| 1. |
- Morales, J. C., et al.
(författare)
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A giant exoplanet orbiting a very-low-mass star challenges planet formation models
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 365:6460, s. 1441-1445
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Tidskriftsartikel (refereegranskat)abstract
- Surveys have shown that super-Earth and Neptune-mass exoplanets are more frequent than gas giants around low-mass stars, as predicted by the core accretion theory of planet formation. We report the discovery of a giant planet around the very-low-mass star GJ 3512, as determined by optical and near-infrared radial-velocity observations. The planet has a minimum mass of 0.46 Jupiter masses, very high for such a small host star, and an eccentric 204-day orbit. Dynamical models show that the high eccentricity is most likely due to planet-planet interactions. We use simulations to demonstrate that the GJ 3512 planetary system challenges generally accepted formation theories, and that it puts constraints on the planet accretion and migration rates. Disk instabilities may be more efficient in forming planets than previously thought.
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| 2. |
- Tinetti, G., et al.
(författare)
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A chemical survey of exoplanets with ARIEL
- 2018
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Ingår i: Experimental Astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 46:1, s. 135-209
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Tidskriftsartikel (refereegranskat)abstract
- Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25–7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10–100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed – using conservative estimates of mission performance and a full model of all significant noise sources in the measurement – using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL – in line with the stated mission objectives – will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.
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| 3. |
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| 4. |
- Vilardell, Anna M., et al.
(författare)
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Mechanical behavior of in-situ alloyed Ti6Al4V(ELI)-3 at.% Cu lattice structures manufactured by laser powder bed fusion and designed for implant applications
- 2021
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Ingår i: Journal of The Mechanical Behavior of Biomedical Materials. - : Elsevier. - 1751-6161 .- 1878-0180. ; 113, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- In the present study, cellular lattice structures for implant applications are reported for the first-time incorporating copper directly by in-situ alloying in the laser powder bed fusion process. The aim to incorporate 3 at.% Cu into Ti6Al4V(ELI) is selected for improved antibacterial properties while maintaining appropriate mechanical properties. Previously, topologically optimized Ti6Al4V(ELI) lattice structures were successfully designed, manufactured and studied for implant applications. The development of a new alloy produced by in-situ alloying of elemental powder mixture of Ti6Al4V(ELI) and pure Cu powders was used here for the production of identical lattice structures with improved antibacterial properties. One of the same as-designed CAD models was used for the manufacturing of these lattices compared to previous work on pure Ti6Al4V(ELI) lattices, making direct comparison of mechanical properties possible. Similar manufacturability highlights the applicability of this alloying technique to other lattice designs. Microstructural characterization was performed by optical and electron microscopies, as well as microCT. Mechanical characterization was performed by means of compression tests and hardness measurements. Results showed that in-situ alloying with copper leads to the formation of localized Cu-rich regions, refinement of martensitic phase and the formation of CuTi2 intermetallic precipitates, which increased the hardness and strength of the material. Deviations in wall thickness between the as-designed and as-manufactured lattices led to anisotropy of the mechanical properties of the lattices. Higher compressive strength values were obtained when thicker walls were oriented along the loading direction. Nevertheless, alloying with Cu had a higher impact on the compressive strength of lattice structure than the wall thickness deviations. The direct in-situ alloying of copper in Ti6Al4V(ELI) is a promising mute for direct manufacturing of antibacterial implants.
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| 5. |
- Vilardell, A. M., et al.
(författare)
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B2-structured Fe3Al alloy manufactured by laser powder bed fusion : Processing, microstructure and mechanical performance
- 2023
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Ingår i: Intermetallics (Barking). - : Elsevier. - 0966-9795 .- 1879-0216. ; 156
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Tidskriftsartikel (refereegranskat)abstract
- Prealloyed Fe3Al was successfully manufactured by laser powder bed fusion. The best set of process parameters led to parts with a relative density of 99.5 %, a surface roughness, Sa, of 31.5 ± 5.6 μm and a hardness of 319 ± 14 HV0.1. Its microstructure as well as its mechanical properties at room and high temperatures were analyzed. The results of the chemical composition showed minor variations in aluminum content oscillating between 21 and 28 at.% along the melt pool. Additionally, elongated grains were observed to grow parallel to the building direction, as well as the development of a weak 001 texture along the building direction. The mechanical properties were influenced by the temperature. Compression tests showed a loss in strength with the increase in temperature, from a yield strength of 621 ± 40 MPa at room temperature to 89 ± 20 MPa at 650 °C. Reciprocating sliding wear tests showed that fragmentation of the intermetallic at room temperature occurs, whereas plastic deformation dominated at higher temperatures. For all temperatures, tribochemical wear was also present due to the oxidation of wear debris.
