arXiv subjects

Nuo Yang

Publications and source records attributed to Nuo Yang.

At least 73 records · Page 4Linked to original sources

arXiv2016

The effects of graphite nanoparticles, phase change material, and film cooling on the solar still performance

In this paper, we performed four modifications on the solar still, as (A) adding the graphite nanoparticles, (B) the graphite nanoparticles together with the phase change material (PCM), (C) the graphite nanoparticles together with glass film cooling, and (D) graphite nanoparticles with both PCM and glass film cooling. The effects of modifications are measured and compared with each other. The productivities of modified (A), (B), (C), and (D) solar stills are enhanced by about 50.28%, 65.00%, 56.15% and 73.80%, respectively, as compared with the conventional solar still. The influences of saline water depths on the performance of modifications (A) and (B) are also considered. Results revealed that the best output yield is obtained for 0.5cm water depth for all solar stills.

cond-mat.mes-hall↗

arXiv2016

Enhancing thermal conductivity of bulk polyethylene along two directions by paved crosswise laminate

Recently, some reports show that the ultra-low thermal conductivity of bulk polymers can be enhanced along one direction, which limits its applications. Here, we proposed paved crosswise laminate methods which can enhance the thermal conductivity of bulk polyethylene (PE) along two directions. We find that the thermal conductivity of paved crosswise polyethylene laminate (PPEL) reaches as high as 181 W/m-K along two in-plane directions, which is three orders of magnitude larger than bulk amorphous polyethylene and even more than two times larger than PE single chain (54 W/m-K). The analyses of mechanism indicated that PPEL is a much more crystal-like structure due to the inter-chain van der Waals interactions. Our study may provide guides on the design and fabrication of polymer structures with high thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2016

Manipulate Temperature Dependence of Thermal Conductivity of Graphene Phononic Crystal

By using non-equilibrium molecular dynamics simulations(NEMD), the modulation on temperature dependence of thermal conductivity of graphene phononic crystals (GPnCs) are investigated. It is found that the temperature dependence of thermal conductivity of GPnCs follows $T^{-α}$ behavior. The power exponents ($α$) can be efficiently tuned by changing the characteristic size of GPnCs. The phonon participation ratio spectra and dispersion relation reveal that the long-range phonon modes are more affected in GPnCs with larger size of holes ($L_0$). Our results suggest that constructing GPnCs is an effective method to manipulate the temperature dependence of thermal conductivity of graphene, which would be beneficial for developing GPnCs-based thermal management and signal processing devices.

cond-mat.mes-hall↗

arXiv2015

Unusual Isotope Effect on Thermal Transport of Single Layer Molybdenum Disulphide (MoS2)

Thermal transport in single layer molybdenum disulfide (MoS2) is critical to advancing its applications. In this paper, we use molecular dynamics (MD) simulations with first-principles force constants to study the isotope effect on the thermal transport of single layer MoS2. Through phonon modal analysis, we found that isotopes can strongly scatter phonons with intermediate frequencies, and the scattering behavior can be radically different from that predicted by conventional scattering model based on perturbation theory (Tamura's formula). Such a discrepancy becomes smaller for low isotope concentrations. Natural isotopes can lead to a 30% reduction in thermal conductivity for large size samples. However, for small samples where boundary scattering becomes significant, the isotope effect can be greatly suppressed. It was also found that the Mo isotopes, which contribute more to the phonon eigenvectors in the intermediate frequency range, have stronger impact on thermal conductivity than S isotopes.

cond-mat.mtrl-sci↗

arXiv2015

Enhancing thermoelectric figure-of-merit by low-dimensional electrical transport in phonon-glass crystals

Low-dimensional electronic and glassy phononic transport are two important ingredients of highly-efficient thermoelectric material, from which two branches of the thermoelectric research emerge. One focuses on controlling electronic transport in the low dimension, while the other on multiscale phonon engineering in the bulk. Recent work has benefited much from combining these two approaches, e.g., phonon engineering in low-dimensional materials. Here, we propose to employ the low-dimensional electronic structure in bulk phonon-glass crystal as an alternative way to increase the thermoelectric efficiency. Through first-principles electronic structure calculation and classical molecular dynamics simulation, we show that the $π$-$π$ stacking Bis-Dithienothiophene molecular crystal is a natural candidate for such an approach. This is determined by the nature of its chemical bonding. Without any optimization of the material parameter, we obtain a maximum room-temperature figure of merit, $ZT$, of $1.48$ at optimal doping, thus validating our idea.

cond-mat.mtrl-sci↗

arXiv2015

A Revisit to High Thermoelectric Performance of Single-layer MoS2

Both electron and phonon transport properties of single layer MoS2 (SLMoS2) are studied. Based on first-principles calculations, the electrical conductivity of SLMoS2 is calculated by Boltzmann equations. The thermal conductivity of SLMoS2 is calculated to be as high as 116.8 Wm-1K-1 by molecular dynamics (MD) simulations. The predicted value of ZT is as high as 0.26 at 500K. As the thermal conductivity could be reduced largely by phonon engineering, there should be a high possibility to enhance ZT in the SLMoS2-based materials.

