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Experimental studies for stability of ferroelectric phase in potassium nitrate composite films.

INTRODUCTION

Ferroelectric materials and its composite films are essential components in a wide spectrum of remarkable applications (1), (2). In thin film form, ferroelectrics and, more widely, polar materials have now been used for several years in rf devices and in nonvolatile memories (3-5). Components based on ferroelectric films are also being developed for various sensors and actuator applications and for tunable microwave circuits. Composites and multilayer oxides are prepared in bulk by heating the sample at high temperature to ensure chemically and thermally stable interfaces that result in enhanced properties. However, in the processing of composite and multilayer films, the experimental conditions have to be optimized for enhancement of properties as high-temperature heat treatment triggers many unwanted reactions in thin films.

New ferroelectric composite materials have been fabricated and studied using nanomaterials (6-11). The nanoceramics can be obtained using the sol--gel technology (1), (2), (6-12). These composites show improvement towards higher operating temperature. The flexible composites were fabricated in thin film form (200-400 [micro]m) by hot-pressing (12). Thin film sensors of ceramic/polymer composite with mixed connectivities possess high values of piezo and pyroelectric coefficients and the formability and flexibility which are not attainable in single-phase ferroelectric materials (2), (12), (13).

The efficiency and the piezo- and pyroelectric figures-of-merit are influenced by the temperature dependence of the dielectric properties and the nature of the spatial distribution of polarization of the composite materials (14). The composite with low acoustic impedance matching to water and tissue can be produced. These composites can be useful in biomedical and under water transducer applications (2-14).

Materials Properties

Potassium nitrate (KN[O.sub.3]) is well known ferroelectric material (15-18). It exhibits ferroelectric properties in phase III under atmospheric pressure in the temperature range (110-124[degrees]C) upon cooling (19), (20). KN[O.sub.3] exists in phase II (trigonal crystal structure) at room temperature and transforms to phase I (orthorhombic crystal structure) when heated above 128[degrees]C to phase I. In the cooling cycle phase I does not revert back directly to phase II but passes through phase III (rhombohedral crystal structure) in the temperature range (110-124[degrees]C) with the spontaneous polarization along the c-axis (21) and then to phase II as shown in Fig. 1. Many researchers (15-21) have been deeply engrossed to obtain the stability of ferroelectric phase III in KN[O.sub.3] at room temperature. The ferroelectric phase III of KN[O.sub.3] can be used in large potential applications in the area of switching devices and memory cells, and ferroelectric characteristics. Ferroelectric phase III in thin film of KN[O.sub.3] has also been explained to exist due to surface field effects at room temperature (22), (23). These thin films have been studied for switching properties for memory devices by other workers (1), (2).

Structure of Poly(vinyl fluoride): PVF

PVF polymer crystallizes much like polyethylene because the fluorine are close enough in size to hydrogen so as not to interfere with regular packing. PVF has head--head and tail--tail defects, where successive repeat units are backwards. Typically, these amount to 5% for PVDF and 25-32% for PVF. The dipole moment of PVF could be quite large in the transplanar conformation if all fluorines were on the same side on the carbon--carbon (C--C) plane (isotactic) (14), (24-26).

The nature of the polymer in the fabrication of the composite can play a key role in the modification of the ferroelectric properties. Broadly speaking, polymers can be divided in two categories viz., polar and nonpolar. The polar polymers can further be subdivided into two categories viz., ferroelectric and nonferroelectric (24-26). In this regard in the polar categories, we have chosen polyvinyl fluoride (PVF) which is nonferroelectric.

The presence of polymer can also reduce brittleness and increase cohesion to the composite films (27), (28). The densities of PVF and KN[O.sub.3] are in the same range and this can be helpful in the formation of uniform composite layers. These films will be characterized for electrical, structural, and thermal properties using different techniques.

We have formed the composite films of KN[O.sub.3] and polyvinyl fluoride (PVF) to study the ferroelectric phase III of KN[O.sub.3] at room temperature by simple melt pressed techniques. Thick film of potassium nitrate composite prepared by melt-pressing have certain advantages such as low cost, simple construction, and good ferroelectric properties over other types of composites. In the present study the presence of the ferroelectric phase III of KN[O.sub.3] in the composite films has been investigated using x-ray diffraction (XRD). The ferroelectric hysteresis loop (P-V), the capacitance vs. temperature (C-T), and switching transient characteristics measurements also support the presence of the ferroelectric phase III in the composite film at room temperature.

