Electrorheological effects in urea-doped BaxSr1−xTiO3 suspensions

Scripta Materialia 55 (2006) 671–673 www.actamat-journals.com

Electrorheological effects in urea-doped BaxSr1 xTiO3 suspensions Jian hong Wei,a,b,* Sui li Peng,a,b Li hong Zhao,a,b Jing Shi,a,b Zheng you Liua,b and Wei Jia Wena,c a

Department of Physics, Wuhan University, Wuchang, Luojiashan Mountain, Wuhan, Hubei 430072, PR China Key Laboratory of Acoustic and Photonic Materials and Devices, Ministry of Education, Wuhan 430072, PR China c Department of Physics and Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, PR China b

Received 21 March 2006; revised 9 May 2006; accepted 1 July 2006 Available online 28 July 2006

Urea-doped BaxSr1 xTiO3 particles (BSTU) were prepared by a modified sol–gel method. The structure and morphology of the BSTU particles were characterized. The dielectric properties of the BSTU particles and the electrorheological (ER) effects of the ER fluids based on these particles were investigated. The results show that the ER performance of those fluids is much higher than that of pure BaxSr1 xTiO3(BST)-based ER fluids. The shear stress of the BSTU (doped with 3 wt.% urea) based ER fluids (with a volume fraction of 30%) reaches 17.2 kPa at E = 4 kV/mm. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Electrorheological effect; Dielectric properties; Amorphous oxides

Electrorheological (ER) fluids are known as smart liquids for their apparent viscosity which is capable of experiencing a rapid, reversible change upon application of an electric field. The ability to control the apparent viscosity electrically makes ER fluids potentially important in numerous electromechanical devices, such as valves, dampers, and clutches in the automotive and robotics industries. However, the operative mechanism of ER fluids and how to make high-performance ER fluids for industrial application are unresolved research issues. Much effort has been expended on preparing highly active ER materials. It is well known that dry ferroelectric particle suspensions, such as BaTiO3, with a dielectric constant around 2000 (depending on its crystallization state), have quite low ER responses, usually of only several kPa orders. When these particles absorb a small amount of water, their ER response can be substantially improved [1–3]. The presence of water can dramatically enhance the interaction of the particles because of the high dipole moment of H2O molecules (1.85 Debye).

* Corresponding author. Address: Department of Physics, Wuhan University, Wuchang, Luojiashan Mountain, Wuhan, Hubei 430072, PR China. Tel.: +86 27 68754613; fax: +86 27 68752569; e-mail: [email protected]

But the shortcomings of water (with its high current density, high temperature evaporation and low freezing point) make it unsuitable for many applications. To overcome the problems brought by water [4,5], new materials of high molecular dipole moment and high boiling points have been suggested. A good example is urea, which has a high dipole moment of 4.56 Debye and a high decomposing temperature of 133 °C [6,7]. In this letter, we report on a new type of anhydrous ER fluid consisting of ureadoped BaxSr1 xTiO3 (BSTU) particles suspended in silicone oil. BaxSr1 xTiO3 (BST) is an infinite so solid of BaTiO3 and SrTiO3, which exhibits good insulating behavior with a large relative dielectric constant and a small dielectric loss, has recently attracted great interest as a new type of dielectric material. These properties make urea-doped BST an ideal candidate for an ER fluid [8]. All the chemical reagents in this study were of analytical grade. Titanium butoxide (Ti(OC4H9)4), as an inorganic precursor, was first dissolved in water-free alcohol. Diethanolamine mixed with Ba(NO3)2 and Sr(NO3)2 ([Ba]/[Sr] = 1) was added to form a clear solution. Here, diethanolamine was used as an additive to prevent the precipitation of oxides from the alcoholic titanium butoxide in the presence of excess water. In addition, water-free alcohol and H2O were mixed with urea to form another homogeneous solution. In the

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J. h. Wei et al. / Scripta Materialia 55 (2006) 671–673

preparation, the volume ratio of H2O and C2H5OH was kept at 1:9, and the added urea was 0, 1, 3, and 5 wt.% for different samples. The second solution was then added into the first solution, which resulted in a transparent sol after 1 h of stirring; the particles were formed when the sols were allowed to age for several hours at room temperature. The particles were dried at 100 °C by degassing for 10 h to remove any trace of water. Then the solid particles were milled into powders. The chemical structure was determined by a Nicolet Dx-10 Fourier transform infrared spectroscopy (FT-IR) spectrophotometer in which the IR spectra were recorded by diluting the milled powders in KBr. The per cent concentrations of C, H, N elements in the final products were measured by an Elementar Vario EL-III analytical apparatus. The X-ray diffraction patterns of the particles were recorded on a Rigaku D/Max-A diffractometer with CuKa radiation to identify the structure of the samples. According to the X-ray diffraction pattern, the samples were amorphous. The particles densities were determined by pycnometery using silicone oil as the dispersing medium. The dielectric performance of the particles was measured using an Agilent 4294A precision impedance analyzer. The ER suspension was prepared by grinding and dispersing a weighed amount of particles in a weighed amount of silicone oil in a mortar. The dielectric constant, density, viscosity and conductivity of the silicon oil used in the ER fluids were 2.20, 0.963 g/cm3, 50 cPs and 10 12 S/m at room temperature, respectively. The ER suspensions with 30 vol.% of particles were prepared by magnetic stirring for 8 h. The rheological behavior of the suspensions was investigated using a Rheometric Scientific rheometer (TA ARES model) equipped with a rotational concentric cylinder. The gap between the inner and the outer cylinder was 1 mm. All the experiments reported in this paper were performed at 30 °C. Table 1 shows the elementary analysis results and the densities of the BST particle and the BSTU particles. It can be seen that the densities of the particles decrease with the increase in the urea contents in the particles. In addition, the elementary analysis results show that the per cent concentration of C, H, N element in the final product increases with the increase in the urea content in the final products. Figure 1 shows the FT-IR spectra of 3 wt.% urea-doped BSTU and pure BST particles. The spectrum for pure BST (spectrum (a) in Fig. 1) shows that the broad band around 3450 cm 1 is the asymmetric and symmetric stretching vibrations of O– H group, whereas the band around 1615 cm 1 is the H–O–H bending of the coordinated water. The IR

