- Email: [email protected]
Study of dielectric relaxations in zinc oxide-epoxy resin nanocomposites
Journal of Alloys and Compounds 477 (2009) 316–321
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Review
Study of dielectric relaxations in zinc oxide-epoxy resin nanocomposites Hichem Smaoui a,b,∗ , Lassad E.L. Mir c , Hajer Guermazi b , Serge Agnel d , Alain Toureille d a
Faculté des Sciences de Sfax, Département de Physique, BP. 802 Route de Soukra, 3018 Sfax, Tunisia Unité de Physique des Matériaux Isolants et Semi-isolants, IPEIS, Route Menzel Chaker, km 0.5, BP 1172, 3018 Sfax, Tunisia c Laboratoire de Physique des Matériaux et des Nanomatériaux Appliquée à l’Environnement, Faculté des Sciences de Gabès, Cité Erriadh Manara Zrig, 6072 Gabès, Tunisia d Groupe Energie et Matériaux (G E M), Université Montpellier 2, Place Eugène Bataillon, CC 079, 34095 Montpellier CEDEX 5, France b
a r t i c l e
i n f o
Article history: Received 8 August 2008 Received in revised form 14 October 2008 Accepted 21 October 2008 Available online 9 December 2008 Keywords: Composite materials Polymers Dielectric response Ionic conduction
a b s t r a c t Broadband dielectric relaxation spectroscopy (DRS) (10−3 to 106 Hz) and thermally stimulated depolarization currents (TSDC) techniques were used to investigate dielectric relaxations and molecular mobility in nanocomposites based on epoxy resin (ER) filled with nano-conductive particles. ZnO and Zn97%OAl3% nanoparticles were used as fillers and the dielectric measurements were performed at room temperature. The obtained data were first analysed in terms of the dielectric permittivity and then transformed to electric modulus to highlight conduction process. Interfacial relaxation is found to be strongly dependent on the presence of nano-filler particles. The overall molecular mobility is found to decrease in the glassy state in the nanocomposites as compared to the ER while it increases near the glassy-rubber transition. Heterogeneity introduced by the nano-filler particles has increased the space charge density in epoxy nanocomposites. AC conductivity of both ER-matrix and epoxy nanocomposites is frequency-dependent, follows the exponential law ac ∼ ωs and shows a dc plateau in the low frequency range. An increase in dc conductivity was notified in nanocomposites. © 2008 Elsevier B.V. All rights reserved.
Contents 1. 2.
3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Characterization techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Dielectric relaxation spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Thermal step method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Thermally stimulated depolarisation currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Epoxy resins (ERs) are widely used in insulation such as electrical machinery, in power electronic devices and packing of integrated circuits. They are also employed as coatings and adhesives
∗ Corresponding author at: Unité de Physique des Matériaux Isolants et Semiisolants, IPEIS, Route Menzel Chaker, km 0.5, BP 1172, 3018 Sfax, Tunisia. Tel.: +216 98 65 62 48; fax: +216 74 27 44 37. E-mail address: [email protected] (H. Smaoui). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.084
316 317 317 317 317 317 317 318 321 321 321
because of their good mechanical properties and strength of interaction with materials and metals, respectively. For some interesting applications we need to improve the electrical conductivity of such adhesives. So the dispersion within the epoxy matrix of conductive powder particles with sufficient quantity allows to reach high electrical conductivity values and consequently conductive or semi-conductive composite is then formed. These composite polymeric materials have found a variety of applications because they constitute excellent models for studying the interactions between condensed matter and electromagnetic radiation such as microwaves. They are also used for spatial applications [1].
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
These composites are considered as heterogeneous disordered systems [2,3]. Their performances depend on many factors, electrical properties of their constituents, geometric characteristics, volume fraction of the filler and the spatial distribution of the filler within the polymeric matrix [4–8]. Interactions between the two constituents can also influence dielectric properties of such composites. When the filler size is in the nanometre scale, the obtained heterogeneous systems are known as nanocomposites. Because of the small size of nanoparticles, polymer–filler interface property becomes a dominant factor in determining the properties of nanocomposites [9]. Thus, strong interactions between the polymer and nanoparticles are usually formed producing improved materials with better electrical, mechanical and thermal properties [10]. Compared with the traditional composites containing micron-sized fillers, the nanocomposites have good optical property due to small filler size [11]. In addition to dipolar relaxations related to polymer matrix, interfacial relaxations termed the Maxwell–Wagner–Sillars (MWS) effect [12–14] can also interfere. This phenomenon (MWS) appears generally in heterogeneous media due to the accumulation of charges at the interfaces which leads to the formation of large dipoles. Furthermore, conductivity relaxations are also generally present in these composites and can be investigated as dipolar and interfacial relaxations by dielectric spectroscopy as well as thermally stimulated depolarisation currents (TSDC) technique [15–17]. Because of the presence of conductive particles, dielectric or conductivity relaxations may be obscured [18]. It is then more convenient to use the formalism ‘electric modulus’ first introduced by McCrum et al. [19]. Macedo et al. [20] were the first to exploit the modulus and used it for the investigation of electrical relaxation phenomena in vitreous ionic conductors. It has also used in polymers to study their conductivity relaxation behaviour [21–23]. The use of electric modulus helps to understand bulk relaxation properties at low frequencies. Thus, common difficulties like electrode nature and contact, space charge injection phenomena and absorbed impurity conduction effects, which appear to obscure relaxation in permittivity presentation, can be resolved or even ignored [18,24]. Complex modulus, electric modulus or inverse complex permittivity, M*, is defined by the following equation: M∗ =
1 1 ε ε = = 2 + j = M + jM , ε∗ ε − jε ε + ε 2 ε 2 + ε 2
j = (−1)1/2
(1)
where M is the real and M is the imaginary electric modulus, and ε is the real and ε is the imaginary permittivity. In the present work, broad band dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC) techniques were used to investigate dielectric relaxations in nanocomposites made of nanoparticles (ZnO and Zn97%OAl3%) dispersed in an epoxy-amine. Thermal step method (TSM) was also used to display the influence of nano-fillers on the behaviour of space charge density. Structural, electrical and optical properties of the investigated epoxy matrix have been studied previously [24–28]. It has demonstrated by means of TSDC, TSM and IR spectroscopy techniques that this polymer presents a polar character and the polarization depends strongly on its thermal history. The dielectric relaxation of epoxy-amine systems has been discussed at length by Fitz and Mijovic [29]. The ␣ process is associated with segmental motion of the terminal epoxy groups while the  process which is located in the high mega hertz region results from the localized motions.
317
Table 1 Investigated samples. Sample
Composition
EP ZnO-EP ZnOAl-EP
Pure epoxy resin Epoxy + 2wt.% ZnO Epoxy + 2wt.% Zn97%OAl3%
2. Experimental 2.1. Sample preparation Nanocomposites object of this study were made by dispersing conductive nanoparticles within an epoxy matrix. The epoxy resin was a 2,2-bis(4 glycidyloxyphenyl) propane (molar mass 340 g/mol) and the hardener was a difunctional amine (Maestria). These two components are carefully and homogeneously mixed without any solvent at stoechiometry ratio and at room temperature. Conductive nanoparticles (ZnO and Zn97%OAl3%) were elaborated by sol–gel method using El Mir et al. protocol [30,31]. The obtained nanocrystallites having a prismatic shape with an average grain size of about 30 nm and with a specific surface area of about 25 m2 /g, were used as filler and without any surface treatment. These nano-fillers were hand mixed into the obtained mixture with a ratio of 2% in weight. The final mixture was stirred at room temperature, and then poured into parallelepipedal mould. The hardening was carried out at room temperature and under atmospheric pressure for 5 days. The obtained samples were labelled and listed in Table 1. Sample dimensions are of (60 mm × 60 mm) × 0.6 mm for DRS measurements and of (100 mm × 100 mm) × 1 mm for TSDC and thermal step measurements. All samples were coated on both sides with aluminium electrodes (4 cm diameter), deposited under vacuum.
