Design and Development for Capacitive Humidity Sensor Applications of Lead-Free Ca,Mg,Fe,Ti-Oxides-Based Electro-Ceramics with Improved Sensing Properties via Physisorption

Abstract

Despite the many attractive potential uses of ceramic materials as humidity sensors, some unavoidable drawbacks, including toxicity, poor biocompatibility, long response and recovery times, low sensitivity and high hysteresis have stymied the use of these materials in advanced applications. Therefore, in present investigation, we developed a capacitive humidity sensor using lead-free Ca,Mg,Fe,Ti-Oxide (CMFTO)-based electro-ceramics with perovskite structures synthesized by solid-state step-sintering. This technique helps maintain the submicron size porous morphology of the developed lead-free CMFTO electro-ceramics while providing enhanced water physisorption behaviour. In comparison with conventional capacitive humidity sensors, the presented CMFTO-based humidity sensor shows a high sensitivity of up to 3000% compared to other materials, even at lower signal frequency. The best also shows a rapid response (14.5 s) and recovery (34.27 s), and very low hysteresis (3.2%) in a 33%-95% relative humidity range which are much lower values than those of existing conventional sensors. Therefore, CMFTO nano-electro-ceramics appear to be very promising materials for fabricating high-performance capacitive humidity sensors.

Keywords: hydrophilicity; mechanism; porous; recovery; relative humidity; response; stability; water absorption.

Figure 1

Flow chart for sensor fabrication with the morphology at different sintering temperature.

Figure 2

Experimental setup for the measurement of the capacitive humidity response of the electro-ceramic based sensors.

Figure 3

Pore size distribution (PSD) and relative cumulative frequency (RCF) of (a) unsintered and sintered at (b) 450 °C, (c) 650 °C, (d) 850 °C and (e) 1050 °C materials measuring from the electron micrographs using ImageJ.

Figure 4

Density, open-porosity, water absorption and water contact angle (WCA) of (a) unsintered and sintered at (b) 450 °C; (c) 650 °C; (d) 850 °C and (e) 1050 °C ceramic samples.

Figure 5

The response curves of the capacitance versus relative humidity (RH) at different frequencies of CMFTO electro-ceramic at 25 °C. Inset image represents the variation of capacitance with RH at 25 °C at different frequency in logarithmic scale (log(C) vs. % RH). Note: the capacitance increases monotonically with % RH at different frequencies, but increased rate is faster at 10² Hz.

Figure 6

The variations of capacitance with frequency at different humidity condition (33%–95% RH) for CMFTO based humidity sensor at 25 °C. Inset image represents the variation of capacitance with frequency at different RH in logarithmic scale (log(C) vs. log(RH)). Note: The value of capacitance increases with increased % RH, but decreases with increased frequency. The decreased rate is faster in lower frequency (<10⁴ Hz) and higher humidity range (>85% RH).

Figure 7

The sensitivity (%S) response of CMFTO based capacitive sensor with % RH at different test frequencies at 25 °C. Note: the sensitivity increases monotonically with % RH at different frequencies, but the value of sensitivity is highest (~3000%) at 10² Hz. Hence, 10² Hz is considered as the most suitable frequency for the further analysis.

Figure 8

Schematic representation of the humidity sensing mechanism of CMFTO electro-ceramic at different humidity environment. Note: the adsorption of water molecules on CMFTO nanoceramic is characterized by two processes. The first-layer water molecules (at lower humidity) are attached on the CMFTO electro-ceramic through two hydrogen bonds. As a result, the water molecules are not able to move freely and thus, the impedance value increases. In contrast, from the second layer (at higher humidity), water molecules are adsorbed only through one hydrogen bond. Hence, the water molecules are able to move freely and thus, the impedance value decreases. This insists to increase the capacitance value.

Figure 9

The transformed response curves of logarithmic capacitance (logC) vs. RH of CMFTO electro-ceramic based capacitive sensor. Note: first linear transformation curve (red-line) is well fitted by logC = 0.0102RH − 10.8148 in the RH range from 33% to 75% and the second linear transformation curve (green-line) is well fitted by the formula logC = 0.0532RH − 14.0401 at the higher humidity range (>75% RH). Here, regression, R² represents a best fit of the curves to improve linearity.

Figure 10

The hysteresis property of CMFTO electro-ceramic-based capacitive humidity sensor at 10² Hz under 25 °C. Note: the value of hysteresis is extremely low (~3.2%) compared to other conventional capacitive sensors. The low hysteresis value is mainly due to the fast adsorption and desorption rate of water particles on the surface of the CMFTO electro-ceramic.

Figure 11

Response and recovery times of the CMFTO humidity sensors for humidity levels between 33% RH and 95% RH at 10² Hz. (A) Response time (14.5 s); (B) Recovery time (34.27 s).

Figure 12

Stability analysis of CMFTO electro-ceramic-based humidity sensor measured at a test frequency 10² Hz at 25 °C. Note: The measurement was conducted repeatedly for 30 days at 2-day interval and very negligible changes are observed.

Figure 13

Complex impedance plots and equivalent circuits of CMFTO based electro-ceramic under different humidity levels. (A) At lower humidity range (33%–75% RH), single semicircles are formed; the inset represents an equivalent circuit at lower RH; (B) At higher humidity condition (85%–95% RH), the radii of semicircle decrease and a straight line appears, and the straight lines become longer with increasing of humidity; the inset represents an equivalent circuit at higher RH. R_f and C_f: are the resistance and capacitance of CMFTO electro-ceramics, respectively; Z_i: interface impedance between CMFTO electro-ceramic surface and electrode.