3.1 Electrical Characterization of the DBD
Figure 3.1 and figure 3.2 show the typical measured quasi-pulsed (60 kHz) input
voltage to the electrodes and current waveforms produced in the DBD using N2 and
N2+0.1%O2 of 50 slm and having 500 W of power output from the power supply. In
practice, the electrical property for the nitrogen discharge is not very different from
that for N2+0.1%O2. Filament current increases rapidly as the voltage rises rapidly
and decreases rapidly as voltage increases to some level. The rapid decrease of the
current is mainly due to the charge accumulation on the dielectric surface. The
filament current oscillates between positive and negative values at lower voltage
mainly because of the displacement current (large rate of change of voltage).
Measured peak current for N2 is approximately 0.4 A for 25 cm2 of discharge area,
which is equivalent to ~0.016 A/cm2 in current density and the N2+0.1%O2 is
approximately 0.5 A for 25 cm2 of discharge area, which is equivalent to ~0.02 A/cm2
in current density.
Figure 3.3 and figure 3.4 show the typical electrical properties for N2 and N2+0.1% O2
discharge. The large symbol is represented by breakdown point. Figure 3.5 and Figure
3.6 shows the typical Lissajous figure obtained for the same test conditions as in
Figure 3.1 and Figure 3.2. The shape of the Q-V curve is a distorted version of the
standard parallelogram [Wagner, H.E., et al. 2003] observed in a DBD driven by a
sinusoidal AC power supply. When the voltage reaches the peak value (~ 8 kV), the
maximal effective capacitance (~400 nC) for the N2 and N2+0.1%O2 DBD across the
electrodes is obtained, which extinguishes the discharge (zero current), as observed in
Figure 3.1 and Figure 3.2. Corresponding plasma absorbed power is 175 W and
efficiency is about 35% in this case. As the voltage continues to decrease, the
effective capacitance begins to decrease as expected. The electrical energy consumed
per voltage cycle E and the plasma absorbed power P can be estimated by the
following relations [Wagner, H.E., et al. 2003]:
( ) area of (Q-V) diagram
E
Ñ
V t dQ (1)P 1E fE
T (2) where f is the frequency of AC-pulsed voltage.
3.2 Temperature Analysis of APPJ
This section is to explore for gas (Air) in plasma activation analysis of
temperature with or without stage. By changes the distance from the jet, through
the temperature of changes in the relative strength of the APPJ on qualitative
analysis, this experiment used by the measurement tools for thermocouple.
3.2.1 Air temperature distribution without stage
The temperature distribution without stage means free jet in flow field. The
figure 3.7 is represented by X and Z direction. Figure 3.8 and table 1 are two
dimensional temperature distributions of the APPJ. It is under conditions of air with
50slm (Power= 500W). The maximum value of temperature is 78℃ which is
observed at X= 35mm but the temperature distribution of X axis is relatively uniform
with slightly clear edge effect. Also it can be seen the temperature has decreased with
the increase in Z-axis distance.
3.2.2 Air temperature distribution with stage
Table 2 shows air temperature distribution with stage which is the same
conditions above-mentioned. It can be seen more higher than temperature distribution
without stage. The increase of the temperature is mainly due to the heat accumulation
on the stage. Although the temperature is much higher, it is not over the limit of
material.
3.3 Images and Spectral Analysis of APPJ
Figure 3.9 shows the typical image of post-discharge region for N2, N2+0.03%O2
and N2+0.1%O2 discharges at the condition of 50 slm and 500 W. For pure nitrogen
dischrage, the plume looks longer (~ 2.5 cm) with yellow to orange color, while it
becomes shorter (~ 1.5 cm) with green color with 0.03% of traced oxygen addition.
And it becomes shortest (~ 1 cm) with blue color with 0.1% of traced oxygen addition
finally. These color changes can be clearly explained from the OES measurement, as
typically shown in figure 3.10-3.12, which shows the optical emission spectrum in the
range of 180-900nm for the post-discharge plasma.
UV emission along the channel exit (X-direction) is relatively uniform ether N2
or N2+0.1%O2 considering the experimental uncertainties of OES in figure 3.13.
