Development of Advanced ECAs with Micro/nano Hybrid Filler System: Filler Functionalization, Dispersion, and Conductivity Improvement

During the recent two decades, considerable efforts have been made to explore new generations of interconnecting materials and printed lines to replace the traditionally used toxic lead-based solders in electronic industries. Electrical conductive adhesive (ECA) which consists of conductive metallic particles and a polymeric matrix has attracted a great deal of attention as one of the most promising alternative materials. The conventional ECAs are typically made of silver micro flakes and epoxy resin. The low electrical conductivity of these ECAs is their main drawback compared to traditional lead-based solders, which hinders their applicability in today’s blooming electronic industries. An enormous amount of research works have been conducted to enhance the electrical conductivity of the conventional ECAs, including increasing the polymer shrinkage, surface modification of silver flakes, introduction of low melting components to the ECA formulation, and the use of nano-sized conductive materials inside the formulation of the conventional ECAs. All of these approaches affect the quality of inter filler interaction inside the electrical network, in different ways. The recent progress in nanotechnology helped material scientists to precisely design nanomaterials with different morphologies and surface chemistry. Owing to this capability, incorporation of nano-sized conductive fillers with different natures, morphologies, and surface properties inside the conventional formulation of ECAs has drawn considerable attention to overcome the common drawbacks of conventional ECAs, such as poor electrical and mechanical properties, reliability issues, and large filler content. It has been reported that the introduction of conductive nanomaterials into the conventional ECAs can improve the electrical conductivity of ECAs if their size, morphology, and the ratio between nanofiller and silver flakes is carefully taken under consideration. In this project, we developed new generations of hybrid electrical conductive adhesives (ECAs) by introducing conductive nanofillers (spherical silver nanoparticles (Ag NPs), high aspect-ratio silver nanobelts (Ag NBs), and graphene) into the conventional formulation of ECAs. To harness the characteristic properties of the nanofillers and to facilitate their homogeneous dispersion inside the epoxy, nanofillers were functionalized. In the first step of this project, spherical Ag NPs were synthesized and simultaneously functionalized with thiocarboxilic acids, resulting in the formation of NPs less than 5 nm. Two thiocarboxylic acids with the same chemical structure but different chain lengths (3 and 11 carbons) were used to functionalize the NPs. We showed that the size and the electrical properties of the NPs can be controlled by varying the chain length of their covering organic layer. The diameter of the Ag NPs functionalized with the short-chain acid was two times smaller than those with the long-chain acid. We also found that the short-chain functionalized NPs were electrically conductive while the long-chain functionalized ones were nonconductive. The short-chain functionalized NPs were incorporated into the conventional ECAs. We found that at low NPs contents (< 20 wt %) the electrical conductivity of the hybrid ECAs increased due to the filling of NPs into the interstices of the micron-sized silver flakes, bridging of the NPs among separated flakes, and sintering of the NPs at relatively low curing temperature of 150 °C. However, higher NPs contents reduced the electrical conductivity because they may cluster and increase the gaps between the silver flakes. Furthermore, at higher NPs content, the number of contact points increases, which in turn decreases the electrical conductivity of the final ECAs. The positive effect of the synthesized NPs on the electrical conductivity of the nanocomposite is basically attributed to the increased number of electrical pathways inside the electrical network due to the bridging of the NPs between separated silver flakes. However, a large amount of NPs are needed to form effective bridges inside the network, which increases the number of contact points inside the filler system and also increases the cost of the final ECAs. In the second step, we implement a novel type of high aspect-ratio silver nanostructure, silver nanobelts (Ag NBs), as co-filler inside the conventional formulation of the ECAs. The Ag NBs (10-40 nm thick, 100-400 nm wide and 1-10 µm long) were synthesized through self-assembly and room-temperature joining of hexagonal and triangular silver unit blocks which were synthesized by chemical reduction of silver nitride in the presence of poly(methacrylic acid). The incorporation of a small amount of the Ag NBs (2 wt%, NBs to flakes weight-ratio, K = 0.03) into a conventional ECA with 60 wt% silver flakes resulted in an electrical conductivity enhancement of 1550% in comparison to that of the conventional ECAs with the same total silver weight fraction, while addition of 2 wt% (K = 0.03) NBs into the conventional ECA with 80 wt% silver flakes enhanced the electrical conductivity of the hybrid ECA approximately 240%. These results imply high aspect-ratio NBs are more effective to improve the electrical conductivity of ECAs at concentrations close to percolation threshold. Considering the importance of the aspect-ratio of the nanofillers, in the next step, we implemented graphene, which is known for its exceptional electrical, mechanical and thermal properties, to further reduce the amount of silver flakes while maintaining a high electrical conductivity. Graphene, possessing the highest aspect-ratio among all the nanostructures and also due to its 2D structure, can provide extremely high surface area for electron transformation inside the electrical network. However, to exploit the interesting properties of the graphene, their single layer structure must be preserved inside the polymeric matrix. To achieve this goal, we applied two types of surface modification to exfoliate and stabilize graphene layers. First, we decorated graphene surface with Ag NPs, functionalized with a short chain length thiocarboxylic acid, and introduced this 2D nanostructure into the conventional ECAs. The electrical conductivity measurements revealed that the decorated graphene significantly improves the electrical conductivity of the conventional ECAs only at low filler concentrations, while to achieve high electrical conductivity, elevated curing temperatures are needed. This situation is a result of the increased number of contact points because of Ag NPs on graphene surface. Second, we used a non-covalent approach to stabilize graphene using the surfactant; sodium dodecyl sulfate (SDS). Our results showed that the stabilization of graphene with SDS noticeably enhance the electrical conductivity of the ECAs, which is attributed to the role of SDS in exploiting the high aspect-ratio of graphene. In order to examine this hypothesis, we used a larger size graphene and applied the same SDS modification protocol. The electrical resistivity measurements showed that the electrical conductivity enhancement in the case of hybrid ECAs with large SDS–modified graphene was more significant than that with small SDS–modified graphene. The percolation threshold for the hybrid ECA with 1.5 wt% of both large and small graphene was reduced to an interestingly low value of 10 wt% while this value for conventional ECAs, and hybrid ECAs with non-modified graphenes was 40 wt%. Furthermore, adding 1.5 wt% of large SDS-modified graphene into the conventional ECA with 80 wt% silver flake content resulted in a very low electrical resistivity of 1.6 × 10-5 Ω.cm which is lower than that of eutectic lead-based solders

