Proceedings of ITC/ISHW2007

Dynamic evolution of near-UV emission spectrum observedin the microwave plasma-surface interaction
A. Matsubara1, S. Takayama2, K. Nakayama1, T. Kanaba1, M. Tomimoto1, R. Akiyama2,S. Okajima1, and M. Sato2
1

Chubu University, 1200, Matsumoto, Kasugai, Aichi, 487-8501, Japan2

National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu, 509-5292, JapanThe material heating during the carbothermic reduction of magnetite in a 2.45 GHz microwave multimodefurnace has been investigated with multipoint spectroscopic measurements. The heating mode shifts fromvolumetric heating to plasma heating after sudden rise in temperature of the material from ~700 °C to ~1000 °C
accompanied by the light emission of plasma. Then, the emission spectrum in the near-UV range (240 nm - 310 nm)
changes drastically from a continuous spectrum to a discrete line spectrum with increasing temperature of material,
representing progress of reduction process of iron oxide. The position dependency of the spectral evolution indicatesthe importance of the plasma-surface interaction in the large-scale surface structure on the material.
Keywords: plasma surface interaction, microwave iron making, reduction process, spectroscopy, near-UV emission,
continuous spectrum, cathodoluminescence, surface structure.

1. Introduction
Plasma-surface interaction that is one of crucial issues
in fusion research as well as plasma processing has beenstudied in some cases in field of the microwave material
processing also. The microwave processing is based on theheating of material under the microwave irradiation. The
microwave energy absorbed into the material transfers
consequently to the thermal energy of the material. The
main advantage of that method enables volumetric heatingof materials since the absorption length is comparable orlarger than the scale length of most electrically insulating
materials such as ceramics, polymers, and certain
composite materials, even metal if it is powdered state, as arule, leading significant energy savings and reduction inprocess time. Sometimes electric discharge occurs on thesurface of the material, which has been believed to behelped by the presence of vapours of various substances
and thermal electrons emerging from hot spots on thesurface. The occurrence of the discharge has been routinely
to be avoided, because the volumetric heating is lost; thepart of microwave energy is absorbed as the kinetic energyof the plasma electron. However a few applications utilize
such plasma in order to modify the surface condition or
boost specific chemical reaction. One of the applications is
microwave ironmaking presented here.
High-purity pig irons have been produced
successfully in a multimode microwave test reactor frompowdered iron ores (magnetite) with carbon as a reducing
agent in a nitrogen atmospheric pressure environment [1].
The method has the potential advantage that the CO2
author’s e-mail:Matsubara.Akihiro@sti.chubu.ac.jp

emission can be reduced by tens of percent in conventional
blast furnaces, if the electric power for the microwave isgenerated by renewable energy, such as solar, hydro andnuclear power [2]. A feature of the microwave method is
the sudden rise in temperature of the material from ~700°C to ~1000 °C accompanied by light emission of plasma,called the temperature jump [3]. The nature of thetemperature jump is emergence of an additional energy
flow to the material through the plasma.
The in-situ visible emission spectroscopy has been
introduced to demonstrate the characteristics of the heating
mechanism including volumetric and/or plasma surface
heating [4]. The light emission after the temperature jump
during the microwave ironmaking consists of strong
atomic/molecular lines that can be assigned as those listed
in the spectrum database based on the spark and arc
discharge, indicating the presence of plasma electrons withseveral electron volts in the electron temperature. It was
found that the structure of emission spectrum near-UVrange (240 nm-310 nm) changes drastically from thecontinuous spectrum to discrete line spectra of iron withincreasing surface temperature [3]. The continuousspectrum was assigned as a cathode luminescence due tothe impingement of a plasma electron onto the sample
surface of magnetite. It was also stated that the evolution
from continuous spectrum to lines spectrum was the firstobservation to capture the progress of reduction process ofion oxide via in-situ spectroscopic method.
In those series of experiments, the multi-points
spectroscopic and pyrometric measurements wereProceedings of ITC/ISHW2007
conducted. The purpose of this article, therefore, is topresent the results obtained those multipoint systems.
These results not only supplement the prior statement
associated to the spectral evolution, but also demonstrate
importance of the plasma surface heating in the reductionof iron ore after the temperature jump. Especially, theposition dependency of the evolution of emission spectrum
promotes deeper understanding of the plasma-surface
interaction such as the reaction cycle being in the complex
surface structure on the material.

