Synthesis Of Hollow TiO₂ Microspheres And Their Er Performance

Elecrorheological（ER） fluids, a typical smart material, are usually composed of nano-sized or micro-sized particles with high dielectric constant dispersed in a liquid with low dielectric constant.The system is liquid at room temperature.Upon exposure to an electric field, the suspended particles are polarized and attracted to each other to form a chain structure along the external electric field direction, and these electrically induced structures vanish when the electric field strength is removed. At the moment, the ER fluid produces a continuous and reversible change in the flow state（generally refers to the apparent viscosity which indicates its resistance to flow） and rheological properties in milliseconds, realizing a liquidlike to solidlike phase transition. These fluids have attracted considerable attention from both academic and industrial communities for potential uses because of their distinguishing features, such as low power consumption, fast response time and reversibility. However, until now, there are still several limitations for commercialization of ER fluids because of relatively low polarization force. The density of inorganic disperse material often leads to sedimentation problems, which greatly reduces the ER efficiency by disturbing the formation of the fibril-like structures. Accordingly, for the purpose of improving sedimentation stability, we fabricated hollow Ti O₂ microspheres by sacrificial template and sol-gel methods in this paper. A novel model with detailed explanation about ER mechanism was proposed. The main conslusions are listed as follows:1. In order to obtain TiO₂ hollow microspheres with ideal morphology, the factors that mainly influence the sample preparation were deeply discussed. As a result, the best experimental conditions were established.2. In order to figure out the relationship between particle density and ER property,monodisperse york-shell SiO₂/TiO₂ nanospheres（YSNS） with different SiO₂ core sizes had been successfully synthesized via a sacrificial templating method which combined sol-gel coating and partial etching processes. The ER results showed that the ER performance of the YSNS-based ER fluid prominently enhanced with the decrease of SiO₂ core size, which was attributed to the enhancement of electric field force between YSNSs. Combined with the analysis of dielectric spectroscopy, it was found that a parameter K at low frequency（10-2–100 Hz）（the absolute value of the slope of permittivity curves at 0.01 Hz） could be utilized to characterize the efficiency of structural evolution of polarized particles in ER fluid. Compared with the ER performance, it could be concluded that the value of ?ε′（100 Hz–100 k Hz） just demonstrated the initial intensity of the interface polarization in the ER fluid as the electric field was applied, which ignored the distributing evolution of polarized disperse particles in ER fluid. The polarizability ?ε′（0.01 Hz–100 kHz） obtained in the frequency range of 10-2–105 Hz should be more suitable for analyzing the system of ER fluid. The relationships between polarizability of disperse particles, parameter K and ER properties were discussed in detail.3. For investigating the variation of polarized particles before and after the formation of chainlike structure in ER fluids, the hollow TiO₂ nanospheres were fabricated and adopted as dispersing materials in electrorheological（ER） fluids with different viscosity. Electrorheological results showed that the ER fluid with low viscosity had the better ER efficiency than ER fluid with high viscosity, which was ascribed to the better ability of low viscosity ER fluid in maintaining chainlike structures. Through the analysis of dielectric properties, it was found that the increase of permittivity of ER fluids at low frequency should result from the distribution change of polarized particles in ER fluids. The distribution change of polarized particles would induce the increase of electric field strength in the region of columnar structure, which we called “secondary polarization”. The secondary polarization could lead to the increase of the number of charge on the surface of polarized particles, resulting in further enhancement of electrostatic interactions between polarized particles. Moreover, it was found that the magnitude of polarized charge caused by secondary polarization might be much more than the number of charge caused by the interface polarization, which implied that secondary polarization played an important role in ER phenomenon.4. In order to further improve the ER effect of hollow TiO₂, Ti³⁺-self doped TiO₂ microspheres were prepared by re-calcinating high temperature induced anatase TiO₂ hollow microspheres in a pure H2 environment. According to SEM,TEM, and UV-vis analysis, hydrogenated sample displayed well maintained hollow structure, in which the Ti³⁺ ions and oxygen vacancies generated by electron trapping at the Ti4+ centers through hydrogenation. The ER results showed that hydrogenation could effectively enhance the ER performance. Combining with the analysis of dielectric spectroscopy, it was found that hydrogenated hollow TiO₂ ER fluid exhibited larger interface polarization and fast polarization rate than that of pristine hollow TiO₂ ER fluid, which maybe caused by more surfce charges in hollow TiO₂ microspheres due to Ti³⁺ self-doping, resulting in enhancement of electric field force and strengthening of fibrous structure.