Improvement in thermal conductivity and mechanical properties of ethylene‐propylene–diene monomer rubber by expanded graphite

Thermally conductive fillers are usually employed in the preparation of rubber composites to enhance thermal conductivity. In this work, ethylene‐propylene‐diene monomer rubber (EPDM)/expanded graphite (EG) and EPDM/graphite composites with up to 100 phr filler loading were prepared. Compared to EPDM/graphite compounds with the same filler loading, stronger filler network was demonstrated for EPDM/EG compounds. Thermal conductivity and mechanical properties of EPDM/graphite and EPDM/EG composites were compared and systematically investigated as a function of the filler loading. The thermal conductivity of both EPDM/graphite and EPDM/EG composites increased with increasing volume fraction of fillers, and could be well fitted by Geometric Mean Model. The thermal conductivity as high as 0.910 W · m⁻¹ · K⁻¹ was achieved for the EPDM/EG composite with 25.8 vol% EG, which was ∼4.5 times that of unfilled EPDM. Compared to EPDM/graphite composites, EPDM/EG composites exhibited much more significant improvement in thermal conductivity and mechanical properties, which could be well correlated with the better filler‐matrix interfacial compatibility and denser structure in EPDM/EG composites, as revealed in the SEM images of tensile fracture surfaces. POLYM. COMPOS., 38:870–876, 2017. © 2015 Society of Plastics Engineers     

Preparation of expandable graphite with silicate assistant intercalation and its effect on flame retardancy of ethylene vinyl acetate composite

A halogen‐free intumescent flame retardant expandable graphite composite (EG), with an initial expansion temperature of 202°C and expansion volume of 517 mL g⁻¹, was successfully prepared via a facile two‐step intercalation method, i.e. using KMnO₄ as oxidant and H₂SO₄, Na₂SiO₃·9H₂O as intercalators. The prepared EG flame retardant was characterized by field emission scanning electron microscope, X‐ray diffraction spectroscopy, energy dispersive spectroscopy and Fourier transform infrared spectroscopy. Furthermore, flame retardancy and thermal property of various ethylene vinyl acetate copolymer (EVA) composites, including EVA/EG and EVA/EG/APP (ammonium polyphosphate) specimens, were studied through limiting oxygen index instrument (LOI), vertical combustion UL‐94 rating, thermal gravimetric and differential thermal analysis. The results indicate that the EVA/EG and EVA/EG/APP composites exhibit a better flame retardancy. Addition of EG at a mass fraction of 30% leads LOI of 70EVA/30EG composite improved to 28.7%. Even more, the synergistic effect between EG and APP improves the LOI of 70EVA/10APP /20EG composite to 30.7%. This synergistic efficiency is attributed to the formation of compact and stable layer‐structure, and the prepared EG can make EVA composite reach the UL‐94 level of V‐0. POLYM. COMPOS., 36:1407–1416, 2015. © 2014 Society of Plastics Engineers     

Critical effect and enhanced thermal conductivity of Cu-diamond composites reinforced with various diamond prepared by composite electroplating

Despite the great potential value as heat sink materials, their practical application of high thermal conductivity (TC) Cu-diamond composites is limited since high temperature and high pressure (above 1000 K and 60 MPa) were requisite in the conventional process. In this study, high TC void-free Cu-diamond composites reinforced with various diamond particles were prepared via composite electroplating. The impacts of diamond particle sizes (ranged from 10 to 400 μm) on microstructure, interface and TC of the composites were investigated. The TC of Cu-diamond composites was improved with the increase of diamond particle sizes and well-combined interface. Interestingly, a critical size for improved the TC of Cu-diamond composites was clearly observed and the critical value (22 μm) was derived from Kipitza theory. Based on the TC results and critical analysis, the Cu-diamond composite reinforced with large diamond particles (400 μm) was synthesized, which possessed the TC of 846.52 W m⁻¹ K⁻¹ and the thermal expansion coefficient of 7.2 × 10⁻⁶ K⁻¹. Such attractive thermal properties suggested that electroplating Cu-diamond composites showed the promising application as heat sink materials in microelectronic industry.     

