Anisotropy of functional properties of SiC composites with GNPs,GO and in-situ formed graphene

This paper reports on anisotropy of functional properties of different silicon carbide-graphene composites due to preferential orientation of graphene layers during sintering. Dense silicon carbide/graphene nanoplatelets (SiC/GNPs) and silicon carbide/graphene oxide (SiC/GO) composites were sintered in the presence of yttria (Y₂O₃) and alumina (Al₂O₃) sintering additives at 1800 °C in vacuum by the rapid hot pressing (RHP) technique. It is found that electrical conductivity of SiC/GNPs and SiC/GO composites increases significantly in the perpendicular direction to the RHP pressing axis, reached up to 1775 S/m in the case of SiC/GO (for 3.15 wt.% of rGO). Also, thermal diffusivity was found to increase slightly by the addition of GNPs in the SiC/GNPs composites in the perpendicular direction to the RHP pressing axis. But, in the parallel direction, the addition of GNPs showed a negative effect. The formation of graphene domains was observed in reference sample SiC-Y₂O₃-Al₂O₃ sintered by RHP, without any addition of graphene. Their presence was confirmed indirectly by increasing electrical conductivity about three orders of magnitude in comparison to the reference sample sintered by conventional hot press (HP). Raman, SEM and TEM analysis were used for direct evidence of presence of graphene domains in RHP reference sample.     

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.     

The effect of graphene nanoplatelets on the thermal and electrical properties of aluminum nitride ceramics

The thermal conductivity (κ) of AlN (2.9 wt.% of Y₂O₃) is studied as a function of the addition of multilayer graphene (from 0 to 10 vol.%). The κ values of these composites, fabricated by spark plasma sintering (SPS), are independently analyzed for the two characteristic directions defined by the GNPs orientation within the ceramic matrix; that is to say, perpendicular and parallel to the SPS pressing axis. Conversely to other ceramic/graphene systems, AlN composites experience a reduction of κ with the graphene addition for both orientations; actually the decrease of κ for the in-plane graphene orientation results rather unusual. This behavior is conveniently reproduced when an interface thermal resistance is introduced in effective media thermal conductivity models. Also remarkable is the change in the electrical properties of AlN becoming an electrical conductor (200 S m⁻¹) for graphene contents above 5 vol.%.     

Graphene nanoplatelet/silicon nitride composites with high electrical conductivity

Silicon nitride (Si₃N₄) processed with up to 25 vol.% of graphene nanoplatelets (GNPs) gives conductive composites with the highest electrical conductivity (40 Scm^?1) reported for these ceramics with added conductive particles. During compaction and pressure-assisted densification of the composites in the spark plasma sintering (SPS), a preferred orientation of GNPs occurs. Consequently, the electrical conductivity measured along the direction perpendicular to the SPS pressing axis is more than one order of magnitude higher than the one measured along the parallel direction.Percolation in the composites is observed for 7–9 vol.% of GNPs, depending on the measuring direction, perpendicular or parallel to the pressing axis. Different conduction mechanisms are apparent for the two orthogonal orientations. Charge transport along the direction defined by the graphene ab-plane (perpendicular direction) may be explained by a two dimensional variable range hopping mechanism, whereas conduction in the parallel direction shows a more complex behavior, with a metallic-type transition (dσ/dT < 0) for high GNP contents. A thin amorphous layer was identified at the Si₃N₄/GNPs interface that may affect the conduction for the parallel configuration.     

Boron carbide/graphene platelet ceramics with improved fracture toughness and electrical conductivity

Boron carbide/graphene platelet (B₄C/GPLs) composites have been prepared with a different weight percent of GPLs as sintering additive and reinforcing phase, hot pressed at 2100 °C in argon. The influence of the GPLs addition on fracture toughness (K_IC) and electrical conductivity was investigated. Single Edge V-Notched Beam (SEVNB) method was used for fracture toughness measurements and the four-point Van der Pauw method for electrical conductivity measurements. With increasing amount of GPLs additives, the fracture toughness increased due to the activated toughening mechanisms in the form of crack deflection, crack bridging, crack branching and graphene sheet pull-out. The highest fracture toughness of 4.48 MPa.m^1/2 was achieved at 10 wt.% of GPLs addition, which was ∼50% higher than the K_IC value of the reference material. The electrical conductivity increased with GPLs addition and reached the maximum value at 8 wt.% of GPLs, 1.526 × 10³ S/m in the perpendicular and 8.72 × 10² S/m in the parallel direction to the hot press direction, respectively.     

