Chapter 1. Introduction
1.3 Specific Objectives and Organization of the Thesis
Based on the preceding discussion of the related studies of DBD atmospheric-pressure plasma, it was clear that further experimental study was needed to provide a better fundamental understanding of the properties of DBD atmospheric-pressure plasma and will thus lead to more effective applications.
Therefore, the specific objectives and organization of this thesis were summarized as follows:
1. To develop a parallel-plate DBD operating under the atmospheric-pressure condition that is driven by a high-voltage bipolar quasi-pulsed power supply.
(Chapter 2)
2. To study nitrogen-based and air-based DBD planar plasmas, which include measuring the electrical properties of the discharges, the spectrum intensity distributions and temperature distributions in the post-discharge region, to name a few. (Chapter 3)
3. To interpret the measurements in Item No. 2 to understand the plasma physics in the discharge and post-discharge jet regions. (Chapter 3)
4. To study the surface modification of PP film by applying the post-discharge jet region of a nitrogen-based planar DBD. The effects of oxygen addition into the nitrogen DBD and treating distance between the DBD and PP film on the hydrophilic properties were investigated, including the surface hydrophilic properties, roughness and chemical composition (elements and functional groups). Optical emission intensity from the excited species and ozone concentration in the post-discharge region were measured and then used to explain the measured contact angles for the stationary and non-stationary PP
films. (Chapter 4)
5. To study nitrogen, DBDs with and without the addition of trace oxygen were then applied to clean stationary and non-stationary ITO glass using the post-discharge jet region at different treating distances. Measurements of the concentrations of several reactive species at various spatial locations under different levels of oxygen addition to nitrogen were then used to elucidate the cleaning process. (Chapter 5)
6. To apply a parallel-plate DBD atmospheric-pressure plasma jet (APPJ) to inactivate two typical bacteria, E. coli and B. subtili, in the post-discharge jet region. Various inexpensive working gases, including pure N2, pure O2 and compressed air, were tested and the discharges were characterized accordingly.
(Chapter 6)
7. To apply a parallel-plate DBD atmospheric-pressure plasma jet (APPJ) to inactivate B. subtilis spore in the post-discharge jet region. Various working gases, including compressed air and air mixed with Carbon Fluorine (CF4), were tested and the discharges were characterized accordingly. (Chapter 7)
Chapter 2
Experimental Methods
2.1 Test Facility
The test facility for a planar DBD APPJ measurement in this study included: a planar DBD APPJ, cooling system, distorted sinusoidal voltage (quasi-pulsed) power supply, gas supply system, venting chamber and non-stationary stage. Table 2-1 summarizes various components of the DBD system. Each of these arrangements is described in the following in turn.
2.1.1 Planar DBD APPJ
Figure 2-1 illustrates the schematic diagram of a parallel-plate DBD atmospheric-pressure plasma jet (APPJ) along with a gas supply system and the instrumentation for voltage and current measurements. This APPJ consisted of two parallel copper electrodes (50 × 50 × 8 mm each) with embedded cooling water. Each electrode was covered with a ceramic plate 70 × 70 × 2 mm for the inactivation/sterilization application, and a quartz plate 70 × 70 × 1 mm for the surface hydrophilic modification application. The 5 mm dielectric plates extruded from the end of the electrodes (in the flow direction), and prevented the electrode assembly from arcing. The distance between the two dielectric plates (ceramic/quartz) was 1 mm throughout the study. The assembly of electrodes and dielectrics was then covered by a Teflon insulation layer to provide safety and prevent arcing problems during operation.
2.1.2 Cooling System
The water flow of the cooling electrode was delivered by a cooling system. The inlet cooling temperature control was 20 ± 2℃ for the electrode and the water pressure control was 1.6 ± 0.1 kg/cm2. The high voltage and ground electrode used separate cooling pipes which are controlled separately. The diameter of the cooling pipe was 1/4”.
2.1.3 Distorted Sinusoidal Voltage Power Supply
This DBD assembly was powered by a distorted sinusoidal voltage (quasi-pulsed) power supply (Model Genius-2, EN Technologies Inc.). This power supply facilitated the adjustment of the frequency (20~60 KHz), power density (low/middle/large), peak current (max. 4A), peak voltage (max. 15 kV), and power (max. 2 kW). The distorted sinusoidal voltage power facilitated generation of high voltage with high dv/dt to enhance the discharge intensity and thus radical generation. Another feature of the output electrical waveform is the distorted sinusoidal voltage waveform which can minimize the occurrence of arc (or streamer) to ensure stable power output according to asymmetric load and sudden load change caused by the plasma.
