Ab initio predictions of structural and elastic properties of struvite: contribution to urinary stone research

In the present work, we carried out density functional calculations of struvite--the main component of the so-called infectious urinary stones--to study its structural and elastic properties. Using a local density approximation and a generalised gradient approximation, we calculated the equilibrium structural parameters and elastic constants C(ijkl). At present, there is no experimental data for these elastic constants C (ijkl) for comparison. Besides the elastic constants, we also present the calculated macroscopic mechanical parameters, namely the bulk modulus (K), the shear modulus (G) and Young's modulus (E). The values of these moduli are found to be in good agreement with available experimental data. Our results imply that the mechanical stability of struvite is limited by the shear modulus, G. The study also explores the energy-band structure to understand the obtained values of the elastic constants.     

Thermo-mechanical properties of cubic titanium nitride

The equilibrium structure, elastic constants C_ij and thermodynamic functions of cubic titanium nitride (TiN) were calculated within the temperature range of 0–3100 K and under a pressure range 0–60 GPa. Properties were computed using the generalised gradient approximations (GGA) exchange-correlation functional. Calculated mechanical properties (Elastic constants, Young’s modulus and shear modulus) and phonon spectra of TiN obtained via robust DFT-QHA algorithm, were generally in a good agreement with available experimental and theoretical analogous values. In particular, a well-examined quasi-harmonic approximation method implemented in the Gibbs2 code is utilised herein to provide accurate estimation of thermal expansion coefficients, entropies, heat capacity values (at different combinations of temperature/volume/pressure) and Debye’s temperature. Parameters calculated herein shall be useful to elucidate the superior performance of TiN at harsh operational conditions encompassing elevated temperatures and pressures pertinent to cutting machineries and surface coatings.     

First-principles investigations on structural stability,elastic and electronic properties of Co7M6 (M= W,Mo, Nb) µ phases

The structural, elastic and electronic properties of Co₇M₆ (M?=?W, Mo, Nb) μ phases were investigated by first-principles calculations based on the density functional theory (DFT). The calculated cohesive energy indicates that Co₇M₆ (M?=?W, Mo, Nb) μ phases are thermodynamically stable. Besides, Co₇W₆ owns a higher structural stability than that of Co₇Mo₆ and Co₇Nb₆. The obtained elastic constant demonstrates that Co₇M₆ (M?=?W, Mo, Nb) are mechanically stable. With Voigt-Reuss-Hill (VRH) approximation, the elastic bulk modulus (B), shear modulus (G), Young's modulus (E) and Poisson's ratio (ν) were derived. The ductility and plasticity as well as the elastic anisotropy of the three phases were discussed in details. Finally, the density of states and charge density difference were also analysed to reveal the underlying mechanism of structural stability and the elastic properties.     

Aggregate Elasticity and Tabletability of Molecular Solids: a Validation and Application of Powder Brillouin Light Scattering

Describing the elastic deformation of single-crystal molecular solids under stress requires a comprehensive determination of the fourth-rank stiffness tensor (C_ijkl). Single crystals are, however, rarely utilized in industrial applications, and thus averaging techniques (e.g., the Voigt or Reuss approach) are employed to reduce the C_ijkl (or its inverse S_ijkl) to polycrystalline aggregate mechanical moduli. With increasing elastic anisotropy, the Voigt and Reuss-averaged aggregate moduli can diverge dramatically and, provided that drug molecules almost exclusively crystallize into low-symmetry space groups, warrants a significant need for accurate aggregate mechanical moduli. This elasticity data, which currently is largely absent for pharmaceutical materials, is expected to aid understanding how materials respond to direct compression and tablet formation. Powder Brillouin light scattering (p-BLS) has recently demonstrated facile access to porosity-independent, aggregate mechanical moduli. In this study, we extend our previous p-BLS model for obtaining mechanical properties and validate our approach against a broad library of molecular solids with diverse intermolecular interaction topologies and with previously determined C_ijkl which permits benchmarking our results. Our Young’s and shear moduli determined with p-BLS strongly correlate, with limited bias (i.e., a near 1:1 relation), with the Voigt-averaged Young’s and shear moduli determined using the C_ijkl. Through follow-on tabletability studies, we introduce initial classifications of tabletability behavior based on the results of our p-BLS studies and the apparent elastic anisotropy. With further development, this approach represents a robust and novel method to potentially identify materials for optimum tabletability at early developmental stages.     

