Constant-volume Specific Heat Research Articles

DFT and DFPT calculations of the structural, electronic, optical, vibrational and thermodynamic properties of silicon tetraborate

In this study, the structural, mechanical, electronic, optical vibrational, and thermodynamic properties of a new crystalline material, silicon tetraborate (SiB4O7), were calculated using DFT and DFPT. This material may prove crucial for the development of devices with particular optical and electronic properties. We report here that the band gap calculated for SiB4O7 using the GGA-PBE functional was Eg= 3.922 eV which is consistent with the values found in the literature for similar materials, such as PbB4O7. The band gap energy calculated using the HSE06 hybrid functional was Eg= 5.120 eV. Six prominent peaks were observed in the optical absorption spectrum of SiB4O7. Based on the optimized structures, the elastic stiffness constants and the elastic compliance of the system were calculated using the stress–strain method. The mechanical properties of the orthorhombic-SiB4O7, including the mechanical moduli (bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, and anisotropy), were evaluated using the Voigt–Reuss–Hill approach. The mechanical and vibrational properties, thermodynamic potentials, and calculated constant-volume specific heat can be used to predict the stability of the crystal phase. The breakthroughs and insights presented herein for this new material may help in the development of optoelectronic devices.

First-Principles Study of the Structural, Mechanical and Thermodynamic Properties of Al11RE3 in Aluminum Alloys

The stability and mechanical and thermodynamic properties of Al11RE3 intermetallics (RE = Sc, Y and lanthanide La-Lu) have been investigated by combining first-principles and Debye model calculations. It was found that the formation enthalpies of the Al11RE3 intermetallics are all negative, indicating that they are stable; moreover, the experimental values of Al11La3 and Al11Ce3 are in good agreement with the predicted values, which are −0.40 kJ/mol and −0.38 kJ/mol, respectively. The calculated results of the mechanical properties reveal that the Young’s modulus E and shear modulus G of Al11RE3 (RE = La, Ce, Pr, Nd and Sm) intermetallics are obviously greater than that of Al, implying that the stiffness, toughness, and tensile strength of them are significantly greater than those of aluminum, and that they, as strengthen phases, can effectively improve the mechanical property of aluminum alloys. The Poisson’s ratio v of Al11Sc3 (0.37) is the largest, and the heterogeneity is obvious. All the Al11RE3 intermetallics can enhance the thermostability of the aluminum because of their lower Gibbs free energy F in the range of −5.002~−4.137 eV/atom and thermal expansion coefficient α of Al in the range of 2.34~2.89 × 10−5/K at 300K, as well as higher entropy and constant volume-specific heat than aluminum at finite temperatures. With an increase in the atomic number, different change trends were observed for the formation enthalpy ΔHf, bulk modulus B, Young’s modulus E, and shear modulus G. This paper can provide ideas and help for designing a high-performance, heat-resistant aluminum alloy.

Plastic deformation behavior of a Cu–10Ta alloy under strong impact loading

In this work, a Cu–10Ta alloy with a copper to tantalum mass ratio of 9:1 is prepared using powder metallurgy technology. Physical properties of the alloy, including density, microstructure, melting point, and constant-volume specific heat, are tested. Via the split Hopkinson pressure bar (SHPB) and flyer-plate impact experiments, the relationship between equivalent stress and equivalent plastic strain of the material is studied at temperatures of 298–823 K and under strain rates of 1 × 10−3–5.2 × 103 s−1, and the Hugoniot relationship at impact pressures of 1.46–17.25 GPa and impact velocities of 108–942 m/s is obtained. Evolution of the Cu–10Ta microstructure that occurs during high-strain-rate impact is analyzed. Results indicate that the Cu–10Ta alloy possesses good thermal stability, and at ambient temperatures of up to 50% its melting point, stress softening of less than 15% of the initial strength is observed. A modified J-C constitutive model is employed to accurately predict the variation of yield strength with strain rate. Under strong impact, the copper phase is identified as the primary source of plastic deformation in the Cu–10Ta alloy, while significant deformation of the high-strength tantalum particles is less pronounced. Furthermore, the longitudinal wave speed D is found to correlate linearly with the particle velocity u upon strong impact. Analysis reveals that the sound speed and spallation strength of the alloy increase with increasing impact pressure.

