Preparation and electrorheological properties of triethanolamine-modified TiO2
Graphical abstract
Triethanolamine-modified TiO2 ER fluid was synthesized with the sol–gel method. Under an external electric field of 5 kV/mm, the yield stress is 32.6 kPa, which is about 50 times higher than that of pure TiO2. The leaking current density is lower than 16 μA/cm2. Triethanolamine-modified materials are good for preparing good ER fluids.
Introduction
Electrorheological (ER) fluid is a suspension made of micrometer- or nanometer-sized particles in an insulating liquid. If the external electric field is higher than a certain critical value, the liquid–solid transition happens in several milliseconds, and the transition is reversible as the electric field is turned off [1]. The industry applications of such smart materials are under the design by the engineers [2], [3]. The most urgent mission is to enhance the shear stress of the ER materials.
It is found that the optimized ER materials are not micrometer- but nanometer-sized particles [4], [5] or micrometer particles with nanometer-sized structures, such as pores or layers [6]. The ER fluids composed of nanometer-sized particles or particles with nanometer-sized structures show more advantages such as large yield stress, low current density, and low sedimentation. However, not all ER fluids that are composed of nanometer-sized particles would present large yield stresses. For example, nanometer-sized pure TiO2 show very low ER effect with a yield tress of 0.6 kPa at 4 kV/mm [6]. The yield stress of inorganic ionic modified TiO2 [7], [8] is a few orders of magnitude higher than that of pure TiO2. The characteristics above indicate that the properly modified materials with nanometer size or nanometer-sized structure are crucial to enhance the ER activity. In this paper, we prepare an ER fluid composed of an organic modified titanium dioxide (triethanolamine–TiO2). The concentration of the ER fluid suspensions is denoted by the ratio of the mass of powders in grams to the volume of base oil in milliliters [9]. The rheological experiments show that the shear yield stress of the ER fluid with a concentration reaches 32.6 kPa at which can satisfy most industrial applications. The sedimentation ratio of the ER fluids is 98% in 20 days. The ER properties also present under AC electric field. Some detailed dielectric properties such as molecular dipole moment are used to analyze the ER property.
Section snippets
Preparation of triethanolamine-modified TiO2 particles
The triethanolamine–TiO2 gel powders were prepared by a sol–gel method. Triethanolamine and titanium butoxide (Ti(C4H9O)4) were used as starting materials; acetic acid (HAc, CH3COOH), ethanol (C2H5OH) and deionized water were employed as solvents. First, a solution of triethanolamine and deionized water was mixed in ethanol. Secondly, Ti(C4H9O)4 was dissolved in ethanol at a volume ratio of Ti(C4H9O)4:ethanol=1:2. In order to avoid precipitation, a small amount of acetic acid was added.
Materials characteristics
The SEM shows in Fig. 1(a) that the triethanolamine-modified TiO2 powder morphology is very irregular, and the powder diameter is distributed mainly in the range from 1 to 100 μm. Fig. 1(b) shows that the diameters of powder particles are as small as 20 nm. Comparing the two graphs, one may find out that agglomeration is very serious in our sample. In our experiment, the yield stress is tested with the milled powders whose diameter distribution in Fig. 1(c) is narrow and the average size is about
Conclusions
In this study, we choose TiO2 and achieve its ER enhancement under AC and DC electric field by doping triethanolamine, which is synthesized with the sol–gel technique. Under a DC electric field, the ER fluids shows large yield stress of 32.6 kPa at 5 kV/mm, and low leaking current density of 16 A/cm2. High dielectric constant and high molecular dipole moment are the major condition for the high values of yield stress of the material. However, under an AC electric field, the yield stress decreases
Acknowledgments
This work is supported by the National Natural Science Foundation of China under Grant Nos. 1024402, 10574027, the Tang Zhongying Science Creative Foundation (B2-11-03) and the Fudan University Graduate Student Creative Foundation.
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