26-04-2022 | By Robin Mitchell
Researchers have recently developed a new sapphire fibre that can measure temperatures in extreme environments. What challenges do extreme temperatures present to engineers, what did the researchers develop, and how could it be beneficial in reducing emissions from jet engines?
Of all electronic components that exist, sensors are by far one of the most important. The ability to convert environmental factors (such as heat, light, and sound) into an electrical current, voltage difference, or change in resistance allows circuits to change their behaviour based on what the environment is doing.
In the case of temperature sensors, their importance cannot go understated; they are an essential component in so many different devices ranging from car engines to computers. However, not all environments are friendly to electronic devices, and thus engineers have to consider a wide range of different factors when designing a temperature sensor.
The most significant factor in temperature sensor design is the expected temperature range, and this can be as small as 20˚C for a fridge right up to 1000˚C for an industrial furnace. A direct contact sensor is ideal for applications with a narrow temperature range in room temperature environments (such as phone and CPU sensors), but a furnace would easily melt and destroy such a sensor. Instead, indirect sensors that use IR detectors can be used as black body radiation (the IR emitted from hot objects) is remarkably consistent.
Another challenge to consider is the pressure on the environment. Sea level air pressure is extremely easy to work with, and most basic temperature sensors will perform at this pressure. However, the vacuum of space can easily cause sensors with any liquid, gels or trapped air to break, and the intense pressures found at the bottom of the ocean can crush a sensor.
From there, engineers will also need to consider mechanical vibration from the environment and humidity and maybe even the chemical composition of the surrounding air. Of course, basic sensors used in everyday devices will not require the same degree of consideration, but sensors used in extreme environments must consider anything that may affect them.
Researchers from Oxford University have recently developed a new optic fibre made from sapphire that can be used in extreme temperatures over 2000˚C. To make the sensor, the researchers modified a section of the fibre with a Bragg Grating that reflects a specific wavelength of light depending on the temperature. Thus, a light beam from a safe distance can be shone into the optic fibre, and the resulting reflection can be used to infer the temperature at the Bragg grating.
However, the researchers also included an additional feature into the fibre to allow for extreme selectivity. The fibre itself may only be half an mm in width, but this is significantly larger than the wavelength of light being reflected back, and this means that it is easy for a wide range of frequencies to be reflected back. As such, the researchers created a very narrow channel of just 10um in width down the length of the fibre, and this specifically traps the reflected light from the Bragg grating.
The current sensor is only 1cm in length, meaning that it has very limited applications, but the researchers plan to increase this length to many meters. This fibre length would then be usable in real-world environments that experience substantial temperatures needing direct contact.
One area that the researchers have been targeting, in particular, is jet engines and how their sensors could be used to decrease their emissions. When it comes to reducing the environmental impacts of a jet engine, the best option is to improve the engine's efficiency. A more efficient engine will use less fuel and perform complete combustion producing CO2 and water instead of nitrous compounds.
As such, the new sensor can be used in direct contact with the jet engine (and possible the exhaust), and multiple Bragg gratings can be integrated along the length of the fibre. These readings could then be used to determine if the engine is operating at its maximum efficiency. If not, onboard controllers can make adjustments to the engine to boost the efficiency, and this, in turn, will reduce the emissions from the engine. Of course, the sensor has far more applications than just jet engines, and the ability to have direct contact with a hot body in excess of 2000˚C is a major achievement in the field of sensors.