Abstract
A novel functionalized multi-walled carbon nanotube (FMWCNT)/hydroxyethyl cellulose (HEC) composite-based humidity sensor was successfully developed for humidity monitoring applications. FMWCNTs were synthesized by covalently functionalizing multi-walled carbon nanotubes (MWCNTs) in a mixture of sulfuric and nitric acid to enhance their hydrophilicity. The FMWCNTs were characterized using transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy and dispersion analysis to verify the presence of functional hydroxyl and carboxyl groups. A FMWCNT/HEC (1 : 6 w/w) composite ink was formulated using the solution blending technique with 2.5 wt% FMWCNTs. A multi-layered humidity sensor was fabricated using additive print manufacturing processes on a flexible polyethylene terephthalate (PET) substrate. Screen printing and gravure printing processes were used to deposit the bottom silver (Ag) electrode and FWMCNT/HEC sensing layers, respectively. The capability of the fabricated humidity sensor was investigated by measuring its resistive response towards relative humidity (RH) varying from 20% RH to 80% RH. As the RH was increased from 20% RH to 80% RH in steps of 10% RH at 25 °C, it was observed that the resistance of the printed sensor increased linearly. The printed sensor demonstrated resistance changes as high as ≈290% at 80% RH, when compared to its base resistance at 20% RH. A sensitivity and a response time of 0.048/%RH and ≈20 s were obtained for the printed sensor, respectively. The results thus demonstrated the feasibility of employing additive print manufacturing processes to develop a highly sensitive sensor for humidity monitoring applications.
Highlights
Over the years, the need for monitoring humidity in the automobile, medical and food industries has become a growing requirement.[1,2,3,4] Further, the maintenance of ambient conditions within human habitats for optimum comfort has o en been in uenced by humidity, making it one of the vital parameters that needs to be monitored.[5]
PVPP was chosen as a suitable binder due to its swelling behavior in water and its loading was restricted to 50 wt% functionalized multi-walled carbon nanotube (FMWCNT), maintaining a binder to ller ratio of 1 : 2. The Transmission electron microscopy (TEM) image of the FMWCNT/ HEC ink (Fig. 1(c)) shows the FMWCNTs embedded in the HEC polymer matrix
It was observed that the ends of the nanotubes were closed due to the presence of amorphous carbonaceous impurities around the multiwalled carbon nanotubes (MWCNTs) (Fig. 7)
Summary
Resistive type RH humidity sensors are the most commonly implemented type in the industry, as they are much easier to integrate and use relatively simpler electronics to monitor and control ambient humidity.[8] Most of the resistive type humidity sensors are fabricated by coating hygroscopic polymers including polyimide,[8,9,10] co-polymerized PMMA/PMAPTAC12 and polyelectrolyte[13] as the humidity sensitive lms.[12] these devices are typically slow and suffer from a relatively longer response time (105 s).[14] these sensors have lower humidity detection limits (30–42% RH), making them impractical for use in many industrial and domestic applications.[8,15] research has been focused on the development of novel materials for resistive type humidity sensors to overcome the drawbacks associated with polymeric based resistive type humidity sensors Owing to their large surface area to volume ratio and hollow cylindrical nanostructure,[16] multiwalled carbon nanotubes (MWCNTs) have been. The performance of the printed humidity sensor was investigated by measuring its resistive response towards relative humidity (RH) varying from 20% RH to 80% RH at a constant temperature of 25 C
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