2.1.4 Gas Supply System
Various working gases flowed between the parallel plates, including N2 (99.99%), O2 (99.99%), O2/N2 (0.004-1.6%), O2 (99.99%), compressed air (produced from an oil-less compressor) and CF4/air (2% of CF4). The flow rates were controlled by manually adjustable flowmeters. The gas was introduced through two holes at the top of the parallel plate assembly and then passed through a sieved aluminum plate containing 480 holes (0.5 mm in diameter each) for pressure redistribution, followed by a convergent section with a length of 10 mm to coincide with the channel gap size (1 mm)
at the end of the section.
2.1.5 Venting Chamber
In setting up the planar DBD APPJ in a venting chamber for operation safety, the cylindrical chamber size was D600 × L700 (mm), as shown in Figure 2-2. The planar DBD APPJ was mounted on the top side of the chamber. The exhaust pipe (diameter 210 mm) was mounted on the bottom side of the chamber. The venting flow was produced by a variable blower which was driven by a 1 HP-3phase induction motor (Tatung EBFC) with frequency converter (TECO 7300CV, 1~60 Hz). The blower (frequency 8 Hz) remained on during the experiments as the flow is introduced into the chamber.
2.1.6 Non-Stationary Stage
For the surface treatment, the distance between the bottom edge of the planar DBD and the sample varied in the range of z=2~20 mm; “z” denotes the coordinate in the downstream direction measured from the bottom edge of the DBD assembly throughout the thesis. The sample was either stationary or transported by a pre-programmed non-stationary stage (Unice E-O Service Inc., U-S1-D0-H080378, maximum traveling distance 20 cm, USB driver). The stage plate size was 10×10 cm. The non-stationary variable speed of the sample passing the DBD jet was in the range of 1-9 cm/s. We could preset the passes of the sample treatment using the pre-programmed non-stationary stage.
2.2 Instrumentation
The instruments for a planar DBD APPJ measurement in this study included thermocouples for post-discharge gas temperature measurements, the measurement of
electric properties, the OES for optical spectral measurements, the FTIR for gas and surface sample analysis, contact angles for surface energy measurements, AFM for surface profile measurements, SEM for bacteria morphological observation, visualization of planar DBD APPJ and the sample preparation of bacteria. Table 2-2 summarizes various instruments which were used for the measurements. Each of these arrangements is described in the following in turn.
2.2.1 Thermocouples for Post-Discharge Gas Temperature Measurement
The gas temperatures in the post-discharge jet region were measured using a K-type (-50-500 ℃ ) thermocouple, which was fixed on a non-stationary stage. The thermocouple was connected to a digital indicator (Brainchild BTC-900) for temperature display.
2.2.2 Measurement of Electric Properties 2.2.2.1 Current and Voltage Waveforms
Input voltage and output current waveforms across the electrodes of the parallel-plate discharges were measured by a high-voltage probe (Tektronix P6015A) and a current monitor (IPC CM-100-MG, Ion Physics Corporation Inc.), respectively, through a digital oscilloscope (Tektronix TDS1012B). The current monitor was of the Rogowski coil type. The output sensitivity was 1 volts/Amp, and the diameter of the hole was 0.5 inch to suit the insulating power cable. The Rogowski coil used for fast current changing measurement was proved to be better than the Hall-effect device in terms of sampling speed and accuracy.
2.2.2.2 Power Absorption Estimation based on Lissajous Figure
Plasma power absorption was measured by the technique of the “Lissajous figure”
(Q-V characteristics) [Wagner et al., 2003] using a capacitor with a capacitance of Cm=6.8 nF and a voltage probe (Tektronix P2220).
Figure 2-3 shows the typical Lissajous figure obtained for a DBD. The shape of the Q-V curve is a distorted version of the standard parallelogram [Wagner et al., 2003]
observed in a DBD driven by a sinusoidal AC power source. The electrical energy consumed per voltage cycle, E, and the plasma absorbed power, P, were estimated by the following relations [Wagner et al., 2003]:
E=∮V(t)dQ ≡ area of (Q-V) diagram (1)
P 1 E fE
=T = (2) where f is the frequency of the distorted sinusoidal voltage.
