0012-5008/05/0012- © 2005 Pleiades Publishing, Inc.0255

Doklady Chemistry, Vol. 405, Part 2, 2005, pp. 255–257. Translated from Doklady Akademii Nauk, Vol. 405, No. 6, 2005, pp. 776–778.Original Russian Text Copyright © 2005 by Lyakhov, Vityaz’, Grigor’eva, Talako, Barinova, Letsko.

Self-propagating high-temperature synthesis is apromising industrial method for manufacturing inor-ganic powders. However, the disadvantages of thismethod, such as micron grain size in the nascent prod-ucts and their multiphase character caused by high tem-peratures of their combustion and quenching, severelylimit its applicability. Previously, it was shown formetal–metal systems that the use of mechanochemi-cally synthesized composites from initial metals makesit possible to significantly decrease the combustiontemperature [1–4] and obtain monophase nanosizedintermetallic compounds [5, 6]. However, for highlyexothermic metal–oxide reactions, there is a dangerthat the reaction can proceed in an activator in the ther-mal explosion mode. In this case, the heat released in avery short time should be rapidly removed, which isdifficult to achieve even in the best-cooled planetaryball mills of the AGO type [7].

In the present work, we found an approach to the syn-thesis of precursors suitable for the preparation of interme-tallic compound/oxide nanocomposites by SHS.

For preparing nanocomposites, AGO-2 planetaryball mills with water cooling were used (jar volume,250 cm

3

; ball diameter, 5 mm; ball charge, 200 g; sam-ple weight, 10 g; rotation speed of grinding jars aroundthe common axis,

~1000

rpm). Activation was carriedout in an argon atmosphere.

The activated mixture was compacted at a pressureof 4–6 t in a mold

~17

mm in diameter and

~25

mm inheight (up to a strength sufficient to transfer the sampleinto a reactor). The synthesis was carried out in anargon atmosphere. The sample was ignited by an elec-trically heated tungsten coil.

The microstructure of powders was studied by opti-cal metallography (Polyvar and Neophot optical micro-

scopes at magnifications from 100 to 1000

×

), scanningelectron microscopy (a Camscan scanning electronmicroscope with a Link Analytical AN10000 energydispersive X-ray analyzer; Link ZAF4-FLS softwarewas used for quantitative analysis), and transmissionelectron microscopy (a JEM 1000X microscope).

Previously [8], we showed that, in metallic systemsthat have low enthalpies of formation of intermetalliccompounds, an SHS process can be carried out at lowdegrees of dilution if mechanochemically synthesizedcomposites, which have a large area of contact betweenthe initial components, are used as precursors. Formetal–oxide systems, with considerably higher heats ofchemical reactions, mechanically stimulated SHS reac-tions can be carried out in an activator. The high exo-thermicity of a system suggests that such reactions canalso be carried out at high degrees of dilution.

If a mixture of metals that can form intermetalliccompounds in the course of SHS, one of the compo-nents being able to reduce the initial oxide, is used as adiluent, mechanochemical activation can result inmetallic composites with uniformly distributed oxides.Thus, in the course of mechanical activation of the

O

y

+ Me'' + Me' mixture, the chemical reaction

O

y

+ Me" O

k

yields

Me'/Me"/ O

k

composites and subsequent SHS from these materialswill result in the formation of

/ O

k

com-posites. Previous studies showed that the use of nano-sized metal–metal composites as precursors leads to theformation of nanosized SHS products. Therefore, webelieve that the same will be true for metal–oxide com-posites.

To check the feasibility of this approach, we usedthe

Fe

2

O

3

+ Fe + Al system, in which the reduction ofthe metal from its oxide is a highly exothermic reaction.The heat of the reaction

Fe

2

O

3

+ 2Al = Al

2

O

3

+ 2Fe is

~840

kJ/mol. This reaction is very fast and has noinduction period; i.e., a mechanically stimulated SHSreaction takes place in the activator. According to X-raypowder diffraction, this reaction yields alumina andiron. To accomplish the reaction under dilution condi-tions, the composition of the initial mixture was chosen

Mex'

Mex' Mez'' Mez''

Men' Mem'' Mez''

