参杂缺陷石墨烯的高分子复合材料导热特性分子动力学模拟

Polymers are widely used advanced materials composed of macromolecular chains, which can be found in materials used in our daily life. Polymer materials have been employed in many energy and electronic applications such as energy harvesting devices, energy storage devices, light emitting and sensing devices, and flexible energy and electronic devices. The microscopic morphologies and electrical properties of the polymer materials can be tuned by molecular engineering, which could improve the device performances in terms of both the energy conversion efficiency and stability. Traditional polymers are usually considered to be thermal insulators owing to their amorphous molecular chains. Graphene-based polymeric materials have garnered significant attention due to the excellent thermal conductivity of graphene. Advanced polymeric composites with high thermal conductivity exhibit great potential in many applications. Therefore, research on the thermal transport behaviors in graphene-based nanocomposites becomes critical. Vacancy defects in graphene are commonly observed during its fabrication. In this work, the effects of vacancy defects in graphene on thermal transport properties of the graphene-polyethylene nanocomposite are comprehensively investigated using molecular dynamics (MD) simulation. Based on the non-equilibrium molecular dynamics (NEMD) method, the interfacial thermal conductance and the overall thermal conductance of the nanocomposite are taken into consideration simultaneously. It is found that vacancy defects in graphene facilitate the interfacial thermal conductance between graphene and polyethylene. By removing various proportions of carbon atoms in pristine graphene, the density of vacancy defects varies from 0% to 20% and the interfacial thermal conductance increases from 75.6 MW·m⁻²·K⁻¹ to 85.9 MW·m⁻²·K⁻¹. The distinct enhancement in the interfacial thermal transport is attributed to the enhanced thermal coupling between graphene and polyethylene. A higher number of broken sp² bonds in the defective graphene lead to a decrease in the structure rigidity with more low-frequency (< 15 THz) phonons. The improved overlap of vibrational density states between graphene and polyethylene at a low frequency results in better interfacial thermal conductance. Moreover, the increase in the interfacial thermal conductance induced by vacancy defects have a significant effect on the overall thermal conductance (from 40.8 MW·m⁻²·K⁻¹ to 45.6 MW·m⁻²·K⁻¹). In addition, when filled with the graphene layer, the local density of polyethylene increases on both sides of the graphene. The concentrated layers provide more aligned molecular arrangement, which result in better thermal conductance in polyethylene. Further, the higher local density of the polymer near the interface provides more atoms for interaction with the graphene, which leads to stronger effective interactions. The relative concentration is insensitive to the density of vacancy defects. The reported results on the thermal transport behavior of graphene-polyethylene composites provide reasonable guidance for using graphene as fillers to tune the thermal conduction of polymeric composites.     

石墨烯及其聚合物复合材料

近些年来,石墨烯以其独特的结构和优异的性质成为备受瞩目的研究前沿和热点。石墨烯作为纳米增强组分,少量添加可以使聚合物的物理性能得到大幅地提高。本文就石墨烯及其在聚合物复合材料的研究进展进行了综述,着重阐述了现已工业化制备石墨烯的氧化还原法,以及石墨烯/聚合物复合材料的制备方法(溶液共混、原位聚合和熔融共混)和性能(电学性能、导热性能、力学性能、热性能以及气体阻隔性能),并指出其待解决的关键技术及工业化前景。     

石墨烯层数测量方法的研究进展

石墨烯具有高导电性、高韧度、高强度、超大比表面积等特点,在电子、航天工业、新能源、新材料等领域有广泛应用。对石墨烯层数测量方法的研究有助于深入理解石墨烯性能与微观结构之间的关系。本文着重阐述了包括光学显微镜、拉曼光谱、原子力显微镜和透射电镜等测量石墨烯层数的方法,同时比较了各种测量方法的优点及局限性,并指出石墨烯层数的测量方法还有待进一步完善。     

The addition of functionalized graphene oxide to polyetherimide to improve its thermal conductivity and mechanical properties

