Ballistic Thermal Conductance Research Articles

Quasi-ballistic thermal transport in silicon carbide nanowires

Silicon carbide (SiC) is an important industrial material that enables the thermal stability of power electronics. However, the nanoscale phenomenon of ballistic thermal conduction, which may further improve the thermal performance, remains unexplored in SiC. Here, we reveal the length and temperature scales at which SiC exhibits quasi-ballistic thermal conduction. Our time-domain thermoreflectance measurements probe the thermal conductivity of SiC nanowires as a function of their length and temperature. The deviation of the thermal conductivity from the diffusive limit in nanowires shorter than a few micrometers indicates the transition into a quasi-ballistic thermal conduction regime. Naturally, the deviation is greater at lower temperatures, yet the effect persists even above room temperature. Our Monte Carlo simulations of phonon transport support our experimental results and show how phonons with long mean free paths carry a substantial amount of heat, causing quasi-ballistic conduction. These findings show that quasi-ballistic heat conduction can persist at the microscale at operating temperatures of power devices, and thus may help improve the thermal design in electronics based on SiC.

Molecular dynamics study on the thermal conductivity and ballistic resistance of twisted graphene

The twist as a new degree of freedom induces several angle dependent properties in twisted graphene, such as electronic and optical properties. However, the ballistic resistance and thermal conductivity of twisted graphene remain not well understood and experimentally challenging. Therefore, the effect of interlayer twist angle on the thermal conductivity and ballistic resistance of bilayer graphene (BLG) has been explored using molecular dynamics simulation herein. Results show that the thermal conductivity of graphene is very sensitive to the twist angle of the interlayer plane. As the twist angle increases from 0° to 60°, the in-plane thermal conductivity of twisted bilayer graphene (t-BLG) first decreases and then increases with a periodic interval of 30°, exhibiting an asymmetric double V-shaped curve. And the cross-plane thermal conductivity of t-BLG gradually first decreases from 0° to 30° and then increases from 30° to 60°, also showing an asymmetric V-shaped curve. In contrast, the interlayer twist angle has a limited influence on the ballistic resistance of t-BLG. With the change of the twist angle, the ballistic limit velocity does not change much, and the fluctuation range is less than 1.56%; the relative kinetic energy also changes little with the fluctuation less than 4%. Such a slight difference is insufficient to make a significant change in its ballistic resistance and its impact resistance is mainly affected by the number of membrane layers. Each additional layer increases its ballistic limit speed by about 8%. This study will provide some theoretical guidance for the study of heat dissipation in the anti-ballistic process of twisted graphene and the design of advanced protective materials.

Surface asymmetry induced turn-overed lifetime of acoustic phonons in monolayer MoSSe

Recent successful growth of asymmetric transition metal dichalcogenides via accurate manipulation of different chalcogen atoms in top and bottom surfaces demonstrates exotic electronic and chemical properties in such Janus systems. Within the framework of density functional perturbation theory, anharmonic phonon properties of monolayer Janus MoSSe sheet are explored. By considering three-phonons scattering, out-of-plane flexural acoustic (ZA) mode tends to undergo a stronger phonon scattering than transverse acoustic (TA) mode and the longitudinal acoustic (LA) mode with phonon lifetime of ZA (1.0 ps)<LA (23.8 ps)<TA (25.8 ps). This is sharply different from the symmetric MoS2 where flexural ZA mode has the weakest anharmonicity and is least scattered. Moreover, utilizing non-equilibrium Green function method, ballistic thermal conductance at room temperature is found to be around 0.11nWK-1nm-2, lower than that of MoS2. Our work highlights intriguing phononic properties of such MoSSe Janus layers associated with asymmetric surfaces.

Solution of Bullet Proof Wooden Frame Construction Panel with a Built-In Air Duct

The growing terrorism threats across the world play an important role in the design of civil buildings and living areas. The safety of personnel is a top priority in unclassified buildings, especially military buildings. However indoor air quality and thermal comfort has a direct impact on personal productivity and ability to concentrate on duties and affect the decision making in stress conditions. The use of wooden structures is becoming more common in the building construction, and application of wooden frame structures for the construction of new buildings as well as for retrofitting the existing buildings. Prefabricated wooded frame construction perfectly fits need of unclassified buildings, allowing significant reduction of construction time and integration of various active and passive elements, such as a fresh air supply duct. Within the scope of this paper a 12 mm thick ballistic panel made of aramid was tested. Ballistic panel, thermal conductivity, and fire resistance of wooded construction panel with embedded air duct were analyzed for the various modelled exterior wall solutions. The main advantage of the proposed technology is fast and qualitative modular construction of unclassified buildings, providing all modern requirements not only for safety, but also for the energy efficiency and indoor air quality. It was found that bullet proof aramid panels do not reduce overall fire safety in comparison to traditional construction materials. However embedded outdoor air supply ducts significantly reduces construction heat transfer coefficient.

