Rok Zaplotnik

Application of extremely non-equilibrium plasmas in the processing of nano and biomedical materials

Some applications of extremely non-equilibrium oxygen plasma for tailoring the surface properties of organic as well as inorganic materials are presented. Plasma of low or moderate ionization fraction and very high dissociation fraction is created by high frequency electrodeless discharges created in chambers made from a material of low recombination coefficient. The O atom density often exceeds 1021 m−3 which allows for rapid functionalization of carbon-containing materials. Surface saturation with polar oxygen-rich groups is achieved in a fraction of a second and further exposure leads to etching. The etching is often non-uniform and results in nano-structuring of surface morphology. A combination of rich morphology and saturation with polar functional groups allows for a super-hydrophilic character of originally hydrophobic materials. Polymer composites are etched selectively so the polymer component is removed from the sample surface, leading to modified surface properties. Furthermore, such a treatment allows for distinguishing the distribution and orientation of fillers inside the polymer matrix. The exposure of inorganic materials to non-equilibrium oxygen plasma causes one-dimensional growth of metal oxide nanoparticles, thus representing a unique technique for the rapid catalyser-free growth of nanowires.

Microwave discharge as a remote source of neutral oxygen atoms

The late flowing afterglow of an oxygen plasma was used as a remote source of neutral oxygen atoms. Plasma was created via a microwave discharge in a narrow quartz glass tube with an inner diameter of 6 mm at powers between 50 W and 300 W. The tube was connected to a wider perpendicular tube with an inner diameter of 36 mm. The density of neutral oxygen atoms was measured in the wide tube about 70 cm from the discharge using a classical nickel catalytic probe. The oxygen atom density as a function of gas pressure had a well-defined maximum. The oxygen atom density can be as large as 11 × 1020u2009m-3. At the lowest power tested (50 W), the maximum was obtained at a pressure of about 30 Pa. However, at higher powers, the maximum shifted to higher pressures. As a result, at 300 W the maximum appeared at 60 Pa. The results can be explained through collision phenomena in gas phase and surfaces in both discharge and flowing afterglow regions, and strong pressure gradients along the narrow tube.

Controlling the oxygen species density distributions in the flowing afterglow of O2/Ar–O2 surface-wave microwave discharges

The evolution of species densities along a reactor radially positioned on an O2 surface-wave microwave discharge is investigated by means of modelling with the aim to define the density tuning possibilities. The validity of the models is shown by the comparison of the calculated and measured axial distribution of O-atoms. The calculations revealed that due to the perpendicular injection of the plasma into the reactor, the gas temperature is close to the room temperature in most of it, except for a 5?cm region around the inlet. It is shown that the pressure drop along the discharge tube, which results in the change of pressure in the discharge region with the gas flow rate, induces the variation of the relative density of active species entering the reactor, where the pressure is kept constant. The surface recombination probability of atoms varies along the afterglow tube due to the surface temperature gradient, as well as due to the conditioning of the surface resulting from the continuous operation of the system. The system is shown to be very practical in applications where surfaces/porous materials are to be treated homogeneously by pumping active species through them, since by tuning the gas flow rate equidensity surfaces can be obtained in the case of the two most abundant species, the O-atoms and O2(a) molecules. In the case of O-atoms the densities obtained at the two pressures investigated, i.e. 100 and 50?Pa, are very similar, as well as their evolution along the reactor, while the density of O2(a) molecules decreases considerably with pressure.

A Fiber Optic Catalytic Sensor for Neutral Atom Measurements in Oxygen Plasma

The presented sensor for neutral oxygen atom measurement in oxygen plasma is a catalytic probe which uses fiber optics and infrared detection system to measure the gray body radiation of the catalyst. The density of neutral atoms can be determined from the temperature curve of the probe, because the catalyst is heated predominantly by the dissipation of energy caused by the heterogeneous surface recombination of neutral atoms. The advantages of this sensor are that it is simple, reliable, easy to use, noninvasive, quantitative and can be used in plasma discharge regions. By using different catalyst materials the sensor can also be applied for detection of neutral atoms in other plasmas. Sensor design, operation, example measurements and new measurement procedure for systematic characterization are presented.

A catalytic sensor for measurement of radical density in CO2 plasmas.

