Showing papers by "Cochin University of Science and Technology published in 1988"

Reactively evaporated films of indium sulphide

Abstract: s-In2S3 films are prepared by reactively evaporating indium in an atmosphere of sulphur. X-ray diffraction studies show that the grains have no preferred orientation on the substrate surface. Film properties (surface smoothness, optical transmission, and refractive index) are optimized in the substrate temperature interval (425 ± 50) K and this is explained in terms of the concept of critical optimization of Vincett. Fundamental absorption starts at (2.01 ± 0.01) eV and the transition leading to this is a direct forbidden one. Charge carriers in the film are n-type and the activation energy for electronic conduction is (0.26 ± 0.02) eV. s-In2S3-Schichten werden mittels reaktivem Verdampfen von Indium in einer Schwefelatmosphare prapariert. Rontgenbeugungsuntersuchungen zeigen, das die Korner auf der Substratoberflache keine Vorzugsorientierung haben. Die Schichteigenschaften (Oberflachenglattheit, optische Durchlassigkeit und Brechungsindex) werden im Substrat-Temperaturbereich (425 ± 50) K optimiert und dies wird mit dem Konzept der kritischen Optimierung von Vincett erklart. Die Grundgitterabsorption beginnt bei (2,01 ± 0,01) eV und der dazu fuhrende ubergang ist ein direkter verbotener ubergang. Die Ladungstrager in der Schicht sind vom n-Typ und die Aktivierungsenergie fur Elektronenleitung betragt (0,26 ± 0,02) eV.

Production of hydroxyl‐terminated liquid natural rubber—mechanism of photochemical depolymerization and hydroxylation

Abstract: Photochemical degradation of natural rubber yielded hydroxyl-terminated liquid natural rubber (HTNR) when carried out in solution in presence of H2O2. Ultraviolet radiation from a medium pressure mercury vapor lamp and sunlight were found to be almost equally effective in bringing about the depolymerization and hydroxylation of natural rubber. The variations in the composition of the reagents and exposure time on the extent of depolymerization was conducted, and a suitable procedure for the large scale preparation of HTNR was described. A probable mechanism leading to the formation of HTNR as well as the side products is discussed based on the analytical data.

Studies on the Cure Characteristics and Vulcanizate Properties of 50 / 50 NR /SBR Blend

Abstract: Blends of 50/50 natural rubber (NR) and styrene-butadiene rubber (SBR) are vulcanized using several conventional and semi-EV systems. The cure characteristics and vulcanizate properties are compared. The quantity and quality of crosslinks in each case are deciphered by chemical probes to correlated them with the vulcanizate properties.

A Characterization of the Bivariate Exponential Distribution

Abstract: In this article a characterisation of the Gumbel's Bivariate Exponential Distribution is established on the basis of the properties of the conditional expectation of the component variables. The characterising property is proposed as the definition of lack of memory in the bivariate case.

An (s,S) inventory system with rest periods to the server

Abstract: We study the (s,S) inventory system in which the server takes a rest when the level of the inventory is zero. The demands are assumed to occur for one unit at a time. The interoccurrence times between successive demands, the lead times, and the rest times are assumed to follow general distributions which are mutually independent. Using renewal and convolution techniques we obtain the state transition probabilities.

A study of geotechnical properties of Cochin Marine Clays

Abstract: Most of the Greater Cochin area, which is undergoing rapid industrialisation, consists of extremely soft marine clay calling for expensive deep foundations. This paper presents a study on the physical properties and engeering characteristics of Cochin marine clays. These marine clays are characterised by high Atterberg limits and natural water contents. They are moderately sensitive with liquidity indices ranging over 0.46 to 0.87.The grain size distribution shows almost equal fractions of clay and silt size with sand content varying around 20%. Use of a dispersing agent in carrying out grain size distribution test plays an important role. The fabric of these clays had been identified as flocculant. The pore water has low salinity which results in marginal changes in properties on washing.Consolidation test results showed a preconsolidation pressure of up to about 0.5 kg/cm2 with high compression indices. Compression index vs liquid limit yielded a correlation comparable to that of published data. The undisturbed samples have a much larger coefficient of secondary consolidation as a result of flocculant fabric. These clays have very low undrained shear strength.