2.2. Characterization techniques 2.2.1. Dielectric relaxation spectroscopy Dielectric spectroscopy measurements have been performed at room temperature on a Solartron SI 1260 Impedance Gain-Phase analyser in the frequency range 10−3 to 106 Hz, interfaced to a computer by a Solartron 1296 Dielectric Interface. The amplitude of the ac signal was 1 V.
2.2.2. Thermal step method The thermal step method, which has been widely described in previous papers [32,33], allows the determination of the space charge repartition in an insulating sample. This technique is not destructive, based on measurements of the thermodilatation current caused by the application of a so-called thermal step to one side of a plate sample. The presence of charges within the insulator induces image charges on each electrode. Then, the propagation of the thermal step (positive or negative), and the resulting expansion (dilatation or contraction) across the sample thickness, modify the equilibrium of these image charges and consequently, a current appears in the external circuit joining the electrodes. The current is then amplified and recorded with a computer. The deconvolution of the measured current gives the electric field and the space charge distributions. The theory and the numeric treatment have been described in detail in previous papers [34,35].
2.2.3. Thermally stimulated depolarisation currents TSDC technique is based on a sample’s depolarization by thermal activation [17]. At a given temperature Tp , a static electric field is applied to the investigated sample for a time tp that is long enough to permit the different mobile entities of the material to orient themselves within the field. This configuration is then frozen by a rapid decrease in temperature to a temperature T0 low enough for the molecular mobility to be considered zero, keeping the electric field applied in order to avoid any relaxation of dipoles and/or charges. At this temperature, the field is switched off and the sample is short-circuited for a certain time to eliminate the eventual surface charges and stabilise sample at this temperature. The polarised sample is then shortcircuited through a high sensitivity electrometer in an oven, which is programmed to rise linearly with time. During the linear increase of temperature, the return to equilibrium of the previously oriented entities generates a depolarisation current which is recorded as a function of temperature. TSDC provide information on relaxation processes that occur in polymers. In contrast to the experimental simplicity of the TSDC technique, the experimental data analysis is not easy, as the polarization may be due to several microscopic processes (associated with dipolar or trapped charge mechanisms), whose relaxation will contribute to the depolarization current [36]. In the present work, samples were polarised at a temperature Tp = 30 ◦ C for the polarization time (tp ) of 3 h under a polarization field (Ep ) of 3 kV/mm. Depolarization currents were measured at a constant heating rate of 1 ◦ C/min.
318
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
Such result can be explained by the existence of significant interactions between nano-fillers and molecular entities forming the ER-matrix. For the pure epoxy resin, Fig. 1(b) shows the presence of two main relaxation processes located around 1850 and 14 Hz. The first one (1850 Hz) is attributed to interfacial relaxation process known as MWS effect. In fact, epoxy resin networks present a lacunar structure with microspherical voids produced during mixing process when air liberation is restricted due to the system viscosity [1,37]. Consequently two phases with different permittivities are then present in these systems, air and polymeric matter. Furthermore, in previous work [28] we have demonstrated by means of X-ray reflectometry measurements that the ER-network presents locally a periodic character with various spacing structures (from 32 to 74 nm) due to preparation mode of samples which makes the structure more heterogeneous and favours the presence of interfaces in the epoxy network. The small relaxation peak around 14 Hz is due to the relaxation of internal stress produced during the preparation. Similar relaxation has been reported for other epoxy resins [38–40]. It is known, that when the hardening temperature is lower than the glass temperature Tg (45 ◦ C for the investigated epoxy resin), the motion of chain segments is restricted which is converted to shrinkage internal stress [41,42]. The appeared peaks on the epoxy nanocomposites spectra of Fig. 1(b) were attributed to interfacial process. We note that these relaxation peaks shift to the lower frequencies in epoxy nanocomposites, suggesting a decrease of charges and/or dipoles mobility at the interfaces. This is due to the presence of significant interactions at the interface which decrease the aptitude of dipoles to be oriented under the effect of ac electric field. This effect is more prominent in the ZnO-epoxy nanocomposite. Fig. 1(b) shows also a very small relaxation around 42 kHz attributed to dipolar relaxation which is not influenced by the nano-filler particles. The stability of the relaxation frequency suggests that there are no significant interactions between nano-filler particles and the relaxing polar entities in the ER-matrix. In the low frequency range the slope of log(ε ) versus log(f) is −1, which is typical for dc conductivity effect [11,15,43]. These results are confirmed by the corresponding ac conductivity plots, ac (f) (Fig. 1(c)), which exhibit a dc conductivity plateau at low frequency. It has been calculated from dielectric losses according to the following relation: ∗ (ω) = jε0 ωε∗ (ω) = ε0 ωε + jε0 ωε
Fig. 1. Real part of dielectric permittivity ε (a), dielectric losses ε (b) and real part of ac conductivity ac (c) against frequency of samples indicated on the plot.
3. Results and discussion The overall dielectric behaviour of the investigated samples, obtained by DRS measurements at room temperature is shown in Fig. 1: Real part of dielectric permittivity ε , dielectric losses ε and real part of ac complex conductivity ac as functions of frequency. In the frequency range 10−2 to 2 × 103 Hz, ε is lower in nanocomposites than in the pure epoxy resin (Fig. 1(a)), indicating a decrease of the molecular mobility in the glassy state.
