Showing papers on "Debye model published in 2007"

Investigation of cohesive energy effects on size-dependent physical and chemical properties of nanocrystals

Abstract: The intrinsic factor to dominate the size-dependent properties of nanocrystals was investigated through applying cohesive energy to determine the physical-chemical properties. With understanding of the nature of the factor, a model for size-dependent melting temperature, Debye temperature, diffusion activation energy, and vacancy formation energy of nanocrystals was established. The accuracy of the developed model was verified by using the available experimental data of gold nanocrystals. It was found that the above properties have the same size-dependent trend which is contributed by the essential effects of surface/volume ratio. The study reveals that the vacancy formation determined by the cohesive energy is the intrinsic factor to dominate the size-dependent physical-chemical properties.

Modified Debye model parameters of metals applicable for broadband calculations.

Abstract: The finite-difference time-domain method can provide broadband results if the excitation source is a pulse. This demands that the parameters of modeled materials have to be applicable over broad frequency bands. We optimize the modified Debye model parameters for gold, silver, copper, platinum, and aluminum using a large-scale nonlinear optimization algorithm. The complex relative permittivities calculated using the optimized parameters agree well with experimental values over broad frequency bands. The associated root-mean-square deviations are 0.49%, 3.52%, 4.13%, 1.64%, and 0.66%, respectively. We also provide an example of broadband calculations. The obtained broadband results are verified by a series of steady-state calculations.

Computation of Interfacial Thermal Resistance by Phonon Diffuse Mismatch Model

Abstract: The thermal resistances of 1250 kinds of interface were computed at room temperature based on the phonon diffuse mismatch model. The result shows that the ratio of Debye temperature and the ratio of average sound velocity can be approximately used to characterize the difference of two materials in terms of interfacial thermal resistance. The high interfacial thermal resistances are composed of high and low Debye temperature materials. The low interfacial thermal resistances are composed of both similar Debye temperature materials, and their Debye temperatures are very high. The relation between the interfacial thermal resistance with the ratio of average sound velocity is similar to that of the ratio of Debye temperature. [doi:10.2320/matertrans.MAW200717]

Effect of nickel substitution on structural, infrared and elastic properties of lithium ferrite

Abstract: The structural and elastic properties of Li 0.5(1-x) Ni x Fe 2.5-0.5x O 4 (x = 0.0-1.0) spinel ferrite system have been studied by means of X-ray diffraction and infrared spectroscopic measurements at 300K. The X-ray diffraction data has been used to determine the lattice constant, X-ray density, distributions of cations among the tetrahedral and octahedral sites of spinel lattice, anion parameters, site radii, ionic radii, bond angle and bond length. The nature and change in the position of IR bands have been explained on the basis of cations involved in the system. The force constants have been used to calculate elastic moduli like bulk modulus, rigidity modulus, Young's modulus, Poisson's ratio, Debye temperature and corrected to zero porosity. The observed variation of elastic constants with nickel substitution has been explained on the basis of strength of interatomic bonding. The applicability of heterogeneous metal mixture rule for estimating elastic constants has been tested. The Debye temperature obtained from elastic constant data is higher than that of the X-ray diffraction analysis, mainly due to existence of peaks in the vibrational spectra at lower frequencies.

Size, temperature, and bond nature dependence of elasticity and its derivatives on extensibility, Debye temperature, and heat capacity of nanostructures

Abstract: With the miniaturization of a solid down to nanometer scale, the elasticity, extensibility, Debye temperature, and specific heat capacity of the solid are no longer constant but change with variation of size. These quantities also change with the temperature of the measurement and the nature of the chemical bond involved. The mechanism behind the intriguing tunability and the interdependence of these quantities remain yet a high challenge. A set of analytical solutions is presented herewith showing that the observed trends could be reproduced by taking the fact of bond order deficiency into consideration. Agreement between predictions and observations clarifies that the shortened and strengthened surface bonds dictate intrinsically the observed tunability, yet atoms in the core interior remain as they are in the bulk. The thermally softening of a specimen arises from bond expansion and bond vibration due to the internal energy increases.