The thermal and electrical conductivity and mechanical properties of polyetherimide (PEI) containing either alkyl‐aminated (enGO) or phenyl‐aminated graphene (pnGO) oxides were studied. A solution casting method was used to prepare functionalized graphene oxide/PEI composites with different filler contents. The introduction of functionalized graphene oxide to the PEI matrix improved the thermal conductivity, electrical conductivity, and mechanical properties. The thermal conductivities of the enGO 3 wt%/PEI and pnGO 3 wt%/PEI composites were 0.324 W/mK and 0.329 W/mK, respectively, due to the high thermal conductivity of the graphene‐based materials and the strong interface adhesion due to the filler surface treatment between the fillers and the matrix. The electrical conductivities of the functionalized graphene oxide/PEI composites were larger than that of PEI, but the electrical conductivity values were generally low, which is consistent with the magnitude of the insulator. The strong interfacial adhesion between the fillers and the matrix led to improved mechanical properties. Copyright © 2014 John Wiley & Sons, Ltd.     

衬底上石墨烯制备及改性研究

大面积高质量石墨烯的制备对石墨烯电子特性及石墨烯基纳器件相关研究有重要意义。本文综述了近几年来衬底上制备石墨烯的相关实验以及衬底与石墨烯相互作用研究的重要进展。目前,采用化学气相沉积、外延生长等方法可在衬底表面上制备出较大面积、高质量的石墨烯材料。衬底与石墨烯相互作用和界面间晶格匹配、原子成键及电荷转移等密切相关,其对吸附石墨烯的几何结构、能带结构及电子特性等产生明显影响。实验与理论计算的结合可望加深衬底与石墨烯作用机理的理解,指导衬底上石墨烯制备及改性的进一步研究。     

石墨烯导热性能及其测试方法

石墨烯以其独特的结构和优异的性能引起人们广泛的关注。本文通过总结近期国内外石墨烯导热工作研究进展,梳理了石墨烯的导热机理,根据导热机理指出缺陷、基底、石墨烯边缘是影响石墨烯热导率的主要因素。综述了采用分子动力学及非平衡态格林函数方法进行石墨烯热导率模型计算的研究成果,并在模型的基础上,全面阐述了实验方面针对不同层数石墨烯热导率的测试方法及结果。     

磺化石墨烯/天然橡胶复合材料的性能研究

以磺化石墨烯为填料,将其填充到天然胶乳中,混合均匀后共沉淀,采用传统橡胶加工方法制备了天然橡胶复合材料。对磺化石墨烯的结构和形貌进行了表征,测定了硫化胶的力学性能、耐磨性能、透气性能和导热性能。研究结果表明:磺化石墨烯表面含有丰富的活性官能团,为少层片状结构,硫化胶的力学性能、耐磨性得到了提高,而透气性和导热性有所下降。当石墨烯添加量为2.0%(wt)时,复合材料的拉伸强度最大为27.06MPa;磨耗体积仅为0.08cm~3;导热系数最小为0.42W/(m·K);透气量最低,为1.98×10~(-4)cm~3/(m~2·d·Pa)。     

石墨烯导热研究进展

石墨烯具有目前已知材料中最高的热导率,在电子器件、信息技术、国防军工等领域具有良好的应用前景。石墨烯导热的理论和实验研究具有重要意义,在最近十年间取得了长足的发展。本文综述了石墨烯本征热导率的研究进展及应用现状。首先介绍应用于石墨烯热导率测量的微纳尺度传热技术,包括拉曼光谱法、悬空热桥法和时域热反射法。然后展示了石墨烯热导率的理论研究成果,并总结了石墨烯本征热导率的影响因素。随后介绍石墨烯在导热材料中的应用,包括高导热石墨烯膜、石墨烯纤维及石墨烯在热界面材料中的应用。最后对石墨烯导热研究的成果进行总结,提出目前石墨烯热传导研究中存在的机遇与挑战,并展望未来可能的发展方向。     

Thermal properties of multilayer graphene and hBN reinforced copper matrix composites

