Microfabricated sol-gel relative humidity sensors for soil suction measurement during laboratory tests
Dr. Rafaela Cardoso, Instituto Superior Técnico, University of Lisbon, Portugal
Currently there are no small sensors to be incorporated inside soil samples for laboratory testing, to monitor water transport during loading. This is an important limitation for a better understanding of the hydro-mechanical coupled behaviour of soils.
A sol-gel relative humidity sensor (11 mm ×11 mm), microfabricated in clean room environment, was conceived to be incorporated in soil specimens during standard laboratorial tests. The sensor operates based on changes in electrical resistivity detected by a cerium doped silica‑titania film deposited using a sol-gel technique over interdigitated aluminium electrodes (spacings varying between 700 μm and 100 μm - see Figure ).
The measurement of the water retention curve allowed selecting the best electrode spacing and showed that the measurements can be considered acceptable for suction ranges between 1 MPa and 10 MPa, which extends the suction interval covered by tensiometers, normally operating up to 2MPa. The selected sensors were then incorporated inside oedometer cells and showed promising results.
Although the sensors require improvements in terms of sol-gel deposition and calibration protocol, the results confirm their scientific potential for being used in testing and characterization of unsaturated soils.
Figure. Sol-gel sensors: a) Top view with the geometry of the interdigitated electrodes (length 8500µm, width 100µm) defined with spacing ranging from 100-700 µm; b) Top view schematics and pictures of the sensors after sol-gel deposition and wiring (welding joins covered with silicone).