Shell Nanocomposite Based on the Local Polarization and Its

Core/Shell Nanocomposite Based on the Local Polarization and Its Electrorheological Behavior .... Min Wook Kim , Il Jae Moon , Hyoung Jin Choi , Yongs...
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Langmuir 2005, 21, 6553-6559

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Core/Shell Nanocomposite Based on the Local Polarization and Its Electrorheological Behavior Baoxiang Wang and Xiaopeng Zhao* Institute of Electrorheological Technology, Department of Applied Physics 141#, Northwestern Polytechnical University, Xi’an, 710072, P. R. China Received November 7, 2004. In Final Form: April 24, 2005 Aimed at the increase of electrorheological effect, a novel core/shell material was prepared by the combination of mechanochemical activity and sol-gel technique. The structure analyses X-ray diffraction, Fourier transform infrared spectrometry, scanning electron microscopy, and energy-dispersive spectrometry showed that a modified kaolinite/titanium oxide nanocomposite consisted of the mechanochemically activated kaolinite/NaCl complex coated by titanium oxide. A distinct enhancement of the electrorheological activity was found by using such particles dispersed in silicone oil than those of kaolinite or titanium oxide alone under a direct current electric field. Modified kaolinite/titanium oxide electrorheological fluid has a larger dielectric constant enhancement ∆′, and a strong interfacial polarization occurs with a clear dielectric loss peak around 2 kHz. Doping NaCl into the core (kaolinite) by the mechanochemical activation and limiting the transferring of the ions by the shell (titanium oxide) may increase the interfacial polarizability of particles and induce a high electrorheological effect.

Introduction Electrorheological (ER) fluids are suspensions of dielectric particles in a nonconducting liquid, and they exhibit drastic changes in their rheological properties, which include a large increase in apparent viscosity and the formation of reversible suspension microstructures under an applied electric field.1-7 ER characteristics find practical applications in many devices, for example, actuators, shock absorbers, active devices, human muscle simulators, ER tactile displays, photonic crystal, and various other control systems.8 Particle polarization is now widely thought to be responsible for the interaction forces that lead to the rheological change of ER fluid.9-12 The parameters in connection with particle polarization, such as high dielectric constant and dielectric loss and suitable conductivity, have been accepted as the basic factors that dominate the ER effects. Furthermore, the dielectric and conduction properties are closely related to the molecular or crystal structures of the materials. Thus, it is possible to modify dielectric properties to increase ER activity by designing the molecular and crystal structures of ER materials.13-16 * To whom should be correspondence should be addressed. Telephone: 86-29-88495950. E-mail: [email protected] (1) Block, H.; Kelly, J. P. J. Phys. D: Appl. Phys. 1988, 21, 16611677. (2) Halsay, T. C. Science 1992, 258, 761-766. (3) Sim, I. S.; Kim, J. W.; Choi, H. J.; Kim, C. A.; Jhon, M. S. Chem. Mater. 2001, 13, 1243-1247. (4) Tajiri, K.; Ohta, K.; Nagaya, T.; Orihara, H.; Ishibashi, Y.; Doi, M.; Inoue, A. J. Rheol. 1997, 41, 335-341. (5) Klingenberg, D. J.; Zukoki, C. F. Langmuir 1990, 6, 15-17. (6) Park, J. H.; Lim, Y. T.; Park, O. O. Macromol. Rapid Commun. 2001, 22 (8), 616-619. (7) Kim, J. W.; Liu, F.; Choi, H. J.; Hong, S. H.; Joo, J. Polymer 2003, 44 (1), 289-293. (8) Hao, T. Adv. Mater. 2001, 13 (24), 1847-1857. (9) Sung, J. H.; Choi, H. J.; Sohn, J. I.; Jhon, M. S. Colloid Polym. Sci. 2003, 281 (12), 1196-1200. (10) Lengalova, A.; Pavlinek, V.; Saha, P.; Quadrat, O.; Kitano, T.; Steiskal, J. Eur. Polym. J. 2003, 39 (4), 641-645. (11) Lim, Y. T.; Park, J. H.; Park, O. O. J. Colloid Interface Sci. 2002, 245 (1), 198-203. (12) Yin, J. B.; Zhao, X. P. Chem. Mater. 2002, 14, 4633-4640. (13) Zhao, X. P.; Yin, J. B. Chem. Mater. 2002, 14, 2258-2263.

