Atomic radius | EPFL Graph Search

The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. Since the boundary is not a well-defined physical entity, there are various non-equivalent definitions of atomic radius. Four widely used definitions of atomic radius are: Van der Waals radius, ionic radius, metallic radius and covalent radius. Typically, because of the difficulty to isolate atoms in order to measure their radii separately, atomic radius is measured in a chemically bonded state; however theoretical calculations are simpler when considering atoms in isolation. The dependencies on environment, probe, and state lead to a multiplicity of definitions. Depending on the definition, the term may apply to atoms in condensed matter, covalently bonding in molecules, or in ionized and excited states; and its value may be obtained through experimental measurements, or computed from theoretical mode

Atomic force microscopy studies of nanotribology and nanomechanics

Ismaël Palaci

This thesis comes within the scope of tribology studies at the nanometer scale. The experimental techniques used in this work are essentially related to atomic force microscopy, which gives a direct access to the topography and forces of the studied systems. Two principal aspects of the nanotribology have kept our attention. They are the nanofriction and the nanomechanics. These two domains divide the thesis in two main parts that remain however intimately bound. The first experimental part of the thesis describes the friction of a nanoscopic tip sliding on hydrophilic surfaces. The dependence of the friction force on the sliding velocity is studied for various applied normal loads and surrounding relative humidity levels. We found a logarithmic dependence of the friction force on both the scanning velocity and the relative humidity. For the first time, a transition from a positive to a negative slope of the friction force versus the logarithm of the sliding velocity has been observed for the very same tip-surface contact, by varying the relative humidity. The role of the relative humidity on the friction force has then been studied more deeply, leading to a two-thirds power law dependence of the capillary force on the normal load. Finally, analytical expressions for the friction phenomenon and the capillary condensation between the asperities of the tip and the surface have been developed to explain the phenomenological behaviors. The second experimental part describes themechanics of carbon nanotubes and tobacco mosaic viruses. In order to stay in the linear elasticity regime, small indentation amplitudes have been applied in the radial direction of multiwalled carbon nanotubes adsorbed on a silicon oxide surface. Using a theory based on the Hertz model, the radial stiffness has been evaluated and compared to molecular dynamics simulations. We found a radial Young modulus strongly decreasing with increasing radius and reaching an asymptotic value of 30 ± 10 GPa. The tobacco mosaic viruses have been adsorbed on a polyimide porous membrane. Evidence for the softness of the viruses has been obtained by imaging the tubes with the atomic force microscope in non-contact mode. Viruses hanging like ropes over the pores of the surface served as basis to measure their bending Young modulus. Using a model of a clamped beam loaded by a discrete gradient of van der Waals forces, we found a bending Young modulus of 3.1 ± 0.1 MPa.

EPFL2007

Nanoscale modelling of ionic transport in the porous C-S-H network

Khalil Ferjaoui

To reduce the CO2 footprint of construction materials, concrete producers blend their cement with Supplementary Cementitious Materials (SCMs). SCMs such as fly ash or blast furnace slag are mostly the byproducts of other industries. And while SCMs are chosen to match the properties of the common Ordinary Portland cement (OPC), their addition to the cement recipe may alter the chemistry of the system. These changes may lead to different mechanical and transport properties of the cement-based structure which may, in turn, affect its long-term resistance to harmful external agents. Therefore, understanding the relationship between cementâs microstructure and the degradation mechanisms is key for optimizing the design of new cementitious materialsIn this context, chloride attack is the most common reason for steel rebars to corrode especially when exposed to external chloride (seawater, deicer saltsâŠ). At low w/c ratios (typically 13) pore solution, the C-S-H surface develops a negative surface charge density. Electrostatic interactions between the surface and the ions in the pore solution result in a redistribution of the ionic species in two layers of charge at the interface of the solid i.e. the Electrical double layer (EDL). In nanoscopic pores, the EDL is dominated by atomic phenomena which are thought to interfere with the mobility of ions and chloride in particular. In order to understand and quantify the surface effects and their influence on ionic transport, we firstly propose an atomistic model of the EDL formation based on the use of the Metropolis Monte Carlo algorithm. This model is used to compute the ionic distributions and electrochemical potentials of electrolytes at equilibrium in nanoscopic pores. These quantities constitute the main driving forces of ionic transport at the pore scale. The microstructure parameters including the surface charge density of C-S-H, the ionic strength (and pH) of the pore solution, and the pore size are equally investigated and their effect on chlorideâs behavior quantified. Among the other parameters, the model also provides quantitative information on the effect of calcium ions which are thought to play a major role in the binding of chloride on C-S-H. The next step consists in using the calculated atomic-scale properties of the EDL in order to resolve the transport problem at the pore scale and compute microscopic diffusivities of chloride. This is achieved by using the molecular computations from the Monte Carlo (MC) engine in order to implement a modified version of the classical Poisson-Boltzmann system. The method is compared to the classical Finite element analysis of the Poisson-Nernst-Planck (PNP) equations and the data are discussed in the light of established experimental results in the literature.

