Phonon Conductivity Research Articles

Energy Transport by Radiation in Hyperbolic Material Comparable to Conduction

AbstractRadiation as a heat transfer mode inside a bulk material is usually negligible in comparison to conduction. Here, the contribution of radiation to energy transport inside a hyperbolic material, hexagonal boron nitride (hBN), is investigated. With hyperbolic dispersion, i.e., opposite signs of dielectric components along principal directions, phonon polaritons contribute significantly to energy transport due to a much greater number of propagating modes compared to that in a normal material. A many‐body model is developed to account for radiative heat transfer in a material with a nonuniform temperature distribution. The total radiative heat transfer through hBN is found to be largely contributed by the high‐κ modes within the Reststrahlen bands, and is comparable to phonon conduction. Experimental measurements of temperature‐dependent thermal transport also show that radiative contribution to thermal transport is of the same order as that from phonons. Therefore, this work shows, for the first time, radiative heat transfer inside a material can be comparable to phonon conductive heat transfer.

Effectiveness of Geant4 in Monte Carlo Simulation Studyofphonon Conduction in Sn Host with Si Nanowire Interface

Abstract We have explored the effectiveness of Geant4 by using it to simulate phonon conduction in Sn Host with Si Nanowire Interface. Our Monte Carlo Simulation shows that the effectiveness of the phonon conduction Geant4 simulation increases when the system attained a steady state of 100 time steps. We have simulated phonon conduction in Sn host with Si nanowire interface using a Geant4Condensed Matter Physics Monte Carlo simulation toolkit in a low cost and less powerful processing computer machine. In the simulation, phonons were displaced inside a computation domain from their initial positions with the velocities and direction vectors assigned to them. A time step was selected so that a phonon can move at most the length of one sub-cell in one time step. Our phonon conduction analysis of SiSn based alloy using Geant4 showed performance enhancement and reasonable predicted thermal values. Numerical predictions of the thermal profile simulations of the values of the temperature in each cell were all within ten percent of the average temperature of Silicon – Tin.

Numerical and experimental investigation of heat transfer across a nanoscale gap between a magnetic recording head and various media

With the emergence of Heat-Assisted Magnetic Recording and Microwave-Assisted Magnetic Recording, understanding nanoscale heat transfer at the head-media interface is crucial for developing reliable hard disk drives. There is a need to develop a methodology that uses a spacing-dependent nanoscale heat transfer coefficient, determined by using wave-based radiation and van der Waals force driven phonon conduction theories to predict head temperatures in hard disk drives. We present a numerical model to simulate the head temperature due to heat transfer across a closing nanoscale gap between the head and the media (nonrotating) and compare our results with static touchdown experiments performed with a head resting on three different media (Si, magnetic disks with AlMg, and glass substrates). The Thermal Fly-Height Control (TFC) heater in the head is powered to create a local protrusion, leading to contact of a resistive Embedded Contact Sensor (ECS) that is used to measure the temperature change. As the ECS approaches the media, enhanced phonon conduction heat transfer causes a drop in the ECS temperature vs TFC power curve. Our model shows that the introduction of van der Waals forces between the head and the media during computation of the head's thermal protrusion causes a steeper drop in the simulated ECS temperature curve, ensuring a good quantitative match with experiments for all of the media materials tested and different initial ECS-media spacings. We isolate the effect of air conduction on ECS cooling by comparing our simulations with experiments performed in air vs vacuum.

Low melting point alloy segregated network construction in thermosetting polymer matrix for the electromagnetic interference shielding and thermal conductivity enhancement

With the development of high power, miniaturization and integration of electric devices, materials with effective electromagnetic interference (EMI) shielding and high thermal conductivity have become a hot topic. In this paper, cross-linked epoxy microspheres (CEMs) were firstly synthesized by precipitation polymerization, and then the CEMs were physically mixed with low melting point alloy (LMPA) SiBi58. Finally, the segregated epoxy/SnBi58 composites were prepared via hot-pressing technique. The SEM results showed that the aggregated SnBi58 particles formed a continuous network microstructure in the epoxy matrix, which was beneficial to the conduction of electrons and phonons. The epoxy/SnBi58 composites with 50 vol% alloy presented an outstanding electromagnetic interference (EMI) shielding effectiveness (SE) of 72 dB at 10.5 GHz and a satisfactory thermal conductivity of 1.6 W mK−1. Furthermore, this study demonstrated the potential application of LMPA in both thermal management and EMI shielding materials in future, and also widened the application of epoxy resin.