Interpretation of mechanical properties and structure of TeO2-Li2O-B2O3 glasses

Abstract: Lithium borate glasses containing TeO2 as Li0.6 TexB1.4� 2xO2.4� x have been prepared by the conventional rapid quenching method over a wide range of composition (x ¼ 0, 0.1, 0.2, 0.3, and 0.35). Ultrasonic velocities (longitudinal and shear) were measured in these glasses at room temperature. The elastic moduli, and the Debye temperature, were calculated and discussed quantitatively in terms of the glass transition temperature, the cross-link density, and the packing density. The monotonic decrease in the velocities, the glass transition temperature, and the elastic moduli as a function of TeO2 modifier content reveals the loose packing structure, which is attributed to the increase in the molar volume and the reduction in the vibrations of the borate lattice. The compositional dependence of these parameters suggested that TeO2 changes the rigid character of Li0.6B1.4O2.4 to a matrix of ionic behavior bonds by breaking down the lithium borate structure. This was attributed to the creation of more and more discontinuities and defects in the glasses. r 2007 Elsevier B.V. All rights reserved.

Thermoelectric properties and electronic structure of Zintl compound BaZn2Sb2

Abstract: Polycrystalline sample of the title compound was prepared and its thermoelectric properties from 2to675K were investigated. This Zintl compound shows rather low thermal conductivity, 1.6Wm−1K−1, at room temperature. The value of its thermoelectric figure of merit ZT reaches 0.31 at 675K. Its electronic structure, calculated by ab initio methods, suggests that the electrical transport are mainly ascribe to [Zn2Sb2] framework for p-type BaZn2Sb2. The heat capacity curve at low temperature was fitted lineally to obtain Debye temperature (about 208K). It provides the authors with a host lattice for modification and optimization the thermoelectric properties through substitution and/or doping.

The temperature dependence of the isothermal bulk modulus at 1 bar pressure

Abstract: It is well established that the product of the volume coefficient of thermal expansion and the bulk modulus is nearly constant at temperatures higher than the Debye temperature. Using this approximation allows predicting the values of the bulk modulus. The derived analytical solution for the temperature dependence of the isothermal bulk modulus has been applied to ten substances. The good correlations to the experiments indicate that the expression may be useful for substances for which bulk modulus data are lacking.

Synthesis and several features of the Na 2 O-B 2 O 3 -Bi 2 O 3 -MoO 3 glasses

Abstract: Glasses in the system Na2−2xB4−4xBixMo0.5xO7−4x, 0 ≤ x ≤ 0.4, have been prepared by the melt quenching technique. Elastic properties and IR spectroscopic studies have been employed to study the role of Bi2O3 and MoO3 on the structure of Na2B4O7 glass. Elastic properties and Debye temperature have been investigated using sound velocity measurements at 4 MHz. The results showed that the density and the molar volume increase while both sound velocities and the determined glass transition temperatures decrease with increase in x. Infrared spectra of the glasses reveal that the strong borate network consists of diborate units and is affected by the increase in the concentration of Bi2O3, and MoO3. These results are interpreted in terms of the increase in the number of non-bridging oxygen atoms, substitution of longer bond lengths of Bi–O, and Mo–O in place of shorter B–O bond and the change in Na+ ion concentration. The results indicate that bismuth and molybdenum ions have been substituted for boron ions as network modifier ions. The elastic moduli are observed to increase with the increase in Bi2O3 and MoO3 content. This contradiction in the elastic moduli–molar volume relation is attributed to the role of the respective bonds.

