Enhanced Electrorheological Performance of Nb-Doped TiO2

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Enhanced Electrorheological Performance of Nb Doped TiO2 Microspheres Based Suspensions and Their Behavior Characteristic in Low Frequency Dielectric Spectroscopy Xiaosong Guo, Yulu Chen, Ming Su, Dong Li, Guicun Li, Chengdong Li, Yu Tian, Chuncheng Hao, and Qingquan Lei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08155 • Publication Date (Web): 16 Nov 2015 Downloaded from http://pubs.acs.org on November 23, 2015

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ACS Applied Materials & Interfaces

Enhanced Electrorheological Performance of Nb Doped TiO2 Microspheres Based Suspensions and Their Behavior Characteristic in Low Frequency Dielectric Spectroscopy Xiaosong Guo, Yulu Chen, Ming Su, Dong Li, Guicun Li, Chengdong Li, Yu Tian, Chuncheng Hao* , Qingquan Lei Laboratory of Functional and Biological Nanomaterials, College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao, China. *E-mail: [email protected] Fax: 86-532-84022814; Tel: 86-532-84022632 ABSTRACT Titanium dioxide and Nb-doped titanium dioxide microspheres with the same size were fabricated by a simple sol-gel method and the formation mechanism of Nb-doped titanium dioxide microspheres was proposed. Titanium dioxide and Nb-doped titanium dioxide microspheres were adopted as dispersed materials for electrorheological (ER) fluids to investigate the influence of the charge increase introduced by Nb doping on the ER activity. The results showed that Nb doping could effectively enhance the ER performance. Combining with the analysis of dielectric spectroscopy, it was found that the interface polarization of Nb

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doped TiO2 ER fluid was larger than that of TiO2 ER fluid, which might be caused by more surface charges in Nb-TiO2 microspheres due to Nb5+ doping, resulting in enhancement of electric field force and strengthening of fibrous structure. In addition, by comparing and analyzing the permittivity curves of Nb-TiO2/LDPE solid composite and Nb-TiO2/silicone oil fluid composite, it could be concluded that the enhancement of permittivity in the low frequency resulted from the increase of the order degree of dispersed particles in ER fluid rather than from the quasi-dc (QDC) behavior. Moreover, the absolute value of slope of permittivity curves (Κ) at 0.01 Hz could be utilized as the standard for judging the ability to maintain the chain-like structure. The relationships between polarizability of dispersed particles, dielectric spectrum, parameter Κ and ER properties were discussed in detail. KEYWORDS electrorheological response, Nb-doped, low frequency dielectric spectroscopy, secondary polarization, TiO2 microspheres

INTRODUCTION Electrorheological (ER) fluids composing of polarizable solid particles dispersed in an insulating medium are suspensions whose viscosity or shear strength can reversibly and quickly change under an applied external electric field.1-3 This phenomenon has attracted much attention from both academic and industrial communities for potential uses in various mechanical devices, such as clutches, valves and damping devices.4-6 However, until now, there are still several limitations for commercialization of ER fluids because of relatively low polarization force.7-9 In order to overcome the poor ER effect, many efforts have been made to improve the ER performance, such as doping rare-earth (RE) ions,10-12 changing the morphology13-14 and designing complex structure.15-19 Among these efforts, doping element is a simple and effective method to improve the ER activity. Nevertheless, this method often induces the change of the


