Magnetic Nanocomposites

Copyright © 2009 American Scientific Publishers All rights reserved Printed in the United States of America

Science of Advanced Materials Vol. 1, 262–268, 2009

Single Step Synthesis and Properties of M/MFe2O4 and PVDF/M/MFe2O4 (M = Co, Ni) Magnetic Nanocomposites Sasanka Deka1 ∗ † , P. A. Joy1 , and A. Pratheep Kumar2 1

RESEARCH ARTICLE

2

Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411008, India Polymer Science and Engineering Division, National Chemical Laboratory, Pune 411008, India

Metal/ferrite nanocomposites (M/MFe2 O4 ; M = Co, Ni), consisting of nanoparticles of a metal and the corresponding metal ferrite, have been synthesized under in situ conditions by a simple and Delivered by Ingenta to: in a polymer matrix have been single step process. The metal/ferrite nanocomposites embedded prepared by a simple hot-pressing method. The performance parameters such as magnetization, Guest User coercivity, permeability and dielectric constant of the two- and three-phase nanocomposite systems IP : 94.97.23.89 are compared. The coercivity increases permeability decreases with increasing polymer Sat, 24and Octthe 2009 21:11:35 content in the three-phase composite. Enhancement of the dielectric constant is observed at low frequencies.

Keywords: Nanocomposite, Nanocrystalline, Ferrite, Magnetic Properties, Permeability, Dielectric.

1. INTRODUCTION Metal-ceramic composites have been studied extensively for the past many decades because of their unusual mechanical as well as tunable thermal, electrical and magnetic properties suitable for various applications. Inorganic composites containing a magnetic metal and ceramic components are extremely useful for electromagnetic interference (EMI) shielding and many other applications. Similarly, spinel type soft ferrites are generally known for their microwave absorbing properties and is used in many applications.1 Composites containing nanocrystalline components or nanocomposites are of much current research interest because of their enhanced properties and possible applications.2 Polymer based magnetic composites have drawn much attention recently because of their flexibility, compatibility with printed wiring boards, and their easy fabrication into various shapes. It is possible to develop materials for special purposes and totally new material morphologies as well as device geometries using magnetopolymer composites.3 4 Most of the research work carried out are on two-phase magneto-polymer composites based on bulk metals or ferrites and only few reports are ∗

Author to whom correspondence should be addressed. Present address: National Nanotechnology Laboratory of CNR-INFM, Unita di Ricerca IIT, Via Arnesano Km 5, 73100 Lecce, Italy †

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available on the polymer based magnetic nanocomposite systems.5–8 Several properties such as magnetic, dielectric, ferroelectric, conductivity, mechanical properties, etc. have been investigated on ferrite/polymer,9–15 metal or metal oxide core–shell/polymer,16–18 and metal/polymer19–22 composites. There are few studies reported in the literature on three-phase magnetic-metal/magnetic-oxide/polymer composites.23–25 It has been suggested that these composites, after little modification, can be used as capacitors and inductors. Moreover, these composites have various advantages over the currently used ferromagnetic–ferroelectric ceramic composites so that they can be used as electromagnetic wave absorbers, biocompatible magnetic nanofibres, etc. For the synthesis of these composites, the metal and metal oxide powders are mixed together and added to the polymer matrix and this is followed by hot molding or mixing. There is a disadvantage in the processing methods of polymer/metal/ferrite nanocomposites if the metal and ferrite particles are added separately to the polymer matrix during the processing. In such cases, nonuniform mixing of metal and ferrite particles may occur giving rise to defective properties. In this manuscript we report a simple in situ method for the synthesis of metal/ferrite nanocomposites and studies on the magnetic and dielectric properties of some magneto-polymer composites. 1947-2935/2009/1/262/007

doi:10.1166/sam.2009.1053

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Single Step Synthesis and Properties of M/MFe2 O4 and PVDF/M/MFe2 O4 (M = Co, Ni) Magnetic Nanocomposites

