Paper PS-TuP13
Particle Temperature Histories in a Tubular Low Temperature Plasma Reactor: Relevance to the Synthesis of Amorphous Metal Alloys

Tuesday, October 23, 2018, 6:30 pm, Room Hall B

The nonequilibrium environment of low temperature plasma (LTP) allows it to incorporate a significant amount of specific free energy to materials with which it is in contact. It has been shown recently that LTP is capable of synthesizing materials that are far away from equilibrium, as in the case of hyperdoped silicon nanocrystals [1]. Furthermore, LTP can also process pre-synthesized materials in such a way that the material is pushed far away from equilibrium, as demonstrated with in-flight size focusing of polydisperse aerosols [2]. However, examples on the transformation of materials with equilibrium atomic structure to materials with non-equilibrium structure are scarce. In this work, we propose that the distinct nanoparticle (NP) temperature histories in tubular LTP reactors can be utilized to transform crystalline metals into amorphous metals. Spatial characterization of an Ar plasma in a capacitively coupled tubular reactor revealed the existence of a zone with sharply elevated ion density and gas temperature in the vicinity of the powered electrode. Theory suggests that such an intense zone would heat NPs to temperatures above 1000 K, and rapid cooling would follow as NPs leave the zone. In the characterized reactor, an aerosol processing scenario was simulated, where pre-synthesized crystalline NPs were sent into the LTP. Copper-zirconium (CuZr), which is a well-established glass former and is of interest for low temperature electro-catalysis applications, was taken to be the particle material. Calculations showed that the temperature history of a NP is strictly dependent on diameter, and on the intensity of the zone. CuZr melted in the intense zone, and subsequent cooling of the melt in the low intensity plasma downstream lead to quenching rates on the order of 10⁵ K/s, all while particles maintaining a unipolar negative charge. Quenching rates of this magnitude are known to be sufficient to arrest an amorphous atomic structure [3].

[1] Zhou Shu et al., “Boron‐ and Phosphorus‐Hyperdoped Silicon Nanocrystals,” Part. Part. Syst. Charact., vol. 32, no. 2, pp. 213–221, Aug. 2014.

[2] N. B. Uner and E. Thimsen, “In-Flight Size Focusing of Aerosols by a Low Temperature Plasma,” J. Phys. Chem. C, vol. 121, no. 23, pp. 12936–12944, Jun. 2017.

[3] F. Gillessen and D. M. Herlach, “Crystal nucleation and glass-forming ability of Cu-Zr in a containerless state,” Mater. Sci. Eng., vol. 97, pp. 147–151, Jan. 1988.