Functional Ink Formulation for Printing and Coating of Graphene and Other 2D Materials: Challenges and Solutions

The properties of 2D materials are unparalleled when compared to their 3D counterparts; many of these properties are a consequence of their size reduction to only a couple of atomic layers. Metallic, semiconducting, and insulating types can be found and form a platform for a new generation of devices. Among the possible methods to utilize 2D materials, functional printing has emerged as a strong contender because inks can be directly formulated from dispersions obtained by liquid-phase exfoliation. Printed graphene-based devices are shifting from laboratory applications toward real-world and mass-producible systems going hand in hand with a good understanding of suitable exfoliation methods for the targeted type of ink. Such a clear picture does not yet exist for hexagonal boron nitride (h-BN), the transition metal dichalcogenides (TMDs), and black phosphorous (BP). Rather, reports of applications of these 2D materials in printed devices are scattered throughout the literature, not yet adding to a comprehensive and full understanding of the relevant parameters. This perspective starts with a summary of the most important features of inks from exfoliated graphene. For h-BN, the TMDs, and BP, the characteristic properties when exfoliated from solution and strategies to formulate inks are summarized.

2D Material Based Inks For Room Temperature Printed Electronics

Sina Abdolhosseinzadeh

Functional printing is a versatile mass production method that has gained considerable scientific and industrial attention in the past few years. A wide range of electronics from passive circuit components (e.g., resistors and interconnects) to high-performance transistors and high-quality displays can now be printed energy-, resource-, and cost-efficiently. In spite of the recent progresses in the formulation of inks and significant reductions in their production costs, most of the commercially available inks are still very expensive (e.g., silver inks), and limited to a few conductive and insulator materials that usually require high-temperature thermal treatments to become fully functional. To enable new applications, and further reduce the production costs, it is necessary to develop novel inks based on semiconductive and new conductive materials (with improved chemical or optical properties) that can become functional after drying (dry-only inks). Development of such inks is very challenging, and the few successful results have been mostly obtained by polymer-based inks, which suffer from cost and chemical-stability issues.Since 2004 and after the first successful isolation of single-layer pristine graphene, 2D materials have disrupted almost all fields of science and technology, from medicine, to electronics and beyond. Their unique morphology, diverse and widely-tunable physical and chemical properties, ease of synthesis, low-cost of production, and abundance of raw materials have brought up new possibilities, which were not conceivable before. Formulation of additive-free inks for room-temperature printing of electronics is one of such possibilities. In spite of their great potential, formulation of additive-free 2D-material-based inks has so far been limited to low-concentration inks that are not suitable for high-throughput manufacturing, which is one of the main incentives for printing of electronics. Furthermore, current 2D materials' synthesis and processing methods have either low yield, or are not scalable, which further limit their real-world applications. Here in this work, several processing and ink formulation strategies are reported, which can address the aforementioned problems and pave the way towards out of lab application of 2D materials.In chapter 1, after a brief introduction on different device fabrication methods, basics of liquid-phase processing, especially the topics that are related to ink formulation and printing, are concisely reviewed. In chapter 2, a universal strategy for additive-free formulation of pristine 2D materials-based inks, and their scalable production are reported. Being a universal approach, this method allows us to print advanced electronic devices consisting different types of semiconductive and conductive materials. MXenes are a recently discovered group of 2D materials with exceptional optical and electronic properties which can revolutionize the printed electronics industry. Due to their importance and being at their early stages of discovery, 2 chapters of this thesis (chapters 3 & 4) have been dedicated to their ink formulation challenges. In chapter 3, the possibility of using byproducts of MXene synthesis as a sustainable ink formulation strategy has been investigated, which can simultaneously address the financial and environmental issues of printing electronics. In chapter 4, the conventional MXene ink formulation strategies are revisited to enable their scalable printing.

