Sensing Performance of Nanocrystalline Graphite-Based Humidity Sensors

Environmental sensors play a crucial role in a wide range of applications. Among them, humidity sensors that are stable and operational in harsh environments are incredibly important for process control and monitoring. Nanocrystalline graphite (NCG) is a type of carbon-based thin film material. Previous work has shown that NCG has excellent mechanical properties and is able to withstand high radiation doses. The granular structure of the NCG film makes it a good candidate for humidity sensing as the film consists of conductive graphitic grains with a high density of sp(2) bonds and amorphous grain boundaries with high resistivity, and adsorption of water molecule onto the film forms conductive pathways between grains through the Grotthuss mechanism which lowers the resistance of the film by a measurable amount. Here, we report for the first time a working humidity sensor with linear response, fabricated using NCG as the sensing material for harsh, real-world environments, which include exposure to weak acids via rainfall, UV radiation, mechanical wear, and high-humidity environments. The calculated sensitivity of the best-fabricated sensor is S = 0.0334%, with a maximum resistance change of -4.4 k Omega, over the range of 15% RH to 85% RH. The response time of the sensor is 20 ms with the current measurement setup. The baseline resistance value of the sensor at 15% RH is 210 k Omega. The sensor has the potential to be used as a humidity sensor for harsh environments due to the chemical, thermal, and mechanical stability of the NCG film.