Besides, UV emission (180-280nm) is greatly enhanced by adding only trace of O2
(0.1%) into the N2 discharge (figure 3.14). However, further addition of O2 (1.6%)
greatly reduces the UV emission, which is probably caused by the electro-negativity
of O2 plasma. Underlying plasma chemistry should be studied using more detailed
measurement or simulation in the near future (figure 3.15, 3.17). In addition, visible
emission, especially in the range of 450-550nm, is greatly enhanced by adding trace
of O2, which causes the yellow-orange plume to become blue as observed in figure
3.16.
UV emission (200-300nm) decreases in the downstream direction as expected
(figure 3.18). And near-UV emission (300-400nm) also decreases further downstream
(figure 3.19).
3.4 Contact Angle Measurements
The change of wettability is usually characterized experimentally by the contact
angle θ, which is formed on the solid surface along the linear solid-liquid borderline
of air in figure 3.20. An increase of wettability or making a polymer more hydrophilic
leads to a decrease of the contact angle.
In this work, distilled water was used as the working liquid. The values of the
static contact angle shown here were the average of five measured values obtained
using Laplacian curve fitting based on the imaged sessile water drop profile, with the
drop sizes of 2 l .
3.4.1 Contact angle measurements of stationary PP film
The changes of the contact angle are presented in figure 3.21-3.22 and table 3 &
4, respectively. It show the evolution of the contact angle data measured on the
polypropylene (PP) film treated in the DBD nitrogen based environment, for various
working gases, as a function of the Z axis after treatment. In the same manner, after
fixing up other plasma treatment conditions except Z direction, the change of the
contact angle to plasma treatment distance was measured with increasing the distance
from 2 to 20 mm. The contact angle observed is found to 103o for the untreated
sample to the lowest value 23o found associated with 6 mm treated sample except
N2+1.6%O2. The contact angles measured over an extended area of the treated sample
show a dispersion of about ±5°, which is within experimental error. It can be seen that
there is an optimal distance rather than closer from the jet. Consequently, the surface
modification of PP film by atmospheric pressure plasma is the most ideal at Z=
6~8mm.
3.4.2 Contact angle measurements of moving PP film
The changes of the contact angle are presented in figure 3.23 and table 5 & 6,
respectively. After fixing up other plasma treatment conditions except various
working gases, the change of the contact angle to plasma treatment gas was measured
with increasing the percent from 0.05% to 1.6% when stage velocity was 0.5cm/s in
table 5. The contact angle corresponding to treatment in N2+0.1%O2 is evidently
small, indicating excellent adhesion properties. In the same manner, it measured
contact angle which 0.1% to 0.8% when stage velocity was 1cm/s to 8cm/s. Figure
3.23 and table 6 shows the evolution of the contact angle data measured on the
polypropylene (PP) film treated in the DBD nitrogen based environment, for various
working gases, as a function of stage velocity after treatment. We observe that results
indicate contact angle corresponding to treatment in N2+0.1%O2 is also evidently
small, due to hydrophilization, by incorporation of polar functional groups. After
increasing stage velocity that was 2cm/s, the contact angle obviously large than
velocity was 1cm/s.
3.4.3 Aging effect
The lower contact angle of the plasma-processed PP film is then found to
partially recover following aging of the samples in the air. At longer aging times, the
contact angle increases more slowly and finally reaches a plateau value. As can be
seen in figure 3.24, the increase in contact angle is the higher for the treated sample
with moving PP film case and the lower for the N2+0.1%O2 plasma treated sample
after the one day of aging.
3.5 AFM analysis
The physical modifications occurring on the PP surfaces during plasma treatment
can be detected using AFM. The conditions of nitrogen plasma treatment were fixed
at 500W and 50slm. Only distance from jet was changed from 2 to 20mm.
As shown in table 7, at first, the surface of untreated is more roughly. Gradually,
it is seen that the RMS roughness decreases with plasma processing distance from
40.369nm for the untreated surface to 19.628nm for Z is 2mm treated surface.
