Thermal conductivity of polymer composites based on the length of multi-walled carbon nanotubes

The anisotropic development of thermal conductivity in polymer composites was evaluated by measuring the isotropic, in-plane and through-plane thermal conductivities of composites containing length-adjusted short and long multi-walled CNTs (MWCNTs). The thermal conductivities of the composites were relatively low irrespective of the MWCNT length due to their high contact resistance and high interfacial resistance to polymer resins, considering the high thermal conductivity of MWCNTs. The isotropic and in-plane thermal conductivities of long-MWCNT-based composites were higher than those of short-MWCNT-based ones and the trend can accurately be calculated using the modified Mori-Tanaka theory. The in-plane thermal conductivity of composites with 2 wt% long MWCNTs was increased to 1.27 W/m·K. The length of MWCNTs in polymer composites is an important physical factor in determining the anisotropic thermal conductivity and must be considered for theoretical simulations. The thermal conductivity of MWCNT polymer composites can be effectively controlled in the processing direction by adjusting the length of the MWCNT filler.     

预制体结构对针刺石英纤维/环氧树脂复合材料导热性能的影响

为了探索预制体结构对针刺石英纤维/环氧树脂复合材料导热性能的影响,采用逐层针刺技术和树脂传递模塑工艺制作了针刺石英纤维/环氧树脂复合材料。利用瞬态热线法测量了环氧树脂和不同预制体结构的针刺石英纤维/环氧树脂复合材料的导热性能。结果表明:随着纤维体积分数的提高,针刺石英纤维/环氧树脂复合材料的导热性能得到了提升。其中,用石英纤维短切毡增强环氧树脂的导热性能比环氧树脂提高了35.9%。当针刺石英纤维/环氧树脂复合材料中的无纬布纤维平行于热线时,采用石英纤维短切毡与石英纤维无纬布共同增强的2种针刺石英纤维/环氧树脂复合材料的导热性能分别比环氧树脂提高了45.5%和46.4%;而当无纬布纤维垂直于热线时,导热性能比环氧树脂分别提高了56.4%和61.8%。针刺石英纤维/环氧树脂复合材料的导热性能不仅受石英纤维体积分数影响,也受到预制体中无纬布纤维体积分数和取向的影响。      

Optimum mixing ratio of epoxy for glass fiber reinforced composites with high thermal stability

The optimum condition of glass fiber/epoxy composites was investigated according to mixing ratio of two epoxy matrices. Novolac type epoxy and isocyanate modified epoxy were used as composites matrix. Based on chemical composition of mixing matrix, optimum mixing ratio of epoxy resins was obtained through FT-IR instrument. In order to investigate thermal stability and interface of epoxy resin, glass transition temperature was observed by DSC instrument, and static contact angle was measured by reflecting microscope. Change of IR peak and T_g was conformed according to different epoxy mixing ratios. After fabrication of glass fiber/epoxy composites, tensile, compression, and flexural properties were tested by UTM by room and high temperature. The composites exhibited best mechanical properties when epoxy mixing ratio was 1:1.     

Morphology, dynamic mechanical and thermal studies on poly(styrene-co-acrylonitrile) modified epoxy resin/glass fibre composites

Poly(styrene-co-acrylonitrile) (SAN) was used to modify diglycidyl ether of bisphenol-A (DGEBA) type epoxy resin cured with diamino diphenyl sulfone (DDS) and the modified epoxy resin was used as the matrix for fibre reinforced composites (FRPs) in order to get improved mechanical and thermal properties. E-glass fibre was used as the fibre reinforcement. The morphology, dynamic mechanical and thermal characteristics of the systems were analyzed. Morphological analysis revealed heterogeneous dispersed morphology. There was good adhesion between the matrix polymer and the glass fibre. The dynamic moduli, mechanical loss and damping behaviour as a function of temperature of the systems were studied using dynamic mechanical analysis (DMA). DMA studies showed that DDS cured epoxy resin/SAN/glass fibre composite systems have two T_gs corresponding to epoxy rich and SAN rich phases. The effect of thermoplastic modification and fibre loading on the dynamic mechanical properties of the composites were also analyzed. Thermogravimetric analysis (TGA) revealed the superior thermal stability of composite system.     

Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes

Aligned carbon nanotubes (CNTs) are implemented into alumina-fiber reinforced laminates, and enhanced mass-specific thermal and electrical conductivities are observed. Electrical conductivity enhancement is useful for electrostatic discharge and sensing applications, and is used here for both electromagnetic interference (EMI) shielding and deicing. CNTs were grown directly on individual fibers in woven cloth plies, and maintained their alignment during the polymer (epoxy) infiltration used to create laminates. Using multiple complementary methods, non-isotropic electrical and thermal conductivities of these hybrid composites were thoroughly characterized as a function of CNT volume/mass fraction. DC and AC electrical conductivity measurements demonstrate high electrical conductivity of >100 S/m (at 3% volume fraction, ∼1.5% weight fraction, of CNTs) that can be used for multifunctional applications such as de-icing and electromagnetic shielding. The thermal conductivity enhancement (∼1 W/m K) suggests that carbon-fiber based laminates can significantly benefit from aligned CNTs. Application of such new nano-engineered, multi-scale, multi-functional CNT composites can be extended to system health monitoring with electrical or thermal resistance change induced by damage, fire-resistant structures among other multifunctional attributes.     

Effect of aspect ratio on thermal conductivity of high density polyethylene/multi-walled carbon nanotubes nanocomposites

This study aims to investigate experimentally the effects of aspect ratio (length/diameter ratio) and concentration of multiwalled carbon nanotubes (MWCNTs) on thermal properties of high density polyethylene (HDPE) based composites. The aspect ratios of two types of MWCNT fillers are in the range of 200–400 and 500–3000. Composite samples were prepared by melt mixing up to weight fraction of 19% filler content, followed by a compression molding. Measurements of density, specific heat and thermal diffusivity (by modulated photothermal radiometry, PTR) were performed and effective thermal conductivities k_e of nanocomposites were calculated using these values. The results show that the composites containing MWCNTs with higher aspect ratio have higher thermal conductivities than the ones with lower aspect ratio. In terms of conductivity enhancement k_e/k_m − 1, the results indicate that MWCNTs with higher aspect ratio provide three to fourfold larger enhancement than the ones with lower aspect ratio, at low filler concentrations.     

Mechanical and thermal behavior of non-crimp glass fiber reinforced layered clay/epoxy nanocomposites

Mechanical and thermal properties of non-crimp glass fiber reinforced clay/epoxy nanocomposites were investigated. Clay/epoxy nanocomposite systems were prepared to use as the matrix material for composite laminates. X-ray diffraction results obtained from natural and modified clays indicated that intergallery spacing of the layered clay increases with surface treatment. Tensile tests indicated that clay loading has minor effect on the tensile properties. Flexural properties of laminates were improved by clay addition due to the improved interface between glass fibers and epoxy. Differential scanning calorimetry (DSC) results showed that the modified clay particles affected the glass transition temperatures (T_g) of the nanocomposites. Incorporation of surface treated clay particles increased the dynamic mechanical properties of nanocomposite laminates. It was found that the flame resistance of composites was improved significantly by clay addition into the epoxy matrix.     

Conducting polymer coated graphene oxide reinforced C–epoxy composites for enhanced electrical conduction

Enhanced electrical conductivities were achieved in C–epoxy composites by integrating them with conducting polymers (CPs), namely poly pyrrole (PPY), poly(3,4-ethylene dioxythiophene) (PEDOT) and graphene oxide (GO) enwrapped by CPs. By in-situ polymerization of pyrrole or 3,4-ethylenedioxythiophene (EDOT) in the presence of the GO (template), sodium bis(2-ethylhexyl) sulfosuccinate (structure directing agent), ferric chloride (oxidant), the electrically conductive sheets of GO enwrapped CPs were obtained. The formation of CP coating on GO was confirmed by Raman spectroscopy, scanning electron microscopy and thermo gravimetric analysis studies. Different wt% of CP and CP coated GO were added to the epoxy resin and this resin was used to prepare the 2D laminated C–epoxy composites by hand layup method. DC electrical conductivity of the prepared C–epoxy composites were analyzed using current–voltage (I–V) characteristics and impedance measurements. Typical results showed that CP coated GO, at 0.5 wt% addition to epoxy imparted highest DC electrical conductivity for C–epoxy composite.     

