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The Nd:YAG laser-induced breakdown of 20 μm glass microspheres was investigated using time-resolved optical shadow and Schlieren images. Time-resolved imaging showed the location of the initial breakdown and the shockwave motion over its first 400 μm of expansion. Measured shockwave velocities were in the range of 1–10 km/s and showed a linear dependency on laser fluence within 30 ns. 相似文献
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Using the approach of Rulla (1996 SIAM J. Numer. Anal. 33, 68-87)for analysing the time discretization error and assuming moreregularity on the initial data, we improve on the error boundderived by Barrett and Blowey (1996 IMA J. Numer. Anal. 16,257-287) for a fully practical piecewise linear finite elementapproximation with a backward Euler time discretization of amodel for phase separation of a multi-component alloy. 相似文献
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Nichols William T. Malyavanatham Gokul Henneke Dale E. O'Brien Daniel T. Becker Michael F. Keto John W. 《Journal of nanoparticle research》2002,4(5):423-432
Silver nanoparticles were produced by laser ablation of a continuously flowing aerosol of microparticles in nitrogen at varying laser fluences. Transmission electron micrographs were analyzed to determine the effect of laser fluence on the nanoparticle size distribution. These distributions exhibited bimodality with a large number of particles in a mode at small sizes (3–6-nm) and a second, less populated mode at larger sizes (11–16-nm). Both modes shifted to larger sizes with increasing laser fluence, with the small size mode shifting by 35% and the larger size mode by 25% over a fluence range of 0.3–4.2-J/cm2. Size histograms for each mode were found to be well represented by log-normal distributions. The distribution of mass displayed a striking shift from the large to the small size mode with increasing laser fluence. These results are discussed in terms of a model of nanoparticle formation from two distinct laser–solid interactions. Initially, laser vaporization of material from the surface leads to condensation of nanoparticles in the ambient gas. Material evaporation occurs until the plasma breakdown threshold of the microparticles is reached, generating a shock wave that propagates through the remaining material. Rapid condensation of the vapor in the low-pressure region occurs behind the traveling shock wave. Measurement of particle size distributions versus gas pressure in the ablation region, as well as, versus microparticle feedstock size confirmed the assignment of the larger size mode to surface-vaporization and the smaller size mode to shock-formed nanoparticles. 相似文献
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Marowsky G. Schäfer E. P. Keto J. W. Tittel F. K. 《Applied Physics A: Materials Science & Processing》1976,9(2):143-146
Fluorescence studies on electron-beam excited mixtures of POPOP dye vapor and various buffer gases are reported. Xenon has
been found to be a promising candidate as a buffer gas for efficient energy deposition of the electron-beam and energy transfer
to the POPOP dye vapor. These experiments may be an important step towards realization of an electrically excited dye vapor
laser.
Work supported in part by the National Aeronautics and Space Administration and the Energy Research and Development Administration. 相似文献
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Michael Gammage Hugo Celio Michael F. Becker John W. Keto Desiderio Kovar 《Journal of nanoparticle research》2017,19(8):276
Bare Ag nanoparticles (~10 nm) and Ag nanoparticles (1–20 nm) on the surfaces of larger TiOx nanoparticles were prepared by laser ablation of microparticle aerosols (LAMA). The behaviors of the nanoparticles during high temperature annealing were then studied with ex situ and in situ transmission electron microscopy. For the ex situ heating experiments, Ag and Ag-on-TiOx NPs were collected onto gold TEM grids and subjected to annealing treatments at 500 °C in argon, vacuum, and air. At this temperature, bare Ag NPs on carbon TEM supports coarsened rapidly in both air and argon atmospheres. In contrast, Ag-on-TiOx NPs that were heated to 500 °C in flowing argon or in a vacuum did not coarsen significantly and were remarkably stable. Ag-on-TiOx NPs that were heated to 500 °C in air, however, behaved quite differently. The TiOx crystallized upon heating and a significant loss of Ag were observed from the surfaces of the TiOx, likely due to sublimation. These results demonstrate that the surface defect structure and chemistry of the oxide support strongly influence the thermal stability of Ag NPs produced by LAMA. 相似文献
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Ekoko Bakambo Gracien Zhou Ruimin Li Jian Xin Li Hui Tondozi Keto 《Journal of Radioanalytical and Nuclear Chemistry》2004,262(3):751-754
Gamma-irradiation has been applied to synthesize the nanocomposite semiconductor constituted of zinc and cadmium sulfide Cd1-xZnxS ultra fine particles at room temperature by utilizing homogeneous release of S2– ions from the decomposition of sodium thiosulfate. The structure, morphology, size and optical properties of that compound have been studied by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and ultraviolet-visible spectrometer (UV-visible). The product obtained from irradiated solutions containing zinc ions, cadmium ions and the sulfur source has been characterized as a composite of (CdxZn1-x)S, with spherical morphology and with a diameter of average size of about 5.3 nm. The possible mechanism of formation of the composite is proposed.The financial support of Shanghai Science and Technology Foundation through No. 0252nm 095 is gratefully acknowledged.The authors would thank the staff of Shanghai Applied Radiation Institute of Shanghai University for their valuable contributions. 相似文献