Showing papers by "Indian Institute of Technology Madras published in 2000"

Structure and properties of aerated concrete: a review

Abstract: Aerated concrete is relatively homogeneous when compared to normal concrete, as it does not contain coarse aggregate phase, yet shows vast variation in its properties. The properties of aerated concrete depend on its microstructure (void–paste system) and composition, which are influenced by the type of binder used, methods of pore-formation and curing. Although aerated concrete was initially envisaged as a good insulation material, there has been renewed interest in its structural characteristics in view of its lighter weight, savings in material and potential for large scale utilisation of wastes like pulverised fuel ash. The focus of this paper is to classify the investigations on the properties of aerated concrete in terms of physical (microstructure, density), chemical, mechanical (compressive and tensile strengths, modulus of elasticity, drying shrinkage) and functional (thermal insulation, moisture transport, durability, fire resistance and acoustic insulation) characteristics.

Picosecond optical nonlinearity in monolayer-protected gold, silver, and gold-silver alloy nanoclusters

Abstract: Monolayer-protected Au, Ag, and Au:Ag alloy nanoclusters have been synthesized using octanethiol and octadecanethiol as capping agents. The particle-size distribution is narrow with an average core size of 3--4 nm. Optical nonlinearity induced by 35 ps pulses at 532 nm has been investigated in these samples using the Z-scan technique. It is found that in general, they behave either as saturable absorbers or reverse saturable absorbers depending on the intensity of excitation. Au and Ag clusters show nearly the same efficiency for optical limiting, but the alloy clusters are found to be less efficient in limiting and are less photostable. The observed effects are explained in terms of the electron dynamics of the excited-state species.

Extraction of bamboo fibers and their use as reinforcement in polymeric composites

Abstract: Few investigations have been carried out with bamboo fibers despite its high strength, biodegradability, and low cost. The overall objective of this work was to investigate fiber extraction from bamboo and the use of these bamboo fibers as reinforcement in polymeric composites. A combination of chemical and mechanical methods was used for the extraction of bamboo fibers. Conventional methods of compression molding technique (CMT) and roller mill technique (RMT) were explored for the mechanical separation. Fiber population from both the techniques were characterized. Mechanical properties of the fibers also were evaluated. Bamboo fibers obtained from CMT and RMT were used to make unidirectional composites of polyester. High values of tensile strength were observed in all the composites. The predominant mode of failure for the composite was shown to be the cracking of the fiber–matrix interface. Quantitative results from this study will be useful for further and more accurate design of bamboo reinforced composite materials. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 83–92, 2000

Enhancement of reverberant speech using LP residual signal

Abstract: We propose a new method of processing speech degraded by reverberation. The method is based on analysis of short (2 ms) segments of data to enhance the regions in the speech signal having a high signal-to-reverberant component ratio (SRR). The short segment analysis shows that SRR is different in different segments of speech. The processing method involves identifying and manipulating the linear prediction residual signal in three different regions of the speech signal, namely, high SRR region, low SRR region, and only reverberation component region. A weight function is derived to modify the linear prediction residual signal. The weighted residual signal samples are used to excite a time-varying all-pole filter to obtain perceptually enhanced speech. The method is robust to noise present in the recorded speech signal. The performance is illustrated through spectrograms, subjective and objective evaluations.

Microstructural features of dissimilar welds between 316LN austenitic stainless steel and alloy 800

Abstract: For joining type 316LN austenitic stainless steel to modified 9Cr–1Mo steel for power plant application, a trimetallic configuration using an insert piece (such as alloy 800) of intermediate thermal coefficient of expansion (CTE) has been sometimes suggested for bridging the wide gap in CTE between the two steels. Two joints are thus involved and this paper is concerned with the weld between 316LN and alloy 800. These welds were produced using three types of filler materials: austenitic stainless steels corresponding to 316, 16Cr–8Ni–2Mo, and the nickel-base Inconel 182 1 . The weld fusion zones and the interfaces with the base materials were characterised in detail using light and transmission electron microscopy. The 316 and Inconel 182 weld metals solidified dendritically, while the 16–8–2 (16%Cr–8%Ni–2%Mo) weld metal showed a predominantly cellular substructure. The Inconel weld metal contained a large number of inclusions when deposited from flux-coated electrodes, but was relatively inclusion-free under inert gas-shielded welding. Long-term elevated-temperature aging of the weld metals resulted in embrittling sigma phase precipitation in the austenitic stainless steel weld metals, but the nickel-base welds showed no visible precipitation, demonstrating their superior metallurgical stability for high-temperature service.

