High thermal conductivity of flexible polymer composites due to synergistic effect of multilayer graphene flakes and graphene foam

Research on flexible thermal interface materials (TIMs) has shown that the interconnected network of graphene foam (GF) offers effective paths of heat transportation. In this work, a variant amount of multilayer graphene flakes (MGFs) was added into 0.2 vol% GF/polydimethylsiloxane (PDMS) composite. A remarkable synergistic effect between MGF and GF in improving thermal conductivity of polymer composites is achieved. With 2.7 vol% MGFs, the thermal conductivity of MGF/GF/PDMS composite reaches 1.08 W m⁻¹ K⁻¹, which is 80%, 184% and 440% higher than that of 2.7 vol% MGF/PDMS, GF/PDMS composites and pure PDMS, respectively. The MGF/GF/PDMS composite also shows superior thermal stability. The addition of MGFs and GF decreases slightly the elongation at break, but observably increases the Young’s modulus and tensile strength of composites compared with pure PDMS. The good performance of MGF/GF/PDMS composite makes it a good TIM for possible application in thermal management of electronics.     

Thermal and mechanical properties of waste grass broom fiber-reinforced polyester composites

The main focus of this study is to utilize waste grass broom natural fibers as reinforcement and polyester resin as matrix for making partially biodegradable green composites. Thermal conductivity, specific heat capacity and thermal diffusivity of composites were investigated as a function of fiber content and temperature. The waste grass broom fiber has a tensile strength of 297.58 MPa, modulus of 18.28 GPa, and an effective density of 864 kg/m³. The volume fraction of fibers in the composites was varied from 0.163 to 0.358. Thermal conductivity of unidirectional composites was investigated experimentally by a guarded heat flow meter method. The results show that the thermal conductivity of composite decreased with increase in fiber content and the quite opposite trend was observed with respect to temperature. Moreover, the experimental results of thermal conductivity at different volume fractions were compared with two theoretical models. The specific heat capacity of the composite as measured by differential scanning calorimeter showed similar trend as that of the thermal conductivity. The variation in thermal diffusivity with respect to volume fraction of fiber and temperature was not so significant.The tensile strength and tensile modulus of the composites showed a maximum improvement of 222% and 173%, respectively over pure matrix. The work of fracture of the composites with maximum volume fraction of fibers was found to be 296 Jm⁻¹.     

Tribological behavior of poly (phenyl p-hydroxybenzoate)/polytetrafluoroethylene composites filled with hexagonal boron nitride under dry sliding condition

The sliding friction and wear behavior of polytetrafluoroethylene (PTFE) composites filled with poly (phenyl p-hydroxybenzoate) (PHBA) and hexagonal boron nitride (h-BN) was investigated with a pin-on-disc tester. The tensile properties, ball indentation hardness, impact strength and thermal diffusivity were measured. The test results in this paper indicate that the tensile strength, elongation at break, and impact strength decreased, however, the ball indentation hardness and thermal diffusivity were increased when the content of h-BN was increased. PTFE composites filled with 20 wt% PHBA and 20 wt% h-BN exhibited a comparative friction coefficient to pure PTFE. Meantime, the wear rate of the composite decreased about 15 times compared to pure PTFE. The synergistic effect of h-BN with low friction and PHBA with high bearing ability promoted the low friction coefficient and wear rate of h-BN/PHBA/PTFE composites.     

Enhanced thermal conductivity of SiCp/PS composites by electrospinning–hot press technique

Silicon carbide particle/polystyrene (SiCp/PS) electrospun mats are firstly prepared by electrospinning technology, then to be fabricated the corresponding thermally conductive SiCp/PS composites by the method of “laminating-hot press”. The mass fraction of SiCp and laminating mode of SiCp/PS electrospun mats affecting on the thermal conductivities, dielectric and thermal properties of the composites are investigated. The addition of 32.8 vol% SiCp improves the thermally conductive coefficient λ of pure PS from 0.182 to 0.566 W/m K and thermal diffusivity of pure PS from 0.169 to 0.376 mm²/s, whereas the dielectric constant values still remain at relatively low levels. The thermal stabilities of the SiCp/PS composites are increased with the increasing addition of SiCp. For a given SiCp loading, the SiCp/PS composites from warp–weft arrangement of SiCp/PS electrospun mats possess relative higher thermally conductive coefficient λ and dielectric constant values than those of SiCp/PS composites from warp–warp arrangement of SiCp/PS electrospun mats.     

