Detection of extremely large magnetoresistance in a ring-shaped array of magnetic quantum dots with very high performance and controllable parameters

By means of Green’s function technique, we study the magnetoresistance (MR) effect in a ring-shaped array of magnetic quantum dots (QDs), with or without magnetic leads, while the magnetic QDs...

Attenuation and rheological analysis of magnetic field-induced responsiveness for the magnetorheological elastomer-based composite

A novel self-supporting multi-layer magnetorheological elastomer-based (MRE-based) composite with large magnetic field-induced responsiveness has been designed and fabricated. We characterized its morphological properties, evaluated the impact of fabrication conditions on its field-induced responsiveness, investigated attenuation of its field-induced responsiveness under different storage temperatures along with time and analyzed this mechanism from the perspective of rheology. The results showed that the MRE-based composite had homogeneous dispersing of the magnetic fillers and a clear interface between different layers. The field-induced responsiveness of the MRE-based composite could be affected by the fabrication conditions, and it attenuated at different rates when subjected to different storage temperatures along with time; its attenuation period lasted a few days under room temperature while over one month under low temperature (4℃). The rheological analysis results indicated a long-term cross-linking process over the storage period along with the attenuation of field-induced responsiveness, which might lead to increasing elasticity (indicated by the loss factor tan δ) and rigidity (indicated by the storage modulus G') of the MRE-based composite along with the storage period. What's more, emerging feature of Payne effect could be found on MRE-based composite during cyclic shear, which indicated decline of the mechanical properties due to strain-induced inherent friction. On the other hand, the iron fillers in MRE layer could enhance the shear modulus and lead to MR effect (up to 187%) for the whole composite, which benefits to the magnetic field-induced responsiveness, due to the relative strengthen of the MRE layer against the assist layer. This work presents a better understanding on the attenuation of the field-induced responsiveness, which is important for the future application of the MRE-based composite.

Dual Properties of Polyvinyl Alcohol-Based Magnetorheological Plastomer with Different Ratio of DMSO/Water

Polyvinyl alcohol (PVA)-based magnetorheological plastomer (MRP) possesses excellent magnetically dependent mechanical properties such as the magnetorheological effect (MR effect) when exposed to an external magnetic field. PVA-based MRP also shows a shear stiffening (ST) effect, which is very beneficial in fabricating pressure sensor. Thus, it can automatically respond to external stimuli such as shear force without the magnetic field. The dual properties of PVA-based MRP mainly on the ST and MR effect are rarely reported. Therefore, this work empirically investigates the dual properties of this smart material under the influence of different solvent compositions (20:80, 40:60, 60:40, and 80:20) by varying the ratios of binary solvent mixture (dimethyl sulfoxide (DMSO) to water). Upon applying a shear stress with excitation frequencies from 0.01 to 10 Hz, the storage modulus (G′) for PVA-based MRP with DMSO to water ratio of 20:40 increases from 6.62 × 10−5 to 0.035 MPa. This result demonstrates an excellent ST effect with the relative shear stiffening effect (RSTE) up to 52,827%. In addition, both the ST and MR effect show a downward trend with increasing DMSO content to water. Notably, the physical state of hydrogel MRP could be changed with different solvent ratios either in the liquid-like or solid-like state. On the other hand, a transient stepwise experiment showed that the solvent’s composition had a positive effect on the arrangement of CIPs within the matrix as a function of the external magnetic field. Therefore, the solvent ratio (DMSO/water) can influence both ST and MR effects of hydrogel MRP, which need to be emphasized in the fabrication of hydrogel MRP for appropriate applications primarily with soft sensors and actuators for dynamic motion control.

