Expanded Graphite Sheets Research Articles

Flexible low-profile shield for reducing back radiation of wrist-worn antenna

A flexible conducting lossy shield is developed and demonstrated for a reduction in the back radiation of a linearly polarized X-band patch antenna for Wireless Body Area Network (WBAN) applications. The proposed structure comprises a flexible patch antenna backed by a laminated sheet of Expanded Graphite (EG). This flexible sheet is prepared from EG powder pressed uniformly over a thin adhesive tape. The conducting nature of EG is exploited, and the EG sheet is used as a protective shield to prevent back radiation emanating out of the WBAN antenna from propagating toward the human body. The antenna performance is tested over human wrist phantoms experimentally and through full wave simulation studies. The results indicate the marginal variation in the −10 dB impedance bandwidth with a slight increase in the gain and directivity values for a resonant frequency in the range 9.7–10.3 GHz (bending radius variation of 40, 30, and 20 mm and along two bending planes). A measured on-body gain of 6.7 dBi is exhibited by the antenna backed by the EG shield and bent over a 20 mm radius human wrist phantom with an S11 of −19 dB, a −10 dB % bandwidth of 8%, and a directivity of 9.4 dBi. All the other bending configurations exhibit values higher than these. Electric field and Specific Absorption Rate (SAR) studies reveal a reduction in wave penetration by the EG shield antenna inside the wrist phantom with a decrease in the SAR by a maximum of 5%. The measured front-to-back ratio values indicate the lowering of back radiation with a maximum average increase of 76% exhibited by an EG backed antenna. The developed EG shield has an additional advantage of being a standalone structure, which can be integrated with any wearable antenna for a possible reduction in the back radiation.

Ultra‐wideband electromagnetic interference shielding properties of parallel expanded graphite/Fe3O4 composites

AbstractThe rapid development of electronic devices in various fields has stimulated an increased demand for electromagnetic interference (EMI) shielding materials working in ultra‐wideband frequencies of gigahertz. However, the aggregation problem and single attenuation mechanism severely limit the broadband shielding applications of nanomaterials. To address these challenges, a novel composite of parallel expanded graphite (EG) embedded in the densified Fe3O4 matrix is designed and successfully synthesized through the in situ chemical coprecipitation followed by pressure sintering. This unique structure creates heterogeneous interfaces by combining the conductive EG and magnetic Fe3O4. The significant dielectric and magnetic loss then arise from conduction and polarization loss, as well as the eddy current and resonance effects. The multiple reflections of electromagnetic waves between the oriented EG sheets further cause the “absorb–reflect–reabsorb” effect that prolongs the propagation paths and enhances the dissipation of electromagnetic waves. Based on these multiple mechanisms over an ultrawide range of frequencies (8.2–40 GHz), the EG/Fe3O4 composites exhibit exceptional EMI shielding performance, with an average shielding effectiveness of 29.3 dB, indicating the potential for multifunctional EMI shielding applications.

Enhanced thermal conductivity of polyamide nanocomposites involving expanded graphite–carbon nanotube network structure using supercritical CO2

In this study, supercritical carbon dioxide (scCO2) was used to form a network structure of expanded graphite (EG) and multi-walled carbon nanotube (MWCNT) to achieve high thermal conductivity. Continuous processes, such as supercritical drying and rapid expansion of supercritical solutions were applied to pristine graphite and MWCNT. Polyamide 6 (PA6) nanocomposites with scCO2-treated EG and MWCNT were prepared by twin-screw extruder-based melt compounding. In the PA6 nanocomposites, the EG sheets were homogenously dispersed, while the MWCNTs were located between the EG sheets, thereby acting as a bridge for the EG. Subsequently, the 35 wt.% imbedding in PA6 nanocomposite had an effective heat transfer pathway and high thermal conductivity (2.12 W m−1 K−1). It was observed that supercritical fluid processing is a facile and effective strategy to improve the thermal conductivity of polymer nanocomposites.

