Showing papers on "Liquid metal published in 2008"

Eutectic Gallium-Indium (EGaIn) : A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature

Abstract: This paper describes the rheological behavior of the liquid metal eutectic gallium-indium (EGaIn) as it is injected into microfluidic channels to form stable microstructures of liquid metal. EGaIn is well-suited for this application because of its rheological properties at room temperature: it behaves like an elastic material until it experiences a critical surface stress, at which point it yields and flows readily. These properties allow EGaIn to fill microchannels rapidly when sufficient pressure is applied to the inlet of the channels, yet maintain structural stability within the channels once ambient pressure is restored. Experiments conducted in microfluidic channels, and in a parallel-plate rheometer, suggest that EGaIn’s behavior is dictated by the properties of its surface (predominantly gallium oxide, as determined by Auger measurements); these two experiments both yield approximately the same number for the critical surface stress required to induce EGaIn to flow (~0.5 N/m). This analysis–which shows that the pressure that must be exceeded for EGaIn to flow through a microchannel is inversely proportional to the critical (i.e., smallest) dimension of the channel–is useful to guide future fabrication of microfluidic channels to mold EGaIn into functional microstructures.

Eutectic gallium-indium (EGaIn): a moldable liquid metal for electrical characterization of self-assembled monolayers.

Abstract: Herein we describe the formation of conformal electrodes from the fluid metal eutectic, Ga–In (which we abbreviate “EGaIn” and pronounce “e-gain”; 75% Ga, 25% In by weight, m.p.= 15.5 8C), and their use in studying charge transport across self-assembled monolayers (SAMs). Although EGaIn is a liquid at room temperature, it does not spontaneously reflow into the shape with the lowest interfacial free energy as do liquids such as Hg and H2O: as a result, it can be formed into metastable, nonspherical structures (e.g., cones, and filaments with diameters 1 mm). This behavior, along with its high electrical conductivity (3.4 4 10 Scm ) and its tendency to make low contact-resistance interfaces with a variety of materials, makes EGaIn useful for forming electrodes for thin-film devices. We discuss the convenience and precision of measurements of current density (J, Acm ) versus applied voltage (V, V) through SAMs of n-alkanethiolates on template-stripped, ultraflat Ag (Ag–SCn Ag–SCnH2n+1, n= 10, 12, 14, 16) using EGaIn. An ideal electrode for physical-organic studies of SAMs would 1) make conformal, but nondamaging, physical contacts, 2) readily form small-area (micrometer diameter) contacts, to minimize the contribution of defects in the SAM to J, 3) form without specialized equipment, and 4) be nontoxic. Point 3 is particularly important: elimination of procedures such as evaporating metals or lithographic patterning would allow a wide range of laboratories—including those without access to clean rooms—to survey relationships between structure and electrical conductivity. There are currently three general techniques for forming top contacts for large-area (i.e., more than a few molecules) electrical measurements on SAMs of organic molecules: 1) The direct deposition of metals such as Au or Ti by using electron-beam or thermal evaporation ensures atomic-level contact, but results in low yields of devices owing to damage to the organic monolayer by reaction with hot metal vapors, and in the formation of metal filaments that short the junctions. 2) The installation of an electrically conducting polymer between the SAM and a metallic top contact inhibits formation of metal filaments, but the instability of SAMs of alkanethiolates to the temperatures required to anneal most electroactive polymers limits the broad application of this approach. 3) The use of Hg allows formation of conformal contacts at room temperature, but Hg is toxic, amalgamates with metals, tends to form junctions that short, is difficult to form into small contacts, and measurements with Hg must be performed under a solvent bath. EGaIn does not flow until it experiences a critical surface stress (0.5 Nm ), at which point it yields (i.e., flows). EGaIn 1) makes conformal, nondamaging contacts at room temperature, 2) can be molded into nonspherical shapes with micrometer-scale (or larger) dimensions, 3) is commercially available, 4) can be deposited with a pipette or syringe without high temperatures or vacuum, 5) has a low vapor pressure, and 6) is nontoxic. The work function of EGaIn (4.1–4.2 eV) is close to that of Hg (4.5 eV), but EGaIn does not alloy with many metals. It is therefore an ideal replacement for Hg, especially in devices that incorporate SAMs (which are generally formed on Au or Ag). Auger spectroscopy on samples of EGaIn in air show that its surface is principally composed of oxides of Ga (see the Supporting Information); gallium oxide is an n-type semiconductor. There is undoubtedly an adsorbed film of water on this surface, as EGaIn has a high surface free energy (ca. 630 dynescm ), as do oxides formed from similar metals. During our measurements, there were no observable changes in the average magnitude or range of J when EGaIn was allowed to sit in air for extended periods before we deposited it on the SAM, or when we performed the measurements using the same drop of EGaIn to form between three and five junctions, or while we flowed dry N2 over the sample: therefore the contribution of the surface oxide to J was probably constant for the duration of the experiments. We formed EGaIn electrodes by suspending a drop of EGaIn from a metal 26s-gauge needle affixed to a 10-mL syringe, bringing the drop into contact with the bare surface of a sacrificial film of Ag using a micromanipulator, and retracting the needle slowly (ca. 50 mms ); the EGaIn adhered to both the needle and the Ag (Figure 1). The drop of EGaIn pinched into to an hour-glass shape until it bifurcated into two structures, one attached to the syringe (a cone approximately 0.05 mL in volume) and one (which was discarded) attached to the Ag. We produced conical tips of EGaIn with diameters ranging from less than 1 mm to 100 mm; the larger the bore of the needle, and the more rapidly we [*] Dr. R. C. Chiechi, Dr. E. A. Weiss, Dr. M. D. Dickey, Prof. G. M. Whitesides Department of Chemistry and Chemical Biology Harvard University 12 Oxford St., Cambridge, MA 02138 (USA) Fax: (+1)617-495-9857 E-mail: gwhitesides@gmwgroup.harvard.edu