Optical absorption and thermal diffusivity in GexTe100−x glasses by the photoacoustic technique

Abstract: The variation in the optical energy gap with composition in the GexTe100-x system (15≤x≤28) of glasses in bulk form has been studied using photoacoustic techniques. It is found that the optical energy gap increases with increasing composition factor x, with the rate of increase showing a slowing-down trend for compositions with x > 20. The composition dependence of the thermal diffusivity of these samples has also been measured. The thermal diffusivity is found to peak around the composition corresponding to x = 20. The observed phenomena are explained on the basis of chemical bond formation in this system of glasses. The change in the short-range order and increase in the number of Te-Te bonds with decreasing x are interpreted as being responsible for the behaviour exhibited by these materials.

Effect of dissolved NaCl, MgCl(2), and Na(2)SO(4) in seawater on the optical attenuation in the region from 430 to 630 nm.

Abstract: The split-pulse laser method is used to reinvestigate the optical attenuation of distilled water in the region from 430 to 630 nm. The studies are then extended to ionic solutions of NaCl, MgCl(2), and Na(2)SO(4), these salts forming the major constituents of seawater. The effect of the concentration of these constituents on optical attenuation is investigated. Further, optical attenuation studies are carried out for the region from 430 to 630 nm for an aqueous solution prepared with all the major constituents in the same proportions as in natural seawater. These values are then compared with values obtained for natural seawater. The relative role of dissolved salts and suspended particles on optical attenuation in seawater is discussed. The lowest attenuation is observed at ~450 nm for all solutions and is found to coincide with that for distilled water.

Percolation threshold of thermal conduction in A x IV B 1 − x VI chalcogenide semiconducting glasses

Abstract: The thermal diffusivity of three members of the ${A}_{x}^{\mathrm{IV}{B}_{1\mathrm{\ensuremath{-}}x}^{\mathrm{VI}}}$ family of semiconducting alloy chalcogenide glasses with 0.1\ensuremath{\le}x\ensuremath{\le}0.28 have been measured by the photoacoustic technique. It is found that in all the systems studied, the thermal diffusivity and hence the thermal conductivity exhibit a threshold maximum value at the critical composition ${x}_{c}$=0.20. The observation is attributed to the threshold percolation of rigidity in the glass network at this critical composition.

On characterizing the bivariate exponential and geometric distributions

Abstract: In this note, a characterization of the Gumbel's bivariate exponential distribution based on the properities of the conditional moments is discussed. The result forms a sort of bivariate analogue of the characterization of the univariate exponential distribution given by Sahobov and Geshev (1974) (cited in Lau and Rao ((1982), Sankhyā Ser. A, 44, 87)). A discrete version of the property provides a similar conclusion relating to a bivariate geometric distribution.

Transient solution for a finite capacity M/G a,b /1 queueing system with vacations to the server

Abstract: In this paper we consider a single server queueing system with Poisson input, general service and a waiting room that allows only a maximum of ‘b’ customers to wait at any time. A minimum of ‘a’ customers are required to start a service and the server goes for a vacation whenever he finds less than ‘a’ customers in the waiting room after a service. If the server returns from a vacation to find less than ‘a’ customers waiting, he begins another vacation immediately. Using the theory of regenerative processes we derive expressions for the time dependent system size probabilities at arbitrary epochs.

Optical energy gap and thermal diffusivity of GeSe semiconducting glasses

Abstract: The photoacoustic (PA) technique is used to study the variation of the optical energy gap and to measure the thermaldiffusivity of GexSe100−x (10 ≦ x ≦ 38) glasses. The optical energy gaps of these samples with different values of x are determined from the PA spectrum which also gives features of the optical absorption in the subband gap region. The variation of the thermal diffusivity with composition is also investigated. The observed behaviour is explained on the basis of chemical bonding between the constituents and the chemical threshold and mechanical percolation threshold behaviour of the network. Die photoakustische (PA) Technik wird benutzt, um die Anderung der optischen Energielucke zu untersuchen und das thermische Diffusionsvermogen von GexSe100−x (10 ≦ x ≦ 38)-Glasern zu messen. Die optische Energielucke dieser Proben mit verschiedenen x- Werte wird aus dem PA-Spektrum bestimmt, das ebenfalls die Einzelheiten der optischen Absorption im Energieluckenbereich der Subbander wiedergibt. Die Anderung des thermischen Diffusionsvermogens mit der Zusammensetzung wird ebenfalls-untersucht. Das beobachtete Verhalten wird auf der Grundlage der chemischen Bindung zwischen den Konstituenten und der chemischen Schwelle und dem mechanischen Perkolationsschwellenverhalten des Netzwerks erklart.

