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Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries. The intellectual origins of materials science stem from the Age of Enlightenment, when researchers began to use analytical thinking from chemistry, physics, and engineering to understand ancient, phenomenological observations in metallurgy and mineralogy. Materials science still incorporates elements of physics, chemistry, and engineering. As such, the field was long considered by academic institutions as a sub-field of these related fields. Beginning in the 1940s, materials science began to be more widely recognized as a specific and distinct field of science and engineering, and major technical universities around the world created dedicated schools for its study. Materials scientists emphasize understanding how the history of a material (processing) influences its structure, and thu

Nanocrystalline metals: transient testing during in situ X-ray diffraction and molecular dynamics

Zhen Sun

Nanocrystalline (NC) metals have attracted widespread interest in materials science due to their high strength compared to coarse-grained counterparts. It is well know that during uniaxial deformation, the stress-strain behaviour exhibits an extraordinary work-hardening followed by an early observation of constant flow stress. Information on possible deformation mechanisms have been gathered by extensive research combining in situ deformation experiments, electron microscopy observations and computational modelling. Generally, these mechanisms are categorized into two types: dislocation slip and grain boundary (GB) accommodation processes, as for instance, GB sliding based mechanisms and GB migration eventually coupled to the shear stress. However, the interplay between these mechanisms resulting in the constant deformation resistance is not fully understood. Transient testing has proven to be a suitable tool to gather information on the rate limiting deformation mechanisms that are activated during the deformation path. In particular in stress reduction experiments, after an intermediate/large stress drop thermally activated dislocation slip is suppressed so that other underlying mechanisms are brought into foreground during subsequent transient creep. Those mechanisms may play a minor role in the determination of the flow stress but might still be essential to the development of a constant deformation resistance. Within this thesis, transient testing is combined with in situ X-ray diffraction at the Swiss Light Source. Therefore, the transient responses are captured in terms of evolution of macrostrain as well as diffraction peak broadening. Since dislocation slip and GB accommodation have an opposite footprint on the peak broadening, the presence of these two types of mechanisms can be distinguished. Three electrodeposited NC materials with different grain sizes are investigated: two NC Ni batches and one NC Ni50Fe50 batch. The results reveal that the constant flow stress reached during uniaxial deformation of electrodeposited NC metals reflects a quasi-stationary balance between dislocation-based mechanisms and GB-mediated accommodation. The latter plays an important role in producing plastic strain and recovering the defects and internal stress. Depending on the magnitude of the stress drop, a non-monotonic behaviour of the diffraction peak width is observed, suggesting an alternation of mechanisms. Also, by comparing transient responses among different NC materials, the relative contributions of dislocation slip and GB accommodation mechanisms are discussed in terms of grain size and alloying. Finally, different magnitudes of stress reduction are carried out by molecular dynamics (MD) simulations with the aim to verify in situ experimental results and explore the mechanisms responsible for GB accommodation. MD simulations confirm that dislocation slip is reduced after a moderate stress drop, however can continue to operate after adaption of the GB structures by a variety of GB accommodation mechanisms explaining the non-monotonic behaviour of the peak broadening during transient creep.

EPFL2016

Submicro- and Nano-Structured Porous Materials for Production of High-Intensity Exotic Radioactive Ion Beams