Thermal diffusivity and thermal conductivity of granitoids at 283–988 K and 0.3–1.5 GPa

AbstractThe thermal diffusivity and thermal conductivity of four natural granitoid samples were simultaneously measured at high pressures (up to 1.5 GPa) and temperatures (up to 988 K) in a multi-anvil apparatus using the transient plane–source method. Experimental results show that thermal diffusivity and thermal conductivity decreased with increasing temperature (<600 K) and remain constant or slightly increase at a temperature range from 700 to 988 K. Thermal conductivity decreases 23–46% between room temperature and 988 K, suggesting typical manifestations of phonon conductivity. At higher temperatures, an additional radiative contribution is observed in four natural granitoids. Pressure exerts a weak but clear and positive influence on thermal transport properties. The thermal diffusivity and thermal conductivity of all granitoid samples exhibit a positive linear dependence on quartz content, whereas a negative linear dependence on plagioclase content appears. Combining these results with the measured densities, thermal diffusivity, and thermal conductivity, and specific heat capacities of end-member minerals, the thermal diffusivity and thermal conductivity and bulk heat capacities for granitoids predicted from several mixing models are found to be consistent with the present experimental data. Furthermore, by combining the measured thermal properties and surface heat flows, calculated geotherms suggest that the presence of partial melting induced by muscovite or biotite dehydration likely occurs in the upper-middle crust of southern Tibet. This finding provides new insights into the origin of low-velocity and high-conductivity anomaly zones revealed by geophysical observations in this region.

Scattering of phonon by stacking faults and dislocation cores in plastically deformed LiF crystal: An analysis

The present work studies and analyses the effect of compression on LiF crystal. The compression is of the order that it deforms the LiF crystal plastically. The sequence of closed packed structure of plastically deformed crystal is disturbed and we expect stacking faults i.e., faults in the crystal along a plane. Plastically deformed crystal contains high density of dislocations. The depression of phonon conductivity in plastically deformed crystal is associated with interaction of the phonon with high density of dislocations. Phonon or the quantised lattice wave scatters with these stacking faults and dislocation cores, as a result, there is depression of lattice thermal conductivity near the peak. The number of stacking faults/cm N[Formula: see text] has been calculated which is of the order of 101/cm, and dislocation core size which is of the order of burger vector if dislocation density is of the order of 108/cm2. Separate contributions of relaxation rates for various phonon scattering processes have been calculated for both longitudinal and transverse phonons. Present work strongly supports that phonons are mainly scattered by stacking faults and dislocation cores in plastically deformed LiF crystal.

(Invited) Mie-like Scattering of Phonons by a Single Vacancy in IV-VI Compounds

Defects engineering has been a widely used approach for developing low thermal conductivity materials. Also, for some materials (e.g., IV-VI compound), considerably large amount of vacancies was often observed in single crystalline samples. The atomic vacancy in dilute case was assumed as a point defect that can cause the Rayleigh scattering with its rate proportional to the fourth power of phonon frequency. Due to the large dependence of scattering rate on phonon frequency, a vacancy was considered to only weakly suppress the mode thermal conductivity of low frequency acoustic phonons. We show that this does not hold for IV-VI materials. We compare the phonon scattering by a single vacancy in group IV (diamond and Si) and IV-VI (PbTe and GeTe) crystals using an exact ab initio Green’s function approach. The phonon-vacancy scattering rates in diamond and Si follow the well-known fourth-power dependence on phonon frequency. However, PbTe and GeTe exhibit phonon scattering rates that have much weaker frequency dependence. As a result, a vacancy scatters low frequency acoustic phonons more strongly in PbTe and GeTe than in Si and diamond. This unusual behavior in PbTe and GeTe originates from the significant changes of interatomic force constants that extend up to the distance of 10 angstrom from the vacancy site. Therefore, a vacancy in IV-VI materials cannot be assumed as a point defect but its finite size should be considered for phonon scattering. In contrast, a vacancy in Si and diamond changes interatomic force constants only up to 3 angstrom range. Our study provides new insights on the phonon spectral contribution to thermal transport when vacancies exist in IV-VI materials and will help to develop low thermal conductivity materials by engineering vacancies.

Cross property connection between the electric and the thermal conductivities of copper graphite composites

The cross property relationship between thermal and electrical conductivity of the copper matrix–graphite composite was characterised in the composition range up to 100 vol.% of graphite. It was observed that the investigated cross property is linear in the whole composition range. This linearity averages out nonlinear dependence of single conductivity on composition. It means also that the behaviour of investigated composite under applied external electric potential and thermal gradient are qualitatively identical. Both conductivities are realised by free electrons in copper and in basal plane of graphite. The only difference between both cases is phonon thermal conductivity in graphite phase. The possible explanation for linear cross property is that all non linear effects are probably ruled by the scattering of valence electrons on copper–graphite interface via converting of some part of kinetic electron energy to the vibrations of phonons in graphite phase. Further Wiedemann–Franz law was used to determine composition threshold above which the phonon conductivity in graphite phase starts to play significant role. This threshold was observed around 25 vol.% of graphite. As linearity of cross property is valid also above the composition threshold it means probably that above this composition electrical conductivity composition dependence start to be nonlinear as well.

