[en] The transient shear behavior of magnetic powder (MP) excited by a stepwise magnetic field in a rotational magnetic powder clutch (MPC) was experimentally studied. The experiment showed that the stable shear stress was approximately proportional to the applied magnetic flux density. The characteristic rising time of the shear stress was independent of the strength of the magnetic field and was affected by the shear rate. It took less than 0.1 s for the shear stress to rise to 63% (1 − e⁻¹) of the stable value. The transient shear stress rising process consisted of two subprocesses: the chain forming process which was less than 100 ms, and the chain coarsening process lasting for dozens of seconds. Upon switching the field off, the shear stress fell rapidly to zero in 0.1 s. Control methods to improve the transient response time of the MPC were discussed and experimentally verified: applying a low voltage in advance; applying a high level voltage for the torque to rise to the target torque and then a desired voltage. These transient characteristics of MP were compared with those of electrorheological (ER) and magneto-rheological (MR) fluids and actuators. The study provides a better understanding of MP excited by a magnetic field and the implications for application. (technical note)

[en] The dynamic compressive and static normal stresses of magnetic powders (MPs) with the constant volume squeezed between two parallel plates were experimentally studied. The compressive stress increased in a power law as the gap distance decreased, with an exponent range of −0.73 (0.04 T) to −2.63 (0.77 T). The values of the scale factor to normalize the compressive curves were mainly dominated by the applied magnetic field. The compressive behavior of the MPs showed a less significant velocity effect and initial gap distance effect than those of the magnetorheological (MR) and electrorheological (ER) fluids because of the absence of a host liquid. The compressive stress was generated by the serious particle aggregation, which was induced by a magnetic field and friction during compression. This study is designed to further the understanding of the behavior of smart ER/MR materials and the application of MP actuators in squeeze mode. (technical note)

[en] Recently, there are two independent approaches related to a class of nonlinear Lie algebras of three generators; and the realizations of these generators are achieved respectively in Schwinger-boson and position representation. However, by use of the representation transformation between these two representations, the equivalence of the two realizations is therefore proved. (general)

[en] The rheological properties of magnetic powders (MPs) excited by magnetic fields under parallel-plate shear were experimentally studied. The results showed that the shear stress increased with increasing magnetic field strength and volume fraction. It was also influenced by the plate gap distance, and was independent of the shear rate. The shear stress could be ascribed to the contributions of magnetic force and friction force, dominated by the normal force, the shear strain and the friction coefficient. The ratio of shear stress to normal stress, a form of friction coefficient, was influenced by volume fraction, magnetic field strength and gap distance. These results provide a better understanding of the mechanisms of magnetic field excited magnetic powders under parallel-plate shear. (paper)

[en] We report the results of angle dependent resistivity of NdFeAsO_0.82F_0.18 single crystals in the superconducting state. By doing the scaling of resistivity within the frame of the anisotropic Ginzburg-Landau theory, it is found that the angle dependent resistivity measured under different magnetic fields at a certain temperature can be collapsed onto one curve. As a scaling parameter, the anisotropy Γ can be determined for different temperatures. It is found that Γ(T) increases slowly with decreasing temperature, varying from Γ ≅ 5.48 at T = 50 K to Γ ≅ 6.24 at T = 44 K. This temperature dependence can be understood within the picture of multi-band superconductivity

[en] The first-order Raman spectroscopy of diamond exhibits splitting and redshift after the burst of high-pressure (160–200 GPa) and high-temperature (∼2000 K). The observed longitudinal optical (LO) and the transverse optical (TO) splitting of Raman phonon is related to the tensile-strain induced activation of the forbidden or silent Raman modes that arise in the proximity of the Brillouin zone center. (paper)

[en] The rheological properties of magnetorheological (MR) fluids are usually determined by particle structures and polarized particle interactions. However, the particle structures may undergo various evolutions at different shear states and history; this evolution leads to shear stress hysteresis. Therefore, the shear history effect of MR fluids was experimentally investigated in this study. In a shear rate ramp test, the shear stress at low shear rate was higher in the shear rate ramp-down process than in the shear rate ramp-up process. If the next shear test started after a rest time, the start shear stress decayed slowly and approached the original value of the first test when the interval was long enough. The MR fluids also displayed obvious hysteresis loops during the current ramp test. A high shear rate and magnetic field could reduce the shear history effect by accelerating particle structure evolutions, and then hysteresis decreased. This effect was ascribed to the evolution of particle structures during different test modes and durations, and the evolution is governed by interparticle interactions, viscous forces, and the Brownian motions of particles. These results indicated that the accuracy of the force control of MR fluids could be enhanced under high magnetic fields and high shear rates. Thus, these factors should be considered in MR actuator designs. (paper)

[en] The linear dynamic viscoelastic properties of carbonyl iron particle suspensions under external magnetic fields were studied. Results revealed a frequency-independent viscoelasticity under a certain critical magnetic field. The characteristic magnetic field intensity decreased as the increase of particle volume fraction of the suspension, corresponding to a similar value of characteristic loss tangent. By considering the competitions among hydrodynamic, friction and magnetic forces of particles under different magnetic fields, a theoretical linear viscoelastic model was proposed to describe and analyze the tested frequency dependence of viscoelasticity of the suspensions. The model could also be applied to polymer gels with similar viscoelastic behaviors and inspire the design of novel broad frequency vibration damping materials with frequency-independent viscoelasticity. (paper)

[en] We have studied the angular magnetoresistance of iron pnictides BaFe_2−x Ni_x A_s2, which shows clear 180 degree periodicity as fitted by a cosine function. In the x = 0.065 sample, the phase of the two-fold symmetry changes 90 degrees above the tetragonal-to-orthorhombic structural transition temperature T_s. Since the phase at low temperature is associated with the rotation of orthorhombic domains by magnetic field, we show that even vacuum grease can push the presence of orthorhombic domains at temperatures much higher than T_s. Our results suggest that residual stress may have significant effects in studying the nematic orders and its fluctuations in iron pnictides. (paper)

[en] Rheology of a suspension is mainly determined by particle interactions and the host fluid rheology. By scaling rheological properties of different magnetorheological (MR) suspensions prepared with Newtonian and non-Newtonian (including shear thickening and shear thinning) host fluids, the competition between the particle interaction and the host fluid rheology could be clearly revealed. A simple normalizing method by considering the ratio of the magnetic force to viscous force experienced by particles is introduced. The shear stress curves of the three kinds of MR suspensions could be well scaled into the same master curve. The magnetic force maintains the particles structures under magnetic fields, while the viscous force is a destructive factor to the particle structures. Therefore, the magnetic force dominates the rheology of MR suspensions at low shear rates with small viscous force. While as the increase of shear rate to a very high value, particle chain structures under external magnetic fields might get similar to that under zero magnetic field because of the dominant role of the viscous force. This work clearly demonstrates the competition between the particle interaction and the viscous force experienced by particles, which finally dominates the particle structure inside of the MR suspension and its rheological behavior. (paper)