Dr. Robert Okojie, Research Electronics Engineer at the NASA Glenn Research Center, develops harsh-environment microelectromechanical systems (MEMS). Okojie currently processes, fabricates, tests, and packages silicon carbide pressure sensors, accelerometers, and fuel injectors.
NASA Tech Briefs: What kinds of work have you done with MEMS, particularly the silicon carbide sensors?
Dr. Robert Okojie: I have focused on the area of MEMS-based pressure sensors using silicon carbide, which allows us to extend the operational capability of the pressure sensor from the conventional silicon pressure sensors that operate around 200 °C. We are looking at applying MEMS–based silicon carbide pressure sensors in temperatures that exceed 600 °C.
NTB: Right, you demonstrated reliable sensors at 600 °C. What was the breakthrough there?
Dr. Okojie: The breakthrough was the development of a robust contact metallization that is stable. The second breakthrough was the development of a packaging that is compatible with the silicon carbide device itself.
The main issue with the packaging: You have differences in the coefficient of thermal expansion between materials. So when you go to a high temperature, this difference in CTE begins to manifest itself in the form of fatigue. When you go to cycle the device from hot to cold, the fatigue generally leads to the degradation of the device performance. What we’ve done is engineered a package so that each coefficient of thermal expansion is essentially closer to the coefficient of thermal expansion of silicon carbide. So you now have a harmonious functionality between the packaging and sensor.
NTB: Why are sensors for high-temperature environments so important? How will we use them currently?
Dr. Okojie: They are very important for several reasons. As we continue to work toward the reduction of hydrocarbons during combustion, it becomes very important to know exactly how to control the production of these hydrocarbons, and to do that will require that you have the sensor as close as possible to the source of the production of the hydrocarbons. So that means that you basically want to put the sensors as close to the combustion chamber of the engine as possible, so that you can have a real-time monitoring of the generation of the hydrocarbons; then you can apply the necessary control mechanisms to minimize the production of hydrocarbons.
Another significant reason is as engines continue to progress toward lean burn — when you burn lean, you’re working to reduce the emission of NOx, which again helps in the efficient combustion of fuel — you run into what’s called lean blowout, which means there’s always a tendency for the flame to go off. Think about it: You don’t want to be at 35,000 feet and have your combustion go off in your engine. In order to prevent this kind of problem, you need to have a pressure sensor as close as possible in the combustion chamber that will sense the onset of lean blowout, and then activate the necessary control mechanism to suppress the lean blowout, and allow the engine to continue to run effectively. Otherwise, you may run into a situation, where in fact, this lean blowout situation could potentially damage some critical engine parts.
There are many other applications again in the area of renewable energy, as we continue to explore new sources of energy. You need high-temperature sensors for geothermal energy exploration. You want to go down into the Earth perhaps as deep as six kilometers, where the temperature is between 350 and 400 °C in a supercritical liquid. You need sensors like silicon carbide pressure sensors to survive in that harsh environment in order to monitor the supercritical pressure. Silicon devices, or devices that are based on conventional silicon, will not cut it. So that’s why we need to develop this new family of MEMS pressure sensors, based on silicon carbide, that will essentially survive such harsh environment.
NTB: Is that the main advantage of the silicon carbide? That it can survive the harsh environment?
Dr. Okojie: Yes, and it’s due to its far superior thermal mechanical properties over silicon. Its chemistry is almost near inert; in other words, it does not react with the environment, as, for example, silicon will do. That gives it stability. It’s also very simple mechanically. It has a very high Young’s Modulus, which allows us to handle heavier loads than the equivalent thickness of silicon. What we’re doing now is leveraging these superior properties of silicon carbide and using the material as a basis for the sensor.
Other applications include the planned mission to Venus, where the atmospheric temperature is pretty much close to 500 °C. The atmospheric pressure is about 90 bar. These are the kind of devices that we are going to be potentially using when we go to conduct science missions. The same idea applies to any planet that has high temperatures and high pressure — a combination of which will require the use of devices that are based on silicon carbide.