EXPERIMENTAL

Purification of Potassium Nitrate (KN[O.sub.3])

The purification of KN[O.sub.3] means to remove the unwanted impurities. For this purpose, the crystal grown method was adopted in which KN[O.sub.3] powder was dissolved in double distilled water and saturated solution was prepared. The crystals of KN[O.sub.3] were grown by keeping the solution at a constant temperature for about 100 h in closed environment for its slow growth. Good and larger size crystals of KN[O.sub.3] were obtained and excessive water was removed by using the Whatmann 1 filter paper. These crystals were kept in vacuum oven at pressure [10.sup.-3] mbar at 60[degrees]C for 2 days. The crystals were ground in a mortar and pestle to obtain the powder and this powder was passed through standard sieves [mesh No. 240] (particle size [approximately equal to] 60 [micro]m). This powder was further dried and used to fabricate the composite films with different polymer matrix.

Mixing With Polymer and Fabrication of Composite Films

The purified potassium nitrate powder of Merck India and polyvinyl fluoride (PVF) powder of Aldrich U.S.A were used for preparation of the composite films. The fine powder of KN[O.sub.3] was filtered through test sieves mesh No. 350 (particle size [approximately equal to] 45 [micro]m). The same method was employed by other workers (29) when the KN[O.sub.3] crystals were ground in a mortar and pestle and then passed through a series of standard sieves to restrict the particle size to various ranges from macroscopic (~1.1 mm) crystals to powders with particle size <38 [micro]m. The purified powder of KN[O.sub.3] (50% by weight ratio) was mixed in PVF powder. The powder mixture was uniformly spread on a brass foil and covered with another part of the foil and kept in a stainless steel die in the melt-press machine. Meanwhile the mixture sandwich between the brass foils with stainless steel die. The mixture was heated up to temperature 218[degrees]C [+ or -] 2[degrees]C and then a stress of 250 kg c[m.sup.-2] was applied for 30 s. The temperature of die was brought down to room temperature and then the pressure was released. In potassium nitrate the optimization of experimental conditions play a very crucial role to obtain the stability of ferroelectric phase III. The existence of phase III strongly depends upon the applied pressure and temperature (15-18). We have optimized the experimental conditions particularly for applied pressure and temperature to obtain the stability of ferroelectric phase III at room temperature. The thickness of composite film measured was about 40 [micro]m. The circular indium electrodes (20), (27), (28) having an area of 0.20 c[m.sup.2] were vacuum deposited on both the surfaces of the sample.

RESULTS AND DISCUSSIONS

Structural Properties: X-ray diffraction (XRD)

The X-ray diffraction measurements were performed to obtain the structural information about the existence of ferroelectric phase III in the composite films. The X-ray diffraction scans for ferroelectric composite films were undertaken using Bruker a AXS diffractometer with Ni-filtered Cu K[alpha] radiation of wavelength 1.54 [Angstrom]. Figures 2-4 show the X-ray scan of 100% KN[O.sub.3] (powder form), 100% PVF (film form), and the 50 wt% KN[O.sub.3] composite films.

X-ray Diffraction (XRD) Pattern of Potassium Nitrate (KN[O.sub.3])

The X-ray diffraction scan of 100% KN[O.sub.3] in powder form was also taken and it indicated a diffraction peak of phase II at 2[theta] = 29.2[degrees] of (012) reflection, but the diffraction peak at 2[theta] = 29.85[degrees] of (003) reflection for ferroelectric phase III was not observed (29).

This indicates that the 100% KN[O.sub.3] powder contained phase II only, which is not ferroelectric as shown in Fig. 2.

X-ray Diffraction (XRD) Pattern of Poly(vinyl fluoride) (PVF)

The X-ray diffraction pattern of 100% PVF is quite similar to that of PVDF film (5). In 100% PVF samples the diffraction peaks at 2[theta] = 18.95[degrees] and 2[theta] = 20.51[degrees] belong to reflections from planes (110) and (110)/(200), respectively. Another diffraction peak seen at 2[theta] = 27.17[degrees]corresponds to the reflection of plane (120) as shown in Fig. 3.

