simulations and theory. Recent theoretical work claims, however, that at low enough temperatures the free surfaces of all liquids should become layered, unless preempted by bulk freezing. Using x-ray reflectivity and diffuse scattering measurements we show that there is no observable surface-induced layering in water at T=298 K, thus highlighting a fundamental difference between dielectric and metallic liquids. The implications of this result for the question in the title are discussed.

Formation of the Taylor cone on the surface of liquid metal in the presence of an electric field

The hydrodynamic evolution of the surface of a liquid metal in the presence of an electric field is investigated using both analytical and numerical techniques. It is established that a free liquid surface with axial symmetry evolves with time into a cone-like shape, with the cone angle equal to Taylor's static value of 98.6°. The mechanism behind such interesting flow behaviour is that the system of electrohydrodynamic (EHD) equations has a self-similar asymptotic solution that generalizes Taylor's static result. The asymptotic solutions are found and the time-dependent behaviours of basic physical quantities (electric field strength, fluid velocity and surface curvature) near the singularity are established. The results and the analytical and numerical techniques used are thought to be useful in the development of time-dependent models of operating liquid metal ion sources and EHD sprayers.

Examination of liquid metal surfaces through angular and energy measurements of inert gas collisions with liquid Ga, In, and Bi

Gas–liquid scattering experiments are used to measure the recoil directions and energies of neon, argon, and xenon atoms scattering from liquid gallium, indium, and bismuth. The angular and energy distributions vary systematically with the identity of the gas and liquid and with the incident gas energy and liquid temperature. We find that the gas atoms scatter into a narrower angular range from liquids with higher surface tension γ (γGa>γIn>γBi), while they transfer less energy to liquids of higher atomic mass (mBi>mIn>mGa). Comparisons of the angular distributions with scattering models suggest that lower surface tension liquids possess atomically rougher surfaces that redirect impinging atoms more broadly into space. The trend we observe linking broader angular distributions with lower energy transfer appears to be a microscopic manifestation of a general trend between decreasing surface tension and increasing atomic mass for main group metals such as Ga, In, and Bi.

Liquid-metal based coal gasification

Significant environmental benefits are secured in the gasification of coal, petroleum coke, biomass and similar carbonaceous materials, if the ash content is rendered non-leachable by the production of a vitreous slag. By combining float glass and generic melt circulation technologies, a new liquid-metal based process for gasification is proposed. The first case considered focuses on low sulphur feeds for which the proposed basic flowsheet involves a pressurised horizontal liquid-metal transport gasifier, to which solid charge materials are simply introduced onto the moving liquid metal surface via a barometric leg entrained in liquid metal. The solids are transported in a straight line through an air and steam top-blown zone to gasify the solids and produce a liquid sheet of molten slag, analogous to float glass manufacture, except that the liquid-metal is force circulated through the gasifier and external heat and mass transfer functions, involving direct countercurrent contacting in irrigated packed or moving beds to remove solid particulates and gaseous pollutants at high temperature. In the context of advanced power generation, the fuel gas is cooled to around 240°C to permit straightforward filtration and then is re-heated again by direct liquid-metal contacting to at least 730°C for transmission to the combustors of a gas turbine, which yields enhanced combined cycle efficiencies. More complex flow-sheets are then explored in which metallic zinc is added non-intrusively in a separate gas circuit independent of the fuel gas circuit to effect desulphurisation and to yield a valuable by-product, zinc sulphide. It is shown that not only can a slag be produced in a single stage air-blown cycle without the need for air separation and oxygen production, but also toxic components in air,in general, can be removed; in the particular case of mercury, this can be recovered directly in elemental form.

Experimental and theoretical studies of the motion generated by a two-frequency magnetic field at the free surface of a gallium pool

The behavior of a free surface of liquid metal in the presence of a periodic magnetic field pulsating at two frequencies (a midrange frequency f 1 and a low frequency f 2, referred to as the modulation frequency) has been investigated. An expression for the forces induced in an electroconductive material by such a field is established for f 2 ≪ f 1, and a heuristic model is developed to describe the free surface axisymmetric deformations of an inviscid liquid metal submitted to modulated electromagnetic forces. The electromagnetic forces generate standing waves with an oscillation frequency of f 2 or 2f 2. Experiments performed on a cylindrical gallium pool show that the observed surface waves are concentric. The maximum amplitude of the surface oscillations and their frequency are measured by means of a contact probe and analyzed with respect to the modulation frequency f 2. The oscillation amplitude exhibits resonance peaks for values of f 2 corresponding to the eigenfrequencies of the free surface modes. The various free surface oscillation modes are identified. Depending on the value of f 2, the free surface oscillates at the frequency f 2 or at the frequency 2f 2. The experimental features of the free surface oscillations agree qualitatively with those predicted by the proposed heuristic model.

Stability of a liquid metal interface affected by a high-frequency magnetic field

The stability of a free liquid metal surface influenced by an alternating magnetic field is investigated both experimentally and analytically. The experimental set-up consists of an annulus filled with liquid metal. The outer wall of the annulus is made of quartz glass to allow visualization using a high-speed camera system. The magnetic field is generated by ring-like inductor located at some distance above the free surface of the liquid metal. The inductor is supplied by an alternating current of variable frequency between 5 kHz and 50 kHz. The alternating magnetic field induces Lorentz forces within the liquid and generates an electromagnetic pressure acting on the free surface. Two different types of instabilities in the are detected. At lower inductor currents we observe sinusoidal surface waves, while at higher currents, above a critical value, an electromagnetic pinch is generated. During the experiments we measure the critical current and the amplitude of the surface waves for various frequencies. A simple analytical model has been developed in order to predict the instability of a plane liquid metal surface affected by an alternating electromagnetic field. The hydrodynamic equations are simplified by applying the Hele-Shaw approximation for a thin annulus gap. The electromagnetic equations are simplified by using the skin depth approximation. Here we exploit the fact that for high frequencies the penetration depth of the magnetic field is small compared to the liquid layer height. Within this assumption, the electromagnetic pressure on the free surface can be calculated by using the method of mirror charges. Linear stability analysis shows that the plane surface becomes unstable to perturbations of wave number k whenever the effect of the electromagnetic pressure exceeds the stabilizing effects of surface tension and gravity. The critical inductor current predicted by the model is in good agreement with the critical value of the instability observed in experiment.