Evaluation of Hybrid Electrically Conductive Adhesives

An electrically conductive adhesive (ECA) is a composite material acting as a conductive paste, which consists of a thermoset loaded with conductive fillers (typically silver (Ag)). Many works that focus on this line of research were successful at making strides to improve its main weakness of low electrical conductivity. Most research focused on developing better silver fillers and co-fillers, or utilizing conductive polymers to improve its electrical conductivity, however, most of these works are carried out on small scale. In this work, we aim to produce larger quantities of hybrid ECA to successfully test its properties. Industry is interested in materials with superior physical properties. As such, rheological behavior and mechanical strength were explored as it has been theoretically hinted that incorporation of exfoliated graphene within the composite could impact those factors listed in a positive manner. In the first step of this project, pre-treated sodium dodecyl sulfate (SDS)-decorated graphene’s rheological properties were examined. An epoxy resin diglycidylether of bisphenol-A (DGEBA) was the main polymer used for this study: a well-known material that can behave either as a shear-thinning or shear-thickening material depending on the supplier. We showed how composites that contain graphene (Gr) had higher viscosities than ones that contained SDS decorated graphene Gr(s). Not only did we confirm that surfactant was a key factor in the decrease of viscosity, but we also report how Gr and Gr(s) had a special effect that suppresses the intrinsic shear thickening behavior of epoxy resin at weight concentrations (wt%) higher than 0.5 wt%. The results showed that Gr(s) is not only beneficial in terms of improving the conductivity of conventional ECAs, but it also acts as a solid lubricant that decreases the viscosity of the composite paste at higher weight concentrations. In the second step of the project, pre-treated SDS decorated graphene’s mechanical properties were examined. In specific, its lap-shear strength (LSS) as well as the effect of residual solvent when present in our hybrid ECA system were studied in order to follow up on the thermal results obtained from a previous study. We showed that our initial suspicion was correct as the LSS did decrease for all of the solvent-assisted formulations that contained Gr(s) ranging from 66 to 84%, however, we were not able to tell whether or not that decrease was caused by lower crosslinking density. Instead, we uncovered another reason for this decrease: bubble formation during the curing step. This suspicion was confirmed qualitatively through light microscopy and quantitatively through optical profilometry, where we present an increase in surface roughness for the solvent-assisted samples. Furthermore, by using SEM, we also confirmed that this bubble formation extends throughout the entire bulk material rather than just at the interface. Lastly, we investigated whether the use of solvent to assist in the mixing process significantly improves the electrical conductivity at a lower weight loading of Ag, and compared the electrical conductivity with that of the products prepared under the same higher weight loading of Ag using a solvent-free mixing method from previous work. Thirdly, we investigated another mechanical property of our hybrid ECAs through indentation tests, where we use Hertizan equations to characterize elastic modulus. Since we learned that the addition of Ag flakes is detrimental to the mechanical strength, we focused on the difference between the elastic moduli for Gr and Gr(s) in a solvent-free environment. In the last step of this project, we explored the use of a liquid-suspended co-filler (instead of carbon filler-based materials) in Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS): a conductive polymer that is frequently in conductive thin-films. We report that by using PEDOT:PSS as a conductive co-filler into the conventional ECA with 60 wt% of Ag, we observed higher conductivity equivalent to adding an extra 20 wt% of Ag into the system. Furthermore, we report that an increase of PEDOT:PSS in the composite appears to decrease the LSS of the material by 20%.

