47 research outputs found
Influence of Yb:YAG laser beam parameters on Haynes 188 weld fusion zone microstructure and mechanical properties
The weldability of 1.2 mm thick Haynes 188 alloy sheets by a disk Yb:YAG laser welding was examined. Butt joints were made, and the influence of parameters such as power, size, and shape of the spot, welding speed, and gas flow has been investigated. Based on an iconographic correlation approach, optimum process parameters were determined. Depending on the distribution of the power density (circular or annular), acceptable welds were obtained. Powers greater than 1700 W, welding speeds higher than 3.8 m mm1, and spot sizes between 160 and 320 lm were needed in the circular (small fiber) configuration. By comparison, the annular (large fiber) configuration required a power as high as 2500 W, and a welding speed less than 3.8 m min�1. The mechanical properties of the welds depended on their shape and microstructure, which in turn depended on the welding conditions. The content of carbides, the proportion of areas consisting of cellular and dendritic substructures, and the size of these substructures were used to explain the welded joint mechanical properties
Exact results for hydrogen recombination on dust grain surfaces
The recombination of hydrogen in the interstellar medium, taking place on
surfaces of microscopic dust grains, is an essential process in the evolution
of chemical complexity in interstellar clouds. The H_2 formation process has
been studied theoretically, and in recent years also by laboratory experiments.
The experimental results were analyzed using a rate equation model. The
parameters of the surface, that are relevant to H_2 formation, were obtained
and used in order to calculate the recombination rate under interstellar
conditions. However, it turned out that due to the microscopic size of the dust
grains and the low density of H atoms, the rate equations may not always apply.
A master equation approach that provides a good description of the H_2
formation process was proposed. It takes into account both the discrete nature
of the H atoms and the fluctuations in the number of atoms on a grain. In this
paper we present a comprehensive analysis of the H_2 formation process, under
steady state conditions, using an exact solution of the master equation. This
solution provides an exact result for the hydrogen recombination rate and its
dependence on the flux, the surface temperature and the grain size. The results
are compared with those obtained from the rate equations. The relevant length
scales in the problem are identified and the parameter space is divided into
two domains. One domain, characterized by first order kinetics, exhibits high
efficiency of H_2 formation. In the other domain, characterized by second order
kinetics, the efficiency of H_2 formation is low. In each of these domains we
identify the range of parameters in which, the rate equations do not account
correctly for the recombination rate. and the master equation is needed.Comment: 23 pages + 8 figure
Grain Destruction in Interstellar Shocks
Interstellar shock waves can erode and destroy grains present in the shocked
gas, primarily as the result of sputtering and grain-grain collisions.
Uncertainties in current estimates of sputtering yields are reviewed. Results
are presented for the simple case of sputtering of fast grains being stopped in
cold gas. An upper limit is derived for sputtering of refractory grains in
C-type MHD shocks: shock speeds v_s \gtrsim 50 \kms are required for return
of more than 30\% of the silicate to the gas phase. Sputtering can also be
important for removing molecular ice mantles from grains in two-fluid MHD shock
waves in molecular gas. Recent estimates of refractory grain lifetimes against
destruction in shock waves are summarized, and the implications of these short
lifetimes are discussed.Comment: To appear in Shocks in Astrophysics, ed. T.J. Millar. Talk given at
conference Shocks in Astrophysics, Manchester, Jan. 1995. 13 pages with 6
figures: uuencoded compressed postscript. Also available as POPe-633 on
http://astro.princeton.edu/~library/prep.htm
IONIZATION DETECTION OF RYDBERG LINESHAPES IN STRONTIUM VAPOUR PERTURBED BY FOREIGN GASES
L'excitation à deux photons laser à été utilisée pour obtenir des paramètres de forme des profils de raie pour des niveaux Rydberg du Sr perturbés par des collisions. Les élargissements et les déplacements des niveaux 5sns 1S0 et 5snd 1D2 (7 ≤ n ≤ 52) en présence des gaz He, Ar, Xe à des pressions allant jusqu'à une atmosphère, sont donnés et comparés aux valeurs théoriques. L'observation de structures satellites pour les niveaux de Rydberg peu élevés est aussi présentée.Using laser two-photon excitation and ionization detection, we report lineshape parameters for collisionally perturbed Rydberg levels of Sr. With He, Ar, Kr and Xe as perturber gases at pressures up to one atmosphere, the broadening and shift rates of 5sns 1S0 and 5snd 1D2 (7 ≤ n ≤ 52) are given and compared with theoretical predictions. The observation of satellite structures on low lying Rydberg levels are also presented