Copper does not instantly lose its solid structure when a laser heats it extremely abruptly. Its atoms remain partially organized for longer than expected.
To observe this phenomenon, the researchers sent an extremely brief laser pulse onto a thin layer of copper. The laser first heats the metal’s electrons. These then transfer some of their energy to the atoms, which begin to move more vigorously.
Natural copper.
Image Wikimedia
At this scale, everything takes place over a few femtoseconds to a few picoseconds. A femtosecond is one millionth of one billionth of a second. The team therefore used very short electron pulses to directly track how the atoms’ organization evolved.
The experiments used absorbed energies representing two to four times the threshold required for ultrafast melting. Disorganization begins at the surface, slightly before the normal melting temperature. It then rapidly spreads into the material.
The most surprising result appears when the copper is strongly overheated. Some simulations predicted that its crystal structure would then collapse very rapidly. The measurements instead show that this atomic order disappears progressively.
New simulations better explain this discrepancy. Older models notably assumed overly static pressure conditions. In the experiment, the atoms can move and relax more freely. They therefore retain some of their organization beyond the overheating limit predicted by certain models.
The study also shows that energy transfer between the laser-heated electrons and the atomic lattice remains relatively weak under these conditions. This transfer directly influences how quickly the material heats up and transforms.
This work could improve the models used to describe matter subjected to extremely rapid energy inputs. This question is of particular interest in nuclear fusion research, where certain materials may be exposed to extreme temperatures and energy fluxes.
The researchers now want to test copper under other pressure conditions. They also plan to study copper alloys, which are being considered for certain components that must absorb or dissipate large amounts of heat in fusion systems.