2. EXPERIMENTAL DETAILS

Co/CoFe2 O4 and Ni/NiFe2 O4 nanocomposites were synthesized by the glycine-nitrate method of combustion.26 It is known that a larger glycine/nitrate ratio gives rise to a mixed phase with the required ferrite and a reduced phase as impurity phase.27 Hence the synthesis is performed by taking the glycine/nitrate ratio as 0.8 to get the ferrite and the metal phases in a single step. Appropriate amounts of Co(NO3 )2 · 6H2 O (or Ni(NO3 )2 · 6H2 O) and Fe(NO3 )3 · 9H2 O were weighed in the molar ratio of 1:1 to obtain approximately 1 g of the mixed phases. The metal nitrates were dissolved separately in minimum amount of distilled water in separate beakers and then mixed thoroughly in a crystallizing dish. To the mixed solution of the metal nitrates, a water solution of glycine was added, taking the glycine/nitrate ratio as 0.8. The final mixed solution was evaporated on a hot plate at 200 C till it forms a thick mass, which subsequently underwent autocombustion in 3. RESULTS AND DISCUSSION Delivered few seconds to give a fine powder. For comparison of by Ingenta to: Guest User the properties of Co/CoFe2 O4 and Ni/NiFe2 O4 nanocomPowder X-ray diffraction (XRD) patterns of the as-syntheIP : 94.97.23.89 posites, single phase CoFe2 O4 and NiFe2 O4 nanoparticles sized ferrites and the corresponding metal/ferrite nanoSat,by24taking Oct 2009 21:11:35 were also synthesized by the same procedure, composites are shown in Figure 1. For NFO, all the the metal to Fe ratio as 1:2 and a glycine/nitrate ratio of observed reflections correspond to the spinel structure 0.67. For convenience, the as-synthesized CoFe2 O4 and of NiFe2 O4 . On the other hand, along with the reflecNiFe2 O4 are named as CFO and NFO and the composites tions from NiFe2 O4 , reflections from Ni metal are also Co/CoFe2 O4 and Ni/NiFe2 O4 are named as C-CFO and observed in the case of N-NFO (Ni/NiFe2 O4 ). Similarly, N-NFO, respectively. The main advantage of the present the XRD pattern of CFO showed reflections corresponding method of synthesis is that it is possible to synthesize to CoFe2 O4 and along with the reflections from CoFe2 O4 , metal oxide nanoparticles with controlled particle size by reflections from Co metal are also observed in the XRD varying the amount of glycine used for the synthesis.27 28 pattern of C-CFO. The XRD patterns of Ni and Co correMoreover, by controlling the oxidizer (nitrate) to fuel spond to the fcc phase of these metals. The mass fractions molar ratio, one can synthesize metal nanoparticles along of different crystalline phases present in a composite can with oxides in a single step. be approximately calculated from the ratios of the most Poly(vinylidene fluoride) (PVDF) was used as the polymer matrix in the present study, whose dielectric constant is around 10.24 PVDF is having a melting point of 170 C, upper service temperature of 130 C and very low thermal conductivity (30 × 10−4 K/cm). Composites were made with different polymer to magnetic nanocomposites ratio. The PVDF powder was ground thoroughly with CoFe2 O4 (CFO), Co/CoFe2 O4 (C-CFO), NiFe2 O4 (NFO) or Ni/NiFe2 O4 (N-NFO) separately and then blended. The mixture was poured into a toroidal mould (toroids for the permeability measurements) and into a disk mould (pellets for dielectric measurements), and hot pressed at 200 C for 20 min. Disks and toroids of C-CFO and N-NFO were also made by cold uniaxial pressing of the as-synthesized powders. The numbers in the sample codes of the polymer/ metal/ferrite nanocomposites (CFO-50, C-CFO-50, C-CFO-75, NFO-50, N-NFO-50, and N-NFO-75) indicate the weight percentage of the polymer used. The ferrite and metal/ferrite nanocomposites were characterized for their phase purity by powder X-ray diffraction (XRD) measurements with CuK radiFig. 1. Powder XRD patterns of NiFe2 O4 (NFO), Ni/NiFe2 O4 (N-NFO), CoFe2 O4 (CFO) and Co/CoFe2 O4 (C-CFO). ation ( = 15418 Å) using Ni filter (Philips, PW-1830). Sci. Adv. Mater. 1, 262–268, 2009

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Microstructures of the magneto-polymer nanocomposites obtained using a Leica Stereoscan-440 scanning electron microscope. Hot pressed samples were fractured to expose the inner portion and mounted on a specimen mounting stub. Silver paste was used to connect the sample to sample holder for electrical conduction. A thin layer of gold was coated on the surface of the sample to avoid charging of the specimen. Magnetization measurements at room temperature, up to a maximum magnetic field of 10 kOe, were made on a PAR EG&G 4500 vibrating sample magnetometer. For the dielectric measurements, silver paste was coated on the polished surfaces of the pellets to provide electrical contacts. Dielectric measurements were made on a General Radio LCR bridge model 1608-A. The complex permeability and quality factor have been determined using a HP 4342A Q-meter.

