Acoustic Phonon Transmission Research Articles

Local resonance mechanism for enhancing the thermoelectric performance of PBCF-graphene nanoribbons

Phonon local resonance mechanism can effectively block phonon transport and thus reduce phonon thermal conductance. Inspired by this, we investigated the thermoelectric performance of Armchair-PBCF-graphene nanoribbons (A-PBCF-GNRs) and its branched structures by using non-equilibrium Green's function method. The results show that the thermoelectric performance of branched A-PBCF-GNRs can be significantly improved. Especially, the room-temperature ZT value of single branch A-PBCF-GNRs (SB-A-PBCF-GNRs) and double branch A-PBCF-GNRs (DB-A-PBCF-GNRs) can reach to 0.56 and 0.80, which is 1.5 and 2 times greater than that of A-PBCF-GNRs. Analysis shows that this is mainly attributed to the significant decrease in thermal conductance of branched A-PBCF-GNRs caused by the localized electronic and phonon states. Moreover, the mode resolved phonon transmission probabilities demonstrated that the local resonance mechanism of branched structure blocks the low-frequency acoustic phonon transmission. This work indicates the broad application prospects of the local resonance mechanism of branched structure in the thermoelectric field.

Quasi-Casimir coupling can induce thermal resonance of adsorbed liquid layers in a nanogap.

In a vacuum nanogap, phonon heat transfer can be induced by quasi-Casimir coupling in the absence of electromagnetic fields. However, it is unknown whether phonons can be transmitted across a nanogap via solid-like liquid layers adsorbed on solid surfaces. Here, we elucidate that phonon transmission across a nanogap can be induced by quasi-Casimir coupling via adsorbed liquid layers using classical nonequilibrium molecular dynamics simulation. We modulated the gap distance to verify the existence of quasi-Casimir coupling between interfacial solid-liquid or liquid-liquid layers. Thermal resonance can be induced between two liquid layers by quasi-Casimir coupling, agitating the co-occurrence of thermal resonance between interfacial solid layers, while a liquid monolayer disturbs the resonance. The thermal resonance between liquid layers results in a larger heat flux and thermal gap conductance compared with those in the vacuum gap case, while the liquid layer limits the acoustic phonon transmission in the interfacial solid layers. The fundamental understanding of quasi-Casimir heat transfer at the solid-liquid interface could pave the way for future nanoscale energy transport and thermal management.

Interference, scattering, and transmission of acoustic phonons in Si phononic crystals

The transient processes of phonon scattering and transmission in Si phononic crystals with periodic pores are simulated using the concurrent atomistic-continuum method. The nature of phonon transport is demonstrated to be dependent on the relation between the phonon wavelength, λ, the period length, p, and the neck width, n, of the phononic crystal. Three distinct regimes have been observed: (1) phonons with wavelengths equal to 2.9p or larger propagate predominantly ballistically, which is accompanied by specular reflection of varying extents, depending on the wavelength of the phonons; the properties of these phonons are consistent with the phonon dispersion relations of the phononic crystal modelled as a homogenous and harmonic system; (2) phonons with wavelengths < n are partly reflected by the pore boundaries and partly propagate ballistically in the solid regions of the necks; two types of vibrational modes are identified: the single crystal Si phonon modes, and the vibrational modes resulting from internal surface scattering; (3) phonons with wavelength close to p are most strongly scattered, and the internal surface-related modes dominate the transport; these modes have the slowest group velocities and lowest energy transmission, and the transport is predominantly diffusive even for specimens originally at zero temperature. For phononic crystals that contain a single crystal heater at their center, the interface between the single crystal heater and the phononic structure provides a strong resistance to short wavelength phonons, leading to predominantly diffusive phonon transport and an average energy flux that is two orders of magnitude lower than a same-sized single crystal specimen.

Quasi-Casimir coupling induced phonon heat transfer across a vacuum gap

In vacuum, thermal energy is transported by photons (thermal radiation) but not phonons. Recent studies, however, indicated that phonon heat transfer across a vacuum gap is mediated by the quantum fluctuation of electromagnetic fields. Specifically, in the heat exchange between two objects separated by a nanoscale vacuum gap, phonons carry thermal energy more efficiently than photons. However, it remains unclear if phonons can propagate without electromagnetic fields. Here, we demonstrate that phonon transmission across a sub-nanometer vacuum gap can be induced by quasi-Casimir force subjected to the Lennard–Jones atoms using classical molecular dynamics simulation. The net heat flux across the vacuum gap increases exponentially as the gap distance decreases, owing to acoustic phonon transmission. The local heat flux, evaluated using the Irving–Kirkwood method, increases singularly at the interfacial layers, while that at the inner layers agrees well with the net heat flux. These findings provide evidence of the strong thermal resonance induced by quasi-Casimir coupling between the interfacial layers. Thus, we conclude that the quasi-Casimir coupling induced by intermolecular interaction is a heat transfer mode for phonon heat transfer across a vacuum gap in nanoscale.

