High Debye Temperature Research Articles

Thermal conductance of metal-diamond interfaces at high pressure.

The thermal conductance of interfaces between metals and diamond, which has a comparatively high Debye temperature, is often greater than can be accounted for by two-phonon processes. The high pressures achievable in a diamond anvil cell (DAC) can significantly extend the metal phonon density of states to higher frequencies, and can also suppress extrinsic effects by greatly stiffening interface bonding. Here we report time-domain thermoreflectance measurements of metal-diamond interface thermal conductance up to 50 GPa in the DAC for Pb, Au0.95Pd0.05, Pt and Al films deposited on type 1A natural [100] and type 2A synthetic [110] diamond anvils. In all cases, the thermal conductances increase weakly or saturate to similar values at high pressure. Our results suggest that anharmonic conductance at metal-diamond interfaces is controlled by partial transmission processes, where a diamond phonon that inelastically scatters at the interface absorbs or emits a metal phonon.

High-pressure phase stability, mechanical properties and bonding characteristics of Ti4GeC3 compound

Using first-principles calculation, we have studied the influence of pressure on the phase stability, elastic, mechanical properties and bonding characteristics of bulk form ternary-layered carbide Ti4GeC3 compound. Present results indicate that the thermodynamic stability and elastic stability can be maintained up to the pressure of 500 GPa for the bulk Ti4GeC3. The transition from the brittle to ductile phase occurs at about 60 GPa. It is predicted that Ti4GeC3 possesses good thermal conductivity due to a relative high Debye temperature (731.6 K). Moreover, a metallic bonding character is found in Ti4GeC3 compound by analysis of electronic structures. Our results show that this compound is a material with excellent mechanical, ceramic and elastic isotropic properties for future applications.

The effect of macroscopic polarization on intrinsic and extrinsic thermal conductivities of AlN

The effect of macroscopic polarization on thermal conductivity of bulk wurtzite AlN has been theoretically investigated. Our results show that macroscopic polarization modifies the phonon group velocity, Debye frequency and Debye temperature of the AlN. Using revised phonon velocity and Debye temperature, various phonon scattering rates and combined scattering rate are calculated as functions of the phonon frequency at room temperature. The intrinsic and extrinsic thermal conductivities of AlN have been estimated using these modified parameters. The theoretical analysis shows that up to a certain temperature the polarization effect acts as negative effect and reduces the intrinsic and extrinsic thermal conductivities. However, after this temperature both thermal conductivities are significantly enhanced. High phonon velocity and Debye temperature are the reason of this enhancement which happens due to the polarization effect. The revised thermal conductivities at room temperature are found to be increased by more than 20% in AlN due to macroscopic polarization phenomenon. The method we have developed can be taken into account during the simulation of heat transport in optoelectronic nitride devices to minimize the self heating processes.

Mode dependent lattice thermal conductivity of single layer graphene

Molecular dynamics simulation is performed to extract the phonon dispersion and phonon lifetime of single layer graphene. The mode dependent thermal conductivity is calculated from the phonon kinetic theory. The predicted thermal conductivity at room temperature exhibits important quantum effects due to the high Debye temperature of graphene. But the quantum effects are reduced significantly when the simulated temperature is as high as 1000 K. Our calculations show that out-of-plane modes contribute about 41.1% to the total thermal conductivity at room temperature. The relative contribution of out-of-plane modes has a little decrease with the increase of temperature. Contact with substrate can reduce both the total thermal conductivity of graphene and the relative contribution of out-of-plane modes, in agreement with previous experiments and theories. Increasing the coupling strength between graphene and substrate can further reduce the relative contribution of out-of-plane modes. The present investigations also show that the relative contribution of different mode phonons is not sensitive to the grain size of graphene. The obtained phonon relaxation time provides useful insight for understanding the phonon mean free path and the size effects in graphene.

High pressure phase stability, mechanical and optical properties of Ti4SiC3 compound: ab initio study

High pressure phase stability and physical properties of bulk ternary-layered carbide Ti4SiC3 compound are studied through ab initio calculations. Present results indicate that the thermodynamic stability and elastic stability can be maintained up to about 200 GPa for this compound. And it is predicted to be a quasi-brittle ceramic and possesses good thermal conductivity due to a relative high Debye temperature (849.7 K). Moreover, the analysis of optical properties suggests that Ti4SiC3can be used as good coating materials in the ultraviolet region. These findings identify the compound of Ti4SiC3 as a material with excellent mechanical, ceramic and elastic isotropic properties for future applications.

