The Performance of Zinc Alloy Based Metal Matrix Composites Produced Through Squeeze Casting

Zinc-aluminum cast alloys (ZA alloys) have good castability, mechanical properties and excellent tribological characteristics. Of all the ZA alloys, ZA-27 (containing 27% aluminum) has the highest strength and optimum wear resistance. However all the ZA alloys, including ZA-27, suffer from lack of creep resistance and high temperature stability. One probable solution to improve these properties is to reinforce the alloys with ceramic particles or fibers to result in metal matrix composites (MMCs). MMCs can be economically produced through squeeze casting which involves infiltration. This paper presents the salient features of an experimental study on ZA-27 alloy based MMCs produced through Squeeze Casting.     

Influence of TiB2 content on microstructure and hardness of TiB2-B4C composite

In this study a composite ceramic powder mixture using fine powders of titanium diboride (TiB₂) and boron carbide (B₄C) was consolidated to theoretical density using the technique of plasma pressure compaction. Achieving rapid consolidation of the composite powder mixture is an essential requirement for achieving microstructural control and better mechanical properties in the consolidated end product. The variables chosen in making the composite samples was the titanium diboride (TiB₂) content. The microstructure and hardness of the ceramic composite sample made by consolidating various powder mixtures at a temperature of 1700 °C are compared. Microhardness measurements reveal a gradual increase in hardness with an increase in TiB₂ content in the starting powder mixture. The specific role of TiB₂ content in influencing microstructural development and hardness is presented and discussed.     

Post-traumatic responses to aerial bombing

A refugee population exposed to aerial bombing was assessed for psychosocial sequelae within two months. Forty-three members over 15 years were interviewed using a structured questionnaire. Seventy-four percent had experienced an immediate but transient stress reaction. Subsequently, PTSD, anxiety, depressive and somatic symptoms were common. Forty-four percent met the DSM III diagnostic criteria for PTSD. In addition, social withdrawal, irritability and hostility, interpersonal relationship problems and functional disability were found. Although a variety of psychosocial symptoms were seen, it is suggested that part of this response be considered as manifestation of a healthy, normal attempt to cope with a severely traumatizing experience. The effects of collective trauma and social methods of treatment are also described.     

Mechanical spectroscopy of nanocrystalline aluminum films: effects of frequency and grain size on internal friction

Energy dissipation by internal friction is a property of fundamental interest for probing the effects of scale on mechanical behavior in nanocrystalline metallic films and for guiding the use of these materials in the design of high-Q micro/nanomechanical resonators. This paper describes an experimental study to measure the effects of frequency, annealing and grain size on internal friction at room temperature in sputter-deposited nanocrystalline aluminum films with thicknesses ranging from 60 to 120 nm. Internal friction was measured using a single-crystal silicon microcantilever platform that calibrates dissipation against the fundamental limits of thermoelastic damping. Internal friction was a weak function of frequency, reducing only by a factor of two over three decades of frequency (70 Hz to 44 kHz). Annealing led to significant grain growth and the average grain size of 100 nm thick films increased from 90 to 390 nm after annealing for 1 h at 450?(°)C. This increase in grain size was accompanied by a decrease in internal friction from 0.05 to 0.02. Taken together, these results suggest that grain-boundary sliding, characterized by a spectrum of relaxation times, contributes to internal friction in these films.     

Degradation of rice straw by Pleurotus flabellatus

Degradative changes in the constituents of rice straw substrate during the growth and commercial production of fruit bodies by Pleurotus flabellatus were investigated. There was a continuous increase in the free sugars from the time of inoculation until the end of harvesting of the fruit bodies, whereas the free amino acids reached a maximum at the end of mycelial growth and showed a slight decrease after fructification. Glucose, xylose, arabinose, cellobiose and a higher oligosaccharide represented the spectrum of sugars produced. Of the 12 amino acids (apart from two peptides) identified, aspartic acid, histidine and tyrosine were the major components. Solubilities and spectral characteristics of lignin degraded in the straw substrate were studied using alkali-extracted lignin. Laccase and protease activities in the straw substrate at different stages of fungal degradation could be correlated to the decreases in the staw lignin and protein contents, respectively. Colour reactions of the fungus on several phenolic media, demonstration of phenolic oxidases in the straw substrate, and ability of the fungus to degrade lignin conclusively proved that the fungus under study is a typical ‘white rot’ fungus. Degradation of the straw substrate by the fungus reached a maximum at the end of harvest of the fruit bodies. The spent straw substrate (left over after harvesting the fruit bodies) contained free sugars and amino acids, respectively 5 and 2 times the concentration of those in the undegraded straw substrate, and lignin decreased to about 25% of the level in the straw before fermentation. Potential applications of the spent straw substrate as an upgraded form of rumen feed and for the production of single-cell protein are discussed.     

Controllability of nonlinear systems consisting of a bilinear mode with time-varying delays in control

Sufficient conditions are established for the global relative controllability of nonlinear system consisting of a bilinear mode with time-varying delays in control. The results are a generalization of previous results and are obtained by using Schauder's fixed point theorem.     

Concentrations of four essential trace elements in breast milk of mothers from two socio-economic groups: preliminary observations

Concentrations are reported for Cu, Mn, Mo and Zn in human milk samples obtained from mothers belonging to low and middle per capita family income groups. The trace element levels in milk of both groups are compared for samples obtained at two stages, viz. 3-5 days and 4-6 weeks post partum. Although the Mn level during the first stage appears to be lower in samples from the low income group, the difference is not statistically significantly (p less than or equal to 0.01). However, at 4-6 weeks post partum, Zn is significantly lower (p less than or equal to 0.01) in milk of mothers belonging to this group.     

Effect of manufacturing errors on the performance of aerostatic journal bearings

The results of a theoretical analysis of the effect of manufacturing errors on the performance of aerostatic journal bearings is reported. Investigations carried out to study the influence of (a) geometrical form errors such as ovality, lobing and cylindricity and (b) roughness patterns such as one-dimensional longitudinal roughness and two-dimensional isotropic roughness show that manufacturing errors which cause deviations from ideal surface geometry significantly affect the performance characteristics of aerostatic journal bearings.