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Graphene keeps nanoparticles apart
Materials Today Volume 18, Number 1 January/February 2015
X-ray spectroscopic findings on YBCO (YBa2Cu3O7 x). Their findings are published in the Nature journal Science Reports and point to a better understanding of how superconductivity arises in these materials. They carried out X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) experiments on YBCO at room temperature and at a chilly 2588C, a temperature much colder than the material’s critical temperature. YBCO contains two types of structural units: stacked ‘planes’ of copper oxide, which are thought to carry the superconducting current and separate ‘chains’ of copper oxide that lie in between these planes. The role of these chains has puzzled scientists since the discovery of YBCO in 1987. However, a hint lay in the fact that doping the chains with oxygen and so changing their length can alter the critical temperature. That said, most researchers assumed that the doping level of the mate-
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rial was solely determined by the structure of the chains at the time of synthesis. Now the team from Sweden and Switzerland has demonstrated that the chains in YBCO react to cooling by supplying the copper oxide planes with positive charges (electron holes) through a self-doping mechanism. Model calculations incorporating the RIXS data revealed that this self-doping process is accompanied by changes in the copper –oxygen
bonds that link the chains to the planes. The finding challenges the conventional wisdom regarding how YBCO becomes a superconductor wherein a constant doping level in the copper oxide planes is assumed. The team suggests that earlier temperature-dependent experiments may have to be re-evaluated, which could add to clues to solve the puzzle of high-temperature superconductivity. The team is now working on a more detailed temperature-dependent study that they hope will show whether restructuring and redistribution of the occupation of orbitals occurs at the phase transition to superconductivity or if this is a change that happens at higher temperature in the so-called pseudogap region. If the latter, then the implication would be that the critical temperature might be nudged higher by manipulating the chemistry to this end. David Bradley
the ‘push’ toward accumulation at grain boundaries. The innovative approach has been developed by researchers at the Universities of Wollongong and Technology in Australia, Northeastern University in China, and Sahand University of Technology and Islamic Azad University in Iran. ‘The most important novelty of this work is reaching toward a uniform distribution of nanoparticles in aluminum-based composites for the first time using the encapsulation capacity of graphene sheets,’ researcher Zhengyi Jiang of the University of Wollongong told Materials Today. The result is an improvement in yield strength and ductility of 45% and 84%, respectively, using just 1 vol.% of graphene nanosheets. ‘The advantages of these composites are higher tensile properties and especially tensile elongation,’ explains Zhengyi Jiang. ‘This work demonstrates a new roadmap for the implementation of graphene sheets in enhancing mechanical properties of metal matrix composites.’
The boost in tensile properties could be the result of more than one mechanism at work, suggest the researchers. The onionlike shells of graphene around the nanoparticles could reduce the susceptibility of SiC to cracking, which would in turn increase the threshold stress limit for the composite. The graphene could also block the movement of dislocations through the matrix, making propagation difficult. Fiber pull-out toughening, where growing cracks come across reinforcements in the matrix that require additional energy to move past, could also be having an effect. The composites could be useful for aerospace applications where high tensile properties, combined with low weight, are highly desirable. ‘Some modifications of this approach are needed before scaling up to mass production,’ says Zhengyi Jiang. ‘But this is a completely practical approach to the production of advanced composites using a simple ball milling method.’ Cordelia Sealy
Graphene keeps nanoparticles apart Automotive, aerospace, and thermal management applications rely on ceramicreinforced metal matrix composites for safety reasons. Despite the strength and toughness of these composites, the materials tend to lack ductility, which limits more widespread employment. Adding nanoparticles can overcome this limitation, but it has proven challenging to distribute the particles evenly throughout a metal matrix. Now, however, researchers think they have come up with a novel solution to the problem using graphene [Fadavi Boostani, et al., Composites A (2014), doi:10.1016/j.compositesa.2014.10.010]. Ceramic SiC nanoparticles are wrapped in graphene nanosheets, rather like the layers of an onion, before being added into the metal matrix. Ball milling, an industrial process for grinding materials into very fine powders, is used to encase nanoparticles with highly flexible sheets of graphene. The encapsulation prevents the agglomeration of nanoparticles once incorporated into the molten alloy matrix. The coated particles also seem to resist
