Field-Assisted 3D-Printing of Aligned Composites

Abstract eng:
The deposition of multi-phase materials with microstructural control enables the integration of microstructure and structural features, which creates new pathways towards optimizing component performance. Here, we demonstrate that acoustic microfluidic print nozzles are an effective pathway to control SiC whisker orientation and packing density in printed composites. In addition to the ability to tune microstructure along a single print line, the overall concentration of reinforcements can be increased by selectively isolating a stream of focused fibers, enabling the deposition of material with higher fiber volume fractions than the initial ink composition. We show that even modest volume fractions of acoustically-focused and concentrated SiC fibers can produce printed composites with unprecedented control over microstructural ordering that exhibit strengths rivaling polymer-matrix composites with higher volume fractions of stiffer fibers. The ability to control fiber alignment at high volume fractions would create new opportunities to print ultra-strong composite materials with engineered anisotropic properties (mechanical, electrical, thermal, and optical). A promising approach for achieving top-down control over microstructure is to combine field-assisted assembly with direct deposition. While electrostatic or electromagnetic fields are suitable for a narrow range of ink compositions and particle types, acoustic fields are broadly applicable to a wide class of colloids, spanning a broad range of composition, particle shape, and size We demonstrate acoustic-fieldassisted deposition of two-phase aligned composites, such as SiC fibers in epoxy, using microfluidic print nozzles coupled to inexpensive piezoelectric actuators. Mechanical characterization of printed materials with varying microstructures show that significant gains in strength with even modest additions of SiC fibers in epoxy as a result of unprecedented control of the printed microstructure. Figure 1: (A) Conceptual illustration of acoustic print nozzle. Piezoelectric The two-stage acoustic nozzle actuation generates acoustic forces that align fibers during printing. Multiple schematic shown in Figure 1 illustrates piezos in series enable enhancement of fiber volume fraction of the printed via the alignment of fibers in a viscoelastic removal of flash. (B) Printed line with continuously varied microstructure. The matrix, which results from an applied focusing frequency is tuned to induce focusing of SiC fibers in epoxy; when the acoustic force generated by piezoelectric frequency is tuned away from resonance (“focus off”), the fibers spread to the elements mounted above the nozzle width of the deposited line. pathway. These nozzles are microfabricated in silicon (150 µm deep) with a bonded glass capping layer according to the procedures in Ref. [1], with piezoelectrics bonded to the nozzle to generate standing waves in the channel. Critically, acoustic focusing enables control of the microstructure of the deposited line “on the fly” by modulating acoustic excitation parameters, as demonstrated in Figure 1B for SiC fibers in epoxy. In this case, when the excitation frequency is tuned to induce focusing (“focus on”), fibers collapse to the center of the print line. The focusing transition occurs quickly enough to tune the microstructure from densely packed to distributed fibers within approximately one filament width. When the excitation frequency is changed (“focus off”), fibers spread nearly uniformly throughout the print line. The transition between focused and de-focused microstructures, and vice versa, is approximately symmetric; this transition is influenced by the shape of the focus zone in the (x,z)-plane and the flow speed in the channel. Figure 2A is a schematic of the single-channel nozzle geometry; in all cases, the print nozzle is stationary and the print substrate moves relative to the printhead. The effect of excitation voltage on microstructure is shown in Figure 2B for 0 and 23 V (peak-to-peak). For the control condition (0 V), no fiber focusing is apparent, and cross-sectional imaging reveals a a)

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