Electrolyte humidity sensors

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. 

The range of available ceramic sensors for measuring humidity in industrial process environments can be classified into four categories (1) ionic, (2) electronic, (3) solid electrolyte, and (4) rectifying-junction types. A solid electrolyte humidity sensor that uses doped strontium cerate has been reported... 

As the film is conductive owing to electrolytic layers that contain water, the conductivity should depend on the quantity of water in these layers, and thus on the humidity in the air. This dependence for the Li+ form of LB film is shown in Figure 7.3. The conductivity increases within three orders ofmagnitude when the humidity rises from 0 to 100%. So, this film can be used in humidity sensors. 

ISO 15106-1 2003 Plastics - Film and sheeting - Determination of water vapour transmission rate - Part 1 Humidity detection sensor method ISO 15106-2 2003 Plastics - Film and sheeting - Determination of water vapour transmission rate - Part 2 Infrared detection sensor method ISO 15106-3 2003 Plastics - Film and sheeting - Determination of water vapour transmission rate - Part 3 Electrolytic detection sensor method... 

Cho N, Lim T, Jeon Y, Gong M (2008) Inkjet printing of polymeric resistance humidity sensor using UV-curable electrolyte inks. Macromol Res 16 149-154... 

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. 

Doping of a trivalent rare earth into SrCeOa is a key point in applying the cerate as the electrolyte for a humidity sensor. 

In order to increase the electric p-type conductivity, several rare-earth cations were doped into the Ce site of the SrCeOs electrolyte. The highest conductivity was observed for the cerate with Yb substitution (Iwahara et al. 1981). Of the three rare-earth cations, Yb3+, Y +, and Sc +, Yb has the closest ionic radius to Ce. It is easily concluded that the best candidate dopant is Yb. Since the hole concentration produced by the substitution is closely related to the humidity sensing mechanism, the hole concentration in the solid electrolyte directly influences the applicability for the humidity sensor. The... 

In a humidity sensor, ytterbium doping into the rare-earth cerium base material is the key point to obtaining a suitable solid electrolyte for humidity detection. Cerium site substitution of the ytterbium trivalent cation produces holes in the cerate and it becomes an excellent protonic conductor in the presence of hydrogen or water vapor. Rare-earth doping in the cerate is essential to obtain a solid electrolyte for the humidity sensor. 

Zaromb, S., M.E. Moehlenkamp and J.X. Tull, 1993, Humidity-resistant ambient-temperature solid-electrolyte amperometric sensors, in Proc. Symp. on Chemical Sensors II, 1993, eds M. Butler, A. Ricco and N. Yamazoe (Electrochemical Society, Pennington) pp. 162-173. 

For example, based on solid electrolytes, electronically conducting ceramic films, or humidity sensors... 

M.J. Yang, Y. Li, N. Camaioni, G. Casalbore-Miceli, A. Martinelli, G. Ridolfi, Polymer electrolytes as humidity sensors progress in improving an impedance device, Sensors and Actuators B, 2002,86, pp. 229-234. 

For micro PS a decrease in the specific resistivity by two or three orders of magnitude is observed if the dry material is exposed to humid air [Ma8] or vapors of polar solvents, e.g. methanol [Be6]. This sensitivity of PS to polar vapors can be used to design PS-based gas sensors, as discussed in Section 10.4. This change in resistivity with pore surface condition becomes dramatic if the pores are filled with an electrolyte. From the strong EL observed under low anodic as well as low cathodic bias in an electrolyte it can be concluded that micro PS shows a conductivity comparable to that of the bulk substrate under wet conditions [Ge8]. Diffusion doping has been found to reduce the PS resistivity by more than five orders of magnitude, without affecting the PL intensity [Ell]. 

Figure 9-1 Sensor device for measurement of relative humidity, containing an ionic liquid as electrolyte (with permission of Novasina S.A.)...

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