Silicon-Based Microchemical Systems: Characteristics and Applications
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Silicon-Based
Microchemical Systems: Characteristics and Applications Klavs F. Jensen
Abstract Microfabrication techniques and scale-up by replication promise to transform classical batch-wise chemical laboratory procedures into integrated systems capable of providing new understanding and control of fundamental processes. Such integrated microchemical systems would enable rapid, continuous discovery and development of new products with the use of fewer resources and the generation of less waste. Additional opportunities exist for on-demand and on-site synthesis, with perhaps the first applications emerging in portable energy sources based on the conversion of hydrocarbons to hydrogen for miniaturized fuel cells. Microchemical systems can be realized in a wide range of materials including stainless steel, glass, ceramics, silicon, and polymers. The high mechanical strength, excellent temperature characteristics, and good chemical compatibility of silicon combined with the existing fabrication infrastructure for microelectromechanical systems (MEMS) offer advantages in fabricating chemical microsystems that are compatible with strong solvents and operate at elevated temperatures and pressures. Furthermore, silicon-based microsensors for flow, pressure, and temperature can readily be integrated into the systems. Microsystems for broad chemical applications should be discovery tools that can easily be applied by chemists and materials scientists while also having a convincing “scale-out” to at least small production levels. The interplay of both these capabilities is important in making microreaction technology successful. Perhaps the largest impact of microchemical systems will ultimately be the ability to explore reaction conditions and chemistry at conditions that are otherwise difficult to establish in the laboratory. Case studies are selected to illustrate microfluidic applications in which silicon adds advantages, specifically, integration of physical sensors and infrared spectroscopy, highthroughput experimentation in moisture-sensitive organic synthesis, controlled synthesis of nanoparticles (quantum dots), multiphase and heterogeneous catalytic reactions at elevated temperatures and pressures, and thermal management in the conversion of hydrocarbons to hydrogen. Keywords: chemical synthesis, fluidics, microelectromechanical systems (MEMS), phase transformation, sensor, Si.
Introduction Microfluidic systems for chemical transformations (“microreactors”) have been realized in many different materials in-
MRS BULLETIN • VOLUME 31 • FEBRUARY 2006
cluding stainless steel,1–4 glass,5–7 ceramics,8 silicon,9 and polymers.10–13 The choice of the structural material depends on
chemical compatibility, temperature, and pressure, as well as ease of fabrication and integration. Integration of sensors places additional constraints, such as transparent layers for optical spectroscopy and tailored conductive layers for resistancebased sensors. Metal microchannels can be created by several techniques, including mechanica
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