Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W24: Glassy and Amorphous Systems |
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Sponsoring Units: DCMP Chair: Bruno Uchoa, Boston University Room: Morial Convention Center 216 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W24.00001: Dynamics of a polymer nanocomposite under stress Robert Riggleman, Juan de Pablo The response of polymer glasses to deformation has received much attention in recent years. Previous simulation studies have shown that strain applied to a polymer glass can destroy energy barriers that impede relaxation on the potential energy landscape. However, less attention has been placed on the response of a polymer glass to an applied stress, as in a creep experiment. We have performed molecular simulations of the non-linear creep of a coarse-grained polymer glass and a polymer nanocomposite in shear, compression, and extension. We study the effects of the applied stress on the dynamics of our system and find dynamic enhancement up to a factor of 100 compared to the quiescent polymer glass below its glass transition temperature, Tg. We find that the free volume changes induced by the stress are not indicative of the changes in dynamics; however, the strainrate is strongly correlated with the dynamics. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W24.00002: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W24.00003: Out-of-Equilibrium to In-Equilibrium Dynamics of SiO$_2$ Katharina Vollmayr-Lee, Jake Roman, Juergen Horbach We study the aging dynamics of SiO$_2$ (modeled by the BKS model) via molecular dynamics simulations. The system is well equilibrated at temperature $T_{\rm high}$, then quenched to $T_{\rm low}$ and observed after a waiting time $t_{\rm wait}$. We present results for the structure factor, for the mean squared displacement, and for the intermediate scattering function. The resulting relaxation times show for the largest investigated $T_{\rm low}$ that during the simulation run the system is first out-of equilibrium and then reaches equilibrium with relaxation times independent of $T_{\rm high}$ and $t_{\rm wait}$. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:18PM |
W24.00004: Double Percolation in the Intermediate Phase of Network Glasses J.C. Phillips, Gerry Lucovsky Intermediate Phases (IP) in network glasses, discovered by Boolchand and coworkers, exhibit two striking properties: a nearly reversible glass transition, and nearly zero internal network stress. Double percolation, a well-established phenomenon in polymer blend-carbon black composites and numerical simulations, explains quantitatively the ranges of the intermediate phase observed in ternary chalcogenide alloys. The mechanism underlying IP double percolation in network glasses is spinodal enthalpy-entropy balance. Triple percolation also explains many aspects of the phase diagrams of cuprate high-temperature superconductors. [Preview Abstract] |
Thursday, March 13, 2008 3:18PM - 3:30PM |
W24.00005: Effects of relaxation on the energy landscape of amorphous silicon Houssem Kallel, Normand Mousseau, Francois Schiettekatte Amorphous silicon is used in many devices around us, included as a thin-film transistor in most flat screens, it also serves as the reference for the study of disordered network systems. Recently, differential scanning calorimetry and nanocalorimetry measurements (DSC) $^1$ have shown that the heat released as the temperature of the sample is raised following implantation, is temperature independent. To understand this behaviour, we characterize the energy landscape of model \emph{a}-Si. Using the activation-relaxation technique (ART nouveau) with the modified Stillinger-Weber potential, we generate models at four levels of relaxation and identify the relaxation mechanisms by analysing 100 000 events for each model. We find that while the distribution of the activation barriers shifts to higher energy as the system is relaxed, the distribution of the relaxation energies is almost unchanged. The relation between these two phenomena is consistent with the DSC measurements. This work is supported, in part, by NSERC, FQRNT and the CRC Foundation. HK is grateful for a scholarship from the Tunisian Ministry of Higher Education, Scientific Research and Technology. \newline $^1$ R. Karmouch \emph{et al.}, Phys. Rev. B \textbf{75}, 075304 (2007) [Preview Abstract] |
Thursday, March 13, 2008 3:30PM - 3:42PM |
