Chemical, mechanical, and thermal control of substrate-bound carbon nanotube growth

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 323-357).Carbon nanotubes (CNTs) are long molecules having exceptional properties, including several times the strength of steel piano wire at one fourth the density, at least five times the thermal conductivity of pure copper, and high electrical conductivity and current-carrying capacity. This thesis presents methods of CNT synthesis by atmospheric-pressure thermal chemical vapor deposition (CVD), where effective choice of the catalyst composition and processing conditions enables growth of tangled single-wall CNTs or structures of aligned multi-wall CNTs, on bare silicon, microstructured silicon, and ceramic fibers. Applying mechanical pressure during growth controls the structure of a CNT film while causing significant defects in the CNTs. This mechanochemisty approach is used to "grow-mold" CNTs into 3D-shaped microforms. A new reactor apparatus featuring a resistively-heated suspended platform enables rapid ( 100 °C/s) temperature control and versatile in situ characterization, including laser measurement of CNT film growth kinetics, and imaging of stress-induced film cracking. By thermally pre-treating the reactant mixture before it reaches the substrate platform, aligned CNTs are grown to 3 mm length in just 15 minutes.(cont.) A microchannel array is created for combinatorial flow studies of nanomaterials growth, having velocity range and resolution far exceeding those of conventional furnaces. A detailed design methodology considers compressible slip flows within the microchannels and flow leaks across the array, and the devices are fabricated by KOH etching of silicon. Initial experiments with this system demonstrate chemically-driven transitions in CNT yield and morphology along the microchannels, and flow-directed alignment of isolated CNTs and CNT strands. Applications of aligned CNTs in reinforced composites and electromechanical probes are enabled by the CNT synthesis technologies presented here, and show significant initial promise through collaborative research projects. Overall, controlling the packing density and matrix reinforcement of aligned CNTs gives material attributes spanning from those of energy-absorbing foams to stiff solids; however, significant increases in CNT length, growth rate, and packing density must be achieved to realize macroscopic fibers and films having the properties of individual CNTs. New machines can be created for studying the limiting aspects of growth reactions, for exploring new reaction regimes, and for producing exceptionally long nanostructures, looking ahead to fabrication of CNT-based materials in a continuous and industrially-scalable fashion.by Anastasios John Hart.Ph.D

Explaining Evaporation-Triggered Wetting Transition Using Local Force Balance Model and Contact Line-Fraction

Understanding wettability and mechanisms of wetting transition are important for design and engineering of superhydrophobic surfaces. There have been numerous studies on the design and fabrication of superhydrophobic and omniphobic surfaces and on the wetting transition mechanisms triggered by liquid evaporation. However, there is a lack of a universal method to examine wetting transition on rough surfaces. Here, we introduce force zones across the droplet base and use a local force balance model to explain wetting transition on engineered nanoporous microstructures, utilizing a critical force per unit length (FPL) value. For the first time, we provide a universal scale using the concept of the critical FPL value which enables comparison of various superhydrophobic surfaces in terms of preventing wetting transition during liquid evaporation. In addition, we establish the concept of contact line-fraction theoretically and experimentally by relating it to area-fraction, which clarifies various arguments about the validity of the Cassie-Baxter equation. We use the contact line-fraction model to explain the droplet contact angles, liquid evaporation modes, and depinning mechanism during liquid evaporation. Finally, we develop a model relating a droplet curvature to conventional beam deflection, providing a framework for engineering pressure stable superhydrophobic surfaces

High-speed roll-to-roll manufacturing of graphene using a concentric tube CVD reactor