cond-mat.mes-hall↗

arXiv2015

Extremely High Thermal Conductivity of Aligned Bulk Carbon Nanotube-Polyethylene Composites

The poor thermal conductivity of bulk polymers may be enhanced by combining them with high thermal conductivity materials such as carbon nanotubes. Different from random doping, we find that the aligned carbon nanotube-polyethylene composites (ACPCs) has a high thermal conductivity by non-equilibrium molecular dynamics simulations. The analyses indicate that the aligned structure can not only take advantage of the high thermal conduction of carbon nanotubes, but enhance thermal conduction of polyethylene (PE) chains. Our predictions may inspire manufacturing aligned polymer-based composites for a wide variety of applications.

cond-mat.mtrl-sci↗

arXiv2015

The Vacancy Effect on Thermal Interface Resistance between Aluminum and Silicon by Molecular Dynamics

Thermal transport across interfaces is an important issue for microelectronics, photonics, and thermoelectric devices and has been studied both experimentally and theoretically in the past. In this paper, thermal interface resistance (1/G) between aluminum and silicon with nanoscale vacancies was calculated using non-equilibrium molecular dynamics (NEMD). Both phonon-phonon coupling and electron-phonon coupling are considered in calculations. The results showed that thermal interface resistance increased largely due to vacancies. The effect of both the size and the type of vacancies is studied and compared. And an obvious difference is found for structures with different type/size vacancies.

cond-mat.mtrl-sci↗

arXiv2014

Thermoelectric Properties of Nanoscale three dimensional Si Phononic Crystal

The thermoelectric properties of n type nanoscale three dimensional (3D) Si phononic crystals (PnCs) with spherical pores are studied. Density functional theory and Boltzmann transport equation under the relaxation time approximation are applied to study the electronic transport coefficients, electrical conductivity, Seebeck coefficient and electronic thermal conductivity. We found that the electronic transport coefficients in 3D Si PnC at room temperature (300 K) change very little compared with that of Si, for example, electrical conductivity and electronic thermal conductivity is decreased by 0.26 to 0.41 and 0.39 to 0.55 depending on carrier concentration, respectively, and the Seebeck coefficient is similar to that of bulk Si. However, the lattice thermal conductivity of 3D Si PnCs with spherical pores is decreased by a factor of 500 calculated by molecular dynamics methods, leading to the ZT of 0.76, which is about 30 times of that of porous Si. This work indicates that 3D Si PnC is a promising candidate for high efficiency thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2014

Nanoscale Graphene Disk: A Natural Functionally Graded Material --The Thermal Conductivity of Nanoscale Graphene Disk by Molecular Dynamics Simulation

In this letter, we investigate numerically (by non-equilibrium molecular dynamics) and analytically the thermal conductivity of nanoscale graphene disks (NGDs), and discussed the possibility to realize FGM with only one material, NGDs. We found that the NGD has a graded thermal conductivity and can be used as FGM in a large temperature range. Moreover, we show the dependent of NGDs' thermal conductivity on radius and temperature. Our study may inspire experimentalists to develop NGD based FGMs and help heat removal of hot spots on chips by graphene.

cond-mat.mtrl-sci↗

arXiv2014

Significant Reduction of Graphene Thermal Conductivity by Phononic Crystal Structure

We studied the thermal conductivity of graphene phononic crystal (GPnC), also named as graphene nanomesh, by molecular dynamics simulations. The dependences of thermal conductivity of GPnCs on both length and temperature are investigated. It is found that the thermal conductivity of GPnCs is significantly lower than that of graphene and can be efficiently tuned by changing the porosity and period length. For example, the ratio of thermal conductivity of GPnC to thermal conductivity of graphene can be changed from 0.1 to 0.01 when the porosity is changed from about 21% to 65%. The phonon participation ratio spectra reveal that more phonon modes are localized in GPnCs with larger porosity. Our results suggest that creating GPnCs is a valuable method to efficiently manipulate the thermal conductivity of graphene.

cond-mat.mtrl-sci↗

arXiv2014

Thermal Interface Conductance between Aluminum and Silicon by Molecular Dynamics Simulations

