- Email: [email protected]
Effect of swift heavy ion irradiation on dielectrics properties of polymer composite films
Materials Science and Engineering B 137 (2007) 85–92
Effect of swift heavy ion irradiation on dielectrics properties of polymer composite films N.L. Singh a,∗ , Anjum Qureshi a , F. Singh b , D.K. Avasthi b a
b
Physics Department, M.S. University of Baroda, Vadodara 390002, India Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
Received 25 September 2006; received in revised form 27 October 2006; accepted 30 October 2006
Abstract Ferric oxalate was used as organometallics fillers in polyvinyl chloride (PVC) to form polymer matrix composite films at different concentration of filler. These films were irradiated with 80 MeV O6+ ions at the fluences of 1 × 1011 and 1 × 1012 ions/cm2 . The radiation induced modifications in dielectric properties, microhardness, surface morphology and surface roughness of polymer composite films have been investigated at different concentration (i.e. 5%, 10% and 15%) of filler. It was observed that hardness and electrical conductivity of the films increase with the concentration of the dispersed ferric oxalate and also with the fluence. From the analysis of frequency, f, dependence of dielectric constant, ε, it has been found that the dielectric response in both pristine and irradiated samples obey the Universal law given by ε ∝ fn−1 . The dielectric constant/loss is observed to change significantly due to the irradiation. This suggests that ion beam irradiation promotes (i) the metal to polymer bonding and (ii) convert the polymeric structure into hydrogen depleted carbon network. Thus irradiation makes the polymer harder and more conductive. Atomic force microscopy (AFM) shows that average roughness (Ra ) of the irradiated films is lower than that of unirradiated films. Surface morphology of irradiated polymer composite films is observed to change. Scanning electron microscopy (SEM) results show that partial agglomeration of fillers in the polymer matrix. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyvinyl chloride (PVC); Composites; Ion irradiation; Microhardness; ac electrical frequency response; AFM; SEM
1. Introduction Traditionally, polymer matrix composites materials have been thought of primarily as insulating materials and, as such they have been used in applications like power tool handles, cable, jackets, capacitor films and electronic packaging materials. More recently, however, there is a growing market need for polymer composites materials with increased electrical conductivity. In recent years, there is an increasing interest in the polymer matrix composites prepared by mixing two or more constituents to make up some disadvantages in single material [1–4]. The presence of filler in polymer matrix affects both electrical as well as mechanical properties. To prepare the composites with excellent dielectric property, therefore the effective utilization of filled polymers depends strongly on loading fillers homogeneously throughout polymer [5] and effective microstructure
∗
Corresponding author. Tel.: +91 265 2783924; fax: +91 265 2795569. E-mail addresses: singhnl [email protected] (N.L. Singh), [email protected] (A. Qureshi). 0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.10.013
in the composites. Furthermore, interfaces among the different phases in composites play also an important role on deciding the dielectric property. The composites are regarded as heterogeneous and many relations have been proposed to depict their dielectric property in terms of physical properties of their constituents [6]. The dielectric property of the composites depends on the volume fraction, size and shape of conducting fillers, and also on other factors such as, preparation method, interface and interaction between the fillers and the polymers [7]. Ion beam irradiation has long been recognized as an effective method for modifying the properties of diverse materials, including polymers and polymer composites. Important properties of polymer composites, i.e. mechanical property, thermal stability, chemical resistance, melt flow, process ability and surface properties significantly improved by ion beam irradiation [8,9]. Ion irradiation of such polymer composites can result in significant changes in the chemical structure of filled filler and polymer surface layers including (i) degradation of chemical bonds and backbone structure and (ii) crosslinking of polymer chains. A filled polymer differs substantially from the free one in wide range of properties. These materials can be crosslinked to obtain
86
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
