Electrophysical properties of composition based on natural flake graphite and binder

The electrophysical properties of a highly filled composition based on natural flake graphite and coal tar pitch as a binder have been studied. Blanks have been obtained in a laboratory broaching press. The conductivity of pitch binder interlayers in the composition has been varied by heat treatment, and the electric resistivity and magnetoresistance along and across the texture axis have been measured. The thickness and imperfection of surface layers of graphite flakes have been assessed. The formation of structural defects of the surface layers is associated with a decrease in the size of mosaic blocks as a result of friction between flakes during mass movement under pressure in the conical part of the die. It has been shown that friction forces facilitate consolidation of the material and ensure directed orientation of stacks.     

Single-walled carbon nanotube buckypaper and mesophase pitch carbon/carbon composites

Carbon/carbon composites consisting of single-walled carbon nanotube (SWCNT) buckypaper (BP) and mesophase pitch resin have been produced through impregnation of BP with pitch using toluene as a solvent. Drying, stabilization and carbonization processes were performed sequentially, and repeated to increase the pitch content. Voids in the carbon/carbon composite samples decreased with increasing impregnation process cycles. Electrical conductivity and density of the composites increased with carbonization by two to three times that of pristine BP. These results indicate that discontinuity and intertube contact barriers of SWCNTs in the BP are partially overcome by the carbonization process of pitch. The temperature dependence of the Raman shift shows that mechanical strain is increased since carbonized pitch matrix surrounds the nanotubes.     

碳纳米管/天然橡胶复合材料的结构与性能

通过机械混炼法制备了碳纳米管(CNT)／天然橡胶(NR)复合材料,研究了CNT的预处理方式对复合材料结构与性能的影响。结果表明,与NR相比,CNT／NR复合材料的硫化返原现象减轻,硫化后凝胶质量分数降低,硫化剂用量应适当增加,由混酸氧化处理的CNT填充橡胶复合材料的硫化迟滞效应明显;复合材料内部存在CNT的富集区域和CNT含量很少的橡胶区域,CNT与NR之间的界面结合作用不好;由HF处理的CNT填充橡胶复合材料的整体性能最好,但受CNT在橡胶基体中的不良分散状态及界面性质的影响,其力学性能不高。     

Thermophysical properties of carbon/graphite fibers and MOD-3 fiber-reinforced graphite

The flash method is extended to measure directly the thermal diffusivity of graphite/carbon fiber in unidirectionally fiber-reinforced composites and also in fiber bundles. The thermal diffusivity was measured using both composities and fiber bundles for Morganite II and Thornal 50 S graphite fiber and for composite samples containing PX 505 carbon fiber. In addition, the thermal diffusivity of Morganite II and Thornel 50 S graphite fiber was calculated from the effective thermal conductivity of composite samples measured by an absolute method. The thermal diffusivity of MOD-3 fiber-reinforced graphite was measured and the results were used to compute the thermal conductivity in the three orthogonal directions.     

The effect of pyrolysis on the mechanical properties and microstructure of carbon fiber-reinforced and stabilized fiber-reinforced phenolic resins for carbon/carbon composites

Carbon/carbon composites were prepared with phenol-formaldehyde resin, one kind of commercial carbon fiber, and a stabilized fiber that was developed in our laboratory. The effect of pyrolysis on the microstructure, fracture behavior, and flexural strength of the composites during the carbonization process was studied. During the pyrolysis of the composites a chemical reaction at the fiber/resin interface apparently took place. A thermogravimetry (TG) study indicated that the use of stabilized fiber reinforced composites inhibited decomposition reactions and thermal fragmentation in the matrix resin, and reduced the weight loss of the final composites. The X-ray reflection of the resin and the two composites showed a reflection appearing at 2θ ≈ 12° when the samples were carbonized above 600°C. The intensity of this reflection in the composites made with stabilized fiber was higher than that of the composite made with carbon fiber. Because of the formation of strong bonding in the fiber-matrix interface, the composites made with stabilized fiber showed catastrophic failure and low flexural strength below carbonization temperatures of 600°C. Above 600°C, the flexural strength of the composites increased with an increase in the carbonization temperatures, even if the fracture behaviors showed catastrophic failure. The flexural strength of the composites made with carbon fiber showed pseudo-plastic patterns and debonding with very little fiber pullout. Above 800°C, these composites showed a catastrophic failure and smooth failure surfaces. During pyrolysis the flexural strength decreased with an increase in the carbonization temperature.     

