Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

Using the Landauer formulation of transport theory, we predict that dielectric quantum wires should exhibit quantized thermal conductance at low temperatures in a ballistic phonon regime. The quantum of thermal conductance is universal, independent of the characteristics of the material, and equal to ${\ensuremath{\pi}}^{2}{k}_{B}^{2}T/3h$ where ${k}_{B}$ is the Boltzmann constant, $h$ is Planck's constant, and $T$ is the temperature. Quantized thermal conductance should be experimentally observable in suspended nanostructures adiabatically coupled to reservoirs, devices that can be realized at the present time.

Quantized Thermal Conductance of Dielectric Quantum Wires

With the innovation of microelectronics technology, the heat dissipation problem inside the device will face a severe test. In this work, cellulose aerogel (CA) with highly enhanced thermal conductivity (TC) in vertical planes was successfully obtained by constructing a vertically aligned silicon carbide nanowires (SiC NWs)/boron nitride (BN) network via the ice template-assisted strategy. The unique network structure of SiC NWs connected to BN ensures that the TC of the composite in the vertical direction reaches 2.21 W m-1 K-1 at a low hybrid filler loading of 16.69 wt%, which was increased by 890% compared to pure epoxy (EP). In addition, relying on unique porous network structure of CA, EP-based composite also showed higher TC than other comparative samples in the horizontal direction. Meanwhile, the composite exhibits good electrically insulating with a volume electrical resistivity about 2.35 × 1011 Ω cm and displays excellent electromagnetic wave absorption performance with a minimum reflection loss of - 21.5 dB and a wide effective absorption bandwidth (< - 10 dB) from 8.8 to 11.6 GHz. Therefore, this work provides a new strategy for manufacturing polymer-based composites with excellent multifunctional performances in microelectronic packaging applications. 

https://link.springer.com/content/pdf/10.1007/s40820-022-00863-z.pdf

Vertically Aligned Silicon Carbide Nanowires/Boron Nitride Cellulose Aerogel Networks Enhanced Thermal Conductivity and Electromagnetic Absorbing of Epoxy Composites

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Unlike the wide range control of electrical conductivity, the tuning of thermal conductivity is difficult especially for the increase or bidirectional tuning. Traditional methods typically only dec...

Bidirectional Tuning of Thermal Conductivity in Ferroelectric Materials Using E-Controlled Hysteresis Characteristic Property

Graphene, due to its atomic layer structure, has the highest room temperature thermal conductivity k for all known materials. Thus, it is expected that graphene based materials are the best candidates for thermal management in next generation electronic devices. In this perspective, we first review the in-plane k of monolayer graphene and multilayer graphene obtained using experimental measurements, theoretical calculations and molecular dynamics (MD) simulations. Considering the importance of four-phonon scattering in graphene, we also compare the effects of three-phonon and four-phonon scattering on phonon transport in graphene. Then, we review phonon transport along the cross-plane direction of multilayer graphene and highlight that the cross-plane phonon mean free path is several hundreds of nanometers instead of a few nanometers as predicted using classical kinetic theory. Recently, hydrodynamic phonon transport has been observed experimentally in graphitic materials. The criteria for distinguishing the hydrodynamic from ballistic and diffusive regimes are discussed, from which we conclude that graphene based materials with a high Debye temperature and high anharmonicity (due to ZA modes) are excellent candidates to observe the hydrodynamic phonon transport. In the fourth part, we review how to actively control phonon transport in graphene. Graphene and graphite are often adopted as additives in thermal management materials such as polymer nanocomposites and thermal interface materials due to their high k. However, the enhancement of the composite's k is not so high as expected because of the large thermal resistance between graphene sheets as well as between the graphene sheet and matrix. In the fifth part, we discuss the interfacial thermal resistance and analyze its effect on the thermal conductivity of graphene based materials. In the sixth part, we give a brief introduction to the applications of graphene based materials in thermal management. Finally, we conclude our review with some perspectives for future research.