useful properties. Our interest in dispersion of organometallic compound is mainly because of ion irradiation induced enhancement of both electrical conductivity and mechanical hardness of polymers. In this study, the PVC films were dispersed with different concentration (i.e. 5%, 10% and 15%) of ferric oxalate compound and irradiated with 80 MeV O6+ ions at the fluences of 1 × 1011 and 1 × 1012 ions/cm2 . We have studied the mechanical (i.e. microhardness), surface roughness, surface morphology and dielectric properties by means of Vickers’ microhardness indentation, atomic force microscopy (AFM), scanning electron microscopy (SEM) and LCR meter, respectively. 2. Experimental detail As an organometallic compound we took ferric oxalate. It was formed by taking 6.24 g of oxalic acid and 5.24 g of ferric chloride with ethanol as a solvent in a round bottom flask, and it was refluxed for 4 h at 60 ◦ C. The excess of ethanol was then distilled out and the substance was dried at 75 ◦ C for 3 h in an oven. The PVC and ferric oxalate compound of 5%, 10%, and 15% were dissolved in acetone and the solutions were mixed and stirred thoroughly for about an hour and poured on clean glass trough. The solvent was evaporated at room temperature (25 ± 1 ◦ C) to get thin films (thickness ∼50 m) of dispersed PVC with 5%, 10%, and 15% concentration of ferric oxalate compound. The films were used for irradiation. All films were irradiated with 80 MeV O6+ ions at the fluences of 1 × 1011 and 1 × 1012 ions/cm2 from the Pelletron of the ˙Inter University Accelerator Center (˙IUAC), New Delhi. A Carl Zeiss optical microscope and accessories were used to investigate Vickers’ microhardness. The surface morphology of pristine and irradiated surfaces was obtained using an atomic force microscope (AFM) in the contact mode (Digital Nanoscope IIIa Instrument Inc.) and scanning electron microscopy (SEM) (Model: JEOL JSM 5600). ac electrical properties of all samples were measured in the frequency range 50 Hz–30 MHz at room temperature using variable frequency LCR meter (General Radio, USA; model-1689/Hewlett Packard 4284A). The conductivity was calculated using the relation σ = t/RA (−1 cm−1 ), where R is the resistance measured, A the cross-sectional area of the electrode and t is the thickness of the polymeric film. The dielectric constant was calculated using the relation ε = εp /C0 , where εp is capacitance measured using the LCR meter; C0 = ε0 A/t, where ε0 is the permittivity of vacuum. 3. Results and discussion 3.1. ac electrical frequency response ac electrical measurement was performed for pristine and irradiated samples. Fig. 1a–c shows the variation of conductivity with log of frequency for the pristine and irradiated samples at different ferric oxalate concentration. A sharp increase in conductivity was observed in pristine as well as irradiated samples. It was also observed that conductivity increases with increasing concentration of dispersed ferric oxalate compound (Fig. 1a, pristine) as well as those irradiated at the fluence of
Fig. 1. (a) ac conductivity vs. log frequency for pristine pure and dispersed ferric oxalate in PVC films. (b) ac conductivity vs. log frequency for irradiated (at the fluence of 1 × 1011 ions/cm2 ) pure and dispersed ferric oxalate compound in PVC films. (c) ac conductivity vs. log frequency for irradiated (at the fluence of 1 × 1012 ions/cm2 ) pure and dispersed ferric oxalate compound in PVC films.
1 × 1011 ions/cm2 (Fig. 1b) and 1 × 1012 ions/cm2 (Fig. 1c), respectively. The increase in conductivity with different ferric oxalate concentration for pristine samples may be attributed to the conductive phase formed by dispersed organometallic compound in polymer matrix. It is known that electrical conductivity of such composites depends on the type and concentration of the dispersed compound [10,11]. As a result the conductivity of dis-
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
persed films increases on increasing the concentration of ferric oxalate compound in the polymer matrix. It is also observed that after the irradiation the conductivity increases with fluence (Fig. 1). Irradiation is expected to promote the metal to polymer bonding and convert the polymeric structure to a hydrogen depleted carbon network. It is this carbon network that is believed to make the polymers more conductive [8]. Fig. 2a–c shows the plot of dielectric constant versus log frequency for pristine and irradiated samples of pure PVC and different concentration of ferric oxalate dispersed PVC films. When the fillers are dispersed in the insulating polymer, the dielectric constant of composites investigated increases with concentration of fillers. Such results have been observed experimentally [12,13]. The partial agglomerations also increase with increasing the filler concentration as shown in Fig. 5 (AFM). As evident from Fig. 2, the dielectric constant remains almost constant up to 100 kHz. At these frequencies, the motion of the free charge carriers is constant and so the dielectric constant presumably remains unchanged. It is also observed that