Alignment Of Magnetic Particles Research Articles

Electrically/Magnetically Dual‐Driven Shape Memory Composites Fabricated by Multi‐Material Magnetic Field‐Assisted 4D Printing

AbstractShape memory polymers (SMPs) are smart materials that enable to transform back to their original shape from the deformed state when subjected to external stimuli. They have shown great potential used as sensors and actuators in diverse applications. However, current research on SMPs primarily focuses on the utilization of a single source of stimuli (e.g., electricity, magnetism, light, etc.), which heavily restricts their potential in complex circumstances. In this study, a novel approach is developed to fabricate multi‐layer electrically/magnetically dual‐driven shape memory composites (ML‐EMSMCs) based on a magnetic field‐assisted digital light processing (MF‐DLP) 4D printing technique. The fabricated ML‐EMSMCs contain alternating high electric conductive layers (up to 5.37 × 10−3 S cm−1) and magnetic responsive layers (10.7 emu g−1), enabling Joule heat‐based and high‐frequency magnetic field induction‐based stimuli. Furthermore, the ML‐EMSMCs exhibited excellent shape memory behavior, good formability, and magnetic properties. The developed 4D printing techniques allows for the alignment of magnetic particles with a unidirectional magnetic field, significantly improving their shape recovery speed. The developed electrically/magnetically dual‐driven and photocurable SMP composites shed light on the development of actuators and sensors with multiple functionalities.

Finite element modeling of the rotational dynamics of a single magnetic particle in a strong magnetic field and liquid medium

We have performed a comparative computational study of magnetic particle alignment in a strong magnetic field and ambient viscous fluid using the first-principles Navier–Stokes equation as well as numerical modeling using the finite element method (FEM). FEM solution has been compared with the solution of the unsteady rotations of a magnetic particle in a viscous fluid during the alignment process described with nonlocal integro-differential equations (IDEs) for torque and angular velocity. The assumption of nonlocality comes from the history term of acceleration torque with a non-Basset kernel function, which has its origin in the presence of vortices in the flow of a particle fluid environment due to its unsteady rotation and ambient fluid inertia and friction. The flow vortices of the ambient fluid are explicitly shown in the solution of the FEM model. Moreover, the solution of the time evolution of the alignment angle and angular velocity for the FEM model are in good agreement with an IDE model which we have recently developed. The obtained results are justification for interchangeability of the FEM and IDE models, which may have important consequences for large-scale simulations of magnetic microparticles.

Development of stimuli-responsive flexible micropillar composites via magneto-induced injection molding and characterization of magnetic particle alignment

Magnetic soft composites have emerged as a promising soft actuator with a high degree of precision in the magnetic field stimuli system. But, their mass manufacturing strategies have yet to be developed, and most studies focus on small-scale near-net shaping production. Here, we first apply injection molding (which only takes a few seconds to shape) to fabricate the stimuli-responsive flexible micropillar composites with magnetic particle alignment design using an external magnetic field. The arrays exhibit magnetically anisotropic particle alignment up to 82.57 % along the longitudinal direction, showing a magnetic bending actuation response. We achieve a structural novelty of the magnetic micropillar with a high aspect ratio of up to 10 and pattern sizes of 50 μm via sacrificial LIGA insert mold and magneto-induced injection molding. For advanced mass production, the permanent metal mold is also applied to develop the micropillar composites based on the mechanical demolding approach; successful manufacturing is achieved by fabricating defect-free micropillar with 200 μm cylindrical pattern size and aspect ratio of 6. Further, the effect of powder volume fraction on the magneto-rheological behavior and corresponding magnetic performance is characterized in the injection molding process. The magnetic particle alignment trend is confirmed by the torque balance and the criteria of critical solids loading. Finally, we establish an injection molding process for magnetic soft composites, and verify the optimal powder fraction for the particle alignment.

Structure-property-actuation relationships of shape-fixable magnetic vitrimer micropillar arrays

Magnetic actuation of micropillar arrays has been exploited for contactless and programmable deformation according to pre-programmed alignment of magnetic particles within the micropillars. However, once the external magnetic field is removed, the magnetically deformed structure reverts to its initial state. Herein, we synthesized magnetic vitrimer composites as micropillar arrays capable of shape fixation via dynamic covalent bonds. In the magnetic vitrimer composites, we systematically varied the concentration of magnetic particles and evaluated their structure-property-performance relationships. An increase in the concentration of the magnetic particles increases both magnetization and modulus of the composites. Although a higher magnetization of the composites will enhance magnetic responsivity, a larger modulus represents a higher bending stiffness, reducing the deformation strain by magnetic actuation. Due to this trade-off relationship, the magnetic actuation of the micropillars is determined by two variables. In competition between two variables, embedding 20 vol% of magnetic particles achieves the maximum bending angle of 45° for the micropillars. By understanding the structure-property relationships, we can optimize the concentration of magnetic particles for a high magnetic responsivity of the shape-fixable micropillar arrays.