(2)
The real part of *(ω) is given by ac (ω) = ε0 ωε , where ε0 = 8.85 × 10−12 F m−1 is the permittivity of the free space and ω = 2f the angular frequency. It can also be seen from Fig. 1(c) that the dc conductivity value in epoxy nanocomposites is slightly higher than in the pure epoxy resin. At high frequency ac conductivity is frequency dependent, increases with increasing frequency and follows the law ac (ω) ≈ ωs with 0 ≤ s ≤ 1 characterizing hopping conduction [3,44]. The real and imaginary parts of permittivity were then transformed into electric modulus formalism via Eq. (1). Fig. 2(a)–(c) is obtained, giving the real part (M ), imaginary part (M ) and the complex plane representation (M (M )) of the electric modulus for pure epoxy resin and epoxy nanocomposites. In addition to the interfacial relaxation process, Fig. 2(b) shows another relaxation peak in the low frequency range due to dc conduction, which is not evident in ε (f) presentation. It can also be seen that the dc conduction peak shifts to higher frequencies in epoxy nanocomposites as compared to pure ER-matrix (Table 2), suggesting an increase in the mobility of charge carriers which contribute effectively in the conduction process. The two processes interfacial and dc conduction give also two distinct semicircles in the complex presentation (Fig. 2(c)). An additional weak circle arc appears on the diagram of pure epoxy resin
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
319
Table 3 (a) Parameters evaluated by fitting data according to the Havriliak–Negami approach for pure epoxy resin and epoxy nanocomposites. (b) Parameters evaluated by fitting data experimental points, describing the internal stress relaxation, to the Debye approach for pure epoxy resin. (a)
Sample
Ms
M∞
1−˛
Interfacial relaxation
EP ZnO-EP ZnOAl-EP EP ZnO-EP ZnOAl-EP
0.1743 0.1877 0.1772 0.0011 0.0013 0.0018
0.2274 0.2354 0.2242 0.1539 0.1610 0.1705
1 1 0.377 0.986 0.964 0.961
1 0.882 0.5985 0.890 0.893 0.897
Conduction
(b)
Sample
Ms
M∞
Internal stresses
EP
0.1630 0.1645
0.1706 0.1715
which has been attributed to the internal stresses relaxation. The variation of the position and semicircles radius reflect the influence of the nano-filler particles. Experimental data were fitted using Havriliak–Negami equation given by the following [45]: ε ∗ (ω) = ε∞ +
εs − ε∞ (1 + (jω)
1−˛
)
,
0 < (1 − ˛),
≤1
(3)
where εs and ε∞ are the limiting values of dielectric constant sides the relaxation, ˛ and are parameters describing the symmetric and asymmetric broadening of the relaxation time distribution, is the central relaxation time and ω is the angular frequency. In the electric modulus formalism the Havriliak–Negami equations have the following form [18]: M = M∞ Ms M = M∞ Ms
[Ms A + (M∞ − Ms ) cos ]A Ms2 A2
+ 2A (M∞ − Ms )Ms cos + (M∞ − Ms )2
Ms2 A2
+ 2A (M∞
[(M∞ − Ms ) sin ]A − Ms )Ms cos + (M∞ − Ms )2
(4)
(5)
where Ms =1/εs , M∞ =1/ε∞ , A=[1+2(ω)(1−␣) sin(˛/2)+(ω)2(1−␣) ]1/2 and = arctg[(ω)(1−␣) cos(˛/2)/(1 + (ω)(1−␣) sin(˛/2))]. The curves with solid lines on Fig. 3(a)–(c) are produced by best fitting experimental points to the Havriliak–Negami equation. All parameters evaluated by fitting data according to Havriliak–Negami approach for pure epoxy resin and epoxy nanocomposites are listed in Table 3(a). The dashed semicircle curve on the inset of Fig. 3(a) was obtained by fitting experimental points, describing the so-called internal stress relaxation, to the Debye equation [18]:
Fig. 2. Real part of electric modulus M (a), imaginary part of electric modulus M against frequency (b) and Cole–Cole plots (c) of the investigated samples.
Table 2 Frequencies corresponding to the maximum of M relative to dc conduction process. Sample
f (Hz)
EP ZnO-EP ZnOAl-EP
1.13 × 10−3 2.90 × 10−3 2.75 × 10−3
M −
M∞ + Ms 2
2
+ M = 2
M + M 2 ∞ s 2
(6)
It was produced by a superposition of two semicircles represented with dash dot lines. The corresponding evaluated parameters were given in Table 3(b). It can be seen from exponent values corresponding to interfacial relaxation process that heterogeneity introduced by the incorporation of nano-filler particles in the epoxy resin matrix leads to an asymmetric distribution of relaxation times and more particularly in ZnOAl epoxy nanocomposite. While the evaluated ˛ and parameter values for the pure epoxy resin show a Debye behaviour of the interfacial relaxation. Influence of nano-filler particles on interfacial relaxation can also be seen through the corresponding relaxation strength values defined by ε = εs − ε∞ . This relaxation strength takes the value 1.34 for pure epoxy resin and the values 1.08 and 1.18 for ZnO and ZnOAlepoxy nanocomposite, respectively. We remark a decrease of this parameter in nanocomposites, which indicates a decrease of the relaxed dipole number at interfaces. In the low frequency, the evaluated parameter values (Table 3(b)) corresponding to the conduction
320
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
Fig. 4. Space charge distribution in pure epoxy resin and epoxy nanocomposites after 3 h of polarization under 3 kV mm−1 at 30 ◦ C.
space charge distribution has the same shape presenting a polarization character and the amplitude of the space charge density is clearly amplified in epoxy nanocomposite samples than in the pure epoxy resin. This demonstrates that, in nanocomposites, the polarisation was amplified by the contribution of interfacial polarisation component due to the inclusion of nano-filler particles. Moreover, the DRS results show a diminution in the relaxed dipoles quantity (decrease of ε value) at interfaces in nanocomposites; this proves that the interfacial dipoles were stabilised by charge interactions, emphasized by dielectric behaviour. Samples were the depolarised by TSDC measurements. The corresponding TSDC spectra of pure epoxy resin and epoxy nanocomposites are represented in Fig. 5. They present a main current peak around 44 ◦ C attributed to the ␣ relaxation, associated with the glass transition of the soft phase of ER-matrix. This figure shows a small shift of the ␣ peak to lower temperature in epoxy nanocomposites. It can be also seen that dipole relaxation associated with the ␣ relaxation were more pronounced in epoxy nanocomposites. Such results can be explained in terms of increased free volume due to loosened packing of chains which leads to an increase of chain mobility in nanocomposites in the glass–rubber state. In fact, near the glass transition temperature the segmental mobility of the polymer molecules increases and becomes a dominating factor causing important
Fig. 3. Cole–Cole plots of the investigated samples: PE (a); ZnO-EP (b); ZnOAl-EP (c). The solid lines are produced by best-fitting the experimental points to the Havriliak–Negami approach. The dashed semicircle on the inset (of (a)) was produced by a superposition of the two dash dot semicircles obtained by fitting the corresponding data to the Debye approach.
process for pure epoxy resin and epoxy nanocomposites are nearly constant and close to the unity indicating a near Debye process. Indeed, in the low frequency range conductivity is governed by charge carriers of the matrix rather, than the fillers. Similar results were found by Psarras et al. [46] on epoxy resin amine filled with nickel powder at various amounts. Fig. 4 shows space charge repartition in the pure epoxy resin and epoxy nanocomposites after polarization under an electric field of 3 kV/mm for 3 h at a temperature of 30 ◦ C. In all samples the
Fig. 5. TSDC spectra of pure epoxy resin and epoxy nanocomposites submitted to an electric polarization (Ep = 3 kV mm−1 , Tp = 30 ◦ C, tp = 3 h).
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
TSDC dipolar relaxations in epoxy nanocomposites. In contrast dielectric measurements have shown a decrease of molecular mobility in the glassy state resulting from a decrease of dielectric permittivity in epoxy nanocomposites (Fig. 1(a)). An interfacial Maxwell–Wagner–Sillars relaxation associated with accumulation of charges at the interfaces appears at higher temperature on the TSDC spectra (Fig. 5). So, in the polarisation process two mechanisms are likely operative, which are the orientation of polar groups with the polarising electric field and the space charge polarisation. The disorientation of polar groups during the TSDC measurement give rise to a current peak (␣) which appears at lower temperature (than the space charge polarisation peak which appears at higher temperature). At higher temperature the space charge motion is accompanied by a creation of carriers which is due to ionisation of impurities as well as the breaking of chemical bonds (epoxy ring-opening reaction, braking of N–H bonds, etc.). Indeed, in previous works [26,27] we have studied the effect of heating temperature on the space charge and structure of the epoxy polymer. We have demonstrated that space charge density increases inside the material with increasing temperature of heating. This was related to charge carriers creation due to structural changes. The thermally created carriers become more mobile at higher temperature and constitute a homocurrent in the internal field. This current is opposite to the classical TSDC current due to dipolar disorientation (heterocurrent). As the temperature is raised, the homocurrent starts increasing, as the number of thermally generated carriers increases. So a reversal is observed from heterocurrent to homocurrent. Comparing the TSDC spectra of different samples around the interfacial relaxation, it can be concluded that nano-filler composition influence well charges nature and their interactions at the interfaces. 4. Conclusion In this paper, dielectric relaxations of an epoxy resin matrix and epoxy nanocomposites made by dispersing conductive nanoparticles (ZnO and Zn97%OAl3%) in the epoxy matrix were investigated by means of DRS and TSDC techniques. Interfacial or MWS relaxation is evident in the dielectric spectrum of the pure epoxy resin matrix. The origin of this interfacial process is may be due to the lacunar and/or stratified structure of the epoxy network. Heterogeneity induced by the presence of nano-fillers in the epoxy matrix has magnified the space charge density. The distribution of relaxation times associated with the interfacial process becomes asymmetric in nanocomposites. In the low frequency range conductivity is governed by charges carriers of the epoxy matrix rather, than of the nano-fillers (Hopping conduction). Acknowledgments The authors acknowledge the financial support of the «Ministère de l’Enseignement Supérieur, de la Recherche Scientifique et de la Technologie», Tunisia.
