35

POSSIBILITY OF THE CORE-IN-SHELL IRON PARTICLES FORMATION VIA

MA SHS TECHNOLOGY

T.Yu.Kiselevaa, A.I.Letsko

b, T.L.Talako

b, S.A.Kovaleva

c, T.F.Grigorieva

d,

A.A.Novakovaa, N.Z.Lyakhov

d

a Department of Physics, Moscow M.V.Lomonosov State University, Moscow, 119991 Russia

b State Scientific Institution “The Power Metallurgy Institute”, Minsk, BY-220005 Republic of Belarus c State Scientific Institution “The Joint Institute of Mechanical Ingineering of the National Academy of

Sciences of Belarus”, Minsk, 220072 Republic of Belarus d Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch, Russian Academy of

Sciences,

Novosibirsk, Russia

e-mail: Kiseleva.TYu@gmail.com Abstract. Iron and zirconia containing powdered composite materials are of great interest for

use as functional coatings, catalysts, for medical and biotechnological purposes.

Mossbauer spectroscopy, X-ray diffraction, Transmission and Scanning electron Microscopy

have been performed to develop the exact condition of powders with Fe@ZrO2 structure formation in

the course of Self-Propagated High temperature synthesis (SHS) on mechanically activated (MA)

precursors. Study of peculiarities of local structure transformation and its influence on the structure of

the resulting SHS structure let to synthesize powdered material with core-in-shell structure with

promising functionality.

Introduction.

The core-shell nanomaterials and nanostructures have gained considerable

attention and become an important research area due to their potential applications in

industry and medicine [1]. Up to now it is found that particles with nanoshells can be

synthesized practically using any material, like semiconductors, metals and insulators.

Among these materials particles with ferromagnetic/antiferromagnetic iron containing

core covered by dielectric shell exhibit very interesting electrical, optical and

magnetic properties [2-3]. Core shell particles can be assembled and further utilized

for creation of another type of novel materials.

Iron and zirconia containing particulate materials are of great interest for use

as functional coatings, catalysts, for medical and biotechnological purposes [4].

Possibility of using ZrO2 for purposes of iron particles encapsulation due to its good

insulating properties, chemical inertness, wear resistance, high fracture toughness

extend applying of composite material in improvement of special composite

technology.

Numerous techniques have been developed to synthesize nanoshell containing

particles. Preparation of such particles in most cases involves multistep synthesis

procedure. The possibility of metal particles encapsulation by mechanically activated

self-propagating high-temperature synthesis MASHS by finding of exact synthesis

conditions has been recently reported [5].

Self-propagating high-temperature synthesis SHS is based on the exothermal

reaction between several chemical elements which passes in a combustion mode after

a local initiation of the process. The preliminary mechanoactive treatment of materials

involved in the reaction is considered efficient for purposefully influencing the

structure of the reactive powder batch and SHS parameters that allows the

mechanisms of phase and structural formation of materials to be regulated during

synthesis. Nevertheless, for high energy oxide–active metal systems, a synthesis of

36

composite materials via the above methods is very difficult, since oxide reduction is

high exothermic and pass usually in the thermal explosion mode, which was earlier

shown using the example of CuO–Me (Me are Al, Ti, and Zr) systems [6–9]. It was

shown that twin-stage mechanical activation including the synthesis of Cu–Ме (where

Ме are Ti and Zr) mechanocomposites at the first stage and a subsequent joint

mechanical activation of the reactive mixture with a formation of triple CuO–Cu/Me

mechanocomposites allows the intensive dispersion of materials to be avoided during

SHS and its translation from the thermal explosion mode into the combustion mode

with keeping the stoichiometric ratio constant between copper oxide and reducing

metal. It was revealed that, during the mechanical activation of interacting Cu–Ti and

Cu–Zr formation of nanocomposites with no intermetallic compounds which

correspond to the equilibrium diagram of these systems [10]. AFM data on Cu/Zr

mechanocomposite revealed the presence of 20- to 50-nm-thick layers at the

interfaces with different structure, that allowed some mechanochemical interaction

between metals to be suggested.

