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FeCl3–CoCl2 mixed fillers effects on the structural, electrical and magnetic properties of PVDF films
Journal of Magnetism and Magnetic Materials 262 (2003) 203–211
FeCl3–CoCl2 mixed fillers effects on the structural, electrical and magnetic properties of PVDF films A. Tawansi*, A.H. Oraby, H.I. Abdelkader, M. Abdelaziz Department of Physics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Received 3 May 2002; received in revised form 25 July 2002
Abstract Films of polyvinylidene fluoride (PVDF) filled with (x)FeCl3(20x)CoCl2 mixture were prepared by casting method and studied by X-ray diffraction (XRD), differential thermal analysis (DTA), infrared transmission (IR), ultraviolet/ visible optical absorption, DC electrical conduction, DC magnetic susceptibility and electron spin resonance (ESR). XRD implied an amorphous structure for x ¼ 0:0% and 15% and a semicrystalline structure (containing a- and bPVDF phases at x ¼ 5% and 10% and a phase only at x ¼ 20%). Melting and transition temperatures were identified using DTA. Conjugated double bonds and head-to-head defects were detected by IR spectra, which suggested the presence of polarons and/or bipolarons in the polymeric matrix. The DC electrical conduction was discussed using the modified interpolaron hopping model. The optical absorption spectra implied the presence of high spin tetrahedral forms of both Fe3+ and Co2+. The ESR revealed the existence of cluster of both Fe3+ and Co2+ for xX5% within the PVDF matrix. The temperature dependence of the DC magnetic susceptibility obeyed Curie–Weiss law, indicating the role of the localized energy states. The obtained negative values of the paramagnetic Curie temperature, yp ; for xo8:5% indicated an antiferromagnetic interaction, while positive yp obtained for x > 8:5% suggested the ferromagnetic interaction, at lower temperatures. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyvinylidene fluoride; FeCl3; CoCl2; XRD; DTA; ESR; IR; Optical spectra; Electrical conduction; DC magnetic susceptibility
1. Introduction Polyvinylidene fluoride (PVDF) contains approximately equal amounts of amorphous and crystalline components. The amorphous region plays a small role in macroscopic ferroelectrical properties. The crystalline component may exist in four different phases; a; b; g; and d: The b-phase is the most important in piezoelectric applications of *Corresponding author.
PVDF [1]. Besides its pyro- and piezoelectric properties and high elasticity, its high permittivity, relatively low dissipation factor and high dielectric strength have made this polymer also very useful as a capacitor dielectric [2]. Since the discovery of ferroelectricity in the PVDF, extensive research has been carried out to understand the ferroelectric behaviour. The electromechanical properties are highly dependent upon the structural parameters such as molecular orientation, crystallinity and the state of
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 5 9 0 - 5
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polarization. Various methods such as high temperature annealing [3], stretching [4] and high electric field poling [5] have been employed to introduce a high degree of crystallinity and perfect alignment of dipoles in polymer films [6]. Most studies considered the FeCl3 filler as an efficient electron acceptor and this was attributed to the formation of Fe2Cl2 4 [7]. CoCl2 was selected as a filler for PVDF. Besides its interesting magnetic properties, Co2+ may exist in different structural forms (with fourfold or sixfold coordinations): tetrahedral, octahedral and isolated forms [8]. In previous works, our research group investigated the physical properties of PVDF films filled with FeCl3 [9] and CoCl2 [10]. These investigations revealed that the used fillers modified the electric and magnetic properties of PVDF. However, a nonlinear (even nonmonotonic) filler level (FL) dependence of the contents of the active structural forms was found. This exerted strong influences on the electrical and magnetic properties of the filled PVDF films. Seeking for a linear (or monotonic) FL dependence of the electrical and magnetic properties, in a fair FL range, it was thought to use the intrasubstituted (x)FeCl3(20x)CoCl2 mixed fillers. Therefore, the present work was devoted to investigate the effect of filling PVDF with a mixture of FeCl3 and CoCl2 on crystallinity, electric and magnetic properties.
Differential thermal analysis (DTA) was carried out using an equipment type (Shimadzu DTA-50) with measuring temperature range 30–4001C and the heating rate of 51C/min. The infrared (IR) spectrophotometer (Perkin-Elmer 883) was used for measuring the IR spectra in the wavenumber range of 4000–400 cm1. Ultraviolet/visible (UV/ VIS) absorption spectra were measured in the wavelength range of 200–900 nm using a spectrometer (Perkin-Elmer UV/VIS). The DC electrical resistivity was measured using an autorange multimeter (Keithley 175) with an accuracy of 0.2%. The ESR spectra were recorded on JEOL spectrophotometer (type JES-FE2XG) at a frequency of 9.45 GHz, using 1,1 diphenyl-2-pierylhydrazyl (DPPH) as a calibrant. The DC magnetic susceptibility was measured using the Faraday pendulum balance technique, which provides an accuracy better than 3.0%.
