J. Santander

Tolerant Chalcogenide Cathodes of Membraneless Micro Fuel Cells

The most critical issues to overcome in micro direct methanol fuel cells (μDMFCs) are the lack of tolerance of the platinum cathode and fuel crossover through the polymer membrane. Thus, two novel tolerant cathodes of a membraneless microlaminar-flow fuel cell (μLFFC), Pt(x)S(y) and CoSe(2), were developed. The multichannel structure of the system was microfabricated in SU-8 polymer. A commercial platinum cathode served for comparison. When using 5 M CH(3)OH as the fuel, maximum power densities of 6.5, 4, and 0.23 mW cm(-2) were achieved for the μLFFC with Pt, Pt(x)S(y), and CoSe(2) cathodes, respectively. The Pt(x)S(y) cathode outperformed Pt in the same fuel cell when using CH(3)OH at concentrations above 10 M. In a situation where fuel crossover is 100 %, that is, mixing the fuel with the reactant, the maximum power density of the micro fuel cell with Pt decreased by 80 %. However, for Pt(x)S(y) this decrease corresponded to 35 % and for CoSe(2) there was no change in performance. This result is the consequence of the high tolerance of the chalcogenide-based cathodes. When using 10 M HCOOH and a palladium-based anode, the μLFFC with a CoSe(2) cathode achieved a maxiumum power density of 1.04 mW cm(-2). This micro fuel cell does not contain either Nafion membrane or platinum. We report, for the first time, the evaluation of Pt(x)S(y)- and CoSe(2)-based cathodes in membraneless micro fuel cells. The results suggest the development of a novel system that is not size restricted and its operation is mainly based on the selectivity of its electrodes.

Optimized technology for the fabrication of piezoresistive pressure sensors

In this work a technology for the fabrication of piezoresistive pressure sensors is presented, based on the use of silicon BESOI (bonded and etch-back silicon on insulator) wafers. The main purpose of the proposed technology is the optimization of the thin silicon diaphragm definition process that is one of the most critical steps in the fabrication of silicon pressure sensors. The buried silicon oxide layer of the BESOI wafers is used as an automatic etch stop of the silicon anisotropic etching, making it possible to obtain very precise control of the sensor diaphragm thickness. In addition, the type and thickness of the layers acting as masking materials on the backside of the wafers have been optimized in order to get a high-yield process. The experimental results obtained when using the proposed technology are presented and discussed.

Results on the reliability of silicon micromachined structures for semiconductor gas sensors

Abstract Thin film semiconductor gas sensors fabricated on thermally isolated silicon substrates have been proposed as good alternative to thick film devices that are on the market as they show low power consumption. However, for their industrial success, it is necessary to assess good yield and high reliability for maintaining the functionality of the device during a long period of time. In this paper, a set of thermo-mechanical tests has been applied to gas sensors based on silicon micromachined structures with dielectric membranes. The aim of the tests is to determine the survivability of the devices under aggressive conditions of use. The tests have been carried out on two specific structures, a single Si 3 N 4 membrane; and the same device that also includes a silicon plug below the sensor active area. Results are compared as a tool for improving the structure in the future.

Localized growth and in situ integration of nanowires for device applications

Simultaneous localized growth and device integration of inorganic nanostructures on heated micromembranes is demonstrated for single crystalline germanium and tin oxide nanowires. Fully operating CO gas sensors prove the potential of the presented approach. With this simple CMOS compatible technique, issues of assembly, transfer and contact formation are addressed.

Bump bonding of pixel systems

Abstract A pixel detector consists of an array of radiation sensing elements which is connected to an electronic read-out unit. Many different ways of making this connection between these two different devices are currently being used or considered to be used in the next future. Bonding techniques such as flip chip technology can present real advantages because they allow very fine pitch and a high number of I / O s. This paper presents a review of the different flip chip technologies available and their suitability for manufacturing pixel detectors. The particular problems concerning testing of pixel detectors and thermal issues related to them are pointed out.

Feasibility of a flip chip approach to integrate an IR filter and an IR detector in a future gas detection cell

In this work we have studied the feasibility of integrating an infrared filter and an infrared detector by means of a flip-chip technique. This filter and detector combination should be the heart of a future gas detection cell based on infrared absorption. In our case the filter is a surface micromachined Fabry-Perot interferometer, and the infrared detector is a bulk micromachined thermopile. The flip-chip technique is an elegant solution to assure the optical micro-alignment of both devices and allows the electrical contact needed to actuate active optical filters.

A highly sensitive IR-optical sensor for ethylene-monitoring

Precise and continuous ethylene detection is needed in various fruit storage applications. The aim of this work is the development of a miniaturised mid-infrared filter spectrometer for ethylene detection at 10.6 μm wavelength. For this reason optical components and signal processing electronics need to be developed, tested and integrated in a compact measurement system. The present article describes the proposed system set-up, the status of the development of component prototypes and results of gas measurements performed using a first system set-up. Next to a microstructured IR-emitter, a miniaturised multi-reflection cell and a thermopile-array with integrated optical filters and microstructured Fresnel lenses for the measurement of ethylene, two interfering gases and one reference channel are proposed. Recently a miniaturised White cell as absorption path is tested with various commercial and a self-developed thermal emitter. First ethylene measurements have been performed with commercial twofold thermopile detectors and a Lock-in-amplifier. These showed significant absorption at an ethylene concentration of 100ppm. For the detection module different types of thermopiles were tested, first prototypes of Fresnel lenses have been fabricated and characterised and the parameters of the optical filters were specified. Furthermore a compact system electronics for signal processing containing a preamplification stage and Lock-in-technique is in development.

Accurate extraction of contact resistivity on Kelvin D-resistor structures using universal curves from simulation

An accurate procedure to extract contact resistivity of metal-semiconductor contacts from contact resistance measurements made on a D-resistor-type Kelvin cross test structure is presented. The effects of spreading currents are taken into account through simulation and a set of universal curves that eliminates the need of further simulations is computed. The values given can be incorporated in any contact resistance measurement routine to extract the actual value of contact resistivity along with an estimation of the error. The two-dimensional model is validated with experimental results. >

A micro alkaline direct ethanol fuel cell with platinum-free catalysts

This paper presents the fabrication and characterization of a micro alkaline direct ethanol fuel cell. The device has been conceived as a feasibility demonstrator, using microtechnologies for the fabrication of the current collectors and traditional techniques for the membrane electrode assembly production. The fuel cell works in passive mode, as expected for the simplicity required for micro power systems. Non-noble catalysts have been used in order to implement the main advantage of alkaline systems, showing the feasibility of such a device as a potential very-low-cost power device at mini- and micro scales. (Some figures may appear in colour only in the online journal)

Hybrid polymer electrolyte membrane for silicon-based micro fuel cells integration

A novel approach for a hybrid polymer electrolyte membrane compatible with silicon-based fuel cells is proposed in this study. The membrane consists of a polymer matrix of polydimethylsiloxane (PDMS) filled with a proton-conducting polymer. The fabrication steps of the hybrid membrane as well as its electrochemical characterization are explained in detail. The obtained proton conductivities demonstrate the validity of the present approach as a proof of concept for the obtaining of a new generation of fully integrated micro proton-exchange membrane fuel cells.