Lowest Formation Enthalpy Research Articles

Stability, Mechanical Properties and Anisotropic Elastic Properties of GaxMgy Compounds

The stability, mechanical properties and anisotropic properties of sound velocities of Ga2Mg5, GaMg2, GaMg, O-Ga2Mg, H-Ga2Mg and Ga5Mg2 are investigated systematically by the first-principles calculation. The cohesive energy and formation enthalpy are obtained and used to estimate the stability of the Ga-Mg binary compounds. GaMg compound is the most stable and has the lowest formation enthalpy as -0.162eV/atom of those GaxMgy compounds. The elastic constants of single crystal, hardness, bulk, shear, Young's modulus and Poisson's ratio of the polycrystalline crystal are obtained and used to estimate the mechanical properties. Ga5Mg2 and H-Ga2Mg have the lager bulk, shear and Young’s modulus and corresponding B/G is low. H-Ga2Mg is harder than the other compounds from the results of Poisson’s ratio. The anisotropic mechanical properties are discussed using the anisotropic index, two-dimensional planar projections on different planes of the bulk and Young's modulus. The Young's modulus of H-Ga2Mg shows the strongly anisotropy of mechanical properties and GaMg2 has the weakest anisotropy among all the compounds.

Accounting for the thermo-stability of PdHx (x = 1–3) by density functional theory

We calculate the formation enthalpies of PdHx (x = 0–3) by cluster expansion (CE) and calculations based on density functional theory. CE predicts the stable palladium hydride structures PdH, PdH2.67, and PdH2.75. The band structures and density of states indicate that the amount of hydrogen in the palladium lattice does not alter the metallic character of the palladium significantly. However, all PdHx structures with x > 1 have greater formation enthalpies than that of the given reaction path 4PdH2=2PdH+2Pd+3H2 and thus they are thermodynamically unstable. The shorter bond length of Pd–H and the smaller bond angle of Pd–H–Pd imply a higher cohesive energy in zincblende (ZB) PdH than that in rocksalt (RS) PdH. Bader charge analysis shows a stronger electronegativity of H atoms in ZB-PdH than that in RS-PdH. This results in a stronger Pd–H bond in ZB-PdH than that in RS-PdH. Thus ZB-PdH has lower formation enthalpy than that of RS-PdH. However, regarding the dynamic stability, we conclude that hydrogen atoms prefer to occupy the octahedral sites of the palladium lattice because of the lower zero-point energy and vibration free energy than that of occupying the tetrahedral sites.

Stability and superconductivity properties of metal substituted aluminum diborides (M0.5Al0.5B2)

The stability table and superconducting properties such as screened Coulomb potential, electron-phonon coupling and critical temperature of planar boron sheet included ternary crystalline compounds of MgB2-type M0.5Al0.5B2 (M = Li, Be, Na, Mg, K, Ca, Sc, Ti, V, Y, Zr, Nb, Mo, Tc, Ru, Hf, Ta, W, Re, and Os) have been investigated by first-principles density functional theory calculations with spin-orbit coupling. It is shown that the M = Li, Mg, Ca, Sc, Ti, V, Y, Zr, Nb, Mo, Tc, Hf, Ta, and W-substituted compounds are thermodynamically stable at the ambient conditions. Among them, Nb0.5Al0.5B2 has the lowest cohesive energy while Ti0.5Al0.5B2 has the lowest formation enthalpy. Except for Be0.5Al0.5B2, all compounds are mechanically stable, and fourteen of them are dynamically stable also. So, a dozen of M0.5Al0.5B2 (M = Li, Mg, Ca, Sc, Ti, V, Zr, Nb, Mo, Tc, Hf, and Ta) compounds are satisfying all three stability conditions. The calculated electronic properties show that all structures have metallic character and interestingly the radii of the substituted atoms correlate with surface area of regular B6 hexagons and volumes of metal-boron pyramids. All of them are hard materials, and V0.5Al0.5B2 has the highest semi-empirical microhardness (27 GPa). According to the precise phonon dispersion and electron-phonon coupling calculations, Tc’s of the stable compounds are lower than that of MgB2 (39 K), and Tc0.5Al0.5B2 has higher Tc (∼7 K) than the others.

