| 1. |
- 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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| 2. |
- 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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| 3. |
- Joelsson, Torbjörn, 1972-, et al.
(författare)
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Phase stability tuning in the NbxZr1-xN thin-film system for large stacking fault density and enhanced mechanical strength
- 2005
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 86:13
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Tidskriftsartikel (refereegranskat)abstract
- The phase stability of hexagonal WC-structure and cubic NaCl-structure 4d transition metal nitrides was calculated using first-principles density functional theory. It is predicted that there is a multiphase or polytypic region for the 4d transition metal nitrides with a valence electron concentration around 9.5 to 9.7 per formula unit. For verification, epitaxial NbxZr1-xN (0 <= x <= 1) was grown by reactive magnetron sputter deposition on MgO(001) substrates and analyzed with transmission electron microscopy (TEM) and x-ray diffraction. The defects observed in the films were threading dislocations due to nucleation and growth on the lattice-mismatched substrate and planar defects (stacking faults) parallel to the substrate surface. The highest defect density was found at the x=0.5 composition. The nanoindentation hardness of the films varied between 21 GPa for the binary nitrides, and 26 GPa for Nb0.5Zr0.5N. Unlike the cubic binary nitrides, no slip on the preferred [1 (1) over bar0]{110} slip system was observed. The increase in hardness is attributed to the increase in defect density at x=0.5, as the defects act as obstacles for dislocation glide during deformation. The findings present routes for the design of wear-resistant nitride coatings by phase stability tuning.
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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
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| 8. |
- 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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| 9. |
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| 10. |
- Söderberg, Hans, et al.
(författare)
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Nanostructure formation during deposition of TiN/SiNx nanomultilayer films by reactive dual magnetron sputtering
- 2005
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 97:11, s. 114327-1
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Tidskriftsartikel (refereegranskat)abstract
- Multilayer thin films consisting of titanium nitride (TiN) and silicon nitride (SiNx) layers with compositional modulation periodicities between 3.7 and 101.7 nm have been grown on silicon wafers using reactive magnetron sputtering. The TiN and SiNx layer thicknesses were varied between 2-100 nm and 0.1-2.8 nm, respectively. Electron microscopy and x-ray diffraction studies showed that the layering is flat with distinct interfaces. The deposited TiN layers were crystalline and exhibited a preferred 002 orientation for layer thicknesses of 4.5 nm and below. For larger TiN layer thicknesses, a mixed 111/002 preferred orientation was present as the competitive growth favored 111 texture in monolithic TiN films. SiNx layers exhibited an amorphous structure for layer thicknesses ≥0.8 nm; however, cubic crystalline silicon nitride phase was observed for layer thicknesses ≤0.3 nm. The formation of this metastable SiNx phase is explained by epitaxial stabilization to TiN. The microstructure of the multilayers displayed a columnar growth within the TiN layers with intermittent TiN renucleation after each SiNx layer. A nano-brick-wall structure was thus demonstrated over a range of periodicities. As-deposited films exhibited relatively constant residual stress levels of 1.3±0.7 GPa (compressive), independent of the layering. Nanoindentation was used to determine the hardness of the films, and the measurements showed an increase in hardness for the multilayered films compared to those for the monolithic SiNx and TiN films. The hardness results varied between 18 GPa for the monolithic TiN film up to 32 GPa for the hardest multilayer, which corresponds to the presence of cubic SiNx. For larger wavelengths, ≥20 nm, the observed hardness correlated to the layer thickness similar to a Hall-Petch dependence, but with a generalized power of 0.4. Sources of the hardness increase for shorter wavelengths are discussed, e.g., epitaxial stabilization of metastable cubic SiNx, coherency stress, and impeded dislocation activity.
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