X-ray Diffraction (XRD) Pattern of 50 wt% (KN[O.sub.3] :PVF) Composite Films

The diffraction peaks of KN[O.sub.3] at 2[theta] = 29.85[degrees]of (003) reflection for ferroelectric phase III and 2[theta] = 29.2[degrees] of (012) reflection for phase II have already been reported in literature (30), (31). These reflections have also been observed in the X-ray diffraction scan of the 50 wt% composite film as shown in Fig. 4. X-ray diffraction scan expanded around the 20 30e for ferroelectric phase III in 50 wt% composite film has also been shown in Fig. 4a. X-ray diffraction measurements have been done by various researchers (27), (30), (32) to know the crystal structure and values of different phases of KN[O.sub.3].

There are reports on the X-ray diffraction and infrared spectroscopy studies carried out on the phase transition in the doped KN[O.sub.3] samples (22), (23). When starch was mixed with KN[O.sub.3] powder, some of the X-ray peaks were affected (22), (30), (32), (33). It may be possible that KN[O.sub.3] in the composite form with PVF gives rise to enhanced (003) reflection of phase III. However, 100% PVF film exhibited no diffraction peaks in the expanded X-ray scan around 20 = 29.85[degrees]. Therefore, it can be inferred that the diffraction peaks in the expanded X-ray diffraction scans belong to KN[O.sub.3] phases.

Crystallite Sizes

The crystallite sizes or the composite films were calculated from the X-ray diffraction peak broadening using Scherrer's equation, assuming no crystallite orientation,

[t.sub.hkl] = (0.89[lambda]) / [beta]cos[[theta].sub.hkl] (1)

where [t.sub.hkl] is the crystallite size perpendicular to the (hkl) plane, [lambda] is the X-ray wavelength, [beta] is the half-peak width in radian, and [[theta].sub.hkl] is the Bragg's angle.

The optimum crystallite size of~60 nm corresponding to the diffraction peak of phase III was found for the 50 wt% composite sample. The crystallite size in the 100% KN[O.sub.3] of phase II corresponding to (012) reflection at 2[theta] = 29.2[degrees] was estimated to be ,100 nm and the X-ray diffraction peak of phase III was absent. It is interesting to note here that these samples contain phase III content with small crystallite sizes (32). The crystallite size seems to play an important role in the existence of phase III in the composite at room temperature. The crystallite size at 2[theta] = 29.80[degrees] of reflection (003) vs. doping percentage of KN[O.sub.3] in PVDF was calculated by Neeraj et al. (27). The large crystallize size of ferroelectric phase III in 50 wt% composite films may be responsible for producing stable ferroelectric phase at room temperature.

Ferroelectric Polarization Switching (FPS)

Ferroelectric polarization switching is the process by which the remanent polarization is reoriented into a new position of remanent polarization ([P.sub.r]). It is possible to induce switching both by an electric field and mechanical stress. At zero applied field, there are two states of remanent polarization, [+ or -] [P.sub.r] furthermore; these two states of remanent polarization are equally stable. Either of these two states could be encoded as a "1" or a "0" and since no external field is required to maintain these states, the memory device is nonvolatile. To switch the state of the device, a threshold field greater than coercive field [+ or -] [E.sub.c] is required. Additionally, to reduce the required applied voltage (to within 5V) for a given coercive field [+ or -] [E.sub.c], the ferroelectric materials need to be processed in thin films (1), (2).

In most ferroelectric memories, the memory cell is read destructively by sensing the current transient that is delivered to a small load resistor when an external voltage is applied to the cell. For example, if a memory cell is in a negative state of remanent polarization (-[P.sub.r]) and a positive switching voltage is applied to it, there will be a switching charge given by (4), (11), (20).

Q = A[[epsilon].sub.0][epsilon][E.sub.a] + A [[integral].sub.0.sup.[infinity]] dP / dt dt (2)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

where A is the area of the cell, [epsilon] is the dielectric constant of the ferroelectric materials, [E.sub.a] is the applied electric field, [t.sub.s] is the switching time, i is the switching current, [[epsilon].sub.0] is the permittivity of free space, and [P.sub.s] is the spontaneous polarization of the ferroelectric material. The nonswitching current arises from a linear dielectric response (A[[epsilon].sub.0][epsilon][E.sub.a]) and the switching current response arises from the displacement current.

The kinetics of polarization reversal may be described by measuring the switching time (ts) for different amplitude of electric pulse. Because the current decrease exponentially, it is difficult to measure the total switching time ([t.su .s]), and [t.sub.s] is usually taken as the value where current falls to 0.1 [i.sub.max] (20).