First-principles study of phase stability and elastic properties in metastable Ti-Mo alloys with cluster structure

This paper provided a novel approach for evaluating phase stability and elastic properties in metastable Ti–Mo alloys with low Mo content by first-principles combined with cluster structure. In 54-atom body-centered-cubic supercell by substituting Ti atoms with 2–7 Mo atoms (7.1–23.0?wt% Mo), individual cluster structure of β-phase was constructed by ‘-Mo-Ti-Mo-’ cluster unit having the lowest cohesive energy. The distorted supercell was more stable than undistorted one at a low Mo content. With increasing Mo content, the density of state at Fermi level decreased, and bonding electron number increased, indicating β-phase stability was gradually promoted. Tetragonal shear elastic constant (C′?=?(C₁₁?–?C₁₂)/2), shear modulus (G₁₁₁) and anisotropy factor (A?=?C₄₄/C′) exhibited a fluctuation with Mo addition, while the change trend of A was opposite to C′ and G₁₁₁. Calculated Young’s modulus exhibited similar changing trend to the C′, implying that the softening of C′ resulted in low Young’s modulus of β-phase. Measured Young’s modulus exhibited significant difference from calculated one, which was mainly caused by formation of α″-martensite and ω-phase. The values of C′, G₁₁₁ and A were considered to associate with not only elastic properties of β-phase itself but also transition from β-phase to α″-martensite and/or ω-phase.     

Computational simulation study on the anion recognition properties of functionalized tetraphenyl porphyrins

Dedicated bond force constant and bulk modulus of C _n fullerenes (n = 20, 28, 36, 50, 60) are computed using density functional theory (DFT). DFT predicts bond force constants of 611, 648, 675, 686, and 691 N/m, for C₂₀, C₂₈, C₃₆, C₅₀, and C₆₀, respectively, indicating that the bond force constant increases for larger fullerenes. The bulk modulus predicted by DFT increases with decreased fullerene diameter, from 0.874 TPa for C₆₀ to 1.830 TPa for C₂₀. The bond force constants predicted by DFT are then used as an input for finite element analysis (FEA) of the fullerenes, considered as spatial frames in structural models where the bond stiffness is represented by the DFT-computed bond force constant. In agreement with DFT, FEA predicts that smaller fullerenes are stiffer, and underestimates the bulk modulus with respect to DFT. The difference between the FEA and DFT predictions of the bulk modulus decreases as the size of the fullerene increases, from 20.9 % difference for C₂₀ to only 4 % difference for C₆₀. Thus, it is concluded that knowing the appropriate bond force constant, FEA can be used as a plausible approximation to model the elastic behavior of small fullerenes.

     

First principles investigation of pressure dependent stability,phonon, Debye temperature,physical, mechanical and thermodynamic properties of Rh3Al intermetallic compound

ABSTRACT

Pressure dependence of stability, phonon, Debye temperature, physical, mechanical and thermodynamic properties of Rh₃Al intermetallic compound were investigated by first-principles The calculated cohesive energy (E_c), formation enthalpy (ΔH) show that Rh₃Al is a thermodynamically stable compound. Properties related to the phonons of Rh₃Al were also obtained. In addition, the transverse sound velocity (ν_s), longitudinal sound velocity (ν_l), average sound velocity (ν_m) and Debye temperature (Θ_D) of Rh₃Al were calculated by using the VRH method along with pressure range from 0 to 60?GPa. The values of lattice parameters, bulk modulus and its first-order pressure derivative are consistent well with other works. The band structure indicates that Rh₃Al compound exhibits a metallic character. Moreover, the total density of states, partial density of states, Mulliken charges and electron density difference have been analysed to explain the physical properties. Based on the stress–strain approach and the Born stability criteria, the mechanical properties were evaluated by elastic constants (C_ij), other modulus (B, E, G), (B/G) ratio, Poisson’s ratio (ν), the anisotropic index (A), hardness (H) and compressibility (K) for this intermetallic compound. Finally, the thermodynamic properties, including enthalpy, free energy, entropy and heat capacity are discussed range from 0 to 1000?K.     

Phase stability,elastic, anisotropic and thermodynamic properties of HoT2Al20 (T = Ti,V, Cr) intermetallic cage compounds

The structural stability, elastic properties, anisotropy, dynamics stability and thermodynamics properties were explored for pure Al and HoT₂Al₂₀ intermetallics from the first-principles method. The formation enthalpy and phonon frequencies indicate that these HoT₂Al₂₀ intermetallics maintain structural stability. The elastic constants C_ijs and moduli B, G, E and H_v indicate these intermetallics possess higher hardness and the better resistance to deformation. The values of Poisson’s ratio and B/G demonstrate that HoT₂Al₂₀ intermetallics are brittle materials. The anisotropic constants and anisotropic acoustic velocities confirm that HoT₂Al₂₀ intermetallics exhibit anisotropic properties. Importantly, the calculated thermal quantities demonstrate that these new HoT₂Al₂₀ intermetallics possess the better thermodynamic properties at high temperature.     

