Insights into plasma‐assisted polymerization at atmospheric pressure by spectroscopic diagnostics

Abstract

In the present work, a polydiagnostic study of an atmospheric pressure plasma jet specifically designed for the deposition of coatings by using an Ar/tetraethyl orthosilicate (TEOS) mixture is undertaken. Both passive and active diagnostic methods are utilized to shed light on the complexity of gas-phase mechanisms of precursor fragmentation. A detailed characterization is performed by (fast imaging) optical emission spectroscopy (OES) to clarify the spatial-temporal behavior of excited species. The influence of the TEOS admixture on gas temperature is studied by OES and Rayleigh scattering techniques. The evolution of streamers and their influence on radical formation in the treatment zone is also discussed. K E Y W O R D Sactive species, laser scattering, plasma diagnostics, plasma jet, plasma polymerization

Studying the plasma‐assisted polymerization at atmospheric pressure in Ar/TEOS by active laser diagnostics

“…The discharge appears in a form of numerous narrow filaments (visible under high‐resolution imaging) combined in a strong filamentary jet, propagating from the tip of the nozzle directly to the target. [ 12 ] As reported elsewhere, the discharge evolution along the voltage phase has a strong impact on the electron density, gas temperature, and excited species production in Ar/TEOS plasmas. [ 12 ] Moreover, TEOS fragmentation is known to be governed by electron impact reactions as well as by collisions with Ar metastables (Ar m ), whose formation is strongly influenced by the pulsed discharge dynamics.…”

“…[ 12 ] As reported elsewhere, the discharge evolution along the voltage phase has a strong impact on the electron density, gas temperature, and excited species production in Ar/TEOS plasmas. [ 12 ] Moreover, TEOS fragmentation is known to be governed by electron impact reactions as well as by collisions with Ar metastables (Ar m ), whose formation is strongly influenced by the pulsed discharge dynamics. Taking this into account, three characteristic time delays, namely, at 2, 22, and 42 µs, have been considered in regard to the positive voltage peak position, defined as the time zero.…”

“…An Argon flow of 2 standard liters per minute (slm) was injected in the discharge region through the primary gas channel, whereas, simultaneously, a second flow of 3 slm of Ar was introduced through the secondary gas channel (see Figure 1b) in the region downstream of the high‐voltage (HV) electrode tip. Similar to our previous investigation, [ 12 ] no additional gas shielding was applied to protect the discharge from the air impurities diffusion into the discharge core. However, we have to emphasize that the amount of ambient air impurities is expected to be low in the core of the plasma jet due to the relatively high total flow rate used (5 slm).…”

“…The value of 6 kV has been chosen as the minimum voltage sustaining a stable discharge, whereas 12 kV was the maximum voltage not yet resulting in overheating the nozzle. The gas temperature, estimated in our previous work using laser Rayleigh scattering for the same operational conditions, [ 12 ] was in the range 310 ± 15 K for the lowest power of 2.5 W and 390 ± 15 K for 11.5‐W power, which is considered to be sufficiently low for the deposition of the coatings even on heat‐sensitive materials.…”

“…Although the influence of TEOS molecules on the spatial–temporal behavior of excited species has been already discussed in our recent work, predominant ground‐state radical production and recombination mechanisms remain so far unexplored. [ 12 ] Such a misbalance between the wide practical use of TEOS and the rather obscured knowledge of the related kinetic processes hinders the construction of an appropriate model of the polymerization processes and the precise prediction of the composition of the deposits. Taking into account the low ionization degrees typically achieved in APPJs (10 −6 –10 −7 [ 13 ] ), the investigation of the ground‐state active species, including radicals and atomic species, is of key importance, as they are the main factors defining TEOS fragmentation and plasma polymerization.…”