Nonmetallic Solids Research Articles

Effects of Localized Interface Phonons on Heat Conductivity in Ingredient Heterogeneous Solids

Phonons are the primary heat carriers in non-metallic solids. In compositionally heterogeneous materials, the thermal properties are believed to be mainly governed by the disrupted phonon transport due to mass disorder and strain fluctuations, while the effects of compositional fluctuation induced local phonon states are usually ignored. Here, by scanning transmission electron microscopy electron energy loss spectroscopy and sophisticated calculations, we identify the vibrational properties of ingredient-dependent interface phonon modes in Al x Ga1 – x N and quantify their various contributions to the local interface thermal conductance. We demonstrate that atomic-scale compositional fluctuation has significant influence on the vibrational thermodynamic properties, highly affecting the mode ratio and vibrational amplitude of interface phonon modes and subsequently redistributing their modal contribution to the interface thermal conductance. Our work provides fundamental insights into understanding of local phonon-boundary interactions in nanoscale inhomogeneities, which reveal new opportunities for optimization of thermal properties via engineering ingredient distribution.

Phonon modes involved in triggering threshold phenomenon characterising sliding-velocity dependence of wearless nanofriction

ABSTRACT Wearless nanofriction between nonmetallic solids is investigated in terms of molecular dynamics (MD). The results of this study concern wearless-nanofrictional characteristics of the MD models in which the thermal-buffer zone is located far enough from the sliding interface so as not to disturb the heat generation mechanism. An enormous number of simulation runs and frequent validation of their results were carried out so that the pressure and the interfacial temperature, which directly affect the heat generation processes, were made common at different sliding velocities even under the above constraint. Sliding-velocity dependence of the ensemble-averaged frictional force density under the steady sliding condition is characterised by the threshold phenomenon. The dramatic increase in rate of the frictional-heat generation at sliding speeds above the threshold is triggered by resonant coherent excitation of certain phonon modes. Spatiotemporal patterns of such elastic waves hidden in random thermal motion of atoms near the sliding interface were clarified by the sampling and averaging. Features of these patterns are discussed in relation to the threshold characteristics of wearless nanofriction.

Определение теплопроводности кремния с детальным учетом кинетики взаимодействия фононов

The thorough study of the heat carriers --- quasiparticles --- phonons interaction resulted in a pioneering method for calculating the thermal conductivity of nonmetallic solids. As the interactions of phonons are much more complicated than those of usual atoms and molecules, it is necessary to take into account the presence of two types of phonons with different properties; the decay of one phonon into two or the fusion of two phonons into one as a result of interaction; the presence of two types of interaction of phonons, one of which is elastic, the other is inelastic (moreover, the type of interaction results from solving the energy and quasi-momentum conservation equations). The existing methods for determining thermal conductivity, which typically involve solving the Boltzmann transport equation, use the iteration method, whose parameter is the average time between successive phonon interactions, and the calculation results provide little information on all types of interactions. In this research, we developed a method of direct Monte Carlo simulation of phonon diffusion with strict account for their interaction owing to the energy and quasi-momentum conservation laws. Calculations of the thermal conductivity coefficient for pure silicon in the temperature range of 100---300 K showed good agreement with the experiment and calculations of other authors, and also made it possible to consider the phonon kinetics in detail

Charge State Effect of High Energy Ions on Material Modification in the Electronic Stopping Region

It has been observed that modifications of non-metallic solids such as sputtering and surface morphology are induced by electronic excitation under high-energy ion impact and that these modifications depend on the charge of incident ions (charge-state effect or incident-charge effect). A simple model is described, consisting of an approximation to the mean-charge-evolution by saturation curves and the charge-dependent electronic stopping power, for the evaluation of the relative yield (e.g., electronic sputtering yield) of the non-equilibrium charge incidence over that of the equilibrium-charge incidence. It is found that the present model reasonably explains the charge state effect on the film thickness dependence of lattice disordering of WO3. On the other hand, the model appears to be inadequate to explain the charge-state effect on the electronic sputtering of WO3 and LiF. Brief descriptions are given for the charge-state effect on the electronic sputtering of SiO2, UO2 and UF4, and surface morphology modification of poly-methyl-methacrylate (PMMA), mica and tetrahedral amorphous carbon (ta-C).