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material composition, crystal structure, inter-microstructure, the size of particles, defects and charge carrier state. All these factors usually affect the ER performance and complicate the analysis of ER mechanism.10 Hence, designing experiment to minimize the factors impacting on ER activity is necessary for in-depth study of the intrinsic mechanism of the ER effect. Herein, titanium dioxide and Nb-doped titanium dioxide microspheres were fabricated by solgel method for studying the influence of the increase of charges introduced by doping Nb on the ER activity. Nb-doped titanium dioxide microspheres were chosen for three reasons. First, compared with large-radius rare-earth ions (approximate 100 pm), the ionic radius of Nb5+ (64 pm) is similar with that of Ti4+ (61 pm) so that defects arising from lattice expansion is relatively less, which can effectively reduce the effects of defect arising from lattice expansion on the ER performance.20 Second, Nb substitution on the Ti site creates a Nb5+ state located upon the conduction band minimum, which contributes more electrons to the unoccupied Ti 3d orbital without introducing impurity states in the bandgap, finally resulting in the enhancement of interface polarization.21-24 Third, Nb and Ti precursors have similar hydrolytic process and hydrolysis speed. Thus, Nb-doped titanium dioxide amorphous microspheres having the same size as pure titanium dioxide microspheres can be simply obtained through a simple sol-gel method by adjusting the molar ratio of Ti and Nb precursors. Therefore, the influence of particle size on the ER activity can be eliminated for reliable analysis of the influence of the charge increase on ER performance. Moreover, the dielectric spectroscopy of titanium dioxide and Nbdoped titanium dioxide microspheres based ER fluids were investigated in order to gain further insight into the effect of Nb doping. The detailed relationship between the variation of low frequency permittivity curve and behavior of particles in solid and fluid medium were analyzed,


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respectively. The results of this work might expand the application of the low frequency dielectric spectrum in analyzing the behavior of ER fluid.

RESULTS AND DISCUSSION Structure and Morphology

Figure 1. (a) low and (b) high magnification SEM image of TiO2 microspheres; (c) low and (d) high magnification SEM image of Nb-TiO2 microspheres. It is well known that the size, surface roughness and dispersion degree of particles play an important role in ER activity.25-27 The morphologies and microstructures of the samples were observed by scanning electron microscopy (SEM), as shown in Figure 1. The results present that the TiO2 and Nb-TiO2 microspheres almost have similar particle size and dispersity. The average


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diameter of the TiO2 and Nb-TiO2 microspheres is approximately 500 nm. Therefore, the factors like the size, surface roughness and dispersion degree of particles will no longer be considered in analysis of ER property. EDS Elemental Analysis

Figure 2. EDS elemental mapping of TiO2 microspheres. White, red and green colours represent O, Ti and Nb, respectively. Figure 2 shows representative EDS elemental mapping of the TiO2 microspheres. It reveals that the TiO2 samples are only composed of Ti and O, and no detectable Nb element is found. Meanwhile, the EDS elemental mapping of the Nb-TiO2 microspheres is shown in Figure 3. In comparison with TiO2 microspheres, it can be observed that the atomic ratio of Ti to Nb is 1:7 and the distribution of Nb is homogeneous throughout the entire of microspheres, which can be concluded that Nb dopant is uniformly distributed in TiO2 microspheres.


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Figure 3. EDS elemental mapping of Nb-TiO2 microspheres. White, red and green colours represent O, Ti and Nb, respectively. FTIR

Figure 4. The FTIR spectrum of (a) undoped and (b) Nb-doped TiO2 microspheres.


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FTIR spectrum of the as-synthesized microspheres is shown in Figure 4, in which the absorption at around 3422 cm-1 and 1630 cm-1 can be attributed to the O-H stretching vibrations and bending vibrations of hydroxyl groups and adsorbed water on the surface of amorphous microspheres.28-30 The CH2 asymmetric and symmetric stretching bands are located at 2918 and 2847 cm-1, respectively, which can be attributed to the alkyl chains of residual Hexadecylamine (HDA) in colloid microsphere.31-32 The broad intense band in the range of 450-700 cm-1 is due to the bending vibration of Ti-O bonds.33-36 Moreover, the stretching vibration of the Nb-O bond is centered at 600 cm-1, which might overlap with the original peak of Ti-O-Ti vibration, resulting in no additional peaks presented due to Nb doping.37-39 Proposed Mechanism for the Formation of Nb-doped TiO2 Microspheres

Figure 5. Schematic of the formation mechanism of the Nb-doped TiO2 microspheres. The formation mechanism of the Nb-TiO2 microspheres is proposed in Figure 5, which illustrates the role of HDA as a structure-directing agent affecting the morphology and


