Browsing by Subject "Nano"
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Item Combustion behavior of sol-gel synthesized aluminum and tungsten trioxide(Texas Tech University, 2006-05) Prentice, DanielCalcined (to remove impurities) and non-calcined tungsten trioxide (WO3) aerogels as well as micron-scale and nano-scale commercial WO3 powders were mixed with nano-scale aluminum (Al) and their combustion performance in the form of combustion wave speeds was compared in loose powder and pressed pellet configurations. Results show that both the calcined and non-calcined, aerogel based mixtures outperformed the commercial based mixtures in both configurations. Combustion wave speed was also found as a function of mixture bulk density. Results show that conduction is the dominant energy transfer mechanism in pressed pellets while convection is the dominant mechanism in loose powder form. This causes material purity to be the most important factor for pressed pellets and oxidizer particle size to be the most important factor for loose powders. A preliminary aging study was conducted which showed a 7% performance reduction after 4 days of laboratory air exposure and 91-98% performance reduction after 22 months of exposure.Item Fabrication and characterization of open celled micro and nano foams(2013-08) Srinivas Sundarram, Sriharsha, 1985-; Li, Wei, doctor of mechanical engineeringOpen 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.Item Hierarchical three-dimensional Fe-Ni hydroxide nanosheet arrays on carbon fiber electrodes for oxygen evolution reaction(2014-05) O'Donovan-Zavada, Robert Anthony; Manthiram, ArumugamAs demands for alternative sources of energy increase over the coming decades, water electrolysis will play a larger role in meeting our needs. The oxygen evolution reaction (OER) component of water electrolysis suffers from slow kinetics. An efficient, inexpensive, alternative electrocatalyst is needed. We present here high-activity, low onset potential, stable catalyst materials for OER based on a hierarchical network architecture consisting of Fe and Ni coated on carbon fiber paper (CFP). Several compositions of Fe-Ni electrodes were grown on CFP using a hydrothermal method, which produced an interconnected nanosheet network morphology. The materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Electrochemical performance of the catalyst was examined by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The best electrodes showed favorable activity (23 mA/cm², 60 mA/mg), onset potential (1.42 V vs. RHE), and cyclability.Item Modeling the melt dispersion mechanism for nanoparticle combustion(2007-12) Francis, Andrew; Pantoya, Michelle; Levitas, Valery; Oler, James W.Thermite particles have long been known to increase in reactivity as they decrease in size. However, during fast heating (106 - 108 K/s) of Al nanothermites, the diffusion mechanism that explains micron size thermite reactions cannot explain the extremely fast ignition times and much higher flame propagation velocities. A new mechanism known as the melt dispersion mechanism has recently been introduced to explain the fast oxidation of these Al nanothermites. A model has been created dependant upon key parameters to predict the reactivity of Al nanothermites. In this study, flame propagation velocities are statistically evaluated in terms of an integral that employs a probability density function (pdf) for key parameters and a flame velocity equation dependent on relative particle size (Al core radius divided by oxide shell thickness), oxide shell formation temperature, and oxide shell strength. It is shown that flame propagation velocity depends sensitively on relative particle size, relative particle size distribution, oxide shell formation temperature, and shell strength. It is also dependant upon particle size, and oxide shell thickness but not as sensitively. Both single and bimodal particle sizes were studied. Combining smaller nanoparticles with larger nanoparticles in a bimodal mixture significantly increases the flame propagation velocity as compared to a composite consisting of only the larger particles. The results presented here suggest that better reproducibility of the flame velocity may be achieved experimentally by selecting a material with a narrow relative particle size distribution. A combination of increased oxide shell formation temperature and increased oxide shell strength could be used to maximize the flame velocity in particles with increased relative particle size.Item Scale effects on the latent heat of phase change & the effect of dynamic contact angles on dynamic capillary pressure(2014-12) Shin, Jeong-Heon; Deinert, Mark; Shi, Li; DiCarlo, David; Halil Berberoglu; Bogard, David G.Surface tension is an important material property that affects the behavior of micro/nano size thermal-fluid systems. In this dissertation, I investigate how surface tension affects the latent heat of a phase change in nanoscale systems as well as on the movement of water in microstructures. Classical thermodynamic models were developed to describe how the latent heat of melting in nano-pores depends on scale and were extended to the melting of metallic nano-particles. The results from these models were verified by comparison with experimental data from the open literature for hydrocarbons and water in nano-size pores, as well as for free standing metallic nano particles. A classical thermodynamic model was also developed to describe how the latent heat of vaporization depends on scale. This was verified experimentally using a Thermogravimetric Analysis/Differential Scanning Calorimeter available in the core facilities of the Texas Materials Institute. This verified that the latent heat of vaporization for water confined nano-pores decreases with pore size. A model for dynamic capillary pressure in porous media was analyzed using experimentally derived data for the velocity dependent contact angle of water on SiO₂ glass. The data were derived from images of microfluidic flows in capillary tubes, obtained using high speed digital microscopy.