, Sensors, metal oxide, chemiresistor - UT Arlington – UTA Technologies/ISA NOx Pap… · NOx, Sensors, metal oxide, chemiresistor ABSTRACT ... renewed emphasis on the development

Copyright 2003 by ISA – The Instrumentation, Systems and Automation Society Presented at ISA Expo 2003 Technical Conference; http:/www.isa.org

2021 Miller Dr., Suite B, Longmont, CO 80501 Phone: 720.494.8401 Fax: 720.494.8402 Web: www.synkera.com

Debra J. Deininger Stephen S. Williams Senior Scientist President & CTO Synkera Technologies, Inc. Synkera Technologies, Inc. Longmont, CO 80501 Longmont, CO 80501 Clayton J. Kostelecky Senior Engineer Synkera Technologies, Inc. Longmont, CO 80501 KEYWORDS NOx, Sensors, metal oxide, chemiresistor ABSTRACT The environmental and health-related impacts of NOx emissions are widely known. Efforts to further reduce acid rain have led to a number of recent rulings specific to NOx reduction, which have then prompted regulatory agencies, engine manufacturers, and utility plants to place a renewed emphasis on the development of new monitoring and control technologies. Potential applications for NOx sensing include industrial health and safety, monitoring of emissions in on- and off-road vehicles, flue gas monitoring of large combustion sources (e.g., power plants), and control of gas-fired furnace atmospheres for use in materials processing. Extensive knowledge of materials chemistry and nano-engineering has led to the development of a mixed metal oxide semiconductor that is extremely sensitive to NOx vapors, offering sub-ppm detection limits and enhanced selectivity. This NOx sensor technology has demonstrated good performance and long-term durability in a rugged low-cost package. The development and testing of this new sensor technology will be discussed in detail, and compared to other NOx sensing technologies. The paper will conclude with a summary of the advantages and disadvantages of using chemiresistive sensors for the detection of NOx in a variety of applications.



INTRODUCTION In the world of chemical sensors, solid-state metal oxide sensors are widely regarded as a low-cost option with questionable performance characteristics. Recent development work has significantly improved the performance of solid-state sensors, without increasing the sensor cost. Additionally, metal oxide gas sensors possess inherent advantages for many process control applications because the sensors are extremely robust, long-lived, and capable of withstanding elevated temperatures. SOLID STATE SENSING BACKGROUND The development of metal oxide semiconductor sensors traces back to 1952, when Bardeen and Brattain [i] discovered that the electrical conductance of oxide semiconductors could be modulated by surrounding gases. Seiyama et al. [ii] were among the first to point out that semiconducting oxides can be used as gas sensors. The last three decades have seen significant development of empirical and trial-and-error techniques to develop novel solid-state sensors. Recently, work has begun to identify the basic principles and phenomena that occur in a semiconducting sensing material [iii].

Most semiconductor metal oxides undergo surface interactions, such as physisorption, and chemisorption, with gas molecules at elevated temperatures (300 to 600°C). Since most semiconductor sensors are polycrystalline (composed of multiple crystallite grains pressed or sintered into a continuous structure incorporating grain boundaries), the adsorbed gases have significant electronic effects on the individual crystalline particles. These gas-solid interactions, illustrated in Figure 1, result in a change in electron (or hole) density at the surface (i.e., a space charge forms), which in turn results in a change in overall conductivity of the semiconductor oxide. This behavior has been widely exploited to produce rugged, low cost ceramic sensors that are widely used in many applications with varying degrees of

success. The best-known example is tin oxide (SnO2), which has been used for the detection of many different gases, including carbon monoxide (CO) and VOCs. However, this sensing

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FIGURE 1: SENSING MECHANISM IN SOLID STATE METAL OXIDE SENSORS.



