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Conduction and relaxation mechanisms in gadolinium oxide nanoparticle doped polyvinyl alcohol films
Journal Pre-proof Conduction and Relaxation Mechanisms in Gadolinium Oxide Nanoparticle Doped Polyvinyl Alcohol Films S N Madhuri, M V Murugendrappa, K Rukmani
PII:
S2352-4928(19)31569-7
DOI:
https://doi.org/10.1016/j.mtcomm.2020.100942
Reference:
MTCOMM 100942
To appear in:
Materials Today Communications
Received Date:
31 December 2019
Accepted Date:
17 January 2020
Please cite this article as: Madhuri SN, Murugendrappa MV, Rukmani K, Conduction and Relaxation Mechanisms in Gadolinium Oxide Nanoparticle Doped Polyvinyl Alcohol Films, Materials Today Communications (2020), doi: https://doi.org/10.1016/j.mtcomm.2020.100942
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Conduction and Relaxation Mechanisms in Gadolinium Oxide Nanoparticle Doped Polyvinyl Alcohol Films. S N Madhuri1, M V Murugendrappa2 and K Rukmani1,* Department of Physics, Bangalore University, Bengaluru 560056 2 Department of Physics, BMS College of Engineering, Bengaluru 560019 1
*
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Corresponding author: [email protected]
Pure PVA PVA- 2wt % Gd2O3
120
3x107
3x107
PVA- 4wt % Gd2O3
15
90
PVA- 6wt % Gd2O3
2x107
2x107
30
1x107
0 2
3
5
4
5
6
log f
re
10
Z()
60
303K 323K 343K 363K 383K 403K 423K Fit
Z()
150
T = 423 K
-p
20
ro
Graphical abstract
1x107
0
2x107
3
Highlights
5
6
4x107
7
6x10 Z ()
0
0
2x107
Z()
4x107
6x107
Doping in nano form decreases in contrast to doping with ions which were reported to increase . Activation energy decreases monotonically where as resistivity increases with increasing concentration. The films exhibit negative resistance co-efficient with temperature. Relaxation is seen to be non-Debye in nature. Correlated Barrier Hopping is the preferred mechanism for conduction.
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4 log f
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2
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0
0
Abstract
Concentration dependent dielectric and electrical properties of PVA-Gd2O3 nanocomposite films over a wide range of frequencies (100Hz-1MHz) and temperatures (303K-423K) is reported here. TEM of the Gd2O3 nanoparticles showed that they could be indexed to the cubic phase. The frequency variation of the dielectric permittivity and dielectric loss could be explained by the Maxwell Wagner Sillar’s model. The dielectric permittivity decreased in the composite films contrary to observations in other similar systems. The complex impedance showed Arrhenius behavior and the activation energies decreased monotonically with increasing concentration.
of
However the resistivity of the nanocomposite films shows an increase with concentration, the 4wt% film showing the highest resistivity which indicates other contributions to conductivity. The Nyquist plot could be fitted to a simple parallel RC circuit showing the presence of long
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range conduction in these samples. The relaxation times varied from 10-4 to 10-6s and indicated a non-Debye type of relaxation. The master curve showed good overlap indicating that the same
-p
relaxation processes were active at all temperatures. The ac conductivity obeys Jonscher’s law and the exponent shows a monotonic decrease with temperature. This indicates that Correlated
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Barrier Hopping is the probable mechanism for conduction in these samples.
Correlated Barrier Hopping.
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1. Introduction
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Keywords PVA-Gd2O3 Nanocomposite films, FTIR, Activation energy, Jonscher’s power law,
The combination of inorganic nanoparticles and organic polymers forms an interesting class of materials called polymer nanocomposites. Polymers contribute properties such as ease of processing, low cost and flexibility while small quantities of the dopant nanoparticles add useful properties which help to tailor the material for specific
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applications [1-4]. Nanocomposites using a wide variety of polymers have been studied [1-7] and the ones using the polymer poly vinyl alcohol (PVA) are biodegradable and environment friendly. PVA has high mechanical strength, good environmental stability and forms nanocomposites with ease. There is abundant literature reporting the effect of inorganic dopants like ZnBr2, nano ZnO, nano CeO2, CdS, Gd3+, Mn2+, EuCl3, TbCl3, YCl3, Ag [8-16] etc. on the structural, optical, dielectric and electrical properties of PVA so as to enhance the suitability of the material for particular applications. The addition of
semiconductor nanoparticles such as ZnO and HgSe to PVA have shown increased conductivity in these films paving the way for possible applications in the electronic industry [9,16]. The addition of nanoparticles of the rare earth oxide CeO2 to PVA gave rise to enhanced photoluminescence even at low concentrations of the dopant [10]. Rare earth sesquioxides are one of the highly studied materials that have a wide range of applications in optoelectronics [17-19]. They are chemically and thermally very stable. Among them Gd2O3 is of particular interest due to its completely half filled 4f electronic is expected to give rise to interesting optical, dielectric and electrical
of
shell which
properties. The photoluminescence (PL) in lanthanides are in general sharp and intense as these arise from transitions within the f shell energy levels as well as from defect centers,
ro
and this is true in gadolinium also. Amongst the lanthanides, half filled shell has the maximum magnetic moment of I=7/2 as all the electron spins are parallel. This makes it
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useful as a contrast agent in MRI [18]. The large magnetic moment also affects its electrical properties [20]. Recently Sudip Mukherjee et.al. [21] reported high dielectric
re
constant and low loss in a system of Gd2O3 nanocrystal dispersed in a silica matrix. Other reports with similar results are also available in literature [22]. These types of materials
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are promising candidates for memory device applications. Researchers such as Taha A. Hanafy [23] and M. Obula Reddy et.al.[20] have reported the effect of lanthanide ions like La3+, Gd3+ and Er3+ on the dielectric relaxation and ac conductivity of PVA matrix.
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They observed significant increase in the dielectric permittivity and conductivity of PVA. The effect of Gd2O3 nanoparticles and Sm2O3 nanoparticles on PVA films was to give rise to good photoluminescence in the blue and red region with sufficient color purity for optoelectronic applications [24,25]. Because these materials have such a multitude of applications it is of interest to study the conductivity and relaxation mechanisms
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operative in them. The ac conductivity in such nanocomposites can be modeled in many ways, the main mechanisms being quantum mechanical tunneling (QMT), small and large poloron assisted tunneling and Correlated Barrier Hopping (CBH). The most suitable model can be inferred from the temperature dependence of the exponent obtained from fitting the dielectric data to Jonscher’s power law. An analysis of the dielectric data in the light of various formalisms helps delineate the relaxation processes in the material and gives an idea of the mechanisms occurring in these materials. Hence it was felt that a
detailed dielectric analysis of this material will enhance our understanding of the processes taking place and lead to a better design of materials for new applications. With this view, a concentration dependent study of the dielectric and electrical properties of nanocomposite films of Gd2O3 with PVA was undertaken and to the best of our knowledge this is the first report on such a rare earth system. The results of the study point to an uncommon decrease in the value of dielectric permittivity and an increase in
2. Experimental alcohol
((C2H4O)x,
average
molecular
weight
13,000-23,000,
98%
ro
Polyvinyl
of
the resistivity of the films with concentration of Gd2O3.
hydrolyzed), Gadolinium(III) nitrate hexahydrate [Gd(NO3)3.6H2O], glycine [C2H5NO2]
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of purity 99.99% were obtained from Sigma Aldrich and used as obtained. Gd2O3 nanoparticles were synthesized by solution combustion method and PVA-Gd2O3
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nanocomposite films were prepared by solution casting technique. The detailed procedure for sample preparation is given elsewhere [24]. The structural details of the pure PVA and PVA-Gd2O3 (2wt%, 4wt% and 6wt%) nanocomposite films were analyzed by Fourier
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transmission infrared (FTIR) spectra recorded in ATR mode at room temperature by using Nicolet 6700 FTIR with a resolution of 4cm-1. The morphology of the nanoparticles
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was analyzed by TEM images recorded using Titan Themis 300kV, FEI with ultra-bright XFEG gun. The dielectric measurements over a wide range of frequencies (100Hz1MHz) and temperatures (303K-423K) with step size 20K were carried out by Precision Impedance Analyzer (6505 B), Wayne Kerr. 3. Results and Discussion
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A. FTIR analysis
The intermolecular interaction and the structural modifications induced by the dopant in the polymer matrix were analyzed by FTIR spectra. The FTIR spectra of pure PVA and PVA-Gd2O3 nanocomposites with dopant concentration of 2wt%, 4wt% and 6wt% are shown in fig.1. All the composites exhibit the characteristic bands of pure PVA along with few other new bands. The assignments of all the bands are tabulated in Table 1 and are in good agreement with previously reported work [9,25]. A broad and strong band is
observed at 3268 cm-1 which may be assigned to the –OH group present in the backbone
(d)
PVA - 6wt% Gd2O3
(c)
PVA - 4wt% Gd2O3
(b)
PVA-2wt%Gd2O3
of 916 847
-p
1089
1658
ro
1417 1328
2851 2917
3000
2000
Wavenumber (cm-1)
re
4000
1712 1574
Pure PVA
(a)
3268
Transmittance %
of the PVA matrix.
1000
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Fig. 1. FTIR spectra of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
Table 1. Assignments of Infrared bands of PVA-Gd2O3 nanocomposite films.
Assignments
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Wagging of -OH C-C stretching -CH2 rocking of PVA C-O stretching of acetyl group C-O stretching of secondary alcohol Wagging vibration of -CH or C-C stretching Bending (CH-OH) or -CH2 stretching Wagging of -CH2 vibrations -CH bend of -CH2 Bending of -CH2 vibrations Small conjugated polyene sequence -OH and -CH bending vibration Unsaturated C=C stretching mode Acetyl C=O groups of PVA ester -CH symmetric stretching vibration -CH symmetric stretching vibration -CH asymmetric stretching vibration -CH asymmetric stretching vibration -OH group in the polymer background -OH stretching vibration of hydroxyl group of PVA
-p
6wt.% (cm-1) 676 847 917 1092 1143 1238 1335 1379 1412 1547 1652 1713 2853 2920 2929 2987 3272 -
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4wt.% (cm-1) 680 845 916 1090 1143 1238 1330 1378 1435 1555 1656 1708 2851 2919 3269 -
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847 916 1089 1143 1239 1328 1377 1417 1555 1574 1658 1712 2851 2917 2941 3268 -
2wt.% (cm-1) 682 844 916 1090 1143 1238 1328 1378 1434 1558 1654 1713 2852 2921 3260 3617
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PVA
The -CH symmetric stretching is observed at 2917 cm-1. If one looks at the ratio of the
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intensities of the peaks at 3268cm-1 to the peak at 2917 cm-1, it changes from 1.420 in PVA to 0.852, 1.19 and 1.05 for 2wt%, 4 wt% and 6 wt% of Gd2O3 respectively. Assuming that the –CH symmetric peak is unaffected by the addition of Gd2O3, the –OH peak decreases in intensity in the nanocomposites. This indicates decrease in the number of free –OH groups of PVA matrix due to the formation of hydrogen bond between dopant Gd2O3 and –OH group [26,27]. This decrease is maximum in the 2wt% sample and minimum in the 4wt% sample. Sharp and intense bands were observed at 1417cm-1, 1089cm-1 and 847cm-1. These bands correspond to the -CH bending of CH2 and -CO and
C-C stretching of acetyl group respectively. A moderate band is observed at 1574 cm-1 in PVA which corresponds to the bending vibration of –OH group and is absent in the composites. This absence may be due to the formation of hydrogen bonds or metal-ligand complex within the PVA matrix in the composites [12,23]. There is a slight shift and decrease in the intensity of few bands and formation of new bands in the composite which are due the incorporation of Gd2O3 nanoparticles in the PVA matrix. The FTIR also shows bands around 680cm-1 which is due to the wagging of –OH and are seen to be
of
shifted from those in pure PVA. These bands show a blue shift with increasing concentration which could be because of the proximity of the large Gd atoms increasing the free space between the polymer chains. Bands from the Gd2O3 itself have not been
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seen as these occur at low frequencies below our range of measurement. However, the XRD results published by us earlier [24] show the peaks of Gd2O3 confirming the
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incorporation of the nanoparticle into the PVA film. The XPS results also show the presence of Gd3+ in the films while the Raman spectra shows the presence of modes
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corresponding to the nanoparticle in the case of the 6wt% composite film [24]. These prove conclusively that the nanoparticle has got incorporated into the PVA matrix and a
B. TEM analysis
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nanocomposite has been formed.
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The morphology of the synthesized Gd2O3 nanoparticle was analyzed by TEM micrograph (fig.2(a)). The particles were seen to be slightly agglomerated and quasi spherical in shape. The interplanar distance (fig.2(b-c)) was estimated to be 4.413Å and 2.88Å, which correspond to [2 1 1] and [1 2 3] planes of the Gd2O3 nanoparticle respectively. The average particle size was obtained to be 21.20nm. The Selected Area
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Electron Diffraction (SAED) pattern (fig.2(d)) shows that the synthesized nanoparticles are crystalline in nature without any irregularity and the diffraction pattern is well indexed to the cubic structure of the nanoparticle and is in good agreement with the XRD results reported elsewhere [24].
