Today, the use of humidity sensors to monitor and control relative humidity (RH) has gained a lot of interest in many industrial processes. Thus, in this work, mullite planar substrates were printed and partially sintered to be used as humidity sensing materials.
First, mullite powder (Baikowski SA 193 CR) with 1 wt% MgO was dispersed in a photocurable resin (ADMATEC Europe BV). To achieve a high solid content and a satisfactory viscosity, a dispersant (Disperbyk-103) was also added. Phase composition and particle size distribution of as received mullite powder were assessed by XRD analysis and laser granulometry, respectively.
Samples were manufactured using a stereolithography printer (ADMAFLEX 130). The main printing parameters (layer thickness, exposure time and LED power) were optimized to achieve a good quality printing. After printing, the green bodies were submitted to a water debinding process for 4 h to remove extra slurry. Then, the samples were oven-dried at 70 °C before thermal debinding process and sintering at 1400 °C (Sensor 1) or 1450 °C (Sensor 2) for 1 h. Finally, platinum interdigitated electrodes were screen printed on the substrates.
The SEM micrographs of the sintered samples showed that mullite was highly porous, as expected.
During tests under humid air flow, the sensors’ impedance was monitored by means of an LCR meter (Hioki 3533-01). The measurements were done at room temperature under an alternating voltage of 2 V at the frequency of 1 kHz. When exposed to water vapor, the impedance value decreased from 155 MΩ under dry air to 480 kΩ under 85 RH% for Sensor 1 and from 115 MΩ under dry air to 410 kΩ for Sensor 2.
Initially, at low humidity level, the impedance change was limited and above 19% RH, the impedance of the sensors decreased sharply with the increase in humidity level. The sensor response (R) was defined in accordance to the Equation 1:

where Z₀ and Z_g are the impedance of the sensor under dry and under humid air, respectively. Comparing the sensors’ performance under 89% RH, Sensor 1 showed 13% higher sensor response compared to Sensor 2 (322.9 and 280.5, respectively). At the same time, the response and recovery times (the time required by the sensor to achieve 90% of the total impedance variation when the atmosphere was changed) of Sensor 1 (91 and 167 s, respectively) are longer than those of Sensor 2 (64 and 119 s, respectively). This could be due to the higher porosity of Sensor 1.
In addition, three consecutive measurements under dry and humid air (85% RH) showed that both sensors had a great repeatability and displayed the same sensor response, as well as the response and recovery times of the previous tests.
Finally, tests were carried out towards 44 ppm ammonia, 100 ppm methane, 500 ppm carbon dioxide and 2.5 ppm nitrous oxide. No interferences towards these gases were noticed, so mullite sensor exhibited an excellent selectivity with respect to humidity.