321
References [1] Y. Baziard, S. Breton, S. Toutain, A. Gourdenne, Eur. Polym. J. 24 (6) (1988) 521–526. [2] G.C. Psarras, E. Manolakaki, G.M. Tsangaris, Compos. Part A: Appl. Sci. Manuf. 34 (2003) 1187–1198. [3] J.C. Dyre, T.B. Shrfder, Rev. Mod. Phys., 72. 3 (2000) 873–892. [4] F. Carmona, C. Mouney, J. Mater. Sci. 27 (1992) 1322–1336. [5] I.G. Chen, W.B. Johnson, J. Mater. Sci. 26 (1991) 1565–1976. [6] G. Boiteux, J. Fournier, D. Issotier, G. Seytre, G. Marichy, Synth. Met. 102 (1999) 1234–1235. [7] J. Fournier, G. Boiteux, G. Seytre, J. Mater. Sci. Lett. 16 (1997) 1677–1679. [8] Ye.P. Mamunya, V.V. Davydenko, P. Pissis, E.V. Lebedev, Eur. Polym. J. 38 (2002) 1887–1897. [9] Y. Sun, Z. Zhang, K.S. Moon, C.P. Wong, J. Polym. Sci. Part B: Polym. Phys. 42 (2004) 3849–3858. [10] C. Zhang, R. Mason, G. Stevens, IEEJ Trans. FM 126 (11) (2006) 1105–1111. [11] Y. Sun, Z. Zhang, C.P. Wong, Polymer 46 (2005) 2297–2305. [12] J.C. Maxwell, Electricity and Magnetism, vol. 1, Clarendon Press, Oxford, 1892, p 452. [13] K.W. Wagner, Arch. Electrotech., Berlin, 1914. [14] R.W. Sillars, J. Inst. Eng. 80 (1937) 378. [15] A. Kanapitsas, P. Pissis, R. Kotsilkova, J. Non-Cryst. Solids 305 (2002) 204–211. [16] P. Pissis, G. Georgoussis, V.A. Bershtein, E. Neagu, A.M. Fainleib, J. Non-Cryst., Solids 305 (2002) 150–158. [17] V.M. Gun’ko, V.I. Zarko, E.V. Goncharuk, L.S. Andriyko, V.V. Turov, Y.M. Nychiporuk, R. Leboda, J. Skubiszewska-Zieba, A.L. Gabchak, V.D. Osovskii, Y.G. Ptushinkii, G.R. Yurchenko, O.A. Mishchuk, P.P. Gorbik, P. Pissis, J.P. Blitz, Adv. Colloid Interface Sci. 131 (2007) 1–89. [18] G.M. Tsangaris, G.C. Psarras, N. Kouloumbi, J. Mater. Sci. 33 (1998) 2027– 2037. [19] N.G. McCrum, B.E. Read, G. Williams, Anelastic and Dielectric Effects in Polymeric Solids, Wiley, London, 1967, 108-111. [20] P.B. Macedo, C.T. Moynihan, R. Bose, Phys. Chem. Glasses 13 (1972) 171–179. [21] A.A. Bakr, A.M. North, Eur. Polym. Sci. 13 (1977) 799–803. [22] H.W. Starkweather, P. Avakian, J. Polym. Sci. B, Polym. Phys. 30 (1992) 637– 641. [23] C. show-an, S.L. Chien, Macromolecules 26 (1993) 2210–2216. [24] F.S. Howell, R.A. Bose, P.B. Macedo, C.T. Moynihan, J. Phys. Chem. Glasses 14 (1973) 122. [25] H. Smaoui, H. Guermazi, S. Agnel, Y. Mlik, A. Toureille, Polym. Int. 49 (2000) 1513–1518. [26] H. Guermazi, H. Smaoui, S. Agnel, Y. Mlik, A. Toureille, F. Schué, Polym. Int. 50 (2001) 743–747. [27] H. Guermazi, H. Smaoui, S. Agnel, Y. Mlik, A. Toureille, F. Schué, Polym. Int. 52 (2003) 1287–1293. [28] H. Medhioub, C. Zerrouki, N. Fourati, H. Smaoui, H. Guermazi, J.J. Bonnet, J. Appl. Phys. 101 (2007) 043509. [29] B.D. Fitz, J. Mijovic, Polym. Adv. Technol. 9 (1998) 721. [30] L. El Mir, A. Amlouk, C. Barthou, S. Alaya, Physica B Condens. Matter 388 (2007) 412–417. [31] Z. Ben Ayadi, L. El Mir, K. Djessas, S. Alaya, Nanotechnology 18 (2007) 445702. [32] A. Toureille, J. Electrostat. 32 (1994) 277–286. [33] P. Notingher, S. Agnel, A. Toureille, IEEE Trans. Dielec. Elec. Insul. 8 (6) (2001) 985–994. [34] A. Toureille, J.P. Reboul, P. Merle, J. Appl. Phys. III (1991) 111–123. [35] A. Cheri, M. Abou Daka, A. Toureille, IEEE Trans. on Dielec. and Elec. Insul. 27 (6) (1992) 1152–1158. [36] C. Bucci, R. Fieschi, C. Guidi, Phys. Rev. 148 (2) (1966) 816–823. [37] L. Ramajo, M. Reboredo, M. Castro, Compos.: Part A 36 (2005) 1267–1274. [38] J. Mijovic, K.F. Lin, J. Appl. Polym. Sci. 32 (1986) 3211. [39] C. Brahatheeswaran, V.B. Gupta, Polymer 34 (1993) 289. [40] X. Chen, S. Li, Macromol. Rapid Commun. 22 (2001) 349–352. [41] G. Yang, S.Y. Fu, J.P. Yang, Polymer 48 (2007) 302–310. [42] Y.M. Cao, D.H. Yu, L. Chen, J. Sun, Polym. Test 20 (2001) 685–691. [43] P.S. Anantha, K. Hariharan, Mater. Sci. Eng. B 121 (2005) 12–19. [44] G.C. Psarras, Compos. Part A: Appl. Sci. Manuf. 37 (2006) 1545–1553. [45] S. Havriliak, S. Negami, Polymer 8 (1967) 161–210. [46] G.C. Psarras, E. Manolakaki, G.M. Tsangaris, Compos. Part A: Appl. Sci. Manuf. 33 (2002) 375–384.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Review
Study of dielectric relaxations in zinc oxide-epoxy resin nanocomposites Hichem Smaoui a,b,∗ , Lassad E.L. Mir c , Hajer Guermazi b , Serge Agnel d , Alain Toureille d a
Faculté des Sciences de Sfax, Département de Physique, BP. 802 Route de Soukra, 3018 Sfax, Tunisia Unité de Physique des Matériaux Isolants et Semi-isolants, IPEIS, Route Menzel Chaker, km 0.5, BP 1172, 3018 Sfax, Tunisia c Laboratoire de Physique des Matériaux et des Nanomatériaux Appliquée à l’Environnement, Faculté des Sciences de Gabès, Cité Erriadh Manara Zrig, 6072 Gabès, Tunisia d Groupe Energie et Matériaux (G E M), Université Montpellier 2, Place Eugène Bataillon, CC 079, 34095 Montpellier CEDEX 5, France b
a r t i c l e
i n f o
Article history: Received 8 August 2008 Received in revised form 14 October 2008 Accepted 21 October 2008 Available online 9 December 2008 Keywords: Composite materials Polymers Dielectric response Ionic conduction
a b s t r a c t Broadband dielectric relaxation spectroscopy (DRS) (10−3 to 106 Hz) and thermally stimulated depolarization currents (TSDC) techniques were used to investigate dielectric relaxations and molecular mobility in nanocomposites based on epoxy resin (ER) filled with nano-conductive particles. ZnO and Zn97%OAl3% nanoparticles were used as fillers and the dielectric measurements were performed at room temperature. The obtained data were first analysed in terms of the dielectric permittivity and then transformed to electric modulus to highlight conduction process. Interfacial relaxation is found to be strongly dependent on the presence of nano-filler particles. The overall molecular mobility is found to decrease in the glassy state in the nanocomposites as compared to the ER while it increases near the glassy-rubber transition. Heterogeneity introduced by the nano-filler particles has increased the space charge density in epoxy nanocomposites. AC conductivity of both ER-matrix and epoxy nanocomposites is frequency-dependent, follows the exponential law ac ∼ ωs and shows a dc plateau in the low frequency range. An increase in dc conductivity was notified in nanocomposites. © 2008 Elsevier B.V. All rights reserved.
Contents 1. 2.
