Kinetics of microwave-enhanced solid-phase reaction of NiFe 2O4 formation

Article abstract of DOI:10.1134/S0036023608040013

Microwave-enhanced solid-phase reaction between iron(III) oxide and nickel(II) oxide has been studied at 850-900°C. The formal-kinetic approach to data processing showed that microwave treatment considerably increases the rate of the solid-phase reactions

Full text of DOI:10.1134/S0036023608040013

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 4, pp. 495–498. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © A.S. Vanetsev, A.E. Baranchikov, Yu.D. Tret’yakov, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 4, pp. 549–552.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Kinetics of Microwave-Enhanced Solid-Phase Reaction
of NiFe O Formation
2
4
A. S. Vanetsev, A. E. Baranchikov, and Yu. D. Tret’yakov
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
Received July 31, 2007
Abstract—Microwave-enhanced solid-phase reaction between iron(III) oxide and nickel(II) oxide has been
studied at 850–900°C. The formal-kinetic approach to data processing showed that microwave treatment con-
siderably increases the rate of the solid-phase reactions and changes its rate-controlling stage.
DOI: 10.1134/S0036023608040013
Microwave heating is a promising way to increase
the rates of solid-phase reactions [1, 2]. Microwave
heating makes it possible to carry out important physi-
cochemical processes, such as dehydration, decompo-
sition of salt and hydroxide precursors, synthesis of
multicomponent compounds, and sintering of ceram-
ics, considerably decreasing time and energy consump-
tion compared to conventional versions of these pro-
cesses [2]. The amount of literature regarding the
chemical applications of microwave heating has grown
by several times during the last ten or fifteen years.
Among others, it includes numerous works on the
microwave-assisted synthesis of individual and multi-
component oxide compounds [3]. Unfortunately, most
EXPERIMENTAL
The starting iron(III) oxide was synthesized by the
thermal decomposition of Fe(NO ) · 9H O (pure for
3
3
2
analysis grade) in a burner flame. The resulting product
was in addition subjected to isothermal annealing at
8
00°ë for 3 h. Nickel oxide was synthesized by an iden-
tical procedure with nickel nitrate hexahydrate
Ni(NO ) · 6H O (pure for analysis) as the precursor.
3
2
2
Stoichiometric mixtures of the precursor oxides,
after being ground with an agate mortar, were homoge-
nized in a laboratory Fritsch Pulverizette 7 planetary
mill (agate bowls and milling bodies; heptane). After
this, the powders were dried and pelletized on a Carver
2
works are merely experimental: the choice of precur- Model C hydraulic press (120 kg/mm ). The high-tem-
sors is not substantiated, and discussion only concerns perature treatment of compacted powders was carried
the microstructure and structure-sensitive properties of out at 850 and 900°ë for 10–60 min in a Linn Therm
products, but not fundamental inferences about the Multilabor 2.4/2.45 laboratory microwave oven (output
effects of the physical and chemical properties of pre- power, 2 kW; operation frequency, 2.45 GHz). The
cursors and the organization of the reaction zone or the heating rate used to bring samples to the isothermal
mechanism of the interaction of microwaves with test exposure temperature was 50 K/min. After the isother-
compounds.
mal exposure was over, samples were quenched in air.
References were processed in an ordinary resistor fur-
nace under the same conditions.
In this context, an experimental study of the kinetics
and mechanism of solid-phase reactions in powdered
reagent mixtures and formal-kinetic analysis are of
X-ray powder diffraction was measured on a
considerable interest for suggesting a reaction mecha- DRON-3M diffractometer (CoK
radiation, 0.03°
α
nism, at least, its rate-controlling stage.
In this context, this work studies the mechanism of
microwave-enhanced solid-phase reactions in the system
increments, 5-s exposure). X-ray diffraction patterns
were analyzed with reference to the JCPDS-PDF2 data-
base.
The degree of solid-phase reaction (1), α, was deter-
Fe O + NiO
NiFe O .
(1)
2
3
2
4
mined by quantitative X-ray powder diffraction analy-
sis. The references used were (1 – x){NiO + α-Fe O } +
In choosing this reaction, we were guided by the con-
siderable experience of our team in studies of ultra- xNiFe
2
3
O mechanical mixtures with various molar
2
4
sound-enhanced solid-phase reactions in this system ratios between an equimolar NiO + iron(III) oxide mix-
Fe O –NiO [4, 5] and in the presence of salt additives ture and nickel ferrite NiFe O . The single-phase nickel
2
3
2
4
[
6]. In addition, the mechanism of nickel ferrite forma- ferrite sample used as a component of reference mix-
tion can be considered to be reliably determined [7], tures was synthesized as follows: after being homoge-
which makes it possible to consider this reaction as a nized in a planetary mill, a stoichiometric iron(III)
model one.
oxide + nickel oxide mixture was brought to 1000°ë,
4
95

