Materials Research Bulletin 80 (2016) 321–328
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Structural and magnetic characteristics of PVA/CoFe2O4 nano-composites prepared via mechanical alloying method S. Rashidi, A. Ataie* School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran
A R T I C L E I N F O
Article history: Received 23 December 2015 Received in revised form 15 March 2016 Accepted 14 April 2016 Available online 20 April 2016 Keywords: A. Composites A. Nanostructure B. Magnetic properties C. Electron microscopy C. X-ray diffraction
A B S T R A C T
In this research, polyvinyl alcohol/cobalt ferrite nano-composites were successfully synthesized employing a two-step procedure: the spherical single-phase cobalt ferrite of 20 4 nm mean particle size was synthesized via mechanical alloying method and then embedded into polymer matrix by intensive milling. The results revealed that increase in polyvinyl alcohol content and milling time causes cobalt ferrite particles disperse more homogeneously in polymer matrix, while the mean particle size and shape of cobalt ferrite have not been significantly affected. Transmission electron microscope images indicated that polyvinyl alcohol chains have surrounded the cobalt ferrite nano-particles; also, the interaction between polymer and cobalt ferrite particles in nano-composite samples was confirmed. Magnetic properties evaluation showed that saturation magnetization, coercivity and anisotropy constant values decreased in nano-composite samples compared to pure cobalt ferrite. However, the coercivity values of related nano-composite samples enhanced by increasing PVA amount due to domain wall mechanism. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction In the last decade, synthesis and properties of polymeric magnetic nano-composites composed of organic polymer and magnetic materials in nano-metric dimensions have intensively attracted scientific and technological interests [1,2]. Combination of desired physical and chemical inherent features of these components not only provides multifunctional nano-composites with remarkable properties but also improves the sum of their individual properties [3,4]. For instance, encapsulation of magnetic nano-particles within non-magnetic polymer matrix improves the intrinsic properties of magnetic particles such as their chemical stability and dispersibility, prevents the aggregation of magnetic particles and reduces toxicity [5–7]. Among various types of magnetic materials, spinel cobalt ferrite with a chemical formula of CoFe2O4 is emerging as one of the most promising magnetic materials in terms of high coercivity, moderate saturation magnetization, excellent chemical stability, mechanical hardness and large magneto-crystalline-anisotropy compared to other spinel ferrites [8,9]. These properties make it a remarkable candidate for applications in high density magnetic recording medium, catalysts, ferro-fluid [10,11] and particularly in
* Corresponding author. E-mail addresses:
[email protected],
[email protected] (A. Ataie). http://dx.doi.org/10.1016/j.materresbull.2016.04.021 0025-5408/ ã 2016 Elsevier Ltd. All rights reserved.
biomedical fields, including magnetic resonance imaging (MRI), targeting drug delivery, magnetic separation, biosensors and hyperthermia [6,12]. Polyvinyl alcohol (PVA) is semi-crystalline, non-toxic, biocompatible, biodegradable polymer with an excellent chemical resistance which is extensively used for biomedical applications [9,13]. Therefore, embedding CoFe2O4 nano-particles in PVA matrix due to their significant potential applications have been investigated earlier and the effects of polymer matrix and the synthesis methods on size, shape and magnetic properties of cobalt ferrite particles have been studied [7,9,14,15]. However, no effort has been made to preparation and characterization of this nano-composite by mechanical alloying so far. Generally, two methods are being used for formation of polymer matrix nano-composites. In the first method, known as ex-situ synthesis, previously formed nano-particles are incorporated and mixed into a polymer matrix while in the second route, nano-composites prepared by in-situ synthesis of nano-particles and the polymerization of the monomers [16,17]. Even though it is possible to obtain very homogenous dispersion with the second method, this method is mostly expensive, bases on complex solution or melting process and obtaining homogenous dispersion with high load of filling particles is infeasible. One sustainable alternative to overcome these thermal and solvent problems is using solid-state mixing method such as mechanical alloying [3,4,16]. Mechanical alloying is a well-established technique with
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many advantages including low cost, high yield, good ecological stability, no limitations on the material and low temperature synthesis [16], which is being successfully used to synthesize novel materials at the nano-scale [11]. For the first time, Shaw et al. [18] reported that the mechanical alloying could be employed in preparation of polymer matrix composites. Up to now, some investigations in the case of polymer/ magnetic metal [3,4,19] and polymer/soft magnetic ferrite nanocomposites [17,20] have been done by applying this method. However, to the best of our knowledge, no detailed study has been carried out on preparation of polymer/hard magnetic ferrite nanocomposite by mechanical alloying technique. In the present work, the mechanical alloying method is applied for synthesis of PVA/CoFe2O4 nano-composite and the effects of milling time and various amounts of magnetic nano-particles on the characteristics of nano-composite samples are investigated in detail. The results would be helpful for the further studies on physical and biocompatibility properties of these nano-composites for applying in biomedical field. 2. Experimental procedure 2.1. Materials and methods Hematite (a-Fe2O3, Merck), cobalt carbonate (CoCO3, VWR) and polyvinyl alcohol (PVA, (-C2H4O)n, Merck) with a molecular weight of 72,000 LR were used as starting materials without further purification. To synthesize CoFe2O4 nano-particles, iron oxide and cobalt carbonate powders were mixed according to Eq. (1) and then milled in a high energy planetary ball mill PM2400 with a rotation speed of 400 rpm and ball to powder weight ratio of 30:1 rpm for 25 h at room temperature.
