MISiCFET Chemical Sensors for Applications in Exhaust Gases and Flue Gases

Abstract

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 300°C and for at least three days at 700°C. 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 NH 3 sensors with low cross sensitivity to NO x have been demonstrated as potential sensors for control of selective catalytic reduction (SCR) of NO x by urea injection into diesel exhausts. Here we show that sensors with a porous platinum or iridium gate show different temperature ranges for NH 3 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.

SiC sensors: a review

“…Whilst large boiler and commercial premises assess their pollution using Fourier transform infrared (FTIR) spectroscopy, the cost of these systems is too high for small installations, such as domestic central heating boilers. It has been shown that silicon carbide sensors can be used to determine the mode of operation for a boiler operating on wood pellets [49]. This showed that the CO concentration in the exhaust varies between 80 and 500 ppm, with an NO variation between 60 and 100 ppm, which correlated well with the measured oxygen concentration.…”

“…The sensitivity of the catalytic sensor to a gas is determined by both the choice of the metal and the temperature of operation [48]. For example, comparisons between the ammonia senisitivity of sensors with Pt and Ir gates over the temperature range 150-400 • C have shown that, although both metallizations give a maximum sensor signal of 1200 mV, the Ir device has a maximum sensitivity at 300 • C and the Pt device at 225 • C [49]. The temperature dependence is further complicated when the operational temperature exceeds 600 • C, where the sensors begin to operate in a binary fashion, where the reaction rate at the metal surface increases dramatically [50].…”

Thermal and flow analysis of SiC-based gas sensors for automotive applications

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