This section will analyse if the used hydrogen flow, porosity of the iron oxide powder, the presence of water in the cavity or small leaks could influence hydroxyl emission peaks. Conclusions on the influence of these processes will be supported by measurement results. In this way, the first sub-question of this report can be answered adequately.
4.4.1 Hydrogen flow
The hydrogen flow which is set can influence the spectrum as well. A part of the spectrum is measured for different flows of hydrogen gas; the pressure is kept as constant as possible in this experiment. The result is give inFigure 4.7.
Figure 4.7: The measured spectra for different flows of hydrogen. The pressure is kept constant. The flow is depicted in sccm (standard cubic centimetres per minute, that is, at 1 atm and 273 K), with an uncertainty of 0.5 sccm for each flow. Left: Enlarged spectrum between 300 and 325 nm. Right: Wide spectrum.
Apart from the spectrum at a flow of 8 sccm (which is probably an incorrect measurement) the baselines of the spectra match very well. If a closer look is taken to the region containing the hydroxyl peak (around 310 nm), it can be seen that the nitrogen peak at 315 nm is strongly influenced by the flow of hydrogen. This may have to do with small leakages which are still present in the cavity through which nitrogen can leak to the inside. The pressure is kept constant.
When the flow of hydrogen is low, the flow of nitrogen leaking inside the cavity is relatively large compared to the flow of hydrogen. For high flows of hydrogen, the nitrogen leakage flow is relatively small.
In the experiments, the flow can be set digitally and will stay substantially more constant than the pressure (approximately 10% versus 100% relative fluctuations). Therefore, in the experiments, it is assumed that the flow of hydrogen is constant and that it will not change much in the measured spectra. In the experiments, a hydrogen flow of 10 sccm is used, unless indicated otherwise.
4.4.2 Porous iron oxide powder
Another process that could contribute to the creation of hydroxyl radicals is the presence of water in porous iron oxide powder. Porous iron powder is often obtained after the combustion of iron particles [44]. After combustion, and during storage the iron oxide powder comes in contact with the humid air in the environment. The water in the air could be absorbed by the porous iron oxide powder, which in turn can dissociate to hydroxyl radicals when present in the hydrogen plasma.
To determine if there is water present in the porous iron oxide particles, an experiment involving a TGA (as described insubsection 3.1.3) is conducted. A sample of iron oxide is placed in the small bucket which is continuously weighed and conserved in a nitrogen environment with a relative humidity of 0 at a temperature of 26.4 ± 0.1◦C for 4 hours. If water is present in the porous iron powder, this will slowly evaporate, and the total sample mass will decrease. The results of this experiment are given inFigure 4.8.
Figure 4.8: The evolution of the mass of the sample of iron oxide as a function of time.
The graph on the left displays the results of the magnetite, the graph on the right displays the results of the mixed iron powder.
It can be seen that the mass of the magnetite powder decreases by approximately 15 µg. Com-pared to the mass of the sample, which is around 11.7 mg, this is quite low. Using these masses and the molecular weight of water and magnetite, it can be calculated that this decrease in mass corresponds with 5 · 1017 water molecules. The sample contains 3 · 1019 molecules of magnetite.
Therefore, there are 60 times more magnetite molecules than water molecules. However, also low concentrations of water can change the plasma characteristics.
The measurements on the mixed powder have more remarkable results: its mass increases over time, which is hard to explain. This could be caused by the bucket in which the sample is con-tained oscillates. Thus, the possible contamination of the powder with water has to be considered when this powder is used.
Another issue that is possible is the presence of air (in particular nitrogen) in the porous iron oxide. If the spectrum on the right in Figure 4.2(in which the cavity only contains a hydrogen plasma) is compared withFigure 4.9(in which the cavity contains both the hydrogen plasma and iron oxide), it can be seen that the nitrogen emission peaks (as inFigure 4.3) are slightly higher in Figure 4.9, relative to the hydrogen emission peaks. This indicates that there probably is a small amount of nitrogen gas present in the porous iron oxide.
Figure 4.9: The full emission spectrum of a hydrogen plasma with the mixed iron powder.
4.4.3 Other causes
Apart from the fact that the pressure, hydrogen flow and porous iron oxide particles can cause changes in hydroxyl emission peaks, there are some other causes to which a change can be assigned:
1. The vacuum chamber always contains a little bit of water, because it is opened regularly to take the cavity out (to observe or replace the iron powder for example) and humid air can enter the vacuum chamber. It is also aerated in this way. This water can then dissociate to hydroxyl radicals after the discharge of the hydrogen plasma, and therefore cause an increase in hydroxyl emission peaks, independent of the cavity or the sample it contains. This can be prevented by heating the vacuum chamber to over 100◦C, so the water evaporates and leaves the chamber. However, this is a task which costs much effort to do and will not evaporate all of the water. This measurement is not taken before conducting the measurements in this report.
2. Even after the helium leakage test, there could be (very) small leakages present in the vacuum chamber. Air could dissipate through these small holes. Thus, nitrogen, oxygen and possibly even a small amount of water can enter the vacuum chamber. Nitrogen in the chamber could cause the nitrogen emission peaks to rise and could disturb the ratios between a nitrogen and a hydroxyl emission peak in the next sections. Oxygen could recombine with the hydrogen radicals in the plasma, and could potentially form hydroxyl radicals, which would influence the height of the hydroxyl emission peak, which would rise as well.
In conclusion, the hydrogren flow used in the experiments stays constant, and is assumed not to influence the hydroxyl emission peaks. The number of water molecules in the porous iron oxide is 60 times less than the number of iron oxide molecules. However, this small amount could still influence the plasma characteristics and therefore its emission peaks. Also, the presence of water in the cavity could not be ruled out. Small leaks may also still be present, although before the experiment several leaks have been stopped.