The combined effect of mechanical and thermal energy on the solid-state formation of NiFe2O4 from the system 2NiCO3·3Ni(OH)2·4H2O-FeC2O4·2H2O

Article abstract of DOI:10.1016/j.tca.2007.11.025

Spinel-type ferrites MFe₂O₄ (M = Ni, Zn, Mn, Co, Mg) are among the most important magnetic materials and they are prepared mostly by solid-state reaction at elevated temperatures. We proposed in the past [V. Sepelak, I. Bergmann, A. Feldhoff, P. Heitjans, F. Krumeich, D. Mmenzel, F.J. Litterst, S.J. Campbell, K. Becker, J. Phys. Chem. C 111 (2007) 502612; S. Bid, P. Sahu, S.K. Pradhan, Physica E 39 (2007) 175] a mixed synthetic route that uses both mechanical and thermal energy. In the present work the strategy is applied to the preparation of NiFe₂O₄ starting from mixtures of 2NiCO₃·3Ni(OH)₂·4H₂O-FeC₂O₄·2H₂O subjected to both mechanical and thermal annealing. TG/DSC measurements allowed to obtain information on the reaction mechanism and also a ΔH for the reaction between the constituent oxides yielding NiFe₂O₄ in good agreement with the deduced from the data of ΔG° versus T. The XRPD of the samples recovered after the TG/DSC runs shows the reflexions of NiFe₂O₄ only when starting from milled mixture. NiFe₂O₄ can be prepared by thermal treatment of physical mixtures 2NiCO₃·3Ni(OH)₂·4H2O-10FeC₂O₄·2H₂O by 36 h at 1100 °C. On the contrary 12 h at 400 °C yield XRPD-phase pure NiFe₂O₄. Samples of NiFe₂O₄ obtained from the milled mixture show a surface area that decrease by increasing the synthesis temperature from 400 °C to 700 °C. On the basis of measurements of molar thermal capacity, it is demonstrated that NiFe₂O₄ can be obtained by a 12 h annealing at 450 °C of the mechanically activated mixture.

Full text of DOI:10.1016/j.tca.2007.11.025

Available online at www.sciencedirect.com
Thermochimica Acta 469 (2008) 86–90
The combined effect of mechanical and thermal energy on
the solid-state formation of NiFe2O4 from the system
2NiCO3·3Ni(OH)2·4H2O–FeC2O4·2H2O
Vittorio Berbenni∗, Chiara Milanese, Giovanna Bruni, Amedeo Marini
CSGI, Dipartimento di Chimica Fisica dell’Universita` di Pavia,
Via Taramelli 16, 27100 Pavia, Italy
Received 10 September 2007; received in revised form 26 November 2007; accepted 27 November 2007
Available online 15 December 2007
Abstract
Spinel-type ferrites MFe2O4 (M = Ni, Zn, Mn, Co, Mg) are among the most important magnetic materials and they are prepared mostly
by solid-state reaction at elevated temperatures. We proposed in the past [V. Sepelak, I. Bergmann, A. Feldhoff, P. Heitjans, F. Krumeich, D.
Mmenzel, F.J. Litterst, S.J. Campbell, K. Becker, J. Phys. Chem. C 111 (2007) 502612; S. Bid, P. Sahu, S.K. Pradhan, Physica E 39 (2007)
175] a mixed synthetic route that uses both mechanical and thermal energy. In the present work the strategy is applied to the preparation of
NiFe2O4 starting from mixtures of 2NiCO3·3Ni(OH)2·4H2O–FeC2O4·2H2O subjected to both mechanical and thermal annealing. TG/DSC mea-
surements allowed to obtain information on the reaction mechanism and also a ꢀH for the reaction between the constituent oxides yielding
NiFe2O4 in good agreement with the deduced from the data of ꢀG◦ versus T. The XRPD of the samples recovered after the TG/DSC runs
shows the reflexions of NiFe2O4 only when starting from milled mixture. NiFe2O4 can be prepared by thermal treatment of physical mixtures
2NiCO3·3Ni(OH)2·4H2O–10FeC2O4·2H2O by 36 h at 1100 ◦C. On the contrary 12 h at 400 ◦C yield XRPD-phase pure NiFe2O4. Samples of
NiFe2O4 obtained from the milled mixture show a surface area that decrease by increasing the synthesis temperature from 400 ◦C to 700 ◦C. On the
basis of measurements of molar thermal capacity, it is demonstrated that NiFe2O4 can be obtained by a 12 h annealing at 450 ◦C of the mechanically
activated mixture.
