Samara L. Firebaugh

Design and fabrication of microfluidic devices for multiphase mixing and reaction

Using silicon microfabrication technology, microchemical devices have been constructed for the purpose of conducting heterogeneously catalyzed multiphase reactions. The motivation behind the design, the fabrication approach, and the experimental characterization are presented for two classes of devices. The first design involves multiple parallel channels with integrated filter structures to incorporate standard catalytic materials. These catalysts are in the form of finely divided porous particles in a packed-bed arrangement. The second device involves the incorporation of porous silicon as a catalyst support, in the form of a thin layer covering microstructured channels. These microstructured channels simulate the structure of a packed bed and enhance mass transfer relative to an open channel. The ability to incorporate features at the tens-of-microns scale can reduce the mass-transfer limitations by promoting mixing and dispersion for the multiple phases. Directly integrating the catalyst support structures into the channels of the microreactor allows the precise definition of the bed properties, including the supports size, shape and arrangement, and the void fraction. Such a design would find broad applicability in enhancing the transport and active surface area for sensing, chemical, and biochemical conversion devices. Reaction rates for the gas-liquid-solid hydrogenation of cyclohexene using the integrated catalyst with porous silicon as a support compare favorably to those rates obtained with the packed-bed approach. In both cases, the mass transfer coefficient is at least 100 times better than conventional laboratory reactors.

Investigation of high-temperature degradation of platinum thin films with an in situ resistance measurement apparatus

Many microfabricated systems require metallizations that can withstand high temperatures. In particular, a microfabricated chemical reactor system which we are investigating needs thin metal films for heating and temperature sensing that can withstand prolonged 1000/spl deg/C exposure. The current microreactor metallization, a 100-nm platinum film with a 10-nm titanium adhesion layer, degrades at temperatures greater than 700/spl deg/C. This degradation was examined with a custom-built high-temperature resistance measurement apparatus in addition to chemical analysis, scanning electron microscopy (SEM), atomic-force microscopy (AFM) and wafer curvature measurements. Thicker films and coating layers increased the lifetime of these films while exposure to oxygen decreased lifetime, consistent with the hypothesized degradation mechanism of agglomeration.

Miniaturization and integration of photoacoustic detection

Photoacoustic spectroscopy is an absorption spectroscopy technique that is currently used for low-level gas detection and catalyst characterization. It is a promising technique for chemical analysis in mesoscale analysis systems because the detection limit scales favorably with miniaturization. This work focuses on the scaling properties of photoacoustic spectroscopy, and on the miniaturization of gas-phase photoacoustic detection of propane in a nitrogen ambient. The detection system is modeled with a transmission line analogy, which is verified experimentally. The model includes the effects of acoustic leaks and absorption saturation. These two phenomena degrade the performance of the photoacoustic detector and must be controlled to realize the scaling advantages of photoacoustic systems. The miniature brass cells used to verify the model employ hearing aid microphones and optical excitation from a mechanically chopped, 3.39 μm He–Ne laser, transmitted into the cells with an optical fiber. These cells a...

Miniaturization and integration of photoacoustic detection with a microfabricated chemical reactor system

Photoacoustic spectroscopy is a useful technique for monitoring chemical composition in mesoscale analysis systems because the detection limit scales favorably with miniaturization. The key element of a photoacoustic spectrometry system is the detector. This work focuses on the miniaturization of photoacoustic detection. In particular, we are using 3.4 /spl mu/m light to detect propane in a carbon dioxide background-a system that is useful for monitoring combustion reactions. Two systems have been developed. In the first, a miniature photoacoustic cell has been machined into the mounting block of a microfabricated chemical reactor, demonstrating the integration of a photoacoustic detector with a microsystem. The cell used a hearing aid microphone and an infrared diode that was modulated at the first acoustic resonance of the cell. As the gas composition of the cell changed from carbon dioxide to propane the resonance peak was observed to shift and increase, as was expected from theory. This work also presents the first demonstration of a microfabricated photoacoustic detection cell. The cell used an optical microphone and laser excitation brought into the cell via an optical fiber. The light was modulated at a frequency far below the first acoustic resonance, and a signal of 0.05 Pa was observed in the presence of propane.

Chemical performance and high temperature characterization of micromachined chemical reactors

Chemical throughput, product distribution, and lifetime studies have been conducted in Si micromachined chemical reactors (microreactors) for catalytic partial oxidation reactions. Experiments using Pt-catalyzed NH/sub 3/ oxidation as model reaction show that conversion and selectivity behavior of conventional reactors can be reproduced in the microreactor. Highly flammable gases that explode in conventional reactors have been safely oxidized in the microreactor. Packaging and high-temperature materials degradation have been identified as technical challenges to commercial chemical production using microreactor systems.

Novel Liquid Phase Microreactors for Safe Production of Hazardous Specialty Chemicals

A novel liquid-phase microreactor with thin film temperature sensing, good thermal management, and fast mixing has been fabricated for the production of hazardous specialty chemicals, specifically for organic peroxides. The reactor has been characterized and shown to be suitable for these types of applications. Moreover, fast mixing at 10ms and good heat transfer based on an overall heat transfer coefficient of 1445 W/m2°C were demonstrated. The established linear resistive response of the temperature sensors was confirmed experimentally, and the reactor was shown to operate at 11 psi at the 1.0 ml/min design flow rate.

Expansion of Microreactor Capabilities through Improved Thermal Management and Catalyst Deposition

Capabilities of a gas phase membrane-based microreactor have been expanded, enabling new chemistries and applications. The expanded capabilities include improved control of temperature and selectivity for highly exothermic partial oxidation reactions, a larger thermal and pressure operating window, and the development of a catalyst library.

Integrated Gas Phase Microreactors

Microfabricated chemical reactors could be efficient experimental platforms by allowing process development on a single unit and subsequent scale-up by replication. The ability to integrate control, sensor, and reactor functionality offers advantages over the current complex, multi-unit pilot-plant environment. Assemblies of microfabricated reactors may provide point-of-use generation of hazardous chemical intermediates with storage and transportation restrictions. Fabrication, application, and materials issues in the development of microchemical reactor technology are presented. Microreactor feasibility is demonstrated with examples of partial oxidation reactions. The utility of physically-based, predictive simulation tools in guiding microreactor design is addressed as well.