Paper SE+PS-ThM11
Optical Emission Spectroscopy of He Dielectric Barrier Discharges at Atmospheric-Pressure Applied to the Functionalization of Wood Surfaces

Thursday, October 31, 2013, 11:20 am, Room 203 C

Application of dielectric barrier discharges (DBD) to the modification of “novel” materials such as nanostructured polymers is much more challenging than for conventional substrates such as Si or SiO2. This can be attributed not only to the highly anisotropic nature of this polymer which can introduce spatial inhomogeneities of the electric field near the substrate surface, but also to its highly porous microstructure which can release impurities either from plasma-substrate chemical reactions or from sample outgassing. In this work, a porous wood sample (sugar maple, acer saccharum) was placed on the bottom electrode of a DBD operated in nominally pure helium to examine the influence of plasma-wood interactions and substrate outgassing on the evolution of the plasma properties. Optical emission spectroscopy revealed strong emission from N2, N2+, O and OH impurities. While the nitrogen and oxygen emission can be attributed to air outgassing, the OH emission was ascribed to etching of the weak boundary layer and humidity desorption from wood. We have calculated various line ratios from the time-resolved optical emission spectra. The He-588 nm-to-He-707 nm line ratio, I588/707, was found to decrease from when going from right after ignition of the first few discharges where substrate outgassing is important to longer treatment times where “pumping” of the wood samples is nearly complete. Assuming that the He 3D and 3S levels (L•S coupling) giving rise to the emission at 588 and 707 nm are populated by stepwise excitation through the most populated metastable He 3S1 level and are lost by spontaneous emission, the I588/707 line ratio becomes link to the ratio of the rate for stepwise excitation of the He 3D and 3S levels, which is only a function of the electron temperature Te. Accounting for collisional energy transfer reactions between the He 3D, 3P, and 3S states in atmospheric-pressure plasmas and using the set of cross sections reported in literature for stepwise excitation, energy transfer reactions, and collisional quenching, it was found that the observed decrease of I588/707 can be ascribed to an decrease of the electron temperature. Further analysis of the time evolution of the emission spectra and of the current-voltage characteristics indicated that the release of products from the wood substrate also yields to a significant quenching of He metastables. This method was further used to examine the detailed influence of hexamethyldisiloxane (HMDSO) and titatnium isoproxyde (TTIP) injection on the plasma characteristics during plasma enhanced chemical vapor deposition of functional, nanostructured coatings on wood.