Iron Nanocomposite

May 2, 2002 - State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China,...
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Articles One-Step Synthesis of Cobalt-Phthalocyanine/Iron Nanocomposite Particles with High Magnetic Susceptibility Jian Guo Guan,*,†,§ Wei Wang,† Rong Zhou Gong,† Run Zhang Yuan,† Leong Huat Gan,‡ and Kam Chiu Tam§ State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China, Division of Chemistry, National Institute of Education, Nanyang Technological University, Nanyang Walk, Singapore, 637616, and Division of Manufacturing Engineering, School of Mechanical and Product Engineering, Nanyang Technological University, Nanyang Avenue, Singapore, 639798 Received August 9, 2001. In Final Form: March 6, 2002 Low-density cobalt-phthalocyanine (Co-Pc)/Fe nanocomposite particles with both adjustable electric and magnetic properties were synthesized using one-step thermolysis. The nanocomposite particles were fully characterized by Fourier transform infrared spectroscopy, X-ray diffraction analysis, scanning electron microscopy, high-resolution electron microscopy, and thermogravimetric-differential thermal analysis. The magnetic hysteresis loop and microwave electromagnetic parameters of the nanocomposite particles were measured. The magnetorheological (MR) properties of the MR suspensions based on the nanocomposite particles dispersed in methyl silicone oil were investigated as functions of weight percent of particles, magnetic field strength (H), temperature (T), and shear rate (γ). The results indicated that the Co-Pc/Fe nanocomposite consisted of micrometer-sized regular spheroids with hundreds of thousands of Co-Pccoated R-Fe nanoparticles on the inside and Co-Pc layers on the surface of the spheroids. It showed good characteristics of antioxidation and high magnetic susceptibility. For nanocomposites containing 82.7 wt % Fe, with increasing microwave frequency, the complex permittivity (r) gradually reduced while the real part minimum and imaginary part maximum of the complex permeability (µr) occurred. In comparison with conventional carbonyl iron powders, the density of the nanocomposite particles was much lower, and r decreased significantly while µr hardly changed. Magnetic field induced shear stress of the MR suspension based on nanocomposites with 89.7 wt % Fe was basically independent of T and γ but increased abruptly with increasing H. With increasing T, response time of the MR suspension to an external magnetic field seemed to decrease. The results suggested that the synthesized Co-Pc/Fe nanocomposite particles with different compositions could be utilized as an excellent microwave absorber and as dispersed particles of high-performance MR suspensions.

1. Introduction Nanocluster-based materials or nanocomposites have received significant attention in the past two decades because they exhibit unusual, even unique, properties (optic, electric, and magnetic) that are size-dependent and are different from those of the atomic and bulk counterparts. The most studied materials with size-dependent properties have been, of course, semiconductors and metals. For example, many previous papers1-5 have dealt * To whom correspondence should be addressed. E-mail [email protected]. † State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology. ‡ Division of Chemistry, National Institute of Education, Nanyang Technological University. § Division of Manufacturing Engineering, School of Mechanical and Product Engineering, Nanyang Technological University. (1) Wilcoxon, J. P.; Provencio, P. P. J. Phys. Chem. B 1999, 103, 9809. (2) Carpenter, E. E.; Sims, J. A.; Wienmann, J. A.; Zhou, W. L.; O’Connor, C. J. J. Appl. Phys. 2000, 87, 5615. (3) Martino, A.; Stoker, M.; Hicks, M.; Bartholomew, C. H.; Sault, A. G.; Kawola, J. S. Appl. Catal., A 1997, 161, 235. (4) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J.; Hajipanayis, G. C. J. Mater. Res. 1999, 14, 1542.

with the preparation and properties of magnetic metals and alloys as they are of practical and fundamental importance. Some4,5 reported that by changing the size of magnetic particles, the coercive force of these magnetic colloids could be changed while the high saturation magnetization was maintained. The potential use of magnetic colloidal particles lies in magnetic biological separation, microwave absorbers, magnetic fluids, magnetorheological (MR) suspensions, magnetic recording devices, giant magnetoresistors, catalysts, and so forth. However, these nanoparticles have the problems of poor processability and easy oxidation, combined with large density. These limit some of the above-mentioned applications. To exploit the full potential of the technological applications of these materials, it is important to address the above problems. As organic substances have low density, good processibility and chemical stability, scientists have been guided to integrate one or more inorganic nanoparticles with suitable organic substances (usually polymers), leading to a new special class of nanomaterials, namely, organic-inorganic hybrids or nanocomposites (5) Douglass, D. C.; Cox, A. J.; Bucher, J. P.; Bloomfield, L. A. Phys. Rev. 1993, B47, 12874.

