Sensitive side of low cost sensors

Low-cost solid state sensors can measure NOx levels very accurately, at high temperature and pressure. Automotive companies are assessing whether the technology could determine the quality of air in the cabin of vehicles. The sensors, which were launched in May, could also be used to measure exhaust levels in car parks or garages and in combustion control areas. “They require significant power input [several hundred mW], so are not suitable for portable applications that require battery operation,” explains Deborah Deininger, product manager at Synkera Technologies, which developed the sensor. The company is based in Colorado, USA. According to Deininger, the ‘P/N 718’ nitrogen dioxide sensors are also not suitable for applications where levels of NO and NO2 must be measured independently. But for applications that require measurement of total NOx emissions, the sensor response can be tailored via variations in sensing and heating potential, in order to optimise the sensor response for intended applications. In this way, she says, both high sensitivity and high range detection can be achieved with a single, low cost sensor. Solid-state metal oxide sensors are widely regarded as being low cost, but with questionable performance characteristics. Recent work has improved their performance, but without increasing sensor cost. Also, metal oxide gas sensors possess inherent advantages for many process control applications because they are robust and reliable, and can withstand high temperatures. This sensor was developed with Patrick Moseley, one of the founders of UK-based company Capteur that first commercialised MOS sensors and was later sold to City Technology. Synkera’s sensor is the only one released commercially so far from the Moseley-Synkera collaboration, but more are likely to follow. “The careful selection of sensing materials is critical for improving sensor performance. Synkera has focused on several specific aspects of materials chemistry in its recent development,” says Deininger. Several researchers have recently reported a substantial performance increase in semiconducting metal oxide sensors as the grain size is reduced to the ‘nano-scale’ level, she adds. “Perhaps the simplest explanation for this phenomenon rests in the fact that chemisorption [the underlying principle of the sensors] is primarily a surface effect. Chemisorbed oxygen ions act as surface acceptors, trapping electrons and forming a space charge layer.” Synkera used tungsten oxide (WO3) based materials to create its NOx sensor. Adding ‘dopants’ to this base material enhanced the properties of the sensor by modifying its conductivity and microstructure. Although Synkera would not reveal the exact material and chemical properties that make up the sensor, Deininger did reveal that the sensor contained “a proprietary blend of dopants and catalysts to optimise the sensitivity and selectivity of the sensor’s response to NOx”. The solid-state NOx sensors are packaged in a commercial electronics package. Because they operate at 200-350°C, the sensor element is suspended in order to minimise heat transfer between it and the package. The sensors are heated via a resistive material that has been screen-printed on the undersurface of the sensor elements. The electrical connections between the sensor element and the package are formed via resistance welding, leading to strong, reliable connections which are unaffected by temperature and chemical environment. Each sensor has four electrical connections. Two are used to measure the resistance of the sensing material, while two are used to provide power to the resistive heater. A comparison of the response of a WO3-based sensor prepared from commercial coarse grained powders and one which had been prepared from a nano-structured material (grain size <100nm) were tested. The sensors were held in an air background, and 8ppm NO2 was introduced from 200 to 400 seconds. The introduction of NO2 caused an increase in the sensor resistance. The tests clearly showed that the sensor produced from the nano-structured sensor had a significantly larger and faster response to the challenge gas of 8ppm NO2. The response of a typical sensor to 1ppm of NO and NO2 was also tested. The sensors were held in a background of dry air, with challenge gas introduced for 300 second exposures at 300 and 900 seconds. At the operating temperature of around 250°C, the sensor’s response to both of these gases was very large, with a shift in resistance of more than an order of magnitude on switching from air to challenge gas. The sensors typically showed a stronger response to NO2 compared to NO. “The sensors have a complete recovery after gas exposure, although both response and recovery time are somewhat slow,” says Deininger. The response of the sensor to a slightly larger concentration of 10ppm NO and NO2 was also tested. The operating temperature was around 250°C. At these higher concentrations, the response of the sensor was faster, while the recovery was significantly slower. Complete recovery was obtained after around 10 minutes. Synkera recently began evaluating the stability of the sensors over much longer time periods (months-years). This work is ongoing. “Preliminary results are very encouraging. The sensors show a strongly increasing resistance with increasing gas concentration,” Deininger concludes.