ELECTROCHEMICAL GAS SENSORS 67 The chip-type device was fabricated using a small bar of YSZ (8 YSZ, 6 × 2 × 1.5 mm), as shown in Figure 1.36, and applying two kinds of oxide pastes, one at each end of the zirconia bar, leaving about 1 mm space in between, followed by calcining at 650°C for 2 h, to form a couple of belt-shaped oxide electrodes (width about 2 mm; thickness about 30 mm). A Pt wire was wound around each oxide elec- trode as an electrical collector (Miura et al. 1008). The test of sensing characteristics has showed that the EMF values varied almost logarithmically with the concentrations of H 2 in the tested range of 20­200 ppm. Furthermore, the authors could operate this chip-type device at 600°C for approximately 400 h as a stability test. The EMF value was relatively stable during the test period. Due to its good sensing characteristics, the YSZ-based device has the potential to be used as a H 2 sensor that can operate in high- temperature combustion exhaust. In fact, this device does not require a reference gas for operation. 9. MEMS AND NANOTECHNOLOGY IN ELECTROCHEMICAL GAS SENSOR FABRICATION Microelectromechanical systems (MEMS), especially micro working electrodes, with very small elec- trode surface area, have increasingly been employed in the fabrication of electrochemical sensors. Microfabrication of electrochemical devices has numerous advantages over standard fabrication pro- cesses. These advantages include precise reduced sensor size, reduced cost, smaller sample size, faster response time, higher concentration sensitivity, well-defined geometric features, and potential for mass production. These advantages can be obtained without degradation of the signal-to-noise ratio, as the sensor size is reduced using careful design. Progress in the development of microamperometric sensors was slow compared to the production of micropotentiometric sensors. Microamperometric sensors of the early 1980s consisted only of micro- fabricated electrodes on a suitable substrate (Stetter et al. 1988). These electrodes were then coated with a liquid electrolyte solution that also carried the sample to the electrode area for analysis. An example of an early microamperometric sensor was presented by Sleszynski and Osteryoung in 1984. That sensor, which used very small electrodes constructed from nonconductive epoxy and reticulated vitreous carbon (RVC), was shown to yield desirable results compared to standard fabricated sensors. However, the goal of constructing complete, practical, and commercially successful amperometric gas sensors using the microfabrication approach has still not been achieved. During the late 1980s, microfabrication technology for the construction of amperometric sensors was investigated with the introduction of novel sensor electrode designs. In 1988, Maclay and co-workers (Maclay et al. 1988; Buttner et al. 1990) introduced a series of Nafion-based microfabricated amperometric gas sensors us- ing gold sensing electrodes in the shape of a square grid. The newly designed sensors were evaluated by comparing their analytical characteristics with those of conventional sensors. The response time of the miniaturized sensors was more than one order of magnitude faster than conventional sensors, although they had lower sensitivity. This work also elucidated the fact that the sensitivity of the device depended not only on the chemical nature of the electrode surface but also on the specific structure of the electro- catalytic surface and the interface created by gas/electrode/electrolyte. In an effort to improve the sensitivity of microamperometric sensors, Buttner et al. (1990) con- structed devices with an integrated design in which the working electrodes, counter electrodes, and