三维石墨烯-碳纳米管复合结构热导率的分子动力学模拟

采用非平衡分子动力学方法模拟了三维石墨烯-碳纳米管复合结构的法向热导率。结果表明相比于多层石墨烯,其法向热导率提高了一个量级,其界面热阻相比碳纳米管的接触热阻降低了一个量级,但是石墨烯和碳纳米管的界面形变又阻碍了三维石墨烯-碳纳米管复合热导率的进一步提高。通过其振动态密度和重叠能进一步探究了三维石墨烯-碳纳米管复合结构结构能量的传递及声子的局域化情况。结果表明,碳管的添加激发了更多中高频声子振动参与传热,但是依然是低频声子占据主导;验证了界面处的形变是阻止法向热导率进一步提升的主要因素。     

Coupled thermal‐electrical analysis of carbon nanotube/epoxy composites

The electrical and thermal behavior of epoxy composites reinforced with different contents of multi‐walled carbon nanotubes (from 0.1 to 0.4 wt% CNT) is studied when they are subjected to relatively high DC voltages (from 1 to 100 V). These materials obey Ohm's law, reaching values of electrical conductivity in the range of 0.01–0.5 S/m. The transported electric current leads to a significant increase of temperature, which is a result of the Joule heating effect. The temperature increases to 40ºC in CNT/epoxy composites when applying 100 V. The study of heating due to Joule's effect gives information about the electrical transport mechanisms implied. It is also confirmed that both, electrical conductivity and Joule's heating effect depend on the morphological features of the composites. The functionalization of CNTs decreases the electrical conductivity of composites but increases their corresponding Joule heating, due to the strong interface between the nanotubes and matrix, which hinders the formation of pathways in CNT in direct contact. The technique of CNT dispersion applied also affects to the increase of temperature induced by the electrical current. POLYM. ENG. SCI., 54:1976–1982, 2014. © 2013 Society of Plastics Engineers     

Latex technology as a simple route to improve the thermal conductivity of a carbon nanotube/polymer composite

A novel route was revealed to reduce the interfacial phonon scattering that was considered as the bottleneck for carbon nanotubes (CNTs) to highly improve the thermal conductivity of CNT/polymer composites. Semicrystalline polyurethane (PU) dispersions were used as latex host to accommodate multi-walled carbon nanotubes (MWCNTs) following the colloidal physics method. The thermal conductivity increased from 0.15 W m⁻¹ K⁻¹ to 0.47 W m⁻¹ K⁻¹, by ∼210%, as the addition of the MWCNTs increases to 3 wt%. The morphology of the composites that was characterized by optical microscopy, scanning electron microscopy and differential scanning calorimeter suggested that the continuous nanotube-rich phase existing in the interstitial space among the latex particles and the crystallites nucleated at the nanotube–polymer interface were the main factors for the effective reduction of interfacial phonon scattering.     

A novel nano-configuration for thermoelectrics: helicity induced thermal conductivity reduction in nanowires

In this article, we propose a novel helical nano-configuration towards the designing of high ZT thermoelectric materials. Non-equilibrium molecular dynamics (NEMD) simulations for 'model' bi-component nanowires indicate that a significant reduction in thermal conductivity, similar to that of flat superlattice nanostructures, can be achieved using a helical geometric configuration. The reduction is attributed to a plethora of transmissive and reflective phonon scattering events resulting from the steady alteration of phonon propagating direction that emerges from the continuous rotation of the helical interface. We also show that increasing the relative mass ratio of the two components lowers the phonon energy transmission at the interface due to differences in vibrational frequency spectra, thereby relatively 'easing' the phonon energy propagation along the helical pathway. While the proposed mechanisms result in a reduced lattice thermal conductivity, the continuous nature of the bi-component nanowire would not be expected to significantly reduce its electrical counterpart, as often occurs in superlattice/alloy nanostructures. Hence, we postulate that the helical configuration of atomic arrangement provides an attractive and general framework for improved thermoelectric material assemblies independent of the specific chemical composition.     

