Indian Journal of Pure & Applied Physics
Vol. 42, December 2004, pp.926-933
Structural and electrical properties of nano-sized magnesium aluminate
Siby Kurien, Shajo Sebastian, Jose Mathew & K C George*
Department of Physics, SB College, Changanacherry, Kottayam, Kerala
*E-mail: [email protected]
Received 17 December 2003; revised 20 August 2004; accepted 30 September 2004
The effects of temperature, frequency, and molarity on the dielectric behaviour and ac electrical conductivity have been
studied for nanosized samples of MgAl2O4 prepared by chemical co-precipitation technique. X-ray powder diffraction
(XRD) study shows that the particle size is 8 nm and the system is orthorhombic. FTIR studies have been used to confirm
the formation of the resultant compound. The variations of dielectric constant (∈′), the loss tangent (tanδ) as a function of
frequency (200 Hz – 4 MHz) and the temperature (303-450 K) have also been studied. The experimental results indicate that
the real dielectric constant (∈′), tanδ, and logσ increase with temperature and then decrease. The dielectric constant (∈′),
and tanδ, decrease while the ac conductivity (σac) increases as the frequency increases.
[Keywords: Nanosized magnesium aluminate, Electrical conductivity, Molarity]
IPC Code: B82B
1 Introduction
Owing to small size, nanoparticles show properties,
which are surprisingly different from those of the bulk
material. Since their properties can be engineered
during synthesis and processing steps, the cluster
assembled nanophase materials appear to have great
technological potential, due to their grain size
dependent properties1. Nanophase materials exhibit
greatly altered electrical properties; compared to their
normal, bulk counterparts. Spinel aluminates and their
related structures have been studied thoroughly
because of their theoretical and technological
relevance. Spinel ferrites are widely used for specific
applications, such as magnetic powders for massive
storage device2. There are reports regarding the study
of ionising irradiation effects in MgAl2O4, showing its
high resistance to displacing radiation damage, which
makes it a good candidate for some fusion technology
applications3. MgAl2O4 is a mixed oxide whose
physical properties range between those of
magnesium oxide and aluminium oxide. Magnesium
aluminate is a very good refractory oxide due to its
high melting point. It shows some excellent physical,
chemical, electrical and thermal properties from room
temperature to elevated temperatures. It is very much
resistive towards acids and alkalies4.
The physical properties of ceramic materials are
influenced by the nature of grains, such as shape, size,
orientations, grain boundaries, voids, inhomogeneities
etc. The electronic properties and electrical
conductivity of mesoscopically small crystals, change
compared to their bulk counterparts owing to their small
grain sizes. The change also depends on the large
percentage of their atoms in grain boundary
environments and the interactions between grains5. The
dielectric properties change with the temperature and
frequency of the applied field6. The study of ac
conductivity at different temperatures and frequencies
gives valuable information on conduction in materials
based on localised electric charge carriers7. In the
present study the spinel precursor is characterised by
XRD and FTIR. The effect of temperature and
frequency on the dielectric and ac electrical properties
of nanostructured MgAl2O4 system has also been
studied.
2 Experimental Details
The nanoparticles of MgAl2O4 were prepared by
the chemical route. In this method, MgNO3 and
AlNO3 were dissolved in distilled water in the
required ratio, so that the molar concentration was
0.1 M. Then these solutions were thoroughly mixed
using a magnetic stirrer. Ammonia was added drop by
drop to the solution until the precipitation was
complete. The precipitate thus filtered out, was
washed several times by dissolving it in distilled
water. After removing excess ammonia, it was
allowed to dry by placing it for three hours in a
furnace maintained at 100°C. It was then kept at a
temperature of 300°C for one hour and powdered
using an agate mortar immediately. It was sintered at
900°C for six hours and powdered to ultra fine form.
KURIEN et al.: NANOSIZED MAGNESIUM ALUMINATE
The same experimental steps were carried out to
prepare 0.05 M concentrations by changing the
weight of chemicals and volume of the distilled water.
