Improved chemical sensors track and control emissions

Controlling combustion and industrial processes requires sensors that can operate in complex, high-temperature environments while remaining selective to specific gases. Silicon carbide field-effect-transistor (SiC-FET) sensor systems can control environmental factors and emissions in a variety of settings, such as power plants.¹ By using appropriate sensing layers and operation temperatures, SiC-FET sensors can monitor and control many types of toxic emissions, such as carbon monoxide, nitric oxides, sulfur dioxide, hydrogen sulfide, volatile organic compounds, and ammonia. However, the selectivity and long-term stability of these sensors must be improved for them to be useful in many applications. To combat these selectivity and stability issues, we developed a new SiC-based sensor.

In addition to monitoring and controlling toxic gases, particulate matter is another important safety concern. Particulate can be hazardous depending on its size, concentration, shape, and chemical content. Unfortunately, current particulate detectors are rather costly and cannot detect chemical content. Thus, we designed a separate new sensor and an accompanying low-cost method to identify dangerous particulate matter.

A SiC-based transistor can be used as a gas sensor when its gate contact is made from a catalytic metal. When using a catalytic metal, gas molecules will decompose and react with the catalytic gate contact, changing the amount of current passing through the transistor. Some gas species, such as hydrogen, can even diffuse to the underlying oxide exposed by the porous catalytic metal. This chemical activity will charge the gate area, also changing the current through the channel of the transistor.

Our newly designed silicon carbide-based sensors (see Figure 1) have greatly improved selectivity, tunability, and long-term stability.² While an enhancement-mode transistor needs a certain applied voltage to conduct current, a depletion-mode transistor conducts current without an applied voltage. Ours is a depletion-mode sensor that can be operated at a much smaller gate voltage, improving the structural stability of the catalytic metal. The depletion-mode design allows us to finely tune the current level during operation to change the sensitivity and dynamic range of the sensor for different gases.

Figure 1. Silicon carbide-based gas sensors on a 4" silicon carbide wafer. Insert: Mounted gas sensor and a temperature sensor (blue) on a 16-pin header.

Additionally, our new sensor is useful for detecting ammonia in power plants. Earlier results suggested that SiC-based sensors could detect ammonia in diesel engine exhaust by using an iridium gate contact. Thus, these sensors are useful for controlling the rate of urea injection into catalytic converters, a process that reduces nitric oxides in the diesel exhaust into nitrogen gas and water. When we attempted to use the same sensor in a power plant, we found that it could only track carbon monoxide (CO) because of the plant's low-oxygen atmosphere (about 4% in modern plants compared to 8% in diesel engines). We found that our sensor is selective to ammonia even in a low-oxygen environment with a high level of CO.

Because SiC/catalytic-metal-gate sensors often respond poorly to sulfur dioxide (SO₂), a dangerous power-plant effluent, we used a temperature-cycling operating mode and advanced data evaluation to considerably improve our sensor's SO₂ detection capabilities.^3,4 Chemical reactions occurring on the surface of the catalytic metal change slightly at different temperatures. Thus, the selectivity of our sensor for a certain gas molecule (or different concentrations of the same molecule) were improved by evaluating the magnitude and speed of the sensor response at selected times during many temperature cycles. By choosing a proper temperature cycle based on these results, we improved the response of our sensor to specific gases, such as SO₂.

In addition, we developed a sensor to better detect hazardous particulate. The chemical content of a particle influences its toxicity, but no currently available commercial instruments can detect the chemical content of particulate matter. We developed a method in which our sensor detects the gases emitted from heated particulate.⁴ We studied the detection of fly ash with a known content of ammonia. Once we cycled the temperature of the particles up to 430°C, they released the ammonia to be detected by an ammonia sensor.⁵ The released gas acted as a ‘fingerprint’ for the chemical content of the particulate.

Environmental and health concerns drive the development of both gas and particle sensors. We demonstrated the selective, low-level detection of two toxic gases: ammonia and sulfur, both present in power-plant effluent. We also developed a method to detect the chemical content of particles by heating the particles to high temperatures and detecting the emitted gases, which act as a fingerprint of the chemical content. In the future, we hope to extend this fingerprint method to detect other particles.