mechanism also tends to result in poor selectivity and excessive baseline drift. Modification of the sensor materials and processing methods can significantly reduce these problems. The careful selection of sensing materials is critical for improving sensor performance. The authors have focused on several specific aspects of materials chemistry in recent work. REDUCED GRAIN SIZE Several researchers [iv,v,vi] have recently reported a substantial performance increase in semiconducting metal oxide sensors as grain size is reduced to the “nano-scale” level. Perhaps the simplest explanation for this phenomenon rests in the fact that chemisorption (i.e., the underlying principle of these sensors) is primarily a surface effect. Chemisorbed oxygen ions act as surface acceptors, trapping electrons and forming a space charge layer. For example, in the case of SnO2 in the presence of CO, these ions react to form CO2, leading to a measurable conductivity change. To maximize the opportunities for such reactions to occur, a high ratio of surface area to volume is needed. An inverse relationship exists between surface area and particle size; hence, ultra-fine-grained materials that offer very high surface area are desirable. From a more theoretical perspective, one must consider the conduction mechanisms in semiconducting oxides. In polycrystalline materials, grain boundaries typically contribute most of the resistance, and conduction is controlled by the height of the energy barrier established at the grain boundary due to the conduction band bending into the space charge layer. Large grain size significantly reduces the concentration of grain boundaries, which in turn reduces sensitivity to changes in the gaseous environment. Boundary layer effects are also present where “necking” of individual grains occurs. In this case, the material’s resistance is controlled by the width of the bulk conduction channel, which narrows as the space charge layer forms. The population of such sites also increases with decreasing grain size, as does the likelihood that such interconnections will be of small enough size to enable the presence of boundary layer effects. Consequently, both mechanisms contribute best when the individual grains are very small, thereby increasing response. For sensor related materials, it has been proposed that intra-grain resistance dominates when the individual grain size is less than twice the Debye length (a function of the space charge layer thickness). For sputtered films of SnO2, this length has been determined to be 3 nm by Hall effect measurements [iv]. Therefore, one can postulate that reducing the size of the sensing material to less than 100 nm increases response, while further reductions to the sub-10-nm regime can have a substantial effect on performance. The solid state NOx sensor described here is based upon nano-structured metal oxide materials.



MATERIALS CHEMISTRY In the present work, tungsten oxide (WO3) based materials have been used to create a NOx sensor. WO3 is well known for its affinity for NOx [vii,viii]. The addition of dopants to this base metal oxide material can enhance the properties of the sensor through modification of the conductivity (via the addition of excess electrons in the conduction band, or through the addition of holes) and/or influence on microstructure (porosity, grain size and grain boundaries). Catalysts are used in conjunction with conventional solid-state sensors to promote a response to a particular gas, thereby improving sensitivity and selectivity. For example, noble-metal catalysts are often used to detect non-polar organics because they form strong homopolar or ligand field bonds with the adsorbate [ix]. It should be noted that it is very important that the catalyst material be well dispersed over the semiconductor surface to ensure good performance. The present sensor contains a proprietary blend of dopants and catalysts to optimize the sensitivity and selectivity of the sensor’s response to NOx. EXPERIMENTAL SENSOR PACKAGING AND ELECTRONICS: The solid-state NOx sensors described herein are packaged in a commercial electronics package, referred to as a TO-39 header and cap. Sensors with and without a protective cap are shown in Figure 2. Because the sensors operate at relatively high temperatures of 200-350°C, the sensor element is suspended to minimize heat transfer between the sensor element and the package. The sensors are heated via a resistive material that has been screen-printed on the undersurface of the sensor elements. In this application, a commercially available ruthenium oxide paste is used to form the heater. 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.

FIGURE 2: SOLID-STATE NOX SENSOR



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. Solid-state metal oxide sensors are typically operated using a simple voltage divider, as shown in Figure 3. This requires two voltage supplies: heater voltage (VH) and circuit voltage (VC). VH is applied to the heater in order to maintain a constant, elevated temperature, for optimum sensing performance. VC is applied to allow a measurement of the output voltage (Vout) across a load resistor (RL). Sensor resistance (Rs) is calculated using the following formula:

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In some cases, as discussed below, a potentiostatic circuit is preferred, in which a constant voltage is applied to the sensor. The applied sensing voltage may range from less than 100 mV to more than 6V and the optimum input voltage may depend on the sensor chemistry. DISCUSSION EFFECT OF NANOMATERIALS The use of nano-structured materials as the active sensing element has been shown to significantly improve sensor performance. Figure 4 is a comparison of the response of a WO3 based sensor prepared from commercial coarse grained powders and one which has been prepared from a nano-structured material (grain size <100 nm). The sensors were held in an air background, and 8 ppm NO2 was introduced from 200 to 400 seconds. The introduction of NO2 causes an increase in the sensor resistance. In order to

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FIGURE 4: WO3 BASED SENSOR RESPONSE TO 8 PPM NO2.