Study on thermal properties of graphene foam/graphene sheets filled polymer composites

The polymer composites composed of graphene foam (GF), graphene sheets (GSs) and pliable polydimethylsiloxane (PDMS) were fabricated and their thermal properties were investigated. Due to the unique interconnected structure of GF, the thermal conductivity of GF/PDMS composite reaches 0.56 W m⁻¹ K⁻¹, which is about 300% that of pure PDMS, and 20% higher than that of GS/PDMS composite with the same graphene loading of 0.7 wt%. Its coefficient of thermal expansion is (80–137) × 10⁻⁶/K within 25–150 °C, much lower than those of GS/PDMS composite and pure PDMS. In addition, it also shows superior thermal and dimensional stability. All above results demonstrate that the GF/PDMS composite is a good candidate for thermal interface materials, which could be applied in the thermal management of electronic devices, etc.     

Enhanced thermal-conductive and anti-dripping properties of polyamide composites by 3D graphene structures at low filler content

In this work, 3D graphene structures constructed by graphene foam (GF) were introduced into polyamide-6 (PA6) matrix for the purpose of enhancing the thermal-conductive and anti-dripping properties of PA6 composites. The GF were prepared by one-step hydrothermal method. The PA6 composites were synthesized by in-situ thermal polycondensation method to realize PA6 chains covalently grafted onto the graphene sheets. The 3D interconnected graphene structure favored the formation of the consecutive thermal conductive paths or networks even at relatively low graphene loadings. As a result, the thermal conductivity was improved by 300% to 0.847 W·m⁻¹·K⁻¹ of PA6 composites at 2.0 wt% graphene loading from 0.210 W·m⁻¹·K⁻¹ of pure PA6 matrix. The presence of self-supported 3D structure alone with the covalently-grafted PA6 chains endowed the PA6 composites good anti-dripping properties.     

Manufacture and characterisation of thermoplastic composites made from PLA/hemp co-wrapped hybrid yarn prepregs

PLA/hemp co-wrapped hybrid yarns were produced by wrapping PLA filaments around a core composed of a 400 twists/m and 25 tex hemp yarn (Cannabis sativa L) and 18 tex PLA filaments. The hemp content varied between 10 and 45 mass%, and the PLA wrapping density around the core was 150 and 250 turns/m. Composites were fabricated by compression moulding of 0/90 bidirectional prepregs, and characterised regarding porosity, mechanical strength and thermal properties by dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC). Mechanical tests showed that the tensile and flexural strengths of the composites markedly increased with the fibre content, reaching 59.3 and 124.2 MPa when reinforced with 45 mass% fibre, which is approximately 2 and 3.3 times higher compared to neat PLA. Impact strength of the composites decreased initially up to 10 mass% fibre; while higher fibre loading (up to 45 mass%) caused an increase in impact strength up to 26.3 kJ/m², an improvement of about 2 times higher compared to neat PLA. The composites made from the hybrid yarn with a wrapping density of 250 turns/m showed improvements in mechanical properties, due to the lower porosity. The fractured surfaces were investigated by scanning electron microscopy to study the fibre/matrix interface.     

In situ preparation and mechanical properties of CNTs/MCMBs composites

By adding carbon nanotubes (CNTs) into medium temperature coal tar pitch, mesocarbon microbeads (MCMBs) were obtained via thermal condensation, then CNTs/MCMBs composites were in situ prepared using compression molding. The morphology, structure and mechanical properties of CNTs/MCMBs composites were characterized by optical microscope, digital camera, scanning electron microscope (SEM) and mechanical test machine. Results showed that CNTs were used as the nucleating agent and could inhibit the growth and coalescence of MCMBs. The optical textures of CNTs/MCMBs composites showed similar characteristics to the thermal condensation products from coal tar pitch with CNTs. The mass ratio of CNTs to coal tar pitch played an important role in the mechanical properties of CNTs/MCMBs composites. The density and bending strength of CNTs/MCMBs composite first increased and then decreased with the increase of the proportion of CNTs. When the proportion of CNTs was 5 wt%, the density of the composite reached the maximum (1.76 g/cm³). In addition, the bending strength of the composite reached the maximum (79.6 MPa) as adding 2 wt% CNTs into coal tar pitch.     