Electromagnetized Graphene Facilitates Direct Lineage Reprogramming into Dopaminergic Neurons

AbstractGraphene is a carbon nanomaterial that has unique characteristics, including ballistic conduction, thermal and electrical conductivity, and biocompatibility. It is emerging as a promising tool for controlling various cell behaviors, such as viability, development, and differentiation. Here, it is reported that the magnetized graphene nanosheets facilitate direct lineage reprogramming of induced dopaminergic (iDA) neurons. The graphene nanosheets are exposed to specific intensities and frequencies of electromagnetic fields, which lead to the accumulation of histone acetylation, including H3K27ac and H4K12ac, for the robust direct reprogramming of DA neurons. Remarkably, electromagnetized graphene nanosheet‐mediated in vivo reprogramming significantly enhances the generation of iDA neurons in the mouse models of Parkinson's disease (PD), which efficiently ameliorate PD symptoms. Taken together, the results provide evidence that magnetized graphene can be used as a novel therapeutic application for PD which expands the applications of graphene as biomaterials for regenerative therapeutics.

Unified ballistic transport relation for anisotropic dispersions and generalized dimensions

An analytical formula is derived for particle and energy densities of fermions and bosons, and their ballistic momentum and energy currents for anisotropic energy dispersions in generalized dimensions. The formulation considerably simplifies the comparison of the statistical properties and ballistic particle and energy transport currents of electrons, acoustic phonons, and photons in various dimensions in a unified manner. Assorted examples of its utility are discussed, ranging from blackbody radiation to Schottky diodes and ballistic transistors, quantized electrical and thermal conductance, generalized ballistic Seebeck and Peltier coefficients, their Onsager relations, the generalized Wiedemann-Franz law and the robustness of the Lorenz number, and ballistic thermoelectric power factors, all of which are obtained from the single formula. The new formulation predicts a thermoelectric power factor behaviour of 3D Dirac bands which has not been observed yet.

The important role of strain on phonon hydrodynamics in diamond-like bi-layer graphene

In this work, combining first-principles calculation and the phonon Boltzmann transport equation, we explored the diffusive thermal conductivity of diamond-like bi-layer graphene. The converged iterative solution provides high room temperature thermal conductivity of 2034 W mK−1, significantly higher than other 2D materials. More interesting, the thermal conductivity calculated by relaxation time approximation is about 33% underestimated, revealing a remarkable phonon hydrodynamic transport characteristic. Significant strain dependence is reported, for example, under 5% tensile strain, room temperature thermal conductivity (1081 W mK−1) of only about 50% of the strain-free sample, and under 20% strain, it reduces dramatically to only about 11% of the intrinsic one (226 W mK−1). Unexpectedly, in addition to the remarkable reduction in the absolute value of thermal conductivity, tensile strain can impact the hydrodynamic significance. For example, under 5% strain, the underestimation of relaxation time approximation in thermal conductivity is reduced to 20%. Furthermore, using a non-equilibrium Green’s function calculation, high ballistic thermal conductance (2.95 GW m−2 K–1) is demonstrated, and the mean free path is predicted to be 700 nm at room temperature. The importance of the knowledge of phonon transport in diamond-like bi-layer graphene goes beyond fundamental physics owing to its relevance to thermal management applications due to the super-high thermal conduction.