EPFL2022

APT/TEM characterization of the element segregation in ferritic/martensitic steels after irradiation in SINQ

Lijuan Cui

Ferritic /martensitic (FM) steels are among the most important materials in various nuclear applications. As radiation-induced segregation (RIS) is one of the major factors for embrittlement of nuclear materials, thereofre it is essential to study the RIS in FM steels. A comprehensive study has been performed on F82H, Eurofer 97, and ODS Eurofer steels after irradiation at SINQ by combining APT and TEM techniques. The study reveals that after irradiation, the RIS at low-angle GBs in F82H is less pronounced as compared to the random high-angle GBs, while in ODS Eurofer the RIS is similar. It was also found that the element segregation extent is not influenced by the GB CSL structures. The results confirmed that, as reported in literature, the small interstitial elements, such as C, N, P, exhibit GB segregation behaviour. Whereas, the GB segregation of substitutional elements depends on their atomic size. The undersized elements are enriched, and the oversized elements V and W are generally depleted at GBs. In some special cases, oversized elements are also enriched at GBs after irradiation, which may be caused by the âsolute drag effectâ of O or N. For the elements with giant sizes (with a volume over 2 times of Fe) such as Ca, Sc and K, their RIS behaviour was not known in literature, and in this work they showed strong enrichment at GBs. Further, this study also revealed that the segregation of these elements at interfaces between phases of chromium carbides, vanadium nitrides, and the matrix is essentially similar to that at GBs, except for Ti. The radiation-induced clustering of solutes was also studied. Two kinds of clusters were investigated in this study, including oxide clusters in ODS Eurofer and nano-clusters (N-clusters) with Si, Mn, Ti, Sc, Ca and K. In F82H, with increasing irradiation temperature, the N-clusters are changed from dilute, nonuniform-size and -shape to more uniform-size, compacted, and sphericial shape. Comparing with that of F82H, N-clusters have a lower number density and bigger size in Eurofer 97 irradiated under the same condition ~20 dpa/ ~300 Â°C. In ODS Eurofer, N-clusters were only observed in specimen irradiated at 300 Â°C. Some oxide clusters in the specimen irradiated at 480 Â°C contain a high portion of Si, Mn, Ti, Sc, Ca, and K. The number density of oxide clusters increases and their size decreases with irradiation temperature. Last but not least, the concentrations of spallation transmutants were quantified in detail. The results agree well with that of the neutronic calculations. This comprehensive study of irradiation effects on GB segregation and solute clustering revealed a number of novel features in FM steels. The investigation on the transmutation elements not only greatly contributes to spallation materials technology, but also broadens the basic understanding of atomic size effect on GB segregation. The comparison between the RIS behaviors in F82H / Eurofer 97 and ODS Eurofer demonstrated that the oxide clusters or oxygen could promote the segregation of some elements such as V, W and Ti, which is of significance for the application of ODS steels in intensive irradiation environments.

EPFL2021