Molecular dynamics simulations of nanoscale mechanical processes

Fracturing and friction are fundamental processes with microscopic origin and macroscopic consequences, but our basic understanding of the processes is still limited. The development of a better understanding of both fracturing and friction is important for many practical applications, spanning from understanding the strength of technological materials to determining the occurrence of earthquakes. Significant effort has been made to study fracturing and friction phenomena using modeling techniques spanning from the atomic to the macroscopic scale. For example, billion-particle molecular dynamics models can now routinely be applied to address material deformation processes. However, our understanding of the effect of various types and symmetries of chemical bonds on the qualitative behavior of nanoscale cracks and surface contacts is still in its infancy. I have addressed the difference in behavior in systems with ionic bonds and with covalent bonds using the sum of an LJ potential and a Coulomb potential to model the ionic bonds in NaCl, and a three-body SW potential for covalent bonds in Si. These two materials are assumed to represent members of two different material classes, classified according to the symmetry of the interactions. The two material classes behave differently in response to tensile and shear stress. While NaCl undergoes plastic deformations by slip transformations which preserve its crystal structure, Si has a more random atom arrangement and behaves more like a liquid when compressed

Effect of carbonation on bulk resistivity of cement/carbon nanofiber composites

The conductivity of cement/carbon nanofiber (CNF) composite materials has previously been shown to be affected by parameters such as e.g. CNF content or water to cement (w/c) ratios, water saturation and temperature. However, whether and to what extent chemical processes like cement carbonation can affect the electrical conductivity of cement/CNF materials remains unexplored. To investigate this the resistivity changes upon carbonation of Portland G cement/CNF composites were followed for more than 4 months. An increase in resistivity was observed within the first weeks of carbonation followed by a plateau and a subsequent decrease after 4 months. The changes in resistivity were correlated with the progress of the carbonation front followed using X-ray tomography. The magnitude of the resistivity changes was found to be related to w/c ratio. Volumetric changes affecting the connectivity between the CNFs can explain the resistivity changes.publishedVersio

Clustering and Precipitation during Early-Stage Artificial Aging of Al–Si–Mg(–Cu) Foundry Alloys

High-Si aluminum foundry alloys are an important material class for products with complex 3D geometries where casting is the most suitable production method. With Mg and/or Cu additions, these alloys gain strength upon heat treatment due to the formation of nanoprecipitates. These precipitated phases are of the same kind as in the wrought Al–Mg–Si(–Cu) alloys having much lower Si contents, which have been the subject of a high number of studies. Some of these studies indicate that atomic clusters formed during storage at room temperature have a strong effect on the phases that evolve during artificial aging. In this work, foundry alloys containing Si, Mg, and Cu are investigated. Room-temperature storage is found to have a great influence on kinetics during early aging. Cu additions accelerate the formation of hardening precipitates during early aging, but 1 month of room-temperature storage negates the positive effect of Cu. The maximum achievable strength is found to be limited mainly by the solubility limits of Si and Mg at the solution heat treatment temperature. With insights derived from transmission electron microscopy and atom probe tomography results, this study contributes to the understanding of the solute balance and early aging kinetics and how wrought and foundry alloys differ in these respects.publishedVersio

Portland cement hydration in the vicinity of electrically polarized conductive surfaces

Hardening of Portland cement-based materials in vicinity of electrically conductive surfaces, especially when the surfaces are electrically or galvanically polarized, can lead to both morphological and chemical changes in cement close to the surfaces due to combined electrochemical and electrophysical processes. Cement hydration products close to graphite and steel surfaces being positively (anode) and negatively (cathode) electrically polarized (direct current) were studied. Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy were used to compare structure and atomic composition of cement hydration products on cathode, anode and a reference surface with no electrical polarization. The application of direct current (DC) potential in aqueous Portland G cement dispersion significantly affects cement hydration products close to cathode and anode and different products were found at the anode compared to the cathode surfaces. At the graphite anode, calcium sulphate crystals along with calcium hydroxide were most abundant, while the graphite cathode was mainly covered with calcium hydroxide. The calcium hydroxide carbonated upon exposure to air during drying. When steel electrodes where used, the most significant adsorption occurred at the anode, in contrast to graphite where the largest amount of the adsorbed material was found on the cathode. The observed differences were explained in view of electrophysical (electrophoresis, electroosmosis) and electrochemical (reduction and oxidation) processes occurring at electrode surfaces upon application of DC current. The knowledge gained in this work is important for engineering of electrically conductive cement nano-composites where typically the contact surface of an electrically conductive filler and a cementitious matrix is high.publishedVersio

Correlation between Differential Fast Scanning Calorimetry and Additive Manufacturing Results of Aluminium Alloys

