Hard Fillers Research Articles

Possibilities in Recycling Magnetic Materials in Applications of Polymer-Bonded Magnets

Polymer-bonded magnets have increased significantly in the application of drive technology, especially in terms of new concepts for the magnetic excitation of synchronous or direct current (DC) machines. To satisfy the increasing demand of hard magnetic filler particles and especially rare earth materials in polymer-bonded magnets, different strategies are possible. In addition to the reduction in products or the substitution of filler materials, the recycling of polymer-bonded magnets is possible. Different strategies have to be distinguished in terms of the target functions such as the recovery of the matrix material, the filler or both materials. In terms of polymer-bonded magnets, the filler material—especially regarding rare earth materials—is important for the recycling strategy due to the limited resource and high costs. This paper illustrates two different recycling strategies relative to the matrix system of polymer-bonded magnets. For thermoset-based magnets, a thermal strategy is portrayed which leads to similar magnetic properties in terms of the appropriated atmosphere and process management. The mechanical reusage of shreds is analyzed for thermoplastic-based magnets. The magnetic properties are reduced by about 20% and there is a change in the flow conditions and with that, an influence on the pole accuracy.

Correlation between the Flow and Curing Behavior of Hard Magnetic Fillers in Thermosets and the Magnetic Properties

Polymer bonded magnets based on thermoplastics are economically produced by the injection molding process for applications in sensor and drive technology. Especially the lack of orientation in the edge layer, as well as the chemical resistance and the creep behavior limit the possible implementations of thermoplastic based polymer bonded magnets. However, thermoset based polymer bonded magnets have the opportunity to expand the applications by complying with the demands of the chemical industry or pump systems through to improved chemical and thermal resistance, viscosity and creep behavior of thermosets. This paper investigates the influence of hard magnetic particles on the flow and curing behavior of highly filled thermoset compounds based on an epoxy resin. The basic understanding of the behavior of those highly filled hard magnetic thermoset systems is essential for the fabrication of polymer bonded magnets based on thermosets in the injection molding process. It is shown that several factors like the crystal structure, the particle shape and size, as well as the thermal conductivity and the adherence between filler and matrix influence the flow and curing behavior of highly filled thermoset compounds based on epoxy resin. However, these influencing factors can be applied to any filler system with respect to a high filler amount in a thermoset compound, as they are based on the material behavior of particles. Further, the impact of the flow and curing behavior on the magnetic properties of polymer bonded magnets based on thermosets is shown. With that, the correlation between material based factors and magnetic properties within thermosets are portrayed.

Carbon Nanotube Composites with Bottlebrush Elastomers for Compliant Electrodes.

Wearable electronics and biointerfacing technology require materials that are both compliant and conductive. The typical design strategy exploits polymer composites containing conductive particles, but the addition of a hard filler generally leads to a substantial increase in modulus that is not well-matched to biological tissue. Here, we report a new class of supersoft, conductive composites comprising carbon nanotubes (CNT) embedded in bottlebrush polymer networks. By virtue of the bottlebrush polymer architecture, these materials are several orders of magnitude softer than comparable composites in the literature involving linear polymer networks. For example, a CNT content of 0.25 wt % yields a shear modulus of 66 kPa while maintaining a typical conductivity for a CNT composite (ca. 10-2 S/m). An added benefit of this bottlebrush matrix chemistry is the presence of dynamic polyester bonds that facilitate thermal (re)processing. This unique strategy of designing soft composites provides new opportunities to tailor the structure and properties of sustainable advanced materials.

Injection Moulding of Multipolar Polymer-Bonded Magnets into Soft Magnetic Inserts for Rotors in Reluctance Motors

Due to lower magnetic properties of polymer-bonded magnets compared to sintered magnets, a complete redesign of the multipolar soft magnetic flux barriers in rotors with alignment guidelines was carried out to eliminate the frequently used rare-earth magnets, causing a new influence of the outer magnetic field on the cavity by using soft magnetic inserts. Within this new process, the main influencing factors on the magnetic flux density such as filler content, tool temperature, holding pressure and injection velocity were analysed and correlated. The studies were based on the compound of Polyamide 12 and up to a 60 vol.-% of the hard magnetic filler, strontium ferrite. Based on the study, the injection moulding of multipolar-bonded magnets into soft magnetic inserts for rotors and, in turn, into complex geometries can be optimized in terms of the orientation of the filler, the microstructure and the magnetic flux density. The investigations show no significant influence of the process parameters known from the literature such as the mass temperature Tm, which affects the magnetic flux density, as well as the orientation and the microstructure similar to tool temperature Tt, but is less efficient. The main influencing factors identified during the investigations are the tool temperature Tt, the injection velocity vin and the holding pressure ph. As known influencing factors are only based on simple geometries such as ring structures or plates, new factors were determined for complex rotor geometries.