(a)
(b)
d= 2.88 Å (h k l)=(1 2 3)
(50 nm)
(5 nm)
(d)
of
(c)
(8 5 3)
ro
(1 2 3)
(1 3 4)
-p
d=4.4128 Å (h k l)=(2 1 1)
(8 3 1)
(6 1 1)
(5.00 1/nm)
re
(5 nm)
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Fig. 2. (a-c) TEM micrograph of the Gd2O3 nanoparticle, (d) SAED diffraction pattern of Gd2O3 nanoparticle indicating different (h k l) planes
C. Dielectric studies (i)
Temperature and frequency dependent dielectric permittivity and tangent loss
The complex permittivity of the material can be expressed as
j
(1)
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where and are the real and imaginary parts of the complex permittivity and j 1. The real part of the complex permittivity can be given by
where
0
0 r is the permittivity of vacuum ( 8.85 10
(2) 12
F / m ) and r is the relative
permittivity of the material. The magnitude of indicates the ability of the material to store energy from the applied electric field.
The imaginary part of the complex permittivity is given by
tan
(3)
where tan is the dissipation factor. The temperature and frequency dependent dielectric permittivity ( ) and tangent loss (tan ) for all the samples are shown in fig. 3(a-d) and fig. 4(a-d) respectively. It is seen that the dielectric permittivity is high at lower frequencies, decreases as the frequency increases and reaches saturation at very low values after 10 kHz. This phenomenon of larger dielectric permittivity at lower
of
frequencies is well explained using Maxwell-Wagner-Sillar’s polarization (MWS) model [28,29]. The dielectric permittivity increases monotonically with respect to increasing
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temperature in all the samples. The only exception is the curve for the 2wt% sample at the temperature of 303K (room temperature) which does not show an increase in the
-p
value of at low frequencies roughly below 1kHz. The origin of this anomaly is not clear and could be coming from the presence of space charge effect at the low
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frequencies. As a function of concentration (fig. 3(e)) the dielectric permittivity of the nanocomposites decreases drastically with increase in concentration of dopant upto 4wt% Gd2O3 and then increases for 6wt% Gd2O3. This variation is in consonance with the size
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of the nanocrystallites in these films reported by us earlier [24], the 2wt% and 4wt% being smaller around 6nm and the 6wt% being larger around 16nm. The composites
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made from the much larger Gd3+ ions in PVA are reported to show an increase in compared to pure PVA by Hanafy et. al. [20,23]. Their data is however from 50kHz onwards and the actual values obtained in the nanocomposites are similar to those quoted by us. The PVA used by them is of similar average weight yet they quote lower values of
for these films maybe because of their higher frequency of observation. Size
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dependent dielectric permittivity where decreases with decreasing particle size has been observed by S. Chattopadhyay et. al. in antiferroelectric PbZrO3 nanocrystals [30] and ferroelectric PbTiO3 nanocrystals [31]. In these bulk systems, decrease in permittivity was attributed earlier to the occurrence of a diffused phase transition in the material from (anti)ferroelectric to paraelectric which reduced . However, Chattopadhyay et. al. observe this effect by just reducing the size of the particle to the nano regime. Some reports attribute the quench in to random disorder which destroys the long range polar
order [30]. The nano size of the dopant (Gd2O3) in our samples seems to limit the contribution from the dipoles which orient in the electric field. PVA is a semi crystalline polymer made up of amorphous regions and a small amount of crystalline regions [32]. Some type of random disorder, mainly in the amorphous regions of PVA, may be reducing the contribution to . The 4wt% nanocomposite film exhibits very low values of as compared to the other two and this may be due to some random disorder quenching the formation of dipoles or freezing of the dipoles in these films [30]. Hanafy
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et.al. also suggest the possibility of the dopant forming metal ligand type of structure which could quench the dielectric permittivity [12,23]. A small decrease in dielectric
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permittivity as a function of increasing doping has been reported in PVA-Ag nanocomposite at high frequencies [15]. The dielectric tangent loss as a function of frequency (fig. 4(a)) shows a peak that shifts with temperature in pure PVA. In the
-p
composites (fig.4(b-d)), the peak is seen to shift to the lower side with increasing concentration and is below the range of the measurement. Tangent loss increases
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monotonically with increasing temperature in all the samples suggesting the presence of thermally active dielectric process in the samples. The concentration dependent tangent
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loss at 423K is shown in fig.4(e).
When DC conductivity is not negligible, the relationship between total electrical conductivity ( * ) and complex permittivity ( ) is given by the following equation
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s * j 1 ( j )1 0 n *
(4)
where * r j im , r is the real part and im is the imaginary part of the
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conductivity. The real part of the complex permittivity is given by
1 ( s )(1 ( ) sin 2 im n 2 (1 ) 1 2( )1 sin 0 ( ) 2
(5)
Where is the relaxation time, ‘ n ’ is an exponent and ‘ ’ is a parameter whose value lies between 0 and 1. When 0 , the above equation reduces to Debye equation. is
an empirical parameter introduced to account for the changes when there is a spread of relaxation times in the system [33]. The larger the value of the more is the spread in
16
303K 323K 343K 363K
3
3
12
90
(b)
(a)
6
4
383K 403K 423K
383K 1.8 403K 423K
4
2
303K 323K 343K 363K
1.2
303K 323K 343K 363K
(c)
120
20
6
383K 403K 423K
150
8
60 0 102
103
104 f(Hz)
30
105
106
102
103
104 f(Hz)
4
105
0.6 102
2
106
103
104 f(Hz)
105
106
FIT
FIT
0 104
105
102
106
103
104
T = 423 K
15
(d)
2
12
10
PVA- 2wt % Gd2O3 PVA- 4wt % Gd2O3 PVA- 6wt % Gd2O3
(e)
0 2 10
10
3
10
5 10
4
3
4
10
5
10 f(Hz)
10
10
FIT 0 102
10
3
10
4
10
5
4
10
0
10
6
5
10
6
f(Hz)
6
10
2
10
3
4
10 f(Hz)
f(Hz)
105
106
f(Hz)
60
8 2
104
Pure PVA 120
4
20
103
of
16
303K 323K 343K 363K
383K 6 403K 423K
FIT
0 102
106
f(Hz)
f(Hz)
20
105
ro
103
10
5
-p
0 102
10
6
re
Fig. 3. Modified Cole-Cole fit of dielectric permittivity vs frequency at different temperatures of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% and (e) concentration dependent dielectric permittivity of all the samples at 423K
4
4
3
tan
tan
6
303K 323K 343K 363K 383K 403K 423K
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303K 323K 343K 363K 383K 403K 423K
8
2
2
(b) 2wt% Gd2O3
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(a) Pure PVA
303K 323K 343K 363K 383K 403K 423K
3
tan
10
(c) 4wt% Gd2O3
1
1
2
0
0
0 2
3
4 log f
5
2
6
3
4 log f
5
6
2
3
4
5
6
log f
10
303K 323K 343K 363K 383K 403K 423K
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tan
2
8
3
4
log f
PVA- 6wt % Gd2O3
4 2
(e) T = 423 K
0
2
PVA- 4wt % Gd2O3
6
(d) 6wt% Gd2O3
1
Pure PVA PVA- 2wt % Gd2O3
tan
3
5
6
0 2
3
4
5
6
log f
Fig. 4. Temperature and frequency dependent dielectric loss of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% and (e) concentration dependent dielectric loss of all the samples at 423K
relaxation times. A spread in the relaxation times indicates the non-Debye nature of the relaxation, and may arise because the dipoles in the polymer nanocomposite experience slightly varying environments giving rise to different relaxation times. The experimental data has been least square fitted to modified Cole-Cole equation (eqn. 5) in fig. 3 and the parameters tabulated in Table 2. Both and n values were obtained to be between 0 and 1 indicating the non-Debye nature of the relaxation in the films. (ii)
Temperature and frequency dependence of real part (Z ) and imaginary part (Z ) of
of
the complex impedance ( Z * )
The complex impedance can be represented by the following equation
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Z Z jZ
(6)
-p
where Z Z cos is the real part and Z Z sin is the imaginary part of the complex impedance.
The real part of complex impedance Z , is seen to be high at low frequencies, decreases
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as the frequency increases and saturates beyond that (fig.5(a-d)). This may be attributed to the accumulation of charges at the grain boundaries at low frequencies whereas at high
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frequencies the release of space charges gives rise to a decrease in Z value. It can be seen that the Z value is maximum for 4wt% sample compared to the 2wt% and 6wt%
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samples (fig 5(e)).
The Cole-Cole equation for imaginary part of the impedance Z is given by
Z
R
1 ( j )
(7)
where R is resistance of the material, the angular frequency, the relaxation time and
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the fitting parameter whose value lies between 0 and 1. 1 corresponds to the
Debye type of relaxation mechanism; however values of obtained for all the samples were less than 1, indicating the presence of non-Debye type of relaxation mechanism in
) the samples (Table 2). It is seen from the graph that Z reaches a maximum value ( Z max at particular frequency known as relaxation frequency ( max ) and later decreases with
value respect to frequency. It is also seen that as the temperature increases Z max
7
2.0x10
303K 323K 343K 363K 383K 403K 423K
7
1.2x10
6
8.0x10
7
1.0x10
6
5.0x10
4.0x10
(c) 4wt% Gd2O3
(b) 2wt% Gd2O3
(a) Pure PVA 0.0
0.0 3
4
5
0.0
2
6
3
4
5
2
6
3
3x106
6
303K 323K 343K 363K 383K 403K 423K
6
4.0x10
2x106
4x104
0 2
1x10
6
4 log f
6
6
2wt% Gd2O3
(d) 6wt% Gd2O3
4wt% Gd2O3
0.0 4
5
6wt% Gd2O3
(e)
0 3
6
Pure PVA
2.0x10
2
5
8x104
Z'()
6
6.0x10
T = 423 K Z'()
8.0x10
4
log f
log f
log f
of
2
Z'()
7
1.5x10
6
6
4.0x10
303K 323K 343K 363K 383K 403K 423K
2
6
3
4
5
ro
Z'()
6
8.0x10
1.6x10
Z'()
7
1.2x10
303K 323K 343K 363K 383K 403K 423K
7
Z'()
7
1.6x10
6
log f
log f
0 102
103
104
303K 323K 343K 363K
105
106
4x106
103
(d)
105
2x106
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Z''()
104 f (Hz)
103
0 102
1x106
103
103
104
f (Hz)
105
105
(e)
5x105
104
106
105
106
303K 323K 343K 363K 383K 403K
'' Z ( )
2x106
1x106
1x107
0 102
383K 403K 423K FIT
f (Hz)
103
104
105
106
f (Hz)
423K FIT
0 102
103
104 f (Hz)
105
106
Pure PVA 4x104
2x104
0 2 10
103
104
105
106
f(Hz)
2wt% Gd2O3
1x106
0 102
104
f (Hz)
106
303K 323K 343K 363K 383K 403K 423K FIT
3x106
0 102
1x107
Z''()
0 102
2x107
4x105
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6
383K 403K 423K FIT
(c)
Z''()
2x107
f (Hz)
2x10
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'' Z ( )
2x105
6
4x106
303K 323K 343K 363K
Z''()
6x10
3x107
1x106
(b)
8x105
Z''()
Z''()
8x106
3x107
4x105
(a)
'' Z ( )
1x107
re
-p
Fig. 5. Temperature and frequency dependent real part (Z ) of complex impedance of (a) Pure PVA, (b) 2wt% PVA-Gd2O3, (c) 4wt% PVA-Gd2O3, (d) 6wt% PVA-Gd2O3 nanocomposite and (e) concentration dependent Z vs. log f of all the samples at 423K
4wt% Gd2O3 6wt% Gd2O3
T = 423 K
106
0 102
103
104 f(Hz)
105
106
Fig. 6. Modified Cole-Cole fit to variation of imaginary part (Z ) of the complex impedance for temperature range 303K-423K of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% and (e) concentration dependent Z vs. log f of all the samples at 423K
decreases and shifts to higher frequency indicating the reduction in the bulk resistance and the decrease in the relaxation time with temperature. Cole-Cole fitting to the above equation (eqn. 7) and the concentration dependence of imaginary part of complex impedance Z versus frequency are shown in fig. 6(a-d) and fig. 6(e) respectively. If it is found that the imaginary part of complex impedance follows Arrhenius behavior
can be fitted for the relaxation process then the frequency f max corresponding to the Z max Ea kT
, where f 0 is the characteristic
of
(fig. 7) to the equation given by f max f 0 e
frequency, Ea is the activation energy, k is the Boltzmann constant and T is the absolute
ro
temperature. The values of activation energy obtained for all the samples are given in Table 3. It is seen that the activation energy of the samples decreases monotonically with
-p
increase in dopant concentration. This activation energy is a measure of the potential barrier that the molecular dipole has to overcome to go from one orientation to another in
re
the applied field. This could also be the charge hopping between two different sites whose energies are in the form of two potential minima with a potential barrier between them. The latter is more likely in a polymer nanocomposite as there are charges located at
lP
the grain boundaries. The monotonic decrease in the activation energy with increasing dopant concentration indicates that the charges are coming closer as concentration of
ur na
Gd2O3 nanoparticle is increased and need lesser energy to move from one site to another. The resistivity on the other hand does not follow this trend, increasing in going from 2wt% to 4wt% and then decreasing in the 6wt% sample. This behavior indicates that the resistivity depends on factors other than the activation energy, such as distribution of nanoparticles in the film [34]. The resistivity of all the nanocomposites is more than that
Jo
of pure PVA . (iii)
Nyquist complex impedance plot or Cole-Cole plot
Nyquist plot of all the samples for wide range of temperatures (303K-423K) and frequencies (100Hz-1MHz) is shown in fig.8(a-d). A depressed semicircle is observed for all samples which indicate a non-Debye type of relaxation in these materials [29,33]. The experimental data are fitted to semicircle using Zview version 3.2b software and the
equivalent circuit of a parallel RC circuit fits the data well as shown in fig.8. The values of R and C obtained are tabulated in Table 4. It is seen that the capacitance value of the Pure PVA PVA- 2wt% Gd2O3
5
log max
PVA- 4wt% Gd2O3 PVA- 6wt% Gd2O3
4
FIT
of
3
2 2.8
3.2
3.6
ro
2.4
1000/T
-p
Fig. 7. Graph of log max vs. 1000/T (Arrhenius plot) for PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% 4x107
(b)
re
9x106
(a)
3x107
Z()
Z()
lP
3x106
2x107
ur na
7 7 1x10 3x10
0
0
5x106
0
1x107
2x107
7 2x107 2x10
2x107
0
Z()
6x107
8x107
Z()
6
(d)
0
Z()
Z()
2x107
1x107
2x107
0 2x106
4x107
7
6x10 Z ()
1x106
0 0
4x107
3x106
Jo
3x10
4x10 1x107
(c)
7
Z()
6x10
6
2x107
Z()
4x107
6x107
0 0
2x106
4x106
6x106
8x106
Z()
Fig. 8. Nyquist plot for PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
303K 323K 343K 363K 383K 403K 423K Fit
samples increases and resistance decreases with increasing temperature in all the samples. This may be attributed to the increase in the conducting electrons due to thermal agitation caused by increasing temperature. The resistance is more in composites than the pure PVA with the 6wt% showing the least value.
