Photochemical behaviour of 4-styrylcoumarin polymorphs in the crystalline state

Addition of a small amount of coumarin during crystallization produces new polymorphic modifications in 4-styrylcoumarin and 4-(3-fluorostyryl)coumarin, which are photolabile. Interestingly, upon irradiation polymorphic modification of 4-(3-fluorostyryl)coumarin produces a mirror-symmetric photodimer in contrast to the centrosymmetric photodimer obtained without addition of coumarin during crystallization.

Study of formation of silicon carbide in the Acheson process

Formation of silicon carbide in the Acheson process was studied using a mass transfer model which has been developed in this study. The century old Acheson process is still used for the mass production of silicon carbide. A heat resistance furnace is used in the Acheson process which uses sand and petroleum coke as major raw materials.: It is a highly energy intensive process. No mass transfer model is available for this process. Therefore, a mass transfer model has been developed to study the mass transfer aspects of the process along with heat transfer. The reaction kinetics of silicon carbide formation has been taken from the literature. It has been shown that reaction kinetics has a reasonable influence on the process efficiency. The effect of various parameters on the process such as total gas pressure, presence of silicon carbide in the initial charge, etc. has been studied. A graphical user interface has also been developed for the Acheson process to make the computer code user friendly.

Structural characterization of DC magnetron-sputtered TiO2 thin films using XRD and Raman scattering studies

Titanium dioxide films have been deposited using DC magnetron sputtering technique onto well-cleaned p-silicon substrates at an oxygen partial pressure of 7 x 10(-5) mbar and at a sputtering pressure (Ar + O-2) Of I X 10(-3) mbar. The deposited films were calcinated at 673 and 773 K. The composition of the films as analyzed using Auger electron spectroscopy reveals the stoichiometry with an 0 and Ti ratio 2.08. The influence of post-deposition annealing at 673 and 773 K on the structural properties of the titanium dioxide thin films have been studied using XRD and Raman scattering. The structure of the films deposited at the ambient was found to be amorphous and the films annealed at temperature 673 K and above were crystalline with anatase structure. The lattice constants, grain size, microstrain and the dislocation density of the film are calculated and correlated with annealing temperature. The Raman scattering study was performed on the as-deposited and annealed samples and the existence of Raman active modes A(1g), B-1g and E-g corresponding to the Raman shifts are studied and reported. The improvement of crystallinity of the TiO2 films was also studied using Raman scattering studies. (C) 2003 Elsevier Ltd. All rights reserved.

Reactive Diffusion Between Vanadium and Silicon

Interdiffusion study is conducted in the V-Si system to determine integrated diffusion coefficients of the phases. Activation energy values are calculated from the experiments conducted at different temperatures. The average values are found to be 208, 240 and 141 kJ/mol, respectively, for the V(3)Si, V(5)Si(3) and VSi(2) phases. The low activation energy for the VSi(2) phase indicates very high concentration of defects or the significant contribution from the grain boundary diffusion. The error in calculation of diffusion parameters from a very thin phase layer in a multiphase diffusion couple is discussed. Further the data available in the literature in this system is compared and the problems in the indirect methodology followed previously to calculate the diffusion parameters are discussed.

Severe wear of a near eutectic aluminium-silicon alloy

The severe wear of a near eutectic aluminium silicon alloy is explored using a range of electron microscopic, spectroscopic and diffraction techniques to identify the residually strained and unstrained regions, microcracks and oxidized regions in the subsurface. In severe wear the contact pressure exceeds the elastic shakedown limit. Under this condition the primary and eutectic silicon particles fragment drastically. The fragments are transported by the matrix as it undergoes incremental straining with each cyclic contact at the asperity level. The grains are refined from similar to 2000 nm in the bulk to 30 nm in the near surface region. A large reduction in the interparticle distance compared with that for a milder stage of wear gives rise to high strain gradients which contribute to an enhancement of the dislocation density. The resulting regions of very high strain in the boundaries of the recrystallized grains as well as within the subgrains lead to the formation of microvoidskracks. This is accompanied by the formation of brittle oxides at these subsurface interfaces due to enhanced diffusion of oxygen. We believe that the abundance of such microcracks in the near surface region, primed by severe plastic deformation, is what distinguishes a severe wear regime from mild wear. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Topology Optimization of Micromachined Structures with the Manufacturing Constraints of KOH Etching of (110) Silicon

