Decrease In Cohesive Energy Research Articles

Influences of size, shape, and wall thickness on melting entropy and enthalpy of metallic nanostructures

From the perspective of a bond-order-length-strength correlation, we put forward an analytical solution to describe the size, shape, and wall thickness dependency of melting temperature, entropy, and enthalpy for metallic nanostructures. Theoretical reproduction of measurements clarified that (i) when the crystal size reduces, the atomic coordination number lowers, the atomic cohesive energy decreases, and the surface-to-volume ratio increases; (ii) at the same equivalent radius, with the decrease in the number of sides for polyhedral nanoparticles and polygonal nanowires or nanotubes, the melting temperature, entropy, and enthalpy depress; and (iii) the melting temperature, entropy, and enthalpy of nanotubes are always lower than those of nanowires with the same cross-sectional radius. The present formulation is accurate and convenient, which not only shows deeper insight into the physical origins of a melting thermodynamic property response to perturbations but also provides guidance for the design and optimization of electronic nanodevices.

First-Principles Study of Structural Stability and Mechanical Properties of Ta–W–Hf Alloys

In order to obtain the effect of W and Hf elements on the mechanical properties of Ta–W–Hf alloys, the structural, mechanical, and electronic properties of Ta–xW–6.25Hf (x = 6.25, 12.50, 18.75, 25.00, 31.25, 50.00) alloys were studied using first-principles calculation based on density functional theory, and the supercell method. The calculated formation enthalpy and elastic constants clarify that Ta–W–Hf alloys have structural and dynamical stability. The formation enthalpy and the cohesive energy decrease with the increase in W content, and the cohesive energy increases when Hf element is added to Ta–W alloy. In addition, bulk modulus (B), shear modulus (G), and Young’s modulus (E) for each of the Ta–xW alloys increase gradually with the increase in W concentration. The B, G, and E of Ta–xW–6.25Hf alloys is lower than that of Ta–xW alloy under the same W content conditions, suggesting that Hf alloying with higher Ta–W concentration becomes softer than the Ta–W alloy. Based on the mechanical characteristic, the B/G and Poisson’s ratio of Ta–W–Hf alloys are higher than those of Ta–W alloys with W content over 25%, the ductility of Ta–W–Hf alloys improves with the addition of Hf, and Hf can reduce the anisotropy of Ta–W–Hf alloys. Furthermore, the electronic density of states shows that alloying W and Hf improves the metallicity of Ta. The results in this work provide the underlying insights needed to guide the design of Ta–W–Hf alloys with excellent mechanical properties.

Ternary Synergy of Lys, Dopa, and Phe Results in Strong Cohesion of Peptide Films.

Synergistic interactions between 3,4-dihydroxyphenylalanine (Dopa, Y*), cationic residues, and the aromatic rings have been recently highlighted as influential factors that enhance the underwater adhesion strength of mussel foot proteins and their derivatives. In this study, we report the first ever evidence of a cation-catechol-benzene ternary synergy between Y*, lysine (Lys, K), and phenylalanine (Phe, F) in adhesive peptides. We synthesized three hexapeptides containing a different combination of Y*, K, and F, i.e., (KY*)3, (KF)3, and (KY*F)2, respectively, exploring the relationship between the cohesive performance and molecular architecture of peptides. The peptide with the (KY*F)2 sequence displays the strongest underwater cohesion energy of 10.3 ± 0.3 mJ m-2 from direct nanoscale surface force measurements. Combined with molecular dynamics simulation, we demonstrated that there are more bonding interactions (including cation-π, π-π, and hydrogen bond interactions) in (KY*F)2 compared to the other two peptides. In addition, peptide (KY*F)2 still shows the strongest cohesive energies of 7.6 ± 0.7 and 3.7 ± 0.5 mJ m-2 in acidic and high-ionic strength environments, respectively, although the cohesive energy decreases compared to the value in pure water. Our results further explain the underwater cohesion mechanisms combining multiple interactions and offer insights on designing Dopa containing underwater adhesives.

