Phonon Conductivity Research Articles

Spectral phonon conduction and dominant scattering pathways in graphene

In this paper, we examine the lattice thermal conductivity and dominant phonon scattering mechanisms of graphene. The interatomic interactions are modeled using the Tersoff interatomic potential and perturbation theory is applied to calculate the transition probabilities for three-phonon scattering. The matrix elements of the perturbing Hamiltonian are calculated using the anharmonic interatomic force constants obtained from the interatomic potential as well. The linearized Boltzmann transport equation is applied to compute the thermal conductivity of graphene for a wide range of parameters giving spectral and polarization-resolved information. The complete spectral detail of selection rules, important phonon scattering pathways, and phonon relaxation times in graphene are provided. We also highlight the specific scattering processes that are important in Raman spectroscopy-based measurements of graphene thermal conductivity, and provide a plausible explanation for the observed dependence on laser spot size.

Heat transport of the quasi-one-dimensional alternating spin chain material (CH3)2NH2CuCl3

We report a study of the low-temperature heat transport in the quasi-one-dimensional S = 1/2 alternating antiferromagnetic-ferromagnetic chain compound (CH_{3})_{2}NH_{2}CuCl_{3}. Both the temperature and magnetic-field dependencies of thermal conductivity are very complicated, pointing to the important role of spin excitations. It is found that magnetic excitations act mainly as the phonon scatterers in a broad temperature region from 0.3 to 30 K. In magnetic fields, the thermal conductivity show drastic changes, particularly at the field-induced transitions from the low-field N\'{e}el state to the spin-gapped state, the field-induced magnetic ordered state, and the spin polarized state. In high fields, the phonon conductivity is significantly enhanced because of the weakening of spin fluctuations.

Lattice dynamics and thermal properties of phononic semiconductors

An enhanced adiabatic bond charge model has been employed to study the lattice dynamics of phononic metamaterials based on group-IV and group--III-V semiconductors. Using the full phonon spectrum and a realistic Brillouin zone summation method, we have developed theories of phonon scattering rates from interface formation and anharmonicity. Numerical results for specific-heat capacity and phonon conductivity of thin Si/Ge and GaAs/AlAs superlattices are then presented and compared with available experimental measurements. The roles of various phonon scattering mechanisms in controlling the thermal conductivity in different temperature ranges have been quantified.

Absence of Casimir regime in two-dimensional nanoribbon phonon conduction

In stark contrast with three-dimensional (3D) nanostructures, we show that boundary scattering in two-dimensional (2D) nanoribbons alone does not lead to a finite phonon mean free path. If combined with an intrinsic scattering mechanism, 2D boundary scattering does reduce the overall mean free path; however, the latter does not scale proportionally to the ribbon width, unlike the well known Casimir regime occurring in 3D nanowires. We show that boundary scattering can be accounted for by a simple Mathiessen-type approach for many different 3D nanowire cross sectional shapes; however, this is not possible in the 2D nanoribbon case, where a complete solution of the Boltzmann transport equation is required. These facts have strong implications for the thermal conductivity of suspended nanostructures.

Surface roughness and thermal conductivity of semiconductor nanowires: Going below the Casimir limit

By explicitly considering surface roughness at the atomic level, we quantitatively show that the thermal conductivity of Si nanowires can be lower than Casimir's classical limit. However, this violation only occurs for deep surface degradation. For shallow surface roughness, the Casimir formula is shown to yield a good approximation to the phonon mean free paths and conductivity, even for nanowire diameters as thin as 2.22 nm. Our exact treatment of roughness scattering is in stark contrast with a previously proposed perturbative approach, which is found to overpredict scattering rates by an order of magnitude. The obtained results suggest that a complete theoretical understanding of some previously published experimental results is still lacking.

