Preparation and enhanced electrorheological activity of TiO2 doped with chromium ion

TiO2 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 TiO2 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 Cr6+ and Cr3+ are codoped in TiO2 crystal and induce a shift of the binding energy of TiO2. Rheological experiments show that Cr-doping significantly enhances the ER activity of TiO2. The yield stress tau(E) of a typical 10 mol % Cr-doped TiO2 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 ((tau(E) - tau(0))/tau(0)) of a typical Cr-doped TiO2 suspension is about 280 (10.55 s(-1)) at 3 kV/mm, which is 18 times higher that of pure TiO2 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 TiO2 to the improvement in the dielectric and conduction properties, induced by the activated internal structure, including defect and impurities of TiO2 due to doping with different valent Cr ions.