Cohesive traction–separation relations for tearing of ductile plates with randomly distributed void nucleation sites

Cohesive zone traction-separation relations, and the related phenomenological parameters, for steady-state ductile plate tearing, are strongly tied to the micro-mechanics governing the void nucleation and growth process leading to localized deformation and micro-crack formation. The effects of such local variations on the damage evolution and cohesive zone parameters, respectively, are brought out in this study. A 2D plane strain model setup, first considered in Nielsen and Hutchinson (Int J Impact Eng 48:15–23 (2012)], is adopted, but here by discretely modeling a finite number of finite-size void nucleation sites distributed randomly in the plate material. It is found that the heterogeneous material conditions, resulting from the nucleation process, strongly affect the localization of damage and fracture, which influence the cohesive energy. By considering a number of realizations of the random distribution for each material configuration, it is concluded that: (i) the peak force in the cohesive traction-separation relation is, essentially, unaffected by the heterogeneity coming into play through the damage-related microstructure, while (ii) the cohesive energy decreases when either increasing the number or the size of the nucleation sites. The cohesive energy is found to be in the range of those previously reported for homogeneous materials, but a direct comparison should be made with caution. The results imply that care should be taken if the actual material configuration diverges from a homogeneous microstructure such as when considering very thin plates and for plates with a few void nucleation sites.

Magnetically Enhanced Mechanical Stability and Super‐Size Effects in Self‐Assembled Superstructures of Nanocubes

AbstractArtificial materials from the self‐assembly of magnetic nanoparticles exhibit extraordinary collective properties; however, to date, the contribution of nanoscale magnetism to the mechanical properties of this class of materials is overlooked. Here, through a combination of Monte Carlo simulations and experimental magnetic measurements, this contribution is shown to be important in self‐assembled superstructures of magnetite nanocubes. By simulating the relaxation of interacting macrospins in the superstructure systems, the relationship between nanoscale magnetism, nanoparticle arrangement, superstructure size, and mechanical stability is established. For all considered systems, a significant enhancement in cohesive energy per nanocube (up to 45%), and thus in mechanical stability, is uncovered from the consideration of magnetism. Magnetic measurements fully support the simulations and confirm the strongly interacting character of the nanocube assembly. The studies also reveal a novel super‐size effect, whereby mechanically destabilization occurs through a decrease in cohesive energy per nanocube as the overall size (number of particles) of the system decreases. The discovery of this effect opens up new possibilities in size‐controlled tuning of superstructure properties, thus contributing to the design of next‐generation self‐assembled materials with simultaneous enhancement of magnetic and mechanical properties.

Atomistic investigation of hydrogen embrittlement effect for symmetric and asymmetric grain boundary structures of bcc Fe

An atomistic simulation study has been carried out to understand that the grain boundary hydrogen (H) will have a significant effect on grain boundary (GB) energy, surface (FS) energy and cohesive energy. Molecular statics simulations at zero temperature for pure a-Fe with a single H atom and two H atoms placed at the crystal interfaces of 〈1 0 0〉 symmetric tilt grain boundary (STGB) systems and asymmetric tilt grain boundary (ATGB) systems were performed. Simulation results show that the segregation energy of solute H atom on the GB was greater than zero, i.e. the system energy with H located at the GB is larger than the energy with H at the free surface, which implies that the failure mode should be intergranular. Moreover, only when H is located within the distance of 2a (lattice constant) from the grain boundary will H have a significant effect on grain boundary destruction. Finally, the calculated GB energy, surface energy and cohesive energy of bcc Fe 〈1 0 0〉 STGB and ATGB systems are affected by the hydrogen segregation at the GB structure. Both the surface energy and the grain boundary cohesive energy decrease with the increase of hydrogen coverage. The impacts of H or 2H atoms at the GB plane on the surface energy and the cohesive energy are explored as quadratic polynomial functions. The promoting effect of 2H atoms on the intergranular fracture is more obvious than H atoms’. Both H and 2H have more effect on surface energy than cohesive energy.

