Small, lightweight sensors that exploit the unique characteristics of nanoceramics are being developed for use in measuring air/fuel ratios in the exhaust gas flows of advanced aircraft engines. Adaptive control systems would process the readings from these sensors, along with other sensory and control inputs, to generate engine commands to maximize fuel efficiency and minimize emissions of undesired exhaust products. Sensors and control systems of this type could also be used to enhance the operation of automotive and other internal-combustion engines that burn a variety of fuels, and of stationary combustors like furnaces and kilns.
The developmental sensors are called "lambda sensors" because the fuel/air ratio of particular relevance in this context is denoted by the symbol λ (lambda). The defining equation is λ ≡ [actual value of (density of oxygen/density of fuel)] ÷ [stoichiometric value of (density of oxygen/density of fuel)]. Ordinarily, a monitoring and control system would be designed to keep λ as close as possible to 1. Heretofore, commercially available sensors have not offered the rapid response and high sensitivity needed for monitoring and controlling λ when λ varies from substoichiometry (0.5) to super-stoichiometry (1.5).
Nanomaterials and, in particular, nanoceramics, are novel high-precision materials that are synthesized with selected crystalline sizes Equally important, the complex of electrical and chemical interactions at the interfaces and within the bulk crystalline material can be exploited for sensing chemical species of interest. The net effect of these interactions is that the electrical resistance of a piece of nanoceramic exposed to pure air or oxygen differs from its electrical resistance during exposure to air or oxygen with a fuel or other analyte gas mixed in, and the ratio between the two resistances can be taken as a measure of the concentration of the analyte in the gas mixture. A major portion of the present effort to develop λ sensors is devoted to optimizing crystalline structures, mostly by tailoring grain sizes, in order to exploit combinations of bulk and surface effects to obtain rapid response and high sensitivity.
Initial experiments have been performed on λ sensors made from nanoscale Ga2O3 powders that were synthesized by a solvent-precipitation technique and by a thermal-quench technique. Fabrication of each sensor began with squeezing of Ga2O3 powder at a pressure of 2,000 psi (14 MPa) to form a pellet. The pellets were sintered for one hour in air at a temperature of 1,300 °C. By use of silver paste as a bonding agent, the pellets were bonded to an alumina substrate with platinum electrodes for measuring the electrical resistance of the pellets.
The figure is a plot of data from an experiment in which a sensor was exposed to a synthetic exhaust mixture of CO (regarded as the fuel gas), O2, and N2. In this experiment, λ was made to alternate between 0.5 and 1.5. The plot indicates the feasibility of λ sensors made from Ga2O3 nanoceramics, though the response time is still longer than the maximum (about 10 ms) allowable in engine-control applications. Refinements in the sensor materials and designs will be necessary to reduce response times. Further research will also be needed to ensure stability and reproducibility of sensor characteristics, to increase durability, to enhance sensitivity, to achieve further miniaturization, and to establish temperatures and develop temperature controls for reliable sensor operation.
This work was done by Chuanjing Xu and Tapesh Yadav of Nanomaterials Research Corp. for Lewis Research Center. No further documentation is available. Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135.
Refer to LEW-16700.