Applied Physics A (2018) 124:70 https://doi.org/10.1007/s00339-017-1487-z

Polymeric phase change nanocomposite (PMMA/Fe:ZnO) for electronic packaging application Pranabi Maji1 · Ram Bilash Choudhary1 · Malati Majhi1 Received: 3 July 2017 / Accepted: 16 December 2017 © Springer-Verlag GmbH Germany, part of Springer Nature 2017

Abstract This paper reported the effect of Fe-doped ZnO (Fe:ZnO) nanoparticles on the structural, morphological, thermal, optical and dielectric properties of PMMA matrix. Fe-doped ZnO nanoparticle was synthesized by co-precipitation method, after its surface modification incorporated into the PMMA matrix by free radical polymerization method. The phase analysis and crystal structure were investigated by XRD and FTIR technique. These studies confirmed the chemical structure of the PMMA/Fe:ZnO nanocomposite. FESEM image showed the pyramidal shape and high porosity of PMMA/Fe:ZnO nanocomposite. Thermal analysis of the sample was carried out by thermo-gravimetric analyzer. PMMA/Fe:ZnO nanocomposite was found to have better thermal stability compared to pure one. Broadband dielectric spectroscopic technique was used to investigate the transition of electrical properties of Fe-doped ZnO nanoparticle reinforced PMMA matrix in temperature range 313–373 K. The results elucidated a phase transition from glassy to rubbery state at 344 K.

1 Introduction Since last few decades the pace of research on the investigation of nanostructured metal oxide particles such as ZnO, ­ZrO2, ­TiO2, have been accelerated due to their large surface area and enhanced quantum confinement effect. In particular, nanoscale ZnO particles are n-type metal oxide particles with a large exciton binding energy (60 meV) and bandgap (3.37 eV). These are markedly suitable for optoelectronic applications such as ultraviolent lasers, light emitting diodes, p–n junction devices, solar cells, biological and chemical gas sensors and high-frequency electronic devices [1–3]. The literature review suggests that the surface of ZnO nanoparticle plays an important role in carrier transport. The unbound oxygen on the surface of ZnO traps the charge carrier, enhances interfacial potential and decreases the charge mobility [4]. To enhance the versatility of ZnO, the structural modification process has been exercised. For this purpose, metal ion doping is selectively employed as one of the most appropriate approach [5]. The structural modification * Ram Bilash Choudhary [email protected] 1



Nanostructured Composite Materials Labaratory, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand 826004, India

of ZnO by metal ion (Co, Mn, Fe) doping controls the carrier concentration, adjust the energy band structure and provides high dielectric constant and low loss factor. Fe 3þ doped metal oxide is predicted to improve electrical properties, thereby exchanging an electron between ­Zn+2 and F ­ e+3. It results into higher value of dielectric constant and electrical conductivity [1, 6]. On the other hand, high dielectric constant and low loss polymeric nanocomposites have been the focus of research interest for fabricating high-frequency capacitors and energy storage devices. A large number of polymer nanocomposites have been synthesized by introducing metal oxide nanoparticles (ZnO, Z ­ rO2 and ­TiO2) into polymer matrices such as poly(vinylidene fluoride) (PVDF) polycarbonate (PC), epoxy resin, polypropylene (PP), polystyrene (PS) and poly (methyl methacrylate) (PMMA) to improve their structural and dielectric properties. Their dielectric behavior depends not only on the type of filler but also on the interface bonding between the polymer and metal oxide as well as on the thermal properties of the individual components [7–9]. It is therefore essential to understand the thermal behavior of the polymeric composites utilized for the said purpose. The thermal properties and dielectric properties are simultaneously used in electronic packaging for device performance. Materials with low dielectric loss and high thermal stability are necessarily required for the dissipation of heat generated in the device and to reduce the thermal failure.