© 2008 Published by Elsevier B.V.
Keywords: Solid-state synthesis; NiFe2O4; Mechanical activation; TG/DSC; Heat capacity
1. Introduction
for example, the synthesis of powder of Ni ferrite has been
accomplished by a self-propagating high-temperature synthe-
sis [4] and, moreover, several wet chemical methods have been
developed to synthesize oxide ceramic powders to improve
their properties. Indeed hard- and soft-magnetic ultrafine fer-
rite powders have been synthesized using a combination of
coprecipitation and mechanical milling [5] and low-temperature
synthesis of NiFe2O4 has been performed by hydrothermal
methods [6–8]. Furthermore Rashad and Fouad [9] synthe-
sized Ni ferrite powders from fly ash via a chemical synthesis
route (coprecipitation method) where the precursor had to
be treated at T > 800 ◦C for 2 h to obtain NiFe2O4. Again
nickel ferrite powders were synthesized by pulse wire dis-
charge method and characterized for their magnetic properties
[10].
Spinel ferrites MFe2O4 (M = Ni, Zn, Mn, Co, Mg) are among
the most important magnetic materials and have been widely
used for various applications. In particular Ni ferrite powders
(NiFe2O4) have been extensively investigated for several appli-
cations such as ferrofluids, catalysts, microwave devices and
magnetic materials [1–3].
Usually NiFe2O4 powders are prepared by solid-state reac-
tion method and this approach requires heating at very high
temperatures for long periods of time so that the energy con-
sumption is high and the production rate quite limited. For
these reasons, alternative routes have been searched for. So,
∗
It has however to be noted that these methods have also draw-
backs: high pH sensitivity, stringent drying conditions, complex
Corresponding author. Tel.: +39 02 987211; fax: +39 0382 987575.
E-mail address: berbenni@matsci.unipv.it (V. Berbenni).
0040-6031/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.tca.2007.11.025

V. Berbenni et al. / Thermochimica Acta 469 (2008) 86–90
87
equipment and expensive precursors [11]. To circumvent these
problems the preparation of NiFe2O4 has been performed by
direct milling of equimolar mixtures of NiO and Fe2O3: it
has been recently demonstrated that, by milling these mixtures
for times between 1 h and 20 h, nanometric NiFe2O4 could be
obtained [12,13].
As an attempt to take opportunity of the advantages of the
conventional ceramic method (simple equipment, no stringent
drying condition, no expensive precursors) and to do away with
its main drawbacks (very high temperature, lack of microstruc-
tural homogeneity), we developed a “mixed” synthetic route to
some technologically important ferrites such as BaFe12O19 [14]
and ZnFe2O4 [15]. In this sense we used starting mixtures of
precursors (metal, metal carbonates and metal oxalates) that are
of low cost and not very sensitive to air moisture and subjected
them, prior to the annealing stage, to mechanical activation by
high-energy milling. We were able to show that the mechanical
activation reduces the temperature and duration of the ther-
mal activation (annealing) and that such a mechanical–thermal
preparation succeeds in obtaining the desired compound with
interesting magnetic properties [14,15].
Fig. 1. DR/FT-IR spectrum of 2NiCO3·2Ni(OH)2·Ni(OH)HCO3·4H2O.
