This section will give a characterisation of the iron powders which are used in the experiments and give an explanation why these powders are used.
In the conducted experiments, two types of iron oxide powders are used:
1. ’Mixed’ iron oxide: this iron oxide powder is obtained from the combustion of iron powder in a cyclonic burner, designed by Tim Spee [38]. This kind of burner can establish a self-propagating iron flame, with a temperature of about 800◦C. Because not all iron particles
have reacted completely, according toEquation 2.6, a mixture of different types of iron oxide are present in this powder. The XRD analysis of this powder shown in Figure 3.8supports this statement. This powder is chosen because it is an actual powder which could be used in the future to store energy in. This iron powder is the result from an actual combustion, which makes it a realistic example with regard to the future in which iron powder may serve as an energy carrier.
The particle diameters are not measured but are estimated to be approximately equal to the diameter of 30 µm of the iron powder which was used.
2. Magnetite: this powder can also be denoted by iron(II,III) oxide or Fe3O4. This powder is bought on Sigma Aldrich, has a purity of 95% and consists of particles with a diameter of approximately 5 µm [39]. This type of iron oxide is an intermediate type of iron oxide which can be a result of incomplete combustion of iron oxide, as shown in Equation 2.6.
This powder is chosen because its composition is significantly better defined compared to the mixed iron powder. Also, the particle size is still in the range of what would be realistic if iron powder will serve as an energy carrier.
These particles are significantly smaller than the powders (or pellets) used in other experiments.
Sabat et al. used iron oxide pellets of approximately 500 µm in size [13] and in other experiments pellets in the form of disks with a diameter of 40 mm and a height of 3 to 9 mm [35]. Smaller particles will have a larger specific area (when assuming the mass of the sample is the same) that will be in contact with the plasma and therefore the hydrogen radicals compared to larger particles. This will positively influence the reduction process. No research has been done on the effect of different particle sizes of iron oxide on their reduction in a hydrogen plasma, which is why it cannot be ruled out there are other differences between small and large particles. However, many experiments report a positive correlation between a smaller particle size and higher reaction rate [40,41].
Figure 3.8: An XRD measurement of the mixed iron oxide powder. It is clear that this powder consists of several types of iron oxide. Courtesy: Martijn Weijers.
Using this equipment the experiments as described in the next section can be conducted. Both the pressure and the flow of the hydrogen gas in the cavity is varied to investigate their influence on the intensity of the light emitted by the plasma. The measurements on the intensity of the emitted light of the hydrogen plasma when iron oxide is present are conducted using different gap widths (between the electrodes) and different voltages in order to create different power densities.
Furthermore, the intensity is always measured over a specified period so that the reduction process can be monitored accurately.
Results and discussion
In the previous part of this report, the theory describing plasma behaviour and optical emission and the experimental set-up including calibration measurements are discussed. This chapter begins with a characterisation of a hydrogen plasma without iron oxide in order to create a reference point for the measurement in which iron oxide is included. For this purpose, the results of the measured Paschen curve of the hydrogen plasma, a full spectrum and the dependency of the spectrum on the pressure are analysed and discussed. When it is clear what the reference point is, first several other causes for hydroxyl emission peaks to change (except for the reduction of iron oxide of course) are discussed in order to answer the first sub-question. Thereafter, using the analysis of the other causes, the results of the experiments in which iron oxide was included are discussed to answer the second sub-question of this report. In this final part, experiments with different power densities and different powders are conducted.
4.1 The Paschen curve
As discussed in section 2.2the breakdown voltage of a certain plasma is governed by Paschen’s law. In this report, this is done for a hydrogen plasma. The breakdown voltage is plotted against the product of the pressure and the distance between the two electrodes inFigure 4.1.
Figure 4.1: The experimentally measured Paschen curve for a hydrogen plasma. Meas-urement conducted by both Philemon Koolen and Sam Tennebroek.
It can be seen that when pd decreases (starting from the minimum) there is an abrupt change in breakdown voltage, whereas that when pd increases there is a slow rise in breakdown voltage. This is according to the behaviour of Paschen curves in general. Equation 2.2is used to fit the measure-ments (actually, an adapted version of this equation - in which pd is replaced by
- is used to avoid fitting problems with the negative part of the function). The numerical valueA of the experimentally determined minimum is VB = (1.5 ± 0.1) · 102 mVpp at (pd)min= 3.4 ± 0.1 mbar·cm.
Literature suggests that this minimum should occur at about 2.55 ± 0.05 Torr·cm, which is equal to 3.40 ± 0.07 mbar·cm [20], with a breakdown voltage of (3.2 ± 0.2) · 102 V. The measured pd matches the one from the literature very well. However, the breakdown voltages cannot be compared because the measured voltage is from the signal generator (hence expressed in mVpp), whereas the voltage from the literature is the one which, in this set-up, would be measured by the Octiv (a device which measures voltage and power). SeeFigure 3.1. Nevertheless, the Paschen curve will still provide useful information for next sections.