A new approach for electronic heat conduction in molecular dynamics simulations

We present a new approach for the two-temperature molecular dynamics (MD) model for coupled simulations of electronic and phonon heat conduction in nanoscale systems. The proposed method uses a master equation to perform heat conduction of the electronic temperature eschewing the need to use a basis set to evaluate operators. This characteristic allows us to seamlessly couple the electronic heat conduction model with MD codes without the need to introduce an auxiliary mesh. We implemented the methodology in the large-scale atomic/molecular massively parallel simulator code, and through multiple examples, we validated the methodology. We then study the effect of electron–phonon interaction in high energy irradiation simulations and the effect of laser pulse on metallic materials. We show that the model provides an atomic level description in complex geometries of energy transfer between electrons and phonons. Thus, the proposed approach provides an alternative way to the two-temperature MD models. The parallel performance and some aspects of the implementation are presented.

Improved Thermoelectric Properties of Re-Substituted Higher Manganese Silicides by Inducing Phonon Scattering and an Energy-Filtering Effect at Grain Boundary Interfaces.

In this study, the effect of the grain boundary density on the transport properties of the Re-substituted higher manganese silicide Mn30.4Re6Si63.6 has been investigated. The efficiency of electrical energy conversion from waste heat, mainly in thermoelectric generators, depends on how the thermal conduction is reduced, while the charge-carrier electrons/holes contribute to possess a large magnitude of both the electrical conductivity σ and Seebeck coefficient S. In this work, we tried to obtain such a condition with a novel approach of merging the energy-filtering effect at the grain boundaries to improve thepower factor (PF) =S2σ. The nanostructuring and heavy-element substitution were also employed to greatly scatter the phonon conduction. As a result, enhancement of the PF was observed in the diffused nanostructure of annealed ribbon samples, and the enhancement was correlated with the formation of Schottky barriers at the grain boundary interface. Together with a reduction of the thermal conductivity to very low magnitude 1.27 W m-1 K-1, we obtained a maximum ZT = 1.15 at 873 K for the annealed ribbon samples.

Quasiperiodic Branches in the Thermoelectricity of Nanowires

Branches attached to a nanowire may significantly modify the transport of excitations along it due to wave interference. In this article, we study the thermoelectric properties of thin nanowires with quasiperiodically placed branches by means of a real-space renormalization plus convolution method developed for the Kubo–Greenwood formula, in which tight-binding and Born models are respectively used for the calculation of electrical and lattice thermal conductivities. The results show a substantial growth of the thermoelectric figure of merit (ZT) induced by the long-range quasiperiodic disorder, because it diminishes the thermal conduction of long wavelength acoustic phonons because such phonons are usually not altered by local defects nor by impurities. A remarkable reduction of the thermal conduction can be further achieved by using low sound velocity materials.

Stress Controllability in Thermal and Electrical Conductivity of 3D Elastic Graphene‐Crosslinked Carbon Nanotube Sponge/Polyimide Nanocomposite

AbstractStress controllability in thermal and electrical conductivity is important for flexible piezoresistive devices. Due to the strength‐elasticity trade‐off, comprehensive investigation of stress‐controllable conduction in elastic high‐modulus polymers is challenging. Here presented is a 3D elastic graphene‐crosslinked carbon nanotube sponge/polyimide (Gw‐CNT/PI) nanocomposite. Graphene welding at the junction enables both phonon and electron transfer as well as avoids interfacial slippage during cyclic compression. The uniform Gw‐CNT/PI comprising a high‐modulus PI deposited on a porous templated network combines stress‐controllable thermal/electrical conductivity and cyclic elastic deformation. The uniform composites show different variation trends controlled by the porosity due to different phonon and electron conduction mechanisms. A relatively high k (3.24 W m−1 K−1, 1620% higher than PI) and suitable compressibility (16.5% under 1 MPa compression) enables the application of the composite in flexible elastic thermal interface conductors, which is further analyzed by finite element simulations. The interconnected network favors a high stress‐sensitive electrical conductivity (sensitivity, 973% at 9.6% strain). Thus, the Gw‐CNT/PI composite can be an important candidate material for piezoresistive sensors upon porosity optimization based on stress‐controllable thermal or electrical conductivity. The results provide insights toward controlling the stress‐induced thermal/electrical conductivities of 3D interconnected templated composite networks for piezoresistive conductors or sensors.