Table 1 shows the details of XRD-study. FTIR
spectra were also used for the characterisation. The
powder form of MgAl2O4 was consolidated into
pellets of diameter 13 mm and thickness
approximately 2 mm by using a hand-operated
hydraulic press, applying a pressure of 0.3 GPa.The
pellets were again sintered at 900°C for two hours to
obtain maximum density. The surfaces of the samples
were polished and painted with silver paste for good
electrical contact. The painted pellets were
encapsulated with non conducting epoxy powder to
have a proper protection from probable environmental
distractions.
The dielectric measurements as a function of
frequency in the range 100 Hz-4 MHz and
temperature in the range 303-450 K were carried out
using an impedance meter by immersing the
encapsulated pellets in an oil bath, the temperature of
which was regulated with an accuracy of ± 0.1°C. The
capacitance (CP) and dielectric loss (tanδ) were also
measured. The dielectric constant was calculated
using the formula ∈ ′ = C p d . The ac conductivity was
∈0 A
calculated using the equation σac = ∈′∈0ω tanδ, where
ω is the angular frequency.
3 Results and Discussion
The X-ray diffraction patterns of nanoparticles of
MgAl2O4 at 0.1 M concentration, and at different
compositions are shown in Fig. 1. The average
particle size determined from X-ray line broadening
using the Debye-Scherrer equation, is found to be 8
nm for 0.1 M and 6 nm for 0.05 M. According to the
Cheometric model suggested by Rondinone, the size
of the particle decreases when the concentration of the
metal ions is low8,9. The structural study of MgAl2O4
with composition change was also carried out. In
MgxAl2−xO4, where x=0.2, 0.4, 0.6, and 0.8, the peaks
in the XRD pattern (Fig. 1) shift to right as x
increases. The compounds having the crystal structure
of the form AB2X4, where A and B are cations and X
is an anion for example O, S, Se, Te belongs to spinel
group. Because these compounds crystallise with the
same structure as the mineral spinel, MgAl2O4, which
is the parent compound in this group10. Magnesium
aluminate (MgAl2O4) because of its high melting
point and chemical stability, shows remarkable
electrical, magnetic and other physical characteristics.
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Table 1—Details of XRD study
2θ
d(actual)
d(observed)
hkl
%change in d
37.2
45
66.4
2.428
2.058
1.409
2.417
2.014
1.408
302
204
405
0. 453
0.2137
0.07
Fig. 1 —(a) XRD spectra of MgAl2O4 0.1 M. (b), (c), (d) & (e) XRD
spectra of MgxAl2-xO4. (b) x = 0.2, (c) x = 0.4, (d) x = 0.6,(e) x = 0.8
Most of the spinel compounds have the space group
Fd3m. The spinel structure AB2O4 consists of almost
closely packed oxygen anions, in which the
tetrahedral and octahedral sites are occupied by
cations. The unit cell can be obtained by doubling this
fcc oxygen sub lattice, along each of three directions. If
the cations A and B are occupied at the ‘8’ tetrahedral
‘A’ sites, and 16 octahedral ‘B’ sites, out of the 64 and
32 available sites, the arrangement is referred to as the
stoichiometric or normal spinel. In the normal spinel,
Mg2+ ions occupy the tetrahedral positions, and Al3+
ions occupy octahedral positions. There are some
spinel systems where the tetrahedral positions are
occupied by both trivalent and divalent ions and is
known as the inverse spinel structure11 . It may be due
to the arrangement of metal ions in different A and B
sites while the composition changes. An X-ray
diffraction shows that all the samples are in a single
MgAl2O4 phase. The FTIR spectra corresponding to the
nanophase MgAl2O4 are shown in Fig. 2 (a & b). The
symmetry predicts four infrared lattice vibrations ν1, ν2,
ν3, and ν4 .The vibrations ν1 and ν2, are sensitive to the
changes in the interaction between oxygen and cations,
and these frequently appear in the wavelength ranges 600540 and 450-400 cm−1. The vibration ν3 normally appear
close to 330 cm−1 and ν4 is lower12 than 270 cm−1. In
case of MgAl2O4, observed vibrations appear at
wave numbers 535,482,361 and 66 cm−1. But for the
single crystal MgAl2O4, the reported values for different
vibrational frequencies13 are 538,480,309 and 68 cm−1.