PVA-4wt% Gd2O3
0.70 0.81 0.40 0.68 0.96 0.80 0.70 0.60 0.81 0.89 0.71 0.79
0.67 0.80 0.74 0.43 0.95 0.83 0.40 0.51 0.90 0.75 0.75 0.81
0.90 0.87 0.87 0.92 0.84 0.77 0.95 0.82 0.76 0.79 0.79 0.73
ro
303K 363K 423K 303K 363K 423K 303K 363K 423K 303K 363K 423K
Parameters (Z ) Z (s) 5.9 10-4 8.7 10-5 2.9 10-6 2.9 10-4 5.3 10-5 7.9 10-4 1.3 10-4 2.5 10-4 1.6 10-4 3.8 10-5
0.70 0.76 0.81 0.64 0.74 0.76 0.54 0.67 0.78 0.70 0.72 0.75
M (s)
3.9 10-4 2.2 10-5 1.4 10-6 1.6 10-4 2.2 10-5 3.1 10-5 4.3 10-6 1.6 10-4 5.9 10-5 1.6 10-5
ur na
PVA-6wt% Gd2O3
n
-p
PVA-2wt% Gd2O3
( )
re
Pure PVA
Temperature (K)
lP
Sample
of
Table 2. Values of fitting parameter ( ) and exponent n (fig. 3(a-d)), fitting parameter (Z ) and relaxation time Z (fig. 6(a-d)), fitting parameter and relaxation time M (fig. 11(a-d)) for PVA-Gd2O3 nanocomposite films.
This increase in resistance of the composites may be because the Gd2O3 bonds with the PVA backbone and reduces the number of charge carriers available for conduction. Fig. 9
Jo
shows the concentration dependent Cole-Cole plot of all the samples at 423K.
Table 3. Activation energy of pure PVA and nanocomposite films. Sample Activation energy (eV) Pure PVA 0.55 PVA-2wt%Gd2O3 0.40 PVA-4wt%Gd2O3 0.34 PVA-6wt%Gd2O3 0.23
1x106
1x10
T = 423 K
6
8x105
5x105
4wt% Gd2O3 6wt% Gd2O3
5x105
Fit
3x105
3x105 0 5x105
0
2x106
ro
Z() 5x10 1x10 2x106 2x106 Fig. 9. Concentration dependent Nyquist plot of all the samples at 423K 5
6
-p
0
(iv)
2x106
Temperature and frequency dependent real and imaginary part of electric modulus The electric modulus is given by
re
0
1x106
of
Z()
8x10
Pure PVA 2wt% Gd2O3
5
lP
M * M jM
(8)
where M is the real part of the electric modulus, given by
( 2 ) 2
ur na
M
(9)
and M is the imaginary part of the electric modulus, given by
M
( 2 ) 2
(10)
The variation of M with respect to frequency at different temperatures for the samples is
Jo
shown in fig. 10(a-d). The magnitude of M is negligible at the lower frequencies. This may be due to the negligibly small contribution of electrode polarization in the samples. A continuous dispersion on increasing frequency may be due to the short-range mobility of the charge carriers. The anomaly in in the 2wt% sample noted at 303K is reflected in fig.10(b) also as they represent the same set of data.
1.5
1.0
0.8
(a)
303K 323K 343K 363K
(b) 2wt% Gd2O3 0.6
M
M
Pure PVA
303K 323K 343K 363K 383K 403K 423K
0.4
0.5
0.2
383K 403K 423K 0.0
0.0 2
3
4 log f
5
6
2
2.0
3
4 log f
5
6
1.2
(c) 4wt% Gd2O3
(d) 6wt% Gd2O3 1.0
1.6
0.8
3
4 log f
5
of
0.4 0.2
0.0 2
303K 323K 343K 363K 383K 403K 423K
0.6
0.0
6
2
3
ro
0.4
M
303K 323K 343K 363K 383K 403K 423K
0.8
4 log f
5
6
-p
M
1.2
Fig. 10. M vs. f plot of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% 303K 323K
re
0.3
0.6
303K 323K 343K 363K
423K FIT
383K 403K
343K 363K
0.2
383K 403K 423K FIT
M
M
lP
0.4
0.1
0.2
ur na
(a)
0.0 102
10
3
10
4
10
5
10
0.0 102
6
(b) 103
0.20 0.16
303K 323K 343K 363K
0.5
FIT
10
3
10
4
f (Hz)
106
343K 363K
383K 403K 423K FIT
M
M
Jo 0.00 102
105
0.3
0.08
383K 403K 423K
303K 323K
0.4
0.12
0.04
104
f (Hz)
f (Hz)
0.2 0.1
(c) 10
5
10
6
0.0 102
(d) 103
104
105
106
f (Hz)
Fig. 11. Bergman modified Kohlraush-Williams-Watts function fit for M vs. f plot of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
The plot of M as a function of frequency (fig. 11(a-d)) shows a well defined peak
), which shifts to the higher frequency with temperature. The peak indicates a shift ( M max in the mobility from large range movement to short range movement of the charges and also indicates that the charge carriers undergo thermally activated hopping mechanism. The asymmetry and broadening of the peak suggests the spread of relaxation with different time constants. The solid line in the graph (fig. 11(a-d)) indicates the theoretical fitting of Bergman modified Kohlraush-Williams-Watts (KWW) function to the
M max max 1 1 max
ro
M ( )
of
experimentally obtained data. The KWW function is given by
(11)
-p
where denotes the amount of deviation from the Debye behavior. 1 indicates the ideal Debye nature and 0 indicates the maximum interaction between the dipoles.
re
is the angular frequency and max ( 2f max ) is the angular frequency at M max
lP
(maximum value of M ). The maximum value of M occurs when max 1 , where is the relaxation time which can be calculated by
1 . The calculated value of max
and the fitting parameter are tabulated in Table 2. The values are in the range 10-4-
ur na
10-6 s and are seen to decrease with increasing temperature. This indicates that the faster relaxation is due to increased side group movement of PVA with increasing temperature. It is seen that value lies below 1, indicating the non-Debye type of relaxation mechanism of the samples.
vs. f plot and M verses f plot are complementary to each other. The comparison of
Jo
Z
these two is shown in fig.12. It can be seen that the peaks of Z and M don’t coincide with one another. This result suggests the presence of both long range conduction and of localized relaxation mechanism in the sample [35,36]. The asymmetric nature of the peaks indicates the deviation from ideal Debye nature of the samples. Further the master curve of M
M max
has been shown in fig.13. All the peaks overlap on verses log f f max
one master curve in all the samples, indicating that the same type of relaxation mechanisms are active at all the temperatures. A slight broadening of curve can be seen in 4wt% film.
105
M Z
(a) 0.4
423K
0.18
M Z
(b)
M
Z
M
0.12
0.2 4
10
10 f (Hz)
10
5
10
6
10
105
12
2
0.4
(c)
M Z
423K
103
104 f (Hz)
105
5
(d)
M Z
423K
8
4
Z
3
M
Z
0.08
0 106
105
0.3
lP
0.12
M
1
re
0.16
2
2wt% Gd2O3
0.00
0
4
-p
Pure PVA 3
3
ro
0.06
0.1 0.0 102
423K
8
0.3
4
of
0.5
12
0.2 2
ur na
4
0.04
0.00 102
4wt% Gd2O3 103
104 f (Hz)
105
0 106
0.1
0.0 102
1
6wt% Gd2O3 103
104 f (Hz)
105
Fig. 12. Comparison plot of Z and M with respect to the varying frequency
Jo
Z
105
0 106
303K 323K 343K 363K 383K 403K 423K
0.6 0.4 0.2
0.8
0.0 -2
-1
0
1
0.6 0.4 0.2
Pure PVA -3
2
303K 323K 343K 363K 383K 403K 423K
2wt% Gd2O3
0.0
3
-2
-1
0
0.6
3
ro
0.8 0.6 0.4
re
0.4
(d)
1.0
0.2
lP
MMmax
0.8
4wt% Gd2O3
2
4
303K 323K 343K 363K 383K 403K 423K
-p
303K 323K 343K 363K 383K 403K 423K
MMmax
(c)
0.2
1
log(f/fmax)
log(f/fmax)
1.0
of
MMmax
0.8
(b)
1.0
MMmax
(a)
1.0
6wt% Gd2O3
0.0
0.0 -2
-1
0
1
ur na
log(f/fmax)
2
3
-2
-1
0
1
2
log(f/fmax)
f Master curve of M M vs. log f for PVA-Gd2O3 max max nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% 13.
Jo
Fig.
3
(v)
AC conductivity The ac conductivity ( ac ) can be calculated by the empirical relation ac r o tan , where the symbols have their usual meanings. The ac conductivity of pristine PVA is obtained to be of the order of 10-7 S/m to10-8 S/m and these values are in agreement with those quoted in literature [5,13]. The ac conductivity increases with increasing frequency and also increases with temperature (fig. 14). This may be due to the increase in the mobility of the charge carrier with respect to increase in the temperature. The
of
conductivity is seen to be decreasing with increase in dopant concentration upto 4wt% and again increasing slightly in 6wt% sample. This indicates that the incorporation of
ro
nanoparticles of rare earth oxide in the polymer matrix increased the resistivity of the material.
To find the conduction mechanism in the sample, the conductivity vs. angular frequency
-p
curve has been fitted to the Jonscher’s power law (fig.14), ( ) dc A s , where dc is the dc conductivity, A is a constant which determines the strength of the polarizability,
re
and s is the exponent which gives the information about the degree of interaction between the lattice and mobile ions. It is seen that the power law equation fits the experimental
lP
data well (Fig.14). The variation of exponent s with respect to the temperature indicates the type of conduction mechanism involved in the samples. The exponent s (Fig.15) is found to be decreasing monotonically with increase in the temperature in pure PVA and
ur na
all the nanocomposites. This suggests that the correlated barrier hopping (CBH) is the
Jo
major type of conduction mechanism involved in all the samples [37].
-5
-5
-6
log ac Sm-1
-7
(a) -9 3
4
log
343K 363K 383K
-8
403K 423K FIT
5
-9
4
5
303K 323K
383K 403K 423K
log ac Sm-1
-6
-7
re
-7
-8
3
lP
-8
(c)
FIT
4
log
6
log
-5
-6
log ac Sm-1
(b) 3
6
-5
303K 323K 343K 363K
343K 363K
383K 403K 423K FIT
of
303K 323K
ro
-8
-7
-p
log ac Sm-1
-6
-9
303K 323K
5
6
-9
343K 363K
(d) 3
4
5
383K 403K 423K FIT 6
log
Jo
ur na
Fig. 14. Jonscher’s fit for variation of log ac as a function of temperature and frequency for PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
0.6
s
0.4
0.2
Pure PVA PVA- 2wt% Gd2O3 PVA- 4wt% Gd2O3
0.0 300
330
360
390
420
ro
T(K)
of
PVA- 6wt% Gd2O3
-p
Fig. 15. Variation of ‘s’ with temperature for PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
Sample
Temperature (K)
R ( )
303K 363K 423K 303K 363K 423K 303K 363K 423K 303K 363K 423K
2.08 10 1.88 106 8.80 104 7.93 107 6.81 106 1.02 106 5.64 107 1.35 107 2.06 106 8.28 106 3.60 106 9.33 105
ur na
Pure PVA
lP
re
Table 4. Values of resistance (R) and capacitance (C) (fig. 8), dc (Sm 1 ) (fig. 14).
