Carboxylic‐Acid Functionalized Multiwalled Carbon Nanotube‐Alkane‐Based Resistive Temperature Sensor for Cold Chain Applications

A portable PET‐ITO (indium tin oxide coated polyethylene terephthalate electrode) based thermal sensor is developed using carboxylic acid modified multiwalled carbon nanotube (MWCNT‐COOH) based dispersion in an alkane. This work demonstrates the use of MWCNT‐COOH in hexadecane dispersions with 1‐hexadecanol as a stabilizing agent for temperature‐sensitive electronic sensors. On cooling, pure hexadecane exhibits a phase transition from liquid to solid at ∼18°C. A switching ratio (resistance OFF state/resistance ON state) of more than ∼10 ² order is achieved for a PET‐ITO electrode‐based device. The OFF state corresponds to well‐dispersed phase of MWCNT‐COOH in hexadecane above 15°C. In the OFF state, there is limited overlap of nanotubes, giving rise to a highly resistive path for electrons. However, at temperature below 11°C, the crystallization of hexadecane leads to phase separation of MWCNT‐COOH tubes, resulting in the creation of conductive pathways through overlap. Thus, the whole dispersion acts as a descending temperature sensor. A hexadecane‐based dispersion is chosen for upscaling it to an LED proof‐of‐concept due to favorable switching temperature (8°C–12°C). A maximum switching ratio of ∼10 ³ is reported here for (C16‐D4) 100 mg of MWCNT‐COOH and 300 mg of 1‐hexadecanol in 10 g of hexadecane‐based dispersion.