2. Experimental setup
The multimode test furnace at the National Institutefor Fusion Science is shown in Fig. 1. According to theconcepts developed in Germany in order to improve thehomogeneity of the electromagnetic field, the applicator
shape is hexagonal [5]. The furnace is equipped with fivemagnetrons. The microwave power of one magnetron is
2.5 kW at a frequency of 2.455 ± 0.030 GHz. Two mode
stirrers scatter the standing waves. Before starting theprocess, the chamber was evacuated by the rotary pumpand refilled with nitrogen gas. During processing, acontinuous nitrogen gas flow of about one litter/minute
was used with a pressure a little higher than ambientpressure.
A multi-point emission spectroscopic diagnostic wasconducted through the viewing port on the furnace. Tensightlines with a distance of 2.2 mm and a spot diameter of
1.0 mm were provided by means of the fiber bundle andmagnifying lens located above the furnace. Figure 2 shows
the schematic of the sight lines for the spectroscopic
measurement, as well as for the pyrometry. Sevensightlines, labeled as f1, f2… f7, are directed to the sample
surface through the hole of the insulator board as thetop-cover, and the remaining three (f8 - f10) are terminatedby the surface of that insulator. The spectrometer was a
Czerny-Tuner imaging polychromator with a focal lengthof 250 mm. The exposure time was 2 s and the cycle time
for capturing was 5 s [3].
InGaAs pyrometers (labeled as IR’s in Figs.1 and 2)
were taken for the temperature measurement. IR1, IR2, andIR3 were directed to the sample surface, the crucible outersurface and the crucible bottom, respectively. The detection
wavelength range for IR1 and IR3 was 1.95 µm - 2.5 µm,
that for IR2 was 0.8 µm - 1.6 µm. The spot size was 10mm for IR1, 4 mm for IR2, and 3 mm for IR3. As shownin Fig.2, IR1 and f1 (mentioned before) were adjusted insuch a way that they cross each other at the point at 15 mmabove the bottom of the crucible along its central axis. It isnoted that the position of the observation spot (i.e.,intersection of the sight lines and surface of material) slides
on the surface with increasing the surface temperature,since the powdered material shrinks and melts as shown
schematically in Fig.2; the height of the powdered material

Fig.1

Schematic diagram of the multi-mode test furnaceand diagnostics system. The opposite side distanceof hexagonal cross-section is 1.1 m.

Fig.2

Sight lines for multipoint spectroscopic
observation and configuration of material, crucible
and insulator (left). Video camera pictures of thesample surface without top insulator (upper right)
and with top insulator with the sighthole (lowerright).

was set before the heating at 34 mm (light-shaded area),
and this height changed to around 10 mm after the heating(dark-shaded area). The photograph at upper right inFig.2 shows the surface of the material captured before
heating by video camera located above the furnace (Seealso Fig.1). The central point of the cross-mark withcircle of 10 mm in actual diameter indicates the sight axisof the IR1, and the left cross-mark is the observation spotof f1 for the initial height of the material.
The purified reagent of Fe3O4 with graphite wasprepared according to the preparation performed in theprior experiment [3]. The mix ratio was MFe3O4 : MC = 90 :10 ( = 54.0 g : 6.0 g) by weight. Corresponding mol ratio
was nFe3O4 : nC = 1.0 : 2.0. The crucible was thermallyinsulated by alumina-silica fiberboards as illustrated inFigs. 1 and 2.