PVA-2wt% Gd2O3
Jo
PVA-4wt% Gd2O3
PVA-6wt% Gd2O3
7
Parameters C (F) dc Sm 1 3.08 10-12 3.14 10-8 1.23 10-10 3.46 10-7 -9 3.32 10 7.41 10-6 -14 5.26 10 2.33 10-9 -11 1.93 10 2.77 10-8 2.19 10-10 1.85 10-7 -13 1.33 10 6.73 10-9 -12 4.42 10 2.84 10-8 1.01 10-10 1.36 10-7 -11 1.80 10 2.41 10-8 -11 5.44 10 1.54 10-8 -10 2.74 10 2.14 10-7
(
)
4. Conclusion Dielectric studies on PVA-Gd2O3 nanocomposite films as a function of frequency at various temperatures is reported here. FTIR and TEM were used to characterize the films and nanoparticles respectively and are seen to agree with the XRD and Raman data published by us earlier [24] and confirm the formation of the nanocomposite films. Dielectric permittivity of the nanocomposite films decrease with increasing dopant concentration up to 4wt% and increases again for 6wt%. This is contrary to the usual
of
trend observed in these systems and seems to be coming from the insulating nature of the rare earth oxide dopant. The permittivity is low in all the nanocomposites and the decrease with increasing Gd2O3 may arise from a decrease in the number of dipoles or
ro
their random orientation in the nanocomposites. This effect is enhanced in the 4wt% film and may be due to ligand formation and more amorphous nature of the sample as seen in
-p
the XRD data reported elsewhere by us [24]. The 2wt% film shows an anomaly at 303K whose origin is not clear. The values of increase with increasing temperature. As a
re
function of dopant concentration dielectric loss of the samples shows a monotonic decrease. Fitting to modified Cole-Cole plot showed non-Debye type of relaxation in
lP
with respect to temperature follows the Arrhenius the films. The variation of Z max behavior and curve fitting gives decreasing activation energies in the range 0.55eV to 0.23eV as a function of dopant concentration. The resistivity however, shows an increase
ur na
in the nanocomposites with the 4wt% film showing the maximum resistivity. This result points to factors other than the activation energy contributing to the conduction in these samples. The Nyquist plot shows depressed semicircles and points to a non-Debye type of relaxation. A single RC parallel circuit fits the data and indicates the presence of long range conductivity. The imaginary part of the complex electric modulus shows peaks
Jo
shifting to the high frequency side with temperature and can be fit to KWW function. The fitting parameter is less than 1, once again indicating a non-Debye type of relaxation. The maxima of Z and M occur at different frequencies indicating the presence of both localized relaxation and long range conduction. The master curve indicates that the same relaxation mechanisms are active at all temperatures. The AC conductivity can be fit to Jonscher’s law and the exponent s shows a monotonic decrease with temperature indicating that Correlated Barrier Hopping is the mechanism for conduction.
In summing up it is observed that the 4wt% film exhibits the maximum increase or decrease in the properties with the 6wt% showing the minimum change. The lack of gradation in the properties with increase in dopant makes it difficult to point out the origin of these properties. The permittivity shows a decrease in the nanocomposites while the dc and ac conductivity also show a decrease, the result being a little surprising. Gd2O3 is an insulator and PVA is an insulating polymer and the effect of the addition of Gd2O3 seems to reduce the number of available charges both for orientation in the field as well
of
as for conduction.
ro
Declaration of interests
Acknowledgement
lP
re
-p
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
ur na
S. N. Madhuri is thankful to INUP, IISc, CeNSE, Bengaluru funded by Meity, Govt. of India for providing characterization facilities for FTIR and TEM. Reference [1]
K. Gupta, P.S. Mukherjee, A.K. Meikap, P.C. Jana, Effect of samarium nanoparticles on
Jo
the electrical transport properties of polyaniline, Adv. Nat. Sci.: Nanosci. Nanotechnol., 5 (2014) 025003. doi:10.1088/2043-6262/5/2/025003.
[2]
S. Sugumaran, C.S. Bellan, Transparent nano composite PVA-TiO2 and PMMA-TiO2 thin films: Optical and dielectric properties, Optik, 125 (2014) 5128–5133. doi:10.1016/j.ijleo.2014.04.077.
[3]
J. Luo, Y. Xu, H. Mao, Magnetic and microwave absorption properties of rare earth ions (Sm3+, Er3+) doped strontium ferrite and its nanocomposites with polypyrrole, J. Magn. Magn. Mater., 381 (2015) 365–371. doi:10.1016/j.jmmm.2015.01.019.
[4]
T. Hanemann, D.V. Szabó, Polymer-nanoparticle composites: from synthesis to modern applications, Materials, 3 (2010) 3468-3517. doi:10.3390/ma3063468.
[5]
N. B. R. Kumar, V. Crasta, B.M. Praveen, Dielectric and electric conductivity studies of PVA (Mowiol 10-98) doped with MWCNTs and WO3 nanocomposite films, Mater. Res.
[6]
of
Express. 3 (2016) 055012. doi:10.1088/2053-1591/3/5/055012. A.M. El Sayed, S. El-Gamal, Synthesis and investigation of the electrical and dielectric
doi: 10.1007/s10965-015-0732-4.
M. Abdelaziz, M.M. Ghannam, Influence of titanium chloride addition on the optical and dielectric
properties
of
PVA
films,
doi:10.1016/j.physb.2009.10.030.
B,
405
(2010)
958–964.
J. Naik, R.F. Bhajantri, S.G. Rathod, T. Sheela, V. Ravindrachary, Synthesis and
re
[8]
Phys.
-p
[7]
ro
properties of Co3O4/(CMC+PVA) nanocomposite films, J. Polym. Res., 22 (2015) 97.
characterization of multifunctional ZnBr2/PVA polymer dielectrics, J. Adv. Dielec., 4
[9]
lP
(2016) 1650028. doi:10.1142/S2010135X16500284.
K.S. Hemalatha, K. Rukmani, N. Suriyamurthy, B.M. Nagabhushana, Synthesis , characterization and optical properties of hybrid PVA–ZnO nanocomposite: A dependent
study,
ur na
composition
Mater.
Res.
Bull.
51
(2014)
438–446.
doi:10.1016/j.materresbull.2013.12.055. [10]
K.S. Hemalatha, K. Rukmani, Synthesis, characterization and optical properties of polyvinyl alcohol–cerium oxide nanocomposite films, RSC Adv., 6 (2016) 74354–74366. doi:10.1039/C6ra11126b.
G. N. H. Kumar, J. L. Rao, N. O. Gopal, K. V. Narasimhulu, R. P. S. Chakradhar, A. V.
Jo
[11]
Rajulu,
Spectroscopic
investigations
of
Mn2+
ions
doped
polyvinylalcohol
films, Poly., 45 (2004) 5407. doi:10.1016/j.polymer.2004.05.068.
[12]
T.A. Hanafy, Dielectric relaxation and alternating-current conductivity of gadoliniumdoped poly(vinyl alcohol), Journal of Applied Polymer Science, 108 (2008) 2540–2549. doi:10.1002/app27567.
[13]
F.H.A. El-kader, W.H. Osman, K.H. Mahmoud, M.A.F. Basha, Dielectric investigations and ac conductivity of polyvinyl alcohol films doped with europium and terbium chloride, Phys. B, 403 (2008) 3473–3484. doi:10.1016/j.physb.2008.05.009.
[14]
E.F.M. El-Zaidia, S.I. Qashou, A.A.A. Darwish, T.A. Hanafy, Structural, optical, electrical and dielectric properties of PVA-YCl3 films, Surf. Rev. Lett. 26 (2019) 1850149. doi:10.1142/S0218625X18501494.
[15]
S. Mahendia, A.K. Tomar, S. Kumar, Electrical conductivity and dielectric spectroscopic
of
studies of PVA-Ag nanocomposite films, J. Alloys Compd., 508 (2010) 406–411. doi:10.1016/j.jallcom.2010.08.075. [16]
S. Sinha, S.K. Chatterjee, J. Ghosh,
A. K. Meikap, Dielectric relaxation and ac
ro
conductivity behaviour of polyvinyl alcohol–HgSe quantum dot hybrid films, J. Phys. D: Appl. Phys., 47 (2014) 27530. doi:10.1088/0022-3727/47/27/275301.
A.J. Kenyon, Recent developments in rare-earth doped materials for optoelectronics,
-p
[17]
Prog. Quant. Elec. 26 (2002) 225–284. doi: 0079-6727/02/$.
N. Luo, X. Tian, C. Yang, J. Xiao, W. Hu, D. Chen, L. Li, Ligand-free gadolinium oxide
re
[18]
for in vivo T1-weighted magnetic resonance imaging, Phys. Chem. Chem. Phys., 15
[19]
lP
(2013) 12235–12240. doi:10.1039/c3cp51530c.
L. Pereira, Organic light emitting diodes: The use of rare earth and transition metals. Pan Stanford, CRC Press, Taylor & Francis Group, 2012. ISBN:978-9-81426-795-3 M.O. Reddy, B.C. Babu, Structural , optical , electrical , and magnetic properties of
ur na
[20]
PVA:Gd3+ and PVA:Ho3+ polymer films, Indian J. Mater. Sci., 2015 (2015) 927364. doi:10.1155/2015/927364. [21]
S. Mukherjee, P. Dasgupta, P.K. Jana, Size-dependent dielectric behaviour of magnetic Gd2O3 nanocrystals dispersed in a silica matrix, J. Phys. D: Appl. Phys., 41 (2008)
Jo
215004. doi:10.1088/0022-3727/41/21/215004.
[22]
S. El-Sayed, Optical properties and dielectric relaxation of polyvinylidene fluoride thin films
doped
with
gadolinium
chloride,
Phys.
B.
454
(2014)
197–203.
doi:10.1016/j.physb.2014.07.076. [23]
T.A. Hanafy, Dielectric relaxation and alternating current conductivity of lanthanum , gadolinium , and erbium-polyvinyl alcohol doped films, J. Appl. Phys., 112 (2012) 034102. doi: 10.1063/1.4739752.
[24]
S.N. Madhuri, K.S. Hemalatha, K. Rukmani, Preparation and investigation of suitability of gadolinium oxide nanoparticle doped polyvinyl alcohol films for optoelectronic applications, J. Mater. Sci.: Mater. Elec., 30 (2019) 9051–9063. doi:10.1007/s10854-01901237-9.
[25]
S.N. Madhuri, K. Rukmani, Synthesis and concentration dependent tuning of PVASm2O3 nanocomposite films for optoelectronic applications, Mater. Res. Express, 6 (2019) 075017. doi:10.1088/2053-1591/ab1326. R. F. Bhajantri, V. Ravindrachary, A. Harisha, V. Crasta, S. P. Nayak, B. Poojary,
of
[26]
Microstructural studies on BaCl2 doped poly (vinyl alcohol), Poly., 47 (2006) 3591.
[27]
ro
doi:10.1016/j.polymer.2006.03.054.
I. Omkaram, R. S. Chakradhar, J. L. Rao, EPR, optical, infrared and Raman studies of
-p
VO2+ ions in polyvinylalcohol films, Physica B: Condensed Matter, 388 (2007) 318. doi:10.1016/j.physb.2006.06.134.
I. Shivaraja, S. Matteppanavar, S.K. Deshpande, S. Rayaprol, B. Angadi, Investigation of
re
[28]
space charge polarization behavior in Pb0.9Bi0.1Fe0.7W0.3O3 ceramic, J. Alloys Compd.
[29]
lP
800 (2019) 334–342. doi:10.1016/j.jallcom.2019.06.010. K.S. Hemalatha, G. Sriprakash, M.V.N.A. Prasad, R. Damle, K. Rukmani, Temperature dependent dielectric and conductivity studies of polyvinyl alcohol-ZnO nanocomposite by
impedance
spectroscopy,
ur na
films
J.
Appl.
Phys.,
118
(2015)
154103.
doi:10.1063/1.4933286. [30]
S. Chattopadhyay, P. Ayyub, V. R. Palkar, A. V. Gurjar, R. M. Wankar, M. Multani, Finite-size effects in antiferroelectric nanoparticles, J. of Phys.: Condensed Matter, 9
Jo
(1997) 8135. doi: 0953-8984/97/388135+11$19.50. [31]
S. Chattopadhyay, P. Ayyub, V. R. Palkar, M. Multani, Size-induced diffuse phase
transition in the nanocrystalline ferroelectric PbTiO3, Physical review B, 52 (1995) 13177. doi: 0163-1829/95/52(18)/13177(7)/$06. 00.
[32]
Z. W. Abdullah, Y. Dong, I. J. Davies, S. Barbhuiya, PVA, PVA blends, and their nanocomposites for biodegradable packaging application, Polymer-Plastics Tech. and Engi., 56 (2017) 1307. doi: 10.1080/03602559.2016.1275684.
[33]
K. C. Koa, Dielectric Phenomena in Solids with Emphasis of Physical Concepts of Electronic Processes, Elsevier Academic Press, UK , 2004. ISBN: 0-12-396561-6.
[34]
V.S. Shanthala, S.N.S. Devi, M. V Murugendrappa, Synthesis, characterization and DC conductivity studies of polypyrrole/copper zinc iron oxide nanocomposites, J. Asian Ceramic Soc., 5 (2017) 227–234. doi:10.1016/j.jascer.2017.02.005
[35]
S. Thakur, R. Rai, I. Bdikin, M.A. Valente, H. Pradesh, C.U. De Santiago, Impedance and Modulus Spectroscopy Characterization of Tb modified Bi0.8A0.1Pb0.1Fe0.9Ti0.1O3
[36]
of
Ceramics, Mater. Research, 19 (2016) 1–8. doi:10.1590/1980-5373-MR-2015-0504.
R. Gerhardt, Impedance and dielectric spectroscopy revisited : distinguishing localized
ro
relaxation from long-range conductivity, J. Phys. Chem. Solids. 55 (1994) 1491–1506. doi:10.1016/0022-3697(94)90575-4.