Seismicity of the North-East India Region

Abstract: The north-east India region is seismically one of the most active regions In the world. An earthquake data base for the region from the earliest time to 1984 is presented here. These data would be utilized for investigating the earthquake phenomenon in the region.

Lack of correlation between DST results and urinary MHPG in depressed inpatients.

Abstract: Abnormalities of noradrenaline metabolism and of the activity of hypothalamic-pituitary adrenal axis (HPA) have been reported in depression. To study the possible relationship between these 2 parameters, urinary excretion of 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG) and Dexamethasone Suppression Test (DST) were analyzed in 58 depressed patients. A positive correlation was found between the age of depressed patients and 24-h urinary excretion of MHPG. Twenty-two patients (38%) were DST non suppressors. Pre-DST plasma cortisol levels were significantly higher in non suppressors than suppressors. No difference was found however between urinary MHPG levels in supressors and non suppressors. There was no correlation between pre-DST plasma cortisol and levels of urinary excretion of MHPG. These results do not support the hypothesis of a relationship between these 2 parameters. However, when depressed patients were separated into two groups according to urinary excretion of MHPG (“high MHPG” and “low MHPG”), the “high MHPG” group included significantly more non suppressors then the “low MHPG” one. This result is not sufficient to demonstrate of link between HPA system activity and central noradrenaline metabolism.

Sublethal effects of copper and mercury on some biochemical constituents of the estuarine clam Villorita cyprinoides var. cochinensis (Hanley).

Abstract: The usefulness of sentinal organisms in environmental pollution monitoring or surveillance programs has been well established. A particularly significant attribute of sublethal physiological response is that it is amenable to both laboratory and field measurements unlike traditional toxicant testing. Such methods could be employed to develop environmental quality models to predict the biological effects of potential pollutants as well as to directly monitor the effects in the environment. This paper describes the effect of sublethal amounts of two well known aquatic pollutants namely Cu and Hg on an estuarine clam Villorita cyprinoides var. cochinensis. The changes in carotenoid content (total and unsaponifiable), metabolic rate, lactic acid as well as glycogen contents of the tissues of the clam exposed to sublethal amounts of Cu(II) and Hg(II) were investigated over a range of time.

Effect of vulcanization temperature on the cure characteristics and vulcanizate properties of natural rubber and styrene‐butadiene rubber

Abstract: Gum compounds of natural rubber (NR) and styrene-butadiene rubber (SBR) are vulcanized at different temperatures and the cure characteristics and vulcanizate properties are compared. The optimum cure time at a particular vulcanization temperature could be expressed empirically in terms of that temperature using an exponential relation. The crosslink density of the vulcanizates is found to decrease with an increase in the vulcanization temperature. For a given compound there is a particular vulcanization temperature which gives the best compromise of properties for specific applications. Mischungen von Elastomeren wie Naturkautschuk und Styrol-Butadien-Kautschuk werden bei verschiedenen Temperaturen vulkanisiert und die Behandlungscharakteristika und Vulkanisateigenschaften verglichen. Die optimale Behandlungszeit bei einer bestimmten Vulkanisationstemperatur konnte empirisch durch eine Exponentialbeziehung dieser Temperatur ausgedruckt werden. Die Vernetzungsdichte der Vulkanisate nimmt mit steigender Vulkanisationstemperatur ab. Fur eine vorgegebene Mischung gibt es eine bestimmte Vulkanisationstemperatur, bei der die Materialien die besten Eigenschaften fur spezifische Anwendungen besitzen.

