Researchers deployed lasers to overheat gold samples, measuring temperatures with ultra-bright X-ray pulses
Greg Stewart/SLAC National Accelerator Research Institute
A thin layer of wafers that fire laser shots can heat to 14 times the melting point, pushing solids well beyond predicted limits, with some solids potentially lacking any defined melting point.
Overheating is a well-known phenomenon where a solid can exceed its melting point or even its boiling point without altering its state. For instance, water heated in a microwave can reach temperatures above 100°C (212°F) if the container is smooth and undisturbed, but will boil vigorously as soon as disturbed.
Many physicists have suggested an upper overheating limit of approximately three times the standard melting point in Kelvin, termed the entropy catastrophe. This occurs because the entropy, often viewed as the degree of disorder in a system, is greater when the material transitions to a liquid state. Remaining solid at temperatures above this point contradicts the second law of thermodynamics.
Recently, Thomas White from the University of Nevada, Reno, and his team found that gold can heat up to 14 times its melting point, exceeding even the entropy catastrophe.
White and his colleagues directed a powerful laser at a 50-nanometer-thick gold sheet for 45 seconds, then measured the temperature using reflective X-rays through a novel approach. By analyzing shifts in the frequency of the reflected X-rays, they determined the excess energy transferred to the gold, allowing them to calculate the heating effect on the sheet.
“The temperatures we measured were astonishingly high,” White remarked.
After ensuring no errors in their measurements, White and his team revisited existing theories, realizing that the rapid heating of gold likely reduced the solid’s entropy compared to its potential liquid state, surpassing predicted temperature limits. “It’s crucial to state that we haven’t violated the second law of thermodynamics,” White clarified.
The actual limitations of overheating are still open questions, he noted. “I may have thought the overheating limit was resolved in the 1980s, but now I believe it’s a question worthy of further exploration. Can we heat materials beyond their melting point?”
Utilizing this X-ray technology for measuring solid heating may also help simulate the effects of extreme heat and pressure from planetary interiors on materials within very short timescales, according to Sam Vinco at Oxford University. “Currently, there isn’t an effective thermometer for solids that operates on such brief timescales,” he noted.
It will also be fascinating to ascertain if this phenomenon applies to other solids beyond gold, Vinco stated, particularly regarding whether there’s a limit to heating before melting occurs. “What’s captivating is considering whether rapid heating could essentially circumvent traditional thermodynamic principles,” he added.
Source: www.newscientist.com