a-Fe2O3 + CoCO3 ! CoFe2O4 + CO2
(1)
The vial and balls were both made of hardened chromium steel. Details of synthesis and reaction mechanism of CoFe2O4 nanoparticles were reported in our previous work [21]. The synthesized CoFe2O4 nano-particles and PVA powders were mixed with different weight ratios of magnetic nano-particles, i.e. 20, 50 and 80%. The mixed powders were milled up to 30 h without
Fig. 1. XRD patterns of PVA, pure CF and their corresponding nano-composites milled for 10 h.
Fig. 2. XRD patterns of PVA/CF2 sample milled for various periods of time.
using any surfactant/capping agent under air atmosphere. The rotation speed and ball to powder ratio were 300 rpm and 15:1, respectively. To avoid excessive heating, periodic interruption in milling was set for 10 min, twice an hour. Cobalt ferrite and the corresponding nano-composites containing 20, 50 and 80 wt.% cobalt ferrite nano-particles were labeled as CF, PVA/CF2, PVA/CF5 and PVA/CF8, respectively. 2.2. Characterization The phase identification of the samples was investigated by Xray diffraction (XRD) at room temperature using a Philips PW3710 diffractometer with Cu-Ka radiation (l = 0.15406 nm) in a 2u range between 10 and 100 and step size of 0.02 . The mean crystallite size and the lattice strain of the milled samples were estimated by XRD peak broadening analysis using Williamson-Hall equation [22,23]. Morphology and chemical composition of milling products were examined by both field emission scanning electron microscope (FESEM, Hitachi S4160) and high-resolution transmission electron microscope (HRTEM, JEOL, JEM-2100, operating at 200 kV) equipped with an energy dispersive spectrometer (EDS) point chemical analysis. Also, the particle size of cobalt ferrite was analyzed by MIP (Microstructural Image Processor) software. Formation of cobalt ferrite nano-particles and corresponding
Fig. 3. Mean crystallite size and strain changes as a function of milling time in PVA/ CF2 sample (the data of 0 h is related to CF sample).
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nano-composites were confirmed by Fourier transform infrared spectroscopy (FTIR, Bruker Equinox 55) using 16 scants at 4 cm1 resolution in the 400–4000 cm1 range with samples dispersed in KBr pellets. Magnetic properties of the prepared samples were studied using vibrating sample magnetometer (VSM) at a maximum applied field of 10 kOe at room temperature. 3. Results and discussion 3.1. XRD analysis X-ray diffraction patterns of pure PVA, cobalt ferrite and 10 h milled PVA/CF nano-composites with different cobalt ferrite contents are presented in Fig. 1. XRD pattern of cobalt ferrite powder shows that all of the obtained diffraction peaks are well
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matched with the cubic structure of CoFe2O4 inverse spinel phase (ICDD card no. 001–1121) with Fd–3 m space group. The calculated mean crystallite size and lattice parameter were 15 nm and 8.374 A , respectively. XRD pattern of PVA shows its semicrystalline structure by a broad diffraction peak at 2u = 19.81 due to the strong intermolecular interaction between PVA chains through intermolecular hydrogen bonds [14,24]. Analysis of the XRD patterns of PVA/CF nano-composites revealed that the crystalline structure of CoFe2O4 is preserved in all samples but the intensity of peaks weakens compared with that of the pure composition in proportion to PVA content. The broad peak of PVA is clearly discernable in PVA/CF2 sample; however, its intensity decreases in PVA/CF5 and PVA/CF8 samples due to its relatively low amount and probable amorphization phenomenon. Partially decomposition of polymer chains to its constitution such as C, O
Fig. 4. FESEM images of (a) pure CF and (b) PVA/CF8, (c) PVA/CF5, (d) PVA/CF2 nano-composites milled for 10 h. PVA/CF2 sample milled for (e) 20 h and (f) 30 h.
