arXiv subjects

Nuo Yang

Publications and source records attributed to Nuo Yang.

At least 55 records · Page 3Linked to original sources

arXiv2018

High efficient solar evaporation by airing multifunctional textile

Solar evaporation is important for many applications such as desalination, power generation and industrial drying. Recently, some studies on evaporation reported obtaining high energy efficiency and evaporation rate, which are based on floating evaporation setup (FES) with nanomaterials. Here, we proposed a new cheap and simple setup, named as airing evaporation setup (AES). It shows that the energy efficiency of AES reaches up to 87 % under 1 kW/m2 of solar irradiation, which is 14% higher than that of FES. Meanwhile, the total evaporation rate of AES is about 20% higher than that of FES. The theoretical analysis reveals that the main reason for a better performance of AES is the increasing evaporation area. More interesting, AES could be used for designing portable systems due to its simplicity and flexibility. Furthermore, we show that AES and the corresponding wick material can be used in solar desalination, textile quick-drying and warm-keeping.

physics.app-ph↗

arXiv2018

Unexpectedly High Cross-plane Thermoelectric Performance in Layered Carbon Nitrides

Organic thermoelectric (TE) materials create a brand new perspective to search for high-efficiency TE materials, due to their small thermal conductivity. The overlap of pz orbitals, commonly existing in organic π-stacking semiconductors, can potentially result in high electronic mobility comparable to inorganic electronics. Here we propose a strategy to utilize the overlap of pz orbitals to increase the TE efficiency of layered polymeric carbon nitride (PCN). Through first-principles calculations and classical molecular dynamics simulations, we find that A-A stacked PCN has unexpectedly high cross-plane ZT up to 0.52 at 300 K, which can contribute to n-type TE groups. The high ZT originates from its one-dimensional charge transport and small thermal conductivity. The thermal contribution of the overlap of pz orbitals is investigated, which noticeably enhances the thermal transport when compared with the thermal conductivity without considering the overlap effect. For a better understanding of its TE advantages, we find that the low-dimensional charge transport results from strong pz-overlap interactions and the in-plane electronic confinement, by comparing π-stacking carbon nitride derivatives and graphite. This study can provide a guidance to search for high cross-plane TE performance in layered materials.

cond-mat.mes-hall↗

arXiv2018

Superior thermal conductivity of poly (ethylene oxide) for solid-state electrolytes: a molecular dynamics study

Solid-state lithium-ion batteries (SSLIBs) are considered to be the new generation of devices for energy storage due to better performance and safety. Poly (ethylene oxide) (PEO) based material becomes one of the best candidate of solid electrolytes, while its thermal conductivity is crucial to heat dissipation inside batteries. In this work, we study the thermal conductivity of PEO by molecular dynamics simulation. By enhancing the structure order, thermal conductivity of aligned crystalline PEO is obtained as high as 60 W/m-K at room temperature, which is two orders higher than the value (0.37 W/m-K) of amorphous structure. Interestingly, thermal conductivity of ordered structure shows a significant stepwise negative temperature dependence, which is attributed to the temperature-induced morphology change. Our study offers useful insights into the fundamental mechanisms that govern the thermal conductivity of PEO but not hinder the ionic transport, which can be used for the thermal management and further optimization of high-performance SSLIBs.

physics.app-ph↗

arXiv2018

Unexpected Thermal Conductivity Enhancement in Pillared Graphene Nanoribbon with Isotopic Resonance

Thermal transport in nanoribbon based nanostructures is critical to advancing its applications. Wave effects of phonons can give rise to controllability of heat conduction in nanostructures beyond that by particle scattering. In this paper, by introducing pillars to form structural resonance, we systematically studied the thermal conductivity of graphene nanoribbon based phononic metamaterials (GNPM) through non-equilibrium molecular dynamical simulation. Interestingly, it is found that the thermal conductivity of GNPM is counter intuitively enhanced by isotope engineering, which is strong contrast to the common notion that isotope engineering reduces thermal conductivity.Further mode analysis and atomic Green function calculation reveal that the unexpected increasing in thermal conductivity originates from the breaking of the resonant hybridization wave effect between the resonant modes and the propagating modes induced by isotope engineering. Besides, factors including the system width and pillar height can also efficiently tune the thermal conductivity of GNPM. This abnormal mechanism provides a new dimension to manipulate phonon transport in nanoribbon based nanostructures through wave effect.

