The microstructure and mechanical properties of ball-milled stainless steel powder: The effect of hot-pressing vs. laser sintering

The microstructure and mechanical properties of sintered stainless steel powder, of composition AISI 420, have been measured. Ball-milled powder comprising nanoscale grains was sintered to bulk specimens by two alternative routes: hot-pressing and microlaser sintering. The laser-sintered alloy has a porosity of 6% and comprises a mixture of delta ferrite and tempered martensite, and the relative volume fraction varies along the axis of the specimen due to a thermal cycle that evolves with progressive deposition. In contrast, the hot-pressed alloy has a porosity of 0.7% and exhibits a martensitic lath structure with carbide particles at the boundaries of the prior austenite grains. These differences in microstructure lead to significant differences in mechanical properties. For example, the uniaxial tensile strength of the hot-pressed material is one-half of its compressive strength, due to void initiation at the carbide particles at the prior austenite grain boundaries. Nanoindentation measurements reveal a size effect in hardness and also reveal the sensitivity of hardness to the presence of mechanical polishing and electropolishing. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Fabrication of high-aspect-ratio polymer microstructures and hierarchical textures using carbon nanotube composite master molds

Scalable and cost effective patterning of polymer structures and their surface textures is essential to engineer material properties such as liquid wetting and dry adhesion, and to design artificial biological interfaces. Further, fabrication of high-aspect-ratio microstructures often requires controlled deep-etching methods or high-intensity exposure. We demonstrate that carbon nanotube (CNT) composites can be used as master molds for fabrication of high-aspect-ratio polymer microstructures having anisotropic nanoscale textures. The master molds are made by growth of vertically aligned CNT patterns, capillary densification of the CNTs using organic solvents, and capillary-driven infiltration of the CNT structures with SU-8. The composite master structures are then replicated in SU-8 using standard PDMS transfer molding methods. By this process, we fabricated a library of replicas including vertical micro-pillars, honeycomb lattices with sub-micron wall thickness and aspect ratios exceeding 50:1, and microwells with sloped sidewalls. This process enables batch manufacturing of polymer features that capture complex nanoscale shapes and textures, while requiring only optical lithography and conventional thermal processing. © 2011 The Royal Society of Chemistry.

Matching an oscillator model to a phase response curve

The Phase Response Curve (PRC) has proven a useful tool for the reduction of complex oscillator models. It is also an information often experimentally available to the biologist. This paper introduces a numerical tool based on the sensitivity analysis of the PRC to adapt initial model parameters in order to match a particular PRC shape. We illustrate the approach on a simple biochemical model of circadian oscillator. © 2011 IEEE.

Controlling the phase of an oscillator: A phase response curve approach

The paper discusses elementary control strategies to control the phase of an oscillator. Both feedforward and feedback (P and PI) control laws are designed based on the phase response curve (PRC) calculated from the linearized model. The performance is evaluated on a popular model of circadian oscillations. ©2009 IEEE.

Shock amplification, curve veering and the role of damping

The circumstances are investigated under which high peak acceleration can occur in the internal parts of a system when subjected to impulsive driving on the outside. Previous work using a coupled beam model has highlighted the importance of veering pairs of modes. Such a veering pair can be approximated by a lumped system with two degrees of freedom. The worst case of acceleration amplification is shown to occur when the two oscillators are tuned to the same frequency, and for this case closed-form expressions are derived to show the parameter dependence of the acceleration ratio on the mass ratio and coupling strength. Sensitivity analysis of the eigenvalues and eigenvectors indicates that mass ratio is the most sensitive parameter for altering the veering behaviour in an undamped system. Non-proportional damping is also shown to have a strong influence on the veering behaviour. The study gives design guidelines to allow permissible acceleration levels to be achieved by the choice of the effective mass and damping of the indirectly driven subsystem relative to the directly driven subsystem. © 2013 Elsevier Ltd.

Picosecond pulse generation in narrow stripe mode-locked quantum dot master oscillator power amplifier

This paper reports a monolithically integrated mode-locked narrow stripe QD MOPA operating at 1300nm generating a stable 20GHz pulse train with an average power of 46.4mW and a pulse duration of 8.3ps. © Optical Society of America.