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Now showing 1 - 10 of 236
  • Publication
    Calculation of the Positions of the α- and β-bands in the Electronic Spectra of Benzenoid Hydrocarbons Using the Method of Limited Configuration Interaction
    (1963) Koutecký, Josef; Paldus, Josef; Vitek, Vaclav
    The positions of the α- and β-bands in the electronic absorption spectra of twenty aromatic benzenoid hydrocarbons were calculated by the semiempirical method of limited configuration interaction in the π-electron approximation using the Huckel molecular orbitals. The agreement of the experimental and calculated values is good for the β-band whereas a systematic deviation is observed for the α-band. This deviation cannot be removed by extending the configuration interaction of the monoexcited states constructed from the molecular orbitals considered. However, the consideration of electronic repulsion enables us to explain the character of the dependences of the experimental excitation energies on the excitation energies obtained by the simple Huckel method of molecular orbitals. Using a suitable choice of semiempirical parameters different for various electronic transitions (showing no large mutual differences) yields semiempirical interpolation formulas for the; p-, α-, and β-bands which give very good agreement with the corresponding experimental excitation energies for the compounds studied.
  • Publication
    Monte Carlo Analysis of Stress-Directed Phase Segregation in Binary Thin Film Alloys Under Nonisothermal Annealing
    (2008-01-01) Nieves, Alex M; Vitek, Vaclav; Sinno, Talid
    The use of patterned stress fields to direct phase separation in thin film alloys is investigated computationally with Monte Carlo simulations in which atomic interactions are represented by a Lennard-Jones potential. We show that careful design of annealing schedules based on consideration of the system phase diagram can lead to vastly enhanced patterning kinetics. In particular, by avoiding the low temperature formation of highly stable nuclei within the entire system, the kinetics of patterning are accelerated by rapid monomerdiffusion, rather than classical Ostwald ripening in which small precipitates must dissolve to feed larger ones.
  • Publication
    Grain Boundary Structures in f.c.c, and b.c.c. Metals and Sites for Segregated Impurities
    (1979-07-01) Pond, R. C; Vitek, Vaclav; Smith, D. A
    The key features leading to low-energy grain boundaries in metals are discussed by reference to computer-simulated structures and geometrical analysis in terms of atom packing. Low energy is found to be associated with boundary structures consisting of relatively dense packing, and this can in turn be expressed in terms of the space-filling packing of coordination polyhedra. The geometrical method of analysis is shown to be well suited to the identification of interfacial sites for segregated impurities.
  • Publication
    Comment on "Glide Systems and Peierls Stresses in f.c.c. and b.c.c. Metals From Phonon Energies"
    (1974-07-01) Taylor, G.; Vitek, Vaclav
    The theory of Boffi et al. of Peierls stresses in crystals is criticized on physical grounds on a number of points.
  • Publication
    Dextran grafted silicon substrates : preparation, characterization and biomedical applications
    (2003-04-21) Eckmann, David M; Ombelli, Michela; Composto, Russell J
    Biodevices used in the cardiovascular system suffer from well-known problems associated with surface-induced gas embolism and thrombosis. In order to improve the biocompatibility of these devices, biomimetic coatings show good promise. We recently synthesized a coating layer of dextran, a relatively simple and well characterized neutral polysaccharide, with the purpose of mimicking the cells' glycocalyx layer, that prevents non-specific cells-protein interactions. Systematic physical chemical characterization was performed on coatings obtained both from commonly used polydisperse dextrans and low-dispersity dextrans in the 1-100 kDalton molecular weight range. We have combined standard surface analysis techniques, such as ellipsometry, contact angle measurements and AFM, with less traditional vibrational spectroscopy techniques in the characterization of our biomimetic coatings. FTIR, micro-FTIR and micro-Raman spectroscopies were utilized to correlate the conformational and molecular aspects of the grafted poly- and monodisperse dextran chains to their attractive biological properties.
  • Publication
    Temperature-Sensitive Nanocapsules for Controlled Drug Release Caused by Magnetically Triggered Structural Disruption
    (2008-12-01) Liu, Ting-Yu; Liu, Kun-Ho; Liu, Dean-Mo; Chen, San-Yuan; Chen, I-Wei
    Self-assembled nanocapsules containing a hydrophilic core and a crosslinked yet thermosensitive shell have been successfully prepared using poly(ethylene-oxide)-poly(propylene-oxide)-poly(ethylene-oxide) block copolymers, 4-nitrophenyl chloroformate, gelatin, and 1-ethyl-3-(3- dimethylaminopropyl) carbodiimide. The core is further rendered magnetic by incorporating iron oxide nanoparticles via internal precipitation to enable externally controlled actuation under magnetic induction. The spherical nanocapsules exhibit a hydrophilic-to-hydrophobic transition at a characteristic but tunable temperature reaching 40ºC, triggering a size contraction and shrinkage of the core. The core content experiences very little leakage at 25ºC, has a half life about 5 h at 45ºC, but bursts out within a few minutes under magnetic heating due to iron oxide coarsening and core/shell disruption. Such burst-like response may be utilized for controlled drug release as illustrated here using a model drug Vitamin B12.
