Showing papers by "Carol Livermore published in 2007"

A MEMS Singlet Oxygen Generator—Part II: Experimental Exploration of the Performance Space

Abstract: This paper reports the quantitative experimental exploration of the performance space of a microfabricated singlet oxygen generator (muSOG). SOGs are multiphase reactors that mix H2O2, KOH, and Cl2 to produce singlet delta oxygen, or O2 (a). A scaled-down SOG is being developed as the pump source for a microfabricated chemical oxygen-iodine laser system because scaling down a SOG yields improved performance compared to the macroscaled versions. The performance of the muSOG was characterized using O2 (a) yield, chlorine utilization, power in the flow, molar flow rate per unit of reactor volume, and steady-state operation as metrics. The performance of the muSOG is measured through a series of optical diagnostics and mass spectrometry. The test rig, which enables the monitoring of temperatures, pressures, and the molar flow rate of O2 (a), is described in detail. Infrared spectra and mass spectrometry confirm the steady-state operation of the device. Experimental results reveal O2 (a) concentrations in excess of 1017 cm-3, O2 (a) yield at the chip outlet approaching 80%, and molar flow rates of 02(a) per unit of reactor volume exceeding 600 times 10-4 mol/L/s.

A MEMS Singlet Oxygen Generator—Part I: Device Fabrication and Proof of Concept Demonstration

Abstract: This paper reports the design, fabrication, and proof of concept demonstration of a singlet oxygen generator (SOG) that operates on the microscale. The micro-SOG (muSOG) chip is implemented in a three-wafer stack using deep reactive ion etching (DRIE) and wafer bonding as key technologies. The device creates singlet delta oxygen (O2(a)) in an array of packed-bed reaction channels fed by inlet manifolds with pressure drop channels that ballast the flow. An integrated capillary array separates the liquid and gas by-products, and a microscale heat exchanger removes excess heat of reaction. The fabrication process and package are designed to minimize collisional losses and wall deactivation of O2(a). The design, fabrication, and package of the device are documented. Proof of concept demonstration of the device is given by optical emission measurements of the spontaneous decay of the O2 (a) molecule into its triplet state and by the observation of the emission from dimol pairs of O2 (a) molecules.

Templated Assembly by Selective Removal

Abstract: Templated assembly by selective removal (TASR) is an effective technique for site-selective multi-component assembly at the nanoand micro-scales. In this project, the TASR approach has been created and quantitatively modeled; work to expand the technology and demonstrate practical applications is now underway. The TASR approach offers great promise for assembling arbitrary (not necessarily periodic) systems of multiple different types of nanoscale components, such as electronics and biological or chemical sensing devices. It also offers a path to a new kind of shape and size selective chromatography.

Singlet oxygen generator on a chip for MEMS-based COIL

Abstract: Microelectromechanical systems (MEMS) offer a promising approach for creating compact, efficient chemical oxygen iodine lasers. In this paper we report the demonstration and characterization of a chip-scale, MEMS-based singlet oxygen generator, or microSOG. The microSOG is a batch-fabricated silicon chip that is micromachined to form reactant inlets and distribution system, an array of microstructured packed bed reaction channels to ensure good mixing between the BHP and the chlorine, a gas-liquid separator that removes liquid from the output stream by capillary effects, integrated heat exchangers to remove the excess heat of reaction, and product outlets. The microSOG has successfully generated singlet delta oxygen, and the resulting singlet delta concentrations were measured in a quartz test cell downstream of the chip using absolutely-calibrated near-infrared emission measurements made by an InGaAs array spectrometer. A kinetics analysis was used to determine the concentration at the chip's outlet from the concentration at the measurement point. Singlet delta yield at the outlet was determined to be about 81% at 150 Torr plenum pressure with a 25 sccm flow of chlorine. The corresponding output flow carries about 1.4 W of power at the chip's outlet.

A Portable Power Source Based on MEMS and Carbon Nanotubes

Abstract: There is a growing need for small, lightweight, reliable, highly efficient and fully rechargeable portable power sources. The focus of this project is the design and modeling of a system in which energy is stored in the elastic deformation of carbon nanotube (CNT)-based springs. The CNTs are coupled to a MEMS electric generator. When the CNT deformation is released, the stored energy actuates the generator, which then converts the energy into electricity. The MEMS generator may be operated in reverse, as a motor, in order to wind the CNT springs and recharge the system. Alternatively, the stored elastic energy may be used to supply a mechanical load directly. This project is motivated by recent research into the mechanical properties of CNTs. The CNTs have a high stiffness, low defect density, and a consequently high yield strain that enables them to store elastic energy with significantly greater energy density than typical spring materials such as high-carbon steel. Models suggest that CNTs can be reversibly stretched by up to 15% [1]; lower strains of up to 6% have been demonstrated experimen-tally to date [2-3].This type of system offers several important potential advantages. First, due to CNTs’ high strength, high flexibility, and low defect density, they can store energy at very high energy density. Con-sidering just the CNT-based spring itself, the energy density of an array of CNTs stretched to a reversible 15% strain is about 1500 W-hr/kg, about ten times the energy density of Li-ion batteries. The energy density of the final system will be lower because of the finite conversion efficiency of the generator and the weight of both the supporting structure and the generator hardware. In addition, because energy storage in the CNT system is based on stretching chemical bonds rather than breaking and reforming chemical bonds as in batteries, the CNT-MEMS generator sys-tem has the potential to operate at higher power densities, un-der harsher conditions, to deeper discharge levels, and through a greater number of charge-discharge cycles than a chemical bat-tery.The system architecture consists of a CNT-based energy storage element, an energy release rate mechanism, and a MEMS gen-erator. This project is examining and modeling different varia-tions on this system architecture that incorporate different modes of deformation of the CNT-based energy storage element, vari-ous types of generators, different types of coupling between the storage element and the generator, and different size scales for the various components. One conceptual example is illustrated below, in which the axial relaxation of an axially-stretched CNT-based storage element is converted to rotational motion of a wheel. The wheel is coupled to a piezoelectric generator through a mechanism that regulates the rate of energy release, much as in a mechanical watch.

A MEMS Steam Generator

Abstract: Previous work [1] has shown that MEMS technology has significant potential to create more compact, higherperforming hardware for chemical oxygen iodine lasers (COIL). In COILs, the laser medium is a flowing gas that must be pumped through the system at high mass flow rates to ensure proper system operation. As a result, compact pumps with high pumping rates are a key element of the COIL system. One promising component of a MEMS COIL system would be a compact MEMS pump system in which the pump action is provided in part by micro steam ejectors and the micro steam generators that supply their driving fluid. This work describes the design and modeling of a microscale hydrogen peroxide (H2O2)-based steam generator to supply such a MEMS pump system. Hydrogen peroxide is a readily available, inexpensive, nontoxic, and environmentally friendly fluid that may be catalytically decomposed to form steam. Steam generation by the catalytic decomposition of H2O2 also finds other important applications in the MEMS field beyond pumping, particularly in the area of thrust generation. Compared to their macroscale counterparts, MEMS H2O2-based steam generators offer better performance, notably improved mixing, and higher uniformity due to the absence of moving parts [2-3].