A unified model for the cohesive enthalpy, critical temperature, surface tension and volume thermal expansion coefficient of liquid metals of bcc, fcc and hcp crystals

Abstract: First the cohesive enthalpy of pure liquid metals is modeled, based on experimental critical temperatures of alkali metals. The cohesive enthalpies are scaled to the melting points of pure metals. The temperature coefficient of cohesive enthalpy is the heat capacity of the liquid metal. The surface tension and its temperature coefficient for pure liquid metals are modeled through the excess surface enthalpy, excess surface entropy and molar surface area supposing that the outer two surface layers of liquid metals are similar to the {1 1 1} plane of fcc crystals. The volumetric thermal expansion coefficient of liquid metals is scaled to the ratio of the heat capacity and cohesion enthalpy. From known values of melting point, heat capacity and molar volume the following calculated properties of liquid metals are tabulated: (i) cohesive enthalpy at melting point, (ii) cohesive energy of the solid metal at 0 K, (iii) critical temperature, (iv) surface tension at melting point, (v) volume thermal expansion coefficient, and (vi) temperature coefficient of surface tension. The present models are valid only for liquid metals of bcc, fcc or hcp crystals as only their structure and nature of bonding are similar enough to be treated together.

Diamond-Based Metal Matrix Composites for Thermal Management Made by Liquid Metal Infiltration — Potential and Limits

Abstract: Diamond-based metal matrix composites have been made based on pure Al and eutectic Ag-3Si alloy by gas pressure infiltration into diamond powder beds with the aim to maximize thermal conductivity and to explore the range of coefficient of thermal expansion (CTE) that can be covered. The resulting composites covered roughly the range between 60 and 75 vol-% of diamond content. For the Al-based composites a maximum thermal conductivity at room temperature of 7.6 W/cmK is found while for the Ag-3Si based composites an unprecedented value of 9.7 W/cmK was achieved. The CTE at room temperature varied as a function of the diamond volume fraction between 3.3 and 7.0 ppm/K and 3.1 and 5.7 ppm/K for the Al-based and the Ag-3Si-based composites, respectively. The CTE was further found to vary quite significantly with temperature for the Al-based composites while the variation with temperature was less pronounced for the Ag-3Si-based composites. The results are compared with prediction by analytical modeling using the differential effective medium scheme for thermal conductivity and the Schapery bounds for the CTE. For the thermal conductivity good agreement is found while for the CTE a transition of the experimental data from Schapery’s upper to Schapery’s lower bound is observed as volume fraction increases. While the thermophysical properties are quite satisfactory, there is a trade-off to be made in these materials between high thermal conductivity and low CTE on the one side and surface quality and machinability on the other.

Liquid metal thermal interface material system

Abstract: A metal thermal interface structure for dissipating heat from electronic components comprised a heat spreader lid, metal alloy that is liquid over the operating temperature range of the electronic component, and design features to promote long-term reliability and high thermal performance.

