Fluorinated graphene reinforced polyimide films with the improved thermal and mechanical properties

In order to explore the addition effect of fluorinated graphene (FG) on the mechanical and thermal performances of polyimide (PI) matrix, FG sheets are first prepared and employed as the nanofillers to construct PI/FG nanocomposite films. The prepared film is optically transparent at low content of FG and experimental results demonstrate that the addition of FG can effectively enhance the properties of PI matrix. Especially, compared with pure PI matrix, the addition of 0.5 wt% FG in PI can endow 30.4% increase in tensile stress and 115.2% increase in elongation at break. Experimental analyses considering the morphology and microstructure are also conducted, and the results indicate that the improved mechanical properties of the PI/FG nanocomposite films are mainly attributed to the good dispersibility of FG sheets in PI host, and the effective stress transfer between the polymer and the FG.     

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.     

Enhanced conductivity behavior of polydimethylsiloxane (PDMS) hybrid composites containing exfoliated graphite nanoplatelets and carbon nanotubes

Polydimethylsiloxane (PDMS) hybrid composites consisting of exfoliated graphite nanoplatelets (xGnPs) and multiwalled carbon nanotubes functionalized with hydroxyl groups (MWCNTs-OH) were fabricated, and the effects of the xGnP/MWCNT-OH ratio on the thermal, electrical, and mechanical properties of polydimethylsiloxane (PDMS) hybrid composites were investigated. With the total filler content fixed at 4 wt%, a hybrid composite consisting of 75% × GnP/25% MWCNT-OH showed the highest thermal conductivity (0.392 W/m K) and electrical conductivity (1.24 × 10⁻³ S/m), which significantly exceeded the values shown by either of the respective single filler composites. The increased thermal and electrical conductivity found when both fillers are used in combination is attributed to the synergistic effect between the fillers that forms an interconnected hybrid network. In contrast, the various different combinations of the fillers only showed a modest effect on the mechanical behavior, thermal stability, and thermal expansion of the PDMS composite.     

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.     

Enhanced thermal conductivity and dielectric properties of Al/β-SiCw/PVDF composites

Polymeric composites with high thermal conductivity, high dielectric permittivity but low dissipation factor have wide important applications in electronic and electrical industry. In this study, three phases composites consisting of poly(vinylidene fluoride) (PVDF), Al nanoparticles and β-silicon carbide whiskers (β-SiC_w) were prepared. The thermal conductivity, morphological and dielectric properties of the composites were investigated. The results indicate that the addition of 12 vol% β-SiC_w not only improves the thermal conductivity of Al/PVDF from 1.57 to 2.1 W/m K, but also remarkably increases the dielectric constant from 46 to 330 at 100 Hz, whereas the dielectric loss of the composites still remain at relatively low levels similar to that of Al/PVDF at a wider frequency range from 10⁻¹ Hz to 10⁷ Hz. With further increasing the β-SiC_w loading to 20 vol%, the thermal conductivity and dielectric constant of the composites continue to increase, whereas both the dielectric loss and conductivity also rise rapidly.     

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.     

Graphene woven fabric-reinforced polyimide films with enhanced and anisotropic thermal conductivity

Due to the growing needs of thermal management in modern electronics, polyimide-based (PI) composites are increasingly demanded in thermal interface materials (TIMs). Graphene woven fabrics (GWFs) with a mesh structure have been prepared by chemical vapor deposition and used as thermally conductive filler. With the incorporation of 10-layer GWFs laminates (approximate 12 wt%), the in-plane thermal conductivity of GWFs/PI composite films achieves 3.73 W/mK, with a thermal conductivity enhancement of 1418% compared to neat PI. However, the out-of-plane thermal conductivity of the composites is only 0.41 W/mK. The in-plane thermal conductivity exceeds its out-of plane counterpart by over 9 times, indicating a highly anisotropic thermal conduction of GWFs/PI composites. The thermal anisotropy and the enhanced in-plane thermal conductivity can be attributed to the layer-by-layer stacked GWFs network in PI matrix. Thus, the GWFs-reinforced polyimide films are promising for use as an efficient heat spreader for electronic cooling applications.     