cond-mat.mes-hall↗

arXiv2018

Quantifying phonon particle and wave transport in nanostructures--The unexpectedly strong particle effect in silicon nanophononic metamaterial with cross junction

Understanding phonon transport mechanisms in nanostructures is of great importance for delicately tailoring thermal properties. Combining phonon particle and wave effects through different strategies, previous studies have obtained ultra-low thermal conductivity in nanostructures. However, phonon particle and wave effects are coupled together, that is their individual contributions to phonon transport cannot be figured out. Here, we present how to quantify the particle and wave effects on phonon transport by combining Monte Carlo and atomic green function methods. We apply it to 1D silicon nanophononic metamaterial with cross-junctions, where it has been thought that the wave effect was the main modulator to block phonon transport and the particle effect was negligibly weak. Surprisingly, we find that the particle effect is quite significant as well and can contribute as much as 39% to the total thermal conductivity reduction. Moreover, the particle effect does not decrease much as the cross section area (CSA) of the structure decreases and still keeps quite strong even for CSA as small as 2.23 nm2. Further phonon transmission analysis by reducing the junction leg length also qualitatively demonstrates the strong particle effect. The results highlight the importance of mutually controlling particle and wave characteristics, and the methodologies for quantifying phonon particle and wave effect are important for phonon engineering by nanostructuring.

cond-mat.mes-hall↗

arXiv2018

Energy and exergy analysis of solar stills with micro/nano particles: A comprehensive study

In this paper, a comparative study between modified solar stills (with graphite or copper oxide micro/nano particles) and classical solar still is carried out, based on the productivity and the thermal performance. Exergy destructions in various components of the solar stills have been calculated, analyzed and discussed. Evaporation is faster and the exergy of evaporation is higher at the modified solar stills than that of the classical one. Furthermore, the energy and exergy efficiencies of the modified stills are enhanced compared with the classical one. A brief discussion regarding the effect of different parameters on solar stills efficiency is also presented. The daytime energy efficiency of graphite/water and copper oxide/water mixtures are 41.18% and 38.61%, respectively, but for the classical still is only 29.17%. Moreover, the daytime exergy efficiencies of graphite, copper oxide nanofluid based stills and classical still are 4.32%, 3.78% and 2.63%, respectively.

physics.app-ph↗

arXiv2018

High thermal conductivity of bulk epoxy resin by bottom-up parallel-linking and strain: a molecular dynamics study

The ultra-low thermal conductivity (~0.3 Wm-1K-1) of amorphous epoxy resins significantly limits their applications in electronics. Conventional top-down methods e.g. electrospinning usually result in aligned structure for linear polymers thus satisfactory enhancement on thermal conductivity, but they are deficient for epoxy resin polymerized by monomers and curing agent due to completely different cross-linked network structure. Here, we proposed a bottom-up strategy, namely parallel-linking method, to increase the intrinsic thermal conductivity of bulk epoxy resin. Through equilibrium molecular dynamics simulations, we reported on a high thermal conductivity value of parallel-linked epoxy resin (PLER) as 0.80 Wm-1K-1, more than twofold higher than that of amorphous structure. Furthermore, by applying uniaxial tensile strains along the intra-chain direction, a further enhancement in thermal conductivity was obtained, reaching 6.45 Wm-1K-1. Interestingly, we also observed that the inter-chain thermal conductivities decrease with increasing strain. The single chain of epoxy resin was also investigated and, surprisingly, its thermal conductivity was boosted by 30 times through tensile strain, as high as 33.8 Wm-1K-1. Our study may provide a new insight on the design and fabrication of epoxy resins with high thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2017

