8 results found
D'Elia E, Ahmed HS, Feilden E, et al., 2019, Electrically-responsive graphene-based shape-memory composites, Applied Materials Today, Vol: 15, Pages: 185-191, ISSN: 2352-9407
Shape memory materials can open new design opportunities in fields as diverse as healthcare, transportation or energy generation. In this respect, shape memory polymers (SMPs) have attracted much attention due to their advantages over metals in terms of weight and reliability. However, they are still marred by slow reaction times and poor mechanical performance. In this work we show how, by integrating a graphene network in a SMP matrix, it is possible to create composites with very low carbon contents (below 1 wt%) able to change shapes in short times (10 s of seconds) in response to low electric voltages (<10 V). This is possible because the conductive network is highly interconnected at the microscopic scale, acting as a very efficient Joule heater. The composites exhibit excellent shape fixity (>0.95 ± 0.03) and shape recovery ratios (>0.98 ± 0.03). Due to the 2D nature of graphene, this network directs crack propagation during fracture resulting in materials that retain bending strengths close to 100 MPa and exhibit significant extrinsic toughening (with toughness that reach values up to 3 times the initiation value). Furthermore, changes in conductivity can be used to follow the formation and growth of damage in the material before catastrophic failure, allowing the use of this material as a damage sensor. These results provide practical guidelines for the design of reliable shape memory composites for structural and sensing applications.
Rocha VG, Garcia-Tunon E, Botas C, et al., 2017, Multimaterial 3D Printing of Graphene-Based Electrodes for Electrochemical Energy Storage Using Thermoresponsive Inks, ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 37136-37145, ISSN: 1944-8244
The current lifestyles, increasing population, and limited resources result in energy research being at the forefront of worldwide grand challenges, increasing the demand for sustainable and more efficient energy devices. In this context, additive manufacturing brings the possibility of making electrodes and electrical energy storage devices in any desired three-dimensional (3D) shape and dimensions, while preserving the multifunctional properties of the active materials in terms of surface area and conductivity. This paves the way to optimized and more efficient designs for energy devices. Here, we describe how three-dimensional (3D) printing will allow the fabrication of bespoke devices, with complex geometries, tailored to fit specific requirements and applications, by designing water-based thermoresponsive inks to 3D-print different materials in one step, for example, printing the active material precursor (reduced chemically modified graphene (rCMG)) and the current collector (copper) for supercapacitors or anodes for lithium-ion batteries. The formulation of thermoresponsive inks using Pluronic F127 provides an aqueous-based, robust, flexible, and easily upscalable approach. The devices are designed to provide low resistance interface, enhanced electrical properties, mechanical performance, packing of rCMG, and low active material density while facilitating the postprocessing of the multicomponent 3D-printed structures. The electrode materials are selected to match postprocessing conditions. The reduction of the active material (rCMG) and sintering of the current collector (Cu) take place simultaneously. The electrochemical performance of the rCMG-based self-standing binder-free electrode and the two materials coupled rCMG/Cu printed electrode prove the potential of multimaterial printing in energy applications.
Natural structural materials like bone and shell have complex, hierarchical architectures designed to control crack propagation and fracture. In modern composites there is a critical trade-off between strength and toughness. Natural structures provide blueprints to overcome this, however this approach introduces another trade-off between fine structural manipulation and manufacturing complex shapes in practical sizes and times. Here we show that robocasting can be used to build ceramic-based composite parts with a range of geometries, possessing microstructures unattainable by other production technologies. This is achieved by manipulating the rheology of ceramic pastes and the shear forces they experience during printing. To demonstrate the versatility of the approach we have fabricated highly mineralized composites with microscopic Bouligand structures that guide crack propagation and twisting in three dimensions, which we have followed using an original in-situ crack opening technique. In this way we can retain strength while enhancing toughness by using strategies taken from crustacean shells.
