Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B39: Focus Session: Imaging & Modifying Materials Under Extreme Conditions of Radiation, Temperature, and at the Limits of Space and Time Resolution |
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Sponsoring Units: DMP GIMS Room: 348 |
Monday, March 18, 2013 11:15AM - 11:51AM |
B39.00001: Quantifying transient dynamics in materials using time resolved \textit{in situ} TEM Invited Speaker: Geoffrey Campbell The dynamic transmission electron microscope (DTEM) is a standard TEM that has been modified such that the electron beam can be operated with a single intense pulse of electrons (\textgreater\ 10$^9$ e$^{-})$ with a pulse duration of just 15 ns. The short pulse of electrons is created via photoemission at the microscope cathode and enables time resolved observations of \textit{in situ} experiments. However, it can also be operated in thermionic emission mode for normal operation of the microscope for alignment and experiment setup. Additional modifications have also been made to the optical design of the condenser lens system. The \textit{in situ} experiments currently use a second laser to initiate the dynamic response of interest in the specimen. The relative timing of the pulses from the two laser systems sets the time of the observation relative to the initiation of the event under study. The DTEM has been used to investigate a number of rapid phenomena in materials We have studied the rapid nucleation and growth at the nanoscale of crystalline phases from an initially amorphous metal alloy parent phase and in amorphous Ge. DTEM has also been used to study reactive multilayer films of Ni and Al that sustain a reaction front speed greater than 10 m/s. We have also investigated rapid solidification of nanoscale films of liquid Al-Cu alloys. This work performed under the auspices of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B39.00002: Studying dynamic processes in liquids by TEM/STEM/DTEM Patricia Abellan, James E. Evans, Taylor J. Woehl, Katherine L. Jungjohann, Lucas R. Parent, Ilke Arslan, William D. Ristenpart, Nigel D. Browning In order to study dynamic phenomena such as corrosion or catalysis, extreme environmental conditions must be reproduced around the specimen - these include high-temperatures, high-pressures, specific oxidizing/reducing atmospheres or a liquid environment. The use of environmental stages specifically designed to fit in any transmission electron microscope (TEM) allows us to apply the distinct capabilities of each instrument to study dynamic processes. Localized gas/fluid conditions are created around the sample and separated from the high vacuum inside the microscope using hermetically sealed windowed-cells. Advanced capabilities of these techniques include spatial resolutions of $\sim $1 Angstrom or better in aberration corrected instruments or temporal resolutions in the microsecond-nanosecond range in a dynamic TEM (DTEM). Here, unique qualities of the DTEM that benefit the \textit{in-situ} experiments with gas/fluid environmental cells will be discussed. We also present our results with a liquid stage allowing atomic resolution imaging of nanomaterials in a colloidal suspension, core EEL spectra acquisition, continuous flow, controlled growth of nanocrystals and systematic calibration of the effect of the electron dose on silver nuclei formation. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B39.00003: Imaging Lead Dendrite Formation and Ion Diffusion in Aqueous Solution with Scanning Transmission Electron Microscopy Edward White, Scott Singer, Veronica Augustyn, William Hubbard, Matthew Mecklenburg, Bruce Dunn, B. C. Regan Using a scanning transmission electron microscope, we image the formation of lead dendrites and the local Pb$^{2+}$ concentration in an electrochemical cell containing a saturated solution of lead(II) nitrate. We control the morphology of the lead deposits with the rate of potential change, which can result in dendrites or compact layers. The processes are reversible and can be repeated. During lead stripping and plating the local Pb$^{2+}$ concentration can be measured as an increase or decrease in signal intensity, respectively, as ions come into and out of solution. Quantitative digital image analysis reveals excellent correlation between changes in the Pb$^{2+}$ concentration, the rate of lead deposition, and the current passed by the electrochemical cell. Furthermore imaging the ionic concentration as a function of time and distance from the electrode provides a measurement of the diffusion coefficient of the Pb$^{2+}$ ion. Real-time electron microscopy of dendritic growth dynamics and the associated local ionic concentrations can provide new insight into the functional electrochemistry of batteries and related energy storage technologies. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:51PM |
B39.00004: Imaging and measuring the evolution of solid density within a thermal explosion Invited Speaker: Laura Smilowitz Explosives have been used for millennia. All materials are energetic, but high explosives have the ability to release their stored energy in a very short period of time- nanoseconds in the case of detonations. Many explosives have an as-designed behavior that is well understood and controlled. However, the off-nominal behavior, such as would occur in an accident scenario, is typically much less understood. The subject of our research has been the energy release mechanisms for secondary high explosives heated to thermal explosion. The study of thermal explosions poses several difficulties including extreme temperature, pressure, and rate of change. In addition, thermal explosions pose the difficulty of being spontaneous dynamic events with limited ability to predict the time of the event. Typically, event durations are tens of microseconds and timing jitter is tens of seconds- essentially a one in a million duty cycle. These difficulties have precluded the use of many standard laboratory diagnostics to the study of the phenomena. In the past years, we have developed diagnostics which can survive the extremes of the thermal explosion with sufficient response time and the ability to remain armed and be triggered by the onset of the spontaneous event. In addition to microsecond temporal resolution, the diagnostics need to be spatially resolved with 100 micron spatial resolution and centimeter field of view in order to capture the spatial heterogeneity of the event. Our work has focused on the important secondary high explosive PBX 9501 which is a formulation of the organic crystalline nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). Our evolving understanding of this material has enabled us to develop a table-top x-ray imaging experiment providing millisecond time resolution with duration of minutes and sensitivity to density changes of better than 1{\%}. This quasistatic regime provides images of material thermal expansion, phase transitions, and thermal decomposition leading to the onset of thermal ignition. A second technique provides microsecond scale time resolution with duration of milliseconds and contrast sensitivities of a few percent. This technique allows us to observe the propagation of ignition which determines the overall violence of the thermal explosion. In this talk, I will describe our current understanding of thermal explosions, and the evolution of the radiographic diagnostics that we have developed to study thermal explosions. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B39.00005: Radiographic imaging of solidification in Al-Cu alloys Jason Cooley, Amy Clarke, Seth Imhoff, Brian Patterson, Wah-keat Lee, Kamel Fezzaa, Alexander Deriy, Tim Tucker, Martha Barker, Kester Clarke, Robert Field, Dan Thoma, David Teter Until the advent of third generation synchrotrons the ability to image the microstructure of metals during solidification was non-existent. Today's sources have sufficient energy and flux to perform real time radiographic imaging of solidification in thin samples with resolution sufficient to image dendrites, eutectic lamellae, and the density change across the solidification front. Feedback control of the solidification interface is also possible. We report on the radiographic imagining of Al-Cu eutectic alloys during solidification at the Argonne National Laboratory Advanced Photon Source. Cooling rates of up 10 degrees C / sec and, temperature gradients of up to 150 degrees C / cm were used to control the solidification. The samples were $\sim$ 100 microns thick and the field of view was $\sim$ 1.4 x 1.7 mm. The experimentally accessible phase space included both plane front and cellular growth regimes. The experimental resolution in the micron range was adequate to quantify cellular radii, cellular interface angles, lamellar interface angles, and lamellar spacing. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B39.00006: Pressure-induced antiferrodistortive phase transition and phonon softening in SrTiO$_3$ Shih-Chang Weng, Ruqing Xu, Ayman Said, Shih-Lin Chang, Tai-Chang Chiang SrTiO$_3$, at room temperature, undergoes an antiferrodistortive transition under pressure with a critical pressure of P$_c \sim 9.6$ GPa. This transition is accompanied by a cubic--to--tetragonal structural distortion, and the same distortion can be induced at ambient pressure by lowering the sample temperature to below T$_c \sim 105$ K. The temperature-induced transition is known to involve a soft phonon at the R point in the Brillouin zone based on neutron scattering, inelastic x-ray scattering, and thermal diffuse scattering studies. The same soft mode is expected for the pressure induced transition, and we report herein the first direct measurement using inelastic x-ray scattering and a diamond-anvil pressure cell. The phonon softening behavior follows a power law and is accompanied by a central peak. The results are analyzed theoretically and correlated with those for temperature-induced transition. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B39.00007: Femtosecond laser fabrication of micro/nano-channel array devices for parallelized fluorescence detection Brian Canfield, William Hofmeister, Lloyd Davis Cost-effective pharmaceutical drug discovery depends on increasing assay throughput while reducing reagent needs. Ultrasensitive, highly parallelized fluorescence-based platforms that incorporate a nano/micro-fluidic chip with an array of closely spaced channels would meet this need. We discuss the use of direct femtosecond laser machining to fabricate prototype fluidic chips with arrays of more than one hundred closely spaced channels. Traditional machining techniques involve overlapping focal spots from many laser pulses while scanning the substrate in order to create channels. However, this procedure is not only lengthy but may allow thermal effects to accumulate that degrade the quality of both the channel profile and surrounding substrate material. We are developing a different method for machining a line with just a single pulse, using a combination of cylindrical lenses and an aspheric lens to reshape a near-Gaussian beam into a tight line focus. Channels on the order of 1 micron wide, 5 microns deep, and nearly 2000 microns long may be made this way. We also address the critical issue of mitigating the high autofluorescence responses that arise from the creation of defects by fs-laser machining in fused silica. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B39.00008: Bond dissociation of small molecules on the silver tip under the influence of local electric field Haiyan He, Mayukh Banik, Vartkess Apkarian, Ruqian Wu The manipulation of chemical bonds at metallic nano-junctions, such as at scanning tunneling junctions, and under laser irradiation is currently of great interest, motivated by both fundamental considerations and applications in nanoeletronics, nanophotonics and nanocatalysis. In this work, we systematically investigate bond formation and dissociation of small molecules (e.g., oxygen and carbon monoxide) at the junction of two silver (111) tipped surfaces, through first principles molecular dynamics simulations. The electronic structures and vibrational frequencies are a sensitive function of the gap size, and significantly modified by the local electric fields. The calculated results are compared with recent experiments. \textbf{Acknowledgement.} This work was supported by the National Science Foundation under CHE-0802913 and computing time at XSEDE. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B39.00009: Understanding the ultrafast electron photoemission process, from simulation to experiment Jenni Portman, He Zhang, Zhensheng Tao, Chong-yu Ruan, Martin Berz, Philip Duxbury The ongoing efforts to develop a reliable ultrafast electron diffraction and imaging system require a stable source of photoemitted electrons and an understanding of how the properties of the generated bunch depend on the photocathode. In order to gain more understanding of this process, we combine the three-step photoemission model with N-particle electron simulations. By using the Fast Multipole Method to treat space charge effects, we are able to follow the time evolution of pulses containing over $10^6$ electrons and investigate the role of laser fluence and extraction field on the total number of electrons that escape the surface. The results of these simulations are compared to experimental images of the photoemission process collected using the shadow imaging technique. We are able to show good quantitative agreement both for the number of electrons generated and the pulse parameters. We also see evidence of a virtual cathode limit, which gives an upper limit to the number of electrons that is is possible to extract. The extension of these results to various extraction fields, laser pulse shapes and photocathode material parameters, represents a very interesting future development, allowing to better optimize the materials used in electron pulse generation. [Preview Abstract] |
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