dielectric constant increases for irradiated films with fluence. The increase in dielectric constant may be attributed to the chain scission and as a result the increase in the number of free radicals, etc. As frequencies increases further (i.e. beyond 100 kHz), the charge carriers migrate through the dielectric and get trapped against a defect sites and induced an opposite charge in its vicinity. At these frequencies, the polarization of trapped and bound charges cannot take place and hence the dielectric constant decreases [14]. The dielectric constant decreases at higher frequencies (i.e. beyond 100 kHz) obeys the universal law [15] of dielectric response given by ε ∝ fn−1 , where n is power law exponent and varies from zero to one (0 < n < 1), n = 0.76 for PVC and 0.74 for all dispersed ferric oxalate pristine films. The value of n = 0.85 for PVC and 0.76 for all dispersed ferric oxalate irradiated samples were obtained at the fluence of 1 × 1011 ions/cm2 (Fig. 2b) and 1 × 1012 ions/cm2 (Fig. 2c), respectively. Fig. 2a–c clearly shows that the frequency dependence of dielectric constant, ε, obeys Universal law. The observed nature of the fluence dependence of dielectric constant in studied frequency range can be explained by the prevailing influence of the enhanced free carriers due to irradiation [16]. Fig. 3a–c shows the variation of dielectric loss with log frequency for pristine and irradiated samples of pure PVC and ferric oxalate dispersed PVC films at the concentration of 5%, 10% and 15%, respectively. The dielectric loss decreases with increasing log frequency. It is noticed that dielectric loss increases on increasing the concentration of filler and also with the fluence. 3.2. Microhardness Fig. 4a–c shows the plot of the Vickers’ microhardness (Hv ) versus applied load (P) for pristine and irradiated films of pure PVC, and dispersed ferric oxalate compound of 5%, 10% and 15% in PVC. The microhardness indentations were carried out on the surface of the pristine and irradiated films at room temperature under the different applied loads from 50 to 1000 mN and at a constant loading time of 30 s. It was observed that the microhardness (Hv ) value increases with the load up to 100 mN
87
Fig. 2. (a) Plot of dielectric constant vs. log frequency for pristine pure and dispersed ferric oxalate compound in PVC films. (b) Plot of dielectric constant vs. log frequency for irradiated (at the fluence of 1 × 1011 ions/cm2 ) pure and ferric oxalate dispersed compound in PVC films. (c) Plot of dielectric constant vs. log frequency for irradiated (at the fluence of 1 × 1012 ions/cm2 ) pure and ferric oxalate dispersed compound in PVC films.
and then decreases and become saturated beyond the load of 400 mN. Hardness can be defined as the resistance to indenter penetration, or as the average pressure under the indenter, calculated as the applied load divided by the projected area of contact incorporating the plastic component of displacement. The hardness is known to be influenced by surface effects.
88
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
Fig. 3. (a) Plot of dielectric loss vs. log frequency for pristine pure and dispersed ferric oxalate compound in PVC films. (b) Plot of dielectric loss vs. log frequency for irradiated (at the fluence of 1 × 1011 ions/cm2 ) pure and ferric oxalate dispersed compound in PVC films. (c) Plot of dielectric loss vs. log frequency for irradiated (at the fluence of 1 × 1012 ions/cm2 ) pure and ferric oxalate dispersed compound in PVC films.
Fig. 4. (a) Plot of hardness (Hv ) vs. applied load (P) for pristine pure and dispersed ferric oxalate compound in PVC films. (b) Plot of hardness (Hv ) vs. applied load (P) for irradiated (at the fluence of 1 × 1011 ions/cm2 ) pure and dispersed ferric oxalate compound in PVC films. (c) Plot of hardness (Hv ) vs. applied load (P) for irradiated (at the fluence of 1 × 1012 ions/cm2 ) pure and dispersed ferric oxalate compound in PVC films.
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
89
Fig. 5. (a) AFM image of pure PVC film (pristine). (b) AFM image of dispersed ferric oxalate (5%) in PVC film (pristine). (c) AFM image of dispersed ferric oxalate (15%) in PVC film (pristine). (d) AFM image of pure PVC film (irradiated at the fluence of 1 × 1011 ions/cm2 ). (e) AFM image of dispersed ferric oxalate (5%) in PVC film (irradiated at the fluence of 1 × 1011 ions/cm2 ). (f) AFM image of dispersed ferric oxalate (15%) in PVC film (irradiated at the fluence of 1 × 1011 ions/cm2 ). (g) AFM image of pure PVC film (irradiated at the fluence of 1 × 1012 ions/cm2 ). (h) AFM image of dispersed ferric oxalate (5%) in PVC film (irradiated at the fluence of 1 × 1012 ions/cm2 ). (i) AFM image of dispersed ferric oxalate (15%) in PVC film (irradiated at the fluence of 1 × 1012 ions/cm2 ).
90
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
Fig. 5. (Continued ).