Introducing the ion into the particles as a means to increase the polarizable ability and the ER effect has been researched in the electrorheological field. For example, Na+ was contained in the zeolite, and the ion compound as an additive was added into the ER fluid.17,18 However, the ion introduced by these methods is easily moved under the electric field; moreover, the movement of the ion is occurring not only inside the particle but also among the particles. So the leaking current intensity will increase rapidly, and even electric breakdown happens. All of these will decrease the ER effect. It is a research difficulty to fix the ions so that the ions could only move inside the particle. On the other hand, a core/shell material, as a material that is tightly associated with the interfacial polarization, had been also researched, for example, metal particles having a nonconductive metal oxide shell, polyaniline (PANI)-coated polymethyl methacrylate, etc.19,20 On the basis of these results, the advantages of core/shell material and ion polarization are integrated and a novel core/shell material, by using the combination of the mechanochemical method and sol-gel technique, was designed to largely enhance the ER effect of kaolinite. Kaolinite, as opposed to montmorillonite, has very low cation exchange capacities. Although some methods are adopted to improve its ER effect, its ER activity is still weak.21,22 So the choice of mechanochemical activated kaolinite with alkali halide (for example, NaCl) is aimed at modifying the dielectric and polarizable properties of kaolinite to enhance the ER activity. Studies of the (14) Duan, X.; Zhao X. P. J. Colloid Interface Sci. 2002, 251 (2), 376380. (15) Yin, J. B.; Zhao, X. P. J. Colloid Interface Sci. 2003, 257 (2), 228-236. (16) Sohn, J. I.; Cho, M. S.; Choi, H. J.; Jhon, M. S. Macromol. Chem. Phys. 2002, 203 (8), 1135-1141. (17) Gamota, D.; Filisko, F. E. J. Rheol. 1991, 35, 399-415. (18) Conrad, H.; Sprecher, A. F.; Choi, Y.; Chen, Y. J. Rheol. 1991, 35, 1393-1410. (19) Cho, M. S.; Cho, Y. H.; Choi, H. J.; Jhon, M. S. Langmuir 2003, 19, 5875-5881. (20) Davis, L. C. J. Appl. Phys. 1993, 73, 680-683. (21) Wang, B. X.; Zhao, X. P. J. Mater. Chem. 2002, 12 (10), 28692871. (22) Wang, B. X.; Li, J.; Zhao, X. P. Acta Chim. Sinica 2003, 61 (2), 240-244.

10.1021/la047261x CCC: $30.25 © 2005 American Chemical Society Published on Web 06/01/2005

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Figure 1. Schematic illustration of fabrication process of a novel core/shell material and its polarization behavior.