Destabilizing the dehydriding thermodynamics of MgH2 by reversible intermetallics formation in Mg−Ag−Zn ternary alloys

In this study, Mg−Ag−Zn ternary alloys are employed as carriers for hydrogen storage, which show improved thermodynamic and kinetic properties compared with pure Mg. Mg90Ag7.5Zn2.5 alloy exhibits a reversible phase transformation during hydriding-dehydriding process with a maximum hydrogen storage capacity of approximately 4.2 wt%. Two-step reactions occur in the dehydriding process of Mg90Ag7.5Zn2.5. In the first step, a portion of MgH2 reacts with MgAg and MgZn1.8Ag0.2 phases to transform to Mg54(Ag, Zn)17 solid solution and release hydrogen. After that, the remaining MgH2 decomposes to Mg. Because of the modified reaction pathway in the first step dehydrogenation, the dehydriding equilibrium pressure at 300 °C increases to 0.28 MPa. Besides, the apparent activation energy (Ea) for dehydrogenation of the Mg90Ag7.5Zn2.5 alloy is lowered to 118.7 kJ mol−1. By fully utilizing the first step reaction, another alloy Mg78Ag16.5Zn5.5 is designed, which presents intact destabilized dehydriding thermodynamics and a lower dehydrogenation Ea of 116.5 kJ mol−1. Our work demonstrates that alloying Mg with elements that can form compounds with low formation enthalpies, and the hydrides of the elements have low stabilities, are the main guidelines for the designing of the Mg-based hydrogen storage alloys with destabilized dehydriding thermodynamics by reversible intermetallics formation.

Properties of Fe–Mn–Al alloys with different Mn contents using density functional theory

The chemical stability, electronic structures, mechanical properties and Debye temperature of Fe–Mn–Al alloys were investigated using first-principles calculations. The formation enthalpy and cohesive energy are negative for Fe–Mn–Al alloys, showing that they are thermodynamically stable. FeAl has the lowest formation enthalpy, indicating that FeAl is the most stable alloy in the Fe–Mn–Al system. The partial density of states, total density of states and electron density distribution maps were used to analyze the physical properties of the Fe–Mn–Al alloys. A combination of mainly covalent and metallic bonds exists in these Fe–Mn–Al alloys, resulting in good electronic conductivity, high melting points, and high hardness. These alloys display disparate anisotropy due to the calculated different shapes of the 3D curved surface of the Young’s modulus and anisotropic index. FeAl has the highest bulk modulus, shear modulus and Yong’s modulus of 187.1, 119.8 and 296.2 GPa, respectively. Further, the Debye temperatures and sound velocity of these Fe–Mn–Al compounds were explored.

Possible Applications of Nanomaterials for Nuclear Fusion Devices

Abstract Conditions of nuclear fusion and nuclear fusion devices were described, and some possible applications of nanomaterials for nuclear fusion devices were presented in the present article. Muon-catalyzed fusion is one of methods for nuclear fusion to cause even at room temperature or lower, and protons or heavy ions with huge energy are irradiated to metals such as beryllium or copper, which results in emission of negative or positive charged muons from the metals. An experiment using a pyroelectric power source using lithium tantalite crystal was also reported to achieve nuclear fusion in a desktop-like device. Hydrogen storage is also important for the fusion devices, and the possibility of hydrogen storage in hydrogen storage metallic alloys was studied by diffusion calculation and potential calculation of deuterium fusion. Enhancement of deuterium diffusion in the Pd alloys would be one of the key points for energy materials. Carbon(C)/copper(Cu)-based composite materials with high thermal conductivity and good stability at high temperatures were also developed by adding a small amount of titanium, which has a low enthalpy of alloy formation with C and Cu. These carbon-based materials could be a candidate material for the plasma facing components of fusion devices.

First principles investigation of Mg7XH16 (X = Ti, Zn, Pd, and Cd) ternary hydrides for hydrogen storage applications

We present the electronic structure, energetic stability, mechanical, and electronic properties of Mg7XH16 (X = Ti, Zn, Pd, Cd) ternary hydride systems using a first principles approach in the framework of density functional theory. The possible usage of these systems in hydrogen storage applications is discussed. The systems under investigation have lower formation enthalpy than conventional MgH2 material indicating lower thermodynamic stability and improved hydrogen releasing capacity. All the systems are mechanically stable and compressible materials with relatively low elastic moduli values. In the electronic aspect, Mg7XH16 (X = Ti, Zn, Cd) systems are metal, but Mg7PdH16 is a zero-gap semiconductor with bands nearly touching at Fermi level (EF) and zero density of state at EF. This material can also have possible applications in optoelectronics.

