Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X30: Composite and Porous Media |
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Sponsoring Units: DCMP Chair: Stefano Curtarolo, Duke University Room: Morial Convention Center 222 |
Friday, March 14, 2008 8:00AM - 8:12AM |
X30.00001: Modeling of multiscale porous media B. Biswal, P.-E. Oren, R.J. Held, S. Bakke, R. Hilfer A continuum geometrical model for reconstructing three dimensional pore scale microstructure of multiscale porous media is presented. Pore scale geometries of different carbonate textures are successfully reconstructed with informations from thin sections. The reconstructed models incorporate correlations with the primordial depositional textures, scale dependent intergranular porosity over many decades, vuggy porosity, a percolating pore space, a fully connected matrix space, strong resolution dependence and wide variability in the permeabilities and other properties. A method to generate synthetic micro-CT images at arbitrary resolutions is also developed. The model can be extended to a wide class of multiscale porous media. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X30.00002: Direct observation of pore blocking and advanced adsorption in nanoporous alumina: cooperative effects at the origin of hysteresis Felix Casanova, Casey E. Chiang, Chang-Peng Li, Ivan K. Schuller We tailor anodized alumina with independent pores with well-defined, simple geometries (inkbottle, funnel), in order to study the effects of pore morphology in hysteretical capillary condensation, independently from other cooperative processes such as network effects. We confirm, by direct observation using optical interferometry, the occurrence of two cooperative phenomena: the classical pore blocking effect in nearly ideal `inkbottle' pores (which has usually been employed to describe hysteresis loops in disordered/interconnected porous materials) and the advanced adsorption in pores with a change in the cross section. Both effects have been predicted in theoretical and simulation works, but not directly observed experimentally before. They are relevant for the development of a theory of the poorly understood hysteresis in complex porous materials. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X30.00003: Acoustic Band Pass, Band Gap and Dispersion in Discrete Media at Micro and Nano Scales Hasson Tavossi Acoustic properties of models of crystals, when measured at macroscopic scale, are found experimentally to have remarkable similarities with the same wave properties observed at atomic and nano-scales. It can be shown that, elastic moduli and other wave properties such as; band-pass, forbidden band, wave tunneling, attenuation, cutoff-frequency, and dispersion, depend on the similar structural factors as for phonons in crystals. Acoustic properties of the macroscopic models of discrete media, in the length scale range; 1.5 mm to 30 micrometers, and the frequency range; audible to ultrasonic are studied. The Band-pass, band-gap, attenuation, and dispersion expressed in wave-number (ka), show similar characteristics as the phonons in solids. These findings can lead to a better understanding of the wave properties of solids at nanoscales. The readily analyzable wave models at large scale are convenient tools to verify experimentally the models for complex binary composites. Experimental findings and numerical results for wave properties of discrete structures at large scale are compared with atomic scale wave behavior of solids, for a wide range of frequencies, from audible to ultrasound, to show the common characteristics with the phonons behavior in solids. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X30.00004: Conducting-tip AFM Studies of Multi-Walled Carbon Nanotube/Polyimide Nanocomposites A. Trionfi, D. Scrymgeour, J.W.P. Hsu, M.J. Arlen, D. Wang, L.-S. Tan, R.A. Vaia Electrical transport studies of multi-walled carbon nanotube (c-MWNT)/polymer nanocomposites have shown metallic behavior with conductivity $\sigma =\sigma _0 \left( {\phi -\phi _c} \right)^t$ above the percolation threshold. The conductivity depends on three aspects of the conducting network (CN): the conductivity of the constituent c-MWNT, the number of c-MWNT making up the CN, and the detailed interconnectivity of the CN. Using conducting-tip atomic force microscopy (C-AFM), we have studied the density and conductivity of the c-MWNT CN as a function of c-MWNT loading between 0.5 - 5.0 wt {\%} in a polyimide matrix. Using the Principle of Delesse, the volume fraction of the c-MWNT CN can be calculated from the conducting areal density measured in the C-AFM scans. The results of the C-AFM tests have shown localized areas of electrical transport associated with c-MWNT as well a clear dependence of conducting areal density and conductivity on the c-MWNT loading. This work was performed in part at the US Department of Energy, Center for Integrated Nanotechnologies, at Los Alamos and Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X30.00005: Systematic Study of Microwave Absorption, Heating, and Microstructure Evolution of Porous Copper Powder Metal Compacts Darin Zimmerman, John Diehl, Earnie Johnson, Kelly Martin, Nicholas Miskovsky, Charles Smith, Gary Weisel, Brock Weiss, JunKun Ma We present a systematic study\footnote{ J. Ma, J. F. Diehl, E. J. Johnson, K. R. Martin, N. M. Miskovsky, C. T. Smith, G. J. Weisel, B. L. Weiss, and D. T. Zimmerman, J. Appl. Phys. \textbf{101}, 074906 (2007)} of the absorption, heating behavior, and microstructure evolution of porous copper powder metal powder compacts subjected to 2.45 GHz microwave radiation and explain our observations using known physical mechanisms. Using a single mode microwave system, we place the compacts in pure electric (E) or magnetic (H) fields and compare the heating trends. The observed trends in the E- and H-field heating reflect the dramatic changes in the conductivity, permittivity, and permeability of the samples caused by the microstructure evolution during heating in the two types of fields. The observed dependence of the initial microwave heating of the samples suggests that the microwave absorption in the sample is dominated by the properties of the individual metal particles composing the sample. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X30.00006: Do pores strengthen materials? -- A pore size effect analysis Catalin Picu, Mohan Nuggehally, Mark Shephard Pores with radius larger than several microns are known to reduce the yield and flow stress of ductile materials and to increase their toughness. In this work we discuss a new mechanism leading to an increase of the strength of a material by nanosized pores. We show that voids grow by the emission of dislocations. As the void radius is reduced, while their volume fraction is kept constant, the mean spacing between voids decreases and their number increases. This makes the concurrent dislocation nucleation from neighboring voids more difficult. The situation is equivalent to increasing the density of dislocation sources in the material. Furthermore, we show that the critical stress for dislocation nucleation form an isolated void also increases as the pore size is reduced. The analysis is performed using a computationally efficient adaptive atomistic-continuum method. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X30.00007: First-Principles Calculations for the Dislocations in Titanium Nitride Ramkumar Gudipati, Y.G. Shen, Wentao Xu, A.S. Rao, H.L. Dang, Sanwu Wang Nanostructured$^{ }$superhard materials have been successfully synthesized in recent years. The hardness of nanosuperlattices$^{ }$and nanocomposites significantly exceeds that of the component materials. While it is believed that the nanodimensions are needed to impede dislocation activity and grain-boundary sliding, relevant calculations are rare. We report first-principles density-functional calculations for the core structures and energetics of various dislocations including the [110]{\{}110{\}}, [110]{\{}111{\}}, [110]{\{}100{\}}, [100]{\{}100{\}} and [100]{\{}110{\}} edge dislocations in bulk TiN. We found that the formation energies of the core dislocations were continuously increased when their sizes decreased. We also found that the most common types of dislocations in TiN are the [110]{\{}110{\}} and [110]{\{}111{\}} edge dislocations. The obtained results are helpful for elucidating the atomic-scale mechanism for the superhardness of nanocomposites. [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X30.00008: Interaction of RDX Explosive Molecules with Metal-Organic Khorgolkhuu Odbadrakh, James Lewis We have studied interactions of cyclotrimethylene trinitramine (RDX), a highly energetic explosive's molecule, with metal-organic framework of composition Zn$_4$O (1,4-benzenedicarboxylate)$_3$ (MOF-5), within {\it ab initio} density functional theory method. The structures were optimized with the Fireball atomic orbital basis sets to a good agreement with experimental values. Optimal surface geometries have been obtained for MOF-5-RDX system and first principles estimates of the binding energies, charge transfer, and activation barriers are given. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X30.00009: Control of Microcellular Structure in Polymeric Foams via Nanofiller Size and Surface Chemistry Kerem Goren, Linda Schadler, Rahmi Ozisik Polymeric foams are used in many applications, where thermal insulation, selective sound inhibition or low density materials are needed. They are generally produced by blending polymer with a chemical blowing agent, which releases inert gas at processing temperatures. There are two major drawbacks to this process: chemical residues that form during the decomposition of blowing agent and formation of large (macroscopic) pores that weaken the material. To overcome these drawbacks, supercritical carbon dioxide (scCO2) is used. Use of scCO2 along with nanofillers provide heterogeneous nucleation, and present the opportunity to control pore size and pore distribution. In the current study, the effect of silica size and silica-scCO2 interaction on microcellular structure was investigated. Two different silica fillers were synthesized: 15 and 150 nm. These silica nanofillers were surface modified with tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilanes. Dynamic Light Scattering, FT-IR, TGA, and SEM were used to characterize the samples. Results indicate that there is a strong correlation between surface chemistry, and hence scCO2 and filler interaction on pore size and size distribution. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X30.00010: Diffusion in an array of cavities in two and three dimensions Mykyta V. Chubynsky, Francis Torres, Gary W. Slater We consider diffusion of point-like particles in arrays of cavities separated by infinitely thin walls with holes. We show that in the small-hole limit, the effective diffusion coefficient $D$ is related to the electrical capacitance of a plate of the same shape as the hole. Applying this result to elongated holes in 3D, we find an interesting logarithmic dependence of $D$ on the hole width. A logarithmic dependence is also obtained in 2D. As a consequence, the diffusion rate reaches 10\% of the free diffusion rate when the hole widths are only about $10^{-6}$ of the cavity size. The theoretical predictions are validated using a numerically exact computational method. In the opposite limit, when holes span the cavity boundaries nearly completely and only small pieces of walls remain, analytical calculations are also possible. The result in this case is interesting, since a finite reduction in the diffusion coefficient is obtained even for infinitely thin walls, contrary to predictions of various effective-medium theories that this reduction should be proportional to the excluded volume fraction. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X30.00011: First principles modeling of structure and properties of multi-component amorphous steels Viatcheslav Kazimirov, Despina Louca, Michael Widom Amorphous steels (iron based metallic glasses) exhibit unique physical properties that have opened the venue to new commercial applications as well as renewed the interest in this field. To provide a realistic description of the three dimensional structures and associate the coordinated environment of atoms to physical properties, first principles quantum mechanical molecular dynamics (MD) simulations along with the pair density function (PDF) analysis of neutron and X-ray diffraction data were used by way of considering chemical effects, ionic size ratio and concentration. Direct comparison of the simulated atomic structures obtained from MD with the local atomic structures determined experimentally show a very good agreement between the two, indicating that this theoretical approach can be applied towards simulating multi-component alloys. The atomic coordinates were used to develop the building blocks of cluster-like structures that give rise to the short-range order. The diffusion rates of different atom species were modeled at several temperatures that allowed us to describe the quenching process. In addition, the bulk moduli as a function of chemical composition were simulated and showed a very good agreement with the ones obtained experimentally. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X30.00012: Structural, dynamic and electronic properties of amorphous Al$_2$O$_3$: ab--initio molecular dynamics calculations Gonzalo Gutierrez, Sergio Davis First principles molecular dynamics (MD) calculations of amorphous Al$_2$O$_3$ in a system consisting of a supercell of 80 atoms is reported. A detailed analysis of the interatomic correlations allows us to conclude that the short range order is mainly composed by AlO$_4$ tetrahedra, but in contrast to classical MD results, also an important number of AlO$_5$ units are present. The vibrational density of state, calculated by means of the velocity autocorrelation function, present two main bands, a low frequency one related to the inter-tetrahedron vibration and a high frequency band related to the intra-tetrahedron vibration. By means of a geometry relaxation we obtain a fully relaxed system, and calculated its elastic properties. The reported bulk modulus is 193.4 GPa, the smallest among the several phases of alumina. The electronic properties were characterized by means of both the total and partial electronic density of states as well as by means of the electron localization function. The system present a rather small gap of 2.4 eV. The consequences of these results will be discussed. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X30.00013: Disorder Induced Transition into a One-Dimensional Wigner Glass Shimul Akhanjee, Joseph Rudnick The destruction of quasi-long range crystalline order as a consequence of strong disorder effects is shown to accompany the strict localization of all classical plasma modes of one- dimensional Wigner crystals at $T=0$. We construct a phase diagram that relates the structural phase properties of Wigner crystals to a plasmon delocalization transition recently reported. Deep inside the strictly localized phase of the strong disorder regime, we observe ``glass-like'' behavior. However, well into the critical phase with a plasmon mobility edge, the system retains its crystalline composition. We predict that a transition between the two phases occurs at a critical value of the relative disorder strength. This transition has an experimental signature in the AC conductivity as a local maximum of the largest spectral amplitude as function of the relative disorder strength. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X30.00014: Structural and electronic properties of amorphous silicon carbide: A first principles and experimentally constrained molecular relaxation approach Parthapratim Biswas, Raymond Atta-Fynn We present first-principles modeling of amorphous silicon carbide within a localized basis density functional formalism to study the electronic, vibrational and structural properties for system containing 1000 atoms. Our work for Si$_{.5}$-C$_{.5}$ system shows that the short range chemical order is dominated by heteronuclear Si-C bonds with coordination defect with a degree of chemical disorder. We calculate the electronic density of states that shows a presence of clean optical gap in the spectrum and study the localization nature of the electronic band tail states and vibrational eigenstates. We compare our results with existing models and experimental data available in the literature. Finally, we presents some preliminary results for models obtained by experimentally constrained molecular relaxation technique [1,2] that directly uses experimental data in conjunction with a classical force-field. \newline [1] P. Biswas, D.N. Tafen and D.A. Drabold, Phys Rev B71, 54204 \newline [2] P. Biswas, R. Atta-Fynn, S. Chakraborty, and D.A.Drabold, J.Phys.:Condens. Matter 19 (2007) 455202 [Preview Abstract] |
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