mechanochemical method with treatment with clay have been attached with importance recently in much of the literature, especially applying to kaolinite.24-29 Such studies showed that the surface areas of the materials were increased, which resulted in an increase of the ion exchange capacity in the clay. Frost et al. showed that the dry grinding of kaolinite, termed mechanochemical activation, resulted in the delamination of kaolinite as a function of grinding time.30 The mechanochemical technique was also applied in the study of the intercalation of alkali halides, such as CsF, CsCl, and CsBr, into kaolinite.31 The results that mechanochemical activation causes significant changes in the kaolinite structure in surface energy and in chemical reactivity are prompting us to use mechanochemically activated kaolinite in the study of electrorheological fluid. We have primarily focused on the preparation of new inorganic/inorganic core/shell particles for unique anhydrous ER materials that consist of mechanochemically activated kaolinite (core) and titanium oxide (shell) to improve the ER effect. The rheological properties are studied as a function of electric field strength, shear rate, and the mass percent content of the titanium oxide component. Design and Polarization Behavior of a Novel Core/Shell Material The electrorheological material containing limited ions may induce the local-area polarization of ions, thus enhancing the polarizability of particles, especially interfacial polarizability. The limited ions can be introduced into the ER material by intercalation, doping, coating, etc. This provides a novel and operable path for the design of the ER material. In this novel core/shell material, the ions are introduced into the core (kaolinite) by a mechanochemical method, so the polarizability of the core is increased. The sol-gel method is adopted to form the shell of titanium oxide, so the movement of ions (such as Na+, Cl-, etc.) is effectively limited by the shell. The interfacial polarization of the core/shell material is enhanced such that it is suitable for the ER effect. The schematic illustration of the design of this core/shell material and its polarization behavior are shown in Figure 1. (23) Wang, B. X.; Zhao, X. P. J. Mater. Chem. 2002, 12 (6), 18651869. (24) Frost, R. L.; Kristof, J.; Mako, E.; Martens, W. N. Langmuir 2002, 18 (17), 6491-6498. (25) Lapides, I.; Yariv, S.; Golodnitsky, D. J. Therm. Anal. Calorim. 2002, 67 (1), 99-112. (26) Frost, R. L.; Kristof, J.; Mako, E.; Kloprogge, J. Langmuir 1999, 15, 8787-8794. (27) Ovadyahu, D.; Shoval, S.; Lapides, I.; Yariv, S. Thermochim. Acta 1996, 282-283, 369-383. (28) Yariv, S.; Lapides, I. J. Mater. Synth. Process. 2000, 8 (3-4), 223-233. (29) Frost, R. L.; Horvath, E.; Mako, E.; Kristof, J.; Cseh, T. J. Colloid Interface Sci. 2003, 265 (2), 386-395. (30) Frost, R. L.; Mako, E.; Kristof, J.; Horvath, E.; Kloprogge, J. J. Colloid Interface Sci. 2001, 239, 458-466. (31) Michaelian, K. H.; Zhang, S. L.; Yariv, S.; Lapides, I. Appl. Clay Sci. 1998, 13, 233-243.

Experimental Section Materials. The kaolinite sample (Al2Si2O5(OH)4) employed in this work was from Shanghai, China. It was received as a finely divided white powder of high purity. The specific surface area is 26 m2/g. The kaolinite used to prepare the nanocomposites was used without further purification. Tetrabutyl titanate (Ti(OBu)4, Cheng Du Associated Chemical Institute, China), dimethyl sulfoxide (DMSO, Beijing Yatai Co., China), sodium chloride (NaCl, Tan Jin Chemical Third Co., China), and waterfree alcohol (AnHui Te Jiu Factory, China) were used as received. Synthesis of Modified Kaolinite/Titanium Oxide Nanocomposite ER Fluids. Kaolinite (10 g) was mixed with 200 mL of DMSO and stirred at 80 °C. After 8 h, the resulting material (kaolinite/DMSO) was filtered and the wet clay was air-dried for a week. Then an amount of 10 g of kaolinite/DMSO composite was placed into 200 mL of sodium chloride solution at 25 °C. The weight ratio of the kaolinite/DMSO composite to sodium chloride is 1:2. The mixture was stirred using a magnetic stirrer for 8 h. After that, the solution was heated to 90 °C with stirring until all the solvent had evaporated. The sample was dried and ground for 2 h in a ball mill, and a grayish-white powder was obtained. The powder was pressed into i11.3 mm × 5.0 mm spherical disks. The disks were heated at 100 °C for 0.5 h, 200 °C for 0.5 h, and 300 °C for 2 h. After thermal treatment, the disks were ground for 3 h in a ball mill. The product was washed with denionized water repeatedly until no further chloride was detectable by the silver nitrate solution. Afterward, the kaolinite/ NaCl (referred to as “modified kaolinite” in this work.) composite particles was achieved by sequentially drying in a vacuum oven at 80 °C for 2 h, then grinding for 2 h in a ball mill and drying in a vacuum oven at 80 °C for 2 h again. An amount of 3 g of modified kaolinite composite was mixed with 15 mL of water-free alcohol and stirred for 2 h at 25 °C to get a uniform suspension. At the same time, 7 mL of tetrabutyl titanate solution was dispersed in 7 mL of water-free alcohol. Then the diluted tetrabutyl titanate was slowly added by dropping into the suspension of kaolinite/NaCl composite and further stirred for 5 h at 25 °C. At last, 5 mL of water-free alcohol solution and 0.2 mL of deionized water were added slowly to suspension. The suspension was further stirred for 5 h at 25 °C. Then the suspension was aged overnight (10 h) at room temperature to obtain precipitates of modified kaolinite coated with titanic gel. The precipitates were carefully dehydrated in a vacuum oven for 4 h at 80 °C and 2 h at 100 °C to form a loose dry powder. The modified kaolinite/titanium oxide composite so obtained was then crushed in a mortar. The silicone oil was first dried at 100 °C for 2 h, and then modified kaolinite/titanium oxide composite ER fluids (the particle fractions were 25 and 20 vol %) were prepared by dispersing the composite particles in the silicone oil under irradiation from an infrared lamp. The properties of the silicone oil were as follows: (f ) 2.60-2.80, σf ) 10-12-10-13 S/m, F ) 0.975-0.985 g‚cm-3, η ) 500 mPa‚s, 25 °C).32 Simultaneously kaolinite (10 g) was reacted using the above procedure (not including reaction with sodium chloride) to obtain the kaolinite/titanium oxide composite particle. Kaolinite/ titanium oxide, pure kaolinite, and pure titanium oxide were further dehydrated in a vacuum for 4 h at 80 °C and 2 h at 100 °C for the preparation of ER fluids and then were mixed quickly with silicone oil at a volume fraction of 25%. Characterization. X-ray diffraction (XRD) patterns were obtained with a Riguku diffractometer (D/III-γA, Japan), using (32) Zhao, X. P.; Wang B. X.; Zuo, Z. Y. Chinese Patent 03114668.8, 2003.

