According to the dielectric design, a new kind of rare-earth (RE)-doped TiO 2 particle was synthesized by means of sol-gel technique for use in electrorheological (ER) fluids. A distinct enhancement in the yield stress under dc electric field was found by using such particles, when compared with that of pure TiO 2 . The yield stress of typical cerium-doped TiO 2 suspension was about 5.0 kPa at 3 kV/mm and 7.0 kPa at 4 kV/mm, which were 10 times higher than that of a pure TiO 2 suspension. The yield stress showed a marked dependence on RE-doping degree. Substitution of 10 mol % cerium or 8 mol % lanthanum for Ti could obtain the highest yield stress. These were well explained by the dielectric measurements that showed an increase in the dielectric loss and the dielectric constant at low frequency and their regular change with rare earth content. The lattice distortion and defects in the TiO 2 crystal caused by substitution for Ti with large-radius RE ions may be responsible for the obvious improvement noted above.

A new kind of mesoporous rare-earth-doped TiO2 particle with a well-defined crystalline framework was synthesized under a general temperature state for use as electrorheological (ER) active material. The neutral surfactant dodecylamine (DDA) was used as a template to direct the mesoporous structure. The low-angle X-ray diffraction peak showed that the mesoporous rare-earth-doped TiO2 had an interlayer distance of about 3.4 nm and the high-angle peaks showed that the material possessed an anatase crystalline framework. ER properties of the suspension based on this material and silicone oil were measured in a dc electric field. It had been shown that the extraordinary high yield stress over 8.1 kPa could be induced when a 3 kV/mm electric field was applied. It was 20 times higher than that of pure TiO2 ER suspension and twice as high as that of single-doped TiO2 ER suspension as reported in our previous works. Interestingly, the shear stress of this suspension was found to continuously increase with temperature elevation in the range 10−100 °C. These improvements were attributed to the pursuit of slow polarization and suitable conduction properties of this mesoporous-doped TiO2 suspension in terms of the measured results of dielectric and conduction properties. The present experimental result suggests that both the active internal structure and interface or surface structure of ER particles, which merit the pursuit of suitable dielectric and conduction properties, are the key to high ER activity.

TiO 2 is very important electrorheological (ER) material due to its high dielectric constant. But its weak ER activity is amazing. We report here a novel approach to enhance the electrorheological activity of TiO 2 by doping it with chromium (Cr) ion. X-ray diffraction shows that the material possesses the anatase phase when the Cr/Ti molar ratio is smaller than 10%, while phase separation appears upon further increasing the Cr/Ti molar ratio. X-ray photoelectron spectroscopy shows that Cr 6+ and Cr 3+ are codoped in TiO 2 crystal and induce a shift of the binding energy of TiO 2 . Rheological experiments show that Cr-doping significantly enhances the ER activity of TiO 2 . The yield stress τ E of a typical 10 mol % Cr-doped TiO 2 suspension is about 2.3 and 7.0 kPa at 4 kV/mm when the volume fraction is 18% and 34%, respectively. The ER efficiency ((τ Eτ 0 )/τ 0 ) of a typical Cr-doped TiO 2 suspension is about 280 (10.55 s -1 ) at 3 kV/mm, which is 18 times higher that of pure TiO 2 suspension. Especially, the yield stress shows a marked dependence on Cr doping degree, and ER activity tends to decline with phase separation when the doping degree is higher than 10 mol %. On the basis of the structure analysis and dielectric and conduction measurements, we attribute the enhancement of ER activity of TiO 2 to the improvement in the dielectric and conduction properties, induced by the activated internal structure, including defect and impurities of TiO 2 due to doping with different valent Cr ions.

To enhance electrorheological (ER) activity by improving interfacial polarization, we prepared a new mesoporous Cr-doped TiO2 ER material by a copolymer-templated sol-gel method. The material was characterized by differential scanning calorimeter and thermogravimetric (DSC-TG) analysis, Fourier transform infrared (FT-IR), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption, and X-ray photoelectron spectroscopy (XPS) techniques. The ER activity was studied by the rheological curve and yield stress under an electric field. The results showed that the mesoporous Cr-doped TiO2 ER material possessed a high surface area over 200 m2/g and a crystalline anatase pore wall doped by different valent Cr ions. The ER activity of mesoporous Cr-doped TiO2 was higher than that of nonporous Cr-doped TiO2. The yield stress and ER efficiency of the mesoporous Cr-doped TiO2 ER suspension was 3 times as high as that of the nonporous Cr-doped TiO2 ER suspension, 7 times as high as that of the mesoporous undoped TiO2 ER suspension, and 20 times as high as that of the nonporous pure TiO2 ER suspension. Furthermore, the ER activity of mesoporous Cr-doped TiO2 showed a dependence on surface area, and the high porosity or surface area samples showed higher ER activity. The dielectric spectra analysis showed that the mesoporous Cr-doped TiO2 ER suspension possessed a significantly larger interfacial polarizability compared with the nonporous Cr-doped TiO2 ER suspension, and the regular change of polarizability with surface area or porosity was in accordance with the change of ER activity with surface area or porosity. The improvement of dielectric properties or polarization could well explain the enhancement of the ER activity of mesoporous Cr-doped TiO2.