The unique properties of graphene make it a very attractive application, although there are still no commercial products in which graphene would play a key role. Good thermal conductivity is undoubtedly one of the attributes which can be easily used both in materials involving large monoatomic layers, that are very difficult to obtain, as well as multilayer graphene flakes, which have been commercially available on the market for several years. The article presents the results of tests on the characteristic thermal properties of composites with the addition of 2–15% of multilayer graphene (MLG) crystals. The motivation of the study was literature reports showing the possibility of increasing the thermal conductivity of composites with MLG participation in the copper matrix. Since the production of composites with increased properties is associated with obtaining a strong orientation of the flakes in the structure, composites with hBN flakes exhibiting significantly worse but also directional thermal properties were produced for comparison. The paper showed a strong influence of flake morphology on the possibility of creating a directional structure. The obtained Cu/MLG composites with the addition of only 2% MLG were characterized by an increase in the thermal conductivity coefficient of about 30% in relation to sinters without the participation of MLG.

     

点缺陷石墨烯的电导

Graphene is one of the most promising materials in nanotechnology and has attracted worldwide attention and research interest owing to its high electrical conductivity, good thermal stability, and excellent mechanical strength. Perfect graphene samples exhibit outstanding electrical and mechanical properties. However, point defects are commonly observed during fabrication which deteriorate the performance of graphene based-devices. The transport properties of graphene with point defects essentially depend on the imperfection of the hexagonal carbon atom network and the scattering of carriers by localized states. Furthermore, an in-depth understanding of the effect of specific point defects on the electronic and transport properties of graphene is crucial for specific applications. In this work, we employed density functional theory calculations and the non-equilibrium Green's function method to systematically elucidate the effects of various point defects on the electrical transport properties of graphene, including Stone-Waals and inverse Stone-Waals defects; and single and double vacancies. The electrical conductance highly depends on the type and concentration of point defects in graphene. Low concentrations of Stone-Waals, inverse Stone-Waals, and single-vacancy defects do not noticeably degrade electron transport. In comparison, DV585 induces a moderate reduction of 25%–34%, and DV55577 and DV5555-6-7777 induce significant suppression of 51%–62% in graphene. As the defect concentration increases, the electrical conductance reduces by a factor of 2–3 compared to the case of graphene monolayers with a low concentration of point defects. These distinct electrical transport behaviors are attributed to the variation of the graphene band structure; the point defects induce localized states near the Fermi level and result in energy splitting at the Dirac point due to the breaking of the intrinsic symmetry of the graphene honeycomb lattice. Double vacancies with larger defect concentrations exhibit more flat bands near the Fermi energy and more localized states in the defective region, resulting in the presence of resonant peaks close to the Fermi energy in the local density of states. This may cause resonant scattering of the carriers and a corresponding reduction of the conductance of graphene. Moreover, the partial charge densities for double vacancies and point defects with larger concentrations exhibit enhanced localization in the defective region that hinder the charge carriers. The electrical conductance shows an exponential decay as the defect concentration and energy splitting increase. These theoretical results provide important insights into the electrical transport properties of realistic graphene monolayers and will assist in the fabrication of high-performance graphene-based devices.     

Fabricated wearable and flexible chip composed strain of gallium and silver metals composites assembled on graphene inside PDMS matrix

With a view to creating a specific unique chip, its wearable, flexible and conductor.utilizing conventional physical methods like grafting and assembling metals (silver, copper, and gallium indium alloy) on graphene as composite, then immersed inside PDMS matrix. However, there is an incompatibility between liquid gallium and graphene sheet. So we used the abridge of metallic nanoparticles as silver and copper as a boundary barrier to its different charges to maximize interfacial surface interaction in between amorphous carbon and liquid gallium. to evaluate the chip conduct during the fabrication process we utilize various characterization such as electrochemical EIS and CVT to justify conductivity besides electrochemical reaction and oxidation and reduction, addition to measure the dielectric constant (?) of a composite at a different range of frequencies which is equal to 3.73 compared to PDMS 2.69.moreover its thermal stability (DSC) of prepared composite and tensile stress as an inductor for enforcement and enhancement physical properties, as well as, surface Morphology techniques characterize using TEM IR, and UV absorption.     