A. K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983.
-p
[37]
Jo
ur na
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ISBN 0-9508711-0-9
PII:
S2352-4928(19)31569-7
DOI:
https://doi.org/10.1016/j.mtcomm.2020.100942
Reference:
MTCOMM 100942
To appear in:
Materials Today Communications
Received Date:
31 December 2019
Accepted Date:
17 January 2020
Please cite this article as: Madhuri SN, Murugendrappa MV, Rukmani K, Conduction and Relaxation Mechanisms in Gadolinium Oxide Nanoparticle Doped Polyvinyl Alcohol Films, Materials Today Communications (2020), doi: https://doi.org/10.1016/j.mtcomm.2020.100942
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Conduction and Relaxation Mechanisms in Gadolinium Oxide Nanoparticle Doped Polyvinyl Alcohol Films. S N Madhuri1, M V Murugendrappa2 and K Rukmani1,* Department of Physics, Bangalore University, Bengaluru 560056 2 Department of Physics, BMS College of Engineering, Bengaluru 560019 1
*
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Corresponding author: [email protected]
Pure PVA PVA- 2wt % Gd2O3
120
3x107
3x107
PVA- 4wt % Gd2O3
15
90
PVA- 6wt % Gd2O3
2x107
2x107
30
1x107
0 2
3
5
4
5
6
log f
re
10
Z()
60
303K 323K 343K 363K 383K 403K 423K Fit
Z()
150
T = 423 K
-p
20
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Graphical abstract
1x107
0
2x107
3
Highlights
5
6
4x107
7
6x10 Z ()
0
0
2x107
Z()
4x107
6x107
Doping in nano form decreases in contrast to doping with ions which were reported to increase . Activation energy decreases monotonically where as resistivity increases with increasing concentration. The films exhibit negative resistance co-efficient with temperature. Relaxation is seen to be non-Debye in nature. Correlated Barrier Hopping is the preferred mechanism for conduction.
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4 log f
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2
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0
0
Abstract
Concentration dependent dielectric and electrical properties of PVA-Gd2O3 nanocomposite films over a wide range of frequencies (100Hz-1MHz) and temperatures (303K-423K) is reported here. TEM of the Gd2O3 nanoparticles showed that they could be indexed to the cubic phase. The frequency variation of the dielectric permittivity and dielectric loss could be explained by the Maxwell Wagner Sillar’s model. The dielectric permittivity decreased in the composite films contrary to observations in other similar systems. The complex impedance showed Arrhenius behavior and the activation energies decreased monotonically with increasing concentration.
of
However the resistivity of the nanocomposite films shows an increase with concentration, the 4wt% film showing the highest resistivity which indicates other contributions to conductivity. The Nyquist plot could be fitted to a simple parallel RC circuit showing the presence of long
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range conduction in these samples. The relaxation times varied from 10-4 to 10-6s and indicated a non-Debye type of relaxation. The master curve showed good overlap indicating that the same
-p
relaxation processes were active at all temperatures. The ac conductivity obeys Jonscher’s law and the exponent shows a monotonic decrease with temperature. This indicates that Correlated
re
Barrier Hopping is the probable mechanism for conduction in these samples.
Correlated Barrier Hopping.
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1. Introduction
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Keywords PVA-Gd2O3 Nanocomposite films, FTIR, Activation energy, Jonscher’s power law,
The combination of inorganic nanoparticles and organic polymers forms an interesting class of materials called polymer nanocomposites. Polymers contribute properties such as ease of processing, low cost and flexibility while small quantities of the dopant nanoparticles add useful properties which help to tailor the material for specific
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applications [1-4]. Nanocomposites using a wide variety of polymers have been studied [1-7] and the ones using the polymer poly vinyl alcohol (PVA) are biodegradable and environment friendly. PVA has high mechanical strength, good environmental stability and forms nanocomposites with ease. There is abundant literature reporting the effect of inorganic dopants like ZnBr2, nano ZnO, nano CeO2, CdS, Gd3+, Mn2+, EuCl3, TbCl3, YCl3, Ag [8-16] etc. on the structural, optical, dielectric and electrical properties of PVA so as to enhance the suitability of the material for particular applications. The addition of
semiconductor nanoparticles such as ZnO and HgSe to PVA have shown increased conductivity in these films paving the way for possible applications in the electronic industry [9,16]. The addition of nanoparticles of the rare earth oxide CeO2 to PVA gave rise to enhanced photoluminescence even at low concentrations of the dopant [10]. Rare earth sesquioxides are one of the highly studied materials that have a wide range of applications in optoelectronics [17-19]. They are chemically and thermally very stable. Among them Gd2O3 is of particular interest due to its completely half filled 4f electronic is expected to give rise to interesting optical, dielectric and electrical
of
shell which
properties. The photoluminescence (PL) in lanthanides are in general sharp and intense as these arise from transitions within the f shell energy levels as well as from defect centers,
ro
and this is true in gadolinium also. Amongst the lanthanides, half filled shell has the maximum magnetic moment of I=7/2 as all the electron spins are parallel. This makes it
-p
useful as a contrast agent in MRI [18]. The large magnetic moment also affects its electrical properties [20]. Recently Sudip Mukherjee et.al. [21] reported high dielectric
re
constant and low loss in a system of Gd2O3 nanocrystal dispersed in a silica matrix. Other reports with similar results are also available in literature [22]. These types of materials
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are promising candidates for memory device applications. Researchers such as Taha A. Hanafy [23] and M. Obula Reddy et.al.[20] have reported the effect of lanthanide ions like La3+, Gd3+ and Er3+ on the dielectric relaxation and ac conductivity of PVA matrix.
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They observed significant increase in the dielectric permittivity and conductivity of PVA. The effect of Gd2O3 nanoparticles and Sm2O3 nanoparticles on PVA films was to give rise to good photoluminescence in the blue and red region with sufficient color purity for optoelectronic applications [24,25]. Because these materials have such a multitude of applications it is of interest to study the conductivity and relaxation mechanisms
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operative in them. The ac conductivity in such nanocomposites can be modeled in many ways, the main mechanisms being quantum mechanical tunneling (QMT), small and large poloron assisted tunneling and Correlated Barrier Hopping (CBH). The most suitable model can be inferred from the temperature dependence of the exponent obtained from fitting the dielectric data to Jonscher’s power law. An analysis of the dielectric data in the light of various formalisms helps delineate the relaxation processes in the material and gives an idea of the mechanisms occurring in these materials. Hence it was felt that a
detailed dielectric analysis of this material will enhance our understanding of the processes taking place and lead to a better design of materials for new applications. With this view, a concentration dependent study of the dielectric and electrical properties of nanocomposite films of Gd2O3 with PVA was undertaken and to the best of our knowledge this is the first report on such a rare earth system. The results of the study point to an uncommon decrease in the value of dielectric permittivity and an increase in
2. Experimental alcohol
((C2H4O)x,
average
molecular
weight
13,000-23,000,
98%
ro
Polyvinyl
of
the resistivity of the films with concentration of Gd2O3.
hydrolyzed), Gadolinium(III) nitrate hexahydrate [Gd(NO3)3.6H2O], glycine [C2H5NO2]
-p
of purity 99.99% were obtained from Sigma Aldrich and used as obtained. Gd2O3 nanoparticles were synthesized by solution combustion method and PVA-Gd2O3
re
nanocomposite films were prepared by solution casting technique. The detailed procedure for sample preparation is given elsewhere [24]. The structural details of the pure PVA and PVA-Gd2O3 (2wt%, 4wt% and 6wt%) nanocomposite films were analyzed by Fourier
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transmission infrared (FTIR) spectra recorded in ATR mode at room temperature by using Nicolet 6700 FTIR with a resolution of 4cm-1. The morphology of the nanoparticles
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was analyzed by TEM images recorded using Titan Themis 300kV, FEI with ultra-bright XFEG gun. The dielectric measurements over a wide range of frequencies (100Hz1MHz) and temperatures (303K-423K) with step size 20K were carried out by Precision Impedance Analyzer (6505 B), Wayne Kerr. 3. Results and Discussion
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A. FTIR analysis
The intermolecular interaction and the structural modifications induced by the dopant in the polymer matrix were analyzed by FTIR spectra. The FTIR spectra of pure PVA and PVA-Gd2O3 nanocomposites with dopant concentration of 2wt%, 4wt% and 6wt% are shown in fig.1. All the composites exhibit the characteristic bands of pure PVA along with few other new bands. The assignments of all the bands are tabulated in Table 1 and are in good agreement with previously reported work [9,25]. A broad and strong band is
observed at 3268 cm-1 which may be assigned to the –OH group present in the backbone
(d)
PVA - 6wt% Gd2O3
(c)
PVA - 4wt% Gd2O3
(b)
PVA-2wt%Gd2O3
of 916 847
-p
1089
1658
ro
1417 1328
2851 2917
3000
2000
Wavenumber (cm-1)
re
4000
1712 1574
Pure PVA
(a)
3268
Transmittance %
of the PVA matrix.
1000
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Fig. 1. FTIR spectra of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
Table 1. Assignments of Infrared bands of PVA-Gd2O3 nanocomposite films.
Assignments
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Wagging of -OH C-C stretching -CH2 rocking of PVA C-O stretching of acetyl group C-O stretching of secondary alcohol Wagging vibration of -CH or C-C stretching Bending (CH-OH) or -CH2 stretching Wagging of -CH2 vibrations -CH bend of -CH2 Bending of -CH2 vibrations Small conjugated polyene sequence -OH and -CH bending vibration Unsaturated C=C stretching mode Acetyl C=O groups of PVA ester -CH symmetric stretching vibration -CH symmetric stretching vibration -CH asymmetric stretching vibration -CH asymmetric stretching vibration -OH group in the polymer background -OH stretching vibration of hydroxyl group of PVA
-p
6wt.% (cm-1) 676 847 917 1092 1143 1238 1335 1379 1412 1547 1652 1713 2853 2920 2929 2987 3272 -
re
4wt.% (cm-1) 680 845 916 1090 1143 1238 1330 1378 1435 1555 1656 1708 2851 2919 3269 -
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847 916 1089 1143 1239 1328 1377 1417 1555 1574 1658 1712 2851 2917 2941 3268 -
2wt.% (cm-1) 682 844 916 1090 1143 1238 1328 1378 1434 1558 1654 1713 2852 2921 3260 3617
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PVA
The -CH symmetric stretching is observed at 2917 cm-1. If one looks at the ratio of the
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intensities of the peaks at 3268cm-1 to the peak at 2917 cm-1, it changes from 1.420 in PVA to 0.852, 1.19 and 1.05 for 2wt%, 4 wt% and 6 wt% of Gd2O3 respectively. Assuming that the –CH symmetric peak is unaffected by the addition of Gd2O3, the –OH peak decreases in intensity in the nanocomposites. This indicates decrease in the number of free –OH groups of PVA matrix due to the formation of hydrogen bond between dopant Gd2O3 and –OH group [26,27]. This decrease is maximum in the 2wt% sample and minimum in the 4wt% sample. Sharp and intense bands were observed at 1417cm-1, 1089cm-1 and 847cm-1. These bands correspond to the -CH bending of CH2 and -CO and
C-C stretching of acetyl group respectively. A moderate band is observed at 1574 cm-1 in PVA which corresponds to the bending vibration of –OH group and is absent in the composites. This absence may be due to the formation of hydrogen bonds or metal-ligand complex within the PVA matrix in the composites [12,23]. There is a slight shift and decrease in the intensity of few bands and formation of new bands in the composite which are due the incorporation of Gd2O3 nanoparticles in the PVA matrix. The FTIR also shows bands around 680cm-1 which is due to the wagging of –OH and are seen to be
of
shifted from those in pure PVA. These bands show a blue shift with increasing concentration which could be because of the proximity of the large Gd atoms increasing the free space between the polymer chains. Bands from the Gd2O3 itself have not been
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seen as these occur at low frequencies below our range of measurement. However, the XRD results published by us earlier [24] show the peaks of Gd2O3 confirming the
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incorporation of the nanoparticle into the PVA film. The XPS results also show the presence of Gd3+ in the films while the Raman spectra shows the presence of modes
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corresponding to the nanoparticle in the case of the 6wt% composite film [24]. These prove conclusively that the nanoparticle has got incorporated into the PVA matrix and a
B. TEM analysis
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nanocomposite has been formed.
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The morphology of the synthesized Gd2O3 nanoparticle was analyzed by TEM micrograph (fig.2(a)). The particles were seen to be slightly agglomerated and quasi spherical in shape. The interplanar distance (fig.2(b-c)) was estimated to be 4.413Å and 2.88Å, which correspond to [2 1 1] and [1 2 3] planes of the Gd2O3 nanoparticle respectively. The average particle size was obtained to be 21.20nm. The Selected Area
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Electron Diffraction (SAED) pattern (fig.2(d)) shows that the synthesized nanoparticles are crystalline in nature without any irregularity and the diffraction pattern is well indexed to the cubic structure of the nanoparticle and is in good agreement with the XRD results reported elsewhere [24].
(a)
(b)
d= 2.88 Å (h k l)=(1 2 3)
(50 nm)
(5 nm)
(d)
of
(c)
(8 5 3)
ro
(1 2 3)
(1 3 4)
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d=4.4128 Å (h k l)=(2 1 1)
(8 3 1)
(6 1 1)
(5.00 1/nm)
re
(5 nm)
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Fig. 2. (a-c) TEM micrograph of the Gd2O3 nanoparticle, (d) SAED diffraction pattern of Gd2O3 nanoparticle indicating different (h k l) planes
C. Dielectric studies (i)
Temperature and frequency dependent dielectric permittivity and tangent loss
The complex permittivity of the material can be expressed as
j
(1)
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where and are the real and imaginary parts of the complex permittivity and j 1. The real part of the complex permittivity can be given by
where
0
0 r is the permittivity of vacuum ( 8.85 10
(2) 12
F / m ) and r is the relative
permittivity of the material. The magnitude of indicates the ability of the material to store energy from the applied electric field.