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intense peaks of the individual phases, or with more accuoften used to give comparable results.27 28 32 Hence, it is 29 racy by the least-squares fit method. The approximate reasonable to assume that the particle sizes of the differweight percentages of the metal and the corresponding ent samples and components are the same as the average ferrite are calculated from least-squares refinement of the crystallite size. Thus, X-ray diffraction studies show that XRD patterns using the PowderCell program.30 In the case the present glycine-nitrate combustion synthesis procedure of N-NFO the mass fractions of nickel metal and nickel gives metal/ferrite nanocomposites when the metal nitrates are taken in proper ratios. ferrite are calculated as approximately 50% each, corTypical microstructures of the polymer/metal/ferrite responding to the ratios of the starting metal ions used nanocomposites are shown in Figure 2. The Co/CoFe2 O4 for the synthesis. However, in case of the C-CFO, the and Ni/NiFe2 O4 particles are agglomerated and are dismass fractions of Co and CoFe2 O4 are obtained as 35% persed almost homogeneously in the polymer matrix. Both and 65%, respectively. Since no other impurity peaks are N-NFO and C-CFO formed cluster like particles within observed in the XRD pattern of C-CFO, it may be assumed the PVDF matrix, where individual particles are not disthat the ferrite phase formed is a Co-rich composition tinguishable. Since the metal and ferrite particles are not Co1+x Fe2−x O4 . capped, the agglomeration of the metal and ferrite partiThe reflections in all the XRD patterns are found to be cles in the nanocomposites is likely to affect the magnetic slightly broad and this is due to the smaller size of the characteristics of the nanocomposite. particles in the as-synthesized compositions. The average Figure 3 shows the magnetization (M) versus field (H ) crystallite size of each phase is calculated from X-ray line curves ofto: the different Co and Ni based nanocomposites, broadening, using the Scherrer formula, afterDelivered correctingby Ingenta 31 measured at room temperature. For CFO, the magnetifor the line broadening due to instrumental contribution. Guest User zation is not saturated at 10 kOe and the value of M The average crystallite sizes are obtained as 34 IP nm: for 94.97.23.89 (6621:11:35 emu/g) is less than the saturation magnetization of bulk CoFe2 O4 in CFO, 36 nm for NiFe2 O4 in NFO, 2924 nmOct and 2009 Sat, CoFe2 O4 (∼80 emu/g).33 On the other hand NFO shows 35 nm for CoFe2 O4 and Co phases in C-CFO, 35 nm and saturation of magnetization at 10 kOe, although the mag59 nm for NiFe2 O4 and Ni phases in N-NFO, respectively. netization is slightly lower than that reported for the bulk. The average crystallite size calculated from XRD data and The lower magnetization is due to the smaller particle size the particle sizes obtained from TEM measurements for of the ferrites. It is known that the saturation magnetization ferrite particles synthesized by the glycine-nitrate process

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Cross sectional micrographs of C-CFO-50 (a, b) and N-NFO-50 (c, d) under two different magnifications.

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Single Step Synthesis and Properties of M/MFe2 O4 and PVDF/M/MFe2 O4 (M = Co, Ni) Magnetic Nanocomposites (b)

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Fig. 4. Variation of the (a) magnetization at 10 kOe per gram of sample and (b) coercivity as a function of the polymer wt% in the magnetopolymer composites. The dashed and dotted lines are the expected magnetization for the C-CFO and N-NFO polymer composites, respectively. (c, d) Magnetization at 10 kOe per gram of the magnetic component as a function of the amount of magnetic component.

