*Article* **Plasma Treatment of Thermally Modified and Unmodified Norway Spruce Wood by Diffuse Coplanar Surface Barrier Discharge**

**Zuzana Košelová, Jozef Ráhel' and Oleksandr Galmiz \***

> Department of Physical Electronics, Masaryk University, Kotlarska 2, 61137 Brno, Czech Republic; zuzanakoselova@gmail.com (Z.K.); rahel@mail.muni.cz (J.R.)

**\*** Correspondence: oleksandr.galmiz@gmail.com

**Abstract:** This work deals with the treatment of wood surfaces by diffuse coplanar surface barrier discharge (DCSBD) generated at atmospheric pressure. The effect of the distance of the sample from the electrode surface and the composition of the working gas in the chamber was studied. Norway spruce (*Picea abies*) wood, both unmodified and thermally modified, was chosen as the investigated material. The change in the surface free energy (SFE) of the wood surface was investigated by contact angles measurements. Chemical and structural changes were studied using infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Activation at a 0.15 mm gap from the electrode led in all cases to an increase in the SFE. The largest change in SFE components was recorded for wood thermally modified to 200 ◦C. At a 1 mm gap from the electrode increase of SFE occurred only when oxygen (O2) and argon (Ar) were used as working gas. Treatment in air and nitrogen (N2) resulted in an anomalous reduction of SFE. With the growing temperature of thermal modification, this hydrophobization effect became less pronounced. The results point out the importance of precise position control during the DCSBD mediated plasma treatment. A slight reduction of SFE on thermally modified spruce was achieved also by short term ultra-violet (UV) light exposure, generated by DCSBD.

**Keywords:** Norway spruce; thermally treated wood; DCSBD; plasma treatment; surface free energy
