Atmosphere effects in the processing of silicon carbide and silicon oxycarbide thin films and coatings

Silicon carbide and silicon oxycarbide films were prepared from solutions of polycarbosilane and methyldimethoxysilane + tetraethoxysilane, respectively, and deposited on different substrates (Si wafers, stainless steel plates, sapphire and SiC fibers). The coatings were heated at different temperatures and in different atmospheres, such as regular grade argon, ultra high purity and argon vacuum. The films were characterized using different techniques (FT-IR, XRD, SIMS, Ellipsometry).The influence of the processing parameters (heat treatment temperature and atmosphere) on the final microstructure of the coatings is discussed in this article.     

Simulation of laser generated ultrasound with application to defect detection

Laser generated ultrasound holds substantial promise for use as a tool for defect detection in remote inspection thanks to its ability to produce frequencies in the MHz range, enabling fine spatial resolution of defects. Despite the potential impact of laser generated ultrasound in many areas of science and industry, robust tools for studying the phenomenon are lacking and thus limit the design and optimization of non-destructive testing and evaluation techniques. The laser generated ultrasound propagation in complex structures is an intricate phenomenon and is extremely hard to analyze. Only simple geometries can be studied analytically. Numerical techniques found in the literature have proved to be limited in their applicability, by the frequencies in the MHz range and very short wavelengths. The objective of this research is to prove that by using an explicit integration rule together with diagonal element mass matrices, instead of the almost universally adopted implicit integration rule to integrate the equations of motion in a dynamic analysis, it is possible to efficiently and accurately solve ultrasound wave propagation problems with frequencies in the MHz range travelling in relatively large bodies. Presented results on NDE testing of rails demonstrate that the proposed FE technique can provide a valuable tool for studying the laser generated ultrasound propagation. PACS 02.70.Dh; 43.35.+d; 42.62.-b     

Extraction methods of red blood cell membrane proteins for Multidimensional Protein Identification Technology (MudPIT) analysis

Since red blood cells (RBCs) lack nuclei and organelles, cell membrane is their main load-bearing component and, according to a dynamic interaction with the cytoskeleton compartment, plays a pivotal role in their functioning. Even if erythrocyte membranes are available in large quantities, the low abundance and the hydrophobic nature of cell membrane proteins complicate their purification and detection by conventional 2D gel-based proteomic approaches. So, in order to increase the efficiency of RBC membrane proteome identification, here we took advantage of a simple and reproducible membrane sub-fractionation method coupled to Multidimensional Protein Identification Technology (MudPIT). In addition, the adoption of a stringent RBC filtration strategy from the whole blood, permitted to remove exhaustively contaminants, such as platelets and white blood cells, and to identify a total of 275 proteins in the three RBC membrane fractions collected and analysed. Finally, by means of software for the elaboration of the great quantity of data obtained and programs for statistical analysis and protein classification, it was possible to determine the validity of the entire system workflow and to assign the proper sub-cellular localization and function for the greatest number of the identified proteins.     

A low numerical dissipation immersed interface method for the compressible Navier–Stokes equations

A numerical method to solve the compressible Navier–Stokes equations around objects of arbitrary shape using Cartesian grids is described. The approach considered here uses an embedded geometry representation of the objects and approximate the governing equations with a low numerical dissipation centered finite-difference discretization. The method is suitable for compressible flows without shocks and can be classified as an immersed interface method. The objects are sharply captured by the Cartesian mesh by appropriately adapting the discretization stencils around the irregular grid nodes, located around the boundary. In contrast with available methods, no jump conditions are used or explicitly derived from the boundary conditions, although a number of elements are adopted from previous immersed interface approaches. A new element in the present approach is the use of the summation-by-parts formalism to develop stable non-stiff first-order derivative approximations at the irregular grid points. Second-order derivative approximations, as those appearing in the transport terms, can be stiff when irregular grid points are located too close to the boundary. This is addressed using a semi-implicit time integration method. Moreover, it is shown that the resulting implicit equations can be solved explicitly in the case of constant transport properties. Convergence studies are performed for a rotating cylinder and vortex shedding behind objects of varying shapes at different Mach and Reynolds numbers.     

A numerical study of reignition induced by a diffusion flame

We present a numerical study of the reignition of a cold reactant mixture by the interaction with a nearby diffusion flame. This reignition mechanism may be an important process in turbulent non-premixed flames at high rates of strain where quenched sections of the stoichiometric surface are folded by the turbulent flow and come in close proximity with other burning flame sections. We consider an idealized one-dimensional setup containing the fundamental ingredients that are expected to contribute to this reignition mode. One- and two-step irreversible chemical mechanisms with heat release levels typical of practical hydrocarbon fuels are considered. It is observed that a slow moving reignition kernel originates on the high-temperature region of the burning flame in the one-step chemistry case owing to small leakage of oxidizer from the cold-mixture side. This kernel gradually moves, increasing the local temperature above that provided by diffusion and eventually leads to thermal runaway with the formation of a deflagration wave. The reignition time depends on the chemistry details, the Damköhler number, but in any case it cannot exceed the mixing time. This implies that the flame-induced reignition time is essentially bounded from above by mixing. Unless one of the free streams is hotter, in which case auto-ignition (as opposed to reignition) may proceed first, the reignition time is chemistry dependent. In the case of two-step chemistry, the reignition pathway is different initially owing to leakage of the radical species, but it approaches that of the one-step chemistry case shortly thereafter. It is observed that the only difference between the two cases is in the initial phase of the evolution of the reignition kernel. This phase appears to be very sensitive to the chemistry details, a general aspect of ignition. A parametric study is carried out to elucidate the effect of each non-dimensional quantity on the reignition time for the one-step chemistry case.     

Environmental effects on initiation and propagation of surface defects on silicate glasses: scratch and fracture toughness study

The glass composition and surrounding environment can play an important role in the initiation and propagation of surface defects, which affect the practical strength of glass. We have studied how the environment and glass composition affect the tribological and indentation properties of multicomponent silicate glasses. Soda lime silica and aluminosilicate glasses were studied to compare the effects of the alkali ion and glass network type on surface defect formation. Although both glasses contained leachable sodium ions, the surface wear of soda lime glass decreased with increasing humidity while sodium aluminosilicate glass had an observable increase in surface wear. This indicated that sodium ion and water activity on/in glass surfaces vary depending on the glass network structure. The exchange of Na⁺ with K⁺ in aluminosilicate glass increased the elastic modulus, hardness, and resistance to fracture substantially; however, it did not improve the surface scratch resistance in humid environments. This suggested that the improved fracture toughness for the K-exchanged aluminosilicate glass is mainly due to the improved bulk properties; surface wear can readily take place regardless of Na/K-exchange.     

Characterization of boroaluminosilicate glass surface structures by B K-edge NEXAFS

Techniques traditionally used to characterize bulk glass structure (NMR, IR, etc.) have improved significantly, but none provide direct measurement of local atomic coordination of glass surface species. Here, we used Near-Edge X-ray Absorption Fine Structure (NEXAFS) as a direct measure of atomic structure at multicomponent glass surfaces. Focusing on the local chemical structure of boron, we address technique-related issues of calibration, quantification, and interactions of the beam with the material. We demonstrate that beam-induced adsorption and structural damage can occur within the timeframe of typical measurements. The technique is then applied to the study of various fracture surfaces where it is shown that adsorption and reaction of water with boroaluminosilicate glass surfaces induces structural changes in the local coordination of boron, favoring B^IV species after reaction.