An ab initio study of ZrB2–SiC interface strength as a function of temperature: Correlating phononic and electronic thermal contributions

This work focuses on understanding correlations between thermal conduction and mechanical strength in a model high temperature material interface. Analyses examine single crystal ZrB₂, single crystal SiC, and a 〈0 0 0 1〉–〈1 1 1〉 ZrB₂–SiC interface using a framework based on Car Parrinello molecular dynamics (CPMD) ab initio simulation method from 500 K to 2500 K. Analyses indicate that the strength reduction with increase in temperature is strongly correlated to phonon and electron thermal diffusivity change. With increase in temperature, phonon thermal diffusivity increases in the case of ZrB₂ and reduces in the cases of SiC as well as the interface. Electron contribution to thermal diffusivity increases with temperature increase in the case of interface. Examination of change in thermal properties at different mechanical strain levels reveals that the mechanisms of strength and thermal property change with increase in temperature may be similar to the mechanisms responsible for property change with change in applied strain.     

Enhanced Charge Transport in ZnO Nanocomposite Through Interface Control Using Multiwall Carbon Nanotubes

Hybrid strategy of ZnO with carbon nanotube (CNT) has been attempted, and synergistic effects have been demonstrated in ZnO‐CNT hybrid nanostructures owing to the advantageous effects of interface modification on the charge transport process. Here, we report the effects of interface control using multiwall CNTs (MWCNTs) on the charge transport properties in Al‐doped ZnO (AZO) nanocomposite. Although the AZO‐MWCNT nanocomposite is composed of numerous nanograins, it shows single crystalline charge transport behavior due to significantly weakened grain‐boundary scattering at room temperature. The dominant charge transport mechanism is converted from lattice vibration scattering to grain‐boundary scattering at 873 K due to the variation in the charge distribution at the grain boundary. The results demonstrate that interface control using carbon nanomaterials has a significant effect on the charge transport behavior in AZO nanocomposite.     

Effect of carbon nanotube dispersion and network formation on thermal conductivity of thermoplastic polyurethane/carbon nanotube nanocomposites

Carbon nanotubes (CNTs) were dispersed without any solvent in poly(tetramethylene ether glycol), (PTMEG) well above its melting point by ultrasonication in the pulse mode and different times. The polyol/CNT suspensions were used to prepare in situ polymerized thermoplastic polyurethane TPU/CNT nanocomposites with the CNT concentration of ～ 0.05 vol%, much below the CNT geometrical percolation threshold calculated at 0.43 vol%. Results of rotational rheological measurements and ultraviolet–visible (UV‐Vis) spectroscopy analysis revealed improvement in the nanoscale CNT dispersion with sonication time. Moreover, the optical microscopic images and sedimentation behavior for these samples pointed out to the formation of segregated CNT networks with different microstructures at different sonication times. Through‐plane thermal conductivity measurements showed an increase in thermal conductivity of the in‐situ polymerized TPU/CNT nanocomposites from polyol/CNT suspensions with increasing sonication time followed by a decrease at long sonication times. Different models were used to evaluate the role of CNT dispersion state and created microstructure on thermal conductivity of nanocomposites. The formation of a segregated network at medium sonication times consisting of large CNT aggregates and small bundles increased the nanocomposite thermal conductivity up to 99.7%, while at longer sonication times, an increase in interfacial area with a corresponding increase in kapitza boundary resistance, effectively decreased the system thermal conductivity. POLYM. ENG. SCI., 56:394–407, 2016. © 2016 Society of Plastics Engineers     