Electrical discharge machining of boron carbide-graphene nanoplatelets composites

Boron carbide (B₄C) composites containing 0–10 wt.% graphene nanoplatelets (GNPs) were consolidated using hot pressing at 1950 °C for 60 min under 30 MPa in an argon atmosphere. Their electrical discharge machining (EDM) characteristics were evaluated for the first time. Additionally, the effects of GNPs on the microstructure, electrical conductivity, material removal rate (MRR), and surface roughness (R_a) of B₄C composites were investigated. The results show that the B₄C composite containing 10 wt.% GNPs exhibited the highest electrical conductivity of 4997 S·m^?1 because of the formation of GNPs conductive networks. Under a fine machining condition, the MRR of the B₄C composite was enhanced by 43.5 % (from 6.55 to 9.40 mm³ min^?1) compared with that of monolithic B₄C. Furthermore, the R_a decreased to 1.12 μm, which was significantly lower than that of pure B₄C (2.44 μm). Scanning electron microscope and energy disperse spectroscopy analysis of the EDM surfaces were used to determine that the main material removal mechanisms for B₄C composites with less than 2 wt.% GNPs were spalling and melting. As the GNPs content increased, B₄C grain fallout, melting, and evaporation became more dominant. The other mechanisms, including thermal shock and oxidation, are also discussed in detail.     

Highly conductive interconnected graphene foam based polymer composite

In this study, the impact of graphene sheet size on the electrical conductivity of interconnected graphene foam polymer composite is thoroughly investigated. Graphene oxide solution is produced from small flake graphite (SFG) (2–15 μm) and large flake graphite (LFG) (>100 μm), respectively. Each solution is used to produce three-dimensional GO foam, which is subsequently heat-treated to produce reduced graphene oxide (RGO) foam. The RGO foams are then infiltrated with poly(dimethylsiloxane) (PDMS) to produce graphene-PDMS (G-PDMS) composites. The in-plane electrical conductivity of the G-PDMS composite (0.4 wt%) from LFG reaches ∼3.2 S/m, which is more than two orders of magnitude greater than that of G-PDMS (1.9 wt%, 1.4 × 10⁻² S/m) from SFG. This value is also four orders of magnitude higher than that of the G-PDMS composite prepared from mechanical mixing of 4 wt% RGO powder made from SFG with PDMS (4.2 × 10⁻⁵ S/m). The though-plane electrical conductivity followed the same trend for SFG and LFG. This reveals that the interconnected graphene foam supplies more efficient paths for electron transfer inside the polymer than conventional graphene powder and the use of large sized graphene sheets can significantly improve the electrical properties of G-PDMS.     

Multifunctional polyimide/graphene oxide composites via in situ polymerization

In this article, we detail an effective way to improve electrical, thermal, and gas barrier properties using a simple processing method for polymer composites. Graphene oxide (GO) prepared with graphite using a modified Hummers method was used as a nanofiller for r‐GO/PI composites by in situ polymerization. PI composites with different loadings of GO were prepared by the thermal imidization of polyamic acid (PAA)/GO. This method greatly improved the electrical properties of the r‐GO/PI composites compared with pure PI due to the electrical percolation networks of reduced graphene oxide within the films. The conductivity of r‐GO/PI composites (30:70 w/w) equaled 1.1 × 10¹ S m^?1, roughly 10¹⁴ times that of pure PI and the oxygen transmission rate (OTR, 30:70 w/w) was reduced by about 93%. The Young's modulus of the r‐GO/PI composite film containing 30 wt % GO increased to 4.2 GPa, which was an approximate improvement of 282% compared with pure PI film. The corresponding strength and the elongation at break decreased to 70.0 MPa and 2.2%, respectively. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40177.     