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monodispersity control in the sol-gel synthesis. Monodispersed precursor beads are proposed to form through a cooperative assembly process involving long-chain alkylamine and species/oligomers.40-42 During the hydrolysis process of TBOT and Nb(OC2H5)5, three main kinds of resultant species and their oligomers may be contained as follows: (1) Ti species/oligomers First hydrolysis (eq 1) produces unstable hydroxyalkoxides Ti(OH)x(OC4H9)4-x, then polycondensation reactions (eq 2) follow via olation or oxolation (i.e., preferential elimination of water or of butanol, respectively), leading to an extensive Ti-O-Ti network.43-44 Ti(OC4H9)4+xH2O→Ti(OH)x(OC4H9)4-x+xC4H9OH

(1)

(OC4H9)4-x(OH)x-1Ti-OH+C4H9O-Ti(OH)x(OC4H9)3-x→ (OC4H9)4-x(OH)x-1Ti-O-Ti(OH)x(OC4H9)3-x+C4H9OH

(2)

(2) Nb species/oligomers The hydrolysis and polycondensation reactions of Nb(OC2H5)5 (eq 3 and 4) are similar to TBOT, which induce the formation of Nb-O-Nb network. Nb(OC2H5)5+xH2O→Nb(OH)x(OC2H5)5-x+xC2H5OH

(3)

(OC2H5)5-x(OH)x-1Nb-OH+C2H5O-Nb(OH)x(OC2H5)4-x→ (OC2H5)5-x(OH)x-1Nb-O-Nb(OH)x(OC2H5)4-x+ C2H5OH

(4)

(3) Ti/Nb species/oligomers The polycondensation reaction (eq 5 and eq 6) occurs between unstable hydroxyalkoxides about titanium and niobium, resulting in Ti-O-Nb network. (OC4H9)4-x(OH) x-1Ti-OH+C2H5O-Nb(OH)x(OC2H5)4-x→ (OC4H9)4-x(OH) x-1Ti-O-Nb(OH)x(OC2H5)4-x+ C2H5OH

(5)

(OC2H5)5-x(OH) x-1Nb-OH+C4H9O-Ti(OH)x(OC4H9)3-x→ (OC2H5)5-x(OH) x-1Nb-O-Ti(OH)x(OC4H9)3-x+ C4H9OH


(6)

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The resultant species and oligomers most likely participate in hydrogen-bonding interactions with amino groups of the HDA to form inorganic-organic composites. Such hybrid composites contain hydrophobic long-chain alkyl groups and thus tend to self-organize into hybrid micelles to reduce the interfacial free energy. Meanwhile, further hydrolysis and condensation of the species/oligomers associated with the hybrid micelles result in interconnection and short-range packing of the hybrid micelles to form a new liquid condensed phase rich in HDA and titanium/niobium oligomers. As the titanium/niobium oligomers further polymerize, the condensed phase becomes denser with time, accompanies by the formation of an inorganic framework as a result of the gelation transformation, and finally precipitates from the solvent. In order to minimize the surface free energy, the condensing phase takes a spherical shape as in conventional colloid formation processes. ER property The flow curves of the shear stress as function of shear rate under various electric fields for the TiO2 and Nb-doped TiO2 microspheres based ER fluids are plotted in Figure 6. The suspensions have good rheological properties in the range of shear rates used. While external electric field is applied, a dramatic increase in the shear stress is observed and each suspension exhibits a plateau, demonstrating the formation of a chainlike structure among the polarized particles.45-46 The values of shear stress increase with the electric field strength due to the enhancement of electrostatic interactions between particles.