High-strength aluminium alloy powders modified with different nanoparticles by ball milling (7075/TiC, 2024/CaB6, 6061/YSZ) have been investigated in-situ during rapid solidification by differential fast scanning calorimetry (DFSC). Solidification undercooling has been evaluated and was found to decrease with an increasing number of nanoparticles, as the particles act as nuclei for solidification. Lower solidification undercooling of individual powder particles correlates with less hot cracking and smaller grains in the material produced by powder bed fusion of metals by a laser beam (PBF-LB/M). Quantitatively, solidification undercooling less than about 10–15 K correlates with almost crack-free PBF-LB/M components and grain sizes less than about 3 µm. This correlation shall be used for future purposeful powder material design on small quantities before performing extensive PBF-LB/M studies.publishedVersio

Biofilms, nontuberculous mycobacteria and infection

As micobactérias não tuberculosas (MNTs) são agentes infeciosos emergentes responsáveis por infeções diversas, nomeadamente infeções associadas aos cuidados de saúde. Neste trabalho foi avaliada a capacidade de formação de biofilmes por duas MNTs (M. smegmatis e M. chelonae). Os biofilmes foram caracterizados utilizando microscopia eletrónica e a eficácia de diversos desinfetantes foi avaliada contra MNTs recuperadas de biofilmes. Os resultados obtidos demonstram que as MNTs são capazes de formar biofilmes em materiais presentes em ambiente hospitalar e de resistir à ação de diversos desinfetantes.Non-tuberculous mycobacteria (NTMs) are emerging infectious agents responsible for various infections, namely, health-care-associated infections. In this work, biofilms assembly by two NTMs (M. smegmatis and M. chelonae) was assessed. Biofilms were characterized using electron microscopy and the ef ficacy of several disinfectants was determined against NTMs recovered from biofilms. The results obtained demonstrate that NTMs are able to assemble biofilms on materials present in a hospital environment and to resist the action of various disinfectants.info:eu-repo/semantics/publishedVersio

Local mechanical properties and precipitation inhomogeneity in large-grained Al–Mg–Si alloy

Al–Mg–Si (6xxx series) alloys show excellent mechanical properties due to the precipitates formed during heat treatment. However, heat treatment of these alloys results in a soft precipitation free zone (PFZ) close to grain boundaries that weakens them and promotes fracture, and thereby reduces the ductility of the material. This study provides quantitative insights into the mechanical properties and underlying plasticity behavior of Al–Mg–Si (6xxx series) alloys through combined nanoindentation hardness measurements and in-depth characterization of the microstructure adjacent to the PFZ region and in the grain interior. Experimental nanoindentation, transmission microscopy (TEM) and electron channeling contrast imaging results confirm the weakening effect from PFZ by means of a reduced hardness close to grain boundaries. The nanoindentation hardness mapping also revealed an increase in hardness a few micrometers from the grain boundary with respect to the grain interior. Precipitate quantification from TEM images confirms that the hardness increase is caused by a locally higher density of precipitates. To the authors’ best knowledge, this harder zone has not been recognized nor discussed in previously reported findings. The phenomenon has important implications for the mechanical properties of large-grained ( µm) aluminium alloys.publishedVersio

μSR study of Al-0.67%Mg-0.77%Si alloys

Zero-field muon spin relaxation measurements were carried out with Al-0.67%Mg- 0.77%Si alloys in the temperature range from 20 K to 300 K. Observed relaxation spectra were compared with the relaxation functions calculated by a Monte Carlo simulation with four fitting parameters: the dipolar width, trapping rate, detrapping rate and fraction of initially trapped muons. From the fitting, the temperature variations of the trapping rates reveal that there are three temperature regions concerning muon kinetics. In the low temperature region below 120 K, muons appeared to be trapped in a shallow potential yielded by dissolved Mg atoms, and thus little effect of heat treatment of the samples was observed, while in the mid and hightemperature regions, the trapping rates clearly depended on the heat treatment of the samples suggesting muon-cluster and/or muon-vacancy interactions

Effect of pulse-current-based protocols on the lithium dendrite formation and evolution in all-solid-state batteries

Understanding the cause of lithium dendrites formation and propagation is essential for developing practical all-solid-state batteries. Li dendrites are associated with mechanical stress accumulation and can cause cell failure at current densities below the threshold suggested by industry research (i.e., >5 mA/cm2). Here, we apply a MHz-pulse-current protocol to circumvent low-current cell failure for developing all-solid-state Li metal cells operating up to a current density of 6.5 mA/cm2. Additionally, we propose a mechanistic analysis of the experimental results to prove that lithium activity near solid-state electrolyte defect tips is critical for reliable cell cycling. It is demonstrated that when lithium is geometrically constrained and local current plating rates exceed the exchange current density, the electrolyte region close to the defect releases the accumulated elastic energy favouring fracturing. As the build-up of this critical activity requires a certain period, applying current pulses of shorter duration can thus improve the cycling performance of all-solid-solid-state lithium batteries.publishedVersio