Numerical Investigation of the Orientability of Single Reinforcement Fibers in Polymer Matrices.

Fiber-reinforced polymers are increasingly being used, especially in lightweight structures. Here, the effective adaptation of mechanical or physical properties to the necessary application or manufacturing requirements plays an important role. In this context, the alignment of reinforcing fibers is often hindered by manufacturing aspects. To achieve graded or locally adjusted alignment of different fiber lengths, common manufacturing technologies such as injection molding or compression molding need to be supported by the external non-mechanical process. Magnetic or electrostatic fields seem to be particularly suitable for this purpose. The present work shows a first simulation study of the alignment of magnetic particles in polymer matrices as a function of different parameters. The parameters studied are the viscosity of the surrounding polymer as a function of the focused processing methods, the fiber length, the thickness and permeability of the magnetic fiber coatings, and the magnetic flux density. The novelty of the presented works is in the development of an advanced simulation model that allows the simulative representation and reveal of the fluid–structure interaction, the influences of these parameters on the inducible magnetic torque and fiber alignment of a single fiber. Accordingly, the greatest influence on fiber alignment is caused by the magnetic flux density and the coating material.

A Theoretical Analysis of Magnetic Particle Alignment in External Magnetic Fields Affected by Viscosity and Brownian Motion

The interaction of an external magnetic field with magnetic objects affects their response and is a fundamental property for many biomedical applications, including magnetic resonance and particle imaging, electromagnetic hyperthermia, and magnetic targeting and separation. Magnetic alignment and relaxation are widely studied in the context of these applications. In this study, we theoretically investigate the alignment dynamics of a rotational magnetic particle as an inverse process to Brownian relaxation. The selected external magnetic flux density ranges from 5μT to 5T. We found that the viscous torque for arbitrary rotating particles with a history term due to the inertia and friction of the surrounding ambient water has a significant effect in strong magnetic fields (range 1–5T). In this range, oscillatory behavior due to the inertial torque of the particle also occurs, and the stochastic Brownian torque diminishes. In contrast, for weak fields (range 5–50μT), the history term of the viscous torque and the inertial torque can be neglected, and the stochastic Brownian torque induced by random collisions of the surrounding fluid molecules becomes dominant. These results contribute to a better understanding of the molecular mechanisms of magnetic particle alignment in external magnetic fields and have important implications in a variety of biomedical applications.

Effect of alignment of magnetic particles on the rheological properties of natural rubber composite

Effect of alignment of magnetic particles on the rheological properties of natural rubber composite

Particle organization versus volume fraction in magneto-active elastomer composites

Magneto-active elastomers (MAEs) are a subclass of smart materials that have the ability to actuate and alter their mechanical and magnetic response when subjected to an external magnetic field. Past research has shown that higher net magnetic remanence of an MAE is desirable to increase actuation and produce the most magnetic work. To create a magnetically aligned MAE, a hard magnetic material is mixed in an elastomer solution, then cured in a strong external magnetic field causing particle alignment. A high degree of alignment of magnetic particles is expected to result in improved remanence. However, in MAEs with soft magnetic particles cured in a field, past research has shown that the degree of alignment, vs. less ordered clustering, varies with particulate volume content. Consequently, it is important to study the degree to which volume content of magnetic material affects physical particle alignment, bulk magnetic properties, and the coupling between the two in MAEs with hard magnetic filler particles. In this study MAEs with a hard magnetic filer (barium ferrite) are investigated with varying volume content of magnetic material ranging from 5 to 30% by volume. Batches of MAEs were tested using X-ray diffraction and vibrating sample magnetometry to gain information on crystal structure and bulk magnetic properties. The results of the collected data suggested all poled samples had both higher physical particle orientation as well as a larger remanence than the unpoled counterparts. As volume fraction of magnetic filler was increased the net magnetic properties per volume of magnetic material remained fairly constant. However, the degree of physical particle orientation varied with a local minimum at medium volume fractions. Collected data was used to calculate two orientation parameters, the degree of preferred alignment parameter, η, and the orientation of distributions of magnetic domains parameter, σ. The two parameters were compared to show a relationship between physical particle alignment and net magnetic properties as volume of magnetic material increases. Based on the collected results, a hypothesis is presented on how particle interaction is speculated to evolve as volume fraction of magnetic material is increased.

