Additive manufacturing of capacitive humidity sensors by screen printing for condition monitoring of FRP composites

Abstract

Screen printing as a method of additive manufacturing applicable for the production of highly sensitive humidity sensors is presented. For this purpose, manufacturing, benchmarking and optimization of sensitive micro-composite dielectric paste systems suitable for screen printing are demonstrated. Furthermore, commercially available silver paste systems were rheologically and electrically characterized and benchmarked. Thereby bulk silver conductivities of up to 15 % were measured with average layer thicknesses in the range between 3 - 10 μm and an occurring surface roughness Ra between 0.7 - 3.7 μm. The printed sensors capacitance was measured along with the characterization of its humidity sensing capability. The capacitance of the printed sensors varied between 6.2 - 8.5 pF depending on the layer thickness of the micro-composite dielectric materials and increased up to 26 pF while detecting 90 % relative humidity. Differences between water molecule absorption and desorption are determined by the occurring hysteresis between the detection of increasing and decreasing humidity. Malfunctions of the sensors due to the arising cross contact between top and bottom electrode could be addressed by varying the layer thickness or selection of materials.