3. Results and discussion 3.1. XRD Fig. 1 depicts the XRD scans for PVDF films of various mixed filler fractions. The observed spectra of x ¼ 0:0% and 15% characterize the amorphous
2. Experimental Films of PVDF filled with (x)FeCl3(20x)CoCl2 were prepared by the casting method. Dimethylformamide (DMF) was used to dissolve the used materials. A mixture of the dissolved polymer with mixed halides was cast to a glass dish and kept in a dry atmosphere at 323 K for 2 weeks to remove the solvent traces. Different concentrations of the fillers were obtained where x ¼ 0:0; 1,5, 10, 15, 19 and 20 wt%. The thickness of films was in the range of 100–150 mm. The X-ray diffraction (XRD) scans were obtained using a Seimens type F diffractometer with CuKa radiation and an LiF monochromator.
Fig. 1. XRD scans of PVDF films with various mass fractions of (x)FeCl3(20x)CoCl2.
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205
Table 1 The assigned [11] X-ray diffraction peaks characterizing the crystalline PVDF phases FeCl3 (wt%)
2y (degree)
Assignments
5
20.7 38.8
(1 1 0) b, (2 0 0) b (0 0 2) a
10
20.5 39.5
(1 1 0) b, (2 0 0) b (0 0 2) a
20
39.5
(0 0 2) a
structures. The spectrum of x ¼ 20% indicates the presence of a phase while the spectra of x ¼ 5% and 10% contain the main peaks of a and b phases [11]. The assigned crystalline peaks are listed in Table 1. 3.2. DTA Fig. 2 shows the DTA thermograms of the present system. An endothermic peak is observed at Tm ; which is attributed to the PVDF melting. The Tm values at various filler levels are listed in Table 2. The small endothermic peaks, observed at temperaturesoTm ; are assigned to the ferroelectric to paraelectric phase transitions [12]. The FL dependences of the absorbed heat, H; and the height, h; of the melting peak are shown in Fig. 3. It is clear that, both H and h exhibit minima at x ¼ 15%: The values of the order of reaction, n; at Tm calculated using Kissinger method [13], were E1. This indicates that all the reactants are melted at Tm : 3.3. IR analysis The IR transmission spectra for the present system are shown in Fig. 4. The main PVDF characterizing frequencies are observed, where b phase occurs most notably at 510 and 840 cm1 and a phase at 430, 610 and 950 cm1. The structural disorder can be identified by investigation of the filling level dependence of certain IR absorption peaks depicted in Fig. 5. The band at 1665 cm1 was assigned to the CQC stretching in the difluorinated alkenes [14]. This band indicates
Fig. 2. The DTA thermograms (x)FeCl3(20x)CoCl2.
of
PVDF
filled
with
Table 2 The FL dependence of the melting temperature (Tm ) FeCl3 (wt%)
Tm (1C)
0.8 5 10 15 20
182 172 168 172 168
the presence of polarons in the polymeric matrix. The band at 1665 cm1 shows a minimum value at x ¼ 15%: The band at 1628 cm1 was assigned to the CQC stretching in the monofluorinated alkenes. The intensity of this band decreases as x increases. The band at 750 cm1, which refers to the head-to-head and tail-to-tail defects, revealed a
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Fig. 3. The filler level dependence of: (J) the heat absorbed at, and () the peak height of, the melting point. The solid lines are only to guide the eyes.
minimum value at x ¼ 5%: The intensity of the band at 840 cm1 (which refers to the b phase) sharply decreases as x increases up to 5%. For 5%pxp15%; the intensity at 840 cm1 was almost constant, while at x > 15%; this intensity increases.
3.4. Optical absorption The optical absorption spectra measured at room temperature for various FL in PVDF are shown in Fig. 6. The bands in the range of 370– 500 nm are due to crystal field transitions of the tetrahedral isolated Fe3+ ions [15]. The observed band positions and their assignments are given in Table 3. The dependence of the absorbance of the band at 373 nm on FLs is shown in Fig. 7. This band exhibits a maximum peak value at x ¼ 10% indicating maximum content of tetrahedral Fe3+. Moreover, three visible absorption bands are noticed at 625, 666 and 690 nm for 0:0%pxp10%; characterizing the high spin tetrahedral Co2+ ions [16]. The FL dependence of the intensity of these three bands is shown in Fig. 7. These bands reveal minimum intensity at x ¼ 5%; indicating minimum tetrahedral content of Co2+, while the case of x ¼ 1% exhibits the maximum absorption intensity, indicating maximum tetrahedral content of Co2+.
Fig. 4. The IR transmission spectra of PVDF filled with various FLs.
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207
Table 3 The observed band positions and their assignments [15] for the tetrahedral Fe3+ FeCl3 (wt%) 1 5
10
Fig. 5. The FL dependence of the IR peaks at (1) 750 cm1, (2) 1628 cm1 (3) 840 cm1, (4) 950 cm1 and (5) 1665 cm1. The solid lines are only to guide the eyes.
