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Mean Free Path (mean + free_path)
Selected AbstractsDetermination of mean free path for energy loss and surface oxide film thickness using convergent beam electron diffraction and thickness mapping: a case study using Si and P91 steelJOURNAL OF MICROSCOPY, Issue 2 2006D. R. G. MITCHELL Summary Determining transmission electron microscope specimen thickness is an essential prerequisite for carrying out quantitative microscopy. The convergent beam electron diffraction method is highly accurate but provides information only on the small region being probed and is only applicable to crystalline phases. Thickness mapping with an energy filter is rapid, maps an entire field of view and can be applied to both crystalline and amorphous phases. However, the thickness map is defined in terms of the mean free path for energy loss (,), which must be known in order to determine the thickness. Convergent beam electron diffraction and thickness mapping methods were used to determine , for two materials, Si and P91 steel. These represent best- and worst-case scenario materials, respectively, for this type of investigation, owing to their radically different microstructures. The effects of collection angle and the importance of dynamical diffraction contrast are also examined. By minimizing diffraction contrast effects in thickness maps, reasonably accurate (±15%) values of , were obtained for P91 and accuracies of ±5% were obtained for Si. The correlation between the convergent beam electron diffraction-derived thickness and the log intensity ratios from thickness maps also permits estimation of the thickness of amorphous layers on the upper and lower surfaces of transmission electron microscope specimens. These estimates were evaluated for both Si and P91 using cross-sectional transmission electron microscopy and were found to be quite accurate. [source] Thermal Conductivity of the Rare-Earth Strontium AluminatesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2010Chunlei Wan The thermal conductivity of a series of complex aluminates, RE2SrAl2O7, with different rare-earth (RE) ions, has been measured up to 1000°C. There is a strong dependence on the atomic number of the RE ion, ranging from an approximately 1/T dependence for the lanthanum strontium aluminate to an almost temperature-independent behavior of the dysprosium strontium aluminate. The latter conductivity is comparable with that of yttria-stabilized zirconia, the current material of choice for thermal barrier coatings. The temperature dependence of the thermal conductivities of all the aluminates studied can be fit to a standard phonon,phonon scattering model, modified to account for a minimum phonon mean free path, in which the difference in behavior is attributed to increased phonon,phonon scattering with the atomic mass of the RE ion. Although a satisfactory parametric fit is obtained, the model does not take into account either the detailed layer structure of the aluminates, consisting of alternating rock-salt and perovskite layers in a natural superlattice structure, or the site preferences of the RE ion. This suggests that further model development is warranted. [source] On spherical harmonics expansion type modelsfor electron,phonon collisionsMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 3 2003J.-P. Bourgade Abstract In this paper, we give the rigorous derivation of a diffusion model for semiconductor devices, the starting point being a microscopic description of electron transport by means of a kinetic equation of Boltzmann type. The limit of a small mean free path at a large time leads to a diffusion equation of ,SHE' type (spherical harmonics expansion). We deal with a collision operator that models interactions between electron and phonons. This induces a peculiar form for the diffusion tensor: electron,phonon collisions happen to be discontinuous in energy and inelastic, and, as a consequence, the diffusion tensor appears as an infinite dimensional matrix. Copyright © 2003 John Wiley & Sons, Ltd. [source] Modification of the 21-cm power spectrum by X-rays during the epoch of reionizationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009L. Warszawski ABSTRACT We incorporate a contribution to reionization from X-rays within analytic and seminumerical simulations of the 21-cm signal arising from neutral hydrogen during the epoch of reionization. The relatively long X-ray mean free path (MFP) means that ionizations due to X-rays are not subject to the same density bias as UV ionizations, resulting in a substantive modification to the statistics of the 21-cm signal. We explore the impact that X-ray ionizations have on the power spectrum (PS) of 21-cm fluctuations by varying both the average X-ray MFP and the fractional contribution of X-rays to reionization. In general, prior to the epoch when the intergalactic medium (IGM) is dominated by ionized regions (H ii regions), X-ray-induced ionization enhances fluctuations on spatial scales smaller than the X-ray MFP, provided that X-ray heating does not