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Figure 6. The flow curves of shear stress as a function of shear rate for (a) TiO2 and (b) Nbdoped TiO2 microspheres based ER fluid. Although all suspensions show ER effect, the difference between them is significant. Firstly, the ER performance of Nb-TiO2 microspheres based ER fluid is obviously better than that of undoped TiO2 microspheres. For instance, at 3.0 kV mm-1 and 1.0 s-1, the shear stress of Nb-TiO2


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and TiO2 suspension is 323 Pa and 193 Pa, respectively. At 3.0 kV mm-1 and 100 s-1, the shear stress of Nb-TiO2 and TiO2 based ER fluid is 351 Pa and 137 Pa, respectively. In addition to shear stress, ER efficiency is another parameter for evaluating ER performance. The ER efficiency can be defined as [(τE - τ0)/τ0], in which τ0 is the shear stress under zero electric field strength and τE is the shear stress under different external electric field strengths. Here, at 3 kV mm-1 and 1.0 s-1, the ER efficiency for the suspension of Nb-TiO2 and TiO2 is 52.8 and 31.2, respectively. At 3 kV mm-1 and 100 s-1, the ER efficiency for the suspension of Nb-TiO2 and TiO2 is 57.5 and 21.8, respectively. All these results show that Nb doping can enhance the ER performance. Secondly, unlike the Nb-TiO2 ER fluid, the TiO2 ER fluid exhibits a slight decrease in shear stress in the low shear-rate region until a critical shear rate is reached. Then, the shear stress increases with shear rate. Previous work have been tried to provide rheological explanation for such a deviating tendency.27 Particularly, Cho-Choi-Jhon (CCJ) model provides sufficient fitting for decreasing phenomenon of shear stress in the low shear rate region by introducing additional parameters into the Bingham and De Kee-Turcotte model.27 Under the shear flow, the fibrillated structures of the ER particles are experiencing a breaking and reformation process, and the rate of this process is thought to depend on the competition between attractive electrostatic and repulsive hydrodynamic interactions. Figures 6a1 and 6a2 show the state of the chains composing of polarized TiO2 microspheres in the low shear-rate region and in the high shear-rate region, respectively. At 1.0 s-1, the rate of deformation may be slightly faster than the rate of reformation, which induces slight decomposition of the aligned chains in TiO2 microsphere based ER fluid. As the shear rate increases to 100 s-1, hydrodynamic interactions become more dominant in this process, resulting in more damage of the aligned chains (Figure 6a2). Moreover, this further disruption will increase the polarized particles distance (dP), leading


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to the reduction of polarized charge, which can weaken the electrostatic interactions. Consequently, shear stress presents a decreasing trend. As shown in Figure 6b, the stable flow curve and wider plateau level of shear stress versus shear rate reflect that the electric fieldinduced electrostatic interaction between Nb-TiO2 is so strong that it can effectively rebuild particle chains in wide shear rate range. Compared with TiO2, the states of the chains of Nb doped TiO2 in ER fluid are shown in Figures 6b1 and 6b2. More polarized charge on the surface of Nb-TiO2 microspheres due to Nb doping enhance the electrostatic interactions and the rate of reformation. As a result, compared with TiO2 based ER fluid, the chain structure in Nb-TiO2 based ER fluid can be effectively maintained. Therefore, the Nb-TiO2 based ER fluid presents a more stable flow curve in wide shear rate region.47 Dielectric Property of ER Fluids To gain more insight into the relationship of ER behavior with dielectric property, the permittivity (ε′) and dielectric loss factor (ε′′/ε′) were investigated by broadband dielectric spectroscopy (Figure 7). According to the proposed polarization mechanism by Hao et al., the polarizability and the polarization response of particles play an important role in ER performance, and a good ER effect requires that ER fluids should have a large interfacial polarizability ∆ε′102-105 Hz (∆ε′=ε′102 Hz-ε′105 Hz).48-49 Moreover, the relaxation times (τ) which is related to the interfacial polarization are estimated by the following equation:27, 50

τ=

1 2π f max

where ƒmax is the frequency at maximum dielectric loss factor.


