Enhanced thermal conductivity and retained electrical insulation for polyimide composites with SiC nanowires grown on graphene hybrid fillers

Polyimide (PI) composites containing one-dimensional SiC nanowires grown on two-dimensional graphene sheets (1D–2D SiC_NWs-GSs) hybrid fillers were successfully prepared. The PI/SiC_NWs-GSs composites synchronously exhibited high thermal conductivity and retained electrical insulation. Moreover, the heat conducting properties of PI/SiC_NWs-GSs films present well reproducibility within the temperature range from 25 to 175 °C. The maximum value of thermal conductivity of PI composite is 0.577 W/mK with 7 wt% fillers loading, increased by 138% in comparison with that of the neat PI. The 1D SiC nanowires grown on the GSs surface prevent the GSs contacting with each other in the PI matrix to retain electrical insulation of PI composites. In addition, the storage modulus and Young’s modulus of PI composites are remarkably improved in comparison with that of the neat PI.     

Influence of pores on the thermal insulation behavior of thermal barrier coatings prepared by atmospheric plasma spray

The defects in materials play very important role on the effective thermal conductivity. Especially, the spatial and geometrical characteristics of pores are significant factors for the thermal insulation behavior of thermal barrier coatings (TBCs). In this paper, finite element method was employed to simulate the thermal transfer behavior of TBCs with different spatial and geometrical characteristic of pores. The simulation results indicate that the thermal insulation effect of TBCs would be enhanced when the pore size, pore volume fraction and pore layers which are perpendicular to the thickness direction increase and the space between the adjacent pores decreases. It is predicted that the effective thermal conductivity is different at different directions for the atmospheric plasma spray (APS) TBCs. A novel method, Computational Micromechanics Method (CMM), was utilized to depict the thermal transferring behavior of actual coatings. At the same time, model with different kinds of defects were established, and the effective thermal conductivity as the function of defect orientation angle, defect volume fraction and defect shape coefficient was discussed in detail. The simulation results will help us to further understand the heat transfer process across highly porous structures and will provide us a powerful guide to design coating with high thermal insulation property.     

Polypyrrole/poly(vinyl alcohol-co-ethylene) nanofiber composites on polyethylene terephthalate substrate as flexible electric heating elements

Polypyrrole/poly(vinyl alcohol-co-ethylene) (PPy/PVA-co-PE) nanofiber composites on polyethylene terephthalate (PET) substrates were prepared using spray coating technique and in situ polymerization process. The electric heating behaviors of composites were investigated as functions of the amounts of nanofiber and PPy. It was observed that, the electrical resistivity of composites decreased significantly with increasing nanofiber and PPy contents. Scanning electron microscope images and infrared spectrum studies confirmed the formation of well dispersed network-like structure of PPy/PVA-co-PE nanofibers on PET substrate. Furthermore, maximum temperature attained at a given applied voltage for the composites could be well controlled by changing nanofibers and PPy amounts. PPy/PVA-co-PE nanofiber/PET composites exhibited excellent electric heating performance in aspects of rapid temperature response, long retaining behavior, thermal and operational stability. The incorporation of PPy on PVA-co-PE nanofibers/PET nonwoven substrates resulted in high conductivity and enhanced heating behavior, which have potential to be used as efficient electric heating elements.     

极端湿热环境下隔热的超疏水聚偏氟乙烯/聚酰亚胺纳米纤维复合气凝胶

优异的隔热材料在建筑、航空航天和体育设备等领域有着广泛的应用需求.然而,在实际应用中,隔热材料在不同温度和湿度条件下,其性能往往会恶化.因此,构建在极端湿热环境下仍具有出色的隔热性能的块状材料是非常重要的.在本工作中,我们构思了一种绿色制备策略,即通过静电纺丝和冷冻干燥技术来制备超疏水且可压缩的聚偏氟乙烯/聚酰亚胺(PVDF/PI)纳米纤维复合气凝胶. PVDF纳米纤维和PI纳米纤维分别充当疏水性纤维骨架和机械支撑骨架,形成具有良好机械柔韧性的坚固的三维框架. PVDF/PI气凝胶在室温下具有出色的超疏水特性(水接触角为152°)和低导热性(31.0 m W m^-1K^-1).同时,在100%湿度(80℃)下, PVDF/PI气凝胶仅显示出48.6 m W m^-1K^-1的低热导率,其性能优于大多数商业绝热材料.因此,新型的PVDF/PI复合气凝胶有望成为高温和高湿环境中应用的优良隔热材料.     

