A. John Hart

Massachusetts Institute of Technology | MIT · Department of Mechanical Engineering

Qualification of high-performance metal components produced by laser powder bed fusion (LPBF) must identify process-induced porous defects that reduce ductility and nucleate fatigue cracking. Detecting such defects via optical monitoring of LPBF provides a path towards in-process quality control without downstream testing such as by computed tomogr...

Sustainable manufacturing systems must have sufficient throughput to meet demand, be cost-effective, and have an acceptably low environmental impact. For additive manufacturing by laser powder bed fusion (LPBF), part designers and process engineers must understand the underlying scalings among energy utilization, process parameters, and part geomet...

With the increasing adoption of metal additive manufacturing (AM), researchers and practitioners are turning to data-driven approaches to optimise printing conditions. Cross-sectional images of melt tracks provide valuable information for tuning process parameters, developing parameter scaling data, and identifying defects. Here we present an image...

Development and qualification of process parameters in laser powder bed fusion (LPBF) commonly involves many variables. At the outset of development, whether transferring known parameters to a new machine, or exploring a new material, single-track and single-layer experiments are a convenient means of down-selecting key variables and exploring para...

Additively manufactured (AM) structural components with complex geometries and tailored properties at voxel-size resolution will lead to significant leap in performance in various critical engineering applications. However, at each voxel, we first need to be able to design the alloy efficiently and reliably. We demonstrate a hybrid approach combini...

Tactile sensors, or sensors that collect measurements through touch, have versatile applications in a wide range of fields including robotic gripping, intelligent manufacturing, and biomedical technology. Hoping to match the ability of human hands to sense physical changes in objects through touch, engineers have experimented with a variety of mate...

Ultraviolet (UV) printing of photopolymers is a widely adopted manufacturing method because of its high resolution and throughput. However, available printable photopolymers are typically thermosets, resulting in challenges in postprocessing and recycling of printed structures. Here, we present a new process called interfacial photopolymerization (...

Mechanically flexible surface structures with embedded conductive electrodes are attractive in contact-based devices, such as those used in reversible dry/adhesion and tactile sensing. Geometrical shapes of the surface structures strongly determine the contact behavior and therefore the resulting adhesion and sensing functionalities; however, avail...

Polyetherimides (PEI) are high‐performance thermoplastic polymers featuring a high dielectric constant and excellent thermal stability. In particular, PEI thin films are of increasing interest for use in solid‐state capacitors and membranes, yet the cost and thickness are limited by conventional synthesis and thermal drawing techniques. Here, a met...

Development of a refractory selective solar absorber (RSSA) is the key to unlock high-temperature solar thermal and thermochemical conversion. The fundamental challenge of RSSA is the lack of design and fabrication guidelines to simultaneously achieve omnidirectional, broadband solar absorption and sharp spectral selectivity at the desired cutoff w...

A variety of tools can be used for spreading metal, ceramic, and polymer feedstocks in powder bed additive manufacturing methods. Rollers are often employed when spreading powders with limited flowability, as arises in powders comprising fine particle sizes or high surface energy materials. Here, we study roller-based powder spreading for powder be...

Spreading uniform and dense layers is of paramount importance to creating high-quality components using powder bed additive manufacturing (AM). Blade-like tools are often employed for spreading powder metal feedstocks, especially in laser powder bed fusion and electron beam melting, where powders are characterized by a D50 of 30 microns or greater....

Polymer matrix nanocomposites (PNCs) incorporating high volume fractions (Vf in excess of 10 vol%) of aligned carbon nanotubes (A-CNTs) are promising for high-performance structural composite applications leveraging texture for multifunctionality and performance-to-weight ratios. However, to enable manufacturing of scalable structures using A-CNT P...

Wrinkles invariably form during graphene growth and post-growth transfer, limiting graphene films' large-scale uniformity for electronic applications. We report a transfer-free synthesis route for highly-uniform bilayer graphene directly on dielectric substrates—SiO2, sapphire, and MgO—by interfacial carbon precipitation. Ultrathin Pd leaves having...

