Sep 13, 2019
Brauer Hall, room 12
Dr. Elijah Thimsen, Assistant Professor
Department of Energy, Environmental & Chemical Engineering
Washington University in St. Louis
Beyond Equilibrium in Low Temperature Plasma Processes
A great challenge of the 21st century is developing general methods to predict the direction of processes that operate very far from local equilibrium. The proposal is that beyond equilibrium thermodynamics can be used to predict stationary states in systems very far from local equilibrium, similar to how classical thermodynamics can be used to predict stationary states in systems governed by local equilibrium. The knowledge is necessary to engineer reactions of importance to society such as CO2 conversion and synthesis of advanced materials. Low temperature plasmas are an attractive experimental system in this context. Within low temperature plasma, different species have temperatures that can differ by two orders of magnitude at the same location in space, for example molecules near room temperature and extremely hot electrons in the range from 10,000 to 100,000 K. In this talk, two examples of highly unexpected processes that occur in this unique environment will be presented. The first example is spontaneous splitting of carbon dioxide (CO2) into carbon monoxide and oxygen at a background temperature and total pressure where CO2 is the only significant carbonaceous species present at equilibrium. In other words, the chemical speciation begins in its equilibrium state and spontaneously moves away from that state in the low temperature plasma. The idea is that the nonequilibrium flux in the reaction extent is caused by heat transfer between very hot electrons and relatively cool gas molecules. The second example involves feeding an elemental source aerosol into a low temperature plasma at a background temperature and total pressure under which the material has negligible saturation pressure. Interestingly, the elemental aerosol spontaneously vaporizes in the plasma, despite the negligible vapor pressure at the background temperature. The phenomenon has been observed for a variety of different materials, including Bi, Ga, Sb, and In. The nonequilibrium vaporization-condensation process causes a redistribution of mass as a function of size and a narrowing of the size distribution. Furthermore, the mean size after vaporization can be made to be either larger or smaller than the source aerosol. The idea is that the nonequilibrium evaporation flux is caused by highly energetic ion bombardment. Furthermore, high quality compound semiconductor nanocrystals can be synthesized from the vapor generated in the plasma including GaN, GaSb, and InN. Moving forward, these experimental observations can be used to develop thermodynamically admissible kinetic models to predict stationary configurations as a function of plasma state variables.
Organizer / Host: Dr. Biswas