María Francisca López

Fullerenes from aromatic precursors by surface-catalysed cyclodehydrogenation

Graphite vaporization provides an uncontrolled yet efficient means of producing fullerene molecules. However, some fullerene derivatives or unusual fullerene species might only be accessible through rational and controlled synthesis methods. Recently, such an approach has been used to produce isolable amounts of the fullerene C60 from commercially available starting materials. But the overall process required 11 steps to generate a suitable polycyclic aromatic precursor molecule, which was then dehydrogenated in the gas phase with a yield of only about one per cent. Here we report the formation of C60 and the triazafullerene C57N3 from aromatic precursors using a highly efficient surface-catalysed cyclodehydrogenation process. We find that after deposition onto a platinum (111) surface and heating to 750 K, the precursors are transformed into the corresponding fullerene and triazafullerene molecules with about 100 per cent yield. We expect that this approach will allow the production of a range of other fullerenes and heterofullerenes, once suitable precursors are available. Also, if the process is carried out in an atmosphere containing guest species, it might even allow the encapsulation of atoms or small molecules to form endohedral fullerenes.

In vitro corrosion behaviour of titanium alloys without vanadium

Abstract The corrosion behaviour of three new non-toxic titanium alloys for use as biomaterials has been investigated. The corrosion current densities of these Ti alloys have low values, indicating a passive state that is stable with time. These values were calculated by using both direct and alternate current methods. The comparison with Ti–6Al–4V, widely used as biomaterial, shows slightly lower corrosion rates for the non V-containing alloys. To determine the pitting corrosion resistance, anodic polarisation curves were performed. Those alloys containing Zr, with very low passivation current densities, show the best behaviour. This indicates a low susceptibility to localised corrosion in these alloys. The chemical surface analysis performed on the samples suggests an influence of the passive layer composition on the passivation current density.

Electrochemical growth of Acidithiobacillus ferrooxidans on a graphite electrode for obtaining a biocathode for direct electrocatalytic reduction of oxygen

An aspect in microbial fuel cell research that is currently of great interest is the development of bacterial cathodes. Bacterial cathodes that catalyze oxygen reduction to water at low pH have the advantage of overcoming the kinetic limitations due to the requirement of 4 protons per molecule reduced. In this work we have studied the performance of a biocathode using as electrocatalyst an acidophile microorganism: Acidithiobacillus ferrooxidans. Growth of the microorganism directly on the electrode took place using an applied voltage of 0 V vs. SCE as the only energy source and without adding redox mediators to the solution. Current densities of up to 5 A m(-2) were measured for O2 reduction in the At. ferrooxidans cathode at pH 2.0 and the electrocatalytic wave was shifted 300 mV to higher potential compared to the control graphite electrodes without the bacterium.

Surface characterization of new non-toxic titanium alloys for use as biomaterials

Abstract The surface oxide layers formed spontaneously through air contact on three new, non-toxic titanium alloys have been investigated by X-ray photoelectron spectroscopy (XPS). The alloys investigated were Ti–7Nb–6Al, Ti–13Nb–13Zr and Ti–15Zr–4Nb. The results show a large surface enrichment in Al oxides and Zr oxides, respectively, which indicates a considerable outwards diffusion of these two elements. However, an analysis of the Nb 3d spectra reveals that the formation of Nb oxides at the surface is less favoured than in the case of the other alloying elements. The XPS data show for all three Ti alloys a passive layer formed by a mixture of Ti, Al or Zr and Nb oxides.

CHEMICAL ANALYSIS OF PASSIVE FILMS ON TYPE AISI 304 STAINLESS STEEL USING SOFT X-RAY ABSORPTION SPECTROSCOPY

Centro National de Investigaciones Metalurgicas, CSIC, Avda. Gregorio de1 Amo 8, 28040 Madrid, Spain Abstract-X-ray absorption spectroscopy has been employed to investigate the differences between passive films on type AISI 304 stainless steel produced by anodic polarization at two scan rates and four potential values. Cr L,,,,,, spectra show that in all cases a Cr,O, film is formed at the surface. Fe and Ni signals have a mainly metallic lineshape with a possible small contribution from hydroxides, which is maximum for the polarized sample at the slowest scan rate and the maximum potential. Also for this sample a decrease in the Mn signal indicates Mn dissolution. 0 1998 Elsevier Science Ltd. All rights reserved Keywords: A. stainless steel, B. polarization, C. pitting corrosion, soft x-ray absorption spectroscopy (XAS).