Efficient thermoelectric energy conversion on quasi-localized electron states in diameter modulated nanowires

It is known that the thermoelectric efficiency of nanowires increases when their diameter decreases. Recently, we proposed that increase of the thermoelectric efficiency could be achieved by modulating the diameter of the nanowires. We showed that the electron thermoelectric properties depend strongly on the geometry of the diameter modulation. Moreover, it has been shown by another group that the phonon conductivity decreases in nanowires when they are modulated by dots. Here, the thermoelectric efficiency of diameter modulated nanowires is estimated, in the ballistic regime, by taking into account the electron and phonon transmission properties. It is demonstrated that quasi-localized states can be formed that are prosperous for efficient thermoelectric energy conversion.

Thermoelectric Properties of Scaled Silicon Nanowires Using the sp 3 d 5 s*-SO Atomistic Tight-Binding Model and Boltzmann Transport

As a result of suppressed phonon conduction, large improvements of the thermoelectric figure of merit, ZT, have been recently reported for nanostructures compared to the raw materials' ZT values. It has also been suggested that low dimensionality can improve a device's power factor as well, offering a further enhancement. In this work the atomistic sp3d5s*-spin-orbit-coupled tight-binding model is used to calculate the electronic structure of silicon nanowires (NWs). The linearized Boltzmann transport theory is applied, including all relevant scattering mechanisms, to calculate the electrical conductivity, the Seebeck coefficient, and the thermoelectric power factor. We examine n-type nanowires of diameters of 3nm and 12nm, in [100], [110], and [111] transport orientations at different carrier concentrations. Using experimental values for the lattice thermal conductivity in nanowires, the expected ZT value is computed. We find that at room temperature, although scaling the diameter below 7nm can be beneficial to the power factor due to banstructure changes alone, at those dimensions enhanced phonon and surface roughness scattering degrades the conductivity and reduces the power factor.

Phononic conductance of quasi-one-dimensional waveguides with an atomic hole defect

A theoretical approach based on the matching method has been developed to investigate the local phonon density of states (DOS) and the coherent phonon transmission near the atomic hole defect. The model system containing the defect is composed of three monatomic triple parallel chains, with atomic hole defect introducing them in the central chain. The matching method is used, in the harmonic approximation, to calculate the local Green's functions for the irreducible set of sites that constitute the inhomogeneous perturbed domain. The model system is studied for different cases of elastic hardening and softening in the perturbed zone. The purpose is to investigate how the local dynamics can respond to changes in the microscopic environment on the perturbed domain. The analysis of the total phonon conductance spectra and the densities of vibration states identify characteristic features and demonstrate the central role of the atomic hole defect localized in the perfect structure and the possible use, of them, as filters of phonon.

Doping induced electronic structure and estimated thermoelectric properties of CaMnO 3 system

The electronic properties of Sr doped CaMnO 3 are studied using the first principle density functional theory calculation based on a plane wave basis and pseudopotentials. The thermoelectric properties are analyzed on the basis of electronic properties. The band structure results show that the doped system undergoes a semiconductor-to-conductor transition and the bands near Fermi level experience a significant distortion; the density of states results show that the density of states near Fermi level is increased. The combination of Mnd and Op orbitals exhibits enhanced covalence nature. It is estimated that the thermopower and carrier conduction capability should be enhanced, and the phonon conduction should be depressed, indicating the improved thermoelectric properties for Sr doped CaMnO 3 system.

Theory of phonon conductivity of semiconductor superlattices

ABSTRACTWe present a single-mode relaxation-time theory of phonon conductivity of semisonductor superlattices with nanoscale periodicities. Analytic expressions have been obtained for phonon-interface scattering and phonon-phonon scattering taking into consideration the effects of interfaces and the presence of two materials in superlattices. Numerical calculations have been performed by using phonon eigensolutions obtained from an enhanced adiabatic bond charge model and by carrying out Brillouin zone integration using the special q-points scheme. The experimental measured conductivity results for Si(19)/Ge(5) and Si(72)/Ge(30) superlattices have been successfully explained.