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Figure 2. XRD patterns of the raw kaolinite (a), modified kaolinite (b), titanium oxide (c), and modified kaolinite/titanium oxide composite (d). Cu KR radiation with a scanning rate of 4 deg min-1. All measurements were taken using a generator voltage of 40 kV and a current of 20 mA. Fourier transform infrared (FTIR) spectra between 400 and 4000 cm-1 were obtained by an EQUINOX55 FTIR spectrometer (Bru¨cker Co., Germany) using 16 averaged scans at 4 cm-1 resolution. The solid samples were prepared as KBr pellets. Morphological study was carried out with a JSM5800 scanning electron microscope operating at 20 kV. With this aim, particles were suspended in water-free alcohol with manual stirring, deposited by casting directly in the copper sample holder, and dried at room temperature for some hours. After that, they were coated with a thin gold film by sputtering to avoid charge buildups because of their low conductivity. In addition, the particles of kaolinite and modified kaolinite/titanium oxide were also pressed into i11.3 mm × 2.0 mm spherical disks under 12 MPa pressure. Then the disks were broken, and a fresh crosssection was obtained. The cross-section was sputter-coated with a thin gold film in order to do a morphological study on the crosssection. Energy dispersive spectrometry (EDS) patterns were obtained by Link ISIS (Oxford Co., England), using an SiLi detector. It was used to do element analyses (starting with boron, since for elements of lower atomic weight large errors are incurred). Furthermore, the dielectric relaxation spectra of all prepared ER fluids were examined using the HP 4284A Precision LCR meter. The frequency of ac electric fields ranged from 20 to 1 MHz. The shear stress of ER fluids was measured by a parallel plate force transducer (shear rate is about 5 s-1; transducer from China). A rotary viscometer (NXS-11A; China; the gap between the outer cup and the inner bob is 2 mm) and a high-voltage DC power source (GYW-0/0; China, 0-10 kV) were used to research the rheological properties of the ER fluids (shear rate range is from 0 to 105 s-1). The ER fluid was placed into the gap between the stationary cup and the rotating bob. The static yield stress was read from the controlled shear stress mode measurement. The ER fluid is sheared by an applied mechanical torque until the particle chain structure is broken so that slipping occurs between the cup and bob. Thus, the shear rate is observed when the flow of the ER fluid starts. The viscometer equipped with a temperature controller was used to measure the temperature dependence of the shear stress, and the thermal rate was ca. 1 °C‚min-1.

