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EXTRAORDINARILY LOW
MÖSSBAUER-FRACTION IN AEROSOLED Fe FINE PARTICLES
K. Haneda, A. Morrish
To cite this version:
K. Haneda, A. Morrish. EXTRAORDINARILY LOW MÖSSBAUER-FRACTION IN AEROSOLED Fe FINE PARTICLES. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-91-C2-93.
�10.1051/jphyscol:1979232�. �jpa-00218630�
JOURNAL DE PHYSIQUE Colloque C2, supplkment au n o 3, Tome 40, mars 1979, page C2-91
EXTRAORDINARILY LO!d MOSSBAUER-FRACTION
IN
AEROSOLED FeFINE
PARTICLES K. ~aneda+ and A.H. MorrishDepartment of Physics, University of Manitoba, Winnipeg, MB, Canada R3T 2N2
Rbsum6.- Le spectre Mgssbauer de particules de fer prbparses par une technique asrosol a 6td btudi6, et en particulier la fraction sans recul. Les rssultats suggsrent un "adoucissement" des vibrations de rbseau.
Abstract.- ~gssbauer spectra of Fe small particles prepared by an aerosol technique have been studied with special reference to the recoilless fraction. It is suggested that a softening of the lattice vibrations occurs in these mist-like particles.
1 . Introduction.- An aerosol, or so-called evapora-
tion method, has been developed over the past decade as a new technique to prepare very fine metal par- tlcles 11-31. In this method, metals are evaporated into an inert gas atmosphere of relatively low pres- sure. At first the metal atoms float around inside the chamber. However, because the mean-free-path of these metallic atoms is reduced, they eventually condense into the form of fine particles. The par- ticle size can be controlled to some extent by chan- ging the pressure or the species of the inert gas.
This paper reports on a ~Essbauer study of particles prepared by this method; special reference is made to the recoilless fraction of these mist-like par- ticles.
2. Experiments and Results.- Several samples, with particle sizes ranging from a few hundred angstrEms to about 0.1 pm have been prepared. The average par- ticle size ofeach sample was determined by the BET nitrogenadsorption method assuming a spherical shape, and was found to be 238, 377,and 912 for samples labelled A, B,and C respectively. X-ray diffraction patterns indicate that these samples consist of two phases, one with the crystal structure of ordinary a-Fe, and the other, a significant fraction for the smaller particles, with the structure of spinel iron oxide-either magnetite, y-FenOs, or a mixture of bdth oxides. The 912
1
sample was almost pure metal- lic iron. MEssbauer spectra, taken with a fly-back drive, are shown in figure 1 for samples A and B.To make an absorber, the powder was dispersed in acetone, dried in air, and then pressed gently bet- ween two plates of perspex or beryllium. The impor- tant feature to note is that only a small absorption is observed for sample A to 300 K (Fig. la), which
+ Present address : Research Institute for Scienti- fic Measurements, Tohoku University, Sendai, Japan.
implies that the majority of the particles experien- ce a recoil upon the adsorption of a gamma quantum.
-10 - 5 0 5 10
V E L O C I T Y ( m m / s )
Fig. 1 : Mijssbauer spectra for two aerosoled samples.
a) sample A (238
1)
at 300 K, b) sample B (3771)
at300 K, c) sample A at 77 K, and d) sample B at 77 K.
This result is in contrast to the well-developed hy-
0
~erfine split spectrum observed for the 912 A sample (not shown) at 300 K. Even for sample B, after col- lecting 12 megacounts per channel, the adsorption is relatively small although hyperfine-split and super- paramagnetic patterns are discernable. Ln order to eliminate the parabolic background produced by the solid angle effect, a spectrum for sample A at 300 K was obtained with a triangular-shaped drive using a wider velocity range and then folded. Only a very
small central absorption was observed. Upon lowering the temperature to 77 K, all the samples exhibited a well-defined absorption spectrum. It is important to mention that for fine particles of various iron
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979232
C2-92 JOURNAL DE PHYSIQUE
oxides, prepared by other methods, such as chemical sample has an average particle diameter of 2 9 3
i,
precipitation, hyperfine-field split MEssbauer absor- ptions accompanied by central superparamagnetic li- nes have usually been observed at room temperature, even for smaller particle sizes ( < I 0 0
k).
It is li- kely that chemically prepared particles adhere to each other by van der Wall's forces or by intersti- tial water to form agglomerates during the chemical processing, and these agglomerates may share the gamma-ray recoil / 4 / . This conjecture may be the reason why several Gssbauer studies of ultrafine particles have been made successfully without using supporting materials. Thus, it seems very plausible that the aerosoled powders consist essentially of isolated particles, presumably because they are pre- pared in a completely water-free chamber. Instead, these particles form only necklace-like arrangements/ 1 , 3 / ; in other words, the particles are relatively
well dispersed.