EPFL2021

3D Printing and Supercritical Foaming of Hierarchical Cellular Materials

Matteo Gregorio Modesto Marascio

Hierarchical porous structures have been gathering interest in different fields owing to their unique properties associated with their multi-scale features. The observation of natural materials brought new insights into the functionality of cellular materials and inspired new processes to produce synthetic hierarchical structures. Such hierarchical cellular materials have shown significant potential in many applications, as filtering, tissue engineering and drug delivery. Polymers in particular gathered a burgeoning interest thanks to their ease of processing, which allows to produce structures at high strength/density ratio and high surface area with defined porosity. In the present study, it is proposed to develop novel technologies to manufacture polymer cellular structures. From the application of Supercritical Carbon Dioxide Foaming (ScCO2) to Fused Deposition Modelling / Fused Filament Fabrication (FDM / FFF), an extrusion based additive manufacturing technology, hierarchical porous structures were created. The material transformation phenomena and the processing window of this homothetic foaming processed were studied. The fine tuning of the foaming parameters allowed creating a micro cellular porosity with-in a 3D printed structure, without modifying the 3D topology. This allowed producing a wide range of cellular struc-tures with controlled multi scale porosity and stiffness reduced up to hundred times the stiffness of the 3D printed cellular structures. Homothetic foaming was then applied to biopolymers in order to create gradient hierarchical porous structures. In particular, articular cartilage and bone (osteochondral) defects were targeted. Cartilage repair is a challenging clinical problem because large defects do not regenerate. Tissue engineering offers a solution by implanting a material, defined as scaffold, loaded with cells to induce a regeneration in the tissue that otherwise would not occur. Multi material Poly(lactide-co-caprolactone) â Poly(lactide-BTCP) cellular structures were successfully processed into a scaffold able to replicate the complex gradient mechanical properties of the osteochondral tissue. In particular, the processing windows to process multi material 3D printed and foamed structures was established. The mechanical properties under compression of these scaffolds were compared to the values measured by nanoindentation on human articular cartilage, showing a good correlation between scaffold and target application. Furthermore, from the developed knowledge, a novel additive manufacturing method is proposed from the inte-gration of FDM/FFF and ScCO2 into a single process, named 3D Foam Printing. 3D Foam Printing was applied to process structures with hollow filaments or filaments with radial porosity with live porosity control during the process. The influence of processing parameters on foam morphology was investigated. Different cellular structures achievable by tuning the printing temperature and speed were described for different biomaterials. The processes described in this work allowed to mimic the complex mechanical properties of the osteochondral tissue, a natural hierarchical material. However, they demonstrated to be relevant to a wide range of materials, as polymers, blends and composites. This is particularly true for 3D Foam Printing, which could positively influence many engineering applications, as aerospace, medicine and energy, where tuneable cellular polymers are highly demanded.

EPFL2018

Reusable support for additive manufacturing

Ziqi Wang

Additive manufacturing (AM) processes such as material extrusion and vat photopolymerization all require supports to print parts with overhang features. These additional supports using the same or different materials are a waste of materials since they need to be removed after the three-dimensional (3D) printing process and cannot be reused. The printing of supports is also time-consuming for the nozzle-based material extrusion processes. A new type of reusable support has been developed to address the support-related challenges in AM. The main idea is to use a set of dynamically controlled metal pins as a programmable building platform. In the layer fabrication, the metal pins will move up one-layer thickness after the printing of each layer. Also, each metal pin will automatically stop at a specified height that is determined by a combination of metal tubes, magnetic discs, and magnetic rings. Additional supports can be 3D-printed on the top surface of the metal pins, while the amount of the supports is dramatically reduced. After the printing process, the metal rods can be separated from the part and reset for the next printing job. A prototype system has been constructed to demonstrate the reusable support principle. The layout optimization and toolpath generation algorithms for the reusable support are also presented. The experimental results of several test cases show an average of nearly 40% saving on the printing time and material with increased reliability and robustness. The reusable support provides a support generation strategy that could be beneficial to other AM processes such as stereolithography and selective laser melting.