Dense graphene nanoplatelet/yttria tetragonal zirconia composites: Processing,hardness and electrical conductivity

Yttria tetragonal zirconia ceramic composites with 1, 2.5, 5 and 10 vol% nominal contents of graphene nanoplatelets (GNPs) were fabricated and characterized. First, the GNP dispersion in isopropanol was optimized to de-agglomerate the GNPs without damaging their structure. Then, submicrometric fully dense composites were obtained via spark plasma sintering (SPS) at 1250 °C with a 5 min holding time. The processing routine produced a nearly homogeneous GNP dispersion in the ceramic matrix, and the GNPs preferential orientation was perpendicular to the sintering compression axis. A ceramic grain refinement due to the GNPs was also detected. The Vickers hardness measured on the plane perpendicular to the sintering compression axis (basal plane) was lower than on the cross sections. This anisotropy increased with the increasing GNP content, while the average hardness decreased. The electrical conductivity was also highly anisotropic, up to seven times higher for the basal planes. The electrical percolation threshold for these composites was estimated to be between 2.2 and 4.4 vol% of the GNP measured content.     

Characterisation of graphite nanoplatelets and the physical properties of graphite nanoplatelet/silicone composites for thermal interface applications

Thermally conducting and highly compliant composites were developed by dispersing graphite nanoplatelets (GNPs) into a silicone matrix by mechanical mixing. X-ray diffraction (XRD) indicates that the average thickness of the GNPs decreased from 60 to 35 nm during mechanical mixing. XRD-texture analysis demonstrated that GNP/silicone composites at 8 wt.% GNPs have a higher degree of basal plane alignment than at 20 wt.%. Differential scanning calorimetry showed that GNPs raised the curing temperature of silicone with no significant effect on the glass transition temperature. The thermal conductivity of the 20 wt.% composites reached 1.909 W/m.K, an 11-fold increase over silicone suggesting an improved dispersion compared to similar composites prepared by dual asymmetric centrifuge mixing. The percolation threshold for electrical conductivity of the composites was at ∼15 wt.%. The compressive modulus of the composite increased to twice that of silicone at 20 wt.%. The corresponding strength decreased by a factor of two compared to silicone and this can be attributed to the weak bonding at the GNP-silicone interface. Overall, these GNP/silicone composites, with a high thermal conductivity, low electrical conductivity and compliant nature are promising materials for use as thermal pads for thick gap filling thermal interface applications.     

Enhanced mechanical and electrical properties of polyimide film by graphene sheets via in situ polymerization

In this study, polyimide/graphene nanocomposite films which exhibited significant enhancements in mechanical properties and electrical conductivity were successfully fabricated. Graphene oxide (GO) synthesized by Hummer’s method was chemically modified with ethyl isocyanate to give ethyl isocyanate-treated graphene oxide (iGO), which is readily dispersed in N,N′-dimethylformamide (DMF). The iGO dispersion in DMF was then used as media for synthesis of polyimide/functionalized graphene composites (PI/FGS) by an in situ polymerization approach. It was shown that addition of only 0.38 wt% of FGS, Young’s modulus of the PI/FGS composite film was dramatically increased from 1.8 GPa to 2.3 GPa, which is approximately 30% of improvement compared to that of pure PI film, and the corresponding tensile strength was increased from 122 MPa to 131 MPa. In addition, the electrical conductivity of the PI/FGS with this graphene content was increased by more than eight orders of magnitude to 1.7 × 10⁻⁵ S m⁻¹.     

In-situ synthesis and characterization of electrically conductive polypyrrole/graphene nanocomposites

Polypyrrole (PPy)/graphene (GR) nanocomposites were successfully prepared via in-situ polymerization of graphite oxide (GO) and pyrrole monomer followed by chemical reduction using hydrazine monohydrate. The large surface area and high aspect ratio of the in-situ generated graphene played an important role in justifying the noticeable improvements in electrical conductivity of the prepared composites via chemical reduction. X-ray photoelectron spectroscopy (XPS) analysis revealed the removal of oxygen functionality from the GO surface after reduction and the bonding structure of the reduced composites were further determined from FTIR and Raman spectroscopic analysis. For PPy/GR composite, intensity ratio between D band and G band was high (∼1.17), indicating an increased number of c-sp² domains that were formed during the reduction process. A reasonable improvement in thermal stability of the reduced composite was also observed. Transmission electron microscopy (TEM) observations indicated the dispersion of the graphene nanosheets within the PPy matrix.     