Heat transport across the metal–diamond interface

Thermal conductivity of the aluminium–diamond (Al–diamond) composites, prepared by the gas pressure infiltration method, is measured by steady state technique. A detailed theoretical investigation on the heat conduction mechanism across the Al–diamond interface is presented. It was confirmed that both electrons and phonons actively take part in the flow of heat at the interface. In the Al side, electrons of Al couple with the phonons and carry the heat up to the interface. This electron–phonon pair which predominantly carries heat in the Al, breaks down at the Al–diamond interface. The coupling between phonons of both Al and diamond takes place at the interface which eventually leads the heat conduction across the interface to the diamond. The phonon–phonon coupling across the interface is discussed by scattering mediated acoustic mismatch model (SMAMM). It is shown that for Al–diamond composite, the implementation of the SMAMM yields an interface thermal resistance (ITR) value of 4.44 × 10^− 9 m²K/W, which is in fairly good agreement with values derived from experimental thermal conductivity values of this composite implemented in the Hasselman–Johnson (HJ) mean field scheme.     

Modelling of the effective thermal conductivity of carbon nanotube nanofluids based on dimensionless groups

A model has been developed to predict the effective thermal conductivity of carbon nanotube (CNT) suspensions. This model uses a set of dimensionless groups based upon the properties of the base fluid, the CNT‐fluid interface, and characteristics of the nanotubes themselves, such as diameter, aspect ratio, and thermal conductivity. According to the model, the thermal conductivity of the CNT suspension increases in a nonlinear way as the CNT concentration is increased. The model is in good agreement with experimental data that was obtained for a series of CNT–R113 (Cl₂FC‐CClF₂) nanofluids.     

Electrophoretic deposition and characterization of Eu2O3 nanocrystal—Carbon nanotube heterostructures

Electrophoretic deposition (EPD) was employed to assemble a layered architecture of multi-walled carbon nanotube (CNT) mats and europium oxide nanocrystal (NC) films. CNT mats were produced via low field–high current EPD, while NC films were deposited via high field–low current EPD. The two EPD techniques were integrated in an alternating sequence to create CNT mat–NC film–CNT mat heterostructures with sharp interfaces. The EPD techniques produced uniformly porous CNT mats and densely packed NC films. Morphology and surface coverage of the films were investigated using scanning electron microscopy (SEM), while elemental characterization was performed using energy dispersive spectroscopy (EDS). Photoluminescence (PL) spectroscopy measurements were conducted on the NC suspension and the NC films to confirm film formation. Capacitance–voltage (CV) measurements were performed to probe energy-storage capabilities of the layered architecture.     

Finite size gap effects on the modeling of thermal contact conductance at polymer‐mold wall interface in injection molding

Heat transfer in polymer processing by injection molding is affected by the thermal contact conductance at the interface between the polymer and the metal mold. The modeling of thermal contact conductance at such interfaces is simplified by the assumption of an isothermal condition at the two contacting surfaces. In this study we examine the validity of such an assumption for the case of an interface involving plastic (a low thermal conductivity material) and metal (a high thermal conductivity material). The study shows that at such an interface between materials of widely varying thermal conductivity, the conditions at the interface depart from the isothermal assumption, with the heat flux becoming more uniform and the temperature difference varying by a larger magnitude across the contact plane. This effect is more pronounced as the width of the gaps increases for the same area of contact. This suggests that the modeling of the contact conductance should be based on average temperatures for the contacting surfaces. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1776–1782, 2000     

Thermal interface tailoring in composite materials

The thermal transport in heterogeneous materials systems, such as in composites, is essentially controlled by the phonon scattering phenomena at the materials interface due to the interface materials property mismatch. Such phenomena are also prevalent in joints or component interfaces. The thermal property mismatch at the materials interface, in the molecular scale, is primarily dictated by the phonon density of state across the interface. In this paper, the interface materials configuration for tailoring the thermal properties of composite materials with nano constituents is presented. The materials modeling using both the finite element analysis (FEM) and molecular dynamics (MD) simulations is performed to identify the effect of materials constituent scale as well as the nano constituent surface functionalization on the interface thermal transport phenomena. It is observed that the effect of surface functionalization towards establishing covalent bonding between the nano constituent surface the matrix (such as polymers) is extremely important in enhancing the interface thermal conductance.     

