Preparation and characterization of BiFeO3 ceramic V' Fruth, L' Mitoseriua, D' Bergerb, A' Ianculesc

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Preparation and characterization of BiFeO3
ceramicV. Fruth, L. Mitoseriua, D. Bergerb, A.
Ianculescub, C. Mateib, S. Preda and M.
ZaharescuInstitute of Physical Chemistry I.G.
Murgulescu, 202 Splaiul Independentei, 060021
Bucharest, Romaniaa Al. I. Cuza University, 11
Carol I, 700506 Iasi, Romaniab Politehnica
University, 1-7 Ghe. Polizu, 011061 Bucharest,
Romania
Abstract From the technological point of view,
the mutual control of electric and magnetic
properties is an attractive possibility, but the
number of candidate materials is limited. One of
them, BiFeO3, has critical conditions for
synthesizing single phase since the temperature
stability range of the phase is very narrow.
Hence, various aspects of BiFeO3 system have to
be studied. Fine particles of BiFeO3 are obtained
using a wet chemical route (combustion technique)
and compared with the same product prepared by
classic solid state reaction. The sintering
programs have been varied in order to investigate
the mechanism reactions and to show the relation
between the microstructures and the
magnetoelectric behavior. The samples are
characterized by using various techniques X-ray
diffraction (XRD) study is carried out for phase
determination and lattice parameter calculations
scanning electron microscopy (SEM-EDX) and
TEM-SAED to find out grain size and morphology
differential scanning calorimeter (DSC) to
determine Néel temperature (TN) and Curie
temperature (TC) of the samples combined with
differential thermal analysis (DTA). The obtained
bulk materials were characterized (density,
porosity, etc) and correlated with the phase
composition present in the samples. Electric and
magnetic properties were evaluated. This study
underlines the role of the preparation route on
the structural and morphological characteristic
of the obtained materials and their influence on
the magnetoelectric behavior.
Experimental Bismuth ferrite was prepared by
solution combustion method from ?-alanine
containing precursor. The precursor was prepared
in aqueous solution from metal nitrates and
?-alanine, at molar ratio Bi(NO3)3Fe(NO3)3?-alan
ine 112. Oxidiser (metal nitrates)-fuel
(?-alanine) compositions were calculated using
oxidising valence of metal nitrates and reducing
valence of ?-alanine. X-ray diffraction data were
collected using a Shimadzu XRD 6000
diffractometer with CuK? radiation at a step of
1.2/min in the range 2? 10 - 80. Scanning
electron micrographs were obtained using a
HITACHI S2600N scanning electron microscope.
Thermal analysis (DTA-TG) was performed in air,
in the temperature range 25-1000C, at a heating
rate of 10C /min using SHIMADZU TG-60
equipment. Bismuth ferrite samples were prepared
by annealing the combustion product in air at
500C and 600C, 1-3h. The obtained nanopowder
was die pressed and sintered at 700C in air for
1 hour. The annealed powders were characterised
by X-ray diffraction (XRD), scanning electron
microscopy (SEM) and transmission electron
microscopy (TEM) combined with selected area
electron diffraction (SAED). FT-IR spectra were
performed on a Nicolet Magna IR Spectrometer
550. As reference, BiFeO3 was prepared by
conventional classic ceramic route starting from
Bi2O3 and Fe2O3 (99 in purity). The mixture of
the two oxides, in molar ratio 11, was
thoroughly mixed and grain sizes smaller than 25
?m were used. Density and porosity were
determinate by immersion technique.
For cooperation DTA profile of Bi2O3 was
presented with red line. One may notice any
exothermal event characteristic to the ? ? ?
Bi2O3 polymorph transition. DTA/TG curves of the
nitrides precursor were presented in the insert.
Characteristic vibration of the perovskite
structures can be observed.
After sintering the nanopowder at 700C/1h (Fig.
4) a high tendency of densification can be
observed. A high interconnection of micron grains
can be noticed. For comparison the SEM image of a
sintered body of a Bi2O3Fe2O3 mixture, after
different annealing treatments, are presented in
Figure 5-6.
(a)
Fig. 3 XRD patterns of the BiFeO3 powders after
different annealing treatment. In isothermal
condition and cumulated treatments (a) the BiFeO3
phase start to form over 700C and is well formed
at 825C after a few minutes treatments. Traces
of a cubic phase were detected and can be
assigned to Bi36Fe2O57 phase. This phase
accompanied as secondary phase BiFeO3 formation.