10.1021/la011265+ CCC: $22.00 © 2002 American Chemical Society Published on Web 05/02/2002

Synthesis of (Co-Pc)/Fe Nanocomposite Particles

(OIHs). OIHs can take unique properties of the entrapped nanoparticles together with the existing qualities of the organic substance and even show novel properties due to synergistic effects derived from the interaction between them. Most of the works about OIHs focus on polymer nanocomposites in the form of films or bulks. There are two main techniques to prepare them. One is to disperse the preformed nanoparticles, usually after modification with suitable surfactants, in polymer or polymeric monomer solution. For example, Mark and his colleagues6 prepared monolithic poly(methyl methacrylate)/SiO2 nanocomposites by dispersing surface-modified silica nanoparticles in methyl methacrylate, followed by polymerization of the monomeric continuous phase. Tang et al.7 produced a superparamagnetic nanocomposite freestanding film with appreciable electrical conductivity, which was derived from the dispersion of ultrafine γ-Fe2O3 particles coated by anionic surfactants in the polyaniline solution. The film was found to possess a saturation magnetization of 25 emu/g γ-Fe2O3. Surface modification of nanoparticles used in this method, however, affects surface structure and makes surface and interface activity of the nanoparticles passive. The other technique is to directly create nanoparticles in a hydrocarbon solution containing the polymer or polymeric monomers. For example, Rajeshwar and co-workers8,9 succeeded in dispersing Pt nanoparticles in polypyrrole thin films using a modified electrochemical method. Sohn and Cohen10 obtained superparamagnetic free-standing copolymer films containing uniformly distributed magnetic iron oxide nanoparticles by static casting. Wizel et al.11 ultrasonically radiated solutions containing methylacrylate and Fe(CO)5 and obtained superparamagnetic nanocomposites containing amorphous metal nanoparticles. They reported that the nanocomposites have a magnetic moment of 1.5 emu/g at 15 kG. Recently, Pavel and Mackay12 succeeded in producing transparent cadmium sulfide nanoparticle/ polymer composites using a one-system reverse micellar synthesis. In contrast, there has been relatively little work on colloidal particles containing nanoclusters prior to Armes’ work13 in 1992. To the best of our knowledge, the stream of colloidal OIHs was pioneered by Kirkland14 and Iler et al.15 at Dupont, who reported the synthesis of uniform spherical polymer-silica composites with average diameters ranging from 500 up to 20 000 nm. Mann et al.16,17 have synthesized magnetite/ferritin and cadmium sulfide/ ferritin nanoparticles using a biomimetic approach. To obtain advanced materials with good processability, a number of papers reported synthesis of such conducting polymers as polyaniline or polypyrrole in the presence of (6) Pu, Z. C.; Mark, J. E.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1997, 9, 2442. (7) Tang, B. Z.; Geng, Y.; Lam, J. W. Y.; Li, B.; Jing, X.; Wang, X.; Wang, F.; Pakhomov, A. B.; Zhang, X. X. Chem. Mater. 1999, 11, 1581. (8) Bose, C. S. C.; Rajeshwar, K. J. Electroanal. Chem. 1992, 333, 235. (9) Chen, C. C.; Bose, C. S. C.; Rajeshwar, K. J. Electroanal. Chem. 1993, 350, 161. (10) Sohn, B. H.; Cohen, R. E. Chem. Mater. 1997, 9, 264. (11) Wizel, S.; Prozorov, R.; Cohen, Y.; Doron, A.; Shlomo, M.; Gedanken, A. J. Mater. Res. 1998, 13 (1), 211. (12) Pavel, F. M.; Mackay, R. A. Langmuir 2000, 16, 8568. (13) Gill, M.; Mykytiuk, J.; Armes, S. P.; Edwards, J. L.; Yeates, T.; Moreland, P. J.; Mollett, C. J. J. Chem. Soc., Chem. Commun. 1992, 108. (14) Kirkland, J. J. U.S. Patent No. 3,782,075, 1974. (15) Iler, R. K.; McQueston, H. J. U.S. Patent No. 4,010,242, 1977. (16) Wong, K. K. W.; Douglas, T.; Gider, S.; Awschalom, D. D.; Mann, S. Chem. Mater. 1998, 10, 279. (17) Wong, K. K. W.; Mann, S. Adv. Mater. 1996, 8, 928.