Zwitterionic surfaces are increasingly explored as antifouling coatings due to their propensity to resist protein, bacterial, and cell adhesion and are typically applied as polymeric systems. Here, the self‐assembly of strongly interacting small molecule amphiphiles is reported to produce nanoribbons for antifouling applications. Synthesized amphip...

Chlorosulfonic acid and oleum are ideal solvents for enabling the transformation of disordered carbon nanotubes (CNTs) into precise and highly functional morphologies. Currently, processing these solvents using extrusion techniques presents complications due to chemical compatibility, which constrain equipment and substrate material options. Here,...

The mechanical experience of consumption (i.e., feel, softness, and texture) of many foods is intrinsic to their enjoyable consumption, one example being the habit of twisting a sandwich cookie to reveal the cream. Scientifically, sandwich cookies present a paradigmatic model of parallel plate rheometry in which a fluid sample, the cream, is held b...

Due to their exceptional mechanical and chemical properties and their natural abundance, cellulose nanocrystals (CNCs) are promising building blocks of sustainable polymer composites. However, the rapid gelation of CNC dispersions has generally limited CNC-based composites to low CNC fractions, in which polymer remains the dominant phase. Here we r...

Scalable production of carbon nanotubes (CNTs) requires catalysts and reaction conditions that provide high nucleation efficiency. In situ characterization methods such as environmental transmission electron microscopy (ETEM) can reveal fundamental mechanisms of synthesis, but to date have primarily provided qualitative observations on small sample...

Molecularly organized nanocomposites of polymers and carbon nanotubes (CNTs) have great promise as high-performance materials; in particular, conformal deposition of polymers can control interfacial properties for mechanical load transfer, electrical or thermal transport, or electro/chemical transduction. However, controllability of polymer-CNT int...

Binder jet additive manufacturing (BJAM) enables processing of metals, ceramics, polymers and composites through the deposition of a binding agent onto a powder bed in a layer-by-layer sequence. Most commonly, a polymer binder is utilized to provide temporary strength to the printed component enabling part removal and handling from the printer; how...

Spreading of uniform, dense powder layers is a critical requirement for powder bed additive manufacturing (AM) processes, and layer quality is influenced by powder feedstock selection and spreading parameters. In prior work, quantification of powder layer density has required disruption of the powder layer after spreading, or compromise in the prec...

Metal additive manufacturing (AM) by laser powder bed fusion (L-PBF) builds upon fundamentals established in the field of laser welding which include the influence of gas and plume dynamics on weld depth and quality. L-PBF demands a thorough investigation of the complex thermophysical phenomena that occur where the laser interacts with the metal po...

The layer-by-layer building in the laser powder bed fusion (LPBF) process can be exploited to achieve graded alloy compositions along the build direction. The local control over alloying element composition would allow for tailored material properties. This work demonstrates the use of LPBF to achieve gradient structures by mixing two austenitic st...

Due to their exceptional mechanical and chemical properties and their natural abundance, cellulose nanocrystals (CNCs) are promising building blocks of sustainable polymer composites. However, the rapid gelation of CNC dispersions has generally limited CNC-based composites to low CNC fractions, in which polymer remains the dominant phase. Here we r...

Laser-based metal processing including welding and three dimensional printing, involves localized melting of solid or granular raw material, surface tension-driven melt flow and significant evaporation of melt due to the applied very high energy densities. The present work proposes a weakly compressible smoothed particle hydrodynamics formulation f...

A carbon nanotube (CNT)-mediated three-dimensional (3D) vanadium pentoxide (V2O5) nanoarchitecture with tunable morphology and translatable electrical and electrochemical functionality is developed via the step-wise chemical vapor deposition. Controlling the pressure, gas flow, and growth time, based on the collective understanding of the vapor-sol...

Powder bed fusion additive manufacturing (PBFAM) of metals has the potential to enable new paradigms of product design, manufacturing and supply chains while accelerating the realization of new technologies in the medical, aerospace, and other industries. Currently, wider adoption of PBFAM is held back by difficulty in part qualification, high prod...