APPLICATION OF WC GRAINS BY DEPOSITION BRAZING

The article describes problems of hard depositional brazing of wolfram-carbide grains in various matrixes. Especially iron and nickel base matrix on base material from constructional carbon steel. The experimental part was brazed by oxyacetylene flame (method 912 according to EN ISO 4063). The major goals of the experiment were to describe the technological procedure of hard depositional brazing of this specific part. Consequently, were proved wolfram-carbide deposit on two test coupons. These test coupons were metallographically and qualitatively evaluated. The advantage of wolfram-carbide depositional brazing is a service life of deposit, that could be multiple higher, than standard weld deposit with carbon, chromium and vanadium filler material. The advantage is also costly because this kind of braze is cheaper than hard surfacing filler material. This specific application of the wolfram-carbide layer is one of the hardest coatings that could be by flame brazing technology performed. Usage of this application is for example in the mining, woodcutting industry, where it is necessary to improve abrasion and resistance, service life, and guaranteed durability or hardness for cutting tools.

Mechanical design and analytic solution for unfolding deformation of locomotive ferromagnetic robots

Magnetically actuated robots are attracting much interest due to the advantages of fast response, remote manipulation and enabling operations in enclosed spaces. Recent advances in fabricating ferromagnetic polymeric matrices embedded with hard magnetic fillers provide routes to multimodal locomotion for soft-bodied robots. One limitation of these matrix-based robot designs is that it requires low volume fraction of hard magnetic fillers to achieve soft and compliant robot body such that moderate magnetic fields are sufficient for actuation. However, low volume fraction of functional magnetic fillers leads to magnetically weak soft robots that are difficult to actuate. Here, we propose a compliant and high-performance robot design operating at magnetic fields down to 1 mT by utilizing a high-quality ferromagnetic film and mechanics-guided three-dimensional (3D) assembly technique. A parylene coating is deposited to keep the assembled arch shape, allowing releasing and actuating the structure as a freestanding robot. The robot would unfold and fold periodically under cyclic magnetic fields, driving the robot in a desired direction. To illustrate the versatile applicability of this approach, robots in two different representative geometries are presented, one in traditional straight configuration and the other in serpentine configuration. Through theoretical analysis and finite element analysis, fundamental results are offered for the proposed robot design, including concise solutions to the unfolding deformation, the effects of coating thickness on spring back, the maximum strain in the hard ferromagnetic film and a comparison of unfolding deformation of both designs. The results clearly show the effect of geometry/material parameters, external magnetic field and prestrain in assembly process, providing essential design guidelines to compliant and fast-moving magnetic robots via the proposed method.

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.

Simultaneously enhancing the ultimate strength and ductility of high-entropy alloys via short-range ordering

Simultaneously enhancing strength and ductility of metals and alloys has been a tremendous challenge. Here, we investigate a CoCuFeNiPd high-entropy alloy (HEA), using a combination of Monte Carlo method, molecular dynamic simulation, and density-functional theory calculation. Our results show that this HEA is energetically favorable to undergo short-range ordering (SRO), and the SRO leads to a pseudo-composite microstructure, which surprisingly enhances both the ultimate strength and ductility. The SRO-induced composite microstructure consists of three categories of clusters: face-center-cubic-preferred (FCCP) clusters, indifferent clusters, and body-center-cubic-preferred (BCCP) clusters, with the indifferent clusters playing the role of the matrix, the FCCP clusters serving as hard fillers to enhance the strength, while the BCCP clusters acting as soft fillers to increase the ductility. Our work highlights the importance of SRO in influencing the mechanical properties of HEAs and presents a fascinating route for designing HEAs to achieve superior mechanical properties.

Insight into the diffusivity of particulate composites considering percolation of soft interphases around hard fillers: From spherical to polyhedral particles

The properties of particulate composites are generally conditioned by the interphase configurations in the materials, among which its percolation may trigger the change of transport properties. The three-dimensional particle with its morphology from spherical to polyhedral is used to represent the fillers in composites and the three-phase structures with hard core-soft shell networks are generated. By using the model of hard core-soft shell, the percolation of interphases in three-phase systems is simulated and an empirical function is derived to quantify the dependence of the critical percolation threshold ϕcore,c on the characteristics of multi-sized polyhedral cores and soft interphases. An analytical approximation of diffusivity that involves interphase percolation is further presented and the impact of various factors on the diffusivity of composites is predicted. The results indicate that the change of the diffusivity of composites near the percolation threshold is affected by the synergetic effect of particle morphology, fineness of fillers and interphase thickness.

