MA and MA SHS production of nanocomposites metal / oxides and intermetallics / oxides
Grigoryevaa T.F., Talakob T.L., Novakovac A.A., Vorsinaa I.A.,
Barinovaa A.P., Kiselevac T.Yu., Sepelakd V., Beckerd K.D.,
Lyakhova N.Z., Vityazb P.A.
a Institute of Solid State Chemistry and Mechanochemistry, Kutateladze str., 18, Novosibirsk 630128, Russia.
b Institute of Powder Metallurgy, Platonova str., 41, Minsk 220071, Belarus
c Lomonosov Moscow State University, Vorobyevy Gory, Moscow, Russia
d Institute of Physical and Theoretical Chemistry, Hans-Sommer-str. 10,D-38106 Braunschweig, Germany
Abstract
At present, nanocomposites metal (a mixture of metals) / oxide (carbide, boride, nitride) are widely used as precursors for self-propagating high-temperature synthesis (SHS), sintering, deposition, etc.
It is known that the intermetallide / oxide nanocomposites provide substantially better mechanical characteristics of materials for use at high temperature. For instance, a three-fold increase in the flow stress is observed in oxide dispersion hardened intermetallide FeAl in comparison with sintered FeAl. The stability of nanometer dispersion and of the mechanical characteristics of the material is conserved after thermal treatment at 700 oC for 1000 hours [[1]].
SHS is known to be a good method to obtain powdered intermetallic and oxide materials; however, classical SHS does not allow one to ensure nanometer particle size for the material, monophase intermetallic component and homogeneous distribution of other phases [[2]].
It was shown with a number of metal systems that SHS-preceding mechanical activation during which nanocomposites are formed from the initial components allows obtaining nano-sized monophase intermetallic materials by means of SHS [[3], [4], [5]].
Two approaches were considered in the present work to obtain the intermetallide / oxide nanocomposites:
· mechanochemical synthesis;
· a combination of mechanical activation and SHS.
Sample preparation and examination procedures
Water-cooled ball planetary mills AGO-2 were used to manufacture nanocomposites [[6]]. Mill drum volume was 250 cm3, ball diameter 5 mm, the weight of balls loaded into the mill was 200 g, a sample weight was 10 g, the frequency of rotation of the drums around their common axis was ~1000 rpm, activation was carried out in argon.
The activated mixture was pressed at a pressure of 4-6 t in a press mold until the diameter was ~ Æ 17 mm and the height ~ 25 mm (until the strength was sufficient for carrying the sample into the reactor). Synthesis was carried out in argon; the sample was set on the fire with a spiral filament made of tungsten and heated with electric current.
Powder microstructure was investigated by means of optical metallography (optical microscopes: “Polyvar”, Austria, and “Neophot”, Germany) at the magnification of 100-1000, scanning electron microscopy (certified scanning electron microscope “Camscan” with a microspectral X-ray analyzer AN 10000 of Link Analytic company, according to the quantitative analysis program ZAF4-FLS), and with the help of a JEM-1000X transmission electron microscope.
X-ray phase analysis was carried out with a high-resolution diffractometer D8 ADVANCE of BRUKER AXS GMBH company with software package DIFRACplus.
Differential thermal analysis was carried out with a Q-1500D derivatograph (Hungary) in argon. Measurements were carried out within temperature range 20 to 1000oС at the heating rate of 10 deg/min.
Mossbauer spectra were recorded with a YaGRS-4 spectrometer equipped with a 57Со source in chromium matrix in the constant acceleration mode. Gamma quanta were recorded with a scintillation detector with pulse accumulation in a multi-channel analyzer AI-1024.
I. Mechanochemical preparation of intermetallide / oxide nanocomposites.
It is possible to obtain the intermetallide / oxide nanocomposites mechanochemically by reducing the oxide in an excess of the metal as a reducing agent. Reduction of iron oxides with aluminium was chosen for investigation.
At first, the mechanochemical reduction in the reaction mixture of the stoichiometric composition was investigated. The IR spectra of the products of MA of a mixture of iron oxide with aluminium (Fe2O3 + 2 Al ® Al2O3 + 2 Fe) are shown in Fig. 1. These data provide evidence of the formation of a-Al2O3 phase as rapidly as after MA for only 40 s. Mossbauer spectra after MA for 40 s reveal the presence of reduced iron, a small amount of the initial iron oxide Fe2O3 (hematite), a small amount of the new oxide phase Fe3O4 (magnetite), insignificant amount of intermetallides of the system Fe-Al (Fe2Al5, FeAl2) and a substantial amount (comparable with the amount of free a-fe) of the complex oxide – spinel FexAl2-xO4 (hercynite) (Fig. 2). An increase in the time of MA to 2 minutes causes almost complete consumption of the initial iron oxide, an increase in the content of a-Fe phase, and a decrease in spinel content. The amount of intermetallide mixture with high aluminium content is conserved.