The mechanically activated mixtures were prepared by dry
milling lots of 2 g of physical mixtures: the powders were put
into zirconia jars (12.5 mL) of a planetary mill (Pulverisette 7
by Fritsch, Germany) with five zirconia balls (12-mm diameter;
the mass ratio between the milling balls and the sample powder
was 7:1). The mill was operated at 450 rpm rotation speed for
times up to 90 h.
In the present work, we applied the same synthetic strat-
egy to nickel ferrite starting from mixtures of 2NiCO3·
3Ni(OH)2·4H2O and FeC2O4·2H2O.
2. Experimental
2.2. Experimental techniques
2.1. Starting chemicals and sample preparation
The specific surface area of the starting reactants, of
the mixtures and of the different samples of NiFe2O4 pre-
pared was determined by N2 adsorption (BET method).
The nitrogen adsorption curve was recorded by a Sorp-
tomatic 1990 (Thermo Electron Corporation, operating with
the static volumetric principle). The correction for the volume
of the sample was introduced by measuring the He adsorp-
tion.
TG/DSC measurements were performed with a TG/DSC
simultaneous thermogravimetric analyser (Q600, TA Instru-
ments Inc., USA) connected to a computer (TA 3000) fitted
with an appropriate software. Samples of ≈50 mg of the
2NiCO3·3Ni(OH)2·4H2O–FeC2O4·2H2O mixtures were placed
in an alumina pan and heated at 10 K/min (under air flow of
100 mL/min) from 25 ◦C up to the temperature (600 ◦C) where
a constant mass value is reached.
X-ray powder diffraction patterns were recorded in step scan
mode (step 0.015◦, 1 s/step, 40 kV, 30 mA, 2θ = 10–55◦, Cu K␣)
with an X-ray powder diffractometer (Bruker D5005) equipped
with a position sensitive detector (PSD, Braun).
The specific heat capacity of the NiFe2O4 samples were
determined by the Q2000 DSC (Tzero technology, TA Instru-
ments Inc., USA) connected to a computer (TA 3000) fitted with
an appropriate software. The samples (≈10 mg) were placed
in aluminium pans that were closed and heated at 20 K/min
(under nitrogen flow of 50 mL/min) from 0 ◦C to 400 ◦C. The
instrument, before the measurements, was calibrated, under
the same experimental conditions, with a sapphire disk (mass
25.93 mg).
The starting chemicals were purchased from Aldrich
Chimica (Italy): 2NiCO3·3Ni(OH)2·4H2O (purity 99+%) and
FeC2O4·2H2O (purity 99.9%).
TG measurements have been performed on samples of the
precursors FeC2O4·2H2O and 2NiCO3·3Ni(OH)2·4H2O. In the
case of FeC2O4·2H2O the mean residual mass attained at 400 ◦C
is 44.63 0.01% that is in good agreement with the value
expected (44.38%) for Fe2O3 formation and the mean resid-
ual mass attained at 600 ◦C from 2NiCO3·3Ni(OH)2·4H2O is
58.79 0.17% that is much lower than that expected (63.56%)
for the formation of NiO.
This experimental expected difference can be explained
by allowing for the fact that 2NiCO3·3Ni(OH)2·4H2O has
converted during storage at room temperature to 2NiCO3·
2Ni(OH)2·Ni(OH)HCO3·4H2O: in this case the expected value
of the residual mass becomes 58.34% that is close to the
mean experimental value attained at 600 ◦C (58.79 0.17%).
Actually such a conversion is supported by observing that
the DR/FT-IR spectrum of the NiO precursor shows a broad
band between 1500 cm−1 and 1200 cm−1 with two maxima
at ≈1450 cm−21−and 1390 cm−1 that are characteristic, respec-
−
tively of CO3 and HCO3 (see Fig. 1). This fact has been
taken into consideration in the preparation of the physical mix-
ture of 2NiCO3·3Ni(OH)2·4H2O–FeC2O4·2H2O (molar ratio
Fe/Ni = 2.0) were prepared by weighing the appropriate amounts
of the reactants and by stirring them in acetone suspension for
3 h. Then the solvent was allowed to evaporate in an oven at
60 ◦C overnight.

