$ 0,&52)$%5,&$7(' 6863(1'('78%( &+(0,&$/ 5($&725 )25 )8(/ 352&(66,1* Leonel R. Arana * , Samuel B. Schaevitz **† , Aleksander J. Franz * , Klavs F. Jensen * and Martin A. Schmidt **† * Dept. of Chemical Engineering, ** Dept. of Electrical Engineering and Computer Science † Microsystems Technology Laboratories Massachusetts Institute of Technology, Cambridge MA 02139 $%675$&7 A microfabricated suspended-tube reactor has been developed and demonstrated to operate at temperatures over 900°C for efficient thermal processing of chemical fuels. This reactor uses thin-walled SiN tubes to directly address the most significant problem in small-scale fuel processors: thermal management. It efficiently isolates a high-temperature zone while maintaining a temperature gradient of up to 2000°C/mm. This design is ideally suited to serve as a combustor/recuperator for thermoelectric (TE) and thermophotovoltaic (TPV) generators, and as a reformer to produce hydrogen for portable fuel cell systems. Using the integrated heaters, catalytic ammonia cracking has been carried out to produce up to 1.6 W (9 sccm) of hydrogen with 97% fuel utilization. ,1752'8&7,21 Combustible liquid fuels store up to hundreds of times more energy than state-of-the-art batteries, and are second only to nuclear fuels in the amount of energy stored per unit weight [1]. This explains why a great deal of research has focused on miniaturizing generators to convert chemical energy to electricity in low power (< 100 W) systems. Batteries still dominate, however, because generators, particularly those with moving parts and with high-temperature fuel processors, are difficult to miniaturize. Examples of such generators include fuel cell systems, in which liquid fuels are reformed into hydrogen, heat engines (e.g., TE and TPV), in which high- temperature combustion is required. Several groups have explored different approaches to high-temperature fuel processing on the small scale. Examples range from a membrane-based TE device [2] to combustion-driven mechanical engines [3]. Most efforts have focused on chemical conversions in microfluidic systems with less emphasis on thermal management and scaling. Thermal efficiency remains the key issue in these systems. The most important requirement for a fuel processor in a power generation system, whether as burner or hydrogen generator, is that it be thermally efficient. Any heat loss to the environment is wasted energy and therefore directly undermines the efficiency of the overall process. Thermal isolation of the hot zone is very difficult in miniaturized electric generators (producing < 100 W), and even more so in MEMS generators (~ 1 W), since heat loss relative to heat generation is inversely proportional to characteristic length. )LJXUH  6(0 RI )  UHOHDVHG UHDFWRU VKRZLQJ IRXU VXVSHQGHG 6L1 WXEHV FRQQHFWLQJ WR WKH 6L UHDFWLRQ ]RQH 6L VODEV WKHUPDOO\ OLQNLQJ WKH IRXU WXEHV DQG D PHDQGHULQJ 7L3W UHVLVWRU We have developed a suspended-tube reactor/heat exchanger (Fig. 1) that is designed specifically to isolate a high-temperature zone and allow heat recuperation from process streams for efficient thermal processing of chemical fuels. The applications include on-demand hydrogen production and micro-TE and TPV generators. Our initial reaction studies have focused on ammonia cracking for hydrogen generation. 5($&725 '(6,*1 The suspended-tube reactor, as shown in the schematic in Fig. 2, consists of four thin-walled (2 μm) silicon nitride tubes, comprising two separate U-shaped fluid channels. )LJXUH  6FKHPDWLF RI VXVSHQGHGWXEH UHDFWRU On one end, the tubes are fixed into a silicon substrate containing fluidic channels and ports; on the other end, the channels are free. The free end (hot zone) is partially encased in silicon to form a thermally isolated region of high thermal conductivity in which the chemical reactions Si Reaction Zone 6LGH Si Slabs 7RS Fuel SiN Tubes with 2 μm Wall Thickness Si Reaction Zone (Hot)  PP 0-7803-7185-2/02/$10.00 ©2002 IEEE 232