4. Conclusions
Mechanically robust mesoporous gadolinium-doped ceria
(CGO) humidity sensors were fabricated by the attachment of the
sensing layer athightemperatures (900 ◦C) keeping themicrostructure
intact. The layers are stable to temperature cycles, the
self-cleaning required in commercial ceramic sensors at high temperature
could be performed, if required, without detriment of
the sensing properties. The resistance of the sensing film changes
around four orders of magnitude within the measured humidity
levels (0–2.4% H2O) at the selected optimum frequency (10 Hz)
and the response and recovery times were 2.9 and 44.0 s, respectively.
No drift was measurable during the 13 cycles of the dynamic
response experiment performed. Maximum hysteresis was only
∼0.1% indicating very high reliability. Impedance spectra were analyzed
and the parameters obtained fromthefitting ofthe equivalent
circuits verified an ionic type of conduction mechanism.
Moreover,the influence oftemperature,thickness ofthe sensing
layer and pore size were studied in order to check the consistency
of the sensing mechanism and explore the possibility of adjusting
sensing parameters to different applications.
Higher temperatures imply higher mobility of ions and therefore,
lower sensitivity and response and higher recovery. The thinner film sensor studied presents a lower sensitivity and
response rate, although a higher recovery rate. Pore sizes of ∼11 nm
showed a faster response and recovery, while pore sizes of ∼4 nm
seem to limit the diffusion of the water molecules.
The CGO mesoporous sensors fulfilled all the required characteristics
of high performance humidity sensors. Moreover, the
synthesis route can be extrapolated to fabricate any other metal
oxide.