Neutron diffraction study of Li4Ti5O12 at low temperatures

The crystal structure of the “zero-strain” positive electrode material Li4Ti5O12 was characterized by neutron powder diffraction in the temperature range 3.4 K–300 K. No phase transition was detected, and the thermal evolution of lattice parameters has been evaluated by the 2nd order Grüneisen approximation using the Debye formalism for internal energy and intrinsic anharmonicity contributions. A relatively high Debye temperature θD = 689 ± 71 K was determined. The thermal behavior of cation-anion bond lengths in octahedral and tetrahedral environments is discussed. The lithium diffusion pathway in Li4Ti5O12 was discussed on the basis of bond-valence modeling.

Prediction and measurement of thermal transport across interfaces between semiconductor and adjacent layers

The thermal boundary conductance between multilayer structures including Al film, semiconductors with high Debye temperatures (GaN, AlN, Si, diamond) and dielectric substrates (sapphire) has been measured using a two-color femtosecond laser pump-probe system (a variation of transient time-domain thermoreflectance, TDTR). The thermal boundary conductance for the combinations of semiconductors and dielectrics falls within a relatively narrow range, 10–20 MW m−2 K−1, at room temperature. The measured thermal boundary conductance between Al film and semiconductor or dielectric substrates is one order of magnitude larger than that between semiconductor and dielectric substrates. A modified diffuse mismatch model (DMM) is used to interpret the data and extract the phonon transmissivity at the interface. The predicted results of the DMM corrected by attenuation constant agree well with the experimental values. Over a wide phonon velocity, both the measured and predicted results decrease with the increasing average phonon velocity. Both the vibration mismatch and changes in the localized phonon transport near the interface contribute to the reduction in thermal boundary conductance. Other scattering mechanisms are discussed which may explain the failure of the DMM at room temperature.

Structural Stability and Elastic and Electronic Properties of W2X (X = B, C, N): First-Principles Investigations

The structural stability, elastic property, and mechanical stability of W2X (X = B, C, N) are investigated using first-principles calculations with density functional theory. The calculations show that Re2N-type W2N is both thermodynamically and elastically stable. The calculated elastic properties indicate that W2X compounds are potential low-compressibility materials. The large elastic modulus, low Poisson’s ratios, small B/G ratios, and high Debye temperatures of thecompounds indicate that they are potential hard materials. The analysis of the density of states and electronic localization function provides further understanding of the elastic and chemical properties of these compounds.

Average and Local Structural Origins of the Optical Properties of the Nitride Phosphor La3–xCexSi6N11 (0 < x ≤ 3)

Structural intricacies of the orange-red nitride phosphor system La(3-x)Ce(x)Si6N11 (0 < x ≤ 3) have been elucidated using a combination of state-of-the art tools, in order to understand the origins of the exceptional optical properties of this important solid-state lighting material. In addition, the optical properties of the end-member (x = 3) compound, Ce3Si6N11, are described for the first time. A combination of synchrotron powder X-ray diffraction and neutron scattering is employed to establish site preferences and the rigid nature of the structure, which is characterized by a high Debye temperature. The high Debye temperature is also corroborated from ab initio electronic structure calculations. Solid-state (29)Si nuclear magnetic resonance, including paramagnetic shifts of (29)Si spectra, are employed in conjunction with low-temperature electron spin resonance studies to probes of the local environments of Ce ions. Detailed wavelength-, time-, and temperature-dependent luminescence properties of the solid solution are presented. Temperature-dependent quantum yield measurements demonstrate the remarkable thermal robustness of luminescence of La2.82Ce0.18Si6N11, which shows little sign of thermal quenching, even at temperatures as high as 500 K. This robustness is attributed to the highly rigid lattice. Luminescence decay measurements indicate very short decay times (close to 40 ns). The fast decay is suggested to prevent strong self-quenching of luminescence, allowing even the end-member compound Ce3Si6N11 to display bright luminescence.

A heuristic approach to the detection of solar neutrinos

In this paper we consider the coherent scattering of the solar neutrino flux off target matter in a terrestrial detector. We propose a heuristic approach that targets the apparent abnormal neutrinos scattering on high Debye temperature monocrystals. The possibility of observing the diurnal effect of solar neutrinos using the presumed coherent scattering on high Debye temperature monocrystals, such as sapphire, is presented and discussed. A ‘Cavendish-type’ torsion balance is used for the measurement of the presumed neutrino-induced torques. An effect correlated with the direction of the sun was observed.