Mechanical performance of CNT-filled glass fiber/epoxy composite in in-situ elevated temperature environments emphasizing the role of CNT content

Carbon nanotubes (CNTs) are one of the prime choice nano-filler reinforcement for fibrous polymeric composites. But the stability of the CNT/polymer interface is yet to be ensured for elevated temperature engineering applications. Present study deals with the assessment of elevated temperature durability of glass fiber/epoxy (GE) composite with various level of multi walled carbon nanotube (MWCNT) loading. Flexural testing at room temperature revealed that addition of 0.1% MWCNT yielded maximum strength (+32.8% over control GE) and modulus (+11.5% over control GE) amongst all the CNT modified composite systems. Further, MWCNT–GE composites resulted in accelerated degradation of mechanical performance with increasing temperature as compared to GE composite. Dynamic mechanical thermal analysis (DMTA) was carried out to study the viscoelastic behavior of all composites over a range of temperature. The design parameters were evaluated by Weibull probability function. Fractographic analysis figured out various failure modes in all composites at various temperatures.     

Effects of aluminium nitride inclusions on thermal and electrical properties of epoxy and polypropylene: An experimental investigation

Thermal and dielectric properties of polymers reinforced with micro-sized aluminium nitride (AlN) particles have been studied. A set of epoxy–AlN composites, with filler content ranging from 0 to 25 vol% is prepared by hand lay-up technique. With similar filler loading, polypropylene -AlN composites are fabricated by compression molding technique. Density (ρ_c), effective thermal conductivity (k_eff), glass transition temperature (T_g), coefficient of thermal expansion (CTE) and dielectric constant (ε_c) of these composites are measured experimentally. The various experimental data were interpreted using appropriate theoretical models. Incorporation of AlN in both the resin increases the k_eff and T_g whereas CTE of composite decreases favourably. The dielectric constant of the composite also found to get modified with filler content. With improved thermal and modified dielectric characteristics, these AlN filled polymer composites can possibly be used for microelectronics applications.     

Synchronous effects of multiscale reinforced and toughened CFRP composites by MWNTs-EP/PSF hybrid nanofibers with preferred orientation

MWNTs-EP/PSF (polysulfone) hybrid nanofibers with preferred orientation were directly electrospun onto carbon fiber/epoxy prepregs and interlaminar synchronously reinforced and toughened CFRP composites were successfully fabricated. With MWNTs-EP loading increasing, the oriented nanofibers were obtained accompanying with enhanced alignment of inner MWNTs-EP. Flexural properties and interlaminar shear strength of composites were improved with increasing MWNTs-EP loadings, whereas fracture toughness attained maximum at 10 wt% MWNTs-EP loading and then decreased. Based on these results, multiscale schematic modeling and mechanism schematic of hybrid nanofibers reinforced and toughened composites were suggested. Due to the preferred orientation of nanofibers, MWNTs-EP was inclined to align vertically to carbon fiber direction along the in-plane of interface layer. The proposed network structures, containing four correlative phases of MWNTs-EP/PSF sphere/carbon fiber/epoxy matrix, contributed to simultaneous improvement of strength and toughness of composites, which was realized by crack pinning, crack deflection, crack bridging and effective load transfer.     

On the Lewis–Nielsen model for thermal/electrical conductivity of composites

Several models have been proposed in the literature to describe the thermal and electrical conductivities of particulate composites. Among the proposed models, the Lewis–Nielsen model appears quite attractive as it is simple to use and it predicts the correct behavior when filler concentration () approaches the maximum packing concentration (_m). In this paper, the Lewis–Nielsen model is evaluated in light of a vast amount of experimental data available on thermal and electrical conductivities of particulate composites. The Lewis–Nielsen model is found to describe the experimental data for both thermal and electrical conductivities reasonably well.     