Celanese to expand acetyl chain capacities to strengthen high value, downstream vinyls portfolio

Wearless nanofriction between nonmetallic solids is investigated in terms of molecular dynamics. Sliding-velocity dependence of the frictional force density is characterized by the threshold phenomenon. Further insights into this phenomenon were gained by analyses of ensemble-averaged behavior under the steady sliding condition. An enormous number of simulation runs and frequent validation of their results were carried out so that the interfacial temperature and the pressure, which directly affect the heat generation processes, were made common at different sliding velocities. The dramatic increase in rate of the frictional-heat generation at sliding speeds above the threshold is triggered by excitation of certain vibrational modes. Such elastic waves hidden in random thermal motion of atoms near the sliding interface were separated by the sampling and averaging. The atomistic reason for the dependence of the threshold speed on the combination of the lattice constants along the sliding direction is discussed based on the patterns of the orderly atomic-displacement fields.

Velocity-dependent threshold phenomenon predicted by nonequilibrium molecular dynamics study on wearless nanofriction

ABSTRACT Wearless nanofriction between nonmetallic solids is investigated in terms of molecular dynamics. Sliding-velocity dependence of the frictional force density is characterised by the threshold phenomenon. Universal features of the ensemble-averaged threshold behaviour under the steady condition have become clearer through the following effort. An enormous number of simulation runs and frequent validation of their results were carried out so that the interfacial temperature and the pressure, which directly affect the heat generation processes, were made common at different sliding velocities. The energy dissipation mechanism underlying these universal features is discussed with an emphasis on the constraint on wearless-nanofrictional characteristics due to the spatial translational symmetry of the interfacial atomic configuration along the sliding direction.

Phononic mechanism determining threshold speed of wearless sliding nanofriction clarified based on molecular dynamics simulations

Wearless nanofriction between nonmetallic solids is investigated in terms of molecular dynamics. Sliding-velocity dependence of the frictional force density is characterized by the threshold phenomenon. Further insights into this phenomenon were gained by analyses of ensemble-averaged behavior under the steady sliding condition. An enormous number of simulation runs and frequent validation of their results were carried out so that the interfacial temperature and the pressure, which directly affect the heat generation processes, were made common at different sliding velocities. The dramatic increase in rate of the frictional-heat generation at sliding speeds above the threshold is triggered by excitation of certain vibrational modes. Such elastic waves hidden in random thermal motion of atoms near the sliding interface were separated by the sampling and averaging. The atomistic reason for the dependence of the threshold speed on the combination of the lattice constants along the sliding direction is discussed based on the patterns of the orderly atomic-displacement fields.

Phonon hydrodynamics and ultrahigh-room-temperature thermal conductivity in thin graphite.

Allotropes of carbon, such as diamond and graphene, are among the best conductors of heat. We monitored the evolution of thermal conductivity in thin graphite as a function of temperature and thickness and found an intimate link between high conductivity, thickness, and phonon hydrodynamics. The room-temperature in-plane thermal conductivity of 8.5-micrometer-thick graphite was 4300 watts per meter-kelvin-a value well above that for diamond and slightly larger than in isotopically purified graphene. Warming enhances thermal diffusivity across a wide temperature range, supporting partially hydrodynamic phonon flow. The enhancement of thermal conductivity that we observed with decreasing thickness points to a correlation between the out-of-plane momentum of phonons and the fraction of momentum-relaxing collisions. We argue that this is due to the extreme phonon dispersion anisotropy in graphite.

