Application of Ultrasonic Measurement on Small‐Sized Ceramic Sample

The longitudinal and transverse wave velocities of polycrystalline MgO were successfully measured simultaneously and five types of elastic moduli, the Debye temperature, and the Grüneisen parameter were evaluated as functions of temperature from 298 to 1764 K. An aluminum foil was used as a couplant between the MgO specimen and waveguide. Both acoustic waves were successfully propagated into the specimen through the solid and liquid aluminum layers. In particular, no influence of melted aluminum foil on the acoustic wave velocities was observed. With increasing temperature, the Young's, shear, and bulk moduli decreased monotonically and the Grüneisen parameter and Poisson's ratio increased monotonically, which indicated a decrease in the ionic bonding strength. The Debye temperature decreased almost linearly with increasing temperature, which showed the effect of thermal expansion. Although previous investigators concluded that the Grüneisen parameter was almost independent of temperature, our data showed an obvious gradual increase in this parameter with temperature. In addition, our results implied the possibility of an abrupt increase in the Grüneisen parameter at temperatures less than 400 K. The slope of the Lamé parameter–temperature plot was almost zero over the entire investigated temperature range. On the basis of three‐dimensional volume‐nonpreserving shear, this behavior was interpreted as being caused by interlocking associated with the network of short Mg–O bonds.     

The relationship between the Grüneisen and other thermodynamic parameters and intermolecular forces in polymers

A simple relationship between the Grüneisen parameter and Beyer's non-linearity parameter in the case of polymers has been obtained in terms of Mie potential parameters from the pressure dependence of bulk modulus. Using this simple model, it is shown that Beyer's parameter of non-linearity, Carnevale and Litovitz's acoustical parameter, Moelwyn-Hughes parameter and the Grüneisen parameter are all related to each other and express the same physical quantity for polymers. The contribution of interchain vibrations to the total specific heat and the specific heat ratio has been found from the Grüneisen parameter in case of twelve polymers. It is proposed that the dependence of the isobaric specific heat of polymeric materials on the interchain forces is stronger than of the isochoric specific heat.     

On the presence of states with a negative Grüneisen parameter in overdriven explosion products

Experimental data on the properties of explosion products in overdriven detonation for 50/50 TNT/RDX and PBX-9502 explosives indicate conditions characterized by a negative Grüneisen parameter. Given a possible relationship of this anomaly with the properties of explosion product components, available experimental data for nitrogen, carbon monoxide, carbon dioxide, and water are analyzed. The first three of these (carbon dioxide specifically) are capable of manifesting a negative Grüneisen parameter at pressures and temperatures characteristic of the anomaly of overdriven explosion products.     

A quantum mechanical formulation of electron transport induced wind forces in metallic single-walled carbon nanotubes

In this paper the forces induced on the atoms of the lattice due an electric current (electron transport induced wind forces) are calculated based on quantum mechanics. These forces are calculated in metallic single-walled armchair carbon nanotubes (SWCNTs) from the momentum transfer between the charge carriers and the lattice in a quantum mechanical framework. Energy and phonon dispersion relations are the main input for the formulation proposed. Scattering of electrons with longitudinal acoustic and longitudinal optical phonons are considered to be the only scattering mechanisms that are responsible for the momentum exchange. The current-voltage characteristics are also predicted using the same framework and show good agreement with experimental data. The study shows that using the constant effective charge number for SWCNT is inaccurate at higher electric field forces due to saturation of the lowest energy subband. The thermal effect on the effective charge number appears to be very important, due to the increasing scattering probabilities at higher temperatures.     