Rational design of electrically conductive polymer composites for electronic packaging

Electrically conductive polymer composites, i.e. polymers filled with conductive fillers, may display a broad range of electrical properties. A rational design of fillers, filler surface chemistry and filler loading can tune the electrical properties of the composites to meet the requirements of specific applications. In this dissertation, two studies were discussed. In the first study, highly conductive composites with electrical conductivity close to that of pure metals were developed as environmentally-friendly alternatives to tin/lead solder in electronic packaging. Conventional conductive composites with silver fillers have an electrical conductivity 1~2 orders of magnitude lower than that of pure, even at filler loadings as high as 80-90 wt.%. It is found that the low conductivity of the polymer composites mainly results from the thin layer of insulating lubricant on commercial silver flakes. In this work, by modifying the functional groups in polymer backbones, the lubricant layer on silver could be chemically reduced in-situ to generate silver nanoparticles. Furthermore, these nanoparticles could sinter to form metallurgical bonds during the curing of the polymer matrix. This resulted in a significant electrical conductivity enhancement up to 10 times, without sacrificing the processability of the composite or adding extraneous steps. This method was also applied to develop highly flexible/stretchable conductors as building block for flexible/stretchable electronics. In the second study, a moderately conductive carbon/polymer composite was developed for use in sensors to monitor the thermal aging of insulation components in nuclear power plants. During thermal aging, the polymer matrix of this composite shrank while the carbon fillers remained intact, leading to a slight increase in filler loading and a substantial decrease in the resistivity of the sensors. The resistivity change was used to correlate with the aging time and to predict the need for maintenance of the insulation component according to Arrhenius’ equation. This aging sensor realized real-time, non-destructive monitoring capability for the aging of the target insulation component for the first time.Ph.D

Mechanical, Thermal, and Electrical Properties of Graphene-Epoxy Nanocomposites—A Review

Monolithic epoxy, because of its brittleness, cannot prevent crack propagation and is vulnerable to fracture. However, it is well established that when reinforced—especially by nano-fillers, such as metallic oxides, clays, carbon nanotubes, and other carbonaceous materials—its ability to withstand crack propagation is propitiously improved. Among various nano-fillers, graphene has recently been employed as reinforcement in epoxy to enhance the fracture related properties of the produced epoxy–graphene nanocomposites. In this review, mechanical, thermal, and electrical properties of graphene reinforced epoxy nanocomposites will be correlated with the topographical features, morphology, weight fraction, dispersion state, and surface functionalization of graphene. The factors in which contrasting results were reported in the literature are highlighted, such as the influence of graphene on the mechanical properties of epoxy nanocomposites. Furthermore, the challenges to achieving the desired performance of polymer nanocomposites are also suggested throughout the article

Cure and Conductivity: Investigation of Thermal Processing of Silver Nanobelt - Epoxy Composites

