International
Journal of MechanicalJOURNAL
Engineering andOF
Technology
(IJMET), ISSN 0976 –
INTERNATIONAL
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6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 1, January- April (2012), © IAEME
ENGINEERING AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 3, Issue 1, January- April (2012), pp. 258-266
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IJMET
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INVESTIGATION OF THERMOELECTRET STATE
POLYURETHANE-CDS NANOCOMPOSITE
Suruchi* and Ramvinode
Department of Physics
*Department of Chemistry
Hindustan College of Science and Technology, Farah,
Mathura (Uttar Pradesh)-281122, India
*Corresponding author: Email:sur_pranika@yahoo.co.in
Fax: +91-565 2763364
ABSTRACT
25 micrometer thin film of polyurethane-CdS nanocomposites was prepared by solution
mixing method to investigate the thermoelectret state by means of thermally stimulated
discharge current measurement. The TSDC is characterized by two well defined peaks
showing that nanocomposites are follow at least two type of polarization mechanism says
dipolar and space charge polarization, while possibility of interfacial polarization along
with these polarization is prominent in nanocomposite samples. The peak near glass
transition region is having broad shoulder ascribed the space charge polarization. The
several feature of peaks and TSDC parameters shows that thermoelectret state in PU is
significantly affected by incorporation of CdS.
Keywords: TSDC, space charge polarization, nanocomposites, CdS, polyurethane
1. INTRODUCTION
Organic–inorganic nanocomposites have attracted a lot of interests in recent years
since they usually not only occupy the combined properties of organic polymers (e.g.,
flexibility, ductility, dielectric) and inorganic materials (e.g., rigidity, high thermal
stability, strength, hardness, high refractive index) [1–9]
There are many nanocomposite polymers, especially containing nano SiO2 or nano TiO2,
which were prepared by the sol–gel approach and investigated with a focus on how the
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nanoparticles influence the mechanical, thermal, and optical properties and so on of the
nanocomposite polymers and the relationship between structure and properties [10-14].
Important aspects of the chemistry involved in the formation of these systems are the
uniformity, phase continuity, domain sizes and the molecular mixing at the phase
boundaries, which all have a direct influence on optical, physical, and mechanical
properties [15].
A growing interest exists in the development of new material for charge storage and
insulating electrical/ electronic devices. Although polyurethanes exhibit good mechanical
properties such as high abrasion resistance tear strength, flexibility and elasticity,
polyurethanes have some disadvantages, for example, poor thermal stability and low
barrier properties. Now days the polymer nanocomposites are so popular due the creation
of nano-interface between organic polymers and inorganic nanofillers. This new era of
science and technology motivate the researcher to develop new in improved polymer
nanocomposite to develop the Nanoelectromechanical-System (NEMS) to be a future
technology. The performance of NEMS is several times better than MEMS.
In view of above background we have chosen to study the thermoelectret state in CdS
nanoparticle embedded polyurethane samples. The objective of present work is to
investigate the thermoelectret state in CdS embedded polyurethane samples by using
thermally stimulated discharge current (TSDC) technique.
2. MATERIALS AND METHOD
Cadmium acetate, cadmium chloride, carbon disulphide and N, N-dimethylformamide
were purchased from Merck India ltd. Polyurethane is supplied by Redox India. The thin
film of polyurethane and polyurethane-CdS nanocomposites was prepared by solution
mixing method. 5g PU dissolved in 100 ml of N, N-dimethylformamide (DMF) and
stirred for the period of 4 h at 333K. The homogeneous milky solution was prepared. The
milky solution was used to prepare pristine PU film. The optically plane glass plates were
rinse with acetone and distillated water and then slowaly drawn from PU solution. Now
the glass plates were vacuum dried at 313K. After 24h of vacuum drying, the samples of
pristine PU were easily pealed off from glass plates. The PU-CdS nanocomposite thin
film is also prepared by similar procedure, however, the nanopartices are prepared
separately. In order to prepare CdS nanoparticles, the cadmium acetate (Cd (CH3COO)2,
ammonium thiocynate (NH4CNS) are taken according to weight percent ratio and
dissolve in DMF under stirring at 313K using magnetic stirrer for the period of 1h. This
solution is added drop by drop in a solution of pristine PU as prepared.. The remaining
method for preparation of PU-CdS nanocomposite thin film is the same as discussed
above. The 25 µm thin samples were used for TSDC study. The samples were vacuum
aluminized over the central circular area of 3.5 cm diameter using High Vacuum Coating
Unit (Vacuum Equipment Company Ltd, Noida, India) for the TSDC measurements. The
TSDC was recorded by using Keithley (model 6514) system electrometer at a linear
heating rate of 30/min. The detailed procedure of TSDC is reported in our article [16].
