Humidity-Sensitive Materials

There are many types of devices such as mirror-chilled hygrometers, psychrometers, optical sensors, and solid-state sensors which can be used for humidity control (Nitta 1981 Rittersma 2002). Mirror-based dew point sensors have better accuracy, but these devices are more costly to make and use (Chen and Lu 2005). Solid-state humidity sensors can be fabricated at low cost and used for moisture control more conveniently. Solid-state humidity sensors are broadly classified into three types gravimetric (QCM and SAW), resistive, and capacitive, according to the output form of the sensor 

Korotcenkov, Handbook of Gas Sensor Materials Properties, Advantages and Shortcomings for Applications Volume 1 Conventional Approaches, Integrated Analytical Systems, 

Source Reprinted from Chen et al. (2009). Published by MDPl 

In addition to adsorption-chemisorption processes, capillary condensation phenomena are also involved when considering the fine structure of the porous matrix. According to the basic theory of adsorption on a porous matrix (Adamson and Cast 1997), when the vapor molecules are first physico-sorbed onto the porous material, capillary condensation will occur if the micropores are narrow enough. The critical size of pores for a capillary condensation effect is characterized by the Kelvin radius. In the case of water the condensation of vapor into the pores can be expressed by a simplified Kelvin equation (Ponec et al. 1974)  

The values of Kelvin radius, calculated for water vapors in accordance with Eq. (18.1), are presented in Table 18.1. For these calculations, the following data and assumptions were used the contact angle between the liquor and the wall of concave (0) is constant the and y are 1.8x10 L/mol and 72.8 mN/m (20 °C), respectively the temperature (7) is about 293 K and x=cos 0. It is seen that the Kevin radius increases with the relative humidity, and the rate of change (the slope) also increases with relative humidity. This means that the pores with smaller diameter are filled first, while bigger pores are filled later. 

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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. 

An important issue in the construction of humidity sensors is the selection of materials. Among the polymeric humidity-sensitive materials, polyimides represent a good choice because they exhibit excellent thermal stability, low dielectric constant, and low equilibrium moisture content. Conversely, polyimides are highly resistant to irradiation and are mechanically strong and chemically stable in the presence of most common contaminants. 

Xin, Y. and Wang, S. (1994). An investigation of sulfonated polysnUbne humidity-sensitive materials. Sensors <8 Actuators A. 40 147. 

Chlorinated polyolefins (CPOs) also are a mainstay of adhesion promotion to TPO. The CPO generally is applied as a dilute solution (5-35 wt. %) in aromatic solvents to the TPO to act as a tie-coat, to which the topcoat is applied. These materials are often humidity sensitive and only adhere to TPOs with EPDM levels of greater than 5 percent. 

Another problem with these instruments is determination of mechanical properties of moisture sensitive materials (10-12). Such materials have tendency to gain moisture when subjected to mechanical excitation at constant relative humidity and temperature. This makes single frequency tests impractical for isomoisture studies over a range of frequencies. 

If conduction or radiation is the predominant mode of heat transfer, the surface (and possibly the interior) moisture may literally boil regardless of the temperature or the humidity of the environment. This may be readily demonstrated by microwave drying. Thus, if control of granulation temperature is important, direct heat (convection) dryers usually offer greater control and product safety since the material s surface does not exceed the wet-bulb temperature during the steady state period. However, it will be shown later in this chapter that properly controlled dielectric drying may also be used to dry heat sensitive materials. 

This section deals with mechanisms of electrostatic charging and charge relaxation in powders and with the important variables, such as powder size, quantity, temperature, humidity, etc., which affect these phenomena. Also considered are experimental techniques for investigating the electrostatic characteristics of sensitive materials, with particular reference to lead azide. Finally, the application of such information to the safe handling of sensitive substances is discussed. 

Temperature and humidity are controlled where required, to safeguard sensitive materials (raw materials, drugs, samples, reference standards etc.)... 

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... 

According to Nijhuis et al. (1996), freeze drying (known as a suitable dehydration process for pharmaceutical and food products) is not suitable for the production of homogeneous films because the films obtained are generally very spongy. Also, a freeze-dried product tends to be porous and the problem of rapid rehydration may arise once the product is exposed to a more humid environment. Moreover freeze drying is very energy intensive. The equipment is also more expensive than atmospheric pressure dryers. It is best suited for heat-sensitive materials, or when solvent recovery is required, or if there are risks of fire and/or explosion. 

Zhang WG, Yan J, Wang G, Li H-X, Zhang G-S (2009) A natural humidity sensitive two dimensional tunable photonic band gap material and its optic properties. J Inorg Mater 24(l) 57-60 Zhao Y, Zhang Y-N, Wang Q (2011) Research advances of photonic crystal gas and liquid sensors. Sens Actuators B Chem 160 1288-1297... 

Zou J, He HY, Dong JP, Long YC (2004) A guest/host material of LiCl/H-STI (stilbite) zeolite assembly preparation, characterization and humidity-sensitive properties. J Mater Chem 14 2405-2411... 

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