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| 6. |
- Vilardell, Anna M., et al.
(författare)
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Dense nanostructured calcium phosphate coating on titanium by cold spray
- 2017
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Ingår i: Journal of the European Ceramic Society. - : Elsevier. - 0955-2219 .- 1873-619X. ; 37:4, s. 1747-1755
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Tidskriftsartikel (refereegranskat)abstract
- This article deals with the understanding of building-up mechanisms of bioactive nanocrystalline hydroxyapatite coatings by Cold Spray, revealing very promising results in contrast to more conventional techniques such as Plasma Spray. A full characterization of feedstock and coatings is provided. The agglomerated structure of the powder proved to be suitable to obtain successfully thick hydroxyapatite coatings. A crystallite size below similar to 20 nm in the powder and the as-sprayed coatings is calculated by the Rietveld X-ray refinement method and agreed by Transmission Electron Microscopy. Some wipe tests were carried out on Ti6Al4V substrates in order to study the deposition of single particles and the nanoscale features were evaluated. The resulting structure indicates that there is no delimitation of particle boundaries and the overall coating has been formed by effective compaction of the original nanocrystallites, leading to consistent and consolidated layers. (C) 2016 Elsevier Ltd. All rights reserved.
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| 7. |
- Vilardell, Anna M., et al.
(författare)
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Manufacturing and characterization of in-situ alloyed Ti6Al4V(ELI)-3 at.% Cu by laser powder bed fusion
- 2020
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Ingår i: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 36, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- Biofunctionalization of Ti6Al4V alloy with metallic agents like Ag or Cu is a promising approach to add antibacterial properties and thus to reduce the risk of implant failure. This research investigates the in-situ alloying of Ti6Al4V(ELI) with 3 at.% Cu powders using Laser Powder Bed Fusion (L-PBF). The morphology and geometrical characteristics of the single tracks and layers were studied. Laser powers of 170 W and 340 W, and scanning speeds ranging from 0.4 to 1.4 m/s and 0.8-2.8 m/s were implemented. Single track results showed balling effect and humping at high scanning speeds, 1.4 m/s and 1.6 m/s, for each laser powder respectively. Conversely, keyhole formation occurred at lower scanning speeds of 0.4-0.6 m/s for 170 W laser power, and below and 0.8 m/s for 340 W laser power. For both laser powers, single layers resulted in smoother surfaces at lower scanning speeds. These results were used for the development of optimal process parameters for 3D cubes with 99.9 % density. Optimal process parameters were found for 170 W and 340W laser powders at 0.7-0.9 and 1.0-1.2 m/s scanning speeds, respectively.In-situ alloying by L-PBF was challenging and a homogeneous distribution of Cu within the alloy was hard to achieve. The increase in laser power from 170 to 340 W resulted in small increase in homogenization. Microstructural analyses after stress-relieving treatment showed the presence of alpha' and beta phases, as well as CuTi2 intermetallic precipitates. The finer microstructure together with CuTi2 intermetallic precipitates resulted in an increase in hardness. This study demonstrates the potential for printing in-situ alloyed Ti6Al4V(ELI)- 3 at.% Cu for biomedical applications. However, further studies are required to determine the effectiveness of antibacterial properties.
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| 8. |
- Vilardell, Anna M., et al.