Improved thermal conductivity of epoxy resins using silane coupling agent‐modified expanded graphite

AbstractA silane coupling agent was used to modify the surface of expanded graphite (EG), which was subsequently used as a thermally conductive filler to fabricate diglycidylether of bisphenol‐A (DGEBA)/EG composites with high thermal conductivity via hot blending and compression‐curing processes. The surface characteristics of silane coupling agent‐modified EG (Si@EG) were characterized by a variety of analytical techniques. The effects of the Si@EG content on the thermal conductivity, thermal stability, impact strength, and morphology of the DGEBA/Si@EG composites were investigated. The results revealed that the addition of 80 wt.% Si@EG increased the thermal conductivity of the composites from 0.17 to 10.56 W/m K, which was 61.1 times higher than that of pristine DGEBA. The initial decomposition temperature of the composite containing 80 wt.% Si@EG was 60.6°C higher than that of pristine DGEBA. The impact strength of the composites decreased from 2.0 to 0.87 kJ/m2 when the Si@EG content increased from 0 to 80 wt.%. The scanning electron microscopy images of the fractured surfaces revealed that the EG sheets in the DGEBA matrix formed a continuous thermally conductive path at high Si@EG contents.

Cost-Effective Fabrication of High-Density Polyethylene/Graphene Nanocomposites by an In Situ Exfoliation Method Based on a Series Explosion Effect

As ideal nanofillers, graphene nanosheets (GNS) are widely used to prepare high-performance graphene-reinforced nanocomposites owing to their excellent properties. However, the high manufacturing cost and low production efficiency of GNS severely restrict their large-scale application. Herein, a cost-effective melt blending approach was demonstrated for facilely preparing high-density polyethylene (HDPE)/GNS nanocomposites by using industrial-grade expanded graphite (EG). The exfoliation and dispersion of GNS in the matrix under different processing temperatures and times have been studied. A series explosion effect generated by the synergy of volumetric extensional rheology and free water can violently and fastly exfoliate pristine EG into few-layer GNS. The in situ exfoliation mechanism revealed that the synergistic effect of the explosive forces of free water and the adhesion between HDPE and GNS can destroy the van der Waals forces between the EG sheets during melt blending, thereby realizing ultrafast exfoliation and uniform dispersion of EG in the HDPE matrix. As expected, the HDPE/GNS nanocomposites showed high crystallinity, excellent thermal conductivity, high dielectric constant, and extremely low dielectric loss. This work provides a new idea for facilely fabricating high-performance polymer-based two-dimensional layered nanocomposites.

Layered-structure N-doped expanded-graphite/boron nitride composites towards high performance of microwave absorption

High-efficiency microwave absorbers with broadband absorption are strongly desired for electromagnetic protection. Herein, we successfully synthesized a hybrid microwave absorbing material with two-dimensional layered structure, which consisted of expanded graphite (EG) and boron nitride (BN). The introduction of BN is to regulate the conductivity of EG and also to improve the thermal stability of the composite material. The ultrathin BN nano-sheets were uniformly wrapped on the EG sheets via chemical vapor deposition. Attributed to the dielectric loss and conductive loss, the as-prepared hybrid material exhibited high performance for microwave absorption. The effective absorbing bandwidth (reflection loss value < -10 dB) was achieved up to 9.37 GHz with the thickness of 2.4 mm in the frequency range of 5.75–6.98 GHz and 9.86–18 GHz. And the minimum reflection loss was -51.58 dB at the thickness of 3 mm. The excellent performance of microwave absorption was attributed to the dielectric loss, interfacial polarization loss and optimized impedance matching. Moreover, compared to pure EG, the thermal decomposition temperature of EG/BN composite materials had increased by about 100 °C which was up to 714 °C. Taking the advantages of high thermal stability of BN as well as the fine microwave absorbability of EG, the EG/BN composites would be applied in high-temperature microwave absorbing fields.

