Researchers from the IISc Bangalore developed highly sensitive strain sensors using thin resonating structures made of graphene.
A strain sensor or strain gauge is a device used to measure strain (deformation) on an object. They are useful in many applications including personal and structural health monitoring. The most common type of strain sensor consists of a long, thin conductive strip in a zig-zag pattern of parallel lines with a flexible insulator on the back to support this strip. When the object is deformed, the strip or foil also gets deformed, causing its electrical resistance to change. The change in resistance is identified using the Wheatstone bridge and it is related to the strain on that object.
The strain sensor takes advantage of the electrical conductivity, and this zig-zag pattern of parallel lines does not increase sensitivity as the percentage change in resistance for a given strain for the entire zig-zag is the same as for any single trace. To increase the sensitivity, the single linear trace has to be extremely thin or would need to be operated at much lower voltage. Now, thin traces can overheat quickly and operation at lower voltages makes it difficult to measure resistance accurately.
To overcome these challenges, researchers from the Indian Institute of Science have developed a strain sensor from graphene nanoresonators. The sensor employs tiny (sub-10um) structures, made of graphene, that vibrate at ultrahigh frequencies (10-100MHz) and are exquisitely sensitive to changes in the strain. These string-like vibrating structures are called nanoresonators and strain tuning of resonance frequency provides an additional control knob in experiments that use these devices.
Swapnil More, PhD student in Akshay Naik’s lab at the Centre for Nano Science and Engineering (CeNSE), developed a simple technique to fabricate graphene nanoresonators on a thinned circular region on the Si/SiO2 substrate. The research also demonstrates how to tune the strain in these nano-resonators by controllably deforming a small section of the silicon substrate. The researchers used air pressure to apply controlled deformation and studied the changes in the resonant frequency. They found out that strain changes as low as 10^-9 can be measured using these devices.
The researchers say, “Besides their utility as sensors, strain-tunable nano resonators form interesting tools to study some of the most intriguing dynamical phenomena such as synchronization, mode coupling and internal resonance.”
Further information regarding the research can be found at https://iopscience.iop.org/article/10.1088/1361-6439/abe20b.