Humidity Sensors: Types, Nanomaterials and Environmental Monitoring
Title: Humidity Sensors: Types, Nanomaterials and Environmental Monitoring
Editor: Christopher T. Okada
Publisher: Nova Science Pub Inc.
Hardcover: 187 pages
Pubdate: July 31, 2011
There is a substantial interest in the development of relative humidity sensors for applications in monitoring moisture levels at home, in clean rooms, cryogenic processes, medical and food science. This new book presents current research in the study of humidity sensors, including measuring methods and standards of water vapor sorption and humidity; environmental and bio-medical applications using quartz crystal microbalance humidity sensors and surface modified electrospun nanofibrous membranes for humidity detection. (Imprint: Nova Press).
Table of Contents
Chapter 1 - Water Vapour Sorption and Humidity: A Survey on Measuring Methods and Standards pp. 1-92.
Authors / Editors: (Erich Robens, Katrin Rübner, Peter Klobes, Devrim Balköse, Institut für Anorganische Chemie und Analytische Chemie der Johannes Gutenberg-Universität, Duesbergweg, Mainz, Germany, and others)
Under environmental conditions water exists in all three classical states of matter: solid, liquid and gas. The water molecule is non-linear and therefore polar. In comparison with other liquids water has anomalous features; about 63 exceptional properties are recorded. This article starts with reviewing properties of water, typical occurrences and definitions such as relative and absolute humidity and moisture content. Water is present everywhere in nature and engineering; it may be helpful or harmful. The survey concerns both: atmospheric hygrometry and usual measuring methods of the moisture content of solids and liquids as well as water sorption. The determination of the atmospheric humidity is among the more difficult problems in metrology. In contrast, humidity determination of materials is simple; however the definition of the dry state is difficult. Because water is bound at and in solids and liquids in many different ways it turns out that the humidity content of materials is difficult to define and to measure accurately. We provide a survey on the measuring methods, describe the most important ones and discuss advantages and accuracy.
Chapter 2 - Environmental and Bio-Medical
Applications using Quartz Crystal Microbalance (QCM) Humidity Sensors
Modified with Nano-Assembled Thin Films
Authors / Editors: (Serhiy Korposh, Roman Selyanchyn, Seung-Woo Lee, Graduate School of Environmental Engineering, the University of Kitakyushu, Wakamatsu, Kitakyushu, Japan)
Nano-assembled thin films prepared by a layer-by-layer approach on QCM resonators were used as sensitive elements for monitoring relative humidity. Different types of films, polymer-based (PDDA/PSS )n film and porphyrin-based (PDDA/TSPP )n and (PDDA/MnTSPP )n films (where n=5, 10, and 15), showed linear responses to relative humidity changes in the range of 4%–94%. An increase of the number of bilayers deposited on the QCM electrode enhanced the sensitivity to relative humidity. The porphyrin-based films showed ca. 4.5 times higher sensitivity towards relative humidity than the polymer-based films, owing to the presence of the higher number of free sulfonic acid groups. Incorporation of the metal ion into the porphyrin pyrrole ring was not significant in sensitivity. However, all films showed a good reversibility to the stepwise changes of relative humidity and the response and recovery times (t90) were very fast, within 15 sec. Additional examples of the QCM humidity sensors for environmental and medical sensing are presented. In particular, a respiratory monitoring system based on QCM frequency oscillations was designed and investigated. The developed system, in which the sensor response reflects lung movements, was able to track human breathing parameters such as respiratory rate, tidal volume, and inspiratory/expiratory flow, which were tested on healthy volunteers.