Structural and elastic properties of Zr2AlX and Ti2AlX (X = C and N) under pressure effect

Abstract: Using first-principles density functional calculations, the effect of high pressures, up to 20 GPa, on the structural and elastic properties of Zr2AlX and Ti2AlX, with X = C and N, were studied by means of the pseudo-potential plane-waves method. Calculations were performed within the local density approximation to the exchange-correlation approximation energy. The lattice constants and the internal parameters are in agreement with the available results. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's moduli and Poisson's ratio for ideal polycrystalline Zr2AlX and Ti2AlX aggregates. We estimated the Debye temperature of Zr2AlX and Ti2AlX from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of Zr2AlC, Zr2AlN and Ti2AlN compounds, and it still awaits experimental confirmation.

Prediction study of structural and elastic properties under the pressure effect of M2GaC (M=Ti,V,Nb,Ta)

Abstract: Using first-principles density functional calculations, the effect of high pressures, up to 20 GPa, on the structural and elastic properties of M2GaC, with M=Ti, V, Nb, and Ta, were studied by means of the pseudopotential plane-waves method. Calculations were performed within the local density approximation to the exchange-correlation approximation energy. The lattice constants and the internal parameters are in agreement with the available results. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young’s moduli, and Poisson’s ratio for ideal polycrystalline M2GaC aggregates. We estimated the Debye temperature of M2GaC from the average sound velocity. This is a quantitative theoretical prediction of the elastic properties of Ti2GaC, V2GaC, Nb2GaC, and Ta2GaC compounds and it still awaits experimental confirmation.

First-principles calculations of thermodynamic properties of TiB2 at high pressure

Abstract: The equations of state (EOS) and other thermodynamic properties of TiB 2 are investigated using ab initio plane-wave pseudopotential density functional theory method. The obtained results are in good agreement with the experimental measured data and other theoretical calculated ones. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of relative volume V / V 0 on pressure P , cell volume V on temperature T , and Debye temperature Θ and specific heat C V on pressure P are successfully obtained.

Unusual field and temperature dependence of the Hall effect in graphene

Abstract: We calculate the classic Hall conductivity and mobility of the undoped and doped (or at the gate voltage) graphene as a function of temperature, magnetic field, and carrier concentration. Carrier collisions with defects and acoustic phonons are taken into account. The Hall resistivity varies almost linearly with temperature below the Debye temperature. The magnetic-field dependence of resistivity and mobility is anomalous in weak fields at low gate voltage: there is a square-root field contribution in the resistivity and the Hall mobility diverges logarithmically with the field.

Effective 2-Debye-Pole FDTD Model of Electromagnetic Interaction Between Whole Human Body and UWB Radiation

Abstract: We have successfully developed a human body finite difference time domain model based on efficient two-pole Debye dispersion, and analyzed for the first time the electromagnetic interaction between a whole human body and ultra wide band radiation having a wide frequency spectrum. The two-pole Debye dispersion model is obtained for 50 individual human tissue properties from Gabriel's Cole-Cole data by least squares fitting over a wide frequency range from 100 MHz to 6 GHz. For validation, the model is exposed to radiation of a spread spectrum signal modulated by typical binary phase shift keying. Local energy absorption in a human body has been compared between the two-pole Debye model and a conventional model with frequency-independent permittivity and conductivity.

Temperature Dependence of the Thermal Properties of Optical Memory Materials

Abstract: We have measured the temperature dependence of the thermal conductivities of Ge2Sb2Te5 (GST) and ZnS–SiO2 using a nano second thermoreflectance measurement system from room temperature to 500–600 °C. The specific heat capacities of these materials also have been determined from -130 to 500 °C for GST and from room temperature to 600 °C for ZnS–SiO2. The Debye temperature was obtained from specific heat capacity measurement. Using the obtained temperature dependence of the thermal conductivities, a temperature simulation inside a simple structured optical disk with and without considering its temperature dependence was carried out, and the difference in maximum temperature was approximate 80 °C.