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Fig. 5. Schematic illustration showing mechanical milling process of PVA and CF nano-particles.
Fig. 6. (a) TEM image of CF sample, (b) HRTEM image and SAED pattern of CF sample, (c) TEM image of PVA/CF2 sample and (d) HRTEM image and SAED pattern of PVA/ CF2 sample. The inset in (a) and (c) shows the particle size distribution.
and H due to the heat and mechanical forces induced during the prolonged milling time could also be considered as another reason. Based on the above results, PVA/CF2 sample was selected for further investigations. Fig. 2 shows the XRD patterns of PVA/ CF2 nano-composite sample milled for various periods of time. The presence of the PVA peak is clear in all samples. In addition, cobalt ferrite peaks have slightly shifted to the higher-angles side compared to that of pure cobalt ferrite by increasing the milling time. This displacement could be attributed to the possible entering of carbon atoms released from polymer decomposition into cobalt ferrite crystalline structure, which is in agreement with smaller atom radius of C (0.76 A [25]) compared with that of Co2+ (0.82 A [26]). Another reason can be the milling-induced lattice strain. Fig. 3 demonstrates the variation of mean crystallite size and lattice strain as a function of milling time for PVA/CF2 sample. The mean crystallite size of cobalt ferrite in PVA/CF2 sample shows negligible decrease from 15 to 13 nm with continued milling up to 30 h. This observation can be attributed to the presence of polymer chains that acted as heterogeneous nucleation sites in the nanocomposite sample. In addition, by increasing the milling time
Fig. 7. EDX chemical analysis of PVA/CF2 sample milled for 30 h.
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lattice strain decreases. This reduction also can be interpreted by the damping and lubricating nature of PVA, which results in decreasing the mechanical milling energy. It is well known that the energy of colliding balls and rigidity of the solid are two pivotal factors in the amount of energy imparted to the solid in mechanical alloying. 3.2. Micro-structure studies Fig. 4 gives the FESEM micrographs of the CF, PVA/CF2, PVA/ CF5 and PVA/CF8 samples. Agglomerated uniform nano-metric particles of cobalt ferrite with a mean particle size of almost 40 8 nm could be observed in Fig. 4a. FESEM images in Fig. 4b to d revealed that: 1) PVA and cobalt ferrite particles are present in all samples. 2) Cobalt ferrite particles/agglomerates diffuse pervasively in polymer matrix. 3) The initial morphology of the magnetic particles is preserved and the size of individual particles does not change drastically. 4) Increase in polymer amount enhances particles dispersion and deagglomeration of cobalt ferrite agglomerates in the polymer matrix. This can be linked to the changes in ball-to-powder, ball-to-wall or ball-to-ball collision as well as temperature decline during milling process by increasing of polymer amount. Moreover, as PVA/CF nano-composite is classified to a ductile/brittle component system [16], the increment of polymer as a ductile material restricts the tendency of brittle cobalt ferrite particles to cold-welding during milling; thus, the agglomerates size reduces. In Fig. 4d–f, the effect of milling time on dispersion of cobalt ferrite particles in PVA/CF2 sample is shown. As it is seen, agglomerates size decreased and the particles dispersed more homogeneously by increasing the milling time up to 30 h. A proposed model can be used to illustrate the mechanical milling process of PVA and cobalt ferrite nano-particles (Fig. 5), which is almost prevalent in mechanical milling of inorganic nanoparticles with a polymer matrix [16]. In the initial stages of milling, the ductile PVA is flattened, whereas the brittle cobalt ferrite particles are reduced in size. Then, the PVA powders start to weld and the cobalt ferrite particles become embedded in the polymer matrix. Further milling leads polymer lamella become closer and thinner while the cobalt ferrite particles become more fractured. Welding and fracture mechanism then attain to an equilibrium state and randomly originated boundaries promotes the formation of composite particles. Eventually refinement of composite particle takes places in the steady state. TEM and HRTEM images with corresponding SAED patterns of cobalt ferrite and 30 h milled PVA/CF2 sample are shown in Fig. 6. Also, the insets in Fig. 6(a and c) show the particles size distribution histograms of mentioned samples fitted well with the lognormal function. The aggregation of individual cobalt ferrite nano-particles with an average particle diameter in the range of 20 4 nm can be seen in Fig. 6(a and b). SAED pattern (Fig. 6b) sustains the spinel structure of CoFe2O4 and diffraction rings corresponding to the different diffraction planes identified previously by XRD analysis with the following Miller indices: (111), (220), (311), (222), (400), (422), (511), (440), (533) and (731). The ring type SAED pattern provides another support for the conclusion that the nano-sized crystalline cobalt ferrite particles have been formed. In Fig. 6c, cobalt ferrite nano-particles with an average size of 18 3 nm have been embedded in PVA matrix and the polymer surrounded all particles. It is noteworthy that repeatedly fracturing and cold welding process results in homogenous dispersion of cobalt ferrite nano-particles in the polymer matrix. Fig. 6d exhibits cobalt ferrite particle coated with PVA layer; though, the thickness of the coating is not uniform surrounding the particles. The SAED pattern indicates that some diffraction rings corresponding to cobalt ferrite are blurred due to the presence of PVA. It deduces that the sample is composed of a
Fig. 8. FTIR spectra of PVA, CF and their corresponding nano-composites.