High performance thermoelectric module through isotype bulk heterojunction engineering of skutterudite materials

We demonstrate filled CoSb3 skutterudite materials with excellent thermoelectric (TE) performance that results in one of the highest reported single stage module efficiency. The improvement in TE material performance was obtained by creating isotype n/n “bulk heterojunction” structure through assembly of novel skutterudite nanocrystals with different Yb-doping content. Combination of significant increase in carrier transport through heterojunction structure and reduction in long-range acoustic phonon transmission by two-phase mixture resulted in enhanced power factor and reduced lattice thermal conductivity. As a result, the figure-of-merit (zT) of heterojunction TE material is improved by more than 35% compared with pristine single homogeneous material. Using these improved TE materials, a high module conversion efficiency of ~9.15% was obtained when operating between 650 °C and 50 °C. This is one of the highest conversion efficiency among the practically measured single stage modules.

Casein phosphopeptide-biofunctionalized graphene oxide nanoplatelets based cellulose green nanocomposites with simultaneous high thermal conductivity and excellent flame retardancy

In this contribution, a green nanocomposite consisting of casein phosphopeptide-biofunctionalized graphene oxide nanoplatelets (CPGO) and cellulose nanofibers (CNFs) was fabricated via vacuum-assisted self-assembly technique. A ‘natural nacre’ and strong hydrogen bonds network structure was constructed by CNFs and CPGO. In this architecture, the scaffold of one-dimensional CNFs is employed as the structural component to construct hydrogen bonds network and reinforce mechanical strength, while the arranged two-dimensional CPGO in the framework provides a convenient pathway for in-plane acoustic phonon transmission. The as-obtained nanocomposite possesses high in-plane thermal conductivity of 12.75 W m−1 K−1 and favourable tensile strength of 33.45 MPa. The theoretical simulation results indicated it has low thermal resistance between CPGO. Furthermore, a phosphorus-nitrogen flame retardant system was built and fire experiments presented that it could effectively prevent flame spreading. Heat release rate curve results showed that the total heat release and peak heat release rate decreased by 58.6 % and 20 % than CNFs, respectively. Cooling down experiments for LED chips showed that nanocomposites have an excellent cooling rate (8.85 °C min−1) which can transfer efficiently heat energy and demonstrated its potential usefulness in electronic device-cooling applications.

Robust modeling of acoustic phonon transmission in nanomechanical structures

The transmission of acoustic phonons is an important element in the design and performance of nano-mechanical devices operating in the mesoscopic limit. Analytic expressions for the power transmission coefficient, T, exist only in the low-frequency limit, in cases described by thin-plate elasticity theory, and for well-defined elastic waveguiding geometries. We compare two numerical techniques based on finite-element computations to determine the frequency dependence of T for arbitrary phonon scattering structures. Both methods take into account acoustic mode conversion to acoustic and optical modes. In one case, the phase and amplitude of complex-valued reflected waves are determined and related to transmission through a Fresnel equation, while in the other, the magnitude of the transmitted mechanical power is directly calculated. The numerical robustness of these methods is demonstrated by considering the transmission across an abrupt junction in a rectangular elastic beam, a well-known problem of considerable importance in mesoscopic device physics. The simulations presented extend the standard results for acoustic phonon transmission at an abrupt junction, and are in good agreement with analytic predictions in the long-wavelength limit. More generally, the numerical methods developed provide an effective tool for calculating acoustic mode energy loss in nano-mechanical resonators through mode conversion and heat transfer in arbitrary mesoscopic structures.