(författare)
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Topology optimization and characterization of Ti6A14V ELI cellular lattice structures by laser powder bed fusion for biomedical applications
- 2019
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Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 766, s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- Topology optimization approach was used for the design of Ti6A14V ELI lattice structures with stiffness and density close to the human bone for implant applications. Three lattice designs with volume densities of 350/0, 40 % and 45 % and corresponding elastic modulus of 18.6 GPa, 23.1 GPa 27.4 GPa close to the human bone were generated. Laser powder bed fusion (LPBF) technique was used for the manufacturing of the specimens. Physical measurements and mechanical characterization of specimens were assessed by microCT analyses and compression test, perpendicular and parallel to the building direction of the specimens. LPBF Ti6A14V ELI manufactured lattice structures showed deviations in wall thickness in comparison with the generated designs, leading to an increase in relative porosity but also a decrease in elastic modulus in comparison with the original designs. Horizontal walls of the lattice structures showed higher wall thickness in comparison with the vertical walls, leading to anisotropic behaviour of the lattice structures. Higher elastic modulus and compression strength were obtained when thicker walls were oriented along the loading direction of the compression test, showing a complete failure by dividing the specimens into two neighbouring halves. All specimens showed 45 degrees diagonal shear fracture along the structure. On the other hand, higher energy absorption at first maximum compression strength peak was observed when samples were tested parallel to the building direction (when thinner walls were oriented along the loading compression direction). Results showed that designed lattice structures can possess the levels of human bones' stiffness and therefore can reduce/avoid stress shielding on implant applications.
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| 9. |
- Vilardell, Anna M., et al.
(författare)
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Cold spray as an emerging technology for biocompatible and antibacterial coatings : State of art
- 2015
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Ingår i: Journal of Materials Science. - New York : Springer. - 0022-2461 .- 1573-4803. ; 50:13, s. 4441-4462
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Forskningsöversikt (refereegranskat)abstract
- The use of coatings in biomaterials has been fundamental on the applicability of many medical devices and has helped improve mechanical properties such as wear and fatigue and biological properties such as biocompatibility and bioactivity of implant prosthesis, thus, in essence, ameliorating human quality life. The aim of the present paper is to give a review on cold spray (CS) coating systems that are emerging in orthopedics industry (internal fixation systems and prosthesis) as well as those for antibacterial purposes (in body and touch external surfaces). These studies are very new, the oldest dating from the half of last decade and most deal with the improvement of biocompatibility and bioactivity of hard tissue replacement; therefore, research on biocoatings is in constant development with the aim to produce implant surfaces that provide a balance between cell adhesion and low cytotoxicity, mechanical properties, and functionalization. CS offers many advantages over conventional high-temperature processes and seems to be able to become competitive in front of the low-temperature techniques. It is mainly cost effective, appropriate for oxygen-sensitive materials, and environmentally green. It basically involves the use of feedstock material in powder form, which is supersonically sprayed onto the appropriate substrate but without any melting as it occurs in conventional thermal spray processes. Biocompatible metallic materials and polymers have been successfully deposited by this method because it is based on the plasticity of the coating material; pure ceramic deposits, for example of hydroxyapatite, are still a challenge.
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| 10. |
- Vilardell, A. M., et al.
(författare)
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Influence of surface topography on fatigue behavior of Ti6Al4V alloy by laser powder bed fusion
- 2018
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Ingår i: Procedia CIRP. - : Elsevier. ; , s. 49-52
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Konferensbidrag (refereegranskat)abstract
- This article deals with the understanding of the influence of surface topography on fatigue behavior of Ti6Al4V alloy specimens produced by laser powder bed fusion (LPBF). The same laser parameters and scan strategy were used for all specimens, giving a sample density higher than 99.5 %. Two different surface topographies were obtained by using the top and side surfaces of the specimens. The surface topography and morphology were investigated by optical surface profilometry and focus variation microscopy. Four-point bending fatigue test was performed on specimens with top and side surfaces as the highest stressed surface respectively. Machined specimens were used as reference. The features of the fracture surface, such as crack initiation and propagation, were analyzed by focus variation and scanning electron microscopy (SEM). Both, fatigue results and fracture surface investigations, were correlated and discussed in relation to surface topography and microstructure, as well as manufacturing parameters.
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