Dr. Gary Hunter, Intelligence Systems Hardware Lead and Technical Lead of Chemical Sensors, Sensors and Electronics Branch
- Tuesday, 02 March 2010
One of the things one would like to do with a sensor is have it be stable, have it be reliable, be able to give you the same results every time, and be able to – if at all possible – eliminate these types of drifts that are associated with higher temperature chemical reactions. That’s work that we’ve done for a period of time with respect to silicon carbide gas sensors.
One of the approaches to decreasing the chemical reactions that happen at high temperatures is to put barrier or interface layers in between, for example, the silicon carbide and the metal alloy. The barrier layer associated with this particular patent that you mentioned is palladium oxide. The patent describes the idea that one of the ways to solve this problem of higher temperature reactions to give a sensor with better stability and good sensitivity all at the same time, for silicon carbide, is to use this palladium oxide barrier layer. We found what we believe to be good stability with respect to the sensor at higher temperatures for extended periods of time using palladium oxide.
Now, what would you use it for? Well, there are two sets of applications that one might take a look at, two sets of gasses that I mentioned a few moments ago with respect to silicon carbide. One is the measurement of hydrocarbons, which can be used for a range of different purposes. For example, with respect to fire detection applications, hydrocarbons are emitted. With respect to leak detection, a fuel is a hydrocarbon. These sensors can also be used to measure hydrogen or toxic species such as hydrazine. So, the ability to make a stable silicon carbide sensor leads to a range of possible applications from fire to leak detection to measuring toxic gases. That’s some of the type of work that we’ve done with respect to the hydrocarbon detector.
We also work with silicon carbide sensors to make them sensitive to NOx. Again, start with the same platform but modify some the materials on that platform to target NOx instead of hydrocarbons. Reducing engine emissions are a good application for NOx detection. We are also doing work in monitoring a patient’s breath in order to determine their health – the application of a NOx sensor based on silicon carbide or other approaches might allow one to, for asthma conditions, note the amount of NOx in their breath and begin to understand issues and conditions related to asthma.
NTB: Since we’re talking about silicon carbide, what is atomically flat silicon carbide and how does it improve the performance of a Schottky diode sensor?
Dr. Hunter: The advantages of silicon carbide have to do with its ability to operate at high temperatures. Silicon carbide is a high-temperature semiconductor, so it can operate at temperatures in excess of 600 degrees C. We typically operate between 500 – 600 degrees C with our sensors. One can then have high temperature electronics, as it develops, that can be coupled with a silicon carbide gas sensor, or a pressure sensor, or other types of systems. Then one can combine electronics with these high-temperature sensors and start to make smart systems.
One of the challenges with silicon carbide is that it is not as mature an electronic material as silicon. The present silicon carbide wafers can have defects associated with them. Those defects can affect device performance. In particular, they can affect gas sensor device performance. So, one of the things we’ve shown is that if you go to an atomically flat surface, if you go to a surface that doesn’t have defects, then you are able to get better response out of the sensor than you would with a rougher surface with defects. An analogy I’ve used before is that it’s like trying to put a sensor on the surface of the moon versus a putting green — pretty rough with potholes vs smooth and flat. So, the idea is that if you deal with materials that have less defects, that are smoother, that are flatter, that have more uniform surface properties, it allows you to better control the sensor response, from what we’ve seen so far, than if you were to try to deal with a less controlled, defect-filled surface.
NTB: There is a lot of talk these days about "Lick and Stick" sensor systems. What are Lick and Stick sensors?
Dr. Hunter: Well, other folks may have different versions and different interpretations of this approach, and even different names other than ”Lick and Stick”, but what we mean is as follows.
A direction that we’d like to see for sensor technology is to make them small and make them smart — be able to integrate within a sensor system the components one needs to be able to make the sensor operate and make that self-contained and independent. So, for example, within the sensor you have a sensing element, say a hydrogen sensor, but in order to be able to operate that sensor you may also want to have a microprocessor, power, signal conditioning, and communications. You may have multiple sensors. You may want to read several parameters at the same time. So, the bottom line is, what we mean by Lick and Stick sensors are systems that are small, self-contained, the size of a postage stamp, and which we are able to literally place where you need them when you need them without having to rewire the vehicle or without having to provide new power lines for the sensor.
So what we mean by Lick and Stick is the concept of being able to, like a postage stamp, put sensor systems where you need them and when you need them as a self-contained, smart, individual unit. To be clear, we’re not talking about literally gluing things onto surfaces for all aerospace applications. For some applications, the idea that the “stamp” might fall off could cause real issues. Often, they might be screw-mounted or use some other formal mounting system. But we do want to significantly decrease all of the baggage that often comes into play in implementing sensors.