三维机织碳纤维/环氧树脂复合材料在两种测量方法下的热响应机制对比

采用瞬态热线法和闪光法分别测量了多种结构参数的三维机织碳纤维/环氧树脂复合材料的导热系数。通过对3D正交机织碳纤维/环氧树脂复合材料的有限元模拟可以看出,3D正交机织碳纤维/环氧树脂复合材料内经纱、纬纱和Z向纱的导热作用在不同的受热形式下会发生变化。采用瞬态热线法测量时,2.5D机织碳纤维/环氧树脂复合材料的导热系数低于2.5D经向增强结构,同时高于3D正交结构,而采用闪光法测量时,2.5D经向增强和3D正交碳纤维/环氧树脂复合材料的导热系数均小于2.5D机织结构。这是由于在使用不同的测量方法时,三维机织碳纤维/环氧树脂复合材料内部相同的纱线系统在导热过程中所起的作用并不相同。随着纤维体积含量的提高,瞬态热线法和闪光法测得的2.5D机织碳纤维/环氧树脂复合材料的导热系数都在不断提高。由于经纱的屈曲,采用闪光法测量时,导热性能提升更加明显。研究结果表明,三维机织碳纤维/环氧树脂复合材料在不同受热形式下具有不同的热响应机制。     

Enhancement of tensile and thermal properties of epoxy nanocomposites through chemical hybridization of carbon nanotubes and alumina

This paper presents the properties of epoxy nanocomposites, prepared using a synthesized hybrid carbon nanotube–alumina (CNT–Al₂O₃) filler, via chemical vapour deposition and a physically mixed CNT–Al₂O₃ filler, at various filler loadings (i.e., 1–5%). The tensile and thermal properties of both nanocomposites were investigated at different weight percentages of filler loading. The CNT–Al₂O₃ hybrid epoxy composites showed higher tensile and thermal properties than the CNT–Al₂O₃ physically mixed epoxy composites. This increase was associated with the homogenous dispersion of CNT–Al₂O₃ particle filler; as observed under a field emission scanning electron microscope. It was demonstrated that the CNT–Al₂O₃ hybrid epoxy composites are capable of increasing tensile strength by up to 30%, giving a tensile modulus of 39%, thermal conductivity of 20%, and a glass transition temperature value of 25%, when compared to a neat epoxy composite.     

Effect of interface debonding on the thermal conductivity of microencapsulated-paraffin filled epoxy matrix composites

Microcapsules containing phase change materials (microPCMs) can be filled in polymeric matrix forming smart temperature-controlling composites. The aim of this study was to investigate the effect of interface debonding on the thermal conductivity of microPCMs containing paraffin/epoxy composites. The shell thickness and average size of microPCMs were controlled by regulating the core/shell ratios and emulsion stirring rates. Test results indicated that the thermal conductivity (K_e) of all composites decreased after a thermal shock treatment. SEM and thermography measurements were applied to observe the interface behaviors of composites after a violent thermal treatment process. It was proved that the interface debonding was generated because of the mismatch of expansion coefficient between shell and epoxy. A modeling analysis of the relative thermal conductivity (K_r) indicated that the effective approach to decrease the debonding is to enhance the molecule tangling degree between shell and matrix.     

Effect of surface treatment on thermal stability of the hemp-PLA composites: Correlation of activation energy with thermal degradation

The thermal behavior of hemp-poly lactic acid composites with both untreated and chemically surface modified hemp fiber was characterized by means of activation energy of thermal degradation. Three chemical surface modification employed were; alkali, silane and acetic anhydride. Model-free isoconversion Flynn–Wall–Ozawa method was chosen to evaluate the activation energy of composites. The results indicated that composites prepared with acetic anhydride modified hemp had 10–13% higher activation energy compared to other composites. Further, among the three surface modifications, acetic anhydride resulted in higher activation energy (159–163 kJ/mol). Fourier transform infrared spectroscopy supported the findings of thermogravimetric analysis results, wherein surface functionalization changes were observed as a result of surface modification of hemp fiber. It was concluded that, higher bond energy results in higher activation energy, which improves thermal stability. The activation energy data can aid in better understanding of the thermal degradation behavior of composites as a function of composite processing.     