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along with CoFe2 O4 in C-CFO. Co metal has a higher saturation magnetization (∼156 emu/g) than that of cobalt ferrite. Lower magnetization is obtained for the polymer based nanocomposites and this is expected because of the non-magnetic nature of the polymer. Higher coercivities are obtained for C-CFO containing polymer compos– – ites. Similar changes in the magnetization and enhanced – coercivity are also observed for the Ni/NiFe2 O4 (N-NFO) – – and PVDF/Ni/NiFe2 O4 nanocomposites. Relatively higher – magnetization is observed for the Ni/NiFe2 O4 composite – – – – as compared to that of NiFe2 O4 , due to the presence of Ni metal in the composite. Fig. 3. Magnetization as a function of magnetic field, measured at room Figures 4(a) and (b) show the variation of the magtemperature, for (a) the as-synthesized CoFe2 O4 , Co/CoFe2 O4 , and their netization at 10 kOe and coercivity, respectively, as a polymer composites; (b) the as-synthesized NiFe2 O4 , Ni/NiFe2 O4 , and function of the polymer concentration in the compostheir polymer composites. The numbers correspond to the wt% of the polymer in the composites. ites. For both C-CFO and N-NFO based composites, the decrease in the magnetization is almost linear. The magnetizations of the polymer based C-CFO composites are decreases with decreasing particle size due to the increased to:that calculated based on the mass fraction of the lower than contribution from a magnetically dead layer onDelivered the surface by Ingenta Guest User metal/ferrite composite, whereas for N-NFO based comof a particle.34 Although the magnetizations of the differ: 94.97.23.89 posites, the magnetization decreases linearly as expected. ent samples are not saturated at 10 kOe, C-CFOIP shows Sat, to 24CFO. Oct 2009 The21:11:35 lower magnetization of the C-CFO/polymer composa relatively larger magnetization when compared ite is because of the fact that the magnetization is not This is due to the presence of Co metal nanoparticles

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saturated at 10 kOe and therefore it is not a true compariis likely to be due to the larger magnetocrystalline son. The lower magnetization can be explained in terms of anisotropies of both Co and CoFe2 O4 in C-CFO. Higher the larger magnetocrystalline anisotropy as evidenced from magnetocrystalline anisotropy leads to lower permeability the increased coercivity. The coercivity increases with the because of the constraints to domain wall motion in low increasing polymer content, but the increase is not linear. magnetic fields. There is an overall decrease in the permeIt may be seen that there are corresponding changes in ability for the polymer based composites when compared the coercivity and magnetization. For example, for C-CFO, to that of the metal/ferrite composites. This is similar to the coercivity is not much increased when the ferrite is that reported for many ferrite/polymer composites. For all 50% loaded whereas a large increase in the coercivity is the samples, permeability () increases with increasing obtained when the ferrite loading is reduced to 25% and a frequency. Higher values of the permeability are obtained corresponding decrease in the magnetization is observed. for C-CFO, as shown in Figure 5(a). Interestingly, is Similar changes are observed for N-NFO/polymer comfound to decrease with polymer loading. The decrease in posites also. Magnetization at 10 kOe calculated per gram the permeability with polymer loading can be explained of the magnetic component in the composites is comin terms of the increased coercivity. It can be seen from pared in Figures 4(c and d). These figures indicate that Figure 4(b) that these samples have higher coercivities for CFO and C-CFO/polymer composites, higher polymer (Hc ) where Hc increases with increasing polymer conloading leads to decreased magnetization at a given field tent. Higher coercivity means higher magnetocrystalline (at 10 kOe). This is due to the increased magnetocrysanisotropy energy of the system and the later is inversely talline anisotropy of the CFO-based polymer Delivered composites.by Ingenta proportional to: to . Therefore, permeability is reduced as For NFO based polymer composite, the saturated magthe polymer content is increased. Guest User netization is not changed after incorporation of NFO in The contribution to permeability of a material comes IP : 94.97.23.89 the polymer matrix whereas for N-NFO/polymer from the two different mechanisms of magnetization. Sat, compos24 Oct 2009 21:11:35 ite, the magnetization decreases drastically in the polymer These are the contributions from the domain wall motion matrix and is independ on the polymer content. and domain (spin) rotation. The permeability can be For magnetic materials, the magnetic anisotropy is expressed as = 1 + s + d , where s and d are the conknown to increase with decreasing particle size, due to tributions to susceptibility from spin rotation and domain the contribution from increased surface anisotropy. Apart wall motion.39 From the studies on MnZn and NiZn ferritefrom this, dipolar and exchange interactions between polymer composites, Tsutaoka has shown that the domain the particles also contribute to large magnetocrystalline wall component increases and then remains almost conanisotropy.35 The SEM micrographs show that the metal/ stant with decreasing the volume fraction of the ferrites.39 ferrite particles are highly agglomerated and this is likely The spin component shows a reverse trend where it to be the reason for the increased coercivity of the metal/ decreases at lower volume fractions of the ferrites. It has ferrite/polymer composites. The increased coercivity is due been shown that the permeability is suppressed by the to the increased magnetic anisotropy and this increases the demagnetizing field and is independent of the ferrites used. field required for the saturation of the magnetization so The variation in the permeability with the volume fracthat the measured magnetization at 10 kOe is lower. In the tion of the ferrite could be related to a gap parameter /D case of the simple ferrites (CFO and NFO), only one polywhere D is the average size of the ferrite particles and /2 mer based composition is studied. However, in this case is the average thickness of the polymer layer between the also, enhanced coercivity and reduced magnetization are particles. Also, the permeability is known to be proporobserved. Results similar to that observed in the present tional to the magnetization and decreases with increasing work have been reported for iron/iron-oxide/polymer anisotropy. Shen et al. found a decrease in the permeability composites,16 and Fe3 O4 -polymer composite.36 Enhanceof Ni/NiZn-ferrite/PVDF composite when the concentration of Ni metal is decreased and this has been explained ment of coercivity with increasing amount of polymer in terms of a double percolation effect.24 for the strontium hexaferrite-polyvinyl chloride compos37 ite system is reported. The results have been interpreted The normalized initial magnetization curves shown in in terms of the increased magnetic exchange interactions Figures 5(c and d) indicate that the magnetization at between the metal-ferrite particles as the mean interlow fields is reduced for the polymer composites. For particle distance is decreased. Similar behavior observed a magnetic material, the changes in the magnetization for a nickel ferrite-polymer composite has been explained occur through domain wall motion at low magnetic field in terms of the increased dipolar interactions between the strengths and domain rotation at higher fields leading to particles causing a slower decay of the remanence and magnetic saturation. As shown in Figure 5(c), the magnecoercivity.38 tization at low fields is decreased in the order C-CFO > C-CFO-75 > C-CFO-50 and the same trend is observed Figure 5 compares the permeability spectra of the diffor the permeability. Almost similar behavior is observed ferent nanocomposites. Relatively larger permeability is for N-NFO and its composites. Thus, a direct comparison obtained for N-NFO when compared to C-CFO. This 266