Because in structural studies the methods allowing one to get information on

topological and compositional short-range orders, which largely determine the

properties of synthesized composite systems, are of particular importance, the

uncertainties arising in the study of the structural features of nanoparticles forming the

composite system in the establishment of a relation between their structure and

reactivity, as well as in the exploration of a system behavior as a whole, can be

resolved via resonance techniques. To study the changes in the local structure of

particles during the mechanical activation of metallic systems and in MASHS

compounds containing Fe, Mossbauer spectroscopy on 57

Fe nuclei is a powerful

method [11-13] due to its exceptional resolution (~10–8 eV) and high informativity

for getting both qualitative and quantitative information about the local phase

composition, size effects, ultrathin magnetic and interparticle interactions, and

chemical activity of atoms. However, since Mossbauer spectroscopy allows the local

characteristics of iron atoms and their immediate environment to be obtained, unique

information, being important for the development of technological chains of

nanocomposite synthesis, can be revealed via its combination with

macrocharacteristics obtained via other structural methods, such as electron

microscopy and X-ray diffraction.

This work is aimed at synthesizing a Fe@ZrO2 composite powders from a

precursor prepared via MASHS. For this purpose, the influence of the local structures

and structural phase transformations during mechanochemical preparation of

precursors on the final SHS product, have been studied.

Experimental

In this work the SRFeP 3.200 Fe powder (with a particle size of 20–50 μm),

Fe2O3 oxide (TC 6-09-5346-87, particle size is 10–20 μm), and M-41 zirconium (0.5–

1 mm) were used. Mechanical activation (MA) was implemented in high energy

planetary ball mills with water cooling in an argon atmosphere (the volume of the

steel drum was 250 cm3, diameter of steel balls was 5 mm, and the ratio of a mass of

balls to a mass of the prepared material was 20 : 1; the rate of the drum rotation

around the common axis was ~1000 rep/min).

At the first stage the mechanical activation of the Fe–20 wt % Zr powder

system with the formation of Fe/Zr double mechanocomposites was performed; after

that the iron oxide was added to the obtained composites and the joint mechanical

37

activation was implemented for 4 min with the formation of Fe2O3/Fe/Zr triple

composites. The duration of mechanical activation of Fe–Zr was varied from 1 to 30

min. No contamination of the material from the mill drum and the grinding balls with

the used activation time occurred, as was shown in [14].

SHS was performed in an argon atmosphere, and the sample was set on fire by

the tungsten coil heated by the current. Temperature and combustion rate were

estimated via a thermal couple method (chromel–alumel thermal couples with a

diameter of ≈0.2 mm) using an external twin-channel 24-bit analog-to-digital

converter ADSC24-2T.

The local structure of composites at each synthesis stage was probed via

Mossbauer spectroscopy. Mossbauer spectra were recorded using MS1104 Em

spectrometer at temperatures of 300 and 80 K utilizing a low-temperature cryostat in

the transmission geometry. Co57(Rh) was used as gamma-radiation source.

Analytical processing of spectra was performed using Univem MS software.

Structural and phase states of powder composites and the influence of

mechanical activation on a thin structure of crystallites were studied via X-ray

diffraction on Empyrean Panalytical diffractometer in the Bragg-Brentano geometry

with Сu radiation. An analytical treatment of diffraction data was implemented

utilizing HighScorePlus platforms and ICDD PDF-4 standard database.

Morphology and particle sizes at different synthesis stages were studied via

transmission electron microscopy on a LEO 912 microscope with electronography.

The composition of local zones of nanocomposites was probed on MIRA\TESCAN

high-resolution scanning electron microscope equipped with a micro-X-ray spectral

analysis (MXRSA) attachment.

Results and discussion

X-ray structural analysis revealed approximately 90% of α-Fe and about 10%

of Zr in Fe–20 wt % Zr mixture of powders after mechanoactivation for 4 min. A

further increase in the activation time to 30 min leads to the disappearance of

zirconium phase reflexes. The intensity of iron reflexes is almost unchanged, and the

peak shift is weak. The prolonged mechanical activation induces the decrease in the

average size of iron crystallites from 40 to 13 nm and a slight increase in the lattice

parameter from 0.28654 nm to 0.28660 nm. According to X-ray diffraction, no

obvious chemical interaction between iron and zirconium during the mechanical

activation of Fe–20 wt % Zr powder mixture is observed (Fig. 1). After 4 min of

mechanical activation, zirconium is refined and distributed over the volume of

mechanoactivating mixture, which leads to a small intensity and a large width of Zr

reflexes.