Mechanical,thermal, and conductivity performances of novel thermoplastic natural rubber/graphene nanoplates/polyaniline composites

Conventional polymer blending has a shortcoming in conductivity characteristic. This research addresses the preparation of conductive thermoplastic natural rubber (TPNR) blends with graphene nanoplates (GNPs)/polyaniline (PANI) through melt blending using an internal mixer. The effect of PANI content (10, 20, 30, and 40 wt %) on the mechanical and thermal properties, thermal and electrical conductivities, and morphology observation of the TPNR/GNPs/PANI nanocomposites was investigated. The results showed that the tensile and impact properties as well as thermal conductivity of nanocomposite had improved with the incorporation of 3 wt % of GNPs and 20 wt % of PANI as compared to neat TPNR and reduced with further increase of the PANI content. It was observed that the GNPs and PANI acted as a critical component to improve the thermal stability and electrical conductivity of the TPNR/GNPs/PANI nanocomposites. The most improved conductivity of 5.22 E-5 S/cm was observed at 3 wt % GNPs and 40 wt % PANI. Variable-pressure scanning electron microscopy micrograph revealed the good interaction and distribution of GNPs and PANI within TPNR matrix at PANI loadings lower than 30 wt %. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48873.     

Fabrication and properties of reduced graphene oxide reinforced yttria-stabilized zirconia composite ceramics

Fully dense yttria-stabilized zirconia (YSZ) ceramics reinforced with reduced graphene oxide (RGO) were fabricated by spark plasma sintering (SPS), and their electrical, thermal, and mechanical properties were investigated. Graphene oxide (GO) was exfoliated by a short sonification in dimethylformamide (DMF)/water solution and uniformly mixed with ZrO₂ powders. The microstructure of the composites showed that undamaged RGO sheets were homogeneously distributed throughout matrix grains. The electrical conductivity of YSZ composites drastically increased with the addition of RGO, and it reached 1.2 × 10⁴ S/m at 4.1 vol.%. However, the thermal diffusivity increased only 12% with RGO addition. The hardness decreased slightly with RGO addition, whereas the fracture toughness significantly increased from 4.4 to 5.9 MPa^1/2. The RGO pull-out and crack bridging contributed to the improved fracture toughness.     

Sustainable conducting polymer composites: study of mechanical and tribological properties of natural fiber reinforced PVA composites with carbon nanofillers

ABSTRACT

Ball milled jute fiber (JF) was added to Polyvinyl Alcohol (PVA)/20 wt.% multi-layer graphene (MLG) composites in various proportions (0, 5, 10, 15 and 20 wt.%) to prepare sustainable and biodegradable conducting polymer composites. Also, PVA/17.5wt.%MLG/2.5wt.%MWCNT/20wt.% JF composite was prepared for comparison purpose. A dynamic mechanical analysis of the composites was conducted to analyze their viscoelastic nature. The electrical conductivity of the composites was measured to study their suitability for various applications. Jute reinforcement increased the electrical conductivity of PVA/MLG nanocomposites. The PVA/20wt.%JF/17.5wt.%MLG/2.5wt.%MWCNT hybrid composite had the highest electrical conductivity of 3.64 × 10^?4 S/cm among all the composites prepared. Multilayered structures of the hybrid composite films were made by hot-pressing, and their effectiveness in electromagnetic interference shielding was tested. The shielding effectiveness of the composites decreased with jute addition. The wear resistance of PVA/MLG/JF composites increased with an increase in the jute content up to an optimum value of 10 wt.%, and then it started deteriorating.     

Electrical performance of orthotropic and isotropic 3YTZP composites with graphene fillers

3 mol% yttria tetragonal zirconia polycrystal (3YTZP) composites with orthotropic or isotropic microstructures were obtained incorporating few layer graphene (FLG) or exfoliated graphene nanoplatelets (e-GNP) as fillers. Electrical conductivity was studied in a wide range of contents in two configurations: perpendicular (σ_?) and parallel (σ_//) to the pressing axis during spark plasma sintering (SPS). Isotropic e-GNP composites presented excellent electrical conductivity for high e-GNP contents (σ_? ～ 3200 S/m and σ_// ～ 1900 S/m for 20 vol% e-GNP), consequence of their misoriented distribution throughout the matrix. Optimum electrical performance was achieved in the highly anisotropic FLG composites, with high electrical conductivity for low contents (σ_? ～ 680 S/m for 5 vol%), percolation threshold below 2.5 vol% FLG and outstanding electrical conductivity for high contents (σ_? ～ 4000 S/m for 20 vol%), result of the high aspect ratio and low thickness of FLG.     