Interfacial Thermal Transport in Boron Nitride-Polymer Nanocomposite

Polymer composites with thermally conductive nanoscale filler particles, such as graphene and hexagonal boron nitride (h-BN), are promising for certain heat transfer applications. While graphene-polymer composites have been extensively investigated, studies on h-BN-polymer composites has been relatively rare. In this paper, we use molecular dynamics (MD) simulations to study the interfacial thermal conductance (ITC) involved in the h-BN-polymer composites. We first compare the ITC across h-BN/hexane (C6H14) interfaces to that of graphene/hexane interfaces, where we found that the electrostatic interaction due to the partial charge on h-BN atoms can play an important role in such interfacial thermal transport. Based this finding, we further explore the thermal transport across different h-BN interfaces, including h-BN/hexanamine (C6H13NH2), h-BN/hexanol (C6H13OH), h-BN/hexanoic acid (C5H11COOH), where the increasingly polar molecules lead to systematic changes in the electrostatic interactions between h-BN and polymers. Heat flux decomposition and atom number density calculations are performed to understand the role of electrostatic interaction in thermal transport across h-BN-polymer interfaces. It was observed that stronger electrostatic interactions across the interfaces can help attract the polymer molecules closer to h-BN, and the reduced interface distance leads to larger heat flux contributed from both van der Waals and electrostatic forces. These results may provide useful information to guide the design of thermally conductive h-BN-polymer nanocomposites.

physics.app-ph↗

arXiv2017

Generalized two-temperature model for coupled phonons

The design of graphene-based composite with high thermal conductivity requires a comprehensive understanding of phonon coupling in graphene. We extended the two-temperature model to coupled groups of phonon. The study give new physical quantities, the phonon-phonon coupling factor and length, to characterize the couplings quantitatively. Besides, our proposed coupling length has an obvious dependence on system size. Our studies can not only observe the nonequilibrium between different groups of phonon, but explain theoretically the thermal resistance inside graphene.

cond-mat.mes-hall↗

arXiv2017

Low-cost high-efficiency solar steam generation by wick material with graphite micro/nano particles

Generating water steam by solar energy is a significant process for many fields. In this paper, a low-cost high-efficiency wick type steam generator is proposed. It's based on the heat localization and thin-film evaporation. The measurements show that the energy efficiency is 84 % at 1 kw/m2. Besides, the dependence of efficiency on particle concentration and size are discussed. The optimal particle concentration is found at 60 g/m2, and a smaller particle size gives higher efficiency. The experimental results agree well with the theoretical prediction based on thin-film evaporation theory. Our study offers a new in-depth understanding of low-cost high-efficiency solar steam generation.

cond-mat.mes-hall↗

arXiv2017

Ultra-fast Vapor Generation by a Graphene Nano-ratchet

Vapor generation is of prime importance for a broad range of applications: domestic water heating, desalination and wastewater treatment, etc. However, the slow and low efficiency evaporation limits their development. In this paper, we proposed a nano-ratchet, multilayer graphene with cone-shaped nanopores (MGCN), to accelerate the vapor generation. By performing molecular dynamics simulations, we found that the air molecules spontaneously transport across MGCN and form a remarkable pressure difference, 21kPa, between the two sides of MGCN. Besides, we studied the dependence of pressure difference on the ambient temperature and the geometry of MGCN in detail. By further analysis of the diffusive transport, we identified that the pressure difference relates to the competition between ratchet transport and Knudsen diffusion. The significant pressure difference could lead to 15 times enhancement of vapor generation at least, which shows the wide applications of this nano-ratchet.

cond-mat.mes-hall↗

arXiv2017

Thermal Conductivity of PAAm Hydrogel and its Crosslinking Effect

As the interface between human and machine becomes blurred, hydrogel incorporated electronics and devices have emerged to be a new class of flexible/stretchable electronic and ionic devices due to their extraordinary properties, such as soft, mechanically robust and biocompatible. However, heat dissipation in these devices could be a critical issue and remains unexplored. Here, we report the experimental measurements and equilibrium molecular dynamic (EMD) simulations of thermal conduction in polyacrylamide (PAAm) hydrogels at room temperature. The thermal conductivity of the PAAm hydrogels can be modulated from 0.33 to 0.51 Wm-1K-1 by changing the crosslinking density. The crosslinking density dependent thermal conductivity in hydrogels is explained by the competition between the increased conduction pathways and the enhanced phonon scattering effect. The assumption is further supported by both the equilibrium swelling ratio measurement and molecular simulation of hydrogels. Our study offers fundamental understanding of thermal transport in soft materials and provides design guidance for hydrogel-based devices.

cond-mat.soft↗

arXiv2016

The unexpected thermal conductivity from graphene disk, carbon nanocone to carbon nanotube