Theoretical Study on the Mechanism of Anisotropic Thermal Properties of Ti2AlC and Cr2AlC

Temperature dependences of thermal and elastic properties, such as the Grüneisen parameters, thermal expansion, bulk modulus, and heat capacity of Ti₂AlC and Cr₂AlC, are studied by combining first‐principles method and lattice dynamic calculation based on the quasi‐harmonic model. Experimental thermal expansion coefficient is also measured for comparison. Thermal expansion coefficients of Ti₂AlC and Cr₂AlC show different trends: Ti₂AlC exhibits anisotropic thermal expansion while Cr₂AlC shows generally isotropic character. The mechanism is explored by investigating the isotropy or anisotropy of Grüneisen parameters (phonon anharmonicity and thermal pressure) and elastic stiffness (response to thermal pressure) of Ti₂AlC and Cr₂AlC. In addition, the calculated bulk modulus of Cr₂AlC is higher at ambient temperature but decreases faster than the value of Ti₂AlC as temperature increasing.     

Molecular dynamics simulations of thermal transport in porous nanotube network structures

Carbon nanotube based 3D nanostructures have shown a lot of promise towards designing next generation of multi-functional systems, such as nano-electronic devices. Motivated by their recent successful experimental synthesis as well as characterization, and realizing that thermal dissipation is an important concern in proposed devices because of ever-increasing power density, we have investigated the phononic thermal transport behavior in 3D porous nanotube network structures using reverse non-equilibrium molecular dynamics simulations. Based on our study, the length scale associated with the distance between nanotube junctions emerges as the most dominating parameter that governs phonon scattering (hence the characteristic mean free path) and the heat flow in these nanostructures at molecular length scales. However, because of their spatial inhomogeneity, we show that the aerial density of carbon nanotubes (normal to heat flow) is also of critical importance in determining their system-level thermal conductivity. Based on our findings, we postulate that both parameters should be considered while designing nano-devices where thermal management is relevant.     

Anharmonicity of thermal vibrations and grüneisen parameter of chalcogenide glasses in the Ge-As-S system within the approximation of the free volume concept

The Grüneisen parameter and the ultimate strain of “bond breaking” at the ultimate elongation of interparticle bonds have been calculated. The Grüneisen parameters determined from different formulas are compared. It is demonstrated that the volume deformation required to form a microvoid in the glass network only slightly depends on glass composition. The microvoid volume is determined primarily by the ultimate strain of the interatomic bond. In chalcogenide glasses, the ultimate strain of the interatomic bond is governed by the limiting displacement of a bridging chalcogen atom, which varies in inverse proportion to the lattice Grüneisen parameter. The role of the Grüneisen parameter in the fluctuation free volume and soft configuration models is discussed.     

Molecular phonon couplers at carbon nanotube/substrate interface to enhance interfacial thermal transport

A novel assembling process of incorporating carbon nanotubes as thermal interface materials for heat dissipation has been developed by synthesizing vertically aligned carbon nanotubes on a copper substrate and chemically bonding the carbon nanotubes to a silicon surface. The assembling process and the copper/carbon nanotubes/silicon structure are compatible with current flip-chip technique. The carbon nanotubes are covalently bonded to the silicon surface via a thin but effective bridging layer as a “molecular phonon coupler” at the CNT-silicon interface to mitigate phonon scattering. Experimental results indicate that such an interface modification improves the effective thermal diffusivity of the carbon nanotube-mediated thermal interface by an order of magnitude and conductivity by almost two orders of magnitude. The interfacial adhesion is dramatically enhanced as well, which is significant for reliability improvement of the thermal interface materials.     

Energetics of single-wall carbon nanotubes as revealed by calorimetry and neutron scattering

Bundles of (10,10) single-wall carbon nanotubes (SWCNTs) have been studied by high-temperature oxidation calorimetry and inelastic neutron scattering to obtain standard formation enthalpies and entropies at 298 K. SWCNTs are found to be only moderately less stable than graphite, and are significantly more stable than their fullerene counterparts. They are 7 kJ mol⁻¹ metastable in terms of enthalpy relative to graphite, and just 5 kJ mol⁻¹ less stable than diamond. Despite striking differences in vibrational dynamics of carbon atoms in SWCNTs and graphite, their thermodynamic properties at room and higher temperatures are dominated by the same set of high energy vibrations, reflected in very similar vibrational entropies. However, the energetics of SWCNTs are governed by the diameter-dependent enthalpic contributions, but not the specifics of phonon density of states.     

