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Dual mode NOx sensor: measuring both the accumulated amount and instantaneous level at low concentrations.

Groß A, Beulertz G, Marr I, Kubinski DJ, Visser JH, Moos R - Sensors (Basel) (2012)

Bottom Line: Experimental results are presented demonstrating the sensor's integrating properties for the total amount detection and its sensitivity to both NO and to NO(2).The long-term detection of NO(x) in the sub-ppm range (e.g., for air quality measurements) is discussed.Additionally, a self-adaption of the measurement range taking advantage of the temperature dependency of the sensitivity is addressed.

View Article: PubMed Central - PubMed

Affiliation: Department of Functional Materials, Bayreuth Engine Research Center (BERC), University of Bayreuth, 95440 Bayreuth, Germany.

ABSTRACT
The accumulating-type (or integrating-type) NO(x) sensor principle offers two operation modes to measure low levels of NO(x): The direct signal gives the total amount dosed over a time interval and its derivative the instantaneous concentration. With a linear sensor response, no baseline drift, and both response times and recovery times in the range of the gas exchange time of the test bench (5 to 7 s), the integrating sensor is well suited to reliably detect low levels of NO(x). Experimental results are presented demonstrating the sensor's integrating properties for the total amount detection and its sensitivity to both NO and to NO(2). We also show the correlation between the derivative of the sensor signal and the known gas concentration. The long-term detection of NO(x) in the sub-ppm range (e.g., for air quality measurements) is discussed. Additionally, a self-adaption of the measurement range taking advantage of the temperature dependency of the sensitivity is addressed.

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Influence of the temperature on the sensor response to 10 times 5 ppm NO for 25 s.
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f8-sensors-12-02831: Influence of the temperature on the sensor response to 10 times 5 ppm NO for 25 s.

Mentions: In Figure 8, the sensor responses of a 100 μm Pt-IDE sensor on NO steps at various temperatures from 300 to 400 °C are compared. For all investigated temperatures, the electrical behavior in the unloaded and partly NOx-loaded state can be described by an R//C element. The diameter of the semicircle in the Nyquist-plot, reflecting the resistance, decreased during NO loading as shown in Figure 8(a) for 380 °C (orange) and 400 °C (red line). Data are shown for both prior (solid points) and after (open symbols) exposure to 1,250 ppm·s NO. The conductivity of the sensitive layer in the unloaded state depends on temperature (a detailed analysis gives an activation energy of 0.9 eV). Correspondingly, the resistance of the loaded state also decreases with temperature. The sensor response, /ΔR//R0, as a function of time is depicted in Figure 8(b) for ten steps of 5 ppm NO for 25 s alternating with 0 ppm for 100 s. This plot shows that the sensor exhibits integrating properties from 300 °C to 400 °C with a reduced sensor response at lower temperatures. Extracting the characteristic lines from these data (Figure 8(c)) demonstrates the linearity of the sensor response up to 400 °C as well as the enhanced sensitivity at higher temperatures. Plotting the sensitivity S calculated from the characteristic lines according to Equation (6) as a function of the temperature (Figure 8(d)) points out that the sensitivity to NO correlates nearly linearly with the temperature from 300 to 370 °C. Further rise in the temperature up to 400 °C results in a slightly reduced increase in the sensitivity. As reported in [16], the decreased stability of the formed nitrates at higher temperatures deteriorates the holding abilities of the storage sites and therefore the accumulating behavior of the sensitive layer. At lower temperatures, the kinetics limits the storage rate being reflected by the sensitivity, whereas the linear measurement range is restricted by the stability of the nitrates. Therefore, the sensitivity in the linear detection range increases with temperature whereas this range gets smaller at elevated temperatures. To adapt the measurement range of the integrating NOx sensor to the requirements of the application, the measurement temperature is an effective tool as it is an easy parameter to adjust.


Dual mode NOx sensor: measuring both the accumulated amount and instantaneous level at low concentrations.

Groß A, Beulertz G, Marr I, Kubinski DJ, Visser JH, Moos R - Sensors (Basel) (2012)

Influence of the temperature on the sensor response to 10 times 5 ppm NO for 25 s.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3376581&req=5

f8-sensors-12-02831: Influence of the temperature on the sensor response to 10 times 5 ppm NO for 25 s.
Mentions: In Figure 8, the sensor responses of a 100 μm Pt-IDE sensor on NO steps at various temperatures from 300 to 400 °C are compared. For all investigated temperatures, the electrical behavior in the unloaded and partly NOx-loaded state can be described by an R//C element. The diameter of the semicircle in the Nyquist-plot, reflecting the resistance, decreased during NO loading as shown in Figure 8(a) for 380 °C (orange) and 400 °C (red line). Data are shown for both prior (solid points) and after (open symbols) exposure to 1,250 ppm·s NO. The conductivity of the sensitive layer in the unloaded state depends on temperature (a detailed analysis gives an activation energy of 0.9 eV). Correspondingly, the resistance of the loaded state also decreases with temperature. The sensor response, /ΔR//R0, as a function of time is depicted in Figure 8(b) for ten steps of 5 ppm NO for 25 s alternating with 0 ppm for 100 s. This plot shows that the sensor exhibits integrating properties from 300 °C to 400 °C with a reduced sensor response at lower temperatures. Extracting the characteristic lines from these data (Figure 8(c)) demonstrates the linearity of the sensor response up to 400 °C as well as the enhanced sensitivity at higher temperatures. Plotting the sensitivity S calculated from the characteristic lines according to Equation (6) as a function of the temperature (Figure 8(d)) points out that the sensitivity to NO correlates nearly linearly with the temperature from 300 to 370 °C. Further rise in the temperature up to 400 °C results in a slightly reduced increase in the sensitivity. As reported in [16], the decreased stability of the formed nitrates at higher temperatures deteriorates the holding abilities of the storage sites and therefore the accumulating behavior of the sensitive layer. At lower temperatures, the kinetics limits the storage rate being reflected by the sensitivity, whereas the linear measurement range is restricted by the stability of the nitrates. Therefore, the sensitivity in the linear detection range increases with temperature whereas this range gets smaller at elevated temperatures. To adapt the measurement range of the integrating NOx sensor to the requirements of the application, the measurement temperature is an effective tool as it is an easy parameter to adjust.

Bottom Line: Experimental results are presented demonstrating the sensor's integrating properties for the total amount detection and its sensitivity to both NO and to NO(2).The long-term detection of NO(x) in the sub-ppm range (e.g., for air quality measurements) is discussed.Additionally, a self-adaption of the measurement range taking advantage of the temperature dependency of the sensitivity is addressed.

View Article: PubMed Central - PubMed

Affiliation: Department of Functional Materials, Bayreuth Engine Research Center (BERC), University of Bayreuth, 95440 Bayreuth, Germany.

ABSTRACT
The accumulating-type (or integrating-type) NO(x) sensor principle offers two operation modes to measure low levels of NO(x): The direct signal gives the total amount dosed over a time interval and its derivative the instantaneous concentration. With a linear sensor response, no baseline drift, and both response times and recovery times in the range of the gas exchange time of the test bench (5 to 7 s), the integrating sensor is well suited to reliably detect low levels of NO(x). Experimental results are presented demonstrating the sensor's integrating properties for the total amount detection and its sensitivity to both NO and to NO(2). We also show the correlation between the derivative of the sensor signal and the known gas concentration. The long-term detection of NO(x) in the sub-ppm range (e.g., for air quality measurements) is discussed. Additionally, a self-adaption of the measurement range taking advantage of the temperature dependency of the sensitivity is addressed.

Show MeSH