Structural, electronic and optical properties of bandgap-tunable Cu2ZnSixSn1−xSe4 alloys

Density functional theory calculations are performed to investigate the structural, electronic and optical properties of Cu2ZnSnSe4 alloys. The lattice constants linearly decrease with the increment of the Si concentration and the mean atomic weight. The Cu2ZnSixSn1−xSe4 alloys are miscible with low formation enthalpies. A tunable bandgap is obtained in a wide energy range (1.06–2.26 eV) and primarily due to the upward shift of the conduction band minimum. The alloy Cu2ZnSi0.5Sn0.5Se4 has an optimal bandgap (1.49 eV) and a higher optical absorption coefficient than that of the well-studied CuZnSnS4. The bandgap-tunable Cu2ZnSixSn1−xSe4 alloys can be promising for their photovoltaic applications.

Interfacial Self-Assembly and Oriented Attachment in the Family of PbX (X = S, Se, Te) Nanocrystals.

The realization of materials with new optoelectronic properties draws much scientific attention toward the field of nanocrystal superstructures. Low-dimensional superstructures created by interfacial assembly and oriented attachment of PbSe nanocrystals are a striking example because theory showed that PbSe sheets with a honeycomb geometry possess non-trivial flat bands and Dirac cones in the valence and conduction bands. Here, we report on the formation of one-dimensional linear and zigzag structures and two-dimensional (2D) square and honeycomb structures for the entire lead chalcogenide family: PbX (X = S, Se, Te). We observe that PbTe, with a lower bulk melting temperature and enthalpy of formation than those of PbSe, shows a higher nanocrystal surface reactivity, such that the surface must be passivated and the reaction conditions moderated to obtain reasonably ordered superstructures. The present findings constitute a step forward in the realization of a larger family of atomically coherent 2D superstructures with variable IV–VI and II–VI compositions and with electronic properties dictated by the nanogeometry.

The effect of Al content on the structural, mechanical, and thermal properties of B2-FeAl and D03-Fe3Al from atomistic study

We investigate the lattice constant, formation enthalpy, elastic constants, bulk modulus, shear modulus, Young’s modulus, specific heat, thermal expansion coefficient and microhardness as a function of Al content for B2-FeAl and D03-Fe3Al using static and dynamics simulations with a long-range analytical embedded atom method (la-EAM). The lattice constants of both B2-FeAl and D03-Fe3Al crystals increase with Al content, opposite to the elastic constants, bulk modulus and shear modulus. Stoichiometric B2-FeAl has the lowest formation enthalpy and Young’s modulus. The calculation results of Gibbs free energies show that the phase transformation temperature from D03 to B2 decreases with Al content, which is in good agreement with experiments. The microhardness resulting from vacancies for B2-FeAl increases monotonically with Al content, indicating that the resultant microhardness is controlled by the vacancies, which agrees well with experiments.

A first-principles calculation of structural, mechanical, thermodynamic and electronic properties of binary Ni-Y compounds.

The intermetallic compounds between rare earth (RE) elements and transition metal elements have been comprehensively researched due to their appealing magnetic, electronic, optical and thermal properties, in which Ni–Y alloys are one kind of important system. In this work, a systematic investigation concerned with structures, elastic, and thermodynamic properties of Ni17Y2, Ni5Y, Ni7Y2, Ni3Y, Ni2Y, NiY, Ni2Y3 and NiY3 in Ni–Y systems is implemented by means of first-principles calculations. NiY has the lowest formation enthalpy within −0.49 kJ per mol per atom. Ni5Y has the largest bulk modulus, shear modulus and Young's modulus of 181.71 GPa, 79.75 GPa and 208.70 GPa, respectively. Furthermore, the effects of different concentrations of yttrium on the mechanical and thermal properties of Ni–Y compounds are estimated by using the Voigt–Reuss method. The electronic density of states and chemical bonding between Ni and Y are key factors that determine mechanical and thermodynamic properties of these compounds. What's more, results indicate that all compounds are dynamically stable as shown by the calculated phonon dispersions.

Influence of atomic order on the enthalpy of formation and bulk modulus of the sigma phase

We have calculated the enthalpy of formation and bulk modulus for 19 typical binary sigma compounds with different atomic order (i.e. atomic constituent distribution or site occupancy preference on inequivalent sites of a crystal structure) based on the experimental site occupancy as well as completely ordered and hypothetically disordered states by using the EMTO-CPA (Exact Muffin-Tin Orbitals - Coherent Potential Approximation) method. The calculation results show that at 0 K the sigma phase in ordered state bears a lower enthalpy of formation and a larger bulk modulus than the ones in less ordered state.