Garcia-Tunon E, Feilden E, Zheng H, et al., 2017, Graphene Oxide: An All-in-One Processing Additive for 3D Printing, ACS Applied Materials and Interfaces, Vol: 9, Pages: 32977-32989, ISSN: 1944-8244
Many 3D printing technologies are based on the development of inks and pastes to build objects through droplet or filament deposition (the latter also known as continuous extrusion, robocasting, or direct ink writing). Controlling and tuning rheological behavior is key for successful manufacturing using these techniques. Different formulations have been proposed, but the search continues for approaches that are clean, flexible, robust and that can be adapted to a wide range of materials. Here, we show how graphene oxide (GO) enables the formulation of water-based pastes to print a wide variety of materials (polymers, ceramics, and steel) using robocasting. This work combines flow and oscillatory rheology to provide further insights into the rheological behavior of suspensions combining GO with other materials. Graphene oxide can be used to manipulate the viscoelastic response, enabling the formulation of pastes with excellent printing behavior that combine shear thinning flow and a fast recovery of their elastic properties. These inks do not contain other additives, only GO and the material of interest. As a proof of concept, we demonstrate the 3D printing of additive-free graphene oxide structures as well as polymers, ceramics, and steel. Due to its amphiphilic nature and 2D structure, graphene oxide plays multiple roles, behaving as a dispersant, viscosifier, and binder. It stabilizes suspensions of different powders, modifies the flow and viscoelasticity of materials with different chemistries, particle sizes and shapes, and binds the particles together, providing green strength for manual handling. This approach enables printing complex 3D ceramic structures using robocasting with similar properties to alternative formulations, thus demonstrating the potential of using 2D colloids in materials manufacturing.
Saiz Gutierrez E, Picot O, Ferraro C, et al., 2017, Using graphene networks to build bioinspired self-monitoring ceramics, Nature Communications, Vol: 8, ISSN: 2041-1723
The properties of graphene open new opportunities for the fabrication of composites exhibiting unique structural and functional capabilities. However, to achieve this goal we should build materials with carefully designed architectures. Here, we describe the fabrication of ceramic-graphene composites by combining graphene foams with pre-ceramic polymers and spark plasma sintering. The result is a material containing an interconnected, microscopic network of very thin (20–30 nm), electrically conductive, carbon interfaces. This network generates electrical conductivities up to two orders of magnitude higher than those of other ceramics with similar graphene or carbon nanotube contents and can be used to monitor ‘in situ’ structural integrity. In addition, it directs crack propagation, promoting stable crack growth and increasing the fracture resistance by an order of magnitude. These results demonstrate that the rational integration of nanomaterials could be a fruitful path towards building composites combining unique mechanical and functional performances.
D'Elia E, Eslava S, Miranda M, et al., 2016, Autonomous self-healing structural composites with bio-inspired design, Scientific Reports, Vol: 6, ISSN: 2045-2322
Strong and tough natural composites such as bone, silk or nacre are often built from stiff blocks boundtogether using thin interfacial soft layers that can also provide sacrificial bonds for self-repair. Herewe show that it is possible exploit this design in order to create self-healing structural composites byusing thin supramolecular polymer interfaces between ceramic blocks. We have built model brick-andmortarstructures with ceramic contents above 95 vol% that exhibit strengths of the order of MPa(three orders of magnitude higher than the interfacial polymer) and fracture energies that are twoorders of magnitude higher than those of the glass bricks. More importantly, these properties can befully recovered after fracture without using external stimuli or delivering healing agents. This approachdemonstrates a very promising route towards the design of strong, ideal self-healing materials able toself-repair repeatedly without degradation or external stimuli.
D'Elia E, Barg S, Ni N, et al., 2015, Self-healing graphene-based composites with sensing capabilities, Advanced Materials, Vol: 32, Pages: 4788-4794, ISSN: 1521-4095
A self-healing composite is fabricated by confining a supramolecular polymer in a graphene network. The network provides electrical conductivity. Upon damage, the polymer is released and flows to reform the material. Healing is repeatable and autonomous. The composite is sensitive to pressure and flexion and recovers its mechanical and electrical properties even when rejoining cut surfaces after long exposure times.
Responsive graphene oxide sheets form non‐covalent networks with optimum rheological properties for 3D printing. These networks have shear thinning behavior and sufficiently high elastic shear modulus (G′) to build self‐supporting 3D structures by direct write assembly. Drying and thermal reduction leads to ultra‐light graphene‐only structures with restored conductivity and elastomeric behavior.
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