Thermal energy storage performance of hierarchical porous kaolinite geopolymer based shape-stabilized composite phase change materials

Phase change materials (PCMs) are prospective energy materials that are widely applied in building energy conservation, waste heat recovery, infrared stealth technology and solar dynamic power system. The enhancement of heat transfer and leak-proof performance are critical to PCMs. Although geopolymers have been applied in thermal energy storage, meanwhile, hierarchically porous geopolymers have already shown superb performance in various functional applications, to the authors’ knowledge, no report concerning the application of hierarchical porous ones have been issued. This paper concerns the preparation of a shape-stabilized composite PCMs, consisting of hierarchically porous kaolinite-based geopolymer (PKG) embedding polyethylene glycol 4000 (PEG4000), which shows promising prospects in thermal energy storage. Optimized porous geopolymer matrices feature high porosity (>83%), combined with high specific surface area (4.7 m²/g) and thermal conductivity (TC, 1.324 W·m⁻¹·K⁻¹). Furthermore, the shape-stabilized composite PCMs show excellent thermal energy storage properties: loading rate of 80.93 wt%, latent heat of 168.80 J g⁻¹ and TC of ∼0.36 W·m⁻¹·K⁻¹ at 20–30 °C, which is 1.64 times of the TC of pure PEG4000. Finally, the photothermal conversion performances of the shape-stabilized composite PCMs were also simulated.     

Multilayered ultrahigh molecular weight polyethylene/natural graphite/boron nitride composites with enhanced thermal conductivity and electrical insulation by hot compression

A scalable strategy to fabricate thermally conductive but electrically insulating polymer composites was urgently required in various applications including heat exchangers and electronic packages. In this work, multilayered ultrahigh molecular weight polyethylene (UHMWPE)/natural graphite (NG)/boron nitride (BN) composites were prepared by hot compressing the UHMWPE/NG layers and UHMWPE/BN layers alternately. Taking advantage of the internal properties of NG and BN fillers, the UHMWPE/NG layers played a decisive role in enhancing thermal conductivity (TC), while the UHMWPE/BN layers effectively blocked the electrically conductive pathways without affecting the thermal conductive pathways. The in-plane TC, electrical insulation, and heat spreading ability of multilayered UHMWPE/NG/BN composites increased with the increasing layer numbers. At the total fillers loading of 40 wt%, the in-plane TC of multilayered UHMWPE/NG/BN composites with nine layers was markedly improved to 6.319 Wm⁻¹ K⁻¹, outperforming UHMWPE/BN (4.735 Wm⁻¹ K⁻¹) and pure UHMWPE (0.305 Wm⁻¹ K⁻¹) by 33.45% and 1971.80%, respectively. Meanwhile, the UHMWPE/NG/BN composites still maintained an excellent electrically insulating property (volume resistance~5.40×10¹⁴ Ω cm ; breakdown voltage~1.52 kV/mm). Moreover, the multilayered UHMWPE/NG/BN composites also exhibited surpassing heat dissipation capability and mechanical properties. Our results provided an effective method to fabricate highly thermal conductive and electrical insulating composites.     

Improving interfacial compatibility by a micro–nano synergetic structure for high-performance epoxy composites

A multiscale functional filler of micro–nano synergetic structure was successfully prepared via in-situ growth of silica (SiO₂) on biomimetic dopamine modified carbon fiber (CF) surface. The CF-SiO₂ hybrid as a reinforcement possessed lubricating and reinforcing effect to enhance tribological performance, thermal stability and thermomechanical property of epoxy (EP) composites. The micro–nano synergetic structure was of great importance for ameliorating the compatibility and interfacial adhesion between CF and EP matrix, which was conductive to transferring stress from matrix to fiber and alleviating stress concentration. It was concluded that the friction coefficient and wear rate of EP/CF-SiO₂ were 0.382 and 1.12 × 10⁻⁵ mm³/N·m, that is, a decline of 58% and 2.5 times, respectively, compared to EP/CF. The CF-SiO₂ hybrid exhibited excellent friction-reducing and anti-wear performance.     