Electrically conductive adhesive (ECA) composites based on diglycidyl ether of bisphenol-A / triethylenetetramine (DGEBA/TETA) epoxy, including a hybrid nanocomposite containing newly developed silver nanobelts, have been investigated for the relationships between the cure process and the evolving electrical properties. When nanobelt-filled electrically conductive composites are fully developed their advantages may include: 1) reduced total filler content compared to microcomposites, resulting in reduced cost; 2) improved final conductivity; 3) improved longevity due to high nanobelt stability. The current work is focused on several studies of electrically conductive composites, and of silver nanobelts: 1) Traditional investigations, via differential calorimetry, of the effect that the filler additions had on the composite cure behavior. 2) Development of nanobelt synthesis methods to provide consistent and repeatable production of high quality one-dimensional silver nanobelt fillers. 3) Characterization of the particle joining process exhibited by silver nanobelts when heat treated without being inside a polymer matrix. 4) In-situ characterization of the evolving electrical conductivity throughout the cure progression of a microcomposite containing only silver microflake fillers. 5) Similar in-situ electrical characterization of conductivity throughout the cure of several hybrid nanocomposites containing silver nanobelts and silver microflakes. The primary results and scientific contributions from this work are: 1) Determination that addition of silver nanobelts to a composite influences cure and glass transition behavior less than or equal to the presence of small quantities of solvents, demonstrating that traditional cure analysis methods alone are not sufficient for characterizing these conductive nanocomposites. 2) Improvements in the particle aspect ratio and the batch repeatability when synthesizing silver nanobelts in gram-quantities, with the determination that the dominant factors controlling final morphology are the reaction mixture pH and the concentration of the dissolved silver salt in the precursor solution. 3) Identification of a non-diffusional joining process that occurs between silver nanobelts outside of a composite, during heat treatment between 75 °C and 180°C, allowing reductions in network electrical resistance without the detrimental effects of the activation of diffusion. 4) and 5) First observations of path-dependency during cure displayed by the evolving electrical conductivity of hybrid nanocomposites and a microcomposites, due to the onset of vitrification when partially cured. In addition, a number of other contributions were also made, mainly: 1) Design, construction, and verification a custom curing mold with inlaid reusable 4-wire electrical probe integrated with a specimen curing mold, designed to obtain in-situ electrical measurements throughout cure during high precision temperature control. 2) A table-top vacuum-mixing system that mounts onto a common model of vortex mixer, allowing solvent blending by continuous stirring under vacuum of up to 6 simultaneous batches of nanocomposites, to obtain composite batches with low final solvent contents. 3) Demonstration that small additions of high aspect ratio nanoparticles, such as silver nanobelts, into a hybrid nanocomposite has the potential to substantially alter the distribution of fillers, causing standard models of electrical percolation in hybrid systems to be potentially inaccurate. Although the hybrid nanocomposites filled with silver nanobelts has a lower conductivity than anticipated, it is thought that this is due to the path-dependent and resistance-increasing effect of its high glass transition temperature, and the resulting highly significant vitrification during cure. It is hypothesized that additions of solvent will substantially reduce vitrification effects, and improve conductivity further. This system shows potential of desirably high conductivity at filler contents as low as 55 wt% silver. Further, these demonstrations of the influence of vitrification on the evolving electrical properties during cure represent a significant source of bias or variation for research conducted on new conductive composites. Similarly, this may also significantly impact the utilization of conductive adhesives in industry, since industry prefers multi-stage cure treatments that feature significant amounts of vitrification

Mechanical, thermal, and acoustic properties of aluminum foams impregnated with epoxy/graphene oxide nanocomposites

Hybrid structures with epoxy embedded in open-cell aluminum foam were developed by combining open-cell aluminum foam specimens with unreinforced and reinforced epoxy resin using graphene oxide. These new hybrid structures were fabricated by infiltrating an open-cell aluminum foam specimen with pure epoxy or mixtures of epoxy and graphene oxide, completely filling the pores. The effects of graphene oxide on the mechanical, thermal, and acoustic performance of epoxy/graphene oxide-based nanocomposites are reported. Mechanical compression analysis was conducted through quasi-static uniaxial compression tests at two loading rates (0.1 mm/s and 1 mm/s). Results show that the thermal stability and the sound absorption coefficient of the hybrid structures were improved by the incorporation of the graphene oxide within the epoxy matrix. However, the incorporation of the graphene oxide into the epoxy matrix can create voids inside the epoxy resin, leading to a decrease of the compressive strength of the hybrid structures, thus no significant increase in the energy absorption capability was observed.publishe

