American researchers have recently put their finger on a fascinating phenomenon which seems to allow materials to challenge the laws of physics. These extremely counterprint behaviors could lead to a small revolution in material sciences, with very interesting concrete applications.
The vast majority of known materials follow a certain number of almost irremovable rules, anchored in the laws of thermodynamics. The pressure, for example, forces the constituents of matter to get closer to each other, in a certain limit; The more it increases, the more the volume is supposed to decrease. The temperature, for its part, has the opposite effect. When it increases, the same goes for the internal energy of atoms, which start to vibrate with an increasingly important amplitude: the volume of the material therefore tends to increase when heated.
But in a study published in the prestigious newspaper Natureresearchers from the universities of Chicago and San Diego have shown that these two rules were not ultimately engraved in the rock. In fact, they can even reverse when the materials are in a so -called “metastable” state.
A question of energy
Over time, all physical systems tend to evolve towards a state of equilibrium where their energy level will be minimal (see the concept of entropy for more details): we speak of a stable state. If we represent the energy of the system on a curve, this stable state is represented by the lowest point – The overall minimum.
To visualize this concept, you can imagine a bullet that would spontaneously roll towards the bottom of a valley over time: it stops moving when it stabilizes at the bottom of the slope, and therefore arrives in its stable state.
But the situation can also be more complex. Imagine for example that our ball is placed in a hollow located at the top of a peak. Under these conditions, it cannot reach the lowest point, the overall minimum which corresponds to the stable state. Instead, she finds herself Stuck in what is called a local minimum from which she can only come out with the intervention of an external force, like a person who would push her outside the hollow where she is housed.
In thermodynamics, this is called a Metastable balance. It is for example because of this phenomenon that water in surfusion (less zero degrees, but always liquid) does not freeze spontaneously if it is left quiet. On the other hand, at the slightest disturbance, the entire volume quickly turns into ice.
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Extremely counter-intuitive effects
To return to this study, the authors have shown that these metastable states can considerably change the behavior of materials. In their stable state, they perfectly follow the conventional rules of thermodynamics. But under these particular conditions, the situation changes radically, and particularly counter-intuitive phenomena can emerge.
Certain materials in metastable states can for example contract when heated, and conversely, expand under the effect of pressure. The authors speak of “negative compressibility”, And this is something that had never been observed before.
Immense concrete potential
The most interesting thing is that these works are not only promising in basic research. According to the authors, it may be possible toAdjust these metastable statesfor example thanks to oxydo-reduction reactions, to modify the way in which materials react to heat and other forms of energy. In theory, this could allow Design materials with extremely useful properties.
The team quotes a particularly relevant example in the field of construction. To build a structure, the effects of constituent thermal expansion must be rigorously checked; If the structure of a building swells or contracts beyond a critical threshold during a wave of heat or cold, it could collapse with catastrophic consequences. If it were possible to design materials with the null thermal expansion coefficient, this would immediately skip a major constraint from structural engineering.
Another potential scope hides on the side of electric vehicles. Over the charge and discharge cycles, their batteries tend to lose in capacity because lithium ions, used to store energy, are gradually trapped at the anode. This chemical alteration is today irreversible – but with a perfectly calibrated metastable material, it would be theoretically possible to reverse the reaction to restore the battery in its original state.
And it is only the emerged part of a huge iceberg of possibilities. This concept could open the way to lots of materials more revolutionary than each other … on paper, at least. It should be noted that this study remains very exploratory. Currently, there is no guarantee that researchers will one day be able to tame these metastable states to bring out all these so desirable properties.
But the potential of this new field of research is such that many specialists will undoubtedly be interested in it in the near future. It will be very interesting to follow their work which, in the long term, could lead to a small theoretical revolution with a very concrete impact on many industries.
The study text is available here.
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