Polymer film humidity sensors

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. 

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. 

A new Pt(II) polyyne polymer, P15, prepared from the reaction of cfs-[Pt(PPh3)2Cl2] with l,4-diethynyl-2,5-dihexadecyloxybenzene using the extended one pot polymerization route, was tested for its sensing properties and showed fast and reproducible response to relative humidity variations and methanol vapor in surface acoustic-wave (SAW) sensors.46 A SAW sensor was fabricated from polymer P15 as a sensitive membrane, and the polymer was deposited as thin film on the surface of SAW delay lines implemented on three different piezoelectric substrates. High sensitivity and reproducibility were recorded for such devices. The acoustic characterization of the polymer film was also studied with the aid of theoretical results obtained by the perturbation theory. 

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. 

The fabrication of humidity sensors on silicon chips has recently become possible using 1C. production technology [37, 49], This realizes a small, low cost humidity sensor, and makes it possible to integrate the humidity sensor with other sensors or signal-handling circuitry on the same chip. A new integrated temperature and humidity sensor developed by Yamamoto et al., consists of a polymer capacitor on the p-n diode of a temperature sensor [50] as illustrated in Figure 20-33. A thin film of polyimide is used as the moisture-sensitive material... 

An electronic conduction-type humidity sensor was developed by Ishida et al. [56J. The sensor structure is very similar to the one illustrated in Figure 20-41. A cross-linked hydrophilic polymer in which carbon particles are dispersed is used as the humidity-sensitive film. The swelling of the polymer disturbs the ohmic contacts between dispersed carbon particles and thus the electronic resistance of the element increases sharply as the relative humidity approaches 100< o, as shown in Figure 20-41. This element was put into practical use as a dew detector for the VTR cylinder in 1978. 

Water (humidity) sensors Thin film polymer/metal oxide solvent Capacitance or resistance Measures water in gases range 10 -20% water ... 

We have found that a composite film consisting of a conducting polymer and an insulating polymer is useful for a humidity sensor (II), The PANA as... 

The absorption of water by polymers is especially suited to the development of a capacitive humidity sensor based on the high dielectric constant of water, since capacitance depends on the area, the thickness, and the dielectric constant of a dielectric. Uptake of water causes an increase in the dielectric constant followed by an increase in capacitance. Polyimide is a suitable sensitive material, but so are certain inorganic ceramics such as AI2O3 [142] and low-density Ta205, which change their capacitance with the formation of water dipoles. Low-density Ta20s layers can be formed by anodic oxidation of sputtered tantalum films, whereas polyimide films are deposited by spin coating and subsequent polymerization. 

Fig. 3.15 (a) Conductivity responses for three donut-shaped thin-film sensors based on carbon-doped poly 4-vinyl phenol (b, c) conductivity responses of carbon-doped polyvinyl acetate to (b) temperature and (c) air humidity. The polymer films swell in response to the presence of a particular gas which increases the distance between the carbon dopants and changes the resistance of the film. These results were obtained from 3-day experiments in an air ambient where the gas sensors were kept in stagnant air (Data from Buchler and Ryan (1997))... 

Chatzandroulis S, Tegou E, Goustouridis D, Polymenakos S, Tsoukalas D (2004) Capadtive-type chemical sensors using thin sUicon-polymer bimorph membranes. Sens Actuators B 103 392-396 Chen Z, Jin M-C (1992) An alpha-alumina moisture sensor for relative and absolute humidity measurement In Proceedings of the 27th annual conference of IEEE Industry Application Society, Houston, TX, vol 2, pp 1668-1675 Chen Z, Lu C (2005) Humidity sensors a review of materials and mechanisms. Sens Lett 3(4) 274—295 Chen Z, Jin M-C, Zhen C (1990) Humidity sensors with reactively evaporated Al Oj films as porous dielectrics. Sens Actuators B 2 167-171... 

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... 

Sakai Y (1993) Humidity sensors using chemically modified polymeric materials. Sens Actuators B 13(14) 82-85 Sakai Y, Sadaoka Y, Ikeuchi K (1986) Humidity sensors composed of grafted copolymers. Sens Actuators 9 125-131 Sakai Y, Sadaoka Y, Matsuguchi M (1996) Humidity sensors based on polymer thin films. Sens Actuators B 35-36 85-90... 

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