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The introduction of inorganic filler into the polymer matrix enhances its thermal property [10]. Extensive studies have been made to improve the thermal properties of PMMA by the addition of inorganic fillers. Ramanathan et al. had investigated the effect of graphite nanofiller on PMMA and found that the addition of small amount (1–5 wt%) of graphite increased the onset degradation temperature of polymer composite by 20 °C [11]. For organic modified clay-filled PMMA nanocomposite, Hwu et al. had observed that both the thermal stability and glass transition temperature (Tg) of the solid composite increased as compared to the pure matrix [12]. Yuan et al. had found that with the graphene loading as low as 0.07 wt% into the PMMA, the mechanical properties and thermal stability achieved a maximum value [13]. Similarly, for low filler weight fraction (< 1.0 wt%), the glass transition temperature of alumina/PMMA composite reduced by 25 °C as compared to virgin polymer [14]. P. Thomas et al. reported that the addition of 50 wt% ­Sr2TiMnO6 filler enhanced the dielectric permittivity ( 𝜀′ = 30.9 at 100 Hz) as well as thermal stability of PMMA matrix, thereby indicating the possibility of using this material as a suitable candidate for capacitor application [15]. The literature also witnesses a lot of research activities towards improving the properties of ZnO and PMAA-ZnO nanocomposite. Maleki et al. studied the surface modification of Fe-doped ZnO hybrid nanomaterial under mild hydrothermal condition using n-butylamine as surface modifier [16]. Sawalha et al. reported the electrical conductivity measurement of Z ­ n1−xFexO and found that conductivity decreased with increase in Fe content [17]. Colak et al. studied the effect of heat treatment and variation of Fe dopant concentration (mol%) on ZnO particle [18]. Dinesha et al. reported that the dielectric permittivity and ac conductivity of Fe-doped ZnO nanomaterial decreased with Fe content [6]. NIR reflectance efficiency of PMMA matrix reinforced with nano polyurethane (PU) modified ZnO was investigated by Soumya et al. and they observed that it exhibited 55% NIR reflectivity at 810 nm wavelength [19]. Wacharawichanant et al. found that the addition of ZnO improved the thermal stability and dielectric constant of PMMA polymer blend [20]. Literature search also shows that thermal stability of a polymer can be increased by the replacement of H-atom. If a polymer backbone is composed of Si-atom then its stability is increased. If a metal ion interacts with this Si atom then thermal stability also increased [21, 22]. However, detailed studies on the thermal and electrical properties of KH-570 (3-(methacryloxy) propyl trimethoxysilane) treated nano Fe-doped ZnO incorporated PMMA nanocomposite are scantly reported. In the present work, authors aim to fabricate and characterize PMMA-Fe-doped ZnO nanocomposite (PMMA/Fe:ZnO) as a promising material for its

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use in electronic packaging, high-frequency capacitors and energy storage devices. The nanoscale ZnO particles were modified by Fe doping to achieve low loss and high thermal stability. Broadband dielectric spectroscopic (BDS) technique was used to examine the effect of temperature on the phase transition of PMMA/Fe:ZnO nanocomposite in the temperature range 313–373 K. Two stage glassy–rubbery phase transition was explained and the mechanism of action was elaborated.

2 Experimental 2.1 Synthesis of KH‑570 modified (Fe:ZnO) nanoparticles The Fe-doped ZnO powder was synthesized by co-precipitation process using stoichiometric amount of zinc acetate as starting material. Dopant ion was provided by transition metal nitrate (Fe(NO3)3, ­9H2O) (Merck). To synthesize Fedoped (5%) ZnO nanocrystal (Fe:ZnO), zinc acetate and dopant salt were dissolved in water of PH below 2 at 80 °C. Thereupon ammonium solution (­ NH4OH) was added dropwise with constant stirring. After washing with distilled water, the precipitate was dried at 120 °C for overnight in vacuum. To obtain the sample in crystalline phase, the powder was further kept in vacuum oven at 250 °C. Finally, Fe:ZnO nanoparticles were formed and it was modified with KH-570 silane coupling agent as reported in our previous paper [23]. Fe:ZnO and KH-570 (1:2) was added into 30 ml solution of water and ethanol (1:1) and it was stirred at 60 °C for 1 h. The precipitate was collected and heated in an oven at 80 °C for 12 h.