15
19
20
Observed Wavelength (nm)
Transitions A1g(S)-
6
305 360
4
310 361 409
4
305 363 405 435
4
300 360 405 447
4
300 359 405 431
4
301 364 405 442
4
T1g(P) T2g(D)
4
T1g(P) T2g(D) 4 Eg(G) 4
T1g(P) T2g(D) 4 Eg(G) 4 A1g(G) 4
T1g(P) T2g(D) 4 Eg(G) 4 A1g(G) 4
T1g(P) T2g(D) 4 Eg(G) 4 A1g(G) 4
T1g(P) T2g(D) 4 Eg(G) 4 A1g(G) 4
3.5. DC electrical conduction The DC electrical resistivity, r; was measured in the temperature range of 290–403 K for PVDF filled with (x)FeCl3(20x)CoCl2 mixed halides, where x ¼ 0:0%; 1%, 5%, 10%, 15%, 19% and 20%. The mono- and difluorinated alkenes (detected by IR analysis) indicating the presence of conjugated polyene, may evidence the formation of polarons and/or bipolarons in the polymeric matrix. Therefore, the present results can be discussed on the basis of the Kuivalainen et al. [17] modified interpolaron hopping model, in which the conduction is attributed to phononassisted hopping between polaron and/or bipolaron bound states in the polymer. According to this model, the electrical resistivity can be expressed as Fig. 6. The UV/VIS absorption spectra of PVDF films filled with various FLs.
r ¼ ½kT=A1 e2 gðTÞðR20 =zÞ ½ðYp þ Ybp Þ2 =Yp Ybp expð2B1 R0 =zÞ;
ð1Þ
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Fig. 7. The FL dependence of the Co2+ absorbance peaks at: 690 nm (K), 666 nm (m), 626 nm (J) and the dependence of the Fe3+ absorbance peak at 373 nm ( * ). The solid lines are only to guide the eyes.
where A1 ¼ 0:45; B1 ¼ 1:39; Yp and Ybp are the concentration of polarons and bipolarons, respectively; R0 ¼ ð3=4PCimp Þ1=3 is the typical separation between impurities whose concentration is Cimp ; z ¼ ðz8 z2> Þ1=3 is the average decay length of a polaron and bipolaron wave function, z8 and z> are the decay lengths parallel and perpendicular to the polymer chain, respectively. Bredas et al. [18] reported that the extension of defect should be the same for polaron and bipolaron. The electronic transition rate between polaron and bipolaron states can be expressed as gðTÞ ¼ g0 ðT=300 KÞnþ1 ;
ð2Þ
where n is a constant B10, and the pre-factor g0 ¼ 1:2 1017 S1, was estimated by Kivelson [19]. Using a computer-aided program, the order of magnitude of r in the present system was adjusted with the impurity concentration Cimp ; which actually was the fitting parameter. The parameter z8 ¼ 1:06 nm while z> ¼ 0:22 nm [20], which depends on the interchain resonance energy and the interchain distance. Taking Yp ¼ Ybp for simplicity, which is an acceptable approximation [21], using Eqs. (1) and (2), we can obtain the values of the hopping distance R0 : A linear decrease of R0 as the temperature increases, for
Fig. 8. The temperature dependence of hopping distance R0 for various FLs: (1) 0.0%, (2) 1%, (3) 5%, (4) 10%, (5) 15%, (6) 19% and (7) 20%. The solid lines represent the linear fitting.
various FLs, is noticed in Fig. 8. This indicates that the concentration of thermally activated polarons (acting as hopping sites for the charge carriers) increases gradually as the temperature increases. Accordingly R0 can be represented, as a function of temperature by the following linear formula: R0 ¼ MT þ N;
ð3Þ
where M is a negative factor which is the slope of the lines observed in Fig. 8, and N is the value of R0 at zero absolute temperature. Fig. 9 shows the FL dependence of log r measured (and R0 calculated) at T ¼ 373 K. Two maxima were observed at x ¼ 1% and 19% for log r (and R0 ). The calculated values of R0 were in the range of 1.8–6.7 nm corresponding to 7.2–26.8 monomer units, where the monomer unit length of PVDF E0.25 nm [22]. This suggests the predominance of an interchain one-dimensional hopping mechanism. It is noteworthy that there is a slight increase of R0 (and log r) as a function of x in the range of 5%pxp15%: This may imply that the hopping sites (the polarons and/or bipolarons) increase slightly as x increases.
A. Tawansi et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 203–211
209
Fig. 9. The FL dependence of log rðÞ and R0 ðJÞ at temperature T ¼ 373 K. The solid lines are only to guide the eyes.
3.6. ESR Fig. 10 shows the ESR spectra of the present system at 300 K. For the case of x ¼ 0:0%; the spectrum is characterized by the following three features: (i) a broad (incomplete and deformed) Lorentzian signal due to the Co2+–Co2+ exchange interaction indicating the presence of Co2+ aggregated forms [23,24] and (ii) a lot of complicated weak hyperfine lines which are distributed all over the whole spectrum. These lines may arise from residual free carriers and/or neutral defects induced in the PVDF chain structure due to CoCl2 filling [25]. The unpaired electron of some of these structural defects results from a ‘‘domain wall’’ in the bond alternation resulting in a neutral Pelectron free radical [26]. These defects may arise in the isomerization process of converting cis to trans forms. The ESR hyperfine pattern may also arise from radicals having electrons localized in an sp2 or sp3 orbital, where the hyperfine splitting from adjacent 1H or 13C nuclei would be easily distinguished if trans-defects occurred in the Psystem [27]. It is clear from Fig. 10 that for the cases of xX5% the spectra are characterized by strong and broad Lorentzian signals which may be ascribed mainly to the Fe3+ clusters coupled by strong spin–spin interaction. The observed hyperfine lines for the cases of xo20% can be attributed to the
Fig. 10. The ESR spectra (x)FeCl3(20x)CoCl2.