A catalytic sensor for the measurement of radical density in weakly ionized CO2 plasmas, created in a low-pressure electrodeless discharge, is presented. The CO2 plasma was created in a 4 cm wide borosilicate glass tube inside a copper coil connected to a radio frequency generator operating at 27.12 MHz with a nominal power of 250 W. The dissociation fraction of the CO2 molecules was measured in the early afterglow at pressures ranging from 10 Pa to 100 Pa, and at distances of up to 35 cm along the gas stream from the glowing plasma. The radical density peaked (2 × 1020 m−3) at 80 Pa. The density quickly decreased with increasing distance from the glowing plasma despite a rather large drift velocity. The dissociation fraction showed similar behavior, except that the maximum was obtained at somewhat lower pressure. The results were explained by rather intense surface recombination of radicals.

A Large Electrodeless Plasma Reactor as a Source of Radicals

Low-pressure plasma created in gases of two- or multiatom molecules is often used for dissociation of molecules to parent atoms. The dissociation fraction depends on the type of discharge, the discharge power and coupling, the properties of the discharge reactor, and the pumping speed. Large quantities of atoms are produced in large reactors providing poor loss due to heterogeneous surface recombination. The best results are obtained in electrodeless radio-frequency discharges created in glass discharge systems. An example of such a system is presented in this paper.

Tailoring surface morphology of cotton fibers using mild tetrafluoromethane plasma treatment

Surface morphology of woven cotton fabrics was modified by treatment with mild plasma created in tetrafluoromethane (CF4) at the pressure of 60u2009Pa. Samples of dimensions 20u2009cmu2009×u200920u2009cm were mounted into a 27-cm-wide cylindrical plasma reactor powered with a radio frequency generator of frequency 27.12u2009MHz and output power of about 500u2009W. The volume of the reactor was 17u2009l and the corresponding power density was solely 30u2009W/l. The plasma density was estimated with a double electrical probe and was about 2u2009×u2009109u2009cm−3. The type of radicals created in plasma was determined using optical emission spectroscopy (OES). No CFx and a negligible amount of F radicals were observed by OES, and surprisingly the predominant spectral features were CO bands. The evolution of surface morphology vs. plasma treatment time was monitored by scanning electron microscopy. Originally smooth fibers became extremely rough after prolonged treatment. The results were explained by etching of fibers with gases released from the samples upon plasma treatment.

Rapid Hydrophilization of Model Polyurethane/Urea (PURPEG) Polymer Scaffolds Using Oxygen Plasma Treatment

Polyurethane/urea copolymers based on poly(ethylene glycol) (PURPEG) were exposed to weakly ionized, highly reactive low-pressure oxygen plasma to improve their sorption kinetics. The plasma was sustained with an inductively coupled radiofrequency generator operating at various power levels in either E-mode (up to the forward power of 300 W) or H-mode (above 500 W). The treatments that used H-mode caused nearly instant thermal degradation of the polymer samples. The density of the charged particles in E-mode was on the order of 1016 m−3, which prevented material destruction upon plasma treatment, but the density of neutral O-atoms in the ground state was on the order of 1021 m−3. The evolution of plasma characteristics during sample treatment in E-mode was determined by optical emission spectroscopy; surface modifications were determined by water adsorption kinetics and X-ray photoelectron spectroscopy; and etching intensity was determined by residual gas analysis. The results showed moderate surface functionalization with hydroxyl and carboxyl/ester groups, weak etching at a rate of several nm/s, rather slow activation down to a water contact angle of 30° and an ability to rapidly absorb water.

E and H Modes of Inductively Coupled SO 2 Plasma

Inductively coupled plasma (ICP) represents a source of charged as well as neutral reactive particles. Plasma created in sulfur dioxide (SO2) gas is a source of SO radicals suitable for functionalization of delicate organic materials with SO groups, provided the appropriate discharge mode is chosen. The ICP was sustained in either Eor H-mode. In the first case, the plasma glow is rather uniform along the entire reactor and the color is dark blue due to extensive radiation originating from excited SO radicals. In the H-mode, the light intensity is orders of magnitude larger in the coil, and the image is white due to various atomic transitions in the entire spectrum. The plasma density in the H-mode is, therefore, large enough to cause not only dissociation of SO2 to SO but also destruction of SO radicals to S and O atoms, so plasma created in H-mode is not that suitable for functionalization of organic materials with SO groups.