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X-ray spectroscopic findings on YBCO (YBa2Cu3O7 x). Their findings are published in the Nature journal Science Reports and point to a better understanding of how superconductivity arises in these materials. They carried out X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) experiments on YBCO at room temperature and at a chilly 2588C, a temperature much colder than the material’s critical temperature. YBCO contains two types of structural units: stacked ‘planes’ of copper oxide, which are thought to carry the superconducting current and separate ‘chains’ of copper oxide that lie in between these planes. The role of these chains has puzzled scientists since the discovery of YBCO in 1987. However, a hint lay in the fact that doping the chains with oxygen and so changing their length can alter the critical temperature. That said, most researchers assumed that the doping level of the mate-
NEWS
rial was solely determined by the structure of the chains at the time of synthesis. Now the team from Sweden and Switzerland has demonstrated that the chains in YBCO react to cooling by supplying the copper oxide planes with positive charges (electron holes) through a self-doping mechanism. Model calculations incorporating the RIXS data revealed that this self-doping process is accompanied by changes in the copper –oxygen
bonds that link the chains to the planes. The finding challenges the conventional wisdom regarding how YBCO becomes a superconductor wherein a constant doping level in the copper oxide planes is assumed. The team suggests that earlier temperature-dependent experiments may have to be re-evaluated, which could add to clues to solve the puzzle of high-temperature superconductivity. The team is now working on a more detailed temperature-dependent study that they hope will show whether restructuring and redistribution of the occupation of orbitals occurs at the phase transition to superconductivity or if this is a change that happens at higher temperature in the so-called pseudogap region. If the latter, then the implication would be that the critical temperature might be nudged higher by manipulating the chemistry to this end. David Bradley
the ‘push’ toward accumulation at grain boundaries. The innovative approach has been developed by researchers at the Universities of Wollongong and Technology in Australia, Northeastern University in China, and Sahand University of Technology and Islamic Azad University in Iran. ‘The most important novelty of this work is reaching toward a uniform distribution of nanoparticles in aluminum-based composites for the first time using the encapsulation capacity of graphene sheets,’ researcher Zhengyi Jiang of the University of Wollongong told Materials Today. The result is an improvement in yield strength and ductility of 45% and 84%, respectively, using just 1 vol.% of graphene nanosheets. ‘The advantages of these composites are higher tensile properties and especially tensile elongation,’ explains Zhengyi Jiang. ‘This work demonstrates a new roadmap for the implementation of graphene sheets in enhancing mechanical properties of metal matrix composites.’
The boost in tensile properties could be the result of more than one mechanism at work, suggest the researchers. The onionlike shells of graphene around the nanoparticles could reduce the susceptibility of SiC to cracking, which would in turn increase the threshold stress limit for the composite. The graphene could also block the movement of dislocations through the matrix, making propagation difficult. Fiber pull-out toughening, where growing cracks come across reinforcements in the matrix that require additional energy to move past, could also be having an effect. The composites could be useful for aerospace applications where high tensile properties, combined with low weight, are highly desirable. ‘Some modifications of this approach are needed before scaling up to mass production,’ says Zhengyi Jiang. ‘But this is a completely practical approach to the production of advanced composites using a simple ball milling method.’ Cordelia Sealy
Graphene keeps nanoparticles apart Automotive, aerospace, and thermal management applications rely on ceramicreinforced metal matrix composites for safety reasons. Despite the strength and toughness of these composites, the materials tend to lack ductility, which limits more widespread employment. Adding nanoparticles can overcome this limitation, but it has proven challenging to distribute the particles evenly throughout a metal matrix. Now, however, researchers think they have come up with a novel solution to the problem using graphene [Fadavi Boostani, et al., Composites A (2014), doi:10.1016/j.compositesa.2014.10.010]. Ceramic SiC nanoparticles are wrapped in graphene nanosheets, rather like the layers of an onion, before being added into the metal matrix. Ball milling, an industrial process for grinding materials into very fine powders, is used to encase nanoparticles with highly flexible sheets of graphene. The encapsulation prevents the agglomeration of nanoparticles once incorporated into the molten alloy matrix. The coated particles also seem to resist
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