杏吧原创

Winnipeg, Canada

Less is most definitely more. The vital ingredient responsible for endowing
mundane metals with a range of valuable new properties is not a new compound or
a new material鈥攊t is the holes. 杏吧原创s are taking ordinary metals like
aluminium and frothing them up to create amazing metal foams.

Lightweight, fireproof, sound and heat absorbent, foamy metals could soon be
cropping up in cars, ships and aeroplanes. They are already damping down noise
in road tunnels, and even orbiting the planet aboard the space shuttle. The
foams use less material to fill the same space, saving metal as well as weight.
And yet they retain a lot of the strength of the original materials. Nature
realised the benefits of such a design a long time ago. Many natural materials,
such as bone, have a foamy kind of structure, making them light but tough.

The idea of foamy metals has been around since the 1950s, almost as long as
foamy plastics, such as the polyurethane foam found in furniture and car
upholstery, and the blown polystyrene of Styrofoam cups and packaging chips. But
while filling plastics with a mass of bubbles is straightforward, it鈥檚 not so
easy when it comes to doing the same to, say, a lump of aluminium. Only in the
past ten years have researchers managed to master the trick. Now they are eager
to exploit the potential of metal foams. 鈥淲e want to take these materials
developed during the Cold War and bring them out into the light of day,鈥� says
Brian Leyda, manager of engineering at ERG Materials and Aerospace Corporation
in Oakland, California, a leading commercial supplier of metal foams.

One way to form these foams was developed in Germany in the early 1990s by
Joachim Baumeister and his team at the Fraunhofer Institute for Applied
Materials Research in Bremen. They start not with solid metal but with a powder.
The researchers first mix powdered metal with a foaming agent鈥攁 compound
that decomposes to give off gas when it is heated. After squashing them together
by rolling and extruding the mixture, they then heat it. This softens the metal
and simultaneously makes the foaming agent decompose and give off gas, which
forms bubbles in the semimolten metal to create a foam-like structure. The
technique is used mainly to produce aluminium foam, but foams of other
metals鈥攊ncluding tin and zinc and alloys such as bronze鈥攃an also be
made. As a foaming agent, the researchers use a metal hydride, which produces
hydrogen gas when heated.

Meanwhile, at the Alcan Corporation in Ontario, Canada, Il Joon Yin has come
up with a quite different way to make foamy aluminium. Around ten years ago,
when Yin was carrying out research with baths of molten aluminium, he often
encountered problems with unwanted foaming. 鈥淚 thought, why not use this
negative aspect as a positive one,鈥� he says. Yin鈥檚 brainchild is a stirred bath
of molten aluminium with air jets bubbling up through the metal. A related
technique uses an impeller to create a vortex that draws bubbles down into the
bath.

The air bubbles create a 鈥渉ead鈥� on the vat of molten metal, and this foamy
top layer is continuously drawn off onto a conveyer belt and cooled into sheets.
Molten aluminium is normally too runny to stay frothy: the bubbles collapse and
the metal goes flat. Yin鈥檚 trick is to add alumina or silicon carbide fibres,
which keep the molten metal thick enough to preserve the bubbly structure as it
cools.

Both methods have their place, Baumeister believes. 鈥淚n the Alcan process it
is difficult to obtain three-dimensional, shaped parts, which is easily done in
our process by foaming our material inside a mould,鈥� he says. 鈥淥n the other
hand, production of large flat panels seems to be easier using the Alcan
辫谤辞肠别蝉蝉.鈥�

A third foaming method is being investigated by the American aerospace
company McDonnell Douglas. In this technique, a container is packed with
powdered metal and pressurised with an inert gas such as argon, so that the gas
fills all the tiny gaps between the particles. The container is then heated to
fuse the metal particles to each other, trapping the gas in the spaces between
them. The resulting chunk of metal is then rolled out and heated in a furnace,
where the trapped gas expands to produce a foam.