Polymer-derived ceramic/graphene oxide architected composite with high electrical conductivity and enhanced thermal resistance

A low temperature method for the fabrication of architected ceramic composites contining graphene is developed based on the infiltration of lightweight graphene oxide (GO) micro-lattices with a preceramic polymer. Self-supported highly porous three-dimensional (3D) GO structures fabricated by direct ink writing are infiltrated with a liquid organic-polysilazane (a compound of Si, C, H, N), and subsequently pyrolyzed at temperatures of 800–1000?ºC to activate the ceramic conversion. These ceramic composites replicate the patterned GO skeleton and, whereas the graphene network provides the conductive path for the composite (electrical conductivity in the range 0.2–4?S?cm^?1), the ceramic wrapping serves as a protective barrier against atmosphere, temperature (up to 900?°C in air) and even direct flame. These structured composites also show hydrophobicity (wetting angle above 120°) and better load bearing capacity than the corresponding 3D GO lattice. The process is very versatile, being applicable to different liquid precursors.     

Synthesis of conducting graphene/Si3N4 composites by spark plasma sintering

Graphene/silicon nitride (Si₃N₄) composites with high fraction of few layered graphene are synthesized by an in situ reduction of graphene oxide (GO) during spark plasma sintering (SPS) of the GO/Si₃N₄ composites. The adequate intermixing of the GO layers and the ceramic powders is achieved in alcohol under sonication followed by blade mixing. The reduction of GO occurs together with the composite densification in SPS, thus avoiding the implementation of additional reduction steps. The materials are studied by X-ray photoelectron and micro-Raman spectroscopy, revealing a high level of recovery of graphene-like domains. The SPS graphene/Si₃N₄ composites exhibit relatively large electrical conductivity values caused by the presence of reduced graphene oxide (∼1 S cm⁻¹ for ∼4 vol.%, and ∼7 S cm⁻¹ for 7 vol.% of reduced-GO). This single-step process also prevents the formation of highly curved graphene sheets during the thermal treatment as the sheets are homogeneously embedded in the ceramic matrix. The uniform distribution of the reduced GO sheets in the composites also produces a noticeable grain refinement of the silicon nitride matrix.     

Electrical conductivity maps in graphene nanoplatelet/silicon nitride composites using conducting scanning force microscopy

Graphene nanoplatelet (GNP)/silicon nitride (Si₃N₄) ceramic composites containing 12 and 15 wt.% of GNPs are prepared by mixing the nanoplatelets and the ceramic powders in a liquid medium followed by densification of the dried mixture by spark plasma sintering. The electrical conductivity of these composites is investigated at the nanoscale by conducting scanning force microscopy to understand the influence of the carbon phase content when above the percolation threshold. The establishment of a conducting network is revealed from the conduction measured at GNPs emerging at the surface of the composites. Current maps obtained for two orientations of the composites, parallel and perpendicular to the press sintering axis, show a preferential orientation of the nanoplatelets within the ceramic matrix. The effective current per conducting pixel, determined from the corresponding maps, is four times larger for the 15 wt.% GNP composite than for the 12 wt.% one. For the same composite (15 wt.%), differences in the effective current are measured for each of the two probing configurations. These results are interpreted in terms of unbalanced weight and nature of the resistors forming the percolated network.     

Preparation and properties of reduced graphene oxide/fused silica composites

An in situ strategy for fabrication of reduced graphene oxide/fused silica (rGO/FS) composites using 3-aminopropyltriethoxysilane as surfactant is reported. GO nanosheets were bound to FS particles by an electrostatic assembly between ultra thin negatively charged GO sheets and positively charged amino-modified FS particles. After spark plasma sintering, rGO/FS bulk composites have been produced from the GO and FS composite particles with GO being reduced to rGO in vacuum at high temperatures. Results show that rGO sheets were well dispersed in the matrix, and conductivity of these rGO/FS composites at room temperature was strongly dependent on the rGO nanosheet concentration. i.e., the conductivity of rGO/FS was increased to 10⁻⁴ S/cm when a conducting network was formed inside the composites. The effect of GO nanosheets on the mechanical properties of rGO/FS bulk composites was also investigated. The addition of 1 wt.% GO sheets to FS resulted in 72% increase in Vickers hardness, indicating the stress transfering from the FS matrix to the rigid rGO sheets. With the same rGO content, the fracture toughness of the as-prepared composites was increased by 74%. The main toughening mechanisms were thought to be crack deflection, crack branching, pulling-out and bridging of the rGO sheets.