Thermal conductivity improvement in electrically insulating silicone rubber composites by the construction of hybrid three-dimensional filler networks with boron nitride and carbon nanotubes

In this study, we constructed hybrid three-dimensional (3D) filler networks by simply incorporating a relatively low content of one-dimensional carbon nanotubes (CNTs; 0.0005–0.25 vol %) and a certain content of two-dimensional boron nitride (BN; 30 phr) in a silicone rubber (SIR) matrix. As indicated by transmission electron microscopy observation, flexible CNTs can serve as bridges to connect BN platelets in different layers to form hybrid 3D thermally conductive networks; this results in an increase in thermally conductive pathways, and the isolation between CNTs can prevent the formation of electrically conductive networks. Compared to the SIR–BN composite with the same BN content, the SIR–BN–CNT composites exhibited improved thermal conductivity, slightly increased volume resistivity, and comparable breakdown strength without a largely decreased flexibility. When 0.25 vol % CNTs were incorporated, the SIR–BN–CNT composite exhibited 75 and 25% higher thermal conductivities relative to the neat SIR and SIR–BN composite with 30 phr BN, respectively, and a thermal conductivity that was even comparable to SIR–BN composite with 40 phr BN. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46929.     

Fabrication and characterization of silicon nanostructures based on metal-assisted chemical etching

We present a facile method to fabricate one-dimensional Si nanostructures based on Ag-induced selective etching of silicon wafers. To obtain evenly distributed Si nanowires (SiNWs), the fabrication parameters have been optimized. As a result, a maximum of average growth rate of 0.15 μm/min could be reached. Then, the fabricated samples were characterized by water contact angle (CA) experiments. As expected, the as-etched silicon samples exhibited a contact angle in the range of 132°–136.5°, whereas a higher contact angle (145°) could be obtained by chemical modification of the SiNWs with octadecyltrichlorosilane (OTS). Additionally, Raman spectra experiments have been carried out on as-prepared nanostructures, showing a typical decreasing from 520.9 cm^?1 to 512.4 cm^?1 and an asymmetric broadening, which might be associated with the phonon quantum confinement effect of Si nanostructures.     

Thermal and electrical properties of a high entropy carbide (Ta,Hf, Nb,Zr) at elevated temperatures

The thermal and electrical properties were measured for a high entropy carbide ceramic, consisting of (Hf, Ta, Zr, Nb)C. The ceramic was produced by spark plasma sintering a mixture of the monocarbides and had a relative density of more than 97.6%. The resulting ceramic was chemically homogeneous as a single-phase solid solution formed from the constituent carbides. The thermal diffusivity (0.045–0.087 cm²/s) and heat capacity (0.23–0.44 J/g•K) were measured from room temperature up to 2000°C. The thermal conductivity increased from 10.7 W/m•K at room temperature to 39.9 W/m•K at 2000°C. The phonon and electron contributions to the thermal conductivity were investigated, which showed that the increase in thermal conductivity was predominantly due to the electron contribution, while the phonon contribution was independent of temperature. The electrical resistivity increased from 80.9 μΩ•cm at room temperature to 114.1 μΩ•cm at 800°C.     

Bi2Te3-MWCNT nanocomposite: An efficient thermoelectric material

Bi₂Te₃–MWCNT nanocomposite has been synthesized by hydrothermal technique and demonstrate the role of MWCNT for thermoelectric properties. Herein, MWCNT has been used as conducting filler, which leads to the enhancement in the electrical conductivity in the case of nanocomposite. Bi₂Te₃–MWCNT nanocomposite shows ~22% decrease in the thermal conductivity as compared to Bi₂Te₃ nanostructures, which is attributed to the enhanced phonon scattering at the interfaces of Bi₂Te₃–MWCNT nanocomposite. Due to the increase in the electrical conductivity and decrease in the thermal conductivity, the overall enhancement in the figure of merit is ~45% in Bi₂Te₃–MWCNT nanocomposite as compared to Bi₂Te₃ nanostructures.     