peak of kaolinite (0.715 nm).33 The effect of grinding causes the diminution of the d001 spacing (corresponding to 1/2 of the raw kaolinite). The loss of intensity of the d001 peak means that the stacking between the kaolinite layers is disrupted and partly lost. The mechanochemical treatment has partly broken the hydrogen bonding between adjacent kaolinite layers. The peak width (as FWHM) of the d001 peak for the mechanochemically activated kaolinite/NaCl complex is increased to 0.424 from 0.329 of raw kaolinite. The broadening of the peak may be connected to the reduction of crystallite size. Figure 2b also shows the loss of intensity in the second- and third-order diffractions between 20° 2θ and 25° 2θ and between 35° 2θ and 40° 2θ.34 However, the peaks of NaCl phase did not appear in the Figure 2b; this may indicate that NaCl is doped into the kaolinite. Figure 2c is the XRD pattern of the dry-gel particle of titanium oxide prepared by the sol-gel technique. A low broad peak occurred between 20° 2θ and 30° 2θ. Furthermore, there are no other peaks shown in Figure 2c. All these data indicate that titanium oxide is amorphous. From Figure 2d the XRD patterns of the modified kaolinite/titanium oxide composite shows that the coat of titanium oxide has further decreased the intensity of the d001 peak and that of other peaks such as the second- and third-order diffractions between 20° 2θ and 25° 2θ and between 35° 2θ and 40° 2θ. Figure 3 shows the FTIR spectra of the raw kaolinite (a), modified kaolinite (b), modified kaolinite/titanium oxide (c), and titanium oxide (d). The IR absorption bands of kaolinite at 3695, 3669, 3654, and 3621 cm-1 are attributed to O-H stretching vibrations. Those at 1114 and 1033 cm-1 are attributed to Si-O stretching vibrations, and the hydroxyl deformation modes observed at 937 and 913 cm-1 are attributed to the inner-surface and inner hydroxyls, respectively.35,36 After mechanochemical treatment, the change can be seen in the Figure 3b. The 3695, 3669, and 3654 cm-1 bands attributed to O-H stretching vibration bands of the interlayer surface of layered silicates are varied. The 3669 and 3652 cm-1

Results and Discussion Structure Characteristics. The X-ray diffraction spectra of kaolinite (a), modified kaolinite (b), titanium oxide (c), and modified kaolinite/titanium oxide composite (d) are illustrated in Figure 2. The characteristic maximum of raw kaolinite was observed at 12.6° (very intense, sharp and narrow), which corresponds to the basal spacing d001

(33) Frost, R. L.; Kristof, J.; Kloprogge, J. T.; Horvath, E. Langmuir 2000, 16 (12), 5402-5408. (34) Frost, R. L.; Mako, E.; Kristof, J.; Horvath, E.; Kloprogge, J. T. Langmuir 2001, 17 (16), 4731-4738. (35) Kristof, J.; Frost, R. L.; Felinger, A.; Mink, J. J. Mol. Struct. 1997, 410-411, 119-122. (36) Frost, R. L.; Kristof, J.; Paroz, G. N.; Tran, T. H.; Kloprogge, K. T. J. Colloid Interface Sci. 1998, 204, 227-236.

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Figure 3. FTIR spectra of the raw kaolinite (a), modified kaolinite (b), titanium oxide (c), and modified kaolinite/ titanium oxide composite (d).

Figure 4. SEM images of kaolinite and modified kaolinite/ titanium oxide composite: (a) kaolinite; (b) modified kaolinite/ 34%titanium oxide composite.

disappeared, and the 3695 cm-1 was diminished. In harmony with the decrease in intensity of the hydroxyl stretching vibration bands, the intensity of the kaolinite hydroxyl deformation modes decreased after the mechanochemical treatment; in particular, the 937 cm-1 also disappeared. Furthermore, the Si-O stretching vibration was shifted to lower frequencies from 1114 to 1104 cm-1. Figure 3c is the IR spectra of pure titanium oxide. From 900 to 400 cm-1 the very strong and broad peak is attributed to Ti-O band sorption. The 3399 cm-1 peak is attributed to O-H stretching. The peak at 2958 cm-1 is the C-H stretching vibration. The peaks at 1463 and 1377