3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Characterization techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Dielectric relaxation spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Thermal step method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Thermally stimulated depolarisation currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Epoxy resins (ERs) are widely used in insulation such as electrical machinery, in power electronic devices and packing of integrated circuits. They are also employed as coatings and adhesives
∗ Corresponding author at: Unité de Physique des Matériaux Isolants et Semiisolants, IPEIS, Route Menzel Chaker, km 0.5, BP 1172, 3018 Sfax, Tunisia. Tel.: +216 98 65 62 48; fax: +216 74 27 44 37. E-mail address: [email protected] (H. Smaoui). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.084
316 317 317 317 317 317 317 318 321 321 321
because of their good mechanical properties and strength of interaction with materials and metals, respectively. For some interesting applications we need to improve the electrical conductivity of such adhesives. So the dispersion within the epoxy matrix of conductive powder particles with sufficient quantity allows to reach high electrical conductivity values and consequently conductive or semi-conductive composite is then formed. These composite polymeric materials have found a variety of applications because they constitute excellent models for studying the interactions between condensed matter and electromagnetic radiation such as microwaves. They are also used for spatial applications [1].
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
These composites are considered as heterogeneous disordered systems [2,3]. Their performances depend on many factors, electrical properties of their constituents, geometric characteristics, volume fraction of the filler and the spatial distribution of the filler within the polymeric matrix [4–8]. Interactions between the two constituents can also influence dielectric properties of such composites. When the filler size is in the nanometre scale, the obtained heterogeneous systems are known as nanocomposites. Because of the small size of nanoparticles, polymer–filler interface property becomes a dominant factor in determining the properties of nanocomposites [9]. Thus, strong interactions between the polymer and nanoparticles are usually formed producing improved materials with better electrical, mechanical and thermal properties [10]. Compared with the traditional composites containing micron-sized fillers, the nanocomposites have good optical property due to small filler size [11]. In addition to dipolar relaxations related to polymer matrix, interfacial relaxations termed the Maxwell–Wagner–Sillars (MWS) effect [12–14] can also interfere. This phenomenon (MWS) appears generally in heterogeneous media due to the accumulation of charges at the interfaces which leads to the formation of large dipoles. Furthermore, conductivity relaxations are also generally present in these composites and can be investigated as dipolar and interfacial relaxations by dielectric spectroscopy as well as thermally stimulated depolarisation currents (TSDC) technique [15–17]. Because of the presence of conductive particles, dielectric or conductivity relaxations may be obscured [18]. It is then more convenient to use the formalism ‘electric modulus’ first introduced by McCrum et al. [19]. Macedo et al. [20] were the first to exploit the modulus and used it for the investigation of electrical relaxation phenomena in vitreous ionic conductors. It has also used in polymers to study their conductivity relaxation behaviour [21–23]. The use of electric modulus helps to understand bulk relaxation properties at low frequencies. Thus, common difficulties like electrode nature and contact, space charge injection phenomena and absorbed impurity conduction effects, which appear to obscure relaxation in permittivity presentation, can be resolved or even ignored [18,24]. Complex modulus, electric modulus or inverse complex permittivity, M*, is defined by the following equation: M∗ =
1 1 ε ε = = 2 + j = M + jM , ε∗ ε − jε ε + ε 2 ε 2 + ε 2
j = (−1)1/2
(1)
where M is the real and M is the imaginary electric modulus, and ε is the real and ε is the imaginary permittivity. In the present work, broad band dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC) techniques were used to investigate dielectric relaxations in nanocomposites made of nanoparticles (ZnO and Zn97%OAl3%) dispersed in an epoxy-amine. Thermal step method (TSM) was also used to display the influence of nano-fillers on the behaviour of space charge density. Structural, electrical and optical properties of the investigated epoxy matrix have been studied previously [24–28]. It has demonstrated by means of TSDC, TSM and IR spectroscopy techniques that this polymer presents a polar character and the polarization depends strongly on its thermal history. The dielectric relaxation of epoxy-amine systems has been discussed at length by Fitz and Mijovic [29]. The ␣ process is associated with segmental motion of the terminal epoxy groups while the  process which is located in the high mega hertz region results from the localized motions.
317
Table 1 Investigated samples. Sample
Composition
EP ZnO-EP ZnOAl-EP
Pure epoxy resin Epoxy + 2wt.% ZnO Epoxy + 2wt.% Zn97%OAl3%
2. Experimental 2.1. Sample preparation Nanocomposites object of this study were made by dispersing conductive nanoparticles within an epoxy matrix. The epoxy resin was a 2,2-bis(4 glycidyloxyphenyl) propane (molar mass 340 g/mol) and the hardener was a difunctional amine (Maestria). These two components are carefully and homogeneously mixed without any solvent at stoechiometry ratio and at room temperature. Conductive nanoparticles (ZnO and Zn97%OAl3%) were elaborated by sol–gel method using El Mir et al. protocol [30,31]. The obtained nanocrystallites having a prismatic shape with an average grain size of about 30 nm and with a specific surface area of about 25 m2 /g, were used as filler and without any surface treatment. These nano-fillers were hand mixed into the obtained mixture with a ratio of 2% in weight. The final mixture was stirred at room temperature, and then poured into parallelepipedal mould. The hardening was carried out at room temperature and under atmospheric pressure for 5 days. The obtained samples were labelled and listed in Table 1. Sample dimensions are of (60 mm × 60 mm) × 0.6 mm for DRS measurements and of (100 mm × 100 mm) × 1 mm for TSDC and thermal step measurements. All samples were coated on both sides with aluminium electrodes (4 cm diameter), deposited under vacuum.
