| 1. |
- Neidhardt, J., et al.
(författare)
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Transmission electron microscopy studies and simulation of the indentation response of superelastic fullerenelike carbon nitride thin films
- 2008
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 103:12
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Tidskriftsartikel (refereegranskat)abstract
- The highly elastic behavior of fullerenelike carbon nitride thin films on single crystal Si (001) substrates has been studied by nanoindentation. The films exhibit a microstructure of frequently bent and intersecting graphene sheets. Transmission electron microscopy of indented areas revealed no tendency to plastic deformation in the highly elastic and compliant film. Also, a unique deformation pattern in the Si substrate is observed, where a {111} facetted inverted pyramid of untransformed Si remained intact under the indenter with a retained CNx /Si (001) interface. Analytical approaches using the effective indenter method, supported by finite element methods, are employed to understand the nonlinear but fully elastic behavior of the coatings as well as to describe the critical parameters for the deformation and phase transformation of the Si substrate. © 2008 American Institute of Physics.
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| 2. |
- Cordoba Gallego, Jose Manuel, et al.
(författare)
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Morphology influence of the oxidation kinetics of carbon nanofibers
- 2009
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Ingår i: CORROSION SCIENCE. - : Elsevier BV. - 0010-938X. ; 51:4, s. 926-930
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports the stability and oxidation rate of five types of carbon nanofiber (CNF) with distinctly different orientation of their graphite sheets based on conversion to CO2 when heated in the presence of oxygen. A non-isothermal technique was used to determine the oxidation kinetic parameters including the activation energy (E-a) Graphite shows a similar activation energy (E-a = 158 kJ/mol(-1)) to CNF with longitudinal alignment (E-a = 156kJ/mol(-1)). CNF type herringbone (E-a = 126kJ/mol(-1)) and platelet (E-a = 145 kJ/mol(-1)) show the lowest oxidation resistance which improved dramatically after a heat treatment at 3023 K of the herringbone (E-a = 216 kJ/mol(-1)) and platelet (E-a = 174 kJ/mol(-1)) structures.
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| 3. |
- Molina-Aldareguia, J.M., et al.
(författare)
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Deformation structures under indentations in TiN/NbN single-crystal multilayers deposited by magnetron sputtering at different bombarding ion energies
- 2002
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Ingår i: Philosophical magazine. A. Physics of condensed matter. Defects and mechanical properties. - 0141-8610. ; 82:10 SPEC., s. 1983-1992
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Tidskriftsartikel (refereegranskat)abstract
- Work elsewhere has suggested that multilayer films with layer thicknesses of a few nanometres can be much harder than monolithic films, although there is considerable variation in the observed magnitude of this effect. To investigate this, multilayer TiN/NbN films have been deposited by reactive magnetron sputtering on to MgO single crystals. The hardnesses measured were similar to those of the TiN and NbN alone, which is consistent with the observation by transmission electron microscopy (TEM) that deformation across the interfaces was not prevented. Varying the electrical potential at which the film was grown from -10 to -200 V and the corresponding ion energy from 10 to 200 eV increased the hardness from 19 to 25 GPa, further decreases in the potential caused the hardness to decrease. Using TEM, deformation was observed to occur along the apparent columnar boundaries within the films, suggesting that the effect of the electrical potential on the measured hardness was caused by changes in the apparent strength of the columnar boundaries, possibly associated with the variations in the volume fraction of voids that were observed on these boundaries.
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| 4. |
- Tamayo-Ariztondo, J., et al.
(författare)
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Effect of heat treatment of carbon nanofibres on electroless copper deposition
- 2010
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Ingår i: Composites Science And Technology. - : Elsevier BV. - 0266-3538 .- 1879-1050. ; 70:16, s. 2269-2275
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Tidskriftsartikel (refereegranskat)abstract
- Cu is a well known heat sink material due to its high thermal conductivity. However, its coefficient of thermal expansion (CTE) is high. One of the most promising solutions for reducing it is to reinforce copper with carbon nanofibres (CNF) because of their low CTE. To exploit the properties of the CNFs a good dispersion of the reinforcement within the matrix must be achieved. One of the processing methods used to obtain a homogeneous CNF distribution is coating the CNF with Cu using electrochemical deposition. In this paper, the effect of the carbon structure on electroless deposition technique is studied. Different CNF have been compared: herringbone (HB), platelet (PL) and longitudinally aligned (previously heat treated) (LAHT). Herringbone and Platelet CNF were heat treated at 2750 °C for 30′ which resulted in a structure resembling graphite with loops at the fibre surface. These loops are responsible for an enhancement of the copper coating. It is shown that the Cu coverage in electroless deposition is high for the graphene plane and poor at the edges of the plane.
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| 5. |
- Högberg, Hans, 1968-, et al.
