Exploring Mechanisms of Hydration and Carbonation of MgO and Mg(OH)2 in Reactive Magnesium Oxide-Based Cements

Prediction of Zn2(V, Nb, Ta)N3 Monolayers for Optoelectronic Applications

A new family of ternary nitride materials, Zn2(V, Nb, Ta)N3 monolayers, is predicted. A fabrication mechanism of the Zn2(V, Nb, Ta)N3 monolayers is proposed based on the chemical vapor deposition approach used for their bulk counterparts. The calculations show that these monolayers are thermodynamically and environmentally stable and that the Zn2VN3 monolayer is the most stable and the easiest to synthesize. The Zn2VN3 monolayer also has the highest strength and elasticity. The Zn2(V, Nb, Ta)N3 monolayers are semiconductors with nearly equal direct and indirect band gaps. Considering optoelectronic properties, the predicted monolayers are transparent to the visible light and provide shielding in the ultraviolet region. Thus, the predicted Zn2(V, Nb, Ta)N3 monolayers are promising for applications in LED devices and as blocking layers in tandem solar cells.

Interaction of Nitrite Ions with Hydrated Portlandite Surfaces: Atomistic Computer Simulation Study

The nitrite admixtures in cement and concrete are used as corrosion inhibitors for steel reinforcement and also as anti-freezing agents. The characterization of the protective properties should account for the decrease in the concentration of free NO₂^- ions in the pores of cement concretes due to their adsorption. Here we applied the classical molecular dynamics computer simulation approach to quantitatively study the molecular scale mechanisms of nitrite adsorption from NaNO₂ aqueous solution on a portlandite surface. We used a new parameterization to model the hydrated NO₂^- ions in combination with the recently upgraded ClayFF force field (ClayFF-MOH) for the structure of portlandite. The new NO₂^- parameterization makes it possible to reproduce the properties of hydrated NO₂^- ions in good agreement with experimental data. In addition, the ClayFF-MOH model improves the description of the portlandite structure by explicitly taking into account the bending of Ca-O-H angles in the crystal and on its surface. The simulations showed that despite the formation of a well-structured water layer on the portlandite (001) crystal surface, NO₂^- ions can be strongly adsorbed. The nitrite adsorption is primarily due to the formation of hydrogen bonds between the structural hydroxyls on the portlandite surface and both the nitrogen and oxygen atoms of the NO₂^- ions. Due to that, the ions do not form surface adsorption complexes with a single well-defined structure but can assume various local coordinations. However, in all cases, the adsorbed ions did not show significant surface diffusional mobility. Moreover, we demonstrated that the nitrite ions can be adsorbed both near the previously-adsorbed hydrated Na⁺ ions as surface ion pairs, but also separately from the cations.

Prediction and Characterization of Two-Dimensional Zn2VN3

A two-dimensional (2D) monolayer of a novel ternary nitride Zn₂VN₃ is computationally designed, and its dynamical and thermal stability is demonstrated. A synthesis strategy is proposed based on experimental works on production of ternary nitride thin films, calculations of formation and exfoliation energies, and ab initio molecular dynamics simulations. A comprehensive characterization of 2D Zn₂VN₃, including investigation of its optoelectronic and mechanical properties, is conducted. It is shown that 2D Zn₂VN₃ is a semiconductor with an indirect band gap of 2.75 eV and a high work function of 5.27 eV. Its light absorption covers visible and ultraviolet regions. The band gap of 2D Zn₂VN₃ is found to be well tunable by applied strain. At the same time 2D Zn₂VN₃ possesses high stability against mechanical loads, point defects, and environmental impacts. Considering the unique properties found for 2D Zn₂VN₃, it can be used for application in optoelectronic and straintronic nanodevices.

Supercritical CO2-Induced Evolution of Alkali-Activated Slag Cements

The phase changes in alkali-activated slag samples when exposed to supercritical carbonation were evaluated. Ground granulated blast furnace slag was activated with five different activators. The NaOH, Na₂SiO₃, CaO, Na₂SO₄, and MgO were used as activators. C-S-H is identified as the main reaction product in all samples along with other minor reaction products. The X-ray diffractograms showed the complete decalcification of C-S-H and the formation of CaCO₃ polymorphs such as calcite, aragonite, and vaterite. The thermal decomposition of carbonated samples indicates a broader range of CO₂ decomposition. Formation of highly cross-linked aluminosilicate gel and a reduction in unreacted slag content upon carbonation is observed through ²⁹Si and ²⁷Al NMR spectroscopy. The observations indicate complete decalcification of C-S-H with formation of highly cross-linked aluminosilicates upon sCO₂ carbonation. A 20-30% CO₂ consumption per reacted slag under supercritical conditions is observed.