2.2 Synthesis of PMMA/ZnO and PMMA/Fe:ZnO nanocomposite For the sake of preparing PMMA/ZnO and PMMA/Fe:ZnO nanocomposite, 8 ml of methyl methacrylate (MMA) monomer and 0.6 g of benzoyl peroxide (BPO) were taken in 100 ml of water. 0.1 gm surface modifier ZnO/Fe:ZnO, 0.2 g of polyvinyl alcohol (PVA) and 2 g of disodium hydrogen phosphate were taken in 200-ml three-necked round-bottom flask as reported in our previous paper [23]. The solution was stirred continuously for 2 h at 80 °C so as to expedite the radical polymerization reaction. A reflux condenser was fitted with the flask throughout reaction process. The schematic reaction for the preparation of KH-570 modified (Fe:ZnO) nanoparticles reinforced PMMA nanocomposite has been shown in Fig. 1.

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Fig. 1  Reaction scheme for a KH-570 modified (Fe:ZnO) nanoparticles and b PMMA/ Fe:ZnO nanocomposite

2.3 Measurement

3 Results and discussion

The phase analysis was conducted using X-ray powder diffractometer (XRD) (Bruker-D8, Cu–Kα radiation, λ = 1.5408 Å) in the angular range (2θ) of 10°–80°. Fourier transform infrared (FTIR) spectra were recorded using Perkin-Elmer-RX1 instrument in the wave number range 4000–400 cm−1. The surface morphology of the nanocomposite was examined by electron microscopic image using field effect scanning electron microscope (FESEM) (ZEISS, SUPRA55). The thermal degradation was studied by thermogravimetric measurement technique using TGA instrument (NETZSCH-Gerätebau GmbH, STA 449 F3 Jupiter). For TGA study, 1 mg sample was heated in pure nitrogen atmosphere from room temperature to 800 °C in pure nitrogen atmosphere. Photo luminescent (PL) technique was used for examining the sample quality using LS 55 PerkinElmer spectrophotometer to record the spectra with a xenon discharge lamp (240 nm excitation wavelength) as source. The temperature-dependent dielectric spectra of the prepared samples were recorded using LCR meter (HIOKI-3532-50) in the frequency range 100 Hz–5 MHz.

3.1 XRD analysis To investigate the influence of Fe:ZnO on the crystalline behavior of PMMA matrix, X-ray diffraction patterns of PMMA, ZnO, Fe:ZnO, PMMA/ZnO and PMMA/Fe:ZnO nanocomposite were recorded. Pure wurtzite phase of ZnO was obtained for Fe:ZnO as shown in Fig. 2A(c). Figure 2A(c) also revealed that no change took place in the structure of ZnO after Fe doping. The XRD spectra for nanocomposite showed a broad peak (10–20°) of PMMA with few sharp peaks (20°–70°) of Fe:ZnO. The incorporation of Fe:ZnO into PMMA matrix produced neither an additional peak nor any peak shift, thereby indicating that both PMMA and Fe:ZnO maintained their phase in the PMMA/Fe:ZnO nanocomposite as shown in Fig. 2A(c). Moreover, high intense peaks at 26° and 29° corresponding to (111) and (200) planes of Si structure were observed in Fe:ZnO incorporated PMMA nanocomposite [24, 25]. The crystallite size of the PMMA/Fe:ZnO nanocomposite

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was calculated from Scherrer’s formula and was estimated ~ 8 nm. Consequently, some strains due to crystal imperfection and distortion of the host lattice were generated and these were calculated using Williamson–Hall plots. We observed a positive slope with magnitude 0.027 for PMMA/ZnO nanocomposite and a negative slope with magnitude 0.024 for PMMA/Fe:ZnO nanocomposite as shown in Fig. 2b, c. This was the consequence of increase in crystal size after inclusion of Fe-doped ZnO nanoparticles. The calculated crystal size showed slight difference with that of the result obtained from Scherrer’s formula as shown in Table 1. This was probably due to the internal stain which was not considered in Debye–Scherrer’s model.