PA6/CF/EG导电复合材料的制备与性能

采用熔融共混法制备了不同碳纤维/热膨胀石墨(CF/EG)比例的尼龙6/碳纤维/热膨胀石墨(PA6/CF/EG)导电复合材料并研究其性能。结果表明,CF的加入能显著提高复合材料的力学性能;而随着EG含量的提高,复合材料的导电性能和导热性能显著提高,但力学性能在一定程度上得到降低。当CF质量分数为20%时,复合材料具有最优的力学性能,当EG质量分数为20%时,复合材料体积电导率可显著提高至0.262 S/m,热导率可达1.3379W/(m·K)。     

Highly electrically and thermally conductive silicon carbide-graphene composites with yttria and scandia additives

Dense silicon carbide/graphene nanoplatelets (GNPs) and silicon carbide/graphene oxide (GO) composites with 1 vol.% equimolar Y₂O₃–Sc₂O₃ sintering additives were sintered at 2000 °C in nitrogen atmosphere by rapid hot-pressing technique. The sintered composites were further annealed in gas pressure sintering (GPS) furnace at 1800 °C for 6 h in overpressure of nitrogen (3 MPa). The effects of types and amount of graphene, orientation of graphene sheets, as well as the influence of annealing on microstructure and functional properties of prepared composites were investigated. SiC-graphene composite materials exhibit anisotropic electrical as well as thermal conductivity due to the alignment of graphene platelets as a consequence of applied high uniaxial pressure (50 MPa) during sintering. The electrical conductivity of annealed sample with 10 wt.% of GNPs oriented parallel to the measuring direction increased significantly up to 118 S·cm⁻¹. Similarly, the thermal conductivity of composites was very sensitive to the orientation of GNPs. In direction perpendicular to the GNPs the thermal conductivity decreased with increasing amount of graphene from 180 W·m⁻¹ K⁻¹ to 70 W·m⁻¹ K⁻¹, mainly due to the scattering of phonons on the graphene – SiC interface. In parallel direction to GNPs the thermal conductivity varied from 130 W·m⁻¹ K⁻¹ up to 238 W·m⁻¹ K⁻¹ for composites with 1 wt.% of GO and 5 wt.% of GNPs after annealing. In this case both the microstructure and composition of SiC matrix and the good thermal conductivity of GNPs improved the thermal conductivity of composites.     

Thermal conductivity and electromagnetic shielding effectiveness of composites based on Ag‐plating carbon fiber and epoxy

Thermally conductive and electromagnetic interference shielding composites comprising low content of Ag‐plating carbon fiber (APCF) were fabricated as electronic packing materials. APCF as conductive filler consisting of carbon fiber (CF) employed as the structural component to reinforce the mechanical strength, and Ag enhancing electrical conductivity, was prepared by advanced electroless Ag‐plating processing on CF surfaces. Ag coating had a thickness of 450 nm without oxide phase detected. The incorporation of 4.5 wt % APCF into epoxy (EP) substrate yielded thermal conductivity of 2.33 W/m·K, which is approximately 2.6 times higher than CF–EP composite at the same loading. The APCF–EP composite performed electromagnetic shielding effectiveness of 38–35 dB at frequency ranging from 8.2 to 12.4 GHz in the X band, and electromagnetic reflection was the dominant shielding mechanism. At loading content of APCF up to 7 wt %, thermal conductivity of APCF–EP composites increased to 2.49 W/m·K. Volume resistivity and surface resistivity decreased to 9.5 × 10³ Ω·cm and 6.2 × 10² Ω, respectively, which approached a metal. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42306.     

Synergistic enhancement of thermal conductivity by addition of graphene nanoplatelets to three-dimensional boron nitride scaffolds for polyamide 6 composites

Three-dimensional boron nitride/graphene nanoplatelets (3D-BN-GNP) scaffolds were fabricated using an ice-templating method and polyamide 6 (PA6)-based composites were prepared by vacuum impregnation of caprolactam monomers into the scaffolds, followed by polymerization. The BN sheets in the PA6/3D-BN and PA6/3D-BN-GNP composites display a predominant parallel alignment along the ice-crystal formation constructing thermally conductive paths. The addition of few GNPs assists the dispersion of BN sheets in the PA6/3D-BN-GNP composites and repair the broken thermal paths caused by local agglomeration of the BN sheets. Consequently, GNPs play a morphology-promoted synergistic role in the enhancement of the thermal conductivity of the PA6/3D-BN-GNP composites. The PA6/3D-BN-GNP composite prepared with 23.40 wt% BN sheets and 2.60 wt% GNPs exhibits the highest thermal conductivity of 2.80 W m⁻¹ K⁻¹, which is 833% and 33% higher than the values recorded for the pure PA6 and the PA6/3D-BN composite at BN loading of 26.18 wt%, respectively. Infrared imaging analysis revealed that the surface of the PA6/3D-BN-GNP composite has a fast response to heating and cooling, suggesting the potential of the composites in thermal management applications.     