Thermal,mechanical and electrical properties of polyurethane/(3-aminopropyl) triethoxysilane functionalized graphene/epoxy resin interpenetrating shape memory polymer composites

Functionalized graphene (FG) was successfully synthesized by treating graphene oxide with (3-aminopropyl) triethoxysilane (KH-550) and then reduced by hydrazine hydrate. Subsequently, significant reinforcement of polyurethane/epoxy resin (PU/EP) composites in situ synthesized on the FG is prepared. Morphologic study shows that, due to the formation of chemical bonding, the FG was dispersed well in the PU/EP matrix and the mechanical performance is improved. Meanwhile, the thermal degradation temperature was enhanced almost 50 °C higher than that of PU/EP. The conductivity of PU/FG/EP nanocomposites was 82.713 × 10⁻⁶ S/m at 2.0 wt% loadings. The resulting composites exhibited 96% shape fixity, 94% shape recovery, enhanced shape recovery force to realize thermo-electric dual-responsive property. Comparing with the results in literature, the composites used in this study have shown a progress between electrical conductivity and shape memory property.     

Improved cryogenic interlaminar shear strength of glass fabric/epoxy composites by graphene oxide

The cryogenic interlaminar shear strength (ILSS) at cryogenic temperature (77 K) of glass fabric (GF)/epoxy composites is investigated as a function of the graphene oxide (GO) weight fraction from 0.05 to 0.50 wt% relative to epoxy. For the purpose of comparison, the ILSS of the GF/epoxy composites is also examined at room temperature (RT, 298 K). The results show that the cryogenic ILSS is greatly improved by about 32.1% and the RT ILSS is enhanced by about 32.7% by the GO addition at an appropriate content of 0.3 wt% relative to epoxy. In addition, the ILSS of the composite at 77 K is much higher than that at RT due to the relatively strong interfacial GF/epoxy adhesion at 77 K compared to the RT case.     

Thermal shock behavior of nano-sized SiC particulate reinforced AlON composites

Aluminum oxynitride (AlON) has been considered as a potential ceramic material for high-performance structural and advanced refractory applications. Thermal shock resistance is a major concern and an important performance index of high-temperature ceramics. While silicon carbide (SiC) particles have been proven to improve mechanical properties of AlON ceramic, the high-temperature thermal shock behavior was unknown. The aim of this investigation was to identify the thermal shock resistance and underlying mechanisms of AlON ceramic and 8 wt% SiC–AlON composites over a temperature range between 175 °C and 275 °C. The residual strength and Young's modulus after thermal shock decreased with increasing quenching temperature and thermal shock times due to large temperature gradients and thermal stresses caused by abrupt water-quenching. A linear relationship between the residual strength and thermal shock times was observed in both pure AlON and SiC–AlON composites. The addition of nano-sized SiC particles increased both residual strength and critical temperature from 200 °C in the monolithic AlON to 225 °C in the SiC–AlON composites due to the toughening effect, the lower coefficient of thermal expansion and higher thermal conductivity of SiC. The enhancement of the thermal shock resistance in the SiC–AlON composites was directly related to the change of fracture mode from intergranular cracking along with cleavage-type fracture in the AlON to a rougher fracture surface with ridge-like characteristics, crack deflection, and crack branching in the SiC–AlON composites.     

Preparation of low-density polyethylene/low-temperature expandable graphite composites with high thermal conductivity by an in situ expansion melt blending process

A facile strategy with the advantages of low cost and ease of mass production was presented to prepare low-density polyethylene (LDPE)/low-temperature expandable graphite (LTEG) composites with relatively high thermal conductivity by an in situ expansion melt blending process. LTEGs were expanded and delaminated into graphite multi-layers and graphite nanoplatelets during processing which synergistically created more thermo-conducting paths in the composites and hence led to great improvements in thermal conductivity. Thermal conductivity of the composite with 60 wt% of LTEG loading was increased by 23 times as compared to the pure LDPE, increasing from 0.47 to 11.28 W/mK. The incorporation of LTEG decreased the melting temperature and the degree of crystallinity of LDPE. Percolation threshold of both the electrical conductivity and rheological measurements was observed at about 8 vol% of LTEG loading. Moreover, the LDPE/LTEG composites showed better thermal stability compared to the pure LDPE.     

