OPTICAL EMISSION CHARACTERIZATION OF EXTREMELY REACTIVE OXYGEN PLASMA DURING TREATMENT OF GRAPHITE SAMPLES

KARAKTERIZACIJA EKSTREMNO REAKTIVNE KISIKOVE PLAZME Z OPTIČNO EMISIJSKO SPEKTROSKOPIJO MED OBDELAVO KOMPOZITA POLIMER – GRAFIT

Zlatko Kregar1, Marijan Bišćan1, Slobodan Milošević1, Kristina Eleršič2, Rok Zaplotnik2, Gregor Primc2, Uroš Cvelbar3

1Institut za fiziku, Bijenička 46, 10000 Zagreb, Croatia
2Institut "Jožef Stefan", Jamova cesta 39, 1000 Ljubljana, Slovenia
3Center of Excellence for Polymer Materials and Technologies, Tehnološki park 24, 1000 Ljubljana, Slovenia

zkregar@ifs.hr

Prejem rokopisa – received: 2011-06-01; sprejem za objavo – accepted for publication: 2011-07-19

Characteristics of oxygen plasma during treatment of polymer graphite composite were monitored by optical emission spectroscopy. Plasma was created in a rather small volume of about 3×10–5 m–3 within a quartz glass tube by an electrode-less radiofrequency discharge in the H mode. The discharge was established using an RF generator with the frequency of 13.56 MHz and the output power of 550 W. The composite samples were discs with a diameter of 25 mm and the thickness of 8 mm. Plasma was created in pure oxygen and characterized by an optical spectrometer through an optical fiber. A low-resolution spectrometer Avantes AvaSpec 3648 was adjusted to the lowest acquisition time due to intensive radiation. The optical spectra revealed several atomic lines originating from radiative transitions from highly excited states, CO bands from 3rd positive as well as Angstrom transitions, and a broad continuum between 400 and 700 nm. Weak carbon atomic lines in the UV and infra-red part of spectrum were detected as well. The intensity of major spectral features was measured versus treatment time for 30 s. The results showed continuous decrease of oxygen lines and simultaneous increase of a CO line at 266 nm and C line at 911 nm. Spectral features were explained by intensive oxidation of the graphite sample. The unusually high intensity of CO bands were explained by interaction of highly excited oxygen atoms with graphite samples, while the appearance of the broad continuum was explained by partial overlapping of radiative transitions within CO molecule.

Keywords: optical emission spectroscopy, oxygen plasma, surface modification, functional groups, graphite

Z optično emisijsko spektroskopijo smo spremljali lastnosti kisikove plazme med obdelavo kompozita polimer-grafit. Plazmo smo ustvarili v razmeroma majhni prostornini približno 3×10–5 m–3 znotraj kvarčne cevi. Za vzbujanje plazme smo uporabili brez-elektrodno visokofrekvenčno plinsko razelektritev v H načinu. Uporabili smo radiofrekvenčni generator, ki je deloval pri frekvenci 13,56 MHz in izhodni moči 550 W. Kompozitni vzorci so imeli valjasto obliko s premerom 25 mm in višino 8 mm. Plazmo smo vzbujali v čistem kisiku, njene karakteristike pa merili z optičnim spektrometrom. Uporabili smo spektrometer Avantes AvaSpec 3648, ki ima nizko ločljivost v širokem območju med okoli 180 nm in 1100 nm. Zaradi izredne svetilnosti plazme smo uporabili najmanjši integracijski čas. Optični spektri so pokazali značilne črte, ki izvirajo iz atomskih prehodov, molekularne pasove, ki izvirajo iz prehodov v CO radikalih (3. pozitivni in Angstromov sistem), kakor tudi širok kontinuum v območju med 400 in 700 nm. Opazili smo tudi šibke črte, ki izvirajo iz prehodov v C atomih. Intenziteto najpomembnejših spektralnih vrhov smo spremljali med obdelavo vzorcev, ki je trajala 30 s. Rezultati so pokazali stalno padanje intenzitete kisikovih črt, obenem pa rast CO črte pri 266 nm in C črte pri 911 nm. Rezultate smo pojasnili z intenzivno oksidacijo kompozitnih vzorcev. Nenavadno visok CO vrh smo pojasnili z močno kemijsko interakcijo, ki je posledica prisotnosti visoko vzbujenih kisikovih atomov, medtem ko smo pojav kontinuuma pripisali prekrivanju spektralnih pasov, ki izvirajo iz različnih prehodov znotraj molekule CO.

Ključne besede: optična emisijska spektroskopija, kisikova plazma, modifikacija površine, funkcionalne skupine, grafit