Alignment of magnetic particles in anisotropic Nd–Fe–B bonded magnets

High-performance anisotropic bonded magnets need a high volumetric filling fraction of magnetic powder in a polymer binder and good alignment while possessing adequate mechanical properties. The correlation between the degree of alignment (DOA) and the volumetric filling fraction of magnetic powder, the type of binder and the processing parameters have been studied using Nd–Fe–B anisotropic bonded magnets. Thermomagnetic analysis and differential scanning calorimetry results show that the magnetic powder can be well aligned in a bonded magnet under an external magnetic field at a temperature higher than the binders’ melting point by 20–60 K, depending on the type of magnetic powder and binder. The DOA results mainly from the interplay between the Zeeman energy (E app) and the inter-particle static energy (E stat) in bonded magnets at the chosen alignment temperature and the magnetic fields. A good alignment can be achieved by an alignment magnetic field which is about twice its coercivity in an anisotropic Nd–Fe–B bonded magnet, which is confirmed by our experimental and modeling results.

Alignment of magnetic particles in hydrogel matrix: A novel anisotropic magnetic hydrogels for soft robotics

Magnetic hydrogels are composed of magnetic particles and hydrogel matrix. In recent years, the magnetic hydrogels have been developed rapidly because they have shown promising applications in drug release and artificial muscle. In this paper, we proposed a study to develop novel anisotropic magnetic hydrogels and investigate their mechanical and sensing properties for possible applications in soft robotics. In preparing the anisotropic magnetic hydrogels, the polyacrylamide (PAAm) hydrogel is chosen as a model hydrogel because of its popular application in soft electronics and ionic conductors. A method of free radicals copolymerization is employed to fill (polyacrylic acid/acrylamide) polymers in preparing anisotropic hydrogels under the magnetic field. Unlike most of the previous studies which incorporated magnetic nanoparticles into hydrogels, we mixed the micro-size carbonyl iron particles (CIPs) with the hydrogel and cured them under a magnetic field to form anisotropic structures within its crosslinking polymer chains. The particles and formed particle chains will not only improve the mechanical properties of the hydrogels but also provide sensing function as the electrical resistance changed from mechanical deformation referred to piezoresistivity. We experimentally evaluated the magnetorheological and the piezoresistive behaviors of the magnetic hydrogels, and demonstrated their potential use in soft robots as flexible touch sensors and variable-stiffness devices.

Mechanical Orientation of Fine Magnetic Particles in Powders by an External Magnetic Field: Simulation‐Based Optimization

A numerical algorithm for predicting the optimal conditions for the effective alignment of magnetic particles in dense powders during the compactization process using an externally applied field is presented. This task is especially important for the development of permanent magnets due to the fact that the alignment of anisotropy axes of nanocomposite grains increases both remanence and coercivity of magnetic materials. In contrast to previously known methods where the magnetic moment of each particle is assumed to be “fixed” with respect to the particle itself, the approach considers the (field‐dependent) deviation of this moment from the particle anisotropy axis that occurs even for magnetically “hard” particles possessing a strong mechanical contact. It is shown that this deviation leads to the existence of the optimal value of the applied field for which the particle orientation (or alignment) time is minimal. The influence of the external pressure and internal mechanical friction on the details of the compactization/orientation process is also studied.

Micropillar Arrays: Enhancement of Magneto‐Mechanical Actuation of Micropillar Arrays by Anisotropic Stress Distribution (Small 38/2020)

In the article number 2003179, Jeong Jae Wie and co‐workers report enhanced twisting or bending actuation of rectangular micropillars containing pre‐programmed horizontal or vertical magnetic particle alignment. Anisotropic stress distributions of rectangular pillars drastically enhance magneto‐mechanical actuation of micropillars in comparison to square micropillars. Magnetically deformed micropillars are selectively shape‐fixed by local evaporative assembly of polymeric solution even after removal of the magnetic field.