The disappearance of Zr reflexes in the X-ray profile after 20 min mechanical

activation is probably due to its grinding and homogeneous distribution in the formed

composite. Iron is known to be more plastic than zirconium, and this favors a rapid

milling of zirconium and its distribution in the composite mixture. Because of the low

solubility of zirconium and iron in each other [10], a micro structure of particles with

nanoscale inclusions is formed in the metallic matrix that is obvious in the TEM

images of particles (Fig. 1 e). According to electron diffraction, a composite particle

with a size of ~300 nm has a bcc Fe and fcc Zr inclusions (the light spots in the dark-

field pattern) with sizes of 6–14 nm.

38

60000

80000

100000

-F

e -F

e

à)

4000

6000

8000

10000

b)

c)

ZrZ

rZrZr

Zr

-F

e

-F

e

-F

e

-F

e

-F

e

-Fe

30 40 50 60 70 80 90 100

4000

6000

8000

10000

d)

2 grad

Inte

nsi

ty, im

p

-F

e

-F

e

-F

e

e)

Inte

nsi

vit

y,

imp

Figure 1. X-ray diffraction pattern of initial iron particles (a), MC Fe/Zr after 4 min (b),20

min (c), 30 min (d) activation and TEM images in dark field of MC Fe/Zr after 4 min

activation (e)

Figure 2 displays the Mossbauer results for Fe/Zr mechanical nanocomposites

obtained by the mechanical activation of Fe and Zr mixture for 4 (а), 20 (b), and 30

(c) min. A phase composition of the composites determined from Mossbauer spectra

is shown as diagrams (Fig. 2d–2f). The main component of spectra is a sextet with

parameters Нeff = 330 kOe, which corresponds to the particles with bcc α-Fe

structure. The amount of this component with an increasing time of activation is

almost unchanged (82 ± 2%). The spectra also contain components–sextets with

magnetic hyperfine field values lower than for bcc α-Fe: 303, 298, 278, and 240 (±3

kOe). These components can be interpreted in the model [15], according to which a

spectrum of nc iron is described by the component corresponding to the grain of iron

crystalline phase and by components which are due to the disordered state with the

presence of Zr atoms in the grain boundary zone of the nanocrystalline iron. With a

decreasing size of grain of mechanically activated particles, the number of subspectra

from the grain boundaries is expected to rise, as follows from this model [15, 16].

Nonequivalent iron positions will be highlighted by the spectral components with

reduced Нeff values if the iron particles contain small zirconium particles, as is

observed in Fig. 2. This is due to the modified environment of iron atoms in the local

zones of the particle. With increased activation time, the intensity of components

which are due to the interaction between iron and zirconium particles also rises.

39

0,96

0,98

1,00

Inte

nsi

ty, ar

b.u

nit

sFe/Zr 4 min

a)

0,94

0,96

0,98

1,00

1,02

c)

b)

Fe/Zr 20 min

-12 -8 -4 0 4 8 12

0,88

0,90

0,92

0,94

0,96

0,98

1,00

1,02

Fe/Zr 30 min

velocity, mm/s

0

10

20

30

40

50

60

70

80

90

100

phase composition

phase composition

, S

, %

phase composition

-Fe (83%)

-Fe(Zr) (16%)

grain boundary

Zr(Fe) (2%)

0

10

20

30

40

50

60

70

80

90

100

S, %

Zr(Fe) (2%)

-Fe(Zr) (13%)

grain boundary

-Fe (80%)

FeZr2 (5%)

0

10

20

30

40

50

60

70

80

90

100 S

, %

FeZr2 (3%)

Zr(Fe) (3%)

-Fe(Zr) (12%)

grain boundary

-Fe (82%)

Figure 2. Mossbauer spectra of mechanically activated Fe-Zr 4, 20, 30 min (left) calculated

phase composition (right) .