(författare)
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Growth and characterization of MAX-phase thin films
- 2005
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Ingår i: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 193, s. 6-10
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Tidskriftsartikel (refereegranskat)abstract
- We report that magnetron sputtering can be applied to synthesize MAX-phase films of several systems including Ti–Si–C, Ti–Ge–C, Ti–Al–C, and Ti–Al–N. In particular, epitaxial films of the known phases Ti3SiC2, Ti3GeC2, Ti2GeC, Ti3AlC2, Ti2AlC, and Ti2AlN as well as the newly discovered thin film phases Ti4SiC3, Ti4GeC3 and intergrown structures can be deposited at 900–1000 °C on Al2O3(0001) and MgO(111) pre-seeded with TiC or Ti(Al)N. From XTEM and AFM we suggest a growth and nucleation model where MAX-phase nucleation is initiated at surface steps or facets on the seed layer and followed by lateral growth. Differences between the growth behavior of the systems with respect to phase distribution and phase stabilities are discussed. Characterization of mechanical properties for Tin+1Si–Cn films with nanoindentation show decreased hardness from about 25 to 15 GPa upon penetration of the basal planes with characteristic large plastic deformation with pile up dependent on the choice of MAX material. This is explained by cohesive delamination of the basal planes and kink band formation, in agreement with the observations made for bulk material. Measurements of the electrical resistivity for Ti–Si–C and Ti–Al–N films with four-point probe technique show values of 30 and 39 μΩ cm, respectively, comparable to bulk materials.
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| 6. |
- Lloyd, S.J., et al.
(författare)
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Multilayered materials : A palette for the materials artist
- 2003
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Ingår i: Philosophical Transactions. Series A. - : The Royal Society. - 1364-503X .- 1471-2962. ; 361:1813, s. 2931-2949
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Tidskriftsartikel (refereegranskat)abstract
- Developments in the understanding of how materials behave enable us to design material structures to display specified properties. We introduce multilayered materials as systems in which new properties, not found in their constituents in bulk form, can emerge. The importance of transmission electron microscopy to determine structure-property relationships in nanoscale multilayers through characterization of their atomic and electronic structure is emphasized. Two examples of technologically useful multilayer systems are considered in more detail: hard coatings made from nitride multilayer films and the new structures and magnetic properties that are found in some metal multilayer systems. Finally, we discuss the future developments that are required to fully exploit the novel properties found in multilayered materials.
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| 7. |
- Molina-Aldareguia, J.M., et al.
(författare)
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Kink formation around indents in laminated Ti3SiC2 thin films studied in the nanoscale
- 2003
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Ingår i: Scripta Materialia. - 1359-6462 .- 1872-8456. ; 49:2, s. 155-160
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Tidskriftsartikel (refereegranskat)abstract
- The deformation mechanisms in ductile Ti3SiC2(0 0 0 1) single-crystal films have been analysed by nanoindentation and cross-sectional transmission electron microscopy. Permanent deformation includes formation of kink bands, as the nanolaminated material buckles out at the perimeter of the contact area, and delamination cracks. Evidence is presented for incipient kink-band formation.
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| 8. |
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| 9. |
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| 10. |
- Vigueras, G., et al.
(författare)
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An XFEM/CZM implementation for massively parallel simulations of composites fracture
- 2015
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Ingår i: Composite structures. - : Elsevier BV. - 0263-8223 .- 1879-1085. ; 125, s. 542-557
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Tidskriftsartikel (refereegranskat)abstract
- Because of their widely generalized use in many industries, composites are the subject of many research campaigns. More particularly, the development of both accurate and flexible numerical models able to capture their intrinsically multiscale modes of failure is still a challenge. The standard finite element method typically requires intensive remeshing to adequately capture the geometry of the cracks and high accuracy is thus often sacrificed in favor of scalability, and vice versa. In an effort to preserve both properties, we present here an extended finite element method (XFEM) for large scale composite fracture simulations. In this formulation, the standard FEM formulation is partially enriched by use of shifted Heaviside functions with special attention paid to the scalability of the scheme. This enrichment technique offers several benefits since the interpolation property of the standard shape function still holds at the nodes. Those benefits include (i) no extra boundary condition for the enrichment degree of freedom, and (ii) no need for transition/blending regions; both of which contribute to maintaining the scalability of the code.Two different cohesive zone models (CZM) are then adopted to capture the physics of the crack propagation mechanisms. At the intralaminar level, an extrinsic CZM embedded in the XFEM formulation is used. At the interlaminar level, an intrinsic CZM is adopted for predicting the failure. The overall framework is implemented in ALYA, a mechanics code specifically developed for large scale, massively parallel simulations of coupled multi-physics problems. The implementation of both intrinsic and extrinsic CZM models within the code is such that it conserves the extremely efficient scalability of ALYA while providing accurate physical simulations of computationally expensive phenomena. The strong scalability provided by the proposed implementation is demonstrated. The model is ultimately validated against a full experimental campaign of loading tests and X-ray tomography analyzes.
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