Effects of temperature and clay content on water absorption characteristics of modified MMT clay/cyclic olefin copolymer nanocomposite films: Permeability,dynamic mechanical properties and the encapsulated organic device performance

In the present study, amino-silane modified layered organosilicates were used to reinforce cyclic olefin copolymer to enhance the thermal, mechanical and moisture impermeable barrier properties. The optimum clay loading (4%) in the nanocomposite increases the thermal stability of the film while further loading decreases film stability. Water absorption behavior at 62 °C was carried out and compared with the behavior at room temperature and 48 °C. The stiffness of the matrix increases with clay content and the recorded strain to failure for the composite films was lower than the neat film. Dynamic mechanical analysis show higher storage modulus and low loss modulus for 2.5–4 wt% clay loading. Calcium degradation test and device encapsulation also show the evidence of optimum clay loading of 4 wt% for improved low water vapor transmission rates compared to other nanocomposite films.     

Preparation and characterization of poly(3-octylthiophene)/carbon fiber thermoelectric composite materials

Poly(3-alkylthiophene) (P3AT) with a high Seebeck coefficient has recently been reported. However, P3AT/inorganic conductive composites exhibit relatively poor thermoelectric performance because of their low electrical conductivity. In this work, carbon fiber sheets with a high electrical conductivity were chosen as the inorganic phase, and poly(3-octylthiophene)(P3OT)/carbon fiber composites were prepared by casting P3OT solution onto the carbon fiber sheets. The carbon fiber sheets incorporated into the composites can provide good electrical conductivity, and P3OT can provide a high Seebeck coefficient. The highest power factor of 7.05 μW m⁻¹ K⁻² was obtained for the composite with 50 wt% P3OT. This work suggests a promising method for preparing large-scale thermoelectric composites with excellent properties.     

Synergistic effect of boron nitride flakes and tetrapod-shaped ZnO whiskers on the thermal conductivity of electrically insulating phenol formaldehyde composites

Tetrapod-shaped zinc oxide (T-ZnO) whiskers and boron nitride (BN) flakes were employed to improve the thermal conductivity of phenolic formaldehyde resin (PF). A striking synergistic effect on thermal conductivity of PF was achieved. The in-plane thermal conductivity of the PF composite is as high as 1.96 W m⁻¹ K⁻¹ with 30 wt.% BN and 30 wt.% T-ZnO, which is 6.8 times higher than that of neat PF, while its electrical insulation is maintained. With 30 wt.% BN and 30 wt.% T-ZnO, the flexural strength of the composite is 312.9% higher than that of neat PF, and 56.2% higher that of the PF composite with 60 wt.% BN. The elongation at break is also improved by 51.8% in comparison with that of the composite with 60 wt.% BN. Such a synergistic effect results from the bridging of T-ZnO whiskers between BN flakes facilitating the formation of effective thermal conductance network within PF matrix.     

Characterization and properties of transparent cellulose nanowhiskers-based graphene nanoplatelets/multi-walled carbon nanotubes films

Transparent cellulose nanowhiskers (CNW)/graphene (GN) and CNW/multi-wall carbon nanotube (MWCNT) films were obtained by ultrasonication assisted mechanically stirring followed by solvent casting methods. GN has more significant influence on the properties of CNW film than MWCNT does because GN exhibits strong interaction with CNW by its adsorption on the surface of GN. Thermal behaviors of CNW-based composite films were greatly affected by addition of GN or MWCNT. The melting peak and initial degradation temperature increase by 23.5 and 24 °C, and by 78 °C and 94 °C for the composite films containing 5 wt% MWCNT and 5 wt% GN, respectively. The composites show the contact angles of 61.9° for GN included film and 46.9° for MWCNT included film, which is higher than that of pure CNW film (42.8°).     

