Fabrication and characterization of open celled micro and nano foams
Srinivas Sundarram, Sriharsha, 1985-
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Open celled micro and nano foams fabricated from polymers and metals have attracted tremendous attention in the recent past because of their applications in numerous areas such as catalyst carriers, filtration media, ion exchange membranes and tissue engineering scaffolds. In this study open celled polymer micro- and nano foams with controllable pore size and porosity were fabricated via solid state foaming of immiscible blends. The polymer foams were used as templates for fabricating nickel foams using an ethanol based electroless plating process. Thermal conductivity of micro- and nano foams was studied as a function of pore size and porosity using finite element and molecular dynamics based models. The effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management was investigated via numerical models. Open celled micro foams were fabricated via solid state foaming of ethylene acrylic acid (EAA) and polystyrene (PS) co-continuous blends. Blending temperature was the main parameters affecting the formation of co-continuous structure. Gas saturation and foaming studies were performed to determine ideal processing conditions for the blend. The results indicated that saturation pressure and foaming temperature were major process parameters determining the porosity of the foamed samples. Open celled polymer templates were obtained by selective extraction of PS phase using dichloromethane (DCM). Foaming resulted in faster extraction of PS and also in a higher porosity. Open celled nano foams were fabricated via solid state foaming of polyetherimide (PEI) and polyethersulfone (PES). The effect of process parameters namely saturation pressure and temperature, desorption time, and foaming temperature and time on porosity and pore size was studied. A high gas concentration and foaming temperature were required to obtain nano pore-sized foams. Throughout the cross section there existed regions with varying pore size and porosity and solid skins at the surface regions of the foam. A solvent surface dissolution process using dimethylformamide (DMF) was employed to access the internal porous structure. Micro- and nano cellular nickel foams were fabricated from EAA and PES templates via electroless plating. The structure of the nickel foams was an inverse of the polymer templates. Ethanol based electroless plating solutions were used to ensure infiltration into the porous structure because of the small pore sizes. Finite element and molecular dynamics based models were developed to predict thermal conductivity of polymer foams as a function of pore size and porosity. Pore sizes ranging from 1 nm to 1 mm were studied. Models were partially validated using experimental data. The results showed that pore size has significant effect on thermal conductivity even for microcellular and conventional foams. When the pore size is reduced to the nanometer scale, the thermal conductivity of the nano foam dramatically reduces and the value could be lower than that of air for certain porosity levels. The extremely low thermal conductivity of polymer nanofoams is possibly due to increased phonon-phonon scattering in the solid phases of the polymer matrix in addition to low thermal conductivity of gas trapped in nano sized pores. Finite element based models were also developed to study the effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management applications. The results showed that foams with smaller pore sizes can delay the temperature rise of the heat source for an extended period of time by rapidly dissipating heat in the phase change material. The lower temperatures resulting from the use of a smaller pore size metal foam could significantly increase the lifetime of IC chips.