Theoretical calculations of the linear, nonlinear elastic constants and the Grüneisen parameters of cubic perovskites (Sr, Ba, Pb)TiO3

The second-order, third-order and fourth-order elastic constants of SrTiO3, BaTiO3 and PbTiO3 cubic perovskites are calculated by using the first-principles total-energy method combined with the means of homogeneous deformation. The predicted results provide reasonable agreement with the reported experimental data, other theoretical results and Cauchy relations. The curved surfaces of Grüneisen parameters at various acoustic propagation modes are plotted by using predicted results. In addition, the Grüneisen parameters at low temperature limit, high temperature limit and Debye temperature are also calculated. Since nonlinear elastic constants are very difficult to be measured in experiments, the methods and results presented here provide a valuable guidance for experiments and the investigation of anharmonic properties for perovskite materials.

Metallic conduction and large electron-phonon-impurity interference effect in single TiSi nanowires

We report on the first electrical characterizations of single-crystalline TiSi nanowires (NWs) synthesized by chemical vapor deposition reactions. By utilizing the focused-ion-beam-induced deposition technique, we have delicately made four-probe contacts onto individual NWs. The NW resistivities have been measured between 2 and 300 K, which reveal overall metallic conduction with small residual resistivity ratios in the NWs. Surprisingly, we find that the effect due to the interference processes between the elastic electron scattering and the electron-phonon scattering largely dominates over the usual Boltzmann transport even at room temperature. Such prominent electron-phonon-impurity interference effect is ascribed to the presence of large amounts of disorder and high Debye temperatures in TiSi NWs.

Thermal properties of high-k Hf1−xSixO2

Classical atomistic simulations based on the lattice dynamics theory and the Born core-shell model are performed to systematically study the crystal structure and thermal properties of high-k Hf1−xSixO2. The coefficients of thermal expansion, specific heat, Grüneisen parameters, phonon densities of states and Debye temperatures are calculated at different temperatures and for different Si-doping concentrations. With the increase of the Si-doping concentration, the lattice constant decreases. At the same time, both the coefficient of thermal expansion and the specific heat at a constant volume of Hf1−xSixO2 also decreases. The Grüneisen parameter is about 0.95 at temperatures less than 100 K. Compared with Si-doped HfO2, pure HfO2 has a higher Debye temperature when the temperature is less than 25 K, while it has lower Debye temperature when the temperature is higher than 50 K. Some simulation results fit well with the experimental data. We expect that our results will be helpful for understanding the local lattice structure and thermal properties of Hf1−xSixO2.

Specific heat of ferroelectric Pb(Zr1−xTix)O3 ceramics across the morphotropic phase boundary

Ferroelectric ceramic materials have a wide range of applications because of their piezoelectric and pyroelectric properties. One of their most important physical properties is the specific heat. In this study, the specific heats of a series of Pb(Zr1−xTix)O3 (x = 0.41 to 0.48) (PZT) compositions in the vicinity of and across the morphotropic phase boundary (MPB) were measured and analyzed. The temperature range was from 1.8 to 300 K. It is believed that these are the lowest temperature specific heat measurements ever made on PZT. Absolute entropy and enthalpy were computed from the specific heat data. Differences between the specific heats of the different compositions were very small. However, the calculated Debye temperatures were slightly different. The composition with the highest Debye temperature (x = 0.47, ΘD = 274 K) was within the morphotropic phase boundary. The results are useful in computing design parameters for technical devices.

DETECTION OF SOLAR NEUTRINOS BY COHERENT SCATTERING IN HIGH-DEBYE TEMPERATURE MONOCRYSTALS

The possibility of detecting solar neutrinos by coherent scattering in high-Debye temperature monocrystals such as sapphire is presented and discussed. Preliminary experimental results indicate that the solar neutrinos flux (0–430 keV) produces an observable torque of a high-sensitive torsion balance. Our experiments give a result for the diurnal force as predicted by Weber's theory of enhanced solar neutrinos coherent scattering.

Electronic, optical and lattice dynamic properties of the novel diamond-like semiconductorsLi2CdGeS4 andLi2CdSnS4

Li2CdGeS4 and Li2CdSnS4 are novel quaternary diamond-like semiconductors (DLSs) which have been synthesized recently.We present first-principles calculations of their electronic, optical and lattice dynamic propertieswith the plane-wave pseudopotential method. We have found an indirect band gap of 2.78 eV forLi2CdGeS4 and a direct bandgap of 2.50 eV for Li2CdSnS4. The serious stretching vibrations of the Ge/Sn–S and Li–S bonds may enhance theirphonon energies, and cause them to exhibit high heat capacities and Debye temperatures,which are promising for nonlinear optical applications. Compared with Cu-based DLSs, Liplays a key role in enlarging the band gaps and increasing the lattice phonon energies,which would increase the thermal conductivity accompanied by an increase of the opticaldamage threshold.

