J Supercond Nov Magn (2012) 25:1185–1193 DOI 10.1007/s10948-011-1396-x
O R I G I N A L PA P E R
Poly(vinyl phosphonic acid) (PVPA)–BaFe12 O19 Nanocomposite Z. Durmus · H. Kavas · H. Sozeri · M.S. Toprak · A. Aslan · A. Baykal
Received: 22 September 2011 / Accepted: 13 December 2011 / Published online: 3 January 2012 © Springer Science+Business Media, LLC 2011
Abstract We present a method for the fabrication of PVPA/ BaFe12 O19 nanocomposite by in-situ polymerization of vinyl phosphonic acid, VPA in the presence of synthesized BaFe12 O19 NPs. Nanoparticles and the nanocomposite were analyzed by XRD, FTIR, TGA, SEM, TEM, VSM, and conductivity techniques for structural and physicochemical characteristics. Nanoparticles, identified as BaFe12 O19 from XRD analysis, were successfully coated with PVPA and the linkage was assessed to be via P–O bonds. Electron microscopy analysis revealed aggregation of BaFe12 O19 particles and dominantly platelet morphology upon composite formation. TGA analysis revealed the composition of the nanocomposite as 65% BaFe12 O19 and 35% polymer. Magnetic evaluation revealed that adsorption of PVPA anions during the preparation of the nanocomposite strongly influenced the magnetic properties resulting in much lower saturation magnetization values. DC conductivity measurements were used to calculate activation energy of PVPA/BaFe12 O19 nanocomposite and it was obtained as 0.37 eV. Z. Durmus () · A. Aslan · A. Baykal Department of Chemistry, Fatih University, 34500 Buyukcekmece, Istanbul, Turkey e-mail:
[email protected] H. Kavas Department of Physics, Gebze Institute of Technology, 41400 Cayirova, Izmit, Turkey H. Sozeri TUBITAK-UME, National Metrology Institute, PO Box 54, 41470 Gebze-Kocaeli, Turkey M.S. Toprak Functional Materials Division, Royal Institute of Technology—KTH, SE16440 Stockholm, Sweden
Keywords Magnetic nanomaterials · Nanocomposites · Conductivity · Dielectric properties
1 Introduction Materials consisting of nanometer-sized particles have attracted broad interest in fundamental sciences and technological applications due to the novel and/or enhanced physicochemical properties different form micron-grained counterparts [1]. M-type barium hexaferrite with hexagonal molecular structure BaFe12 O19 (Ba hexaferrite, BaM) is a promising material for permanent magnet, advanced recording, and microwave absorption because of its fairly large magnetocrystalline anisotropy, high Curie temperature, relatively large magnetization, excellent chemical stability, and corrosion resistivity, whereas its magnetic and electrical properties should be modulated to satisfy different applications [2–10]. Barium hexaferrite may be the most extensively investigated material among the family of hard ferrites having the general formula MeFe12 O19 , where Me = Ba, Sr, Pb [11]. There are many approaches to manufacture Ba-M-type ferrites, including the traditional ceramic sintering, coprecipitation, microwave plasma, reverse micelle method, etc. as well as in the foam form [12]. Poly(vinyl phosphonic acid) (PVPA) is a simple polymeric acid which can be produced by free-radical polymerization of vinyl phosphonic acid, VPA [13]. Recently, it gained increasing interest as a model system to study the proton conduction mechanism of phosphonated polymers suggested for applications in fuel cells [14]. Other applications are concerned with the protection of metal surfaces as a primer [15–18] or as a reactive component in dental cements [19–21].
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Conducting polymers are attractive materials, as they cover a wide range of functions from insulators to metals and retain the mechanical properties of conventional polymers [22, 23]. The considerable electrochemical and physicochemical properties result in conducting polymers having various practical applications [24–27]. Organic–inorganic composite materials are receiving growing research interests in recent years, since they usually provide a new functional hybrid with synergetic or complementary activity between organic and inorganic materials, which are not available from their single components, and can provide novel or enhanced properties for various applications. Especially, composite materials composed of conducting polymers and magnetic particles have attracted even more attention. In the literature, an in-situ polymerization method was proven a simple and efficient strategy to fabricate polymer capped BaFe12 O19 nanocomposite with the electromagnetic properties [28, 29]. However, there is no work related with the synthesis and detailed conductivity investigation of the nanocomposite in the literature. Here, we present on the synthesis and detailed physicochemical characterization of PVPA/BaFe12 O19 nanocomposite. To the best of our knowledge, this is the first report on its synthesis and electrical transport property (electronic/dielectric) characterization.