Functionalizing magnet additive manufacturing with in-situ magnetic field source

Additive manufacturing via 3-D printing technologies have become a frontier in materials research, including its application in the development and recycling of permanent magnets. This work addresses the opportunity to integrate magnetic field sources into 3-D printing process in order to enable printing, alignment of anisotropic permanent magnets or magnetizing of magnetic filler materials, without requiring further processing. A non-axisymmetric electromagnet-type field source architecture was designed, modelled, constructed, installed to a fused filament commercial 3-D printer, and tested. The testing was performed by applying magnetic field while printing composite anisotropic Nd-Fe-B + Sm-Fe-N powders bonded in Nylon12 (65 vol.%) and recycled Sm-Co powder bonded in PLA (15 vol.%). Magnetic characterization indicated that the degree-of-alignment of the magnet powders increased both with alignment field strength (controlled by the electric current applied to the magnetizing system) and the printing temperature. Both coercivity and remanence were found to be strongly dependent on the degree-of-alignment, except for printing performed below but near the Curie temperature of Nd-Fe-B (310 ° C). At applied field of 0.15 kOe, Sm-Co and hybrid Nd-Fe-B/Sm-Fe-N printed samples showed degrees-of-alignment of 83 % and 65 %, respectively. The variations in coercivity were consistent with previous observations in bonded magnet materials. This work verifies that integration of magnetic field sources into 3-D printing processes will result in magnetic alignment of particles while ensuring that other advantages of 3-D printing are retained.

Ferromagnetic particle structuring in material jetting - Manufacturing control system and software development

This paper presents a novel integrated material-manufacturing-control system to print parts with oriented ferromagnetic particles. The proposed, designed, and constructed Additive Manufacturing (AM) system, based on material jetting, photo polymerization, real-time control systems, and active magnetic particle alignment control, allows local orientation control of a specialized ferromagnetic suspension in photosensitive resin to fabricate objects with anisotropic ferromagnetic properties. The proposed system builds upon the standard slicing and path planning algorithms used by opensource 3D printing software and extends it by adding two extra dimensions related to the orientation of the magnetic particles within the printing process itself, enabling the online control of magnetic orientation within the printed parts. The result is a successful prototype device and software which produces cured composite materials whose microscopic grain orientations are validated by microscopy. The prototype device shows excellent potential to 3D print materials with complex directional magnetic properties.

Characterization of magnetic particle alignment in photosensitive polymer resin: A preliminary study for additive manufacturing processes

Material jetting 3D printing is an additive manufacturing technique that allows producing complex parts without tooling and minimum material wastage. In this study, orientation control of randomly shaped, anisotropic hard magnetic ferrite particles is demonstrated for material jetting-based additive manufacturing processes using a developed particle alignment configuration. Strontium ferrite and PR-48 photosensitive resin were used as the base materials. An automated experimental setup with two neodymium permanent cube magnets capable of generating a dipolar magnetic field was built to align magnetic particles in the resin. Particle alignment was characterized for directionality using images obtained through real time optical microscopy. The orientation of magnetic particles was observed to be dependent on the distance of separation between the cube magnets and the magnetization time. X-ray diffraction was used to indicate the c-axis alignment of the hexagonal strontium ferrite particles in the cured specimens. The influence of process parameters on particle orientation was evaluated, employing a full factorial experiment analysis. This fundamental research serves as a basis for constructing and optimizing the magnetic particle alignment setup for additive manufacturing processes.

Design of Microscale Magnetic Tumbling Robots for Locomotion in Multiple Environments and Complex Terrains.

This paper presents several variations of a microscale magnetic tumbling (TUM) robot capable of traversing complex terrains in dry and wet environments. The robot is fabricated by photolithography techniques and consists of a polymeric body with two sections with embedded magnetic particles aligned at the ends and a middle nonmagnetic bridge section. The robot’s footprint dimensions are 400 m × 800 m. Different end geometries are used to test the optimal conditions for low adhesion and increased dynamic response to an actuating external rotating magnetic field. When subjected to a magnetic field as low as 7 mT in dry conditions, this magnetic microrobot is able to operate with a tumbling locomotion mode and translate with speeds of over 60 body lengths/s (48 mm/s) in dry environments and up to 17 body lengths/s (13.6 mm/s) in wet environments. Two different tumbling modes were observed and depend on the alignment of the magnetic particles. A technique was devised to measure the magnetic particle alignment angle relative to the robot’s geometry. Rotational frequency limits were observed experimentally, becoming more prohibitive as environment viscosity increases. The TUM’s performance was studied when traversing inclined planes (up to 60°), showing promising climbing capabilities in both dry and wet conditions. Maximum open loop straight-line trajectory errors of less than 4% and 2% of the traversal distance in the vertical and horizontal directions, respectively, for the TUM were observed. Full directional control of TUM was demonstrated through the traversal of a P-shaped trajectory. Additionally, successful locomotion of the optimized TUM design over complex terrains was also achieved. By implementing machine vision control and/or embedding of payloads in the middle section of the robot, it is possible in the future to upgrade the current design with computer-optimized mobility through multiple environments and the ability to perform drug delivery tasks for biomedical applications.

