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of a concentrically layered and uniaxially pressed tri- layered structure containing yttria-stabilized zirconia (YSZ) and platinum. The YSZ was derived from two sources. The first source is Tosoh spray dried zirconia (TZY8) and the second is a finely synthesized zirco- nia produced using a proprietary precipitation/glycine- nitrate process. Similarly, the platinum was also pro- duced as a fine, nanometer powder using the glycine nitrate process. Upon producing the necessary precursor powders for the sensor, electrodes containing an YSZ-platinum composite were pressed into thin (0.2 mm thick) pellets using a 10 mm die. The electrolyte layer of the ampero- metric cell was produced by pressing YSZ powder into the center of a 13 mm die. By carefully removing the punch from one end of the die, the pre-pressed 10 mm composite electrode was placed on top of the electrolyte and the remaining circumference of the die was filled with YSZ. Repeating the process for the opposite side yielded the structural basis for the amperometric sen- sor. The sensor pellets were fired to a temperature of 1500◦C for 4 h. The surface of the pellets were polished to provide a smooth surface for zeolite deposition and to expose the back of the sensing electrode for wire attachment. 2.2. Zeolite synthesis Zeolite crystals were grown by hydrothermal synthe- sis, beginning from aluminum flake (Aldrich), sodium hydroxide (Baker), tetramethylammonium hydroxide (TMA, Aldrich) and HS40, a colloidal silicate solution. Silicate and aluminate solutions were combined to form the crystallization solution similar to the method em- ployed by Tsapatsis and Boudreau [13]. This solution was immediately placed in a Teflon container inside a pre-heated stainless steel pressure vessel (Parr 200 cc) with a magnetic stir bar and sealed. The vessel was then placed in an oil bath on a hot plate modified to control the oil temperature. The oil bath was controlled at 80◦C and the solution was stirred continuously inside the sealed pressure vessel via a magnetic stirring bar. The solution was recovered from the pressure ves- sel after 2 to 3 days and separated by centrifuge. The centrifuge separated the zeolite crystals from the re- maining growth solution, which was then discarded. Distilled water was added to the zeolite crystals, which were put into suspension with the aid of an ultrasonic probe. The separation and dilution process continued until the suspended solution had a pH between 10.0 and 10.5. 2.3. Zeolite deposition The zeolite solutions produced are diluted to a level of 1 g of zeolite per 150 g of distilled water. The dilute zeolite solutions were placed on the active sensor sur- face by the use of a dropper. Typically, a 1 cm diameter sample received 1 to 1.5 g of dilute solution to create near monolayer zeolite coverage. The sensor with the solution covering its surface was placed in a covered petri dish for a period of 3 days to allow slow settling of the particles on the surface. The dish was opened for 30 min each morning then resealed. On the third morning, the dish was opened and the remaining solution was allowed to air dry. On the fourth day, the dry sample was rinsed with de-ionized water from a wash bottle for about one minute. 2.4. Zeolite film growth The zeolite regrowth solution is similar to the origi- nal growth solution except that it contains more silica and water. The solution is 4.1 (TMA)2O:0.35 Na2O:1 Al2O3:4.4 SiO2:706 H2O. The solution was prepared in a manner similar to that used to grow the crystals, first producing a sodium aluminate solution to which a silicate solution is added. The sensor substrates, which have already undergone the deposition of a zeolite seed layer, are placed in the bottom of the pressure vessel with their zeolite side up. About 75 grams of regrowth solution was added to the vessel. The pressure vessel was then sealed and placed in an oven for 2 days at 80◦C. After 2 days, the growth solution was replaced with fresh growth so- lution and the vessel was sealed and heated another two days. Sensor samples were rinsed thoroughly with deionized water before adding silver lead wire exten- sions and testing. 2.5. Characterization A variety of characterization techniques were used throughout the sensor fabrication to assure that the fi- nal product was a viable sensor. X-ray diffraction was carried out using a Philips PW 1800 on the precursor powders used for the base sensor as well as the zeolite crystals, before and after deposition. A Hitachi S-800 field emission microscope was crucial to the analysis of the zeolite crystals, depositions and films produced and was also used for analysis of the sensor microstructures. 2.6. Performance testing Sensors were evaluated for sensitivity to gaseous species and selectivity. The apparatus for testing con- tained mass flow controllers for portioning the gaseous mixtures and a tube furnace to maintain the sensor tem- perature in a uniform atmosphere. The sensor sample was placed in the furnace and heated to a stable 600◦C. The atmosphere was controlled through three mass flow controllers and switching valves which allow vari- ous gases to flow to each mass flow controller. The nom- inal scale on two of the mass flow controllers was 0 to 200 sccm and 0 to 20 sccm for the third. The gases available for use included high purity (Air Products UHP, 99.995% purity) oxygen, nitrogen, and argon. Also used were 1% oxygen in nitrogen, air, 1000 ppm NO in argon and CO2. All gases were supplied at 40 psi, and were calibrated in each mass flow control before experimentation started. Lead wires from the sensor exited the sealed tube and were attached to an EG&G 273A potentiostat. The po- tentiostat was used to apply a bias to the cell while mea- suring the current flow, the basic amperometric sensor measuring technique. A bias of 1.8 V was used for all CHEMICAL SENSORS 4309PDF Image | Development of a selective gas sensor utilizing zeolite membrane
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