A kind of magnetoresistance material was prepared, named as magnetorheological gel (MRG). The MRG samples were fabricated by dispersing carbonyl iron particles (CIP) into the polyurethane gel, and several experimental devices were prepared to investigate the magnetoresistance characteristics of MRG. The magnetoresistance characteristic was systematically tested and the influence of the magnetic field was analyzed. It is found that the resistance value of MRG with the CIP content of 70 wt % can be decreased from 7.56 to 2.44 MΩ with increasing of magnetic field from 0.1 T to 1 T. The experimental results have also proved that CIP content and the composition of matrix have a greater impact on the magnetoresistance characteristics. Because of the interaction between the magnetic force of CIP and motion resistance under a magnetic field, the obvious hysteresis phenomenon was observed and recorded. Lastly, it was observed that the magnetoresistance can be changed reversibly by controlling the magnetic field.

•The flower-like carbonyl iron particles (CIPs) were synthesized through in situ reduction method.•A new kind of dimorphic MRGs based on conventional and flower-like CIPs are prepared.•The dimorphic MRGs exhibit an increase in the maximum dynamic yield stresses.•The maximum dynamic yield stresses of the dimorphic MRGs can reach 58.11 kPa.•This kind of dimorphic MRGs have broad potential applications in vibration reduction.In this study, a new kind of dimorphic magnetorheological gels (MRGs) based on the conventional carbonyl iron particles (CIPs) and flower-like CIPs have been prepared for improving the yield stress and dynamic mechanical properties. The flower-like CIPs are synthesized by a simple and facile in situ reduction method. Characterization results indicate that the flower-like CIPs are synthesized successfully and a layer of uniform and continuous Fe nanosheets are grown on the surface of the raw microsphere CIPs. In addition, the flower-like CIPs exhibit excellent magnetic properties, which the saturated mass magnetization (Ms) can achieve 168.76 emu/g. In order to study the influence of mass fraction of flower-like CIPs on the rheological properties of this dimorphic MRGs, a series of polyurethane-based dimorphic MRGs are prepared by partial substitution of the CIPs with as-synthesized flower-like CIPs, and the MR properties of them are systematically investigated under both oscillatory and rotational shear modes. The experimental results indicate that, with 8 wt% flower-like CIPs, the maximum dynamic yield stresses and magneto-induced shear yield stress of dimorphic MRGs are 58.11 kPa and 54.53 kPa, ∼1.39 and ∼1.37 times of the MRG without flower-like CIPs at the same magnetic particle content. Moreover, the average loss factor and the loss factor under 1 T of the sample (flower-like CIPs weight content 8 wt%) are 0.36 and 0.07, which are approximately 1.71 and 2.71 times than that in the non-substitution sample. The increased loss factor is beneficial to improving the vibration reduction effect of MRGs of damping devices in the whole magnetic field region. Furthermore, the possible mechanism for the enhanced MR properties in dimorphic MRGs is proposed. In summary, this work is expected to promote the design and application of MRG devices.Download high-res image (213KB)Download full-size image

•Hierarchical Flower-like Fe was synthesized by in situ reduction method assisted with appropriate external magnetic field.•The obtained α-Fe was assembled by the iron nanosheets and diameter is about 3 μm.•A plausible mechanism for the formation of the flower-like α-Fe have been proposed.•The minimum RL value as low as almost −33.1 dB at 17.46 GHz can be obtained.Self-assembly of flakes to form three-dimensional flower-like structure is an effective way to strengthen electromagnetic absorption capability. However, it still remains a great challenge to synthesis of such structure, especially the preparation of flower-like Fe particles. In this paper, hierarchical flower-like α-Fe particles with a diameter of ∼3 μm are prepared by combining a simple in situ reduction method and assisted with an appropriate external magnetic field at normal atmosphere. A possible mechanism for the formation of flower-like structure related to the synergy of magnetic field and hydrogen bubble which generated by in-situ reduction reaction is proposed. The electromagnetic properties of flower-like α-Fe-paraffin composites were measured with the coaxial reflection/transmission technique at 2–18 GHz. The minimal reflection loss value of −33.1 dB is obtained at 17.46 GHz. Moreover, the effective absorption bandwidth with reflection loss exceeding −10 dB is up to 6.2 GHz (7.7–13.9 GHz) at a thickness of 3 mm, which covers whole of the X-band (8–12 GHz) and part of the Ku-band (12–18 GHz). These excellent absorbing performances suggesting that the flower-like α-Fe particles synthesized here could be an extremely promising electromagnetic absorbing material.