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a different metal or metal oxide resulting in a host of nanocomposites in stable colloidal form.18 Armes et al.19 first reported that this host of conducting polymer nanocomposites has unusual “raspberry” morphologies rich in inorganic component at the surface. Recently, they synthesized a colloidal dispersion of poly(4-vinylpyridine)/ silica nanocomposite particles in high yield by homopolymerizing 4-vinyl pyridine in the presence of an ultrafine silica sol using a free-radical initiator in aqueous media at 60 °C. Transmission electron microscopy (TEM) and aqueous electrophoresis measurements confirmed that the poly(4-vinylpyridine)/silica nanocomposite particles exhibited “currant-bun” particle morphologies with the surface polymer-rich.20 Hirai and Komasawa21 developed a simple method of in situ condensation polymerization in reverse micellar systems to prepare CdS/polyurethane nanocomposite particles. The integration of magnetic nanoparticles with conducting or semiconducting organic substances has been of particular interest.22-27 The fact that the coercive forces for magnetic nanoparticles with retention of high magnetization will be changed when the particle diameter decreases from the micrometer to the nanometer range has directed our research toward the design, preparation, and application of microsized organic-inorganic nanocomposite particles with adjustable electrical and magnetic properties.28-30 In this paper, cobalt(II)-phthalocyanine (Co-Pc) and liquid Fe(CO)5 are employed as the starting materials to synthesize Co-Pc/Fe nanocomposite particles by one-step thermolysis. Co-Pc is chosen because it is one of the most studied classes of organic functional materials. It shows interesting optical and electronic properties, which make it suitable for a wide range of applications.31,32 It is an 18-π electronic aromatic system. π-Orbital conjugation provided by the benzo moieties and the orbital perturbation caused by the nitrogen atoms at the four meso positions (Chart 1) are expected to have a strong interaction with the nanoparticles. This is helpful for hindering the agglomeration of nanoparticles and the combustion or oxidation of magnetic iron nanoparticles. The results show that those low-density magnetic nanocomposite particles with both high saturation magnetization and high stability can be prepared using the simple one-step thermolysis as long as an appropriate organic (18) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (19) Gill, M.; Armes, S. P.; Fairhurst, D.; Emmett, S. N.; Idzorek, G.; Pigott, T. Langmuir 1992, 8, 2178. (20) Percy, M. J.; Barthet, C.; Lobb, J. C.; Khan, M. A.; Lascelles, S. F.; Vamvakaki, M.; Armes, S. P. Langmuir 2000, 16, 6913. (21) Hirai, T.; Komasawa, I. J. Mater. Chem. 2000, 10, 2234. (22) Pokhodenko, V. D.; Krylov, V. A.; Kurys, Y. I.; Posudievsky, O. Y. Phys. Chem. Chem. Phys. 1999, 1, 905. (23) Butterworth, M. D.; Armes, S. P.; Simpson, A. W. J. Chem. Soc., Chem. Commun. 1994, 2129. (24) Butterworth, M. D.; Bells, S. A.; Armes, S. P.; Simpson, A. W. J. Colloid Interface Sci. 1996, 183, 91. (25) Nguyen, M. T.; Diaz, A. Adv. Mater. 1994, 6, 858. (26) Wan, M.; Zhou, W.; Li, J. Synth. Met. 1996, 78, 27. (27) Guan, J. G.; Zhao, S. L.; Hu, H. J.; Yuan, R. Z. Chinese Patent No. 99 1 16542.X, 1999. (28) Huang, J. Ph.D. Dissertation, Wuhan University of Technology, Wuhan, China, 1998. (29) (a) Guan, J. G.; Huang, J.; Zhao, S. L.; Yuan, R. Z. Proceedings of the 6th International Conference on Electro-Rheological Fluids, Magneto-Rheological Suspensions and Their Applications; Tao, R., Ed.; World Scientific: Singapore, 2000; p 53. (b) Guan, J. G.; Huang, J.; Zhao, S. L.; Yuan, R. Z. Int. J. Mod. Phys. B 2001, 15, 599. (30) Huang, J.; Jiang, D. S.; Guan, J. G.; Yuan, R. Z. J. Wuhan Univ. Technol., Mater. Sci. Ed. 2000, 15, 19. (31) (a) Emmelius, M.; Pawlowski, G.; Vollman, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 1445. (b) Law, K.-Y. Chem. Rev. 1993, 89, 2652. (c) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (32) (a) Andre´, J.-J.; Simon, J. Molecular Semiconductors; SpringerVerlag: Berlin, 1985. (b) Nazeeruddin, M. K.; Humphry-Baker, R.; Gra¨tzel, M.; Murrer, B. A. Chem. Commun. 1998, 719.

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Chart 1. Structure of Cobalt-Phthalocyanine

Figure 1. X-ray diffraction pattern for Co-Pc/R-Fe nanocomposite particles.