2 Experimental 2.1 Materials Vinyl phosphonic acid (95% Fluka), DMF (99% Fluka), α-α -Azodiisobutyramidin dihydrochlorid (AIBHC, 98% Fluka), Methanol (99% Merck), Iron (III) chloride hexahydrate (FeCl3 ·6H2 O, Merck), Barium nitrate (Ba(NO3 )2 , Merck), Ammonia (NH3 ), Citric acid (C6 H8 O7 Merck) were used as received without further purification. 2.2 Characterization X-ray powder diffraction (XRD) analysis was conducted on a Rigaku Smart Lab diffractometer operated at 40 kV and 35 mA using Cu Kα radiation (λ = 1.54059 Å). Fourier transform infrared (FT-IR) spectra of the samples were recorded with a Perkin–Elmer BX FT-IR infrared spectrometer in the range of 4000–400 cm−1 . Transmission electron microscopy (TEM) analysis was performed using FEI Tecnai G2 Sphera microscope. A drop of diluted sample in alcohol was dripped on a TEM grid and dried prior to insertion to TEM column. Scanning Electron Microscopy (SEM) analysis was performed, in order to investigate the microstructure of the sample, using FEI XL40 Sirion FEG Digital Scanning Microscope. Samples were coated with gold at 10 mA
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for 2 min prior to SEM analysis. The thermal stability was determined by thermogravimetric analysis (TGA, Perkin– Elmer Instruments model, STA 6000). The TGA thermograms were recorded for 5 mg of powder sample at a heating rate of 10 ◦ C/min in the temperature range of 30–800 ◦ C under nitrogen atmosphere. The electrical conductivity of the PVPA and PVPA–BaFe12 O19 nanocomposite was studied in the range of 20–150 ◦ C with 10 ◦ C steps. The samples were used in the form of circular pellets of 13 mm diameter and 3 mm thickness. The pellets were sandwiched between gold electrodes and the conductivities were measured using Novocontrol dielectric impedance analyzer in the frequency range 1 Hz–3 MHz. The temperature was controlled with a Novocool Cryosystem, between 100 ◦ C and 250 ◦ C. The dielectric data (ε and ε ) were collected during heating as a function of frequency. Room temperature VSM measurements were performed by using a Quantum Design Vibrating sample magnetometer (QD-VSM). 2.3 Synthesis of BaFe12 O19 NP’s and PVPA/BaFe12 O19 Nanocomposite For the citrate sol-gel synthesis of BaFe12 O19 nanoparticles, stoichiometric amounts of Fe(NO3 )3 ·9H2 O and Ba(NO3 )2 were dissolved in a minimum amount of deionized H2 O by stirring at 50 ◦ C with Fe/Ba ratio of 12. Citric acid was then added to the mixture solution of Ba2+ and Fe3+ to chelate these ions. The molar ratios of citric acid to metal ions used were 1:1. Ammonia was added to adjust the pH value to 7. The clear solution was slowly evaporated at 80 ◦ C under constant stirring, forming a viscous gel. By increasing the temperature up to 200 ◦ C, the gel precursors were combusted to form brown loose powders. The precursor was precalcined at 450 ◦ C for 4 h followed by sintering at 1100 ◦ C for 1 h. The hexaferrite BaFe12 O19 particles were thus obtained. PVPA/BaFe12 O19 nanocomposite was produced by free radical polymerization of vinyl phosphonic acid in DMF using AIBHC (0.1 mol%) as initiator. Then VPA/hexaferrite molar ratio (40:1) was added to reaction mixture. Afterward, the reaction mixture dialyzed against water to remove excess monomer and solvent. The reaction mixture was purged with nitrogen and the polymerization reaction was performed at 85 ◦ C for 2 h. The resulting sample filtered and washed several times with toluene, dried in vacuum and stored in a glove box. 3 Results and Discussion 3.1 FTIR Analysis FTIR spectra of PVPA, BaFe12 O19 NPs, and PVPA/ BaFe12 O19 nanocomposite along with the suggested linkage of PVPA to BaFe12 O19 surface are given in Fig. 1a–1d.