Bi2Fe4O9 was not detected. In nonisothermal
condition (b) also the BiFeO3 phase formed very
fast at 825C but one can noticed the changed
intensities of the peaks especially that
positioned at 57.06 degrees. Following the
combustion route, very well crystallized BiFeO3
phase can be obtained (c) at 600C and 3 hours
annealing.
The dielectric data were obtained at f1kHz on
the Au-electroded samples in plan-parallel
configuration, by using an impedance analyzer
Solartron SI1260 in the range of 30?200C with a
heating/cooling rate of 0.5C/min. The dielectric
constant measured at two cycles heating/cooling
(Fig. 9) shows a continuous increasing with
temperature and a small thermal hysteresis (in
the sense that the values on cooling are slightly
higher than on heating). Only small values of the
permittivity (??50 at room temperature and ??200
at T200C) are observed, like often reported in
BiFeO3-based materials. The higher values of ?
for Tgt150C are not related to some dielectric
transition (the ferro-para phase transition
temperature being much higher Tc830C), but is
just apparent due to the increasing with
temperature of the dielectric losses (Fig. 10).
In fact, a material with tan?gt1 cannot be
considered anymore a dielectric. Therefore, at
room temperature and below 125C, the losses are
tan?lt10. It is worth to mention the good
reproducibility of the dielectric data found
after a number of cycles heating-cooling both in
the real part of permittivity and losses, showing
that the loss mechanisms are not related to the
possible humidity adsorbed at the ceramic pores
changing the electrical properties of the grain
boundaries. The magnetic moments of the ceramic
samples were measured using a superconducting
quantum interferometric device SQUID magnetometer
(Quantum Design) in the range of temperatures
5?350 K and fields of ?50kOe. The results of the
magnetic measurements are presented in Figs.
11-12. A small non-linearity is visible
particularly at T5 K. This can be associated to
a field-induced weak ferromagnetic state that
transforms into antiferromagnetic one for Tgt10 K
(this transition is better visible in Fig. 12,
where the M(T) dependence is shown). Another
magnetic phase transition seems to take place
around 350 K (Fig. 12).
TEM micrographs of BiFeO3 (600C/3h) at different
scales showing aggregation state (a) and
particles consisting of simple crystalline
domains (b). Although it was difficult to define
a shape, the morphology of the particles seemed
to be approximately spherical. The BiFeO3
particles are aggregated into clusters in the
range of several hundred nanometers. First rings
(d) are associated to (010), (110), (111), (120)
and (121) reflections of the BiFeO3 phase. SAED
studies are in good agreement with the XRD
measurements.
Conclusions Conditions for synthesizing single
phase BiFeO3 using a combustion route were
established. After annealing 3 hours at 600C the
precursor transformed in nanopowder containing
the BiFeO3 compound with parameters of the
hexagonal elementary cell a 5.577 Å and c13.866
Å at room temperature. After the sintering
process dense bodies (95 of the theoretical
density) were obtained. The microstructure shows
a compact structure with grains of nanosize
dimension well interconnected. It is worth to
mention the good reproducibility of the
dielectric data found after a number of cycles
heating-cooling both in the real part of
permittivity and losses, showing that the loss
mechanisms are not related to the possible
humidity adsorbed at the ceramic pores changing
the electrical properties of the grain
boundaries. By the present experiments it was
confirmed the complex nature of the magnetic
activity in BiFeO3 ceramics, with at least two
magnetic transitions in the range of (5, 350) K,
most probable from a weak ferromagnetism to
antiferromagnetism with two types of order below
and above 350 K, respectively. The reaction
mechanisms in the subsolidus region of the
Bi2O3-Fe2O3 system was analized. BiFeO3 formed
above 700C and can be obtain in a few minutes at
temperatures around 800C. Trace of a secondary
phase, with a cubic structure and assigned to a
Bi-rich compound (Bi36 Fe2O56) was observed. This
study underlines the role of the preparation
route on the structure characteristic of the
obtained nanopowders and sintered bodies at lower
temperature.
The main features of the data inserted in the
Fig.8 can be summarised as follow (i) starting
from nanopowders, dense bodies (gt 95 of the
theoretical one) at lower temperature (700C) are
obtained. The same density was reached at 825ºC
when started with oxide mixture. The results
show that sintering of BiFeO3, via combustion
route, has a profound effect on densification and
microstructure. (ii) the reaction following the
classic route is much more dependent on
temperature than time above 750C. In the
temperature range 600-750C the reactions in the
solid are accompanied with a decreasing of the
density with a corresponding increase of the
porosity. Also, for the same density, a higher
contraction in diameter is noticed for the
classic route.