ПОВЕРХНОСТЬ. РЕНТГЕНОВСКИЕ, СИНХРОТРОННЫЕ И НЕЙТРОННЫЕ ИССЛЕДОВАНИЯ, 2014, № 12, с. 16-20
УДК 539
PHYSICS OF AGING OF NANOPHASE ALLOYS: MOSSBAUER INVESTIGATION OF NANOCRYSTALLINE IRON-COPPER PSEUDOALLOY
© 2014 г. A. G. Gavriliuk12*, S. N. Aksenov1**, R. A. Sadykov1, V. P. Filonenko3
1Institute for Nuclear Reserch, Russian Academy of Science, 142190, Troitsk, Moscow, Russia 2Institute of Crystallography, Russian Academy of Science, 119333 Moscow, Russia 3Institute for High Pressure Physics, Russian Academy of Science, 142190 Troitsk, Moscow, Russia *E-mail:gavriliuk@mail.ru, **E-mail: axenov@inr.ru Received February 24, 2014
This communication considers structural features of nanocrystals synthesized at high pressure consolidated mixed nanopowders. This system presents a pseudoalloy of mutually insoluble elements Fe and Cu, and contains pores. The study aims to demonstrate quasi-stable state of nanocrystals and factors preventing the aging. Mossbauer and X-ray analyses show a strong dependence of phase composition of Fe—Cu nanocrystalline pseudo-alloy on pressure and on aging duration. It is found pressure to coarsen phase inhomogeneity which exists initially in an as-synthesized Fe—Cu pseudoalloy. Aging duration is shown to affect the nanophase state in the same manner. It is agued that aside with typical parameters as grain size, density, phase composition of nanocrystalline substances one should know a prehistory of every sample: synthesis duration, temperature and pressure, which have a marked influence on physical properties of nanophase compacts. The Fe disorder in the Cu lattice revealed by Mossbauer spectra substantially depends on applied pressure.
DOI: 10.7868/S0207352814120087
I. HISTORY AND SAMPLE PREPARATION
Ultrafine copper—iron alloyed particles were obtained by levitation method in an inert gas flow [1]. In this method the droplet of metal is suspended and heated to the molten state in the electromagnetic field of rf inductor. The molten droplet is blown by the inert gas (helium or argon). The ultrafine metal particles appearing near the surface of a droplet are carried away by the transport gas and collected in the filter. If one controls velocities of copper and iron wires and flow of an inert gas then one can produces Fe—Cu alloys with different contents of Cu and Fe.
Electron microscopic measurements showed that the average diameter of the particles in our experiments was about 80 nm. There were many experiments for Fe—Cu alloy with two different compositions: Fe015Cu085 and Fe019Cu081. These two kinds of samples had different prehistory:
Sample (I) — Fe015Cu085 have been oxidized in air at normal conditions after production.
Sample (II) — Fe019Cu0 81 after production have been placed into rubber shell and then treated at hydrostatic pressure up to 2 GPa in piston-cylinder high pressure camera.
There are another different methods for production of iron-copper nanocrystalline pseudoalloy: mechanical alloying (MA) boll-milling method or repeated rolling [2, 3]; sol-gel route (SGR) [4]; evaporation-condensation in an inert atmosphere (ECIA) [5]. Copper—
iron pseudoalloys produced by these methods reveal close properties and reveal the shapes of Mossbauer absorption spectra like ones for the levitation method.
II. EXPERIMENTS
From X-ray emission spectroscopy relative atomic concentrations of iron 15 at % and 19 at % in Cu matrix were found for sample (I) and (II) respectively.
The Mossbauer and X-ray investigations have been made for these two types of alloys.
Il.a. Aging and high pressure treatment of the sample type (I). It was found that the behavior of the Fe—Cu pseudoalloy sample (I) depends on aging and high pressure treatment.
As a result of aging at 150°C during 1 h of the sample (I), the appearance on X-ray diffractogram ofweak copper oxide reflexes is observed (Fig. 1). The main oxidation process is finished in some minutes after placing of the sample from inert gas into air. The appearance of oxides due to long aging seems to be the result of formation enough structured oxide phases from oxygen solution in copper matrix.
Mossbauer spectra of the sample (I) measured just after production is shown in Fig. 2a. From Mossbauer measurements one can conclude that the iron atoms occupy three different positions [5]. The first position corresponds to bulk a-Fe phase. In the second one iron atom replaces Cu in diamagnetic copper lattice,
Intensity
I.6 1.8 2.0 2.2 2.4 2.6 2.8
d, A
Fig. 1. X-ray patterns for not aged sample (CuSTa radiation)
(a); sample aging during 8 moths (Fe^a radiation) (b).
and, finally, in the third position iron atoms form small iron clusters (two atoms in cluster (dimer), three ... and so on) incorporated into diamagnetic copper lattice. Thus, it is possible to split Mossbauer absorption spectrum into three subspectra. Sextet of a-Fe phase — magnetic subspectrum. Single line corresponding to solitary iron atoms in the copper matrix and doublet line corresponding to small iron clusters in the copper matrix — paramagnetic subspectra. Fitting of experimental spectrum for not aging sample was done for sextet, singlet and doublet (Fig. 2a). Algorithm of fitting is described in [7]. Such shapes of Mossbauer spectra are common for different techniques of nanocrystal-line pseudoalloy production [2—5].