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3. RESULT AND DISCUSSION
In order to gain a more complete understanding of the effect of nanofillers in
thermoelectrets state, charge relaxation and charge storage properties of PU+ CdS
nanocomposites, the TSDC of nanocomposite samples have been undertaken. It is usual
to report that TSDC data in three different levels of presentation: the direct experimental
results, the relaxation map, and the distribution of the kinetic parameters. We present the
direct experimental results of TSDC in Pristine PU and PU+CdS nanocomposite samples.
The depolarization current density is a function of temperature. Since, the
depolarization current density (current intensity per unit area J (T)) is the rate of
decreasing of the polarization, we have
(1)
where I(T) is the current intensity at temperature T (or at time t) of the constant rate
heating rate, P(T) is the remaining polarization at temperature T (or at time t) and A is
the effective area of the electrodes. The analysis of the TSDC results could be explain on
the basis of Debye relaxation theory. According to this theory at each temperature of
linear heating rate, the decay of the polarization with time is a first-order rate process. For
an elementary single motional process, we can thus write:
(2)
where P(T) = P(t) is the polarization at temperature T (at time t) of the heating rate and
τ(T) is a temperature-dependent relaxation time, characteristic of the elementary mode of
motion under consideration. Combining esq. (1) and (2), it comes out that
(3)
An important feature of the TSDC technique is that it allows the study of elementary or
single relaxation processes using the so-called thermal polarizing procedure, however, it
also allows to study the multiple relaxation process. The importance of eq. (3) is that it
allows the calculation of the temperature-dependent relaxation time of a single relaxation
process from the experimental result. As the temperature rises linearly with time in the
depolarization step, temperature and time are related by T = T0 + rt, where T0 (the socalled freezing temperature) is the temperature at the beginning of the heating rate (at t =
0) and r is the heating rate. In this way the remaining polarization at temperature T, P(T),
is given by:
(4)
where Tf is a temperature well above the temperature of the maximum of the TSDC peak,
where the sample is already completely depolarized. The temperature-dependent
relaxation time associated with a given mode of motion can thus be calculated from:
(5)
where I (T) is as noted before, the depolarisation current intensity measured at constant
heating rate. The capability of directly calculating the relaxation time from the TSDC
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results is an essential quantitative feature of the TSDC technique and that is why
thermally stimulated depolarisation currents is an experimental technique that provides
important information on molecular mobility in solids, i.e. on the kinetics of relaxation
processes and the distribution of relaxation times.
In the present experiment PU & PU+CdS nanocomposite of 3%, 7%, 12%, 20% weight
ratios have been prepared. The TSDC of all samples were recorded with poling field of
75 &150 kV/cm at 298K, 373K and 423K. The TSDC for PU pristine polarized at 298K
with the polarizing field of 75kV/cm shows single peaks 423K, while TSDC of
nanocomposite sample shows single peak located at 456±8K in (Fig. 3). The peak of 12%
CdS nanocomposite sample is very clear as compare to peaks of other nanocomposites.
The TSDC of these samples show negative current initially and attain the positive value
after 384K. Similar trends are observed for TSDC recorded for 75kV/cm at 298K
polarizing temperature (Fig.2). Thermally stimulated current for PU samples at 298K
with polarizing field of 150kV/cm (Fig. 3) gives positive current peak at 413K, however,
nanocomposites samples shows two well resolved negative and positive peaks
respectively. This behaviour of TSDC at polarizing field of 298K is explained on the
basis of hetero and homocharge theory.