Phonon stability and phonon transport of graphene-like borophene

Recent decades have seen tremendous progress in quantitative understanding of phonon transport, which is critical for the thermal management of various functional devices and the proper optimization of thermoelectric materials. In this work, using a first-principles based calculation combined with the non-equilibrium Green’s function and a phonon Boltzmann transport equation, we provide a systematic study of the phonon stability and phonon transport of a monolayer boron sheet with a honeycomb, graphene-like structure (graphene-like borophene) in both ballistic and diffusive regimes. For free-standing graphene-like borophene, phonon instabilities occur near the centre of the Brillouin zone, implying elastic instability. Investigation of the electronic structures shows that the phonon instability is due to the deficiency of electrons. Our first-principles results show that with net charge doping and in-plane tensile strain, graphene-like borophene becomes thermodynamically stable in ideal planar nature, because the bonding characteristic is modified. At room temperature, the ballistic thermal conductance of graphene-like borophene (7.14 nWK−1 nm−2) is higher than that of graphene (4.1 nWK−1 nm−2), due to high phonon transmission. However, its diffusive thermal conductivity is two orders of magnitude lower than graphene, because the phonon relaxation time is dramatically reduced compared with its carbon counterpart. Although the phonon group velocity and phonon anharmonicity are comparable with those of graphene, the suppressed phonon space results in dramatically strong phonon–phonon scattering. These thermal transport characteristics in both ballistic and diffusive regimes are of fundamental and technological relevance and provide guidance for applications of boron-based nanomaterials in which thermal conduction is the major concern.

Heat Propagation in a One-Dimensional Harmonic Crystal on an Elastic Foundation

A closed system of differential equations has been derived to describe thermal processes in a one-dimensional harmonic crystal on an elastic foundation. It is shown that the evolution of thermal perturbation in such a crystal is described by a discrete unsteady-state equation, a special case of which is the hyperbolic equation of ballistic heat conduction. This equation remains valid with negative stiffness of bonds between particles of the crystal in its entire stability range. The thermal perturbation front propagates with the maximum group velocity of mechanical waves. The propagation of a short-term thermal perturbation in the crystal on the elastic foundation is determined by the equation of ballistic thermal conductivity of the same type as in the crystal without an elastic foundation. The only parameter of this equation is the maximum group velocity (in absolute value), i.e., the maximum rate of energy propagation in the crystal on the elastic foundation. This quantity is proportional to the absolute value of the half-difference of the upper and lower cutoff frequencies. The rate of heat wave propagation in the crystal on the elastic foundation with positive stiffness is always lower than that in the crystal without an elastic foundation. The obtained equation is found to be valid both for positive stiffness values and for negative ones, for which the chain stability condition is satisfied. As an example, a dynamic problem of heat distribution is solved exactly for a parabolic initial temperature profile to model heating of a one-dimensional crystal on a foundation by a short laser pulse. Due to the dispersion of mechanical waves in the chain on the foundation, their group velocity depends on the wave number and the ratio of bond stiffnesses in the chain and the elastic foundation. The thermal front propagates with the maximum possible group velocity in the system, which depends only on this ratio.

Interfacial heat transport across multilayer nanofilms in ballistic–diffusive regime

In recent years, investigations of nanoscale heat transfer have been regarded as a critical challenge for thermal engineering applications. Therefore, improving the thermal conduction inside nanodevices is required to ensure the reliability of future-generation nanoelectronics. Owing to the fast improvement in the semiconductor industry, researchers have been strongly motivated to study interfacial phonon transport to reduce the self-heating mechanism caused by ballistic thermal conduction, which induces size-dependent effective thermal conductivity. Based on the phonon Boltzmann equation (BTE), we obtain the ballistic–diffusive equation and Cattaneo–Vernotte models to explain the heat conduction in multiple interfaces of silicon and silicon dioxide nanolayers. Both the temperature-jump boundary condition and thermal boundary resistance, also referred to as Kapitza resistance, can control the interfacial heat transport in the ballistic–diffusive regime. In the present study, our analytical models are compared with the phonon BTE to validate the performance of our methodology. We find that if the thermal boundary resistance reduces, the heat transport across the interfaces can be enhanced, which is necessary to limit the heat dissipation in silicon nanolayers.

Semiballistic thermal conduction in polycrystalline SiGe nanowires

Ballistic heat conduction remains a controversial nanoscale phenomenon because of its occurrence and strength depending on the material, alloy composition, and temperature. Here, we discuss the impact of ballistic thermal conduction and compare the results with theoretical predictions. We experimentally investigate ballistic thermal transport in SiGe polycrystalline nanowires by measuring the length dependence of thermal conductivity for different alloy compositions and temperatures. At room temperature, our experiments show that ballistic effects are negligible in nanowires made of pure polycrystalline Si but become stronger as the Ge composition increases. As we decreased the temperature, we observed that ballistic contribution becomes even stronger.

Probing ballistic thermal conduction in segmented silicon nanowires.