CFD Modelling of Liquid Metal Flow and Heat Transfer in Blast Furnace Hearth

Abstract: An in-depth understanding of the liquid metal flow and heat transfer is essential in order to identify the key mechanisms for the hearth erosion of a blast furnace. In this study, a comprehensive computational fluid dynamics model is described which predicts the flow and temperature distributions of liquid iron in blast furnace hearth, and the temperature distribution in the refractories. The new model addresses conjugate heat transfer, natural convection and turbulent flow through porous media, with its main features including improved transport equations (a modified k–e turbulence model and thermal dispersion term) and a three-dimensional, high-resolution grid. The new turbulence model and terms take account of the effect of microscopic flows around coke particles and allow unified treatment of coke bed and coke-free layer. The predicted results show a well-organized flow pattern: two large-scale recirculation zones are separated vertically at the taphole level. This flow pattern controls the temperature distribution in the liquid phase, so that the temperature remains nearly uniform in the upper zone, but changes mainly across the lower zone. The effects of several factors were examined, such as cases comparing fluid buoyancy with constant fluid density as well as the shape and position of the coke free zone (i.e. based on reported dissection studies). Natural convection is found to be most important for the liquid metal flow patterns observed. Comparison with the plant data shows that the refractory pad temperature is under-predicted when assuming intact hearth lining. The pad temperature is very sensitive to the erosion of protection layer in the hearth lining.

Fast processes in liquid metal foams investigated by high-speed synchrotron x-ray microradioscopy

Abstract: Rupture of an individual film in an evolving liquid metal foam is investigated by means of high-speed x-ray radioscopy using white synchrotron radiation. At a frame rate of 5000frames∕s, the rupture event is spread over three to four images. The images show that the remnants of the rupturing film are pulled into the surrounding plateau borders in 600±100μs which conforms well with a liquid movement governed by inertia and not by viscosity. Within one order of magnitude, the viscosity of the liquid involved must be similar to the viscosity of pure liquid aluminium.

Dissolutive wetting of Si by molten Cu

Abstract: Dissolutive wetting occurs when a liquid spreads over a solid surface with simultaneous dissolution of the solid into the liquid. This process is of great interest for both fundamental research and several industrial processes, an important example being soldering in microelectronics fabrication processes [1]. Several studies, performed for various liquid metal/solid metal systems, have shown that for millimetresized droplets the spreading time in dissolutive wetting ranges from a few to several hundred seconds [2–6]. This time is orders of magnitude higher than the spreading time found in liquid metal/solid metal systems with negligible miscibility, which is typically around 10 ms [7–11]. Despite the progress made over the last 10 years in the understanding of dissolutive wetting, several points remain obscure concerning both the driving force and kinetics of this type of wetting. The aim of the work reported in this paper is to contribute to this subject by performing a comparative study of spreading on the same substrate (monocrystalline Si) at 1100 C with pure copper and Si-presaturated copper. In the past, a similar attempt to study spreading in the same system in equilibrium and non-equilibrium conditions was made using Ag/Cu couple [12]. However, with the sessile drop technique used in these experiments, the initial stages of spreading were obscured by metal melting. In the present study this difficulty is overcome by using the dispensed drop technique, which enables the processes of melting and spreading to be separated (see for instance [13]) According to the Cu–Si phase diagram, at 1100 C a liquid CuSi alloy containing 52 at %Si is in equilibrium with solid Si (the solubility of Cu into solid Si is negligible) [14]. As for the surface tension of pure Cu at 1100 C, 1280 mN/m [15], it is much higher than that of molten Si, which, extrapolated to 1100 C from the melting point of Si, is close to 800 mN/m [16, 17]. As a consequence, dissolution of Si into Cu is expected to decrease the surface tension of the liquid. Wetting was studied in a metal furnace under a vacuum of (1–5) 9 10 Pa. The experiment involved heating pure Cu or the CuSi alloy (purity higher than 99.999%) in an alumina crucible placed above the Si substrate. At the experimental temperature, the liquid was extruded from the crucible through a capillary, forming droplets with a diameter ddr lying between 1.3 and 2 mm. In view of the high sensitivity of Si to oxidation and to improve surface cleaning, a prior heat treatment of the substrates was performed at 1250 C before depositing the drop at 1100 C. The wetting process was filmed by a camera (500 frames per second) connected to a computer, enabling automatic image analysis. The characteristic dimensions of the drop (drop base diameter d and visible contact angle h) were extracted with an accuracy of ±2 for h and ±2% for d. The (111) surfaces of electronic purity Si have an average roughness of 1–4 nm after polishing with diamond paste up to 0.1 lm. Figure 1 gives the temporal variation in contact angle h and the normalized drop base diameter d/ddr for the wetting of Si by a Cu droplet. Due to the resolution of 2 ms, most of the non-reactive spreading, which occurs at t \ 2 ms, is missing. However, on the drop base diameter curve, it can be clearly seen that the triple line velocity vanishes at point A, corresponding to time t & 4 ms. This is just a tendency because, after slightly receding to point B, d starts to P. Protsenko Department of Colloid Chemistry, MSU, Moscow, Russia