3.2 FTIR analysis The FTIR spectra for pure PMMA, pure ZnO, Fe:ZnO, PMMA/ZnO and PMMA/Fe:ZnO nanocomposite were recorded as shown in Fig. 3. Figure 3b, d showed a dominant absorption peak between 400 and 480 cm−1 which Table 1  Crystallite size and strain value of PMMA/ZnO and PMMA/ Fe:ZnO composites Polymeric nanocomposite Crystalline size (nm)

Micro strain (×10−3)

Debye– WilliamScherrer son–Hall PMMA/ZnO PMMA/Fe:ZnO

12 8

Fig. 2  a X-ray diffraction pattern for a PMMA, b ZnO, c Fe-doped ZnO, d PMMA/ZnO nanocomposite and e PMMA/ Fe:ZnO nanocomposite, b W–H plots for PMMA/ZnO nanocomposite and c W–H plots for PMMA/Fe:ZnO nanocomposite

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10 5

27 (tensile) 24 (compressive)

corresponded to the transverse optical stretching mode of ZnO [26]. The broad peak in the region 3200–3400 cm−1 was ascribed to the stretching vibration of OH group on the surface of inorganic nanoparticles. The peaks between 2850 and 2900 cm−1 were assigned to C–H stretching vibration of KH-570. The FTIR spectra shows absorption peak at 1635 cm−1 and it was ascribed to the characteristic peak for C=O stretching vibration. The peak at 800 cm−1 corresponded to Zn–O–Si symmetrical stretching vibration. In addition, the characteristic peak at 1120–1010 cm−1 was observed and it was ascribed to the stretching vibration of C–O–C in PMMA. Further, an additional peak was obtained in the range 800–1500  cm−1. This corresponded to the incorporation of ­Fe+3 ions into the lattice position of ZnO nanostructures. These studies suggested a strong interaction between PMMA and Fe doped ZnO nanoparticles [27].

3.3 FESEM analysis FESEM imaging technique was used to examine the surface morphology of the polymeric nanocomposites. Figure 4a–c represent the FESEM images for PMMA, PMMA/ ZnO nanocomposite and PMMA/Fe:ZnO nanocomposite, respectively. Figure 4c showed a pyramid-like structure of the nanocomposite with few micron wide and slowly growing plane faceted. The PMMA/ZnO nanocomposite was relatively more dense as compared to polymer with a small porosity. In addition, high porosity in PMMA/ Fe:ZnO nanocomposite was observed. Further, distinct grain boundaries were also observed in the FESEM image. The particles were found to be uniformly dispersed and no agglomeration was observed. The EDX image for PMMA/

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3.5 Photoluminescence (PL) analysis

Fig. 3  FTIR spectra for a PMMA, b ZnO, c Fe-doped ZnO, d PMMA/ZnO nanocomposite and e PMMA/Fe:ZnO nanocomposite

Fe:ZnO nanocomposite has been shown in Fig. 4d. This corresponded to the presence of C, Fe, Zn, O and Pt atoms in the PMMA/Fe:ZnO nanocomposite. The Pt peak in the EDS spectrum originated from the platinum sputtered onto the sample of nanocomposite. However, the trace of any other element was not observed in the FESEM image, thereby confirming the purity of the sample.

3.4 UV–Vis analysis To study the effect of Fe-doped ZnO nanoparticles on PMMA polymer, optical study was conducted using UV–Vis spectroscopy. The UV–Vis absorbance spectra for PMMA and PMMA/Fe:ZnO nanocomposite have been shown in Fig. 5. From the spectra it was clear that the strong absorbance found for wavelength below 400 nm of PMMA/Fe:ZnO nanocomposite while a very low absorbance in the visible region. An absorption in the UV region (200–280 nm) was observed due to n→σ* and π→π* transition related to C–H and C=O bonds present in the polymeric chain. The broad peak at 390 nm was attributed to the transition of electrons from valence band to conduction band of ZnO. The absorption peak was found to shift towards higher wavelengths (red shift) which mean the bandgap decreases which may be caused by defects and crystallite size after doping [28]. This affirmed that the synthesized specimen could be used as UV-shielding materials.