Synthesis and Application of Graphene Decorated Nanocomposites for Photovoltaics and Printed Electronics

Improving the yield performance of commercial solar cells has been a major topic of interest for many research studies as different processing parameters are extensively explored. Screen printing metallization is the most widely-used technique to form the metallic contacts in commercial silicon solar cells. Among various process parameters in screen printing technology, firing step is considered to be a major cost-determining stage as a high amount of thermal budget is consumed during high temperature sintering process. Bending of the cell (deflection) due to residual stress produced in firing step is also considered as a very serious problem for reliability of the module. Another important concern in conventional metallic pastes used in screen printing technique is the presence of lead which is not an environmentally friendly material. Lead free Electrically Conductive Adhesives (ECAs) which have been of particular interest in electronic packaging applications are considered as an interesting alternative and their possibility to replace conventional metallic pastes in screen printing technology is systematically investigated in this research. In addition to be environmentally friendly, ECAs provide low processing temperature which is beneficial for developing ultra-thin solar cells, decreasing cost and increasing the yield performance of the solar cells. After establishing the screen printing process to print conductive materials, different conductive nanocomposites were synthesized using ECA and silver nanowires. Conductivity measurements revealed that addition of even a very small amount of silver nanowires could improve the conductivity of the nanocomposite for around 70%. Morphology characterization analysis confirmed the influence of silver nanowires in improving the electrical properties of the paste by tunneling effect. Despite the significant improvement in conductivity of the nanocomposite which made it an appealing alternative for printed electronics, its conductivity was still lower than the conventional metallic pastes used in the solar cell fabrication. To reach a comparable conductivity, two dimensional graphene sheets that have outstanding electrical and structural properties were used. Material synthesis and curing profile are optimized to improve the electrical property of the developed nanocomposites which resulted in the highest boost in conductivity, over 2 orders of magnitude of ECA. Different characterization techniques were utilized to study the curing kinetics and thermal stability of the developed materials. A detailed residual stress analysis was also conducted on all developed nanocomposites as well as the metallic paste printed substrates and results revealed that the amount of residual stress induced in substrates was one order of magnitude lower in nanocomposite printed wafers in comparison with the metallic printed ones. Also, the amount of increase in residual stress by decreasing the wafer thickness was small in the case of using nanocomposite paste. It was also shown that the amount of bending increased with decreasing the thickness of substrate in all samples; however, it was much lower in nanocomposite printed wafers confirming that the developed nanocomposites could be a promising alternative in screen printing metallization of ultra-thin wafers. Effective Work Function (EWF) was introduced for the multi-phase nanocomposite being used as electrodes in electronic devices. Based on the theoretical studies on tunneling phenomenon and the potential barrier height measurement analysis on MOS devices, IV method was illustrated and used for the first time to measure the EWF of the multi-phase graphene-decorated nanocomposite. The results were verified using two other techniques: UPS spectroscopy and TLM. The results from all measurements were in reasonable agreement confirming the accuracy of results obtained from all three methods. However the existing small difference between the EWF values was attributed to the heterogeneous nature of the nanocomposite at the interface confirming the necessity of using the effective work function definition. Finally, highly conductive graphene-decorated nanocomposite as well as the evaporated Al were used as the back contact of two solar cells. Solar cells were fabricated and characterized using different techniques. Quantum efficiency, illuminated IV and Dark IV results from both cells were in an excellent agreement with each. In general, the obtained close performance results as well as the equal obtained efficiencies for both cells confirmed a very satisfactory achievement for a standard technology baseline development