3. Results and Discussion
3.1 Heating characteristics
Fig. 3(h) shows temperature traces obtained for IR1,IR2, and IR3. The heating mode is volumetric heating
initially, but changes in plasma surface heating after thetemperature jump at 740 s -.840 s. The series of the pictureProceedings of ITC/ISHW2007
at the lower of Fig. 3 depicts the surface condition capturedby the video camera of which the field of view is indicatedin Fig. 2. During the volumetric heating the bright crack
structure on the surface as shown in the second picturelabeled as t = 185 s continues, but the micro-scale
discharge spark often occurs along the crack. After t = 700s, the size of spark grows approximately each time it
appears (see third picture labeled t = 743 s), and finally it
triggers the large-scale emission like flame covering whole
top surface (see fourth picture labeled t = 840 s). At thesame time, the increase in temperatures by IR1 and IR2
becomes rapid, and the intense emission spectrum begins
to be observed, which shows typical phenomena of thetemperature jump. There is no jumping behavior in thetemperature trace obtained by IR3, indicating the heating is
mainly due to the plasma generated on the top surface ofthe material. In Fig.3(h) the dashed-dotted line by the labelof SiC-TC, which means the temperature trace for thethermocouple tip covered with the silicon-carbide powder(see Fig. 1), represents the relative change in the measureof the spatial averaged microwave energy density. The
temperature of the SiC-TC decreases at the temperature
jump, implying the emergence of the plasma being themicrowave absorber.

Fig.3

Time evolution of the emission spectrum for various
sight lines (a)-(g). Temperature traces obtained for IR1,IR2, and IR3 (h). The temperature of SiC-TC (seetext) is also included in (h). A series of the picture of theappearance of the surface (see also Fig.2).

spectral radiance

200x1015
180 (a) f1, t = 840 s160 ToIR1 = 1018 C
140120100806040200240 260 280 300(photons m-2 nm-1 sr-1 s-1 )3.2 Evolution of the spectrum
Figures 3(a)–3(g) show the time evolution of theemission spectrum as a contour map for various position of
the sight line of f1 - f7. It is found that the drastic transition
from continuous spectrum to discrete spectrum is seen in
every sight line. The temperature range for the transition isapproximately 1050 °C -1250 °C that is consistent withthat stated in prior report [3]. This range is coincident withthe temperature range of high reduction rate for magnetite
and wustite [1]; therefore, the decay of the continuous
spectrum suggests the significant progress of the reduction
process of magnetite.
Typical continuous and discrete spectrums appearing
Almost all thein the evolution are shown in Fig. 4.
discrete spectra are well assigned to the spectra of ironatom observed in arc and spark discharges, and the originof the continuous spectrum has been considered to becathodoluminescence of magnetite [3]. It should be notedthat the spectral intensity of the continuous spectrum is atleast three orders of magnitude larger than that of black
body emission for present surface temperature around1000 °C. The wavelength range of the continuous spectrumof the present results coincides with that ofcathodoluminescence [6] and absorption spectrums [7] ofmagnetite. Therefore, the transition from continuous
spectrum to discrete spectrum can represent the progress ofreduction process from magnetite to iron.
From Figs. 3(a)–3(g), it is also noticed that the decayof the continuous spectrum begins earlier for larger number

wavelength λ (nm)Fig.4(c) f1, t = 1255 s
TIR1 = 1332 oC(b) f1, t = 1120 s
TIR1 = 1205 oC

240260280300

wavelength λ (nm)240260280300

wavelength λ (nm)Emission spectrums appearing in the evolution .
The peaks around 308 nm in (a) are OH spectrum.
Almost all the peaks in (b) and (c) are spectra of ironneutral.