Utilization of residual CdCl2 in CBD-CdS to realize grain growth in CdTe: A novel route

CdTe films were deposited by thermal evaporation onto chemical bath deposited CdS (CBD-CdS) films. The composite films were subjected to rapid thermal annealing (RTA) to observe simultaneous grain growth in both the CdS and CdTe layers. The films were characterized by measuring the compositional, microstructural and photoluminescence (PL) properties. PL spectra is dominated by the characteristic peaks (∼1.42 eV and ∼1.26 eV) associated with the virgin CdTe film. Additional features located at ∼2.56 eV and ∼1.99 eV could also be detected. The Fourier Transform Infra Red (FTIR) peak at ∼482 cm⁻¹ appeared due to the simultaneous presence of absorption peaks for CdTe stretching mode as well as Cd-S modes. Appearance of the broad peak between 1000 cm⁻¹ and 1165 cm⁻¹ may be an indication of interfacial alloying. Secondary ion mass Spectroscopy (SIMS) measurements were done to observe the compositional uniformity in the film and to measure the interfacial mixing behaviour.     

Pyroelectric and dielectric characterisation of alternate layer Langmuir–Blodgett films incorporating ions

Non-centrosymmetric Langmuir–Blodgett (LB) thin films exhibit a temperature-dependent electric polarisation, which is known as the pyroelectric effect. Alternate layer LB films were prepared by transferring polysiloxane alternately with eicosylamine from a subphase of Millipore water (18 MΩ cm^− 1) incorporating different metallic ions onto a 50 nm thick thermally evaporated aluminium substrate. Pyroelectric and dielectric measurements were performed on LB films with a sandwich structure comprising film. Pyroelectric coefficients of these LB films are measured at several temperatures. The dielectric constant and dielectric losses were obtained to determine the pyroelectric figure of merit values of these LB films, which were calculated in the range of 24.9 to 56.7 μC m^− 2 K^− 1.     

Transparent and flexible cellulose nanofibers/silver nanowires/acrylic resin composite electrode

In this study, we report a novel, eco-friendly and simple method to fabricate cellulose nanofibers (CNFs)/silver nanowires (AgNWs)/acrylic resin (AR) composite electrode. CNFs with average diameter of 15 nm were disintegrated only by one time-pass grinding. Aqueous dispersion of AgNWs was embedded onto the surface of CNFs film by simple vacuum filtration. The final composite electrode was obtained by impregnating CNFs/AgNWs film to AR with the assist of adhesive tape. This electrode with AgNWs density of 134 mg/m² showed low sheet resistance (4 Ω/sq), and high light transmittance (85%) which was 6% lower than that of neat AR. The coefficient of thermal expansion of the composite electrode was as low as 25.32 ppm K⁻¹. The tensile strength and Young’s modulus of CNFs/AgNWs/AR composite film were 35.71 MPa and 1.63 GPa, which were about 8 and 5.8 times larger than neat AR film, respectively.     

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.     

Highly mechanical strength and thermally conductive bismaleimide–triazine composites reinforced by Al2O3@polyimide hybrid fiber

Bismaleimide–triazine (BT) resins have received a great deal of attention in microelectronics due to its excellent thermal stability and good retention of mechanical properties. Thereafter, developing BT based composites with high mechanical strength, thermal conductivity and dielectric property simultaneously are highly desirable. In this study, one hybrid fiber of Al₂O₃ nanoparticle (200 nm) supported on polyimide fiber (Al₂O₃@PI) with core–shell structure was introduced into BT resin to prepare promising Al₂O₃@PI–BT composite. The results indicated that the resultant composites possessed high Young’s modulus of 4.06 GPa, low dielectric constant (3.38–3.50, 100 kHz) and dielectric loss (0.0102–0.0107, 100 kHz). The Al₂O₃@PI hybrid film was also conductive to improve thermal stability (T_d5% up to 371 °C), in-plane thermal conductivity (increased by 295% compared to that of the pure BT resin). Furthermore, the Al₂O₃@PI–BT composite were employed to fabricate a printed circuit substrate, on which a frequency “flasher” circuit and electrical components worked well.     