2.2. Characterization techniques 2.2.1. Dielectric relaxation spectroscopy Dielectric spectroscopy measurements have been performed at room temperature on a Solartron SI 1260 Impedance Gain-Phase analyser in the frequency range 10−3 to 106 Hz, interfaced to a computer by a Solartron 1296 Dielectric Interface. The amplitude of the ac signal was 1 V.
2.2.2. Thermal step method The thermal step method, which has been widely described in previous papers [32,33], allows the determination of the space charge repartition in an insulating sample. This technique is not destructive, based on measurements of the thermodilatation current caused by the application of a so-called thermal step to one side of a plate sample. The presence of charges within the insulator induces image charges on each electrode. Then, the propagation of the thermal step (positive or negative), and the resulting expansion (dilatation or contraction) across the sample thickness, modify the equilibrium of these image charges and consequently, a current appears in the external circuit joining the electrodes. The current is then amplified and recorded with a computer. The deconvolution of the measured current gives the electric field and the space charge distributions. The theory and the numeric treatment have been described in detail in previous papers [34,35].
2.2.3. Thermally stimulated depolarisation currents TSDC technique is based on a sample’s depolarization by thermal activation [17]. At a given temperature Tp , a static electric field is applied to the investigated sample for a time tp that is long enough to permit the different mobile entities of the material to orient themselves within the field. This configuration is then frozen by a rapid decrease in temperature to a temperature T0 low enough for the molecular mobility to be considered zero, keeping the electric field applied in order to avoid any relaxation of dipoles and/or charges. At this temperature, the field is switched off and the sample is short-circuited for a certain time to eliminate the eventual surface charges and stabilise sample at this temperature. The polarised sample is then shortcircuited through a high sensitivity electrometer in an oven, which is programmed to rise linearly with time. During the linear increase of temperature, the return to equilibrium of the previously oriented entities generates a depolarisation current which is recorded as a function of temperature. TSDC provide information on relaxation processes that occur in polymers. In contrast to the experimental simplicity of the TSDC technique, the experimental data analysis is not easy, as the polarization may be due to several microscopic processes (associated with dipolar or trapped charge mechanisms), whose relaxation will contribute to the depolarization current [36]. In the present work, samples were polarised at a temperature Tp = 30 ◦ C for the polarization time (tp ) of 3 h under a polarization field (Ep ) of 3 kV/mm. Depolarization currents were measured at a constant heating rate of 1 ◦ C/min.
318
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
Such result can be explained by the existence of significant interactions between nano-fillers and molecular entities forming the ER-matrix. For the pure epoxy resin, Fig. 1(b) shows the presence of two main relaxation processes located around 1850 and 14 Hz. The first one (1850 Hz) is attributed to interfacial relaxation process known as MWS effect. In fact, epoxy resin networks present a lacunar structure with microspherical voids produced during mixing process when air liberation is restricted due to the system viscosity [1,37]. Consequently two phases with different permittivities are then present in these systems, air and polymeric matter. Furthermore, in previous work [28] we have demonstrated by means of X-ray reflectometry measurements that the ER-network presents locally a periodic character with various spacing structures (from 32 to 74 nm) due to preparation mode of samples which makes the structure more heterogeneous and favours the presence of interfaces in the epoxy network. The small relaxation peak around 14 Hz is due to the relaxation of internal stress produced during the preparation. Similar relaxation has been reported for other epoxy resins [38–40]. It is known, that when the hardening temperature is lower than the glass temperature Tg (45 ◦ C for the investigated epoxy resin), the motion of chain segments is restricted which is converted to shrinkage internal stress [41,42]. The appeared peaks on the epoxy nanocomposites spectra of Fig. 1(b) were attributed to interfacial process. We note that these relaxation peaks shift to the lower frequencies in epoxy nanocomposites, suggesting a decrease of charges and/or dipoles mobility at the interfaces. This is due to the presence of significant interactions at the interface which decrease the aptitude of dipoles to be oriented under the effect of ac electric field. This effect is more prominent in the ZnO-epoxy nanocomposite. Fig. 1(b) shows also a very small relaxation around 42 kHz attributed to dipolar relaxation which is not influenced by the nano-filler particles. The stability of the relaxation frequency suggests that there are no significant interactions between nano-filler particles and the relaxing polar entities in the ER-matrix. In the low frequency range the slope of log(ε ) versus log(f) is −1, which is typical for dc conductivity effect [11,15,43]. These results are confirmed by the corresponding ac conductivity plots, ac (f) (Fig. 1(c)), which exhibit a dc conductivity plateau at low frequency. It has been calculated from dielectric losses according to the following relation: ∗ (ω) = jε0 ωε∗ (ω) = ε0 ωε + jε0 ωε
Fig. 1. Real part of dielectric permittivity ε (a), dielectric losses ε (b) and real part of ac conductivity ac (c) against frequency of samples indicated on the plot.
3. Results and discussion The overall dielectric behaviour of the investigated samples, obtained by DRS measurements at room temperature is shown in Fig. 1: Real part of dielectric permittivity ε , dielectric losses ε and real part of ac complex conductivity ac as functions of frequency. In the frequency range 10−2 to 2 × 103 Hz, ε is lower in nanocomposites than in the pure epoxy resin (Fig. 1(a)), indicating a decrease of the molecular mobility in the glassy state.