Wang and Zhao

cm-1 are -CH2- and -CH3- deformation vibration, respectively. The 1039 cm-1 peak is the C-O stretching vibration of butyl alcohol. All these data show that titanium oxide should contain some residual butyl alcohol. The IR spectra of the modified kaolinite/titanium oxide composite in Figure 3d show that the peaks at 3399, 2960, 1463, 1383 cm-1, etc. are the vibrations of residual trace butyl alcohol. From 900 to 400 cm-1 the very broad peak is attributed to Ti-O band absorption. However, the peaks at 756 and 698 cm-1 attributed to Al-OH deformation vibration and the peaks at 539 cm-1 attributed to Al-O deformation vibration are all decreased. This may indicate that Al-O-Ti band sorption may exist and there are strong interactions between AlO2(OH)4 octahedral sheets of modified kaolinite and titanium oxide. The morphology of pure kaolinite and modified/titanium oxide composite is illustrated in Figure 4. It can be seen that kaolinite (a) is composed of small platelets. However, the modified kaolinite/34%titanium oxide composite (b) revealed significant morphological differences. On the surface of kaolinite layer there exist fine grayish particles. These nanoscale particles and their clumps are titanium oxide, which is formed on the coating shell. Figure 5 shows the scanning electron microscopy (SEM) results for a sample disk. The difference is distinctive between that of pure kaolinite (Figure 5a) and that of the modified kaolinite/34% titanium oxide composite (Figure 5b). The photos reveal that the edge of kaolinite is unclear because of the coating of titanium oxide and grinding. Furthermore, the surface distribution of the Ti element (shown as red dots) about the cross-section of the modified kaolinite/titanium oxide nanocomposite is shown Figure 5c). It also shows that titanium oxide is dispersed at nanoscale into the composite. The results from element analysis of kaolinite, modified kaolinite, and modified kaolinite/titanium oxide composite were examined. The chemical components of pure kaolinite are mainly O, Al, Si, Ti, Fe, etc. The element mass fraction is O/Al/Si ) 51.6:22.5:25.5, which is close to the ideal composition Al2Si2O5(OH)4. The main elements of modified kaolinite are O, Al, Si, Na, Cl, etc. The mass fraction of NaCl in the modified kaolinite is 1.04%. The results also show that the main elements of modified kaolinite/ titanium oxide composite are O, Al, Si, Ti, Na, Cl, C, etc. This composite through thermal treatment (200 °C for 0.5 h, 300 °C for 0.5 h, and 550 °C for 2 h) could be transformed into a modified kaolinite/TiO2 composite, and the latter shows that the C element disappears because of calcinations. However, the mass fraction of the Na

Figure 5. SEM images of the fresh cross-section of sample: (a) kaolinite; (b) modified kaolinite/34%titanium oxide composite; (c) the distribution of Ti element on the surface of the composite (Ti shown as red dots).

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Figure 7. Dynamic shear stress of modified kaolinite/34 wt % titanium oxide composite (20 vol % in silicone oil) ERF as a function of shear rate under various electric fields.

Figure 6. Shear stress (a) and leaking current density (b) of ERFs under different electric field strengths.

element is increased and that of the Cl element is decreased compared to that of the former. This may indicate that the Cl elements are mainly adsorbed into the interlayer of kaolinite and are easy to diminish by calcination. So by the combination of the mechanochemical method and the sol-gel method, a certain amount of ion is limited into the core by the shell and provides the material basis for the local-area polarization. Electrorheological Properties. Figure 6 shows the behavior of the shear stress of the modified kaolinite/ titanium oxide ERF, modified kaolinite ERF, titanium oxide ERF, and kaolinite ERF with an increase of the DC electric field. The particle fraction is 25 vol % in silicone oil. The shear stress is weak in pure kaolinite ERF and titanium oxide ERF. The ER fluid of the modified kaolinite/ 34 wt % titanium oxide composite particle, however, displays notable ER effects due to a synergetic effect. The shear stress of modified kaolinite/34 wt % titanium oxide ERF is about 9.5 kPa at 4 kV/mm, which is 11.5 times that of pure kaolinite ERF and 2.2 times that of kaolinite/ titanium oxide ERF. Because no ions are introduced into the kaolinite/titanium oxide core/shell material, its leaking current density is very low (5 µA/cm2 under 4 kV/mm in Figure 6b) and thus the ER effect is relevantly low. On the other hand, if the modified core (modified kaolinite) is not coated, the ions in the core may freely migrate among the particles under the electric field. The leaking current density of modified kaolinite ERF is enormously increased