Multiphysics modelling of the spray forming process

Abstract: An integrated, multiphysics numerical model has been developed through the joint efforts of the University of Oxford (UK), University of Bremen (Germany) and Inasmet (Spain) to simulate the spray forming process. The integrated model consisted of four sub-models: (1) an atomization model simulating the fragmentation of a continuous liquid metal stream into droplet spray during gas atomization; (2) a droplet spray model simulating the droplet spray mass and enthalpy evolution in the gas flow field prior to deposition; (3) a droplet deposition model simulating droplet deposition, splashing and re-deposition behavior and the resulting preform shape and heat flow; and (4) a porosity model simulating the porosity distribution inside a spray formed ring preform. The model has been validated against experiments of the spray forming of large diameter IN718 Ni superalloy rings. The modelled preform shape, surface temperature and final porosity distribution showed good agreement with experimental measurements.

The use of ionic liquid ion sources in focused ion beam applications

Abstract: A new monoenergetic, high-brightness ion source can be constructed using an arrangement similar to liquid metal ion sources by substituting the liquid metal with an ionic liquid or room-temperature molten salt. Ion beams produced by these ionic liquid ion sources (ILISs) have energy deficits and distributions that closely resemble their metallic counterparts, with the exception that they can be stably operated at current levels as low as a few nanoamperes if needed. ILISs are here presented as having two further key advantages: (1) the ability to obtain both positive and negative ion beams and (2) the ability to produce very diverse molecular ions in terms of their masses, compositions, and properties due to the fact that the number of available ionic liquids is large. In this article an overview of ILISs is presented, as well as preliminary results of their performance in a focused ion beam column.

Particulate Wetting and Metal: Ceramic Interface Phenomena

Abstract: An experimental technique for evaluation of wettability of ceramic particulate with liquid metal was developed. Wettability tests were conducted by pressure infiltration through uniformly packed powder specimens. The threshold pressures for infiltration can either be used as quantitative measure of wettablity or be converted to wetting angles by using the capillary force equation. With this technique, wettability was measured for SiC and B4C particulates with liquid aluminum alloys. The effects of temperature, time, and alloying element on wettabilty were investigated. Theoretical calculations of solid-liquid interfacial energy were also reviewed briefly. 61 references.

System and method for a substrate with internal pumped liquid metal for thermal spreading and cooling

Abstract: A circuit board includes a pump and a channel. The channel includes a liquid metal and a coating. The liquid metal is pumped through the channel by the pump and the coating reduces diffusion and chemical reaction between the liquid metal and at least portions of the channel. The liquid metal can carry thermal energy to act as a heat transfer mechanism between two or more locations on the substrate. The substrate may include electrical interconnects to allow electrical components to be populated onto the substrate to form an electronics assembly.

Interfacial phase transitions in conducting fluids.

Abstract: We present a review, largely based on recent experimental work of our group, on phase transitions at interfaces of fluid metals, alloys and ionic liquids. After a brief analysis of possible experimental errors and limitations of surface sensitive methods, we first deal with first-order wetting transitions at the liquid/vapour and liquid/wall interface in systems such as Ga-based alloys, K–KCl melts, and fluid Hg. The following chapter refers to surface freezing or surface induced crystallization in different metal alloys. The respective surface phase diagrams are discussed in comparison with their bulk counterpart. In the last part we present very recent investigations of ionic liquid interfaces, including order–disorder transitions at the liquid/vapour interface and examples of two-dimensional phase transitions at the electrified ionic liquid/metal interface. Finally, a simple electrowetting experiment with an ionic liquid droplet under vacuum is described which gives new insight into the contact angle saturation problem. The article ends up with a few perspective remarks on open problems and potential impact of interfacial phenomena on applied research.

Self-organization in a chromium thin film under laser irradiation

Abstract: Self-organization of chromium on glass was observed during laser ablation of the metal film with partially overlapping laser pulses. The beam of a nanosecond pulse laser tightly focused to a line was applied to the back-side ablation of the chromium thin film on a glass substrate. While the line ablated with a single laser pulse had sharp edges on both sides with ridges of the melted metal, the use of partially overlapping pulses formed a complicated structure made of the metal remaining from the ridges. Regular structures of ripples were developed in a certain range of laser fluence and pulse overlap. The ripple period could be controlled from 2.5 to 4 μm by variation of the processing parameters. Various experimental techniques were applied to test the structures, and different models of the ripple formation in the thin metal film were considered. The initial quasi-periodical formation started because of dewetting of thin liquid metal films on the glass substrate after its melting. Similar to the evaporation of liquid films, the small perturbation in the ridge thickness was able to induce instability in evaporation of the thin melted metal film. Freezing of the nonequilibrium state between laser pulses was one of the stabilizing factors in self-organization of the metal.