Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z26: Composite and Porous Media |
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Sponsoring Units: DCMP Room: 328 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z26.00001: Flexibility of zeolite frameworks Vitaliy Kapko, Michael Treacy, Michael Thorpe Zeolites are an important class of industrial catalysts because of their large internal surfaces and molecular-sieving properties. Recent geometric simulations (1) show that almost all of the known zeolites can exist without distortion of their tetrahedra within some range of densities, which we call the flexibility window. Within this window, the framework accommodates density changes by rotations about the shared tetrahedral corners. We argue that the presence of a flexibility window can be used as a topological criterion to select potential candidates for synthesis from millions of hypothetical structures. We also investigate the exceptions to the rule, as well as the shape of the flexibility window and the symmetric properties of zeolites inside it. (1) A. Sartbaeva, S.A. Wells, M.M.J. Treacy and M.F. Thorpe The flexibility window in zeolites, Nature Materials 5, 962-965 (2006); I. Rivin, commentary 931-932. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z26.00002: Electronic and ionic conduction in oxo-vanadium arsenates Victoria Soghomonian Electrically conducting microporous zeolite-like or zeoate frameworks are largely unstudied as electronic materials, but may offer new avenues in catalysis and in electrical energy storage applications. Zeolites and zeoates are characterized by the presence on nanoscale channels and cavities delineated by their crystalline framework, and occur naturally or can be synthesized by inorganic methods. The material under discussion here, an oxo-vanadium arsenate system, combines the properties of the well known but electrically insulating microporous zeolites, with an electronically active framework. We present the structure and the experimentally measured electronic and ionic conductivities of the materials system, the temperature dependences of the conductivities, and discuss possible electronic and ionic conduction mechanisms at play in empty frameworks and ion-exchanged frameworks. We also discuss how microporous electrically conducting frameworks can find use in electrical energy storage, and compare the zeoate frameworks in such applications to other materials systems such as mesoporous carbon. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z26.00003: Synthesis of hybrid zeolite materials with TiO2 nanocrystals using solid-solid method Corina Orha, Carmen Lazau, Cornelia Ratiu, Paula Sfirloaga, Paulina Vlazan, Alexandra Ioitescu, Florica Manea, Paul Barvinschi, Ioan Muscutariu, Ioan Grozescu Zeolite seems to be a promising support for TiO2 photocatalyst because of its regular pores and good adsorption ability. TiO2 supported on zeolite integrates the photocatalytic activity of TiO2 with the adsorption properties of zeolites. The aim of this paper was the syntheses and characterizations of functionalized zeolite materials with undoped, Fe-doped and N-doped TiO2 nanocrystals. The zeolite hybrid materials impregnation with titanium dioxide was achieved through solid-solid method. TiO2 doped with metallic (Fe) and non-metallic (N) ions was obtained directly from precursors by sol-gel and hydrothermal methods. The hybrid materials were characterized by XRD, SEM with EDAX, IR and AFM. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z26.00004: Carbon dioxide and methane transport in DDR zeolite: insights from molecular simulations into carbon dioxide separations in small pore zeolites Sang eun Jee, David Sholl Zeolites are good candidates as a membranes for chemical separations because of their excellent chemical and thermal stability. Cage type zeolites are promising materials for gas separation since their narrow windows are expected to control molecular transport. DDR is one of the strongest candidates for light gas separations because of its narrow 8MR window. In our study, we examined the separation selectivity of DDR for CO$_{2}$/CH$_{4}$ separation using atomistic simulation methods. We introduced new force fields which can reproduce experimental single component adsorption and diffusion data for this material for the first time. Previously interatomic potentials that have been applied to DDR overestimate experimental diffusivities at least one order of magnitude. We characterized single-component and binary adsorption using Grand Canonical Monte Carlo, and single-component. diffusion using a combination of Molecular Dynamics and Transition State Theory. The most important observation from our calculation is that CO$_{2}$/CH$_{4}$ diffusion in DDR is very different from the usual situation in nanoporous materials, where the presence of a slowly diffusing species retards transport rates of a more rapidly diffusing species. In DDR, we show that CO$_{2}$ diffusion rates are only weakly affected by the presence of CH$_{4}$, despite the very slow diffusion of the latter species. The physical origins of this unusual behavior are explained by analyzing the adsorption sites and diffusion mechanism for each species. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z26.00005: Effects of inhomogeneous partial absorption and the geometry of the boundary on the population evolution of molecules diffusing in general porous media Seungoh Ryu, David L. Johnson We consider the diffusion-relaxation dynamics in porous media with partially absorbing boundary conditions. Spectral analysis of Helmholtz equation for the uniform boundary condition has been widely used as a probe of geometry of the medium. The NMR relaxation of the fluid magnetization, for example, is used for a variety of media such as rocks, cement, bones, and cheese. While direct relationship between their geometry and the spectrum forms the basis for such applications, little attention has been paid to the spatial variation of the boundary condition. We report on the way the geometry and such inhomogeneity become intertwined and affect the spectrum. It is often impossible to gauge how severe such interference is in the biological or geophysical experiments. We develop a perturbative theory and numerical techniques and test for cases for which exact solution is obtained. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z26.00006: Decomposing First Passage Random Walks Lawrence Schwartz, David Johnson, Sidney Redner We develop a simulation method to model the time dependence of diffusion in composite materials with a wide range of pore sizes. Here, first passage techniques are useful because they allow a walker to move efficiently through the large open regions of the pore space.\footnote{Toumelin et at. J. Mag. Res. 188, 83 (2007).} However, because one does not keep track of each intermediate position, these techniques are not well suited to calculating the time development of the effective diffusion coefficient, D(t). To address this problem we show that first passage propagation can be decomposed in terms of a sequence of intermediate probability distributions. For example, given a first passage walk from the origin to the surface of a sphere of radius R in a time t, we can evaluate the probability distributions for the particle's location at any earlier time t$'$. We will illustrate the behavior of these intermediate distributions with a series of examples in one and three dimensions. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z26.00007: Thermally Stable Mesoporous Silica Spheres synthesized under Mild Conditions Christopher Ziegler, Eunyoung You, James Watkins Thermally stable, mesoporous silica spheres were synthesized using a one-pot technique under mild conditions. As-calcined silica spheres were shown to be highly porous with surface areas greater than 1000 m$^{2}$/g and pore volumes on the order of 1 cc/g. Pore walls were found to be highly resistant to collapse as a consequence of thermal treatment at temperatures exceeding 750\r{ }C or hydrothermal treatment in boiling water at temperatures exceeding 100\r{ }C for over 100 hours. $^{29}$Si-$^{1}$H cross polarization NMR data indicate that the silica is highly condensed at the surface providing rationale for the exceptional pore wall stability observed. The mesoporous silica spheres were synthesized from tetraethyl orthosilicate (TEOS) at room temperature and near-neutral pH using cysteamine and cetyltrimethylammonium bromide (CTAB) in a mixed water and ethanol system. Sphere size was shown to be tunable by altering the relative amounts of ethanol, CTAB, or TEOS. Sphere diameters ranging from 30 nm to 560 nm were observed. The preparation method and characterization of these highly condensed, thermally stable, mesoporous silica spheres for applications including sensing, catalysis, purification, and payload encapsulation is presented. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z26.00008: Ordered and disordered pores in porous anodic alumina: Partial equilibrium results from hexagonal lattice Arief Budiman, Daniel Lo Cluster variation method is employed to find equilibrium states of pore arrangement in porous anodic alumina. Our work is motivated by a need to predict the extent of pore ordering in the anodic alumina. The pores are assumed to occupy a hexagonal two-dimensional lattice and interact with each other through their elastic deformations. Alumina-electrolyte interface energy and metal-oxide interface stress are included in the analysis. Equilibrium ordered and disordered states are obtained. Spatial pore arrangements of these states will be presented. Incorporation of ionic transports in the electrolyte and aluminum-to-alumina reactions to the model will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z26.00009: Nanoporous Substrate with Mixed Nanoclusters for Surface Enhanced Raman Scattering. Sehoon Chang, Hyunhyub Ko, Srikanth Singamaneni, Ray Gunawidjaja, Vladimir Tsukruk Rapid detection of plastic and liquid explosives is an urgent need due to various societal and technological reasons. We employed a novel design of surface enhanced Raman scattering (SERS)-active substrate based on porous alumina membranes decorated with mixed nanoclusters of gold nanorods and nanoparticles. We demonstrated trace level detection of several important explosives such as dinitrotolene (DNT), trinitrotoluene (TNT), and hexamethylenetriperoxidediamine (HMTD) by fast, sensitive, reliable Raman spectroscopic method. We achieved near molecular-level detection (about 15$\sim $ 30 molecules) of DNT and TNT utilizing the SERS substrate. However, trace level detection is challenging due to the lack of common optical signatures (fluorescence, absorption in UV-vis range) or chemical functionality of peroxide-based explosives such as HMTD. To overcome this, we employed photochemical decomposition approach and analyzed chemical fragments using SERS. We suggest that tailored polymer coating, mixed nanoclusters, and laser-induced photocatalytic decomposition are all critical for achieving this unprecedented sensitivity level.. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z26.00010: Sorption Isotherms in Networked Nanoporous Media: Do the Parts Equal the Whole? Casey E. Chiang, Felix Casanova, Anne M. Ruminski, Michael J. Sailor, Ivan K. Schuller Sorption isotherms are a very important tool in the characterization of nanoporous materials. However, there is still controversy as to how capillary condensation and evaporation transitions along the isotherm correlate with the pore morphology (shape) and topology (within a network). We combined narrow ($<$10 nm) and wide ($>$30 nm) pores in nanoporous silicon to tailor the simplest networks: narrow above wide (ink-bottle) and vice versa (funnel). In addition, we compared these against their single-layered constituents. From sorption isotherms measured by optical interferometry, we observe that capillary \textbf{condensation} occurs identically in all pores with direct access to the gas reservoir and slightly delayed (delayed adsorption) when access is blocked, while capillary \textbf{evaporation} occurs identically in all pores with direct access to the gas reservoir and is delayed (pore-blocking) until direct access is allowed otherwise. These experimental results allow us to understand the global capillary behavior in nanoporous silicon. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z26.00011: Mechanical stability of Metal Organic Framework-5. Wei Liu, Ju Li Metal organic frameworks (MOFs) are crystalline structures of metal ions bridged by organic linkers. They have been proved to be highly useful in gas storage, separation, purification and catalysis. Mechanical stability is very important for their applications in industry. We studied the stress-strain relations of MOF-5 (the prototypical MOF) under different temperatures via Molecular Dynamics method (MD). It has been found that under normal stress MOF-5 is relatively strong; while under shear stress it is easy to collapse. Furthermore, under both normal and shear stress condition, its stability becomes weaker as temperature increases. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z26.00012: Smart nanoporous preconcentrator of explosives based on MOF5 Khorgolkhuu Odbadrakh, James Lewis We present investigations of interactions of explosive molecules RDX and TATP with metal organic framework MOF-5, using DFTY based ab-initio simulation method FIREBALL. Energetics studies in bulk shows that only one of the binding sites of RDX in MOF-5 suggested by quantum chemistry calculations confirm. The absorption site is on a linker of the framework through 2-(OH) bond. However, surface interactions are stronger, with significantly higher binding barriers. We confirmed two adsorption sites on the surface: one with the linker and the other on a connector of the framework through 3-(OH) bonds. The stronger interactions on the surface suggest importance of size, and surfaces of MOF nanoparticles in precontentrating the explosive molecules in the framework. Ab-initio Molecular Dynamics simulations show that the absorption of the RDX in MOF-5 is highly sensitive to temperatures, suggesting high diffusion rates for the explosive molecules at room temperature. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z26.00013: Optical birefringence in Compressed Aerogels P. Bhupathi, J. Hwang, R. M. Martin, L. Jaworski, D. B. Tanner, Y. Lee, J. Blankstein, N. Mulders We performed optical birefringence measurements on 98{\%} porosity silica aerogel samples subjected to various degrees of uniaxial compression up to 15{\%} strain over a wide range of wavelength, 200 to 800 nm. Silica aerogels are composed of an entangled network of 3 - 5 nm diameter SiO$_{2}$ strands and can be synthesized in a wide range of porosity, especially in the high porosity limit close to 99.9{\%}, yet maintaining good mechanical stability. Uncompressed aerogels exhibit no or minimal degree of birefringence, indicating the isotropic nature of the material. Uniaxial compression of aerogel introduces global anisotropy, which produces optical activity in the material. We observed a quasi-linear strain dependence in \textit{$\vert \Delta $n$\vert =\vert $n}$_{e} - n_{o}$\textit{$\vert $} in compressed aerogels, where $n_{e(o)}$ is the index of refraction for the extraordinary (ordinary) ray which has its polarization parallel to the compression axis. This effect has potential applications for aerogels as tunable waveplates operating in a broad spectral range. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z26.00014: Microwave Absorption in Percolating Metal-insulator Composites Darin Zimmerman, Jeremy Cardellino, Kyle Cravener, Kelly Feather, Nicholas Miskovsky, Gary Weisel We measure several electromagnetic properties of tungsten-Teflon composites as a function of metal volume concentration. The electric (E) and magnetic (H) loss tangents at 2.45 GHz and the dc conductivity each exhibit a percolation transition at a different critical value of the metal volume fraction, $p$. Moreover, the transition behavior depends on the average particle size and size distribution of the metal component. We explain the variation in each case by a schematic model derived from established percolation theory and the distinct response of conducting particles to microwave electric and magnetic fields. [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z26.00015: Anisotropy of induced polarization in the context of the generalized effective-medium theory. Vladimir Burtman, Michael S. Zhdanov, Alexander Gribenko The rock samples are examples of heterogeneous complex structure material. Modeling of electromagnetic response of this medium makes it possible to study the anisotropy of induced polarization (IP) effect. The IP effect is studied it in the context of the developed generalized effective-medium theory of induced polarization (GEMTIP). The effective-medium conductivity defined by the GEMTIP model, in a general case, is represented by a tensor function. This tensorial property of the effective-medium conductivity provides a new insight in the anisotropy phenomenon in the IP effect. As an example, we consider a multiphase composite polarized model of a rock formation with ellipsoidal inclusions. We demonstrate that the effective conductivity of this formation may be anisotropic, even if the host rock and all the grains are electrically isotropic. [Preview Abstract] |
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