Theoretical study on the stability,electronic, mechanical,vibrational and thermodynamic properties of rare-earth intermetallic compound Rh3Ce

The structural, stability, electronic, mechanical, vibrational and thermodynamic properties of rare-earth intermetallic compound Rh₃Ce have been explored systematically by using first-principle calculations. The evaluation of the equilibrium lattice parameters were obtained firstly. Remarkably, the result of calculated unit cell volume, derived by the total energies as a function of volume, is consistent with other results. Next, the values of cohesive energy (E_c), formation enthalpy (ΔH) have verified that Rh₃Ce is a stable compound. In addition, the band structure and the total density of states indicate a metallic behaviour. Furthermore, the Mulliken charges were calculated to understand the bonding in Rh₃Ce compound. Otherwise, the elastic constants(C_ij) as well as other modulus were also calculated to evaluated the mechanical properties of Rh₃Ce. Phonon dispersion curves for Rh₃Ce were depicted to access the vibrational properties. Finally, the thermodynamic properties of Rh₃Ce were summarised range from 0 to 60?GPa, 0 to 1800?K, respectively. We also pointed out that the thermal expansion(α), heat capacity(C_v), entropy(S), Debye temperature(Θ) and Güneisen parameter (γ) change under pressure and temperature.     

First-principles investigation on structural,elastic, electronic and thermodynamic properties of filled skutterudite PrFe4P12 compound for thermoelectric applications

Filled skutterudite compound PrFe₄P₁₂ is studied using the full potential linear muffin-tin orbital method with the local density approximation for the exchange correlation potential to investigate the systematic trends for structural and elastic properties of the cubic PrFe₄P₁₂ skutterudite. The calculated ground state quantities such as the lattice constant and internal free parameters are in fairly good agreement with the available experimental data. The elastic constants and their pressure dependence are obtained by calculating the total energy versus volume-conserving strains using the Mehl model. Pressure and temperature effects on the lattice constant, bulk modulus, thermal expansion coefficient, Debye temperature and heat capacity are obtained in the range of 0–30 GPa and 0–1000 K. Reduction of bulk modulus and Debye temperature with temperature essentially indicates the thermal softening of the rare earth-filled skutterudites lattice.     

First-principles study of the structural,elastic and electronic properties of SbXI (X=S,Se, Te) crystals

The structural, elastic, elastic anisotropy and electronic properties of ferroelectric SbSI and paraelectric SbSI, SbSeI and SbTeI crystals were computed using the local density approximation with first-principle calculations, based on density functional theory. The independent elastic constants of SbXI compounds were computed and the results reveal that they are mechanically stable. Some polycrystalline quantities such as bulk modulus, shear modulus, acoustic velocities, Young’s modulus, Poisson’s ratio, elastic anisotropy and elastic Debye temperatures of these compounds were derived from computed elastic constants. Energy band structures show that these compounds have an indirect band gap. The electronic charge distribution and partial density of states of SbXI compounds indicate that the Sb-X bond is typically covalent with a strong hybridization as well as Sb-I compounds that have strong ionic character. The results obtained were compared with experimentally measured values and other theoretical data.     

Roles of electrostatic interaction and dispersion in CH···CH,CH···π, and π···π ethylene dimers

The structural, mechanical, electronic, and optical properties of orthorhombic Bi₂S₃ and Bi₂Se₃ compounds have been investigated by means of first principles calculations. The calculated lattice parameters and internal coordinates are in very good agreement with the experimental findings. The elastic constants are obtained, then the secondary results such as bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, anisotropy factor, and Debye temperature of polycrystalline aggregates are derived, and the relevant mechanical properties are also discussed. Furthermore, the band structures and optical properties such as real and imaginary parts of dielectric functions, energy-loss function, the effective number of valance electrons, and the effective optical dielectric constant have been computed. We also calculated some nonlinearities for Bi₂S₃ and Bi₂Se₃ (tensors of elasto-optical coefficients) under pressure.