The dynamic mechanical response of SiC particulate reinforced 2024 aluminum matrix composites

The compression properties of the aluminum alloy 2024 metal matrix composites reinforced with 50 vol.% SiC particles were investigated using Instron testing machine and split Hopkinson pressure bar (SHPB) in this paper. The compression stress–strain curves were obtained at the strain rates ranging from 1 × 10^− 3 to 2.5 × 10³/s. The fracture surfaces were characterized by scanning electron microscopy. The results showed that SiCp/2024 Al composites exhibited high strain-rate sensitivity. The strength of composites tended to increase–decrease with increasing of strain rates. The effect of the strain rate on elongation was also discussed.     

Mechanical properties of natural fibre reinforced polyester composites: Jowar,sisal and bamboo

In this paper, the experiments of tensile and flexural tests were carried out on composites made by reinforcing jowar as a new natural fibre into polyester resin matrix. The samples were prepared up to a maximum volume fraction of approximately 0.40 from the fibres extracted by retting and manual process, and compared with established composites like sisal and bamboo developed under similar laboratory conditions. Jowar fibre has a tensile strength of 302 MPa, modulus of 6.99 GPa and an effective density of 922 kg/m³. It was observed that the tensile strength of jowar fibre composite is almost equal to that of bamboo composite, 1.89 times to that of sisal composite and the tensile modulus is 11% and 45% greater than those of bamboo and sisal composites, respectively at 0.40 volume fraction of fibre. The flexural strength of jowar composite is 4%, 35% and the flexural modulus is 1.12 times, 2.16 times greater than those of bamboo and sisal composites, respectively. The results of this study indicate that using jowar fibres as reinforcement in polyester matrix could successfully develop a composite material in terms of high strength and rigidity for light weight applications compared to conventional sisal and bamboo composites.     

Thermal conductivity of polystyrene–aluminum nitride composite

The thermal conductivity of polymer composites having a matrix of polystyrene (PS) containing aluminum nitride (AlN) reinforcement has been investigated under a special dispersion state of filler in the composites: aluminum nitride filler particles surrounding polystyrene matrix particles. Data for the thermal conductivity of the composites are discussed as a function of composition parameters (aluminum nitride concentration, polystyrene particle size) and temperature. It is found that the thermal conductivity of composites is higher for a polystyrene particle size of 2 mm than that for a particle size of 0.15 mm. The thermal conductivity of the composite is five times that of pure polystyrene at about 20% volume fraction of AlN for the composite containing 2 mm polystyrene particle size. The relationship between thermal conductivity of composites and AlN filler concentrations has been compared with the predictions of two theoretical models from the literature.     

Mechanical properties of a diamond–copper composite with high thermal conductivity

Using pressureless infiltration of copper into a bed of coarse (180 μm) diamond particles pre-coated with tungsten, a composite with a thermal conductivity of 720 W/(m K) was prepared. The bending strength and compression strength of the composite were measured as 380 MPa. As measured by sound velocity, the Young's modulus of the composite was 310 GPa. Model calculations of the thermal conductivity, the strength and elastic constants of the copper–diamond composite were carried out, depending on the size and volume fraction of filler particles. The coincidence of the values of bending strength and compressive strength and the relatively high deformation at failure (a few percent) characterize the fabricated diamond–copper composite as ductile. The properties of the composite are compared to the known analogues — metal matrix composites with a high thermal conductivity having a high content of filler particles (~ 60 vol.%). In strength and ductility our composite is superior to diamond–metal composites with a coarse filler; in thermal conductivity it surpasses composites of SiC–Al, W–Cu and WC–Cu, and dispersion-strengthened copper.     