W24.00006: Models of Ge$_x$Se$_{1-x}$ Marc-Andr\'e Malouin, Normand Mousseau We present numerical models of chalcogenide glasses constructed using the effective two and three body interaction potential developed by Mauro and Varshneya [1] combined with the activation-relaxation technique (ART nouveau) [2]. Structures are prepared starting from a random distribution, avoiding biases and crystalline remnants. Structural properties are studied mainly via characteristic system measurements including partial and total radial distribution functions, bond angle distributions, mean coordinations and bonds population. Results are shown for $Ge_{x}Se_{1-x}$ for various \emph{x} concentrations and compared to both experimental measurements and \emph{ab initio} simulation results. \newline [1] J.C. Mauro and A.K. Varshneya, J. Am. Ceram. Soc., 89 [7] 2323-6 (2006). \newline [2] R. Malek and N. Mousseau, Phys. Rev. E 62, 7723 (2000). [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W24.00007: Growth and properties of amorphous Ge$_{x}$Se$_{1-x}$ films prepared by pulsed laser deposition W.C. Liu, W. Zhou, G. Hoffman, R. Reano, R. Sooryakumar, P. Boolchand Chalcogenide glasses such as Ge$_{x}$Se$_{1-x}$ are interesting materials for their light induced effects. These phenomena include photdarkening, photofluidity, anisotropic optomechanical response and giant reversible photosoftening. Some of these effects have stimulated device applications that have led to mature technologies. In order to further investigate these systems, we report on the growth of thin films of Ge$_{x}$Se$_{1-x}$ (x $\sim $0.2) chalcogenide glass by pulsed laser deposition. The films were prepared using rotating targets of chalcogenide glasses that were prepared by conventional direct synthesis from elements in evacuated silica ampoules followed by melt quenching. Deposition was carried out in a vacuum chamber at room temperature on thermally oxidized silicon substrates. We will present results on the homogeneity, composition, and loss properties of these films, as well as their response to electron beams. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W24.00008: Particle Jumps in SiO$_2$: A Computer Simulation Robin Bjorkquist, Katharina Vollmayr-Lee, Juergen Horbach We use molecular dynamics simulations to investigate the aging dynamics of SiO$_2$ modeled by the BKS potential. The system is well equilibrated at temperature $T_{\rm high}$ and then quenched to $T_{\rm low}$. We characterize the dynamics by means of jumps in single-particle trajectories and measure the number of jump events, the number of jumping particles, and jump size. All measured quantities increase with increasing $T_{\rm low}$ and decrease with increasing waiting time. For the largest investigated temperatures $T_{\rm low}$ and for long enough waiting times the system reaches equilibrium and all measured quantities become independent of $T_{\rm high}$ and waiting time. [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W24.00009: Flexible-Rigid elastic transition in sodium germanate glasses: A Brillouin light scattering study. W. Zhou, W.C. Liu, R. Sooryakumar, D.I. Novita, P. Chen, P. Boolchand Global structures in network glasses are characterized by their connectedness or mean co-ordination number, $r$, and have been classified as being flexible ($r\sim $2), intermediate($r\sim $2.4) or stressed rigid ($r\sim $2.7) . Recently these ideas have been extended from covalent networks to ionically bonded ones, as evidence of a reversibility window in (Na$_{2}$O)$_{x}$(GeO$_{2})_{1-x }$ glasses has emerged$^{2}$ in the 14{\%} $<$ x $<$ 19{\%} range. Glasses at x $<$ 14{\%} are viewed as stressed-rigid and those at x $>$ 19{\%} to be flexible. We have performed Brillouin light scattering measurements on this glass system, and will present results on the dependence of the longitudinal (LA) and transverse (TA) acoustic phonon velocities across a broad range of compositions, 0 $<$ x $<$ 30{\%}. Variations in C$_{11}$(x) and C$_{44}$(x) elastic constants across the three elastic phases will be discussed. $^{2}$ K.Rompicharla et al. (unpublished) [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W24.00010: Nature of electrical conductivity threshold in bulk (Ag$_{2}$Se)$_{x}$(GeSe$_{4}$)$_{1-x}$ glasses C. Holbrook, P. Boolchand, P. Chen, A. Pradel, A. Piarristeguy Bulk glasses were synthesized over the 0 $<$ x $<$ 25{\%} range, and examined in FT-Raman, m-DSC, Electric Force Microscopy (EFM) and complex impedance experiments. Ag$_{2}$Se as an additive to GeSe$_{4}$ base glass leads to macroscopic phase separation as revealed by bimodal T$_{g}$s ( base glass-T$_{g}$ =168\r{ }C, additive glass T$_{g}$ = 230\r{ }C at low x $<$ 15{\%} . In addition, at higher x ($>$ 16{\%}), a third T$_{g}$ near 211\r{ }C is observed, and its strength increases with increasing x. EFM confirms the heterogeneous character of the glasses displaying a conducting phase that is segregated at low x ($<$12{\%}), and which percolates at higher x ($>$16{\%}). Electrical conductivity results show a step-like jump of nearly 2 orders of magnitude in the 16{\%} $<$ x $<$ 20{\%} range. Frequency of the Raman active corner-sharing mode of GeSe$_{4 }$units (200 cm$^{-1})$ steadily decreases with increasing x, suggesting that the third phase (T$_{g}$=211 C) most likely is a conducting GeSe$_{4}$-Ag$_{2}$Se phase. These data are consistent with a volume percolation$^{2}$ of solid electrolyte phases near x$\sim $ 16{\%} contributing to the step-like jump in conductivity of glasses. $^{1}$ V.Balan et al. J.Optoelectronics Adv. Mater. 