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Korotcenkov, G.; Simonenko, N.P.; Simonenko, E.P.; Sysoev, V.V.; Brinzari, V. Capacitance-Based Humidity Sensors. Encyclopedia. Available online: (accessed on 23 June 2024).
Korotcenkov G, Simonenko NP, Simonenko EP, Sysoev VV, Brinzari V. Capacitance-Based Humidity Sensors. Encyclopedia. Available at: Accessed June 23, 2024.
Korotcenkov, Ghenadii, Nikolay P. Simonenko, Elizaveta P. Simonenko, Victor V. Sysoev, Vladimir Brinzari. "Capacitance-Based Humidity Sensors" Encyclopedia, (accessed June 23, 2024).
Korotcenkov, G., Simonenko, N.P., Simonenko, E.P., Sysoev, V.V., & Brinzari, V. (2023, May 08). Capacitance-Based Humidity Sensors. In Encyclopedia.
Korotcenkov, Ghenadii, et al. "Capacitance-Based Humidity Sensors." Encyclopedia. Web. 08 May, 2023.
Capacitance-Based Humidity Sensors

Capacitance-based humidity sensors consist of a moisture-absorbing dielectric material placed between a pair of electrodes that form a small capacitance. Most capacitive sensors use plastic or polymers as the dielectric material with typical dielectric constants ranging from 2 to 15.

capacitive conductometric impedance

1. Capacitance-Based Humidity Sensors

A wide variety of paper types have been tested to manufacture capacitive humidity sensors. In particular, Courbat et al. [1] used p_e: smart paper type 2 from Felix Schoeller, Mraovic et al. [2] used Vimax (recycled paper) (70 g/m2), M-Liner (cardboard) (230 g/m2), and PackPro (food-packaging paper) (50 g/m2), and Andersson et al. [3] tested HP advanced photo paper and Canon PT-101 photo paper. Furthermore, Balde et al. [4] employed a coated paper (65 g/m2), Wawrzynek et al. [5] fabricated sensors using 300-µm-thick glossy coated paper from Kelly Paper, Ullah et al. [6] suggested using tissue paper for the fabrication of capacitive humidity sensors, and Gaspar et al. [7] fabricated sensors based on Lumi silk (90 g/m2) from Stora Enso (Helsinki, Finland).
Although parallel-plate (PP) capacitive sensors generally have a higher sensitivity than IDE-based capacitive sensors [8], these structures are surprisingly reported to be rather sporadic [6]. Much more often, humidity sensors are manufactured using interdigitated electrodes, which may be due to their easier availability in microelectronic laboratories and enterprises. For the manufacture of ID electrodes, various types of printing technology are commonly used. However, Rahimi et al. [9] showed that the laser-ablation method can also be used to form IDE on the paper surface, which can become a simple and scalable alternative to conventional photolithography-based processes and printing technologies. They tested two laser-processing methods for the selective removal of the conductive aluminum film (25 nm) of a metallized paper (MP) substrate, namely direct (DLA) and indirect laser ablation (ILA), for operation at wavelengths of 1.06 µm (Nd-YAG) and 10.6 µm (CO2), respectively. Metalized paper (MP) is a commodity in which a thin layer of aluminum offers both a decorative appearance and protective/controlled gas permeation. Rahimi et al. [9] found that the utilization of a Nd:YAG (1.06 µm) fiber laser, which had a greater absorption coefficient for metal (Al) films than the paper substrate, can provide more direct laser ablation (DLA), enabling higher selectivity in the removal of metallized layers, with minimal thermal effects on the porous paper substrate.
In general, the humidity-sensing materials employed in PB sensors might be classified into two types: (i) the paper itself, and (ii) non-paper hydrophilic humidity-sensing materials, such as ceramics, polymers, carbon materials, and their composites.

