Plasma activation
Importantly, the plasma activation can be performed at atmospheric pressure using air or typical industrial gases including hydrogen, nitrogen and oxygen.
Thus, the surface functionalization is achieved without expensive vacuum equipment or wet chemistry, which positively affects its costs, safety and environmental impact.
[1] Many industries employ surface preparation methods including wet chemistry, exposure to UV light, flame treatment and various types of plasma activation.
Arc discharges at atmospheric pressure are self-sustained DC electric discharges with large electric currents, typically higher than 1 A, in some cases reaching up to 100.000 A, and relatively low voltages, typically of the order of 10 – 100 V. Due to high collision frequencies of plasma species, atmospheric pressure arcs are in thermal equilibrium having temperatures of the order of 6.000 – 12.000 °C.
This process heats the cathode stimulating thermal electron emission, which sustains the high discharge currents.
On the cathode surface the electric currents concentrate at fast moving spots with sizes of 1 – 100 μm.
[2] Pulsed atmospheric arc technology improves the arc stability at low electric currents, maximizes the discharge volume, and together with it the production of reactive species for plasma activation, while at the same time reducing the size of the driving high voltage electronics.
The gas is very reactive allowing high surface treatment speeds when only a short-time contact with the substrate is sufficient to achieve the activation effect.
In this case, the substrate is subject not only to the reactive chemical species, but also to their ions with energies of up to 10 – 20 eV, to high temperatures reaching within the cathode spots 3000 °C, and to UV light.
It reduces metal oxides by their reactions with hydrogen species and leaves the surface free from organic contaminants.
Moreover, the fast moving multiple cathode spots create a microstructure on the substrate improving mechanical binding of the adhesive.
When the field in the rest space is negligible – this happens at large distances to the electric grounds – the corona discharge can be ignited.
On the other hand, electrons initiating the avalanches in the positive corona are produced by the photoionization of the gas, surrounding the high voltage anode.
Due to the presence of the dielectric barrier, such plasma sources operate only with sine-wave or pulsed high voltages.
[7] Piezoelectric direct discharge can be considered as a special technical realization of the dielectric barrier discharge, which combines the alternating current high voltage generator, high voltage electrode and the dielectric barrier into a single element.
In addition, when operated in far from the electric ground, it also produces corona discharges on the sharp edges of the piezo-transformer.
The goal of the plasma generators is to convert the electric energy into the energy of charged and neutral particles – electrons, ions, atoms and molecules – which then would produce large quantities of chemical compounds of hydrogen, nitrogen and oxygen, in particular short-lived highly reactive species.
In addition, at the contact points of discharge filaments the surface can locally reach high temperatures.
In the thin cathode and anode layers, the ions and the electrons reach average energies up to 10 times higher, corresponding to temperatures of 100,000 °C.
They include atomic H, N and O species, OH and ON radicals, ozone, nitrous and nitric acids, as well as various other molecules in metastable excited states.
[11] Moreover, when the discharge directly contacts the substrate, the ions of these species as well as the electrons, both having high energies, bombard the surface.