Thermal Energy Storage and Heat Transfer of Nano-Enhanced Phase Change Material (NePCM) in a Shell and Tube Thermal Energy Storage (TES) Unit with a Partial Layer of Eccentric Copper Foam

Thermal energy storage units conventionally have the drawback of slow charging response. Thus, heat transfer enhancement techniques are required to reduce charging time. Using nanoadditives is a promising approach to enhance the heat transfer and energy storage response time of materials that store heat by undergoing a reversible phase change, so-called phase change materials. In the present study, a combination of such materials enhanced with the addition of nanometer-scale graphene oxide particles (called nano-enhanced phase change materials) and a layer of a copper foam is proposed to improve the thermal performance of a shell-and-tube latent heat thermal energy storage (LHTES) unit filled with capric acid. Both graphene oxide and copper nanoparticles were tested as the nanometer-scale additives. A geometrically nonuniform layer of copper foam was placed over the hot tube inside the unit. The metal foam layer can improve heat transfer with an increase of the composite thermal conductivity. However, it suppressed the natural convection flows and could reduce heat transfer in the molten regions. Thus, a metal foam layer with a nonuniform shape can maximize thermal conductivity in conduction-dominant regions and minimize its adverse impacts on natural convection flows. The heat transfer was modeled using partial differential equations for conservations of momentum and heat. The finite element method was used to solve the partial differential equations. A backward differential formula was used to control the accuracy and convergence of the solution automatically. Mesh adaptation was applied to increase the mesh resolution at the interface between phases and improve the quality and stability of the solution. The impact of the eccentricity and porosity of the metal foam layer and the volume fraction of nanoparticles on the energy storage and the thermal performance of the LHTES unit was addressed. The layer of the metal foam notably improves the response time of the LHTES unit, and a 10% eccentricity of the porous layer toward the bottom improved the response time of the LHTES unit by 50%. The presence of nanoadditives could reduce the response time (melting time) of the LHTES unit by 12%, and copper nanoparticles were slightly better than graphene oxide particles in terms of heat transfer enhancement. The design parameters of the eccentricity, porosity, and volume fraction of nanoparticles had minimal impact on the thermal energy storage capacity of the LHTES unit, while their impact on the melting time (response time) was significant. Thus, a combination of the enhancement method could practically reduce the thermal charging time of an LHTES unit without a significant increase in its size.     

Thermal diffusivity measurement of ceramic materials used in spraying of TBC systems

The basic goal of this article was thermal diffusivity characterization of ceramic materials used in thermal barrier coating (TBC) systems for depositions of the insulation layer and characterization of the materials’ morphology and remanufacturing process. The base material was oxide 8YSZ (ZrO_2? ×?8Y₂O₃), which is usually dedicated to deposition of an insulating top layer in TBC systems. The data related to thermal properties such as thermal diffusivity and thermal conductivity are widely presented in the literature, but there is lack of information about the morphological form of investigated materials, and the presented results vary widely. Data on thermal properties based on the literature sources are inadequate for the real morphological form of materials used in the experiment (e.g., massive or single crystalline material vs. plasma-sprayed coatings), which consequently gives an unsatisfactory accuracy of the obtained numerical simulations by MES methods. This article presents the characterization of thermal diffusivity of the commercial 8YSZ ceramic material synthesized or remanufactured by different routes, which is investigated in the forms of pressed powder pellet (two commercial nano-sized powders with different morphologies), sintered pellets (one commercial powder, solid-state co-precipitated reacted powder of 8YSZ type), and a two-layered coating system of In625?+?NiCrAlY/8YSZ type. The range of analysis included morphological investigations of different types of powders in initial conditions and after remanufacturing (sintering, thermal spraying) as well as the thermal diffusivity analysis by the laser flash method. The obtained data were corrected by porosity factor and compared to each other. The best similarity for obtained thermal diffusivity data was found for commercial powers of HOSP^TM type after pressing and sintering processes and calculated (2-layered model) value of thermal diffusivity for two-layered system of In625/8YSZ TBS system. The results showed that there are significant differences in thermal diffusivity values for materials with different morphological forms.

