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.
NTB: You’ve also done work with silicon carbide accelerometers. I was curious about what the advantages are of these accelerometers, and what kinds of extreme-impact applications can they be used for.
Dr. Okojie: We had an earlier collaboration with the Air Force Research Laboratory, in which we were essentially exploring the use of a silicon-carbide-based accelerometer to measure extreme impacts. We’re talking about impacts that could be as high as 200 kilogees. You will need silicon carbide for such extreme impact environments. Again, you want a material that has the mechanical stability that will survive that environment and transmit the necessary data that you require to conduct whatever decision that you want to conduct.
NTB: What is the Lean Direct Injection Array, and what kinds of work did you do that?
Dr. Okojie: As we continue to march toward lean burn, in order to improve fuel combustion efficiency, and in order to reduce the level of NOx that’s being generated, it will require that we also investigate the fuel injector itself: how the injector delivers fuel into the combustion chamber of the engine. We then realized that, in order to do that, you need to develop what’s called a lean direct injector.
It is an injector that will provide you the opportunity to burn lean. Of course, lean burning comes with higher temperatures also. Now traditional lean direct injectors use metal plates, but the problem with metal plates is that when these plates go through thermal excursion, they tend to warp, or the fuel cokes inside the injector orifice. This orifice could be as small as 8 thousandths of an inch. Once it blocks the orifice, fuel does not flow into the combustion chamber. Of course, in that situation, you have an inefficient conversion of fuel. What I have done now is to introduce silicon carbine as a basis for the lead direct injector. Why am I using silicon carbide? Again, I’m taking advantage of the near inert surface chemistry of the material.
It also begins to offer us a new opportunity to create a divergent technology path that will allow us to introduce silicon carbide pressure sensors, silicon carbide floor sensors, and silicon carbide electronics on to the lead direct injector platform. If we do all that, we are now beginning to talk about a smart fuel injector. This is an injector that will have a high degree of autonomous functionality. It will sense the temperature of the combustion chamber. It will sense pressure in the combustion chamber. With all the information that it gets, it sends the information to the silicon carbide-based electronics, that is pretty much close to the combustion chamber, and now the electronics will instruct the actuator, which is again based on silicon carbide, to control or modulate the fuel in order to bring the combustion process to within operation. You cannot implement this in a metal-based lead direct injector because of the obvious thermal and mechanical limitations of the material.
NTB: What are the biggest challenges with working with high-temperature electronics?
Dr. Okojie: The biggest challenge is that, first of all, you don’t have that many options in terms of the devices that you want to use to monitor the behavior of the sensor. As a matter of fact, there are no high-temperature electronics out there that one can use as a basis for signal conditioning of the pressure sensor. Until we are able to develop silicon carbide electronics that can survive, say, 600 °C, then it becomes a challenge to be able to condition the signal of my pressure sensor. So that’s my challenge right now. I consider that to be the technological gap that currently exists. It limits the functional capability of the pressure sensor because you don’t have a signal conditioning function that goes along with the sensor itself.
NTB: Can you take me through the HyFly program, and how the silicon carbide sensors were used there?
Dr. Okojie: In the HyFly program, we also want to bring to bear the robust functionality of the silicon carbide pressure sensor that we developed. The HyFly program is essentially a hypersonics program, where the combustion temperature could go as high as 3000 °F or higher for a few minutes. In most hypersonic engines, the duration of flight is relatively short because basically you want to use the engine to access space. Going from ground to space takes about a few minutes, but it’s a few minutes of aggressive combustion, and you need to monitor all the functional health of the engine as you launch. In doing so, you want pressure sensors in the flow path of the engine to give you real-time measurement of the flow velocity, and the overall flow conditions of the combustion process to allow you to understand how to continuously improve the performance of the engine in the future. The HyFly program offered us an opportunity to utilize the high-temperature silicon carbide that we developed to test in a hypersonic test rig.
NTB: What are you working on currently as a research electronics engineer?
Dr. Okojie: My main focus right now is how to extend the temperature environment of my sensor from 600 °C to 1000 °C. I feel that if I can develop a pressure sensor that operates reliably at 1000 °C, it will extend its lifetime to several years, if such a device operates at a lower temperature. In other words, if I develop a 1000 °C pressure sensor, that, say, will survive maybe a few months, it implies that if I were to operate the device at 600 or 700 °C, the functional life time of the device will be extended to more than 10 years — a prediction based on the mean time of failure analysis. My primary focus now is to make the pressure sensor perform much better than it currently does.
NTB: Do you have a team that you’re working with?
Dr. Okojie: We have a bunch of individual researchers here that work on specific aspects of the device. On my team that focuses on the silicon carbide pressure sensors, I have about a half dozen people that primarily work on my technology, the silicon carbide pressure sensor, accelerometer, and fuel injector.
NTB: With a sensor that potentially operates at 1000 degrees °C, what do you see as some interesting applications for that?
Dr. Okojie: It actually allows the device to come even closer to the combustion chamber than the 600 °C device. It also gives you the confidence that when you insert the device in a remote location, say 6 kilometers into the Earth in a 400 °C environment for several years, that you have a functional, stable device.
NTB: Can you take us through a typical day of yours?
Dr. Okojie: A typical day will be coming in first thing, and processing my devices in our own class-100 cleanroom. I spend half of my day mostly in the cleanroom, helping in the fabrication of the different devices. Then I step out and focus on analyzing the test results that are going on in my lab.
I also have my test lab, where I test the devices that I fabricate in the cleanroom. It’s a combination of trying to bring a device to life, and making sure that the device is functioning properly. We’ve now sent some of the devices out for packaging. They come back from packaging, and then we have to do the reliability analysis testing here. So: a combination of fabrication, post-fabrication, lab testing, packaging, and post-packaging, long-term characterization.
NTB: What would you say is the most satisfying part of your job?
Dr. Okojie: Expecting the unknowns. Sometimes your prediction does not essentially reproduce itself. In other words, you have your hypothesis that you’re going with, and you think you’ve taken care of all of your physics and the math and the calculations, and at the end of the day, you’re surprised by some new phenomena that you encounter, which actually makes it very interesting. Because any new phenomena that we encounter opens up an opportunity for new ideas and new inventions. As a matter of fact, a lot of our inventions were based on accidental discoveries. That, for me, is that most fun part of this work. It can be quite fascinating when you’re confronted with surprising phenomena that you never anticipated would be there.
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