(as high as 85 µA/cm2 under 4 kV/mm); even electric breakdown occurs, and thus, the ER effect of modified kaolinite is still low. So we adopt the mechanochemical method to modify the core and the sol-gel method to form the shell. Not only is the ER effect largely improved (9.5 kPa) but also the leaking current intensity of modified kaolinite/34 wt % titanium oxide ERF is suitable for the ER effect (10 µA/cm2 under 4 kV/mm). The intrinsic reasons for these improvements are the introduction of limited ions and the increase of local-area polarizability. The shear stresses of kaolinite/34 wt % titanium oxide composite ERF (20 vol % in silicone oil) as a function of shear rate under various electric fields are shown in Figure 7. The suspension had good rheological properties in the range of shear rates used; the shear stress of the suspension increases with the shear rate. In the absence of an electric field, the flow behavior of suspensions made of modified kaolinite/titanium oxide composite only shows a slight departure from Newtonian fluid. In the presence of an electric field, we can observe Bingham plastic behavior in the modified kaolinite/titanium oxide composite suspension, which is the typical rheological characteristic of ERFs under an electric field.37-40 In addition, the ER effects of modified kaolinite/titanium oxide composite ER fluids show a strong dependence on the content of titanium oxide in Figure 8. There seems to be an optimum content of titanium oxide for maximizing the shear stress of ER fluids (under γ˘ ) 5 s-1, 25 vol % in silicone oil). The ER fluid of modified kaolinite/34 wt % titanium oxide composite has the highest value of shear stress among the ER fluids of five modified kaolinite/ titanium oxide composites. Furthermore, the shear stress of the modified kaolinite/titanium oxide ER fluid starts to decline with a further increase of the titanium oxide content. Temperature Effects of ERFs. The temperature effects of modified kaolinite/titanium oxide composite ERF, kaolinite/titanium oxide ERF, kaolinite ERF, and titanium oxide ERF are shown in Figure 9 (E ) 1 kV/mm, γ˘ ) 1.6 s-1, 20 vol %). The shear stress of modified kaolinite/ titanium oxide composite ERF increases with an increase of temperature at first. Then at a critical temperature it starts to drop. Its maximum stress appears at around 90 (37) Chin, B. D.; Park, O. O. J. Rheol. 2000, 44 (2), 397-412. (38) Ginder, J. M.; Cecio, S. L. J. Rheol. 1995, 35 (1), 211-235. (39) Cho, M. S.; Choi, H. J.; Ahn, W. S. Langmuir 2004, 20, 202-207. (40) Sung, J. H.; Choi, H. J.; Jhon, M. S. Mater. Chem. Phys. 2002, 77, 778-783.

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Figure 8. Shear stress of modified kaolinite/titanium oxide composite ERF as a function of the content of titanium oxide.

Figure 9. Temperature effects of modified kaolinite/titanium oxide composite ERF, kaolinite/titanium oxide ERF, kaolinite ERF, and titanium oxide ERF (E ) 1 kV/mm, γ˘ ) 1.6 s-1).

°C, and even at 110 °C it remains at a significant value that only change 8% compared with that at 90 °C,which also is far larger than the shear stress at 25 °C. As far as titanium oxide ERF and kaolinite ERF are concerned, their operating temperature range is rather narrow. With the variation of temperature, the shear stresses of titanium oxide ERF and kaolinite ERF were changed only slightly and declined with an increase of the temperature under low shear stress. Furthermore, Figure 9 shows that the largest shear stress shifts to lower temperature (45 °C). These results show that the temperature dependence of modified kaolinite/titanium oxide composite ERF has shown obvious optimization. Its operating temperature has broadened within the range 25-110 °C, and the temperature effect was obvious improved, which is also better than that of kaolinite/titanium oxide ERF (maximum at about 80 °C). Dielectric Properties of ER Fluids. The particle polarization controlling the ER response depends on the dielectric properties of the dispersions. The dependence of dielectric properties on electric field frequency is presented in Figure 10. We noticed that ∆′ values (∆′ ) ′100Hz - ′100kHz) of pure titanium oxide and kaolinite ER fluid were small and that there was not a clear dielectric relaxation between 20 Hz and 1 MHz. Through mechanochemical activation and coating, slow polarization was induced and charge carriers were enhanced, which help to induce ER enhancement. Comparing modified kaolinite/titanium oxide with single component or kaolinite/titanium oxide, we find that modified kaolinite/ titanium oxide has a larger dielectric constant enhancement ∆′ and a strong interfacial polarization occurs with a clear dielectric loss peak around 2 kHz (fmax). This clearly