Manufacturing processes is a key subject in undergraduate engineering curricula, and ideally blends theory with hands-on activities and exposure to manufacturing practice. Therewith, the emergence of scalable, versatile digital learning tools and techniques suggests that manufacturing courses should explore how to maximize the use and value of in-p...

Metal additive manufacturing (AM) by laser powder bed fusion (L-PBF) builds upon fundamentals established in the field of laser welding which include the influence of gas and plume dynamics on weld depth and quality. L-PBF demands a thorough investigation of the complex thermophysical phenomena that occur where the laser interacts with the metal po...

Uniform powder spreading is a requisite for creating consistent, high-quality components via powder bed additive manufacturing (AM), wherein layer density and uniformity are complex functions of powder characteristics, spreading kinematics, and mechanical boundary conditions. High spatial variation in particle packing density, driven by the stochas...

In article number 2100245, Crystal E. Owens, Gareth H. McKinley, A. John Hart and co‐workers present a direct‐write printing process controlled by two parameters to extrude aqueous carbon nanotube (CNT) solutions with liquid crystalline behavior, drawing highly conductive and flexible traces, sensors, and circuits with integrated circuit elements o...

Binder jet additive manufacturing (BJAM) is capable of fabricating complex three-dimensional components from a variety of material classes. Understanding the fundamentals of BJAM, including spreading of thin layers of powder, powder-binder interactions, and post-processing is critical to develop robust process parameters for BJAM. Toward meeting th...

Printed electronics rely on the deposition of conductive liquid inks, typically onto polymeric or paper substrates. Among available conductive fillers for use in electronic inks, carbon nanotubes (CNTs) have high conductivity, low density, processability at low temperatures, and intrinsic mechanical flexibility. However, the electrical conductivity...

Manufacturing of printed electronics relies on the deposition of conductive liquid inks, typically onto polymeric or paper substrates. Among available conductive fillers for use in electronic inks, carbon nanotubes (CNTs) have high conductivity, low density, processability at low temperatures, and intrinsic mechanical flexibility. However, the elec...

The top surface of carbon nanotube (CNT) “forests” produced by thermal chemical vapor deposition (CVD) often has a tangled morphology, owing to the self-organization of the CNTs at the initial stage of the CVD process. Removal of this top “crust” layer, without damaging the intrinsic microstructure of the CNT forest is often a key step for further...

Arsenic is a widespread trace groundwater contaminant that presents a range of health risks and has an acceptable level of only 10 μg L-1 in drinking water. However, in many countries arsenic quantification in water is limited to centralized laboratories because it requires the use of elemental analysis techniques with high capital cost. As a resul...

Powder bed fusion additive manufacturing (PBFAM) of metals has the potential to enable new paradigms of product design, manufacturing and supply chains while accelerating the realization of new technologies in the medical, aerospace, and other industries. Currently, wider adoption of PBFAM is held back by difficulty in part qualification, high prod...

Uniform powder spreading is a requisite for creating consistent, high-quality components via powder bed additive manufacturing (AM), wherein layer density and uniformity are complex functions of powder characteristics, spreading kinematics, and mechanical boundary conditions. High spatial variation in particle packing density, driven by the stochas...

Cluster expansion approximates an on-lattice potential with polynomial regression. We show that using a convolutional neural network (CNN) instead leads to more accurate prediction due to the depth of the network. We construct our CNN potential directly on cubic lattice sites, representing voxels in a 3D image, and refer to our method as the voxela...

In article number 2005370, Benedetto Marelli and co‐workers design a silk‐based microneedle technology to provide food quality and information without opening packages. The microneedles can pierce through commercial food films and transport food fluids to biosensors printed on the backside. By reading the sensors' colorimetric responses, non‐expert...

Surfaces with switchable adhesive properties are employed by robots to quickly grip and release objects and thereby to perform dexterous manipulation and locomotion tasks. Robotic grippers with switchable adhesion have been developed using structured polymers and electrostatic mechanisms. However, manipulating delicate items can be challenging as t...

Laser-based metal processing including welding and three dimensional printing, involves localized melting of solid or granular raw material, surface tension-driven melt flow and significant evaporation of melt due to the applied very high energy densities. The present work proposes a weakly compressible smoothed particle hydrodynamics formulation f...