First-principles calculation of multiple hydrogen segregation along aluminum grain boundaries

The segregation of multiple hydrogen atoms along aluminum (Al) grain boundaries (GBs) and fracture surfaces (FSs) was investigated through first-principles calculations considering the characteristics of GBs. The results indicate that hydrogen segregation is difficult along low-energy GBs. The segregation energy of multiple hydrogen atoms along GBs and FSs and the cohesive energy was obtained for three types of high-energy Al GBs. With increasing hydrogen segregation along the GBs, the cohesive energy of the GB decreases and approaches zero with no decrease in GB segregation energy. The GB cohesive energy decreases in parallel with the volume expansion of the region of low electron density along the GB.

Diffusion of Au and its influence on the coercivity of [FePt/Au/FePt]2x thin films during annealing in different atmospheres

Influence of heat treatment atmosphere on magnetic properties of post-annealed [FePt(15 nm)/Au(7.5 nm)/FePt(15 nm)]2x thin films was investigated. Thin films were deposited by dc magnetron sputtering at room temperature onto SiO2(100 nm)/Si(100) substrates and post-annealed in a temperature range of 500 °C–800 °C for 30 s in flowing Ar and Ar + H2 (3 vol%) atmospheres. It was shown that hydrogen addition into annealing atmosphere allows to achieve high value of coercivity ~1671.1 kA/m after annealing at 600 °C. Such coercivity could be reached by heat treatment in pure Ar just after annealing at 700 °C. This effect could be related to acceleration of Au grain boundary diffusion into L10-FePt phase. Such acceleration could be possible due to two reasons: hydrogen incorporation into Au crystal lattice, leading to interatomic bounds break and point defects formation; and hydrogen segregation at most defective areas, leading to cohesive energy decrease and upraise of significant stresses. As a result, nonmagnetic grid in grain boundaries of ferromagnetic material was formed at temperature lower by 100 °C.

Physical and optical properties of thermally deposited Ge-S-Ga thin films

Optical properties of Ge20S80−xGax thin films, using well established Swanepoel's method, have been reported. Thin films of Ge20S80−xGax glassy alloy were deposited on glass substrates by thermal evaporation technique under high vacuum conditions (~10−5Torr). FE-SEM images show that the films are amorphous and surface smoothness improves with an increase in Ga concentration. Presence of excess number of lone pair's electrons (L) and increase in average coordination number ⟨Z⟩, show that the material under study is a good glass former. Refractive index (n1) is found to increase whereas the extinction coefficient (K) decreases with an increase in Ga content. Increase in refractive index is a consequence of increased polarization and density of the glassy alloy. Addition of Ga augments the non-linear response of GeS binary alloy. Obtained values of non-linear refractive index (nnl) are higher than the values reported in the literature for other chalcogenide glasses. Optical band gap decreases with an increase in Ga concentration. Decrease in cohesive energy and formation of enough localized states in the band gap is liable for the decrease in optical band gap. Correlation between the refractive index dispersion and the microstructure of films has been elucidated by the single effective oscillator model. Obtained optical properties are consistent with predicted physical properties of Ge20S80−xGax glassy alloy.

