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Foams Group
Trinity College Dublin,
Physics Department, Trinity College, Dublin 2.
Contact: Professor Denis Weaire
Tel: +353-1-6081055
Fax: +353-1-6711759
Email: dweaire@tcd.ie
Web: www.tcd.ie/physics/foams
Foams Group, Physics Department, Trinity College Dublin
Many materials can be foamed, that is they can be brought into a mixed-phase cellular structure with solid or liquid cell walls and gaseous cells. The production of metallic foams reaches back to the 1940s, although much progress has been made since.
Our collaborator in this Prodex/Elips/ESA project Professor John Banhart (Hahn-Meitner-Institut Berlin, formerly at the Fraunhofer Laboratory in Bremen, Germany) contributed to this with the sophistication of a powder-compact foaming route and novel foam imaging techniques using X-ray spectroscopy (figure 1).
In view of the requirement for uniform properties of a metallic foam, we investigate theoretically the degree of homogeneity that can be reached from a hot liquid metal foam that is allowed to cool and solidify.
Commencing from the expanded (homogeneous) liquid foam, we model the competition between the processes of solidification and liquid drainage. Rapid solidification indicates that the foam remains uniform, while drainage will redistribute material leading to spatial inhomogeneity. This has been confirmed using numerical solutions of the relevant differential equations. Figure 2 shows the calculated final relative density of a metallic foam sample that was cooled and frozen from all sides.
These types of calculations also enabled us to derive a predictive 'homogeneity criterion' which relates the physical and material parameters in a form that shows when the solidified foam will be uniform and could be employed as a design guideline.
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