The PL spectra for PMMA/ZnO and PMMA/Fe:ZnO nanocomposite have been shown in Fig. 6a. The PL spectra for pure PMMA/ZnO nanocomposite showed emission peak around 430 nm. This strong blue emission occurred probably due to exiton recombination attributed to near-bandedge (NBE) emission of ZnO nanoparticles [29]. Instead of intense peaks at PL spectra, weak blue peak (450–485 nm) and blue green peak (494 nm) were also observed. In fact, visible emission in PL spectra was responsible for premier defects in ZnO nanoparticles, which included Zn vacancies (VZn), oxygen vacancies (Vo), zinc interstitial (­ Zni), oxygen interstitial (Oi) and substitution of O at Zn position (OZn) [30]. An intense broad green peak at 543 nm represented deep centers of emission and it was related to the zinc (VZn), oxygen (Vo) vacancies and zinc interstitial ­(Zni) [31–33]. An extra peak at 422 nm (2.9 eV) was assigned to n → 𝜋 ∗ transition in the unsaturated aldehyde or ketone group [34]. For Fe-doped ZnO polymer nanocomposite, a blue shift in PL spectra was observed due to Burstein–Moss effect. Due to the Burstein–Moss effect in the Fe-doped ZnO, the Fermi level shifted into the conduction band (CB). The absorption transition changed the Fermi level within the CB from the bottom of the CB. The change in the transition level leads to a broadening of the energy gap and result in blue shift [35, 36].

3.6 Thermal analysis The thermal properties of pure PMMA, PMMA/ZnO and PMMA/Fe:ZnO nanocomposites were examined by thermogravimetric analysis (TGA) as shown in Fig. 7a. The weight loss in PMMA/Fe:ZnO nanocomposite was shifted to the higher temperature as compared to pure PMMA and PMMA/ZnO. This is confirmed that Fe:ZnO nanoparticles were successfully incorporated into the polymer matrix. The weight loss started at 250 °C and ended at 400 °C. The rapid weight loss at 300 °C for PMMA/ZnO composite corresponded to the burning of organic moieties. The value of ­T1/2 (the temperature at which 50% original mass of polymer was lost) was found to be shifted from 356 to 242 °C for PMMA/Fe:ZnO nanocomposite. It was observed that the thermal stability of PMMA/Fe:ZnO nanocomposite was relatively higher than that of PMMA/ZnO nanocomposite. PMMA/Fe:ZnO nanocomposite showed only 8% weight loss at 500 °C while PMMA/ZnO showed 85% weight loss at 500 °C. This occurred due to the strong interaction between PMMA and Fe:ZnO. This restricted the thermal motion of PMMA and enhanced the thermal stability. The thermal activation energy was calculated using Broido method with the assumption that thermal degradation followed first order reaction as shown by the relation [37];

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Fig. 4  FESEM images for a PMMA, b PMMA/ZnO nanocomposite and c PMMA/Fe:ZnO nanocomposite followed by EDX spectra of PMMA/ Fe:ZnO nanocomposite (d)

activation energy depends neither on the value of heating rate nor on the maximum reaction temperature. It was found that PMMA/Fe:ZnO nanocomposite owned higher activation energy (30KJ/mol) as compared to PMMA/ZnO nanocomposite (13 KJ/mol) because of high degradation temperature. Thus, higher thermal stability of the as-prepared PMMA/ Fe:ZnO nanocomposite ensured its utility for high temperature application purpose [38]. The DTA analysis (Fig. 7b) showed that the thermal degradation was an endothermic process. As the energy of the polymer was lower than that of monomer and also an additional heat was supplied during degradation, this results an endothermic process. The glass transition temperature estimated from DTA curve was ~ 71 °C. Fig. 5  UV–Vis spectra for a PMMA, b PMMA/ZnO nanocomposite and c PMMA/Fe:ZnO nanocomposite

ln[−ln(1 − 𝛼)] = lnK −

ΔE , RT

W −W

where α = W 0−W t  = amount of polymer degraded in time t, 0

∞

W0 = initial mass, Wt  = mass at time t, W∞ = mass after infinite time. K = apparent activation energy, R = universal gas constant (8.314 J/mol K) and T = temperature in Kelvin. The main advantage of this method is that the calculated

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3.7 Dielectric analysis A comparative study on dielectric permittivity of PMMA, PMMA/ZnO and PMMA/Fe:ZnO nanocomposite at room temperature have been shown in Fig. 8. It infers that the permittivity decreases with the increase of Fe concentration and it is quite high for undoped ZnO sample. A trivalent ion has been doped in ZnO which acts as a shallow donor and increases the value for permittivity and conductivity as well [39, 40]. The present study revealed that when Fe was doped into ZnO, it behaved as deep donor and depressed the intrinsic donor concentration. Hence, the permittivity