The human erythrocyte membrane skeleton may be an ionic gel

Biochemical and biophysical observations indicate that the erythrocyte membrane skeleton is composed of a swollen network of long, flexible and ionizable macromolecules located at the cytoplasmic surface of the fluid membrane lipid bilayer. We have analyzed the mechanochemical properties of the erythrocyte membrane assuming that the membrane skeleton constitutes an ionic gel (swollen ionic elastomer). Using recently established statistical thermodynamic theory for such gels, our analysis yields mathematical expressions for the mechanochemical properties of erythrocyte membranes that incorporate membrane molecular parameters to an extent not achieved previously. The erythrocyte membrane elastic shear modulus and maximum elastic extension ratio predicted by our membrane model are in quantitative agreement with reported values for these parameters. The gel theory predicts further that the membrane skeleton modulus of area compression, K _G, may be small as well as large relative to the membrane elastic shear modulus, G, depending on the environmental conditions. Our analysis shows that the ratio between these two parameters affects both the geometry and the stability of the favoured cell shapes.     

Ab initio DFT study of structural and mechanical properties of methane and carbon dioxide hydrates

The structural and mechanical properties of methane and carbon dioxide hydrates were investigated using density functional theory simulations. Well-established equations of state of solids and exchange-correlation functionals were used for fitting the unit lattice total energy as a function of volume, and the full second-order elastic constants of these two gas hydrates were determined by energy–strain analyses. The polycrystalline elastic properties were also calculated from the unit lattice results. The final results for methane hydrate agree well with available experimental data and with other theoretical results. The two gas hydrates were found to be highly elastically isotropic, but they differed significantly in shear properties. The presented results for carbon dioxide hydrates are the first complete set reported so far. The results are a significant contribution to the ab initio material characterisation of gas hydrates required for ongoing fundamental studies and technological applications.     

Effects of water on the mechanical properties of gelatin films

The mechanical properties of gelatin films were studied in relation to the effect of water, and compared with those of collagen films. The S-shaped sorption isotherm was separated into an adsorption curve C₁ and dissolution curve C₂. From the C₂ curve, the interaction parameter χ₁ of Flory–Huggins' equation was calculated. The χ₁ of gelatin were larger than those of collagen at low relative humidities (RH), while they coincided with each other at high RH. It was found that a composite curve was made by shifting stress relaxation curves obtained at different humidities along the log time axis. The shift factor for the formation of the composite curve was analyzed by Fujita–Kishimoto's equation, which was based on the free volume theory. The parameter β, which expressed the extent of the contribution of sorbed water to the increase in the free volume of the system, was 0.05 in the range of C₂ from 0 to 0.08 (0–65% RH). This value was much smaller than 0.16 for collagen. The value was 0.16 in the range of C₂ higher than 0.08, which was equal to that of the collagen. Dynamic shear modulus G′, loss modulus G″, and tan δ were determined as functions of RH. The gelatin film extended more than 100% at 73% RH under the very small stress of about 10⁷ dyn/cm². This corresponds to the region where β changes from 0.05 to 0.16, although such a phenomenon was not observed in the collagen film. The wide-angle X-ray pattern of extended gelatin was similar to that of renatured collagen fiber.     

NpH-MD-Simulations of the Elastic Moduli of Cellulose II at Room Temperatue

We have used molecular dynamics modeling to investigate the stucture and mechanical properties of regenerated cellulose fibres. This work is motivated by continued interest in replacing the environmentally hazardous viscose process by alternative spinning methods. An important input parameter for any realistic model of the elastic properties is the stiffness tensor of the crystalline constituent, cellulose II. Conventional molecular mechanics techniques can be used to estimate the elastic reaction of a material with respect to small external stresses or strains, i.e. the compliance and stiffness tensors, and the elastic moduli derived therefrom, at zero temperature. In order to access non-zero temperatures, it is necessary to use either the quasi-harmonic approximation for the vibrational free energy or molecular dynamics (MD) simulations. In the present work, Parrinello-Rahman constant-stress MD was performed to generate trajectories in constant particle number (N), constant external stress tensor (p or t) and constant enthalpy H (NpH or HtN) ensemble. This was found to be less time consuming than working with isothermal conditions, as done by other authors. The fluctuations in kinetic energy and MD cell vectors were then used to calculate adiabatic elastic constants, thermal expansion coefficients and heat capacity. The isothermal elastic constants were found by applying a standard thermodynamic relation. The Youngs modulus along the chain direction, E_l, was determined to be 155 GPa, whereas the values in the perpendicular directions vary between 51 and 24 GPa. These results are of the same order of magnitude as those obtained by Tashiro and Kobayashi [1] with the static (T = 0K) method, but our value of E_l is 5% lower and, unexpectedly, the lateral values are up to six times higher. A strong anisotropy is found for shear along the chains in planes containing the chain axis, the shear modulus ranging from 5 to 20 GPa. Convergence was achieved in the simulations, to the extend that the elastic constants become stationary, but significant internal stresses remain, pointing to shortcomings in the software used. Further work is necessary to resolve these problems, although the major conclusions should be unaffected.     