     

Investigation of nanocomposite PNIPAm-g-graphene with pre-lower critical solution temperature rapid thermal response function for chip adaptive cooling: A molecular dynamics and computational fluid dynamics simulations

Temperature-sensitive hydrogels have been widely used for rapid adaptive cooling in electronic device thermal management with promising applications. However, existing temperature-sensitive hydrogels can only regulate the flow in the chip cooling system after the ambient temperature reaches their lower critical solution temperature (LCST). Before reaching LCST, effective rapid heat dissipation for electronic chips is not achievable. This study aims to develop a temperature-sensitive hydrogel that can provide assisted adaptive cooling for electronic chips before reaching its LCST. This requires the hydrogel to have a thermal conductivity far surpassing existing hydrogel materials. Using the temperature-sensitive hydrogel PNIPAm and graphene molecules as base materials, this research utilized molecular dynamics simulations to graft graphene molecules onto PNIPAm molecules in different ways, resulting in the temperature-sensitive hydrogel material PNIPAm-g-graphene. Non-equilibrium molecular dynamics (NEMD) was employed to calculate the thermal conductivity of this material under different temperature conditions. The results indicate that the thermal conductivity of PNIPAm-g-graphene can reach up to 1.95474 W/m K (graphene grafted at  CH₃ functional group, temperature at 280 K). Compared to the thermal conductivity of PNIPAm under the same conditions (0.45 W/m K), the increase in thermal conductivity is significant, demonstrating excellent thermal conductivity compared to PNIPAm. Subsequently, this study analyzed the underlying mechanisms of different thermal conductivities in materials obtained by grafting graphene molecules at different points using the method of overlap in Phonon Density of States Curves (PDOS) from the perspective of interfacial thermal conduction. Finally, through computational fluid dynamics (CFD) simulations, this study investigates the chip's adaptive cooling performance with PNIPAm-g-graphene material. The results show that, compared to traditional temperature-sensitive hydrogels, PNIPAm-g-graphene can achieve efficient adaptive cooling of chip hotspots before the cooling fluid temperature reaches its LCST value. This finding is significant for the field of chip cooling. The study proposes a new method for rapid, adaptive cooling of chip hotspots and explores its feasibility from the perspectives of molecular dynamics and CFD simulation. It holds importance in the thermal management of electronic devices and the rapid adaptive cooling of electronic chips.     

氧化石墨烯的可控还原及结构表征

采用氧化还原法, 通过控制还原时间制备了不同还原程度的石墨烯; 用红外光谱、 紫外光谱、 拉曼光谱、 X射线衍射、 热重分析、 电导率测量等多种手段系统研究了不同还原程度石墨烯的结构与性能; 采用透射电子显微镜、 扫描电子显微镜和原子力显微镜比较了氧化石墨烯和石墨烯的形貌. 结果表明, 随着还原程度的增加, 石墨烯中含氧基团减少, 紫外吸收峰逐渐红移, D带与G带的强度比增加, 热稳定性和导电性提高. 微观结构表征说明石墨烯比氧化石墨烯片的厚度增加, 褶皱增多.     

Tailoring thermal conductivity of bulk graphene oxide by tuning the oxidation degree

The thermal conductivity of graphene oxides can be tailored by tuning oxidation degree due to the introduction of atomic- and nano-scale phonon scattering centers.     

Surface‐Coated Thermally Expandable Microspheres with a Composite of Polydisperse Graphene Oxide Sheets

Surface‐modified thermally expandable microcapsules (TEMs) hold potential for applications in various fields. In this work, we discussed the possible surface coating mechanism and reported the properties of TEMs coated with polyaniline (PANI) and polydisperse graphene oxide sheets (ionic liquid‐graphene oxide hybrid nanomaterial (ILs‐GO)). The surface coating of PANI/ ILs‐GO increased the corresponding particle size and its distribution range. The morphologies analyzed by scanning electron microscopy indicated that no interfacial gap was observed between the microspheres ink and substrate layer during the substrate application. The thermal properties were determined by thermogravimetric and differential thermal analyses. The addition of ILs‐GO to the polyaniline coating significantly improved the thermal expansion and thermal conductivity of the microcapsules. The evaporation of hexane present in the core of TEMs was not prevented by the coating of PANI/ ILs‐GO. The printing application of TEMs showed excellent adaptability to various flexible substrates with great 3D appearance. By incorporating a flame retardant agent into TEMs coated by PANI/ILs‐GO, finally, these TEMs also demonstrated a great flame retardant ability. We expect that these TEM‐coated PANI/ ILs‐GO are likely to have the potential to improve the functional properties for various applications.     