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Fig. 1 FTIR spectra of (a) PVPA, (b) BaFe12 O19 and (c) PVPA/BaFe12 O19 nanocomposite and (d) suggested linkage of PVPA to BaFe12 O19 surface
Fig. 2 XRD powder pattern of PVPA/BaFe12 O19 nanocomposite
As prepared powder presents characteristic peaks that are exhibited by the BaFe12 O19 powder: characteristic absorption bands for BaFe12 O19 at around 591 and 438 cm−1 corresponding to vibrations of the tetrahedral and octahedral sites for BaFe12 O19 in Fig. 1a [30, 31]. The absorption peak at 1143 cm−1 for the PVPA sample (Fig. 1c) is assigned to P=O stretching vibration and the absorbances at 939, 1004, and 2320 cm−1 were assigned to PO–H vibration [32–38]. The absorption due to P=O stretching is not observed in the spectrum for the nanocomposite (Fig. 1b). Furthermore, additional absorbances at 2998 and 2800 cm−1 due to P–O–H bonds in PVPA are not observed for the nanocomposite which suggests that oxygen atoms are coordinated to the metal centers on the nanoparticles. The disappearance of the P=O absorption together with the P–O–H bands may indicate that all three oxygens can interact with the nanopar-
ticle surface. However, since the ranges for the different P–O stretching peaks greatly overlap and depend on the degree of hydrogen bonding or metal binding, the definitive assignment of these bands is difficult. Suggested conjugation schemes of PVPA to BaFe12 O19 nanoparticle surface is given in Fig. 1d. These are possible schemes contributing to the observed FTIR spectra of the nanocomposite with different ratios of occurrence. 3.2 XRD Analysis Phase investigation of the crystallized product was performed by XRD and the diffraction pattern is presented in Fig. 2. The XRD pattern indicates that the product is M-type BaFe12 O19 and the diffraction peaks are broadened owing to very small crystallite size. All of the observed diffraction peaks are indexed by the cubic structure of BaFe12 O19
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Fig. 3 SEM micrographs of (a) BaFe12 O19 particles and (b, c) PVPA/BaFe12 O19 nanocomposite
Fig. 4 (a) TEM micrographs at different magnifications, and (b) EDX spectrum of PVPA/BaFe12 O19 nanocomposite
(JCPDS no. 84-0757) revealing a high phase purity of hexaferrite [39, 40]. The average crystallite size of BaFe12 O19 using Scherrer’s relation was found to be 15 ± 6 nm for observed 20 peaks with the following miller indices: (110), (107), (114), (200), (202), (203), (204), (205), (206), (1011), (213), (209), (217), (2011), (1014), (219), (220), (1114), (2111), and (2014). 3.3 SEM and TEM Analysis The microstructure and morphology of the obtained BaFe12 O19 particles and PVPA/BaFe12 O19 nanocompos-
ite has been studied by SEM and TEM, and representative micrographs are presented in Figs. 3 and 4, respectively. The SEM (Fig. 3a) images of BaFe12 O19 particles indicate that strong agglomeration of particles in the size range of ∼100 nm to ∼900 nm. Aggregation appears unavoidable due to higher annealing temperature and interaction between magnetic particles. Compared with the crystallite size obtained from Scherrer’s relation this reveals polycrystalline nature of BaFe12 O19 particles. Nanocomposite images in Fig. 3b and 3c reveal the plate-like morphology with random orientation. Plate-like morphology is attributed to the
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Fig. 5 TGA thermograms of (a) BaFe12 O19 NP’s (b) PVPA/BaFe12 O19 nanocomposite and (c) PVPA
PVPA polymerization causing islands of platelets around the hexaferrite aggregates. Boundaries between the grains in Fig. 3a are filled with the polymer, smoothing out the surface features. TEM images of PVPA/BaFe12 O19 nanocomposite are shown in Fig. 4a. Observed morphologies are similar to those of hexaferrite particles in Fig. 3a and nanocomposite in Fig. 3b. Plate-like morphology observed in SEM is also observed here and it is attributed to the PVPA polymerization forming islands of platelets around the BaFe12 O19 aggregates due to its sticky nature. PVPA layer is visible in TEM although its low density and EDX analysis reveal signals of P and C that are from the PVPA coating while Fe/Ba ratio is quantified as 12 as expected from the materials’ stoichiometry (Fig. 4b). The synthetic method in this study is proven applicable to the synthesis PVPA/BaFe12 O19 composite. 