The imaginary part of the complex permittivity is given by
tan
(3)
where tan is the dissipation factor. The temperature and frequency dependent dielectric permittivity ( ) and tangent loss (tan ) for all the samples are shown in fig. 3(a-d) and fig. 4(a-d) respectively. It is seen that the dielectric permittivity is high at lower frequencies, decreases as the frequency increases and reaches saturation at very low values after 10 kHz. This phenomenon of larger dielectric permittivity at lower
of
frequencies is well explained using Maxwell-Wagner-Sillar’s polarization (MWS) model [28,29]. The dielectric permittivity increases monotonically with respect to increasing
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temperature in all the samples. The only exception is the curve for the 2wt% sample at the temperature of 303K (room temperature) which does not show an increase in the
-p
value of at low frequencies roughly below 1kHz. The origin of this anomaly is not clear and could be coming from the presence of space charge effect at the low
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frequencies. As a function of concentration (fig. 3(e)) the dielectric permittivity of the nanocomposites decreases drastically with increase in concentration of dopant upto 4wt% Gd2O3 and then increases for 6wt% Gd2O3. This variation is in consonance with the size
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of the nanocrystallites in these films reported by us earlier [24], the 2wt% and 4wt% being smaller around 6nm and the 6wt% being larger around 16nm. The composites
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made from the much larger Gd3+ ions in PVA are reported to show an increase in compared to pure PVA by Hanafy et. al. [20,23]. Their data is however from 50kHz onwards and the actual values obtained in the nanocomposites are similar to those quoted by us. The PVA used by them is of similar average weight yet they quote lower values of
for these films maybe because of their higher frequency of observation. Size
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dependent dielectric permittivity where decreases with decreasing particle size has been observed by S. Chattopadhyay et. al. in antiferroelectric PbZrO3 nanocrystals [30] and ferroelectric PbTiO3 nanocrystals [31]. In these bulk systems, decrease in permittivity was attributed earlier to the occurrence of a diffused phase transition in the material from (anti)ferroelectric to paraelectric which reduced . However, Chattopadhyay et. al. observe this effect by just reducing the size of the particle to the nano regime. Some reports attribute the quench in to random disorder which destroys the long range polar
order [30]. The nano size of the dopant (Gd2O3) in our samples seems to limit the contribution from the dipoles which orient in the electric field. PVA is a semi crystalline polymer made up of amorphous regions and a small amount of crystalline regions [32]. Some type of random disorder, mainly in the amorphous regions of PVA, may be reducing the contribution to . The 4wt% nanocomposite film exhibits very low values of as compared to the other two and this may be due to some random disorder quenching the formation of dipoles or freezing of the dipoles in these films [30]. Hanafy
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et.al. also suggest the possibility of the dopant forming metal ligand type of structure which could quench the dielectric permittivity [12,23]. A small decrease in dielectric
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permittivity as a function of increasing doping has been reported in PVA-Ag nanocomposite at high frequencies [15]. The dielectric tangent loss as a function of frequency (fig. 4(a)) shows a peak that shifts with temperature in pure PVA. In the
-p
composites (fig.4(b-d)), the peak is seen to shift to the lower side with increasing concentration and is below the range of the measurement. Tangent loss increases
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monotonically with increasing temperature in all the samples suggesting the presence of thermally active dielectric process in the samples. The concentration dependent tangent
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loss at 423K is shown in fig.4(e).
When DC conductivity is not negligible, the relationship between total electrical conductivity ( * ) and complex permittivity ( ) is given by the following equation
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s * j 1 ( j )1 0 n *
(4)
where * r j im , r is the real part and im is the imaginary part of the
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conductivity. The real part of the complex permittivity is given by
1 ( s )(1 ( ) sin 2 im n 2 (1 ) 1 2( )1 sin 0 ( ) 2
(5)
Where is the relaxation time, ‘ n ’ is an exponent and ‘ ’ is a parameter whose value lies between 0 and 1. When 0 , the above equation reduces to Debye equation. is
an empirical parameter introduced to account for the changes when there is a spread of relaxation times in the system [33]. The larger the value of the more is the spread in
16
303K 323K 343K 363K
3
3
12
90
(b)
(a)
6
4
383K 403K 423K
383K 1.8 403K 423K
4
2
303K 323K 343K 363K
1.2
303K 323K 343K 363K
(c)
120
20
6
383K 403K 423K
150
8
60 0 102
103
104 f(Hz)
30
105
106
102
103
104 f(Hz)
4
105
0.6 102
2
106
103
104 f(Hz)
105
106
FIT
FIT
0 104
105
102
106
103
104
T = 423 K
15
(d)
2
12
10
PVA- 2wt % Gd2O3 PVA- 4wt % Gd2O3 PVA- 6wt % Gd2O3
(e)
0 2 10
10
3
10
5 10
4
3
4
10
5
10 f(Hz)
10
10
FIT 0 102
10
3
10
4
10
5
4
10
0
10
6
5
10
6
f(Hz)
6
10
2
10
3
4
10 f(Hz)
f(Hz)
105
106
f(Hz)
60
8 2
104
Pure PVA 120
4
20
103
of
16
303K 323K 343K 363K
383K 6 403K 423K
FIT
0 102
106
f(Hz)
f(Hz)
20
105
ro
103
10
5
-p
0 102
10
6
re
Fig. 3. Modified Cole-Cole fit of dielectric permittivity vs frequency at different temperatures of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% and (e) concentration dependent dielectric permittivity of all the samples at 423K
4
4
3
tan
tan
6
303K 323K 343K 363K 383K 403K 423K
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303K 323K 343K 363K 383K 403K 423K
8
2
2
(b) 2wt% Gd2O3
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(a) Pure PVA
303K 323K 343K 363K 383K 403K 423K
3
tan
10
(c) 4wt% Gd2O3
1
1
2
0
0
0 2
3
4 log f
5
2
6
3
4 log f
5
6
2
3
4
5
6
log f
10
303K 323K 343K 363K 383K 403K 423K
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tan
2
8
3
4
log f
PVA- 6wt % Gd2O3
4 2
(e) T = 423 K
0
2
PVA- 4wt % Gd2O3
6
(d) 6wt% Gd2O3
1
Pure PVA PVA- 2wt % Gd2O3
tan
3
5
6
0 2
3
4
5
6
log f
Fig. 4. Temperature and frequency dependent dielectric loss of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% and (e) concentration dependent dielectric loss of all the samples at 423K
relaxation times. A spread in the relaxation times indicates the non-Debye nature of the relaxation, and may arise because the dipoles in the polymer nanocomposite experience slightly varying environments giving rise to different relaxation times. The experimental data has been least square fitted to modified Cole-Cole equation (eqn. 5) in fig. 3 and the parameters tabulated in Table 2. Both and n values were obtained to be between 0 and 1 indicating the non-Debye nature of the relaxation in the films. (ii)
Temperature and frequency dependence of real part (Z ) and imaginary part (Z ) of
of
the complex impedance ( Z * )
The complex impedance can be represented by the following equation
ro
Z Z jZ
(6)
-p
where Z Z cos is the real part and Z Z sin is the imaginary part of the complex impedance.
The real part of complex impedance Z , is seen to be high at low frequencies, decreases
re
as the frequency increases and saturates beyond that (fig.5(a-d)). This may be attributed to the accumulation of charges at the grain boundaries at low frequencies whereas at high
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frequencies the release of space charges gives rise to a decrease in Z value. It can be seen that the Z value is maximum for 4wt% sample compared to the 2wt% and 6wt%
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samples (fig 5(e)).
The Cole-Cole equation for imaginary part of the impedance Z is given by
Z
R
1 ( j )
(7)
where R is resistance of the material, the angular frequency, the relaxation time and
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the fitting parameter whose value lies between 0 and 1. 1 corresponds to the
Debye type of relaxation mechanism; however values of obtained for all the samples were less than 1, indicating the presence of non-Debye type of relaxation mechanism in
) the samples (Table 2). It is seen from the graph that Z reaches a maximum value ( Z max at particular frequency known as relaxation frequency ( max ) and later decreases with
value respect to frequency. It is also seen that as the temperature increases Z max
7
2.0x10
303K 323K 343K 363K 383K 403K 423K
7
1.2x10
6
8.0x10
7
1.0x10
6
5.0x10
4.0x10
(c) 4wt% Gd2O3
(b) 2wt% Gd2O3
(a) Pure PVA 0.0
0.0 3
4
5
0.0
2
6
3
4
5
2
6
3
3x106
6
303K 323K 343K 363K 383K 403K 423K
6
4.0x10
2x106
4x104
0 2
1x10
6
4 log f
6
6
2wt% Gd2O3
(d) 6wt% Gd2O3
4wt% Gd2O3
0.0 4
5
6wt% Gd2O3
(e)
0 3
6
Pure PVA
2.0x10
2
5
8x104
Z'()
6
6.0x10
T = 423 K Z'()
8.0x10
4
log f
log f
log f
of
2
Z'()
7
1.5x10
6
6
4.0x10
303K 323K 343K 363K 383K 403K 423K
2
6
3
4
5
ro
Z'()
6
8.0x10
1.6x10
Z'()
7
1.2x10
303K 323K 343K 363K 383K 403K 423K
7
Z'()
7
1.6x10
6
log f
log f
0 102
103
104
303K 323K 343K 363K
105
106
4x106
103
(d)
105
2x106
Jo
Z''()
104 f (Hz)
103
0 102
1x106
103
103
104
f (Hz)
105
105
(e)
5x105
104
106
105
106
303K 323K 343K 363K 383K 403K
'' Z ( )
2x106
1x106
1x107
0 102
383K 403K 423K FIT
f (Hz)
103
104
105
106
f (Hz)
423K FIT
0 102
103
104 f (Hz)
105
106
Pure PVA 4x104
2x104
0 2 10
103
104
105
106
f(Hz)
2wt% Gd2O3
1x106
0 102
104
f (Hz)
106
303K 323K 343K 363K 383K 403K 423K FIT
3x106
0 102
1x107
Z''()
0 102
2x107
4x105
ur na
6
383K 403K 423K FIT
(c)
Z''()
2x107
f (Hz)
2x10
lP
'' Z ( )
2x105
6
4x106
303K 323K 343K 363K
Z''()
6x10
3x107
1x106
(b)
8x105
Z''()
Z''()
8x106
3x107
4x105
(a)
'' Z ( )
1x107
re
-p
Fig. 5. Temperature and frequency dependent real part (Z ) of complex impedance of (a) Pure PVA, (b) 2wt% PVA-Gd2O3, (c) 4wt% PVA-Gd2O3, (d) 6wt% PVA-Gd2O3 nanocomposite and (e) concentration dependent Z vs. log f of all the samples at 423K
4wt% Gd2O3 6wt% Gd2O3
T = 423 K
106
0 102
103
104 f(Hz)
105
106
Fig. 6. Modified Cole-Cole fit to variation of imaginary part (Z ) of the complex impedance for temperature range 303K-423K of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% and (e) concentration dependent Z vs. log f of all the samples at 423K
decreases and shifts to higher frequency indicating the reduction in the bulk resistance and the decrease in the relaxation time with temperature. Cole-Cole fitting to the above equation (eqn. 7) and the concentration dependence of imaginary part of complex impedance Z versus frequency are shown in fig. 6(a-d) and fig. 6(e) respectively. If it is found that the imaginary part of complex impedance follows Arrhenius behavior
can be fitted for the relaxation process then the frequency f max corresponding to the Z max Ea kT
, where f 0 is the characteristic
of
(fig. 7) to the equation given by f max f 0 e
frequency, Ea is the activation energy, k is the Boltzmann constant and T is the absolute
ro
temperature. The values of activation energy obtained for all the samples are given in Table 3. It is seen that the activation energy of the samples decreases monotonically with
-p
increase in dopant concentration. This activation energy is a measure of the potential barrier that the molecular dipole has to overcome to go from one orientation to another in
re
the applied field. This could also be the charge hopping between two different sites whose energies are in the form of two potential minima with a potential barrier between them. The latter is more likely in a polymer nanocomposite as there are charges located at
lP
the grain boundaries. The monotonic decrease in the activation energy with increasing dopant concentration indicates that the charges are coming closer as concentration of
ur na
Gd2O3 nanoparticle is increased and need lesser energy to move from one site to another. The resistivity on the other hand does not follow this trend, increasing in going from 2wt% to 4wt% and then decreasing in the 6wt% sample. This behavior indicates that the resistivity depends on factors other than the activation energy, such as distribution of nanoparticles in the film [34]. The resistivity of all the nanocomposites is more than that
Jo
of pure PVA . (iii)
Nyquist complex impedance plot or Cole-Cole plot
Nyquist plot of all the samples for wide range of temperatures (303K-423K) and frequencies (100Hz-1MHz) is shown in fig.8(a-d). A depressed semicircle is observed for all samples which indicate a non-Debye type of relaxation in these materials [29,33]. The experimental data are fitted to semicircle using Zview version 3.2b software and the
equivalent circuit of a parallel RC circuit fits the data well as shown in fig.8. The values of R and C obtained are tabulated in Table 4. It is seen that the capacitance value of the Pure PVA PVA- 2wt% Gd2O3
5
log max
PVA- 4wt% Gd2O3 PVA- 6wt% Gd2O3
4
FIT
of
3
2 2.8
3.2
3.6
ro
2.4
1000/T
-p
Fig. 7. Graph of log max vs. 1000/T (Arrhenius plot) for PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% 4x107
(b)
re
9x106
(a)
3x107
Z()
Z()
lP
3x106
2x107
ur na
7 7 1x10 3x10
0
0
5x106
0
1x107
2x107
7 2x107 2x10
2x107
0
Z()
6x107
8x107
Z()
6
(d)
0
Z()
Z()
2x107
1x107
2x107
0 2x106
4x107
7
6x10 Z ()
1x106
0 0
4x107
3x106
Jo
3x10
4x10 1x107
(c)
7
Z()
6x10
6
2x107
Z()
4x107
6x107
0 0
2x106
4x106
6x106
8x106
Z()
Fig. 8. Nyquist plot for PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
303K 323K 343K 363K 383K 403K 423K Fit
samples increases and resistance decreases with increasing temperature in all the samples. This may be attributed to the increase in the conducting electrons due to thermal agitation caused by increasing temperature. The resistance is more in composites than the pure PVA with the 6wt% showing the least value.