First-principles calculations of the dynamical and thermodynamic properties of lead chalcogenides PbX (X = S, Se, Te)

We perform first-principles calculations of the dynamical and thermodynamic properties of lead chalcogenides PbX (X = S, Se, Te ). Using density functional perturbation theory, we obtain the phonon frequencies and the phonon dispersion curves, as well as corresponding density of states. The calculated phonon frequencies at the Γ, X, and L points of the Brillouin zone show good agreement with the experimental values for most vibration modes. The thermodynamics properties including the phonon contribution to the Helmholtz free energy ΔF, the phonon contribution to the internal energy ΔE, the entropy S, and the constant-volume specific heat Cv are determined within the harmonic approximation based on the calculated phonon dispersion relations. The difference values of H-H298 have been also calculated and compared with the available experimental data.

Structural phase transition, elastic constants and thermodynamic properties of TmAs: A DFT study

Employing the all-electron full-potential linearized augmented plane wave method and the local density approximation, we report on the results of the structural stability, phase transition pressure, thermodynamic, elastic, electronic and magnetic, properties of thulium arsenide (TmAs). The computed structural parameters, bulk modulus are compared with experimental and other theoretical values. It is found that TmAs undergoes a first-order phase transition from the NaCl-type structure to the CsCl-type structure at approximately 32.96 GPa, corresponding to a volume collapse of around 6.8% during this transition. We have also calculated some elastic properties such as Young modulus, anisotropy factor, and Poisson ratio. The Vickers hardness was predicted using two empirical expressions, which give the shame result ∼5.40 GPa. To gain further information, we used the quasi-harmonic approximation to calculate some thermodynamic properties such as the constant-volume specific heat capacity, the Debye temperature and the entropy of our material of interest. The authors’ results show a monotonous change of the thermodynamic properties of TmAs against temperature. These accords well with the general behavior commonly reported in the literature. The band structure and corresponding density of states have also been determined and analyzed using LSDA, LSDA + U and mBJ-LSDA + U approaches.

Study of dynamic and thermodynamic properties of zinc-blend and wurtzite cadmium sulfide (CdS) using density functional theory

Abstract The dynamic and thermodynamic properties of wurtzite (wz) and zinc-blend (zb) CdS are investigated within the density functional theory using different approximation methods such as LDA, PBE, and DFT+U. Hellmann–Feynman approach is implemented for the relaxation of atomic position for both phases. To guarantee the accuracy of calculation, the convergence test of total energy with respect to energy cutoff and k-point sampling is performed. The dynamic properties such as phonon dispersion, phonon density of state, frequency along with high symmetry points, static and dynamic polarizability, and dielectric constants are calculated. The obtained values are compared with previous theoretical results. DFT + U approximation gives a good result that is consistent with the available theory. Moreover, the vibrational energy, vibrational free energy, entropy, electron chemical potential, and constant-volume specific heat are obtained within LDA, PBE, and DFT + U approximations.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Recent Advances in Molecular Design of Organic Thermoelectric Materials

A comparative ab initio study of the mechanical, dynamical and thermophysical characteristics of XC2 (X = La, Ce, Tb, Ho)