Effects of Joint Vibrational States on Thermal Boundary Conductance

Abstract: The accuracy of predictions of the thermal boundary conductance (hBD ) made using traditional models such as the diffuse mismatch model (DMM) varies depending on the types of material comprising the interface. At interfaces of materials with very different material properties, the discrepancy between the measured hBD and the DMM results has been associated with inelastic scattering. In this study, a new model, the joint frequency diffuse mismatch model (JFDMM), is derived under the assumption that the phonon flux approaching the interface is altered by phonons vibrating at joint frequencies around the interface affected by phonons on both sides of the interface. This model yields improved hBD predictions for a wide range of materials over a temperature range of several 100s of Kelvin, indicating that at these temperatures, substrate phonons are participating in interfacial thermal transport.

The dc electrical conductivity of semiconducting TeO 2 V 2 O 5 MoO 3 bulk glasses

Abstract: The direct current electrical conductivity of ternary 40TeO2–(60−x)V2O5–xMoO3 glasses prepared by press-melt quenching technique was studied at temperatures between 90 and 403 K. From the conductivity–temperature relation, it was found that the small polaron hopping (SPH) model was applicable at the temperature above Θ D/2 (Θ D: the Debye temperature), and the electrical conduction was due to the non-adiabatic SPH of electrons between vanadium ions for all glasses. The hopping carrier mobility and carrier density were determined at different temperatures. At temperatures lower than Θ D/2, a T−1/4 dependence of the conductivity was found, which can be described by the variable-range hopping (VRH) conduction mechanism. The density of states at (or near) the Fermi level, N(EF), was found from Mott parameters analysis, which was a function of V2O5 content. All the semiconducting glass compositions exhibited a crossover from VRH to SPH conduction at a characteristic temperature

Calculation of the thermodynamic properties of B2 AlRE (RE=Sc, Y, La, Ce Lu)

Abstract: First-principles calculations for the total energy and elastic constants of the B2-type AlRE (RE=Sc, Y, lanthanide) have been performed at T =0 K by using the projector augmented wave (PAW) method within the generalized gradient approximation (GGA). The Debye temperatures, Gruneisen constants, the temperature dependences of the Gibbs free energy, coefficients of thermal expansion, heat capacities are obtained for the B2-AlRE within the Debye–Gruneisen model. The activation energy of self-diffusion, Poisson's ratio, Debye sound velocities are also evaluated for the B2-AlRE in the present work.

Temperature dependence of the pyroelectric coefficient and the spontaneous polarization of AlN

Abstract: Using the Debye model and existing experimental data of the pyroelectric coefficient of AlN, the temperature dependence of the pyroelectric coefficient as well as the spontaneous polarization of AlN is calculated over a wide temperature range from 0to1000K. The pyroelectric coefficient is proportional to T3 at low temperature and increases acutely from 0 to around 400K, and then increases gently from 400to1000K. It makes AlN uniquely suitable for application in high temperature pyroelectric sensors. The spontaneous polarization of AlN changes a little from 0to1000K, which indicates that the features of III-nitrides based devices will hardly be degraded by the change of the spontaneous polarization.

First-principles study of the relations between the elastic constants, phonon dispersion curves, and melting temperatures of bcc Ta at pressures up to 1000 GPa

Abstract: The possibility to relate analytically elastic shear modulus to melting temperature in the framework of Lindemann melting criterion is investigated here in the particular case of the body-centered cubic phase of tantalum, which is identified as a problematic case. Equation of state, elastic constants, and full phonon dispersion curves (PDCs) are first gathered for a wide pressure range using density functional theory and its perturbation within the generalized gradient approximation. A global fair agreement is found with previous experimental studies. Anomalies in PDCs tend to disappear with compression. Various equivalent Debye temperatures ${\ensuremath{\theta}}_{\mathrm{D}}(n)$ are then deduced and compared for increasing compression. The initial Debye model for atomic vibration is found to stand well above $120\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Under this pressure a possibly significant difference up to 10% is found between elastic Debye temperature ${\ensuremath{\theta}}_{\mathrm{D}}(\ensuremath{-}3)$ and ${\ensuremath{\theta}}_{\mathrm{D}}(\ensuremath{-}2)$ required in Lindemann melting criterion. As for all the theoretical melting curves proposed in the past, the one found here using ${\ensuremath{\theta}}_{\mathrm{D}}(\ensuremath{-}2,V)$ completely overpasses the melting curve established by static measurements in diamond anvil cells, but agrees well with the shock melting experiment available. This fact is extensively discussed in terms of evolution of PDCs and explaining hypotheses to be tested in the future are proposed.

