Andres M. Cardenas-Valencia

A finite element method modeling approach for the development of metal/silicon nitride MEMS single-use valve arrays

The problem of controlled liquid delivery to a microelectromechanical system (MEMS) device is one that is being continuously investigated. There are many types of valves being fabricated for different applications. For single-use systems, burst valves are an ideal choice. These types of valves ensure that there is no leaking of the fluid to the rest of the system. Models have been developed using COMSOL multiphysics incorporated in an optimization routine to study and predict the behavior of valves made with metallic resistors over silicon nitride membranes, with the goal of developing a tool that can be used to design low-power valves. Power and temperature data for valves made in our MEMS facility have been used to obtain an average overall heat transfer coefficient, which in turn is used to predict the voltage, temperature and stress of the breaking point of the valves. The simulation results were found to be adequate to represent the behavior of valves for two ohmic heater designs fabricated with gold and platinum. The models can be used to compare different valve designs to minimize the energy required for actuation. Experimental data showed that the valves could be broken with between 15 and 50 mJ.

A Review of the Emerging Field of Underwater Mass Spectrometry

Mass spectrometers are versatile sensor systems, owing to their high sensitivity and ability to simultaneously measure multiple chemical species. Over the last two decades, traditional laboratory-based membrane inlet mass spectrometers have been adapted for underwater use. Underwater mass spectrometry has drastically improved our capability to monitor a broad suite of gaseous compounds (e.g., dissolved atmospheric gases, light hydrocarbons, and volatile organic compounds) in the aquatic environment. Here we provide an overview of the progress made in the field of underwater mass spectrometry since its inception in the 1990s to the present. In particular, we discuss the approaches undertaken by various research groups in developing in situ mass spectrometers. We also provide examples to illustrate how underwater mass spectrometers have been used in the field. Finally, we present future trends in the field of in situ mass spectrometry. Most of these efforts are aimed at improving the quality and spatial and temporal scales of chemical measurements in the ocean. By providing up-to-date information on underwater mass spectrometry, this review offers guidance for researchers interested in adapting this technology as well as goals for future progress in the field.

Evaluation of reagentless pH modification for in situ ocean analysis: determination of dissolved inorganic carbon using mass spectrometry

RATIONALE In situ analytical techniques that require the storage and delivery of reagents (e.g., acidic or basic solutions) have inherent durability limitations. The reagentless electrolytic technique for pH modification presented here was developed primarily to ease and to extend the longevity of dissolved inorganic carbon (DIC) determinations in seawater, but can also be used for other analytical methods. DIC, a primary carbon dioxide (CO(2)) system variable along with alkalinity, controls seawater pH, carbonate saturation state, and CO(2) fugacity. Determinations of these parameters are central to an understanding of ocean acidification and global climate change. METHODS Electrodes fabricated with electroactive materials, including manganese(III) oxide (Mn(2)O(3)) and palladium (Pd), were examined for potential use in electrolytic acidification. In-line acidification techniques were evaluated using a bench-top membrane introduction mass spectrometry (MIMS) setup to determine the DIC content of artificial seawater. Linear least-squares (LLSQ) calibrations for DIC concentration determinations over a range between 1650 and 2400 µmol kg(-1) were obtained, using both the novel electrolytic and conventional acid addition techniques. RESULTS At sample rates of 4.5 mL min(-1), electrodes clad with Mn(2)O(3) and Pd were able to change seawater pH from 7.6 to 2.8 with a power consumption of less than 3 W. Although calibration curves were influenced by sampling rates at a flow of 4.5 mL min(-1), the 1σ measurement precision for DIC was of the order of ±20 µmol kg(-1). CONCLUSIONS Calibrations obtained with the novel reagentless technique and the in-line addition of strong acid showed similar capabilities for DIC quantification. However, calculations of power savings for the reagentless technique relative to the mechanical delivery of stored acid demonstrated substantial advantages of the electrolytic technique for long-term deployments (>1 year).