At the higher loads, beyond 400 mN, the interior of the bulk specimen is devoid of surface effects. Hence the hardness value at higher loads represents the true value for the bulk and it is consequently independent of the load. It was found that hardness
increases as ferric oxalate concentration increases. This may be due to the improvement in bonding properties [17]. The hardness of polymer composites also increases on irradiation with fluences of 1 × 1011 and 1 × 1012 ions/cm2 , respectively. This
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
may be attributed to the growth of a hydrogen depleted carbon network which makes the polymer harder [8]. 3.3. Morphology of the composites The surface morphology of pristine and irradiated films of pure PVC, and dispersed ferric oxalate compound of 5% and 15% in PVC was measured by AFM on 5 m × 5 m area as shown in Fig. 5. Each AFM image was analyzed in terms of surface average roughness (Ra ). The average roughness values are 5.7, 9.3, and 22 nm for unirradiated samples and those of irradiated samples the roughness values are 2.7 nm, 6.7 nm, 19.3 nm at the fluence of 1 × 1011 ions/cm2 and 2.5 nm, 5.4 nm, 10.6 nm at the fluence of 1 × 1012 ions/cm2 , respectively. It was found that
91
roughness increases as ferric oxalate concentration increases. The increase in roughness may be due to the increase of density and size of metal particles on the surfaces of the PVC films [18,19]. It is also observed that after irradiation the roughness of the surface decreases and the surface becomes significantly smoother. This relative smoothness is probably due to defect enhanced surface diffusion. Fig. 6 shows the surface morphology of composite films with 15% ferric oxalate dispersed pristine and irradiated at the fluences of 1 × 1011 and 1 × 1012 ions/cm2 , respectively. After irradiation partial agglomeration of fillers and flake-like structures could be seen in the micrographs of SEM. 4. Conclusions Ion irradiation has been shown to significantly enhance both electrical and microhardness of organometallic compound dispersed PVC films. It may be attributed to (i) metal to polymer bonding and (ii) conversion of the polymeric structure to hydrogen depleted carbon network. Thus irradiation makes the polymer harder, more conductive. Dielectric loss and constant are observed to change significantly with the fluence. This might be attributed to breakage of chemical bonds and resulting in the increase of free radicals, unsaturation, etc. It is also observed that dielectric constant obeys Universal law of dielectric response. The surface roughness increases as ferric oxalate concentration increases and decreases on irradiation as observed from AFM studies. After irradiation partial agglomeration of fillers and flake-like structures were observed in the micrographs of SEM. Acknowledgments Authors are thankful to ˙Inter University Accelerator Center ˙ (IUAC), New Delhi for providing irradiation facility and IUCDAFE Indore for providing AFM and SEM facility. The financial support given by ˙IUAC, New Delhi is gratefully acknowledged. References
Fig. 6. (a) SEM micrograph of dispersed ferric oxalate (15%) in PVC film (pristine). (b) SEM micrograph of dispersed ferric oxalate (15%) in PVC film (irradiated at the fluence of 1 × 1011 ions/cm2 ). (c) SEM micrograph of dispersed ferric oxalate (15%) in PVC film (irradiated at the fluence of 1 × 1012 ions/cm2 ).
[1] B. Russell, V. Valerity, et al., Science 292 (2001) 2469. [2] W.M. Chen, Y. Yuan, Mater. Res. Bull. 35 (2000) 807. [3] S. Etienne, C. Stochil, J.I. Bessede, J. Alloys Compd. 310 (2000) 368. [4] T. Pan, J.P. Haung, Z.Y. Li, Phys. B 301 (2001) 190. [5] P. Xiao, M. Xiao, K.C. Gong, Polymer 42 (2001) 4813. [6] G.M. Tsangarris, G.C. Psarras, N. Kouloumbi, Mater. Sci. Technol. 12 (1996) 533. [7] C. Brosseau, P. Talbot, J. Appl. Phys. 89 (2001) 4532. [8] Y.Q. Wang, M. Curry, E. Tavenner, N. Dobson, R.E. Giedd, Nucl. Instrum. Meth. B 219–220 (2004) 798. [9] V. Zaporojtchenko, T. Strunskus, K. Behnke, C. Von Bechtolsheim, M. Keine, F. Faupel, J. Adhesion Sci. Technol. 14 (2000) 467. [10] M. Abu-Abdeen, G.M. Nasr, H.M. Osman, A.I. Abound, Egypt. J. Sol. 25 (2002) 275. [11] Ye. P. Mamunya, V.V. Davydenko, P. Pissis, E.V. Lebedev, Eur. Polym. J. 38 (2002) 1887. [12] G.C. Psarras, E. Manolakaki, G.M. Tsangarris, Composites A 33 (2001) 375. [13] A.E. Ghany, A.E. Salam, G.M. Nasr, J. Appl. Polym. Sci. 77 (2000) 1816.
92
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
[14] A.K. Jonscher, Dielectric Relaxation in Solids, Chesla Dielectric Press, London, 1983. [15] A.K. Jonscher, Nature 267 (1977) 673. [16] T. Phukan, D. Kanjilal, T.D. Goswami, H.L. Das, Nucl. Instrum. Meth. B 234 (2005) 520.
[17] W.H. Bowyer, M.G. Bader, J. Mater. Sci. 7 (1972) 1315. [18] X. Yan, T. Xu, S. Xu, X. Wang, S. Yang, Nanotechnology 15 (2004) 1759. [19] A. Qureshi, N.L. Singh, A.K. Rakshit, F. Singh, D.K. Avasthi, V. Ganesan, Nucl. Instrum. Meth. B 244 (2006) 235.
