Temperature-Dependent Debye Temperature and Specific Capacity of Graphene

The shot-range interaction and the atomic anharmonic vibration are both considered, and then the analytic functions of the Debye temperature, the specific capacity and the thermal conductivity of graphene with the temperature are obtained. The influence of anharmonic vibration on these thermal physical properties is also investigated. Some theoretical results are given. If only the harmonic approximation is considered, the Debye temperature of the graphene is unrelated to the temperature. If the anharmonic terms are considered, it increases slowly with the increasing temperature. The molar heat capacity of the graphene increases nonlinearly with the increasing temperature. The mean free path of phonons and the thermal conductivity of the graphene decrease nonlinearly with the increasing temperature. The relative changes of the Debye temperature, the specific heat capacity and the thermal conductivity caused by the anharmonic terms increase with the increasing temperature. The anharmonic effect of atomic vibration becomes more significant under higher temperature.

Structural, mechanical, electronic properties, and Debye temperature of quaternary carbide Ti3NiAl2C ceramics under high pressure: A first-principles study

Quaternary carbide Ti3NiAl2C ceramics has been investigated as a potential nuclear fusion structural material,and it has advantages in certain aspects compared with Ti2AlC,Ti3AlC2,and Ti3SiC2 structural materials.In this paper,quaternary carbide Ti3NiAl2C ceramics is pressurized to investigate its structural,mechanical,electronic properties,and Debye temperature.Quaternary carbide Ti3NiAl2C ceramics still maintains a cubic structure under pressure(0–110 GPa).At zero pressure,quaternary carbide Ti3NiAl2C ceramics only has three bonds:Ti–Al,Ni–Al,and Ti–C.However,at pressures of 20 GPa,30 GPa,40 GPa,60 GPa,and 70 GPa,new Ti–Ni,Ti–Ti,Al–Al,Ti–Al,and Ti–Ti bonds form.When the pressure reaches 20 GPa,the covalent bonds change to metallic bonds.The volume of quaternary carbide Ti3NiAl2C ceramics can be compressed to 72%of its original volume at most.Pressurization can improve the mechanical strength and ductility of quaternary carbide Ti3NiAl2C ceramics.At 50–60 GPa,its mechanical strength can be comparable to pure tungsten,and the material changes from brittleness to ductility.However,the degree of anisotropy of quaternary carbide Ti3NiAl2C ceramics increases with the increasing pressure.In addition,we also investigated the Debye temperature,density,melting point,hardness,and wear resistance of quaternary carbide Ti3NiAl2C ceramics under pressure.

EFFECT OF HYDROGEN ON DEBYE TEMPERATURE AND LATTICE PARAMETER OF Ti-30Mo ALLOY

The variations of Debye temperature and lattice parameter of Ti-30Mo alloy with different hydrogen contents were measured by means of X-ray diffraction.It was found that as the hydrogen content increases,the Debye temperature decreases,but the lattice parameter of the aUoy increases.The above results experimentally support the decohesion theory that the in- crease of hydrogen in metals will weaken the cohesive force between the atoms and make met- als brittle.

DEBYE TEMPERATURE AND VALENCE ELECTRON STRUCTURE IN INTERMETALLIC TiAl

The Debye temperature 0D of intermetallic compound TiAl at room temperature was determined by means of X-ray diffraction.The experiments show that the Debye temperature of TiAl is 515 K,which is obviously higher than that of Al(394 K)or Ti(380 K).The experimental results indicate that the bonding in the intermetallic compound TiAl is stronger than that in pure metal Ti or Al which is in good agravement with the calculation of its valence electron structures.The relationship between the Debye temperature of TiAl and its brittle-ductile transition temperature is also dealt with in the paper.

Effects of temperature and pressure on thermodynamic properties of Cd_(0.25)Zn_(0.75)Se alloy

Thermodynamic properties of Cd0.25Zn0.75Se alloy are studied using quasi harmonic model for pressure range of 0 GPa-10 GPa and temperature range 0 K-1000 K. The structural optimization is obtained by self-consistent field calculations and full-potential linearized muffin-tin orbital method with GGA＋U as an exchange correlation functional where U=2.3427 eV is Hubbard potential. The effects of temperature and pressure on bulk modulus, Helmholtz free energy, internal energy, entropy, Debye temperature, Grüneisen parameter, thermal expansion coefficient, and heat capacities of the material are observed and discussed. The bulk modulus, Helmholtz free energy, and Debye temperature are found to be decreased on increasing temperature while there is an increasing behavior with rise of the pressure. Whereas the internal energy has increasing trend with the rise in temperature and it almost remains insensitive to pressure. The entropy of the system increases （decreases） with rise of pressure （temperature）.