substance is chosen. The synthesized Co-Pc/Fe nanocomposite particles have distinctive currant-bun particle morphologies with the surface organic-substance-rich. Changing compositions of the nanocomposites can control its magnetic properties. Uses of the synthesized Co-Pc/ Fe nanocomposite particles with different compositions as an excellent microwave absorber and as dispersed particles of high-performance MR suspensions are exploited. 2. Experimental Section 2.1. Synthesis of Co-Pc/Fe Nanocomposite Particles. Into a closed system containing a solution of 5.0 g of cobaltphthalocyanine (Co-Pc) ultrasonically dispersed in 250.0 mL of N,N-dimethyl formamide (DMF), 50.0 mL of liquid iron pentacarbonyl (purchased from Shaanxi Siping Chemical Engineering Factory) was added. The mixture was mechanically stirred for several minutes, after which the temperature was raised to above 120 °C to initiate the decomposition of liquid iron pentacarbonyl while continuously stirring. The carbon monoxide produced was absorbed by KMnO4 aqueous solution. Decomposition lasted for several hours until no observable gas was produced at the mixture refluxing temperature of above 120 °C. The suspension was then cooled to room temperature and centrifuged at 1500 rpm for 6 min to obtain the expected Co-Pc/Fe nanocomposite particles. The product was dried under vacuum at a temperature of not more than 100 °C for 24 h. The Co-Pc/Fe nanocomposite particles with different compositions were available by altering feed formulations, for example, Co-Pc concentration in DMF solution. 2.2. Characterization of Co-Pc/Fe Nanocomposite Particles. The crystal form of Fe in the obtained Co-Pc/Fe nanocomposite particles was identified at room temperature with Cu KR radiation and a graphite monochromator on a Rigaku D/MAX-RB diffractometer. A scan rate of 1°/min was used, and the scan interval was 0.01°/2θ. The Fourier tranform infrared (FTIR) spectra were recorded on a Nicolet 60 SXB spectrophotometer using a KBr disk. The microstructure and diameter of the nanocomposite particles were observed and estimated, respectively, using a high-resolution electron microscope, model H-900NAR (Hitachi Co. Lt., Japan), and a JEOL JSM-35CF. For high-resolution electron microscopy (HREM), the powder samples were prepared as described in a patent33 as follows: powder samples were encrusted using electrocoppering and then double polished and attenuated with ions. Differential thermal analysis (DTA) and thermal gravimetry (TG) were carried out with the sample in air using a TAS-100 thermal analyzer in a temperature range of 20-900 °C and at a scanning rate of 10 °C/min; from this, the content of Fe component in the particles was calculated according to a method described previously.34 Nanocomposite particle density was determined in a 10 mL pycnometer using paraffin oil as a dispersing medium. 2.3. Measurement of Electrical, Magnetic, and Magnetorheological Properties. According to the GB3217-92 procedure,35 the magnetic hysteresis loops of the Co-Pc/Fe nano(33) Fang, K. M. Chinese Patent No. 85 1 05876.1, 1985. (34) Armes, S. P.; Gottesfeld, S.; Beery, J. G.; Garzon, F.; Agnew, S. F. Polymer 1991, 32, 2325.

composite particles were determined by a YEW 3257-15 dc magnetic hysteresis tracking instrument. The coercive force and specific saturated magnetization of the samples were calculated from the magnetic hysteresis loops. Microwave electromagnetic parameters including complex permeability (µr ) µ′ + iµ′′) and complex permittivity (r ) ′ + i′′) in the X band of a frequency of 8.2-12.4 GHz were determined with a waveguide cell in a Hewlett-Packard 8510C vector network analyzer and calculated from the reflection parameter, S11, and the transmission parameter, S12, according to Hewlett-Packard’s standard procedures. Pressing the well-dispersed mixtures containing 75% (by weight) Co-Pc/Fe nanocomposite particles and 25% (by weight) solid paraffin as an adhesive made the rectangle samples with a thickness of 4.0 mm. The MR suspension was prepared by grinding in a mortar a weighed amount of the Co-Pc/Fe nanocomposite particles and dispersing well into a weighed amount of methyl silicone oil of 1.32 mPa s at a temperature (T) of 25 °C. The field-induced shear stress (∆τ) and time response of the MR suspension to an external magnetic field were determined with a self-made coaxial cylindrical rotational EMR rheometer29 with a computer for data acquisition. However, the maximum magnetic field upon application of samples to be determined, which was produced by an iron core coil linking with a stabilized current supply, was not more than 120 kA/m due to a limitation of the design. The zerofield viscosity (η0) was determined at 25 °C with an NDJ-11 coaxial cylindrical viscometer (Chengdu Instrument Factory, Sichuan, China). The centrifugation stability (or settlement stability) of the MR suspension was estimated by the volume percentage of the upper pure liquid layer of the MR suspension after it was centrifuged at 1000 rpm for different periods of time.

3. Results and Discussion 3.1. Microstructure of Nanocomposite Particles. At the experimental temperature, the liquid iron pentacarbonyl was decomposed into Fe nanoparticles, which agglomerated to form an organic-inorganic nanocomposite with diameters in the micrometer range only after their complex formation with cobalt-phthalocyanine happened owing to the highly reactive surface of Fe nanoparticles. The FTIR spectrum of Co-Pc/Fe nanocomposite particles is somewhat different from that of Co-Pc, despite no characteristic absorption peaks of metal iron. The two stretch vibration peaks of CdN at 1469 cm-1 and CdC at 1425 cm-1 in Co-Pc shift to 1399 and 1363 cm-1 in Co-Pc/Fe nanocomposites, respectively. This result suggests that the electrons in CdC and CdN of cobalt-phthalocyanine form chemical bonds with the active surface of Fe nanoparticles so that the electrons in CdC and CdN are dispersed and red shifts of the absorption peaks occur. Figure 1 shows the X-ray diffraction pattern for the Co-Pc/Fe nanocomposite particles. Obviously, the main peak at 2θ ) 45° was attributed to the (110) peak of the R-Fe crystallite (d ) 2.003 nm). No (35) Gong, R. Z.; Guan, J. G.; Fang, L.; Yuan, R. Z. Chin. J. Chem. Phys. 2000, 13, 354.