Proceedings of ITC/ISHW2007of the sight line. Figures 5(a) and 5(b) depict the time
evolution of the spectral intensities, ISR, at the wavelengthof λ = 289 nm and λ = 302 nm. The former and latterwavelengths are associated to the continuous spectrum andthe iron spectrum, respectively. In the intensity of the ironspectrum, the component of the continuous spectrum isalready subtracted. It can be seen that the steady decreasein ISR of the continuous spectrum (λ = 289 nm) is earlier
for f5 than f1. The beginning time of the decreasing is
t~950 s for f5, but t~1050 s for f1. On the other hand, thevalue of ISR of iron spectrum (λ = 302 nm) for f1 increases
moderately, but that of f5 decreases briefly at t~1100 s, andthen increases rapidly. These behaviors depending on theposition vary smoothly spatially, which can be recognized
by Fig. 5(c) for the continuous spectrum and by Fig. 5(d)for iron spectrum.
The possible interpretation for the position
dependence is presented as follows. First, it is assumed thatafter the temperature jump the position of the observation
spot for each sight line slides immediately for the edge ofthe material (toward the right hand side of Fig.2), because
the material should shrink rapidly and its height drops
substantially. This assumption has been based on theobservation that the emission band spectrum of CN decays
rapidly just after the temperature jump, implying theconsumption of the space in the powdered material.
Fundamental process on the surface is likely that when apart of the surface layer melts resulting from the reductionof iron oxide induced by the plasma bombardment, itclumps together and drops for the bottom. Then, in thecentral part of the material (near the axis of the crucible),the new surface layer of the powder material of iron oxide
comes to the surface. The reaction cycle, such as
reduction, clumping, and dropping, completes faster fornearer the edge of material, because the reaction cycle canalso progress from the side surface of the material. Thisis reason why the continuous spectrum decays faster fornearer edge. The brief drop and the significant increases in
the iron spectrum in the final stage of the heating can berelated to the growth and decay of the large-scale crack
structure that is found for the present case. The cracks
grow via erosion with the reaction cycle, but they decay
via melt meaning the end-state of the reaction. In present
case the brief drop can be due to escaping of material fromthe observation spot with growing the crack, andsignificant increase can be due to returning of the material
to the observation spot with spreading the melt region.

4. Summary
We presented the experimental results obtained by themultipoint spectroscopic and pyrometric measurements
during the microwave ironmaking. Both results
demonstrate the importance of the plasma surface heatingin the reduction of iron ore after the temperature jump.

Fig.5 The time evolution of the spectral intensity, ISR,
(spectral radiance in the unit [photons m-2 s-1 sr-1nm-1]) at the wavelength of λ = 289 nm and λ = 302
nm for the sight line of f1 (y = 0 mm) in (a) and f5(y = 8.8 mm) in (b), and for various position of thesight line as the contour map in (c) for λ = 289 nm
and (d) for λ = 302 nm, respectively.

Especially, the position dependencies of the evolution ofcontinuous spectrum and iron spectrum make us address
the reaction cycle on the large-scale surface structure in thematerial.

Acknowledgement
This research is supported in part by a Grant-in-Aidfor Scientific Research on Priority Areas (465) of the
Ministry of Education, Culture, Sports, Science andTechnology in Japan.

References
[1]
[2]

[3]
[4]

[5]
[6]
[7]

K. Ishizaki, et al., ISIJ, 46, 1403 (2006).
K. Nagata, et al., Proc.11th International Conference on
Microwave and High Frequency Heating, (3-6 Sep 2007,Oradea, Romania) pp. 87-90.
A. Matsubara, et al., submitted to JMPEE.
M. Sato, et al., Proc.10th International Conference on
Microwave and High Frequency Heating, (11-15 Sep 2005,
Modena, Italy) p.p. 272-275.
L. Feher, et al., DE-PS 19633245C1 (27.11.1997);
L. Feher, et al., DE-PS 10329411 (31.1.2005).
I. Balberg, et al., Phys. Rev. Lett. 27, 1371 (1971).
P.A. Milies, et al., Rev. Mod. Phys. 29, 279 (1957).