Elastic properties of amorphous T0.75Y0.75B14 (T = Sc, Ti, V, Y, Zr, Nb) and the effect of O incorporation on bonding, density and elasticity (T′ = Ti, Zr)

We have systematically studied the effect of transition metal valence electron concentration (VEC) of amorphous T0.75Y0.75B14 (a-T0.75Y0.75B14, T = Sc, Ti, V, Y, Zr, Nb) on the elastic properties, bonding, density and electronic structure using ab initio molecular dynamics. As the transition metal VEC is increased in both periods, the bulk modulus increases linearly with molar- and mass density. This trend can be understood by a concomitant decrease in cohesive energy. T′ = Ti and Zr were selected to validate the predicted data experimentally. A-Ti0.74Y0.80B14 and a-Zr0.75Y0.75B14 thin films were synthesized by high power pulsed magnetron sputtering. Chemical composition analysis revealed the presence of up to 5 at.% impurities, with O being the largest fraction. The measured Young’s modulus values for a-Ti0.74Y0.80B14 (301 ± 8 GPa) and a-Zr0.75Y0.75B14 (306 ± 9 GPa) are more than 20% smaller than the predicted ones. The influence of O incorporation on the elastic properties for these selected systems was theoretically studied, exemplarily in a-Ti0.75Y0.75B12.75O1.25. Based on ab initio data, we suggest that a-Ti0.75Y0.75B14 exhibits a very dense B network, which is partly severed in a-Ti0.75Y0.75B12.75O1.25. Upon O incorporation, the average coordination number of B and the molar density decrease by 9% and 8%, respectively. Based on these data the more than 20% reduced Young’s modulus obtained experimentally for films containing impurities compared to the calculated Young’s modulus for a-Ti0.75Y0.75B14 (without incorporated oxygen) can be rationalized. The presence of oxygen impurities disrupts the strong B network causing a concomitant decrease in molar density and Young’s modulus. Very good agreement between the measured and calculated Young’s modulus values is obtained if the presence of impurities is considered in the calculations. The implications of these findings are that prediction efforts regarding the elastic properties of amorphous borides containing oxygen impurities on the at.% level are flawed without taking the presence of impurities into account.

Physics and chemistry of producing silicon-hydroxylapatite-titanium composite materials

In the research, Si-HA poweders have been synthesized using SBF solution with different content of silicon. It was found that all the samples synthesized from the model solution of extracellular fluid under varying concentration of silicate ions are single-phase and repesent hydroxyapatite. The nature of the reagent containing SiO4-4 ions does not affect the hydroxyapatite structure. In the study of the surface and morphological characteristics of the phosphate coatings modified by silicon ions, it was found that as the degree of phosphate calcium substitution by SiO4-4 ions increases, the surface wettability on the titanium substrates deteriorates and cohesive energy decreases. It is shown that the titanium coating is formed in three stages. After irradiation of titanium substrates coated with Si-HA, the crystals can keep growing and the surface can keep regenerating.

Coordination-resolved local bond relaxation, electron binding-energy shift, and Debye temperature of Ir solid skins

Numerical reproduction of the measured 4f7/2 energy shift of Ir(100), (111), and (210) solid skins turns out the following: (i) the 4f7/2 level of an isolated Ir atom shifts from 56.367eV to 60.332eV by 3.965eV upon bulk formation; (ii) the local energy density increases by up to 130% and the atomic cohesive energy decreases by 70% in the skin region compared with the bulk values. Numerical match to observation of the temperature dependent energy shift derives the Debye temperature that varies from 285.2K (Surface) to 315.2K (Bulk). We clarified that the shorter and stronger bonds between under-coordinated atoms cause local densification and quantum entrapment of electron binding energy, which perturbs the Hamiltonian and the core shifts in the skin region.

Effect of cohesive energy on thermal expansion behavior of T6 tempered 7A09 aluminum alloy

The dynamic effect of cohesive energy on the CTE of T6 (the highest strength temper) tempered 7A09 aluminum alloy was studied in this paper. Transmission electron microscopy (TEM) was combined with differential scanning calorimetry (DSC) to analyze the phase transitions of T6 tempered 7A09 aluminum alloy taking place in a heating process. The corresponding CTE curve was recorded by a thermal dilatometer. The ion-bonding model was utilized to estimate the variation of cohesive energy caused by the phase transitions. Results indicate that GPII zones and η′ phase are main precipitates in T6 tempered 7A09 alloy. During the heating process, the cohesive energy of alloy system increases with the transition from η′ to η phase and the precipitation of η phase. The dissolution of η phase leads to a decrease of cohesive energy. Although volume changes of alloy system have effects on the CTE, the effect of the cohesive energy change is more obvious than that of volume change after the precipitation of η phase begins in the Ostwald ripening stage. Effects of cohesive energy and volume changes can be expressed in the nonlinearity of CTE curve.