A single component of the spectrum with parameters δ = –0.08 mm/s, being typical of

the Zr(Fe) phase (the dilute solid solution of iron in zirconium) slightly increases and,

herewith, an additional doublet component with Mossbauer parameters δ = –0.15

mm/s and Δ= 0.24 mm/s corresponding to FeZr2. phase parameters arises [17, 18].

Such a structural state is not detected via X-ray diffraction, since its content does not

exceed 3%, as follows from Mossbauer spectroscopy. During the mechanical

activation of a Fe–20 wt % Zr powder mixture even after 30 min treatment, the main

identified phase is α-Fe; Zr is dispersed to the nanosizes, being predominantly in the

form of independent inclusions in both grain boundaries and the volume of iron

particles. At the same time, a small quantity (on the order of 2–5%) of solid solutions

of iron in zirconium, disordered states of zirconium-in-iron solid solutions, and FeZr2

intermetallic compound are observed.

As was shown in our papers [19–23], the mechanical activation of Fe2O3

mixtures with reducing metals (for example, Al and Ti) can occur in the heating

explosion mode, leading to both the complete reduction of iron oxide and the partial

formation of composite structures containing metal oxides, depending on the mutual

concentration and activation energeticity. The activity of metal as a reductant and

energeticity of mechanical activation favors the rates and the degree of passing

40

mechanoactive reaction. The dilution of mixtures by inactive to the reduction metals

(in particular, iron) allows controlling the rates and the mechanisms of interaction. In

the series of metal activity, zirconium is a good reductant: Al < Ti < Zr < Ga < Fe.

The complete passage of the mechanically activated reaction 3Fe2O3 + 3Zr →3ZrO2 +

6Fe (+ΔH) in the planetary ball mill is obvious in the Mossbauer spectrum shown in

Fig. 3.

An analysis of phase composition using the Mossbauer spectrum of the sample after

the mechanical activation of Fe2O3 (6.4 g) and Zr (2.6 g) mixture for 1 min, which is

displayed in the diagram, reveals that the result of the interaction is a composite

containing particles with bcc-Fe (a component with magnetic ultrathin splitting Нeff =

330 kOe (75%) (shown in gray), ZrO2 zirconium oxide with two- and three-valent

iron (a doublet component (33%)—Fe(2+)/ZrO2 with parameters δ = 0.94 mm/s, Δ =

0.85 mm/s, and Fe(3+)/ZrO2 with parameters δ = 0.33 mm/s, Δ = 0.80 mm/s) [24,25].

A spectrum also contains a nonmagnetic singlet with parameters which correspond to

Zr (Fe) (3%) phase (unreacted zirconium fraction). When performing the mechanical

activation of Fe2O3 with Fe/Zr composite particles instead of pure zirconium particles,

the interacting mixture is diluted by a poor reductant metal, namely, iron. Such

dilution, enables us to regulate the process of particle grinding and manage the rates

of the interaction with iron oxide. The interaction of iron oxide with Fe/Zr double

mechanical composites in 4-, 20- and 30-min mechanical activation, studied by us via

transmission electron microscopy, revealed the formation of a composite material

(Fig. 4) in which the size of the smallest fraction of composite particles is reduced and

reaches 100–200 nm. Inclusions with sizes of 6–8 nm are observed in these particles.

SEM study (Fig.4) reveal that large composite particles are characterized by

several heterogeneity scales. An element analysis of the local zones reveals particles

with a size of 3–4 μm (with a round shape, almost unformed and deformed after MA)

that are the pristine iron; the small particles were mainly located at the grain

boundaries of metallic particles and the zones enriched with zirconium (the light

zones) have small sizes. The SEM image highlights the areas around the metallic

particles with reduced heterogeneity scale. Inside these layers it shows the thin zones

with a mixed color which can testify to the subtle mixing of components and/or some

mechanochemical interaction, since zirconium is an active reductant.