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V. Berbenni et al. / Thermochimica Acta 469 (2008) 86–90
Table 1
Exothermic enthalpy values obtained in the DSC scans performed on samples
of mixtures milled for increasing times
Milling time (h)
ꢀH (J g−1
)
Tonset (◦C)
Tmax (◦C)
12
30
32
48
90
−32.0
−39.5
−44.4
−47.6
−49.6
464
476
474
493
519
505
522
522
533
554
expected (48.22%) for the formation of a mixture of Fe2O3
(5 mol) and NiO (5 mol). The importance of this second stage
decreases with increasing milling time: it is no longer present in
the TG curve of the mixture milled 90 h.
As concerns the calorimetric part of the thermal measure-
ments, the DSC curve up to ≈400 ◦C is constituted by the
superimposition of endothermic peaks, due to the dehydration
and decomposition processes, and of exothermic effects, due to
the secondary oxidation CO → CO2. A broad exothermic peak
is present between ≈470 ◦C and ≈520 ◦C in the DSC runs per-
formed on milled mixture. The enthalpy values are reported in
Table 1 and they show an increasing trend by increasing the
milling time.
Fig. 2. TG curves for samples of physical (solid line) and milled (dashed line)
mixture 2NiCO3·2Ni(OH)2·Ni(OH)HCO3·4H2O–FeC2O4·2H2O.
3. Results and discussion
3.1. TG/DSC measurements
TG measurements have been performed on samples of either
physical and milled mixtures of 2NiCO3·3Ni(OH)2·4H2O–
FeC2O4·2H2O.
By considering that the standard free energy of the reaction
NiO + Fe2O3 → NiFe2O4 as a function of T (K) is expressed by
the relationship [16]:
Fig. 2 compares TG run performed on samples of physical
mixture and of milled mixture.
In the case of the sample of physical mixture, three mass loss
processes can be individuated. The residual mass value obtained
at the end of the first process (T ≈ 215 ◦C) is 80.06 0.18%
(mean of three runs). Such a mass value is in fair agreement
with that expected (79.89%) for the reaction:
◦
ꢀ
reacG (J mol−1) = −19, 900 − 3.8T
the value ꢀreacH◦ = −20 kJ mol−1 is calculated at 800 K that
corresponds to −41.1 J g−1. From the calorimetric data it can be
seen that that a milling of ≈30 h is enough to trigger the complete
reaction NiO + Fe2O3 → NiFe2O4 to occur within 600 ◦C.
As a matter of fact XRPD patterns have been recorded on
the residuals recovered after the TG/DSC runs: the reflexions of
NiO and Fe2O3 only are present in the samples coming from the
physical mixtures while the reflexions present in the residuals
of the milled mixture are only those of NiFe2O4 (see Fig. 3).
2NiCO3·2Ni(OH)2·Ni(OH)HCO3·4H2O(s)
+ 10FeC2O4·2H2O(s) → 10FeC2O4(s)
+ 3NiCO3(s) + 2NiO(s) + 27H2O(v)
(1)
The second stage ends at T ≈ 290 ◦C and with a mass value of
53.61 0.28% (mean of three runs) in good agreement with that
expected (53.42%) if the following reaction would take place:
10FeC2O4(s) + (5/2)O2(g)
→ 5Fe2O3(s) + 10CO(g) + 10CO2(g)
(2)
The last process ends with a mean mass value of 48.48 0.04%
that is in fair agreement with the value expected (47.96%) on
the basis of the following reaction:
3NiCO3(s) → 3NiO(s) + 3CO2(g)
(3)
In the TG runs performed on the milled mixture reaction (1) can-
not be individuated. The results show that a first stage of mass
loss occurs whose final mass value decreases with increasing
milling time: namely the mixture sample milled 90 h shows at
T ≈ 300 ◦C a mass value of 50.15% that corresponds to the %
mass expected for the formation of a mixture of Fe2O3 (5 mol),
NiO (4 mol) and NiCO3 (1 mol). The second stage of mass loss
occurs between ≈300 ◦C and ≈400 ◦C and shows a mean final
mass of 48.16 0.20% that very well agrees with the value
Fig. 3. Residuals recovered after TG run up to 650 ◦C in air on samples of
physical mixtures (lower curve) and of milled mixtures (upper curve).