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Figure 7. (a) Permittivity curves of TiO2 and Nb-doped TiO2 microspheres based ER fluid. The inset corresponds to an enlarged view of the permittivity curve in the range of 101-106 Hz. (b) dielectric loss factor ε′′/ε′ for TiO2 and Nb-doped TiO2 microsphere based suspensions. It can be found from the inset of Figure 7a that the ∆ε′102-105 Hz of Nb-TiO2 based ER fluid was larger than that of TiO2 based ER fluid, indicating that a relatively stronger interfacial polarizability occurred in Nb-TiO2 based ER fluid, which was attributed to the increase of charge carriers caused by Nb doping. Therefore, compared with the TiO2 ER fluid, the Nb-doped TiO2 ER fluid had a relatively higher yield stress. The dielectric constant and the dielectric loss are usually not independent. Therefore, putting emphasis on the interfacial polarization is essentially the same as that on the dielectric loss. The suspension has a larger dielectric loss peak, meaning that it has stronger interfacial polarization. In this case, it can be obviously observed in Figure 7b that Nb-TiO2 based ER fluid has a larger dielectric loss peak than that of TiO2 based ER fluid, also implying that Nb-TiO2 based ER fluid should have relatively stronger interfacial polarization. Moreover, it could be concluded that the relaxation time of Nb-TiO2 ER fluid (τNb-TiO2) is a little longer than τTiO2.


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Furthermore, it can be clearly seen in Figure 7a that the permittivity of Nb-TiO2 and TiO2 based ER fluid increased significantly, especially in the low frequency range of 10-2-100 Hz. Initially, this phenomenon was considered to be caused by quasi-dc (QDC) behavior.51 The QDC is essentially different from the direct current, which is related to polarization and is a complex re-equilibrium process of the space charge under the outside field, including the transition, migration, trapping, and even electrochemical process.52 In order to verify whether this increase of permittivity in low frequency resulted from QDC, TiO2/low-density polyethylene (LDPE) and Nb-TiO2/LDPE solid composites were fabricated to compare their results of dielectric spectrum with ER fluids.

Figure 8. Permittivity curves of pure LDPE, TiO2/LDPE and Nb-TiO2/LDPE solid composites. Figure 8 presents the dielectric spectrum of LDPE, TiO2/LDPE, Nb-TiO2/LDPE composites. As expected, the permittivity of pure LDPE exhibits almost no variation with frequency in the range of 10-2-105 Hz due to no interface polarization in pure LDPE. Compared with the


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permittivity curve of LDPE, both the permittivity curves of TiO2/LDPE composite and NbTiO2/LDPE composite present an increase trend in the range of 102-105 Hz, which can be attributed to the interface polarization. These results are similar to that of ER fluid (Figure 9a). Interestingly, in the range of 10-2-100 Hz, there are significant differences between their ER fluid and solid composites. Compared with the TiO2 based ER fluid and Nb-TiO2 based ER fluid, their solid composites exhibit no further enhancement of permittivity in low frequency. Thus, it can be concluded that the increase of permittivity of ER fluid in the range of 10-2-100 Hz should not be attributed to the QDC behavior. The reason that ER fluid and solid composites present a quite different behavior in the low frequency dielectric spectroscopy would be further analyzed below. Dielectric spectroscopy measures the dielectric properties of a medium as a function of frequency. It is based on the interaction of an external field with the electric dipole moment of the sample, often expressed by permittivity. Usually, the external field in dielectric test can not affect the internal structure of detected object. For instance, in Nb-TiO2/LDPE solid composites, the Nb-TiO2 particles will maintain the original random state in LDPE medium, even under an external field. However, one significant distinction between ER fluid and solid composite in the above description is that the dispersed particle in ER fluid will tend to form the chain-like structures along the electric-field direction when the electric field is applied. Therefore, the influence of distribution change of polarized particles in the fluid medium should be considered in the dielectric analysis. This distinction between ER fluid and solid composite can lead to different tendency of permittivity curve in the range of 10-2-100 Hz (Figure 9b).


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Figure 9. Permittivity curves of Nb-TiO2/silicone oil and Nb-TiO2/LDPE in the range of (a) 101107 Hz; (b) 10-2-101 Hz.

Figure 10. The schematic illustration for polarization behavior of Nb-TiO2 in (a) LDPE and (b) ER fluid in the different frequency range.