Microstructure and mechanical properties of carbon/carbon composites with PyC/SiC/TiC multilayer interphases

Carbon/carbon composites with PyC/SiC/TiC multilayer interphases (CCs-PST) have been successfully prepared by a joint process of chemical vapor deposition and carbothermal reduction. Effect of the Ti(OC₄H₉)₄/C₆H₄(OH)₂ molar ratio on the morphology of TiC particles was investigated and the ratio was optimized as 8:1. When the Ti(OC₄H₉)₄/C₆H₄(OH)₂ molar ratio was 8:1, many homogeneously distributed TiC nanoparticles with the sizes of 100–500 nm on the fibers were observed. The structural evolution of CCs-PST was discussed and the mechanical properties of as-prepared materials were investigated by flexural and interlaminar shear tests. The resulted composites demonstrated a PyC and SiC mixed inner interphase with the thickness of 0.5–1 $\mathrm{\mu }\mathrm{m}$ and a TiC outer interphase with a thickness about 0.5 µm. Flexural strength of 201.45 ± 5.27 MPa and modulus of 21.21 ± 1.58 GPa showed a 41.7% and 7.83% improvement respectively as compared with that of the neat CCs. The interlaminar shear strength of CCs-PST was 66.71 ± 4.87 MPa, which was 51.20% higher than that of the CCs. The improved mechanical properties were attributed to the enhanced interface bond between fibers and matrix induced by the PST.     

Microstructure and mechanical properties of continuous carbon fiber-reinforced ZrB2-based composites via combined electrophoretic deposition and sintering

A process combining electrophoretic deposition (EPD) with hot pressing (HP) was developed to fabricate continuous carbon fiber-reinforced ZrB₂-based composites (C_f/ZrB₂-based composites). ZrB₂-based ultra-high temperature ceramic (UHTC) particles were uniformly pre-coated on continuous carbon fibers via EPD. Then, the UHTC-coated carbon fibers were stacked and hot pressed to prepare the C_f/ZrB₂-based composites. Microstructure observations revealed that almost no micro-pores were found in the inter-bundle and intra-bundle regions of fibers after HP. The flexural strength, fracture toughness and the work of fracture of the C_f/ZrB₂-based composite were measured as 199 ± 26 MPa, 6.71 ± 1.29 MPa·m^1/2, and 754 ± 58 J/m², respectively. Based on the observations of non-brittle fracture behavior, fractured morphology and crack propagation, the enhanced fracture properties were mainly attributed to the multiple toughening mechanisms, such as fiber pull-out, fiber bridging, crack deflection and branching along the interfaces.     

Effect of chemical treatment on the thermal properties of hybrid natural fiber-reinforced composites

The main objective of this work is to study the effect of chemical treatment on the thermal properties of hybrid natural fiber-reinforced composites (NFRCs). Different chemical treatments [i.e., alkalized and mixed (alkalized+ silanized)] were used to improve the adhesion between the natural fibers (jute, ramie, sisal, and curauá) and the polymer matrix. A differential scanning calorimetry, thermogravimetry, and a dynamic mechanical analysis were performed to study the thermal properties of hybrid NFRC. It was found that the chemical treatments increased the thermal stability of the composites. Scanning electron microscopy images showed that the chemical treatments altered the morphology of the natural fibers. A rougher surface was observed in case of alkali treated fiber, whereas a thin coating layer was formed on the fiber surface during the mixed treatment. However, for some fibers (i.e., sisal and rami), the chemical treatment has a positive impact on the composite properties, whereas for the jute and curauá composites, the best behavior was found for untreated fibers. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47154.     

The effect of multiscale hybridization on the mechanical properties of natural fiber-reinforced composites

The main objective of this work was to investigate the effect of reinforcements at different scales on the mechanical properties of natural fiber-reinforced composites. Pure jute and interlaminar hybrid jute/glass fiber-reinforced polymer composites were fabricated. Different types of fillers in two weight fractions (1 and 3 wt. %) were used as second reinforcements in the hybrid jute/glass composites. Tensile, flexural, and impact tests were performed. It was found that the macroscale inter-play hybridization significantly improved the mechanical properties of the pure jute fiber based composites. When the fillers are used as second hybridization, the modified composites presented higher mechanical properties when compared to pure jute composites. However, the effect of fillers on the mechanical properties of the hybrid composites presented various trends due to the interaction between several factors (i.e., particle scale, content, and nature), which cannot always be separated. Increasing the synthetic filler content improved the tensile properties of the filled hybrid composites, while increasing the natural filler content worsen the tensile properties. The flexural strength of the multiscale hybrid composites was improved, while the impact properties were negatively affected.     

