FORMATION OF IRON–CARBON NANOPARTICLES BEHIND SHOCK WAVES
317
3
increasing temperature. The optical density minimum
and the smallest radius deduced from LII data (no LII
signal is observed between 2000 and 2300 K) suggest
that, above 2000 K, particles are in abundance and their
size is ~1 nm or below. After a run carried out above
N , cm
p
016
1
1
1
1
1
1
1
2
3
015
2
000 K, particles as large as 700 nm [3] can be found in
014
the shock tube. Apparently, they result from coagula-
tion at room temperature, which is unrelated to the
high-temperature processes in question. The second
peak in the temperature dependences of optical density
and particle radius is due to the two-step process that
includes the formation of condensation nuclei behind
the incident shock wave (in the temperature range
1400–1700 K, where the first peak occurs) and the con-
densation of small carbon particles on these nuclei at
013
012
011
5
00
1000 1500 2000 2500 3000
2
800–3100 K behind the reflected shock wave. Similar
T, K
processes take place in the formation of iron–carbon
particles. Fe(CO) pyrolysis behind the incident shock
5
Fig. 9. Temperature dependence of the concentration of
nanoparticles resulting from the pyrolysis of the mixtures (1)
wave yields iron particles up to the highest temperature
examined (1560 K). Note that, at 1400–1560 K, carbon
atoms resulting from ë é pyrolysis can play some
3
%C O + Ar, (2) 0.5%Fe(CO) + Ar, and (3) 1%Fe(CO) +
3 2 5 5
3
%C O + Ar.
3
2
3
2
role in particle formation. Next, ë é pyrolysis behind
3
2
the reflected shock wave yields carbon vapor up to the
increasing temperature. The concentration of carbon highest temperature examined (3150 K). This vapor
particles shows a more complicated behavior related to condenses on iron particles, which are the most favor-
the above-mentioned two-peak temperature depen- able condensation nuclei. Here, neither the temperature
dence of optical density. Note the obvious similarity dependence of optical density nor that of particle size
between the behaviors of the iron and binary particle shows an extremum. These quantities gradually
concentrations. The higher value of the iron particle decrease with increasing temperature because of the
concentration in the temperature range 1700–2500 K is decreasing size of the primary iron particles and the
due to the considerable reduction in the size of the iron decreasing rate of condensation.
3
particles. From the density of iron (7700 kg/m ) and the
LII size of iron particles between 2000 and 2500 K
ACKNOWLEDGMENTS
(
~0.5 nm), we deduce that, at an iron particle concen-
1
5
–3
tration of ~5 × 10 cm , there may be less than ten
This work was supported by the Russian Foundation
atoms in one iron particle. This deduction is consistent for Basic Research, project no. 04-03-32163.
with the total number of iron atoms in the mixture.
Thus, from the behaviors of the iron and iron–carbon
particle concentrations and particle size data (Fig. 7), it
REFERENCES
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strongly on the size of the iron clusters serving as car-
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1
2
. Tanke, D., Wagner, H.Gg., and Zaslonko, I.S., Proc.
Combust. Inst., 1998, vol. 27, p. 1597.
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CONCLUSIONS
3
. Emelianov, A., Eremin, A., Jander, H., and Wag-
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2002, p. 102.
The above experimental data suggest the following.
Iron nanoparticles form at a high rate behind the inci-
dent shock wave starting at 600 K. As the temperature
is raised to 1500 K, their radius decreases rapidly from
4. Melton, L.A., Appl. Opt., 1986, vol. 23, p. 2201.
5
. Roth, P. and Filippov, A.V., J. Aerosol Sci., 1996, vol. 27,
3
0 to 1.5 nm, while the optical density of the medium
no. 1, p. 95.
does not fall sharply. This finding probably indicates an
increase in the number of fine particles. In considering
the formation of carbon particles from ë é , it is neces-
6
. Schramml, S., Dankers, S., Bader, K., et al., Combust.
Flame, 2000, vol. 120, p. 439.
3
2
7
8
. Van der Wall, R.L., Appl. Phys., 1998, vol. 67, p. 115.
. Woiki, D., Giesen, A., and Roth, P., Proc. Combust. Inst.,
sary to take into account that the pyrolysis of this com-
pound begins at a much higher temperature of 1400 K.
Large soot particles form between 1500 and 1800 K. At
higher temperatures, the combination of growing parti-
cles into large agglomerates is suppressed; as a conse-
2
000, vol. 28, p. 2531.
9. Starke, R., Kock, B., and Roth, P., Shock Waves, 2003,
vol. 12, p. 351.
quence, the size of resulting particles decreases with 10. Smirnov, V.N., Kinet. Katal., 1993, vol. 34, no. 4, p. 591.
KINETICS AND CATALYSIS Vol. 46 No. 3 2005