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Figure 2. SEM micrograph (a) and HREM micrographs (b,c) of Co-Pc/Fe nanocomposite particles.

peak in the pattern corresponds to ferric or ferrous oxide. Figure 1 also indicated that the Co-Pc in the nanocomposite particles is amorphous because there are not any peaks at 2θ E 30°. This is similar to our previous report,30 where metal-Pc in metal-Pc/Fe3O4 nanocomposites was also confirmed to be amorphous. As the (110) peak of the R-Fe crystallite widened significantly, the average diameter of the R-Fe crystallite was calculated to be about 15 nm from the half-height-width of the diffraction peak according to the Scherrer equation. Figure 2a shows a typical SEM micrograph of the CoPc/Fe nanocomposite particles containing 89.7% (by weight) R-Fe. The Co-Pc/Fe nanocomposite particles have an almost regularly spherical shape and a relatively smooth surface. The average diameter is about 1.2 ( 0.2 µm. The density of the nanocomposite particles was determined to be 3.66 g/cm3, which is much lower than that of pure iron powders, about 7.8 g/cm3. The value of 3.66 g/cm3 is also much lower than 5.46 g/cm3, which is based on the assumption that the density of the nanocomposite particles obeys a linear law of mixture, and the density values of pure iron and organic Co-Pc are 7.8 and 1.52 g/cm3, respectively. Obviously, the lower density of the Co-Pc/Fe nanocomposite particles may be attributed to their special structure and organic Co-Pc in the nanocomposites. This can be further confirmed from the HREM micrograph. Figure 2b,c shows a HREM micrograph of a thin section cut from the Co-Pc/Fe nanocomposite particles. The nanocomposite particles have a shellcore structure, and the observed morphologies are reminiscent of the typical currant bun20 rather than the raspberry.19 The R-Fe nanoparticles derived from the thermal decomposition of the liquid Fe(CO)5 were primarily encapsulated into the interior of composite particles. Figure 2b indicates that the surface of the nanocomposite particle is completely covered by Co-Pc layers due to the existence of the white ring, which represents the organic coating Co-Pc because it has a lower conductivity than metal iron. The average thickness of Co-Pc layers on the surface of nanocomposite particles, differing with feed concentration of Co-Pc before thermolysis occurs, is certified to be about 100 nm by HREM. In contrast, previous studies18,19 indicated that the distinctive raspberry particle morphologies were formed in many conducting polymer/metal oxide nanocomposite systems, in which metal oxide nanoparticles were present mainly on the surface of the nanocomposite particles rather than within the interior, especially when inorganic oxide contents were higher than 50% by mass. Both the Fe cluster nanoparticles and aggregates of the cluster nanoparticles were completely covered by Co-Pc during the formation of the Co-Pc/Fe nanocomposite particles. From the 15 nm nanoparticle size obtained from the X-ray diffraction (XRD) data in Figure 1, it was calculated that a micrometer-sized magnetic nanocomposite particle consists of hundreds of thousands of R-Fe nanoparticles on the inside dispersed in organic Co-Pc. A separation of about 5 nm between R-Fe nanoparticles, indicating the presence of a thinner layer, possibly a monolayer of Co-

Figure 3. TG and DTA curves of Co-Pc/Fe nanocomposite particles.

Pc on R-Fe nanoparticles, was observed in Figure 2c, which implicates an almost molecular level dispersion of the organic component. This preserves a barrier that prevents Fe nanoparticles from growing into bigger particles. The diameters of the Fe cluster nanoparticles ranged from 10 to 15 nm. This is almost in accordance with the result calculated from the XRD. 3.2. Antioxidation Properties of Nanocomposite Particles. Antioxygenation and content of the organic substance (Co-Pc) in the Co-Pc/Fe nanocomposite particles were analyzed and operated with the sample in air using TG and DTA with a TAS-100 thermal analyzer. Figure 3 indicates that weight change of Co-Pc/Fe nanocomposite particles started from 150 °C and ended at 545 °C. First, the weight goes down due to loss of the organic component, and later weight goes up due to oxidation of iron. This suggests that thermal stability and antioxidative stability of the nanocomposite particles is in excess of 150 °C, whereas Fe cluster nanoparticles may self-ignite in air. It is reasonable to suggest that the superior antioxygenation of nanocomposites may be attributed to the currant-bun morphology and covering organic layers of cobalt-phthalocyanine. However, comparing with standard data of thermal analysis for normal iron powders, we find that the temperature of the Feoxidizing exothermic reaction of the nanocomposite particles is still 80 K lower than that of normal micrometersized Fe powders. And the nanocomposite particles will completely oxidize into Fe2O3 once the Co-Pc layers are lost at a temperature of above 314 °C. 3.3. Electric and Magnetic Properties of Nanocomposite Particles. The magnetic hysteresis loop curves of Co-Pc/Fe nanocomposite particles recorded in a YEW 3257-15 dc magnetic hysteresis tracking instrument demonstrated that Co-Pc/Fe nanocomposites would easily reach their saturation magnetic induction upon application of a relatively low magnetic field strength. This suggested that using these materials as dispersed particles in MR suspensions would be useful for improving MR properties. For Co-Pc/Fe nanocomposite particles containing 89.7 wt % R-Fe crystallites, the specific saturation magnetization values and coercive forces were calculated from the magnetic hysteresis loop curves to be 76.3 A m2/kg and 4.15 kA/m, respectively. This confirmed that the magnetic susceptibility of the Co-Pc/Fe nanocomposites was by far higher than that of other organic nanocomposites containing ferrite nanoparticles reported in the literature.7,11,22-26 In addition, our preliminary experiments showed that the saturation magnetization

