San Mamés de Burgos, Spain
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Borrell A.,Nanomaterials and Nanotechnology Research Center | Fernandez A.,Fundacion ITMA | Merino C.,Grupo Antolin Ingenieria | Torrecillas R.,Nanomaterials and Nanotechnology Research Center
International Journal of Materials Research | Year: 2010

Graphitic materials obtained at low temperatures are interesting for a wide range of industrial applications including bipolar plates. In this work, graphite based nanocomposites have been obtained starting from carbon nanofibers and a mixture of carbon nanofibers with 20 vol.% of alumina nanopowders. High density carbon components were obtained by using Spark Plasma Sintering at temperatures as low as 1500-1800°C for this kind of materials. The effect of spark plasma sintering parameters on the final density, and the mechanical and electrical properties of resulting nanocomposites have been investigated. Pure carbon nanofibers with around 90% of theoretical density and fracture strength of 60 MPa have been obtained at temperatures as low as 1500°C applying a pressure of 80 MPa during sintering. It has been proved that attrition milling is a suitable method for preparing homogeneous mixtures of carbon nanofibers and alumina powders. © Carl Hanser Verlag GmbH & Co. KG.

Varela-Rizo H.,University of Alicante | Rodriguez-Pastor I.,University of Alicante | Merino C.,Grupo Antolin Ingenieria | Martin-Gullon I.,University of Alicante
Carbon | Year: 2010

Graphene oxide nano-platelets were produced from helical-ribbon carbon nanofibers by oxidation with KMnO4/H2SO4 and further exfoliation by ultrasonication. The KMnO4 to carbon nanofiber ratio is 1:1. TEM and AFM were used to characterize the samples. TEM shows individual nanocrystals with straight edges and SAED yields a hexagonal spot pattern arrangement, attributed to graphene layers. The thickness of the nanocrystals, measured by AFM, is approximately 1.7 nm, which corresponds to a single layer of hydrated graphene oxide. © 2010 Elsevier Ltd. All rights reserved.

Bortz D.R.,University of Alicante | Merino C.,Grupo Antolin Ingenieria | Martin-Gullon I.,University of Alicante
Composites Part A: Applied Science and Manufacturing | Year: 2011

Susceptibility to matrix driven failure is one of the major weaknesses of continuous-fiber composites. In this study, helical-ribbon carbon nanofibers (CNF) were dispersed in the matrix phase of a continuous carbon fiber-reinforced composite. Along with an unreinforced control, the resulting hierarchical composites were tested to failure in several modes of quasi-static testing designed to assess matrix-dominated mechanical properties and fracture characteristics. Results indicated CNF addition offered simultaneous increases in tensile stiffness, strength and toughness while also enhancing both compressive and flexural strengths. Short-beam strength testing resulted in no apparent improvement while the fracture energy required for the onset of mode I interlaminar delamination was enhanced by 35%. Extrinsic toughening mechanisms, e.g., intralaminar fiber bridging and trans-ply cracking, significantly affected steady-state crack propagation values. Scanning electron microscopy of delaminated fracture surfaces revealed improved primary fiber-matrix adhesion and indications of CNF-induced matrix toughening. © 2011 Elsevier Ltd. All rights reserved.

Bortz D.R.,University of Alicante | Merino C.,Grupo Antolin Ingenieria | Martin-Gullon I.,University of Alicante
Composites Science and Technology | Year: 2011

This study investigates the monotonic and dynamic fracture characteristics of a discontinuous fiber reinforced polymer matrix. Specifically, small amounts (0-1. wt.%) of a helical-ribbon carbon nanofiber (CNF) were added to an amine cured epoxy system. The resulting nanocomposites were tested to failure in two modes of testing; Mode I fracture toughness and constant amplitude of stress tension-tension fatigue. Fracture toughness testing revealed that adding 0.5 and 1.0. wt.% CNFs to the epoxy matrix enhanced the resistance to fracture by 66% and 78%, respectively. Fatigue testing at 20. MPa peak stress showed a median increase in fatigue life of 180% and 365% over the control by the addition of 0.5 and 1.0. wt.% CNF, respectively. These results clearly demonstrate the addition of small weight fractions of CNFs to significantly enhance the monotonic fracture behavior and long-term fatigue performance of this polymer. A discussion is presented linking the two behaviors indicating their interdependence and reliance upon the stress intensity factor, K. © 2010 Elsevier Ltd.

Bortz D.R.,University of Alicante | Merino C.,Grupo Antolin Ingenieria | Martin-Gullon I.,University of Alicante
Composites Science and Technology | Year: 2012

We report the results of an extensive multi-stress ratio experimental study on the axial fatigue behavior of an all-carbon hierarchical composite laminate, in which carbon nanofibers (CNFs) are utilized alongside traditional micron-sized carbon fibers. Primary carbon fibers were arranged in matrix-dominated biax ±45° lay-ups in order to establish matrix and matrix/fiber interaction based performance. CNFs were matrix dispersed by three-roll calender milling. Results indicate that the CNF-reinforced composites collectively possess improved fatigue and static properties over their unmodified counterparts. Large mean lifetime improvements of 150-670% were observed in fully compressive, tensile and tensile dominated loadings. Enhancements are attributed to the high interface density and damage shielding effect of the CNFs within the matrix. Further improvements are believed to occur when the nanofibers arrest and redistribute small scale, slowly propagating matrix cracks at low applied stresses. These results highlight the ability of a nanometer-sized reinforcing phase to actively participate and enhance matrix properties while moving toward a cost effective alternative to current material solutions. © 2011 Elsevier Ltd.

Bortz D.R.,University of Alicante | Heras E.G.,Grupo Antolin Ingenieria | Martin-Gullon I.,University of Alicante
Macromolecules | Year: 2012

Epoxy systems have proven popular having important applications in aerospace and wind energy, but fracture and fatigue resistance of this polymer remain less than desired. Graphene oxide, a form of atomically thin carbon, possessing impressive multifunctional properties and an ideal interface for interacting with polymer matrices, has emerged as a viable reinforcement candidate. In this work, we report enhancements of 28-111% in mode I fracture toughness and up to 1580% in uniaxial tensile fatigue life through the addition of small amounts (≤1 wt %) of graphene oxide to an epoxy system. Graphene oxide was uniquely synthesized by unraveling and splaying open helical-ribbon carbon nanofibers. The resulting oxygenated basal planes and edges of the graphene oxide sheets were observed to promote onset of the cross-linking reaction and led to an increase in total heat of reaction effecting slightly higher glass transition temperatures of the cured composites. Measured improvements were also detected in quasi-static tensile and flexural stiffness and strength. The addition of only 0.1 wt % graphene oxide yielded a ∼12% increase in tensile modulus. At 1 wt %, flexural stiffness and strength were 12 and 23% greater than the unmodified epoxy. Sheets were observed to be well-dispersed and at various orientations within the matrix, enabling their large, 2D, and zero bulk dimensions to pin incipient matrix cracks, a toughening mechanism not typically detected in nanocomposites. © 2011 American Chemical Society.

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