  • Publication
    Processing of single wall carbon nanotubes and implications for filling experiments
    (2001-11-26) Russo, Richard M; Chikkannanavar, Satishkumar B; Stercel, Ferenc; Smith, Brian W; Luzzi, David E
    Single wall carbon nanotubes (SWNTs) have been processed in different schemes to get clean material for use in various filling experiments. The SWNTs synthesized by different methods require different processing schemes, and this is presumably due to heterogeneous nature of the various contaminants present along with the carbon nanotubes. For the pulsed laser synthesized SWNTs, a combination of nitric acid, hydrogen peroxide and hydrochloric acid treatment gives best results and the purified SWNTs give best ever filling fraction for fullerene, C60 of ~90%. The processing improves the surface cleanliness of SWNTs, in turn giving greater access for the target molecules, and hence the higher filling fraction. For the carbon arc produced SWNTs, air oxidation followed by treatment with nitric acid has been found to work best and the processed SWNTs have been used for filling experiments with metal chlorides. Both these processing schemes still leave a small fraction of catalyst impurities in the final material, thus the material quality of filled material and hence its properties depend on the processed material used for the filling experiments.
  • Publication
    Carbide-Derived Carbons: Effect of Pore Size on Hydrogen Uptake and Heat of Adsorption
    (2006-10-27) Yushin, Gleb; Dash, Ranjan; Jagiello, Jacek; Fischer, John E; Gogotsi, Yury
    Cryoadsorption is a promising method of enhancing gravimetric and volumetric onboard H2 storage capacity for future transportation needs. Inexpensive carbide-derived carbons (CDCs), produced by chlorination of metal carbides, have up to 80 % open-pore volume with tunable pore size and specific surface area (SSA). Tuning the carbon structure and pore size with high sensitivity by using different starting carbides and chlorination temperatures allows rational design of carbon materials with enhanced C-H2 interaction and thus increased H2 storage capacity. A systematic experimental investigation of a large number of CDCs with controlled pore size distributions and SSAs shows how smaller pores increase both the heat of adsorption and the total volume of adsorbed H2. It has been demonstrated that increasing the average heat of H2 adsorption above 6.6 kJ mol-1 substantially enhances H2 uptake at 1 atm (1 atm = 101 325 Pa) and -196 °C. The heats of adsorption up to 11 kJ mol-1 exceed values reported for metal-organic framework compounds and carbon nanotubes.
  • Publication
    Breakdown of Dynamic Scaling in Thin Film Binary Liquids Undergoing Phase Separation
    (2004-05-06) Chung, Hyun-joong; Composto, Russell J
    The kinetics of phase separation in thin polymer blend films displaying discrete and bicontinuous domain morphologies are examined. For discrete domains, the correlation length ξ grows as t1/3, in agreement with a coalescence model. By plotting ξ/d vs t/ti (initiation time), universal growth behavior is obtained for thickness values (d) from 1000 to 190 nm. In contrast, bicontinuous domains grow with a decreasing exponent, 0.62 to 0.28, as d decreases from 900 to 90 nm (i.e., no universal growth). This slowing down with reduced dimensionality suggests suppression of lateral hydrodynamic pumping.
  • Publication
    Hydrogenation of Mg film and Mg nanoblade array on Ti coated Si substrates
    (2008-10-23) He, Yuping; Zhao, Yiping; Huang, Liwei; Wang, Howard; Composto, Russell J
    The hydrogenation of Mg film and Mg nanoblade array fabricated on Ti coated Si substrates has been studied and compared. The nanoblades start to absorb hydrogen at a temperature between 250 and 300 degrees C, which is much lower than 350 degrees C for Mg film. However, the saturated total hydrogen uptake in nanoblades is less than half of that in the film, resulting from MgO formation by air exposure. The nanoblade morphology with large surface area and small hydrogen diffusion length, and the catalytic effect of Ti layer, are two main reasons for the nanoblade hydrogenation behavior.