Why are intelligent materials important

Smart materials

“Invar”, the invariable, unchangeable, an iron alloy with 36 percent nickel, does not change its length or at least hardly changes when the temperature fluctuates. Other alloys have a "memory", some lose their electrical resistance at low temperatures ...

Eiffel Tower in Paris

For Charles Edouard Guillaume, the inventor, it must have been a moment of triumph: over a hundred meters of his alloy, dubbed “Invar”, hung like a wire from the second platform of the Eiffel Tower. The lower end of the wire was fixed to the ground, the other end on the platform connected to a movable lever, which in turn moved a pen that pressed on a recording drum. “Invar”, the invariable, unchangeable, an iron alloy with 36 percent nickel, should show its specialty: it does not change its length or at least hardly changes when the temperature fluctuates. But the Eiffel Tower itself stretched on the morning of June 8, 1912 in the warmer air, millimeter by millimeter the second platform pushed upwards. With the invar wire as an unchangeable reference, the whole thing could be precisely registered. In the late afternoon, the entire tower, cooled by a rain shower, collapsed by almost four centimeters.

Invar made a career as a replacement for the expensive platinum iridium in "original meters", as a material for balance springs in chronometers - wherever low thermal expansion was required. Invar is widespread in today's technology, for example also in the "shadow masks" of TV picture tubes. Guillaume received the Nobel Prize in 1920 for his discovery.

Spirited nickel-titanium

Twenty years later, the Swede Arne Olander made a metallurgically equally important discovery: an alloy of gold and cadmium showed something like a memory. If you deformed it and then heated it, it would revert to its old shape. Cadmium is poisonous, gold is expensive - the alloy had no future.

But the effect is. In 1962, when the US Naval Ordnance Laboratory was looking for an alloy that could withstand corrosive seawater and was difficult to locate magnetically, they found a mixture of nickel and titanium and named it nitinol. The "-nol" stood for the Naval Ordnance Laboratory, the "Niti-" for nickel and titanium. When plates of this material were riveted to the bow of a submarine, it is said to have happened: a worker heated a plate, as is usual with steel, to make it more malleable - the part cracked into a different shape, damaging the rivets, the shape of his birth: Nitinol was discovered as a metal with memory.

Many other metal combinations later became known to have the same effect; Nickel-titanium, however, is still the most important alloy alongside copper-zinc-aluminum. NASA developed a wire mesh from such a “memory metal” that unfolded into a satellite dish in space; At many universities and companies, small heat engines have been developed in which Nitinol rhythmically stretches and compresses. Today, Nitinol has hit the ground, for example in the form of sprinkler valves that let water out in the event of a fire. Memory metals may also take off in a really futuristic way, as metallic muscles for robots, for example.

Matter with two faces

The wonderful thing about the nickel-titanium alloys has not yet been exhausted, the right mixture is also fantastically elastic. A stick as thick as a pencil can be bent like hard rubber and then returns to its old shape. The effect is used, among other things, for heavy-duty spectacle frames, but also for tooth brackets and highly elastic tubes for widening narrowed vessels. The reason for the wonderful variability of such alloys: Their atomic lattices can take two different forms. At low temperatures a so-called martensitic structure, which is characterized by a zigzag pattern. When heated, this grid changes into a different, “austenitic” shape. Depending on the temperature, the metal can jump back and forth between the two states - kink, creak, similar to a pressed-in shoe polish lid.

resistance is futile

As impressive as the properties of purely metallic compounds are, the upper floor of the material universe is only opened up together with non-metals. When ordinary aluminum combines with oxygen, the hardest known substance after diamond is formed, aluminum oxide, which - colored by traces of other metals - is also valued as ruby ​​or sapphire.

High temperature superconductors

A complex compound of the metals yttrium, barium and copper with oxygen, YBa2Cu3O7, was even able to acquire fame as the first “high-temperature superconductor”. This material already loses its electrical resistance at the temperature of liquid nitrogen - at minus 196 degrees Celsius. For us, this is a rather frosty value, but almost tropical compared to the temperatures below which normal superconductors conduct electricity without loss: a few degrees above absolute zero temperature of minus 273 degrees Celsius.

What is different with the high-temperature superconductors? The rules of quantum mechanics only allow electrons to move undisturbed in a superconductor if they occur in pairs, in so-called Cooper pairs. Since the electrons charged in the same direction repel each other, a force must come to the rescue to hold the pair together. In conventional superconductors, this force is provided by common oscillations of the atomic nuclei.

Superconductor in a locomotive hovers over magnetic rails

This is why the whole thing only works at extremely low temperatures - when the temperature-related tremors of the atomic lattice no longer disturb this mechanism. However, high-temperature superconductors apparently work according to a completely different mechanism. Recent experiments suggest that the Cooper pairs are supported here by a moving pattern of small magnetic fields that arise inside the material. If so, the underlying theory should lead to materials that become superconducting at even higher temperatures. Another effect of high-temperature superconductors: when they are superconducting, they tend to allow a magnetic field to penetrate into them. The magnetic field is tied into small bundles, so-called flux tubes, which penetrate the material. Since the superconductor can “cling to” these tubes, friction-free magnetic bearings can be produced in this way. The high-temperature superconductor yttrium-barium-copper oxide (YBa2Cu3O7) favored.

The ice-cold goalkeepers

Finally, with the new superconductors, highly sensitive "SQUIDs" have become possible, with which one can still measure magnetic fields that are only one billionth of the strength of the earth's magnetic field. The abbreviation stands for "Superconducting Quantum Interference Device". The devices contain a superconducting ring with a thin point, a "gate", through which the external magnetic field can be let in in small portions ("flux quanta"). At the beginning of the measurement, many quanta crowd in front of the gate. If the "pressure" is great enough, the gate opens - but only to let exactly one quantum in. The supercurrent in the ring collapses for a short time. This can be determined by electronic circuits that are coupled to the ring. Then the effect repeats itself: gate open - gate closed, until the number of quanta outside and inside is the same. The strength of the magnetic field can be determined by counting the “door openings”. SQUIDs are used in geophysics, materials testing and medicine, among other things. There the doctors examine the magnetic fields that arise in the patient's brain and heart.