Papers by Author: Nuno Franco

Abstract: Aiming to improve the nanotribological response of Si-based materials we implanted silicon wafers with different fluences of iron ions (up to 2x10¹⁷ cm^-2). Implantation was followed by annealing treatments at temperatures from 550°C to 1000°C. The implanted surfaces were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and wettability tests. Then, samples were submitted to AFM-based nanowear tests. We observe an increase of both hidrophobicity and and wear resistance of the implanted silicon, indicating that ion implantation of Si can be a route to be deeper explored in what concerns tribomechanical improvement of Si.

Abstract: Beryllium and carbon are foreseen as materials for plasma facing components of future fusion devices. Erosion, re-deposition and thermal annealing events arising from heat-loads during reactor operation will produce mixed material layers and compounds on the plasma facing surfaces, leading to changes in the material properties. In order to mimic the erosion/deposition and compound formation processes, carbon layers have been evaporated onto beryllium plates and annealed in the 373 to 1073 K range for 90 min. Ion beam measurements revealed a smooth beryllium and carbon interdiffusion at the samples interface up to 773 K. A carbide formation reaction front became apparent for higher temperatures in scanning electron microscopy observations, with the volume fraction of Be₂C crystals resulting also evident in X-ray diffraction patterns. The annealing route induced delamination of large surface areas of the samples through telephone cords blistering, attributed to strain energy release. At 973 and 1073 K, cracking occurred preferentially along the blisters boundaries and evolved in time, leading to their final removal. This fracture behaviour seems caused by the different thermal expansion coefficients of the phases. The superficial films remain unchanged at lower temperatures. The results prove that the compound formation promotes by itself the occurrence of dust emission events in tokamaks.

Abstract: The Hotbird is a state of the art X-ray laboratory for advanced materials characterisation, installed at ITN since 1999. Several major improvements in its capabilities have been implemented. On the one hand, new hardware developments have extended the applications that can be studied and on the other hand, new software has enabled both enhanced automated control of the system, and improved data analysis that leads to extraction of further precise information from the data. One improvement was the implementation of the x-ray reflectometry (XRR) technique, which is a major expansion of the Hotbird capabilities. XRR is well-suited to characterise film thickness and roughness with high resolution. Furthermore, several optics improvements, such as a Göbel mirror and monochromators were introduced. The combination of this optics allows one to use either a higher intensity beam (orders of magnitude better) or a higher resolution beam configuration. A new high-temperature chamber was developed, which allows one to perform in-situ experiments with excellent temperature control up to 800 °C, in all possible configurations. Data simulation/fitting analysis software for XRR was developed. Also, to control the diffractometer and perform experiments, a new user-friendly software package was developed. In order to illustrate the Hotbird capabilities improvements, several experimental examples will be described.

Abstract: High temperature phase transformations in EUROFER reduced activation ferritic martensitic (RAFM) steel were studied in-situ by means of X-ray diffraction. Results show that, during slow cooling, the austenite to ferrite transformation takes place around 755 oC. Full transformation of the austenitic phase into pure martensite is observed for cooling above 5 oC/min. This transformation was found in samples annealed at 950 oC for 3 h and quenched in liquid nitrogen. TEM analyses reveal a high concentration of carbides along the grain boundaries of the martensitic structure. The thermal expansion coefficient derived from the measurements was 12.7x10-6 K-1.

Abstract: Ga0.81In0.19As0.14Sb0.86 layers were grown on (100)-Te doped GaSb substrates 2º missoriented towards (110), (111)A and (111)B directions by metalorganic vapour deposition (MOVPE) at 540 °C. X-ray reciprocal space maps done in symmetric (224) and asymmetric (115) directions show a super-lattice structure due to the phase separation with a 5 nm period and independent of substrate orientation. The x-ray maps show different stage of relaxation of the films and in same cases an interdiffusion region near the substrate. Despite of the phase separation, channelling experiments with H ions as projectiles showed a good quality of the films. Channelling experiments show that the crystalline quality gets worse with increasing the In and As concentration.

Abstract: Optical and structural properties of single crystalline α-Al2O3 were changed by the implantation of high fluences of Ni ions. Sapphire single crystals with <0001> orientation were implanted at room temperature with 150 keV nickel ions. Implantation fluences were in the range 0.3×1015 to 1.8×1017 cm-2. After implantation the optical absorption spectra reveal the presence of a band peaking in the region 300 - 500 nm, depending on the retained fluence. This is usually related to the presence of metallic particles. X-ray diffraction (XRD) studies show the presence of metallic Ni after implantation. Annealing in oxidizing atmosphere promotes the ecrystallization of the host matrix along with the formation of NiAl2O4 as deduced from Rutherford Backscattering Spectrometry (RBS) and confirmed through XRD. In vacuum the particles formed are metallic like with some Ni spinel also present. The control of the implantation fluence, temperature and annealing atmosphere allows tailoring the component phases.

Abstract: Antiferromagnets (AF) such as MnPt and MnNi can be used in spin valves and tunnel junctions due to their high exchange coupling, high blocking temperature, and fair corrosion resistance. They are used as pinning layer in a AF/FM/barrier/FM structure, where FM is a ferromagnet such as CoFe and the barrier is an ultra-thin insulating layer. However, as deposited MnPt and MnNi films with thickness around 20 nm are in the fcc phase, and show no exchange bias in AF/FM bilayers. A transition from fcc to fct is required, and takes place upon annealing. We present an X-ray diffraction study of the phase transition in glass/Ta 7nm/Ru 3 nm/MnPt 20 nm/CoFe 5 nm/Ta 3 nm AF/FM bilayers. We observe the MnPt phase transition from fcc to fct around 250°C. We correlate the phase transition with the change of the bilayers magnetic properties.