Mechanical and tribological properties of SiC whisker-reinforced SiC composites via oscillatory pressure sintering

SiC whisker (SiCw)-reinforced SiC composites were prepared by an oscillatory pressure sintering (OPS) process, and the effects of SiCw content on the microstructure and mechanical and tribological properties of such composites were investigated. The addition of SiCw could promote the formation of long columnar α-SiC, and the aspect ratio of α-SiC grains first increased and then decreased with the increase of SiCw content. When the SiCw content was 5.42 wt%, the relative density of the SiC–SiCw composite reached up to 99.45%. The SiC–5.42 wt% SiCw composite possessed the highest Vickers hardness, fracture toughness, and flexural strength of 30.68 GPa, 6.66 MPa·m^1/2, and 733 MPa, respectively. In addition, the SiC–5.42 wt% SiCw composite exhibited the excellent wear resistance when rubbed with GCr15 steel balls, with a friction coefficient of .76 and a wear rate of 4.12 × 10⁻⁷ mm³·N⁻¹·m⁻¹. This could be ascribed to the improved mechanical properties of SiC–SiCw composites, which enhanced the ability to resist peeling and micro-cutting, thereby enhancing the tribological properties of the composites.     

Simultaneously improving thermal conductivity and dielectric properties of poly(vinylidene fluoride)/expanded graphite via melt blending with polyamide 6

Poly(vinylidene fluoride)/polyamide 6/expanded graphite (PVDF/PA6/EG) composite is prepared via one-step melt extrusion. The EG is well dispersed with the addition of PA6 and mainly distributed in the PA6 phase due to the stronger affinity between them. As a result, the PVDF/PA6/EG sample presents higher dielectric permittivity than the PVDF/EG sample and maintain a low dielectric loss due to its sea-island phase structure, which impedes the formation of conductive path in the composite. The mean interlayer spacing of the EG in the polymer matrix decreases obviously due to its improved dispersion state, which is in favor of the phonon transportation in the composite. As a result, the PVDF/PA6/EG sample exhibits a significantly improved thermal conductivity of about 0.48 W m⁻¹ K⁻¹, which is 140% higher than that of the PVDF sample and 37% higher than that of the PVDF/EG sample. Moreover, the PVDF/PA6/EG sample presents higher Young's modulus and tensile strength than the PVDF/EG sample. While the elongation at break of the PVDF/PA6/EG sample is only a little lower than that of the PVDF/EG sample. This means that the tensile properties of the composite are not destroyed obviously by melt blending with the immiscible PA6.     

Simultaneously Improved Thermal Conductivity and Dielectric Properties of NBR Composites by Constructing 3D Hybrid Filler Networks

This work aims to address the heat accumulation issue in electronic components during high-frequency operation through the preparation of novel thermally conductive composites. First, polydopamine (PDA) and in-situ growth of silver (Ag) nanoparticles are applied for the surface modification of graphene oxide (GO) and carbon nanotube (CNT) to prepare pGO@Ag and pCNT@Ag hybrid filler, respectively. Then, nitrile butadiene rubber (NBR) is chosen as the polymeric matrix and simultaneously incorporated with both pGO@Ag and pCNT@Ag to prepare polymeric composites with excellent thermal conductivity (TC) and dielectric constant (ɛ_r). Due to the construction of 3D heat conduction networks by utilizing 2D pGO@Ag and 1D pCNT@Ag, the fabricated NBR composites achieved the maximum TC of 1.0112 W/(mK), which is 636% higher than that of neat NBR (0.1373 W (mK)⁻¹). At the filler loading of 9 vol%, the TC of pGO@Ag/pCNT@Ag/NBR composite is 152% that of GO/CNT/NBR composite (0.6660 W (mK)⁻¹). Moreover, due to electron polarization effect of GO and CNT and micro-capacitor effect of Ag nanoparticles, a large ɛ_r of 147.12 is attained at 10 Hz for NBR composites. Overall, the development of dielectric polymer materials with high TC is beneficial for enhancing the service life and safety stability of the electronic components.     

