3D Printing of Nanotwinned Copper

Nanotwinned (NT)-metals exhibit superior mechanical and electrical properties when compared to their coarse-grained and nano-grained counterparts. NT-metals, either as a film or in bulk, are usually obtained by pulsed electrodeposit­­­­ion (PED), plastic deformation and sputter deposition. However, 3D printing of NT-metals has not been explored. Researchers at Department of Mechanical Engineering and Alan G. MacDiarmid NanoTech Institute, The University of Texas at Dallas, USA have reported a localized pulse electrodeposition (L-PED) process for 3D printing of NT-Cu, which is fully dense, almost free of impurities and low microstructural defects, with no obvious interface between deposited layers, with good mechanical and electrical properties and without the requirement of any post-treatment.

The process is based on L-PED that occurs at the tip of an electrolyte-filled nozzle steered in 3D-XYZ directions, by precisely positioning the stage relative to the substrate for omnidirection (complex 3D, Fig. 1(a)) or layer-by-layer (lateral, Fig. 1(b))) metal printing. The 3D printed Cu structures and patterns are shown in Figs. 1(c) – 1(f). The FIB image of a micropillar printed by vertical deposition at 0.5 V and at an average current density of ≈0.018 A/cm² is shown in Fig. 2(a). The SEM image of a four-layer hollow square shape printed by layer-by-layer process at 0.7 V and at an average current density of 0.045 A/cm² is shown in Fig. 2(b). Both structures exhibit the presence of high density aligned twin boundaries (TBs) within their grains in which most of the TBs are aligned perpendicular to the electric field direction. The TEM image (Fig. 2(c)) confirmed the formation of densely packed nanotwins, mostly aligned in one direction in almost all grains. No noticeable interlayer is observed in the 3D printed Cu by L-PED. With the exception of a few grains, stacking faults, in general, are not frequently observed in 3D-printed Cu.

Fig. 1 (a) Schematic of L-PED process; (b) Schematic side-view of the meniscus between the nozzle tip and the growth front for layer-by-layer deposition of nt-metals; (c-f) SEM images of several 3D-printed Cu structures. (c) A 24-layer structure printed by layer-by-layer DC-ED process, printing time ≈200 min; (d) UTD letter printed, printing time ≈16 min; (e) A micropillar with diameter of ≈10 μm 3D printed by PED, printing time ≈60 min; (f) A helical structure fabricated by pulsed voltage, printing time ≈12 min

Fig. 2 (a) FIB ion channeling contrast image of cross-section of a 3D printed micropillar, printing time ≈60 min; (b) SEM image of a layer-by-layer structure (four-layer) cross-sectioned by FIB, printing time ≈35 min; and (c) FIB ion channel image of the cross-section that shows high-density parallel TBs.

The formation and presence of inter-layers during a step-wise 3D printing of six-layer structure of Cu using a nozzle diameter of 10 μm at 0.5 V pulsed voltage with a duty cycle of 1/100 is studied (Fig. 3(a)). Each layer is printed starting from a point with a shift to the right from the starting point of the former layer. FIB ion channeling contrast images acquired at the cross section (Figs. 3(b) and 3(c)) indicate that the grains in each new layer has continued growing from the grains in the previous layer without the formation of any new grains at each layer. One columnar grain initiated from the first layer grew to the fifth layer (boundary of the grain is shown by a dashed line) with no interlayer could be observed at any step and between any two layers. In L-PED process, the preceding layer functions as the seed layer for the deposition of the next layer, which helps development of an interface-free structure, which is further confirmed by the EDS map of Cu (Fig. 3(d).

Fig. 3 (a) Plan-view FIB image of a six-layer printed Cu structure, printing time ≈35 min; (b, c) Zoomed-in views of the layers that show each layer is deposited on the previous layer, without any noticeable interlayer; (d) EDS map of Cu

The high quality of 3D printed NT-Cu with minimal porosity and structural defects as well as the absence of any interface between the printed layers offer good mechanical property (elastic modulus: 128.2 ± 10.9 GPa; hardness: 2.0 ± 0.19 GPa) and electrical resistance (3.9×10⁻⁷ Ω.m). By modifying the geometry of the nozzle, the geometry of the meniscus between the nozzle tip and the substrate can be varied, which would facilitate fabrication of geometries with more sharp corners and helical pitches. Layer-by-layer deposition seems to be necessary to fabricate entangled structures. The reproducibility of the L-PED process largely depends on the stability of the meniscus, which is influenced by humidity and temperature. In addition, moving speed of the nozzle also influences the reproducibility; high speed could result in breakage of the meniscus while too slow displacement might lead to clogging of the nozzle tip by the deposited metal. The L-PED process can be used for direct 3D printing of layer-by-layer and complex 3D microscale NT-Cu structures for various applications including electronics, micro/nano electromechanical systems (MEMS, and NEMS), metamaterials, plasmonic, and sensors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Adv. Mater. 2017, 1705107, DOI: 10.1002/adma.201705107

Electrochemical 3D Printing of Copper

Additive manufacturing (AM), also referred attractively as 3D printing, is a well-established process of creating complex 3D geometries through a layer-by-layer building approach. 3D printing has received considerable attention in automotive, aerospace, and biomedical sectors. Direct metal laser sintering (DMLS) is the most commonly used method for 3D printing of metals which involves selective laser sintering of layers of metal powders. However, the high capital cost, defects in the manufactured components and inability to work with multiple materials, limit 3D printing of metals by DMLS and warrant the development of novel non-laser based 3D printing techniques.