Dielectric Spectroscopy of Hybrid Magnetoactive Elastomers.

Dielectric properties of two series of magnetoactive elastomers (MAEs) based on a soft silicone matrix containing 35 vol% of magnetic particles were studied experimentally in a wide temperature range. In the first series, a hybrid filler representing a mixture of magnetically hard NdFeB particles of irregular shape and an average size of 50 μm and magnetically soft carbonyl iron (CI) of 4.5 μm in diameter was used for MAE fabrication. MAEs of the second series contained only NdFeB particles. The presence of magnetically hard NdFeB filler made it possible to passively control MAE dielectric response by magnetizing the samples. It was shown that although the hopping mechanism of MAEs conductivity did not change upon magnetization, a significant component of DC conductivity appeared in the magnetized MAEs presumably due to denser clustering of interacting particles resulting in decreasing interparticle distances. The transition from a non-conducting to a conducting state was more pronounced for hybrid MAEs containing both NdFeB and Fe particles with a tenfold size mismatch. Hybrid MAEs also demonstrated a considerable increase in the real part of the complex relative permittivity upon magnetization and its asymmetric behavior in external magnetic fields of various directions. The effects of magnetic filler composition and magnetization field on the dielectric properties of MAEs are important for practical applications of MAEs as elements with a tunable dielectric response.

Interface Structure and Dynamics in Polymer‐Nanoparticle Hybrids: A Review on Molecular Mechanisms Underlying the Improved Interfaces

AbstractIn a nano‐assembly of soft polymer chains and hard filler nanoparticles (NPs), the intermolecular interactions at the polymer‐filler interface lead to variations in viscoelastic properties in the vicinity of the NP surface. However, unravelling the molecular parameters controlling the entropic and enthalpic interactions, including polymer chain conformation, dynamics, dispersion mechanisms and confinement effects across the interfacial region, has been remote and challenging. Even though the polymer‘s molecular structure and the NPs surface chemistry contribute to these interfacial interactions, the underlying mechanism behind the enhanced dynamic‐mechanical performance is still a hot topic of debate. By controlling the chemistry of NPs and polymer matrix in model nanocomposites, many efforts have been made in elucidating the physical origin of the structure and dynamics of the interfacial polymer layer with estimates varying from 1 to 10 nm around the spherical NPs. Therefore, understanding the interface structure and dynamics influenced by polymer‐NP interaction is of great technological importance. In this article, the latest polymer‐particle interface literature developments are reviewed where computational simulations, calorimetry, and dielectric spectroscopy studies have been used to examine the constraints levied by the interface structure on segmental relaxation dynamics of polymer chains in the vicinity of NP surface.

Prediction of local resonance band gaps in 2D elastic metamaterials via Bloch mode identification

This paper investigates methods of band gap formation in two-dimensional locally resonant elastic metamaterials and proposes a technique for computing such gaps as algebraic functions of integrated Bloch modes and metamaterial parameters. Mode shapes associated with the deformation of unit cells consisting of a hard matrix, soft filler, and hard resonator are investigated near the lower and upper bounds of the first local resonance band gap. Similarities reveal that Bloch modes at these bounds have the same non-dimensional geometries and locations within the irreducible Brillouin Zone irrespective of material properties, matrix and resonator thicknesses, and unit cell size. A theoretical model for the unit cell incorporating Mindlin plate theory is employed to compute the frequency of these modes in transverse vibration using conservation of energy. A least squares system identification algorithm is then described to generate surface equations for non-dimensional Bloch mode shapes computed by finite element analysis, employing a computation-saving Kronecker factorization and yielding continuous surface equations for the non-dimensional Bloch modes when rectangular cell components are utilized. Nine sample metamaterials are analyzed with this approach generating reliable predictions of the first band gap bounds. The presented framework offers insights into band gap formation, opens avenues in inverse unit cell design, and provides versatile algebraic relationships between band gap frequencies and unit cell parameters in such resonant metamaterials.