Thermodynamic properties of ZnSe under pressure and with variation in temperature

The thermodynamic properties of Zn Se are obtained by using quasi-harmonic Debye model embedded in Gibbscode for pressure range 0–10 GPa and for temperature range 0–1000 K. Helmholtz free energy, internal energy, entropy,Debye temperature, and specific heat are calculated. The thermal expansion coefficient along with Gruneisen parameter are also calculated at room temperature for the pressure range. It is found that internal energy is pressure dependent at low temperature, whereas entropy and Helmholtz free energy are pressure sensitive at high temperature. At ambient conditions,the obtained results are found to be in close agreement to available theoretical and experimental data.

First-principles calculation of elastic and thermodynamic properties of Ni_2 MnGa Heusler alloy

The equilibrium lattice parameter, heat capacity, thermal expansion coefficient and bulk modulus of Ni2MnGa Heusler alloy are successfully obtained using the first-principles plane-wave pseudopotential （PW-PP） method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus B and temperature T up to 800 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure and decreases with increasing temperature. The pressure dependence of heat capacity Cv and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperature of Ni2MnGa is determined from the non-equilibrium Gibbs function. Our calculated results are in excellent agreement with the experimental data.

Calculated Structure,Elastic and Electronic Properties of Mg_2Pb at High Pressure

The effects of high pressure on structure, elastic and electronic properties of the intermetallic MgzPb were calculated by the first-principles plane wave pseudo-potential method in the scheme of density functional theory （DFT） within the generalized gradient approximation. The elastic constants and Debye temperature obtained at 0 GPa are in good agreement with the available experiment data and other theoretical results. The electronic properties calculated suggest that the electronic density of states （DOS） at the Fermi level decreases under high pressure.

Theoretical study of the elastic properties of titanium nitride

The equilibrium lattice parameter, relative volume V/V0, elastic constants Cij, and bulk modulus of titanium nitride are successfully obtained using the ab initio planewave pseudopotential （PW-PP） method within the framework of density functional theory. The quasi-harmonic Debye model, using a set of total energy vs molar volume obtained with the PW-PP method, is applied to the study of the elastic properties and vibrational effects. We analyze the relationship between the bulk modulus and temperature up to 2000 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonously with increasing pressure and decreases with increasing temperature. Moreover, the Debye temperature is determined from the non-equilibrium Gibbs functions.

X-Ray and Neutron Diffraction Studies on Thermal Parameters of Thalous Bromide

Thermal parameters of TIBr were determined using both X-ray and neutron diffraction techniques. The data was analysed by Rietveld profile refinement procedure. From the neutron diffraction data, due to weak odd-order reflections, it was not possible to determine the individual thermal parameters. TheX-ray diffraction measurements yielded BT1=0.296(5) nm2 and BBr=0.162(5) nm2. The overall isotropic value, B was 0.252(7) nm2 which is in good agreement with B=0.230(8) nm2 obtained from present neutron diffraction measurements. The present values are also in good agreement with theoretical estimates obtained from the shell models.

Advances of ferrous and ferric Mossbauer recoilless fractions in minerals and glasses

Mössbauer spectroscopy has been used widely to characterize the ferric(Fe^(3+))and ferrous(Fe^(2+))proportions and coordination of solid materials.To obtain these accurately,the recoilless fraction is indispensible.The recoilless fractions(f)of iron-bearing minerals,including oxides,oxyhydroxides,silicates,carbonates,phosphates and dichalcogenides,and silicate glasses were evaluated from the temperature dependence of their center shifts or absorption area with the Debye model approximation.Generally,the resolved Debye temperature(θ_(D))of ferric iron in minerals,except dichalcogenides,through their center shifts ranging from 400 to 550 K,is significantly larger than ferrous iron ranging from 300 to 400 K,which is consistent with the conclusion from previous work.The resolved f(Fe^(3+))RT with the center shift model(CSM)ranges from 0.825 to 0.925,which is larger than that obtained for f(Fe^(2+))RT,which ranges from 0.675 to 0.750.Meanwhile,the θ_(D) and f resolved from temperature-dependence of absorption are generally lower than from center shifts,especially for ferric iron.The significant difference between f(Fe^(3+))and f(Fe^(2+))indicates the necessity of recoilless fraction correction on the Fe^(3+)/(Fe^(3+)+Fe^(2+))resolved from Mössbauer spectra.