Mechanochemically Synthesized Precursors for the Preparation of Intermetallic Compound/Oxide

Nanocomposites by SHS

Corresponding Member of the RAS

N. Z. Lyakhov*,

Academician of the NAS of Belarus

P. A. Vityaz’**, T. F. Grigor’eva*, T. L. Talako**, A. P. Barinova*, and A. I. Letsko**

Received August 25, 2005

* Institute of Solid-State Chemistry and Mechanochemistry, Siberian Division, Russian Academy of Sciences, ul. Kutateladze 18, Novosibirsk, 630128 Russia

** Institute of Powder Metallurgy, National Academy of Sciences of Belarus, ul. Platonova 41, Minsk,220005 Belarus

CHEMICALTECHNOLOGY

256

DOKLADY CHEMISTRY

Vol. 405

Part 2

2005

LYAKHOV

et al

.

taking into account the subsequent formation of theFeAl intermetallic compound. The X-ray powder dif-fraction pattern of the 12.5 wt %

Fe

2

O

3

+ 60.9 wt % Fe +26.6 wt % Al mixture after 2 min of activation showedthe presence of two phases: Fe and Al (Fig. 1a). Nooxide phases, including the initial iron oxide, wererevealed by X-ray diffraction. The absence of reflec-tions due to the initial phase of ferric oxide implies thatit was reduced by aluminum, and the lack of reflectionsdue to alumina is associated either with the formationof an X-ray amorphous modification or with the factthat fine alumina particles are coated and spatially sep-arated by plastic metals. Transmission electron micros-copy showed that all components of the mechanochem-

ically synthesized composite are nanosized (Fig. 2a).These data allowed us to assume that, in the course ofmechanical activation, ferric oxide is reduced with alu-minum to give highly dispersed alumina and the

Fe/Al/Al

2

O

3

composite is formed. This composite has alayered structure (Fig. 3a). The mechanochemicallyprepared nanocomposite was used as the initial materialin the SHS process leading to the formation of the FeAlintermetallic compound (Fig. 1b). The SHS productinherits the morphology of the initial composite(Fig. 3b), although the initial material contained a mix-ture of the metals while the product contained theirchemical compound. The retention of morphology canbe evidence of a highly uniform distribution of the ini-

10

4030 50 60 70 802

θ

, deg

0

20

30

40

50

60

70

I

, arb. units

FeAl

FeAl

(b)

(a)

Fig. 1.

X-ray powder diffraction patterns of Fe

2

O

3

+ Fe + Al samples after (a) mechanical activation and (b) SHS.

0.5

µ

m

(a)

0.5

µ

m

(b)

Fig. 2.

High-resolution microphotographs of a Fe

2

O

3

+ Fe + Al sample after (a) mechanical activation and (b) SHS.

DOKLADY CHEMISTRY

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Part 2

2005

MECHANOCHEMICALLY SYNTHESIZED PRECURSORS 257

tial metals, Fe and Al, in the mechanically obtainednanocomposite. Transmission electron microscopyshowed that the use of nanocomposites as the initialmaterial ensures a high dispersion of the resulting SHSproducts (Fig. 2b).

Thus, our findings show that high degrees of dilu-tion are admissible for highly exothermic reactions ofoxide reduction that proceed in the course of mecha-nochemical synthesis and the rates of these reactionscan remain very high. High degrees of dilution ensurehigh dispersion of the oxides formed upon mecha-nochemical synthesis since secondary aggregation ofthe nascent oxides is most likely prevented. Short-termactivation of such mixtures makes it possible to reducethe oxides and create a nanocomposite that is a highlyhomogeneous mixture of the metals with the mechani-cally synthesized nanosized oxides. Using these mecha-nocomposites as precursors in SHS ensures the forma-tion of intermetallic compound/oxide nanocomposites.

ACKNOWLEDGMENTS

This work was performed under the aegis of RASintegration program no. 8-15, “Fundamental Problemsof the Physics and Chemistry of Nanosized Systemsand Nanomaterials.”

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(‡) (b)

3

µ

m 3

µ

m

A6

A4

A5

Fig. 3.

Microphotographs of Fe

2

O

3

+ Fe + Al samples after (a) mechanical activation and (b) SHS.