Effect of tungsten addition on thermal conductivity of graphite/copper composites

Graphite/copper composites with high thermal conductivity were fabricated by tungsten addition, which formed a thin tungsten carbide layer at the interface. The microstructure and thermal conductivity of the composite material were studied. The results indicated that the insertion of tungsten carbide layer obviously suppressed spheroidization of copper coating on the graphite particles during the sintering process, and decreased the interfacial thermal resistance of the composites. Compared with the graphite/copper composites without tungsten, the thermal conductivity of the obtained composites was increased by 43.6%.     

Highly thermally conductive POSS-g-SiCp/UHMWPE composites with excellent dielectric properties and thermal stabilities

Polyhedral oligomeric silsesquioxane grafting thermally conductive silicon carbide particle (POSS-g-SiCp) fillers, are performed to fabricate highly thermally conductive ultra high molecular weight polyethylene (UHMWPE) composites combining with optimal dielectric properties and excellent thermal stabilities, via mechanical ball milling followed by hot-pressing method. The POSS-g-SiCp/UHMWPE composite with 40 wt% POSS-g-SiCp exhibits relative higher thermal conductivity, lower dielectric constant and more excellent thermal stability, the corresponding thermally conductive coefficient of 1.135 W/mK, the dielectric constant of 3.22, and the 5 wt% thermal weight loss temperature of 423 °C, which holds potential for packaging and thermal management in microelectronic devices. Agari’s semi-empirical model fitting reveals POSS-g-SiCp fillers have strong ability to form continuous thermally conductive networks.     

Preparation of ultra-low dielectric constant silica/polyimide nanofiber membranes by electrospinning

Ultra-low dielectric constant silica/polyimide (SiO₂/PI) composite nanofiber membranes are prepared by the combined sol–gel and electrospinning techniques. The emulsion composed of partially hydrolyzed tetraethoxysilane (TEOS) and polyamic acid (PAA) is spun to yield the precursor of the SiO₂/PI fibers with a core–shell structure due to phase separation. The dielectric constant (k) of the composite membranes varies from 1.78 to 1.32 with increasing content of SiO₂. The fibers accumulate and form the film with a large amount of pores leading the lower k. In addition, the interfacial reaction between SiO₂ and the PI matrix reduces the value of k as the SiO₂ concentration is increased. The thermal stability of PI increase after mixing with SiO₂ and the SiO₂/PI composite fibers have large commercial potential in the electronics industry.     

Electrospun nanofibre toughened carbon/epoxy composites: Effects of polyetherketone cardo (PEK-C) nanofibre diameter and interlayer thickness

Polyetherketone cardo (PEK-C) nanofibres were produced by an electrospinning technique and directly deposited on carbon fabric to improve the interlaminar fracture toughness of carbon/epoxy composites. The influences of nanofibre diameter and interlayer thickness on the Mode I delamination fracture toughness, flexure property and thermal mechanical properties of the resultant composites were examined. Considerably enhanced interlaminar fracture toughness has been achieved by interleaving PEK-C nanofibres with the weight loading as low as 0.4% (based on weight of the composite). Finer nanofibres result in more stable crack propagation and better mechanical performance under flexure loading. Composites modified by finer nanofibres maintained the glass transition temperature (T_g) of the cured resin. Increasing nanofibre interlayer thickness improved the fracture toughness but compromised the flexure performance. The T_g of the cured resin deteriorated after the thickness increased to a certain extent.     

Elevated temperature tensile properties and thermal expansion of CNT/2009Al composites

1.5 vol.% and 4.5 vol.% carbon nanotubes reinforced 2009Al (CNT/2009Al) composites with homogeneously dispersed CNTs and refined matrix grains, were fabricated using powder metallurgy (PM) followed by 4-pass friction stir processing (FSP). Tensile properties of the composites between 293 and 573 K and the coefficient of thermal expansion (CTE) from 293 to 473 K were tested. It was indicated that load transfer mechanism still takes effect at temperatures elevated up to 573 K, thus the yield strength of the 1.5 vol.% CNT/2009Al composite at 423–573 K, was enhanced compared with the 2009Al matrix. However, for the 4.5 vol.% CNT/2009Al composite, the yield strength at 573 K was even lower than that for the matrix, due to the quicker softening of ultrafine-grained matrix. Compared with the 2009Al matrix, the CTEs of the composites were greatly reduced for the zero thermal expansion and high modulus of the CNTs and could be well predicted by the Schapery’s model.     