First-principles calculations of elasticity and thermodynamic properties of LaNi_5 crystal under pressure

This paper investigates the equilibrium lattice parameters, heat capacity, thermal expansion coefficient, bulk modulus and its pressure derivative of LaNi5 crystal by using the first-principles plane-wave pseudopotential method in the GGA-PBE generalized gradient approximation as well as the quasi-harmonic Debye model. The dependences of bulk modulus on temperature and on pressure are investigated. For the first time it analyses the relationships between bulk modulus B and temperature T up to 1000 K, the relationship between bulk modulus B and pressure at different temperatures are worked out. The pressure dependences of heat capacity Cv and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperatures of LaNi5 at different pressures are successfully obtained. The calculated results are in excellent agreement with the experimental data.

Pseudo-potential investigations of structural,elastic and thermal properties of tungsten disilicide

The plane-wave pseudopotential method using the generalized gradient approximation within the density functional theory is used to investigate the structure and bulk modulus of WSi2. The quasi-harmonic Debye model, using a set of total energy versus cell volume obtained with the plane-wave pseudopotential method, is applied to the study of the elastic properties and vibrational effects. We have analysed the bulk modulus of WSi2 up to 1600 K. The major trend shows that the WSi2 crystal becomes more compressible when the temperature rises and the increase of compressibility leads to the decrease of Debye temperature. The predicted temperature and pressure effects on the thermal expansion, heat capacity and Debye temperatures are determined from the non-equilibrium Gibbs functions and compared with the data available.

New ordered MAX phase Mo_2 TiAlC_2: Elastic and electronic properties from first-principles

First-principles computation on the basis of density functional theory(DFT) is executed with the CASTEP code to explore the structural, elastic, and electronic properties along with Debye temperature and theoretical Vickers’ hardness of newly discovered ordered MAX phase carbide Mo2TiAlC2. The computed structural parameters are very reasonable compared with the experimental results. The mechanical stability is verified by using the computed elastic constants. The brittleness of the compound is indicated by both the Poisson’s and Pugh’s ratios. The new MAX phase is capable of resisting the pressure and tension and also has the clear directional bonding between atoms. The compound shows significant elastic anisotropy. The Debye temperature estimated from elastic moduli(B, G) is found to be 413.6 K. The electronic structure indicates that the bonding nature of Mo2TiAlC2is a mixture of covalent and metallic with few ionic characters. The electron charge density map shows a strong directional Mo–C–Mo covalent bonding associated with a relatively weak Ti–C bond.The calculated Fermi surface is due to the low-dispersive Mo 4d-like bands, which makes the compound a conductive one.The hardness of the compound is also evaluated and a high value of 9.01 GPa is an indication of its strong covalent bonding.

Mechanical and thermodynamic properties of the monoclinic and orthorhombic phases of SiC_2N_4 under high pressure from first principles

First principles calculations are preformed to systematically investigate the electronic structures, elastic and thermodynamic properties of the monoclinic and orthorhombic phases of Si C2N4 under pressure. The calculated structural parameters and elastic moduli are in good agreement with the available theoretical values at zero pressure. The elastic constants of the two phases under pressure are calculated by stress–strain method. It is found that both phases satisfy the mechanical stability criteria within 60 GPa. With the increase of pressure, the degree of the anisotropy decreases rapidly in the monoclinic phase, whereas it remains almost constant in the orthorhombic phase. Furthermore, using the hybrid density-functional theory, the monoclinic and orthorhombic phases are found to be wide band-gap semiconductors with band gaps of about 2.85 e V and 3.21 e V, respectively. The elastic moduli, ductile or brittle behaviors, compressional and shear wave velocities as well as Debye temperatures as a function of pressure in both phases are also investigated in detail.

