Materials Science in Space

- What is a Metal Foam ? -

A metal foam, as one might expect, is a dispersion of gas bubbles in a metallic bulk material. These extremely lightweight materials (density of around 0.25 g/cm³) have some very promising properties; including high specific stiffness, very good energy-absorption, low thermal conductivity and outstanding absorption of electromagnetic waves. Because of these interesting properties, metal foams have a number of potential applications - such as crash protection material for cars, building material for wall cladding and sound-proofing.

Metal foams can be produced by a wide variety of foaming methods. Generally speaking, all one has to do is release a gas into molten metal, ensure that the gas cannot escape, and then finally stabilise and solidify the foam by quickly cooling it at exactly the right time.

One way of achieving this is to produce a precursor material, such as aluminium mixed with a small amount of titanium hydride (0.5 wt% of TiH₂). Once heated up in the region of 660°C the aluminium melts and becomes liquid, while the TiH₂, known as the blowing agent, decomposes to release hydrogen gas. This gas thus forms the bubbles and creates a highly porous structure.

The characterisation of metal foams, in terms of morphology and properties, has received much attention in recent years. However, little research has appeared which tackles the true mechanisms of metal foam formation - that is to say, how the foam emerges from the liquid, how it changes with time and how gravity influences the mechanisms of foam formation.

A good analogy of metal foam formation is the head on top of a freshly-poured glass of beer. Pouring the beer causes the dissolved CO₂ to form gas bubbles. The beer itself forms very thin liquid films in between these gas bubbles. With time, smaller bubbles begin to coalesce (i.e. join together) to form bigger ones. Gravity also plays a role since it causes the thin liquid films to drain to the bottom of the glass. This liquid drainage explains why eventually the foam on a beer collapses and disappears !

In order to get a better understanding of the various mechanisms of foam growth, coalescence and gravity-induced liquid drainage, a number of scientists are now investigating inside metal foams using X-ray radioscopy and ultrasound techniques. These in-situ techniques allow one to see inside the material and observe the internal configuration of the metal foam in real-time - something that was previously impossible.