facilitate comparison between two different sensor materials, the sensor response is plotted as RG/RA, the resistance at any point divided by the average resistance in air. Figure 4 clearly shows that the sensor produced from the nano-structured sensor has a significantly larger and faster response to the challenge gas of 8 ppm NO2. EFFECT OF TEMPERATURE: For a given sensing material composition, the operating temperature defines the single most effective control of sensor performance. The response of the sensors can be tailored via control of the temperature at the sensor surface. The effect of temperature on the WO3 based NOx sensor is shown in Figure 5. The sensors are held in a background of dry air, with 5 ppm NO2 introduced for two 300 second exposures at times of 300 and 900 seconds. Again, the introduction of challenge gas causes an increase in sensor resistance. In the case of the WO3 based NOx sensor presented here, lower temperatures are preferred for higher sensitivity, particularly at the ppb level, while higher temperatures can be used to reduce both the sensor resistance in air and the response to gas, allowing a larger dynamic range to be achieved. For this sensor, upper detection limits are defined by saturation of response and

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instrumental limitations of measuring very high resistance values. Particularly at the lower temperatures, the measurement of high resistance values is an area of great concern. EFFECT OF OPERATING POTENTIAL: Although the sensors can be operated using a simple voltage divider circuit as shown in Figure 3, the sensors are non-linear devices, and the voltage at which resistance measurements are made will impact the apparent sensitivity. Figure 6 shows the sensor resistance in air as the applied potential is swept from 10 mV to 6V and back. The sensor resistance in air is a constant, regardless of the potential applied to make the measurement; however, the sensor response in challenge gas is strongly dependent on applied voltage below 4 V. For this reason, in order to achieve the highest sensitivity (and therefore, lowest detection limits), the applied sensing voltage should be kept at 4 -6 V. For applications where lower detection limits are not critical, the simple voltage divider circuit shown in Figure 3 has worked very well. The response of the sensor to a range of NOx concentrations using the voltage divider circuit depicted above has been carried out, and is shown in Figure 7. In this case, the sensor signal is reported as a voltage. Across this range of test concentrations, the sensors show a strong, reproducible response, with excellent signal to noise ratio. TYPICAL RESPONSE DATA The response of a typical sensor to 1 ppm of NO and NO2 is shown in Figure 8. The sensors are 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 ~250°C, the sensor’s response to both of these gases is very large, with a shift in resistance of more than an order of magnitude on switching from air to challenge gas. The sensors typically show a somewhat stronger response to NO2 as compared to NO, as is seen in Figure 8. The sensors have a complete recovery after gas exposure, although both response and recovery time are somewhat slow.

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Figure 9 shows the response of another typical sensor to the slightly larger concentration of 10 ppm NO and NO2. The operating temperature is approximately 250°C. The sensors are again held in air with challenge gas applied at 300 and at 900 seconds. At these higher concentrations, the response of the sensor is faster, while the recovery is significantly slower. Complete recovery is obtained after ~10 minutes.

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FIGURE 9: NOX SENSOR RESPONSE TO 10 PPM CHALLENGE.

The response and recovery time of the sensors is dependent on concentration. Response time (T90) is defined as the time required for the sensor to reach 90% of its full response, while recovery time (T10) is the time required to return to within 10% of the original baseline. Typical response times are 1-3 minutes, depending on the concentration of challenge gas applied, while recovery times (T10) are generally less than 2 minutes, even at higher concentrations. The sensor response is very reproducible, as shown in Figure 10. 10 ppm NO2 was introduced for 300 seconds into a background of dry air at 300 and 900 seconds, several hours apart. The sensor responses are almost identical in terms of baseline, sensitivity and response time.



An evaluation of the stability of the sensors over much longer time periods (months-years) has recently begun and is currently ongoing. Preliminary results are very encouraging. The sensors show a strongly increasing resistance with increasing gas concentration. This response is linear when viewed on a log (resistance) versus log (concentration) plot. A typical plot is shown in Figure 11. The response to low levels of both NO2 and NO is shown at concentrations ranging from 1 to 20 ppm. It is evident from this plot that there is a very large sensor response to these relatively low gas concentrations. An analysis of the response function in conjunction with measured noise levels shows that the sensors are capable of theoretical detection limits of 30 ppb NO2 and 60 ppb NO, at a signal to noise ratio of 3, under laboratory conditions. In practice, the achievable detection limits will obviously be considerable higher due to variations in flow, environmental temperature, and humidity impacting the stability of the sensor. Actual process detection limits are likely to be approximately 0.5 ppm and depend significantly upon instrumentation and frequency of calibration. Although solid state chemiresistive sensors are not normally known for their selectivity, the selectivity of the sensors can be tuned to a large extent through modifications in materials,