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Figure 10 shows schematic illustration of polarization behavior of Nb-TiO2 in LDPE and ER fluid in three kinds of typical frequency range during the dielectric test process. It can be seen in Figure 10a that for the Nb-TiO2/LDPE solid composite, at f >106 Hz, there is only a small number of polarization charges on the surface of the polarized Nb-TiO2 (Figure 10A1), which is attributed to the Debye, the atomic, and the electronic polarizations. As the frequency of dielectric testing decreases to the range of 101-102 Hz, the number of polarization charge on the surface of Nb-TiO2 microspheres increases (Figure 10A2), which might be resulted from the slow polarization, i.e., the interfacial polarization in this case. In the range of 10-2-100 Hz, no more polarization occurs in composite so that the number of polarization charge remains unchanged (Figure 10A3). However, ER fluid is a kind of liquid composite, in which the dispersed polarized particles can tend to form the chain-like structure along the electric-field. At the relatively high frequency (f >10 Hz), there is not enough time for the polarized particles to transfer to another location. As a result, the particles will still keep the original disorder state. Consequently, the polarization behavior of Nb-TiO2 in ER fluid is almost similar to that in LDPE in the high-frequency range (shown in Figures 10B1 and 10B2), which can be reflected by dielectric spectrum (Figure 9a). Figure 10B3 illustrates that in the process of low frequency dielectric test, Nb-TiO2 in ER fluid will tend to form the chain-like structures. When dielectric spectroscopy equipment tests the data point at a certain frequency ƒ, the period of the alternating electric field (T) is given by T=1/ƒ. At the low frequency, the period of the alternating electric field is much more than the relaxation time of interface polarization. Therefore, the polarized particles in ER fluid have long time to form the chain-like structures after interface polarization completes. While electric field direction changes, the surface polarity of dispersed particles reverses immediately. The polarized


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particles will continue to form the chain-like structures based on the previous half-cycle of the alternating electric field rather than come back to a disorder state, which is due to the viscosity of the medium. Thus, the time for polarized particles to form the chain-like structures increases with the decrease of testing frequency, resulting in the increase of the order degree of fibrous structure in ER fluid. The structural evolution from disorder to relative order in the ER fluid will reduce dP along the electric field direction, which can induce the increase of the electric field intensity (E) between polarized particles along the electric field direction. The magnitude of charge on the surface of polarized particles (Qs) can be estimated by the following equation: Qs =EεrA where εr is the dielectric permittivity of fluid, A is the area of the dispersed particles. In the case, the value of εr and A is invariable. Therefore, the magnitude of charge on the surface of polarized particles will increase with the polarized particles distance along the electric field direction. We call this phenomenon as “secondary polarization” and the polarized charge due to secondary polarization as “CSP” (seen in Figure 10B3). The emergence of CSP may induce the increase of permittivity. Therefore, the permittivity curve of ER fluid revealed an increasing trend with the decrease of frequency in the low frequency range (Figure 9b). Additionally, it can be concluded that the CSP should increase with the order degree of dispersed particles in ER fluid. Through the above analysis, it can be concluded that the enhancement of permittivity in the low frequency results from the increase of the order degree of chain-like structure in ER fluid rather than from the QDC behavior. Moreover, the CSP plays an important role in the ER performance, which is closely related to the distribution of dispersed particles in ER fluid. Thus, the ability to maintain the chain-like structure in ER system is of significant importance. The ER fluid having a stronger ability to maintain the chain-like structure should have a larger dynamic


