Grantee Research Project Results
Final Report: SiC-Microhotplate Conductometric Sensor Array for NOx, CO, and Hydrocarbon Monitoring of Hot Engine Emissions
EPA Contract Number: 68D02075Title: SiC-Microhotplate Conductometric Sensor Array for NOx, CO, and Hydrocarbon Monitoring of Hot Engine Emissions
Investigators: Doppalapudi, Dharanipal
Small Business: Boston MicroSystems Inc.
EPA Contact: Richards, April
Phase: I
Project Period: October 1, 2002 through July 31, 2003
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2002) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , SBIR - Monitoring , Small Business Innovation Research (SBIR)
Description:
As the number of mobile source emissions in the United States increases, it is necessary to further reduce the emission of CO, NOx, hydrocarbon, and other pollutants from such sources to provide safe air quality, especially in urban environments. Significant reductions in pollution emissions can be achieved using advanced engine controls based on real-time measurements of CO, NOx, and hydrocarbon concentrations in the exhaust stream. To meet this need, Boston MicroSystems, Inc., proposed to develop a CO, NOx, and hydrocarbon engine emissions sensor that is capable of operating in hot engine exhaust gases for advanced engine emissions controls.
Summary/Accomplishments (Outputs/Outcomes):
In Phase I, Boston MicroSystems, Inc., proposed to integrate its silicon carbide microhotplate conductometric gas sensor platform with high-temperature semiconducting metal oxide (SMO) gas-sensitive films and harsh environment-compatible Ti3SiC2 electrical contacts, to allow operation in high-temperature, corrosive engine exhaust gases. A prototype microhotplate sensor, with appropriately chosen SMO film(s) was fabricated and tested at temperatures and in gases representative of engine exhaust.
During the 10-month Phase I research project, Boston Microsystems, Inc., first completed fabricating and packaging its four microhotplate gas sensor arrays for use. It was demonstrated that the microhotplates could be heated to 1,100°C with only 21 mW steady-state power consumption and held for hours at that temperature without any noticeable change in performance. The temperature of the microhotplates was controlled by varying the input power. SMO films were deposited onto these microhotplates by self-lithographic chemical vapor deposition (SLCVD) in a chamber especially built for this project. Preliminary testing showed that the sensors exhibit sensitivity to NO, NO2, and CO. This first batch of microhotplates was tested at room temperature in ambient air, keeping electrodes at near-room temperature while heating the microhotplate and the sensor films to the desired elevated temperatures. This allowed for use of Boston MicroSystems, Inc.'s well-established low-temperature Ti/Al metal contacts as electrodes.
Conclusions:
In this first design, the microhotplates have multiple through-plate perforations to increase the exposed p-n junction area and thereby increase sensitivity. During the Phase I studies, important additional insight was gained into the effect of these perforations on hotplate cooling at high temperatures and atmospheric pressures. Perforations were found to induce gas flow through the plate, enhancing convective cooling and decreasing the maximum operating temperature and thermal uniformity over the sensor. Therefore, SLCVD deposition of SMO films was performed at reduced pressures (typically 1-10 torr), and measurements of microhotplate gas sensor response were taken over a wide pressure range from 0.1-760 torr. Based on these findings, a modified hotplate optimized for high-temperature operation at pressures of 1 atm was designed. Furthermore, the new design incorporated a temperature sensor on the microhotplate, enabling good control and monitoring of the sensor operation. At the time of this Final Report, Boston MicroSystems, Inc., has completed the first four stages of the fabrication process.
During Phase I, Boston MicroSystems, Inc., also investigated refractory metal contacts that would enable sensor operation in harsh environments. Two different contact systems based on Ti3SiC2 and Ti/TaSi2/Pt systems have been established. These refractory electrodes will be used in the fabrication of the next generation hotplates during Phase II. Based on the results so far, Boston MicroSystems, Inc., is confident that the company will be able to successfully develop SiC microhotplate-based conductometric sensor arrays for monitoring hot engine emissions during Phase II.
Supplemental Keywords:
SiC, microhotplate conductometric sensor array, NOx, CO, hydrocarbons, monitoring, engine emissions, mobile source emissions, air, real-time measurement, sensor, semiconducting metal oxide, SMO, self-lithographic chemical vapor deposition, SLCVD, small business, SBIR., Scientific Discipline, Air, air toxics, Analytical Chemistry, Environmental Monitoring, mobile sources, Atmospheric Sciences, Engineering, Chemistry, & Physics, Environmental Engineering, particulate matter, particulates, engine exhaust, atmospheric particles, air pollutants, vehicle emissions, motor vehicle emissions, SiC microhotplate conductometric sensor, automotive emissions, emissions measurement, particulate emissions, air sampling, automotive exhaust, emissions, automobiles, emissions analyzer, atmospheric aerosols, exhaust, nitrogen oxides (Nox)The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.