Self-Assembled Monolayers and Polymer Films
This project focused on the preparation of alkane, dicarbamate, and partially fluorinated monolayers and polymer films in carbon dioxide and traditional organic solvents. Monolayers of partially fluorinated n-alkanethiols with varying degrees of fluorination were synthesized and characterized so that the influence of the solvent on the monolayer could be understood. To predict monolayer properties as a function of the degree of fluorination, models were created and it was found that many monolayers formed in carbon dioxide had better electrochemical and barrier properties than those formed in traditional liquid solvents. Atom transfer radical polymerization of 2-hydroxyethyl methacrylate from a surface followed by acylation with fluorinated acid chlorides was observed to produce surface-initiated films with fluorocarbon side chains. Notable achievements included the successful preparation of polymer films with extremely low critical surface energies (9 mN/m) and the determination of the fluorocarbon structure within the film. The film resistance (> 100 Ω•cm2) is the highest ever reported for a surface-initiated polymer film. This work was sponsored by the National Science Foundation.
Graphite Nanofibers as Catalysts
Three types of graphite nanofibers (GNFs), with varying orientations of the graphene sheets (herringbone, platelet, and ribbon), are used as catalysts for the gas-phase reactions. We study the effect of concentrations, temperature, residence time as well as fiber type on the selectivity and conversion of a variety of reactions. Our first study was on the oxidative dehydrogenation of ethanol to acetaldehyde and ethyl acetate in the presence of oxygen. The effects of fiber type, temperature, oxygen concentration, and residence time on conversion and product ratio were explored. When identical processing conditions were employed, herringbone fibers produced the highest conversions to ethanol. To increase conversion, the concentration fo oxygen used was increased which also tended to increase the percentage of acetaldehyde produced. As expected, temperature had a large effect on conversion and product ratios. We concluded that the oxygen groups terminating the prismatic edge sites of the graphene planes were responsible for the catalytic activity in oxidative dehydrogenation.
Graphite Nanofibers in Thermal Interface Materials (TIMs)
Thermal Interface Materials (TIMs) are designed to fill in poorly conducting air gaps between components such as heat sinks and microprocessors to allow for efficient thermal transport and prevent the overheating of critical electronic components. TIMs have come in many forms including thermal greases or pastes, phase change materials (PCMs), solders, and thermally conductive adhesive pads or tapes. Although thermal greases and pastes have the highest thermal conductivities they are usually oily-based so their application is messy, making them difficult to apply in a thin, uniform coat and they are not reusable. Moreover, in applications that involve high temperatures or extensive thermal cycling, the thermal greases have suffered from drying out or voiding from pumping out of the interface which increases the thermal barrier. Pads have become a more common TIM to avoid the difficulties with pastes; however, the thermal conductivity of commercially available pads are significantly lower (typically 2 orders of magnitude) then available pastes and hence do not provide for fast and effective thermal transport.
Our research group has developed a process for generating flexible TIMs using binder, plasticizer and graphite nanobifers (GNFs). GNFs are similar to carbon nanotubes (CNTs) in that they are made entirely of carbon; however, GNFs are significantly cheaper to manufacture, can be grown in a larger range of diameters and lengths significantly longer than CNTs, and have the ability to be highly conductive in more than one direction. Furthermore, with a simple change of growth catalyst and production conditions we can easily alter the direction in which the sheets of graphite orient themselves in GNFs. This work is currently sponsored by the National Science Foundation and the PA Keystone Innovation Zone.
Graphite Nanofibers in Phase Change Materials (PCMs)
Phase Change Materials (PCMs) are used to absorb and release thermal energy during transient heating. PCMs have been used successfully at small scales for solar energy storage, in energy efficient building materials, and in cooling portable electronics. In many larger systems, however, the low thermal conductivity of most PCMs results in poor utilization of the PCM mass. Recently, a nanoenhanced PCM with embedded, low cost graphite nanofibers (GNF) characterized large thermal conductivities, has been developed. The unique nanofibers are synthesized using a catalytic decomposition process and take the form of concentric basal planes along the fiber axis. Preliminary results have shown that, when blended into a PCM, the nanofibers improve thermal performance of the PCM. This research examines the fundamental nature of thermal transport in nanoenhanced PCMs during melting and solidification in (1) thin layers (0.5 mm to 5 mm) for use in building materials, thermal interface materials as well as other thin film applications, and in (2) thicker layers (20 mm to 200 mm) for solar energy storage, electronics thermal management and other high power energy systems.
The fundamental energy transport mechanisms responsible for the observed improvement in thermal performance of the nanoenhanced PCM will be determined. Computational research will be conducted by applying molecular dynamics (MD) modeling to various nanofiber styles. The MD model will be used to predict the nature of energy transport in the different fiber styles, and characterize the thermal conductivity of a single fiber. Stochastic simulations involving specific matrix configurations will be conducted to determine how the fiber styles affect the thermal performance of the nanoenhanced PCMs.
Experiments involving PCMs that are embedded with different types of nanoparticles allow direct comparison of the influence of each nanoparticle type on the PCM’s effective thermal conductivity. This provides the basis for which the influence of GNFs on the enhancement of the effective thermal may be compared to that brought about by other more expensive nanoparticles. This work is currently sponsored by the National Science Foundation and the Office of Naval Research.