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Stacking graphene sheets
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Materials Today Volume 17, Number 6 July/August 2014
3D vascular system allows for self-healing of composite materials Scientists have turned to nature to develop a 3D vascular system that permits highperformance composite materials such as fiberglass to heal both autonomously and repeatedly. Damage to such fiber-reinforced composites, commonly used in aerospace, automotive, civil, and naval fields due to their effective strength-to-weight ratio, can be difficult to repair using traditional approaches. The team, from the University of Illinois at Urbana-Champaign, whose research was published in the journal Advanced Materials [Patrick, et al., Adv. Mater. (2014), doi:10.1002/adma.201400248], were looking to solve the problem of small cracks in composites that become irreversibly damaged by delamination, limiting the wider deployment of such materials in industry. They demonstrated the first repeated healing in a fiber-reinforced composite system using vasculature patterns of micro-channels that
integrate two isolated networks – an epoxy resin and hardener acting as liquid healing agents sequestered in two different microchannel networks. As fiber-composite laminates are produced by the weaving and stacking of multiple layers, it is comparatively easy for the structure to separate between the layers. In this new 3D vascular system, when a fracture breaks apart the separate networks, the healing agents are automatically released into the crack plane. On coming into contact with one another in situ, or within the material, they polymerize to form a structural glue at the damage site and were shown to heal the material over multiple cycles. It is important the vascular networks do not run in straight lines to allow the healing agents to mix properly once released. Therefore the vessels overlap, significantly improving their resilience and life span.
The team introduced the same process used for making laminates to stitch in a line made from a bio-friendly polymer (termed ‘‘sacrificial fiber’’) within the composite. Once this was achieved, the system was heated to melt and evaporate the sacrificial fibers so that hollow microchannels remained, which became the vasculature for the self-healing system. The method therefore integrates seamlessly with standard manufacturing processes for polymer composites, and is also highly scalable. The approach could be used in structures prone to cyclic damage and are critically important for the safety and performance of engineered systems. The team is now continuing to explore biomimetic vasculatures through more advanced fabrication techniques, which could lead to even more complex vascular architectures, including multi-scale and branched networks. Laurie Donaldson
Stacking graphene sheets An electric field can change the structure of a tri-layer of graphene, converting it from a metallic form to a semiconductor form, according to physicists at the University of Arizona. The discovery might open up the applications of the all-carbon material being heralded as the natural successor to silicon in microelectronics [LeRoy, et al., Nat. Mater. (2014), doi:10.1038/nmat3965]. Brian LeRoy and his collaborators point out that changing the crystal structure of most materials requires the application of a high temperature, high pressure or both, but an electric field is all that is required to alter the stacking of the three layers between their two stable forms. The first layer is the familiar graphene monolayer. In the second layer, half the atoms sit over the center of the hexagon in the bottom layer and the other half sit over an atom in the bottom layer. For the third layer, half the atoms again sit over the hexagon in the second layer. Now, there is a choice for the other half of the atoms, they can either sit directly over atoms in the second layer or over atoms in the bottom layer. The second layer sits over half the holes in the bottom layer. The third layer either sits directly above the first layer (ABA) or over the other half of the holes in the bottom layer (ABC). The team explains that both stacking patterns can thus exist in the same trilayer 264
Image credit: Pablo San-Jose ICMM-CSIC.