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catalyst and operating conditions, including temperature and applied potential. Through careful consideration of materials and conditions, the solid state NOx sensor presented here has been shown to have relatively high selectivity to oxides of nitrogen. The response of the sensors to some typical challenge gases is shown in Table 1. In order to facilitate comparison among challenge gases, gases which causes an increase in sensor resistance (NO2, NO) have their response reported as RG/RA, while gases which cause a decrease in resistance (H2, NH3) have their response reported as –RA/RG. The sensors show a very small response to hydrogen (H2) and ammonia (NH3), which is the opposite direction of the response to nitrogen oxides. The sensors show no response to carbon dioxide (CO2), sulfur dioxide (SO2) and methane (CH4), and it should be noted that the exposure to low levels of sulfur gases does not inhibit the sensors’ response to NOx.

TABLE 1: SELECTIVITY OF THE NOX SENSOR

CHALLENGE GAS CONCENTRATION RG/RA OR –RA/RG NO2 10 ppm 200

NO 10 ppm 45

H2 100 ppm -2.2

NH3 25 ppm -1.1

CO2 5% No response

SO2 5 ppm No response

CH4 1% No response

CONCLUSION Inexpensive solid-state sensors with good performance characteristics, including extremely high sensitivity have been produced. The sensors are rugged, inexpensive and unaffected by relatively high temperatures. The sensors may be useful in applications where inexpensive, long-lived and durable sensors are required, such as process control. The sensors do require significant power input (several hundred mW) and thus are not suited for portable applications requiring battery operation. The sensors are also not suitable for applications where levels of NO and NO2 must be measured independently. For applications requiring measurement of total NOx, the sensor response can be tailored via variations in sensing and heating potential, in order to optimize the sensor response for intended applications. In this way, both high sensitivity and high range detection can be achieved with a single, low-cost sensor.



ACKNOWLEGEMENTS The authors wish to recognize the support of NSF, NASA and DOE-OIT in the development of this sensor technology. i. Bardeen, J. and Brattain, W.H., “Surface Properties of Germanium”, Bell Syst. Tech. J.,

32,1,1953 1.

ii. Seiyama, T.; Kato, A.; Fujishi, K.; Nagatani, M., “A New Detector for Gaseous Components using Semiconductor Thin Films”, Anal. Chem., 34, 11, 1962, 1502.

iii Morazzoni, F.; Canevali, C.; Chiodini, N.; Mari, C.; Ruffo, R.; Scotti, R.; Armelao, L.; Tondello, E.; Depero, L.E.; Bontempi, E., “Nanostructured Pt-doped tin oxide films: Sol-gel preparation, spectroscopic and electrical characterization”, Chem. Mater., 13, 11, 2001, 4355.

iv. Jin, Z.H.; Zhou, H.J.; Jin, Z.L.; Savinelli, R.F.; Liu, C.C., “Application of nano-crystalline porous tin oxide thin film for CO sensing”, Sensors and Actuators B, 52, 1-2, 1998, 188.

v. Varghese, O.K.; Malhotra, L.K.; Sharma, G.L., “High ethanol sensitivity in sol-gel derived SnO2 thin films”, Sensors and Actuators B, 55, 2-3, 1999, 161.

vi. Wu, N.L.; Wang, S.Y.; Rusakova, I.A., “Inhibition of crystallite growth in the sol-gel synthesis of nanocrystalline metal oxides” Science, 285, 5432, 1999, 1375.

vii. LeGore, L.J.; Lad, R.J.; Moulzouf, S.C.; Vetelino, J.F.; Frederick, B.G.; Kenik, E.A., “Defects and Morphology of tungsten trioxide films”, Thin Solid Films, 406, 2002, 79.

viii. Yang, Jong-In; Lim, H.; Han, Sang-Do, “Influence of binders on the sensing and electrical characteristics of WO3-based gas sensors”, Sensors and Actuators B, 60, 1999, 71.

ix. Madou, M.J.; Morrison, S.R Chemical Sensing with Solid State Devices; Academic Press: New York, 1989.

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, Sensors, metal oxide, chemiresistor - UT Arlington – UTA Technologies/ISA NOx Pap… · NOx, Sensors, metal oxide, chemiresistor ABSTRACT ... renewed emphasis on the development