Tunability of acoustic phonon transmission and thermal conductance in three dimensional quasi-periodically stubbed waveguides

We investigate acoustic phonon transmission and thermal conductance in three dimensional (3D) quasi-periodically stubbed waveguides according to the Fibonacci sequence. Results show that the transmission coefficient exhibits the periodic oscillation upon varying the length of stub/waveguide at low frequency, and the period of such oscillation is tunably decreased with increasing the Fibonacci number N. Interestingly, there also exist some anti-resonant dips that gradually develop into wide stop-frequency gaps with increasing N. As the temperature goes up, a transition of the thermal conductance from the decrease to the increase occurs in these systems. When N is increased, the thermal conductance is approximately decreased with a linear trend. Moreover, the decreasing degree sensitively depends on the variation of temperature. A brief analysis of these results is given.

Hierarchical Graphene–Carbon Fiber Composite Paper as a Flexible Lateral Heat Spreader

As a low dimensional crystal, graphene attracts great attention as heat dissipation material due to its unique thermal transfer property exceeding the limit of bulk graphite. In this contribution, flexible graphene–carbon fiber composite paper is fabricated by depositing graphene oxide into the carbon fiber precursor followed by carbonization. In this full‐carbon architecture, scaffold of one‐dimensional carbon fiber is employed as the structural component to reinforce the mechanical strength, while the hierarchically arranged two‐dimensional graphene in the framework provides a convenient pathway for in‐plane acoustic phonon transmission. The as‐obtained hierarchical carbon/carbon composite paper possesses ultra‐high in‐plane thermal conductivity of 977 W m−1 K−1 and favorable tensile strength of 15.3 MPa. The combined mechanical and thermal performances make the material highly desirable as lateral heat spreader for next‐generation commercial portable electronics.

Band gaps of acoustic waves propagating in a solid/liquid phononic Fibonacci structure

We study the acoustic–phonon transmission spectra in quasiperiodic (Fibonacci type) superlattices made up from the solid crystal quartz and the liquid mercury (Hg). The phonon dynamics is described by a coupled elastic equations within the static field approximation model. We use a transfer-matrix treatment to simplify the algebra, which would be otherwise quite complicated, allowing a neat analytical expression for the phonon transmission coefficients. Numerical results is presented and discussed for both the transmittance spectra as well as the localization factor derived from the Lyapunov exponent, showing that the Fibonacci quasiperiodic structure acts as a filter for the phonon's transmission spectra.

Eigenfrequency and decay factor of the localized torsional phonon in a nanowire superlattice with a defect layer

AbstractWe theoretically study the localized vibrational modes in a cylindrical nanowire superlattice with a defect layer. We focus on torsional modes whose dispersion relations can be obtained analytically. We calculate the eigenfrequencies of the localized mode and corresponding decay factor. In particular, we examined how the eigenfrequencies depend on the width of the defect layer. It is shown that the localized modes cannot exist below a critical frequency due to the effect of the radial confinement of phonons. We also study the resonant transmission of acoustic phonons as a result of the interaction with these localized modes. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Heat transport in a four-perpendicularity-bend quantum waveguide

Using the scattering matrix method, we investigate acoustic phonon transmission and thermal conductance in a four-perpendicularity-bend quantum waveguide at low temperatures. The transmission spectrum of the quantum waveguide displays a series of resonant peaks and dips; and when one of the bend heights is larger than or equal to the minimum of the dimensions of the phonon channel in the quantum waveguide, a stop-frequency gap will appear; and some single four-perpendicularity-bend quantum waveguides with larger bend heights exhibit narrower width or smaller number of the stop-frequency gaps than that with smaller bend heights. The thermal conductivity is much sensitive to the change of the smaller heights and longitudinal lengths of the bend section; and the thermal conductivity decreases with the increasing of the temperature first, then increases after it reaches a minimum. The investigations of multiple four-perpendicularity-bend waveguides connected in series indicate that the first additional waveguide suppresses the transmission coefficient and forms the stop-frequency gap; and two additional resonance peaks will be formed when each four-perpendicularity-bend waveguide is added in the series. The results could be useful for controlling thermal conductance artificially and the design of phonon devices.

Low-Temperature Thermal Conductance in Superlattice Nanowire with Structural Defect

Using the scattering-matrix cascading method, we investigate the effect of structural defect on the acoustic phonon transmission and thermal conductance in the superlattice nanowire at low temperatures. In the present system, the phonon transmissions exhibit quite complex oscillatory behaviour. It is found that a lateral defect in an otherwise periodic structure significantly decrease the thermal conductance and completely washes away the transmission quantization. However, the appreciable transmission quantization survives in the presence of a longitudinal defect whereas a good quantization plateau of thermal conductance emerges below the universal level in a wide temperature range with the lateral defect.