The structural, mechanical, lattice dynamics and thermophysical characteristics of XC2 (X = La, Ce, Tb, Ho) have been theoretically investigate by means of density functional theory (DFT). The optimized lattice constants and elastic constants of XC2 (X = La, Ce, Tb, Ho) are in an excellent agreement with the available experimental data. The elastic moduli including elastic constants Cij, bulk modulus B, shear modulus G, Young's modulus E, Debye temperature θ, wave velocities and anisotropy indexes have been predicted successfully. The brittle/ductile properties of four compounds are determined by the ratio of B/G. According to the values of anisotropic indexes, three dimensional (3D) surface constructions indicate that XC2 shows mechanical anisotropy. Results indicate that four compounds can be stabilized mechanically and dynamically in the ground state phase of the tetragonal structure. The thermophysical characteristics such as internal energy, Helmholtz free energy, entropy and the constant volume specific heat under high temperatures are also predicted and analyzed comprehensively based on phonon density of states.

Thermodynamic instability of 3D Einstein-Born-Infeld AdS black holes * *Supported in part by the National Natural Science Foundation of China (11005016, 11875196, 11375121)

Super-entropic black holes possess finite-area but noncompact event horizons and violate the reverse isoperimetric inequality. It has been conjectured that such black holes always have negative specific heat at constant volume or negative specific heat at constant pressure whenever , making them unstable in extended thermodynamics. In this paper, we describe a test of this instability conjecture with a family of nonlinear electrodynamic black holes, namely 3D Einstein-Born-Infeld (EBI) AdS black holes. Our results show that when nonlinear electrodynamics effects are weak, the instability conjecture is valid. However, the conjecture can be violated in some parameter region when nonlinear electrodynamics effects are strong enough. This observation thus provides a counter example to the instability conjecture, which suggests that super-entropic black holes may be thermodynamically stable.

Electronic structure, elastic, optical and thermal properties of chalcopyrite CuBY2 (B = In, Ga, In0.5 Ga0.5; Y = S, Se, Te) solar cell compounds

The electronic structure, elastic, optical and thermal properties of chalcopyrite solar cell compounds, CuBY2 (B = In, Ga, In0.5Ga0.5; Y = S, Se, Te) are systematically studied. The crystal structure of ternary semiconductors is the chalcopyrite structure with space group I4¯2d. Density functional calculations are carried out within the local density approximation and generalized gradient approximation. The calculated lattice constant values are in good agreement with the available data. The electronic band structure shows that these chalcopyrite compounds are direct band gap semiconductors. To design a photovoltaic material with the band gap between 1.0 eV and 2.4 eV, indium (In) and gallium (Ga) are mixed to form CuInY2 (Y = S, Se, Te). It is observed that the energy band gap increases as the Ga concentration increases in CuIn1-xGaxY2 (Y = S, Se, Te). The computed optical parameters indicate that these materials can be the effective candidates for the solar cell devices. Phonon dispersion curves show that these compounds are dynamically stable. Moreover, the thermal properties, such as the entropy and the constant-volume specific heat are also calculated.

New Zirconium Diboride Polymorphs-First-Principles Calculations.

Two new hypothetical zirconium diboride (ZrB) polymorphs: (hP6-P6/mmc-space group, no. 194) and (oP6-Pmmn-space group, no. 59), were thoroughly studied under the first-principles density functional theory calculations from the structural, mechanical and thermodynamic properties point of view. The proposed phases are thermodynamically stable (negative formation enthalpy). Studies of mechanical properties indicate that new polymorphs are less hard than the known phase (hP3-P6/mmm-space group, no. 191) and are not brittle. Analysis of phonon band structure and density of states (DOS) also show that the phonon modes have positive frequencies everywhere and the new ZrB phases are not only mechanically but also dynamically stable. The estimated acoustic Debye temperature, , for the two new proposed ZrB phases is about 760 K. The thermodynamic properties such as internal energy, free energy, entropy and constant-volume specific heat are also presented.

