Humidity sensors materials

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. 

A similar perovskite oxide, Bai iLa TiOs, has also been examined as humidity sensor material. The oxide was deposited on a quartz substrate as a film. The performance of a representative sensor film of BaTiOs is presented in fig. 42 (Ardakani et al. 1993). With an increase of relative humidity, the sensor resistance greatly decreases. Also, the sensor response was essentially independent of the film thickness. This indicates that the sensing... 

Multilayered structures play an important role in the production of, e.g., biomaterials, catalysts, corrosion protectors, detectors/diodes, gas and humidity sensors, integral circuits, optical parts, solar cells, and wear protection materials. One of the most sophisticated developments is a head-up-display (HUD) for cars, consisting of a polycarbonate substrate and a series of the layers Cr (25 nm), A1 (150 nm), SiO, (55 nm), TiO, (31 nm), and SiO, (8 nm). Such systems should be characterized by non-destructive analytical methods. 

Fundamental concepts of the molecular layering method have been developed and applied by the team headed by Professor Valentine Aleskovsky in Russia. This method, similar to atomic layer epitaxy and atomic layer deposition, has been used to create monolayers on oxides and polymers as humidity sensors, flame retardants, and agents to enhance sintering in ceramic materials. 

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. 

Nieuwenhuizen and Harteveld [92] have realized a nerve agent dosimeter gas sensor based on the strong interaction between certain metal ions and organophosphorus compounds. In this case, the sensor material contains La(III) 2-bis(carboxymethyl)amino hexadecanoic acid and different factors such as humidity, concentration and layer thickness have been studied and optimized. Using a combination of a metallic complex with a molecular-imprinted polymer, a very sensitive sensor was developed for the detection of soman, a chemical warfare agent (the detection limit was 7 ppt) [93]. The biosensing material is based on a polymer coated onto a fiber-optic probe modified with a luminescent europium detection complex. This complex was... 

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. 

Ceramic materials have an advantage in terms of mechanical strength and thermal and chemical stability. A number of investigations have been carried out on humidity sensors utilizing porous ceramic elements. There have been a number of reviews of ceramic humidity sensors [1, 5], and some examples of recent developments are introduced in this chapter. 

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. 

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... 

P. Su, Y. Sun, C. Lin, Novel low humidity sensor made of Ti02 nanowires/poly(2-acrylamido-2-methylpropane sulfonate) composite material film combined with quartz crystal microbalance , Talanta, 69, 946-951, (2006). 

P. Su and C. Wang, Flexible humidity sensor based on Ti02 nanoparticles-polypyrrole-poly-[3-(methacrylamino)propyl] trimethyl ammonium chloride composite materials. Sens. Actual. B,... 

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