3.6 XPS analysis
To analyze the change of chemical compositions on the PP film surface and the chemical binding state, XPS (ESCA PHI 1600, A1/Mg dual anode, 1486.6eV &
1253.6eV) was used. We compared untreated PP (a) with N2+0.1%O2-treated PP at
Z= 6mm (b), as shown in Figure 3.25. The plasma treatment conditions of 5s, 500 W,
and 50slm which showed the lowest value of the contact angle, were used for
preparing a sample (b).
In table 8 shows quantitative atomic percent concentration and ratio of untreated
and plasma treated sample. The obvious change between untreated and N2+0.1%O2 at
Z= 6mm is that oxygen contents of 10.06% in the untreated PP film increased to
oxygen contents of 36.46% for plasma-treated PP film. Namely, after plasma
treatment, the percent ratio of O/C increased from 0.125% to 0.898%. From Table 7,
we can confirm that the polar functional groups containing oxygen are introduced on
the PP film surface. In figure 3.25 presents the survey spectra of the untreated PP (a)
with N2+0.1%O2-treated PP at Z= 6mm (b). From figure 3.25, we can know that C1s
peak decreases a little while O1s peak relatively increases.
3.7 O
3and NO
2measurement
Figure 3.26 shows the O3 concentration measured using ozone analyzer (Model
450) as a function of the flow rate ratio of O2 to N2 at different distance from jet. It
can be seen the highest concentrations of ozone when Z is 8mm. We also observed
that the O3 concentration closely between 12mm to 20mm. Besides, the O3
concentration increased linearly with the increase in the O2 and N2 ratio except
N2+0.04%O2 to N2+0.09%O2. In this plasma, dense O3 and contact angle degree are
present in the same region. O3 has a weak absorption band in the contact angle degree
(N2+0.04%O2 to N2+0.09%O2). The region of contact angle degree is overlapped with
the absorption band of O3.
Figure 3.27 shows the NO2 concentration measured using multi gas monitor as a function of the flow rate ratio of O2 to N2 at different distance from jet. We observed
that highest concentrations of NO2 when Z is 8mm. It also can be seen NO2
concentration closely between 12mm to 20mm. But it was different from O3
concentration of various nitrogen based gases that the contact angle degree and O3
concentration were dependent. The NO2 concentration of various nitrogen based gases
was independent of contact angle degree.
3.8 Surface reaction mechanism for PP
PP is a saturated hydrocarbon polymer with a carbon backbone containing hydrogen and methyl (–CH3) groups arranged in an alternating fashion in figure 3.28.
The reactivities of the hydrogen groups in PP depend on the nature of the C atom to
which they are attached. There are three types of C atoms in any given monomer unit
of PP, primary C, to which only one C atom is bonded; secondary C, to which two C
atoms are bonded; and tertiary C, to which three C atoms are bonded (see figure ). In
general, the reactivities of hydrogen bound to these C atoms scale as: Htert > Hsec >
Hpri [Dorai R. and Kushner M. J., 2003].
In Figure 3.29, the change which can occur on the PP film surface during plasma
treatment is simply presented by three steps. It is likely that PP film surface forms
cross-linked network structure or is oxidized by the mechanism, as shown in Figure
[Kwon OJ., et al. 2004].
At first step, because the stability by the electrondonating effect of the near alkyl
group is the highest in the tertiary carbon atom, the tertiary carbon radical is quickly
formed by the abstraction of the tertiary hydrogen. The quickly formed tertiary carbon
radicals react with radicals in the near polymer chain. And then cross-linked network
structure is formed on the polymer surface [Kwon OJ., et al. 2004].
At the second step, after the hydrogen on the secondary carbon is abstracted by
plasma, oxygen containing functional groups, such as –C–OH, –CO–OH, and –C=O
are introduced on the polymer surface. It is considered that surface oxidization by
plasma is mainly occurred at secondary carbon site in this step [Kwon OJ., et al.
2004].
At the third step, there are two possible reactions. One is that the radical
generated by the abstraction of tertiary methyl group forms three dimensionally
cross-linked structures by reacting with the near polymer chain. The other is that the
process of surface oxidization proceeds a little more by the abstraction of hydrogen in
the methyl group [Kwon OJ., et al. 2004].