Sandrina Fernandes

ISOLDE, the CERN Isotope Separator On-line DEvice is a unique source of low energy beams of radioactive isotopes - atomic nuclei that have too many or too few neutrons to be stable. The facility is like a small 'chemical factory', giving the possibility of changing one element to another, by selecting the atomic mass of the required isotope beam in the mass separator, rather as the 'alchemists' once imagined. It produces a total of more than 1000 different isotopes from helium to radium, with half-lives down to milliseconds, by impinging a 1.4 GeV proton beam from the Proton Synchrotron Booster (PSB) onto special targets, yielding a wide variety of atomic fragments. Different components then extract the nuclei and separate them according to mass. The post-accelerator REX (Radioactive beam EXperiment) at ISOLDE accelerates the radioactive beams up to 3 MeV/u for many experiments. A wide international user radioactive ion beam (RIB) community investigates fundamental aspects of nuclear physics, particle and atomic physics, solid-state physics, materials science, astrophysics, biophysics and medicine. However, there are still a number of radioactive isotopes which are not accessible for experimental physics, either because it has been impossible to produce the element of interest, too low yields (number of isotopes per second measured in the beam line) are obtained or due to the higher post-acceleration energies required for the user's experiments. The aim of this thesis was to synthesize oxide and carbide porous materials, to be used as thick targets to increase the intensity of exotic RIBs. This was done by the evaluation of their radioactive isotope release properties and of their stability under irradiation conditions at high temperatures. This thesis is focused on the study of the chemical and physical processes occurring in the target system where the isotopes are produced, before these enter the ion-source system, are mass-separated and are sent to the user beam lines. The products formed in nuclear reactions between a proton beam and the target must, at a first step, diffuse from the interior of the target out to the surface and evaporate from it. Minimum delay times for the diffusion process can be realized by achieving the shortest diffusion lengths of highly-permeable, low-density, open-structure targets and by operating at the highest possible temperature so that release times are minimized and commensurate with the half-life of the isotopes of interest. Whereas until now target materials with micro-scale structures have been synthesized and used for RIB production, new porous submicro- and nano-structured materials offer the advantage of having lower diffusion activation energy of atoms and thus larger diffusion coefficient than the corresponding bulk counterpart, thanks to the increase of surface to volume ratio of these materials. The potential of this phenomenon for isotope mass separation online (ISOL) targets and many other applications is an evident drop of temperature to maintain a fast diffusion process, and higher release efficiency for the production of intense exotic isotope beams. In Chapter 1 the ISOL technique and the RIB production issues are introduced, followed by a review of recent progresses on target materials to increase yield production in the principal existing and next-generation ISOL and In-Flight facilities worldwide. Chapter 2 gives the theoretical fundamentals of diffusion and effusion in solid materials, and addresses specifically diffusion, effusion and release properties of ISOL targets. The study presented in Chapter 3 consists in the evaluation of release properties in different polycrystalline silicon carbide samples. The diffusion coefficients for Be, Na, Mg and F isotopes were extracted from off-line release measurements done at ISOLDE. The full characterization of the release time structure inherent to the physical and chemical processes of the ISOL RIB production on a submicrometric SiC target material (target SiC #334) used at ISOLDE, was made. This last target development resulted in improvement of magnesium short-lived isotopes yields. This was the first demonstration of the successful use of a submicrometric porous material as an ISOL target material, which delivered unprecedented 8x104 21Mg ions per µC. In Chapter 4, the synthesis of porous submicro- and nano-structured alumina to be used as an ISOL target, was followed by materials characterization in terms of microstructure and related with selected isotope release properties. Whenever possible, the release mechanisms were explained or proposed. The approach is relevant for the study of microstructure effects in other solid porous oxide and carbide targets, for optimization of their release performance. Chapter 5 shows the results of the TARPIPE experiment, set up within the design study task of the foreseen ISOL-type European facility called EURISOL, to investigate proton irradiation damage on typical and novel ISOL target materials under continuous proton irradiation at high temperature (> 1273 K). Materials such as SiC, Al2O3, Y2O3, C and Ta in a variety of forms were irradiated at the 72 MeV proton irradiation station of injector 1 at PSI (Paul Scherrer Institute). The effects of radiation damage on these materials were evaluated from microstructure and chemical analyses before and after irradiation by visual inspection, gamma spectroscopy, scanning electronic microscopy and electron dispersive X-ray analysis on a case-by-case basis. The moderate structure evolution of the porous oxide and carbide samples tested confirms that these materials are good candidates for high-power solid targets to be used at the existing and next generation high-power ISOL facilities, such as TRIUMF (TRI-University Meson Facility) and EURISOL (EURopean Isotope Separator On-line). In Chapter 6, the development and testing of a composite high-power alumina target for neon isotope production associated to a cold FEBIAD ion-source exposed to unprecedented proton beam intensities is reported. A 0.5 GeV proton beam from TRIUMF was used up to 25 µA, a factor 10 with respect to the present oxide targets in operation. The yields obtained were compared to the ones delivered by using a 70 µA proton beam in SiC targets at this facility. The target performance in terms of diffusion properties was evaluated. A general conclusion and discussion are given in Chapter 7. The release studies obtained with irradiation and ion implantation techniques on prospective SiC and Al2O3 matrices resulted in the identification of alternative target matrices for fast radioisotope release; this was confirmed experimentally from Na and Mg elements release efficiency determined online for a SiC target. A high-power oxide/metal composite target made of micrometric structural length scale Al2O3 pellets and Nb foils was operated online at unprecedented beam power intensities. Composite targets made of Nb foils and submicro- and nano-structured porous Al2O3 are expected to further increase yields of exotic isotopes. This ISOL target concept can be applied in the future for many other refractory insulating ceramics by using gradient microstructures and/or different ceramic/metallic combinations. Special care has to be taken, in order to minimize the stress induced by a mismatch of the coefficient of thermal expansion in the different ceramic layers and/or the underlying metallic substrate. Micro- and nano-technology fabrication techniques can be used to tailor pore and grain size distribution in ceramic and metallic complex-shaped composite materials and to shape these in compacts, monoliths and fenestrated structures. A balance between large surface area and large macropore volume for optimum target radioisotope release performance must be achieved. The resulting target material must in addition be stable at high temperatures, have good thermo-mechanical resistance and high thermal conductivity under irradiation for long irradiation time. The understanding of isotope release mechanisms in different solid materials, including the porous and dense fast-ion conductors will contribute to the production of exotic RIB at unprecedented high-intensity.

EPFL2010