Effects of pitch fluoride on the thermal conductivity of carbon foam derived from mesophase pitch

Carbon foams with high thermal conductivity were obtained from mixtures of mesophase pitch and pitch fluoride. The addition of pitch fluoride in mesophase pitch could significantly increase the specific thermal conductivity of as-prepared carbon foams. After graphitization at 2873 K, the specific thermal conductivity of carbon foams increased from 82 up to 155.4 (W/mK)/(g/cm³) when the content of pitch fluoride was 3% in the raw material.     

Improving mechanical and thermal properties of short carbon fiber/polyamide 6 composites through a polydopamine/nano-silica interface layer

Carbon fiber (CF) reinforced matrix composites have been applied widely, however, the interfacial adhesion of composites is weak due to smooth and chemically inert of CF surface. To solve this problem, A polydopamine/nano-silica (PDA-SiO₂) interfacial layer on carbon fiber surface was constructed via polydopamine and nano- SiO₂ (CF-PDA-SiO₂) by a facile and effective method to reinforce polyamide 6 composites (CFs/PA6). The effects of PDA-SiO₂ interfacial layer on crystallization structure and behavior, thermal properties, and mechanical properties of CFs/PA6 composites were investigated. Furthermore, interfacial reinforcement mechanism of composites has been discussed. This interfacial layer greatly increased the number of active groups of CF surface and its wettability obviously. The tensile strength of CF-PDA-SiO₂/PA6 composites increased by 28.09%, 19.37%, and 26.22% compared to untreated-CF/PA6, CF-PDA/PA6, and CF-SiO₂/PA6 composites, respectively, which might be caused by the increased interfacial adhesion between CF and PA6 matrix. The thermal stability, crystallization temperature, crystallinity, and glass transition temperature (T_g) of CF-PDA-SiO₂/PA6 composites improved correspondingly, attributing to the heterogeneous nucleation of nano-SiO₂ in the crystalline area and hydrogen bonds with molecular chains of PA6 in the amorphous area. This work provides a novel strategy for the construction of interfaces suitable for advanced CF composites with different structures.     

Swelling and related mechanical and physical properties of carbon nanofiber filled mesophase pitch for use as a bipolar plate material

Mesophase pitch was investigated as a melt processable precursor to a compression or injection moldable all carbon bipolar plate. After shaping, carbonization to 1000 °C or greater is required to achieve the desired electrical and mechanical properties, but gases evolved during this step lead to swelling. Carbon nanofiber was added to suppress swelling during carbonization and bypass the typical oxidation steps used when processing mesophase pitch. The addition of carbon nanofiber decreased swelling by increasing the viscosity of the melt. Carbonized materials with carbon nanofibers can show strengths (30-50 MPa) and conductivities (20-80 S cm⁻¹) consistent with composite bipolar plate materials. The materials show conductivities below Department of Energy target values at the current carbonization temperatures, which were limited to 1000 °C. The use of glass fibers as a secondary filler led to reduced gas permeability in porous samples.     

Microstructure and thermophysical properties of graphite foam/glass composites

Mesophase pitch based graphite foams with different thermal properties and cell structures were infiltrated with glass by pressureless infiltration to prepare potential alternative composites for cooling electronics. Microstructure, thermal diffusivity and coefficient of thermal expansion (CTE) of the obtained composites were investigated. It was demonstrated that there was excellent wettability of the graphite foam by molten glass, and the foam framework was retained well after infiltration, which could facilitate good heat transfer throughout the composites. The highest thermal diffusivity of the composites reached 202.80 mm²/s with a density of 3.81 g/cm³. And its CTE value was 4.53 ppm/K, much lower than the corresponding calculated result (7.46 ppm/K) based on a simple “rule of mixtures” without considering the space limitations of the graphite foams. Thus, the mechanical interlocking within the space limitations of the graphite network played a crucial role in limiting the thermal expansion of the glass. The CTEs of the graphite foam/glass composites varied from 4.53 to 7.40 ppm/K depending on the graphite foam density which varied from 0.82 to 0.48 g/cm³. The CTEs were a good match to those of semiconductor chips and packaging materials.     