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Figure 5. The effect of the external magnetic field strength on the field-induced shear stress for a MR suspension of 25 wt % Co-Pc/Fe nanocomposite particles in methyl silicone oil at T of 25 °C and γ of 500 s-1. Figure 4. Influence of f on complex permittivity and complex permeability of Co-Pc/Fe nanocomposite particles containing 82.7 wt % R-Fe component.

of Co-Pc/Fe nanocomposite particles depended on their compositions and increased with the weight percent of R-Fe nanoparticles. This was in agreement with what was expected. Figure 4 shows the influence of microwave frequency (f) on the complex permeability (µr) and complex permittivity (r) of Co-Pc/Fe nanocomposite particles containing 82.7 wt % R-Fe crystallites in the entire X band. It indicates that both the real part (′) and imaginary part (′′) of the complex permittivity show an overall tendency of gradual decrease with increasing f; that is, ′ reduces from 7.0 to 5.5 and ′′ from 1.5 to 0.26. This relationship between complex permittivity and f is exactly what a superior microwave absorber requires. Increasing f, µ′′ increases before the existence of a maximum and then decreases and fluctuates around 0.5; at the same time, µ′ at first decreases abruptly from 1.7 and then fluctuates around 1.5. In comparison with the values for conventional carbonyl iron powders,36 which are among the most popular microwave absorbers in radar absorbing coating (RAC),37-39 both ′ and ′′ of Co-Pc/Fe nanocomposite particles decrease significantly more. µr is approximately equal to that of carbonyl iron powders except that the maximum µ′′ shifts to 9 GHz from 10 GHz of carbonyl iron powders. These results encourage one to speculate that using the synthesized Co-Pc/Fe nanocomposite particles as microwave absorbers will improve matching microwave impedance of RAC with that of free space and will further enlarge microwave absorption bandwidth of RAC. This is because we notice that until now, the scarcity of materials having high µr and low r has been preventing the achievement of thin, lightweight RAC with broadband absorption.40 As the permeability tangent δ () µ′′/µ′) represents the magnetic loss value of a material, the fact that µ′ of the nanocomposites abruptly decreases with increasing f with the maximum µ′′ shifting to low frequency (36) Fang, L.; Gang, R. Z.; Guan, J. G.; Yuan, R. Z. Acta Phys.-Chim. Sin. 2001, 17, 364. (37) Oveniariste, K. Microwave Absorbent Material; Science Publishing House: Moscow, 1982. (38) Ruan, S. P.; Xu, B. K.; Suo, H.; Wu, F. Q.; Xiang, S. Q.; Zhao, M. Y. J. Magn. Magn. Mater. 2000, 212, 175. (39) (a) Whitney, L. R.; Hoyle, C. D.; Seemann, R. W. U.S. Patent No. 4,814,546, 1989. (b) Dawson, M. H.; Suffredini, L. P.; O’Neal, J. R. U.S. Patent No. 4,173,038, 1979. (40) Wu, M. Z.; He, H. H.; Zhao, Z. S.; Yao X. J. Phys. D: Appl. Phys. 2000, 33, 2398.