A Comparison Study on the Magneto-Responsive Properties and Swelling Behaviors of a Polyacrylamide-Based Hydrogel Incorporating with Magnetic Particles

This work investigates the mechanical properties, microstructures, and water-swelling behavior of a novel hydrogel filled with magnetic particles. The nanoparticles of magnetite (Fe3O4) and the micro-particles of carbonyl iron (CI) were selected and filled into a polyacrylamide (PAAM) hydrogel matrix to create two types of magnetic hydrogels. The isotropy and anisotropy of magnetic hydrogels are also presented in this study. The isotropic samples were cured without applying a magnetic field (MF), and the anisotropic samples were cured by applying an MF in the direction perpendicular to the thickness of the samples. The effects of the size, content, and inner structures of magnetic particles on the magneto-responsive and swelling properties of magnetic hydrogels were investigated. It was found that the magnetorheological (MR) effect of anisotropic samples was apparently higher than that of isotropic samples, and the hydrogels with CI exhibited a noticeable MR effect than those with Fe3O4. The storage modulus can be enhanced by increasing the filler content and size, forming an anisotropic structure, and applying an external MF. In addition, the magnetic hydrogels also have a swelling ability that can be tuned by varying the content and size of the particle fillers.

Studies with Rheological Behavior of Composite Lithium-Based Magnetorheological Grease

Magnetorheological (MR) grease has advantages of the anti-settlement behavior and excellent sealing property compared with conventional MR materials. However, the rheological behaviors of MR effect and shear yield stress are too small to limit the further applications with MR grease. This paper proposes a composite lithium-based MR grease with boric acid-hydroxyl stearic acid to improve the rheological behaviors. Eight different samples of composite lithium-based MR grease with different ratios of mass between lithium stearate and lithium borate are prepared by the saponification method. The rheological behaviors are tested and discussed with qualitative and quantitative analysis. The experimental results show that the off-state viscosity reduces with the decrement of the ratio of mass under low shear rate which changes from 68.7 Pa·s to 16.5 Pa·s. Lithium stearate content has more effects with off-state viscosity. Based on the Herschel-Bingham model, the shear stress of composite lithium-based MR grease can be improved dramatically by adjusting the ratio of mass which is increased by 170% under the magnetic flux density of 0.2 T. Compared with single lithium-based MR grease, the maximum yield shear stress is increased by 166.7% at off-state and the maximum MR effect is also increased by 19.1%. The MR effect can reach 23,600% with a specific mass ratio of the composite MR grease. The experimental results validate that the feasibility of the performance improvement by the composite lithium-based MR grease.

Simulations on the rheology of dry magneto-rheological fluid under various working modes

This work studied the rheological properties and magnetorheological (MR) mechanism of dry magnetorheological fluid (MRF) under various working modes. A novel simulation method combining the discrete element method and computational fluid dynamics was developed, in which the bilateral coupling between particles and the flow field of the matrix (air) was considered. The microstructures and mechanical properties in the redispersion process, shear mode, and valve mode were systematically simulated for the first time. The results indicated that dry MRF presented superior redispersion property and response time (several μs) than liquid-based MRFs. In shear mode, the magnetic dipolar force and friction force dominated the evolution of microstructures. In valve mode, the magnetic dipolar force and viscous drag force of air became the main interactions. Magnetic particles aggregated into sturdy chain structures and hindered the airflow. The MR effect in valve mode was the pressure gradient of the matrix, which increased up to 1.08×105 Pa/m with the increasing particle volume fractions and decreased under a large inflow velocity. The best MR effect in valve mode was achieved under a magnetic field of B=63 mT. Simulations revealed the influence of dimensionless Mn and Re number on the MR effect. The pressure gradient of the matrix was controlled by the external field and can be utilized to design a dry MRF valve for precious and transient vibration control. Simulated dimensionless shear stress in shear mode agreed well with experiments. This work will promote the development and applications of novel high-performance MRFs.

