Belousov–Zhabotinsky dissipative reactions in Ti–B and Ni–Al alloy systems

During unidirectional micropyretic synthesis, a high-temperature reaction front propagates in a steady, oscillatory or unstable manner, depending on the reaction conditions chosen, thereby converting a reactant mix to products which display features of the wave propagation conditions. Bands are sometimes noted in the synthesized solid, often with a fixed periodicity along the propagation direction. This banded structure is thought to be the signature of an oscillating wave front. In previous articles by us and others, banded structures and other residual instabilities have been thought to arise from variations in the combustion front velocity caused by a mismatch between the heat diffusion rate and the heat production rate from the product synthesis (i.e. by exceeding the critical Lewis number). Such thermal oscillations have been modeled in the past by a single overall reaction formulation and a corresponding heat flow solution. Spiral fronts have also been noted. It is likely that the banded structures could also have originated from entropy-generating, dissipative, Belousov–Zhabotinsky (BZ) type reactions. The overall BZ reaction progresses in the form of a non-linear oscillator with several intermediary steady-state sub-reactions contained in a reaction volume. One possible sub-reaction sequence by which a BZ reaction may occur during micropyretic synthesis is discussed. Micropyretic synthesis can invoke both types of oscillations, i.e. the Lewis type and the BZ type. Two binary systems where the BZ oscillations are possible, namely the Ti–B and Ni–Al alloy systems, are examined. Spiral fronts are thought to contain both types of oscillations. A comparison with experiments shows that the micropyretic product chemistry is adequately predicted for the first time by a BZ formulation in both systems. Modeling methods for both types of oscillations are presented and integrated for the special case where an oscillation in the liquid metal concentration is noted. Forward reaction rate constants are calculated for the sub-reactions.     

Influence of process parameters on the formation of Silicalite-1 zeolite particles

Silicalite-1 particles with minimum twinning have been synthesized inside the polar core of non-ionic surfactant/co-surfactant-stabilized water-in-oil (w/o) type emulsions at 150° ± 1 °C within a short reaction time of 5 h. The non-ionic surfactants of varying hydrophilic–lipophilic balance (HLB) values, i.e. sorbitan monooleate (Span 80, HLB: 4.3), sorbitan monolaurate (Span 20, HLB: 8.6), polyoxyethylene (4) lauryl ether (Brij 30, HLB: 9.7) and polyoxyethylene sorbitan monooleate (Tween 80, HLB: 15), the cationic surfactant, i.e. cetyl trimethyl ammonium bromide (CTAB), surfactant concentration, co-surfactant, synthesis temperature and time have been found to play significant role in controlling size and characteristics of Silicalite-1. It has been observed that the crystallinity and size of Silicalite-1 can be tailored by adjusting the interactions between the polar surfactant head groups at the w/o interface and the growing crystallographic surfaces (or silicate/TPA ions) in the aqueous medium of the emulsion.     

Thermal stresses in an infinite non-isotropic plate having a penny-shaped crack

Of concern in the paper is the distribution of thermal stresses in the vicinity of a penny-shaped crack in a thick elastic plate made of a non-isotropic material. The problem pertains to the situation where the crack is opened by a prescribed normal pressure and a prescribed heat-flux or a prescribed temperature.     

MHD Flow past a circular cylinder – a numerical study

 The steady, two-dimensional, incompressible MHD flow past a circular cylinder with an applied magnetic field parallel to the main flow is studied using the finite difference method. The magnetic Reynolds number is assumed to be small. Results are presented up to the Reynolds number R=500 and interaction parameter N=1.3. As N increases suppression of the separation is observed. Drag coefficient is decreasing for the small values of N and then increasing as N increases. It is found that a smaller value of far field distance is required as N increases. Received 6 March 2000     

Anomalous quantum transport through a thin film of varying impurity density with the distance from the film surface

We explore a novel transport phenomenon by studying the effect of disorder on electron transport through a thin film of varying disorder strength with the distance from its surface. A simple tight-binding model is used to describe the film which is attached to two metallic electrodes, where the coupling of this film to the electrodes is treated through the use of Newns-Anderson chemisorption theory. It is found that, in the strong disorder regime the current amplitude through the film increases with the increase of the disorder strength, while it decreases in the weak disorder regime. This strange behavior is completely opposite to that of a conventional disordered system. Our results also predict that the electron transport is significantly influenced by the finite size of the thin film.     