8, 2112(2006). $^{2 }$H. Scher and R.Zallen, J. Chem. Phys.53,3759(1970). * Supported by NSF grant DMR 04-56472 [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W24.00011: Synthesis of dry AgPO$_{3}$ glass and characterization by Raman, IR and m-DSC D. Novita, P. Boolchand Glass transition temperature of titled glass apparently vary over a wide range, 163\r{ }C$<$ T$_{g}<$ 254\r{ }C depending on the humidity of the ambient environment in which precursors are handled$^{1}$. We have examined a set of 4 samples (1(181\r{ }C),2(203\r{ }C), 3(242.7\r{ }C), 4(254\r{ }C) with different T$_{g}$s indicated in parenthesis, in m-DSC, Raman and IR experiments to elucidate the role of bonded water. Our results show that the glass transition endotherm of sample 4 (dry) is characteristic of a stressed-rigid glass, while that of sample 1 (wet) of a flexible glass. Although Raman scattering of samples 1 and 4 look superficially similar, they are different in details; the Boson peak in sample 4 has a stronger intensity than in sample 1. IR reflectance signal strength in the mid-IR range is weaker for sample 4 than for sample 1, and furthermore differs in details. These results show that presence of water disrupts the P-O-P chain network by replacing bridging O with terminal OH$^{-}$ ends. And once water is bonded, it is difficult to remove it completely. These results will be compared to previous reports in the field. $^{1}$ D.I.Novita and P.Boolchand Phys. Rev. B (in press) * Supported by NSF grant DMR 04-56472 [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W24.00012: Evidence for Quasi Tetrahedral S=As(S$_{1/2}$)$_{3}$ local structures in As$_{x}$S$_{1-x}$ glasses Ping Chen, P. Boolchand, C. Holbrook, K. Jackson, D. Georgiev, M. Micoulaut Raman scattering and modulated DSC on bulk As$_{x}$S$_{1-x}$ glasses in the 0.08 $<$ x $<$ 0.41 range is studied$^{1}$. Vibrational spectroscopy results supported by first principles cluster calculations reveal that modes near 335, 380 and 490 cm$^{-1}$ represent respectively symmetric stretch, asymmetric stretch and As=S stretch of Quasi- Tetrahedral (QT) S=As(S$_{1/2})_{3}$ local structures. Mode at 365 cm$^{-1}$ is attributed to both symmetric and asymmetric stretch of pyramidal (PYR) As(S$_{1/2})_{3}$ units. In addition, modes of S$_{n}$ chains near 460 cm$^{-1}$ and those of S$_{8 }$ rings near 475, 430, 217,and 150 cm$^{-1}$ are observed at x $<$ 0.25. Scattering strengths of modes of QT units are found to display a broad global maximum in the 0.20 $<$ x $<$ 0.30 composition range, while modes of PYR units increase monotonically with x, and modes of S$_{n}$ and S$_{8}$ rings monotonically decrease with x in the range examined. These findings are in harmony with the existence of a reversibility window (RW) in the 0.20 $<$ x $<$ 0.30 range. $^{1}$ D.G. Georgiev Ph.D. Thesis, Univ. of Cincinnati (unpublished) 2003. $^{2}$ D.G.Georgiev and P.Boolchand Phil. Mag.\underline {83}, 2941(2003). * Supported by NSF grant DMR 04-56472 [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W24.00013: Evidence of Intermediate Phase in (Na$_{2}$O)$_{x}$(GeO$_{2}$)$_{1-x}$ glasses V. Rompicharla, Ping Chen, D. Novita, P. Boolchand, M. Micoulaut, W. Huff Intermediate phases have been observed in covalent glasses, but ionically bonded network systems have received much less attention in this respect. We have now examined titled glasses in m-DSC, Raman scattering, IR reflectance and Birefringence experiments over wide range of soda concentration, 3 $<$ x $<$ 30{\%}. Thermal experiments reveal a sharp reversibility window (RW) in the 14{\%} $<$ x $<$ 19{\%} soda range, which correlates well with a broad global maximum in molar densities (germanate anomaly) . Raman and IR reflectance TO and LO mode frequencies exhibit anomalies between x$_{c}$(1) = 14{\%} (\textit{stress} transition) and x$_{c}$(2) = 19{\%} (\textit{rigidity} transition), with optical elasticity power-laws confirming the nature of the transitions. Birefringence measurements dramatize the macroscopically stress-free nature of the Intermediate Phase (IP) in the RW. These data also suggest that the germanate anomaly can be understood as a direct consequence of the multiscale structural self-organization of glasses in the IP. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W24.00014: Competing effects in the photo-crystallization of a-Se HARP films R.E. Tallman, B.A. Weinstein, A. Reznik, J.A. Rowlands, M. Kubota, K. Tanioka Photo-crystallization of amorphous (a-) Se is investigated as a function of temperature (T = 250--340 K) and exposure time by Raman scattering in photo-active films used in high-gain avalanche rushing photoconductor (HARP) cameras. We observe different temperature regimes that reflect the competition of viscosity-driven and strain-driven processes. No photo-crystallization is seen below 260K, and above 318K the crystallization rate is thermally enhanced. For 260--302 K the onset time for photo-crystallization decreases with increasing temperature, but the crystallization rate is T-independent. \textit{Surprisingly}, photo-crystallization is strongly suppressed in a 15 degree range around the glass transition at T$_{g} \quad \sim $ 310K. Our results are in qualitative accord with a theory by Stephens[1] that treats the effects of local strain on the growth of crystalline-Se domains within an a-Se matrix -- the strain arising from the density difference between the crystalline and amorphous phases. [1] R.B. Stephens, J. Appl. Phys. \underline {\textbf{51}}, 6197 (1981). [Preview Abstract] |
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