Sintering of metallic nano-aggregates in an atmospheric pressure non-thermal plasma

Abstract

It has been recently demonstrated that non-thermal plasmas (NTP) can function as nanoparticle processing reactors by selectively heating particles, inducing chemical reactions and structural transformations. However, lacking in prior studies is direct comparison between NTP particle-processing and more traditional, equilibrium thermal processing. With tandem differential mobility analysis, we studied the sintering of metallic nano-aggregates as they traversed through an atmospheric pressure NTP. As a reference, sintering experiments with a tube furnace were also conducted. Silver and bismuth nano-aggregates were synthesized via gas phase reactors for experiments and were selected to represent high and low melting point metals, respectively. We found that the silver aggregate sintering behavior was similar in the plasma and the furnace systems. With increasing plasma power/furnace temperature, the sintering process can be divided into to an aggregate compaction stage, a constant mobility stage and a vaporization stage. The mobility size shift of the silver aggregates was used to estimate their equivalent temperatures in the plasma. With an aggregate sintering model, an equivalent particle sintering temperature was estimated to be 165–270 °C in the plasma as the silver aggregate mobility diameter decreased from 100 nm to 95-65 nm. In contrast to silver, due to the early onset of particle vaporization, the three stages of bismuth aggregate sintering were not as clearly discernible. The constant mobility stage was only observed in a narrow range of plasma powers for the 100 nm aggregates and was not evident for the 20 nm aggregates. Additionally, distinct patterns of bismuth particle mobility shifts were observed during plasma sintering and furnace sintering, with the plasma reactor being a more effective sintering unit. The reducing environment within the plasma and the partial oxidation of the bismuth aggregates could be the reason for the effectiveness of plasma sintering. Further measurements of the particle transmission efficiency through the plasma reactor shows that it is particle size and material dependent.