Figure 1. The FT-IR spectra of pure BST particles and BSTU particles.

absorption band at 657 cm 1 is attributed to the Ti–O–Ti stretching vibrations. In comparing the two spectra in Figure 3, it is noted that although the two spectra are similar as a whole, there are some observable differences as marked on spectrum (b). In spectrum (b), the absorption band at 3259 cm 1 is attributed to the H–O stretching vibrations of the absorbent water of the urea molecules, the newly appeared bands at 3400 cm 1 and 1630 are attributed to the N–H stretching vibrations and the band at 1710 cm 1 is attributed to unsaturated C@O vibration absorption [9,10]. Spectrum (b) confirms the existence of urea molecule in BST particles and suggests that BSTU particles possess more unsaturated groups than pure BST particles do. We speculate that BSTU particles are made of Ti– O–Ti polymeric networks and the Ba2+ and Sr2+ ions are contained in this network. The O–H groups and the C@O group may combine metal ions by coordination bonds or with oxygen atoms by hydrogen bonds. Sufficient active groups on the surfaces of particles would promote surface activity because they may produce synergetic effects or might react with each other [11,12]. Further work on this is in progress. Figure 2 shows the shear stress for 3 wt.% BSTU samples at a fixed shear rate of 1.0 s 1, varying with time

Table 1. The elementary analysis results of urea-doped BaxSr1 xTiO3 particles Elements

Pure BST 1 wt.% BSTU 3 wt.% BSTU 5 wt.% BSTU

C (%)

H (%)

N (%)

20.55 21.68 22.92 23.96

5.132 5.925 6.601 6.657

0 2.265 5.335 7.345

Particles density (g/cm)3 1.780 1.742 1.725 1.632

Note: 1 wt.% BSTU means 1 wt.% urea-doped BST particles, etc.

Figure 2. The measured shear stress variation under a pulsed electric field in 3 wt.%-BSTU particle-based ER fluids.

J. h. Wei et al. / Scripta Materialia 55 (2006) 671–673

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Table 2. The dependence of dielectric constant and dielectric loss of BSTU particles at 1 kHz on urea content

Dielectric constant Dielectric loss

Figure 3. The relationship between shear stress and electric field for ER fluids of pure BST particles and BSTU particles.

upon the application of step-voltage pulses;, the pulse width is 40 s while the height varies from 1 to 4 kV/ mm. The zero field viscosity of the 30% suspension is about 500 MPa s. In the figure, we note that a stable shear stress could be observed when the electric field varied from 1 to 4 kV/mm and that the maximum shear stress in the 3 wt.% urea-doped BST is 17.2 kPa at E = 4 kV/mm with a volume fraction of 30%. Figure 3 shows the dependence of the shear stress of the BSTU particle ER fluids on the external electric field strength at the fixed shear rate of 1.0 s 1. In the case of pure amorphous BST particles, the shear stress is rather low and the maximum shear stress is 2.5 kPa at E = 4.0 kV/mm, while the urea-doped particles show a noticeably stronger ER effect. The maximum shear stress of 1 wt.% BSTU is 11.04 kPa at E = 4 kV/mm, while the maximum shear stress of 5 wt.% BSTU is 6.87 kPa at E = 4 kV/mm, and the maximum shear stress of 3 wt.% BSTU is 17.20 kPa at E = 4 kV/mm. According to Hao et al. and others [13,14], interfacial polarization is considered to be responsible for the interaction force that leads to the rheological change in ER fluids. Under the same experimental conditions, because of the interaction of urea and BST, the doped BST particles show a stronger polarization strength and there are more polarization charges on the surfaces of the particles when an external electric field is applied. The ER effect of the BSTU particle is much better than that of the pure BST. Figure 3 also shows that the shear stress is dependent of the urea content, the optimal concentration of urea being about 3 wt.%. When the concentration is above the critical point, the urea molecules are more likely to be resolved in the fluid phase rather than adsorbed on the particle surface in the absence of an electric field. But in the presence of the electric field, the enriched urea molecules in the fluid phase will gather into the interstices formed by neigh-

Pure BST

1 wt.% BSTU

3 wt.% BSTU

5 wt.% BSTU

37.8

41.2

92.3

44.5

0.03

0.07

0.15

0.25

boring particles in which a non-uniform high electric field is generated. However, the more polar groups lead to too much conductivity and dielectric loss (see Table 2), which is relevant to the degradation of the ER properties at the high urea concentration limit. ER fluids based on BSTU particles and silicone oil were investigated at room temperature under an applied direct current electric field. The BSTU particles have an amorphous structure and irregular morphology. The ER performance of BSTU-based ER fluid is much better than that of pure BST, the shear stress of 3 wt.% ureadoped BST can reach 17.2 kPa at E = 4 kV/mm with a volume fraction of 30%. We are grateful for the financial support from the National Natural Science Foundation of China (Grant Nos. 10418014 and 10474074). [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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