Final shape and formative time of protrusions on liquid-metal cathodes

This paper presents new results of the study on the formation of protrusions on the surface of liquid-metal cathodes of the capillary and needle types. The problems associated with the shaping and growth time of a protrusion are considered in relation to the dimensions of the liquid-metal pool on the cathode top, the length of the needle extension, and the value and waveform of the voltage applied to the gap. It has been shown that the time of growth of a protrusion can be shortened by an order of magnitude and simultaneously its stability can be increased by applying to the gap an additional dc voltage, close in magnitude to the threshold voltage at which electrohydrodynamic instability develops at the liquid-metal surface.

The liquid surface/interface spectrometer at ChemMatCARS synchrotron facility at the Advanced Photon Source

We discuss results from the first experiments on a new liquid surface/interface X-ray spectrometer recently commissioned by ChemMatCARS at Sector 15 of the Advanced Photon Source. These experiments include studies of liquid/liquid interfaces, monolayers supported on the water surface, and liquid metal surfaces.

Dynamic surface tension measurements on molten metal-oxygen systems: model validation on molten tin

A theoretical model on oxygen transport at the surface of liquid metals has been validated by dynamic surface tension measurements performed on liquid tin as test metal. The oxygen contamination conditions have been obtained at different oxygen partial pressures under low total pressure conditions (Knudsen regime), 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 new 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 carefully underlined.

X-ray study of the liquid potassium surface: Structure and capillary wave excitations

We present x-ray reflectivity and diffuse scattering measurements from the liquid surface of pure potassium. They strongly suggest the existence of atomic layering at the free surface of a pure liquid metal with low surface tension. Prior to this study, layering was observed only for metals like Ga, In and Hg, the surface tensions of which are 5-7 fold higher than that of potassium, and hence closer to inducing an ideal "hard wall" boundary condition. The experimental result requires quantitative analysis of the contribution to the surface scattering from thermally excited capillary waves. Our measurements confirm the predicted form for the differential cross section for diffuse scattering, $d\sigma /d\Omega \sim 1/q_{xy}^{2-\eta}$ where $\eta = k_BT q_z^2/2\pi \gamma $, over a range of $\eta$ and $q_{xy}$ that is larger than any previous measurement. The partial measure of the surface structure factor that we obtained agrees with computer simulations and theoretical predictions.

Liquid lithium surface research and development

A liquid metal surface made with a capillary porous structure (CPS) (solid base) filled with lithium (liquid) has been offered for application in a magnetic confinement fusion reactor. The approach is investigated experimentally for divertor and first wall relevant conditions. The CPS ensured stability of the liquid surface under pulsed plasma impact in disruption simulation and tokamak experiments. Continuous operation of lithium capillary target was studied under electron beam load in stationary thermal conditions in the range 1–10 MW/m 2 of energy flux in steady state. Lithium evaporation was shown to dominate at temperatures higher than 400 °C and it removed up to 0.7 of incident power. Heat flux redistribution at the liquid lithium surface was analyzed. Lithium ionization, lithium plasma parameters near the liquid surface were evaluated. The importance and possibility of prompt lithium removal from the near surface layer in divertor conditions are emphasized.

Hydrogen and helium entrapment in flowing liquid metal plasma-facing surfaces

The ability to use liquids as plasma-facing component (PFCs) depends on their interaction with the plasma and the magnetic field. One important issue for the moving liquid is the ability to entrain particles that strike the PFC surface (helium and hydrogen isotopes) while accommodating high heat loads. To study this problem, a numerical model has been developed using the HEIGHTS computer simulation package. The model was used to investigate pumping of He particles by the flowing liquid rather than requiring a standard vacuum system. Hydrogen isotope (DT) particles are likely be trapped in the liquid metal surface (e.g., lithium) due to the high chemical solubility of hydrogen. The incident He particles in the established low-recycling regime at PFCs could be harder to pump using standard vacuum pumping techniques. The analysis results indicate, however, a reasonable chance of adequate helium self-trapping in flowing lithium as PFC without active pumping.

Concept design of a new liquid metal target station for high power neutron scattering facilities

A new liquid metal target station without a window wall for high power charged particle beams to pass through is proposed for neutron-scattering sources and/or neutron-spallation sources. The beams are irradiated directly from above into the free surface of the liquid metal in a target tube soaked vertically in a reservoir tank of the liquid metal. A large hollow chamber is located above the target tube. This chamber works as a getter pump to evacuate the system and as a heat siphon to remove the heat fed by the beams, where the liquid metal works as the working liquid for both functions. Bismuth, Bi in the liquid state at temperatures above 300 °C, is chosen as the target medium. Experimental data for a cylindrical bismuth target are used to calculate the temperature distribution along the liquid metal target column. The evaporation rate of Bi atoms due to boiling under reduced pressure is calculated under a simple assumption in order to evaluate its activity as the working medium of a getter pump and a heat siphon.