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yield stress and wider plateau level. Therefore, a method to characterize the ability should be required. According to the essential of dielectric testing, a parameter Κ can be used to quantify the absolute value of slope of permittivity curves at 10-2 Hz (Figure 7a), which indicates the increment of permittivity per unit time. The permittivity curve of ER fluid with a larger Κ indicates that the ER fluid has stronger ability to form fibrous structure, which implies that fibrous structure of polarized particle is harder to destroy. In some sense, the intensity of the interface polarizability ∆ε′(102-105 Hz) often determines and influences the value of Κ. The ER fluid with large interface polarizability usually should have a large Κ. Therefore, it can be founded in Figure 7a that the K of Nb-TiO2 fluid is larger than that of TiO2 fluid, which indicates that NbTiO2 fluid has a stronger ability to form the chain-like structures. However, besides the interface polarizability, there are some other factors which might affect K, such as density, size of particle. In order to elucidate the mechanism, more research should be required. In conclusion, the rheological behavior can be presumed as follow. The surface charges associated with interfacial polarization make the ER particles turn along the direction of an external electric field. During the process, the dP along the electric field direction decreases, which induces the increase of the CSP, resulting in further enhancement of electrostatic interactions between polarized particles. As the shear rate increases to some extent, shear fieldinduced hydrodynamic interaction can destroy particle chains. This disruption of particle chains will increase of dP along the electric field direction, leading to the reduction of CSP, which will weaken the electrostatic interactions. Consequently, shear stress of TiO2 based ER fluid presents a decrease trend with increasing frequency. To the Nb-TiO2 based ER fluid, more surface charges of Nb-TiO2 due to the Nb5+ doping enhances the interface polarization, which can strengthen the ability of forming fibrous structure. In other words, under the same shear rate,


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fibrous structure composing of polarized particles in Nb-TiO2 in ER fluid should be more difficult to destroy, which might be due to more CSP on the surface of Nb-TiO2 microspheres. As a result, the yield stress of Nb-TiO2 based ER fluid is larger than that of TiO2 based ER fluid. In addition, for destroying particle chains, relative high shear rate is needed to promote hydrodynamic interaction. Therefore, Nb-TiO2 based ER fluid displays wider plateau level, which can be clearly seen in Figure 6.

CONCLUSION Titanium dioxide and Nb-doped titanium dioxide microspheres with the same size were fabricated by simple sol-gel method, which were adopted as dispersed materials for electrorheological (ER) fluids to investigate the influence of the charge increase introduced by Nb doping on the ER activity. The rheological results showed that Nb doping could effectively enhance the ER performance. The improvement of ER activities was contributed to the enhancement of polarization force between particles. The dielectric results showed the Nb-doped TiO2 ER fluid had a stronger interface polarization than TiO2 ER fluid, which was attributed to more surface charges of Nb-TiO2 caused by Nb5+ doping, resulting in strengthening of formed fibrous structure. In addition, by comparing and analyzing the dielectric result of Nb-TiO2/LDPE solid composite and Nb-TiO2/silicone oil composite in the low frequency, it was found that the increase of permittivity in the low frequency was resulted from the increase of the order degree of fibrous structure in ER fluid rather than from the QDC behavior. Moreover, the absolute value of slope of permittivity curves (Κ) could be utilized as the standard for judging the ability to maintain the chain-like structure. The result was important in expanding the application of the low frequency dielectric spectrum for ER study.

ACKNOWLEDGEMENTS


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This work is supported by National Natural Science Foundation of China (no. 51407098), Natural Science Foundation of Shandong Province of China (ZR2014EEM006), the ScienceTechnology Foundation for Middle aged and Young Scientist of Shandong Province, China (no. BS2014CL018), Development Program in Science and Technology of Qingdao (no. 14-2-4-50jch), A Project of Shandong Province Higher Educational Science and Technology Program (no. J14LA14).

MATERIALS AND METHODS Materials. Titanium tebrabutyl titanate (TBOT, 98%) was purchased from Jiangshu Qiangshen Functional chemical co., LTD of China. Niobium(v) ethoxide (99.99%) was purchased from Beijing lark technology co., LTD of China. Ethyl alcohol (99.7%) were purchased from Laiyang Fine Chemical Plant of China. 1-Hexadecylamine (HDA, 90%) and Polydimethyl siloxane fluid (viscosity (η) =(100±8) mPa·s, and specific density (ρ) = 0.966-0.974 g·cm-3 at 25 °C ) were purchased from Aladdin Industrial Corporation. Low-Density Polyethylene (LDPE) was purchased from Borealis. All of the reagents were used as received, without further purification. All of the reagents were used as received, without further purification. Preparation of the TiO2 and Nb-doped TiO2 Microspheres. In a typical experiment, 1.5 g HDA was fed into a reactor, in which a mixture of 200 ml ethanol, 1 ml of H2O had been loaded, followed by vigorous stirring with the speed at 1400 rpm for 30 min to get a uniform solution. After complete dissolution of HDA, tetrabutyl titanate (TBOT, 4 mL) dissolved in ethanol (6 ml) was introduced rapidly in the above transparent mixture under stirring, and the stirring speed was adjusted to 500 rpm immediately and then last for 3min. After aging for 24 h, the white precipitate was harvested by centrifugation, followed by washing with ethanol several times to remove residual HDA from the surface of the titanium particles, then dried at 80 °C for 12 h. The


