Like Jekyll and Hyde – such condition applies also to some materials. An international team led by Hanus Seiner from the Institute of Thermomechanics has discovered that in some directions the alloy of nickel, manganese and gallium is able to transmit elastic waves faster than steel, but in other directions the pulses propagate more slowly than in air. To describe the alloy's behaviour, scientists had to analyse every single atom. The study was published in the prestigious Advanced Materials journal.
Take a mallet and hit the anvil with it with all your might. The mallet bounces off, the anvil remains seemingly motionless. But the force that bounces the mallet is generated inside the hard material of the anvil in response to the blow. It is made possible by elasticity, which is specific to each material. And it is elasticity that is studied by scientists from the Institute of Thermomechanics and Institute of Physics of the Czech Academy of Sciences.
"It demonstrates the forces by which the atoms in the lattice are bound to each other. You need to know this if the material is to be worked with further" explains Kristýna Repček from the Department of Ultrasonic Methods of the Institute of Thermomechanics. It sounds simple, but there is a catch. Materials can behave differently in different directions. The so-called anisotropy can be demonstrated on graphite – it is very soft in one direction, so simply brushing it against paper breaks the bonds of the carbon atoms in its lattice, leaving a trace. In the perpendicular direction, on the other hand, it is very hard due to its strong bonds. A similar property is exhibited by the nickel-manganese-gallium alloy (Ni-Mn-Ga), which has been of interest to the scientific community for over twenty years.
But only now have Czech physicists, in collaboration with Lappeenranta University of Technology in Finland, succeeded in deciphering the elasticity conditions of Ni-Mn-Ga in detail. To achieve this, they had to perfect a method of transient grating spectroscopy that allows them to record the mechanical response of crystals to laser pulses shorter than one nanosecond. There are fewer than ten laboratories in the world that can work with this technology, and only the Institute of Thermomechanics of the Czech Academy of Sciences has developed a variant with an ultratransient grid, capable of analysing many times shorter material responses. "Thanks to it, we are able to obtain the most information on material elasticity in the whole world. We can find out its elasticity without touching it. And even for layers that are only a few micrometers thin, that's really unique. I consider the development of the method itself to be an important contribution of our paper," says Kristýna Repček.
Magnet-controlled movements
The anisotropy of elasticity in Ni-Mn-Ga is so extreme because an unusual deformation mechanism takes place in the alloy at the atomic level. The same mechanism also allows the development of supermobility, i.e. high strain mobility in the crystal lattice. Thanks to this, even a very weak mechanical force or a small change in the magnetic field can cause a visible change in the shape of a Ni-Mn-Ga crystal.
Ni-Mn-Ga belongs to the family of shape memory alloys, i.e. metallic materials whose shape can be changed in a controlled manner by changing external conditions. Its "cousin", the nickel-titanium alloy or nitinol, is now routinely used in a number of industries, such as orthodontics. It can also be found in eyeglass frames, hydraulic systems in fighter jets or bra reinforcements. In the case of nitinol, however, the change in shape is achieved by changes in temperature.
Being able to achieve the same effect with Ni-Mn-Ga using magnetism is a huge advantage, as Kristýna Repček points out: "Imagine that you need the material to react very quickly and repeatedly. Ordinary nitinol would have to be heated and cooled over and over again, which takes time – energy needs to be added and removed. Whereas if you could just move a magnet over the material, you'd achieve orders of magnitude higher speeds." Other positives of Ni-Mn-Ga alloy can be seen in its stability or resistance to vibration.
Subcutaneous drug delivery
It can be used in a variety of sensors, where the material responds to a change in conditions and provides information about it, or in actuators, where a human provides the impulse and the material makes the change. A very concrete application could one day be a micropump for drug delivery directly under the skin. The chip measures blood glucose levels and, if it detects an overshoot, instructs the micropump to draw a drop of medication from a reservoir and deliver it to the site where it is needed. The development of micropumps using supramobility in Ni-Mn-Ga is being pursued at several facilities around the world. "I have seen a working prototype of such a device even as a high school project, so I believe that such an application is not unrealistic," says the scientist.
Nickel-manganese-galium, however, still has a long way to go. The measured constants will be used by other experts to create a model of the material and make suggestions on how to improve the alloy. It is already known that in its current form it does not have very suitable thermal properties for applications.
The direct observation of the mechanical instability of Ni-Mn-Ga alloy crystals is the first groundbreaking result of the half-billion-CZK OP JAK FerrMion project, and the team of scientists led by Hanuš Seiner from the Institute of Thermomechanics of the CAS will now focus on other phases of the alloy or the aforementioned thermal properties.
Magnetic shape memory
It is well known that magnetic fields can be used to move some metal objects at a distance. But can you also change their shape? Magnetic alloys with shape memory can. It looks like a magic trick: the Ni-Mn-Ga-based alloy crystal does not need to be touched; all that needs to be done is change the direction of the external magnetic field and the metal crystal deforms to the point where it is visible to the naked eye. Why is this? The crystal lattice tries to arrange itself so that its own magnetic moments lie parallel to the external field. In conventional materials, this can only be achieved by rotating the whole object. In an alloy with supermobility, however, it is easier to deform according to the magnetic moments. This phenomenon was first described at the turn of the millennium by groups from the Helsinki University of Technology in Finland and the Massachusetts Institute of Technology in the USA.