Ballistic heat conduction in semiconductors is a remarkable but controversial nanoscale phenomenon, which implies that nanostructures can conduct thermal energy without dissipation. Here, we experimentally probed ballistic thermal transport at distances of 400-800 nm and temperatures of 4-250 K. Measuring thermal properties of straight and serpentine silicon nanowires, we found that at 4 K heat conduction is quasi-ballistic with stronger ballisticity at shorter length scales. As we increased the temperature, quasi-ballistic heat conduction weakened and gradually turned into diffusive regime at temperatures above 150 K. Our Monte Carlo simulations illustrate how this transition is driven by different scattering processes and linked to the surface roughness and the temperature. These results demonstrate the length and temperature limits of quasi-ballistic heat conduction in silicon nanostructures, knowledge of which is essential for thermal management in microelectronics.

Ballistic and Diffusive Thermal Conductivity of Graphene

In electronics, cooling a device would seem to be as straightforward as putting a good thermal conductor on top. Graphene is one of the best thermal conductors and thus a natural choice, but realize that its thermal conductivity is a function of sample size $L$, and also strongly depends on temperature. Through extensive phonon-mode calculations, the authors show that graphene's maximum thermal conductivity occurs around 100 K, and that $L$ of just 10 $\ensuremath{\mu}$m is sufficient for maximum effect. With these results, one can engineer a device with optimal $L$ for a required thermal conductivity at a given temperature.

Ballistic electronic and thermal conductance of monolayer and bilayer black phosphorus

We performed first principle calculations based on density functional theory and GW approximation with Landauer approach to study ballistic electronic conductance and ballistic thermal conductance of black phosphorus (BP) whose dimensions are smaller than the mean free path of carriers. Ballistic electronic conductance of monolayer BP and bilayer BP with four different stacking styles, and ballistic thermal conductance of monolayer BP were investigated. The electronic transmission value along armchair direction is found to be a little higher than that along zigzag direction near the band gap energy range. Our calculations showed that, the Г point hole effective mass along zigzag direction is stacking-style sensitive, while other Г point effective mass is independent of stacking style. We also reported the anisotropic ballistic thermal conductance of monolayer BP: the anisotropic ballistic thermal conductance ratio of zigzag-to-armchair approaches a constant (about 2.0) when the temperature is beyond 200 K. The reasons for these properties were discussed.

Fast electron transfer and enhanced visible light photocatalytic activity using multi-dimensional components of carbon quantum dots@3D daisy-like In2S3/single-wall carbon nanotubes

One of current ideal thought to reduce the recombination of photogenerated electrons and holes from semiconductors is constructing Multi-dimensional semiconductor-carbon (S-C) heterostructures. As well as acting to photo-degrade the organic contaminant and decrease the toxicity, β-In2S3 is chosen as a potential semiconductors for photocatalysts. Relative good photocatalytic properties delivered on β-In2S3, however, the fast recombination of photogenerated charge carriers are often appeared, causing reduced further application. Carbon quantum dots (CQDs) involve the introduction of plasma effect into the zero-dimensional nanomaterials in order to arouse an effect: either promoting the charge carriers transfer or plasma energy conversion of the photo-excited CQDs. However, state of the two components also encompass the poor quantum yield existed in the CQDs by its serious agglomeration. Thereby, the delivery of extraordinary ballistic electrical and thermal conductivity on single-walled carbon nanotubes (SWNTs), have shown potential for use in a variety of semiconductors decorating application. Herein, we show that a novel CQDs@In2S3/SWNTs composite in the Multi-dimensional (3D) hierarchical superstructures with an enhanced photocatalytic efficiency. The ESR analysis and free radicals trapping experiments indicated that the O2•− and h+ were the main active species for the photocatalytic degradation. The potential photocatalytic mechanism of the three components is discussed and the direction in the plasma effect exhibited in CQDs is also considered, with a particular focus on photocatalytic area.

Low-temperature heat transport of spin-gapped quantum magnets

This article reviews low-temperature heat transport studies of spin-gapped quantum magnets in the last few decades. Quantum magnets with small spins and low dimensionality exhibit a variety of novel phenomena. Among them, some systems are char-acteristic of having quantum-mechanism spin gap in their magnetic excitation spectra, including spin-Peierls systems, S = 1 Haldane chains, S = 1/2 spin ladders, and spin dimmers. In some particular spin-gapped systems, the XY-type antiferromagnetic state induced by magnetic field that closes the spin gap can be described as a magnon Bose-Einstein condensation (BEC). Heat transport is effective in probing the magnetic excitations and magnetic phase transitions, and has been extensively studied for the spin-gapped systems. A large and ballistic spin thermal conductivity was observed in the two-leg Heisenberg S = 1/2 ladder compounds. The characteristic of magnetic thermal transport of the Haldane chain systems is quite controversial on both the theoretical and experimental results. For the spin-Peierls system, the spin excitations can also act as heat carriers. In spin-dimer compounds, the magnetic excitations mainly play a role of scattering phonons. The magnetic excitations in the magnon BEC systems displayed dual roles, carrying heat or scattering phonons, in different materials.