Electronic Band Structure and Heat Capacity Calculation of Some TlX (X = Sb, Bi) Compounds

The tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA) has been used to calculate structural and electronic properties of thallium pnictides TlX (X = Sb, Bi). As a function of volume, the total energy is evaluated. Apart from this, equilibrium lattice parameter, bulk modulus, first order derivative, electronic and lattice heat co-efficient, Debye temperature and Grüneisen constants, band structure and density of states are calculated. From energy band diagram, we observed metallic behaviour in TlSb and TlBi compounds. The equilibrium lattice constants agreed well with the available data.

Mechanical properties,thermophysical properties and electronic structure of Yb^(3+)or Ce^(4+)-doped La_(2)Zr_(2)O_(7)-based TBCs

First-principles calculations based on density functional theory were perfo rmed to investigate the cohesive energies,elastic modulus,Debye temperatures,thermal conductivities and density of states of La_(2-x)Yb_(x)Zr_(2)O_(7),La_(2)Zr_(2-x)Ce_(x)O_(7)and La_(2-x)Yb_(x)Zr_(2-x)Ce_(x)O_(7)(x=0.00,0.25,0.50,0.75,1.00)ceramics.The results show that doping Yb~(3+)or Ce~(4+)into La_(2)Zr_(2)O_(7)reduces its elastic modulus,thermal conductivity and Debye temperature.Compared with La_(2-x)Yb_(x)Zr_(2)O_(7)(x≠0.00),La_(2)Zr_(2-x)Ce_(x)O_(7)compounds have better ductility and lower Debye te mperature.The Debye temperature values of La_(2)Zr_(2-x)Ce_(x)O_(7)(x≠0.00)co mpounds are in the range of 485.0-511.5 K.Among all components,the fluorite-type La_(2-x)Yb_(x)Zr_(2-x)Ce_(x)O_(7)(x=0.75,1.00)compounds exhibit better mechanical and thermophysical properties,and their thermal conductivity values are only 1.213-1.246 W/(m·K)(1073 K),which are 14.5%-16.7%lower than that of the pure La_(2)Zr_(2)O_(7).Thus,our findings open an entirely new avenue for TBCs.

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-MnAl 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-MnAl 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.

Structural,mechanical,thermodynamic and electronic properties of Pt_(3)M(M=Al,Co,Hf,Sc,Y,Zr)compounds under high pressure

In this work,the impacts of pressure on the structural,mechanical,thermodynamic and electronic properties of typical Pt_(3)M(M=Al,Co,Hf,Sc,Y,Zr)compounds were investigated systematically by the firstprinciples density function theory calculations.The calculated lattice parameters,volume and elastic constants of Pt_(3)M compounds are in good agreement with available experimental and calculation values.With the increase in pressure,the lattice parameters and volume of Pt_(3)M compounds decrease,while the elastic constants,bulk modulus,shear modulus and Young’s modulus increase.The variations in Pugh’s ratio and Poisson’s ratio indicate that these Pt_(3)M compounds are mechanically stable and ductile.The mechanical anisotropy of these Pt_(3)M compounds is enhanced by rising pressure.Thermodynamic analysis indicates that sound velocity and Debye temperature increase with the increase in stress.The charge distribution does not change obviously,implying that no phase transition occurs in the range of 0-100 GPa.

First-principle calculation on mechanical and thermal properties of B2-NiSc with point defects

Using the first-principles plane-wave pseudo-potential method based on density functional theory,the effect of vacancy and anti-position defect on the mechanical and thermal properties of B2-NiSc intermetallics were discussed in detail.Several parameters,such as the shear modulus,bulk modulus,modulus of elasticity,C11-C12,the Debye temperature and Poisson＇s ratio,have been calculated to evaluate the effect of vacancy and anti-position defect on the hardness,ductility and thermal properties of B2-NiSc intermetallics.The results show that VNi,ScNi,VSc and NiSc the four point defects all make the crystal hardness decrease and improve plasticity of B2-NiSc intermetallics.The entropy,enthalpy and free energy of VNi,ScNi,VSc and NiSc are monotonously changed as temperature changes.From the perspective of free energy,NiSc is the most stable,while ScNiis the most unstable.Debye temperature of NiSc intermetallics with four different point defects shows VNi,ScNi,VSc and NiScthe four point defects all reduce the stability of B2-NiSc intermetallics.