2.2.3 OES for Spectral Measurements
The spectral optical emission intensities of the APPJ were measured using a monochromator (PI Acton SP 2500) with a Photomultiplier tube (Hamamatsu R928), which was mounted on a mobile 3-D table (see Figure 2-4). The spectral range was 180~900 nm with 1200-g/mm grating (Holographic, 300-nm Blaze and 500-nm Blaze).
When the center wavelength of the emission line is 400 nm the linear dispersion is 1.489 nm/mm. Both sides of the fiber (Ocean Optic. BTW-FPC-600-0.22-1.5-UV, 600 μm) were separately connected to the SMA adapter of the monochromator and the collimating lenses (Ocean Optics 74-UV).
2.2.3.1 Species Identification
Measured optical spectra were used to determine the types of species in the
discharge and post-discharge jet region. In addition, the absolute intensity was proportional to the number density in the region of interest. One aspect of OES measuring is identifying which particle species emits radiation. From the line position, the wavelength is a characteristic of an element/compound; it is sufficient to use a wavelength calibrated (survey) monochromator in combination with wavelength tables for atoms, ions and molecules [Fantz 2006]. A spectroscopy software package, named Plasus SpecLine, was used to evaluate the spectral data, i.e. finding specific lines in the spectra (ex. Figure 2-5) and excited species level change (ex. Table 2-3), and identifying unknown peaks or comparing data from the OES measurements.
2.2.3.2 Gas Temperature Measurements
Metastable states of molecules play an important role in different discharges due to their ability to accumulate a great amount of energy, which can be effective in various chemical and physics processes. For example, from measured line intensities, the concentrations of NO as functions of the average discharge power and estimations of the electron density in the discharge could be derived. Variations in the measured rotational temperature of NO-γ(245-249 nm) can be attributed to changes in the gas heating and, consequently, the gas dynamics in DBD. Since the collision frequency is very high in a DBD under atmospheric-pressure condition, we can assume thermal equilibrium between translational and rotational temperatures. Therefore, in the spectrum of NO- γ transition, the rotational distribution of excited emission (N2(A3∑u+)+NO X( 2∏ )→NO A( 2∑+)+N2(X1∑g+)) corresponds to the ambient gas temperature [Bibinov et al., 2000]. We used LIFBASE [Luque et al., 1999] to simulate gas temperature in the discharge region (ex. Figure 2-6) throughout the study.
2.2.3.3 2nd Order Radiation in OES spectra
Diffraction gratings produce a multiple order of diffracted light where constructive interference permits the light of one wavelength to appear at more than one angle of diffraction. For example, 236 nm light passing through a monochromator normally appears as 236nm “first order” light. Some of the 236nm light will also appear at 472nm as “second order” light and so on. We removed second order emission lines from measured OES spectra by long-pass filter verification. See Appendix A for details.
2.2.4 Measurement of Ozone Concentrations 2.2.4.1 Ozone Monitor
The concentrations of ozone were measured using an ozone monitor (API Model 450) which was mounted on a mobile 3-D table in the post-discharge jet region. The detection of ozone molecules is based on the absorption of 254 nm UV light due to an internal electronic resonance of the ozone molecule. The API Model 450 used a mercury lamp so that a large majority of the light emitted was at the 254nm wavelength. The operation range was 1 to 1000 ppm. The O2/N2 (<1.6%) DBD plasma produced a concentration of ozone well under 1000 ppm based on our measurements. However, as the oxygen exceeds some amount, the concentration of ozone is over the range of the ozone monitor and cannot be measured properly.
2.2.4.2 FTIR
Infrared spectroscopy exploits the fact that the chemical bounds of molecules have specific frequencies at which they rotate or vibrate corresponding to discrete energy levels (vibrational modes). In order for a vibrational mode in a molecule to be IR active, it must be associated with changes in the permanent dipole. Ozone belongs to the dipole molecule (106.8°). In pure O2 and compressed air DBD plasma, the ozone concentration
is over 1000 ppm, which renders the above-mentioned ozone monitor inappropriate.
The ozone infrared spectrum absorbance of the APPJ was measured using a FT-IR spectrometer (Bruker Tensor 27). For example, Figure 2-7 shows the infrared absorption spectra (800-3600 cm-1) of the planar air DBD APPJ. A strong absorption peak of 1055 cm-1 for the ozone was clearly observed in the compressed air discharges because of abundant oxygen species in the discharges. We defined the 1055 cm-1 line intensity of IR spectrum as the relative ozone concentration in the post-discharge jet region.