Micro hole drilling of high frequency printed circuit board containing hard fillers

High frequency printed circuit board (PCB), as the significant support for 5G technology, has been receiving intense attention from the increasing number of scholars. Concerning precision fabrication of high frequency PCBs, micro hole drilling is deemed as one of the extremely crucial challenges. In this study, the coated drill bits’ wear and micro hole quality were investigated through the drilling experiments of high frequency PCBs. Results showed that two kinds of coated drill bits exhibited well in terms of hole registration accuracy. Using diamond like carbon coated drill bits could obtain fewer burrs than drilled with diamond coated drill bits before drilling 200 holes, but the burr height increased very slowly with the increment of hole number when using diamond coated drill bits. Taking the combination of aluminum sheet and phenolic aldehyde board as the entry board was conducive to obtaining less burr, and the micro drill bits with helix angles of 40° and 42° were contributed to achieving less burr too. Additionally, decreasing the feed rate could lessen the hole blocking.

Assessments of Process Parameters on Cutting Force and Surface Roughness during Drilling of AA7075/TiB2 In Situ Composite.

Reinforced aluminum composites are the basic class of materials for aviation and transport industries. The machinability of these composites is still an issue due to the presence of hard fillers. The current research is aimed to investigate the drilling topographies of AA7075/TiB2 composites. The samples were prepared with 0, 3, 6, 9 and 12 wt.% of fillers and experiments were conducted by varying the cutting speed, feed, depth of cut and tool nose radius. The machining forces and surface topographies, the structure of the cutting tool and chip patterns were examined. The maximum cutting force was recorded upon increase in cutting speed because of thermal softening, loss of strength discontinuity and reduction of the built-up-edge. The increased plastic deformation with higher cutting speed resulted in the excess metal chip. In addition, the increase in cutting speed improved the surface roughness due to decrease in material movement. The cutting force was decreased upon high loading of TiB2 due to the deterioration of chips caused by fillers. Further introduction of TiB2 particles above 12 wt.% weakened the composite; however, due to the impact of the microcutting action of the fillers, the surface roughness was improved.

A magneto-active soft gripper with adaptive and controllable motion

Bionic soft grippers that can gently conform to the objects of any profiles and softly hold them with uniform pressure have been receiving considerable attention. To realize rapid and non-contact driving, a novel magneto-active soft gripper is developed in this paper. The gripper is embedded with magneto-active elastomer (MAE) actuators that are made from a mixture of a soft material substrate and hard magnetic fillers. Firstly, the MAE actuators doped with different contents of magnetic fillers are manufactured and magnetized by applying an external magnetic field. An experiment is designed to measure the magnetically induced deformation of MAE actuators under different external magnetic fields. On basis of the experimental study of MAE actuators, the magneto-active soft gripper is then designed and successfully fabricated. The gripping function and efficiency of the magneto-active soft gripper is demonstrated through an experimental study. Meanwhile, the maximum gripping force of grippers is measured and the effects of applied magnetic field and mass faction of magnetic fillers are examined. It is found that the gripping force increases as either magnetic field strength or magnetic fillers’ concentration increases but tends to be stable when the magnetic field is greater than 80 mT. When executing a gripping motion, the magneto-active soft gripper has a significantly short response time than the existing ones with other driving methods.

Ring-like structures in magnetoactive elastomers based on magnetic hard powder

In this study, we report on the experimentally observed phenomenon of ring-like structures formation from chains of magnetic microparticles in magnetoactive elastomers based on magnetic hard powder. In order to find an explanation for the specific macroscopic magnetic properties of composites, microstructural observations on the elastic composite based on a thin layer of magnetic hard particles have been carried out. Particles embedded in such magnetic composites can move inside the matrix with some restrictions and form chain-like structures on being subjected to magnetizing and turn over as first the strength of the external magnetic field decreases and then the polarity of the field switches to opposite. The quantitative parameters of these processes depend on a number of factors including coercivity and remanence of particles, polymer matrix elasticity, and external field intensity. In zero-field, the magnetized particles can assemble into ring-like structures, thus tending to bring the free energy of the overall system to a minimum. The study is primarily aimed to shed a light on understanding the specific magnetic properties of magnetic polymers, such as magnetoactive or magnetorheological elastomers with a magnetic hard filler. Furthermore, the phenomenon of ring-like structures formation can be used for controlled remote patterning of particles in magnetic elastic composite thin films, which is attractive for various applications.

Magneto-mechanical properties of elastic hybrid composites

Abstract The paper gives an overview of tunable elastic magnetic composites based on silicon rubber matrix highly filled with a magnetic soft and hard filler. The magnetic soft phase, which is represented by iron microparticles, allows active control of the physical properties of the composites, while the magnetically hard phase (e.g. neodymium–iron–boron alloy microparticles) is mainly responsible for passive adjustment of the composite. The control is performed by the application of an external magnetic field in situ, and passive adjustment is performed by means of pre-magnetization in order to change material remanent magnetization, i.e. the initial state. The potential and limits of active control and passive tuning of these composites in terms of their magneto-mechanical behavior are presented and discussed.