A partial oxide reduction after the mechanical activation of Fe2O3 with Fe/Zr

is proven by X-ray diffraction: in these samples after 4 min mechanical activation (in

addition to reflexes from Fe2O3 (R-3c) hematite and α-Fe (Im-3m)), reflexes of

0

20

40

60

80

100

phase composition

Fe(2+)/ZrO2

Fe(2

+)

in Z

rO2

_F

e (

57%

)

Fe(3

+)

in Z

rO2

Zr(

Fe)

S,%

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,94

0,95

0,96

0,97

0,98

0,99

1,00

1,01

Inte

nsity, arb

.units

Fe(3+)/ZrO2 Zr(Fe)

Fe2O

3+Zr (1 min)

velocity, mm\s

-Fe

Figure 3. Mossbauer spectrum of the

full reduction result

41

tetragonal ZrO2 zirconium dioxide (P42/nmc) were identified. It worth mentioning

that the tetragonal phase in zirconium dioxide is metastable, and its stabilization is

possible by Fe ions [25]. Mossbauer results on the interaction between Fe2O3 and

mechanically composite Fe/Zr particles with various times of preliminary treatment

are shown in Fig. 4. The analytical processing of spectra revealed that the durability

of the first MA stage in the case of Fe–20 wt % Zr powder system exerts a small

influence on the structure of the forming composites: the formation of (Fe2+)/ZrO2 is

observed in Fe/Zr samples with a large MA time. According to spectroscopy data, if

the reduction reaction passes during MA, it is not too active. Herewith, the active

grinding of iron particles is observed. If we compare the SFe/SFe(Zr) ratio (the

quantity of bcc iron particles to the number of disordered state on the iron surface), it

is reduced, characterizing the increase in the fraction of the atoms on the particle

surface. A characteristic broadening of the Mossbauer line is also observed.

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,92

0,96

1,00

Inte

nsity

, arb

.uni

ts

a)

0

10

20

30

40

50

60

70

80

90

100

phase composition

phase composition

phase composition

, S %

_Fe(Zr) (16%)

(grain boundary

Fe2O

3 (7%)

_Fe (73%)

Zr(Fe) (4%)

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,90

0,92

0,94

0,96

0,98

1,00

1,02

c)

b)

0

10

20

30

40

50

60

70

80

90

100

S %

Fe-ZrO2 (3%)

Zr(Fe) (4%)

_Fe(Zr) (16%)

grain boundary

_Fe (67%)

Fe2O

3 (9%)

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,92

0,94

0,96

0,98

1,00

velocity, mm\s

0

10

20

30

40

50

60

70

80

90

100

S %

_Fe(Zr) (14%)

grain boundaryZr(Fe)4%

_Fe (67%)

Fe2O

3 (8%)

Fe-ZrO2 3%

Mossbauer studies reveal that, in the case of 4 min mechanical activation of the iron

oxide and Fe/Zr mechanical composites synthesized at various MA times, the

composition of the forming local iron-containing structures is quite similar.

Nevertheless, the parameters and subsequent SHS mechanism with the use of these

composites as precursors differably for various durability of the first MA stage of iron

with zirconium. Figure 5 displays the comparative SHS thermograms depending on

the durability of the first MA stage. In the use of a Fe2O3/Fe/Zr mechanical composite

as a precursor synthesized from Fe/Zr mechanical composite after 4 min MA, the rate

of the system selfheating is 60.7C/s. At a temperature on the order of 584C, the

isothermal plateau is observed, followed by the reducing temperature. According to

the parameters found, the process can be characterized as low temperature combustion

or smoldering. When using a Fe/Zr mechanical composite prepared by 30 min MA,

the rate of the system self-heating decreases to 3.7C/s; the maximum temperature is

anomaly low, being only 142C; and the durability of the isothermal plateau is

prolonged. This is a case of a self-annealing.

Figure 4. Mossbauer spectra

of the samples after the second

MA stage of Fe2O3/Fe/Zr

(x=4,20 and 30 min) (a,b,c) for

4 min, derived phase

composition (bar diagrams),

dark field TEM image and SEM

image of the first sample (d,e).

d)

e)

42

Figure 6 depicts the Mossbauer spectra of SHS composites based on

mechanically synthesized precursors.The main spectral component belongs to α-Fe.