Fundamental limits for near-junction conduction cooling of high power GaN-on-diamond devices

The integration of differing materials can enable breakthrough performance for semiconductor devices. One example is the integration of gallium nitride (GaN) and diamond to form GaN-on-diamond, which enables high-power GaN devices to achieve extreme power densities and, arguably, approaches fundamental limits for conduction cooling. Here, we examine the fundamental limits for near-junction phonon conduction cooling of GaN-on-diamond devices via finite element calculations of their lowest possible thermal resistance. A semi-classical transport theory for phonons interacting with interfaces and defects is used to calculate the in-plane thermal conductivity of a GaN epilayer and thereby accurately account for the thermal spreading resistance of the GaN layer. The device thermal resistance of a state-of-the-art GaN-on-diamond structure is predicted to be ∼13.0 K mm W−1 for a 12 finger device with 30 μm gate-to-gate spacing and a power dissipation of 5 W mm−1. For the same multifinger cell geometry and dissipated power, device thermal resistances as low as ∼10.0 K mm W−1 may be possible with assuming anisotropic but homogeneous diamond, as well as the absence of phonon scattering by external defects in the GaN layer and interface.

Effect of microscopic-ordered structures on intrinsic thermal conductivity of liquid-crystalline polysiloxane

Intrinsic insulating and thermal conductive polymers were prepared by synthesizing liquid-crystalline polysiloxanes elastomer (LCPE) with chemical cross-linking (0.83 W/m K and 0.81 W/m K). The microscopic-ordered structures were found in LCPE and considered affording high thermal conductivity. Chemical structures were observed by hydrogen nuclear magnetic resonance spectroscopy (1H-NMR) and fourier transform infrared (FT-IR) spectroscopy. Microscopic-ordered structures were analyzed by polarizing optical microscopy, scanning electron microscope and X-ray diffraction. Thermal conductivities were calculated according to equation: λ = α·ρ·Cp. Thermal properties were researched by differential scanning calorimeter and thermal gravimetric analyzer. The results revealed that LCPE showed high thermal conductivity because of the microscopic-ordered structures existing. The phonon scattering was suppressed and the mean free path of phonon was extended maximally. In addition, phonon conduction path was more complete. Our study provided a useful method to enhance the thermal conductivities of intrinsic thermal conductivity polymer themselves.

Transient Response of h-BN-Encapsulated Graphene Transistors: Signatures of Self-Heating and Hot-Carrier Trapping.

We use transient electrical measurements to investigate the details of self-heating and charge trapping in graphene transistors encapsulated in hexagonal boron nitride (h-BN) and operated under strongly nonequilibrium conditions. Relative to more standard devices fabricated on SiO2 substrates, encapsulation is shown to lead to an enhanced immunity to charge trapping, the influence of which is only apparent under the combined influence of strong gate and drain electric fields. Although the precise source of the trapping remains to be determined, one possibility is that the strong gate field may lower the barriers associated with native defects in the h-BN, allowing them to mediate the capture of energetic carriers from the graphene channel. Self-heating in these devices is identified through the observation of time-dependent variations of the current in graphene and is found to be described by a time constant consistent with expectations for nonequilibrium phonon conduction into the dielectric layers of the device. Overall, our results suggest that h-BN-encapsulated graphene devices provide an excellent system for implementations in which operation under strongly nonequilibrium conditions is desired.

Phonon thermal conductance across GaN-AlN interfaces from first principles

The vibrational thermal conductances ($G$) across GaN-AlN interfaces are computed using a nonequilibrium Green's function formalism in the harmonic limit with bulk and interfacial interatomic force constants (IFCs) fully from density functional theory. Several numerical methods and supercell configurations are employed to examine the sensitivity of $G$ to variances of IFCs. In particular, the effects of supercell size, the enforcement of symmetry constraints, and truncation of IFCs near the interface, and atomic relaxation on phonon transmission and conductance are explored. Our fully first-principles calculations are compared with common approximations and measured $G$ values inferred from thermal conductivity measurements for GaN-AlN superlattices. Our calculated value, $G\ensuremath{\sim}300\phantom{\rule{0.16em}{0ex}}\mathrm{MW}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$, is nearly half that from measurements. This discrepancy is critically analyzed in terms of the physical assumptions of the calculations and the derivation of the experimental values. This work provides guidelines to determine ``physically correct'' sets of interfacial IFCs from first principles for thermal conductance calculations using minimal computational resources. It also contributes toward developing predictive calculations and a more complete picture of thermal conduction across interfaces, a step toward first-principles multiscale thermal transport.