First-principles calculations of the electronic structure, chemical bonding, and thermodynamic properties of USiO4

The density functional theory (DFT) + U method is adopted to study the electronic structure, magnetism, chemical bonding, and thermodynamic properties of USiO4. A bandgap of 3.1 eV is obtained by analyzing the band diagram of USiO4. The calculated structural parameters are consistent with the recent experimental results. The charge density and charge density differences are studied in order to analyze the chemical bonds in USiO4. The results indicate that interactions within USiO4 are mostly ionic and exhibit weak covalent character. In addition, phonon behavior is studied in detail. We predict phonon frequencies and assign and analyze the Raman-active and infrared-active modes at the Γ point. Furthermore, thermodynamic properties such as the internal energy ΔE, Helmholtz free energy ΔF, constant-volume specific heat CV, and entropy S are investigated in the region between 0 K and 1000 K. The results are expected to provide useful information for subsequent experiments on USiO4.

Theoretical investigation of structural, mechanical, electronic and thermal behavior of platinum group metals and their intermetallic alloys PtRhX (X = Pd, Ir, Os, Ru)

The first-principles study of platinum group metals (Pt, Pd, Rh, Ir, Os, Ru) and their intermetallic alloys PtRhX (X = Pd, Ir, Os and Ru) are presented. Total energy was calculated with two different correlation functionals i.e. the local density approximation (LDA) and the generalized gradient approximation (GGA). The ground state quantities such as bulk modulus and lattice parameter have been evaluated. The structure stability of the different alloys was investigated by the three independent elastic constants (C11, C12, and C44). We have also calculated the electronic band structures and densities of states. The electronic calculations revealed that the alloys show transitions of electrons from valence to conduction bands there by making these alloys metallic. To study the fundamental characteristics of these alloys, we have also analyzed the thermodynamic characteristics such as Debye temperatures, vibrational energy, entropy, and constant-volume specific heat at different temperatures range from 0 K to 1000 K.

Investigation of phonon properties and thermal behavior of UO2 crystal using density-functional theory

Understanding thermal behavior and processes underlying the heat transport of UO2 nuclear fuel in nuclear reactor plays a key role in predicting the efficiency of the fuel. If the heat transport, which is an important parameter in temperature distribution of the fuel, does not occur properly, the continuous increase of temperature would lead to the melting of the fuel and therefore, environmental hazards. In this work, by using a non-spin-polarized calculation for the simple description of the paramagnetic state and ignoring the Hubbard correction, the thermal properties and phonon properties of bulk UO2 are calculated. These calculations are based on the density-functional theory (DFT) and density-functional perturbation theory (DFPT). To determine the lattice-vibration properties by the finite-displacement method, we have calculated the second-order and third-order force constants based on which such quantities as constant-volume specific heat, Gruneisen parameter, three-phonon scattering rate, scattering rate due to different levels of isotopic enrichment, and cumulative thermal conductivity are calculated. The results of the calculated specific heat based on the harmonic approximation show a good agreement with the experimental values, specifically for temperatures lower than 400 Kelvin. The results obtained for three-phonon scattering rate reveal that the scattering rate increases with temperature, thereby leading to the decrease of thermal conductivity. The results related to different levels of isotopic enrichments do not show any sensible changes in the scattering rates.

Dimensionless parameters determining the effect of dilution on ignition delay of syngas and hydrocarbon fuels