V. Berbenni et al. / Thermochimica Acta 469 (2008) 86–90
89
Table 2
Phases present in samples of physical mixtures after the annealing
T annealing (◦C)
Time annealing (h)
Phases
600
700
800
900
1000
1100
12
12
12
12
80
24
NiO, Fe2O3
NiO, Fe2O3
NiO, Fe2O3
NiO, Fe2O3, NiFe2O4
NiO, Fe2O3, NiFe2O4
NiFe2O4
3.2. Annealing of mixtures and characterization of the
products
The specific surface area of both precursors has been mea-
sured [2NiCO3·3Ni(OH)2·4H2O: 51.8735 m2 g−1; FeC2O4·
2H2O: 1.8597 m2 g−1]. The specific surface area of the phys-
ical mixture is 14.9133 m2 g−1 that is in good agreement with
the value calculated as the weighted mean of the specific sur-
faceareasofthetwoprecursors(14.1124 m2 g−1). Millingofthis
mixture causes the surface area to increase from 14.9133 m2 g−1
to 18.4580 m2 g−1 (≈31%).
Fig. 4. XRPD patterns of sample of 2NiCO3·3Ni(OH)2·4H2O–FeC2O4·2H2O
milled mixture annealed 12 h at temperatures of 400 ◦C (b); 450 ◦C (c); 500 ◦C
(d); 600 ◦C (e) and 700 ◦C (f). The patterns (a) of the precursors mixture annealed
36 h at 1100 ◦C are reported for sake of comparison.
Table 3
Data of specific surface area of samples of mixtures 2NiCO3·3Ni(OH)2·
4H2O–FeC2O4·2H2O subjected to different mechanical and/or thermal
treatments
About 500 mg of physical mixture were put into alumina
boats and heated in air at 10 K/min up to different temperatures
between 600 ◦C and 1100 ◦C where they have been annealed
for different times. Table 2 reports the different phases found
by XRPD after the annealing at each temperature and, from the
data reported in Table 2, it can be concluded that NiFe2O4 can be
prepared starting from a physical mixture only by a prolonged
(24 h) thermal treatment in air at 1100 ◦C.
Mechanical/T (◦C)/time (h)
A (m2 g−1
)
Milled/–/–
18.4580
89.9488
47.7094
33.9285
14.1273
6.4454
Milled/400/12
Milled/450/12
Milled/500/12
Milled/600/24
Milled/700/12
–/1100/36
About 500 mg of the milled mixture were put into alumina
boats and heated in air at 10 K/min up to different temperatures
between 400 ◦C and 700 ◦C where they have been annealed for
12 h. Fig. 4 shows the patterns of the annealed samples: XRPD-
pure NiFe2O4 is obtained after the annealing of 12 h annealing at
400 ◦C. Furthermoreitcanbeseenthat, byincreasingtheanneal-
ing temperature, the peaks of NiFe2O4 become increasingly
narrow. The patterns of the mixture of the precursors annealed
36 h at 1100 ◦C is reported for sake of comparison.
0.3032
450 ◦C, 500 ◦C, 600 ◦C and 700 ◦C. It can be seen that the
annealing at 400 ◦C induces a very important increase of such a
parameter that, then, decreases by annealing at higher temper-
atures. The surface area of NiFe2O4 obtained at 700 ◦C is ≈20
times that of NiFe2O4 obtained at the sample.