Effect of swift heavy ion irradiation on dielectrics properties of polymer composite films N.L. Singh a,∗ , Anjum Qureshi a , F. Singh b , D.K. Avasthi b a
b
Physics Department, M.S. University of Baroda, Vadodara 390002, India Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
Received 25 September 2006; received in revised form 27 October 2006; accepted 30 October 2006
Abstract Ferric oxalate was used as organometallics fillers in polyvinyl chloride (PVC) to form polymer matrix composite films at different concentration of filler. These films were irradiated with 80 MeV O6+ ions at the fluences of 1 × 1011 and 1 × 1012 ions/cm2 . The radiation induced modifications in dielectric properties, microhardness, surface morphology and surface roughness of polymer composite films have been investigated at different concentration (i.e. 5%, 10% and 15%) of filler. It was observed that hardness and electrical conductivity of the films increase with the concentration of the dispersed ferric oxalate and also with the fluence. From the analysis of frequency, f, dependence of dielectric constant, ε, it has been found that the dielectric response in both pristine and irradiated samples obey the Universal law given by ε ∝ fn−1 . The dielectric constant/loss is observed to change significantly due to the irradiation. This suggests that ion beam irradiation promotes (i) the metal to polymer bonding and (ii) convert the polymeric structure into hydrogen depleted carbon network. Thus irradiation makes the polymer harder and more conductive. Atomic force microscopy (AFM) shows that average roughness (Ra ) of the irradiated films is lower than that of unirradiated films. Surface morphology of irradiated polymer composite films is observed to change. Scanning electron microscopy (SEM) results show that partial agglomeration of fillers in the polymer matrix. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyvinyl chloride (PVC); Composites; Ion irradiation; Microhardness; ac electrical frequency response; AFM; SEM
1. Introduction Traditionally, polymer matrix composites materials have been thought of primarily as insulating materials and, as such they have been used in applications like power tool handles, cable, jackets, capacitor films and electronic packaging materials. More recently, however, there is a growing market need for polymer composites materials with increased electrical conductivity. In recent years, there is an increasing interest in the polymer matrix composites prepared by mixing two or more constituents to make up some disadvantages in single material [1–4]. The presence of filler in polymer matrix affects both electrical as well as mechanical properties. To prepare the composites with excellent dielectric property, therefore the effective utilization of filled polymers depends strongly on loading fillers homogeneously throughout polymer [5] and effective microstructure
∗
Corresponding author. Tel.: +91 265 2783924; fax: +91 265 2795569. E-mail addresses: singhnl [email protected] (N.L. Singh), [email protected] (A. Qureshi). 0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.10.013
in the composites. Furthermore, interfaces among the different phases in composites play also an important role on deciding the dielectric property. The composites are regarded as heterogeneous and many relations have been proposed to depict their dielectric property in terms of physical properties of their constituents [6]. The dielectric property of the composites depends on the volume fraction, size and shape of conducting fillers, and also on other factors such as, preparation method, interface and interaction between the fillers and the polymers [7]. Ion beam irradiation has long been recognized as an effective method for modifying the properties of diverse materials, including polymers and polymer composites. Important properties of polymer composites, i.e. mechanical property, thermal stability, chemical resistance, melt flow, process ability and surface properties significantly improved by ion beam irradiation [8,9]. Ion irradiation of such polymer composites can result in significant changes in the chemical structure of filled filler and polymer surface layers including (i) degradation of chemical bonds and backbone structure and (ii) crosslinking of polymer chains. A filled polymer differs substantially from the free one in wide range of properties. These materials can be crosslinked to obtain
86
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
useful properties. Our interest in dispersion of organometallic compound is mainly because of ion irradiation induced enhancement of both electrical conductivity and mechanical hardness of polymers. In this study, the PVC films were dispersed with different concentration (i.e. 5%, 10% and 15%) of ferric oxalate compound and irradiated with 80 MeV O6+ ions at the fluences of 1 × 1011 and 1 × 1012 ions/cm2 . We have studied the mechanical (i.e. microhardness), surface roughness, surface morphology and dielectric properties by means of Vickers’ microhardness indentation, atomic force microscopy (AFM), scanning electron microscopy (SEM) and LCR meter, respectively. 2. Experimental detail As an organometallic compound we took ferric oxalate. It was formed by taking 6.24 g of oxalic acid and 5.24 g of ferric chloride with ethanol as a solvent in a round bottom flask, and it was refluxed for 4 h at 60 ◦ C. The excess of ethanol was then distilled out and the substance was dried at 75 ◦ C for 3 h in an oven. The PVC and ferric oxalate compound of 5%, 10%, and 15% were dissolved in acetone and the solutions were mixed and stirred thoroughly for about an hour and poured on clean glass trough. The solvent was evaporated at room temperature (25 ± 1 ◦ C) to get thin films (thickness ∼50 m) of dispersed PVC with 5%, 10%, and 15% concentration of ferric oxalate compound. The films were used for irradiation. All films were irradiated with 80 MeV O6+ ions at the fluences of 1 × 1011 and 1 × 1012 ions/cm2 from the Pelletron of the ˙Inter University Accelerator Center (˙IUAC), New Delhi. A Carl Zeiss optical microscope and accessories were used to investigate Vickers’ microhardness. The surface morphology of pristine and irradiated surfaces was obtained using an atomic force microscope (AFM) in the contact mode (Digital Nanoscope IIIa Instrument Inc.) and scanning electron microscopy (SEM) (Model: JEOL JSM 5600). ac electrical properties of all samples were measured in the frequency range 50 Hz–30 MHz at room temperature using variable frequency LCR meter (General Radio, USA; model-1689/Hewlett Packard 4284A). The conductivity was calculated using the relation σ = t/RA (−1 cm−1 ), where R is the resistance measured, A the cross-sectional area of the electrode and t is the thickness of the polymeric film. The dielectric constant was calculated using the relation ε = εp /C0 , where εp is capacitance measured using the LCR meter; C0 = ε0 A/t, where ε0 is the permittivity of vacuum. 