FABRICATION AND CHARACTERIZATION OF CARBON NANOTUBES FOR PRESSURE SENSING APPLICATION

Carbon nanotubes (CNTs) can be utilized to replace Si as the pressure sensing element since they have better electromechanical properties that remain stable even at 250oC. With Si-based sensor, the intrinsic limitation of P-N junction causes it to operate below 120oC. Thus the work of developing CNTs as the pressure sensing element would require synthesizing good quality, highly aligned CNTs array with specific characteristics to enhance their sensing properties before integrating them into the testing configuration in order to establish the working principle of the pressure sensor.

Aligned multiwalled carbon nanotubes (MWCNTs) array was synthesized via thermal chemical vapor deposition (CVD) and characterized using Raman spectroscopy, scanning electron spectroscopy (SEM) and transmission electron microscopy (TEM). Studies were performed to investigate the effect of different methods of Al2O3 buffer layer preparations and Fe precursor used on the MWCNTs produced. It was found that direct deposition of Al2O3 produced MWCNTs with highest degree of crystallinity (ratio of disorder peak to graphitic peak, ID/IG = 0.98) whilst oxidation of Al thin film in air exhibited the highest growth rate resulting in the thickest MWCNTs film (~60 m). Both forms of Fe catalyst, pellet and powder produced similar film thickness (typically 47 m) but Fe pellet gives cleaner MWCNTs array without the presence of particles.

Piezoresistive measurement was performed on the as-grown MWCNTs array without transferring to a conductive substrate as the buffer layer is made electrically conductive. Electrical resistance of the MWCNTs array, R decreased with applied weights (50 g to 500 g) and is more responsive towards rapid changes in the applied loading (interval of 10 s). At constant weight loading (100 g and 500 g), the resistance decreases linearly with temperature, T (from 30oC to 180oC) while the slope, dR/dT remains independent of the weight.
In summary, good quality, highly aligned MWCNTs array has been successfully synthesized, characterized and developed as the pressure sensing element. Results of piezoresistive measurement have established its prospect as the pressure sensing element suitable for operation beyond the temperature limit of Si-based sensors.