Characteristics, humidity sensors

Kusano, H. Kimura Shin, L Kitagawa, M. Kobayashi, H. Application of Cellulose Langmuir-Blodgett Films as Humidity Sensors, and Characteristics of the Sorption of Water Molecules into Polymer Monolayers. Thin Solid Films 1997, 295, 53-59. 

Water evaporates from or condenses onto an electrolyte solution depending on the relative humidity of the surrounding atmosphere. Electrolyte humidity sensors utilize this phenomenon. Lithium chloride is a typical material [7]. An electric hydrometer using lithium chioride was developed in 1939 by Dunmore [7] and is still widely used. In a typical lithium chloride humidity sensor the lithium chloride solution is impregnated into a plant pith substrate (10 mm x 4 mm x 0.2 mm) [8] and Pt electrodes are applied to both faces of the element. The plant pith possesses a fine reticulate structure and is therefore a suitable porous binder for the electrolyte. Lithium chloride solution trapped in such a porous binder is too stable to flow out even under very humid conditions. Since the humidity range covered by one such unit is about 30 7o r.h., a wider humidity range from 10 to 100% can only be measured by using a number of elements with different characteristics. 

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]. 

Humidity sensors using cross-linked styrene-sulfonate have been developed by Pope [41] using this mechanism. Cross-linked copolymers prepared from styrene-sulfonate are fabricated on polystyrene substrates furnished with Ag electrodes. This sensor responds to water vapor at temperatures below 100 °C and rinsing the element is enough to restore the sensor characteristics in case of contamination. A similar sensor which consists of polystyrene-sulfonate containing 4-10% divinylbenzene, has been developed by Musa [42]. This sensor exhibits extremely low hysteresis, 3% r.h., but the long term stability is poor. 

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. 

Capacitance-humidity characteristics at various temperatures of the temperature-humidity sensor. 

The range of sample characteristics and manner of their detection, is much larger than can be realistically addressed in the space of a single chapter. We will confine this chapter mainly to the chemical sensor research areas discussed in other chapters in this volume, dividing them into electrical, optical, and mass and thermal measurements. Our focus will furthermore be on the generic chemical and physical phenomena upon which such measurements can be based, as opposed to the alternative organization that would address chemical sensors in the context of their application (i.e, auto exhaust sensor, clinical diagnostic sensor, environmental sensor) or of the kinds of samples detected (i.e, CO sensors, humidity sensor, biosensor, etc.), as used in a previous ACS Symposium Series volume on Chemical Sensors (D. Schuetzle, R. Hammerle, Eds., ACS Sympos. Ser. 309, 1986). 

A problem with these conductors is that humidity in the ambient atmosphere affects the conductivities and hence sensing characteristics when an amperometric mode of operation is adopted, as described later. The sensing electrodes have been provided mostly by Pt, and the counter electrodes by Ag, PdH, TiH, H0.35M0O3, H,W03, etc. The gases to be measured extended from H2 in inert gases to in air in 1982, and now seem to also include CO, NH3 and NO, all diluted in air, as well as O2 in the gas phase or condensed phase. Besides the examples given in Table 35.1, proton conductors have been used in humidity sensors, e.g. NAFION membrane in room temperature operation type , antimonic... 

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... 

Sensor performance for different applications is defined by various features of the ceramic. For example, the electrical output of most pressure sensors is dependent on the bulk piezoelectric properties of a PZT ceramic. Oxygen gas sensor performance is defined by the conductivity behavior of Zr02 ceramics, which is in turn dependent on the oxygen vacancy content of the material. The performance of still other sensors, for example, ceramic thermistors, is dependent on the grain boundary characteristics of doped BaTi03 ceramics. For humidity sensors based on NiO/ZnO, the p—n junction characteristics of the interface define sensor performance. 

