Publications

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The mass-sensitive smart preconcentrator (SPC) consists of a Lorentz-Force-actuated MEMS resonator with an integral heater and surface coating for the collection of chemical analytes. Control circuitry is used to drive the SPC to resonance and measure its oscillation frequency. The frequency shift produced by adsorption of analyte on the SPC surface is inversely proportional to the mass of analyte collected. Thus, the SPC can measure when it has collected sufficient analyte for a downstream detection system. The limit of detection (LOD) of the SPC is less than 50 ppb for DMMP (dimethyl-methyl- phosphonate). At 1 ppm, less than 1 second collection of DMMP is sufficient to trigger analysis. An analytical model of operation of the SPC is used to predict the motion of the paddle and the shear modulus of silicon. © The Electrochemical Society.

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We have built and tested a miniaturized, thermoelectric power source that can provide in excess of 450 {micro}W of power in a system size of 4.3cc, for a power density of 107 {micro}W/cc, which is denser than any system of this size previously reported. The system operates on 150mW of thermal input, which for this system was simulated with a resistive heater, but in application would be provided by a 0.4g source of {sup 238}Pu located at the center of the device. Output power from this device, while optimized for efficiency, was not optimized for form of the power output, and so the maximum power was delivered at only 41mV. An upconverter to 2.7V was developed concurrently with the power source to bring the voltage up to a usable level for microelectronics.

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The natural gas industry seeks inexpensive sensors and instrumentation to rapidly measure gas heating value in widely distributed locations. For gas pipelines, this will improve gas quality during transfer and blending, and will expedite accurate financial accounting. Industrial endusers will benefit through continuous feedback of physical gas properties to improve combustion efficiency during use. To meet this need, Sandia has developed a natural gas heating value monitoring instrument using existing and modified microfabricated components. The instrument consists of a silicon micro-fabricated gas chromatography column in conjunction with a catalytic micro-calorimeter sensor. A reference thermal conductivity sensor provides diagnostics and surety. This combination allows for continuous calorimetric determination with a 1 minute analysis time and 1.5 minute cycle time using air as a carrier gas. This system will find application at remote natural gas mining stations, pipeline switching and metering stations, turbine generators, and other industrial user sites. Microfabrication techniques will allow the analytical components to be manufactured in production quantities at a low per-unit cost.

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A microflame-based detector suit has been developed for sensing of a broad range of chemical analytes. This detector combines calorimetry, flame ionization detection (FID), nitrogen-phosphorous detection (NPD) and flame photometric detection (FPD) modes into one convenient platform based on a microcombustor. The microcombustor consists in a micromachined microhotplate with a catalyst or low-work function material added to its surface. For the NPD mode a low work function material selectively ionizes chemical analytes; for all other modes a supported catalyst such as platinum/alumina is used. The microcombustor design permits rapid, efficient heating of the deposited film at low power. To perform calorimetric detection of analytes, the change in power required to maintain the resistive microhotplate heater at a constant temperature is measured. For FID and NPD modes, electrodes are placed around the microcombustor flame zone and an electrometer circuit measures the production of ions. For FPD, the flame zone is optically interrogated to search for light emission indicative of deexcitation of flame-produced analyte compounds. The calorimetric and FID modes respond generally to all hydrocarbons, while sulfur compounds only alarm in the calorimetric mode, providing speciation. The NPD mode provides 10,000:1 selectivity of nitrogen and phosphorous compounds over hydrocarbons. The FPD can distinguish between sulfur and phosphorous compounds. Importantly all detection modes can be established on one convenient microcombustor platform, in fact the calorimetric, FID and FPD modes can be achieved simultaneously on only one microcombustor. Therefore, it is possible to make a very universal chemical detector array with as little as two microcombustor elements. A demonstration of the performance of the microcombustor in each of the detection modes is provided herein.

The microcombustor described in this report was developed primarily for thermal management in microsystems and as a platform for micro-scale flame ionization detectors (microFID). The microcombustor consists of a thin-film heater/thermal sensor patterned on a thin insulating membrane that is suspended from its edges over a silicon frame. This micromachined design has very low heat capacity and thermal conductivity and is an ideal platform for heating catalytic materials placed on its surface. Catalysts play an important role in this design since they provide a convenient surface-based method for flame ignition and stabilization. The free-standing platform used in the microcombustor mitigates large heat losses arising from large surface-to-volume ratios typical of the microdomain, and, together with the insulating platform, permit combustion on the microscale. Surface oxidation, flame ignition and flame stabilization have been demonstrated with this design for hydrogen and hydrocarbon fuels premixed with air. Unoptimized heat densities of 38 mW/mm{sup 2} have been achieved for the purpose of heating microsystems. Importantly, the microcombustor design expands the limits of flammability (Low as compared with conventional diffusion flames); an unoptimized LoF of 1-32% for natural gas in air was demonstrated with the microcombustor, whereas conventionally 4-16% observed. The LoF for hydrogen, methane, propane and ethane are likewise expanded. This feature will permit the use of this technology in many portable applications were reduced temperatures, lean fuel/air mixes or low gas flows are required. By coupling miniature electrodes and an electrometer circuit with the microcombustor, the first ever demonstration of a microFID utilizing premixed fuel and a catalytically-stabilized flame has been performed; the detection of -1-3% of ethane in hydrogen/air is shown. This report describes work done to develop the microcombustor for microsystem heating and flame ionization detection and includes a description of modeling and simulation performed to understand the basic operation of this device. Ancillary research on the use of the microcombustor in calorimetric gas sensing is also described where appropriate.