(38d) Synthesis and Online Characterization of Metallic Nanoparticles By Spark Ablation

Aerosol synthesis is a facile route for the continuous production of nanoparticles. Among the many existing gas phase processes, inert gas condensation routes are outstanding as they can yield particularly small and pure nanoparticles. Compared to other methods like flame spray pyrolysis or CVD, no by-products, e.g. carbonaceous species, are formed. Among the various inert gas condensation processes, spark ablation is a facile method to produce small metallic nanoparticles by creating a high voltage discharge between two electrodes. This leads to heating and partial evaporation of the electrode tips and, after recondensation, the formation of nanoparticles. These can be used as seeds for heterogeneous nucleation processes or to study aerosol mechanics. Spark ablation already proved to be suitable both for the production of a number of different materials [1] as well as for scalability [2]. Within this work, particles made from a number of different metals are synthesized with controlled size.

The setup used in this work adapts the design used in [1] and comprises of a high current power supply, rotameters to adjust the carrier (He or Ar, 99.999 % purity) and quench gas (N₂, 99.999 % purity) flows and a sixfold vacuum flange. Exchangeable capacitors and resistors are integrated into the spark circuit. Thereby, both spark frequency and energy can be influenced. The electrode rods typically have a diameter of 2 mm and hemispherically shape tips and are held by brass adapters allowing for quick exchange. The produced aerosols are measured by a combination of a TSI 3080 electrostatic classifier equipped with a TSI 3077A ⁸⁵Kr neutralizer, a TSI 3085 Nano DMA and a TSI 3776 condensation particle counter.

In the present study, the influence of both process and material parameters on the produced particles was investigated. Therefore, eleven different elements were tested, namely Ag, Al, C, Cu, Hf, Mo, Nb, Sn, Zn, Ti and W. These metals represent a wide range of material properties - from very low (Sn, Zn) to very high melting (W, C) elements and from highly (Cu, Ag) to weakly conductive (Ti, C) ones. Furthermore, the circuit current and capacitance as well as the carrier gas flow and the electrode diameter were investigated for their influence on the produced particles’ properties.

The circuit capacity C and current I turned out to be the most important process parameters to tune the size of the produced particles. A higher capacity leads to a higher dissipated energy per spark discharge and reduces the spark frequency at the same time. The fact that larger particles are produced when increasing C indicates a strong influence of energy dissipation along the electrodes and thus, their thermal conductivity as a change in circuit capacitance has no influence on the time-averaged energy input into system. With higher capacitance, an increasing amount of the energy from the spark discharge cannot be dissipated be means of thermal conduction and is instead used for heating and evaporation of electrode material. An increased circuit current also leads to larger particles; the dependency can be fitted best with a power law function which further supports the hypothesis of the strong influence of heat dissipation. Consequently, in an approach to nondimensionalize the problem, a modified Fourier number was defined that also includes the melting and boiling point as well as the enthalpies of fusion and evaporation of the respective elements. Correlating the mass yield of different electrode materials against their modified Fourier number rat otherwise the same process conditions showed an inversely proportional dependency for the majority of elements [3].

Summarizing, an overview of the most important process parameters for spark ablation synthesis and their influence on the produced particles’ properties is given. It is pointed out that heat dissipation along the electrodes is a crucial phenomenon when it comes to correlating material properties of different elements with the mass yield achieved by using the respective electrode material.

This work was supported by the German Research Council (DFG) and the Cluster of Excellence “Engineering of Advanced Materials” (EAM).

[1] N. S. Tabrizi, M. Ullmann, V. A. Vons, U. Lafont, and A. Schmidt-Ott, “Generation of nanoparticles by spark discharge,” J. Nanoparticle Res., vol. 11, no. 2, pp. 315–332, Feb. 2009.

[2] J. Feng, G. Biskos, and A. Schmidt-Ott, “Toward industrial scale synthesis of ultrapure singlet nanoparticles with controllable sizes in a continuous gas-phase process,” Sci. Rep., vol. 5, no. 1, p. 15788, Dec. 2015.

[3] M. Domaschke, M. Schmidt and W. Peukert, "Aerosol synthesis of ultrafine monometallic nanoparticles by spark ablation" J Aerosol Sci., submitted