Microwave Discharges: Fundamentals and Applications
Microwave discharges: fundamentals and applications
Plasma chemical activity of non-equilibrium microwave discharges is analyzed. Non-uniformity is the inherent property of majority of electrical discharges and microwave discharges are no exception. Reasons of non-uniformity of microwave discharges are analyzed. Peculiarities of physical-chemical processes in strongly non-uniform microwave discharges are demonstrated placing high emphasis on the influence of small gas additions to the main plasma gas. This problem is directly related to the possibility of using actinometry-method. For the first time, the author outlines the economic theory through the View Product.
Bibliography of Microwave Optical Technology. Although microwaves and coherent optics, being two of the largest and most useful branches of Although microwaves and coherent optics, being two of the largest and most useful branches of electrical engineering to emerge technologically, are usually considered as distinct subjects, many of the underlying fundamental principles, scientific achievements, and practical applications have common features.
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10 th International Workshop on Microwave Discharges: Fundamentals and Applications
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In the experiment, the initial pH of 5. It has to be mentioned that the amount of water in the Petri dish was reduced during the treatment due to evaporation and nebulisation. Therefore the slope of the curve in Figure 10 left is not linear. The correlation of acidification and antimicrobial effects is shown in the right diagram of Figure In this experiment an initial number of B.
It is known that nitride oxides in contact with wet films or droplets lead to an acidification through different mechanisms. Another possibility are reactions or processes involving hydrated ions. However, the interaction between a plasma and a liquid is quite complex and further investigation are necessary on this issue. The basic scheme of the plasma source here described and the treatment process is given in Figure 11 Fig. The device consists of a multimode waveguide structure which serves as process chamber and an ignition device which is mounted on a moveable lance.
The microwave radiation is generated by a magnetron frequency 2. The alignment of the magnetron frequency to the process chamber geometry is performed via a moveable shorting plunger. The PET bottle is placed in the centre of the process chamber, the lance with the ignition device is driven into the bottle and by applying the microwave field the plasma is ignited in their bottom region see Figure 11 Fig.
After the ignition of the plasma the lance is moved to its initial position outside of the bottle and the plasma moves freely to the neck of the bottle [[ 10 ]. The plasma is generated in ambient air i. The shape of the plasma is predominantly given by the field configuration in the process chamber, respectively the modes which can propagate in the waveguide. Therefore simulations of the field distribution in the device by means of the CST program MicroWave Studio allow a rapid development and optimization of new designs.
A very stable plasma ignition at atmospheric pressure is performed by an ignition pin similar to the electrode microwave discharges, described e. This is mainly driven by convection. After the launching the plasma expands and moves self propagating through the waveguide and thus the hollow compartment.
In Figure 12 Fig. The plasma luminosity in the centre of the process chamber as a function of position and time is shown in the left plot of Figure 12 Fig. At certain times, the snapshot of the full waveguide is shown plots at the right.
As can be seen in the snapshots, the propagating plasma has a diameter of about 40 mm and a length of 50 to 80 mm. The propagation velocity is about 0. The modulation of the plasma propagation and luminosity is clearly seen in the left plot of Figure 12 Fig. In the vicinity of the ignition pin the plasma is localized and expansion takes place after launching during the movement to the antenna.
The velocity of the plasma propagation can be controlled by the amount of absorbed power in the plasma which heats the plasma and leads therefore to convection. Beside this movement by material transport there is also a propagation of the plasma state. This is caused by the strong absorption of the microwave field in the front of the plasma and therefore a high ionisation rate in this plasma region is generated [ 12 ].
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This effect can be easily demonstrated by coupling the microwave power from the bottom of the process chamber. Then, the plasma starts after ignition to move towards the microwave source. During this movement the plasma expands and as soon as the convection force becomes dominant, the direction of movement changes.
The microwave field distribution has to be adapted to the specific compound and purposes of the treatment; otherwise a local thermal deformation of the bottles occurs. Optical emission spectroscopy was carried out to characterize the plasma. The radiation emitted by the plasma is coupled to an optical fibre leading to an optical multichannel analyzer spectrometer with a ICCD-camera as detector. The overview spectrum of the UV-range is shown in Figure 14 Fig. For the conditions being considered high pressure and thus high collision rates T rot is close to the gas temperature.
To estimate T rot , the OH-spectrum at nm is measured with a resolution of about 0.
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Although this method has previously been shown to be unqualified to examine small temperature gradients in a plasma, the described approach is a simple and easy method to get information on the gas temperature [ 17 ]. This result is in agreement with the observed propagation of the plasma forced by convection. Although the temperature of the microwave excited plasma is such high, the treatment of plastic materials with an upper limit for the treatment temperature of about K is possible. This effect may be explained by the diffuse shape of the plasma with a low heat capacity and the fast movement of the plasma.
The propagating microwave plasma has many similarities to plasma torches driven at atmospheric pressures [ 14 ]. NO is formed due to the plasma chemical reactions at high power input [ 18 ].
Thus, the following possible lethal components of the plasma can be identified namely i UV-radiation, ii low molecular chemical reaction products mainly nitride oxides NO , iii reactive oxygen species O-atoms and OH-radicals and heat. Furthermore the NO quickly forms other species by reaction with oxygen molecules present in the background air gas. This is confirmed by FTIR-analysis of the residual gas in the plastic bottles several minutes after plasma treatment see Figure 16 Fig. The dominant species present in the residual gas is nitrogen dioxide NO 2 , which is formed via reaction of NO and O 2.
About ppm of NO 2 is present and visible as brown gas in the plastic bottle. NO 2 is one of the most prominent air pollutants, a poisonous gas and therefore another microbicidal component. NO 2 is partly converted to nitrogen peroxide N 2 O 4. Acidification can be a candidate for antimicrobial effects. The formation of nitrous oxide N 2 O is of minor importance.
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Thus, the microwave discharge has the ability to perform a very effective decontamination. For microbiological tests, microorganism containing dilution Escheria coli, Staphylococcus aureus or Aspergillus niger is sprayed into the plastic bottles and dried under aseptic conditions.
After plasma treatment the bottles are scrubbed with 10 ml of a rinsing solution water with NaCl and Tween The resulting elution undergoes the classical micro-biological test procedures count plate method or membrane filtration. The antimicrobial effect of the plasma is demonstrated in Figure 17 Fig. Depending on the microorganism, 5 to 7 orders of magnitude of reduction are observed. Note that the total plasma-on time is about 1.
Therefore, with respect to further needed improvements, the device is applicable in an in-line aseptic filling procedure. In order to simulate the filling process the time between plasma treatment and the elution more precisely between the end of the last plasma cycle and filling of the bottle with scrubber solution was varied. The results of this experiment are shown in Figure 18 Fig. The reduction of vegetative bacterium S.