3. Results and discussion 3.1. ac electrical frequency response ac electrical measurement was performed for pristine and irradiated samples. Fig. 1a–c shows the variation of conductivity with log of frequency for the pristine and irradiated samples at different ferric oxalate concentration. A sharp increase in conductivity was observed in pristine as well as irradiated samples. It was also observed that conductivity increases with increasing concentration of dispersed ferric oxalate compound (Fig. 1a, pristine) as well as those irradiated at the fluence of
Fig. 1. (a) ac conductivity vs. log frequency for pristine pure and dispersed ferric oxalate in PVC films. (b) ac conductivity vs. log frequency for irradiated (at the fluence of 1 × 1011 ions/cm2 ) pure and dispersed ferric oxalate compound in PVC films. (c) ac conductivity vs. log frequency for irradiated (at the fluence of 1 × 1012 ions/cm2 ) pure and dispersed ferric oxalate compound in PVC films.
1 × 1011 ions/cm2 (Fig. 1b) and 1 × 1012 ions/cm2 (Fig. 1c), respectively. The increase in conductivity with different ferric oxalate concentration for pristine samples may be attributed to the conductive phase formed by dispersed organometallic compound in polymer matrix. It is known that electrical conductivity of such composites depends on the type and concentration of the dispersed compound [10,11]. As a result the conductivity of dis-
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
persed films increases on increasing the concentration of ferric oxalate compound in the polymer matrix. It is also observed that after the irradiation the conductivity increases with fluence (Fig. 1). Irradiation is expected to promote the metal to polymer bonding and convert the polymeric structure to a hydrogen depleted carbon network. It is this carbon network that is believed to make the polymers more conductive [8]. Fig. 2a–c shows the plot of dielectric constant versus log frequency for pristine and irradiated samples of pure PVC and different concentration of ferric oxalate dispersed PVC films. When the fillers are dispersed in the insulating polymer, the dielectric constant of composites investigated increases with concentration of fillers. Such results have been observed experimentally [12,13]. The partial agglomerations also increase with increasing the filler concentration as shown in Fig. 5 (AFM). As evident from Fig. 2, the dielectric constant remains almost constant up to 100 kHz. At these frequencies, the motion of the free charge carriers is constant and so the dielectric constant presumably remains unchanged. It is also observed that dielectric constant increases for irradiated films with fluence. The increase in dielectric constant may be attributed to the chain scission and as a result the increase in the number of free radicals, etc. As frequencies increases further (i.e. beyond 100 kHz), the charge carriers migrate through the dielectric and get trapped against a defect sites and induced an opposite charge in its vicinity. At these frequencies, the polarization of trapped and bound charges cannot take place and hence the dielectric constant decreases [14]. The dielectric constant decreases at higher frequencies (i.e. beyond 100 kHz) obeys the universal law [15] of dielectric response given by ε ∝ fn−1 , where n is power law exponent and varies from zero to one (0 < n < 1), n = 0.76 for PVC and 0.74 for all dispersed ferric oxalate pristine films. The value of n = 0.85 for PVC and 0.76 for all dispersed ferric oxalate irradiated samples were obtained at the fluence of 1 × 1011 ions/cm2 (Fig. 2b) and 1 × 1012 ions/cm2 (Fig. 2c), respectively. Fig. 2a–c clearly shows that the frequency dependence of dielectric constant, ε, obeys Universal law. The observed nature of the fluence dependence of dielectric constant in studied frequency range can be explained by the prevailing influence of the enhanced free carriers due to irradiation [16]. Fig. 3a–c shows the variation of dielectric loss with log frequency for pristine and irradiated samples of pure PVC and ferric oxalate dispersed PVC films at the concentration of 5%, 10% and 15%, respectively. The dielectric loss decreases with increasing log frequency. It is noticed that dielectric loss increases on increasing the concentration of filler and also with the fluence. 3.2. Microhardness Fig. 4a–c shows the plot of the Vickers’ microhardness (Hv ) versus applied load (P) for pristine and irradiated films of pure PVC, and dispersed ferric oxalate compound of 5%, 10% and 15% in PVC. The microhardness indentations were carried out on the surface of the pristine and irradiated films at room temperature under the different applied loads from 50 to 1000 mN and at a constant loading time of 30 s. It was observed that the microhardness (Hv ) value increases with the load up to 100 mN
87
Fig. 2. (a) Plot of dielectric constant vs. log frequency for pristine pure and dispersed ferric oxalate compound in PVC films. (b) Plot of dielectric constant vs. log frequency for irradiated (at the fluence of 1 × 1011 ions/cm2 ) pure and ferric oxalate dispersed compound in PVC films. (c) Plot of dielectric constant vs. log frequency for irradiated (at the fluence of 1 × 1012 ions/cm2 ) pure and ferric oxalate dispersed compound in PVC films.