This study explored the prediction of the effect of dilution on the ignition delay of syngas and hydrocarbon fuels from dimensionless parameters. In this regard, the dilution effect was categorized into two sub-effects. The first is a non-chemical effect, which mainly represents the effect of decreasing the fuel-air concentration and the effect of heat-release damping. The second is the chemical effect induced by participation of the diluent in the reaction pathway. Then, we derived the determining parameters of each sub-effect based on chemical kinetics. The non-chemical effect was found to be related to the normalized sensitivity of the ignition delay to the total concentration, n˜; the ratio of the molar constant volume specific heat of the diluent and the fuel-air mixture, cv,D*; and the average Zel'dovich number during the ignition delay, Z˜em, which represents the effect of temperature rise during the ignition delay on the reaction rate. Meanwhile, the extent of the chemical effect was found to be related to the sensitivity of the ignition delay to the reaction rate coefficient of the three-body reactions, Sτ0−kj, multiplied by the sensitivity of the reaction rate coefficients to the third body concentration, Skj−[M]j, and the ratio of the collision efficiency of the diluent and the fuel-air mixture, ηD,j*. Based on this relationship, we investigated the correlation between the extent of dilution effect and the aforementioned dimensionless parameters, n˜,cv,D*,Z˜em,Sτ0−kj,Skj−[M]j,andηD,j*, under the temperature range of 600–1400 K, pressure of 1–80 bar, and equivalence ratio of 0.5–2 with 13 different syngas or hydrocarbon fuels. Consequently, a strong correlation was found between the dilution effect and the dimensionless parameters, with the deterministic coefficient over 0.95. Furthermore, we suggested a proper form of the empirical correlation for the ignition delay that can cover a wide range of diluent concentrations, considering the extent of the chemical effect appropriately. This study presents the first dimensionless analysis for the dilution effect on ignition delay, developing a general framework for prediction of the effect based on the thermochemical characteristics of a fuel-air mixture.

Structural, lattice dynamics and thermodynamic properties of GaAs1-xPx from first-principles calculations

The structural, lattice dynamics and thermodynamic properties of GaAs1-xPx with different concentration of the components have been investigated via density functional perturbation theory. The results of equilibrium lattice parameters of GaAs1-xPx indicate that our calculated values are relatively in good agreement with the available experimental and theoretical results. The calculated phonon-dispersion curves of GaAs1-xPx along high symmetry lines in the Brillouin zone display a significant hardening as the concentration of x is enhanced. Besides broadening phonon states under the definite wave vector, GaAs1-xPx alloy system expands the optic branches and raises the longitudinal modes (LO) more than the transverse modes (TO) with the elevation of the concentration of x. The LO-TO splitting degree at Γ-point reveals that the ionic strength of GaAs1-xPx will increase with the raise of the concentration of x. Moreover, using the calculated phonon density of states, the thermodynamic properties of GaAs1-xPx including the constant-volume specific heat Cv, the internal energy △E, the Helmholtz free energy △F and the entropy S are calculated within the harmonic approximation.

Lattice theory for binding of linear polymers to a solid substrate from polymer melts: I. Influence of chain connectivity on molecular binding and adsorption.

Most theories of the binding of molecules to surfaces or for the association between molecules treat the binding species as structureless entities and neglect their rigidity and the changes in their stiffness induced by the binding process. The binding species are also taken to be "ideal," meaning that the existence of van der Waals interactions and changes in these interactions upon molecular binding are also neglected. An understanding of the thermodynamics of these multifunctional molecular binding processes has recently come into focus in the context of the molecular binding of complex molecules, such as dendrimers and DNA grafted nanoparticles, to surfaces where the degree of binding cooperativity and selectivity, as well as the location of the binding transition, are found to be sensitive to the number of binding units constrained to a larger scale polymeric scaffold. We address the fundamental problem of molecular binding by extending classical Langmuir theory to describe the particular example of the reversible binding of semiflexible polymer chains to a solid substrate under melt conditions. The polymer chains are assumed to have a variable number N of binding units (segments) and to exhibit variable bending energies and van der Waals interactions in the bulk and on the surface, in addition to strong directional interactions with the surface. The resulting generalized Langmuir theory is applied to the examination of the influence of the chain connectivity of ideal polymers on the surface coverage Θ, transition binding temperature T1/2 at which Θ = 1/2, and on the derivative |dΘ/dT|T=T1/2 and the constant volume specific heat of binding, Cv bind, measures of the cooperativity and "sharpness" of the binding transition, respectively. Paper II is devoted to the impact of the van der Waals attractive interactions and chain stiffness on the reversible binding of nonideal polymer chains to a solid surface, including the enthalpy-entropy compensation phenomenon observed experimentally in many molecular and particle binding processes.