Basic moments of phonon density of states spectra and characteristic phonon temperatures of group IV, III–V, and II–VI materials

Abstract: We have redigitized a large variety of phonon density of states (PDOS) spectra, that have been published by diferent researchers for group IV (diamond, 3C-SiC, Si, and Ge), III–V (BN, BP, BAs, BSb, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb), and II–VI materials (ZnO, ZnS, ZnSe, ZnTe, CdS, and CdTe), including calculations of their moments, ⟨en⟩, of orders n=−1, 1, 2, and 4. Notwithstanding the obvious differences in concrete shapes of spectra presented for one and the same material by different authors, the respective magnitudes of estimated moments have been found in most cases to be nearly the same (to within uncertainties of some few percent). For most materials under study, the average phonon temperatures of the lower and upper sections of PDOS spectra, ΘL and ΘU, are found to be by factors of order 0.6 lower or 1.4 higher, respectively, than the average phonon temperature, ΘP, of the total PDOS spectra. The estimated high-temperature limits of Debye temperatures, ΘD(∞), are found to be significantly higher (by factors of order 1.4) than ΘP, implying an order-of-magnitude equality, ΘD(∞)≈ΘU (within differences not exceeding an order of ±10%, for all materials under study). The phonon temperatures, Θg, that are effective in controlling the observable temperature dependences of fundamental energy gaps, Eg(T), are found to be usually of the same order as the respective average phonon temperatures, Θg≈ΘP. The existing differences between these two qualitatively different types of characteristic phonon temperatures are seen to be limited, for diamond, 3C-SiC, Si, Ge, AlN, GaN, GaP, GaAs, GaSb, InP, InSb, ZnS, ZnSe, ZnTe, and CdTe, to an order of ±12%. We design an exemplary way for precalculating harmonic parts of isochoric heat capacities on the basis of the estimated quadruplets of PDOS spectra moments. This novel calculation scheme is exemplified for silicon and germanium.

Phase transition and elastic constants of CaO from first-principle calculations

Abstract: The structural B1-B2 phase transition of CaO and the elastic properties of the B1 phase of CaO are investigated by ab initio plane-wave pseudopotential density functional theory method. The dependences of the elastic constants c(ij), the aggregate elastic modulus B-s and G, the elastic anisotropic parameter A, and the Debye temperature Theta(D) on pressure are successfully discussed. From the usual condition of equal enthalpies, we find that the structural B1-B2 phase transition of CaO occurs at 62.8GPa, consistent with the experimental value 61 GPa. From our elastic constants of CaO under pressure, we find that the B1-B2 structural transformation occurs at about 178 GPa by the elastic instability, which is far higher than the transition pressure 62.8 GPa. (c) 2006 Elsevier B.V. All rights reserved.

Correlation between mechanical, thermal and electronic properties in Zr–Ni, Cu amorphous alloys

Abstract: We show that mechanical properties (stiffness and hardness) of Zr–Ni, Cu amorphous alloys increase linearly with Ni, Cu content over a wide composition range (22 ≤ x Ni,Cu ≤ 65 at%). This correlates with the observed increase in the Debye temperatures and densities with x and shows that the strength of interatomic bonding increases with x in these alloys. Accordingly, the thermal stability (e.g. the crystallization and glass transition temperatures) of these alloys also increases with x. Since the electronic density of states at the Fermi level decreases linearly with x within the same x-range, a very simple relationship exists between the electronic structure and mechanical and thermal properties. We also deduce the mechanical properties of hypothetic amorphous Zr and briefly discuss the possibility of its preparation.