Thin graphite gets cool fast

Thermal Conductivity In nonmetallic solids, heat is transported primarily through crystal vibrations called phonons. These phonons can have wavelike properties under certain conditions, which increases the thermal conductivity of the material. Machida et al. found that making graphite samples thin expands the hydrodynamic regime from cryogenic to room temperatures. The researchers measured an extremely high thermal conductivity in the very thin graphite samples, which may be important for a variety of electronics applications. Science , this issue p. [309][1] [1]: /lookup/doi/10.1126/science.aaz8043

Evolutionary algorithm-based design of acoustic metamaterials for thermal insulation

Heat transfer at the nanoscale is an acoustic problem. In nonmetallic solids, heat is primarily carried by lattice vibrations (phonons), and its transfer can similarly be impeded by methods for manipulating vibrations familiar to acousticians. Metamaterials have seen increasing use in altering and controlling vibration. Metamaterial concepts, including periodic structuring of materials and introduction of resonant inclusions, have been used to control vibration on the macroscale, and can also affect heat transfer when implemented at nanoscale. Locally resonant materials appear to be able to address the frequency range responsible for heat transfer most easily. Control of heat transfer is of particular importance because heating and cooling buildings are among the most extensive human uses of energy, thus presenting a significant opportunity for societal benefit through increased efficiency. Even modest improvements in building insulation will result in significant energy and cost savings. In pursuit of a superior and cost-effective insulating material for building applications, an optimization of the nanostructure of a polymer metamaterial panel to reduce heat transfer is carried out using simulation in COMSOL and a genetic algorithm. Results of this research are presented here. [Funded by USDOE Office of Energy Efficiency and Renewable Energy (EERE)—Office of Building Technologies.]

Nanoceramics in Bone Tissue Engineering: The Future Lies Ahead

Nanoceramics are composed of ceramics and are classified as inorganic, heat-resistant, nonmetallic solids made of both metallic and nonmetallic compounds. Bone tissue engineering applies bioactive scaffolds, host cells and osteogenic signals for restoring damaged or diseased tissues. Composites of bioactive ceramics closely match the properties of bone. In the present review paper, an attempt has been made to emphasize the suitability of nanoceramics in the field of bone tissue engineering. Toxicity of these synthesized nanomaterials should be checked before their real application. Nanoceramics, in future, will surely prove to be important nanomaterials in the field of tissue engineering.

Vaporization of water droplets with non-metallic inclusions of different sizes in a high-temperature gas

Experimental research on mechanisms of vapor production by boiling and evaporation was carried out on heterogeneous droplets of water containing graphite particles of different sizes and concentrations, heated by high-temperature environment (up to about 1373 K). Different steps have been observed, such as free surface evaporation of the droplet, bubbles boiling at the solid inclusion/liquid interface, the explosive disintegration of a drop into a cluster of small droplets and efficient evaporation of the produced smaller droplets. We proposed the conditions for the appearance of this explosive breakup of droplets. The present paper reports that the addition of small graphite particles into heterogeneous droplets can reduce their lifetimes by about 50% in this type of high-temperature environment. Moreover, this facilitated the explosive breakup of droplets, allowing intense vaporization. This behavior led us to make the hypothesis that an insulating layer of vapor appears at the outside droplet surface and at the interface between the solid inclusions and the droplet. Measurements on the explosive breakup of heterogeneous water droplets have allowed establishing an increase of the evaporation surface area by almost fifteen times, as compared to the initial surface of droplets. However, when using graphite suspension, the measured increase of the evaporation surface was not larger than threefold. The evaporation surface area during the explosive breakup of water-graphite suspensions was 5.4 smaller as compared to water without impurities. We reported on the reasons for the above differences. The results are useful for developing technologies of fire extinguishing by using water sprays containing non-metallic solids. Practical implementation of the explosive breakup of heterogeneous droplets can eventually permit extinguishing fire on larger areas with an identical water load.