4
96
VANETSEV et al.
α
Analyzing the degree of reaction as a function of
1
.0
.9
.8
.7
.6
.5
.4
.3
.2
synthesis time in formal kinetic terms, we found that
the rate-controlling stage of the reaction changed in a
microwave field. In the references, ion diffusion
through the product layer was the rate-controlling stage
of the solid-phase reaction over the entire range of the
temperatures studied. This is proven by the satisfactory
goodness of fit provided by the Jander model:
0
0
0
0
0
0
0
0
0
2
3
(
1 – 1 – α ) = k τ,
(5)
J
where τ is the reaction time and k is the apparent rate
J
constant of the reaction described by the Jander model.
We should note that the above description of exper-
imental data is in good agreement with previous kinetic
studies of the solid-phase reaction (1) [5]. The Jander
model describes the kinetics of solid-phase reactions
controlled by the volume diffusion of one component
through the product layer; i.e., this is a diffusion model.
On the other hand, the rate curves for microwave-
treated reaction mixtures are best fitted by the soft
sphere (ss) equation, which describes a solid-phase
reaction controlled by an interfacial chemical reaction:
.1
0
0
10
20
30
40
50
60
τ, min
Fig. 1. Rate curves for reaction (1) under microwave heat-
ing (solid lines) and thermal heating (dashed-and-dotted
lines): ꢀ 850 and ꢁ 900°C.
which was followed by isothermal annealing for 10 h at
this temperature.
3
1 – 1 – α = k τ.
(6)
ss
α was calculated from the relationship
A distinguishing feature of this type of reaction is
(2) almost instantaneous nucleation, and the surface of
2
2
x = 1.67(2)Y – 0.67(2)Y (R = 0.998),
where
each particle becomes coated with a continuous prod-
uct layer in short reaction times; the process rate is pro-
I
2
20
portional to the surface area of the unreacted reagent. In
the absence of diffusion hindrances and with the provi-
sion that iron oxide particles are isotropic, the descrip-
tion in terms of the soft sphere model agrees with the
current ideas about the mechanism of this reaction [7];
it is known that, in the reaction of nickel and iron
oxides, a ferrite layer forms only on Fe O particles.
Y = -------------------- -- .
(3)
I
+ I
104
2
20
Here, I104 is the integrated intensity of the (104) diffrac-
tion maximum from the α-Fe O phase, I is the inte-
grated intensity of the (220) diffraction maximum from
the NiFe O phase, and x is the nickel ferrite mole frac-
2
3
220
2
4
2
3
tion (the degree of reaction).
Regarding the dynamics of alteration of the real
structure of reaction components during the reaction
Mean crystallite sizes for iron(III) oxide and nickel
ferrite were estimated as coherence lengths (CLs) from
the Selyakov–Scherrer relationship
(
Fig. 2), we note that broadening values for both the
starting α-Fe O and the reaction product decrease with
2
3
λ
increasing degree of reaction, indicating an increase in
the coherence lengths for these phases. We failed to
obtain reliable data on the real structure of iron oxide
during 1-h microwave exposure because of the too low
intensity of the respective reflections in the X-ray dif-
fraction pattern.
DCL = --------------------,
(4)
Bcosθ
hkl
where B = βhkl – s (βhkl is the overall broadening of the
hkl) diffraction maximum fitted by the Lorentzian, and
(
s is the instrumental broadening (for the diffractometer
used, 0.09° ± 0.01° 2θ).
This increase in the coherence length is evidently
due to the coherent intergrowth of primary crystallites
of the new phase; the coherent intergrowth rate is suffi-
ciently high at high temperatures (commensurate with
Tammann’s temperature). This behavior of the system
agrees with the postulates of the Jander and soft sphere
models; these models consider both the starting reagent
and the reaction product as compact phases, in which
there are all conditions for the coherent intergrowth of
crystallites.
RESULTS AND DISCUSSION
To study the kinetics and mechanisms of micro-
wave-enhanced solid-phase reactions, we carried out a
set of experiments on the synthesis of nickel ferrite
from oxides with microwave heating. The rate curves in
Fig. 1 imply that microwave heating noticeably enhan-
ced the rate of the solid-phase reaction. The degree of
reaction α within 1-h exposure in a microwave oven
reached 0.98 at 900°ë and 0.92 at 850°ë; in references,
the respective values were 0.31 and 0.22.
The mean crystallite size for the ferrite phase
increases far more appreciably with treatment time
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008