Graphene and single-wall carbon nanotube (SWCNT) have attracted great attention because of their ultra-high thermal conductivity. However, there are few works exploring the relations of their thermal conductivity quantitatively. The carbon nanocone (CNC) is a graded structure fall in between graphene disk (GD) and SWCNT. We perform non-equilibrium molecular dynamics (NEMD) simulation to study the thermal conductivity of CNC with different apex angles, and then compare them with that of GD and SWCNT. Our results show that, different from the homogeneous thermal conductivity in SWCNT, the CNC also has a natural graded thermal conductivity which is similar to the GD. Unexpectedly, the graded rate keeps almost the same when the apex angle decreases from 180° (GD) to 19°, but then suddenly declines to zero when the apex angle decreases from 19° to 0° (SWCNT). What is more interesting, the graded effect is not diminished when the interatomic force constant is weakened and mean free path is shorten. That is, besides nanoscale, the graded effect can be observed in macroscale graphene or CNC structures.

cond-mat.mes-hall↗

arXiv2016

A Modified Theoretical Model to Predict the Thermal Interface Conductance Considering Interface Roughness

The acoustic mismatch model and the diffuse mismatch model have been widely used to predict the thermal interface conductance. However, the acoustic mismatch model (diffuse mismatch model) is based on the hypothesis of a perfectly smooth (completely disordered) interface. Here, we present a new modified model, named as the mixed mismatch model, which considers the roughness/bonding at the interface. By taking partially specular and partially diffuse transmissions into account, the mixed mismatch model can predict the thermal interface conductance with arbitrary roughness. The proportions of specular and diffuse transmission are determined by the interface roughness which is described by the interfacial density of states. It shows that the predicted results of the mixed mismatch model match well with the values of molecular dynamics simulation and experimental data.

cond-mat.mes-hall↗

arXiv2016

Nano-Cross-Junction Effect on Phonon Transport in Silicon-Nanowire-Cages

Wave effects of phonons can give rise to controllability of heat conduction beyond that by particle scattering at surfaces and interfaces. In this work, we propose a new class of 3D nanostructure: a silicon-nanowire-cage (SiNWC) structure consisting of silicon nanowires (SiNWs) connected by nano-cross-junctions (NCJs). We perform equilibrium molecular dynamics (MD) simulations, and find an ultralow value of thermal conductivity of SiNWC, 0.173 Wm-1K-1, which is one order lower than that of SiNWs. By further modal analysis and atomistic Green's function calculations, we identify that the large reduction is due to significant phonon localization induced by the phonon local resonance and hybridization at the junction part in a wide range of phonon modes. This localization effect does not require the cage to be periodic, unlike the phononic crystals, and can be realized in structures that are easier to synthesize, for instance in a form of randomly oriented SiNWs network.

cond-mat.mes-hall↗

arXiv2016

Enhancement of interfacial thermal conductance of SiC by overlapped carbon nanotubes and intertube atoms

We proposed a new way, adding intertube atoms, to enhance interfacial thermal conductance (ITC) between SiC-carbon nanotube (CNT) array structure. Non-equilibrium molecular dynamics method was used to study the ITC. The results show that the intertube atoms can significantly enhance the ITC. The dependence of ITC on both the temperature and the number of intertube atoms are shown. The mechanism is analyzed by calculating probability distributions of atomic forces and vibrational density of states. Our study may provide some guidance on enhancing the ITC of CNT-based composites.

cond-mat.mes-hall↗

arXiv2016

The adjustable thermal resistor by reversibly folding a graphene sheet

Phononic (thermal) devices are studied such as thermal diode, thermal transistors, thermal logic gates, and thermal memories. However, the thermal resistor has not been demonstrated yet. Here, we propose an instantaneously adjustable thermal resistor by folded graphene. Through theoretical analysis and molecular dynamics simulations, we studied the phonon folding effect and the dependent of thermal resistivity on the length between two folds and the overall length. Further, we discuss on the possibility to realize the instantaneously adjustable thermal resistor in experiment. Our studies bring insights in designing thermal resistor and understanding thermal modulation of 2D materials by adjusting its basic structure parameters.

cond-mat.mes-hall↗

arXiv2016

A series circuit of thermal rectifiers: an effective way to enhance rectification ratio

The low rectification ratio limits the application of thermal rectifiers. In this paper, it is found that a series circuit of thermal rectifiers in asymmetric graphene/graphene phononic crystal (GPnC) structures can improve rectification significantly. The simulations by using non-equilibrium molecular dynamics (NEMD) are performed. Moreover, the size effect on thermal rectification is also studied and discussed.

cond-mat.mes-hall↗