Fabrication of micro sensors for simultaneous temperature and humidity measurements via inkjet printing technology

In this research, material used in thermistor and capacitive humidity sensor will be investigated. First we choose silver nanoparticle to fabricate the electrode, nickel oxide for thermistor, polyaniline for capacitive humidity sensor. After 175℃ post-treatment and package, we investigate the single sensor and optimize them by adjust the inkjet parameters. The well-adjusting thermistor will obey Arrhenius equation and has a sensitivity higher than commercial uses. The capacitive humidity sensor will obey Frendlich isotherm. For commercialized temperature and humidity control system there two sensors with independent circuit to monitor temperature and humidity respectively, however in some extreme condition such as freezer or reflow oven these two sensors may interrupt to each other that results in decreased sensitivity. Therefore, in order to overcome this challenge, this research is aimed at integrating sensing material into one parallel circuit on flexible substrates via a low cost direct writing technology. A layer-by-layer strategy is used to integrate the temperature-sensitive NiO layer and humidity-sensitive polyaniline (PANI) layer together as a highly responsive device. The sensing elements, which have an adjustable dimension with a submillimeter scale, can operate over a wide range from temperature lower than 20°C to 100 °C and 20% RH to 90% RH with a great sensitivity. After adjust of inkjet parameters and adding silver nanoparticle into nickel oxide ink for the purpose of optimization for this sensor circuit, this circuit will be analyzed carefully to read temperature and humidity measurements at once. From the water adsorption on PANI thin film and temperature variation in NiO layer, the resistance and capacitance readings of the device in AC mode can be directly correlated to the environmental conditions. A correlation formula combining Arrhenius equation and Frendlich isotherm will be developed to accurately describe the sensor responses.