Novel High Temperature Materials for In-Situ Sensing Devices

Abstract

Abstract:The overriding goal of this project was to develop gas sensor materials and systems compatible with operation at temperatures from 500 to 700 °C. Gas sensors operating at these temperatures would be compatible with placement in fossil-energy exhaust streams close to the combustion chamber, and therefore have advantages for process regulation, and feedback for emissions controls. The three thrusts of our work included investigating thin film gas sensor materials based on metal oxide materials and electroceramic materials, and also development of microhotplate devices to support the gas sensing films. The metal oxide materials NiO, In 2 O 3 , and Ga 2 O 3 were investigated for their sensitivity to H 2 , NO x , and CO 2 , respectively, at high temperatures (T > 500 °C), where the sensing properties of these materials have received little attention. New ground was broken in achieving excellent gas sensor responses (>10) for temperatures up to 600 °C for NiO and In 2 O 3 materials. The gas sensitivity of these materials was decreasing as temperatures increased above 500 °C, which indicates that achieving strong sensitivities with these materials at very high temperatures (T ! 650 °C) will be a further challenge. The sensitivity, selectivity, stability, and reliability of these materials were investigated across a wide range of deposition conditions, temperatures, film thickness, as using surface active promoter materials.We also proposed to study the electroceramic materials BaZr (1-x) Y x O (3-x/2) and BaCe (2-x) Ca x S (4-x/2) for their ability to detect H 2 O and H 2 S, respectively. This report focuses on the properties and gas sensing characteristics of BaZr (1-x) Y x O (3-x/2) (Y-doped BaZrO 3 ), as significant difficulties were encounter in generating BaCe (2-x) Ca x S (4-x/2) sensors. Significant new results were achieved for Y-doped BaZrO 3 , including sensitivities of more than 60 atm -1 for H 2 O vapor at 400 °C. These results were achieved despite significant difficulties with a strong Ba deficiency in the deposited films, and difficulties with stress in the targets and films. Ultimately, these films achieved good sensitivity, selectivity, and reliability in our gas sensing tests.The final thrust of our project was to develop microhotpates. We proposed the use of SiC thin films for the heater of the microhotplate, but despite extensive efforts we were not able to secure a reliable source of SiC. An alternative microhotplate architecture using SiO 2 and Si 3 N 4 suspended membrane structures, and a polysilicon heater were developed, which could be fabricate at commercial MEMs foundries. These microhotplates were fabricated at Microtechnology Services Frankfurt (MSF) in Germany. The fabricated heaters were able to achieve temperatures > 600 °C using ~ 0.25 W, and when combined with In 2 O 3 films demonstrated sensor systems with sensor responses up to 50 for 25 ppm NO x , and time constants of less than 10 s. I. Executive SummaryThe most important goals of this project was to extend the performa...