Magnetite and increased plasma powder

The final experiment aims to increase the power density even more and to use an iron oxide sample which is defined and characterised better than the mixed iron oxide sample which was obtained by the direct combustion of iron powder. The mixed iron oxide sample is a combination of different iron oxides (Fe2O3, Fe3O4and FeO), which can also have different crystal structures, for example, Fe₂O₃ can be α-Fe₂O₃, β-Fe₂O₃ or γ-Fe₂O₃. The hydroxyl radicals react with all these different iron oxides, but they do not have to do so in an evenly divided way. It could be that some iron oxides are reduced more easily than others. Moreover, an XRD measurement of a sample as can

be seen inFigure 3.8is a good qualitative measurement, but not a quantitative one. It cannot be derived from this XRD measurement in what proportions the individual iron oxides are contained in the mixed iron oxide.

For this experiment, the magnetite as described in section 3.3 is used. Magnetite has only one crystal structure [39] and the magnetite powder that is used in 95% pure. In this case, it is very clear what the properties are of the iron oxide powder that is reduced. In addition to this, a larger amplifier (at the place of the amplifier in Figure 3.1) with higher capacities is used, so the amplified signal can be made stronger, and the power (and therefore the power density) will be increased. Also, the cavity is filled up even more: 75% of the cavity is filled with disks of stainless steel (resulting in d = 10 ± 1 mm). This results in a power density of 3.9 W/cm³, which is significantly more than the values of 0.32 and 0.96 W/cm³. The results of the experiment with magnetite and a high power density are shown inFigure 4.16.

Figure 4.16: The measured spectra in the last experiment. Magnetite and a power density of 3.9 W/cm³ are used. Measurement conducted by Philemon Koolen.

Again, neither of the spectra inFigure 4.16match as well as inFigure 4.10. The values of IN₂/IOH ·

will be used to analyse the results. An enlarged spectrum of the region around the hydroxyl peaks is given inFigure 4.17.

Figure 4.17: Enlarged figure of the part where the hydroxyl emission peaks are located.

The time depicted is in minutes. Measurement conducted by Philemon Koolen.

In this spectrum it can be seen that the overall height of the spectrum is very high at first, then decreases to the lowest one at t = 60 min, and then increases again. This change in the overall spectrum height is again probably due to changes in pressure over time. Unlike insubsection 4.5.2, the height of the spectra attains a minimum if the spectra are observed chronologically. This means that, according toFigure 4.5, the product pd should have been between approximately 1.4 and 4.0 mbar·cm during the whole experiment. The minimum is either passed from left to right or from right to left depending on if the pressure was in- or decreasing respectively. Again, a table with the ratios between the nitrogen and hydroxyl emission peaks can be constructed. See Table 4.3 andFigure 4.18below.

Table 4.3: The ratios IN₂/IOH · of the nitrogen and hydroxyl emission peaks in a hydrogen plasma containing magnetite powder.

t (min) 0 20 40 60 80 100

ratio sharp peaks w.r.t. zero 1.1 1.1 1.1 1.1 1.1 1.1 ratio mean peaks w.r.t. zero 1.1 1.1 1.1 1.1 1.1 1.0 ratio sharp peaks w.r.t. baseline 4.8 1.8 1.6 1.8 1.6 1.4 ratio mean peaks w.r.t. baseline 2.3 1.4 1.3 1.5 1.3 1.2

Figure 4.18: The ratios of the nitrogen and hydroxyl emission peaks in a hydrogen plasma containing magnetite powder as a function of time.

If Table 4.1is compared with Table 4.3it can be seen that the behaviour of the ratios with re-spect to the baseline either matches or completely differs in these two tables, depending on if the pressure increased or decreased during the experiment. Both situations will be analysed.

One possibility is that the pressure would have decreased over time, in the region 1.4 < pd < 4.0. It could then be argued that the rate decrease in ratios inTable 4.3is a lot higher than inTable 4.1.

A high rate in decrease between t = 0 and t = 20 min could indicate that iron oxide is being reduced, because both the reduction process and the pressure lower the ratios, and this rate is higher than the pressure on its own can account for (seeTable 4.1). However, in this explanation,

there is not accounted for the exceptional ratio of 4.8 at t = 0, which does not occur inTable 4.1.

The ratios at t = 0 should lie between the minimum and maximum value in this table, because it should not matter if iron oxide powder is present or not at t = 0. This high ratio could have two causes: t = 0 is not the real time when the measurement started but is somewhat later than where t = 0 should have been, due to inaccuracy in noting the start of the experiment. Secondly, this can be caused because the product pd lied outside the range in which the influence of the pressure on the intensity is measured, so pd < 0.4 or pd > 5.6. In this unknown region, it might be possible to reach ratios of 4.8.

The other possibility is that the pressure increased over time. If this would have been the case, the decrease in ratios inTable 4.3can be explained by concluding that the reduction of iron oxide has a more significant influence on the hydroxyl emission peak (which would let it decrease) than the increasing pressure (which would let it increase). This is in accordance with the measurement in subsection 4.5.2 because it can be seen that after 10 minutes, the ratio starts to decrease. It is plausible that the first 10 minutes are ’starting-up time’, and after that, the reduction rate is high enough to become dominant over the change in pressure, which is trying to bring the ratios up. Furthermore, the ratios in the experiment with a power density of 3.9 W/cm³ are generally lower (meaning a relatively higher hydroxyl emission peak) than those in the experiment with a power density of 0.96 W/cm³, which is probably caused by this higher power density, or, possibly by the fact that magnetite is reduced more easily than other iron oxides. Lower ratios mean that the hydroxyl emission peaks are relatively high with respect to the nitrogen emission peaks, thus many hydroxyl radicals are created, therefore it could mean that the reduction process is taking place at a higher rate.

In conclusion, it cannot be said with certainty that the reduction process takes place in a hydrogen plasma with a power density of 3.9 W/cm³ and magnetite powder as iron oxide. Despite, there are indications that reduction is taking place, and in that case, it can be seen that more hydroxyl radicals are produced thanks to the higher power density (or the difference in iron oxide powder) as compared to the previous experiments. The first measurement however, provides a remarkable spectrum in which obvious changes in the hydroxyl emission peak can be seen, while the rest of the spectrum stayed constant. Also, the second measurement gave indications of the reduction process taking place. Although the possible influences, like the presence of water in the porous powder or in the cavity should be considered and bring a slight uncertainty to this statement.

With this in mind, a considered answer can be given on the research question of this report.