Chapter 3 - Holographic Humidity Sensors pp. 117-142 (Free download Available)
Authors / Editors: (I. Naydenova, J. Raghavendra, S. Martin, V. Toal, Dublin Institute of Technology, Dublin, Ireland)
This chapter presents holographic devices for sensing the relative humidity in the environment. The principle of operation of two main types of holographic sensors based on transmission and reflection holograms is described and their properties are theoretically modelled. The effect of different factors such as change of the overall refractive index, change of the hologram‘s thickness due to swelling/shrinkage of the layer and change in the refractive index modulation due to absorption/desorption of moisture are considered. In the experimental studies emphasis is given to reflection holographic humidity sensors which can provide a visual indication of the relative humidity observed as a change of the colour of the reflected light. The work presented focuses on holograms recorded in acrylamide based photopolymers. The results from studies of their sensitivity, reversibility, relative humidity range of operation and response time are summarised. The operational temperature range is also studied. The potential use of the humidity sensors as user-verifiable security holograms is described as well as applications in packaging and environmental sensing.
Chapter 4 - Surface Modified Electrospun Nanofibrous Membranes for Humidity Detection Based on Quartz Crystal Microbalance Technique pp.143-160.
Authors / Editors: (Xianfeng Wang, Bin Ding, Jianyong Yu, Gang Sun, State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, China, and others)
Electrospinning exhibits the unique ability to produce diverse forms of fibrous assemblies with remarkable specific surface area and high porosity, making electrospun nanomaterials highly attractive to ultra-sensitive sensors and other nanotechnological applications. In this chapter, electrospun fibrous chitosan (CS) membranes modified with polyelectrolytes (polyacrylic acid (PAA) and polyethyleneimine (PEI)) were used as sensing materials coated on quartz crystal microbalance (QCM) for humidity detection. A series of nanofibrous membranes containing polyelectrolytes are fabricated on QCM and characterized regarding their morphology, sensitivity and hysteresis. Sensing experiments are carried out by measuring the resonance frequency shifts of QCM due to the additional mass loading. The results indicated that the response of the sensors increased by three to four orders of magnitude with increasing the relative humidity (RH) from 8 to 95% at room temperature, exhibiting high sensitivity and that, in the range of 20-95 % RH, the showed a good linearity. Additionally, the prepared sensors exhibited remarkably enhanced sensitivity with increasing the coating loads of polyelectrolytes on QCM. Furthermore, the dynamic Langmuir adsorption model was used to determine the kinetic parameters such as adsorption rates and Gibbs free energy for various RH between 70 and 95 %.
Chapter 5 - Monitoring Relative Humidity using ZnO Based Film Bulk Acoustic-Wave Resonator pp. 161-176.
Authors / Editors: (Xiaotun Qiu, Hongyu Yu, School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, Arizona, USA, and others)
Humidity is a dynamic parameter that is essential for various fields of industry as well as human lives. There is a substantial interest in the development of relative humidity (RH) sensors for applications in monitoring moisture level at home, in clean rooms, cryogenic processes, medical and food science, and so on. Humidity sensors based on changes in the capacitance or resistance of the sensing element from absorption of water vapor have been investigated extensively. Alternatively, surface acoustic wave (SAW) resonant sensors have also been developed with polymer films deposited on top of a SAW resonator as the sensing layer. In this chapter, a novel RH sensing device using ZnO based film bulk acoustic-wave resonator (FBAR) was described. The resonant frequency of the FBAR decreased in a two-stage manner as RH increased in the environment. For low RH (RH < 50 %), a frequency downshift of 2.2 kHz per 1 % RH change was observed. This effect was attributed to water molecules replacing the adsorbed oxygen on the ZnO surface, thus increasing the density of the film, resulting in a frequency drop. For high RH (RH > 50%), a frequency downshift of 8.5 kHz per 1% RH change was obtained, which was due to the mass loading effect of the water layers formed on the ZnO surface. It was demonstrated that the two-stage response of the FBAR can be interpreted using the power law theory for semiconductor gas sensors and the mass loading effect of the resonator. UV light was applied to monitor its effects on the humidity sensing performance of the FBAR. UV can enhance the sensitivity at low RH (frequency downshift increased to 3.4 kHz per 1 % RH change), while degrading the sensitivity at high RH (frequency downshift decreased to 5.7 kHz per 1% RH change). The mechanism of the influence of the UV illumination was interpreted.
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