Low temperature plasma as a means to transform nanoparticle atomic structure

Low temperature plasma (LTP) is a highly nonequilibrium substance capable of increasing the specific free energy of mass that flows through it. Despite this attractive feature, there are few examples of the transformation of solid material with an equilibrium atomic structure into a material with a nonequilibrium atomic structure. As a proposed example of such a transformation, in this work, it is argued that the transformation of crystalline metal nanoparticles into amorphous metal nanoparticles is feasible using LTP. To inform the feasibility calculations, detailed characterization was performed to determine the electron temperature, ion density, and background gas temperature as a function of axial position in a typical flow-through, radiofrequency, capacitively-coupled plasma reactor. Measurements revealed the existence of an intense zone with sharply elevated ion density and gas temperature in the vicinity of the powered electrode. The high intensity zone, amidst an otherwise low-intensity plasma, provides a means by which to transform the atomic structure of nanoparticles while maintaining unipolar negative charge to suppress coagulation. Theory suggests that such an intense zone would provide intense heating of nanoparticles, and subsequent rapid cooling. Calculations for copper-zirconium (CuZr) alloy show that the temperature history of a nanoparticle depends primarily on the intensity of the zone in the vicinity of the powered electrode, and on particle size. If one considers melting CuZr nanoparticles in the intense zone and then rapidly cooling them in the low-intensity plasma downstream, then the quenching rates are found to be high, on the order of 10⁵ K s^-1. Since quenching rates of this magnitude are sufficient to arrest an amorphous atomic structure, LTP reactors can be used to transform crystalline metal nanoparticles into amorphous metal nanoparticles via a highly nonequilibrium quenching process.