strongly suppress galaxy formation. Conversely, at later times when H ii regions dominate, small-scale fluctuations in the 21-cm signal are suppressed by X-ray ionization. Our modelling also shows that the modification of the 21-cm signal due to the presence of X-rays is sensitive to the relative scales of the X-ray MFP and the characteristic size of H ii regions. We therefore find that X-rays imprint an epoch and scale-dependent signature on the 21-cm PS, whose prominence depends on fractional X-ray contribution. The degree of X-ray heating of the IGM also determines the extent to which these features can be discerned. We further show that the presence of X-rays smoothes out the shoulder-like signature of H ii regions in the 21-cm PS. For example, a 10 per cent contribution to reionization from X-rays translates to a 20,30 per cent modulation in the 21-cm PS across the scale of H ii regions. We show that the Murchison Widefield Array will have sufficient sensitivity to detect this modification of the PS, so long as the X-ray photon MFP falls within the range of scales over which the array is most sensitive (,0.1 Mpc,1). In cases in which this MFP takes a much smaller value, an array with larger collecting area would be required. As a result, an X-ray contribution to reionization has the potential to substantially complicate analysis of the 21-cm PS. On the other hand, a combination of precision measurements and modelling of the 21-cm PS promises to provide an avenue for investigating the role and contribution of X-rays during reionization. [source] Spectroscopic ellipsometry study of thin diffusion barriers of TaN and Ta for Cu interconnects in integrated circuitsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 4 2008S. Rudra Abstract The objective of this work is to study the optical and electrical properties of tantalum nitride and tantalum barrier thin films used against copper diffusion in Si in integrated circuits using spectroscopic ellipsometry in the VUV and UV,visible range. Single layers of tantalum nitride and bilayer films of Ta/TaN were produced by reactive magnetron sputtering on Si(100) substrates covered with a native oxide layer. Ellipsometric measurements were performed in the energy range from 0.73,8.7 eV and the dielectric functions were simulated using Drude,Lorentz model and effective medium approximation (EMA) in order to obtain information regarding film thickness, film composition, free carrier plasma energy, mean relaxation time and electrical resistivity. The film thickness clearly affects the electrical resistivity and the electron mean free path. It was observed that for films of Ta on TaN even after maintaining the deposition condition suitable for the ,-phase of Ta, it turned out to be a mixture of ,- and ,-phases with higher contribution of the ,-phase. It is shown that even a very small intermixture of two different phases of Ta can be determined accurately using ellipsometry. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Water uptake behavior of layered silicate/starch,polycaprolactone blend nanocompositesPOLYMER INTERNATIONAL, Issue 2 2008C Javier Pérez Abstract The water uptake behavior of biodegradable layered silicate/starch,polycaprolactone blend nanocomposites was evaluated. Three different commercial layered silicates (Cloisite Na+, Cloisite 30B and Cloisite 10A) were used as reinforcement nanofillers. Tests were carried out in two different environments: 60 and 90% relative humidity using glycerol solutions. The clay/starch,polycaprolactone blend nanocomposites were obtained by melt intercalation and characterized by gravimetric measurements and tensile tests. The intercalated structure (determined by wide-angle X-ray diffraction) showed a decrease in water absorption as a function of clay content probably due to the decrease of the mean free path of water molecules. The diffusion coefficient decreased with clay incorporation but a further increase in the clay content did not show an important effect on this parameter. Elongation at break increased with exposure showing matrix plasticization. Mechanical properties of the nanocomposites deteriorated after exposure whereas they remained almost constant in the case of the neat matrix. Copyright © 2007 Society of Chemical Industry [source] Instrument and data analysis challenges for imaging spectropolarimetryASTRONOMISCHE NACHRICHTEN, Issue 6 2010C. Denker Abstract The next generation of solar telescopes will enable us to resolve the fundamental scales of the solar atmosphere, i.e., the pressure scale height and the photon mean free path. High-resolution observations of small-scale structures with sizes down to 50 km require complex post-focus instruments, which employ adaptive optics (AO) and benefit from advanced image restoration techniques. The GREGOR Fabry-Pérot Interferometer (GFPI) will serve as an example of such an instrument to illustrate the challenges that are to be expected in instrumentation and data analysis with the next generation of solar telescopes (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||