H. Thomas Hahn

Intercalation and exfoliation routes to graphite nanoplatelets

Graphite nanoplatelets with thicknesses down to 2–10 nm are synthesized by alkali metal intercalation followed by ethanol exfoliation and microwave drying. Graphite that has already been intercalated and exfoliated with an oxidizing acid is reintercalated with an alkali metal to form a first stage compound, as confirmed by powder X-ray diffraction. This can be achieved either by heating graphite and potassium or caesium at 200 °C, or at room temperature using a sodium–potassium alloy. Reaction of the intercalated graphite with ethanol causes exfoliation of the graphene layers. Microwave radiation aids in drying and results in further separation of the sheets. Thermogravimetric analysis indicates that the graphite nanoplatelets are approximately 150 °C less stable in air than pristine graphite. High aspect ratio graphite nanoplatelets offer promise as reinforcements for high strength carbon–carbon composites.

Surface functionalized alumina nanoparticle filled polymeric nanocomposites with enhanced mechanical properties

Alumina nanoparticles were successfully functionalized with a bi-functional coupling agent, (3-methacryloxypropyl)trimethoxysilane (MPS), through a facile neutral solvent method. MPS was found to be covalently bound with the nanoparticles. The linked MPS was polymerized with a vinyl-ester resin monomer through a free radical polymerization. Atomic force microscope phase images showed a uniform distribution of nanoparticles. Microtensile test results revealed the Youngs modulus and strength increasing with particle loading. Microscopic examinations revealed the presence of large plastic deformations at the micron scale in the nanocomposites in agreement with the observed strengthening effect of functionalized nanoparticles. Thermo-gravimetric analysis (TGA) did not show any significant change in the thermal degradation of the nanocomposite as compared with the neat resin. The polymer matrix effectively protected the alumina nanoparticles from dissolution in basic and acidic solutions.

Particle surface engineering effect on the mechanical, optical and photoluminescent properties of ZnO/vinyl-ester resin nanocomposites

Zinc oxide (ZnO) nanoparticles functionalized with a bi-functional coupling agent methacryloxypropyl-trimethoxysilane (MPS) were used to fabricate a vinyl-ester resin polymeric nanocomposite, which shows an improved interfacial interaction between the particle and matrix. As a result, in comparison to the unmodified particle-filled nanocomposites, the functionalized particle-filled composites possessed higher resistance to thermal degradation, and demonstrated improved UV shielding and enhanced photoluminescent properties. The more uniform particle dispersion, passivation of the particle surface with MPS and increased oxygen vacancies were justified to contribute to the increased thermal stability and the enhanced photoluminescent properties. Significant tensile strength improvement was closely related to the observed uniform particle distribution and the intimate interfacial interaction through the strong chemical bonding.

Formation of microvoids during resin-transfer molding process

Abstract Voids in a composite part are deleterious because they degrade its strength and modulus. In resin-transfer molding (RTM), voids result mainly from inhomogeneous fiber architecture. Such inhomogeneity leads to non-uniform permeability of the fiber preform, which in turn causes the resin velocity to vary from point to point at a micro scale. The capillary pressure, which also prevails at this length scale, exacerbates the spatial variation of the resin velocity. The combined effect of pressure gradient and capillary pressure can be described by the capillary number. The resulting microscopic perturbations in the resin-flow front allow voids to form. The present paper proposes a mathematical model to describe the mechanisms of void formation. The existing data are used to validate the assumptions introduced. The model is then used to analyze new data from one-dimensional RTM experiments.

Towards Development of a Self-Healing Composite using a Mendable Polymer and Resistive Heating

Mendomers are a group of polymers that are mendable upon heating. Specifically, cracks in these polymers have been shown to heal themselves when heated close to the glass transition temperature. The main mechanism behind the healing is the thermally reversible Diels—Alder reaction, where a dicyclopentadiene unit in the polymer backbone breaks apart into two cyclopentadiene terminal groups, which then reunite upon heating. The present study investigates the feasibility of using a mendomer as a matrix for re-mending composites reinforced with graphite fibers. The graphite fibers are used as electrical conductors to provide the necessary heat to the polymer. Specimens were prepared by spreading a monomer, called mendomer, powder on a graphite/epoxy laminate substrate and curing in a vacuum oven. Microcracks were introduced by bending the substrate coupon, and the latter was heated by applying electric currents. The healing behavior was confirmed by disappearance of microcracks that were observed with an optical microscope and a scanning electron microscope (SEM).