Science and technology of graphene-based inks for polymer-composite applications

openMany of our modern technologies require materials with unusual combinations of properties that cannot be met by the conventional metal alloys, ceramics, and polymeric materials. This is especially true for materials that are needed for aerospace, underwater, and transportation applications. For example, researchers, engineers and material scientists are increasingly searching for structural materials that have low densities, are strong, stiff, as well as abrasion and impact resistant, in addition not to be easily corroded. Material property combinations and ranges have been, and are yet being, extended by the development of composite materials. Polymer-based composites are the combinations of two or more organic and inorganic materials, mixed together to create a new material, the composite, with enhanced physical cumulative properties of the constituents. The polymer acts as the matrix, while the filler is dispersed in order to improve the physical properties of the final composite. Polymer matrices and fillers are chosen to create composites with tailored properties; e.g., high-modulus but brittle carbon fibres are added to low-modulus polymers to create a stiff, lightweight composite with increased toughness compared to the bare polymer. Recently, however, researchers have reached the limits of optimizing composite properties of traditional micrometre-scale composite fillers, because the properties achieved usually involve compromises: stiffness is traded for toughness, or toughness is obtained at the cost of optical transparency. In addition, macroscopic defects due to regions of the high or low volume fraction of filler often lead to breakdown or failure. Recently, a large window of opportunities has opened to overcome the limitations of traditional micrometre-scale polymer composites: nanoscale filled polymer composites, in which the filler is <100 nm in at least one dimension. Examples of nanoscale fillers are carbon black, carbon nanotubes (CNTs), exfoliated clays, and two-dimensional (2D) crystals, such as graphene. In particular, graphene has a Young’s Modulus of 1 TPa and intrinsic strength of 130 GPa, electrical conductivity, σ, of up to 108 S m−1, thermal conductivity of ~ 5×103 W m−1·K−1, and a specific surface area of 2630 m2 g–1, being, therefore, a promising filler for polymer matrices. Graphene/polymer composites possess not only increased stiffness and strength, compared to the pristine matrices, but can be useful for multi-functional applications such as in the electronic field, as wearable strain sensors, printed electrodes, conductive adhesives, and supercapacitors.Other 2D crystals, with their own and peculiar properties, can be used as fillers for different kind of applications. As an example, hexagonal-boron nitride (h-BN) has similar mechanical and thermal properties compared to those of graphene, but it is an electrical insulator. Therefore, h-BN/polymer composites can cover applications in which high electrical conductivity is undesirable, e.g. for thermal management or food packaging. Whereas, 2D semiconductors, such as Black Phosphorous (BP) and transition metal dichalcogenides (TMDs), can be exploited as fillers for developing composites useful for optoelectronic applications, such as pulsed fibre lasers and photo-actuators. In spite of the recent development of 2D crystals/polymer composites, there are many questions yet to be answered. In particular, many technical barriers involving structure control, dispersion of 2D crystals in the matrix, the interfacial interaction between 2D crystals and matrix, and re-aggregation issues between 2D crystal flakes must be taken into account to target the wide applications of these advanced composites. The aim of my PhD work, presented in this Thesis, was indeed to investigate 2D crystals as potential fillers for the development of future polymer-based composites, trying to meet the requirements set by the aforementioned open-questions, linking the morphology of the 2D crystals and their dispersion in the polymer matrix to the final properties of the as-produced composites. In my PhD work, I focused my attention, particularly on graphene, h-BN, and WS2. The 2D crystals used in this work have been synthesized exploiting sonication-assisted LPE and wet-jet milling (WJM)-assisted LPE, the latter being a completely novel approach developed in our group that allows an industrial rate production of 2D crystals, meeting the demand of large-scale filler production required for the composite field. The morphology of the as-synthesized 2D crystals, in terms of surface area (A), lateral size (l) and thickness (t), has been tuned by exploiting sedimentation-based separation (SBS). The size-sorted 2D crystals are subsequently used as fillers in polymer composites, investigating several polymer matrices: polycarbonate (PC), acrylonitrile butadiene styrene (ABS), and polylactic acid (PLA). Composites are produced by means of solution blending technique, allowing a thorough dispersion of the fillers inside the matrix, and then coagulated or cast in order to obtain composite pellets or composite films. Mechanical, thermal and electrical characterizations of composite films have been performed and the obtained results are linked to the morphological properties of the fillers.openXXXI CICLO - SCIENZE E TECNOLOGIE DELLA CHIMICA E DEI MATERIALI - NanochemistryBONACCORSO, F. (IIT) PELLEGRINI, V. (IIT)Lago, Emanuel

Equilibrium and Transport Properties of Systems Containing Graphene Sheets (-Oil Nanocomposites and Membranes) from Molecular Dynamics Simulations