We present the design of a concentric tube (CT) reactor for roll-to-roll chemical vapor deposition (CVD) on flexible substrates, and its application to continuous production of graphene on copper foil. In the CTCVD reactor, the thin foil substrate is helically wrapped around the inner tube, and translates through the gap between the concentric tubes. We use a bench-scale prototype machine to synthesize graphene on copper substrates at translation speeds varying from 25 mm/min to 500 mm/min, and investigate the influence of process parameters on the uniformity and coverage of graphene on a continuously moving foil. At lower speeds, high-quality monolayer graphene is formed; at higher speeds, rapid nucleation of small graphene domains is observed, yet coalescence is prevented by the limited residence time in the CTCVD system. We show that a smooth isothermal transition between the reducing and carbon-containing atmospheres, enabled by injection of the carbon feedstock via radial holes in the inner tube, is essential to high-quality roll-to-roll graphene CVD. We discuss how the foil quality and microstructure limit the uniformity of graphene over macroscopic dimensions. We conclude by discussing means of scaling and reconfiguring the CTCVD design based on general requirements for 2-D materials manufacturing.National Science Foundation (U.S.). Science, Engineering, and Education for Sustainability (Postdoctoral Fellowship Award 1415129

Ultrathin high-resolution flexographic printing using nanoporous stamps

Since its invention in ancient times, relief printing, commonly called flexography, has been used to mass-produce artifacts ranging from decorative graphics to printed media. Now, higher-resolution flexography is essential to manufacturing low-cost, large-area printed electronics. However, because of contact-mediated liquid instabilities and spreading, the resolution of flexographic printing using elastomeric stamps is limited to tens of micrometers. We introduce engineered nanoporous microstructures, comprising polymer-coated aligned carbon nanotubes (CNTs), as a next-generation stamp material. We design and engineer the highly porous microstructures to be wetted by colloidal inks and to transfer a thin layer to a target substrate upon brief contact. We demonstrate printing of diverse micrometer-scale patterns of a variety of functional nanoparticle inks, including Ag, ZnO, WO[subscript 3], and CdSe/ZnS, onto both rigid and compliant substrates. The printed patterns have highly uniform nanoscale thickness (5 to 50 nm) and match the stamp features with high fidelity (edge roughness, ~0.2 μm). We derive conditions for uniform printing based on nanoscale contact mechanics, characterize printed Ag lines and transparent conductors, and achieve continuous printing at a speed of 0.2 m/s. The latter represents a combination of resolution and throughput that far surpasses industrial printing technologies.Massachusetts Institute of Technology. Department of Mechanical EngineeringNational Science Foundation (U.S.) (Grant CMMI-1463181)United States. Air Force Office of Scientific Research. Young Investigator Program (Grant FA9550-11-1-0089)National Institutes of Health (U.S.) (Grant 1R21HL114011-01A1

Conformal Robotic Stereolithography

Additive manufacturing by layerwise photopolymerization, commonly called stereolithography (SLA), is attractive due to its high resolution and diversity of materials chemistry. However, traditional SLA methods are restricted to planar substrates and planar layers that are perpendicular to a single-axis build direction. Here, we present a robotic system that is capable of maskless layerwise photopolymerization on curved surfaces, enabling production of large-area conformal patterns and the construction of conformal freeform objects. The system comprises an industrial six-axis robot and a custom-built maskless projector end effector. Use of the system involves creating a mesh representation of the freeform substrate, generation of a triangulated toolpath with curved layers that represents the target object to be printed, precision mounting of the substrate in the robot workspace, and robotic photopatterning of the target object by coordinated motion of the robot and substrate. We demonstrate printing of conformal photopatterns on spheres of various sizes, and construction of miniature three-dimensional objects on spheres without requiring support features. Improvement of the motion accuracy and development of freeform toolpaths would enable construction of polymer objects that surpass the size and support structure constraints imparted by traditional SLA systems.American Society for Engineering Education. National Defense Science and Engineering Graduate FellowshipNational Institute of Mental Health (U.S.) (University of Michigan Microfluidics in Biomedical Sciences Training Program. 5T32-EB005582)Singapore-MIT Alliance for Research and Technology (SMART

Design and analysis of kinematic couplings for modular machine and instrument structures

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2002.Includes bibliographical references (p. 278).by Anastasios John Hart.S.M

