Humidity-Sensing Materials
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Humidity-Sensing Materials Thomas Hubert
Adsorption of Water Vapor onto Porous Solid Materials Introduction The measurement and control of hum id ity—the concentration of water vapor in air or other gases—is important in many areas, including meteorology, the domestic environment (air conditioners), medicine (respiratory equipment), food production (drying processes), industry (paper, textile, electronics, and automotive), and agriculture (cereal stocking and greenhouses). So, it is not surprising that it has generated a vast number of publications.1 The concentration of water vapor in air or other gases depends on pressure and temperature and must be measured over a wide ränge occupying some nine decades from 1 ppb v to 106 ppm v . Three concentration ranges can bedefined: trace humidity (1 ppb v -l,000 ppm v ); ambient (environmental) humidity (1 X 103 ppm v to 5 X 104 ppm v ); and high humidity (5 X 104 ppm v to 106 ppm v ). Also, water vapor concentration can be expressed in various ways. For example, the absolute concentration can be expressed as the ratio of the mass of water vapor present in a mass or volume unit of moist gas (g kg"' or g m~3). If the water vapor partial pressure rises to the Saturation vapor pressure, a thermodynamic equilibrium exists between water vapor and liquid water at a given total pressure and tem perature. This temperature is the dew point (fd), or frost point, at which dew or ice is formed on cooling a gas in the ränge of -110°C to + 100°C. The relative humidity (RH) can be expressed by the ratio of actual vapor pressure to the Satu ration vapor pressure, expressed as a percentage from 0% to 100% RH. (For ex ample, an RH of 65% at 20°C and 1 atm corresponds to a dew point of 13.2°C, a partial pressure of 1,520 Pa, and an abso lute humidity of 11.3 g irT 3 ).A precise measurement of humidity is comparatively difficult to achieve in practice. Whereas, for example, mass can be mea sured in a typical laboratory to an accuracy of ±1 X 10" 5 , a n d a t m o s p h e r i c pressure to ±5 X 10~4, the accuracy of
MRS BULLETIN/JUNE 1999
humidity determination is commonly limited to ±3% RH.2 The main methods of humidity mea surement are compiled in Table I,2'3 and several other techniques have been proposed which use properties of air that change with humidity, such as the transmission of sound waves (acoustic), absorption and emission of ultraviolet light (Lyman-alpha) or infrared waves, ther mal conductivity, and optical refractive index. Mass spectrometry, gas chromatography, and Carl-Fischer titration down to 30 ppm v or less may also be employed. Of the various hygrometers, only those
The interaction of a solid material and water vapor molecuies is characterized by physical adsorDtion (physisorption) due to van der Waals forces and chemical adsorption (chemisorption) with chemi cal bonds. Most organic polymers can adsorb remarkable amounts of water from the atmosphere. The water molecuies interact with internal hydrogen bonds and polar groups of the macromolecular Compound. A
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