Its narrowing Mossbauer line testifies to the increasing particle sizes after SHS. The

spectrum is better described in the presence of a segment with parameters of Fe2Zr

phase (Нeff = 220 kOe), which indicates the formation of this phase during SHS. A

paramagnetic doublet with parameters being characteristic of a two-valent iron ions in

ZrO2, as well as a doublet caused by Zr(Fe) environment of iron atoms, is also

observed in the spectra. The interaction in the mixture is also proven by the changed

parameters of the components associated with the oxide component. An analysis of

spectral parameters revealed the presence of α-Fe2O3 phase (a sextet with parameters

Нeff = 515 kOe) in the spectra. The sextet subspectra with parameters being close to

both those of Fe3O4 and γ-Fe2O3 are also

resolved in the spectra. It is worth

mentioning that X-ray diffraction does not

allow Fe3O4 and γ-Fe2O3 to be

distinguished.

Mossbauer parameters of these

oxides in the bulk materials are different,

which allows uniquely determining the

oxide structure. Nevertheless, in the case of

decreasing particle sizes, structural

defectiveness, and the nonstoichiometry

caused by the synthesis method, a typical

Mossbauer spectrum of Fe3O4 (γ-Fe2O3)

takes the form of the distributed ultrathin

magnetic fields, which leads to uncertainty

in the structure identification.

It is also possible that one result of

the mechanochemical reduction of hematite

is the heterogeneous mixture containing a set of structural states with various

stoichiometry. In order to reveal the type of these oxides, a Mossbauer spectrum was

Figure 5. SHS thermograms with the

use of Fe2O3/Fe/Zr mechanical

composite precursors depending on the

durability of the first MA stage; 1)

4min; (2) 30 min.

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,92

0,96

1,00

velocity, mm\s

c)

0

10

20

30

40

50

60

70

80

90

100

phase composition

S%

ZrO

2/F

e (

2%

)

-Fe (77%)

Fe

3O

4/

fe2O

3 (

11%

)

-F

e2O

3 ((2

%)

Fe 2

Zr

Zr(

Fe)

)2%

0

10

20

30

40

50

60

70

80

90

100

фазы

S%

Zr(

Fe)

2%

)Z

r(F

e)

3%

)

-Fe (69%)

Fe

3O

4/

fe2O

3

-F

e2O

3(4

%)

ZrO

2/F

e (

2%

)

-F

e(Z

r)(5

%)

Fe

2Z

r (4

%)

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,92

0,94

0,96

0,98

1,00

mmsFe

0

10

20

30

40

50

60

70

80

90

100

S%

phase composition

ZrO

2/F

e (

2%

)

-F

e(Z

r)

Fe

3O

4/

fe2O

3 (

10%

) -Fe (72%)

-F

e2O

(6%

) 3

Fe

2Z

r (5

%)

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,92

0,94

0,96

0,98

1,00

b)

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,94

0,96

0,98

1,00

Inte

nsi

ty, ar

b.u

nit

s

а)

Figure 6. Mossbauer spectra (at 300 K) of

SHS products synthesized on mechanically

activated Fe2O3/Fe/Zr precursors (x=4,20

and 30 min) (a,b,c) phase composition (bar

diagrams), the insert shows a spectrum

registered at T=80 K).

43

recorded at a temperature of Т = 80 K. Figure 8 in the insert proves the presence of γ-

Fe2O3. An additional component is most likely due to the size effects, defectiveness,

and even Zr replacement. According to Mossbauer data, for SHS synthesis on

precursors with 20 and 30 min mechanical activation, there is much less iron and the

amount of the complex oxides increases. Figure 7 displays TEM images (a light-field

one and a dark-field one) of the microstructure of the composite SHS particles based

on the mechanically activated (4 min) Fe/Zr precursor.

a) b) c)

A complex structure inheriting that of mechanically activated precursor is

observed in the image and, herewith, according to the dark-field pattern, the

inclusions of Zr-containing phase are also retained. SEM images of the SHS product

(Fig. 7b,c) reveal the layered structure of the metallic zones and the zones highly

enriched with zirconium. However, the concentration curves testified to the lower

gradient of the metal content in these visually distinguished zones when compared to

the mechanical composites. An analysis of element distribution in the oxide zones has

proven the presence of iron oxides near zirconium ones.

To change the velocity of the SHS process we performed the reducing of the

period of Fe/Zr composite mechanochemical interaction with Fe2O3 in twice (from 4

to 2 min).

