Response humidity sensors

Electrospun nanofibers of several polymers, among others PVB were used in a surface acoustic wave resonator (30). The surface acoustic wave resonator is used as an ultrafast response humidity sensor. 

Fig. 13.12c. The estimated response time (baseline to 90% signal saturation) of this sensor was about 25 30 ms, which is one or two orders of magnitude faster than those of existing relative humidity sensors. The remarkably fast response of this...

Conventional humidity sensors of the electric resistance variable type use hydrocarbon polyelectrolyte as a moisture sensing material. Therefore, the sensors usually have insufficient heat resistance, and cannot be used at temperatures of 60°C or more. Another problem is that they deteriorate when in contact with cigarette smoke and oil contained in the air [64,65]. When the fluorinated pitch-deposited coating was breathed upon, the electrical resistance quickly decreased, but electrical resistance quickly recovered when this action was stopped. Then, how to develop a humidity sensor excelling in humidity response sensitivity, heat resistance and durability was attempted [66]. Two kinds of comb-like electrodes with different electrode gaps were made, and a thin film was formed on the surfaces by vacuum deposition of fluorinated pitch. The obtained fluorinated pitch sensors were left at rest in a thermostatic chamber, and electrical resistance was determined under the following conditions. 

Senaratne and Baker designed a humidity sensor based on the fact that Ag" "-ZME in dimethylformamide will exhibit no electrochemical response related to Ag" /Ag reduction, unless water is present to promote ion exchange. Using this technique, 100 ppb of water in DMF could be detected [172]. 

Suitably modified fiber optic sensors can also be used for detecting gas vapors, humidity, ions, and organic compounds. Fiber inclusions that show length variation were used to develop humidity sensors, whereas ion-responsive lipid bilayers formed the basis for the detection of inorganic ions. Immobilized neutral and ionic crown ethers in polymeric membranes were designed as sensors for determination of barium and copper (Wolfbeis 2000). 

Several challenges remain for the ultimate practical use of these sensors. The response time of the solid state sensors are short (seconds) for initial sensing, but recovery times range from minutes to hours at room temperature. The stability of the sensor to drift associated with accumulation of fixed charge at interfaces, as well as the high sensitivity to ubiquitous urban pollutants ozone and N02 are problematic. All MPc OTFTs show some response to moisture, and conductivity is also temperature sensitive so that humidity and temperature compensation are essential. On a basic research level, the detailed characterization of charge trapping states, electronic structure, and the interactions with analytes is not yet fully understood on a quantitative theoretical basis. The time response of sensor initiation and recovery is also not understood in a detailed manner. In spite of these limitations, the intrinsic chemical stability of MPc compounds and their compatibility with microsensor array fabrication make these candidate OTFTs for further research and development. 

A humidity sensor has to satisfy the following practical requirements (1) high sensitivity over a wide humidity range, (2) quick response, (3) good reproducibility and no hysteresis, (4) robustness and long life, (5) resistance to contaminants, (6) insigniHcant dependence on temperature, and (7) simple structure and low cost. For particular applications, other requirements should be satisHed, such as low power, low weight, or microprocessor compatibility. 

As shown in Ikble 20-2, lithium chloride humidity sensors have rather slow responses to humidity compared to other types of humidity sensors. This type of humitidy sensor cannot, therefore, be used in automatic control systems which require a rapid response. They are, however, widely used for meteorological observations such as radiosonde because of their reproducibility, long term stability, and low cost [7]. Care should be taken when using this type of sensor in very humid environments because the accuracy and lifetime are lowered. Thin films of potassium metaphosphate [9] and barium fluorite [10] are also used as humidity sensors. The film of potassium metaphosphate is deposited on a glass substrate by vacuum sublimation and Ag paste is used as the electrodes. Such sensors show a response to humidity within 2 s, but are not stable [9]. 

When water molecules are adsorbed on semiconducting oxides, the conductivity increases or decreases according to whether the oxides are n- or p-type. In response to demands for operation at elevated temperatures, semiconductor humidity sensors using metal oxides, such as perovskite-type oxide, have been proposed as shown in Table 20-2. 

Figure 20-24 shows the resistance of Zr02-MgO as a function of water vapor content (ppmw). The resistance decreases rapidly with an increase in water vapor from 10 to 10 ppmw. Compared with the ionic-type humidity sensor, the response of the semiconductor-type is rather slow because of the slow rate of chemisorption or the subsequent electron transfer process on the oxide surface. The microstructure of the elements as defined by surface area and average particle size, has a less pronounced effect on sensing characteristics than is the case in the ionic-type humidity sensors [31]. 

The impedance-type humidity sensor has the advantages of simple structure and rapid response. Moreover, long term operation becomes possible when the element has been stabilized by coating with a resin film. This type of sensor is also easily compatible, depends on the chemical structure of the polymers, and, in principle, enhancement of the water sensitivity can be achieved by cross-linking or copolymerisation. 

A capacitance-type humidity sensor developed by Vaisala consists of a comb shaped Au electrode and cellulose acetate dissolved in ethylene dichloride as humidity-sensitive materials. A schematic view of this sensor is shown in Figure 20-31 [47]. This sensor is now widely used in meteorological observations and in many other humidity measuring instruments. As illustrated in Figure 20-32, the capacitance-humidity characteristics show a linear relation from 0 to 100<7o r. h. [48]. This sensor has the advantages of good accuracy, low hysteresis, and fast response time. 