Electrochemical additive manufacturing (ECAM) (electrochemical 3D printing) involves deposition of thin and highly adherent layers of metal with a layer-by-layer approach onto a conducting surface through reduction of metal ions from the electrolyte. It combines the basic principles of electrodeposition and 3D printing. Two different approaches, viz., localized electrochemical deposition (LCD) and meniscus confined electrode (MCE), have been used for electrochemical 3D printing of metals. These methods, however, require the use of expensive piezo-based movement stages and nanopippettes/ultrafine electrodes. The slow rate deposition rate of metals (Cu: from ≈100 nm/s to ≈0.18 μm/s), inability to develop components of complex geometries, porosity and roughness are some of the major limitations. Moreover, majority of the structures fabricated using these methods are simple wire-based architectures. Methods for 3D electrochemical printing of metals with improved deposition rates and capable of printing complex structures are warranted. In this perspective, researchers at Dyson School of Design Engineering and Department of Earth Science and Engineering, Imperial College London, UK, have reported a novel strategy for ECAM using a MCE approach.

The electrochemical 3D printer assembly comprised of a plastic syringe and nozzle (diameter 400 μm) with a porous sponge filled with 1 M CuSO₄ was mounted on a carriage and its movement in the x-, y- and z- directions was precisely controlled using computer controlled stepper motors. Two copper rods suspended in the CuSO₄ electrolyte served as the counter and reference electrodes while a copper plate served as the working electrode. The print head was moved to contact the copper plate and retract back by a small amount. The sufficient back pressure provided by the porous sponge to the hydraulic head enabled the formation of a stable meniscus (localized electrolyte). When a positive potential was applied between the working and counter electrodes, deposition of Cu occurs on the working electrode through reduction of Cu²⁺ ions with a simultaneous replenishment of Cu²⁺ ions from the counter electrode. To create 3D printed structures, the print head was moved in x-, y- and z-directions with varying traverse speed as directed by the 3D model. The various components of the electrochemical 3D printer assembly is schematically represented in Figs. 1 (a), 1(b) and 1(c). Optical image of the 3D printed Cu with the shape of the letters “I”, “C”, and “L” using 1 M CuSO₄ at 4 V at a print head speed of 0.4 mm/s is shown in Fig. 1(d).

Fig. 1 Schematic illustration of the electrochemical 3D printer assembly: (a) Print head set-up; (b) electrode arrangement; (c) print nozzle and sponge in the tip, highlighting how they act during the deposition of Cu; and (d) optical images of the printed Cu structures featuring the letters “I”, “C”, and “L” printed using 1 M CuSO₄ at 4 V at a print head speed of 0.4 mm/s.

Electrochemical 3D printing of Cu dots and lines (lateral print head velocity: 0.4 mm/s) were made using 1 M CuSO₄ as the electrolyte at varying voltages from 1 to 6 V for 1 h. The Cu dots printed at 1 V exhibited a dense structure with a high degree of concentricity. In spite of a faster rate of growth along with a dense structure, those printed at 2 V exhibited a convex shape due to preferential deposition at the center. Formation of Cu dendrites becomes apparent at 3V due to the limitations imposed by the mass transport and the a continuous increase in porosity is observed with a further increase in deposition potential (Fig. 2(a)). Hence, achieving dimensionally accurate structures would be difficult using electrochemical printing of Cu dots at potentials beyond 2 V. Irrespective of the deposition potentials in the rage of 1-6 V, electrochemically printed Cu lines fails to exhibit any Cu dendrites since the relative position of the print head aids mass transport of Cu²⁺ ions or might have assisted mechanical removal of the dendrites (Fig. 2(b)). The thickness of the Cu lines is increased from 3 μm at 1 V to 15 μm at 4 V, beyond which it decreased due to the decrease in deposition efficiency. For a given lateral speed of 0.4 mm/s for 3600 s, Cu lines can be printed for 144 passes over a distance of 10 mm.

Fig. 2 SEM images of (a) single Cu dot; and (b) Cu lines (lateral print head speed: 0.4 mm/s) electrochemically printed using 1 M CuSO₄ at 3-6 V for 1 h

The electrochemical 3D printed Cu dots and Cu lines exhibit a higher hardness than cold worked cast Cu. The Vickers hardness of Cu dots varies from 184 to 196 MPa, Cu lines ranging from 211 to 228 MPa when compared to the hardness of cold worked cast Cu, which varies from 50 to 176 MPa; the smaller the grain size, the higher is the hardness. The electrical conductivity of Cu lines is ranging between 1.31×10⁶ and 6.86×10⁶ S/m, which agrees well with that of nanocrystalline Cu (5.41×10⁶ S/m) but relatively lower than coarse grained Cu.

Electrochemical 3D printing of Cu builds a foundation for future work to achieve high speed deposition as well as printing of multi-metal functional structures. The z-height resolution opens up new avenues for the fabrication of functional electronics such as sensors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Xiaolong Chen et al., A Low Cost Desktop Electrochemical Metal 3D Printer, Adv. Mater. Technol. 2017, 1700148, DOI: 10.1002/admt.201700148