A scheme for finite element analysis of mode I and mixed mode stable crack growth and a case study with AISI 4340 steel

A new scheme for elastic–plastic finite element analysis has been proposed for the study of stable crack growth (SCG) from initiation to instability in both mode I and mixed modes (I and II). The scheme is based on node-release technique and helps to determine the variation of fracture load with crack extension without requiring much computer storage and time. The scheme permits predictions of load variation with load line displacement (LLD), maximum fracture load, crack tip current plastic zone and crack edge profile. In the analysis the condition for crack extension at every stage of the SCG is considered to be governed by CTOA/COD reaching a critical value. The scheme of analysis is different from the ones proposed by earlier investigators. The whole SCG is analysed in a few stages using the ANSYS software and a single discretization. Element arrangement in the discretization is decided from the very beginning; it has a capability of accommodating changes in boundary conditions arising out of crack extension in the later stages. Each stage is analysed afresh ignoring state of stress–strain reached at a material point at the end of the previous stage. Case studies on both mode I and mixed mode presented considering AISI 4340 steel, which is widely used in nuclear power industry, indicate that the SCG through it can be characterized in terms of a single COD or CTOA. Predictions for the initiation and maximum fracture loads in both the cases compare very closely with the experimental data reported. The results presented also include the value of critical COD/CTOA (0.035 mm/0.0875 rad) characterizing the SCG through the steel and show that the initiation load is not significantly affected by crack tip radius up to 0.05 mm.     

Effect of Active Raw Material Variability on Tablet Production: Physicochemical, Physiocomechanical and Pilot Plant Studies

Batch-to-batch variation in an active raw material was evaluated following a production process change by the raw material supplier. Physicochemical and physicomechanical tests were applied and indicated that polymorphism or solvate formation was not occurring but changes in crystal properties in terms of size and degree of aggregation were involved. Other more subtle differences in crystal properties were also possible

The deleterious effect of the raw material process change on tablet production was noted and a method of overcoming the problem identified

The necessity for comprehensive? reformulation studies including physicomechanical screening is underlined. Such studies are also necessary if changes in raw material production methods occur to avoid process problems related to the marked hatch-to-batch variation that can occur in these instances     

Microstructure and Mechanical Characterization of Stainless Steel and Copper Joint by Metallurgical Modification in GTAW Process

Metallurgical and Materials Transactions A - The dissimilar metal joining of stainless steel (SS)/copper (Cu) was done cost-effectively with Gas Tungsten Arc Welding (GTAW). A combination of...     

Mechanics and FEM estimation of gaps generated in star-ring active contacts of ORBIT motor during operation

International Journal of Mechanics and Materials in Design - We estimate deformations or gaps occurring at all the ideally form-closed contacts, that separate the chambers, in epitrochoid...     

Simulation studies on model reference adaptive controller based speed estimation technique for the vector controlled permanent magnet synchronous motor drive

This paper presents a speed estimation technique for the permanent magnet synchronous motor drive. A Model Reference Adaptive System (MRAS) has been formed using the instantaneous and steady-state reactive powers to estimate the speed. It has been shown that such unique MRAS offers several desirable features. The proposed technique is completely independent of stator resistance and is less parameter sensitive, as the estimation-algorithm is only dependent on q-axis stator inductance. Also, the method requires less computational effort as the simplified expressions are used in the MRAS. The stability of the proposed system is achieved through Popov’s Hyperstability criteria. Extensive simulation results are presented to validate the proposed technique. The system is tested at different speeds including zero speed and a very satisfactory performance has been achieved.