Application of Wavelet Transform to the study of Lattice Dynamics of two-dimensional Nanostructures

Abstract Theory of vibration of atoms that is the theory of lattice dynamics now forms an important part of study in Nanotechnology as these vibrations determine thermal and mechanical characteristics of a material and also govern phenomena like diffuse scattering of X-rays, neutron scattering, spin-lattice relaxation, etc. Lattice Dynamical analysis is helpful in the design and engineering of Nanomaterial and nanostructures for phononic and acoustic applications. Unlike current techniques that are based on Fourier analysis that gives results for frequency distribution along the wave vector, the wavelet transform can be used to determine how the energy of a simulated system is distributed in time, space, and among wave numbers, simultaneously. This is important in the study of interfacial phonon (quanta of crystal vibration) conductance, where the time the phonon packet interacts with the interface is preserved in the transformation. Wavelet transform is a unique tool for probing localized phonon physics at Nano scales. Two- dimensional Nanostructures like Graphene and hexagonal Boron Nitride has unique thermal properties which make them suited for a variety of applications. Wavelet Transform is applied to the lattice dynamics of a two- dimensional lattice and phonon transport is studied for industrially important materials like Graphene and its variations.

First-Principles Investigation on Type-II Aluminum-Substituted Ternary and Quaternary Clathrate Semiconductors R8Al8Si128 (R = Cs, Rb), Cs8Na16Al24Si112

Structural and vibrational properties of the aluminium-substituted ternary and quaternary clathrates R8Al8Si128 (R = Cs, Rb), Cs8Na16Al24Si112 are investigated. The equilibrium volume of R8Si136 expands when all Si atoms at the 8a crystallographic sites are replaced by Al. Formation of the Al–Si bond is thus anticipated to correlate with decreased guest vibration modes. Underestimation of the predicted lattice phonon conductivity κL (1.15 W m−1 K−1) compared to a previous experiment (1.9 W m−1 K−1) in Cs8Na16Si136 is thought to arise from our evaluation on the phonon mean free path λ using the “scattering centers” model. Accordingly, we expect that the “three-phonon” processes dominate the determination of the phonon relaxation time, leading to a more reasonable λ in the R8Al8Si128 system. Additionally, the “avoided-crossing” effect causes no appreciable difference in the sound speed for acoustic phonons in this framework. Starting with configuration optimization about aluminium arrangements in Cs8Na16Al24Si112, the calculated lattice parameter agrees well quantitatively with the experiment. The reduced Uiso of Cs from this calculation is anticipated to be primarily related to temperature-dependent quartic anharmonicity. Meanwhile, the predicted κL for Cs8Na16Al24Si112 remains not sensitive to the Al arrangement on 96g Wyckoff sites.

Odd-even phonon transport effects in strained carbon atomic chains bridging graphene nanoribbon electrodes

Based on first-principles approaches, we study the ballistic phonon transport characteristics of finite monatomic carbon chains stretched between graphene nanoribbons, an sp1-sp2 hybrid carbon nanostructure that has recently seen significant experimental advances in its synthesis. We find that the lattice thermal conductance anomalously increases with tensile strain for the even-numbered carbon chains that adopt the alternating bond-length polyyne configuration. On the other hand, in the odd-numbered carbon chain cases, which assume the equal bond-length cumulene configuration, phonon conductance decreases with increasing strain. We show that the strong odd-even phonon transport effects originate from the characteristic longitudinal acoustic phonon modes of carbon wires and their unique strain-induced redshifts with respect to graphene nanoribbon phonon modes. The novel phonon transport properties and their atomistic mechanisms revealed in this work will provide valuable guidelines in designing hybrid carbon nanostructures for next-generation device applications such as nano-biosensors.

Photon‐Mediated Phonon Heat Current Between Distant Graphene Sheets Inserted Into a Cavity

In this work, a mechanism of achieving phonon heat current between distant graphene sheets inserted into a cavity with two reflectors at different temperatures are presented. The photon‐mediated pseudospin interaction between two graphene sheets and electron–phonon interaction in each graphene sheet lead to the generation of this phonon heat current. We calculate the phonon heat current, thermal conductivity, and current–current correlation functions between distant graphene sheets. This phonon heat current can be manipulated by the energy flow density and frequency of photons. The giant heat current and thermal conductivity of phonons induced by intense photon field indicate that this mechanism has potential applications on the chip cooling, and far‐range heat transfer.