David J. Kubinski

Comparison of sol-gel and ion beam deposited MoO3 thin film gas sensors for selective ammonia detection

Abstract This paper presents a comparative study of the gas-sensing behavior of MoO3 thin films prepared by ion beam deposition and sol–gel techniques, respectively. The sensing response of these films to ammonia in the presence of interfering gases (such as NO2) is assessed. The microstructural characteristics of the sensing materials prepared by these two techniques are determined using electron microscopy and microanalysis techniques. MoO3 is shown to be highly sensitive to ammonia, and the sensitivity depends on processing. The sensing properties of MoO3 thin films and the origin of the p–n-type transition occurring in sol–gel processed films are correlated with the type of the MoO3 polymorph(s) used for sensing.

Using a MISiC-FET sensor for detecting NH/sub 3/ in SCR systems

One way to decrease the emitted levels of NO/sub x/ from diesel engines is to add NH/sub 3/ in the form of urea to the exhausts after combustion. NH/sub 3/ will react with NO/sub x/ in the catalytic converter to form N/sub 2/ and water, which is called selective catalytic reduction (SCR). The amount of NH/sub 3/ added may be regulated through closed-loop control by using an NH/sub 3/ sensor. The metal-insulator silicon-carbide field-effect transistor (MISiC-FET) sensor has previously been tested for this application and has been shown to be sensitive to NH/sub 3/. Here, the sensors have been further studied in engine SCR systems. Tests on the cross sensitivity to N/sub 2/O and NO/sub 2/, and studies concerning the influence of water vapor have been performed in the laboratory. The difference between Ir and Pt films, with regard to catalytic activity, has also been investigated. The sensors were found to be sensitive to NH/sub 3/ in diesel engine exhausts. The addition of urea was computer controlled, which made it possible to add NH/sub 3/ in a stair-like fashion to the system and detect it with the MISiC-FET sensors. The presence of water vapor was shown to have the largest effect on the sensors at low levels and the NH/sub 3/ response was slightly decreased by a background level of NO/sub 2/.

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

The accumulating-type (or integrating-type) NOx sensor principle offers two operation modes to measure low levels of NOx: 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 NOx. Experimental results are presented demonstrating the sensor’s integrating properties for the total amount detection and its sensitivity to both NO and to NO2. We also show the correlation between the derivative of the sensor signal and the known gas concentration. The long-term detection of NOx 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.

The Effect of the Thickness of the Sensitive Layer on the Performance of the Accumulating NOx Sensor

A novel and promising method to measure low levels of NOx utilizes the accumulating sensor principle. During an integration cycle, incoming NOx molecules are stored in a sensitive layer based on an automotive lean NOx trap (LNT) material that changes its electrical resistivity proportional to the amount of stored NOx, making the sensor suitable for long-term detection of low levels of NOx. In this study, the influence of the thickness of the sensitive layer, prepared by multiple screen-printing, is investigated. All samples show good accumulating sensing properties for both NO and NO2. In accordance to a simplified model, the base resistance of the sensitive layer and the sensitivity to NOx decrease with increasing thickness. Contrarily, the sensor response time increases. The linear measurement range of all samples ends at a sensor response of about 30% resulting in an increase of the linearly detectable amount with the thickness. Hence, the variation of the thickness of the sensitive layer is a powerful tool to adapt the linear measurement range (proportional to the thickness) as well as the sensitivity (proportional to the inverse thickness) to the application requirements. Calculations combining the sensor model with the measurement results indicate that for operation in the linear range, about 3% of the LNT material is converted to nitrate.

Aging Studies of Sr-Doped LaCrO3 ∕ YSZ ∕ Pt Cells for an Electrochemical NO x Sensor

The stability and NO x sensing performance of electrochemical cells of the structure Sr-doped LaCrO 3-δ (LSC)/yttria-stabilized zirconia (YSZ)/Pt are being investigated for use in NO x aftertreatment systems in diesel vehicles. Among the requirements for NO x sensor materials in these systems are stability and long lifetime (up to 10 years) in the exhaust environment. In this study, cell aging effects were explored following extended exposure to a test environment of 10% O 2 at operating temperatures of 600-700°C. The data show that aging results in changes in particle morphology, chemical composition, and interfacial structure. Impedance spectroscopy indicates an initial increase in the cell resistance during the early stages of aging, which is correlated principally to densification of the Pt electrode. Also, X-ray photoelectron spectroscopy indicates formation of SrZrO 2 solid-state reaction product in the LSC, a process which is of finite duration. Subsequently, the overall cell resistance decreases with aging time, due in part to roughening of YSZ-LSC interface, which improves interface adherence and enhances charge transfer kinetics at the gas phase/YSZ/LSC triple-phase boundary. This study constitutes a first step in the development of a basic understanding of aging phenomena in solid-state electrochemical systems with applications not only to sensors, but also to fuel cells, membranes, and electrolyzers.

