Analysis of External-Field-Driven Surface Stabilization and Patterning

The ability to form, manipulate, and stabilize fine-scale materials features by controlling macroscopic forces is essential to advancing microelectronics and nanotechnology. For example, externally applied electric fields and thermal gradients drive atomic motion through the mass transport phenomena of electromigration or thermomigration, respectively, that have been considered for several decades as major sources of materials reliability problems in microelectronics. However, when tuned optimally, the same destructive forces can provide paradigm shifts in surface engineering and nanofabrication. Toward this end, developing a fundamental understanding and multi-physics, multi-scale mathematical models capable of predicting external-field-driven surface morphological evolution constitutes the main focus of this thesis. We have developed a surface mass transport model that accounts for electromigration and thermomigration, diffusional anisotropy, and the temperature dependence of atomic diffusivity to examine the surface morphological response of stressed electrically and thermally conducting crystalline elastic solids under the simultaneous action of an electric field and/or a temperature gradient; these solid materials include uniaxially stressed bulk crystals and coherently strained epitaxial films on substrates that may undergo stress-induced surface morphological instabilities such as the Asaro-Tiller/Grinfeld instability and the Stranski-Krastanow growth instability, respectively. Using linear stability analysis, and validated by self-consistent dynamical simulations, we have found that properly directed external fields of magnitude higher than a critical value can stabilize the planar surface morphology and derived the conditions for synergistic action of multiple external fields, as well as the criticality conditions for surface stabilization. We have also minimized the critical external field strength requirements by combining the external field action with substrate engineering techniques. Furthermore, we have investigated systematically the complex asymptotic states reached in the electromigration-driven void morphological evolution in thin films of face-centered cubic metals with-oriented film planes under the simultaneous action of a general biaxial mechanical stress. In addition, we have developed and experimentally validated a model for analyzing the current-driven dynamics of single-layer epitaxial islands on crystalline substrates. Simulations based on the model have shown that the dependence of the stable steady island migration speed on the inverse of the island size stops being linear for larger-than-critical island sizes. In this nonlinear regime, we have discovered morphological transitions, Hopf bifurcations, and necking or fingering instabilities for various surface crystallographic orientations, island misfit strains, and diffusional anisotropy parameters. Consistent with the predictions of linear stability theory, dynamical simulations show that, under certain conditions, large-size islands undergo a fingering instability which following finger growth and, depending on the substrate orientation, necking instability, leads to formation of single or multiple nanowires. The nanowires have constant widths, on the order of 10 nm, which can be tuned by controlling the externally applied electric field strength. Moreover, we have studied systematically the patterns formed due to the current-driven evolution of pairs of different-size islands driven to coalescence

Dynamic simulation of activated sludge based wastewater treatment processes: Case studies with Titagarh Sewage Treatment Plant, India

STOAT has been extensively used for the dynamic simulation of an activated sludge based wastewater treatment plant in the Titagarh Sewage Treatment Plant, near Kolkata, India. Some alternative schemes were suggested. Different schemes were compared for the removal of Total Suspended Solids (TSS), b-COD, ammonia, nitrates etc. A combination of IAWQ#1 module with the Takacs module gave best results for the existing scenarios of the Titagarh Sewage Treatment Plant. The modified Bardenpho process was found most effective for reducing the mean b-COD level to as low as 31.4 mg/l, while the mean TSS level was as high as 100.98 mg/l as compared to the mean levels of TSS (92 62 mg/l) and b-COD (92.0 mg/l) in the existing plant. Scheme 2 gave a better scenario for the mean TSS level bringing it down to a mean value of 0.4 mg/l, but a higher mean value for the b-COD level at 54.89 mg/l. The Scheme Final could reduce the mean TSS level to 2.9 mg/l and the mean b-COD level to as low as 38.8 mg/l. The Final Scheme looks to be a technically viable scheme with respect to the overall effluent quality for the plant. (C) 2009 Elsevier B.V. All rights reserved

Focus on plasma-facing materials in nuclear fusion reactors

International audienceFusion energy is a promising, safe, and reliable green energy solution to the increasing energy demand. However, there are several materials challenges that need to be overcome to increase the technical readiness to a level that enables a fusion pilot plant on the grid. This focus issue aims to identify and address a set of such key impediments for realizing deuterium-tritium (D–T) fusion power in a tokamak reactor and highlight the most recent progress on those research frontiers. The main emphasis of this collection is on materials development challenges resulting from helium irradiation, neutron-induced degradation, thermomechanical loading, and the corrosive environment faced by the divertor and first-wall materials, commonly known as plasma-facing components, and blanket systems for tokamak fusion reactors