N-doped porous carbon chain with 3D interconnected network structure to modify expanded graphite for efficient thermal energy storage materials

N-doped porous carbon-based material with a 3D interconnected network structure was synthesized by carbonizing the amino-functionalized metal-organic framework (NH2-MOF-5) combined with expanded graphite (EG). NH2-MOF-5 grows in situ between the EG sheet layers by solvent heat method. After 1000 °C carbonization, N-doped porous carbon (NPC) was supported in the pores of EG in the form of a 3D interconnected carbon network. SA/NPC@EG, which anchored stearic acid (SA) in NPC@EG by capillary action, surface tension, and the additional hydrogen bonding between SA molecules and doped nitrogen, forming a shape-stabilized phase change material. The layer structure of EG reduces the interface thermal resistance. NPC serves as a phonon transfer channel to connect the heat transfer between the EG sheets. The ‍results showed that NPC@EG loaded with 85 wt% SA owned excellent thermal conductivity (2.134‍ W∙m−1∙K−1) while retaining sufficient phase change enthalpy (177.7 J∙g−1). SA/NPC@EG exhibited high thermal stability owing to the combined effect of the expanded graphite sheet layer structure and the N-doped porous carbon chain network structure. This material can block and absorb high-energy-density heat flow to meet the stability and reliability requirements of the working environment.

Constructing 3D carbon-metal hybrid conductive network in polymer for ultra-efficient electromagnetic interference shielding

In the present work, an effective and versatile strategy is adopted to fabricate ultra-efficient electromagnetic-interference (EMI) shielding composites by constructing perfect 3D carbon-metal hybrid conductive network in polyamide 6 (PA6) matrix. The surface of PA6 granules is first functionalized with expanded graphite (EG) sheets as conductive substrate and subsequently coated with very thin nickel (Ni) layer to form core-shell PA6/EG/Ni complex particles, then highly efficient PA6 shielding composites with well-connected 3D Ni-EG hybrid conductive network can be assembled by a conventional hot compression of the complex particles. The lamellar EG sheets not only serve as deposition template to induce a homogeneous and compact deposition of thin Ni plating, but also facilitate the formation of perfect 3D Ni-EG conductive network by restricting the diffusion between Ni and low-viscosity PA6 melt during hot compression. Benefit from this unique 3D carbon-metal hybrid network and a synergistic effect of the Ni-EG multilayer structure, the PA6/EG/Ni composite shows an ultra-efficient EMI shielding performance of 77.3 dB with extremely low filler content (0.9 vol%). This feasible approach of constructing 3D carbon-metal hybrid network in polymer matrix provides a new possibility for developing affordable high-performance EMI shielding composites for next-generation communication applications.

Amorphous NiFe–OH/Ni–Cu–P supported on self-supporting expanded graphite sheet as efficient bifunctional electrocatalysts for overall water splitting

With the serious intensification of energy shortage and greenhouse effect, people begin to look for the sustainable energy sources to replace fossil energy sources. Herein, self-supporting expanded graphite sheet (SSEGS) was developed as an ideal catalyst support through electrochemically intercalating flexible graphite sheet in alkaline solution. Electroless deposition was employed to synthesize Ni–Cu–P alloy on SSEGS and then an amorphous NiFe hydroxide/Ni–Cu–P/SSEGS (NiFe–OH/Ni–Cu–P/SSEGS) composite catalyst was further constructed through electrodeposition. Benefitting from the unique structural advantage of SSEGS and the synergistic effect between two amorphous Ni-based materials (Ni–Cu–P alloy and NiFe–OH), the resulting electrode exhibited superior bifunctional electrocatalytic performance in 1 M KOH. For H2 evolution reaction and O2 evolution reaction, the NiFe–OH/Ni–Cu–P/SSEGS composite catalyst could reach 10 mA cm−2 at low overpotentials of 75 and 240 mV, respectively. Remarkably, the two-electrode system driven by NiFe–OH/Ni–Cu–P/SSEGS as the anode and cathode could afford 10 mA cm−2 at a low cell voltage of 1.56 V vs. RHE. And after the 12 h stability test, the cell voltage at 10 mA cm−2 increased by only 7 mV, indicating that the two-electrode system had excellent stability. The preparation of NiFe–OH/Ni–Cu–P/SSEGS material with superior bifunctional electrocatalytic performance has a significance influence to the development and expansion of hydrogen production technology.