Physiology of phytoplankton in relation to metal concentration: effect of cadmium on Scenedesmus bijugatus and Nitzschia palea

Abstract: The effect of Cd at sublethal concentrations on pure cultures of S. bijugatus and N. palea was studied with special reference to their physiology during exponential growth phase. Photosynthetic productivity, respiration, pH, biomass, pigments, carbohydrate, protein, lipid as well as nutrient uptake were selected to study the comprehensive effect of the metal. The physiology of the species was significantly affected by Cd during the exponential growth phase and altered the ratio of carbohydrate, protein and lipid in the test species.

Transient solution of a finite‐capacity M/Ga,b/1 queueing system

Abstract: In this article we consider a single-server, bulk-service queueing system in which the waiting room is of finite capacity. Arrival process is Poisson and all the arrivals taking place when the waiting room is full are lost. The service times are generally distributed independent random variables and the distribution is depending on the batch size being served. Using renewal theory, we derive the time-dependent solution for the system-size probabilities at arbitrary time points. Also we give expressions for the distribution of virtual waiting time in the queue at any time t.

Effect of vulcanization temperature on the cure characteristics and vulcanizate properties of natural rubber and styrene-butadiene rubber

Abstract: La densite de reticulation du vulcanisat diminue quand augmente la temperature de vulcanisation

Scanning Electron Microscopy Examination of the Fracture Surface of NR/SBR and NR/BR Blends

Abstract: Blending of two or more rubbers is carried out for three main reasons,1 (1) improvement in technical properties (2) better processing (3) lower compound cost. Many products in the rubber industry are based on blends in all or part of their construction. Tyres are typical examples of products in large-scale volume production. Natural rubber (NR)/styrene-butadiene rubber (SBR) and natural rubber/polybutadiene rubber (BR) blends are extensively used in the manufacture of tyres.2 Scanning electron microscopy (SEM) is now widely employed to observe the microstructure of fracture surfaces and hence to study the fracture mechanism3-5. Fracture may be caused due to faults in the material or operating conditions. The fracture surfaces of polymeric materials show many characteristic features which enable the destructive influence to be recognized. Since the mechanical behaviour of blended rubbers is very sensitive to the filler distribution and level,6-8 curing aspects,9-11 compatibility of the rubber12-14...

Transition to chaos in a driven pendulum with nonlinear dissipation

Abstract: The Melnikov-Holmes method is used to study the onset of chaos in a driven pendulum with nonlinear dissipation. Detailed numerical studies reveal many interesting features like a chaotic attractor at low frequencies, band formation near escape from the potential well and a sequence of subharmonic bifurcations inside the band that accumulates at the homoclinic bifurcation point.

Saturable absorbers for low power optical phase conjugation

Abstract: Properties of a few saturable absorbers as candidates for low power optical phase conjugation (OPC) are examined. The specific systems studied are the dyes eosin, erythrosin B and Rose Bengal, doped in gelatin, polyvinyl alcohol and boric acid glass films. A method for quickly checking saturated absorption is suggested and demonstrated. Examples for OPC with these systems are cited and limitations are briefly discussed.

Energy gap studies on As-Sb-Se glasses

Abstract: Variation of optical energy gapE 0 with composition in several glasses belonging to the As-Sb-Se system have been studied. The photoacoustic technique, well-suited for highly absorbing samples, has been used for the measurements. Results reported on the As x Sb5Se95−x , As x Sb10Se90−x , As x Sb15Se85−x and As x Sb20Se80−x glasses of the As-Sb-Se family are explained on the basis of the chemically ordered network model.

Use of Padé approximants in the evaluation of α and δ for one-dimensional maps

Abstract: Recently an analytic algorithm for evaluating the Feigenbaum indices of one-dimensional maps was developed using a perturbative expansion. We find that the use of Pade approximants in the resulting asymptotic series, significantly improves the technique.

Medium-term earthquake prediction

Abstract: There is a worldwide effort to study and identify spatial and temporal variation of seismicity and recognize patterns that precede earthquakes “Mogi, 1969, 1984; Ohtake et al, 1977a, b; Habermann, 1981; Wyss et al, 1983; Liu Puxiong et al, 1984; Ohnaka, 1984–1985; and many others”. Gupta and Singh “1986” carried out a detailed study of earthquake swarms and quiescence which precede major earthquakes in the northeast India region and concluded: 1. Moderate magnitude to great earthquakes in the northeast India region are found to be preceded, generally, by well-defined earthquake swarms and quiescence periods.