Now that foamy metals can be made in large quantities, it is time to put them
to use. At Harvard University, materials scientist Tony Evans is already a year
into a three-year project to study foamed metals funded by a number of American
defence agencies. Collaborators include researchers at MIT, the University of
Virginia and Cambridge University in England. 鈥淥ur main thrust is to investigate
mechanical properties and to try to understand applications,鈥� says John
Hutchinson, a Harvard engineer involved with the project. 鈥淭hey have kind of
remarkable properties.鈥�

Pick up a chunk of metal foam and one of the most important properties is
obvious. Because the materials are mostly hollow cavities, they can be
incredibly light compared to a lump of metal of the same size. Metal foams can
be anything from one half to one fiftieth the density of the bulk metal,
depending on the foaming method. However, the material is still strong enough to
be used in body parts for aeroplanes. 鈥淲e鈥檙e looking into using low-density foam
metals as cores in sandwich panels for aircraft,鈥� says Hutchinson.

Fused sandwich

Sandwich panels are composed of two metal skins enclosing a lightweight
material such as a polymer foam. These three components are glued together.
鈥淏onding at the interfaces is the weak link,鈥� says Hutchinson. But a sandwich of
aluminium foam between two skins of solid aluminium could be joined together by
heating鈥攖he layers would simply fuse together, forming a very secure bond.
Using these panels, manufacturers could make lighter, stronger planes and
rockets鈥攚ith obvious advantages for fuel consumption, carrying capacity
and safety.

Shipbuilders are also interested in metal foams, especially for the
above-decks structures on high-performance naval vessels. Saving weight helps to
increase a ship鈥檚 speed, and a lighter above-decks structure makes it more
stable, too鈥攚ith less weight above the water-line, the ship is less
inclined to capsize. But while aircraft manufacturers have been using aluminium
alloys for many years, shipbuilders are still cautious. 鈥淭he Navy takes a lot of
persuading to use aluminium because it catches fire readily,鈥� Evans says. That鈥檚
the last thing you want if your ship is hit by a missile.

But here aluminium foam has sprung a welcome surprise. Unlike the bulk metal,
it is essentially fireproof. Aluminium foam stands up to a blowtorch that would
turn normal aluminium into a puddle and set it alight. No one has studied this
phenomenon in depth yet, but one theory is that the 鈥渃ells鈥� in the foam prevent
heat from being conducted into the metal. Heating also coats the surface of the
foam with a naturally protective layer of aluminium oxides, leaving very little
unprotected aluminium that could catch fire.

Such a lightweight, fireproof material could also have a number of roles in
cars. European motor manufacturers are planning to use aluminium foam as part of
the 鈥渇ire wall鈥� that separates the engine from the passenger compartment. As a
bonus, it could also cut down engine noise. By controlling the shape and size of
the bubbles, aluminium foam can be made an extremely good sound dampener.
鈥淪heets of aluminium foam are already used in Japan to line the insides of
traffic tunnels,鈥� says Hutchinson. These muffle noise inside the tunnels, while
at the same time being very durable linings鈥攁luminium should be resistance
to corrosion.

Not only could foams make driving quieter and cars safer, but they could make
having an accident safer too. Just like a foam-rubber cushion, a piece of metal
foam squashes easily, soaking up the energy of an impact. 鈥淓nergy absorption is
an obvious application,鈥� says Evans. And there are few occasions when absorbing
energy is more important than in a car accident. Baumeister sees this leading to
a major use for metal foams. 鈥淭he most promising applications include energy
absorbers for frontal and side impact in the transport industry,鈥� he says.

In a crash, the myriad cells in the foam would collapse in on themselves and
gradually soak up the force of impact. Several car companies are interested in
making side impact beams and knee shields out of aluminium foam. By tailoring
the density of the foam, designers will be able to fine-tune mechanical
properties such as rigidity to suit each part.

So will the next new model you see in car showrooms be built with metal-foam
components? 鈥淐ost comes into this, of course,鈥� says Evans. The high-tech
applications will have to wait until metal foams with specific tailored
properties can be churned out cheaply and easily. 鈥淲e have to determine which
manufacturing methods are affordable,鈥� he says.