The thermal interface conductance between Al and Si was simulated by a non-equilibrium molecular dynamics method. In the simulations, the coupling between electrons and phonons in Al are considered by using a stochastic force. The results show the size dependence of the interface thermal conductance and the effect of electron-phonon coupling on the interface thermal conductance. To understand the mechanism of interface resistance, the vibration power spectra are calculated. We find that the atomic level disorder near the interface is an important aspect of interfacial phonon transport, which leads to a modification of the phonon states near the interface. There, the vibrational spectrum near the interface greatly differs from the bulk. This change in the vibrational spectrum affects the results predicted by AMM and DMM theories and indicates new physics is involved with phonon transport across interfaces. Keywords:

cond-mat.mtrl-sci↗

arXiv2013

Extreme Low Thermal Conductivity in Nanoscale 3D Si Phononic Crystal with Spherical Pores

Thermoelectric material provides a high hope for converting harmful and useless heat into useful energy, electricity. It can also be used as solid state Peltier coolers in integrated circuits, an outstanding challenge for electronic engineers. The desire for a high efficient thermoelectric material has never been so keen. Although many works have been done, we are still far from having a recipe for thermoelectric materials. Nano-structuring provides an effective way to increase figure of merit (ZT) by reducing the thermal conductivity without affecting electronic property.1 Here, we propose a novel nanoscale three-dimensional (3D) Si phononic crystal (PnC) with spherical pores, which can reduce the thermal conductivity of bulk Si by a factor up to 10,000 times at room temperature. The extreme-low thermal conductivity could lead to a larger value of ZT than unity. The thermal conductivity changes little when temperature increases from room temperature to 1100 K. The phonon participation ratio spectra show there are more phonon localizations as the porosity of PnC increases.

cond-mat.mes-hall↗

arXiv2013

Modulation of Thermal Conductivity in Kinked Silicon Nanowires: Phonon Interchanging and Pinching Effects

We perform molecular dynamics simulations to investigate the reduction of the thermal conductivity by kinks in silicon nanowires. The reduction percentage can be as high as 70% at room temperature. The temperature dependence of the reduction is also calculated. By calculating phonon polarization vectors, two mechanisms are found to be responsible for the reduced thermal conductivity: (1) the interchanging effect between the longitudinal and transverse phonon modes and (2) the pinching effect, i.e a new type of localization, for the twisting and transverse phonon modes in the kinked silicon nanowires. Our work demonstrates that the phonon interchanging and pinching effects, induced by kinking, are brand new and effective ways in modulating heat transfer in nanowires, which enables the kinked silicon nanowires to be a promising candidate for thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2013

Reduction of Thermal Conductivity by Nanoscale 3D Phononic Crystal

The thermal conductivity of nanostructures needs to be as small as possible so that it will have a greater efficiency for solid-state electricity generation/refrigeration by thermoelectrics. We studied how the period length and the mass ratio affect the thermal conductivity of isotopic nanoscale 3D phononic crystal of Si. Simulation results by equilibrium molecular dynamics show isotopic nanoscale 3D phononic crystal can make a significance reduction on the thermal conductivity of bulk Si at high temperature,1000 K. Size and mass effects are obvious in manipulating thermal conductivity. The thermal conductivity decreases as the period length and mass ratio increases. The phonon dispersion curves show the decrease of group velocities in 3D phononic crystals. The phonon's localization and band gap is clearly shown in spectra of normalized inverse participation ratio in nanoscale 3D phononic crystal structure.

cond-mat.mes-hall↗

arXiv2013

Thermal transport in nanostructures

This review summarizes recent studies of thermal transport in nanoscaled semiconductors. Different from bulk materials, new physics and novel thermal properties arise in low dimensional nanostructures, such as the abnormal heat conduction, the size dependence of thermal conductivity, phonon boundary/edge scatterings. It is also demonstrated that phonons transport super-diffusively in low dimensional structures, in other words, Fourier's law is not applicable. Based on manipulating phonons, we also discuss envisioned applications of nanostructures in a broad area, ranging from thermoelectrics, heat dissipation to phononic devices.

cond-mat.mes-hall↗

arXiv2012

How Does Folding Modulate Thermal Conductivity of Graphene

We study thermal transport in folded graphene nanoribbons using molecular dynamics simulations and the non-equilibrium Green's function method. It is found that the thermal conductivity of flat graphene nanoribbons can be modulated by folding and changing interlayer couplings. The analysis of transmission reveals that the reduction of thermal conductivity is due to scattering of low frequency phonons by the folds. Our results suggest that folding can be utilized in the modulation of thermal transport properties in graphene and other two dimensional materials.

cond-mat.mes-hall↗

arXiv2010

Violation of Fourier's Law and Anomalous Heat Diffusion in Silicon

We study heat conduction and diffusion in silicon nanowires (SiNWs) systematically by using non-equilibrium molecular dynamics. It is found that the thermal conductivity of SiNWs diverges with the length, even when the length is up to 1,100 nm which is much longer than the phonon mean free path. Moreover, an anomalous heat diffusion is observed which is believed to be responsible for the length dependent thermal conductivity. Our results provide strong evidence that Fourier's law of heat conduction is not valid in low dimensional nanostructures.

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