Dynamic viscoelastic properties of solutions of shear-degraded deoxyribonucleic acid

Calf thymus and salmon sperm deoxyribouncleie acid were degraded by high-shear stirring to molecular weights M in the range of 1.3–3.2 × 10⁶ and purified by chromatography on methylated bovine serum albumin. Dynamic viscoelastic properties of the fragmented products, in aqueous glycerol solutions in the concentration range of c = 0.003–0.01 g./ml., were investigated with the apparatus of Birnboim and Ferry. At values of the product cM higher than 4 × 10³, the frequency dependence of the components of the complex shear modulus, G′ and G″, displayed a plateau region in which G′ > G″ – ων₁η_S, similar to that observed in concentrated solutions of coiling polymers where it is attributed to an entanglement network (ω is radian frequency, ν₁ volume fraction of solvent, and η₈, solvent viscosity). The width of this plateau region on the logarithmic frequency scale is given by Δ = 3.8 (log cM – 3.56). At lower values of cM, the frequency dependence is intermediate between those predicted by the theory of Zimm for flexible coiled macromolecules and by the theory of Kirkwood and Auer for rods. Fitting to the Zimm theory gives highly discrepant values for molecular weights, while fitting the low-frequency end of the dispersion to the Kirkwood-Auer theory gives reasonable agreement for both molecular weight and rotary diffusion coefficient. It is concluded that the helical fragments appear as nearly rigid rods in their behavior at very low frequencies, but at higher frequencies reveal substantial bending flexibility.     

Prediction on elastic properties of Nb-doped Ni systems

On the basis of first-principles simulation, the structure, formation enthalpy and mechanical properties (elastic constant, bulk and shear modulus and hardness) of five Nb-doped Ni systems are systematically studied. The calculated equilibrium volume increases with the Nb concentration increasing. The computational elastic constants and formation enthalpy indicate that all Nb-doped Ni systems are mechanically and thermodynamically stable in our research. The hardness of these systems was predicted after the bulk modulus and shear modulus had been accurately calculated. The results show that the hardness increases with the Nb concentration increasing when the Nb concentration was below 4.9%, beyond which the hardness will decrease; this is within the scope of our study.     

Methane fermentation of xylan by mesophilic and thermophilic methane sludges

The degradation of xylan during methane fermentation proceeded as a first-order reaction. The rate constants were calculated to be 0.40–0.09 day^–1 at 37° C and 0.341 day^–1 at 55° C. From calculations based on the experimental data, K _A and C _A values in the expression of the velocity of xylose consumption changed as the fermentation progressed. In the mesophilic fermentation, the degradation of xylan slowed down after 2 days of incubation, but the rate of consumption of xylose increased between days 3 and 4 of incubation and slow again at the 5th day of incubation. In the thermophilic fermentation, the degradation of xylan proceeded at a constant rate and the rate of consumption of xylose increased slightly on the 3rd day of incubation. When the velocity of gas evolution was determined, the C _G value for acetate at 55° C was about 1.8 times larger than the value at 37° C.     

A New Computational Method for Membrane Compressibility: Bilayer Mechanical Thickness Revisited

Because lipid bilayers can bend and stretch in ways similar to thin elastic sheets, physical models of bilayer deformation have utilized mechanical constants such as the moduli for bending rigidity $\left({\kappa }_C\right)$ and area compressibility (K_A). However, the use of these models to quantify the energetics of membrane deformation associated with protein-membrane interactions, and the membrane response to stress is often hampered by the shortage of experimental data suitable for the estimation of the mechanical constants of various lipid mixtures. Although computational tools such as molecular dynamics simulations can provide alternative means to estimate K_A values, current approaches suffer significant technical limitations. Here, we present a novel, to our knowledge, computational framework that allows for a direct estimation of K_A values for individual bilayer leaflets. The theory is based on the concept of elasticity and derives K_A from real-space analysis of local thickness fluctuations sampled in molecular dynamics simulations. We explore and validate the model on a large set of single and multicomponent bilayers of different lipid compositions and sizes, simulated at different temperatures. The calculated bilayer compressibility moduli agree with values estimated previously from experiments and those obtained from a standard computational method based on a series of constrained tension simulations. We further validate our framework in a comparison with an existing polymer brush model and confirm the polymer brush model’s predicted linear relationship with proportionality coefficient of 24, using elastic parameters calculated from the simulation trajectories. The robustness of the results that emerge from the method allows us to revisit the origins of the bilayer mechanical (compressible) thickness and in particular its dependence on acyl-chain unsaturation and the presence of cholesterol.