Magnetic and electromagnetic evaluation of the magnetic nanoparticle filled polyurethane nanocomposites

The magnetic and electromagnetic wave absorption behavior of a flexible iron-nanoparticle reinforced polyurethane nanocomposite is reported. Surface-initiated-polymerization (SIP) method was utilized to fabricate high-quality nanocomposites with uniform particle distribution and tunable particle loading (up to 65wt%). The enhancement of coercive force is observed when the nanoparticles are embedded into the polymer matrix. Electromagnetic wave absorption performance at a discrete frequency as studied by metal-backed reflection loss indicates that the SIP nanocomposites can save the weight up to 50% compared to the composite counterpart with micron-size particles.

Giant magnetoresistance behavior of an iron/carbonized polyurethane nanocomposite

This letter describes the magnetoresistance (MR) behavior of the heat treated polyurethane composites reinforced with iron nanoparticles. The flexible nanocomposites were fabricated by the surface-initiated-polymerization method. The uniformly distributed nanoparticles within the polymer matrix, well characterized by field emission scanning electron microscopy, favor a continuous carbon matrix formation, rendering the transition from insulating to conductive composites. The coercive forces reflect strong particle loading and matrix dependent magnetic properties. By simply annealing in a reducing environment, the obtained nanocomposites possess a MR of 7.3% at room temperature and 14% at 130K occurring at a field of 90kOe.

Processing and properties of SiC/vinyl ester nanocomposites

The feasibility of improving polymer composites was investigated using 30 nm SiC nanoparticles in a vinyl ester resin. Even when the particle loading was less than 4% by weight, the viscosity of the nanoparticle suspension was found to increase much higher than that of microparticle suspension. This phenomenon may be the result of association between nanoparticles and polymer molecules, effectively making the nanoparticles larger. The resulting reduction in the mobility of polymer molecules also led to delayed curing. Ultrasonic mixing did not fully disperse the particles. As a result, the composite strength did not improve although the modulus increased. The use of a dispersant, methacryloxy propyl trimethoxy silane (MPS), improved the dispersion quality and hence the composite strength. The paper discusses the issues involved with processing, characterization and properties of SiC/vinyl ester nanocomposites. Methods of improving the nanocomposite quality are proposed in the paper as well.

A finite element simulation of resin transfer molding based on partial nodal saturation and implicit time integration

Abstract A finite element (FEM) approach using an implicit time integration algorithm has been developed to simulate isothermal resin transfer molding process. The finite element formulation is based on the concept of partial saturation at the flow front. The discretized equation is essentially the same as is obtained by a finite element/control volume formulation. The FEM approach is easier to implement for different types of elements. Unlike the explicit time integration schemes, the proposed implicit algorithm uses fixed time steps and allows the flow to advance farther than a layer of flow front elements in a single time step. The resulting code RTMSIM has been validated by comparing its predictions with closed-form solutions for flat plates. A wicking analysis is provided to show the codes ability to deal with the effect of capillary pressure. Several other simulations are provided to demonstrate the accuracy and efficiency of the code.

Flexible high-loading particle-reinforced polyurethane magnetic nanocomposite fabrication through particle-surface-initiated polymerization

Flexible high-loading nanoparticle-reinforced polyurethane magnetic nanocomposites fabricated by the surface-initiated polymerization (SIP) method are reported. Extensive field emission scanning electron microscopic (SEM) and atomic force microscopic (AFM) observations revealed a uniform particle distribution within the polymer matrix. X-ray photoelectron spectrometry (XPS) and differential thermal analysis (DTA) revealed a strong chemical bonding between the nanoparticles and the polymer matrix. The elongation of the SIP nanocomposite under tensile test was about four times greater than that of the composite fabricated by a conventional direct mixing fabrication method. The nanocomposite shows particle-loading-dependent magnetic properties, with an increase of coercive force after the magnetic nanoparticles were embedded into the polymer matrix, arising from the increased interparticle distance and the introduced polymer?particle interactions.