Three-dimensional carbon nanotube based polymer composites for thermal management

In this study, the porous multiwall carbon nanotube (MWCNT) foams possessing three-dimensional (3D) scaffold structures have been introduced into polydimethylsiloxane (PDMS) for enhancing the overall thermal conductivity (TC). This unique interconnected structure of freeze-dried MWCNT foams can provide thermally conductive pathways leading to higher TC. The TC of 3D MWCNT and PDMS composites can reach 0.82 W/m K, which is about 455% that of pure PDMS, and 300% higher than that of composites prepared from traditional blending process. The obtained polymer composites not only exhibit superior mechanical properties but also dimensional stability. To evaluate the performance of thermal management, the LED modulus incorporated with the 3D MWCNT/PDMS composite as heat sink is also fabricated. The composites display much faster and higher temperature rise than the pristine PDMS matrix, suggestive of its better thermal dissipation. These results imply that the as-developed 3D-MWCNT/PDMS composite can be a good candidate in thermal interface for thermal management of electronic devices.     

Yttria-stabilized zirconia-based composites with adaptive thermal conductivity

Thermal conductivity trends in a “chameleon coating” thin film were characterized with a time-domain thermoreflectance (TDTR) technique. A yttria-stabilized zirconia (YSZ)-based nanocomposite material containing ∼21 vol.% silver (Ag) was employed for this study. The thermal conductivity (k) of the as-deposited composite film was measured with TDTR and found to have a value of 7.4 ± 1.4 W m⁻¹ K⁻¹. The film was then annealed at 500 °C for 1 h to stimulate Ag flow from within the composite to the surface via diffusion. The Ag that coalesced on the surface during annealing was removed to expose the underlying porous YSZ matrix, and the sample was reexamined with the TDTR technique. The thermal conductivity of the porous nanocomposite YSZ material was then measured to be 1.6 ± 0.2 W m⁻¹ K⁻¹, which is significantly lower than a fully dense control sample of pure nanocrystalline YSZ (2.0 ± 0.1 W m⁻¹ K⁻¹). The annealed film displayed a 20% reduction in thermal conductivity as compared to the control sample and a 4–5-fold reduction in thermal conductivity as compared to the as-deposited material. The experiments demonstrate temperature triggering of a composite material, resulting in self-modifying thermal conductivity and diffusion-controlled porosity. These aspects can be used to enhance or restrict thermal transport (i.e., a thermal switch). The applicability of the TDTR technique to measurements of thin, nanoporous film materials is also demonstrated.     

Improving interlaminar fracture toughness of carbon fibre/epoxy laminates by incorporation of nano-particles

Double-cantilever-beam tests were applied to investigate the mode I interlaminar fracture toughness of carbon fibre/epoxy laminates, in which the epoxy matrices were incorporated with rubber and silica nano-particles, either singly or jointly. It is shown that the toughness is improved owing to the presence of these nano-particles although nano-rubber is more effective than nano-silica. Further, by keeping the total particle weight percentage constant in epoxies (e.g., at 8 and 12 wt.%) filled with equal amount of nano-silica and nano-rubber, the interlaminar toughness values of the hybrid laminates are always higher than those with nano-silica filled epoxies but lower than those with nano-rubber filled matrices. Scanning electron microscopy examination of the delaminated surfaces of composite laminates filled with nano-particles revealed that cavitation of nano-rubber particles/void growth and debonding of nano-silica from epoxy matrix are responsible for the improved interlaminar toughness observed. It is also shown that the bulk toughness of nano-particle filled epoxies cannot be fully transferred to the interlaminar toughness of composite laminates, being limited by the constraint effect imposed by the carbon fibres. Finally, the role of fibre-bridging on the delaminated crack and hence delamination toughness is discussed.     

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.     

Multivariable dependency of thermal shrinkage in highly aligned polypropylene tapes for self-reinforced polymer composites

Self-reinforced composites offer a unique combination of properties such as high specific strength, high impact resistance, and recyclability by incorporating highly aligned fibers within a random matrix of the same polymer. However, high temperatures will shrink the system to recover randomness in the aligned segments, compromising the composite thermal stability during processing as self-reinforced tapes are consolidated into the final composite through heating and pressure. Hence, the dynamic nonlinear multivariable (i.e., time, temperature, stress) shrinkage exhibited by self-reinforced polypropylene (SRPP) tapes was measured and modeled at the maximum shrinkage limit achieved in the proximity of the composite processing temperature [∼140 to160 °C]. At high stress (∼7.5 MPa) the thermal shrinkage of the SRPP tapes was reduced and a parallel creep mechanism was activated. The modeling, and prediction of the main factors governing the thermal shrinkage expand and diversify the dynamic design window for new SRPP composites.     