First‐principles investigations on elevated temperature elastic and thermodynamic properties of ZrB2 and HfB2

As promising candidates for ultrahigh temperature applications, high‐temperature properties, which are quite rare and fragmentary, have great significance to ZrB₂ and HfB₂. In this work, thermodynamic and mechanical properties of ZrB₂ and HfB₂ from 0 K to 2000 K were investigated by a combination of first principles calculations and quasi‐harmonic approximations. The ground‐state properties, including lattice parameters, elastic constants, phonon dispersion, and mode‐Grüneisen parameters are calculated. The theoretical thermal expansion, elastic and thermodynamic properties at elevated temperatures show good agreement with experiments. By discussing Grüneisen parameters anisotropy, the mechanism for the thermal expansion anisotropy of ZrB₂ and HfB₂ is uncovered. The influence of direction‐dependent sound velocities on the anisotropy of thermal conductivity is also discussed.     

Elucidating the effect of heating induced structural change on electrical and thermal property improvement of single wall carbon nanotube

We have clarified that the electrical and thermal properties of single-walled carbon nanotubes (SWCNTs) are improved by multiple structural changes (wall number, diameter, and crystallinity) induced by high temperature treatment. Focusing on the relationship between structural change and electrical and thermal properties, high-purity SWCNTs were fabricated using the water-assisted CVD method and treated at high temperatures (1500–2000 °C) in an argon atmosphere. We showed that the electrical and thermal properties of the SWCNTs were improved by ∼2.9 and 3.0, respectively, which required lower treatment temperatures than for multi-walled CNTs (MWCNTs). In addition to the crystallinity improvement, the wall number and diameter increased with treatment temperature. When compared to as-grown SWCNTs of similar wall number and diameter, the heat treated SWCNTs exhibited higher electrical and thermal properties, which suggested that the property improvements could be attributed to not only to the wall number and diameter but also to the improvement in crystallinity.     

Theoretical exploration of the abnormal trend in lattice thermal conductivity for monosilicates RE2SiO5 (RE = Dy,Ho, Er,Tm, Yb and Lu)

Rare earth monosilicates RE₂SiO₅ have been considered as promising environmental barrier coating materials for silicon-based ceramics due to their low thermal conductivity and good high-temperature stability. We herein performed a systematic study of the lattice dynamics for RE₂SiO₅ (RE?=?Dy, Ho, Er, Tm, Yb and Lu) using first-principles calculations. The loosely bound rare earth atoms provide large Grüneisen parameters and low phonon group velocities, both of which determine the low thermal conductivity. Theoretical exploration predicts an anomalous increase of lattice thermal conductivity with increment of RE atomic number and the mechanism is explained by the stronger atomic bonding and weaker phonon anharmonicity. Although incorporating heavier atoms has long been considered as an effective way to reduce lattice thermal conductivity, this work addresses the importance of bonding heterogeneity and anharmonicity rather than atomic mass variation. This theoretical study suggests an alternative approach towards the design of new thermal insulating materials.     