Role of matrix modification on interlaminar shear strength of glass fibre/epoxy composites

Interlaminar shear properties of fibre reinforced polymer composites are important in many structural applications. Matrix modification is an effective way to improve the composite interlaminar shear properties. In this paper, diglycidyl ether of bisphenol-F/diethyl toluene diamine system is used as the starting epoxy matrix. Multi-walled carbon nanotubes (MWCNTs) and reactive aliphatic diluent named n-butyl glycidyl ether (BGE) are employed to modify the epoxy matrix. Unmodified and modified epoxy resins are used for fabricating glass fibre reinforced composites by a hot-press process. The interlaminar shear strength (ILSS) of the glass fibre reinforced composites is investigated and the results indicate that introduction of MWCNT and BGE obviously enhances the ILSS. In particular, the simultaneous addition of 0.5 wt.% MWCNTs and 10 phr BGE leads to the 25.4% increase in the ILSS for the glass fibre reinforced composite. The fracture surfaces of the fibre reinforced composites are examined by scanning electron microscopy and the micrographs are employed to explain the ILSS results.     

Thermal conductivity of epoxy composites with a binary-particle system of aluminum oxide and aluminum nitride fillers

Aluminum oxide and aluminum nitride with different sizes were used alone or in combination to prepare thermally conductive polymer composites. The composites were categorized into two systems, one including composites filled with large-sized aluminum nitride and small-sized aluminum oxide particles, and the other including composites filled with large-sized aluminum oxide and small-sized aluminum nitride. The use of these hybrid fillers was found to be effective for increasing the thermal conductivity of the composite, which was probably due to the enhanced connectivity offered by the structuring filler. At a total filler content of 58.4 vol.%, the maximum values of both thermal conductivities in the two systems were 3.402 W/mK and 2.842 W/mK, respectively, when the volume ratio of large particles to small particles was 7:3. This result was represented when the composite was filled with the maximum packing density and the minimum surface area at the same volume content. As such, the proposed thermal model predicted thermal conductivity in good agreement with experimental values.     

Analytical and Numerical Investigation on Effective Thermal Conductivity of Polymer Composites Filled with Conductive Hollow Particles

The thermal behavior of hollow conductive particles filled in epoxy resin has been investigated using 3D finite element computation. The effect of the filler concentrations associated with the particle/matrix interfacial resistance on the effective thermal conductivity of the composites was considered. The relationship between the out-of-plane effective thermal conductivity, the wall thickness of the hollow particles, and the ratio of the thermal conductivities of the filler to the matrix material were also taken into account. The numerical results show an increase of the effective thermal conductivity with increasing wall thickness of the hollow particles. However, for a large contact resistance and/or for a high effective thermal conductivity, it is shown that the contact resistance has a dominant influence on the effective thermal conductivity of the composites. The numerical results were also compared to some well-known analytical effective thermal conductivity models.     

Influence of electrolessly silver-plated multi-walled carbon nanotubes on thermal conductivity of epoxy matrix nanocomposites

In this work, multi-walled carbon nanotubes (MWCNTs) were electrolessly Ag-plated in order to investigate the effect of plating time on the thermal conductivity of Ag-plated MWCNTs-reinforced epoxy matrix composites. MWCNT surfaces were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). The thermal conductivity of Ag-plated MWCNT-reinforced epoxy nanocomposites was measured using the thermal equilibrium method with ASTM D5470. From the results, it was found that the thermal conductivity of the composites enhanced with increasing plating time. In particular the Ag-10/EP sample showed more than 150% enhancement of the thermal conductivity compared to the as-received CNTs/EP sample. These results were attributed to the high contents of Ag particles and the increase of the interfacial adhesion between the Ag-CNTs and EP matrix in the composites.