Composites Reinforced in Three Dimensions by Using Low Magnetic Fields

Abstract

The orientation and distribution of reinforcing particles in artificial composites are key to enable effective reinforcement of the material in mechanically loaded directions, but remain poor if compared to the distinctive architectures present in natural structural composites such as teeth, bone, and seashells. We show that micrometer-sized reinforcing particles coated with minimal concentrations of superparamagnetic nanoparticles (0.01 to 1 volume percent) can be controlled by using ultralow magnetic fields (1 to 10 milliteslas) to produce synthetic composites with tuned three-dimensional orientation and distribution of reinforcements. A variety of structures can be achieved with this simple method, leading to composites with tailored local reinforcement, wear resistance, and shape memory effects.

Self-shaping composites with programmable bioinspired microstructures

“…1i,j, Supplementary Movie 4). This reversible bending response is similar in nature to that of bimetallic thermostats and of a family of synthetic bilayer structures comprised of two distinct materials 7,20,21 .…”

“…Thus, low or high CMF angle reinforcement can be recreated simply with horizontally or vertically oriented platelets, respectively, with respect to the long axis of the object (Supplementary Figure S1). To gain control over the orientation of these microplatelets, 12-nm-diameter superparamagnetic iron oxide nanoparticles were adsorbed onto the platelet surface electrostatically 21 . As the nanoparticles are on the outside of the platelet in position to maximize torque, these coated platelets exhibit a UHMR effect enabling facile orientation under low external magnetic fields.…”

“…Uniform magnetic fields thus pin one of the long axes of the platelets axially parallel to the field direction. In addition, the second long axis can be fixed by employing rotating magnetic fields of sufficiently high rotational frequency 21,22 . ARTICLE Bulk hygroscopic composites were produced by mixing UHMR alumina platelets into fluid polymer solutions that were then allowed to gel in the presence of a magnetic field.…”

“…To gain control over reinforcement architectures, we exploit the ultra-high magnetic response (UHMR) exhibited by discontinuous, anisotropic reinforcement microparticles coated with superparamagnetic iron oxide nanoparticles 21 . By applying weak external magnetic fields only during sample preparation, reinforcement particles can be oriented into architectures akin to the natural systems or into a wide range of completely unique and programmable shape-changing microstructures.…”

Biomimetic Materials by Freeze Casting

“…The enhanced mechanical properties arise from the directional alignment of pore channels induced by the magnetic field. 88 Potential applications for magnetic field aligned freeze casting include: spiral-reinforced structures with exceptional torsional rigidity, structural alignment of high aspect ratio nanoparticles to create ''nanobridges'' between adjacent lamellae for increased strength and toughness, or local two-dimensional and threedimensional reinforced structures, such as those described by Erb et al 102 Fig . 5.…”