Acoustic and Eddy Current Methods of Nondestructive Testing of Thermally Expanded Graphite Sheets

The paper presents noncontact methods for testing the density of thermally expanded graphite (TEG) sheets: acoustic shadow method, based on the evaluation of the transmission coefficient of the acoustic wave in high frequency range, and eddy current method, based on the evaluation of the complex resistance of parametric surface-mounted eddy-current transducer. Rresults of investigation of the sensitivity curves for the sheets and mats of TEG in a wide range of their density variations. According to the results of the experiment, there is a decrease in the amplitude parameter U in the implementation of the acoustic shadow method with an increase in the density of TEG sheets. Increasing the density of TEG sheets also leads to a linear decrease in the complex resistance of eddy current transducer.

Study of reinforcement mechanism and structural elucidation of expanded graphite‐carbon black hybrid filler‐SBR nanocomposites through comprehensive analysis of mechanical properties and small angle X‐ray data

AbstractUse of hybrid fillers as a reinforcing agent for polymers is found to be critical step toward developing a high‐performance composite material. However, limited know‐how on the nature of interaction of the hybrid fillers with the polymer chains resulted in a major impediment toward large‐scale transmissibility of such a technology. Herein, we report about a strategy, wherein the polymer composite (free of curatives), comprising of hybrid filler and its gel was leveraged to effectively understand the physics involved toward reinforcement. Styrene‐butadiene random copolymer as the matrix, and combination of expanded graphite and carbon black (N220) as the model hybrid filler were selected. The hybrid filler containing composite (SG22) demonstrated significant improvement in terms of the physico‐mechanical properties such as tensile strength, modulus and so forth compared to the neat carbon black‐filled system (S22). Stress‐relaxation studies indicated that SG22 registered minimal decay in the force with time compared to S22. SG22 demonstrated a gel fraction of 68 ± 1% while 56 ± 1% was noted for S22. Further, rheometric studies like strain sweep, frequency sweep, complex viscosity of the gel fragments indicated the formation of fractal network of the hybrid fillers inside the polymer matrix. Small angle X‐ray studies corroborated the crucial role played by the expanded graphite sheets in determining the microstructure of the composite owing to their lubrication effect and segregation of carbon black agglomerates by cutting through their sharp edges resulting in a well‐distributed filler network.

Utilizing ammonium persulfate assisted expansion to fabricate flexible expanded graphite films with excellent thermal conductivity by introducing wrinkles

Traditional expanded graphite (EG) films exhibit excellent electric and thermal conductivities, due to weak oxidation degree induced low defect density. However, the stiff sheet structure makes EG films very brittle, which greatly limits its applications in next generation flexible electric devices. To overcome this challenge, ammonium persulfate ((NH4)2S2O8) is employed as both expansion agent and weak oxidation agent. As a result, the weak oxidation can effectively reduce the surface energy mismatching between EG and water, making it possible for EG to be exfoliated in water by high-speed shearing combined with sonication. Most particularly, the expansion step introduces plenty of wrinkles into the EG sheets, which notably improve the flexibility of EG films. Besides, using raw graphite with large lateral size and less (NH4)2S2O8 can greatly improve the final overall performances. The prepared EG films exhibit good electric and thermal conductivities of 2977 S/cm, 854 W/mK, respectively. 33.1 dB EMI shielding property is obtained with the film thickness of only 10 μm. Moreover, 800 times direct bending can be overcome without any structure break. This facial large scale and environmentally friendly method endows the films with great potential applications in flexible electric devices.