of
PVDF
filled
with
PVDF structural defects induced by Co2+. This interpretation is accepted, considering the disappearance of the hyperfine lines in the case of x ¼ 20% (which is free from Co2+). The aggregated Fe3+ and Co2+ forms evidenced by the present ESR spectra support the tetrahedral clusters of Fe3+ and Co2+ revealed from the optical absorption spectra. 3.7. DC magnetic susceptibility The temperature dependence of the reciprocal values of the DC magnetic susceptibility, w; measured in the temperature range of 95–285 K for various FLs is shown in Fig. 11. It is clear that
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Fig. 12. The FL dependence of yp (J) and meff ().
Fig. 11. The temperature dependence of the reciprocal magnetic susceptibility for variously filled PVDF films: (1) 0.0%, (2) 1%, (3) 5%, (4) 10%, (5) 15% (6) 19% and (7) 20%. The solid lines represent the linear fitting.
w obeys Curie–Weiss law in which w ¼ C=ðT yp Þ;
ð4Þ
where C is the Curie’s constant and yp is the paramagnetic Curie temperature. Negative yp values obtained for xo8:5%; suggest an antiferromagnetic exchange interaction between the magnetic centres. On the other hand, positive yp values were found for x > 8:5% indicating a ferromagnetic interaction at lower temperatures. The Curie–Weiss plots were used, together with the equation meff ¼ 2:84½wm ðT yp Þ1=2
4. Conclusions The present work tried to explore, more explicitly, the role of most of the microstructural forms on the electrical and magnetic properties of PVDF films filled with (x)FeCl3(20x)CoCl2, in the mass fraction range of 0%pxp20%: This mixture of Fe3+ and Co2+ ions enhanced the magnetoactivity of PVDF. The fair intrasubstituted range of 5%pxp15% revealed a monotonic FL dependence for most of the investigated parameters, except for the Fe3+ and Co2+ tetrahedral contents. The various structures and physical properties found for the present films suggest some useful technical applications as follows: *
ð5Þ
to calculate the effective magnetic moment, meff ; where wm is the molar susceptibility of the used sample. The FL dependences of yp and meff are shown in Fig. 12. It is remarkable that meff has a minimum value at x ¼ 15% and the maximum value is observed at x ¼ 1%: Correlating the maximum value of meff (at x ¼ 1%) with the maximum content of tetrahedral Co2+ ions (see Fig. (7)) it could be implied that the tetrahedral Co2+ ions exhibit strong magnetic moments.
*
A film of the present system having x ¼ 1% exhibits a maximum content of tetrahedral Co2+ form, maximum values of magnetic moment and electrical resistivity. This sample can be used as an electromagnetic wave sensor and modulator, especially for infrared and microwaves. The sample of x ¼ 10% contains maximum contents of b phase and tetrahedral Fe3+ and Co2+ forms. Thus, it can serve as a piezoelectric and paramagnetic semiconductor, which can be used to produce surface acoustic waves with variable frequency, acting as a diffraction grating of variable order for laser beams. This
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type of gratings is very important for the modern optical communications.
References [1] M.A. Dovspike, M.S. Conradi, J. Appl. Phys. 65 (1989) 2. [2] R. Gregorio, E.M. Ueno, J. Mater. Sci. 34 (1999) 4489. [3] J.S. Green, B.L. Farmer, J.F. Robalt, J. Appl. Phys. 60 (1986) 2690. [4] K. Omote, H. Ohigashi, K. Koga, J. Appl. Phys. 81 (1997) 2760. [5] U. Bharti, T. Kaura, R. Nath, IEEE Trans. Dielectr. Electr. Insul. 2 (1995) 1106. [6] V. Bharti, H.S. Xu, G. Shanthi, Q.M. Zhang, J. Appl. Phys. 87 (2000) 452. [7] P. Kuivalainen, Phys. Rev. B 31 (12) (1985) 7900. [8] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Wiley, London, 1962. [9] A. Tawansi, H.I. Abdelkader, E.M. Abdelrazek, J. Mater. Sci. Tech. 13 (1997) 194. [10] A.H. Oraby, Polym. Testing 19 (2000) 865. [11] S. El Hefnawy, Study of physical properties of some polymeric sensors for electromagnetic waves and its applications, Ph.D. Thesis, Mansoura University, Egypt, 1995.
