N.W. Makau

Bio: N.W. Makau is an academic researcher from Moi University. The author has contributed to research in topics: Local-density approximation. The author has an hindex of 1, co-authored 1 publications receiving 49 citations.

Performance of various density-functional approximations for cohesive properties of 64 bulk solids

Abstract: Accurate and careful benchmarking of different density-functional approximations (DFAs) represents an important source of information for understanding DFAs and how to improve them. In this work we have studied the lattice constants, cohesive energies, and bulk moduli of 64 solids using six functionals, representing the local, semi-local, and hybrid DFAs on the first four rungs of Jacob’s ladder. The set of solids considered consists of ionic crystals, semiconductors, metals, and transition-metal carbides and nitrides. To minimize numerical errors and to avoid making further approximations, the full-potential, all-electron FHI-aims code has been employed, and all the reported cohesive properties include contributions from zeropoint vibrations. Our assessment demonstrates that current DFAs can predict cohesive properties with mean absolute relative errors of 0.6% for the lattice constant and 6% for both the cohesive energy and the bulk modulus over the whole database of 64 solids. For semiconducting and insulating solids, the recently proposed SCAN meta-GGA functional represents a substantial improvement over the other functionals. However, when considering the different types of solids in the set, all of the employed functionals exhibit some variance in their performance. There are clear trends and relationships in the deviations of the cohesive properties, pointing to the need to consider, for example, long-range van der Waals (vdW) interactions. This point is also demonstrated by consistent improvements in predictions for cohesive properties of semiconductors when augmenting GGA and hybrid functionals with a screened Tkatchenko-Scheffler vdW energy term. Submitted to: New J. Phys.

Ultra-High Temperature Materials II: Refractory Carbides I (Ta, Hf, NB and Zr Carbides)

Abstract: The work represents a thorough treatment of ultra-high temperature materials with melting points around or over 2500 °C. The second volume included physical (structural, thermal, electromagnetic, optical, mechanical and nuclear) and chemical (binary, ternary and multicomponent systems, solid-state diffusion, wettability, interaction with chemicals, gases and aqueous solutions) properties of refractory carbide materials: tantalum carbides (monocarbide TaC1–x and semicarbide a/b-Ta2±xC), hafnium monocarbide HfC1–x, niobium carbides (monocarbide NbC1–x and semicarbide a/b/c-Nb2±xC) and zirconium monocarbide ZrC1–x. It will be of interest to researchers, engineers, postgraduate, graduate and undergraduate students alike. The reader/user is provided with the full qualitative and quantitative assessment for the materials, which could be applied in various engineering devices and environmental conditions in the wide range from cryogenic to ultra-high temperatures, on the basis of the latest updates in the field of physics, chemistry, nanotechnology, materials science and engineering.

Structural, mechanical and electronic properties of 3d transition metal nitrides in cubic zincblende, rocksalt and cesium chloride structures: a first-principles investigation.

Abstract: We report systematic results from ab initio calculations with density functional theory on three cubic structures, zincblende (zb), rocksalt (rs) and cesium chloride (cc), of the ten 3d transition metal nitrides. We computed lattice constants, elastic constants, their derived moduli and ratios that characterize mechanical properties. Experimental measurements exist in the literature of lattice constants for rs-ScN, rs-TiN and rs-VN and of elastic constants for rs-TiN and rs-VN, all of which are in good agreement with our computational results. Similarly, computed Vickers hardness (HV) values for rs-TiN and rs-VN are consistent with earlier experimental results. Several trends were observed in our rich data set of 30 compounds. All nitrides, except for zb-CrN, rs-MnN, rs-FeN, cc-ScN, cc-CrN, cc-NiN and cc-ZnN, were found to be mechanically stable. A clear correlation in the atomic density with the bulk modulus (B) was observed with maximum values of B around FeN, MnN and CrN. The shear modulus, Young’s modulus, HV and indicators of brittleness showed similar trends and all showed maxima for cc-VN. The calculated value of HV for cc-VN was about 30 GPa, while the next highest values were for rs-ScN and rs-TiN, about 24 GPa. A relation (HV/ 2 D ) between HV and Debye temperature ( D) was investigated and verified for each structure type. A tendency for anti-correlation of the elastic constant C44, which strongly influences stability and hardness, with the number of electronic states around the Fermi energy was observed. (Some figures may appear in colour only in the online journal)

Physical Properties of the NbC Carbide

Abstract: Transition metal carbides are interesting materials with a singular combination of properties, such as high melting points, high hardness, good transport properties and relatively low costs, which makes them excellent candidates for several technological applications. The possible applications of NbC carbide remained unexplored as it was in the past expensive and available in limited volumes. In order to guide investigations of the applicability of NbC, a deeper understanding of the physical properties of this carbide is fundamental. In this review paper, key physical properties of NbC are compiled with emphasis on its chemical bonding, a careful description of the C-Nb phase diagram, the phases formed and the crystal structures. Thermal properties are discussed and correlated with the intrinsic and extrinsic features of NbC. Finally, elastic properties are discussed.