Thermal conductivity enhancement and heat transport mechanism of carbon fiber z-pin graphite composite structures

This paper successfully constructed an innovative carbon fiber z-pin graphite composite with three-dimensional thermal conductive paths by taking advantages of high conductivities of both graphite sheet and z-pin. The effects of structural parameters and temperature on thermal conductivity (TC) were investigated, and thermal transport mechanism was analyzed. The results show that pyrolytic graphite sheet (PGS) and expanded graphite sheet (EGS) composites both show considerable increase in through-thickness TCs reaching up to 1.80–6.20 W m−1K−1 after implanting carbon fiber z-pins. Infrared thermography demonstrates the formation of three-dimensional heat conductive paths in-between graphite sheet and z-pin. Our results indicate that the synergistic effect of the heat exchange becomes more evident with the increase in the in-plane TC of graphite sheet and particularly the axial TC of z-pin. The maximum through-thickness TC of 6.20 W m−1K−1 is achieved, which is 647% higher than the control sample.

Performance of Lead Acid Battery Using Graphite Based Composite As Cathode Current Collector

Introduction Lead acid batteries have been widely used for more than 100 years. They are used for vehicles and backup power supplies. However, when overdischarged, lead acid battery cannot be recharged and its performance is greatly declined. Recently, Iwai et al. had revealed that the cause of not being able to be recharged is the nucleation of α-PbO2 on cathode material, β-PbO2, and that, when gold or platinum is used as the current collector, formation of α-PbO2 is prevented due to local cell reaction, then that lead acid battery can be recharged even after overdischarge [1,2]. In this study, we investigated the possibility of realizing the lead acid battery with high resistance to overdischarge using graphite materials, such as natural expanded graphite sheet as a current collector. We think that graphite materials will be advantageous for cost reduction, weight reduction and improvement of processability for lead acid batteries. Experiment We developed graphite based composite material as a current collector which was prepared by the following procedure, in order to improve water repellency. Resin materials such as polyethylene, polypropylene (PP), ethylene propylene copolymer and fluororesin were dispersed with graphite sheet, preheated in air at 100 degrees for 10 minutes, and then heated at 160 degrees to 200 degrees for 10 minutes. Then, the graphite-based composite material was immersed in sulfuric acid (H2SO4) and the weight variation was measured. Also, the surface electric resistance of the graphite composite material was measured by four-terminal method. We decided to use PP as resin for graphite based composite materials, because graphite based composite material using PP have a low electric resistance and PP is easily dispersed in graphite sheet. Then, we chose PP contents of 5% from the viewpoint of water resistance. We used two-electrode glass cell for charge-discharge experiments. The cathode paste was prepared by mixing purchased β-PbO2 powder as an active material, acetylene black as a conducting additive and PTFE as a binder at the ratio of 80:15:5 in weight. The cathode was fabricated by pressing the paste on a current collector. In this study, we tested three kinds of current collectors, Pb plate, graphite sheet and graphite-based composite material. Each current collectors were used in 35 wt% H2SO4. Lead plate and 35 wt% H2SO4 were employed as anode and electrolyte, respectively. Firstly, we discharged the cell at 9 mAg-1 for 30 min and charged at 180 mAg-1 for 20 min. We repeated this cycle 20 times in order to stabilize the charge-discharge reaction. Secondly, we deeply discharged the cell down to 0 V at 9 mAg-1 and opened the circuit for 48 h. Finally, we once again charged the cell at 180 mAg-1 for 60 min, then we discharged the cell at 9 mAg-1 for 30 min and charged at 180 mAg-1 for 20 min and repeated the cycle to study the cell performance. Results and discussion Figure 1 (A) and (B) show the charge-discharge curves for the lead acid battery using Pb plate and graphite sheet as a current collector, respectively. In the case of Pb plate, the lead acid battery cannot be recharged after the open circuit. On the other hand, in the case of graphite sheet, it can be recharged even after the open circuit. Electrode potential does not come out for carbon, so it is thought that carbon has a role of stopping the local cell reaction. From this reason, it can be explained that the formation of α-PbO2 is prevented on cathode β-PbO2 then that the similar performance was shown when using gold or platinum as a current collector. In figure 2, performance of lead acid battery using graphite based composite as cathode current collector is shown. We discharged the cell at 9 mAg-1 to 0 volt and charged at 180 mAg-1 for 60 min and repeated the cycle between 0 volt and 2 volt. Between 1.9 V and 1.3 V, potential flat area is clearly observed. Just only inserting a thin graphite based composite, lead acid battery repeating charge / discharge cycles between 0 volt and 2 volt so many times without decay was realized. This result may be caused by improvement of water repellency of current collector by impregnate with PP into graphite sheet.