Selection of optimal conditions in multi-pass face-milling using a genetic algorithm

Abstract: In a Computer-Aided Process Planning (CAPP) system, one of the important steps is the selection of machining parameters which yield optimum results. In this paper, a face-milling operation has been considered. The machining parameters such as number of passes, depth of cut in each pass, speed and feed are obtained using a genetic algorithm, to yield minimum total production cost while considering technological constraints such as allowable speed and feed, dimensional accuracy, surface finish, tool wear and machine tool capabilities.

New dispatching rules for shop scheduling: A step forward

Abstract: This paper provides a set of new dispatching rules for the minimization of various performance measures such as mean, maximum and variance of flow time and tardiness in dynamic shops. A static rule...

Understanding the morphology of fungi

Abstract: Filamentous fungi comprise an industrially very important collection of microorganisms, since they are used for the production of a wide variety of products ranging from primary metabolites to secondary metabolites and further on to industrial enzymes (such as proteases, lipases and antibiotics) It is known that fungal morphology is often considered as one of the key parameters in industrial production For the production of fungal metabolite products, the desired morphology varies from one product to another Many parameters affect the morphology of fungi during the process of fermentation, among them speed of agitation, specific growth rate, dissolved oxygen, number of spores or conidia per liter of fermentation broth are important and should be considered when higher yield is desired in the process It is, therefore, of considerable importance to understand the mechanism underlying the morphology of the cell, its growth and product formation by filamentous fungi Such knowledge may be used in the optimization of the microbial process Several literatures with various fungi to study their morphology, relating enzyme or product production to the character of the fungi in the study is reviewed It is also considered that how the process parameters affects the morphology The aim of this communication is to review the relevant literature to understand the morphology of filamentous fungi

Characterization of surface topography by confocal microscopy : I. Principles and the measurement system

Abstract: Surface topography and, in particular, roughness and form, plays an important role in determining the functional performance of engineering parts. The measurement and understanding of surface topography is rapidly attracting the attention of the physicist, the biologist and the chemist as well as the engineer. Optics in general played an important role in measurement and, with the advent of opto-mechatronics, it is once again at the forefront of measurement. In this paper, the principles and performance of a confocal microscope, together with the measurement system, are described. Suitable fixtures are developed and integrated with the computer system for generating three-dimensional surface and form data. Software for data acquisition, analysis of various parameters including new parameters and visualization of surface geometrical features has been developed. Both the intensity and the auto-focus methods are used to measure two-dimensional surface roughness by use of the system and results are presented. The measurement and characterization of three-dimensional surface topography and form error will be presented in part II of this paper.

Evaluation of circularity from coordinate and form data using computational geometric techniques

Abstract: Data for evaluating circularity error can be obtained from coordinate measuring machines or form measuring instruments. In this article, appropriate methods based on computational geometric techniques have been developed to deal with coordinate measurement data and form data. The computational geometric concepts of convex hulls are used, and a new heuristic algorithm is suggested to arrive at the inner hull. Equi-Distant (Voronoi) and newly proposed Equi-Angular diagrams are employed for establishing the assessment features under different conditions. The algorithms developed in this article are implemented and validated with the simulated data and the data available in the literature.

Statistical optimization of lipase catalyzed hydrolysis of methyloleate by response surface methodology

Abstract: Hydrolysis of methyloleate was optimized using lipase from Chromobacterium viscosum immobilized on IRC-50. The optimization was studied by a statistical methodology using response surface methodology (RSM). The cumulative interactive effect of substrate, water concentration and time was studied in optimizing the hydrolysis of methyloleate. The interactive effect of substrate-time was found to be significant compared to substrate-water and time-water interactions. A well correlation was observed between the optimum values obtained from the response surface contour plots and from the quadratic regression model equation. The optimal values obtained for substrate, water and time were found to be in the experimental range chosen.

Production of microbial chitinases : A revisit

Abstract: Production of microbial chitinases have received increased attention in recent years due to its potential application in various fields. There has been an all around effort to increase the production of chitinases by using genetically engineered organisms and by incorporating modern fermentation techniques. This review presents a brief outline on the production of microbial chitinases which includes information on random screening and selection of chitinolytic organisms and the rationale behind the use of genetically engineered organisms. The problems and perspectives involved in large scale production of chitinases is discussed with special reference to bioreactor studies and new fermentation methods. The present limitations in the understanding of the chitinase fermentation process is also discussed and the scope for future investigation is outlined in this communication.

