Anis Zribi

CO2 detection using carbon nanotube networks and micromachined resonant transducers

A CO2 sensor using single-walled carbon nanotubes (CNT) and a microelectromechanical system (MEMS) resonator is demonstrated. The MEMS transducer comprises a membrane driven into transverse vibrations by means of Lorentz forces. A downshift in the resonant frequency of the device is measured by a laser vibrometer when changes in the stress state of the CNT film∕membrane structure are caused by adsorption of CO2 onto the nanotubes. The sensor has shown excellent sensitivity, linearity, and recovery over a broad range of concentrations (0–15vol%). In comparison to resistive, dielectric, and gravimetric CNT transducers, this sensor displayed remarkable intrinsic selectivity in the presence of interferences.

Synthesis of novel micro- and mesoporous zeolite nanostructures using electrospinning techniques

Micro- and mesoporous nanostructured zeolites are fabricated for rapidly responding, selective detection of individual gases in a multigas environment. These nanostructures are synthesized by an electrospinning technique wherein the process parameters are varied through a design of experiment (DoE) methodology. Various morphologies, namely, high aspect ratio nanofibers, nanoparticles, and mixed microstructures, were obtained at different DoE variable settings. The zeolites of interest in this paper are silica and silicalite, wherein the aluminum content is zero. In the case of mesoporous silica, the sol-gel solution consisting of the silica source and the structure-directing agent is directly electrospun to obtain the nanostructured material on a device substrate. The structure-directing agent is later removed by calcination to obtain the porous nanostructures. In the case of microporous silica, the nanostructured silica is first synthesized and then co-electrospun with a polymer to obtain the desired nanostructure. The structure-directing agent and polymer are later removed by calcination.

A Hybrid MEMS–Fiber Optic Tunable Fabry–Perot Filter

This paper describes a bulk-micromachined electrostatically tunable Fabry-Perot interferometric filter. The device is a hybrid optical filter combining fiber optics and microelectromechanical systems (MEMS) technologies. The static mirror is a Bragg grating mirror built on the end face of an optical fiber that is integrated with a MEMS device, which includes the tunable gold-coated mirror. The MEMS device is fabricated in ¿100¿-oriented silicon wafers using modified and optimized batch MEMS processes. A two-stage approach was used to achieve high finesse (F) and broad tunability simultaneously. In the first stage, actuator issues and mirror structural defects were corrected for by optimizing the fabrication-process parameters. In this stage, near-theoretical performance has been achieved with a pure Si MEMS tunable etalon. In stage two, optical fibers with dielectric stack mirrors from Micron Optics, Inc. have been integrated with the MEMS devices to form tunable cavities. The device insertion loss was below 15 dB and was mostly attributed to absorption losses in the gold coating. We measured a pass bandwidth of around 0.54 nm and a tuning range of nearly 120 nm resulting in an F of over 220.

Probabilistic Analysis of a Comb-Drive Actuator

In the present paper, we discuss about design methodology to study the reliability of a microelectromechanical systems device. The proposed methodology was illustrated for design of a comb-drive actuator. In particular, effect of variations introduced in the design parameters due to the fabrication process and their impact on reliability of a comb-drive actuator is reviewed in detail. We use Crystal Ball® (Decisioneering, Inc., 2000), a probabilistic analysis tool to analyze the performance of comb-drive actuator. The present method requires an analytical model or transfer function derived using experimental data to study the variation in output due to variations in input parameters. We developed an analytical model for displacement of actuator and verified the analytical model using finite element model. This analytical model was used to study the variation in displacement of comb-drive actuator. This methodology uses a combination detailed experimental studies done to establish fabrication limitations or capabilities. The analysis and final design selection was based on a combination of Crystal Ball® studies and fabrication constraints. The same methodology could be extended to study the reliability of MEMS sensors and actuators due to variations on process parameters.

Photonic bandgap fiber enabled Raman detection of nitrogen gas

Raman detection of nitrogen gas is very difficult without a multi-pass arrangement and high laser power. Hollow-core photonic bandgap fibers (HC-PBF) provide an excellent means of concentrating light energy in a very small volume and long interaction path between gas and laser. One particular commercial fiber with a core diameter of 4.9 microns offers losses of about 1dB/m for wavelengths between 510 and 610 nm. If 514nm laser is used for excitation, the entire Raman spectrum up to above 3000 cm-1 will be contained within the transmission band of the fiber. A standard Raman microscope launches mW level 514nm laser light into the PBF and collects backscattered Raman signal exiting the fiber. The resulting spectra of nitrogen gas in air at ambient temperature and pressure exhibit a signal enhancement of about several thousand over what is attainable with the objective in air and no fiber. The design and fabrication of a flow-through cell to hold and align the fiber end allowed the instrument calibration for varying concentrations of nitrogen. The enhancement was also found to be a function of fiber length. Due to the high achieved Raman signal, rotational spectral of nitrogen and oxygen were observed in the PBF for the first time to the best of our knowledge.

Sensor Design Guidelines

This chapter focuses on introducing fundamental design principles of transducers, familiarizing readers who are new to this field with the common vocabulary used in describing transducer performance, and providing a succinct historical background about the implementation of thin films and nanostructures in sensors and analytical instruments. A systematic methodology and a sequence of guiding steps to follow in designing a transducer beginning with a concept, through materials selection, and transducer design and fabrication are presented. These steps are covered in more detail in subsequent chapters with concrete examples.

Oil-free stress impedance pressure sensor for harsh environment

We report on the design, fabrication and demonstration of a pressure sensor using a magnetic material strain gauge. The gauge consists of a soft magnetic thin foil or ribbon patterned in various geometries. It is attached to machined stainless steel diaphragm by means of an adhesive layer. Upon application of pressure to the diaphragm, strain develops in the magnetic gauge leading to a change in its impedance phase and amplitude, which are measured and correlated to pressure. The sensor demonstrated high sensitivity (4 10-5 degrees/Pa in phase angle and 5.4 10-5 Ohm/Pa in amplitude) and low hysteresis (0.26% in phase angle, 0.03% in amplitude). Compared to piezoresistive strain gauges, the magnetic gauge provides approximately 5times to 9times improvement in strain gauge factor. In addition to higher performance, the magnetic pressure sensor is an attractive alternative to silicon-micromachined sensors for harsh environment high-pressure sensors