The TSDC for PU samples polarized at 423 K with polarizing field of, 75 and
150kV/cm are characterized by peak appeared at 450±2K (Fig.5). However,
nanocomposite samples are characterized by one unstable peak (i.e say β peak) in the
temperature range from 366 to 388K and other stable peak (i.e. say α peak) in the
temperature range from 443 to 458K. α – peak is strong as compare to β peak. Similar
TSDC charteristics are observed when samples are polarized at 373K with different
polarizing field as shown in Fig. 3 and 4.
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The appearance of TSDC peak, characteristic of the microphase-separated morphology of
PU is appeared in the nanocomposite samples at 450±2K, suggesting that the microphase
separation into hard-segment microdomains and soft-segment microphase is affected by
interaction of CdS with PU. Interestingly, similar effects are observed in epoxy
resin/layered silicate nanocomposites, where results to be reported elsewhere show that
the long range heterogeneity of epoxy resin is destroyed in the nanocomposites [9]. The
TSDC peak at high temperature (i. e. α-relaxation peak) is corresponding to glass
transition temperature. The theoretically reported Tg of PU is well agreed with the
difference of ±5K. In addition, a broad shoulder appeared on the high-temperature side of
the peak in the composites, which was attributed to the α-relaxation in an interfacial
region where the glass transition is increased due to interaction with the particle surface.
When the PU+CdS nanocomposites sample were polarized at room temperature, a current
reversal is obtained. The magnitude of currents varied from experiment to experiment.
This fact is due to change in poling condition of samples as well as density of CdS in PU
samples. During the cooling from high temperature to room temperature, it is expected
that large temperature gradient will exists with in sample, which result in the disturbance
of trapped charges. Therefore, sign of the TSDC was opposite to that of the charging
current (-ve). The TSDC characteristics for samples polarized above room temperature
are qualitatively similar. In PU+CdS nanocomposites samples shows an unstable new
peak in the temperature range from 366 to 388K, however, this new peak is not appeared
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in pristine PU for any condition of polarization. In other words the TSDC characteristics
are similar for all samples at room temperature polarization and also similar above room
temperature polarization for all samples. It has been noticed that the current is smaller for
low temperature polarization than the high temperature polarization. It has been observed
that the peak is shifted with respect to the polarizing field.
The relationship between the peak current and polarizing field is shown in Fig. 10.The
peak current in some cases fall on a straight, and extrapolations of this curve passes
through a point at no vanishing applied field Eo. From the present experiment it may be
possible that the charge injected into PU+CdS nanocomposites in polarization at
temperature above the room temperature for all polarizing field is ionic in nature.
In other hand the dipole alignment, which is corresponding to first peak (β- peak)
observed in all PU+CdS nanocomposites at all condition of polarization, however, the
dipole polarization is no fully observed in pristine PU because activation is varying
between 0.23-1.01 eV, which corresponding to both dipolar (β-relaxation) relaxation and
space charge relaxation (α-relaxation). The calculated value of activation energy for βpeak (i.e. dipolar relaxation) of PU+CdS nanocomposites samples is well agreed with
value reported for other polymers [11]. The activation energy was calculated by using
initial rise method [12]. The α-relaxation peak (high temperature peak of nanocomposite
samples is not only alone due to α-relaxation process. The dipolar relaxation and
interfacial polarization can not ignored fully, because i) peak current of α-peak is the
function of field shown in (Fig. 7to 8) and ii) The calculated value of activation energy
is not fully agreed with α-relaxation process these results suggested that the existence of
multiple relaxation process in PU+CdS nanocomposite. Since no any complete theory for
relaxation process in polymer nanocomposites is developed, therefore, we conclude that
multiple relaxation process are the origin of polarization in nanocomposite samples for
which the present study is an evidence.