Magnetic states of iron in copper matrix are changed with aging time. Spectrum of the sample (I) that was aged during 8 months is shown in the Fig. 2b. Fitting of this spectrum was done for sextet and two doublets. It is evidently that the paramagnetic sub-spectrum is strongly changed. The relative content of single iron atoms in copper is decreased approximately by 10%. The relative contents of dimers, small clusters and bulk clusters are increased. It seams appeared small clusters and increased bulk clusters at aging can occur through mechanism of diffusion.
To demonstrate that the paramagnetic subspec-trum is not the result of the supperparamagnetic mechanism Mossbauer, spectra for sample (I) was measured at helium temperature after aging during 4 months (Fig. 2c). Because it was difficult to distinguish single and doublet lines for Mossbauer spectra measured at T = 10 K fitting of experimental spectrum was done for sextet and one doublet.
II.b. Sample type (I) annealing. Annealing of the sample (I) at 150°C during 1 h leads to almost full oxidation of iron and copper. Even under annealing at
Intensity 1.002
1.000
(a)
0.992
10 -8 -6 -4 -2 0 2 4 6 8 10
Velocity, mm/s
Intensity 1.002 г
1.000
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(b)
0.994
10 -8 -6 -4 -2 0
2 4 6 8 10 Velocity, mm/s
Intensity 1.002
(c)
10-8 -6 -4 -2 0
2 4 6 8 10 Velocity, mm/s
Fig. 2. Mossbauer spectrum of the sample (I): just after production (a); aging during 8 months (b); at T = 10 K, aging 4 months (c).
low pressure (~10-4 Torr) strong oxidation is observed. Therefore annealing of the sample (I) is worth technology for production of Fe—Cu nanocrystalline pseudoalloy. Under annealing at atmospheric pressure copper is oxidized to tenorite phase (CuO) and under annealing at pressure ~10-4 Torr copper is oxidized to
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GAVRILIUK и др.
Intensity
40
45
50
55
29, deg
Fig. 3. X-ray diffractograms of the sample (I) annealed at atmospheric pressure and at pressure of ~10-4 Torr (Fe^a radiation).
tenorite and cuprite phase (Cu2O) as well. The X-ray diffractograms of the sample (I) annealed at different pressure is shown in Fig. 3. There are no any iron oxide reflexes on this pattern. It can be explained by the small amount of iron and by the small size of iron oxide particles leading to strong broadening of reflexes.
Mossbauer spectroscopy for annealed sample (I) reveales narrow doublets with quadrupole splitting 0.66 mm/s that corresponds to paramagnetic x-Fe2O3 phase described in work [8]. As it was found by Yu.F. Krupyansky and I.P. Suzdalyev [8] Mossbauer spectrum of paramagnetic x-Fe2O3 phase is quite narrow doublet with the value of quadrupole splitting in the range of 0.63—0.90 mm/s. The constant of the
quadrupole splitting depends on the size of iron oxide clusters. Mossbauer spectrum of annealed sample (I) is shown in Fig. 4. From the discussion of x-Fe2O3 structure [8] and from the analysis of Mossbauer spectra for small iron clusters in copper matrix [5] one can conclude that the iron is fully oxidized to paramagnetic x-Fe2O3 phase with the average size of clusters ~50 A. The X-ray reflexes of the paramagnetic x-Fe2O3 should be strongly broadened and should be located in the positions of a-Fe2O3 and y- Fe2O3 phases [8].
II.c. Sample type (II) annealing. The sample (II) was not oxidized at annealing during 1 h at 150°C. Fig. 5 shows X-ray diffractograms measured before and after annealing of the sample (II). There are no any oxide reflexes in X-ray diffractograms of annealed sample. The broad reflexes on X-ray diffractograms for non annealed sample is the result of apparatus broadening line width (CoKa radiation was used). Nevertheless, it is evident that in the range of Bragg angles 23°— 28° there are no any copper or iron oxide reflexes.
But Mossbauer spectra of annealed sample (II) is strongly unusual (Fig. 6). Spectrum peaks are strongly broadened and asymmetrical. This fact is wonderful if one takes into account that there are no visible changes in X-ray diffractograms. Mossbauer spectrum in Fig. 6 have been fitted by two subspectra: sextet with small quadrupole interaction and sextet with large value of the quadrupole interaction ~5 mm/s. But this fitting perhaps cannot explain adequately a real situation. It seems, more probably, such a shape of the spectrum is the result of copper atoms diffusion into a-Fe lattice without substantial deformation of the crystal structure. Alike mechanism is responsible for the mechanical alloying of iron and copper powdered mixture that was carefully investigated by J. Eckert et
Intensity
1.00
Intensity
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0.96
Sample (I)
A
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