Promoted electron transport and sustained phonon transport by DNA down to 10 K

This work reports on a pioneering study of the electron transport in nanometer-thick Ir film supported by a DNA fiber, and the phonon transport sustained by the DNA itself. By evaluating the electrical resistivity (ρ_e)∼temperature relation based on the Block-Grüneisen theory, we find the Ir film has weak phonon softening indicated by 7–15% Debye temperature reduction. The Ir film's intrinsic ρ_e is promoted by DNA electron thermal hopping and quantum tunneling, and is identical to that of bulk Ir. Although the nanocrystalline structure in ultrathin metallic films intends to give a higher Lorenz number since it reduces the electrical conductivity more than thermal conductivity, the DNA-promoted electron transport in the Ir film preserves a Lorenz number close to that of bulk crystalline Ir. By defining a new physical parameter entitled “thermal reffusivity”, the residual phonon thermal resistivity of DNA is identified and evaluated for the first time. The thermal reffusivity concept can be widely used to predict the phonon thermal transport potential of defect-free materials. We predict that the thermal diffusivity of defect-free DNA fiber could be 36–61% higher than the samples studied herein. The structure domain size for phonon diffusion/scattering is determined as 0.8 nm in DNA.     

Thermal and mechanical properties of CeO2

The thermal and mechanical properties of cerium dioxide (CeO₂) were assessed using a range of experimental techniques. The oxygen potential of CeO₂ was measured by the thermogravimetric technique, and a numerical fit for the oxygen potential of CeO₂ is derived based on defect chemistry. Mechanical properties of CeO₂ were obtained using sound velocity measurement, resonant ultrasound spectroscopy and nanoindentation. The obtained mechanical properties of CeO₂ are then used to evaluate the Debye temperature and Grüneisen constant. The heat capacity and thermal conductivity of CeO₂ were also calculated using the Debye temperature and the Grüneisen constant. Finally, the thermal conductivity was calculated based upon laser flash analysis measurements performed on pellets fabricated using a range of feedstock purities to resolve discrepancies in the existing literature.     

Thermal expansion and Grüneisen parameter of polyethylene between 5 and 320 K

The linear thermal expansion coefficient, α, of polyethylene with crystallinity X=0.44, 0.62, 0.77 and 0.98 has been measured between 5 and 320 K with an experimental accuracy of about 5% above 15 K and 10% below 15 K using an interferometric dilatometer system. In the temperature range 40 to 100 K the thermal expansion is found to be independent of crystallinity, whereas at lower as well as higher temperatures, α increases with increasing noncrystallinity. By linear extrapolation the thermal expansion coefficient of the noncrystalline phase can be estimated. The influence of the phonon spectrum on the thermal expansion is discussed in terms of the Grüneisen parameter. For completely crystalline polyethylene, a bulk-Grüneisen parameter of $\text{γ}{}_{\mathrm{<mtext>b</mtext>}}\text{?2}$ is found below 20 K. This value is smaller than expected from theories for polymer crystals. For noncrystalline polyethylene, γ_b is constant ($\text{γ}{}_{\mathrm{<mtext>b</mtext>}}\text{?1.6}$) between 30 and 100 K, showing a rapid increase at lower temperatures with $\text{γ}{}_{\mathrm{<mtext>b</mtext>}}\text{?12.5}$ at 5 K. This high value for γ_b can be related to excess modes in the vibration spectrum of the noncrystalline phase, which have previously been found to contribute also to the heat capacity at these temperatures.     

Spectroscopic evaluation of individually dispersed single-walled carbon nanotubes

We prepared the individually dispersed solutions of various single-walled carbon nanotubes (SWCNTs) synthesized by electric arc discharge, catalytic chemical vapor deposition, and high-pressure carbon monoxide process using the biocompatible chitosan derivative, chitosan hydroxyphenylacetamide, and evaluated their dispersion efficiencies. We propose a new spectroscopic parameter for quantitative evaluation of the individually dispersed SWCNTs and present the optimum SWCNT-to-dispersant ratio for high-quality dispersion of each SWCNT type based on the proposed parameter.     

