Tribological Challenges in Micromechanical Systems

Despite much progress in surface micromachining technology, adhesion, friction and wear remain key issues, severely limiting the realization and reliability of many microelectromechanical systems (MEMS) devices. In this article, we focus on the use of molecularly thin organic films as release and anti-stiction coatings for MEMS. The various classes of organic films explored for MEMS are reviewed here, followed by a discussion of the current limitations and areas for improvements for this coating technology.     

Oligodeoxyribonucleotide association with single-walled carbon nanotubes studied by SPM

Studies have been performed on both as-received and chemically oxidized single-walled carbon nanotubes (SWCNTs) grown by two different growth methods to better understand the preferential association of the oligodeoxyribonucleotide T30 (ODN) with SWCNTs. Samples of T30 ODN:SWCNT were examined under ambient conditions using non-contact scanning probe microscope (SPM) techniques. The resulting images show different morphologies ranging from tangled networks of SWCNTs to individual, well-dispersed isolated SWCNTs as the sonication time is increased. SPM images of well-dispersed, as-received SWCNTs reveal isolated features that are 1.4 to 2.8 nm higher than the bare SWCNT itself. X-ray photoemission spectroscopy (XPS) confirmed these features to be T30 ODN in nature. Chemically oxidizing the SWCNTs before sonication is found to be an effective way to increase the number of T30 ODN features.     

Project governance and stakeholders: a literature review

Despite the importance of stakeholders in project governance, project management literature lacks from an inclusive framework which defines the roles, relationships and positions of internal and external stakeholders inside and outside of the organization's governance structure. This paper has the purpose to report a review on project governance literature to draw attention to the context within which the stakeholders are positioned, to extract their roles and relationships inside and outside of the organization and to develop new avenues for research regarding stakeholders in project governance. The conducted thematic analysis reveals that there are three contexts influencing organization's approaches towards stakeholders: success, megaprojects and ethics. The developed conceptual framework illustrates that organizations are in direct contact with external stakeholders at the organizational level and project level. Strategic decisions made at the organizational level are operationalised at the portfolio level and influence the approach towards external stakeholders at the project level. Considering the lack of theories to support general doctrine of stakeholder theory, this research suggests that future governance researchers adopt a broader view in selection of theoretical lenses in order to include the social and psychological aspects of the management of external stakeholders.     

A Novel Low Power Architecture for DLL-Based Frequency Synthesizers

This paper presents a novel DLL-based frequency synthesizer architecture to generate fractional multiples of reference frequency and reduce the power consumption of the frequency synthesis block. The architecture is adopted for French VHF application as an example. The DLL architecture allows for minimal area, while consuming low power. The proposed circuit can operate at a substantially low supply voltage. The circuit level and system level designs are presented. It was shown that for the mentioned standard, a mere 27 delay stages for VCDL are sufficient to cover French VHF band. Simulation results confirm the analytical predictions. The proposed DLL-based frequency synthesizer is implemented in a 0.13 μm CMOS technology. This fractional DLL-based frequency synthesizer is adopted for 176 MHz to 216 MHz with maximum power consumption of 2.62 mW and RMS jitter of 10 ps @ 216 MHz.     

Effect of processing parameters and glycerin addition on the properties of Al foams

Aluminum foam has been produced by sintering and dissolution processes using NaCl powders as a space holder. In this research, glycerin is used as a novel lubricant along with acetone. The effects of the processing parameters including compacting pressure, sintering temperatures (620, 640 and 650 °C), size, and volume fraction of the space holder, on the physical and mechanical properties of the produced foams have been investigated. Due to segregation of the Al and NaCl powders at high compaction pressures, spalling of Al foams was observed. Meanwhile, adding small amounts of acetone and glycerin to the mixture ensures homogeneity and prevents segregation of dissimilar powders at varying pressure. Moreover, the addition of glycerin provides an improved homogenous stress distribution within the produced foams during mechanical testing, which in turn halts crack propagation. Meanwhile, an alternative technique to remove NaCl particles during the dissolution stage has been proposed. The results showed that high quality foams were successfully produced under a compaction pressure range of 250–265 MPa and sintering temperature of 650 °C.     