Ab initio investigations of the phase stability in group IVB and VB transition metal nitrides

Using a combination of electronic structure density functional theory and an evolutionary algorithm, we predicted the thermodynamically stable structures of the group IVB and VB transition metal nitrides at 0K. Our results demonstrate that in the group IVB nitrides there is a competition between nitrogen interstitial ordered forms of the hexagonal close packed structure and nitrogen vacancy ordered forms of the B1 structure. The low metal vacancy formation energies in the B1 structure even allowed metal vacancy ordered forms of the B1 HfN structure to be thermodynamically stable. The work also revealed how bond distortion and bond angles allowed the predicted Ti2N and Zr2N P4/mnm phases to be stable as compared to the Pnnm structure for Hf2N. This work also predicted several metal-rich phases to be present in the Zr-N system, which have yet to be observed potentially owing to this system being the least experimentally studied of the group IVB nitrides. In the group VB nitrides, the low formation enthalpies of the ε-TaN prototype and Bh structures, as well as the C6 structure, suppress the dominance of the B1 phase at low temperatures. The collective results provide in-depth and inclusive examination of the similarities and differences for phase stability in these two classes of transitional metal nitrides at 0K.

Main reinforcement effects of precipitation phase Mg2Cu3Si, Mg2Si and MgCu2 on Mg-Cu-Si alloys by ab initio investigation

To predict and compare the main reinforcement effects of the key precipitation phases Mg2Cu3Si, Mg2Si and MgCu2 in Mg-Cu-Si alloy, the structural, mechanical and electronic properties of these phases have been studied by ab initio calculations. The lowest formation enthalpy and cohesive energy indicate that Mg2Cu3Si has the strongest alloying ability and structural stability. The mechanical modulus indicates that Mg2Cu3Si has the strongest resistance to reversible shear/volume distortion and has maximum hardness. The characterization of brittle (ductile) behavior manifests that MgCu2 has favorable ductility. Meanwhile the evaluation of elastic anisotropy indicates that Mg2Si possesses elastic isotropy. Debye temperature prediction shows that Mg2Si and Mg2Cu3Si have better thermal stability. To achieve an unbiased interpretation on the phase stability and mechanical behavior of these precipitation phases, the density of states and differential charge densities are also analyzed. The current study deepens the comprehensive understanding of main reinforcement effects of these precipitation phases on Mg-Cu-Si alloys, and also benefits to optimize the overall performances of Mg-Cu-Si alloy from the hardness, ductility and thermal stability by controlling these second precipitation phases during the heat treatment process.

Electrode modulated capacitance-electric field nonlinearity in metal-insulator-metal capacitors

Metals with low enthalpy of oxide formation (ΔHox) are used to examine the influence of the metal/dielectric interface, in the absence of a significant interfacial layer oxide (ILO), on the voltage nonlinearity of capacitance for metal-insulator-metal capacitors. For both atomic layer deposited Al2O3 and HfO2 dielectrics, Ag electrode devices show the lowest quadratic electric field coefficient of capacitance (αECC), followed in increasing order by Au, Pd, and Ni. The difference between the metals is greater for thinner dielectrics, which is consistent with increased influence of the interface. In addition, with decreasing dielectric thickness the quadratic voltage field coefficient of capacitance increases, whereas αECC decreases. It is proposed that the thickness dependencies are due to an interaction between vertical compression of the dielectric under an applied bias and the concomitant lateral expansion induced stress that is concentrated near the interface. Through this interaction, the metal interface inhibits lateral expansion of the dielectric resulting in a reduced αECC. Indeed, αECC is found to increase with the increasing lattice mismatch at the metal/dielectric interface, likely due to edge dislocations. Finally, Al, a high ΔHox metal, is found to fit the trend for Al2O3 but not for HfO2, due to the formation of a thin reduced-k ILO at the HfO2/Al interface. These results suggest that minimization of metal/dielectric lattice mismatch may be a route to ultra-low nonlinearity in highly scaled metal-insulator-metal devices.

Assessment on the structural, elastic and electronic properties of Nb3Ir and Nb3Pt: A first-principles study

The pressure dependent behaviors on the structural, elastic and electronic properties of the A15 structure Nb3Ir and Nb3Pt were studied using first-principles calculations based on the density functional theory within generalized gradient approximation and local density approximation methods. Initially, the optimized lattice constants of Nb3Ir and Nb3Pt are consistent with the available experimental and theoretical results. Furthermore, Nb3Ir is found to be more thermodynamically stable than Nb3Pt due to its lower formation enthalpy and higher melting temperature. In addition, the elastic constants of Nb3Ir and Nb3Pt show an increasing tendency, and keep mechanically stable structures under pressures to 40 GPa. Besides, the increasing Cauchy pressures and B/G values have indicated that higher pressures can improve their ductility in both Nb3Ir and Nb3Pt. Finally, the pressure-dependent behaviors on the density of states, Mulliken charges and bond lengths are discussed for both compounds.