Fabrication of single-wall carbon nanotubes within the channels of a mesoporous material by catalyst-supported chemical vapor deposition

Diameter-controlled single-wall carbon nanotubes (SWCNTs) have been synthesized using Co, Fe/Co and Rh/Pd alloy nanoparticles trapped within the one-dimensional channels of a mesoporous materials (Folded Sheets Mesoporous material: FSM-16) by catalyst-supported chemical vapor deposition (CCVD) using ethanol as carbon source at 973-1173 K. The SWCNTs synthesized are characterized by transmission electron microscopy, Raman spectroscopy and photoluminescence spectroscopy. The yield, diameter distribution and quality of the SWCNTs strongly depend on the reaction temperature during CCVD. The product synthesized at 1173 K contains only SWCNTs, in marked contrast to those synthesized at lower temperatures. As the reaction temperature decreases, the relative abundance of multi-wall carbon nanotubes against SWCNTs significantly increases, whereas the mean diameter of SWCNTs increases as reaction temperature increases. The results show that a careful control of the reaction temperature is crucial to fabricate diameter-controlled SWCNTs from the channels of FSM-16.     

Thermal behavior of mullite between 4 K and 1320 K

The evolution of metric parameters of 2:1 and 3:2 mullites have been measured between 4 K and 1320 K using neutron and X‐ray powder diffraction. Negative thermal expansion was observed at low temperature for the a‐cell parameter and consequently for the cell‐volume, which is more pronounced for 2:1 mullite than those for 3:2 mullite. Each parameter is simulated using Grüneisen first‐order approximation for the zero pressure equation of state at 0 K, where the vibrational energy was calculated using microscopic approach. While the b‐ and c‐cell parameters require only one Debye term, a second Debye spectrum with negative Grüneisen parameter was required to fit the a‐cell parameter as well as the cell volume. At 4 K, 300 K and 1320 K the model, respectively, calculates the volume thermal expansion coefficients of 0.09x10^?6 K^?1, 9x10^?6 K^?1, and 17.3x10^?6 K^?1 for 2:1 mullite, and 0.09x10^?6 K^?1, 8.7x10^?6 K^?1, and 17.3x10^?6 K^?1 for 3:2 mullite. Temperature‐dependent Raman spectra and phonon density of states hint for the possible microscopic sources of the cell contraction at low temperature. A simple polynomial approach is presented to calculate the elastic stiffness coefficients of the 3:2 mullite, which are not available from experiments.     

Thermodynamic property of ternary compound MgCaSi: A study from ab initio Debye-Grüneisen model

The thermodynamic properties of MgCaSi and its mother phase Ca₂Si are comparatively investigated from ab initio calculations and quasi-harmonic Debye-Grüneisen model. At 0 K, MgCaSi is more thermodynamically stable. Under high temperature, the advantage of higher thermodynamically stability of MgCaSi is reduced, originating from the less negative entropy contribution because the thermodynamic entropy of MgCaSi increases more slowly with temperature and the entropy values are slightly smaller. With increasing temperature, the anti-softening ability for MgCaSi is slightly smaller due to the slightly faster decrease trend of bulk modulus than that of Ca₂Si, although the bulk modulus of MgCaSi is higher in the whole temperature range considered. The thermal expansion behaviors of both MgCaSi and Ca₂Si exhibit similar increase trend, although thermal expansion coefficient of MgCaSi is slightly lower and the increases is slightly slower at lower temperature. The isochoric heat capacity and isobaric heat capacity of MgCaSi and Ca₂Si rise nonlinearly with temperature, and both are close to the Dulong–Petit limit at high temperature due to the negligibly small electronic contribution. The Debye temperature of both phases decrease with increasing temperature, and the downtrend for MgCaSi is slightly faster. However, MgCaSi possess slightly higher Debye temperature, implying the stronger chemical bonds and higher thermal conductivity than the mother phase Ca₂Si. The Grüneisen parameter of MgCaSi and Ca₂Si increase slightly with temperature, the values of MgCaSi are slightly larger. The investigation of electronic structures shows that with substitution of partial Ca by Mg in Ca₂Si, the stronger MgSi, MgCa and SiSi covalent bonds are formed, and plays a very significant role for the structural stability and mechanical properties.