Several papers have been published in connec- tion with the Gssbauer fraction f of ultrafine par- ticles 1 5 1 . The subject has been discussed in terms of the packing factor, the weak binding of the sur- face atoms, the phonon spectrum as a function of size, and the effect of the suspension medium. Gene- rally speaking, the dependence of f on particle size is complex, especially since it is difficult to dis- tinguish between size and surface'effects. However, for particles with sizes of 2 3 8 and 3 7 7
i,
the sur-face to volume ratio is not that large, and the contribution from the surface atoms may not be the dominating factor. Indeed, none of the factors dis- cussed in the literature may be the primary origin of the present observations.
','he Debye temperature of the aerosoled small Fe particles has been estimated by the following approach. The Debye model gives for the recoilless fraction f at the temperature T
where ER, k, BD, and T represent the recoil energy, the Boltzmann constant, the Debye temperature, and the absolute temperature, respectively 1 6 1 . Table I gives the Ksssbauer fraction at 300, 1 3 0 , and 7 7 K for various values of 8 calculated from this equa-
D
tion. From Figure 1 , the use of table I leads to a rough estimate of the Debye temperature as between
1 5 0 and 1 0 0 K, possibly about 1 3 0 K. Figure 2 shows
a spectrum taken at 1 3 0 K for an additional sample prepared in He gas under 2 nun Hg pressure; this
that is, intermediate in size between samples A and B. The medium absorption intensity observed is con- sistent with the analysis.
Table I : Calculated recoilless-fraction at 3 0 0 , 1 3 0 , and 7 7 K for various Debye temperatures, OD'
V E L O C I T Y (rnm/s) OD
(K) 250 200 150 130 100 50
Fig. 2 : Mssgbauer spectrum at 1 3 0 K for another sample ( 2 9 3 A).
Since the Debye temperature for bulk iron and iron oxide is 4 6 7 and 6 6 0 K respectively / 7 / , a conside- rable reduction is 8D would imply a weaker interato- mic bonding in aerosoledparticles. This conclusion is supported by Afanas'ev et al. 181, who observed a similar phenomenon for 1 2 0 and 1 9 0 Fe-Ni particles prepared by an aerosol technique. Another model which should be looked at is a lattice of small particles in which the bonding can be produced by magnetic in- teractions or be influenced by the presence of a binder; namely the effect of packing factor or the effect of the suspension medium. However such a sys- tem eventually would have a small Debye temperature and may imply what we are seeing.
Therefore, it appears that an aerosoled Fe par- ticle, whether it is pure iron, or partially oxidized, has a reduced Debye temperature when in the few hun- dred angstrom size range. The Fe atoms seem to be more loosely bound, and to have a larger vibrational ,amplitude than in the bulk. This weak binding may be a characteristic of particles that form and grow during the evaporation and subsequent condensation in the aerosol method.
Recoil l e s s - f r a c t i o n a t T = 7 7 K
0.81 0.73 0.59 0.50 0.32 0.01
1
T = 3 0 0 K 0.51 0.35 0.12 0.07 0.02 0.00
T = 1 3 0 K 0.73 0.62 0.43 0.33 0.16 0.00
References
We are grateful to Professor Tasaki's group at /I/ Kimoto, K., Kamiya, Y., Nonoyama, M., and uyeda, R., Japan, J. Appl. Phys.
2
(1963) 702.Osaka University, Japan, for the samples. This re-
/2/ Gen, M., Ya, Velichenkova, E.A., Eremina, I.V., search was partially funded by the National Research and Ziskin, M.S., Sov. Phys.-Solid State
5
(1964)Council of Canada. 1274.
/3/ Tasaki, A., Tomiyama, S., Iida, S., Wada, N., and Uyeda, R., Japan. J. Appl. Phys. (1965) 707.
/4/ Coey, J.M.D., and Khalafalla, D., Phys. Status Solidi (a)
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(1972) 229./5/ Marshall, S.W., and Wilenzick, R.M., Phys. Rev.
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(1966) 219; Roth, S., and ~ Z r l , E.M., Phys. Lett.25A
(1967) 299; van Wieringen, J.S., Phys. Lett.26A
(1968) 370; MacRae, A.U., and Ger- mer, L.H., Phys. Rev. Lett.8
(1962) 489; Wiegers, M.P.A., and Trooster, J.M., Phys. Rev. B E (1977)72; Matsushita, E., and Matsubara, T., Prog. Theor.
Phys.
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(1978) 15.161 See for example "Topics in Applied Physics", Volume 5, Edited by U. Gonser (Springer-Verlag, Berlin, Heildelberg, and New York) 1975, p. 15.
/7/ "American Institute of Physics Handbook" (McGraw- Hill Book Company, New York) 1972.
/8/ Afanas'ev, A.M., Suzdalev, I.P., Gen, M.Ya., Gol' danskii, V.I., Korneev, V.P., and Manykin, E.A., Sov. Phys.-JETP
