Who's Who at NASA

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.

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