Printed, Fully Metal Oxide, Capacitive Humidity Sensors Using Conductive Indium Tin Oxide Inks

Abstract

In this research, fully metal oxide, capacitive humidity sensors for printed electronic applications have been designed and fabricated through the development of conductive indium tin oxide and dielectric aluminum oxide inks for the screen-printing process. Sensors were printed in a parallel plate configuration in 4 and 9 cm2 conductive plate areas. Typically, commercially available discrete humidity sensors have a sensitivity in the range of 0.2–0.5 pF/RH%, whereas the printed humidity sensors presented in this paper have sensitivities of 0.85–7.76 pF/RH% depending on the sensor size, allowing for customizable properties. Response times were measured using a weighted average and found to be 21.4 s on average and recovery times were 4.8 s on average. The sensing performance was highly linear (R 2 > 0.97) for sensors of all sizes across the measured humidity range of 5–95%. Impedance spectroscopy was used to determine the sensing mechanism, and the mechanism was simulated and matched with experimental data. The sensing mechanism analysis shows that the sensing is primarily dictated by alumina at a lower relative humidity. The ITO contributes through increasing the ionic conductivity at a higher relative humidity, contributing to the high linearity of the sensor.

Electrospun Nickel Manganite (NiMn2O4) Nanocrystalline Fibers for Humidity and Temperature Sensing

“…Monitoring humidity and moisture levels is essential in many industrial processes, such as the food, medical and semiconductor industries, as high-quality products often need to be produced in controlled conditions [ 1 ]. Humidity sensors are components of smart monitoring systems [ 2 ]. In the food industry, besides humidity monitoring in the food production process, humidity sensors can be incorporated as components of intelligent food packaging [ 3 ].…”

“…A good sensing material needs to show high sensitivity, good selectivity, and be low-cost, reliable and easy to fabricate [ 8 ]. The rapid development of flexible electronics increases the need for developing sensing materials suited for such applications [ 11 , 12 ], such as metal oxide capacitive humidity sensors on paper [ 2 ]. Recent research has focused on developing a self-powered, flexible humidity sensor [ 13 ].…”

MEMS Humidity Sensors

“…For a quick response time and better sensitivity, the sensing material thickness should be minimized. Furthermore, the thickness of the electrodes should be minimized for better sensitivity and to reduce the parasitic capacitance, but that means additional fabrication steps that are needed to form the bonding pads [10][11][12]. Capacitive sensors can measure the humidity over the entire range from 0-100%, which makes them preferable in most applications [9].…”

A Review on Humidity, Temperature and Strain Printed Sensors—Current Trends and Future Perspectives

“…Similarly, cellulose acetate butyrate and parylene have been utilized as sensing—passivation layer in paper-based humidity sensors [ 40 ]. McGhee et al [ 41 ] presented a capacitive printed humidity sensor by using in-house indium tin oxide (ITO) and aluminum oxide (Al 2 O 3 ) inks for screen printing. The sensors’ properties were tailored by using different design sizes.…”