A factor group analysis of the spinel structure
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INDIAN J PURE & APPL PHYS, VOL 42, DECEMBER 2004
Fig. 2a—FTIR spectra of MgAl2O4 0.1 M
Fig. 2b—FAR FTIR spectra of MgAl2O4 0.1 M
Fig. 2c—FTIR spectra of MgxAl2-xO4. (a) x = 0.2, (b) x = 0.4,
(c) x = 0.6
yields the following vibrational modes for k=0.
Γ=A1g+Eg+T1g+3T2g+2A2u+2Eu+4T1u+2T2u. The modes
A1g, Eg, and T2g, modes are Raman active while T1u is IR
active14. In the FTIR spectrum of MgAl2O4, the band
at 1400 cm-1 corresponds to γNO3 vibrations, which is
present because of the inadequate decomposition of
nitrates. The band at 3469 cm−1 corresponds to
valence vibrations of the occluded water15. Our
measurement of the spectrum of powered sample
yielded the bands, which are in almost good
agreement with the reported values of single crystals.
The changes in the observed values are due to the
formation of nanophase. In MgxAl2−xO4 for x = 0.2,
0.4, and 0.6 the corresponding ν1 values are 535,
534.7, and 535.1 cm−1 [Fig. 2c]. Actually, at x = 0.6
the vibration level splits into 701 and 535 cm−1. The
splitting of the high frequency band may be explained
by the assumption that a certain number of Al3+ ions
occupies the tetrahedral sites in the spinel type
structure16.
The frequency dependence of the ac electrical
conductivity (σac), dielectric constant (∈′), and
dielectric loss (tanδ) at selected temperatures for the
ceramic material MgAl2O4, have also been studied. The
frequency variation of ∈′ and tan δ in the temperature
range 303-450 K of both 0.1 M and 0.05 M samples
are shown in Figs 3 & 4. At low frequency, ∈′ has a
high value and it decreases with increase in frequency
in both samples and finally becomes independent of
frequency. The dielectric constant ∈′ and tan δ
decrease rapidly at lower frequencies and slowly at
higher frequencies. In normal dielectric behaviour, the
values of dielectric constant and dielectric loss
decrease with increasing frequency reaching a
constant value. Beyond a certain frequency of the
electric field, the electron exchange interaction
between the ions at the octahedral sites cannot follow
the alternation of the applied ac electric field7,15 . In
MHz region, the charge carriers would barely have
started to move before the field reversal occurs and ∈′
falls to a small value at higher frequencies. Fig. 4
shows the variation of tanδ with frequency. In both
samples, the value of tanδ decreases with increasing
frequency. The variation of tan δ with frequency is
similar to the variation of ∈′ with frequency.
According to the two-layer model, the space-charge
polarization arises due to the inhomogeneous
dielectric structure of the material. It is composed of
large well conducting grains separated by thin poorly
conducting intermediate grain boundaries. The
KURIEN et al.: NANOSIZED MAGNESIUM ALUMINATE
Fig. 3—Variation of ε'with log f for 0.1 M sample. Inset: Variation for 0.05 M sample
Fig. 4—Variation of tanδ with log f for 0.1 M sample. Inset: Variation for 0.05 M sample
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INDIAN J PURE & APPL PHYS, VOL 42, DECEMBER 2004
variations of the dielectric loss with frequency at
different temperatures are almost in the same form for
samples with molarities 0.05 M and in 0.1 M. The
change in magnitude may be due to the change in
concentration and hence the change in particle size.
Figure 5 shows the temperature dependence of ∈′
at different frequencies. In both samples, the value of
∈′ is low at room temperature and increases slowly to
its maximum value around 360 K and then decreases
slowly as the temperature increases. The variation of
∈′ with temperature is very much pronounced in the
low frequency regions. This sharp increase of ∈′ at
high temperature in a low frequency region, may be
due to the development of space charge polarisation
arising from defects and impurities present in the
surface of ceramic materials17. The dielectric constant
of a material results from electronic, ionic, and space
charge polarisation. The electronic contribution is
always present and is the main contributor in the
optical range of frequencies. But for higher dielectric
constant value, other factors become dominant. The
effects of frequency and temperature on conduction
are not independent. For the electronic and ionic
polarisations up to 1010 Hz, the frequency as well as
the temperature effect is negligible. At a higher
temperature, however, there is an increasing
contribution resulting from ion mobility and crystal
imperfection mobility. Also, at a higher temperature,
conductivity effect becomes dominant. The combined
effect is to give a sharp rise in the apparent dielectric
constant at low frequencies with increasing
temperature, corresponding to both the ion jump
orientation effects and the space charge effects18. The
variation of ∈′ with temperature at different
frequencies may be due to this reason.
