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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
    Influence of Cation Order on the Dielectric Properties of Pb(Mg1/3Nb2/3)O3-Pb(Sc1/2Nb1/2)O3 (PMN-PSN) Relaxor Ferroelectrics
    (2003-11-01) Farber, Leon; Davies, Peter K
    The effect of the B-site cation chemistry and ordering on the dielectric properties of solid solutions in the (1-x)Pb(Mg1/3Nb2/3)O3-Pb(Sc1/2Nb1/2)O3 (PMN-PSN) perovskite system was examined in samples with 0.1 ≤ x ≤ 0.9. Thermal annealing treatments were effective in inducing long-range B-site order in the samples within this compositional range. The well-ordered, large chemical domain ceramics exhibit relaxor behavior up to x = ~0.5; for higher values of x, normal ferroelectric behavior was observed. For x ≤ 0.5 reductions in the chemical domain, size had no significant effect on the weak-field dielectric properties, but induced a transition to relaxor behavior for x > ~0.6. The disordered PSN-rich samples undergo a spontaneous zero-field relaxor to ferroelectric transition similar to that reported previously for PSN. The field-dependent properties of compositions lying closest to the relaxor to ferroelectric crossover exhibited the highest sensitivity to alterations in the chemical order. The properties of this system are consistent with a "random site" description of the 1:1 ordered Pb(β′1/2 β1/2)O3 structure with β′ = (Mg(2-2x)/3Nb(1-x)/3Scx) and β″ = Nb.
  • Publication
    Dynamical Behavior of Heat Conduction in Solid Argon
    (2011-04-01) Kaburaki, Hideo; Li, Ju; Yip, Sidney; Kimizuka, Hajime
    Equilibrium molecular dynamics is performed to obtain the thermal conductivity of crystalline argon using the Green-Kubo formalism, which permits the study of dynamical details of the transport process. A large system run to longer times is used to derive the heat flux autocorrelation functions from the low temperature solid to the liquid state. The power spectrum of an autocorrelation function reveals the change in the nature of the underlying atomic motions across the temperature range.
  • Publication
    Effect of Seeding on the Microstructure and Mechanical Properties of alpha-SiAlON: I, Y-SiAlON
    (2002-05-01) Shuba, Roman; Zenotchkine, Misha; Chen, I-Wei; Kim, Joo-Sun
    The effect of seeding on the microstructure and mechanical properties of single-phase Y-α-SiAlON ceramics with elongated grains has been studied. Seeds of the intended α-SiAlON compositions but with different size, shape, and number of grains have been compared for their effects. The microstructure, resistance (R-curve) behavior, and Weibull modulus are strongly correlated to the number density of the seeds. The highest fracture toughness reached is ~12 MPa∙m1/2 and can be obtained with as little as 1% seeding. The thermodynamic stability of seeds has been examined and is attributed to their chemical composition.
  • Publication
    The Effect of Powder Mixing Procedures on α-SiAlON
    (2005-10-03) Shuba, Roman; Chen, I-Wei
    Various procedures of mixing starting powders for hot-pressing α-SiAlON ceramics were studied. They included different milling methods (attrition milling, ball milling, and sonication), liquid medium (water, isopropyl alcohol, and pyridine), and atmospheres (ambient air and nitrogen). These mixing procedures resulted in markedly different densification behavior and fired ceramics. As the powders experienced increasing oxidation because of mixing, the densification temperature decreased, the amount of residual glass increased, and α-SiAlON was destabilized and replaced by ß-SiAlON and AlN polytypes during hot pressing. These effects were mitigated when pyridine, nitrogen, and sonication were used. Several protocols that yielded nearly phase-pure, glass-free dense α-SiAlON were thus identified.
  • Publication
    Unusually low thermal conductivity of gallium nitride nanowires
    (2008-03-28) Guthy, Csaba; Nam, Chang-Yong; Fischer, John E
    We report measurements of thermal conductivity κ on individual gallium nitride nanowires (GaN NWs) with diameters ranging from 97 to 181 nm grown by thermal chemical vapor deposition. We observed unexpectedly small kappa values, in the range of 13–19 W/m K at 300 K, with very weak diameter dependence. We also observe unusual power law κ~Tn behavior with n=1.8 at low temperature. Electron-energy-loss-spectroscopy measurements indicate Si and O concentrations in the ranges of 0.1–1 and 0.01–0.1 at. %, respectively. Based on extensive numerical calculations, we conclude that both the unexpectedly low κ and the T1.8 dependence are caused by unusually large mass-difference scattering, primarily from Si impurities. Our analysis also suggests that mass-difference scattering rates are significantly enhanced by the reduced phonon group velocity in nanoscale systems. Planar defects running the length of the NW, previously characterized in detail, may also play a role in limiting the phonon mean free path.
  • Publication
    High-resolution characterization of defects in oxide thin films
    (2008-11-03) Nikiforov, Maxim N; Brukman, Matthew J; Bonnell, Dawn A
    Nanometer sized defects in thin HfOx films are detected by atomic force microscopy facilitated leakage current measurements. Differences in the electrical properties of individual defects were distinguished. The effects of two mechanisms that localize the tip-sample interaction and increase spatial resolution were calculated. The expected increase in tip-sample current due to stress induced phase transformations and band gap narrowing has been calculated, and a behavior diagram is presented that shows the pressure necessary to generate a detectable current increase as a function of tip radius.