The polarization switching transient current characteristics in the composite film at room temperature were studied using alternate square wave pulses as shown in Fig. 5. The sweep voltage range varies from -15V to +15V to detect the switching current pulses. The polarization switching current was observed across a resistance (0.35 k[OMEGA]) in series with the sample. The polarization switching current peak ([i.sub.m]) occurs at time ([t.sub.m]). From the measured switching currents, the spontaneous polarization can be calculated as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

where i is the switching current, Q is the switching charge, and A is the area of the electrode.

The value of the switching charge density in the range between 1.5 and 7 [micro]C [cm.sup.-2] calculated by integrating the switching current data, is in reasonable agreement with the value of spontaneous polarization obtained by P-V hysteresis loop measurements as shown in Fig. 6. Same type of switching current behavior in ferroelectric PZT capacitor was discovered by other workers (34), (35). The switching current peaks show some asymmetry which may be due to the incomplete switching of polarization during pulse measurements (1), (2). Another reason may be due to the space charge relaxation and conduction effects (1), (2).

Ferroelectric Properties

Hysteresis Loop. The ferroelectric hysteresis loop characteristics of the composite films were recorded using a standard Sawyer--Tower circuit along with a storage oscilloscope connected to a computer. In Fig. 6 the ferroelectric hysteresis loop was observed in the 50 wt% KN[O.sub.3] composite films at room temperature using 50 Hz sine wave signal. The value of remanent polarization ([P.sub.r] = 5.50 [micro]C [cm.sup.-2]) and the value of spontaneous polarization ([P.sub.s] = 6.50 [micro]C [cm.sup.-2]) were obtained in the composite film at room temperature.

However, the value of spontaneous polarization ([P.sub.s] = 6.30 [micro]C [cm.sup.-2]) at 121[degrees]C was discovered by other researchers (16-19), (22). It indicates that the polarization versus voltage (P-V) characteristics in 50 wt% (KN[O.sub.3]: PVF) composite films predominantly support the stability of ferroelectric phase III at room temperature. Ferroelectric hysteresis loops of different KN[O.sub.3] compositions have been carried out by Neeraj et al. (27). The hysteresis loop of 50 wt% KN[O.sub.3] shows the maximum value of remanent polarization ([P.sub.r]). The value of [P.sub.r] increases up to 50 wt% KN[O.sub.3] in the composite and beyond this composition the remanent polarization ([P.sub.r]) indicates a decreasing tendency.

The composite film greater than 50 wt% KN[O.sub.3] mixing were found to be more brittle. Therefore, 50/50 wt% compositions were thought to be better composite films for ferroelectric applications (27).

Phase Transition

Capacitance vs. Temperature (C-T) Measurements.

The capacitance vs. temperature behavior of the composite films as temperature cooled through the paraelectric--ferroelectric phase revealed anomalies in the vicinity of the Curie point ([T.sub.c]). This type of behavior obeyed the Curie--Weiss law just above [T.sub.c] in the paraelectric region (36-39). The presence of the paraelectric-to-ferroelectric phase transition of the composite films has been confirmed by capacitance vs. temperature (C-T) measurements (14-17). The C-T measurements showed the phase transition temperature according to the Curie-Weis law, [epsilon](T) = C/(T - [T.sub.c]) where [epsilon] is a dielectric constant, C is Curie--Weis constant, T is a measurement temperature, and [T.sub.c] is the transition temperature. Also the C-T showed thermal hysteresis upon heating and cooling. In potassium nitrate there are two Curie transition temperatures, the upper range is ([T.sub.c] = 124[degrees]C) and the lower range is ([T.sub.c] = 110[degrees]C) upon cooling.

Capacitance vs. Temperature (C-T) With Zero Bias.

The capacitance has been measured as a function of temperature and shows a sharp change during heating and cooling cycles. LCRQ Bridge NO.6018 was used at fixed frequency of 1 kHz for capacitance vs. temperature measurements. The capacitance versus temperature (C-T) measured at 1 kHz, without applied dc bias voltage is shown in Fig. 7 for the 50 wt% composite films.

Discussion on the Paraelectric-to-Ferroelectric Phase Transition

The capacitance dependence upon temperature in the composite film shows the transition from paraelectric-to-ferroelectric phase.