Fast Desktop-Scale Extrusion Additive Manufacturing

Significant improvements to the throughput of additive manufacturing (AM) processes are essential to their cost-effectiveness and competitiveness with traditional processing routes. Moreover, high-throughput AM processes, in combination with the geometric versatility of AM, will enable entirely new workflows for product design and customization. We present the design and validation of a desktop-scale extrusion AM system that achieves a much greater build rate than benchmarked commercial systems. This system, which we call ‘FastFFF’ is motivated by our recent analysis of the rate-limiting mechanisms to conventional fused filament fabrication (FFF) technology. The FastFFF system mutually overcomes these limits, using a nut-feed extruder, laser-heated polymer liquefier, and servo-driven parallel gantry system to achieve high extrusion force, rapid filament heating, and fast gantry motion, respectively. The extrusion and heating mechanisms are contained in a compact printhead that receives a threaded filament and augments conduction heat transfer with a fiber-coupled diode laser. The prototype system achieves a volumetric build rate of 127 cm 3 /hr, which is approximately 7-fold greater than commercial desktop FFF systems, at comparable resolution; the maximum extrusion rate of the printhead is ∼14-fold greater (282 cm 3 /hr) than our benchmarks. The performance limits of the printhead and motion systems are characterized, and the tradeoffs between build rate and resolution are assessed and discussed. High-speed desktop AM raises the possibility of new use cases and business models for AM, where handheld parts are built in minutes rather than hours. Adaptation of this technology to print high-temperature thermoplastics and composite materials, which require high extrusion forces, is also of interest

Synthese de composes mineraux monocristallins par reduction electrochimique en milieu fondu

SIGLECNRS RP 440 (25) / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Molecular Gastronomy Meets 3D Printing: Layered Construction via Reverse Spherification

The potential use of additive manufacturing (AM) techniques for processing of food can span from satisfaction of basic necessities to high-end cuisine and fine dining. The purpose of this study was to explore how AM, specifically extrusion-based layer-wise deposition, can be combined with the reverse spherification technique that is widely used in molecular gastronomy. First, by manual extrusion, we identify suitable recipes and ingredient concentrations to form freestanding features in a liquid bath. Subsequently, a desktop extrusion is adapted for the deposition of a calcium solution into an alginate bath first as a two-dimensional (2D) pathway and then as three-dimensional (3D) geometry by layer-wise deposition. The 2D geometries are measured and compared to a nominal geometry, to elucidate how tool speed and extrusion rate influence form and dimensional accuracy. We demonstrate that motorized extrusion-based AM can be combined with reverse spherification to form stable objects by gelation of fruit-based solutions. In addition, a wider set of manual experiments shows the possibility of combining different flavors and the creation of complex multilayer and multiflavor objects. Additional studies on the deposition precision are required to optimize the process of creating a full 3D geometry. This study shows that 3D printing via reverse spherification can bridge the gap between culinary art and AM technology, and enable new capabilities for creation of dining experiences. This is a step toward the digital design and manufacturing of unique edible objects with complex flavors, textures, and geometries

Twist-coupled Kirigami cells and mechanisms

Manipulation of thin sheets by folding and cutting offers opportunity to engineer structures with novel mechanical properties, and to prescribe complex force–displacement relationships via material elasticity in combination with the trajectory imposed by the fold pattern. Here we study the mechanics of a cellular Kirigami that rotates and buckles upon compression, presenting an example of a design strategy that we call ”flexigami”. The addition of diagonal cuts to an equivalent closed cell permits the cell to collapse reversibly without incurring significant tensile strains in its panels. Using finite-element modeling and experiments we show how the mechanical behavior of the cell is governed by the coupled rigidity of the panels and hinges and we design cells to achieve reversible force response ranging from smooth mono-stability to sharp bi-stability. We then demonstrate the cell-based construction of laminates with multi-stable behavior and a rotary-linear boom actuator, as well as self-deploying cells with shape memory alloy hinges. Advanced digital fabrication methods can enable the realization of this and other so-called flexigami designs that derive their overall mechanics from fold and panel elasticity, for applications including deployable structures, soft robotics and medical devices.National Science Foundation (Grant EFRI-1240264)U. S. Army Research Office (Contract W911NF-13-D-0001