Comparative study of the corrosion behavior of MA‐956 and conventional metallic biomaterials

In this work the corrosion behavior of a new biomaterial, the MA-956 superalloy, immersed in Hanks solution is evaluated. A comparison with conventional metallic alloys used as articular implants is established. To determine the corrosion behavior we employed electrochemical methods: evaluation of corrosion potential Ecorr, electrochemical impedance spectroscopy (EIS), and anodic polarization curves. The corrosion resistance of the MA-956 superalloy preoxidized at 1100 degrees C during 100 h is at least two orders of magnitude higher than for the other alloys. This satisfactory behavior is stationary with time. Also the probability of the appearance of the pitting corrosion process is very low. When cracking is generated in the alpha-alumina layer the repassivation process is assured because of the high Cr content in the superalloy. This study is the first step in proposing this new alloy as a biomaterial. The low toxicity of these metallic alloys in the physiological environment suggests that in vivo their biocompatibility could be satisfactory.

X-ray absorption spectroscopy study of pulsed-laser-evaporated amorphous carbon films

Amorphous carbon (a-C) films obtained by pulsed-laser ablation of graphite have been investigated by X-ray Absorption Spectroscopy (XAS). The onset of 1s →σ* transitions in the films lies in the gap between theπ* andσ* bands in graphite and very close to the absorption edge of diamond, indicating a high content ofsp3 hybridization. A sharp feature at this onset is observed and assigned to a core exciton insp3-hybridized disordered C atoms. Its shift of 0.5 eV with respect to the core exciton in diamond is probably due to a higher localization of the excited electron induced by disorder. A small peak coming from C−H bonds at the surface is observed and its intensity inereases with the amount ofsp3-hybridized atoms in the sample. This can be easily explained by associating a higher amount of dangling bonds at the surface to a highersp3 content. Polarization-dependent XAS measurements show that the angular distribution of these C−H bonds has a mean value close to the normal to the surface.

Corrosion behaviour of an Fe3Al-type intermetallic in a chloride containing solution

Abstract The corrosion behaviour of an Fe 3 Al-base intermetallic compound with different crystal structures in a chloride containing solution has been investigated. The corrosion current densities of this intermetallic were independent of the material crystal structure showing a passive state stable with time. These corrosion rates were of the same order of magnitude as for 316L stainless steel. The pitting corrosion resistance evaluated by means of cyclic anodic polarization curves was high for all different states. Amongst the different crystal structures of this intermetallic alloy, the two ordered states present the lowest pitting probability. This Fe 3 Al intermetallic shows higher pitting corrosion resistance than the 316L stainless steel but its capacity for repassivation is lower. A damaging factor of influence on the pitting corrosion behaviour is the presence of non-metallic inclusions on the surface which reduce the pitting corrosion resistance by almost a half.

Corrosion behaviour of amorphous Fe-Cr-Ni-(Si,P) alloys

Abstract The passive layers formed spontaneously via air contact on the surface of amorphous Fe62 Cr10 Ni8 X20 (X = Si,P) alloys were studied by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The results for Fe62 Cr10 Ni8 Si20 showed the formation of a protective film, mainly consisting of oxidized chromium, that greatly enhanced the corrosion resistance of this alloy, whereas in Fe62 Cr10 Ni8 p20 the film was less corrosion resistant. To evaluate the influence of the passive layer composition on the corrosion resistance, standard electrochemical measurements on both alloys in 0.01 M HCl solution were performed. The study reveals that the best corrosion behaviour is given by the Si containing amorphous alloy confirming XPS and AES results.

Resonant Photoemission vs. Coster-Kronig Auger Decay At the LIII Thresholds of Ni Metal and CuO

We report on a resonant photoemission study of Ni metal and CuO at the LIII soft-X-ray absorption thresholds. The valence band spectra exhibit a strong enhancement of electron emission at resonance, which is shown to be mostly due to an incoherent superposition of a photoemission signal with a more intense signal from L3M4,5M4,5 Coster-Kronig decay of the core-ionized state. The resonant enhancement of the valence band photoemission satellite amounts to a factor of 12, far short of the recent claim of a giant resonance. Implications of these results with respect to the nature of the 3d states in these materials are briefly discussed.