R. Gonzalez-Arrabal

H trapping and mobility in nanostructured tungsten grain boundaries: a combined experimental and theoretical approach

The trapping and mobility of hydrogen in nanostructured tungsten grain boundaries (GBs) have been studied by combining experimental and density functional theory (DFT) data. Experimental results show that nanostructured W coatings with a columnar grain structure and a large number of (1 1 0)/(2 1 1) interfaces retain more H than coarsed grained W samples. To investigate the possible influence of GBs on H retention, a complete energetic analysis of a non-coherent W(1 1 0)/W(1 1 2) interface has been performed employing DFT. Our results show that this kind of non-coherent interface largely attracts point defects (both a H atom and a metallic monovacancy separately) and that the presence of these interfaces contributes to a decrease in the migration energy of the H atoms with respect to the bulk value. When both the W monovacancy and H atom are introduced together into the system, the HV complex becomes the most stable configuration and one of the mechanisms explaining the H retention in the radiation damaged GB observed experimentally.

Size and dimensionality effects in superconducting Mo thin films

Molybdenum is a low Tc, type I superconductor whose fundamental properties are poorly known. Its importance as an essential constituent of new high performance radiation detectors, the so-called transition edge sensors (TESs) calls for better characterization of this superconductor, especially in thin film form. Here we report on a study of the basic superconducting features of Mo thin films as a function of their thickness. The resistivity is found to rise and the critical temperature decreases on decreasing film thickness, as expected. More relevant, the critical fields along and perpendicular to the film plane are markedly different, thickness dependent and much larger than the thermodynamic critical field of Mo bulk. These results are consistent with a picture of type II 2D superconducting films, and allow estimates of the fundamental superconducting lengths of Mo. The role of morphology in determining the 2D and type II character of the otherwise type I molybdenum is discussed. The possible consequences of this behaviour on the performance of radiation detectors are also addressed.

Modelling the thermomechanical behaviour of the tungsten first wall in HiPER laser fusion scenarios

The behaviour of a tungsten first wall is studied under the irradiation conditions predicted for the different operational scenarios of the European laser fusion project HiPER, which is based on direct drive targets and an evacuated dry wall chamber. The scenarios correspond to different stages in the development of a nuclear fusion reactor, from proof of principle (bunch mode facility) to economic feasibility (pre-commercial power plant). This work constitutes a quantitative study to evaluate first wall performance under realistic irradiation conditions in the different scenarios. We calculated the radiation fluxes assuming the geometrical configurations reported so far for HiPER. Then, we calculated the irradiation-induced evolution of first wall temperature and the thermomechanical response of the material. The results indicate that the first wall will plastically deform up to a few microns underneath the surface. Continuous operation in a power plant leads to fatigue failure with crack generation and growth. Finally, crack propagation and the minimum tungsten thickness required to fulfil the first wall protection role is studied. The response of tungsten as a first wall material as well as its main limitations will be discussed for the HiPER scenarios.

Compositional, structural and morphological modifications of N-rich Cu3N films induced by irradiation with Cu ions at 42 MeV

N-rich Cu3N films were irradiated with Cu at 42 MeV in the fluences range from 4 × 1011 to 1 × 1014 cm−2. The radiation-induced changes in the chemical composition, structural phases, surface morphology and optical properties have been characterized as a function of ion fluence, substrate temperature and angle of incidence of the incoming ion by means of ion-beam analysis (IBA), x-ray diffraction, atomic force microscopy, profilometry and Fourier transform infrared spectroscopy techniques. IBA methods reveal a very efficient sputtering of N whose yield (5 × 103 atom/ion) is almost independent of substrate temperature (RT-300 °C) but slightly depends on the incidence angle of the incoming ion. The Cu content remains essentially constant within the investigated fluence range. All data suggest an electronic mechanism to be responsible for the N depletion. The release of nitrogen and the formation of Cu2O and metallic Cu are discussed on the basis of existing models.

Thermal stability of Mo/Au bilayers for TES applications

Mo/Au bilayers are among the most suitable materials to be used as transition-edge sensors (TES) in cryogenic microcalorimeters and bolometers, developed, among other fields, for space missions. For this purpose the thermal stability of TES at temperatures below 150 °C is a critical issue. We report on the dependence of functional properties (superconducting critical temperature, residual resistance and α) as well as on microstructure, chemical composition and interface quality for optimized high quality Mo/Au bilayers on annealing temperature and time. Data show that the functional properties of the bilayers remain stable at T < 150 °C, but changes in microstructure, interface quality and functional properties were observed for layers heated at T ≥ 200 °C. Microstructural and chemical composition data suggest that the measured changes in residual resistance ratio (RRR) and TC at T ≥ 200 °C are mainly due to an increase in the average Au grain size and to Au migration along the Mo grain boundaries at the Au/Mo interface. A way to stabilize the functional properties of the Mo/Au bilayers against temperature enhancements is proposed.

First principles study of the behavior of hydrogen atoms in a W monovacancy

We report on the structural, energetic, and electronic properties which explain the behavior of atomic hydrogen in a W monovacancy. Based on the density functional theory, we calculate the most stable configurations of up to eleven hydrogen atoms in a monovacancy using the SIESTA and the VASP codes. We discuss about the deformation produced in a unit cell when a vacancy is formed and subsequently filled with different number of H atoms. We estimate the formation and the binding energies as a function of hydrogen accumulation in the monovacancy, comparing to data previously reported and highlighting the origin of the disparity of the results. Our calculations show that the maximum number of hydrogen atoms that the monovacancy can trap depends on the procedure followed for its filling, being 10 when it is filled in sequentially and larger than 11 when it is filled in via simultaneous insertion of the hydrogen atoms. Finally, we study the influence of the valence charge density and charge redistribution in the formation of the most stable structures.