Optical parameters of ternary Te15(Se100−xBix)85 thin films deposited by thermal evaporation

Thin films of Te15(Se100−xBix)85 (x=0, 1, 2, 3, 4 and 5 at.%) glassy alloys were deposited by thermal evaporation (at 10−4 Pa) from bulk samples. Optical characterization of the films was done by analysing their transmission spectra taken in the spectral range 400–2300 nm. Swanepoel's method was used to calculate the refractive index (n) and extinction coefficient (k). It was found that the refractive index increases with an increase in Bi content. The Wemple–DiDomenico single-oscillator approach was used to calculate the average band gap energy (Eo), dispersion energy (Ed) and static refractive index (no). The absorption coefficient (α) and film thickness were calculated from the transmission spectra of the films. The optical band gap (Eg) was estimated using Tauc's extrapolation and was found to decrease from 1.37 to 1.21 eV with Bi addition from 0 to 5 at.% in glassy alloys. The decrease in optical band gap is explained on the basis of the decrease in cohesive energy of the samples and the difference of electronegativity of the atoms involved. The real (εr) and imaginary parts (εi) of the dielectric constant for the films were also calculated and reported.

Theorectical study of the oxide-ion conductorLa2Mo2-xMxO9(M=Cr,W)

We perform the first-principles study of the structural properties and the high temperature phase stabilization of Cr, W-doped β-La2Mo2O9. We find that Cr prefers the site with four coordination numbers and W atom preferentially occupies the site with five coordination numbers. The nonlinear dependence of cell parameter on W content in W-doped systems results from the nonlinear change in Mo(W)-O and La-O bond lengths with W content and the decrease of coordination number around W occupied site. The decrease of cohesive energy with the W-doped concentration is conducive to the understanding of the stabilization of the β-La2Mo2-xWxO9 at lower temperature. The appearance of stronger W-O bond leads to the increase of the energy barrier of oxygen-ion concerted motion in W-dopedβ-La2Mo2O9.

First principle study of structural, phase stabilization and oxygen-ion diffusion properties of β-La 2 − xL xMo 2O 9 (L = Gd, Sm, Nd and Bi) and β-La 2Mo 2 − yM yO 9 (M = Cr, W)

We have performed a first principle study of structural and phase stabilization of β-La 2 − x L x Mo 2O 9 (L = Gd, Sm, Nd and Bi) and β-La 2Mo 2 − y M y O 9 (M = Cr, W). The substitutional-site properties were discussed in terms of the empirical parameter, bond valence sums (BVS), which characterizes the interactions between atoms and its nearest-neighbor atoms and correlates well with the stability of the structure. We found that Gd, Sm and Nd atoms prefer the crystallographic sites with largest BVS values. The nonlinear dependence of cell parameter on W content in W-doped systems results from the nonlinear change in Mo/W–O bond length with W content. The decrease of cohesive energy and the deviation of BVS values from the expected values upon the Gd, Sm, Nd and W-doped concentration help us understand the experimentally observed stabilization of the β phase to lower temperatures in these doped system. The O ion diffusion properties in W-doped systems have been studied using the nudged elastic band method and the dimer method. We found that, W-doping leads to the obvious increase in the energy barriers of O ion concerted diffusion. In addition, there is a remarkable decrease in the difference of energy barriers between two diffusion channels involving O(1) ion, which sheds light on only one relaxation peak in the mechanical relaxation measurement in W-doped system, compared to undoped system.