Single-walled carbon nanotube–epoxy composites for structural and conductive aerospace adhesives

Single-walled carbon nanotubes (SWCNTs) were incorporated at low loading (up to ∼1 wt%) into an unfilled aerospace-grade epoxy system, to impart electrical conductivity while maintaining structural bonding capability, as a route for development of a structural and conductive adhesive. At these low SWCNT loadings the tensile properties were maintained or improved, while strength decreased in a higher loading case. The structural bonding performance of composite-to-composite joints, evaluated in lap-shear and peel tests, was reasonably maintained for adhesives containing 0.5 wt% or 1 wt% SWCNTs. In the case of the 0.5 wt% SWCNT–adhesive, peel and lap-shear strength were unchanged while the addition of 1 wt% resulted in 30% increase of peel strength but the lap-shear strength was reduced by 10–15%. For 1 wt% SWCNT–adhesives, conductivities as high as 10⁻¹ S m⁻¹ and typically ∼10⁻³ S m⁻¹ were achieved. Joint electrical resistance measured between aluminum adherends was larger than predicted by the bulk conductivity, but was reduced by a post-treatment step resulting in apparent joint conductivities within one order of magnitude of the bulk samples.     

Growing polystyrene chains from the surface of graphene layers via RAFT polymerization and the influence on their thermal properties

The polystyrene (PS) macromolecular chains were grafted on the surface of graphene layers by reversible addition-fragmentation chain transfer (RAFT) polymerization. In this procedure, a RAFT agent, 4-Cyano-4-[(dodecylsulfanylthiocarbonyl) sulfanyl] pentanoic acid, was used to functionalize the thermal reduced graphene oxide (TRGO) to obtain the precursor (TRGO-RAFT). It can be calculated that the grafting density of PS/graphene (PRG) composites was about 0.18 chains per 100 carbons. Successful in-plain attachment of RAFT agent to TRGO and PS chain to TRGO-RAFT was shown an influence on the thermal property of the PRG composites. The thermal conductivity (λ) improved from 0.150 W m⁻¹ K⁻¹ of neat PS to 0.250 W m⁻¹ K⁻¹ of PRG composites with 10 wt% graphene sheets loading. The thermal property of PRG composites increased due to the homogeneous dispersion and ordered arrangement of graphene sheets in PS matrix and the formation of PRG composites.     

Effects of poly(oxyethylene)-block structure in polyetheramines on the modified carbon nanotube/poly(lactic acid) composites

The hybrids of multi-walled carbon nanotube and poly(lactic acid) (MWCNT/PLA) were prepared by a melt-blending method. In order to enhance the compatibility between the PLA and MWCNTs, the surface of the MWCNTs was covalently modified by Jeffamine® polyetheramines by functionalizing MWCNTs with carboxylic groups. Different molecular weights and hydrophilicity of the polyethermaines were grafted onto MWCNTs with the assistance of a dehydrating agent. The results showed that low-molecular-weight Jeffamine® polyetheramine modified MWCNTs can effectively improve the thermal properties of PLA composites. On the other hand, high-molecular-weight and poly(oxyethylene)-segmented polyetheramine could render the modified MWCNTs of well dispersion in PLA, and consequently affecting the improvements of mechanical properties and conductivity of composite materials. With the addition of 3.0 wt% MWCNTs, the increment of E′ of the composite at 40 °C was 79%. For conductivity, the surface resistivity decreased from 1.27 × 10¹² Ω/sq for neat PLA to 8.30 × 10⁻³ Ω/sq for the composites.     

A facile assembly of polyimide/graphene core–shell structured nanocomposites with both high electrical and thermal conductivities

Polyimide/reduced graphene oxide (PI/r-GO) core–shell structured microspheres were fabricated by in-situ reduction of graphene oxide (GO), which was coated on the surface of PI microspheres via hydrogen bonding and π–π stacking interaction. The highly ordered 3D core–shell structure of PI/r-GO microspheres with graphene shell thickness of 3 nm was well characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM) and Raman spectra. The glass transition temperature (T_g) of PI/r-GO microspheres was slightly increased because of the interaction of r-GO and PI matrix while the temperature at 5% weight loss (T_5%) of PI/r-GO microspheres was decreased due to the side effect of reductant hydrazine hydrate. The PI/r-GO nanocomposites exhibited highly electrical conductivity with percolation threshold of 0.15 vol% and ultimate conductivity of 1.4 × 10⁻² S/m. Besides, the thermal conductivity of PI/r-GO nanocomposites with 2% weight content of r-GO could reach up to 0.26 W/m K.