Figure 10. Dielectric spectra as a function of electric field frequency: (a) dielectric constant of ERFs; (b) dielectric loss factor of ERFs.

shows the contributions from doping NaCl by the mechanochemical activation on the enhancement of interfacial polarization. It was well-known that two important parameters in interfacial polarization, including the difference in dielectric constant ∆′ at 102 and 105 Hz related to the magnitude of polarization and a local maximum of dielectric loss factor between 102 and 105 Hz related to the proper polarization response, are responsible for a “good” ER effect.41-44 The large ∆′ and proper polarization response in this modified kaolinite/titanium oxide ER fluid induce a strong and stable interaction between particles for high ER activity. Furthermore, Ikazaki and Hao et al. had experimentally and theoretically determined that the maximum value of the dielectric loss tangent of a dispersed particle should be larger than 0.1 as a criterion for the ER effect, which is concluded as follows:

tan δ >

sp - ∞p > 0.1 ∞psp

(1)

sp and ∞p are the dielectric constants of the particle at the low frequency and high frequency, respectively.45,46 Our experiment is also consistent with that theory. We consider that the doping of NaCl into the core (kaolinite) (41) Klingenberg, D. J.; Zukoski, C. F. Langmuir 1990, 6, 15-24. (42) Hao, T.; Kawai, A.; Ikazaki, F. Langmuir 2000, 16, 3058-3066. (43) Hao, T.; Kawai, A.; Ikazaki, F. Langmuir 1998, 14, 1256-1262. (44) Ikazaki, F.; Kawai, A.; Uchida, K.; Kawakami, T.; Edamura, K.; Sakurai, K.; Anzai, H.; Asako, Y. J. Phys. D: Apply. Phys. 1998, 31, 336-347. (45) Hao, T.; Kawai, A.; Ikazaki, F. Langmuir 1999, 15, 918-921. (46) Hao, T.; Kawai, A.; Ikazaki, F. J. Colloid Interface Sci. 2001, 239 (1), 106-112.

Core/Shell Nanocomposite

by the mechanochemical activation, the limiting of the transfer of the ions by the shell (titanium oxide), and the higher concentration of charge carriers at the interface or surface regions may have increased the interfacial polarizability of particles under high electric field and induced high ER effect. Conclusions A novel kind of modified kaolinite/titanium oxide nanocomposite, as an anhydrous ER material, was prepared by the combination of mechanochemical activity and the sol-gel technique. The material was composed of a core of mechanochemically activated kaolinite/NaCl composite and a shell of amorphous titanium oxide. The suspension made of this material dispersed in silicone oil shows a strong ER activity due to a synergetic effect. Its shear stress could be up to 9.5 kPa at 4 kV/mm, which is 11.5 times that of pure kaolinite ERF and 2.2 times that of kaolinite/titanium oxide ERF. Furthermore, the working temperature range of this suspension was found to be increased from 25 to 110 °C.

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Moreover, the characteristics of the ER fluids were explained by assuming that a mechanochemically activated kaolinite/NaCl core interfered with the interfacial polarization of the amorphous titanium oxide shell under an applied electric field, and this was subsequently confirmed by analysis of the dielectric spectra for the ER suspensions. So the modified kaolinite/titanium oxide nanocomposite as a novel core/shell material helps to achieve the conclusion that the movement of ions is limited inside the shell and gives rise to local polarization, which is beneficial to the ER effect. Acknowledgment. This work was supported by the National Natural Science Foundation of China for Distinguished Young Scholar under Grant No. 50025207, the National Natural Science Foundation of China under Grant No. 50272054, the “863” Foundation under Grant No. 2001AA327130, and the Doctor’s Foundation from Ministry of Education of China. LA047261X