The result of interaction revealed from Mossbauer spectroscopy analysis

(Fig.8 (a)) shows that it slightly different from the result of the 4 minute activation.

The difference lies in the amount of transformed Fe2O3 and the Zr(Fe) phase.

Figure 7. TEM (a) and SEM (b,c) images of SHS composites synthesized with

the use of different duration of the first step 4min (a), 20 min (b), 30

min (c).

44

The comparison of the thermograms of the SHS processes obtained on the precursors Fe2O3/(Fe/Zr) 4 min (Fig.9(1)) and Fe2O3/(Fe/Zr) 2 min (Fig.9 (2)) testify the that the rate of

temperature rise of almost the same in this two samples, but ignition temperature and the

maximum heating temperature are quite different.

SEM images (Fig.10) of the synthesized SHS composite obtained in the characteristic

radiation of Fe, Zr and O showed that large Fe particles (1-3 mkm, green color) are

coated with zirconium oxide (blue). Between large particles there are smaller ones

with the structure of mixed iron-oxides (violet). Mossbauer spectrum of the SHS

composite (Fig.8(b)) have shown that particulate composite mainly in ferromagnetic

state with small addition of antiferromagnetic and paramagnetic phases . Spectrum

revealed that along with Fe-component, there is a small amount of intermetallic Fe2Zr

and about 2% of a solid solution of Fe in Zr (probably at the interface zone) .

Figure 9. Comparison of

SHS thermograms for the

samples with precursors

Fe2O3/(Fe/Zr) 4 min (1) and

Fe2O3/(Fe/Zr) 2 min (2)

Figure 8. Mossbauer spectra of Fe2O3/(Fe/Zr) mixture mechanically activated during 2

min (a) and the result of SHS on this precursor (b)

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,90

0,93

0,96

0,99

1,02

Re

l.In

ten

sity

Velocity, mm/s

Fe

FeZr

Fe2+

:ZrO2

Fe2O

3

Fe3O

4

b)

-12 -8 -4 0 4 8 12

0,96

0,98

1,00

Velocity, mm/s

Re

l.In

ten

sity, a

rb.u

nits

Fe2O

3+Fe/Zr 2 min

a)

0

10

20

30

40

50

60

70

80

90

100

Zr(

Fe

)(2

%)

g

rain

bo

un

da

ry (

16

%)

Fe

(74

%)

S%

phase composition

Fe

2O

3(8

%)

0

10

20

30

40

50

60

70

80

90

100

phase composition

ZrO

2/F

e+

(5

%)

Fe

2Z

r (7

%)

Fe

(50

%)

g-F

e2O

3/F

e3O

4(1

1%

)

S%

Fe

2O

3 (

11

%)

45

a) b)

Conclusions

To sum up the research, the considerable changes in the parameters and SHS

mechanism in mechanically activated composite precursors with increasing durability

of the first MA stage up to 30 min can be explained by the grinding of the internal

structure of the composite precursors and the dispersion of nanoscale zirconium

particles in iron leading to a decrease in the contact surface of zirconium and iron

oxide, the reduced size of the local self-heating zones as a result of metallothermal

reaction, and thus to the more rapid heat scattering in the bulk of the composite

particles with the refractory components, as well as to the participation of some

zirconium fraction in the reactions of the intermetallide formation (Fe2Zr) and the

complex oxides and significantly reduced heat effect.

Crystalline, almost spherical Fe nanoparticles were successfully synthesized

and encapsulated in zirconia shell by MASHS synthesis when the degree of

mechanical interaction at the stages of precursor formation has been reduced. Despite

of multi-step procedure and complexity of the synthesis, it represents a step forward

in the ability to synthesize zirconia shell on ferromagnetic iron particles that may

serve as functional protective coating regarding to particles stability and magnetic

behavior.

Acknowledgements

This work was supported by the Russian Academy of Sciences (project no. 19), the

National Academy of Science of Belorussia (project no. 5.45.2.7), and the Russian

Foundation for Basic Research (project no. 13-02-00838). Authors thanks also

Moscow University Program of Development for technical support.

Figure 10. SEM images of

cross section of the

incapsulated Fe particles (a)

and elements mapping (b)

46

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