PVA-4wt% Gd2O3
0.70 0.81 0.40 0.68 0.96 0.80 0.70 0.60 0.81 0.89 0.71 0.79
0.67 0.80 0.74 0.43 0.95 0.83 0.40 0.51 0.90 0.75 0.75 0.81
0.90 0.87 0.87 0.92 0.84 0.77 0.95 0.82 0.76 0.79 0.79 0.73
ro
303K 363K 423K 303K 363K 423K 303K 363K 423K 303K 363K 423K
Parameters (Z ) Z (s) 5.9 10-4 8.7 10-5 2.9 10-6 2.9 10-4 5.3 10-5 7.9 10-4 1.3 10-4 2.5 10-4 1.6 10-4 3.8 10-5
0.70 0.76 0.81 0.64 0.74 0.76 0.54 0.67 0.78 0.70 0.72 0.75
M (s)
3.9 10-4 2.2 10-5 1.4 10-6 1.6 10-4 2.2 10-5 3.1 10-5 4.3 10-6 1.6 10-4 5.9 10-5 1.6 10-5
ur na
PVA-6wt% Gd2O3
n
-p
PVA-2wt% Gd2O3
( )
re
Pure PVA
Temperature (K)
lP
Sample
of
Table 2. Values of fitting parameter ( ) and exponent n (fig. 3(a-d)), fitting parameter (Z ) and relaxation time Z (fig. 6(a-d)), fitting parameter and relaxation time M (fig. 11(a-d)) for PVA-Gd2O3 nanocomposite films.
This increase in resistance of the composites may be because the Gd2O3 bonds with the PVA backbone and reduces the number of charge carriers available for conduction. Fig. 9
Jo
shows the concentration dependent Cole-Cole plot of all the samples at 423K.
Table 3. Activation energy of pure PVA and nanocomposite films. Sample Activation energy (eV) Pure PVA 0.55 PVA-2wt%Gd2O3 0.40 PVA-4wt%Gd2O3 0.34 PVA-6wt%Gd2O3 0.23
1x106
1x10
T = 423 K
6
8x105
5x105
4wt% Gd2O3 6wt% Gd2O3
5x105
Fit
3x105
3x105 0 5x105
0
2x106
ro
Z() 5x10 1x10 2x106 2x106 Fig. 9. Concentration dependent Nyquist plot of all the samples at 423K 5
6
-p
0
(iv)
2x106
Temperature and frequency dependent real and imaginary part of electric modulus The electric modulus is given by
re
0
1x106
of
Z()
8x10
Pure PVA 2wt% Gd2O3
5
lP
M * M jM
(8)
where M is the real part of the electric modulus, given by
( 2 ) 2
ur na
M
(9)
and M is the imaginary part of the electric modulus, given by
M
( 2 ) 2
(10)
The variation of M with respect to frequency at different temperatures for the samples is
Jo
shown in fig. 10(a-d). The magnitude of M is negligible at the lower frequencies. This may be due to the negligibly small contribution of electrode polarization in the samples. A continuous dispersion on increasing frequency may be due to the short-range mobility of the charge carriers. The anomaly in in the 2wt% sample noted at 303K is reflected in fig.10(b) also as they represent the same set of data.
1.5
1.0
0.8
(a)
303K 323K 343K 363K
(b) 2wt% Gd2O3 0.6
M
M
Pure PVA
303K 323K 343K 363K 383K 403K 423K
0.4
0.5
0.2
383K 403K 423K 0.0
0.0 2
3
4 log f
5
6
2
2.0
3
4 log f
5
6
1.2
(c) 4wt% Gd2O3
(d) 6wt% Gd2O3 1.0
1.6
0.8
3
4 log f
5
of
0.4 0.2
0.0 2
303K 323K 343K 363K 383K 403K 423K
0.6
0.0
6
2
3
ro
0.4
M
303K 323K 343K 363K 383K 403K 423K
0.8
4 log f
5
6
-p
M
1.2
Fig. 10. M vs. f plot of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% 303K 323K
re
0.3
0.6
303K 323K 343K 363K
423K FIT
383K 403K
343K 363K
0.2
383K 403K 423K FIT
M
M
lP
0.4
0.1
0.2
ur na
(a)
0.0 102
10
3
10
4
10
5
10
0.0 102
6
(b) 103
0.20 0.16
303K 323K 343K 363K
0.5
FIT
10
3
10
4
f (Hz)
106
343K 363K
383K 403K 423K FIT
M
M
Jo 0.00 102
105
0.3
0.08
383K 403K 423K
303K 323K
0.4
0.12
0.04
104
f (Hz)
f (Hz)
0.2 0.1
(c) 10
5
10
6
0.0 102
(d) 103
104
105
106
f (Hz)
Fig. 11. Bergman modified Kohlraush-Williams-Watts function fit for M vs. f plot of PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
The plot of M as a function of frequency (fig. 11(a-d)) shows a well defined peak
), which shifts to the higher frequency with temperature. The peak indicates a shift ( M max in the mobility from large range movement to short range movement of the charges and also indicates that the charge carriers undergo thermally activated hopping mechanism. The asymmetry and broadening of the peak suggests the spread of relaxation with different time constants. The solid line in the graph (fig. 11(a-d)) indicates the theoretical fitting of Bergman modified Kohlraush-Williams-Watts (KWW) function to the
M max max 1 1 max
ro
M ( )
of
experimentally obtained data. The KWW function is given by
(11)
-p
where denotes the amount of deviation from the Debye behavior. 1 indicates the ideal Debye nature and 0 indicates the maximum interaction between the dipoles.
re
is the angular frequency and max ( 2f max ) is the angular frequency at M max
lP
(maximum value of M ). The maximum value of M occurs when max 1 , where is the relaxation time which can be calculated by
1 . The calculated value of max
and the fitting parameter are tabulated in Table 2. The values are in the range 10-4-
ur na
10-6 s and are seen to decrease with increasing temperature. This indicates that the faster relaxation is due to increased side group movement of PVA with increasing temperature. It is seen that value lies below 1, indicating the non-Debye type of relaxation mechanism of the samples.
vs. f plot and M verses f plot are complementary to each other. The comparison of
Jo
Z
these two is shown in fig.12. It can be seen that the peaks of Z and M don’t coincide with one another. This result suggests the presence of both long range conduction and of localized relaxation mechanism in the sample [35,36]. The asymmetric nature of the peaks indicates the deviation from ideal Debye nature of the samples. Further the master curve of M
M max
has been shown in fig.13. All the peaks overlap on verses log f f max
one master curve in all the samples, indicating that the same type of relaxation mechanisms are active at all the temperatures. A slight broadening of curve can be seen in 4wt% film.
105
M Z
(a) 0.4
423K
0.18
M Z
(b)
M
Z
M
0.12
0.2 4
10
10 f (Hz)
10
5
10
6
10
105
12
2
0.4
(c)
M Z
423K
103
104 f (Hz)
105
5
(d)
M Z
423K
8
4
Z
3
M
Z
0.08
0 106
105
0.3
lP
0.12
M
1
re
0.16
2
2wt% Gd2O3
0.00
0
4
-p
Pure PVA 3
3
ro
0.06
0.1 0.0 102
423K
8
0.3
4
of
0.5
12
0.2 2
ur na
4
0.04
0.00 102
4wt% Gd2O3 103
104 f (Hz)
105
0 106
0.1
0.0 102
1
6wt% Gd2O3 103
104 f (Hz)
105
Fig. 12. Comparison plot of Z and M with respect to the varying frequency
Jo
Z
105
0 106
303K 323K 343K 363K 383K 403K 423K
0.6 0.4 0.2
0.8
0.0 -2
-1
0
1
0.6 0.4 0.2
Pure PVA -3
2
303K 323K 343K 363K 383K 403K 423K
2wt% Gd2O3
0.0
3
-2
-1
0
0.6
3
ro
0.8 0.6 0.4
re
0.4
(d)
1.0
0.2
lP
MMmax
0.8
4wt% Gd2O3
2
4
303K 323K 343K 363K 383K 403K 423K
-p
303K 323K 343K 363K 383K 403K 423K
MMmax
(c)
0.2
1
log(f/fmax)
log(f/fmax)
1.0
of
MMmax
0.8
(b)
1.0
MMmax
(a)
1.0
6wt% Gd2O3
0.0
0.0 -2
-1
0
1
ur na
log(f/fmax)
2
3
-2
-1
0
1
2
log(f/fmax)
f Master curve of M M vs. log f for PVA-Gd2O3 max max nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt% 13.
Jo
Fig.
3
(v)
AC conductivity The ac conductivity ( ac ) can be calculated by the empirical relation ac r o tan , where the symbols have their usual meanings. The ac conductivity of pristine PVA is obtained to be of the order of 10-7 S/m to10-8 S/m and these values are in agreement with those quoted in literature [5,13]. The ac conductivity increases with increasing frequency and also increases with temperature (fig. 14). This may be due to the increase in the mobility of the charge carrier with respect to increase in the temperature. The
of
conductivity is seen to be decreasing with increase in dopant concentration upto 4wt% and again increasing slightly in 6wt% sample. This indicates that the incorporation of
ro
nanoparticles of rare earth oxide in the polymer matrix increased the resistivity of the material.
To find the conduction mechanism in the sample, the conductivity vs. angular frequency
-p
curve has been fitted to the Jonscher’s power law (fig.14), ( ) dc A s , where dc is the dc conductivity, A is a constant which determines the strength of the polarizability,
re
and s is the exponent which gives the information about the degree of interaction between the lattice and mobile ions. It is seen that the power law equation fits the experimental
lP
data well (Fig.14). The variation of exponent s with respect to the temperature indicates the type of conduction mechanism involved in the samples. The exponent s (Fig.15) is found to be decreasing monotonically with increase in the temperature in pure PVA and
ur na
all the nanocomposites. This suggests that the correlated barrier hopping (CBH) is the
Jo
major type of conduction mechanism involved in all the samples [37].
-5
-5
-6
log ac Sm-1
-7
(a) -9 3
4
log
343K 363K 383K
-8
403K 423K FIT
5
-9
4
5
303K 323K
383K 403K 423K
log ac Sm-1
-6
-7
re
-7
-8
3
lP
-8
(c)
FIT
4
log
6
log
-5
-6
log ac Sm-1
(b) 3
6
-5
303K 323K 343K 363K
343K 363K
383K 403K 423K FIT
of
303K 323K
ro
-8
-7
-p
log ac Sm-1
-6
-9
303K 323K
5
6
-9
343K 363K
(d) 3
4
5
383K 403K 423K FIT 6
log
Jo
ur na
Fig. 14. Jonscher’s fit for variation of log ac as a function of temperature and frequency for PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
0.6
s
0.4
0.2
Pure PVA PVA- 2wt% Gd2O3 PVA- 4wt% Gd2O3
0.0 300
330
360
390
420
ro
T(K)
of
PVA- 6wt% Gd2O3
-p
Fig. 15. Variation of ‘s’ with temperature for PVA-Gd2O3 nanocomposite films with dopant concentration (a) 0wt%, (b) 2wt%, (c) 4wt%, (d) 6wt%
Sample
Temperature (K)
R ( )
303K 363K 423K 303K 363K 423K 303K 363K 423K 303K 363K 423K
2.08 10 1.88 106 8.80 104 7.93 107 6.81 106 1.02 106 5.64 107 1.35 107 2.06 106 8.28 106 3.60 106 9.33 105
ur na
Pure PVA
lP
re
Table 4. Values of resistance (R) and capacitance (C) (fig. 8), dc (Sm 1 ) (fig. 14).