2.2.5 Contact Angles for Surface Energy Measurement
The hydrophilic property (contact angle) of the PP film and the ITO glass surface were measured using a contact angle machine (KRŰSS GH100) with a 2 µL drop of de-ionized water placed on the glass surface using a micropipette. The values of the static contact angle were obtained using Laplace-Young curve fitting to measure the 10 average values in the plasma treatment region.
2.2.6 AFM for Surface Profile Measurement
The AFM images for measuring the roughness of the sample surface were obtained using a Veeco Dimension 5000 Scanning Probe Microscope (D5000).When the probe scan is near the sample surface, the probe with sample interaction force reflects testing-arm deformation through laser-beam detection and feeds the information to a PC.
The Z-axis information showed the roughness of the sample, and the X-axis and Y-axis showed the AFM scanning area. Three sets of data can be output: 2D/3D morphology, step profile, and roughness analysis.
2.2.7 SEM for Bacteria Morphological Observation
The bacteria of the control (untreated) and the plasma-treated samples were rinsed with distilled water and fixed with 2.5% glutaraldehyde solution overnight. The fixed bacteria were dehydrated in a series of graded acetone concentrations and then dried in a CO2 atmosphere (HCP-2, Hitachi, Japan) under critical conditions, and then coated with a thin layer of gold. The effects of the plasma treatment on the structure of the bacteria were examined using a scanning electron microscopy (Hitachi S-470 type II, electron voltage: 5kV, amplification factor: 30k).
2.2.8 XPS for Surface Chemical Composition
The surface chemical composition of the PP film and ITO glass before and after application of the AP plasma jet was measured using an X-ray photoelectron spectrometer (PHI Quantera SXM, Scanning Monochromated Aluminum anode, chamber pressure below 5×10-10 torr for ITO glass; ESCA PHI 1600, Mg anode at 250W and 15KV, 1253.6 eV, the electron take-off angle respect to the PP film surface was 45°, chamber pressure below 2×10-8 torr for PP film).
2.2.9 Visualization of Planar DBD APPJ
The different discharge images of the planar DBD APPJ were taken using a digital camera (Nikon D1H) and lens (Nikon Micro-Nikkor 105mm f/2.8). The image sensor was 12-bit RGB CCD with 2.74 million pixels. The typical images of the post-discharge region for DBD plasma were taken after 0.2 sec (side view: width 1mm) and 2 sec (front view width: 50 cm) of exposure. The typical images of the discharge region for DBD plasma were taken after 0.2 sec of imaging.
2.2.10 Sample Preparation of Bacteria
2.2.10.1 E. coli and B. subtilis
E. coli (BCRC 13014) and B. subtilis (BCRC 14716) cultures were grown in 100 mL of nutrient broth which was maintained for 12 h and 18 h at 37◦C, respectively. This allowed the bacteria to reach the exponential-log phase. Bacteria (10 mL) were harvested and transferred from the broth under sterile conditions to the phosphate buffer solution (90 mL and pH 7.0). The solution was then diluted further to the required concentration level (3.2×107 CFU/mL), which was very high as compared to previous studies [Sun et al., 2007]. Then 0.1 mL of the diluted solution was added to sterile petri dishes (diameter 80 mm) containing 20 mL of nutrient agar, and spread on the central area of the petri dishes (4 cm × 4 cm) to ensure that all bacteria were treated by a plasma jet 5 cm in width. During the bacteria preparation, we made sure that the petri dishes were level in order to prevent any of the bacteria flowing out of the central area of the petri dish. The petri dishes with nutrient agar were exposed to the parallel-plate DBD APPJ following the planned test conditions, as will be shown later. After the plasma jet treatment, the petri dishes were incubated at 37°C for 24 h, prior to determining the resulting number of colony-forming units (CFU/mL) by NIH ImageJ [Sheffield et al., 2007]. This software can easily calculate the area of grown bacteria based on a pixel count, which resulted in the survival rate of the bacteria in the petri dish.