Peak intrinsic thermal conductivity in non-metallic solids and new interpretation of experimental data for argon

The inelastic nature of 3-phonon processes is investigated within the framework of perturbation theory and linearized Boltzmann transport equation (BTE). By considering the energy conservation rule governing this type of interaction in a statistical average sense, the impact of different forms of the regularized energy-conserving Dirac delta function on 3-phonon scattering rates was evaluated. Strikingly, adopting Lorentz distribution, in accordance with the shape of eigenenergy broadening of phonon normal modes due to the leading term of crystal anharmonicity, was found to play a critical role in activating umklapp processes at low temperature, leading to intrinsic lattice thermal conductivity peak at finite temperature for perfect crystal. This characteristic behavior, unique to the Lorentzian, lays foundation for developing adjustable-parameter-free computational models for reliable prediction of the finite lattice conductivity at low temperature, even in the absence of extrinsic scattering processes (e.g., by crystal imperfections and boundary). An iterative solution scheme for BTE was used to compute the intrinsic thermal conductivity of solid argon over the entire temperature range (2–80 K). For the first time, the experimentally observed T2 behavior in the low temperature, T, limit and the peak temperature (∼8 K) were successfully recovered, in addition to the classical high temperature T−1 behavior above 20 K by the sole use of 3-phonon processes. The good agreement with experiment indicates that phonon–phonon interactions dominate over the entire temperature range in argon, contrary to previous hypotheses that the subpeak regime is dominated by phonon-defect scattering. Anisotropy in thermal conductivity of single crystal at low temperature due to phonon focusing was observed. In addition, argon conductivity is underestimated by an order of magnitude in single mode relaxation time approximation, where the collective nature of phonon mode relaxation is ignored.

Estimation of the ionic charge of non-metallic species into an electrical discharge through a web application

Estimate the ionic charge generated in electrical discharges will allow us to know more accurately the concentration of ions implanted on the surfaces of nonmetallic solids. For this reason, in this research a web application was developed to allow us to calculate the ionic charge generated in an electrical discharge from the experimental parameters established in an ion implantation process performed in the JUPITER (Joint Universal Plasma and Ion Technologies Experimental Reactor) reactor. The estimated value of the ionic charge will be determined from data acquired on an oscilloscope, during startup and shutdown of electrical discharge, which will then be analyzed and processed. The study will provide best developments with regard to the application of ion implantation in various industrial sectors.

<i>In-Situ</i> Observation of Radiation Damage Using a Combined Facility of Electron Microscope and Heavy-Ion Accelerator(s) —History and Future Prospectives—

Radiation effects in materials for fission and fusion reactors are caused by fast neutrons which produce cascade damage, which has been known to be considerably different from simple Frenkel pairs produced by relativistic electrons. In-situ observation of radiation damage using a combined facility of electron microscope and heavy-ion accelerators has been a powerful tool to investigate the nature of the cascade damage. In this overview, brief historical survey of this experimental technique will be given, followed by the results and discussions on cascade damage mainly in gold will be reviewed. Careful considerations will be required to generalize the results to other metals as well as to other experimental conditions. Finally, there are a number of points to be developed for better understanding of cascade damage and for extending the “in-situ” techniques to other materials as non-metallic solids and to other intriguing phenomena. [doi:10.2320/matertrans.MD201304]

Asymptotic correction schemes for semilocal exchange-correlation functionals

Aiming to remedy the incorrect asymptotic behavior of conventional semilocal exchange-correlation (XC) density functionals for finite systems, we propose an asymptotic correction scheme, wherein an exchange density functional whose functional derivative has the correct $(\ensuremath{-}1/r)$ asymptote can be directly added to any semilocal density functional. In contrast to semilocal approximations, our resulting exchange kernel in reciprocal space exhibits the desirable singularity of the type $O(\ensuremath{-}1/{q}^{2})$ as $q\ensuremath{\rightarrow}0$, which is a necessary feature for describing the excitonic effects in nonmetallic solids. By applying this scheme to a popular semilocal density functional, PBE [Perdew, Burke, and Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)], the predictions of the properties that are sensitive to the asymptote are significantly improved, while the predictions of the properties that are insensitive to the asymptote remain essentially the same as PBE. Relative to the popular model XC potential scheme, our scheme is significantly superior for ground-state energies and related properties. In addition, without loss of accuracy, two closely related schemes are developed for the efficient treatment of large systems.