Effect of Mould Orientation on the Field-Dependent Properties of MR Elastomers under Shear Deformation

In this study, magnetorheological elastomer (MRE) was fabricated using an electromagnetic device with a new configuration mold at the orientation of 0°, 45° and 90°. This new curing concept enhanced the alignment of carbonyl iron particles (CIPs) within the silicone matrix in the presence of silicone oil (SO) during solidifying, by eliminating air gaps to prevent magnetic flux losses. Using a mold made of steel, which is a magnetic material, the mold functions as a guide for concentrated magnetic flux of 0.315 T to pass through the MRE sample. Scanning electron microscopy (SEM) was used to observe the surface morphology of the fabricated MRE samples particularly the alignment of the CIPs. The field-dependent dynamic properties of the MREs were measured using a rheometer. The analysis implied that the effectiveness of the MRE operating under shear deformation with this curing concept provided the highest magneto-induced modulus of 1.01 MPa when a 45° orientation mold is used, with relative magnetorheological (MR) effect value up to 918%, followed by 0° mold orientation with 0.79 MPa magneto-induced modulus and 646% relative MR effect. The high modulus properties offered by this MRE are believed to be potentially useful in industrial applications where a high range of stiffness is required particularly in the shear direction.

Study on the dynamic mechanical properties of magnetorheological elastomer (MRE) with Fe@C

Magnetorheological elastomer (MRE) is a promising type of smart material, suitable for shock absorbers in tank engine mount systems. In order to improve the performance of the shock absorber, we improve the magnetorheological (MR) effect of the MREs and reduce its loss factor by enhancing the interaction force between the particles and the substrate. In this work, a novel carbon-coated iron material was prepared by heat treatment, and then carbon-coated iron (Fe@C) of different proportions was added to natural rubber to prepare MRE samples. The tensile strength and elongation at break of the MRE samples were tested by a universal electronic tension machine. The results indicated that better bonding strength between the particles and the matrix increased in tensile strength. The MR effect and the loss factor of the MRE samples were tested by a rheometer. The results demonstrated that the MR effect of MREs modified with Fe@C was significantly improved, and the loss factor of this kind of MRE decreased markedly.

Huge magnetoresistance in topological insulator spin-valves at room temperature

Topological insulators (TI) have extremely high potential in spintronic applications. Here, a topological insulators thin-film (TITF) spin valve with the use of the segment gate-controlled potential exhibits a huge magnetoresistance (MR) value higher than 1000% at room temperature which is more than 50 times the MR of typical topological insulators (TI) spin-valves. A high spin-polarized current is provided by the band structure generated by the tunable segment potential. The results reveal a very large resistance difference between the parallel and antiparallel configurations. The MR effect is strongly influenced by the thin-film thickness, the gate potential, the gate size, and the distribution. The proposed results will help to not only improve the room-temperature performance of the spin-valves but also enhance the applications of magnetic memories and spintronic devices.

Experimental Investigation on the Effect of Graphene Oxide Additive on the Steady-State and Dynamic Shear Properties of PDMS-Based Magnetorheological Elastomer

Isotropic polydimethylsiloxane (PDMS)-based magnetorheological elastomers (MREs) filled with various contents of graphene oxide (GO) additive were fabricated by the solution blending-casting method in this work. The morphologies of the produced MREs were characterized, and the results indicate that the uniform distribution of GO sheets and carbonyl iron particles (CIPs) becomes difficult with the increase of GO content. The steady-state and dynamic shear properties of the MREs under different magnetic field strengths were evaluated using parallel plate rheometer. It was found that the physical stiffness effect of GO sheets leads to the increase of the zero-field shear modulus with increasing GO content under both the steady-state and dynamic shear loads. The chemical crosslinking density of PDMS matrix decreases with the GO content due to the strong physical crosslinking between GO and the PDMS matrix. Thus, the MREs filled with higher GO content exhibit more fluid-like behavior. Under the dynamic shear load, the absolute MR effect increases with the GO content due to the increased flexibility of the PDMS matrix and the dynamic self-stiffening effect occurring in the physical crosslinking interfaces around GO sheets. The highest relative MR effect was achieved by the MREs filled with 0.1 wt.% GO sheets. Then, the relative MR effect decreases with the further increase of GO content due to the improved zero-field modulus and the increased agglomerations of GO and CIPs. This study shows that the addition of GO sheets is a possible way to prepare new MREs with high MR effect, while simultaneously possessing high zero-field stiffness and load bearing capability.