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[12] I.S. El Ashmawi, The physical properties of some polymeric films with some silver compounds, M.Sc. Thesis, Mansoura University, Egypt, 2000. [13] H.E. Kissinger, Anal. Chem. 29 (11) (1957) 1703. [14] I. Fleming, D.H. Williams, Spectroscopic Methods in Organic Chemistry, McGraw-Hill, New York, 1966. [15] B. Hannoyer, M. Lenglet, R. Corles, J. Non-Cryst. Solids 151 (1992) 209. [16] D. Nicholls, Complexes and First Row Transition Elements, London, 1974, p. 97. [17] P. Kuivalainen, H. Stubb, H. Isotlo, Phys. Rev. B 31 (1985) 7900. [18] J.L. Bredas, R.R. Chance, R. Silbey, Phys. Rev. B 31 (1985) 7900. [19] S. Kivelson, Mol. Cryst. Liq. Cryst. 77 (1981) 65. [20] N.F. Mott, R.W. Gurrey, Electronic Processes in Ionic Crystals, Oxford University Press, London, 1940. [21] A. Tawansi, H.I. Abdelkader, M. Elzalabany, E.M. Abdelrazek, J. Mater. Sci. 29 (1994) 3451. [22] R. Hazegawa, Y. Takahashi, Y. Chatwi, H. Tadakoro, J. Polym. 13 (1972) 600. [23] A. Tawansi, A.H. Oraby, E.M. Abdelrazek, M.I. Ayad, M. Abdelaziz, J. Appl. Polym. Sci. 70 (1998) 1437. [24] H. Toriumi, R.A. Weiss, H.A. Frank, Macromolecules 17 (1984) 9104. [25] A. Tawansi, E.M. Abdelrazek, H.M. Zidan, J. Mater. Sci. 32 (1997) 6243. [26] I.B. Goldberg, H.R. Crow, B.R. Newman, A.J. Heeger, M. Diarmid, J. Chem. Phys. 70 (1979) 1132. [27] A.J. Stone, Molec. Phys. 6 (1963) 509.
FeCl3–CoCl2 mixed fillers effects on the structural, electrical and magnetic properties of PVDF films A. Tawansi*, A.H. Oraby, H.I. Abdelkader, M. Abdelaziz Department of Physics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt Received 3 May 2002; received in revised form 25 July 2002
Abstract Films of polyvinylidene fluoride (PVDF) filled with (x)FeCl3(20x)CoCl2 mixture were prepared by casting method and studied by X-ray diffraction (XRD), differential thermal analysis (DTA), infrared transmission (IR), ultraviolet/ visible optical absorption, DC electrical conduction, DC magnetic susceptibility and electron spin resonance (ESR). XRD implied an amorphous structure for x ¼ 0:0% and 15% and a semicrystalline structure (containing a- and bPVDF phases at x ¼ 5% and 10% and a phase only at x ¼ 20%). Melting and transition temperatures were identified using DTA. Conjugated double bonds and head-to-head defects were detected by IR spectra, which suggested the presence of polarons and/or bipolarons in the polymeric matrix. The DC electrical conduction was discussed using the modified interpolaron hopping model. The optical absorption spectra implied the presence of high spin tetrahedral forms of both Fe3+ and Co2+. The ESR revealed the existence of cluster of both Fe3+ and Co2+ for xX5% within the PVDF matrix. The temperature dependence of the DC magnetic susceptibility obeyed Curie–Weiss law, indicating the role of the localized energy states. The obtained negative values of the paramagnetic Curie temperature, yp ; for xo8:5% indicated an antiferromagnetic interaction, while positive yp obtained for x > 8:5% suggested the ferromagnetic interaction, at lower temperatures. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyvinylidene fluoride; FeCl3; CoCl2; XRD; DTA; ESR; IR; Optical spectra; Electrical conduction; DC magnetic susceptibility
1. Introduction Polyvinylidene fluoride (PVDF) contains approximately equal amounts of amorphous and crystalline components. The amorphous region plays a small role in macroscopic ferroelectrical properties. The crystalline component may exist in four different phases; a; b; g; and d: The b-phase is the most important in piezoelectric applications of *Corresponding author.
PVDF [1]. Besides its pyro- and piezoelectric properties and high elasticity, its high permittivity, relatively low dissipation factor and high dielectric strength have made this polymer also very useful as a capacitor dielectric [2]. Since the discovery of ferroelectricity in the PVDF, extensive research has been carried out to understand the ferroelectric behaviour. The electromechanical properties are highly dependent upon the structural parameters such as molecular orientation, crystallinity and the state of
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 5 9 0 - 5
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polarization. Various methods such as high temperature annealing [3], stretching [4] and high electric field poling [5] have been employed to introduce a high degree of crystallinity and perfect alignment of dipoles in polymer films [6]. Most studies considered the FeCl3 filler as an efficient electron acceptor and this was attributed to the formation of Fe2Cl2 4 [7]. CoCl2 was selected as a filler for PVDF. Besides its interesting magnetic properties, Co2+ may exist in different structural forms (with fourfold or sixfold coordinations): tetrahedral, octahedral and isolated forms [8]. In previous works, our research group investigated the physical properties of PVDF films filled with FeCl3 [9] and CoCl2 [10]. These investigations revealed that the used fillers modified the electric and magnetic properties of PVDF. However, a nonlinear (even nonmonotonic) filler level (FL) dependence of the contents of the active structural forms was found. This exerted strong influences on the electrical and magnetic properties of the filled PVDF films. Seeking for a linear (or monotonic) FL dependence of the electrical and magnetic properties, in a fair FL range, it was thought to use the intrasubstituted (x)FeCl3(20x)CoCl2 mixed fillers. Therefore, the present work was devoted to investigate the effect of filling PVDF with a mixture of FeCl3 and CoCl2 on crystallinity, electric and magnetic properties.