Phonon transport in graphene based materials

The reservoir area dependent thermal transport at nanoscale two-dimensional and one-dimensional interfaces is investigated by the non-equilibrium Green's function method. For the two-dimensional nanoscale interface composed of graphene sheets, the reservoir area is identical to the contact area S at the interface. As S increases from few atoms, the interfacial thermal conductance σ per S (Λ = σ/S) is negatively dependent on S due to the decrease of phonon transmission per S. With S increasing to several square nanometers, Λ converges to a constant value. However, for the one-dimensional nanoscale interface composed of nested carbon nanotubes (NCNTs), it is σ instead of Λ that converges to a constant value because the reservoir in one-dimensional nanoscale NCNTs has a fixed area, which can only provide finite transport channels. There are two competitive factors influencing the thermal transport at the interface in the NCNT model. One is phonon mode coupling and the other is phonon scattering. These two factors lead to an interesting trend of σ that as the overlap between NCNTs increases, σ increases at first and then decreases and converges to a constant value. These findings indicate that the thermal transport behavior has a strong dependence on the contact details and reservoir area at the nanoscale interface.

The reservoir area dependent thermal transport at the nanoscale interface.

Different from diffusive and ballistic phonon transport, the hydrodynamic phonon transport has many unique thermal properties such as super-linearity dependence of thermal conductivity on width and macroscopic phonon motion. In this work, the thickness dependent hydrodynamic phonon transport in layered titanium trisulphide (TiS3) is investigated by firstprinciples calculation and improved Callaway model. From the picture of linearized phonon Boltzmann transport based on first-principles calculation, two indicators at 100 K clearly show the existence of hydrodynamic phonon transport: the displaced phonon distribution with a constant drift velocity regardless of phonon wavevector and phonon polarization as well as the much larger N scattering rates than U scattering rates. The extracted drift velocity decreases significantly from 1954 m/s to 241 m/s under a temperature gradient 107 K/m as the temperature increases from 50 K to 100 K. From the improved Callaway model, the hydrodynamic phonon transport has a non-monotonic dependence on thickness due to the competition between phonon-phonon scattering and phonon-boundary scattering. The non-monotonic dependence can better understand the phonon properties of hydrodynamic transport and help design materials to observe hydrodynamic phonon transport experimentally. The temperature, width and length effects on the phonon transport behavior are also discussed individually.

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Non-Monotonic Thickness Dependent Hydrodynamic Phonon Transport in Layered Titanium Trisulphide: First-Principles Calculation and Improved Callaway Model Fitting

At the nanometer scale, heat (phonon) transport is sensitive to the contact details at the interface due to the phonon wave property. However, the effects of contact atom distribution are ignored. In this work, the atomic Green's function (AGF) method and molecular dynamics (MD) simulation are applied to explore those effects. A parameter named as the average distance d[combining macron] is raised here to measure the distribution of contacted atoms at the interface. Based on the AGF method, phonon transmission profiles at different d[combining macron] (distribution) with the same number of contacted atoms have a coincident point, the reverse frequency fr. If the phonon frequency f is smaller (larger) than fr, smaller d[combining macron] has smaller (larger) phonon transmission. The overlap of the vibrational density of states from the MD simulation and the local density of states from the AGF method indicate that the reverse frequency is caused by the match degree of vibration modes across the interface. The existence of reverse frequency leads to the reverse temperature Tr. Increasing the contact area or the interfacial coupling strength can cause the blue shift of fr and the increase of Tr. The MD simulations observe a larger temperature jump at the interface for larger d[combining macron], similar to that from the AGF method at temperatures higher than Tr due to the high-temperature limit property in MD. The results are independent of the choice of cutoff distance in potential and interfacial coupling strength, indicating that the conclusion here is applicable for the general interface.

Ping Lu

Papers

Bidirectional Tuning of Thermal Conductivity in Ferroelectric Materials Using E-Controlled Hysteresis Characteristic Property

Phonon transport in graphene based materials

The reservoir area dependent thermal transport at the nanoscale interface.

Non-Monotonic Thickness Dependent Hydrodynamic Phonon Transport in Layered Titanium Trisulphide: First-Principles Calculation and Improved Callaway Model Fitting

The effects of contact atom distribution at the interface on the phonon transport.