Review of the mechanical properties of carbon nanofiber/polymer composites

In this paper, the mechanical properties of vapor grown carbon nanofiber (VGCNF)/polymer composites are reviewed. The paper starts with the structural and intrinsic mechanical properties of VGCNFs. Then the major factors (filler dispersion and distribution, filler aspect ratio, adhesion and interface between filler and polymer matrix) affecting the mechanical properties of VGCNF/polymer composites are presented. After that, VGCNF/polymer composite mechanical properties are discussed in terms of nanofibers dispersion and alignment, adhesion between the nanofiber and polymer matrix, and other factors. The influence of processing methods and processing conditions on the properties of VGCNF/polymer composite is also considered. At the end, the possible future challenges for VGCNF and VGCNF/polymer composites are highlighted.     

二氧化硅气凝胶隔热复合材料的制备与应用

介绍了二氧化硅(SiO2)气凝胶的结构特点及隔热性能;对二氧化硅气凝胶隔热复合材料的制备方法及其应用前景进行总结并作了适当的评述;探讨了该领域今后的研究方向。     

Estimation of thermal conductivity for polypropylene/hollow glass bead composites

The thermal conductivity of hollow glass bead (HGB)-filled polypropylene (PP) composites was estimated using the thermal conductivity equation of inorganic hollow microsphere-filled polymer composites published in the previous paper. The estimations were compared with the measured data of the PP composites filled with two kinds of HGB with different size (the mean diameter was respectively 35 μm and 70 μm). The results showed that the predictions of the thermal conductivity were in good agreement with the measured data except to individual data points. Furthermore, both the estimated and measured thermal conductivity decreased roughly linearly with increasing the HGB volume fraction when the HGB volume fraction was less than 20%; the influence of the particle diameter on the thermal conductivity was insignificant.     

Effect of graphene oxide nanosheets on the physico-mechanical properties of chitosan/bacterial cellulose nanofibrous composites

In this work, novel chitosan/bacterial cellulose (CS/BC) nanofibrous composites reinforced with graphene oxide (GO) nanosheets are introduced. As cell attachment and permeability of nanofibrous membranes highly depend on their fiber diameter, the working window for successful electrospinning to attain sound nanofibrous composites with a minimum fiber diameter was determined by using the response surface methodology. It is shown that the addition of GO nanosheets to CS/BC significantly reduces the average size of the polymeric fibers. Their mechanical properties are also influenced and can be tailored by the concentration of GO. Fourier transform infrared spectroscopy reveals hydrogen bonding between the GO nanosheets and the polymer matrix. A decrease in the hydrophilicity of the electrospun nanofibers and their water vapor permeability with the addition of GO are also reported. The prepared nanofibrous composites are potentially suitable candidates for biomedical applications such as skin tissue engineering and wound dressing.     

Enhanced thermal resistance of phenolic resin composites at low loading of graphene oxide

By incorporating graphene oxide (GO) into phenolic resin (PR), GO/PR composites were prepared, and the effects of the content and reduction degree of GO on thermal resistance of GO/PR composites were studied. The peak degradation temperature of the PR was increased by about 14 °C with GO which was heat treated. The char yield of GO/PR composite at a GO weight fraction of 0.5% was about 11% greater than that of PR. The interactions such as covalent bonds and π–π stacking between GO and PR were regarded as the main reason for the enhancement. Located at the GO–PR interface, GO effectively anchored and structured PR molecular near the surfaces of GO sheets, and thus facilitated the formation of char. The superiority of GO/PR composites over PR in terms of thermal properties enhancement should also be related to the promoting graphitization by the addition of GO.     

Thermal conductivity of graphite flakes–SiC particles/metal composites

A new family of high thermal conductivity composites, produced through infiltration of a metallic alloy into preforms of mixtures of graphite flakes and either ceramic or carbon materials (in the form of particles or short fibers), has been recently developed. Composites microstructure roughly consists of alternating layers of flakes and metal-particles composite. The present work focuses on graphite flakes–SiC particles/Al–12 wt%Si composites. The effects that the relative amounts of the components, as well as the average diameter of SiC particles (varied over the range 13–170 μm), have on the thermal conductivity are investigated. The experimental results are analyzed by means of two model microstructures: (i) alternating layers of flakes and metal-particle composite, and, (ii) oriented discs (graphite flakes) randomly distributed in a metal-particle composite matrix. Fitting experimental data by means of these model microstructures leads to reasonable values of the thermal conductivity of graphite flakes along the transversal and longitudinal directions.