Fluidic packaging of microengine and microrocket devices for high pressure and high temperature operation

The fluidic packaging of Power MEMS devices such as the MIT microengine and microrocket requires the fabrication of hermetic seals capable of withstanding temperature in the range 20-600/spl deg/C and pressures in the range 100-300 atm. We describe an approach to such packaging by attaching Kovar metal tubes to a silicon device using glass seal technology. Failure due to fracture of the seals is a significant reliability concern in the baseline process: microscopy revealed a large number of voids in the glass, pre-cracks in the glass and silicon, and poor wetting of the glass to silicon. The effects of various processing and materials parameters on these phenomena were examined. A robust procedure, based on the use of metal-coated silicon substrates, was developed to ensure good wetting. The bending strength of single-tube specimens was determined at several temperatures. The dominant failure mode changed from fracture at room temperature to yielding of the glass and Kovar at 600/spl deg/C. The strength in tension at room temperature was analyzed using Weibull statistics; these results indicate a probability of survival of 0.99 at an operational pressure of 125 atm at room temperature for single tubes and a corresponding probability of 0.9 for a packaged device with 11 joints. The residual stresses were analyzed using the method of finite elements and recommendations for the improvement of packaging reliability are suggested

A MULTILEVEL APPROACH TOWARD RELATING SUBJECTIVE WORKLOAD AND EFFORT TO PERFORMANCE DURING STABLE AND AFTER SHIFTS IN TASK DEMAND

This laboratory study tested a causal model of the effects of changes in perceptions of subjective workload and effort in relation to performance during stable and after shifts in task demand. Accordingly, this study addressed the need for within-person examinations of how individuals as a function of self-regulation respond to shifts in task demand. Participants were 198 university undergraduates who were trained to perform a computer game representing a complex decision-making environment. Subjective workload, subjective cognitive effort, and objective performance were concurrently measured at regular intervals (i.e., every 60 s) in five 10-minute trials, two of which involved a shift, either an increase or decrease, in task demand. Relationships between variables were examined using a longitudinal, multilevel approach suitable for disaggregating within-person (i.e., state) and between-person (i.e., trait) components. The proposed model reflecting inconsistent mediation was consistently supported when conditions involved stable task demands. Specifically, in trials involving stable demands, changes in subjective workload had positive indirect effects but stronger negative direct effects on performance. However, there was little support for inconsistent mediation in trials involving shifts in task demand. Rather, dynamic effects were observed as a function of the shift in task demands such that changes in subjective workload showed increasingly positive effects on performance after increases in task demand but increasingly negative effects after decreases in task demand. In general, this research demonstrated the need to account for indirect effects such as the volitional aspect of control (i.e., effort) and dynamic effects as a function of shifts in task demand when trying to understand the relationship between changes in subjective workload and performance

Three species of entomopathogenic nematodes of the family Steinernematidae (Nematoda: Rhabditida) new to continental Portugal

Abstract In order to determine the species of entomopathogenic nematodes in Continental Portugal, a survey was conducted in the country. Nematodes were recovered from soil samples collected from Alentejo, Algarve, center and the north of Portugal from 2006-2009. Isolates were identified based on morphology and sequence analysis. Phylogenetic analysis was based on sequences of partial 28S (D2D3), internal transcribed spacer (ITS) and cytochrome oxidase c subunit I gene (COX I). Isolates 59F, 15G, 20F and 2B were characterized in detail. Isolate 2B shows a morphology identical to Steinernema intermedium which is a member of the affine/intermedium-group and is characterized by the presence of strongly curved and robust spicules with a distinct rostrum, in the male, and dorsal tail depression in third-stage infective junveniles (IJ). However, the phylogenies based on the three molecular markers revealed that isolate 2B is more closely related to S. affine than with S. intermedium. The morphological results for isolate 20F identify it as S. kraussei which is a member of kraussei/feltiae–group, characterized by IJs with a straight body of medium length (mean = 700–950 μm), lateral field mostly with eight ridges, rather broad, flatly rounded and continuous cephalic region, excretory pore at level of mid-pharynx; males with mucronate tail, yellowish spicules ca. 50 μm long and wide manubria; females with short conoid tail with pointed non-mucronate tip. Based on morphology and sequence analysis, isolates 59F and 15G were considered conspecific and identified as a species belonging to the glaseri–group. More detailed studies are necessary to solve if these isolates represent a new species

Towards an embeddable composite health monitoring sensor : design and optimization of MEMS piezoelectric vibration energy harvesters

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2006.Includes bibliographical references (p. 123-127).Wireless structural health monitoring (SHM) has gained considerable interest as a potential method of reducing aircraft maintenance costs while increasing safety. Distributed power supplies for the sensing nodes are needed for a practical implementation of this technique. Vibrational energy harvesting using MEMS piezoelectric resonant devices is an appealing solution to this challenge due to their relatively high potential for power generation (compared to other harvesting techniques) and capacity to be integrated into the microfabrication procedure of the remainder of the SHM node. A comprehensive review of energy harvesting devices, particularly piezoelectric harvesters, is presented, including characterization of previous devices in terms of key power performance metrics. An existing coupled electromechanical model is presented and modified for use in optimization studies with both {3-1} Mode and {3-3} Mode piezoelectric cantilevered beam configurations. The modified model accurately represents published experimental results. A series of design optimization studies is presented for four devices operating in an aircraft vibration environment.(cont.) Untapered bimorph {3-1} Mode and unimorph {3-3} Mode MEMS devices with large proof masses, as well as tapered unimorph devices (both with and without proof masses) are studied and a 2- or 3-parameter geometric optimization is performed. Optimization objectives include power output, operating power density, static power density, and specific power with very different optimum device configurations favored for the different objectives. A {3-3} Mode MEMS unimorph device optimized for static power density is conservatively predicted to generate 1.9 mW/cm3 of electrical power. This optimization is presented graphically and the predicted performance of the optimum {3-3} Mode unimorph devices with proof masses is detailed. It is found that the optima occur within the micro-scale design space studied, suggesting that MEMS devices are the best choice for distributed aircraft vibrational energy harvesting. The optimization results presented are for one potential SHM vibration environment using cantilevered beam harvesters, however, the technique presented can be extended to other environments and other harvester geometries.by Anna Marie Mracek.S.M

Locus of Rater-Ratee Race Effects as Influenced by Rating Source

This study examines the impact of both rater and ratee race on job performance ratings. Traditionally, the true nature of race-based distortions to performance ratings is difficult to ascertain due to a lack of true score in performance. By utilizing a series of walk-through performance measures, Cronbach's (1955) accuracy components were used to determine the true nature of race-based distortions. Overall, the majority-member supervisors did not deflate the ratings of minority-members. In fact, the ratings of minority-members were inflated by both source levels; and peers inflated ratings significantly more when compared to supervisors. Moreover, majority members were rated accurately by both supervisors and peers. The implications of this and the potential reasons for it are discussed.  M.A