The value of capacitance shows two-fold increase in the temperature range 134-150[degrees]C as compared to the room temperature in the heating cycles. This change may be attributed to the transition of phase II to I with the transition starting at 134[degrees]C during heating cycle. The DSC curve of KN[O.sub.3] : PVF composite also show a peak at 133[degrees]C (40) during heating cycle. This shows that temperature dependence of capacitance has a correlation with the DSC transition peak.

Therefore, the sharp change in the capacitance of Fig. 7 in the temperature range 134-152[degrees]C may be attributed to the transition of phase II [right arrow] I of KN[O.sub.3] in the composite. Above 170[degrees]C, the value of capacitance decreases with increasing the temperature during heating modes. In the cooling modes the capacitance increases with decreasing the temperature in the temperature range 165-127[degrees]C and then decreases up to room temperature (19).

This Curie--Weiss behavior further confirms the paraelectric-to-ferroelectric phase transition and supports the ferroelectric behavior in the composite films up to room temperature (19) The C-T measurements on lead titanate (PTO) was discovered by Mou et al. (38) and the same trends support the Curie--Weiss and ferroelectric behavior in the composite film (38).

Discussion on the Ferroelectric Phase Transition

During the cooling cycle, the capacitance shows a sharp drop in the temperature range 126-110[degrees]C. This temperature range has been indicated the upper and lower ranges of [T.sub.c] (19). On the other hand the endothermic DSC curve shows a peak at 115[degrees]C, which was attributed to the transition from phase I [right arrow] phase III of KN[O.sub.3] in the composite(19). Therefore in all probability the capacitance variation in the temperature range 126-110[degrees]C in the cooling mode can be related to the phase change I [right arrow] III. The value of capacitance falls down three times during this temperature range and possesses the ferroelectric phase III up to room temperature.

Capacitance vs. Temperature (C-T) With Applied DC Bias Voltage of +2V. Figure 8 shows the capacitance vs. temperature (C-T) measurements with applied DC bias voltage of +2V. The capacitance dependence upon temperature in the composite film with applied DC bias voltage of +2V shows the transition from paraelectric-to-ferroelectric phase. The value of capacitance shows twofold increase in the temperature range 140-152[degrees]C as compared to the room temperature in the heating cycles. This change may be attributed to the transition of phase II to I with the transition starting at 140[degrees]C during heating cycle. The DSC curve of KN[O.sub.3]: PVF composite also show a peak at 133[degrees]C [40] during heating cycle. This shows that temperature dependence of capacitance has a correlation with the DSC transition peak

Therefore, the sharp change in the capacitance of Fig. 8 in the temperature range 140-152[degrees]C may be attributed to the transition of phase II [right arrow] I of KN[O.sub.3] in the composite films.

Discussion on the Ferroelectric Phase Transition

During the cooling cycle, the capacitance shows a sharp drop in the temperature range 124-110[degrees]C. The value of capacitance falls down approximately three times during this transition. On the other hand the endothermic DSC curve shows a peak at 115[degrees]C, which was attributed to the transition of phase I [right arrow] phase III of KN[O.sub.3] in the composite (40). Therefore in all probability the capacitance variation in the temperature range 124-110[degrees]C in the cooling mode can be related to the phase change I [right arrow] III. Above 152[degrees]C, the value of capacitance increases with increasing the temperature during heating modes. In the cooling modes the capacitance decreases with decreasing the temperature up to room temperature.

The temperature dependent capacitance on KNb[O.sub.3]/KTa[O.sub.3] superlattices with applied dc bias was discovered by Sigma et al. (36) and supports the trends of phase transition in the composite film. In the composite films (Figs. 8 and 9) the range of temperature lower than ~100[degrees]C, the capacitance is nearly temperature independent with no bias voltage dependence for dc voltage of +2V. This indicates the paraelectric phase II in the composite films during heating cycles.

However, as the temperature increased above 100[degrees]C the capacitance shows an increase trend with applied dc voltage. This C-T behavior in the composite films with +2V bias indicated some inconsistent with paraelectric-ferroelectric phase transition just above 152[degrees]C during heating modes (36-39). During cooling modes in the composite films with applied bias, the paraelectric-to-ferroelectric phase transition has been observed in the 124 [right arrow] 110[degrees]C transition.