Influence of quantum effect on deviation from linear isotherm regularity

A general regularity was found based on an effective pair potential of Lennard-Jones LJ (12, 6), for both dense, nonmetallic and nonionic fluids and solids according to which (Z−1)v2 linearly varies with respect to ρ2 for each isotherm, and this equation of state (EoS I) is known as LIR. However, despite the fact that Ne is a simple spherical species, unexpectedly, its solid and liquid phases both show a significant deviation from EoS I. In this work, we have investigated the accuracy of the EoS I for other systems, including quantum light molecules, such as D2, H2 and He, in both fluid and solid states at different temperatures. Like Ne, we have noticed that these systems do not well obey the EoS I. Then, using a simple van der Waals equation, it is shown that significant deviations in dense systems of light molecules are because of the unbalance between repulsive and attractive interactions, due to the importance of the quantum effect. Also, we notice that at higher temperatures and for heavier species, LIR is valid due to a decrease in the quantum effect. We have shown that the hard-sphere fluid remarkably deviates from LIR. Two other EoSs have also been examined.

DSMC scheme to study phonon dynamics

In order to solve the Boltzmann transport equation (BTE) of phonons for investigating heat conduction in non-metallic solids, we propose employing a DSMC (direct simulation Monte Carlo) scheme to simulate the dynamics of phonons analogous to rarefied gas. In contrast to treating the BTE with conventional linear approximation, this scheme requires no relaxation times as input parameters. We can directly investigate couplings among phonons with different modes, although we have to assume an appropriate scattering model for phonon-phonon interactions. In this paper, we describe the DSMC scheme for phonon dynamics and present some results with our prototype codes for a simple solid model. In the first case of single-branch four-phonon processes, we carried out simulations of non-equilibrium thin films with a temperature gradient. We found that the temperature jump at the boundaries can be successfully achieved. For the second case of three phonon processes, we developed a simulation code that takes into consideration the different acoustic branches to evaluate the mode-dependent relaxation time and mean free path. This type of DSMC scheme for phonons enables us to include other relevant factors, such as optical branches and phonon-electron interactions.

The Core Structure of Basal Dislocations in Deformed Sapphire (α-Al 2 O 3 )

The atomic structure of dislocation cores is decisive for the understanding of plasticity in crystalline solids. The core structure of dislocations in sapphire introduced by high-temperature plastic deformation has been investigated with the use of the negative spherical-aberration imaging technique. The ability of this technique to discriminate oxygen columns from aluminum (Al) columns, combined with reproduction of subtle contrast features by image simulation, leads to a markedly detailed atomic model of the dislocation cores. The partial dislocations are Al-terminated, with electrical neutrality being achieved because half of the Al columns are missing. These partials also undergo core spreading, which results in random occupancy of both tetrahedrally and octahedrally coordinated sites, though Al in tetrahedral coordination never occurs in a perfect crystal. Unusual dislocation core structures may be present in other technologically important nonmetallic solids.

Excitation of core levels in nonmetallic solids: Some theoretical problems

The radiation from charged particles undergoing centripetal or transverse acceleration in synchrotron or related devices is (a) excellent for spectroscopic studies involving the excitation of electrons from core levels in solids to states in or near the conduction band and vacuum continua, and (b) becoming much more widely available for experimentation. Thus, there is renewed interest in better understanding such excitations and the dynamics of processes initiated by them (Core Hole Initiated Relaxation Processes). This article describes work done by the author and collaborators to calculate wavefunctions and associated energy eigenvalues of the localized states (ground and excited) found to be involved in the absorption of soft x rays near the observed edges (thresholds associated with the onset of absorption from deeper shells as the photon energy is increased). A convenient designation for the exploration of these structures is X-ray Absorption Near Edge Spectroscopy. Other work described involves calculations of the probabilities of the various processes [some electronic—and often Auger-like—and some in which there are changes in the motion of the participating atoms (ions)] that could effect the annihilation of the core-hole generated when a photon is absorbed.