Specific heat and magnetic order of La0.2Ca0.8MnO3

Measurements of specific heat of the electron-doped La0.2Ca0.8MnO3 manganite are reported for 2&amp;lt;T&amp;lt;400 K. It was found that La0.2Ca0.8MnO3 exhibits an orbital ordering at TOO=204 K and a long-range antiferromagnetic order below TN=178 K. The entropy changes (ΔS) around both TOO[ΔS(TOO)≈0.38 J/mol K] and TN[ΔS(TN)≈3.58 J/mol K] were determined. The entropy change associated with the orbital ordering was found to be much smaller than that for charge ordered manganites. The low temperature specific heat indicates that La0.2Ca0.8MnO3 has rather high Debye temperature ΘD=685 K in comparison with existing results for electron-doped La1−xCaxMnO3 manganites. Moreover, the presence of a large 1/T2 hyperfine contribution and of a considerable linear term, unexpected for La0.2Ca0.8MnO3 with antiferromagnetic insulating ground state, was revealed. The possible relationship between the behavior of the heat capacity and the phase separation is discussed.

Rocking curve imaging for diamond radiator crystal selection

The combination of low atomic number, high crystal packing density, and very high Debye temperature makes diamond the best material for use as a bremsstrahlung radiator in the coherent bremsstrahlung (CB) process, a process that is uniquely suited for generating highly polarized high-energy photon beams for photonuclear experiments. The crystal quality of the diamond radiator has a vital effect on the polarization and other properties of the photon beam and the best large-area diamond monocrystals currently available, both natural and synthetic, contain many defects that can degrade their performance as CB radiators. The diamonds used for this study were synthetic type Ib samples produced through the HPHT process by the firm Element Six. They were examined using the double crystal rocking curve imaging method in a synchrotron X-ray beam. Dislocation densities were calculated from the measured rocking curve peak position maps in the way proposed by Ferrari et al. [1]. It is shown that dislocation is one major defect that affects the rocking curve width in local regions. The most significant contribution to the whole-crystal rocking curve width for thin crystals is the systematic variation of the peak position across its surface. This is interpreted in terms of a large-scale bending of the entire crystal. Data supporting this interpretation are presented, and possible explanations for the bending and methods for its mitigation are discussed.

First-Principles Study of the Electronic Structure, Optical Properties, and Lattice Dynamics of BC2N

First-principles calculations of the electronic, optical, and lattice dynamic properties of c-BC2N, w-BC2N, cp-BC2N, z-BC2N, and t-BC2N were performed with the density functional theory (DFT) plane-wave pseudopotential method. It is found that the difference in electronic structures and optical properties is arising from the different numbers and species of chemical bonds in the five phases. The vibration analysis shows three main frequency regions arising from different relative movements among the B, C, and N atoms for the five phases. The calculation demonstrates that z-BC2N and t-BC2N have more vibration states and own much higher vibration frequencies. The experimental Raman peak, well reproduced in z-BC2N and t-BC2N, has been assigned to be the serious relative translational movements of C atoms (C1 and C3 for z-BC2N; C2 and C3 for t-BC2N) along the z direction with the other atoms moving slightly. Though the five phases may not display excellent heat capacities in room temperature (about 30 J/mol/K), the high Debye temperatures, reaching about 1700 K, may lead to some research and application interests in the thermodynamic aspect.

Reducing thermal conductivity of thermoelectric materials by using a narrow wire geometry

The dependence of the thermal conductivity of narrow wires made from bismuth and covalently bonded materials on wire diameter was numerically calculated by considering contributions of mean free paths of carriers and phonons. The results suggest that a reduction in the thermal conductivity should be observable in a bismuth wire having a diameter of less than 1 μm sample. A reduction of nearly 20% in the temperature range of 150–300 K is expected due to the use of a narrow wire geometry. Such a geometry reduces the mobility and the thermal conductivity of the carriers, which is the dominant component, while the thermal conductivity due to phonons was dramatically reduced by using narrow wires at temperatures under 50 K due to the longer mean free path phonons. The thermal conductivity of materials with covalent bonding such as silicon was also estimated, and it is expected that the thermal conductivity of a silicon wire could be reduced due to the mean free path of phonons being longer than that of the carriers. The results suggest that it should be possible to enhance the figure of merit by reducing the thermal conductivity through using wire geometries having diameters of less than 100 nm in materials having low mobilities, high thermal conductivities, and high Debye temperatures.