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CHEMICAL SENSORS electrode is then, the only way to introduce ionic and electronic conducting elements into contact with the chemical environment to allow the sensor to operate. The processing conditions for the zeolite and the op- erating environment limit the usefulness of traditional mixed-conducting single-phase ceramics and lead to a composite of two very durable materials. The compos- ite used is composed of platinum and yttria stabilized zirconia (YSZ). The platinum is an electronic conduc- tor and the YSZ phase conducts oxygen anions from the electrochemical reaction sites at the surface. This composite material suits the need for high chemical durability, smooth surface, and process compatibility with the zeolite. Zeolite is a naturally occurring mineral with the unique property of having a molecularly uniform porous structure. Their general form is that of alumino- silicates with many variants being developed since the early days of synthetic zeolite. In the specific case of zeolite A, the crystals are grown in a hydrothermal syn- thesis process that involves dissolved aluminum and silica sol. Sodium hydroxide is necessary as the ba- sic templating agent around which the zeolites assem- ble. In typical processing, the crystals are near perfect cubes, well under 1 micron in width. Under condi- tions of added growth medium or nucleation deterrents, larger crystals can be formed since the growth of crys- tals follows the laws of thermodynamics as they apply to crystal growth [8]. Under the right conditions, crys- tals as large as 50 microns can be grown on earth or up to 85 microns in low earth orbit micro-gravity [9]. Several other research groups have worked with ze- olites to create selective gas sensors. One interesting approach was the measurement of dielectric constant of zeolite crystals with selectively adsorbed gases within the channels [10]. Another approach [11] used a zeolite membrane to selectively limit gases involved in a con- trolled combustion to determine concentration of var- ious hydrocarbons. Similar to the latter approach, our sensor uses the concept of gas flow through a selective zeolite membrane, but utilizes the direct measurement of an amperometric sensor. Amperometric sensor designs have not yet seen as widespread use as either potentiometric electrochemi- cal sensors or sensors based on semi-conducting oxides in spite of their marked advantages. Most designs are based on yttria-stabilized zirconia used as an oxygen anion conductor. As such, oxygen-containing species that encounter the electrode surface of the sensor may dissociate and allow the oxygen anions to permeate through the YSZ electrolyte. This permeation is slow at room temperature and only becomes significant at tem- peratures above 400◦C. Electrons freed in the transition from oxygen molecule to oxygen anion then travel from the electrode through an external circuit where they may be counted and attributed to the gas interaction at the sensor surface. To facilitate that all gaseous species of interest that encounter the sensor surface do dissociate and get counted, an electrical bias is placed across the two sides of the cell which drives the oxygen through. The amplitude of the applied voltage depends on the energy required to dissociate the specific oxygen containing gaseous species. By choosing a set voltage or sweeping the voltage applied, the response to a sin- gle oxygen-containing component can be determined. The signal measured is a current, which is proportion- ally linear to the number of gaseous species dissociated into oxygen anions. The measurement, however, must be performed under the conditions that the current pass- ing through the cell is limited by mass transfer, hence the name, diffusion limiting sensor. This requires that the gas contacting the sensor be of low partial pressure for the species of interest and others that may interfere. To accomplish this, two approaches have been used. The first is to utilize a diffusion-limiting hole in a solid membrane, which sits above the active electrode of the sensor. This allows the cell to pump away oxygen faster than it can diffuse through the hole. The second approach is to use a two-chambered cell. Gas entering the first chamber is pumped at a con- stant rate to reduce the amount of oxygen or interfering gas species. The remaining gas then enters the second chamber where the gas partial pressure is measured. Both of these approaches will prolong response time and produce complex responses as the gas composition changes. A new approach has been realized through the uti- lization of a mixed conducting membrane that serves not only as the electrode, but also as the diffusion bar- rier, eliminating diffusion holes or secondary pumping chambers [12]. In this case, the sensing electrode is a mixed conducting composite consisting of platinum and YSZ. The composite is developed to have connec- tivity of both phases in three dimensions, allowing for the simultaneous transport of both electrons and oxygen anions as is required at the electrode of an electrochem- ical cell. The gas interactions will only be viable if they occur near the phase boundaries on the surface of the electrode. 2. Experimental 2.1. Amperometric sensor fabrication The creation of a new sensor begins with new mate- rials. Many of the materials used in the sensor were created expressly for this application using novel syn- thesis techniques to obtain uniquely tailored properties. Schematically shown in Fig. 1 is the sensor developed in this study. The basic amperometric sensor is composed Figure 1 Schematic of amperometric sensor design with perm-selective zeolite membrane. 4308PDF Image | Development of a selective gas sensor utilizing zeolite membrane
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