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Fig. 6  PL spectra for a PMMA/ ZnO nanocomposite, PMMA/ Fe:ZnO nanocomposite and b schematic diagram for probable emission involved in PL spectrum

Fig. 7  a TGA curves of PMMA, PMMA/ZnO nanocomposite and PMMA/Fe:ZnO nanocomposite, b DTA curve for PMMA/Fe:ZnO nanocomposite and c glass transition temperature of PMMA/Fe:ZnO nanocomposite

porosity and permittivity owned lower value as compared to pure ZnO sample. 3.7.1 Dielectric and modulus analysis

Fig. 8  Dielectric permittivity for PMMA, PMMA/ZnO nanocomposite and PMMA/Fe:ZnO nanocomposite

was lowered by the presence of Fe doping. In the case of permittivity, density of materials also plays an important role. Material with high porosity and low density results into low dielectric permittivity and dielectric losses [41]. SEM images confirmed that Fe-doped sample exhibited high

Figure 9a, b show the frequency-dependent real and imaginary part of dielectric permittivity in the temperature range 313–343 K for PMMA/Fe:ZnO nanocomposite. The dielectric constant showed lower value and weak frequency dependent behavior at low temperature. As the temperature enhances, it is increased rapidly attaining maximum value (ε′~23) at 343K and then decreased. The increase in dielectric constant with decrease in frequency revealed that the nanocomposite exhibited a strong interfacial polarization [42, 43]. The as-synthesized nanocomposite comprised a semiconducting island on insulating polymer matrix and dipoles were developed at the interface due to large difference in conductivity. In the lower frequency region, all the functional dipoles of the polymer chain oriented themselves along the field direction and resulted in to high value of dielectric permittivity. As the frequency of the field increased, it was difficult for the dipole moment to orient along the

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Fig. 9  Frequency dependent a dielectric constant, b dielectric loss, c real part of electrical modulus and d imaginary part of electrical modulus for PMMA/Fe:ZnO nanocomposite at 313–343 K

oscillating field and hence the permittivity decreased. Thus, increase in dielectric constant was ascribed to the increase in charge carrier concentration as well as increase in polarization. The existence of oxygen vacancies (Vo) and zinc interstitial ­(Zni) confirmed by PL study, are the possible source of polarization. In the presence of an external field the zinc ions and oxygen vacancies in the neighborhood can change their position and try to align along the field direction which can be described by Kröger–Vink notation [44]:

ZnZn → Zni 2+ + 2e− OO → Vo⋅⋅ + 2e− + 1∕2 O2 𝜀 ′ also increased with increase in temperature. In case of PMMA, a polar polymer, the increase in dielectric constant was due to the rapid orientation of dipole along the field which increased the mobility of the charge carrier due to thermal energy.  𝜀′′ increased with increase in temperature.

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As the temperature increased large numbers of dipoles were trying to orient them and gave a high value of dielectric loss. This phenomenon was well supported by dielectric modulus formalism as shown in Fig. 9c, d. This enabled for the interpretation of dielectric formalism which was independent of the nature of electrode material, contact, space-charge injection and absorption of impurity and obscured the relaxation phenomena [45]. The presence of two relaxation peaks was attributed to the α-relaxation and MWS effect. The peaks were shifted to higher frequency region with increase in temperature, thereby implying that the relaxation time decreased and charge mobility increased with temperature. Figure 10a, b show frequency dependent real ( 𝜀′ ) and imaginary part ( 𝜀′′ ) of dielectric permittivity in the temperature range 343–373 K. It is evident from figure that dielectric permittivity decreases with increase in temperature. Since the composite is admixture of amorphous and crystalline regime. In the amorphous region, polymer chain is irregular,