Rapid Design, Fabrication and Optimization of a Single Event Thermo-Pneumatic Microactuation for the Delivery of Minute Amounts of Liquids

The need for efficient metering control of liquids in small devices has led to a boom in the advent of different micro-fluidic actuation mechanisms. Here we present a brief study on a thermal-pneumatic actuation mechanism for an on-demand delivery of minute amounts of liquids. A closely coupled, iterative design-fabrication strategy is used for optimization of a system in which no freely moving membranes are included. Special consideration was given to the heating device, minimizing the energy consumed. The fabrication method and performance of two types of fabricated resistors are compared herein. The first, a conventional Nickel-Chromium resistor using, lift-off micro-fabrication techniques, was initially tested. The second, a Copper cladded liquid crystal polymer in conjunction with a novel mask-less patterning system was used to produce nonconventional heating micro-ohmic heaters. The heating efficiency was proven to be superior using the latter approach. Various micro-fabricated fluidic devices have been designed as case studies and have been fabricated and integrated using a variety of materials to illustrate the functionality of the approach. The combination of design and fabrication steps, the simplicity of the resistive device, and the materials selected combined, yield a direct path to making fluidic transport devices for micro-analytical and power systems.Copyright

Semi-fuel cell studies for powering underwater devices: integrated design for maximized net power output

Use of sensor systems in water bodies has applications that range from environmental and oceanographic research to port and homeland security. Power sources are often the limiting component for further reduction of sensor system size and weight. We present recent investigations of metal-anode water-activated galvanic cells, specifically water-activated Alcells using inorganic alkali peroxides and solid organic oxidizers (heterocyclic halamines), in a semi-fuel cell configuration (i.e., with cathode species generated in situ and flow-through cells). The oxidizers utilized are inexpensive solid materials that are generally (1) safer to handle than liquid solutions or gases, (2) have inherently higher current and energy capacity (as they are not dissolved), and, (3) if appropriately packaged, will not degrade over time. The specific energy (S.E.) of Al-alkali peroxide was found to be 230 Wh/kg (460 Wh/kg, considering only active materials) in a seven-gram cell. Interestingly, when the cell size was increased (making more area of the catalytic cathode electrode available), the results from a single addition of water in an Al-organic oxidizer cell (weighing ~18 grams) showed an S.E. of about 200 Wh/kg. This scalability characteristic suggests that values in excess of 400 Wh/kg could be obtained in a semi-fuel-cell-like system. In this paper, we also present design considerations that take into account the energy requirements of the pumping devices and show that the proposed oxidizers, and the possible control of the chemical equilibrium of these cathodes in solution, may help reduce this power requirement and hence enhance the overall energetic balance.

Novel, MEMS-Fabricated, Reserve, Galvanic Cells for Deployable Underwater Sensors: Depth-Sensitive Activation & In-Situ Generation of Electron-Acceptors Species

With the aim of benefiting miniaturized sensing systems in environmental applications a discussion on the preliminary development of portable galvanic batteries of the reserve type, i.e. activated on-demand via specific-depth immersion, is presented. Two major aspects in the design of the presented batteries are discussed. First, two possible novel chemistries are presented herein that allow for the batteries to be activated using water. Electrochemical characterization, in the form of polarographic curves obtained from tests of the fabricated cells, is presented. Using aluminum as an anode, the cells have open voltage circuits of 1.6 and 1.8 volts, respectively, and are capable of providing power densities up to 3 and 15 mAmps/cm . The fabrication routes of batteries with thin-form factors are detailed. Second, a micro fabricated pressure sensitive membrane is described, realized and tested as an automated way to activate the cells, via the immersion depth. So far the fabricated silicon nitride membranes (2times2 mm2 times 2 mum) can withstand up to 30 meters of depth underwater. Additional benefits of the proposed novel cells such as simplicity in their fabrication and use, safety, and their potential for powering underwater environmental sensors are also outlined.

Novel Microfabricated Batteries for Marine Sensors: In-situ Catholyte Generation via Water Addition

Flat micro fabricated galvanic cells have been reported in the recent literature. In this work we present an aluminum-anode cell, which has been loaded with an oxyhalogenated organic compound (sodium dichloro-s-triazinetrione) that releases hypochlorite ions via a hydrolysis reaction when water is added. A catalytic metal is used as the current collector. This novel concept in micro cells and the electrochemical characterization of the fabricated cells are presented herein. The results show that the fresh in-situ formation of hypochlorite ions, using the mentioned hydrolysis reaction, constitute a packaging strategy for the oxidizer that leads to advantageous features, especially for battery systems to be used in marine applications. Cells with improved operational life and capacities (about six to ten times larger than in a cell activated with the 10% sodium hypochlorite solution, recently reported in literature) are described herein.