Differential thermal analysis (DTA) was carried out using an equipment type (Shimadzu DTA-50) with measuring temperature range 30–4001C and the heating rate of 51C/min. The infrared (IR) spectrophotometer (Perkin-Elmer 883) was used for measuring the IR spectra in the wavenumber range of 4000–400 cm1. Ultraviolet/visible (UV/ VIS) absorption spectra were measured in the wavelength range of 200–900 nm using a spectrometer (Perkin-Elmer UV/VIS). The DC electrical resistivity was measured using an autorange multimeter (Keithley 175) with an accuracy of 0.2%. The ESR spectra were recorded on JEOL spectrophotometer (type JES-FE2XG) at a frequency of 9.45 GHz, using 1,1 diphenyl-2-pierylhydrazyl (DPPH) as a calibrant. The DC magnetic susceptibility was measured using the Faraday pendulum balance technique, which provides an accuracy better than 3.0%.
3. Results and discussion 3.1. XRD Fig. 1 depicts the XRD scans for PVDF films of various mixed filler fractions. The observed spectra of x ¼ 0:0% and 15% characterize the amorphous
2. Experimental Films of PVDF filled with (x)FeCl3(20x)CoCl2 were prepared by the casting method. Dimethylformamide (DMF) was used to dissolve the used materials. A mixture of the dissolved polymer with mixed halides was cast to a glass dish and kept in a dry atmosphere at 323 K for 2 weeks to remove the solvent traces. Different concentrations of the fillers were obtained where x ¼ 0:0; 1,5, 10, 15, 19 and 20 wt%. The thickness of films was in the range of 100–150 mm. The X-ray diffraction (XRD) scans were obtained using a Seimens type F diffractometer with CuKa radiation and an LiF monochromator.
Fig. 1. XRD scans of PVDF films with various mass fractions of (x)FeCl3(20x)CoCl2.
A. Tawansi et al. / Journal of Magnetism and Magnetic Materials 262 (2003) 203–211
205
Table 1 The assigned [11] X-ray diffraction peaks characterizing the crystalline PVDF phases FeCl3 (wt%)
2y (degree)
Assignments
5
20.7 38.8
(1 1 0) b, (2 0 0) b (0 0 2) a
10
20.5 39.5
(1 1 0) b, (2 0 0) b (0 0 2) a
20
39.5
(0 0 2) a
structures. The spectrum of x ¼ 20% indicates the presence of a phase while the spectra of x ¼ 5% and 10% contain the main peaks of a and b phases [11]. The assigned crystalline peaks are listed in Table 1. 3.2. DTA Fig. 2 shows the DTA thermograms of the present system. An endothermic peak is observed at Tm ; which is attributed to the PVDF melting. The Tm values at various filler levels are listed in Table 2. The small endothermic peaks, observed at temperaturesoTm ; are assigned to the ferroelectric to paraelectric phase transitions [12]. The FL dependences of the absorbed heat, H; and the height, h; of the melting peak are shown in Fig. 3. It is clear that, both H and h exhibit minima at x ¼ 15%: The values of the order of reaction, n; at Tm calculated using Kissinger method [13], were E1. This indicates that all the reactants are melted at Tm : 3.3. IR analysis The IR transmission spectra for the present system are shown in Fig. 4. The main PVDF characterizing frequencies are observed, where b phase occurs most notably at 510 and 840 cm1 and a phase at 430, 610 and 950 cm1. The structural disorder can be identified by investigation of the filling level dependence of certain IR absorption peaks depicted in Fig. 5. The band at 1665 cm1 was assigned to the CQC stretching in the difluorinated alkenes [14]. This band indicates
Fig. 2. The DTA thermograms (x)FeCl3(20x)CoCl2.
of
PVDF
filled
with
Table 2 The FL dependence of the melting temperature (Tm ) FeCl3 (wt%)
Tm (1C)
0.8 5 10 15 20
182 172 168 172 168
the presence of polarons in the polymeric matrix. The band at 1665 cm1 shows a minimum value at x ¼ 15%: The band at 1628 cm1 was assigned to the CQC stretching in the monofluorinated alkenes. The intensity of this band decreases as x increases. The band at 750 cm1, which refers to the head-to-head and tail-to-tail defects, revealed a
206
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Fig. 3. The filler level dependence of: (J) the heat absorbed at, and () the peak height of, the melting point. The solid lines are only to guide the eyes.
minimum value at x ¼ 5%: The intensity of the band at 840 cm1 (which refers to the b phase) sharply decreases as x increases up to 5%. For 5%pxp15%; the intensity at 840 cm1 was almost constant, while at x > 15%; this intensity increases.