Several other materials have been developed for humidity sensors and the electrical responses of three different ones are shown in Figure 30.10. 

FIGURE 30.10 Electrical response of three ceramic humidity sensors at room temperature and 1 kHz. 

Plasma-polymerised humidity sensors were proposed using styrene (PPS) [84]. The sensor was constructed with a 12 nm Au - 164 nm PPS - 12 nm Au sandwich structure. The response time of the capacitance was less than one minute. 

Figure 9.6. A composite of alumina particles in a LiF matrix as a humidity sensor." The graph shows the response of the resistance to the relative humidity. Published with permission from the Journal of Materials Education.

Wang et al. [22] and Wang and Wu [23] prepared mesoporous silica aerogel thin films for use as humidity sensors. They either spin-coated [22] or dip-coated [23] sol-gel-derived silica colloids onto gold electrode-patterned alumina substrates, then used CSCE extraction to obtain aerogel films. The response of the aerogel films to applied electric fields was evaluated over a range of plausible ambient temperamres and relative humidities. When water adsorbed onto the sensor surface, the conductivity of the material increased. 

Among the various categories of smart textiles and flexible materials, which include optically, mechanically, chemically, electrically, and thermally activated substances/ stmctures (Tao, 2001), several have shown large opportunities of applications in PPE. This includes wearable electronics, for example, physiological condition, temperature, and humidity sensors, power and data transmitters, and end-of-life indicators, which are the subject of the next chapter of this book. Various types of smart flexible materials have also found their way into PPE, for instance, as responsive barriers, self-decontaminating membranes, thermoregulating layers, and shock-absorbing patches. 

Innocenzi P, Martucd A, GugUelmi M, Bearzotti A, Traversa E (2001) Electrical and structural characterisation of mesoporous sUica thin films as humidity sensors. Sens Actuators B Chem 76 299-303 Innocenzi P, Falcaro P, Bertolo JM, Bearzotti A, Amenitsch H (2005) Electrical responses of silica mesostructured films to changes in environmental humidity and processing conditions. J Non-Cryst Solids 351 1980-1986 Jacobs PA (1977) Carboniogenic activity of zeolites. Elsevier Scientific, New York... 

Silverstein MS, Tai HW, Sergienko A, Lumelsky YL, Pavlovsky S (2005) PolyHlPE IPNs, hybrids, nanosctile porosity, silica monohths and ICP-based sensors. Polymer 46 6682-6694 Sisk BC, Lewis NS (2003) Estimation of chemical and physical characteristics of analyte vapours through tmalysis of the response data of arrays of polymer-carbon black composite vapour detectors. Sens Actuators B 96 268-282 Suii K, Annapoorni S, Sarkar AK, Tandon RP (2002) Gas and humidity sensors based on iron oxide-polypyrrole nanocomposites. Sens Actuators B 81 277-282... 

The most important considerations with respect to sensor characteristics are surface properties (hydrophilic-hydrophobic), pore-size distribution, and electrical resistance. To ensure adequate sensitivity and response a sensor of this type should consist of a very thin film of porous ceramic with a porosity >30%. The contacting electrodes may be interdigitated or porous sandwich-type structures made from noble metals (e.g., Pd, Pt, Au) and so constructed that they do not obstruct the pores of the oxide film. Humidity affects not only the resistance of a porous ceramic but also its capacitance by extending the surface area in contact with the electrodes. The high dielectric constant of adsorbed water molecules also plays an important role. 

In particular, Connolly et al. (2005) designed NH capacitive sensor with 500-nm-thick porous SiC film. The response in humidity was very low for RH<50 %, which was attributed to the porous dimensions. The exact sensing mechanism is still not clear, but NH levels as low as-0.5 ppm were detected. Porous alumina (AI2O3) has also been examined as a sensing material for capacitive gas sensors and in particular for humidity measurements (Nahar and Khanna 1982 Timar-Horvath et al. 2008). The Al Og-based humidity sensor was a volume-effect device based on physical adsorption. At low humidity, the walls of the pores are lined with one-molecular-thickness liquid layer. As the humidity increases, after saturating the walls, due to a capillary condensation effect, the water starts condensing in the pores (Boucher 1976 Neimark and Ravikovitch 2001). It was established that the water molecules, even at a partial pressure higher than the saturated vapor pressure tend to condense in capillary pores with a radius below the Kelvin radius r, which is defined as function (1) (Boucher 1976) ... 

It should be noted that optical humidity sensors as a rule use similar effects, which were discussed above (Russell and Flecher 1985 Ballantine and Wohltjen 1986 Boltinghouse and Abel 1989 Wang et al. 1991 Kharaz and Jones 1995 Ando et al. 1996 Zhou et al. 1998 Skrdla et al. 1999 Alvarez-Herrero et al. 2004). The water adsorption in a porous matrix produces a variation in the optical response of the device, because the refractive index of the layer changes when the hydration of sensing material takes place and the pores are filled or emptied. The water adsorption isotherms and, therefore, the sensor response depend on the size and shape of the pores. One can find in Posch and Wolfbeis (1988), Otsuki and Adachi (1993), Papkovsky et al. (1994), Costa-Femandez et al. (1997), Costa-Femandez and Sanz-Medel (2000), Choi and Tse (1999), Choi and Shuang (2000), and Bedoya et al. (2001, 2006) a description of optical humidity sensors used and other principles. 

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