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Fig. 10  Frequency dependent a dielectric constant, b dielectric loss, c real part of electrical modulus and d imaginary part of electrical modulus of PMMA/Fe:ZnO nanocomposite at 343–393 K

but in the crystalline regime it is regular and well arranged. When the polymer composite is heated, movement of the main chain sets in and become maximum at glass transition temperature (Tg) and hence loss occurs due to α-relaxation. This can be understood by free volume theory [46]. According to this theory, the molecular mobility near Tg depends on the free volume. At glassy state, temperature is lower than Tg the expansion of polymer separated the fillers and the interface between polymer and filler was increased. The dielectric constant also increased. As the temperature increases, the glassy state expands, this is because of the normal volume expansion of molecule and results into rubbery-like polymer state. That made the fillers easier to connect each other. Interface between polymer and filler was decreased and dielectric constant gets decreased [47]. Most of the researchers believe that this volume expansion of the polymer contributes to the abrupt decrease in dielectric permittivity [48, 49]. The electrical modulus as shown in Fig. 10c, d has been studied for the determination of dielectric response of PMMA/Fe:ZnO nanocomposite. The peaks were found to be shifted to the lower frequency side with increase in temperature. This occurred due to decrease in charge carrier density, as charge mobility increased with increase in temperature.

3.7.2 ac conductivity analysis Figure 11 shows frequency dependent ac conductivity of PMMA/Fe:ZnO nanocomposite in the temperature range 313–373 K. It exhibited frequency-independent behavior in the frequency range 100 Hz–1 KHz although it increased with frequency upto 5 MHz. This occurred due to hopping action of the charge carriers among the defect sites of

Fig. 11  Frequency-dependent ac conductivity for PMMA/Fe:ZnO nanocomposite at temperature 313–393 K

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Table 2  Value of dielectric parameters of PMMA/Fe:ZnO nanocomposite at three different temperatures

P. Maji et al. Dielectric parameters

Dielectric constant (ε′) (at 100 Hz) Dielectric loss (ε″) (at 100 Hz) ac conductivity (σac) (at 5 MHz) Characteristic frequency (fmax)

the polymer chain [50–54]. The hopping transport model describes the charge transport phenomena of amorphous inorganic semiconductor. In amorphous materials there are localized states for electron and holes. Charge carrier on different sites has different energies, due to different in their environment. Charge transport happens when the charge carriers are tunnel from one localized states to other. As the frequency of the electric field is increased the charge carriers jump to lower energy states and are on average move along or against the field, depending on their charge, which results a high value of conductivity. The ac conductivity also showed a rapid increase with the increase of temperature (upto 343 K), following the increase in the ion mobility. The temperature effect was markedly reduced at glass transition temperature with further increase in temperature, the temperature coefficient of PMMA/Fe:ZnO nanocomposite turned negative at rubbery state (T > Tg). Value of dielectric parameters of PMMA/Fe:ZnO nanocomposite at three different temperatures (313 K, 343 K, 373 K) has been shown in Table 2.

4 Conclusions PMMA/Fe:ZnO nanocomposites were prepared via free radical polymerization process. The successful fabrication of polymeric composite was confirmed by XRD, FTIR, FESEM and EDX techniques. FESEM images revealed high porosity in PMMA/Fe:ZnO nanocomposite. TGA study revealed that PMMA/Fe:ZnO nanocomposite imparted enhanced thermal stability. DTA study revealed that the glass transition temperature (Tg) of PMMA/Fe:ZnO nanocomposite was 344 K. The PL spectra for Fe-doped ZnO polymer nanocomposite showed a blue shift of the NBE due to Burstein-Moss effect. The dielectric study revealed an increase in dielectric constant ( 𝜀′~ 23) of PMMA/Fe:ZnO nanocomposite. The temperature-dependent dielectric permittivity showed a phase transition from glassy to rubbery state at 343 K. Such an enhancement in thermal stability and dielectric properties may lead its utility for electronic packaging and related device fabrication. Acknowledgements  The authors are grateful to the Director, IIT (ISM), Dhanbad for his kind support and encouragement. Pranabi Maji

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Temperatures 313 K

343 K

373 K

17 11 1.34 × 10−3 Sm−1 300 Hz

23 16 1.4 × 10−3 Sm−1 2 KHz

13 9 3.8 × 10−4 Sm−1 300 Hz

and Malati Majhi also thankful to IIT (ISM), Dhanbad for providing a Senior Research Fellowship (SRF).

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