Atomistic modeling of the effect of codoping on the atomistic structure of interfaces in alpha-alumina

Sintering aids or dopants have often been used successfully to limit the grain growth of alumina during sintering. Recently codoping of alumina with transition elements has been reported to produce additional effects in comparison to single doping in enhancement of creep and real in-line transmittance of light. The current study attempts to address the issue of the atomistic mechanism behind these experimentally observed codoping effects. The effect of codoping on the atomistic structure of a series of La–Y, Mg–Y, La–Mg codoped -alumina interfaces was studied using energy minimization calculations. The segregation energy for single doping as well as codoping is negative for all the surfaces and grain boundaries. While, there is no significant energetic gain for La–Y cosegregation in comparison to single doping whereas segregation energies for Mg–Y and Mg–La codoping is more negative than single doping. A specific arrangement of dopants (associative effect) is also observed in La–Y codoped interfaces. Both mechanisms can thus contribute to the improved microstructures and properties

Cement and Concrete Research

Experimental work has been done to determine changes in the particle shape of portlandite grown in the presence of different ions. To quantify the experimentally observed changes in morphology a new analysis tool was developed, allowing the calculation of the relative surface energies of the crystal facets. The observed morphology in the presence of chlorides and nitrates was facetted particles of a similar shape, the addition of sulfates leads to hexagonal platelet morphology and the addition of silicates leads to the formation of large irregular aggregates. In addition to the experimental work, the surfaces of portlandite were studied with atomistic simulation techniques. The empirical force field used has first been validated. The equilibrium morphology of portlandite in vacuum and in water was then calculated. The results indicate that the presence of water stabilizes the [20.3] surface and changes the morphology. This is consistent with the experimental observation of [20.3] surfaces

Atomistic simulation of the absorption of calcium and hydroxyl ions onto portlandite surfaces - towards crystal growth mechanisms

Portlandite is an important constituent of cementitious materials. Consequently the growth of portlandite is of great interest to fully understand the hydration of cement, a process still posing many scientific challenges. In this paper the growth of portlandite in aqueous systems is studied by simulating the adsorption of Ca2 + and OH− at different portlandite surfaces. For the simulation an adapted version of the Freeman (Freeman et al., 2007) in combination with the TIP4P/2005 (Abascal and Vega, 2005) force field was used for both molecular dynamics, conventional and well-tempered metadynamics calculations. Depending on the structure of the portlandite–water interface, different adsorption sites were observed. Based on these we were able to propose an atomistic mechanism of portlandite growth in different crystallographic directions. The proposed mechanism indicates that different species control the growth in different directions, consistent with experimental observations reported in literature (Arnold, 2004)

Voronoi tessellation-based algorithm for determining rigorously defined classical and generalized geometric pore size distributions

The geometric pore size distribution (PSD) P(r) as function of pore radius r is an important characteristic of porous structures, including particle-based systems, because it allows us to analyze adsorption behavior, the strength of materials, etc. Multiple definitions and corresponding algorithms, particularly in the context of computational approaches, exist that aim at calculating a PSD, often without mentioning the employed definition and therefore leading to qualitatively very different and apparently incompatible results. Here, we analyze the differences between the PSDs introduced by Torquato et al. and the more widely accepted one provided by Gelb and Gubbins, here denoted as T-PSD and G-PSD, respectively, and provide rigorous mathematical definitions that allow us to quantify the qualitative differences. We then extend G-PSD to incorporate the ideas of coating, which is significant for nanoparticle-based systems, and of finite probe particles, which is crucial to micro and mesoporous particles. We derive how the extended and classical versions are interrelated and how to calculate them properly. We next analyze various numerical approaches used to calculate classical G-PSDs and may be used to calculate the generalized G-PSD. To this end, we propose a simple yet sufficiently complicated benchmark for which we calculate the different PSDs analytically. This approach allows us to completely rule out a recently proposed algorithm based on radical Voronoi tessellation. Instead, we find and prove that the output of a grid-free classical Voronoi tessellation, namely, the properties of its triangulated faces, can be used to formulate an algorithm, which is capable of calculating the generalized G-PSD for a system of monodisperse spherical particles (or points) to any precision, using analytical expressions. The Voronoi-based algorithm developed and provided here has optimal scaling behavior and outperforms grid-based approaches.ISSN:1539-3755ISSN:1063-651XISSN:1095-3787ISSN:1550-237

Atomistic Simulations of Silicate Species Interaction with Portlandite Surfaces

Portlandite (Ca(OH)(2), CH) is the second most abundant hydrate formed in the reaction of Portland cement with water, making it an important component in the built environment. Formation of CH is closely linked to the growth of the main hydrate phase, calcium silicate hydrate (C-S-H), affecting the microstructure and properties of cement. Understanding the interplay between the growth of CH and C-S-H is the key to comprehend the hydration reaction kinetics. This interplay mainly happens via the interaction of the different species present in the pore solution with the hydrates formed. It has been speculated that silicate species poison the growth of portlandite. In this work, we give evidence and propose a mechanism toward this experimentally observed effect using atomistic simulations. We also study the stability of a Ca-Si complex (CaSiO2(OH)(2)) expected to exist in pore solution using metadynamics calculations. We find that the adsorption of this stable Ca-Si complex at the (0001) portlandite surface is energetically favorable, contrary to the previously observed adsorption of the CH growth species Ca2+ and OH-. Additionally, the adsorbed complex retains a certain mobility at the surface. Growth poisoning is thus likely to happen by preferential adsorption of Ca-Si complexes. The interplay of CH and C-S-H growth is likely to be enhanced by the easier polymerization of calcium-silicate species adsorbed at portlandite surfaces

Monolithic resorcinol–formaldehyde alcogels and their corresponding nitrogen-doped activated carbons

Here we report the adaptation of formaldehyde crosslinked phenolic resin-based aerogel and xerogel synthesis to ethanol-based solvent systems. Three specific formulations, namely one resorcinol–formaldehyde (RF) and two resorcinol– melamine–formaldehyde (RMF) systems were studied. As-prepared resins were characterized in terms of envelope and skeletal density. Furthermore, resin samples were pyrolyzed and activated in a CO2 gas atmosphere using a single-step protocol. The corresponding carbon materials featured high surface areas, moderate water uptake capacity and thermal conductivities in the 0.1 W.m−1K−1 range, in line with comparable activated carbons. The amount of formaldehyde in the synthesis of the RMF derived carbons proved to be a critical parameter in terms of both structural features and amount of N dopant in the carbonaceous matrix. Furthermore, a high formaldehyde concentration also has a drastic effect on the pore structure of the corresponding RMF carbons, leading primarily to mesopore formation without almost any macropore formation. Perhaps more importantly, the effect of the ammonia curing catalyst concentration on the material microstructure showed the opposite effect as observed in classical, water-based phenolic resin preparations. The ethanol-based synthesis clearly affects the pore structure of the resulting materials but also opens up the possibility to create inorganic/organic hybrid materials by simple combination with classical alkoxide-based silica sol–gel chemistry

Atomistic Simulation of Y-Doped α-Alumina Interfaces

The exact mechanism of creep resistance enhancement due to yttrium (Y) doping in a-alumina is still subject to speculation, although it is known that dopants segregate strongly to grain boundaries. The current work applies atomistic simulation techniques to the study of segregation to a reasonable number of interfaces in Y-doped a-alumina. Y is shown to segregate stronger to surfaces than grain boundaries and to form ordered structures at the interfaces, which may decrease diffusion coefficients. These Y-ordered regions may act as nucleation sites for YAG precipitates particularly for rapid sintering techniques