Fig. 2. Composition of activated mixture Fe2O3 + 2 Al.
The IR spectra and X-ray diffraction patterns of the products of MA of the mixture Fe2O3 + 4 Al, that is, with a two-fold excess of aluminium, are shown in Fig. 3. One can clearly see that the reduction of the major part of iron oxide with the formation of a-Al2O3 occurred after MA for 40 s, and FeAl intermetallide was formed. The data obtained from Mossbauer spectra also provide evidence of the formation of a substantial amount of FeAl intermetallide (Fig. 4).
Fig. 4. Composition of activated mixture Fe2O3 + 4 Al.
ccording to these data, a small amount of complex oxide FexAl2-xO4 appears after MA for 40 s; an insignificant amount of the initial iron oxide remains; however, only a smaller part of the reduced metal is present as a-iron, while the major part of iron reacts with aluminium to form intermetallic compounds among which the most abundant one is FeAl.
For a three-fold excess of aluminium in the initial mixture, the rate of mechanochemical reduction decreases noticeably; this is not quite clear from the point of view of classical kinetics. According to the IR spectroscopic data (Fig. 5), the a-Al2O3 phase is formed only after MA for 1 min. Mossbauer spectroscopic investigation showed that the MA of this mixture involves partial reduction of a-Fe2O3; Fe3O4 and spinel appear. After MA for 2 minutes, the product is a mixture of a small amount of a-iron and intermetallides Fe2Al5, FeAl2 and FeAl (Fig. 6).
Fig. 6. Composition of activated mixture Fe2O3 + 6 Al.
We suppose that encapsulation of iron oxide aluminium may occur in this case, which results in worsening of the conditions of nucleation necessary for the topochemical reduction process to proceed.
In the case of a four-fold excess of aluminium in the initial mixture, the reduction of iron oxide almost does not proceed during the investigated time of MA (Fig. 7).
Fig. 7. Composition of activated mixture Fe2O3 + 8 Al.
Estimation of the coherent length of the products formed during MA of the mixtures of iron oxide with aluminium, obtained on the basis of X-ray diffraction data, suggests that this value is within the nanometer range. The effective coherent length (without taking micro-strain into account) is 40-50 nm for a-Al2O3 and ~ 20 nm for intermetallides.
So, the results of investigation allow us to conclude that nanocomposites intermetallide / oxide with the high content of oxide phase can be obtained mechanochemically with rather high efficiency; the composition of the intermetallic phase will depend on the amount of the reducing agent in the initial mixture.
In order to estimate the effect of iron on oxide reduction process, we studied the mechanochemical interaction of iron oxide with iron.
The IR spectroscopic investigation carried out for the mixture Fe + a-Fe2O3 showed that a mixture mechanically activated for 1-2 minutes exhibits two intensive bands with the maxima at 555 and 480 cm-1, related to the stretching vibrations nFe-O (in octahedral coordination) (Fig. 8 a). Further activation for 3-4 minutes causes a sharp decrease in the intensity of these bands. After activation for 5 min, a band with the maximum at 420 cm-1 appears, which correspodns to the stretching vibrations nFe-O of ferrous oxide. The intensity of this band increases sharply after activation for 8 minutes (Fig. 8 с); the band related to the stretching vibrations nFe-O of a-Fe2O3 (555 cm-1) is also conserved. An intensive band of the stretching vibrations of ferrous oxide FeO is characteristic of the IR spectra of the samples after activation for 12 min (Fig. 8 d) and 20 min.
The data of X-ray phase analysis confirm these observations (Fig. 9): after activation for 2 min we observe a decrease in the intensity of diffraction reflections of iron and iron oxide a-Fe2O3 (Fig. 9 a), which allows us to assume that this stage involves grinding of the initial components and the formation of contact surface between them. After activation for 3 min, in addition to broadening of the diffraction peaks, we observe asymmetry of the reflections of a-Fe2O3. After activation for 5 minutes, the reflections of the new phase FeO appear; their intensity increases substantially up to 8 minutes of activation (Fig. 9 b). After activation for 12 minutes, the diffraction patterns contain the reflections of FeO and Fe with nanometer-scale coherent lengths (Fig. 9 с).