and then decreases and become saturated beyond the load of 400 mN. Hardness can be defined as the resistance to indenter penetration, or as the average pressure under the indenter, calculated as the applied load divided by the projected area of contact incorporating the plastic component of displacement. The hardness is known to be influenced by surface effects.
88
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
Fig. 3. (a) Plot of dielectric loss vs. log frequency for pristine pure and dispersed ferric oxalate compound in PVC films. (b) Plot of dielectric loss vs. log frequency for irradiated (at the fluence of 1 × 1011 ions/cm2 ) pure and ferric oxalate dispersed compound in PVC films. (c) Plot of dielectric loss vs. log frequency for irradiated (at the fluence of 1 × 1012 ions/cm2 ) pure and ferric oxalate dispersed compound in PVC films.
Fig. 4. (a) Plot of hardness (Hv ) vs. applied load (P) for pristine pure and dispersed ferric oxalate compound in PVC films. (b) Plot of hardness (Hv ) vs. applied load (P) for irradiated (at the fluence of 1 × 1011 ions/cm2 ) pure and dispersed ferric oxalate compound in PVC films. (c) Plot of hardness (Hv ) vs. applied load (P) for irradiated (at the fluence of 1 × 1012 ions/cm2 ) pure and dispersed ferric oxalate compound in PVC films.
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
89
Fig. 5. (a) AFM image of pure PVC film (pristine). (b) AFM image of dispersed ferric oxalate (5%) in PVC film (pristine). (c) AFM image of dispersed ferric oxalate (15%) in PVC film (pristine). (d) AFM image of pure PVC film (irradiated at the fluence of 1 × 1011 ions/cm2 ). (e) AFM image of dispersed ferric oxalate (5%) in PVC film (irradiated at the fluence of 1 × 1011 ions/cm2 ). (f) AFM image of dispersed ferric oxalate (15%) in PVC film (irradiated at the fluence of 1 × 1011 ions/cm2 ). (g) AFM image of pure PVC film (irradiated at the fluence of 1 × 1012 ions/cm2 ). (h) AFM image of dispersed ferric oxalate (5%) in PVC film (irradiated at the fluence of 1 × 1012 ions/cm2 ). (i) AFM image of dispersed ferric oxalate (15%) in PVC film (irradiated at the fluence of 1 × 1012 ions/cm2 ).
90
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
Fig. 5. (Continued ).