KINETICS OF MICROWAVE-ENHANCED SOLID-PHASE REACTION
from 18–20 nm after 10 min to 40–50 nm after 60 min)
β, °2θ
497
(
than for iron oxide (from 12–15 nm after 10 min to
0
.28
1
5−18 nm after 60 min). Most likely, this fact is due to
(a)
the competition of coherent intergrowth in the iron
0
.26
oxide phase with transformation into NiFe O . The
2
4
product layer forming on Fe O particles creates an
additional hindrance to their coarsening.
2
3
0.24
0
0
0
.22
.20
.18
Another interesting feature of microwave treatment
is as follows: in spite of the considerable difference
between the rates of the solid-phase reaction under
microwave heating and thermal heating, the dynamics
of alteration of the real structure is virtually the same in
both cases. The difference between the coherence
lengths determined in the reference and microwave
experiments is in most cases within the measurement
error limits (Fig. 2). In other words, microwave treat-
ment does not additionally enhance grain coarsening.
This result agrees with numerous studies of microwave-
enhanced sintering [1, 3, 8, 9], where grain coarsening
was virtually unobserved in a microwave field despite
the considerable increase in the ceramic compaction
rate. Likely, this effect is due to the fact that microwave
treatment stimulates intercrystallite diffusion more than
volume diffusion.
0.16
.14
0
0.32
0
0
.31
.30
(
b)
0.29
0
0
.28
.27
Thus, we can conclude that the rate-controlling
stage of the process changes as a result of diffusion
enhancement in a microwave field. Comparing the
results of the experiments we carried out in this work
with nickel ferrite synthesis in salt matrices [6], we
state that the microwave effect consists in releasing dif-
fusion hindrances. In all probability, the reaction mech-
anism does not change upon release. These data are in
good agreement with the currently dominating model
of nonthermal effects of microwaves [10]. This model
postulates that a high-frequency electromagnetic field
should generate fluxes of charged species (ions or
vacancies) in the substance; the intensity of these fluxes
changes symbatically with the intensity of the external
electromagnetic field. As a result, unbalanced fluxes of
charged species appear along any extended crystal
defect (free surfaces, interfaces, or grain boundaries)
and an additional driving force of diffusion is created.
Not influencing the mechanism of the chemical reac-
tion, microwaves intensify diffusion; this means that
the greatest effect of microwaves is expected in diffu-
sion-hindered processes.
0.26
0
0
.25
.24
0.23
0
10
20
30
40
50
60
τ, min
Fig. 2. Physical broadening of (a) (104) α-Fe O3 and
2
(b) (220) NiFe O diffraction peaks vs. time of microwave
2 4
heating (solid lines) and thermal heating (dashed-and-dot-
ted lines): ꢀ 850 and ꢁ 900°C.
vation energy of this process in a microwave field was
near one-fourth that during ordinary heating.
In summary, we have shown that the use of micro-
wave fields is an efficient approach to considerably
increasing the rate of a solid-phase reaction at high tem-
peratures. Having studied the kinetics of the solid-
phase reaction between iron oxide and nickel oxide, we
determined that microwave treatment releases diffusion
hindrances, thus changing the rate-controlling stage of
the reaction.
The release of diffusion limitations should most
likely considerably decrease the activation energy of
the reaction. Unfortunately, the data obtained in this
work do not make it possible to calculate the effective
activation energy of the reaction in a microwave field;
this value is estimated at 60–70 kJ/mol, i.e., 2.5–3 times
lower than the activation energy of the process with
ACKNOWLEDGMENTS
This work was supported by the Russian Foundation
ordinary thermal heating [4]. Analogous results were for Basic Research (project no. 06-03-33042), the
obtained in the kinetic study of the reaction between National Science Foundation, and a Presidential grant
titanium dioxide and barium carbonate [11]. The acti- (MK-5317.2007.3).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008

4
98
VANETSEV et al.
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