Impedance-humidity sensors

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. 

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... 

Yarkin DG (2003) Impedance of humidity sensitive metal/porous silicon/n-Si structures. Sens Actuators A 107 1-6 Yeh Y, Tseng T, Chang D (1989) Electrical properties of porous titania ceramic humidity sensors. J Am Ceram Soc 72 1472-1475... 

The solid polymers used for resistance-type humidity sensors with acid and alkaline groups, such as sulfonic acid or quaternary ammonium, absorb water depending on the humidity of the atmosphere. As the electrical resistance decreases in proportion to the absorbed water, the humidity can be determined by measuring the electrical resistance. If the current applied is dc, then polarization or electrolysis may occur. Thus, generally, ac impedance is measured. Figure 1 shows the ehemieal structures of representative polymers that can be used for humidity sensors these are all hydrophobie polymers. In addition, they are either... 

Fig. 3 The temperature dependence of the impedance of an electrical-resistance humidity sensor.

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. 

Fig. I. Resistance characienwiev of typical Dunmore and Pope sensors, la) Dunmure sensors are limited to a narrow range of humidity. This sensor operates between 40 and 6(1% RH tbi Pope sensors operate over a wide humidity range, hut output impedance of the sensor varies tram I(NXI ohms 1100% RH) to several megohms (10% RH). which complicates readout circuitry...

S. Kusanagi, Mechanism of the sensitivity of the planar CO sensor and its dependency on humidity, J. Electrochem. Soc., 1992, 139, 3224-3229 S.B. Lee, A. Cocco, D. Keyvani and G.J. Maclay, Humidity dependence of carbon monoxide rate in a Nation-based electrochemical cell, J. Electrochem. Soc., 1995, 142, 157-160 R.J. Mortimer and A. Beech, AC impedance characteristics of solid-state planar electrochemical carbon monoxide sensors with Nation as solid polymer electrolyte, Electrochim. Acta, 2002, 47, 3383-3387. 

Impedance (Z)-humidity characteristics of the model sensor with pore size distribution as shown in Figure 14 (a) to (d), respectively. 

ABSTRACT Titanium dioxide, or TiO, is a well-known sensor material. We have investigated the impedance characteristics and surface roughness of two different types of devices involving TiOj-coated planar interdigital electrodes. One of them has a silicon dioxide (SiOj) insulation layer, the other one is built up without this layer. In this contribution, the response to selected hydrocarbons and humidity (at a temperature near 300 °C) and the influence of the substrate material will be discussed in some detail. 

Impedance spectroscopy titanium dioxide gas sensor humidity hydrocarbons ... 

It is shown in Fig. 9 that the impedance curves of the sensor operated in a dry environment do not depend very much on time. The same can be said about the curves taken at various humidities. The results also show that the quasi-semidrcular curves shrink continuously with humidity (i.e., the curves for lower humidity lie in between the curves for dry atmosphere and the curves for more humid atmosphere). 

We assume that the effect of the humidity dominates in the locus diagrams of Fig. 8 and Fig. 9, as the impedance does not change comparably when the sensor is subjected... 

The sensitivity of planar TiOj-based IDE sensors to hydrocarbons and humidity was successfully demonstrated. It was shown that the response of the sensor to the analyte gas depends on frequency, which suggests the use of impedance spectroscopy to investigate the sensor mechanism at work. 

In this context, ILs on surfaces can also have an impact on electrochemical sensing applications. The fact that they combine a sufficient to good conductivity with a wide electrochemical window and a negligible vapor pressure, leads directly to the idea of using them as electrolyte in electrochemical cells for sensing applications If a substance like alcohol [2], water, or better humidity, should be analyzed, the resistance (or also impedance, if DC is used) of the electrolyte is changed by the diffusion of water molecules inside the IL Such an electrochemical sensor is already available at the market for years, developed by the company Novasina/Switzerland. 

Among the various interesting and useful properties of the new class of polymers, their switchable electrical conductivity has proven the most attractive to the community of chemists and physicists, and so it is understandable that these polymers are called conducting polymers. Much effort has been spent on measurements of their electrical conductivity and on determinations of the factors that affect its value [44,113,119,124,151,213,314,327,328,344,345,393 17]. The use of the conventional ex situ dc four-point method [44,393,398,399] or the ac impedance technique in a metal [polymer metal sandwich arrangement [119,124,410] for measurements of the conductivity of dry polymer samples is straightforward. However, the conductivities of dry polymers are affected by humidity and any gas present. Indeed, this is the property that is utilized in gas sensors. Conductivity can also be measured in situ, i.e., under controlled electrochemical and chemical conditions [151,394,395,397,400,402,406,408,417]. 

As has also been found in this type of ceramic sensor, the base resistance of the element gradually increases during operation (Yamazoe and Shimizu 1986). This drift seems to be related to the formation of stable chemisorbed OH-groups on the surface of metal oxides, which impede Grotthuss-type proton conduction. The presence of temporal drift is one of the main disadvantages of resistive humidity... 

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