Plasma processes for surface treatments of inorganic materials and polylactic acid hydrophilization at atmospheric pressure

Dielectric barrier discharges (DBD) at atmospheric pressure were used for surface modification by non-equilibrium low-temperature plasma. The plasma interacts with the surface depending on treatment gas composition and treatment conditions. In this work several topics are covered: calculation of H atom concentration in Ar/H₂ plasma, jet development for Ag2_S cleaning in Ar/H₂ post-discharge, silicon surface functionalization by means of Ar/H₂ plasma and polylactic acid (PLA) surface functionalization in ambient air, nitrogen and oxygen. The H atom formation was observed through H₂ continuum emission that is created when metastable Ar_m^* collides with H₂ molecule in ground state and two H atoms are formed. There are also H atoms formed from direct electron excitation of H₂ molecule in ground state. The third major reaction, from which H atoms can be formed, is H₂ collision with Ar₂^* excimer, but the created H atoms concentration is negligible. From a simple reaction model and with help of Bolsig⁺ software, the concentration of H atoms in discharge and its decay behind plasma region were calculated. In order to prevent overheating of the plasma reactor by H atom recombination on the walls and in the volume of the reactor the minimal electrode length, when maximum H atoms concentration is reached, was estimated. The electrode length estimated from theoretical calculations and from an experiment with wedge-shape electrodes were in good agreement. A DBD plasma jet with N₂ shielding gas was developed for restoration and cleaning of Ag₂S. The N₂ shielding gas prevents the oxidizing species from ambient air to react with H atoms and oxidize the surface. The cleaning of Ag₂S was therefore quicker and the treated area was bigger than without shielding gas. No glove box was needed that simplifies handling. The effects of silicon wafer surfaces exposure to direct or afterglow Ar/H2 plasmas were investigated. Direct plasma treatment introduced Si-H but also Si-O functionalities. The oxide thickness increased and saturated with treatment time. The Si-H functionalities were found not only inside oxide on the wafer surface but also in the bulk below oxide, where they were stable in time. The Si-H functionalities could not be introduced neither by H atom treatment in post-discharge, neither in combination with UV radiation from a separate Ar plasma DBD source. Meanwhile the UV radiation oxidized the surface and removed the Si-H functionalities created by HF etching. To improve PLA surface adhesion a plasma-chemical treatment using nonthermal plasma generated in ambient air, N₂ or O₂ via diffuse coplanar surface barrier discharge (DCSBD) was used. The optimal treatment time and power were investigated. The most hydrophilic surface was obtained after only 3-4 s treatment. Treatment up to 10 s did not damage the polymer but longer treatments (30 and 60 s) caused partial degradation. The plasma broke C-C/C-H bonds and formed more C-O, OC= O and C-O-C bonds. During storage the amount of surface oxygen decreased and a negligible amount of nitrogen was adsorbed. (Author)