Production and potential applications of a xylanase from a new strain of Streptomyces cuspidosporus

Abstract: Enzyme production by a new mesophilic Streptomyces isolate was investigated which grew optimally on 1% (w/v) xylan and 10% (w/v) wheat bran at pH 7 and 37 °C. Xylan induced only CMCase (0.29 U/ml) besides xylanase (22–35 U/ml, 40–49 U/mg protein). Wheat bran induced xylanase (105 U/ml, 17.5 U/mg protein), CMCase (0.74 U/ml), β-xylosidase (0.009 U/ml), β-glucosidase (0.026 U/ml), α-L-arabinofuranosidase (0.049 U/ml), amylase (1.6 U/ml) and phytase (0.432 U/ml). The isolate was amenable to solid state cultivation and produced increased levels of xylanase (146 U/ml, 28 U/mg protein). The pH and temperature optima of the crude xylanase activity were 5.5 and 65 °C respectively. The pI was 6.0 as determined by PEG precipitation. The crude enzyme was applied in treatment of paper pulp and predigestion of poultry feed and was found to be effective in releasing sugars from both and soluble phosphorus from the latter.

Microstructural changes during welding and subsequent heat treatment of 18Ni (250-grade) maraging steel

Abstract: Sheet material specimens from 18Ni (250-grade) maraging steel were gas tungsten-arc welded using two different filler wires: one matching the base material in composition and the other with higher cobalt but lower molybdenum and titanium contents. Welding was carried out both in constant-current and pulsed modes. Post-weld aging was performed at three different temperatures, viz., 425, 480 and 520°C. Metallographic characterization revealed pronounced segregation, presumably of Ti and Mo, along interdendritic and intercellular boundaries in the weld metal produced with filler of matching composition. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Segregation and austenite reversion were not noticed when Ti and Mo contents were reduced in the filler wire, except at the highest aging temperature used. The use of pulsed welding resulted in some grain refinement, but the effect was not considerable.

Aqueous two-phase: the system of choice for extractive fermentation.

Abstract: Extractive fermentation in aqueous two-phase systems is a meaningful approach to overcome low product yield in a conventional fermentation process, and by proper design of the two-phase system it is possible to obtain the product in a cell-free stream. The characteristics of an aqueous two-phase system, various polymers used for forming an aqueous two-phase system, the physicochemical parameters of the aqueous two-phase system, partitioning of biomolecules and cell mass and the effect of individual phase forming polymers on cell growth and product formation are reviewed in this article. The various extractive fermentation processes are also summarised here. At the end, the economic viability and scope of aqueous two-phase fermentation are briefly discussed in relation to the wider application of this topic.

Comparative studies on citric acid production by Aspergillus niger and Candida lipolytica using molasses and glucose

Abstract: Citric acid production by Aspergillus niger NCIM 548 and Candida lipolytica NCIM 3472 has been studied in shake culture using glucose and molasses as carbon sources. Methanol addition (3% v/v) at 40 h of fermentation enhanced the production of citric acid by Aspergillus niger whereas a reduction in citric acid production by Candida lipolytica was observed with addition of methanol. Maximum citric acid concentration of 12 kg/m3 was obtained with Aspergillus niger using molasses in the presence of methanol, while maximum citric acid concentration of 8.4 kg/m3 was obtained with Candida lipolytica using glucose without methanol. It appears that product formation by Aspergillus niger is either non-growth associated or partially growth associated depending on the substrate. Methanol addition changes the nature of product formation in case of Candida lipolytica.

On the Landau-Ginzburg description of boundary CFTs and special Lagrangian submanifolds

Abstract: We consider Landau-Ginzburg (LG) models with boundary conditions pre- serving A-type N = 2 supersymmetry. We show the equivalence of a linear class of boundary conditions in the LG model to a particular class of boundary states in the cor- responding CFT by an explicit computation of the open-string Witten index in the LG model. We extend the linear class of boundary conditions to general non-linear bound- ary conditions and determine their consistency with A-type N = 2 supersymmetry. This enables us to provide a microscopic description of special Lagrangian submani- folds in C n due to Harvey and Lawson. We generalise this construction to the case of hypersurfaces in P n . We nd that the boundary conditions must necessarily have vanishing Poisson bracket with the combination (W () W ()), where W ( )i s the appropriate superpotential for the hypersurface. An interesting application considered is the T 3 supersymmetric cycle of the quintic in the large complex structure limit.