Structural, morphological, luminescent and electronic properties of sprayed aluminium incorporated iron oxide thin films

Thin films of aluminium incorporated Fe2O3, synthesized by simple chemical spray pyrolysis on to glass substrates using aqueous solutions of analytical reagent grade ferric trichloride and aluminium nitrate as precursors. The influence of aluminium doping on to morphological properties, contact angle, X-ray photoelectron spectroscopy, photoluminescence and thermal conductivity properties have been investigated. The preparative parameters have been optimized to obtain good quality thin films which are uniform and well adherent to the substrate. The FE-SEM and AFM micrographs depict the films are compact and homogeneous (spindle-shaped hematite nanostructures) with varying grain sizes (average grain size ~20–60nm). Contact angle measurement show the films are hydrophobic in nature. The chemical composition and valence states of constituent elements in Fe2O3 are analyzed by X-ray photoelectron spectroscopy. The excitonic strong violet emission has been observed in photoluminescence. The specific heat and thermal conductivity study shows the phonon conduction behavior is dominant in these polycrystalline films. We studied interparticle interactions like grains, grain boundary effects using complex impedance spectroscopy.

Structural Order-Disorder Transitions and Phonon Conductivity of Partially Filled Skutterudites

Filled skutterudites are high-performance thermoelectric materials and we show how their phonon conductivity is greatly influenced by the topology of the filler species. We predict (ab initio) the phase diagram of Ba(x)Co4Sb12 and find several stable configurations of Ba ordering over the intrinsic voids. The phonon conductivity predicted using molecular dynamics shows a minimum in the two-phase mixture regime, dominated by significantly reduced long-range acoustic phonon transport.

Simulation of thermal conductance across dimensionally mismatched graphene interfaces

This paper considers phonon transport behavior in graphene nanoribbons (GNRs) that bridge semi-infinite graphene contacts. The work employs an atomistic Green’s function (AGF) method to investigate phonon wave effects in GNRs with both zigzag and armchair edges. Thermal conductances are found to be sensitive to the edge shape of the ribbons; a sandwiched zigzag GNR structure has almost twice the thermal conductance of the corresponding armchair structure. Results show that the graphene/GNR interface moderately reduces phonon conductance compared to a freestanding GNR. At fixed device lengths, conductance increases with the width of GNR. On the other hand, conductance decreases with GNR length. The zigzag ribbons show smaller reduction upon increasing of GNR length than armchair ribbons; the conductances of both ribbons converge to a length-independent value. For very short devices, thermal conductance can exceed that of a single graphene-GNR interface.

Investigation of structural, morphological, luminescent and thermal properties of combusted aluminium-based iron oxide

Nanocomposites of aluminium integrated hematite α-Fe 2O 3 are synthesized by combustion route using aqueous solutions of AR grade ferric trichloride and aluminium nitrate as precursors. The influence of aluminium incorporation on to the morphology, XPS, photoluminescence and thermal properties has been investigated. The FESEM and AFM micrographs depict that the samples are compact and have homogeneously distributed grains of varying sizes (∼20–60 nm). Chemical composition and valence states of constituent elements in hematite are analyzed by XPS. In room temperature photoluminescence (PL) study, we observed strong violet emission around 436 nm without any deep-level emission and a small PL FWHM indicating that the concentrations of defects are responsible for deep-level emissions. The specific heat and thermal conductivity study shows the phonon conduction behavior is dominant. We studied interparticle interactions using complex impedance spectroscopy. We report a new potential candidate for its possible applications in optoelectronics and magnetic devices.

Phonon transport in large scale carbon-based disordered materials: Implementation of an efficient order-Nand real-space Kubo methodology

We have developed an efficient order-$N$ real-space Kubo approach for the calculation of the phonon conductivity which outperforms state-of-the-art alternative implementations based on the Green's function formalism. The method treats efficiently the time-dependent propagation of phonon wave packets in real space, and this dynamics is related to the calculation of the thermal conductance. Without loss of generality, we validate the accuracy of the method by comparing the calculated phonon mean free paths in disordered carbon nanotubes (isotope impurities) with other approaches, and further illustrate its upscalability by exploring the thermal conductance features in large width edge-disordered graphene nanoribbons (up to $\ensuremath{\sim}20\text{ }\text{nm}$), which is out of the reach of more conventional techniques. We show that edge disorder is the most important scattering mechanism for phonons in graphene nanoribbons with realistic sizes and thermal conductance can be reduced by a factor of $\ensuremath{\sim}10$.