Bill Clyne at the University of Cambridge agrees: 鈥淥ne can dream up lots of
applications,鈥� he says. 鈥淏ut manufacturing techniques have not matured enough.鈥�
Clyne鈥檚 team is looking at ways of improving and controlling the processes for
producing foams, with an eye to large-scale manufacture. For example, they want
to be able to fine-tune pore size in the foam, which affects the stiffness of
the material and how it absorbs impacts. Haydn Wadley at the University of
Virginia in Charlottesville, Virginia, is tackling similar problems. 鈥淲e want to
predict the outcomes of various manufacturing processes,鈥� Wadley says, to see
which method would be best suited to making foams for different applications.
His group is also developing sensors that will allow manufacturers to monitor
the foaming process.

While others strive to perfect their manufacturing processes, ERG in
California is already producing a specialised metal foam that has many of the
properties of standard foams but has some unique applications. Using a secret
process, ERG concocts a foam and then bursts the walls between adjacent bubbles.
This creates a network of interconnected chambers in which liquids and gases can
flow or be stored. Amazingly, the material keeps about 80 per cent of its
original strength. 鈥淲e are just duplicating what Mother Nature has developed
over millions of years,鈥� says Leyda. Some parts of our skeleton, such as the
upper femur, are made of a thin layer of bone covering an open-cell foam
structure. This gives us bones that are lightweight, tough and effectively
hollow, leaving space for the marrow.

ERG鈥檚 open-cell metal foam is used in what are known as wet-wing structures
for unpiloted aircraft. The bubbles fill with fuel, allowing wings to double as
fuel tanks. And anyone standing on Broadway in New York City can see a chunk of
ERG鈥檚 foam on prominent display in the shape of the MTV logo on top of the
Viacom corporation鈥檚 headquarters.

The foams also have more mundane applications. The interconnected bubbles
form a convoluted path for any gases or liquids flowing through them, and the
twisting and tumbling ensures good mixing鈥攑erfect for delivering a
thorough mixture of fuel and air to an acetylene torch, for example, or for
blending pigments into plastics. Such a complex path also continually brings the
gases or liquids into contact with the metal walls鈥攎aking the foam an
ideal substrate for a catalyst.

Open-cell metal foams have also been used to build the plasma-flow
controllers needed to ensure good mixing and smooth delivery of the gases that
etch semiconductors. Precision lightweight robotic arms are made using foams,
allowing cooling air to be pumped down the arm to prevent the metal changing
temperature and distorting. And in a prototype hydrogen-fuelled bus built by a
consortium of Georgia Tech, local government and industry, ERG鈥檚 foamed
aluminium is used as a kind of fuel tank. It supports the lithium hydride powder
that generates the hydrogen, and the release of the gas is controlled by
adjusting the temperature of the foam.

ERG鈥檚 foams have found their way into space, too. 鈥淥ur materials are being
used on the space shuttle,鈥� says Leyda, 鈥渁s the mechanical support structure and
heat exchanger for the device that selectively removes contaminants from the
cabin atmosphere.鈥�

Staying cool

The cryogenic heat exchangers that are needed to remove the heat that would
otherwise swamp the signals picked up by infrared-sensing satellites also depend
on the foam. The wide-angle infrared explorer currently being built by the
Lockheed Corporation includes aluminium foam for this purpose.

Leyda predicts that foamy materials developed for military projects could
soon start appearing in everyday devices, such as desktop computers. 鈥淚 see
cryogenic heat exchanger foam from military spy satellites being adapted to cool
advanced semiconductor electronic devices,鈥� he explains.

Baumeister and his colleagues at the Fraunhofer Institute foresee even more
down-to-earth applications for metal foams, particularly in the construction
industry. The combination of light weight and resistance to fire could make
aluminium foam ideal for elevator cars, for example. And foams of titanium or
other biocompatible metals could be used to make prosthetic devices, such as
artificial joints and bones.

Once production starts on a grand scale, it looks as though metal foams will
become as popular as their plastic counterparts. In the future, the world will
be a much bubblier place.