This study investigates the processes of interaction of nanosecond pulsed-periodic laser radiation with the flow of aerosol agglomerates of gold nanoparticles synthesized in a spark discharge. Nanoparticles in a gas flow are spatially separated nano-objects whose interaction with each other and with the walls of an experimental cell was insignificant. Therefore, the energy absorbed by nanoparticles was used only for their own heating with further shape and size modification and on heat transfer to the surrounding gas. In the research, we used laser radiation with wavelengths of 527 and 1053 nm at pulse energies up to 900 µJ and pulse repetition rates up to 500 Hz. The dynamics of changes in the nanoparticles size during their sintering process depending on the laser pulses energy is characterized by an S-shaped shrinkage curve. Complete sintering of the initial agglomerates with their transformation into spherical nanoparticles is achieved by a series of impacting laser pulses. The result of nanoparticles’ laser modification is largely determined by the pulse energy and the efficiency of the nanoparticles’ radiation absorption.

In this paper, we investigated the interaction of nanosecond pulsed-periodic infrared (IR) laser radiation at a 50 and 500 Hz repetition rate with aerosol platinum (Pt) and silver (Ag) nanoparticles agglomerates obtained in a spark discharge. Results showed the complete transformation of Pt dendrite-like agglomerates with sizes of 300 nm into individual spherical nanoparticles directly in a gas flow under 1053 nm laser pulses with energy density 3.5 mJ/cm². Notably, the critical energy density required for this process depended on the size distribution and extinction of agglomerates nanoparticles. Based on the extinction cross-section spectra results, Ag nanoparticles exhibit a weaker extinction in the IR region in contrast to Pt, so they were not completely modified even under the pulses with energy density up to 12.7 mJ/cm². The obtained results for Ag and Pt laser sintering were compared with corresponding modification of gold (Au) nanoparticles studied in our previous work. Here we considered the sintering mechanisms for Ag, Pt and Au nanoparticles agglomerates in the aerosol phase and proposed the model of their laser sintering based on one-stage for Pt agglomerates and two-stage shrinkage processes for Au and Ag agglomerates.

Aerosol particle charging is widely used in various technical applications. A model of a needle-plate type charger for efficient charging of aerosol nanoparticles in a corona plasma discharge has been developed and investigated. The main difference from analogues is the modernization of the geometry of the hole in the grounded plate and the number of corona needles. This led to a significant increase in the efficiency of the charger by more than 25%. The effect of two types of discharge plates with cylindrical and conical inner holes on the external charging efficiency of aerosol particles is experimentally investigated. Metallic nanoparticles of Ag with sizes of 20–160 nm and number concentration of 10⁶–10⁸ cm⁻³ were used as a test aerosol. It is found that the maximum efficiency of particle charging is achieved by using a plate with a conical hole by reducing electrostatic losses from 37±3% to 20±2%. This is due to the redistribution of the electrostatic field strength lines when changing the cylindrical hole to a conical one. An additional effect of increasing the efficiency of particle charging was also achieved through the use of a multi-pointed needle, which provided lower electrostatic losses compared to a single needle at the same corona discharge current of 20 µA. It has been experimentally shown that the use of a conical hole of the plate and a multi-pointed needle increased the charging efficiency of particles from 47±3% to 59±4% in comparison with the standard design with a cylindrical hole and a single-pointed needle.