Characteristics of Milk Powders Produced by Spray Freeze Drying

Skim and whole milk powders were manufactured at lab scale by spray freeze drying (SFD), using liquid nitrogen as the cryogen. The polydispersity of droplet/particle sizes was limited using an encapsulator nozzle to atomize the feed. Particle morphology was examined using a scanning electron microscope. Samples were compared with equivalent spray-dried powders in tests of wettability and dissolution in water. The spray freeze-dried powders were found to be highly porous, with a uniform structure of pores throughout the entire particles. When tested in water, SFD skim milk powders wetted roughly three times as fast as industrially spray-dried agglomerated skim milk powders and were observed to dissolve rapidly by breaking down into smaller particles.     

A simplified method for analyzing mold filling dynamics. Part II: Extensions and comparisons with experiment

A dimensional analysis based on four parameters has been developed previously to predict injection pressure; clamp force, and bulk temperature for the injection molding of amorphous materials in center-gated disk-shaped cavities. In this paper geometric and semicrystalline-materials approximations are introduced and tested for extending the previous analysis to include multigated thin cavities and semicrystalline materials. The combination of these approximations and the previous analysis, known hereafter as the Radial Flow Method (RFM), greatly simplifies the analysis of mold filling. The geometric approximation, which is based on a simple model for the axial stress distribution in the cavity, is shown to give reasonable predictions when compared with experimental data and a numerical two-directional flow simulation for the filling of an off-center-gated rectangular cavity with acrylonitrilebutadiene-styrene copolymer (ABS). The semicrystallinematerials approximation, in which heat capacity and viscosity changes during crystallization are neglected, is shown to give good agreement with experimental data for the filling of a center-gated disk-shaped cavity with polypropylene. As an illustration, the Radial Flow Method is used to analyze the molding of a large, thin-wall automobile interior trim panel. The inlet melt temperature, mold-wall temperature, part thickness, injection rate, viscosity and gate locations are varied in a series of calculations to determine the relative effectiveness of these variables in lowering the injection pressure and Clamp force. The results obtained with the Radial Flow Method are in good agreement with those obtained by a finiteelement simulation of two-directional flow.     

Numerical modeling of energy balance equations in quantum well AlxGa1-xAs/GaAs p-i-n photodiodes

The energy balance equations coupled with drift diffusion transport equations in heterojunction semiconductor devices are solved modeling hot electron effects in single quantum well p-i-n photodiodes. The transports across the heterojunction boundary and through quantum wells are modeled by thermionic emission theory. The simulation and experimental current-voltage characteristics of a single p-i-n GaAs/Al _xGa_1-xAs quantum well agree over a wide range of current and voltage, The GaAs/Al_xGa_1-xAs p-i-n structures with multi quantum wells are simulated and the dark current voltage characteristics, short circuit current, and open circuit voltage results are compared with the available experimental data, In agreement with the experimental data, simulated results show that by adding GaAs quantum wells to the conventional cell made of wider bandgap Al_x Ga_1-xAs, short circuit current is improved, but there is a loss of the voltage of the host cell, In the limit of radiative recombination, the maximum power point of an Al_0.35Ga_0.65As/GaAs p-i-n photodiode with 30-quantum-well periods is higher than the maximum power point of similar conventional bulk p-i-n cells made out of either host Al_0.35Ga_0.65As or bulk GaAs material     

Design and performance analysis of MAC schemes for Wireless Sensor Networks Powered by Ambient Energy Harvesting

Energy consumption is a perennial issue in the design of wireless sensor networks (WSNs) which typically rely on portable sources like batteries for power. Recent advances in ambient energy harvesting technology have made it a potential and promising alternative source of energy for powering WSNs. By using energy harvesters with supercapacitors, WSNs are able to operate perpetually until hardware failure and in places where batteries are hard or impossible to replace. In this paper, we study the performance of different medium access control (MAC) schemes based on CSMA and polling techniques for WSNs which are solely powered by ambient energy harvesting using energy harvesters. We base the study on (i) network throughput (S), which is the rate of sensor data received by the sink, (ii) fairness index (F), which determines whether the bandwidth is allocated to each sensor node equally and (iii) inter-arrival time (γ) which measures the average time difference between two packets from a source node. For CSMA, we compare both the slotted and unslotted variants. For polling, we first consider identity polling. Then we design a probabilistic polling protocol that takes into account the unpredictability of the energy harvesting process to achieve good performance. Finally, we present an optimal polling MAC protocol to determine the theoretical maximum performance. We validate the analytical models using extensive simulations incorporating experimental results from the characterization of different types of energy harvesters. The performance results show that probabilistic polling achieves high throughput and fairness as well as low inter-arrival times.     

Sub-5 keV electron-beam lithography in hydrogen silsesquioxane resist

We fabricated 9-30 nm half-pitch nested Ls and 13-15 nm half-pitch dot arrays, using 2 keV electron-beam lithography with hydrogen silsesquioxane (HSQ) as the resist. All structures with 15 nm half-pitch and above were fully resolved. We observed that the 9 and 10-nm half-pitch nested Ls and the 13-nm-half-pitch dot array contained some resist residues. We obtained good agreement between experimental and Monte-Carlo-simulated point-spread functions at energies of 1.5, 2, and 3 keV. The long-range proximity effect was minimal, as indicated by simulated and patterned 30 nm holes in negative-tone resist.     

Analysis of demand‐priority access using a priority queue with server vacations and message dependent switchover times

In this paper, we analyse the message waiting times in a local area network (LAN) that uses the demand‐priority access method. This is a priority‐based round‐robin arbitration method where the central controller (the repeater) polls its connected ports to determine which have transmission requests pending and the requests' priority classes. We model it as a 2‐priority M/G/1 queue with multiple server vacations and switchover time between service periods. The service discipline is nonpreemptive and the length of the switchover time is dependent upon the priority class of the preceding message served as well as that of the message to be served next. We provide an approximate analysis for the waiting times of both message classes and derive expressions for the Laplace–Stieltjes transforms (LST) of the stationary distributions and the mean waiting times. We conclude with numerical and simulation results to show the applicability and accuracy of the analytical model. This revised version was published online in June 2006 with corrections to the Cover Date.     

Engineering Management-a personal perspective

Based on my experience as an engineer and educator, I believe that "engineering leadership" rather than "engineering management" should be the goal of engineers in key roles and the focus of their management education. I would also like to see programs in technical organizations led by engineering leaders rather than nontechnical managers. I believe that it is more beneficial to enhance the leadership potential of an engineer than to educate the nontechnical manager in technology.     

Active Corrosion of Sintered α-Silicon Carbide in Oxygen–Chlorine Gases at Elevated Temperatures

The active corrosion of sintered α-silicon carbide from heat exchanger tubes in the temperature range 900° to 1100°C in gas mixtures containing 2% Cl₂ by volume with additions of O₂ or H₂ has been investigated by thermogravimetric analysis and subsequent examination of the corrosion products. The presence of a small amount of oxygen accelerated rapid active corrosion in chlorine-containing gas mixtures, but the corrosion was suppressed by an active-to-passive transition when the concentration of oxygen in the gas mixture was too high. Low rates of attack were observed in the environments containing H₂ even when the chlorine potential was high. The concentration of oxygen necessary to produce the active-to-passive transition was found to vary from one material to another and may be related to the amount of excess carbon in the sintered silicon carbide.