Masoud Dahmardeh

Transforming carbon nanotube forest from darkest absorber to reflective mirror

Carbon nanotube (CNT) forests are known to be among the darkest materials on earth. They can absorb the entire visible range of electromagnetic wave more efficiently than any other known black material. We have attempted controlled mechanical processing of the CNTs and, surprisingly, observed mirror-like reflection from the processed area with 10%–15% reflectivity, a level higher than typical reflectivity of pure forests by over two orders of magnitude, for a wide range of the spectrum (570–1100 nm). Patterning of micro mirrors in the forest is demonstrated to show its potential application for producing monolithically integrated reflector-absorber arrays in the material.

Field-emission-assisted approach to dry micro-electro-discharge machining of carbon-nanotube forests

This work investigates dry micro-electro-discharge machining (μEDM) of vertically aligned carbon nanotube (CNT) forests that are used as cathodes in the process, as opposed to conventional μEDM where the material to be machined forms the anode, toward achieving higher precision in the patterned microstructures. The new configuration with the reversed polarity is observed to generate higher discharge currents in the process, presumably due to effective field-emission from CNTs. This effect allows the process to be performed at very low discharge energies, approximately 80× smaller than in the conventional normal-polarity case, with the machining voltage and tolerance down to 10 V and 2.5 μm, respectively, enabling high-precision high-aspect-ratio micropatterning in the forests. The new approach is also demonstrated to make the process faster, cleaner, and more stable than conventional processing. Spectroscopic analyses of the forests processed by reverse μEDM show no evidence of significant crystalline det...

Possible mechanism in dry micro-electro-discharge machining of carbon-nanotube forests: A study of the effect of oxygen

The working principle of dry micro-electro-discharge machining of vertically aligned carbon-nanotube forests is investigated by evaluating the effect of oxygen on the process. The machining experiments with controlled oxygen/nitrogen ratios indicate a correlation between the peak current of discharge pulses and the oxygen concentration, suggesting not only a vital role for oxygen in the process, but also a removal mechanism fundamentally different from that in typical electro-discharge machining based on direct melting and evaporation of the sample material. The highest surface quality and uniformity in the machined forest microstructures as well as smooth machining without short circuiting are achieved at an approximate oxygen concentration of 20% under the discharge condition of 30 V and 10 pF, revealing that air is an optimal medium for the removal process. Elemental and molecular analyses show no evidence of significant crystalline deterioration or contamination in the nanotubes processed with the tec...

Selective RF heating of resonant stent toward wireless endohyperthermia for restenosis inhibition

This paper reports a novel active stent targeted at the application to endohyperthermia treatment for in-stent restenosis problems. The stainless-steel stent designed to function as an electrical inductor is integrated with a flexible capacitor strip to form a resonant circuit, which serves as a frequency-selective wireless heater controlled using a tuned radio-frequency (RF) magnetic field applied externally. The fabricated stent device with the initial diameter of 2 mm is expanded up to 6 mm in diameter inside an artificial artery using a balloon catheter. The expanded device is revealed to show efficient heat generation with temperature rise of >30 °C when resonated using an RF power of 320 mW. Temporal and frequency characteristics are evaluated to demonstrate rapid heating ability with strong frequency sensitivity. These promising results validate the feasibility of wireless stent hyperthermia that potentially offers a novel therapeutic path to long-term inhibition and management of stent restenosis.

Cone-shaped forest of aligned carbon nanotubes: An alternative probe for scanning microscopy

A scanning microscopy probe based on three-dimensionally shaped carbon nanotube (CNT) forests and its application to atomic-force microscopy (AFM) are reported. Micro-scale CNT forests directly grown on silicon cantilevers are patterned into cone shapes with the tips of a few individual nanotubes. The CNT-forest-based probes provide significantly higher mechanical stability/robustness than the common single-CNT probes. AFM imaging using the fabricated probes reveals their imaging ability comparable to that of commercial probes. The patterning process also improves the uniformity of the CNT forests grown on each cantilever. The results suggest a promising future for CNT scanning probes and their production approach.