Filler-reduced phonon conductivity of thermoelectric skutterudites: Ab initio calculations and molecular dynamics simulations

The phonon conductivities of CoSb 3 and its Ba-filled structure Ba x (CoSb 3) 4 are investigated using first-principle calculations and molecular dynamics (MD) simulations, along with the Green–Kubo theory. The effects of fillers on the reduction of the phonon conductivity of filled skutterudites are then explored. It is found that the coupling between filler and host is strong, with minor anharmonicity. The phonon density of states and its dispersion are significantly influenced by filler-induced softening of the host bonds (especially the short Sb–Sb bonds). Lattice dynamics and MD simulations show that, without a change in the host interatomic potentials, the filler–host bonding alone cannot lead to significant alteration of acoustic phonons or lowering of phonon conductivity. The observed smaller phonon conductivity of partially filled skutterudites is explained by treating it as a solid solution of the empty and fully filled structures.

Heat conductivity of the spin-Peierls compounds TiOCl and TiOBr

We report experimental results on the heat conductivity \kappa of the S=1/2 spin chain compounds TiOBr and TiOCl for temperatures 5K<T<300K and magnetic fields up to 14. Surprisingly, we find no evidence of a significant magnetic contribution to \kappa, which is in stark contrast to recent results on S=1/2 spin chain cuprates. Despite this unexpected result, the thus predominantly phononic heat conductivity of these spin-Peierls compounds exhibits a very unusual behavior. In particular, we observe strong anomalies at the phase transitions Tc1 and Tc2. Moreover, we find an overall but anisotropic suppression of \kappa in the intermediate phase which extends even to temperatures higher than Tc2. An external magnetic field causes a slight downshift of the transition at Tc1 and enhances the suppression of \kappa up to Tc2. We interprete our findings in terms of strong spin-phonon coupling and phonon scattering arising from spin-driven lattice distortions.

Physical and mechanical metallurgy of high purity Nb for accelerator cavities

In the past decade, high $Q$ values have been achieved in high purity Nb superconducting radio frequency (SRF) cavities. Fundamental understanding of the physical metallurgy of Nb that enables these achievements is beginning to reveal what challenges remain to establish reproducible and cost-effective production of high performance SRF cavities. Recent studies of dislocation substructure development and effects of recrystallization arising from welding and heat treatments and their correlations with cavity performance are considered. With better fundamental understanding of the effects of dislocation substructure evolution and recrystallization on electron and phonon conduction, as well as the interior and surface states, it will be possible to design optimal processing paths for cost-effective performance using approaches such as hydroforming, which minimizes or eliminates welds in a cavity.

Interference effects in phonon scattering across a double atomic well

We develop an analytical and numerical formalism to study the double atomic nanowell configuration influence on coherent transmission and reflection scattering, derived as elements of a Landauer-Buttiker type scattering matrix in quasi-one-dimensional multichannel waveguides. The state densities at neighbourhood of the defect region are investigated. The defect region consists in the presence of a double atomic nanowell occurring in thin film. One solves this problem by the matching method in the harmonic approximation. The theoretical formalism using simultaneously the Green's function and the matching method is detailed in order to describe the complete evanescent and the propagating fields. The phononic coherent conductance is also studied. Numerical calculations are presented and illustrated as function of the force constants occurring in the model. The fluctuations in the conductance spectra are related to Fano resonances due to the coherent coupling between travelling phonons and the localized vibration modes in the neighbourhood of the nanowell domain.

Cluster scattering effects on phonon conduction in graphene

The phonon-scattering cross section associated with isotopic clusters is evaluated from first principles and used to estimate the reduction in thermal conductance of wide graphene samples. A strong sensitivity of the thermal conductivity toward clustering is predicted for micrometer-sized samples at low temperatures. Important differences are obtained between the atomistically computed cross section, and existing analytical approximations, emphasizing the importance of atomistic investigations of thermal transport. Finally, possible techniques are suggested for synthesizing graphene containing isotopic clusters.