Expanded graphite supported p-n MoS2-SnO2 heterojunction nanocomposite electrode for enhanced photo-electrocatalytic degradation of a pharmaceutical pollutant

Wastewater treatment challenges by conventional methods have necessitated the need for alternative/complementary methods that are environmentally benign and efficient especially toward recalcitrant organic pollutants. In this regard, a novel photoanode consisting of a p-n MoS2-SnO2 heterojunction anchored on expanded graphite (EG) was fabricated and employed in the photo-electrocatalytic degradation of ciprofloxacin, a pharmaceutical pollutant, in water using a current density of 0.010 A cm−2. The photoanode material was characterised with transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Raman spectroscopy and X-ray diffraction (XRD) to confirm that the nanocomposite was successfully prepared. Photoelectrochemical studies were carried out with cyclic/linear sweep voltammetry and chronoamperometry. The removal efficiency of the photo-electrocatalytic cell was determined on a UV–Visible spectrophotometer and the extent of mineralisation was measured by a total organic carbon analyser. The results obtained revealed that the SnO2 particles are nanosheets while the MoS2 particles are hierarchical microspheres having nanosheets of MoS2, and they were anchored on the interlayers of the EG sheets. Also, the p-n MoS2-SnO2 heterojunction anchored on expanded graphite (EG) was found to be photoactive and displayed a better removal efficiency and mineralisation in comparison to EG, SnO2-EG and MoS2-EG electrodes. This may be due to the formation of the MoS2-SnO2 p-n heterojunction in the MoS2-SnO2/EG nanocomposite which enhanced the light harvesting ability of the material resulting in its improved photo-electrocatalytic performance. Hence the MoS2-SnO2/EG is potentially a good photoelectrode which may be beneficial for a photo-electrocatalytic treatment of industrial wastewaters and other photo-electrocatalytic applications.

The use of expanded graphite sheet as an efficient method for preventing heat explosion in portable gas stoves

In Korea, a total of 112 explosions related to portable butane gas stoves occurred between 2011 and 2015. This resulted in the injury of 187 people and the death of 8 others. Although safety devices are installed in portable gas stoves or canisters, reliability is low. We conducted experiments to identify the process of the portable butane gas stove’s explosion and performance of an improved portable gas stove. The improved one has an attached expanded graphite sheet inside the gas canister cover which has a directional high thermal conductivity to diffuse heat. As a result of these experiments, we found the process, characteristics, and trends of explosions of portable butane gas stoves. Moreover, we checked that the stoves with an attached graphite sheet help prevent explosions by diffusing heat. In addition, the temperature of canisters was constant until most of the butane gas was consumed. These results provide data to improve safety devices for portable gas stoves as well as to prevent deadly accidents.