Thermophoretic Motion of a Sphere Parallel to an Insulated Plane

An analytical study is presented for the thermophoresis of a sphere in a constant applied temperature gradient parallel to an adiabatic plane. The Knudsen number is assumed to be small so that the fluid flow can be described by a continuum model with a thermal creep and a hydrodynamic slip at the particle surface. A method of reflections is used to obtain the asymptotic formulas for the temperature and velocity fields in the quasisteady situation. The thermal insulated plane may be a solid wall (no-slip) and/or a free surface (perfect-slip). The boundary effect on the thermophoretic motion is found to be weaker than that on the axisymmetric thermophoresis of a sphere normal to a plane with constant temperature. In comparison with the motion driven by gravitational force, the interaction between the particle and the boundary is less significant under thermophoresis. Even so, the interaction between the plane and the particle can be very strong when the gap thickness approaches zero. For the thermophoretic motion of a particle parallel to a solid plane, the effect of the plane surface is to reduce the translational velocity of the particle. In the case of particle migration parallel to a free surface due to thermophoresis, the translating velocity of a particle can be either greater or smaller than that which would exist in the absence of the plane surface, depending on the relative thermal conductivity and the surface properties of the particle and its relative distance from the plane. Not only the translational velocity but also the rotational velocity of the thermophoretic sphere near the plane boundary is formulated analytically. The rotating direction of the particle is strongly dominated by its surface properties and the internal-to-external thermal conductivity. Besides the particle motion, the thickness of the thermophoretic boundary layer is evaluated by considering the thermophoretic mobility. Generally speaking, a free surface exerts less influence on the particle movement than a solid wall. Copyright 2000 Academic Press.     

Epoxy nanocomposites with high thermal conductivity and low loss factor: Realize 3D thermal conductivity network at low content through core-shell structure and micro-nano technology

Dielectric polymers with high thermal conductivity are very promising in the fields of aerospace and electronic device packaging. However, composites with excellent dielectric properties usually have low thermal conductivity. It is usually to fill the polymer with thermal conductivity particles to improve the thermal conductivity, but the high content of filler often reduces the mechanical properties of the polymer. In this paper, the traditional insulating polymer epoxy resin was used as the matrix, by covering the surface of silicon carbide with graphene to form a core-shell structure and co-filled with nano diamonds to achieve the preparation of high-performance epoxy resin at low content. The results showed that at the filling content of 30 wt%, the thermal conductivity of epoxy nanocomposites showed a dramatic thermal conductivity enhancement of 1263%, the energy storage modulus increased by 1.1 GPa, and the dielectric loss remained unchanged at 50 Hz. The advantages of the composite are the structural design and surface modification of the filler, which not only take advantage of its inherent advantages, but also improve the interface area with the epoxy matrix. The composite materials with excellent properties are expected to provide theoretical guidance for the application of high thermal conductivity dielectric materials.     

石墨烯及其复合材料在锂离子电池中的应用

石墨烯是一种单原子层厚度的石墨材料,具有独特的二维结构和优异的电学、力学以及热学性能。同时它也是一种具有良好应用前景的锂离子电池电极材料。电极材料的微观结构对其性能有很大影响,利用石墨烯获得具有特殊形貌和微观结构的电极材料,能有效改善材料的各项电化学性能。本文综述了石墨烯及其复合材料在锂离子电池中的应用研究进展。在负极复合材料中,石墨烯不仅可以缓冲材料在充放电过程中的体积效应,还可以形成导电网络提升复合材料的导电性能,提高材料的倍率性能和循环寿命。通过优化复合材料的微观结构,例如夹层结构或石墨烯片层包覆结构,可进一步提高材料的电化学性能。在正极复合材料中,石墨烯形成的连续三维导电网络可有效提高复合材料的电子及离子传输能力。此外,相比于传统导电添加剂,石墨烯导电剂的优势在于能用较少的添加量,达到更加优异的电化学性能。最后对石墨烯复合材料的研究前景进行了展望。