Preparation of Nylon-6/flake graphite derivatives composites with antistatic property and thermal stability

Nylon-6/flake graphite (FG) composite, Nylon-6/graphene intercalation compounds (GIC) composite and Nylon-6/exfoliated graphite (EG) composite were prepared by FG, GIC, EG and caprolactam via in situ polymerization, and the volume resistivities of Nylon-6/flake graphite derivatives composites were also investigated. Meanwhile, the structure of Nylon-6/EG composite was characterized and the thermal stability of Nylon-6/EG composite was investigated as well. When the mass percents of FG, GIC and EG were 1%, 2–4% and 1%, the volume resistivities of flake graphite derivatives composites would reach 7.5 × 10⁶ Ω cm, 3.6 × 10⁸–1.4 × 10⁶ Ω cm and 2.3 × 10⁶ Ω cm. When the mass percent of EG increases from 0% to 9%, the thermal stability temperature of Nylon-6/EG composite would enhance from 70 to 196 °C. This shows that Nylon-6/flake graphite derivatives composites can have the antistatic property and thermal stability synchronously.     

Influence of alkali treatment on the interfacial and physico-mechanical properties of industrial hemp fibre reinforced polylactic acid composites

The focus of this work was to produce short (random and aligned) and long (aligned) industrial hemp fibre reinforced polylactic acid (PLA) composites by compression moulding. Fibres were treated with alkali to improve bonding with PLA. The percentage crystallinity of PLA in composites was found to be higher than that for neat PLA and increased with alkali treatment of fibres which is believed to be due to the nucleating ability of the fibres. Interfacial shear strength (IFSS) results demonstrated that interfacial bonding was also increased by alkali treatment of fibres which also lead to improved composite mechanical properties. The best overall properties were achieved with 30 wt.% long aligned alkali treated fibre/PLA composites produced by film stacking technique leading to a tensile strength of 82.9 MPa, Young’s modulus of 10.9 GPa, flexural strength of 142.5 MPa, flexural modulus of 6.5 GPa, impact strength of 9 kJ/m², and a fracture toughness of 3 MPa m^1/2.     

Thermal and mechanical behaviour of sisal/phenolic composites

This research proposes the development of polymeric composites reinforced with natural fibres to become stronger the damaged timber structures and proposes thermal and mechanical characterization of these composites. Fibres with larger structural applications are glass and carbon fibres but the use of natural fibres is an economical alternative and possesses many advantages such as biodegradability, low cost and is a renewable source. Woven sisal fabric was submitted to heat treatment before moulding and the influence of moisture content of fibres on the composites behaviour was observed. The paper presents mechanical characterization by tensile and flexural strength of woven sisal fabric composites, with and without thermal treatment (at 60 °C for 72 h) on the fabric, thermal characterization by TGA and the manufacturing process by compression moulding. Experimental results show to sisal/phenolic composites a tensile strength and a flexural strength value of 25.0 MPa and 11.0 MPa, respectively, independent to the use of sisal fibres with or without thermal treatment.     

An innovative process to fabricate copper/diamond composite films for thermal management applications

Diamond dispersed copper matrix (Cu/D) composite films with strong interfacial bonding were produced by tape casting and hot pressing without carbide forming additives. The tape casting process offers an original solution to obtain laminated materials with accurate thickness control, smooth surface finish, material net-shaping, scalability, and low cost. This study presents an innovative process of copper submicronic particles deposition onto diamond reinforcements prior to densification by hot pressing. Copper particles act as chemical bonding agents between the copper matrix and the diamond reinforcements during hot pressing, thus offering an alternative solution to traditionnal carbide-forming materials in order to get efficient interfacial bonding and heat-transfer in Cu/D composites. It allows high thermal performances with low content of diamond, thus enhancing the cost-effectiveness of the materials. Microstructural study of composites by scanning electron microscopy (SEM) was correlated with thermal conductivity and thermal expansion coefficient measurements. The as-fabricated films exhibit a thermal conductivity of 455 W m^?1 K^?1 associated to a coefficient of thermal expansion of 12 × 10^?6 °C^?1 and a density of 6.6 g cm^?3 with a diamond volume fraction of 40%, which represents a strong enhancement relative to pure copper properties (λ_Cu = 400 W m^?1 K^?1, α_Cu = 17 × 10^?6 °C^?1, ρ_Cu = 8.95 g cm^?3). The as-fabricated composite films might be useful as heat-spreading layers for thermal management of power electronic modules.