Magnetic-Field Sensitivity of Storage Modulus for Bimodal Magnetic Elastomers.

The magnetic-field dependence of the storage modulus for bimodal magnetic elastomers consisting of carbonyl iron with a diameter of 2.8 μm (magnetic) and aluminum hydroxide with a diameter of 1.4 μm (nonmagnetic) was measured, and the effect of nonmagnetic particles on the magnetic-field sensitivity of the storage modulus was investigated. The coefficient of the magnetic-field sensitivity for the monomodal magnetic elastomer increased from 0.018 to 0.026 mT-1 for the bimodal one by embedding nonmagnetic particles of 6.6 vol %. At volume fractions above 5.4 vol %, the bimodal magnetic elastomer exhibited significant nonlinear viscoelasticity at 0 mT and a high storage modulus at 500 mT, simultaneously, the coefficient of the magnetic-field sensitivity demonstrated high values. This strongly indicates that both the particles form a particle network at the off-field, and they make a well-developed chain structure under magnetic fields. The time profiles of the storage modulus for bimodal magnetic elastomers can be fitted by a linear combination of two exponential functions with two characteristic times showing the alignment of magnetic particles. The alignment time for the fast and slow processes was distributed around 3.3 ± 0.3 and 176 ± 12 s, respectively. The alignment time was independent of the volume fraction of the nonmagnetic particles; however, the increment in the storage modulus for the fast process significantly increased at volume fractions above 5.4 vol %. It was also revealed that the coefficient of the magnetic-field sensitivity can be scaled by a power function of the increment in the storage modulus divided by the off-field modulus, ΔG'/G'0, not only for the bimodal magnetic elastomers but also for the monomodal ones.

Magnetic alignment of short carbon fibres in curing composites

Abstract Alignment of magnetic particles into a viscous fluid by a homogeneous magnetic field has been studied both experimentally and theoretically, but very few studies have investigated the effects of viscosity changes on the capability of achieving complex fibre assemblies in composite materials. In this paper, we study the alignment of short carbon fibres into a viscous matrix whose viscosity is made dependent on the time, in order to simulate the curing process of the composite. A simple model is derived which gives the evolution of fibre position and orientation in terms of the external field and shows good agreement with the experimental evidence on nickel coated carbon fibres embedded in PDMS. The model is used to predict the fibre distribution at curing and hence could be a useful tool to predict the mechanical, electrical and magnetic properties of the composite. A closed form expression of the minimum magnetic field intensity necessary to achieve the desired orientation is derived in terms of the magnetic and geometric properties of the fibres. This equation can be used to guide experiments, and offers a way to discriminate which matrix/fibre configuration can be most proficiently used to have a highly ordered composite.

Polymeric foam-ferromagnet composites as smart lightweight materials

A new class of lightweight smart materials based on a polymeric matrix with embedded magnetic micro-particles was developed. The application of a magnetic field (MF) during the foaming of samples induced, along the MF lines, the alignment of magnetic particles dispersed in the polymer thus forming chain-like reinforcing structures. The aligned micro-particles induced an anisotropic mechanical behaviour, strongly improving the mechanical stiffness and strength along the MF direction compared to unfilled systems. Most notably, the chain-like structures imparted a magneto-sensitive behaviour to the lightweight materials. In fact, foams showed a direct relationship between the foams elastic response and the intensity as well as the shape of the time dependent MF applied during their magneto-elastic characterisation. This magneto-elastic behaviour has been obtained at low MF strength (below 200 kA m−1).

Enhanced microactuation with magnetic field curing of magnetorheological elastomers based on iron–natural rubber nanocomposites

The incorporation of nanoparticles of iron in a natural rubber matrix leads to flexible magnetorheological (MR) materials. Rod-shaped MR elastomers based on natural rubber and nanosized iron have been moulded both with and without the application of an external magnetic field during curing. These MR elastomer rods and filler material were characterized by X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. Magnetic properties were investigated by using vibrating sample magnetometry. Microactuation studies were carried out by employing a laser Doppler vibrometer. It is seen that microactuation of field cured samples have been enhanced by two times when compared with that of zero field cured samples. The effect of alignment of magnetic particles during field-assisted curing was also studied by using a dynamic mechanical analyzer. A plausible model is put forwarded to explain the observed enhancement of actuation for field cured samples.