3.4. Optical absorption The optical absorption spectra measured at room temperature for various FL in PVDF are shown in Fig. 6. The bands in the range of 370– 500 nm are due to crystal field transitions of the tetrahedral isolated Fe3+ ions [15]. The observed band positions and their assignments are given in Table 3. The dependence of the absorbance of the band at 373 nm on FLs is shown in Fig. 7. This band exhibits a maximum peak value at x ¼ 10% indicating maximum content of tetrahedral Fe3+. Moreover, three visible absorption bands are noticed at 625, 666 and 690 nm for 0:0%pxp10%; characterizing the high spin tetrahedral Co2+ ions [16]. The FL dependence of the intensity of these three bands is shown in Fig. 7. These bands reveal minimum intensity at x ¼ 5%; indicating minimum tetrahedral content of Co2+, while the case of x ¼ 1% exhibits the maximum absorption intensity, indicating maximum tetrahedral content of Co2+.
Fig. 4. The IR transmission spectra of PVDF filled with various FLs.
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Table 3 The observed band positions and their assignments [15] for the tetrahedral Fe3+ FeCl3 (wt%) 1 5
10
Fig. 5. The FL dependence of the IR peaks at (1) 750 cm1, (2) 1628 cm1 (3) 840 cm1, (4) 950 cm1 and (5) 1665 cm1. The solid lines are only to guide the eyes.
15
19
20
Observed Wavelength (nm)
Transitions A1g(S)-
6
305 360
4
310 361 409
4
305 363 405 435
4
300 360 405 447
4
300 359 405 431
4
301 364 405 442
4
T1g(P) T2g(D)
4
T1g(P) T2g(D) 4 Eg(G) 4
T1g(P) T2g(D) 4 Eg(G) 4 A1g(G) 4
T1g(P) T2g(D) 4 Eg(G) 4 A1g(G) 4
T1g(P) T2g(D) 4 Eg(G) 4 A1g(G) 4
T1g(P) T2g(D) 4 Eg(G) 4 A1g(G) 4
3.5. DC electrical conduction The DC electrical resistivity, r; was measured in the temperature range of 290–403 K for PVDF filled with (x)FeCl3(20x)CoCl2 mixed halides, where x ¼ 0:0%; 1%, 5%, 10%, 15%, 19% and 20%. The mono- and difluorinated alkenes (detected by IR analysis) indicating the presence of conjugated polyene, may evidence the formation of polarons and/or bipolarons in the polymeric matrix. Therefore, the present results can be discussed on the basis of the Kuivalainen et al. [17] modified interpolaron hopping model, in which the conduction is attributed to phononassisted hopping between polaron and/or bipolaron bound states in the polymer. According to this model, the electrical resistivity can be expressed as Fig. 6. The UV/VIS absorption spectra of PVDF films filled with various FLs.
r ¼ ½kT=A1 e2 gðTÞðR20 =zÞ ½ðYp þ Ybp Þ2 =Yp Ybp expð2B1 R0 =zÞ;
ð1Þ
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Fig. 7. The FL dependence of the Co2+ absorbance peaks at: 690 nm (K), 666 nm (m), 626 nm (J) and the dependence of the Fe3+ absorbance peak at 373 nm ( * ). The solid lines are only to guide the eyes.
where A1 ¼ 0:45; B1 ¼ 1:39; Yp and Ybp are the concentration of polarons and bipolarons, respectively; R0 ¼ ð3=4PCimp Þ1=3 is the typical separation between impurities whose concentration is Cimp ; z ¼ ðz8 z2> Þ1=3 is the average decay length of a polaron and bipolaron wave function, z8 and z> are the decay lengths parallel and perpendicular to the polymer chain, respectively. Bredas et al. [18] reported that the extension of defect should be the same for polaron and bipolaron. The electronic transition rate between polaron and bipolaron states can be expressed as gðTÞ ¼ g0 ðT=300 KÞnþ1 ;
ð2Þ
where n is a constant B10, and the pre-factor g0 ¼ 1:2 1017 S1, was estimated by Kivelson [19]. Using a computer-aided program, the order of magnitude of r in the present system was adjusted with the impurity concentration Cimp ; which actually was the fitting parameter. The parameter z8 ¼ 1:06 nm while z> ¼ 0:22 nm [20], which depends on the interchain resonance energy and the interchain distance. Taking Yp ¼ Ybp for simplicity, which is an acceptable approximation [21], using Eqs. (1) and (2), we can obtain the values of the hopping distance R0 : A linear decrease of R0 as the temperature increases, for
Fig. 8. The temperature dependence of hopping distance R0 for various FLs: (1) 0.0%, (2) 1%, (3) 5%, (4) 10%, (5) 15%, (6) 19% and (7) 20%. The solid lines represent the linear fitting.
various FLs, is noticed in Fig. 8. This indicates that the concentration of thermally activated polarons (acting as hopping sites for the charge carriers) increases gradually as the temperature increases. Accordingly R0 can be represented, as a function of temperature by the following linear formula: R0 ¼ MT þ N;
ð3Þ
where M is a negative factor which is the slope of the lines observed in Fig. 8, and N is the value of R0 at zero absolute temperature. Fig. 9 shows the FL dependence of log r measured (and R0 calculated) at T ¼ 373 K. Two maxima were observed at x ¼ 1% and 19% for log r (and R0 ). The calculated values of R0 were in the range of 1.8–6.7 nm corresponding to 7.2–26.8 monomer units, where the monomer unit length of PVDF E0.25 nm [22]. This suggests the predominance of an interchain one-dimensional hopping mechanism. It is noteworthy that there is a slight increase of R0 (and log r) as a function of x in the range of 5%pxp15%: This may imply that the hopping sites (the polarons and/or bipolarons) increase slightly as x increases.