Facile synthesis of high-performance Ni(OH)2/expanded graphite electrodes for asymmetric supercapacitors

Cost-effective commercial expanded graphite (EG) was used as a raw material, and a facile in-situ electrodeposition method was adopted to synthesize a layered Ni(OH)2/EG composite electrode in an N,N-dimethylformamide-water system. Scanning electron microscopy images show that expanded graphite sheets, Ni(OH)2 nanoparticles and carbon nanotubes construct a layered structure, which not only effectively restrains restacking of EG sheets but also prevents aggregation of nickel hydroxide particles. The electrode delivers a satisfactory initial specific capacitance of 1719.5 F/g at 1 A/g with a total mass loading of 5.0 mg/cm2. Even at 10 A/g, the capacitance only decreases to 1181.3 F/g, showing a remarkable rate capability. Moreover, an optimized asymmetric supercapacitor (ASC) device was fabricated, in which the Ni(OH)2/EG electrode was used as a positive electrode and commercial activated carbon (AC) was used as a negative electrode. The ASC device can deliver a prominent energy density of 32.3 Wh/kg at power density 504.7 W/kg, and long cycling life with 79% original capacitance after 1000 cycles at 5 A/g, which can be prospective to be applied in practical devices for energy storage and conversion.

THE EFFECT OF SEALING SYSTEM ON ZERO FUGITIVE EMISSIONS AND GLOBAL WARMING

Nowadays keeping the air clean is one of the most important aims in environmental protection. The fugitive emissions especially volatile organic compounds (VOC’s) and the ozone-precursors nitrogen oxides (NOX) have a great impact on the quality of the atmospheric air. Worldwide fugitive emissions from leaking valves, pumps and flanges amount to over a million metric tonnes per year. Elimination or reduction of fugitive emissions could save industry many millions of dollars and prolong scarce resources. Sealing system is the major contributing factor will be through the lowering of fugitive emissions. In this study various kind of fibre reinforced Nitrile Butadiene Rubber (NBR) sheet sealing materials and new developed expanded graphite sheets were investigated. Experimental results shown that, the sealing performance has been improved by the development in material technologies. Fibre base materials have higher permeability than expanded graphite for both liquid and gas environments. They have adequate sealing performance to need for all over applications. Expanded graphite shows better sealing performance than fibre base materials but it has lower recovery. As a result, for every specific application, careful selection, correct installation and operation according to the performance envelope, regular inspection and maintenance must be considered for low or zero emission requirements.

Prediction of thermal conductivity of composite polymer materials filled with expanded graphite sheet fillers

Adding fillers of high thermal conductivity to the base polymer materials is a solution to make composites of high thermal conductivity. Expanded graphite sheets have been recognized as an economic and efficient filler material to make composite polymers of high conductivity. However, the prediction of the effective thermal conductivity of the composite materials is a difficult task due to the random nature in fillers distribution. The anisotropic properties of the sheet fillers make the heat transfer analysis difficult. In this study, a conjugate heat conduction model considering the random distribution nature of fillers is set up and numerically solved. A validated commercial software FLUENT and finite volume method was used in the analysis. Then an experiment is done to validate the model. The numerical results are used to summarize a correlation for the prediction of the effective thermal conductivity of the composite materials. It is found that the filler height to thickness ratio plays an important role in intensifying the heat conduction in the composite materials. At a given filler content, the graphite sheets should be fabricated thin enough to have higher height to thickness ratios and consequently higher performance in effective conductivity.

Expanded graphite as a filler for epoxy matrix composites to improve their thermal, mechanical and electrical properties

Expanded graphite (EG)-reinforced epoxy composites were prepared by a solution mixing method. The structure and morphology of the EG/epoxy composites were investigated by XRD, FE-SEM and HR-TEM. The EG prepared by acid oxidation and thermal expansion shows good compatibility with the epoxy resin that enters the EG layers to decrease their thickness to 60–70 nm, owing to its abundant oxygen-containing functional groups. With the addition of 9 wt% EG, the thermal decomposition temperature of the composite increases from 340 to 480 °C, the electrical conductivity from 10−15 to 10−5 S/cm and the tensile stress is increased by more than 30%. These improvements are attributed to the good dispersion of EG sheets in the epoxy matrix.