Grafting of poly(ε-caprolactone) on electrospun gelatin nanofiber through surface-initiated ring-opening polymerization

Gelatin–polycaprolactone hybrid materials are being used in medicinal applications such as drug delivery, tissue engineering applications nowadays. In this study, gelatin–polycaprolactone graft copolymer was synthesized through ring-opening polymerization process instead of conventional grafting methods using potentially cytotoxic crosslinkers, or applying plasma, aminolysis reactions. Gelatin nanofiber (GNF) was produced by electrospinning process before immobilization of poly(ε-caprolactone). Ring-opening polymerization of ε-caprolactone was successfully initiated on the surface of GNF using tin(IV) isopropoxide and gelatin (primary amine functional groups) as an initiator and a coinitiator, respectively. The growing of polymer chains on GNF was indicated by both scanning electron microscope and energy-dispersive X-ray spectroscopy. Chemical structure was characterized through infrared spectrum, thermal behaviors and the material composition was analyzed with thermogravimetric analysis, wettability and hydrophilicity were determined by water contact angle measurements.     

The effect of a CNT interface on the thermal resistance of contacting surfaces

Experimental and theoretical analyses were used to study the effect of thermal contact resistance in two materials, aluminum and graphite. Experimental investigation included the use of a modern laser flash device to measure the effective thermal conductivity of each material for three different cases: in direct contact, with a graphite coating and with a thin sheet of carbon nanotube (CNT) thermal interface material (TIM). For both materials total thermal resistance values were determined corresponding to different cases for same contact pressure. Results showed that the CNT TIM produced the minimum thermal contact resistance. A theoretical study was carried out to compare the experimental results with thermal resistance models from the literature. Based on the surface roughness of the materials tested, two models were used. Both models showed reasonable agreement with the experimental results with an error of less than 6.5%. The results demonstrate the effectiveness of CNT materials in improving the thermal conductance of contacting surfaces.     

Molecular dynamics simulations of thermal transport in porous nanotube network structures

Carbon nanotube based 3D nanostructures have shown a lot of promise towards designing next generation of multi-functional systems, such as nano-electronic devices. Motivated by their recent successful experimental synthesis as well as characterization, and realizing that thermal dissipation is an important concern in proposed devices because of ever-increasing power density, we have investigated the phononic thermal transport behavior in 3D porous nanotube network structures using reverse non-equilibrium molecular dynamics simulations. Based on our study, the length scale associated with the distance between nanotube junctions emerges as the most dominating parameter that governs phonon scattering (hence the characteristic mean free path) and the heat flow in these nanostructures at molecular length scales. However, because of their spatial inhomogeneity, we show that the aerial density of carbon nanotubes (normal to heat flow) is also of critical importance in determining their system-level thermal conductivity. Based on our findings, we postulate that both parameters should be considered while designing nano-devices where thermal management is relevant.     

Multiple nanostructures in high performance Cu2S0.5Te0.5 thermoelectric materials

Cu₂(S,Te) solid solution material has shown excellent thermoelectric performance with a maximum zT value up to 2.1. Previous work has observed specific mosaic nanocrystals in this solid solution which is considered to be responsible for its ultra-low thermal conductivity. In the present study, we performed in-depth microstructural examinations by transmission electron microscopy and revealed additional multiform nanostructures in Cu₂S_0.5Te_0.5 solid solution. Apart from widely existed mosaic nanocrystals, nano-domains of ordered structure, nanoscale periodic antiphase domains, stacking faults and polytype have been observed. Formation of such abundant nanoscale substructures could be related to the large mismatch in atomic size between S and Te solute elements. These nanostructures are all coherently embedded in the matrix lattice. Thereafter, continuous carrier transportation is still guaranteed while in the meantime phonon scattering is enhanced by such high density of structural imperfection. Thus, solid solution strategy in Cu₂S_0.5Te_0.5 matches well with the phonon-glass electron-crystal concept for high performance thermoelectric materials.