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209
Fig. 9. The FL dependence of log rðÞ and R0 ðJÞ at temperature T ¼ 373 K. The solid lines are only to guide the eyes.
3.6. ESR Fig. 10 shows the ESR spectra of the present system at 300 K. For the case of x ¼ 0:0%; the spectrum is characterized by the following three features: (i) a broad (incomplete and deformed) Lorentzian signal due to the Co2+–Co2+ exchange interaction indicating the presence of Co2+ aggregated forms [23,24] and (ii) a lot of complicated weak hyperfine lines which are distributed all over the whole spectrum. These lines may arise from residual free carriers and/or neutral defects induced in the PVDF chain structure due to CoCl2 filling [25]. The unpaired electron of some of these structural defects results from a ‘‘domain wall’’ in the bond alternation resulting in a neutral Pelectron free radical [26]. These defects may arise in the isomerization process of converting cis to trans forms. The ESR hyperfine pattern may also arise from radicals having electrons localized in an sp2 or sp3 orbital, where the hyperfine splitting from adjacent 1H or 13C nuclei would be easily distinguished if trans-defects occurred in the Psystem [27]. It is clear from Fig. 10 that for the cases of xX5% the spectra are characterized by strong and broad Lorentzian signals which may be ascribed mainly to the Fe3+ clusters coupled by strong spin–spin interaction. The observed hyperfine lines for the cases of xo20% can be attributed to the
Fig. 10. The ESR spectra (x)FeCl3(20x)CoCl2.
of
PVDF
filled
with
PVDF structural defects induced by Co2+. This interpretation is accepted, considering the disappearance of the hyperfine lines in the case of x ¼ 20% (which is free from Co2+). The aggregated Fe3+ and Co2+ forms evidenced by the present ESR spectra support the tetrahedral clusters of Fe3+ and Co2+ revealed from the optical absorption spectra. 3.7. DC magnetic susceptibility The temperature dependence of the reciprocal values of the DC magnetic susceptibility, w; measured in the temperature range of 95–285 K for various FLs is shown in Fig. 11. It is clear that
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Fig. 12. The FL dependence of yp (J) and meff ().
Fig. 11. The temperature dependence of the reciprocal magnetic susceptibility for variously filled PVDF films: (1) 0.0%, (2) 1%, (3) 5%, (4) 10%, (5) 15% (6) 19% and (7) 20%. The solid lines represent the linear fitting.
w obeys Curie–Weiss law in which w ¼ C=ðT yp Þ;
ð4Þ
where C is the Curie’s constant and yp is the paramagnetic Curie temperature. Negative yp values obtained for xo8:5%; suggest an antiferromagnetic exchange interaction between the magnetic centres. On the other hand, positive yp values were found for x > 8:5% indicating a ferromagnetic interaction at lower temperatures. The Curie–Weiss plots were used, together with the equation meff ¼ 2:84½wm ðT yp Þ1=2
4. Conclusions The present work tried to explore, more explicitly, the role of most of the microstructural forms on the electrical and magnetic properties of PVDF films filled with (x)FeCl3(20x)CoCl2, in the mass fraction range of 0%pxp20%: This mixture of Fe3+ and Co2+ ions enhanced the magnetoactivity of PVDF. The fair intrasubstituted range of 5%pxp15% revealed a monotonic FL dependence for most of the investigated parameters, except for the Fe3+ and Co2+ tetrahedral contents. The various structures and physical properties found for the present films suggest some useful technical applications as follows: *
ð5Þ
to calculate the effective magnetic moment, meff ; where wm is the molar susceptibility of the used sample. The FL dependences of yp and meff are shown in Fig. 12. It is remarkable that meff has a minimum value at x ¼ 15% and the maximum value is observed at x ¼ 1%: Correlating the maximum value of meff (at x ¼ 1%) with the maximum content of tetrahedral Co2+ ions (see Fig. (7)) it could be implied that the tetrahedral Co2+ ions exhibit strong magnetic moments.
*
A film of the present system having x ¼ 1% exhibits a maximum content of tetrahedral Co2+ form, maximum values of magnetic moment and electrical resistivity. This sample can be used as an electromagnetic wave sensor and modulator, especially for infrared and microwaves. The sample of x ¼ 10% contains maximum contents of b phase and tetrahedral Fe3+ and Co2+ forms. Thus, it can serve as a piezoelectric and paramagnetic semiconductor, which can be used to produce surface acoustic waves with variable frequency, acting as a diffraction grating of variable order for laser beams. This
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type of gratings is very important for the modern optical communications.
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