arXiv subjects

Nicholas Makau

Publications and source records attributed to Nicholas Makau.

3 recordsLinked to original sources

arXiv2024

Thermoelastic Properties Of The Ti2AlC MAX Phase: An Ab Initio Study

MAX phases are used on an industrial scale in the transportation, armour, and furnace development sectors, among others. However, data on the dynamical properties of these materials under varying temperature and pressure conditions are rare or unavailable. This study reports on the dynamical properties of the elastic constants of Ti2AlC, under these conditions, obtained from first-principles calculations. Both static and dynamical results are presented and discussed. The dynamical results show that the elastic moduli are degraded; specifically, the bulk and shear moduli show a reduction ranging from 15 to 29% and 13 to 31%, respectively, between pressures of 10 - 30 GPa and in the temperature range of 300 - 1200 K. The reduction in these moduli is likely caused by anharmonic lattice effects that lead to thermal-induced softening, particularly in the high-pressure and temperature range. The lattice parameters of this material under the conditions of study did not vary significantly. Such data is useful as part of decision support that can inform applications as well as the limitations of use.

cond-mat.mtrl-sci↗

arXiv2020

Two-Dimensional graphene-HfS$_2$ van der Waals heterostructure as electrode material for Alkali-ion batteries

Poor electrical conductivity and large volume expansion during repeated charge and discharge is what has characterized many battery electrode materials in current use. This has led to 2D materials, specifically multi-layered 2D systems, being considered as alternatives. Among these 2D multi-layered systems are the graphene-based van der Waals heterostructures with transition metal di-chalcogenides (TMDCs) as one of the layers. Thus in this study, graphene-Hafnium disulphide (Gr-HfS$_2$) system, has been investigated as a prototype Gr-TMDC system for application as battery electrode. Density functional theory calculations indicate that Gr-HfS$_2$ van der Waals heterostructure formation is energetically favoured. In order to probe its battery electrode applications capability, Li, Na and K intercalants were introduced between the layers of the heterostructure. Li and K were found to be good intercalants as they had low diffusion barriers as well as positive open circuit voltage. A comparison to bilayer graphene and bilayer HfS$_2$ indicate that Gr-HfS$_2$ is a favourable battery electrode system.

cond-mat.mtrl-sci↗

arXiv2017

Partial Density of States Ligand Field Theory (PDOS-LFT): Recovering a LFT-Like Picture and Application to Photoproperties of Ruthenium(II) Polypyridine Complexes

Gas phase density-functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are reported for a data base of 98 ruthenium(II) polypyridine complexes. Comparison with X-ray crystal geometries and with experimental absorption spectra measured in solution show an excellent linear correlation with the results of the gas phase calculations. Comparing this with the usual chemical understanding based upon ligand field theory (LFT) is complicated by the large number of molecular orbitals present and especially by the heavy mixing of the antibonding metal e*$_{g}$ orbitals with ligand orbitals. Nevertheless, we show that a deeper understanding can be obtained by a partial density-of-states (PDOS) analysis which allows us to extract approximate metal t$_{2g}$ and e*$_{g}$ and ligand π* orbital energies in a well-defined way, thus providing a PDOS analogue of LFT (PDOS-LFT). Not only do PDOS-LFT energies generate a spectrochemical series for the ligands, but orbital energy differences provide good estimates of TD-DFT absorption energies. Encouraged by this success, we use frontier-molecular-orbital-theory-like reasoning to construct a model which allows us in most, but not all, of the cases studied to use PDOS-LFT energies to provide a semiquantitative relationship between luminescence lifetimes at room temperature and liquid nitrogen temperature.

physics.chem-ph↗