First‐principles calculations of thermodynamic properties and planar fault energies in Co3X and Ni3X L12 compounds

AbstractWe do Density Functional Theory based total‐energy calculations of the phase in X and X compounds, X being a transition metal element. The lattice parameters, magnetic moments and formation enthalpies are determined and compared with the available experimental data. The (111) superlattice intrinsic stacking fault energy (SISF), a crucial factor affecting materials strength and their mechanical behavior, is calculated using the axial interaction model. We have applied the quasiharmonic Debye model in conjunction with first‐principles in order to establish the temperature dependence of the lattice parameters and the (111) SISF energies. We investigate our prediction of a low formation enthalpy in the system Ni −25 at.%Zn by doing auxiliary simulations for the fcc random alloy at the composition 25 at.%Zn. Our simulations indicate that the elements Ti, Zr, Hf, Nb, and Ta can help stabilizing the promising and extremely important alloy.

A New Ternary Alloy of Cr2CuAl and its Structures

Using the new material structure prediction software CALYPSO, the possible structures of Cr2CuAl were studied; they are P4mm, Pmmm, Immm, P4mmm, I4/mmm, and F-43m crystal structures, all parameters of which were obtained. The F-43M structure, also called Hg2CuTi-type structure, is a high-ordered structure; with this structure, Cr2CuAl is a ferrimagnet with a magnetic moment of 0.45 μB. The F-43M structure is a cubic structure, and the others are all tetragonal structures. Are there phase transitions between the structures as to that in Mn2NiGa? That is an interesting problem to investigate. There has been effort to synthesize Cr2CuAl, but because Al4Cu9 and AlCrCu2 have lower formation enthalpy, Cr2CuAl is difficult to synthesize.

Effects of alloying elements such as Ti, Zr and Hf on the mechanical and thermodynamic properties of Pd-Base superalloy

Abstract From first-principle calculations with the Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) approaches, the structural, elastic, electronic and thermodynamic properties of the PdTM and Pd 31 TM(TM = Ti, Zr, Hf) alloy have been investigated in this work. The calculated cohesive energy and formation enthalpies of these alloys, the PdZr and Pd 31 Zr have the lowest formation enthalpy with −0.57 and −0.13 eV/atom, respectively. It is calculated the elastic constants of crystal, bulk shear, Young's modulus and Poisson's ratio of the PdTM and Pd 31 TM(TM = Ti, Zr, Hf) compounds. The elastic constants show that these alloys are mechanically stable, and PdTi has the largest shear modulus and Young's modulus within 103.2 GPa and 36.9 GPa, respectively. Pd 31 Ti also has the largest shear modulus i.e. 70.5 GPa, and Pd 31 TM (TM = Ti, Zr, Hf) compounds have almost similar Young's modulus. The anisotropic mechanical properties of the PdTM and Pd 31 TM(TM = Ti, Zr, Hf) are discussed by the anisotropic index, three-dimensional (3D) surface contours and the planar projections on (001) and (110) planes of the Young's modulus. Vogit and Reuss method are used to estimate the effects of different concentrations of transition elements on the mechanical properties. The electronic structures and chemical bonding characteristics are analyzed by the band structures and the density of states. When compared with Ni-Base superalloys, Pd-Base alloys have excellent properties with great potential application as next generation superalloy at higher temperature.

Effect of oxygen-to-metal flux ratio on incorporation of metal species into quaternary BeMgZnO grown by plasma-assisted molecular beam epitaxy

Owing to its large bandgap covering the UV region of the optical spectrum, the quaternary BeMgZnO is of interest, particularly the collective effect Be and Mg fluxes on the solid composition. Incorporation of Be, Mg, and Zn in the wurtzite BeMgZnO quaternary alloy was found to depend strongly on the reactive-oxygen to metal flux ratio during growth by plasma-assisted molecular beam epitaxy under metal-rich conditions. For a given set of metal fluxes, reducing the VI/II (oxygen to metal flux) ratio from 1.0 to 0.6 increased the bandgap from 4.0eV to 4.5eV and decreased the c lattice parameter from 5.08Å to 5.02Å. The corresponding change in composition from Be0.07Mg0.21Zn0.72O to Be0.10Mg0.34Zn0.56O was consistent with a systematic reduction in the Zn incorporation coefficient from 0.23 to 0.12, while those of Be and Mg remained at ∼1. This behavior was explained by the substantially lower formation enthalpies of wurtzite BeO and MgO, −5.98eV and −5.64eV, respectively, compared to that of ZnO, −3.26eV, determined using first principles calculations, as well as the high equilibrium vapor pressure of Zn, which results in re-evaporation of excessive Zn from the growing surface, details of which are the topic of this manuscript.