Fig. 9. XRD patterns of activated mixture Fe + a-Fe2O3
(а) 1, (b) 8 и (с) 12 min.
A more detailed X-ray diffraction investigation of the mixture activated for 20 minutes showed that ferrous oxide FeO with the effective coherent length of ~ 13 nm had been formed by this moment (Fig. 10). Two reflections of not very high intensity appeared at small angles; they may be attributed to the basic lines of the metastable phase b-Fe2O3 (40-1139). It may be assumed that the appearance of this phase is connected with the decomposition of unstable FeO phase. Further activation does not bring any changes about.
Fig. 10. XRD patterns of mixture Fe + a-Fe2O3, activated during 20 min
The performed investigations showed that the role of iron as a reducing agent in the presence of aluminium is insignificant.
II. A combination of mechanical activation and SHS.
It was shown previously that mechanochemiсally obtained Ме'/Ме'' nanocomposites in the systems with negative enthalpies of mixing can serve as precursors for obtaining nano-sized intermetallic compounds, for example, NiAl, FeAl, by means of SHS [[7], [8], [9]].
To investigate the possibility of making nanocomposites intermetallide / oxide by means of MA + SHS, we chose the system Fe2O3 + Fe + Al in which the content of iron oxide in the initial mixture is much less that that of the metal components (12.5 wt.% Fe2O3 + 60.9 wt.% Fe + 26.6 wt.% Al). It is shown, that for the stoichiometric composition and 2-3-fold excess Al chemical reaction it can be easily carried out mechanochemically. In the mixtures under investigation, the reaction is to proceed not only under a substantial excess of the reducing agent (Al) but also in the case of dilution with the second metal (Fe). Due to rapid removal of heat under the conditions of mechanical activation, the mechanism of the process can change.
The DTA investigation of a mixture of 12.5 wt.% Fe2O3 + 60.9 wt.% Fe + 26.6 wt.% Al showed (Fig. 11, curve 1) that two exothermal peaks with the maxima at 678°С and 982°С are characteristic for this mixture. The first peak corresponds to the exothermal reduction of iron oxide, the second one to the formation of FeAl intermetallide. X-ray phase analysis of the products of mechanochemical interaction in the mixture 12.5 wt.% Fe2O3 + 60.9 wt.% Fe + 26.6 wt.% Al after activation for 2 minutes exhibited the presence of two phases: Fe and Al (Fig. 12 a). X-ray phase analysis did not reveal any oxide phases, including the initial iron oxide. The absence of diffraction reflections of the initial iron oxide phase allows us to assume that its reduction with aluminium proceeded till completion; the absence of the reflections of aluminium oxide is connected either with the formation of X-ray amorphous modification or with the fact that fine aluminium oxide particles are coated, spatially separated and screened by plastic metals.
Fig. 12. XRD patterns of mixture Fe2O3 + Fe + Al after MA (a) and after SHS (b).
In any case, the absence ofdiffraction reflections of aluminium oxide is an indirect confirmation of its nano-dispersed character and its homogeneous distribution over a mixture of metals. The DTA investigation of this mixture after MA for 2 minutes (Fig. 11, curve 2) also provides evidence in favour of complete reduction of iron oxide because now heat evolution is observed only in one point – at 382°С, which corresponds to the formation of intermetallide FeAl. It should be noted that the temperature of the start of formation of FeAl intermetallide decreases substantially after activation. Such a decrease in the temperature of reaction start after preliminary mechanical activation was described in the works of many authors, including us. This decrease is believed to be connected with a decrease in the size of particles of the initial metals, removal of oxide films from their surfaces, formation of tight contacts between them and a substantial increase in the contact surface.
Transmission electron microscopy showed that all the components of mechanochemically synthesized composite have nanometer size (Fig. 13). These data allowed us to assume that mechanical activation of the mixture was accompanied by the chemical reaction involving reduction of iron oxide with the formation of fine aluminium oxide; thus, Fe/Al/Al2O3 nanocomposite was formed. This composite has a layered structure (Fig. 14 a). The concentration curves for iron and aluminium, obtained from the data of x-ray structural microanalysis on this composite, are shown in Fig. 15.