Håkan Wilhelm Hugosson

Theory of the effects of substitutions on the phase stabilities of Ti1−xAlxN

We have performed ab initio studies of the effect of substitutions on the phase stabilities of Ti1−xAlxN, x=0−1. The nonmetal substitutions studied include B, C, O, and Si. Metal substitutions studied include Sc, Zr, V, Cr, and Mn. The main objective has been to suggest substitutions that increase the thermal stability of the NaCl structure of Ti1−xAlxN at high Al contents. From these extensive and consistent calculations, some possible avenues for such stabilization present themselves, among which substitution with nonmetal C and Si, and metal V, Cr, and Mn are found to be the most promising.We have performed ab initio studies of the effect of substitutions on the phase stabilities of Ti1−xAlxN, x=0−1. The nonmetal substitutions studied include B, C, O, and Si. Metal substitutions studied include Sc, Zr, V, Cr, and Mn. The main objective has been to suggest substitutions that increase the thermal stability of the NaCl structure of Ti1−xAlxN at high Al contents. From these extensive and consistent calculations, some possible avenues for such stabilization present themselves, among which substitution with nonmetal C and Si, and metal V, Cr, and Mn are found to be the most promising.

Theory of phase stabilities and bonding mechanisms in stoichiometric and substoichiometric molybdenum carbide

First principles, total energy methods have been applied to predict the relative stabilities of the four experimentally verified MoC phases: the cubic delta(NaCl) phase and the three hexagonal gamm ...

Phase stability diagrams of transition metal carbides, a theoretical study

The experimental phase diagrams at T=0 of the refractory compounds ZrC, NbC and MoC have been reproduced with great accuracy from first principles theory. The energy of formation for these compounds has been calculated for several phases and stoichiometries in order to understand, for example, the differences and changes in homogeneity ranges found in these systems. This determination of relative phase stabilities for a wide range of concentrations is necessary for first principles determination of phase diagrams for these compounds with complex bonding and structural properties as well as technological importance.

Phase stability tuning in the NbxZr1-xN thin-film system for large stacking fault density and enhanced mechanical strength

The phase stability of hexagonal WC-structure and cubic NaCl-structure 4d transition metal nitrides was calculated using first-principles density functional theory. It is predicted that there is a multiphase or polytypic region for the 4d transition metal nitrides with a valence electron concentration around 9.5 to 9.7 per formula unit. For verification, epitaxial NbxZr1−xN (0⩽x⩽1) was grown by reactive magnetron sputter deposition on MgO(001) substrates and analyzed with transmission electron microscopy (TEM) and x-ray diffraction. The defects observed in the films were threading dislocations due to nucleation and growth on the lattice-mismatched substrate and planar defects (stacking faults) parallel to the substrate surface. The highest defect density was found at the x=0.5 composition. The nanoindentation hardness of the films varied between 21GPa for the binary nitrides, and 26GPa for Nb0.5Zr0.5N. Unlike the cubic binary nitrides, no slip on the preferred ⟨11¯0⟩{110} slip system was observed. The inc...

The connection between the electronic structure and the properties of binderless tungsten carbides

Using first principles density functional calculations, we here connect the physical properties of binderless tungsten carbides with the theoretical electronic structure as calculated from density functional theory calculations. We show that electronic structure calculations predict and explain various known phenomena in these systems, an example of how such theoretical studies can be a valuable tool in materials science. For example, changes in the energy of formation from 25% substitution with Mo or Ti is used to explain the differences in intermixing between binderless tungsten carbides using TiC or Mo2 Ca sc binder phase. Substitutions with Ti atoms are also predicted to stabilize tungsten carbide in the NaCl-structure. A study of the charge density redistribution after substitution is also made. 2003 Published by Elsevier Science Ltd.

Solvent dependence on conformational transition, dipole moment, and molecular geometry of 1,2-dichloroethane: insight from Car-Parrinello molecular dynamics calculations.