0D surface modification and 3D silicon carbide network construction for improving the thermal conductivity of epoxy resins

With rapid advances in electronic device miniaturization and increasing power density, high thermal conductivity polymer composites with excellent properties are becoming increasingly significant for the progress of next-generation electronic apparatuses. In this work, a new type of three-dimensional (3D) network silicon carbide (SiC) frame and core-shell SiC@SiO₂ (SiC@SiO₂) were successfully prepared. The effects of different filler forms (dispersed particle filler and three-dimensional continuous filler network) on the thermal conductivity of the composites were compared. The composites based on the three-dimensional filler network exhibited evidently better thermal conductivity improvement rates, compared to their traditional counterparts. The thermal conductivity of the epoxy/SiC@SiO₂ composite having a total filler content of 17.0 vol% was 0.857 W/m/K, 328.5% higher than that of pure epoxy resin. Similarly, the thermal conductivity of the EP/3D-SiC composite having a total filler content of 13.8 vol% was 1.032 W/m/K, 416.0% higher than that of pure epoxy resin. The abovementioned stats were proven via molecular simulations. We estimated the interfacial thermal resistance (ITR) of the EP/3D-SiC composite to be 5.98 × 10^?8 m² K/W, which was an order of magnitude lower than that of the epoxy composites without a 3D network. Simultaneously, computerized molecular simulation technology was used to verify the feasibility of the experiment, which provided new ideas for the preparation of other highly thermally conductive materials.     

In situ sintered silver decorated 3D structure of cellulose scaffold for highly thermoconductive electromagnetic interference shielding epoxy nanocomposites

This study presents a 3-dimensional (3D) network structure of cellulose scaffold (CS), which was in situ decorated with silver nanoparticles (AgNPs). The scaffold was then infiltrated with epoxy matrix and cured at elevated temperature to sinter the AgNPs; finally, highly thermoconductive epoxy composites (Ag@CS/epoxy) was obtained. The resultant Ag@CS20/epoxy composite reached a thermal conductivity of 2.52 W·m⁻¹·K⁻¹ at 2.2 vol% of filler loading, which shows an enhancement of over 11-folds in the thermal conductivity compared to the neat epoxy. The superb electrical conductivity value of over 53,691 S·m⁻¹ of the Ag@CS20/epoxy was achieved, which led to exceptional EMI SE values of 69.1 dB. Furthermore, surface temperatures during heating and cooling were also investigated to demonstrate the superior heat dissipating capacity of the Ag@CS/epoxy composite, which can be potentially put an application as thermal dissipating material in the next generation of electronics.     

环氧树脂/二烷基次膦酸铝阻燃体系的制备和性能

着重研究了环氧树脂/二乙基次膦酸铝（EP/OP930）阻燃材料的阻燃性能、热分解性能和力学性能。结果表明,OP930的含量仅需15 %（质量分数,下同）就可以使EP/OP930体系的极限氧指数达到29.8 %,垂直燃烧实验达到UL 94 V-0级标准;此外,EP/OP930体系的综合性能良好,不同OP930含量的阻燃材料的力学性能、热稳定性能与原材料相比变化不大。     

A low-temperature prepared composite coating to protect SiC-coated C/C composites against oxidation in a wide temperature range for long-life service

To improve the oxidation resistance of carbon/carbon (C/C) composites in a wide temperature range (1173–1773 K), a composite coating containing rich B₂O₃ glass was prepared on SiC-coated C/C composites by slurry dipping-densifying at low temperature. Borosilicate and SiO₂ glasses acted as oxygen barriers at low and medium-high temperatures, respectively. Besides, Hf-oxides (HfO₂, HfSiO₄) ceramic particles improved the thermal stability of the glass and enhanced the crack resistance of glass layer. Therefore, the composite coating can effectively protect C/C composites against oxidation for 403 h at 1173 K, 723 h at 1473 K and 403 h at 1773 K with the mass gain of 3.77 g·m⁻², 21.41 g·m⁻² and 0.42 g·m⁻², respectively. After 50 times thermal cycles between room temperature and 1773 K, the mass gain of the coated sample was 3.95 g·m⁻² and the mass retention rate was up to 98.19 % during the thermos-gravimetric test from room temperature to 1773 K.     