(2)
The real part of *(ω) is given by ac (ω) = ε0 ωε , where ε0 = 8.85 × 10−12 F m−1 is the permittivity of the free space and ω = 2f the angular frequency. It can also be seen from Fig. 1(c) that the dc conductivity value in epoxy nanocomposites is slightly higher than in the pure epoxy resin. At high frequency ac conductivity is frequency dependent, increases with increasing frequency and follows the law ac (ω) ≈ ωs with 0 ≤ s ≤ 1 characterizing hopping conduction [3,44]. The real and imaginary parts of permittivity were then transformed into electric modulus formalism via Eq. (1). Fig. 2(a)–(c) is obtained, giving the real part (M ), imaginary part (M ) and the complex plane representation (M (M )) of the electric modulus for pure epoxy resin and epoxy nanocomposites. In addition to the interfacial relaxation process, Fig. 2(b) shows another relaxation peak in the low frequency range due to dc conduction, which is not evident in ε (f) presentation. It can also be seen that the dc conduction peak shifts to higher frequencies in epoxy nanocomposites as compared to pure ER-matrix (Table 2), suggesting an increase in the mobility of charge carriers which contribute effectively in the conduction process. The two processes interfacial and dc conduction give also two distinct semicircles in the complex presentation (Fig. 2(c)). An additional weak circle arc appears on the diagram of pure epoxy resin
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
319
Table 3 (a) Parameters evaluated by fitting data according to the Havriliak–Negami approach for pure epoxy resin and epoxy nanocomposites. (b) Parameters evaluated by fitting data experimental points, describing the internal stress relaxation, to the Debye approach for pure epoxy resin. (a)
Sample
Ms
M∞
1−˛
Interfacial relaxation
EP ZnO-EP ZnOAl-EP EP ZnO-EP ZnOAl-EP
0.1743 0.1877 0.1772 0.0011 0.0013 0.0018
0.2274 0.2354 0.2242 0.1539 0.1610 0.1705
1 1 0.377 0.986 0.964 0.961
1 0.882 0.5985 0.890 0.893 0.897
Conduction
(b)
Sample
Ms
M∞
Internal stresses
EP
0.1630 0.1645
0.1706 0.1715
which has been attributed to the internal stresses relaxation. The variation of the position and semicircles radius reflect the influence of the nano-filler particles. Experimental data were fitted using Havriliak–Negami equation given by the following [45]: ε ∗ (ω) = ε∞ +
εs − ε∞ (1 + (jω)
1−˛
)
,
0 < (1 − ˛),
≤1
(3)
where εs and ε∞ are the limiting values of dielectric constant sides the relaxation, ˛ and are parameters describing the symmetric and asymmetric broadening of the relaxation time distribution, is the central relaxation time and ω is the angular frequency. In the electric modulus formalism the Havriliak–Negami equations have the following form [18]: M = M∞ Ms M = M∞ Ms
[Ms A + (M∞ − Ms ) cos ]A Ms2 A2
+ 2A (M∞ − Ms )Ms cos + (M∞ − Ms )2
Ms2 A2
+ 2A (M∞
[(M∞ − Ms ) sin ]A − Ms )Ms cos + (M∞ − Ms )2
(4)
(5)
where Ms =1/εs , M∞ =1/ε∞ , A=[1+2(ω)(1−␣) sin(˛/2)+(ω)2(1−␣) ]1/2 and = arctg[(ω)(1−␣) cos(˛/2)/(1 + (ω)(1−␣) sin(˛/2))]. The curves with solid lines on Fig. 3(a)–(c) are produced by best fitting experimental points to the Havriliak–Negami equation. All parameters evaluated by fitting data according to Havriliak–Negami approach for pure epoxy resin and epoxy nanocomposites are listed in Table 3(a). The dashed semicircle curve on the inset of Fig. 3(a) was obtained by fitting experimental points, describing the so-called internal stress relaxation, to the Debye equation [18]:
Fig. 2. Real part of electric modulus M (a), imaginary part of electric modulus M against frequency (b) and Cole–Cole plots (c) of the investigated samples.
Table 2 Frequencies corresponding to the maximum of M relative to dc conduction process. Sample
f (Hz)
EP ZnO-EP ZnOAl-EP
1.13 × 10−3 2.90 × 10−3 2.75 × 10−3
M −
M∞ + Ms 2
2
+ M = 2
M + M 2 ∞ s 2
(6)
It was produced by a superposition of two semicircles represented with dash dot lines. The corresponding evaluated parameters were given in Table 3(b). It can be seen from exponent values corresponding to interfacial relaxation process that heterogeneity introduced by the incorporation of nano-filler particles in the epoxy resin matrix leads to an asymmetric distribution of relaxation times and more particularly in ZnOAl epoxy nanocomposite. While the evaluated ˛ and parameter values for the pure epoxy resin show a Debye behaviour of the interfacial relaxation. Influence of nano-filler particles on interfacial relaxation can also be seen through the corresponding relaxation strength values defined by ε = εs − ε∞ . This relaxation strength takes the value 1.34 for pure epoxy resin and the values 1.08 and 1.18 for ZnO and ZnOAlepoxy nanocomposite, respectively. We remark a decrease of this parameter in nanocomposites, which indicates a decrease of the relaxed dipole number at interfaces. In the low frequency, the evaluated parameter values (Table 3(b)) corresponding to the conduction
320
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
Fig. 4. Space charge distribution in pure epoxy resin and epoxy nanocomposites after 3 h of polarization under 3 kV mm−1 at 30 ◦ C.
space charge distribution has the same shape presenting a polarization character and the amplitude of the space charge density is clearly amplified in epoxy nanocomposite samples than in the pure epoxy resin. This demonstrates that, in nanocomposites, the polarisation was amplified by the contribution of interfacial polarisation component due to the inclusion of nano-filler particles. Moreover, the DRS results show a diminution in the relaxed dipoles quantity (decrease of ε value) at interfaces in nanocomposites; this proves that the interfacial dipoles were stabilised by charge interactions, emphasized by dielectric behaviour. Samples were the depolarised by TSDC measurements. The corresponding TSDC spectra of pure epoxy resin and epoxy nanocomposites are represented in Fig. 5. They present a main current peak around 44 ◦ C attributed to the ␣ relaxation, associated with the glass transition of the soft phase of ER-matrix. This figure shows a small shift of the ␣ peak to lower temperature in epoxy nanocomposites. It can be also seen that dipole relaxation associated with the ␣ relaxation were more pronounced in epoxy nanocomposites. Such results can be explained in terms of increased free volume due to loosened packing of chains which leads to an increase of chain mobility in nanocomposites in the glass–rubber state. In fact, near the glass transition temperature the segmental mobility of the polymer molecules increases and becomes a dominating factor causing important
Fig. 3. Cole–Cole plots of the investigated samples: PE (a); ZnO-EP (b); ZnOAl-EP (c). The solid lines are produced by best-fitting the experimental points to the Havriliak–Negami approach. The dashed semicircle on the inset (of (a)) was produced by a superposition of the two dash dot semicircles obtained by fitting the corresponding data to the Debye approach.
process for pure epoxy resin and epoxy nanocomposites are nearly constant and close to the unity indicating a near Debye process. Indeed, in the low frequency range conductivity is governed by charge carriers of the matrix rather, than the fillers. Similar results were found by Psarras et al. [46] on epoxy resin amine filled with nickel powder at various amounts. Fig. 4 shows space charge repartition in the pure epoxy resin and epoxy nanocomposites after polarization under an electric field of 3 kV/mm for 3 h at a temperature of 30 ◦ C. In all samples the
Fig. 5. TSDC spectra of pure epoxy resin and epoxy nanocomposites submitted to an electric polarization (Ep = 3 kV mm−1 , Tp = 30 ◦ C, tp = 3 h).
H. Smaoui et al. / Journal of Alloys and Compounds 477 (2009) 316–321
TSDC dipolar relaxations in epoxy nanocomposites. In contrast dielectric measurements have shown a decrease of molecular mobility in the glassy state resulting from a decrease of dielectric permittivity in epoxy nanocomposites (Fig. 1(a)). An interfacial Maxwell–Wagner–Sillars relaxation associated with accumulation of charges at the interfaces appears at higher temperature on the TSDC spectra (Fig. 5). So, in the polarisation process two mechanisms are likely operative, which are the orientation of polar groups with the polarising electric field and the space charge polarisation. The disorientation of polar groups during the TSDC measurement give rise to a current peak (␣) which appears at lower temperature (than the space charge polarisation peak which appears at higher temperature). At higher temperature the space charge motion is accompanied by a creation of carriers which is due to ionisation of impurities as well as the breaking of chemical bonds (epoxy ring-opening reaction, braking of N–H bonds, etc.). Indeed, in previous works [26,27] we have studied the effect of heating temperature on the space charge and structure of the epoxy polymer. We have demonstrated that space charge density increases inside the material with increasing temperature of heating. This was related to charge carriers creation due to structural changes. The thermally created carriers become more mobile at higher temperature and constitute a homocurrent in the internal field. This current is opposite to the classical TSDC current due to dipolar disorientation (heterocurrent). As the temperature is raised, the homocurrent starts increasing, as the number of thermally generated carriers increases. So a reversal is observed from heterocurrent to homocurrent. Comparing the TSDC spectra of different samples around the interfacial relaxation, it can be concluded that nano-filler composition influence well charges nature and their interactions at the interfaces. 4. Conclusion In this paper, dielectric relaxations of an epoxy resin matrix and epoxy nanocomposites made by dispersing conductive nanoparticles (ZnO and Zn97%OAl3%) in the epoxy matrix were investigated by means of DRS and TSDC techniques. Interfacial or MWS relaxation is evident in the dielectric spectrum of the pure epoxy resin matrix. The origin of this interfacial process is may be due to the lacunar and/or stratified structure of the epoxy network. Heterogeneity induced by the presence of nano-fillers in the epoxy matrix has magnified the space charge density. The distribution of relaxation times associated with the interfacial process becomes asymmetric in nanocomposites. In the low frequency range conductivity is governed by charges carriers of the epoxy matrix rather, than of the nano-fillers (Hopping conduction). Acknowledgments The authors acknowledge the financial support of the «Ministère de l’Enseignement Supérieur, de la Recherche Scientifique et de la Technologie», Tunisia.