High-precision dry micro-electro-discharge machining of carbon-nanotube forests with ultralow discharge energy

This work investigates reverse-polarity dry micro-electro-discharge machining (μEDM) of pure carbon-nanotube (CNT) forests that are used as cathodes in the process, as opposed to conventional μEDM where the material to be machined forms the anode. The new configuration with the reversed polarity is observed to generate higher discharge currents, most likely due to effective field-emission from CNTs. This effect allows the process to be performed at very low discharge energies, ~80× smaller than in the conventional normal-polarity case, with the machining voltage and tolerance down to 10 V and 2.5 μm, respectively, enabling high-precision high-aspect-ratio micropatterning in the forests.

High-aspect-ratio, 3-D micromachining of carbon-nanotube forests by micro-electro-discharge machining in air

This paper reports micro-electro-discharge machining of vertically aligned carbon nanotube forests for the formation of high-aspect-ratio, three-dimensional microstructures in the material. The developed forest machining method is a dry process performed in air, generating high-frequency pulses of electrical discharge to locally machine the nanotubes in order to create target shapes in a forest. With this approach, forest microstructures can be fabricated to have varying shapes along their height, unachievable with conventional pre-patterned chemical vapor deposition growth techniques. The use of the pulses with a minimized discharge energy defined with 35 V and 10 pF in the discharge generation circuit leads to an aspect ratio of 20 with the smallest feature of 5 µm in forests without disordering the vertical orientation of the nanotubes. Micromachining of multilayer geometries as well as arrayed needle-like microstructures with angled surfaces is demonstrated.

Active integrated antenna enhanced using photonic bandgap and defected ground structure

This paper presents a slow-wave low-noise active receiving antenna with defected ground structure (DGS) and photonic bandgap (PBG). The integrated design of a low noise amplifier (LNA) stage and an enhanced patch antenna is presented. The antenna consists of a microstrip patch integrated with one-dimensional DGS in ground plane and two-dimensional PBG. It is demonstrated that application of DGS and PBG eliminates the second and third harmonics and improve the return loss level. Moreover, the combination use of PBG and DGS decreases the occupied area by 61%. Also, by adopting DGS pattern on the ground plane, the length of the output matching network of the LNA shortened by about 39%, while maintaining the matching and performances of the amplifier. This, also, eliminates the third harmonic.

The effects of three-dimensional shaping of vertically aligned carbon-nanotube contacts for micro-electro-mechanical switches

A micro-electro-mechanical switch integrated with vertically aligned carbon nanotubes (CNTs) as the contact material is presented. Arrays of the CNTs are three-dimensionally micropatterned using a pulsed micro-discharge process to have tapered contact surfaces with controlled angles, achieving maximized contact areas, while providing contact resistances in the 10 Ω range with an enhanced current capacity. A shape-memory-alloy actuator is integrated to demonstrate stable switching for ∼1.4 × 106 ON-OFF cycles with no sign of damage. The results prove that post-growth micropatterning of CNTs is a promising path to improved and reliable micro contact switches enabled by arrayed CNT contacts for high-power applications.

Optimal platform design using non-dominated sorting genetic algorithm II and technique for order of preference by similarity to ideal solution; application to automotive suspension system

ABSTRACT Unlike conventional approaches where optimization is performed on a unique component of a specific product, optimum design of a set of components for employing in a product family can cause significant reduction in costs. Increasing commonality and performance of the product platform simultaneously is a multi-objective optimization problem (MOP). Several optimization methods are reported to solve these MOPs. However, what is less discussed is how to find the trade-off points among the obtained non-dominated optimum points. This article investigates the optimal design of a product family using non-dominated sorting genetic algorithm II (NSGA-II) and proposes the employment of technique for order of preference by similarity to ideal solution (TOPSIS) method to find the trade-off points among the obtained non-dominated results while compromising all objective functions together. A case study for a family of suspension systems is presented, considering performance and commonality. The results indicate the effectiveness of the proposed method to obtain the trade-off points with the best possible performance while maximizing the common parts.