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In hierarchical sensor networks, higher powered relay nodes can be deployed to act as cluster heads. The lifetime of such a network is primarily determined by the lifetime of the relay nodes. Recently, it has been shown that the use of a Mobile Data Collector (MDC), which moves through the sensor network to collect data, can improve the performance of the network in a number of ways. We propose a model for hierarchical networks using a MDC, where the MDC travels along a pre-determined trajectory at a known speed, and collects data from each relay node. In this model, the relay nodes need to buffer their data until they can be uploaded to the MDC. To reduce the buffer size, as well as the delay for the data communication, it is desirable to minimize the length of the trajectory. In our model the MDC "visits" a relay node, to collect data, at any point within the permitted transmission distance of the relay node. There is a trade-off in this approach, as a large permitted transmission distance means that the relay node will expend higher energy, and hence, have a reduced lifetime. In this paper, we present two formulations for computing the minimal trajectory of the MDC. Our approaches allow us to control the transmission distances of the relay nodes and hence, achieve the lifetime objectives. Minimizing the length of the trajectory reduces the buffer size requirements at each node as well as the delay between successive visits.

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Zr oxide coatings were deposited using plasma electrolytic oxidation (PEO) on Zr-2.5wt%Nb (Zr-2.5Nb) alloy, which is currently used for pressure tubes in the CANDU nuclear reactor. The effects of two PEO processing factors, electrolyte concentration and current density, on the microstructure and properties of the coatings were systematically investigated. The coating morphology and chemical composition were determined using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Potentiodynamic polarization corrosion testing was used to examine the corrosion resistance of the coatings in a dilute aqueous, LiOH solution. Pin-on-disc wear tests were performed under dry and lubricated sliding conditions. SEM was also used to characterize the wear traces on the coated and uncoated surfaces. A 30-day autoclave experiment was carried out to study the corrosion performance of the Zr-2.5Nb substrate, PEO coatings and a commercial autoclaved black oxide coating. Based on the results of this research, recommendations are made as to processing parameters for the production of oxide coatings with improved wear and corrosion resistance.

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