Graphene sheets belong to an interesting class of materials. Their exceptional properties, including high thermal and electrical conductivity, mechanical strength etc., could play an important role in multiple applications, suggesting the possible use of graphene sheets in, e.g., electronic devices, nanocomposites, energy storage, and membranes for water desalination, to name just a few. Understanding the properties of graphene is essential to secure progress in all these areas. Molecular dynamics simulations were performed to provide molecular level insights of the equilibrium and transport properties of several systems containing graphene sheets.In the first part of the thesis graphene-oil nanocomposites are considered. Results show that it is possible to obtain stable dispersions of graphene sheets in oils such as n-hexane, n-octane and n-dodecane by appropriately functionalizing the edges of the graphene sheets with short branched alkanes. Excluded-volume effects, generated by the branched architecture of the functional groups grafted on the graphene sheets, are responsible for the stabilization of small graphene sheets in the organic systems considered here. Using non-equilibrium molecular dynamics, the Kapitza resistance at the graphene-octane and graphene-graphene interfaces was calculated. Our results demonstrate that it is possible to reduce the Kapitza resistance at the graphene sheet-octane interface by using the functionalized graphene sheets, but the functional groups must show vibrational modes compatible with those of the organic matrix. A higher value of Kapitza resistance for graphene sheets in vacuum compared to that in octane was found because the graphene-graphene interface has larger Kapitza resistance than the graphene-octane interface, which is consistent with observations for carbon nanotube – carbon nanotube contacts. More importantly, the Kapitza resistance for the graphene-graphene contact can be 30% lower than values reported for the carbon nanotube – carbon nanotube contact. Equilibrium and non-equilibrium molecular dynamics simulations to assess the effective interactions between dispersed graphene sheets, the self-assembly of graphene, and the heat transfer through the graphene-octane nanocomposite. Evidence is provided for the formation of nematic phases when the graphene sheets volume fraction increases within octane. The atomic-level results are input for a coarse-grained Monte Carlo simulation that predicts anisotropic thermal conductivity for graphene-based composites when the graphene sheets show nematic phases. Overall, these results suggest that it might be possible to produce nanocomposites containing graphene sheets. Such materials could show exceptional mechanical and thermal-transport properties (due to the inclusion of graphene sheets), while maintaining the lightweight typical of polymeric materials.In the second part of the thesis umbrella sampling simulations were employed to study the transport of water molecules and ions through the membranes incorporating bare and functionalized graphene pores. By calculating the potential of mean force for ion and water translocation through the bare graphene pores, we show that ions face a large energy barrier and will not pass through the narrower pore studied (Ø ~ 7.5 Å) but can pass through the wider pores (Ø ~ 10.5 and 14.5). Water, however, faces no such impediment and passes through all the pores studied with little energy barrier. When charged groups are grafted to the pore rim, the results show that the charges can help to prevent the passage of ions. Comparison of results for graphene pore to that of carbon nanotube pore reveals that COO- groups are more effective when grafted to the rim of GS pore in preventing Cl- ions from passing through the membrane compared to that of carbon nanotube pore. The results presented could be useful for the design of water desalination membranes

Multi-Layer-Graphene-Nanoclay-Epoxy Nanocomposites – Theory and Experimentation

The influence of Multi-Layer Graphene (MLG) and nanoclay on the performance of epoxy based nanocomposites has been studied. First, the theoretical aspects of nano-fillers and their impact on mechanical, thermal, and electrical properties of nanocomposites have been discussed. Then, nanocomposites were produced with varying weight fraction of nano-fillers (0.05, 0.1, 0.3, 0.5, and 1.0 wt%). It was observed that organic solvent, if not completely removed, causes porosity which acts as stress raiser and deteriorates the mechanical properties. The influence of reinforcement morphology on the mechanical properties of epoxy nanocomposites was studied using two nano-fillers: MLG and nanostructured graphite (NSG). It was observed that mechanical properties of nanocomposites were higher when the filler had corrugated and fluted topography. Modeling and simulation of epoxy nanocomposites were carried out using finite element method. It was observed that graphene based nano-fillers are efficient in scattering and dissipation of heat flux thereby increasing the thermal stability of epoxy nanocomposites. The macro-topography of bulk samples of monolithic epoxy and nanocomposites was modified by treating the samples with the abrasive papers. It was observed that surface notches, when exceed certain depth, cause degradation in mechanical properties. It was further observed that tensile properties are more sensitive to topography than flexural properties