MISiCFET chemical sensors for applications in exhaust gases and flue gases

A chemical gas sensor based on a silicon carbide field effect transistor with a catalytic gate metal has been under development for a number of years. The choice of silicon carbide as the semiconductor material allows the sensor to operate at high temperatures, for more than 6 months in flue gases at 300degreesC and for at least three days at 700degreesC. The chemical inertness of silicon carbide and a buried gate design makes it a suitable sensor technology for applications in corrosive environments such as exhaust gases and flue gases from boilers. The selectivity of the sensor devices is established through the choice of type and structure of the gate metal as well as the operation temperature. In this way NH3 sensors with low cross sensitivity to NOx have been demonstrated as potential sensors for control of selective catalytic reduction (SCR) of NOx by urea injection into diesel exhausts. Here we show that sensors with a porous platinum or iridium gate show different temperature ranges for NH3 detection. The hardness of the silicon carbide makes it for example more resistant to water splash at cold start of a petrol engine than existing technologies, and a sensor which can control the air to fuel ratio, before the exhaust gases are heated, has been demonstrated. Silicon carbide sensors are also tested in flue gases from boilers. Efficient regulation of the combustion in a boiler will decrease fuel consumption and reduce emissions.

Dosimeter-Type NOx Sensing Properties of KMnO4 and Its Electrical Conductivity during Temperature Programmed Desorption

An impedimetric NOx dosimeter based on the NOx sorption material KMnO4 is proposed. In addition to its application as a low level NOx dosimeter, KMnO4 shows potential as a precious metal free lean NOx trap material (LNT) for NOx storage catalysts (NSC) enabling electrical in-situ diagnostics. With this dosimeter, low levels of NO and NO2 exposure can be detected electrically as instantaneous values at 380 °C by progressive NOx accumulation in the KMnO4 based sensitive layer. The linear NOx sensing characteristics are recovered periodically by heating to 650 °C or switching to rich atmospheres. Further insight into the NOx sorption-dependent conductivity of the KMnO4-based material is obtained by the novel eTPD method that combines electrical characterization with classical temperature programmed desorption (TPD). The NOx loading amount increases proportionally to the NOx exposure time at sorption temperature. The cumulated NOx exposure, as well as the corresponding NOx loading state, can be detected linearly by electrical means in two modes: (1) time-continuously during the sorption interval including NOx concentration information from the signal derivative or (2) during the short-term thermal NOx release.

Investigating the Stability and Accuracy of the Phase Response for NOx Sensing 5% Mg-modified LaCrO3

Impedance spectroscopy measurements were carried out on LaCr{sub 0.95}Mg{sub 0.05}O{sub 3} (LCM) asymmetric interdigitated electrodes supported on fully stabilized 8-mol% Y{sub 2}O{sub 3}-stabilized ZrO{sub 2} (YSZ) electrolytes. Experiments were carried out using 0-50 ppm NO{sub x}, 5-15% O{sub 2} with N{sub 2} as the balance, over temperatures ranging from 600-700 C. AC measurements taken at a constant frequency between 1-100 Hz indicated the phase response of the sensor was less sensitive to fluctuations in the O{sub 2} concentration and the baseline drift was limited. Specific frequencies were observed where the sensor response was essentially temperature independent.

Cosputtered Metal and

High-temperature metal-insulator-silicon-carbide (MISiC) sensors are currently under development for use as NH3 sensors in selective-catalytic-reduction (SCR) systems in diesel engines or non-SCR (NSCR) systems in boilers. The detection of NH3 by these sensors requires the presence of triple points where the gas, the metal, and the insulator meet. These triple points have traditionally been located at the interface between the insulator and a porous metal. However, to facilitate the long-term stability of the devices when used in a harsh environment, a nonporous gate material would be preferred. Here, the behavior of the samples where such triple points have been introduced in a dense film through cosputtering of the insulator (SiO 2), and either Pt or Ir is studied. The NH3 sensitivity of the materials was found to be in accordance with the earlier investigations on Si-based samples with cosputtered gate materials. Several metal-to-insulator ratios for each of the metals Pt and Ir were studied. The sensitivity of the layers as well as their selectivity to different concentrations of NH3 at temperatures ranging from 150 degC to 450 degC was investigated. The films containing 60%-70% Pt or Ir were found to give a high sensitivity toward NH3. These samples were shown to be sensitive also to propylene and H2 but were rather insensitive to NO and CO

hboxSiO_2

59Co NMR and magnetization study has been performed in two series of multilayer stacks: [Ru(2.5 nm)/Co(x)/Cu(2.5)]10 and [Cu(2.5 nm)/Co(x)/Ru(2.5)]10. The amount of Co involved in the interfaces of the two respective series was found to be 0.6 nm and 0.75 nm per Co layer. This asymmetry of intermixing as a function of deposition sequence is related to differences in spreading energy between Co on Ru and Ru on Co. The resulting difference in the effective thickness of the Co layer was shown to affect the magnetoresistive properties of the mixed Co/Cu/Co/Ru multilayers.