Thermomechanical analysis of two-dimensional boron monolayers

Using density functional theory calculations (both perturbed and unperturbed) as well as thermodynamic and ballistic transport equations, what follows investigates thermal and mechanical properties of two-dimensional boron monolayers (${\ensuremath{\delta}}_{6}, \ensuremath{\alpha}, {\ensuremath{\delta}}_{5}$, and ${\ensuremath{\chi}}_{3}$ sheets with respective vacancy densities $\ensuremath{\eta}=0$, 1/9, 1/7, 1/5) as they relate to the vacancy density. The triangular $({\ensuremath{\delta}}_{6})$ sheet's room-temperature phonon and electron thermal conductances are found, respectively, to be roughly 2.06 times and 6.60 times greater than those of graphene. The Young's moduli, calculated from longitudinal and transverse sound velocities are in good agreement with those obtained from elastic constants. Values range from 171 to 619 N/m, two of which (619 N/m for $\ensuremath{\alpha}$ sheet and 546 N/m for ${\ensuremath{\delta}}_{5}$ sheet) exceed graphene's Young's modulus ($\ensuremath{\sim}340\phantom{\rule{0.16em}{0ex}}\mathrm{N}/\mathrm{m}$). It is determined that the vacancy density has a diminishing effect on both the phonon heat capacity at constant volume and the phonon ballistic thermal conductance, but no regular correlation on the electron heat capacity and electron ballistic thermal conductance.

Axial strain effects on ballistic phonon thermal transport in silicon nanowires

We study the effects of axial strain on phonon thermal transport in silicon nanowires (Si NWs). The thermal conductance of strained Si NWs in the ballistic regime is evaluated with the phonon dispersion relation derived through an atomistic approach. Compressive strain increases the ballistic thermal conductance of [100]-oriented Si NWs having a square cross section with a side length of about 3 nm. In contrast, tensile strain reduces it. These strain effects mainly result from two factors: a change in the phonon energy per unit length of a NW directly due to the length change of the NW, and a change in the phonon group velocity. We also investigate the relationships of the strain effects with the wire size, surface orientation, and wire orientation. Among the three factors, the wire orientation has the most significant effect on the strain effects.

Thermal Transport in Single-Layer MoS2and Black Phosphorus Transistors

Ballistic thermal conductance in single-layer MoS2 and black phosphorus (BP) is calculated based on the phonon dispersion. In single-layer MoS2, the ballistic thermal conductance is almost isotropic of a value of $10.7\times 10^{8}$ W/ $\text{m}^{2}\text{K}$ ; while in single-layer BP, the in-plane ballistic thermal conductance is anisotropic with a value of $12.8\times 10^{8}$ W/ $\text{m}^{2}\text{K}$ in the zigzag direction and $7.1\times 10^{8}$ W/ $\text{m}^{2}\text{K}$ in the armchair direction. Thermal transport behaviors at both steady state and transient state in double-gated MOSFET-like transistors with single-layer MoS2 and BP used as the channel material are studied. The conventional Fourier’s law with contact temperatures as the boundary conditions and modified boundary conditions that can capture quasi-ballistic phonon transport effects are used. The simulation results show that Fourier’s law with contact temperatures as the boundary conditions underestimate the peak temperature rise by 64% in single-layer MoS2 transistors and 56% in single-layer BP transistors when the channel length is comparable to the phonon mean free path. And the strong anisotropic thermal properties of single-layer BP lead to higher temperature rise in the armchair direction than that in the zigzag direction.

Tunable ballistic thermal conductance of electrons in strained graphene nanoribbons

The thermal transport properties of electrons in graphene nanoribbon (GNR) subjected to strain fields are studied by non-equilibrium Green's function approach. The results show that the stretching of the carbon–carbon bond in strained GNR can enhance the ballistic thermal conductance of electrons. The enhancement originates from that the change of C–C bond length can modulate the nearest-neighbour π-orbital overlap and reduce the threshold energies of electron transport modes. A brief analysis of these results is given.