The quantitative explanation of TSDC could be explained with the help of Fig. 9
(a) to Fig. 9 (f). Fig. 9(a) and Fig. 9(b) show a systematic model for polarization in
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polymers activity. The arrows indicate the direction of the polarization current. The
polarization due to the dipole alignment is shown by
, the charge due to the
microscopic displacement of the ions with trapping by
, the space charge built up
, and the space charge
by the migration of ions over macroscopic distances by
injected from the electrodes by
. All these four kinds of charge co-exist in the
polymer and polymer nanocomposite samples. The formal two kinds give a uniform
volume polarization, which is the heterocharge, the third gives a non-uniform
heterocharge, and a fourth is nonuniform homocharge. With increasing temperature the
space charge due to free ions,
disappears by the recombination with counter ions
as shown in Fig. 9 (c). The direction of the current is opposite to that of the polarization
current. Further, disorientation of dipoles as shown in Fig. 9 (d), and liberation of ions
from traps (i.e. Fig. 9 (e)) takes place and direction of these currents is also opposite to
that of the polarization current.
If the electrodes are deposited in vacuum and fully contacted with both surfaces of the
polymer sample, the homocharge should be drawn towards the nearest electrode by the
electrical image force. The direction of TSDC should than be the same as that of the
decay of hetrocharge. This situation is corresponding to the negative polarity of TSDC as
observed in present study. However, if there is an air gap between the electrode and
surface of the sample, the magnitude of image force become negligible and the
homocharge may diffuse into the sample due to the internal field. Therefore, the direction
of TSDC is opposite to that of the charging current / polarization current as shown in Fig.
9 (f).
Gaur et al [10] recently pointed out that nanofillers modified the trap structure of
polymers due to which large variation of activation energy is obtained.
In present case the semiconductor behaviour of CdS nanoparticle may have quite less
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energy band gap as compare to traditional CdS. This low energy band gap of CdS is very
less as compare to polyurethane subsequently when molecular species of CdS attached
with polymer molecules and farming interface of nanomatric order. These interfaces itself
act as new trapping levels formed at different depth. The high temperature TSDC peaks
may be controlled by the trap concentration. This trap concentration is the function of wt
% of CdS nanoparticle in PU matrix. The trapped charge carrier concentration was
calculated by using relation as reported in literature [14]
nt = 2.7 Jm kTm2 / edγA
where Jm is the maximum value of the current density, e the electronic charge, k the
Boltzmann constant, Tm the peak temperature, γ the heating rate, A the activation energy
and d the sample thickness. It has been found to be that carrier concentration nt (i.e.
varying from 3x 1018 m-3 to 5x 1018 m-3 ) increases with concentration of CdS nanofillers,
which is an evidence for new trapping sites are introduced by incorporation of
nanofillers.
It is evident from characteristics behaviour of TSDC that the new traps introduced in PU
matrix can act as electrons and hole traps with various energy levels. The current start
increasing at a certain temperature due to frequent trapping of charge carriers and
exponentional decrease of current after attaining the maximum value is caused by
trapping of charge carrier at deeper and deeper level. It is evident from higher value of
activation energy of α-peak, (i.e. 1.5 to 3.5eV) for nanocomposite samples, which is
normally not obtained in case of polymer like polyurethane. These results suggested that
the structure of pristine PU is modified by filling of CdS nanoparticals which causes the
behaviour of TSDC and modified the various parameters such as charge released,
relaxation time, and activation energy particularly as shown in table 1. In the present
work we have demonstrated that nanofillers additives modified the TSDC behavior as it
is also reported in literature [13-16]. It was found that a strong correlation between CdS
wt % and TSDC parameters exist. In order to obtain excellent theroelectret properties, the
formation of a new interfaces between polymer and additives, which acts as pathway for
charge drift and neutralization, has to be prevented and isolated additive domains need to
be generated to act as efficient charge traps.
4. CONCLUSION
It is concluded that the dipolar, space charge, interfacial relaxation processes are
simultaneously operative in nanocomposite system because of its heterogeneous
structure, however TSDC characteristics of pristine PU samples follow the dipolar and
space charge relaxation mechanism. The new trapping sites introduced by nanofillers
demonstrate the potentiality of nanofillers to modify the thermoelectret behaviour of
polymer in general.
Acknowledgements
We are thankful to Dr.M.S.Gaur,Prof(Department of Physics, HCST,Mathura)for his full
co operation & guidance during this work.
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