Browsing by Subject "Nucleation"

Now showing 1 - 5 of 5
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    Early stages of zeolite growth.
    (2010-08) Kumar, Sandeep
    Zeolites are crystalline nonporous aluminosilicates with important applications in separation, purification, and adsorption of liquid and gaseous molecules. However, an ability to tailor the zeolite microstructure, such as particle size/shape and pore-size, to make it benign for specific application requires control over nucleation and particle growth processes. But, the nucleation and crystallization mechanisms of zeolites are not fully understood. In this context, the synthesis of an all-silica zeolite with MFI-type framework has been studied extensively as a model system. Throughout chapters 2, 4 and 5, MFI growth process has been investigated by small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). Of fundamental importance is the role of nanoparticles (~5 nm), which are present in the precursor sol, in MFI nucleation and crystallization. Formation of amorphous aggregates and their internal restructuring are concluded as essential steps in MFI nucleation. Early stage zeolite particles have disordered and less crystalline regions within, which indicates the role of structurally distributed population of nanoparticles in growth. Faceting occurs after the depletion of nanoparticles. The chapter 6 presents growth studies in silica sols prepared by using a dimer of tertaprpylammonium (TPA) and reports that MFI nucleation and crystallization are delayed with a more pronounced delay in crystal growth.
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    Fully-renewable and degradable thermoplastic elastomers.
    (2009-01) Wanamaker, Carolyn Leigh
    The most common polymers derived from renewable feedstocks, poly(3-hydroxybutyrate), polyglycolide, and polylactide (PLA), have high stiffness and tensile strength, but are inherently brittle, thus limiting the potential for these polymers to replace elastic and ductile polymers derived from fossil fuels. The work described in this thesis was directed toward expanding the properties of renewable resource polymers through the investigation of completely-biorenewable thermoplastic elastomers. Polymenthide (PM), a soft biorenewable polymer derived from (-)-menthol, is immiscible with PLA and was utilized as the middle block in a PLA-containing ABA triblock copolymer. Tensile measurements demonstrated impressive elongations and elastomeric properties typical of thermoplastic elastomers, however, the materials were relatively weak. The tensile properties of the polymers were found to be highly dependent on the molecular weight and crystallinity of the polylactide blocks. Substituting the amorphous PLA with semi-crystalline PLLA or PDLA significantly improved the strength of the material. Blends of the enantiomeric triblock copolymers further increased the strength through stereocomplexation of the enantiomeric polylactide segments. These results led to the investigation of stereocomplexed micelles as nucleating agents for PLLA. Quantifiable improvements in the nucleation efficiency of PLLA were observed when blending PLLA with PDLA-containing triblock copolymers. Finally, potential applications of these all-biorenewable triblock copolymers were investigated through hydrolytic degradation and adhesion studies. During hydrolytic degradation, the triblock copolymers were able to maintain a significant amount of their mechanical properties for many weeks.
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    Modeling and simulation of homogeneous nucleation in turbulent flows: physics, methods and realizable solutions
    (2013-03) Murfield, Nathan James
    Numerical simulations of nanoparticle nucleation in turbulent shear flows are performed. We consider the homogeneous nucleation of dibutyl-phthalate (DBP) nanoparticles via direct numerical simulation (DNS) and large-eddy simulation (LES). The flows consist of a high-temperature, DBP-laden stream issuing into a low-temperature, faster or slower moving, DBP-free environment. As the flows cool, via molecular and large- scale convective mixing, the DBP vapor becomes highly supersaturated and particles are formed by nucleation. This particle formation takes place in the absence of condensation or coagulation. Classical nucleation theory is used to model particle nucleation and the Navier-Stokes equations are coupled with the scalar transport equations to provide the fluid, thermal, and chemical fields. The effects of large-scale mixing and vapor concentration on homogeneous nucleation rates are investigated via DNS in three-dimensional planar jets. The simulation results provide a demonstration of how nucleation takes place in narrow regions where molecular mixing of the two streams occurs. When maximum nucleation rates occur in conditions where the nucleation rates are sensitive to ambient conditions, islands of nucleation form. There are two possible nucleation events: initial shear layer nucleation, and later nucleation in coherent structures or eddies generated by the velocity difference between the jet and the co-flow. A scatter plot diagram of observed dilution paths in temperature versus condensable vapor concentration space where nucleation rates are superimposed is shown to be a convenient tool for analyzing nucleation events. Convection by large-scale eddies gradually spreads the range of mixing paths in this space towards higher nucleation rates. The results also show that boundary conditions, including inlet concentration and velocity ratio, have both qualitative and quantitative effects on particle nucleation. The effects of Lewis number on the homogeneous nucleation of DBP particles are also studied via DNS. Simulations at two Lewis numbers are performed to investigate the effects of molecular mixing on nucleation. These simulations are also carried out at two co-flow velocities to assess the effects of large-scale mixing. The results show that the Lewis number as well the level of large-scale mixing inherent in the flow have substantial effects on particle nucleation. The effects of the subgrid-scale (SGS) scalar interactions on