Yadav BC, Pandey NK, Srivastava A, Sharma P (2007) Optictil humidity sensors based on titania films fabricated by sol-gel and thermal evaporation methods. Meas Sd Technol 18 260-264 Yajima T, Iwahara H, Uchida H, Koide K (1990) Relation between proton conduction and concentration of oxide ion vacancy in SrCe03 based sintered oxides. SoUd State Ionics 40-41 914-917 Yajima T, Iwahara H, Koide K, Yamamoto K (1991) CaZrOj-type hydrogen and steam sensors trial fabrication and their characteristics. Sens Actuators B 5 145-147... 

Delapierre G, Grange H, Chambaz B, Destannes L (1983) Polymer based capacitive humidity sensor—characteristics and experimental results. Sens Actuators A 4 97-104... 

Regarding response and recovery times of polymer-based humidity sensors, one can say that the porosity of the humidity-sensitive polymer is important to obtain quick responses, because the response and recovery times are determined by the time it takes for water vapor to diffuse into or out from a polymer film to reach an equilibrium (Yamazoe and Shimizu 1986). The polymer film should therefore be as thin as possible for quick response. Tsuchitani et al. (1985) believe that, besides the film thickness, an increase in hydrophobicity of the constituent ionic monomer appears to shorten the response time, while the mole fraction of the constituent ionic monomer to non-ionic monomer strongly affects the sensing characteristics. 

Fig. 18.6 Operating characteristics of a-Al203-based capacitance humidity sensors during long-term stability test (Data from Chen and Jin 1992)...

Poor linearity of operating characteristics can be added to the disadvantages of porous semiconductor-based RH sensors (see Fig. 18.8). Research carried out by O Halloran et al. (1999) showed that this parameter of humidity sensors can be improved by changing the current density used to form the porous layer. It has been shown that it is possible to form a device that is linear over the humidity range 10-90 % RH. However, porous silicon layers formed at such low current density values show a long response time and poor sensitivity in comparison to layers formed using higher values of current density. 

Nahar RK, Khanna VK (1998) Ionic doping and inversion of the characteristic thin film porous Al Oj humidity sensor. Sens Actuators B 46 35-41... 

O HaUoran GM, van der Vlist W, Sarro PM, French PJ (1999) Influence of the formation parameters on the humidity sensing characteristics of a capadtive humidity sensor based on porous silicon. In CD proceedings of the 13th European conference on solid-state transducers, EUROSENSORS XIII, The Hague, September 12-15, pp 117-120 Otsuki S, Adachi K (1993) Effect of humidity on the fluorescence properties of a medium-sensitive fluorophore in a hydrophilic polymer film. J Photochem Photobiol A Chem 71 169-173... 

She Y-E, Jiang B-Y, Liu J-Z (1994) Si-Mg-Al-O system ceramic humidity sensor. Sens Actuators A 40 151-153 Shimizu Y, Arai H, Seiyama T (1985) Theoretical studies on the impedance-humidity characteristics of ceramic humidity sensors. Sens Actuators 7 11-22... 

Typical operating characteristics of OTFTs are shown in Fig. 20.2. These results were obtained by Zhu et al. (2002), who studied pentacene-based TFT as a humidity sensor. 

Besides their excellent characteristics, polyimides can be considered fully compatible with standard electronic processing procedures, an important cost control consideration (Strijkova and Georgieva 2013). On the other hand, iron oxides have been used for the manufacture of humidity sensors devices. Therefore, maghemite appears as a humidity sensor material chosen by many researchers due to its high sensitivity, simple design, and low cost. 

Since the 1980 s, a new type of humidity sensor, based on a solid electrolyte, has been under development. The sensor uses a protonic conductor as a base component and makes a galvanic cell of a water vapor gas concentration type. When the characteristics of this new type of sensor are compared with conventional humidity sensors, two representative advantages of this sensor are seen. The sensor output is an EMF change, and is suitable for continuous operation with a fast response. The sensor can operate at higher temperatures, because the solid electrolyte is stable even at elevated temperatures. These features are expected to accelerate the development of this type of sensor. The base material is a perovskite-type strontium cerate SrCeOa. The pure cerate is not a protonic conductor. 

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