To try and discriminate between the different samples of
nickel ferrite prepared at the different temperatures, the molar
heat capacity of NiFe2O4 has been determined in the temper-
ature range 0–400 ◦C. The Cp data of the NiFe2O4 sample
obtained from the precursors subjected to different thermal
and/or mechanical treatments have been fitted by the quadratic
polynomial Cp = A + BT + CT2. The values of the parameters A,
B and C are reported in Table 4 for all the NiFe2O4 samples.
´
The mean value of the Neel temperature (TN determined by
DSC) [17] of the samples of NiFe2O4 prepared from the milled
mixture is TN = 589 2 ◦C that does not significantly differ from
that of the NiFe2O4 sample obtained from the physical mixture
(590 ◦C).
Table 3 reports the surface area of the NiFe2O4 samples
obtained from the milled mixture after 12 h annealing at 400 ◦C,
Table 4
Cp data (mean of three independent measurements) of the samples of NiFe2O4 obtained by different mechanical and thermal treatment of the mixture
2NiCO3·3Ni(OH)2·4H2O–FeC2O4·2H2O
A
B
C
Sample
133.50 11.25
139.70 6.48
136.68 7.75
138.83 2.26
136.52 5.08
131.96 7.27
0.1999 0.0136
0.2405 0.0153
0.2417 0.0140
0.2450 0.0026
0.2462 0.0049
0.2602 0.0093
−3 × 10−4
Milled + 12 h 400 ◦C
Milled + 12 h 450 ◦C
Milled + 12 h 500 ◦C
Milled + 12 h 600 ◦C
Milled + 12 h 700 ◦C
-/36 h 1100 ◦C
−2.8 × 10−4 0.4 × 10−4
−3 × 10−4
−3.7 × 10−4 0.5 × 10−4
−3.3 × 10−4 0.4 × 10−4
−3.6 × 10−4 0.5 × 10−4

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V. Berbenni et al. / Thermochimica Acta 469 (2008) 86–90
It can be observed that the values of the parameters of all the
samples but that annealed at 400 ◦C coincide within the rele-
vant standard deviation. The Cp results act as a confirmation
that NiFe2O4 is obtained, provided high-energy milling of the
mixture, by a thermal treatment of the mixture at temperatures
as low as 450 ◦C while a thermal treatment at 1100 ◦C is needed
to obtain the same compound when starting from a mixture not
previously subject to mechanical activation.
The surface area of NiFe2O4 obtained from the milled mix-
ture is from ≈160 (450 ◦C) to ≈20 times (700 ◦C) higher than
that of the physical one (1100 ◦C).
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The TG curves recorded on samples of physical mixture show
that the reaction occurs in three steps in the case of the physical
mixture: the dehydration of the mixture (stage 1) is followed
from the decomposition of iron(II) oxalate to yield Fe2O3 (stage
2) and finally from the decomposition of NiCO3 to NiO. Stage 1
cannot be individuated in the TG curves recorded on sample of
mechanically activated mixture: the TG curves only show two
stages. The first of them ends at a mass value that corresponds to
a mixture constituted by 5 mol of Fe2O3, 4 mol of NiO and 1 mol
of NiCO3. The second stage, that nearly vanishes by increasing
milling time, shows a final mass value in good agreement with
that expected for the formation of a mixture 5NiO–5Fe2O3.
Furthermore DSC data and XRPD of the residuals recov-
ered after the TG measurements demonstrated that NiFe2O4 is
produced within 600 ◦C only starting from the milled mixture.
XRPD patterns of samples of both physical and milled mix-
ture annealed at temperatures between 400 ◦C and 1100 ◦C show
that NiFe2O4 is obtained by 12 h annealing at temperatures as
low as 400 ◦C while 24 h at 1100 ◦C are needed to yield NiFe2O4
when starting from the physical mixture. Actually Cp measure-
ments indicated that NiFe2O4 is obtained from the mechanically
activated mixture by 12 h annealing at 450 ◦C.
[16] O. Kubaschewski, High Temp. High Press. 4 (1972) 1.
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