Hybrid magnetic elastomers prepared on the basis of a SIEL-grade resin and their magnetic and rheological properties

Abstract Hybrid magnetic elastomers (HMEs) belong to a novel type of magnetocontrollable elastic materials capable of demonstrating extensive variations of their parameters under the influence of magnetic fields. Like all cognate materials, HMEs are based on deformable polymer filled with a mixed or modified powder. The complex of properties possessed by the composite is a reflection of interactions occurring between the polymer matrix and the particles also participating in interactions among themselves. For example, introduction of magnetically hard components into the formula results in the origination of a number of significantly different behavioral features entirely unknown to magnetorheological composites of the classic type. Optical observation of samples based on magnetically hard filler gave the opportunity to establish that initial magnetization imparts magnetic moments to initially unmagnetized grains, as a result of which chain-like structures continue to be a feature of the material even after external field removal. In addition, applying a reverse field causes them to turn into the polymer as they rearrange into new ring-like structures. Exploration of the relationship between the rheological properties and magnetic field conducted on a rheometer using vibrational mechanical analysis showed an increase of the relative elastic modulus by more than two orders of magnitude or by 3.8 MPa, whereas the loss factor exhibited steady growth with the field up to a value of 0.7 being significantly higher than that demonstrated by elastomers with no magnetically hard particles. At the same time, measuring the electroconductivity of elastomers filled with a nickel-electroplated carbonyl iron powder made it possible to observe that such composites demonstrated an increase of variation of the resistivity of the composite influenced by magnetic field in comparison to elastomers containing untreated iron particles. The studies conducted indicate that this material exhibits both magnetorheological and magnetoresistive effect and does indeed have the potential for use in various types of devices.

Modeling of Structural Damage Evolution in Dispersion-Filled Elastomeric Nanocomposites with Regard for Interfacial Interaction

Computer modeling of internal damage evolution in elastomeric nanocomposites with high structural phase inhomogeneity (hard dispersed filler and soft elastomeric matrix) has been carried out. The concentration of filler particles was chosen to be sufficiently high, so that their mutual interaction affected significantly the strength properties of the material. Dispersed inclusions were assumed to be absolutely rigid and durable. Only a finitely deform able incompressible matrix (the mechanical properties of which were set using the neo-Hookean elastic potential) could be damaged. The model takes into account the following specific features of the composite structure: a high stress concentration in the gaps between closely located inclusions, the presence of elastomeric layers with increased stiffness on the surface of filler particles, different interphase contact conditions (full adhesion or slip at the matrix–inclusion interface), and the possibility of anisotropic hardening during uniaxial stretching (due to the reorientation of molecular chains in the elongation direction). The latter factor made it possible to study theoretically the mechanism of formation of high-strength microstrands in the gaps between adjacent particles. The occurrence of such formations in filled elastomers, which was observed in numerous experiments, is a proven fact. A new (anisotropic) fracture criterion has been developed to describe it, because this phenomenon cannot be simulated within the generally accepted strength criteria. The calculations based on this new approach showed that local matrix discontinuities occur not in the gaps between particles (sites of highest stress concentration) but at a certain distance, forming a hollow ring (microstrand) around it. The link between neighboring inclusions is not violated, and the material retains its load-carrying capacity at the macrolevel. Thus, the presence of microstrands is a possible reason for hardening an elastomer when a hard dispersed filler is introduced into it.

An investigation on wear behavior of UHMWPE/carbide composites at elevated temperatures

AbstractUltra‐high molecular weight polyethylene (UHMWPE) is extensively used in frictional applications due to its advanced wear resistance. This advanced polymer is reinforced with hard particulate fillers for further developments against wear conditions. Since elevated temperatures prevail in the service conditions, wear behavior of UHMWPE composites is an important issue for the engineering applications. In the present work, UHMWPE‐based composites including silicon carbide (SiC) fillers were fabricated in a compression molding chamber. In the specimen preparation stage, molding pressure, filler amount, and filler particle size were varied to investigate the influence of these variables. Upon deciding the optimum parameters from the wear tests conducted at room temperature, the wear experiments were repeated for the optimum specimen at elevated temperatures, such as 40 and 60°C. According to the results, the wear behavior of the SiC/UHMWPE composites is heavily changed by the effect of elevated temperature. Adhesive effect is pronounced at elevated temperatures while the wear characteristics possess the abrasive effect in the sliding path. In addition, the composites exhibit an accelerated material loss as temperature increases during the frictional system.