Effect of structural defect on phonon transmission quantization in low-dimensional superlattices

Using the scattering-matrix cascading method, we investigate the effect of structural defect on the acoustic phonon transmission quantization in a low-dimensional superlattice quantum waveguide. In the present system, the phonon transmission exhibits rather complex resonant behaviors. It is found that a lateral defect in an otherwise periodic structure modifies the transversal phonon transmission spectra nontrivially and completely washes away the transmission quantization due to the strong additional scattering by defect. However, the appreciable quantization survives in the presence of a longitudinal defect. Our results also show that by adjusting the geometric parameters of the structural defect, one can control the phonon transport of the structure to match practical requirements in devices.

Resonant transmission of acoustic phonons in a nanowire superlattice with a defect layer

We study theoretically phonon transmission in a nanowire superlattice with a defect layer. We focus on azimuthally symmetric torsional modes and calculate the phonon transmittance. In frequency gaps, there exist transmission peaks originating from the resonant interaction with vibrational modes localized at the defect layer. It is shown that these peaks can be generated only above the critical frequency determined by the radius of the nanowire superlattice.

Acoustic phonon transport through a quantum waveguide with two stubs

Using the scattering matrix method, we investigate the acoustic phonon transmission and thermal conductance in a quantum waveguide with two stubs at low temperatures. It is found that the threshold frequency of the onset of each phonon mode travelling through the quantum waveguide depends on the stub heights. The thermal conductance exhibits oscillatory decaying behaviour with the width between the two stubs due to the coupling effects between them. In addition, the transmission coefficient and thermal conductance are also sensitive to the widths and heights of the stubs. It is suggested that changing the geometric parameters of the stubs could provide an efficient way to control the thermal conductance of the proposed micro-structures artificially.

Acoustic phonon transmission spectra in piezoelectric AlN/GaN Fibonacci phononic crystals

We study the acoustic-phonon transmission spectra in periodic and quasiperiodic (Fibonacci type) superlattices made up from the III-V nitride materials AlN and GaN. The phonon dynamics is described by a coupled elastic and electromagnetic equations within the static field approximation model, stressing the importance of the piezoelectric polarization field in a strained condition. We use a transfer-matrix treatment to simplify the algebra, which would be otherwise quite complicated, allowing a neat analytical expressions for the phonon transmission coefficients. Numerical results, for the normal incidence case, show a strike self-similar pattern for both hexagonal (class 6 mm) and cubic symmetries crystalizations of the nitrides.

Acoustic phonon transport and thermal conductance in a multiple channels nanostructure

Abstract Using the scattering-matrix method, we investigate the acoustic phonon transmission and thermal conductance in a multiple channels nanostructure at low temperature. Our results show that phonon transport is strongly influenced by mode coupling effect between channels, and the effect is different for different types of boundary conditions between channels.

Phonon heat transport through periodically stubbed waveguides

We investigate the acoustic phonon band structure, transmission spectrum and thermal conductance in a periodically stubbed waveguide structure by use of the transfer matrix method and the scattering matrix method. We find that the existence of stop-frequencies or dips in the transmission spectrum, which corresponds to the stop bands or gaps in the acoustic band structure. The dependence of the stop band width and the dip width on the stub height is also demonstrated. We also find that the universal quantum thermal conductance can be clearly observed and the thermal conductance increases monotonically with increasing temperature. Our results show that the acoustic phonon band structure, transmission spectrum and thermal conductance can be artificially controlled by adjusting the height of the stub.

Acoustic-phonon transmission and thermal conductance in a double-bend quantum waveguide

Acoustic-phonon transmission and thermal conductance in a double-bend quantum waveguide at low temperatures are investigated with the use of the scattering matrix method. The calculated results show that the total transmission coefficient versus the reduced phonon frequency exhibits a series of resonant peaks and dips. The stop-frequency gap can be observed for certain structural parameters due to the mode-mode coupling in the bend region. The universal quantum thermal conductance and the decrease of the thermal conductance at very low temperatures can be clearly observed. However, for higher temperatures where the higher transverse modes are excited, the reduced thermal conductance K∕T is proportional to temperature T. The transmission coefficient and thermal conductance sensitively depend on the geometric parameters of the double bend, which provide an efficient way to control thermal conductance artificially by adjusting the parameters of the proposed microstructures.