Physical and optical properties of binary amorphous selenium-antimony thin films

Amorphous thin films with compositions Se1−xSbx (x=0, 0.025, 0.05, 0.075, and 0.10 at. %) have been deposited by thermal evaporation (at ∼10−4 Pa) from bulk samples. The compositional dependence of their optical properties, refractive index, extinction coefficient, absorption coefficient, and optical band gap with increasing Sb content is investigated using transmission spectra in the range of 400–1200 nm. The refractive-index dispersion has been analyzed on the basis of the Wemple–DiDomenico single-oscillator approach. It has been found that the refractive index increases with increasing Sb content. The behavior of the optical band gap, when the composition of the material is varied, shows, as expected, just the opposite trends. The optical band gap decreases from 2.025 to 1.753 eV with ±0.001 eV uncertainty. Band gap calculated theoretically also shows a decrease with the increase in Sb content. The optical behavior is supported by physical properties, i.e., decrease in optical band gap is supported by the decrease in cohesive energy of the system. Some other physical properties, viz., coordination number, lone-pair electrons, and glass transition temperature, are also investigated theoretically. The optical results may lead to yield more sensitive detectors based on amorphous selenium, and physical properties may be useful in achieving more stable alloys which are favorable in x-ray imaging applications.

Structural and thermodynamic properties of Fe based compounds with NaZn13-type

A series of Fe based compounds with NaZn13-type structure is studied by using inversed interatomic potentials. Calculated results show that Si atoms and Co atoms are preferentially substituted for Fe atoms at 96i site. The addition of Si or Co makes the cohesive energy decrease. It is found that the lattice constants of LaFe13-x-yCoySix and NdFe13-x-yCoy Six decrease with the increase of Co content. The investigation on the phonon density of states indicates that the lower frequency modes are mostly excited by the rare-earth atoms, and the higher frequency modes are mostly excited by Si atoms. Moreover, the addition of Co atoms makes the Debye temperature of LaFe11.5-yCoySi1.5 compounds increase.

Phase stability and elastic properties ofXMgB14studied byab initiocalculations(X=Al,Ge,Si,C,Mg,Sc,Ti,V,Zr,Nb,Ta,Hf)

The effect of valence electron concentration (VEC) and size of the $X$ element in $X{\text{MgB}}_{14}$ (space group $Imma$ $X=\text{Al}$, C, Si, Ge, Mg, Sc, Ti, V, Zr, Hf, Nb, Ta) on stability and elastic properties was studied using ab initio calculations. Generally, icosahedral bonds, present in these compounds, are electron deficient. Based on the Bader charge analysis [Bader, Atoms in Molecules: A Quantum Theory (Oxford University Press, New York, 1990)] and density of states, $X$ elements and Mg are shown here to transfer electrons to the boron network. Hence, the stability of the compounds studied increases as more electrons are transferred. As the VEC of the $X$ element increases, fewer electrons are transferred to the boron network, and therefore the phase stability decreases. The bulk moduli of all compounds are in the range from 205 to 220 GPa. This can be understood analyzing the cohesive energy thereof. As the bulk modulus increases, the cohesive energy decreases.

Atomistic simulation of phonon dispersion for body-centred cubic alkali metals

Atomistic simulations of phonon dispersion for body-centred cubic alkali metals were carried out using the modified analytic embedded atom potentials. The expressions for atomic force constants are derived, the cohesive energy and elastic constants are calculated, and the phonon dispersion curves of Li, Na, K, Rb, and Cs are calculated along five principal symmetry directions. The calculated results are in good agreement with the available experiments. For all of the five alkali metals, in the same direction, a similar phonon dispersion curve is obtained in spite of the successive phonon frequency decreases for Li, Na, K, Rb, and Cs, which may be related to the atom mass increases or the cohesive energy decreases. PACS Nos.: 63.20.Dj, 71.20.Dg, 31.15.Ct