3.4 Thermal Analysis Thermogravimetric analysis curves of PVPA, BaFe12 O19 and PVPA/BaFe12 O19 nanocomposite from room temperature up to 750 ◦ C are presented in Fig. 5. BaFe12 O19 shows no weight loss in the temperature range of TG analysis. On the other hand, two-stage degradation is seen in the TGA curves of PVPA and three-step decomposition for the PVPA/BaFe12 O19 nanocomposite samples. Weight loss at temperatures up to 150 ◦ C can be assigned to the evaporation of adsorbed water. Combustion of PVPA starts around 230 ◦ C reaching a total weight loss of ∼70% at 750 ◦ C [41]. Degradation of PVPA over BaFe12 O19 takes place in two steps, the first one begins around 250 ◦ C and the second one around 400 ◦ C reaching a total weight loss of 20%. The additional decomposition in the nanocomposite around 280 ◦ C
may be originated from the fact that BaFe12 O19 particles behave as catalysts causing breakdown of PVPA backbone at lower temperatures. Derivative weight loss (DTG) curves of the PVPA and PVPA/BaFe12 O19 are also illustrated in the inset of Fig. 5 which shows a similar behavior in the degradation of these two samples. Based on the thermogram, PVPA content of the nanocomposite is ∼35% which means an inorganic (BaFe12 O19 ) content of about 65%. 3.5 Magnetization Room temperature M–H hysteresis curves of bulk (i.e., as annealed) BaFe12 O19 particles and PVPA/BaFe12 O19 are shown in Fig. 6. It is observed that PVPA/BaFe12 O19 nanocomposite has very low saturation magnetization (Ms ) of about 5 emu/g compared to that of the bulk sample (55 emu/g). According to the TGA analysis results, real mass of the magnetic core is just 65% of the total mass of PVPA/ BaFe12 O19 nanocomposite. Thus, if mass of the magnetic core is used to find the real Ms of the composite, it becomes ∼9.1 emu/g, which is still very far from the bulk value. If there was no coating on the surface of magnetic particles, this low Ms value could be partially attributed to the presence of very small particles with superparamagnetic behavior [42, 43] and also other non-magnetic phases such as α-Fe2 O3 . On the other hand, in the presence of PVPA coating, the sharp decrease in Ms can be explained by the adsorption of PVPA anions during the preparation of the composite. This means that free spins on the surface of the BaFe12 O19 are pinned during the polymerization and hence they cannot contribute to total magnetization of the sample. Contrary to the saturation magnetization, coercivities of the powders, both coated and as annealed, are nearly the same. It
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Fig. 6 M–H curves measured at temperature below Tc for PVPA/BaFe12 O19 nanocomposite and uncoated BaFe12 O19 nanoparticles
Fig. 7 The angular frequency dependence of the ac conductivity of PVPA/BaFe12 O19 nanocomposite in the temperature range of 20 to 120 ◦ C
should be noted that coercivity mainly depends on the particle size, which does not change with coating. Therefore, this observation is in accordance with our expectations. Finally, Fig. 6 also shows that bulk BaFe12 O19 powders do not saturate up to 15 kOe as expected because BaFe12 O19 has a very high anisotropy field [44]. 3.6 Temperature and Frequency Dependent Conductivity and Dielectric Permittivity Measurements Figure 7 shows the angular frequency dependence of the ac conductivity at different temperatures ranging from 20 to 120 ◦ C. The ac conductivity increases with increasing temperature and frequency. Total conductivity σT of ferrites, conducting polymers and their composites can be analyzed with the equation of σT = σDC (T ) + σ (ω)
(1)
[45] due to their semiconducting nature at elevated temperatures and frequencies. The first term in this equation is the σ dc , dc electrical conductivity which is related to the drift of electric charge carriers via band conduction mechanism and is temperature-dependent by following an Arrhenius relation: σDC = σ0 + exp(−Ea /kT ).