•A theoretical model of magneto-rheological gel was developed to precisely predict the magnetic field-dependent conductivity of MRG.•Several samples of magneto-rheological gel were fabricated and their magneto-resistance properties were explored at different magnetic fields.•The effectiveness of the proposed model was validated through a comparative work between measured results and theoretically obtained results.•It is shown that the proposed model can reproduce well experimental results without causing significant difference.•This theoretically model is expected to provide guidance for magneto-rheological gel applied to many potential fields, such as the magnetic sensor.A magneto-rheological gel (MRG) is a kind of smart material fabricated by dispersing ferromagnetic particles in high viscosity polymer gel. The internal particles of MRG are uniformly dispersed under off-state. While with the precondition of an external applied magnetic field, the particles will form a reversible chain-like structure, in which process the electrical resistivity of MRG has an immense change. In order to deeply understand the changing mechanism of the magneto-resistance of MRG, a theoretical model based on both the magnetic-dipole behavior and the percolation theory was proposed in this work. This model incorporates the parameters of the magnetic field intensity and carbonyl iron powder (CIP) volume fraction. As a further effort to explore the magneto-resistance properties of MRG, a series of experimental testing were conducted. The magneto-resistance properties of MRG with different CIP volume fractions and under different magnetic fields were investigated. The experimental results indicated that the conductivity of MRG was significantly increased with the increasing magnetic field and CIP volume fraction. In addition, in order to verify the effectiveness of the proposed theoretical model, the experimental measured results and theoretically obtained results were compared. The validation results demonstrated that the theoretical model proposed in this work is capable of reproducing the field-dependent magneto-resistance properties of MRG.

Selection cooperation is an attractive cooperative strategy for wireless networks due to its simplicity and efficiency. In this paper, we consider a heterogeneous cooperative network consisting of different kinds of nodes with low-cost radios where the activities of one kind of nodes are triggered by the other kinds of nodes. This is a common scenario for many networks, such as wireless sensor networks. By exploiting the transmission relationship between heterogeneous nodes, we propose a selection cooperation protocol where inducing nodes can cooperate with the following nodes after their own transmissions for improving the communication reliability of the latter nodes. Through performance analysis, we show an interesting feature that the diversity-multiplexing tradeoff of the proposed protocol does not rely on the best relay selection method and the protocol always achieves the full diversity gain. We further develop an energy-efficient best relay selection method based on power control where the power consumption is minimized without decreasing the full diversity order. Simulation results demonstrate the good performance of the protocol and the remarkable energy reduction of the proposed best relay selection method.

Magnetorheological gel (MRG) is a new kind of MR material, and it can be regarded as the analogue of MR fluid. In this study, a MRG sample based on a polyurethane matrix was fabricated. The normal force of MRG was investigated by using an advanced commercial rheometer under rotational and oscillatory shear modes. The influence of time history, strain amplitude, frequency, shear rate, and temperature in the presence of a variable and constant magnetic field on the normal force of MRG was systematically studied. The experimental results indicated that the magnetic field has a greater impact on the normal force of MRG, and the increment of magnetic field enhanced the normal force obviously. However, the other factors in the presence of a constant magnetic field only have weak effects on the normal force. This behavior can be regard as the magnetic field-dependent normal force, and the mechanism of interaction between the magnetic field and normal force was investigated by microstructure analysis.