Cellular network configuration with co-channel and adjacent-channel interference constraints

Design of cellular networks has drawn much recent interest from the OR scientific community. A challenging issue is the handling of channel interference constraints. Co-channel interference occurs when the same channel is reused within a threshold distance. Adjacent-channel interference occurs when two channels with adjacent or nearby frequencies are used in the same cell tower. We present a mathematical programming formulation for this channel allocation problem with both types of interference constraints—it also includes decisions on location of cell towers. Our focus is on the special case where a cell tower and/or channel can interfere with at most two other towers/channels. By establishing theoretical properties for channel allocation amongst towers under this circumstance, we develop an efficient solution procedure. An iteration of the procedure uses a heuristic to locate the cell towers, then allocates the channels to the towers using a polynomial-time algorithm, and finally improves this allocation using a simulated annealing procedure. The iterative steps are embedded within an external simulated annealing method. This nested simulated annealing procedure provides encouraging computational results compared to a standard commercial solver like ILOG CPLEX 8.1. The major contribution of the work is the simultaneous consideration of co-channel and adjacent-channel interference constraints.     

Online monitoring of the thermal degradation of POM during melt extrusion

Because of its high stiffness, chemical resistance, and low viscosity, Poly (oxymethylene) (POM) is of high relevance for technical applications. The thermal degradation of POM during processing affects its final properties and decreases the long‐term stability. The degradation is indicated by the emission of formaldehyde (FA) gas. The aim of this study is to monitor the thermal degradation of POM online, during the melt extrusion in a co‐rotating twin screw extruder (TSE). The effect of the processing conditions on the thermal stabilisation of the POM is observed by FA emission and online viscosity measurements. The effect of processing conditions on the compounding of POM with two different FA scavengers is also studied. Fourier transform infrared (FTIR) spectroscopy is used for the online measurement of FA gas and acetyl acetone colour measurement for the offline characterisation. The online viscosity is measured by passing the melt through a slit die at constant volume flow rate. An enhanced thermal degradation is found with decreasing throughput and increasing screw speed. A good correlation between the online viscosity and offline FA measurement is observed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Studies on gas–solid heat transfer during pneumatic conveying

Interactions between solids and gas during pneumatic conveying can be utilized for variety of applications including flash drying, solids preheating etc. Experiments on air–solid heat transfer were carried out in a vertical pneumatic conveying heat exchanger of 54 mm inside diameter, using gypsum as the solid material. The effect of solids feed rate (0.6–9.9 g/s), air velocity (4.21–6.47 m/s) and particle size (231–722.5 μm) on air–solid heat transfer rate, heat transfer area and air–solid heat transfer coefficient has been studied. Empirical correlations have been proposed for the prediction of Nusselt number based on the present experimental data. The proposed correlations predict Nusselt number within an error of ±15% for the present data.     

Growth of low dislocation density single crystals of nickel

Low dislocation density single crystals of nickel have been grown at high ambient pressure by the Czochralski method. X-ray Laue picture shows that the crystals are strain-free. The dislocation density was determined to be <10³/cm² by the etching procedure. It was found that the necking and cone regions are very critical in the dislocation introduction in the crystals. An increase in the ambient pressure used during the growth seems to aid the crystal quality.     

Dielectric properties of the system Pb2 −x Nd x Li0·5Nb1·5 O6+δ

The dielectric properties of the ceramic defect pyrochlore-structured system Pb_2−x Nd _x Li_0·5Nb_1·5 O_6+δ has been studied as a function of concentrationx, frequency and temperature. The results have been analyzed for relaxation process and conduction mechanism. Now deceased.     

Facile synthesis of poly(4‐hydroxy styrene) from polystyrene

A facile methodology to modify polystyrene, to incorporate alkali soluble hydroxy groups, is reported. Polystyrene of well‐defined number‐average molecular weights in the range of 3000 to 50,000 were prepared by suspension polymerization at 80°C by varying the relative mole ratio of carbon tetrachloride, used as chain transfer agent. Polystyrenes were acetylated; the acetyl groups were converted to acetoxy groups, by Baeyer–Villiger oxidation, and hydrolyzed in a nonaqueous alkaline medium to generate hydroxy groups. The extents of modifications were monitored by infrared spectroscopy. The glass transition temperatures of poly(4‐acetylstyrene‐co‐styrene), poly(4‐acetylstyrene‐co‐4‐acetoxystyrene‐co‐styrene), and poly(4‐hydroxystyrene‐co‐4‐acetylstyrene‐co‐styrene) were 107, 134, and 142°C, respectively. The polymers were evaluated in photoresist formulations. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3192–3201, 2004     

Developments in Processing of Functionally Gradient Metals and Metal–Ceramic Composites:A Review

Functionally gradient/graded materials(FGMs), an emerging new class of materials, are the outcome of the recent innovative concepts in materials technology. FGMs are in their early stages of evolution and expected to have a strong impact on the design and development of new components and structures with better performance. FGMs exhibit gradual transitions in the microstructure and/or the composition in a specific direction, the presence of which leads to variation in the functional performance within a part. The presence of gradual transitions in material composition in FGMs can reduce or eliminate the deleterious stress concentrations and result in a wide gradation of physical and/or chemical properties within the material. Functionally graded metal–ceramic composites are also getting the attention of the researchers. Among the fabrication routes for FGMs such as chemical vapour deposition, physical vapour deposition, the sol–gel technique, plasma spraying, molten metal infiltration, self propagating high temperature synthesis, spray forming,centrifugal casting, etc., the ones based on solidification route are preferred for FGMs because of their economics and capability to make large size products. The present paper discusses and compares various solidification processing techniques available for the fabrication of functionally gradient metals and metal–ceramic composites and lists their properties and possible applications. The other processing methods are briefly described.