Enhanced out-of-plane thermal conductivity of polyimide composite films by adding hollow cube-like boron nitride

Boron nitride nanosheet (BNNS) is widely used in electronic thermal management due to its excellent planar thermal conductivity and insulating properties. However, it is challenging to improve the out-of-plane thermal conductivity of BNNS-doped composites due to the anisotropy of the thermal conductivity of BNNS. Therefore, the BNNS in the matrix must be oriented to obtain composites with high out-of-plane thermal conductivity. In this study, BNNS powders with directional structures were synthesized directly using sodium chloride templates. The as-obtained BNNS powders have a unique hollow cube-like structure with an ultra-low density of 2.67 × 10⁻² g/cm³ and nearly 8 times the volume of the same mass of two-dimensional (2D) BNNS, making it easy to form the out-of-plane thermal conductivity paths in the polymer matrix. In addition, the high out-of-plane thermal conductivity of 4.93 W m⁻¹ K⁻¹ at 23.3 wt% loadings was obtained by doping it into a polyimide (PI) matrix. This value is 9.7 times higher than that of 2D BNNS-doped PI at the same loadings, 17.6 times higher than pure PI, and 6.1 times higher than the thermally conductive PI film sold by DuPont. Therefore, the prepared composite film has great potential for application in electronic thermal management.     

Core–shell spherical graphite@SiC attenuating agent for AlN-based microwave attenuating ceramics with high–efficiency thermal conduction and microwave absorption abilities

A core–shell structured spherical graphite (SG)@SiC attenuating agent with a tunable silicon carbide (SiC) shell thickness was synthesized via in-situ solid-liquid reaction of SG and Si. Then, fully dense 10 wt%SG@SiC/AlN microwave attenuating composite ceramics were prepared through hot-pressing sintering, and the morphology of SG@SiC particle was well maintained. By moderately modulating the thickness of the SiC shell with relatively low complex permittivity and thermal conductivity, an effectively inhibited solid solution of SiC into AlN, weakened dipole and electron polarization, enhanced conduction loss, and an improved impedance matching, thermal conductivity and microwave loss capacity were simultaneously achieved. Thus, the SG@SiC/AlN composite exhibit excellent and impressive thermal conductivity of 63.92 W m⁻¹·K⁻¹ and minimum reflection loss of −34.2 dB. The outstanding performance of SG@SiC/AlN composite indicates that the composite is promising microwave attenuating ceramic with excellent thermal conduction and microwave absorption ability. This work opens up a new core–shell structure strategy for designing and developing a high-efficiency attenuating agent and microwave attenuating ceramic.     

Matching micro- and nano-boron nitride hybrid fillers for high-thermal conductive composites

Incorporating hybrid fillers into polymer has been considered as one of the effective ways to obtain composites with high-thermal conductivities (TCs). Herein, we fabricated polytetrafluoroethylene (PTFE) composites by using micro-boron nitride nanosheets (mBNNs) and nano-BNNs (nBNNs) as fillers, and studied the optimum ratios of mBNNs to nBNNs (i.e., mBNNs:nBNNs) for obtaining high-thermal conductive composites at different filler's contents. The results indicated that for the composites with total BNNs contents of 10, 20, and 30 wt%, the optimum mBNNs:nBNNs for obtaining the highest TC were 9:1, 9:1, and 5:5, respectively. The highest TC of the composites with 30 wt% BNNs could reach 1.46 W·m⁻¹·K⁻¹, which was 356% higher than that of PTFE. The reasons for optimum mBNNs:nBNNs values were interpreted by observing the composite's microstructures. Moreover, the fabricated composites also exhibited excellent electrical insulation properties. This study has important implications for obtaining high-thermal conductive composites using hybrid fillers.