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Single Step Synthesis and Properties of M/MFe2 O4 and PVDF/M/MFe2 O4 (M = Co, Ni) Magnetic Nanocomposites (a)

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Delivered by Ingenta to: Guest User IP : 94.97.23.89 Sat, 24 Oct 2009 21:11:35

Fig. 5. Variation of permeability with frequency for (a) Co/CoFe2 O4 and its polymer nanocomposites and (b) Ni/NiFe2 O4 and its polymer nanocomposites. Variation of the normalized initial magnetization of (c) Co/CoFe2 O4 and (d) Ni/NiFe2 O4 and their polymer nanocomposites.

of the changes in the permeability and the initial magnetization at low magnetic fields indicates the contribution from domain wall motion to the permeability. The contribution from wall motion to the permeability is suppressed with increasing volume fraction of the polymer. The variation of the room temperature dielectric constant ( ) of the magneto-polymer nanocomposites, as a function of ac frequency, is shown in Figure 6. Differences in the concentrations of the inorganic materials or the polymers do not affect the dielectric properties of the

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C-CFO nanocomposites. decreases with increasing ac frequency. Maximum is seen for all samples at the lowest frequency studied (200 Hz). On the other hand, a noticeable difference can be seen in the case of N-NFO samples. The Ni/Ferrite/polymer nanocomposites show a slight increase in the permittivity value as compared to the ferrite/polymer composite. This is due to the presence of larger amounts Ni particles (evidenced from XRD measurements) and hence the corresponding increase in the conductivity.24 Though the same feature has not been observed in Figure 6(a), but the increase in at low frequency for the C-CFO samples can be accounted as due to the presence of cobalt metal. But the overall high value of permittivity for the N-NFO samples corroborates the higher amount of nickel metal in the N-NFO sample than that of cobalt in C-CFO samples. Similar trends of decreasing dielectric constant with increasing frequency have been reported for several polymer based magnetic composite systems.5 21 23–25

4. CONCLUSIONS

Fig. 6. Variation of dielectric constant with frequency for (a) PVDF/Co/ CoFe2 O4 nanocomposites and (b) PVDF/Ni/NiFe2 O4 nanocomposites.

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The present studies show that it is possible to synthesize two phase nanocomposites, consisting of nanoparticles of a metal and a corresponding metal ferrite, by a simple and single step process by the glycine nitrate autocombustion reaction. Preliminary studies were made on 267


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the magneto-polymer composites, where the metal/ferrite nanocomposites are embedded in a PVDF matrix by a simple hot-pressing method. The performance parameters obtained for these three-phase nanocomposite systems are comparable to the values reported in the literature for other similar systems.

Received: 25 September 2008. Accepted: 10 December 2008.

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