PVA-2wt% Gd2O3
Jo
PVA-4wt% Gd2O3
PVA-6wt% Gd2O3
7
Parameters C (F) dc Sm 1 3.08 10-12 3.14 10-8 1.23 10-10 3.46 10-7 -9 3.32 10 7.41 10-6 -14 5.26 10 2.33 10-9 -11 1.93 10 2.77 10-8 2.19 10-10 1.85 10-7 -13 1.33 10 6.73 10-9 -12 4.42 10 2.84 10-8 1.01 10-10 1.36 10-7 -11 1.80 10 2.41 10-8 -11 5.44 10 1.54 10-8 -10 2.74 10 2.14 10-7
(
)
4. Conclusion Dielectric studies on PVA-Gd2O3 nanocomposite films as a function of frequency at various temperatures is reported here. FTIR and TEM were used to characterize the films and nanoparticles respectively and are seen to agree with the XRD and Raman data published by us earlier [24] and confirm the formation of the nanocomposite films. Dielectric permittivity of the nanocomposite films decrease with increasing dopant concentration up to 4wt% and increases again for 6wt%. This is contrary to the usual
of
trend observed in these systems and seems to be coming from the insulating nature of the rare earth oxide dopant. The permittivity is low in all the nanocomposites and the decrease with increasing Gd2O3 may arise from a decrease in the number of dipoles or
ro
their random orientation in the nanocomposites. This effect is enhanced in the 4wt% film and may be due to ligand formation and more amorphous nature of the sample as seen in
-p
the XRD data reported elsewhere by us [24]. The 2wt% film shows an anomaly at 303K whose origin is not clear. The values of increase with increasing temperature. As a
re
function of dopant concentration dielectric loss of the samples shows a monotonic decrease. Fitting to modified Cole-Cole plot showed non-Debye type of relaxation in
lP
with respect to temperature follows the Arrhenius the films. The variation of Z max behavior and curve fitting gives decreasing activation energies in the range 0.55eV to 0.23eV as a function of dopant concentration. The resistivity however, shows an increase
ur na
in the nanocomposites with the 4wt% film showing the maximum resistivity. This result points to factors other than the activation energy contributing to the conduction in these samples. The Nyquist plot shows depressed semicircles and points to a non-Debye type of relaxation. A single RC parallel circuit fits the data and indicates the presence of long range conductivity. The imaginary part of the complex electric modulus shows peaks
Jo
shifting to the high frequency side with temperature and can be fit to KWW function. The fitting parameter is less than 1, once again indicating a non-Debye type of relaxation. The maxima of Z and M occur at different frequencies indicating the presence of both localized relaxation and long range conduction. The master curve indicates that the same relaxation mechanisms are active at all temperatures. The AC conductivity can be fit to Jonscher’s law and the exponent s shows a monotonic decrease with temperature indicating that Correlated Barrier Hopping is the mechanism for conduction.
In summing up it is observed that the 4wt% film exhibits the maximum increase or decrease in the properties with the 6wt% showing the minimum change. The lack of gradation in the properties with increase in dopant makes it difficult to point out the origin of these properties. The permittivity shows a decrease in the nanocomposites while the dc and ac conductivity also show a decrease, the result being a little surprising. Gd2O3 is an insulator and PVA is an insulating polymer and the effect of the addition of Gd2O3 seems to reduce the number of available charges both for orientation in the field as well
of
as for conduction.
ro
Declaration of interests
Acknowledgement
lP
re
-p
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
ur na
S. N. Madhuri is thankful to INUP, IISc, CeNSE, Bengaluru funded by Meity, Govt. of India for providing characterization facilities for FTIR and TEM. Reference [1]
K. Gupta, P.S. Mukherjee, A.K. Meikap, P.C. Jana, Effect of samarium nanoparticles on
Jo
the electrical transport properties of polyaniline, Adv. Nat. Sci.: Nanosci. Nanotechnol., 5 (2014) 025003. doi:10.1088/2043-6262/5/2/025003.
[2]
S. Sugumaran, C.S. Bellan, Transparent nano composite PVA-TiO2 and PMMA-TiO2 thin films: Optical and dielectric properties, Optik, 125 (2014) 5128–5133. doi:10.1016/j.ijleo.2014.04.077.
[3]
J. Luo, Y. Xu, H. Mao, Magnetic and microwave absorption properties of rare earth ions (Sm3+, Er3+) doped strontium ferrite and its nanocomposites with polypyrrole, J. Magn. Magn. Mater., 381 (2015) 365–371. doi:10.1016/j.jmmm.2015.01.019.
[4]
T. Hanemann, D.V. Szabó, Polymer-nanoparticle composites: from synthesis to modern applications, Materials, 3 (2010) 3468-3517. doi:10.3390/ma3063468.
[5]
N. B. R. Kumar, V. Crasta, B.M. Praveen, Dielectric and electric conductivity studies of PVA (Mowiol 10-98) doped with MWCNTs and WO3 nanocomposite films, Mater. Res.
[6]
of
Express. 3 (2016) 055012. doi:10.1088/2053-1591/3/5/055012. A.M. El Sayed, S. El-Gamal, Synthesis and investigation of the electrical and dielectric
doi: 10.1007/s10965-015-0732-4.
M. Abdelaziz, M.M. Ghannam, Influence of titanium chloride addition on the optical and dielectric
properties
of
PVA
films,
doi:10.1016/j.physb.2009.10.030.
B,
405
(2010)
958–964.
J. Naik, R.F. Bhajantri, S.G. Rathod, T. Sheela, V. Ravindrachary, Synthesis and
re
[8]
Phys.
-p
[7]
ro
properties of Co3O4/(CMC+PVA) nanocomposite films, J. Polym. Res., 22 (2015) 97.
characterization of multifunctional ZnBr2/PVA polymer dielectrics, J. Adv. Dielec., 4
[9]
lP
(2016) 1650028. doi:10.1142/S2010135X16500284.
K.S. Hemalatha, K. Rukmani, N. Suriyamurthy, B.M. Nagabhushana, Synthesis , characterization and optical properties of hybrid PVA–ZnO nanocomposite: A dependent
study,
ur na
composition
Mater.
Res.
Bull.
51
(2014)
438–446.
doi:10.1016/j.materresbull.2013.12.055. [10]
K.S. Hemalatha, K. Rukmani, Synthesis, characterization and optical properties of polyvinyl alcohol–cerium oxide nanocomposite films, RSC Adv., 6 (2016) 74354–74366. doi:10.1039/C6ra11126b.
G. N. H. Kumar, J. L. Rao, N. O. Gopal, K. V. Narasimhulu, R. P. S. Chakradhar, A. V.
Jo
[11]
Rajulu,
Spectroscopic
investigations
of
Mn2+
ions
doped
polyvinylalcohol
films, Poly., 45 (2004) 5407. doi:10.1016/j.polymer.2004.05.068.
[12]
T.A. Hanafy, Dielectric relaxation and alternating-current conductivity of gadoliniumdoped poly(vinyl alcohol), Journal of Applied Polymer Science, 108 (2008) 2540–2549. doi:10.1002/app27567.
[13]
F.H.A. El-kader, W.H. Osman, K.H. Mahmoud, M.A.F. Basha, Dielectric investigations and ac conductivity of polyvinyl alcohol films doped with europium and terbium chloride, Phys. B, 403 (2008) 3473–3484. doi:10.1016/j.physb.2008.05.009.
[14]
E.F.M. El-Zaidia, S.I. Qashou, A.A.A. Darwish, T.A. Hanafy, Structural, optical, electrical and dielectric properties of PVA-YCl3 films, Surf. Rev. Lett. 26 (2019) 1850149. doi:10.1142/S0218625X18501494.
[15]
S. Mahendia, A.K. Tomar, S. Kumar, Electrical conductivity and dielectric spectroscopic
of
studies of PVA-Ag nanocomposite films, J. Alloys Compd., 508 (2010) 406–411. doi:10.1016/j.jallcom.2010.08.075. [16]
S. Sinha, S.K. Chatterjee, J. Ghosh,
A. K. Meikap, Dielectric relaxation and ac
ro
conductivity behaviour of polyvinyl alcohol–HgSe quantum dot hybrid films, J. Phys. D: Appl. Phys., 47 (2014) 27530. doi:10.1088/0022-3727/47/27/275301.
A.J. Kenyon, Recent developments in rare-earth doped materials for optoelectronics,
-p
[17]
Prog. Quant. Elec. 26 (2002) 225–284. doi: 0079-6727/02/$.
N. Luo, X. Tian, C. Yang, J. Xiao, W. Hu, D. Chen, L. Li, Ligand-free gadolinium oxide
re
[18]
for in vivo T1-weighted magnetic resonance imaging, Phys. Chem. Chem. Phys., 15
[19]
lP
(2013) 12235–12240. doi:10.1039/c3cp51530c.
L. Pereira, Organic light emitting diodes: The use of rare earth and transition metals. Pan Stanford, CRC Press, Taylor & Francis Group, 2012. ISBN:978-9-81426-795-3 M.O. Reddy, B.C. Babu, Structural , optical , electrical , and magnetic properties of
ur na
[20]
PVA:Gd3+ and PVA:Ho3+ polymer films, Indian J. Mater. Sci., 2015 (2015) 927364. doi:10.1155/2015/927364. [21]
S. Mukherjee, P. Dasgupta, P.K. Jana, Size-dependent dielectric behaviour of magnetic Gd2O3 nanocrystals dispersed in a silica matrix, J. Phys. D: Appl. Phys., 41 (2008)
Jo
215004. doi:10.1088/0022-3727/41/21/215004.
[22]
S. El-Sayed, Optical properties and dielectric relaxation of polyvinylidene fluoride thin films
doped
with
gadolinium
chloride,
Phys.
B.
454
(2014)
197–203.
doi:10.1016/j.physb.2014.07.076. [23]
T.A. Hanafy, Dielectric relaxation and alternating current conductivity of lanthanum , gadolinium , and erbium-polyvinyl alcohol doped films, J. Appl. Phys., 112 (2012) 034102. doi: 10.1063/1.4739752.
[24]
S.N. Madhuri, K.S. Hemalatha, K. Rukmani, Preparation and investigation of suitability of gadolinium oxide nanoparticle doped polyvinyl alcohol films for optoelectronic applications, J. Mater. Sci.: Mater. Elec., 30 (2019) 9051–9063. doi:10.1007/s10854-01901237-9.
[25]
S.N. Madhuri, K. Rukmani, Synthesis and concentration dependent tuning of PVASm2O3 nanocomposite films for optoelectronic applications, Mater. Res. Express, 6 (2019) 075017. doi:10.1088/2053-1591/ab1326. R. F. Bhajantri, V. Ravindrachary, A. Harisha, V. Crasta, S. P. Nayak, B. Poojary,
of
[26]
Microstructural studies on BaCl2 doped poly (vinyl alcohol), Poly., 47 (2006) 3591.
[27]
ro
doi:10.1016/j.polymer.2006.03.054.
I. Omkaram, R. S. Chakradhar, J. L. Rao, EPR, optical, infrared and Raman studies of
-p
VO2+ ions in polyvinylalcohol films, Physica B: Condensed Matter, 388 (2007) 318. doi:10.1016/j.physb.2006.06.134.
I. Shivaraja, S. Matteppanavar, S.K. Deshpande, S. Rayaprol, B. Angadi, Investigation of
re
[28]
space charge polarization behavior in Pb0.9Bi0.1Fe0.7W0.3O3 ceramic, J. Alloys Compd.
[29]
lP
800 (2019) 334–342. doi:10.1016/j.jallcom.2019.06.010. K.S. Hemalatha, G. Sriprakash, M.V.N.A. Prasad, R. Damle, K. Rukmani, Temperature dependent dielectric and conductivity studies of polyvinyl alcohol-ZnO nanocomposite by
impedance
spectroscopy,
ur na
films
J.
Appl.
Phys.,
118
(2015)
154103.
doi:10.1063/1.4933286. [30]
S. Chattopadhyay, P. Ayyub, V. R. Palkar, A. V. Gurjar, R. M. Wankar, M. Multani, Finite-size effects in antiferroelectric nanoparticles, J. of Phys.: Condensed Matter, 9
Jo
(1997) 8135. doi: 0953-8984/97/388135+11$19.50. [31]
S. Chattopadhyay, P. Ayyub, V. R. Palkar, M. Multani, Size-induced diffuse phase
transition in the nanocrystalline ferroelectric PbTiO3, Physical review B, 52 (1995) 13177. doi: 0163-1829/95/52(18)/13177(7)/$06. 00.
[32]
Z. W. Abdullah, Y. Dong, I. J. Davies, S. Barbhuiya, PVA, PVA blends, and their nanocomposites for biodegradable packaging application, Polymer-Plastics Tech. and Engi., 56 (2017) 1307. doi: 10.1080/03602559.2016.1275684.
[33]
K. C. Koa, Dielectric Phenomena in Solids with Emphasis of Physical Concepts of Electronic Processes, Elsevier Academic Press, UK , 2004. ISBN: 0-12-396561-6.
[34]
V.S. Shanthala, S.N.S. Devi, M. V Murugendrappa, Synthesis, characterization and DC conductivity studies of polypyrrole/copper zinc iron oxide nanocomposites, J. Asian Ceramic Soc., 5 (2017) 227–234. doi:10.1016/j.jascer.2017.02.005
[35]
S. Thakur, R. Rai, I. Bdikin, M.A. Valente, H. Pradesh, C.U. De Santiago, Impedance and Modulus Spectroscopy Characterization of Tb modified Bi0.8A0.1Pb0.1Fe0.9Ti0.1O3
[36]
of
Ceramics, Mater. Research, 19 (2016) 1–8. doi:10.1590/1980-5373-MR-2015-0504.
R. Gerhardt, Impedance and dielectric spectroscopy revisited : distinguishing localized
ro
relaxation from long-range conductivity, J. Phys. Chem. Solids. 55 (1994) 1491–1506. doi:10.1016/0022-3697(94)90575-4.
A. K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983.
-p
[37]
Jo
ur na
lP
re
ISBN 0-9508711-0-9