graphene flake. Nevertheless, there is a sharp boundary between the arrangements involving strain among the hexagons of carbon atoms in each graphene layer. ‘‘Due to the different stacking configurations on either side of the domain wall, one side of the material behaves as a metal, while the other side behaves as a semiconductor,’’
explains LeRoy. The team was probing these trilayer materials with the metal tip of a scanning tunneling microscope and discovered that they could move the position of the domain wall within a flake of graphene, as they moved the domain wall, the crystal structure of the trilayer graphene changed in its wake. ‘‘We had the idea that there would be interesting electronic effects at the boundary, and the boundary kept moving around on us,’’ LeRoy adds. ‘‘At first it was frustrating, but once we realized what was going on, it turned out to be a most interesting effect.’’ By applying an electric field to move the boundary, it is now possible for the first time to change the crystal structure of graphene in a controlled fashion. ‘‘This basically gives us an on-off switch, which had not been realized yet in graphene,’’ he suggests. While silicon’s crown is unlikely to be usurped in the very near future given its vast industrial legacy and prominence, developments such as this nudge graphene another step forward in its accession to the throne. ‘‘The next step in the work is to show that we can move the domain walls using electrostatic gates instead of needing the STM tip. This ability would open the way to using the motion of the domain walls in devices,’’ LeRoy told Materials Today. David Bradley
Materials Today Volume 17, Number 6 July/August 2014
3D vascular system allows for self-healing of composite materials Scientists have turned to nature to develop a 3D vascular system that permits highperformance composite materials such as fiberglass to heal both autonomously and repeatedly. Damage to such fiber-reinforced composites, commonly used in aerospace, automotive, civil, and naval fields due to their effective strength-to-weight ratio, can be difficult to repair using traditional approaches. The team, from the University of Illinois at Urbana-Champaign, whose research was published in the journal Advanced Materials [Patrick, et al., Adv. Mater. (2014), doi:10.1002/adma.201400248], were looking to solve the problem of small cracks in composites that become irreversibly damaged by delamination, limiting the wider deployment of such materials in industry. They demonstrated the first repeated healing in a fiber-reinforced composite system using vasculature patterns of micro-channels that
integrate two isolated networks – an epoxy resin and hardener acting as liquid healing agents sequestered in two different microchannel networks. As fiber-composite laminates are produced by the weaving and stacking of multiple layers, it is comparatively easy for the structure to separate between the layers. In this new 3D vascular system, when a fracture breaks apart the separate networks, the healing agents are automatically released into the crack plane. On coming into contact with one another in situ, or within the material, they polymerize to form a structural glue at the damage site and were shown to heal the material over multiple cycles. It is important the vascular networks do not run in straight lines to allow the healing agents to mix properly once released. Therefore the vessels overlap, significantly improving their resilience and life span.
The team introduced the same process used for making laminates to stitch in a line made from a bio-friendly polymer (termed ‘‘sacrificial fiber’’) within the composite. Once this was achieved, the system was heated to melt and evaporate the sacrificial fibers so that hollow microchannels remained, which became the vasculature for the self-healing system. The method therefore integrates seamlessly with standard manufacturing processes for polymer composites, and is also highly scalable. The approach could be used in structures prone to cyclic damage and are critically important for the safety and performance of engineered systems. The team is now continuing to explore biomimetic vasculatures through more advanced fabrication techniques, which could lead to even more complex vascular architectures, including multi-scale and branched networks. Laurie Donaldson
Stacking graphene sheets An electric field can change the structure of a tri-layer of graphene, converting it from a metallic form to a semiconductor form, according to physicists at the University of Arizona. The discovery might open up the applications of the all-carbon material being heralded as the natural successor to silicon in microelectronics [LeRoy, et al., Nat. Mater. (2014), doi:10.1038/nmat3965]. Brian LeRoy and his collaborators point out that changing the crystal structure of most materials requires the application of a high temperature, high pressure or both, but an electric field is all that is required to alter the stacking of the three layers between their two stable forms. The first layer is the familiar graphene monolayer. In the second layer, half the atoms sit over the center of the hexagon in the bottom layer and the other half sit over an atom in the bottom layer. For the third layer, half the atoms again sit over the hexagon in the second layer. Now, there is a choice for the other half of the atoms, they can either sit directly over atoms in the second layer or over atoms in the bottom layer. The second layer sits over half the holes in the bottom layer. The third layer either sits directly above the first layer (ABA) or over the other half of the holes in the bottom layer (ABC). The team explains that both stacking patterns can thus exist in the same trilayer 264
Image credit: Pablo San-Jose ICMM-CSIC.
graphene flake. Nevertheless, there is a sharp boundary between the arrangements involving strain among the hexagons of carbon atoms in each graphene layer. ‘‘Due to the different stacking configurations on either side of the domain wall, one side of the material behaves as a metal, while the other side behaves as a semiconductor,’’
explains LeRoy. The team was probing these trilayer materials with the metal tip of a scanning tunneling microscope and discovered that they could move the position of the domain wall within a flake of graphene, as they moved the domain wall, the crystal structure of the trilayer graphene changed in its wake. ‘‘We had the idea that there would be interesting electronic effects at the boundary, and the boundary kept moving around on us,’’ LeRoy adds. ‘‘At first it was frustrating, but once we realized what was going on, it turned out to be a most interesting effect.’’ By applying an electric field to move the boundary, it is now possible for the first time to change the crystal structure of graphene in a controlled fashion. ‘‘This basically gives us an on-off switch, which had not been realized yet in graphene,’’ he suggests. While silicon’s crown is unlikely to be usurped in the very near future given its vast industrial legacy and prominence, developments such as this nudge graphene another step forward in its accession to the throne. ‘‘The next step in the work is to show that we can move the domain walls using electrostatic gates instead of needing the STM tip. This ability would open the way to using the motion of the domain walls in devices,’’ LeRoy told Materials Today. David Bradley