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A study of piezoresistive sensing based on carbon-nanotube forests Bsoul, Anas Amjad Mohammad

Abstract

Carbon nanotubes have attracted considerable attention since their discovery due to their exceptional electrical, mechanical, and optical properties. Piezoresistance of carbon nanotubes is promising, and can be utilized to enable various types of devices. This work investigates devices functionalized with vertically aligned multi-walled carbon-nanotube forests, with a focus on pressure and strain sensors. A fabrication process based on Si-micromachining techniques that overcomes the challenges associated with using carbon-nanotube forests was developed for the devices construction. A pressure sensor is fabricated to have a multi-walled carbon-nanotube forest supported by a deflectable 8-µm-thick Parylene-C membrane suspended by a silicon frame. The responses of the fabricated sensors are experimentally characterized. The sensitivities to positive and negative gauge pressures are found to be comparable in magnitude with the average values of -986 ppm/KPa and +816 ppm/KPa, respectively. The measurement also reveals that the temperature coefficient of the resistance for a forest suspended with a Parylene membrane is -515 ppm/ºC, ~3x smaller than that for a forest fixed onto a silicon substrate. A strain gauge is also fabricated to have a multi-walled carbon-nanotube forest supported by an 8-µm-thick Parylene-C membrane that is supported by two silicon substrates at both ends. The response of the fabricated strain gauge is experimentally characterized. The experiments show that the fabricated device has two sensitivity regions: a sensitive region with a gauge factor of 4.52, about 3.76x more than that for a previously reported carbon-nanotube forest/PDMS based strain gauge, and a less sensitive region with a gauge factor of 0.87. Moreover, the response to gradual strain decreases is very similar to that for gradual strain increases, and the measured gauge factors are 4.4 and 0.77 for both sensitivity regions. The results are analyzed and the source of piezoresistance is explained. Finite element analysis is performed for the strain gauge. The results show that the change in lateral separations between the carbons nanotubes, which are transversal to the direction of the applied force, are not equal in the center region, whereas the change in longitudinal separations between the carbon nanotubes, which are parallel to the applied force, are more equal.

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