Hu Zhide,Yan Hua,Yang Jianjian,Wang Xuemei,Yu Rongsheng
Magnetorheological (MR) fluid has been widely used in many engineering field because of its unique magnetorheological effect. However, the poor lubrication of it needs to be considered seriously. Otherwise, the engineering applicability of MR fluid may be hindered. The friction property of several MR fluids based on different components was investigated on a modified four-ball tester. Two kinds of experimental conditions were designed, namely off-field and steady magnetic field. The results show that tribological properties of MR fluid are mainly decided by base oils. The friction properties of mineral oil-based MR fluid are better than that based on synthetic oil. The effect of two kinds of thixotropic agents on the tribological properties of MR fluid is exact opposite with and without magnetic field. Silica is superior to clay without magnetic field. Under steady magnetic field condition, the friction coefficient of MR fluid is over three times larger than that of MR fluid in off-field. The effect of base oil on the friction property is similar to that without magnetic field. But clay is better than silica due to the effect of magnetic field.
Magnetorheological fluid (MRF) is a widely used smart material that suffers from sedimentation. Since sedimentation is unavoidable, it is crucial to study and improve the redispersibility of MRFs. However, previous redispersibility testing methods have problems, such as complicated operation and low precision. Simultaneously, a simple and effective method is urgently needed for high-precision modeling of MRF sedimentation to test the rheological properties of settled MRFs at different depths. After systematically analyzing the redispersion problem, this paper proposes decoupling the energy required for redispersing settled MRFs into two parts, which are related to different factors. These two parts are the energy required to separate the agglomerated particles (related to the MRF formula) and that to redisperse the settled MRF uniformly vertically against gravity (related to the solid concentration and packing limit). The energy that separates the agglomerated particles is proportional to the shear stress of slowly shearing the corresponding agglomerated samples, i.e., the yield stress. Thus, this paper proposes a simple microdamage quasi-static indentation method to measure the yield stresses of settled MRFs at different depths to characterize the redispersibility of the corresponding MRFs. Herein, this method is applied to study the mechanisms of the influences of surfactants, thixotropic agents, and their networks on the redispersibility of MRFs. The results indicate that a well-dispersed plate-like thixotropic agent network can effectively improve redispersibility, while surfactants with poor compatibility degrade redispersibility. In summary, this redispersibility test method will greatly facilitate studies of MRFs, such as optimizing the formulas and establishing sedimentation models.
Silver powders were prepared by a chemical reduction method. Effects of different surfactants during this preparation process were examined. Using these silver powders, silver pastes were prepared for low temperature co-fired ceramics (LTCC) applications. Another batch of different surfactants were added into the silver paste and evaluated. Moreover, various organic thixotropic agents were used to modify the thixotropy behavior of the silver paste. It was found that the surfactants of PVP (Polyvinyl pyrrolidone, for preparation of the silver powder) and Span-85 (for the preparation of silver paste) exhibited the best performance, with an electrical resistivity of 0.11 mΩ mm. The silver paste with the thixotropic agent of hydrogenated castor oil shows the largest thixotropic index of 1.56. Using different organic additives, the microstructural, rheological and electrical properties of the silver paste can be considerably improved, and our results shed light on the optimization of the silver paste for LTCC applications.
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