At the higher loads, beyond 400 mN, the interior of the bulk specimen is devoid of surface effects. Hence the hardness value at higher loads represents the true value for the bulk and it is consequently independent of the load. It was found that hardness
increases as ferric oxalate concentration increases. This may be due to the improvement in bonding properties [17]. The hardness of polymer composites also increases on irradiation with fluences of 1 × 1011 and 1 × 1012 ions/cm2 , respectively. This
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
may be attributed to the growth of a hydrogen depleted carbon network which makes the polymer harder [8]. 3.3. Morphology of the composites The surface morphology of pristine and irradiated films of pure PVC, and dispersed ferric oxalate compound of 5% and 15% in PVC was measured by AFM on 5 m × 5 m area as shown in Fig. 5. Each AFM image was analyzed in terms of surface average roughness (Ra ). The average roughness values are 5.7, 9.3, and 22 nm for unirradiated samples and those of irradiated samples the roughness values are 2.7 nm, 6.7 nm, 19.3 nm at the fluence of 1 × 1011 ions/cm2 and 2.5 nm, 5.4 nm, 10.6 nm at the fluence of 1 × 1012 ions/cm2 , respectively. It was found that
91
roughness increases as ferric oxalate concentration increases. The increase in roughness may be due to the increase of density and size of metal particles on the surfaces of the PVC films [18,19]. It is also observed that after irradiation the roughness of the surface decreases and the surface becomes significantly smoother. This relative smoothness is probably due to defect enhanced surface diffusion. Fig. 6 shows the surface morphology of composite films with 15% ferric oxalate dispersed pristine and irradiated at the fluences of 1 × 1011 and 1 × 1012 ions/cm2 , respectively. After irradiation partial agglomeration of fillers and flake-like structures could be seen in the micrographs of SEM. 4. Conclusions Ion irradiation has been shown to significantly enhance both electrical and microhardness of organometallic compound dispersed PVC films. It may be attributed to (i) metal to polymer bonding and (ii) conversion of the polymeric structure to hydrogen depleted carbon network. Thus irradiation makes the polymer harder, more conductive. Dielectric loss and constant are observed to change significantly with the fluence. This might be attributed to breakage of chemical bonds and resulting in the increase of free radicals, unsaturation, etc. It is also observed that dielectric constant obeys Universal law of dielectric response. The surface roughness increases as ferric oxalate concentration increases and decreases on irradiation as observed from AFM studies. After irradiation partial agglomeration of fillers and flake-like structures were observed in the micrographs of SEM. Acknowledgments Authors are thankful to ˙Inter University Accelerator Center ˙ (IUAC), New Delhi for providing irradiation facility and IUCDAFE Indore for providing AFM and SEM facility. The financial support given by ˙IUAC, New Delhi is gratefully acknowledged. References
Fig. 6. (a) SEM micrograph of dispersed ferric oxalate (15%) in PVC film (pristine). (b) SEM micrograph of dispersed ferric oxalate (15%) in PVC film (irradiated at the fluence of 1 × 1011 ions/cm2 ). (c) SEM micrograph of dispersed ferric oxalate (15%) in PVC film (irradiated at the fluence of 1 × 1012 ions/cm2 ).
[1] B. Russell, V. Valerity, et al., Science 292 (2001) 2469. [2] W.M. Chen, Y. Yuan, Mater. Res. Bull. 35 (2000) 807. [3] S. Etienne, C. Stochil, J.I. Bessede, J. Alloys Compd. 310 (2000) 368. [4] T. Pan, J.P. Haung, Z.Y. Li, Phys. B 301 (2001) 190. [5] P. Xiao, M. Xiao, K.C. Gong, Polymer 42 (2001) 4813. [6] G.M. Tsangarris, G.C. Psarras, N. Kouloumbi, Mater. Sci. Technol. 12 (1996) 533. [7] C. Brosseau, P. Talbot, J. Appl. Phys. 89 (2001) 4532. [8] Y.Q. Wang, M. Curry, E. Tavenner, N. Dobson, R.E. Giedd, Nucl. Instrum. Meth. B 219–220 (2004) 798. [9] V. Zaporojtchenko, T. Strunskus, K. Behnke, C. Von Bechtolsheim, M. Keine, F. Faupel, J. Adhesion Sci. Technol. 14 (2000) 467. [10] M. Abu-Abdeen, G.M. Nasr, H.M. Osman, A.I. Abound, Egypt. J. Sol. 25 (2002) 275. [11] Ye. P. Mamunya, V.V. Davydenko, P. Pissis, E.V. Lebedev, Eur. Polym. J. 38 (2002) 1887. [12] G.C. Psarras, E. Manolakaki, G.M. Tsangarris, Composites A 33 (2001) 375. [13] A.E. Ghany, A.E. Salam, G.M. Nasr, J. Appl. Polym. Sci. 77 (2000) 1816.
92
N.L. Singh et al. / Materials Science and Engineering B 137 (2007) 85–92
[14] A.K. Jonscher, Dielectric Relaxation in Solids, Chesla Dielectric Press, London, 1983. [15] A.K. Jonscher, Nature 267 (1977) 673. [16] T. Phukan, D. Kanjilal, T.D. Goswami, H.L. Das, Nucl. Instrum. Meth. B 234 (2005) 520.
[17] W.H. Bowyer, M.G. Bader, J. Mater. Sci. 7 (1972) 1315. [18] X. Yan, T. Xu, S. Xu, X. Wang, S. Yang, Nanotechnology 15 (2004) 1759. [19] A. Qureshi, N.L. Singh, A.K. Rakshit, F. Singh, D.K. Avasthi, V. Ganesan, Nucl. Instrum. Meth. B 244 (2006) 235.