The wall-slip effect is observed in areas with magnetorheological fluids (MRFs). A slip layer is formed, which reduces the friction between the solid particles and working surface that causes relative movement of the particles. This leads to errors in the measurement of rheological parameters and an inaccurate braking torque model. Thus, here, a rheometer with a sandpaper on the rotor is used to change the working surface roughness to analyze the wall-slip effect of the MRFs. Based on the experimental results, the influence patterns of wall-slip effect on fluid viscosity and yield stress are obtained. Furthermore, a MRF model is established that considers wall-slip effect, which is different from the conventional models. The model is employed to establish a magnetorheological (MR) braking torque model. To verify the braking torque model, a prototype was manufactured, and its mechanical properties were tested. When compared with a smooth rotor, the braking torque of MR brakes with rectangular grooves is increased. This confirms the existence of the wall-slip effect and shows that the wall-slip effect of MRF can be effectively suppressed by incorporating grooves on the rotor surface.
We have measured the magnetic hysteresis loops of an epitaxial YBa2Cu3O7−δ thin film using our recently developed device which can provide the field magnitude in the range of 0–1000 Oe and the field sweep rate up to 107 Oe/s. The shape of the hysteresis loop measured changes with the field-sweep rate up to the critical sweep rate; and over the critical sweep rate the ac magnetization reaches its real critical state where magnetization does not change even with a further increase in the field-sweep rate. The critical sweep rate is about 106 Oe/s at 77 K. With the hysteresis loops and Bean model, we have calculated the magnetization critical current density (Jcac) which is consistent with that obtained by I–V measurements. We have also studied flux motion and activation energy under the high sweep rate magnetic field. At temperature 77 K, the velocity of the flux motion is of the order 10 m/s and the pinning energy U0/k is about 339 K which is much smaller than the magnetization decay measurement.
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