implicates that the nanocomposites have a maximum microwave absorption at a lower frequency than the conventional iron powders. 3.4. MR Properties of Co-Pc/Fe Nanocomposite Particles. The desired suspended particles in MR suspensions should, in general, meet the requirements of high saturated magnetization, narrow hysteresis loop (at best, no coercive force), low density matching that of the dispersing medium, and good dispersibility and nonabrasion.41,42 However, most of the MR suspensions reported in the literature to date were based upon the micrometersized inorganic particles such as iron, cobalt, nickel, its alloys, or ferrite and have the problems of poor stability, slow response rate, and/or incomplete irreversibility. This is because these magnetic particles, almost all with a density of more than 7-8 g/cm3, have both relatively great coercive forces and poor compatibility with the dispersing medium. The pure magnetic metal particles suspended in MR suspensions when used over a period of time in air are prone to oxidation, hence deteriorating MR properties. The high magnetic susceptibility and low coercive force of the Co-Pc/Fe nanocomposite particles containing 89.7 wt % Fe as well as their relatively low density prompt their use in preparing high-performance MR suspensions. To investigate MR properties of the Co-Pc/Fe nanocomposite particles, MR suspensions with different weight percentages were prepared by triturating and ultrasonically dispersing the Co-Pc/Fe nanocomposite particles in methyl silicone oil with a density of 0.98 g/cm3. Figure 5 shows the effect of the external magnetic field strength (H) on magnetic field induced shear stress (∆τ) for the MR suspension of 25% (by weight) Co-Pc/Fe nanocomposite particles in methyl silicone oil at shear rate (γ) of 500 s-1. Obviously, the MR suspension based on Co-Pc/Fe nanocomposite particles shows significant MR effect. Its ∆τ increases linearly with increment of H. The relationship between ∆τ and H fits well with the equation of ∆τ/Pa ) 17.5H/kA m-1 - 58 at H ranging from 30 to 110 kA/m. Figure 6 shows the response characteristics of the shear stress for the MR suspension of 40 wt % Co-Pc/Fe nanocomposite particles at different temperatures upon application of a stepwise external magnetic field. It indicates that the MR suspension responds to the external magnetic field soon, though it responds more slowly to the external field than ER fluids, which are often (41) Guan, J. G. Advanced Materials of Electrorheological Fluid. In Advanced Materials and Technology of Functional Polymer; Hu, H. J., He, T. B., Eds.; Chemical Industry Press: Beijing, 2000; Chapter 19. (42) Huang, J.; Guan, J. G.; Chen, W. Y.; Yuan, R. Z. J. Wuhan Univ. Technol., Mater. Sci. Ed. 1998, 13, 1.

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Figure 6. The response of shear stress for a MR suspension of 40 wt % Co-Pc/Fe nanocomposite particles in methyl silicone oil at different temperatures of 25 °C (O), 40 °C (b), 60 °C (4), and 85 °C (9) upon application of a stepwise magnetic field. γ ) 500 s-1, H ) 100 kA/m.

Figure 7. The dependence of the shear rate on the field-induced shear stress for the MR suspensions of Co-Pc/Fe nanocomposite particles with different weight percentages of 10 wt % (O) or 40 wt % (4) in methyl silicone oil at T of 25 °C and H of 100 kA/m.

Table 1. Influence of the Weight Percentage of the Co-Pc/Fe Nanocomposite Particles on the Properties of the MR Suspensions at a Temperature of 25 °C

Table 2. Influence of the Weight Percentage of Co-Pc/Fe Nanocomposite Particles on the Maximum Field-Induced Shear Stress and Its Corresponding Shear Rate of MR Suspensionsa

centrifugation stability, % w, %

∆ τ/Paa

η0/mPa s

5 min

35 min

90 min

10 25 40

970 1760 2470

15.3 121 435

0 0 0

3.5 3.0 2.0

13.0 12.0 9.0

a

H ) 100 kA/m, γ ) 500 s-1.

a

w, %

τmax/Pa

γmax/s-1

10 25 40

970 1800 2560

500 640 740

H ) 100 kA/m, T ) 25 °C.

considered to respond to an external electrical field within milliseconds. Figure 6 also shows that the time response of the MR suspension to the external magnetic field seems to decrease with increasing temperature up to 85 °C. It is reasonable to suggest that increasing temperature is helpful, to some extent, for expediting the formation of particle chains or columns parallel to the direction of the external magnetic field. This may possibly be attributed to decreasing coercive force of magnetic nanocomposite particles at higher temperature. The value of ∆τ of the MR suspension, however, increases only a little or hardly changes with increasing temperature. This implies that the kind of MR suspension based on Co-Pc/Fe nanocomposite particles is almost temperature independent. This is the desired property for superior MR suspensions. From Table 1, it is obvious that increasing the weight percentage of nanocomposite particles (w) used in the MR suspensions, ∆τ, zero field viscosity (η0), and centrifugation stability for MR suspensions all increase greatly, though η0 increases more rapidly by far than ∆τ. In an attempt to improve the sedimentation stability and redispersibility of MR suspensions, such colloidal ceramic particles as silica and fibrous fiber were respectively incorporated into MR suspensions in the pioneering works, but with only limited success.43,44 A highly stable MR suspension based on nano-ferrite particles through the use of polyelectrolytes has been developed by Kormann et al.,45 but its τy values are relatively small and are strongly temperature de-

pendent. However, there has been no significant progress in developing stable MR suspensions with still high τy. But Phule et al. have recently prepared a relatively stable and easily redispersible MR suspension based on mesoscale carbonyl iron and nickel ferrite particles suspended in polar organic liquids using colloidal and other additives.46 Table 1 shows that the MR suspensions have satisfactory stability, even after a rigorous centrifugation was applied. In fact, there is a long-term stability of antisedimentation of the present MR suspension system. No phase separation was observed even for the MR suspensions that have been kept quiescently at normal room temperature for about 6 months. The MR effect also did not change after the MR suspension has been kept for a long time. The superior stability of the MR suspension is believed to be related to both the low density of the Co-Pc/Fe nanocomposite particles and the strong interaction between methyl silicone oil and the Co-Pc/Fe nanocomposite particles. Figure 7 illustrates the dependence of the shear rate on field-induced shear stress for a MR suspension of CoPc/Fe nanocomposite particles with different weight percentages. ∆τ for all the suspensions containing different contents of dispersed particles is not always constant but increases gradually before leveling off and finally gradually decreases with increasing shear rate. However, as shown in Table 2, the tiny maximum peak (τmax) and the shear rate corresponding to the maximum peak (γτmax)