mixture of crystalline cobalt ferrite and semi-crystalline form of PVA at the nano-scale, which has been also confirmed by the XRD results. Additionally, The EDX point chemical analysis (Fig. 7) proves the existence of Co, Fe, O and C as the key elements of PVA/ CF2 sample. It should be noted that no contamination exists from the balls, vials or cleaning sand used. (The Cu peaks are related to the copper grid used in imaging). 3.3. FTIR analysis FTIR spectra of original PVA, cobalt ferrite nano-particles and their corresponding nano-composites are depicted in Fig. 8. Two vibrational bands at 554 cm1 and 485 cm1 are observed in IR spectra of cobalt ferrite. The higher frequency band is correlated to the stretching (n) intrinsic vibration of metal-anion at the tetrahedral site (Mth O) and the lower frequency band attributes to octahedral metal stretching (Moh O) of spinel ferrite lattice [20,27]. These vibration bands in cobalt ferrite sample exhibit another clue for formation of pure CoFe2O4 structure. In the case of PVA, the main characteristic bands are located at 3379 cm1 for OH group, 2941 cm1 for C H, 1718 cm1 for C¼O, 1436 cm1 for H C H bending (d), 1142 cm1 for C O, 1095 cm1 for COC 1 and 851 cm for CH2 rocking [7,13]. As can be seen in Fig. 8, all the characteristic bands of PVA and cobalt ferrite exist in all PVA/CF nano-composites. The detailed series of the characteristic bands for PVA/CF2 sample are summarized in Table 1. In comparison with that of the PVA spectrum, all the bands in nano-composites shift to lower frequencies. Furthermore, the intensity of PVA and cobalt Table 1 Characteristic FTIR vibrations for PVA/CF2 nano-composite sample. Sample
PVA/CF2
IR bands(cm1)
Description
3290 2922 1714 1420 1090 849 550 472
n (OH) n (CH) n (C¼O) d (HCH) n (COC) CH2 rocking n (MthO) n (MohO)
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Fig. 9. Hysteresis loops for (a) pure CF and 10 h milled nano-composite samples and (b) PVA/CF2 sample milled at different time. The insets show the enlarged view of loops at center as well as fitting curves to LAS for the all samples.
ferrite bands is proportionate to their amounts, where the intensity of cobalt ferrite is diminished by increasing the polymer content. These results confirm the interaction between cobalt ferrite nanoparticles and PVA chains [5,6].