The focus of this paper is on designing useful compliant micro-mechanisms of high-aspect-ratio which can be microfabricated by the cost-effective wet etching of (110) orientation silicon (Si) wafers. Wet etching of (110) Si imposes constraints on the geometry of the realized mechanisms because it allows only etch-through in the form of slots parallel to the wafer's flat with a certain minimum length. In this paper, we incorporate this constraint in the topology optimization and obtain compliant designs that meet the specifications on the desired motion for given input forces. Using this design technique and wet etching, we show that we can realize high-aspect-ratio compliant micro-mechanisms. For a (110) Si wafer of 250 µm thickness, the minimum length of the etch opening to get a slot is found to be 866 µm. The minimum achievable width of the slot is limited by the resolution of the lithography process and this can be a very small value. This is studied by conducting trials with different mask layouts on a (110) Si wafer. These constraints are taken care of by using a suitable design parameterization rather than by imposing the constraints explicitly. Topology optimization, as is well known, gives designs using only the essential design specifications. In this work, we show that our technique also gives manufacturable mechanism designs along with lithography mask layouts. Some designs obtained are transferred to lithography masks and mechanisms are fabricated on (110) Si wafers.

Studies on the deposition and characterization of silicon oxide films for optical coating applications

Silicon oxide films were deposited by reactive evaporation of SiO. Parameters such as oxygen partial pressure and substrate temperature were varied to get variable and graded index films. Films with a refractive index in the range 1.718 to 1.465 at 550 nm have been successfully deposited. Films deposited using ionized oxygen has the refractive index 1.465 at 550 nm and good UV transmittance like bulk fused quartz. Preparation of graded index films was also investigated by changing the oxygen partial pressure during deposition. A two layer antireflection coating at 1064nm has been designed using both homogeneous and inhomogeneous films and studied their characteristics.

Brillouin Scattering Investigation of Solvation Dynamics in Succinonitrile-Lithium Salt Plastic Crystalline Electrolytes

Temperature dependent Brillouin scattering studies have been performed to ascertain the influence of solvent dynamics on ion-transport in succinonitrile-lithium salt plastic crystalline electrolytes. Though very rarely employed, we observe that Brillouin spectroscopy is an invaluable tool for investigation of solvent dynamics. Analysis of various acoustic (long wavelength) phonon modes observed in the Brillouin scattering spectra reveal the influence of trans-gauche isomerism and as well as ion-association effects on ion transport. Although pristine SN and dilute SN-LiClO(4) samples show only the bulk longitudinal-acoustic (LA) mode, concentrated SN-LiClO(4) (similar to 0.3-1 M) electrolytes display both the bulk LA mode as well as salt induced brillouin modes at ambient temperature. The appearance of more than one brillouin mode is attributed to the scattering of light from regions with different compressibilities (''compactness''). Correspondingly, these modes show a large decrease in the full width at half-maximum (abbreviated as nu(f)) as the temperature decreases. Anomalous temperature dependent behavior of nu(f) with addition of salt could be attributed to the presence of disorder or strong coupling with a neighbor. The shape of the spectrum was evaluated using a Lorentzian and Fano line shape function depending on the nature and behavior of the Brillouin modes.

Insulating State and Breakdown of Fermi Liquid Description in Molecular-Scale Single-Crystalline Wires of Gold

Electrical transport measurements on ultrathin single-crystalline Au nanowires, synthesized via a wet chemical route, show an unexpected insulating behavior. The linear response electrical resistance exhibits a power-law dependence on temperature. In addition, the variation of current over a wide range of temperature and voltage obeys a universal scaling relation that provides compelling evidence for a non-Fermi liquid behavior. Our results demonstrate that the quantum ground state In ultrathin nanowires of simple metallic systems can be radically different from their bulk counterparts and can be described In terms of a Tomonaga-Luttinger liquid (TLL), in the presence of remarkably strong electron-electron interactions.