A comparative study of the vibrational and thermodynamic properties of α-RDX and γ-RDX under ambient conditions.

The structural parameters, Raman/IR spectra, and vibrational and thermodynamic properties of α-RDX and γ-RDX were investigated by performing first-principles calculations using the CASTEP program. The obtained structural parameters and vibrational frequencies of the internal modes of α-RDX are consistent with previous results. The vibrational bands of γ-RDX in the region 300-3113cm-1 were assigned to vibrational modes for the first time. Furthermore, the thermodynamic properties of α-RDX and γ-RDX were calculated, including the constant-volume specific heat Cv, entropy S, Helmholtz free energy F, enthalpy H, and Gibbs free energy G.

A first-principles study of novel cubic AlN phases

By a first-principles approach combined with an evolutionary method, four novel cubic AlN phases (cP16, cF40, cI16, and cI24 AlN) are proposed as promising phases after detailed stability analysis. These four cubic AlN phases exhibit a hardness range of 7.8–17.0 GPa. The anisotropy of their mechanical properties, including Young's modulus, shear modulus, and Poisson's ratio, is systematically studied. Moreover, they are all semiconductors with band gaps smaller than those of experimental phases, and consist of polar covalent Al–N bonds with sp3 hybridization. Their thermodynamic functions, including enthalpy, Gibbs free energy, and vibrational entropy, are calculated in the temperature range from 0 to 2000 K. Their thermodynamic properties, including Debye temperature and constant-volume specific heat, are investigated. The anisotropy of their minimum thermal conductivities is discussed on the basis of Clarke's model. We hope that this study of the mechanical, electronic, and thermodynamic properties of novel cubic AlN phases will provide guidance for the industrial applications of these four cubic AlN phases.

First-principles study of lattice dynamical and thermodynamic properties of Li<sub>2</sub>NH

<sec>One of the key issues for scale applications of hydrogen energy is the availability of safe, efficient and ecnomicical hydrogen storage technologies. In the past few years, light metal hydrides have attracted considerable attention due to their high hydrogen capacity. With a hydrogen capacity up to ~6.5 wt%, Li<sub>2</sub>NH is regarded as one of the most promising hydrogen storage materials. Although the hydrogen physical and thermodynamic properties of Li<sub>2</sub>NH have been studied, the electronic structure, phonon vibration mode and thermodynamic properties of Li<sub>2</sub>NH have not yet been resolved. In this paper, by using the first principles based on the density functional theory (DFT), we investigate the electronic structure, lattice dynamical and thermodynamic properties of Li<sub>2</sub>NH in detail.</sec><sec>Firstly, the structure of Li<sub>2</sub>NH is optimized and the lattice parameters and total energy of the crystals are calculated. As shown by the calculation results, the lattice parameters are in good agreement with previous theoretical and experimental results. Our lowest-energy structure of Li<sub>2</sub>NH has orthorhombic <i>Pnma</i> symmetry at <i>T</i>=0 K for all of the proposed structures. Secondly, the electronic band-structure studies reveal that Li<sub>2</sub>NH has a small band gap of about 2.0 eV. The analysis of total and partial density of states of Li<sub>2</sub>NH show that the bonding between the N and H has a covalent character. Thirdly, the lattice dynamical properties of Li<sub>2</sub>NH are investgated at the corresponding equilibrium states. These results show that only the phonon dispersion curves of Li<sub>2</sub>NH (<i>Pnma</i>) without negative frequencies are calculated along the high-symmetry points. The optical modes of phonon frequencies at <i>Γ</i> point are assigned as Raman and Infrared-active modes. Based on the calculated phonon density of states, the thermodynamic properties are computed, such as the Helmholtz free energy, internal energy, entropy and the constant-volume specific heat versus temperature. The calculation results may explore the applications in areas of hydrogen storage for Li-N-H, which is of great importance forusing hydrogen in the future.</sec>