(2)
The second term, σ (ω), is frequency-dependent and related to the dielectric relaxation caused by the localized electric charge carriers and obeys the empirical formula of frequency dependence given by ac power law [46]. σ = B(T )w n(T )
(3)
where B(T ) and n(T ) ≤ 1 are constants at a certain temperature. The logarithm of ac conductivity versus that of angular frequency curves fitted according to (3) and fitting
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Fig. 8 The temperature dependency of fitting parameters n and B
Fig. 9 The ε as a function of frequency of PVPA/BaFe12 O19 nanocomposite in the temperature range of 20 to 120 ◦ C
curves are shown as continuous lines in Fig. 7. The temperature dependency of fitting parameters of n and B are shown in Fig. 8. While B is increasing, n is decreasing with increasing temperature. The conduction mechanism can be assigned as conduction due to correlated barrier hopping (CBH) [46] with decreasing n values by increasing temperature. This type of conductivity in conducting polymer– nanoferrite composites were also reported in our previous studies [47, 48]. The ε as a function of frequency in the temperature range of 20–120 ◦ C for PVPA/BaFe12 O19 nanocomposite is shown in Fig. 9. The ε increases with increasing temperature up to 105 Hz but it decreases with increasing temperature above this frequency. The frequency dependency at all temperatures is same and it decreases with increasing frequency. Figure 10 shows the ε as a function of frequency in the temperature range of 20 to 120 ◦ C. The ε linearly decreases
by frequency at all temperatures and it increases with increasing temperature. The range between ε curves at different temperatures are slightly narrowing down at higher frequencies. This change can be explained on the basis of Koop’s theory, which is based on the Maxwell–Wagner model for the homogeneous double structure [49] in which the highly conducting grains are separated by relatively poor conducting grain boundaries and are found to be more effective at higher frequencies, while the conducting grains are more effective at lower frequencies [50]. The conductivity difference between grains and grain boundaries means different resistance causing the accumulation of charge carriers in separated boundaries and increase in dielectric constants. The DC conductivities are found by extrapolation of frequency independent part of (lower frequency regimes) curves to the zero frequency. Figure 11 shows the variation of DC conductivity with respect to temperature for PVPA/BaFe12 O19 nanocomposite. The DC conductivity
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Fig. 10 The ε as a function of frequency of PVPA/BaFe12 O19 nanocomposite in the temperature range of 20 to 120 ◦ C
Fig. 11 The Arrhenius plot of DC conductivities with linear fits for PVPA/BaFe12 O19 nanocomposite
curve is linearly fitted according to the Arrhenius law and the activation energy of PVPA/BaFe12 O19 nanocomposite is found as 0.37 eV. Anke et al. [51] found the activation energy of PVPA as 65 kJ/mol (0.674 eV) and also reported its conductivity is strongly dependent on amount of the phosphonic acid sites which is the potential source of water molecules. The conductivity of PVPA can be increased two times by addition of BaFe12 O19 nanoparticles.
4 Conclusion We presented a method for the fabrication of PVPA/ BaFe12 O19 nanocomposite by in-situ polymerization of PVPA in the presence of as-synthesized BaFe12 O19 nanoparticles. Nanoparticles were identified as BaFe12 O19 from
XRD analysis and were successfully coated with PVPA and the linkage was assessed to be via P–O bonds by FTIR analysis. SEM and TEM analysis revealed aggregation of BaFe12 O19 particles, and that nanocomposite had dominantly plate like morphology, due to the presence of PVPA. TGA analysis revealed the composition of the nanocomposite as 65% BaFe12 O19 and 35% polymer. Magnetic evaluation revealed that adsorption of PVPA anions during the preparation of the nanocomposite strongly influenced the magnetic properties resulting in much lower saturation magnetization values. DC conductivity measurements were used to calculate activation energy of PVPA/BaFe12 O19 nanocomposite and it was obtained as 0.37 eV, in agreement with earlier reported values. The method reported hereby is rather versatile and can be used for the fabrication of a variety of nanocomposites. Acknowledgements The authors are thankful to the Fatih University, Research Project Foundation (Contract No: P50021104-B), Scientific and Technological Research Council of Turkey (TÜB˙ITAK) (Project No: 110T487).
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