Flexible Conductive Composites with Programmed Electrical Anisotropy Using Acoustophoresis

Abstract

Developing mechanically flexible composite materials with high electrical conductivity is currently hindered by the need to use high loading of conductive filler, which severely limits flexibility. Here, acoustic focusing is used to control arrangement of conductive particles in photopolymer matrices to create composites with both tunable conductivity and flexibility. Acoustophoresis patterns filler particles into highly efficient percolated networks which utilize up to 97% of the particles in the composite, whereas the inefficient stochastic networks of conventional dispersed‐fiber composites utilize <5%. These patterned materials have conductivity an order of magnitude higher than conventional composites made with the same ink, reaching 48% the conductivity of bulk silver within the assembled silver‐particle networks (at 2.6 vol% loading). They also have low particle loading so that they are flexible, withstanding >500 bending cycles without losses in conductivity and changing conductivity only 5% within cycles on average. In contrast, conventional unpatterned composites with the same conductivity require such high loading that they are prohibitively brittle. Finally, modulating the applied acoustic field controls the anisotropy of the conductive networks and produces materials which are either 2D conductive, 1D conductive, or insulating, using the same nozzle and ink, paving the way for versatile multifunctional 3D printing.

A 3D Printed Morphing Nozzle to Control Fiber Orientation during Composite Additive Manufacturing

“…An important advantage of the presented PolyJet 3D printing-based nozzle fabrication methodology is the reliance on electronically transferrable models, which researchers can readily download (see Supporting Information) to modify and/ or print on site. In contrast to prior methods associated with complex system apparatuses for controlling fiber orientation (e.g., via external fields), [28][29][30][31][32][33][34] reproducing the approach in this work is a relatively facile endeavor, with access to a PolyJet 3D printer and a single pressure valve representing the only critical barriers. We expect that AM technologies that allow for printing of soft materials, such as stereolithography, [42] could also be employed to fabricate nozzles comprising a single, flexible material; however, the nozzle design would likely need to be adapted to prevent elements intended to be functionally rigid from introducing undesired deformations that compromise operational performance.…”

“…An important caveat to the results for fiber alignment as a function of P A is that the overall degree of reorientation control did not appear to be as drastic as those of other strategies, such as methods based on acoustic fields, [28][29][30][31][32] magnetic fields, [33,34] or RDIW. [35] Although the proportion of fibers exhibiting tight alignment with the print direction decreased with increasing P A , the majority of fibers remained moderately aligned regardless of P A for those examined.…”

“…In particular, the Begley and Gianola groups have employed acoustophoretic methods to focus and concentrate numerous microparticles, including SiC fibers, within printed filaments. [ 28–32 ] Similarly, researchers have reported the ability to apply magnetic fields of varying magnitude to tune fiber alignment during material extrusion. [ 33,34 ] Motivated to bypass challenges associated with applied fields (e.g., restrictions of fiber concentration, geometry, and/or material), the Lewis group introduced the concept of “rotational direct ink writing (RDIW)”—an approach in which the printhead spins with varying rotational rates during extrusion‐based deposition to disrupt the alignment of embedded fibers as desired.…”

“…In particular, the Begley and Gianola groups have employed acoustophoretic methods to focus and concentrate numerous microparticles, including SiC fibers, within printed filaments. [28][29][30][31][32] Similarly, researchers have Fiber-filled composite materials offer a unique pathway to enable new functionalities for systems built via extrusion-based additive manufacturing (or "3D printing"); however, challenges remain in controlling the fiber orientations that govern ultimate performance. In this work, a multi-material, shape-changing nozzle-constructed by means of PolyJet 3D printing-is presented that allows for the spatial distribution of short fibers embedded in polymer matrices to be modulated on demand throughout extrusion-based deposition processes.…”

Additive Manufacturing of Polymer Matrix Composite Materials with Aligned or Organized Filler Material: A Review

“…For instance, it is well known that aligning micro-and nanoscale filler material in the direction of mechanical loading improves its mechanical properties by providing reinforcement [33] and enhances its electrical [34] and thermal [35] conductivity by reducing the percolation threshold in the alignment direction. [36,37] Several methods exist to spatially arrange and/or align discontinuous filler material in the polymer matrix. Electric [38] and magnetic [39] fields orient the filler material in the field direction but require ultrahigh field strengths (of the order of 20 kV m À1 [40] and 8000 mT, [41] respectively), thus limiting dimensional scalability of the specimens.…”