Roey Nadiv

Graphene-Based Hybrid Composites for Efficient Thermal Management of Electronic Devices

Thermal management has become a critical aspect in next-generation miniaturized electronic devices. Efficient heat dissipation reduces their operating temperatures and insures optimal performance, service life, and efficacy. Shielding against shocks, vibrations, and moisture is also imperative when the electronic circuits are located outdoors. Potting (or encapsulating) them in polymer-based composites with enhanced thermal conductivity (TC) may provide a solution for both thermal management and shielding challenges. In the current study, graphene is employed as a filler to fabricate composites with isotropic ultrahigh TC (>12 W m(-1) K(-1)) and good mechanical properties (>30 MPa flexural and compressive strength). To avoid short-circuiting the electronic assemblies, a dispersion of secondary ceramic-based filler reduces the electrical conductivity and synergistically enhances the TC of composites. When utilized as potting materials, these novel hybrid composites effectively dissipate the heat from electronic devices; their operating temperatures decrease from 110 to 37 °C, and their effective thermal resistances are drastically reduced, by up to 90%. The simple filler dispersion method and the precise manipulation of the composite transport properties via hybrid filling offer a universal approach to the large-scale production of novel materials for thermal management and other applications.

Optimal nanomaterial concentration: harnessing percolation theory to enhance polymer nanocomposite performance

A major challenge in nanocomposite research is to predict the optimal nanomaterial concentration (ONC) yielding a maximal reinforcement in a given property. We present a simple approach to identify the ONC based on our finding that it is typically located in close proximity to an abrupt increase in polymer matrix viscosity, termed the rheological percolation threshold, and thus may be used as an indicator of the ONC. This premise was validated by rheological and fractography studies of composites loaded by nanomaterials including graphene nanoribbons or carbon or tungsten disulfide nanotubes. The correlation between in situ viscosity, the rheological percolation threshold concentration and the nanocomposite fractography demonstrates the utility of the method.

Cement Reinforcement by Nanotubes

Loading a matrix with nano-sized particles such as nanotubes (carbon or tungsten di-sulfide) is expected to improve the mechanical properties of composite materials better than traditional (macroscopic) fillers due to extra-ordinary mechanical properties accompanied by high surface area. One of the major challenges towards achieving this goal is an effective dispersion of the as-produced aggregated nanotubes. In this work we demonstrate a novel dispersion method, facilitating the integration of individual nanotubes in cement paste matrix. We demonstrate the effectiveness of our nanotubes dispersion method by enhancing both flexural strength and compressive strength of cement paste using carbon and tungsten di-sulfide nanotubes. Finally, a comprehensive fractography study indicates that both types of nanotubes fail via pull-out mechanism with an intermediate state of bridging mechanism.

Improved Bonding of Carbon Fiber Reinforced Cement Composites by Mineral Particle Coating

The low penetrability of cement product into the interior of multifilament yarns hinders the utilization of textile reinforced concrete (TRC). This inferior fiber-matrix bond often leads to premature failure. Therefore, improved bonding of such textile reinforced composite is much needed. This study focuses on the improvement of multifilament carbon yarns’ bond to cement matrix by mineral particle coatings. A carbon multi-filament was impregnated by micro-sized alumosilicate, nano- and micro-sized silica. The penetrability of the particles into the multifilament was characterized and the effects of the coatings on the fiber/matrix bond were compared to an epoxy-polymer coated-carbon multifilament. The micro-sized silica and alumosilicate coatings coated the yarn efficiently and enhanced both pull-out strength and toughness as compared to both nano-silica and epoxy coatings. This was mainly attributed to superior bonding due to a postulated pozzolanic effect. Contrarily, the nano-silica failed to show the same beneficial effects due to extensive agglomeration. It is concluded that coating of carbon multifilament with mineral fillers should be highly considered for TRC. However, the filler has to be thoroughly deagglomerated prior to the coating step.