Modeling the in-plane thermal conductivity of a graphite/polymer composite sheet with a very high content of natural flake graphite

Natural flake graphite/polymer composite sheets were prepared using tape casting method. The in-plane thermal conductivities, i.e. the thermal conductivities along the tape casting plane, of the composites were measured and the results compared to the predictions by the Maxwell and Agari models. The comparison indicated that both models cannot predict the thermal conductivity of the natural flake graphite/polymer composite at a very high graphite concentration where the flakes were shown from scanning electron microscopy (SEM) images to have a good orientation parallel to the tape casting direction. The SEM images also illustrated that the well oriented flakes formed a continuous graphite–graphite network. Based on the observed structure, a new thermal conductivity model was constructed. The new model applies well to the composites at a large content of highly oriented graphite flakes. The model was also validated by the experimental results from samples with the same graphite content but with different degrees of graphite flake orientation.     

Mechanical and thermal properties of graphite platelet/epoxy composites

Anhydride-cured diglycidyl ether of bisphenol A (DGEBA) reinforced with 2.5-5% by weight graphite platelets was fabricated. The structural, mechanical, viscoelastic and thermal properties of these composites were studied and compared. XRD studies indicated that the processing of composites did not change the original d-spacing of pure graphite. Tensile property measurements of composites indicated higher elastic modulus and tensile strength with increasing concentration of graphite platelets. The storage modulus and glass transition temperatures (T_g) of the composites also increased with increasing platelet concentration, however, the coefficient of thermal expansion decreased with the addition of graphite platelets. The thermal stability was determined using thermogravimetric analysis. The composites showed higher thermal stability in comparison with pure epoxy and increased char concentration for higher graphite concentration. The effects of reinforcement on the damage mechanisms of these composites were investigated by scanning electron microscopy.     

Formation of porosity and change in binder pitch properties during thermal treatment of green carbon materials

The formation of porosity during heat treatment of an extruded mixture of coke particles and a binder pitch was investigated using image analysis of polished cross-sections of shaped samples. The porosity due to pitch devolatilisation takes place during a rather narrow interval of temperature when the sample is heated at 12 K/hour. Only a small fraction of pores is formed when the shaped sample is submitted to an intermediate isothermal step which allows the departure of volatile compounds from the binder. Physico-chemical changes occurring in the pitch during pyrolysis were followed by measuring glass transition temperature in relationship with weight loss. The results indicate that in sufficiently slow heating conditions, low molecular compounds essentially diffuse through the fluid pitch and evaporate at the external surface of the sample during pyrolysis. As a result, no marked porosity is formed during baking. In contrast, a high heating rate allows low molecular mass compounds to accumulate inside the sample and the formation of a significant porosity is due to the evaporation inside the shaped sample of accumulated low molecular mass compounds.     

The mechanical and thermal properties of graphite fiber reinforce polyphenylquinoxaline and polyimide composites

Comparative studies were made on the fabrication characteristics, mechanical properties and thermal stabilities of three commercially available composite systems based on linear monoether polyphenylquinoxaline, P13N polyimide, and Skybond 703 polyimide, reinforced with Modmor II graphite fibers. Fabrication parameters and prepreg conditions were related to the properties of the laminates. Interlaminar shear strength and flexural properties were evaluated before and after the thermal aging at 600°F for periods up to 500 hrs. Thermal degradation of composites at various stages of thermal' aging was studied using optical and scanning electron microscopes.     

Effect of surfactant and electron treatment on the electrical and thermal conductivity as well as thermal and mechanical properties of ethylene vinyl acetate/expanded graphite composites

This study presents an investigation of the electrical and thermal conductivities of composites based on an ethylene vinyl acetate (EVA) copolymer matrix and nanostructured expanded graphite (EG). To improve the EG dispersion in EVA, EG sheets were modified by treating them with the anionic surfactant sodium dodecyl sulphate (SDS) in water. The modified SDS‐EG platelets, after being filtered and dried, were melt‐mixed with EVA to prepare the composites. Finally, both EVA/EG and EVA/SDS‐EG composites were subjected to 50 kGy electron beam (EB) irradiation. SEM images confirm that the irradiated EVA/EG samples had improved interfacial adhesion, while the irradiated EVA/SDS‐EG samples showed even better interfacial adhesion. The gel contents of the irradiated samples without and with SDS treatment increased with increase in EG loading. The EVA/EG composites exhibited a sharp transition from an insulator to a conductor at an electrical percolation threshold of 8 wt %, but with SDS‐EG the electrical conductivity was extremely low, showing no percolation up to 10 wt % of filler. The EB irradiation had no influence on electrical conductivity. The thermal conductivity linearly increased with EG content, and this increase was more pronounced in the case of SDS‐EG, but decreased after EB irradiation. The thermal properties were little influenced by EB irradiation, while better polymer–filler interaction and better filler dispersion as a result of SDS treatment, and the EB irradiation initiated formation of a cross‐linked network, had a positive effect on the tensile properties. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42396.