Numerical study of effective parameters on the drying process in a vertical porous channel

Comprehensive understanding of evaporation in porous channels is important for the design of modern heat exchangers and thermal systems such as heat pipes. This paper presents a numerical study of evaporation in a vertical porous channel. The volume of the fluid method was applied to capture the interface between phases. The multi‐dimensional limiter for explicit solution method, in which there are additional constraints based on the flux‐corrected transfer method, was used to solve the interface transfer equation. Pressure drop, Nusselt number, and the drying rate were considered as performance parameters. Comparing simulation results with experimental data showed that this new solver can precisely solve the thermal phase change in a porous medium. The influence of parameters effective on the drying process, such as porosity, permeability, the dynamic contact angle was investigated. Based on the results of the pressure profile in the final state, four different regions are visible. Because of the close fit of the wall temperature to the average cross‐sectional temperature, the Nusselt number increases sharply at transition points from the liquid to the vapor phase. By increasing the dynamic contact angle, the average drying rate in the channel is reduced.     

Leadership: Why gender and culture matter.

For decades, understanding of leadership has been largely based on the results of studies carried out on White men in the United States. We review major theories and models of leadership as they pertain to either gender or culture. We focus on 3 approaches to leadership: trait (including leadership categorization or implicit leadership theory), behavioral (including the two-factor, transformational–transactional leadership, and leader–member exchange models), and contingency (i.e., contingency model of leadership effectiveness and normative decision making). We discuss how dynamics related to either culture or gender (e.g., stereotypes and schemas, ingroup–outgroup interaction, role expectations, power and status differentials) can have an important impact on many aspects of leadership. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Adhesion Characteristics of PDMS Surfaces During Repeated Pull‐Off Force Measurements

To mimic the adhesive effects of gecko toes, artificial surfaces have been manufactured recently using polydimethylsiloxanes (PDMS). However, the effects of repeated contacts on the adhesive properties remain largely unexplored. In this paper we report on the effect of repeated pull‐off force measurements on the adhesion behavior of PDMS (polymer kit Sylgard 184, Dow Corning) tested with a borosilicate glass probe. A decrease in pull‐off force with increase in number of test cycles is found until a plateau is reached. The initial value and the rate of change in pull‐off force strongly depend on the sample preparation procedure, including curing time and cross‐linking. It is proposed that the behavior is due to steady coverage of the probe with free oligomers. The results are crucial for developing reusable, durable, and residue‐free bioinspired adhesives.     

A simple soft lithographic nanopatterning of gold on gallium arsenide via galvanic displacement

Nanoscale patterning of gold layers on GaAs substrate is demonstrated using a combination of soft lithographic molding and galvanic displacement deposition. First, an electroless deposition method has been developed to plate gold on GaAs with ease and cost-effectiveness. The electroless metallization process is performed by dipping the GaAs substrates into a gold salt solution without any reducing agents or additives. The deposition proceeds via galvanic displacement in which gold ions in the aqueous solution are reduced by electrons arising from the GaAs substrate itself. The deposition rate, surface morphology and adhesion property can be modulated by the plating parameters such as the choice of acids and the immersion time. Second, soft lithographic patterning of nanodots, nanorings, and nanolines are demonstrated on GaAs substrates with hard-polydimethylsiloxane (h-PDMS) mold and plasma etching. This method can be easily applied to the metallization and nanopatterning of gold on GaAs surfaces.     

Dynamic response of thick laminated annular sector plates subjected to moving load

The dynamic response of thick laminated annular sector plates with simply supported radial edges subjected to a radially distributed line load, which moves along the circumferential direction, is studied. A three-dimensional hybrid method composed of series solution, the layerwise theory and the differential quadrature method in conjunction with the finite difference method is employed. The fast rate of convergence and high accuracy of the method are demonstrated through different examples. Additionally, as a limit case, the out-of-plane dynamic responses of circular curved beams is obtained and compared with those of an unconstrained higher order shear deformation curved beam theory, which is formulated here. Then, the effects of different parameters such as the sector angle, thickness-to-outer radius ratio, ply lay out and the load velocity on the out-of-plane response of the symmetric and antisymmetric cross-ply laminated sector plates are investigated. The results can be used as benchmark solutions for future works.