A New Class of Engineering Materials: Particle-Stabilized Metallic Emulsions and Monotectic Alloys
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tectic alloys are produced by solidifying liquid metallic emulsions, i.e., small droplets of one liquid metal dispersed in the matrix of another immiscible liquid metal.[1] However, once the droplets are nucleated, they immediately approach each other and coalesce under the influence of gravity or the interfacial gradient force, making the production of monotectic alloys with homogeneously distributed second phase almost impossible, especially for thick castings with considerable cooling times.[2–6] Here, we show for the first time that solid particles with appropriate wettability can be used to stabilize liquid metallic emulsions, opening a principally new route to produce thick wall monotectic alloys with homogeneously distributed second-phase droplets (particles). There has been common knowledge in colloid chemistry for more than a century[7] regarding how to stabilize water- or oil-based liquid foams[8–17] and emulsions[13,18–22] by solid particles. On the other hand, solid particle-stabilized metallic foams[23–28] were originally discovered quite empirically by metallurgists and independently from the aforedescribed common knowledge of colloid chemists. However, it has become clear that the law of stabilization of water foams and metallic foams by particles is a common law.[10,20]
ISTVA´N BUDAI is Researcher, Department of Nano-composites, BAY-NANO Research Institute on Nanotechnology, Miskolc, Hungary-3515, and Assistant Professor, Faculty of Engineering, University of Debrecen, Otemeto utca 2-4, Debrecen, Hungary-4028. GEORGE KAPTAY, Head, Department of Nano-composites, and Vice-Director, Scientific Affairs, BAY-NANO Research Institute on Nanotechnology, is Head, Department of Nanotechnology, University of Miskolc, Hungary-3515. Contact e-mail: [email protected] Manuscript submitted March 9, 2009. Article published online May 14, 2009 1524—VOLUME 40A, JULY 2009
Due to this gap in ‘‘common knowledge’’ of the colloid chemists’ and metallurgists’ communities, it is not surprising that the metallic analogue of particlestabilized oil/water emulsions does not exist today (Figure 1). The goal of this article is to show that particle-stabilized liquid metallic emulsions (PSLMEs) can be produced, at least in a laboratory scale. In addition to the scientific curiosity, this might have a serious impact on further materials development. Solidifying the PSLMEs, a new route opens to produce monotectic alloys with homogeneously distributed second-phase particles in virtually any size, which is impossible to do when the monotectic alloys are not stabilized by particles. Let us summarize in Figure 2 the experience gathered by colloid chemists for the last century.[7–22] Figure 2(c) is a theoretical emulsion stability diagram[20] for particle-stabilized emulsions. One can see that the contact angle (H) and the liquid-phase ratio (/B) are the two independent parameters that determine whether, in the given three-phase (liquid A + liquid B + solid C) combination of materials, any emulsion can be stabilized, and, if yes, the stable e
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