We have investigated the molecular geometry and dipole moment distribution for the major conformational states of 1,2-dichloroethane (DCE) in three different solvents under ambient conditions using the Car-Parrinello mixed quantum mechanics/molecular mechanics method. The solvents studied were water, DCE, and chloroform. Within the time scale investigated, we find a conformational equilibrium existing between the gauche and trans forms of DCE in all three solvents. In the chloroform solvent, the conformational transition occurs more frequently than in water solvent and in liquid DCE (i.e., DCE solute in DCE solvent). The population of gauche conformer is more in the case of water solvent, while the trans conformer dominates in chloroform solvent. We report a bimodal nature of the dipole moment distribution for DCE in all three solute-solvents studied, where the peaks are attributed to the trans and gauche conformational states. The dipole moments of both of the conformational states increase with increasing polarity of the solvent. Also, with an increase in solvent polarity, an increase in the C-Cl bond length and magnitude of atomic charges in DCE has been observed. The increase in atomic charges of DCE is almost twice when the solvent is changed from chloroform to water.

Investigations into conformational transitions and solvation structure of a 7-piperidino-5,9-methanobenzo[8] annulene in water

Solvation shell structure of a 7-piperidino-5,9-methanobenzo[8] annulene (PMA) in water has been investigated in ambient conditions using both molecular dynamics (MD) and Car-Parrinello molecular dynamics (CPMD) calculations. From the MD calculations, we find that this molecule exists in three major conformational states out of which two are in twist-boat forms and one in chair form. Due to the limited time scale accessible in CPMD simulations, we have studied all the three conformational states separately using CPMD. The molecular geometry, electronic charge distribution and solvation structure for all three forms are investigated. The stability order of the chair and twist-boat conformations in water solvent has been reversed when compared to the gaseous phase results and in the case of polar aprotic solvents (J. Org. Chem., 1999, 61, 5979). From the radial distribution function, we find that the solvent density around the chair form is significantly lower, which has to be directly related to the smaller solvent accessible area for this conformation and this is in complete agreement with earlier reports. Among the findings are that the solvation shell structure around the nitrogen atom in the chair form of PMA is considerably different from the open conformational forms or the twist-boat forms. The dipole moment for the closed form is found to be significantly larger when compared to the twist-boat forms.

Density functional study of carbon vacancies in titanium carbide

It is well established that TiC contains carbon vacancies not only in carbon-deficient environments but also in carbon-rich environments. We have performed density functional calculations of the vacancy formation energy in TiC for C- as well as Ti-rich conditions using several different approximations to the exchange-correlation functional, and also carefully considering the nature and thermodynamics of the carbon reference state, as well as the effect of varying growth conditions. We find that the formation of carbon vacancies is clearly favorable under Ti-rich conditions, whereas it is slightly energetically unfavorable under C-rich conditions. Furthermore, we find that the relaxations of the atoms close to the vacancy site are rather long-ranged, and that these relaxations contribute significantly to the stabilization of the vacancy. Since carbon vacancies in TiC are also experimentally observed in carbon-rich environments, we conclude that kinetics may play an important role. This conclusion is consistent with the experimentally observed high activation energies and sluggish diffusion of vacancies in TiC, effectively causing a freezing in of the vacancies.

Temperature dependence of band gaps and conformational disorder in PEDOT and its selenium and tellurium derivatives: Density functional calculations

The conducting polymer poly(3,4-ethylenedioxythiophene), or PEDOT, is an attractive material for flexible electronics. We present combined molecular dynamics and quantum chemical calculations, based on density functional theory, of EDOT oligomers and isoelectronic selenium and tellurium derivatives (EDOS and EDOTe) to address the effect of temperature on the geometrical and electronic properties of these systems. With finite size scaling, we also extrapolate our results to the infinite polymers, i.e., PEDOT, PEDOS, and PEDOTe. Our computations indicate that the most favourable oligomer conformations at finite temperature are conformations around the flat trans-conformation and a non-flat conformation around 45° from the cis-conformation. Also, the dihedral stiffness increases with the atomic number of the heteroatom. We find excellent agreement with experimentally measured gaps for PEDOT and PEDOS. For PEDOT, the gap does not increase with temperature, whereas this is the case for its derivatives. The conformational disorder and the choice of the basis set both significantly affect the calculated gaps.

Experimental and density functional theory studies of some novel piperidine-containing acetylene glycols

Synthesis routes of novel piperidine-containing acetylenes are presented. The new molecules are expected to exhibit plant growth stimulation properties. In particular, the yield in a situation of d ...