2. Capacitive Humidity Sensors with Paper as the Sensing Material

Table 1 lists the characteristics of PB humidity sensors with paper as the sensitive material. To characterize capacitive humidity sensors, such parameters as sensory response (C/C0) and sensitivity (ΔC/ΔRH) are usually used, which describe the change in capacitance upon interaction with humid air, as well as the response time (τres) and recovery time (τrec), which characterize the kinetics of the sensory response. The response/recovery time is usually defined as the time taken to reach 90% of the final equilibrium value. A parameter such as hysteresis is also used, which characterizes the possible differences between sensor readings when measuring in conditions of increasing or decreasing humidity. The methods for measuring these parameters do not differ from the methods used in the testing of gas sensors and described in many publications, including [10][11][12][13][14][15][16].
Table 1. Summary of paper-based capacitance humidity sensors with paper sensing elements.
It was also found that the capacitive response is reversible, and the sensitivity, rate of response, and flexibility of the sensors are sufficient for a variety of applications [18][20]. For example, such sensors can be utilized in measuring rates and patterns of breathing. Most humidity sensors are fairly stable for at least 7 days [6]. This may be acceptable for cheap sensors, which are not intended for long-term operation, especially in aggressive environments.
However, the analysis of published results indicates that despite the significant progress in paper-based humidity sensors, most sensors still exhibit poor repeatability, high drift (increase in base value (C0) above 10%), low sensitivity (C/C0 is typically below 200%), and slow response/recovery (from 4–16 s to several minutes) [2][7]. In addition, some of the technology routes (protocols) used in the course of producing the humidity sensors, such as tape pasting, vacuum filtration, and hand drawing, are not sufficiently scalable and reliable for mass production. Meanwhile, recent studies revealed that many of the problems associated with paper-based humidity sensors can be successfully resolved. For example, Zhang et al. [19], developed capacitive humidity sensors with a maximum change of 1480% (C/C0) at 1 kHz, and a maximum change of 411.4% at 10 kHz, using robust and precise screen- and gravure-printing technologies. The sensors were highly repeatable, exhibiting a standard deviation in sensitivity of 1.05% for three continuous runs, stable, and insensitive to mild temperature changes (15–30 °C). It is worth noting their fast response, of ca. 0.8 s, and recovery, of ca. 0.78 s. To achieve these results, Zhang et al. [19] subjected the copy paper to a treatment in 200 mM of hydrochloric acid (Sigma-Aldrich, St. Louis, MO, USA) to remove the calcium-carbonate filler. Next, the as-prepared porous paper was pressed and dried for 180 s at 115 °C. Compared with the conventional paper, the paper prepared in this manner had a larger specific surface area, which provided, according to Zhang et al. [19], remarkable sensor performance. The sensors, fabricated by Rahimi et al. [3], also showed a high degree of repeatability, with a capacitance variability of less than 4%.
However, the information given in Table 1, especially regarding the response/recovery times, should be treated with caution, because these measurements are often carried out in large measuring chambers. This means that these data may receive a substantial contribution from the dynamics of atmospheric changes in the chamber. Furthermore, high sensitivity, measured as ∆C/% RH (pF/% RH), does not always take into account the area of the capacitive sensor, which masks the real value. Frequently, to achieve high sensitivity, it is sufficient to simply enhance the sensor area.

3. Humidity Sensors with Solid-State Sensing Elements

Some solid-state materials, such as carbon nanotubes (CNTs), graphene oxide (GO), porous silicon (PSi), and alumina oxide (AO), have been applied to produce capacitive paper-based humidity sensors. The parameters of these sensors are listed in Table 2.
Table 2. Summary of paper-based capacitance humidity sensors.
As can be seen from the results presented in Table 2, sensors designed with GO and CNTs, except for the sensors developed by Song et al. [25], are not highly sensitive to water vapor [22][24], which coincides with the results of studies performed on polymers and solid substrates [16][28]. Moreover, their parameters are noticeably inferior to those of Al2O3-based detectors [4] and PSi [27]. However, this is quite understandable because the nanoporous structures of Al2O3 and PSi encourage the capillary condensation of water vapor, which has a significant effect on the material capacitance (or dielectric permittivity) compared to the adsorption of H2O molecules on the surfaces of GO and CNTs only. Regarding the sensors developed by Song et al. [25], high sensitivity was achieved by optimizing the IDE structure and choosing the correct paper substrate.
Balde et al. [4] used the opposite approach. Instead of applying a sensitive layer to the surface of the paper substrate, they formed a layer of porous aluminum directly on the paper. For this purpose, anodic oxidation of the aluminum layer deposited on the surface of coated paper (65 g/m2) was used. The ID-electrode capacitor was fabricated by an evaporation of 300-nm-thick aluminum onto the top of a thin AAO film through a Pt mask.
It is important to note here that the observed sensory response and hysteresis in paper-based sensors using an additional coating with a humidity-sensitive material should be considered as the combined response of the paper substrate with pronounced hydrophilic properties and the sensitive material, which inhibits the adsorption/desorption of H2O molecules on the substrate [24]. In these structures, the passivation of the paper substrate prior to the deposition of additional humidity-sensitive materials may be a rational solution to improve the sensory-response kinetics. This conclusion is in good agreement with the results of studies conducted by Kafy et al. [26], who found that GO/CNC-based sensors fabricated on a PET polymer substrate performed faster than paper-based devices (see Table 2).


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