3.4. Magnetic analysis The room temperature M-H loops of cobalt ferrite and nanocomposite samples are shown in Fig. 9 and the magnetic data are
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also tabulated in Table 2. The saturation magnetization of CF sample is 52.2 emu/g, which is in close agreement with previous results reported by Beitollahi et al. [28,29]. Although this value is higher than that obtained by other preparation techniques with comparable particle size [10], it is much lower than that of the bulk sample (80 emu/g) [29]. This reduction can be attributed to the distortion of the magnetic moment in the surface of the nano-sized cobalt ferrite as well as residual strain in the ball milled powder [28,30]. As expected, the saturation magnetization of nanocomposite samples decreased with increasing PVA content. The existence of the PVA dead layer on the surface of magnetic nanoparticles can cause surface anisotropy, crystalline disorder, reduction in the particle-particle interaction and exchange coupling energy which resulting in a significant decrease in Ms values [7,31]. Besides, it is considered that the saturation magnetization of the magnetic composites depends mainly on the total mass of the magnetic materials and varies as a function of the loaded weight [17]. It is observed that the coercivity of the nano-composites reduced comparing to pure cobalt ferrite from 832 to 506 Oe. As earlier reported, particle size and residual strain are the key factors in the coercivity of cobalt ferrite [32]. Fig. 10 illustrates the relationship between the mean particle size of samples (CF and PVA/CF2) and their coercivity in this study. Where the magnetic data in the right side of the graph belongs to CF sample milled for 20 h. It is observed that the coercivity increases by decreasing the particle size and hits a peak value around 37 nm which is in well consistent with single domain size of cobalt ferrite (Dcritical 40 nm) [32], then gets the same direction with diminution in particle size. As seen, the particles sizes of pure CF sample and PVA/ CF2 nano-composite samples are closely similar and all of them are in the single domain range. Therefore, it can be concluded that the decrease of coercivity in nano-composite samples is correlated to the release of induced residual strain. Fig. 9b also evidences that prolonging of the milling time does not have significant effect on the saturation magnetization of PVA/CF2 sample but the millinginduced coercivity decreases due to the above mentioned reason. Cobalt ferrite nano-particles with larger residual strain demonstrate larger coercivity value which can be related to the strain induced magnetic anisotropy [32,33]. For a more precise investigation, the anisotropy constant of samples was determined by fitting the high field regions (H » Hc) to the Law of Approach to Saturation (LAS), using the equation of ! 0:07619K 2 0:0384K 3 ð2Þ MðHÞ ¼ Ms 1 H2 Ms2 H3 Ms3 (where K is the effective magnetic anisotropy, Ms the saturation magnetization, M the magnetization and H the applied magnetic field), the first numerical coefficient 0.07619 is used for random polycrystalline samples with cubic anisotropy [34]. In the present work, the experimental magnetic data above 6 kOe (high field regions) was fitted to Eq. (2) and Ms and K were the fitting parameters. The values of saturation magnetization computed by fitting were close to magnetization obtained from the magnetic loops. The magnetic anisotropy constants are listed in Table 2 for Table 2 Magnetic properties of pure CF and PVA/CF nano-composite samples. Samples
Hc (Oe)
Mr (emu/g)
Ms (emu/g)
Keff (J/m3)
CF PVA/CF8-10h PVA/CF5-10h PVA/CF2-10h PVA/CF2-30h
832 506 519 540 507
17.9 10.3 7.4 2.8 2.7
52.2 37.8 25.3 9.6 9.2
2.2 104 1.6 104 1 104 0.43 104 0.42 104
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Fig. 10. The relationship between the mean particle size and the coercivity of CF and PVA/CF2 samples.
Fig. 11. Coercivity and anisotropy constant change as a function of milling time in PVA/CF2 sample (the data of 0 h is related to CF sample).
the all samples and fitting curves to LAS are shown in Fig. 9. The obtained anisotropy values confirm the direct relationship between residual strain and magnetic anisotropy. As discussed in the XRD section, presence of polymer results in release of residual strains (Fig. 3) and subsequently magnetic anisotropy reduces in nano-composites, which is well match with our results; Thereby, the composite samples exhibit lower coercivity values compared to that of pure cobalt ferrite (Fig. 11). Besides, as observed the coercivity of nano-composite samples enhances as the polymer content increases. This can be related to the domain wall mechanism, as the domain walls of magnetic ferrite decrease by more PVA, the magnetization and demagnetization require more energy and the wall domain movements are harder; therefore, the coercivity force increases [20]. 4. Conclusion Magnetic nano-composites, composed of cobalt ferrite nanoparticles and PVA polymer were obtained using a two-step mechanical milling and the effects of milling time and polymer content were investigated. It was found that single-phase cobalt ferrite of 20 4 nm particle size distributed uniformly by increasing PVA amount and milling time up to 80 wt.% and 30 h, respectively; however, the size and shape of particles were not changed drastically. The interaction of PVA chains and magnetic phase has been confirmed in nano-composite samples. Moreover, magnetization and coercivity of nano-composite samples decreased in comparison to pure cobalt ferrite due to the presence of non-magnetic polymer layer. The obtained results in this work
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