321
References [1] Y. Baziard, S. Breton, S. Toutain, A. Gourdenne, Eur. Polym. J. 24 (6) (1988) 521–526. [2] G.C. Psarras, E. Manolakaki, G.M. Tsangaris, Compos. Part A: Appl. Sci. Manuf. 34 (2003) 1187–1198. [3] J.C. Dyre, T.B. Shrfder, Rev. Mod. Phys., 72. 3 (2000) 873–892. [4] F. Carmona, C. Mouney, J. Mater. Sci. 27 (1992) 1322–1336. [5] I.G. Chen, W.B. Johnson, J. Mater. Sci. 26 (1991) 1565–1976. [6] G. Boiteux, J. Fournier, D. Issotier, G. Seytre, G. Marichy, Synth. Met. 102 (1999) 1234–1235. [7] J. Fournier, G. Boiteux, G. Seytre, J. Mater. Sci. Lett. 16 (1997) 1677–1679. [8] Ye.P. Mamunya, V.V. Davydenko, P. Pissis, E.V. Lebedev, Eur. Polym. J. 38 (2002) 1887–1897. [9] Y. Sun, Z. Zhang, K.S. Moon, C.P. Wong, J. Polym. Sci. Part B: Polym. Phys. 42 (2004) 3849–3858. [10] C. Zhang, R. Mason, G. Stevens, IEEJ Trans. FM 126 (11) (2006) 1105–1111. [11] Y. Sun, Z. Zhang, C.P. Wong, Polymer 46 (2005) 2297–2305. [12] J.C. Maxwell, Electricity and Magnetism, vol. 1, Clarendon Press, Oxford, 1892, p 452. [13] K.W. Wagner, Arch. Electrotech., Berlin, 1914. [14] R.W. Sillars, J. Inst. Eng. 80 (1937) 378. [15] A. Kanapitsas, P. Pissis, R. Kotsilkova, J. Non-Cryst. Solids 305 (2002) 204–211. [16] P. Pissis, G. Georgoussis, V.A. Bershtein, E. Neagu, A.M. Fainleib, J. Non-Cryst., Solids 305 (2002) 150–158. [17] V.M. Gun’ko, V.I. Zarko, E.V. Goncharuk, L.S. Andriyko, V.V. Turov, Y.M. Nychiporuk, R. Leboda, J. Skubiszewska-Zieba, A.L. Gabchak, V.D. Osovskii, Y.G. Ptushinkii, G.R. Yurchenko, O.A. Mishchuk, P.P. Gorbik, P. Pissis, J.P. Blitz, Adv. Colloid Interface Sci. 131 (2007) 1–89. [18] G.M. Tsangaris, G.C. Psarras, N. Kouloumbi, J. Mater. Sci. 33 (1998) 2027– 2037. [19] N.G. McCrum, B.E. Read, G. Williams, Anelastic and Dielectric Effects in Polymeric Solids, Wiley, London, 1967, 108-111. [20] P.B. Macedo, C.T. Moynihan, R. Bose, Phys. Chem. Glasses 13 (1972) 171–179. [21] A.A. Bakr, A.M. North, Eur. Polym. Sci. 13 (1977) 799–803. [22] H.W. Starkweather, P. Avakian, J. Polym. Sci. B, Polym. Phys. 30 (1992) 637– 641. [23] C. show-an, S.L. Chien, Macromolecules 26 (1993) 2210–2216. [24] F.S. Howell, R.A. Bose, P.B. Macedo, C.T. Moynihan, J. Phys. Chem. Glasses 14 (1973) 122. [25] H. Smaoui, H. Guermazi, S. Agnel, Y. Mlik, A. Toureille, Polym. Int. 49 (2000) 1513–1518. [26] H. Guermazi, H. Smaoui, S. Agnel, Y. Mlik, A. Toureille, F. Schué, Polym. Int. 50 (2001) 743–747. [27] H. Guermazi, H. Smaoui, S. Agnel, Y. Mlik, A. Toureille, F. Schué, Polym. Int. 52 (2003) 1287–1293. [28] H. Medhioub, C. Zerrouki, N. Fourati, H. Smaoui, H. Guermazi, J.J. Bonnet, J. Appl. Phys. 101 (2007) 043509. [29] B.D. Fitz, J. Mijovic, Polym. Adv. Technol. 9 (1998) 721. [30] L. El Mir, A. Amlouk, C. Barthou, S. Alaya, Physica B Condens. Matter 388 (2007) 412–417. [31] Z. Ben Ayadi, L. El Mir, K. Djessas, S. Alaya, Nanotechnology 18 (2007) 445702. [32] A. Toureille, J. Electrostat. 32 (1994) 277–286. [33] P. Notingher, S. Agnel, A. Toureille, IEEE Trans. Dielec. Elec. Insul. 8 (6) (2001) 985–994. [34] A. Toureille, J.P. Reboul, P. Merle, J. Appl. Phys. III (1991) 111–123. [35] A. Cheri, M. Abou Daka, A. Toureille, IEEE Trans. on Dielec. and Elec. Insul. 27 (6) (1992) 1152–1158. [36] C. Bucci, R. Fieschi, C. Guidi, Phys. Rev. 148 (2) (1966) 816–823. [37] L. Ramajo, M. Reboredo, M. Castro, Compos.: Part A 36 (2005) 1267–1274. [38] J. Mijovic, K.F. Lin, J. Appl. Polym. Sci. 32 (1986) 3211. [39] C. Brahatheeswaran, V.B. Gupta, Polymer 34 (1993) 289. [40] X. Chen, S. Li, Macromol. Rapid Commun. 22 (2001) 349–352. [41] G. Yang, S.Y. Fu, J.P. Yang, Polymer 48 (2007) 302–310. [42] Y.M. Cao, D.H. Yu, L. Chen, J. Sun, Polym. Test 20 (2001) 685–691. [43] P.S. Anantha, K. Hariharan, Mater. Sci. Eng. B 121 (2005) 12–19. [44] G.C. Psarras, Compos. Part A: Appl. Sci. Manuf. 37 (2006) 1545–1553. [45] S. Havriliak, S. Negami, Polymer 8 (1967) 161–210. [46] G.C. Psarras, E. Manolakaki, G.M. Tsangaris, Compos. Part A: Appl. Sci. Manuf. 33 (2002) 375–384.