Figure 6 shows the temperature dependence of
dielectric loss tanδ at selected frequencies. The value
of tanδ increases with temperature and reaches a
maximum value at around 360 K and then decreases
with temperature. As the temperature increases, ac
electrical conductivities increase due to the increase in
the drift mobility of thermally activated electrons19.
As a result, the dielectric polarisation increases
causing an increase in tanδ. The frequency
dependence of ac electrical conductivity is shown in
Fig. 7. The conductivity increases with frequency and
temperature in both the samples. According to
theoretical derivations, the real electrical conductivity
is directly proportional to the frequency20. The
derivation was based on the inhomogeneous nature of
the dielectric structure which was considered as a
double-layer dielectric model. The first layer consists
of fairly conducting grains, which are separated by a
second layer of grain boundaries of poorly conducting
material. The grain boundaries are effective at lower
frequencies and grains at higher frequencies21.
Fig. 5—Variation of ε' with temperature for 0.1 M sample. Inset: Variation for 0.05 M sample
KURIEN et al.: NANOSIZED MAGNESIUM ALUMINATE
931
Fig. 6—Variation of tanδ with temperature for 0.1 M sample. Inset: Variation for 0.05 M sample
Fig. 7—Variation of log10 σac with log f for 0.1 M sample. Inset: Variation for 0.05 M sample
Figure 8 shows the variation of conductivity
(log10σac) with 1000 at different frequencies for the
T
prepared samples of MgAl2O4 at two molarities. It is
clear from the figures that the conductivity is high at
higher frequencies at a given temperature, thus
confirming small polaron hopping in the present
samples. In the low-temperature region, the
conductivity increases with temperature and in the
high temperature region it decreases with the increase
in the temperature. The same variation is observed at
all frequencies. This form of variation is already
reported. The total conductivity of the compound is
given by σtotal = σo + σac. The value of σo, which is
frequency-independent and temperature-dependent,
arises because of the drift mobility of electric charge
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INDIAN J PURE & APPL PHYS, VOL 42, DECEMBER 2004
Fig. 8—Variation of log10 σac with 1000/T for 0.1 M sample. Inset: Variation for 0.05 M sample
carriers. It is actually dc electrical conductivity. σac is
frequency- and temperature-dependent and is related
to dielectric relaxation caused by localised electric
charge carriers.
σac = Bωs
… (1)
B and s are composition- and temperature-dependent
parameters. B has conductivity unit (Ω−1cm−1), while s
is dimensionless, and its value is 0 < s < 1. The
conductivity increases with frequency. According to
Eq. (1), the logarithmic relation between σac and the
angular frequency ω(2πf) represents a straight line with a
slope, equal to the exponent s, and intercepts a part equal
to logB on the y-axis at log f = 0 for each sample at the
designated temperature22. The conductivity increases as
the frequency increases and becomes frequencyindependent at a higher temperature. In this case, the
material may be changing to its disordered form from its
ordered form.
4 Conclusion
Nanophase MgAl2O4 was prepared by the
conventional chemical co-precipitation method. By
using the X-ray diffraction pattern the particle sizes
were measured. The structural characterisation was
made using the FTIR spectrum. The variation of
electrical properties, such as conductivity, dielectric
loss, and dielectric constant with temperature and
frequency was studied. The dielectric constant and the
loss decrease rapidly with increasing frequency and
reach a constant value at all temperatures, showing
that it is independent of frequency at higher
frequency. The dielectric constant increases with
temperature, and reaches a maximum value and after
that it decreases. This type of variation was observed
for all frequencies. The dielectric loss also shows the
same type of variation as observed in the case of the
dielectric constant. The ac conductivity is found to be
high for a higher frequency, and shows a trend
necessary for a small polaron hopping.
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