(43) Kordonski, W. I.; Gorodkin, S. R.; Novikova, Z. A. Proceedings of the 6th International Conference on Electro-Rheological Fluids, Magneto-Rheological Suspensions and Their Applications; Nakano, M., Koyama, K., Eds.; World Scientific: Singapore, 1998; p 535. (44) Chen, Z. Y.; Tang, X.; Zhang, G. C. Proceedings of the 6th International Conference on Electro-Rheological Fluids, MagnetoRheological Suspensions and Their Applications; Nakano, M., Koyama, K., Eds.; World Scientific: Singapore, 1998; p 486.

(45) Kormann, C.; Laun, M.; Richter, R. J. Proceedings of the 5th International Conference on ER Fluids, MR Suspensions and Associated Technology; Bullough, W. A., Ed.; World Scientific: Singapore, 1996; p 362. (46) Phule, P. P.; Ginder, J. M. Proceedings of the 6th International Conference on Electro-Rheological Fluids, Magneto-Rheological Suspensions and Their Applications; Nakano, M., Koyama, K., Eds.; World Scientific: Singapore, 1998; p 445.

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Guan et al.

Figure 8 shows the relationship between shear stress and time of MR suspensions containing various contents of Co-Pc/Fe nanocomposite particles in methyl silicone oil under a stepwise external magnetic field. It is obvious that the MR suspension has the characteristics of a rapid response and superior reversibility of changes in viscosity induced by the external magnetic field, and these are little related to the concentration of the nanocomposite particles. Further experiments also show that the response time is always lower than 100 ms and is almost independent of shear rate and external magnetic field strength. 4. Conclusion

Figure 8. Switching characteristics of the MR suspensions of Co-Pc/Fe nanocomposite particles with different weight percentages of 10 wt % (b), 25 wt % (O), or 40 wt % (2) in methyl silicone oil at T of 25 °C and stepwise H of 100 kA/m.

increase with increasing the content of dispersed nanocomposite particles. This indicates that MR suspensions with a high concentration of dispersed particles will give rise to a stronger MR effect at a higher shear rate. Under the influence of an external magnetic field, Co-Pc/Fe nanocomposite particles will very quickly be magnetized and form particle chains and columns along the direction of the external magnetic field, producing the field-induced shear stress. When a shear stress is applied to the MR suspension, the structure of the particle chains or columns will certainly change. For example, particle chain or column deformation comprising dislocation, distortion, and even breakage may occur, which obviously depends on not only the concentration of the dispersed particles but also the shear rates. The field-induced shear stress of the MR suspension may possibly decrease unless the rate of destroying particle chains or columns matches with that of reforming them. It is reasonable to suppose that the higher concentration of the dispersed particles would be helpful to reform particle chains or columns along the direction of the external magnetic field within a shorter time period. On the other hand, a higher shear rate would increase the rate of destroying the chain or column structure. Therefore, the results presented in Figure 7 and Table 2 are in accordance with chain-structure model. Table 2 also indicates that a MR suspension containing 40 wt % Co-Pc/Fe nanocomposites shows the peak maximum of 2560 Pa at γ of 740 s-1 when an external magnetic field strength of 100 kA/m is applied.

Cobalt(II)-phthalocyanine/iron nanocomposite particles with high magnetic susceptibility have been prepared by organic-inorganic in situ nanocomposite technology. From the results of HREM, the diameters of R-Fe nanoparticles filling in Co-Pc/Fe nanocomposite particles are found to be in the range of 10-15 nm. The surfaces of the nanocomposite particles are completely covered by Co-Pc layers with a thickness varying from 50 to 150 nm. The structure of the organic layers covering surfaces of both the Fe nanoparticles and nanocomposite particles gives rise to a density much lower than that of iron powders and their good dispersability, for example, in adhesives of RAC or an organic medium of MR suspensions. In comparison with conventional carboxyl iron powders, the complex permittivity of the nanocomposite particles, which gradually reduces with increasing f in the X band, decreases significantly while the complex permeability remains almost unchanged. This result suggests that CoPc/Fe nanocomposite particles have a potential application of microwave-absorbing coatings. The MR suspensions containing Co-Pc/Fe nanocomposite particles in methyl silicone oil with good stability show rapid, reversible, and significant changes in shear stress upon application of an external magnetic field. The field-induced shear stress of MR suspensions is found to be almost independent of temperature and shear rate but increases with increasing external magnetic field strength and the concentration of the nanocomposite particles in methyl silicone oil. The response rate of the MR suspensions to an external magnetic field seems faster at higher temperature. Acknowledgment. Dr. J. G. Guan expresses thanks for the financial support from Chinese National Natural Science Foundation Grants No. 29674021, 59832090, and 29904005 and from Nanyang Technological University. LA011265+