Capacitance vs. Temperature (C-T) With Applied DC Bias Voltage of -2V. The capacitance dependence upon temperature in the composite film with applied dc bias voltage of -2V shows the transition from paraelectric-to-ferroelectric phase. The value of capacitance shows two and half-fold increase in the temperature range 133-156[degrees]C as compared to the room temperature in the heating cycles. This change may be attributed to the transition of phase II to I with the transition starting at 133[degrees]C during heating cycle. The DSC curve of KN[O.sub.3]: PVF composite also shows a peak at 133[degrees]C (40) during heating cycle. This shows that temperature dependence of capacitance has a correlation with the DSC transition peak. Therefore, the sharp change in the capacitance of Fig. 9 in the temperature range l33-156[degrees]C may be attributed to the transition of phase II [right arrow] I of KN[O.sub.3] in the composite film.

Discussion on the Ferroelectric Phase Transition

During the cooling cycle, the capacitance shows a sharp drop in the temperature range 124-110[degrees]C. On the other hand the endothermic DSC curve shows a peak at 115[degrees]C, which was attributed to the transition of phase I [right arrow] phase III of KN[O.sub.3] in the composite [40]. Therefore in all probability the capacitance variation in the temperature range 124-110[degrees]C in the cooling mode can be related to the phase change I [right arrow] III.

The capacitance vs. temperature measurements with/or without dc bias voltage in the composite films indicated the value of ferroelectric curie point ([T.sub.c]) at 124[degrees]C [+ or -] 2[degrees]C (upper range) and 110[degrees]C [+ or -] 2[degrees]C (lower range) during cooling modes (39). We observe that the composite film shows the ferroelectric behavior below [T.sub.c]. (up to room temperature) and paraelectric behavior above [T.sub.c], during cooling cycles. The C-T curves of the composite films as shown in Figs. 7-9 supports the ferroelectric behavior. The capacitance increases with increasing the temperature during heating modes as shown in Table 1. During the cooling modes, the transition temperature range 124[degrees]C [+ or -] 2[degrees]C [right arrow] 110[degrees]C [+ or -] 2[degrees]C strongly support the paraelectric (Phase II)-to-ferroelectric (phase III) transition and support the stability of ferroelectric phase III in the composite films at room temperature (36-39).

TABLE 1. Shows the values of capacitances during heating and
cooling modes.

            Healing mode                     Cooling mode

Applied     Variation of    Variation of     Variation of  Variation of
bias     the temperature             the  the temperature           the
            ([degrees]C)     capacitance     ([degrees]C)   capacitance
                                    (pF)                           (pF)

Zero        (134-150~16)  (47-96~49) [up     (126-110~16)    (93-40~53)
bias          [up arrow]          arrow]     [down arrow]         [down
                                                                 arrow]

+2V         (140-152~12)  (46-96~50) [up     (124-110~14)    (94-34~60)
bias          [up arrow]          arrow]     [down arrow]         [down
                                                                 arrow]

-2V         (133-156~23)     (41-105~64)     (124-110~14)   (106-40~66)
bias          [up arrow]      [up arrow]     [down arrow]         [down
                                                                 arrow]


CONCLUSION

The XRD experiments show that ferroelectric phase III in the composite films is retained at room temperature. The remanent polarization 5.50 [micro]C [cm.sup.-2] was obtained from the hysteresis loop measurements. The switching current characteristics have been observed in the composite films which are usually seen in ferroelectric materials. The transitions in the C-T measurements have supported the Curie-Weiss behavior. Further advanced research work is essentially required to develop the ferroelectric nanocomposite films for ferroelectric applications.

ACKNOWLEDGMENT

The authors thankfully acknowledge the contribution of Shri Aseem Chauhan, Chancellor, Amity University Rajasthan for the continuous inspiration, moral support throughout the advanced research work.

Correspondence to: Dr. Neeraj Kumar; e-mail: nkumar@jpr.amity.edu or neeraj.phy@rediffmail.com

Contract grant sponsor: Shri Aseem Chauhan, Chancellor, Amity University Rajasthan.

DOI 10.1002/pen.23446

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2012 Society of Plastics Engineers

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Neeraj Kumar, (1) Rabinder Nath (2)

(1) Department of Physics, Amity School of Engineering and Technology, Amity University Rajasthan, Jaipur 302001, Rajasthan, India

(2) Ferroelectric Materials and Devices Research Lab, Department of Physics, Indian Institute of Technology, Roorkee, Roorkee 247667, India
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