(537c) Annealing Carbon By Pulsed Laser Light
AIChE Annual Meeting
Wednesday, November 18, 2020 - 8:30am to 8:45am
Perhaps the most over looked application of laser annealing carbon is the ability to do so continuously and rapidly. Graphitization furnaces operate as a batch processing system. Time from start to completion is on the order of a day due to the slow (~25 °C/min) heating rates, long hold times, and slow cooling of the heavily insulated furnace. Additionally, these furnaces require routine maintenance and replacement of expensive specialty graphite heating elements. CO2 laser annealing provides equivalent material transformation on the order of seconds and modern CO2 lasers are designed for years of maintenance free use . Although laser annealing is limited to thinner material due to limited heating depth, materials can be annealed continuously and with potentially high throughput.
The material was heated to 2,600 °C in 1.4 ms Under the action CO2 laser radiation carbon can be heated to 2,600 °C in 1.4 ms. After 20 s, the structure is equivalent to that obtained from furnace annealing at 2,600 °C for 1 hr. as has been shown elsewhere . In contrast a Q-switched Nd:YAG laser with pulse width ~ 8 ns heats carbon materials to graphitization temperatures with a heating rate on the order of a few 1011 °C/s. The peak temperature is controlled by the laser pulse energy; typical temperatures range between 2,500 °C to the C2 sublimation temperature of 4,184 °C [2,3]. The time at elevated temperature is limited (time above 2,000 °C is 1.5 µs). On this time scale, the long-range material motions are kinetically restricted. Consequently the Nd:YAG laser annealing trajectory deviates from traditional furnace pathways. The limited time at elevated temperature can be used for the purpose of surface modification. Surface modification via kinetically limited oxidation is one potential application. Another application is making connections between carbons without additional material. Laser sintering a mixture of carbon materials could result in a wide range of potential applications.
Heat-Treatment-Temperature (HTT) and resulting carbon structural transformation has been extensively studied. A detailed quantification of HTT and resulting material annealing was provided by Oberlin in 1984 . The four stages outlined in Oberlin's HTT diagram (temperature dependent) are believed to be separated by "very rapid" transitions . So rapid, in fact, that intermediate structures are not observable on furnace heating timescales (several minutes minimum) due to slow ramp rates and have thus been ignored. These intermediate structures remain unexplored and represent an entirely new class of carbon materials. It has been demonstrated that CO2 laser heating results in equivalent end structure as compared to furnace annealing . The high temporal control of CO2 laser annealing allows for exploration of such intermediate structures that can be captured by the rapid heating and cooling timescales. In summary lasers are poised to be instrumental in the advancement of carbon science and technology.
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