Evaporative self-assembly of semiconducting polymers is a low-cost route to fabricating micrometer and nanoscale features for use in organic and flexible electronic devices. However, in most cases, rate is limited by the kinetics of solvent evaporation, and it is challenging to achieve uniformity over length- and time-scales that are compelling for...

Metal additive manufacturing (AM) will impact many industries, including those producting of advanced structural, propulsion, electronic, and thermal systems. Many established metal AM processes involve layer-by-layer deposition and selective binding or melting of metal powders. However, these processes do not easily permit multi-material printing...

At an estimated cost of $8 billion annually in the United States, revision surgeries to total joint replacements represent a substantial financial burden to the health care system. Fixation failures, such as implant loosening, wear, and mechanical instability of the poly(methyl methacrylate) (PMMA) cement, which bonds the implant to the bone, are t...

Composites of polymers and organized carbon nanotube (CNT) networks have been proposed as next‐generation lightweight structural materials, yet polymer infiltration of CNT networks often results in stress‐concentrating heterogeneities, due to local CNT aggregation or incomplete infiltration. Herein, it is demonstrated that dense CNT‐polymer composi...

Food quality monitoring, particularly, the detection of bacterial pathogens and spoilage throughout the food supply chain, is critical to ensure global food safety and minimize food loss. Incorporating sensors into packaging is promising, but it is challenging to achieve the required sampling volume while using food‐safe sensor materials. Here, by...

Flexographic printing, which involves high-speed contact of an inked stamp against a substrate, is of increasing interest for scalable manufacturing of electronics in new formats. However, the adaptation of flexography to thinner, finer features generally required for printed electronics requires improved understanding of stamp-substrate contact me...

3D printing is used to self‐assemble surface‐functionalized nanoparticles into supercrystalline macrostructures. The obtained columns feature high strength, favorably comparing to hierarchical composite materials, known for their exceptional mechanical properties. Further details can be found in the article, number 2000352, by A. John Hart, Gerold...

To translate the exceptional properties of colloidal nanoparticles (NPs) to macroscale geometries, assembly techniques must bridge a 10⁶‐fold range of length. Moreover, for successfully attaining a final mechanically robust nanocomposite macroscale material, some of the intrinsic NPs’ properties have to be maintained while minimizing the density of...

To enable robust rheological measurements of the properties of yield stress fluids, we introduce a class of modified vane fixtures with fractal-like cross-sectional structures. A greater number of outer contact edges leads to increased kinematic homogeneity at the point of yielding and beyond. The vanes are 3D printed (3DP) using a desktop stereoli...

Advances in three-dimensional nanofabrication techniques have enabled the development of lightweight solids, such as hollow nanolattices, having record values of specific stiffness and strength, albeit at low production throughput. At the length scales of the structural elements of these solids—which are often tens of nanometers or smaller—forces r...

2020 IOP Publishing Ltd. Aseptic loosening, or loss of implant fixation, is a common complication following total joint replacement. Revision surgeries cost the healthcare system over $8 billion annually in the United States. Despite the prevalence of aseptic loosening, timely and accurate detection remains a challenge because traditional imaging m...

Further understanding of how nanoparticle catalyst composition influences the yield of carbon nanotubes (CNTs) is key to their scalable, cost-effective manufacture. In particular, the role of trace carbon deposits on promoting CNT nucleation from the catalyst has been studied recently by our team. Here, we show that deposition of solid carbon onto...

Micro- and nanotextured surfaces with reconfigurable textures can enable advancements in control of wetting and heat transfer, directed assembly of complex materials, and reconfigurable optics, among many applications. However, reliable and programmable directional shape change in large scale is significant for prescribed applications. Herein, we d...

Accurate quantification of trace contaminants currently requires collection, preservation, and transportation of large volumes (250-1000 mL) of water to centralized laboratories, which impedes monitoring of trace-level pollutants in many resource-limited environments. To overcome this logistical challenge, we propose a new paradigm for trace contam...