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preparation process of Nb-doped titanium microsphere was almost the same as that of titanium particles except that the amount of 4 ml TBOT was replaced by the mixture of 3.5 mL TBOT plus 0.5 mL Niobium(v) ethoxide. The titanium and Ni-doped titanium microspheres have a narrow particle size distribution with an average particle size of 500 nm as shown in SEM image. Preparation of TiO2 Microspheres/LDPE Composite and Nb-TiO2 Microspheres/LDPE Composite. LDPE and TiO2 particles were dried for 8 h at 80 °C and 100 °C, respectively, before melt compounding. LDPE pellets and were melt-blended at 110 °C and 65 rpm for 3 min, using a two-roller mixer (RM-200C, HAPRO Rheometer). Then, TiO2 particles and LDPE (weight ratio=1:9) were mixed under strenuous stirring for 7 min. The obtained TiO2/LDPE composite was inserted between two steel boards (150 mm×150 mm×2.2 mm) and molded to a 1 mm-thick sheet (sample) by the hot-press method (25T/50T). The hot-press was performed at 10 MPa and 150 °C for 3 min. The sample under the steel boards was cooled in air after the hotpress. The cooling time from 150 °C to room temperature was 45 min. The preparation process of Nb-TiO2 microspheres/LDPE was similar to that of TiO2 microspheres/LDPE composite, in which Nb-TiO2 instead of TiO2. Characterization. The surface morphology of the particles was observed by field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F). FTIR spectrum was determined on a Nicolet Magna IR-750 spectrophotometer using KBr pressed disk. Investigation of Electrorheological (ER) and Dielectric Properties. The silicone oils, obtained TiO2 and Nb-doped TiO2 microspheres were dried in a vacuum oven at 80 °C for 12 h before the experiments to avoid the influence of moisture. The dried particles were then dispersed in silicone oil [Aladdin Industrial Corporation; dielectric constant (ε) = 2.72-2.78, viscosity (η) = (100±8) mPa·s, and specific density (ρ) = 0.966-0.974 g·cm-3 at 25 °C] to form the ER fluids (10


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wt% particle concentration). The concentration of the ER fluids was denoted as the ratio of the nanoparticle weight to the total weight of the ER fluid. Finally, the obtained ER fluids were further heated at 80 °C for 12 h to remove residual water, which may influence the ER behavior. The ER properties of the suspensions were measured by an electrorheometer (HAAKE Rheo Stress 6000, Thermo Scientific, Germany) with a parallel-plate system (PPER35), and a WYZ020 DC highvoltage generator (0-5 kV, 0-1 mA). The steady-flow curves ofshear stress-shear rate were measured by the controlled shear rate (CSR) mode at room temperature. A dielectric analysis was carried out using a Novocontrol broadband dielectric spectrometer (Novolcontrol Technologies GmbH & Co.KG) over a frequency range of 10-2-107 Hz. All experiments were performed at 25 °C. The calibration of the permittivity cell is described in the supporting information.

ASSOCIATED CONTENT Supporting Information Available: [The description about the calibration of the permittivity cell] This material is available free of charge via the Internet at http://pubs.acs.org.

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The schematic illustration for polarization behavior of Nb-TiO2 in (a) LDPE and (b) ER fluid in the different frequency range 356x341mm (150 x 150 DPI)


Enhanced Electrorheological Performance of Nb-Doped TiO2

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