nanoparticle nucleation are investigated via a priori analysis of DNS data. To assess the effect of SGS scalar interactions on DBP particle nucleation, the temperature and mass-fractions are filtered and the resulting quantities are used to compute the nucleating particle field. Two filter widths are used to obtain varying levels of SGS interactions. Particle size distributions are computed to examine the particle fields produced. This work shows that the SGS interactions' effect on nucleation has two distinct trends. In the proximal region of the flow, the unresolved interactions act to decrease particle formation. However, as the flow transitions or becomes turbulent the effect of the SGS interactions acts to increase particle formation. In the DNS, all relevant length and time-scales are resolved while in LES a closure is used to represent the SGS stress, and fluid-scalar fluxes. We perform the LES at two resolutions to illustrate the effect of "resolving less" and "modeling more". Additionally, to illustrate the effects of the SGS interactions on homogeneous nucleation in turbulent flows, the unresolved scalar-scalar interactions appearing in nucleation source term are neglected. The results again show that nucleation initially occurs in the shear layers where molecular transport dominates and across the span of the wake once the core collapses and the flow transitions to turbulence. Pre-transition, the saturation ratio - representative of the driving force for particle formation - predicted by the LES and DNS is quite similar. Post-transition, the saturation ratios predicted by the LES are significantly greater than those predicted by the DNS, and the discrepancy increases as the filter-width increases (and resolution decreases). This dynamic is also reflected in the nucleation rate. The LES predicts nucleation rates between one and two orders of magnitude greater than the DNS and the discrepancy increases as the resolution decreases. There is also a shift towards the nucleation of smaller nanoparticles in the LES compared to the DNS. The results suggests that the SGS interactions act to decrease the rate of nanoparticle nucleation and increase nuclei size. The compute time between the DNS and LES decreased by three orders of magnitude, suggesting that SGS closures for nucleation would be a significant addition to simulation capabilities and tools. The work concludes with a discussion of a probabilistic method able to resolve these issues, which are inherent to LES.
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    New particle formation: sulfuric acid and amine Chemical nucleation photochemical reaction chamber studies and the laboratory cluster-CIMS.
    (2012-02) Titcombe, Mari E.
    The formation of new particles from gas phase condensation has been shown to significantly enhance concentrations of cloud condensation nuclei (CCN) in the Earth's atmosphere. Particles that have grown to CCN size contribute significantly to Earth's radiation balance. And particle nucleation has been observed throughout the atmosphere in varying meteorological conditions. Yet the chemical processes involved in particle nucleation are not well understood. Sulfuric acid has long been recognized as a contributor to new particle formation. However, sulfuric acid condensation alone cannot account for high particle production and growth rates observed in many regions of the atmosphere. Scientific understanding of these processes has been limited by available instrumentation and the chemical complexity of the atmosphere. A novel Chemical Ionization Mass Spectrometer (cluster-CIMS) has been developed, in collaboration with colleagues at the National Center for Atmospheric Research, to characterize homogeneously nucleated molecular clusters produced in a controlled laboratory environment. The cluster-CIMS gently ionizes neutral molecular clusters for quadrupole mass filtration with a minimum of disturbance to cluster composition. It is capable of characterizing particles from molecular sizes up to 1.5 nm in diameter with a resolution of +/- 1 amu. A climate controlled photochemical reaction chamber, designed as a 1000 L batch reactor, was built to produce nucleated molecular clusters at atmospherically relevant conditions. Laboratory experiments were conducted to elucidate potential molecular candidates for particle nucleation. The role of amines in particle formation was experimentally examined after atmospheric observations revealed enhanced sulfuric acid nucleation rates in the presence of amine compounds. Experimental results obtained with the cluster-CIMS, as well as other aerosol instrumentation, support the hypothesis that amines enhance sulfuric acid nucleation rates.
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    Study of the mechanism of nucleation in the polluted atmospheric boundary layer
    (2013-04) Chen, Modi
    Atmospheric aerosols can affect human health and earth's radiation balance. The formation of these aerosols has been shown to cast high uncertainty in current global climate modeling. Most observed nucleation events in the boundary layers are correlated with high sulfuric acid concentration. Nucleation rates are usually proportional to sulfuric acid concentration up to the third power. After atmospheric aerosol particles are formed, they often grow at a speed faster than can be explained by sulfuric acid condensation, suggesting that other chemical species also participate in this process. The detailed mechanisms of how these particles are formed and their subsequent growth are still unclear. This work is focused on furthering our understanding of atmospheric nucleation. My contribution is mainly on the following three topics: (1) characterizing condensation particle counters (CPCs) for accurate particle measurements down to 1 nm, the size close to the smallest stable sulfuric acid clusters; (2) developing a method of estimating time and size resolved particle growth rates and atmospheric nucleation rates based on data from both atmospheric and laboratory studies; (3) deriving of a simple semi-empirical acid-base reaction model for atmospheric nucleation in the polluted atmospheric boundary layer.