2.2.10.2 B. subtilis Spore
B. subtilis (BCRC 14716) precultures were grown in 100 mL of nutrient broth which was maintained for 18 h at 37℃; then 0.1 mL culture suspension was added to sterile petri dishes containing 20 mL of nutrient agar (beef extract, 3 g/liter; peptone, 5 g/liter;
agar, 15 g/liter; with the pH adjusted to 6.8±0.2). After being incubated at 37℃ for 2 weeks, the sporulated bacteria were collected by suspension in approximately 5 mL of
water per plate. The spores were washed five times by centrifugation and suspension in sterile water, and finally stored at 4℃ in water at concentrations of about 1.24×109 spores/mL, diluted according to the demand thickness of the experiment. Then, 0.1 mL of the diluted solution was added to sterile petri dishes (diameter 80 mm) containing 20 mL of nutrient agar and spread on the central area of the petri-dishes (4 cm × 4 cm) to ensure that all bacteria were treated by a plasma jet 5 cm in width. During the bacteria preparation we made sure that the petri dishes were level; otherwise, some bacteria would flow out of the central area of the petri dish. The petri dishes with nutrient agar were exposed to the parallel-plate DBD APPJ following the planned test conditions, as will be shown later. After the plasma jet treatment, the petri dishes were incubated at 37°C for 24 h, prior to determining the resulting number of spore (spore/mL) by NIH ImageJ, which was described earlier.
2.3 Test Conditions
The test conditions for the planar DBD APPJ measurements in this study included surface modification applications and bacteria sterilization/inactivation applications. Each of these test conditions has been summarized in Tables 2-4 – 2-7.
2.3.1 Surface Modification Applications 2.3.1.1 PP Film Modification
Various working gases flowed between the parallel plates, including N2 (99.99%
purity) and its mixture with trace oxygen (99.99% purity) with different volume fractions in the range of 0.004-1.6%. The flow rates were controlled by manually adjustable flowmeters. The total flow rate was fixed at 50 slm throughout the study. The output power from the power supply was fixed at 500 W (plasma-absorbed power: 175
W). For the PP film treatment, the distance between the planar-DBD APPJ bottom edge and the PP film was varied, while the PP film (0.31 mm in thickness, density of 0.91~0.92 g/cm3, Nan-Ya Inc.) was either stationary or transported by a pre-programmed non-stationary stage. The moving speed of the PP film was in the range of 0.5-8 cm/s throughout the study. The testing conditions of the PP film modification have been summarized in Table 2-4.
2.3.1.2 ITO Glass Surface Cleaning
Discharge gases, which included pure nitrogen (99.99%) and mixtures of gases with 0.004~1% of oxygen in nitrogen flowed from the top to the bottom between the parallel plates for the ITO glass surface treatment under the conditions of 60 kHz (power supply), 50 slm (flow rate) and 175 W (plasma absorbed power). In addition, the gas temperatures (measured by a K-type thermocouple) in the jet region (z=2~20 mm) were generally low in the range of 50-80°C under the typical operating condition, which was safe for ITO glass cleaning and other applications. The data on the contact angle for the stationary case presented here were obtained after 5 s of plasma jet stream impinging onto a stationary ITO glass. The non-stationary speed of the ITO glass passing the DBD jet was in the range of 1-9 cm/s. The testing condition of ITO glass surface cleaning has been summarized in Table 2-5.
2.3.2 Bacteria Sterilization/Inactivation Applications 2.3.2.1 E. coli and B. subtilis
Various working gases flowed between the parallel plates, including N2 (99.99%), O2 (99.99%) and compressed air (produced from an oil-less compressor). The flow rates were controlled by manually adjustable flowmeters. For clarity of presentation, all of
the results presented in this paper were performed under the conditions of 30 kHz (power supply) and 10 slm (flow rate). The petri dishes which contained the bacteria were transported by a pre-programmed moving stage. For the treatment of the bacteria, the distance between the bottom edge of the planar DBD and the bacteria varied in the range of z=4-20 mm. The moving speed of the petri dishes was kept at 1 cm/s and the number of passes of the DBD jet pass varied in the range of 1-18. Note that “a pass” is defined as the motion of the APPJ traveling back and forth over the petri dish. Resulting
the results presented in this paper were performed under the conditions of 30 kHz (power supply) and 10 slm (flow rate). The petri dishes which contained the bacteria were transported by a pre-programmed moving stage. For the treatment of the bacteria, the distance between the bottom edge of the planar DBD and the bacteria varied in the range of z=4-20 mm. The moving speed of the petri dishes was kept at 1 cm/s and the number of passes of the DBD jet pass varied in the range of 1-18. Note that “a pass” is defined as the motion of the APPJ traveling back and forth over the petri dish. Resulting